Transformation Behaviour Of High Valent Iron Species During Ferrate Oxidatoin Of Contaminants And The Mechanism Of Enhanced Ferrate Oxidation

Potassium ferrate is a potassium salt of high-valent iron oxyanion with multiple functions of oxidation,disinfection,coagulation and adsorption,and its reaction products are nontoxic and harmless.Therefore,ferrate has become a research hotspot in the field of water treatment in recent years.The Fe atom in the ferrate molecule is hexavalent,and it undergoes intermediate valence such as Fe（Ⅴ）and Fe（IV）during its reduction to final product Fe（Ⅲ）.It has been reported that the reactivity of intermediate iron species is several orders of magnitude higher than that of Fe（VI）species.But they are extremely unstable in water and easy to decompose,resulting in low utilization of the oxidation capacity.Due to the short life and low steady-state concentration of intermediate iron species,it is difficult to observe intermediate iron species directly,causing many troubles to study it.Until now,it is still not clear about the oxidation contribution of intermediate iron species during ferrate oxidation of organic pollutants in water,and the corresponding reaction characteristics or oxidation mechanisms have not been mastered.In this paper,the effect of phosphate anion（a common buffer anion to stabilize pH）on the removal of organic compounds in aquatic environment by ferrate was studied at first.The reaction kinetics model was developed by using a one-electron transfer model compound ABTS（2,2’-biazo-bis（3-ethylbenzothiazoline-6-sulfonic acid）diammonium salt）to reveal the roles of intermediate iron species in ferrate oxidation.And another kinetics model was constructed by using a two-electron transfer model compound PMSO（methyl phenyl sulfoxide）to investigate the transformation behavior of high-valent iron species,to study the relationship between ferrate self-decomposition and ferrate oxidation of organic compounds,and to further reveal the generation rules of intermediate iron species.Finally,based on the idea of using intermediate iron species to enhance ferrate oxidation,the enhanced effect of Fe（Ⅲ）and Na₂SO₃ on pollutants removal by ferrate in aquatic environment were investigated respectively,and the activization mechanisms were explained.Inorganic phosphate is one of the most common buffer salt to stabilize pH in K₂FeO₄oxidation experiments,but the effect caused by phosphate on K₂FeO₄ oxidation has always been ignored.In this study,it was found that phosphate had a significant inhibitory effect on removal of typical novel pollutants by K₂FeO₄ oxidation in aquatic environment,such as carbamazepine,bisphenol S,ciprofloxacin,sodium dichlorophenolate,etc.At pH8.0,10 m M phosphate could reduce the corresponding second-order reaction rate constants(k_app)by 1.7-2.4 times.The stoichiometry of reaction between K₂FeO₄ and ABTS was close to 1:1 in phosphate buffer but close to 1:2 in borate buffer.Based on this phenomenon,it was speculated that the inhibitory effect of phosphate on K₂FeO₄ is related to its inhibition to intermediate iron species.Because the reaction of K₂FeO₄ and ABTS is fast（within 200 ms）,kinetic curves were obtained by stopped-flow spectrometer here and kinetic model was constructed for fitting.The fitting results showed that phosphate had no effect on reactivity of Fe（VI）with ABTS,but greatly inhibited reaction activity of Fe（Ⅴ）with ABTS(the value of k_app decreased by 1-2 orders of magnitude).Pyrophosphate,an inorganic complexing agent,also has obvious inhibitory effect on oxidation of organic compounds treated by K₂FeO₄,indicating that phosphate inhibition may be realized via its complexation with intermediate iron species.In view of wide application of phosphate buffer in K₂FeO₄ oxidation experiments,previous studies may underestimate the actual oxidation capacity of K₂FeO₄.At the same time,this phenomenon also proves that intermediate iron species could play an indispensable role in decontamination process of K₂FeO₄ oxidation.Due to the instability of K₂FeO₄ in aquatic environment,its self-decomposition will consume oxidizing agent,but highly active intermediate iron species may be generated here,such as Fe（Ⅴ）/Fe（IV）.In this paper,reaction kinetics of K₂FeO₄ and PMSO in different buffers and with different K₂FeO₄ dosages at pH 7.0 was systematically studied to explore the relationship between K₂FeO₄ self-decomposition and K₂FeO₄ oxidation.It is found that apparent second-order rate constants(k_app)of K₂FeO₄ and PMSO increased with the increasing K₂FeO₄ concentration under the condition of excessive oxidant.This effect could be greatly weakened when target compound was excessive,and it could be eliminated completely after adding pyrophosphate to inhibit Fe（Ⅴ）oxidation.The above results indicated that this effect was caused by the reaction of PMSO and Fe（Ⅴ）generated via self-decomposition of K₂FeO₄.Kinetics model showed that Fe（Ⅴ）was the main active species during oxidation of PMSO by K₂FeO₄,and its contribution ratio was up to 98%.Hence,self-decomposition of K₂FeO₄ is a self-activation process essentially,which could transform Fe（VI）species into highly active Fe（Ⅴ）species to participate in removal of target compounds.This study also showed that oxidative activity of Fe（VI）species with target compounds was not affected by phosphate,but the reactivity of Fe（Ⅴ）species was inhibited significantly by phosphate.In-situ formed Fe（Ⅲ）or exogenous Fe（Ⅲ）could promote self-decomposition of K₂FeO₄ and catalyze the oxidation reaction of Fe（Ⅴ）species,thus it showed enhanced effect on K₂FeO₄ oxidation.Fe（Ⅲ）could strengthen the removal of organic pollutants with electron-rich groups by K₂FeO₄,such as sulfamethoxazole and carbamazepin,shown as acceleration of its removal rate and increase of final degradation efficiency of pollutants,which proved the feasibility of enhanced effect of Fe（Ⅲ）on K₂FeO₄ decontamination.K₂FeO₄ is more stable under alkaline conditions with slower rate of its self-decomposition,so the accumulation of intermediate iron species is less here.Under this condition,Fe（Ⅲ）could promote decomposition of K₂FeO₄ to form highly active intermediate iron species,and then improve the steady state concentration of intermediate iron species.Hence,the enhanced effect of Fe（Ⅲ）on K₂FeO₄ oxidation under alkaline conditions was more obvious than that under acidic conditions.The main active species in Fe（Ⅲ）-K₂FeO₄system was high-valent iron species,and its oxidation could be inhibited by humic acid in water,but it was not affected by common aquatic anions and cations,such as Ca²⁺,Mg²⁺,HCO₃^-.Compared with Fe（Ⅲ）,Na₂SO₃ could promote the formation of intermediate iron species more quickly and directly,and greatly increase the steady state concentration of intermediate iron species,so rates of pollutants removal in Na₂SO₃-K₂FeO₄ system was faster.Meanwhile,when the concentration of Na₂SO₃ was excessive compared with K₂FeO₄,active iron species initiated the chain reaction to produce sulfate radical with stronger activity to participate in oxidation of pollutants.Na₂SO₃-K₂FeO₄system also has perfect removal efficiency on recalcitrant organic pollutants,and it could expand the application of K₂FeO₄.