Study On The Degradation Kinetics And Mechanism Of Sulfasalazine And The Synergist Trimethoprim By Non-activated Peroxymonosulfate

Sulfonamide antibiotics(SAs)are produced and consumed largely in China.They are widely applied in the medical and breeding industries.SAs have been detected in multiple water bodies due to their enormous applications.It is well-known that SAs and their metabolites might exhibit toxic effects on aquatic organisms through bioaccumulation.Traditional treatment processes on wastewater are not capable of removing antibiotics at a relatively low concentration,leading to their release.Activated peroxymonosulfate(PMS)has become a promising technology to remove SAs,while the background reaction,namely,the natural oxidation process of PMS is always overlooked.Thus,it is of great importance to study the oxidation capability and mechanism of PMS between the target contaminates.In this research,sulfasalazine(SSZ)and trimethoprim(TMP)were selected as the target pollutants degraded by non-activated PMS process in aqueous solution.The degradation kinetics of SSZ and TMP were investigated in detail.Radical quenching experiment and electron paramagnetic resonance(EPR)method were used to clarify the main active species which led the degradation of target pollutants.Based on mass spectrometry analysis and density functional theory(DFT)calculations,the degradation pathways and mechanisms of PMS direct oxidation towards TMP as well as SSZ were proposed.Above all,a comprehensive evaluation of the effectiveness and mechanism of PMS direct oxidation towards target pollutants in water was executed.The main research contents and conclusions were listed as follows.(1)The oxidation ability of PMS was clarified by conducting the degradation on SSZ and TMP by non-activated PMS.EPR detection and radical quenching experiment revealed that direct oxidation of PMS was responsible for the degradation rather than radicals.The kinetics experiment results showed that the degradation of SSZ increased by 37%when PMS concentration increased from 1 mmol·L-1 to 8 mmol·L-1.Basic condition was found to be beneficial to SSZ degradation.At pH 9.0,the second-order reaction rate constant between SSZ and PMS was estimated at 0.1822 L·(mol s)-1,which was 3.25 times higher than that at pH 5.0.In the case of TMP,the enhanced degradation efficiency of TMP would take place at a higher PMS level,and a linear relationship could be obtained for TMP degradation rate constant,which was linear versus PMS concentration.The second-order reaction rate constant was estimated at 0.043 L·(mol·s)-1.With pH rising from 5.0 to 9.0,the degradation rate of TMP increased by 28 times.Basic condition was also favorable for the oxidation.It was found that SO52-had a higher reactivity with TMP,while HTMP+ had almost no reactivity with PMS.Employed SSZ as the main pollutant,the degradation efficiency on water quality parameters was also evaluated.The results showed that the increase of ionic strength would enhance the SSZ degradation.When the ionic strength of system reached 233 mmol·L-1,the degradation of SSZ was 67%.Chloride ions could significantly promote the removal of SSZ.It could be observed that SSZ of 20μmol·L-1 was completely degraded in 30 minutes when chloride ions increased to 20 mmol·L-1.It is necessary to pay attention to the subsequent treatment of halogenated products.A realistic water sample from Zixia Lake was introduced into the reaction system,which was found to decline SSZ removal efficiency by 15%compared with purified water.In addition,dissolved oxygen was also found to play an important role for the removal on SSZ.(2)By frontier molecular orbital(DFT)calculation as well as kinetic experiments,the reactive sites were determined by two SSZ structural analogues.Results suggested that the hydroxybenzoic acid group and sulfonamide group were the reactive sites.Attacked by PMS,the hydroxybenzoic acid group and the sulfonamide group would undergo hydroxylation and SO2 extrusion reaction respectively to generate SSZ+OH,SSZ-SO2 and SSZ-SO2+OH.In the case of TMP,the two amine groups on pyrimidine ring of TMP were verified as the reactive sites.The oxidation of the amine groups would lead to the generation of hydroxylamine products labelled as N8-OH-TMP and N9-OH-TMP.The DFT calculation results showed that water molecules would participate in the reaction to lower the energy barrier.The hydroxyl group would be transferred from PMS to the amino group to form an intermediate with an N-OH bond,which underwent rearrangement and electron transfer to generate the products N8-OH-TMP and N9-OH-TMP.In addition,the energy barrier for N8-OH-TMP and N9-OH-TMP to form toxic nitroso-products were 19.0 and 20.0 kcal·mol-1,which were beyond the capability of PMS oxidation.(3)ECOSAR predictive model calculation indicated that with the application of non-activated PMS treatment,all the oxidation products of SSZ and TMP were less toxic than the parent compound.