Alice Pradel

Charlotte Catrouillet

Julien Gigault

The environmental fate of nanoplastics: What we know and what we need to know about aggregation

Keywords: Conceptualization, Formal analysis, Methodology, Writing-Original draft preparation. Charlotte Catrouillet Conceptualization, Writing-Reviewing

The presence of nanoplastics in the environment has been proven. There is now an urgent need to determine how nanoplastics react in the environment and to assess the risks they may pose. Here, we examine nanoplastic homo-and heteroaggregation, with a focus on environmentally relevant nanoplastic particle models. We made a systematic analysis of experimental studies, and ranked the environmental relevance of 377 different solution chemistries, and 163 different nanoplastic particle models. Since polymer latex spheres are not environmentally relevant (due to their monodisperse size, spherical shape, and smooth surface), their aggregation behavior in natural conditions is not transferable to nanoplastics. A few recent studies suggest that nanoplastic particle models that more closely mimic incidentally produced nanoplastics follow different homoaggregation pathways than latex sphere particle models. However, heteroaggregation of environmentally relevant nanoplastic particle models has seldom been studied. Despite this knowledge gap, the current evidence suggests that nanoplastics may be more sensitive to heteroaggregation than previously expected. We therefore provide an updated hypothesis about the likely environmental fate of nanoplastics. Our review demonstrates that it is essential to use environmentally relevant nanoplastic particle models, such as those produced with top-down methods, to avoid biased interpretations of the fate and impact of nanoplastics. Finally, it will be necessary to determine how the heteroaggregation kinetics of nanoplastics impact their settling rate to truly understand nanoplastics' fate and effect in the environment.

Introduction

Plastics have become an inherent component of the modern industrial world, bringing both significant societal benefits and widespread environmental contamination [START_REF] Andrady | Applications and societal benefits of plastics[END_REF]. By 2015, plastics were the third most-produced material, with a production estimated at 8.3 billion metric tonnes [START_REF] Geyer | Production, use, and fate of all plastics ever made[END_REF], roughly twice the mass of all animals on Earth [START_REF] Elhacham | Global human-made mass exceeds all living biomass[END_REF]. Plastic debris originates from mismanaged plastic waste and from the use and aging of plastic materials (e.g., textiles, tire wear, paint coatings, etc.), which incidentally release plastic particles [START_REF] Mitrano | Microplastic regulation should be more precise to incentivize both innovation and environmental safety[END_REF]. As plastics degrade, they produce microplastics (1 µm to 5 mm) [START_REF] Chamas | Degradation Rates of Plastics in the Environment[END_REF][START_REF] Efimova | Secondary Microplastics Generation in the Sea Swash Zone With Coarse Bottom Sediments: Laboratory Experiments[END_REF][START_REF] Gewert | Pathways for degradation of plastic polymers floating in the marine environment[END_REF][START_REF] Song | Rapid Production of Micro-and Nanoplastics by Fragmentation of Expanded Polystyrene Exposed to Sunlight[END_REF], and nanoplastics (<1 µm) [START_REF] Dawson | Turning microplastics into nanoplastics through digestive fragmentation by Antarctic krill[END_REF][START_REF] Gigault | Marine plastic litter: the unanalyzed nano-fraction[END_REF][START_REF] Lambert | Characterisation of nanoplastics during the degradation of polystyrene[END_REF][START_REF] Mattsson | Nanofragmentation of Expanded Polystyrene Under Simulated Environmental Weathering (Thermooxidative Degradation and Hydrodynamic Turbulence)[END_REF]. Consequently, all of the Earth's environmental compartments are contaminated by microplastics: including freshwater lakes [START_REF] Eriksen | Microplastic pollution in the surface waters of the Laurentian Great Lakes[END_REF], rivers [START_REF] Lechner | The Danube so colourful: a potpourri of plastic litter outnumbers fish larvae in Europe's second largest river[END_REF], sediments [START_REF] Klein | Occurrence and Spatial Distribution of Microplastics in River Shore Sediments of the Rhine-Main Area in Germany[END_REF], soils [START_REF] Zubris | Synthetic fibers as an indicator of land application of sludge[END_REF], seas, oceans close to anthropogenic activity [START_REF] Thompson | Lost at Sea: Where Is All the Plastic?[END_REF], and those that are more remote [START_REF] Lusher | Microplastics in Arctic polar waters: the first reported values of particles in surface and sub-surface samples[END_REF], as well as some of the deepest ocean sediments [START_REF] Peng | The ocean's ultimate trashcan: Hadal trenches as major depositories for plastic pollution[END_REF], the atmosphere [START_REF] Allen | Atmospheric transport and deposition of microplastics in a remote mountain catchment[END_REF], and the biosphere [START_REF] Provencher | Assessing plastic debris in aquatic food webs: what we know and don't know about uptake and trophic transfer[END_REF]. Nanoplastics are present in soils [START_REF] Wahl | Nanoplastic occurence in a soil amended with plastic debris[END_REF], freshwater [START_REF] Li | Evaluating the Occurrence of Polystyrene Nanoparticles in Environmental Waters by Agglomeration with Alkylated Ferroferric Oxide Followed by Micropore Membrane Filtration Collection and Py-J o u r n a l P r e -p r o o f Journal Pre-proof GC/MS Analysis[END_REF], seawater (Materić et al., 2022a;[START_REF] Ter Halle | Nanoplastic in the North Atlantic Subtropical Gyre[END_REF][START_REF] Li | Evaluating the Occurrence of Polystyrene Nanoparticles in Environmental Waters by Agglomeration with Alkylated Ferroferric Oxide Followed by Micropore Membrane Filtration Collection and Py-J o u r n a l P r e -p r o o f Journal Pre-proof GC/MS Analysis[END_REF], sea ice (Materić et al., 2022b), and snow [START_REF] Materić | Nanoplastics transport to the remote, high-altitude Alps[END_REF]. They are suspected to be present in many other environmental compartments also, but it is technically challenging to extract, characterize, and quantify plastic particles smaller than ~20 µm from environmental matrices [START_REF] Ivleva | Chemical Analysis of Microplastics and Nanoplastics: Challenges, Advanced Methods, and Perspectives[END_REF].

As nanoplastics have only recently been discovered in the environment [START_REF] Gigault | Marine plastic litter: the unanalyzed nano-fraction[END_REF][START_REF] Ter Halle | Nanoplastic in the North Atlantic Subtropical Gyre[END_REF], and they are difficult to quantify in environmental matrices [START_REF] Gigault | Nanoplastics are neither microplastics nor engineered nanoparticles[END_REF], their effects on organisms and environmental processes is less clear than for microplastics. The colloidal properties of nanoplastics (Brownian motion and high surface reactivity) make them prone to aggregation [START_REF] Gigault | Nanoplastics are neither microplastics nor engineered nanoparticles[END_REF]. They are also prone to leach additives and move through physiological barriers [START_REF] Allan | Examining the Relevance of the Microplastic-Associated Additive Fraction in Environmental Compartments[END_REF][START_REF] Paul | Micro-and nanoplasticscurrent state of knowledge with the focus on oral uptake and toxicity[END_REF] more quickly than larger plastic particles, which can increase organisms' exposure to toxins, compared to the dangers of microand macroplastics that we knew about before. Consequently, the scientific community, the public, and policymakers expressed the urgent need to elucidate the environmental fate and impact of nanoplastics [START_REF] Allan | Global Summit on Regulatory Science 2019 Nanotechnology and Nanoplastics[END_REF]GESAMP, 2015;[START_REF] Sapea | Science Advice for Policy by European Academies)[END_REF].

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In this study, we elucidate relevant points about the nanoplastics ecological crisis [START_REF] Koelmans | Risks of Plastic Debris: Unravelling Fact, Opinion, Perception, and Belief[END_REF][START_REF] Persson | Outside the Safe Operating Space of the Planetary Boundary for Novel Entities[END_REF]Rockström et al., 2009): i) what is known about the environmental fate of nanoplastics; ii)

what further needs to be known (including more research on aggregation mechanisms); and iii) how to improve our understanding in a targeted way.

It is essential to assess the processes by which nanoplastics are transformed, transported, and accumulated in the environment, to assess their environmental fate. This study proposes up-to-date hypotheses of where nanoplastics are located, and where they accumulate in the environment based on a systematic review of nanoplastic homo-and heteroaggregation rates and processes. Aggregation (i.e., both homo-and heteroaggregation) is the focus of this study, because it is the primary process that dictates the transport of colloidal species in the environment. We focus on the behavior of environmentally relevant nanoplastic particle models. We interpret experiments through classical colloidal theory and compare them to studies of engineered nanomaterials. We also highlight a few knowledge gaps and identify strategic and efficient ways to move research forward.

Nanoplastic transport processes

The colloidal properties of nanoplastics

With decreasing size, particles transition away from motion dictated by gravitational forces (i.e., macro-to microparticle behavior) and towards motion dictated by Brownian motion and intermolecular forces (i.e., colloidal behavior) [START_REF] Elimelech | Particle deposition and aggregation: measurement, modelling and simulation[END_REF][START_REF] Hiemenz | Principles of colloid and surface chemistry[END_REF]. The transition to Brownian motion in water occurs when carbon-based particles are smaller than ~1 µm [START_REF] Hiemenz | Principles of colloid and surface chemistry[END_REF]. Given a favorable (attractive) balance of surface interactions and hydrodynamic forces, nanoplastics sorb onto particles and/or molecules. The increase in dimension caused by the sorption of such species onto nanoplastics can cause them take on a gravity-driven motion again, the same as larger particles. Therefore, the colloidal stability of nanoplastics (i.e., their capacity to remain dispersed) is the primary determinant of their environmental fate [START_REF] Filella | Colloidal Properties of Submicron Particles in Natural Waters[END_REF]IUPAC, 1997;[START_REF] Stumm | Aquatic chemistry : chemical equilibria and rates in natural waters[END_REF], and the focus of this review. The review focuses on nanoplastic stability in the hydrosphere, since water has been identified as one of the major transport pathways of plastic debris [START_REF] Brahney | Constraining the atmospheric limb of the plastic cycle[END_REF]. Atmospheric transport processes will not be investigated here, because there are not yet many studies on this topic and since atmospheric plastics are J o u r n a l P r e -p r o o f

Journal Pre-proof likely to re-enter the hydrosphere through precipitation [START_REF] Allen | Microplastics and nanoplastics in the marine-atmosphere environment[END_REF][START_REF] Allen | Atmospheric transport and deposition of microplastics in a remote mountain catchment[END_REF][START_REF] Bergmann | White and wonderful? Microplastics prevail in snow from the Alps to the Arctic[END_REF][START_REF] Wik | Occurrence and effects of tire wear particles in the environment -A critical review and an initial risk assessment[END_REF].

Physicochemical properties of environmental compartments

The stability of colloids is sensitive to the interplay between the properties of the dispersed phase (colloid) and the continuous phase (fluid) [START_REF] Buffle | A Generalized Description of Aquatic Colloidal Interactions: The Three-colloidal Component Approach[END_REF][START_REF] Filella | Colloidal Properties of Submicron Particles in Natural Waters[END_REF][START_REF] Hiemenz | Principles of colloid and surface chemistry[END_REF][START_REF] Stumm | Aquatic chemistry : chemical equilibria and rates in natural waters[END_REF]. It is therefore practical to categorize the environment into different compartments, defined by common physicochemical properties, to predict the fate of nanoplastics. Key parameters to consider are the composition, concentration, shape, and size of colloids and the ionic composition, pH, temperature, and velocity gradients of fluids [START_REF] Baalousha | Effect of nanomaterial and media physicochemical properties on nanomaterial aggregation kinetics[END_REF][START_REF] Hotze | Nanoparticle Aggregation: Challenges to Understanding Transport and Reactivity in the Environment[END_REF][START_REF] Peijnenburg | A Review of the Properties and Processes Determining the Fate of Engineered Nanomaterials in the Aquatic Environment[END_REF]. In the aqueous environment, nanoplastics are more likely to encounter naturally occurring species in concentrations ranging from μg L -1 to mg L -1 (Table 1) [START_REF] Benner | Chapter 3 -Chemical Composition and Reactivity[END_REF][START_REF] Fraters | Extraction of soil solution by drainage centrifugation-effects of centrifugal force and time of centrifugation on soil moisture recovery and solute concentration in soil moisture of loess subsoils[END_REF][START_REF] Libes | Introduction to marine biogeochemistry[END_REF][START_REF] Stumm | Aquatic chemistry : chemical equilibria and rates in natural waters[END_REF], rather than other nanoplastics, since current and anticipated nanoplastic concentrations are currently in the range of pg L -1 to µg L -1 [START_REF] Lenz | Microplastic exposure studies should be environmentally realistic[END_REF][START_REF] Li | Evaluating the Occurrence of Polystyrene Nanoparticles in Environmental Waters by Agglomeration with Alkylated Ferroferric Oxide Followed by Micropore Membrane Filtration Collection and Py-J o u r n a l P r e -p r o o f Journal Pre-proof GC/MS Analysis[END_REF]Materić et al., 2022). Therefore, the stability of nanoplastics is mainly affected by the size and composition of natural species and the concentration and valence of electrolytes. In contaminated environments, anthropogenic species such as pesticides and pharmaceuticals can also interact with nanoplastics to modify their environmental fate and their toxicity [START_REF] Velzeboer | Strong Sorption of PCBs to Nanoplastics, Microplastics, Carbon Nanotubes, and Fullerenes[END_REF][START_REF] Zarfl | Are marine plastic particles transport vectors for organic pollutants to the Arctic?[END_REF].

Natural species have historically been classified based on size, as either solutes (<1 nm and in thermodynamic equilibrium with the solvent), colloids (1 nm to 1 µm), or particles (>1 µm) [START_REF] Buffle | Environmental particles[END_REF][START_REF] Lead | Environmental Colloids and Particles: Current Knowledge and Future Developments[END_REF][START_REF] Stumm | Aquatic chemistry : chemical equilibria and rates in natural waters[END_REF]. However, operational definitions have been adopted and consider molecules that pass through 0.45 μm or 0.22 μm filters as "dissolved". This definition is problematic since this size cut-off randomly splits the colloidal fraction into the dissolved and particulate fractions. Since the literature reviewed does not distinguish particulate, colloidal and truly dissolved species, we chose to remain coherent with the terminology used in the reviewed articles. Therefore, we define dissolved species as electrolytes and molecules smaller than 0.22 or 0.45 μm. Particulate species are defined as solid species with defined geometrical dimensions and a high molar mass (approximately > 10 6 g mol -1 ). This broadened definition includes species smaller than 0.22 m (e.g., engineered nanomaterial).

J o u r n a l P r e -p r o o f

A range of processes can lead to the aggregation of colloids in environmental systems.

The probability of aggregation occurring can be predicted by the classical DLVO (Derjaguin Landau Verwey Overbeek) theory of colloidal stability and its extended version (XDLVO), which model surface interaction energies. The DLVO theory states that colloidal particles can aggregate when the repulsive electrostatic force is less strong than the attractive van der Waals force [START_REF] Derjaguin | Theory of the stability of strongly charged lyophobic sols and of the adhesion of strongly charged particles in solutions of electrolytes[END_REF][START_REF] Verwey | Theory of the stability of lyophobic colloids : the interaction of sol particles having an electric double layer[END_REF]. Repulsive electrostatic force originates from the overlapping of the electronic double layer surrounding particles of equal charge. The global (i.e., average) surface charge of incidental nanoplastics is negative [START_REF] El Hadri | Nanoplastic from mechanically degraded primary and secondary microplastics for environmental assessments[END_REF][START_REF] Blancho | Metal-binding processes on nanoplastics: rare earth elements as probes[END_REF].

The charge of particles is pH-dependent if it originates from hydrolyzable surface functional groups, such as carboxylic acids. As pH nears the point zero of charge, the absolute zeta potential decreases, causing a decrease in electrostatic repulsion [START_REF] Israelachvili | Intermolecular and surface forces. IUPAC, 1997. Colloidally Stable, in: Compendium of Chemical Terminology (the "Gold Book[END_REF][START_REF] Stumm | Aquatic chemistry : chemical equilibria and rates in natural waters[END_REF]. Increasing ionic strength (IS) also screens the electrostatic repulsion between particles of equal charge, thereby reducing their stability. The XDLVO incorporates other interactions, such as hydrophobic (Lewis Acid-Base) interactions which originate from the fact that water molecules form intramolecular hydrogen bonds with other water molecules and with hydrophilic surfaces (e.g., carboxylic acids and phenols). Hydrophobic nanoplastics tend to be less stable in water, and their surface tend to sorb other hydrophobic species. However, not all aggregation processes are covered by the DLVO theory. For example, multivalent counterions (i.e., ions with an opposite charge than the particle) can also attract two oppositely charged surfaces and cause ion-specific interactions with surfaces [START_REF] Labbez | Ion-Ion Correlation and Charge Reversal at Titrating Solid Interfaces[END_REF]. As will be discussed below, the surface roughness of particles and surfaces and steric interactions are not entirely integrated in XDLVO models.

Table 1 presents typical concentrations of major chemical elements in surface water for five generic types of rivers, which were grouped to reflect a diversity of geologies, soil types, land use and climates [START_REF] Hammes | Geographically distributed classification of surface water chemical parameters influencing fate and behavior of nanoparticles and colloid facilitated contaminant transport[END_REF], as well as two soil pore water types [START_REF] Fraters | Extraction of soil solution by drainage centrifugation-effects of centrifugal force and time of centrifugation on soil moisture recovery and solute concentration in soil moisture of loess subsoils[END_REF], and seawater [START_REF] Benner | Chapter 3 -Chemical Composition and Reactivity[END_REF][START_REF] Libes | Introduction to marine biogeochemistry[END_REF]. As illustrated in Figure 1A and B, due to electrostatic repulsion, nanoplastics with a given electronegativity and hydrophobicity are more stable in fresh water types with low IS and with many monovalent ions (e.g., Rivers 1&2 in Table 1) compared to brackish water (e.g., River 6 in Table 1) or seawater (Table 1).

J o u r n a l P r e -p r o o f The effect of (macro)molecules on nanoplastic stability depends on the nature of the (macro)molecules and their concentration, as well as the type and concentration of electrolytes [START_REF] Buffle | A Generalized Description of Aquatic Colloidal Interactions: The Three-colloidal Component Approach[END_REF][START_REF] Yu | Interactions between engineered nanoparticles and dissolved organic matter: A review on mechanisms and environmental effects[END_REF]. While natural organic matter generally has an overall negative charge, some humic substances and proteins contain positively charged sections [START_REF] Buffle | A Generalized Description of Aquatic Colloidal Interactions: The Three-colloidal Component Approach[END_REF].Z. Dong et al., 2020;[START_REF] Dubin | Protein Purification by Selective Phase Separation with Polyelectrolytes[END_REF]X. Li et al., 2021b;[START_REF] Wheeler | Environmental dimensions of the protein corona[END_REF]. On one hand, negatively charged molecules entirely sorbed onto a nanoplastic particle, can stabilize it against aggregation by electrostatic and steric repulsion (i.e., the reduction of entropy and increase in osmotic repulsion as two layers of molecules overlap) [START_REF] Napper | Steric stabilization[END_REF]Fritz et al., 2002) (Figure 1D, top). This layer of sorbed materials, called an eco-corona, is formed spontaneously as nanoplastics enter the environment [START_REF] Wheeler | Environmental dimensions of the protein corona[END_REF]. It lends nanoplastics new properties (i.e., size, shape, surface charge & composition), which can modify its surface interactions and toxicity [START_REF] Kihara | Reviewing nanoplastic toxicology: It's an interface problem[END_REF][START_REF] Paul | Micro-and nanoplasticscurrent state of knowledge with the focus on oral uptake and toxicity[END_REF]. On the other hand, macromolecules can destabilize nanoplastics when their sorption reduces their diffusion coefficient and increases their gravity-driven behavior [START_REF] Buffle | A Generalized Description of Aquatic Colloidal Interactions: The Three-colloidal Component Approach[END_REF][START_REF] Enfrin | Nano/microplastics in water and wastewater treatment processes -Origin, impact and potential solutions[END_REF]. This J o u r n a l P r e -p r o o f occurs particularly when (macro)molecules form bridges between two particles (Fritz et al., 2002) (Figure 1D, bottom). These processes will cause nanoplastics to remain dispersed in water types that contain high dissolved organic carbon (DOC) content and low alkalinity, such as in Rivers 2 and 3 (Table 1). However, this stabilization depends on the geographic and seasonal variability of organic matter concentrations [START_REF] Stumm | Chemical processes regulating the composition of lake waters[END_REF], as well as the chemical properties of the organic matter [START_REF] Buffle | A Generalized Description of Aquatic Colloidal Interactions: The Three-colloidal Component Approach[END_REF].

Finally, particulate species (whether organic, mineral or a mixture of both) can act as surfaces onto which nanoplastics can be attracted and attached in a process called heteroaggregation (Figure 1C). As is the case for engineered nanomaterials, heteroaggregation destabilizes nanoplastics and can accelerate their settling [START_REF] Li | Influences of water properties on the aggregation and deposition of engineered titanium dioxide nanoparticles in natural waters[END_REF][START_REF] Quik | Heteroaggregation and sedimentation rates for nanomaterials in natural waters[END_REF]). Yet despite the importance of particles in the environment, measurement of particle concentrations in natural water is not standard practice and is sometimes only assessed indirectly, for example with measurements of turbidity or dry weight.

J o u r n a l P r e -p r o o f [START_REF] Hammes | Geographically distributed classification of surface water chemical parameters influencing fate and behavior of nanoparticles and colloid facilitated contaminant transport[END_REF], soil pore water [START_REF] Fraters | Extraction of soil solution by drainage centrifugation-effects of centrifugal force and time of centrifugation on soil moisture recovery and solute concentration in soil moisture of loess subsoils[END_REF] and seawater [START_REF] Benner | Chapter 3 -Chemical Composition and Reactivity[END_REF][START_REF] Stumm | Aquatic chemistry : chemical equilibria and rates in natural waters[END_REF]. Ion concentration and ionic strength (IS) are in mmol L -1 , and dissolved organic carbon (DOC) concentration is in mg L -1 (*surface + deep ocean). IS was calculated as J o u r n a l P r e -p r o o f

1 2 ∑ 𝑧 𝑖 2 𝑛 𝑖 𝑛 𝑖=1

Approaches to assess nanoplastic fate

Thorough assessment of the environmental fate of nanoplastics requires three complementary approaches:

i) Undertaking experiments that model the transport of nanoplastics in environmental systems; this approach can identify and quantify the processes that control the environmental fate of nanoplastics [START_REF] Hendren | A functional assay-based strategy for nanomaterial risk forecasting[END_REF]. ii) Numerically modeling the behavior of nanoplastics by combining the rates and probabilities obtained experimentally with estimates of nanoplastic concentrations; numerical simulations can either predict the environmental fate of nanoplastics [START_REF] Besseling | Fate of nano-and microplastic in freshwater systems: A modeling study[END_REF] or model all possible scenarios to determine the key processes controlling the fate of nanoplastics [START_REF] Clavier | Determination of nanoparticle heteroaggregation attachment efficiencies and rates in presence of natural organic matter monomers[END_REF]. iii) Quantifying and characterizing nanoplastics in environmental samples; this field data can be used to assess the validity of experimental and numerical studies.

These approaches have been used successfully to study natural, engineered, and incidental particles [START_REF] Hochella | Natural, incidental, and engineered nanomaterials and their impacts on the Earth system[END_REF]. However, not all approaches are feasible for nanoplastics. For instance, characterization and quantification of nanoplastics in environmental samples is challenging because both nanoplastics and natural organic matter (NOM) are carbon-based, and NOM is orders of magnitude more abundant than nanoplastics (c.f.: Table 1). While most methods cannot quantify nanoplastics in environmental matrices due to the presence of NOM [START_REF] Gigault | Nanoplastics are neither microplastics nor engineered nanoparticles[END_REF][START_REF] Ivleva | Chemical Analysis of Microplastics and Nanoplastics: Challenges, Advanced Methods, and Perspectives[END_REF], recent analytical developments have enabled such quantification (Materić et al., 2022a;[START_REF] Li | Evaluating the Occurrence of Polystyrene Nanoparticles in Environmental Waters by Agglomeration with Alkylated Ferroferric Oxide Followed by Micropore Membrane Filtration Collection and Py-J o u r n a l P r e -p r o o f Journal Pre-proof GC/MS Analysis[END_REF]. Therefore, experimental approaches are currently the dominant way of investigating the environmental fate of nanoplastics. Indeed, 82 out of the 84 studies reported here were experimental studies. Of the two remaining studies, one characterized nanoplastics in the environment and the other modeled nanoplastic interaction with a peptide. For the first field study, polyethylene, polystyrene and polyvinyl chloride nanoplastics were extracted from contaminated soil. This study showed that the nanoplastics formed heteroaggregates with naturally occurring species, but did not quantify nanoplastics [START_REF] Wahl | Nanoplastic occurence in a soil amended with plastic debris[END_REF]. The numerical modeling study showed that oligoalanine, a small model peptide, can undergo conformational changes (denaturation) when it aggregates with low molecular weight PE and polyamide (Nylon) particles [START_REF] Hollóczki | Evidence for protein misfolding in the presence of nanoplastics[END_REF]. The insights obtained from field samples and numerical models concur with the experimental J o u r n a l P r e -p r o o f observations described below. However, more evidence from these approaches would be beneficial to develop a complete understanding of nanoplastic fate. The following section first highlights the major trends in how experiments were designed, then describes the main findings of the experimental studies.

Lessons learned from experiments with nanoplastic particle models

As when studying other nanomaterials in the environment [START_REF] Dale | Modeling Nanomaterial Environmental Fate in Aquatic Systems[END_REF][START_REF] Hammes | Geographically distributed classification of surface water chemical parameters influencing fate and behavior of nanoparticles and colloid facilitated contaminant transport[END_REF], two complementary types of experimental designs are used for nanoplastics: 1) those that aim to gain insight into aggregation mechanisms by incrementally modifying certain parameters of the particle (e.g., size, type of function)

or the matrix (e.g., cations, anions, pH); and 2) those aim to mimic the fate of nanoplastics by selecting environmentally representative nanoplastic particle models and natural matrices (e.g., natural river water, seawater).

On one hand, mechanistic studies identify conditions that cause aggregation, and provide explanations for more environmentally relevant studies. On the other hand, experiments performed with environmentally realistic conditions can be extrapolated to true environmental conditions and can validate mechanistic studies. To bridge the gap between these two types of experimental designs and to better identity where more knowledge is needed, we mapped the environmental relevance of the experiments performed, based on the proposed Framework for Relevance and Methods Evaluation (FRAME) [START_REF] Surette | What is "Environmentally relevant"? A framework to advance research on the environmental fate and effects of engineered nanomaterials[END_REF].

Environmental relevance of nanoplastic particle models and solution chemistries

To assess the nature of the evidence gathered, we reviewed the 82 experimental studies, which assessed the environmental relevance of 163 particles and 377 solution chemistries (i.e., different electrolytes, dissolved and particulate species, without accounting for the different concentrations and pH). The environmental relevance was mapped in a two-dimensional matrix that included one score for the particles' environmental relevance and one score for the solution's environmental relevance. The number of experiments performed with a specific combination of these scores was reported in the matrix (Figure 2 and detailed in Tables 2 and3).

J o u r n a l P r e -p r o o f

The more diverse the mixture of species in a solution, the better it mimics the real environment. This forms the basis of our environmental relevance score. The criteria used to develop the score are primarily a function of the electrolyte composition. The presence of each (macro)molecule or particle adds one point to the score of environmental relevance (Table 2). Artificial water is defined as a mixtures of several electrolytes which mimic natural water. Biological growth media, which are used to culture organisms (e.g., cells, algae etc.) are considered to be artificial water, since they contain a mixture of several electrolytes. In some cases they contain organic molecules, such as vitamins. However, these are present in low concentrations, so the score remained 5, unless (macro)molecules or particles were added. Finally, natural water has a score of 7. Since most natural water types were filtered before being used, the score of environmental relevance was also increased to 8 when (macro)molecules and/or particles were added. While this ranking does not precisely measure increases in environmental relevance, it allows a rapid assessment of the types of experiments performed. The environmental relevance scores for nanoplastic particle models (Figure 2 and Table 3) were based on our understanding of the processes that generate incidental (i.e., secondary) nanoplastics in the environment. Primary nanoplastics (e.g. intentionally produced in the medical or agricultural sectors) were excluded, since their total mass in the environment is expected to be negligible compared to the incidentally released nanoplastics [START_REF] Gigault | Nanoplastics are neither microplastics nor engineered nanoparticles[END_REF]. Nanoplastic particle models are either synthesized with bottom-up or top-down methods.

The bottom-up approach uses molecular components as starting material and links them with chemical reactions to form nanostructures. Lower scores were given to particle models produced from bottom-up synthesis methods for two reasons. First, these methods use surfactants and/or additives which are not expected in incidental J o u r n a l P r e -p r o o f

Journal Pre-proof nanoplastics. The presence of surfactants and additives in the solution reduces the affinity of nanoplastics for other surfaces by masking surface charges and/or causing steric repulsion [START_REF] Fan | Effects of surfactants on graphene oxide nanoparticles transport in saturated porous media[END_REF][START_REF] Jódar-Reyes | Electrokinetic behavior and colloidal stability of polystyrene latex coated with ionic surfactants[END_REF][START_REF] Romero-Cano | Electrosteric Stabilization of Polymer Colloids with Different Functionality[END_REF].

Therefore, this modifies aggregation behavior in experiments and leads to erroneous hypotheses about the environmental fate of nanoplastics. Second, the most common bottom-up synthesis method, emulsion-polymerization, generates smooth, spherical particles with monodisperse sizes, called latex spheres. However, incidental nanoplastics are expected to have rough surfaces, to be non-spherical and polydisperse since they originate from a variety of primary materials and are generated by a variety of processes (e.g., photo-degradation or mechanical fragmentation).

Concerning particles produced from bottom-up methods, some studies use latex spheres grafted with chemical moieties such as carboxylate, amine, and sulfonate. We name latex spheres according to their composition, the functional group that they contain, and their charge. For example, PSL-COOH(-) is a negatively charged polystyrene latex sphere with carboxylic acid (COOH) functional groups on its surface. Each surface functional group has specific ionization and complexation constants. This generates a specific surface charge depending on the solution composition (mainly its pH, IS and the presence of ions that can form complexes). Therefore, the surface functionalization of nanoplastics governs the environmental relevance of studies.

 Particles produced from bottom-up processes that have sulfonate (SO3) and amine (NO2) surface-functionalization were given a score of 1. Additionally, a score of 1 is given to latex spheres that are stabilized by sulfate (SO4) ions (e.g., sodium dodecyl sulfate), although the distinction between SO3 and SO4 is not always clearly indicated. Sulfur is not representative of nanoplastics since it is not a component of plastic polymers, nor is it generated during mechanical abrasion, photo-oxidation or hydrolysis of plastics [START_REF] Mattsson | Nano-plastics in the aquatic environment[END_REF].

Nitrogen is a component of a few polymers (e.g., polyurethane, polyamide, acrylonitrile and acrylonitrile butadiene styrene) [START_REF] Geyer | Production, use, and fate of all plastics ever made[END_REF]International Organization for Standardization, 2013). However, amine functional groups are often grafted in high surface concentrations onto polymer latex spheres compared to concentrations expected in environmental nanoplastics (Molecular Probes, 2004), which warrants giving them a low score of environmental relevance.

 Nanoplastic particle models synthesized without surface functionalization (NF for non-functionalized) or with carboxylic functional groups have a score of 2, J o u r n a l P r e -p r o o f since nanoplastics generated from mechanical degradation retain their original polymer chemistry and those formed by photo-oxidation (and to a lesser extent digestion, and hydrolysis) have carboxylic functional groups [START_REF] Gewert | Pathways for degradation of plastic polymers floating in the marine environment[END_REF][START_REF] Rowenczyk | Microstructure characterization of oceanic polyethylene debris[END_REF].

 Particles produced by bottom-up methods and that were aged in a way that mimics environmental aging processes (e.g., UV-irradiation) have a score of 3.

The top-down approach starts with large objects and reduces their dimensions (generally with physical methods) to achieve nanoscale materials [START_REF] Cao | Nanostructures and nanomaterials: Synthesis, properties and applications[END_REF]. The environmental relevance scores of nanoplastic particle models increases when they are produced from top-down methods since those methods mimic environmental degradation processes. Nanoplastic particle models generated with top-down processes from pre-production plastics or un-aged plastic objects have a score of 4. Nanoplastic particle models generated with top-down methods using plastics found in the environment have a score of 5, since they are formed by environmental degradation and transformation processes. The highest score, 6, was given to nanoplastics extracted from the environment. Using this classification, Figure 2

reveals that :

 There is a consistent lack of environmentally relevant nanoplastic particle models: 91% of the experiments used non-aged nanoplastic particle models produced by bottom-up processes (particle environmental relevance scores of 1 and 2).

 Particles produced with bottom-up processes (scores 1 and 2) have been studied in a range of solution chemistries, to understand mechanisms and also to mimic environmental conditions.

J o u r n a l P r e -p r o o f  Only 6% of nanoplastic particle models produced from bottom-up methods were aged (score 3). 3% of studies used nanoplastic particle models produced with top-down methods and un-aged plastics (score 4).

 No experiment was performed with particle models produced with top-down methods from environmental plastic (score 5). Nor were experiments performed with real nanoplastics (score 6) since these cannot yet be extracted from environmental matrices for experimental investigation. The chemical composition of the nanoplastic dispersions is not environmentally relevant because 83% of the polymer latex spheres dispersions contained surfactants (80% of the nanoplastic dispersions). Furthermore, the composition of the nanoplastic particle models does not reflect the composition of plastic debris. As illustrated in Figure 3, 94% of nanoplastic particle models are composed of PS, and only 6% of plastic waste is composed of PS. Conversely, less than 2% of nanoplastic particle models were composed of either polypropylene (PP) or polyethylene (PE), although these polymers compose about half of the plastic waste generated [START_REF] Geyer | Production, use, and fate of all plastics ever made[END_REF] and are the dominant types of plastic polymer found in the environment [START_REF] Schwarz | Sources, transport, and accumulation of different types of plastic litter in aquatic environments: A review study[END_REF]. 55% of latex spheres used in the literature were functionalized. Carboxylic and amine functional groups were the most common (24% and 26%, respectively). Four studies artificially aged PSL particles by photo-oxidation to increase their environmental relevance [START_REF] Liu | Aggregation kinetics of UV irradiated nanoplastics in aquatic environments[END_REF][START_REF] Mao | Nanoplastics display strong stability in aqueous environments: Insights from aggregation behaviour and theoretical calculations[END_REF][START_REF] Wang | UV-induced aggregation of polystyrene nanoplastics: effects of radicals, surface functional groups and electrolyte[END_REF][START_REF] Xu | Influence of dissolved black carbon on the aggregation and deposition of polystyrene nanoplastics: Comparison with dissolved humic acid[END_REF].

While these particles were increasingly hydrophilic and had a rougher surface, compared to the original PSL, they generally remained spherical (Figures 4a and Figure 4b).

Six nanoplastic particle models were produced using top-down methods: two nanoplastic particle models were produced from laser ablation of un-aged PET and PS (Magrì et al., J o u r n a l P r e -p r o o f 2018; [START_REF] Yu | Aggregation kinetics of different surface-modified polystyrene nanoparticles in monovalent and divalent J o u r n a l P r e -p r o o f Journal Pre-proof electrolytes[END_REF], while four fragmental nanoplastic particle models were produced from mechanical abrasion of un-aged polymers [START_REF] Astner | Effects of soil particles and convective transport on dispersion and aggregation of nanoplastics via small-angle neutron scattering (SANS) and ultra SANS (USANS)[END_REF]S. Dong et al., 2020;[START_REF] Pradel | Stabilization of Fragmental Polystyrene Nanoplastic by Natural Organic Matter: Insight into Mechanisms[END_REF][START_REF] Venel | Environmental Fate Modeling of Nanoplastics in a Salinity Gradient Using a Lab-on-a-Chip: Where Does the Nanoscale Fraction of Plastic Debris Accumulate?[END_REF]. [START_REF] Astner | Effects of soil particles and convective transport on dispersion and aggregation of nanoplastics via small-angle neutron scattering (SANS) and ultra SANS (USANS)[END_REF] Since top-down methods are less controlled than emulsion-polymerization, the morphology of the particles is more heterogenous than that of latex spheres. This is illustrated in Figure 4, which shows high polydispersity in sizes, and the irregular shapes and surface roughness of the fragmental PS nanoplastics (Fig. 4c), compared to aged (Fig. 4b) and non-aged PSL (Fig. 4a). PBAT nanoplastic particle models have a surface roughness of 12 nm, which is significantly higher than PSL's surface roughness of 1 nm [START_REF] Astner | Effects of soil particles and convective transport on dispersion and aggregation of nanoplastics via small-angle neutron scattering (SANS) and ultra SANS (USANS)[END_REF]. This roughness can increase the surface area onto which dissolved species can sorb, thereby modifying the sensitivity of particles to dissolved species. Furthermore, top-down processes do not permit a controlled surface chemistry. This explains the small but significant differences in zeta potential and stability of fragmental PS nanoplastics produced using the same method [START_REF] El Hadri | Nanoplastic from mechanically degraded primary and secondary microplastics for environmental assessments[END_REF][START_REF] Pradel | Stabilization of Fragmental Polystyrene Nanoplastic by Natural Organic Matter: Insight into Mechanisms[END_REF]. So nanoplastic particle models produced from top-down processes must be characterized in depth to gain thorough understanding of what particle properties (e.g., charge, roughness, hydrophobicity) control their behavior. Furthermore, modeling the surface interaction energies of particles that are non-spherical and have rough surfaces, requires the use of the surface element integration (SEI) method [START_REF] Huang | Is Surface Roughness a "Scapegoat" or a Primary Factor When Defining Particle-Substrate Interactions?[END_REF][START_REF] Vold | Van der Waals' attraction between anisometric particles[END_REF]. So far, to the best of our knowledge, the SEI method is only applicable to the van der Waals and electrostatic forces (classical DLVO) and does not integrate hydrophobic forces (XDLVO). So far, all the nanoplastics produced by top-down processes used clean, new plastic objects. On the contrary, nanoplastics in the environment are produced from aged plastics objects, and therefore have different properties (e.g., leached additives, oxidized surface, etc.). A recent method developed by [START_REF] Blancho | A reliable procedure to obtain environmentally relevant nanoplastic proxies[END_REF] to extract nanoplastics from the weathered surface of marine plastic debris is a promising method to generate environmentally relevant nanoplastic particle models. However, one of the important difficulties of extracting nanoplastics from environmental matrices is the process of removing the NOM coating without disturbing the nanoplastic chemistry [START_REF] Treilles | Impacts of organic matter digestion protocols on synthetic, artificial and natural raw fibers[END_REF].

Rapid aggregation of nanoplastic particle models produced with top-down methods

PSL have been used for decades to investigate colloidal processes and develop classical colloidal theory. Unsurprisingly, the conclusions drawn from mechanistic studies of nanoplastic aggregation are largely in agreement with classical colloidal theory. These aggregation mechanisms have been reviewed extensively for PSL [START_REF] Alimi | Microplastics and Nanoplastics in Aquatic Environments: Aggregation, Deposition, and Enhanced Contaminant Transport[END_REF] and for engineered nanomaterials [START_REF] Baalousha | Effect of nanomaterial and media physicochemical properties on nanomaterial aggregation kinetics[END_REF] and therefore will only be briefly described here. To quantify particles' susceptibility to aggregation, studies often determine the critical coagulation concentration (CCC), which is the minimal concentration of a salt required to provoke the maximum aggregation rate of particles.

Conversely, the attachment efficiency of particles, which is the aggregation rate normalized by the maximum aggregation rate in a specific solution chemistry, is useful a useful metric to use in numerical models [START_REF] Grolimund | Aggregation and deposition kinetics of mobile colloidal particles in natural porous media[END_REF]. Although homoaggregation is less environmentally relevant than heteroaggregation, determining the CCC for homoaggregation of nanoplastic particle models is useful to compare their sensitivity to the solution chemistry (e.g., ionic composition, IS, and pH).

Due to their high surface electronegativity, latex spheres are stable up to hundreds of mmol L -1 of monovalent ions in the pH range of natural water types (5 to 8), in agreement with the DLVO theory of colloidal stability. At pH 5, PSL-NF(-) have a CCC of 311 mmol L -1 NaCl, which is typical for CCC values of bottom-up particle models (X. Li et al., 2021b). As the pH increases, particles are increasingly negatively charged, which increases electrostatic repulsion and therefore, increases the CCC. PSL-NF(-) have a

J o u r n a l P r e -p r o o f

CCC of 450 mmol L -1 and 591 mmol L -1 NaCl at pH 6 [START_REF] Liu | Aggregation kinetics of UV irradiated nanoplastics in aquatic environments[END_REF] and 7.5 (Y. Mao et al., 2020), respectively. Also, as PSL-NF(-) were increasingly photo-oxidized (environmental relevance score of 3), their electronegativity increased and their hydrophobicity decreased. This increased their stability in monovalent salts, as predicted by DLVO theory [START_REF] Liu | Aggregation kinetics of UV irradiated nanoplastics in aquatic environments[END_REF][START_REF] Mao | Nanoplastics display strong stability in aqueous environments: Insights from aggregation behaviour and theoretical calculations[END_REF][START_REF] Xu | Influence of dissolved black carbon on the aggregation and deposition of polystyrene nanoplastics: Comparison with dissolved humic acid[END_REF]. However, surfactants can cause PSL to remain stable when the electrical double layer (EDL) at the particle' surface is entirely screened (e.g., > 1000 mmol L -1 monovalent electrolytes) [START_REF] González-Fernández | Do transparent exopolymeric particles (TEP) affect the toxicity of nanoplastics on Chaetoceros neogracile?[END_REF][START_REF] Mao | Nanoplastics display strong stability in aqueous environments: Insights from aggregation behaviour and theoretical calculations[END_REF][START_REF] Yu | Aggregation kinetics of different surface-modified polystyrene nanoparticles in monovalent and divalent J o u r n a l P r e -p r o o f Journal Pre-proof electrolytes[END_REF].

Multivalent ions strongly destabilized nanoplastics with opposite charge. A few mmol L - 1 of multivalent cations can effectively destabilize negatively charged polymer latex spheres, even if they are non-functionalized (NF) (X. Li et al., 2020Li et al., , 2021b;;[START_REF] Liu | Aggregation kinetics of UV irradiated nanoplastics in aquatic environments[END_REF][START_REF] Mao | Nanoplastics display strong stability in aqueous environments: Insights from aggregation behaviour and theoretical calculations[END_REF][START_REF] Wang | Aggregation and stability of sulfate-modified polystyrene nanoplastics in synthetic and natural waters[END_REF][START_REF] Wang | UV-induced aggregation of polystyrene nanoplastics: effects of radicals, surface functional groups and electrolyte[END_REF]. In CaCl2, BaCl2, and MgCl2, PSL-NF(-) had CCCs ranging from ≈ 30 to 40 mmol L -1 at pH 6, and ≈ 60 to 70 mmol L -1 at pH 7.5 [START_REF] Liu | Aggregation kinetics of UV irradiated nanoplastics in aquatic environments[END_REF]Mao et al. (2020). In trivalent cations, destabilization is even stronger with a CCC that is one order of magnitude lower than with divalent ions (Y. Mao et al., 2020). However, as will be explained below, if the speciation of cations is not measured, the actual concentration of electrolytes (especially trivalent cations) is often overestimated, which introduces some error in the determination of the CCC.

Divalent or trivalent ions can bridge particles of opposite charge by forming surface complexes. For example, as the surface oxidation of UV-aged PSL-NF(-) increased, their stability in CaCl2 decreased. This shows that Ca(II) was more likely to create bridges because of the increase in carboxylic functional groups produced by oxidation (Liu et al., 2019).

Nanoplastic particle models produced by top-down methods are globally less stable than polymer latex spheres in both monovalent and divalent electrolytes. For example, the CCC of fragmental PET particles was 54 and 55 mmol L -1 NaCl and KCl, respectively and 2 mmol L -1 for both CaCl2 and MgCl2, which is consistently one order of magnitude lower than for PSL-NF(-) (S. Dong et al., 2020). This reduced stability can be attributed to several differences in particle properties:

i)

The absence of surfactants in the more environmentally relevant particle models decreases stability. For example, after removing surfactants from PSL-NF(-), the CCC of 140 mmol L -1 NaCl at pH 6 [START_REF] Singh | Understanding the stability of nanoplastics in aqueous environments: effect of ionic strength, temperature, dissolved organic matter, clay, and heavy metals[END_REF], whereas PSL-NF(-)with surfactants had a CCC of 591 and 310 mmol L -1 NaCl at pH 7.5 and 7.4, respectively (Y. Mao et al., 2020;[START_REF] Yu | Aggregation kinetics of different surface-modified polystyrene nanoparticles in monovalent and divalent J o u r n a l P r e -p r o o f Journal Pre-proof electrolytes[END_REF].

J o u r n a l P r e -p r o o f

Journal Pre-proof ii)

Mechanical abrasion processes used to produce fragmental nanoplastic particle models generate lower surface charge than functionalized nanoplastic particle models [START_REF] El Hadri | Nanoplastic from mechanically degraded primary and secondary microplastics for environmental assessments[END_REF]. This lower electronegativity reduces electrostatic repulsion and makes them more prone to aggregate. [START_REF] Astner | Effects of soil particles and convective transport on dispersion and aggregation of nanoplastics via small-angle neutron scattering (SANS) and ultra SANS (USANS)[END_REF] noticed aggregation of fragmental PBAT nanoplastic particle models with slight agitation in deionized water. This could be attributed to the particles' relatively low zeta potential (-22 ± 3.6 mV) [START_REF] Astner | Effects of soil particles and convective transport on dispersion and aggregation of nanoplastics via small-angle neutron scattering (SANS) and ultra SANS (USANS)[END_REF]. However, [START_REF] Magrì | Laser Ablation as a Versatile Tool To Mimic Polyethylene Terephthalate Nanoplastic Pollutants: Characterization and Toxicology Assessment[END_REF] noted that PET produced from laser ablation were stable in monovalent salts (CCC = 700 mmol L -1 NaCl). This is explained by the fact that laser ablation can provoke photochemical interactions that mimic environmental photo-degradation [START_REF] Zafiropoulos | UVlaser Ablation of Polymerized Resin Layers and Possible Oxidation Processes in Oil-Based Painting Media[END_REF]. This generates carboxylic functional groups which increase the electronegativity of the particles (-36 mV). iii)

Particles that have nonspherical and irregular shapes are suspected to have lower stability. [START_REF] Veclin | Effect of the Surface Hydrophobicity-Morphology-Functionality of Nanoplastics on Their Homoaggregation in Seawater[END_REF] found that surfactant-free PSL with varying degrees of electronegativity, hydrophobicity and surface roughness are significantly more stable in synthetic seawater than fragmental PS particles. This showed that shape had a greater influence on homoaggregation compared to charge or hydrophobicity. However, in other experiments it is often difficult to separate the effects of surfactants, charge and shape. For example, laser-ablated PS were systematically less stable than PSL-NF(-), PSL-COOH(-) and PSL-NH2(-) containing surfactants, in NaCl and CaCl2 [START_REF] Yu | Aggregation kinetics of different surface-modified polystyrene nanoparticles in monovalent and divalent J o u r n a l P r e -p r o o f Journal Pre-proof electrolytes[END_REF]. Similarly, [START_REF] Pradel | Stabilization of Fragmental Polystyrene Nanoplastic by Natural Organic Matter: Insight into Mechanisms[END_REF] observed that PSL-COOH (-) with surfactants did not aggregate even at 770 mmol L -1 NaCl (at pH 5, 6.5 and 8), whereas fragmental PS nanoplastics had a CCC of 59 and 67 mmol L - 1 NaCl at pH 6.5 and 8, respectively. Interestingly, fragmental PET particles had similar CCC values to fragmental PS particles: 54 and 110 mmol L -1 NaCl at pH 6 and 10, respectively (S. Dong et al., 2020).

So, the CCC of 2 mmol L -1 for both CaCl2 and MgCl2 for fragmental PET particles suggests that in freshwater, nanoplastics are expected to aggregate rapidly in more alkaline conditions (e.g., river water types 5 and 6). The highest aggregation rate is expected in seawater, due to the combination of high IS (≈ 0.7 mol L -1 ), a high concentration of multivalent ions (e.g., Mg 2+ ), and a low NOM content (Table 1). However, the behavior of environmentally relevant nanoplastic particle models in artificial water types (environmental relevance score of 5 and 6) has barely been studied.

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Only one study of laser-ablated PET showed that, similar to polymer latex spheres, PET rapidly aggregated in a biological growth medium containing multivalent ions [START_REF] Magrì | Laser Ablation as a Versatile Tool To Mimic Polyethylene Terephthalate Nanoplastic Pollutants: Characterization and Toxicology Assessment[END_REF]. However, it is unclear whether all top-down particle models follow the same trend. For example, in CaCl2 fragmental PET aggregates less as the pH increases (CCC = 2.1, 4.5 and 5.6 mmol L -1 CaCl2 at pH 6, 8 and 10, respectively) despite having carboxyl and hydroxyl surface groups which can be deprotonated and could bridge with Ca 2+ . This is contrary to what is observed with polymer latex spheres, and shows that we need more evidence on the aggregation of environmentally relevant nanoplastic particle models, especially in artificial water types.

What about elemental speciation?

When performing aggregation experiments, it is imperative to consider the speciation of elements in solution, since ions can form different aqueous complexes as a function of solution pH and ionic composition. This complexation modifies IS and the reactivity and solubility of the species, which in turn modifies aggregation. When ions form covalent bonds, the overall charge of the newly formed aqueous complex is lower than that of the free ions, which reduces the solution's ionic strength. As an example, the IS of an electrolyte composed of Na2SO4 is calculated as

𝐼𝑆 = 0.5([Na + ] + [SO 4 2-] × 2 2 + [NaSO 4 ] + [HSO 4 -] + [OH -] + [H + ]) (1)
whereas without considering ion speciation, the ionic strength is calculated as

𝐼𝑆 = 0.5([Na + ] + [SO 4 2-] × 2 2 ) (2)
Comparing Equations 1 and 2 shows that forgetting to account for aqueous complexes overestimates IS. Furthermore, the presence of HSO4 -shows that the solution pH is an essential parameter of speciation.

Trivalent cations tend to precipitate as hydroxides at intermediate and high pHs [START_REF] Brown | Hydrolysis of metal ions[END_REF]. Therefore, aggregation of PSL-NF(-) in these solutions is more likely caused by heteroaggregation rather than homoaggregation. Mao et al. (2020) performed experiments at pH 7.5 with Al concentrations ranging from 0.25 to 10 mmol L -1 and Fe(III) concentrations from 0.1 to 10 mmol L -1 . We performed geochemical modeling of these experimental conditions using PHREEQC software and sit database, and without considering the potential Al and Fe complexation onto PSL. This predicted that >95% and 100% of Al(III) and Fe(III) are present in solid form due to precipitation. Similarly, Fe(III) is present in a solid form in 0.01 and 1 mmol L -1 FeCl3 at pH 5.4 (data from [START_REF] Cai | Effects of inorganic ions and natural organic matter on the aggregation of nanoplastics[END_REF].

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Journal Pre-proof Geochemical speciation models provide information about solids or complexes that can form in different environments if they contain the equilibrium constants of all complexes and solids considered. For example, [START_REF] Singh | Understanding the stability of nanoplastics in aqueous environments: effect of ionic strength, temperature, dissolved organic matter, clay, and heavy metals[END_REF] calculated that Hg(II) occurred mainly in the Hg(0) form at pH 6, whereas using an updated database [START_REF] Lian | The AQUA-MER databases and aqueous speciation server: A web resource for multiscale modeling of mercury speciation[END_REF], we calculated that Hg(II) is mainly in the HgCl2 and HgOHCl forms. Also, Ramirez

Arenas et al. ( 2020) calculated that Fe(III) mostly occurred as Fe(OH)2 + at pH 8.2. However, by including precipitation as a possible process in the models, we calculated that Fe(III) is mainly present in solid form.

Heteroaggregation must be further investigated

When nanoplastics are incidentally generated from a plastic object and released into the environment, they are most likely to encounter naturally occurring species. In the case where they are released in the technosphere (e.g., landfills or wastewater treatment plants) they will encounter anthropogenic species, such as contaminants or sludge. However, literature shows that nanoplastic aggregation is most often investigated without the addition of (macro)molecules or particles (64% of the experimental conditions reported). A quarter of the experimental conditions contained (macro)molecules, while only 7% contained particulate species and 3% contained both (macro)molecules and particulate species. In solutions with added (macro)molecules and/or particles, figure 5 shows that environmental (macro)molecules are most studied (31%), followed by biological (macro)molecules (30%) and anthropogenic (macro)molecules (16%). Environmental and anthropogenic particles were studied in 16 and 7% of these solutions, respectively. The shortage of heteroaggregation studies is particularly notable for environmentally relevant nanoplastic particle models (scores 4 and 5). Particulate species are present in all ecosystems and abundant in terrestrial ecosystems, so their impact on nanoplastic aggregation must be better understood.

J o u r n a l P r e -p r o o f Nanoplastics heteroaggregate with mineral species of opposite charge due to attractive electrostatic forces. Yu et al. ( 2021) compared the heteroaggregation rates of PSL with various surface functionalizations (NF(-), COOH(-) and NH2(+)), with either hematite (positively charged) or kaolinite (negatively charged). As expected, hematite and kaolinite strongly heteroaggregated with nanoplastics of opposite charge, in both NaCl and in CaCl2. In the pH range of natural water, PSL-COOH(-) only aggregated with the positively charged magnetite, goethite, and Al2O3 minerals, but did not aggregate with negatively charged kaolinite, montmorillonite, MnO2 and SiO2 [START_REF] Zhang | Charge mediated interaction of polystyrene nanoplastic (PSNP) with minerals in aqueous phase[END_REF].

Nanoplastics also heteroaggregate with particles of equal but lower surface charge. [START_REF] Singh | Understanding the stability of nanoplastics in aqueous environments: effect of ionic strength, temperature, dissolved organic matter, clay, and heavy metals[END_REF] noted that PSL-NF(-) heteroaggregated with bentonite (montmorillonite) clay which had a lower negative charge [START_REF] Singh | Understanding the stability of nanoplastics in aqueous environments: effect of ionic strength, temperature, dissolved organic matter, clay, and heavy metals[END_REF]. This heteroaggregation can also be due to the presence of some positively charged edges on the bentonite clay, and to the hydrophobic nature of bentonite [START_REF] Worrall | Clays: their nature, origin and general properties[END_REF]. PSL-NF(-) were also found to sorb onto slightly negatively charged (-7 mV) flocs of biologically active sludge from a wastewater treatment plant, through van der Waals attraction and hydrophobic interactions (H. Li et al., 2020). Interestingly a more environmentally

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Journal Pre-proof relevant nanoplastic particle model (fragmental PBAT, score 4) strongly heteroaggregates with vermiculite even in the absence of electrolytes [START_REF] Astner | Effects of soil particles and convective transport on dispersion and aggregation of nanoplastics via small-angle neutron scattering (SANS) and ultra SANS (USANS)[END_REF].

With the addition of (macro)molecules, heteroaggregation processes are less easy to predict, but generally point to increased stability. Suspended sediments, which are natural mixtures of organic and mineral species extracted from a river bottom, significantly heteroaggregated with PSL-SO4(-) and induced settling in 50 to 500 mmol L -1 NaCl. The addition of 10 mg L -1 humic acid (HA) decreased the heteroaggregation rate due to steric repulsion [START_REF] Li | Interactions between nano/micro plastics and suspended sediment in water: Implications on aggregation and settling[END_REF]. Even the addition of low concentrations of HA (1 mgC L -1 , which is lower than the dissolved organic carbon content of rivers in Table 1) reduced heteroaggregation rates between negatively charged PSL and positive hematite, in up to 300 mmol L -1 NaCl [START_REF] Yu | Heteroaggregation of different surfacemodified polystyrene nanoparticles with model natural colloids[END_REF]. Similar behavior was observed for multi walled carbon nanotubes, since 0.5 mg L -1 HA inhibited their heteroaggregation with hematite nanoparticles in 100 mmol L -1 NaCl [START_REF] Huynh | Heteroaggregation of Multiwalled Carbon Nanotubes and Hematite Nanoparticles: Rates and Mechanisms[END_REF].

Overall, current evidence suggests most natural rivers have NOM concentrations that can effectively slow down heteroaggregation, at least in monovalent salts (i.e., >1mgC L -1 in Table 1). However, in soil pore water and in freshwater bodies that contain high concentrations of suspended sediments and low concentrations of HA (e.g., after high intensity rainfall events, in agricultural watersheds with exposed soils), nanoplastics could quickly heteroaggregate with naturally occurring particles [START_REF] Praetorius | Development of Environmental Fate Models for Engineered Nanoparticles-A Case Study of TiO2 Nanoparticles in the Rhine River[END_REF].

In terrestrial ecosystems, nanoplastics are expected to be closely associated with wastewater treatment for biosolids, for example, which can lead to important cotransport of nanoplastics with biosolids, in sludge-amended soils [START_REF] Keller | Transport of nano-and microplastic through unsaturated porous media from sewage sludge application[END_REF]. The recently developed suspended particulate matter models could be used to provide environmentally-relevant rates of heteroaggregation [START_REF] Walch | Generation of Reproducible Model Freshwater Particulate Matter Analogues to Study the Interaction with Particulate Contaminants[END_REF].

Interactions between nanoplastics, dissolved and particulate matter have not been investigated with divalent or multivalent cations, although these cations can have strong destabilizing effects even in the presence of environmental or biological (macro)molecules (c.f.: following section). Furthermore, these conclusions are drawn from nanoplastic particle models with low environmental relevance (score 1 or 2). The absence of heteroaggregation studies using environmentally relevant nanoplastic particle models leads to uncertainty about true heteroaggregation and settling rates.

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(Macro)molecules can either stabilize or destabilize nanoplastics

Since the stabilizing effect of (macro)molecules against heteroaggregation has seldom been investigated, we can gain insight by investigating the stabilizing effect of (macro)molecules against homoaggregation. In monovalent electrolytes, (macro)molecules have a stabilizing effect on nanoplastics due to electrostatic repulsion and, when sorbed onto particles, due to steric repulsion [START_REF] Liu | Aggregation kinetics of UV irradiated nanoplastics in aquatic environments[END_REF][START_REF] Mao | Nanoplastics display strong stability in aqueous environments: Insights from aggregation behaviour and theoretical calculations[END_REF][START_REF] Singh | Understanding the stability of nanoplastics in aqueous environments: effect of ionic strength, temperature, dissolved organic matter, clay, and heavy metals[END_REF]. Various types of organic macromolecules effectively increased the stability of 10 mg L -1 PSL-NF(-) in 500 mmol L-1 NaCl at pH 6 [START_REF] Liu | Influence of environmental and biological macromolecules on aggregation kinetics of nanoplastics in aquatic systems[END_REF]. The stabilizing effect was most pronounced for HA, followed by bovine serum albumin (BSA), a protein which is abundant in blood plasma, extracellular polymeric substances, and sodium alginate (SA). Stabilization was attributed to steric repulsion, as supported by the measurement of the eco-corona thickness by XPS, while differences in stability were attributed to the conformation of the different macromolecules. Interestingly, extracellular polymeric substances, which contain a mixture of proteins and polysaccharides, have a stabilizing effect intermediate between BSA and the polysaccharide SA [START_REF] Liu | Influence of environmental and biological macromolecules on aggregation kinetics of nanoplastics in aquatic systems[END_REF]. Similar trends in stability were observed for more environmentally relevant nanoplastic particle models. Fragments of PS particles were more effectively stabilized by HA than SA [START_REF] Pradel | Stabilization of Fragmental Polystyrene Nanoplastic by Natural Organic Matter: Insight into Mechanisms[END_REF]. The addition of fetal bovine serum, which contains a mixture of biological molecules (e.g., hormones, lipids, sugars, etc.), increased the CCC of laser-ablated PET from 700 to over 4 000 mmol L -1 NaCl [START_REF] Magrì | Laser Ablation as a Versatile Tool To Mimic Polyethylene Terephthalate Nanoplastic Pollutants: Characterization and Toxicology Assessment[END_REF].

In the presence of divalent counterions, macromolecules either stabilized or destabilized nanoplastic particle models, depending on the relative concentration of divalent counterions, macromolecules and nanoplastics, the surface functional groups of the nanoplastics, and the macromolecule chemical and physical properties. For example, in 20 mmol L -1 CaCl2 at pH 6, the homoaggregation rate of 10 mg L -1 PSL-NF(-) alone was lower than with (macro)molecules. Then, homoaggregation rate was higher in 1 and 2 mgC L -1 of BSA and HA compared to 5 mgC L -1 [START_REF] Liu | Influence of environmental and biological macromolecules on aggregation kinetics of nanoplastics in aquatic systems[END_REF]. This was due to bridging at low concentrations of (macro)molecules, and by a more complete coverage of particle surface at higher (macro)molecule concentration which increases steric repulsion. Similarly, at pH 7.4, 1 mgC L -1 HA increased aggregation of 10 mg L -1 PSL-NF(-) and larger concentrations (5 and 10 mgC L -1 ) reduced aggregation in CaCl2 [START_REF] Yu | Aggregation kinetics of different surface-modified polystyrene nanoparticles in monovalent and divalent J o u r n a l P r e -p r o o f Journal Pre-proof electrolytes[END_REF].

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Journal Pre-proof For more environmentally relevant particles, 1 mg L -1 HA was enough to increase stability of 10 mg L -1 fragmental PET in both NaCl (Figure 6b) and CaCl2 at pH 6. This suggests that even in rivers with higher IS and hardness (3, 4 and 5), and in soil porewater, low concentrations of NOM can increase nanoplastic stability. However, whether these molecules effectively reduce heteroaggregation rates remains to be verified. A couple of considerations should be kept in mind about the stabilizing effect of (macro)molecules. First, it is essential to reason in terms of particle number and surface area to compare studies. Collision rates are particle-number-dependent and (macro)molecule's sorption capacity is surface area-dependent. For nanoplastic particle models produced from top-down processes, a rigorous analysis of size distribution using several complementary techniques is essential to convert the mass of particles to the number and surface area of particles [START_REF] Baalousha | Rationalizing Nanomaterial Sizes Measured by Atomic Force Microscopy, Flow Field-Flow Fractionation, and Dynamic Light Scattering: Sample Preparation, Polydispersity, and Particle Structure[END_REF][START_REF] Caputo | Measuring particle size distribution and mass concentration of nanoplastics and microplastics: addressing some analytical challenges in the sub-micron size range[END_REF]. Second, while studying a range of (macro)molecule concentrations provides insight into J o u r n a l P r e -p r o o f mechanisms, ultimately, mass concentrations of naturally occurring species that are lower or equal to nanoplastic mass concentrations has little environmental relevance. Indeed, bridging two nanoplastics by an organic (macro)molecule is an unlikely event, except at the surface of an aging plastic object. Finally, the eco-corona should also be tightly adsorbed onto the particle surface for steric repulsion to be effective in the environment, since other naturally occurring species (e.g., ligands, metal oxides, etc.) can compete for these (macro)molecules. Modeling steric repulsion provided by the ecocorona with the XDLVO theory is complicated by the fact that the solubility of molecules and the thickness of the coating must be known, but are difficult to determine for natural molecules in environmental systems [START_REF] Fritz | Electrosteric Stabilization of Colloidal Dispersions[END_REF][START_REF] Napper | Steric stabilization[END_REF]. Indeed, as nanoplastics travel through different compartments (e.g., rivers, estuaries, ingestion by organisms, biofilms) their eco-corona evolves, thereby modifying their chemical identity and their environmental fate [START_REF] Wheeler | Environmental dimensions of the protein corona[END_REF].

Nanoplastic aggregation in natural water types

Studying nanoplastic aggregation and settling in natural water types is essential to validate the mechanisms investigated in simpler solutions and to confirm our hypothesis about the environmental fate of nanoplastics. 37% of the experiments used natural water types (with environmental relevance scores of 7 and 8). Most (65%) of the studies were done with freshwater (mostly lakes, rivers, and aquifers, with only a few studies of rainwater and wastewater) and 35% of natural water samples came from seas and oceans (Figure 7). However, all of the experiments involved non-aged polymer latex spheres as the nanoplastic particle model (scores of 1 and 2).

J o u r n a l P r e -p r o o f While negatively charged PSL aggregate rapidly in seawater [START_REF] Grassi | Interplay between extracellular polymeric substances (EPS) from a marine diatom and model nanoplastic through eco-corona formation[END_REF][START_REF] Manfra | Comparative ecotoxicity of polystyrene nanoparticles in natural seawater and reconstituted seawater using the rotifer Brachionus plicatilis[END_REF][START_REF] Tallec | Surface functionalization determines behavior of nanoplastic solutions in model aquatic environments[END_REF], their aggregation in freshwater is very dependent on the IS, on the concentrations of divalent cations, and of environmental and biological (macro)molecules (X. Li et al., 2021a;[START_REF] Wang | Aggregation and stability of sulfate-modified polystyrene nanoplastics in synthetic and natural waters[END_REF]. Indeed, [START_REF] Manfra | Comparative ecotoxicity of polystyrene nanoparticles in natural seawater and reconstituted seawater using the rotifer Brachionus plicatilis[END_REF] observed rapid aggregation of PSL COOH(-) in Mediterranean seawater, which had a high organic carbon concentration (13 mg L -1 ) despite having been filtered at 0.45 m.

Other studies found that extracellular polymeric substances, decreased the aggregation rate of both PSL-NF(-) [START_REF] Summers | Agglomeration of nano-and microplastic particles in seawater by autochthonous and de novo-produced sources of exopolymeric substances[END_REF] and PSL-COOH(-) [START_REF] Grassi | Interplay between extracellular polymeric substances (EPS) from a marine diatom and model nanoplastic through eco-corona formation[END_REF] in seawater. Interestingly, PLS-NH2(+) was often stable in seawater [START_REF] Della Torre | Accumulation and Embryotoxicity of Polystyrene Nanoparticles at Early Stage of Development of Sea Urchin Embryos Paracentrotus lividus[END_REF][START_REF] Tallec | Surface functionalization determines behavior of nanoplastic solutions in model aquatic environments[END_REF]. This stresses the fact that using nanoplastic particle models that are not environmentally relevant can lead to incorrect conclusions about their environmental fate.

In the presence of CeO2 nanoparticles, negatively charged PSL aggregated rapidly in seawater. In freshwater they were generally more stable: no heteroaggregation was observed in river water, while some heteroaggregation was observed in lake water that had a low pH (5.35) and groundwater that had a very high IS (712 mmol L -1 ) (X. Li et al., 2020). In another study, PSL-NF(-) was more stable against homoaggregation in groundwater than in river water. While the IS of the groundwater was approximately J o u r n a l P r e -p r o o f double the IS of the river water, the organic carbon concentration was almost double in the groundwater (3.33 mg L -1 vs. 1.90 mg L -1 in the river), which provided a stabilizing effect [START_REF] Singh | Understanding the stability of nanoplastics in aqueous environments: effect of ionic strength, temperature, dissolved organic matter, clay, and heavy metals[END_REF]. Therefore, nanoplastics are expected to have longer residence times in freshwater types that are near neutral, and have a high NOM content, and a low water hardness (e.g., rivers 1, 2 and 3).

What we know about nanoplastic aggregation and what we need to know

We use the evidence gathered, to propose some hypotheses about nanoplastic aggregation pathways and their subsequent environmental fates. These are shown in Figure 8. Some uncertainty is caused by current knowledge gaps and illustrated with question marks. Homoaggregation has been shaded to illustrate that it has a low probability of occurring in the environment due to low nanoplastic concentrations (parts A and B, Fig. 8). However, studying homoaggregation in the presence of (macro)molecules has shown that the homoaggregation rate of nanoplastics is highly sensitive to the nature and concentration of dissolved species (part D, Fig. 8). While it has been shown that (macro)molecules, such as HA, can effectively stabilize these particles in simple solution chemistries, the effect of (macro)molecules on heteroaggregation in artificial and natural water types must be investigated. There are very few studies of heteroaggregation, and only one study with environmentally relevant nanoplastic particle models. Despite this, current evidence suggests that particulate species strongly heteroaggregate with nanoplastics and may accelerate the settling rate (part C, Fig. 8).

J o u r n a l P r e -p r o o f To gain a full picture of the environmental fate of nanoplastics, we suggest focusing on environmentally relevant nanoplastics, instead of exploring all particle properties. Nanoplastic particle models produced from pre-production plastics or un-aged plastic objects (environmental relevance score 4) are more sensitive to aggregation than commonly used polymer latex spheres (scores 1 to 3). To understand whether nanoplastics that are released from weathered plastic debris are also more sensitive to aggregation would require nanoplastic particle models produced with top-down methods from environmental plastics (score of 5). Methods to generate these nanoplastic particle models are being developed and provide hope for a better quantification of aggregation rates [START_REF] Blancho | A reliable procedure to obtain environmentally relevant nanoplastic proxies[END_REF][START_REF] Villacorta | A new source of representative secondary PET nanoplastics. Obtention, characterization, and hazard evaluation[END_REF].

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Important insight can be gained by comparing the aggregation and settling rates of nanoplastics with those of natural colloids with similar physicochemical properties. For example, the body of knowledge about the environmental fate of engineered or natural carbon particles (e.g., graphene or carbon black, respectively) can inform us about the fate of nanoplastics [START_REF] Sigmund | Environmental transformation of natural and engineered carbon nanoparticles and implications for the fate of organic contaminants[END_REF]. To develop a better understanding of aggregation processes requires a detailed characterization of the dissolved, colloidal, and particulate species present. Studies should move beyond considering filtration as a method that creates homogenous solutions and dispersions, and instead characterize the speciation of ionic species and the molecular weight, surface potential, and polydispersity of colloids.

A relationship between the nanoplastic heteroaggregation state and settling rates needs to be established to quantify nanoplastic flux to sediments [START_REF] Waldschläger | Learning from natural sediments to tackle microplastics challenges: A multidisciplinary perspective[END_REF][START_REF] Zhang | Derivation of settling velocity, eddy diffusivity and pick-up rate from field-measured suspended sediment concentration profiles in the horizontally uniform but vertically unsteady scenario[END_REF]. Indeed, the speed at which aggregates settle is highly dependent on aggregate porosity and fractal dimension [START_REF] Johnson | Settling Velocities of Fractal Aggregates[END_REF]. Without agitation, some aggregates can remain stable in a water column for a certain time [START_REF] Rowenczyk | Heteroaggregates of Polystyrene Nanospheres and Organic Matter: Preparation, Characterization and Evaluation of Their Toxicity to Algae in Environmentally Relevant Conditions[END_REF][START_REF] Wegner | Effects of nanopolystyrene on the feeding behavior of the blue mussel ( Mytilus edulis L.)[END_REF]. With turbulence, shear stress breaks aggregates, thereby resuspending them [START_REF] Adler | A Study of disaggregation effects in sedimentation[END_REF][START_REF] Chen | The impact of nanoplastics on marine dissolved organic matter assembly[END_REF][START_REF] Summers | Agglomeration of nano-and microplastic particles in seawater by autochthonous and de novo-produced sources of exopolymeric substances[END_REF]. Therefore, investigating nanoplastic aggregation in dynamic systems can reveal insights about natural settling rates.

Developing the XDLVO model by incorporating the effects of surface roughness, nonspherical shape, and steric stabilization by macromolecules will help to advance the study of homo-and heteroaggregation. The XDLVO model is limited to van der Waals, electrostatic and hydrophobic interactions for smooth and spherical particles.

Interactions between nonspherical and/or rough particles does not integrate hydrophobic or steric interactions. Coupling chemical interactions (e.g., surface complexation) with the XLDVO physical interactions would greatly improve our ability to predict particle aggregation in natural water.

Finally, the development of numerical models underpinned by high-quality experimental research is a promising way to assess the fate of nanoplastics in the environment [START_REF] Besseling | Fate of nano-and microplastic in freshwater systems: A modeling study[END_REF][START_REF] Li | Synergistic effect of polystyrene nanoplastics and contaminants on the promotion of insulin fibrillation[END_REF][START_REF] Lins | Nanoplastic State and Fate in Aquatic Environments: Multiscale Modeling[END_REF][START_REF] Wang | Aggregation and stability of sulfate-modified polystyrene nanoplastics in synthetic and natural waters[END_REF]. The experimental designs and the metrics used (e.g., collision rate and attachment efficiency) should be discussed with modelers to make sure results can be used in their models. Experimental studies should report the experimental and analytical conditions used J o u r n a l P r e -p r o o f according to the suggested guidelines [START_REF] Kokalj | Quality of nanoplastics and microplastics ecotoxicity studies: Refining quality criteria for nanomaterial studies[END_REF] to facilitate comparison of results.

Our review of homoaggregation studies shows that nanoplastic particle models that are more environmentally relevant have higher attachment efficiencies. This is also expected to be the case for heteroaggregation. This has important implications since numerical simulations of the transport of nanoplastics are sensitive to attachment efficiencies. For example, a study that modeled the transport and retention of micro-and nanoplastic heteroaggregates in a river showed that increasing the attachment efficiency above 0.01 significantly reduced the retention of plastic particles smaller than 4 m in a riverbed [START_REF] Besseling | Fate of nano-and microplastic in freshwater systems: A modeling study[END_REF]. In parallel to these experimental and modeling efforts, overcoming the analytical challenges to quantify nanoplastics in environmental matrices will be essential to verify our hypothesis about the environmental fate of nanoplastics [START_REF] Ivleva | Chemical Analysis of Microplastics and Nanoplastics: Challenges, Advanced Methods, and Perspectives[END_REF].

Method

All original research articles that were present in the Web of Science search engine on August 16 th 2021 and containing the words nanoplastic(s) and stability, or nanoplastic(s) and aggregation, or nanoplastic(s) and agglomeration in either the article's title, abstract or keywords were collected.

This resulted in 87 articles, of which 3 were excluded. Two of them were excluded because they did not actually investigate the aggregation of nanoplastics, while one pertained to the aggregation at the interface between plant leaves and air. Of the remaining 84 articles, only two were not experimental studies. Within the 82 experimental studies a total of 163 different particle types were studied [START_REF] Alimi | Exposure of nanoplastics to freeze-thaw leads to aggregation and reduced transport in model groundwater environments[END_REF][START_REF] Astner | Effects of soil particles and convective transport on dispersion and aggregation of nanoplastics via small-angle neutron scattering (SANS) and ultra SANS (USANS)[END_REF][START_REF] Bergami | Polystyrene nanoparticles affect the innate immune system of the Antarctic sea urchin Sterechinus neumayeri[END_REF][START_REF] Cai | Effects of inorganic ions and natural organic matter on the aggregation of nanoplastics[END_REF][START_REF] Canesi | Interactions of cationic polystyrene nanoparticles with marine bivalve hemocytes in a physiological environment: Role of soluble hemolymph proteins[END_REF][START_REF] Catrouillet | Speciation of metals in solution: a new paradigm for the nanoplastics aggregation evaluation[END_REF][START_REF] Chen | The impact of nanoplastics on marine dissolved organic matter assembly[END_REF][START_REF] Chen | Sedimentation of nanoplastics from water with Ca/Al dual flocculants: Characterization, interface reaction, effects of pH and ion ratios[END_REF][START_REF] Cunha | Microalgal-based biopolymer for nano-and microplastic removal: a possible biosolution for wastewater treatment[END_REF][START_REF] De Oliveira | Interaction of Cyanobacteria with Nanometer and Micron Sized Polystyrene Particles in Marine and Fresh Water[END_REF][START_REF] Della Torre | Accumulation and Embryotoxicity of Polystyrene Nanoparticles at Early Stage of Development of Sea Urchin Embryos Paracentrotus lividus[END_REF]S. Dong et al., 2020;Dong et al., 2019aDong et al., , 2019b;;Z. Dong et al., 2020;[START_REF] Eliso | Toxicity of nanoplastics during the embryogenesis of the ascidian Ciona robusta (Phylum Chordata)[END_REF][START_REF] Fadare | Humic acid alleviates the toxicity of polystyrene nanoplastic particles to Daphnia magna[END_REF][START_REF] Frankel | Controlled protein mediated aggregation of polystyrene nanoplastics does not reduce toxicity towards Daphnia magna[END_REF][START_REF] Ganguly | Ice Nucleation of Model Nanoplastics and Microplastics: A Novel Synthetic Protocol and the Influence of Particle Capping at Diverse Atmospheric Environments[END_REF][START_REF] González-Fernández | Do transparent exopolymeric particles (TEP) affect the toxicity of nanoplastics on Chaetoceros neogracile?[END_REF][START_REF] Grassi | Interplay between extracellular polymeric substances (EPS) from a marine diatom and model nanoplastic through eco-corona formation[END_REF][START_REF] Khoshnamvand | Toxic effects of polystyrene nanoplastics on microalgae Chlorella vulgaris: Changes in biomass, photosynthetic pigments and morphology[END_REF][START_REF] Kihara | Soft and Hard Interactions between Polystyrene Nanoplastics and Human Serum Albumin Protein Corona[END_REF][START_REF] Li | Synergistic effect of polystyrene nanoplastics and contaminants on the promotion of insulin fibrillation[END_REF]H. Li et al., 2020;X. Li et al., 2021aX. Li et al., , 2021bX. Li et al., , 2020;;[START_REF] Li | Interactions between nano/micro plastics and suspended sediment in water: Implications on aggregation and settling[END_REF][START_REF] Li | Impacts of nanoplastics on bivalve: Fluorescence tracing of organ accumulation, oxidative stress and damage[END_REF][START_REF] Liu | Aggregation and Deposition Kinetics of Polystyrene Microplastics and Nanoplastics in Aquatic Environment[END_REF][START_REF] Liu | Sorption of polycyclic aromatic hydrocarbons to polystyrene nanoplastic: Sorption of PAHs to nanoplastic[END_REF][START_REF] Liu | Influence of environmental and biological macromolecules on aggregation kinetics of nanoplastics in aquatic systems[END_REF][START_REF] Liu | Aggregation kinetics of UV irradiated nanoplastics in aquatic environments[END_REF][START_REF] Liu | Functionalized polystyrene nanoplastic-induced energy homeostasis imbalance and the immunomodulation dysfunction of marine clams ( Meretrix meretrix ) at environmentally relevant concentrations[END_REF]Q. Liu et al., 2021;[START_REF] Magrì | Laser Ablation as a Versatile Tool To Mimic Polyethylene Terephthalate Nanoplastic Pollutants: Characterization and Toxicology Assessment[END_REF][START_REF] Manfra | Comparative ecotoxicity of polystyrene nanoparticles in natural seawater and reconstituted seawater using the rotifer Brachionus plicatilis[END_REF][START_REF] Mao | Nanoplastics display strong stability in aqueous environments: Insights from aggregation behaviour and theoretical calculations[END_REF][START_REF] Mekaru | Effect of Agitation Method on the Nanosized Degradation of Polystyrene Microplastics Dispersed in Water[END_REF][START_REF] Mishra | Distinctive impact of polystyrene nano-spherules as an emergent pollutant toward the environment[END_REF][START_REF] Murano | Interplay Between Nanoplastics and the Immune System of the Mediterranean Sea Urchin Paracentrotus lividus[END_REF] et al., 2017;[START_REF] Okshevsky | Biofilm formation by marine bacteria is impacted by concentration and surface functionalization of polystyrene nanoparticles in a species-specific manner[END_REF][START_REF] Oriekhova | Heteroaggregation of nanoplastic particles in the presence of inorganic colloids and natural organic matter[END_REF][START_REF] Pradel | Stabilization of Fragmental Polystyrene Nanoplastic by Natural Organic Matter: Insight into Mechanisms[END_REF][START_REF] Ramirez Arenas | Coagulation of TiO 2 , CeO 2 nanoparticles, and polystyrene nanoplastics in bottled mineral and surface waters. Effect of water properties, coagulant type, and dosage[END_REF][START_REF] Ramirez | Behavior of TiO2 and CeO2 Nanoparticles and Polystyrene Nanoplastics in Bottled Mineral, Drinking and Lake Geneva Waters. Impact of Water Hardness and Natural Organic Matter on Nanoparticle Surface Properties and Aggregation[END_REF][START_REF] Reynolds | Evaluation of non-invasive toxicological analysis of nano-polystyrene in relative in vivo conditions to D. magna[END_REF][START_REF] Ripken | Response of Coral Reef Dinoflagellates to Nanoplastics under Experimental Conditions Suggests Downregulation of Cellular J o u r n a l P r e -p r o o f Journal Pre-proof Metabolism[END_REF][START_REF] Rowenczyk | Heteroaggregates of Polystyrene Nanospheres and Organic Matter: Preparation, Characterization and Evaluation of Their Toxicity to Algae in Environmentally Relevant Conditions[END_REF][START_REF] Saavedra | Influence of nanoplastic surface charge on ecocorona formation, aggregation and toxicity to freshwater zooplankton[END_REF][START_REF] Schampera | Exposure to nanoplastics affects the outcome of infectious disease in phytoplankton[END_REF][START_REF] Sendra | Immunotoxicity of polystyrene nanoplastics in different hemocyte subpopulations of Mytilus galloprovincialis[END_REF][START_REF] Sendra | Are the primary characteristics of polystyrene nanoplastics responsible for toxicity and ad/absorption in the marine diatom Phaeodactylum tricornutum?[END_REF][START_REF] Seoane | Polystyrene microbeads modulate the energy metabolism of the marine diatom Chaetoceros neogracile[END_REF]Shiu et al., 2020aShiu et al., , 2020b;;[START_REF] Shupe | Effect of Nanoplastic Type and Surface Chemistry on Particle Agglomeration over a Salinity Gradient[END_REF][START_REF] Silva | Behavior and biochemical responses of the polychaeta Hediste diversicolor to polystyrene nanoplastics[END_REF][START_REF] Singh | Metal oxide nanoparticles and polycyclic aromatic hydrocarbons alter nanoplastic's stability and toxicity to zebrafish[END_REF][START_REF] Singh | Understanding the stability of nanoplastics in aqueous environments: effect of ionic strength, temperature, dissolved organic matter, clay, and heavy metals[END_REF][START_REF] Song | Fate and transport of nanoplastics in complex natural aquifer media: Effect of particle size and surface functionalization[END_REF][START_REF] Summers | Agglomeration of nano-and microplastic particles in seawater by autochthonous and de novo-produced sources of exopolymeric substances[END_REF]H. Sun et al., 2020;X.-D. Sun et al., 2020;[START_REF] Tallec | Surface functionalization determines behavior of nanoplastic solutions in model aquatic environments[END_REF][START_REF] Trevisan | Nanoplastics Decrease the Toxicity of a Complex PAH Mixture but Impair Mitochondrial Energy Production in Developing Zebrafish[END_REF][START_REF] Vaz | Can the sonication of polystyrene nanoparticles alter the acute toxicity and swimming behavior results for Daphnia magna?[END_REF][START_REF] Venel | Environmental Fate Modeling of Nanoplastics in a Salinity Gradient Using a Lab-on-a-Chip: Where Does the Nanoscale Fraction of Plastic Debris Accumulate?[END_REF][START_REF] Wang | Aggregation and stability of sulfate-modified polystyrene nanoplastics in synthetic and natural waters[END_REF][START_REF] Wang | UV-induced aggregation of polystyrene nanoplastics: effects of radicals, surface functional groups and electrolyte[END_REF][START_REF] Wegner | Effects of nanopolystyrene on the feeding behavior of the blue mussel ( Mytilus edulis L.)[END_REF][START_REF] Wu | Impact of different modes of adsorption of natural organic matter on the environmental fate of nanoplastics[END_REF][START_REF] Wu | Effect of salinity and humic acid on the aggregation and toxicity of polystyrene nanoplastics with different functional groups and charges[END_REF][START_REF] Xu | Influence of dissolved black carbon on the aggregation and deposition of polystyrene nanoplastics: Comparison with dissolved humic acid[END_REF][START_REF] Yu | Heteroaggregation of different surfacemodified polystyrene nanoparticles with model natural colloids[END_REF][START_REF] Yu | Aggregation kinetics of different surface-modified polystyrene nanoparticles in monovalent and divalent J o u r n a l P r e -p r o o f Journal Pre-proof electrolytes[END_REF][START_REF] Zhang | Aquatic behavior and toxicity of polystyrene nanoplastic particles with different functional groups: Complex roles of pH, dissolved organic carbon and divalent cations[END_REF][START_REF] Zhang | The combined toxicity effect of nanoplastics and glyphosate on Microcystis aeruginosa growth[END_REF][START_REF] Zhang | Charge mediated interaction of polystyrene nanoplastic (PSNP) with minerals in aqueous phase[END_REF]Zheng et al., 2021). A few of the particles could come from the same provider and have the same properties but it was impossible to verify if they came from the same batches, therefore they were all considered to be distinct. These particles were dispersed in a total of at least 377 different solution chemistries. A solution chemistry corresponds to a given combination of dissolved and particulate species but does not take into account all of the pH values or the varying concentrations .

The primary focus of most of the articles was aggregation and sedimentation processes (48% particles and 70% solution chemistries); followed by studies of (eco)toxicity, (35% particles and 21% solutions); transport and deposition (9% particles and 5% solutions); and interaction with biogeochemical processes (8% particles and 4% solution chemistries). A summary of the 82 articles studied is presented in the supplementary information.

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Figure 1 :

 1 Figure 1: Nanoplastic aggregation pathways investigated in the literature. A) Homo-aggregation caused by electrostatic screening at high ionic strength.B) Enhanced homo-aggregation due to electrostatic screening and cation bridging by multivalent cations C) Hetero-aggregation of nanoplastics with particulate species (bottom) and reduced hetero-aggregation in the addition of (macro)molecules (top) D) Destabilization by high molecular weight (macro)molecules bridging nanoplastics (bottom), stabilization by (macro)molecule at high IS dominated by monovalent salts (top left) and stabilization with a complete coating of (macro)molecules in the presence of divalent cations (top right)

Figure 2 :

 2 Figure 2: Two-dimensional matrix illustrating the number of times aggregation data was obtained from a given combination of particle environmental relevance score and solution environmental relevance score.

Figure 3 :

 3 Figure 3: Distribution of the composition of nanoplastic particle models used in the reviewed studies (inner circle, with PET = 1% , PE = 1%, PP = 1% and Others = 3% ) and of plastic waste composition in 2015 according to Geyer et al (2017) (outer circle).

  produced fragmental nanoplastic particle models from a biodegradable agricultural mulch composed of polybutyrate adipate-co-terephthalate (PBAT). Dong et al. (2020) studied fragmental PET, whereas Venel et al. (2021) and Pradel et al. (2021) studied fragmental PS particles, using the same method as El Hadri et al. (2020).

Figure 4 :

 4 Figure 4: SEM images of a) non-aged PSL-NF(-), b) PSL-NF(-) that were aged by UV-irradiation and c) fragmental nanoplastic particle models produced from mechanical abrasion of pre-production PS pellets

Figure 5 :

 5 Figure 5: Frequency at which different types of natural and anthropogenic species were studied in experiments that contain natural or anthropogenic model materials.

Figure 6 :

 6 Figure 6: The effect of humic acid (HA) on attachment efficiencies of PSL-NF(-) (top) and fragmental PET (bottom) as a function of NaCl concentration. The arrows indicate increased stability. (Reproduced with permission from Yu et al. (2019) and S.Dong et al., (2020))

Figure 7 :

 7 Figure 7: Relative proportion of different types of water in which nanoplastic particle models were studied.

Figure 8 :

 8 Figure 8: Nanoplastics homoaggregation is not likely to occur in A) monovalent and B) divalent electrolytes. Investigating homoaggregation is only relevant to characterize environmentally relevant nanoplastic particle models and understand the effect of naturally occurring species. C) The effect of particles on nanoplastic aggregation must be investigated in more depth, especially in the presence of (macro)molecules. D) Bridging of nanoplastics is a an unlikely event (bottom). It remains to be understood the extent to which nanoplastics can be stabilized by an eco-corona in the presence of monovalent cations (top left) and divalent cations (top right).

  ; Nolte J o u r n a l P r e -p r o o f

Table 1 :

 1 Typical chemical characteristics of river water

Table 2 :

 2 Details of parameters used for the solution environmental relevance scores

	Solution Environmental Relevance Score	Solution Properties
	1	DI water only
	2	DI water, with (macro)molecules and/or particulate species
	3	One electrolyte only (e.g., just NaCl)
	4	One electrolyte with (macro)molecules and/or particulate species
	5	Artificial water only
	6	Artificial water with (macro)molecules and/or particulate species
	7	Natural water only
	8	Natural water with added (macro)molecules and/or particulate species

Table 3 :

 3 Details of parameters used for the particle environmental relevance scores

	Particle Environmental Relevance Score Particle Properties
	1	Bottom-Up, NH2 or SO3
	2	Bottom-Up, non-functionalized (NF) or COOH
	3	Bottom-Up particles aged
	4	Top-Down from pristine plastic
	5	Top-Down from environmental plastic
	6	Environmental nanoplastic

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Acknowledgements

This work was supported by the French Ministry of Higher Education and Research, the French ANR through the program PLASTI-SCARE and the French Brittany Council through MONANO (SAD-Attractivité). Thank you to Mandy Ducy from Agence IMPAKT Scientifik Inc. for the illustrations. Thank you to Christa Bedwin for editing our work.

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Highlights

 What is known about nanoplastics' environmental fate, and what needs to be known;  A systematic review of nanoplastic aggregation rates and processes.  Up-to-date hypotheses of where nanoplastics are accumulated in the environment  Determine how nanoplastics' aggregation state impacts their settling rate.  Nanoplastics models needed for the interpretations of nanoplastics' fate and impact.

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