Nitin Khandelwal

Nisha Singh

Ekta Tiwari

Remi Marsac

Dieter Schild

Thorsten Schäfer

Krishna Gopala

Darbha
email: gkdarbha80@yahoo.com

Gopala Krishna Darbha
email: gkdarbha@gmail.com

Varying growth behavior of redox-sensitive nanoparticles on 1:1 and 2:1 clay surfaces: mechanistic insights on preferential toxic ions removal in mono, co and multi-metal contaminated waters

Keywords: nZVI, sorption, kaolinite, bentonite, metals removal, reductive co-precipitation

Highlights

 Dominant interlayer growth of Fe 0 NPs on 2:1 bentonite form B-nZVI (-ve charge)  Non-swelling 1:1 kaolinite with surface coverage of Fe 0 NPs led to K-nZVI (+ve)  Preferential sorption of toxic cations on B-nZVI (63 mg/g) than K-nZVI (33 mg/g)  Higher removal of oxy-anions with K-nZVI (248 mg/g) than B-nZVI (149 mg/g)  Removal mechanisms were redox transformation, complexation and co-precipitation

Abstract

In the current study, contrasting growth behaviour of redox sensitive Fe 0 nanoparticles (nZVI) was observed on different clay surfaces i.e., 1:1 non-swelling kaolinite (K-nZVI) and 2:1 swelling bentonite (B-nZVI). Osmotic swelling of bentonite led to Fe 0 nucleation and growth of 5-7 nm size particles in the broadened interlayer spaces. B-nZVI had negative zeta potential due to the domination of the surface charge of bentonite clay. Whereas kaolinite has shown dominant surface growth of nZVI particles (>24.8±7.4 nm) and positive zeta potential, suggesting domination of Fe 0 nanoparticles (nZVI) characteristics. This surface-dependent variation led to higher and faster removal of oxy-anions with K-nZVI, i.e., chromium and arsenic (87.5 and 157.35 mg/g) than B-nZVI (18.4 and 86.9 mg/g). In comparison, B-nZVI has shown higher sorption of cations i.e., nickel and cadmium (36 mg/g and 46 mg/g) than K-nZVI (25 and 27 mg/g). XPS and pXRD analysis of reaction precipitates confirmed reductive sorption of chromium, co-precipitation/ complexation of arsenic, electrostatic attraction and complexation of nickel and cadmium as major removal mechanisms. Drastically higher total contaminant sorption capacities of B-nZVI (327 mg/g) and K-nZVI (372 mg/g) in multicontaminant (Cr+As+Ni+Cd) solutions than individual capacities in mono-ionic solutions was due to co-operative effects and newer sites induced via sorption and redox-transformation of other ionic species. K-nZVI removed chromium and arsenic to below drinking water permissible limits whereas B-nZVI succeeded in separating nickel and cadmium to drinkable levels in groundwater, freshwater, river water, and wastewater samples, emphasizing their applicability in high cationic-low anionic and low cationic-higher anionic species contaminated waters, respectively.

Introduction

Water is a unique entity of the planet and one of the most critical resources required for balanced ecosystem functioning. According to recent estimates, three out of every ten people do not have access to contamination-free drinking water [START_REF] Unesco | The United Nations world water development report 2019: Leaving no one behind[END_REF]. Tremendously increasing water use by nearly 1% per year has resulted in high water stress for over 2 billion people [START_REF] Boretti | Reassessing the projections of the World Water Development Report[END_REF]. In an extensive range of contaminants, increasing concentrations of heavy metals and metalloids in water bodies represent an alarming environmental threat because of their non-degradability and environmental persistence [START_REF] Ali | Environmental Chemistry and Ecotoxicology of Hazardous Heavy Metals: Environmental Persistence, Toxicity, and Bioaccumulation[END_REF]. Once introduced in the environment, these ions tend to bioaccumulate, impact the whole food chain and ultimately threaten human health [START_REF] Engler | The Complex Interaction between Marine Debris and Toxic Chemicals in the Ocean[END_REF][START_REF] Rajeshkumar | Bioaccumulation of heavy metals in fish species from the Meiliang Bay, Taihu Lake, China[END_REF]. In a prevailing environmental condition, these ions can speciate in various ionic species [START_REF] Reeder | Metal Speciation and Its Role in Bioaccessibility and Bioavailability[END_REF]. For example-as shown in fig. S1 a-d, in circum-neutral pH conditions and oxidizing environment, arsenic (As) and chromium (Cr) speciates in anionic species, i.e., arsenate and chromate ions [START_REF] Hug | Solar Oxidation and Removal of Arsenic at Circumneutral pH in Iron Containing Waters[END_REF][START_REF] Wang | Simultaneous Redox Conversion of Chromium(VI) and Arsenic(III) under Acidic Conditions[END_REF], whereas nickel (Ni) and cadmium (Cd) stay in their cationic +2 form [START_REF] Hu | Effect of Nickel and Cadmium Speciation on Nitrification Inhibition[END_REF]. This variation in ionic behavior requires selectivity or a component additive approach as a critical component in the remediation technique [START_REF] Barakat | New trends in removing heavy metals from industrial wastewater[END_REF]. Moreover, the co-contamination of ions with similar charge behavior can induce site competition in the system and may make the remediation even more challenging. Similarly, solutions contaminated with multiple ions of varying charge can also impact the treatment efficiency [START_REF] Reddy | Technical Challenges to In-situ Remediation of Polluted Sites[END_REF]. Several explored removal techniques include conventional methods that suffer from limited selectivity and sorption capacity, such as filtration, sedimentation, coagulation-flocculation, aerobic and anaerobic degradation, etc. [START_REF] Bolisetty | Sustainable technologies for water purification from heavy metals: review and analysis[END_REF]. Bioremediation, including phytoremediation, suffers from slower reaction kinetics [START_REF] Azubuike | Bioremediation techniques-classification based on site of application: principles, advantages, limitations and prospects[END_REF]. Other remediation techniques are distillation, solvent extraction, microfiltration, ultra-filtration, reverse and forward osmosis, electrolysis, etc., with high power consumption and tedious setup and maintenance [START_REF] Le | Materials and membrane technologies for water and energy sustainability[END_REF]. Sorption-assisted water treatment techniques involve cost-effectiveness, environmentally friendly nature, high selectivity, removal capacity, process simplicity, no secondary contamination, and low power consumption, etc., and are therefore of prime interest to the scientific community [START_REF] Gisi | Characteristics and adsorption capacities of low-cost sorbents for wastewater treatment: A review[END_REF]. Redoxsensitive nano-adsorbents, such as magnetic nanoparticles, Fe 3 O 4 , graphene, etc., have recently emerged and explored for their added electron transferring ability at the surface-water interface [START_REF] Khandelwal | Impact of long-term storage of various redox-sensitive supported nanocomposites on their application in removal of dyes from wastewater: mechanisms delineation through spectroscopic investigations[END_REF][START_REF] Mondal | Reduced Graphene Oxide/Fe3O4/Polyaniline Nanostructures as Electrode Materials for an All-Solid-State Hybrid Supercapacitor[END_REF]. It can lead to simultaneous redox transformation of toxic metals, forming coprecipitate with insoluble metal oxy-hydroxides on the composite surface [START_REF] Der Heyden | Application, Chemical Interaction and Fate of Iron Minerals in Polluted Sediment and Soils[END_REF][START_REF] Zou | Environmental Remediation and Application of Nanoscale Zero-Valent Iron and Its Composites for the Removal of Heavy Metal Ions: A Review[END_REF].

Instant inert surface oxide shell formation and self-aggregation of these redox-sensitive nanoparticles in the natural environment generally limits their applicability [START_REF] Guan | The limitations of applying zerovalent iron technology in contaminants sequestration and the corresponding countermeasures: the development in zerovalent iron technology in the last two decades (1994-2014)[END_REF]. Research showed that supporting surfaces, such as clays, graphene, biochar, charcoal, etc., can preserve their redox state [START_REF] Khandelwal | Impact of long-term storage of various redox-sensitive supported nanocomposites on their application in removal of dyes from wastewater: mechanisms delineation through spectroscopic investigations[END_REF][START_REF] Khandelwal | Novel synthesis of a clay supported amorphous aluminum nanocomposite and its application in removal of hexavalent chromium from aqueous solutions[END_REF]. Clays dominate out of several supporting surfaces because of their natural abundance, eco-friendly nature, and high cation exchange capacity that can prevent secondary contamination [START_REF] Ezzatahmadi | Claysupported nanoscale zerovalent iron composite materials for the remediation of contaminated aqueous solutions: A review[END_REF]. Clays are hydrous alumino-silicates and have a flake-like structure. Based on the crystal structure, i.e., a combination of tetrahedral (T) and octahedral (O) layers, kaolinite (1:1-T:O type), and bentonite containing around 80% of montmorillonite (2:1-T-O-T type) are two major clays [START_REF] Sposito | Surface geochemistry of the clay minerals[END_REF]. Both kaolinite and bentonite have been explored to support redox-sensitive nanoparticles like-nanoscale zerovalent iron (nZVI) [START_REF] Shi | Synthesis, characterization and kinetics of bentonite supported nZVI for the removal of Cr(VI) from aqueous solution[END_REF][START_REF] Wang | Zero-Valent Iron Nanoparticles (NZVI) Supported by Kaolinite for Cu-II and Ni-II Ion Removal by Adsorption: Kinetics, Thermodynamics, and Mechanism[END_REF].

Despite a drastic variation in the crystal structure and material properties, i.e., kaolinite with non-swelling nature and less interlayer spacing leading to limited cation exchange capacity (CEC), while bentonite having swelling (>10 times) nature, and very high CEC, etc., surfaceassociated variations in growth behavior of redox-sensitive nanoparticles and its impact on contaminant sorption behavior are still unknown.

Recently researchers have synthesized different clay-nZVI composites for the removal of various contaminants. For example, Baldermann et al. supported NANOFER 25 nZVI suspension on bentonite clay and used it to remove trichloroethylene (TCE), where nanoparticles were attached only on the surface [START_REF] Baldermann | A novel nZVI-bentonite nanocomposite to remove trichloroethene (TCE) from solution[END_REF]. Shi et al. showed B-nZVI formed using a liquid-phase reduction technique had around 18.7 mg/g of CrO 4 2-sorption capacity [START_REF] Shi | Synthesis, characterization and kinetics of bentonite supported nZVI for the removal of Cr(VI) from aqueous solution[END_REF].

Whereas, Üzüm et al. found that kaolin-supported nZVI can remove Cu 2+ and Co 2+ ions with 140 mg/g and 25 mg/g of sorption capacity. Reductive co-precipitation of contaminants due to electron transfer from Fe 0 and its oxidation oxy-hydroxides [START_REF] Sheng | Enhanced Removal of Uranium(VI) by Nanoscale Zerovalent Iron Supported on Na-Bentonite and an Investigation of Mechanism[END_REF]. So far, clays are only considered a supporting material to prevent Fe0 nanoparticle agglomeration and preserve their reactivity [START_REF] Ezzatahmadi | Clay-supported nanoscale zerovalent iron composite materials for the remediation of contaminated aqueous solutions: A review[END_REF].

However, dominating role of variation in crystal structure and properties of clays in deciding or controlling the sorption behavior of nZVI composites for various ionic contaminants is unknown. Here, based on the fundamental variations in crystal structures variation of different clays, we hypothesized that 1:1 clay having limited or no interlayer spacing and non-swelling nature could result in the growth of nZVI mostly on the surface. In contrast, swelling is widespread in 2:1 montmorillonite clay due to hydration [START_REF] Aksu | Swelling of clay minerals in unconsolidated porous media and its impact on permeability[END_REF]. It is reported that Na-bentonite can show two different swellings, i.e., crystalline swelling (limited d-spacing increase) and osmotic swelling resulting in a larger d-spacing increase (even >50Å) [START_REF] Amorim | Effect of clay-water interactions on clay swelling by X-ray diffraction[END_REF]. Osmotic swelling happens at a lower salt concentration (<0.5M) of monovalent ions [START_REF] Amorim | Effect of clay-water interactions on clay swelling by X-ray diffraction[END_REF]. Therefore, supporting nZVI on highly swelling 2:1 bentonite clay can lead to the growth of nanoparticles in interlayer spaces. The variation in the growth behavior of Fe 0 nanoparticles on different clay surfaces can further alter the surface properties such as nanoparticles size, charge, and surface area, etc., leading to varying sorption behavior for ionic contaminants.

To test the hypothesis, B-nZVI and K-nZVI composites were synthesized and sorption behavior was tested in mono, co, and multi-contaminant systems by utilizing different contaminants, i.e., oxy-anions (chromium, arsenic), and metal cations (nickel, cadmium).

Detailed batch sorption studies and surface characterization using dynamic light scattering (DLS), powder X-ray diffraction (pXRD), Fourier-transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) were done to delineate preferential sorption of contaminants and associated removal mechanisms of the composites.

Materials and methods

Materials and chemicals

Clays used include-extra pure Na-bentonite (Loba Chemie Pvt. Ltd., India) with composition (wt%)-54.9 % SiO

Synthesis of nanocomposites

To synthesize bentonite and kaolinite supported nZVI (B-nZVI and K-nZVI, respectively), nanocomposites, clay, and iron salt with a 1:1 mass ratio were introduced to 4:1 ethanol: water mixture [START_REF] Khandelwal | Impact of long-term storage of various redox-sensitive supported nanocomposites on their application in removal of dyes from wastewater: mechanisms delineation through spectroscopic investigations[END_REF]. The suspension was sonicated for 30 minutes, followed by vigorous stirring. Then NaBH 4 (1M) was added dropwise at a flow rate of 4.5 mL/min to the suspension under constant stirring.

The reaction mixture was left for another 20 minutes on vigorous stirring after NaBH 4 addition to ensure complete reduction. Then the obtained precipitate was filtered, washed multiple times with ethanol and water, and vacuum dried.

Characterization of nanocomposites

Clay surface morphology was visualized using field emission scanning electron microscopy (FESEM, Carl Zeiss SUPRA-55VP), and size distribution was obtained using particle size analyzer (ZEN3600, Malvern, UK). Whereas synthesized nanocomposites were analyzed using FESEM and ultra-high-resolution transmission electron microscopy, i.e., UHR-FEG-TEM (JEOL, JEM 2100 F). Elemental composition, mapping, and line scans were obtained using EDAX analysis through INCA software (Oxford Instruments, X-Max) coupled with UHR-FEG-TEM. Specific surface area and pore size distribution using Micromeritics Gemini VII BET-N 2 surface area analyzer. The nitrogen adsorption/desorption isotherms are reported by BJH (Barret Joyner-Halenda) surface analysis method. Before analysis, samples were degassed in vacuum at 150 o C for 12 h. Crystallinity and phase characteristics were obtained by powder X-ray diffraction (pXRD) of materials. pXRD measurements were taken using benchtop powder X-ray diffractometer (Rigaku mini flex) having Cu Kα = 1.54059 Å radiation at 40 kV/15 mA. Samples were scanned in the range of 5 0 -65 0 2θ at 5 0 per minute and step size of 0.02 0 . Surface functionality was interpreted using Fourier transform infrared spectroscopy (FTIR) by analyzing KBr pallets using Thermo-Scientific Nicolet iS5 coupled with the iD1 transmission accessory. Zeta Sizer (ZS-90, Malvern) coupled with MPT-2 pH auto-titrator was used to obtain zeta potential and point of zero charges (pH PZC ) of composites. Reaction precipitates were also analyzed using a PHI 5000 VersaProbe II (ULVAC-PHI Inc.) equipped with a monochromatic Al Kα X-ray source (1486.7 eV) for XPS measurements. Elemental lines are charge referenced to C1s (C x H y ) at 284.8 eV.

Batch sorption experiments and analytical techniques

At pH 6.5, 0.01 M NaNO 3 solutions were introduced with (i) mono contaminant system, i.e., arsenic, chromium, nickel, and cadmium individually, (ii) co-contaminant systems, i.e., anionic (An), i.e., arsenic-chromium and cationic (Ct), i.e., nickel-cadmium and (iii) multi-contaminant systems (MM), i.e., arsenic-chromium-nickel-cadmium. With a fixed weight of adsorbent, i.e., 20 mg, and a solution volume of 25 mL, sorption kinetics was performed at 20 mg/L concentrations of respective contaminants by varying the interaction time. At the same time, sorption isotherm experiments were performed by varying contaminant concentrations in 10-60 mg/L range by keeping interaction time constant to 12 h, which is more than the obtained equilibrium time. Reaction mixtures were kept on shaking (200 RPM) at 25 0 C. After the interaction, mixtures were centrifuged, and reaction precipitates were dried for further characterization. Whereas, supernatants were again filtered using 0.22 µm filter paper, acidified, and analyzed for respective contaminant concentrations using ICP-OES (Thermo iCAP-7400 series). Samples were also analyzed for released iron concentration after the interaction to ensure minimal secondary contamination. Sorption behavior was also evaluated (i) as a function of pH and (ii) in different types of water compositions, i.e., synthetic freshwater, groundwater, wastewater, and natural river water (compositions provided in table-S1a-b in supplementary information), to ensure environmental applicability of the composites.

For that, 20 mg of adsorbent was taken in 25 mL of the solution with having 10 mg/L concentration of each metal ion. All the experiments were performed in duplicates, and errors are shown wherever significant (>5%) and required. Performed blanks include (i) all stated combinations of contaminated solutions without adding adsorbents and (ii) solutions containing composites without adding contaminants.

Results and discussion

Surface characteristics and particles growth behavior

Morphology, composition, and growth mechanisms

The sheet-like structure of both bentonite and kaolinite can be visualized through SEM images in fig. S2a, andS3a. FESEM images showed that most of the clay flakes were of size <2µm while obtained hydrodynamic diameter using particle size analyzer (Fig. S2b, and S3b), showed a size distribution upto 100µm with most particles <10µm. This increased size corresponds to aggregates formed in suspension. After nZVI growth, clay surfaces show the presence of spherical nanoparticles, as observed in HRTEM images in fig. 1 a-c. FESEM images of B-nZVI (Fig. S2 c-d) showed a clear variation having bright interlayer cross-sections compared to K-nZVI where Fe 0 nanospheres were clearly visible only attached on the surface (Fig. S3 c-d). S3 e-f) showed two distinct size distributions of Fe 0 nanoparticles in B-nZVI, i.e., 5.83±2.6 nm and 20.65±8.7 nm corresponding to growth of particles in the interlayers and surface of bentonite, respectively. In contrast, iron nanoparticles covered the surface of kaolinite with a diameter of 24.8±7.4 nm. Elemental mapping in fig. S4 a,b showed the enhanced intensity of iron signal over spherical particles and the presence of oxygen and silicon in the background supporting the growth of iron nanoparticles on clay surfaces. Fig. 1d shows the pXRD pattern of clays and synthesized nanocomposites. The characteristic d-001 peak of bentonite around 2θ= 7.14 0 corresponding to an interlayer spacing of 1.27 nm [START_REF] Araujo | Polyamide 6 Nanocomposites with Inorganic Particles Modified with Three Quaternary Ammonium Salts[END_REF] shifted and broadened after iron loading, suggesting the broadening of interlayer space which can be attributed to Fe 0 nucleation and growth in interlayers. Earlier, Son et al. have also observed a peak shift of d001 in bentonite after iron-oxide loading due to formation of polycations and particles in the interlayers [START_REF] Son | Structure-Property Correlation in Iron Oxide Nanoparticle-Clay Hybrid Materials[END_REF]. In K-nZVI, the d-001 peak of kaolinite around 2θ= 12 0 was present with no shifting or broadening, supporting no significant change in the interlayer spacing [START_REF] Sachan | Identification of Microfabric of Kaolinite Clay Mineral Using X-ray Diffraction Technique[END_REF].

Simultaneously, the generation of characteristic Fe 0 peak near 44.8 0, along with the absence of iron oxide peaks [START_REF] Bae | Advances in Surface Passivation of Nanoscale Zerovalent Iron: A Critical Review[END_REF][START_REF] Khandelwal | Biochar-nZVI nanocomposite: optimization of grain size and Fe0 loading, application and removal mechanism of anionic metal species from soft water, hard water and groundwater[END_REF], suggests that iron particles were in preserved redox state on both clay surfaces with no dominant crystalline iron-oxide phases or thick oxide shells.

Scheme 1 Synthesis and growth mechanisms of nZVI on bentonite and kaolinite surfaces and

respective contaminant removal mechanisms.

The surface of composites was further evaluated using XPS spectra and by fitting Fe2p and O1s regions (fig. 1 e-f). Obtained peaks around 706.9 eV, 711.1 eV, 724.9 eV, and 719.9 eV of binding energies corresponded to Fe(0) 2p 3/2 , Fe(III) 2p 3/2 , Fe(III) 2p 1/2 and satellite peak of Fe(III) 2p 3/2 . Fitting reveals the presence of Fe(II) and Fe(III) along with Fe 0 on the surface in both kaolinite and bentonite [START_REF] Huber | U(VI) removal kinetics in presence of synthetic magnetite nanoparticles[END_REF]. It can be attributed to a very thin shell of iron-oxyhydroxides on the surface of elemental iron nanoparticles. Higher Fe 0 content on bentonite surface compared to kaolinite can be attributed to nanoparticles coverage with bentonite flakes as observed in TEM images.

In summary, two distinct size fractions of nZVI in bentonite, broadening of interlayer spacing as obtained from pXRD, and higher Fe 0 content in B-nZVI compared to no change in d001 peak of kaolinite after nZVI growth, and >20nm particles attached on the surface, suggest the following , iron nanoparticles growth mechanisms (Scheme 1): initially, the addition of bentonite in an interfacial solution can result in its crystalline swelling, which allows iron ions to enter andadsorb in the inter-layer spaces and on the surface as well. Once the addition of NaBH 4 begins, iron particles start to nucleate, and simultaneously interlayer spacing of bentonite clay increases due to its osmotic swelling [START_REF] Yotsuji | Effect of interlayer cations on montmorillonite swelling: Comparison between molecular dynamic simulations and experiments[END_REF][START_REF] Madsen | The swelling behaviour of clays[END_REF]. It may easily allow iron nanoparticles to grow in the interlayer spaces and on the surface. On the other hand, kaolinite is 1:1 clay and does not swell, which results in the surface sorption and growth of iron nanoparticles [START_REF] Wang | Zero-Valent Iron Nanoparticles (NZVI) Supported by Kaolinite for Cu-II and Ni-II Ion Removal by Adsorption: Kinetics, Thermodynamics, and Mechanism[END_REF].

Surface area and pore distribution

BET-N 2 adsorption-desorption isotherms are shown in fig. S5 a,c. Pertaining to limited interlayer space, kaolinite clay showed a BET-N 2 surface area of 3.15 m 2 /g, whereas it was 24.5 m 2 /g for bentonite [START_REF] Zhang | Kaolinite-supported nanoscale zerovalent iron for removal of Pb2+ from aqueous solution: Reactivity, characterization and mechanism[END_REF][START_REF] Shi | Removal of Chromium (VI) from wastewater using bentonitesupported nanoscale zerovalent iron[END_REF]. Literature reports show that bare nZVI particles have a surface area of around 20-25 m 2 /g [START_REF] Reginatto | Use of Nanoscale Zero-Valent Iron for Remediation of Clayey Soil Contaminated with Hexavalent Chromium: Batch and Column Tests[END_REF][START_REF] Tarekegn | Nano zero valent iron (nZVI) particles for the removal of heavy metals (Cd(2+), Cu(2+) and Pb(2+)) from aqueous solutions[END_REF]. After nZVI growth, bentonite surface area increased further to 31.1 m 2 /g, corresponding to the growth of very small (5-7 nm) iron nanoparticles. . In contrast, K-nZVI showed a surface area of 19.4 m 2 /g which was higher than kaolinite but was in range with the surface area of bare nZVI particles [START_REF] Üzüm | Synthesis and characterization of kaolinite-supported zerovalent iron nanoparticles and their application for the removal of aqueous Cu2+ and Co2+ ions[END_REF][START_REF] Adusei-Gyamfi | Carriers for nano zerovalent iron (nZVI): synthesis, application and efficiency[END_REF]. A decrease in the pores (≈ 4 nm) having a maximum volume in B-nZVI compared to bentonite suggests nZVI growth in available pores (fig. S5b). While an increase in the pore volume (fig. S5d) in K-nZVI can be attributed to nanoscale particles growth on the surface. These observations further support the proposed Fe 0 particles growth mechanisms on different clay surfaces. Further details of obtained surface area, average pore diameter, and cumulative pore volume are provided in table S2. B-nZVI has shown a slightly higher cumulative pore volume than bentonite, which may be due to the growth of some nZVI particles on the clay surface.

Surface functionality and charge

Both kaolinite and bentonite shared almost similar surface functionality. FT-IR spectra (fig. S5e) showed a broad peak around 3430 cm -1 for -OH group stretching. 1044 cm -1 peak was due to Si-O bending vibration, while antisymmetric Si-O-Si stretching vibration caused an adsorption peak near 797 cm -1 [START_REF] Kumar | Sodium bentonite and kaolin clays: Comparative study on their FT-IR, XRF, and XRD[END_REF]. Kaolinite group minerals show -OH bands at 3696, 3669 and 3651 cm -1 beside the 3619 cm -1 inner hydroxyl group band while 1632 cm -1 peak of O-H reflected crystal water in clays lattice [START_REF] Wei-Min | Preparation and swelling properties of poly(acrylic acid-co-acrylamide) composite hydrogels[END_REF]. A shift in the 1632 cm -1 O-H peak and the generation of an adsorption band at 670 cm -1 was due to the symmetric Fe-O stretch of goethite (α-FeOOH) [START_REF] Liu | Fine structural features of nanoscale zerovalent iron characterized by spherical aberration corrected scanning transmission electron microscopy (Cs-STEM)[END_REF]. This variation in the growth behavior of nZVI particles, i.e., the domination of bentonite in B-nZVI and of nZVI in K-nZVI, can also be correlated with variation in the zeta potentials of various composites (fig. 2a). The negative zeta potential of both the clays, i.e., <-30 mV, can be attributed to isomorphic substitution in their crystal structures [START_REF] Niriella | Comparison Study of Zeta Potential Values of Bentonite in Salt Solutions[END_REF]. It decreased but remained negative even after nZVI growth in the case of B-nZVI, confirming bentonite domination. Whereas, after nZVI growth, K-nZVI shows the point of zero-charge at pH=8 i.e., at pH<8 K-nZVI has positive zeta potential. The positive zeta potential can be attributed to partial oxidation of nZVI particles and protonation of surface oxide layer. Therefore, in the environmentally relevant pH conditions (pH= 6-8) B-nZVI holds a negative charge, whereas K-nZVI was dominantly positive. This variation resulted from the drastic difference in nZVI growth behavior on both surfaces and is crucial for the adsorbent's selectivity towards specific toxic ions, as observed in later sections.

Contaminant sorption behavior and mechanisms

Preliminary sorption experiments with raw constituents of the composites i.e., bentonite and kaolinite clays and bare nZVI particles, were performed for all the studied contaminants and compared for respective sorption capacities with the synthesized nanocomposites. Results in fig. 2b show minimal sorption of oxy-anions (<2.5 mg/g) with both clays. Bare nZVI show significant sorption of oxy-anions but limited sorption of cationic species (< 5.5 mg/g). In comparison, synthesized nanocomposites showed higher removal of both cations (>18 mg/g) and oxy-anions (>13.5 mg/g). In all the cases, iron release from bare nZVI particles was higher than the WHO-defined permissible limit of 2 mg/L for drinking water. Whereas no significant iron release was observed in the case of nanocomposites. Results confirm that nanocomposites advance their raw constituents regarding contaminant sorption capacities and prevent the secondary release of pollutants. Interestingly, both B-nZVI and K-nZVI showed a drastic variation in contaminant-specific sorption capacities. At a fixed concentration of toxic anions and cations, B-nZVI showed comparatively higher removal of nickel and cadmium, whereas K-nZVI separated chromium and arsenic with high efficiency (fig. 2b). This variation in sorption capacities might be directly correlated with the positive and negative zeta potentials of K-nZVI and B-nZVI (fig. 2a), respectively, allowing faster interaction and bulk transfer of oppositely charged ions on the surface.

After the sorption of toxic ions, reaction precipitates were collected and thoroughly characterized for change in surface charge, crystallinity, functionality, and redox composition to delineate various involved removal mechanisms. Interaction and removal mechanisms were found to be varying for different contaminants for B-nZVI and K-nZVI and are described below in detail.

Chromium

Chromium which speciate in the form of CrO 4 2-oxy-anions, had varying interaction with B-nZVI and K-nZVI. Zeta potential (fig. 3 a-b) of B-nZVI in 0.01M NaNO 3 solution was -16.7±1.3 mV which decreased to -13.3±0.4 mV after interaction with chromate ions. The sole involvement of electrostatic attraction should have resulted in either increased negative zeta potential or electrostatic repulsion with chromate ions, suggesting the involvement of other removal mechanisms. Similarly, zeta potential of K-nZVI was +12.1±0.7 mV which decreased to +8.5±1.3 mV after interaction with chromate ions supporting the involvement of electrostatic attraction. But, a slight decrease in zeta potential points out the presence of different species of chromium on K-nZVI surface as well. These observations were further supported by pXRD spectra (fig. 3c-d), which showed a slight decrease in the intensity of Fe 0 peak in B-nZVI and little oxidation of iron on B-nZVI surface, suggesting limited electron transfer and redoxtransformation in the system leading to limited interaction of B-nZVI with chromium. K-nZVI showed a drastic decrease in 44.8 0 Fe 0 peak and generation of 30 0 and 35 0 intense ironoxide peaks suggesting intense electron transfer and iron oxidation. FTIR spectra (fig. 3 e-f) also showed the enhanced intensity of 530 cm -1 Fe-O stretching. Redox transformation of chromium and composites was further confirmed by obtaining XPS survey scans (fig. S6a-b).

High-resolution scans for Fe2p (fig. S6c-d), O1s (Fig. 4a) and Cr2p regions (fig. 4f). Fe 0 associated peak near 707 eV disappeared after interaction with chromium in both B-nZVI and K-nZVI confirming electron transfer and consumption of Fe 0 . In correspondence, Fe(II) and Fe(III) contribution increased on the surface.

Cr2P spectra showed a dominant presence (72.8% and 79.6%) of Cr(III) near 577 eV on B-nZVI and K-nZVI surfaces, confirming the reduction of chromium on both surfaces [START_REF] Moulder | Handbook of X-Ray Photoelectron Spectroscopy[END_REF][START_REF] Kraut-Vass | NIST X-ray photoelectron spectroscopy database[END_REF]. K-nZVI showed higher intensity of Cr2p-associated peaks compared to B-nZVI. O1s spectra showed the dominance of -OH -functional groups in B-nZVI and both -OH -and -O 2-groups in K-nZVI (fig. 4a) after interaction with chromium, suggesting the simultaneous formation of iron oxides/oxy-hydroxides and chromium oxide/oxy-hydroxide. In combination, zeta potential, pXRD, and XPS data confirm strong interaction and higher redox-transformation of chromate ions with K-nZVI and co-precipitation with formed iron-oxy-hydroxides compared to B-nZVI, which was also supported by experimental sorption capacities data provided in later sections. Results indicate that in the case of B-nZVI, slower CrO 4 2-sorption corresponds to initial electrostatic repulsion followed by electron release from Fe 0 and chromate ions reduction and sorption or co-precipitation of Cr(III) on B-nZVI surface. In contrast, K-nZVI showed strong electrostatic attraction of chromate ions on K-nZVI surface, followed by its reduction on the surface via electron transfer from Fe 0 core and attachment of Cr(III) with FeOOH. In multi-metallic solutions, both K-nZVI and B-nZVI showed a decrease in Cr2p peaks intensity with a decrease in Cr(III) abundance to 71% and 68%, respectively (Fig. 4f).

Arsenic

After the sorption of arsenic, both B-nZVI and K-nZVI showed a drastic decrease of zeta potential from -16.7±1.3 mV and +12.1±0.7 mV to -33.9±1.1 mV and -17.9±2.4 mV, respectively (fig. 3a-b). pXRD spectra (fig. 3c-d) showed a decrease in 44.8 0 Fe 0 peak with a slight increase in 35 0 iron oxide peak and 46 0 FeOOH peak in B-nZVI whereas K-nZVI showed the generation of strong 30 0 and broad 35 0 iron oxide peaks. FTIR spectra showed the generation of 805cm -1 stretching due to FeOOH in B-nZVI along with 530cm -1 and 580cm -1 Fe-O stretching (fig. 8e-f). The XPS data indicate that arsenic is trivalent as added to solution present in the form of As(OH) 3 (aq) or HAsO 2 (aq) specie. Arsenic can get oxidized to As(V) in a surface water environment where Eh is higher. The presence of nZVI may lower Eh, and arsenic can be reduced to As(III) at contact with nZVI. Presence of arsenic dominantly in the form of As(OH) 3 leads to ligand exchange, and switching from Fe-OH 2 + to Fe-O-As(OH) 2 might explain the decrease in zeta potential.

In summary, dominant removal mechanisms were reductive co-precipitation of chromium, complexation of arsenic, electrostatic attraction and co-precipitation of nickel and cadmium with generated iron-oxy-hydroxide shell.

Sorption behavior in different contaminant systems: removal kinetics

Mono-ionic contaminant exposure

Aqueous solutions contaminated with single ionic species were reacted with both B-nZVI and K-nZVI to test the comparative preference of the composites. Results in fig. 5 a-d show that sorption equilibrium was achieved within 3h of interaction time for all contaminants. K-nZVI achieved sorption equilibrium faster for chromium (~1h) and arsenic (~10min) compared to B-nZVI (~6h and 1h, respectively). In addition, 75% removal of chromium was achieved within 2 min of interaction with K-nZVI, whereas it was only 24% for B-nZVI.

Arsenic removal was 78% within 2 min of interaction with K-nZVI compared to 52% for B-nZVI. After equilibration, near complete removal of chromium and arsenic was observed with K-nZVI but not with B-nZVI. Oppositely, B-nZVI showed faster removal of cadmium (56%) and nickel (45%) within 2 min of interaction compared to K-nZVI (27% of nickel and 33% of cadmium). These observations conclude preferential and quasi-instantaneous removal of anionic species with positively charged K-nZVI and cationic species with negatively charged B-nZVI due to both favored chemical binding and electrostatic attractions.

Insights on adsorption kinetics were modelingobtained through kinetic modeling of experimental data usingpseudo-second order and general order models. The first one presumes the order of a reaction to be 2 whereas the latter suggests that the order of a reaction shouldn't be presumed but calculated using experimental data [START_REF] Lima | Kinetic and Equilibrium Models of Adsorption[END_REF]. Therefore, the general order model provides information about equilibrium sorption capacity, rate constant, and reaction order and is a three-parameter model [START_REF] Prola | Adsorption of Direct Blue 53 dye from aqueous solutions by multi-walled carbon nanotubes and activated carbon[END_REF]. Models fit in fig. 5 a-d and obtained parameters in table S3 show that the general order model fits well compared to pseudo-second order for all contaminants and therefore justifies varying reaction orders for different contaminants. The reaction order ranged from 2 to 5, suggesting that both composites follow higher order kinetics for contaminants removal. Intra-particle diffusion (IPD) model in fig. 5 e-h show three different linear segments corresponding to (i) initial bulk transfer to the adsorbent surface, (ii) diffusion of adsorbate in the meso, macro, and micropores, and (iii) final equilibrium stage and, in mono-ionic contaminant systems, the same trend was observed in most of the cases. Here the three segments can be attributed to faster adsorption followed by redox transformation (for chromium and arsenic) and/or co-precipitation and final equilibrium stage [START_REF] Khandelwal | Impact of long-term storage of various redox-sensitive supported nanocomposites on their application in removal of dyes from wastewater: Mechanisms delineation through spectroscopic investigations[END_REF]. Intercept values for IPD were in order Cr< As< Ni< Cd for B-nZVI while it was As< Cr< Cd< Ni for K-nZVI suggesting limited bulk transport and comparatively longer IPD stage for chromium and arsenic with B-nZVI than K-nZVI suggesting slower redox transformation and co-precipitation on B-nZVI surface. Whereas, Ni 2+ and Cd 2+ exhibited limited bulk transfer and shorter IPD stage on K-nZVI surface compared to B-nZVI, suggesting limited metal cations removal capacity of K-nZVI than B-nZVI.

Co-ionic contaminant exposure

Remediation of water bodies contaminated with multiple similar charged toxicant ions can be of higher environmental relevance compared to mono-ionic contamination. The presence of multiple contaminants can either induce competitive sorption or co-operative sorption and, therefore can impact the total metal sorption capacity of the composites.

To understand the behavior of B-nZVI and K-nZVI in co-ionic systems, composites were reacted with aqueous solutions contaminated with Cr-As and Ni-Cd to represent anionic (An) and cationic (Ct) systems, respectively. Similar to mono-ionic system, K-nZVI showed faster and higher removal of both chromium and arsenic (fig. S7 a-b). Parameters obtained from well fitted general order model provide reaction order in the range of 3-5 (table S3b). IPD modeling in fig. 6 a-d also show higher bulk transport of chromium and arsenic to the K-nZVI surface compared to B-nZVI. Equilibrium was not achieved completely in case of B-nZVI for both chromium and arsenic suggesting removal at a slower rate. In comparison to mono-ionic system, removal of Ni 2+ and Cd 2+ decreased slightly for B-nZVI in co-contaminant system (fig. S7 c-d). Corresponding to near equal bulk transfer and similar IPD curve slope, both K-nZVI and B-nZVI showed removal of Ni 2+ and Cd 2+ at a near equal rate (fig. 6 c-d). These observations further suggest a simultaneous increase in the cations removal capacity for K-nZVI in co-ionic containing nickel and cadmium.

Multi-ionic contaminant exposure

Water streams can receive toxic ions through several anthropogenic activities including wastes from different industrial outlets that can contaminate river and groundwater bodies with multiple ionic species. Therefore, it is essential to test the efficiency and behavior of synthesized composites in multi-ionic contaminant solutions. Here, multi-contaminant solutions constitute of equal concentrations of chromium, arsenic, nickel, and cadmium.

The simultaneous presence of both cationic and anionic contaminant species led to drastic variations in the sorption behavior of B-nZVI and K-nZVI compared mono-ionic solutions (fig. S7 e-h). Enhanced sorption of oxy-anions was observed but with the limited bulk transfer of adsorbate and longer and slower IPD stage in B-nZVI. In addition, no sorption equilibrium was achieved in B-nZVI in the studied time frame of 12h (fig. S7 e-h). IPD model (fig. 6 e-h) showed the change in intercept values of B-nZVI and K-nZVI from 3.4 mg/g and 14.6 mg/g to 4.6 mg/g and 16.8 mg/g for chromium in multi-contaminant system, suggesting enhanced bulk transport of chromium on both B-nZVI and K-nZVI surfaces. Whereas arsenic showed a slight decrease in IPD intercept i.e., from 10.4 mg/g and 16.6 mg/g to 8.6 mg/g and 15.8 mg/g. At the same time, B-nZVI showed a drastic decrease in bulk transport of Ni 2+ (from 10.9 mg/g to 2.7 mg/g) and Cd 2+ (14.4 mg/g to 5.7 mg/g), suggesting initial sorption sites competition between nickel and cadmium on B-nZVI surface [START_REF] Boparai | Cadmium (Cd2+) removal by nano zerovalent iron: surface analysis, effects of solution chemistry and surface complexation modeling[END_REF]. K-nZVI showed enhanced intercept values from 6.3 mg/g to 11.3 mg/g for Ni 2+ and 8.3 mg/g to 12.6 mg/g for Cd 2+ suggesting co-operative effects. The mechanism can further be supported using observations from mono-ionic solutions, which showed faster sorption of oxy-anions on K-nZVI that caused B-nZVI zeta potential to decrease from +12.1±0.7 mV to +8.5±1.3 mV for chromium and -33.9±1.1 mV for arsenic. Similarly, in multi-ionic contaminated solutions, initial faster attraction of oxy-anions to K-nZVI surface turned the surface charge to negative and creates an electrostatically favorable environment for heavy metal cations. In stage-2, electron transfer from nZVI led to redox-transformation of chromium, and strong complexation of arsenic and co-precipitation of nickel and cadmium as confirmed using XPS and FTIR analysis. Fitting of experimental data in non-linear kinetic models also showed comparatively best fit with general order where the order of reaction was found in range of 2-6 (table S3c).

Impact of varying contaminant concentrations

Mono-contaminant aqueous solutions of varying concentrations of respective contaminant were reacted with the composites to understand the impact of increasing contaminant concentration the sorption capacities of B-nZVI and K-nZVI. Results in fig. 7 a-d show an increase in the sorption capacity of the composites with increasing contaminant's concentration followed by plateau formation with the saturation of sorption sites. K-nZVI showed continuous increase in sorption capacities for chromium and arsenic oxy-anions when B-nZVI formed a plateau for chromium and showed a comparatively slower increase in arsenic sorption capacity.

In contrast, K-nZVI showed site saturation and formation of plateau for Ni 2+ and Cd 2+ whereas higher removal capacity and continuous increase in sorption capacity of B-nZVI was observed for cationic species.

To obtain the information about maximum sorption capacities, nature of adsorbent surface, and removal mechanisms, non-linear sorption isotherm modeling using Langmuir, Freundlich, and Sips models was performed. In general, Langmuir isotherm assumes monolayer sorption with homogeneous nature of adsorbent surface, whereas Freundlich assumes a heterogeneous surface and multi-layer stacking of adsorbate molecules [START_REF] Ayawei | Modeling and Interpretation of Adsorption Isotherms[END_REF]. Both of these isotherms are twoparameter models whereas Sips isotherm which is a combination of Freundlich (at low concentrations) and Langmuir (at higher concentrations) isotherms, is a three-parameter model [START_REF] Woo | Applicability assessment of functional adsorption zeolite materials in adsorption desalination cum cooling systems driven by low-grade heat source[END_REF]. Further description of sorption isotherm models is provided in the section 3 of supplementary information. Results in fig. 7 a-d and parameters in table S4a show that Sips isotherm best fits all the contaminants, confirming the heterogeneous nature of both composites. In all cases, 'n'≠1 further confirmed the heterogeneous nature of composites surface. Obtained maximum sorption capacities of B-nZVI for chromium and arsenic were 18.4 mg/g and 86.8 mg/g which were comparatively lesser than K-nZVI i.e., 87.4 mg/g for chromium and 157.3 mg/g for arsenic. In contrast, B-nZVI showed higher sorption capacities for Ni 2+ (36 mg/g) and Cd 2+ (46 mg/g) compared to K-nZVI (25 mg/g and 27 mg/g, respectively). These experimental observations further confirm preferential sorption cationic species on B-nZVI and anionic species on K-nZVI surface. Similar preferential behavior of B-nZVI and K-nZVI was observed in co-ionic contaminant solutions (fig. 7 e-h). While B-nZVI showed a slight increase in sorption capacities for chromium (51.1 mg/g) and arsenic (98.2 mg/g), it was still lower than K-nZVI (126.9 mg/g and 121.3 mg/g, respectively). This may be attributed to enhanced aging of B-nZVI in the presence of co-anionic solutions and suggests co-operative effects of oxy-anions. In contrast, a decrease in respective sorption capacities of Ni 2+ and Cd 2+ was observed for both the composites suggesting competitive effects in the presence of metal cations. The simultaneous presence of Cr(III) and As(III) can lead to the formation of ternary complexes leading to their higher sorption capacities. Therefore, total anion sorption capacities were 149.33 mg/g and 248.3 mg/g for B-nZVI and K-nZVI, respectively, which were comparatively higher than the individual chromium and arsenic removal capacities of the composites. Similarly, total cation sorption capacities were 32.9 mg/g (B-nZVI) and 63.4 mg/g (K-nZVI) and were slightly higher than the individual cation sorption capacities of respective composites. This slight increase can be attributed to the impact of enhanced total contaminant load in the system leading to higher sorption on heterogeneous surfaces of composites.

Similarly, the simultaneous presence of both anionic and cationic contaminants led to increased total metal sorption capacities of composites (fig. 7 e-l). Obtained parameters from well-fitted Sips isotherm model (table S4c) have shown higher sorption capacities of K-nZVI for anions i.e., chromium (62.2 mg/g and 98.1 mg/g) and arsenic (131.6 mg/g and 169.6 mg/g), respectively for B-nZVI and K-nZVI. In the case of cationic species, B-nZVI showed higher sorption capacities than K-nZVI for both Ni 2+ (20.1 mg/g and 17.7 mg/g) and Cd 2+ (113.2 mg/g and 87.1 mg/g). Sorption capacity for cadmium increased drastically of both the composites, compared to mono and co-ionic solutions. Interestingly, total maximum contaminant sorption species at very alkaline conditions (pH=10). Interestingly, in the multi-ionic system, K-nZVI has shown sorption of all the contaminants in the studied pH range (3.5-10) that can be attributed to all the cooperative effects as described in previous sections. In the case of a highly acidic environment (pH=3.5), composites have shown a decrease in the sorption capacity for all the contaminants, which may be due to the leaching of formed Fe 2+ , resulting in slight desorption of contaminants.

Environmental applications: contaminant preference in complex water matrices

It is a critical challenge for adsorbents to remove the trace amounts of contaminants and to keep the contaminant concentration below the drinking water permissible limit in environmental water samples. The observed variation in the growth behavior of nZVI particles on different clay surfaces that led to their respective preferential behavior and higher sorption capacities for electrostatically favored contaminants allowed further investigation on their behavior in different complex water matrices. For that, four different waters, i.e., synthetic groundwater, synthetic freshwater, synthetic wastewater, and natural river water, were spiked with 10 mg/L of each contaminant (Cr, As, Ni, and Cd) together. Interestingly, results have shown (fig. 8 a-d) that both composites were able to remove all the to >98% in all studied water matrices. Observed concentration of different contaminants in various elutes shown that K-nZVI was able to remediate water for chromium and arsenic oxy-anions below permissible limits, whereas B-nZVI succeeded in sequestering Ni 2+ and Cd 2+ up to drinking water standards as set by WHO, in all studied types of complex aqueous matrices.

Environmental applications: secondary iron release

Fe 0 oxidation and secondary release of iron ions and sorbed contaminants is a big challenge for the commercialization of nZVI based solutions for water purification and wastewater treatment. Here, we have monitored iron release during all sorption experiments with both the composites. Results in fig. S9 show that both the composites have prevented iron release below its permissible limit of 2 mg/L. Moreover, the mean values of iron release were 0.012 mg/L for B-nZVI and 0.038 mg/L for K-nZVI. Obtained most extreme outliers were in case of high acidic solutions i.e., pH= 3.5 in both cases. Results confirm that considering preferential sorption of specific ionic species, B-nZVI and K-nZVI can be applied to higher cationic-low anionic contamination and higher anionic-low cationic contamination systems, respectively for sustainable water treatment.

Conclusions

The study concludes that varying crystal structures and surface properties of clays lead to different growth behavior of redox-sensitive nZVI particles on these supporting surfaces. Here, swelling bentonite showed Fe 0 nanoparticles growth both in the interlayers and on the surface while only surface attachment was observed in non-swelling kaolinite. Variation in growth behavior altered various surface properties of formed B-nZVI and K-nZVI composites such asnanoparticles size, surface area, zeta potential, etc. These surface variations resulted in selectively higher sorption of electrostatically favored species on respective composites, i.e., positively charged K-nZVI chromium and arsenic oxy-anions (87.47 and 157.35 than of negatively charged B-nZVI (18.44 and 86.88 mg/g). Oppositely, B-nZVI has shown higher sorption of metal cations Ni 2+ and Cd 2+ (36 mg/g and 46 mg/g) than K-nZVI (25 and 27 mg/g).

Detailed characterization of reaction precipitates suggested reductive co-precipitation of Cr(VI), complexation of As(III), complexation and co-precipitation of Ni 2+ and Cd 2+ with generated iron-oxy-hydroxides were the major contaminant removal mechanisms with cooperative effects in multi-ionic contaminated solutions. Sorption capacities ranged to 327 mg/g and 372 mg/g for B-nZVI and K-nZVI, respectively in multi-contaminant solutions, justifying higher total metal sorption capacities of both composites. nZVI growth variation on different clay surfaces has also shown a significant implication in drinking water purification and wastewater treatment, where we observed the ability of B-nZVI and K-nZVI to remediate cationic and anionic metal species, respectively, below the WHOdefined maximum permissible limits from freshwater, groundwater, river water and wastewater like complex aqueous matrices. Overall, this study concludes that the -dependent variation in the growth of redox-sensitive nanoparticles can lead to a variation in their surface properties, resulting in variation in selectively higher removal of specific ionic species than others. Designed composites i.e., B-nZVI and K-nZVI can be used to remediate higher metal cationslow oxy-anions and low metal cations-high oxy-anions contaminated water bodies, respectively.

Fig. 1

 1 Fig.1 TEM images of (a) and inset (b) B-nZVI, (c) K-nZVI, (d) pXRD spectra of clay and

Fig. 2

 2 Fig.2 (a) Zeta potential at varying pH for clays and synthesized nanocomposites and (b)

Fig. 3

 3 Fig. 3 (a, b) zeta potential, (c, d) pXRD and (e-f) FTIR spectra obtained different reaction

Fig. 4

 4 Fig. 4 XPS data for different composites and reaction precipitates showing fitted (a-e) O1s

Fig. 5

 5 Fig.5 (a-d) effect of interaction time, reaction kinetics modeling and (e-h) intra-particle

Fig. 6

 6 Fig. 6 Intra particle diffusion modeling for B-nZVI and K-nZVI in (a-d) co-ionic and (e-h)

Fig. 7

 7 Fig. 7 Impact of varying concentration on sorption capacity of B-nZVI and K-

Fig. 8

 8 Fig.8 removal of various metallic species from different aqueous matrices [m= 20 mg, V= 20

  2 , 19.2 % Al 2 O 3 , 8.3 % Fe 2 O 3 , 3.79 % Na 2 O, 1.99 % MgO, 1 % K 2 O and 0.17 % CaO [21], and kaolinite (Charco Chemicals, India) with composition-55.51 % SiO 2 , 28.6 % Al 2 O 3 , 0.6 % Fe 2 O 3 , 0.1 % Na 2 O, 0.02 % MgO, 4.55 % K 2 O and 0.06 % CaO. Chemicals purchased from Merck, Germany, are Ethanol, NaAsO 2 0.05N solution, Ni(NO 3 ) 2 .6H 2 O, and FeCl 3 .6H 2 O. Merck, India supplied CdCl 2 .H 2 O, K 2 Cr 2 O 7 , HNO 3 , NaOH, NaBH 4 , and NaNO 3 . 18.2 MΩ.cm Milli-Q water was used for preparing all solutions.

Acknowledgments

Authors would like to acknowledge the financial support from the Ramanujan Fellowship grant (SB/S2/RJN-006/2016) from Department of Science and Technology (DST), India. Central instrumentation facility of the Indian Institute of Science Education and Research Kolkata

Nickel

Both B-nZVI and K-nZVI showed a change in zeta potential to higher positive value. A drastic increase in zeta potential from -16.7±1.3 mV to +34.2±1.2 mV in B-nZVI justifies high electrostatic attraction of Ni 2+ (fig. 3a-b). Further, pXRD spectra (fig. 3c-d) showed the generation of 30 0 and 35 0, and 43 0 intense iron oxide peaks and complete disappearance of Fe 0 peak in B-nZVI and presence with very low intensity in K-nZVI. Observation suggests formation of a very thick iron oxide shell on Fe 0 core. FTIR spectra (fig. 4e-f) further supported this observation and showed generation of 530 cm -1 , 570 cm -1 , and 630 cm -1 Fe-O stretching along with 705cm -1 and 805 cm -1 bands due to FeOOH. Standard redox potential of Ni 2+ /Ni 0 is -0.26 V at 298 K and is well above Fe 2+ /Fe 0 (-0.44 V, 298 K) potential which suggest probable reduction of Ni 2+ in Ni 0 [START_REF] Acharya | Chemical Structure of Fe-Ni Nanoparticles for Efficient Oxygen Evolution Reaction Electrocatalysis[END_REF]. But, Ni2p XPS spectra (fig. 4h) showed presence of Ni(II) peak in the region of 854.9 eV corresponding to Ni(OH) 2 suggesting complexation and coprecipitation of nickel.

Cadmium

Similar to nickel, Cd 2+ sorption also resulted in an increase in zeta potential from -16.7±1.3 mV to +9.7±0.2 mV in B-nZVI suggested strong electrostatic attraction (fig. 3a). Whereas, only a slight increase in zeta potential value from +12.1±0.7 mV to +13.1±1.1 mV was observed in K-nZVI (fig. 3b). pXRD and FTIR observations were also similar to nickel, where intense iron-oxide shell generation was observed in the system. Cd3d XPS spectra (fig. 4i) showed the presence of Cd(II) in the system. Standard redox potential of Cd 2+ /Cd 0 is -0.40 V at 298 K and is very near to Fe 2+ /Fe 0 (-0.44 V, 298 K) potential suggesting no reduction of cadmium and probable removal via electrostatic attraction, complexation, and co-precipitation of cadmium. capacities of both B-nZVI (327 mg/g) and K-nZVI (372 mg/g) were several folds higher than mono-ionic sorption capacities. A comparison of sorption capacities of earlier reported adsorbents is provided in Table S6 of supplementary information. This behavior can again be attributed to enhanced co-operative effects due to the presence of both cations and oxy-anions in the system that can facilitate multi-layer stacking, ternary complexation, and co-precipitation in the system, leading to enhanced removal of contaminants. Thermodynamic calculations (Table S5) showed that ΔG values were negative in all the cases and mostly <-20 kJ/mol suggesting reaction spontaneity and chemisorption characteristics [START_REF] Raghav | Adsorption Equilibrium, Kinetics, and Thermodynamic Studies of Fluoride Adsorbed by Tetrametallic Oxide Adsorbent[END_REF][START_REF] Cantu | Thermodynamics, Kinetics, and Activation energy Studies of the sorption of chromium(III) and chromium(VI) to a Mn(3)O(4) nanomaterial[END_REF].

Effect of solution pH

Attributing to the high sorption affinity of B-nZVI for cationic species and K-nZVI for anionic contaminants, similar preferential sorption behavior was observed at varying pH as well (fig. S8 a-l). Therefore, in the studied pH range of 3.5 to 10 K-nZVI showed comparatively higher sorption capacity for chromium and arsenic, whereas B-nZVI showed higher removal of Ni 2+ and Cd 2+ in mono-ionic and co-ionic contaminant exposure. A slight decrease in the sorption of anionic species at alkaline pH can be attributed to negative surface charge of both B-nZVI and K-nZVI that led to electrostatic repulsion in the system with anionic contaminant species and unfavorable ligand exchange (OH -) reactions in basic conditions. For example, As(III) prevails as As(OH) 3 at pH < 9 which behaves like a ligand and can undergo ligand exchange reactions with OH -on Fe-OH surface sites. This reaction is facilitated on positively charged (i.e. protonated) surface sites (Fe-OH 2 + ) because surface H 2 O groups are more easily exchanged than OH -leading to pH-dependent decreasing adsorption of As(III) on composites surface.

In contrast, we observed an increase in the sorption capacity for cationic species with an increase in pH, which may also be attributed to the formation of insoluble metal hydroxide should be acknowledged for providing composites characterization instruments (BET, ICP-OES, FESEM, TEM, pXRD, and XRF). Nisha Singh Nitin Khandelwal also want to acknowledge the research fellowships.

Appendix A. Supplementary Information (SI)

Supplementary information associated with this article can be found, in the online version, at-


Maria G Tsoutsouva

Thierry Duffar
email: thierry.duffar@grenoble-inp.fr

Christian Garnier

G Fournier

Undercooling measurement and nucleation study of silicon droplet solidification

diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The most popular commercial method for producing silicon based solar cells involves a process in which molten silicon is directly cast into a mould and allowed to solidify into an ingot. However, the fact that molten silicon reacts with almost every known materials creates severe obstacles to the photovoltaic (PV) technology. The interfacial interactions between the molten silicon and the mould and the subsequent heterogeneous nucleation that takes place on the crucible surface are determinant for the beginning of solidification. Grains nucleate on the crucible walls and elongate from the periphery towards the center of the ingot, leading to a polycrystalline material. Grains and grain boundaries play a crucial role in PV technology since they directly determine the electrical performance of the solar cell.

In literature there are some research teams who have investigated the silicon-crucible interaction by measuring the wetting angle between a silicon droplet and the nucleating substrate (sessile drop experiments) [1][2][3]. This technique gives interesting information mainly about adhesion and infiltration phenomena rather than nucleation. Unfortunately there has not been found yet a way to directly relate the nucleation process to the wetting results.

Concerning the investigation of nucleation phenomena, a more efficient method is to measure the undercooling temperature during the solidification process since it gives information about whether or not a certain material favors the heterogeneous nucleation. In bibliography there are some research works dealing with undercooling measurements in an electromagnetic levitation setup either concerning pure silicon or silicon containing a controlled percentage of impurities. Those kinds of experiments give information about the role that the presence of nucleation centers, inside the droplet, has on the final microstructure of silicon but do not take under consideration the role that the substrate (crucible or mould walls) plays on the nucleation of silicon [4][5][6][7][8][9].

The influence that the substrate has on the undercooling of silicon has not been studied thoroughly, but some investigations in the area have been done. Appapillai et al. [10,11] determined the nucleation properties using differential scanning calorimetry to measure the undercooling below the melting temperature of molten 1 silicon in contact with silicon oxides and nitrides. Various coatings on the silicon samples were tested for their influence on nucleation. Both dry and wet oxides coatings presented higher interfacial stability resulting in higher undercooling values than the nitrides coatings. However the undercooling measurements presented significant variations probably due to the coatings nonuniformity and the overall samples deformations during melting and solidification process.

A scientific research closely related to the present work has been done by Brynjulfsen and Arnberg [12]. They studied the nucleation undercooling phenomena during the solidification of silicon on SiO 2 , coated with oxidized Si 3 N 4 ,a n dA l 2 O 3 by the sessile drop method. Substrates provided by the commercial crucible producer for photovoltaic applications, Vesuvius, were also investigated. They concluded that the coating parameters such as thickness, roughness and the amount of oxygen in the coating have no significant influence on the undercooling of silicon. However they used a thermocouple placed below the sample holder in order to measure the undercooling temperature and this method cannot give such precise values of the undercooling. In the present work, it is planned to improve these experiments in three directions: a silicon piece is placed and melted directly on a substrate (this avoids the problems of nonuniformity of the coating and the deformation of the sample), use of pyrometers for measuring the temperature directly on the silicon and in addition the meltingsolidification process is visualized in-situ. The final crystalline structure of the solidified silicon droplet is also investigated.

In order to perform these experiments a special installation has been designed, developed and optimized and is presented extensively in the experimental part. Representative results concerning the solidification of silicon on zyarock and silica substrates are discussed in the second part of the article.

Experimental procedure

A special experimental setup has been implemented, as shown in figure 1, to measure the undercooling temperature and give access to the investigation of the nucleation phenomena during the solidification of silicon on a substrate. An electronic grade silicon square piece is placed on a substrate which is housed in a silica crucible. The silica crucible has been chosen among other materials since it is known that it does not favor silicon nucleation [START_REF] Sylla | European Solar Energy Conference[END_REF]. The substrate is hold and centered on the bottom of the crucible and its dimensions are 10 × 10 mm. The silicon mass is 0.8 ± 0.2 gr, depending on the material of the substrate so as after total melting it becomes hemispherically shaped and covers the whole surface of the substrate. This is in order to keep the Si/substrate area constant from one experiment to another. The heating and thus the melting of silicon is realized by electromagnetic induction with the aid of a Celes Generator with 50 kW power operating at 50 kHz. Since silicon and silica are bad conductors of electricity at room temperature, a graphite susceptor is used in order to heat the silicon by conduction through the silica crucible and the substrate and also by radiation through the crucible. As it can be seen in figure 1a the height of the graphite susceptor is smaller than the height of the silica crucible. This choice has been made for the two following reasons: i) C propagation inside the silica crucible, that would cause silicon sample pollution, should be avoided. It is well known that C interacts with Si leading either to the precipitation of SiC or to carbon solidification in interstitial positions in the silicon, affecting the nucleation process in both cases [START_REF] Beaudhuin | [END_REF]. ii) Graphite crucible radiates when heated above 700 °C. The reflection of the graphite radiation on the surface of the hemispherical melted silicon affects the temperature measurement by pyrometers as will be discussed below. The decrease of the graphite height eliminates this non desired vertical reflection of the radiation on the silicon surface, allowing the measurement of the exact Si temperature during the whole process.

The above described system is housed inside a water cooled stainless steel vacuum chamber evacuated to a pressure of 3 × 10 -6 mbar with a turbomolecular pump (TMP on figure 1b) and backfilled with ultra high purity argon gas (99.999%) to a pressure of 1000 mbar (figure 1b). The temperature inside the graphite crucible is measured during the whole process by two mobile type C thermocouples (W5%Re -W26%Re) operating in the range of temperature 0 °C to 2320 °C. As already mentioned, these thermocouples are placed too far from the silicon sample to measure precisely the rapid temperature recalescence significant of nucleation. Then, silicon temperature during the melting-solidification process and the undercooling is measured in parallel by means of an IRCON Modline 5 52-2020 monochromatic pyrometer operating in the temperature range 600 °Cto 2000 °C (spectral range 0.85 μmt o1 . 1μm) and an IR-CON Modline 5 5R-1810 bichromatic pyrometer operating in the temperature range 700 °C to 1800 °C (spectral range 0.75 μmt o1 . 0 5μm). They are both placed on a window at the top of the vacuum chamber. The use of two different pyrometers helps in estimating accurately the actual temperature of the silicon. A monochromatic pyrometer gives the mean temperature value, influenced by the emissivity factor specified to the pyrometer [15]. In case of silicon the temperature value given by the monochromatic pyrometer can vary significantly since the emissivity of solid silicon is 0.6, greater than that of liquid silicon, 0.2 [16]. In these experiments, the emissivity of the monochromatic pyrometer has been adjusted to 0.6. On the other hand a bichromatic pyrometer gives the maximum sample temperature value without having to take into account the emissivity variation [15].

From the first experiments, it became obvious that the silicon surface change, from solid to liquid and vice-versa, influences significantly the temperature values given by the pyrometers. Thus a camera was installed above the monochromatic pyrometer as shown in figure 1b and the whole melting-solidification process has been recorded in real time, through the pyrometer viewing window. Two degassing steps are taking place, the first one at 150 °C and the second one at 1000 °C. At 1000 °C the chamber is rinsed twice and then is backfilled to atmospheric pressure with Ar in order to start the melting process. Approaching the silicon melting temperature the heating rate is decreased in order to detect the melting plateau due to the enthalpy of melting absorbed by the Si. After melting the residual power gives an increase of temperature up to [110 °C to 200 °C] which depends on the material of the substrate. Then the sample is maintained about 5-10 min at this temperature in order to assure its full melting. The electrical power is cut abruptly and the silicon droplet starts to cool down at 300 °C/min, reproducibly from one substrate to the other.

In summary, the output of each experiment is the two recorded temperature profiles, the video film showing the melting and solidification process and the solidified silicon droplets samples. In the frame of this research different substrates (oxides, carbides, borides and nitrides) have been studied, however, in the present paper we focalize on the results obtained after the solidification of Si on zyarock and fused silica substrates. Zyarock is the standard material used for the industrial crucibles in the photovoltaic industry. It is a fused silica product with the following typical composition: silica 99.6%, alumina 0.12%, iron oxide 0.10%, titania 0.027%, magnesia 0.013% sodium oxide 0.008%, calcia 0.007% and potassium oxide 0.005% [17].

Fig. 3 Melting process of silicon on a zyarock substrate as recorded by the camera.

Results and discussion

Figure 3 illustrates a top view of the melting process of silicon on a zyarock substrate as recorded by the camera. Since the camera looks at the sample through the pyrometer window, the black ring that appears in all images is the monochromatic pyrometer measurement spot. The solid Si appears in black, the liquid silicon in white and the square substrate in grey. As it can be seen in the second image (b) the melting process starts from the bottom where the sample is in contact with the substrate. In the third image (c), a floating solid piece of silicon can be seen on the top surface of the droplet. In the last image (d) when the specimen is totally melted it can be remarked a bright zone around the black spot in the middle of the droplet. Silicon in a liquid state behaves as a mirror and thus this bright zone is due to the vertical reflection of the graphite radiation on the droplet surface. On the contrary, the dark central circle is the reflection of the crucible opening. It can be seen in image (d) that the pyrometerspotisinsidethiscircle,assuringthecorrectness of the temperature measurement values. The whole melting process of silicon was found to be the same in the case of the fused silica substrates.

Figure 4a shows a representative temperature profile of the silicon sample during the melting and solidification process on a zyarock substrate as recorded by both the monochromatic and the bichromatic pyrometers.

Zyarock is the material of the crucible that the PV industry uses but it needs to be mentioned that in the present case it is not coated with the anti-adherent Si 3 N 4 coating. During melting the sample absorbs the heating energy, due to the needed enthalpy of fusion, and the temperature remains constant as it can be seen from the presence of a temperature plateau in the bichromatic pyrometer temperature curve. However, in the monochromatic temperature curve the beginning of the melting process is detected by the decrease in the slope of the curve, despite the fact that the heating rate provided by the external supply remains constant. The fact that Fig. 4 Temperature profile of the silicon sample during the melting and solidification process on a zyarock substrate.

the monochromatic temperature curve does not present a melting plateau is probably due to the smaller spot size compared to the bichromatic one. This means that the given temperature represents the temperature of the solid silicon that floats on the surface of the liquid, as presented in figure 3c. At the end of the plateau some temperature fluctuations can be observed. As confirmed by the recorded video, this is due to the fact that the non melted solid pieces of silicon are moving on the top of the droplet and thus the viewing field of the pyrometers is partially occupied with some varying solid and liquid phases.

After complete melting of the solid, a successive temperature increase is detected at about 20 min due to the fact that the enthalpy of fusion is no more absorbed by the sample. The temperature is stabilized and after 7 minutes the electrical power is cut abruptly and the silicon stars to cool down. The abrupt temperature rise observed at 28 min is the recalescence event caused by the latent heat of solidification release during the nucleation and growth of the silicon crystal.

Figure 4b shows in more details the solidification period. Concerning the bichromatic pyrometer measurements, it can be seen a typical isothermal plateau during solidification. Nucleation occurs at temperature T n = 1318 °C, at the beginning of the recalescence. After recalescence the sample remains almost isothermal for approximately 35 sec at T g = 1370 °C until the solidification process is completed. This duration is in agreement with the recorded video during solidification as it will be discussed below. The thermal plateau after recalescence corresponds to the solidification temperature of the nucleated crystals, which is theoretically 1410 °C. The maximum nucleation undercooling temperature ( T) can be approximated as the difference between T g and T n and thus in the present experiment T = 53 °C. There should be no further nucleation event after T n , and further solidification can only be achieved by crystal growth. The experiments have been repeated three times and the measured undercooling values were in the range of 41 °Cto 53 °C. In the same figure, the monochromatic pyrometer curve does not present this abrupt increase of temperature caused by the creation of the first nucleus in the melted silicon as observed in the previous case. This is not surprisingly at all since, as it has already been discussed, the monochromatic pyrometer measures the mean value of sample temperature, then averaging the light emitted by the liquid and solid phases However, there is an obvious correlation between the two curves and both measurements are needed in order to better understand the nucleation and crystal growth phenomena and verify the measured undercooling temperature and solidification time.

The temperature profile is found to present a similar tendency, when the silicon sample is melted and solidified on a fused silica substrate (figure 5). However, the measured undecooling value is 89 °C and it is higher in comparison to the case of silicon solidification on a zyarock substrate.

Figure 6 shows the morphologies of the growing crystal during the solidification of silicon on zyarock and Fig. 5 Temperature profile of the silicon sample during the melting and solidification process on a fused silica substrate. fused silica substrates as observed by the installed camera. The dark gray and bright parts in the figures are the undercooled liquid and the solid that has been solidified from the undercooled liquid, respectively. At the beginning of solidification, thin solid strips crossing the whole droplet can be observed. They develop to several continuous lines as time goes on. Jian et al. [18] have observed similar structures performing rapid solidification experiments in an electromagnetic levitation setup. They suggested that the crystal growth depends on the nucleation undercooling and in the nucleation undercooling region from 40 to 100 + 10 K (similar to the present study) the initial crystal seed solidifies as dendrites that form a number of plates. Apparently the process is the same in case of solidification on a substrate. The "horn" appearing at the end of solidification is due to the 10% decrease of density between molten and solid silicon.

Figure 7 shows the macroscopic views of silicon droplets solidified on fused silica and zyarock, substrates. The undercooling measurements for a zyarock substrate ranged from [41 to 49 K], while, in the case of polished fused silica, the range was [45 to 89 K]. It can be seen that the nature of the material of the substrate plays a role on the solidification process and droplet formation. These experiments confirm that silica is a bad nucleating agent for Si, then validating the choice of a silica crucible in order to protect the molten droplet from external contamination. The significantly smaller undercooling in the case of zyarock also shows that the chemical and/or roughness of the substrate impact the nucleation behavior of Si. Experimental results are currently under interpretation. A correlation between the measured undercooling temperature, the microstructure of the solidified droplet, the interactions at the silicon/substrate interface and the precipitation that takes place and the grain competition during the initial stage of the solidification process of the droplets will be discussed in details in a forthcoming paper.

Conclusions

An experimental setup has been designed and developed in order to investigate the nucleation phenomena during the solidification of electronic grade silicon on different substrates. It has been found that the melting process starts from the bottom of the sample where there is a contact with the substrate. The temperature profile of the silicon sample during the melting and solidification process on a zyarock substrate presented a temperature plateau during melting and a typical cooling curve, with undercooling, recalescence and plateau, during solidification. The thermal plateau after recalescence denotes that the solidification is carried out under isothermal conditions. As during experiments performed under electromagnetic levitation, the formation of continuous strips, presumably dendrite tips intersecting the drop surface, was observed at the beginning of solidification and they develop to several continuous lines as time goes on. Finally, the macroscopic views of the solidified silicon droplets showed the important influence that the material of the substrate has on the nucleation and solidification process. This research is still in progress. The crystalline structure, the microstructure and the grain competition during the initial stage of the solidification process of the droplets is currently under investigation. As the present results show that the set-up and experimental procedure allow emphasizing differences in nucleating power of different substrates, the investigation will be extended to other substrate materials such as carbides, borides, nitrides and Si 3 N 4 -coated zyarock.

We strongly believe that the interpretation of the obtained experimental results and sample characterization will give supplementary information to better understand the heterogeneous nucleation of silicon and its impact on the ingot grain structure.

Fig. 1

 1 Fig. 1 Schematic illustration of the experimental set-up. a) Central part of the setup. b) General view of the experiment with measurement facilities.

Fig. 2

 2 Fig. 2 Schematic temperature profile as measured by the thermocouple placed inside the graphite susceptor.

Figure 2

 2 Figure2presents the experimental meltingsolidification procedure giving the heating and cooling rates in the different process steps.Two degassing steps are taking place, the first one at 150 °C and the second one at 1000 °C. At 1000 °C the chamber is rinsed twice and then is backfilled to atmospheric pressure with Ar in order to start the melting process. Approaching the silicon melting temperature the heating rate is decreased in order to detect the melting plateau due to the enthalpy of melting absorbed by the Si. After melting the residual power gives an increase of temperature up to [110 °C to 200 °C] which depends on the material of the substrate. Then the sample is maintained about 5-10 min at this temperature in order to assure its full melting. The electrical power is cut abruptly and the silicon droplet starts to cool down at 300 °C/min, reproducibly from one substrate to the other.In summary, the output of each experiment is the two recorded temperature profiles, the video film showing the melting and solidification process and the solidified silicon droplets samples. In the frame of this research different substrates (oxides, carbides, borides and nitrides) have been studied, however, in the present paper we focalize on the results obtained after the solidification of Si on zyarock and fused silica substrates. Zyarock is the standard material used for the industrial crucibles in the photovoltaic industry. It is a fused silica product with the following typical composition: silica 99.6%, alumina 0.12%, iron oxide 0.10%, titania 0.027%, magnesia 0.013% sodium oxide 0.008%, calcia 0.007% and potassium oxide 0.005%[17].

Fig. 6

 6 Fig. 6 Morphologies of growing crystals during the solidification of silicon on a zyarock and fused silica substrates.

Fig. 7

 7 Fig. 7 Macroscopic top/side views of solidified silicon droplets on fused silica and zyarock substrates.

Acknowledgements. Through numerous discussions, Drs. N. Eustathopoulos, B. Drevet and D. Camel took an active part in the design and optimization of the experimental setup and procedure. Special thanks are due to Professor Fukuyama Hiroyuki for his comments on the pyrometric temperature measurements. This research has been funded by the ANR-10-HABISOL-007-03 "Mosaïque" project. Key words. silicon, solidification, nucleation, undercooling, silica substrates.