Hydrogen Gas Production from the Injection of Nanoscale Zero-Valent Iron and Sodium Borohydride Solutions: Potential Effects Near Injection Wells

Abstract

The injection of nano-scale zero-valent iron (nZVI) is a remediation technique for the treatment of organic and metal contamination in soil and groundwater. The hydrogen gas (H2) produced during the reaction of nZVI and excess sodium borohydride (NaBH4) used in nZVI synthesis with water can inhibit nZVI transport in the subsurface, potentially limiting solution delivery to the target contaminant zone. Laboratory experiments were completed in a thin flow cell using NaBH4 and nZVI solutions injected into watersaturated medium sands, in which local gas saturations were quantified using a light transmission technique to calculate H2 gas volumes. Hydraulic conductivity, under water-saturated and quasi-saturated conditions, after gas exsolution and throughout gas dissolution, was measured. The results showed that H2 gas volume produced as a result of the reaction of nZVI with water was more than the H2 gas volume produced by the selfhydrolysis of NaBH4 solution regardless of similar NaBH4 concentration used as excess during nZVI synthesis. Pools of H2 gas were formed after injecting nZVI prepared with excess 5 g/L NaBH4 or after injecting 5 g/L NaBH4 without nZVI. Gas accumulated predominantly in a vertical layer of coarse sand, illustrative of a sand pack surrounding an injection well. Lower hydraulic conductivity measurements were linked to higher gas saturations and further reductions were evident as a result of gas pool accumulation at the top of the flow cell. These results show that gas production during the application of nZVI is an important process that must be considered during remediation design and operation to ensure effective delivery to target zones.