Competitive adsorption between phosphate, tartrate and oxalate was studied on two hydroxy aluminum montmorillonite complexes (AlMt1.6 and AlMt6), which were prepared by adding a base to pH 5.5 to samples containing 1.6 and 6.0 mol Al per kg clay. The quantities of phosphate, tartrate and oxalate adsorbed were more closely related to the amount of OH-Al species coatings on the montmorillonite than to the surface area of the complexes. The adsorption capacity of phosphate was much greater than that of tartrate or oxalate for both samples. Adding molar amounts of oxalate and tartrate resulted in an oxalate/tartrate adsorption ratio (Rf) of ∼1. However, in the presence of phosphate, Rf values were <1.0, and the Rf values decreased with increasing amounts of added phosphate, indicating that tartrate competed with phosphate more effectively than oxalate. The presence of tartrate also reduced phosphate adsorption by the complexes. The efficiency of tartrate in reducing phosphate adsorption increased by increasing the initial tartrate/phosphate molar ratio and by adding tartrate 2 h before phosphate addition. Tartrate and oxalate added as a mixture in equimolar quantities were much more effective in inhibiting phosphate sorption than tartrate alone under the same organic ligand concentrations, probably because more sites with high affinity for both the organic ligands were occupied by tartrate and oxalate than by tartrate alone. The efficiency of tartrate alone, or combined with oxalate, in preventing phosphate adsorption was greater for the complex containing a lesser amount of OH-Al species coating the montmorillonite surfaces. This result may be attributable to a greater proportion of sites specific for organic ligands present on AlMt1.6 compared to AlMt6 complex.
