Environ. Sci. Technol. 2005, 39, 8490-8496
Polyethylenimine-Modified Fungal Biomass as a High-Capacity Biosorbent for Cr(VI) Anions: Sorption Capacity and Uptake Mechanisms S H U B O D E N G † A N D Y E N P E N G T I N G * ,‡ Department of Environmental Science and Engineering, Tsinghua University, Beijing, P.R. China 100084, and Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260
Heavy metal pollution in the aqueous environment is a problem of global concern. Biosorption has been considered as a promising technology for the removal of low levels of toxic metals from industrial effluents and natural waters. A modified fungal biomass of Penicillium chrysogenum with positive surface charges was prepared by grafting polyethylenimine (PEI) onto the biomass surface in a twostep reaction. The presence of PEI on the biomass surface was verified by FTIR and X-ray photoelectron spectroscopy (XPS) analyses. Due to the high density of amine groups in the long chains of PEI molecules on the surface, the modified biomass was found to possess positive zeta potential at pH below 10.4 as well as high sorption capacity for anionic Cr(VI). Using the Langmuir adsorption isotherm, the maximum sorption capacity for Cr(VI) at a pH range of 4.3-5.5 was 5.37 mmol/g of biomass dry weight, the highest sorption capacity for Cr(VI) compared to other sorbents reported in the literature. Scanning electronic microscopy (SEM) provided evidence of chromium aggregates formed on the biomass surface. XPS results verified the presence of Cr(III) on the biomass surface in the pH range 2.5-10.5, suggesting that some Cr(VI) anions were reduced to Cr(III) during the sorption. The sorption kinetics indicated that redox reaction occurred on the biomass surface, and whether the converted Cr(III) ions were released to solution or adsorbed on the biomass depended on the solution pH. Sorption mechanisms including electrostatic interaction, chelation, and precipitation were found to be involved in the complex sorption of chromium on the PEI-modified biomass.
Introduction Biosorption has been considered as a promising technology for the removal of low levels of toxic metals from industrial effluents and natural waters. Since fungal biomass is produced in large quantities as byproducts or wastes from food, beverage, and pharmaceutical production, it may serve as a viable source for the development of inexpensive * Corresponding author phone: (65)6874-2190; fax: (65)6779-1936; e-mail:
[email protected]. † Tsinghua University. ‡ National University of Singapore. 8490
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biosorbents. Although most researchers use pristine mycelia directly for the sorption of heavy metals, results usually show that the mycelia exhibit low sorption capacity (1, 2). In recent years, interest has been focused on increasing the sorption capacity of the biomass. Some researchers investigated surface modification of the biomass via physical and chemical modification. Klimmek and Stan (3), for instance, reported that the maximum sorption capacities of the alga Lyngbya taylorii were increased significantly after phosphorylation of the biomass. Bai and Abraham noted that the sorption capacity of the fungus Rhizopus nigricans for Cr(VI) was enhanced after the introduction of carboxyl and amino groups (4). The pretreatment of Penicillium chrysogenum biomass with surfactants and cationic polyelectrolyte was also found to improve the sorption of As(V) anion (5). All these modifications were aimed at increasing the density of the effective functional groups for sorption. Various functional groups, including carboxylate, hydroxyl, sulfate, phosphate, amide, and amino groups, have been found to be responsible for metal sorption (6, 7). Among these, the amine group is very effective in removing heavy metals. It not only chelates cationic metal ions, but also adsorbs anionic metal species through electrostatic interaction or hydrogen bonding. Polyethylenimine (PEI) which is composed of a large number of primary and secondary amine groups in a molecule, exhibits good sorption ability for heavy metals when they are adsorbed or cross-linked on the sorbent surface (8, 9). Electrostatic interaction between adsorbent and adsorbate plays an important role in the process of adsorption or desorption. Since most colloids in water treatment have negative charges, adsorbents with positive zeta potential are favorable for sorption of these pollutants in waters. For instance, several attempts have been made to modify sand through surface coating of positive metallic hydroxides and metallic peroxides, but the coated surface so-prepared was prone to dissolution (10). Polypyrrole-modified glass beads were reported to possess high positive surface charges over a wide pH range with enhanced removal of the negatively charged kaolin particles and humic acid (11). Since the adsorption of metal ions takes place mainly on the fungal biomass surface, increasing the sorption active sites on the surface would be an effective approach to enhance the sorption capacity. One technique is the grafting of long polymer chains onto the biomass surface via direct grafting, or the polymerization of the monomer. In this study, PEI was grafted onto the biomass of Penicillium chrysogenum through a simple two-step reaction. The biomass of P. chrysogenum was chosen in this study as this strain is widely used in penicillin production in the pharmaceutical industry, and a large amount of the biomass is produced as a byproduct. Anionic Cr(VI) was selected as the adsorbate since the PEI-modified biomass exhibits positive surface charge over a wide pH range. It is also known that hexavalent Cr(VI) is commonly present in wastewater emanating from dye, electroplating, leather, and mining industries. The sorption performance (in particular the sorption capacity) was investigated in detail, and the sorption mechanisms were elucidated through XPS analysis and sorption behavior.
Materials and Methods Materials. The strain Penicillium chrysogenum (No. 3.3890) was purchased from China General Microbiological Culture Center, and details of the mycelium preparation have been described earlier (12). Polyethylenimine (molecular weight 10.1021/es050697u CCC: $30.25
2005 American Chemical Society Published on Web 09/30/2005
25 000, branched polymer (-NHCH2CH2-)x(-N(CH2CH2NH2)CH2CH2-)y), 4-bromobutyryl chloride (BBC), and tertamyl alcohol were purchased from Sigma-Aldrich Company. Other chemicals were of reagent grade. Surface Modification. An amount of 10 g of dried biomass was placed in 2.5 mL of pyridine in 95 mL of chloroform, followed by dropwise addition of 5 mL of 4-bromobutyryl chloride. The reaction mixture was sealed and gently stirred at 25 °C for 12 h. The acylated biomass was rinsed with chloroform to remove any unreacted 4-bromobutyryl chloride before being immersed in a mixture containing 10 g of PEI and 0.1 g of KOH in 90 mL of tert-amyl alcohol. After the mixture was stirred at 75 °C for 24 h, the modified biomass was rinsed with copious quantities of methanol and deionized water. Finally, the biomass was freeze-dried to constant weight. FTIR Spectroscopy. Samples of pristine and modified biomass were analyzed with a Bio-Rad FTS-3500 ARX FTIR spectrophotometer under ambient conditions. Before the analysis, the wet samples were freeze-dried. Each lyophilized sample was placed on a gold mirror and determined using reflection mode in the wavenumber range of 400-4000 cm-1. ζ-Potential Measurement. A 0.1 g portion of freeze-dried biomass was placed into 100 mL of deionized water and stirred for 2 h. The pH of the solution was adjusted with 0.1 M NaOH or 0.1 M HCl. After 1 h of stabilization, the final solution pH was recorded, and the supernatant with small fragments was then decanted and used to conduct ζ potential measurements with a Zeta-Plus4 instrument (Brookhaven Corp., USA). All data were determined five times, and the average value was adopted. XPS Analysis. The surfaces of the pristine modified biomass and Cr-laden biomass were analyzed using X-ray photoelectron spectroscopy (XPS). The biomass with Cr was obtained through contact with 3.85 mmol/L Cr(VI) at different pH for 6 h before being washed with deionized water. All samples were freeze-dried until constant weight before analyses with an AXIS HIS spectrometer (Kratos Analytical Ltd, England) with an Al KR X-ray source (1486.71 eV of photons). 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 surface charging effects. A software package, XPSpeak 4.1, was used to fit the XPS spectra peaks, and the full width at half-maximum (FWHM) was maintained at 1.4 for all components in a particular spectrum. Adsorption Experiments. Batch adsorption experiments were carried out in 250-mL flasks, each of which contained 100 mL of chromium solution prepared with K2Cr2O7. A 0.05 g amount of biosorbent was added to a flask and shaken at 120 rpm in a thermostatic shaker at 25 °C for 6 h. The investigation on the effect of solution pH on chromium adsorption was conducted at an initial chromium concentration of 3.85 mmol/L, with the final equilibrium solution pH varying from 2 to 11. Samples were taken at various time intervals, and the total chromium concentrations in the samples were analyzed using an inductively coupled plasma emission spectrometer (ICP-ES, Perkin-Elmer Optima 3000). A colorimetric method was used to analyze the Cr(VI) concentration; the pink complex formed between 1,5diphenycarbazide and hexavalent chromium was measured at 540 nm using a spectrophotometer (13). The Cr(III) concentration was then calculated from the difference between the total Cr and Cr(VI) concentrations. The total adsorbed Cr on the biomass surface was calculated from the difference between initial and final concentration of total Cr in solution, while the adsorbed Cr(III) was calculated from the Cr(III)/Cr(VI) ratios obtained from XPS spectra. After the sorption experiment, the biomass was filtered, rinsed with
FIGURE 1. FTIR spectra of (a) pristine biomass, (b) biomass reacted with 4-bromobutyryl chloride, and (c) PEI-modified biomass. deionized water, freeze-dried, and then prepared for XPS or FTIR analysis.
Results and Discussion Surface Modification Reactions. SEM images (not shown) revealed that the pristine biomass surface was rugged with surface protuberances, but became much smoother after PEI modification, and no obvious pores were observed on the biomass surface. The pristine fungal biomass had a diameter of about 1.5 µm, and specific surface area of about 2 m2/g. After the modification, the mycelia diameter and surface area increased to 1.9 µm and 3.2 m2/g, respectively. Figure 1 shows the FTIR spectra of the pristine and modified biomass. Peaks at 3360, 1663, 1551, 1161, and 1115 cm-1 are observed in the pristine biomass spectrum (Figure 1a). The broad and strong band ranging from 3200 to 3600 cm-1 may be due to the overlapping of OH and NH stretching, which is consistent with the peaks at 1115 and 1161 cm-1 assigned to alcoholic C-O and C-N stretching vibration (14), thus showing the presence of hydroxyl and amine groups on the biomass surface. The strong peak at 1663 cm-1 can be assigned to a CdO stretching in carboxyl or amide groups, and the band 1551 cm-1 is attributed to N-H bending (14). After the biomass reacted with 4-bromobutyryl chloride (BBC), a new band at 1740 cm-1 attributed to ester (-COO-) is observed (14), indicating BBC successfully reacted with the hydroxyl groups on the biomass surface. Figure 1c shows the spectrum of the PEI-modified biomass. The disappearance of the ester peak at 1740 cm-1 may be related to a large number of the PEI macromolecules on the biomass surface, which rendered the ester group undetectable in the FTIR analysis. The broad overlapping peak shifts to 3491 cm-1 because a large number of amine groups were introduced on the surface (9). The peak evident at 1153 cm-1 attributed to the C-N stretching indicates that PEI reacted with BBC on the biomass surface. Additionally, the peaks at 1663 and 1551 cm-1 assigned to CdO stretching and N-H bending shift to 1666 and 1560 cm-1, respectively. To further verify the chemical reactions taking place in the preparation, XPS was used to analyze the pristine and modified biomass. Figure 2a and b show the O1s core-level spectra for the pristine biomass and PEI-modified biomass, respectively. For the pristine biomass, only one peak at 532.8 eV attributed to alcoholic C-O groups can be fitted to the spectrum, which further verified the presence of a large number of the hydroxyl groups on the biomass surface. After PEI modification, the O1s spectrum can be fitted to three peaks. The new binding energies at 531.8 and 533.6 eV can be assigned to the O in the CdO or OdCsO and OdCsO groups, respectively (15, 16), indicating that part of the hydroxyl groups were involved in the reaction with BBC. The acylchlorine group in BBC molecule is very active and can VOL. 39, NO. 21, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. XPS O1s spectra of the pristine biomass and PEI-modified biomass. easily react with hydroxyl and amine groups, while the hydrogen in the amine groups in PEI molecules can replace the bromine in BBC. Thus, the BBC actually serves as a bridge to graft the macromolecular PEI onto the biomass surface. As shown in Figure 3, the new groups of OdCsO and CdO were from -OOC(CH2)3-PEI and -NHCO(CH2)3-PEI, respectively. These XPS results indicate that PEI molecules were successfully grafted onto the biomass. The nitrogen atom concentration increases from 2.93% to 38.56% after PEI modification according to the results of the XPS wide-scan spectra. From the FTIR and XPS analyses, the hydroxyl groups on the pristine biomass surface are involved in the reaction, and the corresponding change in the molecular structure during the preparation of the adsorbent may be proposed as shown in Figure 3. In addition, a few amine groups were present on the pristine biomass and may also participate in the reaction as shown in Figure 3 (with amino group as a representative). The NHCdO group formed increased the concentration of CdO on the biomass surface, resulting in a higher peak at 531.8 eV than that at 533.7 eV (see Figure 2b), as the OdCsO group formed by hydroxyl group with BBC can provide equal contribution to the two peaks. After the modification, long chains of PEI are produced on the biomass surface, which provide many more sites for metal adsorption. Zeta Potential Measurement. The ζ potential of the pristine and modified biomass in solutions at different pH values are shown in Figure 4. The pristine biomass has a zero point of ζ potential at pH 3.1. In contrast, the point of zero ζ potential for the modified biomass is increased to a much higher value of 10.4, which is attributed to the protonation of amine groups in PEI molecules on the biomass surface. Thus, the ζ potential of the modified biomass is positive at pH < 10.4, while the ζ potential of the pristine biomass is positive at pH < 3.1. From the electrostatic interaction point of view, the PEI-modified biomass can be expected to provide better adsorption performance for anionic adsorbates than the pristine biomass since the surface interactions between the adsorbent and the adsorbate in the solutions are enhanced. Other researchers also reported that the zero point of ζ potential was increased significantly after the addition of 8492
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PEI on the solid surface. When PEI was covered on silicon carbide powder, its isoelectric point changed from pH 2 to pH 10.5 (17). Tang et al. reported that the zero point of charge of nano-zirconia powder shifted from pH 6 to pH 10.5 when PEI was used as a dispersant to stabilize the powder suspension (18). Trimaille et al. reported that poly(D,L-lactic acid) nanoparticles exhibited positive ζ potential at pH < 10 when PEI was adsorbed on the particle surface (19). The zero point of ζ potential of PEI-modified fungal biomass in our study corroborates these studies. Sorption Performance. The sorption kinetic experiments (data not shown) reveal that, depending on the solution pH, the sorption equilibrium occurred within 3-6 h. In the following batch sorption experiments, the sorption time was set at 6 h. An important parameter affecting metal ions sorption is pH. It not only influences the properties of the sorbent surface, but also affects the metal speciation in solution. Figure 5 shows the sorption of Cr(VI) on the PEI-modified biomass and pristine biomass as a function of the final equilibrium solution pH. Sorption by the modified biomass increases up to pH 4.6 and then decreases with increasing pH; the maximum sorption of 4.27 mmol/g for Cr(VI) occurred at pH 4.6. Some researchers reported an optimum pH ranging from 2 to 3 for the maximum sorption of Cr(VI) using the seaweed biomass of Sargassum and Ecklonia sp. (20, 21), but the optimum pH increased with increasing contact time (22). The Cr (VI) sorption at pH 7.4 was 2.53 mmol/g, thus indicating that the PEI-modified biomass was capable of adsorbing anionic chromium in neutral pH solution. In contrast, most sorbents show low sorption capacity for anionic Cr(VI) ions at pH above 5 (20, 23). The difference in performance is closely associated with the functional groups on the sorbent surface. As the most functional groups on the sorbent surface, such as the carboxyl group, are negatively charged at pH above 5, electrostatic repulsion would prevent Cr(VI) from approaching the groups, whereas amine groups can be protonated even at pH above 10, thus allowing sorption to occur through electrostatic attraction at higher solution pHs. Figure 5 also shows the consumption of H+ during chromium removal at different pHs. The H+ depletion was about 9-10 mmol/g in the pH range of 2.0-4.6, which may participate in the protonation of amine groups on the biomass and the reduction of Cr(VI). At pH above 4.6, the H+ consumption decreased significantly, and no H+ consumption was observed at pH 11. The correlation of H+ consumption and Cr(VI) reduction during the sorption process will be elucidated in the section Sorption Mechanisms. The sorption capacities (q, mmol/g) of chromium on the modified biomass as a function of residual chromium concentration (c, mmol/L) at the equilibrium pH ranges of 4.3-5.5 and 7.7-8.1 are shown in Figure 6a and b. To predict the sorption capacity of the modified biomass, Langmuir isotherm (q ) qmc/[1/b + c]) was used to fit the experimental data, and the corresponding equations are also shown in Figure 6. At the pH ranges of 7.7-8.1 and 4.3-5.5, the maximum chromium sorption capacities (qm) are 3.06 and 5.37 mmol/g of dry modified biomass, respectively. In contrast to the maximum sorption capacity (0.56 mmol/g) of the pristine biomass at pH range of 4.3-5.5 shown in Figure 6c, the maximum sorption capacity of the PEImodified biomass for Cr(VI) anions increased by 8.6-fold. The Langmuir constant (b) decreased from 1.49 to 0.68 L/mmol for the modified biomass when solution pH increased, indicating the stronger affinity of the modified biomass for chromium ions at lower solution pH. Table 1 compares the sorption capacities of the PEImodified biomass for Cr(VI) with that of several adsorbents reported in the literature. It can be seen that the sorption
FIGURE 3. Schematic diagram illustrating the grafting process of branched PEI on the biomass surface.
FIGURE 4. Zeta potential of the pristine and modified biomass as a function of solution pH.
from 123.45 mg/g (for the pristine biomass) to 200 mg/g. Among other sorbents, sorption capacities of about 150 mg/g have been reported for activated carbon (filtrosorb 400) and composite chitosan biosorbent (25, 26). Sorption Mechanisms. The sorbent surface properties, in particular the functional groups on the biomass surface, determine the sorption mechanisms. The most commonly reported mechanisms for sorption of metal ions include ion exchange, electrostatic interaction, chelation, precipitation, and complexation (7, 18). For anions, electrostatic interaction plays an important role in allowing the approach of the ions to the sorbent surfaces. The amine groups on the sorbent surface are easily protonated under acidic condition and are favorable for anion sorption (31). To understand the sorption mechanisms of Cr(VI) anions on the modified biomass, it is necessary to consider the Cr(VI) speciation and biomass surface properties at different solution pH. The speciation of Cr(VI) is determined by both pH and chromium concentration. In our study, CrO42- anion is the predominant species at pH above 6.5; HCrO4- and Cr2O7 2- anions account for about 80% and 20%, respectively, in the pH range of 2-5; and H2CrO4 and HCrO4- are major species at pH below 2 (20). For the PEI-modified biomass, the surface exhibited positive ζ potential at pH below 10.4, according to the ζ potential measurement. The amine groups (-NH-, -N
