Analytical Properties of the Soluble, Metal‐Complexing Fractions in Sludge‐Soil Mixtures: I. Extraction and Purification of Fulvic Acid

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Abstract
A laboratory method is described for extracting and purifying the fulvic acid fraction of sludge‐soil mixtures. The method, which does not include dialysis or the use of destructive chemical reagents, consists of: (i) extraction with 0.5N NaOH and acidification to pH 1.0 with HCl, (ii) repeated high‐speed centrifugation to remove Al‐Fe‐hydroxy phosphates, and (iii) use of both chelating and H‐form cation exchange resins to remove complexed metal impurities. The existing procedures for preparing fulvic acid, based upon research on natural soil organic matter, were concluded to be not fully adequate for studies on sludge‐soil mixtures. Primarily, this inadequacy is the result of the loss, through dialysis, of important reactive fractions during purification and the lack of specific steps that treat the greater variety of trace elements to be found in sludge materials. In addition, the presence of significant amounts of P in these mixtures complicates the removal of colloidal and complexed metal impurities. The application of the present method to representative sludge‐soil mixtures showed that ash contents are reduced to I‐2%. The procedure should be generally applicable in soil or sediment organic matter studies as well as to studies on the chemistry of sludge‐amended soil.

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