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.