Study of a Heavy Metal Biosorption onto Raw and Chemically

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Langmuir 2006, 22, 8906-8914

Study of a Heavy Metal Biosorption onto Raw and Chemically Modified Sargassum sp. via Spectroscopic and Modeling Analysis J. Paul Chen*,†,‡ and Lei Yang‡ DiVision of EnVironmental Science and Engineering and Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260 ReceiVed March 22, 2006. In Final Form: July 5, 2006 In this study, raw and formaldehyde-modified Sargassum sp. are used for heavy metal removal. A series of experiments shows that the chemical modification by formaldehyde improves biosorption capacity by approximately 20%. Solution pH plays an important role in the metal uptake. According to X-ray photoelectron spectroscopic and Fourier transform infrared spectroscopic analysis, the possible organic functional groups in the metal binding include carboxyl, ether, alcoholic, hydroxyl, and amino functional groups. A new model that includes a series of coordination reactions among a generalized functional group, alkaline earth metal ions and heavy metal ions, is developed for simulation of biosorption process. The model well describes the single- and multiple-species metal biosorption process under different conditions such as pH. The biosorption of heavy metals is due to the ion exchange between the heavy metals and alkaline earth metals and their adsorption onto the free sites of the seaweeds. Slightly more than half of the metal uptake is due to ion exchange. The metal affinity for the functional groups follows a descending order of lead > copper > alkaline earth metal.

1. Introduction Heavy and precious metals are used in operations of various industries such as the semiconductor industry. As a result, waste streams from the industries contain metal ions. Many studies have shown that they are highly toxic and can seriously damage our aqueous environment. Several engineering processes such as ion exchange, adsorption, and precipitation have been used to treat the metal contaminants.1 However, they all have their own limitations. Biosorption is one of the potentially promising technologies for the removal and recovery of heavy metal ions from industrial waste streams. It can be an alternative to the above conventional processes. Biosorbents can effectively sequester metal ions from aqueous solutions. Marine algal biomass is a good biosorbent due to its high uptake capacity and the ready abundance of biomass in many parts of the world.2-5 The biosorption capability of algae is attributed mainly to the cell wall, which contains various polysaccharides and other highly complex organic compounds. Biosorbents have a high affinity for divalent cations.2-5 Metal sorption performance depends greatly on the chemistry of solutions.6-15 Solution pH plays an * Corresponding author. Fax: +1-831-303-8636; +65-6872-5483. E-mail: [email protected]; [email protected]. † Division of Environmental Science and Engineering. ‡ Department of Chemical and Biomolecular Engineering. (1) Khan, E.; Huang, C. P.; Reed, B. E. Hazardous waste treatment technologies. Water EnViron. Res. 2004, 76 (6), 1872-1966. (2) Figueira, M. M.; Volesky, B.; Mathieu, H. J. Instrumental analysis study of iron species biosorption by Sargassum biomass. EnViron. Sci. Technol. 1999, 33 (11), 1840-1846. (3) Schiewer, S.; Volesky, B. Modeling multi-metal ion exchange in biosorption. EnViron. Sci. Technol. 1996, 30 (10), 2921-2927. (4) Sheng, P. X.; Ting, Y. P.; Chen, J. P.; Hong, L. Sorption of Lead, Copper, Cadmium, Zinc and Nickel by Marine Algal Biomass: Characterization of Biosorptive Capacity and Investigation of Mechanisms. J. Colloid Interface Sci. 2004, 275 (1), 131-141. (5) Chen, J. P.; Hong, L. A.; Wu, S. N.; Wang, L. Elucidation of interactions between metal ions and Ca alginate-based ion-exchange resin by spectroscopic analysis and modeling simulation. Langmuir 2002, 18 (24), 9413-9421. (6) Buschmann, J.; Sigg, L. Antimony(III) binding to humic substances: Influence of pH and type of humic acid. EnViron. Sci. Technol. 2004, 38 (17), 4535-4541. (7) Dzombak, D. A.; Morel, F. M. M. Surface Complexation Modeling: Hydrous Ferric Oxide; Wiley-Interscience: New York, 1990.

important role. It can change the nature of metal speciation and thus affect the sorption. With a higher pH, cationic metal ion uptake is generally enhanced. When raw biosorbents such as seaweeds are used for metal waste treatment and recovery, high organic leaching has been observed. The leaching can be quantified by total organic carbon (TOC, in milligrams per liter). This problem has posed a great challenge in its industrial application. Several chemical approaches were adopted in our laboratory to modify locally derived raw seaweed (RSW), Sargassum sp., to achieve prevention of organic leaching and enhancement of metal biosorption.9 It was found that the modification by 0.2% formaldehyde led to the best performance of the modified seaweed (MSW). The organic content of the filtrated water samples was only 3.84 mg/L TOC, 80% less than that of the RSW. The metal biosorption capacity was greatly improved, while the uptake kinetics was similar to that of the RSW. To develop a biosorption process by using the above sorbents for industrial applications, it is desirable to better understand the metal uptake process. Advanced instruments such as X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) can provide good tools to achieve the goal (8) Dixit, S.; Hering, J. G.; Comparison of arsenic(V) and arsenic(III) sorption onto iron oxide minerals: Implications for arsenic mobility. EnViron. Sci. Technol. 2003, 37 (18), 4182-4189. (9) Chen, J. P.; Yang, L. Chemical Modification of Sargassum sp. for Prevention of Organic Leaching and Enhancement of Uptake during Metal Biosorption. Ind. Eng. Chem. Res. 2005, 44, 9931-9942. (10) Ravat, C.; Dumonceau, J.; Monteil-Rivera, F. Acid/base and Cu(II) binding properties of natural organic matter extracted from wheat bran: Modeling by the surface complexation model. Water Res. 2000, 34 (4), 1327-1339. (11) Esposito, A.; Pagnanelli, F.; Veglio, F. pH-related equilibrium models for biosorption in single metal systems. Chem. Eng. Sci. 2002, 57 (3), 307-313. (12) Ma, W.; Tobin, J. M. Development of multimetal binding model and application to binary metal biosorption onto peat biomass. Water Res. 2003, 37 (16), 3967-3977. (13) Kaulbach, E. S.; Szymanowski, J. E. S.; Fein, J. B. Surface complexation modeling of proton and Cd adsorption onto an algal cell wall. EnViron. Sci. Technol. 2005, 39 (11), 4060-4065. (14) Schiewer, S.; Volesky, B. Modeling of the proton-metal ion exchange in biosorption. EnViron. Sci. Technol. 1995, 29 (12), 3049-3058. (15) Yun, Y.-S.; Volesky, B. Modeling of lithium interference in cadmium biosorption. EnViron. Sci. Technol. 2003, 37 (16), 3601-3608.

10.1021/la060770+ CCC: $33.50 © 2006 American Chemical Society Published on Web 09/13/2006

HeaVy Metal Biosorption on Sargassum sp.

because they can provide insight into environmental processes at the molecular level.5,16,17 XPS and FT-IR have been adopted to identify the major functional groups in a biopolymer and coordination types between the metal ions and the surface ligands.2,6,17 For example, the involvement of ion exchange, sorption, and reduction was determined in the biosorption of Cu(II) and Cr(VI) on chitosan according to XPS analysis.17 It has been observed that carboxyl, ether, alcoholic, hydroxyl, and amino functional groups are involved in the metal binding by various sorbents.2,4,9,17 Both instrumental analytical techniques cannot provide a quantitative description of sorption processes, which is important in both the design of engineering treatment systems and environmental assessment. Nonetheless, XPS and FT-IR measurements are essential to provide evidence for further supporting the developed mathematical model(s). The biosorption process is usually described by several empirical models (e.g., Freundlich and Langmuir isotherms) and mechanistic models.7-15 The empirical models have been successfully used to describe experimental results under certain conditions. However, they cannot predict the effect of such important factors as pH. Some efforts have been made to improve the models. For example, Esposito et al. modified the extended Langmuir equation by introducing a term for pH effect.11 The model parameters, however, are pH-dependent. Therefore, mechanistic models must be developed to describe biosorption phenomena. Two mechanistic models have been reported in the literature: surface complex formation (SCF)/coordination models7-10,12,13 and ion exchange models.3,14,15 Ravat and co-workers used an SCF model to describe the acid/base and copper binding properties of a natural biomass.10 Ma and Tobin used a one-site coordination reaction for multiple-species metal binding onto peat biomass.12 The model showed good fitting of biosorption isotherm data. The effect of pH was not simulated by the model. An ion exchange model was developed and used to illustrate the metal biosorption onto the protonated Sargassum.3,14,15 The above models reported in the literature are able to simulate the interactions of specified biomass and metals. In the ion exchange model,3,14,15 hydrogen ions are assumed to exchange with the cationic heavy metal ions in aqueous solutions. The surface formation/coordination models, on the other hand, assume that the heavy metal uptake is due to the presence of organic functional groups. The presence of alkaline earth metal ions is not considered, the assumption of which could be valid since the original organisms to be used as biosorbents grow in freshwater environments. Raw seaweeds (e.g., Sargassum sp. in this study) are essentially saturated with various light metal ions (e.g., calcium). When the biosorption process is simulated, it is important to take the presence of light metal ions into consideration. A combination of coordination reaction and ion exchange may better describe the biosorption processes. In the present work, the RSW and the MSW were used to remove heavy metals from aqueous solutions. A series of experiments was first conducted to obtain the biosorption properties of both biosorbents. Instrumental analysis by FT-IR and XPS was performed to obtain the chemistry in which the metal ions and the functional groups of both biosorbents were involved. The changes in the sorption chemistry were then compared. An equilibrium model capable of identifying the (16) Taboada-Serrano, P.; Vithayaveroj, V.; Yiacoumi, S.; Tsouris, C. Surface charge heterogeneities measured by atomic force microscopy. EnViron. Sci. Technol. 2005, 39 (17), 6352-6360. (17) Dambies, L.; Guimon, C.; Yiacoumi, S.; Guibal, E. Characterization of metal ion interactions with chitosan by X-ray photoelectron spectroscopy. Colloid Surf., A 2001, 177 (2-3), 203-214.

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important sorption chemistry was developed, which was subsequently used to predict the biosorption process as a function of operational parameters. 2. Materials and Methods 2.1. Materials. The raw biomass of Sargassum sp. (the RSW) was harvested from the coasts in Singapore. The biomass was first washed several times with deionized water to remove impurities, and then dried overnight in an oven with the temperature controlled at 60 °C. The dried seaweed was ground to fine particles with sizes from 500 to 800 µm. Reagent-grade lead nitrate and copper nitrate from Merck (Germany) were used to prepare the stock metal solutions. The modified seaweed (the MSW) was prepared as follows. One gram of the RSW was soaked in 100 mL of 0.2% formaldehyde solution for 24 h. The resulted MSW was then filtered from the mixture, washed with 300 mL of deionized water several times, and dried overnight in an oven at 60 °C. Protonated RSW and MSW were prepared by soaking the biosorbent in a 0.2 M HNO3 solution. The mass concentrations were 10 g/L, while the reaction time was 3 h. The biosorbents were rinsed with deionized water until the solution pH reached a constant value of 4.5. The protonated biosorbents were dried at 60 °C in an oven overnight. 2.2. Potentiometric Titration. A 0.2 g sample of protonated biosorbent (RSW or MSW) was dispersed in 100 mL of 0.1 M NaNO3 solution. The potentiometric titration of biomass was carried out with an automatic titrator (Metrohm 716 DMS Titrino) by an NaOH solution. The same procedure was repeated for the MSW. 2.3. Desorption of Light Metal Ions in the RSW and the MSW. A 0.2 g sample of the RSW was added to 100 mL of solution, the pH of which was varied by HNO3 or NaOH. The flasks containing the solutions were shaken in an orbital shaker (DK-OS010, Daiki Sciences Co. Ltd., Korea) overnight. The solution pH after the experiment was recorded. The concentrations of alkali metal ions and alkaline earth metal ions in the supernatants were analyzed by inductively coupled plasma emission spectroscopy (ICP-ES) (PerkinElmer Optima 3000) to determine the calcium and magnesium contents as a function of pH. The same procedure was repeated for the MSW, except that the biomass concentration (m) was changed to 1 g/L. 2.4. Batch Biosorption Experiments. A series of metal biosorption experiments was conducted; the factors in the investigation included pH, adsorption capacity, and competitive effect. The data were subsequently used for the model development as well as its validation. In the pH effect experiment, the desired solution pH was first adjusted by HNO3 or NaOH. The biosorbent (RSW or MSW) was added to the solutions while being shaken at 200 rpm in the orbital shaker. The experiment was performed at room temperature of 22 ( 1 °C. The pH was frequently measured and adjusted accordingly by HNO3 or NaOH. After the experiments, the supernatants were taken from each flask, acidified, and filtered. The metal concentrations of the samples were determined by ICP-ES. In the adsorption isotherm experiments, the solution pH was controlled at 5.0. The initial concentration of heavy metal ions was varied. Other procedures were the same as those used in the above pH effect experiment. The metal uptake at equilibrium was calculated by the following equation: qe )

V(Ci - Ce) W

(1)

where qe is the metal uptake, V is the solution volume, W is the amount of sorbent, and Ci and Ce are the initial and the final (or equilibrium) sorbate concentrations, respectively. In the competitive metal sorption experiments, two heavy metal ions were studied. In each experiment, the initial concentration of one heavy metal ion was fixed while that of the other one was varied. Other procedures were the same as those used in the adsorption isotherm experiments.

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Figure 1. Potentiometric titration of raw and modified Sargassum sp. 2.5. Fourier Transform Infrared Spectroscopy. Fourier transform infrared spectroscopy was used to determine the changes in vibration frequency in the functional groups in the fresh, protonated, and metal-loaded biosorbents. The spectra were collected by an FTS-135 spectrometer (Bio-Rad, USA) within the wavenumber range of 400-4000 cm-1. Specimens of various biosorbents were first mixed with KBr and then ground in an agate mortar (Merck, for spectroscopy) at an approximate ratio of 1/100 for the preparation of pellets (weight of 100 mg). The resulting mixture was pressed at 10 tons for 5 min. Sixteen scans and 8 cm-1 resolutions were applied in recording the spectra. The background obtained from the scan of pure KBr was automatically subtracted from the sample spectra. All spectra were plotted using the same scale on the absorbance axis. 2.6. X-ray Photoelectron Spectroscopy. XPS spectra were obtained with an Axis His spectrometer (Kratos Analytical, Japan) using monochromatized Al KR radiation (1486.6 eV); the source was operated at 15 kV and 10 mA. The RSW, MSW, protonated RSW, protonated MSW, copper-loaded RSW (shown as Cu+RSW in Figures 7-10), and copper-loaded MSW (shown as Cu+MSW in the figures) were analyzed. In the preparation of both copperloaded RSW and MSW, the biosorbents were completely saturated in the concentrated copper ion solution. Prior to XPS measurement all the biosorbent samples were dried for 2 h at 60 °C. The vacuum in the analysis chamber was always better than 5 × 10-8 Pa. Survey scans were collected from 0 to 1200 eV with a pass energy of 80 eV. High-resolution scans for the element Cr were performed over the 570-582 eV range, with the pass energy adjusted to 0.1 eV. Each spectral region was scanned between 15 and 200 times, depending on the intensity of the signal, to obtain an acceptable signal-to-noise ratio at reasonable acquisition times. The spectral deconvolution was performed using the curve-fitting program with the subtraction of Shirley background; the line width (full width at half-maximum) of the Gaussian peaks was maintained constant for all components in a particular spectrum. Surface elemental compositions were determined from XPS peak area ratios, after correction with the experimentally determined sensitivity factors.

3. Results and Discussion 3.1. Potentiometric Titration. As shown in Figure 1, protonated RSW and MSW have a weak acid property. Some hydrogen ions are released from the biosorbents to neutralize the hydroxide when the NaOH is added. Before the sodium hydroxide was added, the pH values were 3.5 and 3.8 for the solutions containing the MSW and the RSW, respectively. It can be found that the RSW and the MSW feature several pK values. From the diagram it can be seen that the MSW has a slightly higher capacity for neutralizing hydroxide ions than the RSW. The titration data were used to determine the model parameters discussed later.

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Figure 2. Desorption of alkaline earth metal ions in the biomass as a function of pH.

Figure 3. pH effect on copper uptake onto biosorbents.

3.2. Desorption of Light Metal Ions from the RSW and the MSW. Desorption experiments were performed to study the leaching of alkali metal ions and alkaline earth metal ions from the RSW and the MSW as a function of pH. Figure 2 shows that the release of alkaline earth metal ions decreases with an increase in pH. The exchange between calcium and magnesium ions and hydrogen ions contributes to the changes. The presence of both metal ions can play an important role in heavy metal biosorption, which is confirmed below in the sorption isotherm experiments and modeling study. The figure also demonstrates that the chemical modification leads to more alkaline earth metal ions leached out from the solids. This may be attributed to the increase in the content of functional groups during the modification, which provides more adsorption sites for alkaline earth metal ions and thus enhances the metal sorption. When the solution pH is shifted to a lower pH, more alkaline earth metal ions are thus released into the solution. Around 2 mM alkali metal ions (Na+ and K+) are released from the RSW into the solution. The leaching was found to be independent of solution pH (data not shown). This indicates that these alkali metal ions are not involved in the ion exchange with protons. 3.3. Effect of pH on Metal Biosorption. Figure 3 shows the biosorption of copper onto the RSW and the MSW as a function of pH. The metal uptake increases from a lower pH and reaches a plateau at equilibrium pH >4.5. The biosorption of other metal ions such as lead also demonstrated a similar behavior (data not shown). It was observed that the alkaline earth metal ions were released during the heavy metal ion sorption.

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Figure 4. Copper isothermal biosorption onto RSW and MSW.

The dependence of copper uptake on pH is related to the surface functional groups on the cell walls and the metal speciation. At lower pH (e.g., pH 2), more hydrogen ions compete with the heavy metal ions for the functional groups, which reduces the chances for the formation of heavy metal ion-functional group complexes and thus fewer heavy metal ions are removed. At higher pH (e.g., pH >5), more ligands become available for metal ion binding, and hence the biosorption is enhanced. 3.4. Biosorption Isotherms. Isothermal experiments of heavy metal biosorption at various pHs were carried out. Figure 4 demonstrates a typical isothermal biosorption of copper ions onto the MSW and the RSW at pH 5. The metal biosorption is enhanced by approximately 20% after the chemical modification is applied. This clearly demonstrates that the chemical modification improves the biosorption capacity. As shown in Figure 2, the contents of alkaline earth metal ions are 6.22 × 10-4 and 1.18 × 10-3 mol/g for the RSW and the MSW, respectively. Figure 4 shows that the maximum sorption capacities for the RSW and the MSW are 0.99 × 10-4 and 1.2 × 10-3 mol/g, respectively, which are higher than the contents of alkaline earth metal ions. This indicates that, in addition to the ion exchange mechanism, there are other reactions contributing to the heavy metal binding. 3.5. Competitive Effect on Metal Biosorption. The metal uptake can be affected by the presence of other competitive metal ions. Figure 5 demonstrates simultaneous lead and copper biosorption onto the RSW. Due to the presence of the competitive metal ions, the uptake of the metal ions is negatively affected. The influence on the metal binding, however, is dependent upon the binding strength of the metal ions onto the sorbent. As shown in Figure 5a, the presence of copper ions slightly affects the biosorption of lead ions; however, lead ions greatly hinder the copper uptake as demonstrated in Figure 5b. The maximum metal adsorption capacity for lead is 1.16 mmol/g RSW, while that for copper is 0.99 mmol/g RSW. Lead ions can form stronger metal complexes than copper ions. As a result, the competitive effect from the lead ions is more obvious than that from the copper ions. 3.6. Fourier Transform Infrared Spectroscopy. Infrared spectra of the protonated, Cu-loaded, and pristine RSW and MSW are shown in Figure 6. The protonated seaweeds (RSW-H and MSW-H) can be treated as a reference; the biosorbents before metal sorption are termed “pristine biosorbents”. The protonated RSW and MSW display similar spectra: the absorbance at 1737.9 cm-1 and that at 1234.4 cm-1 correspond to stretching vibrations of carbonyl double bond (υC)O) and carbon-oxygen single bond (υC-O), respectively.18 This indicates the typical carboxylic

Figure 5. Simultaneous biosorption of lead and copper ions onto RSW.

Figure 6. FT-IR spectra of RSWs and MSWs.

absorption. After the copper ions are adsorbed onto the biosorbents, the carbonyl double bond stretching band exhibits a clear shift to a lower frequency at 1649.1 cm-1, while the carbon-

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Table 1. Carboxyl Stretching Frequencies for Different Species of Sargassum sp. species of Sargassum sp.

υC)O

υC-O

∆ ) υC)O - υC-O

protonated RSW RSW copper-loaded RSW

1737.9 1651.1 1649.1

1234.4 1419.6 1419.6

503.5 231.5 229.5

protonated MSW MSW copper-loaded MSW

1737.9 1649.1 1649.1

1234.4 1423.5 1419.6

503.5 225.6 229.5

oxygen single bond band increases to 1419.6 cm-1, corresponding to the complexation of copper to CdO and C-O bonds. Similar shifts are also found in the pristine RSW and MSW, which is due to the binding of alkaline earth ions (e.g., Ca2+) with the carboxyl groups. Not much difference is found in the IR spectra between the pristine RSW/MSW and copper-loaded RSW/ MSW. The frequencies of CdO or C-O bond stretching and their differences (∆ ) υC)O - υC-O) in different forms of Sargassum sp. are summarized in Table 1. The difference between CdO and C-O bond stretching is related to the relative symmetry of these two carbon-oxygen bonds and reflects the nature of carboxyl group binding status. Table 1 shows that the protonated biosorbent has much larger ∆ values (503.5 cm-1) than the metal chelated biosorbents (225.6-231.5 cm-1). The lower values of ∆ in the presence of metal ions clearly indicate more involvement of pendant carboxylate groups forming complexes with metal ions. The four types of complexation of divalent metal (M2+) and carboxyl groups in the biosorbents can be illustrated as follows.

The infrared frequency at 1033.8 cm-1 is associated with the stretching of alcoholic groups.18 There is no shift among protonated, pristine, and copper-loaded biosorbents, indicating alcoholic groups (-OH) participate less in the binding of metal ions (alkaline metal and copper ions). The infrared frequencies at 1530-1560 cm-1 can be assigned to amine groups. Comparing with protonated seaweeds, the absorbency at this range for calcium- and copper-loaded seaweeds is less obvious, indicating the binding of metal ions onto the amine groups. A shift of 3358 cm-1 to 3396-3445 cm-1 is observed when one compares the spectra of protonated seaweeds with those of metal-loaded seaweeds. The frequencies between 3200 and 3500 cm-1 may contains stretching of both amine (-NH2) and alcoholic groups (-OH).18 One explanation is that part of the amine group may be involved in the metal binding. In the presence of formaldehyde, amine could react with the aldehyde group. Another possibility is that formaldehyde can react with an alcoholic group to form a dative linked structure according to the literature:19

2R-OH + HCHO f (R-O)2CH2 + H2O

(2)

3.7. X-ray Photoelectron Spectroscopy. Binding energy (BE) profiles of copper atom (Cu 2p3/2), carbon atom (C 1s), and oxygen atom (O 1s) in Sargassum sp. before and after the metal biosorption are shown in Figure 7. The wide scan clearly shows a small peak around 933 eV after the copper biosorption onto both the RSW and the MSW, indicating the accumulation of (18) Clothup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: London, 1990. (19) Clayden, J.; Greeves, N.; Warren, S.; Wothers, P. Nucleophilic substitution at C)O with loss of carbonyl oxygen. Organic Chemistry; Oxford University Press: Oxford, U.K., 2001; Chapter 14.

Figure 7. XPS wide-scan spectra of Cu-loaded Sargassum sp.

copper on the biosorbents. It is shown in Figure 8 that the peak area of Cu 2p3/2 in the copper-loaded MSW is larger than that in the copper-loaded RSW, reflecting the higher copper biosorption capacity of the MSW. This is consistent with the observations in both sorption isotherm and pH effect experiments. A less obvious peak around 952 eV is observed in both the Cu-MSW and the Cu-RSW samples. The peak can be assigned to the copper oxide (CuO) crystal.20 In the preparation of samples for the XPS analysis, the copper ion solution with higher concentration was used so that the RSM and the MSW were completely saturated with the copper ions. At a result, copper hydroxide (Cu(OH)2) may be formed, which was then converted to copper oxide (CuO). The C 1s spectra of all the samples shown in Figures 9 and 10 comprise four peaks with BEs of approximately 284.6, 286.1, 287.7, and 289.1 eV differentiated via deconvolution. These peaks can be assigned to C-C, C-O, O-C-O, and OdCsO bonds; the last three can be assigned to alcoholic, ether, and carboxylate groups.20 The carbon atoms of these three respective organic (20) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray photoelectron spectroscopy; Perkin-Elmer Corp.: Eden Prairie, MN, 1992.

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This indicates a decrease in electron density in C-O, probably due to the deformation of the ether groups. From the viewpoint of the change in the geometric shape of organic functional groups, the ether group would undergo the greatest change among the three functional groups (i.e., OH, COOH, and O-C-O) on the biosorbents, when they form the metal complexes. The C-O-C angle of an ether group would expand a certain extent upon its coordination with a metal ion. This adjustment of geometric shape could be another factor responsible for the shift in binding energy of the involved ether carbons. 3.8. Formulation of Biosorption Model. Carboxyl, ether, alcoholic, hydroxyl, and amino functional groups are involved in the metal binding onto the biosorbent as observed from the above instrumental analysis. The metal-functional group complexes can be three-dimensional and highly complicated. For example, according to the preceding discussions on the FT-IR and XPS spectra of the copper-loaded biomass, the triple-ligand complexation manner may be associated with the tetrahedral or square planar geometry as sketched below since they can be formed between alginate-like polymer chains.

Figure 8. XPS spectra of Cu 2p3/2.

functional groups of the seaweeds are typical in algae polysaccharides and have different electron densities.5 The area distribution of the three peaks represents relative popularities of these functional groups in the biosorbents. The areas of carbon bonds (C 1s) among protonated, pristine, and metal-loaded seaweed were calculated and are shown in Table 2. The content of C-C of the RSW is 0.50; it drops to 0.47 after protonation (RSW-H). This indicates a loss in the organic content after the acid is applied. On the other hand, the C-C content in the MSW (0.49) is the same as that in the protonated MSW, showing very limited organic leaching during the protonation. The relative quantity in OdCsO decreases slightly in descending order of the protonated (RSW-H and MSW-H), pristine (RSW and MSW), and copper-loaded (RSW-Cu and MSW-Cu) seaweeds as demonstrated in Table 2. It indicates the formation of carboxyl-metal complexes, in which oxygen atom donates electrons to metal ions and thus the electron density at the adjacent carbon atom in CdO and C-O decreases. Figures 9 and 10 show the XPS spectral changes in the O 1s peak of metal-bound, pristine, and protonated biomass. BE peaks of 529.8, 531.0, and 532.3 eV can be assigned to metal oxide, CdO, and C-O (carboxyl and ether), respectively.20 The BE of the peak at 533.6 eV is beyond the BE range of most organic functional groups; it is tenable to accept this peak as a result of the coordination of hydroxyl groups to metal ions. It should be noticed that metal oxide content decreases after the biosorption process, indicating that part of the alkaline compounds were washed out. After the heavy metals are adsorbed, the quantity of CdO groups decreases, while that of the C-O groups increases. As the metal ions are bound onto the carboxyl groups, the electron density toward the carbon atoms decreases. The content variation of CdO and C-O can also be explained by the four types of complexes shown in the XPS analysis. Copper has a higher affinity to carboxyl than calcium and hydrogen ions. The exchange of copper with Ca or H results in the conversion of the carboxyl groups from protonated type to bridging type, which reflects in a decrease of CdO content with an increase in C-O content. Table 2 shows that the relative quantity of the ether group (O-C-O) in both RSWs and MSWs decreases with a descending order of heavy metal loaded, pristine, and protonated biosorbents.

Because of the high complicacy of the adsorption chemistry, a new simplified mathematical model is developed in this study. In the modeling simulation, the metal biosorption onto the RSW and the MSW is due to two mechanisms: (1) ion exchange between the alkaline earth metal ions and the heavy metal ions, and (2) coordination reactions between the organic functional groups and the heavy metal ions. The process can be visualized as follows. The alkaline earth metal ions are initially bound onto the organic functional groups in the biosorbents. When the heavy metal ions are introduced into the solution, they compete with the alkaline earth metal ions for the adsorption sites (i.e., organic functional groups). As the binding strength (affinity) between the heavy metal ions and functional groups is greater than that between the alkaline earth metal ions and functional groups, the heavy metal ions are adsorbed onto the biosorbents. The alkaline earth metal ions are meanwhile stripped from the biosorbents and released to the solution phase. In addition, some of the functional groups that initially do not form coordination with the alkaline earth metal ions can form metal complexes when the heavy metal ions are introduced into the solution. In the model development, an engineering approach was applied in order to identify a few generalized important functional groups that are responsible for the metal binding. They are able to form complexes with hydrogen ions, alkaline earth metal ions, and heavy metal ions according to the following reactions:

H+ + Li- ) HLi M2+ + jLi- ) MLi2-j

i ) 1, 2, ..., m j ) 1, 2, ..., n

(3a) (3b)

where Li- represents the functional groups and the metal ions. M2+ can be alkaline earth metal ions and/or heavy metal ions to be removed. FITEQL 4.0 was used in this study for the determination of model parameters and representation of experimental observations.21 The experimental data from titration and adsorption/ desorption studies provided the input data for the model so that

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Figure 9. XPS spectra of C 1s and O 1s of metal-loaded seaweed.

the model parameters were determined. After the parameters were obtained, the program was employed to represent the experimental data and predict the sorption behavior. 3.9. Determination of Model Parameters. It was first assumed that few types of functional groups existed in the biomass. When the biomass was protonated, the functional groups had weak acid/base properties. The experimental data from the potentiometric titration of protonated biomass provided input for the determination of reaction constants and total concentrations of functional groups. A modeling simulation with assumptions of one or two functional groups was conducted. The results, however, could not match the experimental data. When the biosorbents with three functional groups were assumed and used in the (21) Herbelin, A.; Westall, J. FITEQL: A Computer Program for Determination of Chemical Equilibrium Constants from Experimental Data, Version 4.0; Technical Report; Department of Chemistry, Oregon State University: Corvallis, OR, 1999.

calculation (i.e., i ) 3 in eqs 3), the simulation results were significantly improved as demonstrated in Figure 1. These three reactions are listed in Table 3; the content of each important group (L1-L3) and the constants of the coordination reactions between the functional groups and hydrogen ions are also given in the table. The reaction constants are quite close to those found in the titration of algal cells.5,13 The protonated RSW and MSW behave like a typical inorganic sorbent with three weak acidic sites. The apparent dissociation constant of HL1 (pK of 4.78 and 3.93 for the RSW and the MSW) is slightly higher than that of pure alginate (pK of 3.38 for polymannuronic acid and pK of 3.65 for polyguluronic acid).22 The pK of the RSW or the MSW is quite consistent with that for the carboxylic acids; this suggests (22) Percival, E.; McDowell, R. H. Chemistry and enzymology of marine algal polysaccharides; Academic Press: London, New York, 1967.

HeaVy Metal Biosorption on Sargassum sp.

Langmuir, Vol. 22, No. 21, 2006 8913

Figure 10. XPS spectra of C 1s and O 1s of protonated seaweed.

that the COO- groups play a key role in the formation of the complexes. The pK of the MSW is lower than that of the RSW, as shown in Table 3. This demonstrates that the affinity of hydrogen for the functional group (L1) is reduced or the acidity of HL1 is enhanced due to the surface reactions such as eq 2 during formaldehyde modification. Based on the calculation by MINEQL+ (version 4.5),23 the metal ions exist as free metal species (M2+) in the concentration and pH ranges in this study. Thus, metal precipitation and aqueous solution reactions (e.g., formation of metal hydroxides) are negligible. The pK value of HL3 is far above the pH range in the experiments; thus, only L1- and L2- can be considered to participate in the alkaline earth metal binding and L1- participates in the heavy metal binding. Since the alkaline earth metal ions (Ca2+ and Mg2+ represented as Alk2+) are originally present in both the RSW and the MSW, the metal binding is determined by the desorption experimental data given in Figure 2. In the modeling, eqs i-vi of Table 3 were used. As shown, the model well represents the experimental data. It must be pointed out that eq vi is used in the modeling for the MSW, but not for the RSW. Good matching between the modeling result and the experimental data is demonstrated in the figure. The additional reaction may result from the changes in chemical structures during the modification. The characterization of binding between the heavy metal ions and the functional group was subsequently determined by using biosorption isotherm data (e.g., copper adsorption in Figure 4). The reaction constants for the heavy metal binding onto the functional group are given in Table 3. Figure 4 shows an example of modeling simulation for the copper adsorption isotherm at pH 5. The computation results by the model developed in this study well match the experimental observations. Comparison of the constants for metal-functional (23) Schecher, W. D. MINEQL+: A chemical equilibrium program for personal computers, users manual Version 4.5; Environmental Research Software: Hallowell, ME, 2002.

group interaction reactions in the table indicates the following descending sequence for the RSW: lead > copper > alkaline earth metal. The content of each functional group is enhanced due to the surface modification by formaldehyde as observed in Table 3. Comparison of the contents show that the modification leads to an increase by 94-455%. This can explain the enhancement in the buffering capacity for base and the metal biosorption by the MSW illustrated in Figures 1, 4, and 8. It is also noted that the metal biosorption capacity does not increase proportionally as the content of functional groups is increased (e.g., Figure 4), which is due to the decrease in the complex formation constants between the metal ions and the functional groups. For example, the amine groups react with aldehyde groups, leading to the amine groups being occupied and less available for the binding metals. As a result, one can see that the chelating potential of the MSW is not significantly enhanced. From the above simulation results, it is clearly shown that the biosorption is attributed to the ion exchange and coordination reactions. The ion exchange mechanism can be described by the combination of several simple equations listed in Table 3. Comparison of the total content of functional group HL1 with that of alkaline earth metal ions indicates that slightly more than half of the metal uptake is due to the ion exchange; however, the coordination reactions cannot be neglected. The good fitting by the model again suggests that the COO- groups play a significant role in the formation of the metal complexes. 3.10. Prediction of Biosorption Behavior. To test its applicability, the model was used to “predict” the pH effect on the metal uptake and the competitive metal biosorption. Figures 3 and 5 show that the experimental observations are well described. The validity of the model and its successful prediction of the biosorption performance further prove that a generalized type of function group (L1) is a valid assumption responsible for the heavy metal-biomass interaction. The binding of metal ions by the MSW and the RSW is a result of not only ion exchange with alkaline earth metal ions but also the adsorption onto the free

8914 Langmuir, Vol. 22, No. 21, 2006

Chen and Yang

Table 2. Summary of Binding Energy and Area Ratios of Sargassum sp. sample surface

proposed components

RSW-H

binding energy (eV) C 1s Valence State 284.6 286.2 287.7 289.0 284.6 286.1 287.7 289.1 284.6 286.1 287.7 289.1 284.6 286.2 287.6 289.1 284.5 286.1 287.6 289.0 284.6 286.1 287.7 289.1 O 1s Valence State 529.8 531.0 532.3 529.8 531.0 532.3 529.8 531.0 532.3 529.8 531.0 532.3 529.8 531.0 532.3 529.8 531.0 532.3

C-C C-O O-C-O OdC-O C-C C-O O-C-O OdC-O C-C C-O O-C-O OdC-O C-C C-O O-C-O OdC-O C-C C-O O-C-O OdC-O C-C C-O O-C-O OdC-O

RSW

RSW+Cu

MSW-H

MSW

MSW+Cu

RSW-H

metal oxide CdO OH-; O-C-O metal oxide CdO OH-; O-C-O metal oxide CdO OH-; O-C-O metal oxide CdO OH-; O-C-O metal oxide CdO OH-; O-C-O metal oxide CdO OH-; O-C-O

RSW RSW+Cu MSW-H MSW MSW+Cu

Table 3. Chemical Reactions in the Biosorption Modeling Simulation Processa,b reaction constant no.

reaction

RSW

MSW

i ii iii iv v vi vii viii ix x

H+ + L1- ) HL1 H+ + L2- ) HL2 H+ + L3- ) HL3 Alk2+ + 2L1- ) Alk(L1)2 Alk2+ + L1- ) AlkL1+ Alk2+ + L2- ) AlkL2+ Cu2+ + 2L1- ) Cu(L1)2 Cu2+ + L1- ) CuL1+ Pb2+ + 2L1- ) Pb(L1)2 Pb2+ + L1- ) PbL1+

104.78 M-1 107.51 M-1 1010.02 M-1 108.69 M-2 102.69 M-1

103.93 M-1 107.21 M-1 1010.10 M-1 106.99 M-2 102.47 M-1 106.83 M-1 107.92 M-2 103.93 M-1

THL1 (mol/g) THL2 (mol/g) THL3 (mol/g) TAlk (mol/g) a

109.01 M-2 105.05 M-1 1010.24 M-2 105.63 M-1 RSW

MSW

1.01 × 10-3 1.98 × 10-4 4.37 × 10-3 6.22 × 10-4

1.96 × 10-3 9.01 × 10-4 1.64 × 10-3 1.18 × 10-3

Alk2+ ) Ca2+ + Mg2+. b T represents total concentration.

sites of the biomass. The metal ions are bound to the biomass via forming ML1+ and M(L1)2 metal complexes.

4. Conclusions Higher solution pH causes higher heavy metal biosorption onto both the RSW and the MSW. Chemical modification by

intensity (counts/s)

relative quantity

1521.2 1192.3 400.7 152.1 2930.9 1898.5 757.4 249.2 2441.4 1641.3 502.2 135.8 1656.6 1122.0 455.9 173.8 1783.8 1239.2 475.2 146.2 2504.8 1993.0 703.6 178.4

0.47 0.37 0.12 0.05 0.50 0.33 0.13 0.04 0.52 0.35 0.11 0.03 0.49 0.33 0.13 0.05 0.49 0.34 0.13 0.04 0.47 0.37 0.13 0.03

938.7 1851.0 161.3 1941.5 2949.6 357.4 959.4 1855.1 389.0 777.6 2355.0 456.2 834.4 2135.3 353.8 1043.9 2427.6 706.2

0.32 0.63 0.05 0.37 0.56 0.07 0.30 0.58 0.12 0.22 0.66 0.13 0.25 0.64 0.11 0.24 0.56 0.16

formaldehyde enhances the metal uptake capacity by approximately 20%. IR and XPS analysis demonstrates that carboxyl, ether, alcoholic, hydroxyl, and amino functional groups are involved in the metal binding onto the biosorbent. A new model is developed for the simulation of the biosorption process. It includes a series of coordination reactions among generalized functional groups, alkaline earth metal ions, and heavy metal ions. Through several examples, it is shown that the model well describes the single- and multiple-species metal biosorption process under different operational conditions such as pH. The binding of metal ions by the biosorbents is due to: (1) ion exchange with alkaline earth metal ions, and (2) formation of metal complexes with the free sites of the biomass. The first mechanism results in more than half of the heavy metal biosorption. The metal affinity for the functional groups follows a decreasing order of lead > copper > alkaline earth metal. Through the modeling simulation, it is demonstrated that the COO- group is the most important functional group in the formation of the metal complexes, while other groups are less important. The chemical modification enhances the content of functional groups, but reduces their affinities for hydrogen and metal ions. Acknowledgment. The financial support provided to J.P.C. by the National University of Singapore (NUS) is appreciated. LA060770+