Analytical Properties of the Soluble, Metal‐complexing Fractions in Sludge‐soil Mixtures: II. Comparative Structural Chemistry of Fulvic Acid

The fulvic acid fractions of three representative sludge‐soil mixtures were extracted, purified, and then studied for their analytical properties. The fulvic acids extracted from sewage sludge (SS fulvic acids) were found to contain from 20 to 40 times as much S as a typical fulvic acid extracted from natural soil organic matter (SOM fulvic acid). Two of the SS fulvic acids also had somewhat less O than a typical SOM fulvic acid. Ultraviolet and visible spectra of the SS fulvic acids suggested that they may be highly aromatic materials and that they may have condensed structures, comparable even with those in SOM humic acids, in those cases where the O content is relatively low. The aromatic nature of the SS fulvic acids could be inferred indirectly from their X‐ray diffraction patterns, which yielded a γ‐band centered more nearly at 4Å than 5Å, thus implying a relatively small content of aliphatic components. The infrared spectra of the SS fulvic acids were generally similar to the Type III spectrum of SOM fulvic acid: The absorption due to C = O in carboxyl groups is not pronounced and the presence of polysaccharides and protein decomposition products is indicated. The spectra differed importantly from the Type III spectrum in showing bands due to the sulfone group and to functional groups substituted on aromatic rings. Metal‐SS fulvic acid solutions were extracted directly from the three sludge‐soil mixtures and were fractionated on Sephadex G‐10 gel. It was found that Cu, Fe, and Zn were complexed by SS fulvic acid, but that Cd and Ni were not. The relative order of complex stability for Ni and Zn implied by these results is different from what has been reported for SOM sulvic acids.