Biogeochemical Behavior of Metals Along Two Permeable Reactive Barriers in a Mining‐Affected Wetland

Abstract

The biogeochemistry of two alkaline permeable reactive barriers (PRBs) installed for remediation in a mining-affected wetland was investigated in order to assess the importance of colloidal particles on metal removal processes in such systems. At the time of investigation, both PRBs were effective in removing U, Cu, and Zn (>95%) from groundwater but were slightly less efficient for Ni and Co (<90%). Previously installed groundwater wells allowed an in-depth analysis of groundwater passing through the first PRB. Here, in an alkaline environment (pH 6.0-9.7), 11-14% of Ni, 36-37% of Co, 77-81% of Cu, 14-17% of U, and 10-19% of Fe were associated with organic matter and inorganic colloids, while upgradient in the more acidic environments (pH <6.0), ionic species and complexes (e.g., Co 2+ , Ni 2+ , Cu 2+ , and UO 2 H 3 SiO 4 + ) dominated. Copper and U preferentially bound to larger colloidal fractions (>1 kDa), which might have promoted their sequestration. Uranium removal was likely further enhanced by U (VI) reduction in the alkaline and oxygen-depleted conditions of the PRBs. The less efficient removal of Ni and Co, being target metals for remediation, was explained by a combination of their high solubility, unfavorable redox and pH conditions created by the alkaline PRBs, and their limited association with colloidal particles. These considerations are critical in the design of future PRBs for the remediation of similar systems.Plain Language Summary Permeable reactive barriers are a technology often applied to treat contaminated groundwater. In this study, the biogeochemistry of two alkaline barriers, placed in a wetland near a decommissioned uranium mine, was investigated. At the time of investigation, both reactive barriers were effective in removing uranium, copper, and zinc from groundwater but were less efficient for nickel and cobalt. Using previously installed groundwater wells, we conducted an in-depth analysis of groundwater passing through the first barrier. The alkaline and oxygen-depleted environment promoted the formation of colloids, which potentially contributed to the effective removal of copper and uranium, while the surrounding acidic wetland promoted the transport of ionic metal species. At the reactive barriers, uranium removal was further enhanced by uranium reduction to more immobile uranium forms through reduced chemical species and the activity of bacteria. The less efficient removal of nickel and cobalt was explained by their high solubility, unfavorable conditions created by the alkaline reactive barriers, and their limited association with colloids. This work provides new insights into future uses of permeable reactive barrier technology for metal remediation in wetlands.

Environmental effects of leachate extracts from reclaimed asphalt pavement: determination of metals, polycyclic aromatic hydrocarbon and acute toxicity to Daphnia magna

“…The high concentration of Zn obtained in the leachate extracts can be explained by the fact that it was relatively mobile and is readily leached in acidic media due to their amphoteric leaching pattern (BERTHELSEN;OLSEN;STEINES, 1995;LIM et al, 2004;CETIN et al, 2014;GUNTEN et al, 2019). It is worth mentioning that the pH of the leached extracts was between 4.95 and 5.07 due to the acidic characteristics of the extraction solution, which was around 4.90.…”

“…The properties of Cd are similar to those of Zn, having greater mobility in acidic media (BERTHELSEN;OLSEN;STEINES, 1995). The occurrence of Cd in RAP can be due to the burning of oils and greases used as lubricants in vehicles, by the abrasion of brake pads or because the Cd can be part of the bitumen used as a binder in pavements (AKPOVETA; OSAKWE, 2014;JIANG et al, 2015;GUNTEN et al, 2019).…”