Sulphate sorption by variable charge soils

SUMMARY The sorption of sulphate (SO2−4) by three variable charge soils from the Canary Islands (Spain) was studied. Sulphate sorption decreased with increasing pH. Only negligible amounts of SO2−4 were sorbed above pH 6.5. When the soils were washed with an indifferent electrolyte (0.01 M KCl), more SO2−4 was recovered than had been sorbed. This indicated a release of native SO2−4 Sulphate replaced hydroxyl ions (OH) and co-ordinated H2O molecules, as well as very small amounts of silicate (Si). No measurable amount of phosphate (P) was released. On average hydroxyl release accounted for 50% of SO2−4 sorbed, the rest being accounted for by the increase in negative charge as measured by K+ adsorption. The results presented here are consistent with the sorption of SO2−4 through a ligand exchange mechanism, but in a different plane of sorption to that of phosphate.