Sulphate sorption by variable charge soils

The sorption of sulphate (SO) by three variable charge soils from the Canary Islands (Spain) was studied. Sulphate sorption decreased with increasing pH. Only negligible amounts of SO were sorbed above pH 6.5. When the soils were washed with an indifferent electrolyte (0.01 M KCl), more SO was recovered than had been sorbed. This indicated a release of native SO Sulphate replaced hydroxyl ions (OH) and co‐ordinated HO 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 SO 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 SO through a ligand exchange mechanism, but in a different plane of sorption to that of phosphate