Metal-organic associations in two extracts from nine soils amended with three sewage sludges

An aerobic sewage sludge, and two anaerobic sewage sludges, one of which was pretreated with chlorine, were separately incubated with nine northeastern US soils to evaluate sludge-metal complexation reactions with soil organic matter. Soil organic matter was separately extracted with sodium pyrophosphate and dimethylformamide (DMF). The extracts were fractionated on non-polar, non-ionic macroreticular and ion exchange resins into acidic, neutral, and basic hydrophilic solute fractions and acidic, neutral, and basic hydrophobic solute fractions. There were distinct differences in metal complexes present in the extracts between the two extractants, especially for Cr and Cd. Also, the total amount of metals recovered from the sludges varied, usually in the order: aerobic > anaerobic > pretreated anaerobic sludge. In the pyrophosphate extracts, hydrophilic neutrals (carbohydrates) were associated with Ni and Pb, and hydrophilic bases (amino acids) were associated with Pb, and hydrophobic bases (porphyrins) were associated with Cu, Ni, and Pb. Zinc was associated with all fractions, but none predominated. However, in the DMF extracts, hydrophilic acids (uronic acids) were associated with Cr, hydrophilic bases (amino acids) with Ni, Cr, and Pb, hydrophobic acids (phenols) with Pb and Zn, and hydrophobic bases (prophyrins) were associated with Cd. Only very few metals were associated with hydrophobic neutrals (hydrocarbons). The kinds of Ni-organic fractions present in the DMF extracts from most of the soils were more closely related to plant uptake by vegetables in a previous study--with the Ni-amino acid fraction being absorbed by plants more easily then the Ni-porphyrin fraction. Differences between the extractants and among the soils can be ascribed to the inherent properties of these two variables.