Audrey E Cartraud
email: cartraud.audrey@hotmail.com

Matthieu Le Corre
email: lecorre@univ-reunion.fr

Jean Turquet

Julie Tourmetz
email: tourmetzj@seor.fr

Plastic ingestion in seabirds of the western Indian Ocean

ABSTRAC T

We investigated seabird plastic ingestion in the western Indian Ocean by analyzing the stomach contents of 222 individuals belonging to nine seabird species (including two endangered species endemics to Reunion Island). The most affected species were tropical shearwaters (79%) and Barau's petrels (59%). The average number of plastic particles per contaminated bird was higher in Barau's petrels (6.10 ± 1.29) than in tropical shearwaters (3.84 ± 0.59). All other studied species also showed plastic presence in their stomach contents. The mass of plastic particles was significantly higher both in juvenile's Barau's petrels and tropical shearwaters than in adults. These results demonstrate the foraging areas of seabirds of the western Indian Ocean have a high level of plastic pollution. In Reunion Island, hundreds of tropical shearwaters and Barau's petrels are attracted by urban lights and die each year. We suggest taking advantage of this situation by using these species as long-term indicators of plastic marine pollution in the region.

Introduction

The increasing rate of plastic pollution in the oceans worldwide is well documented [START_REF] Derraik | The pollution of the marine environment by plastic debris: a review[END_REF]Plastics Europe, 2013;[START_REF] Wilcox | Threat of plastic pollution to seabirds is global, pervasive, and increasing[END_REF] with concentrations of plastic fragments up to 580,000 pieces per square kilometre [START_REF] Barnes | Accumulation and fragmentation of plastic debris in global environments[END_REF]. This emerging pollution is now a major threat to marine wildlife, and this includes entanglement, ingestion and chemical contamination. Ingestion of plastic by marine fauna has been reported all around the world [START_REF] Derraik | The pollution of the marine environment by plastic debris: a review[END_REF][START_REF] Talsness | Component of plastics: experimental studies in animals and relevance for human health[END_REF][START_REF] Verlis | Marine debris is selected as nesting[END_REF] and affects a large number of seabirds including albatrosses, petrels, shearwaters, seagulls and boobies [START_REF] Azzarello | Marine birds and plastic pollution[END_REF][START_REF] Van Franeker | Seabirds, gyres and global trends in plastic pollution[END_REF]. The consequences of plastic ingestion by seabirds include stomach obstruction and internal wounds resulting in low body conditions and death [START_REF] Ryan | Effects of ingested plastic on seabird feeding: evidence from chickens[END_REF]. Plastic ingestion can also impact the physical condition of the bird, reduce their reproduction potential and cause mortality due to toxins transfer [START_REF] Teuten | Transport and release of chemicals from plastics to the environment and to wildlife[END_REF][START_REF] Lavers | Plastic ingestion by flesh-footed shearwaters (Puffinus carneipes): implications for fledgling body condition and the accumulation of plastic-derived chemicals[END_REF]. However the effects of plastic can vary among species, as some species have the ability to regurgitate (e.g., skuas, albatross) thus limiting undesired effects [START_REF] Azzarello | Marine birds and plastic pollution[END_REF]. The species with a low capacity of regurgitation, except when feeding youngs (e.g., petrels), are particularly susceptible to plastic accumulation [START_REF] Fumess | Ingestion of plastic particles by sea-birds at Gough Island, South Atlantic Ocean[END_REF]. The effects of plastic ingestion can also vary in terms of residence time in seabirds' gastro-intestinal tracks depending on the size of the ingested items, the type of plastic and also the quantity and composition of other elements in birds stomachs [START_REF] Ryan | How quickly do albatrosses and petrels digest plastic particles?[END_REF]. Most of the studies on plastic ingestion realized so far have been focusing on the northern hemisphere [START_REF] Yamashita | Physical and chemical effects of ingested plastic debris on short-tailed shearwaters, Puffinus tenuirostris,in the North Pacific Ocean[END_REF][START_REF] Avery-Gomm | Northern fulmars as biological monitors of trends of plastic pollution in the eastern North Pacific[END_REF][START_REF] Van Franeker | Seabirds, gyres and global trends in plastic pollution[END_REF], but also in South Africa [START_REF] Ryan | The incidence and characteristics of plastic particles ingested by seabird[END_REF][START_REF] Ryan | Regional differences in plastic ingestion among southern ocean fur seals and albatrosses[END_REF], South America [START_REF] Copello | Marine debris ingestion by Southern Giant petrels and its potential relationships with fisheries in the Southern Atlantic Ocean[END_REF] and around eastern Australia [START_REF] Lavers | Plastic ingestion by flesh-footed shearwaters (Puffinus carneipes): implications for fledgling body condition and the accumulation of plastic-derived chemicals[END_REF][START_REF] Gilbert | Plastic ingestion in marine and coastal bird species of Southeastern Australia[END_REF]. Only few studies have been focusing on the tropical areas and all of them were realized in the Pacific Ocean [START_REF] Spear | Incidence of plastic in seabirds from the tropical Pacific, 1984-91: relation with distribution of species, sex, age, season, year and body weight[END_REF][START_REF] Verlis | Marine debris is selected as nesting[END_REF][START_REF] Verlis | Seabirds and plastic don't mix: examining the differences in marine plastic ingestion in wedge-tailed shearwaters chicks at near-shore and offshore locations[END_REF].

So far no information has been published on plastic ingestion rates by seabirds in the western Indian Ocean. A few studies have shown plastic concentration in the water column [START_REF] Barnes | Natural and plastic flotsam stranding in the Indian Ocean[END_REF], on the beaches [START_REF] Duhec | Composition and potential origin of marine debris stranded in the Western Indian Ocean on remote Alphonse Island, Seychelles[END_REF][START_REF] Bouwman | The flip-or-flop boutique: marine debris on the shores of St Brandon's rock, an isolated tropical atoll in the Indian Ocean[END_REF] and on marine turtles [START_REF] Hoarau | Ingestion and defecation of marine debris by loggerhead sea turtles, Caretta caretta, from by-catches in the south-West Indian Ocean[END_REF]. In this study we analyzed plastic ingestion in nine seabird species breeding or foraging at Reunion Island and at Juan de Nova (Mozambique Channel), among which two species are endemic and endangered (the Barau's petrel, Pterodroma baraui) or critically endangered (the Mascarene petrel, Pseudobulweria aterrima). The four other breeding species of Reunion were included in the study: the tropical shearwater (Puffinus bailloni), the wedge-tailed shearwater (Ardenna pacifica), the white-tailed tropicbird (Phaethon lepturus) and the brown noddy (Anous stolidus). We also included the lesser noddy (Anous tenuirostris), which does not breed in Reunion but which use the island

Methods

All samples used for this study were found dead in Reunion Island or Juan de Nova. Petrels and shearwaters from Reunion Island are attracted by urban lights and a large scale rescue program has been conducted since 1996 by the Société d'Etudes Ornithologique de La Réunion (SEOR) to monitor the number of birds affected and find solutions to reduce the impact [START_REF] Le Corre | Light-induced mortality of petrels: a 4-year study from Réunion Island (Indian Ocean)[END_REF][START_REF] Salamolard | Impacts des éclairages urbains sur les pétrels de Barau, Pterodroma baraui, sur l'Ile de la Réunion et mesures pour réduire ces impacts[END_REF][START_REF] Gineste | Tropical shearwater population stability at Reunion Island, despite light pollution[END_REF][START_REF] Rodriguez | Seabird mortality induced by land-based artificial lights[END_REF]. The vast majority of the birds found are rescued and released safely but 10% die as a consequence of fatal injuries. We used these dead birds for plastic ingestion analysis. Similarly, seabirds affected by light pollution have previously been used to perform plastic ingestion studies in the Canary Islands [START_REF] Rodriguez | Factors affecting mortality of shearwaters stranded by light pollution[END_REF] and a recent study has demonstrated the interest of using birds disoriented by light for such a monitoring [START_REF] Rodriguez | Seabird mortality induced by land-based artificial lights[END_REF]. Other seabird species are regularly found by people and brought to the wildlife rescue center of the SEOR. These birds are generally injured or weakened by bad weather or marine conditions. Some birds are rehabilitated and released while others die and are kept frozen for later analysis. Sooty terns collected in Juan de Nova all died from cat predation.

We decided to cover a wide range of species in order to capture the variability of the vulnerability to plastic pollution in relation to trophic ecology, foraging behavior, foraging ranges and migration strategies of each species. Samples from Reunion Island were obtained over a period of 14 years (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016) and samples from Juan de Nova over a period of four years (2004)(2005)(2006)(2007)(2008). Samples were kept frozen until necropsies were performed in 2016. Before necropsy, birds were defrosted at room temperature. Necropsies were performed by removing the rib cage and pectoral muscles, exposing the abdominal viscera. Body condition of the birds was analyzed by recording the state of the pectoral muscles categorized as "good" (muscles appear convex), "medium" (muscle appear flat) or "poor" (muscles appear concave). The content of the gizzard and proventriculus were then examined for the presence of plastic using a dissecting microscope. Plastics were counted and classified by type: hard plastics, films and fibers (Fig. 1). Items were placed in Eppendorf tubes and dried in an oven at 50 °C for 48 h and then weighed on a digital balance ( ± 0.00001 g). The color of plastic particles was not recorded.

Statistical analyses

Data were analyzed using RStudio version 0.99.446 with univariate tests to analyze differences of plastic ingestion rates between species and age classes. Data were analyzed to check for Normality with a Shapiro test. When data were normal we used t-tests, ANOVAs and Tukey HSD tests. When data were not normal we used Kruskal Wallis and Pairwise Wilcox tests.

Results

Considering all species together, 50% of the birds analyzed had plastic in their gizzard or proventriculus, but there was strong variation in relation to species and age (Figs. 2 and3). The two most contaminated species were the tropical shearwater (79% of the birds had plastic in their gut, n = 56) and the Barau's petrel (63%, n = 62), followed by the lesser noddy (43%, n = 21), the brown noddy (33% n = 9), the wedge-tailed shearwater (33%, n = 9), the white-tailed tropicbird (29% n = 35) and the sooty tern (15%, n = 27). We analyzed only one critically endangered Mascarene petrel and we found plastic in its gut (100%, n = 1). Finally, we found one piece of fiber in one of the Cape Gannet analyzed (50%, n = 2, Table 1). There was no relation between plastic ingestion and muscular condition.

The average number of plastic particles per contaminated bird was higher in Barau's petrel (6.10 ± 1.29) and tropical shearwaters (3.84 ± 0.59) and lower in other species (Fig. 2). We investigated the effect of age on plastic ingestion of tropical shearwaters, Barau's petrels and white-tailed tropicbirds (Fig. 3). There were no significant differences in the proportion of juveniles and adults having ingested plastic in the three studied species (Barau's petrels: Chi-square = 1.22, p = 0.269; tropical shearwaters: Chi-square = 1.59, p = 0.208; whitetailed tropicbird: Chi-square = 1.03, p = 0.310). The number of particles per contaminated bird was higher in juveniles than in adult Barau's petrels (7.06 ± 1.57 and 2.38 ± 0.8 particles, respectively, p < 0.001), tropical shearwaters (4.09 ± 0.62 and 3.33 ± 1.15 respectively, p = 0.20), and white-tailed tropicbird (2.67 ± 1.20 and 1.71 ± 0.29 respectively, p = 0.06) but the difference was significant for Barau's petrels only.

We also tested differences in the mass of the plastic particles ingested by Barau's petrels, tropical shearwaters and white-tailed tropicbirds. There were no significant differences between species (p = 0.2735) but the mass of plastic particles was higher in juveniles than in adults in both tropical shearwaters (t = 2.1539, p = 0.0405) and Barau's petrels (t = 2.1109, p = 0.0429, Fig. 4). The number of juveniles and adults having ingested plastic was too low in white-tailed tropicbirds to investigate the effect of age. Regarding the amount and the mass of plastic ingested over time, no significant differences could be observed. This study has only been performed on tropical

A B C

8 mm 4 mm 2.5 mm as a roosting place during its post-breeding migration, probably originating from nearby Mauritius or Seychelles populations. Finally we also included the sooty tern (Ornithoprion fuscatus) from Juan de Nova as it is one of the most abundant and widespread species of the region and of the tropical oceans as a whole [START_REF] Feare | An inventory of sooty terns (Sterna fuscata) in the western Indian Ocean with special reference to threats and trends[END_REF] as well as two Cape Gannets (Morus capensis) found in Reunion Island and probably originating from South Africa.

shearwaters as it was the only species where enough birds were analyzed over time.

Hard plastic fragments, fibers and films were the main debris recorded in this study (Fig. 1). Plastics found were mainly fibers, reaching a total of 100% for sooty terns and brown noddies (Fig. 5). Hard fragments and films also represented an important part of plastics in Barau's petrels with on average 1.56 ± 1.15 and 1.51 ± 1.5 particles per contaminated bird respectively. Hard fragments and films were also present in tropical shearwaters, tropicbirds and the wedge-tailed shearwaters' stomach contents (Fig. 5). There were no significant

Discussion

Species-specific differences in plastic ingestion

Our study shows that the nine seabird species included in this study ingest plastic accidentally or opportunistically when foraging or when fed by their parents, but there were significant differences between species. Tropical shearwater and Barau's petrel were the two most impacted species, both in terms of ingestion occurrence and in terms of number and mass of ingested plastic particles per bird. Petrels and shearwaters are known to ingest plastic frequently in various parts of the world [START_REF] Azzarello | Marine birds and plastic pollution[END_REF][START_REF] Moser | A fourteen years survey of plastic ingestion by western North Atlantic seabirds[END_REF][START_REF] Robards | Increasing frequency of plastic particles ingested by seabirds in the subarctic North Pacific[END_REF]. In the northern Pacific Ocean, 88.5% of sooty shearwater (Puffinus griseus) and 81.1% of short-tailed shearwater (Puffinus tenuirostris) had ingested plastic particles [START_REF] Ogi | Migration, wintering distribution and habitat use of an endangered tropical seabird, Barau's petrel Pterodroma baraui[END_REF]). Ogi's study also revealed a significant increase in plastic ingestion from 1975 to 1987. More recently, short-tailed shearwaters have been studied on Stradbroke Island in Australia between 2010 and 2012, showing a plastic ingestion rate of 67% (Acampora et al., 2014). Our study shows that it is also the case in the western Indian Ocean. No previous study has been done on plastic ingestion by tropical shearwaters or Barau's petrel, although [START_REF] Danckwerts | Isolation and characterization of 15 polymorphic microsatellite loci for the Barau's petrel (Pterodroma baraui), an endangered endemic of Réunion Island (Indian Ocean)[END_REF] noted opportunistically in a study on foraging ecology and diet that 16.7% of the fledglings Barau's petrels had plastic particles in their gut (n = 36 birds analyzed). The level of plastic ingestion was lower in the wedge-tailed shearwater, which may be explained by the fact that we sampled mostly adults (7 out of 9) and adults are known to accumulate less plastic than fledglings (Acampora et al., 2014;[START_REF] Van Franeker | Seabirds, gyres and global trends in plastic pollution[END_REF] and this study, Table 2). No conclusions could be drawn regarding muscular condition of the individuals compared to mean number or weight of particles. A recent study recorded similar results on wedge-tailed shearwaters on Lord Howe Island in Australia where there was no obvious influence of ingested plastic on the body condition of the individuals [START_REF] Lavers | Ingestion of marine debris by wedge-tailed shearwaters (Ardenna pacifica) on Lord Howe Island, Australia during 2005-2018[END_REF].

The fact that petrels and shearwaters have more plastic in their gut may be explained by the presence of a gizzard in the Procellariidae which can accumulate non-digested food items such as squid beaks or plastic particles. The foraging ecology of Procellariidae could also explain this high ingestion rate. Petrels and shearwaters are often scavengers and are susceptible to feed at the surface of the water on dead preys or floating organic fragments and can therefore mistake plastic for preys. Besides, [START_REF] Savoca | Marine plastic debris emit a keystone infochemical for olfactory foraging seabirds[END_REF] suggested that plastic debris emit the scent of marine infochemicals (dimethyl sulfide) creating an olfactory trap for olfactory foraging seabirds such as petrels and shearwaters. The rate of plastic ingestion was less important in the three species of terns studied and on the white-tailed tropicbird, as found in other studies [START_REF] Hyrenbach | First evidence of plastic ingestion by white-tailed tropicbirds from O'ahu, Hawai'i[END_REF][START_REF] Roman | Anthropogenic debris ingestion by avifauna in Eastern Australia[END_REF], Table 2).

Terns and tropicbirds perform plunge diving to catch their prey and they do not scavenge. This probably reduces the risk of confusion with plastic items [START_REF] Harrison | Seabirds of Hawaii: Natural History and Conservation[END_REF]. Furthermore, the different species studied do not forage in the same areas, which may result in different plastic ingestion rates due to different level of plastic pollution in these areas. Tracking studies have been realized in the western Indian Ocean (Pinet et al., 2011;[START_REF] Le Corre | Tracking seabirds to identify potential marine protected areas in the tropical western Indian Ocean[END_REF][START_REF] Jaeger | Geolocation reveals year-round at-sea distribution and activity of a superabundant tropical seabird, the sooty tern Onychoprion fuscatus[END_REF] unpublished data) and show different feeding areas and migration patterns for most of the species studied here. Similarly, previous studies have revealed an important plastic difference found in stomach contents of a Fulmar species, Fulmarus glacialis, between individuals from Alaska (average of 2.8 particles per stomach) and individuals from California (average of 11.3 particles per stomach). These differences were attributed to different feeding areas, coastal areas in California being more polluted due to high population [START_REF] Van Franeker | Seabirds, gyres and global trends in plastic pollution[END_REF].

Differences of plastic ingestion in relation to age

Our results show a higher number and a higher mass of plastic particles in juvenile Barau's petrels, compared to adults and a higher mass of plastic particles in juvenile tropical shearwaters compared to adults. These differences are most probably due to food regurgitation from the adults to feed the chicks. Indeed, juveniles do not regurgitate and thus accumulate a large proportion of the plastic particles ingested by adults. Several other studies have shown similar patterns, especially among petrels and shearwaters [START_REF] Rodriguez | Factors affecting mortality of shearwaters stranded by light pollution[END_REF][START_REF] Verlis | Ingestion of marine debris plastic by the wedge-tailed shearwater Ardenna pacifica in the Great Barrier Reef, Australia[END_REF][START_REF] Acampora | Comparing plastic ingestion in juvenile and adult stranded short-tailed shearwaters (Puffinus tenuirostris) in eastern Australia[END_REF], but also on albatrosses [START_REF] Rapp | Community-wide patterns of plastic ingestion in seabirds breeding at French Frigate Shoals, Northwestern Hawaiian Islands[END_REF].

Types of ingested particles

In this paper we also briefly analyzed the types of plastic ingested. Seabirds tend to select distinct shapes and colored plastic, confusing it for potential preys [START_REF] Shaw | Colour-and form-dependent loss of plastic micro-debris from the North Pacific Ocean[END_REF]. More recent studies showed the type of plastic ingested by seabirds was dependent on the plastic fragment characteristic (UNEP, 2016). In this paper, some species such as O. fuscatus or A. stolidus showed a distinct attraction for plastic fibers with an ingestion rate reaching 100% compared to other plastic materials. Fibers are also frequently found in every other species and come from the fishing industry (fishing lines and ropes).

Implications for conservation

Our study shows that the level of plastic ingestion and accumulation is particularly high in the endangered Barau's petrel (and possibly the Critically endangered Mascarene petrel), compared to the other species studied. The impacts of plastic debris on both individual survival and population dynamics are difficult to measure, however other studies have suggested that plastic ingestion may lead to gastrointestinal blockage, reduced storage volume of the stomach or uptake of hazardous chemicals, impacting their reproduction potential and causing mortality [START_REF] Azzarello | Marine birds and plastic pollution[END_REF][START_REF] Ryan | Effects of ingested plastic on seabird feeding: evidence from chickens[END_REF][START_REF] Teuten | Transport and release of chemicals from plastics to the environment and to wildlife[END_REF]. This is particularly worrying for the endemic petrels of Reunion Island which dynamics is already weakened by invasive predators [START_REF] Faulquier | Feral cats Felis catus threaten the endangered endemic Barau's petrel Pterodroma baraui at Reunion Island (Western Indian Ocean)[END_REF], light-induced mortality [START_REF] Le Corre | Light-induced mortality of petrels: a 4-year study from Réunion Island (Indian Ocean)[END_REF] and possibly global warming [START_REF] Legrand | Current wintering habitat of an endemic seabird of Réunion Island, Barau's petrel Pterodroma baraui[END_REF].

A call for a long term monitoring of plastic ingestion in the western Indian Ocean

Our study is the first to quantify plastic ingestion in a range of tropical seabird species of the western Indian Ocean. We have shown that in this part of the world the oceanic food webs are polluted by plastic debris and this is very worrying as it may impact a wide range of marine animals. We have also shown that fledgling tropical shearwaters and Barau's petrels are highly impacted. We suggest to use these two species as bioindicator species to monitor plastic pollution in this region. These two species are attracted by artificial light and hundreds of birds are found stranded annually and rescued [START_REF] Le Corre | Light-induced mortality of petrels: a 4-year study from Réunion Island (Indian Ocean)[END_REF][START_REF] Gineste | Tropical shearwater population stability at Reunion Island, despite light pollution[END_REF]. Although on average 90% of the rescued birds are released successfully, 10% are found dead or die as a consequence of injuries. This represent a regular and predictable number of dead birds, estimated between 50 and 80 fledglings Barau's petrels and 50 to 100 fledgling tropical shearwaters which can be used for necropsies and plastic ingestion monitoring as we did in this study (see also [START_REF] Rodriguez | Seabird mortality induced by land-based artificial lights[END_REF]. In the northern hemisphere, the level of plastic ingestion in Northern Fulmars is routinely used by European policies as a longterm indicator of environment quality and plastic pollution [START_REF] Van Franeker | Seabirds, gyres and global trends in plastic pollution[END_REF]. The same monitoring should be implemented in the western Indian Ocean and tropical shearwaters and Barau's petrels are very appropriate for such a monitoring (see also [START_REF] Ryan | Monitoring the abundance of plastic debris in the marine environment[END_REF][START_REF] Rodriguez | Seabird mortality induced by land-based artificial lights[END_REF].

Fig. 1 .

 1 Fig. 1. (A) Plastic fibers, (B) hard fragments and (C) films collected from tropical shearwaters.

Fig. 2 .

 2 Fig. 2. Percentage of individuals per species having ingested plastic and average number of plastic particles retrieved in affected seabird ± standard error. (TS = tropical shearwater, BP = Barau's petrel, LN = lesser noddy, WS = wedge-tailed shearwater, BN = brown noddy, WT = white-tailed tropicbird, ST = sooty tern). Species with different letters are significantly different from each other.

Fig. 4 .Fig. 5 .

 45 Fig. 4. Average weight of plastic particles retrieved in affected seabird ± standard error. (TS = tropical shearwater, BP = Barau's petrel). * represents significant differences.

Table 1

 1 Seabird species and life stages analyzed in this study.

Table 2

 2 Previous records of plastic ingestion in white tailed tropicbirds, sooty tern, brown noddy and wedge tailed shearwaters.

Matthieu Le Corre: Designed the study and wrote the paper. Jean Turquet: Designed the study. Julie Tourmetz: Managed the collection of the analyzed seabirds.

Acknowledgments

Audrey Cartraud benefited a MSc internship grant from DEAL (Direction de l'Environnement, de l'Aménagement et du Logement). We thank all volunteers and permanent staff of SEOR as well as the local population of Reunion Island for their continuous effort to save endemic seabirds and Region Reunion and Life + Petrels for their work regarding the organization of the seabirds rescues. We are also very thankful to Morgane Manoury and Baptiste Rivolier that helped with the necropsies. This work is part of the program PP609 leaded by MLC and authorized by the Centre de Recherche sur la Biologie des Populations d'Oiseaux.

Declaration of interest

None.


Thomas Thiebault
email: thomas.thiebault@ephe.psl.eu

Comment on "The multi-mechanisms and interlayer configurations of metoprolol uptake on montmorillonite"

Keywords: Comment, Speciation, Metoprolol, Adsorption, Ion-dipole interaction

In the work "the multi-mechanisms and interlayer configurations of metoprolol uptake on montmorillonite", published in Chemical Engineering Journal, montmorillonite are used as an an adsorbent for the uptake of metoprolol under various experimental conditions. The results are interesting but the authors has made a mistake when assessing the speciation of metoprolol. This β-blocker is mostly neutral rather than zwitterionic for pH values up to pKa1. This mistake may induce confusion and false conclusions for the readers of Chemical Engineering Journal unfamiliar with such organic contaminants, especially when assessing the impact of pH on the adsorption mechanisms of metoprolol onto clay minerals.

In a recent work published in Chemical Engineering Journal, Li and co-authors [START_REF] Li | The multi-mechanisms and interlayer configurations of metoprolol uptake on montmorillonite[END_REF] reported the adsorption of metoprolol onto montmorillonite under various experimental conditions. Even if the scientific quality of this paper cannot be disclaimed, the presented speciation diagram of metoprolol and several consequent discussions presented in this article appear to be inappropriate.

As described in the manuscript (Section 2, Fig. 1b), metoprolol displays two pKa, pKa1 = 9.67 and pKa2 = 14.09 respectively [START_REF] Li | The multi-mechanisms and interlayer configurations of metoprolol uptake on montmorillonite[END_REF]. However, when the equilibrium solution pH is between pKa1 and pKa2, metoprolol is neutral rather than zwitterionic, as confirmed by numerous studies in the literature [START_REF] Niedbala | Influence of competing inorganic cations on the ion exchange equilibrium of the monovalent organic cation metoprolol on natural sediment[END_REF][START_REF] Caron | Structure-Lipophilicity Relationships of Neutral and Protonated β-Blockers, Part I, Intra-and Intermolecular Effects in Isotropic Solvent Systems[END_REF][START_REF] Kibbey | Adsorption of Beta Blockers to Environmental Surfaces[END_REF][START_REF] Amirdehi | Solvent Effects on Acid-Base Equilibria of Propranolol and Atenolol in Aqueous Solutions of Methanol: UV-Spectrophotometric Titration and Theory[END_REF].

The zwitterionic charge state is indeed not possible for metoprolol because pKa,base < pKa,acid [START_REF] Schaffer | A guideline for the identification of environmentally relevant, ionizable organic molecule species[END_REF]. Hence, the speciation diagram should be presented as described in Figure 1. The consequences of this mistake are obvious in the sections dealing with the impact of pH and temperature on the adsorption capacity of metoprolol onto montmorillonite.

The adsorption capacity of metoprolol as a function of pH value displays a strong decrease for pH values up to 8.0. Hence, for pH values close or above pKa1 value, the uptake of metoprolol is strongly diminished as it is well-explained in the manuscript. However, this uptake drop cannot be presented as a shift from pure cation exchange (i.e. protonated form) to a mix between electrostatic interaction and ion-dipole interaction (i.e. zwitterionic form), because above pKa1, metoprolol is mostly neutral. As a result, the decrease in the adsorption capacity should be understood through the shift from a protonated to a neutral form as the pH increase. Generally, the adsorption mechanisms of neutral organic compounds onto clay minerals are considered to be weak electrostatic interactions which not lead to high adsorption capacities or favorable adsorption behaviors [START_REF] Thiebault | Clayey-sand filter for the removal of pharmaceuticals from wastewater effluent: percolation experiments[END_REF][START_REF] Zhang | Sorption and desorption of carbamazepine from water by smectite clays[END_REF]. As raised in the manuscript, at pH = 11, 7% of metoprolol is cationic whereas 93% is neutral. This indicates that even if the whole cationic species are sorbed, a significant part of neutral metoprolol is also adsorbed probably through weak electrostatic interactions. This potential adsorption mechanisms of neutral metoprolol are also visible on the Fig. 6 (Section 3.5.) of the manuscript, in which the impact of temperature on the uptake of metoprolol at different pH is presented. The uptake variation at pH 10 (i.e. cationic/neutral form) and pH 7 and 3 (i.e. cationic form) displays distinct patterns. Hence, at pH 3 and 7 the uptake increases as the temperature decreases and the opposite pattern is observed at pH 10. Even if the impact of temperature on the cation exchange extent is still discussed [START_REF] Chen | Tramadol hydrochloride/montmorillonite composite: Preparation and controlled drug release[END_REF][START_REF] Thiebault | Adsorption mechanisms of emerging micro-pollutants with a clay mineral: Case of tramadol and doxepine pharmaceutical products[END_REF], it is very surprising that the uptake of neutral metoprolol (i.e. pH = 10) is improved at higher temperatures, because such weak electrostatic interactions are generally considered to be more significant at lower temperatures [START_REF] Berhane | Sorption-desorption of carbamazepine by palygorskite-montmorillonite (PM) filter medium[END_REF]. However, the discussion of the adsorption mechanisms as a function of the temperature is not performed in the paper, except the modification of thermodynamic parameters. The shift from positive to negative enthalpy values at pH 10 and the two other values (i.e. 3 and 7) respectively, could be assume as an indicator of such modifications.

As a reminder, this comment was motivated by the willingness to confine the propagation of incorrect information in the literature rather than criticize estimated colleagues. The correction could therefore be published by the authors.

Figure 1 :

 1 Figure 1: Speciation of metoprolol under different pH values