The influence of redox potential on speciation of metals and nutrients in compost-amended upland and wetland soils

Increasingly, municipal solid waste (MSW) and sewage sludge composts containing heavy metals are being applied to agricultural lands. Agricultural soils may experience wide seasonal fluctuations in redox potential with a potential to change metal mobility. To assess the influence of redox potential on the speciation of metals (Cd, Cu, Ni, Pb and Zn) and elements regulating metal behavior (Fe, Mn, Al, P and S) in two soils (Typic Fragiaquept and Aeric Fragiochrept) amended with sewage sludge and MSW composts, metals and regulating elements were sequentially extracted from soil-water slurries maintained at four redox levels ({dollar}-{dollar}200, 0, +225 and +450 mV). Results indicated that redox-induced changes in the physical and chemical characteristics of compost-amended soils strongly influence metal speciation and availability.; Oxides of Fe, Mn and Al in soils and composts control metal behavior through chemisorption. However, oxide quantities in the compost-amended soils generally responded to redox potential changes, suggesting that metal sorption capacities may change with soil oxidation conditions. Phosphorus concentrations were high in the sewage sludge compost, and soluble P levels were elevated under reduced conditions to levels of environmental concern. Both composts contributed significant amounts of soluble sulfur, but the sewage compost sulfur was apparently non-reducible and thus would not be involved in control of metal availability under reduced conditions through formation of insoluble metal sulfides.; Soluble and exchangeable metal concentrations generally increased with oxidation, peaking at an Eh of +225mV. This was attributed to dissolution of insoluble metal-sulfides and metal-organic matter complexes formed under reduced conditions. Significantly, field measurements indicated that redox conditions in the Fragiaquept were close to the +225mV level for a significant portion of the year. Changes with oxidation in soluble+exchangeable and organic-bound metal concentrations in the sewage sludge compost treatments suggested that organic matter in this compost was more easily degraded than soil or MSW organic matter. Further, degradation contributed soluble organic matter which complexed and solubilized metals.; During this investigation, it was noted that methods typically used to evaluate Pt redox electrodes could not discriminate between functioning and non-functioning electrodes. A test method using soil-water slurries was developed, evaluated and found to effectively screen Pt electrodes.