Degradation Mechanism of Benzo[a]pyrene Initiated by the OH Radical and (1)O(2): An Insight from Density Functional Theory Calculations

[Image: see text] The degradation mechanism of benzo[a]pyrene (BaP) initiated by (•)OH and (1)O(2) in aqueous solution is investigated by density functional theory calculations. The main degradation products are BaP-1,6-quinone, BaP-3,6-quinone, BaP-4,6-quinone, and BaP-6,12-quinone. (•)OH and HO(2) are the main intermediate radical species. At a low initial concentration of (•)OH, (1)O(2) could be a primary driver for BaP degradation. The degradation mechanism includes six consecutive elementary reactions: (1) (1)O(2) initiation forming BaP-6-OO. (2) 1,3 H-shift (H atom shifts to the OO group) that is promoted by H(2)O, forming BaP-6-OOH. (3) BaP-6-OOH decomposes into the (•)OH radical and BaP-6-O. (4) (•)OH addition to BaP-6-O forming BaP-6-O-1(3,4,12)-OH. (5) Extracting the H atom from the carbon with the OH group by (1)O(2). (6) Extracting the H atom from the OH group by HO(2). At a high initial concentration of (•)OH, the (•)OH-initiated and (1)O(2)-initiated degradation reactions of BaP are both feasible. The degradation mechanism includes six consecutive elementary reactions: (1) (•)OH initiation forming BaP-6-OH or (1)O(2) initiation forming BaP-6-OO. (2) (1)O(2) addition to BaP-6-OH forming BaP-6-OH-12(1,3,4)-OO or (•)OH addition to BaP-6-OO forming BaP-6-OO-12(1,3,4)-OH. (3) Extracting the H atom from the carbon with the OH group by (1)O(2), forming HO(2). (4) 1,3 H-shift (H-shift from the carbon to the OO group), promoted by H(2)O. (5) The loss of the OH radical. (6) Abstracting the H atom from the OH group by HO(2). In this paper, the formation of BaP-4,6-quinone via the BaP degradation is first reported. Water participates in the elementary reaction in which the H atom attached on the aromatic ring shifts to the OO group, serving as a bridge that stabilizes the transition state and transports the proton. A comprehensive investigation explains the degradation mechanism of BaP initiated by (•)OH and (1)O(2) in aqueous solution.