Individual and combined effects of humic acid, bicarbonate and calcium on TCE removal kinetics, aging behavior and electron efficiency of mZVI particles

Uncertainties of aging after the injection of microscale zero valent iron (mZVI) into subsurface hinder the widespread application of this technology into in-situ groundwater remediation. As TCE removal by mZVI is usually accompanied by mZVI anaerobic reaction with water, this study investigated the hydrogen evolution kinetics, TCE removal performance, and electron efficiency (EE) of mZVI particles under single and combined effects of organic geochemical constituents (humic acid (HA)), inorganic constituents (hardness (Ca2+) and alkalinity (HCO3−)) in groundwater. The results showed that both H2 generation and TCE removal were enhanced in the presence of HA (5–20 mg L−1 as DOC), and significant delay of aging precipitates formation was supported by SEM-EDS, XRD and XPS depth-profiling characterization. With increasing Ca2+-HCO3− concentrations (0.4–2 mM), the mZVI corrosion were inhibited for the formation of a thin compact passive film on mZVI surface. Furthermore, in co-presence of organic and inorganic constituents, the negatively-impacted iron corrosion kinetics suggested that promotional effects of organic constituents were dwarfed by the inhibitory effects of inorganic constituents. In addition, the EE values in different systems ranged among 1.31–7.40%, and decreased significantly with time. Hence, these findings provide insight into aging mechanism and performance prediction of mZVI under different groundwater geochemical conditions.