Improved Removal Of Sulfamethoxazole And Cr（Ⅵ） By The Addition Of MoS₂ Into The Fe²⁺/PMS Process

Peroxymonosulfate（PMS）can be activated by ferrous ion(Fe²⁺)to effectively degrade organic contaminants in aqueous solution,but the reduction of ferric ion(Fe³⁺)by PMS to Fe²⁺is of low efficiency.As a result,a large amount of Fe²⁺is essential for degradation of organic compounds in Fe²⁺/PMS process,limiting its application in water treatment.To accelerate the slow transformation from Fe³⁺to Fe²⁺,molybdenum sulfide（MoS₂）was added to Fe²⁺/PMS process as a co-catalyst.And the removal efficiency and mechanism of SMX degradation or simultaneous reduction of Cr（Ⅵ）and degradation of SMX in MoS₂/Fe²⁺/PMS process was studied.The main conclusions are as follows:（1）MoS₂ significantly improved the degradation efficiency of SMX in Fe²⁺/PMS process,and the degradation rate of SMX within 6 min in MoS₂/Fe²⁺/PMS process was 51.8%higher than that of Fe²⁺/PMS process.In this process,the degradation efficiency of SMX reached the optimal reaction condition at the pH of 3（88.5%）,when[MoS₂]₀=0.3 g/L,[Fe²⁺]₀=70μmol/L,[PMS]₀=75μmol/L,[SMX]₀=25μmol/L.The degradation rate of SMX increased with the elevation of MoS₂ or PMS concentration.When Fe²⁺/MoS₂≤1:20,the degradation rate of SMX increased with the increase in Fe²⁺dosages.However,as the ratio Fe²⁺/MoS₂>1:20,the increase of Fe²⁺concentration inhibited SMX degradation.And an equation was simulated to express the relationship between the degradation rate of SMX and the concentration of Fe²⁺,MoS₂ and PMS.Chloride ion and humic acid showed promoted and inhibited effects on the degradation process,while nitrate and bicarbonate ions had no significant influences on the degradation of SMX.The degradation of SMX in real water matrices also had good efficiency（>80%）.And the stability of MoS₂ in structure and as a co-catalyst in MoS₂/Fe²⁺/PMS process remained good after reused for 6 times.（2）In the MoS₂/Fe²⁺/PMS process,MoS₂ accelerated the PMS activation and Fe²⁺/Fe³⁺cycle.It is speculated that MoS₂ significantly accelerated the Fe²⁺/Fe³⁺cycle through the reaction of Mo⁴⁺site and Fe³⁺.The analysis of quenching experiments and electron paramagnetic resonance（EPR）have proved that both sulfate radicals and hydroxyl radicals are the dominant reactive species for the degradation of SMX in the MoS₂/Fe²⁺/PMS process.Based on the 8 intermediates identified by LC/MS,the degradation pathway of SMX in the MoS₂/Fe²⁺/PMS process was proposed mainly to be S-N bond cleavage,aromatic ring hydroxylation and amine oxidation.（3）Moreover,MoS₂/Fe²⁺/PMS process can effectively be applied to simultaneous reduction of Cr（Ⅵ）and degradation of SMX.The removal rate of SMX and Cr（Ⅵ）was98.8%and 99.2%within 30 min at[MoS₂]₀=0.9 g/L,[Fe²⁺]₀=0.3 mmol/L,[PMS]₀=1mmol/L,[Cr（Ⅵ）]₀=100μmol/L,[SMX]₀=25μmol/L,and initial pH of 3.0.The reduction rate of Cr（Ⅵ）increased with the elevation of MoS₂ or Fe²⁺concentration,while excessive PMS inhibited the removal of Cr（Ⅵ）.Chloride ion and humic acid had negative effects on the Cr（Ⅵ）reduction.The Fe²⁺generated through the reaction of Mo⁴⁺and Fe³⁺have been proven to account for the removal of Cr（Ⅵ）in the MoS₂/Fe²⁺/PMS process,the Cr（Ⅵ）can also be directly removed by MoS₂ or PMS.