Kinetic analysis of the photodegradation of polycyclic aromatic hydrocarbons in aqueous solution

Abstract

The mechanisms by which polycyclic aromatic hydrocarbons (PAHs) photodegrade in dilute (£25 nM) aqueous solutions under varying O 2 concentrations were examined. Employing previously measured photodegradation quantum yields (acquired in the absence and presence of electron donors to the PAH cation radicals, P + ), excited singlet state ( 1 P*) lifetimes, 1 P*/O 2 quenching rate constants and other parameters from the literature, a kinetic model was developed to analyze the fraction of PAH photodegradation that proceeds via the 1 P* or excited triplet state ( 3 P*), as well as to determine the relative contribution of electron transfer to O 2 versus the direct reaction with O 2 from each state. The analysis supports the Aquatic Sciences following conclusions: 1) 1 P* is more reactive with O 2 than 3 P*; 2) reaction via 1 P* proceeds predominantly through electron transfer to produce the P + intermediate, whereas reaction via 3 P* proceeds primarily through direct reaction with O 2 within the collision complex, consistent with the thermodynamic driving forces for electron transfer from these states; 3) although 3 P* is involved significantly in the degradation of many PAHs, such as anthracene, 9-methylanthracene, acenaphthene, and perylene, it is not involved in the degradation of others such as pyrene and benzo[a]pyrene. Under aerobic conditions, photodegradation is likely to be controlled largely by 1 P* lifetime and reactivity.

Light changes the atmospheric reactivity of soot

“…The electronically activated species (P Ã ), as PAHs, aromatic structures with carbonyl and carboxyl functional groups (3,4,15,35) can undergo electron transfer followed by hydrolysis and/or nitro compounds formation, both pathways leading to HONO release (21,22,36). In addition, photoexcited PAHs species can react with O 2 (37), via triplet ( 3 P Ã ) and singlet ( 1 P Ã ) excited states, the latter if the concentration of the reactant is sufficiently high (37,38). Besides, singlet oxygen formation can not be excluded (39).…”

Computational chemistry applied to studies of organic contaminants in the environment: Examples based on benzo[a]pyrene

“…Photo-excitation to the 1 P* (rather than forming a radical cation) state may be a step that facilitates oxidation by singlet oxygen for benzo [a]pyrene (Fasnacht and Blough, 2003) whether or not 3 P* makes a contribution depends on the lifetime of the more reactive 1 P* state (Fasnacht and Blough, 2003). See Schwarzenbach and others (1993, pp.…”

Adsorption of naphthalene onto the carbon adsorbent from waste ion exchange resin: Equilibrium and kinetic characteristics

“…Presently, various methods have been investigated to treat PAHs such as catalytic oxidation, biological degradation and adsorption [1][2][3][4][5][6][7], but there are only a few technologies that are economically feasible for the removal of trace organic contaminants from aqueous solution. Owing to the high concentrating ability of typical adsorbents, adsorption is proved to be one of the most attractive and effective techniques to remove organic contaminants from aqueous streams in water/wastewater treatment.…”