In situ speciation studies of copper in the electroplating sludge under an electric field

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1 Spectrochimica Acta Part A 6 (24) In situ speciation studies of copper in the electroplating sludge under an electric field S.-H. Liu, H. Paul Wang Department of Environmental Engineering, National Cheng Kung University, Tainan 711, Taiwan, ROC Received 22 October 23; accepted 9 December 23 Abstract Speciation of copper in the electrokinetic treatments of an industrial sludge was studied by in situ extended X-ray absorption fine structural (EXAFS) and X-ray absorption near-edge structural (XANES) spectroscopies in the present work. The least-square fits (LSF) of the XANES spectra indicated that the main copper species in the sludge were Cu(NO) 3 (82%) and adsorbed copper (Cu/SiO 2 ) (17%). Copper in the sludge possessed a Cu O bond distance of 1.97 Å with a coordination number (CN) of 3.8. In the second shells, the bond distance of Cu (O) Si was 3.3 Å with a CN of 1.5. Most of Cu/SiO 2 was dissolved in the aqueous phase since little Cu (O) Si species in the sludge was found after 18 min of the electrokinetic treatments. About 69% of total was dissolved into the aqueous phase and 51% of which was migrated to the cathode under the electric field (5 V cm 1 ) for 18 min. 24 Published by Elsevier B.V. Keywords: EXAFS; XANES; Electrokinetic treatments; Copper 1. Introduction Generally, non-hazardous industrial sludge wastes are disposed in landfills in Taiwan. However, cost of landfilling is becoming very expensive (US$ 5 25 t 1 ). Toxic metals in sludges might contaminate soils and ground water in a long-term landfilling process. Copper, for instance, may accumulate continuously in natural environment. Excessive absorption of copper has been suspected carcinogenic and may cause breast and brain cancers [1]. The potential accumulation in human tissues and biomagnification through food-chain cause both human health and environmental concerns [2]. The main scientific issues concerning the speciation (or chemical forms) of contaminants ultimately depend on their molecular-scale structures. Basic understanding at this scale is essential in the management of environmental contaminants that may also help the development of effective methods for decontamination or recycling. Speciation data such as bond distance, coordination number (CN) and chemical identity of elements in the complex matrix can be determined Corresponding author. Tel.: ; fax: address: wanghp@mail.ncku.edu.tw (H.P. Wang). by extended X-ray absorption fine structural (EXAFS) spectroscopy. X-ray absorption near-edge structure (XANES) can also provide information of oxidation states of an excited atom, the coordination geometry, and the bonding of its local environment in environmental solids. By EXAFS, we found that nano copper oxides (in ZSM-5 or ZSM-48) involved in the catalytic decomposition of NO and oxidation of chlorophenols in supercritical water [3 6]. Mineralization of CCl 4 with CuO was also determined by in situ EXAFS spectroscopy [7]. These EXAFS data were very useful in revealing the speciation of copper and its possible reaction pathway during mineralization. The electrokinetic technology is a promising and cost-effective remediation method for in situ treatments of contaminated soils [8 11]. During the past decades, the electrokinetic process has been developed and applied in the field tests. The electrokinetic method was also used in the treatments of contaminated sludges for decontamination or recycling of noble metals from sludges. Heavy metals incorporated into the sludges can be classified into several chemical forms including dissolved ionic, electrostatically adsorbed, and surface complexed forms [12]. A high content of copper in the electroplating sludge is generally found, that has caused a major problem in the final disposal processes. It is of great importance to investigate /$ see front matter 24 Published by Elsevier B.V. doi:1.116/j.saa

2 2388 S.-H. Liu, H.P. Wang / Spectrochimica Acta Part A 6 (24) the speciation of interest metals in the electrokinetic treatments of sludges. Thus, the main objective of the present work was to investigate the speciation of copper in the electrokinetic treatments of the sludge by X-ray absorption spectroscopy (XAS). An in situ EXAFS cell was used to reveal the structural change of copper during electrokinetic process. 2. Experimental Cu-rich sludges were sampled from an electroplating plant in the Tainan industrial park. The samples was dried in air at 3 K for 3 days, ground with a mortar and sieved with a 2-mm screen. The electrokinetic experiments of the sludge samples were carried out in a home-made in situ EXAFS cell [13]. The sludge samples were packed in the cell and filled with.1 M potassium nitrate as a conductive solution. A dc voltage of 1 V was constantly applied to electrodes (voltage gradient is 5 V cm 1 ). After 18 min of the electrokinetic treatments, the sludge was sliced into five portions of equal length immediately. Copper concentrations in each portion were pretreated by a general acid digestion procedure [14] and analyzed by flame atomic absorption (AA, 51 Perkin-Elmer) spectroscopy. Clay minerals of the sludge were determined by X-ray powder diffraction (XRD, RIGAKU Model D/MAX III-V) spectroscopy with monochromatic Cu K radiation that was scanned from 5 to 6 (2θ) at a scan rate of 4 min 1. Some chemical properties of the sludge were summarized in Table 1. The in situ XAS spectra of copper were carried out by a home-made in situ EXAFS cell which could be moved precisely to obtain a similar optical path. In present study, the X-ray beam was fixed at the point near the anode section (1.5 cm from the anode). The XANES and EXAFS spectra of copper under the electric field were collected on the Wiggler beamline at the Taiwan Synchrotron Radiation Research Center (SRRC). The electron storage ring was operated at the energy of 1.5 GeV. A Si (1 1 1) double-crystal monochromator was used for selection of energy with an energy resolution of (ev ev 1 ). The absorption spectra were collected in ion chambers that were filled with helium gas. Beam energy was calibrated by the adsorption edge of Cu foil at energy of 8979 ev. The standard deviation calculated from the averaged spectra was used to estimate the statistical noise and error associated with each structural parameter. Table 1 Chemical and mineralogical properties of the electroplating sludge Major mineral components a Initial ph 6.3 Total copper content (mg kg 1 ) 3,1 Water content (%) 75 a Determined by XRD. Quartz, kaolonite The EXAFS data were analyzed using the UWXAFS 3. and FEFF 8. programs [15]. The isolated EXAFS data was normalized to the edge jump and converted to the wave number scale. The Fourier transform was performed on k 3 -weighed EXAFS oscillations in the range of Å 1. To reduce the number of fit variables, the many-body factor (S 2 ) was fixed at.9. Fits of model compounds (such as CuO) have an error of ±.1 Å in radius and of ±1% in coordination number for the first shell atoms and of ±.2 Å and ±25% for the second shell atoms. The absorption edge was determined at the half-height (precisely determined by the derivative) of the XANES spectra after pre-edge baseline subtraction and normalization to the maximum post-edge intensity. Principal component analysis (PCA) and least-squares fitting (LSF) were used in the data treatment to optimize the quantitative extraction of relative concentrations of copper species [16,17]. XANES spectra of model compounds were also measured. Analyses were done qualitatively by comparing spectra from model compounds with those from in situ spectra. Semi-quantitative analyses of the edge spectra were conducted by the least-square fitting of linear combinations of model compound spectra to the spectrum of the sample. The height and area of the near-edge band in the copper spectrum were quantitatively proportional to the amount of copper species. The near-edge 3d 4p mixing peaks from the experimental absorption edge were fitted. A calibration curve could, therefore, be obtained. On the average, an uncertainty limit of 5% corresponds to an error of ca. 2.% in the fit results. 3. Results and discussion After 18 min of the electrokinetic treatments, variations of Cu contents in the sludge and ph values in the electrolyte between electrodes are shown in Fig. 1. Contents of Cu in the sludge were decreased by about 51 and 25% near the anode and the cathode, respectively. Copper near Cu Content (mg/kg) Initial Cu = 31 mg/kg Initial ph = Normalized Distances from Anode Fig. 1. Copper contents and the ph profile across the sludge after 18 min of the electrokinetic treatments (5 V cm 1 ) ph

3 S.-H. Liu, H.P. Wang / Spectrochimica Acta Part A 6 (24) Exp. Fit CuNO 3 (.82) Normalized Absorbance Cu/SiO 2 (.17) Exp. Fit CuNO 3 (.31) Cu(H 2 O) 6 2+ (.18) Fig. 2. In situ XANES spectra and their first derivative of copper in the electroplating sludge before and after (18 min) electrokinetic treatments. Solid lines and circles represent experimental data and the least-square fits, respectively. the cathode might dissolve and migrate to the cathode reservoir. A cation-selective membrane was used to reduce the precipitation of copper hydroxide under electric field. To further investigate the chemical structure of copper during electrokinetic treatments, in situ XANES and EXAFS spectra were also measured. The in situ Cu K-edge XANES spectra and their first derivatives of the sludge during the electrokinetic process are shown in Fig. 2. The pre-edge XANES spectra of copper in the sludge exhibit a very weak 1s-to-3d transition (8977 ev), which is forbidden by the selection rule in the perfect octahedral symmetry (Lin et al. [3,5] and Huang et al. [4,6]). A shoulder at ev and an intense band at ev, characteristic features of, may be attributed to the 1s-to-4p z and 1s-to-4p xy transitions, respectively. The XANES spectra of copper were also expressed mathematically in a LC XANES fit vectors, using the absorption data within the energy range of ev (Fig. 2). XANES spectra of standard copper compounds (Cu/SiO 2, CuO, Cu(NO) 3, CuCO 3, CuSO 4, CuCl 2, Cu(OH) 2,Cu 2 O, and Cu foil) were also measured on the Wiggler beamline. It was found that CuNO 3 (82%) and Cu/SiO 2 (adsorbed copper, 17%) were the main copper species in the sludge. About 69% of the species were dissolved into the aqueous phase and 51% of which were migrated to the cathode after 18 min of electrokinetic treatments. Time dependence for dissolution and migration of in the soil was shown in Fig. 3. Due to the high solubility of CuNO 3 and weak bonding of copper with surfaces of SiO 2, most adsorbed copper (Cu/SiO 2 ) and CuNO 3 were dissolved in the aqueous phase and migrated to the cathode. The in situ EXAFS spectra of the copper in the sludge were also recorded and analyzed in the k range of Å 1. An over 95% reliability of the Fourier transformed EXAFS data fitting for copper was obtained. In all EXAFS data analyses, the Debye Waller factors ( σ 2 ) were less than.1 Å 2 (Table 2). Cu-edge weighted chi spectra and corresponding radial structure functions (RSFs) of the sludge are shown in Fig. 4. The feature centered at about 1.61 Å was arisen from the first shell Cu O bonding, while a feature at about 2.62 Å may be attributed to the second shell Cu (O) Si bonding. Quantitative analysis of the Table 2 Speciation changes of copper in the sludge during electrokinetic treatments Contact time (min) Shell R (Å) CN σ 2 (Å 2 ) Cu O Cu (O) Si Cu O Cu (O) Si Cu O Cu (O) Cu Cu O Cu (O) Cu R: bond distance; CN: coordination number; σ 2 : Debye Waller factor.

4 239 S.-H. Liu, H.P. Wang / Spectrochimica Acta Part A 6 (24) Dissolved (%) Migrated (%) Time (minutes) Fig. 3. Time dependence for dissolution and migration of in the sludge during electrokinetic treatments. in situ spectra can identify bond distance and coordination number for the copper in the sludge during the electrokinetic treatments. In Table 2, copper in the sludge possessed a Cu O bond distance of 1.97 Å with a CN of 3.8. In the second shell, the bond distance of Cu (O) Si was 3.3 Å with a CN of 1.5, indicating that adsorbed as inner-sphere complexes to HO SiO surface groups. Prolonging the treat- ment time, the bond distance of Cu (O) Si was increased with a decrease of the CN. After 18 min of electrokinetic treatments, the dissolution of the Cu (O) Si species may occur since little Si atoms in the second shells was observed. Combined XANES and EXAFS observations, the possible reaction mechanism of copper involved in the electrokinetic Cu-O Cu-O-Si χ (k)*k 3 Magnitude (arb. Unit) Cu-O Cu-(O)-Cu k (Å -1 ) R (Å) Fig. 4. Normalized Cu-EXAFS k 3 -weighed chi (left panel) and corresponding Fourier transforms (right panel) of the electroplating sludge before and after (18 min) electrokinetic treatments. The solid line and dotted lines represent the experimental data and best fitted result, respectively.

5 S.-H. Liu, H.P. Wang / Spectrochimica Acta Part A 6 (24) treatments at the early stage (the first 18 min) may be addressed. in the sludge had a Cu O bond distance of 1.97 Å with its CN of 3.8. In the second shells, Cu (O) Si (3.3 Å) (with a CN of 1.5) was found, indicating an interaction of copper with sludge surfaces. However, because of the perturbation by electric field, about 69% of were dissolved in the aqueous solution and migrated to the cathode under the electric field. 4. Conclusions By least-square fits of XANES, the main Cu species in the sludge were Cu(NO) 3 (82%) and Cu/SiO 2 (adsorbed copper) (17%). Copper in the sludge possessed a Cu O bond distance of 1.97 Å with a CN of 3.8. In the second shells, the bond distances of Cu (O) Si was 3.3 Å withacnof 1.5. The dissolution of Cu (O) Si species under the electric field may occur since little Si atoms in the second shells was observed after 18 min of electrokinetic treatments. About 69% of was dissolved into the aqueous phase and 51% of which was migrated to the cathode after 18 min of electrokinetic treatments. This work exemplified the utilization of in situ EXAFS and XANES for revealing the speciation and the possible reaction path in the electrokinetic treatments. References [1] I. Yucel, F. Apraci, A. Ozet, B. Doner, T. Karayilanoglu, A. Sayar, O. Berk, Biol. Trace Elem. Res. 4 (1994) 31. [2] U. Krogmann, L.S. Boyles, W.J. Bamka, S. Chaiprapat, C.J. Martel, Water Environ. Res. 71 (1999) 692. [3] K.-S. Lin, H.P. Wang, Appl. Catal. B: Environ. 22 (1999) 261. [4] Y.-J. Huang, H.P. Wang, J.-F. Lee, Chemosphere 39 (1999) [5] K.-S. Lin, H.P. Wang, Environ. Sci. Technol. 34 (2) [6] Y.-J. Huang, H.P. Wang, J.-F. Lee, Appl. Catal. B: Environ. 4 (23) 111. [7] Y.-C. Chien, H.P. Wang, Y.W. Yang, Environ. Sci. Technol. 35 (21) [8] Y.B. Acar, A.N. Alshawabkeh, Environ. Sci. Technol. 27 (1993) [9] R. Lageman, Environ. Sci. Technol. 27 (1993) [1] R.F. Probstein, R.E. Hicks, Science 26 (1993) 498. [11] J. Trombly, Environ. Sci. Technol. 28 (1994) A289. [12] S.-O. Kim, S.-H. Moon, K.-W. Kim, S.-T. Yun, Water Res. 36 (22) [13] S.-H. Liu, H.P. Wang, H.-C. Wang, Y.W. Yang, J. Synchrotron Radiat. 8 (21) 919. [14] A.L. Page, Methods of Soil Analysis, second ed., Soil Science Society of America, Madison, Wisconsin, [15] E.A. Stern, M. Newville, B. Ravel, Y. Yacoby, D. Haskel, Physica B 29 (1995) 117. [16] M.J. Fay, A. Proctor, D.P. Hoffmann, M. Houalla, D.M. Hercules, Mikrochim. Acta 19 (1992) 281. [17] A.C. Scheinost, R. Kretzschmar, S. Pfister, Environ. Sci. Technol. 36 (22) 521.