Elucidating triplet-sensitized photolysis mechanisms of sulfadiazine and metal ions effects by quantum chemical calculations.

Sulfadiazine (SDZ) mainly proceeds triplet-sensitized photolysis with dissolved organic matter (DOM) in the aquatic environment. However, the mechanisms underlying the triplet-sensitized photolysis of SDZ with DOM have not been fully worked out. In this study, we investigated the mechanisms of triplet-sensitized photolysis of SDZ(0) (neutral form) and SDZ(-) (anionic form) with four DOM analogues, i.e., fluorenone (FL), thioxanthone (TX), 2-acetonaphthone (2-AN), and 4-benzoylbenzoic acid (CBBP), and three metal ions (i.e., Mg(2+), Ca(2+), and Zn(2+)) effects using quantum chemical calculations. Results indicated that the triplet-sensitized photolysis mechanism of SDZ(0) with FL, TX, and 2-AN was hydrogen transfer, and with CBBP was electron transfer along with proton transfer (for complex SDZ(0)-CBBP2) and hydrogen transfer (for complex SDZ(0)-CBBP1). The triplet-sensitized photolysis mechanisms of SDZ(-) with FL, TX, and CBBP was electron transfer along with proton transfer, and with 2-AN was hydrogen transfer. The triplet-sensitized photolysis product of both SDZ(0) and SDZ(-) was a sulfur dioxide extrusion product (4-(2-iminopyrimidine-1(2H)-yl)aniline), but the formation routs of the products for SDZ(0) and SDZ(-) were different. In addition, effects of the metal ions on the triplet-sensitized photolysis of SDZ(0) and SDZ(-) were different. The metal ions promoted the triplet-sensitized photolysis of SDZ(0), but inhibited the triplet-sensitized photolysis of SDZ(-).