Describing the effect of time on sorption of phosphate by iron and aluminium hydroxides

Iron and aluminium hydroxides were precipitated both in the presence and absence of kaolinite. The reaction between phosphate and these hydroxides was measured for periods which ranged from 5 min to 72 h. The effect of time on phosphate sorption was examined by plotting the sorption data according to different, simple, kinetic equations such as the first order, second order, the parabolic diffusion equation, the Elovich equation and the modified Freundlich equation. The effect of time on sorption was also examined by the mechanistic model recently developed by Barrow (1983b) for the sorption and desorption of phosphate by soils. The sorption of phosphate by iron and aluminium hydroxides increased with time and the reaction continued through the period of observation without reaching a true equilibrium. Curvilinear relationships were obtained when the data were plotted according to the simple kinetic equations. These simple kinetic equations fail to describe the effect of time on sorption partly because the mechanism is different from that postulated and partly because they do not consider electrostatic effects when phosphate ions react with a charged surface. The mechanistic model of Barrow (1983b), which takes this effect into account, described effect of both concentration and time on phosphate sorption. According to this model, the increase in phosphate sorption with time was caused by a redistribution of adsorbed phosphate into the interior of the particles of iron and aluminium hydroxides by solid-state diffusion.