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I read a document in which the authors stated "Sulfate and sulfide ( bisulfide and dihydrogen sulfide) both exist within a system just in varying proportions, and aqueous speciation is dependent on Eh/pH. If one side of the equation changes, the other will transform to balance it out".

The system they are referring to is an engineered wetland, with low amounts of labile carbon but was amended with gypsum and therefore has high sulfate concentrations.

The authors have stated that in deeper zones of the wetland (5-10m), where it is fully saturated, and where oxygen penetration is minimal, aqueous bisulfide would be the main form of sulfur that exists. However, they have provided no evidence, and the carbon that exists in that zone is highly recalcitrant (meaning it is not readily degradable by sulfate reducing bacteria). So how can they say that the main form of sulfur is sulfide in this zone if the sulfate reduction process is not occurring, or very limited, due to low amounts of labile carbon? Even if it is not reacting with metals to form precipitates.

Isn't it common for sulfate to be main form of S even in anaerobic zones? The sulfate reduction process sure is usually carbon limited in high sulfate systems (e.g. marine sediments and even salt lakes). As far as I know, the Eh/pH tell you about the stability of the species, but it does not "magically" transform the sulfate to sulfide.

Thanks

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  • $\begingroup$ I'm not sure what your question is, exactly. Are you asking whether disequilibrium occurs in natural redox systems (yes)? Whether there commonly is sufficient reductive capacity to reduce the majority of sulfate to sulfide? Is the majority of the reduced sulfur aqueous? Are you asking about all systems or just (freshwater??) wetlands? Please edit to focus on the question rather than just stating your opinion. $\endgroup$ – haresfur Mar 26 '18 at 1:35
  • $\begingroup$ My question is can dissolved sulfate (source is gypsum) be more abundant than dissolved sulfide (metals are not present), under anaerobic conditions in which organic carbon is limited? $\endgroup$ – Don Mar 26 '18 at 5:29
  • $\begingroup$ So nothing to do with anaerobic bacteria that breakdown sulfate to hydrogen sulfide ? $\endgroup$ – blacksmith37 Mar 26 '18 at 22:23
  • $\begingroup$ Yes @blacksmith37 $\endgroup$ – Don Mar 27 '18 at 4:12

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