Carboxymethyl cellulose thiol-imprinted polymers: Synthesis, characterization and selective Hg(II) adsorption

JES-01667; No of Pages 18 JOUR N AL OF EN VI R ONM E NT AL S CI E N CE S XX ( XXX X) X XX

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Tarisai Velempini1 , Kriveshini Pillay1,2,3,⁎, Xavier Y. Mbianda1,2,3 , Omotayo A. Arotiba1,2

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Carboxymethyl cellulose thiol-imprinted polymers: Synthesis, characterization and selective Hg(II) adsorption

1. Department of Applied Chemistry, University of Johannesburg, Doornfontein Campus, Johannesburg 2028, South Africa 2. Centre for Nanomaterials, University of Johannesburg, Doornfontein Campus, Johannesburg 2028, South Africa 3. Department of Science and Technology/National Research Foundation Centre of Excellence in Strong Materials, University of the Witwatersrand, Johannesburg 2050, South Africa

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Article history:

Sulfur containing ion imprinted polymers (S-IIPs) were applied for the uptake of Hg(II) from

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Received 21 April 2018

aqueous solution. Cysteamine which was used as the ligand for Hg(II) complexation, was

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Revised 17 November 2018

grafted along the epichlorohydrin crosslinked carboxylated carboxymethyl cellulose

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Accepted 29 November 2018

polymer chain through an amide reaction. The adsorption ability of S-IIPs towards Hg(II)

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Available online xxxx

was investigated by kinetic and isotherm models, which, corresponding, showed that the

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Keywords:

with a maximum adsorption capacity of 80 mg/g. Moreover, thermodynamic studies

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Ion-imprinted polymers

indicated an endothermic and spontaneous reaction with the tendency of an enhanced

Carboxymethyl cellulose

randomness at the surface of the S-IIPs with temperature increases. S-IIPs indicated a high

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Adsorption

degree of selectivity towards Hg(II) in the presence of Cu2+, Zn2+, Co2+, Pb2+ and Cd2+.

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Thiol

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adsorption process followed a pseudo-second-order, fitted well with the Langmuir isotherm

Mercury (II)

Furthermore, the efficiency of S-IIPs was also evaluated against real samples showing

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86.78%, 91.88%, and 99.10% recovery for Hg(II) wastewater, ground water and tap water, respectively. In this study, the adsorbent was successfully regenerated for five cycles, which

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allows for their reuse without significant loss of initial adsorption capability.

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© 2018 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.

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Introduction

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The World Health Organization (WHO) recommends a mercury (Hg) concentration of 6 μg/L in drinking water (Awual, 2017). Although the overall global use of mercury has decreased, its use in the developing countries has increased mainly due to artisanal gold mining (Huber and Leopold, 2016; Kumar et al., 2014; Reilly et al., 2016). Sources of mercury can be from anthropogenic activities such as coal combustion, steel production, chlorine and alkali manufacture in chlor-

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alkali industries and cement production (Marley, 2014). Mercury contamination can also be traced to natural sources such as volcanic eruptions (Marley, 2014). Elemental mercury can be oxidized into inorganic mercury compounds such as salts and oxides, including mercuric sulphide (HgS) mercuric oxide (HgO) and mercuric chloride (HgCl2) (Reilly et al., 2016). These mercury compounds tend to be reactive and soluble in water. Mercury salts are also used as preservatives in certain cosmetic products and therefore are eventually discharged as effluent into the aquatic system (Aronson, 2016). Acute

⁎ Corresponding author at: Department of Applied Chemistry, University of Johannesburg, Doornfontein Campus, Johannesburg 2028, South Africa. E-mail address: [email protected]. (K. Pillay).

https://doi.org/10.1016/j.jes.2018.11.022 1001-0742 © 2018 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Please cite this article as: T. Velempini, K. Pillay, X.Y. Mbianda, et al., Carboxymethyl cellulose thiol-imprinted polymers: Synthesis, characterization and selective Hg(II) a..., Journal of Environmental Sciences, https://doi.org/10.1016/j.jes.2018.11.022

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1.2. Preparation of epichlorohydrin crosslinked polymers

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CMC was first stabilized by crosslinking with epichlorohydrin according to the following procedure: a solution of 1% CMC was prepared by mixing NaOH pellets (5 g) and CMC (2 g) in deionized water (200 mL). Epichlorohydrin (5 mL) was slowly added dropwise under stirring conditions over a few minutes. The final solution was poured into a large amount of methanol. The corresponding residue that was formed was put into a 250 mL round bottomed flask containing ethanol (100 mL) ethanol and NaOH (2 g). Epichlorohydrin (20 mL) was added and the resulting mixture was stirred for 24 hr at 45°C. The crosslinked carboxymethyl cellulose (CL-CMC) product that formed was purified by dropwise additions of 0.1 mol/L HCl followed by washing with deionized water. The CL-CMC was thereafter dried at 50°C in a vacuum oven.

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1.3. Grafting of thiol ligand and ion imprinting

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The carboxylic moieties of CL-CMC were modified by the thiol ligand, with the aid of EDC/NHS coupling agent (Yang et al., 2014). The crosslinked CMC (2 g) was dispersed in deionized water and transferred to a three necked flask. To this mixture was added NHS (0.36 g; 3.0 mmol), EDC (0.54 g; 2.8 mmol) and cysteamine (0.86 g; 11.0 mmol) or cysteamine hydrochloric acid (1.25 g; 11.0 mmol) or 5 amino-1,3,4-thiadiazole-2-thiol (1.47 g; 11.0 mmol) and the pH was thereafter adjusted to 4.5. The mixture was purged with N2 for 5 min, and then stirred at room temperature for 24 hr. After addition of 1000 mg/L Hg(II) solution (10 mL), the pH of the mixture was adjusted to 5 and the reaction was allowed to occur at 35°C under nitrogen for 4 hr. Meanwhile, MBA (200 mg) was sonicated in DMF (20 mL) for 30 min and thereafter added to the three necked flask followed by APS (140 mg). Nitrogen was bubbled into the mixture for 30 min. After increasing the temperature to 70°C, the solution was stirred for 12 hr. When the crosslinking reaction was complete, Hg was leached out with a solution of 0.5 mol/L HNO3. The resulting polymers, sulfur ion imprinted CMC (denoted as S-IIPs) were washed with a mixture of deionized water and ethanol (2:1 V/V), dried at 50°C and crushed to powder with a mortar and pestle.

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Sodium carboxymethyl cellulose (degree of substitution of 0.9 and average molecular weight ~ 250,000), 99% epichlorohydrin (ECH), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC), N-hydroxysulfosuccinimide (NHS), cysteamine (CS), cysteamine hydrochloride (CS·HCl), 5-amino-1,3,4-thiadiazole-2thiol (ATT), methylenebis(acrylamide) (MBA), ammonium per sulphate (APS), dimethylformamide (DMF) and mercury(II) chloride were purchased from Sigma Aldrich and were used without further purification. Sodium hydroxide pellets, 65% HNO3, methanol and mercury standard solution (1000 mg/L) were obtained from Merck.

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1.1. Materials

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1. Materials and methods

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CMC were synthesized and applied for the extraction of Hg (II) 125 from aqueous media. 126

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exposure to elemental Hg may cause liver failures and skin diseases. Health complications also extend to adverse effects on the central nervous system, pulmonary kidney functions and the chromosomes (Punrat et al., 2014). Therefore, it is important to remove or at least reduce the amount of Hg(II) from water. Adsorption has emerged as a suitable technique for the removal of heavy metals from water because it offers the requisite robustness and reliability. In addition, adsorption is cost effective when compared to current conventional water treatment methods such as reverse osmosis, ion exchange, chemical precipitation, and ultrafiltration (Chaudhry et al., 2017; Khalid et al., 2017a). The low cost associated with the adsorption technique is attributable to the type of adsorbing material used in the adsorption process. Although low cost adsorbents such as naturally occurring lignocellulosic materials, activated carbons (Anirudhan et al., 2008; Hadi et al., 2015) and coal fly ash (Attari et al., 2017) have previously been used in the uptake of mercury, poor regeneration coupled with reduced physical stability and lack of selectivity limit their application in the adsorption of mercury adsorption (Arshadi et al., 2017; Attari et al., 2017). It is for this reason that ion-imprinting has emerged as a technology for the removal of mercury from aqueous media. Due to the three-dimensional complementary structure of ligand-target ion, the materials synthesized by the ion imprinting technique show highly selective properties towards their target pollutant (Lenoble et al., 2016). This high selectivity is due to the ligand-target ion three-dimensional complementary structure (Roy et al., 2017). The most commonly used imprinting method is bulk polymerization. However, bulk polymerization has problems such as incomplete template removal, and high density crosslinking, which can lead to poor accessibility of imprinting sites (Mafu et al., 2013; Monier, 2013a). Surface imprinting is an alternative method, which produces polymers with high adsorption capacity, quick mass transfer, high accessibility of active sites and capacity for complete removal of target ions (Wang et al., 2016; Zhu et al., 2018). A suitable material for ion-imprinting is carboxymethyl cellulose (CMC), a low cost, non-toxic natural derivative of native cellulose used in a variety of industries such as the cosmetic, food, and pharmaceuticals industries (Monier et al., 2016). CMC possesses numerous functional groups that can be available for modifications and tailoring towards desired functionalities or usage. Monier (2013b) tested the adsorption of uranyl(VI) on ion-imprinted CMC and obtained good adsorption capacities of 180 mg/g. Selection of ligand has proven to be very important in the imprinting technique; a number of authors have, for example, used thiol-based adsorbents for the extraction of Hg because it is known to have high affinity towards sulfur (Gong et al., 2014; Monier, 2013b; Zhang et al., 2016). Essentially, the high electron density of the sulfur atom allows for the easy electron donation to the cationic Hg through simple reaction conditions, based on the theory of Hard and soft acid–base. Although mercury adsorption using CMC-based material has been reported (Gong et al., 2014), the authors are not aware of any work involving the use of ion imprinted CMC for the removal of Hg(II) pollutants. In this work, ion-imprinted

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Please cite this article as: T. Velempini, K. Pillay, X.Y. Mbianda, et al., Carboxymethyl cellulose thiol-imprinted polymers: Synthesis, characterization and selective Hg(II) a..., Journal of Environmental Sciences, https://doi.org/10.1016/j.jes.2018.11.022

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Scheme 1 – Sulfur ion-imprinted polymers (S-IIPs) synthesis. EDC: 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, NHS: Nhydroxysulfosuccinimide; MBA: methylene bis(acrylamide); DMF: dimethylformamide; APS: ammonium per sulphate; CMC: carboxymethyl cellulose.

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1.4. Carboxylic and amine group estimation

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Carboxylic acid content in CL-CMC was determined as follows: 0.2 g of the polymer was sonicated with 20 mL of 0.1 mol/L NaOH aqueous solution for 1 h at room temperature, then centrifuged. The excess concentration of NaOH was titrated against 0.1 mol/L HCl using phenolphthalein as an indicator (El-Sheikh, 2016), a blank experiment was also carried out. The following equation was used to calculate the carboxylic content ([COOH]):

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Non-imprinted polymers (NIPs) were synthesized as S-IIPs but without the Hg(II). Scheme 1 shows the procedure for the synthesis of the S-IIPs.

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½COOH ¼ 191 190 192 193 194 195 196

ðV 1 −V2 Þ  0:1 W

ð1Þ

where, W (g) is the weight of sample, and V1 (L) and V2 (L) represent the blank and sample experimental volumes of HCl consumed respectively. The active amine content of the NIPs and S-IIPs was determined in a similar manner; however, the polymer was in this case sonicated in HCl and thereafter titrated against NaOH (Tolba et al., 2017).

1.5. Characterization of the polymers

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The polymers were characterized using Fourier transform infrared (FT-IR) spectroscopy (Model 4000, Midac, USA) by the KBr pellet method to identify functional groups after modification. An X-ray photoelectron spectroscope (XPS) (Phoibos 100, SPECS, Germany) with monochromatic Al Kα X-radiation (1486.71 eV, 150 W) and analyzer work function of 4.3979 was used to validate binding energies of the S-IIPs before and after leaching of the Hg ions. Raman analysis was conducted using a confocal Raman microscope (alpha 300R, WITec, Germany) to determine interactions between thiol functional groups and imprinted Hg(II) on the surfaces of adsorbent. A thermal gravimetric analyzer (TGA) (TGA 4000, PerkinElmer, USA) was operated at a temperature range of 30 to 600°C to investigate the thermal stability of the polymers. The samples were run under nitrogen at a heating rate of 10°C/min. X-ray diffraction (XRD) studies were carried out using CuKα radiation (Ultima IV, Rigagu, Japan) for 2θ of between 5 and 90° to evaluate crystallinity of the polymer samples. Surface areas and porosities were determined through Brunauer–Emmett–Teller (BET) analysis; the analysis was undertaken at − 198°C via N2

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Please cite this article as: T. Velempini, K. Pillay, X.Y. Mbianda, et al., Carboxymethyl cellulose thiol-imprinted polymers: Synthesis, characterization and selective Hg(II) a..., Journal of Environmental Sciences, https://doi.org/10.1016/j.jes.2018.11.022

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To determine the effect of varying the amount of adsorbent on the adsorption of Hg(II) ions, various amounts of the adsorbent (5, 10, 15, 20 or 50 mg) were used in the extraction of a 50 mg/L aqueous solution of Hg(II). The adsorption was carried out at 25°C and pH of 5. To study the effect of pH on the adsorption capacity of the adsorbent, a 150 mg/L solution of Hg(II) was used and the pH was varied over a range of 2 to 7; solutions of 0.5 mol/L NaOH or NH4OH and 0.1 mol/L HNO3 were used for the adjustment of the pH. The adsorption kinetics was determined as follows: a 50 mg/L solution of Hg(II) was made to come into contact with the adsorbent at various contact periods and temperatures. Upon establishment of the optimum contact time, the solutions were separated from the adsorbent and the residual Hg(II) was determined. To study the effect of initial Hg concentration of the metal ion (5, 10, 20, 30, 40, 50, 100, 150, 200, 300, and 400 mg/L) the adsorption was carried out at different temperatures of either 25, 35, 45, or 55°C.

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1.7. Hg(II) analysis

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All filtrates from the monitoring of leaching process, adsorption experiments and desorption experiments were analyzed for Hg(II) concentration by square wave anodic stripping voltammetry technique using an Autolab (PGSTAT 302 N, Metrohm, Switzerland). A three-electrode consisting of glassy carbon as a working electrode, a platinum wire as a counter electrode and Ag/AgCl (3 mol/L KCl) as a reference electrode was used. Nitric acid (0.1 mol/L) was employed as the supporting electrolyte. The pH of the filtrate was adjusted to 5 using NaOH and HNO3. The following analytical conditions were used: conditioning time 600 s, conditioning potential − 1.2 V, condition duration 5 sec, deposition potential − 1.4 V, deposition time 60 sec, equilibration time 5 sec, initial potential − 1.0 V, final potential 1.0 V, step 0.005, amplitude 0.002 mV, and frequency 20 Hz. The amount of

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ðC0 −Ce ÞV m

qt ¼

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ð3Þ 284 283

1.8. Selectivity adsorption studies

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The adsorption of 50 mg/L Hg(II) in the presence of other competing cations was investigated. A mass, 150 mg of the adsorbent (NIPs or S-IIPs) was mixed with an aqueous solution (100 mL) containing 50 mg/L of each of the following elements: Pb, Co, Cu, Cd and Zn. The pH of the solution was adjusted to 5 and adsorption was carried out for 24 hr at 25°C and shaking rate of 200 r/min. The mixture was then filtered, and the concentration of the cations was determined using an inductively coupled plasma-optical emission spectrometry (ICP-OES) (iCAP6500 Duo, Thermo Scientific, UK). The selectivity of S-IIPs for Hg over other ions was calculated by the selectivity coefficient (β), which is expressed as follows (Zhu et al., 2017):

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where, qe (mg/g) is the equilibrium adsorption capacity of adsorbent, C0 (mg/L) and Ce (mg/L) are the initial and final Hg (II) concentrations, respectively, V (L) is the volume of Hg(II) solution, and m (g) is the weight of the adsorbent. At various times periods t (min), the adsorption capacity, qt (mg/g) was obtained using Eq. (3).

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ð2Þ

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qe ¼

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adsorbed Hg(II) was calculated using Eq. (2) or (3).

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adsorption/desorption using a Micrometrics Tristar (Model ASAP 2020, Micrometrics Tristar, USA) surface area and porosity analyzer. Morphology and elementary composition studies were carried out using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), respectively. The micrographs were recorded using a TESCAN VEGA3 (VEGA3, TESCAN, Czech Republic) SEM. A Bruker Avance III (500 MHz, Avance III, Bruker, USA) nuclear magnetic resonance (NMR) spectrometer was used for 13C solid state analysis. NMR was used to evaluate the structural transformation of the polymer backbone before and after Hg(II) leaching. Elemental analysis was measured by a Thermo fisher Scientific CHNS-O 2000 elemental analyzer (CHNS-O 2000, Thermo fisher Scientific, UK). The zeta potentials of the polymers were determined using a Malvern nanosizer (Model ZENN 3600, Malvern, UK) to determine the surface charge of polymer samples; samples were dispersed in conical flasks and solutions of 0.1 mol/L HNO3 and 0.1 mol/L NaOH were used to adjust the pH.

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β¼

DHgðIIÞ DMðIIÞ

C0 −Ce V  M Ce

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ð4Þ

where, DHg(II) and DM(II) are the distribution coefficients of Hg (II) and another ion M(II), respectively. D (L/g) is the distribution coefficient which is calculated using the following equation: D¼

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ð5Þ

Where, M (g) is the mass of adsorbent. The relative selectivity 304 305 coefficient (βr) of binding of Hg to S-IIPs against NIPs is defined by: 306 βr ¼

βimpr: βnon‐impr:

ð6Þ

where, βimpr. and βnon-impr. Are the selectivity coefficients of S-IIPs 307 308 and NIPs, respectively. 309

1.9. Desorption studies

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The adsorbent reuse and regeneration experiments were conducted by subjecting an aqueous solution (10 mL) of 20 mg/L Hg(II) to the adsorption process using IIPs (20 mg) under the following reaction conditions: 120 min, 25°C, and pH 5. The mixture was filtered and the adsorbed Hg(II) was analyzed. Hg loaded adsorbents were thoroughly washed with deionized water, filtered and transferred to the bottle containing the desorbing agent. Following an analysis of the desorbed Hg(II) ions, the adsorbent was regenerated with 0.5 mol/L HCl. The process was repeated four more times using the same adsorbent.

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Please cite this article as: T. Velempini, K. Pillay, X.Y. Mbianda, et al., Carboxymethyl cellulose thiol-imprinted polymers: Synthesis, characterization and selective Hg(II) a..., Journal of Environmental Sciences, https://doi.org/10.1016/j.jes.2018.11.022

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2.1.1. Effect of different ligands

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Sulfur IIPs (S-IIPs) functionalized with cysteamine (CS) were found to possess the highest adsorption percentage compared to cysteamine HCl (CS·HCl) functionalized S-IIPs and 5 amino1,3,4-thiadiazole-2-thiol (ATT) functionalized S-IIPs (Table 1). On the other hand, Appendix A Fig. S1 which shows a type IV isotherm with hysteresis loops at relative pressures of between 0.8 and 1 for CS·HCl and ATT IIPs, signifies the presence of mesopores in the polymers (Fu et al., 2016; Meouche et al., 2017). S-IIPs functionalized with ATT gave the lowest adsorption percentage because, being cyclic (Appendix A Fig. S2), this compound seems too bulky to fit in the matrices of the polymer. The low adsorption percentage associated with the cysteamine hydrochloride (CS·HCl) functionalized S-IIPs is attributable to the hydrochloride moiety, which prevents the amine groups from reacting with the carboxylic functionalities, thus inhibiting the formation of the crucial amide bond (Luong et al., 2015). Additionally, as indicated in Table 1, IIPs with CS were found to possess the highest surface area compared to ATT functionalized S-IIPs and CS·HCl functionalized S-IIPs. It is for this reason that CS was adopted in this study for the synthesis of NIPs and S-IIPs.

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Table 1 – Effect of various ligands on the adsorption percentages of the adsorbents. Ligand

Cysteamine hydrochloride (CS·HCl) 5-Amino-1,3,4thiadiazole-2thiol (ATT) Cysteamine

Surface area (m2/g)

Pore volume (cm3/g)

Pore Adsorption diameter (nm)

1.55

0.0052

17.06

63.5%

0.553

0.0032

56.79

56.7%

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0.0668

40.13

97.3%

2.2. Characterization of polymers

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A peak appearing at 1728 cm−1 in the FT-IR spectrum of CLCMC (Fig. 2a) is ascribed to the CO stretching bands of the COOH group (Luong et al., 2015; Meouche et al., 2017) and grafting of the ligand was confirmed when this peak disappeared in the FT-IR spectra of unleached sulfur IIPs (Un.S-IIPs) and leached sulfur IIPs (L.S-IIPs). The FT-IR spectra of NIPs, unleached and leached S-IIPs shown in Fig. 2b shows the stretching bands of the alkene 1618 cm−1 (Yari et al., 2015) which overlap with the bending band associated with the N–H of the amide group present in both L.S-IIPs and Un.S-IIPs. Since this band does not appear to have shifted, such an observation indicates that the NH functionality is not involved in Hg (II) coordination because the –CO– of the amide group tends to withdraw electrons from the nitrogen. The stretching bands associated with the amine group is prominent at 3415 and 3414 cm−1 for Un.S-IIPs and L.S-IIPs, respectively. These bands overlap with the characteristic OH stretching band of the hydroxyl groups. A shift in the stretch bands is indicative of an electron deficit in the Hg cations, which draws lone pair of electrons from N and thus providing evidence for the involvement of N in coordination. Furthermore, such a shift indicates successful incorporation of the cysteamine moiety into the CL-CMC. The OH bending of the hydroxyl overlapping with the symmetry vibration of COO − (Yu et al., 2017) on the surface of the polymers occurs at different positions; Un.S-IIPs (1426 cm− 1) and L.S-IIPs (1417 cm−1). The shift to a lower frequency for L.S-IIPs suggest participation of oxygen atoms of the hydroxyl in the interaction with Hg ions. Other peaks were an asymmetric stretching vibration of C–O–C at 1113 cm−1 (Lin et al., 2015), C–H asymmetrical stretching at 2929 cm−1 and stretching vibrations of CN bonds at 1264 cm− 1. It should be mentioned that no bands disappeared after Hg unleaching, nevertheless this does not dismiss the complexation of Hg with the thiol groups. Thiol-Hg bonds can occur at lower frequencies than those used in IR analysis (Gong et al., 2014). The elemental compositions of the CMC, L.S-IIPs, Un.S-IIPs and NIPs were analyzed by an elemental analyzer and the results are shown in Appendix A Table S1. The CMC as expected does not contain any nitrogen and sulfur, but the L. S-IIPs, Un.S-IIPs and NIPs all show various amounts of S and N emanating from the cysteamine ligand. Of note is the slight decrease in the proportions of the S (1.89%) and N (1.96%) in L. S-IIPs compared to the Un.S-IIPs (S-2.35% and N-2.77%). It can be assumed that some of the N and S was leached out during the Hg leaching process with the nitric acid. However, the leaching did not affect the selectivity adsorption and

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The coupling of amino groups of the CS to the carboxyl groups of the crosslinked CMC with EDC/NHS was carried out at pH 4. This pH was chosen since EDC crosslinker reacts more effectively in the pH range of 4–6 (Gök et al., 2017). Furthermore, the undesirable disulphide bonds that forms between – SH moieties of the cysteamine in the presence of oxygen was minimized by performing the experiment under an inert atmosphere (purging with nitrogen) during the grafting of the ligand and the subsequent Hg imprinting processes. During the imprinting process, Hg strongly binds to thiols mainly because of the hard–soft acid–base (HSAB) theory. According to the HSAB theory, softer transition-metal ions are prone to form stable complexes with ligands carrying softer donor atoms. The complexes formed by a combination of the same acid–base class would show strong binding properties. In this instance, the SH group act as a soft base, thereby easily combining with the softly acidic Hg2+.

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The COOH content of the crosslinked CMC was found to be 3.71 mmol/g. The ligand ratios were varied to optimize the amount of the ligand required for reaction with the COOH (Fig. 1 for a summary of the findings). Fig. 1 shows that the amount of amino groups initially increased exponentially with the carboxyl-ligand ratio before it almost leveled off. To this end, the optimum carboxyl-ligand ratio was found to be 1:1.5; beyond this ratio any further increases in the carboxyl-ligand ratio resulted in minimal increases in the amine content.

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2.1.2. Effect of carboxyl-ligand ratio on amine content

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2. Results and discussion

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Please cite this article as: T. Velempini, K. Pillay, X.Y. Mbianda, et al., Carboxymethyl cellulose thiol-imprinted polymers: Synthesis, characterization and selective Hg(II) a..., Journal of Environmental Sciences, https://doi.org/10.1016/j.jes.2018.11.022

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3.2 2.8 2.4

1:1.5

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COOH:Ligand ratio Fig. 1 – Amine concentrations at different carboxyl and ligand ratios.

a

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NIPs

Transmittance (%)

1426

3414 1264

2929

3414

1618

500

1000 1500 2000 2500 3000 3500 4000 4500 -1

-1

Wavenumber (cm )

Wavenumber (cm )

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1113

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Intensity

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adsorption capacity of the L.S-IIPs for the Hg(II) as discussed in Sections 1.3, 1.4, and 1.5. The percentage compositions of C and H elements increased from the starting material (CMC) in

Transmittance (%)

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1:0.5

F

2.0

comparison to the NIPs, L.S-IIPs, and Un.S-IIPs (Appendix A Table S1) due to the immobilization of the methylene bis acrylamide, in the latter three polymers. XPS is a valuable technique for the analysis of the elemental valence state on the surfaces of the polymers. Furthermore, it is useful in validating interaction between functional groups and metal ions on adsorbent surfaces. The XPS wide spectra of CMC and Un.S-IIPs shown in Appendix A Fig. S3a confirmed the imprinting of the Hg onto the polymers. While the Hg 4f peak is clear in Un.S-IIPs, it is evidently absent in CMC. The narrow XPS scan of the Hg 4f orbital in Appendix A Fig. S3b for Un.S-IIPs depicts two peaks at 100.8 and 104.7 eV belonging to the 4f7/2 and 4f5/2 of the Hg, respectively (Das et al., 2017; Kumar and Jiang, 2014; Zhang et al., 2015). The Hg 4f7/2 binding energy at 100.8 eV is indicative of the presence of Hg(II) which is typically at 100.7 ± 0.1 eV (Behra et al., 2001). This means that the mercury was imprinted as Hg(II). The characteristic XPS narrow scan peaks of N1 s and O1s for Un.S-IIPs and L.S-IIPs are shown in Appendix A Figs. S4 and S5, respectively. When comparing the Un.S-IIPs and L.SIIPs peaks of the N1 s spectra, a decrease in binding energy from 397.6 to 396.1 eV, is noticed after leaching due to the free lone pair of electrons which were previously engaged in complexation with Hg(II). The leaching of the Hg results in

D

Amine content (mmol/g)

3.6

S-Hg-S

Un.S-IIPs

2000

CMC

1000 L.S-IIPs

0 200

400

600

NIPs

800

1000

1200

-1

Wavenumber (cm )

Fig. 2 – Fourier transform infrared (FT-IR) spectra of (a) crosslinked CMC (CL-CMC, (b) non-imprinted polymers (NIPs), leached sulfur ion imprinted polymers (L.S-IIPs), unleached sulfur ion imprinted polymers (Un.S-IIPs), and (c) Raman spectra of CMC, Un.S-IIPs, L.S-IIPs, and NIPs.

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425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448

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JOUR N AL OF EN VI R ONM E NT AL S CI E N CE S XX ( XXX X) X XX

463 464 465 466 467 468 469 470 Q9 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488

F

O

462

R O

461

P

460

D

459

E

458

a

being recorded at 357.2°C; the weight losses for Un.S-IIPs and NIPs were observed at 328.6 and 319.8°C, respectively (Fig. 3b). The decline in thermal stability of Un.S-IIPs compared to L.SIIPs is attributed to the Hg co-ordination, which weakened the internal hydrogen bonds in the structural matrixes of the material. Finally, weight losses of 80.57% for NIPs, 72.56% for Un.S-IIPs and 72.61% for L.S-IIPs were observed. The XRD diffractograms of the polymers display one peak at 2θ ≈ 24° as depicted in Fig. 4a. Un.S-IIPs have a peak intensity shorter than both L.S-IIPs and NIPs indicating a change in the structural order of the material. In this case, Un. S-IIPs is less crystalline and lacks uniformity. Interaction of the ligand and the target Hg ions in imprinted adsorbents impacted a non-orderly structure. This observation is further supported by the decomposition profile of the TGA showing greater stability of the L.S-IIPs compared to the Un.S-IIPs. The nitrogen adsorption–desorption isotherms for the SIIPs and NIPs samples are shown in Fig. 4b and c, respectively. According to the IUPAC classification, the adsorption isotherms are similar to Type 4, which is indicative of the mesoporous nature of the polymers (Zhou et al., 2016). In addition, pore diameters of S-IIPs were found to be between 2 and 50 nm (Table 1 cysteamine results), which is characteristic of mesoporous materials. However, at high pressure of approximately 0.8–1 an H3 hysteresis loop can be noted, suggesting the presence of large mesopores and macrospores (Lu et al., 2015). BET pore volume for S-IIPs (Table 1, cysteamine pore volume results) was considerably higher compared to that of NIPs (0.0165 cm3/g) and this can be explained as follows: crosslinking with MBA caused a network of bonds to form around Hg ions which after leaching left voids, thus raising the sorbent porosity of S-IIPs. The average pore diameter of S-IIPs (40.13 nm) was found to be larger than that of NIPs (37.82 nm) and this indicates that the pores of S-IIPs are large enough to accommodate the Hg cation. Furthermore, NIPs surface area of 2.121 m2/g was recorded, which is inferior to that of IIPs (7.049 m2/g). The surface morphology of the polymers was probed and is shown in Fig. 4d. The surface of the starting material (i.e. CMC) is

b

0.0

100

NIPs -0.2

80

Un.S-IIPs L.S-IIPs

L.S-IIPs

60

dW (%)

457

T

456

C

455

E

454

R

453

O R

452

N C

451

increase in electron density of the O and thus decreasing the binding energy (Hande et al., 2016). This shift in binding energy was also observed in the O1s peaks. The narrow scans of the C1s of Un.S-IIPs and L.S-IIPs are shown in Appendix A Fig. S6a. The C1s XPS represent the C–N, C–O, C–C (Appendix A Fig. S6b and c) groups of the polymers (Tolba et al., 2017; Das et al., 2017). The simultaneous shifting of the C1s peak to lower binding energy and curve broadening for L.S-IIPs is due to changes in the electron densities of the oxygen, nitrogen and sulfur atoms attached to the carbon atoms. However, for the S2p peak, although it was observed in the XPS narrow scan of the Un.S-IIPs (Appendix A Fig. S7) it was too little and could not be resolved in the XPS narrow scan of L.S-IIPs, as observed and discussed from the elemental composition analysis. The interactions between thiol groups and mercury was verified using Raman. The Raman spectra of CMC, Un.S-IIPs, L. S-IIPs and NIPs from wavenumber of 200 to 1250 cm− 1, is shown in Fig. 2c. The spectra of Un.S-IIPs shows a band at about 280 cm−1 assigned to the bonding of S–Hg–S (Pillay et al., 2013; Firouzzare and Wang, 2012), which is absent in the spectra of CMC, NIPs, and L.S-IIPs. The characteristic stretching of the C–O band at 1073 cm−1 (Attala and Isogai, 2010), is prominent in the spectra of Un.S-IIPs compared to the spectra of CMC, L.S-IIPs, and NIPs, further confirming the interactions of Hg with the O of the C–O. Appendix A Fig. S8 depicts ring torsions domination in the Un.S-IIPs spectra, suggesting that the Hg causes polymer rings to undergo different conformations. This further confirms the presence of Hg in the Un.S-IIPs. Fig. 3a illustrates the TGA analyses of NIPs, unleached SIIPs and leached S-IIPs. The initial dip in the weight percentage of all the polymers at temperatures between 25 and 100°C, associated with the removal of adsorbed water was observed. A noticeable decrease in stability at temperatures between 250 and 300°C was noted on the Un.S-IIPs curve. This may have been caused by decomposition of mercury chloride, oxide or sulphide salts. The next stages were rapid weight losses characterized by decomposition of the polymer backbone. The L.S-IIPs curve displayed the greatest stability when comparing the three polymers, with the major weight loss

U

450

Weight (%)

449

Un.S-IIPs 40

NIPs

-0.4 357.2

-0.6

328.6

-0.8

319.8

20 0 100

200

300

400 o

Temperature ( C)

500

600

-1.0 100

200

300

400

500

600

o

Temperature ( C)

Fig. 3 – (a) Thermal gravimetric analysis (TGA) and (b) derivative thermal gravimetric analysis (DTGA) of NIPs, Un.S-IIPs, and L. S-IIPs. dW: derivative weight.

Please cite this article as: T. Velempini, K. Pillay, X.Y. Mbianda, et al., Carboxymethyl cellulose thiol-imprinted polymers: Synthesis, characterization and selective Hg(II) a..., Journal of Environmental Sciences, https://doi.org/10.1016/j.jes.2018.11.022

489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528

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JOUR N AL OF EN VI RON M EN T AL S CI E NC ES XX ( XXX X) X XX

b

a

12

Intensity (a.u)

3

Quantity adsorbed (cm /g)

10

Un.S-IIPs

L.S-IIPs

8 6 Desorption 4 2

20

40

60

0 0.0

80

0.2

0.4

Adsoprtion 1.0

O

R O

50

40 3

Quantity adsorbed (cm /g)

0.8

Relative pressure (P/Po)

2θ (degree)

c

0.6

F

NIPs

P

30

20

D

Desorption

0.2

0.4

T

0 0.0

E

10

0.6

Adsorption 0.8

1.0

RelativeN pressure (P/Po ) isotherms of (b) Un.S-IIPs and (c) S-IIPs, and (d) Fig. 4 – (a) X-ray diffraction pattern of NIPs, Un.S-IIPs, and L.S-IIPs, adsorption

C

2

529

R R

E

scanning electron microscopy (SEM) images of CMC, Un.S-IIPs, and L.S-IIPs.

545

2.3. Adsorption studies

546

Parameters affecting the adsorption performance process such as contact time, pH, initial ion concentration and dosage of the adsorbent were investigated. To this end, the experimental adsorption data was analyzed using the kinetic, isotherm and thermodynamics models. Modeling the

534 535 536 537 538 539 540 541 542 543

547 548 549 550

C

533

N

532

U

531

O

544

characteristically smooth whereas the rough surfaces of the IIPs emanating from the imprinting step were observed. The L.S-IIPs appears to be made of some elongated sheetlike porous structure that possesses more space compared to the Un.S-IIPs. This noticeable difference is due to the fact that cavities remain in the L.S-IIPs after the leaching of Hg ions. Evidence of cysteamine grafting is supported by EDS pictures of CMC and S-IIPs (Appendix A Fig. S9a and b), which shows that the sulfur peak is prominent in S-IIPs and absent in CMC. Fig. 5 shows solid state 13C NMR peak. While there was no appearance of a new peak as expected, major changes in peak positions were observed between Un.S-IIPs and L.S-IIPs. All the peaks shifted to lower values because structural transformation occurred when Hg was leached out of the mercury polymer matrix.

530

experimental adsorption data is important since it allows the prediction and comparison of the performance of the adsorbent, which is critical for optimization of the adsorption mechanism pathways, expression of the adsorbent capacities, and effective design of the adsorption systems.

551

2.3.1. Effect of pH on the adsorption of Hg(II) by IIPs and NIPs

556

Fig. 6a shows an increase in the adsorption of the Hg(II) ion when the pH of metal ion solution as increased up to pH 5. Adsorption of metal ions then decreases after reaching the maximum. Low retention of Hg at low pH is due to the protonation of the active groups on the surfaces of adsorbent, thereby inducing an overall positive charge on the surfaces of adsorbent. Thus, by determining the zeta potential as a function of pH, the polymer surface charge or point zero charge pH (pHpzc) were found to be 3.6 for NIPs and 3.7 for SIIPs (Fig. 6b). At pH < pHpzc, the surfaces of the adsorbent are positively charged while at pH > pHpzc the adsorbent is mainly negatively charged. Therefore, at low pH values, repulsion occurs between the protonated charged surfaces of the adsorbent and the positively charged Hg(II). Additionally, at low pH, competition arises between the high concentration of H+ found in the solution and the Hg(II) ion to bind to the active sites on the surface of the adsorbent. As pH is increased

557

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552 553 554 555

558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573

9

575 576 577 578 579 580 581

the concentration of H+ is reduced resulting in favorable Hg adsorption. The uptake of Hg on polymer adsorbents can be summarized occurring by electrostatic interactions, chemical interactions and ion exchange or substitution between thiol group, negatively charged oxygen containing functional groups or lone electrons on the amine groups with Hg ions forming various complexes, as shown in Eqs. (7)–(10) (Khalid et al., 2017a, 2017b; Saha et al., 2016).

U

574

N C

O R

R

E

C

T

E

D

P

R O

O

F

JOUR N AL OF EN VI R ONM E NT AL S CI E N CE S XX ( XXX X) X XX

RSH þ Hg2þ →RS‐Hg‐SR þ 2Hþ 583 582 585 584 587 586 589 588

2RCOOH þ Hg

2þ

→RCOO‐Hg‐OOCR þ 2H

ð7Þ þ

2ROH þ Hg2þ →RO‐Hg‐OR þ 2Hþ € þ Hg 2RH2 N

2þ

→RH2 N‐Hg‐NH2 R

€ where, RSH, RCOOH, ROH, and RH2 N

ð8Þ ð9Þ ð10Þ represent thiol,

carboxylic, hydroxy, and amine functional groups respectively, on the surfaces of the adsorbent. However, further pH increments consequently result in a decrease in mercury adsorption. The mercury Eh-pH diagram (Appendix A Fig. S10) shows that the main mercury species occurring at approximately pH 6–10 is Hg(OH)2. Therefore, Hg (II) is precipitated as Hg(OH)2 at pH > 5 and the generated species cannot fit within the crevices of imprinted polymer because of the unrecognizable shape, size and geometry of the Hg(OH)2 molecules (Zhang et al., 2016).

590

2.3.2. Adsorbent dosage effect

600

The increases in the dosage amounts of 5, 10, 15 and 20 mg resulted in a gradual and significant increase in the percentage Hg uptake from aqueous solutions, as shown in Fig. 7. When the dosage of the adsorbent was increased, the number of active sites in the polymer that were available for Hg adsorption also

601

Please cite this article as: T. Velempini, K. Pillay, X.Y. Mbianda, et al., Carboxymethyl cellulose thiol-imprinted polymers: Synthesis, characterization and selective Hg(II) a..., Journal of Environmental Sciences, https://doi.org/10.1016/j.jes.2018.11.022

591 592 593 594 595 596 597 598 599

602 603 604 605

10

T

E

D

P

R O

O

F

JOUR N AL OF EN VI RON M EN T AL S CI E NC ES XX ( XXX X) X XX

613 614

E

a 80 70

2.3.3. Effect of contact time and adsorption kinetics for the 618 adsorption of Hg(II) using S-IIPs 619 The uptake of Hg (II) at various temperatures was monitored 620 over a period of contact time (Fig. 8a). In general, the initial 621 adsorption increased exponentially under all temperatures 622

b 25 S-IIPs NIPs

S-IIPs NIPs

20 15

60 50 40 30 20 10 0

Additionally, the surface area of S-IIPs was found to be far 615 superior to that of NIPs, and this translated into a greater 616 number of sites for Hg adsorption. 617

Zeta potential (mV)

612

R R

611

O

610

C

609

N

608

increased. The maximum percentage removal of Hg that was achieved for S-IIPs was found to be 99.8%; from adsorbent dosage of 20 mg and beyond, no further improvement in the Hg uptake was observed. The few remaining vacant sites available at higher dosages were difficult to access because of repulsion between the few Hg ions occurring in solution and the highdensity volume of the already adsorbed Hg ions. On the other hand, the difference in the S-IIPs and NIPs adsorption is due to the higher affinity of the Hg ion for the imprinted sites.

U

607

Recovery (%)

606

C

Fig. 5 – Solid state 13C nuclear magnetic resonance (NMR) peaks of L.S-IIPs and (inset) Un.S-IIPs; C1-C10: carbon atoms on positions 1–10 for S-IIPs.

10 3.7

5 0 -5 3.6

-10 -15

2

3

4

5 pH

6

7

-20

2

4

6 pH

8

10

Fig. 6 – Effect of pH on (a) Hg(II) adsorption by S-IIPs and NIPs and (b) zeta potential of NIPs and S-IIPs.

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11

JOUR N AL OF EN VI R ONM E NT AL S CI E N CE S XX ( XXX X) X XX

60 40 S-IIPs NIPs

20 0

0

10

20

30

40

50

logðqe−qtÞ ¼ logqe− t 1 t ¼ þ qt k2 qe 2 qe

Fig. 7 – Effect of amount of adsorbent on Hg(II) adsorption.

qt ¼ kp t1=2 þ c

ð13Þ

P

D b

T

50

C

40

E

30

0

0

20

40

O R

10

o

25 C o 35 C o 45 C o 55 C

R

20

60

80

100 120 140 160 180 200

Time (min)

c

d

50

4.5

40

4.0

30

3.5 o

25 C

20

o

35 C

3.0

o

45 C

10

o

55 C 0

0

2

4

6

8 1/2

10

12

14

2.5 0.0030

0.0031

0.0032

629 630 631 632 633 634 635 636 637 638

640 639

E

a

N C

626

U

625

qe (mg/g)

624

628

ð12Þ

where, qe (mg/g) and qt (mg/g) are the amounts of metal ion adsorbed at equilibrium and time (t) per unit weight of the sorbent, respectively, t1/2 (min1/2), and k1 (min−1)) and k2 ((g/ (mg·min)) are are pseudo-first-order and pseudo-second-

conditions; this is due to the high concentration gradient between the sorbate in solution and the adsorption sites on the adsorbent surface. Thereafter, the adsorption rate seems to de-escalate until it reaches an equilibrium, with the time

q t ( m g /g )

623

627

ð11Þ

R O

Mass (mg)

k1 t 2:303

O

Uptake (%)

80

F

required to reach the equilibrium decreasing with temperature. For example, equilibrium is reached within 20 min at 55°C and in 50 min at 25°C. This difference can be described in terms of the kinetic energy of the Hg molecules been adsorbed; at higher temperatures Hg molecules have greater kinetic energy and can hence move faster with less diffusion resistance towards the adsorption sites. The adsorption of ions onto the adsorbents is better explained by the adsorption kinetics, i.e. the pseudo-firstorder (shown in Eq. (11)), pseudo-second order (summarized in Eq. (12)) and the intra-particle diffusion model, represented by the Webber-Morris Eq. (13).

100

0.0033

0.0034

1/2

t (min ) Fig. 8 – (a) Effect of contact on time on Hg(II) adsorption and kinetics for Hg adsorption: (b) linear pseudo-second order, (c) multilinear plots of the Weber and Morris plot (intra-particle diffusion model), and (d) Arrhenius plot for activation energy (Ea) determination. qe: adsorption capacity at equilibrium; qt: adsorption capacity at time t; T: temperature; k2: pseudo-secondorder rate constant.

Please cite this article as: T. Velempini, K. Pillay, X.Y. Mbianda, et al., Carboxymethyl cellulose thiol-imprinted polymers: Synthesis, characterization and selective Hg(II) a..., Journal of Environmental Sciences, https://doi.org/10.1016/j.jes.2018.11.022

642 641 644 643 645 646 647

12

t2:4

Model

t2:5

Pseudo-first-order

t2:6 t2:7 t2:8 t2:9 t2:10 t2:11 t2:12 t2:13 t2:14 t2:15 t2:16 t2:17 t2:18 t2:19

qe (mg/g) k1 (min−1) R2

655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683

50.08 0.00811 20.33 0.9991

49.80 0.0145 36.00 0.9992

49.60 0.0379 93.28 1

Intraparticle kp (mg/(g·min1/2)) c (mg/g) R2

4.403 21.56 0.8982

6.492 18.02 0.9445

3.954 33.24 0.8037

1.374 43.34 0.8886

qe: equilibrium adsorption capacity; k1: pseudo-first-order rate constant; k2: pseudo-second-order rate constant; kp: intraparticle diffusion rate constant; ho: initial adsorption rate; c: boundary layer thickness constant; R2: correlation coefficient.

order rate constants, respectively. In Eq. (13), kp (mg/(g·min1/2)) and c (mg/g) are the intraparticle diffusion rate constant and the constant that is related to the magnitude of boundary layer thickness boundary layer thickness constant, respectively. The linear and non-linear curve fittings (Fig. 8b and c and Appendix A Fig. S11) from which different kinetic parameters were calculated are represented in Table 2. The initial segment shown in Fig. 8c is due to the film diffusion; Hg is transported from boundary film to the exterior surface of adsorbent. The second linear segments are related to gradual adsorption stage, where there is transfer of Hg from the surface to the pores of the adsorbent (intra- particle pore diffusion) whereby the intraparticle is the rate-limiting step. The third segment is related to equilibrium; when intraparticle diffusion slows down. The plot of qt against t1/2 passes through the origin and is linear throughout, when intra-particle is the singular rate controlling step for Hg adsorption. The graphs were not linear for the whole range and instead showed multi-linearity thus suggesting that other chemical reactions such as external mass transfer are involved in the rate controlling step. The correlation coefficients of the pseudo-first and second-order kinetics given in Table 2 were compared and pseudo-second-order kinetics values were higher (0.999–1) compared to pseudo-first-order values (0.513–0.905). In addition, theoretical qe value for second model closely matched the experimental qe value; whereas the pseudo first, the theoretical qe and experimental qe values were not close. Therefore, pseudo-second-order kinetics dominates the adsorption process. The rate constant (k2) and the initial adsorption rate (ho) are also indicated in Table 2. The trend is that both these parameters increase with temperature, thus concluding that adsorption of Hg onto S-IIPs is enhanced with temperature. In order to determine whether the adsorption of Hg is a physical or chemical process, the pseudo-second-order rate

F

50.05 0.00538 13.48 0.9992

O

Pseudo-second–order qe (mg/g) k2 (g/(mg·min)) ho (mg/(g·min)) R2

where, A (g/(mg·min)) is the frequency factor, Ea (kJ/mol) is the activation energy of a reaction, R (0.008314 kJ/(mol·K)) is gas constant and T (K) is the temperature. The plot of ln k2 vs. 1/T is shown in Fig. 8d. The parameter Ea was calculated from the slope of this plot and was found to be 51.54 kJ/mol. This indicates a chemical sorption between the IIPs and mercury ions since activation energies under 40 kJ/mol are typically indicative of a physisorption process, while chemisorption requires activation energies higher than 40 kJ/mol (Wang et al., 2009).

687 686 688 689 690 691 692 693 694 695 696

2.3.4. Influence of initial ion concentration and adsorption 697 isotherms 698

R O

3.82 0.0387 0.6146

ð14Þ

The maximum amount of Hg ions adsorbed per unit mass of adsorbent, which was calculated from adsorption equilibrium, is important in optimizing adsorbent efficiency and applicability. The adsorption capacity of S-IIPs towards various concentrations of Hg and temperatures, which is depicted in Fig. 9a, shows an increase with initial metal ion concentration. Initial qe increments are very rapid followed by steady qe increases before they level off. This is due to the fact that the strength of the driving force for the mass transfer is increased and hence Hg ion uptake is enhanced (Anirudhan et al., 2007). However, adsorption eventually slows down almost to a constant because the active sites available for adsorption become nearly occupied with the adsorbed ions. In order to assess the type of Hg binding, isotherms models were used to simulate the experimental data. Adsorption isotherm studies also give an indication of relationship between the amount of the adsorbate on the adsorbent surface and the adsorbate remaining in solution at equilibrium. Herein, the Langmuir, Freundlich, Temkin and the Dubinin–Radushkevich (D–R) isotherm models described by linear Eqs. (15), (16), (17), and (18) respectively were used. The corresponding linear plots are presented in Fig. 9b, c, d, and e, respectively.

P

5.11 0.0421 0.5134

Ea RT

T

654

11.74 0.06755 0.7216

C

653

20.88 0.0499 0.9049

E

652

55

R R

651

45

O

650

35

C

649

lnk2 ¼ ln A−

25

N

648

Temperature (°C)

U

t2:20 t2:21 t2:22 t2:24 t2:23

constant k2 (g/(mg·min)) at different temperatures was ex- 684 trapolated from the Arrhenius equation: 685

D

t2:3

Table 2 – Kinetic parameters of the pseudo-first-order and second-order for Hg(II) adsorption.

E

t2:1 t2:2

JOUR N AL OF EN VI RON M EN T AL S CI E NC ES XX ( XXX X) X XX

Ce 1 Ce ¼ þ qe KL qm qm logqe ¼ qe ¼

1  logCe þ logK f n

RT RT  lnKt þ  lnCe b b 0

lnqe ¼ ln q −KDR ε2

699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721

ð15Þ 723 722

ð16Þ 725 724

ð17Þ ð18Þ

727 726

where, ε is the Polanyi potential (Debnath et al., 2014), and is 728 729 derived through: 730   1 ε ¼ RT ln 1 þ Ce

ð19Þ

where, qm (mg/g) gives the Langmuir theoretical maximum adsorption capacity, KL (L/mg) is a Langmuir constant that describes the free energy of adsorption, Kf and 1/n are Freundlich constants representing adsorption capacity and

Please cite this article as: T. Velempini, K. Pillay, X.Y. Mbianda, et al., Carboxymethyl cellulose thiol-imprinted polymers: Synthesis, characterization and selective Hg(II) a..., Journal of Environmental Sciences, https://doi.org/10.1016/j.jes.2018.11.022

732 731 733 734 735

13

JOUR N AL OF EN VI R ONM E NT AL S CI E N CE S XX ( XXX X) X XX

a

b 90 80 70

qe (mg/g)

60 50 o

40

25 C o 35 C o 45 C o 55 C

30 20 10

0

100

200

300

F

0 400

d

90

R O

c

O

C0 (mg/g)

25°C 35°C 45°C 55°C

1.95 o

1.80

70 60

P

1.85

qe (mg/g)

1.90

Log qe

80

25 C o 35 C o 45 C o 55 C

1.75

D

50

1.70

40

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

-2

4.5

4

6

35°C 45°C

R

4.3

55°C

In qe

O R

2

25°C

E

4.4

N C

0

InCe (mg/L)

C

e

3.0

T

Log Ce

2.5

E

1.65 -1.5

4.2 4.1 4.0

0

1000 2000 3000 4000 5000 6000 ε2

U

Fig. 9 – (a) Effect of initial ion concentration and temperature on Hg(II) adsorption; Isotherm linear plots at various temperatures for: (b) Langmuir, (c) Freundlich, (d) Temkin, and (e) Dubinin–Radushkevich. C0: initial metal ion concentration; Ce: equilibrium metal ion concentration; ε: Polanyi potential.

736

adsorption intensity, respectively, b is the Temkin constant related to the heat of sorption, Kt is the Temkin isotherm 738 Q10 constant (Ahamed et al., 2013), KDR is the activity coefficient 739 related to adsorption energy, and q' is the D–R theoretical 740 saturation capacity. 741 Calculated isotherm constants are shown in Table 3. The 742 correlation coefficients related to the Langmuir isotherms 743 were much closer to 1 in comparison to the other applied 744 adsorption isotherms for the corresponding temperatures. 745 This suggests that the adsorption of mercury was predomi746 nately a monolayer process without mutual interactions 737

between the adsorbed molecules. Other adsorption processes described by the Freundlich, Temkin, and Dubinin– Radushkevich isotherms are less likely to occur. A dimensionless separation factor (RL) defines the favourability of an adsorption process and is described by Eq. (20). RL ¼

1 1 þ KL C0

747 748 749 750 751

ð20Þ

753 754 If 0 < RL < 1, the reaction is favorable, whereas RL = 0 is 752 indicative of an irreversible reaction. Calculated RL values 755 greater than 1 indicate an unfavorable reaction (Khalid et al., 756

Please cite this article as: T. Velempini, K. Pillay, X.Y. Mbianda, et al., Carboxymethyl cellulose thiol-imprinted polymers: Synthesis, characterization and selective Hg(II) a..., Journal of Environmental Sciences, https://doi.org/10.1016/j.jes.2018.11.022

14

Langmuir

t3:6 t3:7 t3:8

qm (mg/g) KL (L/mg) RL

t3:9 t3:10 t3:11 t3:12 t3:13 t3:14 t3:15 t3:16 t3:17 t3:18 t3:19 t3:20 t3:21 t3:22 t3:23

R2

25

35

81.03 6.567 3.8 × 10−3 0.38 × 10−3 0.9970

45

55

80.32 6.452 3.9× 10−3 0.39 × 10−3 0.9978

82.99 6.887 3.6× 10−3 0.36 × 10−3 0.9977

84.89 7.206 3.5× 10−3 0.35 × 10−3 0.9980

Freundlich 35.212 Kf (mg/g) 1/n 0.14362 0.9403 R2

45.745 0.09829 0.9424

47.022 0.09953 0.9540

56.918 0.06642 0.9336

Temkin Kt (L/g) b R2

1305.3 0.4158 0.9188

1302.9 0.4151 0.9284

6.74 × 105 0.6322 0.8934

81.31 5.63 × 10−5

82.94 3.97 × 10−5

0.9372

0.9014

8.067 0.2422 0.9577

F

t3:5

Temperature (°C)

O

Models

R O

t3:4

Fig. 10 – Thermodynamic fittings for S-IIPs adsorption of Hg (II). Kd: thermodynamics equilibrium constant.

t3:25 t3:26 t3:27 t3:28 t3:29 t3:30 t3:31

qm: theoretical maximum adsorption capacity; KL: free energy of adsorption constant; RL: dimensionless separation factor; Kf: adsorption capacity constant; 1/n: adsorption intensity constant; Kt: isotherm constant; b: heat of adsorption constant; KDR: adsorption energy activity coefficient; q': theoretical saturation capacity.

760 761 762

t4:1 t4:2 t4:3

cations. As a result, a strong surface interaction between metal ions and adsorbent occurs, which leads to an ideal metal ion sorption process. The qm was relatively high compared to other adsorbents reported in the literature for Hg adsorption (Table 4). Deb et al. (2017) reported higher adsorption capacities compared to our S-IIPs. However, it should be noted that the major material in the reported study were multiwalled carbon nanotubes (MWNTs), which are quite expensive relative to the CMC used in this study.

T

C

E

759

R R

758

2017b; Ahamed et al., 2013). According to Table 3, RL range (calculated using lower limit C0 40 mg/L and upper limit C0 400 mg/L) were above 0 but less than 1, showing favorable adsorption of Hg. In this case, the abundant sulfur or nitrogen groups on the surfaces of the adsorbent with delocalized electrons, donate electrons to form complexes with the metal

Table 4 – Comparison of carboxymethyl S-IIPs for this study with other adsorbents.

O

757

P

t3:24

Dubinin–Radushkevich (D–R) q' (mg/g) 77.17 78.86 6.56 × 10−5 KDR (mol2/ 9.62 × 10−5 kJ2) R2 0.9740 0.9535

D

t3:3

Table 3 – Isotherms for Hg(II) adsorption on S-IIPs and NIPs at different temperatures.

E

t3:1 t3:2

JOUR N AL OF EN VI RON M EN T AL S CI E NC ES XX ( XXX X) X XX

t4:4

Adsorbent

t4:5

Silk protein sericin

t4:6

Eucalyptus bark

Q1 t4:7

Fly ash-Zeolite A

94%

Room

t4:8

IIP Fe3O4@SiO2

78.3 mg/g

25

t4:9

101.35 mg/g

25

t4:10 t4:11

Amidoaminemultiwalled carbon nanotubes Aminothiol IIP Thiourea IIP

28 mg/g 110.3 mg/g

25 30

t4:12

S-IIPs

80 mg/g

25

Temperature (°C)

References

64.3%

25

34.60 mg/g

40

Koley et al., 2016 Ghodbane and Hamdaoui, 2008 Attari et al., 2017 Zhang et al., 2016 Deb et al., 2017

U

N

C

Adsorption capacity

Firouzzare and Wang, 2012 Monier et al., 2014 This study

Table 5 – Thermodynamic adsorption of Hg(II).

parameters

for

S-IIPs

Temperature (K)

Δ G° (kJ/mol)

Δ S° (kJ/(mol·K))

Δ H° (kJ/mol)

298.15 308.15 318.15 328.15

−4.12141 −8.01111 −11.9008 −15.7905

0.38897

111.85

Table 6 – S-IIPs and NIPs selectivity results. Ionic radius (pm)

Hg Cu Zn Cd Pb Co

102 73 74 95 120 69

S-IIPs

765 766 767 768 769 770 771

t5:1 t5:2 t5:3

t5:4

t5:9 t5:10 t5:11

t6:1 t6:3 t6:2

t6:4

NIPs β

βr

1.581 2.374 1.505 2.254 0.9639

2.374 2.046 2.186 2.362 2.221

t6:5 t6:6 t6:7 t6:8 t6:9 t6:10 t6:11

D: distribution coefficient; β: selectivity coefficient; βr: relative selectivity coefficient.

t6:12 t6:13 t6:14

D (L/g) 2.272 0.5345 0.4452 0.6509 0.3155 0.9868

β

764

t5:5 t5:6 t5:7 t5:8

ΔG°: Gibbs free energy; ΔS°: entropy of sorption; ΔH°: enthalpy of sorption.

Metal ion

763

4.252 5.105 3.492 7.205 2.303

D (L/g) 0.8758 0.4733 0.3689 0.5818 0.3885 0.9086

Please cite this article as: T. Velempini, K. Pillay, X.Y. Mbianda, et al., Carboxymethyl cellulose thiol-imprinted polymers: Synthesis, characterization and selective Hg(II) a..., Journal of Environmental Sciences, https://doi.org/10.1016/j.jes.2018.11.022

15

JOUR N AL OF EN VI R ONM E NT AL S CI E N CE S XX ( XXX X) X XX

0.5 mol/L HNO3- 1% thiourea

0.5 mol/L HNO3

0.1 mol/L HNO3

1% thiourea

Table 7 – S-IIPs application for the adsorption of Hg(II) in real water. Samples

Adsorption (%)

100

Hg amount before spiking (μg/L)

Wastewater 245.8 Ground Not detected water Tap water Not detected

80 60

1

2

3

4

5

The mechanism of adsorption with increased temperature can be explained in terms of changes in standard thermodynamic parameters such as Gibbs free energy (ΔG°), enthalpy of sorption (ΔH°), and entropy of sorption (ΔS°). The thermodynamic parameters were evaluated by the following equations: ΔHo ΔSo þ lnKd ¼ − RT R

C

ο

ð21Þ

ο

ΔG ¼ ΔH −TΔS

ð22Þ

E

779 778 781 780

U

N C

O R

R

where, Kd is the equilibrium constant obtained from the propor782 tion of adsorbate on the polymer (qe) to that of the adsorbate in 783 solution (Ce) at equilibrium. The slope and intercept of linear plot 784 of Eq. (21) (Fig. 10) gives values of ΔH° and ΔS° respectively and 785 results are shown in Table 5. Basically, ΔG° becomes more negative 786 as temperature increases, thus demonstrating a more spontane787 ous reaction. The ΔS° (388.97 J/(mol K)) value was found to be 788 positive, showing that randomness is heightened during the 789 adsorption process. The calculated enthalpy of sorption value was 790 positive indicating the endothermic nature of the Hg(II) adsorption 791 process. Usually ΔH° values that are less than 80 kJ/mol point to 792 the physical nature of the adsorption process (Parashar et al., 793 2016), otherwise chemisorption occurs. ΔH° was found to be 794 greater than 80 kJ/mol and this further confirms the chemisorp795 tion nature of the adsorption process, which is in agreement with 796 the conclusion drawn from the calculated activation energy value 797 Q11 of 51.54 kJ/mol (Section 1.3.3). 798

2.4. Selective uptake Hg(II)

799

When an adsorbent is non-selective its adsorption capacity towards the specific ion dramatically decreases when other ions with similar charge, size or shape are present because of the competition for the active site of adsorbent that arises. Thus, a selectivity study of Hg was carried out in the presence of other divalent cations and results thereof are presented in Table 6. SIIPs were found to possess the largest distribution coefficient for

800 801 802 803 804 805

3.2%

t7:7 t7:8 t7:9

Hg(II), which indicates that the polymer still possessed high adsorption for the cation. Obviously, the selectivity coefficient of S-IIPs is higher than that of NIPs, thus pointing to the effectiveness of the imprinting process. The highest S-IIPs selectivity coefficient was observed for Pb cations, which has an ionic radius of 120 pm. The lowest selectivity coefficient was found for Co (ionic radius is 69 pm). It appears that Co ions, which have the smallest ionic radii of all the ions, easily fits into Hg cavities while Pb ions possess the largest ionic radii and are as a result too large for the predefined cavities.

806

2.5. Reuse of S-IIPs

816

The ability of a material to be reused is one of the most important factors to consider when deciding on the suitability of a material for use as an adsorbent. A number of desorbing agents were tested for elution of adsorbed Hg over five cycles and the findings are summarized in Fig. 11. It is evident from Fig. 11 that 1 mol/L HNO3 and 1% thiourea in 0.5 mol/L HCl were the most effective eluents. Thiourea in HCl is generally used for the desorption of Hg ions from adsorbent surfaces (Pillay et al., 2013; Dakova et al., 2012) because the sulfur and nitrogen present in thiourea facilitates the removal of Hg from the active sites by binding with the cation. The HCl provides chloride ions which can draw the Hg from the adsorbent and result in formation of HgCl2 (Pillay et al., 2013). It is for this reason that thiourea dissolved in HCl is expected to be a highly effective desorbent. However, when thiourea was repeatedly used as the only desorbing agent continual loss of the adsorbent was observed, that is, the mass of the S-IIPs at the end of each cycle was significantly decreased. This indicates that thiourea is possibly responsible for the partial destruction of the carboxymethyl cellulose and/or interfered with the cysteamine–CMC interactions. In contrast, it was possible to use S-IIPs for five consecutive adsorption cycles without any significant loss in the initial adsorption capacity when nitric acid was employed as a desorbent. This is because the surface imprinting technique allows for the almost complete removal of the template ion (Appendix A Fig. S12) from the imprinted sites during the first three cycles, which suggests that high adsorption capacities were possible in the two subsequent cycles.

817

2.6. Application of S-IIPs in real samples

846

D

773

777

99.10%

E

2.3.5. Adsorption thermodynamics of Hg(II) by S-IIPs

T

772

776

t7:5 t7:6

P

Fig. 11 – Adsorption percentages of the consecutive adsorption–desorption cycles by S-IIPs for Hg(II) removal using different desorbing agents.

775

3.9% 2.4%

R O

Cycle number

774

86.78% 91.88%

O

0

t7:4

RSD (n = 3)

F

20

t7:3

Removal (after spiking)

RSD: relative standard deviation; n: number of replicas.

40

t7:1 t7:2

807 808 809 810 811 812 813 814 815

818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845

The synthesized S-IIPs were applied in the adsorption of real 847 samples of wastewater, groundwater and tap water. The 848 results presented in Table 7 show that the S-IIPs were efficient 849

Please cite this article as: T. Velempini, K. Pillay, X.Y. Mbianda, et al., Carboxymethyl cellulose thiol-imprinted polymers: Synthesis, characterization and selective Hg(II) a..., Journal of Environmental Sciences, https://doi.org/10.1016/j.jes.2018.11.022

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Acknowledgments

865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881

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Ion-imprinted polymers that showed selective adsorption of Hg(II) were synthesized from low cost CMC. Functionalization of the CMC by adding epichlorohydrin and grafting of the ligand were confirmed by various techniques such as EDS, FTIR and TGA. Of the three ligands that were studied for the complexation of Hg(II), cysteamine was found to be more compatible with the metal ion in aqueous solutions. It was established that the optimum pH for the adsorption of the Hg (II) ions is 5. Temperature increases resulted in decreased equilibration time and slightly higher adsorption capacities; adsorption equilibrium was reached within 50 min at a temperature of 25°C. As evidenced by the high calculated activation energy, the mechanism of uptake was found to be through a chemical reaction occurring between the surface of the IIPs and the Hg(II). The imprinting process played a crucial role in the selectivity of polymers for Hg over other cations. Furthermore, it was shown that the size of Zn, Cd, Pb, Co, and Cu influenced their uptake by S-IIPs. The ability of 0.5 mol/L HCl in 1% thiourea and 1 mol/L HNO3 to remove adsorbed Hg (II) was comparable. Furthermore, the IIPs we synthesized were found to be efficient adsorbents for the removal of water spiked with Hg(II). Finally, the synthesized IIPs have demonstrated their potential to be used as selective adsorbents in polluted waters.

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3. Conclusions

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in adsorbing Hg(II) (the water samples spiked with 20 mg/L of Hg before adsorption experiments were conducted). Compared to ground and tap water, wastewater contains many contaminants including metallic compounds (Zhang et al., 2017, 2018; Zeng et al., 2018) that compete with Hg for the adsorption sites on the S-IIPs. This study has also revealed the high selectivity of IIPs towards Hg.

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885

This work is based on the research supported in part by the National Research Foundation of South Africa (No. 93205). The 887 authors would also like to acknowledge the Water Research 888 Commission (WRC) of South Africa for the financial support 889 (No. K5/2387) and Dr. Brian Doyle and Dr. Emanuela Carleschi 890 Q12 for the XPS analysis.

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892 891

Appendix A. Supplementary data

893 894

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jes.2018.11.022.

895

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Zhu, C., Hu, T., Tang, L., Zeng, G., Deng, Y., Lu, Y., et al., 2018. Highly efficient extraction of lead ions from smelting wastewater, slag and contaminated soil by two-dimensional montmorillonite-based surface ion imprinted polymer absorbent. Chemosphere 209, 246–257.

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Please cite this article as: T. Velempini, K. Pillay, X.Y. Mbianda, et al., Carboxymethyl cellulose thiol-imprinted polymers: Synthesis, characterization and selective Hg(II) a..., Journal of Environmental Sciences, https://doi.org/10.1016/j.jes.2018.11.022