Efficient Remediation of an Aquatic Environment Contaminated by Cr

Article pubs.acs.org/jced

Efficient Remediation of an Aquatic Environment Contaminated by Cr(VI) and 2,4-Dinitrophenol by XG‑g‑Polyaniline@ZnO Nanocomposite Rais Ahmad* and Imran Hasan Environmental Research Laboratory, Department of Applied Chemistry, Aligarh Muslim University, Aligarh, 202002, India S Supporting Information *

ABSTRACT: Xanthan gum (XG) grafted polyaniline@zinc oxide nanocomposite (XGP@ZnO) was synthesized by an oxidative free-radical graft copolymerization reaction. The synthesized nanocomposite was approved by different analytical processes such as Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and thermogravimetric analysis. The material was further employed for efficient remediation of aqueous solutions contaminated by Cr(VI) and 2,4-dinitrophenol (2,4-DNP) under batch operations. Solution pH values of 2.0 and 4.0 were found to be optimum pH for the adsorption of Cr(VI) and 2,4,-DNP, respectively. On the basis of higher R2 value and lower χ2 value, equilibrium data was found to be best fitted by the Langmuir model followed by the Redlich−Peterson model using nonlinear regression analysis. The maximum monolayer adsorption capacity was found to be 346.18 mg g−1 for Cr(VI) and 123.15 mg g−1 for 2,4-DNP. With a higher R2 and low χ2 value, pseudo-second-order model was established as the best model to describe the equilibrium data suggesting the chemical adsorption to be the rate-determining step. The value of ΔH° (+13.06 kJ mol−1) for Cr(VI) and (+10.28 kJ mol−1) for 2,4-DNP and negative value of ΔG° (−1.56 to −3.00 kJ mol−1) for Cr(VI) and (−1.75 to −2.94 kJ mol−1) for 2,4DNP in the temperature range of 30−60 °C indicated the overall adsorption process to be endothermic and spontaneous in nature. The mechanism involved in the removal of Cr(VI) and 2,4-DNP was electrostatic adsorption coupled reduction which is also evident from EDX analysis. tion, precipitation, ion exchange, and electroflotation.31−38 Among these methods, the adsorption process has been widely recognized as most efficient for the removal of Cr(VI) and 2,4DNP from contaminated groundwater39−43 due to ecofriendly, economic, and simpler operation.44−46 Among the various polysaccharides, XG was found to have additional high viscosity even at low concentrations, high thermal stability, and higher shelf life.47 XG (Xanthan gum) is mainly composed of glucose units with added side chains of α-Dmannose. The presence of pH tunable groups such as −COOH and −OH on XG can be utilized to bind various organic and inorganic pollutants through electrostatic interaction.48Recently, a conducting polymer with metal oxide nanocomposite has emerged as an attractive alternative for the treatment of wastewater.49−51 These compounds mainly provide a larger surface area for the adsorption, and interfacial adhesion between the surface of the nanocomposite and metal ions and are also easily tractable and economic.52,53Among various conducting polymers polyaniline (PANI) due to presence of abundant amine

1. INTRODUCTION Pollution caused by toxic heavy metals and organic compounds has received widespread attention owing to the high toxicity and carcinogenic effects to both the ecological environment and living organisms.1−10 Cr(VI) has been categorized as one of the toxic heavy metal ions present in effluents produced by various industries such as chemical, mining, aerospace, steel fabrication, and electroplating, etc.11−20 Dinitrophenolic, a group of persistent organic compounds, has received widespread attention due to its detrimental effects on the environmental and human health.21,22 2,4-Dinitrophenol (2,4-DNP) has been broadly observed in industrial effluents owing to its extensive use as a crude substance for specialty chemicals or as a median in pharmaceutical dye and textile industries23−30 which can hinder the biological the cell growth even at low concentration (1 mg L−1). The high toxicity and carcinogenicity of 2,4-DNP and Cr(VI) compounds make essential their effective removal from the wastewater streams. Now, it has become of a matter of great concern to remove these pollutants from groundwater as well as industrial wastewater due to the adverse effect on human health. Important methods for scavenging Cr(VI) and 2,4-DNP from contaminated water include electrocoagulation, adsorption, ultrafiltra© 2017 American Chemical Society

Received: November 18, 2016 Accepted: March 31, 2017 Published: April 11, 2017 1594

DOI: 10.1021/acs.jced.6b00963 J. Chem. Eng. Data 2017, 62, 1594−1607

Journal of Chemical & Engineering Data

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v) solution of XG in deionized water was mixed followed by the addition of 10 mL solution of 0.1 M aniline in 0.1 M HCl. The mixture was allowed to stir vigorously (900 rpm) for 2 h for complete mixing at 5 °C. The copolymerization reaction proceeded with a drop-by-drop addition of 30% APS solution. The reaction was allowed to stand for 12 h at 5 °C, and the product obtained was washed with 0.2 M HCl solution five to six times and then with deionized water in order to remove an excess amount of unreacted aniline and dried at 60 °C for 3 h. 2.4. Characterization of Adsorbent. FTIR spectra of XGP@ZnO was obtained by using the PerkinElmer (PE1600, USA) FTIR spectrophotometer in the range of 4000−400 cm−1. The powder XRD patterns of nanocomposite and individual constituents were recorded by using XRD (Rigaku Ultima IV, Xray diffractometer). SEM, JEOL GSM 6510LV (Japan) at 15 kV accelerated voltage, and 10−15 mm working distance was used to assess the morphological characterization of the nanocomposite. Gold-coated samples were prepared before their introduction into the SEM characterization cell. The particle size and distribution of nanoparticles in the polymer matrix of the prepared nanocomposite were perceived by using JEM 2100 (Japan) TEM technique. The nature of the synthesized nanocomposite with respect to temperature was analyzed by TGA (PerkinElmer model, STA 6000, USA). The concentration of Cr(VI) in the supernatant was measured by flame atomic absorption spectrophotometer. The adjustment of solution pH was accomplished by Elico Li 120 pH meter. A PerkinElmer lambda-25 UV−vis spectrophotometer (USA) was used for the determination of the concentration of 2,4-DNP in the supernatant. 2.5. Adsorption Experiments. Batch experimental design was applied for the evaluation of optimum conditions of adsorption of Cr(VI) and 2,4-DNP over XGP@ZnO. A 0.04 g sample of adsorbents was added to 20 mL of Cr(VI) and a 2,4DNP solution of 120 mg L−1, and the mixture was shaken in a thermostatic water-bath shaker operated at 120 rpm. The concentrations of Cr(VI) and 2,4-DNP in the supernatant was measured using flame atomic absorption spectrophotometer (AAS) and UV−vis spectrophotometer. The effect of time on the adsorption of Cr(VI) and 2,4-DNP on XGP@ZnO was observed in a time span of 10−300 min. The adsorption capacity of XGP@ ZnO was calculated by a mass balance relationship:

and imine groups has been widely used for Cr(VI) and other organic pollutant sequestration.54,55 PANI has good resistance against acid and alkali which makes it more suitable for removal of both organic and inorganic pollutants as they can bind with amine and imine groups.56 However, due to its small size, there is a recycling and reuse challenge for PANI application. So the grafting of PANI with XG was one of the strategies to improve its reuse and recycling capabilities. Recent studies have suggested that PANI and its composite have been used as an adsorbent for the removal of Cr (VI, anionic dye, and phenolic compounds.57−59 The stability of polymer matrix can further be enhanced by the reinforcement of various nanofillers such as natural clays, metal oxide nanoparticles, etc.60 Zinc oxide has been recognized as an effective adsorbent as well as photocatalyst for water pollutant owing to its high activity and economic and ecofriendly features.61,62 So the reinforcement of ZnO nanoparticles in the polymer matrix may enhance thermal, mechanical, electrical, and optical properties as well as improve the adsorption properties. This improvement was imputed to increase the number and distribution of active surface −OH groups of ZnO nanoparticles and XG with amine groups of PANI. However, there are no descriptions for the use of XGP@ZnO nanocomposite as adsorbent for Cr(VI) and 2,4-DNP in the literature. The main objective of this study was to evaluate the XGP@ZnO nanocomposite as an effective adsorbent for the remediation of wastewater contaminated by Cr(VI) and 2,4DNP. The influence of various adsorption parameters such as agitation time, solution pH, adsorbent dose, initial adsorbate concentration, and temperature was observed and optimized by preliminary experiments. The material was distinguished by different analytical methods such as Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM)−transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The nanocomposite material was further employed in efficient remediation of wastewater contaminated by Cr(VI) and 2,4-DNP. 2. Materials and Methods. 2.1. Material. Ammonium persulfate and Xanthan gum were purchased from Sigma-Aldrich (India). Potassium dichromate and zinc acetate were purchased from Merck (India). Aniline monomer was purchased from Fisher Scientific (India) and was distilled before use. 2,4Dinitrophenol solid powder was purchased from SD. Fine Chemicals (Mumbai, India). Stock solutions of Cr(VI) and 2,4DNP (1000 mg L−1 ) were made by dissolving the proper amount of adsorbate material in deionized water. 2.2. Synthesis of Zinc Oxide Nanoparticles. ZnO nanoparticles were synthesized by using zinc acetate and KOH as precursors as reported elsewhere.63 The aqueous solution of zinc acetate (0.2 M) and the solution of KOH (0.4 M) were prepared using deionized water, respectively. The aqueous solution of KOH was slowly poured into zinc acetate solution at 25 °C followed by vigorous stirring (900 rpm), resulting in the development of a white suspension. The obtained colloidal suspension was centrifuged at 8000 rpm for 30 min, washed four times with deionized water followed by absolute alcohol, and then calcined at 600 °C in air atmosphere for 3 h to remove any impurity. 2.3. Synthesis of XG-g-Polyaniline/ZnO. The nanocomposite was prepared by a previously reported method.64 A prescribed amount of ZnO nanoparticles was taken in 100 mL of 0.2 M HCl solution and sonicated for 1 h at 30 °C to obtain a completely dispersed colloidal solution. To this solution, 1% (w/

qe =

(Co − Ce)V W

R% =

Co − Ce × 100 Co

(1)

(2)

where qe (mg g−1) is the adsorption capacity of XGP@ZnO for Cr(VI) and 2,4-DNP; Co and Ce are the initial and equilibrium Cr(VI) and 2,4-DNP concentrations (mg L−1) respectively; V is the volume of the Cr(VI) and 2,4-DNP solution taken (L); W is the amount of the XGP@ZnO (g). 2.6. Adsorption Isotherms. Analysis of the equilibrium data is very important to develop an equation which can represent the results and could be used for design purpose. The equilibrium data obtained for adsorption of Cr(VI) and 2,4-DNP was analyzed by the use of well-known models given by Freundlich, Langmuir, Temkin, Redlich−Peterson and D−R.65−68 The nonlinear adsorption isotherms for Cr(VI) and 2,4-DNP adsorption onto XGP@ZnO are given by eqs 3−7): 1595



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Figure 1. XRD spectra of (a) ZnO NPs, (b) neat XG, (c) PANI, and (d) XGP@ZnO.

qe =

qmKLce 1 + KLce

whose value lies in the range 0−1. qs is the D−R adsorption capacity and kD‑R is the D-R constant related to the heat of adsorption. 2.7. Kinetic Studies of Adsorption. Evaluation of kinetic variables for the sequestration process is a necessary step for the application of adsorption at higher scale. To investigate the mechanism that controls the adsorption process, the kinetic data were explained by Lagergren pseudo-first-order70 and pseudosecond-order model.71 The nonlinear form of both the models are given eqs 10 and 11:

(3)

qe = KFce1/ n

(4)

qe =

RT ln(A Tce) bT

(5)

qe =

KR Ce 1 + aR Ce g

(6)

qe = qs exp(kD ‐ R ε 2)

qt = qe(1 − e−k1t )

(7)

ε (Polanyi potential) can be calculated by eq 8: qt =

⎛ 1⎞ ε = RT ln⎜1 + ⎟ Ce ⎠ ⎝

(8)

1 −2kD ‐ R

(9) −1

k 2qe 2t 1 + k 2qet

(11)

where qt (mg g−1) and qe (mg g−1) are the adsorption capacities of Cr(VI) and 2,4−DNP at time t (min) and equilibrium state, respectively, while k1 and k2 are related to the rate constants for pseudo-first- and -second-order models. 2.8. Thermodynamic Studies. The feasibility of the adsorption process was predicted based on the change of Gibbs free energy (ΔG°, kJ mol−1), enthalpy change (ΔH°, kJ mol−1) and entropy change (ΔS°, J K−1 mol−1) through brief thermodynamic experiments. The equations for the abovementioned parameters are given as72

where R and T are gas constant and absolute temperature (K), respectively. The constant (kD‑R) provides the value of mean free energy (E) per molecule when it transfers to the exterior of the adsorbent from the bulk in the solution and can be computed by using eq 9: E=

(10)

−1

where Ce is the equilibrium concentration (mg L ), qe (mg g ) is the adsorption capacity of adsorbent for Cr(VI) and 2,4-DNP at equilibrium, qm (mg g−1) is the maximum monolayer adsorption capacity, KL (L mg−1) is the Langmuir constant related to the heat of adsorption, n is a numerical value used to distinguish the adsorption favorability, and KF indicates the Freundlich adsorption capacity, AT (L g −1) is the Temkin constant representing adsorbent−adsorbate interaction and the other constant bT is related to heat of adsorption (J mol−1). KR (L g−1) and aR are Redlich−Peterson constants and g is an exponent

ΔGo = −RT ln Kc

(12)

ΔGo = ΔH o − T ΔS o

(13)

From eqs 12 and 13, we obtain eq 14: ln Kc = − 1596

ΔH o ΔS o + R RT

(14) DOI: 10.1021/acs.jced.6b00963 J. Chem. Eng. Data 2017, 62, 1594−1607


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where Kc is defined as the Cad/Ce, Cad concentration of adsorbed adsorbate (mg L−1) and R = 8.314 J mol−1 K−1 is the universal gas constant. 2.9. Nonlinear Chi-Square (χ2) Test. Nonlinear chi-square, χ2 test, a contraption for statistical error evaluation, used to analyze the fitness of the data involving the minimization or maximization of error dispensation between the predicted and experimental data based on its convergence boundary.73 The fitness of the isotherm was evaluated based on the value of χ2 as smaller value indicates a good precision between predicted and experimental values while a larger values suggests the deviation of the experimental values. n

2

χ =

∑i = 1 (qe,cal − qe,exp)i 2 qe,exp

(15)

2.10. Desorption and Regeneration Studies. The regeneration of XGP@ZnO was tested up to five cycles by treating the Cr(VI) and 2,4-DNP adsorbed XGP@ZnO with 0.2 M NaOH solution and desorption was calculated by using eq 16: % desorption =

Figure 2. FT-IR spectra of (a) neat XG, (b) PANI, (c) ZnO, and (d) XGP@ZnO.

may correspond to the interaction of the −OH groups of XG and ZnO with N−H groups of PANI through the formation of hydrogen bonding. The peak at 1742 cm−1 (−CO stretching) is a typical saccharide absorption which concludes that XG is grafted with PANI through the interaction between −OH and N−H groups. Thermal stability of PANI and XGP@ZnO was analyzed by TGA and the thermograms are given in Figure 3. In the TGA

amount of metal ion desorbed to desorbing media 100 amount of metal ion adsorbed on GPCM

(16)

3. RESULTS AND DISCUSSION 3.1. Characterization of XGP@ZnO. The characteristic peaks of ZnO nanoparticles were obtained at 2θ values of 8.03, 11.87, 31.71, 34.38, 38.18, 47.47, 58.51, 62.77, and 68.98 given in Figure 1. The crystalline size calculated using the Scherer formula was found to be 31.2 nm which is also confirmed by TEM micrograph. The XRD spectra of neat XG shows a pure amorphous character with a peak at 2θ = 73.26 with a very low intensity. The characteristic peak for polyaniline was seen at 25.72° showing low crystallinity owing to repetition of quinoid and benzenoid rings in the polyaniline.74 The XRD spectrum of XGP@ZnO shows that the two distinct sharp peaks at 2θ = 19.311 and 2θ = 25.721 respectively with high intensity are characteristics peaks of ZnO nanoparticles in XG-g-PANI matrix. Additionally, the peak occurring at 2θ = 23.21 is correlated to PANI with a higher intensity due to the addition of ZnO nanoparticles. The XRD pattern of XGP@ZnO represents a high-order crystal structure having peaks with increased intensity by incorporation of ZnO nanoparticles.75 Figure 2 represents the FTIR spectra of ZnO nanoparticles, XG, PANI, and XGP@ZnO. The characteristic peaks of ZnO due to hydroxyl groups were recorded at 3416 cm−1, peaks at 516 and 604 cm−1 were due to stretching of the Zn−O bond.76 Neat XG showed a wide transmission peak at 3401 cm−1 (stretching vibration of −OH groups), 2917 cm−1 (stretching vibration of alkyl C−H groups), 1731 cm−1 (CO stretching), 1415 cm−1 (C−C stretching of pyranose ring), and 1059 cm−1 (C−O−C stretching of anhydroglucose units).77 The characteristic peaks of polyaniline are 509 cm−1 (C−N−C bonding of aromatic ring), 695 cm−1 (C−C, C−H bonding mode of aromatic ring), 823 cm−1 (C−H out of plane bending in benzenoid ring), 1042 cm−1, 1303 cm−1, 1501 cm−1, and 1518 cm−1 (C−N stretching of benzenoid and quinoid ring).78 The FTIR spectrum of XGP@ ZnO represents most of the peaks characteristic to PANI with some shifts due to the reinforcement of ZnO and grafting with XG. These shifts include 1518−1580 cm−1, 1501−1502 cm−1, 1145−1144 cm−1 and 823−826 cm−1. Furthermore, a broad peak with reduced intensity appeared at 3230−3455 cm−1 that

Figure 3. TGA thermogram of PANI, ZnO nanoparticles, and XGP@ ZnO nanocomposite.

thermogram of pure PANI, a three step degradation was observed in a temperature span of 30−800 °C. The first weight loss at 109.47 °C is due to the desiccation of water. The second stage of weight loss starting at around 140 °C up to 370 °C almost 60% substance weight loss which represents the degradation of low molecular weight polymers and almost 45% weight loss for PANI was observed at 700 °C due to presence of a higher amount of protonic acid components of the polymer trapped in a matrix which degrades at high temperature.79 From 370 °C onward, degradation of PANI chains takes place up to 800 °C in which almost 90% mass loss is observed. With only 7.3% of weight loss, pure ZnO nanoparticles were found to be more stable in the temperature range of 50−700 °C. Below 125.31 °C, weight loss is accredited toward the dispersal of free water molecules on the surface and the successive weight loss 1597



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Figure 4. SEM micrograph of (a) XG, (b) PANI, and (c) XGP@ZnO before adsorption. (d) EDX spectra of XGP@ZnO, (e) XGP@ZnO after adsorption of Cr(VI), and (f) EDX spectra of XGP@ZnO after adsorption.

Figure 4 shows the surface morphologies of XG, PANI, and XGP@ZnO before and after adsorption of Cr(VI). From Figure 4a,b, XG reveals a granular, smooth, and homogeneous morphology while close packed particles with amorphous morphology were shown by PANI. Before adsorption, XGP@ ZnO shows rough, heterogeneous, and fibrillar structure owing

over higher temperature was due to the dead sorption of bound water.60 The XGP@ZnO also shows the same stages of weight loss but at high temperature representing a higher thermal stability as compared to pure PANI due to the incorporation of ZnO in the XG-g-PANI matrix, and a weight loss of 28% was observed at 700 °C. 1598



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Figure 5. TEM micrograph of XGP@ZnO.

to grafting and cross-linking reactions (Figure 4c). After adsorption of Cr(VI), there is a small drift in the heterogeneity of the material due to adsorption of water molecules on the surface (Figure 4 e). The SEM images lead us to the conclusion that XG has been successfully grafted with PANI with reinforcement of ZnO nanoparticles in the graft copolymer matrix. Energy dispersive X-ray (EDX) analysis of XGP@ZnO before and after adsorption was executed to affirm the cohesion of Cr(VI) ions onto XGP@ZnO and given in Figure 4d,f. The appearance of the Cr(VI) peak in the EDX spectra of Cr(VI) adsorbed XGP@ZnO affirms the Cr(VI) cohesion onto the adsorbent. Transmission electron microscopy is a suitable technique to figure out the size of nanomaterials in the polymer matrix. According to the TEM micrographs given in Figure 5, XG-gPANI and ZnO nanoparticles have interacted in such a way to form a nanocomposite having a well-organized dispersion of nanoparticles in the matrix. It is apparent that ZnO nanoparticles were capsulated in an orderly manner by XG-g-PANI. The average size of ZnO nanoparticles was observed as 31.2 nm. To study the surface properties of the synthesized material as a function of pH, the zeta potential test was conducted, and the results are presented in Figure 6. Considering the graph it was concluded that the surface of the adsorbent is positive in the pH range of 1−5. So the zeta potential of the XGP@ZnO was found to be 5. 3.2. Influence of Solution pH. The influence of solution pH on adsorption of Cr(VI) and 2,4-DNP onto XGP@ZnO in a range of 1−6 with 20 mL of 120 mg L−1 adsorbate concentration is illustrated in Figure 7a. As shown, the adsorption capacity for

Figure 6. Zeta potential curve for XGP@ZnO nanocomposite.

Cr(VI) increases from pH 1.0 to pH 2.0 then decreases with further increase in pH of the solution, while for 2,4-DNP adsorption capacity increases as the pH of the solution increases from pH 1 to pH 4 then decreases with further increase in pH of the solution. At pH 2.0, the Cr(VI) is found in the primary form of HCrO4−. So at pH 2.0, better sequestration of Cr(VI) indicated that the interaction of negatively charged HCrO4− ions with the positively charged XGP@ZnO surface through electrostatic attractive forces as observed by ZP test. But the lowering of the magnitude of adsorption capacity at higher solution pH may be attributed to the competitiveness of CrO42− and OH− ions for the surface functional groups of adsorbent.80,81 Further based on the pKa value of 2, 4-DNP (4.4) and ZP of 1599



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Figure 7. Influence of various adsorption parameters such as (a) pH, (b) contact time, and (c) adsorbent dose on the adsorption of Cr(VI) and 2,4-DNP onto XGP@ZnO.

3.3. Effect of Contact Time and Initial Adsorbate Concentration. Figure 7b shows the adsorption capacity for Cr(VI) and 2,4-DNP as a function of time. The influence of contact time (10−300 min) on the adsorption of Cr(VI) and 2,4DNP onto XGP@ZnO was carried out for initial adsorbate concentration of 40, 80, and 120 mg L−1, while other variables were held constant. It was observed that the adsorption capacity was found to increase with an increase in contact time and attained the equilibrium at 120 min. Further increase in time results in an infinitesimal increment in adsorption capacity suggesting that the adsorbent surface has been fully doped with adsorbate molecules61,83 and no more functional groups in XGP@ZnO are available to react with Cr(VI) and 2,4-DNP. Therefore, 120 min was taken as the equilibrium time for the adsorption of Cr(VI) and 2,4-DNP. The results given in Figure 7b also revealed that the adsorption capacity of XGP@ZnO nanocomposite increases with an

adsorbent (5.0), the adsorption of 2,4-DNP was also based on the strong electrostatic interactions between cationic groups of the XGP@ZnO (amino, hydroxyl groups) and anionic groups (phenolate anions) of 2,4-DNP. At pH < 4, phenolate anions are easily adsorbed on the positively charged adsorbent surface, but as the solution pH increases beyond 4.0, the adsorption capacity tends to decrease due to adsorptive competition between OH− and phenolate anions.82 So pH 4.0 is considered as the optimum pH for 2,4-DNP and pH 2.0 for Cr(VI) with an optimum temperature of 60 °C (Figure S1). R−NH 2 + H+ ⇄ R−NH3+ H 2O

(NO2 )2 PhO−H ⎯⎯⎯→ (NO2 )2 PhO− + H+ R−NH3+ + (NO2 )2 PhO− ⇄ R−NH3+−OPh(NO2 )2 1600



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Figure 8. Nonlinear (a) Freundlich, (b) Langmuir, (c) Temkin, and (d) Redlich−Peterson adsorption isotherms for Cr(VI) and 2,4-DNP onto XGP@ ZnO at 60 °C and optimum pH.

increase in the initial adsorbate concentration from 40 to 120 mg L−1. This is because, at higher concentration, the ratio of an initial number of moles of adsorbate to the available XGP@ZnO surface area is high.83 This may be attributed to an increase in initial adsorbate, that is, Cr(VI) and 2,4-DNP concentration gradient with an increase in initial adsorbate concentration.84 Therefore, 120 mg L−1 was taken as the initial adsorbate concentration for further experiments. 3.4. Influence of XGP@ZnO Dose. The dependence of Cr(VI) and 2,4-DNP on XGP@ZnO was conducted in various adsorbent dosages ranging from 0.5 to 3.5 g L−1 while keeping other parameters such as initial concentration (120 mg L−1), pH, and contact time (120 min) as constant. Figure 7 (c) represents the adsorption capacity of XGP@ZnO for Cr(VI) and 2,4-DNP as a function of adsorbent dose. It can be seen that the magnitude of adsorption efficiency for Cr(VI) and 2,4-DNP increases as the amount of adsorbent increase up to 0.04 g, but on further increasing the adsorbent dose the adsorption capacity decreases. This may be because as the amount of adsorbent increases, the

surface to volume ratio also increases, which provides better adhesion of the adsorbate molecules on the adsorbent surface by facilitating more active sites, but on further increase in adsorbent dose, adsorption capacity decreases due to partial aggregation of adsorbent particles. Therefore, 0.04 g was taken as the optimum adsorbent dose for further experiments as it exhibits appreciable adsorption capacity. 3.5. Adsorption Isotherms Data Analysis. Figure 8 (a−d) illustrates the nonlinear isotherms that are fitted to the experimental data obtained at 60 °C. The values of R2, χ2, and all the parameters obtained by the nonlinear regression analysis of isotherm models applied for Cr(VI) and 2,4-DNP adsorption on XGP@ZnO nanocomposite are summarized in Table 1. The qm values obtained for Cr(VI) and 2,4-DNP from the nonlinear Langmuir plot (Figure 8a) were 346.18 and 123.15 mg g−1, respectively. According to the fitting results listed in Table 1, the Langmuir isotherm model having the highest R2 (0.99, 0.99) as well as lowest χ2 (0.64, 0.69) values, appeared to be best fitted followed by the Redlich−Peterson model (R2 = 0.99, 0.99, and χ2 1601



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The value of n (1.26, 1.04) obtained from the nonlinear Freundlich plot (Figure 8b) was found to be in the range 0 < n < 10 for the adsorbates,69 further indicating favorable adsorption. The values of the binding constant (AT) obtained from the nonlinear Temkin curve (Figure 8c) 1.25 L mg−1 for Cr(VI) and 0.90 L mg −1 for 2,4-DNP suggest a higher degree of interaction between XGP@ZnO and Cr(VI). The values of heat of adsorption bT (143.67, 172.71 J mol−1) reveal that the method of removal of Cr(VI) and 2,4-DNP by XGP@ZnO is chemical adsorption. The g values calculated by the R−P model (Figure 8d) are equal to unity indicating that the Langmuir model is the best model to represent the equilibrium data. The data calculated for the D−R model is given in Table 1 and the graph in Figure S2. From Table 1, the value of mean free energy per molecule (E) 22.71 kJ mol−1 for Cr(VI) and 28.08 kJ mol−1 for 2,4-DNP suggested that the sequestration follows a chemisorption mechanism.86 3.6. Kinetic Data Analysis. Figure 9 panels a and b were used for fitting the equilibrium data obtained from the adsorption of Cr(VI) and 2,4-DNP (120 mg L−1) onto XGP@ZnO at optimum pH and temperature (60 °C). The values of the kinetic variables, R2 and χ2 calculated by the nonlinear regression analysis of kinetic equations are listed in Table 2. It can be seen

Table 1. Adsorption Parameters of Cr(VI) and 2,4-DNP Removal by XGP@ZnO at 60 °C Obtained through Nonlinear Regression Analysis model Langmuir

Freundlich

Temkin

Redlich−Peterson

D−R

parameters −1

qm (mg g ) KL (L mg−1) R2 χ2 n KF [(mg g−1) (L mg−1)]1/n R2 χ2 AT (L mg−1) bT (J mol−1) R2 χ2 g KR (L g−1) aR (L mg−1) R2 χ2 qs (mmol g−1) kD‑R E (kJ mol−1) R2 χ2

Cr(VI)

2,4-DNP

346.18 0.08 0.99 0.64 1.26 9.86 0.98 2.09 1.25 143.67 0.97 0.81 1.04 8.59 4.5 × 10−3 0.99 1.03 3.19 2.89xe−8 22.71 0.98 7.93

123.15 0.01 0.99 0.69 1.04 4.95 0.99 0.85 0.90 172.71 0.98 2.67 1.16 4.77 1.98 × 10−4 0.99 0.73 1.45 1.89xe−8 28.08 0.98 11.50

Table 2. Kinetic Parameters for Cr(VI) and 2,4-DNP Removal by XGP@ZnO at 60 °C Obtained through Nonlinear Regression Analysis model pseudo-first-order

= 1.03, 0.73), Freundlich model (R2 = 0.98, 0.99 and χ2 = 2.09, 0.85) and Temkin model (R2 = 0.97, 0.98 and χ2 = 0.81, 2.67). The fitness of the Langmuir model to the adsorption process implies that Cr(VI) and 2,4-DNP from bulk solution were adsorbed on a specific monolayer which is homogeneous in nature. As can also be seen from Table S1, all the RL values lie between 0 and 1 which confirms the adsorption processes to be favorable under the optimum conditions.85 The higher value of KL, 0.08 L mg−1 for Cr(VI) and 0.01 L mg−1 for 2,4-DNP, supports the high affinity of Cr(VI) as compared to 2,4-DNP.

pseudo-second-order

parameters −1

qe (exp) (mg g ) qe (cal) (mg g−1) k1 (min−1) R2 χ2 qe (exp) (mg g−1) qe (cal) (mg g−1) k2 (g mg−1 min−1) R2 χ2

Cr(VI)

2,4-DNP

37.89 36.72 0.15 0.62 3.81 37.89 38.04 1.20 × 10−3 0.92 0.94

25.11 24.57 0.12 0.84 1.06 25.11 25.27 5.10 × 10−4 0.99 0.08

Figure 9. Nonlinear (a) pseudo-first-order and (b) pseudo-second-order plot for adsorption of Cr(VI) and 2,4-DNP onto XGP@ZnO at optimum pH and temperature. 1602



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that the R 2 values (0.92, 0.99) are adequately large with low values of χ2 (0.94, 0.08) for the pseudo-second-order equation (Figure 9b) followed by that of the pseudo-first-order equation (R2 = 0.68, 0.84 and χ2 = 3.81, 1.06) (Figure 9a). Meanwhile, the qe,cal values (38.04, 25.27 mg g−1) obtained from the pseudosecond-order equation are found to agree slightly with the qe,exp values (37.89, 25.11 mg g−1). The qe,cal values (36.72, 24.57 mg g−1) appraised from the pseudo-first-order kinetic equation have the least agreement with the experimental values, qe,exp (37.89, 25.11 mg g−1). So the adsorption process for Cr(VI) and 2,4DNP onto XGP@ZnO can be best described by pseudo-secondorder kinetics, and the rate limiting step may be chemisorption. The higher value of k2 for Cr(VI) (1.20 × 10−3 g mg−1 min−1) as compared to that for 2,4-DNP (5.10 × 10−4 g mg−1 min−1) indicates an accelerated transport of Cr(VI) from the bulk to XGP@ZnO surface that is higher than that of 2,4-DNP. 3.7. Adsorption Thermodynamic Data Analysis. The adsorption experiments of Cr(VI) and 2,4-DNP onto XGP@ ZnO were carried out at 30, 40, 50, and 60 °C. The values of enthalpy change (Δ H°), entropy change (ΔS°), and Gibbs’s free energy change (ΔG°) of the process were determined from the slopes and intercepts of the plot between log Kc vs 1/ T (Figure 10) by using linear regression analysis and are summarized in

3.8. Mechanism of aAdsorption. The adsorption mechanisms of Cr(VI) and 2,4-DNP onto XGP@ZnO nanomaterial are not yet completely perceived. The genre of adsorbate− adsorbent communication may be contingent on the nature of adsorbate, adsorbent, and pH of the solution. To identify the procedure of the Cr(VI) and 2,4-DNP adsorption onto XGP@ ZnO nanocomposite, a number of different spectroscopic techniques such as FTIR, EDX, and X-ray diffraction were used. There may be various adsorption mechanism through which adsorption is taking place such as surface complexation, ion exchange, and electrostatic interaction. For example, at lower pH than zeta potential, the positively charged amino groups and −OH groups of XGP@ZnO have a strong coordinative affinity toward Cr(VI) and 2,4-DNP by surface complexation through electrostatic interaction, while this affinity decreases at higher pH than the zeta potential due to the repulsive force between negatively charged adsorbate and lone pairs of the −NH2 and −OH groups of XGP@ZnO. The kinetic results in Table 2 show that the removal process is chemical adsorption and may be controlled by the adsorption-coupled reduction mechanism, by forming a complex between phenolate ions and Cr(VI) with XGP@ZnO. The positively surface charged XGP@ZnO nanocomposite due to the presence of adequate surface functional groups (−OH, −NH2) can configure a complex with Cr(VI) and 2,4-DNP at lower pH values. 3.9. Desorption and Regeneration Studies. The regeneration of the XGP@ZnO nanocomposite is a necessary step to improve the cost-effective properties of the adsorbent. The material was applied to five continuous adsorption− desorption cycles (Figure 11). As it was observed that lower

Figure 10. Linear thermodynamic plot for adsorption of Cr(VI) and 2,4DNP at optimum pH at 30, 40, 50, and 60 °C.

Table 3. The ΔH ° values for the adsorption of Cr(VI) and 2,4DNP onto XGP@ZnO were found to be 13.06 and 10.28 kJ mol−1 while ΔS° values are 48.25 and 39.72 J mol−1 K−1. The obtained value of ΔH° is positive which suggested that the adsorption follows an endothermic mechanism. The value of ΔS° is positive which indicates the increment in the randomness at the solid−solute interface during the removal of Cr(VI) and 2,4-DNP onto XGP@ZnO caused by the release of water molecules around Cr(VI) and 2,4-DNP with some structural changes occurred in [email protected] The negative ΔG° values indicated that the adsorption of Cr(VI) and 2,4-DNP onto XGP@ZnO was thermodynamically feasible and spontaneous.

Figure 11. Adsorption−desorption cycles for Cr(VI) and 2,4-DNP onto XGP@ZnO.

values of solution pH are in favor of the Cr(VI) and 2,4-DNP adsorption, the desorption of Cr(VI) and 2,4-DNP from the XGP@ZnO surface was attained at high solution pH values. Therefore, for the regeneration studies, NaOH was used as the desorbing agent throughout the process. The regeneration of XGP@ZnO from the removal of Cr(VI) and 2,4-DNP is

Table 3. Thermodynamic Parameters for Cr(VI) and 2,4-DNP Adsorption on XGP@ZnO at Various Temperature Range Obtained by Linear Regression Analysis ΔH° −1

adsorbate

kJ mol

Cr(VI) 2,4-DNP

13.06 10.28

ΔG° (kJ mol−1)

ΔS° −1

J mol

−1

K

48.25 39.72

30 °C

40 °C

50 °C

60 °C

−1.56 −1.75

−2.04 −2.15

−2.52 −2.55

−3.00 −2.94

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for the adsorption of Cr(VI) and 2,4-DNP from wastewater and could be reused for five or more than five consecutive sequences.

depicted in Figure 11. The adsorptive efficiency for Cr(VI) still remained at 51.67% and 45.97% for 2,4-DNP after five cycles, which suggests that XGP@ZnO has good regeneration capability for Cr(VI) and 2,4-DNP adsorption. The adsorptive efficiency of XGP@ZnO for Cr(VI) and 2,4-DNP decreased by 21.35% in the fourth adsorption−desorption experiment, reflecting the effectiveness and stability of XGP@ZnO. The reduction in adsorption capacity may be because, after each successive cycle, some of the chemically adsorbed adsorbates may not be freed by the desorbing agent thereby decreasing the adsorption capacity. 3.10. Comparison with Other Adsorbents. A comparison table consisting of adsorption capacities of various adsorbents with present work has been given in Table 4. The table itself reveals that the present adsorbent shows a high adsorption capacity toward Cr(VI) and 2,4-DNP as compare to other adsorbents.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00963. RL values for removal of Cr(VI) and 2,4-DNP onto XGP@ ZnO at various initial concentration; effect of temperature plot, D−R isotherm plot (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: rais45@rediffmail.com. Tel.: +91- 0571-2700920-23 ext-3000. Fax: +91- 0571- 2400528. ORCID

Table 4. Comparison with Various Adsorbents

Rais Ahmad: 0000-0001-5464-0036 Funding

adsorption capacity (mg g−1) sample no. 1. 2. 3. 4. 5. 6. 7. 8. 9.

adsorbents Ppy decorated Fe3O4/rGO PANI/SiO2 GG/nZnO SA/PANI nanofibre cross-linked starch-based polymers activated carbon from rubber wood amino functionalized imidazolium-modified silica aluminum impregnated fly ash XG-g-PANI/ZnO

Cr(VI)

2,4-DNP

293.30 63.41 55.56 73.34 5.435

346.18

refs 11 55 61 88 89

96.89

90

91.74

91

12.67 123.15

The authors are highly thankful to UGC−New Delhi for providing financial assistance through Maulana Azad National Fellowship (MANF). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the University Sophisticated Instrumentation Facility (USIF) AMU, Aligarh, Sophisticated Test and Instrumentation Centre, Cochin University.

■

92 present study

REFERENCES

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4. CONCLUSIONS In the present study, a hybrid nanocomposite material (XGP@ ZnO) was synthesized by in situ graft copolymerization of xanthan gum with aniline monomer in the presence of initiator ammonium persulfate and ZnO nanoparticles. Batch experimental design was applied for the evaluation of optimum conditions of adsorption of Cr(VI) and 2,4-DNP over XGP@ ZnO. The adsorption process required 120 min as contact time to reach equilibrium using an initial adsorbate concentration of 120 mg L−1 with 0.04 g of adsorbent. The optimum pH for the removal of Cr(VI) and 2,4-DNP was found to be pH 2.0 and 4.0. Among the evaluated isotherms, based on the highest value of R2 and lowest value of χ2, equilibrium data were best fitted by the Langmuir model followed by Redlich−Peterson model using nonlinear regression analysis. The maximum monolayer adsorption capacity was found to be 346.18 mg g−1 for Cr(VI) and 123.15 mg g−1 for 2,4-DNP. With a higher R2 and low χ2 value, pseudo-second-order rate equation was established as the best model to describing the equilibrium data suggesting the chemical adsorption to be rate-determining step. The thermodynamic parameters indicated the overall feasibility, spontaneity, and endothermic nature of the Cr(VI) and 2,4-DNP adsorption on XGP@ZnO. The mechanism involved in the removal of Cr(VI) and 2,4-DNP ions was adsorption-coupled reduction which is proven by EDX. The results disclosed that the XGP@ ZnO nanocomposite can be employed as an effective adsorbent 1604



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