Redox transformation, solid phase speciation and solution dynamics of copper during soil reduction and reoxidation as affected by sulfate availability
Beate Fulda, Andreas Voegelin, Katrin Ehlert, Ruben Kretzschmar
In periodically flooded soils, interactions of Cu with biogenic sulfide formed during soil reduction lead to the precipitation of sparingly soluble Cu-sulfides. In contaminated soils, however, the amounts of Cu can exceed the amount of sulfate available for microbial reduction to sulfide. In laboratory batch experiments, we incubated a paddy soil spiked to ∼4.4 mmol kg-1 (280 mg kg−1) Cu(II) to monitor temporal changes in the concentrations of dissolved Cu and the speciation of solid-phase Cu during 40 days of soil reduction and 28 days of reoxidation as a function of initially available reducible sulfate (0.06, 2.09 or 5.92 mmol kg−1). Using Cu K-edge EXAFS spectroscopy, we found that a large fraction of Cu(II) became rapidly reduced to Cu(I) (23–39%) and Cu(0) (7–17%) before the onset of sulfate reduction. Combination with results from sequential Cu extraction and chromium reducible sulfur (CRS) data suggested that complexation of Cu(I) by reduced organic S groups (Sorg) may be an important process during this early stage. In sulfate-depleted soil, Cu(0) and Cu(I)–Sorg remained the dominant species over the entire reduction period, whereas in soils with sufficient sulfate, initially formed Cu(0) and (remaining) Cu(II) became transformed into Cu-sulfide during continuing sulfate reduction. The formation of Cu(0), Cu(I)–Sorg, and Cu-sulfide led to an effective decrease in dissolved Cu concentrations. Differences in Cu speciation at the end of soil reduction however affected the dynamics of Cu during reoxidation. Whereas Cu(0) was rapidly oxidized to Cu(II), more than half of the S-coordinated Cu fraction persisted over 14 days of aeration. Our results show that precipitation of Cu(0) and complexation of Cu(I) by reduced organic S groups are important processes in periodically flooded soils if sulfide formation is limited by the amount of available sulfate or the duration of soil flooding. The speciation changes of Cu described in this study may also affect the speciation and solubility of other chalcophile metals in redox-dynamic wetland soils.