TIME-DEPENDENT SORPTION OF PHOSPHATE BY SOILS AND HYDROUS FERRIC OXIDES

The sorption of inorganic phosphate (P) by soils and hydrous ferric oxides was studied at times up to 192h. An initially rapid decrease in solution P concentration was followed by a much slower decrease between 48 and 192h with soils, Fe gel. and natural goethite, whereas synthetic goethite gave a well-defined equilibrium condition after only 48h. Resolution of the sorption isotherms showed that the increase in P sorption with time involved an appreciable shift of P from a more-physically sorbed form to a chemisorbed form. This was supported by chemical fractionation which showed that NaOH-extractable P was fairly constant with increasing sorption time, whereas the additional sorbed P was extracted by citrate-dithionite-bicarbonate from soils, and by HC1 from Fe gel and natural goethite. These sorbents contained short-range (amorphous) material, whereas synthetic goethite, from which all sorbed P was NaOH –extractable. did not. It is proposed that the time-dependent sorption of P and the associated shift of P to chemisorbed forms, involves the diffusion of P into “structurally porous”, short-range order material.