Desorption of phosphate from iron oxides in relation to equilibrium pH and porosity

Reactions of phosphated lepidocrocites and goethites with 0.1 M NaCl, 0.1 M NaOH and 0.5 M NH4F solutions have been studied. Solutions of indifferent electrolyte (0.1 M NaCl) at the same pH as used during adsorption of P were used to desorb P so that new apparent equilibria were reached, but a slow readsorption was also observed. Strongly alkaline solutions seemed to cause some breakdown of the solid surface and part of the adsorbed P became occluded. Desorption and isotopic exchange data have been related to porosity of the two oxides, and presence of a component of the exchangeable P released very slowly, has been attributed to P adsorbed on surfaces of micropores. (orig.)