Abstract
The objective of this study was to evaluate the formation of a probable Zn solid phase in soils. Thermodynamic solubility isotherms revealed that ZnFe2O4, Zn2SiO4 and Zn3(PO4)2.4H2O could form in soils. The main species of Zn in soil solution were Zn2+, ZnCl+, ZnOH+, Zn(OH)2 o, and ZnSO4 o. Sorption of 100 mg kg−1 of Zn was investigated in 21 soil suspensions. The activity of Zn2+ was calculated in these suspensions and were superimposed on the thermodynamic solubility isotherms of Zn minerals. The calculated Zn2+ activities were undersaturated with respect to Zn2SiO4 and Zn3(PO4)2.4H2O but were within the range of ZnFe2O4. It was concluded that ZnFe2O4 was the most probable solid phase which could form in soils. In another experiment involving 6 soils, Zn equilibria were approached from supersaturated (100 mg kg−1 Zn added) and undersaturated (franklinite added) directions. Results from both were converging to a fine. Based on the thermodynamic calculations, experimental results and published literature, a model of Zn interactions in soils was proposed. According to this model adsorption of Zn on Fe oxides is the first and fast reaction, followed by a slower diffusion of Zn into the hydrated layer of Fe oxides and subsequently reacting chemically to form a Zn-ferrite type compounds.
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