RESEARCH ARTICLE
Diphenylarsinic acid sorption mechanisms in soils using batch experiments and EXAFS spectroscopy
1. College of Environmental Science and Engineering, Anhui Normal University, Wuhu 241002, China
2. Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China
3. Key Laboratory of Coastal Environmental Processes and Ecological Remediation, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 264003, China
4. Anhui Provincial Engineering Laboratory of Water and Soil Pollution Control and Remediation, Anhui Normal University, Wuhu 241002, China
5. Centre for Novel Agricultural Products, Department of Biology, University of York, York YO10 5DD, UK |
Available online: 2020-04-01
Abstract
DPAA sorption data was found to fit the Freundlich equation. Kf was significantly positive correlated with oxalate-extractable Fe2O3. Ligand exchange was the main mechanism for DPAA sorption on soils. Bidentate binuclear and monodentate mononuclear DPAA bonds were identified. Diphenylarsinic acid (DPAA) is a phenyl arsenic compound derived from chemical warfare weapons. Macroscopic and microscopic work on DPAA sorption will provide useful information in predicting the partitioning and mobility of DPAA in the soil-water environment. Here, batch experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy were used to investigate the sorption mechanisms of DPAA. The DPAA sorption data from 11 soil types was found to fit the Freundlich equation, and the sorption capacity, Kf, was significantly and positively correlated with oxalate-extractable Fe2O3. The Kf values of eight of the 11 untreated soils (1.51–113.04) significantly decreased upon removal of amorphous metal (hydr)oxides (0.51–13.37). When both amorphous and crystalline metal (hydr)oxides were removed from the untreated soils, the Kf values either decreased or slightly increased (0.65–3.09). Subsequent removal of soil organic matter from these amorphous and crystalline metal (hydr)oxide-depleted samples led to further decreases in Kf to 0.02–1.38, with only one exception (Sulfic Aquic-Orthic Halosols). These findings strongly suggest that ligand exchange reactions with amorphous metal (hydr)oxides contribute most to DPAA sorption on soils. EXAFS data provide further evidence that DPAA primarily formed bidentate binuclear (2C) and monodentate mononuclear (1V) coring-sharing complexes with As-Fe distances of 3.34 and 3.66 Å, respectively, on Fe (hydr)oxides. Comparison of these results with earlier studies suggests that 2C and 1V complexes of DPAA may be favored under low and high surface coverages, respectively, with the formation of 1V bonds possibly conserving the sorption sites or decreasing the steric hindrance derived from phenyl substituents.
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