Studies on Cr(VI) Removal from Aqueous Solutions by Nanoalumina

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Studies on Cr(VI) Removal from Aqueous Solutions by Nanoalumina Madona Lien Paul, Jastin Samuel, Sushree Bedatrayee Das, Shikha Swaroop, Natarajan Chandrasekaran, and Amitava Mukherjee* Centre for Nanobiotechnology, VIT University, Vellore 632014, Tamil Nadu, India S Supporting Information *

ABSTRACT: The current study deals with Cr(VI) removal using unmodified alumina nanoparticles of two different sizes (NA1: mean hydrodynamic diameter = 75.3 ± 2.8 nm; NA2: mean hydrodynamic diameter = 229.8 ± 3.3 nm). The equilibrium adsorption capacities, 73.2 and 59.4 mg of Cr(VI)/g of adsorbent, were noted for NA1 and NA2, respectively, under optimized conditions (pH 7.0, temperature = 27 °C, initial Cr(VI) concentration = 20 mg L−1, adsorbent dosage = 0.1g L−1) but different contact times (120 min for NA1, 180 min for NA2). For both sorbents, the equilibrium adsorption data fitted well with the Langmuir isotherm model. The adsorbent NA1 followed a pseudo-second-order kinetics, whereas NA2 followed pseudo-firstorder kinetics. Surface characterization studies (zeta potential measurement, scanning electron microscopy (SEM), energydispersive X-ray spectroscopy (EDX), and Fourier transform infrared (FTIR) spectroscopy) substantiated oxyanionic binding on the sorbent surface. The EPR and XRD spectroscopy confirmed the existence of reduced Cr(III) on the adsorbent surface. The applicability of the sorbent in Cr(VI)-contaminated water was studied.

1. INTRODUCTION Industrial and other anthropogenic processes constantly release heavy-metal ions into the environment.1 Hexavalent chromium, which is one of these metals, is more hazardous to public health, compared to other valence states, such as trivalent chromium, because of its greater mobility and carcinogenic properties.2 The existing reports on Cr(VI) toxicity (1 mg L−1 initial concentration) in human epithelial-like L-41 cells reveals that long-term action (48 h) causes a complete loss of cell viability.3 Often, the contaminated sites have been reported to have a Cr(VI) concentration above this permissible limit (1.56 mg L−1 in chromite mine site, Sukinda Valley, India).4 The European Medicines Agency, in its 2007 report, identified a lethal oral dose of soluble chromates in humans to be 50−70 mg kg−1. Cr(VI) is considered toxic to the environment if present at a level higher than the permissible limit (the permissible limit in drinking water is 0.05 mg L−1).5 Therefore, the removal of hexavalent chromium becomes more imperative for the betterment of human health and environment.6 Conventional physical method for Cr(VI) removal include the use of electrokinetic mechanisms or ion-exchange resins.7−9 However, as a result of poor metal removal and secondary pollution on chemical treatment, adsorption process has emerged as an alternative method for removal of heavy metals from contaminated waters.10,11 This can be substantiated from our previous Cr(VI) adsorption study using Acinetobacter junii bacterial biomass.12 Nanotechnology has contributed to the development of products and process alternatives for water purification. Currently, nanomembranes, nanofibers, bioactive nanoparticles, carbon nanotubes, and nanoparticles are being widely used for wastewater treatment.13−15 Nanosized metal oxides such as iron oxide and manganese oxide and zinc oxides are extensively used as suitable adsorbents, because of their large surface area and high activity via a size quantization effect.16−18 Nanocomposites based on carbon nanotubes, bentonites, and surface-modified © 2012 American Chemical Society

nano metal oxides have demonstrated promising results on the removal of hexavalent chromium from aqueous environments.19−21 It has been well reported in the literature that alumina has excellent adsorption capacity for metal ions such as Cr(VI), Cd(II), and Pb(II).22 Alumina-supported nanoparticles are reported to be used for the removal of Cu.23 Recently, studies using nanoalumina (40−80 nm mean diameter) for preconcentration of Cd(II) and Pb(II) is reported. However, this nanoalumina is modified with sodium dodecyl sulfate-1-(2pyridylazo)-2-naphthol (SDS-PAN) for the adsorption of metals.24 The removal of heavy-metal ions including Cr(III) using nanoalumina (mean diameter of 53 nm) modified with 2,4-dinitrophenylhydrazine has also been reported.25 Nanoalumina has thus been proved to be an efficient adsorbent system for the adsorption of different heavy metals. The use of nanosized aluminum oxides for the removal of fluoride, which is an anionic species, has been reported.26 Alumina generally carries a positive surface charge, and negatively charged anions are sorbed on the positively charged surface of metal oxides by electrostatic attraction. However, to the best of our knowledge, there are no reports available in the literature dealing with the application of nanoscale aluminum oxides for the removal of hexavalent chromium. The present work reports, for the first time, the application of nanoalumina without any surface modifications for the removal of Cr(VI) by means of process optimization. The study also aims to identify the effect of adsorbent size variation on adsorption capacity, which was not attempted for any nanosorbent system previously. The mechanism of chromium removal is deciphered here by detailed microscopic Received: Revised: Accepted: Published: 15242

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and spectral studies. The effect of Cr(III) on Cr(VI) adsorption is studied to determine the specificity of nanoalumina toward the adsorbates. The applicability of the adsorbent in real water matrices and its reusability are also explored.

q=

V (C0 − Ce) m

adsorption (%) =

2. MATERIALS AND METHODS 2.1. Nanosorbent Preparation for Adsorption. Aluminum oxide nanoparticles of two different sizes procured from Sigma−Aldrich, India were used as adsorbents. The nanoparticles were characterized on the basis of their hydrodynamic size distribution measured in distilled deionized (DI) water (Milli-Q, Millipore Corp.), using a particle size analyzer (90 Plus Particle Size Analyzer, Brookhaven Instruments Corporation, NY, USA). The particle size was represented as the mean hydrodynamic diameter (z-average mean). A stable suspension of nanoparticles was achieved by ultrasonicating the samples at 130 W for 5 min by Vibra Cell VCX 130, Sonics, USA. 2.2. Chemicals Used for Adsorption. The standard stock solutions of hexavalent chromium (1000 mg L−1) and trivalent chromium were prepared by dissolving appropriate amounts of K2Cr2O7 and Cr (NO3)3·9H2O in DI water (Milli-Q). NaOH (1 N)/1 N H2SO4 was used to adjust the pH of the solutions. The hexavalent chromium content in solution was analyzed using 1,5-diphenylcarbazide. The reagent was prepared by mixing 0.25% w/v in 50% acetone. A desorption of Cr(VI) from the adsorbents was achieved using 0.01 N NaOH. All the reagents were of analytical grade and were procured from Sigma−Aldrich, India. 2.3. Elemental Analysis. A colorimetric method was used to measure the concentration of Cr(VI) remaining in the solution, before and after adsorption. Samples were reacted at pH 2.8 with 1,5-diphenylcarbazide to obtain a purple color that indicates the presence of Cr(VI). The measurement was performed at a wavelength of 540 nm. Total Cr was measured at a wavelength of 359.9 nm, using a flame atomic adsorption spectrophotometer (Model Analyst400/HGA 900, Perkin− Elmer, USA). 2.4. Adsorption Studies. The interaction of chromium with nanoalumina of different sizes was conducted in a batch system. Each experiment was repeated in triplicate. The results are expressed as the mean value ± standard deviation (SD). 2.4.1. Optimization of Adsorption Conditions. Adsorption experiments were performed with 0.1 g L−1 adsorbent dose and 20 mg L−1 initial Cr(VI) concentration in 50 mL metal solution at 28 °C for 240 min with varying pH (3.0−12.0). Optimal contact time was determined by adsorption study at optimized pH using an initial Cr(VI) concentration of 20 mg L−1 and an adsorbent dose of 0.1 g L−1 in a 50-mL solution at 28 °C. Cr(VI) concentration in the samples were analyzed at different time intervals (15, 30, 60, 120, 150, 180, and 240 min). The effect of temperature on adsorption was evaluated at 27, 30, 35, and 40 °C. Adsorption experiments were performed with different adsorbent dosage (0.01−0.5 g L−1) to determine the optimal amount of sorbent that gives maximum adsorption. Adsorption studies were conducted for different initial Cr(VI) concentrations in the range of 5−100 mg L−1 to determine the changes in adsorption capacity with initial metal ion concentration. 2.4.2. Quantification of Adsorption. Adsorption capacity and the percentage of adsorption by the adsorbent were measured using the following equations.

(1)

C0 − Ce × 100 C0

(2)

where q is the adsorption capacity (in mg g−1), V the volume of chromium in solution (L), C0 the initial Cr(VI) concentration (in mg L−1), Ce the Cr(VI) concentration at the equilibrium, and m the weight of the adsorbent (g). 2.5. Isotherm for Adsorption. To study the mode of interaction of Cr(VI) ions with the adsorbents when the solution phase and sorbent solid phase are in equilibrium, the adsorption equilibrium data obtained at 28 ± 1 °C were modeled using Langmuir, Dubinin−Radushkevich, and Freundlich isotherms. [Details are provided in the Supporting Information.] 2.6. Adsorption Kinetics. Pseudo-first-order and pseudosecond-order models were applied to the adsorption data to determine the metal uptake rate. The best fit was considered for the model with the highest correlation coefficient value (r2). [Details are provided in the Supporting Information.] 2.7. Thermodynamics of Adsorption. Thermodynamic parametersnamely, the standard free energy (ΔG°), standard enthalpy (ΔH°), and standard entropy (ΔS°)were measured. These are considered important parameters to check the feasibility of the adsorption process. [Detailed equations are provided in the Supporting Information.] 2.8. Statistical Analysis. Each set of experiments were conducted in triplicate. Experiments were repeated separately to ensure reproducibility. Experimentally obtained results were checked for statistical significance via one-way analysis of variance (ANOVA) coupled with Tukey’s multiple comparison test or Dunnette’s post test using GraphPad Prism 5.0 software. 2.9. Mechanism of Adsorption. 2.9.1. Zeta Potential Study. Solutions with different pH ranging from 5.0−9.0 were prepared. The conductivity of each preparation was set to a constant value of 10 mS using 0.1 M KCl. Nanoadsorbents were added to the preparations at a concentration of 10 mg L−1. After the preparations were equilibrated for 1 h, zeta potentials were measured by 90Plus particle size analyzer, Brookhaven Instruments Corp., USA. 2.9.2. Scanning Electron Microscopy. Interacted and uninteracted adsorbents were analyzed microscopically after the specific contact time for adsorption. The glutaraldehydefixed and ethanol-dehydrated samples were mounted on 10mm metal stubs using carbon tape. The samples were then sputter-coated with gold in an argon atmosphere. The surface morphology was observed via scanning electron microscopy (SEM) (Model S400, Hitachi, Japan). 2.9.3. Energy-Dispersive X-ray Analysis (EDAX). The surface elemental analysis of uninteracted and Cr(VI) interacted adsorbents was performed using energy-dispersive X-ray spectroscopy. The gold-sputtered samples were analyzed for surface elemental composition. The spectra obtained were recorded using a Model JSM-5510 system (JEOL, Ltd., Tokyo, Japan). 2.9.4. Cr(VI) Reduction Studies by Electron Paramagnetic Resonance (EPR) Spectroscopy. The spectra of Cr(VI) adsorbents were obtained using an electron paramagnetic resonance (EPR) spectrometer (EMX Plus, Brooker Biospin, Germany). The spectrum was recorded under the following conditions: X-band frequency, 100 kHz (modulation fre15243

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Figure 1. Plots of the effect of various parameters on adsorption capacity ((a) pH, (b) contact time, (c) temperature, (d) adsorbent dosage, and (e) initial Cr(VI) concentration).

at 100 mg L−1. The initial and final total Cr in the solution was measured by atomic absorption spectrometry (AAS).

quency); microwave power, 2 mW; and modulation amplitude, 5 G. 2.9.5. Fourier Transform Infrared (FTIR) Analysis. The surface chemical characteristics of the nanosorbents were characterized by Nicolet Model 6700 Fourier transform infrared spectrometer (Thermo Scientific Instruments Groups, USA). Uninteracted and Cr(VI)-interacted adsorbents NA1 and NA2 were mixed with 100 mg of KBr. A pellet was formed by pressing the fine powdered mixture in a mechanical die press by applying a pressure of 1200 psi for ∼5 min. 2.9.6. X-ray Diffraction (XRD) Analysis. The different phases were determined using powder X-ray diffraction (XRD). The XRD spectra were obtained using an XPERT-Pro powder diffractometer operating in the reflection mode with Cu Kα radiation. The XRD diffractogram was recorded over an angular range of 10°−90° (2θ) with a step size of 0.033° and a collection time of 0.4 s. Identification of peaks was undertaken using the Joint Committee on Powder Diffraction Standards (JCPDS) diffraction database. [JCPDS is now known as the International Centre for Diffraction Data (ICDD).] 2.10. Effect of Cr(III) on Cr(VI) Adsorption. The effect of Cr(III) on Cr(VI) adsorption was studied by making synthetic mixtures with various Cr(VI):Cr(III) ratios (5:95, 10:90, 20:80, 50:50). An initial total chromium concentration was maintained

3. RESULTS AND DISCUSSIONS 3.1. Characterization of Nanosorbent for Adsorption. The two different sizes of alumina nanoparticle sorbents used for the study are designated as NA1 and NA2. The hydrodynamic mean diameters of NA1 and NA2 at zero hour (t0) were found to be 75.3 ± 2.8 and 229.8 ± 3.3 nm, respectively, in a distilled DI water medium. The particle size of NA1 and NA2 was analyzed after 3 h (tf) and measured to be 77.9 ± 3.2 nm and 234 ± 4.6 nm, respectively. In both adsorbents, the differences in the sizes obtained at the two time intervals (t0 and tf) were determined to be statistically insignificant by one-way ANOVA. This shows that there was no appreciable aggregation of particles during the reaction time. The aggregation of particle with time leads to a decrease in specific surface area and, thereby, the number of available adsorption sites, which, in turn, could decrease the adsorption capacity. Therefore, it is relevant to study the particle size during the maximum reaction time considered for the experiments. 15244

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3.2. Optimization of Adsorption Parameters. Adsorption experiments were conducted for the nanosorbents NA1 and NA2 at pH ranging from 3.0 to 12.0 and the adsorption capacities were measured (see Figure 1a). The maximum adsorption capacities (72.12 and 59.4 mg g−1 for NA1 and NA2, respectively) were observed at pH 7.0. For higher pH (9.0 and 12.0), the adsorption capacities decreased drastically. The changes in adsorption capacity with increasing pH from 3.0 to 7.0 and from 7.0 to 12.0 were determined to be statistically significant by one-way ANOVA analysis. At lower pH range (1.0−4.0), Cr(VI) predominantly exists in the form of monovalent HCrO4− and at higher pH (above 4.0), Cr(VI) exists in the form of Cr2O72−.27 Thus, at the pH range from 1.0 to 7.0, the oxyanionic species of Cr(VI) are likely to get adsorbed on the positively charged adsorbent surface. The adsorption behavior of negatively charged oxyanionic chromate ions to the positively charged adsorbent through the Coulombic force of attraction are in well accordance with the zeta potential measurements (the zeta potential charge on the adsorbent surface is discussed later, in section 3.5.1). For further experiments, pH 7.0 was considered to be optimal. The difference in adsorption capacities of NA1 and NA2 at this pH was statistically significant. This may be attributed to the difference in the number of available surface sites, which, in turn, depends on the specific surface area of the adsorbent. Optimal pH obtained (pH 7.0) for both the adsorbent system was well in agreement with the previous Cr(VI) adsorption studies, which used CeO2 nanoparticles as a sorbent.28 Experiments conducted for determining the optimal contact time for adsorption by NA1 yielded a high adsorption capacity (58.3 mg g−1) within 15 min. As the contact time increased from 15 min to 150 min, the adsorption capacity increased to a maximum of 71.2 mg g−1 (Figure 1b). There was no appreciable change beyond that interval. This study indicates that the first 15 min of interaction contributed to 81% of maximum adsorption observed, which may be due to the excess availability of vacant sites. After 15 min, the remaining vacant surface sites may not have been occupied because of the increased competition between the solute molecules. Beyond 150 min, the adsorption capacity was almost constant, indicating equilibrium conditions, with no active sites remaining for adsorption. The slight changes in the adsorption capacity beyond 150 min were determined to be statistically insignificant by one-way ANOVA and Tukey’s multiple comparison test (p-value = 0.2418). Further experiments with NA1 were performed, keeping the optimal contact time as 150 min. When NA2 was interacted with Cr(VI), the adsorption capacity observed after 15 min of contact time was 32.5 mg g−1. As the contact time increased from 15 min to 180 min, the adsorption capacity increased and reached a maximum of 55.1 mg g−1 (see Figure 1b). Changes beyond that interval were determined to be statistically insignificant by one-way ANOVA. Further experiments with NA2 were performed, keeping the optimal contact time as 180 min. Clearly, the time required to adsorb 50% of the initial Cr(VI) by NA2 was significantly higher than that of NA1. This may be due to a decrease in the free movement or percolation of Cr ions in the presence of larger-size particles (NA2) in the solution. There was a slight decrease in the adsorption capacity of the sorbents NA1 and NA2 with an increase in temperature from 27 °C to 40 °C (see Figure 1c). This may be attributed to the loss in binding sites at higher temperature. The effect of

temperature on Cr(VI) solubility was tested by control experiments conducted in the absence of an adsorbent. The insignificant difference in Cr(VI) concentration at time 0 and after 3 h indicate the negligible impact of temperature on Cr(VI) solubility. However, the change in adsorption capacities, with respect to the temperature range studied, was determined to be statistically insignificant by one-way ANOVA and Tukey’s multiple comparison test (p-value = 0.8341). Therefore, the optimal temperature of adsorption experiments was considered to be 27 °C. Previous reports on adsorption studies using adsorbents based on ceria nanoparticles also shared a similar trend.28 A range of adsorbent dosage of 0.01−0.5 g L−1 was used to obtain the maximum amount of chromium(VI) binding by NA1 and NA2 under the optimal conditions. Maximum chromium(VI) adsorption capacities of 64.9 mg g−1 and 55.6 mg g−1 were observed when 0.1 g L−1 of adsorbent was used (see Figure 1d). An increase in adsorption capacity with an increase in adsorbent concentration from 0.01 g L−1 to 0.1 g L−1 was observed. This may be due to availability of the metal ion adsorption sites. A decrease in adsorption capacity with the increase in adsorbent dosage from 0.1 g L−1 to 0.5 g L−1 was observed. The variation in the adsorption capacities obtained at the optimal dosage of sorbent is concordant with the specific surface area available for interaction with the respective sorbents. The effect of initial Cr(VI) concentration on adsorption was analyzed using increasing dosages of Cr(VI) from 5 mg L−1 to 100 mg L−1 (see Figure 1e). The adsorption capacity increased as the Cr(VI) concentration increased up to 20 mg L−1 for the constant adsorbent dosage used (1 g L−1), after which it slightly decreased. The initial increase in adsorption capacity may have resulted from the greater availability of the metal ions, compared to that in the later stages, where adsorption takes place. A high solute concentration gradient imparts a driving force to overcome the mass-transfer resistance between the aqueous solution and solid adsorbents.29 The decrease in the adsorption capacities with further increases in Cr(VI) concentration shows that saturation of binding sites occurred. Moreover, this may have resulted because of the aggregation of adsorbents upon interaction with Cr(VI). However, the difference in this decrease was tested and determined to be statistically insignificant. Therefore, 20 mg L−1 of initial Cr(VI) was considered to be the equilibrium metal ion concentration in further adsorption experiments. 3.3. Adsorption Studies under Optimized Conditions. Adsorption studies were conducted to investigate the role of binding sites in the adsorption process. Optimized parameters (pH 7.0, temperature = 27 °C, initial Cr(VI) concentration = 20 mg L−1, nanosorbent dosage = 0.1 g L−1) were used to carry out the experiments to determine the percentage adsorption on NA1 and NA2. The optimized contact time for NA1 and NA2 were 150 and 180 min, respectively. Cr(VI) adsorption of 36.6% and 29.7% was noted for NA1 and NA2, respectively, which corresponds to their respective adsorption capacities (73.2 and 59.4 mg g−1). The results correlate with the report on Cr(III) adsorption by 2,4-dinitrophenylhydrazin-modified nanoalumina, yielding an adsorption capacity of 100 mg g−1.25 3.4. Adsorption Equilibrium Studies. Adsorption equilibrium data obtained were modeled using Langmuir, Freundlich, and Dubinin−Radushkevich isotherms at 27 ± 1 °C. The various isotherm parameters obtained have been tabulated (see Table S1 in the Supporting Information). The 15245

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Langmuir model showed the best fit for both adsorbents, having the highest regression coefficient (r2). Moreover, the q0 value for adsorption obtained by adsorption experiments was similar to that theoretically quantified from the Langmuir plot. The Langmuir mode of Cr(VI) adsorption was reported in studies using iron-based nanoparticles.30 3.5. Adsorption Kinetics. The dynamics of adsorption of metal ions on the adsorbent was investigated by adsorption kinetics. Pseudo-first-order and pseudo-second-order kinetics were modeled to the kinetics data obtained for initial Cr(VI) concentrations (5−100 mg L−1). The corresponding rates (k1 and k2, respectively) are presented in Table S2 in the Supporting Information. In the case of NA1, a pseudosecond-order kinetics holds well. For this system, the kinetic constant decreased as the initial Cr(VI) concentration increased, showing that it depends on the Cr(VI) concentration rather than the adsorbent capacity, as reported in the case of fluoride adsorption by montomorillonite.31 This indicates that, for an adsorbent with an excess number of binding sites due to its small size, the rate-limiting step for adsorption is the concentration of the solute. The correlation coefficient (r2) value suggests that the NA2 system fit best for pseudo-firstorder kinetics than the pseudo-second-order kinetics. This indicates that, in the presence of a large excess of Cr(VI), the rate of adsorption is largely dependent on the capacity of adsorbent, which, in turn, is dependent on the number of available sites for binding or ion exchange. 3.6. Thermodynamics of Adsorption. The temperaturedependent parameters, such as the changes in free energy, enthalpy, and entropy, were quantified for the adsorbents NA1 and NA2 and are presented in Table S3 in the Supporting Information. ΔH° and ΔS° values were obtained from the plot between 1/T verses ln K (using data provided in the Supporting Information). The negative value of ΔG° suggests that the adsorption process is spontaneous for both sorbents. This may be attributed to the availability of adequate metal binding sites for adsorption. The magnitude of ΔG° for adsorbent NA1 was more than that of NA2, indicating a higher spontaneity. These data are well-correlated with the adsorption data observed under optimal conditions. This indicates the competency between the Cr ion molecules for the comparatively lesser binding sites on the NA2, because of lesser specific surface area. The positive ΔH° values of the adsorbents show that the adsorption reactions were endothermic in nature. A positive value of ΔS° indicated an increase in the number of species and the randomness at the solid/liquid interface. This may have been aroused by the release of water molecules, ion exchange, and/or metal binding. In addition, the positive value of ΔS° shows the affinity of Cr(VI) for the adsorbent used.32 3.7. Mechanism Studies on Adsorption. 3.7.1. Surface Charge Analysis. The zeta potential study enables the measurement of surface charge of adsorbents at different pH ranges (pH 5−9) and the corresponding adsorption capacity measured is given in Figure 2. The positive zeta potential values obtained for NA1 and NA2 at pH 7.0 (29 ± 1.3 and 26 ± 1.5 mV, respectively) decreased as the pH increased to 9.0, indicating a higher stability at pH 7.0. The decrease were determined to be statistically significant by one-way ANOVA, followed by Dunnette’s post test (p < 0.05). The zeta potential observed at pH 5.0 is comparatively less than that at pH 7.0 for NA1 as well as NA2. However, the differences in zeta potentials observed at pH 5.0 and 7.0 were determined to be statistically insignificant (p-value = 0.7832, 0.8165) for both the adsorbents.

Figure 2. Plot of the zeta potential surface charge measured at various pH.

The zeta potential measurements are correlated with the adsorption behavior of negatively charged oxyanionic chromate ions to the positively charged adsorbent surface resulting from a Coulombic force of attraction. A net decrease in this attractive force may have contributed in decreasing the adsorption capacities, as observed at pH 5 and pH 9.33 3.7.2. Scanning Electron Microscopy. Scanning electron microscopy (SEM) images of the uninteracted adsorbents NA1 and NA2 showed a uniform dispersion with almost-spherical structures. The uninteracted NA1 had particles with a mean diameter of 250 nm (Figure 3a), while that of uninteracted NA2 revealed particles with a mean diameter of 100 nm (Figure 3b). This resultant sizes correlated with that obtained based on the hydrodynamic particle size analyzed (see section 3.1). Upon interaction with Cr(VI), both NA1 and NA2 formed aggregates of different sizes (Figure 3c, 3d). This may have resulted from the interaction of Cr(VI) to the nanoparticle through a Coulombic force of attraction, as discussed in section 3.7.1.The oxyanionic form of chromium binds to the alumina nanoparticle, thereby neutralizing the positive charge density, leading to aggregation. Moreover, as the size increases, the electrostatic repulsion between similar charged layers decreases, leading to aggregation.34 However, further experiments must be conducted to determine the difference in rate of aggregation between NA1 and NA2. 3.7.3. EDX Spectroscopy. EDX spectroscopy helps to provide information on the elemental composition of the sorbent surface analyzed. The spectra obtained for interacted NA1 and NA2 confirms the presence of Cr ions on the nano adsorbents (see Figures 4a and 4b). The weight percentage of chromium present in the interacted sorbents NA1 and NA2 were 0.34% and 0.23%, respectively. The decrease in the percentage of chromium corresponds to the decrease in the adsorption capacities observed in the case of NA2. The effect of size variation on the percentage removal of solute is quite evident from these data. 3.7.4. Cr(VI) Reduction Studies by EPR Spectroscopy. To detect the possible redox species of chromium after adsorption, EPR analysis was conducted for uninteracted and Cr(VI) interacted (20 mg L−1) NA1 and NA2. A peak (∼50 G) centered at a g-factor of 1.95 was observed in both Cr(VI)interacted sorbents (Figure 5). This corresponds to Cr(III) in Cr2O3 crystallites, as observed from the EPR study of aluminasupported chromium oxide.35 Although a few nanosorbents have been studied for Cr(VI) removal, this is the first time that a possible Cr(VI) reduction on the surface of a nanosorbent is 15246

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Figure 3. Scanning electron microscopy (SEM) images of (a) uninteracted NA1, (b) Cr(VI)-interacted NA1, (c) uninteracted NA2 (inset shows an image of aggregates at higher magnification), and (d) Cr(VI)-interacted NA2 (inset shows an image of aggregates at higher magnification).

Figure 4. SEM-EDAX spectra of (a) Cr(VI)-interacted NA1 and (b) Cr(VI)-interacted NA2.

stretching vibrations; 1634 cm−1, −NH2 bending vibrations; and 1420 cm−1, stretching vibration of C−O.36 These peaks may denote the functional groups, such as carboxyl and amide, that are present on the surface of the adsorbent. The unique peak at 1040 cm−1 corresponds to the Al−O stretching vibrations.26 Upon interaction with Cr(VI), new sharp peaks corresponding to Cr−O vibrations at 826 cm−1 and 791 cm−1 formed in the fingerprint region, confirming the adsorption of Cr on the nanosorbent surfaces. In addition, the intensity of the peaks at 2927, 1634, and 1420 cm−1 decreased (see Figure 6b).

being reported. This must be studied further to explore a newer understanding into the mechanistics of the process. 3.7.5. Surface Chemical Analysis by FTIR Spectroscopy. The FTIR spectra of uninteracted and Cr(VI)-interacted sorbents, NA1 and NA2 were obtained. The spectrum of uninteracted NA1 was compared to that of the interacted (see Figure 6a). In the spectrum of uninteracted NA1, the peak at 3456 cm−1 (−OH stretching vibration) may be attributed to the atmospheric water vapor.26 The major bands for the adsorbent can be assigned as follows: 2927 cm−1, −CH 15247

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Figure 5. EPR spectra of (a) Cr(VI)-interacted NA1 and (b) Cr(VI)interacted NA2.

Figure 7. XRD diffractogram of (a) uninteracted NA1 and (b) Cr(VI)interacted NA1.

structural changes may have occurred at that level of chromium concentration on the surface (measured chromium incorporation was 0.34% by weight, as obtained by EDAX results). However, a peak at 2θ = 73° was also observed with the interacted sample, which corresponded to Cr2O3.37 This confirms the observation from EPR analysis (see section 3.7.4), indicating the reduction of part of the Cr(VI) to Cr(III). 3.8. Effect of Cr(III) on Cr(VI) adsorption. Effect of Cr(III) on Cr(VI) adsorption was analyzed by carrying out Cr(VI) adsorption study in synthetic mixtures of Cr(VI) and Cr(III) in different ratios. The experiments were conducted only for NA1 that had a higher adsorption capacity, compared to NA2. The total chromium removed was estimated by AAS, and the results obtained are given in Table (1). The percentage reduction in Cr(VI) removal was the highest (73.5%) when 95 mg L−1 of Cr(III) was used with 5 mg L−1 of Cr(VI), which decreased as the Cr(III) concentration in the binary mixture

Figure 6. FTIR spectra of (a) uninteracted NA1 and (b) Cr(VI)interacted NA1.

This indicates the probable involvement of the surface functional groups in the interaction with Cr(VI) as observed in sections 3.7.3 and 3.7.4. The decrease in the intensity of peak at 1040 cm−1 indicates the involvement of the Al−O bonds in the interaction and probable complexation of Cr ions with Al. The spectra of uninteracted and Cr(VI)-interacted NA2 were also compared (figure not shown). The changes observed in the spectra closely resembled that observed for NA1. Therefore, we can conclude that the mechanism of Cr(VI) reduction and adsorption is dependent on the sorbent surface groups present, which is a factor that is not altered with the size variation. 3.7.6. XRD Analysis of Adsorbents. The XRD patterns of the nanoalumina sorbent NA1 before and after interaction with Cr(VI) were recorded. Since a similar trend of Cr(VI) interaction was observed with both NA1 and NA2 in the previous experiments, only NA1 was considered for this study. Figures 7a and 7b shows the X-ray diffractograms of uninteracted and Cr(VI)-interacted NA1 sample, respectively. The peaks of the uninteracted samples (observed at angles of 2θ = 32°, 36°, 46°, 67°) correspond to those of γ-alumina.26 The similarity between the diffractograms of the uninteracted and Cr(VI)-interacted NA1 shows that no significant bulk

Table 1. Effect of Cr(III) on Cr(VI) Adsorption on NA1 adsorption capacity (mg g−1)

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initial concentration of Cr(VI) (mg L−1)

initial concentration of Cr(III) (mg L−1)

in the presence of Cr(III)

in the absence of Cr(III)

% reduction in adsorption capacity (%)

5 10 20 50

95 90 80 50

3.5 6.3 30.9 43.1

13.21 22.26 73.21 65.8

73.5 71.7 57.8 34.5

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decreased. A decrease in Cr(III) concentration to 50 mg L−1 increased the Cr(VI) removal by 91%. This variation was proven to be statistically significant by one-way ANOVA. The results show that alumina nanoparticles are capable of binding Cr(III) as well as Cr(VI) nonspecifically. 3.9. Application and Reusability of Adsorbent. The application potential of the alumina nanoparticle for Cr(VI) adsorption from environmental water matrices was analyzed by conducting experiments in water samples collected from three different sources, namely, (1) lake water, VIT University, Vellore, India, (2) domestic waste water, Treatment plant, VIT University, Vellore, India, and (3) ground water, Solur, India. The experiments were conducted only for NA1 for which the adsorption capacity is higher, compared to NA2. The results obtained are given in Table 2. The adsorption capacity

the aggregation of adsorbent. Nanoalumina was able to remove Cr(VI) by interaction through surface functional groups and probable surface reduction to Cr(III). There have been recent concerns on potential toxic effects of alumina nanoparticles in the environment. Our group has demonstrated the growth inhibitory effect of alumina nanoparticles on freshwater algal species (72 h EC50 values, 110.2 mg L−1 for Chlorella sp. and 100.4 mg L−1 for Scenedesmus sp).38 In another study, we reported a mild to moderate growthinhibitory effect of alumina nanoparticles over a wide concentration range (0.010−1 mg L−1) on Escherichia coli.39 However, the bioavailabilityand, therefore, toxicityof alumina nanoparticles entering into the environment from the sorbent system can be minimized. First, the alumina nanoparticles aggregate upon interaction with Cr(VI), thereby reducing the availability in the treated water. This was evident from the SEM images of Cr(VI)-interacted alumina nanoparticles, NA1 and NA2 (see Figures 3c and 3d). Second, the engineered nanoparticles that may enter the environment from the sludge disposal unit interact with natural organic matter and may aggregate further.40 This aggregation and consequent settling of the particles makes them less bioavailable, thereby reducing their toxic effects.40 Furthermore, the TiO2 and NiO nanoparticles are also reported to aggregate in a natural environment upon interaction with algae such as Chlorella vulgaris and Pseudokirchneriella subcapitata, thereby reducing its bioavailability.41,42 Finally, the potential application prospects lie in scaleup of this batch process for large-scale remediation of the contaminated sites. The sorbents based on alumina nanoparticles can be used for the remediation of chromiumcontaminated waters through ex situ treatments (pump and treat process) or impregnation of the nanoparticles on suitable porous supports to develop nanofilters.

Table 2. Cr(VI) Adsorption Capacities Obtained for NA1 in Different Real Water Matrices sample ID LW DWW GW

location lake water, VIT University, Vellore domestic waste water collection plant, VIT University, Vellore ground water, Solur, Vellore

adsorption capacity (mg g−1) 63.41 37.80 36.83

observed for the lake water sample at an initial Cr(VI) concentration of 50 mg L−1 was very similar to that obtained for distilled DI water (65.8 mg g−1). However, for domestic waste water and ground water, a statistically significant decrease in the adsorption capacities was measured. This may be due to the presence of a large amount of inorganic ions that are present in the environmental water matrix interfering with the adsorption.35 The adsorbent was also proved to be capable of adsorbing Cr(III) from a Cr(III)−Cr(VI) mixture solution, reducing Cr(VI) adsorption. Reusability of the adsorbent surface was analyzed by carrying out desorption studies using 0.1 N NaOH. The adsorbent with comparatively high adsorption capacity, NA1 was considered for the analysis. At pH 7.0 and an initial Cr(VI) concentration of 20 mg L−1, the total chromium removed from the solution was determined to be 15.5 mg L−1. With NaOH treatment, the adsorbent system was regenerated by 91%, leaving 0.41 mg g−1 total chromium on the adsorbent. This proves that a large proportion of chromium removed is reversibly bound to the adsorbent system through a Coulombic force of attraction, as described in section 3.7.1. The regeneration capacity of the adsorbent indicates its high efficiency in recycling, which must be studied further.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and additional data regarding adsorption equilibrium, kinetics, and thermodynamics. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91 416 220 2620. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors thank the Department of Science and Technology (DST), Govt. of India for funding this project and also thank the management of VIT University for their support in research.

4. CONCLUSION AND IMPLICATIONS The results from the present study exhibit the application potential of nanoalumina for the removal of chromium from aqueous solutions. The difference in adsorption capacities achieved by nanoalumina of two different sizes was determiend to be statistically significant. This proves that the adsorption capacity can be enhanced by decreasing the particle size further. The smaller the particle size, the greater the spontaneity of the adsorption and lesser the reaction time required. A monolayer of adsorption was observed by Langmuir isotherm, irrespective of the size of the sorbents. The zeta potential study, SEM micrographs, and EDX spectra confirmed the Cr(VI) adsorption by means of oxyanionic binding and, thereafter,

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REFERENCES

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