Computational Studies On Photochemical Behavior Of Sulfonamide Antibiotics In Water

Sulfonamide antibiotics?SAs?are one of the most successful class of antibiotics,which are widely used in aquaculture,in livestock production,and to treat respiratory and urinary tract infections in human.The annual production and consumption of SAs are increasing steadily.Due to their overuse,SAs are largely released into and frequently detected in the natural water environment.The degradation of SAs in environmental water has received intensive attention because of their potential adverse effects on the environmental ecosystem and on the aquatic organisms.SAs are mainly observed to undergo biotic and abiotic degradation pathways,in which photodegradation is an important abiotic pathway and can be affected by many other factors in the environmental water.However,mechanisms of SAs underlying photodegradation are still uncertain.In this study,the photochemical reaction mechanisms of three representatives SAs pollutants in the aquatic environment were studied by using computational chemistry methods.Some important contents and results are as follows:?1?The triplet-sensitized direct photodegradation mechanisms of sulfachloropyridazine?SCP?and the influence of three selected metal ions(Ca²⁺,Zn²⁺,and Cu²⁺)on SCP photodegradation mechanism were studied.We found two reaction pathways for direct photodegradation of SCP and the lowest energy pathway was determined as the main pathway according to the calculated activation energies.The sulfur dioxide extrusion product 4-?3-chloro-6-iminopyridazine-1?6H?-yl?aniline was the main photoproduct of SCP.The influence of metal ions on the rate-determining step?RDS?of the main pathway was investigated.The studied metal ions promoted the triplet-sensitized photodegradation of SCP by reducing the activation energy of RDS of the main pathway.Additionally,degradation mechanisms of SCP by hydroxyl radical ·OH)radical were investigated.Low energy pathways were computed leading to the formation different products.?2?The triplet-sensitized direct photodegradation mechanisms of sulfamethoxypridazine?SMP?and the effects of selected dissol-ved inorganic matters,i.e.,anions?Br-,Cl-,and NO3-?and cations(Ca²⁺,Mg²⁺,and Zn²⁺)on SMP photodegradation mechanisms were studied.The formation of sulfur dioxide extrusion product was accessed with two different reaction pathways in SMP photodegradation.The lowest energy pathway was determined according to the computed activation energies and the effects of selected anions and cations were investigated on the RDS of the main pathway.The selected anions and cations promoted photodegradation of SMP by dropping the activation energy of the main pathway.In the study of the degradation mechanisms of SMP by ·OH radicals,different products were also predicted with low energy pathways.Results indicated that the ·OH radical can eliminate SMP effectively.?3?The triplet-sensitized direct and indirect photodegradation mechanisms of sulfameter?SME?with excited triplet states of dissolved organic matter?3DOM*?were elucidated.Both the neutral?SME0?and the anionic?SME-?forms of SME were considered for photodegradation,and the effect of the selected metal ions(Ca²⁺,Mg²⁺,and Zn²⁺)was estimated for both the forms.Formation of sulfur dioxide extrusion product was studied with different energy paths in photodegradation of SME0 and SME-,and the effect of metal ions on the RDS was estimated.The studied metal ions promoted the triplet-sensitized photodegradation of SME0,but these metal ions showed an inhibitory effect in the triplet-sensitized photodegradation of SME-.The triplet-sensitized indirect photodegradation mechanisms of SME were investigated with three selected DOMs analogues i.e.,2-acetonaphthone?2-AN?,fluorenone?FN?,and thioxanthone?TN?.Results revealed that the selected DOMs analogues are highly responsible for photodegradation via attacking on the amine moiety of SME.According to the natural bond orbital?NBO?analysis,the triplet-sensitized photodegradation mechanism of SME0 with 2-AN,FN and TN was hydrogen-transfer,and the SME-was proton plus electron transfer with these DOM analogues.