Exploring the relationship between polycyclic aromatic hydrocarbons and sedimentary organic carbon in three Chinese lakes

Abstract

Purpose Previous studies have shown a positive correlation between concentrations of polycyclic aromatic hydrocarbons (PAHs) and total organic carbon (TOC) in lake sediments. However, with respect to the complex organic matter in recent sediments, it is still unclear which part of TOC plays a key role in controlling PAHs distributions in natural sediments. The aim of this study was to examine the relationships between PAHs and TOC components of different origins in lake sediments. Materials and methods Sediment cores from three Chinese lakes with different trophic conditions—Lakes Bosten, Dianchi, and Poyang—were collected using a piston core sampler. The cores were sectioned into 1- or 2-cm intervals immediately after collection and transported on ice to the laboratory where they were stored at �20 °C. The subsamples were freeze-dried and ground with a mortar and pestle for analyses. PAHs were analyzed by GC–MS and TOC was determined with a PE elemental analyzer after the removal of carbonates. Rock-Eval 6 pyrolysis technique was used to deconvolute the TOC in the sediments into free and volatile hydrocarbons (S1), kerogen-derived hydrocarbons (S2), and residual carbon (RC); S2 was further separated into thermal less stable macromolecular organic matter (S2a) and high molecular weight kerogens (S2b). Results and discussion Positive correlations between TOC and PAHs were observed in these lakes. Results show that the more labile, minor components of TOC (S1 and S2a) played a more important role in controlling PAH distributions than the major components of TOC (S2b and RC), probably due to the different accessibilities of the organic components. The algae-derived organic carbon had a greater influence on the distribution of low molecular weight PAHs than that of high molecular weight PAHs in sediments. This suggests that PAHs scavenging in the water column by algae is mainly targeted at low molecular weight PAHs, and that preferential scavenging of low molecular weight PAHs may be due to the different atmospheric phases of low and high molecular weight PAHs. Conclusions Algal scavenging had an important influence on the PAH concentrations, especially the lower molecular weight PAH concentrations in the lake sediments. The results suggest that PAH concentrations and their molecular ratios in lake sediments may neither accurately represent the pollution history of PAHs, nor the origin PAHs source differentiation. This study has significant implications for understanding the roles of organic matter in affecting the distributions of PAHs and other similar organic pollutants in lake sediments.