Transformation and fixation of Zn in two polluted soils by changes of pH and organic ligands

Abstract

Although the movement of Zn from the solid to the solution phase of soils is a significant process preceding plant uptake, the quantity of metal that can be solubilised through rhizosphere processes is also extremely important. Therefore, the consequences of plant-derived organic ligands, and changes in pH on the isotopically exchangeable quantity of Zn (E value) were examined in a polluted acid and calcareous soil. Variations in pH were facilitated through the use of dilute NaOH or HNO₃ solutions. The organic ligands studied included 0.25–5 mM concentrations of sodium tartrate, the free acid and sodium salt of citrate, histidine, and deoxymugineic acid. As expected, the organic ligands and a reduction in pH increased the solution concentration of Zn in these soils. Furthermore, through the application of isotopic dilution techniques, it was determined that some of these organic ligands could significantly increase the quantity of isotopically exchangeable Zn. However, with the exception of the 5 mM sodium citrate treatment in the calcareous soil, pH was the overriding parameter that effected changes in the E value. Reducing the pH by approximately 2 units increased the E value by 39 and 80%, respectively, in the acid and calcareous soil. Conversely, evidence for Zn fixation (a decrease of the E value) was observed in the acid soil when pH was increased. The exudation of organic ligands and variations of pH induced by plants have the potential to significantly vary the quantity of phytoavailable Zn in these 2 polluted soils.