Preparation Of Surface-Co(Ⅱ)OH-Rich Catalysts And Uses For The Degradation Of Phenolic Pollutants

Advanced oxidation processes based on cobalt-activated permonosulfate（Co-PMS）exhibit a good application prospect in the field of organic pollutants.Since Co（Ⅱ）OH has a strong affinity for PMS and can promote its efficient decomposition,the construction of a Co（Ⅱ）OH-rich surface is the key to the preparation of a highly active heterogeneous cobalt-based catalyst.Co（OH）₂ has abundant Co（Ⅱ）OH on the surface,which has been used for the activation of PMS,but it is easily oxidized to Co（OH）₃ and deactivated.Part of OH^-in Co（OH）₂ is replaced by Cl^-or CO₃^2-,and resulting in the surface of Co（Ⅱ）OH structure and redox properties of cobalt hydroxychloride（Co₂（OH）₃Cl）and cobalt hydroxycarbonate（Co₂（OH）₂CO₃）have changed,which may have the potential to activate PMS,but have not yet attracted attention.In addition,the use of carbon materials with large specific surface area and good electrical conductivity as the support of cobalt-based catalysts can not only enhance the adsorption of organic pollutants,but also improve the dispersion of cobalt and the circulation of Co（Ⅱ）/Co（Ⅲ）.For this reason,this paper firstly studied the performance of Co₂（OH）₃Cl and Co₂（OH）₂CO₃ in activating PMS to degrade phenolic pollutants,and then using carbon nanotubes as a support to improve the activity of the catalysts.The main results obtained are as follows:（1）Co₂（OH）₃Cl nanoplates were prepared from CoCl₂ and propylene oxide by a precipitation method and used to activate PMS for degradation of tetrabromobisphenol A（TBBPA）.When the concentration of Co₂（OH）₃Cl and PMS were both 0.1 g L^-1,TBBPA(the initial concentration was 40 mg L^-1)could be completely degraded in 2 min.The pseudo-first-order rate constant k of TBBPA degradation in this system was 2.5 min^-1 to be 9,14,and 6 folds larger than those forα-Co（OH）₂,β-Co（OH）₂ and Co₃O₄systems,respectively.The maximum dissolved Co²⁺(0.15 mg L^-1)corresponded to 0.24%of the total cobalt in Co₂（OH）₃Cl,which made almost no contribution to the degradation of TBPPA.In addition,Co₂（OH）₃Cl showed long-term stability（longer than 9 months）with the high activity after the oxidation treatment.EPR and quenching experiments confirmed that ¹O₂was the main active species involved in TBBPA degradation.The high activity of Co₂（OH）₃Cl attributed to the coordinated Cl^-not only enhanced the ability of surface Co（Ⅱ）OH for anchoring both HSO₅^-and SO₅^2-through the intermolecular hydrogen bond,but also improved electron transfer between Co（Ⅱ）and PMS.（2）CNT@Co₂（OH）₂CO₃ composite catalyst was prepared by hydrothermal method with oxidized carbon nanotubes（CNT）,Co（NO₃）₂ and urea as raw materials,which has high ability of activating PMS to degrade bisphenol A（BPA）.In the presence of 0.1 g L^-1PMS,CNT@Co₂（OH）₂CO₃(0.05 g L^-1)could completely degrade BPA(the initial concentration was 20 mg L^-1)within 2 min,and the mineralization efficiency was about50%.The degradation rate constant k of BPA in the CNT@Co₂（OH）₂CO₃/PMS system was 2.7 min^-1,which was 6 and 23 folds that of the Co₂（OH）₂CO₃/PMS and CNT/PMS systems.The rate constant of per-mass cobalt activation of PMS degradation of BPA in CNT@Co₂（OH）₂CO₃ system was about 20 min^-1 mg^-1,which was 44 folds that of Co₂（OH）₂CO₃ system,indicating that the utilization efficiency of Co²⁺in Co₂（OH）₂CO₃was significantly improved by using CNT as the support.EPR and quenching experiments showed that·OH,SO₄^·-,O₂^·-and ¹O₂ both participated in the degradation of BPA in the CNT@Co₂（OH）₂CO₃/PMS system.In addition,CNT@Co₂（OH）₂CO₃has better reusability.The possible reason was that the electron-rich characteristics of the support CNT were conducive to the regeneration of Co（Ⅱ）on the surface of Co₂（OH）₂CO₃.