Effect of Organic Matter on Sorption of Zn on Soil: Elucidation by Wien Effect Measurements and EXAFS Spectroscopy

Soil organic matter (SOM) is the major factor affecting sequestration of heavy metals in soil. The mean free binding energy and the mean free adsorption energy and speciation of Zn in soil, as affected by SOM, were determined by employing Wien effect measurements. The presence of SOM markedly decreased the Zn binding energy in soils in the following order: Top (5.86 kJ mol^-1) < Bottom (8.66 kJ mol^-1) < Top OM-free (9.44 kJ mol^-1) ≈ Bottom OM-free (9.50 kJ mol^-1). The SOM also significantly decreased the adsorption energy of Zn on black soil particles by reducing nonspecific adsorption of Zn on their surfaces. The speciation of Zn in soils was elucidated by extended X-ray absorption fine structure spectroscopy and microfocus X-ray fluorescence. The results obtained by linear combination fitting of EXAFS spectra revealed that the main forms of Zn in soil were outer-sphere Zn, Zn-illite, Zn-kaolinite, and HA-Zn. As the SOM content increased, the proportion of HA-Zn among the total immobilized Zn increased, and the proportion of nonspecific adsorbed Zn decreased. The present results implied that SOM is an important controlling factor for the environmental behavior of Zn in soils.