Chlorinated solvent degradation in groundwater by green rust-bone char composite: solute interactions and chlorinated ethylene competition

Biochar works as a green catalyst for the dechlorination of chlorinated ethylenes (CEs) by green rust (GR). Although the GR-biochar composite shows great potential for groundwater remediation, its performance under simulated field conditions has not been investigated. In this study, a composite of chloride GR and bone char (BC) was used to investigate the effect of groundwater solutes (Cl-, SO42-, HCO3-, and H4SiO4) and the co-existence of one or two CE(s) on GR-BC reactivity with CEs. Furthermore, a contaminant-free groundwater and five CE-contaminated groundwaters were collected to test the reactivity of the GR-BC composite under real groundwater conditions. Among all tested groundwater solutes, HCO3- affected CE dechlorination rates the most, exemplified by a 6.7-fold decrease in trichloroethylene (TCE) reduction rate constant, k(mass), to 0.16 L g(-1) h(-1) in the presence of 10 mM HCO3- solution when compared to the solute free experiment. Silicic acid led to a 1.7-fold decrease in k(mass) at concentrations of 0.5 and 1 mM and a 5.3-fold decrease at 2 mM. TCE reduction rate was also decreased by the co-existence of PCE (by a factor of 1.6), while cDCE had very little impact. Natural groundwater matrices led to up to 52-fold decrease in k(mass)(TCE), depending on the complexity and pollutant profile of the groundwater. However, relatively fast dechlorination with k(mass)(TCE) >= 0.021 L g(-1) h(-1) was seen in all tested CE-contaminated groundwaters where CE concentrations were comparable. For hard groundwaters, HCO3- is recognized as the main inhibitor for dechlorination, while the impact of the other tested solutes is minor. The study provides practical information for the application of the GR-BC composite for remediation of CE-contaminated groundwaters.

Automated summary

This study found that HCO3- has the greatest impact on the dechlorination rates of CE in the reaction process of GR-BC, and the presence of silicic acid leads to varying degrees of reduction in reaction rate. Contaminated groundwater matrices significantly decrease the reactivity of the GR-BC composite.