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Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Preparation, Kinetics, Thermodynamics, and Mechanism Evaluation of Thiosemicarbazide Modified Green Carboxymethyl Cellulose as an Efficient Cu(II) Adsorbent Mudasir Ahmad,* Kaiser Manzoor, Suhail Ahmad, and Saiqa Ikram* Department of Chemistry Faculty of Natural Sciences Jamia Millia Islamia New Delhi 110025, India S Supporting Information *

ABSTRACT: Thiosemicarbazide-modified carboxymethyl cellulose adsorbent (TCMC) was synthesized by the oxidation reaction of carboxymethyl cellulose with sodium periodate followed by a condensation reaction with thiosemicarbazide and stabilized with sodium borohydride. TCMC was characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), thermogravimetric analyzer (TGA), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). FTIR and XPS analysis successfully shows inclusion of the sulfur and Cu(II) adsorption mechanism onto TCMC. It was also confirmed that the maximum adsorption of Cu(II) ions was achieved by the presence of NH2, OH, and COOH groups. The effects of aldehyde content, pH, time, and concentration of Cu(II) ions were analyzed. Kinetics and error analysis suggested that the pseudo-second-order was best fitted for the adsorption of Cu(II) ions than the other models. The maximum Cu(II) ion adsorption was obtained by the Langmuir model is 144.92 (mg g−1) which is greater than the other adsorbents. Furthermore, TCMC also showed a higher regeneration and Cu(II) ion recovery, and the adsorbent can be regenerated without the loss of adsorption efficiency after various adsorption/desorption cycles.

1. INTRODUCTION The growing concern of water pollution due to industries in recent years pose a major threat to the environment1 not only in developing countries, but also in developed countries.2−4 These circumstances also lead to the outbreak of various syndromes, many of them being harmful and infectious. Industrial wastewater consists of various toxic metal ions such as Pb(II), Hg(II), Cu(II), As(VI), As(III), and Cd(II).5−7 Thus, it has become essential to remove these toxic metal ions from industrial wastewater before its reuse. Various methods were developed for removing these toxic metal ions, namely, chemical or electrochemical treatment or precipitation, ionexchange, coagulation-flocculation, flotation process, membrane filtration, solvent extraction, and adsorption.8 All of these methods are costly9 and pose significant problems in disposal of the precipitated wastes.10,11 To overcome these problems, adsorption is choosed as common and cheap methods, mainly when it is used at low metal ion concentrations.12,13 According to the adsorbent material, adsorption is divided into various categories such as bone charcoal,14,15 aluminum salt, rare earth,16 and bioadsorption. It is considered as one of the beneficial techniques, as the adsorbents used in bioadsorption are obtained from renewable natural resources. From the past few decades, it is considered as an alternate technology for the removal of toxic metal ions from the wastewater, but it has had limited usage due to high operation cost. A few other © XXXX American Chemical Society

drawbacks are physical and chemical stability and lesser metal ion removal at low concentration.17,18 Considering the important aspects of environment and cost, several approaches have been followed to introduce new cost efficient bioadsorbents obtained from abundant renewable natural resources.19−22 As an example, carboxymethyl cellulose (CMC) is an anionic semiflexible polysaccharide gained through the acetylation of cellulose.23 It consists of a large number of hydroxyl groups, which are used for metal adsorption and can be attractive as a starting material since it is cheap and an abundant natural biopolymer on earth.24−29 In recent years researchers have tried to increase the adsorption efficiency by grafting chelating agents in the biopolymer which enhances their chemical strength and metal chelation. Hence, the adsorption property was enhanced by the inclusion of thiosemicarbazide in chitosan and cellulose filters30−33 used for the removal of Cu(II) ions from wastewater. In this work CMC was selected as a biopolymer. It has a tendency at C2 and C3 atoms to split and form Schiff base intermediate and undergo a subsequent condensation reaction. The insertion of thiosemicarbazide in CMC increases functionality, which increases metal chelation.32 Chemical modification has gained attention Received: November 17, 2017 Accepted: May 9, 2018

A

DOI: 10.1021/acs.jced.7b01008 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 1. Synthetic pathway for TCMC.

2.2.2. Synthesis of Thiosemicarbazide Carboxymethyl Cellulose (TCMC). A 1 g sample of OCMC (Figure 1) was dissolved in 5 mL of distilled water at 35 °C until a clear solution was obtained. After 1 h, 2 g of thiosemicarbazide was dissolved in 20 mL of distilled water at pH 6. The pH of thiosemicarbazide solution was adjusted by adding a few drops of 0.1 M NaOH. After fixing the pH, it was added in the reaction flask in a dropwise manner with the help of a dropping funnel for 30 min under a nitrogen atmosphere. The color of the reaction became yellowish; the mixture was allowed to stir for 12 h and then left at room temperature. The mixture was filtered and the precipitate was washed with water several times. The obtained product was collected and dried under vacuum for 36 h at 50 °C to obtain the modified product thiosemicarbazide carboxymethyl cellulose denoted as (TCMC). 2.3. Characterization. Different modifications between CMC and thiosemicarbazide were analyzed by different techniques. The structural analysis of CMC and its modified product were carried out by using FTIR with a scan range from 4000 to 500 cm−1 using a Bruker Tensor 37 spectrophotometer. XPS was performed by using ESCA+, (Omicron Nanotechnology, Oxford Instrument Germany) equipped with monochromator aluminum source (Al kα radiation hν = 1486.7 eV) together with adsorption isotherms, and kinetics was used to analyze the adsorption mechanism of the Cu(II) onto TCMC. The elemental composition (C, H, N, and S) was determined using a Vario EL-III elemental analyzer.The surface morphologies were determined with SEM/EDX using a S-4800 (Hitachi, Japan). The thermal properties were analyzed by TGA under nitrogen environment with a TG/DT A6300 instrument (SII Nano Technology Inc. Tokyo, Japan). Samples with an approximate mass of 10 mg were analyzed in the 50− 700 °C temperature range at a heating rate of 10 °C/min. Changes in crystallinity were determined using D8 advance diffractometer (Bruker) with a Cu target ƛ = 0.154 nm at 40 kV, and 2θ was 10°−80°. Adsorption data were obtained with the help of UV spectrophotometry (UV 1800+, Shimadzu, Japan). The pH in the entire experiment was monitored using a Metrex pH meter model E/17/2054 was used. 2.4. Determination of Aldehyde Content. The degree of cross-linking was assessed by determining the aldehyde content in the product by iodometric titration of the excess periodic acid. A 5 mL aliquot of reaction mixture content was taken in a 50 mL conical flask and 0.5 N KI and 20 mL of HCl were added simultaneously. The iodine so formed was titrated against 0.2 N Na2S2O3·5H2O until it reached the end point by using starch as an indicator. The aldehyde content was obtained by using eq 1.37

in previous years, as it decreases solubility in acid solution, water, and in most of the organic solvents. This is because of the increase in molecular weight,34 and increase of the mechanical properties due to decreases in swelling. This might reduce the loss of adsorbent during the adsorption process.35,36 The present work fills the gap and discusses the preparation of thiosemicarbazide-modified CMC based nontoxic bioadsorbent (TCMC) with higher adsorption capacity than other bioadsorbents for the adsorption of Cu(II) ions. To achieve the planned objective, various analyses were carried out: (i) evaluation of adsorption characteristics of Cu(II) onto TCMC from wastewater; (ii) analysis of experimental conditions through various batch methods such as contact time, amount of dose, concentration of Cu(II) ion, effect of pH, and temperature; (iii) thermodynamics and kinetics of TCMC performed for the first time by using various models and their applicability on the basis of error analysis; (iv) regeneration of bioadsorbent, mechanism of adsorption, and recovery of metal ions. The important characteristics of an adsorbent such as higher adsorption efficiency and environment and economic friendliness are used for the selection of a biopolymer for fabrication of the new adsorbent.

2. MATERIAL AND METHODS 2.1. Material. Sodium salt of carboxymethyl cellulose (low viscosity, Mw = 90 kDa) was purchased from Sigma-Aldrich. Sodium metaperiodate (99%), ethanol (98%), ethylene glycol, sulfuric acid (98%), purified sodium hydroxide (98%), and sodium bicarbonate were purchased from Merck. Thiosemicarbazide (99%) of analytical grade was purchased from Himedia. Throughout the experiment double-distilled water was used. 2.2. Methods. 2.2.1. Synthesis of Schiff Base Intermediate (OCMC). A 5 g sample of CMC was dissolve in 35 mL of distilled water and stirred until a clear solution was obtained at room temperature. We then added 10 mL of 0.45 M periodic acid and adjusted the pH to 3.5 using dilute sulfuric acid and sodium hydroxide to increase and decrease the pH of the solution, respectively. The oxidation reaction was carried out at 45 °C, under dark conditions to prevent auto-oxidation and continued for 4 h, the reaction was stabilized to room temperature, and ethylene glycol (1 mL) was added to stop the reaction and remove the excess periodate ions for 1 h, and the mixture was precipitated in ethanol. The precipitate was washed with water−ethanol solution until the product was free from ethylene glycol and sodium periodate. The washed product was dried in a vacuum oven at 30 °C for 24 h; the obtained product was denoted as OCMC, which was subjected for further analysis. B



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where qe,exp and qe,cal correspond to experimental and calculated adsorption capacities, respectively (mg g−1) and n is the number of measurements.

3. RESULT AND DISCUSSION Periodate oxidation of polysaccharides is an important way to convert the vicinal diols (C2−C3) into paired aldehyde groups without the formation of any side product.40 During the course of reaction 1 mole of CMC utilizes 1 mole of sodium periodate (1:1) ratio.41 The amount of aldehyde content is controlled by changing parameters such as pH, temperature, and concentration of the oxidizing agent. The aldehyde content was obtained using thiosulfate.42 It is well-known that one periodic acid forms two dialdehydes, so from the periodate measurement by sodium thiosulfate it is simple to calculate the aldehyde content. For dialdehyde starch the aldehyde content was 80%43 and the rest was converted in hemiacetals, which stop the further reaction of dialdehyde cellulose. The results have showed dialdehyde content as high as 83%, which undergoes further modification from the nucleophilic addition reaction to yield the TCMC adsorbent for the adsorption of Cu(II) from the aqueous media. 3.1. FTIR and CHNS Analysis. The FTIR spectra of CMC, OCMC, and TCMC are shown in Figure 2. The spectra in

2.5. Adsorption Studies. The adsorption of Cu(II) onto TCMC was analyzed by batch adsorption experiments. Duplicate experiments were executed in a series of 100 mL flasks containing 50 mL of aqueous solution with a known amount of TCMC, pH ranging from 1−9; beyond pH 6 Cu(II) precipitates. Acetic acid and sodium acetate were used for adjusting the pH of the solution. Kinetics (time 0−80 min), thermodynamics, and adsorption isotherms (initial concentration 10−100 mg L−1) of the studied metal ion was used. These mixtures were shaken with the aid of a thermostated shaker at 80 rpm for a specific time. For these studies all parameters were fixed and only one parameter is changed. After the proposed time the solution was filtered and the remaining equilibrium concentration of Cu(II) ion was determined colorimetrically using cefixime and 1,4-dioxan at 336 nm with the help of UV analysis.38 The metal ions adsorbed per unit mass of the TCMC were calculated by using eq 2. Removal efficiencies (%R) were calculated from eq 3. qe =

(C i − Cf )v 100 m

(2)

(C i − Cf ) 100 Ci

(3)

%R =

where Ci and Cf correspond to metal ion concentrations (mg L−1), respectively, and v and m are the volume of the solution (mL) and the mass of the TCMC (g). 2.6. Recovery and Regeneration. Cu(II)ion loaded TCMC (0.05g) was stirred with HCl solution (10 mL, 0.1 mol L−1), used to desorb the metal ion from adsorbent surface. The final Cu(II) ion concentration was analyzed by using UV spectrophotometry. Subsequently, the TCMC was neutralized by NaOH and its reusability checked by repeating adsorption− desorption cycles five times. 2.7. Error Analysis. For the validation of kinetic and isothermal models, error analysis was carried out. The following error functions, that is, the sum of the square of errors (SSE) eq 4, sum of the absolute errors (SAE) eq 5, chi square (χ2) eq 6, and standard deviation Δq(%) eq 7 were calculated between experimental and calculated values.39 n

∑

SSE =

i=0

Figure 2. FTIR spectra of (a) CMC, (b) OCMC, and (C) TCMC.

CMC shows the main characteristic peaks at approximately 1070−1150 cm−1 due to C−O stretching, the peaks at 1260− 1410 cm−1 due to OH bending, and 3600−3100 cm−1 are characteristic peaks of OH.44 As depicted in OCMC the major changes in comparison to CMC is the presence of peaks at 1750 and 840 cm−1 which appeared due to oxidation of CMC, resulting in the formation of aldehydic carbonyl and hemiacetal bond formation, respectively.45 In the case of TCMC the peak disappearance of carbonyl frequencies at 1750 cm−1 and the simultaneous appearance of new peaks at 1680 cm−1, 1620 and 1520 cm−1, is attributed to the C-NH2, −NH−CS−NH and CN group, which respectively confirms the insertion of thiosemicarbazide onto the OCMC.46,47 Usually the TCMC derivatives exist in two forms, thiol and thion.48 The absence of the thiol peak S−H at 2300 cm−1 and C−S peak at 1200 cm−1 (although the peaks are present at 1244 and 828 cm−1) is strong evidence of CS, which confirms active thiosemicarbazide is in existence mainly in the form of thion.49,50 All these

(qe,exp − qe,cal)i2 n

(4)

n

SAE =

∑ |q e,exp − q e,cal|i i=1

⎡q − qe,cal ⎤ e,exp ⎢ ⎥ ∑⎢ ⎥⎦ qe,cal i=1 ⎣ i

(5)

n

χ2 =

Δq(%) =

2 ⎡q − qe,cal /qe,cal ⎤ ⎢ e,exp ⎥ 100 n−1 ⎢⎣ ⎥⎦

(6)

(7) C



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temperature, as it is an endothermic reaction40 attaining a maximum value at 45 °C. On further increase in temperature the aldehyde content decreases due to decomposition of periodate above 45 °C. Figure 3b shows the effect of pH on the aldehyde content and shows better oxidation was obtained at low pH 3−4. On further increasing the pH, the degradation or over-oxidation of CMC was taking place.51,52 Figure 3c shows the effect of periodate concentration on oxidation of CMC. Initially when periodate concentration increases, the aldehyde content also increases, as more 1,2-diol groups are available for oxidation reaction. However, when all the vicinal diols groups were converted into dialdehyde, no further increase in dialdehyde content occurred, which demonstrated a higher aldehyde content at the 1:1 ratio (NaIO4/CMC).51 3.3. XRD Analysis. The structure analysis of TCMC was carried through XRD analysis. The XRD pattern of CMC, OCMC, and TCMC is shown in Figure 4. The CMC diffraction pattern shows a broad peak at 2θ 22.40° corresponding to the semicrystalline nature of CMC as reported previously.53 In periodate oxidation crystallinity is decreased and the loss of crystallinity confirms the opening of glucopyranose rings and the destruction of ordered structure.54 The ordered structure was seen in case of TCMC, where the pattern shows two peaks. One is at 2θ 23.47° and the second peak at 2θ 26.39° which is attributed to the crystallinity pattern of the TCMC. These results also confirm the insertion of

results show that a successful insertion of thiosemicarbazide has taken place in OCMC via the addition reaction. Results of the CHNS analysis of CMC, OCMC, and TCMC are listed in Table 1. The nitrogen content appears in TCMC Table 1. Shows the Elemental Analysis of CMC, OCMC, and TCMC no.

name

C%

N%

H%

S%

1 2 3

CMC OCMS TCMC

36.43 36.01 38.72

0 0 8.05

5.89 5.13 7.13

4.23

after thiosemicarbazide modification, which is strong evidence for the successful insertion of thiosemicarbazide in OCMC. In addition, the sulfur content of 4.23% was also seen in TCMC which confirms the successful synthesis of TCMC.51 These results were also further confirmed by XRD and XPS. 3.2. Effect of pH, Temperature, and Concentration on Aldehyde Content. Oxidation of 1,2-hydroxyl groups to dialdehyde groups is a specific reaction without any side reaction.40,41 The percentage of aldehyde content was monitored by varying pH, concentration, and temperature of the reaction. It was calculated by the above procedure given in section 2.4. In Figure 3a, the results showed the effect of temperature on aldehyde content. It increases on increasing the

Figure 3. Effect of (a) temperature, (b) pH, and (c) concentration on aldehyde content. D



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The first stage started from 150 to 250 °C, 30% was lost in the branches of thiosemicarbazide. During the second stage between 250 and 320 °C, the weight loss was approximately 7% and the residue mass was about 40% and in the third stage between 320 and 540 °C the weight lost was 28% and thus the residue mass was 12% which does not decompose at 670 °C, showing that the chemical modification due to thiosemicarbazide makes TCMC thermally stable in comparison to CMC. 3.5. SEM Analysis. The surface morphology of CMC, OCMC, TCMC, and TCMCA are shown in Figure 6. The SEM image of pure CMC shows large molecules with different shapes as shown Figure 6a. After the oxidation reaction the molecules break down due the opening of glycoside ring at C2 and C3 carbons destroying the structure as shown in Figure 6b. In case of Figure 6c the surface is constricted due to an ordered structure that occurs due to the condensation reaction. In Figure 6d the surface was occupied by the Cu(II) ions which appear as white spots onthe surface due to the conduction.

Figure 4. XRD pattern of (a) CMC, (B) OCMC, (C) TCMC.

4. ADSORPTION PROPERTIES OF TCMC FOR CU(II) ION ADSORPTION 4.1. Effect of pH. Influence of pH on the adsorption of the Cu(II) from the aqueous solutions onto the TCMC at various pH 1−8 is shown in Figure 7. As it is seen from Figure 7 there is significant increase in Cu(II) adsorption at pH 1−6. Thereafter the adsorption of Cu(II) is decreased beyond pH 6. The low metal removal efficiency at highly acidic pH 1−3 is due to protonation of hydroxyl and amine groups which causes repulsion with the metal ions. Subsequent increases in pH 4−6 increases removal efficiency due to the decreased H+ ion concentration. At higher pH there is the possibility of metal hydroxide formation which decreases the adsorption. To avoid the risk of hydroxide formation in solution, the pH 6 was adjusted by using NaOH and HCl. 4.2. Adsorption Kinetics. The rate of adsorption is an important characteristic of an adsorbent as shown in Figure 8.

thiosemicarbazide in OCMC where long chains of TCMC were obtained and a crystalline pattern was well formed which is also a characteristic feature of the addition reaction.55 3.4. Thermal Studies. A thermal study was carried out by TGA to investigate the effect of chemical modification on the thermal stability of CMC as shown in Figure 5. The end of the initial straight line, where the decomposition started, was used to calculate the initial decomposition temperature (IDT).56−58 The IDT in the case of CMC was started at 250 °C and it showed a rapid weight loss between 250 and 300 °C of approximately 40%. At 300 °C the residual mass was 35%, the weight loss decreased and showed a different pyrolysis reaction mechanism. This reaction continued up to 600 °C where the residual mass was zero, that is, all CMC had decomposed. In the case of TCMC the IDT starts from 150 °C which was 100 °C lower than the CMC; this is due to the presence of thiosemicarbazide. The pyrolysis was divided into three stages.

Figure 5. TG graphs of CMC and TCMC up to 700 °C. E



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Figure 6. SEM images of (a) CMC, (b) OCMC, (c) TCMC, and (d) TCMCA.

Figure 7. Effect of pH on the adsorption of Cu(II) with contact time 1 h.

Figure 8. Effect of contact time on the adsorption of Cu(II) with 50 mL of test solution.

where qt and qe (mg g−1) are the adsorption at time t (min) and the adsorption capacity, respectively, and k1 and k2 are the rate constants. The adsorption of Cu(II) is kinetically fitted with eq 8 and eq 9, and the calculated data are depicted in Table 2. As it is seen from the Table 2, the regression coefficient (R2) of eq 8 is lower than that in eq 9, obtained from Figure S1 and Figure S2, respectively, which confirms that the pseudo-second-order model is best fitted with the experimental data. The pseudosecond-order applicability with the adsorption data assures that Cu(II) ions are adsorbed by the chemical reaction.61 Further, the validity of the model was checked by calculating the error analysis, the lower value of χ2, and Δq (%). SSE and SAE in Table 3 confirm the pseudo-second-order kinetics is best fitted with the adsorption data.

Initially the increase in the rate of adsorption is due to presence of large number of vacant sites leading the equilibrium between vacant sites and metal ions at 30 min. In fact, about 95% Cu(II) is adsorbed in 25 min as shown in Figure 8. Compared with other adsorbents, TCMC shows fast adsorption.59,60 The fast adsorption is attributed to amine, sulfur, and hydroxyl groups present in the adsorbent carrying higher affinity toward the Cu(II) adsorption. The kinetics of Cu(II) ions onto TCMC was further analyzed by Lagergen pseudo-first-order eq 8 and pseudo-second-order in eq 9. ln(qe − qt ) = ln qe − k1t

(8)

t 1 1 = t+ qt qe k 2qe2

(9) F



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Table 2. Shows the Kinetic Rate Constants for the Adsorption of Cu(II) by TCMC concn mg L−1

no. 1 2 3

25 50 75

1 2 3

25 50 75

K1

K2

R2

qe cal

pseudo-first-order 15.09 0.968 27.485 0.975 41.070 957 pseudo-second-order 17.8 0.993 23.2 0.993 35.4 0.992

0.010 −0.032 −0.036 0.002 0.003 0.001

temp (K)

ΔGo (kJ mol−1)

ΔHo (J mol−1)

ΔSo (J mol−1 K−1)

Cu(II)

303 308 313

−8.56 −8.58 −8.77

1101.00

30.95

4.3. Adsorption Thermodynamics. The endothermic or exothermic nature of adsorption phenomena can be obtained by analyzing the enthalpy (ΔHo) and standard free energy (ΔGo) respectively. The given equations 10, 11, and 12 were used to obtain the different thermodynamic parameters.62 ΔGo = −RT ln Kc

ln Kc = − Kc =

(10)

ΔH o ΔS o + RT R

SAE

X2

Δqe (%)

1.776 0.182 4.114

2.704 7.314 12.329

0.615 1.588 1. 945

21.768 56.15 68.778

1.565 3.023 1.213

5.96 11.6 17.8

4.716 4.527 4.512

1.667 1.600 1.595

The positive values of ΔHo and ΔSo confirm the endothermic nature and randomness at the solid/liquid interface during the adsorption process. 4.4. Adsorption Isotherms of TCMC for Cu(II) Ions. The effect of concentration on adsorption capacities was studied through batch methods at pH (6) and a temperature of 303 K. In these experiments, the solution concentration was varied from (10−100 mg L−1), TCMC (1 g L−1) with constant stirring at 80 rpm for 30 min. After the proposed time the solution was filtered and the remaining equilibrium concentration of Cu(II) ion was determined by using the procedure given in section 2.5.38 The adsorption of Cu(II) ions onto TCMC was finally studied with the Langmuir model63 eq 13 and Freundlich model64 eq 14.

Table 3. Thermodynamic Parameters of Cu(II) at Different Temperatures metal ion

SSE

⎛ 1 ⎞1 1 1 ⎟⎟ = ⎜⎜ + qe qm ⎝ KLqm ⎠ Ce

(13)

1 ln Ce n

(14)

(11)

ln qe = ln KF +

qe Ce

(12) −1

where Ce, qe, and qm have the usual meaning as stated above in the manuscript. KL and KF (L mg−1) are Langmuir and Freundlich constants, respectively, corresponding to adsorption energy, 1/n is the intensity of adsorption. These results are shown in Table 4 and Figure 10 and 11. As it is clear from Figure 10 and 11 the adsorption capacities

−1

where R is the gas constant (8.314 J K mol ), T is the temperature in Kelvin (K), Kc is the thermodynamic equilibrium constant, qe is the solid phase equilibrium concentration (mg g−1), Ce is the equilibrium concentration of the solution (mg/L), ΔHo is enthalpy change, ΔSo is the randomness of the system, which is obtained from the slope and intercept by plotting the graph between ln Kc versus 1/T, respectively, in Figure 9. The values of ΔGo, ΔHo, and ΔSo are shown in Table 3. The ΔGo values ranging from −8.36 to −8.67 kJ mol−1 for Cu(II) at different temperatures confirm that the process is spontaneous.

Table 4. Shows the Adsorption Isotherm Constants for Cu(II) by TCMC Langmuir isotherm model qm (mg g−1)

KL (L/mg)

144.92

0.0005

R2

X2

SAE

0.995 0.085 1.444 Freundlich isotherm model

SSE

Δqe (%)

−15.713

1.019

kf (L/g)

n

R2

X2

SAS

SSE

Δqe (%)

16.86

0.66

0.937

0.144

3.506-

−0.361

2.87

increased with increasing metal ion concentration until equilibrium was reached. The higher value of the correlation coefficient (R2) for the Langmuir model than for the Freundlich model confirms that the adsorption of Cu(II) ions onto TCMC is best fitted with the Langmuir model which is further supported by statistical error analysis showing lower values of χ2, Δq (%), SSE, and SAE compared with the values of the Freundlich mode. These values confirm eq 13 is best fitted with adsorption data.65 The qm value for the Cu(II) from the Langmuir model is reported in Table 4, which is higher than the other adsorbents reported in Table 5. The higher adsorption capacity of Cu(II) onto TCMC is due to the

Figure 9. Vont Hoff plot for adsorption of Cu(II) at different temperatures. G



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presence of heterogroups such as amine, hydroxyl, and sulfur, which show higher affinity toward the metal ions.66 The higher adsorption capacity together with other suitable features such as easy synthesis, availability, reusability, and easy separation and being environmentally friendly makes TCMC an efficient adsorbent.

5. MECHANISM OF CU(II) ADSORPTION ONTO TCMC The coordination reactions between different functional groups and heavy metal ions describe the metal sorption process in different reaction conditions such as pH. The adsorption of metal ions by the biosorbents may be due to either ion exchange or formation of metal complexes with the free sites of the biomass or electrostatic interactions between functional groups and metal ions. It is demonstrated that the −NH2 and −OH groups are the most important in metal complexes formation than other groups. The interaction between the sulfur and heavy metal ions are not fully understood; however, it is theoretically accepted that sulfur functionalities have strong affinity toward metal ions.72−74The present study discusses the exact mechanism between the metal ions and the sulfur moiety in adsorption processes. The FTIR and XPS analysis were carried out to discuss the adsorption mechanism and interaction of functional groups of TCMC and the Cu(II) ions in the solution. The FTIR spectra before adsorption (TCMC) and after adsorption of ion (TCMCA) were presented in Figure 12a*. It can be seen from Figure 12a* that the N−Cu(II) bond can be formed between the N group of NH and Cu(II) ions. Subsequent peaks are seen in the spectra of TCMCA in comparison to TCMC at 3390 cm−1 which became narrow and the peak at 2910 cm−1 almost disappeared.75 In addition, the stretching vibrations of OH and NH at 3390 cm−1 shifted to 3359 cm−1, the peak at 1680 cm−1 attributed to N−H shifted to 1675 cm−1 and the peak at 1620 cm−1 which is assigned for −NH−CS−NH2 also showed a shift. These peak shifts showed a strong evidence for the interaction between the amine, hydroxyl groups, and the Cu(II) ions.76 The peaks at 1244 and 828 cm−1 which are the strong evidence of CS do not show a shift, leading to the conclusion that sulfur is not taking place in the chemical bonding with the metal ions. These results were also filtered with XPS analysis.

Figure 10. Langmuir isotherm model of TCMC at different concentrations at 303 K with constant adsorbent dose 1 g L−1.

Figure 11. Freundlich isotherm model of TCMC at different concentration on 303 K with constant adsorbent dose 1 g L−1.

Table 5. Results of Adsorption Capacities of Cu(II) by Different Adsorbents no.

adsorbent

metal ion

time

capacity(mg g−1)

isotherm

1 2 3

carboxymethylcellulose sulfate poly(vinyl alcohol) and carboxymethyl cellulose hydrogel carboxymethylcellulose based thermoresponsive nanocomposite hydrogel

Cu(II) Cu(II) Cu(II)

120 min 24 h 48 h

127.7 5.5 2.14

Langmuir NA NA

4

Cu(II)

48 h

2.23

NA

5

carboxymethylcellulose based Na montmorillonite thermoresponsive nanocomposite hydrogel chitosan/clay/magnetite composite

Cu(II)

24 h

17.2

Freundlich

6

chitosan−zeolite composite (CZ)

Cu(II)

60 min

25.88

Langmuir

7

chitosan−tripolyphosphate(CTPP)

Cu(II)

100 min

26.06

Langmuir

8

thiocarbohydrazide chitosan and poly(vinyl alcohol)

Cu(II)

20 min

47.16

Langmuir

9

thiosemicarbazide chitosan

Cu(II)

20 min

142.85

Langmuir

10

thiosemicarbazide carboxymethyl cellulose(TCMC)

Cu(II)

30 min

144.90

Langmuir

H

order of reaction NA NA pseudosecond pseudosecond pseudosecond pseudosecond pseudosecond pseudosecond pseudosecond pseudosecond

ref 67 68 69 69 56 57 58 70 71 present work



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Figure 12. Spectra of TCMC before and after Cu(II) adsorption (a*) FT-IR spectra and (a−e) XPS spectra.

The adsorption mechanism was analyzed through the XPS analysis and the results are presented in Figure 12a−e. As shown in Figure 12a the peak at 163.05 eV is S 2p which confirms the successful inclusion of thiosemicarbazide in the TCMC.77 These results are in accordance with those obtained from the elemental analysis and FTIR. The results, Figure 12a, clearly show a small peak around binding energy (BE) 934.84 eV in TCMCA, indicating the accumulation of Cu(II) on the sorbents. The high resolution spectrum of N 1s Figure 12b, shows two peaks with binding energy 397.41 and 400.45 eV, assigned to N atom of −NH−NH2, and the peak at 400.62 eV is assigned to the high oxidation state of N with the positive charge −NH3+.78 After the Cu(II) adsorption, the N 1s binding energy of the two peaks increases up to 398.58 and 40.62 eV, respectively, indicating formation of NH2−Cu(II) complexes as shown in the eq 15. The increase in binding energy results from the electron density decrease on the N atoms by sharing the lone pair of electrons for the formation of a covalent bond between N and Cu(II).79 + ⎧ ⎫ ⎪ NH 2 Cu(II) → NH 2Cu(II) ⎪ ⎬ ⎨ ⎪ + +⎪ ⎩ NH 3 Cu(II) → NH 2Cu(II) + H ⎭

Similarly, the C 1s spectra of TCMC andTCMA in Figure 12c shows three peaks at 283.46, 284.80, and 286.63 eV corresponding to C−C, C−O, and CO, respectively, resulting from the alcoholic and carboxylic groups.80,81 The carbon atoms of the three respective groups of TCMC are typical in polysaccharides with different electron densities.82 After the adsorption of the Cu(II) the binding energy of C−O and CO shifted to 284.96 and 286.70 eV, respectively. It indicates the hydroxyl and the carboxyl groups are involved in the adsorption of metal ions, and thus the electron density in the carbon atom adjacent to C−O and CO is decreased.78 The high-resolution spectrum of O 1s in Figure 12d similarly shows two peaks with binding energy 531.60 and 534.09 eV. The peak at 531.60 assigned to CO representing the carboxyl groups and the peak at 534.09 eV may have resulted from the C−O bond associated with the hydroxyl or carboxyl group.83 After the copper adsorption onto TCMCA, a certain shift can be observed in the binding energy of CO and C−O from 531.76 and 534.20 eV respectively. This confirms the carboxylic and hydroxyl groups in TCMC are involved in the C(II) adsorption, in which the O atom donates the lone pair of electrons to the metal ion and the resulting electron density around the O atom decreases.78 The results obtained are in accordance with FTIR analysis and C 1s analysis.

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Although, the adsorption mechanism of sulfur-based materials was deduced by the XPS analysis, the high-resolution analysis demonstrates S 2p divided into two peaks at 162.24 and 164.32 eV shown in Figure 12e, these peaks are assigned for the S 2p1/2 and the S 2p3/2. After adsorption of the Cu(II) onto the TCMCA, there is a resulting shift of the peak toward the lower binding energy at 161.50 and 163.17 of S 2p1/2 and S 2p3/2, respectively.84 The decrease in the binding energy in the case of TCMCA shown in Figure 12e confirms the accumulation of negative charge or the increase in the polarity of the sulfur, and enhances the negative charge on the surface to attract the positively charged Cu(II) ions.85

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Saiqa Ikram: 0000-0003-2274-7669 Funding

The authors gratefully acknowledge financial support from TERI University under the scheme of United States Agency for International Development (USAID) program strengthening water and sanitation in urban settings. M.A. acknowledges the University Grants Commission (UGC), New Delhi, India, for financial support as senior research fellow (SRF).

6. DESORPTION AND REGENERATION From the economic point of view, repeated use of an adsorbent is an important aspect in heavy metal adsorption from wastewater. To analyze the repeatability of TCMC in use and the possibility of removing the Cu(II) ions from the wastewater, the repeated use of TCMC was obtained by the procedure in section 2.6 and the results are shown in Figure S3. As it is seen from Figure S3, the Cu(II) ion adsorption is almost the same as in the first cycles, which confirms that interaction between the donor sites on the surface of TCMC and Cu(II) ions are reversible. The above results conclude that the TCMC can be used as an adsorbent for the adsorption of Cu(II) ions from real water samples.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Mr. Gulzar Rafiqi Department of Chemistry, IUST Awantipora, India, for support during the research.

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REFERENCES

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7. CONCLUSION An advanced green carboxymethyl cellulose-based adsorbent was prepared by the inclusion of thiosemicarbazide for the adsorption of Cu(II) ions from aqueous solution. Inclusion of thiosemicarbazide was confirmed through FTIR, CHNS, XRD, TGA, and XPS analysis.The adsorbent shows the best adsorption results at a contact time of 30 min and pH 6 at room temperature. Maximum adsorption capacity was found at 144.90 mg g−1, which is higher than the other adsorbents. Mechanism evaluation of Cu(II) adsorption through XPS and FTIR analysis indicated the higher adsorption of TCMC was attributed to the presence of electron donor groups such as nitrogen, sulfur, and oxygen with higher affinity for the complex formation with Cu(II) ion. The presence of sulfur increases. Besides increasing the polarity presence of sulfur groups also increased the overall negative surface charge resulting into the attraction of the positively charged Cu(II) ions. Studies on desorption and regeneration of the adsorbent confirms that TCMC can be completely regenerated without the loss of the adsorption capacity. It can act as an environmentally friendly adsorbent due to its various advantages such as high, fast, and easy separation reducing the use of energy, regeneration, and recovery of the metal ion decreases the rate of waste generation from sources should not exceed the assimilative capacity of the environment.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b01008. Figures describing kinetic models, adsorption isotherms, and adsorbent regeneration (PDF) J



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