In most of the toxicity tests the researchers exposed model organisms to a compound (like CuSO4, ZnSO4, CdCl2, compounds of Pb, and other toxic elements, and chemicals) but they followed the effects of the single part of the interest of that specific compound. for example, if a study aimed to investigate the toxic effects of Cu, in the model organism and exposed the model organism to CuSO4 the researcher follows and investigates the effects of Cu only. Here as we can see that the compound they give as a toxic chemical in the test environment dissociates into Cu+ ions and SO4 _ ions but most of the literature has not investigated the toxic effects of the second part (like SO4 _ this case) of the toxic chemical compound.
The phenomenon is more specific in aquatic toxicology.
Taking sulphate as an example from your question, its toxicity is negligible compared with that of the metal itself. In CuSo4, copper itself is more toxic than sulphate. See the last point under the Regulatory Guidelines section at (1). The maximum permissible limit for copper in drinking water is 1 mg/L, whereas it's 250 mg/L for sulphate. Usually, researchers use copper sulphate in lower concentrations for toxicity testing (2). For example, in 10 mg of CUSO4.5H2O, there is only 3.84 mg of sulphate, while there is 2.5 mg of copper. Studies testing the toxicity of sulphate itself do so at higher concentrations (3). Thus, the toxicity of sulphate is usually not considered.
Hope this helps.
I appreciate the answer it cleared me but as per the literature nowadays there is a very dynamic shift in toxicological studies and the researcher are looking for the toxic effects of chemicals in very lower concentrations. After the development of omics technology, researchers now focusing to detect the effects of chemical in the environment in micro quantity.
While in the case you mentioned sulfate ions make the medium acidic there should be a possible oxidative stress.
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