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The aim of this work was to study: (i) the competitive adsorption of phosphate (P) and oxalate (OX) anions on an Al oxide as influenced by pH and order of anion addition; and (ii) the adsorption of P on and the desorption of OX from an Al precipitation product [Al(OH)x‐OX] formed in the presence of OX ions. The amount of P adsorbed on Al oxide in the absence of OX decreased with increasing pH. In the absence of P, OX adsorption exhibited a broad maximum between pH 5.0 and 7.0, but rapidly decreased at a pH <5.0 or >7.0. The order of anion addition strongly influenced the adsorption of both P and OX. Maximum reduction in P adsorption occurred when OX was introduced before P, whereas reduction was negligible when P was added before OX. The efficiency of OX in reducing P adsorption was relatively high at acidic pH values, but it was poor at a pH >7.0. The presence of P strongly depressed OX adsorption when P was added before OX or when the anions were added together. The efficiency of P in reducing OX adsorption increased with increasing pH. Finally, the ability of P in replacing OX ions from Al(OH)x‐OX was poor at pH values <7.0, but increased in alkaline environments with increasing pH. The data thus reveal that OX ions compete strongly with P for adsorption sites of Al oxides when they are added before the inorganic ions or are coprecipitated with Al in Al precipitation products, and mainly at acidic or neutral pH values.

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