Infrared Spectroscopic Study of the Water‐Soluble Fraction of Sewage Sludge‐Soil Mixtures During Incubation

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Abstract
Two anaerobically digested sewage sludges and two sandy soils having significantly different pH values were mixed to provide four sludge‐soil combinations. Each was maintained under two different conditions of soil water content: field capacity (1/3‐bar soil water tension) and complete saturation. The soil‐sludge mixtures were incubated at 25°C in the laboratory for 100 weeks, during which time saturation extracts were taken periodically. Infrared (IR) spectra of evaporated films prepared from the extracts were interpreted to obtain information about the principal changes which occur in the water‐soluble fraction of sewage sludge‐soil mixtures as the sludge decomposes in soil.For the alkaline soil maintained at 1/3‐bar soil water tension, and for both sludges, IR absorptions assigned to polysaccharides, proteinaceous material, and sulfonate and/or sulfate compounds had disappeared after about 10 weeks of incubation. The important IR bands which increased in intensity or emerged during this period could be assigned to carboxylates and nitrates. For the alkaline soil maintained at water saturation, the IR spectra followed the same trends, but showed markedly slower changes, with absorption bands assigned to normally labile compounds remaining even after 100 weeks of incubation. For the acidic soil maintained at 1/3‐bar soil water tension, the changes in the IR spectra also were retarded, but much less so than in the spectra of extracts from water‐saturated soils. The least change occurred in the spectra of extracts from the acidic, water‐saturated soil mixed with either sludge.