Potential of Agave lechuguilla biomass for Cr(III) removal from aqueous solutions: Thermodynamic studies

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1 Bioresource Technology 97 (2006) Potential of Agave lechuguilla biomass for Cr(III) removal from aqueous solutions: Thermodynamic studies J. Romero-González a, J.R. Peralta-Videa b, E. Rodríguez a, M. Delgado a, J.L. Gardea-Torresdey a,b, * a Environmental Science and Engineering, University of Texas at El Paso, El Paso, TX 79968, United States b Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968, United States Received 7 July 2004; received in revised form 2 January 2005; accepted 2 January 2005 Available online 29 March 2005 Abstract Thermodynamic studies on the bioadsorption of Cr(III) onto Agave lechuguilla biomass were conduced. The experimental results at different temperatures were modeled using the Langmuir and Freundlich isotherms to obtain the characteristic parameters of each model. Both the Freundlich and Langmuir models were found to represent the bioadsorption process. The average adsorption capacities calculated from Freundlich (4.7 mg/g) and Langmuir (14.2 mg/g) isotherms showed A. lechuguilla to be an effective biomass in the removal of Cr(III) from an aqueous solution. Thermodynamic parameters (DG 0, DH 0 and DS 0 ) determined in the temperature range from 10 to 40 C along with the parameters of the Dubinin Radushkevick equation support the idea that the binding of Cr(III) may be caused by interactions with functional groups such as carboxyl groups located on the outer surface of the cell tissue of the bioadsorbent. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Chromium (III); Thermodynamic parameters; Adsorption; Agave lechuguilla 1. Introduction Industrial wastewater effluents from metal-fishing and mining-metallurgical sectors often contain high levels of heavy metal concentrations and thus create serious environmental pollution hazards. Trivalent chromium is an important pollutant introduced into natural waters by a variety of industrial wastewaters including those from the textile, leather tanning, electroplating, and metal finishing industries (Gaballah and Kilbertus, 1998). Current technologies to remove trivalent chromium such as precipitation and ionic exchange with synthetic resins, not only incur operational costs but also create * Corresponding author. Address: Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968, United States. Tel.: ; fax: address: jgardea@utep.edu (J.L. Gardea-Torresdey). sludge disposal problems (Volesky, 2001). These processes also fail to meet the requirements of legislation for its discharge which are from 0.1 to 3 mg/l of metal concentration (World Health Organization, 1988). The removal of heavy metals based on sorption on nonliving biomass surfaces has been suggested as a low-cost substitute for the treatment of wasters containing heavy metals (Bailey et al., 1998). Agave lechuguilla, or lechuguilla, which is one of the most characteristic plants of the Chihuahuan Desert, represents a prospective biomass to be used in the removal of chromium. LechuguillaÕs central bud is an excellent source of hard fibers, known as ixtle, which are used in making rope, sacks, mats, and brushes. Recent research shows that lechuguilla fibers are comparable to glass fibers for their capacity to carry polishing components. In addition, the ixtle is extremely strong and durable, and is resistant to the effect of many /$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi: /j.biortech

2 J. Romero-González et al. / Bioresource Technology 97 (2006) chemical solutions and solvents (Gentry, 1982). Lechuguilla contains two steroidal saponins yuccagenin and ruizgenin (Blunden et al., 1980) which can be used as natural chelating agents to remove heavy metals such as Cr, Cd, Cu, Pb and Zn from soil and waste (Hong et al., 2000, 2002). These characteristics make lechuguilla a potential biomaterial for the removal of chromium from wastewater and to our knowledge no data has appeared in the literature. In the present work, thermodynamic parameters and isotherms models for adsorption of Cr(III) onto lechuguilla biomass have been investigated and reported herein. Lechuguilla samples were collected from mountains surrounding El Paso, Texas. The plants were washed thoroughly using tap water in order to remove any soil or debris. Only the leaves of the plants were utilized in this study because they represent more than 90% of lechuguilla plants. The washed samples were oven-dried at 80 C for three days and the dried samples were ground using a blender (Wiley mill) and sieved to pass through a mm sieve in order to obtain uniform particle size. The biomass preparation procedure followed laboratory techniques similar to those previously reported by Gardea-Torresdey et al. (2000). In summary, a 250-mg sample of lechuguilla leaf biomass was washed four times with 0.01 M HCl and three times with deionized (DI) water in order to remove soluble material or biomolecules that might interact with any sorbed metal ions. The washed biomass was resuspended in 50 ml of deionized (DI) water to obtain a concentration of 5 mg of lechuguilla per ml of water. The suspension was adjusted to ph 4 (using diluted solutions of HCl and NaOH) due to previous batch studies showed that Cr(III) bound better to lechuguilla at ph 4 (data not shown). Aliquots of 5-ml of biomass solutions (5 mg/ ml) were taken and transferred to clean test tubes. The samples were centrifuged (Marathon 6K Fisher Scientific centrifuge) at 3000 rpm and the supernatants were discarded. The biomass pellets were reacted with 5 ml of solutions of Cr(III) aliquots of 5, 10, 15, 20, 25, 30, 35, and 40 mg/l for 12 h and then equilibrated by rocking (Speci-Mix M-26125) at their respective temperatures. These studies were performed at the temperatures of 10 C in a refrigerator, 22 C on the laboratory bench, and 40 C in an oven. The final solutions of chromium concentrations within the test tube samples were determined by using a flame atomic absorption spectrometer (FAAS) (Perkin Elmer model 3110). The analytical wavelength used was nm with a slit width of 0.7 nm. The chromium hollow-cathode lamp current was 30 ma. An impact bead and a reducing flame were used to improve instrument sensitivity. Standards were prepared by diluting a 1000 mg/l Cr stock solution and linear calibration curves were obtained with correlation coefficients of R 2 = 0.99 or better. Three replicates of each sample were analyzed and the mean value and relative standard deviation given by the instrument were recorded. In order to fit the linear calibration range, some samples were diluted using 5% HNO 3. The final metal concentration was subtracted from the initial metal concentration and the difference was assumed to be the amount of chromium adsorbed by lechuguilla biomass. 2. Methods 3. Results and discussion 3.1. Sorption isotherms The influence of temperature on Cr(III) adsorption was investigated at different concentrations. These isotherms relate metal uptake per mass of adsorbent (q e ) to the adsorbate concentration at equilibrium ( ). Fig. 1 shows that the adsorption of Cr(III) onto lechuguilla was favored at high temperatures. The adsorption capacity and affinity of lechuguilla for Cr(III) was determined with two isotherms models (Freundlich and Langmuir), using Cr(III) solutions at 10, 15, 20, 25, 30, 35 and 40 mg/l. The Freundlich isotherm is a nonlinear sorption model. This model proposes a monolayer sorption with a heterogeneous energetic distribution of active sites, accompanied by interactions between adsorbed molecules. The general form of this model is q e ¼ K F C 1=n e ; ð1þ where K F (mg/g) stands for adsorption capacity and n for adsorption intensity. The logarithmic form of Eq. (1) is: log q e ¼ log K F þ 1 n log ; q e (mg/g) (mg/l) ð2þ Fig. 1. Adsorption isotherms plots of Cr(III) on A. lechuguilla biomass at ph 4.

3 180 J. Romero-González et al. / Bioresource Technology 97 (2006) where K F and 1/n can be determined from the linear plot of log(q e ) versus log( ). Experimental values obtained for the adsorption capacity experiments were used to calculate the Freundlich model parameters at different temperatures. Table 1 shows these results. The Langmuir model represents one of the first theoretical treatments of nonlinear sorption and suggests that uptake occurs on a homogeneous surface by monolayer sorption without interaction between adsorbed molecules. In addition, the model assumes uniform energies of adsorption onto the surface and no transmigration of the adsorbate. The Langmuir isotherm is represented in the following equation: q e ¼ Q Lb ; ð3þ 1 þ b where Q L (mg/g) and b are Langmuir constants related to adsorption capacity and the energy of adsorption, respectively. Eq. (3) is usually linearized to obtain the following form: ¼ þ 1 : ð4þ q e Q L bq L The linearized plot of /q e versus for Cr(III) respectively were analyzed, and the results obtained at the experimental temperatures are shown in Table 1. In the Langmuir model the adsorption intensity (R L ) is expressed by the following equation: 1 R L ¼ ; ð5þ 1 þ bc 0 where C 0 (mg/l) is the initial concentration of the metal. If the average of the R L values from the different initial concentrations used is between 0 and 1, it indicates favorable adsorption. The average values of R L for the different initial Cr(III) concentrations at the respective temperature are shown in Table 1. The correlation coefficient values obtained from the Freundlich and Langmuir isotherms are also presented in Table 1. The values indicate that the adsorption pattern for Cr(III) on lechuguilla followed both the Freundlich isotherm (R 2 > ) and the Langmuir isotherm (R 2 > ) at all experimental temperatures. The values obtained for Cr(III) from the Freundlich model at different temperatures showed a maximum adsorption capacity (K F ) of 5.99 mg/g at 40 C withan affinity value (n) equal to 2.84, which represents a favorable adsorption of Cr(III). According to the Langmuir model, the maximum Cr(III) adsorption capacity was obtained at 40 C with a value of Q L of mg/g and with an affinity (R L ) of 0.07, which also represents a favorable Cr(III) adsorption. As seen in Table 1, a decrease in the temperature produced a decrease in the adsorption capacity in both models. The comparison of the Cr(III) adsorption capacities of lechuguilla with other biomass capacities was made at 22 C (temperature at which the most of works reported these capacities). Therefore, even though the comparison of adsorbents is difficult because their experimental set can be different, the adsorption capacities of lechuguilla biomass at 22 C (4.5 mg/g and mg/g for K F and Q L, respectively) are higher than the average values or are in the range of obtained values of other biomasses with similar experimental conditions. Typical values found in the literature are 0.6 < K F < 3.8 mg/g and 1.4 < Q L < 119 mg/g for Cr(III) (Bailey et al., 1998; Machado et al., 2002; Yun et al., 2001). The fact that the adsorption of Cr(III) onto lechuguilla obeyed both the Freundlich and Langmuir isotherms suggested the formation of an homogenous monolayer of Cr(III) on the outer surface of the absorbent (Das et al., 2000). This further supports the idea that the binding of Cr(III) may be caused by interactions with functional groups such as carboxyl groups located on the surface of the cell tissue of the bioadsorbent (Gardea-Torresdey et al., 2002; Parsons et al., 2002) Thermodynamic parameters Thermodynamic parameters such as change in free energy (DG 0 ), enthalpy (DH 0 ) and entropy (DS 0 ) were determined using the following equations: K c ¼ q e ð6þ where K c is the equilibrium constant, q e is the amount of solute (mg) adsorbed on the adsorbent cubic decimeter of the solution at equilibrium and is the equilibrium concentration (mg/dm 3 ) of the solute in solution, T is the temperature in Kelvin and R is the gas constant: DG 0 ¼ RT ln K c ; ð7þ Table 1 Model parameters for the adsorption of Cr(III) on lechuguilla biomass at different temperatures and ph 4 Freundlich Langmuir T ( C) K F (mg/g) n R 2 Q L (mg/g) b (l/mg) R L R

4 J. Romero-González et al. / Bioresource Technology 97 (2006) ln K c ¼ DH 0 1 R T þ DS0 R : ð8þ The equilibrium constants K c were determined from the intercept of Khan and Singh plots of lnq e / versus q e (Fig. 2) (Krishna et al., 2000). In addition, DH 0 and DS 0 were obtained from the slope and intercept of VanÕHoff plots of lnk c versus 1/T (Fig. 3). The values of the thermodynamic parameters at different temperatures in Eq. (8) are presented in Table 2. The negative values of DG 0 at all temperatures indicate the spontaneous nature of the adsorption of Cr(III) on the adsorbent. The positive value of DH 0 suggests the endothermic nature of adsorption. The positive value of DS 0 shows the increased randomness at the solid/solution interface during the adsorption process, which suggests that Cr(III) ions replace some water molecules from the solution previously adsorbed on the surface of lechuguilla. These displaced molecules gain more ln q e / q e (mg/g) Fig. 2. Khan and Singh plots of lnq e / versus q e for Cr(III) adsorption on A. lechuguilla biomass. ln K c y = x R 2 = /T (10-3 ) Fig. 3. VanÕHoff plot for the adsorption of Cr(III) on A. lechuguilla biomass. translation entropy than is lost by the absorbate ions, thus allowing the prevalence of randomness in the system. Another equation that has been used to determine useful thermodynamic adsorption parameters is the Dubinin Radushkevick equation. The Dubinin Radushkevick equation does not assume a homogeneous surface or a constant sorption potential (Gemeay et al., 2002). The linear presentation of this equation is expressed by ln q e ¼ ln q m K E e 02 ; ð9þ e 0 ¼ RT ln 1 þ 1 ; ð10þ where e 0 is the Polanyi potential, q m is the monolayer capacity (mol/g), is the equilibrium concentration (mol/l), K E is the constant related to sorption energy (mol 2 /kj 2 ). The parameters q m and K E can be obtained from the intercept and slope of the plot shown in Fig. 4. The mean free energy of sorption, E, is calculated by the following equation: E ¼ð 2K E Þ 1=2 : ð11þ The Dubinin Radushkevick parameters and mean free energy are given in Table 3. The magnitude of E is useful for estimating the type of sorption reaction. The E values obtained are between and kj/ mol, which are in the energy range of an ion-exchange reaction, i.e., 8 16 kj/mol (Helfferich, 1962). This indicates and supports the idea that the sorption of trivalent ln q e ε02 (KJ/mol) 2 Fig. 4. Dubinin Radushkevick plots for the adsorption of Cr(VI) on A. lechuguilla biomass. Table 2 Thermodynamic parameters for the adsorption of Cr(III) onto lechuguilla at ph 4 T ( C) K c DG 0 (kj/mol) DH 0 (kj/mol) DS 0 (J/mol/K) Table 3 Dubinin Radushkevick parameters for the adsorption of Cr(III) by lechuguilla at ph 4 T ( C) K E (mol/kj) 2 q m (mol/kg) E (kj/mol) R (10 3 ) (10 3 ) (10 3 )

5 182 J. Romero-González et al. / Bioresource Technology 97 (2006) chromium onto lechuguilla biomass may proceed through an ion exchange reaction, most likely via a carboxylic group. 4. Conclusions In this study results showed that high temperatures increased the bioadsorption capacity of Cr(III) by lechuguilla. In addition, the adsorption equilibrium data fitted well with the Freundlich and Langmuir models at all temperatures. The fact that the adsorption of Cr(III) onto lechuguilla obeyed both models showed the monolayer coverage of Cr(III) on the outer surface of the absorbent. A comparison of adsorption capacities calculated from the Freundlich and Langmuir isotherms and those obtained in the literature showed that A. lechuguilla biomass can be effective for the removal of Cr(III) from an aqueous solution. On the other hand, the thermodynamic parameters (DG 0, DH 0 and DS 0 ) for Cr(III) adsorption and the parameters of the Dubinin Radushkevick equation suggested that Cr(III) was bound by functional groups in the external surface of the adsorbent. Acknowledgements The authors would like to acknowledge financial support from the National Institutes of Health (NIH) (Grant S06GM ) and the University of Texas at El PasoÕs Center for Environmental Resource Management (Cooperative agreement CR ) through funding from the Office of Exploratory Research of the EPA. In addition, the authors acknowledge the financial assistance from HBCU/MI ETC that is funded by the Department of Energy. Dr. Gardea- Torresdey acknowledges the funding from the National Institute of Environmental Health Sciences (Grant R01ES ) and the Dudley family for the Endowed Research Professorship in Chemistry. Jaime Romero-González also acknowledges the financial support from the University of Guanajuato, Mexico. References Bailey, S.E., Olin, T.J., Bricka, R.M., Adrian, D.D., A review of potentially low-cost sorbents for heavy metals. Water Res. 33, Blunden, G., Carabot, A., Cripps, A.L., Jewers, K., Ruizgenin, a new steroidal sapogenin diol from Agave lecheguilla. Steroids 35, Das, D.D., Mahapatra, R., Pradhan, J., Das, S.N., Thakur, R.S., Removal of Cr(VI) from solution using activated cow dung carbon. J. Colloid Interface Sci. 232, Gaballah, I., Kilbertus, G., Recovery of heavy metals ions through decontamination of synthetics solutions and industrial effluents using modified barks. J. Geochem. Explor. 62, Gardea-Torresdey, J.L., Tiemann, K.J., Armendàriz, V., Bess-Oberto, L., Chianelli, R.R., Rios, J., Parsons, J., Gamez, J.G., Characterization of Cr(VI) binding and reduction to Cr(III) by the agricultural byproducts of Avena monida (Oat) biomass. J. Hazard. Mater. B 80, Gardea-Torresdey, J.L., Dokken, K., Tiemann, K.J., Parsons, J.G., Ramos, J., Pingitore, N.E., Gamez, G., Infrared and X-ray adsorption spectroscopic studies on the mechanism of chromium (III) binding to alfalfa biomass. Microchem. J. 71, Gemeay, A.H., El-Sherbiny, A.S., Zaki, A.B., Adsorption and kinetic studies of the intercalation of some organic compounds onto Na + -Montmorillonite. J. Colloid Interface Sci. 245, Gentry, H.S., Agaves of Continental North America. The University of Arizona Press, Arizona, USA. Helfferich, F., Ion Exchange. McGraw-Hill, New York, USA. Hong, K.J., Toknagen, S., Ishigami, Y., Kajiuchi, T., Extraction of heavy metals from MSW incinerator fly ash using saponins. Chemosphere 41, Hong, K., Tokunaga, S., Kajiuchi, T., Evaluation of remediation process with plant-derived biosurfactant for recovery of heavy metals from contaminated soils. Chemosphere 49, Krishna, B.S., Murty, D.S.R., Prakash, B.S.J., Thermodynamics of chromium (VI) anionic species sorption onto surfactant-modified montmorillonite clay. J. Colloid Interface Sci. 229, Machado, R., Carvalho, J.R., Correia, M.J.N., Removal of trivalent chromium (III) from solution by biosorption in cork powder. J. Chem. Technol. Biotechnol. 77, Parsons, J.G., Hejazi, M., Tiemann, K.J., Henning, J., Gardea- Torresdey, J.L., An XAS study of the binding of copper(ii), zinc(ii), chromium(iii) and chromium(vi) to hops biomass. Microchem. J. 71, Volesky, B., Detoxification of metal-bearing effluents: biosorption for the next century. Hydrometallurgy 59, World Health Organization (WHO), Health Criteria 61, Chromium, Geneva. Yun, Y., Park, D., Park, J.M., Volesky, B., Biosorption of trivalent chromium on the brown seaweed biomass. Environ. Sci. Technol. 35,