Moisture retention extended enhanced bioelectrochemical remediation of unsaturated soil.

Bioelectrochemical systems (BESs) have demonstrated great promise in augmented biodegradation of petroleum hydrocarbons in water-saturated soils. However, bioremediation of unsaturated soil in vadose zone has been a challenge due to poor mass transfer and low conductivity. This study proposed a moisture retention layer (2 cm thickness) around the BES anodes to enhance soil remediation under unsaturated conditions. The active soil BESs (closed circuit) includes two reactors with anodic moisture-retaining layers of soil-polyacrylamide hydrogel (SHB) and graphite granule-polyacrylamide hydrogel (GHB) mixtures, and another reactor filled with only soil (SB) without moisture-retaining layer. An open circuit SB was served as a control to simulate natural attenuation. This study demonstrated for the first time that moisture retention layers around the BES anodes could significantly extend and enhance hydrocarbon degradation in vadose zone soil. Results showed that SHB reactor could maintain 43-100% longer duration for electricity generation than other reactors. Correspondingly, SHB showed the best removal (average 21-37%) of total petroleum hydrocarbon (TPH) in spatial distribution, which was ~91% and ~164% higher than other BESs and control, respectively. This study demonstrated that by using low-cost and environmentally friendly hydrogel, BESs could become a viable remediation method for vadose zone soil.