Determination of chemical availability of cadmium and zinc in soils using inert soil moisture samplers

Abstract

A rapid method for extracting soil solutions using porous plastic soil-moisture samplers was combined with a cation resin equilibration based speciation technique to look at the chemical availability of metals in soil. Industrially polluted, metal sulphate amended and sewage sludge treated soils were used in our study. Cadmium sulphate amended and industrially contaminated soils all had > 65% of the total soil solution Cd present as free Cd 2+. However, increasing total soil Cd concentrations by adding CdSO 4 resulted in smaller total soil solution Cd. Consequently, the free Cd 2+ concentrations in soil solutions extracted from these soils were smaller than in the same soil contaminated by sewage sludge addition. Amendment with ZnSO 4 gave much greater concentrations of free Zn 2+ in soil solutions compared with the same soil after long-term Zn contamination via sewage sludge additions. Our results demonstrate the difficulty in comparing total soil solution and free metal ion concentrations for soils from different areas with different physiochemical properties and sources of contamination. However, when comparing the same Woburn soil, Cd was much less available as Cd 2+ in soil solution from the CdSO 4 amended soils compared with soil contaminated by about 36 years of sewage sludge additions. In contrast, much more Zn was available in soil solution as free Zn 2+ in the ZnSO 4 amended soils compared with the sewage sludge treated soils.