Highly Effective Removal of Toxic Cr(VI) from Wastewater Using

DOI: 10.1021/ie402627d. Publication Date (Web): January 1, 2014. Copyright © 2014 American Chemical Society. *Phone: +27123826290, Fax: +27123826286 ...
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Highly Effective Removal of Toxic Cr(VI) from Wastewater Using Sulfuric Acid-Modified Avocado Seed Madhumita Bhaumik,*,† Hyoung J. Choi,‡ Mathapelo P. Seopela,† Rob I. McCrindle,*,† and Arjun Maity*,§,¥ †

Department of Chemistry, Tshwane University of Technology, Private Bag X680, Pretoria, Gauteng 0183, South Africa Department of Polymer Science and Engineering, Inha University, Incheon 402-751, South Korea § Smart Polymers Group, Polymers and Composites, Council for Scientific and Industrial Research (CSIR), Pretoria, Gauteng 0001, South Africa ¥ Department of Civil and Chemical Engineering, University of South Africa (UNISA), Pretoria, Gauteng 0002, South Africa ‡

S Supporting Information *

ABSTRACT: Sulfuric acid modified avocado seed (ASSA), as a low-cost carbonized adsorbent, was investigated for the removal of toxic Cr(VI) from water/wastewater in batch experiments. A low temperature (100 °C) chemical carbonization treatment was employed for the production of the adsorbent. FE-SEM and HR-TEM images revealed the formation of agglomerated and rodlike structured particles after carbonization of avocado seed. BET and TGA analyses of ASSA demonstrated its mesoporous structure and thermal stability up to 200 °C. The presence of oxo-functional groups on the ASSA surface was confirmed by ATRFTIR and XPS studies. Adsorption of Cr(VI) onto ASSA was highly pH dependent and found to be an optimum at pH 2.0. Adsorption isotherm results suggested that the capacity increases with an increase in temperature. Nonlinear regression analysis revealed that the Freundlich isotherm model provides a better correlation than the Langmuir isotherm model for Cr(VI) adsorption onto ASSA. The maximum Cr(VI) adsorption capacity of 333.33 mg/g was obtained at 25 °C, which is higher than most of the previously reported carbonized adsorbents used for Cr(VI) removal. Adsorption kinetics was best described by the pseudo-second-order model. The presence of coexisting ions slightly affected the Cr(VI) removal efficiency of ASSA. Experiment with real wastewater sample containing 47.34 mg/L of Cr(VI) demonstrated that by the use of only 0.03 g/25 mL of ASSA, almost 100% removal was achieved at pH 2.0, which suggests its potential application in wastewater treatment plants. The ASSA retained its original Cr(VI) sorption capacity up to three consecutive adsorption−desorption cycles. Finally, from XPS analysis, electrostatic attraction of Cr(VI) species to the adsorbent and its subsequent reduction to Cr(III) were identified as the leading removal mechanisms.

1. INTRODUCTION The increasing existence of toxic heavy metals in aquatic environments introduced by industrial pollution is a serious environmental problem. Among various heavy metal pollutants, chromium (Cr) is one, commonly present as trivalent Cr(III) and hexavalent Cr(VI) forms in aqueous solution. Chromium(VI) species are more toxic than Cr(III) and act as carcinogens, mutagens, and teratogens in biological systems.1 Excessive exposure to toxic Cr(VI) results in various health problems such as chronic ulcers, dermatitis, hemorrhage, and pulmonary cancer.2 Several industrial operations including leather tanning, metallurgical, electroplating, and dyeing discharge effluents contain elevated concentration of Cr(VI) compared to maximum allowable contaminant level. The maximum permissible limits of Cr(VI) in inland surface and drinking water are 0.1 mg/L and 0.05 mg/L, respectively.3 Therefore, it is imperative to remove excess Cr(VI) from wastewater before it is disposed into natural water resources. Conventional treatment processes for the removal Cr(VI) from industrial effluents include chemical precipitation, electrochemical process, ion exchange, reverse osmosis, and adsorption. Among these techniques, adsorption has been proven to be an attractive technique for the treatment of Cr(VI)-laden © 2014 American Chemical Society

wastewater due to its simplicity in operation and effectiveness. To implement the advantages of the adsorption process for Cr(VI) removal, the selection of a highly efficient adsorbent is the most significant starting point.4 Consequently, numerous media including activated carbon, metal oxides, polymeric, and waste materials have been explored as adsorbents for the removal of Cr(VI) from water/wastewater.5−8 Among these adsorbents, activated carbon has been widely used for the adsorption of Cr(VI) owing to its large surface area and multiple types of reactive surface sites.9,10 Over the past few years, researchers have focused on the production of activated carbon using locally available low cost agricultural wastes or byproducts. Activated carbon derived from various agricultural wastes/byproducts such as sawdust activated carbon, almond shell activated carbon, activated tamarind seed, rice straw-derived carbon, bamboo charcoal, and corn stalk have been investigated for the removal of Cr(VI) from water.11−16 However, most of these carbonizations were Received: Revised: Accepted: Published: 1214

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consistent with the temperature range under which intercalation reaction of HClO4 with graphite has been reported.19 2.2. Characterization of the ASSA. The surface morphology and structure of the prepared ASSA were visualized by field emission scanning electron microscope (FE-SEM), LEO Zeiss SEM and high resolution transmission electron microscope (HR-TEM), JEOL JEM-2100 instrument with a LAB6 filament all operated at 200 kV. The HR-TEM specimen was prepared by dispersing the adsorbent in 2propanol, followed by ultrasonication for 10 min. A drop of mixture was then placed on copper grids and allowed to dry. For FE-SEM, carbon coated adsorbents were used to take images. The thermal stability of the ASSA was examined using a thermo gravimetric analyzer (TGA Q500, TA Instruments) under air, with a flow rate of 50 mL/min and heating rate of 10 °C/min. The surface area and pore structure of the ASSA adsorbent was determined by low temperature N2 adsorption− desorption technique using a Micromeritics ASAP 2020 gas adsorption apparatus (USA). Ash content of ASSA was determined after burning at 650 °C for 4 h in a muffle furnace under atmospheric condition. The point of zero charge (PZC) of the ASSA was measured using a Zeta-Sizer, Malvern Ltd., UK. A Perkin-Elmer attenuated total reflectance-Fourier transform infrared (ATR-FTIR) Spectrum 100 spectrometer, equipped with an FTIR microscope accessory and a germanium crystal, was employed for qualitative determination of the functional groups of the ASSA. Elemental composition and chemical oxidation states of the surface species and near surface species of the ASSA were analyzed using X-ray photoelectron spectroscopy (XPS) on a Kratos Axis Ultra device, with an Al monochromatic X-ray source (1486.6 eV). Survey spectra were acquired at 160 eV and region spectra at 20 eV pass energies, respectively. 2.3. Batch Adsorption Experiments. A stock solution of 1000 mg/L Cr(VI) was prepared by dissolving an appropriate amount of potassium dichromate (K2Cr2O7) in deionized water. All experimental solutions of desired Cr(VI) concentrations were obtained by successive dilution of the stock solution. All adsorption equilibrium experiments were performed in a temperature controlled thermostatic shaker operated at 200 rpm. The adsorbent mass was kept at 20 mg for all equilibrium experiments unless otherwise stated. The experiments were carried out by contacting ASSA adsorbent with 25 mL of Cr(VI) solution in 50 mL plastic bottles placed in the shaker agitated at 200 rpm for 24 h. The effects of pH on Cr(VI) removal by the ASSA were studied by varying the solution pH from 2.0 to 11.0. The initial concentration of Cr(VI) solution was 100 mg/L, and the solution pH was adjusted with either 0.1 M HCl or 0.1 M NaOH. The Cr(VI) removal efficiency (% removal) was calculated using eq 1

conducted at high temperatures which elevated the cost of production of activated carbon. The low temperature chemical carbonization has the potential to further reduce the cost of producing activated carbon from agricultural wastes/byproducts. Therefore, a low temperature chemical activation method was employed in this study for the generation of carbonized adsorbent, utilizing agricultural waste for the effective removal of Cr(VI) from water/wastewater. For the preparation of carbonized adsorbent, avocado seed, an agricultural waste, was of particular interest in this study. Avocado (Persea americana) is a tree, classified in the Laurels family. The avocado tree is native to Central America and Mexico but is now cultivated in tropical and subtropical climates throughout the world. The avocado fruit is very popular in vegetarian cuisine, making it an excellent substitute for meats in sandwiches and salads due to its high fat content. During the processing of avocado fruit, the residual seeds are deposited as waste. Since South Africa is one of the major producers and consumer of avocados globally, it faces the problem of disposing of the avocado seeds. Although the seeds are not detrimental to the environment, reuse could help in minimizing the waste. Therefore, efforts have been devoted to utilize the waste seeds, by producing carbonized materials from avocado seeds for removal of methylene blue and for reduction of COD and BOD in wastewater.17,18 To date, carbonized adsorbent derived from avocado seed utilizing the low temperature chemical carbonization method has not been studied for the removal of Cr(VI). The main objectives of this study were to (i) prepare sulfuric acid-modified avocado seed (ASSA) carbonized at low temperature (100 °C) and use as an adsorbent for the removal of Cr(VI) from water/wastewater, (ii) examine the effects of various experimental parameters such as solution pH, initial Cr(VI) concentration, contact time, temperature, and coexisting ions on removal of Cr(VI) by ASSA, and (iii) explore the Cr(VI) removal mechanism by ASSA.

2. MATERIALS AND METHODS 2.1. Preparation of Sulfuric Acid-Modified Avocado Seed (ASSA) Carbonized Adsorbent. Avocado seeds were collected from a local food market. The seeds were washed with deionized water, cut, and dried in sunlight. The dried seeds were ground to a fine powder in a steel mill. The powdered avocado seed was treated with 98% H2SO4, Sigma-Aldrich, South Africa, in a ratio of 1:1 (avocado seed: H2SO4, w/w) and placed in a vacuum oven at 100 °C for 24 h to accomplish low temperature complete chemical carbonization of the raw powder. After carbonization, the materials was mixed with deionized water and agitated for 2 h to remove the unreacted acid. The mixture was filtered through vacuum. The material was washed repeatedly, followed by soaking in 1% NaHCO3 (Sigma-Aldrich, South Africa) solution to remove the remaining acid. Finally, the prepared carbonized adsorbent, ASSA, was dried in a vacuum oven at 100 °C, ground, and stored in an airtight container prior to use. The selection of avocado seed powder to acid ratio, 1:1 was adopted from an already reported procedure for tamarind seed (used for the removal of Cr(VI) from aqueous solutions) activation.13 In addition, the carbonization condition (100 °C) employed in this study is based on the temperature at which intercalation of H2SO4 within the polyaromatic components of avocado seed occurs. This selection of carbonization temperature is

⎛ C − Ce ⎞ %removal = ⎜ 0 ⎟ × 100 ⎝ C0 ⎠

(1)

where C0 and Ce are the initial and the equilibrium Cr(VI) concentrations, respectively, in mg/L. Adsorption isotherms were performed at 25, 35, and 45 °C. At a particular temperature, the initial Cr(VI) concentration was varied from 100 to 500 mg/L, and the solution pH was maintained at 2.0. The equilibrium sorption capacity was determined using eq 2 1215

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Figure 1. (a) and (b) SEM images of the ASSA before and after adsorption of Cr(VI) and (c) and (d) HR-TEM images of the ASSA at two different magnifications.

qe =

⎛ C0 − Ce ⎞ ⎜ ⎟V ⎝ m ⎠

concentration (mg/L) at any time t, and m is the adsorbent mass (g). The competitive effects of various coexisting ions such as Cu2+, Zn2+, Ni2+, Cl−, NO3−, and SO42‑ on Cr(VI) removal were also examined. Adsorption experiments were conducted with 25 mL of 100 mg/L Cr(VI) solutions containing each of various components in two concentrations (50 and 100 mg/L) and 20 mg of adsorbent at pH 2.0. After adsorption reached equilibrium, the adsorbent was separated from the solution. The filtrate was then analyzed for residual Cr(VI) concentration. Finally potential applicability of the ASSA for Cr(VI) removal from real wastewater sample was carried out in batch mode by using a mining industry wastewater sample collected from Rustenburg, South Africa. 2.4. Desorption Studies. Desorption studies were conducted by using the Cr(VI) adsorbed ASSA. Initially, adsorption study was performed using 0.8 g/L of the ASSA and 100 mg/L of Cr(VI) solution at pH 2.0. Then, desorption experiments were carried out by shaking the Cr(VI) loaded adsorbent with 25 mL of NaOH solution with different concentrations (0.5 M−1.0 M) for 24 h. Desorption of Cr(III) species and regeneration of the adsorbent was obtained the by treating the spent ASSA with 25 mL of 1 M HCl for 4 h. Four

(2)

where qe is the equilibrium amount of Cr(VI) adsorbed per unit mass of adsorbent (mg/g), and V is the volume (L) of the sample. From adsorption isotherm information, thermodynamic parameters such as changes in entropy (ΔS°), enthalpy (ΔH°), and standard Gibbs free energy (ΔG°) for Cr(VI) adsorption were determined. Sorption kinetic experiments were conducted by contacting 0.8 g of adsorbent with 1000 mL of Cr(VI) solutions stirred at 200 rpm. The initial Cr(VI) concentration was varied from 50 to 100 mg/L. At time zero and at selected time intervals thereafter, 5 mL samples were withdrawn and filtered through a 0.45 μm cellulose acetate filter. The filtrate was analyzed for residual Cr(VI) concentration. The amount of Cr(VI) adsorbed was calculated using eq 3 qt =

⎛ C0 − Ct ⎞ ⎜ ⎟V ⎝ m ⎠

(3)

where qt is the time-dependent amount of Cr(VI) adsorbed per unit mass of adsorbent (mg/g), Ct is the bulk-phase Cr(VI) 1216

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consecutive adsorption−desorption cycles, using the same adsorbent, were carried out to test the reusability of the adsorbent. 2.5. Chromium Analysis Methods. The concentrations of Cr(VI) ions were analyzed spectrophotometrically using a UV− visible spectrophotometer (PerkinElmer, Lamda-35, Singapore) at 540 nm after treating Cr(VI) solution with the color forming reagent 1,5-diphenylcarbazide solution. Total Cr concentrations were measured by an inductively coupled plasma-optical emission spectrometer (ICP-OES, Spectro-Arcos, Germany). Concentrations of Cr(III) species were obtained from the difference between measured values of total Cr concentrations and Cr(VI) concentrations. 2.6. Statistical Analysis of the Isotherm and Kinetic Parameters. For accuracy and reliability of the collected experimental data, all analytical measurements were duplicated, and the relative standard deviations of all measurements were less than 5%. Statistical evaluation of the adsorption isotherm and kinetic parameters was performed by employing a nonlinear regression method based on the nonlinear fitting facilities of the software GraphPad Prism (version 5.0).

3. RESULTS AND DISCUSSION 3.1. Characterization of the ASSA. The surface morphology and structure of the prepared carbonized ASSA were examined by FE-SEM and HR-TEM images. Figure 1(a) represents the SEM image of the ASSA before adsorption of Cr(VI) and reveals the formation of various shaped agglomerated particles with pores. This also demonstrates that ASSA was composed of both nanoparticles with diameter 6.0.29 At lower pH, due to the protonation of the surface functional groups, such as −OH and −COOH on ASSA, negatively charged Cr(VI) species are easily attracted to the positively charged (positive zeta potential at pH < 3.6) surface through ionic interaction. In contrast, at higher pH values, because of the deprotonation of the surface functional groups (negative zeta potential at pH > 3.6) as well as 1218

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Figure 5. Effect of pH on the adsorption of Cr(VI) by the ASSA. Initial conc. - 100 mg/L, dose - 0.8 g/L, temperature - 25 °C.

competition between OH− and CrO42‑ ions, leads to the gradual decrease of Cr(VI) removal efficiency. Moreover, the total Cr removal efficiencies of ASSA were much lower at pH < 4.0 and slightly lower than that of Cr(VI) removal at pH > 4.0. In particular, at pH 2.0, the Cr(VI) removal efficiency of ASSA was 99.86%, whereas the total Cr removal efficiency was 84.40%. The decrease in total Cr removal efficiency at pH 2.0 is possibly due to the release of some fraction of Cr(III) species into the solution. In addition, about 86.52% of Cr(VI) species were reduced to Cr(III) species at pH 2.0 by ASSA as the total 13.34 mg/L of Cr(VI) was detected in the solid phase (13.2 mg/L-obtained by desorption of 100 mg/L Cr(VI) adsorbed ASSA with 1 M NaOH solution) and in the liquid phase (0.14 mg/L) after adsorption. The comparison between the initial and final pH values of 100 mg/L Cr(VI) solution after adsorption with 0.8 g/L ASSA for 24 h is presented in Table S1 (Supporting Information). The observed increase in the final pH could be due to the presence of oxo-functional groups (CxO and CxO2) on the surface of the ASSA. These groups hydrolyze water molecules, thus provoking the release of OH− ions into the liquid phase and thereby increasing the final pH value.30 3.3. Adsorption Isotherm. To describe the dynamic equilibrium behavior between adsorbent and adsorbate species, adsorption isotherm data are important. The sorption isotherms for Cr(VI) adsorption onto ASSA at three different temperatures (25, 35, and 45 °C) are presented in Figure 6(a). The Cr(VI) adsorption capacity increases with increase in temperature, confirming the sorption process is endothermic. The most appropriate correlation of the experimental data to a theoretical model is necessary for the practical design and operation of an adsorption system. Therefore, the two most extensively used isotherm models namely Langmuir and Freundlich models were applied to fit the isotherm data. The Langmuir isotherm model, based on the monolayer adsorption onto an adsorbent surface with finite number of energetically identical sorption sites, is represented by eq 4 qe =

Figure 6. (a) Equilibrium isotherms of Cr(VI) sorption onto ASSA and nonlinear isotherm plots: (-) Langmuir model, (---) Freundlich model and (b) fit of equilibrium data to linear Langmuir isotherm model.

The dimensionless separation factor (RL), which is an essential feature of Langmuir isotherm, describes the favorability of adsorption process. The separation factor RL can be defined by eq 5 1 RL = 1 + bC0 (5) where b is the Langmuir constant, and C0 is the initial Cr(VI) concentration. The Freundlich isotherm model, which is an empirical model based on the multilayer adsorption on heterogeneous surfaces, can be expressed by eq 6

qe = KFCe1/ n

Q mbCe 1 + bCe

(6)

where KF (mg/g) is the Freundlich adsorption isotherm constant related to the extent of adsorption, and 1/n is the heterogeneity factor related to the intensity of adsorption. Linear regression has been frequently used to evaluate the model parameters. However, transformations of nonlinear

(4)

where Qm (mg/g) is the maximum adsorption capacity of the adsorbent, and b (L/mg) is the Langmuir constant related with the adsorption energy. 1219

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nature of the sorption process, whereas the positive value of ΔS0 implies an increase in disorder at the solid−liquid interface during adsorption. Moreover, the decrease in ΔG0 values with an increase in temperature suggests the spontaneous nature of the adsorption process. 3.5. Sorption Kinetics. The observed Cr(VI) uptake vs time profiles for three different initial concentrations are demonstrated in Figure 7(a). As shown in Figure 7(a), the rate

equations into linear form usually result in parameter estimation error and distort the fit. For this reason, a nonlinear regression method for parameters estimation was also used in this work. The nonlinear and linearized plots of Langmuir and Freundlich isotherms, obtained at three different temperatures, are depicted in Figures 6 (a), 6(b), and S1 (Supporting Information). The isotherm parameters determined from the linear and nonlinear regression of the experimental data are listed in Table S2 (Supporting Information). It can be observed that R2 (correlation coefficient) values obtained from the linear Langmuir isotherm model are higher than those from the Freundlich model, which suggests that the experimental isotherms are better described by the Langmuir model than the Freundlich model. However, from nonlinear regression analysis, R2 values for Langmuir model are less than that of the Freundlich model, suggesting better description of experimental data with the Freundlich model compared to the Langmuir model. This discrepancy of obtained R2 values might be due to the inherent limitations of linear regression method. These results suggest that the Freundlich model presents a more realistic description of Cr(VI) sorption by ASSA because it accounts for sorption to heterogeneous surfaces or surfaces supporting sites of varied affinity. Furthermore, the obtained RL values fall in the range of 0−1 for all the studied temperatures, which is consistent with favorable adsorption of Cr(VI) onto ASSA adsorbent. The Langmuir maximum adsorption capacities (Qm) of ASSA for Cr(VI), as estimated from the linear Langmuir isotherm model, were 333.33, 370.37, and 400 mg/g at 25, 35, and 45 °C, respectively. The Langmuir maximum adsorption capacity (Qm) for the removal of Cr(VI) by different activated carbon derived from agricultural wastes reported in the literature is s um m a r i z e d i n T a bl e S 3 ( Su p p o r t i n g I n f o r m a tion).8,10,11,13,15,31−34 The comparison of the maximum adsorption capacity of the ASSA with other adsorbents reveals that the ASSA possess significantly higher capacity toward Cr(VI), except for hierarchical porous carbon. Therefore, the high adsorption capacity and cost-effective preparation of the carbonized ASSA confirm its real field applicability for Cr(VI) removal from wastewater. 3.4. Thermodynamic Parameters. Three thermodynamic parameters namely, change in Gibbs free energy (ΔG0), entropy (ΔS0), and enthalpy (ΔH0), were determined by eqs 7 and 8 ΔG 0 = − RT ln Kc

ln Kc =

(7)

ΔS 0 ΔH 0 − R RT

Figure 7. (a) Effect of contact time and initial concentration on the adsorption of Cr(VI) onto the ASSA at 25 °C, and fit of experimental kinetic data with nonlinear pseudo-first-order and pseudo-secondorder kinetic models. (b) Linear pseudo-second-order kinetic model for adsorption of Cr(VI) by the ASSA.

(8)

where Kc (qe/Ce) is the equilibrium constant. The values of ΔS0 and ΔH0 were calculated from the linear plot of ln Kc vs 1/T (Figure S2 (Supporting Information)). The values of ΔG0, ΔS0, and ΔH0 for the present adsorption system are given in Table 1. The positive value of ΔH0 is indicative of the endothermic

and the amount of Cr(VI) adsorbed onto ASSA increased with an increase in initial Cr(VI) concentration from 50 to 100 mg/ L. The sorption rate of Cr(VI) onto ASSA was faster at the initial stage of sorption and then progressed at a slower rate to reach the final equilibrium stage. Specifically, the required equilibrium times for removal of 50, 75, and 100 mg/L Cr(VI) were 180, 360, and 480 min, respectively. The observed variation of equilibrium time with different initial concentrations could be explained by the process of Cr(VI) adsorption. Initially, the Cr(VI) species move across the

Table 1. Thermodynamic Parameters for Cr(VI) Uptake by the ASSA temp (°C)

ΔG0 (kJ/mol)

ΔH0 (kJ/mol)

ΔS0 (kJ/mol/K)

25 35 45

−1.238 −2.499 −3.816

37.16

0.1288

1220

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boundary layer and then diffuse from the boundary layer film to the adsorbent surface and finally diffuse into the porous structure of the adsorbent. Therefore, Cr(VI) solution with higher initial concentration will acquire relatively longer contact time to attain equilibrium due to the increased number of Cr(VI) ions. Investigation of the leading kinetic mechanism involved in Cr(VI) removal by ASSA were performed by fitting experimental kinetic data with pseudo-first-order, pseudosecond-order, and intraparticle diffusion models. Pseudo-first-order kinetic model can mathematically be expressed by eq 9 log(qe − qt ) = log qe −

k1 t 2.303

(9)

where qe and qt are the amounts of adsorbed Cr(VI) (mg/g) at equilibrium and at time t (min), respectively, and k1 (1/min) is the adsorption rate constant of a pseudo-first-order kinetic model. Pseudo-second-order kinetic model can be described by eq 10 t 1 t = + 2 qt qe k 2qe

Figure 8. Intraparticle diffusion model for adsorption of Cr(VI) onto the ASSA.

ions on Cr(VI) removal using ASSA, and the results are presented in Figure 9 (a). It is observed that the presence of all those metal ions slightly reduced the Cr(VI) uptake on ASSA.

(10)

where k2 (g/mg·min) is the rate constant of a pseudo-secondorder kinetic model. The nonlinear and linear plots of pseudo-first-order kinetic and pseudo-second-order kinetic models are presented in Figures 7(a), S3 (Supporting Information), and 7(b), respectively. Description of the kinetic data with appropriate kinetic model was assessed by the correlation coefficient (R2). The parameters of pseudo-first-order and pseudo-second-order kinetic models obtained from nonlinear and linear regression analyses are given in Table S4 (Supporting Information). It should be observed that the values of R2 for pseudo-secondorder model are higher than those for the pseudo-first-order model. In addition, qe values obtained from the pseudo-second order model are close to experimental qe values. These results confirm the removal of Cr(VI) by the ASSA followed the pseudo-second-order kinetic model. The rate-determining step of Cr(VI) adsorption onto ASSA was examined by analyzing the kinetic data with intraparticle diffusion model as represented by eq 11 qt = kit 1/2 + Ci

(11) 1/2

where ki (mg/g·min ) is the intraparticle diffusion rate constant, and Ci is the intercept of the stage i related to the thickness of the boundary layer. Figure 8 represents the linear intraparticle diffusion model plots at three initial concentrations. It can be observed from the plots that there are three different linear regions. The initial linear region is attributed to the film diffusion, the second describes the intraparticle diffusion stage, and the final gradual uptake is governed by the pore-diffusion mechanism.7 The rate constants obtained from the plots for the different initial concentrations are presented in Table S4 (Supporting Information). The values of Ki were found to be 1.85, 1.66, and 1.81 mg/g·mim1/2 for initial 50, 75, and 100 mg/L Cr(VI) solutions, respectively. 3.6. Effect of Coexisting Ions. Generally, industrial and mining operations produce Cr(VI) containing wastewater mixed with other cationic metal ions including Cu2+, Zn2+, Ni2+, and anions like Cl−, NO3−, and SO42−. Therefore, it is important to study the competitive influence of these coexisting

Figure 9. (a) Effect of coexisting ions on the adsorption of Cr(VI) using ASSA. (b) Removal of Cr(VI) from mining wastewater at different pH and adsorbent mass; initial Cr(VI) concentration - 47.34 mg/L. 1221

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In particular, in the absence of Ni2+, Zn2+, and Cu2+ ions the Cr(VI) uptake (capacity) of ASSA was obtained 124.77 mg/g, whereas in the presence of 50 mg/L Ni2+, Zn2+, and Cu2+ ions Cr(VI), the uptake was reduced to 115.65 mg/g, 122.15 mg/g, and 114.35 mg/g. In the presence of 100 mg/L all those metal ions, the uptake was reduced to 114.89 mg/g, 121.71 mg/g, and 113.7 mg/g all measured at pH 2.0. These findings suggest that the metal ion species present in a reaction solution compete for binding onto the sorption sites of the ASSA surface. The competitive influence of Cl−, NO3−, and SO42− ions on Cr(VI) removal was insignificant (uptake 124.35 mg/g and 123.22 mg/ g for 50 mg/L of Cl− and NO3−and 122.65 mg/g and 119.45 mg/g for 100 mg/L of Cl− and NO3−), whereas for both the studied concentrations of SO42− ions there was a slight decrease (116.21 mg/g and 114.55 mg/g for 50 mg/L and 100 mg/L of SO42−) in Cr(VI) uptake on ASSA. The slight decrease in Cr(VI) sorption capacity in the presence of SO42− might be due to the consumption of SO42− ions by the surface sites of ASSA and hence reduction of the available adsorption sites for Cr(VI) ions. Furthermore, SO42− ions would also decrease the surface positive charge and thereby decrease the electrostatic interaction between the surface and the Cr(VI) species. 3.7. Application for Cr(VI) Removal from Real Wastewater Sample. Robustness of the ASSA as adsorbent of Cr(VI) was also confirmed by its ability to remove Cr(VI) from real mining industry wastewater samples. Mining wastewater samples with physicochemical characteristics as follows: pH 6.12, conductivity - 15.87 mS/cm, COD - 1537.5 mg/L, total alkalinity as CaCO3 - 35 mg/L, Cr(VI) - 47.34 mg/L, Zn2+ 35.99 mg/L, Ni2+ - 15.50 mg/L, Cu2+ - 1.50 mg/L, and NO3− 9.40 mg/L, were acquired from a mining industry (Rustenburg) in South Africa. Adsorption experiments were carried out with 25 mL of wastewater samples by varying the mass of ASSA from 0.01 to 0.1 g at its original pH (6.12) and at pH 2.0 (optimum pH for Cr(VI) removal by ASSA), and the results are presented in Figure 9(b). It is observed that maximum (99.95%) removal was achieved by utilizing 0.08 g of ASSA at the original pH of the wastewater, whereas by the use of only 0.03 g of ASSA almost 100% removal of Cr(VI) was obtained from wastewater after adjustment of pH to 2.0. A comparison of Cr(VI) removal efficiencies of ASSA using synthetic and real wastewater samples revealed that 0.8 g/L ASSA removed 99.86% of 100 mg/L Cr(VI) solution, whereas 1.2 g/L of ASSA removed almost 100% of the 47.34 mg/L Cr(VI) contained in the wastewater. The slight reduction in Cr(VI) removal efficiency of ASSA for wastewater sample might be due to the competitive effects of all the other ions present in the wastewater sample. 3.8. Desorption Studies. To develop a cost-effective sorption medium for Cr(VI) removal, it is important that the adsorbed Cr(VI) should be easily desorbed for reusability of the media. Desorption of Cr(VI) from loaded ASSA was conducted under different concentrations (0.5−1 M) of NaOH solutions, and the results are shown in Figure 10 (a). In the lower concentrations of NaOH, very small amounts of Cr(VI) were eluted from the media, while in the higher concentration (1 M) of NaOH 13.2% Cr(VI) elution was observed. The desorption efficiencies of Cr(VI) appeared to be very low even at 1 M NaOH concentration. This is due to the reduction of the adsorbed Cr(VI) to Cr(III) species by ASSA (as observed from the ICP analysis), which could not desorb upon treatment with the NaOH solution. Desorption of the Cr(III) species was achieved by treating the adsorbent with 1 M HCl solution.

Figure 10. (a) Desorption of Cr(VI) from the surface of ASSA with different concentrations of NaOH and (b) regeneration of the ASSA for four cycles.

Further use of the regenerated adsorbent for the next adsorption−desorption cycle revealed that (Figure 10 (b)) the adsorption capacity (124.74 mg/g) of the ASSA remained constant for the first three cycles, but in the subsequent fourth cycle there was a decrease in capacity. Thus, the adsorbent can be reused for three consecutive adsorption−desorption cycles without loss of capacity. 3.9. Adsorption Mechanism. The functional groups present on the ASSA surface greatly influence the Cr(VI) removal mechanism. The oxo-functional groups (denoted by CxOH) including −OH and −COOH, on the surface of ASSA undergo protonation and deprotonation processes at different pH values. At low pH values, in the presence of adequate H+ ions, the surface of the ASSA becomes positively charged via protonation of the surface functional groups,35 which adsorb negatively charged Cr(VI) species via electrostatic attraction as follows: Cx OH + H+ ⇌ Cx OH 2+

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Cx OH 2+ + HCrO4 − ⇌ Cx OH 2+HCrO4 −

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The presence of Cr(III) species in the solution at lower pH can be explained by the Cr(VI) ions possessing a very high positive redox potential and are unstable in the presence of electron donor functional groups (−OH and −COOH).36 1222

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Notes

Therefore, in the acidic solution, Cr(VI) ions can be easily reduced to Cr(III) as presented by the following eq HCrO4 − + 7H+ + 3e‐ → Cr 3 + + 4H 2O

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge Tshwane University of Technology (postdoctoral fellowship programme) and National Research Foundation (NRF grant no: 86902), South Africa, for the financial support and the Council for Scientific and Industrial Research (CSIR), South Africa for providing research infrastructure to this project.

The generated Cr(III) species are also bound to the phenolic and carboxyl groups of the ASSA surface at pH 2.0.37 Further understanding of the Cr(VI) removal mechanisms was accomplished by analyzing the XPS spectrum of ASSA after adsorption with Cr(VI) solution, at pH 2.0 for 24 h. The XPS spectrum of Cr2p orbital of ASSA after treatment with Cr(VI) is illustrated in Figure 4(b). The XPS spectrum reveals two Cr2p intensity peaks at 577.0 and 586.8 eV, which are comparable with the binding energy of Cr(III) in Cr2O3 at 576.5 and 587 eV. The observed peaks are significantly different from the Cr(VI) binding energy peaks of CrO3 at 579.3 and 588.3 eV.1 In addition, small shoulders appear at 579.7 and 588.9 eV on the Cr2P3/2 and Cr2p1/2 bands which are similar to the binding energies of Cr(VI) in CrO3. The presence of traces of Cr(VI) on the ASSA surface indicates that the reduction of Cr(VI) to Cr(III) was not complete. These results suggest that removal of Cr(VI) by ASSA is associated with massive reduction of adsorbed Cr(VI) to Cr(III) species and its subsequent partial adsorption onto the ASSA surface.

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4. CONCLUSIONS Sulfuric acid-modified avocado seed (ASSA) as a cost-effective adsorbent was investigated for the removal of Cr(VI). Adsorption experiment results indicated that Cr(VI) removal was strongly pH dependent, and maximum removal was achieved at pH 2.0. Equilibrium isotherm data fitted well to the Freundlich isotherm model. Kinetic data followed the pseudosecond-order model. Thermodynamic parameters confirmed the spontaneous and endothermic nature of the adsorption process. In the presence of other coexisting ions, Cr(VI) adsorption onto ASSA was slightly decreased due to their competition for the sorption sites. The ASSA adsorbent showed its usefulness in real wastewater treatment by removing Cr(VI) from mining industry wastewater contaminated with other metal ions. Desorption studies demonstrated the reusability of ASSA for Cr(VI) removal by retaining original capacity up to three complete adsorption−desorption cycles. All these results further suggest that the carbonized ASSA is a promising alternative sorption media for effective treatment of Cr(VI) containing wastewater.

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ASSOCIATED CONTENT

* Supporting Information S

The detailed information regarding the pH change before and after adsorption (Table S1), isotherm constants (Table S2), comparison of adsorption capacity with other adsorbents (Table S3), kinetics parameters (Table S4), Freundlich model fitting (Figure S1), thermodynamic parameters (Figure S2), and pseudo-first-order kinetic model fitting (Figure S3) are summarized. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: +27123826290, Fax: +27123826286 (R.M. and M.B.). Phone: +27128412658, Fax: +27128413553 (A.M.). E-mail: [email protected] (M.B.), [email protected] (R.I. M.), [email protected], [email protected] (A.M.). 1223

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