Competition in Adsorption between Phosphate and Oxalate on an Aluminum Hydroxide Montmorillonite Complex

A study was conducted to provide information on phosphate and oxalate sorption on an Al(OH) x ‐montmorillonite (chlorite‐like) complex under changing conditions of pH, total solution ligand concentrations, and order of anion addition. The competitive adsorption of phosphate and oxalate on a chlorite‐like complex (3.3 mol Al kg −1 montmorillonite) was influenced by pH (5.0–9.0), order of anion addition, and initial oxalate/phosphate molar ratio ( R i ). In systems containing a constant amount of phosphate (500 or 400 mmol kg −1 ) and increasing quantities of oxalate ( R i from 0.5 to 3.34), more phosphate than oxalate was adsorbed even when the initial concentration of oxalate was much higher than that of phosphate. Of the sites on the clay surface that were available for adsorption by both anions, 51 to 79% were occupied by phosphate at R i = 3.34 in the range of pH studied. Minimum reduction in phosphate adsorption occurred when phosphate was added to the clay mineral before oxalate, whereas maximum reduction occurred when oxalate was introduced before phosphate. The efficiency of oxalate in inhibiting phosphate adsorption was stronger at pH 5.0 than at pH 7.0 or 9.0. At pH 5.0, by increasing R i from 0.5 to 3.34, phosphate sorption decreased from 20 to 44% when OX was added before phosphate. Many sites of the clay mineral were highly specific for phosphate, whereas some others, common to both the anions, had a higher affinity for phosphate than for oxalate. However, some sites were specific for oxalate anions.