Degradation kinetics and mechanisms of phenol in photo-Fenton process

198He et al.1.J Zhejiang Univ SCI 2004 5(2):198- 205hejiang University SCIENCEISSN 1009-3095htp://ww.zju.edu.cn/jzusJzUSE-mail: jzus@zju.edu.cnDegradation kinetics and mechanisms of phenolin photo-F enton processHE Feng(何锋), LEI Le-cheng (雷乐成)(Department of Environmental Engineering, Zhejiang University, Hangzhou 310027, China)E-mail: lclei@mail.hz.zj.cnReceived Feb. 10, 2003; revision accepted May 20, 2003Abstract: Phenol degradation in photochemically enhanced Fenton process was investigated in this work. UV-VIS spectraof phenol degradation showed the difference between photo-Fenton process and UV/H2O2, which is a typical hydroxyl radicalprocess. A possible pathway diagram for phenol degradation in photo-Fenton process was proposed, and a mathematicalmodel for chemical oxygen demand (COD) removal was developed. Operating parameters such as dosage of H2O2 and ferrousions, pH, suitable carrier gas were found to impact the removal of COD significantly. The results and analysis of kineticparameters calculated from the kinetic model showed that complex degradation of phenol was the main pathway for removalof COD; while hydroxyl radicals acted weakly in the photo-Fenton degradation of phenol.Key words: Phenol, Photo-Fenton process, Kinetics, Radical oxidation, Complex oxidationDocument code: ACLC number: TQ150.9; X783INTRODUCTIONFe3++ HO2:-→>Fe2++H++ O2(3)Fenton process has been well studied recentlyThe combination of Fenton reaction in UV (U1-for its prospective applications in unmanageable traviolet) light, the so-called photo-Fenton reaction,wastewater treatment (Legrini et al, 1993; Ollis and had been shown to enhance the efficiency of theAl-Ekabi, 1993; Prousek, 1996). The high effi- Fenton process. Some researchers also attributedciency of this process is traditionally thought to be this to the decomposition of the photo -activedue to the generation of hydroxyl radical (HO)，Fe(OH)+ which lead to the addition of thewhich is of a high oxidation potential (E° = 2.80 V) HO radicals (Sun and Pignatello, 1993).and can mineralize the organic compounds com-pletely to water and carbon dioxide. In acidic me- Fe(OH)2+ + hv→Fe2++ HO-(4)dium, this radical mechanism can be simply de-However, the free radical mechanism had beenscribed by the following equations (Walling, 1975):questioned at times (Walling and Amarnath, 1982;Fe2++ H2O2→Fe3+ + OH+ HO(1)Kremer, 1985). The alternative mechanisms main-tained that complexes and compounds betweenFe2*+H2O2=昔eFe - 0OH*τ Fe* +HO2 (2) F(11) and H2O2 are the actual interediates in thereaction. Optical absorption measurements duringthe reaction also proved the presence of these com-* Projet (No.20176053) supported by the National Natural Science plexes (Kremet中国煤化工ntl, Boss-Foundation of Chinamann's work oM PVA usingYHCNM HG.He et al. /.J Zhejiang Univ SCI 2004 5(2): 198-205199using photo-Fenton process yielded experimentaldistilled quality. All other chemicals were ACSevidence for the formation of supermolcule from grade.Fe1ID)(I)-PVA and no low molecular weight in-termediates were detected during the PVA-degra- Photoreactor and photodegradation proceduredation. The Fenton-type oxidation of PVA pro-All photolysis experiments were carried out inceeded via a ferrylion (+IV) formed by a two- a batch reactor. Fig.1 shows the experimental setupelectron inner-sphere electron transfer reaction consisting of a reservoir (V=1.00 L), a flow-throughwithin the complex PVA/Fe+ and CO2 was released annular photoreactor (V=0.30 L) equipped with adirectly from the super-macromolecule (Bossmann mercury medium pressure lamp (Philips, GGZ 300)et al, 1998; 2001; Lei et al, 1998).and a quartz filter. The solution was recirculatedSpin-trapping experiments showed that free (250 ml/min) by a pump (Masterflex). The phenolhydroxyl radicals exist in Fenton or photo-Fenton solution was continuously purged (in the reservoir)reactions (Jiang et al, 1993). After study of relevant by O2 during the entire reaction time. All photolysis .literature, we consider that hydroxyl radical oxida- experiments were performed for 60 min. The tem-tion and/or some other complex reaction occur in the perature of the solutions was kept at 30 °C.photo-Fenton process. The problem is that hydroxylUV lampradicals can oxidize simple organic pollutants di-rectly, while refractory ones such as PVA can onlybe degraded through complex pathway.The kinetics of pollutant degradation in Fentonor Photo-Fenton process in the hydroxyl radicalreaction mechanism had been well established byAirmany researchers (Gallard and Laat, 2000; Andreo-zzi et al, 2000). However, kinetics studies whichTAirinclude both HO- reaction and complex oxidation inWater bathphoto-Fenton process, have rarely been reported.Recycle pumpThis work, with phenol chosen as the model organicFig.1 Photochemical batch reactor employed in all irradia-compound established the kinetic model, whereintion experimentsHO. reaction pathway or some other complexpathway are involved in the photo-Fenton process;The total volume of the photolysis solution wasone way is through the inner- sphere electron trans- 1.00 L. The initial concentration of the phenol so-fer where phenol directly oxidized to CO2 and H2O; lution was 100 mg/L. FeSO4 7H2O was added intoin the other way, phenol is first converted to organic the phenol solution before H2O2 was added. Theacid under the attack of HO, then oxidized to CO2 stoichiometric amount of H2Oz (Qth) required for theand H2O. Operating parameters such as H2O2 total oxidation of phenol was calculated by usingvariation, Fet+ variation, initial pH, purge-gas were Eq.(5).studied to investigate the validity and feasibility ofthe proposed model.C6H6O + 14H2O2-→6CO2 + 17H2O(5)C6H6O + 7O2→6CO2 + 3H2O(6)EXPERIMENTAL SECTIONThe pH was adjusted by using proper H2SO4.Oxidation was monitored by measurement of chemi-Materialscal oxygen demand (COD). Residual H2O2 wasFeSO4 7H2O, FeCl;:6H2O, H2O2, H2SO4, NaOH,consumed by enzyme catalase to prevent interfer-phenol, l,10-phenanthroline acetonitrile and acetic ence with COD analysis.acid were all analytical grade, catalase was pur-中国煤化工chased from W orthington; water was of double-Analytical meMYHCNMH G.200He et al. /.J Zhejiang Univ SCI 2004 5(2):198. 205UV/VIS (8500 spectrophotometer, Techcomp) tion of intermediates with characterizing absorptionwas used to measure the UV-VIS spectra of the at 320 nm. At 20 min, the character peaks (209 nm,phenol solutions, while pH was measured by Rex- 269 nm) of phenol disappears and its absorption atpH-analyzer (PHS-25).320 nm also decreases. So all phenol have beenCOD analysis was carried out by titrimetric oxidized at this time and the formed intermediatesmethod. Analysis of ferrous irons was conducted by are being further oxidized to CO2 and H2O.using the modified 1,10-phenanthroline colorimet-Fig.3 shows the UV/VIS absorption spectraric method at 510 nm.occurring during photochemically enhanced Fentondegradation of phenol. It is noteworthy that a colloidwith high absorption at 200 nm to 400 nm formed atRESULTS AND DISCUSSIONSthe moment of FeSO4 7H2O addition to phenol andH2O2 mixture. At the same time phenol's charac-Comparison of UV/VIS-Absorption Spectra dur- terizing peaks disappear in the UV/VIS spectra.ing photo-Fenton and UV/H2O2 phenol degrada- Fig.4 and Fig.5 show that no complex with hightionabsorbance can be formed between Fe(II)/Fe(II)UV/VIS spectra of the reaction mixture during and phenol under the irradiation. Iron may appear inthe UV/H2O2 and photo-Fenton degradation of a new valence state (as Fe(+IV) or Fe(+V)) in thephenol are shown in Fig.2 and Fig.3, respectively. formed complex (colloid). With reaction going on,As UV/H2O2 process is a typical hydroxyl radical no new absorption peak could be found in thereaction, its spectra change in phenol degradation photo-Fenton process spectra. Some phenol waswas investigated for comparison with that of degraded through the inner electron transfer inphoto-Fenton process. Fig.2 shows that addition of photo-Fenton process. This observation is greatlyH2O2 in phenol solution does not change the spectral different from the degradation of phenol in UV/H2O2character peak of the reaction solution. No complex process. Both complex and low molecular weightwith high absorbance was formed in this process. intermediates should be generated in photo-FentonLight absorption at phenol's absorption peaks (209 degradation of phenol, wherein photolysis of H2O2nm, 269 nm) decreases evenly with time. Moreover, also occurs.a new peak (320 nm, though not very obvious) ap-pears at 10 min in the spectra. These spectra results Kinetic modelindicate the degradation of the phenol and the forma-The UV/VIS spectra of phenol degradation in1.1.2-0min1.2 ;一0min.....5 min ...... 2 mir....5min---.. 10 min宫0.91---.10min.20 min0.8----- 30 min---- H,O,0.6 I0.40.3 I20200 25030035400Wavelength (nm)Fig.3-spectra during the photo-Fenton reaction in theFig.2 UV-spectra during the UV/H2O2 reaction in the pre-presence of phenolsence of phenolOperational condit中国煤化工D7 mg/L, tem-Operational conditions: H2O2 Qh, temperature 30°C, pH 3perature 30 °C, pHMHCNM HG.He et al. / J Zhejiang Univ SCI 2004 5(2): 198-20501.5厂1.5.2一0min---- 30 min昌0---- 30min& 0.6号0.6.3-0.:200253504000.0200 250 300350 400Wavelength (nm)Fig.4 UV-spectra during the photolysis of phenol and ferr- Fig.5 UV-spectra during the photolysis of phenol and frricous ion solutionion solutionOperational conditions: FeSO4 7H2O 207 mg/L, temperatureOperational conditions: FeCl:6H2O 201 mg/L, temperature 3030°C,pH3。C, pH 3UV/H2O2 and of photo- Fenton process shows thatd[B]= K;[B]- K,[4](8)phenol degradation can probably proceed throughdtwo pathways: complex oxidation and HO oxida-tion. Fig.6 may represent the principal pathways ofwhere [4] represents the concentration of the phenol,phenol degradation.[B] represents the concentration of the organicsexcept A.In a homothermal ideal reactor with periodicalK，A-continuous current, the apparent rate constant K; canbe written as follows:K， K;\CK↔=k°e-E1RT(9)K,=k,°e-BIRT(10)Fig.6 The principal pathways of phenol degradationK;=k,°e-5/RT(11)In the above reaction pathways (Fig.6), Arepresents the original organic compound (phenol),K; is related to reaction parameters such as theB is the organic intermediates such as acetic acid, dosage of H2O2 and ferrous ions, pH, suitable carrieretc., and C is the final substrates, CO2 and H2O. K;(igas, and so on.=1, 2, 3) is the apparent rate constant (min^ '). TheThe integration constants can be evaluatedpathway from A to c represents the complex reac- from the initial conditions:ion; that from A to B to C represents the radicalreaction.At time t=0, [4]=[A]o, [B]=[B]o=0If all reactions can be simplified to pseudo-first-order kinetics, the following set of differentialThe generalized kinetic model is given byequations could be obtained according to the reac-tion diagram:[4]=[4]elK+K) .(12)_d[4]=(K +K2)[4](7)[B]=[B]e中国煤化工2)] (13)YHCNM HG.202He et al.1.J Zhejiang Univ SCI 2004 5(2):198- 205Combining Eq.(12) and Eq.(13) yields:00 rKK-K;-(K.+K2)180■60mg/L[A+B]。 K.+K2-K、° K+K2- K;●100 mg/L .▲200 mg/L(14)食6040[A+ B] represents the concentration of all the organicchemicals and, [4+B]o is the [A+ B] value at reactiontime zero. Applying the kinetic equation to the CODdata and with2Reaction time (min)COD_ [A+ B](1 5) Fig.7 Phenol solution COD degradation at dfferent initialCOD。[A+ B]。phenol concentrationOperational conditions: H2O2 Qi, FeSO4 7H2O 207 mg/L, tem-The COD-t relationship becomes:perature 30°C, pH3Table 1 Calculated results at different initial concentration[COD]=[COD]_K:+K2-K;e-Ks1of phenol[4]。m)K2K3e- (K+K2)1(16)6(0.66300.0800 0.0424 0.7430 4.74K:+K2-K;1000.53650.2013 0.0349 0.7378 2.67This is the equation describing the reduction of2000.27520.2030 0.0350 0.4782 1.36COD with reaction time. Moreover, in order to eva-luate the proportion of complex reaction to radicalreduced from 0.7430 to 0.7378. However, when thereaction, the selectivity a is defined as K|/K2. The initial phenol concentration increased to 200 mg/L,total apparent reaction constant for phenol elimina- constant K sharply reduced to 0.4782. This obser-tion is defined as K=Kp+K2. To explore the feasi- vation indicates that initial phenol concentrationbility of the model, phenol degradation at different affects the rate of phenol removal insignificantlyinitial concentrations was examined.when it ranged from 60 mg/L to 100 mg/L. Themodel could be used well to predict degradation ofEffect of initial phenol concentrationphenol at low concentration.Fig.7 shows the effect of initial concentrationsof phenol (60 mg/L, 100 mg/L, 200 mg/L) on theEffect of initial H2O2 additionevolution of COD. In this figure, the symbols rep-Fig.8 shows the effect of initial addition ofresent the experimental data, while the lines repre-H2O2 on the COD removal in photo-Fenton degra-sent the calculated curves ftted by the model that dation of phenol. The reaction constants obtained byproposed above. Obviously, the curves of COD nonlinear regression are listed in Table 2. Eq.(16)degradation fitted very well for three initial phenol shows that with sufficiently extended reaction time,concentrations (60 mg/L, 100 mg/L, 200 mg/L). The the COD approaches zero. That is, given sufficientgood agreement between the experimental data and time, phenol can be completely mineralized to CO2.the fted lines of the equation supports well the and H2O in photo-Fenton process.proposed degradation model of phenol. CalculatedTable 2 shows that when the H2O2 addition iskinetics parameters such as K; and a are listed in 1/4 Qth, the calculated curve of COD removal de-Table 1.viated from the X-axis significantly. This can beTable 1 shows that when initial phenol con- explained. The中国煤化imited H2O2.centration increased from 60 mg/L to 100 mg/L,K in the initial reincompleteMHCNM H G".He et al. 1.J Zhejiang Univ SCI 2004 5(2): 198- 20503rate. However, this plateau still exists and does not100厂lead to enhanced removal rate of COD when ex-●12Qm .cessively large amount of H2O2 (2 Quh) is employed.80▲Q。This observation suggests that beyond the threhold,H2O2 concentration is not the limiting factor of the区60photo-Fenton process. This observation may beg 40related to the low value of K3, which leads to theinefficient removal of residual COD (acetic acid,201Effect of Fe2+ additionReaction time (min)Fig.9 shows the effect of [H2O2]/[Fe^*] (Q=Qn)Fig.8 Effect of initial concentration of H2O2 on phenolon phenol degradation. Parameters calculated aresolution COD degradationOperational conditions: FeSO4 7H2O 207 mg/L, temperaturelisted in Table 3. Similar to the effect of the amount30°C, pH3of H2O2 added, with increase of iron(I) concentra-tion K1 increases greatly while K2 increases slightly.Table 2 Calculated results at different initial concentrationThe initial concentration of iron(II) cannot affect theof H2O2radical oxidation of phenol significantly.H2O2K1K2KzKaIt also can be seen that the value of K3 is big-gest with lowest iron(I) concentration added. Lower1/4Qn 0.1202 0.1696 0.0046 0.2898 0.71iron(I) will consume less H2O2 in the initial reac-1/2Qm 0.2355 0.1936 0.0277 0.42911.22tion period. Thus in the process of organic acidQtn0.5365 0.2013 0.0349 0.73782.67oxidation (the period B to C in Fig.6), the residual2 Qt0.6116 0.2188 0.0353 0.8304 2.80H2O2 plays an active role. Fig.8 shows no signifi-cant difference of COD removal for [H2O2][Fe2*]oxidation of phenol. When more initial H2O2 wasadded from 1/2 Qh to 2 Qt, K increased from100.1202 to 0.6112. However, K2 kept its value around●[HO']V[Fe' ]-40:1at 0.2000, from 0.1936 to 0.2188. The increase of8■[H°O]V/Fe2]-20:1H2O2 did not lead to a significantly enhanced free▲[HO']/IFe2 ]=10:1radical oxidation of phenol, though better COD区6removal was achieved with the addition of H2O2.The comparison between Ki and K2 shows radicaloxidation is not the efficient way for COD removal二=in photo-Fenton phenol degradation.Fig.8 shows that when the dosage of H2O2 ad-2(dition is 1/4 Qth and 1/2 Qth, 36.5% and 73.1% CODremoval are achieved respectively, which is far Fig.9 Effect of initial concentration of ferrous ion on phenolmore than the assumed rate 25% (1/4 Qth) and 50%Operational conditions: H2O2 Qm, temperature 30 °C, pH 3(1/2 Qth). This phenomenon was mentioned by Wei-chgrebe and Vogelpohl (1994) and Utset et al. Table 3 Calculated results at different initial concentration(2000). It can be interpreted that dioxgen can serve of Feas the radical acceptor in hydroxyl radical oxidation,[H2O][Fe2] KK3K/K2so O2 takes part in the degradation of phenol and40:10.2326 0.1824 0.0606 0.41501.28leads to the additional removal of COD.20:10.5365_ 0.2013 0.0349 0.7378 2.67When H2O2 (Qth) was employed, the experime-10:1中国煤化工742 3.08nt results had a plateau region at 85% COD removalYHCNMHG.204He et al. /.J Zhejiang Univ SCI 2004 5(2):198. 205ratio of 20:1 to 10:1, which means [H2O2]/[Fe2t] pacted by pH significantly if Fenton reaction is=20:1 is preferred.mainly a radical oxidation process. However, K2fluctuates slightly with the change of pH and es-Effect of pHsentially keeps at a constant value of 0.23. RadicalFig. 10 shows the effect of initial pH variation oxidation is weak during Fenton removal of theon phenol degradation. Parameters calculated are phenol-water COD.listed in Table 4.Effect of carrier gas100Fig.ll shows the effect of purge-gas on phenol8(! pH=7degradation. Parameters calculated are listed inTable 5. The purge gas employed in the photo-区▲pH=4▼pH=3Fenton experiments had significant impact on the50COD removal. With the addition of O2 instead of N2,K1 increases tremendously from 0.1618 to 0.5365. .8 4(K1 increases from 0.1618 to 0.3104 when air is in-troduced. Oxygen effect is demonstrated obviously.2(However, no significant increase of K2 values 0C-curred when instead of N2, air and O2 were intro-101duced. The oxygen contained in air is enough forReaction time (min)Fig.10 Efet of itiail concentration of pH on phenol so- active electron transfer in the HO oxidation oflution COD degradationphenol. From Table 5, K3 with N2 introduction isOperational conditions: H2O2 Qtn, FeSO4+7H2O 207 mg/L, tem- very small. This indicates that without O2 added toperature 30 °Creaction mixture organic acid cannot be easily fur-ther oxidized.Table 4 Calculated results at different Initial concentrationof pHHKK30.9337 0.2368 0.0701 1.1705 3.94▲Air●N,0.5365 0.2013 0.0349 0.7378 2.67e 600.5280 0.2654 0.0307 0.7934 1.990.3215 0.2358 0.0221 0.5573 1 .3620The COD removal rate maximizes at pH 3.0and it begins to decrease with increase of pH. At pH1:7.0, iron(I) cannot keep its active form; the removalrate of COD in both reaction way will decrease.Fig.11 Effect of purge gas on phenol solution COD degra-However, since plenty of H2O2 and UV (Ultraviolet) dationexist in the photo-Fenton process, the removal of Operational conditions: H2O2 Qh, FeSO4 7H2O 207 mg/L, tem-COD can still be achieved at pH=7.perature 30 °C, pH 3Table 4 shows that K; and K3 are biggest at pH3.0 among the four pH values and the valueofa(>1)Table 5 Calculated results at different purge carrier gasdecreases with the enhancement of pH. This obser-Carrier gasvation indicates that both complex oxidation ofV20.1618 0.1147 0.0099 0.27651.41phenol and the further oxidation of organic acid areir0.3104 0.1971 0.0280 0.5075 1.57promoted at low pH. Since pH is a primary effect02中国煤化工378 2.67factor on hydroxyl radical reaction, K2 will be im-9HCNMHG.He et al. /.J Zhejiang Univ SCI 2004 5(2): 198-205205CONCLUSIONing atrazine as a model organic compound. Wat. Res,34(12):3107-3116.The degradation of phenol in photo-FentonJiang, J, Bank, J.F, Scholes, C, 1993. Subsecond time-resolved spin trapping followed by stopped-flow EPRprocess is different from that in UV/H2O2 process.of Fenton reaction products. J. Am. Chem. Soc., 115Iron complex plays an important role in this ad-(11):4742-4746.vanced oxidation process. The established model for Kremer, M.L, Stein, G, 1959. The catalytic decompositionphenol degradation is well supported by the ex-of hydrogen peroxide by ferric perchlorate. Trans.periment results. Operating parameters such asKremer, M.L, 1985. Complex visas 'Free Radical' mechan-Faraday Soc., 55(5):959-973.dosage of H2O2， ferrous ion, pH and carrier gasism for the catalytic decomposition of H2O2 by Fest. Int.impact the removal of COD significantly. Under theJ. Chem. Kinet, 17(12):1299-1314.conditions of high initial H2O2 and Fe2+ addition， Legrini, o.. Oliveros, E. Braun, A.M.. 1993. Photochemicallow initial pH and dioxygen carrier gas, the calcu-processes for water treatment. Chem. Rev, 93(2):671-lated model parameters show that phenol is more698.likely degraded through complex oxidation rather Lei, L, Hu, x. Yue, P.L., Bssmann, S.H, Gob, S, Braun,A.M., 1998. Oxidative degradation of polyvinyl-alcoh-than hydroxyl radical oxidation.ol by the photochemically enhanced Fenton reaction. J.Photochem. Photobiol. A, Chem, 116(3):159-166. .ReferencesOllis, D.F, AlI-Ekabi, H, 1993. Photocatalytic PurificationAndreozzi, R., Apuzzo, A.D, Marotta, R., 2000. 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Process, 33(4): 199-203.H2O2 oxidation reactions in dilute aqueous solution us-Welcome visiting our journal website: http://www.zju.edu.cn/jzusWelcome contributions & subscription from all over the worldThe editor would welcome your view or comments on any item in the journal,or related mattersPlease write to: Helen Zhang, Managing Editor of JZUSE-mail: jzus@zju.edu.cn Tel/Fax: 86-571-87952276中国煤化工MHCNMH G.