PAH degradation and redox control in an electrode enhanced sediment cap
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Capping is typically used to control contaminant release from the underlying sediments. However, the presence of conventional caps often eliminates or slows natural degradation that might otherwise occur at the surface sediment. This is primarily due to the development of reducing conditions within the sediment that discourage hydrocarbon degradation. The objective of this study was to develop a novel active capping method, an electrode enhanced cap, to manipulate the redox potential to produce conditions more favorable for hydrocarbon degradation and evaluate the approach for the remediation of PAH contaminated sediment. A preliminary study of electrode enhanced biodegradation of PAH in sediment slurries showed that naphthalene and phenanthrene concentration decreased significantly within 4 days, and PAH degrading genes increased by almost 2 orders of magnitude. In a sediment microcosm more representative of expected field conditions, graphite cloth was used to form an anode at the sediment-cap interface and a similar cathode was placed a few centimeters above within a thin sand layer. With the application of 2V voltage, ORP increased and pH dropped around the anode reflecting water electrolysis. Various cap amendments (buffers) were employed to moderate pH changes. Bicarbonate was found to be the most effective in laboratory experiments but a slower dissolving buffer, e.g. siderite, may be more effective under field conditions. Phenanthrene concentration was found to decrease slowly with time in the vicinity of the anode. In the sediment at 0-1 cm below the anode, phenanthrene concentrations decreased to ~70% of initial concentration with no bicarbonate, and to ~50% with bicarbonate over ~70 days, whereas those in the control remained relatively constant. PAH degrading gene increased compared with control, providing microbial evidence of PAH biodegradation. A voltage-current relationship, which incorporated separation distance and the area of the electrodes, was established to predict current. A coupled reactive transport model was developed to simulate pH profiles and model results showed that pH is neutralized at the anode with upflowing groundwater seepage. This study demonstrated that electrode enhanced capping can be used to control redox potential in a sediment cap, provide microbial electron acceptors, and stimulate PAH degradation.