Treasuring industrial sulfur by-products: A review on add-value to reductive sulfide and sulfite for contaminant removal and hydrogen production

A highly efficient advanced oxidation process for the destruction of organic contaminants in water is reported. The technology is based on the cobalt-mediated decomposition of peroxymonosulfate that leads to the formation of very strong oxidizing species (sulfate radicals) in the aqueous phase. The system is a modification of the Fenton Reagent, since an oxidant is coupled with a transition metal in a similar manner. Sulfate radicals were identified with quenching studies using specific alcohols. The study was primarily focused on comparing the cobalt/peroxymonosulfate (Co/PMS) reagent with the traditional Fenton Reagent [Fe(II)/H2O2] in the dark, at the pH range 2.0-9.0 with and without the presence of buffers such as phosphate and carbonate. Three model contaminants that show diversity in structure were tested: 2,4-dichlorophenol, atrazine, and naphthalene. Cobalt/peroxymonosulfate was consistently proven to be more efficient than the Fenton Reagent for the degradation of 2,4-dichlorophenol and atrazine, at all the conditions tested. At high pH values, where the efficiency of the Fenton Reagent was diminished, the reactivity of the Co/PMS system was sustained at high values. When naphthalene was treated with the two oxidizing systems in comparison, the Fenton Reagent demonstrated higher degradation efficiencies than cobalt/peroxymonosulfate at acidic pH, but, at higher pH (neutral), the latter was proven much more effective. The extent of mineralization, as total organic carbon removed,was also monitored, and again the Co/PMS reagent demonstrated higher efficiencies than the Fenton Reagent. Cobalt showed true catalytic activity in the overall process, since extremely low concentrations (in the range of microg/L) were sufficient for the decomposition of the oxidant and thus the radical generation. The advantage of Co/PMS compared to the traditional Fenton Reagent is attributed primarily to the oxidizing strength of the radicals formed, since sulfate radicals are stronger oxidants than hydroxyl and the thermodynamics of the transition-metal-oxidant coupling.