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Sulfamethoxypyridazine (SMP) is one of the commonly used sulfonamide antibiotics (SAs).SAs are mainly studied to undergo triplet-sensitized photodegradation in water under natural sunlight with other coexisting aquatic environmental organic pollutants.In this work,SMP was selected as a representative of SAs.We studied the mechanisms of triplet-sensitized photodegradation of SMP and the influence of selected dissolved inorganic matter,i.e.,anions (Br-,Cl-,and NO3-) and cations ions (Ca2+,Mg2+,and Zn2+) on SMP photodegradation mechanism by quantum chemical methods.In addition,the degradation mechanisms of SMP by hydroxyl radical (OH’) were also investigated.The creation of SO2 extrusion product was accessed with two different energy pathways (pathway-1 and pathway-2) by following two steps (step-Ⅰ and step-Ⅱ) in the tripletsensitized photodegradation of SMP.Due to low activation energy,the pathway-1 was considered as the main pathway to obtain SO2 extrusion product.Step-Ⅱ of pathway-1 was measured to be the rate-limiting step (RLS) of SMP photodegradation mechanism and the effect of the selected anions and cations was estimated for this step.All selected anions and cations promoted photodegradation of SMP by dropping the activation energy of pathway-1.The estimated low activation energies of different degradation pathways of SMP with OH* radical indicate that OH* radical is a very powerful oxidizing agent for SMP degradation via attack through benzene derivative and pyridazine derivative ring.