Studies On The Activation Of Persulfate By Metal Oxides In The Oxidation Of Phenolic Contaminants Under Alkaline Conditions

In recent years,it has been extensively reported that activation of persulfate(PS)into SO₄^·-and·OH radicals by transition metal oxides under acidic and neutral conditions has been put into application for oxidizing phenolic contaminants.Although it is possible to nonselectively oxidize or even completely mineralize phenolics via hydroxylation and ring-opening pathways by virtue of such advanced oxidation processes on the basis of SO₄^·-and·OH radicals,the abilities of such oxidizing species for oxidation tend to decrease rapidly due to the ever-increasing p H values.Moreover,the oxidants required for the purpose of complete mineralization will rapidly increase in amount accordingly as the molecular weights increase and the carbon chains of substrates prolong,thereby greatly increasing the operational costs.Therefore,the thinking of turning wastes into“resource reutilization”,i.e.,searching for more cost-effective and efficient oxidizing systems to convert phenolic pollutants in the wastewater into polyphenol polymers,has become an important tendency in the treatment of wastewater containing phenolic pollutants.By virtue of kinetics,solid characterizations,and the density functional theory(DFT),in-depth studies have been conducted on the activation mechanisms for different metal oxides to activate the PS and explore the mechanisms for the oxidation of various phenolics under alkaline conditions.First of all,the catalytic oxidation of phenol by CuO and PS under alkaline conditions was investigated in the present studies.It has been found out that high concentrations of phenol can be rapidly removed in the CuO-PS system through oxidation under alkaline conditions with the low PS:phenol ratio of 0.904:1,indicating the highly efficient the CuO-PS system.Through quenching experiments and EPR analysis,contributions of SO₄^·-,·OH,O₂^·-and ¹O₂to phenol oxidation have been ruled out and it has been indicated that H-abstraction from phenol to phenoxyl radical is the primary mechanism for phenol oxidation under alkaline conditions.Characterizations of the oxidation products,i.e.,analyses by use of Time of Flight Mass Spectroscopy(MALDI-TOF-MS),Infrared Spectroscopy(IR),and Nuclear Magnetic Resonance Spectroscopy(NMR),indicate that most phenol(98.2%)has been further polymerized into polyphenol.DFT calculations further reveal that PS has been activated by use of the mechanism for non-free radicals on the CuO(110)plane with the O-O bond being activated and elongated from 1.45 to 1.58(?).The activated O-O bond then abstracts H from phenolic-OH with cleavage of the O-O bond,resulting in the formation of SO₄^·-and phenoxyl radicals that would be then coupled and finally polymerized into polyphenol polymers.Then,studies have been also conducted on the catalytic oxidation of other phenolic contaminants in the CuO-PS system.Apart from phenol,other phenolics difficult to degrade,such as o-nitrophenol,m-nitrophenol,p-nitrophenol,and2,4-dinitrophenol,can also be oxidized to polyphenols in the CuO-PS system,and the optimal p H values for their removal are close to their respective p K_avalues.As illustrated in the Hammett plot,the inversely linear relationship between log k_obsand BDE_O-H(bond dissociation energy)values of various phenolics further confirms H-abstraction from various phenolics with the formation of the corresponding phenoxyl radicals in the CuO-PS system.Besides,phenol added can improve the removal effect of total organic carbon(TOC)in the solution although the presence of phenol does not necessarily accelerate the oxidation rates of different nitrophenols.Analyses by use of FTIR and MALDI-TOF mass spectra further reveal that oxidative coupling between phenoxyl radicals and nitrophenoxyl radicals accelerates the removal effect of recalcitrant phenolic contaminants.In order to conduct further studies on the facet effect on the PS activation process,three Co₃O₄nanoparticles with different facets[rod(110),cube(100),and octahedra(111)]have been synthesized.The Co₃O₄rods show the highest catalytic reactivity in the oxidation of phenol by PS as followed by Co₃O₄cubes and Co₃O₄octahedra.Quenching experiments and EPR analysis rule out the contributions of SO₄^·-,·OH,O₂^·-and ¹O₂to phenol oxidation,and the presence of phenoxyl radicals in EPR analysis suggests that H-abstraction from phenol to phenoxyl radicals is likely an important pathway in all Co₃O₄-PS systems.TOC analysis and solid characterizations indicate that 96.6%of the phenol has been transformed into polyphenols rather than being mineralized into smaller organic molecules or CO₂in the rod-PS-phenol system.It has been further confirmed in DFT calculations that PS is more readily activated via a non-radical pathway on the Co₃O₄(110)plane with the appropriate Co(II)-Co(II)distances,and the activated O-O bonds then abstract H atoms from phenol molecules with the formation of phenoxyl radicals,as well as SO₄^·-and·OH radicals.The produced phenoxyl radicals have been then linked together to form dimers,trimers,oligomers,and eventually insoluble polymers.Finally,comparisons were also made on the differences between the PMS and PS in the catalytic oxidation of phenol by FeCo₂O₄under neutral and alkaline conditions in the present paper.It has been shown in the studies that the oxidation rates in the PMS systems are higher than those in the PS systems with more PMS consumed under similar experimental conditions.Solution and solid TOC analyses indicate that phenol is primarily transformed into solid sediments in the PS system,whereas a large portion of phenol has been oxidized to dissolved oxidation products in the PMS system.Quenching experiments and EPR analyses suggest that apart from the nonradical pathway,SO₄^·-,·OH,and ¹O₂are playing an important role in the oxidation of phenol in the PMS system while SO₄^·-and·OH also somehow contribute in the PS system.From the unique perspective of turning phenolic pollutants into polyphenols,new methods and new insights have been provided for seeking an“environment-friendly oxidation technology”that is economical,effective,and suitable for the treatment of phenolic wastewater.Meanwhile,the present studies are also of important significance for the treatment of other aromatic wastewater containing aniline,thiophene,etc.