Sodium hyroxyethylcellulose adipate: An efficient and reusable sorbent for cadmium uptake from spiked high-hardness ground water

Muhammad Ajaz Hussain; Shahana Zaman; Azhar Abbas; Muhammad Nawaz Tahir; Muhammad Amin; Syed Zajif Hussain; Irshad Hussain

doi:10.1016/j.arabjc.2018.07.007

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Original article

13 (

); 2766-2777

doi:

10.1016/j.arabjc.2018.07.007

Sodium hyroxyethylcellulose adipate: An efficient and reusable sorbent for cadmium uptake from spiked high-hardness ground water

Muhammad Ajaz Hussain^{^a,⁎}, Shahana Zaman^{^a}, Azhar Abbas^{^a}, Muhammad Nawaz Tahir^{^b,⁎}, Muhammad Amin^{^a}, Syed Zajif Hussain^{^c}, Irshad Hussain^{^c}

a Ibn-e-Sina Block, Department of Chemistry, University of Sargodha, Sargodha 40100, Pakistan

b Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

c Department of Chemistry, SBA School of Science & Engineering (SSE), Lahore University of Management Sciences (LUMS), DHA, Lahore Cantt. 54792, Pakistan

⁎Corresponding authors. majaz172@yahoo.com (Muhammad Ajaz Hussain), muhammad.tahir@kfupm.edu.sa (Muhammad Nawaz Tahir)

Received: 2018-1-7, Accepted: 2018-7-8, Published: 2018-01-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Herein, we report on the evaluation of chemically modified hyrdroxyethylcellulose for its sorption efficacy to remove Cd(II) from spiked high-hardness ground water (GW) and distilled water (DW) solutions. Hyrdroxyethylcellulose was chemically modified using adipic anhydride in the presence of DMAP as a catalyst under homogeneous reaction conditions. Hyrdroxyethylcellulose adipate (HEC-Adip) obtained was then converted to its sodium salt as HEC-Adip-Na after treating with saturated solution of NaHCO₃. Structures were thoroughly characterized using FTIR, SEM, EDS and solid-state CP/MAS ¹³C NMR. The surface charge of the sorbent was monitored by pH_ZPC (pH of zero point charge). The sorption data fitted well to Langmuir isotherm and pseudo-second order kinetic model. Maximum sorption capacity of Cd(II) was calculated to be 114.94 mg g⁻¹ and 112.35 mg g⁻¹ from DW and GW, respectively. Thermodynamic parameters, i.e., ΔH°, ΔS° and ΔG°, were also calculated and showed negative values indicating spontaneous and exothermic nature of sorption process. In addition, the synthesized sorbent is more suitable for repeated use because it shows negligible decrease in its sorption capacity for the uptake of Cd(II) from aqueous solution after five regeneration cycles.

Keywords

Hydroxyethylcellulose

Sodium hydroxyethylcellulose adipate

Metal uptake

Cadmium

Kinetics

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1 Introduction

Worldwide, there are a number of water pollution sources: among them organic and inorganic pollutants are more hazardous (Zhang et al., 2016; Ahmad et al., 2018). Globally due to industrial revolution, the rate of inorganic pollutants production increases many times which is being considered as the dominant and permanent pollutant for drinking and ground water (Negm et al., 2018). During past few decades, people learned that industrial processes such as metallurgy, petrochemical refining, leather tanneries, mining activities and paints and pigment manufacturing (Nguyen et al., 2013; Duan et al., 2014) resulted in severe pollution of ground and surface water which have reached to alarming situation (Morales et al., 2016). From the eco-toxicological point of view, the most hazardous inorganic pollutants coming out of above mentioned industries are heavy metals like lead, cadmium, mercury, and chromium, etc. Among heavy metal ions, cadmium is a highly toxic metal which causes breakdown of immune system which further leads to cancer, diarrhea, renal failure, lungs disorder, skeletal deformation and damage of central nervous system (Oelkers et al., 2011; Khan et al., 2013; Tellez-Plaza et al., 2008 ).

A large number of techniques have been used for the removal of heavy metals from water which include solvent extraction, ion-exchange, reverse osmosis, precipitation and nanofiltration (Williams et al., 1998; Kadirvelu et al., 2001; Cheng et al., 2014; Kim et al., 2006 ). But, these expensive technologies are hazardous to environment and efficiency of the method become compromised afterward. Consequently, cost effective and environmental friendly procedures for water purification should be developed. Recently, research consideration has been dedicated to ion-exchange process which is being considered to be more efficient, economical and eco-friendly for the purification of water containing sewage water, tannery and industrial effluents (Guo et al., 2015; Suteu et al., 2015).

Most valuable ion-exchangers/sorbent are chemically modified cellulosic biomaterials because of their economical and biodegradable nature, water insolubility, reusability and non-toxicity (Abbas et al., 2015; Babel and Kurniawan, 2003; Suteu et al., 2015 ). Among chemical modified cellulosic materials, esterified polysaccharides (getting COOH functionalities as termini) are current topic of interests for the removal of metal ions (Abbas et al., 2017; Lima et al., 2006; Zhong et al., 2012 ).

In the present work, we are aimed to synthesize an efficient and novel sorbent of commercially available biomaterial, i.e., hydroxyethylcellulose (HEC), after its modification by esterification with adipic anhydride to form HEC-adipate (HEC-Adip). HEC-Adip will be having single kind of COOH functionalities suitable to be converted to its sodic form (HEC-Adip-Na) after treating with NaHCO₃. We are aimed to report appraisal of HEC-Adip-Na as a new and efficient ion exchanger in terms of Cd(II) uptake from DW and GW. Present investigation also dealt with metal ions uptake under equilibrium conditions using HEC-Adip-Na and their kinetic evaluation to establish the mechanism of metal ion uptake. The present work is also evaluating the reusability of HEC-Adip-Na for Cd(II) uptake. Calculation of different kinetic parameters and thermodynamic triplet (ΔH°, ΔS° and ΔG°) are also aimed to check the nature and spontaneity of the adsorption process.

2 Materials and methods

2.1

2.1 Materials

Hydroxyethylcellulose was obtained from Natrosol hydroxyethylcellulose, HE10K, Belgium. NaHCO₃, NaOH and HNO₃ were obtained from Fluka. Adipic anhydride, solvents and other reagents were procured from Sigma-Aldrich. All the organic solvents used in the laboratory were of analytical grade. Chemical characterization of GW (from District Sargodha) was done using available analytical techniques and following information was obtained: pH 7.2, calcium hardness 490.5 mg L⁻¹ , magnesium hardness 885 mg L⁻¹ , total hardness (MgCO₃ and CaCO₃) 1375.5 mg L⁻¹ , P-alkalinity (CO₃⁻²) 0.00 mg L⁻¹, M-alkalinity (HCO₃⁻¹) 560.2 mg L⁻¹, total alkalinity 560.3 mg L⁻¹, concentration of sulphate (SO₄⁻²) 187.5 mg L⁻¹, nitrates conc. (NO₃⁻¹) 27.5 mg L⁻¹, phosphate conc. (PO₄⁻³) 21.2 mg L⁻¹, chlorides conc. (Cl⁻¹) 1425.2 mg L⁻¹ and iron conc. (Fe) is 0.12 mg L⁻¹.

2.2

2.2 Synthesis and characterization of sorbent (HEC-Adip-Na)

2.2.1

2.2.1 Synthesis of HEC-adipate and its sodium salt

For the synthesis of HEC-Adip, dried (at 10° C for 5 h in vacuum oven) hydroxyethylcellulose (HEC, 10 g, 36 mmol) was dissolved in 80 mL of DMAc and the mixture was continuously stirred for 2 h at 353 K temperature to get its clear solution. Adipic anhydride (29.8 g, 108 mmol) was added into the solution by parts and stirred at the same temperature (353 K) for 24 h. The product of the reaction was then precipitated and washed thrice with diethyl ether (250 mL). The product obtained was dried in vacuum oven at 50 °C for 24 h.

DS: 1.02; Yield: 12.6 g (85%)

FTIR: 3292 cm⁻¹ (O—H), 2889 cm⁻¹ (C—H), 1712 cm⁻¹ (C⚌O_Ester) and 1045 cm⁻¹ (C—O—C).

The HEC-Adip was treated with NaHCO₃ (saturated solution) and stirred for 2.0 h to get the final product HEC-Adip-Na. The HEC-Adip-Na was separated from solution by filtration and then dried.

FTIR: 3483 cm⁻¹ (O—H), 2879 cm⁻¹ (C—H), 1724 cm⁻¹ (C⚌O_Ester), 1564 cm⁻¹ (COO⁻), 1056 cm⁻¹ (C—O—C).

2.2.2

2.2.2 Measurements

FTIR spectrometer (IR Prestige-21, Shimadzu, Japan) was used for the spectroscopic analysis and spectrum was recorded between the scanning range 5000–500 cm⁻¹ on the frequency of 400 MHz. KBr pellets of samples were prepared and dried in an oven for 30 min at 60 °C before analysis. The ¹³C NMR (solid state CP/MAS) of the HEC-Adip-Na (sorbent) was obtained at room temperature, 100 MHz, acquisition time 0.032 s, NS 5000, Delay time 4.85 µs at proton 90° using DRX-400, Bruker instrument. SEM-EDS (scanning electron microscopy-energy dispersive spectroscopy) image of the samples were recorded on 1530 Gemini LEO (Carl Zeiss) by making a thin coating of dry sample pasted on carbon tape. FAAS analysis was performed using flame absorption spectrometer (AA 6300, Shimadzu, Japan) with a supply air-acetylene mixture. The sample was irradiated with radiation of wavelength 228.8 nm for the determination of Cd(II) ion concentration in GW and DW before and after the action of adsorbent.

2.2.3

2.2.3 Determination of degree of substitution (DS)

In order to calculate the degree of substitution (DS) of HEC-Adip, 100 mg of the product was mixed with 100 mL of saturated solution of NaHCO₃ for about 2 h at room temperature. The solution was filtered and a known volume of sodium bicarbonate (NaHCO₃) was titrated against 0.02 M hydrochloric acid (HCl) with methyl orange indicator. The volume of HCl required for the neutralization of NaHCO₃ was used for the determination of DS of HEC-Adip with the help of following (Eq. (2)) relation;

(1)

n_{Adip} = V_{{NaHCO}_{3}} \times M_{{NaHCO}_{3}} - V_{HCl} \times M_{HCl}

(2)

DS = \frac{276.14 \times n_{Adip}}{m_{HEC - A d i p} - 100 \times n_{Adip}}

where m_HEC-Adip is the mass of HEC-Adip, n_Adip is the number of moles of carboxylate moieties conjugated with HEC-Adip, 276.14 is the mass of HEC repeating unit.

2.2.4

2.2.4 Calculation of yield

In order to calculate the theoretical yield of HEC-Adip, following equation is used (Hussain et al., 2010);

(3)

Theoretical Y i e l d = W P + (\frac{WP}{MwofHEC} \times D S \times M_{Adip})

where WP is mass of polymer used for reaction, Mw of HEC denotes the mass of HEC repeating unit, DS is the degree of substitution of adipate moiety and M_Adip is molar mass of adipic acid.

2.2.5

2.2.5 Determination of pH_ZPC (pH corresponding to zero point charge)

The value of pH_ZPC, pH corresponding to zero point charge (ZPC) of the sorbent HEC-Adip-Na was determined by using solid addition method (Kumar et al., 2008). NaNO₃ solution (50 mL, 0.1 M) were taken and their initial pH values (pH_i) were adjusted between the range of 2–10 by using 0.1 M HCl or 0.1 M NaOH. Sorbent (50 mg) was added to each solution and shaken for 2 h at 200 rpm at room temperature. The mixtures were then filtered after 2 h and final pH values (pH_f) of filtrate were again determined. A graph was plotted between pH_i and pH_f -pH_i. The point at which it intersected the X-axis showed the value of pH_ZPC. The surface of the sorbent will be negatively charged at pH greater than its pH_ZPC which favors uptake of Cd(II) ions by the sorbent.

2.3

2.3 Sorption kinetics

Batch experiments were conducted by adding of the sorbent (50 mg) into Cd(II) ions solution (100 mL) of known concentrations (both DW and GW). The extent of Cd(II) ion uptake by the sorbent, q_e in mg g⁻¹ was figured out by using the following relation (Eqs. (4) and (5);

(4)

q_{e} = \frac{C_{i} - C_{e}}{m} \times V

(5)

Percentage u p t a k e = \frac{C_{i} - C_{e}}{C_{i}} \times 100

In this equation, C_i and C_f are the initial and final concentrations of Cd(II) ions in mg L⁻¹, respectively. V (L) and m (g) are the volume of solution and mass of adsorbent, respectively.

2.3.1

2.3.1 Effect of the initial metal ion concentration

For the determination of effect of metal ion concentration on the adsorption of Cd(II) ions, solution of different concentrations, i.e., 20, 40, 60, 80, 100, 120 and 140 mg L⁻¹ were prepared in both GW and DW. Experiments were performed by adding of sorbent (HEC-Adip-Na, 50 mg) in each solution and stirring for 30 min at 200 rpm speed and temperature of 298 K.

2.3.2

2.3.2 Effect of sorbent dosage

For the investigation of the effect of sorbent on sorption capacity of HEC-Adip-Na, varied amounts of sorbent (30–100 mg) were added to Cd(II) ions solution (100 mL) having optimized concentration, i.e., 60 mg L⁻¹ in both GW and DW and stirred continuously for 30 min at 200 rpm and 298 K temperature.

2.3.3

2.3.3 Effect of contact time

Optimum time for maximum sorption capacity was determined by stirring the mixture having metal ions and sorbent at room temperature for time intervals between the range 5–120 min under optimized conditions of initial metal ion concentration (60 mg L⁻¹) and sorbent dosage (50 mg).

2.3.4

2.3.4 Effect of initial pH

The effect of pH on the sorption of HEC-Adip-Na was determined by preparing solutions of Cd(II) ions (both GW and DW) having a range of pH between 2 and 7 using 1 M NaOH or 1 M HCl solutions and then treating with sorbent at optimized conditions. The sorbent was allowed to settle and then it was filtered by applying vacuum suction. The filtrate was analyzed through FAAS.

2.3.5

2.3.5 Effect of temperature

The effect of temperature on the sorption of Cd(II) ions by HEC-Adip-Na was also studied. For this purpose the same sorption process was carried out at different temperatures ranging from 298 to 338 K. Kinetic studies were performed by analyzing the contents of mixture at different time intervals before the achievement of equilibrium. Adsorption isotherms were also plotted for the adsorption of Cd(II) ions by the sorbent (HEC-Adip-Na) at above mentioned optimal parameters.

2.4

2.4 Regeneration studies

Regeneration studies were also performed on the sorbent. Sorbent (50 mg) was stirred with 100 mL of both GW and DW solution with 60 mg L⁻¹ initial concentration of Cd(II) ions for 30 min. The mixture was filtered and recovered sorbent was then agitated with 100 mL of freshly prepared saturated solution of NaCl (brine) for 24 h. The mixture was then filtered and washed with DW repeatedly until it gave negative result with AgNO₃ test. The recovered sorbent was then air-dried. The regenerated sorbent was used for the uptake of Cd(II) ions again. The same sorption and desorption experiment was conducted for five times repeatedly and sorbent was washed with distilled water each time till neutralization, before proceeding to next cycle. Sorption experiment was also carried out on the acidic form of the sorbent (HEC-Adip) by stirring sorbent (50 mg) with GW and DW solutions (100 mL) containing Cd(II) solution for metal-uptake at specified pH and 298 K.

All the experiments were performed in triplicates and the concurrent values were taken to present results.

3 Results and discussions

3.1

3.1 Synthesis and characterization of the sorbent (HEC-Adip-Na)

3.1.1

3.1.1 Synthesis of HEC-Adip and its sodic form

HEC-Adip (hydroxyethylcellulose adipate) was synthesized by the reaction of HEC with adipic anhydride in DMAc solvent in the presence of DMAP (4-dimethylamino pyridine) catalyst. The obtained chemically modified product was then treated with saturated solution of NaHCO₃ to get its sodium salt (HEC-Adip-Na). Fig. 1 shows the schematic diagram of synthesis of HEC-Adip and HEC-Adip-Na.

Schematic diagram for the synthesis of HEC-Adip and its conversion to HEC-Adip-Na.

3.1.2

3.1.2 FTIR spectroscopic analysis

Fig. 2 shows the FTIR (KBr) spectrum of HEC-Adip and HEC-Adip-Na, HEC-Adip-Cd. The spectra show that multiple functionalities are present in the sorbent. The band at 1712 cm⁻¹ in the spectrum of HEC-Adip shows presence of ester carbonyl functional group which indicate effective linkage of HEC with adipic anhydride. Spectrum of HEC-Adip shows that after the formation of sodium salt of HEC-Adip, this signal transferred to a higher value of 1724 cm⁻¹ and a typical peak at 1564 cm⁻¹ confirms the presence of carboxylate anion showing successful formation of sodium salt of HEC-Adip. In FTIR spectrum of HEC-Adip-Cd, a shift in peak positions especially in finger print region and from 1724 to 1548 cm⁻¹ was observed. This change in FTIR spectra in moving from HEC-Adip-Na to HEC-Adip-Cd was due to the binding of cadmium with carboxylic acid group of the adsorbent. Shifting of these peaks is an evidence of successful adsorption of Cd(II) on the surface of the adsorbent. The intense bands appeared at 857 and 571 cm⁻¹ in the spectrum of HEC-Adip-Cd are also characteristic for Cd to oxygen bond.

FTIR (KBr) spectra of HEC-Adip, HEC-Adip-Na and HEC-Adip-Na after Cd(II) uptake.

3.1.3

3.1.3 Solid-state ¹³C NMR spectroscopic analysis (CP/MAS)

Solid state (CP/MAS) ¹³C NMR spectroscopic analysis of HEC-Adip-Na was performed to confirm the adipate formation (Fig. 3). The signal at 174.94 ppm (C-11) showed ester signal for successful adipate formation wherewas signal at 182.54 ppm (C-16) showed sadium salt formation of terminal COOH group of HEC-Adip. Another peak at 27.15 ppm (C-12-15) exhibits CH₂ groups in adipate moiety of HEC-Adip-Na. Peaks at 104.15 ppm and 62.08 ppm show C-1 and C-6 in the HEC-Adip-Na respectively. Signals of methylene protons of hydroxyethyl group are overlapped with signals of polymer backbone. Other signals in the range of 71.28–83.10 ppm showed C-2-5 and C-7-10 of HEC-Adip-Na.

Solid-state CP/MAS 13C NMR spectra of HEC-Adip-Na.

3.1.4

3.1.4 SEM-EDS analyses

SEM analysis was carried out to investigate the surface morphology of acidic form (HEC-Adip), sodic form (HEC-Adip-Na) and sodic form of sorbent after Cd(II) uptake. SEM analyses showed that the surface topology of the sorbent (HEC-Adip-Na) was rigid and rough (Fig. 4a–c). EDS plots show the presence of Na ions and Cd ions in the sorbent before and after sorption, respectively (Fig. 4d–f).

SEM images of HEC-Adip (a), HEC-Adip-Na (b), HEC-Adip-Na after Cd(II) uptake (c), EDS of HEC-Adip (d), HEC-Adip-Na (e) and HEC-Adip-Na after Cd(II) uptake (f).

3.2

3.2 Determination of the pH of zero-point charge (pH_ZPC)

The pH_ZPC of the sorbent was also investigated to determine the charge on the surface of the sorbent (HEC-Adip-Na) and it was found to be 5 (Fig. 5). This value shows that the sorbent has weak acidic character due to COOH groups of adipate moieties of the sorbent. This means that at pH 5, i.e., pH_ZPC, the surface of the sorbent is neutral while above and below this pH surface of the sorbent has negative and positive charge, respectively. Any pH value greater than 5 will favour metal uptake.

Zero point charge pH (pHZPC) of HEC-Adip-Na sorbent.

3.3

3.3 Sorption studies

3.3.1

3.3.1 Effect of the initial metal ion concentration

The effect of initial metal ion concentration on the sorption capacity of HEC-Adip-Na for the uptake of Cd(II) ions was investigated both from GW and DW. Sorbent (50 mg) of was added into the solutions having different metal ion concentration ranging from 20 to 140 mg L⁻¹. A graph was plotted between initial metal ion concentration and sorption capacity (Fig. 6a). It is evident from the graph that in start, the sorption capacity of sorbent increased with increase in metal ion concentration but later it became constant. The reason behind might be that at the start more sites of the sorbent were available for the uptake of metal ions but later on all the sites were occupied so the rate becomes constant. The sorbent showed maximum sorption capacity at 60 mg L⁻¹ for both GW and DW solutions. This optimized concentration was used for further studies.

Sorption capacity of HEC-Adip-Na as a function of initial Cd(II) concentration from DW and GW (sorption conditions: sorbent dose = 50 mg in 100 mL Cd(II) solution, Cd(II) concentration both in DW and GW = 20–140 mg L−1, temperature = 298 K, contact time = 30 min, pH was kept at 6.0 for both DW and GW) (a). Sorption capacity of HEC-Adip-Na as a function of sorbent dosage (sorption conditions: sorbent dose = 30–100 mg in 100 mL Cd(II) solution, Cd(II) concentration both in DW and GW = 60 mg L−1, temperature = 298 K, contact time = 30 min, pH was kept at 6.0 for both DW and GW) (b). Sorption capacity of HEC-Adip-Na as a function of contact time (sorption conditions: sorbent dose = 50 mg in 100 mL Cd(II) solution, Cd(II) concentration both in DW and GW = 60 mg L−1, temperature = 298 K, contact time = 5–120 min, pH was kept at 6.0 for both DW and GW) (c). Sorption capacity of HEC-Adip-Na as a function of pH of solution (sorption conditions: sorbent dose = 50 mg in 100 mL Cd(II) solution, Cd(II) concentration both in DW and GW = 60 mg L−1, temperature = 298 K, contact time = 30 min, pH was changed from 2 to 7 for both DW and GW) (d).

3.3.2

3.3.2 Effect of the sorbent dosage

The effect of sorbent dosage on the sorption capacity was investigated by varying the amount of sorbent in the range 30–100 mg. The graph showed an increase in the sorption capacity with an increase in the sorbent dosage due to the increase in the availability of active sites of the sorbent but decreases at later stage due to the reason that all Cd(II) ions have been adsorbed and exchange sites are still available. Moreover, it was also inferred that uptake of Cd(II) ions from distilled water (DW) was greater than ground water (GW). The reason behind this fact is the competition for the sorption sites between the alkaline earth metal ions and Cd(II) ions. The plot (Fig. 6b) shows that maximum sorption capacity was observed for 50 mg of sorbent for both GW and DW solutions and this amount was used for further studies.

3.3.3

3.3.3 Effect of contact time

Fig. 6c illustrates the effect of contact time on the sorption capacity of sorbent. The uptake of Cd(II) ions was estimated between the time range 5–120 min. The plot showed that maximum level of uptake of about 95% was achieved within the early 25 min. This fact can be explained in such a way that greater amount of sorption sites were present in the start but after some time reaction attained equilibrium because all sorption sites were occupied.

3.3.4

3.3.4 Effect of initial pH on Cd(II) ions uptake

The effect of initial pH on the sorption capacity of sorbent (HEC-Adip-Na) was also observed at 60 mg L⁻¹ of initial Cd(II) ion concentration and 50 mg sorbent dosage (Fig. 6d). The pH of solution was varied between the ranges 2–7. The uptake of Cd(II) was found to be highly pH dependent. At acidic pH, sorption of Cd(II) ions was very low but it accelerated rapidly when the pH increased from 5 onwards. This was due to the reason that lower pH causes the protonation of HEC-Adip-Na to form its acidic derivative (HEC-Adip) leaving no more Na ions to be exchanged with Cd(II) ions. At pH greater than 5.0 (pH_ZPC for HEC-Adip-Na) the carboxylic moiety was deprotonated to favor the uptake of Cd(II) ions because at pH greater than 5 surface of the sorbent become negatively charged which favors metal uptake. The maximum sorption capacity was observed at pH 6.0 for GW and DW both.

3.4

3.4 Kinetic and isothermal modeling

3.4.1

3.4.1 Kinetic modeling

Kinetic study of the sorption process was done to investigate the mechanism of the process because it is necessary to know the exact mechanism of process before designing a system of water purification. Two different kinetic models (pseudo-first order and pseudo-second order) were applied to the sorption data.

Pseudo-first order kinetic model can be explained in terms of the following linear equation (Eq. (6));

(6)

l o g (q_{e} - q_{t}) = {logq}_{e} - \frac{k_{1}}{2.303} t

In this equation, q_e and q_t denotes the amounts of metal ions adsorbed at equilibrium and the time t, respectively; k₁ (g mg⁻¹ min⁻¹) is the rate constant of the reaction. If straight line is obtained by plotting $l o g (q_{e} - q_{t})$ against t then it would indicate that the same model is applicable to the given situation. A graph was plotted between $l o g (q_{e} - q_{t})$ against t (Fig. 7a). Time range chosen was 5–30 min because it was found in different cases that experimental data fitted well for the initial rapid phase to this model.

Pseudo-first order (a), pseudo-second order (b) kinetic models, Freundlich (c) and Langmuir sorption isotherm (d) for Cd(II) uptake by HEC-Adip-Na sorbent from GW and DW solutions.

Values of qe and k₁ were calculated from the intercept of the straight line and its slop, respectively. The values of different constant (qe, R² and k₁) are given in the Table 1. Low value of correlation constant (R2) and deviation of experimental data indicate that pseudo-first order model didn’t fit well for both GW and DW. So pseudo-second order kinetic model was applied, which is given by the following linear relation (Eq. (7)).

(7)

\frac{t}{q_{t}} = \frac{1}{{kq}_{e}^{2}} + \frac{t}{q_{e}}

where q_e and q_t denote the amount of metal adsorbed at equilibrium and time t, k (g mg⁻¹ min⁻¹) is rate constant. Pseudo-second order model was applied to the whole time range of experimental data of the sorption process. A graph was plotted between t/q_t against t. A straight line was obtained with higher value of correlation coefficient (Fig. 7b, Table 1). The values of q_e and k were calculated from the slope and intercept of the straight line, respectively (Table 1). The q_e calculated from the pseudo-second-order kinetic model matched with the experimental values. Therefore, it was inferred that pseudo second order is best model to evaluate the dynamic sorption of Cd(II) ions by HEC-Adip-Na (sorbent). Fitting of pseudo second order kinetic model is also an evidence for involvement of chemisorption as the rate determining step.

Table 1 Pseudo-second order, pseudo-first order, Langmuir, Freundlich, ion-exchange models and thermodynamic parameters for for Cd(II) uptake by HEC-Adip-Na from GW and DW solutions.

Models	Parameters	DW	GW
Pseudo second order	q_e (mg g⁻¹)	120.48	120.48
	k₂ (g mg⁻¹ min⁻¹)	0.0040	0.0018
	R²	0.9997	0.9990

Pseudo first order	q_e (mg g⁻¹)	8.770	10.93
	k₁ (g mg⁻¹ min⁻¹)	0.0878	0.0747
	R²	0.9188	0.9456

Experimental	q_e (mg g⁻¹)	114.94	112.35

Langmuir parameters	Q_max (mg g⁻¹)	112.37	1.0019
	b (mg L⁻¹)	1.2428	1.8937
	R²	0.9998	0.9999
	R_L	0.0132	0.0087

Freundlich parameters	n	−170.7	−488
	k_F	119.43	113.27
	R²	0.3228	0.0147

Ion Exchange model	S (min⁻¹)	0.08070	0.5488
Ion Exchange model	R²	0.9977	0.9978

Thermodynamic parameters	ΔS° (J mol⁻¹ K⁻¹)	−101.90	−80.351
	ΔH° (kJ mol⁻¹)	−38.632	−30.595
	R²	0.9862	0.9844
	ΔG° (kJ mol⁻¹)	−8.73	−6.50

3.4.2

3.4.2 Sorption isotherms

At equilibrium stage, studies of sorption isotherms provide useful information about the mechanism of sorption process. A sorption isotherm exhibits the distribution of adsorbed molecules between the sorbent (solid phase) and the adsorbate (liquid phase) of any sorbate-sorbent system. The experimental data obtained from the sorption of Cd(II) ions by the prepared sorbent (HEC-Adip-Na) from both GW and DW solutions was fitted to both Langmuir and Freundlich isotherms. Freundlich isotherm model can be written in the form of following linear equation (Eq. (8));

(8)

\log q_{e} = l o g k_{F} + \frac{1}{n} l o g C_{e}

where Ce (mg L⁻¹) is equilibrium metal ion concentration, qe (mg g⁻¹) is equilibrium sorption capacity, k_F and n are the Freundlich constant and sorption intensity, respectively. The values of sorption parameters for Freundlich isotherm (R², k_F and n) are obtained by plotting a graph (Fig. 7c) between log qe and Ce and there values are given in the Table 1.

It is evident from the plot that it has low value of correlation coefficient (R2) which indicates that Freundlich isotherm model is not valid for this sorption process. So the data was fitted to Langmuir isotherm model, which is represented by linear equation given as (Eq. (9));

(9)

\frac{C_{e}}{q_{e}} = \frac{C_{e}}{Q_{\max}} + \frac{1}{Q_{\max} \times b}

where C_e (mg L⁻¹) and q_e (mg g⁻¹) are Cd(II) ions concentration at equilibrium and equilibrium sorption capacity, respectively. Q_max (mg g⁻¹) and b are maximum sorption capacity of sorbent and Langmuir constant, respectively. When the graph was plotted between C_e/q_e and C_e, a straight was obtained with high correlation coefficient for both GW and DW (Fig. 7d). The values of Q_max and b were calculated from the slop and intercept of the straight line, respectively (Table 1).

Maximum sorption capacities for Cd(II) ions uptake by the sorbent (HEC-Adip-Na) were calculated to be 112.35 and 114.94 mg g⁻¹ for GW and DW solutions. Validation of this model by the experimental data confirms the formation of monolayer, i.e., chemisorption.

The value of separation factor (R_L) is also used to investigate whether the process of sorption is favorable or not, if its value is between 0 and 1. It is given by the following equation (Eq. (10));

(10)

R_{L} = \frac{1}{{1 + b C}_{i}}

Table 1 shows that R_L has values between 0 and 1 indicating that uptake of Cd(II) by the sorbent is a favorable process (Table 1).

3.5

3.5 Effect of temperature

In order to predict the thermodynamic nature of sorption process, effect of temperature on the sorption capacity of HEC-Adip-Na was also examined under optimized conditions by changing temperature between the range 298–338 K (Fig. 8a). It was observed that the sorption capacity reduced by 22.9 and 19.2 mg g⁻¹ for GW and DW solutions, respectively. Hence, it was inferred that the process of sorption was exothermic in nature. The reason behind this decrease in sorption capacity can be explained on the basis of mobility of Cd(II) ions which increases as the temperature is increased which results in decreased adsorbent-adsorbate interaction.

Sorption capacity of HEC-Adip-Na as a function of temperature (sorption conditions: sorbent dose = 50 mg in 100 mL Cd(II) solution, Cd(II) concentration both in DW and GW = 60 mg L−1, temperature = 298–343 K, contact time = 30 min, pH was kept at 6.0 for both DW and GW) (a), effect of temperature on equilibrium constant (Kc) (b) and fitting of ion exchange model for Cd(II) uptake by HEC-Adip-Na from GW and DW solutions (c).

3.6

3.6 Determination of thermodynamic parameters

Thermodynamic parameters (ΔG°, ΔS° and ΔH°) were also estimated to predict the spontaneity of sorption process by using the data obtained from the effect of temperature on the sorption capacity of HEC-Adip-Na for uptake of Cd(II) ions from GW and DW solutions. These parameters can be measured by using the following relations (Eqs. (11)–(13);

(11)

K_{c} = \frac{C_{ads}}{C_{e}}

(12)

{Δ G}^{o} = {- R l n K}_{c}

(13)

{l n K}_{c} = \frac{{Δ S}^{o}}{R} - \frac{{Δ H}^{o}}{RT}

In Eq. (11), K_c represents equilibrium sorption constant, C_ads shows amount of adsorbed metal and C_e is the concentration of metal ions at equilibrium stage. When a graph was plotted between lnK_c and 1/T (K⁻¹), a straight line was obtained ((Fig. 8b) with slop and intercept giving the value of entropy change (ΔS°) and enthalpy change (ΔH°), respectively. Negative values of entropy (ΔS°) shows spontaneity of the sorption while negative values of ΔG° and ΔH° demonstrate the exothermic nature and feasibility of the process (Table 1).

3.7

3.7 Mechanism of sorption

High rate of Cd(II) uptake during initial phase of sorption indicates that ion-exchange mechanism is involved in the sorption process. It can also be observed that the sorbent has negligible property of Cd(II) uptake (9% and 13% from GW and DW, respectively) in its acidic form (HEC-Adip) while its sodic form (HEC-Adip-Na) has very enhanced activity of Cd(II) uptake (93.3 and 96.6% from GW and DW, respectively) because it has sufficient Na ions on its surface which can be exchanged with Cd(II) ions. Application of pseudo-second order model and Langmuir isotherm model also confirms the involvement of ion-exchange mechanism. Moreover, Fig. 4f shows no signal for Na ions which might be an evidence that all the Na ions in the sorbent are exchanged with Cd(II) ions and ion exchange is the main phenomenon involved in sorption of Cd(II) ions by the sorbent.

Boyd et al in 1947 (Boyd et al., 1947) derived an equation for ion-exchange mechanism (Eq. (14));

(14)

l o g (1 - F) = - \frac{S}{2.303} t

(15)

F = \frac{q_{t}}{q_{e}}

where q_e and q_t are amounts of metal adsorbed at equilibrium and time t, respectively and S (min⁻¹) is a constant. Values of log(1-F) were plotted against t. A Straight line was obtained with high values of correlation coefficient which further verified the involvement of ion-exchange mechanism during sorption of Cd(II) ions from GW and DW solutions (Fig. 8c, Table 1).

HEC-Adip-Na sorbent appeared highly valuable in term of high sorption capacity for Cd(II) uptake and got reasonable place in chemically modified polysaccharide based sorbents (Table 2).

Table 2 Comparison of HEC-Adip-Na for Cd (II) uptake from GW and DW solutions with other reported sorbents.

Sorbent (treatment)	Sorption capacity (mg g⁻¹)	References
Spent green (modified with sodium hydroxide, pH 5.3–5.6, 298 K, 2 h)	17.3	Low et al. (2000)
Bagasse pith (SO₂ and H₂S, heat)	149.9	Krishnan and Anirudhan (2003)
Corncob (modified with citric acid and nitric acid pH 6.0, 298 K)	32.3 and 19.3, respectively	Leyva-Ramos et al. (2005)
Mercerized cellulose (modified with succinic anhydride, 10 min)	250	Gurgel et al. (2008)
Mercerized sugarcane bagasse (functionalized with ethylenetetramine, 20 min)	86 and 106.4	Gurgel and Gil (2009)
Filter aid cellulose (modified with succinic anhydride, sodic)	178.6 from GW	Belhalfaoui et al. (2009)
COOH/TUD-1 mesoporous solid	72.34	Bensacia et al. (2014)
Hydroxyethylcellulose (modified with succinic anhydride, 15 min)	454.5 from GW	Abbas et al. (2015)
Hydroxyethylcellulose (modified with adipic anhydride)	114.94 from DW 112.35 from GW	Present study

Note: All values of sorption capacities given in table are calculated by Langmuir model.

3.8

3.8 Sorption and desorption (Re-generability)

Regenerability studies are performed to check the efficiency of prepared sorbent in real situations. For this purpose, HEC-Adip-Na (sorbent) was first used for Cd(II) uptake and then almost all sorbed Cd(II) were desorbed by treating with brine and then washing with deionized water. This regenerated sorbent was again used for Cd(II) uptake. Same experiment was repeated up to five regeneration cycles for both GW and DW solutions. The results of sorption–desorption experiments have been given in Fig. 9a and 9b, respectively and these show that there is negligible decrease in sorption capacity (10 mg g⁻¹ for DW and 14 mg g⁻¹ for GW) of the sorbent after five cycles.

Adsorption–desorption values for Cd(II) ions by HEC-Adip-Na (sorption conditions: sorbent dosage = 50 mg/100 mL for both DW and GW, Cd(II) concentration = 60 mg L−1, temperature = 298 K, pH = 6.0 for both DW and GW solutions. Desorption conditions: 50 mg Cd(II)-loaded HEC-Adip-Na/100 mL of saturated solution of M NaCl contact time = 24 h at a temperature = 298 K) DW(a), GW (b) solutions and percentage of Cd(II) ions uptake by the acidic (HEC-Adip), and its sodic form (HEC-Adip-Na) and regenerated sodic forms of the sorbent from GW and DW solutions over five cycles (c).

A graph was plotted between percentage values of sorption and number of sorption–desorption cycles for Cd(II) ions removal (Fig. 9c). It indicated a slight decrease in the percentage of Cd(II) ions uptake (11.4 and 8.3% from GW and DW solutions, respectively) after five regeneration cycles.

4 Conclusions

Present study illustrates synthesis, characterization and use of a novel material (HEC-Adip-Na) as a sorbent for the removal of Cd(II) ions from spiked high-hardness ground water (GW) and distilled water (DW). The experimental sorption data was best fitted to Langmuir isotherm and pseudo-second order kinetic model. Negative values of thermodynamic parameters (ΔG°, ΔS° and ΔH°) showed spontaneity and feasibility of the sorption process. The sodic form of sorbent (HEC-Adip-Na) proved a better choice for the adsorption of Cd(II) ions as their sorption capacity is calculated to be 112.35 and 114.94 mg g⁻¹ form GW and DW solutions, respectively. It is also concluded that HEC-Adip-Na have high selectivity for Cd(II) ions in the presence of other bivalent (Ca⁺² and Mg⁺²) metal ions which are generally found in GW. Regeneration studies confirm that the prepared sorbent (HEC-Adip-Na) has negligible decrease in sorption capacity over five regeneration cycles for the uptake of Cd(II) ions which suggests that it can serve as an economical sorbent for GW purification because it can be recycled many times.

References

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Show Sections

[1] Abbas A., Hussain M.A., Amin M., Sher M., Tahir M.N., Tremel W., . Succinate-bonded pullulan: An efficient and reusable super-sorbent for cadmium-uptake from spiked high-hardness groundwater. J. Environ. Sci.. 2015;37:51-58.
[Google Scholar]

[2] Abbas A., Hussain M.A., Sher M., Hussain S.Z., Hussain I., . Evaluation of chemically modified polysaccharide pullulan as an efficient and regenerable supersorbent for heavy metal ions uptake from single and multiple metal ion systems. Desal. Water Treat.. 2017;78:241-252.
[Google Scholar]

[3] Ahmad M., Taweel G.A., Hidayathulla S., . Nano-composites chitosan-curcumin synergistically inhibits the oxidative stress induced by toxic metal cadmium. Int. J. Biol. Macromol.. 2018;108:591-597.
[Google Scholar]

[4] Babel S., Kurniawan T.A., . Low-cost adsorbents for heavy metals uptake from contaminated water: A review. J. Hazard. Mater.. 2003;97:219-243.
[Google Scholar]

[5] Belhalfaoui B., Aziz A., Elandaloussi E.H., Ouali M.S., De Ménorval L.C., . Succinate-bonded cellulose: a regenerable and powerful sorbent for cadmium-removal from spiked high-hardness groundwater. J. Hazard. Mater.. 2009;169:831-837.
[Google Scholar]

[6] Bensacia N., Fechete I., Moulay S., Debbih-Boustila S., Boos A., Garin F., . Removal of cadmium (II) from aqueous media using COOH/TUD-1 mesoporous solid. Kinetic and thermodynamic studies. Environ. Engin. Manag. J.. 2014;13:2675-2686.
[Google Scholar]

[7] Boyd G.E., Adamson A.W., Myers L.S. Jr, . The exchange adsorption of ions from aqueous solutions by organic zeolites. II. Kinetics. J. Am. Chem. Soc.. 1947;69:2836-2848.
[Google Scholar]

[8] Cheng C., Wang J., Yang X., Li A., Philippe C., . Adsorption of Ni(II) and Cd(II) from water by novel chelating sponge and the effect of alkali-earth metal ions on the adsorption. J. Hazard. Mater.. 2014;264:332-341.
[Google Scholar]

[9] Duan J.C., Tan J.H., Hao J.M., Chai F.H., . Size distribution, characteristics and sources of heavy metals in haze episode in Beijing. J. Environ. Sci.. 2014;26:189-196.
[Google Scholar]

[10] Guo H., Zhang S., Kou Z., Zhai S., Ma W., Yang Y., . Removal of cadmium(II) from aqueous solutions by chemically modified maize straw. Carbohydr. Polym.. 2015;115:177-185.
[Google Scholar]

[11] Gurgel L.V.A., Gil L.F., . Adsorption of Cu(II), Cd(II) and Pb(II) from aqueous single metal solutions by succinylated twice-mercerized sugarcane bagasse functionalized with triethylenetetramin. Water Res.. 2009;43:4479-4488.
[Google Scholar]

[12] Gurgel L.V.A., Júnior O.K., de Freitas Gil R.P., Gil L.F., . Adsorption of Cu(II), Cd(II) and Pb(II) from aqueous single metal solution by cellulose and mercerized cellulose chemically modified with succinic anhydride. Biores. Technol.. 2008;99:3077-3083.
[Google Scholar]

[13] Hussain M.A., Shawar D., Hassan M.N., Tahir M.N., Iqbal M.S., Sher M., . An efficient esterification of pullulan using activated carboxylic acid anhydrides with iodine. Collect. Czech. Chem. Commun.. 2010;75(133–143):2010.
[Google Scholar]

[14] Kadirvelu K., Thamaraiselvi K., Namasivayam C., . Removal of heavy metal from industrial wastewaters by adsorption onto activated carbon prepared from an agricultural solid waste. Biores. Technol.. 2001;76:63-65.
[Google Scholar]

[15] Khan K., Lu Y.L., Khan H., Zakir S., Ihsanullah, Khan S., Khan A.A., Wang T., . Health risks associated with heavy metals in the drinking water of Swat, northern Pakistan. J. Environ. Sci.. 2013;25:2003-2013.
[Google Scholar]

[16] Kim H., Baek K., Lee J., Iqbal J., Yang J., . Comparison of separation methods of heavy metal from surfactant micellar solutions for the recovery of surfactant. Desalination. 2006;191:186-192.
[Google Scholar]

[17] Krishnan K.A., Anirudhan T.S., . Removal of cadmium (II) from aqueous solution by steam activated sulphurized carbon prepared from sugar-cane bagasse pith: kinetics and equilibrium studies. Water SA. 2003;29:147-156.
[Google Scholar]

[18] Kumar A., Parasad B., Mishra I.M., . Adsorptive removal of acrylonitrile by commercial grade activated carbon kinetics, equilibrium and thermodynamics. J. Hazard. Mater.. 2008;15:2589-2600.
[Google Scholar]

[19] Leyva-Ramos R., Bernal-Jacome L.A., Acosta-Rodriguez I., . Adsorption of cadmium(II) from aqueous solution on natural and oxidized corncob. Separation Purif. Technol.. 2005;45:41-49.
[Google Scholar]

[20] Lima I.S., Lazarin A.M., Airoldi C., . Cyclic voltammetric investigations on copper α-N, O-succinated chitosan interactions. Carbohydr. Polym.. 2006;64:385-390.
[Google Scholar]

[21] Low K.S., Lee C.K., Liew S.C., . Sorption of cadmium and leadfrom aqueous solutions by spent grain. Process Biochem.. 2000;36:59-64.
[Google Scholar]

[22] Morales M.E., Derbes R.S., Ade C.M., Ortego J.C., Stark J., Deininger P.L., Roy-Engel A.M., . Heavy metal exposure influences double strand break DNA repair outcomes. PLoS ONE. 2016;11:e0151367.
[Google Scholar]

[23] Negm N.A., Abd ElWahed M.G., Hassan A.R.A., Abou Kana M.T.H., . Feasibility of metal adsorption using brown algae and fungi: Effect of biosorbents structure on adsorption isotherm and kinetics. J. Mol. Liq.. 2018;264:292-305.
[Google Scholar]

[24] Nguyen T.A.H., Ngo H.H., Guo W.S., Zhang J., Liang S., Yue Q.Y., Li Q., Nguyen T.V., . Applicability of agricultural waste and by-products for adsorptive removal of heavy metals from wastewater. Bioresour. Technol.. 2013;148:574-585.
[Google Scholar]

[25] Oelkers E.H., Hering J.G., Zhu C., . Water: Is there a global crisis. Elem.. 2011;7:157-162.
[Google Scholar]

[26] Suteu D., Biliuta G., Rusu L., Coseri S., Nacu G., . Cellulose cellets as new type of adsorbent for the removal of dyes from aqueous media. Environ. Engin. Manag. J.. 2015;14:525-532.
[Google Scholar]

[27] Tellez-Plaza M., Navas-Acien A., Crainiceanu C.M., Guallar E., . Cadmium exposure and hypertension in the 1999–2004. National Health and Nutrition Examination Survey (NHANES) Environ. Health Perspect.. 2008;116:51-56.
[Google Scholar]

[28] Williams C.J., Aderhold D., Edyvean G.J., . Comparison between biosorbents for the removal of metal ions from aqueous solutions. Water Res.. 1998;32:216-224.
[Google Scholar]

[29] Zhang L., Zeng Y., Cheng Z., . Removal of heavy metal ions using chitosan and modified chitosan: a review. J. Mol. Liq.. 2016;214:175-191.
[Google Scholar]

[30] Zhong L., Peng X., Yang D., Sun R., . Adsorption of heavy metals by a porous bioadsorbent from lignocellulosic biomass reconstructed in an ionic liquid. J. Agric. Food Chem.. 2012;60:5621-5628.
[Google Scholar]

Sodium hyroxyethylcellulose adipate: An efficient and reusable sorbent for cadmium uptake from spiked high-hardness ground water

Abstract

Keywords

Hydroxyethylcellulose

Sodium hydroxyethylcellulose adipate

Metal uptake

Cadmium

Kinetics

1 Introduction

2 Materials and methods

2.1 Materials

2.2 Synthesis and characterization of sorbent (HEC-Adip-Na)

2.2.1 Synthesis of HEC-adipate and its sodium salt

2.2.2 Measurements

2.2.3 Determination of degree of substitution (DS)

2.2.4 Calculation of yield

2.2.5 Determination of pHZPC (pH corresponding to zero point charge)

2.3 Sorption kinetics

2.3.1 Effect of the initial metal ion concentration

2.3.2 Effect of sorbent dosage

2.3.3 Effect of contact time

2.3.4 Effect of initial pH

2.3.5 Effect of temperature

2.4 Regeneration studies

3 Results and discussions

3.1 Synthesis and characterization of the sorbent (HEC-Adip-Na)

3.1.1 Synthesis of HEC-Adip and its sodic form

3.1.2 FTIR spectroscopic analysis

3.1.3 Solid-state 13C NMR spectroscopic analysis (CP/MAS)

3.1.4 SEM-EDS analyses

3.2 Determination of the pH of zero-point charge (pHZPC)

3.3 Sorption studies

3.3.1 Effect of the initial metal ion concentration

3.3.2 Effect of the sorbent dosage

3.3.3 Effect of contact time

3.3.4 Effect of initial pH on Cd(II) ions uptake

3.4 Kinetic and isothermal modeling

3.4.1 Kinetic modeling

3.4.2 Sorption isotherms

3.5 Effect of temperature

3.6 Determination of thermodynamic parameters

3.7 Mechanism of sorption

3.8 Sorption and desorption (Re-generability)

4 Conclusions

References

Suggested read for related articles:

2.2.5 Determination of pH_ZPC (pH corresponding to zero point charge)

3.1.3 Solid-state ¹³C NMR spectroscopic analysis (CP/MAS)

3.2 Determination of the pH of zero-point charge (pH_ZPC)