Mechanistic perspective on the anomalous sorption behaviors of chlorinated benzenes in four natural geosorbents

This study focused on evaluating the influence of natural organic matter (NOM) rigidity in controlling the non-linear and competitive sorption behaviors and the mass-transfer rates of chlorinated benzenes (chlorobenzene, 1,2-dichlorobenzene, 1,4 dichlorobenzene, 1,2,4-trichlorobenzene and 1,2,3,4-tetrachlorobenzene) on four natural geosorbents.; Sorbents with the largest hard carbon fractions (Ohio shale and Forbes soil) had more non-linear sorption isotherms and higher organic carbon (OC) normalized sorption distribution coefficients than the other two sorbents (Yolo soil and Pahokee peat). The glassy-polymeric nature of the sorbents, which was determined using wet chemical oxidation to measure hard carbon content and elemental analysis to determine atomic H/O ratios, showed close correlation with the Freundlich sorption non-linearity and the maximum adsorption capacity (Vo) estimated by the Polanyi-Manes (PM) model.; The extent of competition was positively correlated with the Vo and the fraction of hard and soot carbon contents. These findings indicate that competition is associated with voids in the NOM structure, that these voids are likely present within the condensed (hard plus soot) carbon domain and therefore that diagenetic alteration of NOM plays a central role in determining competitive sorption characteristics for hydrophobic contaminants.; The fraction of sorbed molecules subject to instantaneous mass-transfer seems to be inversely related to the fraction of the hard carbon domain. In the NOM compartment where mass transfer is rate-limited, markedly slower rate coefficients distributions for the desorption and the competitive displacement of 1,2-DCB were simulated in Ohio shale, followed by Forbes soil.; Loss of some portions of the glassy polymeric character of the hard OC domain due to the penetration of sorbate molecules into NOM matrices (plasticization) was discussed as a possible contributor to anomalous sorption behaviors (non-linear and competitive sorption isotherms, and non-Fickian mass-transfer rates) observed in this study. The existence of a permanently hard OC domain, not subject to plasticization under environmental conditions, was postulated as an additional factor determining the observed sorption behavior.