ADSORPTION OF SULFATE AND FLUORIDE BY VARIABLE CHARGE SOILS

The adsorption of sulphate and fluoride by three variable charge soils was studied. Adsorption increased with increase in the amount of sulphate added at constant pH, and decreased with increase of pH. The ratios of the amount of released OH- to that of adsorbed .**GRAPHIC**. at pH 5.0 were 0.12, 0.14 and 0.20 for the three soils, respectively, much lower than the corresponding OH-/F- ratios which ranged from 0.3 to 1.0. For a ferric acrisol the OH- released accounted, on average, for only 15% of the .**GRAPHIC**. absorbed, leaving more than 60% to be explained by the decrease in positive charge and the increase in negative charge carried by the soil.