For the first time, the fate of radicals generated in heterogeneous chemical oxidation treatment systems has been accounted for and used to assess treatment performance in three reaction compartments; reaction with the target compound, rhodamine B (RhB), the aqueous phase scavengers, and the solid phase scavengers. Radicals formed during the ultra-violet (UV) activation of hydrogen peroxide (H2O2) (UV-AHP) and persulfate (S2O8 2-) (UV-APS) include hydroxyl (•OH) and sulfate radicals (SO4 •-), respectively. •OH and SO4 •-, used in oxidation treatment systems to degrade a broad spectrum of environmental contaminants, may also react with non-target chemical species (scavengers) that limit treatment efficiency. UV-AHP and UV-APS treatment systems were amended with solid phase alumina to assess scavenging by solid surfaces. The overall rate of reaction and rate of radical scavenging was greater for •OH than SO4 •-. Scavenging by dissolved constituents was dominated by the oxidant used (H2O2, S2O8 2-); and the rate of radical scavenging by alumina was greater than the rate of RhB oxidation in all cases. Treatment efficiency was lower in the UV-AHP than in the UV-APS treatment system and was attributed to greater aqueous and solid phase scavenging rates. The cost of commercially available H2O2 ($0.031 mol-1) and PS ($0.24 mol-1) was used in conjunction with the overall treatment efficiency to assess specific cost of treatment. The specific cost to treat the probe compound with UV-AHP was greater than UV-APS and was attributed to the much lower treatment efficiency with UV-AHP. The much-desired high reaction rate constants between •OH and environmental contaminants, relative to SO4 •-, may come at the cost of greater combined scavenging rates, and consequently lower treatment efficiency.
Keywords: Efficiency; Hydroxyl radical; Kinetics; Scavenging; Sulfate radical.