Preparation Of Iron-based Oxide Coating And Its Fenton-like Degradation Of Phenol

Fenton oxidation has attracted much attention from all over the world due to high efficiency,low cost,simplicity,abundant raw materials and environmental benignity.However,the drawbacks of sludges production,narrow working p H range（p H 2 4）,impossible catalyst recyclability constrained its wide industrial applications.Although the Fenton-like catalyst was developed on the basis of Fenton reaction,its potential applications were hindered,due to its poor or no catalytic activity under circumneutral p H,tanglesome and time consuming preparation process and the complex catalyst reclamation which requires the filtration or centrifugation.In order to satisfy the requirements of commercial applications,it is urgent and desirable to develop Fenton-like catalysts with high catalytic activity,low cost and convenient recycling.This paper employed plasma eletrolytic oxidation（PEO）technique,which is green and efficient for fabricating immobilized coating,to prepare immobilized iron-based compound coating on Q235 carbon steel which successfully solved the problem of catalyst recycling.Scanning electron microscopy（SEM）,high resolution transmission electron microscopy（HRTEM）,X-ray diffractometer（XRD）,X-ray photoelectron spectroscopy（XPS）,NH3 temperature programmed desorption（NH3-TPD）were employed to characterize the material’s phase composition,morphology and surface acidic property,etc.To evaluate the Fenton-like catalytic activity of as-prepared PEO coating,phenol was used as target pollutant.In addition,the influences of the degradation conditions and preparation processes on the phenol degradation efficiency were also studied.Fe₃O₄/FeAl₂O₄ and Fe₃O₄/SiO₂ porous PEO coatings were prepared respectively in aluminate and silicate electrolyte system by plasma electrolytic oxidation.The experiment indicated that 100 % phenol could be degraded within 60 min under p H 4.0,H₂O₂ 6.0 mmol/L,35 mg/L phenol by Fe₃O₄/FeAl₂O₄ PEO coating Fenton-like catalyst and the catalytic activity of coating prepared in aluminate electrolyte was significantly greater than that of coating prepared in silicate electrolyte.What’s more,the effects of pH,H₂O₂ dosage and phenol concentration on degradation efficiency were studied and the optimum conditions were confirmed.The effects of preparation process such as current density,frequency,were also studied in order to optimize coating structure.The stability experiment suggested that as-prepared coating exhibited excellent stability.Compared with that of homogeneous Fenton reaction,the PEO coating couldn’t degrade phenol under p H 5.0,but homogeneous Fenton reaction could degrade phenol even under p H 6.0.Hence,the catalytic activity of PEO coating needs to be improved.To further increase the Fenton-like degradation efficiency of phenol by PEO coating,the transition metal M（Ni,Co,Cu,Mn）modified PEO coating Fenton-like catalysts were prepared.The phase analysis indicated that the surface Fe amount of transition metal modified PEO coating decreased,and MAl2O4 and MSi O3 were formed due to the reaction of transition metal ion with aluminate and silicate respectively while the formed Co2 Si O4 was excluded.The Fenton-like degradation of phenol indicated that Ni modified PEO coating could improve its catalytic activity which might be ascribed to the synergistic effect between Ni and Fe active sites on the catalyst surface.However,the increase of Ni modified PEO coating catalytic activity was not so great which might because Ni was not incorporated into the crystal lattice of Fe₃O₄.Hence,the catalytic activity of PEO coating needs further improvement.In view of high organic pollutant degradation efficiency under acidic condition（p H 2 4）of Fenton reaction,solid acid coating Fenton-like catalyst could be prepared in order to make use of acidic microenvironment on the catalyst to enhance Fenton-like degradation efficiency.This strategy can avoid the acidic adjustment of organic pollutants pre-treatment and the neutralization after treatment.Sulfur modified PEO coating was prepared by plasma electrolytic oxidation during which Na2S2O3·H2O and Na2S2O8 were used as sulfur source.The evaluation of the influence of sulfur amount on the phase composition,morphology and catalytic activity was conducted,especially on the as-prepared coating.The results found that after sulfur modification the phase structure of coating was unchanged and sulfur existed in the form of SO42-and SO32-species on the coating surface and formed chemical bond with Fe.The formed SO42-Fe resulted in the formation of Lewis and Br?nsted acid on the catalyst surface.The sulfur modified PEO coating Fenton-like degradation of phenol indicated that phenol could be rapidly degraded within 7 min under p H 6.0,H₂O₂ 6.0 mmol/L,phenol 35 mg/L.Importantly,even under the weak alkaline（p H 9.0）sulfur modified PEO coating still could remove phenol effciently.However,the consecutive phenol degradation experiment showed that sulfur modified PEO coating exhibited poor stability which might be due to the Fe active sites and sulfur acidic sites loss.Therefore,although as-prepared PEO coating maintained excellent catalytic activity,more improvement on the stability are still required.Due to sulfur loss triggering the second pollution during the solid acid Fenton-like reaction,it’s necessary to prepare metal oxide solid acid modified PEO coating.ZrO₂ as strong solid acid among metal oxides has gained great attention.The PEO coating containing ZrO₂ Fenton-like catalyst was successfully prepared by plasma electrolytic oxidation.The influence of zirconium source on the morphology,phase composition of PEO coating was also studied.The phase analysis suggested that the as-prepared PEO coating was amorphous and comprised of ZrO₂,FePO₄ and Fe₃O₄.Fenton-like phenol degradation by PEO coating prepared in phosphate electrolyte containing 0.5 g K2 Zr F6 indicated that 100 % phenol could be degraded within 2 min under the condition of p H 6.0,H₂O₂ 6.0 mmol/L,phenol 35 mg/L.Although ZrO₂/FePO₄/Fe₃O₄ PEO coating presented outstanding catalytic activity,the Zr and Fe loss during Fenton-like reaction make it not stable.N-butanol as ·OH scavenger and electron spin spectra（ESR）analysis showed that the produced ·OH was the main strong oxidant species.The reaction kinetic analysis of PEO coating displayed that the degradation process could be divided into two stages – the induced stage and rapid degradation period,and both of them followed first order kinetics.At the early stage,phenol and H₂O₂ adsorption on the catalyst occurred and then complexation reaction happened between Fe and H₂O₂,finally Fe active sites catalyzed H₂O₂ to produce ·OH for phenol degradation;at rapid degradation stage,on one hand,heterogeneous Fenton reaction occurred;on the other hand,Fe leaching from the catalyst surface led to homogeneous Fenton reaction.The phenol degradation mechanism of sulrfur modified PEO coating revealed that the produced ·OH mainly was derived from catalyst surface which was directly related with the acid sites on the catalyst surface.