Friday, 14 July 2006
110-8

Effects of Kerogen Carbons on Extraction, Sorption Equilibrium and Kinetics of PAHs in Soils and Sediments.

Yong Ran, Guangzhou Inst of Geochemistry, P O Box 1131, 511 Kehuajie, Tianhe District, Guangzhou, 510640, China, Ke Sun, Guangzhou Inst of Geochemistry, P O Box 1131, 511 Kehuajie, Tianhe District, Guangzhou, 510640, China, Baoshan Xing, Dept of Plant, Soil, and Insect Sciences, Univ of Massachusetts, Stockbridge Hall, Amherst, MA 01003, and Peter Grathwohl, Center of Applied Geosciences, Univ of Tübingen, D-72076 Tübingen, Germany.

The Kerogen cCrbons (KC) are important or even dominating components in Borden aquifer, three contaminated soils from the urban area of Guangzhou, and 22 bulk and size-fractionated sediments from the Pearl River Delta and Estuary, China. A selected group of Polycyclic Aromatic Hydrocarbons (PAHs) was extracted by Soxhlet extraction, sequential three-solvent, and standard accelerated solvent extraction (ASE Sum and ASE STD), respectively, and was also extracted repeatedly by water ASE with different temperatures from 25 to 150 oC. The PAH content obtained with ASE Sum was 1.38 times that with ASE STD and more than twice that with Soxhlet extraction. The correlations of these PAH contents with the KC contents were highly significant, and were better than those with the black carbon (BC) and amorphous organic carbon contents. The in-situ KC and OC normalized solid-water distribution coefficients (log KKC and log KOC) of naphthalene, acenaphthene, phenanthrene, anthracene, fluoranthene, and pyrene were 1.3-1.6 and 1.0-1.3 log unit, respectively, higher than previously reported KOC values in the literature. The temperature effects on the aqueous PAH concentrations were described well with the van't Hoff equation and the desorption enthalpies were estimated. The present study for the first time illustrated the importance of kerogen carbon, in addition to BC, to the extraction and distribution of native PAHs in soils and sediments. The equilibrium sorption and long-term sorption kinetics of typical hydrophobic organic contaminants on the bulk Borden aquifer and the isolated kerogen were investigated. The result suggests that the dual-mode modeling, and the combined linear and PD modeling may overestimate the partitioning component. The partition component is not so important as assumed before in sorption of HOCs for the studied sorbent. The small molecules 1,2-dichlorobenzene (DCB) and naphthalene (Naph) have higher adsorption volumes. The lower adsorption volumes for 1,3,5-trichlorobenzene (TCB) and phenanthrene (Phen) suggest that accessibility to the holes of kerogen by large HOC molecules is reduced. The sorption kinetics for phenanthrene on the bulk sand and the kerogen isolate can well be described by the fractional power kinetics equation (qe, Ce, or KOC = ktb). The similar rate parameter b for qe(t) vs. t on the bulk sand and the kerogen isolate respectively ranges from 0.077 to 0.099 and from 0.068 to 0.081, suggesting the similar sorption kinetics rate. The estimated time to reach 95% of equilibrium respectively is 12.3-31.6 years and 11.1-40.8 years at three relative levels of relative solubility for Phen on the isolated kerogen and the bulk sand, demonstrating the similar diffusion length of Phen on the two sorbents. The observed slow sorption kinetics is related to micropore diffusion within kerogen. The investigation supplies new clue for explaining the often observed much longer persistence of organic contaminants in soils and sediments than the prediction based on the short-term laboratory experiment.

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