Kinetics of arsenate sorption/desorption from metal oxides affected by surface coverage and residence time

Sorption and desorption processes control the mobility, toxicity, and availability of As in natural environments. Surface coverage and residence time may affect the kinetics of As sorption-desorption from soil components and the transformation of As from desorbable into resistant or undesorbable forms. We performed kinetic studies on the sorption of As(V) onto crystalline or poorly crystalline metal oxides (noncrystalline Al(OH)(x), gibbsite, ferrihydrite, and goethite) and its desorption by PO4 at pH 6.0 as affected by the residence time and the surface coverage (50 or 100%) of As(V). Significant amounts of As(V) were sorbed during the initial period of 0.167 h, ranging from 37.9 to 71.8% when the surface coverage was about 100%. The kinetic data, explained best by the Elovich kinetic model, indicated the following order in As(V) sorption: gibbsite < AI(OH)(x) < goethite < ferrihydrite. By adding PO4 immediately after complete sorption of As(V) onto the oxides (50% surface coverage; PO4 added/As(V) sorbed molar ratio of 4), a much higher proportion of As(V) was desorbed after 24 h of reaction from Al oxides (48-56%) than from Fe oxides (18-23%). The amount of As(V) desorbed decreased with increasing residence time. The kinetics of As(V) desorption by PO4 as a function of residence time was explained best by the Elovich kinetic model. The kinetics described the rate of rearrangement of As(V) from desorbable into resistant or undesorbable forms, which occurred more rapidly in Al than Fe oxides. After a residence time of 360 h, the percentage of As(V) desorbed from the oxides was reduced significantly (< 13%).