Isotopic variation of dissolved and colloidal iron and copper in a carbonatic floodplain soil after experimental flooding

Many floodplain soils worldwide are contaminated by present and past industrial and mining activities. During flooding redox potential decreases, triggering the release of dissolved and colloidal metals. We used an anaerobic microcosm incubation to simulate flooding of a carbonate-rich floodplain soil for 40 days. Soil solution samples were extracted to determine the release of dissolved (< 0.02 mu m) and colloidal fractions (0.02-10 mu m). We analyzed stable isotope ratios of colloidal and dissolved Fe and Cu representing two groups of metals with different release behavior; release of Fe was steadily increasing, while that of Cu peaked sharply after flooding and decreased afterwards. The temporal trend of delta Fe-56 values of total Fe in solution indicated dissimilatory iron reduction. The apparent isotopic fractionation between dissolved and colloidal Fe (Delta Fe-56(dissolved-colloidal) = delta Fe-56(dissolved)-delta Fe-56(colloidal)) varied from 0.31 +/- 0.04 parts per thousand to - 1.86 +/- 0.26 parts per thousand. Low delta Fe-56(colloidal) (-1.16 +/- 0.04 parts per thousand) values on day 4 of the experiment suggested colloid formation by precipitation of dissolved Fe, while the strong temporal variation in Delta Fe-56(dissolved-colloidal) indicated subsequent changes in colloid mineralogy, sorption to soil components and/or electron transfer-atom exchange. The variations in delta Cu-65 values (Delta Cu-65(dissolved-colloidal) from 0.81 +/- 0.03 parts per thousand to 1.58 +/- 0.09 parts per thousand) are probably linked to the changing oxidation state of colloidal Cu. While at the beginning of the experiment colloidal Cu and solid soil Cu exchange, these systems decouple after the onset of sulfate reduction in the second half of the experiment. The experimental results fit well to findings from redoximorphic soils described in the literature and highlight the importance of colloids for metal release and the isotopic pattern in carbonatic soils.