Submit manuscript...
MOJ
eISSN: 2573-2919

Ecology & Environmental Sciences

Review Article Volume 3 Issue 1

Progress in the Mechanism and Kinetics of Fenton Reaction

Xinliang Liu,1 Yizhou Sang,1,2 Hailiang Yin,1 Aiguo Lin,1,2 Ziqi Guo,1 Zhen Liu1

1Acdemy of Science and Technology, China University of Petroleum, China
2National University Science Park, China University of Petroleum, China

Correspondence: Xinliang Liu, Acdemy of Science and Technology, China University of Petroleum, No. 66 , West Changjiang Road, Qingdao, Shandong, China, Tel 865-468-393-193, Fax 865-468-393-176

Received: December 14, 2017 | Published: January 16, 2018

Citation: liu X, Sang Y, Yin H, Lin A, Guo Z et al. (2018) Progress in the Mechanism and Kinetics of Fenton Reaction. MOJ Eco Environ Sci 3(1): 00060. DOI: 10.15406/mojes.2018.03.00060

Download PDF

Abstract

Fenton reaction which is involved in Fe2+ and H2O2, was observed by H.J.H. Fenton over 120 years ago, can decomposed organic compounds into H2O and CO2 by the production of oxidizing species. This reaction has been widely used in wastewater treatment, remediation of groundwater. However the mechanism of this reaction is still in discussion. Here, recent studies on the mechanism and kinetics of fenton reaction are summarized and discussed; suggestions are given in research in this field.

Keywords: Fenton reaction; Mechanism; Kinetics; Wastewater; Treatment; Oxidizing species

Overview

The oxidative degradation of organic matter by H2O2 in the role of Fe2 + under acidic conditions was first discovered by H.J. Fenton in 1894, and then applied to the oxidation of tartaric acid, of which the catalytic system was called the Fenton reagent, and the reaction called the Fenton method [1]. Early research on the Fenton reaction was mainly focused on organic synthesis, enzymatic reactions and the cell damage mechanism [2]. It was noted that in the presence of a catalyst, H2O2 can be efficiently decomposed to generate oxidative active substances with strong oxidizing power, and degrade a variety of organic compounds into CO2, H2O and other small molecules. In 1964, Canadian researcher H.R. Eisenhaner applied it to the degradation of organic matter in wastewater [3], and after that more and more researchers carried out in-depth studies on its use in the degradation of organic pollutants [4-6]. Thus a series type of Fenton reagents was developed, such as light/Fenton reagent [7], ultraviolet-visible light/H2O2/ferric oxalate [8], microwave/Fenton reagent [9], and ultrasound/ Fenton reagent [10]. The mechanism of Fenton reaction and the intermediate species are still under controversial discussion although the Fenton reagent has been researched by many researchers as a powerful oxidant in wastewater treatment, biological systems and natural water. Two different mechanisms, namely radical and non-radical mechanism have been developed from last century. The first and most popular theory, known as the Haber-Weiss mechanism involves the formation of • OH radical in the process of H2O2 reduction [11], from the 1930s, the radical mechanism has been questioned by several studies suggesting that the reaction between H2O2 and Fe(II) produces the ferryl ion species, which is the active intermediate species [12].

Hydroxyl Free Radical Mechanism

Reaction mechanism

The hydroxyl radical mechanism was first proposed by Haber and Willstatter in 1932 [13], which revealed the existence and role of free radicals in the reaction system, and regarded the essence of the reaction as • OH generated by a catalytic during the chain reaction between Fe2+ and H2O2. Throughout the reaction process, the chain initiation phase consists of a series of single electron transfer reactions between Fe2+ and H2O2, • OH and H2O2, • OOH and H2O2 (reaction 1-4), and the generated oxygen free radicals induce the chain growth process (• OH, • OOH). Based on this theory, many scientists conducted extensive research. George [14] detected the presence of O2-• in the study of the KO2--H2O2 system, noted that the dissolved oxygen in the system significantly inhibits the decomposition of H2O2, and proposed that during the study of the mechanism, the role of dissolved oxygen in the Fenton system cannot be ignored [15,16]. Thereafter, Barb [17] and Weiss [18] introduced O2-• into the reaction system, and amended the reaction mechanism, that is, the dissolved oxygen in the system is related to the generation and reduction process of Fe3+, and proposed the reaction (5). Regarding the oxygen generation in reaction (5), Barb & Baxendale [19,20] studied the generation mechanism and kinetic characteristics of oxygen in the Fenton system and finalized the radical mechanism for the Fenton reaction, with the conclusion that the reaction of the system is composed of the following seven elementary reactions (reaction 1,5,6-10) [21].

F e 2+ + H 2 O 2 F e 3+ +O H +H O         (1)             H O + H 2 O 2 H 2 O 2 +HO O                 (2) H 2 O 2 +HO O    H 2 O+ O 2 +H O          (3) F e 2+ +H O  F e 3+ +H O                        (4) F e 3+ + H 2 O 2 F e 3+ +O2                         (5) H O + H 2 O 2   H 2 O+H+ O                  (6) F e 3+ + H 2 O 2 F e 2+ +HO O  + H + (7) H 2 O 2 + H + + O 2 H 2 O+ O 2 +H O    (8) F e 2+ +HO+H+F e 3+ + H 2 O            (9) H O     +H O H 2 O 2                              (10) MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOabaeqabaqcLbsaca GIgbGaaOyzaSWaaWbaaWqabeaajugWaiaakkdaiiaacqWFRaWkaaqc LbsacqWFRaWkcaGIibWcdaWgaaadbaqcLbmacaGIYaaameqaaKqzGe GaaO4taSWaaSbaaWqaaKqzadGaaOOmaaadbeaajugibiab=jziUkaa kAeacaGILbqcfa4aaWbaaWqabeaajugWaiaakodacqWFRaWkaaqcLb sacqWFRaWkcaGIpbGaaOisaSWaaWbaaWqabeaajugWaiab=jHiTaaa jugibiab=TcaRiaakIeacaGIpbWcdaahaaadbeqaaiab=jHiTaaaju gibabaaaaaaaaapeGaaOiOaiaakckacaGIGcGaaOiOaiaakccacaGI GaGaaOiiaiaakccacaGIOaGaaOymaiaakMcacaGIGcGaaOiOaiaakc kacaGIGcGaaOiOaiaakckacaGIGcGaaOiOaiaakckacaGIGcGaaOiO aiaakckaaOWdaeaajugibiaakIeacaGIpbWcdaahaaqcfayabeaaju gWaiab=jHiTaaajuaGcqWFRaWkjugibiaakIealmaaBaaameaajugW aiaakkdaaWqabaqcLbsacaGIpbWcdaWgaaadbaqcLbmacaGIYaaame qaaKqzGeGae8NKH4QaaOisaKqbaoaaBaaameaajugWaiaakkdaaWqa baqcLbsacaGIpbWcdaWgaaadbaqcLbmacaGIYaaameqaaKqzGeGae8 3kaSIaaOisaiaak+eacaGIpbWcdaahaaadbeqaaiab=jHiTaaajugi b8qacaGIGcGaaOiOaiaakckacaGIGcGaaOiOaiaakckacaGIGcGaaO iOaiaakccacaGIGaGaaOiiaiaakccacaGIGcGaaOiiaiaakccacaGI GaGaaOikaiaakkdacaGIPaaak8aabaqcLbsacaGIibWcdaWgaaadba qcLbmacaGIYaaameqaaKqzGeGaaO4taKqbaoaaBaaameaajugWaiaa kkdaaWqabaqcLbsacqWFRaWkcaGIibGaaO4taiaak+ealmaaCaaame qabaGae8NeI0caaKqzGeWdbiaakckacaGIGcGae8NKH46daiaakIea lmaaBaaameaajugWaiaakkdaaWqabaqcLbsacaGIpbGae83kaSIaaO 4taSWaaSbaaWqaaKqzadGaaOOmaaadbeaajugibiab=TcaRiaakIea caGIpbWcdaahaaadbeqaaiab=jHiTaaajuaGdaahaaadbeqaaKqzGe WdbiaakckaaaGaaOiOaiaakccacaGIGaGaaOiiaiaakccacaGIGaGa aOiiaiaakIcacaGIZaGaaOykaaGcpaqaaKqzGeGaaOOraiaakwgaju aGdaahaaadbeqaaKqzadGaaOOmaiab=TcaRaaajugibiab=TcaRiaa kIeacaGIpbWcdaahaaadbeqaaKqzadGae8NeI0caaKqzGeWdbiaakc kapaGae8NKH4QaaOOraiaakwgajuaGdaahaaadbeqaaKqzadGaaO4m aiab=TcaRaaajugibiab=TcaRiaakIeacaGIpbWcdaahaaadbeqaaK qzadGae8NeI0caaSWaaWbaaWqabeaajugWa8qacaGIGcaaaiaakcka jugibiaakckacaGIGcGaaOiOaiaakckacaGIGcGaaOiiaiaakccaca GIGaGaaOiiaiaakccacaGIGaGaaOiiaiaakccacaGIGaGaaOiiaiaa kccacaGIGaGaaOiiaiaakccacaGIGaGaaOikaiaaksdacaGIPaaak8 aabaqcLbsacaGIgbGaaOyzaSWaaWbaaWqabeaajugWaiaakodacqWF RaWkaaqcLbsacqWFRaWkcaGIibWcdaWgaaadbaqcLbmacaGIYaaame qaaKqzGeGaaO4taSWaaSbaaWqaaKqzadGaaOOmaaadbeaajugibiab =jziUkaakAeacaGILbWcdaahaaadbeqaaKqzadGaaO4maiab=TcaRa aajugibiab=TcaRiaak+eajugWaiaakkdajugib8qacaGIGcGaaOiO aiaakckacaGIGcGaaOiOaiaakccacaGIGaGaaOiiaiaakccacaGIGa GaaOiiaiaakccacaGIGaGaaOiiaiaakckacaGIGaGaaOiiaiaakcca caGIGaGaaOiiaiaakccacaGIGaGaaOiiaiaakccacaGIGaGaaOikai aakwdacaGIPaaak8aabaqcLbsacaGIibGaaO4taSWaaWbaaWqabeaa cqWFsislaaqcLbsacqWFRaWkcaGIibWcdaWgaaadbaqcLbmacaGIYa aameqaaKqzGeGaaO4taSWaaSbaaKGaGfaajugWaiaakkdaaKGaGfqa aKqzGeGae8NKH46dbiaakckapaGaaOisaSWaaSbaaWqaaKqzadGaaO Omaaadbeaajugibiaak+eacqWFRaWkcaGIibGae83kaSIaaO4taSWd bmaaBaaameaajugWa8aacaGIYaWdbiaakckaaWqabaqcLbmacaGIGc qcLbsacaGIGcGaaOiOaiaakckacaGIGcGaaOiiaiaakckacaGIGaGa aOiiaiaakccacaGIGaGaaOiiaiaakccacaGIGaGaaOiiaiaakccaca GIGaGaaOikaiaakAdacaGIPaaakeaajugib8aacaGIgbGaaOyzaSWa aWbaaWqabeaajugWaiaakodacqWFRaWkaaqcLbsacaGIRaGaaOisaK qbaoaaBaaameaajugWaiaakkdaaWqabaqcLbsacaGIpbWcdaWgaaad baqcLbmacaGIYaaameqaaKqzGeGae8NKH4QaaOOraiaakwgalmaaCa aameqabaqcLbmacaGIYaGae83kaScaaKqzGeGae83kaSIaaOisaiaa k+eacaGIpbWcdaahaaadbeqaaiab=jHiTaaajugib8qacaGIGcGae8 3kaSYdaiaakIealmaaCaaameqabaGae83kaScaaKqzGeGae8hiaaIa e8hiaaIae8hiaaIae8hiaaIae8hiaaIae8hiaaIae8hiaaIae8hiaa Iae8hiaaIae8hkaGIae83naCJae8xkaKcakeaajugibiaakIealmaa BaaameaajugWaiaakkdaaWqabaqcLbsacaGIpbWcdaWgaaqccawaaK qzadGaaOOmaaqccawabaqcLbsacqWFRaWkcaGIibqcfa4aaWbaaWqa beaacqWFRaWkaaqcLbsacqWFRaWkcaGIpbWcdaWgaaqccawaaKqzad GaaOOmaaqccawabaqcLbmacqWFsisljugibiab=jziUkaakIealmaa BaaameaajugWaiaakkdaaWqabaqcLbsacaGIpbGae83kaSIaaO4taS WaaSbaaKqbagaajugWaiaakkdaaKqbagqaaKqzGeGae83kaSIaaOis aiaak+ealmaaCaaameqabaqcLbmacqWFsislaaqcLbsapeGaaOiOai aakccacaGIGaGaaOikaiaakIdacaGIPaaak8aabaqcLbsacaGIgbGa aOyzaSWaaWbaaWqabeaajugWaiaakkdacqWFRaWkaaqcLbsacaGIRa GaaOisaiaak+eacqWFsislcqWFRaWkcaGIibGae83kaSIae8NKH4Qa aOOraiaakwgalmaaCaaameqabaqcLbmacaGIZaGae83kaScaaKqzGe Gae83kaSIaaOisaSWaaSbaaWqaaKqzadGaaOOmaaadbeaajugibiaa k+eapeGaaOiOaiaakckacaGIGcGaaOiOaiaakckacaGIGcGaaOiiai aakccacaGIGaGaaOiiaiaakccacaGIGaGaaOikaiaakMdacaGIPaaa k8aabaqcLbsacaGIibGaaO4taKqbaoaaCaaameqabaGae8NeI0scLb sapeGaaOiOaiaakckacaGIGcaaa8aacqWFRaWkcaGIibGaaO4taSWa aWbaaWqabeaacqWFsislaaqcLbsacqWFsgIRcaGIibqcfa4aaSbaaW qaaKqzadGaaOOmaaadbeaajugibiaak+ealmaaBaaameaajugWaiaa kkdaaWqabaqcLbmapeGaaOiOaKqzGeGaaOiOaiaakckacaGIGcGaaO iOaiaakckacaGIGaGaaOiiaiaakccacaGIGaGaaOiiaiaakccacaGI GaGaaOiiaiaakccacaGIGaGaaOiiaiaakccacaGIGaGaaOiiaiaakc cacaGIGaGaaOiiaiaakccacaGIGaGaaOiiaiaakccacaGIGaGaaOii aiaakIcacaGIXaGaaOimaiaakMcaaaaa@F997@

As understanding deepened on the Fe2+/Fe3+ hydrolyzate and ion forms (Fe(H2O)62+/Fe(H2O)5(OH)+/Fe(H2O)4(OH)2/FeOH2+/Fe(OH)2+/Fe(OH)24+), and with the wide use of dynamics, marking and spectroscopy [22-24], extensive research on the ion forms of Fe2+/Fe3+ in the system were also carried out, and it was concluded that the dissociation and electron transfer of the association product are simultaneous processes, and produce a variety of free radicals with strong oxidization. Based on the current understanding of the complication process of high concentration H2O2-H2O systems, Jones et al. [25] introduced the product of Fe2+/Fe3+/hydrolysis/complex/association into the Fenton elementary reaction, and believed that, the association degree of H2O2 and Fe2+/Fe3+ is determined by the concentration of H2O2, which means that at low concentrations, the three substances can generate associatively [Fe2+/3+(H2O)5(H2O2)] in the aqueous solution, and at high concentrations, they can generate bimolecular and even multi molecular association products [Fe2+/3+(H2O)5(H2O2)n](n≥2). At present, the more consistent view on the understanding of Fe3+ and H2O2 is that the associated Fe3+ can further associate with H2O2 through the transfer of its inner electrons, to generate a compound with single-ended linear or ring configuration (reaction 11-12).

Study of the system shows that:

  1. Fe3+ can significantly affect the dissociation rate of H2O2 with the action of its dissociative products, which may be with iron-oxide intermediate Fe3+HO2- or FeO3+ [26-29];
  2. The decomposition rate of H2O2 to produce O2 is related to the types of complexes [30-32];
  3. The intermediates in H2O2 decomposition include Fe3+(H2O)5O2H- and Fe3+(H2O)4(H2O2)O2H-, with the former product as the main type in high concentration [33].
F e 3+ + H 2 O 2 Fe( H O 2 ) 2 +                 (11) FeO H 2 ++ H 2 O 2 Fe( OH ) ( H O 2 )   (12) MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOabaeqabaqcLbsaca GIgbGaaOyzaSWaaWbaaWqabeaacaGIZaGaaO4kaaaajugibiaakUca caGIibWcdaWgaaadbaGaaOOmaaqabaqcLbsacaGIpbqcfa4aaSbaae aajugWaiaakkdaaKqbagqaaGGaaKqzGeGae8hLHSQaaOOraiaakwga juaGdaqadaGcbaqcLbsacaGIibGaaO4taSWaaSbaaWqaaiaakkdaae qaaaGccaGLOaGaayzkaaqcLbsacaGIYaqcfa4aaWbaaeqabaqcLbma caGIRaaaaKqzGeaeaaaaaaaaa8qacaGIGcGaaOiOaiaakckacaGIGc GaaOiOaiaakckacaGIGcGaaOiOaiaakckacaGIGcGaaOiOaiaakcka caGIGcGaaOiOaiaakckacaGIGcGaaOikaiaakgdacaGIXaGaaOykaa GcbaqcLbsacaGIgbGaaOyzaiaak+eacaGIibWcdaWgaaadbaGaaOOm aaqabaqcLbsacaGIRaGaaO4ka8aacaGIibWcdaWgaaadbaGaaOOmaa qabaqcLbsacaGIpbWcdaWgaaadbaGaaOOmaaqabaqcLbsacqWFugYQ caGIgbGaaOyzaKqbaoaabmaakeaajugibiaak+eacaGIibaakiaawI cacaGLPaaajuaGdaqadaGcbaqcLbsacaGIibGaaO4taKqbaoaaBaaa baqcLbmacaGIYaaajuaGbeaaaOGaayjkaiaawMcaaSWaaWbaaWqabe aacaGIRaWdbiaakckaaaqcLbsacaGIGcGaaOiOaiaakIcacaGIXaGa aOOmaiaakMcaaaaa@88CA@

There still remain controversies on the understanding of the association system of Fe2+ and H2O2, and currently the following evidence can confirm indirectly that the association process of Fe2+ and H2O2 is achieved by the transfer of inner electrons:

  1. Fe2+ must provide more than one complex site in complex with a macrocyclic ligand, and in such conditions thermodynamically unstable H2O2 will be produced when outer electrons are transferred, so from the perspective of thermodynamics, the mechanism of inner electron transfer sounds more feasible [34-36];
  2. The modeling analysis on kinetics shows it reasonable to generate the product of Fe2+- H2O2 by inner electron transfer at pH> 4 [37,38];
  3. It can be concluded that Fe (H2O2)2+ is more stable than Fe (H2O2)O+, Fe(OH)2+, and Fe(H2O)2+ in the gas phase by analysis of charge stripping mass spectrometry combined with the as initio method [39].

Kinetics

The Fenton system may cover more complex reactions, and now researchers focus on its kinetics study in order to understand properly the role of various factors in it, further clarify its reaction mechanism, evaluate the impact of water quality factors on the reaction process, and provide valuable references for its practical application. Gallard et al. [40] established an equation of Fe (II) reaction rate (equation 13), and systematically studied the effect of pH on the system, hypothesizing that pH mainly affects the reaction rate of Fe(II) with H2O2 and the ionized form of HO2·/O2·-, and thus established the following three equations (equations 14-16) to simulate the degradation process of organic compounds via this system. The results showed that at low pH (pH <3), the simulation predictions coincided with the experimental results, but as the pH value increased, the predicted concentration of organic compounds was higher than the experimental value, therefore the authors speculated that there might be other possible reaction pathways and mechanisms in the conditions of high pH. Based on the good consistency of this model with the actual situation at pH less than 3, Gallard et al. [41] further simulated the kinetics of Fe3+ and H2O2, and concluded that during the reaction of Fe (III) with H2O2, they formed a complex first and then this complex decomposed to Fe2+ and HO2• (reaction 17-20), and accordingly proposed a more elaborate kinetic model (equations 21-25). By using the model, they simulated the catalytic decomposition reaction of Fe (III) on H2O2 at pH less than 3. The results showed that the pH value and the concentration ratio of Fe (III) to H2O2 can affect the decomposition process of H2O2 significantly, as the pH increases, the decomposition rate of H2O2 increases accordingly. When 1≤CH2O2/C Fe()≤50, the decomposition rate of H2O2 is proportional to it; when 50 C H2O2 / C Fe( ) 500 MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaKqzGeaeaaaaaa aaa8qacaaI1aGaaGimaiabgsMiJkaadoeal8aadaWgaaqaaKqzadWd biaadIeacaaIYaGaam4taiaaikdaaSWdaeqaaKqzGeWdbiaac+caca WGdbWcpaWaaSbaaeaajugWa8qacaWGgbGaamyza8aacaGGOaaaleqa amaaBaaabaWexLMBbXgBd9gzLbvyNv2CaeHbcfgDH52zaGqbcKqzad Wdbiaa=jwipaGaaiykaaWcbeaajugib8qacqGHKjYOcaaI1aGaaGim aiaaicdaaaa@5533@ , the decomposition rate of H2O2 is unaffected substantially; when   C H2O2 / C Fe( ) 500 MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaKqzGeaeaaaaaa aaa8qacaWGdbWcpaWaaSbaaeaajugWa8qacaWGibGaaGOmaiaad+ea caaIYaaal8aabeaajugib8qacaGGVaGaam4qaSWdamaaBaaabaqcLb mapeGaamOraiaadwgapaGaaiikaaWcbeaadaWgaaqaamXvP5wqSX2q Vrwzqf2zLnharyGqHrxyUDgaiuGajugWa8qacaWFIfYdaiaacMcaaS qabaacfaqcLbsapeGaa4Nh=laaiwdacaaIWaGaaGimaaaa@51FA@ the decomposition rate of H2O2 is inversely proportional to it, and the actual degradation process of waste water proved that the model basically agrees with the experimental results [42].

d[ Fe( II ) ] dt =- k F e ( II ) , H 2 O 2 [ F e ( II ) ][ H 2 O 2 ]+ k F e ( III ) , H 2 O [ Fe( III ) ][ H O 2 ]+ k F e ( III ) , O 2 -• [ F e ( III ) ][ O 2 - ]- k F e ( II ) , O 2 -• [ F e ( II ) ][ O 2 - ]K[ F e ( II ) ][ H O 2 ]- k F e ( II ) ,HO• [ F e ( II ) ][ HO• ]     (13) MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOabaeqabaqcfa4aaS aaaOqaaKqzGeGaaOizaKqbaoaadmaakeaajugibiaakAeacaGILbqc fa4aaeWaaOqaaKqzGeGaaOysaiaakMeaaOGaayjkaiaawMcaaaGaay 5waiaaw2faaaqaaKqzGeGaaOizaiaakshaaaGaaOypaiaak2cacaGI RbGcdaWgaaWcbaGaaOOraiaakwgadaahaaadbeqaamaabmaabaGaaO ysaiaakMeaaiaawIcacaGLPaaaaaaaleqaaKqzGeGaaOilaOWaaSba aSqaaiaakIeadaWgaaadbaGaaOOmaaqabaWccaGIpbWaaSbaaWqaai aakkdaaeqaaaWcbeaajuaGdaWadaGcbaqcLbsacaGIgbGaaOyzaOWa aWbaaSqabeaadaqadaqaaiaakMeacaGIjbaacaGLOaGaayzkaaaaaa GccaGLBbGaayzxaaqcfa4aamWaaOqaaKqzGeGaaOisaSWaaSbaaWqa aiaakkdaaeqaaKqzGeGaaO4taSWaaSbaaWqaaiaakkdaaeqaaaGcca GLBbGaayzxaaqcLbsacaGIRaGaaO4AaOWaaSbaaSqaaiaakAeacaGI LbWaaWbaaWqabeaadaqadaqaaiaakMeacaGIjbGaaOysaaGaayjkai aawMcaaaaaaSqabaqcLbsacaGISaWcdaWgaaadbaGaaOisamaaBaaa baGaaOOmaaqabaGaaO4taGGaaiab=jci3cqabaqcfa4aamWaaSqaaK qzGeGaamOraiaadwgajuaGdaqadaWcbaqcLbsacaWGjbGaamysaiaa dMeaaSGaayjkaiaawMcaaaGaay5waiaaw2faaKqbaoaadmaaleaaju gibiaadIeacaWGpbWcdaWgaaadbaGaaGOmaaqabaqcLbmacqWFIaYT aSGaay5waiaaw2faaKqzGeGaaO4kaiaakUgakmaaBaaaleaacaGIgb GaaOyzamaaCaaameqabaWaaeWaaeaacaGIjbGaaOysaiaakMeaaiaa wIcacaGLPaaaaaaaleqaamaaBaaameaacaGISaGaaO4tamaaBaaaba GaaOOmaaqabaGaaOylaiaakkciaeqaaKqbaoaadmaakeaajugibiaa kAeacaGILbGcdaahaaWcbeqaamaabmaabaGaaOysaiaakMeacaGIjb aacaGLOaGaayzkaaaaaaGccaGLBbGaayzxaaqcfa4aamWaaOqaaKqz GeGaaO4taSWaaSbaaWqaaiaakkdaaeqaaSWaaWbaaWqabeaacaGITa aaaKqzadGaaOOiGaGccaGLBbGaayzxaaqcLbsacaGITaGaaO4AaOWa aSbaaSqaaiaakAeacaGILbWaaWbaaWqabeaadaqadaqaaiaakMeaca GIjbaacaGLOaGaayzkaaaaaaWcbeaadaWgaaadbaGaaOilaiaak+ea daWgaaqaaiaakkdaaeqaaiaak2cacaGIIacabeaaaOqaaKqbaoaadm aakeaajugibiaakAeacaGILbGcdaahaaWcbeqaamaabmaabaqcLbma caGIjbGaaOysaaWccaGLOaGaayzkaaaaaaGccaGLBbGaayzxaaqcfa 4aamWaaOqaaKqzGeGaaO4taSWaaSbaaWqaaiaakkdaaeqaaSWaaWba aWqabeaacaGITaaaaKqzadGaaOOiGaGccaGLBbGaayzxaaqcfaOae8 NeI0scLbsacaGIlbqcfa4aamWaaOqaaKqzGeGaaOOraiaakwgakmaa CaaaleqabaWaaeWaaeaacaGIjbGaaOysaaGaayjkaiaawMcaaaaaaO Gaay5waiaaw2faaKqbaoaadmaakeaajugibiaakIeacaGIpbWcdaWg aaadbaGaaOOmaaqabaqcLbmacaGIIacakiaawUfacaGLDbaajugibi aak2cacaGIRbGcdaWgaaWcbaGaaOOraiaakwgadaahaaadbeqaamaa bmaabaGaaOysaiaakMeaaiaawIcacaGLPaaaaaaaleqaamaaBaaame aacaGISaGaaOisaiaak+eacaGIIacabeaajuaGdaWadaGcbaqcLbsa caGIgbGaaOyzaOWaaWbaaSqabeaadaqadaqaaiaakMeacaGIjbaaca GLOaGaayzkaaaaaaGccaGLBbGaayzxaaqcfa4aamWaaOqaaKqzGeGa aOisaiaak+eajugWaiaakkciaOGaay5waiaaw2faaabaaaaaaaaape GaaiiOaiaacckacaGGGcGaaiiOaiaacckacaGGOaGaaGymaiaaioda caGGPaaaaaa@F16C@ [ H O 2 ]= [ H + ]-1 0 -pKa [ H + ] [ H O 2 O 2 -• ]       (14) MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaKqbaoaadmaake aajugibiaakIeacaGIpbWcdaWgaaadbaGaaOOmaaqabaqcLbmacaGI IacakiaawUfacaGLDbaajugibiaak2dajuaGdaWcaaGcbaqcfa4aam WaaOqaaKqzGeGaaOisaSWaaWbaaWqabeaacaGIRaaaaaGccaGLBbGa ayzxaaqcLbsacaGITaGaaOymaiaakcdalmaaCaaameqabaGaaOylai aakchacaGIlbGaaOyyaaaaaOqaaKqbaoaadmaakeaajugibiaakIea lmaaCaaameqabaGaaO4kaaaaaOGaay5waiaaw2faaaaajuaGdaWada Gcbaqcfa4aaSaaaOqaaKqzGeGaaOisaiaak+ealmaaBaaameaacaGI YaaabeaajugWaiaakkciaOqaaKqzGeGaaO4taSWaaSbaaWqaaiaakk daaeqaaKqzadGaaOylaiaakkciaaaakiaawUfacaGLDbaajugibaba aaaaaaaapeGaaOiOaiaakckacaGIGcGaaOiOaiaakckacaGIGcGaaO iOaiaakIcacaGIXaGaaOinaiaakMcaaaa@694E@
[ O 2 -• ]= 1 0 -pKa [ H + ] [ HO2 O 2 -• ]          (15) MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaKqbacbaaaaaaa aapeWaamWaaOqaaKqzGeGaaO4taSWaaSbaaWqaaiaakkdaaeqaaKqz adGaaOylaiaakkciaOGaay5waiaaw2faaKqzGeGaaOypaKqbaoaala aakeaajugib8aacaGIXaGaaOimaSWaaWbaaWqabeaacaGITaGaaOiC aiaakUeacaGIHbaaaaGcpeqaaKqba+aadaWadaGcbaqcLbsacaGIib WcdaahaaadbeqaaiaakUcaaaaakiaawUfacaGLDbaaaaqcfa4aamWa aOqaaKqbaoaalaaakeaajugibiaakIeacaGIpbqcLbmacaGIYaaake aajugibiaak+ealmaaBaaameaacaGIYaaabeaajugWaiaak2cacaGI IacaaaGccaGLBbGaayzxaaqcLbsapeGaaOiOaiaakckacaGIGcGaaO iOaiaakckacaGIGcGaaOiOaiaakckacaGIGcGaaOiOaiaakIcacaGI XaGaaOynaiaakMcaaaa@6650@
k Fe( II ), Η 2 Ο 2 = k F e 2+ , Η 2 Ο 2   [ Fe2+ Fe( II ) ]+ k Fe ( OH ) + , H 2 O 2 [ Fe ( OH ) + ] F e ( II )     (16) MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaKqzGeaeaaaaaa aaa8qacaGIRbWcdaWgaaqaaKqzadGaaOOraiaakwgalmaabmaabaqc LbmacaGIjbGaaOysaaWccaGLOaGaayzkaaqcLbmacaGISaGaaO4LdS WaaSbaaWqaamaaBaaabaGaaOOmaaqabaaabeaajugWaiaak+5almaa BaaameaadaWgaaqaaiaakkdaaeqaaaqabaaaleqaaKqzGeGaaOypai aakUgalmaaBaaameaacaGIgbGaaOyzamaaCaaabeqaaiaakkdaiiaa cqWFRaWkaaGae8hlaWIae83LdG0aaSbaaeaacqWFYaGmaeqaaiab=9 5apnaaBaaabaGae8NmaiJaaOiiaaqabaaabeaajugibiaakccajuaG daWadaGcbaqcfa4aaSaaaOqaaKqzGeGaaOOraiaakwgacaGIYaGaaO 4kaaGcbaqcLbsacaGIgbGaaOyzaKqbaoaabmaakeaajugibiaakMea caGIjbaakiaawIcacaGLPaaaaaaacaGLBbGaayzxaaqcLbsacaGIRa GaaO4AaOWaaSbaaSqaaiaakAeacaGILbWaaeWaaeaacaGIpbGaaOis aaGaayjkaiaawMcaamaaCaaameqabaGaaO4kaaaaliaakYcacaGIib WaaSbaaWqaaiaakkdaaeqaaSGaaO4tamaaBaaameaacaGIYaaabeaa aSqabaqcfa4aaSaaaOqaaKqbaoaadmaakeaajugibiaakAeacaGILb qcfa4aaeWaaOqaaKqzGeGaaO4taiaakIeaaOGaayjkaiaawMcaaSWa aWbaaWqabeaacaGIRaaaaaGccaGLBbGaayzxaaaabaqcLbsacaGIgb GaaOyzaOWaaWbaaSqabeaadaqadaqaaiaakMeacaGIjbaacaGLOaGa ayzkaaaaaaaajugibiaakckacaGIGcGaaOiOaiaakckacaGGOaGaaG ymaiaaiAdacaGGPaaaaa@87AA@

Other relevant reports were also published on the impact of other ions in the system on its reaction kinetics. Based on the elementary reactions of the classical Fenton system, the De Laat research group [43] established a decomposition kinetics model of H2O2 in the presence of SO42- by the complexation reaction of SO42- and Fe3+, and concluded that at higher concentrations of SO42-, SO42- and Fe3+ formed a complex with very low activity and hindered the complication of Fe3+ and H2O2, resulting in a decline in efficiency of the Fenton system. Walling et al. [44] studied the effect of Cu2+ on the oxidation of organic compounds in the Fenton system. The results showed that the presence of Cu2+ will change the next reaction pathway of organic free radicals generated. When the system is free of Cu2+, there are three possible reaction pathways for the oxidization of organic compounds into organic radicals (R •), the first is that Fe2+ is oxidized by R • into Fe3+, the second is that Fe3+ is reduced into Fe2+, the third is that there are mutual polymerizations between radicals. When Cu2+ is introduced into the system, it can react with R • and reduce its concentration in the system, thereby changing the reaction progress.

F e 3+ + H 2 O 2 Fe ( H O 2 ) 2+ + H +                (17) MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaKqzGeGaaOOrai aakwgalmaaCaaameqabaGaaO4maiaakUcaaaqcLbsacaGIRaGaaOis aSWaaSbaaWqaaiaakkdaaeqaaKqzGeGaaO4taSWaaSbaaWqaaiaakk daaeqaaKqzGeGaeSiZHmOaaOOraiaakwgajuaGdaqadaGcbaqcLbsa caGIibGaaO4taSWaaSbaaWqaaiaakkdaaeqaaaGccaGLOaGaayzkaa WcdaahaaadbeqaaiaakkdacaGIRaaaaKqzGeGaaO4kaiaakIealmaa CaaameqabaGaaO4kaaaajugibabaaaaaaaaapeGaaOiOaiaakckaca GIGcGaaOiOaiaakckacaGIGcGaaOiOaiaakckacaGIGcGaaOiOaiaa kckacaGIGcGaaOiOaiaakckacaGIGcGaaOikaiaakgdacaGI3aGaaO ykaaaa@62F2@
Fe ( H O 2 ) 2+ + H 2 O 2 Fe( OH ) ( H O 2 ) + + H       (18) MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaKqzGeaeaaaaaa aaa8qacaGIgbGaaOyzaKqba+aadaqadaGcbaqcLbsacaGIibGaaO4t aSWaaSbaaWqaaiaakkdaaeqaaaGccaGLOaGaayzkaaWcdaahaaadbe qaaiaakkdacaGIRaaaaKqzGeGaaO4kaiaakIealmaaBaaameaacaGI Yaaabeaajugibiaak+eajuaGdaWgaaqaaKqzadGaaOOmaaqcfayaba qcLbsacqWImhYGcaGIgbGaaOyzaKqbaoaabmaakeaajugibiaak+ea caGIibaakiaawIcacaGLPaaajuaGdaqadaGcbaqcLbsacaGIibGaaO 4taSWaaSbaaWqaaiaakkdaaeqaaaGccaGLOaGaayzkaaWcdaahaaad beqaaiaakUcaaaqcLbsacaGIRaGaaOisaSWaaWbaaWqabeaacaGIRa WdbiaakckaaaqcLbsacaGIGcGaaOiOaiaakckacaGIGcGaaOiOaiaa kckacaGIOaGaaOymaiaakIdacaGIPaaaaa@64EE@
Fe ( H O 2 ) 2+ F e 2+ +H O 2 •         (19) MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaKqzGeGaaOOrai aakwgajuaGdaqadaGcbaqcLbsacaGIibGaaO4taSWaaSbaaWqaaiaa kkdaaeqaaaGccaGLOaGaayzkaaWcdaahaaadbeqaaiaakkdacaGIRa aaaKqzGeGaeSiZHmOaaOOraiaakwgalmaaCaaameqabaGaaOOmaiaa kUcaaaqcLbsacaGIRaGaaOisaiaak+ealmaaBaaameaacaGIYaaabe aajugWaiaakkciqaaaaaaaaaWdbiaakckacaGIGcGaaOiOaiaakcka jugibiaakckacaGIGcGaaOiOaiaakckacaGIGcGaaOikaiaakgdaca GI5aGaaOykaaaa@5958@ Fe( OH )( H O 2 )+F e 2+ +H O 2 •+O H -     (20) MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaKqzGeGaaOOrai aakwgajuaGdaqadaGcbaqcLbsacaGIpbGaaOisaaGccaGLOaGaayzk aaqcfa4aaeWaaOqaaKqzGeGaaOisaiaak+ealmaaBaaameaacaGIYa aabeaaaOGaayjkaiaawMcaaKqzGeGaaO4kaiablYCidkaakAeacaGI LbWcdaahaaadbeqaaiaakkdacaGIRaaaaKqzGeGaaO4kaiaakIeaca GIpbWcdaWgaaadbaGaaOOmaaqabaqcLbmacaGIIascLbsacaGIRaGa aO4taiaakIealmaaCaaameqabaGaaOylaaaajugibabaaaaaaaaape GaaOiOaiaakckacaGIGcGaaOiOaiaakIcacaGIYaGaaOimaiaakMca aaa@5ACD@

d[ Fe( II ) ] dt = k Fe ( H O 2 ) 2+ [ Fe ( H O 2 ) 2+ ]+ k Fe( OH )( H O 2 ) +[ Fe( OH ) ( H O 2 ) + ] k Fe( II ), H 2 O 2 [ Fe( II ) ][ H 2 O 2 ] +k   Fe( III ),H O 2 .    [ Fe( III )H O 2 .  ] + k Fe( III ), O 2 . [ Fe( III ) O 2 .   ]     ( 21 ) - k Fe( II ), O 2 .   [ Fe( II ) O 2 .   ]k   Fe( II ),H O 2 . [ Fe( II )H O 2 .  ] k Fe( II )HO.  [ Fe( II )HO.  ] MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOabaeqabaqcfa4aaS aaaOqaaGqaaKqzGeGaa8hzaKqbaoaadmaakeaajugibiaa=zeacaWF Lbqcfa4aaeWaaOqaaKqzGeGaa8xsaiaa=LeaaOGaayjkaiaawMcaaa Gaay5waiaaw2faaaqaaKqzGeGaa8hzaiaa=rhaaaGaa8xpaiaa=Tga juaGdaWgaaqcbasaaKqzadGaa8Nraiaa=vgalmaabmaajeaibaqcLb macaWFibGaa83taSWaaSbaaKGaGeaajugWaiaa=jdaaKGaGeqaaaqc baIaayjkaiaawMcaaSWaaWbaaKGaGeqabaqcLbmacaWFYaaccaGae4 3kaScaaaWcbeaajuaGdaWadaGcbaqcLbsacaWFgbGaa8xzaKqbaoaa bmaakeaajugibiaa=HeacaWFpbWcdaWgaaqcbasaaKqzadGaa8Nmaa qcbasabaaakiaawIcacaGLPaaajuaGdaahaaqabKqbGeaajugWaiaa =jdajuaicqGFRaWkaaaakiaawUfacaGLDbaacqGFRaWkjugibiaa=T gajuaGdaWgaaqcfasaaKqzadGaa8Nraiaa=vgalmaabmaajuaibaqc LbmacaWFpbGaa8hsaaqcfaIaayjkaiaawMcaaSWaaeWaaKqbGeaaju gWaiaa=HeacaWFpbWcdaWgaaqcfasaaKqzadGaa8Nmaaqcfasabaaa caGLOaGaayzkaaaajuaGbeaakiab+TcaRKqbaoaadmaakeaajugibi aa=zeacaWFLbqcfa4aaeWaaOqaaKqzGeGaa83taiaa=HeaaOGaayjk aiaawMcaaKqbaoaabmaabaqcLbsacaWFibGaa83taSWaaSbaaKqbGe aajugWaiaa=jdaaKqbGeqaaaqcfaOaayjkaiaawMcaamaaCaaabeqc fasaaiab+TcaRaaaaOGaay5waiaaw2faaaqaaKqzGeGae4NeI0Iaa8 3AaKqbaoaaBaaajqwaG9FaaKqzadGaa8Nraiaa=vgalmaabmaajeai baqcLbmacaWFjbGaa8xsaaqcbaIaayjkaiaawMcaaKqzadGaa8hlai aa=HealmaaBaaajiaibaqcLbmacaWFYaaajiaibeaajugWaiaa=9ea lmaaBaaajiaibaqcLbmacaWFYaaajiaibeaaaKqaGeqaaKqbaoaadm aakeaajugibiaa=zeacaWFLbqcfa4aaeWaaOqaaKqzGeGaa8xsaiaa =LeaaOGaayjkaiaawMcaaaGaay5waiaaw2faaKqbaoaadmaakeaaju gibiaa=HeajuaGdaWgaaqcfasaaKqzadGaa8NmaaqcfayabaqcLbsa caWFpbqcfa4aaSbaaKqbGeaajugWaiaa=jdaaKqbagqaaaGccaGLBb GaayzxaaqcLbsaqaaaaaaaaaWdbiaa=bkak8aacqGFRaWkjugib8qa caWFRbGaa8hOaSWdamaaBaaajeaibaqcLbmapeGaa8Nraiaa=vgalm aabmaajeaibaqcLbmacaWFjbGaa8xsaiaa=LeaaKqaGiaawIcacaGL PaaajugWaiaa=XcapaGaa8hsaiaa=9ealmaaBaaajiaibaqcLbmaca WFYaaajiaibeaajugWaiaa=5caaKqaGeqaaKqzGeWdbiaa=bkacaWF GcGaa8hOaKqbaoaadmaabaqcLbsacaWFgbGaa8xzaKqbaoaabmaaba qcLbsacaWFjbGaa8xsaiaa=LeaaKqbakaawIcacaGLPaaajugib8aa caWFibGaa83taKqbaoaaBaaajuaibaqcLbmacaWFYaaajuaGbeaaju gibiaa=5capeGaa8hOaaqcfaOaay5waiaaw2faaKqzGeGaa8hOaOWd aiab+TcaRKqzGeWdbiaa=TgalmaaBaaajeaibaqcLbmacaWFgbGaa8 xzaSWaaeWaaKqaGeaajugWaiaa=LeacaWFjbGaa8xsaaqcbaIaayjk aiaawMcaaKqzadGaa8hla8aacaWFpbWcdaWgaaqccasaaKqzadGaa8 NmaaqccasabaqcLbmacaWFUaaajeaipeqabaqcLbsapaGae4NeI0sc fa4aamWaaOqaaKqzGeWdbiaa=zeacaWFLbqcfa4aaeWaaeaajugibi aa=LeacaWFjbGaa8xsaaqcfaOaayjkaiaawMcaaKqzGeWdaiaa=9ea juaGdaWgaaqcbasaaKqzadGaa8NmaaWcbeaajugibiaa=5cajuaGda ahaaWcbeqaaiab+jHiTaaajugib8qacaWFGcaak8aacaGLBbGaayzx aaqcLbsapeGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkajuaGdaqada GcbaqcLbsacaWFYaGaa8xmaaGccaGLOaGaayzkaaaabaqcLbsacaWF TaGaa83AaKqba+aadaWgaaqcbasaaKqzadWdbiaa=zeacaWFLbWcda qadaqcbasaaKqzadGaa8xsaiaa=LeaaKqaGiaawIcacaGLPaaajugW aiaa=XcapaGaa83taSWaaSbaaKGaGeaajugWaiaa=jdaaKGaGeqaaK qzadGaa8Nla8qacaWFGcWcdaahaaqccasabeaapaGae4NeI0caaaWc beaajuaGpeWaamWaaeaajugibiaa=zeacaWFLbqcfa4aaeWaaeaaju gibiaa=LeacaWFjbaajuaGcaGLOaGaayzkaaqcLbsapaGaa83taKqb aoaaBaaajuaibaqcLbmacaWFYaaajuaGbeaajugibiaa=5calmaaCa aajuaibeqaaSWaaWbaaKqbGeqabaGae4NeI0caaaaajugib8qacaWF GcaajuaGcaGLBbGaayzxaaqcLbsapaGae4NeI0Ydbiaa=TgalmaaBe aajeaibaqcLbmacaWFgbGaa8xzaSWaaeWaaKqaGeaajugWaiaa=Lea caWFjbaajeaicaGLOaGaayzkaaqcLbmacaWFSaWdaiaa=HeacaWFpb WcdaWgaaqccasaaKqzadGaa8NmaaqccasabaqcLbmacaWFUaaajeai peqabaqcLbsacaWFGcqcfa4aamWaaeaajugibiaa=zeacaWFLbqcfa 4aaeWaaeaajugibiaa=LeacaWFjbaajuaGcaGLOaGaayzkaaqcLbsa paGaa8hsaiaa=9eajuaGdaWgaaqcfasaaKqzadGaa8Nmaaqcfayaba qcLbsacaWFUaWdbiaa=bkaaKqbakaawUfacaGLDbaajugib8aacqGF sislpeGaa83AaSWdamaaBaaajeaibaqcLbmapeGaa8Nraiaa=vgalm aabmaajeaibaqcLbmacaWFjbGaa8xsaaqcbaIaayjkaiaawMcaaKqz adWdaiaa=HeacaWFpbGaa8NlaaqcbasabaqcLbsapeGaa8hOaKqbao aadmaabaqcLbsacaWFgbGaa8xzaKqbaoaabmaabaqcLbsacaWFjbGa a8xsaaqcfaOaayjkaiaawMcaaKqzGeWdaiaa=HeacaWFpbGaa8Nla8 qacaWFGcaajuaGcaGLBbGaayzxaaaaaaa@7DF6@ d[ H 2 O 2 ] dt = k Fe( II ), H 2 O 2 [ Fe( II ) ][ H 2 O 2 ] k H 2 O 2 ,HO. [ H 2 O 2 ][ HO. ]                                         ( 22 ) + k H O 2 .,H O 2 .=[ H O 2 . ][ H O 2 . ]+ k O 2 . - , O 2 . - [ O 2 . - ][ O 2 . - ]+ k HO.,HO. [ HO. ][ HO. ] d[ HO. ] dt = k Fe( II ), H 2 O 2 [ Fe( II ) ][ H 2 O 2 ] k H 2 O 2 ,HO. Fe( II )[ HO. ]k H 2 O 2  ,HO.[ H 2 O 2 ][ HO. ]      ( 23 ) d[ Fe( III ) ] dt = dFe( II ) dt                                                                                                                      ( 24 ) [ Fe( III ) ]=( F e 3+ )+[ Fe ( H O 2 ) 2+ ]+[ Fe( OH ) ( H O 2 ) + ]+2[ F e 2 ( OH ) 2 4+ ]                                         ( 25 ) MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOabaeqabaqcfa4aaS aaaOqaaGqaaKqzGeGaa8hzaKqbaoaadmaakeaajugibiaa=HeajuaG daWgaaqcfasaaiaa=jdaaKqbagqaaKqzGeGaa83taKqbaoaaBaaaju aibaGaa8NmaaqcfayabaaakiaawUfacaGLDbaaaeaajugibiaa=rga caWF0baaaiaa=1daiiaacqGFsislcaWFRbGcdaWgaaWcbaGaa8Nrai aa=vgadaqadaqaaiaa=LeacaWFjbaacaGLOaGaayzkaaGaaiilaiaa cIeadaWgaaadbaGaaGOmaaqabaWccaGGpbWaaSbaaWqaaiaaikdaae qaaaWcbeaajuaGdaWadaGcbaqcLbsacaWFgbGaa8xzaKqbaoaabmaa keaajugibiaa=LeacaWFjbaakiaawIcacaGLPaaaaiaawUfacaGLDb aajuaGdaWadaGcbaqcLbsacaWFibqcfa4aaSbaaKqbGeaacaWFYaaa juaGbeaajugibiaa=9eajuaGdaWgaaqcfasaaiaa=jdaaKqbagqaaa GccaGLBbGaayzxaaqcLbsacqGFsislcaWFRbqcfa4aaSbaaKqaGeaa jugWaiaa=HealmaaBaaameaacaWFYaaabeaajugWaiaa=9ealmaaBa aameaacaWFYaaabeaajugWaiab+XcaSiaa=HeacaWFpbGae4Nla4ca jeaibeaajuaGdaWadaGcbaqcLbsacaWFibqcfa4aaSbaaKqbGeaaca WFYaaajuaGbeaajugibiaa=9eajuaGdaWgaaqcfasaaiaa=jdaaKqb agqaaaGccaGLBbGaayzxaaqcfa4aamWaaOqaaKqzGeGaa8hsaiaa=9 eacqGFUaGlaOGaay5waiaaw2faaKqzGeaeaaaaaaaaa8qacaWFGcGa a8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkaca WFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa =bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8 hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWF GcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaKqbao aabmaakeaajugibiaa=jdacaWFYaaakiaawIcacaGLPaaaaeaajugi biaa=TcacaWFRbWcdaWgaaqcfasaaKqzadGaa8hsaiaa=9ealmaaBa aameaacaWFYaaabeaajugWaiaa=5cacaWFSaGaa8hsaiaa=9ealmaa BaaameaacaWFYaaabeaaaKqbGeqaaKqzadGae4Nla4scLbsacaWF9a qcfa4aamWaaeaajugibiaa=HeacaWFpbqcfa4aaSbaaKqbGeaajugW aiaa=jdaaKqbagqaaKqzGeGae4Nla4cajuaGcaGLBbGaayzxaaWaam Waaeaajugibiaa=HeacaWFpbqcfa4aaSbaaKqbGeaajugWaiaa=jda aKqbagqaaKqzGeGae4Nla4cajuaGcaGLBbGaayzxaaqcLbsacaWFRa Gaa83AaKqbaoaaBaaajuaibaGaa83taKqbaoaaBaaajuaibaGaa8Nm aaqcfayabaqcfaIae4Nla4scfa4aaWbaaeqajuaibaGaa8xlaaaaca WFSaGaa83taKqbaoaaBaaajuaibaGaa8NmaaqcfayabaqcfaIae4Nl a4scfa4aaWbaaeqajuaibaGaa8xlaaaaaKqbagqaamaadmaabaqcLb sacaWFpbqcfa4aaSbaaKqbGeaajugWaiaa=jdaaKqbagqaaKqzGeGa e4Nla4scfa4aaWbaaeqajuaibaqcLbmacaWFTaaaaaqcfaOaay5wai aaw2faamaadmaabaqcLbsacaWFpbqcfa4aaSbaaKqbGeaajugWaiaa =jdaaKqbagqaaKqzGeGae4Nla4scfa4aaWbaaeqajuaibaqcLbmaca WFTaaaaaqcfaOaay5waiaaw2faaKqzGeGae43kaSIaa83AaKqbaoaa BaaajuaibaGaa8hsaiaa=9eacqGFUaGlcaWFSaGaa8hsaiaa=9eacq GFUaGlaKqbagqaamaadmaabaqcLbsacaWFibGaa83taiab+5caUaqc faOaay5waiaaw2faamaadmaabaqcLbsacaWFibGaa83taiab+5caUa qcfaOaay5waiaaw2faaaGcbaqcfa4damaalaaakeaajugibiaa=rga juaGdaWadaGcbaqcLbsacaWFibGaa83taiab+5caUaGccaGLBbGaay zxaaaabaqcLbsacaWFKbGaa8hDaaaacaWF9aGaa83AaKqbaoaaBaaa jeaibaqcLbmacaWFgbGaa8xzaSWaaeWaaKqaGeaajugWaiaa=Leaca WFjbaajeaicaGLOaGaayzkaaqcLbmacqGFSaalcaWFibWcdaWgaaqc casaaKqzadGaa8NmaaqccasabaqcLbmacaWFpbWcdaWgaaqccasaaK qzadGaa8NmaaqccasabaaaleqaaKqbaoaadmaakeaajugibiaa=zea caWFLbqcfa4aaeWaaOqaaKqzGeGaa8xsaiaa=LeaaOGaayjkaiaawM caaaGaay5waiaaw2faaKqbaoaadmaakeaajugibiaa=HeajuaGdaWg aaqcKvaq=haajugWaiaa=jdaaKqbagqaaKqzGeGaa83taKqbaoaaBa aajqwba9FaaKqzadGaa8NmaaqcfayabaaakiaawUfacaGLDbaajugi biab+jHiTiaa=TgalmaaBaaajqwaG9FaaKqzadGaa8hsaSWaaSbaaK GaGeaajugWaiaa=jdaaKGaGeqaaKqzadGaa83taSWaaSbaaKGaGeaa jugWaiaa=jdaaKGaGeqaaKazba2=cqGFSaaljugWaiaa=HeacaWFpb Gaa8NlaaqcKfay=hqaaKqzGeGaa8Nraiaa=vgajuaGdaqadaqaaKqz GeGaa8xsaiaa=LeaaKqbakaawIcacaGLPaaadaWadaGcbaqcLbsaca WFibGaa83taiab+5caUaGccaGLBbGaayzxaaqcLbsacqGFsislcaWF RbGaa8hsaKqbaoaaBaaajuaibaGaa8NmaaqcfayabaqcLbsacaWFpb qcfa4aaSbaaKqbGeaacaWFYaaajuaGbeaajugib8qacaWFGcGaa8hl aiaa=HeacaWFpbGaa8NlaKqbaoaadmaabaqcLbsapaGaa8hsaKqbao aaBaaajuaibaqcLbmacaWFYaaajuaGbeaajugibiaa=9eajuaGdaWg aaqcfasaaKqzadGaa8NmaaqcfayabaaapeGaay5waiaaw2faamaadm aabaqcLbsacaWFibGaa83taiab+5caUaqcfaOaay5waiaaw2faaKqz GeGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcqcfa4aaeWaae aacaaIYaGaaG4maaGaayjkaiaawMcaaaGcbaqcfa4damaalaaakeaa jugibiaa=rgajuaGdaWadaGcbaqcLbsacaWFgbGaa8xzaKqbaoaabm aabaqcLbsacaWFjbGaa8xsaiaa=LeaaKqbakaawIcacaGLPaaaaOGa ay5waiaaw2faaaqaaKqzGeGaa8hzaiaa=rhaaaGaa8xpaiab+jHiTK qbaoaalaaakeaajugibiaa=rgacaWFgbGaa8xzaKqbaoaabmaakeaa jugibiaa=LeacaWFjbaakiaawIcacaGLPaaaaeaajugibiaa=rgaca WF0baaa8qacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWF GcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=b kacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hO aiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGc Gaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bka caWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOai aa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGa a8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkaca WFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa =bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8 hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWF GcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=b kacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hO aiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGc Gaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkajuaGdaqadaqaaKqzGeGa a8Nmaiaa=rdaaKqbakaawIcacaGLPaaaaOqaaKqbaoaadmaabaqcLb sacaWFgbGaa8xzaKqbaoaabmaabaqcLbsacaWFjbGaa8xsaiaa=Lea aKqbakaawIcacaGLPaaaaiaawUfacaGLDbaajugibiaa=1dajuaGda qadaqaaKqzGeGaa8Nraiaa=vgajuaGdaahaaqabKqbGeaajugWaiaa =ndajuaicaWFRaaaaaqcfaOaayjkaiaawMcaaKqzGeGaa83kaKqbao aadmaabaqcLbsacaWFgbGaa8xzaKqbaoaabmaabaqcLbsacaWFibGa a83taKqbaoaaBaaajuaibaqcLbmacaWFYaaajuaGbeaaaiaawIcaca GLPaaadaahaaqabKqbGeaajugWaiaa=jdajuaicaWFRaaaaaqcfaOa ay5waiaaw2faaKqzGeGaa83kaKqbaoaadmaabaqcLbsacaWFgbGaa8 xzaKqbaoaabmaabaqcLbsacaWFpbGaa8hsaaqcfaOaayjkaiaawMca amaabmaabaqcLbsacaWFibGaa83taKqbaoaaBaaajuaibaqcLbmaca WFYaaajuaGbeaaaiaawIcacaGLPaaadaahaaqabKqbGeaacaWFRaaa aaqcfaOaay5waiaaw2faaKqzGeGaa83kaiaa=jdajuaGdaWadaqaaK qzGeGaa8Nraiaa=vgajuaGdaWgaaqcfasaaKqzadGaa8Nmaaqcfaya baWaaeWaaeaajugibiaa=9eacaWFibaajuaGcaGLOaGaayzkaaWaaS baaKqbGeaajugWaiaa=jdaaKqbagqaamaaCaaabeqcfasaaKqzadGa a8hnaKqbGiaa=TcaaaaajuaGcaGLBbGaayzxaaqcLbsacaWFGcGaa8 hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWF GcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=b kacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hO aiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGc Gaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaKqbaoaa bmaabaqcLbsacaWFYaGaa8xnaaqcfaOaayjkaiaawMcaaaaaaa@B419@

To meet the needs of practical applications, the studies on kinetic models of this system were reported increasingly for degradation of specific organic pollutants. Chen et al. [45] studied the phenol degradation process via this system and presented the corresponding degradation path, with improvement on the degradation mechanism of aromatic compounds and the practicability of the Fenton system. The author believed that the phenol is first attacked by • OH to generate the radical of dihydroxy cyclohexadiene (DHCD•), after that DHCD• will be dehydrogenized into hydroquinone (HQ). HQ with strong reduction ability can react quickly with Fe3+ to produce semiquinone radical (SQ •), and SQ • will continue to reduce Fe3+ into benzoquinone (BQ), at last BQ will be reduced into SQ • by DHCD •. In the above process, the quinone is an intermediate which will accelerate the circulation of Fe3+/Fe2+ and promote the generation of Fe2+. The whole process is shown as an autocatalytic process (Figure 1). The Duesterberg research group [46] investigated the autocatalytic process of the oxidation on the hydroxy acid by the Fenton system. Based on the Pignatello research model, they revised and added some reactions, adjusted the rate constants of radical reactions, and established a kinetic model of the oxidation on the hydroxy acid by the Fenton system. The results also proved the role of quinone intermediates in the catalytic degradation process of aromatic compounds (Figure 1).

Figure 1: Quinone intermediates electron transfer in the process of phenol oxidation.

  • Reaction mechanism of ferric Ion

The mechanism of ferric ion was first proposed by Bary & Gorin [12] in 1932, in which they believed the reaction pathway to generate Fe (IV) might be in the Fenton system and the relevant process were as follows (reaction 26,27):

Fe( II )+ H 2 O 2 Fe( IV )O+ H 2 O 2 MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaGqaaKqzGeGaa8 Nraiaa=vgajuaGdaqadaGcbaqcLbsacaWFjbGaa8xsaaGccaGLOaGa ayzkaaaccaqcLbsacqGFRaWkcaWFibqcfa4aaSbaaKGaGeaajugWai aa=jdaaWqabaqcLbsacaWFpbqcfa4aaSbaaKGaGeaajugWaiaa=jda aWqabaqcLbsacqGFsgIRcaWFgbGaa8xzaKqbaoaabmaakeaajugibi aa=LeacaWFwbaakiaawIcacaGLPaaajugibiaa=9eacqGFRaWkcaWF ibqcfa4aaSbaaKqbGeaacaWFYaaajuaGbeaajugibiaa=9eajuaGda Wgaaqcfasaaiaa=jdaaKqbagqaaaaa@56C0@
Fe(IV)O+ H 2 O 2 Fe( II )+ H 2 O 2 MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaGqaaKqzGeGaa8 Nraiaa=vgacaWFOaGaa8xsaiaa=zfacaWFPaGaa83taGGaaiab+Tca Riaa=HeajuaGdaWgaaqccasaaKqzadGaa8Nmaaadbeaajugibiaa=9 eajuaGdaWgaaqccasaaKqzadGaa8Nmaaadbeaajugibiab+jziUkaa =zeacaWFLbqcfa4aaeWaaOqaaKqzGeGaa8xsaiaa=LeaaOGaayjkai aawMcaaKqzGeGae43kaSIaa8hsaKqbaoaaBaaajiaibaqcLbmacaWF YaaameqaaKqzGeGaa83taKqbaoaaBaaajiaibaqcLbmacaWFYaaame qaaaaa@561C@
Fe( II )+ H 2 O 2 Fe( IV )O+ H 2 O           ( 26 ) Fe( IV )O+ H 2 O 2 Fe( II )+ H 2 O + O 2   ( 27 ) MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOabaeqabaacbaqcLb sacaWFgbGaa8xzaKqbaoaabmaakeaajugibiaa=LeacaWFjbaakiaa wIcacaGLPaaaiiaajugibiab+TcaRiaa=HeajuaGdaWgaaqcfasaai aa=jdaaKqbagqaaKqzGeGaa83taKqbaoaaBaaajuaibaGaa8Nmaaqc fayabaqcLbsacqGFsgIRcaWFgbGaa8xzaKqbaoaabmaakeaajugibi aa=LeacaWFwbaakiaawIcacaGLPaaajugibiaa=9eacqGFRaWkcaWF ibqcfa4aaSbaaKqbGeaacaWFYaaajuaGbeaajugibiaa=9eaqaaaaa aaaaWdbiaa=bkacaWFGcGaa8hOaiaa=bkacaWFGcGaa8hOaiaa=bka caWFGcGaa8hOaiaa=bkacaWFGcqcfa4aaeWaaOqaaKqzGeGaa8Nmai aa=zdaaOGaayjkaiaawMcaaaqaaKqzGeWdaiaa=zeacaWFLbqcfa4a aeWaaOqaaKqzGeGaa8xsaiaa=zfaaOGaayjkaiaawMcaaKqzGeGaa8 3taiab+TcaRiaa=HeajuaGdaWgaaqcfasaaiaa=jdaaKqbagqaaKqz GeGaa83taKqbaoaaBaaajuaibaGaa8NmaaqcfayabaqcLbsacqGFsg IRcaWFgbGaa8xzaKqbaoaabmaakeaajugibiaa=LeacaWFjbaakiaa wIcacaGLPaaajugibiab+TcaRiaa=HeajuaGdaWgaaqcfasaaiaa=j daaKqbagqaaKqzGeGaa83ta8qacaWFGcGae43kaSIaa83taKqbaoaa BaaajuaibaGaa8NmaaqcfayabaqcLbsacaWFGcGaa8hOaKqbaoaabm aakeaajugibiaa=jdacaWF3aaakiaawIcacaGLPaaaaaaa@8D56@

After that, more and more experiments proved that under certain conditions the Fenton system can produce ferric ions with oxidization properties. In the 1940s, George [47] and Abel [48] questioned the hydroxyl free radical mechanism successively and argued that this system may exist other oxidizing substances, such as ferric ions. Kremer [49] questioned the form of ferric ions in solution in the hydroxyl free radical mechanism. According to this mechanism, the iron is in the form of ferrous and ferric ions in the system, and it was speculated that at low concentrations of hydrogen peroxide, the reaction (28) is less likely to occur, and most of the hydroxyl radicals generated in the reaction (29) will react with ferrous ions to produce ferric ions, and then the ferric ions will continue to react with hydroxyl radicals according to the reaction (30). If the reaction does occur in the system, it proves that Fe3+ and hydroxyl radicals cannot exist alone in the system, both will be further reacted into FeOH3+, the protonated form of FeO2+ (FeO2++H+­—FeO2+), which also suggests the possible presence of ferric ions from the other side.

O H + H 2 O 2 H O 2 MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaGqaaKqzGeGaa8 3taiaa=HeakmaaCaaaleqajeaibaaccaqcLbmacqGFIaYTaaqcLbsa cqGFRaWkcaWFibqcfa4aaSbaaKGaGeaajugWaiaa=jdaaWqabaqcLb sacaWFpbqcfa4aaSbaaKGaGeaajugWaiaa=jdaaWqabaqcLbsacqGF sgIRcaWFibGaa83taKqbaoaaBaaajiaibaqcLbmacaWFYaaameqaaa aa@4B25@ (28)

F e 2+ + H 2 O 2 F e 3+ +O H +O H MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaGqaaKqzGeGaa8 Nraiaa=vgalmaaCaaameqajiaibaqcLbmacaWFYaaccaGae43kaSca aKqzGeGae43kaSIaa8hsaKqbaoaaBaaajiaibaqcLbmacaWFYaaame qaaKqzGeGaa83taKqbaoaaBaaajiaibaqcLbmacaWFYaaameqaaKqz GeGae4NKH4Qaa8Nraiaa=vgalmaaCaaameqajiaibaqcLbmacaWFZa Gae43kaScaaKqzGeGae43kaSIaa83taiaa=HeakmaaCaaaleqajeai baqcLbmacqGFsislaaqcLbsacaWFRaGaa83taiaa=HealmaaCaaame qajiaibaqcLbmacqGFIaYTaaaaaa@5712@ (29)

F e 3+ +O H FeO H 3+ MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbb a9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaGqaaKqzGeGaa8 Nraiaa=vgalmaaCaaameqabaGaa83maGGaaiab+TcaRaaajugibiab +TcaRiaa=9eacaWFibGcdaahaaWcbeqcbasaaKqzadGae4NiGClaaK qzGeGae4NKH4Qaa8Nraiaa=vgacaWFpbGaa8hsaSWaaWbaaWqabeaa caWFZaGae43kaScaaaaa@4790@ (30)

In the last two decades, studies have focused a lot of attention on the oxidation of organic pollutants by the Fenton reagent under neutral conditions, and regarding the reactive mechanism under neutral conditions, many researchers have also carried out extensive investigation of the generation and reaction mechanism of ferric ions. During the study on the oxidation process of H2O2 on cytochrome C catalyzed by the complex of Fe (II) and EDTA, Koppenol et al. [50] found that the reaction characteristics of this process are completely different from those of the oxidation process hydroxyl radicals produced by pulse radiolysis on cytochrome C, and they also concluded that the intermediate with oxidation activity may be present as ferric ions. O Pestovsky et al. [51] excluded the possibility of tetravalent iron existence under the conditions of pH≤3 by use of different principles on the respective reactions of (CH3) 2SO to tetravalent iron and hydroxyl radicals. But under the conditions of pH greater than 3, more and more evidence has proved the existence of tetravalent iron in the system. Stephan et al. [52] studied the oxidation process of As3+ in natural water by the Fenton reagent, and found that under conditions of pH=3.5-7.5, the hydroxyl radical scavenger propanol and formate, do not have a significant impact on the oxidation reaction, so they believe that there are other reactive oxidizing substances in this system and presume it is ferric ion. Laurent et al. [53] simulated the conversion conditions of Fe2+ and Fe3+ ions during the Fenton reaction within the droplets in clouds under two different mechanism conditions, and found that both mechanisms can explain well the process of iron conversion. They also believe that during the reaction the concentration of ferric ion is higher than that of hydroxyl radicals in four orders of magnitude, so that the ferric ion dominates in the reaction. Rahhal et al. [54] found some phenomena against the nature of hydroxyl radicals in the catalytic decomposition process of H2O2 by the complex of Fe (II) and diethyl pentaacetic acetic acid that is the active oxidative intermediate produced in the experiment can be inhibited by t-butanol, so they presumed that the active oxidative intermediate in the system is ferric ion. During the study on the role of NO in oxidation of organic compounds by the Fenton reagent, Sharpe et al. [55] noted that in the reaction, Fe2+ and hydrogen peroxide will first produce two different ferric ions: FeO2+ and FeOH3+, then they will hydrolyze or directly break bonds into hydroxyl radicals, but also pointed out that in the reaction no product generated by hydroxyl radicals and tracer was observed. Walling et al. [56] believe that in this system metal ions will first form a complex with H2O2, the nature of which will decide the following reaction mechanisms: (1) if single electron transfer occurs in the complex of Fe and H2O2, Fe will lose an electron, and produce hydroxyl radicals and the reaction will proceed as the hydroxyl free radical mechanism; (2) if double electron transfer occurs in the complex generated, Fe will lose two electrons, and form strong oxidizing ions with higher valence, then the reaction will proceed as ferric ion mechanism; (3) if the generated complex itself has strong oxidation, the complex in the whole system is the main active oxide intermediate, but this theory has not yet been confirmed experimentally and theoretically.

Summary and Outlook

Currently, in the actual study, since hydroxyl radicals are easily observed in the electron spin resonance (ESR) spectrum, and the reaction kinetics may also be described by the oxidation of hydroxyl radicals, most researchers prefer the hydroxyl free radical mechanism, but in the presence of complexing agents or in some neutral systems, the hydroxyl free radical mechanism alone does not adequately explain some of the special phenomena in the experiment. In the above conditions, more and more researchers began to favor the ferric ion mechanism. Actually, the Fenton system is used widely in chemical, biological and environmental systems, and it is unlikely that this is with the same mechanism all the time. Sometimes two mechanisms may co-exist in different systems, or some kind of mechanism may be dominant, depending on the specific nature of the system. Further research on the reaction mechanism of the system should focus on direct evidence for the existence of an active intermediate by use of the latest online analysis and detection combined with tracking tools and techniques. On this basis, the next work should focus on the catalytic activity and the degradation efficiency of H2O2, to research the impact of other assistive technology (photocatalyst/ultrasonic/ultraviolet, etc.) on the active intermediate in the system, and to provide theoretical guidance for the efficient industrial application of this technology.

Acknowledgement

This research project was supported by the Fundamental Research Funds for the Central Universities under Grant 15CX02102A and Shandong Provincial Natural Science Foundation, China under Grant 2013ZRE28029.

Conflict of Interest

None.

References

  1. Krzysztof Barbusinski (2009) Fenton reaction-controversy concerning the chemistry. Ecological Chemistry and Engineering S 16(3):347-358.
  2. Sonntag C (2008) Advanced oxidation processes: mechanistic aspects. Water Sci Technol 58(5): 1015-1021.
  3. Huang CP, Dong C, Tang Z (1993) Advanced chemical oxidation: Its present role and potential future in hazardous waste treatment. Waste Management 13(5-7): 361-377.
  4. Mark Daniel G, de Luna, James I, Colades, Chia-Chi Su, Ming-Chun Lu (2013) Comparison of dimethyl sulfoxide degradation by different Fenton processes. Chemical Engineering Journal 232(10): 418-424.
  5. İpek Gulkaya, Gulerman A Surucu, Filiz B Dilek (2006) Importance of H2O2/Fe2+ ratio in Fenton's treatment of a carpet dyeing wastewater. Journal of Hazardous Materials 136(3): 763-769.
  6. TT Pham, SK Brar, RD Tyagi, RY Surampalli (2010) Optimization of Fenton oxidation pre-treatment for B. thuringiensis – Based production of value added products from wastewater sludge. J  Environ Manage 91(8): 1657-1664.
  7. Helen Barndõk, Laura Blanco, Daphne Hermosilla, Ángeles Blanco (2016) Heterogeneous photo-Fenton processes using zero valent iron microspheres for the treatment of wastewaters contaminated with 1,4-dioxane. Chemical Engineering Journal 284(1): 112-121.
  8. Stefanos Giannakis, Cristina Ruales-Lonfat, Sami Rtimi, Sana Thabet, Pascale Cotton, et al. (2016) Castles fall from inside: Evidence for dominant internal photo-catalytic mechanisms during treatment of Saccharomyces cerevisiae by photo-Fenton at near-neutral pH. Applied Catalysis B: Environmental 185(5):150-162.
  9. Nannan Wang, Tong Zheng, Jiping Jiang, Wu-seng Lung, Xiaojun Miao, et al.  (2014) Pilot-scale treatment of p-Nitrophenol wastewater by microwave-enhanced Fenton oxidation process: Effects of system parameters and kinetics study. Chemical Engineering Journal 239(3): 351-359.
  10. Zhihui Xu,Yaqun Yu, Di Fang, Jiangyan Xu, Jianru Liang, et al.  (2015) Microwave-ultrasound assisted synthesis of β-FeOOH and its catalytic property in a photo-Fenton-like process. Ultrason Sonochem 27(11): 287-295.
  11. Haber F, Weiss J (1934) The catalytic decomposition of hydrogen peroxide by iron salts. Proc R Soc Lond. 147: 332-352.
  12. Bary WC, Gorin MH (1932) Ferryl Ion, A Compound of Tetravalent Iron. J Am Chem Soc 54(5): 2124-2125.
  13. Haber F, Weiss J (1932) Uber die Katalyse des Hydroperoxydes. Naturwissens chaften 20(51): 948-950.
  14. George P (1947) Some experiments on the reactions of potassium superoxide in aqueous solutions. Discussions of the Faraday Society 2: 196-205.
  15. Neuman EW (1934) Potassium Superoxide and the Three-Electron Bond. The Journal of Chemical Physics 2(1): 31-33.
  16. Pauling L (1979) The discovery of the superoxide radical. Trends in Biochemical Sciences 4(11): N270-N271.
  17. Barb WG, Baxendale JH, George P (1949) Reactions of Ferrous and Ferric Ions with Hydrogen Peroxide. Nature 163(4148): 691-694.
  18. Weiss J, Humphrey CW (1949) Reaction between Hydrogen Peroxide and Iron Salts. Nature 163(4148): 691.
  19. Barb WG, Baxendale JH, George P (1951) Reactions of ferrous and ferric ions with hydrogen peroxide. Part 1-The ferrous ion reaction. Transactions of the Faraday Society 47: 462-500.                   
  20. Barb WG, Baxendale JH, George P (1951) Reactions of ferrous and ferric ions with hydrogen peroxide. Part 11.-The ferric ion reaction. Transactions of the Faraday Society 47: 591-616.
  21. Rigg T, Taylor W, Weiss J (1954) The Rate Constant of the Reaction between Hydrogen Peroxide and Ferrous Ions. The Journal of Chemical Physics 2(4): 575-577.
  22. Wells CF, Salam MA (1965) Hydrolysis of Ferrous Ions : a Kinetic Method for the Determination of the Fe(II) Species. Nature 205(4972): 690-692.
  23. Wells CF, Salam MA (1968) The effect of pH on the kinetics of the reaction of iron (II) with hydrogen peroxide in percWorate media. Journal of the Chemical Society A: Inorganic, Physical, Theoretical, China, p. 24-29.
  24. Rabinowitch E, Stockmayer WH (1942) Association of Ferric Ions with Chloride, Bromide and Hydroxyl Ions (A Spectroscopic Study). Journal of the American Chemical Society 64(2): 335-347.
  25. Jones P, Kitching R, Tobe ML (1959) Hydrogen peroxide + water mixtures. Part 4-Catalytic decomposition of hydrogen peroxide. Transactions of the Faraday Society 55: 79-90.
  26. Kremer ML, Stein G (1959) The catalytic decomposition of hydrogen peroxide by ferric perchlorate. Transactions of the Faraday Society 55: 959-973.
  27. Kremer ML (1962) Nature of intermediates in the catalytic decomposition of hydrogen peroxide by ferric ions. Transactions of the Faraday Society 58: 702-707.
  28. Kremer ML (1963) Oxidation reduction step in catalytic decomposition of hydrogen peroxide by ferric ions [J]. Transactions of the Faraday Society 59: 2535-2542.
  29. Kremer ML, Stein G (1977) Kinetics of the Fe3+ ion-H2O2 reaction: Steady-state and terminal-state analysis. International Journal of Chemical Kinetics 9(2):179-184.
  30. Walling C, Goosen A (1973) Mechanism of the ferric ion catalyzed decomposition of hydrogen peroxide. Effect of organic substrates. Journal of the American Chemical Society 95(9): 2987-2991.
  31. Walling C, Weil T (1974) The ferric ion catalyzed decomposition of hydrogen peroxide in perchloric acid solution. International Journal of Chemical Kinetics 6(4):507-516.
  32. Walling C, Cleary M (1977) Oxygen evolution as a critical test of mechanism in the ferric-ion catalyzed decomposition of hydrogen peroxide. International Journal of Chemical Kinetics 9(4): 595-601.
  33. Lewis T, Richards D, Salter D (1963) Peroxy-complexes of inorganic ions in hydrogen peroxide-water mixtures. Part I. Decomposition by ferric ions. Journal of the Chemical Society 1963: 2434-2446.
  34. Goldstein S, Meyerstein D, Czapski G (1993) The Fenton reagents. Free Radic Biol and Med 15(4): 435-445.
  35. Masarwa M, Cohen H, Meyerstein D (1988) Reactions of low-valent transition-metal complexes with hydrogen peroxide. Are they Fenton-like or not. 1. The case of Cu+ and Cr2+. Journal of the American Chemical Society 110(13):4293-4297.
  36. Goldstein S, Meyerstein D. (1999) Comments on the mechanism of the "Fenton-like"reaction. Accoxints of chemical research 32(7): 547-550.
  37. Gallard H, de Laat J, Legube B (1998) Influence du pH sur la vitesse d’oxydation de compose [prmie or minute]s organiques par Fe/H2O2. Me [prime or minute] canismes re [prime or minute] actionnels et mode [prime or minute] lisation. New Journal of Chemistry 22(3): 263-268.
  38. Gallard H, De Laat J (2001) Kinetics of oxidation of chlorobenzenes and phenyl-ureas by Fe(II)/H2O2 and Fe(III)/H2O2. Evidence of reduction and oxidation reactions of intermediates by Fe(II) or Fe(III). Chemosphere 42(4): 405-413.
  39. Schroder D, Barsch S, Schwarz H (2000) Second Ionization Energies of Gaseous Iron Oxides and Hydroxides: The FeOmHn2+ Dications (m= l,2;n≤ 4). The Journal of Physical Chemistry A 104(21): 5101-5110.
  40. Gallard H, de Laat J, Legube B (1998) Effect of pH on the Oxidation Rate of Organic Compounds by FeII/H2O2. Mechanisms and simulation. New Journal of Chemistry 22(3): 263-268.
  41. De Laat J, Gallard H (1999) Catalytic Decomposition of Hydrogen Peroxide by Fe(III) in Homogeneous Aqueous Solution: Mechanism and Kinetic Modeling. Environmental Science & Technology 33(16): 2726-2732.
  42. Gallard H (2000) Kinetic Modelling of Fe(III)/H2O2 Oxidation Reactions in Dilute Aqueous Solution using Atrazine as a Model Organic Compound. Water Research 34(12): 3107-3116.
  43. De Laat J, Le TG (2005) Kinetics and Modeling of the Fe(III)/H2O2 System in the Presence of Sulfate in Acidic Aqueous Solutions. Environmental Science & Technology 39(6): 1811-1818.
  44. Walling C, Kato S (1971) Oxidation of Alcohols by Fenton's Reagent. Effect of Copper Ion. Journal of the American Chemical Society 93(17): 4275-4281.
  45. Chen R, Pignatello JJ (1997) Role of Quinone Intermediates as Electron Shuttles in Fenton and Photo assisted Fenton Oxidations of Aromatic Compounds. Environmental Science & Technology 31(8): 2399-2406.
  46. Duesterberg CK, Waite TD (2007) Kinetic Modeling of the Oxidation of p-Hydroxybenzoic Acid by Fenton’s Reagent: Implications of the Role of Quinones in the Redox Cycling of Iron. Environmental Science & Technology 41(11): 4103-4110.
  47. George P, Irvine DH (1952) Reaction of methmyoglobin with hydrogen peroxide. Biochem 52(3):511-517.
  48. Koppenol WH (2001) The Haber-Weiss cycle - 70 years later. Redox Report 6(4):229-234.
  49. Kremer ML (1999) Mechanism of the Fenton reaction. Evidence for a new intermediate. Phys. Chem. Chem Phys 1: 3595-3605.
  50. Koppenol W (1985) The Reaction of Ferrous EDTA with Hydrogen Peroxide: Evidence Against Hydroxyl Radical Formation. Journal of Free Radicals in Biology and Medicine 1(4): 281-285.
  51. Pestovsky O, Stoian S, Bominaar EL (2005) Aqueous Fe-IV=O: Spectroscopic identification and oxo-group exchange. Angewand Te Chemie-international Edition 44(42):6871-6874.
  52. Stenphan JH, Olivier L (2003) Iron-catalyzed oxidation of arsenic (III) by oxygen and by hydrogen peroxide: pH-dependent formation of oxidants in the fenton reaction. Environmental Science and Technology 37(12):2734-2742.
  53. Laurent D, Maud L, Nadine C (2005) Impact of radical versus non-radical pathway in the fenton chemistry in the iron redox cycle in clouds. Chemosphere 60(5): 718-724.
  54. Rahhal S, Richter HW (1988) Reduction of Hydrogen Peroxide by the Ferrous Iron Chelate of Diethylenetriamine-N,N, N', N",N"-Pentaacetate. Journal of the American Chemical Society 110(10): 3126-3133.
  55. MA Sharpe, SJ Robb, JB Clark J (2003) Nitric oxide and Fenton/Haber–Weiss chemistry: nitric oxide is a potent antioxidant at physiological concentrations. Neurochem 87(2): 386-394.
  56. Walling C (1998) Intermediates in the Reactions of Fenton Type Reagents. Accounts of Chemical Research 31(4): 155-157.
Creative Commons Attribution License

©2018 liu, et al. This is an open access article distributed under the terms of the, which permits unrestricted use, distribution, and build upon your work non-commercially.