Effect of TCE concentration and dissolved groundwater solutes on NZVI-Promoted TCE dechlorination and H2 evolution

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-, SO42-, and HPO42-) should affect the NZVI longevity and reactivity with target contaminants, but these effects are not well understood. This study evaluates the effect of trichloroethylene (TICE) concentration and common dissolved groundwater solutes on the rates of NZVI-promoted TCE dechlorination and H-2 evolution in batch reactors. Both model systems and real groundwater are evaluated. The TICE reaction rate constant was unaffected by TICE concentration for [TCE] <= 46mM and decreased,by less than a factor of 2 for further increases in TICE concentration up to water saturation (8.4mM).For [TCE] >= 0.46 mM, acetylene formation increased, and the total amount of H-2 evolved at the end of the particle reactive lifetime decreased with increasing [TCE], indicating a higher NO utilization efficiency for TICE dechlorination. Common groundwater anions (5mN) had a minor effect on H-2 evolution but inhibited TICE reduction up to 7-fold in increasing order of Cl- < SO42- < 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 H-2 evolution. NO3- present at > 3 mM slowed TCE dechlorination due to surface passivation. NO3- present at 5 mM stopped TCE dechlorination and H-2 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).