Gel Filtration Studies of Trace Metal-Fulvic Acid Solutions Extracted From Sewage Sludges1

Trace metal-fulvic acid solutions extracted with NaOH from two chemically different sewage sludges and separated from humic acid using HCl were fractionated on a column of Sephadex G-10 gel using deionized water at pH 5.65 as the eluant. The effluent from the gel was analyzed in 3-ml increments for organic C, Cl, and the metals Cd, Cu, Fe, Ni, and Zn. Elution diagrams prepared with the analytical data showed multiple maxima in organic C concentration and a single maximum in Cl concentration (indicating the presence of inorganic salts). Fe and Cu showed concentration maxima that followed closely those in both the organic and inorganic fractions, while Cd, Ni, and Zn tended to show maxima following the inorganic fraction peak and only one or two of the organic C maxima. The data indicated that Cu and Fe formed complexes with sludge-derived fulvic acid (FA) under the conditions of the experiment. Cd, Ni, and Zn were associated with the FA only weakly, in a manner dependent on the nature of the sludge, and in the relative order of affinity Zn > Ni > Cd. These data suggested that sludge-derived FA behaves toward trace metals as a relatively harder Lewis base than Cl⁻. Infrared spectra of the eluted organic fractions indicated that protein decomposition products and polysaccharide components appear in the fractions of relatively larger molecular size, as do the components containing SO₃H groups. The IR spectra also gave evidence that COOH and SO₃H functional groups are involved in metal-FA interactions. An important organic fraction of sludge-derived FA was identified that appears to be an aliphatic, carboxylic acid mixture which can complex only Cu significantly.