Permeable reactive barriers (PRBs) are a technology often applied to treat contaminated groundwater. Despite being well established, their application comes with many site-specific challenges, and installations in wetlands are not well understood. Here we studied the biogeochemistry of two alkaline PRBs installed in a boreal fen, which is affected by metal-contaminated groundwater coming from a waste rock pile of a decommissioned uranium mine. The PRBs were most effective in removing U, Cu, and Zn (&gt;95%) with less efficiency for Ni and Co (&lt;90%). In the rather alkaline environment of the first PRB, up to 14% of Ni, 37% of Co, 81% of Cu, 17% of U, and 19% of Fe were associated with organic matter and inorganic colloids, as determined by sequential filtrations and asymmetric flow field flow fractionation. In the more acidic environments (pH &lt;6.0) of the surrounding wetland and the waste stream-affected soil, geochemical modeling predicted dominantly ionic species and complexes of metals (e.g., Co2+, Ni2+, Cu2+, UO2H3SiO4+). It was demonstrated that large colloidal metal fractions of &gt;1 kDa molecular weight favored the removal of Cu and U from the aqueous phase, suggesting metal sequestration through a process similar to the “colloidal pumping” in open water. Uranium removal was likely enhanced by U(VI) reduction through reduced inorganic species and microbes, the latter supported by 16S rRNA gene sequencing. The less efficient removal of Ni and Co was caused by their high solubility and the limited association with colloids, thus the alkaline PRBs likely created unfavorable redox and pH conditions for their removal. Our results highlight critical considerations in the design of alkaline PRBs for the remediation of metals-contaminated wetlands.

Biogeochemistry of an alkaline permeable reactive barrier in a boreal fen

2019 Fall National Meeting

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