Environ Earth Sci DOI 10.1007/s12665-012-1569-3

ORIGINAL ARTICLE

Removal of chromium(III) from aqueous solutions by adsorption on bentonite from Gaomiaozi, China Yong-Gui Chen • Yong He • Wei-Min Ye Cui-hua Lin • Xiong-Fei Zhang • Bin Ye

•

Received: 18 March 2011 / Accepted: 18 January 2012 Ó Springer-Verlag 2012

Abstract The adsorption behaviors of Cr(III) from aqueous solution by Gaomiaozi (GMZ) bentonite were studied using equilibrium batch techniques. The effects of shaking time, pH value, adsorbent dose, ionic strength and temperature on adsorption capacity of GMZ bentonite were investigated. The optimum pH value was defined to be 7.0 at temperature 293.15 K. Kinetic and isotherm experiments were carried out at the optimum pH. It was enough to reach the adsorption equilibrium at 2 h and the maximum adsorption capacity was 4.68 mg/g under the given experimental conditions. The equilibrium data were fitted to pseudo-second-order kinetic equation. The Freundlich adsorption isotherm models were conducted for the description of the adsorption process. Keywords Adsorption  Cr(III)  GMZ bentonite  pH  Ionic strength  Isotherms

Y.-G. Chen (&)  W.-M. Ye  B. Ye Department of Geotechnical Engineering and Key Laboratory of Geotechnical and Underground Engineering of Ministry of Education, Tongji University, 1239 Siping Road, Shanghai 200092, People’s Republic of China e-mail: [email protected] Y.-G. Chen  Y. He School of Civil Engineering and Architecture, Changsha University of Science and Technology, 960 Section II, South Wanjiali Road, Changsha 410114, People’s Republic of China C. Lin  X.-F. Zhang School of Chemistry and Biological Engineering, Changsha University of Science and Technology, 960 Section II, South Wanjiali Road, Changsha 410114, People’s Republic of China

Introduction The increase in environmental contamination as a consequence of industrial development is a challenge that the society must face (Wang et al. 2009b). For instance, in the nuclear industry, radioactive waste contains a variety of radionuclide, which will pollute the environment badly if the pollutants are not treated well. Chromium compounds are extensively used in electroplating, textile dyeing, cement production, mining and photography industries (Nityanandi and Subbhuraam 2009), anodizing operations in the surface finishing industry, corrosion control, oxidation (Atia 2008), the leather industry (Tahir and Naseem 2007; Nityanandi and Subbhuraam 2009) and various other industrial applications. The presence of chromium in the environment is detrimental to a variety of living species, and the chromium has a great influence on the health of humans or animals. A high level of chromium in the environment does not only significantly inhibit the growth of animals and plants, but also affect the absorption of nutrients in plants. However, some chromium compounds are carcinogenic for humans (Li 2008). Thus, it is necessary to reduce the maximum permissible concentration of Cr(III) in waste water and drinking water. Finding effective methods or inexpensive materials for retarding the movement of ion contaminants in groundwater or removing contaminants from aqueous solution has attracted great attention (Li 2004). Treatment of liquid waste is needed to convert the waste into a more stable solid form to decrease its volume. Conventional technologies such as precipitation, co-precipitation, ion exchange, ¨ zcan et al. 2009), solvent adsorption (Wang et al. 2009a; O extraction and reverse osmosis (Bhattacharyya and Gupta 2008), electrodialysis, electrochemical reduction (Balkaya and Cesur 2008; Pe´rez-Marı´n et al. 2007), membrane

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technology (Atalay et al. 2009) and evaporation removal (Pe´rez-Marı´n et al. 2007) have been employed to remove metal ions from aqueous solutions. Most of these methods involve high cost and are not suitable for small-scale industries. Among typical methods for waste water treatment, adsorption with solid adsorbents is an effective method because of its high efficiency, low cost, availability, profitability and convenience to use (Bradl 2004; Adebowale et al. 2006; Bhattacharyya and Gupta 2008; Cha´vez et al. 2010). Bentonite, as a physical and chemical barrier for adsorption, has strong cation exchange capacity and strong adsorption capacity (Muhammad 2004; Zhao et al. 2008); it is harmless to the environment, and easy to operate (AlQunaibit et al. 2005). In addition, bentonite presents a major advantage of allowing for low cost recovery processes which is suitable for using in water treatment (Chegrouche et al. 1997). So bentonite was considered as the best sorbent for ¨ ren 2005; Tahir and Naseem heavy metal ions (Kaya and O 2007). The adsorption of heavy metal ions on bentonite has been studied extensively in the last decade (Saleh et al. 2005;

Fig. 1 Open pit of GMZ bentonite deposit

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Lacin et al. 2005; Xu et al. 2008; Hoda et al. 2009). The results show that the adsorption of heavy metal ions on bentonite was affected by some factors such as pH, ionic strength, temperature, time, etc. However, there are still few references focusing on the effect of the pH values and the ionic strength on Cr(III) adsorption on bentonite (Tahir and Naseem 2007; Wang et al. 2011; Khan et al. 1995; Bayrak et al. 2006), and there is a need to be studied further. The Gaomiaozi (GMZ) bentonite has been extracted from the northern Chinese Nei Mongolia autonomous region, 300 km northwest from Beijing. There are 160 million tons with 120 million tons Na-bentonite reserves in the deposit (Fig. 1) and the mine area is about 72 km2. In China, GMZ bentonite has been selected as one of the candidates of buffer/backfill material for the geological disposal of high radioactive waste (Liu and Chen 2001; Yang et al. 2009; Li et al. 2009). GMZ bentonite has attracted great interest in China because of its outstanding properties, such as the prominent properties of high swelling, sealing ability (Qin et al. 2008), cation exchange capacity and strong adsorption capacity (Ye et al. 2009).

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As for the adsorption on GMZ bentonite, some heavy metal ions have been studied before, such as Pb(II) (Wang et al. 2009a, b), Ni(II) (Yang et al. 2009), Cu(II) (Li et al. 2009), and Cd(II) (Chen et al. 2011). However, the adsorption capacity of the trivalent ions, such as Cr(III), on this bentonite, has not been studied in detail. The basic objectives of the present work are: (a) to study the effect of contact time, pH, ionic strength, bentonite content and temperature on the adsorption of Cr(III) on GMZ bentonite; (b) to determine the Cr(III) adsorption isotherms and simulate the experimental data with the Langmuir and Freundlich adsorption models; and (c) to discuss the adsorption mechanism of Cr(III) adsorption on GMZ bentonite.

Experimental Adsorbent The GMZ bentonite sample was obtained from the Gaomiaozi County (Nei Mongolia, China). The used GMZ bentonite presents a grain size of no more than 160 lm as shown in Fig. 2. The mineralogical composition of the GMZ bentonite has been quantitatively analyzed using the X-ray diffraction method (Wen, 2008). The bulk composition was determined in terms of mass as follows: montmorillonite 75.4%, quartz 11.7%, cristobalite 7.3%, feldspar 4.3%, kaolinite 0.8%, calcite 0.5%. It appears that the proportion of montmorillonite is dominant in the GMZ bentonite, with a high smectite content of 75.4%. The bulk chemical components of the sample was analyzed using the X-ray fluorescence spectrometry as follows (in mass): SiO2 67.43%, Al2O3 14.20%, TFe2O3 2.40%, Na2O 1.75%, CaO 1.13%, K2O 0.73%, FeO 0.29%, TiO2 0.12%, MgO 0.10%, P2O5 0.02%, MnO 0.02%. Besides, GMZ bentonite also contains some lanthanon, like La, Ce, Nd, etc.

The adsorption capacity of adsorbent is mainly controlled by its cation exchange capacity (CEC) and specific surface area (SSA). The CEC of GMZ bentonite is 77.3 mmol/100 g and the SSA is 570 m2/g (Wen 2006). Experimental process All experiments were performed under aerobic conditions and with chemicals of analytical purity. The adsorption capacity of Cr(III) on GMZ bentonite was investigated by using batch technique in polyethylene centrifuge tubes sealed with a screw cap under ambient conditions. In all experiments, no attempt was made to exclude air. The stock solutions of GMZ bentonite and NaCl were preequilibrated for 24 h and then the Cr(III) stock solution was added to obtain the desired concentration of the different components. A stock solution of Cr(NO3)3 (100 mg/L) was prepared by dissolving Cr(NO3)3 in double distilled water. The pH values of the system were adjusted by adding negligible volumes of 0.01 or 0.1 M HCl, 0.01 or 0.1 M NaOH to obtain the desired values, which were determined by PHSJ-3F pH meter. The ionic strengths were adjusted with 0.1 or 1 M NaCl solution to the desired values. The experimental conditions such as contact time, pH, ionic strength, bentonite content and Cr(III) concentration are selected on the basis of previous work related to the adsorption of some heavy metal ions on GMZ bentonite (Yang et al. 2009; Li et al. 2009; Wang et al. 2009b) and the adsorption of Cr(III) on other betonite (Tahir and Naseem 2007; Wang et al. 2011). After the suspensions were shaken for 24 h, the solid and the liquid phases were separated by centrifugation at 400 rpm for 40 min. The concentrations of Cr(III) solution before and after adsorption were measured by using an ultraviolet–visible (UV–VIS) spectrophotometer, WFJ2100, at a wavelength of 540 nm. The working standards for calibration of spectrophotometer were prepared by diluting stock solution of 1,000 mg/L Cr(III). Fresh calibration was made before the analysis of each batch of sample for Cr(III) determination. The adsorbed amount of Cr(III) on GMZ bentonite was calculated from the difference between the initial concentration and the equilibrium one: qe ¼

ðC0  Ce ÞV m

ð1Þ

The percent adsorption of Cr(III) on GMZ bentonite was estimated by employing the following expression:   C0  Ce % removal ¼  100 ð2Þ C0 Fig. 2 Grain size distribution of GMZ bentonite (Qian 2007)

where qe (mg/g) is the amount concentration of Cr(III) on GMZ bentonite, C0 (mg/L) is the initial concentration of

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Cr(III) in suspension, Ce (mg/L) is the concentration of Cr(III) in supernatant after centrifugation, m (g) is the mass of the adsorbent, and V (L) is the volume of the Cr(III) solution.

Results and discussion Effect of shaking time on adsorption The shaking time is an important factor in determining the equilibration rate for further study (Wang et al. 2011). In this work, the effect of equilibration time on the removal of Cr(III) was carried out in the range of 1–6 h. The concentration of Cr(III) was kept as 100 mg/L while the amount of GMZ bentonite added was 1.0 g/(50 mL) and pH was 7.0 at room temperature (293.15 K). The results are shown in Fig. 3. As shown in Fig. 3a, during the first 120 min of the experiment, the amount of Cr(III) adsorbed on GMZ bentonite increases with time; from 120 to 240 min, there is no significant change of Cr(III) adsorbed; after 240 min of shaking time, the amount of Cr(III) adsorbed on GMZ bentonite maintains a high level which increases with shaking time. The results show that the adsorption of Cr(III) on GMZ bentonite occurs quickly and 120 min is enough to achieve adsorption equilibrium. Similarly, the adsorption of bivalent ions such as Cu(II) and Pb(II) on GMZ bentonite also occurs quickly and 240 min shaking time was found to be appropriate for maximum adsorption (Li et al. 2009; Wang et al. 2009a, b). The literature shows that Cr(III) is rapidly and specifically adsorbed by clay minerals, with about 90% of bentonite being adsorbed within 24 h (Bradl 2004). Thus, a shaking time of 120 min

Fig. 3 a Effect of shaking time on Cr(III) adsorption on GMZ bentonite. b The pseudo-second-order equation of t/qe versus t (C0 = 100 mg/L, pH 7.0, m/v = 20 g/L, I = 0.01 M NaCl, T = 293.15 K)

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was used for all further studies to ensure the equilibrium of Cr(III) adsorption to GMZ bentonite. The fast adsorption of these heavy metals on bentonite suggests that the uptake of those heavy metals from solution to bentonite is mainly dominated by chemical adsorption rather than physical adsorption (Li et al. 2009). In this work, the amount of Cr(III) adsorbed on GMZ bentonite is about 4.68 mg/g under the experimental conditions. The pseudo-second-order equation is often successfully used to describe the kinetics of the adsorption process. Its expression is as follows: dqe ¼ k2 ðqe  qt Þ2 dt

ð3Þ

where k2 (g mg-1 min-1) is the second-order rate constant and qt (mg/g) denotes the amount of Cr(III) adsorbed at time t (min). This equation can be integrated for the boundary conditions t = 0 (qe = 0) to (qt = qe) and then liberalized, it leads to Eq. (4) (Ho and Mckay 1998): t 1 t ¼ þ 2 qt 2k2 qe qe

ð4Þ

By plotting t/qt versus t, a straight line is obtained with the intercept of 1/2k2q2e and the slope of 1/qe (Fig. 3b). The values of k2 and qe for Cr(III) and Cd(II) are given in Table 1. The high correlation coefficient indicates that the applicability of the pseudo-second-order rate model to describe the adsorption process. Effect of pH on adsorption Suspension pH is of prime importance for efficient retention of the trace elements on the sorbent (Esposito et al. 2002). Its influence strongly depends on the nature of the adsorbent used. Thus careful optimization of this parameter is crucial to ensure quantitative retention of the trace elements and in some cases selective retention (Wang et al. 2011). The effect of pH in the range of 4–11 was studied by keeping the Cr(III) concentration, volume, shaking time and amount of bentonite as 100 mg/L, 50 mL, 2 h and 1.0 g, respectively. The results are shown in Fig. 4. Figure 4 shows that the adsorption capacity increases from about 0.5 to 4.7 mg/g as pH increases from 2.0 to 7.0. Above pH 7.0, its effect on the adsorption becomes insignificant and reaches a plateau. The pH effect on the Cr(III) adsorption is not only the same as that on the bivalent metal ions [such as Pb(II), Ni(II), Cu(II) and Cd(II)] adsorption by the GMZ bentonite (Wang et al. 2009a, b; Yang et al. 2009; Li et al. 2009; Chen et al. 2011), but also the same as that on the chromium adsorption by clay minerals or zeolite (Tahir and Naseem 2007; Li 2004). These results indicate that the adsorption of Cr(III) is the consequence of complexation of Cr(III) with

Environ Earth Sci Table 1 Parameters of the pseudo-second-order for the adsorption of Cr(III) and Cd (II) on GMZ bentonite

Metals

pH

Experimental qe (mg/g)

Second-order kinetic k2 (g mg min )

qe (mg/g)

R

-1

Reference 2

Cr(III)

7.0

4.64

0.05

4.71

0.999

This work

Cd(II)

6.0

3.16

0.0516

3.21

0.999

Chen et al. (2011)

Fig. 4 The effect of pH on Cr(III) adsorption on GMZ bentonite (C0 = 100 mg/L, m/v = 20 g/L, I = 0.01 M NaCl, T = 293.15 K)

surface functional groups (Chen et al. 1997). In clay– aqueous systems the potential of the surface is determined by the activity of ions (H? or pH) which react with the mineral surface. For clay minerals the potential determining ions are H? and OH- and complex ions formed by bonding with H? and OH- (Tahir and Naseem 2007). According to the experimental results, the strong pHdependent adsorption indicates that adsorption is dominated by surface complexation according to the surface complexation model (Chen and Wang 2007). In addition, the adsorption of Cr(III) can also be attributed to ion exchange between Cr(III) and H?/Na? on the ion exchange sites at low pH values. At pH [7.0, the Cr(III) will disappear in solution by precipitation. The amount of Cr(III) adsorbed on GMZ bentonite maintains a high level and reaches the maximum. This can be explained in terms of pHpzc of bentonite. The value of pHpzc of bentonite is about pH 6.3 ± 0.1 (Wanget al. 2009b). Similar results of Cu(II) adsorption on GMZ bentonite were also reported (Li et al. 2009). Effect of adsorbent dose on adsorption The dependence of Cr(III) adsorption on GMZ bentonite dose was studied by varying the amount of adsorbent from 0.4 to 2.0 g while keeping all other variables (i.e. pH value, shaking time and concentration) constant. The results are presented in Fig. 5, which indicates that the removal of

Fig. 5 The effect of adsorbent dose Cr(III) adsorption on GMZ 7.0, I = 0.01 M NaCl, bentonite (C0 = 100 mg/L, pH T = 293.15 K)

Cr(III) from solution to GMZ bentonite increases with increasing bentonite content. The removal of Cr(III) from aqueous solution changes from about 79% to 96% when the amount of bentonite added changes from 0.4 to 2.0 g in 50 mL solution. With an increasing adsorbent dose, more surface sites are available to bind Cr(III) at GMZ bentonite surfaces. Similar conclusions have been drawn by Li et al. (2009) for Cu(II) adsorption, Yang et al. (2009) for Ni(II) adsorption and Chen et al. (2011) for Cd(II) adsorption on GMZ bentonite. This can be attributed to the fact that the higher content of bentonite in the solution results in greater availability of exchangeable sites for the ions (Vijaya et al. 2008). Thereby, more Cr(III) ions are adsorbed on GMZ bentonite at a high solid content. On the other hand, with an increasing solid content, the extent of adsorption with every gram of bentonite decreases. Similar results have been found for Pb(II) on MX-80 bentonite (Xu et al. 2008) and for Cd(II) on GMZ bentonite (Chen et al. 2011), that is, an increase in bentonite content reduces the amount of metal recovered per dry weight unit of solid soil used. These results suggest that high bentonite content is not necessary to produce a high adsorption yield. Thereby, the adsorbent dose is suggested to be 20 g/L for the adsorption of Cr(III) on GMZ bentonite. Effect of ionic strength on adsorption Experiments for the evaluation of the influence of ionic strength on the Cr(III) adsorption were carried out with

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Fig. 6 The effect of ionic strength on Cr(III) adsorption on GMZ bentonite (C0 = 100 mg/L, m/V = 20 g/L, T = 293.15 K)

Fig. 7 The effect of temperature on Cr(III) adsorption on GMZ bentonite (C0 = 100 mg/L, m/V = 20 g/L, pH = 7.0)

 DH ¼ R

three different concentrations i.e. 0.01, 0.1 and 1 M NaCl solutions at pH of 4–11. As can be seen from Fig. 6 that the adsorption of Cr(III) on GMZ bentonite is obviously affected by ionic strength at pH \7.0, and independent of ionic strength at pH [7.0. From the ionic strength dependence, one can deduce that ion exchange is the main mechanism for Cr(III) adsorption on GMZ bentonite at pH \7.0, which is also in agreement with the pHpzc of GMZ bentonite. At pH 7.0, adsorption is via ion exchange with hydrogen and sodium ions that saturate the exchange sites (Mohapatra and Gupta 2005). The fact that the amount of Cr(III) adsorbed on GMZ bentonite is higher in 0.01 M NaCl than that of Cr(III) in 1 M NaCl, also supports this hypothesis. From the results it is clear that the presence of the competing cations of the salt will reduce the adsorption. So the ionic strength dependent adsorption indicates that cation exchange partly contributes to the adsorption (Baeyens and Bradbury 1997; Xu et al. 2007). Generally, cation exchange is influenced by ionic strength whereas surface complexation is affected by pH values obviously (Zhao et al. 2008). Effect of temperature on adsorption The effect of temperature on the adsorption of Cr(III) to GMZ bentonite is included in Fig. 7. It can be found that the adsorption capacity of Cr(III) increases, as the temperature in solution increases, suggesting that the adsorption process is endothermic, and this result is the same as the result of Cr(III) adsorption on another bentonite clay reported by Tahir and Naseem (2007). According to Clausius–Clapeyron equation (Young and Crowell 1962):

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dðln Ce Þ dð1=TÞ

 ð5Þ

where R is molar gas constant (8.314 J/mol K) and T (in K) is the absolute temperature. By intergrating Eqs. (5) and (6) can be obtained as follows: log Ce ¼

DH 1  þb 2:303R T

ð6Þ

where b is a constant. Plotting log Ce against 1/T gives a straight line with slope equal to DH/2.303R. The enthalpy of adsorption is calculated to be 2.539 kJ/mol. This type of behavior (positive values of DH) has also been observed in some earlier investigations for the adsorption of Cr(III) onto bentonite (Tahir and Naseem 2007). The positive value of DH confirms the endothermic nature of the adsorption process. Adsorption isotherms The effect of Cr(III) concentration on its adsorption was studied under the optimized conditions of shaking time (2 h), pH (7.0), volume of aqueous solution (50 mL) and amount of adsorbent (1.0 g). The concentration of Cr(III) varied from 25 to 200 mg/L after proper dilution of the Cr(III) solution. Figure 8 shows these results for Cr(III) and these results were analyzed in terms of Freundlich and Langmuir isotherms. The equilibrium data for Cr(III) over a concentration range from 25 to 200 mg/L have been correlated with the Freundlich isotherm. The following form of the Freundlich equation was used for this purpose: qe ¼ kF Ce1=n or, in linear form:

ð7Þ

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affinity towards the adsorbed cations, and the magnitude of constant 1/n is an indicator of linearity of adsorption (Xu et al. 2008). The large value of kF indicates that bentonite has a high adsorption affinity towards Cr(III). The value of 0.1 \ 1/n \ 1 shows favorable adsorption of cadmium ions onto adsorbent (Tajar et al. 2009). Many studies have already been reported in the literature that the adsorption data for bivalent ions including Cd(II), Pb(II), Cu(II), and Ni(II) on GMZ bentonite fit with the Langmuir isotherm very well (Li et al. 2009; Wang et al. 2009a; Yang et al. 2009). But in this work, the adsorption data for Cr(III) in the concentration range used did not fit in the Langmuir equation. These results are similar to those previously reported for the adsorption of Cr(III) on a Pakistan bentonite (Khan et al. 1995). Fig. 8 Adsorption isotherm of Cr(III) on GMZ bentonite. (pH = 7.0, m/v = 20 g/L, I = 0.01 M NaCl, T = 293.15 K)

Conclusions Bentonite, due to its physical and chemical properties, has been considered one of the most promising candidates for use in decontamination and disposal of high-level heavy metal wastes. In the present study, GMZ bentonite has been selected as the candidate of buffer/backfill material for Chinese nuclear waste repository. The adsorption of Cr(III) on this bentonite was studied by batch technique conducted under various experimental conditions such as shaking time, pH, ionic strength, adsorbent dose and temperature. The equilibrium batch experiment data indicate that GMZ bentonite is a suitable material for adsorption, with the maximum uptake capacity of 4.68 mg/g for removal of Cr(III) from aqueous solution under the given experimental conditions. The adsorption achieves the equilibration rapidly and it is strongly dependent on pH and ionic strength. The adsorption process is pseudo-second-order reaction following the Freundlich isotherm adsorption.

Fig. 9 Freundlich adsorption isotherm of Cr(III) on the GMZ bentonite (pH = 7.0, m/v = 20 g/L, I = 0.01 M NaCl, T = 293.15 K)

log qe ¼ log kF þ 1=n log Ce

ð8Þ

where qe is the amount of solute adsorbed per unit weight of adsorbent after equilibrium (mg/g), Ce is the equilibrium concentration of metal ions remained in the bulk solution (mg/L), kF is the constant indicative of the relative adsorption capacity of the adsorbent (mg1-1/n L1/n/g) and 1/n is the constant indicative of the intensity of the adsorption. A plot of log qe againsts log Ce gives a straight line, the slope and the intercept of which correspond to 1/n and log kF, respectively. The linear plot of log qe versus log Ce with R2 = 0.912 is shown in Fig. 9. The magnitude of the constant kF provides quantitative information on the relative adsorption

Acknowledgements Financial supports from Natural Science Foundation of China (No.40802064, 41030748), Innovation Program of Shanghai Municipal Education Commission (12ZZ032), Shanghai Leading Academic Discipline Project (No.B308), Scientific Research Fund of Hunan Provincial Education Department (11A010) and Kwang-Hua Fund for College of Civil Engineering, Tongji University are acknowledged.

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