International Journal of Environmental Research University of Tehran ISSN: 1735-6865 EISSN: 2008-2304 Vol. 4, Num. 3, 2010, pp. 491-500

International Journal of Environmental Research, Vol. 4, No. 3, July-September, 2010, pp. 491-500

Article

Removal of hexavalent chromium (VI) from aqueous solutions using surface modified nanozeolite A

Tashauoei , H . R . ^1* , Movahedian Attar , H . ¹ , Kamali , M . ² , Amin, M . M . ¹ , and Nikaeen, M . ¹

¹ Department of Environmental Health Engineering, School of Public Health, Isfahan University of Medical Sciences, Isfahan 14155-6359, Iran
² Faculty of Advanced Sciences and Technologies, University of Isfahan, Isfahan 81746-73441, Iran

Correspondence Address: *Department of Environmental Health Engineering, School of Public Health, Isfahan University of Medical Sciences, Isfahan 14155-6359, Iran
tashauoei@hlth.mui.ac.ir

Date of Submission: 20-Feb-2010
Date of Decision: 10-Mar-2010
Date of Acceptance: 25-Mar-2010

Code Number: er10054

Abstract

Keywords: Adsorption isotherm, Birjand , Chromate, Modification, Nanozeolite A

Introduction

Increasing levels of heavy metals in the environ-ment pose serious threats to water quality, human health and living organisms (An et al. 2001; Wingenfelder et al. 2005; Vinodhini and Narayanan, 2008). Cr, Ni, Zn, Cu and Cd are considered as priority metals from the point of view of potential health haz-ards to human. Hexavalent chromium have high toxic-ity for humans and animals (McBride 1994; Ayuso et al. 2003; Babel and Opiso, 2007) and commonly inter-fere with beneficial use of effluents for irrigation and industrial applications. They are also the groundwater contaminants at industrial installations (Mier et al. 2001; Malakootian et al. 2009). In epidemiological studies, an association has been found between exposure to Cr(VI) by the inhalation and lung cancer. IARC has classified chromium (VI) in Group 1 (human carcinogen) (WHO 2004). Some cities in Iran have high amount of hexavalent chromium contents in ground water re-sources. Birjand, which is located in Southern Khorasan Province at the Eastern region of Iran, is one such ex-ample [Figure - 1]. Ground water resources in this region are contaminated with hexavalent chromium due to geological structure. The amount of this ion is in the range of 0.04 to 0.08 mg/L in autumn and 0.07-0.09 mg/ L in winter, which is higher than WHO guidelines (0.05 mg/L) (WHO 2004).

Conventional method for industrial effluent treatment is physicochemical treatment including ion exchange, vacuum evaporation, solvent extraction and membrane technologies (Applegate 1984; Sengupta and Clifford 1986; Kentish and Stevens 2001). Among these, ion exchange is one of the most effective and economical methods (Tran et al. 1999; Nameni et al. 2008). Use of various sorbents such as bentonite (Zvinowanda et al. 2009), chitosan (Jha et al. 1988)), perlite (Mathialagan and Viraraghavan 2002), coal (Karabulat et al. 2000), and activated carbon (Fan and Anderson 2005; Gueu et al., 2007) have been reported for the removal of heavy metals including chromate from aqueous solutions. Among the different minerals with sorbent properties, zeolites are one of the most promising sorbents. The advantages of zeolites over resins are much lower cost and ion selectivity. Zeolites have many applications such as molecular sieves and water softeners (Breck 1974; Sand and Mumpton 1978; Kallo and Sherry 1988). They appear also as suitable sorbents for heavy metals; hence several investigations on sorbent behavior of natural (Mellah and Chegrouche 1997; Inglezakis et al. 2003; Dal Bosco et al. 2005; Chutia et al. 2009) and synthetic zeolites (Hertzenberg and Sherry 1980; Singh et al. 2000; Rios et al. 2008) is implemented.

Zeolite A has crystalline cubic lattice structure and is usually produced in sodium form. This zeolite is one of the most important molecular sieves, and has a good cation exchange capacity and can easily exchange sodium ion with cations (Breck 1974; Kamali et al. 2009). Nanocrystalline zeolites have unique properties in comparison to conventional micrometer-sized zeolite crystals. These materials have been reduced in size to just a few unit cells, this gives significant change in properties. The expected effects due to crystals' size reduction are: efficiency enhancement of very rapid reactions, and increasing the external surface (Zhan et al. 2002; Mintova et al. 2003).

Recently, interests on the adsorption of anions and neutral molecules by surface-modified zeolites is increased (Zhang et al. 2002). Zeolites in their original form could adsorb only cations, but some heavy metals such as chromium and arsenic are present in cations, anions, non-ionized species (Sengupta 2002). Hence, development of materials with capability to adsorbanions, cations and neutral molecules is of high significance. Zeolites could be treated using a cationic surfactant in order to do this. By treating, an organic layer is created on the external surfaces and the charge is reversed to positive (Li et al. 1998). Generally, HDTMA is applied for zeolite treatment, which is a quaternary amine with a long chain cationic surfactant (Nizam Nik Malek 2007).

In view of the environmental significance, the chromate adsorption onto surface modified nanozeolite A has been investigated under different experimental conditions, i.e. pH: 3, 5, 7, and 8, solid: liquid (S:L) ratio: 0.5, 2, 3.5, and 5 g/L, adsorbent concentration: of 0.1, 1.0, 10.0 and 100.0 mg/L, and interaction time: 30, 60, 90, and 120 min. The objective of this study was to gain an insight into the physico-chemical characteristics of Cr (VI) adsorption on nanozeolite, in order to evaluate the optimal experimental conditions to be used in water treatment.

Materials and Methods

Morphology and particle size of the nanozeolite samples were examined using a Zeiss (LEO) 1455 VP scanning electron microscope (SEM) and a Philips CM10 transmission electron microscope (TEM). A number of microscope images were obtained for the unmodified and modified nanozeolite. X-ray diffraction was performed on Bruker D8 Adavance X-radiation (CuK _a 1.5806 A). Chromate analyses were carried out using the ICP-Integra XL-GBC (Australia). A Metrohm pH meter model 525A was used for pH measurements.

The centrifugation was done with a Sigma 3K30 at 20,000 rpm.

Potassium chromate (K ₂ CrO ₄ ) was obtained as a commercial technical grade solid from Sigma-Aldrich. HCl and NaOH were purchased by Merck as analytical grade reagents. Doubly distilled water was used.

Nanozeolite A was supplied by the Fadak Group, Isfahan Science and Technology Town, I.R. Iran. The chemical composition of the this sample was SiO ₂ 35.58%, _Al2O3 28.07%, Na ₂ O 17.21%, SO ₃ 1.08%, LOI 17.53% (Breck 1974; Kamali, Vaezifar et al. 2009). [Figure - 2] displays the XRD patterns and [Figure - 3] and [Figure - 4] show SEM and TEM images of non-modified and modified nanozeolite A. Based on these figures, the particle size was in the range of 50-100 nm. Total cation exchange capacity (CEC) and the external CEC (ECEC) were measured according to Haggerty and Bowman method (Haggerty and Bowman 1994) and were found to be equivalent to 2.65 meq. of Na ⁺ /g (CEC) and 0.35 meq. of Na+/g zeolite (ECEC).

A homoionic material is required for examination, so 200 milliliters of 0.1M NaCl solution was placed in reflux with 10 g of nanozeolite at 85 ^o C for 18 hours. Thereafter the solid and liquid phase was separated by centrifugation at 20000 rpm for 5 minutes. In order to obtain a chlorine free sample, the solids phase was washed triple and silver nitrate solution was used as a Cl ^- indicator. After third sample washing, the solids phase was dried at 110 ^o C for 18 hours (Ming and Dixon 1987; Diaz-Nava et al. 2005).

Modification of nanozeolite A was carried out using HDTMA-Br. In this stage, 10 g of raw nanozeolite was added to 100 mL of HDTMA-Br solution and shake for 24 h at 60 ^o C. Preliminary experiments showed that a 24h shaking time is sufficient for equilibrium attaining and also HDTMA does not sorb labwares. Liquid and solids phases were separated using centrifugation at 20,000 rpm for 5 min. Resultant materials was washed by two portions of 100 mL deionized water, shaken at 150 rpm for 15 min and centrifuged at 20,000 rpm for 5 min. The pH of the final solutions was 7.0 ± 0.2. After the final washing, the sample was dried at 110 ^o C for 24 hours. The HDTMA loading on the zeolite was 100% of CEC, which is the optimum for contaminant adsorption by SMZ (Li and Bowman 1997).

Stock chromate solutions with 1000 mg/L concentration were prepared by dissolving 0.3773 g of K ₂ CrO ₄ in 1 L of doubly distilled water. Cr(VI) adsorption by surface modified nanozeolite A was performed by a batch technique at room temperature. 0.0250, 0.1000, 0.1750 and 0.2500 g of nanozeolite was shaken with 50.0 mL of 0.1, 1.0, 10.0 and 100.0 mg/L of Cr(VI) solutions for 30, 60, 90, and 120 min at different pHs (3.0, 5.0, 6.0, and 7.0). All of the solution was then centrifuged at 20,000 rpm for 5 min and Cr(VI) in supernatant was determined by ICP. In order to maintain the pH of the medium, 0.01 N NaOH or 0.01 N HNO ₃ was added drop-wise before carrying out adsorption and the pH was monitored both before and after adsorption.

The adsorption amount at equilibrium, _qe (mg/g) was calculated by mass balance equation (Doulia et al. 2009):

where C _o and C _e are the initial and equilibrium chromate concentration (mg/L), V is the solution volume (mL), and m is the adsor-bent weight (g). The adsorption equilibrium data was analyzed in terms of the Freundlich and Langmuir isotherm models (Glasstone 1981). The Langmuir isotherm equation could be written as:

where q _e is the equilibrium concentration on adsorbent (mg/g), C _e is equilibrium concentration in solution (mg/L), Q _m is maximum adsorption capacity (mg/g), and KL is adsorption equilibrium constant (mg/L). The Langmuir isotherm is based on the assumption that the forces of interaction between adsorbed molecules are negligible and once a molecule occupies a site no further adsorption takes place. Also, the logarithmic form of Freundlich equation may be written as:

where q _e is the equilibrium concentration on adsorbent (mg/g), C _e is equilibrium concentration in solution (mg/L), K _F is adsorption capacity, and n is reaction energy. The Freundlich equation could be described by assuming a heterogeneous surface with adsorption on each class of sites. Although this expression is empirical, 1/n reflects the curvature in the isotherm and may represent the energy distribution of adsorption sites. The linearized form of Freundlich adsorption isotherm is:

By plotting ln q _e versus ln C _e , K _F and n can be determined if a straight line is obtained.

Experiments on adsorption isotherm were performed at 25 ^o C and the effect of nanozeolite dosage (S:L ratio), adsorbate concentration, pH, and interaction time on the chromate adsorption was examine. Taguchi method was applied in order to determine the optimum conditions of examinations (using QUALITEK-4 software). This method uses a combination of parameters to determine the optimum conditions. The optimum conditions was determined based on four factors at four levels [Table - 1]. The designed examinations using QUALITEK-4 along with results are presented in [Table - 2].

Results and Discussion

A batch method was applied for sorption experiments based on parameters determined by Taghuci method [Table - 1]. Based on the results, the estimated maximum chromate amount taken up by the nanozeolite was 82% [Table - 2]. The results were analyzed using Taguchi and statistical methods and results showed that the optimum conditions for Cr(VI) removal is pH of 3, 1 mg/L adsorbate concentration, S:L of 5 and 60 min interaction time. Results showed a decrease in Cr (VI) adsorption by increasing pH. Dependence of heavy metal adsorption on medium's pH is known (Panuccio et al. 2009) and it can be observed that the best chromate adsorption is at pH 3 (Mier, Callejas et al. 2001; Bajda and Klapyta 2006; Yusof and Malek 2009). Zeolites influence the solution's acidity, tending to neutralize the solutions by exchange of H ⁺ with the cations initially present in their structures. The competitive exchange of H ⁺ is considered to be the reason of lower retention of metals in more acidic environment. In addition, a decrease in initial pH decreases the probability of metal hydroxide precipitation because it also decreases the final pH resulting from the H ⁺ exchange on zeolite, which explains the higher adsorption decrease shown for chromium. Zeolites, in general, are weakly acidic in nature and therefore sodium form exchangers are selective for hydrogen (R - Na +H ₂ O RH + Na ⁺ + OH^- , and this leads to high pH values when the exchanger is equilibrated with a relatively dilute electrolyte solution (Leinonen and Lehto 2001) making the metal hydroxide precipitation feasible. The extent of precipitation is clearly affected by the acidities of zeolites since they affect the bulk solution pH (Ayuso, Sanchez et al. 2003). As for the amount of zeolite, the highest chromate adsorption per unit mass of nanozeolite is the ratio of 5. This may be attributed to this fact that large adsorbent amount increases the adsorption sites. At low metal ion loading, the ratio of the number of metal ions to the number of available adsorption sites is small and conse-quently, adsorption is independent of initial concentration, but as the concentration of metal ions increases, the situation changes and the competition for adsorption sites becomes fierce.

Effects of pH. Chromate adsorption at different equilibrium solution pH values was studied. Adsorption of chromate on surface modified nanozeolite A showed a decrease with pH increasing from 3 to 8 [Figure - 5].

Effect of solid to liquid S:L ratio on chromate adsorption by surface modified nanozeolite A is illustrated in [Figure - 6]. Results indicate that the highest chromate adsorption per unit mass of nanozeolite occurred at S:L ratio of 5.

Chromate interacted with nanozeolite rapidly and the maximum uptake was observed within 30 minutes [Figure - 7]. Results showed that the increase of interaction time could lead to the decrease of removal efficiency.

Results showed that the best Cr(VI) adsorption occurs at 10 mg/L and then slows down significantly to reach the lowest adsorption rate at 30 mg/L [Figure - 8].

The presence of competitive ions such as sulfates can affect on the adsorption of Cr(VI) (Bowman 1996). The effect of this competition was investigated by adding 100, 200, 500, 1000, 2000 and 10000 mg/L of sulfate ion to a solution containing 100 mg/L of Cr(VI). Results showed that there is no significant competition up to 1000 mg/L of sulfate (up to 99% chromate removal), but the removal efficiency decreases slightly by increasing the competitive ion to 10000 mg/L (decreasing of removal efficiency to 93%) [Figure - 9]. Based on results chromate adsorption could be affected slightly only at very high sulfate concentrations (10000 mg/L) which is several times higher than the contents of natural waters.

Adsorption was determined at 25 ^o C and the adsorption isotherms are shown in [Figure - 10]. According to this figure, it is being shown that data is better fitted by Langmuir isotherms. The linear forms of these isotherms are plotted in [Figure - 11] and [Figure - 12] to calculate respective coefficients. The corresponding Langmuir and Freundlich parameters along with correlation coefficients are given in [Table - 3].

Several investigations have been conducted to obtain the adsorption capacity of chromate on zeolite. Ayuso et al. (Ayuso et al., 2003) found that the maximum Cr (VI) adsorption capacities by synthetic zeolite is 14.16 mg/ g. Ghiaci et al. (Ghiaci et al., 2004) found that the maximum capacity of clinoptilo-lite and ZSM-5 is in the range of 0.8-3.2 and 0.7-2.7 mg/ g, respectively. Other researchers found a 7.3 mg g-1 (Yusof and Malek, 2009) and 2.92 mg/ g (Misaelides et al., 2008).

Results of examinations were analyzed using Pearson correlation test to determine the effects of each experimental parameter on the removal efficiency. Pearson correlation test showed that:

- there is significant reverse correlation between chromate removal and pH (P _value =0.05, R=-0.415)

- there is no significant correlation between chromate removal and S:L ratio (P _value =0.302)

- there is significant correlation between chromate removal and chromate concentration (P _value =0.244)

- there is no significant correlation between chromate removal and interaction time (P _value =0.471) Multiple regression tests were applied in order to remove interaction effects of other factors in correlation between each factor and the removal efficiency. The results confirmed Pearson correlation test and showed that there is no removal disarray effect.

In order to evaluate the efficiency of surface modified nanozeolite A in removing Cr(VI) from natural polluted waters, an examinations were carried out on a samples from one of the most polluted Birjand's water wells. Based on surveys, the highest chromate amount in Iranian polluted waters is about 100 μg/L. The average amount of this ion during sampling from Birjand's water wells was about 85 μg/L, and examinations were carried out twice under optimum conditions based on results from Taguchi designed examination (pH = 3, adsorbate concentration = 1 mg/L, S:L = 5 and 60 min of interaction time). Results showed surface modified nanozeolite A could reduce Cr(VI) concentration to 46.7 μg/L (45 % removal efficiency), which is lower than the WHO guidelines.

Conclusion

The purpose of this work was to study the feasibility of chromate removal from aqueous solutions by adsorption onto surface modified nanozeolite A. It could be assumed that modifying zeolite could enable nanozeolite to adsorb chromate and lead to considerable decrease in its con-centration. The data reported here showed that surface modified nanozeolite A is an effective sorbent for removing chromate from aquatic solution. The adsorption capacity of nanozeolite was higher than the reported value for zeolites and the adsorption ability waqs enhanced by surface modification, which enables zeolite to adsorb Cr(VI). Equilibrium studies were conducted for the adsorp-tion of Cr(VI) from aqueous solutions onto surface modified nanozeolite A. pH was the most important factor in Cr(VI) adsorption, and other factors had little effect. The equilibrium data have been analyzed using Langmuir and Freundlich isotherm models and the results revealed that Langmuir model is in good agreement with experimental re-sults. Results showed that surface modified nanozeolite A is an efficient tool in removing Cr (VI) from natural water containing Cr(VI) ion in concentrations lower than 90 μg/L under optimum conditions. Also the results showed no significant competition between chromate adsorption and chromate and chromate ions.

Acknowledgements

Authors gratefully acknowledge the generous support provided by Isfahan University of Medical Sciences (No. 5141600); Fadak Group and Nanotek Spadana for nanozeolite A and Mr. M.J. Hesami, Faculty of Advanced Sciences and Technologies, University of Isfahan and A.R. Shahnoush, Department of Chemistry, University of Isfahan; Mr. F. Karakani, Iranian Elites Foundation and Mrs. Vahid Dastjerdi for their kind cooperation.^[48]

References

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