Oxidation of organic pollutants on BDD anodes using modulated current electrolysis

Available online at www.sciencedirect.com

Electrochimica Acta 53 (2008) 2289–2295

Oxidation of organic pollutants on BDD anodes using modulated current electrolysis M. Panizza a,∗,1 , Agnieszka Kapalka b , Ch. Comninellis b,∗∗,1 a

b

Department of Chemical and Process Engineering, University of Genoa, P.le J.F. Kennedy 1, 16129 Genova, Italy Institute of Chemical Sciences and Engineering, Ecole Polytechnique F´ed´erale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland Received 8 June 2007; received in revised form 5 September 2007; accepted 24 September 2007 Available online 29 September 2007

Abstract In this paper, a theoretical model is presented for organic pollutants mineralization at high current efficiency (close to 100%) and low energy consumption on boron-doped diamond electrodes. The model is formulated for a perfect mixed electrochemical reactor operated as a batch recirculation system under multiple current steps, in which the applied current is adjusted during the electrolysis to be close to the limiting value. An experimental validation with the anodic oxidation of 3,4,5-trihydroxybenzoic acid is also provided. The results have shown that multiple current steps electrolysis and continuous current control allowed obtaining high oxidation rate and current efficiency. © 2007 Elsevier Ltd. All rights reserved. Keywords: Current steps electrolysis; Theoretical model; Boron-doped diamond electrodes; Electrochemical oxidation; Wastewater treatment

1. Introduction Industries discharge a great variety of organic pollutants in their wastewater which have to be treated to avoid oxygen depletion with potentially severe impacts on the whole ecosystem. There are different methods for the treatment of industrial wastewaters containing organic pollutants including biological oxidation, adsorption, solvent extraction, incineration and chemical oxidation. The choice of treatment depends on economics as well as ease of control, reliability and efficiency. In recent years, there has been an increasing interest in the removal of organic pollutants by electrochemical methods as an alternative to traditional processes [1–3]. The effective and economical anodic incineration of pollutants requires the choice of catalytic electrode materials and appropriate electrolysis conditions [4–8]. In literature, several anode materials have been tested for the direct electrochemical oxidation of organic compounds but the complete mineralization to CO2 and good faradic efficiency was obtained only using high oxygen overpotential anodes, such as ∗

Corresponding author. Tel.: +39 010 3536032; fax: +39 010 3536028. Corresponding author. Tel.: +41 21 693 3674; fax: +41 21 693 3190. E-mail addresses: [email protected] (M. Panizza), [email protected] (Ch. Comninellis). 1 ISE member. ∗∗

0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.09.044

SnO2 [9–11], PbO2 [12–14] and boron-doped diamond [15–20]. In fact, during electrolysis, these electrodes involve the production of oxygen evolution intermediates, mainly hydroxyl radicals that oxidise the pollutants. Despite their notable ability to remove organics, doped-SnO2 anodes have the major drawback of a short service life and PbO2 anodes could release of toxic lead ions during the electrolysis [21]. On the contrary, synthetic boron-doped diamond (BDD) thin films, deposited both on p-silicon substrates or on valve base metal, with its high anodic stability and wide potential window of water discharge is undoubtedly a promising material for the complete combustion of organics for wastewater treatment [22]. It has been reported that in the potential region of water stability only reactions involving simple electron transfer processes are active on diamond electrodes, but polymeric adhesive films, which cause rapid electrode fouling, were also formed in this potential region [23,24]. However, this deactivation can be avoided by performing the electrolysis at high anodic potentials in the region of water discharge due to the participation of intermediates of oxygen evolution [8,25]. In a previous works [26,27], a mathematical model that permits to predict the evolution of chemical oxygen demand (COD) and current efficiency with time, during the oxidation of organics on BDD electrodes has been described. The model is formulated for a perfect mixed electrochemical reactor operated as a batch

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recirculation system under galvanostatic conditions. The theoretical results have shown good agreement with the experimental data during the oxidation of 2-naphthol [27] and 4-chlorophenol [26]. Subsequently, this model was used as a design tool for the prediction of specific energy consumption and the required electrode area for the elimination of a given organic loading on boron-doped diamond electrodes and an experimental validation with the anodic oxidation of phenol was also provided [28]. The present purpose is apply the model, originally formulated for galvanostatic electrolysis, to a multiple current steps electrolysis or to a semi-continuous current control electrolysis and to show how it can be used to achieve the mineralization of organic pollutants with high reaction rate and current efficiency and low energy consumption. An experimental validation with the anodic oxidation of 3,4,5-trihydroxybenzoic acid is also provided. 2. Experimental The solutions were prepared dissolving 1 g dm−3 of 3,4,5trihydroxybenzoic acid (C7 H6 O5 ) in distilled water in the presence of HClO4 0.5 M as supporting electrolyte. The initial COD of the solution was 1100 mg dm−3 . The boron-doped diamond thin-film electrode was supplied by CSEM Centre Swiss d’Electronique et de Microtechnique of Neuchˆatel. It was synthesised by the hot filament chemical vapour deposition technique (HF CVD) on single crystal p-type Si 1 0 0 wafers (1–3 m cm, Siltronix). The doping level of boron in the diamond layer expressed as B/C ratio was about 3500 ppm. The obtained diamond film thickness was about 1 ␮m with a resistivity of 10–30 m cm. In order to stabilise the electrode surface and to obtain reproducible results, the diamond electrode was pre-treated by anodic polarisation in 1 M HClO4 at 10 mA cm−2 during 30 min. This treatment made the surface hydrophilic. Bulk oxidations of 3,4,5-trihydroxybenzoic were performed in a one-compartment electrolytic flow cell under galvanostatic conditions, applying multiple steps current or in continuous current control using an EG&G mod. 273/A potentiostat/galvanostat. The BDD was used as the anode and stainless steel as the cathode. Both electrodes were disks with a geometrical area of 50 cm2 each, with an inter-electrode gap of 1 cm. The solution was stored in a 450 ml thermo-regulated glass tank and circulated through an electrochemical reactor by a centrifugal pump with a flow rates of 300 dm3 h−1 corresponding to a mass-transfer coefficient in the cell, determined using the ferri/ferrocyanide couple, of about 3.5 × 10−5 m s−1 . More details on the equipment used in bulk electrolyses are given elsewhere [27]. The COD of the solution was measured during electrolysis using a Dr. Lange LASA50 system. The instantaneous current efficiency (ICE) for the anodic oxidation was calculated from the values of the COD using the relationship [29]: ICE = 4 F V

(COD)t − (COD)t+t I t

(1)

where (COD)t and (COD)t+t are the chemical oxygen demands at times t and t + t (in mol O2 dm−3 ), respectively, I the current (A), F the Faraday’s constant (96,487 C mol−1 ) and V is the volume of electrolyte (dm3 ). The average current efficiency has been calculated using the equation: τ ICE(t) dt η¯ = 0 (2) τ where τ is the electrolysis time (in s) necessary to reach a target COD conversion X. 3. Results and discussion A theoretical model that permits to predict the evolution of COD and ICE with time, during the electrochemical oxidation of organic pollutants on BDD electrodes has been described in a previous paper [26–28]. This model has been developed for an electrochemical reactor operating in a batch recirculation mode under galvanostatic conditions, assuming that the anodic oxidation of organics is under diffusion control. According to this model, the limiting current density can be estimated from the value of the COD: ilim (t) = 4 Fkm COD(t)

(3)

where ilim is the limiting current density (A m−2 ) at a given time t, 4 the number of exchanged electrons per mol of O2 , F the Faraday’s constant (C mol−1 ), km the average mass transport coefficient in the electrochemical reactor (m s−1 ) and COD(t) is the chemical oxygen demand (mol O2 m−3 ) at a given time t. Working under galvanostatic conditions, depending on the applied current density i (A m−2 ), different operating regimes have been identified [26–28] and schematised in Fig. 1a: (1) i < ilim : the electrolysis is under current control, organic intermediates are formed during the oxidation, the instantaneous current efficiency (ICE) is 100% and COD decreases linearly with time. This behaviour persists until a critical time (tcr ) or to a critical conversion (Xcr ), corresponding to the time, or to the conversion, at which the applied current

Fig. 1. Schematic current–time curves showing operating regions during constant operating current.

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Table 1 Equations that describe COD, ICE, τ and Esp evolution during organics oxidation at BDD electrode

ICE

Under current limited control [iappl. < ilim ]

Under mass transport control [iappl. > ilim ]

ICE = 1

ICE = exp −



COD (mol O2 m−3 )

COD(t) = COD0 1 −

 Ak m

 α A km VR

t

τ=

Esp (kW h kg COD−1 )

Esp =

1−α α

τ = tcr −

1 F (Vd + Rc A α i0lim ) 3600 8

Esp =



 Ak m

COD(t) = α COD0 exp −

XV α A km

τ (s)

VR

t+

VR

  1 − X  V Akm

ln

α

t+

1−α α

=−

V Akm

  1 − X ln

α

−

1−α α



1 F 1 − α[1 + ln(1 − X/α)] (Vd + Rc A α i0lim ) 3600 8 X

VR = reservoir volume (m3 ), km = mass-transfer coefficient in the electrochemical reactor (m s−1 ), A = electrode area (m2 ), COD0 = initial chemical oxygen demand (mol O2 m−3 ), X = COD conversion, α = i/ i0lim and i0lim is the initial limiting current density (A m−2 ).

density is equal to the limiting current density [26–28]:

Esp =

1 − α VR tcr = α Akm

(4)

Xcr = 1 − α

(5)

1 F (Vd + Rc A i0lim ) 3600 8

(9)

Fig. 2 shows the time evolution of COD and current efficiency, while Fig. 3 shows the behaviour of the specific energy consumption (Esp ) and the electrolysis time (τ) as a function of

(2) i > ilim : the electrolysis is under mass transport control, organic compounds are completely combusted to CO2 and secondary reactions (such as oxygen evolution or electrolyte decomposition) commence, resulting in a decreasing of current efficiency. Under these conditions, the instantaneous current efficiency (ICE) is below 100% and the COD removal follows an exponential trend [26–28]. The equations that describe the temporal evolution of COD and current efficiency have been obtained and they are summarized in Table 1. Starting from this model, equations for the prediction of the electrolysis time τ and specific energy consumption (Esp ) expressed in kW h kg COD−1 , to reach a target COD conversion X were also derived for both the regimes [28] and these equations are summarised in Table 1. Working at controlled potential the instantaneous limiting current is maintained thorough the process and the continuous change in input concentration results in the decreasing of the current flowing in the cell:

Fig. 2. Influence of current density on the evolution of COD and ICE (inset) during the electrolyses of 1 g dm−3 of 3,4,5-trihydroxybenzoic acid in HClO4 0.5 M on boron-doped diamond anode. Applied current density: () 10 mA cm−2 ; () i = 60 mA cm−2 . The solid lines represent model prediction.

i = ilim In these conditions, the oxidation of organics is under mass transport control, the instantaneous current efficiency (ICE) is 100% (Eq. (6)) and COD decreases exponentially with time (Eq. (7)): ICE = 1



Akm t COD(t) = COD0 exp − VR



(6) (7)

Under potentiostatic conditions, the electrolysis time τ and specific energy consumption (Esp ) expressed in kWh kg COD−1 , to reach a target COD conversion X can be calculated using the following equations: τ=−

VR ln(1 − X) A km

(8)

Fig. 3. Influence of current density on specific energy consumption and electrolysis time against conversion (X) during the oxidation of 1 g dm−3 of 3,4,5-trihydroxybenzoic acid in HClO4 0.5 M on boron-doped diamond anode. Applied current density: () 10 mA cm−2 ; () i = 60 mA cm−2 . The solid lines represent model prediction.

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conversion obtained during the oxidation of 1 g dm−3 of 3,4,5trihydroxybenzoic acid at different current densities (i.e. 10 and 60 mA cm−2 ) that are below and above the limiting one, that in our experimental conditions is 46.4 mA cm−2 (Eq. (3)). At low current density, the current efficiency remains 100% and the specific energy consumption remains constant with conversion for almost all the oxidation, but the mineralization of the 3,4,5-trihydroxybenzoic acid required a long electrolysis time because some of the reactor capacity is under-used. On the contrary when operating current exceeds the limiting one, the electrolysis is fast but the current efficiency decreases and therefore the energy consumption increases because a portion of the current is wasted on the secondary reaction of oxygen evolution. In Figs. 2 and 3 the theoretical values of COD, ICE, τ and Esp calculated from the model are reported. As can be seen the model can satisfactorily predict the experimental data for all the applied current densities. In order to avoid under-utilization of reactor capacity (i.e. long electrolysis time) and power wastage (i.e. high-energy consumption), the electrolysis should be carried out at instantaneous limiting current throughout the process, that is working at controlled potential. However, the mineralization of organics for wastewater treatment in a batch recirculation system cannot be performed at controlled potential where organic oxidation commence (i.e. in the potential region before oxygen evolution) because during electrolyses of aromatic compounds the current density decreases rapidly to very low values due to blocking of electrode surface by the depositing polymeric adhesive products [8,25]. For this reason all the anodic oxidation of organics with BDD anodes are performed applying high potentials in the region of water discharge (E > 2.0 V vs. SHE) or applying current density corresponding to anode potential in the region of water discharge (i.e. i > 5 mA cm−2 ). In addition, a systematic study that relate the applied current density and concentration of organic in order to avoid the fouling has been performed recently [30]. As proposed by Gherardini [31], a simple way to minimize the secondary reactions and maximize the performance of the reactor could be to operate with multiple current steps electrolysis, remaining always below the instantaneous limiting current (Fig. 4). Deciding to apply n current steps i1 , i2 ,. . .,in , with i1 < i0lim , the equations of the previous models [26–28] can be adapted to calculate the time for each step in order to remain always near but below the instantaneous limiting current, obtaining therefore a fast oxidation and restricted energy consumption. Eqs. (4) and (5) for the critical time and the critical conversion can be generalised: ticr =

VR 1 − αi Akm αi

(10)

1 3600

Xicr = 1 −

i

αj

(11)

j=1

where ticr and Xicr are the critical time, or critical conversion, for the step i, corresponding to the time, or conversion, at which the applied current density ii is equal to the limiting current density, and αi can be obtained with the following equation: αi =

ii i−1 ilim

ii 

=

i−1

4 Fkm COD0

j=1

j=1



(12)

αj

The equations for the prediction of the evolution of COD and current efficiency with time and the evolution of specific energy consumption and electrolysis time with conversion can be also generalised for a batch electrolysis operating with n current steps. For t < tncr or X < Xncr : ICE is 100% (Eq. (13)), COD decreases as line segments with different slopes (Eq. (14)), the electrolysis time increases with conversion as line segments (Eq. (15)) and the specific energy consumption decrease with conversion (Eq. (16)): ICE = 1

(13) ⎛

COD(t) = ⎝

i−1

⎞

⎡

⎞⎤ i−1  Ak 1 − α m j ⎠⎦ αj ⎠ COD0 ⎣1 − αi ⎝ t+ VR αj

⎛

j=1

j=1

(14) ⎡⎛ n−1 ⎤ ⎞ n−1 X − 1 + αj VR ⎢⎝ 1 − αj ⎠ ⎥ j=1 τ= + n ⎣ ⎦ Akm αj αj j=1

 n−1 Esp =

Fig. 4. Schematic current–time curves showing operating regions during current steps electrolysis.

 (Vd + Rc A ij ) · ij · (1 − αj /αj ) + (Vd + Rc A in ) · in · A km X COD0

(15)

j=1



n−1 j=1

 αj

  n −1+X / αj j=1

(16)

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For t > tncr or X > Xncr : ICE and COD decrease exponentially, while specific energy consumption and electrolysis time increase with conversion: ⎞ ⎤ ⎛ ⎡ n n  1 − α Ak m j⎦ COD(t) = ⎝ αj ⎠ COD0 exp ⎣− t+ VR αj j=1

j=1

(17) ⎡

⎤ n  A k 1 − α m j ⎦ ICE = exp ⎣− t+ VR αj

(18)

j=1

⎡⎛ ⎞  ⎤ n V ⎣⎝ 1 − αj ⎠ 1−X ⎦ τ= − ln n Akm αj j=1 αj

(19)

j=1

 + Rc A ij ) ij ((1 − αj )/αj )    − (Vd + Rc A in ) in ln (1 − X)/ nj=1 αj



Fig. 6. Evolution of specific energy consumption () and electrolysis time () against conversion (X) during the oxidation of 1 g dm−3 of 3,4,5trihydroxybenzoic acid in HClO4 0.5 M on boron-doped diamond anode applying multiple steps current as reported in Table 2. The solid lines represent model prediction (Eqs. (15), (16), (19), and (20)), the full circles represent the critical conversion (Eq. (11)).

n j=1 (Vd

Esp =

1 3600

A km X COD0 (20)

Figs. 5 and 6 show the behaviour COD, current efficiency, specific energy consumption (Esp ) and the electrolysis time (τ) obtained during the oxidation of 3,4,5-hydroxybenzoic acid applying four current steps as reported in Table 2. A current lower than 6 mA cm−2 was not applied in order to remain in the potential in the region of water stability and avoid electrode fouling. Comparing the results of the current steps electrolysis (Figs. 5 and 6) with those at constant currents (Figs. 2 and 3) it can be seen that the current steps electrolysis allows to obtain a fast oxidation rate, comparable with the electrolysis at high current density, but with a current efficiency near 100% and low energy consumption, as in the electrolysis at low current den-

sity (Table 3). This means that almost all the reactor capacity is used and few current is wasted on the secondary reactions. In Figs. 5 and 6 it can be also seen that the model agree very well with the experimental data. If the number of steps n is sufficiently high, it is possible to obtain an accurate adjustment of the current density to the instantaneous limiting current density, thereby permitting optimal use of the reactor, minimizing the secondary reactions. Working in this semi-continuous current control electrolysis the performances of the reactor are similar to those obtainable in potentiostatic electrolysis, but electrode fouling is avoided. Fig. 7 shows the evolution of the COD and ICE obtained during a semi-continuous current control electrolysis of 3,4,5hydroxybenzoic acid, applying ten current steps, as reported in Table 4. For a comparison, the trend of COD and ICE in ideal diffusion controlled electrolysis (i.e. potentiostatic electrolysis) calculated with Eqs. (6) and (7), were also reported in Fig. 7. The behaviour of the semi-continuous current control electrolysis is very similar to those of the ideal diffusion controlled one and the current efficiency remained near 100% for almost all the treatment. Furthermore, from the data reported in Table 3, it is possible to observe that the semi-continuous control electrolysis allowed to obtain higher current efficiency and lower energy consumption than the potential step electrolysis, thereby further improving the reactor performance.

Table 2 Conditions of the four current steps electrolysis

Fig. 5. Time evolution of COD () and ICE () during the electrolyses of 1 g dm−3 of 3,4,5-trihydroxybenzoic acid in HClO4 0.5 M on boron-doped diamond anode applying multiple steps current as reported in Table 2. The solid lines represent model prediction (Eqs. (13), (14), (17), and (18)), the full circles represent the critical time (Eq. (10)).

Step

Current density, ii (mA cm−2 )

Time of the step, ti (min)a

Cell potential (V)

1 2 3 4

30 20 10 6

23 22 42 153

4.5 4.2 3.8 3.6

a

Calculated using Eq. (10).

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Table 3 Comparison of the results obtained during constant currents electrolysis and current steps electrolysis for a COD removal of 90% Applied current density

Average current efficiency, η¯

Electrolysis time, τ (min)

Specific energy consumption, Esp (kWh kg COD−1 )

10 mA cm−2 60 mA cm−2 Four current step (Table 1) Theoretical values for a four current step Semi-continuous control electrolysis (Table 3) Theoretical values for a semi-continuous control electrolysis (10 current steps)

0.96 0.28 0.94 0.98a 0.97 0.99a

200 108 126 116b 115 110b

13 68 14.5 13.5c 14.0 13.8c

a b c

Calculated using Eq. (18). Calculated using Eq. (19). Calculated using Eq. (20).

Table 4 Conditions of the semi-continuous control electrolysis (10 current steps) Step

Current density, ii (mA cm−2 ) Time of the step, ti (min)a Cell potential (V) a

1

2

3

4

5

6

7

8

9

40 7.0 4.8

36 5.0 4.7

32 5.5 4.6

28 6.0 4.4

24 7.0 4.3

20 8.5 4.2

16 10.5 4.0

12 14.5 3.9

8 21.5 3.7

10 6 75 3.6

Calculated using Eq. (10).

Furthermore, if the number of current steps is sufficiently high, i.e. working in a semi-continuous control mode, the performances of the reactor are comparable with those achievable in an ideal diffusion controlled process with an efficiency of 100%. A comparison with experimental data for the combustion of 3,4,5-trihydroxybenzoic acid shows good agreement. References

Fig. 7. Time evolution of COD () and ICE () during the electrolyses of 1 g dm−3 of 3,4,5-trihydroxybenzoic acid in HClO4 0.5 M on boron-doped diamond anode in semi-continuous current control electrolysis as reported in Table 4. The full circles represent the critical time for each current step, the solid lines model prediction (Eqs. (13), (14), (17), and (18)), the full circles represent the critical time (Eq. (10)), the dotted line the behaviour of an ideal diffusion controlled electrolysis (i.e. potentiostatic electrolysis) calculated with Eqs. (6) and (7).

4. Conclusion In this paper, a theoretical model for the prediction of the evolution of chemical oxygen demand, current efficiency, electrolysis time and specific energy consumption has been used to optimize the performance of batch recirculation system for the removal of organic pollutants on boron-doped diamond. It has been shown that multiple current steps electrolysis, remaining always below the instantaneous limiting current, and in the potential region of water discharge, allowed to obtain high oxidation rate and low energy consumption.

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