Anionic reverse microemulsion grafting of acrylamide (AM) on HydroxyEthylCellulose (HEC): Synthesis, characterization and application as new ecofriendly low-cost flocculant

Journal of Water Process Engineering 31 (2019) 100807

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Anionic reverse microemulsion grafting of acrylamide (AM) on HydroxyEthylCellulose (HEC): Synthesis, characterization and application as new ecofriendly low-cost flocculant

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Sara Chaoufa, Soufian El Barkanyb, , Issam Jilalc, Youssef El Ouardid, Mohamed Abou-salamab, Mohamed Loutoub, Ablouh El-Houssainee, Hossain El-Ouarghif, Abderahmane El Idrissig, ⁎ Hassan Amhamdia, a

Laboratory of Research and Development in Engineering Sciences, Faculty of Sciences and Techniques, Abdelmalek Essaadi University, 32 003 Al Hoceima, Morocco Multidisciplinary Faculty of Nador, Department of Chemistry, Mohamed1st University, 60700 Nador, Morocco Laboratory of Solid, Mineral and Analytical Chemistry (LSMAC), Faculty of Sciences (FSO), Mohamed1st University, 60000 Oujda, Morocco d Laboratory of Engineering of Organometallic and Molecular Materials (L IMOM), Faculty of Sciences Dhar El Mehraz, Sidi Mohamed Ben Abdellah University, 30000 Fès, Morocco e Laboratory of Bioorganic and Macromolecular Chemistry, Department of Chemistry, Faculty of Sciences and Technologies, Cadi Ayyad University, 40000 Marrakesh, Morocco f Water and Environmental Management Unit, National School of Applied Sciences, Abdelmalek Essaadi University, 32 003 Al Hoceima, Morocco g Laboratory Applied chemistry and environmental (LCAE-URAC18), Faculty of Sciences of Oujda, Mohamed1st University, 60000 Oujda, Morocco b c

A R T I C LE I N FO

A B S T R A C T

Keywords: Anionic grafting Cellulose Reverse microemulsion Coagulation-flocculation Colloidal particles Heavy metal

New water-soluble cellulosic derivative based on HEC was successfully elaborated (HECc), and used as anionic coagulant-flocculant of colloidal systems Fe(OH)3 and to remove of heavy metal ions (Cu(II)) from aqueous media. HECc was synthesized in unconventional reverse microemulsion process using water as ecofriendly solvent and as hydrogen transfer agent, where cyclohexane was employed as continuous phase to reduce the hydrophobicity of the medium. The amide groups grafted on HEC main chain were saponified to increase the negative charge density, and then the zeta potential value of HECc derivative reached -45 mV. The proposed structure of HECc was confirmed by FTIR, 1H, 13C NMR and deept-135. The XRD technique was used to study the effect of grafting process on the crystalline order. Moreover, the morphological changes and the elementary composition of the surfaces were studied using scanning electron microscope equipped with EDX. The new HECc coagulant-flocculant showed a high coagulation-flocculation capacity toward Fe(OH)3 colloidal system at 35 μL as added volume of 0.5% flocculant solution. Moreover, the pH values showed an important effect (species distribution) on the coagulation-flocculation mechanism and on the flocculating capacity to remove (Cu (II).

1. Introduction With the enormous global market demand for cellulosic fibers, estimated at USD 37 billion in 2020 [1], cellulose chemistry has become a global priority of the modern economy. In comparison with the economic market of synthetic diamond, which will not exceed USD 29 billion in 2023 [2], cellulose and its derivatives have become the suitable to be the primary chemical resource of modern industry, where it currently dominates the second priority of the European bioeconomy [3], indicating that the exploitation of cellulose and its derivatives is a global issue in economic and environmental terms. Cellulose is the most abundant, biodegradable, renewable and biocompatible natural

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polymer [4]. Since, its typical and unique chemical structure, resulting from the high density of hydrogen bonds [5,6], confers it a high structural stability, limiting its direct application in many industrial areas. Moreover, its insolubility in common solvents, except that in complex, expensive and toxic solvent systems [7–12] is the major obstacle to homogenous modification process [13]. In addition, the thermal modification is remote because of its degradation before reaching the melting state. Then, some considerable efforts over the last decades have pushed towards new classes of solvents such as ionic solvents [14–20], DES [21–27], and NDES [28–31], which showed great efficiency in solvating the cellulose but still remain expensive, toxic in some conditions [32,33]. Indeed, it was found that an increase

Corresponding authors. E-mail addresses: [email protected] (S. El Barkany), [email protected] (H. Amhamdi).

https://doi.org/10.1016/j.jwpe.2019.100807 Received 30 December 2018; Received in revised form 10 March 2019; Accepted 21 March 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.

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water treatment agent with high coagulation-flocculation efficiency, non-toxicity, low cost, eco friendly and promising candidates for industrial applications.

in length of the alkyl chain in 1-alkyl-3- methylimidazolium salts increases the toxicity and can even lead to substances more toxic than methanol [34–36], and the second drawback that observed is the biodegradability decrease [37]. Various anionic, cationic and nonionic cellulose based flocculants have been synthesized in order to replace those based on synthetic current polymer for water treatment [34–36,38–44], medical and pharmaceutical industries [45–49]. Yongbo Song et al. studied the flocculating properties of cellulose grafted AM monomer (AMC) in the Urea/NaOH aqueous medium. The results reported that the flocculating capacity of the colloidal Fe(OH)3 particles increases over time reaction, promoting the saponification of amide groups into carboxylate form [50]. In addition, polyacrylamide grafted polyvinyl pyrollidone (PVP-gPAM) showed a good flocculation efficacy in coal-fine suspension, and in destabilization of aqueous suspension of nanoparticles (multi-walled carbon nano-tubes) without and in presence of other ions [51]. In the same way, Gum tragacanth grafted poly of diallyldimethylammonium chloride (GT-g-P(DADMAC)) enhanced the water solubility and functionality of GT, which showed a high efficacy as a flocculant in model aqueous suspensions of kaolin and multi-walled carbon nano-tubes (MWCNT) through standard jar test procedure [52]. The copolymers of Polyacrylamide and carboxymethyl cellulose, in aqueous alcohol medium using a ceric ion initiator and batch polymerization, was successfully prepared by Okieimen and tested their efficacy as flocculant against kaolin suspension [53]. Hydroxypropylmethyl cellulose phthalate (HPMCP) has been used as a flocculant for the recovery of silver from industrial wastewater [54]. The xanthates derivatives of cellulose showed also a good flocculation efficiency for calcite and chrysocolla [55]. The cellulosic ether derivatives such as Hydroxypropyl cellulose (HPC) and hydroxyethyl cellulose (HEC) have been widely used as intermediate water-soluble cellulosic polymer to prepare new classes of functional materials. Indeed, Ce(IV) induced free radical polymerization was used by Ahmed A. et al. to prepare pH-sensitive interpenetrating network (IPN) microspheres (MPs) by emulsion-crosslinking method using glutaraldehyde (GA) as a crosslinking agent [56]. The swelling behavior in water of HPC graft-copolymerized with acrylamide (AAm) using benzoyl peroxide as the initiator and crosslinked with glutaraldehyde was also studied [57].While, the removal of heavy metals from aquatic environments by flocculation using modified cellulosic ethers is poorly discussed in literature. In this paper, a new water-soluble cellulosic derivative (HECc) was successfully elaborated and investigated as an effective anionic coagulant-flocculant for colloidal systems Fe(OH)3 and to remove heavy metal ions (Cu(II)) from aquatic environments. HECc was successfully synthesized via an unconventional reverse microemulsion process using water as ecofriendly solvent and as hydrogen transfer agent, where cyclohexane was employed as continuous phase to reduce the hydrophobicity of the medium. The grafting reaction of AM was followed by an alkali saponification reaction, removing an amount of ammonia trapped by boric acid solution and quantified by volumetric dosage using Tashiro indicator. The saponification reaction showed an important increase in the negative charge density where the zeta potential value of HECc reached -45 mV. The proposed structure, effect of grafting process on the crystalline order, morphological changes and the elementary composition were studied using FTIR, 1H, 13C NMR, deept-135, XRD and SEM-EDX respectively. The new HECc flocculant showed a high flocculation capacity to Fe(OH)3 colloidal system at 35 μL as added volume of 1% flocculant solution. Furthermore, the pH variation was investigated to show its effect on the coagulation-flocculation mechanism and on the flocculating capacity to remove Cu (II) from aqueous solution. To our knowledge, based on a literature review, HECc has never been discussed before on the synthesis side and synthesis method, either for application as coagulant-flocculant colloidal systems or for the removal of heavy metals. HECc was successfully developed as a novel

2. Experimental and methods 2.1. Materials HEC (DS 1.5) of 95% purity was obtained from HIMEDIA Company, and dissolution–precipitation method in water–acetone solvents was used for its purification. AM (C3H5NO), cyclohexane (C6H12), copper nitrate (Cu(NO3)2) and iron chloride (FeCl3)were purchased from Sigma Aldrich. All other chemicals and solvents were obtained from Fluka Company and were used without further purification. 2.2. Methods 2.2.1. Instrumental analysis The chemical structures of HEC, HECac and HECc samples were characterized using Fourier Transform Infrared (FTIR) technique. The spectra were recorded on Shimadzu FTIR-8400S spectrometer using a finely ground KBr pellets with 2% of the sample at a resolution of 2 cm−1. An average of 40 scans were taken for each sample and recorded from 4000 to 400 cm−1. 1H and 13C NMR measurements were carried on a BrukerAvance 400 MHz spectrometer at 360 K using TMS as internal standard. DMSO-d6 and D2O were used as solvent for HEC and HECc, respectively. Approximately 10 mg of HEC was dissolved in 1 ml DMSO-d6 and 2 drops of TFA were added to displace the H2O peaks. In the case of 13C NMR for the both samples (HEC and HECc), approximately 50 mg of sample was dissolved in 1 ml of DMSO-d6 (or D2O) without TFA. For each sample, approximately 14,000 scans and a D1 of 2 s were required. The sample crystallinity was evaluated using X-ray diffraction technique and was performed on an X-ray Diffractometer EQUINOX 2000 using copper radiation CuKα (λ = 1.5418 A°), at an accelerating voltage of 40 kV and an operating current of 30 mA. All patterns were recorded in the range of 2θ (5°–35°). 0.25 g of each sample is pressed under 50 MPa to form pellets of 25 mm in average diameter. Thermal behaviors of HEC, HECar and HECc samples were performed using thermogravimetric analysis (TGA) on a Shimadzu DTG-60 simultaneous DTA-TG apparatus. The sample weight was between 8 and 12 mg. Two scans were run from room temperature to 500 °C at a rate of10 °C min−1under nitrogen purge (50 mL. min−1) The evolution of the surface morphologies and the microstructures of samples were investigated, before and after modification, using scanning electron microscopy (TESCAN VEGA III LM), with an accelerating voltage of 10 kV. Moreover, Energy dispersive X-ray analyzer (EDAX) was employed for profiling the different elements present on the polymer surfaces. The zeta potential (ζ), conductivity and electrophoretic mobility as a function of pH (pH: 2–11), of HEC and HECc in Milli-Q water at 0.1% of each sample, were measured using Zetasizer Nano ZS (Malvern Instruments) at 25 ± 0.1 °C. 2.2.2. Coagulation-Flocculation of colloidal Ferric hydroxide solution (Fe (OH)3) Ferric hydroxide colloidal system (Fe(OH)3) is prepared by the hydrolysis of saturated solution of ferric chloride (Fe(Cl)3) with boiling distilled water. The Fe(Cl)3 solution was added drop-wise to the boiling water until the obtaining of a brilliant dark-red colloidal solution appeared. The coagulation-flocculation study was carried out by adding 10–200 μL of an aqueous solution of HECc (1%) to 5 mLof saturated Fe (OH)3 colloidal solution. Then, the transmittance of the surnageant, for each added volume of flocculant solution, is measured using UV-1800 UV–vis Shimadzu spectrophotometer (resolution of 1 nm) between 500 and 600 nm. 2

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of acrylamide on HEC was justified by the appearance of a new absorption band at1672 cm-1 characterizing carbonyl amide (CO)amide on the HECac spectrum (Fig. 2a). Also, the appearance of a new absorption bands (overlapped by OH bands) at 3223 cm-1 attributed to the vibrations of the amide -NH2 indicates the success of the grafting reaction [59]. In addition, the effect of the reaction medium alkalinity on the saponification of amide groups to carboxylate form was confirmed by the appearance of a new absorption band at 1566 cm-1 attributed to the carbonyl of the carboxylate. Moreover, the comparison of the peak intensities at 1672 cm−1 and at 1566 cm−1 shows that the amide groups attached to HEC backbone are predominant in the cellulosic polymer. The absence of the absorption band characteristic of the π binding (C]C) at 1614 cm−1 indicates that HECac obtained is free of any residual product [60,61]. In relation with the applications envisaged, the negative charge on the HECc surface is ensured by the saponification reaction, allowing the conversion of the amide groups to the carboxylate form. This total conversion is justified by the total disappearance of the absorption bands of the group, and by a notable increase of the characteristic absorption band of carboxylate carbonyl vibrations at 1568 cm−1 (Fig. 2b). Furthermore, trapping of the ammonia released, by boric acid solution, allows following the saponification reaction in the presence of the Tashiro indicator, and was titrated with HCl solution. This typical dosage gives the different values of DS reported in Table 1. 1 H and 13C and Dept-135 NMR measurements were carried out using DMSO-d6 and D2O as solvent for HEC and HECc, respectively. In the case of HEC, TFA was added to DMSO-d6 to displace the H2O proton signals. The D2Owas used as solvent to avoid the overlap of AM methylene signals in HECc with those of DMSO. Fig. 3a and b shows 1H, 13 C and Dept-135 NMR spectra of HEC in DMSO-d6/TFA (a) and HECc in D2O (b). In Fig. 3a, the signals of the methylene protons on α of different hydroxyl groups are located at 3.6 ppm, and those attached to the carbon (C6) of the cellulose backbone gives some signals at around 3.5 ppm. The broad ring proton signals of the cellulose skeleton are recorded from 2.8 to 5.6 ppm. Moreover, the typical carbon signals of HEC methylene (Fig. 3a) were detected from 60.7 to 72.7 ppm, and the peaks assigned to the carbons of cellulose main chains are located in the game of 60–105 ppm [4,13,58]. The 1H NMR spectrum of the HECc sample (Fig. 3b) shows the recording of new signals at 2.43 ppm and 3.69 ppm, which are attributed to the methylene proton resonances of the grafted acrylate moieties (1) and (3), respectively. Furthermore, the chemical shifts distribution of the carbons in the HECc sample (Fig. 3b) gives the evidence of the grafting reaction by the detection of new peaks attributed to the sp3 carbons of the grafted acrylate entities (1) and (3) at 37.77 ppm and 67.71 ppm. The significant peak located at 180.04 ppm (7) was assigned to the quaternary carbon of carboxylate form [62,38,63], and the absence of a further signal in this area characterizing quaternary carbon is a strong indication of the total saponification process (amide → carboxylate). However, the persistence of the peak at 60.54 ppm on the 13C NMR spectrum of HECc indicates that there is still a partial amount of unmodified primary alcohols (hydroxyl group), where a DS value less than 1.5 can be expected [4,13,58]. DEPT-135 spectrum of HECc (Fig. 3b) confirms the investigation of the structural changes in the HEC structure. Indeed, the resonance signals of the cellulosic carbons (C1, C2, C3 C4 and C5) change the orientation by 180°, whereas the signal at 180.04 ppm (7) disappeared indicating its quaternary character, where the methylene protons signals (CH2) kept their orientation. The DS value was calculated using the integration of specific proton signals, and it was estimated at 1. In addition, the absence of the proton and the carbon signals of polyacrylamide is a good indication of hydrogen transfer and that the polymerization of AM was avoided. These results show that the grafting reaction was successfully performed and the HECc sample is free of any residual product.

2.2.3. Cu (II) removal capacity The coagulation-flocculation capacity (CFC) of HEC and HECc samples for the removal of Cu (II) metal ion was studied using 2 mL of flocculant aqueous solution at 0.5% and 10 mL of aqueous metal ion. The metal ion concentration ranges from 100 to 800 mg. L−1 and the pH value varied from 2 to 8. The pH of the different solutions was adjusted by adding different concentrations of HCl or NaOH. Indeed, the metal ion concentrations of the supernatant were determined by atomic absorption measurements using Flame Atomic Absorption (FAA) Thermoscienific iCE3000, Type iCE3500AA System. And the FCC (qe (mg. g−1)) was evaluated by the following equation (Eq. 1):

qe =

(C0 − Ce ) × V m

(1) −1

Where, C0 is the initial concentration of metal ion (mg.L ), Ce is the metal ion equilibrium concentration of metal ion (mg. L−1), m is the mass of HECc (g) and V is the volume of the solution (L). 2.2.4. Preparation of HECc HECc sample was obtained by anionic grafting of AM on HydroxyEthylCellulose (HEC) by reverse microemulsion (Water/ Cyclohexane), followed by alkali saponification of amide groups to carboxylate form. Since, 1 g (13.87 mmol) of AM was added to 1 g (4.38 mmol, 13.16 mmol/OH) of alkoxylated HEC in 7 mL of NaOH (1 N) solution. The mixture was kept under stirring, at room temperature, until a transparent solution appeared. Then 15 mL (2/3) of cyclohexane (C6H12) were added to the mixture under stirring, and heated at 60 °C for 2 h, then, the mixture was neutralized using ethanolic HCl solution and the HECac was precipitated in a large excess of cold ethanol, filtered under vacuum and washed with cold ethanol frequently, then it was stocked in a desiccator in the presence of P2O5 for one week. The saponification 1 g of HECac to HECc was carried out in NaOH (4 N) solution for 12 h (Fig. 1). 3. Results and discussions The new cellulosic derivative HECc, as a new low-cost, biodegradable and ecofriendly coagulant-flocculant of positive charged particles such as colloidal systems (Fe(OH)3) and heavy metal ions (Cu (II)), was successfully prepared by anionic grafting of AM on HEC using reverse microemulsion (Water/Cyclohexane). In the first step, HEC alkoxylation was carried out in NaOH (1 N) solution creating anionic sites (alkoxide) at the polymer surface [58], which initiate the grafting reaction by nucleophilic attack of the double bond of AM (Fig. 1). Generally, the reaction of a nucleophilic center with monomers having π system in their molecular structures allows to their anionic polymerization. For this reason, the water was used, on one hand, as ecofriendly solvent, and on the other hand as a hydrogen transfer agent to avoid the formation of polyacrylamide on the HEC main chain, which can limit its aqueous solubility. However, the grafting of HEC with acrylamide increases at a certain level the hydrophobicity to the resulting derivative, consequently its solubility in the aqueous medium was reduced. Then, the reactivity of HEC and consequently low degree of substitution values is expected. At this stage, cyclohexane was introduced to reduce the hydrophobicity, and to increase the solubility and consequently the reactivity of HEC. 3.1. Structural analysis 3.1.1. Fourier-transform infrared spectroscopy (FTIR) Fig. 2a and b shows the FTIR spectra of AM, HEC, HECac and HECc, respectively. The absorption characteristic band of the OH elongations is located around 3400 cm−1, and those detected at 2920–2881 cm−1 are attributed to the symmetric and asymmetric CH vibrations. The deformation vibration characterizing the natural absorbed water in HEC structure (Fig. 2a) is localized at 1640 cm-1 [4,13,58]. The grafting 3

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Fig. 1. Reverse microemulsion synthesis scheme of HECac and HECc and steric phyllotaxy design stability.

3.2. Scanning Electron microscopy

3.3. X-ray diffraction and thermal analysis

Fig. 4 shows SEM images and EDAX spectra of HEC, HECac and HECc. Unmodified HEC displayed a morphology consisting of agglomerated particles. The aggregates thus observed were discontinuously cemented between each other. These morphological entities resulted from the high density of hydrogen bonds that forms the predominant type of intra and interchain interactions in HEC structure. The introduction of the amide groups in the HEC backbone (HECac) causes a significant variation in the crystal order of the polymer structure, therefore, continuous morphology where the agglomerations disappear, and this result is confirmed by XRD. After saponification of HECac, the HECc microstructure has agglomerated again and this can be attributed to the decreasing in the density of the hydrogen bonds. Moreover, the increase of O/C atomic ratio and the appearance of Na peak on the EDX spectrum of HECc is strong indication of that the grafting reaction was successfully occurred. Then, the polymer surface becomes more anionic and enhances its solubility in aqueous medium and its ability to interact with positive charged particles such as heavy metals and colloidal systems.

Fig. 5a shows the XRD diffractograms of acrylamide, HEC and HECac. The XRD diffractograms of microcrystalline cellulose and HECac are presented in Fig. 5b. Fig. 5a demonstrates that the amorphous aspect of HEC disappeared after the introduction of acrylamide entities into its structure, and a new crystalline order appeared on HECac XRD diffractogram. This phenomenon results from substitution of OH by a new group leading to a change in the arrangement and the density of hydrogen bonds in HEC. Indeed, Fig. 5b shows that the new lattice planes characterized by Bragg angles 2θ = 14.20°, 17.02°, 18.70° and 25.84° is similar to that of microcrystalline cellulose with a clear shift to small 2θ angle, where the intercalation of the macromolecular chains, as effect of grafting reaction, will be suggested at supramolecular level. The success of the experimental approach is also proven by a notable destruction of the crystalline order of AM [64].The thermal behavior of HEC, HECac and HECc are shown in Fig. 5c. The thermograms show two phases of mass loss, the first is located at around 100 °C that is attributed to the loss of solvents and to the absorbed moisture. While the second seat of loss of mass is observed between 240 and 4

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Fig. 2. FTIR spectra of AM, HEC, HECac (a) and HECc (b).

45 mV, and indicating a complete deprotonation of the carboxylic functions. Above neutral pH, the value of zeta potential remains constant, justifying the stability of the charge density at the level of the polymer surface. The electrophoretic mobility is proportional to the surface charge density and zeta potential, so it can be useful to confirm the variation obtained in the zeta potential value. It’s worth noticing, the similarity between the variation of zeta potential and electrophoretic mobility is clearly noted (Fig. 6). The study of electrical behavior using conductivity as a function of pH confirmed the results obtained from zeta potential and electrophoretic mobility. Where a decrease in the conductivity of the aqueous HECc solution between pH = 2 and pH = 5 is a strong indication that the saponification reaction of the acid function was occurred. In addition, the point of intersection of the two lines of the variation of the conductivity is found between pH = 5 and pH = 7, indicating the neutralization of the acidic form at the HECc surface. From PH7, the change of the conductivity curve orientation as a function of pH towards a positive slope proves the total consumption of the carboxylic functions, and the alkalinity of the medium increases its conductivity.

Table 1 DS values of HECac and HECc.

DSCOODSNH2 DSTOT N/C

HECac (Volumetric dosage)

HECc (1H NMR)

0.37 0.78 1.15 2.11

1.00 —————— 1.00 0.00

1H and 13C NMR.

250 °C and which corresponds to the temperature of the thermal degradation of the three cellulosic polymers. This result shows that the modification whether by amide groups or by carboxylates does not influence the thermal stability of HECac and HECc in comparison with HEC [65]. 3.4. Zeta potential, electrophoretic mobility and conductivity Zeta potential measurements are used to evaluate the surface charge density of polymers. The dependence of the zeta potential on pH was investigated, and Fig. 6 shows the effect of the incorporation of carboxylate entities in the HEC surface taking into account the pH values. So, no important effect of the pH on Zeta potential was noted. The electrophoretic mobility and conductivity were measured for HEC, and the values of those parameters remain zero throughout the pH range studied, which is acceptable and in accordance with the uncharged structure of HEC. The potentiometric behavior of HECc awakens three stages of variation of zeta potential according to the pH values. The first stage was recorded in the range of pH from 2 to 5, showing a progressive increase in the value of zeta potential from 0 to −20 mV, indicating the deprotonation of carboxylic function leading to the carboxylate form. The second one describes a sharp decrease of zeta potential from pH = 5 to pH = 7 to reaching a limit value around -

3.5. Application 3.5.1. Coagulation-Flocculation of colloidal systems (Fe (OH3) The insoluble colloidal particles of ferric hydroxide were prepared by hydrolysis reaction of FeCl3, where the positive charge characterizing their colloidal surface is due to the ionization of the hydroxyl groups of the Fe(OH)3colloidal particles, and the hydrochloric acid produced during the hydrolysis was removed by heating to avoid destabilization and coagulation of colloidal system. The presence of the opposite charge given by HECc destabilizes the colloidal system to avoid the repulsion of colloidal particles, and therefore coagulationflocculation and clarification of the solution were observed. The coagulation-flocculation of colloidal Fe(OH)3 solution by HECc was studied 5

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Fig. 3. 1H,

13

C and Dept-135 NMR spectra of (a) HEC in DMSO-d6/TFA and (b) HECc in D2O.

according to the added volume of flocculant solution and the results were illustrated on Fig. 7, where the pH of the system was fixed betweenpH6 and pH7 as the optimal value corresponding to the total deprotonation of carboxylic entities grafted on HEC backbone. On the other hand, the pHpzc (point of zero charge) of Fe(OH)3 is largely discussed in literature and is determined at 8.5 value [66], so optimizing the pH value between 6 and 7 assured the electrostatic interactions between colloidal particles of Fe(OH)3 positively charged and the polymer surface of HECc with a negative charge. Fig. 7 shows that the maximum transmittance value (99%) was achieved at 35 μL of flocculant solution, and then the transmittance of the transparency zone increased progressively by adding more of the flocculant solution. In comparison with other flocculants approaching HECc at the structural level, Yongbo Song et al. have studied the flocculation capacity of AM grafted cellulose (DS = 0.67, synthesized in the NaOH/urea/ H2O system) of ferric hydroxide (Fe(OH)3) saturated colloidal solution, and the results (in the same conditions of this study) showed that flocculation was achieved at an optimal dose of 428 mg.L−1 which corresponds to flocculant solution (1% w) volume of 300 μL [50]. However, the new flocculant HECc improved the flocculating capacity 10 times with a volume of the flocculating solution of 35 μL. Also, M. I. Khalil and A. A. Aly have shown that the ability of a ferric laurate solution to flocculate (transmittance %) by anionic starch derivatives, such as Carboxymethyl starch, Carboxyethyl starch, Poly (acrylic acid)starch graft copolymer and Starch-2-hydroxypropyl citrate is between 43 and 82% even at optimum dose of flocculant which reaches 125 mg.L−1 [67]. To better understand the coagulation-flocculation mechanism of colloidal particles of Fe (OH)3 system, it is necessary to know that the electric charge architecture (double electric layer) of the colloidal particles is divided into layers of Stern and Gouy-Chapman. The Stern layer electrically associates with a diffuse layer of free ions with an opposite charge (Gouy-Chapman Layer). However, the two layers give the particles a characteristic potential called the Zeta potential, and to destabilize the colloidal system towards coagulation-flocculation, it is

necessary to provide the particles with sufficient kinetic energy to overcome the repulsive energy barrier. The destabilization of the colloidal particles can be achieved by the double-layer compression mechanism, where the thickness of the double layer decreases in relation to the ionic concentration of the system (e.g. metal salt coagulant) which causes the decrease of the repulsive effect of diffuse layer until the elimination of the energy barrier [68–72]. In addition, charge neutralization and bridging mechanisms are widely envisaged in flocculation/coagulation process using polymeric systems as alternative coagulants/flocculants to overcome the problems associated with the use of inorganic coagulants (aluminum- and iron- based alum) [73–80]. The latter (bridging mechanism) is available for very high molecular weight anionic flocculants (3–30 106 g. mol−1) with a low charge density, which adsorb on more than one particle and the flocculant acts as a bridge and binds several particles to form aggregates [55]. However, the charge neutralization mechanism is based on zeta potential neutralization where the coagulant-flocculant is adsorbed to the colloidal surface causing their electric discharge and reduction of their electrophoretic mobility [81,82]. At this stage, the neutralized particles can be charged by the positively charged adjacent particles, and hence the formation of flocs, where both mechanisms can act simultaneously. With the excess of polymer, there are not enough particle surfaces available for fixation, and the particles become destabilized, which can cause some electrostatic repulsion ensured by the regular distribution of the carboxylate group along the polymeric chain of flocculent. This phenomenon has been observed when the added volume of the flocculant solution exceeds 35 μL as an optimal volume. The degree of polymerization (DP = 220) of HEC, determined by viscometer, allows to estimate the approximate size of the flocculant polymer chain at 71,000 g. mol−1. However, this low molecular weight value in comparison with the colloidal size indicates the predominance of the charge neutralization mechanism. 3.5.2. Coagulation-Flocculation of heavy metal ions (Cu (II)) The incorporation of new carboxylate groups in cellulosic structure 6

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Fig. 4. SEM images and EDX spectra of HEC, HECac and HECc.

flocculation-coagulation phenomenon. However, the flocculation mechanism for removal of heavy metal, in ionic form from aquatic environments, is widely discussed in the literature, using polysaccharidebased polymers such as flocculants. Indeed, Bingzhi Liu et al. have studied the role of ionic groups of carboxylate-rich magnetic chitosan flocculants on the efficiency and kinetics of removal of heavy metal and cationic dye, where the optimal uptake rate was 98.3% and 87.4%, and exhibited satisfactory removal effect in wide range pH range (4.0–8.0 for Ni (II), 5.0–10.0 for MG), and mechanism of flocculation has been explained by a rapid initial adsorption performed by the action of the

was extensively studied to improve its physicochemical properties including surface charge, and to elaborate a large variety of new heavy metals adsorbents. However, these modifications remain at the level of the cellulosic surface or by crosslinking, limiting the elimination of micro pollutants to electrostatic adsorption on the surface of the adsorbent without reaching the chelating groups in the cellulose polymer core. In this paper, the homogeneous insertion of the carboxylate groups in the HEC main chain giving a new water-soluble polymer (HECc) characterized by a high negative charge density useful in removing metal ions from aquatic environments basing on the 7

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Fig. 5. XRD diffractograms of AM, HEC, HECac (a), HECac and MCC (b) and TGA curves of HEC, HECac and HECc (c).

removal capacity, where the mechanism of flocculation was attributed, firstly, to the phenomenon of chelation-flocculation where the metal ions are chelated by the carboxylic groups negatively charged, this alters the electrostatic polymer behavior that leads to the charge and bridging mechanism [87]. Whereas the effect of the flocculant dose plays an early role in the flocculation mechanism, where the optimal dose causes adsorption mechanisms bridging and seep flocculation [88]. Generally, the flocculation of heavy metals by water-soluble polymeric flocculants is attributed, according to numerous recent studies, to the chelation and trapping of the metal ion by chelating groups which are present on the polymer chain, followed by a neutralization of charge, and adsorption bridging and seep flocculation to form the flocs and aggregates to remove heavy metals [89–91]. HECc as flocculant showed an excellent performance for Cu (II) coagulation-flocculation. The initial metal ion concentration effect on the flocculation capacity, at pH value of 6, was studied and illustrated on Fig. 8a. As shown on Fig. 8a, at low concentrations of the metal ion solution, above 200 ppm, the density of the free chelating functions is very important in front of the fraction which complexes the copper metal ions. At this level, metal ions are unable to reach the energy barrier of HECc to neutralize its negative charge and this overcomes the electrostatic repulsion forces that guarantee the stability of the solution. On increasing the concentration of the metal ions, the Cu (II) ions were trapped in intermolecular cavities and form interchain bridges at the supramolecular level, and consequently generate three-dimensional networks (flocs) which precipitate easily under gravitational action. Fig. 8b shows the influence of pH, in a range of pH = 2 to 8, on the coagulation-flocculation of Cu (II) solution with HECc, where the initial concentration ion solution of Cu (II) was determined from the above optimization results to be 800 ppm (Fig. 8a). At low (pH < 4), a low flocculation capacity was noticed and this is due to the proton

Fig. 6. Zeta potential, electrophoretic mobility and conductivity of HECc according to pH.

carboxylic groups which forms complexes with the metal ion, in which the oxygen donates electrons to metal ions, thus the electron density at the adjacent two carbon atoms decreases [83,84]. In addition, Liang Wu et al. have synthesized an amphoteric starch derivative that was applied as a water-soluble polysaccharide ether and effective flocculant for heavy metal-ion (Cu (II) and Zn (II)) removal from wastewater, in which remove rate for Cu (II) and Zn (II) remained at 93% and 91% after three flocculation/regeneration cycles, and the mechanism of flocculation was explained by the extent of negatively charged flocculant and heavy metal cations [85]. In the same sense, L. Liu et al. have proposed the same mechanism explained by chemisorption chelation of metal ions and charge neutralization which facilitated bridge adsorption by P (AM-DMDAAC) polymer chains to form large floccules [86]. Also, Yongjun Sun et al. have studied the application of the new chitosan-based flocculants as good pH resistance and high heavy metals 8

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Fig. 7. The effect of flocculent solution 0.5% on the spectral transmittance of colloidal Fe(OH)3 solution.

charge neutralization mechanism is much more suggestive for a pH higher than 8, instead of complexation bridging in the pH zone below 6. In the other hand, HECc has shown excellent capacity (˜ 400 mg.g−1) for copper removal as a heavy metal from the aquatic systems. In addition, flocculation-coagulation is instantaneously achieved which is desirable for the treatment of industrial water systems. While almost all of the heavy metal removal treatments, using cellulose derivatives, are based on the adsorption technique which requires a long equilibrium time and low adsorption capacity compared to HECc [92–97]. The use of HECc as flocculant can give a new alternative, from an economic and ecological point of view, nevertheless to replace the processes based on aluminum which is the responsible factor for different kind of clinical and neuro-pathological disease, Alzheimer's, Parkinson-Guam's, Parkinson's, diabetes, cancer and amyotrophic lateral sclerosis [98–101].

competition where the protonation of the carboxylic groups grafted on HEC is much envisaged. Moreover, the HECc chain was affected by the pH change, and it was observed the formation of blue aggregates at the pH > pKa (pH = 5 and pH = 6), which is attributed to the high degree of ionization of carboxyl groups, so increases bridging supramolecular binding with metallic ions. By increasing the pH, beyond pH6, a redistribution of the ionic forms of Cu (II) is certainly envisaged, thus generating new ionic species with new electrical properties. Indeed, in acidic conditions, coagulation-flocculation is attributed to the formation of supramolecular bridges resulting from complexation phenomena between metal ions and polymer chains. Whereas under alkaline conditions, between pH = 7 and pH = 8, the conversion of Cu (II) ions to Cu(OH)+ and Cu (OH)2 causes the formation of a new colloidal system increasing its zeta potential, meaning a significant increase in the flocculating activity of HECc (zeta potential = -45 mV). In addition, the formation of the new copper chemical forms in the system as a function of pH was confirmed by the very remarkable changes of the floc coloring, from the blue color characteristic of aqueous Cu(II) to the black characteristic of Cu(OH)+ and Cu(OH)2to pH8 (Fig. 8b). The formation of Cu(OH)+ and Cu (OH)2in the system at the pH = 8 indicates that pH has a significant and high influence on the coagulation-flocculation process, where the

4. Conclusion New water-soluble cellulosic derivative (HECc) based on the incorporation of the carboxylate groups on the HEC structure was successfully synthesized, using reverse microemulsion (water/cyclohexane) method. The polyacrylamide chain extension was stopped

Fig. 8. Initial concentration (a) and pH (b) effects on the Cu (II) ions removal. 9

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using water as a hydrogen transfer agent, where cyclohexane was used as a continuous phase to limit the hydrophobicity of polymer resulting from the grafting reaction, then can decrease the advancement of the reaction. In addition, the saponification of the amide group leading to carboxylate, increased significantly the negative charge density of the polymer chain, which is confirmed by the high negative value of zeta potential (-45 mV) of HECc. The structural changes during the reactions were studied using the spectroscopy techniques (FTIR, 1H, 13C NMR and deept-135). X-ray diffraction patterns have shown that the grafting process is accompanied by the appearance of a new crystalline order due to the high density of the hydrogen bonds. The morphological changes and the composition of the surfaces were studied using SEM images and EDX spectra. The new HECc flocculant showed a high coagulation-flocculation capacity toward Fe(OH)3 colloidal system and Cu(II) ions removal from aqueous solutions. The pH effect has been studied indicating an important influence on the coagulation-flocculation mechanism and on the flocculating capacity.

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