Photolysis of polycyclic aromatic hydrocarbons associated with fly ash particles under simulated sunlight irradiation

Fly ash particle-associated polycyclic aromatic hydrocarbons (PAHs) were produced by combustion of polyvinyl chloride (PVC), wood, high-density polyethylene (HDPE), and polystyrene in a stove. Fly ash particles were deposited on the glass slides (76 mm × 26 mm) placed in the exposure chamber. The concentration of 16 PAHs ranged from 10.1 pg/dm2 fly ash for acenaphthene to 2348.7 pg/dm2 fly ash for fluoranthene. Medium to large molecular weight PAH concentrations were much higher compared to low molecular weight PAHs on fly ash. Photolysis of PAHs associated with fly ash particles deposited on glass slides was investigated under simulated sunlight irradiation for about 550 h. Generally, the photolysis rates of the 16 PAHs follow first-order kinetics. Under simulated sunlight containing UV-B, the photolysis of PAHs includes two phases: fast (0–50 h) and slow phase (50–550 h). The photolysis rate constants of PAHs in the initial fast phase were far greater than those in the slow phase. When UV-B was removed from the simulated sunlight irradiation, the photolysis rate constants of PAHs were significantly lower compared to those observed in the fast phase with UV-B exposure, suggesting UV-B radiation could accelerate photolysis of PAHs adsorbed on the fly ash particles. In addition, partial least squares (PLS) analysis models were applied to explain the molecular structural characteristics that affect the PAH photolysis rates. The results suggested that the photolysis rate constants of PAHs decrease with the increasing of molecular weight (Mw), the energy of the lowest unoccupied molecular orbital (ELUMO), ELUMO + EHOMO, and the standard heat of formation (ΔHf).