Speciation and sorption structure of diphenylarsinic acid in soil clay mineral fractions using sequential extraction and EXAFS spectroscopy
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The mobility of arsenic (As) in soils is fundamentally affected by the clay mineral fraction and its composition. Diphenylarsinic acid (DPAA) is an organoarsenic contaminant derived from chemical warfare agents. Understanding how DPAA interacts with soil clay mineral fractions will enhance understanding of the mobility and transformation of DPAA in the soil-water environment. The objective of this study was to investigate the speciation and sorption structure of DPAA in the clay mineral fractions.
Materials and methods
Twelve soils were collected from nine Chinese cities which known as chemical weapons burial sites and artificially contaminated with DPAA. A sequential extraction procedure (SEP) was employed to elucidate the speciation of DPAA in the clay mineral fractions of soils. Pearson’s correlation analysis was used to derive the relationship between DPAA sorption and the selected physicochemical properties of the clay mineral fractions. Extended X-ray absorption fine structure (EXAFS) LIII-edge As was measured using the beamline BL14W1 at Shanghai Synchrotron Radiation Facility (SSRF) to identify the coordination environment of DPAA in clay mineral fractions.
Results and discussion
The SEP results showed that DPAA predominantly existed as specifically fraction (18.3–52.8%). A considerable amount of DPAA was also released from non-specifically fraction (8.2–46.7%) and the dissolution of amorphous, poorly crystalline, and well-crystallized Fe/Al (hydr)oxides (20.1–46.2%). A combination of Pearson’s correlation analysis and SEP study demonstrated that amorphous and poorly crystalline Fe (hydr)oxides contributed most to DPAA sorption in the clay mineral fractions of soils. The EXAFS results further demonstrated that DPAA formed inner-sphere complexes on Fe (hydr)oxides, with As-Fe distances of 3.18–3.25 Å. It is likely that the steric hindrance caused by phenyl substitution and hence the instability of DPAA/Fe complexes explain why a substantial amount of DPAA presented as weakly bound forms.
DPAA in clay mineral fractions predominantly existed as specifically, amorphous, poorly crystalline, and crystallized Fe/Al (hydr)oxides associated fractions. Amorphous/poorly crystalline Fe rather than total Fe contributed more to DPAA sorption and DPAA formed inner-sphere complexes on Fe (hydr)oxides.
KeywordsDiphenylarsinic acid (DPAA) EXAFS Sequential extraction Speciation Sorption structure
We are thankful to Dr. Xu Wang for his technical assistance during the experiments at SSRF and Dr. Junqing Xu for his useful suggestion during the data analysis at the National Synchrotron Radiation Laboratory (NSRL), University of Science and Technology of China. We would also like to thank Dr. Xueli Wu for the help in HPLC-MS/MS analysis.
This study received financial support from the National Natural Science Foundation of China (no. 41807117 and 41230858), the Key Projects of Natural Science Research of Universities in Anhui Province (no. KJ2018A0315), the Doctoral Research Start-up Funds Project of Anhui Normal University (no. 2018XJJ50), the Talent Cultivation Project of Anhui Normal University (no. 2018XJJ82), and the Cultivation Project on Excellent Undergraduates’ Thesis (design, create) of Anhui Normal University (no. pyjh2018487).
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