Short-term Fe reduction and metal dynamics in estuarine soils impacted by Fe-rich mine tailings

Abstract

Rates of iron (Fe) reduction and oxidation greatly influence the release and retention of metals in redox-dynamic ecosystems such as estuaries. The fate of metals and Fe cycling operate in multiple timescales (days to years), but processes that occur from days to weeks have been poorly investigated, especially in estuarine soils contaminated with mine tailings. Here we tested the hypothesis that Fe reduction would increase rapidly (within days) when Fe-rich mining tailings are exposed to reducing conditions. Fe reduction is then expected to release metals bound/occluded into Fe oxyhydroxides to the estuarine system. We performed a laboratory incubation for 20 days, expositing estuarine soils to either anoxic or oxic conditions and analyzed HCl-extractable FeII and metals to test this hypothesis. We observed a rapid and significant increase in Fe reduction (FeII production) within 20 d under anoxic conditions, but no increase under oxic conditions. Also, during the 20-d of anoxia, a significant increase in the HCl-extractable metals was observed for Al (1.6 × ), Ba (1.4 × ), Cd (3.1 × ), Co (4.1 × ), Cr (2.6 × ), Cu (1.4 × ), Mn (6.1 × ), Ni (1.6 × ), Pb (1.7 × ), and Zn (3.6 × ), whereas no changes were observed in aerobic incubations. Accounting for the total pool of microbially-reducible Fe (i.e., the low-crystallinity or short-range-ordered Fe phases), we estimated that the residence time for a potential release of metals ranged from 22 d for Mn to 121 d for Cr. Our results demonstrate the high speed of Fe biogeochemical cycling and metal dissociation in aquatic ecosystems impacted by upstream Fe-rich tailings. Our study also suggests that chronic contamination in these areas may persist for several months after the deposition of tailings and will likely persist with the continued arrival of tailings from the upstream river basin.