Fungal Biomass with Grafted Poly(acrylic acid) for Enhancement of Cu

3.3890) was purchased from China General Microbiological Culture Center, ..... is a constant representing the sorption capacity, and n is a constant d...
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Langmuir 2005, 21, 5940-5948

Fungal Biomass with Grafted Poly(acrylic acid) for Enhancement of Cu(II) and Cd(II) Biosorption Shubo Deng and Yen Peng Ting* Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received October 28, 2004. In Final Form: March 7, 2005 The biomass of Penicillium chrysogenum was modified by graft polymerization of acrylic acid (AAc) on the surface of ozone-pretreated biomass. The sorption capacity for copper and cadmium increased significantly as a large number of carboxyl groups were present on the biomass surface, especially when the carboxylic acid group was converted to carboxylate ions using NaOH. When modeled using the Langmuir isotherm, the sorption capacities were 1.70 and 1.87 mmol g-1 for copper and cadmium, respectively. The loaded biosorbent was regenerated using HCl solution and used repeatedly over five cycles with little loss of uptake capacity beyond the second cycle. The sorption of the two metals was time-dependent, and the kinetics fitted the pseudo-second-order equation well. The Freundlich, Langmuir, Temkin, and DubininRedushkevich isotherms were used to model the metal sorption isotherms, and the thermodynamic parameters calculated show that the sorption was spontaneous and endothermic under the condition applied and that the biomass has similar sorption affinities for the two metals. Fourier transform infrared and X-ray photoelectron spectroscopy reveal that carboxyl, amide, and hydroxyl groups on the biomass surface were involved in the sorption of copper and cadmium and ion exchange and complexation dominated the sorption process.

Introduction Water pollution caused by heavy metals is a serious worldwide environmental problem with a significant impact on human health and the environment. This concern has led to the development of various technologies for the removal of metals from wastewater generated from industries such as electroplating, leather tanning, and steel making.1 Biosorption has been recognized as a promising technology that shows potential as an alternative or adjunct to conventional processes for the treatment of wastewater with trace levels of metal contaminant. Many microorganisms including algae, fungi, bacteria, and yeast have been investigated in metal sorption studies.2,3 For instance, fungal biomass produced as a byproduct from the fermentation and pharmaceutical industry has been used to adsorb metallic ions from aqueous solution and shows possible application in wastewater treatment.4-6 Functional groups such as carboxylate, hydroxyl, sulfate, phosphate, amide, and amino groups on the biosorbents have been reported to be responsible for metal binding.2,4,7 Among them, the carboxyl group is the most common functional group found on biosorbents including seaweed biomass,2,8 cyanobacteria,7 and sugar beet pulp.9 * Author for correspondence. Tel.: +65- 6874-2190. Fax: +656779-1936. E-mail address: [email protected]. (1) Deng, S. B.; Bai, R. B. Water Res. 2004, 38, 2423-2431. (2) Yun, Y. S.; Park, D.; Park, J. M.; Volesky, B. Environ. Sci. Technol. 2001, 35, 4353-4358. (3) Khoo, K. M.; Ting, Y. P. Biochem. Eng. J. 2001, 8, 51-59. (4) Tan, T. W.; Cheng, P. Appl. Biochem. Biotechnol. 2003, 104, 119128. (5) Gulati, R.; Saxena, R. K.; Gupta, R. World J. Microbiol. Biotechnol. 2002, 18, 397-401. (6) Schneegurt, M. A.; Jain, J. C.; Menicucci, J. A.; Brown, S. A.; Kemner, K. M.; Garofalo, D. F.; Quallick, M. R.; Neal, C. R.; Kulpa, C. F. Environ. Sci. Technol. 2001, 35, 3786-3791. (7) Yee, N.; Benning, L. G.; Phoenix, V. R.; Ferris, F. G. Environ. Sci. Technol. 2004, 38, 775-782. (8) Chen, J. P.; Hong, L. A.; Wu, S. N.; Wang, L. Langmuir 2002, 18, 9413-9421.

All biosorbents have an intrinsic sorption property which is dependent on the concentration and type of functional groups on the sorbent surface. As the density of these groups effective for metal binding is generally low, most biosorbents do not show a high sorption capacity. Surface modification of the biomass by alkaline pretreatment,4 phosphorylation,10 and sorption of some chemicals11 has been found to significantly enhance the sorption capacity after a number of the effective functional groups including amino, carboxyl, and phosphate groups were introduced on the biomass surface. Surface grafting of acrylic acid (AAc) on a polymer surface is a simple and versatile approach to improve the surface properties of polymers for a wide variety of applications.12 Graft chains with a high density may be introduced to the surface through covalent attachment, thus, making the introduced chains more stable compared with physical adsorption and surface coating methods. The polymer surface is usually pretreated by plasma discharge, UV irradiation, and ozone treatment before the copolymerization of AAc.13,14 For instance, graft copolymerization of AAc has been used to modify the polymer membrane and film to produce smart materials applied in environment-sensitive filtration, immobilization of enzymes, and drug release.13,15 However, the use of graft copolymerization of AAc on biomass for the enhancement of its metal sorption capacity has not been reported in the literature. (9) Reddad, Z.; Gerente, C.; Andres, Y.; Le Cloirec, P. Environ. Sci. Technol. 2002, 36, 2067-2073. (10) Klimmek, S.; Stan, H. J.; Wilke, A.; Bunke, G.; Buchholz, R. Environ. Sci. Technol. 2001, 35, 4283-4288. (11) Bai, R. S.; Abraham, T. E. Water Res. 2002, 36, 1224-1236. (12) Gancarz, I.; Pozniak, G.; Bryjak, M.; Frankiewiez, A. Acta Polym. 1999, 50, 317-326. (13) Kato, K.; Uchida, E.; Kang, E. T.; Uyama, Y.; Ikada, Y. Prog. Polym. Sci. 2003, 28, 209-259. (14) Ying, L.; Yu, W. H.; Kang, E. T.; Neoh, K. G. Langmuir 2004, 20, 6032-6040. (15) Cen, L.; Neoh, K. G.; Kang, E. T. Biosens. Bioelectron. 2003, 18, 363-374.

10.1021/la047349a CCC: $30.25 © 2005 American Chemical Society Published on Web 05/13/2005

Cu(II) and Cd(II) Biosorption Enhancement

In the present study, the biomass of Penicillium chrysogenum was modified by thermal polymerization of AAc on the biomass surface to increase its sorption capacity for metal ions. The presence of many side chains of AAc on the sorbent surface provided a high number of binding sites for metal ions and, thus, enhanced the sorption ability. The common metallic pollutants, copper and cadmium, were selected as adsorbates in this study. The sorption performance, including sorption kinetics and equilibrium, was investigated in detail, and sorption models were also used to fit the experimental data. Finally, the sorption mechanisms of the modified biomass for copper and cadmium were investigated through X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) analyses. Materials and Methods Materials. The strain P. chrysogenum (no. 3.3890) was purchased from China General Microbiological Culture Center, Beijing, China. AAc monomer of purity ∼99.9% was purchased from Sigma-Aldrich Co. and was purified by vacuum distillation before use. Other chemicals are of reagent grade. Preparation of Biomass. P. chrysogenum was cultivated on 3.9% (w/v) potato dextrose agar in Petri dishes and was incubated for 7 days at 30 °C. The spores on the Petri dish were added to 100 mL of sterilized liquid medium in a 250 mL conical flask. The composition of the culture medium (g L-1) prepared using deionized (DI) water was as follows: glucose (30), NH4NO3 (2), yeast extract (2), KH2PO4 (1), MgSO4‚7H2O (0.5), and KCl (0.5) at an initial pH of 5.5. After culturing on a rotary shaker at 150 rpm and 30 °C for 3 days, the fungal biomass was autoclaved at 121 °C for 15 min and then filtered and washed with copious amount of DI water to remove the medium. Surface Modification. Wet biomass was freeze-dried and placed into a flask. The treatment of the biomass was carried out in a gas-phase reactor at 25 °C. A continuous stream of O3/O2 mixture generated from an Azcozon RMU16-04EM ozone generator was blown into the reactor at 300 L h-1 to provide an ozone concentration of about 0.027 g L-1 of the gaseous mixture. After a treatment time of about 30 min (further treatment will destroy the biomass surface and break the biomass), the biomass was immediately placed in a 30% (v/v) solution of AAc in a threeneck round-bottom flask equipped with a thermometer, a condenser, and a gas line. The grafting process was performed in a nitrogen atmosphere at 60 °C for 0.5-4 h under stirring. After the reaction, the biomass was thoroughly rinsed with DI water to remove residual monomer and homopolymer and then filtered, freeze-dried, and stored in a desiccator before use. FTIR Spectroscopy. The samples of the biomass before and after chemical modification and biomass after metal sorption were analyzed with a Bio-Rad FTS-3500 ARX FTIR spectrophotometer under ambient conditions. Before the analysis, the wet samples were freeze-dried, and each sample was placed on a gold film and determined in reflection mode over the wavenumber range of 400-4000 cm-1. Morphology Observation. The surface morphologies of the fungal biomass before and after modification were examined with a scanning electron microscope (SEM, JEOL JSM-6400). The biomass was washed several times with DI water and then freezedried in a dryer until constant weight before being finally coated with a JEOL JFC-1300 Auto Fine Coater fitted with a Pt target for the observation. Batch Sorption Experiments. All the sorption experiments were performed at 25 °C and 150 rpm on an orbital shaker with 0.1 g of the biomass in a 250 mL flask containing 100 mL of copper or cadmium solution prepared using the metal nitrate. Batch adsorption experiments were conducted to examine the sorption kinetics and equilibrium. In the sorption kinetic experiments, a 2 mmol L-1 copper or cadmium solution at different initial solution pHs was used. In the sorption isotherm experiments conducted over 5 h, the initial solution pH was adjusted to 5, with 0.1 g of biomass in 100 mL of metal solution at various concentrations. After sorption, the biomass was separated from the solution by membrane filtration, rinsed with

Langmuir, Vol. 21, No. 13, 2005 5941 DI water, and then freeze-dried for the FTIR and XPS analyses, while the metal concentration in the filtrate was analyzed using an inductively coupled plasma optical emission spectrometer (Perkin-Elmer Optima 3000, U.S.A.). All the sorption experiments were conducted in duplicate, and the mean values were reported. Desorption Experiments. In the desorption experiments, the metal-sorbed biomass (after copper or cadmium adsorption at initial pH 5 or 6 with a 0.1 g amount of the biomass in 100 mL of 2 mmol L-1 metal solution for 5 h) was regenerated in 100 mL of 0.2 M HCl solution on a rotary shaker at 120 rpm for 1 h and then washed with 0.2 M NaOH solution and finally with DI water until a neutral pH was obtained. The regenerated biomass was reused in the next cycle of sorption experiments. XPS Analysis. XPS was used in the surface analysis of the biomass before and after metal adsorption. The biomass was freeze-dried before the analyses. The analysis was made with an AXIS HIS spectrometer (Kratos Anlytical, Ltd., U.K.) with an Al K X-ray source (1486.71 eV of photons) to determine the C, N, and O atoms present on the surface of the samples. The X-ray source was run at a reduced power of 150 W, and the pressure in the analysis chamber was maintained at less than 10-8 Torr during each measurement. All binding energies were referenced to the neutral C(1s) peak at 284.6 eV to compensate for the surface charging effects. The software package of XPSpeak 4.1 was used to fit the XPS spectra peaks, and the full width at half-maximum was maintained constant for all components in a particular spectrum.

Results and Discussion Preparation and Characterization of Biomass Adsorbent. The biomass adsorbent was prepared under a nitrogen atmosphere by thermally induced molecular graft copolymerization of AAc with the ozone preactivated biomass surface (see reaction shown below). Ozone treatment has been widely utilized to generate peroxide and hydroperoxide species on the polymer surface.16,17 During the ozone treatment of the biomass, some peroxide and hydroperoxide species form on the biomass surface. Under thermal induction, these labile functional groups decompose to initiate the free-radical graft copolymerization of AAc. In the polymerization experiments, it was found that the sorption capacity of the modified biomass for copper ions increased with reaction time over 0.5-4 h, but decomposition of the biomass occurred and the mechanical strength decreased after 2 h of reaction. Therefore, the reaction time for the polymerization of AAc on the biomass surface was fixed at 2 h.

Although chemical modification may introduce some functional groups on the biomass surface to enhance the sorption capacity, it is difficult to achieve a high concentration of carboxyl groups. However, copolymerization of AAc on the biomass surface can result in a large number of carboxyl groups and, thus, enhance its sorption capacity. As evident in the scheme shown, the long chain of poly(acrylic acid) (PAAc) grafted on the biomass surface can stretch into the solution due to the electrostatic repulsion of the carboxyl groups during metal sorption process and, thus, provide more binding sites beyond the external surface of the biomass for metal sorption. (16) Boutevin, B.; Robin, J. J.; Torres, N.; Casteil, J. Macromol. Chem. Phys. 2002, 203, 245-252. (17) Wang, W. C.; Vora, R. H.; Kang, E. T.; Neoh, K. G.; Liaw, D. J. Ind. Eng. Chem. Res. 2003, 42, 784-794.

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Figure 1. SEM micrographs of biomass of P. chrysogenum before and after modification. (a) Pristine biomass of P. chrysogenum. (b) Biomass with grafted PAAc. Figure 3. Effect of pH on the adsorption capacity of copper on the modified biomass (0.1 g of biomass in 100 mL of a 2 mmol L-1 copper solution for 5 h).

Figure 2. FTIR spectra of (a) the pristine biomass and (b) biomass with grafted PAAc.

Figure 1 shows the SEM micrographs of P. chrysogenum before and after the surface modification. A prominent change in the modified biomass was an increase in the diameter of the mycelia from 1.5 µm to about 2-3 µm and a more uneven and rougher surface morphology compared to the pristine biomass. This result indicates that AAc was copolymerized on the biomass surface. To confirm the type of functional groups on the biomass before and after modification, FTIR spectra were determined and are shown in Figure 2. The peaks at 3402, 2936, 1674, 1558, 1161, and 1115 cm-1 are observed in the pristine biomass spectrum shown in Figure 2a. The broad and strong band ranging from 3200 to 3600 cm-1 may be attributed to OH groups affected by hydrogen bonding, which is consistent with the peak at 1115 cm-1 assigned to alcoholic CsO stretching vibration. The strong peak at 1674 cm-1 can be assigned to a CdO stretching from the amide group.7 The spectrum also shows peaks at 1558 and 1161 cm-1 which correspond to the NsH bending and CsN stretching from the amide group, respectively.18 Carboxylate group may also be present on the biomass surface due to the presence of the peak at 1558 cm-1. In addition, the band at 2936 cm-1 is due to CH stretching vibrations of CH, CH2, and CH3 groups. The spectrum presents some changes after the biomass was modified with AAc. As shown in Figure 2b, a new peak at 1728 cm-1 is observed, which can be assigned to CdO stretching of COOH, and the bands at 1262 and 926 cm-1 are associated with CsO of COOH,7 indicating that PAAc was successfully grafted on the biomass. The asymmetric CsOsC stretching presents a peak at 1085 cm-1, which is consistent with the group formed in the graft reaction presented. Effect of pH on Metal Sorption. pH is an important parameter that affects metal ion sorption; it not only (18) Shriner, R. L.; Hermann, C. K. F.; Morrill, T. C.; Curtin, D. Y.; Fuson, R. C. The systematic identification of organic compounds, 7th ed.; J. Wiley & Sons: New York, 1998.

Figure 4. Copper and cadmium sorption on the modified and pristine biomass at different controlled pHs during the adsorption process.

influences the properties of sorbent surface but also affects metal speciation in solution. In our experiments, the initial solution pHs were less than 5.0 for copper and 6.0 for cadmium in consideration of the metallic species in solution and possible formation of metallic precipitation.19 Figure 3 shows copper adsorption on the modified biomass at different initial solution pHs. The sorption capacity increased with an increase in initial pH. The final solution pH after 4 h of sorption (shown in Figure 3) was lower than the initial pH, thus, indicating hydrogen ions were released into the solution. Over an initial pH range of 2.2-5.0, the final pH after sorption was below 3.5. This result suggested that ion exchange between Cu2+ and H+ took place during the sorption process. As the pKa of -COOH is in the range of 3.5-5.0,2 the lower solution pH would prevent the dissociation of carboxyl groups and (19) Sheng, P. X.; Ting, Y. P.; Chen, J. P.; Hong, L. J. Colloid Interface Sci. 2004, 275, 131-141.

Cu(II) and Cd(II) Biosorption Enhancement

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Figure 5. Sorption kinetics of copper and cadmium ions on the modified biomass with 0.1 g of biomass in 100 mL of a 2 mmol L-1 metal solution. (a) Cu2+ sorption kinetics data; (b) modeled result for Cu2+ sorption using the pseudo-second-order equation; (c) Cd2+ sorption kinetics data; and (d) modeled result for Cd2+ sorption using the pseudo-second-order equation. Table 1. Kinetic Parameters of the Pseudo-Second-Order Equation for Cu2+ and Cd2+ Sorption metal copper cadmium

pH

qe (mmol g-1)

4.0 5.0 4.0 6.0

0.806 1.395 0.798 1.608

k2 (g v0 (mmol mmol-1 min-1) g-1 min-1) 0.121 0.055 0.066 0.038

0.079 0.107 0.042 0.098

R2 0.999 0.999 0.999 0.999

further adsorption of metal ions would cease. The results of the kinetics of copper sorption on the biomass at initial pH 5 without pH adjustment during the sorption process revealed that the sorption was rapid and the equilibrium was achieved within 30 min. To avoid the effect of the released protons on metal sorption, the solution pH was kept constant during the sorption process, and the result is shown in Figure 4. In copper sorption, the sorption capacity increased with increasing solution pH and a rapid increase in sorption uptake was observed at pHs ranging from 3 to 5. Compared with the sorption capacity without pH control (Figure 3), the sorption capacities with pH control increased significantly and reached 1.35 mmol g-1 at pH 5. The effect of pH on cadmium sorption, presented in Figure 4b, shows a similar increase in sorption capacity with solution pH. A sorption capacity of 1.62 mmol g-1 was achieved at pH 6. Figure 4 also shows the sorption capacity of the pristine biomass for copper and cadmium ions at different solution pHs; the sorption capacities were 0.26 mmol g-1 for copper at pH 5 and 0.23 mmol g-1 for cadmium at pH 6. It can be seen that the sorption capacity for the metals increased significantly after the biomass was grafted with PAAc. Some biomass has reportedly been used to remove copper and cadmium from aqueous solution; the maximum sorption capacities of most biomass were less than 1.0 and 0.4 mmol g-1 for copper and cadmium, respectively.20 For the biomass of P. chrysogenum, the values were 0.134

Figure 6. Adsorption isotherms of (a) copper and (b) cadmium on the biomass (0.1 g of biomass in 100 mL of metal solution at pH 5 for 5 h).

mmol g-1 for copper and 0.347 mmol g-1 for cadmium at pH 4.5. Skowronski et al. reported the maximum sorption capacities of 11.7 mg (0.18 mmol) of Cu and 21.5 mg (0.19 (20) Sag, Y. Sep. Purif. Methods 2001, 30, 1-48.

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Table 2. Calculated Equilibrium Constants and Thermodynamic Parameters Langmuir model metal

qm (mmol g-1)

b (L mmol-1)

copper cadmium

1.70 1.87

4.50 7.14

Freundlich model

R2

(mmol1-1/n L1/n g-1)

n

0.992 0.990

-20.84 -21.99

1.30 1.59

4.35 3.64

mmol) of Cd per gram of dry biomass of P. chrysogenum.21 Even with chitosan coated on the biomass of P. chrysogenum, the sorption capacity for Ni2+ reached only 40-45 mg g-1 using 200 mg L-1 metal solution at pH 6-7.22 Compared with these results, the modified P. chrysogenum with grafted PAAc in this work possessed higher sorption capacity for the metal ions. The results show that hydrogen ions in PAAc on the biomass surface were exchanged and released into solution during the metal sorption process, thus decreasing the solution pH and preventing further sorption of metal ions. To overcome this problem, the biomass with grafted PAAc was treated with a 1 M NaOH solution and then rinsed with DI water until neutral pH was obtained. After this treatment, the functional group of COOH was converted into COONa, and this adsorbent was used in the following experiments. Sorption Kinetics Study. Figure 5a,c shows the sorption kinetics of copper and cadmium ions using the modified biomass at different solution pHs. The sorption of both the metals was time-dependent. The sorption kinetics of copper was rapid in the first 60 min, before becoming more gradual until equilibrium was reached. At pH 4 and pH 5, the equilibrium for copper sorption was attained within 120 and 180 min, respectively. In cadmium sorption, the sorption kinetics was similar. As the biomass surface is bare in the initial stage, the sorption kinetics is fast and normally governed by the diffusion process from the bulk solution to the surface. In the later stage, the sorption is likely an attachment-controlled process due to less available adsorption sites. However, the diffusion and attachment-controlled models cannot describe the experimental data well (R2 < 92%),23 thus indicating that the sorption of copper and cadmium on the modified mass is not a conventional two-stage process of an external mass transfer followed by intraparticle diffusion. The specific surface area of the modified biomass was measured as 1.6 m2/g, suggesting that the micropores available for sorption inside the mycelia are limited and the intraparticle diffusion is negligible. A recently applied pseudo-second-order equation was adopted to describe the sorption kinetic data. The equation is24,25

1 t 1 1 1 ) + t) + t qt k q 2 qe v0 qe

Temkin model R2

bT (J g mmol-2)

AT (L mmol-1)

R2

∆G° (kJ mol-1)

0.983 0.969

8.51 7.14

96.03 107.36

0.988 0.991

-28.4 -28.7

a

∆G° (kJ mol-1)

obtained for both the metals at different solution pHs indicates that the biosorption conforms to the pseudosecond-order reaction mechanism, supporting the assumption that the sorption rate is controlled by chemical sorption. Because AAc was copolymerized on the biomass surface, it seems that cation exchange would dominate the sorption process. The corresponding parameters and regression coefficients for the plot are given in Table 1, with qe values at 1.395 mmol g-1 for copper at pH 5 and 1.608 mmol g-1 for cadmium at pH 6. The initial sorption rate (v0) increased from 0.079 to 0.107 mmol g-1 min-1 for copper ions with an increase in pH from 4 to 5, while it increased from 0.042 to 0.098 mmol g-1 min-1 for cadmium ions with a pH increase from 4 to 6. The higher sorption rates at higher pH for both cooper and cadmium are associated with the pKa of -COOH on the biomass surface. As more carboxyl groups are dissociated at pH 5 and 6 than that at pH 4, more sorption sites are available for metal ions in the initial stage. By comparing the initial sorption rate at pH 4, it is evident that the sorption rate of copper is higher than that of cadmium, which may be related to copper’s smaller ionic radius (rCu ) 0.071 nm < rCd ) 0.097 nm), stronger electronegativity (δcu ) 1.9 > δCd ) 1.7), and hydration energy (Ecu ) -2100 kJ/mol < ECd ) -1807 kJ/mol).26 Sorption Isotherms. Many sorption isotherm models have been successfully applied to experimental data.27,28 Among them, two-parameter models including Freundlich, Langmuir, Temkin, and Dubinin-Redushkevich (D-R) isotherms are widely used because of the ease in evaluating the isotherm parameters compared with three-parameter models. These four isotherm models are examined in the present study. Langmuir and Freundlich isotherms have been successfully used to model many sorption processes. The Langmuir isotherms assume monolayer coverage of adsorbate over a homogeneous adsorbent surface, and the sorption of each molecule onto the surface has equal sorption activation energy, while the Freundlich isotherm is derived by assuming a heterogeneous surface with a nonuniform distribution of heat of adsorption over the surface and a multilayer sorption can be expressed. The Langmuir and Freundlich isotherms may be expressed respectively as

(1)

2 e

where k2 is the rate constant for a pseudo-second-order adsorption (g mmol-1 min-1), qe is the adsorption capacity at equilibrium, and v0 represents the initial sorption rate (mmol g-1 min-1). The parameters of qe and k2 can be obtained experimentally from the slope and intercept of the plot of t/qt versus t. Results using the pseudo-secondorder equation for copper and cadmium sorption kinetics are presented in Figure 5b,d. The good fit (R2 ) 0.999) (21) Skowronski, T.; Pirszel, J.; Pawlik-Skowronska, B. Water Qual. Res. J. Can. 2001, 36, 793-803. (22) Su, H. J.; Wang, Z. X.; Tan, T. W. Biotechnol. Lett. 2003, 25, 949-953. (23) Deng, S. B.; Bai, R. B. J. Colloid Interface Sci. 2004, 280, 36-43. (24) Ho, Y. S.; McKay, G. Adsorpt. Sci. Technol. 1999, 17, 233-243. (25) Ho, Y. S.; McKay, G. Water Res. 2000, 34, 735-742.

qe )

qmCe 1/b + Ce

qe ) aCe1/n

(2) (3)

where qm is the maximum amount of adsorption (mmol g-1), b is the sorption equilibrium constant (l mmol-1), Ce is the equilibrium concentration of the copper or cadmium ions in the solution (mmol L-1), a is a constant representing the sorption capacity, and n is a constant depicting the sorption intensity. (26) Lu, L.; Tsoi, G.; Zhao, X. S. Ind. Eng. Chem. Res. 2004, 43, 79007906. (27) Allen, S. J.; Gan, Q.; Matthews, R.; Johnson, P. A. Bioresour. Technol. 2003, 88, 143-152. (28) Zeng, L.; Li, X. M.; Liu, J. D. Water Res. 2004, 38, 1318-1326.

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Unlike the Langmuir equation, the Temkin isotherm takes into account the interactions between adsorbed species and adsorbates to be adsorbed and is based on the assumption that the free energy of sorption is a function of the surface coverage. When more sorbates are adsorbed, the chance for the incoming sorbates in getting adsorbed is correspondingly reduced; that is, adsorption takes place on a nonuniform surface. The Temkin isotherm takes the following form:27,29

qe )

RT ln(ATCe) bT

(4)

where AT is the equilibrium binding constant corresponding to the maximum binding energy, bT is the Temkin isotherm constant, T is the temperature (K), and R is the ideal gas constant (8.3145 J mol-1 K-1). Figure 6 shows the sorption isotherms of copper and cadmium ions on the modified biomass. The sorption capacities for both the metals increased with an increase in the equilibrium metal concentration in solution. The experimental data were modeled according to Langmuir, Freundlich, and Temkin isotherms, and the evaluated constants are given in Table 2. The Langmuir and Temkin isotherm equations gave better fits than the Freundlich isotherm equation for both copper and cadmium sorption on the biomass. According to the Langmuir equation, the maximum uptake capacities (qm) for copper and cadmium are 1.70 and 1.87 mmol g-1, respectively. The Langmuir constant (b) is 7.15 L mmol-1 for cadmium and 4.5 L mmol-1 for copper, indicating the stronger affinity of the modified biomass for cadmium. The stronger affinity may be related to the higher sorption capacity for cadmium. However, the sorption capacities for the two metals are comparable as both copper and cadmium are divalent ions and the same sorption mechanism is involved in the sorption process. Another model, D-R isotherm, was also used to fit the experimental data. This isotherm assumes no homogeneous surface of the adsorbent and takes the form30

{ [

(

qe ) qD exp -BD RT ln 1 +

1 Ce

)] } 2

(5)

where qD is the D-R isotherm constant and the constant BD is related to the mean free energy of sorption per mole of the sorbate when it is transferred from infinite distance in the solution to the surface of the solid. A linear relationship would be obtained in a plot of ln qe as a function of [ln(1 + 1/Ce)]2. This isotherm for copper and cadmium sorption, plotted in Figure 7, provides a good description of the data for the two metals although the data at low and high concentrations do not correlate well with the isotherm. The values of the constant qD were found to be 1.53 and 1.69 mmol g-1 for copper and cadmium, respectively. The qD values were close to the qm values previously calculated from the Langmuir isotherm. In general, it may be argued that the fit between experimental sorption data and the isotherm models is only mathematically meaningful and does not provide any evidence of the actual sorption mechanism. Nonetheless, some parameters (for instance, the maximum sorption capacity, derived from the Langmuir isotherm) are important for optimization in the design of a sorption system. Additionally, the thermodynamic parameter such (29) Ho, Y. S.; Porter, J. F.; Mckay, G. Water, Air, Soil Pollut. 2002, 141, 1-33. (30) Dubinin, M. M. Chem. Rev. 1960, 60, 235-266.

Figure 7. D-R isotherm for copper and cadmium sorption. Table 3. Thermodynamic Parameters for Sorption of Copper and Cadmium at pH 5 for 5 h metal copper

cadmium

T b ∆G° ∆H° ∆S° (°C) (L mmol-1) (kJ mol-1) (kJ mol-1) (kJ mol-1 K-1) 10 20 30 10 20 30

2.29 3.89 7.40 4.25 6.17 7.86

-18.20 -20.14 -22.44 -19.66 -21.26 -22.60

41.856

0.212

21.900

0.147

as the Gibbs free energy (∆G°) can also be deduced from the Langmuir and Temkin isotherm equations.31,32 For instance,

∆G° ) -RT ln K

(6)

where K corresponds to b in the Langmuir equation and AT in the Temkin equation. Using the constant b in the Langmuir equation at 4.50 L mmol-1 for the copper sorption and 7.14 L mmol-1 for the cadmium sorption, the Gibbs free energies of adsorption for Cu(II) and Cd(II) were calculated to be -20.84 and -21.99 kJ mol-1, respectively (see Table 2). In the Temkin isotherm, the equilibrium binding constants AT were found to be 96.03 and 107.36 L mmol-1 for copper and cadmium, respectively, with the corresponding ∆G° as -28.4 and -28.7 kJ mol-1. The negative values of ∆G° indicate that the sorption of copper and cadmium on the biomass was spontaneous under the experimental conditions. The calculated values of ∆G° from the Temkin equation were more negative than that from the Langmuir equation but did not differ significantly between the metals. Because these values were very close, regardless of whether the Langmuir or Temkin isotherm was used, the results suggest that the biomass has similar affinities for both copper and cadmium. In addition, the standard enthalpy change (∆H°) and standard entropy change (∆S°) in the process can be obtained using the equation31

∆G° ) ∆H° - T∆S°

(7)

The plots of ∆G° versus T were found to be linear for both copper and cadmium at 10, 20, and 30 °C, and the values of ∆S° and ∆H° were determined from the slope and intercept of the plots. As shown in Table 3, ∆S° and ∆H° for the sorption of copper are 0.212 kJ mol-1 K-1 and 41.856 kJ mol-1, respectively. In the case of cadmium sorption, the corresponding values are 0.147 kJ mol-1 K-1 and 21.9 (31) Genc-Fuhrman, H.; Tjell, J. C.; McConchie, D. Environ. Sci. Technol. 2004, 38, 2428-2434. (32) Kim, Y. H.; Kim, C. M.; Choi, I. H.; Rengaraj, S.; Yi, J. H. Environ. Sci. Technol. 2004, 38, 924-931.

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Figure 8. Comparative sorption capacities of the modified biomass for copper and cadmium after regeneration.

Figure 10. XPS C(1s) core-level spectra of the (a) pristine biomass, (b) modified biomass without metal sorption, (c) modified biomass with copper sorption, and (d) modified biomass with cadmium sorption. Table 4. Area Ratios of C(1s) Spectra for the Pristine Biomass and Modified Biomass with and without Metal Sorption peak area ratio (%)

Figure 9. FTIR spectra of (a) the modified biomass (treated with NaOH), (b) the modified biomass with copper sorption, and (c) the modified biomass with cadmium sorption.

kJ mol-1, respectively. The positive values of the enthalpy change (∆H°) indicate that the sorption process of the two metals on the biomass is endothermic, while the positive values of the entropy change (∆S°) reflect the affinity of the biomass for the metal ions and suggest an increased randomness at the biomass/solution surface. In the D-R isotherm, the mean free energy of sorption (E) can be calculated from the constant BD using the following equation:

E)

1

x2BD

(8)

As the values of BD were found to be 2.88 × 10-8 mol2 J-2 for copper, and 1.99 × 10-8 mol2 J-2 for cadmium in Figure 7, the values of E calculated are 4.17 and 5.0 kJ mol-1 for copper and cadmium, respectively. Again, the mean free sorption energies for the two metals are not very significantly different. Desorption Study. After sorption of the metals on the biomass, the sorbents were regenerated using hydrochloric acid (0.2 M). After 1 h of treatment, the extent of desorption was 97.2% for copper and 98.6% for cadmium. The desorption process was extremely rapid, with equilibrium achieved within 10-20 min for both metals. During desorption, the hydrogen ions in the solution compete (or exchange) with the metal ions on the sorption sites. The regenerated biomass was treated with NaOH solution before being reused in subsequent cycles. Figure 8 shows the sorption capacity of the biomass for copper and cadmium in five successive cycles. In the first sorption cycle, the sorption capacities were 1.59 and 1.38 mmol g-1 for cadmium and copper, respectively, but decreased to

biomass

284.6 eV (CsC)

286 eV (CsOH)

287.5 eV OdCsN)

288.6 eV (OdCsO)

pristine modified biomass copper sorption cadmium sorption

48.1 56.4 64.9 67.0

40.6 21.0 11.8 13.4

8.9 3.2 4.2 5.9

2.4 19.4 19.1 12.7

1.28 and 1.11 mmol g-1 in the second cycle, which may be due to the incomplete regeneration or the loss of the broken biomass and the homopolymer of PAAc in the mycelial pellets during regeneration. During the preparation of the modified biomass, the homopolymer of PAAc may also be produced and some may remain in the mycelial pellets, although the biosorbent was rinsed thoroughly. During the desorption process, these homopolymers physically adsorbed on the biomass surface would be easily rinsed out in acid and base solutions. However, the sorption capacity remained relatively constant in subsequent cycles; the sorption capacity attained was 1.25 mmol g-1 for cadmium and 1.06 mmol g-1 for copper in the fifth cycle, indicating that the metal-adsorbed biomass may be regenerated completely using hydrochloric acid solution. Evidently, the biomass can be used repeatedly for metal ion sorption from aqueous solution. FTIR and XPS Analyses. As a myriad of functional groups are often present on the biomass surface, various sorption mechanisms including ion exchange, chelation, microprecipitation, and complexation may be involved in the sorption process. To determine the sorption mechanisms, FTIR and XPS spectra are often used to analyze the changes before and after metal sorption.19,33 Figure 9 shows the FTIR spectra of the modified biomass before and after metal sorption. The bands at 1262 and 926 cm-1 in Figure 2 associated with OH in carboxylic acid disappeared in Figure 9a after the carboxylic acid was converted to carboxylate ions using NaOH. The spectrum presents two characteristic bands at 1584 and (33) Deng, S. B.; Bai, R. B.; Chen, J. P. Langmuir 2003, 19, 50585064.

Cu(II) and Cd(II) Biosorption Enhancement

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Figure 11. XPS spectra of (a) Cu(2p3/2) and (b) Cd(3d5/2) for the modified biomass with copper or lead uptake.

1418 cm-1 which can be attributed to CdO asymmetric and symmetric stretching in carboxylate ions, respectively.7,34,35 After copper and cadmium sorption, although the peak at 1418 cm-1 attributed to CdO symmetric stretching shifted marginally, the CdO asymmetric stretching vibration in carboxylate ions was shifted to 1570 and 1577 cm-1, respectively. This specific bond shifted significantly to the lower wavenumber after metal sorption, suggesting that chemical interactions occurred between the metal ions and the carboxylate groups on the biomass surface. Some researchers have reported that the adsorption bands of symmetric and asymmetric Rs COOs stretching vibrations shifted to higher wavenumbers due to the formation of the PAAc/aluminum complex when the PAAc was adsorbed on the alumina surface.33 The broad peak at 3480 cm-1 decreased to 3395 and 3460 cm-1 respectively after copper and cadmium ions were adsorbed on the sorbent, indicating OH groups may be involved in the sorption. It also can be seen that the peak at 1672 cm-1 attributed to amide groups on the biomass surface shifted to the lower wavenumber of 1652 cm-1 for copper sorption and 1655 cm-1 for cadmium uptake. In the case of copper sorption, the intensity of the peaks attributed to amide and carboxylate groups decreased distinctly. These results indicate that the grafted carboxyl and intrinsic hydroxyl and amide groups on the biomass surface were all involved in the sorption of the two metals. To obtain further insights into the metal sorption on the biomass surface, XPS analyses were conducted. Figure 10 shows the C(1s) core-level spectra of the biomass before and after the chemical modification and after metal sorption. As shown in Figure 10a, four peaks (284.6, 286.0, 287.5, and 288.6 eV) can be fitted to the C(1s) spectrum of the pristine biomass, which are attributed to the carbon in CsC, CsOH, OdCsN, and OdCsO, respectively. This result verified that a higher concentration of hydroxyl groups and a lower concentration of amide and carboxylate groups were present on the pristine surface, which are associated with the sorption capacity of the pristine biomass shown in Figure 4. After the biomass surface was grafted with PAAc, the C(1s) spectrum changed significantly (see Figure 10b). Although the same four peaks can be assigned to the spectrum, their ratios are different from those in the pristine biomass. As shown in Table 4, the area ratio for the peak at 288.6 eV attributed to carboxyl group increased significantly from 2.4 to 19.4% after the modification with AAc, thus indicating that PAAc was grafted on the biomass surface. It can also be seen

that the area ratios decreased from 40.6 to 21% for CsOH and from 8.9 to 3.2 for OdCsN, respectively, which may be due to the coverage of polymer on these sites after the reaction. Figure 10c,d shows the C(1s) spectra of the modified biomass after copper and cadmium uptake. It can be observed that the area ratio of the carboxyl group before and after copper binding was almost constant, but the ratio after cadmium sorption decreased, which may be due to ion exchange between the sorbate and sodium ions in carboxylate groups on the sorbent. In the case of copper sorption, the area ratio of CsOH decreased from 21 to 11.8% and a marginal increase of the peak at 287.5 eV was observed. Similar results were obtained for cadmium uptake. This is indicative of the formation of hydroxylmetal complexation, in which the oxygen donates electrons to copper or cadmium ions and, therefore, the electron density at the adjacent carbon atoms decreases and the binding energy of the carbons is lifted up. Thus, it can be concluded that both carboxyl and hydroxyl groups were involved in the sorption of copper and cadmium on the modified biomass. The copper and cadmium ions adsorbed on the biomass were also determined and analyzed as shown in Figure 11a,b. The Cu(2p3/2) spectrum was fitted with one peak, with a binding energy of 933.5 eV. It is known that Cu2+ located in the ionic binding presents a peak at about 935 eV, while Cu2+ located in the coordination binding has a peak with a relatively lower binding energy.19,33 Therefore, the peak with the binding energy of 933.5 eV in Figure 11a could be characterized as the Cu2+ in the coordination form. Because there are carboxyl and hydroxyl groups on the biomass surface, the copper ion adsorbed on the carboxylate group through ion exchange could be coordinated by a hydroxyl group at the same time. As a result, the Cu2+ with lower electron density can gain electrons from the hydroxyl group, and the Cu2+ ions with the lower binding energy were observed. For the Cd(3d5/2) spectrum, the peak at 405.3 could be assigned to cadmium in the carboxylate form. Because Cd2+ has a completely filled d subshell, its sorption was through ionic bonding with the carboxylate groups on the biomass surface.19 From the FTIR and XPS analyses, it can be confirmed that the carboxyl and hydroxyl groups were involved in the metal sorption and the amide group may participate in the adsorption. The sorption mechanisms were complex, and both ion exchange and complexation occurred in the sorption of copper and cadmium on the modified biomass.

(34) Zaman, A. A.; Tsuchiya, R.; Moudgil, B. M. J. Colloid Interface Sci. 2002, 256, 73-78. (35) Romero-Gonzalez, M. E.; Williams, C. J.; Gardiner, P. H. E. Environ. Sci. Technol. 2001, 35, 3025-3030.

Conclusions The sorption capacity of the modified biomass of P. chrysogenum for Cu(II) and Cd(II) increased significantly

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when the biomass was copolymerized with AAc. The pseudo-second-order model described the kinetic data well, indicating that chemical sorption took place. FTIR and XPS results reveal that the sorption mechanisms were complex, and the amide and hydroxyl groups were also involved in the sorption apart from the grafted carboxyl group. Thermodynamic calculations show that the adsorption of copper and cadmium on the biomass was spontaneous and endothermic. The metal-adsorbed biosorbent may be regenerated completely using HCl solution, and the uptake capacity was relatively constant beyond the second cycle after the homopolymer of PAAc was rinsed

Deng and Ting

out in the first cycle. This study shows that grafting effective groups on the fungal biomass may result in a biosorbent with high sorption capacity for metal ions. Such an approach shows promise to be the next step in enhancing biosorption. Acknowledgment. A research fellowship from National University of Singapore to Shubo Deng is appreciated. LA047349A