Citric acid decreased and humic acid increased Zn sorption in soils

Abstract

Zinc (Zn) is the most limiting nutrient for plant growth in calcareous soils and its availability is controlled by both the solid and solution phase characteristics of the soil. Organic acid anions in soil solution may influence Zn sorption by soil particles and thus its availability. To evaluate the influence of humic and citric acids on sorption of Zn, batch technique was used with various concentrations of citric acid (CA) (0, 0.5 and 1mmolL−1) and humic acid (HA) (0, 200 and 500mgL−1) in three soils differing in clay and calcium carbonate contents. A range of Zn concentrations (0 to 450mgL−1) were applied at constant ionic strength (0.05molL−1 NaCl). The sorption data were fitted to Langmuir, Freundlich and Temkin models. All sorption parameters, including maximum sorption in Langmuir equation (qmax), Freundlich capacity and intensity factors (KF, n), and coefficients in Temkin equation (A, KT) increased in the HA treatments and decreased in the CA treatments. Humic acid increased Freundlich sorption capacity (KF) by 73–95%, whereas CA decreased it by 52–68%. It is concluded that CA could significantly reduce Zn sorption by soil particles, probably through formation of soluble CA-Zn complexes, but HA increased Zn sorption and decreased its mobility probably due to increased number of exchangeable sites and formation of insoluble soil-HA-Zn complexes.