Outer-sphere adsorption of Pb(II)EDTA on goethite |
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| Institution: | 1. Stanford Synchrotron Radiation Laboratory, P.O. Box 4349, Stanford, CA 94309, USA;2. Department of Inorganic Chemistry, Umeå University, Umeå, S-901 87 Sweden;1. Department of Economics and Social Science, Catholic University, Via Emilia Parmense 84, 29100, Piacenza, Italy;2. Department of Economics, Quantitative Methods and Management, University of Milano - Bicocca, Piazza Ateneo Nuovo 1, 20126 Milano, Italy;1. School of Hehai, Chongqing Jiaotong University, Chongqing 400074, PR China;2. School of Civil and Environmental Engineering, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen 518055, PR China;3. Shenzhen Key Laboratory of Water Resource Utilization and Environmental Pollution Control, Shenzhen, 518055, PR China;1. College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing, China;2. National Engineering Technique Research Center for Biotechnology, Nanjing, China;3. Jiangsu National Synergetic Innovation Center for Advanced Bio-Manufacture, China;1. Guangzhou Key Laboratory Environmental Catalysis and Pollution Control, Guangdong Key Laboratory of Environmental Catalysis and Health Risk Control, School of Environmental Science and Engineering, Institute of Environmental Health and Pollution Control, Guangdong University of Technology, Guangzhou 510006, China;2. Guangdong Province Solid Waste Recycling and Heavy Metal Pollution Control Engineering Technology Research Center, Guangdong Polytechnic of Environmental Protection Engineering, Foshan 528216, China;3. Institute of Bioresource and Agriculture, Department of Biology, Hong Kong Baptist University, Hong Kong, China;4. Guangdong EFOR Environmental Group Co., Ltd, Dongguan 523073, China |
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| Abstract: | Fourier transform infrared (FTIR) and extended X-ray absorption fine structure (EXAPS) spectroscopic measurements were performed on Pb(II)ethylenediaminetetraacetic (EDTA) adsorbed on goethite as a function of pH (4–6), Pb(II)EDTA concentration (0.11–72 μM), and ionic strength (16 μM–0.5 M). FTIR measurements show no evidence for carboxylate-Fe(III) bonding or protonation of EDTA at Pb:EDTA = 1:1. Both FTIR and EXAFS spectroscopic measurements suggest that EDTA acts as a hexadentate ligand, with all four of its carboxylate and both of its amine groups bonded to Pb(II). No evidence was observed for inner-sphere Pb(II)-goethite bonding at Pb:EDTA = 1:1. Hence, the adsorbed complexes should have composition Pb(II)EDTA2?. Because substantial uptake of PbEDTA(II)2? occurred in the samples, we interpret that Pb(II)EDTA2? adsorbed as outer-sphere complexes and/or as complexes that lose part of their solvation shells and hydrogen bond directly to goethite surface sites. We propose the term “hydration-sphere” for the latter type of complexes because they should occupy space in the primary hydration spheres of goethite surface functional groups and to distinguish this mode of sorption from common structural definitions of inner- and outer-sphere complexes. The lack of evidence for inner-sphere EDTA-Fe(III) bonding suggests that previously proposed metal/ligand-promoted dissolution mechanisms should be modified, specifically to account for the presence of outer-sphere precursor species. |
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