effect of citric acid on characteristics of zinc sorption in calcareous soils

zinc­ (zn) is an essential micro­nutrient needed by plants, animals and humans. bioavailability of zn to plant root is controlled through sorption process in soils. to evaluate the effect of citric acid on zn sorption,­ a batch experiment was conducted with two soil samples (s1, s2), that differed in their clay and calcium carbonate contents. three levels of citric acid (0,­ 0.5, 1 mm) and twelve various zn concentrations (0 to 450 mg ­l-1) in a medium of  0.05 m nacl solution were applied to the soils. 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. the above mentioned sorption parameters related to s1 (low clay and calcium carbonate) were lower than those belonging to s2 (high levels of 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 physical and spontaneous. the results of the experiments revealed that the application of citric acid significantly decreased zn sorption by soil particles, probably due to formation of soluble complexes, and consequently increased zn availability to plants. therefore, through a modification of plants to secrete higher levels of citric acid, one can expect the absorption of zn by this plants.to be further promoted.