Zinc solubility and fractionation in cultivated calcareous soils irrigated with wastewater

The solubility, lability and fractionation of zinc in a range of calcareous soils from Peshawar, Pakistan were studied (18 topsoils and 18 subsoils). The lability (E-value) of Zn was assessed as the fraction isotopically exchangeable with 70Zn2 +; comparative extractions included 0.005 M DTPA, 0.43 M HNO3 and a Tessier-style sequential extraction procedure (SEP). Because of the extremely low concentration of labile Zn the E-value was determined in soils suspended in 0.0001 M Na2-EDTA which provided reliable analytical conditions in which approximately 20% of the labile Zn was dissolved. On average, only 2.4% of soil Zn was isotopically exchangeable. This corresponded closely to Zn solubilised by extraction with 0.005 DTPA and by the carbonate extraction step (F1 + F2) of the Tessier-style SEP. Crucially, although the majority of the soil CaCO3 was dissolved in F2 of the SEP, the DTPA dissolved only a very small proportion of the soil CaCO3. This suggests a superficial carbonate-bound form of labile Zn, accessible to extraction with DTPA and to isotopic exchange. Zinc solubility from soil suspended in 0.01 M Ca(NO3)2 (PCO2 controlled at 0.03) was measured over three days. Following solution speciation using WHAM(VII) two simple solubility models were parameterised: a pH dependent 'adsorption' model based on the labile (isotopically exchangeable) Zn distribution coefficient (Kd) and an apparent solubility product (Ks) for ZnCO3. The distribution coefficient showed no pH-dependence and the solubility model provided the best fit to the free ion activity (Zn2 +) data, although the apparent value of log10 Ks (5.1) was 2.8 log units lower than that of the mineral smithsonite (ZnCO3).