A direct solid-phase assay specific for heavy-metal toxicity .2. Assessment of heavy-metal immobilization in soils and bioavailability to plants

We have used the solid-phase MetPLATE, an enzyme assay that is specific for heavy-metal toxicity, to investigate metal toxicity of soils that have been amended with urban wastewater sludges or contaminated with dry deposition from metal-plating industries. We have shown that soil toxicity, using MetPLATE, ranged from 21 to 72.5% inhibition of enzyme activity. Evin soil, which displayed the highest toxicity, also had the highest concentrations of Pb and Zn. Metal uptake studies with ryegass grown on Evin soil, showed Zn, Cd, and Pb accumulation in the plant that exceeds the standard levels reported for grasses. Solid-phase MetPLATE was also used as a tool to study the reduction of heavy-metal toxicity following soil amendments to immobilize metals in soil and thus reduce their toxicity. It was found that the addition of 1% hydrated manganese oxide significantly reduced dissolved metals in soil, their accumulation by ryegrass, and soil toxicity as shown by MetPLATE.