EFFECT OF HYDROGEN GAS GENERATION ON THE REACTIVITY OF PERMEABLE REACTIVE IRON BARRIERS

Granular iron is commonly used in permeable reactive barriers (PRBs) for the in-situ remediation of a variety of groundwater contaminants including chlorinated solvents (vinyl chloride, trichloroethylene) and dissolved metals (Cr, U, As). Under anaerobic groundwater conditions, iron corrodes to produce hydrogen gas.

Fe_(s) + H₂O_(l) ↔ Fe²⁺ + OH^- + H_2(g)

Corrosion produced hydrogen gas can have detrimental effects on PRB performance. Previous studies have shown that hydrogen gas affects the physical groundwater flow conditions through a PRB by reducing permeability and contact between the contaminant and the reactive medium. Corrosion produced hydrogen can also escape the PRB, representing a lost electron resource that could otherwise be used in reductive remediation reactions. However, in addition to affecting physical flow conditions, hydrogen may also interact with iron surfaces to influence the reactivity of the iron granules. For example, hydrogen may become absorbed by the iron creating a stored electron resource within the iron that may be used in remediation reactions. Hydrogen may also alter iron corrosion rates through processes such as hydrogen induced cracking, hydrogen enhanced iron dissolution, oxide film reduction and hydrogen induced pitting corrosion. The purpose of this study was to investigate how hydrogen-iron interactions influence the reactivity of iron granules. It was observed that iron can absorb a large proportion of the corrosion produced hydrogen (in some cases >80%) and that the proportion of hydrogen absorbed depends on the oxide film properties of the iron. Iron under high pressures of hydrogen (~400 kPa) appeared to corrode more rapidly and develop an oxide film more quickly than iron under low (<100 kPa) hydrogen pressures.