Characterization of Condensed Organic Matter in Soils and Sediments

Condensed organic matter (OM) such as nonhydrolyzable C (NHC) is important in C cycling and the sorption and fate of hydrophobic organic contaminants in soils and sediments, but its structural components are rarely known. Nonhydrolyzable C in three contaminated soils and 22 bulk and size-fractionated sediments from the Pearl River delta and estuary in China were measured following treatment with an HCl-HF-trifluoroacetic acid method. It was characterized using elemental analysis, radiocarbon accelerated mass spectroscopy, and Raman microspectrometry. The results show that NHC is an important or even dominating component of the organic C (OC) in the investigated soils and sediments. The NHC contents in this study are highly correlated with the OC contents, with a slope of 0.647 for the 25 soil and sediment samples, which is higher than previously reported NHC-OC correlations in the soils. The radiocarbon analysis demonstrates the importance of ancient OM in the NHC samples. The NHC fractions are chemically and structurally different from the biopolymer and humic substances in the soils and sediments, and originated from different thermally matured bitumen or kerogens, polymethylene C (algaenan, cutan, cutin, cuticle, and lipid), aged terrigenous OM (lignin and humin), and black carbon.