Effect of citric acid on characteristics of zinc sorption on two calcareous soils

Zinc (Zn) is an essential micronutrient for plants, animals and humans. Bioavailability of Zn for plant root is controlled by sorption process in soils. In order to evaluate the effect of citric acid on Zn sorption, a batch experiment was conducted with two soil samples (S1, S2) which were different in clay and calcium carbonate content. Three levels of citric acid (0, 0.5, 1mM) and various Zn concentration (0 to 450 mg L-1) were applied at background solution 0.05 M NaCl. Adsorption data were fitted to Langmuir (R2=0.76-0.95), Freundlich (R2=0.83-0.95), Temkin (R2=0.62-0.92), Dubinin-Radushkevich (R2=0.44-0.92) and Elovich (R2=0.54-0.90) isotherm models. Sorption parameters including maximum adsorption of Langmuir (qmax), Freundlich capacity and intensity factors (KF, n), coefficients of Temkin equation (A, KT) and maximum adsorption capacity Dubinin-Radushkevich (qm) decreased by application of citric acid; above mentioned sorption parameters of S1 (with low clay and calcium carbonate) were lower than those of S2 (with high clay and calcium carbonate). The sorption energy parameter of D-R isotherm and Gibbs free energy change (ΔGr) indicated that the Zn adsorption processes were physicaly and spontaneity. The results of this experiments showed that the application of citric acid significantly decreased Zn sorption by soil particles probably due to formation of soluble complexes, and consequently increasing Zn availability for plants. Therefore, by modifying plants to secrete higher of citric acid can increase zinc absorption by plants.