Photocatalytic Degradation of DMP Using a Novel Photo-Fenton-Like System

LIU Jinyuan，WANG Bin，LIU Xilong，WU Chunyan，YANG Ting，CHEN Yang，WU Xiuning，HUA Yingjie，WANG Chongtai

（College of Chemistry and Chemical Engineering，Hainan Normal University，Haikou 571158，China）

Photocatalytic Degradation of DMP Using a Novel Photo-Fenton-Like System

LIU Jinyuan，WANG Bin，LIU Xilong，WU Chunyan，YANG Ting，CHEN Yang，WU Xiuning，HUA Yingjie，WANG Chongtai*

（College of Chemistry and Chemical Engineering，Hainan Normal University，Haikou571158，China）

A novel photo-Fenton-like(PFL)system using a Keggin-type iron-substituted heteropoly⁃anion PW11O39Fe（III）（H2O）4-[PW11Fe（III）（H2O）]as photocatalyst was employed to the photocatalyt⁃ic degradation of dimethylphthalate（DMP）.The experimental results showed that a complete DMP degradation and a 43%total organic carbon（TOC）removal was obtained in less than 80 min in a pH 6.86 mixed phosphate buffer solution containing 0.1 mmol·L-1DMP+1.0 mmol·L-1PW11Fe（III）（H2O）+3.0 mmol·L-1H2O2.Effects of solution pH,initial H2O2and PW11Fe（III）（H2O）concentration on the rate of DMP degradation were also examined,respectively.Mechanisms of photo-catalysis in the absence of H2O2and in the presence of H2O2were simultaneously proposed.The PFL system provides a potential application in practical treatment of wastewater with a relaxed pH requirement.

iron-substituted heteropolyanion；dimethylphthalate degradation；photocatalysis；hydroxyl radicals；total organic carbon；photo-fenton

CLC mumber:O 643.3 Document code:A Article ID：1674-4942（2012）01-0056-07

Electro-Fenton and photo-Fenton developed on the basis of Fenton principles are two kinds of ad⁃vanced oxidation methods,which have exhibited their superiority and high efficiency in oxidative removal of aqueous hazardous organic pollutants such as herbi⁃cides,pesticides,dyes,pharmaceuticals and plastic additives etc[1-13].Electro-Fenton process maybe oxi⁃dizes organics more thoroughly than photo-Fenton process,but it needs to consume electric energy.Pho⁃to-Fenton process is more in line with the principles of high efficiency and low consumption because of a potential use of solar energy,thus possesses a betterapplied prospects.

However,both conventional Electro-Fenton and photo-Fenton process has a main drawback of low working pH.If the pH is≥3.0,Fe3+as a catalyst will precipitate from the solution because of formation of amorphous Fe（III）colloids and slow the reaction.So,the wastewater（pH≥3.0）is needed to reduce pH be⁃fore treatment and also recover to neutral after treat⁃ment by chemicals addition or electrochemical method[14],which would bring about a lot troubles in opera⁃tions and much cost.Although some organic chelators such as ethylenediaminetetraacetic acid（EDTA）and diethylenetriaminepentaacetate（DETAPAC）[15]can be used to chelate Fe3+at neutral pH,they appear to be susceptible to oxidation caused by HO·radicals[16-17].To overcome the defect of low working pH and im⁃prove the capacity of wastewater treatment with a re⁃laxed pH requirement,a strategy is that an Fe（III） -substituted heteropolytungstate anion PW11O39Fe（III）（H2O）4-[PW11Fe（III）（H2O）]was used to replace Fe3+in conventional electro-Fenton and photo-Fenton system because PW11Fe（III）（H2O）is stable at the pH range from 2 to 8 in aque⁃ous solution.In our previous works PW11Fe（III）（H2O）was proved to be an excellent electrocatalyst for H2O2reduction to HO·radicals and applied to the degradation of dimethylphthalate（DMP）in which DMP with an initial concentration of 20 mg·L-1was completely degraded in less than 120 min in the pres⁃ence of H2O2at neutral pH[18].Moreover,it was also found that PW11Fe（III）（H2O）has a strong UV light absorption at near 200 and 258 nm as well as a visi⁃ble light absorption at near 400 and 460 nm,respec⁃tively,exhibiting a high light-absorption activity.Therefore,the novel photo-Fenton-like（PFL）system comprised of PW11Fe（III）（H2O）as a photocatalyst was applied to the degradation of nitrobenzene（NB）and a very efficient NB degradation was obtained[19].

To assess the general capacity of this novel PFL system for degradation of various organic pollutants,DMP was chosen as a model contaminant in this study because it is among the most frequently identified phthalate acid esters（PAEs）,endocrine-disrupting chemicals,in diverse environmental samples includ⁃ing surface marine waters,freshwaters and sediments[20-22],and classified as a‘priority pollutant’by the United States Environmental Protection Agency（USE⁃PA）[23].The efficiency for DMP degradation was evalu⁃ated by determining degradation kinetic curves and percentage total organic carbon（TOC）removal.Influ⁃ences of solution pH,initial H2O2,DMP and PW11Fe（III）（H2O）concentration on the rate of DMP degrada⁃tion were also examined in the present work,and the detailed photocatalytic mechanisms were simultane⁃ously proposed.

1 Experimental

1.1 Materials

Dimethylphthalate,hydrogen peroxide（30%w/w）,acetone,Na2WO4·2H2O,Na2HPO4·12H2O,FeSO4·7H2O,Fe（NO3）3·9H2O and NaHSO4,purchased from Guangzhou Chemical Reagent Co.（China）,were of analytical grade and used without further purification.The mixed phosphate buffer（pH6.86）purchased from Shanghai Rex Co-Perfect Instrument Co.,Ltd China,was of analytical grade.Acetonitrile was HPLC grade and purchased from Dima Technology Inc（Richmond Hill,ON,Canada）.Deionized water was used through⁃out the study.

The lacunary Keggin heteropolytungstate Na7PW11O39and the iron-substituted heteropolytung⁃state Na4PW11O39PFe（III）（H2O）were synthesized as described in the literature[24-25]and characterized by el⁃ement analysis,infrared spectra（IR） and cyclic voltammetry（CV）.

The Na2SO4-NaHSO4buffer solution was used in the variable pH experiments and its pH was adjusted using dilute NaOH solution.In other cases of DMP degradation pH6.86 mixed phosphate buffer was used.

1.2 Device

The photo-degradation reaction of DMP in aque⁃ous solution during PFL treatment was performed in a Photochemistry Reactor（Nanjing Xujiang Instrument Co.Ltd.,China）.A high pressure Hg lamp with an en⁃ergy input of 300 W as a UV light source was posi⁃tioned within a cylindrical quartz vessel.The cylindri⁃cal quartz vessel with water circulation was used to cool the lamp.Quartz glass tube（1000 mL）equipped with a magnetic stirrer were utilized as reaction vessel.

1.3 Photo-degradation procedures

250 mL buffer solution containing DMP and PW11Fe（III）（H2O）was added into the quartz glass tube,and then the photo-degradation reaction of DMP commenced under irradiation.For an analysis of DMP concentration the samples of 2 mL each were taken at intervals during reaction.The dark reaction was conducted by wrapping the tubes with black plas⁃tic film to prevent exposure to light.

1.4 Analytical methods

The DMP concentration was analyzed during deg⁃radation with a high performance liquid chromato⁃graph（HPLC,Elitehplc UV230+,China）equipped with a reverse phase column（Phenomenex® C18,5 um,150 mm × 4.6 mm2）and UV-spectrophotometer.The detection wavelength was 276 nm and the mobile phase was a mixture of water and acetonitrile（50/50 v/v）delivered at a flow rate of 1.0 mL·min-1.After di⁃lution of 5 times all samples were immediately ana⁃lyzed to avoid further reactions.The total organic car⁃bon（TOC）concentration was measured by using a to⁃tal organic carbon analyzer（TOC-VCPH,Shimadzu Co,Japan）.

2 Results and discussion

2.1 Photocatalytic degradation of DMP by PFL system

Fig.1 shows the situations of DMP photo-degra⁃dation at different conditions.In the case of that solu⁃tion only containing DMP was exposure to UV light ir⁃radiation no change in DMP concentration was ob⁃served（curve a）.After addition of PW11Fe（III）（H2O）to the DMP solution and under UV light irradia⁃tion a distinct change in DMP concentration was oc⁃curred(curve b),indicating DMP in aqueous solution underwent degradation,which is similar to the situa⁃tion that solution contains DMP and H2O2in which about 40%of DMP degradation was obtained at 120 min under radiation（curve c）because of some hy⁃droxyl radicals formed by photo-decomposition of H2O2[9].However,upon addition of both PW11Fe（III）（H2O）and H2O2to the DMP solution the rate of DMP photo-degradation was greatly accelerated under irra⁃diation（curve d）and a 100%DMP degradation was achieved in less than 80 min,whereas no DMP degra⁃dation for the same solution took place in dark（curve e）,indicating an excellent photocatalysis of PW11Fe（III）（H2O）for DMP degradation in the presence of H2O2.This photocatalytic effect of PW11Fe（III）（H2O）is mainly attributed to its UV absorption result⁃ed from Od→Fe charge-transfer leading to hydroxyl radials.During illumination PW11Fe（III）（H2O）was excited and an electron transition from HOMO to the dFe （III） orbit occurred, leading to Od→Fe charge-transfer that caused H2O coordinated at Fe center to be oxidized and generation of hydroxyl radi⁃als.Reduced Fe（II）center was returned to the oxi⁃dized state of Fe（III）via oxidation by dissolved O2or resulted HOO·and HO·.The process can be summa⁃rized as below（Eqs.1-5）[19]：

HO·and HOO·radicals produced in the reac⁃tion of Eqs.2 and 3 are the main active oxygen species that account for the photo-decomposition of DMP in the solution containing PW11Fe（III）（H2O） under ambient condition.Upon addition of H2O2to the DMP solution containing PW11Fe（III）（H2O）ligand ex⁃change reaction between H2O2and H2O bound at the Fe（III） center occurred first to form PW11Fe（III）（H2O2）（Eq.6）[26],and then HO·radicals were generat⁃ed in accordance with the following mechanism（Eqs 7-9）[19]：

Fig.1 c/c0vs.t plots of photo-degradation of DMP in pH6.86 mixed phosphate buffer under different condi⁃tions of(a)solution only containing 0.1 mmol·L-1DMP under UV light irradiation;(b)solution containing 1.0 mmol·L-1PW11Fe(III)(H2O)+0.1 mmol·L-1DMP under UV light irradiation;(c)solution containing 3.0 mmol·L-1H2O2+0.1 mmol·L-1DMP under UV light irradiation;(d)solution containing 1.0 mmol·L-1PW11Fe(III)(H2O)+3.0 mmol·L-1H2O2+0.1 mmol·L-1DMP under UV light ir⁃radiation;(e)Solution containing 1.0 mmol·L-1PW11Fe(III)(H2O)+3.0 mmol·L-1H2O2+0.1 mmol·L-1DMP in dark图1 不同条件下DMP光催化降解的c/c0～t曲线，pH6.86混合磷酸盐缓冲溶液:(a)0.1 mmol·L-1DMP,300W汞灯;(b)1.0 mmol·L-1PW11Fe(III)(H2O)+0.1 mmol·L-1DMP,300W 汞 灯;(c)3.0 mmol·L-1H2O2+0.1 mmol·L-1DMP,300W 汞 灯;(d)1.0 mmol·L-1PW11Fe(III)(H2O)+3.0 mmol·L-1H2O2+0.1 mmol·L-1DMP,300W 汞灯;(e)1.0 mmol·L-1PW11Fe(III)(H2O)+3.0 mmol·L-1H2O2+0.1 mmol·L-1DMP,暗处

The photo-catalytic cycle in the presence of H2O2enhances significantly the rate of HO·radicals generation,which causes a more efficient degradation of DMP[18].For a better understanding of the genera⁃tion of HO·radicals in the photo-catalytic cycle,de⁃tails of the photo-exciting and photo-electron transfer are illustrated in Scheme 1[27].It is evident from Scheme 1 that under photo-exciting a bonding elec⁃tron of H2O2is transferred from the bonding molecular orbital σ2P to the anti-bonding molecular orbital σ*2P by means of the HOMO and 3dFe（III）orbital of PW11Fe（III）（H2O）,leading finally to the decomposi⁃tion of H2O2to HO·radicals.

Scheme 1 Illustration of the photo-exciting and photo-electron transfer process in photo-catalytic decomposition of H2O2by PW11Fe(III)(H2O)图示1PW11Fe(III)(H2O)光催化降解H2O2的光激发与光电子转移图解

To examine the mineralization degree of DMP during degradation,the concentration in TOC was thus detected.Fig.2 revealed the changes of TOC in the DMP solution containing PW11Fe（III）（H2O）and H2O2under irradiation.It can be seen from Fig.2 that TOC gradually decreased with increasing irradiation time and reached a removal ratio of ca.43%at 120 min,indicating that DMP degradation was accompa⁃nied by a partial mineralization.But it is found that the decline of TOC was quick within the beginning 30 min and then became slow（Fig.2）.This phenomenon can be explained by the formation of PW11O39Fe（III）（CO2HCO2）.Generally oxalic acid is the final product before DMP mineralization[16].The resulted oxalate ion CO2HCO2-reacts readily with PW11Fe（III）（H2O）through ligand exchange leading to the complex PW11O39Fe（III）（CO2HCO2）（Eq.11）that is resistant to the further mineralization by HO·radicals.

Degradations of DMP with different initial con⁃centration are illustrated in Fig.3，which maybe ac⁃count for the capacity of this novel system.It can be seen from Fig.3 that the DMP degradation for differ⁃ent initial concentration has an identical concentra⁃tion decay trend.But it takes longer time for the high⁃er initial DMP concentration to decay into the identi⁃cal concentration value of c/c0compared to the lower initial DMP concentration.This implies that the ap⁃parent rate of DMP degradation decreases with in⁃creasing of initial DMP concentration.

Fig.2 Changes in solution TOC during DMP degrada⁃tion under UV light irradiation.Initial solution:1.0 mmol·L-1PW11Fe(III)(H2O)+0.1 mmol·L-1DMP+3.0 mmol·L-1H2O2in pH6.86 mixed phosphate buffer solution图2DMP降解的TOC变化:1.0 mmol·L-1PW11Fe(III)(H2O)+0.1 mmol·L-1DMP+3.0 mmol·L-1H2O2,pH6.86磷酸盐缓冲溶液

Fig.3 Photocatalytic degradation of DMP with different initial concentration of:(a)0.05;(b)0.10;(c)0.22 mmol·L-1.Other reaction conditions:pH6.86 mixed phosphate buffer solution containing 3.0 mmol·L-1H2O2+1.0 mmol·L-1PW11Fe(III)(H2O),UV irradiation图3 不同初始浓度DMP光催化降解的C/C0～t曲线:(a)0.05;(b)0.10;(c)0.22 mmol·L-1底液:3.0 mmol·L-1 H2O2+1.0 mmol·L-1PW11Fe(III)(H2O),pH 6.86混合磷酸盐缓冲溶液,300W汞灯

2.2 Effect of pH on the photocatalytic degrada⁃tion of DMP

The stability of H2O2is dependent on pH in the aqueous solution.In addition,PW11Fe（III）（H2O）has an acid-base equilibrium as below:

Therefore,pH of the solution is an important vari⁃able affecting efficiency of the PFL system.As shown in Fig.4,the percentage of DMP degradation increas⁃es with increasing pH.A percentage of 72%was achieved at pH3.5 at the reaction time of 15 min.Then the percentage of DMP degradation declined as pH was over 3.5.At low pH H2O2is stable,probably because it solvates a proton to form an oxonium ion（H3O2+）[9]and reduce substantially the complex reac⁃tivity with ferric center.Although the stability of H2O2decreases when pH increases,it is unfavorable for the ligand exchange reaction（Eq.6）according to Eq.12.This makes the photo-electron transfer to H2O2be⁃come difficult（Eq.7）and slows the generation of hy⁃droxyl radicals（Eq.8）,thus affecting the efficiency of DMP degradation.

Fig.4 Effects of pH on the DMP photocatalytic degrada⁃tion in the solution containing 1.0 mmol·L-1PW11Fe(III)(H2O)+0.1 mmol·L-1DMP+3.0 mmol·L-1H2O2under UV light irradiation,the reaction time is 15 min图4 溶液pH对DMP光催化降解速率的影响,底液:1.0 mmol·L-1PW11Fe+0.10 mmol·L-1DMP+3.0 mmol·L-1 H2O2,300W汞灯,反应时间：15min

2.3 Effect of initial H2O2concentration on the photocatalytic degradation of DMP

Since the hydroxyl radials contributing to the DMP degradation come from the decomposing of H2O2,the impact of the change in H2O2concentration on the photocatalytic oxidation of DMP was investi⁃gated.In these experiments,the initial H2O2concen⁃tration was changed from 0 to 10 mmol·L-1,but the PW11Fe（III）（H2O）concentration kept constant.The photocatalytic degradation percentages of DMP under different initial H2O2concentrations during 15 min are shown in Fig.5.

Fig.5 Effects of the H2O2concentration on DMP photo⁃catalytic degradation in pH 6.86 buffer solution contain⁃ing 1.0 mmol·L-1PW11Fe(III)(H2O)+0.1 mmol·L-1DMP under UV light irradiation,the reaction time is 15 min图5H2O2浓度对DMP降解速率的影响.底液：1.0 mmol·L-1PW11Fe+0.10 mmol·L-1DMP，pH 6.86混合磷酸盐缓冲液,300W汞灯,反应时间:15 min

It can be found from Fig.5 that the percentage DMP degradation is about 14%in the absence of H2O2,but after addition of H2O2the percentage in⁃creases with increasing H2O2concentration,e.g.varia⁃tion from 14%to 68%as the H2O2concentration var⁃ies from 0 to 1 mmol·L-1and then declines.This indi⁃cates that the effect of increasing H2O2concentration is first positive for the DMP degradation because the oxidation power of the PFL system is improved with increasing hydroxyl radical amount in solution from the decomposition of increasing H2O2.However,with the continuous increase in the initial H2O2concentra⁃tion,especially beyond 1 mmol·L-1,the percentage DMP degradation begins to decrease,e.g.variation from 68 to 8%when the H2O2concentration varies from 1 to 10 mmol·L-1.This phenomenon can be ex⁃plained by that H2O2with a higher concentration is easily disproportionate[9，15]and the competitive reac⁃tion of the excess hydrogen peroxide with hydroxyl radicals also takes place according to the following re⁃action equation[15]:

The above Eq.13 shows that a part of hydroxyl radicals would be consumed before oxidizing DMP,which is considered to be an inhibitory effect of H2O2on hydroxyl radicals.This negative effect of excess H2O2leads to a low efficiency of DMP degradation.Al⁃though the H2O2concentration must be enough for for⁃mation of a considerable amount of hydroxyl radicals,a high concentration of H2O2existing in the solution is detrimental to DMP degradation[9].

2.4 Effect of initial PW11Fe（III）（H2O）concen⁃tration on the photocatalytic degradation of DMP

PW11Fe（III）（H2O）acts as photocatalyst in the photocatalytic degradation of DMP.Influence of varia⁃tion of initial PW11Fe（III）（H2O）concentration on the percentage of DMP degradation were examined to obtain an optimal initial PW11Fe（III）（H2O）concen⁃tration.The results are showed in Fig.6.

Fig.6 Influence of the initial PW11Fe(III)(H2O)concen⁃tration on the DMP photocatalyticdegradation in pH6.86 buffer solution containing 1.0 mmol·L-1H2O2+0.1 mmol·L-1DMP under UV light irradiation,the reac⁃tion time is 15 min图6PW11Fe(III)(H2O)浓度对DMP降解速率的影响.底液:1.0 mmol·L-1H2O2+0.1 mmol·L-1DMP,pH 6.86 混 合磷酸盐缓冲液,300W汞灯,反应时间:15 min

It can be seen from Fig.6 that the percentage of DMP degradation increases with increasing of the PW11Fe（III）（H2O）concentration and reaches a max⁃imum at 1.0 mmol·L-1,and then declines when the concentration is over 1.0 mmol·L-1.It is obvious that a high concentration of PW11Fe（III）（H2O）can re⁃sult in more hydroxyl radicals according to the pho⁃to-catalytic cycle described in Eq.6 to 9,whereas the concentration more than 1.0 mmol·L-1causes an ex⁃cess of PW11Fe（III）（H2O）compared to H2O2concen⁃tration of 1.0 mmol·L-1.This is unfavorable for DMP degradation due to the competing reaction of PW11Fe（II）（H2O）*with hydroxyl radicals（Eq.5）.There⁃fore,the photocatalytic degradation of DMP has an op⁃timal PW11Fe（III）（H2O）concentration of 1.0 mmol·L-1in the presence of 1.0 mmol·L-1H2O2.

3 Conclusions

The novel PFL system consisting of PW11Fe（III）（H2O）can directly be applied to the removal of DMP with a relaxed pH requirement,thus overcome the defect of low working pH in the conventional PF system.It is also possible to be used for the degrada⁃tion of organic pollutants in aqueous solution in the absence of initial H2O2.In spite of the higher efficien⁃cy of pollutant degradation as addition of H2O2ultiliza⁃tion of O2in air may be more economically viable in practical treatment of water.

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一个新颖的类光芬顿体系光催化降解DMP

刘津嫒，王 彬，刘希龙，吴春燕，杨 婷，陈 暘，吴秀宁，华英杰，王崇太*

（海南师范大学 化学与化工学院，海南 海口 571158）

一个以Keggin型铁取代杂多阴离子PW11O39Fe（III）（H2O）4-[PW11Fe（III）（H2O）]为光催化剂的新颖类光芬顿体系被用于邻苯二甲酸二甲酯（DMP）的光催化降解.实验结果表明，在pH6.86的含有0.1 mmol·L-1DMP+1.0 mmol·L-1PW11Fe（III）（H2O）+3.0 mmol·L-1H2O2.磷酸盐缓冲溶液中，反应80 minDMP的降解率达到100%，总有机碳去除率为43%，并详细考察了溶液pH、初始过氧化氢浓度和PW11Fe（III）（H2O）浓度对降解效率的影响.提出了过氧化氢存在和缺失条件下的光催化机制，这个新颖的类光芬顿体系为实际的水处理提供了潜在的应用.

铁取代杂多阴离子；DMP降解；光催化；羟基自由基；总有机碳；光芬顿

2012-01-18

国家自然科学基金资助项目（20963003）；海南省重点科技项目（090803）；海南省自然科学基金（509009）；吉林省科技发展计划项目（20090595）；海南师范大学开放实验室项目（kfsy11011）

毕和平

*通讯作者