Effects of co-existing nitrate on TCE removal by mZVI under different pollution load scenarios: Kinetics, electron efficiency and mechanisms.

Abstract

Microscale zero-valent iron in situ reaction zone (mZVI-IRZ) has proved to be effective and efficient for the removal of chlorinated aliphatic hydrocarbons (CAHs) from groundwater. However, nitrate (NO3-), which is ubiquitous in groundwater, affects the mZVI-based attenuation of CAHs in a complicated manner. Both the reaction rate constant (k) and electron efficiency (EE) of mZVI must be considered to comprehensively reflect the effects of NO3- on the short and long-term remediation performances of mZVI. Therefore, the influence of NO3- on trichloroethylene (TCE) removal under high-pollution-load (iron limited) and low-pollution-load (iron excess) conditions was investigated. Low concentrations of NO3- (10 and 50mgNL-1) were found to enhance the TCE removal rate and efficiency, whereas high concentrations of NO3- (100mgNL-1) inhibited the reaction. Although TCE removal was increased at low concentrations of NO3-, the EE of mZVI was dramatically decreased in the presence of NO3- at all concentration levels. Therefore, both the short-term TCE removal characteristics and the EE of mZVI should be considered when evaluating the long-term remediation effectiveness of mZVI-IRZ technology. The effects of NO3- on the TCE removal trends under high- and low-pollution-load scenarios were similar, but had different magnitudes. NO3- affected the TCE removal mainly by promoting mZVI corrosion, competing for electrons and affecting passivation product evolution. Our results provide guidance for the practical application of mZVI-IRZ technology.