Methylarsenic Sorption and Speciation Mechanisms in Natural Systems

Arsenic (As) originates in many rocks and minerals throughout the world. Natural phenomena, such as weathering and biological activities, along with industrial activities and agricultural activities are responsible for As introduction to the environment. The most predominate oxidation states for inorganic As species are arsenate As (H3AsO4) and arsenite As (H3AsO3). In addition to inorganic forms, organic forms of As also exist in nature, typically occurring in terrestrial environments as monomethylarsenate, MMA (CH3H2AsO3) and dimethylarsenate, DMA ((CH3)2HAsO2). MMA and DMA have historically been used as herbicides and pesticides. Because of their large application to agricultural fields and the toxicity of MMA and DMA, the behavior of methylarsenics in the soil environment requires investigation. Accordingly, the objectives of this study were 1) characterize MMA and DMA sorption to aluminum oxide, 2) characterize MMA and DMA sorption, speciation, and distribution to a soil, and 3) characterize MMA and DMA sorption and desorption to various types of soils. MMA and DMA sorption on amorphous aluminum oxide (AAO) were investigated using both macroscopic batch sorption kinetics and molecular scale Extended X-ray Absorption Fine Structure (EXAFS) and Fourier Transform infrared (FTIR) spectroscopic techniques. Sorption isotherm studies revealed sorption maxima of 0.183, 0.145, and 0.056 mmol As / mmol Al for arsenate (As), MMA, and DMA, respectively. In the sorption kinetics studies, 100 % of added As was sorbed within 5 min, while 78 % and 15 % of added MMA and DMA were sorbed, respectively.