Journal of Colloid and Interface Science 317 (2008) 5461 www.elsevier.com/locate/jcis
XAS and XPS studies on chromium-binding groups of biomaterial during Cr(VI) biosorption
Donghee Park a , Yeoung-Sang Yun b , Jong Moon Park a,
a Advanced Environmental Biotechnology Research Center, Department of Chemical Engineering, School of Environmental Science and Engineering,
Pohang University of Science and Technology, San 31, Hyoja-dong, Pohang 790-784, South Korea
b Division of Environmental and Chemical Engineering, Research Institute of Industrial Technology, Chonbuk National University, 664-14 1ga,
Duckjin-dong, Jeonju 561-756, South Korea Received 23 July 2007; accepted 13 September 2007 Available online 21 September 2007
Abstract The reduction of toxic Cr(VI) to the less or nontoxic Cr(III) may be an useful detoxication technique for the treatment of Cr(VI)-contaminated waters. Recently, the protonated biomass of brown seaweed, Ecklonia, was shown to completely reduce Cr(VI) to Cr(III) in the pH range 15. The reduction of Cr(VI) to Cr(III) appeared to occur at the surface of the biomass. In this study, abiotic Cr(VI) reduction by the biomass was performed with various contact times, pHs and initial Cr(VI) concentrations, and surface and bulk characteristics of the Cr-laden biomass was then investigated using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The XPS spectra indicated that the Cr(VI) bound to the biomass was completely reduced to Cr(III) at tested various conditions. XANES and EXAFS spectra of the Cr-laden biomass were very similar to those of Cr(III)-acetate, which means that the Cr bound to the biomass during Cr(VI) reduction had an octahedral geometrical arrangement. The bonding distance of the chromium oxygen atoms was approximately 1.971.99. In conclusion, it was obvious that oxygen containing groups, such as carboxyl and phenolic groups, play a major role in the binding of the Cr(III) resulting from the abiotic reduction of Cr(VI) by the biomass. 2007 Elsevier Inc. All rights reserved.
Keywords: Biosorption; Hexavalent chromium; Ecklonia; Adsorption-coupled reduction
1. Introduction Chromium and its compounds are widely used in industry, with the most usual and important sources coming from the electroplating, tanning, water cooling, pulp producing, and ore and petroleum rening processes [1]. The efuents from these industries contain both Cr(VI) and Cr(III), at concentrations ranging from tens to hundreds of mg L1. Cr(VI) is classied as a primary contaminant due to its mobility in soil and groundwater, as well as its reported harmful effects on organisms, including humans [2]. Within living cells, Cr(VI) induces cancer and mutation by damaging DNA-protein cross-links, and causing single-strand breaks [3]. Cr(III), on the other hand, is stable and less toxic or nontoxic, and has been listed as an essential el* Corresponding author. Fax: +279 2699.
E-mail address: jmpark@postech.ac.kr (J.M. Park). 0021-9797/$ see front matter 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.09.049
ement for the good health and nutrition of many organisms [4]. Because of these differences, the discharge of Cr(VI) to surface water is regulated by the US EPA to below 0.05 mg L1 , while total Cr, including Cr(III), Cr(VI), and its other forms, is regulated to below 2 mg L1 [5]. Because the reduction of toxic Cr(VI) leads to the formation of less toxic or nontoxic Cr(III), it is important to study how this reduction may be implemented for achieving detoxication and; therefore, environmental cleanup. The reduction of Cr(VI) can be accomplished by various chemical reductants [6] and microorganisms [7]. However, these types of process are undesirable due to the use of expensive and toxic chemical reductants, cell death due to the toxicity of Cr(VI), and the need of nutrients. Bearing these factors in mind, it is very signicant that the nonliving biomass of the brown seaweed, Ecklonia, an abundant and inexpensive brown seaweed, is able to efciently reduce toxic Cr(VI) to less toxic or nontoxic Cr(III) [812].

D. Park et al. / Journal of Colloid and Interface Science 317 (2008) 5461
Cr(VI), i.e., HCrO and Cr2 O2 , due to their reduction potentials (above +1.3 V at standard condition), can be reduced to Cr(III) by mixing with the Ecklonia biomass, with the reduced Cr(III) appearing in the aqueous phase or being partly bound to the biomass [8]. Since protons are consumed during the reduction of Cr(VI), the rate of Cr(VI) removal decreases with increasing solution pH. However, Cr(VI) can be completely removed from the aqueous solution, even at pH 5, with sufcient contact time. One gram of the Ecklonia biomass can reduce 4.49 mmol of Cr(VI) at pH 2, i.e., the Cr(VI)-reducing capacity of the Ecklonia biomass is 3.7 times higher than that of the conventional chemical reductant, FeSO4 7H2 O. Despite this remarkable advantage, there is only a little information about the redox reaction between Cr(VI) and the biomass, as until recently an anionic adsorption has been considered as the main removal mechanism of Cr(VI) by biomass from aqueous system, not a redox reaction [13,14]. Recently, a new mechanism has been proposed for the biosorption of Cr(VI) by biomaterials (Fig. 1). Regardless of living or not, Cr(VI) can be abiotically removed from an aqueous system by the biomaterials through both direct and/or indirect reduction mechanism(s). In mechanism I (direct reduction mechanism), Cr(VI) is directly reduced to Cr(III) in the aqueous phase by contact with electron-donor groups of the biomaterial, and the reduced Cr(III) remains in the aqueous phase or may form complexes with Cr-binding groups of it. Mechanism II (indirect reduction mechanism) consists of three steps; (i) the binding of anionic Cr(VI) to positively-charged groups present on the biomaterial surface, (ii) the reduction of Cr(VI) to Cr(III) by adjacent electron-donor groups, and (iii) the release of the reduced Cr(III) into the aqueous phase due to electronic repulsion between the positively-charged groups and the Cr(III), or the complexation of the reduced Cr(III) with adjacent groups, i.e., Cr-binding groups. Amino and carboxyl groups may take part in reaction (i) of mechanism II. As the pH of the aqueous phase is lowered, a large number of hydrogen ions can easily coordinate with the amino and carboxyl groups present on the biomaterial surface. Thus, a low pH makes the biomaterial sur-

face more positive. The more positive the surface charge of the biomaterial, the faster the rate of Cr(VI) removal from the aqueous phase, since the binding of anionic Cr(VI) ion species with the positively-charged groups is enhanced. A low pH also accelerates the redox reactions in both mechanisms I and II, since the protons take part in these reactions. Meanwhile, if there are a small number of electron-donor groups in the biomaterial or protons in the aqueous phase, the chromium bound onto the biomaterial surface may remain in the hexavalent state. Therefore, a portion of mechanisms I and II depends on the biosorption system (solution pH, temperature and species on the biomaterial, as well as the biomaterial and Cr(VI) concentrations, etc.). Although this proposed mechanism is now widely accepted [13], little information is available on Cr-binding and electron-donor groups. The identication of these groups would be helpful in the selection process of new biomaterial types, as well as for attempts to improve the Cr(VI) removal capacity of it. The aims of the present investigation were to examine the functional groups concerned with the biosorption of Cr(VI). Core electron X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) were used to investigate the surface and bulk characteristics of the biomaterial during the biosorption of Cr(VI) under various conditions. Finally, a detailed Cr(VI)-biosorption mechanism has been discussed. 2. Materials and methods 2.1. Preparation of the biomass The brown seaweed, Ecklonia sp., was collected along the seashore of Pohang, Korea [15]. After swelling and rinsing with deionizeddistilled water, the sun-dried biomass was cut into approximately 0.5 cm sized pieces. The cut biomass was treated with a 1 M H2 SO4 solution for 24 h, which replaced the natural mix of ionic species with protons and sulfates. The acid-treated biomass was washed several times with deionizeddistilled water, and then dried at room temperature for several days. The resulting dried biomass was later stored in a desiccator and used in the following experiments. 2.2. Cr(VI) biosorption experiments Stock solutions of Cr(VI), made by dissolving analytical grade K2 Cr2 O7 (Kanto) in deionizeddistilled water, were freshly prepared every time. Each trial was performed by bringing into contact the desired amount of biomass with 200 mL of a Cr(VI) solution of known concentration in a 500 mL Erlenmeyer ask. The asks were agitated on a shaker at 200 rpm under room temperature. In the time course experiment, 1 g of biomass was brought into contact with 200 mL of a Cr(VI) solution containing 200 mg L1 Cr(VI), with the solution pH maintained at pH 3 using 0.5 M H2 SO4 or 1 M NaOH solution. Samples for Cr(VI) and total Cr concentration analyses were intermittently removed from the asks and appropriately diluted. It was conrmed from three independent replicates that the Cr(VI) biosorption experiment was reproducible within 5% error.

Fig. 1. Mechanism of Cr(VI) biosorption by biomaterials. This diagram was modied from that proposed by Park et al. [10,14].
2.3. X-ray photoelectron spectroscopy analysis XPS was employed to verify the oxidation state of the Cr bound to the biomass, B. The Cr-laden biomass, Cr-B, was obtained through contact with Cr(VI), under various conditions (2005000 mg L1 of Cr(VI), pH 24) for several days, depending on biosorption conditions. The Cr(III)-laden biomass, Cr(III)-B, was obtained through contact with 200 mg L1 of Cr(III) at pH 4 for 1 day. Prior to mounting for the XPS, the biomass was washed with deionizeddistilled water, and then freeze-dried in a vacuum freeze dryer (Bondiro, ILSHIN Lab Co.). The resulting biomass was transported to the spectrometer in a portable, gastight chamber. Cr2 O3 (Aldrich), (CH3 CO2 )7 Cr3 (OH)2 (Cr(III)-acetate) (Aldrich) and CrO3 (Aldrich) were used as the Cr(III) and Cr(VI) reference compounds, respectively. Spectra were collected on a VG Scientic model ESCALAB 220iXL. A consistent 2 mm sized spot was analyzed on all surfaces, using an MgK (h = 1253.6 eV) X-ray source, at 100 W and pass energy of 0.1 eV, with 10 high-resolution scans. The system was operated at a base pressure of mbar. The calibration of the binding energy of the spectra was performed with the C1s peak of the aliphatic carbons; 284.6 eV. 2.4. X-ray absorption spectroscopy analysis The X-ray absorption spectra were measured at room temperature, at 7C1 beam-line in a storage ring of 2.5 GeV, with the ring current set at 120160 mA in the Pohang Light Source (PLS), which is a third generation synchrotron radiation source. The Si(111) double crystal monochromator has been employed to produce monochromatic the X-ray photon energy. All samples were run using a helium cryostat, at a temperature of about 15 K, to reduce DebyeWaller effects occurring from thermal disorder in samples. High harmonic contamination was eliminated by detuning the monochromator to reduce the incident of the X-ray intensity by 30%. Data were collected with Si 111 (X-18B) monochromator crystals, with slits adjusted to give a resolution of 12 eV. Cr2 O3 , Cr(NO3 )3 (Aldrich), Cr(III)acetate, CrO2 (Samchun) and CrO3 standards were measured as solids, on tape, in the transmission mode. Ecklonia biomass was saturated with 200 mg L1 Cr(VI) solution, at either pH 2 or 4, prior to the analysis. All of the Cr-laden biomass samples were then washed with deionizeddistilled water prior to use, and run as solid powders in the uorescence mode, with a Lytle ionization detector employed. Fluorescence spectra of the Cr-laden biomass were taken using a 30-element Canberra germanium detector. An average of ve scans of each sample was made for each the X-ray absorption near-edge structure (XANES) and extended X-ray absorption ne structure (EXAFS) spectrum in order to improve the signal-to-noise ratio. The absolute energy positions were calibrated using Cr (5989 eV) metal foils. The E0 values were determined from the absorption edge step midpoint. A linear pre-edge background and a least-squares cubic spline EXAFS background were extrapolated to normalize the X-ray absorption near-edge structure (XANES) absorption intensities. Quantitative comparisons between unknown and stan-

dards were accomplished with nonlinear ts based on the general extended X-ray absorption ne structure (EXAFS) equation, and veried using theoretical simulations carried out with FEFF V8.00. Analysis of the EXAFS data of the standards was in good agreement with published X-ray crystallographic data. 2.5. Chromium analysis A colorimetric method [16] was used to measure the concentrations of the different chromium species in aqueous phase. The pink colored complex, formed from 1,5-diphenylcarbazide and Cr(VI) in acidic solution, was spectrophotometrically analyzed at 540 nm (Genesys 5, Spectronic Ins.). To estimate the total Cr concentration, the Cr(III) was rst converted to Cr(VI) at high temperature (130140 C) by the addition of excess potassium permanganate prior to the 1,5-diphenylcarbazide reaction. The Cr(III) concentration was then calculated from the difference between the total Cr and Cr(VI) concentrations. The detection limit of this method was 0.03 mg L1. 3. Results and discussion 3.1. Abiotic reduction of Cr(VI) to Cr(III) by the biomass To examine the Cr(VI) biosorption characteristics of the Ecklonia biomass, the Cr concentrations proles were investigated at pH 3 (Fig. 2). The concentration of Cr(VI) was found to sharply decrease, and was completely removed from the aqueous phase in 240 h. Meantime, the Cr(III), which was not initially present, appeared in the aqueous phase, and increased proportionately to the Cr(VI) depletion. After the complete removal of Cr(VI), the nal Cr(III) concentration in the aqueous phase remained at 23.2 mg L1 , indicating that 176.8 mg L1 of total Cr had been bound to the biomass. To characterize the biosorption of Cr(VI), it is very important to verify the oxidation state of the Cr bound to the biomass [14].
Fig. 2. Dynamics of the removal of Cr(VI) by the protonated Ecklonia biomass. Conditions: 200 mg L1 of initial Cr(VI) concentration, 5 g L1 of biomass dosage and pH 3.
The oxidation state of the Cr bound to the biomass was characterized using XPS. Fig. 3 shows high-resolution spectra, collected from the Cr2p core region, of the standard chemicals (Cr2 O3 , Cr(III)-acetate and CrO3 ), Cr-unloaded biomass (B), Cr(III)-laden biomass (Cr(III)-B) and Cr-laden biomass (Cr-B). For Cr2 O3 and Cr(III)-acetate, signicant bands appeared at binding energies of 577.0578.0 eV and 586.0588.0 eV, respectively; the former corresponds to Cr2p3/2 orbital, the latter to Cr2p1/2 orbital. The CrO3 was characterized by higher binding energies; 580.0580.5 eV and 589.0590.0 eV, since the hexavalent form draws electrons more strongly than trivalent form. The spectra of the Cr-laden biomass and Cr(III)laden biomass were the same as that for the standard Cr(III)-

chemicals. Finally, the Cr(VI) dynamics in the aqueous phase and XPS spectra of the Cr-laden biomass indicate that the Cr(VI) was completely reduced to Cr(III) when brought into contact with the Ecklonia biomass at pH 3. 3.2. Metal binding study using XPS As mentioned above, a portion of mechanisms I and II for the biosorption of Cr(VI) may depend on the conditions within the biosorption system, such as contact time, solution pH and initial Cr(VI) concentration. To examine the change in the oxidation state of the Cr bound to the biomass during Cr(VI) biosorption, 5 g L1 of the biomass was brought into contact with 200 mg L1 of Cr(VI) solution at pH 3 for 6, 24 or 240 h (Fig. 2). The biomass, followed by desorption with 1 M H2 SO4 solution for 24 h, was also obtained for XPS study. As seen in Fig. 4a, the spectra of the Cr-laden biomass show that the Cr bound to the biomass was in the trivalent state during Cr(VI) biosorption. With increasing contact time, the peak for Cr2p3/2 orbital increased since more chromium was bound to the biomass. These results imply that the redox reaction occurring on the biomass surface, i.e., the second step of mechanism II (Fig. 1), might be very fast, thus the Cr(VI) might be immediately reduced to the trivalent form. Meanwhile, the spectra of the biomass obtained following desorption with 1 M H2 SO4 solution indicate that the chromium could not be completely desorbed from the biomass. It means that the reduced Cr(III) might be complexed with some groups very strongly and irreversibly. Fig. 4b shows the high resolution XPS spectra of the biomass obtained from the biosorption of Cr(VI) at pHs 2, 3 and 4. The contact time required for the complete removal of Cr(VI) from the aqueous phase varied according to the different pH. 200 mg L1 of Cr(VI) was completely removed from the aqueous phase in 42 h at pH 2, while 1280 h was required at pH 4 (Fig. S1). This was due to the participation of protons in the
Fig. 3. High resolution Cr2p spectra of Cr-unloaded biomass (B), the Cr-laden biomass (Cr-B), Cr(III)-laden biomass (Cr(III)-B) and standard chromium chemicals. The Cr-laden biomass was obtained after the biosorption of Cr(VI) at pH 3.0, while the Cr(III)-laden biomass was obtained after the biosorption of Cr(III) at pH 4.
Fig. 4. High resolution Cr2p spectra of the Cr-laden biomass obtained after the biosorption of Cr(VI) under various conditions; (a) contact time, (b) solution pH, and (c) initial Cr(VI) concentration.

anionic adsorption of Cr(VI) to the positively-charged groups and in the reduction of Cr(VI) by the electron donor groups (Fig. 1). According to batch experiments, protons were consumed at the ratio of 1.15 mol of proton per mol of Cr(VI) [8]. Therefore, a low concentration of protons may cause only partial reduction of the Cr(VI) bound to the biomass. However, the XPS spectra of the biomasses obtained at various pHs indicate that only Cr(III) existed on the surface of the biomass (Fig. 4b). This result implies that the Ecklonia biomass has a very strong reductive capacity toward Cr(VI); thus easily or spontaneously reduces Cr(VI) to Cr(III), even at pH 4. Meanwhile, the peak for the Cr2p3/2 orbital of the biomass obtained at pH 2 was smaller than those at pH 3 and 4. The intensity of Cr2p orbital of the biomass obtained at pH 3 was similar to that obtained at pH 4. Analysis of total Cr in the aqueous phase following the complete removal of Cr(VI) indicated that the Cr contents bound to the biomass at pHs 2, 3 and 4 were 21.7, 35.2 and 33.0 mg-Cr g1 , respectively. To examine the oxidation state of the Cr bound to the biomass according to initial Cr(VI) concentration, 5 g L1 of the biomass was brought into contact with 200, 1000 or 5000 mg L1 of Cr(VI) at pH 3. While 200 mg L1 of Cr(VI) was completely removed from the aqueous phase in 10 days, the removal efciencies of initial Cr(VI) concentrations of 1000 and 5000 mg L1 at 30 days were 66.4 and 26.2%, respectively. The Cr contents bound to the biomass were 35.2, 98.4 and 138.2 mg g1 , respectively. Fig. 4c shows all of the XPS spectra of these biomasses, obtained under various initial Cr(VI) concentrations, had a signicant band corresponding to the Cr2p orbitals of Cr(III), with increased peak intensities with increasing Cr contents bound to the biomass. Some studies have tried to examine the oxidation state of the Cr bound to the biomass using XPS analysis. Neal et al. [17] reported that the Cr bound to the cell surface of living Shewanella oneidensis was only in the trivalent form, which was due to the enzymatic reduction of Cr(VI). Dambies et al. [18] reported that the entire Cr bound to glutaraldehyde cross-linked chitosan beads at pH 4 was non-enzymatically reduced to Cr(III),

while only 60% of the Cr bound to native chitosan beads was in the trivalent form. Boddu et al. [19] also reported that about 67% of the Cr bound to a composite chitosan biosorbent, prepared by coating the ceramic substrate with chitosan gel, was reduced to Cr(III) at pH 4. On the contrary, Figs. 4a4c clearly indicate that the entire Cr bound to the Ecklonia biomass under various biosorption conditions, such as contact time, pH and initial Cr(VI) concentration, was in the trivalent state. The difference in the extent of Cr(VI) reduction between chitosan beads and Ecklonia biomass may be related to the differences in their redox potentials. The redox potential of the Ecklonia biomass is lower than that of Fe(III) (+0.77 V at standard condition) at pH 2, since Fe(III) is reduced to Fe(II) by contact with it [11]. This means that the Ecklonia biomass is more efcient than Fe(II), which has been the most commonly used chemical reagent for converting Cr(VI) to Cr(III) [20]. Therefore, it can be suggested that since the redox reaction between Cr(VI) ions and the electron donor groups present on the Ecklonia biomass may be fast and spontaneous, as Cr(VI) binds to the positively charged groups due to electronic attraction, it will then be rapidly reduced to the trivalent form. Thus, the rate limiting step of the biosorption of Cr(VI) may be the contact of Cr(VI) ions with the positively charged groups present on the biomass. As mentioned above, the XPS spectra supply information about ligand effects in transition-metal complexes; electrondonating ligands will lower the binding energy (BE) of the core level electrons, and those of the electron-withdrawing ligands will be raised [17,18]. Fig. 5 shows the O1s orbitals of the Cr-laden biomass according to the contact time. Each band was composed of three sub-bands, which are attributed to the electron interaction between electron-donating ligands and electron-withdrawing ligands. The band at 532.3532.5 eV can be assigned to the existence of >CO, OH or bound H2 O, while that at 529.8530.0 eV can be assigned to the existence of a metal oxide (in this case, it may be chromium oxide) [17]. As the biosorption of Cr(VI) proceeded, the O1s band shifted to a low binding energy; the height of sub-band 1 decreased, but that
Fig. 5. O1s orbitals of the Cr-laden biomass according to the contact time.
of sub-band 3 increased. Meanwhile, the O1s band returned to its original binding energy after desorption of chromium by 1 M H2 SO4 solution. These results suggest that chromium might bind to ligands containing oxygen, such as carboxyl or phenolic groups present on the surface of brown seaweed. However, the C1s and N1s bands hardly shifted on the biosorption of Cr(VI) (Fig. S2). According to the XPS study by Dambies et al. [18], a new oxygen band appeared at around 529.5530.1 eV, which was attributed to chromium oxide, but the N1s band hardly shifted on the sorption of Cr, with the exception of the appearance of a band with a BE of 401.9 eV, which might be attributed to protonated amine or oxidized nitrogen atoms in the amine groups. It is well known that amine groups present on chitosan are related with the anionic adsorption of Cr(VI) and its reduction to Cr(III) [18,19,21]. Meanwhile, FTIR and potentiometric titration studies revealed that the main functional group of Ecklonia biomass was a carboxyl group with a pK value of 4.6 [15]. 3.3. Metal binding study using XAS X-ray absorption spectroscopy (XAS) was also used to investigate the mechanism of chromium binding to the Ecklonia biomass. XAS is based on the absorption of high-energy monochromatic X-rays by an element in the characteristic absorption edge region, and supplies two useful types of spectra, i.e. XANES and EXAFS [22,23]. Firstly, the XANES data were used to verify the oxidation state of the Cr bound to the biomass. Figs. 6 and S3 show XANES spectra of ve reference compounds; Cr(III) as Cr2 O3 , Cr(NO3 )3 and Cr(III) acetate, Cr(IV) as CrO2 , and Cr(VI) as CrO3. The reason for originally choosing ve chromium compounds as references is as follows: Cr2 O3 , CrO2 and CrO3 were chosen to compare XANES spectra patterns according to the oxidation state of chromium, while Cr2 O3 , Cr(NO3 )3 and Cr(III) acetate were chosen to examine the effects of the element atom linked to the chromium atom in the XANES spectra. Furthermore, since XANES spectra are mainly associated with the electronic structure of the target element and multiple scattering events, it would be acceptable to choose Cr(III)-acetate as the reference to represent the Cr(III) chemically sorbed by the biomass through ionic exchange, despite the difference in molecular weight and structure between the Cr(III)-laden biomass and Cr(III)-acetate, i.e., (OH)2 Cr3 (OOCCH3 )7. The XANES data for CrO3 show the well-dened pre-edge peak characteristic of Cr(VI) (from 5985 to 6000 eV) (Figs. 6 and S3). The lack of this feature for any of the other model compounds indicates that the chromium was in its reduced valence state, such as Cr(III) and Cr(IV). The XANES data for Cr2 O3 , Cr(NO3 )3 and Cr(III) acetate show a small peak at 5990 eV, while that for CrO2 does not show this peak (Fig. S3). As seen in the XANES spectra, each Cr(III) compound had absorption patterns distinguishable from each other. Figs. 6 and S3 also show XANES spectra of the Cr(III)laden biomass and Cr-laden biomasses. The Cr contents bound to each biomass were 31.0, 21.7 and 33.0 mg g1 , respectively. The absence of a well dened pre-edge peak between 5985 and 6000 eV, coupled with the presence of a small peak at 5990 eV

Fig. 6. XANES spectra of ve reference chemicals and three chromium-laden biomasses.
in the Cr(III)-laden biomass indicate that the chromium bound to the biomass obviously existed in the trivalent form. The biosorption of Cr(III) by Ecklonia biomass has previously been investigated [15]. FTIR and potentiometric titration studies have shown that the carboxyl group with a pK value of 4.6 mmol g1 was the Cr(III)-binding site within the pH range 15. Alginate is well known to contribute to the strength of the cell wall of brown seaweed, which contains many carboxyl and hydroxyl groups [24]. Extensive studies have been carried out on the biosorption of cationic metals, and the cationic exchange mechanism reported; the carboxyl group of brown seaweed corresponds to the binding site of cationic heavy metals, such as Pb(II) and Cd(II) [25]. It is interesting that the XANES spectra obtained for the Crladen biomasses after the biosorption of Cr(VI) at pH 2 and 4 were well matched with that obtained for the Cr(III)-laden biomass. This result indicates that the chromium bound to the biomass during the biosorption of Cr(VI) existed in the trivalent form, as mentioned in the XPS study above. By analyzing the absorption spectra of the XANES region, the geometry of the Cr-biomass complex can be determined if identical or very similar to that of one of the model compounds. As seen in Figs. 6 and S3, the Cr(III)-acetate spectra identically matched those of the Cr(III)-laden biomass and Cr-laden biomasses; showing their identical molecular geometries. This means that the Cr(VI) reacted with the biomass to form a Cr(III)-biomass complex, which may have a reaction mechanism similar to that for creating Cr(III)-acetate. Cr(III)-acetate is known to have an octahedral geometry in aqueous solution; the nearest neighboring atom of Cr(III) is an oxygen atom [23]. However, the XANES spectra of the model chemical, Cr(NO3 )3 , was distinctly different from those of the Cr(III)-laden biomass and Cr-laden biomasses. This result implies that the chromium was not bound
mately the same lengths (as shown in Table S1, 1.96 for the Cr(III)-acetate and 1.971.99 for the Cr(III)- and Cr-laden biomasses, respectively), which corresponds to the CrO bond lengths cited in the literature [2628]. 4. Conclusion The abiotic reduction of toxic Cr(VI) to the less or nontoxic Cr(III) by biomaterials may be an useful detoxication technique for the treatment of Cr(VI)-contaminated waters. The Cr(VI)-reducing capacities of some biomaterials such as the brown seaweed, Ecklonia, biomass are known to be more higher than that of conventional chemical reductants. Despite this remarkable advantage, however, there is only a little information about the redox reaction between Cr(VI) and biomaterials. In this study, the XPS spectra indicated that the Cr(VI) bound to the biomass was completely reduced to Cr(III) at tested various conditions. XANES and EXAFS spectra of the Cr-laden biomass were very similar to those of Cr(III)-acetate, which means that the Cr bound to the biomass during Cr(VI) reduction had an octahedral geometrical arrangement. The bonding distance of the chromium oxygen atoms was approximately 1.971.99. In conclusion, it is obvious that oxygen containing groups, such as carboxyl and phenolic groups, play a major role in the binding of the Cr(III) resulting from the abiotic reduction of Cr(VI) by biomaterials. The identication of these groups would be helpful in the selection process of new biomaterial types, as well as for attempts to improve the Cr(VI) removal capacity of it. Acknowledgments This work was nancially supported by the KOSEF through the AEBRC and the Program for Advanced Education of Chemical Engineers at POSTECH. This work was also partially supported by the MOCIE and KOTEF through the Human Resource Training Project for Regional Innovation at Chonbuk National University. Supplementary material The online version of this article contains additional supplementary material. Fig. S1 shows dynamics of Cr(VI) removal by the protonated Ecklonia biomass at pH 2 and 4. Fig. S2 shows C1s and N1s orbitals of the Cr-laden biomass according to the contact time. Figs. S3 and S4 show the high and low resolution XANES and FT-EXAFS spectra, respectively. Table S1 shows the FEFF ttings of the EXAFS for the model compounds as well as the Cr(III)- and Cr-laden biomasses. Please visit DOI: 10.1016/j.jcis.2007.09.049. References

[1] J. Barnhart, Regul. Toxicol. Pharm. 26 (1997) S3. [2] M. Costa, Toxicol. Appl. Pharm. 188 (2003) 1. [3] X. Shi, A. Chiu, C.T. Chen, B. Halliwell, V. Castranova, V. Vallyathan, J. Toxicol. Environ. Health Part B 2 (1999) 87.
Fig. 7. FT-EXAFS spectra of ve reference chemicals and three chromiumladen biomasses.
to the nitrogen atom of amino and nitrile groups, which was supported by the ligand effects of the chromium on the N1s band of the XPS spectra (Fig. S2). There have been previous XANES studies on the Cr species bound to other biomaterials. Lytle et al. reported that Cr(VI) taken from the ne lateral roots of wetland plants was rapidly reduced to Cr(III) [26]. Dupont et al. [22] studied the adsorption mechanism of Cr(VI) onto a lignocellulosic substrate using XAS. The oxidation of lignin moieties took place concurrently with the reduction of Cr(VI) to Cr(III), which led to the formation of hydroxyl and carboxyl functions [22]. Cardea-Torresdey et al. [27] reported that Cr(VI) could be bound to an oat byproduct, but was easily reduced to Cr(III) by positively charged functional groups, which subsequently adsorbed to available carboxyl groups. Parsons et al. [28] also reported the reaction of Cr(VI) with the biomaterial of hops, which resulted in the reduction of Cr(VI) to Cr(III), with the Cr(III) bound to the biomaterial with the same geometry as Cr(III)-acetate. These studies reached the same conclusions as found in this study, i.e., Cr(III) resulting from the reduction of Cr(VI) was bound to the carboxyl groups present on the brown seaweed, Ecklonia, biomass. Extended X-ray Absorption Fine Structure (EXAFS) is becoming more and more popular as a tool for the determination of the coordination environment of metal ions in biological systems [22,23]. Figs. 7 and S4 show the Fourier transformed EXAFS (FT-EXAFS) spectra of the model compounds as well as the Cr(III)- and Cr-laden biomasses. Interestingly, the FTEXAFS spectra for the Cr(III)-laden biomass and Cr-laden biomasses were very similar that for Cr(III)-acetate. This result means that the coordination environment of the chromium on the biomasses was also similar to that of Cr(III)-acetate, where Cr(III) is in an octahedral geometrical arrangement. The bonding distances of the chromium oxygen atoms were approxi-
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