The effects of soil organic matter heterogeneity on equilibrium sorption by soils and sediments

Sorption of nonionic hydrophobic organic chemicals (HOCs) by soil is a phase distribution process in which HOCs are preferentially accumulated in naturally-occurring soil/sediment organic mater (SOM). The goal of this study was to identify and characterize which SOM component dominates the nonlinear and competitive sorption commonly observed for soils and sediments. I selected five different soils and sediments collected from relatively polluted areas. Different types of SOM were isolated from each sample, and were quantified and characterized using different physicochemical means. These isolated SOM fractions and the five original samples were used as the sorbents. Phenanthrene and naphthalene were selected as the target HOCs for examining single solute and bisolute sorption equilibria. The major findings include the following three aspects. Firstly, distinct nonlinear and competitive sorption equilibria were observed for all the sorbents. The results indicate that the particulate SOM (black carbon and kerogen) makes the major contributions to the overall nonlinear and competitive sorption exhibited by the bulk soils and sediments. Our finding that black carbon and kerogen are equally important in the nonlinear and competitive sorption is very different from the recent studies that described black carbon as the dominant SOM component but ignored the contribution from kerogen. Secondly, the results show that the sorption on humic acid (HA) is relatively less nonlinear with lower sorption capacities and that the solute-solute competition is comparably insignificant for HA. The less important role of HA in the overall sorption by soils is largely due to its low content in soils. This finding is surprisingly unexpected and is very different from a textbook notion that HA would be the major component of SOM and would be primarily responsible for the HOC sorption. Lastly, the results further revealed that the competitive sorption is not only a function of SOM heterogeneous properties, but is also depended up the loading sequence of the coexisting HOC solutes. When preloaded on a soil, the HOCs with larger molecular sizes may occupy the pore network, blocking a fraction of micropores with sizes smaller than the sorbates, lowering sorption capacities for incoming HOCs of smaller sizes.