超顺磁性纳米Fe3O4处理2,4-二氯酚.pdf

Applied Catalysis B Environmental 123 124 2012 117 126 Contents lists available at SciVerse ScienceDirect Applied Catalysis B Environmental jo ur n al homepage Fenton like degradation of 2 4 dichlorophenol using Fe3O4magnetic nanoparticles Lejin Xua Jianlong Wanga b aLaboratory of Environmental Technology INET Tsinghua University Beijing 100084 PR China bBeijing Key Laboratory of Fine Ceramics Tsinghua University Beijing 100084 PR China a r t i c l e i n f o Article history Received 21 January 2012 Received in revised form 26 March 2012 Accepted 20 April 2012 Available online 27 April 2012 Keywords Fe3O4magnetic nanoparticles Hydrogen peroxide Fenton like 2 4 Dichlorophenol Hydroxyl radicals a b s t r a c t Fenton like degradation of 2 4 dichlorophenol 2 4 DCP in aqueous solution was investigated over Fe3O4 magnetic nanoparticles MNPs as catalyst The obtained samples were characterized by X ray diffraction XRD high resolution transmission electron microscopy HRTEM X ray photoelectron spectroscopy XPS nitrogen adsorption desorption isotherms and physical property measurement system PPMS The catalytic results showed that Fe3O4MNPs presented good properties for the degradation and miner alization of 2 4 DCP achieving complete decomposition of 2 4 DCP and 51 of TOC removal after 180 min at reaction conditions of H2O212 mM Fe3O4MNPs 1 0 g L 2 4 DCP 100 mg L pH 3 0 and T 30 C The effect of different reaction parameters such as initial pH H2O2dosage Fe3O4MNPs addition initial concentra tion of 2 4 DCP and temperature on two stage fi rst order kinetics of 2 4 DCP degradation was studied A high utilization effi ciency of H2O2calculated as 73 was observed According to the analyses of iron leaching reactive oxidizing species and degradation intermediates a possible mechanistic steps of 2 4 DCP degradation dominated by OH reactions especially by free OH in the bulk solution were proposed Besides stability and reusability of Fe3O4MNPs were tested 2012 Elsevier B V All rights reserved 1 Introduction Fenton like process using solid catalysts which can gener ate strong oxidants capable of decomposing recalcitrant organic contaminants provides a promising alternative for the oxidative treatment of polluted soil and water In recent years zero valent iron 1 2 iron based clays silicas and zeolites 3 4 iron containing materials 5 6 and iron oxide minerals 7 10 as heterogeneous Fenton like catalysts have been studied Among these catalysts iron oxide minerals such as magnetite Fe3O4 hematite Fe2O3 and goethite FeOOH are of great fundamental and techno logical importance because of their relative wide availability and specifi c structural magnetic and catalytic properties 7 8 11 The mechanism through the decomposition of hydrogen perox ide H2O2 by pure iron oxides has been proposed in the literature 3 12 14 Under acidic conditions the process involves the redox recycling of Fe II Fe III both at the surface of catalysts and in the bulk solution A complex on the oxide surface assigned as FeII H2O2 is initially formed by ligand displacement between the hydrous surface of FeII H2O and H2O2 producing hydroxyl radi cals OH by intramolecularly electron transfer as seen in Eq 1 Corresponding author at Neng Ke Lou Tsinghua University Beijing 100084 PR China Tel 86 10 62784843 fax 86 10 62771150 E mail address wangjl J Wang The surface FeIIIis reduced to FeIIvia reactions 2 and 3 The dis solved iron resulted from dissolution of iron oxides also initiates H2O2decomposition in the bulk solution which proceeds through a chain reaction 4 6 analogous to the Haber Weiss mechanism 15 These hydroxyl radicals produced both on the oxide surface and in the bulk solution are the main oxidants for the degradation of contaminants At circumneutral pH values the contribution of dissolved iron to H2O2activation is minimal and a different oxida tive species such as high valent iron i e FeIV may be formed as shown in Eqs 7 9 14 FeII H2O H2O2 FeII H2O2 FeIII OH OH 1 FeIII H2O2 FeIII H2O2 FeII HOO H 2 FeIII HOO FeII O2 H 3 FeII H2O2 FeIII OH OH 4 FeIII H2O2 FeII HOO H 5 FeIII HOO FeII O2 H 6 FeII H2O2 FeIV 2OH 7 FeIV H2O2 FeII O2 2H 8 FeIV FeII 2 FeIII 9 Magnetite Fe3O4 has been observed to have especially effi cient catalysis in Fenton like system which is assigned to the presence 0926 3373 see front matter 2012 Elsevier B V All rights reserved http dx doi org 10 1016 j apcatb 2012 04 028 118L Xu J Wang Applied Catalysis B Environmental 123 124 2012 117 126 of FeIIspecies in magnetite structure initiating the Fenton reaction 13 With an inverse spinel crystal structure it exhibits unique electric and magnetic properties based on the transfer of elec trons between ferrous ions and ferric ions in the octahedral sites 16 What is more the high catalytic activity of nanoparticles has attracted more attention due to their high surface area and uniform pore size distribution 11 Researchers have shown that Fe3O4nanoparticles can be implicated as a heterogeneous Fenton like catalyst for removal of environmental contaminants including phenol 7 17 aniline 17 p nitrophenol 11 pentachlorophe nol 18 and 17 methyltestosterone 13 However no study has been reported on Fe3O4magnetic nanoparticles MNPs for Fenton like degradation of 2 4 dichlorophenol 2 4 DCP Much work is still needed to explore the reaction kinetics and degrada tion mechanism so that the heterogeneous Fenton operation can be transformed into an effi cient process suitable for practical applica tions The compound under study 2 4 DCP is used in large amounts in the production of some herbicides and preservatives such as 2 4 dichlorophenoxyacetic acid and pentachlorophenol The aim of this paper is to assess the catalytic performances of Fe3O4MNPs for 2 4 DCP degradation and to study the effect of initial conditions such as pH H2O2and Fe3O4MNPs dosage 2 4 DCP concentration as well as reaction temperature The kinetics of the catalytic reaction degradation mechanism and the reusability of Fe3O4MNPs were also evaluated 2 Materials and methods 2 1 Reagents Ferrous sulfate FeSO4 7H2O Shenyang Reagent Factory fer ric sulfate Fe2 SO4 3 Tianjin Yongda Chemical Reagent Co Ltd and NaOH Beijing Chemical Factory for preparation of Fe3O4 MNPs were of analytical grade and used without further purifi cation 2 4 Dichlorophenol was obtained from Beijing Jinlong Chemical Reagent Co Ltd and its degradation intermediates 2 chlorohydroquinone and 4 6 dichlororesorcinol were purchased from J b H2O2dosage c Fe3O4MNPs addition d initial 2 4 DCP concentration Except for the investigated parameter other parameters fi xed on pH 3 0 H2O212 mM Fe3O4MNPs 1 0 g L 2 4 DCP 100 mg L and temperature 30 C L Xu J Wang Applied Catalysis B Environmental 123 124 2012 117 126123 30060 90120150180 0 1 2 3 4 5 6 7 8 3 103 153 203 253 30 3 353 40 3 45 3 503 55 3 60 1 2 3 4 5 6 7 8 9 283 K 288 K 293 K 298 K 303 K 313 K ln C C0 Time min a First stage Second stage b lnk 1 T x10 3 Fig 8 Effect of temperature on 2 4 DCP degradation a and Arrhenius plots for Fe3O4MNPs catalyzed reaction b carried out at pH 3 0 with H2O212 mM Fe3O4MNPs 1 0 g L and 2 4 DCP 100 mg L lower initial concentration of 2 4 DCP resulting in the scavenging of oxidative radicals as described in former sections 3 3 5 Effect of temperature The apparent rate constants of 2 4 DCP degradation were also determined at selected reaction temperatures to investigate the activation energy of the reaction on the Fe3O4surface As shown in Fig 8a the time of induction period is greatly decreased with the increase of temperature from 283 to 313 K possibly signifying that the dissolution of Fe3O4MNPs in the fi rst stage is an endothermic process 30 The increase in rate constant of second stage with temperature could be attributed to the exponentially increasing kinetic constants for both FeII FeIIIregeneration and radical pro duction 38 39 The apparent activation energies were calculated from these rate constants of two stages identifi ed at the selected reaction temperatures and Arrhenius plots for Fe3O4MNPs cat alyzed reactions are shown in Fig 8b It should be noted that the reactions at temperatures of 283 288 and 293 K are very slow which are excluded from the calculation of the apparent acti vation energy in second stage According to Arrhenius equation k A exp Ea RT the activation energy Ea of fi rst stage was cal culated as 66 0 kJ mol while for the second region of reaction Ea was 28 8 kJ mol The Eavalues are higher than that of the diffusion controlled reactions ranged within 10 13 kJ mol which indicates that the apparent reaction rate is dominated by the rate of intrin sic chemical reactions on the oxide surface rather than the rate of mass transfer 12 The apparent activation energy determined for the second stage is closer to Eareported for the homogeneous Fen ton reaction that is 34 8 kJ mol for acidic oxidation of Orange G 40 suggesting that hydroxyl radicals produced through a Fenton reac tion in the bulk solution Eqs 4 6 play a signifi cant role in the second stage of 2 4 DCP degradation As suggested by previous studies the reaction involving the dis solution of Fe3O4MNPs is the rate determining step of this reaction 0 2 4 6 8 10 0306090120150180 4 6 8 10 12 Fe 2 and dissolved iron mg L Dissolved iron Ferrous ion H2O2 H2O2 mM Time min Fig 9 Investigation of H2O2decomposition and iron dissolution during 2 4 DCP degradation with H2O212 mM Fe3O4MNPs 1 0 g L and 2 4 DCP 100 mg L at pH 3 0 and T 30 C 124L Xu J Wang Applied Catalysis B Environmental 123 124 2012 117 126 30 It can be concluded that an slow transfer of iron from the sur face of Fe3O4MNPs into the solution exists in the fi rst stage and the induction period may be due to the fact that the activation of H2O2 by Fe3O4MNPs does not seem to produce an appreciable amount of effi cacious OH that is the main oxidant for 2 4 DCP degradation 9 The OH formed on the catalyst surface may be at an impor tant rate but they are recombined Eq 15 or scavenged quickly by high concentration of H2O2 Eq 10 and large activated area of Fe3O4MNPs Eq 12 before becoming available to oxidize 2 4 DCP 2 41 The second stage predominantly involves the reaction of 2 4 DCP with hydroxyl radicals produced by reaction with dis solved iron species Eq 4 These results can be further supported by the variation of dissolved iron ferrous ion and H2O2described in Section 3 4 and the effect of radical scavengers discussed in detail in Section 3 5 OH OH H2O2 15 3 4 Iron leaching and H2O2decomposition The Fe ions concentration and the decomposition of H2O2in the 2 4 DCP aqueous solution were investigated with H2O212 mM Fe3O4MNPs 1 0 g L and 2 4 DCP 100 mg L at pH 3 0 and T 30 C as shown in Fig 9 Slow linear dissolution of iron in the fi rst 60 min reaction was found in accordance with the fi rst stage of kinetic degradation of 2 4 DCP Afterwards a high concentration of fer rous 0 2 2 5 mg L from 60 to 120 min was observed indicating that homogeneous Fenton reaction in the bulk solution is expected predominantly in the second stage of 2 4 DCP degradation The fer rous concentration reached a peak value at 120 min when 2 4 DCP was almost completely removed Fig 6 and then the concentra tion decreased to 1 5 mg L after 180 min of reaction As reported previously 29 42 the descending of ferrous concentration may be caused by the oxidation of ferrous ions into ferric ions by remain ing oxidants such as OH and H2O2 in the solution However the concentration of total dissolved iron increased as reaction time increased and the descending period did not occur which could be attributed to the continuous leaching of iron from Fe3O4MNPs About 9 8 mg L of the iron dissolved into solution after 180 min equivalently 9 8 of total iron of 1 0 g L catalyst used It can also be seen that the concentration of H2O2decreased gradually during the degradation of 2 4 DCP The mineralization of one mole 2 4 DCP consumes 12 moles of H2O2in stoichiometry as expressed in Eq 16 Based on the study of Luo et al 6 the uti lization effi ciency of H2O2can be calculated by the ratio of H2O2 amount used for 2 4 DCP degradation against the total amount of the consumed H2O2in the reaction About 51 TOC removal was achieved as seen in Section 3 5 which could be used for calcu lation of the H2O2amount for 2 4 DCP degradation As shown in Fig 9 about 4 9 mM H2O2was consumed during 180 min reaction Thus the utilization effi ciency of H2O2was calculated as 73 in our system C6H4OCl2 12H2O2 6CO2 13H2O 2HCl 16 3 5 Analyses of reactive oxidizing species degradation intermediates and reaction mechanism To discriminate the reactive oxidizing species for 2 4 DCP degra dation n butanol and iodide ion were used to scavenge all the OH produced in the process and surface bounded OH formed at the surface of Fe3O4MNPs respectively 2 43 44 The degradation of 2 4 DCP was almost completely inhibited by excess n butanol 300 mM as shown in Fig 10a indicating that free radicals includ ing surface bounded OH and free OH in the bulk solution have a dominant role in the decomposition of 2 4 DCP In the presence of 10 mM KI the degradation of 2 4 DCP was slightly inhibited and 30060 90120150180 0 20 40 60 80 100 30060 90120150180 0 0 0 2 0 4 0 6 0 8 1 0 30060 90120150180 0 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 2 4 DCP removal TOC removal Dechlorination Removal efficiency Time min b None 300 mM n Butanol 10 mM KI C C0 Time min a c 2 4 DCP 2 Chlorohydroquinone 4 6 Dichlororesorcinol Acetic acid Formic acid C mM Time min Fig 10 a Effect of radical scavengers on the degradation of 2 4 DCP b degra dation effi ciency TOC removal and dechlorination of 2 4 DCP c variation of the concentration of degradation intermediates detected by HPLC and IC Reaction con ditions H2O212 mM Fe3O4MNPs 1 0 g L 2 4 DCP 100 mg L pH 3 0 T 30 C about 88 of 2 4 DCP removal was obtained after 180 min reaction which suggested that surface bounded OH are minor oxidants for rapid degradation of 2 4 DCP Therefore 2 4 DCP was predomi nately oxidized by the attack of hydroxyl radicals particularly OH in the bulk solution further confi rming the investigation of degra dation kinetics in Section 3 3 L Xu J Wang Applied Catalysis B Environmental 123 124 2012 117 126125 OH Cl Cl OH OH Cl OH Cl Cl HO OH OH Cl O O Cl OH Cl Cl OH OH OH HOOCCOOH HOOC COOH and Cl OH OH Cl OH CH3COOH HCOOH etc Fig 11 Proposed reaction pathway for the mineralization of 2 4 DCP at pH 3 0 by Fenton like process using Fe3O4MNPs The decrease of TOC concentration was observed to evaluate the mineralization level of 2 4 DCP in our system as depicted in Fig 10b Firstly TOC concentration decay was slow and then 51 of TOC was removed at 180 min indicating that some of the inter mediates derived from 2 4 DCP decomposition such as carboxylic acids remained in solution As chloride substituents are responsible to the toxicity of aromatic compounds the dechlorination degree was also measured to estimate the detoxifi cation degree As seen in Fig 10b the dechlorination degree of 2 4 DCP reached 90 show ing that most of the chlorine on the aromatic ring was released and formed chloride ion Through HPLC and IC analyses the main intermediates were 2 chlorohydroquinone 4 6 dichlororesorcinol acetic acid and formic acid see Fig 10c The concentration of aromatic intermediates increased slowly at fi rst 60 min reached a peak value at about 90 min and then decreased till complete removal within 150 min Acetic acid and formic acid were gradually formed and remained in solution after 180 min reaction Although the dechlorination of 2 4 DCP to monochlorophenols and phenol has been reported by many researchers 45 46 4 chlorophenol phenol and hydroquinone were not detected in our system Considering the experimental results analyzed above and taking into account the information reported in the literature 47 50 a reaction pathway for the degradation of 2 4 DCP was proposed in Fig 11 The chlorine atom located in the para position on the aro matic ring was substituted by OH to yield 2 chlorohydroquinone which was apparently the preferred location for radical collisions on the 2 4 DCP because of the steric effect This chlorinated hydro quinone may separate two hydrogen atoms with further attack of OH to give 2 chloro 1 4 benzoquinone Meanwhile an elec trophilic OH group could be added onto the aromatic ring of the 2 4 DCP resulting in the formation of hydroxylated products such as 4 6 dichlororesorcinol Further oxidation with dechlorination of chlorobenzoquinone and 4 6 dichlororesorcinol yielded maleic and fumaric acids which were degraded to smaller molecular organic acids such as acetic acid and formic acid remaining in solution eventually 3 6 Catalytic stability of Fe3O4MNPs Successive experiments were performed in order to evaluate the possibility of catalyst reuse From Fig 12 it is obvious that the initial activity decreased gradually during fi ve consecutive runs at similar reaction conditions of Fe3O4MNPs 1 0 g L H2O212 mM 2 4 DCP 100 mg L pH 3 0 and T 30 C This initial loss of activity can be ascribed to the decay of active catalytic sites caused by small amounts of leached iron from the catalyst surface as described in Section 3 4 which is further supported by the observation results 21 43 5 0 20 40 60 80 100 120 min 150 min Removal efficiency Run Fig 12 Five consecutive experiments of the degradation of 2 4 DCP with 1 0 g L reused Fe3O4MNPs at reaction conditions of H2O212 mM 2 4 DCP 100 mg L pH 3 0 and T 30 C from XRD Fig 1c and TEM Fig 2c patterns of reused Fe3O4MNPs Similar results were observed by other authors who attributed the catalyst deactivation to a diversity of factors including reduction of the catalyst specifi c area poisoning of the active catalytic sites by adsorbed organic species conglomeration of MNPs and the dis carding of supernatants during the rinsing of MNPs 17 51 More work is needed to explore the causes of catalyst deactivation and to develop some effective regeneration pr