Controlling Bioavailability of Hydrophobic Organics In Soils Using Carbon Amendments.

Bioavailability of hydrophobic organic compounds in soils is impacted by the nature of binding to soil organic matter types, especially to black carbon surfaces.  Our recent work has demonstrated that addition of black carbon in the form of activated carbon or biochars can reduce contaminant bioavailability in sediments.  In this research we explored the bioavailability of a range of hydrophobic compounds in soils and the impact of carbon amendment.

In the first set of experiments, artificial soil was contaminated with PCBs and used in bioaccumulation experiments fresh or after aging for 19 months.  PCB bioaccumulation in earthworms was reduced by 68% when activated carbon (AC) was placed as a layer without mixing and by 94% when the AC was manually mixed into the soil.  Aging of the AC mixed soil for 19 months resulted in an overall reduction of 99% in PCB biouptake.  AC-treated aged soil also showed two orders of magnitude lower equilibrium aqueous concentrations of PCBs compared to untreated aged soils.  The findings from this study indicate that application of engineered sorbents like AC to PCB impacted soils may greatly reduce PCB uptake at the base of the terrestrial food chain.

As a follow up study, laboratory experiments were conducted to evaluate the bioaccumulation of aged polychlorinated dibenzo-p-dioxins and dibenzofurans (dioxins and furans, or PCDD/F) in soil at the base of the terrestrial food chain using earthworms (E. fetida) as model organisms.  This research also assessed the effect of activated carbon (AC) addition to soil on bioaccumulation in the earthworms and passive uptake in polyoxymethylene (POM) samplers.  Two soil samples from a wetland and a levee in the Tittabawassee River floodplain downstream of Midland, MI were used in this study.  In the untreated soils, biota sediment accumulation factors (BSAFs) ranged from 0.17 for 2,3,7,8-TCDD to 0.02 for some of the higher chlorinated congeners, which were substantially lower than would be predicted using a conventional equilibrium partitioning model.  The addition of AC to the two soils generally reduced the BSAF values to lower than 0.02.  Amendment of the wetland soil (having a high organic content) with 2% and 5% AC resulted in 78 and 91% reduction of toxicity equivalent (TEQ) in earthworms, respectively.  More strikingly, amendment of the natural levee soil (having a low organic content) with 2% and 5% AC showed >99% reduction of TEQ in earthworms.  Also, freely dissolved aqueous concentrations of PCDD/Fs in soil slurries, as measured by equilibrium passive samplers, decreased up to 99% with AC treatment.  Results of this study indicate that bioaccumulation of PCDD/Fs in earthworms from historically impacted floodplain soils is low and can be further reduced by amending with a strong sorbent.