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

• Triplet-sensitized photolysis mechanism of SDZ was electron transfer or H transfer.
• SDZ0 and SDZ− showed different triplet-sensitized photolysis routs.
• Presence of Mg2+, Ca2+, or Zn2+ promoted triplet-sensitized photolysis of SDZ0.
• Presence of Mg2+, Ca2+, or Zn2+ inhibited triplet-sensitized photolysis of SDZ−.

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 SDZ0 (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., Mg2+, Ca2+, and Zn2+) effects using quantum chemical calculations. Results indicated that the triplet-sensitized photolysis mechanism of SDZ0 with FL, TX, and 2-AN was hydrogen transfer, and with CBBP was electron transfer along with proton transfer (for complex SDZ0-CBBP2) and hydrogen transfer (for complex SDZ0-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 SDZ0 and SDZ− was a sulfur dioxide extrusion product (4-(2-iminopyrimidine-1(2H)-yl)aniline), but the formation routs of the products for SDZ0 and SDZ− were different. In addition, effects of the metal ions on the triplet-sensitized photolysis of SDZ0 and SDZ− were different. The metal ions promoted the triplet-sensitized photolysis of SDZ0, but inhibited the triplet-sensitized photolysis of SDZ−.