Sunlight-driven environmental photodegradation of 2-chlorobiphenyl (PCB-1) in surface waters: kinetic study and
Arlen Mabel Lastre-Acosta1, Carolina Mendes Rocha2, Maria Anita Mendes3
1Research Group in Advanced Oxidation Processes (AdOx), Chemical Systems Engineering Center, Department of Chemical Engineering, University of São Paulo, Av. Prof. Luciano Gualberto, tr. 3, 380, São Paulo, SP, Brazil. arlenlastre@gmail.com.
Abstract:
Polychlorinated biphenyls (PCBs) are a family of highly toxic, resistant, and persistent organic pollutants, among which 2-chlorobiphenyl (PCB-1) is one of the simplest. Most studies on PCBs' photochemistry are limited to their direct photolysis, while the important role of reactive photo-induced species (RPS) (hydroxyl radicals, HO●; singlet oxygen, 1O2; and triplet excited states of chromophoric dissolved organic matter, 3CDOM*) in removing PCBs in natural waters through indirect photolysis has not yet been evaluated. In this work, the rate constants of the reactions between aqueous PCB-1 and RPS were obtained under simulated solar radiation (450-W Xenon lamp and an AM 1.5 global filter) by competition kinetics, and the effects of the initial pollutant concentration and the physicochemical characteristics of the water were investigated. The direct photolysis quantum yield of PCB-1 in the range 290-800 nm was found as 1.60 × 10-2 mol Einstein-1. The value of kPCB-1,HO● = (6.80 ± 0.09) × 109 L mol-1 s-1 is in good agreement with the literature. For 1O2, kPCB-1,1O2 = (1.13 ± 0.20) × 106 L mol-1 s-1, while for 3CDOM*, kPCB-1,3CBBP* = (2.44 ± 0.04) × 109 L mol-1 s-1 and kPCB-1,3AQ2S* = (3.36 ± 0.04) × 109 L mol-1 s-1 were obtained using 4-benzoylbenzoic acid (CBBP) and anthraquinone-2-sulfonate (AQ2S) as CDOM proxies, respectively. These results show that the main pathways involved in PCB-1 photodegradation are the reactions with HO● and 3CDOM* together with direct photolysis. In addition, the photodegradation of PCB-1 in sunlit waters was simulated using the kinetic model APEX (Aqueous Photochemistry of Environmentally Occurring Xenobiotics). According to simulations, a greater influence of the water depth and dissolved organic carbon concentration (DOC) on the persistence of PCB-1 is expected, being only slightly influenced by the concentrations of nitrite, nitrate, and bicarbonate. Finally, based on data reported for Brazilian surface waters, the average half-life (t1/2) of PCB-1 is expected to vary from 2 to 14 days. In particular, the t1/2 in the Paranapanema River is estimated at 7 to 8 days.
