Cobalt-doped Hydroxyapatite For Effective Activating Peroxymonosulfate To Degrade Rhodamine B

Rhodamine B（Rh B）is a ubiquitous synthetic organic dye,sought after for its outstanding dyeing attributes and unwavering stability,which have earned it a place of prominence in the textile and paper industries.However,the adverse effects of Rh B on the natural environment and human health demand that appropriate measures be taken to treat industrial wastewater contaminated with Rh B.Advanced oxidation processes based on persulfate have the potential to yield a plethora of highly oxidative reactive species,including the hydroxyl radical（·OH）,sulfate radical（SO₄·^-）,superoxide anion radical（·O₂^-）,singlet oxygen（¹O₂）,and others,that can be effectively employed to degrade organic dyes with efficiency.Cobalt,as a transition metal,displays exceptional activation capabilities for persulfate.Nonetheless,it has a high specific surface energy and is prone to agglomeration during the catalytic reaction,leading to a decline in catalytic efficiency.In order to curtail agglomeration,cobalt is typically loaded onto a carrier with a large specific surface area and strong stability.Hydroxyapatite,with its remarkable physical and chemical stability,high thermal stability,substantial specific surface area,and low solubility,proves to be an excellent choice of carrier for cobalt.The incorporation of cobalt onto a hydroxyapatite carrier effectively curbs agglomeration and enhances the catalytic activity of the hydroxyapatite.Furthermore,this cobalt-based catalyst supported on hydroxyapatite exhibits remarkable activity,recyclability,and other benefits.In this study,Co-doped hydroxyapatite（A-Co/HAP）was successfully prepared by an ion exchange method in which Co²⁺replaces Ca²⁺in the HAP structure.The aim of this study is to investigate the efficiency of A-Co/HAP10/PMS system in degrading Rh B in wastewater,while considering the influence of different factors on degradation efficiency.Additionally,the study examines the effect of Co doping amount on the physicochemical properties and activation effect of the catalyst.The stability of the catalyst is evaluated by monitoring cobalt leaching concentration and conducting cyclic degradation experiments.Furthermore,to improve the stability of Co-doped hydroxyapatite,H-Co/HAP is synthesized via one-step hydrothermal synthesis with diammonium hydrogen phosphate,calcium chloride,cobalt nitrate,and ammonia water as raw materials.The catalytic performance of H-Co/HAP is assessed by adjusting the Co doping amount and the calcium to phosphorus ratio.The following research contents and conclusions are presented in this study:（1）A-Co/HAP was found to effectively activate PMS for the degradation of Rh B.The degradation efficiency of Rh B increased with the increasing amount of Co doping,and A-Co/HAP10 showed the highest degradation efficiency and rate constant k,which were97.78%and 0.426 min^-1,respectively.In the A-Co/HAP10/PMS system,p H had a significant impact on the catalytic efficiency and stability of the catalyst.Strong acidic and alkaline conditions were not favorable for the catalytic reaction,and the optimal p H value was found to be 7,at which the efficiency and stability were optimal.The cobalt leaching concentration in the reaction was 0.45 mg/L,which complied with the national standard of 1 mg/L（GB25467-2010）.The quenching experiments and EPR test results indicated that the main reactive species in the system were ¹O₂ and SO₄·^-,while the secondary reactive species were·O₂^-.The degradation process mainly involves N-de-ethylation,carboxylation,de-carboxylation,de-amination,chromophore cleavage,ring-opening,and mineralization of Rh B,ultimately transforming it into H₂O、CO₂、NO₃^-、NH₄⁺,and other products.（2）Compared to A-Co/HAP,H-Co/HAP exhibits higher catalytic activity and stronger stability.Among various H-Co/HAP catalysts,H-Co/HAP10 demonstrates the best efficiency and stability in degrading Rh B,achieving complete degradation for up to eight cycles with a removal rate of 81.96%for the eighth cycle.However,increasing the doping amount of Co to20 at%leads to a decrease in both degradation rate and stability due to excessive Co doping that affects the structure of hydroxyapatite,leading to decreased stability.During the preparation process,the optimal calcium-phosphate ratio is found to be 5:3,and both calcium-poor and calcium-rich environments have an impact on the stability of H-Co/HAP10.For the degradation reaction,the optimal catalyst dosage is 0.2 g/L,and the PMS dosage is0.5 m M,with a p H of 7,and 50 mg/L humic acid having minimal impact on degradation.According to quenching experiments and EPR testing results,two oxidation pathways,namely non-radical oxidation and radical oxidation,coexist in the system,with ¹O₂ being the main active species,and SO₄·^-and·OH as minor active species.Coexisting ions such as Cl^-,NO₃^-,and H₂PO₄^-have little effect on the degradation efficiency of Rh B,while HCO₃^-can clear·OH and SO₄·^-,resulting in a significant inhibitory effect on the degradation of Rh B.Based on the LC-MS results,a possible degradation pathway was proposed where Rh B undergoes N-de-ethylation,de-carboxylation,dea-mination,chromophore cleavage,ring-opening,and mineralization to form smaller molecular compounds,and finally were completely mineralized to H₂O,CO₂,and inorganic salt ions such as NO₃^-and NH₄⁺.