# Design of hybrid Chitosan-Montmorillonite materials for water treatment: Study of the performance and stability

Julie Salvé, Brian Gregoire, Leslie Imbert, Fabien Hubert, Nathalie Karpel Vel

Leitner, Maud Leloup

► To cite this version:

Julie Salvé, Brian Gregoire, Leslie Imbert, Fabien Hubert, Nathalie Karpel Vel Leitner, et al. Design of hybrid Chitosan-Montmorillonite materials for water treatment: Study of the performance and stability. Chemical Engineering Journal Advances, 2021, 6, pp.100087. 10.1016/j.ceja.2021.100087. hal-03178958

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# Design of hybrid Chitosan-Montmorillonite materials for water treatment: Study of the performance and stability

<figure><img src="image_2.png" /></figure>

Julie Salvé, Brian Grégoire, Leslie Imbert, Fabien Hubert, Nathalie Karpel Vel Leitner, Maud Leloup*

Institut de Chimie des Milieux et Matériaux de Poitiers, 4 rue Michel Brunet, Bât. B27, TSA 51106, 86073 Poitiers Cedex 9, France

## ARTICLE INFO

**Keywords:**

Biopolymer-clay

Structure

Coagulation flocculation

Dissolved Organic carbon release

Sorption

## ABSTRACT

Using hybrid chitosan-montmorillonite materials as environmentally-friendly alternatives to metallic salts in water treatment, and especially for drinking water production, is a new challenge. However, the coagulation-flocculation process requires minimization of polymer release and reactivity optimization to guarantee low Dissolved Organic Carbon content of the treated waters. To meet these requirements, hybrid materials were designed in this study to combine structural stability and good sorption efficiency, even in hard waters. The influence of preparation parameters (polymer molecular weight, polymer to clay mass ratio, addition method and post-preparation treatment) was evaluated and the obtained composites were fully characterized by X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). Jar-Test experiments were carried out to assess the performance of the hybrids toward MethylOrange, a model molecule for the hardly removable hydrophilic compounds. The best results were obtained by drying the hybrids prepared at a 0.5:1 polymer to clay mass ratio by the direct addition method. Structural analysis revealed that the polymer chains of the dried-hybrid were more strongly bound to the Montmorillonite particles while remaining accessible to the organic molecules. This structure prevented its destabilization in the presence of high ionic strength. Under these conditions, all chitosan based materials exhibited some reactivity loss. However, the dissolved chitosan release was reduced to 13% of the initial dosage for the optimized dried hybrid compared to 47% for the undried version. To conclude, the optimized hybrids still outperformed pristine chitosan, Montmorillonite and $FeCl_3$ for the removal of MethylOrange, thus highlighting their potential for future use.

## 1. Introduction

The development of new greener and cleaner technologies for water treatment has become one of the biggest challenges in this area for the coming generation [1-4]. These technologies have to be thought in the context of green engineering by making processes or materials without noxious products formation in all the design and operating phases of the treatment, and by taking into account their end-of-life and the possibility of waste valorisation. Among the potential candidates for metallic salts replacement in the coagulation-flocculation processes, composite organic-organic or inorganic-organic materials made of the association of polymers (e.g. poly (N-vinylcaprolactam), carbamoyl benzoic acids) or clay particles and chitosan are of great interest regarding their unique reactivity toward recalcitrant pollutants [5-10]. These composites are now been widely studied in the literature, and in addition to the low-cost, biodegradable and non-toxic nature, their performance toward the uptake of dyes or metallic cations in batch experiments is well recognized [7,8,11-17]. As regards chitosan-clay composites, many studies

highlighted their potential benefits for industrial wastewater treatment, including no need for pH correction, good performances, accelerated settling or recyclability into the process [18,19]. However, these studies have focused on the optimization of pollutants removal efficiency in synthetic or real wastewaters through a better understanding of the influence of a limited number of preparation parameters (polymer to clay ratio, type of clay, polymers properties) on the materials structure.

Nevertheless, the use of such materials for drinking water production has never been envisaged yet, considering the additional requirements that must be satisfied. Although being important from a mechanical point of view, the connection with water matrix composition as well as water treatment processes and their associated specific conditions has not been established so far.

Water treatment chains of drinking water production facilities from surface freshwaters usually include several steps (coagulation and flocculation, sedimentation, filtration and disinfection) [20]. The coagulation-flocculation is the most important step promoting the aggregation and settling of suspended solids, colloidal particles and a part

<footnote>* Corresponding author.
E-mail address: maud.leloup@univ-poitiers.fr (M. Leloup).</footnote>
of the Dissolved Organic Matter (DOM), which are responsible for the water turbidity, colour and organic load [21]. The classical inorganic coagulants such as aluminium or iron salts have demonstrated adverse consequences during the drinking water treatment (water acidification, mineralization changes, selectivity toward the hydrophobic fractions of DOM, tanks and pipes corrosion) [22–24]. They also produce mineral sludge to be landfilled because of the lack of valorisation potential [25–33]. The development of green materials from natural products, such as chitosan-clay composites, is one of the strategies envisaged to reduce the environmental impact of this step. Still, the use of chitosan-clay composite for the coagulation-flocculation process may produce new unintended effects (e.g. membrane filtration fouling, disinfection by-product formation, bacterial re-growth) due to the potential breakdown of the composite itself and in turn to the subsequent transfer of undesirable dissolved chitosan (CS) in the following treatment steps [34–36]. Consequently, the stability in water of the materials, in addition to the optimization of the sorption properties, has to be investigated in the coagulation-flocculation conditions using jar-tests setup.

The unique methodology presented in this paper can therefore be regarded as a first effort to demonstrate the potential of such hybrid materials for drinking water treatment in a green engineering context. Because of the complexity of water matrices and the diversity of DOM components in surface waters, this research requires that synthetic waters and model organic molecule be considered at first glance. Due to its small size and hydrophilicity, MethylOrange molecule was chosen as an example for the hydrophilic fraction of the organic matter that may be hardly removed by the conventional metallic salts. The goal of this study is to correlate the structure of the hybrids to their performance during coagulation-flocculation experiments while maintaining their reactivity and stability so that Dissolved Organic Carbon (DOC) minimization is guaranteed even in hard water.

## 2. Materials and methods

### 2.1. Materials

The clay minerals used in this study are the SWy-3 montmorillonite reference (MMT, (Ca₁₂Na₃₂K₀₅)[Al₃,₀₁Fe(III)-₄₁Mnₐ₀₁Mg-₅₄Ti₀₂][Si₇-₉₈Al-₀₂]O₂₀(OH)₄) from the Source Clay Repository of the Clay Minerals Society with a cationic exchange capacity (CEC) of 76.4 meq/100g. Chitosan (CS, pKa = 6.2–6.7) of High Molecular Weight (310–375 kDa) and Low Molecular Weight (50–190 kDa) with a 75% deacetylation degree (DD), iron (III) chloride solution (FeCl₃, 41% wt, d = 1.45), HCl, NaOH, KBr, CaCl₂ and NaCl were supplied by Sigma-Aldrich. Methyl Orange MO (C₁₄H₁₄N₃NaO₃S, pure, MW = 327.34 g mol⁻¹, pKa = 3.4) was chosen as the target pollutant in order to mimic the hydrophilic and hardly removable fraction of NOM in natural waters. MO was provided by Fischer Scientific.

The chemical structures of MO and CS are presented in Supporting Information (SI), Figure S1. The DD measurements for each of the used polymers are given in SI Text S1, Figure S2 and Table S1.

### 2.2. CS-MMT hybrids preparation

The reference MMT powder was firstly dialysed in osmosed water (Spectrum Labs™ Dialysis membrane 7000 MW) to dissolve and remove the gypsum naturally present. Remaining coarse mineral impurities were removed after size-fractionation by centrifugation (900 g, 4 min 50, Sorvall™ Legend XFR Thermo Scientific™) to obtain a clay suspension of <1 µm particles. Then, homoionic Na-MMT was prepared by contacting the MMT suspension with 100 mL of a 1 M NaCl solution. After a 24 h contact time, the solid fraction was separated by centrifugation and a fresh NaCl solution was added. These steps were repeated three times to guarantee complete cation exchange. Removal of the chloride excess was performed by a thoroughly washing of the solid with

deionized water (Milli-Q, 18.2 MΩ cm⁻¹) until the chloride concentration was negligible as confirmed by the AgNO₃ test.

Chitosan (CS) stock solution was prepared by addition of CS in acid solution (HCl 1 N, pH = 2) under magnetic stirring until complete dissolution. Then, pH was adjusted to 4.9 with NaOH (1 N).

Chitosan-montmorillonite hybrids (CS/MMT) were prepared using two different protocols at room temperature. The ‘Fast protocol’ (F) consisted in directly adding 25 mL of CS solution in 10 mL of Na-MMT suspension. Then, the mixture was handily stirred for a few minutes. The solid/solution (w/w) ratio was 0.34%. The ‘Slow protocol’ (S) consisted in the drop-by-drop addition of the same CS solution with a peristaltic pump (0.3 mL min⁻¹). Then, the mixture was stirred on a shaking table for 48 h. Hybrids were prepared either with chitosan of Low Molecular Weight (LMW) or High Molecular Weight (HMW) and with initial chitosan-montmorillonite mass ratios of w = 0.125:1, 0.5:1 and 1:1 g g⁻¹. Some CS/MMT hybrids were dried (notation (d)) in a ventilated oven (60 °C) for 24 h. Then, the powder was milled and dispersed in ultra-pure water with the same solid/solution ratio as before drying. CS/MMT hybrids without drying step were named ‘undried’ (ud). For example, a ‘CS/MMT’ (F) w = 0.5 (d)’ hybrid corresponds to the addition of CS solution in Na-MMT suspension by the ‘Fast’ protocol, with a mass ratio CS:MMT w = 0.5 followed by drying and dispersion in ultra-pure water.

It was observed that the addition sequence (adding the Na-MMT dispersion into the CS solution: MMT/CS, instead of the reverse order: CS solution added into the MMT suspension CS/MMT) did not influence significantly the structure-reactivity relationship of the hybrids (Supporting Information, Text S2 and Figure S3). The reproducibility of the hybrids preparation is presented in Supporting Information (Text S3 and Figure S4).

### 2.3. Structural characterization

X-ray diffractograms were obtained on a Bruker D8 Advance X-ray diffractometer (XRD) with CuKα radiation (λ = 0.154 nm), 2θ range between 2° and 50° with a step size of 0.025° s⁻¹. Recorded XRD patterns from oriented samples were obtained by pipetting slurries of the suspension on glass slides and drying these suspensions at room temperature. The infrared spectra were recorded in transmission by accumulation of 100 scans at a resolution of 4 cm⁻¹ in the region 400–4000 cm⁻¹ on a Nicolet 6700 spectrometer equipped with a CsI beamsplitter and DLaTGS detector. Pellet samples were produced by weighing 150 mg of spectroscopic grade KBr powder (Aldrich) with around 1 mg of sample. The mixtures were finely ground and pressed to form 13 mm of diameter disks. The background reference was obtained on pure KBr pellet prepared under the same conditions.

After the hybrids preparation, the quantity of ‘free’ chitosan of the reactants was determined by measuring the Dissolved Organic Carbon (DOC) concentrations of a 10 mg-CS L⁻¹ equivalent treatment dose in deionized water (200 mL), after filtration on a 0.45 µm cut-off membrane (Merck Millipore, HVLP04700 MilliporeDurapore®, 0.45 µm pore size, hydrophilic PVDF, 47 mm membrane). ‘Free’ chitosan refers to the polymer chains that are not bound to clay particles or bound to very small clay minerals thus forming structures which size is less than 0.45 µm. Both forms are thus considered as dissolved under the analysis conditions. DOC analyses were performed with a Shimadzu TOC-V<sub>CPN</sub> (Total Organic Carbon Analyser) advice (standard deviation ± 0.02 mg·C·L⁻¹) equipped with an ASI-V autosampler. All results are expressed as the average DOC concentration from triplicates. These measurements were indicators of the CS sorption on Na-MMT particles.

### 2.4. Reactivity tests

Reactivity experiments were performed using a jar-test apparatus (200 mL) at room temperature (298 K) and pH 5.5 to keep the amine
functions of chitosan protonated (pKa = 6.3). Sorption capacity of reactants was assessed within the 2–20 mg L⁻¹ Methyl Orange (MO) concentration range, consistent with typical values of the dissolved organic carbon concentration in surface freshwaters (1.0–36.0 mg·C·L⁻¹) [5,10,37]. The reactants doses were fixed at 10 mg·CS·L⁻¹ for hybrids containing chitosan, 50 mg·Fe·L⁻¹ for FeCl₃ and 10 mg·MMT·L⁻¹ for montmorillonite to maintain the same solid/liquid ratio between each experiment. For jar-tests with hybrids, CS or MMT, the stirring speed was fixed at 45 rpm for 20 min (flocculation). For FeCl₃, there were two agitation steps: first, 3 min at 150 rpm with pH adjustment to 5.5 (NaOH, 0.1 M or 1 M) in order to disperse the reactant for coagulation, then 20 min at 45 rpm for flocculation. The coagulation phase was not fulfilled for other reactants than FeCl₃ to preserve their structure and no pH correction was necessary for CS, MMT and hybrids jar-tests. After a 20 min settling time, solutions were filtered on 0.45 µm membranes and analyzed to determine the dye and chitosan residual concentrations.

In a second series of experiments, jar-tests were conducted with MO solutions enriched by 80 mg L⁻¹ Ca²⁺ to assess the influence of both ionic strength and MMT stability. Indeed, calcium is the most abundant cation in natural freshwaters [38] and can readily replace Na⁺ from the interlayer of MMT. The corresponding jar-tests were called ‘reactant name + Ca’.

## 2.5. Treatment efficiency : dye and residual chitosan quantification

The dye concentration (C) was calculated (Eq. 1) from absorbance measurements given by a UV-Visible 320 SAFAS Monaco spectrophotometer at 463 nm (standard deviation ± 0.001 cm⁻¹), using a 1 cm quartz cell.

$$C = A/\varepsilon l \quad (1)$$

where C is the Methyl Orange concentration (mg·MO·L⁻¹), l is the cell optical path (1 cm), ε is the extinction coefficient (0.0733 L·mg·MO⁻¹·cm⁻¹) and A is the measured absorbance.

The amount of sorbed dye per unit mass of the sorbent (Qe, mg·MO·g⁻¹) was calculated with Eq. 2.

$$Q_e = (C_i - C_e) \cdot V/m \quad (2)$$

where Cᵢ and Cₑ are the initial and equilibrium dye concentrations (mg·MO·L⁻¹), V is the used solution volume (L) and m is the sorbent mass (g·Fe, g·MMT or g·CS).

After treatment, Dissolved Organic Carbon (DOC) from residual chitosan was estimated by combination of carbon balance and calculated MO removal percentage (Eq. 3).

$$\text{DOC}_{\text{residual chitosan}} = \text{DOC}_{\text{e}} + \text{DOC}_{\text{i}}(R/100 - 1) \quad (3)$$

where DOCᵢ and DOCₑ are the initial and equilibrium DOC concentrations (mg·C·L⁻¹) and R is the MO removal percentage (%) determined from UV-Visible spectroscopy results (Eq. 4)

$$R = (C_i - C_e) \cdot 100/C_i \quad (4)$$

Reproducibility of reactivity tests was evaluated from jar-test replicates performed either with a single hybrid preparation or with four independent preparations of the same hybrid under identical conditions to include variability in the material preparation. Results are presented in Supporting Information (SI Figure S4).

## 3. Results and discussion

To evaluate the performance of the prepared hybrid materials and their applicability in the coagulation-flocculation process of the drinking water treatment, the reactivity of the individual components toward MO sorption was assessed. The isotherm displayed in Fig. 1 clearly demonstrate that neither the pristine Montmorillonite nor the FeCl₃ salt was efficient for removal of the dye molecule. At the opposite, at pH = 5.5, chitosan exhibits a higher reactivity toward MO sorption. For a starting

<figure><img src="image_3.png" /><figcaption>Fig. 1. MO sorption isotherms with FeCl₃, Na-MMT and HMW or LMW chitosan.</figcaption></figure>

MO solution concentration of 20 mg L⁻¹, where the quantity of negative charges brought by the sulfonate group of the MO molecule is 1.4 time higher than the available protonated amino-groups of the chitosan chains (DD = 75%), the sorption capacity of the high molecular weight chitosan reaches a value of 123 mg·MO·g·CS⁻¹. Given that the experiments were realized at pH = 5.5, it was considered that 95% of the amine groups were protonated in solution [39]. Then, in the hypothetical case where all protonated amino-groups of CS were coupled to the sulfonate groups of MO through electrostatic interactions, a maximum sorption capacity of 1358 mg·MO·g·CS⁻¹ would have been obtained. This suggests that only 9% of the chitosan active sites are accessible to MO molecules. The same reasoning applied to the low molecular weight chitosan (LMW) (Qe = 153 mg·MO·g·CS⁻¹) leads to a value of 11%, pointing to a better accessibility of the protonated amino-groups of CS. A possible explanation of the lower sorption capacity of HMW CS could be attributed to a greater chain entanglement as expected in the case of longer polymer chains [40]. These interactions contribute to decrease the mobility of the polymer chains and the probability for ion-pairing with anionic molecules. Schatz et al. [41–43] also evidenced that the rigidity increases with the molecular weight of CS for a given DD, which may negatively affect the possibility for the dye sulfonate groups to interact with the amine groups of glucosamine units due to steric hindrance and conformation adaptation.

### 3.1. Tailoring of CS/MMT hybrids in coagulation-flocculation processes

#### 3.1.1. Choosing the appropriate chitosan molecular weight (MW)

Since the chitosan chain length appears to be an important parameter controlling the flexibility and diffusion capacity of the polymer chains, LMW and HMW chitosan were used to prepare hybrids, with a chitosan to clay ratio of 0.5 g·g⁻¹ using either the fast or slow addition method. Fig. 2a presents the XRD patterns of the hybrids prepared along with the pristine Na-MMT. The basal diffraction peaks (001) of smectites is observed at 12.4 Å as expected for monohydrated Na smectites samples at room humidity [44–46]. Hybrids prepared by the fast method present an additional peak located at 22.7 and 21.2 Å for the HMW and LMW chitosan respectively. This suggests that some chitosan chains are accommodated within the interlayer domain of smectites, contributing to an increase in the interlayer distance. However, the presence of pristine Na-MMT basal peak at 12.4 Å indicates that all structural negative charges are not entirely compensated by chitosan and that some interlayer domain sites are not accessible through this preparation route. Low molecular weight chitosan chains penetrate more readily the interlayer domain as deduced from the relative decrease in intensity of the
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** (a) XRD patterns, (b) FTIR spectra (vibrational bands of interest are defined by black bars), (c) MO sorption isotherms and (d) DOC concentration of free chitosan in ultra-pure water (dose: 10 mg-CS L⁻¹) with montmorillonite Na-MMT, chitosan CS and hybrids CS/MMT (F) w=0.5 (ud), CS/MMT (S) w=0.5 (ud) prepared with High Molecular Weight (HMW) or Low Molecular Weight (LMW) chitosan.</figcaption></figure>

Na-MMT peak to the benefit of the Chitosan-MMT XRD peak. Unlike the fast method, the slow addition of chitosan to the clay minerals solution is observed to favor the intercalation process as observed from the very weak intensity of the Na-MMT smectites peaks, independently of the chitosan molecular weight. The diffraction peak assigned to the chitosan intercalated into the smectites clays is observed at 22.3 Å for both polymer sizes. Taking into account the thickness of the silicate layer at about 0.96 nm, the interlayer distance for each hybrid is about 13 Å. Similar interlayer distances were reported by Darder et al. [39] who concluded that chitosan may be accommodated in arrays of parallel sheets of chains. Okyuama et al. [47] refined the structure of fully deacetylated hydrated chitosan and concluded that the unit cell contains arrays of chitosan chains giving a dimension of 8.95 Å, much larger than the one of chitin (i.e. 4.74 Å) [47–48]. The recovered basal spacing reported here is larger than these values and points toward a more loosely packed structure than observed in the corresponding crystal.

Fig. 2.b presents the FTIR spectra of the as-prepared hybrids. By comparison with the CS and Na-MMT spectra, the vibrational bands can readily be assigned. Besides of the characteristic vibrational bands of Na-MMT smectites ($\delta_{Si-O}$ = 400–600 cm⁻¹, $\delta_{MO-H}$ = 700–900 cm⁻¹, $v_{Si-O}$ = 1000–1200 cm⁻¹, $\delta_{HOH}$ = 1650 cm⁻¹, $v_{HOH}$ = 3000–3600 cm⁻¹,

$v_{MO-H}$ = 3720 cm⁻¹), all prepared hybrids present additional vibrational features assigned to chitosan polymer [49–50]. The most striking differences between the various prepared samples are observed in the region 2800–4000 cm⁻¹. Of particular interest, the envelope of the stretching vibration of CH, CH₂ and CH₃ groups of chitosan in the region 2800–3000 cm⁻¹ presents narrower vibrational bands except for samples HMW ‘Fast’ (HMW CS/MMT (F) w = 0.5 (ud)). Well-resolved components are particularly observed in the case of HMW ‘Slow’ samples, for which complete chains intercalation was deduced from their XRD patterns. This spectral feature might therefore be indicative of more ordered chains resulting from interlayer confinement [51]. Although LMW chitosan chains were also observed to fully occupy the clay interlayer sites, the slight broadening of the C-H stretching vibrations is correlated to the higher chain flexibility as a result of shorter chains. On the basis of these observations, the spectral profile of HMW ‘Fast’ samples showing broader C-H stretching bands may be interpreted as rather flexible chitosan chains which contrasts with the chain intercalation concluded from the XRD pattern. This apparent discrepancy may be resolved if one considers that the chitosan chains penetrate only slightly the interlayer domain which mainly contains Na⁺ cations and that the major part of the chitosan chains are not confined to the interlayer domains of clays.
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** (a, b) XRD patterns, (c, d) FTIR spectra (vibrational bands of interest are defined by black bars), (e, f) MO sorption isotherms and (g, h) DOC concentration of free chitosan in ultra-pure water (dose: 10 mg-CS L⁻¹) with montmorillonite Na-MMT, chitosan CS and hybrids prepared with the fast (F) or slow (S) addition method with varying polymer to clay mass ratio w.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** (a) XRD patterns, (b) MO sorption isotherms and (c) DOC concentration of free chitosan in ultra-pure water (dose: 10 mg-CS L⁻¹) with CS and hybrids CS/MMT (F) w=0.5 (ud), CS/MMT (F) w=0.5 (d).</figcaption></figure>

The isotherms presented in Fig. 2.c highlight significant differences between the studied reactants. Since the sorbed MO quantity is normalized to the initial quantity of chitosan in the hybrids, the sites accessibility of the hybrids can be assessed. Regardless of the molecular weight of chitosan, sorbents prepared by the slow addition method exhibit very similar performances, slightly below the sorption capacity of pristine CS. As observed in Fig. 2.d, the quantity of free CS in solution represents about 20–30% of the total chitosan mass used for the hybrids preparation. Assuming that the reactivity of this free chitosan fraction is equivalent to that of pristine chitosan (Fig. 2.c), it could suggest that the interlayer chitosan chains may be poorly reactive toward MO capture, as compared to the performance of pristine CS. By contrast with the trend observed with LMW and HMW chitosan alone, for reactants prepared by the fast addition method, HMW chitosan strongly promotes the sorption of MO whereas LMW chitosan exhibits a strong decrease of its reactivity. Consequently, in the case of hybrids, when chitosan chains are only partially intercalated within the interlayer domain, more reactive sites appeared to be accessible toward MO sorption since the quantity of free chitosan is equivalent to that of sorbents prepared by the slow addition method (Fig. 2.d). It is inferred from these results that the partial intercalation of CS chains promotes a more extended and more open structure, making the reactive sites more accessible toward MO uptake. This molecular conformation may also favor inter-particulates bridging, increasing the probability of interaction between MO molecules and glucosamine groups and promoting the flocculation [52,53]. The low performance

of (LMW) CS/MMT (F) w = 0.5 (ud) sample is also paired with a low quantity of free chitosan, reaching a value of 20% of the total mass. The better flexibility and mobility of the shorter chains may favor the electrostatic interaction with the MMT surface increasing the quantity of sorbed CS and strongly limiting the accessibility of the amino-group toward MO molecules.

### 3.1.2. Optimum polymer-clay mass ratio

The chitosan to clay mass ratio influence was further investigated to highlight the relationship between composition, structure and reactivity. Only reactants prepared with the HMW chitosan will be discussed here because they performed better.

Figs. 3.a and 3.b present the XRD patterns of hybrids which chitosan to clay mass ratio varies from w = 0.125 : 1 to 1 : 1. First, regarding the fast preparation method (F), binary mixtures composed of chitosan intercalated into the interlayer of clay and pristine Na-MMT clay are observed (Fig. 3.a). The former phase XRD peak shifts continuously to low angles when increasing the quantity of chitosan with respect to the clay, suggesting an increase in the interlayer basal spacing. Consequently, the XRD peak of the pristine Na-MMT relatively decreases in intensity. Thus, by increasing the chitosan to clay mass ratio, more chitosan chains are intercalated into the interlayer domain, producing a higher basal spacing and promoting the cationic exchange of Na⁺. At the molecular scale, as provided by FTIR spectroscopy and displayed on Fig. 3.c, the C-H stretching region does not show distinctive features
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** (a) XRD patterns, (b) MO sorption isotherms and (c) DOC concentration from residual CS in deionized water or in presence of calcium (Ca) after a 10 mg-CS L⁻¹ jar-test on MO with chitosan CS or hybrids CS/MMT (F) w=0.5 (ud), CS/MMT (F) w=0.5 (d), CS/MMT (F) w=0.5 (ud) + Ca and CS/MMT (F) w=0.5 (d) + Ca.</figcaption></figure>

among the three samples, confirming that most of the chitosan chains has maintained their flexibility and only few of them are partially intercalated within the interlayer domain of clays. As shown previously, the slow addition method favors the intercalation of chitosan within the interlayer space (Fig. 3.b). The ratio *w* = 0.125:1 presents the characteristic peak of the pristine Na-MMT clay as expected since the initial quantity of chitosan represents only 70% of the clay cationic exchange capacity (Na-MMT CEC : 76.4 meq/100 g). The vibrational signature of the chitosan chains exhibits well-resolved components, indicative of some flexibility loss due to the almost complete intercalation of polymer chains (Fig. 3.d). When the CEC is exceeded in the case of the two highest chitosan to clay ratios, a single phase is observed and assigned to intercalated chitosan. Changes in their corresponding vibrational spectrum are thus observed, with the disappearance of the fine structure of the CH stretching vibration accounting for the increasing contribution of non-intercalated chitosan chains. For the 1:1 chitosan to clay mass ratio, the XRD peak is broadened with a corresponding basal spacing of almost 30 Å, suggesting strongly expanded interlayers. Whichever the relative quantity of chitosan and clays, the preparation method mainly drives the structure of the formed hybrids. The fast addition promotes the formation of binary mixtures where the chitosan chains remain flexible resulting from their very low interlayer penetration while the slow addition preparation routes favor their intercalation within the interlayer domain.

The performances of the as-prepared hybrids were examined toward the MO sorption and the results are displayed on Figs. 3.e and 3.f. The preparation route appears to be the main parameter governing the hybrids reactivity. Whichever the chitosan to clay ratio, the hybrids prepared by the fast addition method outperform pristine chitosan. This

result illustrates the benefit of associating clays and polymer (Fig. 3.e). The best dye uptake is observed for ratio *w* = 0.5, suggesting that the structure of the hybrid is optimum, with the most accessible sorption sites. Since the examination of the XRD and FTIR profiles for these samples showed the systematic presence of binary systems, it is inferred from the sorption isotherms that the hybrids performance is strongly correlated to the relative quantity of intercalated chitosan chains and pristine Na-MMT clay. Given the low interlayer penetration, the clay is possibly responsible for the anchorage of the chitosan chains, which in turn, makes them more open and more accessible toward MO uptake. The reactivity of the sorbents prepared by the slow addition method is below the performance of pristine chitosan for the *w* = 0.125:1 and 0.5:1 chitosan to clay ratios, suggesting that in some cases, the clay particles can have a negative effect on the hybrid performance (Fig. 3.f). The strongly expanded structure for *w* = 1:1 might explain the good reactivity of the sorbent with a higher accessibility of the amine groups due to chains bridging. Despite the good performance offered by the hybrids prepared by the fast addition protocol, some chitosan is systematically released into solution (Figs. 3.g and 3.h). Interestingly, the linear correlation observed between the quantity of free chitosan and the initial chitosan to clay mass ratio suggests that once the particle surfaces are saturated with chitosan chains, the excess of chitosan remains in solution as free chitosan.

## 3.2. Stabilizing CS/MMT hybrids for water treatment processes

Although good performance can be achieved by the rapid addition method, the release of free chitosan and the problems that can occur in the following treatment processes of a drinking water production chain
(e.g. membrane fouling, disinfection by-products formation) are a severe limitation to the use of these composite materials as is (SI Figure S5) [34–36]. Further research was therefore conducted to reduce the amount of chitosan released while maintaining the performance obtained. Owing to the polar nature of the glucose unit of chitosan chains [11], large amount of water are retained within the porosity of the polymer which may explain their release during the jar test experiments. Therefore, to increase the stability of these materials by reinforcing the chitosan-clay interactions, the optimum hybrid was dried at 60 °C for 24 h. In order to enable comparison with the undried hybrid which is kept in solution, the dried reactant was dispersed again in deionized water before sampling for XRD measurements. The XRD patterns of the two reactants are displayed on Fig. 4.a. Although the peaks position appeared to be conserved, the XRD profile of the dried reactant exhibits broader peaks of low intensities resulting from an increasing disorder of the MMT layers. Despite this slight modification of the hybrid structure, its performance toward MO uptake is maintained, suggesting that the accessibility to the sorption sites was conserved (Fig. 4.b). More importantly, drying the prepared hybrids reduces the amount of chitosan released by half compared to undried hybrids, suggesting that the clay-polymer interaction has greatly increased. (Fig. 4.c).

CS sorption properties can be altered by ionic strength, due to bridging flocculation and weakening of the electrostatic interactions by charge screening that leads to destabilization of complexes formed by chitosan [54]. Moreover, due to the cation exchange properties of clay minerals, the presence of other cations in the aqueous medium during the preparation or operating phases can compete strongly with the intercalated chitosan chains, affecting the composite performances [55]. Depending on the water quality and composition, especially for highly mineralized waters, a destabilization of the composite structure is thus expected during the coagulation-flocculation process, likely to lead to considerable reduction of its performance. To evaluate the stability of the prepared materials under conditions closer to those encountered in real water systems, and due to the great selectivity of clay minerals for divalent cations, tests were carried out in solutions containing 80 mg·Ca²⁺·L⁻¹ to mimic hard water.

Fig. 5.a presents the XRD patterns of the dried and undried version of the most efficient hybrid dispersed in either deionized water or calcium solution. Drastic changes in the structure are revealed on the XRD pattern of CS/MMT (F) w = 0.5 (ud). The two XRD peaks giving a distance of 23.3 Å and 12.6 Å attributed to CS/MMT and Na-MMT respectively, have disappeared to the profit of a single XRD peak at 15.3 Å corresponding to bi-hydrated Ca-MMT. This structural modification is accompanied by a drastic loss of reactivity, still remaining above the chitosan isotherm (Fig. 5.b). This phenomenon is equally supported by a strong release of chitosan in solution reaching a value of 47% of the initial chitosan mass at the end of the jar-test experiment (Fig. 5.c). All these observations point toward a cationic exchange process where Ca²⁺ ions have replaced the intercalated chitosan chains and unreacted Na-MMT. This contrasts with the CS/MMT (F) w = 0.5 (d) series of samples. The expected XRD lines corresponding to Ca-MMT are not observed despite the absence of the peaks corresponding to CS/MMT and Na-MMT (Fig. 5.a). This surprising result may indicate that the hybrid is strongly disorganized. The reactivity of this sorbent is closed to that of the undried one, revealing similar accessibility of the sorption sites. Despite a loss of reactivity in presence of Ca²⁺ ions, the most striking effect of the drying treatment is observed on Fig. 5.c where only 13% of the total chitosan mass was released in solution during the reactivity test, which is drastically lower compared to the 47% obtained for the undried reactant.

## 4. Conclusion

To our knowledge, this study reports for the first time the ability to control the structure and performance of CS/MMT hybrids and their potential for drinking water production in connexion with DOC limitation needs.

Results effectively demonstrate that CS/MMT hybrids performances toward MO sorption are closely related to their structure, which can be tailored by choosing appropriate preparation conditions. CS addition kinetics during the materials preparation mainly influenced the accessibility and flexibility of the fixed polymer chains onto MMT, when polymer to clay mass ratio rather impacted the quantity of sorbed CS. Unless drying, the obtained structures were poorly stabilized as CS remained exchangeable and suffered from competitive effect due to the presence of calcium ions into the water matrix leading to CS release. Hybrids preparation by direct addition of high molecular weight CS into a MMT suspension at a 0.5 mass ratio, followed by drying, was considered as optimal and resulted in the highest pollutant removal efficiency. Such conditions also allowed to prevent the destabilization of the hybrids structure due to the ionic strength. Moreover, the removal of the chosen model hydrophilic molecule is enhanced compared to pristine chitosan or iron salts while maintaining the lowest fraction of residual dissolved chitosan in solution compared to other hybrids preparation conditions. Tailoring of CS/MMT hybrids could thus be a promising solution for the removal of the NOM hydrophilic fraction during the coagulation-flocculation process in the drinking water treatment facilities.

Thereafter, the research will be focused on investigating the performances and selectivity of the optimized hybrids towards NOM in different water resources, quantifying the residual dissolved chitosan to determine the involved mechanisms and estimating the risk of unintended effects resulting from dissolved chitosan release (membrane fouling, disinfection by-products).

## Declaration of Competing Interest

The authors declare no competing financial interest.

## Acknowledgments

The authors acknowledge financial support from the European Union (ERDF) and "Région Nouvelle Aquitaine".

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cej.2021.100087.

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