Fate of hydrophobic organic contaminants in sorbent-treated sediments [electronic resource]
Keywords: Civil and Environmental Engineering.

Contaminated sediment management is a global environmental issue. Sediments often act as sinks and long-term reservoirs for persistent hydrophobic organic contaminants (HOCs), which can pose significant risks for food cycles in aquatic and upland environments. Sorbent amendment is a promising, cost-efficient alternative for in situ sediment remediation of HOCs. However, due to the limited practice and research on this relatively new technology, the long-term permanence of sorbent amendment and the treatment's ability to retain the contaminants over time are not fully understood and often questioned. Concerns about possible sorbents entrainment and movement caused by running water (e.g. flood or storm event) cannot be ruled out without further research. The study presented here was designed to increase understanding about the fate of HOCs in sorbent-treated sediments. In the first part of the study, field scenarios of sorbent treatment and winnowing were simulated in the laboratory to investigate the fate of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) in polluted sediments. Tenax® beads were used to simulate the sorbent treatment and to measure the readily available PCBs and PAHs in the sorbent-treated sediments after all sorbents had been removed for a series of time spans. The study demonstrated that a single sorbent treatment could permanently reduce the readily available HOCs in contaminated sediments. Even though repartitioning of PCBs and PAHs were observed after sorbents removal, experimental data indicated that the sorbent treatment substantially and permanently reduced the readily available PCBs and PAHs (over 70% reduced for all compounds tested) compared to controls. A concern about sorbent amendment is that leaving the used sorbent in the remediated sediment may lead to adverse ecological responses and potential HOCs desorption from the sorbent back into the natural environment under certain conditions. Magnetic activated carbon (MAC), a recoverable sorbent, is a promising solution to mitigate this concern. The second part of the study demonstrates MAC's capacity in treating PCB- and PAH-contaminated sediments. MAC amendment at a dose of 10 dry wt% in a 150-rpm-horizontal shaking system reduced up to 80-92% of PCBs and 94-99% of PAHs during the test. Significant desorption was observed for all tested PCBs and PAHs. Varied sorption-desorption kinetics were observed for alkylated-PAHs due to the numbers of fused benzene rings and methyl groups, etc, No obvious different sorption-desorption kinetics were observed for parent-PAHs. The sorbent performance comparison test (with the same sorbent mass) shows that MAC (coconut-MAC and coal-MAC) has a roughly equivalent efficacy to AC (coal-AC) in reducing PCBs and PAHs uptake in polyethylene (PE) samplers and that the performance of carbon cloth is not comparable to that of AC. Carbon cloth's inferior performance in HOCs uptake verifies the potential of MAC as an effective alternative compared to carbon felt (similar to carbon cloth), a current solution for sorbent recovery. In addition, the sorbent-water partitioning coefficient (Ksorbent) was determined for 18 parent-PAHs and 6 alkylated-PAHs for coal-AC, coal-MAC, coconut-MAC and carbon cloth. Results of Ksorbent again indicate that MAC has comparable performance in HOCs uptake to AC. The Ksorbent determined in this study can be used for further study of sorbent performance through modeling of the selected carbon sorbents.