Mechanisms Of Peroxymonosulfate Activation On Nickel Oxyhydroxide-based Composites For Sulfadiazine Degradation In Water

In recent years,the abuse of PPCP（Pharmaceuticals and Personal Care Products）attracted our attention.Sulfadiazine（SD）,as one of typical broad-spectrum antibiotic,has been used as medicine in veterinary and human health field.Frequently,higher concentration of SD has been detected in worldwide water,which causes the potential threat to human health and ecosystem.However,SD can not be effectively removed via traditional water treatment technology,owing to its biological toxicity and accumulation.Thus,in this study,it was investigated that one of sulfate radical-based advanced oxidation processes（SR-AOPs）for SD degradation in water.Compared with energy-based activation（ultraviolet light and heat）,heterogeneous transition metal catalyst is simpler and more cost effective in practice.Among heterogeneous catalysts,nickel oxyhydroxide（NiOOH）is a transition metal oxyhydroxide formed by high-valent nickel ions and hydroxyl groups.It has highly hydroxylated surface with abundant active sites,resulting in great potential for catalysis application.Therefore,in this paper,the catalytic activity of NiOOH as well as the mechanism of PMS activation via NiOOH for SD degradation were investigated.Moreover,in order to further improve the catalytic activity and stability of the NiOOH,NiOOH was fixed on functionalized covalent organic frameworks（COFs）,and the NiOOH@COFs/PMS system was established.Fruthermore,the mechanism of NiOOH for PMS activation was investigated and the synergistic effect between COFs and NiOOH for PMS activation was revealed.Additionally,the mechanism of SD degradation via adsorption-catalyzed oxidation was elucidated.The results were shown as follows:In this paper,NiOOH was prepared by chemical precipitation.It possessed flowerlike microspheres,which were formed via nanosheets accumulation.It owned the specific surface area of 35.2 m²/g,which was beneficial for exposure of adsorbed and catalytic sites.NiOOH/PMS system reached reaction rate(0.0537 min^-1)for 10mg/L SD degradation under the optimized condition of 0.2 g/L NiOOH and[PMS]:[SD]=20:1.NiOOH/PMS system displayed the highest reaction rate among NiO/PMS system(0.0242 min^-1),Ni（OH）₂/PMS system(0.0216 min^-1)and Ni₂O₃/PMS system(0.0102 min^-1).Additionally,Ni ion leaching reached 0.51 mg/L within the reaction processes.（1）Aiming for further enhancing the catalytic activity and stability of NiOOH,functionalized COFs were mixed with NiOOH for preparation of NiOOH@COFs.The COFs were directly synthesized for adding multiple catalytic active sites（carbonyl functional group,edge nitrogen structure）via Schiff base and enol-ketone tautomerization reaction.After COFs played role as carriers of NiOOH,NiOOH@COFs displayed higher specific surface area（52.4 m²/g）,wider distribution of pore sizes and lower surface negative charge,leading to higher exposure of adsorption sites and catalytic sites.Thus,NiOOH@COFs/PMS system decreased 60min reaction time for SD degradation and the degradation efficiency of NiOOH@COFs/PMS system was 2.3 times higher than that of NiOOH/PMS system.Moreover,NiOOH@COFs reduced the Ni ion leaching by 72%.It displayed that NiOOH@COFs possessed higher catalytic performance and stability.（2）In this paper,the mechanism of NiOOH for PMS activation was investigated via XPS characterization,quenching experiments and ESR pattern analysis.It was owing to the cyclic redox reaction of Ni（III）/Ni（II）for generation of multiple active oxygen species（ROS）,such as·OH,SO₄·^-,O₂·^-and ¹O₂.Moreover,¹O₂ was identified as the dominated ROS.Meanwhile,the mechanism of synergistic effect between COFs and NiOOH for PMS activation was revealed.The added catalytic sites of COFs had contributions as follows:（1）Carbonyl functional group（-C=O）generated ¹O₂ via nucleophilic addition reaction with PMS;（2）The edge N structures provided Lewis basic sites,promoting the cleavage of PMS to produce·OH and SO₄·^-and enhancing the process of NiOOH activated PMS for O₂·^-as well as ¹O₂ generation;（3）The two adjacent carbon atoms of the edge N structures had complexation reaction with the O-O bond of PMS for forming NiOOH@COFs-PMS*surface activated complex species.NiOOH@COFs-PMS*played the role as electron transfer intermediate,enhancing the direct oxidation process of adsorbed SD molecules.Thus,COFs and NiOOH synergistically activated PMS,resulting in increasing of amount and generating rate for ROS.Furthermore,¹O₂ was the dominated ROS,thus,non-radical pathway played the major role in NiOOH@COFs/PMS system.（3）Integrated with the adsorption kinetics and the catalytic oxidation reaction mechanism,the mechanism of synergy effect between adsorption and catalytic oxidation was clarified.SD was adsorbed on the surface of NiOOH via hydrogen bonding,increasing the effective contact between SD and ROS on the surface of the catalyst,which further enhanced the catalytic oxidation reaction rate of SD.COFs played as the carrier material,it possessed uniform pore size and ordered structure,which was favor for increasing the adsorption capability of catalyst.It turned out that the adsorption quantity of SD on catalyst increased up to twice,promoting the reaction rate between SD and ROS.Moreover,SD was mainly degraded via four pathways,and intermediate products included p-aminobenzenesulfonic acid,2-aminopyrimidine and aniline.In summary,this study elucidated the construction and optimization of NiOOH-based catalytic oxidation system,providing the theoretical guidance for degradation of recalcitrant organic pollutant such as sulfadiazine in water.