I read in a book Environmental Chemistry that" in anoxic region of water bodies , some bacteria use sulfate ion as an electron receptor where as other bacteria reduce iron(III) to iron (II). These two products react to give a black later of iron (II) sulfide sediment. This frequently occurs during winter, alternating with production of calcium carbonate by product from photosynthesis during the summer. "
Can anyone help me understand why this phenomena occurs during summer and winter??
Dear Samiksha,
during winter months when the water column is oxic, Fe oxide reduction only occurs in the strongly reducing sediment column. Reduced Fe(II) diffuses to the sediment-water interface where it is immediately oxidized to Fe oxide. As a result, there is no buildup of dissolved Fe in the water column during winter. In early summer the lake becomes thermally stratified. As summer progresses, oxygen is consumed by biological processes and the bottom waters become anoxic. Reductive dissolution of Fe oxide continues to occur in the sediments. Fe(II) diffuses into the anoxic bottom water and is oxidized at the oxycline forming a peak in particulate Fe in the water column. Below the oxycline, where the water is strongly reducing, dissolved Fe(II) is stabilized and its concentration increases with time. In early winter, mixing processes re-oxygenate the entire water column and Fe(II) is rapidly oxidized and precipitated.
Here is a classic work on the subject by Mortimer:
https://www.jstor.org/stable/2256395?origin=crossref&seq=1#page_scan_tab_contents
After creating an account (free), you can read the article for free.
Wishing you best of success,
Johannes
As J. Haedrich saids, stratification of water bodies can explain the formation of black iron (II) sulfide. This ends with the "overturn" of water in late autumn, when the water column is mixed again.
In eutrophic bodies the presence of biological detritus at the bottom can maintain an anoxic environment and enhance sulfides formation during the winter.
Most of the process steps have already been explained here. To resume, ferric iron under oxygen depleted conditions and the production of H2S by bacteria can form pyrite. Reaction with water can produce H2SO4, which changes pH and consequently influence soil compounds like reacting with calcite, forming calcium sulphate and CO2. Decomposition (fermentation) of organic matter (OM) in summer enforces anoxic conditions (less oxygen mixing). Carbonate dissolution is dependent of the alkalinity in the water/sediment interface. Higher CO2 production results in elevated concentrations of carbonic acid, which percolates in the soil and dissolves calcite and so higher concentrations of i.a. Ca2+. Calcite precipitation is thus pH dependent and the CO2 production cycle. I think that both processes are closely linked to biological processes occuring in relationship to chemical environmental conditions.
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