Microchemical Journal 71 (2002) 211–219
An XAS study of the binding of copper(II), zinc(II), chromium(III) and chromium(VI) to hops biomass J.G. Parsonsa, M. Hejazia, K.J. Tiemanna, J. Henningb, J.L. Gardea-Torresdeya, * a
Department of Chemistry and Environmental Science and Engineering Ph.D Program at The University of Texas at El Paso, El Paso, TX 79968, USA b Department of Crop and Soil Science, United States Department of Agriculture–Agricultural Research Service, Oregon State University, Corvallis, OR 973311, USA Accepted 10 December 2001
Abstract Due to the increasing concentrations of heavy metals in potable water and industrial wastewater governmental agencies have created stricter regulations for the treatment of metal contaminated waste. However, current technologies are both expensive and time consuming to use for water and waste treatment. As an alternative to the current technologies, phytofiltration has been proposed, but to make phytofiltration effective the process must be further studied to better understand the metal–biomass chemical interactions. This current study on the use of hops biomass to remove heavy metal ions from aqueous solutions uses X-ray absorption spectroscopy (XAS) to investigate the binding mechanism of heavy metals to hops biomass. The XAS studies showed that copper(II), zinc(II) and chromium(III) remained in the same oxidation state as when they were reacted with the hops biomass. However, the reaction of chromium(VI) with the hops biomass resulted in the reduction of chromium(VI) to chromium(III). Analysis of the XAS spectra showed that copper(II) bound to the hops biomass with a molecular geometry of copper(II) acetate; zinc(II) bound to the hops biomass with a molecular geometry of zinc(II) gluconate; and chromium(VI) bound to the biomass with the geometry of chromium(III) acetate. In addition, the XAS studies showed that the nearest neighboring atom of all the bound metal ions was an oxygen atom. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Hops; XAS; XANES; EXAFS; Copper; Chromium; Zinc
1. Introduction Heavy metal contamination has become a large concern in the past decade as potable fresh water supplies are diminished. The leading causes of *Corresponding author. Tel.: q1-915-747-5359; fax: q1915-747-5748. E-mail address:
[email protected] (J.L. Gardea-Torresdey).
heavy metal contamination of fresh water are industries such as electroplating, mining and metal refining w1,2x. The increase in toxic metals such as chromium in water supplies has raised major health and environmental concerns w3,4x. Increases in health concerns caused by increased heavy metal concentrations have made the US Environmental Protection Agency (EPA) regulate metal ions more
0026-265X/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 6 - 2 6 5 X Ž 0 2 . 0 0 0 1 3 - 9
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closely. However, in the reclamation or cleaning process classical techniques such as ion exchange, precipitation and flocculation are commonly utilized. But, these techniques are expensive and methods like ion exchange are some times unselective between cations such as sodium(I), calcium(II), magnesium(II) and chromium(III) and become ineffective at removing the more toxic metals w2x. As an alternative to the afore mentioned water reclamation techniques, bioremediation has been suggested. Bioremediation utilizes the natural ability of plants to remove metal ions from different media. There are many different sub classifications of bioremediation but one of the most promising methods for water remediation is phytofiltration. Phytofiltration utilizes the functional groups present on the biomass of a plant (inactivated or dead) to remove metal ions from solution. The utilization of the dead biomass has certain advantages over other systems that use live plants. For example, the functionality of the dead biomass is not affected by high concentrations of toxic substances that may limit living plants. Alternatively, dead plants do not require nutrients or other care for growth and propagation as living plants do and thus may present a more cost effective means to remediate heavy metal ions from water. The technique of phytofiltration has been studied by several researchers using batch experiments to remediate water contaminated with uranium, chromium(III) and chromium(VI) among other heavy metal ions w4–11x. More specifically, Sargassum seaweed has been studied for its ability to remove uranium from aqueous solution with much success w8x. Alfalfa has shown the ability to remove copper(II), gold(III), iron(II) and iron(III), among other heavy metal ions, from aqueous solution w9–11x. Although different biomasses have shown the ability to remove high concentrations of metals from aqueous solution, the actual metal binding mechanism and the ligands involved in the process have not been determined. More recently, the technique of X-ray absorption spectroscopy (XAS) has been used to study the mechanism of metal binding by dead plant biomass. XAS consists of two complimentary tech-
niques: X-ray absorption near edge structure (XANES); and extended X-ray absorption spectroscopy (EXAFS). XANES provides information on the oxidation state of the element under investigation, and EXAFS provides information on the coordination environment such as the nearest neighboring atom w12x. Using XAS, different researchers have been able to determine that reduction of metal ions on the biomass surface has occurred. Also, the functional group that is involved in the binding process has been elucidated. For example, it has been shown that gold(III) is reduced to gold(0) by alfalfa biomass using XAS w9x. This study focuses on the determination of the actual functional groups on the hops biomass that are responsible for sequestering the heavy metal ions. The studies were performed using XAS at Stanford Synchrotron Radiation Laboratories (SSRL). Elucidation of the functional groups on the hops biomass, which are responsible for metal binding, is imperative in understanding the chemical mechanism of metal sorption by the hop tissues. 2. Methodology 2.1. Biomass collection Hops biomass samples (stems and leaves) used for this study were grown at the USDA-ARS Hop Research Farm located outside of Corvallis, OR. The samples were collected from the Hop-pickers. The stems and leaves were separated and placed into bags and shipped to the University of Texas at El Paso. The stems and leaves were dried at 60 8C for 1 week and ground to pass through a 100mesh sieve (to produce a uniform particle size). 2.2. XAS studies Biomass samples were washed and two 500-mg samples of hops stems and leaves were pH adjusted to pH 2 and 5 wfrom previously performed experiments these were the highest and lowest binding pHs (data not shown)x. The biomass was then centrifuged at 3000 rev.ymin and the supernatants were discarded. Forty milliliters of 1000 ppm of
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the metal of interest weither copper(II), chromium(III), chromium(VI) or zinc(II)x were equilibrated with the biomass for 1 h (some metal ion exchange resins were treated and reacted with metal ions in the same manner as the biomass samples). After equilibration of the metal ion solution and the biomass, the samples were centrifuged and the biomass was saved. The biomass was then frozen in liquid nitrogen for 45 min, and lyophilized using a Labconco Freeze Dry System (Freezone 4.5). After lyophilization, the samples were loaded onto 1.0-mm sample plates with Mylar tape windows for XAS analysis at SSRL. The model compounds of copper(II) sulfide, copper(II) acetate, copper(II) pthacinine, zinc(II) gluconate, zinc(II) acetate, zinc(II) sulfide, potassium dichromate, chromium(III) acetate, chromium(III) sulfide and chromium(III) phosphate were ground and diluted with boron nitride to achieve a composition to give a change of 1 absorbance unit across the absorption edge and were packed into 1.0-mm sample plates as the biomass samples. The XAS studies of the samples was performed on beamline 7-3 using a liquid helium cryostat, cooling the samples to approximately 20 K to reduce Debye–Waller effects of the samples. The XAS data for all the metals studied were collected on their respective K edges with energies of 8.979, 5.989 and 9.569 keV, using metal foils as standards to calibrate the edged energies at the specified energies for copper, chromium and zinc, respectively. Fluorescence spectra were taken for all biomass samples using a Canberra 13-element germanium detector. The operating conditions for the beamline were a beam energy of 3 GeV, a current between 60 and 100 mA with a Si(220) w 90 monochromater. To improve signal-to-noise ratios, several scans of all 13 channels were averaged. 2.3. XAS data analyses The WinXAS software package was utilized to analyze the experimental results of the XAS studies by standard methods w13,14x. The data were calibrated on the edge energy using 1st and 2nd degree derivatives to identify the specific edge energy. After calibration, the background was sub-
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tracted using a two polynomial fit and divided by a spline to normalize the data. The kinetic energy of the photoelectrons ejected from the samples and model compounds were calculated based on edge positions of the metal of interest (8.979, 5.989 and 9.569 keV for copper, chromium and zinc, respectively) and converted to wavevector space (k) values. The resulting EXAFS curve was weighted by K 2, modified by a Hanning window over the first and last 10% of the range, Fourier transformed ˚ y1 and converted into between 2 and 12.2 A interatomic distance space (or R space). The number of atoms coordinated to the absorbing atom and the Debye–Waller factors were ascertained by least squares fits of Fourier filtered EXAFS data. The bond lengths and coordination numbers were calculated using FEFF 8.10, an ab initio code w15–17x. The FEFF output was created by 15 iterations of the ideal crystal structure of the model compounds studied. These files were performed to create the data files for the curve fittings of the EXAFS samples and model compounds. The hops samples were fitted to the model compounds that best matched the XANES and EXAFS spectra of the biomass samples. The FEFF input file was prepared using ATOMS from crystallographic data of the model compounds studied. 3. Results and discussion XAS spectroscopy is an invaluable analytical method for environmentalyanalytical chemists and scientists. The entire spectrum supplies information about the oxidation state of a metal under interest, the coordination environment such as the interatomic distances and the coordination number. Additionally, the molecular geometry of unknown complexes can be determined through comparisons with known model compounds w12x. The determination of the molecular geometry uses the XANES absorption continuum and the features within that continuum w12x. The XANES continuum is defined as the region after the inflection point and before the EXAFS region. This is a very small region of the entire spectrum that can provide invaluable chemical information on a system of interest. Figs. 1 and 2 show the XANES spectra of copper bound to the hops biomass and the copper
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Fig. 1. XANES spectra of copper(II) reacted with the hops stems and leaves at pH 2 and 5.
Fig. 3. EXAFS spectrum of copper(II) reacted with the hops stems and leaves at pH 2 and 5.
model compounds and ion exchange resins, respectively. As can be seen in both figures, the edge energy of copper(II) on the biomass samples and the model compounds is the same (at approx. 8.99 keV) showing that the copper is present on the biomass as copper(II). Also the spectra of copper(II) acetate, copper reacted with the hops biomass (leaves and stems) and carboxyl containing resin are very similar indicating that the coordination environment is similar between all the samples and the copper(II) acetate and carboxylcontaining model compounds. By analyzing the
continuum portion of the XANES region, the geometry of the hops tissue–copper complex can be determined, if it is identical or very similar to one of the model compounds w12x. The copper(II) acetate continuum matches identically to that of the copper(II) reacted with the hops stems and leaves at both pH 2 and pH 5. This indicates that copper(II) reacted with the hops stems and leaves may have the same reaction mechanism as the formation of copper(II) acetate in aqueous solution, as copper(II) acetate has a square pyramidal geometry w18x.
Fig. 2. XANES spectra of copper(II) model compounds used for comparison purposes of the XANES continuum.
Fig. 4. EXAFS spectrum of the copper(II) model compounds used for comparison purposes.
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Table 1 EXAFS fitting using FEFF v. 8.10, for all the hops stems and leaves samples, model compounds and ion exchange resins Sample
Bond
N
˚ R(A)
˚ s2 (A)
Hops stems copper(II) pH 5 Hops stems copper(II) pH 2 Hops leaves copper(II) pH 5 Hops leaves copper(II) pH 2 Copper(II) acetate Copper gluconate Copper pthacinine Copper sulfide Zinc(II) hops leaves Zinc(II) hops stems Zinc(II) acetate Zinc(II) gluconate Sulfonic acid Zinc(II) sulfide Zinc(II) sulfide Chromium(III) acetate Chromium(III) phosphate Chromium(III) sulfide Cr(III) reacted with hops stems Cr(III) reacted with hops leaves Potassium dichromate Cr(VI) reacted with hops stems Cr(VI) reacted with hops leaves
Cu–O Cu–O Cu–O Cu–O Cu–O Cu–O Cu–N Cu–S Zn–O Zn–O Zn–O Zn–O Zn–O Zn–S Zn–Zn Cr–O Cr–O Cr–S Cr–O Cr–O Cr–O Cr–O Cr–O
4.6 2.2 3.2 2.5 2.5 2.8 3.3 2.2 2 2 1 3.1 2 4 12 1.8 2.0 3.2 2.8 2.7 4 1.8 1.8
1.92 1.94 1.92 1.93 1.97 2.00 2.19 2.24 2.03 2.01 2.01 2.06 2.01 2.35 3.87 2.00 1.92 2.38 1.99 1.99 1.64 1.98 1.99
0.021 0.021 0.026 0.016 0.015 0.018 0.0039 0.0055 0.00065 0.0048 0.0015 0.0045 0.0093 0.0026 0.0065 0.0053 0.010 0.0024 0.0014 0.0052 0.025 0.0013 0.0014
N is the coordination number. The metal-coordinating atomic distance (R) determined from first shell coordination EXAFS data. s2 is a relative mean square deviation in r (the square of the Debye–Waller factor).
Figs. 3 and 4 display the EXAFS of the copper reacted with the hops biomass and the model compounds, respectively. As can be seen by comparing Figs. 3 and 4 and Table 1, the ligands that match the hops biomass EXAFS are oxygen and nitrogen at interatomic distances of approximately ˚ indicating that the copper on the biomass 1.98 A is bound to either an oxygen or nitrogen ligand. Although the interatomic distances for the biomass and copper(II) acetate are identical, the coordination numbers differ. The coordination numbers shown in Table 1 are much larger for the copper(II) reacted with the hops stem and leave tissues at pH 5, indicating that the complex may be somewhat different since copper(II) acetate has a coordination number of 2.5 and the hops biomass leaves and stems, at pH 5.0, have coordination numbers of 3.2 and 4.6, respectively. The hops biomass has a larger molecular structure than the acetate molecule, which may cause changes in the coordination numbers as orientations are changed
and EXAFS are measured through a linear interaction between the nearest neighboring atoms. In addition, the orientation of the oxygen atoms on the biomass may lead to the creation of a distorted square pyramidal complex as has been observed with CuO4O and CuO4N chromophores w18x. Fig. 5 shows the XANES spectra of zinc(II) reacted with the hops stems and leave tissue samples, carboxyl containing resin, and the model compounds. The edge energy (at approx. 9.67 keV) for the hops samples shows that the zinc(II), as with the copper(II) on the hops biomass, remains as zinc(II). Qualitatively, observing the XANES spectrum it can be seen that the spectrum of the zinc on the hops biomass appears like the two oxygen ligated model compounds, zinc(II) acetate and zinc(II) gluconate. By investigating the continuum portion of the absorption edge, similarities and differences for the geometry of the zinc hops samples and the model compounds can be observed. The continuum spectra of the zinc on
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Fig. 5. XANES of the zinc(II) reacted with the hops stems and leaves, with the carboxyl containing resin and zinc model compounds used for comparison purposes.
the hops samples follows closest to that of the zinc(II) gluconate, even though the shoulder present in the zinc(II) gluconate, which has an octahedral geometry, a geometry similar to nickel(II) gluconate, appears somewhat depressed in the hops biomass sample spectra w19,20x. This may be a function of the concentration of zinc to other atoms in the samples. Observing the zinc(II) acetate spectrum, one can determine that there is a second shoulder before the EXAFS region, indicating that the geometry of the zinc(II) acetate is different than that of the zinc on the hops biomass. In addition, the continuum for the zinc reacted with the carboxyl-containing resin is missing the shoulder altogether indicating that the geometry of the zinc bound to the resin and hops is different. The EXAFS of the hops samples reacted with zinc(II) and model compounds are shown in Fig. 6. Fig. 6 clearly shows that the nearest neighboring atom is an oxygen atom as all the oxygen-containing compounds have the major peak at 2.03 ˚ and not at the interatomic distance of ("0.03 A) ˚ (fittings are zinc-sulfur of approximately 2.35 A shown in Table 1). The EXAFS shows that the major ligand involved in the binding process is oxygen as the interatomic distances of the samples and oxygen containing model compounds are at the same distances. The coordination numbers are also displayed in Table 1. As observed in Table 1,
for the zinc bound to the hops biomass, a coordination number of 2 was observed while zinc acetate had a coordination number of 1 and the zinc gluconate had a coordination number of 3. This may be indicating that the geometry of the zinc bound to the hops biomass is somewhere in between that of acetate and gluconate. These data may suggest that the binding of the zinc(II) to the hops biomass form a zinc–hops biomass complex similar to the formation of the zinc(II) gluconate complex. However, the complex is limited by the number of free oxygen atoms on the hops biomass giving lower coordination numbers for the biomass compared to the zinc gluconate, and thus a slightly different atomic geometry. Fig. 7 shows the XANES of chromium(VI) reacted with the hops biomass and potassium dichromate and chromium(III) acetate. This figure shows that in hops biomass both the stems and leaves have the ability to reduce chromium(VI) to chromium(III) as has been observed with oat biomass w21x. This ability to reduce chromium(VI) to chromium(III) means that the toxicity of chromium(VI) in contaminated waters can be reduced or eliminated, as chromium(III) is less toxic than chromium(VI). The apparent shift in the edge energy exclusively shows that the chromium(VI) bound to the hops biomass is present as chromi-
Fig. 6. EXAFS of the zinc(II) reacted with the hops stems and leaves, with the model compounds included for comparison purposes.
J.G. Parsons et al. / Microchemical Journal 71 (2002) 211–219
Fig. 7. XANES spectrum for the chromium(VI) reacted with the hops stems and leaves, with the potassium dichromate and chromium(III) acetate model compounds included for comparison purposes.
um(III). Additionally, the reduction is also observed as the pre-edge of chromium(VI) at and energy of 5.99 keV is greatly reduced. The preedge observed in the hops biomass samples and the chromium(III) acetate is actually a function of the electronic states and electron orbital orientation, which allows electron transitions that create a small pre-edge feature when chromium(III) is bound to an oxygen atom w22,23x. Again looking at the XANES continuum of the chromium samples and the chromium(III) acetate, the hops stems reacted with chromium(VI) and the chromium(III) acetate model compound show identical molecular geometry, which is an octahedron, as there are no real differences in the continuums of the two w24x. However, the hops leaves reacted with chromium(VI) show a slightly different geometry, this may be because the oxygen-containing ligands on the hops leaves are more plentiful or in a different orientation than on the hops stems. These data indicate that the chromium(VI) reacted with the hops biomass to form a chromium(III)-hops biomass complex may have a reaction mechanism similar to that of creating chromium(III) acetate. The EXAFS of the spectra for the chromium(VI) and chromium(III) are shown in Figs. 8 and 9, respectively. Fig. 8 shows the chromium(VI) reacted with the hops biomass stems and
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Fig. 8. EXAFS spectrum collected from the XAS experiments for chromium(VI) reacted with the hops stems and leaves and the model compounds used for comparison purposes.
leaves is present as chromium(III) and has a nearest neighboring atom of oxygen with an inter˚ while potassium atomic distance of 1.98"0.01 A, ˚ dichromate has an interatomic distance of 1.64 A which is much shorter than that of chromium(III) oxygen bond and have coordination numbers of 1.8, 1.8, 1.8 and 4 for: chromium(VI) on the hops stems; chromium(VI) on the hops leaves; chromium(III) acetate; and potassium dichromate, respectively (see Table 1). In Fig. 9, the chromi-
Fig. 9. EXAFS spectrum collected from the XAS experiments for chromium(III) reacted with the hops stems and leaves and the model compounds for comparison purposes.
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um(III) reacted with the hops biomass (stems and leaves) show that the chromium(III) reacted with the hops tissues all have interatomic distances that match chromium–oxygen bonds with distances of ˚ In addition, Fig. 9 shows that the 1.98"0.01 A. chromium(III) sulfide model compound has different interatomic distances than the hop tissues and other model compounds. 4. Conclusions Through the use of XANES and EXAFS, this study has shown that copper(II), zinc(II), chromium(III) and chromium(VI) bind to the hops biomass (stems and leaves) through oxygen atoms. The copper is bound to the biomass with a molecular geometry that matches copper(II) acetate, and the zinc(II) has a molecular geometry very similar to that of zinc(II) gluconate. Also, we have shown that the hops biomass has the capacity to reduce chromium(VI) to chromium(III) where it has a molecular geometry identical to that of chromium(III) acetate. The interpretation of the XANES continuum indicates that the biomass may be reacting with the metal ions in aqueous solution similar to that of free ligands to form complexes that have similar geometry. The EXAFS spectra showed that the coordination numbers of the metals bound to the hops biomass are very similar to that of the model compounds, but usually have lower numbers, which may be determined by the availability or the orientation of the free oxygen ligands on the biomass. The orientation and availability of free oxygen atoms on the biomass may also slightly change the geometry of the complex formed. Although there are subtle differences in the geometries of the biomass complexes, all the ˚ of the interatomic distances are within "0.03 A corresponding oxygen-containing model compounds. Acknowledgments The authors would like to acknowledge financial support from the National Institutes of Health (NIH) (Grant S06GM8012-30). We also acknowledge the financial support from the University of Texas at El Paso (UTEP) Center for Environmental
Resource Management (CERM) through funding from the Office of Exploratory Research of the EPA (Cooperative Agreement CR-819849-01-4). We also acknowledge the HBCUyMI Environmental Technology Consortium, which is funded by the Department of Energy. Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory (SSRL), a national user facility operated by Stanford University on behalf of the US Department on Energy (DOE), Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by The DOE, Office of Biological and Environmental Research and by the NIH, National Center for Research Resources, Biomedical Technology Program. In addition, the authors would like to acknowledge the SSRLy DOE funded Gateway Program. The authors also acknowledge financial support from the Southwest Center for Environmental Research and Policy (SCERP) program. References w1x A. Rehman, A.R. Shakoori, Bull. Environ. Contam. Toxicol. 66 (2001) 542–547. w2x J.L. Gardea-Torresdey, K.J. Tiemann, J.G. Parsons, Microchem. J. 69 (2001) 33–44. w3x D.D. Runnels, T.A. Shepard, E.E. Angino, Environ. Sci. Technol. 26 (1992) 2316–2322. w4x B.L. Carson, H.V. Ellis, J.L. McCann, Toxicology and Biological Monitoring of Metals in Humans, 71, Lewis Publishers, Chelsea, Michigan, 1986, p. 133. w5x M.T. Gonzalez-Munoz, ´ ˜ M.L. Merroun, N. Ben Omar, J.M. Arias, Int. Biodeterior. Biodegrad. 40 (1997) 107–114. w6x S. Ishikawa, K. Suyama, I. Satoh, Appl. Biochem. Biotechnol. 77–79 (1999) 521–533. w7x T.A. Davis, B. Volesky, R.H.S.F. Vieira, Water Res. 34 (2000) 4270–4278. w8x J. Yang, B. Volesky, Water Res. 33 (1999) 3357–3363. w9x J.L. Gardea-Torresdey, K.J. Tiemann, G. Gamez, et al., Environ. Sci. Technol. 34 (2000) 4392–4396. w10x K.J. Tiemann, J.L. Gardea-Torresdey, G. Gamez, et al., Environ. Sci. Technol. 34 (2000) 693–698. w11x J.L. Gardea-Torresdey, K.J. Tiemann, J.H. Gonzalez, J.A. Henning, M.S. Townsend, J. Hazard. Mater. 48 (1996) 181–190. w12x D.C. Koningsberger, Chemical analysis, in: R. Prins (Ed.), X-Ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES, 91, John Wiley & Sons, New York, 1988, p. 673. w13x T. Ressler, J. Synch. Rad. 5 (1998) 118–122.
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