This project aims to reduce residual dioxin contamination in soils and sediments from the past use of Agent Orange during the Vietnam War. Dioxins will be removed from contaminated soils and sediments via anaerobic and aerobic metabolism by microorganisms using a novel stepwise procedure of sequential anaerobic-aerobic biodegradation. In anaerobic conditions, certain anaerobic bacteria can remove chlorine atoms from dioxin molecules and convert them into lower chlorinated congeners through reductive dehalogenation. A stepwise process of anaerobic followed by aerobic biodegradation, can ultimately destroy dioxins in soils and sediments, eliminating the need for any additional steps to handle the pre-concentrated dioxins in plants or other media. Nanoscale Zero Valent Iron particles are reportedly efficient at removing arsenic from environmental matrices as well. While it is not the main focus of this project, the team will also monitor arsenic before and after remediation to determine if there are any secondary benefits from using this remediation approach.
Previous experiments conducted at the Asian Institute of Technology (Binh et al. 2016) showed that sequential anaerobic-aerobic biodegradation could remove 60% of 2,3,7,8-TCDD from contaminated soil after 23 weeks by using the enriched indigenous microorganisms from dioxin-contaminated sediments in Bien Hoa airbase alone. Building on past experience, this project will first conduct lab-scale experiments to find optimal conditions for dioxin removal and then conduct pilot-scale experiments onsite, at the Bien Hoa airbase, for the actual contaminated soils and sediments. The aim is to include a range of diffused dioxin levels, from below 1000 ppt to a few ppb. A design for the full-scale treatment will be prepared and proposed for future application for the bioremediation with consideration of the actual pollution situation and local climate.
The contaminated soil with high dioxin concentrations (>1,000 ppt) has been or will be treated to destroy residual dioxins by some attested alternatives (USAID-Vietnam, 2016). However, large volumes of soils and sediments containing dioxins below 1,000 ppt remain at the sites. Current attempts to address these soils and sediments focus on landfill/containment to prevent direct human and ecologic contacts. This project aims to develop technologies to eventually destroy dioxins using enriched indigenous microbes, which have a high potential for application to convert the passive landfills in Phu Cat, Da Nang, and Bien Hoa Airbases into active landfills. It will also consider the effects of any additives used in the treatment on the microbes and the quality of the soils and sediments. The project therefore would supplement USAID’s efforts to remove dioxins from the hotspot airbases in Vietnam and restore the environmental conditions to safeguard ecological and human health in the areas. The project will also build capacity of the local partners in Vietnam by involving research teams from local universities to develop this new treatment approach, hence sustaining the local human resources to deal with the dioxin contamination in soils and sediments in Vietnam in the long term. The US Government-supported (USG) partner from Emory University has extensive experience on bioremediation of heavy metals in soils and sediments and will train the Asian partners on the analysis of dioxins and microbial communities.
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