Characterization of dissolved organic matter extracted from composted municipal solid waste

The properties and transformation of dissolved organic matter (DOM) extracted from municipal solid waste compost at five stages of composting were studied using a chemical fractionation scheme. We fractionated the DOM into hydrophobic or hydrophilic neutrals, acids, and bases, and obtained solid-state cross polarization magic angle spinning (CPMAS) ¹³C-nuclear magnetic resonance (NMR) spectra of the unfractionated DOM and of the different fractions. The hydrophobic acid (HoA) fraction was the dominant hydrophobic fraction, exhibiting a moderate increase during composting. The hydrophobic neutral (HoN) fraction increased sharply while the hydrophobic bases (HOB) fraction decreased during composting. The hydrophilic neutrals (HiN) represented the major fraction of the hydrophiles until 120 d of composting, decreasing thereafter by 38%. The ¹³C-NMR spectra of the unfractionated DOM revealed an increasing level of aromatic structures in the residual DOM with composting time. The ¹³C-NMR spectra of the HoA fraction suggested a polyphenol-humic structure, whereas the HoN spectra exhibited strong aliphatic features. The spectra of the HiN fraction confirmed its polysaccharide nature and the hydrophilic bases (HiB) fraction contained mainly proteins and carbohydrate-amino complexes. A comparison between the ¹³C-NMR spectra of HoA and fulvic acid (FA) indicated the former to be the most soluble FA fraction. The steady DOM concentration and the relative decrease of HiN as the HoA and HoN fractions increased indicates that DOM at the final stage of composting contained less bioavailable organic matter and more macromolecules related to humic substances. The constant level of DOM observed during the curing and maturation stages represents a steady-state situation during which the chemical composition is continually changing.