Hongjia Zhou,Lin Kang,Ming Zhou,Zhaoxiang Zhong,Weihong Xing*

State Key Laboratory of Materials-Oriented Chemical Engineering,National Engineering Research Center for Special Separation Membrane,Nanjing Tech University,Nanjing 210009,China

Keywords:Pulp wastewater Heterogeneous Fenton Membrane Cu2O Advanced oxidation

A B S T R A C T Both activity and stability of the catalyst can be improved in heterogeneous Fenton reaction, in particular,with no limitation for the working pH and no production of the sludge.In this work,a combination of catalyst Cu2O and pore-channel-dispersed H2O2is proposed to treat the pulp wastewater.Degradation degree of CODs in the wastewater was up to 77%in the ceramic membrane reactor using Cu2O powder(2.0 g·L-1)and membrane feeding H2O2(0.8 ml·L-1)within 60 min.Evolution of·OH radical formation in the advanced oxidation process was analyzed with a fluorescent method.Utilization efficiency of H2O2was successfully enhanced by 10%with the membrane distributor.Further on,the catalyst recyclability was evaluated in a five-cycle test.The concentration of copper ions being dissolved in the treated water was monitored with ICP.After Cu2O/H2O2(membrane)treatment the effluent is qualified to discharge with COD concentration lower than 15 mg·L-1with regard to the national standard GB25467-2010.

1.Introduction

Discharged water from the pulp and paper mill manufacture is one of the main polluting sources to the eco-system[1–4].Fenton reagent(Fe2+/H2O2)has been widely applied to decompose toxic and persistent organic contaminants in pulp wastewater through catalytic oxidation reaction.However,working pH condition(acid)and poor consumption efficiency of H2O2are the main drawbacks in the homogeneous Fent on process.Continuous loss of iron-based catalyst and great amount of produced sludge are target to be solved for large-scale application[5–7].Heterogeneous Fenton process using the solid catalyst including Fe2O3,CuxO(x=1,2),Al2O3,CeO2and MnO2has been attempted to operate under mild condition[6,8–11].It has advantages of treating wastewater with improved utilization efficiency of both the catalyst and oxidant reagent.For instance,in a 2 h reaction using octahedral Cu2O catalyst and H2O290%of p-nitrophenol(C=20 mg·L-1)was decomposed in the solution with neutral pH[12].Activity of the catalyst remained within 7 times of recycling use.

Copper(I)oxide,as a p-type semiconductor,has band-gap energy 2.2 eV and could be accordingly photo-activated under the visible light[13].It has redox property as the monovalent state(Cu+)reduces H2O2and the divalent state(Cu2+)oxidizes H2O2,forming·OH and HO2·radicals respectively[11].Bisphenol(BPA)was degraded by using Cu2O/H2O2and visible-light/Cu2O/H2O2with the reaction constant 1.43 h-1and 2.69 h-1respectively[14].More recently,Cu2O has been intensively studied as an effective catalyst in the heterogeneous Fenton reaction[13,15].In the conventional Fenton process hydrogen peroxide is directly added in the reaction batch with low utilization efficiency and high expense due tothe costly reactant,for instance,the utilization rate of H2O2was as low as 25%at pH=7 in one reported work[16].Effective rate of the oxidant reagent could be improved with the activity-enhanced catalyst by means of nanoparticles[17]and dopant[18].Nevertheless,the complicated preparation of modified catalysts would also increase the overall cost of the heterogeneous Fenton process.

A ceramic membrane reactor(CMR)has been applied in many fields for integration flexibility,environmental friendliness,non-toxicity and chemical stability[19–22].Jiang et al.worked with a double-membrane reactor:one membrane distributing the reactant and the other separating the ultra fine catalyst from product[22].The results proved that the use of the membrane distributor for injecting H2O2has significantly improved the selectivity toward dihydroxybenzene.In some other work,a ceramic hollow fiber membrane was applied to distribute H2O2in heterogeneous catalysis process which has improved phenol conversion rate by30%and dihydroxybenzene(DHB)selectivity by20%,in comparison to directly adding H2O2in whole[23].

In the study,CODs in the pulp wastewater were reduced by using membrane-dispersed H2O2and suspended Cu2O microparticles.Activity and stability of the synthesized catalyst have been evaluated in the batch reactor first.Effect of visible light on the catalytic oxidation reaction was also studied.The influence of the membrane distributor on enhancing H2O2utilization efficiency for COD degradation was assessed.The studied wastewater was taken from a paper manufacturer and it is to be treated in reverse osmosis(RO)process.Improved degradation of COD would significantly alleviate the membrane fouling in the following RO process at the treatment plant.

2.Materials and Methods

2.1.Chemicals,membrane and wastewater

Hydrogen peroxide(30 wt%)and terephthalic acid were purchased from Sinopharm Chemical Reagent Co.Ltd.,China.Sodium hydroxide of purity 96%was bought by Xilong Chemical Co.Ltd.,China.The Cu2O catalyst particles were prepared in the lab through the reaction of Benedict's solution with glucose in a membrane reactor at 80°C for 2 h.

A tubular ceramic membrane distributor for H2O2dispersing(with mean pore size 200,500 and 1000 nm)was supplied by Jiangsu Jiuwu High-tech Co.Ltd.,China.The ceramic membrane is composed of Al2O3.The effective length and outer and inner diameters of the tubular membrane are 3 cm,1.2 cm and 0.8 cm respectively.

The wastewater was provided by a pulp and paper mill manufacturer in Nantong,Jiangsu,China.The intrinsic pH=7.41and are latively high conductivity 2.38 mS·cm-1of the wastewater were measured.The concentration of CODs and the original concentration of Cu2+ions in the wastewater were found to be 54 mg·L-1and 0.103 mg·L-1respectively.

2.2.Membrane-enhanced heterogeneous Fenton catalytic reactor

Pulp wastewater of volume 1 L was treated in a reaction batch stirring at rpm=250 r·min-1for 60 min in each experiment(schematic presentation in Fig.1).Microparticles of the catalyst were suspended in the wastewater and the oxidant reagent was distributed through a ceramic membrane to the reactor.The total consumed volume of H2O2varied from 0.6 to 0.9 ml.The feeding flow rate of H2O2in the membrane changed from 0.01 to 0.08 ml·min-1with a peristaltic pump.Working pH was studied at 3,5,7,and 11 at reaction temperature from20to50°C in the experiment.Concentration of CODs in the wastewater was measured at constant reaction duration.

Fig.1.Schematic diagram on heterogeneous Fenton reaction setup using the tubular ceramic membrane distributor[1—stirrer;2—batch reactor;3—tubular membrane(for dispersing H2O2);4— flux pump;5—H2O2tank].

2.3.Measurement on hydroxyl radical(·OH)and CODs

Hydroxyl radical(E·OH/H2O2=2.8 V)is one of the most reactive oxidants and requisitely formed in advanced oxidation processes[24–26].Though it is difficult to quantify radicals due to the short lifetime,chemiluminescence is one method to trace the concentration of·OH in solution[27].For instance,terephthalic acid(TA)is employed to capture·OH radical and a fluorescent compound hydroxy terephthalic acid(HTA)can be formed(Eq.(1)).The oxidant reagent H2O2in volume 0.8 ml was dispersed in 1 L volume of TA solution(2.5 g·L-1).Fluorescent intensity of the mixture solution was measured every 10 min with an Atomic Fluorescence Spectrophotometer(AFS,RF-5301/PC,Japan).The excitation and emission wavelengths were 315 nm and 425 nm respectively.

Concentration of CODs in the wastewater was measured with a UV–visible Spectrophotometer(UV–Vis,DR2800,Germany).The utilization rate of reactant H2O2could be known with regard to the degradation efficiency(E)of CODs calculated as:

where COD0and COD1are the initial concentration and instant concentration of CODs in the treated pulp wastewater,respectively.

2.4.Band-gap

Band-gap energy of synthesized copper(I)oxide(with the direct band-gap)could be determined by a Tauc relation based on the measurement of light absorbance[28]:

where K,A,hν,and Egare the constant,absor bance value,photon energy and optical band gap respectively.Plotting(Ahν)2vs.hν gives the value of Eg=hν by intersecting the slope of linearity line at the x axis.

2.5.Characterization methods

Microstructure of Cu2O particles was investigated with Scanning Electron Microscopy(SEM,Hitachi S-4800,Japan)and Transmission Electron Microscopy(TEM,JEOL JEM 2100,Japan).Adsorbed compounds on the catalyst after treating the wastewater have been analyzed with Fourier Transform Infrared Spectroscopy(FTIR,Nicolet 8700,USA).Crystalline structure was studied with X-ray Diffraction(XRD,MiniFlex 600,Japan)using Cu Kαradiation(λ =0.154 nm)with generator voltage 40 kV and current 15 mA.Thermogravimetric analysis(TGA,STA 449F3)was carried out by heating the catalyst from 20 to 800 °C with heating rate 10 °C·min-1under N2atmosphere.

Copper ions can be potentially dissolved in the water during the treatment process.Concentration of the metal ions in the solution was analyzed with an Inductively Coupled Plasma Emission Spectrometer(ICP,Optima 7000DV,USA).

3.Results and Discussion

3.1.Characterization on Cu2O microparticles

It found that the synthesized Cu2O powders were in microcubes with dimension ca. 1 μm and dense structure as in SEM and TEM images (Fig. 2a). The face-centered cubic phase was discovered in X-ray Diffraction 2θ at 29.8°,36.5°,42.4°,61.5°,73.6°and 77.5°representing crystal planes(111),(200),(211),(220),(311)and(322)as in Fig.2b.The cut-off absorbance wavelength was 655 nm and the consequent band gap energy was 1.91 eV according to UV–vis spectra and Tauc plot(Fig.2c).Thermal property and chemical composition of the catalyst before and after the use will be further discussed in Section 3.4.

Fig.2.Characterization on the prepared Cu2O microparticles.(a)SEM andTEM(inserted),(b)XRD pattern and(c)UV–vis absorbance spectra with Tauc plot(inserted).

3.2.Enhanced degradation rate with membrane distributor

3.2.1.Performance of different membranes

A tubular membrane was applied as a distributor for dispersing H2O2flow in the solid–liquid phase Fenton reaction.The top layer of ceramic membrane could be with meanpore size 200 nm,500 nm and 1000 nm(Fig.3a).A positive “membrane effect”was witnessed on the advanced oxidation reaction by comparing it to the blank test(Fig.3b).The total CODs in the pulp wastewater were eliminated by 65%in the membrane-assisted process with a 60 min reaction.The degradation efficiency was improved by 6.5%,8.0%and 7.1%using the membrane of mean pore size 200 nm,500 nm and 1000 nm respectively.Accordingly,the 500 nm pore-sized membrane was selected in the treatment process.

Fig.3.Improvement on COD removal ratio using the differently pore-sized ceramic membrane distributor(CH2O2=0.8 ml·L-1,FH2O2=0.015 ml·min-1).

Hydrogen peroxide was supplied in the form of microdroplets as penetrating the pore channels of the membrane.The specific surface area of oxidant reagent increased and the liquid–liquid mass transfer could be improved thanks to the micro-dimension of reactant droplets[23].The ceramic membrane has a hydrophilic surface with large amount of the hydroxyl groups on the surface.It is helpful for the generation process of·OH radicals and degradation rate of the organic pollutants.But the mass resistance could turn to be dominative as using the membrane of very small pore size 200 nm.

3.2.2.H2O2feed flux in the membrane

The supply condition of reactant H2O2was studied with different feeding rates in the tubular membrane.Reduction rate of CODs first climbed up and then dropped as the feeding rate was raised from 0.01 to 0.08 ml·min-1(Fig.4).The maximum degradation rate was 77.4%at FH2O2=0.04 ml·min-1.Sufficient amount of H2O2molecules as the reactant is necessary in the reaction,however,the excessive H2O2molecules would consume ·OH and form other radicals such as HO2·[22,29].It explains the decreased reaction rate at the feeding flux higher than 0.04 ml·min-1.As a result,the membrane-feeding speed of H2O2at 0.04 ml·min-1was determined in the treatment process.

Fig.4.Effect of H2O2feed rate on COD degradation in Cu2O/H2O2(membrane)heterogeneous Fenton process.

3.2.3.Improved yield of hydroxyl radicals

Concentration of·OH radical in solution at continuous reaction time was analyzed in a fluorescence test.The fluorescent compoundhydroxyterephthalic acid(HTA)was formed as·OH radical reacts with terephthalic acid (TA) as in Eq. (1). The impact of the membrane distributor on formation kinetics of ·OH radicals was studied in the fluorescent method.Oxidant reagent H2O2was added in TA solution either directly or through a membrane.The catalyst effect was analyzed by whether adding Cu2O or not in the solution.Fluorescent intensity of the mixture solution was recorded at wavelength 425 nm as a function of reaction duration from 0 to 60 min.

Table 1Fluorescence intensity of different samples at 60 min reaction

With 60 min of reaction,the fluorescence intensity from the membrane-integrated system(I=606.1)was 23%higher than the conventional system(I=491.9)as in Table 1.It proves the membrane improved yield of hydroxyl radicals.On the other hand,the addition of catalyst Cu2O enhanced the fluorescence intensity(i.e.radical yield)from 575.0 to 606.1 a.u.The active sites on catalyst could promote the bond cleavage of H2O2molecules and the generation of·OH radicals[14].In summary,the combination of using the catalyst and membrane-dispersed reactant successfully enhanced the yield of·OH by 50%.It could consequently improve the oxidation reaction rate in the heterogeneous Fenton process.

Fluorescence spectra recorded at interval reaction duration from0 to 60 min are summarized in Fig.5.In general,the fluorescence evolution was fast in the first 50 min and then became slow or unchanged.It is clear that the formation rate of·OH radicals was more constant as feeding H2O2via the membrane(Fig.5b and d)than feeding it directly(Fig.5a and c).

The evolution profiles of fluorescence under different conditions and the photograph on the fluorescent solutions are given in Fig.6.Membrane-enhanced mass transfer made a continuous positive effect on·OH radical formation in the 60 min reaction.The “catalyst effect”has independently surpassed the ‘membrane effect’on ·OH productivity from 50 min by comparing the two bars of“Cu2O+H2O2+TA”and“H2O2(membrane)+TA”.And the joint effect in the catalytic membrane reactor as“Cu2O+H2O2(membrane)+TA”resulted in the highest efficiency for·OH generation.

Fig.5.Fluorescence spectra of H2O2–TA mixed solution to investigate ·OH formation kinetics.(a)The controlled experiment(neither catalyst nor membrane),(b)with the membrane distributor,(c)with the catalyst,and(d)with the membrane and catalyst(conditions:CTA=2.5 g·L-1,CCu2O=2 g·L-1and FH2O2=0.04 ml·min-1).

Fig.6.Improved hydroxyl radical formation due to the membrane distributor.(a)Fluorescence intensity of solution and(b)photograph of solutions obtained at 60 min of 1-pure water,2-TA solution,3-H2O2+TA,4-Cu2O+H2O2+TA,5-H2O2(membrane)+TA and 6-Cu2O+H2O2(membrane)+TA.

3.3.Operation parameters in heterogeneous Fenton reaction

3.3.1.Visible light

A controlled experiment was first carried out under the darkness by suspending Cu2O powder in the wastewater in the absence of H2O2.An equilibrium on adsorption was found after 10 min(in Fig.7).As accessible to visible light,the amount of degraded organic compounds in wastewater was increased by 11%.It was due to the photo-catalytic activity of Cu2O powders[30].

Fig.7.Effect of visible light in Cu2O/H2O2heterogeneous Fenton process for COD degradation.

Further on,the effect of visible light on the Cu2O/H2O2heterogeneous Fenton process was investigated.Hydrogen peroxide was input to the reactor through the ceramic membrane.Within 60 min of reaction,COD degradation rate under the solar light was found 1.3 times higher than that in the darkness.The photo-activity of catalyst Cu2O was also observed in the membrane reactor experiment.

3.3.2.H2O2oxidant dosage

Concentration of the oxidant reagent is a key factor affecting the number of formed·OH hydroxyl radicals.Relationship between H2O2dosage and compound degradation rate was studied as keeping the added weight of Cu2O constant(1 g·L-1).The degradation rate of CODs continuously climbed up as increasing the volume ratio of H2O2from 0.6 to 0.8 ml·L-1but then the rate was suppressed at 0.9 ml·L-1(Fig.8).The maximum reduced percentage of CODs was 62%as consuming 0.8 ml H2O2over 1 L wastewater within 60 min.

Fig.8.Effect of H2O2concentration(distributed through the porous membrane)on COD degradation efficiency.

As explained previously,the excessive amount of H2O2could suppress the oxidation rate by reacting with·OH and forming other species radical as in Eqs.(5)and(6)[9,29].The reaction between ·OH and HO2·radicals could result in the decreased number of active oxidant species involved in the degradation reaction.

3.3.3.Cu2O catalyst content

As presented in Fig.9a,H2O2molecules adsorbed on the surface of catalyst can be cleaved due to the interaction between the oxygen and copper atom.Hydroxyl radicals are formed upon the cleavage of O--O bond in the H2O2molecules.Concentration of the catalyst was studied from 0 to 3 g·L-1added in the wastewater.In the controlled experiment,COD was reduced by 10%as merely using H2O2but no Cu2O.The degradation rate went up from 46.0%to 68.0%when the concentration of added Cu2O increased from 0.4 to 2 g·L-1.The reaction rate was not increasing any more with CCu2O=3 g·L-1.It should be noted that excessive solid particles could even scatter the light and increase turbidity in the solution.

3.3.4.Wastewater pH and temperature

Fig.9.(a)Schematic description on·OH formation in the Cu2O/H2O2process and(b)the loaded amount of Cu2O affects the degradation efficiency of COD.

In homogenous Fenton process,the iron-based catalyst works actively only under acid condition so that the pre-acidification treatment is required.Whereas,in the heterogeneous Fenton process the working pH has been attempted at 3,5,7and11in the experiments(Fig.10).The results indicate that the degraded percentage of CODs achieved 68.0%at pH=7.The degradation efficiency reduced by ca.30%when varying from pH=3(E=78.9%)to pH=11(E=49.4%).Different properties of the catalyst maintained in acid and alkaline solution could be the main reason.

Fig.10.The pH effect on degradation efficiency of wastewater COD in the Cu2O/H2O2 heterogeneous Fenton process.

Fig.11.SEM images of cubic Cu2Oparticles.(a)The freshone,the ones used in wastewater with(b)acid,(c)neutral and(d)alkaline condition.

After the wastewater treatment at various pH the microstructure of Cu2O particles was investigated with SEM.The metal oxide grains agglomerated in the acid solution with pH=3(Fig.11b).And the loss of copper ions is more severe in acid solution as the acid compounds could react with Cu2O and form Cu2+ions[15].The degradation percentage of CODs was the highest since formation of·OH is favored under the acid condition.In alkaline solution,H2O2molecules would decompose to H2O and O2and·OH yield is decreased.As seen in Fig.11d,the surface of catalyst particles was corroded at pH=11.Morphology of the catalyst was maintained at pH=7 without either agglomeration or corrosion.

The oxidation reaction rate varied at different reaction temperatures due to thermal motions and molecular collisions.Bond cleavage and radical attack can be enhanced at higher temperature.The COD degradation rate increased by 15%as heating the reaction solution from 20°C(E=46.0%)to 50 °C(E=79.2%)with 60 min of reaction.Heating up the system needs the electric energy and impacts the operation cost for the manufacturer.The working temperature should be comprehensively determined in each real and specific case in the industrial application(Fig.12).

Fig.12.Effect of reaction temperature on the degradation efficiency of COD.

3.4.Stability of Cu2O catalyst in the recycling use

Stability of the catalyst was evaluated in the repetition tests treating the pulp waste water with pH=7at30°C.Three groups of Cu2O catalyst have undergone recycling tests respectively,each test lasting 1 h(Fig.13a).All the recycled catalysts have maintained the activity after 5 times of application.Degradation rate of CODs kept constantly ca.73%using the reused catalyst.

Fig.13.Stability of the prepared Cu2O catalyst.(a)Catalytic degradation rate in the 5-recycle test and(b)dissolved copper ions in the wastewater.

Concentration of the dissolved copper ions in solution was detected with ICP and it was found in the range of 0.397–0.480 mg·L-1after the 5 time experiment.The discharged concentration is lower than the critical index in the national standard GB25467-2010 in China.

After recycling use of the Cu2O catalyst, physical and structural property was investigated and displayed in Fig. 14. The reused catalyst particles maintained in cubic morphology and adsorbed materials were found on the surface(SEM image in Fig.14a).Face-centered cubic crystal phase was maintained after the repetition tests according to the XRD pattern(Fig.14b).

Characteristic absorbance of Cu--O bond at wavenumber 628 cm-1was constantly detected for the original and reused catalyst in the FTIR spectra(Fig.14c).Stronger absorbance in the band of 1500–1000cm-1was found for the reused catalyst. It can be assigned the adhered organic compounds consisting of C=O and C=C stretchings and C--H out-of plane deformation vibrations.Thermal analysis indicated that the adsorbed organic compound could be degraded by heating the reused catalyst to 400 °C.After calcination at 800 °C there was 90%of the weight remained(Fig.14 d).In summary,recyclability of the Cu2O catalyst is proved with the maintained structural properties and catalytic activity after the repeated tests.

4.Conclusions

Synthesized Cu2O catalyst working with membrane-feeding H2O2was proposed to treat the pulp wastewater in the ceramic membrane reactor.Photocatalytic activity of Cu2O was found as degrading organic pollutants in water under visible light.Concentration of Cu2O,feeding rate of H2O2in membrane,wastewater pH and temperature have been investigated in the heterogeneous Fenton process.COD degradation rate in the pulp wastewater was 77%in the membrane reactor,whilst,the degradation rate was only 60.4%in a reactor without membrane.The utilization efficiency of H2O2was also enhanced with the integration of a membrane distributor. The fluorescence studydemonstrated that the yield of hydroxyl radical from hydrogen peroxide was greatly improved in the catalytic membrane reactor.Structural property and catalytic activity of the catalyst were well maintained after 5 recycling tests(each test 1 h).Concentration of the dissolved copper ions in treated water is lower than the critical value.In summary,the Cu2O/H2O2(membrane)route is proved compatible for treating the pulp wastewater at neutral pH with enhanced H2O2utility and COD reduction.