Copper-binding ability of dissolved organic matter derived from anaerobically digested biosolids

The fate of metals in soils where soluble organic compounds are present may be strongly influenced by the degree to which they are complexed by organic ligands. We undertook this study to determine the combined effect of molecular weight (MW) and hydrophobicity on the Cu-binding ability of dissolved organic compounds in biosolids (i.e., sewage sludge). Dissolved organic matter (DOM) from anaerobically digested sewage biosolids was fractionated by using a combination of MW fractionation and XAD-8 resin chromatography (to separate the dissolved organic compounds according to hydrophilic and hydrophobic characteristics). The Cu-binding abilities of the DOM fractions were obtained by using a Cu2+-ion-selective electrode (Cu-ISE) technique. The Cu-binding ability of fractionated DOM decreased significantly with increasing molecular weight, indicating that low-MW DOM had more metal-binding sites than high-MW DOM. Within each MW fraction, the hydrophilic and the hydrophobic components also exhibited differences in Cu-binding ability. For the DOM with MW 500-3500 Da, the hydrophilic fraction showed a greater Cu-binding capacity than did the hydrophobic fraction, whereas the hydrophobic acid components were most important in binding Cu for DOM with MW > 3500 Da. The maximum Cu-binding capacities of different biosolids-derived DOM fractions, estimated by employing a Langmuir model, ranged from 1.85 to 14.3 mmol Cu mol(-1) dissolved organic C (DOC), which is the same order of magnitude as similar measurements of DOM from other sources.