Experimental evaluation of electrode‐based technologies for in situ sediment remediation and industrial brine treatment

This thesis evaluated the feasibility and performance of electrode‐based technologies for managing environmental contaminants. Specifically, research presented here focuses on the use of electrodes for in situ sediment remediation, and for the selective removal of bromide from brines produced in hydraulic fracturing.

Sediment capping is an in situ remediation strategy to contain contaminants. Compared to traditional sand and sorbent‐amended caps, reactive caps capable of transforming contaminants may improve the remediation efficiency. However, few materials that can provide long‐term contaminant degradation are available for use in sediment caps. An electrode‐based reactive cap using carbon electrodes as the reactive capping material is proposed in this study to stimulate abiotic and biotic contaminant degradation in situ. A thorough understanding of factors affecting cap performance is essential to apply such reactive caps in the field. The primary objectives of study presented here for reactive sediment capping are to demonstrate the ability of an electrode‐based reactive cap to degrade sediment contaminants, to identify factors affecting contaminant degradation, and to investigate the impact of powered electrodes on contaminant biodegradation.

Preliminary results in this study demonstrated a laboratory scale simulated sand cap containing carbon electrodes connected to a DC power supply induced and maintained redox gradient in Anacostia River sediment for more than 100 days. Hydrogen and oxygen were produced by water electrolysis at the electrode surfaces and may serve as electron donor and acceptor for potential contaminant degradation. The hydrogen production rate was proportional to the applied voltage between 2.5 and 5 V, and not greatly affected by pH or the presence of metal cations. Increasing ionic strength and addition of natural organic matter promoted hydrogen production.

Complete nitrobenzene (NB) degradation was achieved in batch reactors with graphite electrodes. NB was stoichiomtrically reduced to aniline (AN) at the cathode with nitrosobenzene (NSB) as an intermediate, followed by rapid oxidization of AN at the anode. The reduction rate of NB and NSB were enhanced by increasing the applied voltage between the electrodes from 2V to 3.5V, but diminishing returns were observed above 3.5 V. NB and NSB reduction rate constants were faster at lower initial NB concentrations. Humic acid and simulated Anacostia River sediment porewater both affected the degradation rate, but only to a limited extent (~factor of 3).

The effect of powered electrodes on contaminant biodegradation rates was investigated in sediment slurry using 2,4‐dichlorophenol (DCP) as a probe compound. DCP was reductively transformed to 4‐chlorophenol in sediment slurry with powered or unpowered electrodes. Graphite felt electrodes did not change DCP removal rates in nutrient‐amended sediment slurry and carbon paper electrodes decreased DCP removal rate in unamended sediment slurry. The observed negative effect of powered electrodes on DCP biodegradation rate may be caused by hydrogen production and increase of sediment pH near the cathode, since an increase of either hydrogen concentration or pH was found to depress the dechlorination rate in unamended sediment slurries without electrodes.

Another application of electrode‐based contaminant removal technology evaluated in this study is selective bromide removal from mining brine produced in hydraulic fracturing of shale gas. Such brine (referred as “flowback” and “produced” water) has raised a number of environmental and human health issues. An important health concern associated with the brine is its high bromide concentration (~1g/L). If the brine is discharged to receiving waters that serve as drinking water sources, the bromide in it can lead to the formation of carcinogenic brominated disinfection byproducts (DBPs) during water treatment. However, the co‐existence of other ions in the mining brine, especially chloride as high as 30‐200 g/L, makes selective bromide removal technically challenging.