Thiocarbohydrazide Cross-Linked Oxidized Chitosan and Poly(vinyl


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Thiocarbohydrazide Cross-Linked Oxidized Chitosan and Poly(vinyl alcohol): A Green Framework as Efficient Cu(II), Pb(II), and Hg(II) Adsorbent Mudasir Ahmad,† Kaiser Manzoor,† Ranjana Ray Chaudhuri,‡ and Saiqa Ikram*,† †

Department of Chemistry Faculty of Natural Sciences, Jamia Millia Islamia, New Delhi, 110025, India Department of Regional Water Studies, TERI University, New Delhi, 11007, India



S Supporting Information *

ABSTRACT: The macroporous thiocarbohydrazide crosslinked chitosan-poly(vinyl alcohol) framework (TCPF), prepared via the condensation reaction of thiocarbohydrazide and the oxidized products of chitosan (OCS) and poly(vinyl alcohol) (OPVA) is reported with selective and efficient adsorption of Cu(II), Pb(II), and Hg(II). The adsorption of Cu(II), Pb(II), and Hg(II) onto TCPF was studied through batch adsorption experiments, and the adsorption data were analyzed by using various models. The Langmuir model fits best with the experimental values which yields adsorption capacities of 47.16 mg g−1, 47.39 mg g−1, and 52.63 mg g−1 for Cu(II), Pb(II), and Hg(II), respectively. The calculated thermodynamic parameters ΔGo, ΔSo, and ΔHo suggest that the adsorption of Cu(II), Pb(II), and Hg(II) is thermodynamically favorable and thus a spontaneous process which follows pseudo-second-order kinetics. The plot of qt versus t1/2 suggests that intraparticle diffusion is not only the ratecontrolling step but also the positive value of the intercept in Pb(II) and Hg(II) plots. This indicates that several steps are operational in the adsorption mechanism. Furthermore, the excellent recycle performances also were achieved with desorption and regeneration efficiencies close to 97−99%, permitting the recovery of both metal ions and TCPF. Finally, the complete soil degradability which can be attained in approximately 90 days makes the whole process environmentally friendly and economically feasible. with 2,5-dimercapto-1,3,4-thiodiazol,17 1,2 ethylene disulfide,18 thiosemicarbazide,19 barbital immobilized chitosan,11 and iodine/bromide and sulfuric acid modified chitosan based adsorbents are analyzed for the successful removal of heavy metal ions.20 CS and its derivatives as adsorbents have showed limited dissolution, high swelling, and low mechanical properties. Chemical cross-linking is used to overcome these problems. It causes a decrease in solubility in water and other common organic solutions. It also causes a decrease in swelling, controls the leaching and thereby improves mechanical stability.21 In general, the common documented cross-linking reagents are glutaraldehyde and epichlorohydrin.22 These crosslinking reagents have shown a high risk of carcinogenicity, toxicity, and immunogenicity to living systems for decades.23,24 This work is intended to fill the knowledge gaps and, to the best of our knowledge, to investigate for the first time the

1. INTRODUCTION Industrial processes have been the cause of various environmental problems. Among these is the generation of wastewater containing heavy metal contaminants which are highly toxic, persistent, and bioaccumulative. Therefore, this wastewater must be treated before its reuse or disposal on land or in water bodies.1 Exposure to Cu(II), Pb(II), and Hg(II) causes cardiovascular problems,2 abdominal pain3, and impairment in the central nervous system, respectively, as witnessed by the Minamata.4,5 Various remedial methods have been used for removal of toxic metals such as chemical precipitation, ion exchange, membrane filtration, electrolytic methods, reverse osmosis, and adsorption.6−11 In recent years bioadsorbents have gained much attention due to their natural availability, as well as being eco-friendly and cost-effective. Chitosan (CS) with glucosamine (poly-β-(1→4)-2-amino-2-deoxy-D-glucose) residues has been widely used as a bioadsorbent,12−14 due to the presence of high content of hydroxyl and primary amine groups.15,16 For over a decade ago researchers have been interested in the chemical modification of CS. CS modified © XXXX American Chemical Society

Received: January 26, 2017 Accepted: May 24, 2017

A

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Figure 1. The reaction pathway and probable metal adsorption onto TCPF.

other chemicals such as thiocarbohydrazide, potassium permanganate, sodium borohydride, cupric chloride, mercury chloride, lead chloride, ethyl alcohol, sodium acetate, acetic acid, acetone, and other solvents were purchased from SDFCL and Merck India. Double distilled water was used throughout the experiment.

nontoxic, mechanically strong thiocarbohydrazide cross-linked CS and poly(vinyl alcohol) (PVA) framework (TCPF) with the help of chemical cross-linking as a selective adsorbent for Cu(II), Pb(II), and Hg(II). To achieve the proposed goal the following were carried out: (i) Oxidation of CS and PVA and further condensation with thiocarbohydrazide forming a crosslinked framework (TCPF). The physical and chemical structural studies of TCPF were analyzed by CHNS elemental analysis, Fourier transform infrared spectroscopy (FT-IR), 1H NMR, X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/ EDX) techniques. (ii) Evaluation of TCPF as an adsorbent for Cu(II), Pb(II), and Hg(II) from water. (iii) Assessment of operation conditions such as initial metal concentration, pH of the solution, adsorbent dose, contact time for the metal removal, and effect of temperature. (iv) Determination of kinetic and thermodynamic behavior of the adsorbent and (v) evaluation of desorption−regeneration and biodegradation of the adsorbent.

3. METHODS 3.1. Preparation of Oxidized-Poly(vinyl alcohol) (OPVA). An aqueous solution of PVA (10 wt %) was prepared in 1 L flask at 90 °C to obtain oxidized poly(vinyl alcohol) (OPVA) by adding 5% (w/v) KMnO4 solution with constant stirring for 10 min. The reaction mixture changed to a dark brown color confirming the oxidation reaction. Distilled water was added to dilute the solution and cool the mixture to room temperature. The solid residue was removed by filtration. For removal of water, the filtrate was subjected to the rotatory evaporator at 50 °C. The obtained elastic material was heated in an oven at 120 °C for 12 h to get the final product labeled as OPVA.25 3.2. Preparation of Oxidized Chitosan (OCS). CS (2 g) was dissolved in 100 mL of aqueous solution containing 0.3 mol of sodium periodate (molar ratio 1.613:1) at 30 °C under dark conditions. After 2 h; 20 mL of 0.1 mol ethylene glycol was added to the reaction mixture to remove excess sodium

2. MATERIALS The chemicals and materials used in the experiment were of high analytical grade. CS obtained from crab shells (with a degree of deacetylation ≥75%, MW = 170 kDa), and PVA (MW = 85 kDa), were purchased from Sigma-Aldrich. The B

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6. DETERMINATION OF MOLECULAR WEIGHT BY INTRINSIC VISCOMETRY The molecular weights of samples were determined by viscometry. The viscosities of samples were measured by a Ubbleohode capillary viscometer in a solvent of 0.25 mol HAc/ 0.25 mol NaAc at 25 °C. The classical Mark−Houwink eq 2 is generally applied to determine the viscosity-average molecular weight.26−28

periodate and to stop the reaction. After 1 h, 200 mL of acetone was added to the reaction mixture in order to precipitate out the final product as oxidized chitosan (OCS). The product was washed with acetone and dried at 40 °C. For confirmation of oxidation the final product was tested to the determination of aldehyde content.26 3.3. Preparation of Thiocarbohydrazide Cross-Linked Chitosan-Poly(vinyl alcohol) Framework (TCPF). For the preparation of TCPF, OPVA was reacted with thiocarbohydrazide shown in Figure 1 in a stepwise manner. A 1.2 g sample of OPVA was dissolved in 30 mL of distilled water, and 20 mL of thiocarbohydrazide aqueous solution (1:1 ratio) was added at 35 °C. After 12 h, 20 mL of aqueous solution containing 1 g of OCS (2:1 ratio) was added in the same manner at 40 °C. The reaction mixture was stirred for 2 h, cooled to 20 °C, and then 1 g of NaBH4 was added to form the stable product. The gel-type product confirms the successful cross-linking. After 1 h 200 mL of 0.5 N NaOH solution was added to obtain the modified product which was filtered, washed several times with double distilled water, and dried under vacuum at 30 °C to get the thiocarbohydrazide cross-linked chitosan−polyvinyl framework denoted as TCPF.

[η] = 1.40 × 10−4Mv 0.83

7. MECHANICAL STRENGTH The mechanical properties of various samples were assessed by measurements of tensile strength (TS). A programmable load monitor, MIL 0301P model, manufactured by MICRONIX, Jamia Millia Islamia, New Delhi, was used to determine the TS. We made seven tests of each sample which had the following dimensions; length 2.8 cm and width 6.8 cm. The thicknesses of the samples were measured with a digital caliper (0−150 mm). TS can be measured by using eq 3: TS =

4. CHARACTERIZATION

tw

100

(3)

8. SWELLING STUDIES The swelling studies of CS, CS/PVA, and TCPF were carried out in dilute acid solution, distilled water, and dilute alkaline solution at room temperature over a period of 24 h. The percentage of swelling was calculated by eq 4. w −w % swelling = s 100 (4) w where ws is the swollen weight of sample (g) and w is the dry weight (g). 9. SOIL DEGRADATION TEST Soil is used as natural degradation environment. The weight and size of the test samples are 0.2 g and 1 × 1.5 cm, buried 15 cm beneath soil on Jamia Millia Islamia, respectively. The pH, moisture of air, and the temperature of the soil were also measured as 7.5, 52%, and 18 °C, respectively. After 30 days of burial; the degraded samples were taken out and washed with the distilled water and dried in vacuum at 30 °C for 1 day before chemical-physical characterization.29 The soil burial test was also analyzed by the weight loss. The weight loss was determined every 7 days after the first day of burial. The average weight loss was calculated by eq 5.30 W − Wd weight loss (%) = i 100 Wi (5)

5. DETERMINATION OF ALDEHYDE CONTENT The titration method was used to determine carbonyl content, with slight modifications as reported earlier.27 Briefly, 0.5 g of sample was added in 10 mL of distilled water in a 50 mL flask. The suspension was gelatinized in a water bath for 30 min, cooled to 40 °C, and adjusted to a pH of 3.2 with 0.1 mol/L HCl. Hydroxylamine (1.5 mL) was added and the mixture was kept in a water bath at 40 °C for 4 h. An excess amount of hydroxylamine was titrated against standardized 0.1 mol/L of HCl. A blank determination of reagent was carried out likewise. The carbonly content was calculated from eq 1.

where Wi is the initial weight of the sample and Wd is the final dry weight after being washed with the distilled water.

% carbonyl content [(Vs − Vb) mL × C HCl × 0.028 × 100] sample weight (dry) in g

fmax

where f max is the force where the sample breaks, t is the thickness, and w is the initial width of the sample.

The oxidation reaction and cross-linking between CS and PVA were analyzed by using various analytical and instrumental measurements. The elemental composition (C, H, N, and S) was achieved on a Vario EL-III elemental analyzer and the respective percentages of each were estimated. FTIR spectra were scanned in the range of 3500−500 cm−1 region using a Bruker Tensor 37 spectrophotometer. 1H NMR spectra were recorded on a Bruker 300 MHz 1H NMR spectrometer with D2O as the solvent. The surface morphologies and constituent percentages were performed with FE SEM/EDX using FEI Quanta 200 F SEM (50−50K X). To determine biodegradability, the samples were analyzed with an SEM instrument ZEISS (Nano Center JMI). X-ray diffraction (XRD) measurements were carried with a D8 Advance diffractometer (Bruker) with Cu target ƛ = 0.154 nm at 40 kV, and 2θ was 10°−80°.

=

(2)

10. ADSORPTION AND DESORPTION STUDIES The influence of the pH, metal ion concentration, dose, contact time, and the temperature were investigated. After adsorption was carried out, the TCPF was separated and the solution was collected for Cu(II), Pb(II), and Hg(II) measurement. The concentration of Cu(II), Pb(II), and Hg was determined

(1)

where Vs is the volume of HCl consumed by sample, Vb is the volume of HCl consumed by blank, and CHCl is the concentration of HCl in mol/L. C

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colorimetrically using the pyrophosphate method at 241 nm,31 the standard procedure using dithizone at 510 nm,32 and diphenylthiocarbazone method at 488 nm,33 respectively (by using the UV 1800+, Shimadzu, Japan). The desorption studies were carried by using 0.1 M EDTA17 and the metal ion removal was determined by eq 6.

Qe =

(C0 − Ct )V m

the free OH groups, due to oxidation. The strongest evidence of oxidation is the presence of a peak at 1738 and 1742 cm−1. This is the characteristic frequency of the carbonyl group (C O) in OPVA and OCS, respectively.36 It also shows C−H stretching vibration band at 1091 cm−1, CH2 bending vibration band at 1420 cm−1, and skeletal vibration bands at 916 and 849 cm−1. In the spectra of TCPF, the peaks at 3300 and 2900 cm−1 were due to OH and CH stretching vibrations. The peak at 1020 cm−1 was a C−O−C linkage which showed the same skeletal vibration as the starting material. The strong evidence of addition reaction is the disappearance of the carbonyl peaks at 1738 and 1742 cm−1 and simultaneously new peaks emerging at 1590 and 1456 cm−1 attributed to (−NH−CS−NH−) and C−N group, respectively, which supports the existence of successful cross-linking of OCS and OPVA in the presence of thiocarbohydrazide.26,37 The peak at 1400 and 1250 cm−1 are due to CH bending vibrations and CS stretching vibrations, respectively.38 These peak appearances directly reflect that the inclusion of thiocarbohydrazide was taking place, which supports the formation of targeted product TCPF. The slight change in the above stretching frequencies in Supporting Information, Figure S1 clearly indicates the adsorption of metal ion on the adsorbent by physical forces.39,40 The elemental analyses in Table S1 additionally supplement the FT-IR data. The nitrogen content in CS was 7.61%, which increased up to 18.05% in TCPF when compared with a nil value in OPA and 3.90% in OCS. In addition, the presence of sulfur (11.2%) in TCPF, which is absent in all the remaining samples strongly supports the inclusion of thiocarbohydrazide between OCS and OPVA. 11.2. Carbonyl Content and Molecular Weight. The amount of carbonyl content during the oxidation reaction was estimated by measuring the unused hydroxylamine after reaction with OCS/OPVA. The variation in carbonyl content with the concentration is shown in Figure S2. It is clear that the carbonyl content increases with increasing the concentration of KMNO4/NaIO4.25,41 As the concentration of KMNO4/NaIO4 increases, a large number of ions are available for the formation of diols. The entire reaction process potentially influences the molecular weight from the initial to the final product. For this reason, an estimation of molecular weight was likewise done to affirm the response plan. Table S2 shows the decrease in the molecular weights of PVA and CS continuously as a consequence of oxidation.42 The inclusion of thiocarbohydrazide moiety into the OCS/OPVA is again evident through an increase in the molecular weight, which is the characteristic feature of the addition reaction. 11.3. Swelling and Mechanical Properties. The swelling behavior of CS and TCPF were assessed by placing them in different solvent systems, such as dilute acetic acid, distilled water, and 0.1 M NaOH solution. To determine the swelling, the samples were taken and weighed after a specific time as reported in Table S3. A convenient proof for cross-linking is the swelling behavior of TCPF in dilute acidic solution. This concludes cross-linking is the best treatment to increase the stability of CS derivatives in acidic media. Mechanical properties of CS and TCPF derivatives were analyzed as listed in Table S4. The thickness of the sample was measured in three places, and the mean was considered. In the case of TCPF the tensile strength increases but elongation decreases when OCS and OPVA combine together in the presence of thiocarbohydrazide. This is because there is no

(6)

−1

where Qe (mg g ) amount of metal ion adsorbed, C0 (mg/L) is concentration at t = 0, Ct (mg/L)is concentration at time t (min), V is the volume of solution (L), and m is mass of adsorbent (g) added. Details of the adsorption and desorption procedure can be found in the Supporting Information (section S1 and section S2, respectively).

11. RESULTS AND DISCUSSION The synthesis procedure of TCPF is shown in Figure 1. The whole of the reactions includes the oxidation reaction and cross-linking procedure. According to the methods available in the literature,25,26 the inclusion of thiocarbohydrazide causes cross-linking between the two Schiff’s base intermediates (OPVA and OCS), which readily undergo an addition reaction with the amino groups of thiocarbohydrazide.34 Since two amino groups present at each end of thiocarbohydrazide separately combine with the two Schiff’s base intermediates during cross-linking; the physical appearance is altered from viscous to gel type, confirming the targeted cross-linked TCPF. 11.1. FTIR and Elemental Analysis. The FTIR spectrum of CS, OCS, PVA, OPVA, and TCPF which are shown in Figure 2. CS shows peaks at 3500−3200 cm−1 due to OH and

Figure 2. FT-IR spectra of (a) CS, (b) OCS, (c) PVA, (d) OPVA, and (e) TCPF.

NH groups, respectively. While in the case of OCS there is a decrease in area of absorption bands at 3500−3200 cm−1 (−OH and NH) and from 2000 to 2300 cm−1 (H2O) compared to that of CS. This indicates a decrease in free −OH and −NH2 groups due to their oxidation.35 The frequencies at 2875 cm−1 were assigned to the characteristic peak of −CH groups. In the case of PVA there is also a broad peak at 3300− 3200 cm−1 due to OH groups, but in the case of OPVA there is decrease in the peak at 3300−3200 cm−1 due to the decrease in D

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ray pattern of PVA consists of a strong peak at 2θ = 19.9° and 2θ = 40.7°,48 whereas OPVA consists of numerous peaks which are crystalline in nature due to oxidation.49 Compared with CS and PVA, TCPF showed a less intense peak at 2θ = 20.1° and a broader peak at 2θ = 40.7°. These results might be due to the ordered structure of TCPF arising from cross-linking between OCS and OPVA in the presence of thiocarbohydrazide shown in Figure 1. 11.6. SEM/EDX Characterization. SEM images of TCPF were analyzed in comparison with SEM images after adsorption of metal ions shown in Figure 4a−d. Before adsorption of metal ions, TCPF has lamellar, porous, and large grooves on the surface as shown in Figure 4a. After adsorption of various metal ions the surface was flattened by the adsorption of metal ions on its outer surface, these changes were continuous and compact after adsorption as shown in Figure 4b−d. The EDX graph of TCPF (Figure S4) confirms the adsorption of metal ions and showed that the adsorption of Hg(II), Pb(II), and Cu(II) occurred in the order of 48.92% > 44.51% > 16.68%, respectively. Besides physisorption, different mechanisms have been proposed for affinity-based interactions of heavy metals with sulfur functionalities. Sulfur increases the surface polarity and enhances the negative charge on the surface to attract the positively charged heavy metal cation.50 According to Pearson theory, the affinity of heavy metals toward sulfur is explained by soft acid−soft base interactions,51,52 in which sulfur functionalities act as soft base and Hg(II) metal as soft acids, while the Pb(II) and Cu(II) are included in the borderline. Thus, among the borderline the soft acid character increases in the order Pb(II) > Cu(II).53 The EDX graph of TCPF in Figure S4b also shows the presence of sulfur peak which demonstrates the successful insertion of thiocarbohydrazide had taken place.

intermolecular hydrogen bonding between the two, which is reduced by the cross-linking that increases stiffness but reduces elongation.43 11.4. 1H NMR Characterization. 1H NMR was carried out to verify the aldehydic groups present in OCS and OPVA. 1H NMR of OCS Figure S3 (a) (from Supporting Information) shows small peak at δ 8.31 ppm attributed to 2H of the aldehydic group.44 The peak at δ 3.7 ppm with low intensity confirms oxidation.45 As shown in the spectra, the peaks in the case of OPVA in Figure S3b showed protons at δ 1.3 and 3.6 ppm, due to a hydroxyl (1H of OH) and methylene group (2H of CH2), respectively.46 11.5. XRD. X-ray diffraction pattern of CS, OCS, PVA, OPVA, and TCPF are shown in Figure 3. CS shows a strong

12. ADSORPTION EXPERIMENTS BY USING BATCH METHODS 12.1. Effect of Temperature on Adsorption. It was observed that initially the adsorption of Cu(II), Pb(II), and Hg(II) increased with an increase in temperature and achieved a maximum adsorption at 28 °C with a removal efficiency of

Figure 3. XRD graph of (a) CS, (b) OCS, (c) PVA, (d) OPVA, and (e) TCPF.

peak at 2θ = 20.1° corresponding to crystal form II.47 But in the case of OCS after oxidation, numerous peaks are seen; this is the indication of the highly crystalline nature of OCS. The X-

Figure 4. SEM of (a) TCPF, (b) TCPF−Cu(II), (c) TCPF−Pb(II), and (d) TCPF−Hg(II). E

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Figure 5. (a−e) Effect of temperature, time, pH, concentration, and dose on adsorption of Cu(II), Pb(II), and Hg(II).

fact that initially when the pH is very low, H+ ion concentration is very high, which protonates the hydroxyl and amine groups to a large extent which in turn decreases the binding sites for Cu(II), Pb(II), and Hg(II), leading to a decrease in the adsorption. The protonation of amine and hydroxyl groups induces the electrostatic repulsion toward the Cu(II), Hg(II), and Pb(II), being a cation−cation interaction. On increasing pH, the H+ ion concentration decreases which in succession decreases the competition of metal ions with adsorbent binding sites thereby increasing adsorption.55 12.4. Effect of Metal Ion Concentration on Adsorption. Figure 5d, shows that as the metal ion concentration is increased, the adsorption increases which is predictable because as the concentration of metal ion increases the probability of contacting between the vacant adsorbent sites and metal ions also increases. On further increase in metal ion concentration, adsorption efficiency becomes constant after reaching the highest equilibrium concentration of 50 mg/L for Cu(II), Pb(II), and Hg(II) with removal efficiencies of 78%, 75%, and 95%, respectively. Beyond this concentration, no further increase in adsorption was observed because of nonavailability of the vaccant sites for metal ions adsorption. 12.5. Effect of Dose on Adsorption. The dose has a strong influence on the adsorption of the metal ions from the solution of a particular concentration. The percent removal of Cu(II), Pb(II), and Hg(II) ions increased on increasing the amount of adsorbent, which may occur because an increased amount of adsorbent results in an increase in surface area as well as the number of vacant sites. Figure 5e shows that as the dose increased from 0.2 to 2 g/L, the amount of Cu(II), Pb(II),

67%, 75%, and 40%, respectively (Figure 5a). This is explained by the fact that with an increase in temperature the collision probability of metal ions with the adsorption sites increases. However, with further increase in temperature, desorption phenomena starts occurring as observed in a decrease in adsorption efficiency. 12.2. Effect of Contact Time on Adsorption. The effect of the contact time on the adsorption of Cu(II), Hg(II), and Pb(II) is shown in Figure 5b. Initially when the contact time increases, a number of vacant sites are free to bind to the metal ions, which results in an increase in adsorption. On further increase in contact time all the vacant sites are occupied and the equilibrium is attained at 30 min for Cu(II) and Pb(II). After this point, there is no active site available for metal ion adsorption. Furthermore, the number of metal ions adsorbed becomes equal to a number of metal ions desorbed from the adsorbent; hence, there is no observed net increase in adsorption. It can clearly be observed from the plot that the adsorption of Hg(II) ions takes place faster than the adsorption of Cu(II) and Pb(II) ions and attains equilibrium after 20 min which further supports the fact that Hg(II) being a soft acid interacts more readily with the soft base sulfur (HASAB principle).54 12.3. Effect of pH of Solution on Adsorption. The uptake capacity and the mechanism involved in the adsorption mainly depends on the pH of the solution. The adsorption of Cu(II), Pb(II), and Hg(II) was analyzed by varying the pH from 2 to 7. Initially, when the pH is increasing the adsorption of metal ions increases gradually attaining a maximum at pH 6, 5, and 6, respectively (Figure 5c). This can be explained by the F

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solution concentration (mg/L), R is the universal gas constant (8.314 J/mol ·K), and T is the absolute temperature in Kelvin (K). The values of ΔHo and ΔSo can be obtained from the slope and intercept of the plot of ln Kc vs 1/T, respectively. Table S5 gives the ΔGo values obtained in the range of −5.87 to −6.09, −7.84 to −8.28, and −8.36 to −8.67 kJ/mol for Cu(II), Pb(II), and Hg(II), respectively. The positive values of ΔHo suggest that the observed adsorption process is endothermic. Furthermore, the ΔGo values obtained are in favor of physisorption.57 The more negative values of ΔGo for Hg(II) followed by Pb(II) and Cu(II), respectively, indicate that the metal ion adsorption was spontaneous. Hg(II) being softer is more readily adsorbed followed by Pb(II) and Cu(II), respectively.54 These results were also confirmed by the EDX spectra shown in Figure S4.

and Hg(II) removal efficiency varies from 25 to 85%, 23 to83%, and 30 to 95%, respectively. On further increase in dose, there was no significant increase in adsorption. The optimum amount of TCPF adsorbent reported is 1.5 g/L with an equilibrium removal of Cu(II), Pb(II), and Hg(II) of 85%, 83%, and 95%, respectively.

13. ADSORPTION THERMODYNAMICS The feasibility and the direction of the adsorption phenomena can be predicted by studying the thermodynamics of the

14. ADSORPTION ISOTHERMS Adsorption isotherms are the graphical representations which show the extent and type of adsorption with the change in concentration at constant temperature. In the current study various models were used such as the Langmuir model, Freundlich model, and D−R model as well as the Temkin model in order to understand the adsorption of Cu(II), Pb(II), and Hg(II) by TCPF The Langmuir model is applicable for monolayer adsorption and predicts that the forces of attraction between the active sites are equivalent. The Langmuir model is represented in eq 10.58 ⎛ 1 ⎞1 1 1 ⎟⎟ = ⎜⎜ + qe qm ⎝ KLqm ⎠ Ce

Figure 6. Van’t Hoff plot for adsorption of Cu(II), Pb(II), and Hg(II) at temperatures 288, 303, and 313 K.

where Ce is equilibrium concentration of the adsorbate in the aqueous solution in mg/L, qe is the mass of adsorbate adsorbed per unit mass of adsorbent in mg g−1 and is used for obtaining the maximum adsorption capacity of the adsorbent, qm is the amount of heavy metal present in a monolayer (mg g−1) and KL is related to adsorption energy and is called the Langmuir constant having unit L mg−1. The values of qm and KL are obtained from intercept and slope respectively shown in Figure S5. The adsorption capacity of TCPF is greater for Hg(II) followed by Pb(II) and Cu(II), respectively. The maximum adsorption capacity (mg g−1) for Cu(II), Pb(II), and Hg(II) onto TCPF compared with various adsorbents revealed the higher adsorption capacity for TCPF reported in Table 1. The

adsorption as shown in Figure 6. The following eqs 7 to 9 can be used to obtain these thermodynamic variables.56 ΔGo = −RT ln Kc ln Kc = − Kc =

(7)

ΔH ° ΔS ° + RT R

(8)

qe Ce

(10)

(9)

where Kc is the equilibrium constant, qe is the solid phase equilibrium concentration (mg g−1), Ce is the equilibrium

Table 1. Comparison of Maximum Adsorption Capacities (qm), Isotherm, Order of Reaction, and Contact Time for Cu(II), Pb(II), and Hg(II) by Different Adsorbents adsorbent

metal

time

capacity (mg g−1)

isotherm

order of reaction

ref

chitosan−MAA nanoparticles iodate doped chitosan coconut-shell carbon chitosan/clay/magnetite composite chitosan−zeolite composite (CZ) chitosan-tripolyphosphate(CTPP) thiol-functionalized polysilsesquioxane ECH-chitosan chitosan TCPF TCPF TCPF

Pb(II) Pb(II) Pb(II) Cu(II) Cu(II) Cu(II) Hg(II) Hg(II) Hg(II) Cu(II) Pb(II) Hg(II)

120 min 4h 2h 24 h 60 min 100 min 100 min 24 h 24 h 30 min 30 min 20 min

11.30 22.22 26.50 17.2 25.88 26.06 43.44 45 24 47.16 47.36 52.62

Langmuir Langmuir Langmuir Freundlich Langmuir Langmuir Langmuir Langmuir Langmuir Langmuir Langmuir Langmuir

pseudo-second pseudo-second first-order pseudo-second pseudo-second pseudo-second pseudo-second na na pseudo-second pseudo-second pseudo-second

55 66 67 68 69 70 71 72 73 present study present study present study

G

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Table 2. Adsorption Isotherm Constants for Adsorption of Cu(II), Pb(II), and Hg(II) by TCPF Langmuir

Freundlich

D−R

Temkin

metal Ions

KL (L mg−1)

qm (mg g−1)

R2

KF

n

R2

qd

β

R2

AT

bT

R2

Cu(II) Pb(II) Hg(II

0.638 0.318 0.083

47.16 47.39 52.63

0.9980 0.9981 0.9997

15.703 17.258 19.76

2.227 2.267 2.247

0.944 0.943 0.913

35.481 36.224 38.459

1.138 9.319 8.282

0.850 0.855 0.887

1.870 2.083 2.300

249.025 249.271 239.280

0.985 0.994 0.993

0.662, 0.231, and 0.245 kJ/mol, respectively. The results obtained from the Tempkin model and D−R models indicate that all the metal ions are adsorbed by TCPF through weak interaction, which supports the physisorption process. On comparing the R2 values in Table 2 obtained from the different models, the results clearly seem to best fit with the Langmuir isotherm rather than the Freundlich isotherm, the D−R isotherm, or the Temkin isotherm.

adsorption was analyzed by the Freundlich model which is best applicable for the adsorption process on heterogeneous surfaces. The generalized Freundlich model is given in eq 11.59 ln qe = ln KF +

1 ln Ce n

(11)

where Ce is the equilibrium concentration of the adsorbate in the aqueous solution (mg/L), qe is the amount adsorbed per unit mass of adsorbent (mg g−1), KF is Freundlich constant, 1/n is another Freundlich constant which denotes the intensity of adsorption. The values of KF and 1/n are derived from intercept and slope, respectively, shown in Figure S6 and Table S6. The Temkin model was also used to describe the adsorbent− adsorbate interactions in eq 12.60 qe =

RT RT ln A T + ln Ce bT bT

15. KINETICS OF ADSORPTION To understand the kinetics of adsorption through the various models, pseudo-first-order, pseudo-second-order, and interparticle diffusion were used for analyzing the adsorption experimental data. The pseudo-first-order kinetic model is used to describe the solute adsorption on the adsorbent surface taking adsorption capacity into consideration. The linear form is given in eq 15.74

(12)

where T is the absolute temperature (K), R is the universal gas constant (8.314 J/mol·K), Ce is the equilibrium concentration of the adsorbate in the aqueous solution (mg/L), AT is equilibrium binding constant (L mg−1), bT is Temkin constant (J/mol). AT and bT are calculated from the plot of qe vs ln Ce shown in Figure S7 and Table S7. The Temkin constant bT values are obtained as 0.2490, 0.2492, and 0.2392 kJ/mol for Cu(II), Pb(II), and Hg(II), respectively. As reported earlier, the enthalpy value was below 20 kJ mol−1 as is the characteristic of physisorption.61,62 The low values of bT for Cu(II), Pb(II), and Hg(II) indicate the weaker interactions of the metal ions with the adsorbent surface. Radushkevich63 and Dubinin (D−R)64 have accounted for the relationship between the adsorption and the porous structure of the adsorbent. The D−R model is shown as eq 13. ln qe = ln qDR

2⎫ ⎧ ⎛ ⎪⎡ 1 ⎞⎤ ⎪ − β ⎨⎢RT ln⎜1 + ⎟⎥ ⎬ ⎪⎢ Ce ⎠⎥⎦ ⎪ ⎝ ⎭ ⎩⎣

ln(qe − qt ) = ln qe − k1t

where qe (mg g ) is the amount of metal ion adsorbed per unit mass of adsorbent at equilibrium, qt (mg g−1) is the amount of metal ion adsorbed per unit mass of adsorbent at time t (min), and k1 (min−1) is the pseudo-first-order rate constant. The values of k1 and qe (Table S9) are determined by evaluating the slope and intercept of the plot between ln(qe−qt) vs t (min) shown in Figure S9 and Table S10. The obtained correlation coefficients R2 vary in the range from 0.968 to 0.957, 0.959 to 0.859, and 0.953 to 0.965 for Cu(II), Pb(II), and Hg(II), respectively, at temperatures 288, 303, and 313 K. The qe calculated values were much lower than the experimental values, and hence, it demonstrates that the adsorption of the metal ions does not follow the pseudo-first-order kinetics. The adsorption data were also analyzed by the pseudosecond-order model, which is given by a generalized eq 16.75

(13)

t 1 1 = t+ qt qe k 2qe 2

where qe is the mass of adsorbate adsorbed per unit mass of adsorbent in mg g−1, qDR is the maximum adsorption capacity of the metal ion (mg g−1), β is the D−R constant (mol2/kJ2). T is absolute temperature (K), where Ce is the equilibrium concentration of the adsorbate in the aqueous solution (mg/L), qDR and β are calculated from the intercept and slope, respectively (Figure S8 and Table S8). The D−R isotherm constant β is related to the mean free energy of adsorption E in eq 14 E=

1 2β

(15)

−1

(16)

−1

where qe (mg g ) is the equilibrium concentration of metal ion adsorbed per unit mass of adsorbent, qt (mg g−1) is the amount of metal ions adsorbed at t (min), k2 is the pseudo-secondorder rate constant (g mg−1 min−1). On plotting t/qt vs t (min) (Figure S10), the values of R2 are 0.992, 0.990, and 0.996 for Cu(II), Pb(II), and Hg(II), respectively, which are much higher than that of the pseudo-first-order reaction suggesting that the adsorption of Cu(II), Pb(II), and Hg(II) follow the pseudosecond-order kinetics. As indicated above the two models, pseudo-first-order and pseudo-second-order, do not explain the mechanism of the adsorption process. To understand the mechanism of the adsorption process the data was analyzed by the interparticle diffusion model. The interparticle diffusion is given in eq 17.76

(14)

The mechanism of adsorption can be predicted by observing values of E. If the values of E are less than 8 kJ/mol then the physisorption is taking place, and if the value of E lies in between 8 and 16 kJ/mol then the process is following the ion exchange mechanism.65 In the present study, the values of E calculated for Cu(II), Pb(II), and Hg(II) from β (Table 2) are

qt = K idt 1/2 + C H

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Figure 7. SEM shows the degradation of TCPF (a) at 0 days and (b) after 30 days.

where qt (mg g−1) is the amount of metal ions adsorbed at t (min), C (mg g−1) is the intercept directly proportional to the boundary layer thickness, Kid (mg g−1 min1/2) is the interparticle diffusion rate constant and is calculated from the slope of the plot between qt vs t1/2 (Figure S11 and Table S11). In the case where the regression coefficient of the plot is linear and passing through the origin, interparticle diffusion is the only rate-determining step; however, in the present study the line is not passing through the origin suggesting that besides interparticle diffusion other factors are also involved in the adsorption process. The positive value of the intercept in Pb(II) and Hg(II) plots indicates that several steps are operational in the adsorption mechanism,77 while a negative value of intercept as in the case of Cu(II) suggests that the boundary layer effect is close to minimum values.78,79

16. DESORPTION AND REGENERATION CHARACTERISTICS

Figure 8. Weight loss of the TCPF in the soil.

Desorption studies were conducted separately by using EDTA, to determine the adsorption process and the recovery of Cu(II), Pb(II), and Hg(II) ions from TCPF. The desorption of metal ions were best facilitated by using 0.1 M EDTA with efficiency 97−99% up to five runs. The percent removal of metal ions is shown in the Figure S12. Cu(II) shows the highest desorption rate because it forms a more stable chelate complex with the EDTA. This is because the ionic radius decreases in the order Cu(II) < Pb(II) < Hg(II).

18. CONCLUSION The procedure for successful preparation, thermodynamics, and kinetics of Cu(II), Pb(II), and Hg(II) adsorption onto TCPF have been studied in the present work. The chemical structure and adsorption characteristics of TCPF were analyzed through several instrumental techniques. From batch experiments, it has been concluded that the best adsorption is observed at a pH 6 for Cu(II) and Hg(II) and pH 5 for Pb(II), with a dosage of 1.5 g/L, and a contact time of 20 min for Hg(II) and 30 min for Cu(II) and Pb(II) at room temperature. The equilibrium data have been analyzed by using the Langmuir, Freundlich, Temkin, and D−R isotherm models. The regression coefficient suggested that the Langmuir isotherm model yields the best fit between the experimental and simulated data. Kinetic studies showed that adsorption follows the pseudo-second-order model because the theoretical and experimental adsorption capacities were in excellent agreement. The thermodynamic parameters ΔGo, ΔSo, and ΔHo suggest that the change in Gibb’s free energy was found to be negative for all the adsorption temperatures of Cu(II), Pb(II), and Hg(II) onto TCPF so it is thermodynamically favorable and thus a spontaneous process. The ΔHo values (572.4, 15.4, 1330.7 J/mol) and ΔSo values

17. BIODEGRABILITY TEST The biodegradation occurs through the enzymatic action and involves organisms within the soil. Figure 7 panels a and b show SEM images of sample surfaces before and after the degradation of 30 days. Figure 7a shows the original surface while in Figure 7b degradation is visible through the increasing porosity followed by increased surface area. These results were further authenticated with the weight loss shown in Figure 8. The degradation of the sample was high in the first 7 days due to the composting process as in the initial stage enough oxygen was available, which facilitates the microbial growth and thus degradation of TCPF. I

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(9) Kushwaha, S.; Sreedhar, B.; Padmaja, P. Sorption of Phenyl Mercury, Methyl Mercury, and Inorganic Mercury onto Chitosan and Barbital Immobilized Chitosan: Spectroscopic, Potentiometric, Kinetic, Equilibrium, and Selective Desorption Studies. J. Chem. Eng. Data 2010, 55, 4691−4698. (10) Jiang, Y.-J.; Yu, X.-Y.; Luo, T.; Jia, Y.; Liu, J.-H.; Huang, X.-J. γFe2O3 Nanoparticles Encapsulated Millimeter-Sized Magnetic Chitosan Beads for Removal of Cr(VI) from Water: Thermodynamics, Kinetics, Regeneration, and Uptake Mechanisms. J. Chem. Eng. Data 2013, 58, 3142−3149. (11) Thirumavalavan, M.; Lai, Y.-L.; Lin, L.-C.; Lee, J.-F. CelluloseBased Native and Surface Modified Fruit Peels for the Adsorption of Heavy Metal Ions from Aqueous Solution: Langmuir Adsorption Isotherms. J. Chem. Eng. Data 2010, 55, 1186−1192. (12) Chen, J. H.; Lin, H.; Luo, Z. H.; He, Y. S.; Li, G. P. Cu(II)imprinted porous film adsorbent Cu-PVA-SA has high uptake capacity for removal of Cu(II) ions from aqueous solution. Desalination 2011, 277, 265−273. (13) Li, M.; Zhang, Z.; Li, R.; Wang, J. J.; Ali, A. Removal of Pb(II) and Cd(II) ions from aqueous solution by thiosemicarbazide modified chitosan. Int. J. Biol. Macromol. 2016, 86, 876−884. (14) Yu, Z.; Zhang, X.; Huang, Y. Magnetic Chitosan-Iron(III) Hydrogel as a Fast and Reusable Adsorbent for Chromium(VI) Removal. Ind. Eng. Chem. Res. 2013, 52, 11956−11966. (15) Baba, Y.; Masaaki, K.; Kawano, Y. Synthesis of a chitosan derivative recognizing planar metal ion And its selective adsorption equilibria of copper(II) over iron(III). React. Funct. Polym. 1998, 36, 167−172. (16) Kanai, Y.; Oshima, T.; Baba, Y. Synthesis of highly porous chitosan microspheres anchored with 1,2-ethylenedisulfide moiety for the recovery of precious metal ions. Ind. Eng. Chem. Res. 2008, 47, 3114−3120. (17) Ahmad, M.; Manzoor, K.; Venkatachalam, P.; Ikram, S. Kinetic and thermodynamic evaluation of adsorption of Cu(II) by thiosemicarbazide chitosan. Int. J. Biol. Macromol. 2016, 92, 910−919. (18) Hsien, T.-Y.; Rorrer, G. L. Heterogeneous Cross-Linking of Chitosan Gel Beads: Kinetics, Modeling, and Influence on Cadmium Ion Adsorption Capacity. Ind. Eng. Chem. Res. 1997, 36, 3631−3638. (19) Hande, P. E.; Kamble, S.; Samui, A. B.; Kulkarni, P. S. ChitosanBased Lead Ion-Imprinted Interpenetrating Polymer Network by Simultaneous Polymerization for Selective Extraction of Lead(II). Ind. Eng. Chem. Res. 2016, 55, 3668−3678. (20) Zhang, A.; Xiang, J.; Sun, L.; Hu, S.; Li, P.; Shi, J.; Fu, P.; Su, S. Preparation, Characterization, and Application of Modified Chitosan Sorbents for Elemental Mercury Removal. Ind. Eng. Chem. Res. 2009, 48, 4980−4989. (21) Zhao, F.; Repo, E.; Sillanpaa, M.; Meng, Y.; Yin, D.; Tang, W. Z. Green Synthesis of Magnetic EDTA- and/or DTPA-Cross-Linked Chitosan Adsorbents for Highly Efficient Removal of Metals. Ind. Eng. Chem. Res. 2015, 54, 1271−1281. (22) Ngah, W.S. W.; Endud, C. S.; Mayanar, R. Removal of copper(II) ions from aqueous solution onto chitosan and cross-linked chitosan beads. React. Funct. Polym. 2002, 50, 181−190. (23) Zeiger, E.; Gollapudi, B.; Spencer, P. Genetic toxicity and carcinogenicity studies of glutaraldehyde-a review. Mutat. Res., Rev. Mutat. Res. 2005, 589, 136−151. (24) Wester, P. W.; van der Heijden, C. A.; Bisschop, A.; van Esch, G. J. Carcinogenicity Study with Epichlorohydrin (CEP) by gavage in rats. Toxicology 1985, 36, 325−339. (25) Liu, Y.-L.; Chiu, Y.-C. Novel Approach to the Chemical Modification of Poly (vinyl alcohol): Phosphorylation. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 1107−1113. (26) Ahmad, M.; Ahmed, S.; Swami, B. L.; Ikram, S. Preparation and characterization of antibacterial thiosemicarbazide chitosan as efficient Cu(II) adsorbent. Carbohydr. Polym. 2015, 132, 164−172. (27) Smith, R. J.; Whistler, R. L.; Paschall, E. F. Production and use of hypochlorite oxidized starches. In Starch Chemistry and Technology, 2nd ed.; Academic Press: New York, USA, 1967; pp 620−625.

(9.7, 14.8, 36.3 J/mol·K) indicated the endothermic nature of adsorption and randomness at the liquid−solid interface, respectively. Desorption and regeneration studies showed that the TCPF can be fully desorbed and regenerated without loss in its adsorption capacity. Biodegradability and weight loss data imply that irreversible biodegradation is a reasonable model. Total degradation should take place within approximately 90 days. This proves this approach to be facile, cost efficient, novel, and eco-friendly for simulating the adsorption and biodegradation behaviors of TCPF for management of wastewater containing heavy metal ions.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00088. Figures describing the effect of concentration, adsorption isotherms, kinetic models, elemental analysis, mechanical properties, molecular weight, structural analysis and adsorbent regeneration (PDF)



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] ORCID

Saiqa Ikram: 0000-0003-2274-7669 Funding

The authors gratefully acknowledge financial support from the TERI University under the scheme of USAID program strengthening water & sanitation in urban settings. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS Mudasir Ahmad acknowledges the University Grants Commision (UGC), New Delhi, India, as a Basic Science Research Fellow (BSRF).



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