Hydrogen Gas Production and Effects During Remediation Using Nano-scale Zero-valent Iron

Abstract

The objective of this study was to develop an understanding of the potential effects of hydrogen gas (H2) exsolution during nanoscale zero-valent iron (nZVI) groundwater remediation applications. An improved understanding of gas exsolution and its effects can be applied to remediation technologies that produce gas in excess of solubility limits, including bioremediation and nZVI. Experiments were performed in thin flow cells packed with water- saturated silica sands. Differential pressure measurements were taken under saturated conditions, after H2 gas exsolution and during gas dissolution. Images were processed with a quantitative light transmission technique to visualize and quantify gas production and transport, and to investigate flow diversion around the injection zone. In a larger-scale experiment, images were processed qualitatively to visualize gas production and transport, and to investigate flow diversion around the injection zone. Water samples were collected and analyzed for contaminant mobilization and redistribution to upper portions of the flow cell. The exsolution of H2 gas caused reductions in hydraulic conductivity with lower values associated with higher gas saturations. Images obtained from dye tests in all experiments demonstrated significant reduction in hydraulic conductivity due to H2 gas produced, with flow diversion of dye solution around the injection zone. The study demonstrated that the dissolution of exsolved gas can be used to investigate relative permeability at small saturation increments and at lower trapped gas saturations than can be obtained using external displacement experiments. H2 gas exsolved due to the reaction of nZVI with water was greater than the H2 gas produced by the self- hydrolysis of NaBH4 solution injected into saturated sands, despite using similar NaBH4 concentration as excess for nZVI synthesis. In the large-scale experiment, the exsolved H2 gas volume was sufficient to produce trapped gas in the injection area, and gas that travelled upwards and away from the injection area as gas channels. Vertical gas transport occurred above a high-concentration trichloroethene (TCE) plume, resulting in its redistribution of TCE, as well as ethene and ethane transport along the gas channel fronts.