Phosphate Adsorption and the Displacement of Structural Silicon in an Allophane Clay

Summary Phosphate was adsorbed at a constant pH, on allophane clay samples which had been pre‐treated with selenite to remove adsorbed silicate. The concomitant release of selenium, silicon, and hydroxyl ion on phosphate adsorption was also measured. The results are compared with those obtained from a similar experiment conducted using clay which had not been treated and they substantiate the earlier conclusion that phosphate displaces structural silicon of clays. The amount displaced increases parabolically with increasing levels of phosphate adsorbed over the concentration range studied.