Effect of trace additives on the efficiency of peroxydisulfate regeneration

Direct chemical oxidation (DCO) is an emerging technology in which the orga nic components of hazardous or mixed waste streams are mineralized to carbo n dioxide by the nearly omnivorous oxidant peroxydisulfate. Following treat ment, the expended oxidant may be electrochemically regenerated to lower co sts and minimize secondary waste; however, due to the varied contents of po tential 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 p eroxydisulfate regeneration step has been measured. The organic additives f ormaldehyde, formic acid, and oxalic acid were tested, as well as the inorg anic ions phosphate, nitrate, fluoride, chloride and thiocyanate. Changes i n the peroxydisulfate formation potential in the presence of these additive s 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 re al-time technique using a rotating disc electrode was developed to measure these additives' effect on the formation kinetics of peroxydisulfate. In ad dition, the effect that various additives had on the rate of electrochemica l peroxydisulfate generation was measured in a large-scale electrolysis cel l. None of the additives caused a significant reduction in the potential or the efficiency of the peroxydisulfate generation step, although the presen ce of formic acid appeared to increase the efficiency. In addition, the pre sence of thiocyanate completely blocks the reduction of peroxydisulfate on platinum. This information is currently being applied in scale-up testing o f this technology for use in treating hazardous waste or the organic compon ents of mixed waste.