Removal of toxic ions from aqueous solutions by surfactant-assisted biopolymeric hybrid membrane: Synthesis, characterization and toxic ions removal performance

Journal of Environmental Chemical Engineering 8 (2020) 103717

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Removal of toxic ions from aqueous solutions by surfactant-assisted biopolymeric hybrid membrane: Synthesis, characterization and toxic ions removal performance

T

Perumal Karthikeyan, Sankaran Meenakshi* Department of Chemistry, The Gandhigram Rural Institute - Deemed to be University, Gandhigram, 624 302, Dindigul, Tamil Nadu, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Adsorption Characterization Toxic anions Membrane Isotherm

In this study is to propose that for the removal of phosphate and nitrate ions from aqueous solutions using zirconium burdened carboxymethylcellulose-surfactant assisted kaolin (Zr-CMC-SKa) membrane as an effective adsorbent. The prepared membrane was synthesized by phase inversion method via polyethylene glycol as a flexible reagent and the developed membrane was extensively characterized by the details of the terms like morphological, compositional, thermal, roughness, functional and microscopic techniques. The optimization of various operating experimental conditions like the function of time, dosage, co-anions, pH, and pHZPC was studied. The adsorption kinetic phenomenon followed by pseudo-second-order and intraparticle diffusion models with good correlations. The fitting of the Freundlich adsorption isotherm model confirms multilayer adsorption of heterogeneous surface interaction between the toxic anions. Thermodynamic parameters, including the (ΔGo), (ΔSo) and (ΔHo) indicated that the phosphate and nitrate adsorption was feasible, spontaneity and endothermic in nature. The mechanistic pathway was mainly governed by electrostatic interaction, complexation followed by the ion-exchange mechanism, which was adopted for the removal of phosphate and nitrate anions from aqueous solutions. Furthermore, the applicability of the synthesized membrane towards promising results against the eutrophicated water sample by reducing the concentration of the toxic anions below the acceptable limit.

1. Introduction The world population growth and industrial developments depend on the groundwater for drinking and other purposes. Industries such as fertilizers, detergents, and food and beverage products of phosphate and nitrate-containing products unavoidably release large quantities of wastages onto the environment [1]. Utilization of drinking water which is highly polluted with phosphate and nitrate ions were reported to cause of various health and environmental problems such as the central nervous system of birth defects, initiation of kidney diseases, stomach cancer, Alzheimer's disease, methemoglobinemia, cancer of the colon, Hodgkin's lymphoma and eutrophication of water bodies [2]. The maximum allowable limit of phosphate and nitrate ions in drinking water as suggested by the Bureau of Indian Standards (BIS) is 0.5 and 45 mg of per L and WHO has recommended a guideline value of 0.5 and 50 mg L. Therefore, it is necessary to remove phosphate and nitrate ions from water/ wastewater. Over the past years, the toxic ions are typically removed from water using precipitation [3], flocculation [4],

⁎

membrane [5–7], biological [8], ion-exchange [9], catalytic reduction [10], reverse osmosis [8] and adsorption [11] techniques. But, legislative use of many techniques is restricted because they are complexation, time-consuming, and dependent on product cost, etc. In compare, the adsorption technique is more selectivity due to their high efficiency, suitability, simplicity, sensitivity, low-cost and easy operation under various environmental conditions [12,13]. To date, numerous materials have been developed and evolved for the remediation of toxic ions from the water with high potentials as well as high adsorption capacity. The following materials such as clay [14], resin [15], sawdust [16], LDHs [17], metal oxide [18], biomaterials [19], carbon-based [20] materials and agricultural wastages [21] are the most widely used adsorbents to the removal of phosphate and nitrate ions from water because of their good performance in the aqueous medium due to their porosity, hydrophobicity, and surface function ability. Until now, researchers have been involved in low-cost and traditional adsorbent materials, which include carbon, clays, biopolymers,

Corresponding author. E-mail address: [email protected] (S. Meenakshi).

https://doi.org/10.1016/j.jece.2020.103717 Received 9 November 2019; Received in revised form 4 January 2020; Accepted 23 January 2020 2213-3437/ © 2020 Elsevier Ltd. All rights reserved.

Journal of Environmental Chemical Engineering 8 (2020) 103717

P. Karthikeyan and S. Meenakshi

kaolin was dispersed in double distilled water by sonication for 1 h. Finally, CMC and clay dispersions were mixed at 200 rpm for 12 h. This homogeneous solution was transferred to a water bath at 80 °C for 30 min and agitated by magnetic stirring for 1 h. The mixture of the polymeric colloidal solution was casting on Teflon coated weighing boats and dried at room temperature for 24 h. Before peeling, the membrane was dried in a hot air oven for 6 h at 50 °C and the membrane was dipped into 3 % of ZrOCl2.8H2O solution for 12 h. Finally, the obtained membrane was continuously washed with water to remove unreacted ZrOCl2.8H2O and stored in a desiccator for further studies. The thickness of the prepared membrane was found to be 0.04 cm, respectively.

etc. Kaolin is a clay mineral, with layers of two tetrahedral silica sheets and one octahedral alumina sheets of the arrangement. The high adsorption efficiency of it is because of their high surface area and great cation exchange properties [22]. Clay embedded biopolymers have been applied for the removal of cationic, anionic dyes and heavy metal ions such as Zn2+, Cd2+, Cu2+, Cr6+, etc [23]. But, natural clay adsorbs the toxic ions onto the external surfaces and it is difficult to make it compatible with a polymeric material. Therefore, the surfactant-assisted kaolin is a hydrophobizing which enhance the adsorption capacity. Carboxymethylcellulose (CMC) is derived from cellulose, via alkalization and etherification. It is abundant, transparent, non-toxic, biocompatible and biodegradable [24]. CMC contains many hydroxyl and carboxyl groups in its molecular matrix, both of which can coordinate with metal ions. Among transition metals, zirconium possesses a higher oxidation state towards higher electron affinity and it has the well-coordinating capability to the biopolymeric materials good interactive adsorbent for the water treatment process. Zirconium is not only the most effective non-poisonous, but also good resistant for oxidizing agents, acid and base interactions, high thermal strength, and amphoteric character [25]. However, the traditional adsorbents were usually difficult to be separated from the treated solution. Hence, membrane adsorption techniques have received more significant attention for its appropriate, rapid recovery and low cost in recent years. The main objective of this work is to elucidate the potential application of the Zr-CMC-SKa membrane has been synthesized successfully and employed for the remediation of toxic anions from the aqueous medium. Physicochemical properties of the synthesized Zr-CMC-SKa membrane before and after treatment were characterized by XRD, EDAX with mapping images, SEM, FTIR, AFM and TGA-DSC analysis. In the meantime, to examine adsorption behaviour of the various influencing key parameters like adsorbent dosage, contact time, pH, pHZPC and co-anions. Adsorption kinetics of phosphate and nitrate ions were inspected by reaction and diffusion-based kinetic models respectively. Besides, the equilibrium data were examined for Freundlich, Langmuir and Dubinin-Radushkevich isotherm models, individually. To improve the cost-effective use of the membrane, the reusability of adsorption efficiency was also analyzed. Our results demonstrated that Zr-CMCSKa membrane could provide a cheap and efficient adsorbent for the remediation of phosphate and nitrate ions from aqueous solutions.

2.3. Batch adsorption experiments Adsorption performance of phosphate and nitrate ions onto Zr-CMCSKa membrane was studied via batch mode experiments. A required amount of Zr-CMC-SKa membrane was added into 50 mL of respective anionic solution with the initial concentration of 100 mg/L. The reaction mixture was placed into a temperature controllable shaker at 180 rpm. The solution pH was adjusted using 0.1 M HCl/NaOH solutions. The impact of contact time was also investigated at various time intervals. The isotherm and kinetic adsorption experiments were also conducted at various temperatures with different initial concentrations. The residual phosphate and nitrate ions concentration were analyzed by standard methods via UV–vis spectrophotometric method using 400 and 202 nm, respectively (Spectroquant Pharo 300, Merck) [26]. All experiments were conducted in duplicate, and the averages of the experimental data were reported. Adsorption percentage and capacity were obtained from the following Eqs. (1) and (2).

Adsorption capacity =

(Ci − Ce ) V m

Removal percentage(%) = (

Ci − Ce ) 100 Ci

(1)

(2)

where V is the volume of respective solution (L), m is the mass of the adsorbent (g), Ci and Ce is the initial and equilibrium concentration of respective toxic anions.

2. Experimental section

2.4. Instrumentation and characterization

2.1. Materials

The active functional groups of the prepared membrane were characterized by FTIR spectroscopy (FTIR) JASCO-460 plus spectrophotometer (Japan) using KBr pellets. X-ray diffraction patterns (XRD) were recorded on X’per PRO model-PANalytical (Netherland) make with the scan rate of 0.5° per min using CuKα (1.5418 Å) radiation source for monitoring the crystallographic information of the prepared membrane. Scanning electron microscopy (SEM) images were examined using a VEGA3 TESCAN model and the elemental-mapping analysis was obtained from EDAX with Bruker Nano Gmbh (Germany) during SEM analysis. Thermal analysis (TGA-DSC) was achieved on the specimens with a Shimadzu thermal analyzer at a heating rate of 10 °C min under N2 airflow. AFM analysis is a powerful method for monitoring the surface morphology and roughness of the prepared membranes. The scan area was about 2 μm × 2 μm and employed in a tapping-mode under ambient conditions.

Sodium carboxymethylcellulose (CMC) was purchased from Chemico laboratories (P) Ltd, Mumbai. Zirconium oxychloride (ZrOCl2.8H2O), sodium chloride (NaCl), kaolin, hydrochloric acid (HCl), cetyl trimethyl ammonium bromide (CTAB), sodium hydroxide (NaOH), glycerol, silver nitrate (AgNO3) were procured from Merck (Germany). A 1000 mg/L stock solution of phosphate and nitrate was prepared using potassium dihydrogen phosphate, potassium nitrate (CDH chemicals, Mumbai, India) and were stored in a polyethylene bottle. The working solutions of the necessary concentrations were made up of the respective stock solution by diluting with DI water and all other chemicals and reagents were utilized in analytical grade. 2.2. Preparation of Zr-CMC-SKa membrane About, 2 g of CTAB was dissolved in distilled water (200 mL), and 10 g of kaolin was added slowly. The reaction combinations were continuously stirred at ambient conditions for 24 h. The reaction mixture was filtered and then several times washed with distilled water till no bromide ions were observed through the AgNO3 solution. The product becomes dried in a hot air oven at 80 °C for 6 h. On the other hand, 2 % of CMC solution was prepared by mixing 2 mL glycerol and 2 g CMC with 100 mL distilled water. Simultaneously, surfactant-assisted

2.5. Water uptake studies A water uptake study is a predominant parameter to investigate the hydrophobic/ hydrophilic nature of the prepared membrane. The water uptake studies of the Zr-CMC-SKa membrane was carried out by the following method. The Zr-CMC-SKa membranes were cut into 3 × 3 cm and measured the weight (W0). Then, the Zr-CMC-SKa membranes were immersed in 3 different solution pHs viz., 3, 7, 12 at room temperature 2

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groups existing in the CMC, zirconium oxychloride, Zr-CMC-SKa, phosphate and nitrate adsorbed Zr-CMC-SKa membrane and are presented in Fig. 1. The FTIR spectrum of CMC shows a strong peak at 3443 cm−1 which is observed due to −OH stretching vibration. The absorption peak at 2908 cm−1 is due to C–H stretching of the methylene groups and the band at 1630 cm-1 due to ring stretching of carboxylate groups. Also, −CH2 and −COH groups observed in the region of 1350−1450 cm−1 are owing to symmetrical deformations. The band in the range of 1000−1166 cm−1 is ascribed to the ether groups of CMC matrix [24]. The peaks at 3677 and 3429 cm−1 were due to −OH groups associated with SKa clay. The asymmetric and symmetric stretching vibration modes of Si-O and Si-O-Si in SKa clay was assigned at 1608 and 1049 cm−1. Additionally, the bending vibration mode of −OH in Al−OH was ascribed at 943 cm-1 [27,28]. The characteristic bands of the CeN bond which are between 910 and 1000 cm−1 (very strong for compounds of the type R-N+(CH3)3) are hidden in complex inorganic-clay. The characteristic peaks at 1395 and 1275 cm−1 indicate the asymmetric and symmetric stretching vibration of Zr-O-H, whereas 1030 cm−1 could be related to the bending vibrations of Zr-O and OeH in zirconium oxychloride [29], which also indicated that SKa and zirconium are present in the Zr-CMC-SKa membrane. After phosphate and nitrate adsorption, new intensity peaks appeared at 1378 and 1038 cm−1 for phosphate and nitrate respectively, which pointed out that phosphate and nitrate ions interacted with Zr-CMC-SKa membrane surface [30]. 3.1.2. Surface morphological analysis The SEM observations were used to characterize the surface morphology of the Zr-CMC-SKa membrane, phosphate and nitrate adsorbed Zr-CMC-SKa membrane surface and are represented in Fig.2. From Fig. 2A, it is evident that the synthesized Zr-CMC-SKa membrane, perceived the permeable nature and it may be due to the fact that the surfactant-assisted kaolin particles were uniformly distributed on the membrane surface. After adsorption of phosphate and nitrate ions, the permeable nature was destroyed as shown in Fig. 2B and C. The prominent changes suggest that the adsorption of toxic ions on the Zr-CMCSKa membrane surface.

Fig. 1. FTIR spectra of sodium carboxymethylcellulose, zirconium oxychloride, Zr-CMC-SKa, phosphate and nitrate adsorbed Zr-CMC-SKa.

for 24 h. After a predetermined time, the samples were removed and weighed. The water uptake behaviour was calculated by the following Eq. (3)

Water uptake% = (

Ws − Wd ) 100 Wd

(3)

where Ws and Wd represent the mass of wet and dry membranes, respectively.

3.1.3. Compositional and mapping analysis EDAX technique provides direct proof of compositional investigation of the Zr-CMC-SKa membrane before and after treatment is shown in Fig. S1. From Fig. S1A, the predominance of the zirconium peak depicted in Zr-CMC-SKa membrane, demonstrates the successful incorporation of Zr4+ ions onto the membrane surface. The EDAX spectra also verified the presence of phosphorous peak in the spent membrane surface which, confirms the phosphate adsorption onto Zr-CMC-SKa membrane and is represented in Fig. S1B. Similarly, the existence of

3. Results and discussion 3.1. Characterization of the adsorbents 3.1.1. Functional group analysis The FTIR analysis was utilized to explain the various functional

Fig. 2. SEM images of (A) Zr-CMC-SKa (B) phosphate and (C) nitrate adsorbed Zr-CMC-SKa. 3

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Fig. 3. TGA and DSC curves of Zr-CMC-SKa membrane.

nitrogen and oxygen peaks in the nitrate adsorbed Zr-CMC-SKa membrane surface, which confirms the occurrence of nitrate adsorption is represented in Fig. S1C. Further, the mapping images of the elements present in Zr-CMC-SKa membrane are demonstrated different colour patterns and are given in Fig. S1D. Similarly, the mapping images of the phosphate and nitrate adsorbed Zr-CMC-SKa membrane shows the presence of the phosphorus, nitrogen and oxygen spots with the resultant elements (Fig. S1E, and F). These results enumerate that compositional analysis of Zr-CMC-SKa membrane surface and confirmed the phosphate and nitrate adsorption onto Zr-CMC-SKa membrane surface. 3.1.4. Thermal analysis Thermal analysis, such as differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA) are the powerful thermoanalytical techniques for monitoring the physio-chemical changes of the synthesized membrane. The TGA-DSC curves of the prepared ZrCMC-SKa membrane are displayed in Fig. 3. The TGA thermogram demonstrated that the initial weight loss occurred between 80–130 °C due to the vaporization of water moieties present in the prepared membrane. The second degradation perceived between 170–240 °C is ascribed to the decomposition of polysaccharide rings due to cleavage of CeOeC bonds and elimination of CO2 molecule from the polymeric backbone. After that, the third weight loss was observed between 470–540 °C, which is due to the occurrence of organic moieties and clay minerals present in the membrane. Afterward, in the third decomposition step, there was no substantial change detected from 550 to 800 °C [22,25,28,31]. In DSC thermograms, an exothermic peak around at 100 °C, resulting to the removal of water molecules, is perceived for Zr-CMC-SKa membrane and an endotherm was focused at 210 °C for polyethylene glycol moieties, the CMC is made up of cellulose, it is easily coordinate with metal ions which endorses the endotherm and exotherm peaks at 600 °C and 320 °C, which is related to the decomposition of lignin, carbohydrates and clay minerals respectively.

Fig. 4. XRD pattern of sodium carboxymethylcellulose, zirconium oxychloride, Zr-CMC-SKa, phosphate and nitrate adsorbed Zr-CMC-SKa.

after phosphate and nitrate treatment, the lowered intensity of the peaks was observed which due to electrostatically adsorbed phosphate and nitrate ions onto the membrane surface.

3.1.6. Surface roughness analysis The three-dimensional images of AFM analysis yielded information about the external features of the surface roughness and morphology of the prepared membranes. Fig. 5A shows the AFM image of the Zr-CMCSKa membrane, which has the bright and dark regions symbolise the high point and the valley like the structure of the membrane surface. The surfaces of the membrane hold on the uneven distribution of surfactant-assisted kaolin, which indicates that kaolin incorporated into the membrane. Fig. 5B and C show the AFM images of the phosphate and nitrate adsorbed Zr-CMC-SKa membrane, which possesses smooth surface and lowered roughness area was observed due to phosphate and nitrate adsorption.

3.1.5. Structural analysis To find out more information regarding the structural properties of the synthesized membrane, it was carried out by X-ray diffraction analysis. From Fig. 4, it can be interpreted that the structure of the ZrCMC-SKa membrane is a different form of parent resources. The difference intensity peaks recommend an entirely different form of crystalline and amorphous nature than that of zirconium oxychloride and CMC matrix. The XRD spectrum of CMC showed an intense peak at 2θ = 20.70°, which indicate the semi-crystalline structure of the polysaccharide chain [32] and the pristine zirconium reveals the sharp intensity peaks at 29.6°, 47.2° and 56.1° which matches to the crystalline planes (100), (102) and (110) [JCPDS Card no 89-3045] [29]. In the XRD spectrum of the Zr-CMC-SKa membrane, new diffraction peaks appeared at 2θ = 10 - 40°, which could be attributed to the presence of surfactant-assisted kaolin present in the polymeric matrix. Furthermore,

3.2. Effect of agitation time on anions uptake The effect of toxic anions removal onto the Zr-CMC-SKa membrane was examined at different agitation time ranges from 0 to 90 min. About 100 mg of the synthesized membrane was taken into 50 mL of 100 mg/L of individual phosphate and nitrate solutions and the reaction content was shaken using orbital shaker at 180 rpm. It is clear from Fig. 6A the removal capacity of toxic ions were gradually increased with the rise in the agitation time. During the adsorption studies, the toxic ions have been a tendency to attain dynamic equilibrium and the removal capacity of toxic anions have reached saturation at 40 min, which may be due to the vacant sites of the Zr-CMC-SKa membrane being completely occupied and have subsequent experiments have been performed for 40 min by fixing the time as 40 min. 4

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Fig. 5. AFM images of (A) Zr-CMC-SKa (B) phosphate and (C) nitrate adsorbed Zr-CMC-SKa.

change in the phosphate and nitrate removal percentage which may be due to the low concentration of toxic ions. The optimal dosage was fixed as 100 mg for further studies and this quantity found to minimize the adsorption errors.

3.3. Effect of membrane dosage on anions uptake The adsorbent dosage studies are constructive to quantify the removal percentage of toxic ions from aqueous medium. The different dosage (25–200 mg) of the synthesized membrane was placed into 50 mL of 100 mg/L of individual phosphate and nitrate solutions and shaken at ambient experimental conditions. As evident from Fig. 6B the removal percentage of toxic anions were gradually raised with raise in adsorbent dosage, which is attributed that enhancement of the active sites was available on the membrane surface. But, a plateau was observed after a dosage of 100 mg, and then there was no substantial

3.4. Effect of pH on anions uptake Removal of toxic ions was extremely reliant on the solution pH as it can alter the surface charge is due to temporal, geological and seasonal properties. Therefore, the removal of toxic anions onto the Zr-CMC-SKa membrane was studied at various pH ranges from 2 to 11 at ambient

Fig. 6. (A) Effect of contact time (B) dosage (C) pH (D) interfering-anions for Zr-CMC-SKa membrane on the adsorption capacity and percentage of phosphate and nitrate ions. 5

Journal of Environmental Chemical Engineering 8 (2020) 103717

P. Karthikeyan and S. Meenakshi

conditions. The pH of the operational solution was adjusted by using a sufficient amount of 0.1 N HCl/NaOH solution. Fig. 6C illustrates that the phosphate and nitrate adsorption onto Zr-CMC-SKa membrane is significantly affected at acidic pH range (2–6) and the removal capacity was 35−44 mg/g for phosphate and 32–36.7 mg/g for nitrate respectively. As the, pH increased to be a basic condition, and the removal capacity was gradually declined. The phosphate ions can exist in diverse forms in solution pHs < 2.1, pH 2.1–7.2, pH 7.2–12 and pH > 12, as H3PO4, H2PO4−, HPO42- and PO43-, respectively [11]. Therefore, the electrostatic attraction can be assigned between the negative charge of the phosphate ions and Zr-CMC-SKa membrane, which has positively charged species followed by increasing the adsorption capacity. The decrease in the adsorption capacity at pHs > 8 is because of the OH− ions concentration increases in operational solution and aggressively competes with the active sites of the Zr-CMC-SKa membrane, prominent to reduce the adsorption capacity. At pHs > 2–6, electrostatic attraction between the analyte and Zr-CMC-SKa membrane endorses the high adsorption capacity for nitrate ions are represented in Fig. 6C. The lower adsorption capacity in the basic medium can be described by the fact that the surface attains negative charge in basic medium and hence electrostatic repulsion force occurred [12].

and 17.8 %, respectively. The result demonstrated that neutral and alkaline pH, the electrostatic contributions are the main driving force for the swelling, while the hydrophobic assistances do not play in neutral and alkaline mediums. In the case of acidic pH, the carboxyl and hydroxyl groups are protonated, thereby hydrogen bonding interaction among the CMC matrix.

3.5. Influence of co-existing anions uptake

logq e = logkF +

Natural drinking water contains several anions, such as HCO32−, SO32−, Cl- etc. These co-existing anions could compete with phosphate and nitrate removal capacity and are the effect of these ions are depicted in Fig. 6D. For this study, a standard solution of phosphate and nitrate ions (50 mg/L) containing a fixed amount of 200 mg/L of coexisting anions was prepared. The co-existing anions altered the nitrate and phosphate adsorption, excepting bicarbonate has an insignificant effect. Among the co-existing anions, nitrate possesses weaker electrostatic screening and lower hydration ratio, so the electrostatic attraction acting on nitrate is weaker, foremost to chloride substitution by nitrate ions. The hydration energy of NO3- (ΔG, −314 kJ/mol) is lower than Cl- (ΔG, −363 kJ/mol) and SO42− (ΔG, −1103 kJ/mol) [33]. In the case of phosphate adsorption, SO32- and Cl- possess the opposing results and which occupies onto the adsorbent surface. The influence of sulphate retains a higher charge density than nitrate and phosphate ions. Also, the ionic radius suggests that a lower ionic radius demonstrates the strongest electrostatic attraction on the adsorbent surface [11].

where Ce is the equilibrium concentration of both anions (mg/L) and qe is the amount adsorbed per unit weight of the adsorbent. The kF and n values obtained from plotting between Ce/qe vs Ce for both anions and the values are shown in Table 1. The La@CS membrane possessed n values (0 < n < 1) and 1/n values (0 < 1/n < 1) which indicates that the adsorption was favourable.

3.8. Adsorption isotherms An isotherm model deals with the amount of toxic (phosphate and nitrate) ions adsorbed on the surface of the adsorbent. The most three important isotherm models namely, Freundlich [35], Langmuir [36] and Dubinin-Radushkevich (D–R) [37] isotherm models can be used to explain the correlation between the different concentrations of adsorbate and applied for adsorbent and it’s can be predicting the adsorption capacity of the adsorbent. 3.8.1. Freundlich isotherm model The Freundlich isotherm model assumes nonideal adsorption on heterogeneous surfaces. The Freundlich adsorption isotherm equation can be expressed according to Eq. (4).

1 logCe n

(4)

3.8.2. Langmuir isotherm model The Langmuir isotherm model assumes monolayer adsorption of adsorbate onto the adsorbent surface. The linear form of the Langmuir adsorption isotherm equation can be expressed according to the following Eq. (5).

Ce 1 C = o + eo qe Qb Q

(5)

where Ce is the equilibrium concentration of phosphate and nitrate ions (mg/L) and qe is the amount of phosphate and nitrate adsorbed per unit weight of the adsorbent. b and Qo are Langmuir constants related to the energy of adsorption (L/mg) and adsorption capacity (mg/g), of both anions. RL value was calculated by using Eq. (6),

3.6. Measurement of pHZPC

RL =

The pHZPC is a significant tool, which is utilized to monitoring the surface ionization behaviour of the prepared membrane by using the Drift method [34]. To determination of pHZPC, the initial pH of 0.1 N NaCl was maintained between 2–11 and was controlled by 0.1 N HCl/ NaOH solutions. 50 mL of 0.1 N NaCl solution was poured into 250 mL iodine flask and 100 mg of Zr-CMC-SKa membrane was added to each solution having different pH between and flasks were stirred in a thermostated shaker for 24 h and the final pH of the solutions was measured. The pHPZC value was measured from the plot of ΔpH [pHInitial – pHFinal] versus pHinitial and the intersection between the curve and abscissa was pHzpc. The pHZPC value of the synthesized ZrCMC-SKa membrane was found to be 6.2. Thus, based on pHZPC, the surface of the membrane is projected to be positively charged up to pH 7, which may be accountable for toxic ions adsorption from the aqueous solution.

1 1 + bCo

(6)

where b is the Langmuir isotherm constant and Co is the initial concentration of anions (mg/L). Langmuir isotherm can be confirmed by a separation factor RL. The RL value indicates the possibility of the isotherms to be either favourable (0 < RL < 1), linear (RL = 1), irreversible (RL = 0) and unfavourable (RL > 1). 3.8.3. Dubinin-Radushkevich isotherm model The Dubinin-Radushkevich adsorption isotherm model can demonstrate the linear form by Eq. (7). D–R isotherm model is used to describe the adsorption process either physisorption or chemisorption.

lnq e = ln Xm − kDR ε 2

(7)

where Xm is the adsorption capacity of phosphate and nitrate ions (mg/ g) and k is the constant related to adsorption energy (mol2/kJ2). The values of k and Xm were obtained from the slope and intercept of the plot between ln qe vs ε2 with KDR, Xm and E values are represented in Table 1. Here ε is referred as Polyani potential and this parameter is the mean free energy of adsorption represented by Eq. (8),

3.7. Water uptake studies The water uptake behaviour of the Zr-CMC-SKa membrane was carried out by distinct solution pHs. The Water uptake values of ZrCMC-SKa membrane of the solution pH at 3, 7, and 12 were 25.6, 16.8 6

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Table 1 Isotherms parameters of Zr-CMC-SKa membrane for the removal of phosphate and nitrate ions. Isotherms

Parameters

Freundlich

Langmuir

Dubnin-Radushkevich

1/n n kF (mg/g) (L/mg)1/n r sd χ2 Qo(mg/g) b (L/g) RL r sd χ2 kDR (mol2/J2) Xm (mg/g) E (kJ/mol) r sd χ2

1 ⎞ ε= RTln ⎛1 + Ce ⎠ ⎝ ⎜

Phosphate

Nitrate

303 K

313 K

323 K

303 K

313 K

323 K

0.421 1.558 7.774 0.992 0.013 0.045 88.36 0.042 0.106 0.971 0.035 0.135 4.04E-05 45.12 0.347 0.782 0.294 1.051

0.405 1.712 7.874 0.989 0.017 0.050 89.40 0.051 0.110 0.965 0.042 0.144 3.81E-05 45.65 0.365 0.865 0.257 0.989

0.387 1.825 7.970 0.997 0.019 0.058 90.76 0.067 0.143 0.976 0.054 0.163 3.12E-05 45.73 0.397 0.887 0.202 0.985

0.372 1.814 6.324 0.999 0.016 0.020 71.07 0.040 0.104 0.978 0.064 0.157 8.12E-05 37.14 0.238 0.856 0.161 0.786

0.371 1.900 6.953 0.995 0.019 0.030 71.39 0.044 0.121 0.970 0.070 0.162 8.07E-05 37.71 0.246 0.897 0.156 0.657

0.311 1.205 7.020 0.993 0.021 0.032 72.58 0.044 0.136 0.987 0.076 0.185 7.98E-05 37.57 0.317 0.888 0.145 0.543

Table 2 Thermodynamic parameters of Zr-CMC-SKa membrane for the removal of phosphate and nitrate ions.

⎟

(8)

where T represents the temperature (K), R represents gas constant (8.314 J mol−1 K−1), Ce indicates the equilibrium concentration (mg/ L) and E (kJ/mol) indicates the mean free energy of the adsorption process obtained from the value of k as shown in Eq. (9), E= − (2k)−0.5

∑

Phosphate

Nitrate

ΔG˚ (kJ/mol)

303 K 313 K 323 K

−8.19 −8.55 −8.67 20.1 22.8

−11.07 −11.43 −11.69 17.7 20.6

energy change (ΔG°) of adsorption can be calculated from Eq. (11),

ΔGo = −RT lnK o

(q e − q e,m)2 q e,m

Temperature

ΔH˚ (kJ/mol) ΔS˚ (J/((K/mol))

(9)

where E value indicates the process either (E > 8) physisorption or (E < 8) chemisorption. In order to recognize the most favourable isotherms for phosphate and nitrate ions adsorption on La@CS membrane was calculated with chi-square analysis. The mathematical expression of chi-square analysis is represented in Eq. (10),

χ2 =

Thermodynamic Parameters

(11)

where T is temperature (Kelvin) and R is the universal gas constant (8.314 J/(mol/K)). The Ko value (adsorption distribution coefficient) was determined from the slope of the plot ln(qe/Ce) Vs. Ce at different temperatures (Khan and Singh). The values of enthalpy (ΔH˚) and entropy (ΔS˚) changes were determined from the slope and intercept of the linear Van’t Hoff plot Eq. (12),

(10)

The experimental data of the Freundlich, Langmuir and D–R isotherm models and the predictable parameters are represented in Table 1. According to the results, Qo values were found to be increases with an increase in temperature which demonstrated that the possible mechanism of phosphate and nitrate removal by the Zr-CMC-SKa membrane was mainly due to physisorption. From the results, the lower sd values and higher r values established that the phosphate and nitrate adsorption onto the Zr-CMC-SKa membrane follows the Freundlich isotherm model. It has been illuminating that the Freundlich isotherm model assumes on multilayer adsorption and involved for heterogeneous surface interactions. The lower χ2 values of Freundlich isotherm indicates the feasible adsorption for the toxic ions adsorption onto Zr-CMC-SKa membrane. The investigational data of the D–R isotherm model reveals that E values of the Zr-CMC-SKa membrane were less than 8.0 kJ/mol at 303, 313 and 323 K, which shows physisorption manner.

lnK o =

ΔSo ΔHo − R RT

(12)

The negative values of ΔGo established that the spontaneous nature of phosphate and nitrate removal. The endothermic nature of the adsorption system was exhibited by positive values of ΔHo, indicating that the adsorption process is thermodynamically feasible. Moreover, the positive values of ΔS° values revealed that increased randomness at the interface of respective phosphate and nitrate with that of Zr-CMC-SKa membrane surface [39].

3.10. Adsorption kinetics

3.9. Adsorption thermodynamics

Adsorption kinetic experiments were assessed to examine the contact time needed in each adsorption studies for the solution to attain the equilibrium state. The kinetic adsorption study is important for the practical application for the removal of phosphate and nitrate ions using for Zr-CMC-SKa membrane which is used to determine the mechanism of adsorption and the rate-limiting steps for chemical reactions. Two main types of kinetic models were commonly used to examine various kinetic limitations and they are the reaction and diffusion-based kinetic models.

A thermodynamic experiment was conducted to recognize the effect of temperature on phosphate and nitrate adsorption and regulate the sorption process. The thermodynamic investigations like standard free energy change (ΔGo), standard entropy change (ΔSo) and standard enthalpy change (ΔHo) were assessed to understand the nature of the adsorption process using Khan and Singh method [38] and the thermodynamic parameters are tabulated in Table 2. Standard Gibb’s free 7

Journal of Environmental Chemical Engineering 8 (2020) 103717

P. Karthikeyan and S. Meenakshi

was established to be effective and stable during the adsorption studies where the regeneration cycles were carried out up to 6 cycles. Fig. S2 shows that the four cycles of the membrane regeneration studies very well maintained up to 68 % of adsorption efficiency which indicated that the reusability of Zr-CMC-SKa membrane was moderately stable.

3.10.1. Reaction-based models The reaction-based kinetic models namely pseudo-first-order kinetic model [40,41] and pseudo-second-order kinetic model [42,43]. Pseudo-first-order kinetic (PFO) model as

log(q e − qt) = logq e −

k ad t 2.303

(13)

3.12. Field studies and comparison analysis

Pseudo-second-order (PSO) kinetic model as

t 1 t = + qt h qe

The Zr-CMC-SKa membrane was used to remove phosphate and nitrate ions from real waters, the real water sample collected from nearby eutrophic endemic area. About 50 mL of phosphate and nitrate polluted water sample was shaken with 100 mg of Zr-CMC-SKa membrane for 30 min under ambient conditions. After treatment, the residual toxic anions concentration and the details are depicted in Table S3. In addition to phosphate and nitrate ions removal performance was achieved by less than tolerance able limit. The practical application of the Zr-CMC-SKa membrane showed a substantial contribution toward the removal of major quality parameters like total hardness, dissolved solids and chloride which are commonly existing in the water. Hence Zr-CMC-SKa membrane can be employed at field conditions for the removal of toxic ions from polluted waters. The removal performance of Zr-CMC-SKa membrane was compared with other adsorbent materials reported in the literature and is signified in Table S4. Zr-CMC-SKa membrane possesses considerable removal capacity with that of the reported adsorbents which shows its high removal capacity towards phosphate and nitrate ions.

(14)

In Eq. (13), qe and qt are the adsorption capacities of phosphate and nitrate ions at equilibrium on the surface at time t, respectively (mg/g), kad is the rate constant of pseudo-first-order model (1/min). Linearity graph of log (qe-qt) versus t gives a straight line that shows the favourable of pseudo-first-order model. kad and qe can be calculated from the slope and intercept respectively. In Eq. (14), qe is the amount of adsorption of phosphate and nitrate ions at equilibrium (mg/g) and qt = qe2kt/(1 + qtkt) is the adsorption capacity on the surface at time t (mg/g), k is the pseudo-second-order rate constant ((g(mg/min)), the initial adsorption rate h = kqe2 The value of qe (1/slope), k (slope2/intercept) and h (1/intercept) of the pseudo-second-order equation can be experimentally found by linearity graph of t/qt versus t gives a straight line that shows the favourable of pseudo-second-order model. The kinetic constants and correlation coefficients (r), were obtained from the non-linear regression method and the kinetic parameters are represented in Table S1 and 2. It was identified that the PSO kinetic model was found to be the best fit than PFO kinetic model had lower sd values and higher r values and the calculated qe values of the PSO model was much closer to the experimental values of qe, nearby 0.999, which endorse that the PSO model is a superior fit.

3.13. Possible mechanism of toxic anions removal The possible mechanism of toxic anions removal by the synthesized Zr-CMC-SKa membrane was governed by ion-exchange, complexation and electrostatic attraction mechanism [19,46,47] and is represented in Fig. 7. The presence of Si2+, Al3+ and Zr4+ (hard acids) ions in ZrCMC-SKa membrane could adsorb phosphate and nitrate ions (hard bases) via strong electrostatic attraction. On the other hand, the presence of CMC and surfactant-assisted clay in the membrane contains many hydroxyl and carboxyl groups, which would also be protonated and it easily interacts with toxic anions (phosphate and nitrate) through electrostatic attraction. Phosphate and nitrate ions form complex with the protonated Zr-O−OH2+, Al−OH4+ and Si−OH5+. Furthermore, the ion exchange mechanism was also involved as the Cl– groups present in Zr-CMC-SKa membrane were exchanged for toxic ions, which was established by the EDAX analysis.

3.10.2. Diffusion-based models Adsorption of liquid adsorbates on solid adsorbents can be investigated by diffusion-based models, which can be either particle or intra-particle [44] diffusion models. The particle diffusion model was represented in Eq. (15),

C ln ⎛1 − t ⎞ = −kpt Ce ⎠ ⎝ ⎜

⎟

(15) −1

In Eq. (15), kp is the diffusion of particle rate constant (min ) and its value is calculated from the slope of ln (1 - Ct/Ce) versus t. The diffusion of the intra-particle model was described by Weber and Morris is given in Eq. (16) [44],

qt = k i

t1/2

4. Conclusions

(16)

In this study, the Zr-CMC-SKa membrane was successfully fabricated and their removal performance towards phosphate and nitrate ions were investigated under various conditions. The characteristics of ZrCMC-SKa membrane were evaluated by SEM, XRD, FTIR, TGA-DSC, EDAX with mapping images and AFM analysis. These results indicated that the Zr-CMC-SKa membrane is a potential adsorbent for phosphate and nitrate removal due to the presence of active functional groups was present in the membrane surface. Freundlich isotherm model was well fit, suggests the multilayer adsorption on the heterogeneous surface interaction for both anions. The positive value of both ΔS° and ΔH° indicates endothermic nature and negative value of ΔG° specifies the spontaneity for phosphate and nitrate removal onto Zr-CMC-SKa membrane. The adsorption kinetic models were well fitted with the intra-particle diffusion and pseudo-second-order kinetic models. The prepared membrane holds enhanced phosphate and nitrate capacities of 88.36 and 71.07 mg/g, respectively. The probable mechanism of phosphate and nitrate ions removal by Zr-CMC-SKa membrane was governed via complexation, ion-exchange and electrostatic attraction mechanism. The field studies of Zr-CMC-SKa membrane demonstrates that it could be effectively employed for the remediation of toxic ions

In Eq. (16), ki is the diffusion of the intra-particle coefficient (mg/g min0.5) and the value of it was obtained from the slope of the plot between t1/2 versus qt. The non-linear form of the particle and intraparticle diffusion kinetic model constants and parameters are given in Table S1 and 2. The intra-particle diffusion model was found to be a better fit than the particle diffusion model which has lower sd values and higher r values. The values of r for the intra-particle diffusion kinetic model are closer to unity, which indicates that the intra-particle diffusion model is contributing towards the rate-controlling step, towards the adsorption of toxic ions onto Zr-CMC-SKa membrane. 3.11. Regeneration studies The stability and financial feasibility are the significant parameters to find out the adsorbent nature. The regeneration of exhausted ZrCMC-SKa membrane was studied using 0.1 M NaOH solution [45]. Then, the residual concentration of phosphate and nitrate ions were measured by UV–vis spectrophotometer. The Zr-CMC-SKa membrane 8

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Fig. 7. Schematic representation of phosphate and nitrate adsorption onto Zr-CMC-SKa membrane.

from water/ wastewater treatments.

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CRediT authorship contribution statement Perumal Investigation, Supervision.

Karthikeyan: Conceptualization, Methodology, Writing - original draft. Sankaran Meenakshi:

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors are gratefully acknowledging the Department of Biotechnology [F. No. BT/PR18885/BCE/8/1374/2016], New Delhi, India for providing financial support to carry out this research work. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jece.2020.103717. References [1] A. Sowmya, S. Meenakshi, A novel quaternized chitosan-melamine-glutaraldehyde resin for the removal of nitrate and phosphate anions, Int. J. Biol. Macromol. 64 (2014) 224–232. [2] H. Jiang, P. Chen, S. Luo, X. Tu, Q. Cao, M. Shu, Synthesis of novel nanocomposite Fe3O4/ZrO2/chitosan and its application for removal of nitrate and phosphate, Appl. Surf. Sci. 284 (2013) 942–949. [3] X. Quan, C. Ye, Y. Xiong, J. Xiang, F. Wang, Simultaneous removal of ammonia, P and COD from anaerobically digested piggery wastewater using an integrated process of chemical precipitation and air stripping, J. Hazard. Mater. 178 (2010) 326–332. [4] J.P. Boisvert, T.C. To, A. Berrak, C. Jolicoeur, Phosphate adsorption in flocculation processes of aluminium sulphate and poly-aluminium-silicate-sulphate, Water Res. 31 (1997) 1939–1946. [5] M. Peydayesh, T. Mohammadi, O. Bakhtiari, Water desalination via novel positively charged hybrid nano fi ltration membranes filled with hyperbranched polyethyleneimine modified MWCNT, J. Ind. Eng. Chem. 69 (2019) 127–140.

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