Phototransformation rate constants of PAHs associated with soot particles

Abstract

Photodegradation is a key process governing the residence time and fate of polycyclic aromatic hydrocarbons (PAHs) in particles, both in the atmosphere and after deposition. We have measured photodegradation rate constants of PAHs in bulk deposits of soot particles illuminated with simulated sunlight. The photodegradation rate constants at the surface (k0p), the effective diffusion coefficients (Deff), and the light penetration depths (z0.5) for PAHs on soot layers of variable thickness were determined by fitting experimental data with a model of coupled photolysis and diffusion. The overall disappearance rates of irradiated low molecular weight PAHs (with 2-3 rings) on soot particles were influenced by fast photodegradation and fast diffusion kinetics, while those of high molecular weight PAHs (with 4 or more rings) were apparently controlled by either the combination of slow photodegradation and slow diffusion kinetics or by very slow diffusion kinetics alone. The value of z0.5 is more sensitive to the soot layer thickness than the k0p value. As the thickness of the soot layer increases, the z0.5 values increase, but the k0p values are almost constant. The effective diffusion coefficients calculated from dark experiments are generally higher than those from the model fitting method for illumination experiments. Due to the correlation between k0p and z0.5 in thinner layers, Deff should be estimated by an independent method for better accuracy. Despite some limitations of the model used in this study, the fitted parameters were useful for describing empirical results of photodegradation of soot-associated PAHs.

Photochemical degradation of PAHs in estuarine surface water: effects of DOM, salinity, and suspended particulate matter

“…It has been proposed in the literature that suspended particulate matter can influence the photodegradation rate of PAHs by hindering the light intensity and photosensitizing humic acid contained in the suspended particulate matter (Xia et al 2009;Dong et al 2010;Kim et al 2013). According to Fig.…”

Assessment of Reliability when Using Diagnostic Binary Ratios of Polycyclic Aromatic Hydrocarbons in Ambient Air PM10

“…This reflects greater divergence in the spatial distribution of An/(An + Phe), which can be attributed to several causes. According to a previous study, the overall rate of photodegradation of irradiated 2-3-ring PAHs on soot particles is affected by fast photolysis and fast diffusion kinetics, while that of PAHs with 4 or more rings is evidently governed by either the combination of slow photolysis and slow diffusion kinetics, or by extremely slow diffusion kinetics alone (Kim et al, 2013). Therefore, the comparatively high COD value of An/ (An + Phe) reflects the greater fragility of these LMW congeners in comparison with those of Fluo and Pyr.…”

“…A study by Kim et al (2013) highlighted the importance of both fast photodegradation and fast diffusion kinetics on the reduction of LMW PAHs (with 2-3 rings) in comparison with HMW PAHs (with 4 or more rings). Although the photodegradation rates of 16 U.S. EPA-priority PAHs follow first-order kinetics, UV-B (315-280 nm) exposure can dramatically accelerate the photolysis rate of PAHs adsorbed onto fly ash particles (Niu et al, 2007).…”

Assessing the effects of UVA photocatalysis on soot-coated TiO2-containing mortars

“…Despite soot particles were traditionally considered relatively unreactive, this pyrogenic organic matter can indeed be degraded (De la Rosa and Knicker, 2011). Kim et al (2013) showed that soot-associated PAHs can be degraded by photolysis. Energetic UV photons have sufficient energy to disrupt chemical bonds in many materials including soot particles.…”