Exchange reaction between selenite and hydroxyl ion of variable charge soil surfaces: I. Electrolyte species and pH effects

Hydroxyl release of red soil and latosol surfaces was quantitatively measured using a self-made constant pH automated titration instrument, to study the changes of hydroxyl release with different added selenite amounts and pH levels, and to study the effects of electrolytes on hydroxyl release. Hydroxyl release increased with the selenite concentration, with a rapid increase at a low selenite concentration while slowing down at a high concentration. The pH where maximum of hydroxyl release appeared was not constant, shifting to a lower valus with increasing selenite concentration. Hydroxyl release decreased with increasing electrolyte concentration, and the decrease was very rapid at a low electrolyte concentration but slow at a high electrolyte concentration. For NaClO4, NaCl and Na2SO4, hydroxyl release was in the order of NaClO4 > NaCl much greater than Na2SO4, and the difference was very significant. But for NaCl, KCl and CaCl2, the order of hydroxyl release was NaCl > KCl > CaCl2, and the difference was smaller. The amount of hydroxyl release from Xuwen latosol was greater than that from Jinxian red soil. Hydroxyl release existed in a wider range of pH with Xuwen latosol than with Jinxian red soil, due to their difference in soil properties. However, both soils had similar curves of hydroxyl release, indicating the common characteristics of variable charge soils.