Phosphate Adsorption on an Oxisol

Adsorption isotherms for phosphate on the A and B horizon of an oxisol from Papua New Guinea showed high affinity adsorption at low solution concentrations and a linear increase in adsorption at higher solution concentrations. Adsorption increased with decrease in pH in the range 3 to 8. Comparison with data obtained under similar conditions with synthetic goethite (α‐FeOOH) and gibbsite [Al(OH) 3 ] suggested that phosphate reacted with FeOH and FeOH 2 + groups on soil iron oxide surfaces to form a binuclear bridging FeOP(O 2 )OFe complex and with exposed Al(OH)H 2 O on kaolinite edges and on hydroxy‐aluminum species to form a binuclear or bidentate phosphate complex. At higher phosphate concentrations isotherms showed continued adsorption which may be due to a further reaction involving hydroxy‐aluminum groups.