DMA, and ultimately arsine gas (Smith et al., 1998; Vaughan, 1993). Both
inorganic and organic species of As undergo various biological and chemical
transformations in soils, including adsorption, desorption, precipitation,
complexation, volatilization, and methylation (Fig. 2). Some important
biogeochemical reactions of As and their significance in soil and aquatic
environments are given in Table VI. The most thermodynamically stable
species of As(III) (i.e., H3AsO3 and H2AsO
4 ) and As(V) (i.e., HAsO2
4 )
occur over the normal soil pH range of 4 to 8.