Bioremediation replaces “dig and dump” for treatment of pesticides in soil
Historical usage of organochlorine pesticides (OCPs) such as DDT, toxaphene and dieldrin has resulted in long-term soil contamination, because these substances typically do not degrade naturally at significant rates, so sites are still impacted decades after the end of application. For example, DDT use in Canada ended over 30 years ago, but impacted soil remains at many sites. Such was the case at a former apple orchard in Ontario that was to be developed for housing, but where concentrations of DDT, DDE and dieldrin exceeded the residential use standards.
In the past, the most common way to deal with OCP-impacted soil was “dig and dump”: excavation, trucking and disposal in an approved landfill. In this case, however, the real estate developer’s consultant had identified a potential on-site treatment technology called Daramend® (a registered trademark of Adventus Intellectual Property Inc.). The technology is based on addition of a soil amendment composed of biodegradable carbon in the form of processed plant material and zero-valent iron (ZVI). The amendment is thoroughly mixed into the soil, typically using a rotary tiller, and water is added to increase the soil’s moisture content to just below the saturation point.
Zero-valent iron particles rust to form ferrous iron and release electrons in the process. Simultaneously, organic carbon is fermented by native soil micro-organisms, contributing more free electrons, which are transferred to electron acceptors, such as oxygen and nitrate, thereby driving soil redox conditions well into the negative range. Also through this in situ chemical reduction process, electrons are transferred to the OCPs, resulting in removal of chlorine atoms (reductive dechlorination).