Performance And Mechanism Of Cobalt Based Catalysts Activiting Peroxymonosulfate For Organic Pollutant Control

Advanced oxidation processes have been developing due to the increasing need for wastewater advanced treatment,especially the removal of organic pollutant s.Peroxymonosulfate（PMS）is environmental-friendly,economical and easy to reserve and transport,and thus is generally investigated for advanced oxidation technology.The activation of PMS can produce sulfate radical(SO₄^·-)and other oxidant active species,or else oxidize pollutants through non-radical pathway.There are different kinds of methods to activate PMS.The usage of transition metal is easy to operate and requires no energy input;meanwhile,the reaction between PMS and transition metal is often rapid,among which cobalt shows the best performance.To reduce the toxicity risk of Co²⁺ions,heterogeneous cobalt materials are prepared via precise design and synthesis for PMS activation.The physicochemical properties of cobalt materials are decided by construction of surface morphology and active sites,and composition of the coating and support.These factors directly influence the performance and mechanism of PMS activation.Ever since the introduction of conventional cobalt oxides,a variety of cobalt materials with high catalysis activity have been developed.So far,the main constraint on the application of PMS activation by cobalt material activation is cobalt ion leaching.Simultaneous improvement on the catalytic activity,stability and recycle of cobalt material is required.This study investigated the catalytic oxidation performances and mechanisms of cobalt-based spinel sulfide,single-atom catalyst and ceramic membrane under the interaction of elements.The intrinsic characteristics of structure and morphology were analyzed,and the reaction features of these cobalt materials were addressed.The naturally existent but low earth content spinel carrollite（Cu Co₂S₄）was chosen in this study first.The optimized preparation conditions to catalytic activity and characteristics of obtained Cu Co₂S₄ were investigated.Copper and cobalt uniformly disperse on the surface of hierarchical structure composed of nanosheets,and their content ratio is close to the theoretical value.During the experiment of bisphenol S（BPS）degradation by Cu Co₂S₄/PMS,when the dosage of PMS:BPS was10:1 at the near-neutral p H,BPS was almost completely degraded in 30 min.By the comparison with other copper and/or cobalt oxide/sulfide for degradation of BPS during PMS activation,pure Cu Co₂S₄ was proved of both high catalytic activity and low Co²⁺leaching.The kinetics model of BPS degradation in Cu Co₂S₄/PMS system was built,and the activation energy was calculated.The dominant active species was identified to be SO₄^·-according to radical scavenging experiment,ESR experiment and transformation of the probe organic compound.Because of the chemical properties of Cu Co₂S₄,PMS and SO₄^·-oxidation,BPS has highest degradation rate in Cu Co₂S₄/PMS system at weak acidic to neutral p H.The reaction was hardly affected by Cl^-,but was significantly inhibited as the concentration of coexistent HCO₃^-increased.BPS was mainly degraded through hydrogen abstraction,β-scission,hydroxylation and bimolecular combination.BPS degradation remained consistent during five continuous recycle experiments of Cu Co₂S₄.Materials based on non-metal substrate possess some physicochemical properties beyond metal materials.For example,non-metal skeleton can guarantee the high dispersity and stability of metal dopant;correspondingly,metal elements break the inert electric environment of the non-metal skeleton and enhance its catalytic activity.This study further investigated the performance and mechanism of single-cobalt-atom catalyst（Co-N-C）activating PMS,and broke the limitation of the study conclusion that Co-N-C/PMS generates singlet oxygen for pollutant degradation.This study found that in Co-N-C/PMS system,the sensitivity of degradation towards radical scavenger is pollutant-dependent.Co-N-C/PMS not only has higher pollutant degradation efficiency than Co₃O₄/PMS,the impact by common inorganic anions on Co-N-C/PMS is also weaker.These suggest the dual PMS activation mechanism of radical and non-radical in Co-N-C/PMS system,and the non-radical oxidation is selective about organic pollutants with p henolic hydroxyl.Furthermore,the non-radical oxidation pathway shows significant difference from singlet oxygen oxidation,which is possibly electron transfer by the skeleton.Sulfate radical is generated on the cobalt active sites,while the electron transfer ability of N-C skeleton is largely improved by single cobalt atoms.The dual activation mechanisms integrate the advantage of radical and non-radical oxidation,guaranteeing the rapid degradation of pollutants and efficient utilization of PMS.The Co-N-C/PMS utilizes 100%oxidation capacity of PMS for 2,4-dichlorophenol degradation.During the continuous flow experiment,mixed pollutants were efficiently degraded by Co-N-C/PMS while the Co-N-C kept stable.This study analyzed the generation and transformation ofα,β-UAs during the degradation of typical phenols in cobalt homogeneous and heterogeneous activating PMS systems for the first time.Five differentα,β-UAs in total were detected in Co²⁺/PMS,Cu Co₂S₄/PMS and Co-N-C/PMS systems,including butylene aldehyde,chlorinated butylene aldehyde,pentene dialdehyde,hydroxylated pentene dialdehyde and hydroxylated hexene dialdehyde.However,the kind and yield ratio of specificα,β-UAs were not exactly the same;they depended on the PMS activation mechanism,oxidation degree and structure of the phenols in the reaction systems.Evaluated by the yield ofα,β-UAs,the two heterogeneous cobalt activation PMS systems were found to reduce the health risk of PMS activation by Co²⁺.Having acknowledged the advantage of dual PMS activation mechanism of single-atom catalyst,cobalt was doped onto ceramic membrane based on the synthesis method of the single-atom catalyst.The membrane loaded with highly-dispersed cobalt achieved simultaneous filtration and PMS catalytic oxidation.The characterizations show that the cobalt was doped on the ceramic membrane（Co-CM）as amorphous nanoparticles.Similar to Co-N-C,high-spin octahedral Co（II）took a major proportion,and the surface cobalt was uniformly dispersed at the atom content of 1.32%.Different organics were degraded in the oxidative filtration process and jar test on table concentrator for comparison.Active species generated by Co-CM/PMS were identified by radical scavenger,ESR experiment and transformation product of the probe compound.The reaction between surface amorphous Co（II）and PMS firstly produces a selective non-radical species,i.e.≡Co（II）-PMS complex,as the primary oxidant,and the complex further decompose into SO₄^·-.The generation and transformation of active species is influenced by hydraulic conditions and solution p H.The selectivity of≡Co（II）-PMS complex makes the degradation of certain pollutant not impacted by common anions and natural organic matters in real surface water.During the continuous oxidative filtration of surface water containing 2,4-dichlorophenol,2,4-dichlorophenol was nearly completely removed,and the decrease of permeate flux was more slowly than filtration by Co-CM without PMS.The activation of PMS also promoted the removal of TOC in surface water by Co-CM filtration.