Infrared spectroscopic and X-ray diffraction characterization of the nature of adsorbed arsenate on ferrihydrite |
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| Authors: | Yongfeng Jia Liying Xu George P Demopoulos |
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| Institution: | a Institute of Applied Ecology, Key Laboratory of Terrestrial Ecological Process, Chinese Academy of Sciences, Shenyang 110016, China
b Department of Mining, Metals and Materials Engineering, McGill University, Montreal, QC, Canada H3A 2B2 |
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| Abstract: | Fourier transformed infrared (FTIR) spectroscopy was used to characterize arsenate-ferrihydrite sorption solids synthesized at pH 3-8. The speciation of sorbed arsenate was determined based on the As-O stretching vibration bands located at 650-950 cm−1 and O-H stretching vibration bands at 3000-3500 cm−1. The positions of the As-O and O-H stretching vibration bands changed with pH indicating that the nature of surface arsenate species on ferrihydrite was strongly pH dependent. Sorption density and synthesis media (sulfate vs. nitrate) had no appreciable effect. At acidic pH (3, 4), ferric arsenate surface precipitate formed on ferrihydrite and constituted the predominant surface arsenate species. X-ray diffraction (XRD) analyses of he sorption solids synthesized at elevated temperature (75 °C), pH 3 clearly showed the development of crystalline ferric arsenate (i.e. scorodite). In neutral and alkaline media (pH 7, 8), arsenate sorbed as a bidentate surface complex (in both protonated  FeO2As(O)(OH)− and unprotonated  forms). For the sorption systems in slightly acidic media (pH 5, 6), both ferric arsenate and surface complex were probably present on ferrihydrite. It was further determined that the incorporated sulfate in ferrihydrite during synthesis was substituted by arsenate and was more easily exchangeable with increasing pH. |
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