Electrochemical treatment of industrial wastewater using a novel layer-upon-layer bipolar electrode system (nLBPEs)

Chemical Engineering Journal 215–216 (2013) 157–161

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Electrochemical treatment of industrial wastewater using a novel layer-upon-layer bipolar electrode system (nLBPEs) Lizhang Wang a,⇑, Yunlong Hu a, Peng Li a, Yanle Zhang a, Qian Yan a, Yuemin Zhao b,⇑ a b

School of Environment Science and Spatial Informatics, China University of Mining and Technology, Xuzhou City, Jiangsu 221008, PR China School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou City, Jiangsu 221008, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

" A novel layer-upon-layer bipolar

"

"

"

"

electrode system (nLBPEs) is developed. The economically prepared CA-GAC as insulating layer can avoid short circuit current. The nLBPEs can enhance electrochemical performance compared with traditional TDE. The obtained w parameter can guide proper selection of packed materials to OM kinds. The nLBPEs shows broad applicability for industrial wastewater treatment.

a r t i c l e

i n f o

Article history: Received 20 May 2012 Received in revised form 17 October 2012 Accepted 6 November 2012 Available online 10 November 2012 Keywords: Layer-upon-layer bipolar electrode system (nLBPEs) Industrial wastewater treatment Granular activated carbon (GAC) Cellulose acetate coated GAC (CA-GAC)

a b s t r a c t A novel layer-upon-layer bipolar electrode system (nLBPEs) possessing the ability of increasing space– time yields was developed for efficient treatment of industrial wastewaters. The cellulose acetate coated granular activated carbon (CA-GAC) was used as insulating layer to avoid short circuit current, which could effectively enhance the electro-oxidation. In addition, the excellent performances of the nLBPEs are theoretically described by introducing the fraction of current applied to particulate electrode, w, which relies on the characteristics of carbon particles, organic matters (OMs) and solutions. The kinetics could provide proper preparation and selection of the filings in accordance with the characteristics of OM. Moreover, the presented nLBPEs is superior to the conventional three dimension electrode and shows broad interests and perfect performances in different kinds of industrial wastewater treatment. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction For environmental approaches, several porous materials, such as copper bead, kaolin or carbon are usually packed into electrode gaps to increase space–time yield due to their bipolarity in high gradient electric field [1–3]. This process, so-called three dimension electrode (TDE) proposed in 1960s [4,5] is used for metal ⇑ Corresponding authors. Tel.: +86 0516 83591320; fax: +86 0516 83591329 (L. Wang), tel.: +86 516 83590092; fax: +86 516 83885455 (Y. Zhao). E-mail addresses: [email protected] (L. Wang), [email protected] (Y. Zhao). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.11.026

recovery (electro-reduction) [6] and removal of organic matter (OM) (electro-oxidation) [7,8]. High removal efficiency and current yield could be achieved yielding this way [9,10]. However, aiming to avoid short circuit current, a membrane arranged between the anodic and cathodic compartments is frequently required [11– 13], causing not only the increase of capital investment but also the difficulty in preventing membrane fouling. Although this problem could be eliminated by directly packing these materials into electrode gaps [14,15], low space-time yield, especially carbon saturation (if carbon was used) would commence for the short circuit of the electrodes [16,17]. Many efforts have been devoted to the

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elimination of short circuit current by packing mixed materials e.g. zero valent iron (Fe0) and silica sand [18], carbon and TiO2 coated carbon [19], while the mixture will further result in the stratification affected by the difference of their density, as well as the increase of the production cost. Moreover, few researchers took into account the influence of fillings on the kinetics and they could not explore how microelectrode work and what factors are involved in the reactions. These disadvantages strongly restrict the wide application of this technology in wastewater treatment. In this study, a novel layer-upon-layer bipolar electrode system (nLBPEs) consisted of granular activated carbon (GAC) and cellulose acetate coated GAC (CA-GAC) layers was proposed. The easily and economically prepared CA-GAC was used as isolating layers to avoid direct contact of carbon and electrode, which could effectively enhance the electro-oxidation by eliminating the short circuit current compared with conventional TDE. By inspecting the current transfer mechanisms of the bipolar systems, a kinetic model associated with characteristics of carbon particles, OM and solutions was used to describe the excellent performances of the nLBPEs. Moreover, the superiority of the nLBPEs to the TDE system was experimentally verified by synthetic phenolic solution and different industrial wastewater treatment. 2. Experimental 2.1. Material preparation

consisted of GAC particles. A micropore plate was installed at the bottom of the cell to support the weight of carbon and electrodes, and employed as solution distributor. Metering pump was used to pump the raw water into the reactor. The power was provided by an electric motor (model: KZD300/12) having a capacity of 0– 300 A/0–12 V. The direction of the solutions was perpendicular to that of current flow and the raw water was treated by single pass across the system at room temperature. Phenol obtained in 99.5% pure was used for the preparation of synthetic solution. The ribonucleic acid (RNA), thiophene-2,5dicarboxylic acid (OB acid) and 4,40 -diaminostilbene-2,20 -disulfonic acid (DSD) manufacturing wastewater, and pyridine wastewater were also used. The characters of these solutions were shown in Table 1. The nLBPEs and TDE were operated under conditions of flowrate of 0.8 L h1, applied current density of 50 A m2 and reaction time of 2.06 h. 2.3. Analytical method A spectrophotometer UV–visible (Shimadu UV 1800) was used to analyze the absorbance of phenolic wastewater and samples. The chemical oxygen demand (COD) of each sample was determined by the standard method [20]. Current yield, g (%) and power consumption, Esp (kW h kg1COD) were calculated according to the following equations, respectively [21].

g¼

GAC with an average particle size of about 0.5 cm through screening separation and a specific surface of 562 m2 g1 according to BET method was used in this study. GAC particles were washed several times with distilled water to remove the fines, oven-dried at 105 °C for 2 days to constant weight. The CA-GAC was achieved by firstly soaking GAC into methyl acetate based CA solution with stirring at 30 r min1 for 10 min, then baking 30 min at 110 °C, and repeating the same operation procedure for several times. GAC and CA-GAC particles were shaped into single-layer plates by bonding them together using nylon–novolac binder. All reagents in extra pure condition were used for the preparation. 2.2. Experimental unit Seven quadrangular cells with 10  5  15 cm dimension were constructed from polymethyl methacrylate plastics. In each unit (Fig. 1a), a pair of PbO2/Ti and Ti plates with active area of 100 cm2 (10  10 cm) was used as anode and cathode, respectively. The prepared GAC and CA-GAC plates (10  10  0.5 cm) were layer-upon-layer arranged in the nLBPEs and the TDE system

FqðCOD0  CODt Þ  100 3600  8ð1 þ wÞi

Esp ¼

1000  Ui qðCOD0  CODt Þ

(b)

j

GAC particle

Cathode

Anode

i

i2

i

3. Results and discussion In a carbon-based bipolar system, not only metallic electrode but also GAC particles can behave as electrode, hence, the systems allow dually heterogeneous oxidation by immobilized charge on metallic and particulate anodes [1,7,14,21]. The charge transfer mode shown by Fig. 1b indicates that only the effective current (i1) can be utilized by bipolar GAC particles. Hence, based on Faraday’s law, the overall oxidation rate of the systems is given as follows:

(OM) GAC layer

is (CO2+H2O, R/)

CA-GAC layer

Fig. 1. (a) Schematic diagram of the designed nLBPEs. (b) Charge transfer mode and OM oxidation at bipolar GAC particles. i1 is the effective current for OM oxidation, i2 and is the current transferred by GAC particles and solution, respectively; R0 the intermediates.

j nF

ð3Þ

where rmax is the oxidation rate in the charge transfer control regime, mol m2 s1; j the applied current density, A m2 and n the electron transferred during the electro-oxidation. The equation indicates that the increase of the w values would be beneficial to the OM oxidation at a constant current density. According to our previous paper [21], the w values can be determined by

w¼ i1

ð2Þ

where F is the Faraday’s constant (96,485 C mol1), q the flowrate (L h1), w the fraction of current applied to particulate electrode, i the overall current (A) and U the applied voltage (V), COD0 and CODt (g L1) the initial COD and COD at time t, respectively.

rmax ¼ ð1 þ wÞ

(a)

ð1Þ

RC RO þ RC

ð4Þ

where RC is the contacting resistance of the particles and RO is the oxidative resistance of OM. It can be seen from Eq. (4) that the fractional current applied to the GAC particles is independent of the cell configuration and solution conductivity, while it relies on the kinds of OM and the surface resistivity of fillings. Therefore, when a target OM is oxidized, increasing RC by the employment of the insulating layers (CA-

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L. Wang et al. / Chemical Engineering Journal 215–216 (2013) 157–161 Table 1 The quality of wastewaters used in this study. COD0 (mg L1)

Items

Phenolic wastewater RNA wastewater OB acid wastewater DSD wastewater Pyridine wastewater

Salts

Range

Average

1326–1420 13,178–16,319 4989–7266 13,200–21,800 7980–11,350

1369 14,970 5670 18,600 9980

0.8 RC 0.64

Rs

Na2SO4 NaCl NaCl Na2SO4 NaCl

(a)

nLBPEs

Range

Average

2.8–3.2 3.3–5.9 7.5–9.1 8.9–9.6 8.9–11.3

3.0 4.7 8.6 9.3 10.9

100

TDE

nLBPEs

80

RO

X (% )

0.48

Saline content (% (w/w))

0.32 0.16

60

TDE

40 20

0 a)

b)

c)

d)

Adsorption

e)

Fig. 2. The dependence of w values on the cell configuration (length y0 and width z0 of electrode, and inter-electrode distance x0) during phenol oxidation in nLBPEs and PBEs. Rs is the solution resistance. Cell dimensions: (a) x0 = 5 cm, y0 = 10 cm, z0 = 10 cm. (b) and (c) x0 = 4.5 cm, y0 = 10 cm, z0 = 10 cm. (d) x0 = 5 cm, y0 = 5 cm, z0 = 5 cm. (e) x0 = 10 cm, y0 = 10 cm, z0 = 10 cm. As for the nLBPEs, in case of (a), (d) and (e), the numbers of GAC and CA-GAC layers are equal, while under conditions of (b) and (c) there is one more CA-GAC or GAC layer, respectively.

0 0

30

60

90

(b)

R ¼ wRO þ Rs

ð5Þ

where R is the overall electric resistance of the systems and Rs the solution resistance. Hence, we can conclude that the application of isolating fillings will inevitably increase the applied voltage while

150

80 nLBPEs 64

TDE

Values

48 32 16 0

X

Esp

Fig. 3. (a) The COD removal efficiency using nLBPEs, TDE and adsorption under continuous flow mode during phenolic wastewater treatment. (b) Electrochemical performances of phenol oxidation in nLBPEs and PBEs. The time started once the solutions flowed out of the systems.

3 2.4

Absorbance

GAC) is a preferable way to increase the effective current (i1) for OM oxidation. Fig. 2 shows the dependence of w values on the cell dimensions and layer amount when phenolic solution was used as the reactant. This figure depicts the w values of the nLBPEs are much higher than those of TDE process and the former is 1.9 times of the latter, illustrating the nLBPEs would accelerate the oxidation rate and increase the current yield at the same applied current density. The superiority of the nLBPEs is intuitively reflected in treating phenolic wastewater with longevity operations at a constant flowrate (Fig. 3). This figure also provides the results obtained by TDE and adsorption for comparison. During adsorption experiment, the COD removal efficiency (X, %) decreases from 95.4% to 4.3% after 130 h continuous operation, meaning GAC is almost saturated and its maximum adsorption ability is reached (Fig. 3a). While when a potential is applied, absolutely stable X values (73.5% and 62.4% for the nLBPEs and TDE, respectively) are obtained over 140 h, which confirm the existence of the bipolarity of GAC particles in the two systems and advantages of the employment of the CA-GAC layers. As for the current yield, the two systems all show lower values (g of 54.2% and 53.5% for nLBPEs and TDE, respectively), this is because the oxidation is in the mass transfer control and the side reaction, such as water decomposition would occur, which decreases the current utilization. Meanwhile, the power consumption of the nLBPEs is 15.99 kW h kg1COD, which is slightly lower than that of the TDE (18.32 kW h kg1COD) (Fig. 3b). It is because a higher applied voltage is required in the presence of the CA-GAC layers because of the increase of the overall resistance with expression of

120

Elapsed time (h)

Original TDE nLBPEs

1.8 1.2 0.6 0 190

240

290

340

390

440

Wave length / nm Fig. 4. The absorbance of original phenolic solution and samples after 2.06 h treatment by using the nLBPEs and TDE, respectively.

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L. Wang et al. / Chemical Engineering Journal 215–216 (2013) 157–161

(a)

(b)

0.5 0.4

35 28

0.3

21 X

0.2

14

0.1

7

0

(c)

a)

b)

c)

0

d)

(d)

100 80

a)

b)

c)

d)

a)

b)

c)

d)

12

9.6

60

7.2 Esp

40

4.8

20

2.4

0

0 a)

b)

c)

d)

Fig. 5. The comparison of w values (a), COD removal efficiency (b), current yield (c) and power consumption (d) when RNA (a), OB acid (b), DSD acid (c) and pyridine (d) wastewaters were treated by the nLBPEs (j) and TDE process (h), respectively.

the current can be maximally utilized. Compared with the TDE, the nLBPEs is still power-saving once the increase ratio of COD removal is higher than that of the applied voltage. During a TDE process, phenol would be oxidized to many intermediates, e.g. benzoquinone, maleic acid or oxalic acid, which made the solution color turn to dark brown initially, then to fade, and almost disappear at a relatively longer reaction time, which were in accordance with the previous reports [22–25]. At the same time, this phenomenon was also displayed when the phenol oxidation was carried out using the nLBPEs. Furthermore, the measurement of sample absorbance provides direct validation that the employment of CA-GAC does not change the degradation pathway of phenol but accelerate the oxidation rate through the increase of effective current applied to GAC particles (Fig. 4). The degradation of OM is for the generation of hydroxyl radicals (HO) with high oxidation ability in case of PbO2/Ti, a ‘‘non-active’’ anode is employed (Eq. (6)) [22]. In addition, the formation in situ of hydrogen peroxide (H2O2) at the cathode and carbon surface also enhances the oxidation (Eq. (7)) [26]. Moreover, the existence of Cl in solutions can generate active Chlorine (Eq. (8)) related to the OM removal [27,28]. All these electro-generated oxidants are beneficial to partial or complete degradation of OM absorbed by metallic/particulate electrodes, thus contributing to the rapid water purification.

H2 O ! HO þ Hþ þ e

ð6Þ

O2 þ 2Hþ þ 2e ! H2 O2

ð7Þ



2Cl þ 2e ! Cl2

ð8Þ

The oxidative resistance of OM also strongly affects the fraction of current applied to bipolar GAC particles, and the lower the RO, the higher the w values. When RNA, OB acid and DSD manufacturing wastewaters and pyridine wastewater were treated by the nLBPEs and TDE, the w values of 0.43, 0.45, 0.32, 0.29 and 0.23, 0.25, 0.16, 0.14 are obtained according to their RO of 0.91 X, 0.82 X, 1.45 X and 1.68 X, respectively (Fig. 5a). In view of the results, fillings with higher contacting resistance should be used to

enhance the electro-oxidation of OM that hard to oxidize. The higher w values of the nLBPEs result in the larger COD removal efficiency (Fig. 5b) and current yield (Fig. 5c), verifying the broad applicability of the novel process in dealing with OM existed in different industrial wastewaters. Moreover, the actually applied current of 3.5 A approximates to the theoretical current calculated by the Faradic relation of i = Fq(COD0  CODt)/[8(1 + w)], which indicates the oxidation is in the charge transfer control regime at large COD values. With respect to the power consumption, it would be decreased when the high saline solution was treated because of the reduction of the applied voltage (Fig. 5d). The results shown in this figure indicate that the novel technology is very suitable to high saline wastewater treatment and it is superior to the others techniques used in this field, e.g. photocatalytic, Fenton and wet oxidation (wet air, catalytic wet air, super critical wet and catalytic wet hydrogen peroxide oxidation) [29–32]. Taking into account the comparison of the real industrial wastewater and phenolic solution treatment, we also conclude that high concentration of OM is prone to achievement of best electrochemical performances at the same experimental conditions.

4. Conclusion In this paper, we develop a layer-upon-layer (GAC and CA-GAC layers) bipolar electrode system (nLBPEs) for industrial wastewater treatment. Compared with the conventional TDE, the novel process could effectively avoid the short circuit current by easily and economically prepared CA-GAC layers, thus increasing the COD removal efficiency and current yield and decreasing the power consumption. The enhancement of electrochemical performances in the nLBPEs is due to the increase of the fraction of current applied to the particulate electrode, w, which is only determined by the contacting resistance of fillings and oxidative resistance of OM. During phenol oxidation, we observed the degradation pathway in the nLBPEs is the same to that of TDE. Furthermore, the electrochemical treatment of RNA, OB acid and DSD manufacturing wastewaters and pyridine wastewater indicate the electro-oxidation of OM that hard to oxidize requires fillings with higher contacting

L. Wang et al. / Chemical Engineering Journal 215–216 (2013) 157–161

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