The Correlations Between The Heterogeneity Of Kerogen In Coal And Sorption-Desorption Of Hydrophobic Organic Contaminants

The heterogeneity of the soil and sediment organic matter (SOM) make effect on the sorption and desorption of relatively hydrophobic organic contaminants (HOCs) such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) by the topsoil and sediment. Recent studies reported that widely dispersed kerogen and black carbon materials are important SOM components and may dominate the overall HOC sorption by soils. Due to the parental and maturational differences of kerogen, it exhibits highly chemical and structural heterogeneity. To establish correlation between heterogeneous properties of kerogen and its sorptive-desorptive characteristics for HOCs, will be helpful for better understanding their mechanismic interactions with HOCs.In this study, we simulated diagenetic alterations under laboratory conditions by thermally treating three different types low-grade coals at 200, 250, 300, 350, 400, 450 and 500℃, yielding three series of coals with differences in parental material and maturation. The original and treated samples were characterized using different methods and were used as sorbents for sorption equilibrium study. The results of characterization revealed that, as the treatment temperature increases, kerogen of coal become matured gradually, physically more condensed and rigid, chemically more reduced, structurally more aromatic.The overall of three original samples shows that due to its heterogeneity, natural kerogen exhibit significantly different sorptive characteristics for phenanthrene and 1,3,5- trichlorobenzene (TCB). The sorption isotherms of similar mature kerogens with different type all exhibit strongly nonlinear, but sorptive characteristics are different because of structurally and chemically heterogeneity.The sorptive characteristics of the same parental material and different maturational kerogen show that as the maturation increases, isotherm nonlinearity increase whereas the sorption capacity increase initially and then decrease. Due to its high contents of aliphatic moiety and more flexibility of aromatic matrix, relatively less matured kerogen may have high sorption capacities and moderately nonlinearisotherms. More matured kerogen may exhibit more nonlinear sorption isotherms, but their sorption capacity may be relatively low.The highly condensed black carbon materials play a major role in the isotherm nonlinearity, but their sorption capacity role effect by the surface characteristic and may be much lower than mildly-altered kerogen materials.The sorption and desorption isotherms measured for phenanthrene and TCB are hysteretic. The apparent desorption hysteresis may be related to the heterogeneity of pore structures. As the maturation increases, sorption-desorption hysteresis increases.This study suggests that the highly heterogeneous kerogen materials may have very different elemental compositions, functionalities and matrix rigidity, and make effect on the sorptive-desorptive characteristics of HOCs. The overall sorption of organic chemicals by kerogen particles might be the sum of the adsorption on external and internal surfaces of rigid backbones and partition into aliphatic mobile moieties. For relatively less matured kerogen, the dominated overall are the adsorption on the flexible aromatic matrix and partition into aliphatic mobile moieties. For more matured kerogen, the dominated overall are the adsorption on external and internal surfaces of rigid backbones. To systemically understand the correlations between heterogeneous properties of kerogen and its sorptive-desorptive characteristics for HOCs, will help us to evaluate the transformation of HOCs in environment and can provide scientific evidence for controlling and remedy of HOCs contaminated subsurface systems.