Extraction of arsenic from a soil in the blackfoot disease endemic area with ionic liquids

Speciation of arsenic extracted with room temperature ionic liquids (RTILs) ([bmim][BF4] (1-butyl-3-methylimidazolium tetrafluoroborate) and [bmim][PF6] (1-butyl-3-methylimidazolium hexafluorophosphate)) from an As-humic acid (As-HA) complex contaminated soil (As-HA/soil) in a blackfoot disease endemic area has been studied by X-ray absorption (near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS)) spectroscopy. About 45% of arsenic in the As-HA/soil can be extracted with [bmim][BF4] while the relatively less hydrophilic [bmim][PF6] extracts 25% of arsenic. The extracted arsenic in the [bmim][BF4] and [bmim][PF6] from the As-HA/soil possesses mainly As(III) species, suggesting that at least two reaction paths may be involved in the extraction process: (1) splitting of As-HA and (2) reduction of As(V) to As(III). The refined EXAFS spectra also indicate that the As(III) extracted in the RTILs possesses the AsO2− structure, which has the As–O bond distances of 1.77–1.79 Å and coordination numbers of 4.0–4.2.