Effects of organic ligands on adsorption of phosphate on a noncrystalline al hydroxide

The effects of oxalate (OX), tartrate (TR), and citrate (CT) on the adsorption of phosphate (P) on a synthetic noncrystalline Al-hydroxide [Al(OH)x] were studied at different initial phosphate/ligand molar ratios and different reaction conditions. The quantities of P adsorbed by the Al(OH)x varied with the ligand concentrations and the reaction conditions. When OX was introduced to the reaction system before P (OX before P), the effect of OX in decreasing P adsorption was greater than that when OX was added as a mixture with P (OX+P) or when P was introduced before OX (P before OX). When P was added after OX adsorption and removal of residual OX solution (OX(-) before P), the effect of OX in preventing P adsorption was lower than that in the system of OX before P. When P was added to the Al(OH)x as a mixture with two or three organic ligands, the reduction of P adsorption was related to the nature and concentration of the organic ligands. The combined effect of organic acids on P adsorption was not equal to the sum of the individual effect of each organic ligand. P adsorption increased with the equilibrium time from 1 to 7 days more in the presence of OX than CT. These findings have important implications for P availability in acidic soils.