Effect of TCE Concentration and Dissolved Groundwater Solutes on NZVI-Promoted TCE Dechlorination and H2 Evolution

Abstract

Nanoscale zero-valent iron (NZVI) is used to remediate contaminated groundwater plumes and contaminant source zones. The target contaminant concentration and groundwater solutes (NO₃^-, Cl^-, HCO₃^-, SO₄^2-, and HPO₄^2-) should affect the NZVI longevity and reactivity with target contaminants, but these effects are not well understood. This study evaluates the effect of trichloroethylene (TCE) concentration and common dissolved groundwater solutes on the rates of NZVI-promoted TCE dechlorination and H₂ evolution in batch reactors. Both model systems and real groundwater are evaluated. The TCE reaction rate constant was unaffected by TCE concentration for [TCE] ≤0.46 mM and decreased by less than a factor of 2 for further increases in TCE concentration up to water saturation (8.4 mM). For [TCE] ≥ 0.46 mM, acetylene formation increased, and the total amount of H₂ evolved at the end of the particle reactive lifetime decreased with increasing [TCE], indicating a higher Fe⁰ utilization efficiency for TCE dechlorination. Common groundwater anions (5mN) had a minor effect on H₂ evolution but inhibited TCE reduction up to 7-fold in increasing order of Cl^- < SO₄^2- < HCO₃^- < HPO₄². This order is consistent with their affinity to form complexes with iron oxide. Nitrate, a NZVI-reducible groundwater solute, present at 0.2 and 1 mN did not affect the rate of TCE reduction but increased acetylene production and decreased H₂ evolution. NO₃^- present at >3 mM slowed TCE dechlorination due to surface passivation. NO₃^- present at 5 mM stopped TCE dechlorination and H₂ evolution after 3 days. Dissolved solutes accounted for the observed decrease of NZVI reactivity for TCE dechlorination in natural groundwater when the total organic content was small (<1 mg/L).