Sorption of phosphate on goethite at high concentrations

phosphorus (P) concentrations in soil solution are generally low, whereas t hose near fertilizer particles may reach much greater values. The process g overning P sorption at these high concentrations is obscure. Thus, the sorp tion mechanisms at high P concentrations on a synthetic goethite were inves tigated to assess the influence of ionic strength on P sorption and release . In addition to classic isotherms, the adsorption was investigated by Four ier-transform infrared spectroscopy and electrophoretic mobility. At consta nt ionic strength, the maximum amount adsorbed was about 2.3 mu mol P m(-2) , which remained constant even at the highest concentrations. With variable ionic strength, the sorption isotherm showed plateaus at 2.3 and 5.3 mu mo l P m(-2). Zeta potential and particle size data indicated that the increas ed ionic strength generated by P addition caused a compression of the phosp hate ion swarm closer to the oxide surface, resulting in a reduction of the electrostatic repulsive forces and leading to an increase in sorption, The results of P release and Fourier-transform infrared spectra of the goethit e-P complexes confirm this hypothesis. Thus, at high concentrations, sorpti on is possible beyond the capacity of the goethite singly coordinated surfa ce groups, suggesting that both chemisorption and physical mechanisms are i nvolved.