I’m not a physicist. At best, I’m an armchair physicist, not so much out of choice, but because people have asked me if there is a difference between the two and what that difference is. What follows is the result of some research I did to answer that question.
First the terms. Both sulfate and sulfide have different spellings. Sulfate/sulphate and sulfide/sulfite. In both, the former is the common term used, i.e., sulfate and sulfide.
Sulfide and sulfate are oxyanions of sulfur. Basically this means that these elements are formed by interaction with oxygen and they have a negative charge. SO4 means the sulfate ion and four oxygen atoms. SO4 is the kind of sulfate we're concerned with. In relation to wild rice water quality standards, SO4 is the proper term to use. John Moyle, whose wild rice work in the 40s was used to set the current wild rice quality standard, specifically wrote - SO4; in other words sulfate.
So what about sulfide? In Len Anderson's excellent article, "The Validity of the Minnesota Wild Rice Sulfate Standard,” Anderson quotes a hearing officer regarding the flowage of the Mississippi River and why the higher level of sulfates from the Cohasset plant doesn’t affect the wild rice stands. Because of the river current, "...there is less of a reducing environment in the sediments and therefore, the sulfate is not reduced to hydrogen sulfide, which is the sulfur compound that damages the wild rice roots."
John Pastor, a University of Minnesota Duluth biologist and expert on wild rice, said that the current standard should be maintained, i.e., 10 mg/L. Pastor said that small-scale research at UMD seems to suggest that once sulfates get into the sediments at the bottoms of streams and lakes, microbes convert the sulfates back into sulfides. Sulfides seem to interfere with the development of the roots of wild rice plants, and the stunted roots starve the rest of the plants of nitrogen. That results in fewer and smaller seeds, and the leaves turn yellow.
Thus, scientific evidence strongly suggests that sulfates are converted to hydrogen sulfide and affect both seeds and root symptoms of natural wild rice year-round.
To put it in more understandable terms, sulfide can be said to be a byproduct of sulfate. With metals, sulfate is an oxidizing agent that forms sulfide. Therefore, SO4 is the main agent we're dealing with. Once SO4 leaches into water, hydrogen sulfide is formed. The higher the level of SO4, the higher the level of hydrogen sulfide.
Both sulfate and sulfide are natural agents. But mining increases the levels, hence endangering not only wild rice, but other vegetation as well and, in turn, impacting the ecosystem.
The bottom line? Sulfate, i.e. SO4, is the main issue we’re dealing with and, therefore, the term that should be used. Of course, if we really want to impress the suits in the Capitol and at Polymet, we can impress them with our deeper knowledge regarding hydrogen sulfide. I suspect they haven't a clue as to what that is.