Kinetics of phosphate sorption and desorption by synthetic aluminous goethite before and after thermal transformation to hematite.

Goethite is an important component of many soils and its transformation to hematite by fire may affect phosphate retention. This study deals with the kinetics of phosphate sorption and desorption by synthetic aluminous goethites and their dehydroxylated products. Phosphate sorption and subsequent desorption were measured for periods up to 128 h. Phosphate sorption increased with equilibrium solution concentration, Al substitution and surface area, and decreased with increasing crystal size. The trend in amount of phosphate sorbed was: partially dehydroxylated goethite > hematite > groethite. Sorption and desorption kinetics are well described by the modified Elovich equation. Desorption of a minor proportion of the sorbed P was rapid and this was sometimes followed by minor resorption. The amount of P desorbed directly reflected the amount that was initially sorbed. Acid dissolution of the Fe oxides with sorbed phosphate suggested that most phosphate was present at the crystal surface and not in relatively poorly accessible micropores.