The Condensed Organic Matter Of Soil And Sediment From Pearl River Delta And Its Effect On The Sorption Behavor Of Organic Contaminants

As sorption and desorption of hydrophobic organic contaminants (HOCs) govern their distribution, transport, bioavailability, and degradation, understanding their interaction mechanisms has been one of the environmental research objectives. Sorption of MOCs is directly related to natural organic matter (NOM) presented in the soils and sediments. Due to the heterogeneity of the soil and sediment organic matter, the sorption mechanisms of HOCs are still unclear. To investigate the chemical and structural compositions of NOM and their correlation with sorption characteristics of HOCs will be quite important. The further understanding the interaction mechanism between HOCs and NOM will help us to evaluate and predict the distribution, transport, transformation, and fate of HOCs in the terrestrial and aquatic environment, and will provide scientific basis for regulation, control, and remediatin of HOCs in the contaminated ecosystem.In this study, three surface soil samples from the Hangpu district of Guangzhou city, two surface river sediments (WR and NR) from the Western River and Northern River of the Pearl River Delta (PRD), and six surface sediment samples (0-20 cm) from the Pearl River Estuary are collected. Chemical and physical fractionation procedures were used for the quantitative isolation of soil/sediment organic matter and the following three fractions were isolated: demineralized fraction (DM), nonhydrolyzable carbon (NHC) and black carbon (BC). After characterizing the NOM fractions in the soils and sediments with element analysis, ¹³C-NMR, ¹⁴C accelerated mass spectrometer (ASM), Fourier transformed infrared spectroscopy(FTIR), Raman spectrometer, the sorption isotherms of phenanthrene (Phen) on these original and treated samples were established. In addition, five of the NHC fractions, one of the BC fraction, and four coals collected from China were used as sorbents for gaseous benzene sorption and desorption equilibria.The results of this work showed that the NHC and BC accounted for 25.6-84.7 % and 4.14-17.3%, respectively, of total organic carbon (OC) with averages of 51.9% and 11.2% in the soils and sediments. The NHC fractions play dominant roles to the overall sorption isotherms of Phen in the bulk samples. The bulk soils and their NHC fractions have lower sorption capacity than the bulk sediments and their NHC fractions, relating to the different source of organic matter between the soils and the sediments. The phenanthrene sorption capacity in the NHC and /or bulk samples is related to the content of the aliphatic carbon, and negatively to aromatic carbon, demonstrating the important role of aliphatic carbon to the Phen sorption and fate in the soils and sediments. The distribution of NHC is heterogeneous at different particle size fractions in the two sediments, and high NHC contents are associated with small size fractions (e.g. slit and clay). Moreover, the NHC fractions in the large size fractions are rich in aromatic carbon content, and low in the H/C value, which may be originated mainly from vascular plant and charcoal, but the small size fractions are rich in aliphatic carbon content and high in the H/C value; All the sorption-desorption isotherms measured for benzene are strongly nonlinear, and well fitted to the Freundlich model. The benzene sorption volumes of coals are higher than their phen sorption ones, but almost close to their CO₂-micropore volumes. Except one BC sample, the sorption and desorption isotherms of NHC and coals measured for benzene are strongly hysteretic. The desorption hystersis is positively related to the content of the aromatic carbon, and negatively to aliphatic carbon, their atomic H/C and the content of mineral of NHC and coals, which may suggest that the adsorption (hole-filling) machanism leads to the desorption for benzene on these condensed organic matter.