Removal Performance And Mechanism Of Odorous Ethanethiol In An Alkali Absorption/Electrochemical Oxidation System

Odorous pollutants have gained increasing attention due to their corrosive effect on environment as well as the detrimental effect on human health.Ethanethiol（C₂H₅SH）,a typical volatile organic sulfur compound,is characterized by its offensive odor that can be detected by human at an extremely low concentration.It is an important pesticide intermediate,as well as a component of petroleum and natural gas.The C₂H₅SH odorous gas emitted from pesticide production and petroleum processing exhibits the chracteristics of high pollutant concentration,high concentration fluctuation and discontinuous discharge.At present,developing high-efficient and cost-effevtive treatment method for such malodors is still an urgent problem to be solved.This study developed an alkali absorption/electrochemical oxidation system to remove C₂H₅SH odorous gas,using cast iron,modified GF and NaOH solution as the anode,cathode and electrolyte,respectively.In this system,the C₂H₅SH odorous gas was first absorbed in alkaline electrolyte,and then be removed by the combination of anodic oxidation and cathodic indirect oxidation,achieving the simultaneous treatment of both C₂H₅SH odorous gas and absorbent.This study provides a technology for the high-efficient treatment of mercaptan malodors.The following studies were carried out in this paper.The electrogenerated ferrate（Ⅵ）（Fe（Ⅵ））from cast iron anode could be in-situ utilized to oxidized C₂H₅SH in Na OH electrolyte.The electrochemical generation of Fe（Ⅵ）was first investigated.The results indicated that the accumulated amount of Fe（Ⅵ）in electrolyte depended on both the generation of Fe（Ⅵ）from anode and the self-decomposition of Fe（Ⅵ）.The reaction mechanisum of C₂H₅SH with Fe（Ⅵ）under alkaline condition was then investigated.The results indicated that the reaction kinetics and fraction of different intermediates were highly dependent on pH.The reaction behaved as a 5/2-order reaction at pH=8.0～10.0,and a second-order reaction at pH=10.5～12.0.The apparent rate constants decreased with increased pH.At pH=9.2,sulfonic acid（C₂H₅SO₃H）was the main oxidation product of C₂H₅SH,while sulfinic acid（C₂H₅SO₂H）was the main product at pH=12.0.A plausible mechanism involving two competing reactions for the intermediates was proposed to explain the pH-dependent reaction order.After that,the cast iron was used as the anode to oxidize C₂H₅SH in electrolyte,and the efficiency and mechanism for C₂H₅SH removal were investigated.The oxidation of C₂H₅SH was the joint effort of both oxidations on anode surface and by in-situ electrogenerated Fe（Ⅵ）,and C₂H₅SH was finally oxidized to C₂H₅SO₃H.The oxidation of C₂H₅SH on anode surface dominated in Na OH solution of low concentration,while the contribution of in-situ electrogenerated Fe（Ⅵ）to C₂H₅SH oxidation was increased as the increasing NaOH concentration.In 2～10molL^-1 NaOH solution,the optimum conditions for C₂H₅SH removal were 2molL^-1 NaOH and 6mAcm^-2.To obtain a higher removal efficiency of C₂H₅SH,H₂O₂ was electrogenerated through oxygen reduction reaction（ORR）on cathode and utilized in-situ using odorous gas as the air source.The graphite felt（GF）which was electrochemically modified in H₂SO₄ was used as the cathode.The surface structure and electrochemical properties of modified GFs were characterized,and the ORR behavior in Na OH solution was detailed studied.The results indicated that the atomic oxygen content of GF was increased from 15.3% to 28.3%and above after modification.The wettability and ORR electrocatalytic activity were significantly improved due to the introduction of abundant oxygen-containing functional groups.The optimum conditions for H₂O₂ electrogeneration were 2molL^-1 NaOH and 6mAcm^-2,and 257.8mgL^-1 H₂O₂ solution could be obtained after 120 min electrolysis.In addition,the ORR behavior showed Na OH-concentration-dependent characteristic,which transformed from 2e reduction pathway to 4e pathway as the increased Na OH concentration.The alkali absorption/electro-oxidation system was developed to treat C₂H₅SH odorous gas using cast iron,modified GF and Na OH solution as the anode,cathode and electrolyte,respectively.The removal of C₂H₅SH in electrolyte as well as in gas phase were investigated.The results indicated that the absorbed C2H5SH in electrolyte was oxidized to C₂H₅SSC₂H₅ and C₂H₅SO₃H and finally oxidized to C₂H₅SO₃H by anodic oxidation and various in-situ generated reactive oxygen species including Fe（Ⅵ）,H₂O₂ and superoxide.Under optimum conditions,more than 99% of the gaseous C₂H₅SH(≤600mLmin^-1,≤277mgm^-3)was removed with this system.This system achieved the continuous on-line treatment of C₂H₅SH odorous gas.