Sorbent-Amended “Active” Sediment Caps for in-Place Management of PCB-Contaminated Sediments

Abstract

In situ capping manages contaminated sediment on-site without creating the removal, transportation, and disposal exposure pathways associated with dredging. PCB mass, a perceived surrogate for risk posed by PCB contaminated sediment, remains on site during in situ capping projects creating concerns over the 1) duration of chemical isolation provided by sediment caps in advection dominated sediment systems and 2) the potential for PCB mass reduction by biotic and abiotic means during periods of chemical isolation. This study characterizes and compares commercially available sorbents to amend traditional sand caps by measuring properties relevant to advective-dispersive transport through porous media and uses these properties as inputs to a numerical model that predicts the performance of thin sorbent layers as sediment cap media in advection and diffusion dominated systems. Thin layer (1.25cm) sorbent amendments include thermally altered carbonaceous materials (coke and activated carbon) and soil. Sorbents provide chemical isolation of PCB-contaminated sediment in the same order (activated carbon - coke and soil - sand) as measured (highest to lowest) sorption Freundlich constants and sorption capacity. All sorbents outperform sand, coke and soil behave similarly. Thin (1.25cm) activated carbon layers chemically isolate PCB-contaminated sediment in advection-dominated systems for hundreds to thousands of years allowing time for natural attenuation.