Intercorrelations among Degree of Geochemical Alterations, Physicochemical Properties, and Organic Sorption Equilibria of Kerogen

Recent studies reported that kerogen is an important natural organic material dominating sorption of relatively hydrophobic organic contaminants (HOCs) by topsoils and river sediments collected from industrialized regions. Due to its chemical and structural heterogeneity, kerogen is expected to exhibit a spectrum of sorptive phenomena for HOCs. The goal of this study is to establish correlations between heterogeneous physicochemical properties of kerogen and its sorptive characteristics for HOCs. In this study, we simulated diagenetic alterations under laboratory conditions by thermally treating a low-grade lignite at 200, 250, 300, 350, 400, 450, and 500 °C, yielding a series of type III kerogen samples having the same parental material but different maturations and physicochemical properties. The treated samples and the original lignite were systematically characterized using different methods and were used as the sorbents for sorption equilibrium study. The results of characterization revealed that black carbon or char was formed at 450 °C or above and that, as the treatment temperature (T) increases, both O/C and H/C atomic ratios decrease whereas aromaticity and reflectance index increase. The sorption and desorption isotherms measured for 1,3,5-trichlorobenzene and phenanthrene are nonlinear and hysteretic. The nonlinearity and apparent desorption hysteresis increase as a function of T and correlate well with rigidity and aromaticity of the organic matrix. The sorption capacity for each sorbate increases initially as T increases, reaches a maximum at 300−350 °C, and then decreases rapidly as T increases beyond 350 °C. This study suggests that the highly heterogeneous kerogen-based coal materials may have varied elemental compositions, functionalities, and matrix rigidity and that they could play major roles in the isotherm nonlinearity and the apparent sorption−desorption hysteresis exhibited by soils and sediments.