Enhancement Method And Effect Of Phenol Degradation By Fe³⁺/CaO₂ Fenton-like System

Since the twenty-first century,China’s urban and rural development has become increasingly rapid,but the water environment has been seriously polluted by the massive discharge of wastewater from factories,agricultural spraying of pesticides and the breeding of stored poultry.Fenton technology is an advanced oxidation technology that produces hydroxyl radicals（·OH）with extremely high oxidation capacity（E=2.8V）,which can degrade both organic and inorganic pollutants in water.It has good prospects for application in the environmental field.However,the utilisation rate is low as the reagent used,hydrogen peroxide（H₂O₂）,doesn’t exist stably in the aqueous environment and tends to decompose into H₂O and O₂.To overcome the shortcomings of traditional Fenton technology,scholars have proposed a Fenton-like technology using calcium peroxide（CaO₂）instead of H₂O₂,which can slowly release H₂O₂ in the environmental water,and has the advantages that traditional Fenton technology doesn’t have,such as long reagent validity,high H₂O₂ utilisation and wide pH range.It has been widely studied and applied by scholars and engineers.During the reaction of CaO₂catalyzed by Fe²⁺,Fe²⁺is rapidly oxidized to Fe³⁺,but the reduction rate of Fe³⁺is slow.Meanwhile,Ca（OH）₂ generated by the hydrolysis of CaO₂ raises the pH of the solution,resulting in the easy formation of precipitation of Fe³⁺,which can’t participate in the recycling reaction of iron ions again,thus the utilization rate of iron ions is low.How to promote the conversion of Fe³⁺to Fe²⁺and strengthen the Fe³⁺/Fe²⁺cycle is of great practical importance to achieve the efficient utilization of iron ions and enhance the degradation performance of the system.However,most of the existing studies have focused on the reaction system of Fe²⁺and CaO₂,and relatively few studies have been conducted on the ability to enhance the reaction of Fe³⁺and CaO₂.However,in studies on the reaction between Fe³⁺and H₂O₂,it was found that the addition of ligands such as ethylenediaminetetraacetate（EDTA）,tripolyphosphate（STPP）and oxalate（OA）could inhibit the precipitation of iron ions,enhance the electron transfer rate,promote·OH production and improve the reaction effect;the addition of reducing agents such as ascorbic acid（AA）and hydroxylamine hydrochloride（HA）could accelerate the Fe³⁺to Fe²⁺conversion,promote Fe³⁺/Fe²⁺cycle and improve the degradation effect of Fenton-like reaction.In this paper,based on the study of Fe³⁺/H₂O₂ system,the effects of the introduction of the ligand EDTA and the addition of the reducing agent AA on the degradation of phenol in the Fe³⁺/CaO₂-like Fenton system were investigated comprehensively.The effects of the two reinforcing reagents,the types and effects of the reactive radicals,and the effects of various condition factors were studied.The main conclusions are as follows:（1）In the Fe³⁺/EDTA/CaO₂ system,the addition of EDTA significantly increases the solubility of iron ions and acts as a bridge between CaO₂ and Fe³⁺,accelerating the electron transfer rate between the two systems and promoting Fe³⁺/Fe²⁺cycling,enabling the system to effectively remove pollutants even at neutral pH conditions.The active radical that dominates the degradation of phenol is·OH,and·O₂^-plays a role in reducing Fe³⁺,prompting the system to start the reaction more quickly and increasing the reaction rate.The higher the concentration of Fe³⁺and CaO₂,the better the degradation effect of the system,but excessive CaO₂ will cause a rapid rise in pH,resulting in poor phenol degradation;among the common anions,SO₄^2-and NO₃^-have almost no effect on the reaction,but Cl^-and CO₃^2-compete with phenol for·OH resulting in inhibition of phenol degradation.The Fe³⁺/EDTA/CaO₂ system has a good ability to consistently degrade pollutants and is also feasible for the removal of pollutants in real waters.（2）The Fe³⁺/AA/CaO₂ system can effectively remove pollutants in the pH=3-7range,and the removal effect becomes progressively worse as the pH rises,the system also has good degradation effect on pollutants in actual water.Phenol is best degraded when the molar ratio of Fe³⁺to AA is 2:1;increasing the CaO₂ dosage can improve the phenol removal rate,but excessive CaO₂ will inhibit the degradation of phenol due to high pH rise.AA accelerates the reduction of Fe³⁺to Fe²⁺,while the complexation of AA keeps dissolved iron ions present throughout the reaction.The main radical produced by the Fe³⁺/AA/CaO₂ system is·OH,which consists of two parts:one part is produced by the Fenton-like reaction promoted by AA,and the other part is produced by the direct reaction of AA with CaO₂,and the Fenton-like reaction serves as the main pathway for the production of·OH.The dosing method of AA affects the degradation of phenol,and batch dosing promotes the degradation of pollutants better than a single dose.