To investigate the effects of electrolytic oxidation on nanofiltration in treating dye waste water, we puta mesh catalytic electrode on the intercept side of the membrane and apply a voltage to realize thecoupling of electrolytic oxidation and nanofiltration. The effects of the electroosmosis, electrophoresisand electrochemical oxidation on the flux were investigated. Experiments show that electroosmosismakes the flux increase linearly with the electric intensity. When there is only an electric filed in thecoupling experiments, we get that, with the increase of the electric intensity the flux is acceleratinguntil the electric intensity reach the critical value, after that the flux increase linearly with the electricintensity. With the current density increasing, the degraded organics and the bubbles generated increase,and so the thickness of the concentration polarization and gel layer is reduced in a certain degree. The
ye wastewater flux increases with the decrease of the feed concentration in the coupling experiments. The trend that theflux increases with the pressure slows down. The flux increases to a certain value and then keeps constantwith the increase of the cross flow velocity. The trend that the flux decreases with time slows down withthe increase of the voltage, because of the electroosmosis, electrophoresis and electrochemical oxidation.And when the voltage increases to a certain degree, the flux keeps at a high level and changes less withtime because the thickness of concentration polarization and gel layer is reduced to the minimum.
. Introduction
It is well known that dye wastewater is of high colority, highOD (Chemical Oxygen Demand) and poor biochemical purifica-ion ability, especially the effluent of the dye production and theyeing stages [1,2]. The dye wastewater always contains a varietyf organic matter with biological toxicity or “three-induced” prop-rties (carcinogenicity, teratogenicity, mutagenicity) [3–6]. Thencomplete degradation products, for example the benzidine andome carcinogenic aromatic compounds, have great toxicity. Suchs phenols inhibit the growth of the aquatic plants and variousrganisms, benzene has significant toxic effects on human ner-ous and vascular systems [7–9]. Based on the various hazards,
he improvement of the wastewater treatment technology playsn extremely important role in maintaining ecological balance, pro-ecting the environment and human health.
∗ Corresponding author at: School of Chemical Engineering and Technology, Tian-in University, Tianjin 300072, PR China. Tel.: +86 22 27409839;ax: +86 22 27890515.
The mechanism, characteristic and MWCO (Molecular WeightCut Off) of nanofiltration membrane make it widely recognized inthe world in spite of the existence of concentration polarization andmembrane fouling which reduce the permeate flux and shorten thelife of the NF membrane [10–13]. So seeking for suitable measuresto reduce the concentration polarization and membrane foulingbecomes the focus of the research. In addition, the research of Vlys-sides et al. shows that the electrolytic oxidation technology usedin treating organic wastewater has a lot of advantages (e.g. effi-cient, easy to operate and so on) [14–19]. The organics are degradedon the anode and the wastewater is decolorized at the same time.However, the only problem of the electrolytic oxidation technologyis the high energy consumption [20].
We put the mesh catalytic anode on the intercept side ofthe membrane, and the cathode is put on the other side of themembrane. By the way above, we combine electrolytic oxidationand nanofiltration together. In addition to the improvement ofpermeate flux caused by electroosmosis, the electrophoresis and
electrochemical oxidation reduce the organics concentration nearthe membrane, thereby the thickness of the concentration polar-ization and gel layer is reduced. And the bubbles generated in theelectrochemical oxidation enhance the turbulence of the fluid on
Table 1Characteristics and operation parameters of NF membranes.
Model Charge MWCO pH range Operating pressure
30 L. Xu et al. / Journal of Membra
he membrane surface. All these effects contribute to the reductionf the concentration polarization and membrane fouling, and somproving the permeate flux. That means we can get high perme-te flux under very low cross flow velocity. So it is helpful to reducehe power consumption of the membrane process and extend the
embrane life.In order to study the effects of the electrolytic oxidation on the
oncentration polarization and membrane fouling, we preparedhe simulated dye wastewater, consisting of 500 mg L−1 Reactiveed 118 (purity ≥99%) and 2 g L−1 NaCl. The pH of the simulatedastewater was 3.80. This dye wastewater was used in all of the
oupling experiments unless stated otherwise. Through the cou-ling experiments of the electrolytic oxidation and nanofiltration,e researched the effects of electroosmosis, electrophoresis and
lectrochemical oxidation on nanofiltration, and we also studiedhe effects of the nanofiltration operating conditions on the per-
eate flux.
. Experimental
.1. Materials
The dye was produced by Tianjin Yadong Group and the reagentssed in the experiments were purchased from Jiangtian Chemicalechnology Co., Ltd., Tianjin, China. The chamber type electric resis-ance furnace was the product of Zhonghuan laboratory furnaceso., Ltd., Tianjin, China. All chemicals were used without furtherurification and all solutions were prepared using deionized waterhich was supplied by Chemical Engineering Research Center of
ianjin University.
.2. Preparation of electrode
We chose the Sn–Sb coated titanium electrode (Ti/SnO2 + Sb2O3)s the catalytic anode in the experiments because of the high oxy-en evolution over potential of it [21]. The thermal decompositionechnique was used to prepare the electrode. Before the coat-ng process, pretreatments of titanium mesh, containing grinding,austic and acid etching, need to be done [22]. First we mixed thelycol and citric acid at 60 ◦C and the ratio is 4:1. After completelysterification, we got the glycol solution of glycol citric acid ester.hen the solution was heated to 90 ◦C and added SnCl4·5H2O andbCl3, the molar ratio of 100:11. Completely dissolved and keptt 90 ◦C for 30 min, we got the coating solution [23]. The specificoating process as follows:
1) Coating: Uniformly coat the solution on the titanium mesh,about 0.02 mL cm−2.
2) Drying: Place the well coated titanium mesh in the drying ovenfor 10 min at 130 ◦C.
3) Thermal oxidation: It was annealed for 10 min at 500 ◦C.
It was taken out and cooled to the ambient temperature. Afterhe ashes blowing away, the electrode was coated and roastedgain. The procedure was repeated 10 times. At the last time, thenode was annealed for an hour and cooled to ambient temperaturen the furnace. After taken out and rinsed with deionized water, weot the Sn–Sb coated titanium electrode (Ti/SnO2 + Sb2O3).
After the preparation of the electrode, we carried out the accel-rated life test at 2000 mA cm−2 in 1.0 mol L−1 H2SO4. The timef the cell voltage rise to 5 V was the accelerated life, and after
alculating through the empirical equation, we can estimate theife of the electrode [24]. The result showed that the electrodeife was about 1980 h at general industrial current density (usually00 mA cm−2).
DL Negative More than 150 2–11 483–2758 kPaDK Negative More than 150 2–11 483–2758 kPa
2.3. The equipment and processes of the electrolytic oxidationand nanofiltration coupling experiments
We chose the D series commercial nanofiltration membrane ofGE water & Process Technologies as the experimental membrane.The membranes of this series are composed of the three-layer com-posite structure. The surface layer of the membrane is polyamide,the support layer is polysulfone, and between the two layers is thetransition zone made of special materials. The D series nanofiltra-tion membranes have high retention rate and good heat, acid andalkali resistance. The main characteristics and operating parame-ters of the membrane are listed in Table 1. The DL membranes wereused in the coupling experiments on account of the higher perme-ate flux. The pure water flux of the membrane is tested before andafter the coupling experiments and the difference is less than 5%,meaning that the membrane has not been damaged in the experi-ment. Aim to study the effect of the electrophoresis, we covered theelectrode with insulated paint to restraint the current. The negativepole of the direct-current power supply (for short DC power supply)is connected to the membrane supporting net, thus the stainlesssteel net is used as the cathode in the coupling experiments. Theelectrodes distance is controlled to 1 mm in the coupling exper-iments, and the structure of the membrane pool can be seen inFig. 1(b).
The devices and processes of the electrolytic oxidation andnanofiltration coupling experiments are shown in Fig. 1. The feedliquid from tank 15 is pressurized by the screw pump 14 andpumped into the membrane pool 4, the control valve 7 combinedwith the bypass valve 12 are used to control the operating pressureand the circulation flow rate. Until the system is stable, the pene-trating fluid weight and the corresponding time were recorded bythe electronic balance and computer program. After calculating wecan get the corresponding permeate flux. In the coupling experi-ments, the pressure, voltage and other operating parameters wereseparately regulated to research the effects on the permeate flux.
2.4. Main test parameter of the electrolytic oxidation andnanofiltration coupling experiment
Permeate flux Jv: permeate flux refers to the quantity of perme-ate liquid through unit membrane area in unit time, the formulafollows:
Jv = �W
S�t(2-1)
Jv is the mass flow rate of the membrane, kg m−2 h−1; �W isthe weight of permeate liquid in a certain time �t, kg; S is theeffective area of the nanofiltration membrane, m2, in this paperS = 0.0061 m2; �t is the operating time, h.
COD removal R:
R = CODb − CODp
CODb× 100% (2-2)
R is the COD removal; CODb is the COD of the feed liquid, mg L−1;−1
CODp is the COD of the penetrating fluid, mg L .
The COD removal of the permeate liquid in both the nanofiltra-tion and the coupling experiments are more than 90% and changevery little. The decoloration efficiency reaches almost 100%.
L. Xu et al. / Journal of Membrane Science 409– 410 (2012) 329– 334 331
es of
3
moot
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catlnmv
ar
Fig. 1. Devices and process
. Results and discussion
Placing the mesh catalytic anode on the intercept side of theembrane and the cathode on the other side, we studied the effects
f electroosmosis, electrophoresis and electrochemical oxidationn membrane process and also researched the effects of the nanofil-ration operating conditions on the permeate flux.
.1. Effect of electroosmosis on the nanofiltration process
As water molecules are polar molecule, they will approach to theathode under the power of the electric field. The higher the volt-ge, the faster the water molecules move. Within a certain range,he increment of permeate flux caused by electroosmosis increaseinearly with the electric intensity [25]. The results show that, in theanofiltration of deionized water under the electric field, the per-eate flux of the membrane increases linearly with the additional
oltage, as we can see in Fig. 2.The difference of the permeate flux under different voltages
nd no voltage can be considered as the electroosmosis flow cor-esponding to the different electric intensity. Thus we obtain the
Fig. 2. Effect of electroosmosis on permeate flux.
the coupling experiments.
linear relationship between the electroosmosis flux Qp and theelectric intensity E, the formula follows:
Qp = 0.015E (3-1)
Qp is the electroosmosis flux, kg m−2 h−1; E is the electric intensity,V cm−1.
3.2. Effect of electrophoresis on the nanofiltration process
Seepage flow makes the Reactive Red molecules flow towardsthe surface of the membrane, thereby the concentration polariza-tion and gel layer appears. When we placed the catalytic anodeon the intercept side of the membrane and cathode on the otherside, the Reactive Red molecules under the electric filed will moveaway from the membrane because the Reactive Red molecules iselectronegative. In this way, the electrophoresis makes the con-centration polarization and gel layer thinner and is conducive toincreasing the permeate flux.
The influence of electrophoresis on the permeate flux can bedivided into two stage. When the voltage applied is small, theelectrophoresis cannot yet offset the effect of seepage flow onthe colloidal particles, the concentration polarization and gel layerstill forms [26]. And the thickness of the layer decreases withthe increase of electric intensity. Since the intermolecular forcebetween the colloidal particles in the gel layer is comparativelylarge, the trend that the increase of electric intensity accelerates theelectrophoresis movement of the particles is more obvious undera stronger electric field. The electrophoresis migration rate of thecolloidal particles in the concentration polarization layer is pro-portional to the electric intensity. Therefore, the permeate fluxaccelerated increases with the increase of the electric intensity.When the electrophoresis migration rate of the colloidal particlesis equal to the rate of migration to the membrane surface, the elec-tric intensity is called critical electric intensity. At this point theconcentration polarization and gel layer is reduced to the mini-mum and the permeate flux is close to the flux of the initial stage ofnanofiltration (average of first 30 min). When the voltage applied isrelatively high, means that the electric intensity is higher than thecritical intensity, the permeate flux will be more affected by elec-troosmosis and increase linearly with the intensity. It can be seen
in Fig. 3 that the results of our experiment are consistent with theinference. We can see in Fig. 3 that the increase of the permeate fluxcan be divided into two stages, accelerating and linear stage, andthe critical electric intensity of our experiment is about 100 V cm−1.
332 L. Xu et al. / Journal of Membrane Science 409– 410 (2012) 329– 334
3p
tcobbbdpptotttnbppa
Fig. 3. Effect of electrophoresis on permeate flux.
.3. Effect of electrochemical oxidation on the nanofiltrationrocess
We can see in Fig. 4 that when the current density is low,he electrode surface discharges little charge and so the electro-hemical reaction rate is slow. The amount and extent of oxidizedrganics on the surface of the membrane are limited, and theubbles generated are few. So the concentration near the mem-rane changes little and the promotion of the turbulence by theubbles near the membrane is weak, thus the electrochemical oxi-ation cannot effectively weaken the impact of the concentrationolarization and gel layer. Consequently, the improvement to theermeate flux is not obvious. With the current density increasing,he electrochemical reaction intensifies and the oxidation of therganics on the membrane surface is enhanced. The quantity ofhe bubbles generated increases, enhancing the turbulence nearhe membrane surface and accelerating the permeate flux. Whenhe current density increases to a certain value, the oxidation rateear the membrane reaches the limitation and the quantity of theubbles stops increasing. So far, the thickness of the concentration
olarization and gel layer is reduced to the limitation. And at thisoint increasing the current density cannot improve the perme-te flux obviously, instead excessive current density leads to vast
Fig. 4. Effect of electrochemical oxidation on permeate flux.
Fig. 5. Effect of feed concentration on permeate flux.
energy consumption and may breakdown the NF membrane. Mean-while, with the increase of the current density, there are plenty ofthe chloridion become hypochlorous or chlorine, which is of strongoxidizing property. Therefore the organics are further oxidized bythe hypochlorous or chlorine.
3.4. Dye wastewater treatment by coupling experiments
The interception ratio of the DL membrane for dye is high andthe penetrating fluid could completely meet the recycle require-ments. However, the concentration polarization and membranefouling reduce the permeate flux and shorten the life of the mem-brane, thus limiting the practical application of nanofiltration inwastewater treatment. In our coupling experiments, the electroos-mosis increases the permeate flux of the membrane. What’s moreimportant is that the electrophoresis and electrochemical oxidationreduce the dye concentration in the concentration polarization andgel layer and the bubbles generated enhance the turbulence, sig-nificantly increasing the permeate flux. In order to study the effectsof the nanofiltration operating conditions on the coupling process,we carried out the coupling experiments with the simulated dyewastewater.
3.4.1. Effect of the feed concentration on the permeate fluxThe increase of dye wastewater concentration leads to the raise
of resistance from the concentration polarization and gel layer. Andthe higher the concentration, the more obvious is the increase ofthe resistance. Therefore, when the operating pressure is constant,the increase of the feed concentration accelerates the reductionof the permeate flux [27]. Since the effects of electroosmosis andelectrophoresis depend on the electric intensity, when the voltageis constant, the electroosmosis and electrophoresis effects changelittle with the feed concentration. The current efficiency of electro-chemical oxidation increases with the feed concentration, but thehigher feed concentration leads to higher concentration of resid-ual organics in the boundary layer and higher resistance of theboundary layer, thus the permeate flux reduces more obviously.Therefore when the operating pressure is constant, the increase ofthe feed concentration accelerates the reduction of the permeateflux in the actual coupling process of the electrochemical oxidationand nanofiltration, as Fig. 5 shows.
In addition, the electrophoresis and electrochemical oxidationreduce the resistance from the concentration polarization and gellayer. As Figs. 3 and 4 show, at the same feed concentration, thehigher the voltage, the more obviously the permeate flux increases.
L. Xu et al. / Journal of Membrane Science 409– 410 (2012) 329– 334 333
Acvs
3
aWttirtb
cacaoatgctotsflvwlA1tvtp
3
tbr
mixtures that of poor shear capacity.
Fig. 6. Effect of operating pressure on permeate flux.
nd the higher feed concentration leads to higher current effi-iency, therefore the increment of the permeate flux caused by theoltage is higher than that of lower feed concentration, as Fig. 5hows.
.4.2. Effect of operating pressure on the permeate fluxIn case of no voltage, the permeate flux increases with the oper-
ting pressure, but the concentration polarization becomes serious.ith the pressure increasing, the total resistance of the nanofiltra-
ion increases, so the trend that the permeate flux increases withhe operating pressure slows down. When the operating pressurencreases to a certain point, there forms a dense gel layer of largeesistance. And at this point, raising operating pressure has lit-le effect of increasing the permeate flux, then the permeate fluxecomes stable [28].
Applying an electric field, the increment of the permeate flux isaused by not only the electroosmosis, but also the electrophoresisnd electrochemical oxidation, which lead to the reduction of theoncentration polarization and gel layer. When the voltage appliednd other conditions of nanofiltration are constant, the effectsf electrophoresis and electrochemical oxidation on the bound-ry layer are decided by the boundary layer itself. That is to say,he more serious concentration polarization and gel layer leads toreater improvement caused by the electrophoresis and electro-hemical oxidation. Therefore, the higher the operating pressure,he greater the impact of the electrophoresis and electrochemicalxidation on the permeate flux. The results of the experiment fithe explanations above, as Fig. 6 shows. When the operating pres-ure is 0.4 MPa, the voltage has little influence on the permeateux. This shows that the degree of concentration polarization isery low at this time and there forms no gel layer. So comparedith the total resistance, the effect of the voltage on the boundary
ayer can be neglected, thus the permeate flux difference is little.s the increase of the operating pressure, the permeate flux under0 V increases almost linearly, means that the total resistance ofhe nanofiltration process no longer increases. This shows that theoltage applied plays an important role in inhibiting the concen-ration polarization and gel layer, especially when the operatingressure is relatively high.
.4.3. Effect of cross flow velocity on the permeate flux
When the operating pressure is constant and no voltage applied,
he permeate flux increases with the cross flow velocity andecomes stable finally, as shown in Fig. 7. Analyzing the variousesistances, we find that adding the cross flow velocity makes the
Fig. 7. Effect of cross flow velocity on permeate flux at different voltage.
concentration polarization layer thinned, which due to the increaseof the tangential force near the membrane. Thus the total resistancediminishes and so the permeate flux increases continuously withthe cross flow velocity. However, when the cross flow velocity onthe membrane surface exceeds a certain value, the concentrationpolarization layer almost reaches the minimum thickness. So far,the resistance of concentration polarization no longer decreasesand the permeate flux becomes basically stable. When exceedingthe certain value, the increasing of cross flow velocity not onlycannot improve the permeate flux apparently, but also raises theenergy consumption.
In the coupling process, the permeate flux of the membraneincreases with the voltage and the increment of permeate flux isapparent at low cross flow velocity. When the cross flow velocityis relatively high, the permeate flux increases little with the volt-age. That is because the lower the cross flow velocity, the thickerthe concentration polarization and gel layer, thus the effects fromthe electrophoresis and electrochemical oxidation are more obvi-ous, seen in Section 3.4.2. Because of the contribution of the electricfield, we can get relatively high permeate flux under low cross flowvelocity, and so the coupling experiments are more suitable for the
Fig. 8. Effect of voltage on the permeate flux.
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34 L. Xu et al. / Journal of Membra
.4.4. Effect of voltage on the permeate fluxIt can be seen in Fig. 8 that, when there is no voltage, the per-
eate flux continues to decay with the operating time as a resultf the increasing of the total resistance. And the resistance is notnly caused by the concentration polarization and gel layer butlso the blocking and adsorption occurred in the membrane pores.ith the voltage increasing, the electroosmosis effect increases
nd the impacts of the electrophoresis and electrochemical oxida-ion on the concentration polarization and gel layer also increasebviously. The higher the voltage, the slower the trend that theermeate flux decreases with the operating time. However, whenhe voltage reaches 10 V, the permeate flux stays at a high levelnd changes little with time. This suggests that the thickness of theoncentration polarization and gel layer on the membrane surfaces reduced to a negligible level at this point.
. Conclusion
1) The effect of the electroosmosis makes the permeate fluxincrease linearly with the electric intensity. Both the elec-trophoresis and electrochemical oxidation reduce the adverseeffects of the concentration polarization and gel layer in thecoupling experiments. And the electric intensity and the cur-rent density accelerate the permeate flux within a certain range.
2) In the coupling process, the higher voltage leads to the strongerinhibition of the concentration polarization and membranefouling, but it cannot completely eliminate the effect of the feedconcentration on the permeate flux. The lower the cross flowvelocity and the higher the feed concentration and the oper-ating pressure, the greater the increment of the permeate fluxcaused by increasing the voltage.
In conclusion, the concentration polarization and membraneouling are effectively restrained by the electroosmosis, elec-rophoresis and electrochemical oxidation in the coupling oflectrolytic oxidation and nanofiltration experiments. Thus we canet high permeate flux under relatively low cross flow velocity andressure in the actual coupling wastewater treatment process. Inhis way, we can reduce the operating pressure and the area ofhe membrane, thereby cutting the equipment investment and the
embrane cleaning and replacement costs.
cknowledgement
This work was supported by the Natural Science Foundation ofianjin under Grant No. 10JCYBJC04900.
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