Effect of trace additives on the efficiency of peroxydisulfate regeneration

Direct chemical oxidation (DCO) is an emerging technology in which the organic components of hazardous or mixed waste streams are mineralized to carbon dioxide by the nearly omnivorous oxidant peroxydisulfate. Following treatment, the expended oxidant may be electrochemically regenerated to lower costs and minimize secondary waste; however, due to the varied contents of potential waste streams, trace chemical species may slowly accumulate in the system. To verify that the electrochemical recycling step was not adversely affected, the effect that several species have on the efficiency of this peroxydisulfate regeneration step has been measured. The organic additives formaldehyde, formic acid, and oxalic acid were tested, as well as the inorganic ions phosphate, nitrate, fluoride, chloride and thiocyanate. Changes in the peroxydisulfate formation potential in the presence of these additives were measured using chronopotentiometry, with current densities from 0.5 to 2.0 A cm(-2), and additive concentrations of 0.1 mM to 0.1 M. Also, a real-time technique using a rotating disc electrode was developed to measure these additives' effect on the formation kinetics of peroxydisulfate. In addition, the effect that various additives had on the rate of electrochemical peroxydisulfate generation was measured in a large-scale electrolysis cell. None of the additives caused a significant reduction in the potential or the efficiency of the peroxydisulfate generation step, although the presence of formic acid appeared to increase the efficiency. In addition, the presence of thiocyanate completely blocks the reduction of peroxydisulfate on platinum. This information is currently being applied in scale-up testing of this technology for use in treating hazardous waste or the organic components of mixed waste.