Fractionation and characterization of dissolved organic matter derived from sewage sludge and composted sludge

Dissolved organic matter (DOM) is an important component affecting the availability of heavy metals in sludge-amended soils. In this study, DOM derived from anaerobically digested dewatered sludge and sludge compost were fractionated by a macroreticular exchange resin, and characterized by Fourier Transform-Infrared spectroscopy (FT-IR) technique to evaluate their potential effects on soil metal mobility. The acid fractions of DOM in sludge and sludge compost respectively were 78% and 73%, which included hydrophilic (HiA) and hydrophobic (HoA) acids. The hydrophobic fraction of DOM increased during composting, with the hydrophobic neutral (HoN) and HoA increasing from <1 to 22% and from 38 to 52%, respectively. The relative amount of each component in the hydrophilic fraction of DOM was as follows: hydrophilic acid (HiA) >> hydrophilic base (HiB) < hydrophilic neutral (HiN). The HiA, HiB, and HiN fraction of the sludge DOM was 1.9, 6.7, and 2.2 times higher than the respective fractions of the sludge compost DOM. The FT-IR spectrum shows that the hydrophobic fraction had considerably higher aromatic acids or aromatic phenols, while the hydrophilic fraction was richer in carboxyl and polyhydroxyl groups. Sludge DOM appeared to have more C-N and C-O ligand character, possibly from organic acid, amino acid, and amines than compost DOM, especially for the HiA, HiB, and HoA fractions. Since the hydrophilic fraction of sewage sludge was higher than that of sludge compost, the addition of sewage sludge would likely have a higher ability in reducing the trace metal adsorption capacity of soils as compared to that of sludge compost.