Application of a biodegradable chelate to enhance subsequent chemical stabilisation of Pb in shooting range soils

Purpose: A risk-based approach commonly applied for Pb-contaminated soil is stabilisation by phosphate amendment. However, the proportion of Pb present in a form that is readily immobilised by phosphate addition is often limited. Chelates were examined as a means to mobilise Pb prior to immobilisation to increase the proportion of Pb that could react with phosphate. Four contrasting shooting range soils from around Australia (Murray Bridge (MB), Townsville (TV), Darwin (DA) and Perth (PE)) were used for the study. Materials and methods: Chelating agents such as ethylenediaminetetraacetic acid (EDTA) are able to extract Pb from existing species in the soil, forming complexes. By lowering soil pH or adding Fe³⁺ for the displacement of Pb from the complex, a greater proportion of Pb may be available for reaction with phosphate. The use of biodegradable ethylenediamine-N,N′-disuccinic acid (EDDS) minimises residual effects of chelate treatment in the soil. Results and discussion: EDTA was not found to be suitable due to the stability of the chelate-Pb complex. The addition of EDDS had variable effect between the four shooting range soils. In the strongly weathered oxidic TV and DA soils, applying EDDS prior to phosphate amendment produced greater reduction in bioaccessible Pb, but in the less weathered MB and PE soil, bioaccessible Pb was increased. Water-extractable (WE) Pb increased in the soils by EDDS application, but this could be offset by lime application in DA soil, without negatively affecting Pb bioaccessibility; WE Pb also decreased over time when ammonium nitrate was added to the soil. Conclusions: Application of EDDS may be able to increase the proportion of Pb that reacts with phosphate in the soil, providing greater reduction in bioaccessible Pb than phosphate-only treatments. Additional treatability studies are needed to test amendments that could manage the water-extractable portion of Pb and to optimise application of EDDS for the best outcome.