Heterogeneous Fenton-like Degradation Of Phenanthrene Catalyzed By Schwertmannite Biosynthesized Using Acidithiobacillus Ferrooxidans

Polycyclic aromatic hydrocarbons(PAHs),organic compounds consisting of at least two fused benzene rings,are highly toxic,carcinogenic and resistant to natural degradation,which exists in the wastewater and soil widely and subsequently causes a serious threat to human being health through the food chain.Relavant laws have been formulated to control the PAHs all over the world with the purpose of reducing the damage to the human living environment.At present,the concentration of PAHs in the nature water and industrial wastewater reached up to the range of 0.001 to 10 ?gg/L and 1 mg/L,respectively.The World Health Organization(WHO)limited the acceptable highest concentration of 0.02?g/L in terms of six kinds of typical PAHs,and the European Community as well as the U.S.Environmental Protection Agency have also listed them as the priority pollutants.Wastewater contaminated with PAHs usually enrich at the sites of manufactured gas plant,refineries and many other chemical industries.Owing to their strong persistence and ubiquitous occurrence,substantial studies have been devoted to seek for a high efficient treatment to remove PAHs from industrial wastewater.Advanced oxidation processes(AOPs)including Fenton-catalysis,photo-catalysis and ozonation have become important technologies for the degradation of persistent organic pollutants(POPs).Conventional Fenton treatment is frequently limited by the required optimal pH of around 3.0 to prevent the precipitation of ferrous ion,and the treatment process would produce a large amount of ferric hydroxide sludge during the neutralization stage.Thus,heterogeneous Fenton-like oxidation is developed to overcome the limitations associated with conventional Fenton treatment.Over the last two decades,heterogeneous Fenton-like oxidation catalyzed by goethite,magnetite,ferrihydrite or limonite,etc.,has been used to treat water or soils contaminated by organic pollutants.Therefore,the objectives of the present study are to(I)investigate the effect of reaction conditions including the loading of biosynthetic schwertmannite,concentration of H2O2,solution initial pH,and the presence of inorganic anions on the degradation of phenanthrene catalyzed by biosynthetic schwertmannite and H2O2,(2)reveal the associated catalytic degradation mechanism and possible transformation pathway of phenanthrene in biosynthetic schwertmannite/H2O2 system,(3)expore the adsorption kinetics and isothermal adsorption of phenanthrene onto schwertmannite,and(4)study the stability of biosynthetic schwertmannite during its repeated use in the catalytic degradation of phenanthrene.The obtained results were summarized as follows:1.The XRD patterns of iron precipitates displayed that all diffraction peaks well matched those of the standard diffraction data for schwertmannite(JCPDS No.47-1775),suggesting that the iron precipitates formed are pure schwertmannite.The SEM of iron precipitates formed were consisted of aggregates of spherical particles,and the surface of spherical particles showed a hedge-hog morphology which is typical for schwertmannite.The biosynthetic schwertmannite was further analyzed by FTIR,showing that it included some functional groups such as-OH,SO42-and Fe-O.The above functional groups represented in the IR spectra were the same as that of schwertmannite.The BET analysis revealed that the specific surface area of the biosynthetic schwertmannite was 54.5 m2/g.2.This biosynthetic schwertmannite can be used as a catalyst for the degradation of phenanthrene by the heterogeneous Fenton-like reaction.The degradation efficiency of phenanthrene was above 99.0%within 3-5 h reaction,under the reaction conditions of H2O2 200 mg/L,schwertmannite 1 g/L,phenanthrene 1 mg/L and pH 3.0-4.5.3.Both inorganic anions and DOM could inhibit the degradation of phenantherene by the Fenton-like process catalyzed by schwertmannite to some degree.When the concentrations of inorganic anions and DOM increased,the degradation efficiency of phenantherene gradually decreased.Inorganic anions presented different potential in respect of inhibiting the degradation of phenantherene,and the inorganic anions studied suppressed the degradation of phenantherene in a sequence of SO42->H2PO4->NO3->Cl-.Similary,the DOM studied suppressed the degradation of phenantherene in a sequence of L-DOM>M-DOM>H-DOM.The above studies provide a certain theoretical basis for practical and high salt wastewater.4.The Fenton-like degradation of phenanthrene catalyzed by biosynthetic schwertmannite can be explained by the surface catalytic mechanisms.There are some some intermediates such as octadecanoic acid,phenanthraquinone,phthalate by analyzing the fragmentation pattern of mass spectra in the Fenton-like reaction catalyzed by biosynthetic schwertmannite.Considering the products identified and some reported phenanthrene transformation pathways,the transformation pathway of phenanthrene in the biosynthetic schwertmannite/H2O2 system was temporarily proposed as follows:mainly,hydroxyl radicals and singlet oxygen attacked the benzene rings of phenanthrene at positions 9 and 10 to ketonize phenanthrene,leading to substituted products such as 9,10-phenanthraquinone.Subsequently,the benzene rings of 9,10-phenanthraquinone can be cleaved and then formed compounds of phthalate.Afterward,the rest benzene ring is further cleaved,and the formed products can be attacked by hydroxyl radicals continuously until complete mineralization.5.Schwertmannite biosynthesized using A.ferrooxidans is an excellent catalyst during its repeated use for the degradation of phenanthrene in heterogeneous Fenton-like reactions.The degradation of phenanthrene was as high as 80%even after 12 cycles of reuse as a Fenton-like catalyst under the optimal reaction conditions.During the 12 cycles of schwertmannite,the leaching amount of Fe remained in a range of 0.38-0.58 mg/L.The structure and morphology of schwertmannite after 12 times of repeated use were also investigated through XRD and FTIR,indicating that schwertmannite is an excellent Fenton-like catalyst for the degradation of phenanthrene and its mineral structure did not change during its repeated use.