In-situ sorbent amendments such as activated carbons are being used to sequester contaminants such as TCDD in a form that reduces or eliminates its bioavailability to humans and other target species. This technology has gained rapid acceptance due to the enormous cost savings compared to execution/dredging and disposal in a hazardous waste landfill, and is being utilized to remediate sediments and floodplain soils of the Tittabawassee River. However, in our view the application of this technology is premature since no studies showing reduced bioavailability to humans or mammalian models exist. In this study, molecular dynamics simulations are used to study the adsorption/sequestration of dioxin in the pore structure of activated carbons. A slit model of activated carbons consisting of six carbon layers is used, and three different pore sizes of ~7 Å, ~12 Å and ~20 Å covering the range from micro-pore to meso-pore, are considered to compare pore size effects. The potential of mean force(PMF) during the desorption process of dioxin is calculated using the adaptive biasing force algorithm. Our results show that PMF increases as the pore size decreases, indicating that activated carbons with smaller pore size have a higher adsorption affinity for dioxin compared to those with larger pores. Our simulation shows that when dioxin is adsorbed inside the pore, it’s adsorbed on the surface for all three different pore sizes. Pi-pi interaction between dioxin and activated carbons may account for this. Adsorption of dioxin in the pore structure can be regarded as a competition between dioxin and water since this process requires dioxin to occupy the space of water. To gain insight into this, we study the PMF of water as well. According to our results, the water molecule is more easily to be removed from larger pores which makes it easier for dioxin to occupy the position. Taken together, our results suggest that dioxin adsorbed by activated carbons with smaller pores could be less bioavailable due to higher adsorption energy. We suggest that bioavailability studies be conducted using activated carbons with different pore size distributions to determine if smaller pore size manifests lowered bioavailability.

Molecular dynamics study of the influence of activated carbon pore structures on dioxin adsorption

253rd National Meeting (2017)

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