Mechanism of sulphate adsorption by two tropical SoilS

SUMMARY The mechanism of sulphate (SO 4 ) adsorption was investigated using two highly‐weathered soils with contrasting surface‐charge characteristics (one with a net negative charge and the other with a net positive charge). The addition of SO 4 caused a release of OH and increased the adsorption of cations, in keeping with the widely‐held view that SO 4 is chemisorbed by replacing ‐OH and ‐OH 2 ligands. On the other hand, adsorbed SO 4 was completely recovered by washing with an indifferent electrolyte solution (1 M KC1), an extractant which is usually taken to be specific for ‘exchangeable’ ions. The amounts of SO, adsorbed were several times higher than those of non‐specifically adsorbed Cl. It is suggested that the experimental observations can be interpreted in terms of a model in which SO 4 adsorbs in a plane closer to the surface than monovalent anions (e.g. in the Stern layer). The consequence of higher counter‐ion valence and proximity to the surface is that additional positive charge is induced onto the surface by OH release. Charge reversal in the diffuse layer allows more cations to be adsorbed. This type of adsorption may involve forces other than the purely electrostatic but, in contrast to chemisorption, the adsorbed anion does not become chemically coordinated to the surface metal atoms.