Sorption of polycyclic aromatic hydrocarbon (PAH) to Yangtze River sediments and their components

Sorption of hydrophobic organic chemical (HOC) to natural sediments includescomplex mechanisms. The present work studies the sorption of polycyclic aromatichydrocarbon (PAH) to Yangtze River sediments. The major objectives were toinvestigate the sorption mechanisms of PAH to sediments and the effect of sedimentsheterogeneity. Specifically, the sorption and distribution of PAH in the fractions ofamorphous organic carbon (AOC), black carbon (BC) and mineral surface wereinvestigated. Pyrene and phenanthrene were involved as major target PAH chemicals for examining the sorption equilibriums in single and binary solute systems. The sorbents include model sediment constituents and five sediments collected from the Yangtze River. The preheated sediments according to the chemothermal oxidation method (CTO-375, Gustafsson et al., 1997) were also studied. In which, the remaining organic carbon (OC)is considered as BC. Analytical procedures for the determination of PAH in solution were developed with fluorescence spectroscopy. Especially, the synchronous fluorescence spectroscopy (SFS) was applied for simultaneous analyses of the primary solute and cosolute PAH in binary solute systems. The sorption isotherms were fitted with different types of model, including Freundlich model, Polanyi-based adsorption model and composite models which comprise a partition model and an adsorption model. The major findings include the following aspects. Firstly, the sorption of pyrene to pristine sediments was found to be mainly dependent on the OC content rather than on the specific surface area (SSA) or the microporosity of sorbents. Composite models were applied for distinguishing the partition and adsorption. The adsorption dominates in the low concentration range, while the partition becomesmore important at high concentrations. The fitted partition coefficients are linearly dependent on the AOC content (fAOC) of sediments, which supports a pyrene partition in the AOC phase in sediments at a high equilibrium concentration. Secondly, the fitted adsorption of pyrene to pristine sediments was compared with that to the preheated sediments. Our finding that the adsorption capacity of the BC fraction is similar in pristine and preheated sediments suggests that the low content AOC in the pristine sediments does not affect the adsorption capacity of the BC fraction. However, variations of fitting parameters suppose that the adsorption dynamics may be alteredthrough dispersive or flocculating effect. A distribution model of pyrene between the AOC, BC and mineral fractions of sediment was established according to the fitted results with model and natural sorbents. It was found that the adsorption to BC dominates the overall sorption at low concentrations (relative concentration C$_{r}$ < 0.1). The contribution of AOC becomes more important at high concentrations. In the presence of OC in sediments, the contribution of the mineral fraction to the overall sorption is negligible