Effects of pore water velocity on the transport of arsenate

The potential for AsO4 to exhibit sorption-related nonequilibrium during transport has not been previously determined. Consequently, the objectives of this study were to determine the applicability of transport models based on the advection-dispersion equation (ADE) using various equilibrium and kinetic sorption expressions for describing AsO4 mobility. Saturated column transport experiments were performed using an applied AsO4 pulse at pore water velocities (PWVs) of 0.2, 1.0, 10, and 90 cm h(-1) through a sand where the principal reactive phase was amorphous or poorly crystalline iron oxides. Observed AsO4 breakthrough curves (BTCs) demonstrated sorption-related nonequilibrium as evidenced by a leftward shift in observed BTCs and an increase in observed effluent recovery with increasing PWV. The use of independently derived equilibrium and kinetic sorption parameters in the ADE failed to describe observed AsO4 BTCs at all PWVs. Apparent sorption rate coefficients obtained by fitting observed BTCs to an nth-order kinetic model increased with increasing PWV. The time scales of the fitted rate coefficients are significantly longer than previously determined rate coefficients describing the chemical step of arsenate sorption by iron oxide minerals, suggesting that slower diffusional processes control the rate of arsenate sorption-desorption during transport.