Removal kinetics and mechanisms of chromium(VI) and arsenic(V) in groundwater by permeable reactive barriers with environmental life cycle assessment

The feasibility of using zero-valent iron (Fe0) and iron oxide-coated sand (IOCS) as a combination of reactive media in permeable reactive barriers (PRBs) for removing Cr(VI) and As(V) from groundwater with various geochemical constituents such as hardness, alkalinity and natural organic matter (NOM) was investigated. In addition, the associated environmental impacts of using these materials as the reactive media in PRBs with the effects of the construction methods, reactive materials and the groundwater constituents were also approached by life cycle assessment (LCA). In the batch study of the removal of As(V) by Fe0 in the presence of hardness (Ca2+) and alkalinity (HCO3-), the As(V) removal was significantly enhanced by the formation of CaCO3 as a nucleation seed for the growth of large iron (hydro)oxide particles. In the study of the effects of NOM, a faster Cr(VI) removal rate was observed in the presence of humic acid (HA) compared to fulvic acid (FA). Fluorescence spectroscopy revealed that no redox reaction occurred in the FA and HA. The free Fe(II) concentration was higher in the dissolved metal-NOM complexes significantly affected the aggregation of the iron corrosion products which in turn affected the removal of As(V). In the batch study, the Fe0 and IOCS mixture performed better on the removal of both Cr(VI) and As(V), compared to using Fe0 or IOCS alone. Compared with Fe0 and quartz sand mixture for the column study, the Fe0 and IOCS mixture achieved the highest removal of both Cr(VI) and As(V), whilst the effects of HA were marginal by using these reactive materials. A synergistic effect in these reactive materials occurred as Fe2+ was adsorbed onto the IOCS so that the iron oxides were transformed to magnetite, providing more reactive surface areas for Cr(VI) reduction and reducing the passivation on the Fe0. HA was adsorbed onto the IOCS so that the impacts of the deposition of HA aggregates on the Fe0 surface were reduced, thus enhancing the Fe0 corrosion. Based on the findings of the LCA, the trench-based construction method can reduce the environmental impacts of the PRBs remarkably compared to the caisson-based method due to less material consumption by the funnel. The use of the Fe0 and IOCS mixture can reduce the impacts generated by the reactive media, compared to using the conventional Fe0 and quartz sand mixture. The findings of this study suggest that the use of the combination of Fe0 and IOCS can have a higher removal efficiency in Cr(VI) and As(V), and arouse a consideration in the design of a more environmental sustainable PRB by using Fe0 and IOCS together