The effect of pH on zinc adsorption by a lateritic soil in the presence of citrate and oxalate

A change in the pH of the rhizosphere may alter the effect that organic ligands have on the concentration of Zn in the soil solution. To assess this effect, Zn adsorption by a lateritic soil was measured in the presence of 0, 1 and 3 mM of citrate and oxalate at nine values of pH ranging from pH 2 to 8. The concentrations of Zn and the ligands remaining in the solution were determined after 17 h shaking with the soil at soil:liquid ratio of 1:5. Subsequently, Zn speciation was calculated. The effectiveness of an organic ligand (L) in changing Zn adsorption was influenced by the sign of the charge of the Zn-L complex, the point of zero salt effect (PZSE) of the soil, the pH of the soil solution and the quantity of the complex formed. When the pH was above the PZSE of the soil, citrate decreased Zn adsorption by up to 38% whereas it increased Zn adsorption by as much as 51 % when the pH was below the PZSE. Oxalate had a similar but smaller effect than citrate under the conditions of the experiment. Zn adsorption was correlated with the concentration of zinc citrate in both pH regions (r2>0.98) and with oxalate above pH 2.7. The increase in Zn adsorption at low pH could have been caused by the adsorption of negatively charged Zn-L complexes.