Kinetic Study of the Reactions of Sulfate Radical Anions with Phenantrene in Aqueous Solutions

FABRICIO RAGONE; VERÓNICA C. MORA; JANINA A. ROSSO

Mendoza

Congreso; 21st Inter American Photochemical Society Meeting; 2011

Inter American Photochemical Society

Polycyclic aromatic hydrocarbons (PAH) are ubiquitous environmental contaminants which derived from a variety of anthropogenic sources involving mainly incomplete combustion of organic matter and by industries. It has been reported that many PAH show a high toxic, carcinogenic, and mutagenic potential. They are hazardous to ecosystems and human health. Therefore, it is important to investigate the fate of PAH in aquatic environments and find a way to eliminate them or reduce their toxicity. In-situ chemical oxidation (ISCO) involves the introduction of a chemical oxidant into the subsurface for the purpose of transforming groundwater or soil contaminants into less harmful chemical species. Persulfate (PS) is the newest form of oxidant currently being used for ISCO. PS can be chemically, photochemically, or thermally activated to generate the stronger oxidant sulfate radicals (SO4.-), which are the main oxidant species. Thus, their reactivity with the contaminant studied is the first parameter to be considered. In this work, we investigate the reactivity of SO4.- radicals with phenanthrene, as a model substance, the reaction mechanism and products formed. Flash-photolysis experiments were carried out with conventional Xenon Co. model 720C equipment with modified optics and electronics. To avoid product accumulation, each solution was irradiated only once. Photolysis of PS (λ < 300 nm) was used as a clean source of SO4.- radicals. The rate of decay of SO4.- radical (= 450 nm) increase with [PHEN], as shown in Figure 1, for [PS] = 5  10-3 M without PHEN (upper trace) and with [PHEN] = 0.3 M (lower trace). The experimental traces could be well fitted to pseudo-first order decay, with apparent rate constant (b /s-1). The slopes of the linear plots in inset Figure 1 (b /s-1 vs, [PHEN]) yield the bimolecular rate constant k (SO4.- + FEN) = (3.2 ± 0.8)  109 M-1 s-1. Experiments performed with higher concentration of PHEN (1.3 M ) indicated the formation of a transient species. Figure 2 shows the spectrum at 1  10-3 s after the flash and the decay observed at 300 nm (Inset Figure 2).