Efficiency And Mechanism Of Fe(?) On Removal Of Bisphenol A Analogues And Organicarsenicals

Chemical oxidation with the use of potassium ferrate?Fe????has a great advantage in abating organic micropollutants and the in situ formed Fe???from the decay of Fe???can be used as a benign coagulant to remove some pollutants.This dual function and environmentally friendly property are crucial advantages of Fe???over other commonly used oxidants.This paper mainly studied the oxidation mechanism of Fe???on typical bisphenol A analogues?BPAs?and organicarsenicals in an aqueous environment,and the adsorption mechanism of the in situ formed Fe???on the removal of total organic carbon?TOC?and/or heavy metal derived from the former oxidation residue.Furthermore,an efficient method for removing BPAs and organicarsenicalsals in water environment was presented.Meanwhile,the mechanism of humic acid?HA?effected on the process of Fe???oxidizing BPAs and organicarsenicals were studied,which could be a theoretical support for the application of Fe???in water treatment.Firstly,the removal efficiency of BPS and BPAF by Fe???was investigated in this study,and examined the biodegradability of the oxidation products to evaluate the oxidation mechanism of Fe???in water treatment applications.The second-order reaction rate constants(k_app)of Fe???with BPS and BPAF were 1.3×10³ M^-1s^-1 and3×10² M^-1s^-1,respectively,at pH 7.0.In the oxidation process,a part of BPS molecules dimerized,while other BPS molecules were oxidized through the oxygen-transfer process,leading to the formation of hydroxylation products and benzene-ring cleavage products.The dominant reaction of BPAF with Fe???was oxygen-transfer process,and BPAF was degraded into lower molecular weight products.The variation of assimilable organic carbon?AOC?suggested that the biodegradability of BPAF and BPS be largely improved after Fe???oxidation.Compared with the oxidation products of BPS,the BPAF's were easier to be bio-consumed.Pure culture test showed that BPAF inhibite the growth of Escherichia coli,while Fe???oxidation completely eliminate this toxic effect.Hence Fe???oxidation has a great potential on controlling the BPAs in contaminated water.To further explore the adsorption mechanism of the in situ formed Fe???on TOC in the Fe???oxidation system,Fe???oxidation of BPF was investigated,in terms of oxidation dynamics,theoretical calculation,reaction pathway proposal,and the removal efficiency of TOC from the oxidation system by the in situ formed Fe???.As the solution pH increased from 6.5 to 10.0,the k_app of Fe???with BPF decreased from 2.0×10³ M^-1s^-11 to 1.2×10² M^-1s^-1.The species-specific rate constants of the reactions of BPF with Fe???suggested that HFeO₄^-be the key species for the oxidation of BPF.Theoretical calculation results indicated that C?14?,C?4?,C?11?,and C?21?atoms of BPF molecular was electron-rich.These carbon atoms were easy to be electrophilic attacked by Fe???.Oxidation mechanism study revealed that BPF was transformed into monocyclic products,coupling products,and ring-cleavage products through hydroxylation pathway,bond-cleavage pathway,and coupling pathway.The reaction ratio of Fe???with BPF was 3.33,and 8%of the total organic carbon?TOC?was removed by Fe???oxidation?Fe???:BPF=20:1?,while 30%of TOC was adsorbed by Fe???resulted ferric particles.Besides Fe???oxidation,adsorption by ferric particle is also an effective way for the elimination of dissolved organics in water,which was confirmed by the results of XPS,HR-TEM and SEM-EDX.In addition,the oxidation of organicarsenicals by Fe???,coupling with simultaneous removal of TOC and arsenic with the in situ formed Fe???were studied.It was found that the k_app of Fe???with p-ASA and ROX were 2.0×10³ M^-1s^-1 and3.1×10² M^-1s^-1,respectively,at pH 7.0.The k_app decreased with the increase of pH.Species-specific rate constants analysis showed that HFeO₄^-was the dominant species reacting with p-ASA and ROX.Fe???attacked the As-C bond of ROX and resulted in the formation of arsenate and 2-nitrohydroquinone.As for p-ASA,of which over80%was oxidized,with the formation of As?V?and 4-aminophenol,while the other20%of the p-ASA was oxidized to nitarsone?NIA?.As?V?and NIA could be mostly adsorbed?>85%?by Fe???resultant ferric?oxyhydr?oxides through surface adsorption.Total-As removal ratios decreased with the presence of SRHA,PO₄^3-and SiO₃^2-,due to the fact that they would be coated on the surface of ferric?oxyhydr?oxides or form inner-sphere complexes,decreasing the zeta potential of ferric?oxyhydr?oxides.The electrostatic repulsive force aroused by SRFA,SRHA,PO₄^3-,SiO₃^2-and OH^-attenuated the interaction between ferric particles and arsenate,leading to the desorption of captured arsenate.The removal of TOC is of significance for water treatment since it could improve the microbiological stability of treated water and control the formation of disinfection by-products.The process of Fe???oxidation?Fe???/p-ASA=20:1?eliminated around 18%of TOC,while Fe???resultant particles achieved over 40%of TOC removal in the system,collectively resulting the TOC removal efficiency 1.6 to 38 times higher than those of O₃,HClO,and permanganate treatment.It is concluded that,besides Fe???oxidation,adsorption of organic pollutants with Fe???resultant nanoparticles could also be an effective method for environmental remediation.HA is a ubiquitous natural organic macromolecule in water environment.In order to better study the practical application of Fe???,the influence of HA on the oxidation mechanism of Fe???was explored.The results showed that HA could either inhibit or promote the oxidation of organic micropollutants by Fe???in water.Co-existing humic acids decreased the removal efficiency of BPAs and organicarsenicals with Fe???.This was mainly because HA would competitively consume oxidant and decrease the removal efficiency of target pollutants in water treatment processes.Compared with BPAF,the oxidation intermediates formed in the oxidation of BPS by Fe???might be more rapidly reduced by HA to the parent molecular.Thus,the inhibition effects of HA on the Fe???oxidation of BPS were more obvious than that on BPAF.Compared with ROX,only parts of the oxidized intermediate of p-ASA with Fe???reduced back to its parent by HA,while the residue was further oxidized,getting rid of the influence of HA,and thus HA accelerated the Fe???oxidation of BPF.It is found that the reducing group in HA plays a role in promoting the oxidation of Fe???.Detailed investigation was conducted to obtain the effects of different kinds of HA and different nominal molecular weight?NMW?fractions of HA on the rate constant.In consequence,the ultraviolet absorption of HA at 254 nm positively correlated with the oxidation of BPF.A possibility is that the aromatic rings??-electrons?and unsaturated bonds of HA influence the electron cloud density of BPF,which make it easy to be oxidized by Fe???.