Destruction of EDTA using Ce(IV) mediated electrochemical oxidation: A simple modeling study and experimental verification

Chemosphere 68 (2007) 1067–1073 www.elsevier.com/locate/chemosphere

Destruction of EDTA using Ce(IV) mediated electrochemical oxidation: A simple modeling study and experimental verification Jae-Wook Lee a, Sang-Joon Chung b, Subramanian Balaji b, Vasily V. Kokovkin b, Il-Shik Moon b,* a

Department of Environmental and Chemical Engineering, Seonam University, Namwon 590-711, Republic of Korea b Department of Chemical Engineering, Sunchon National University, Suncheon 540-742, Republic of Korea Received 5 October 2006; received in revised form 24 January 2007; accepted 26 January 2007 Available online 23 March 2007

Abstract Mediated electrochemical oxidation (MEO) is a recent development in the environmental research field for the complete destruction of organic pollutants. This study presents the destruction of EDTA by cerium(IV) MEO process in nitric acid medium. The destruction reaction was carried out in a continuous stirred tank reactor under various conditions. A simple kinetic model was developed to analyze and simulate the organic destruction in the MEO process. The model was based on the calculation of the total mass balance, the component mass balance, and the energy balance in the reactor and also in the heating jacket. The sensitivity to key operating conditions such as the initial EDTA concentration (50–200 mM), EDTA feeding time (30–180 min), reaction temperature (323–363 K), and the rate laws corresponding to zero-, first-, second-, and third-order reaction were analyzed. It was found that the model simulated agreed well with the experimental data for EDTA oxidation. The results obtained showed the suitability of the MEO process for the effective mineralization of high concentrations of EDTA.  2007 Elsevier Ltd. All rights reserved. Keywords: EDTA destruction; Cerium(IV); Mediated electrochemical oxidation; Modeling; CSTR

1. Introduction Ethylenediaminetetraacetic acid (EDTA) is a common chelating agent used for complexing metal ions in many industrial applications such as photographic, textile, paper manufacturing, metal industries, etc. (Emilio et al., 2002; Ghiselli et al., 2004). In addition, it has widely been used for chemical cleaning of the steam generator internals in nuclear power plants (Park et al., 2006). Although EDTA by itself is relatively harmless to humans, it may produce deleterious effects by increasing metal distributions to the surrounding environment through enhanced mobility of metal-EDTA complexes (Means et al., 1978). The presence of EDTA in waste water can increase the level of lead and *

Corresponding author. Tel./fax: +82 61 750 3581. E-mail address: [email protected] (I.-S. Moon).

0045-6535/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.01.073

zinc as much as 200% (Alder et al., 1990). In the US the permissible level of EDTA concentrations in effluent discharge have been reported at 1–72 lg l1. EDTA is not easily biodegradable and therefore degradation of EDTA gets significance and has been attempted by many researchers using several processes such as the conventional thermal and biological treatments, advanced oxidation processes, and heterogeneous photocatalysis (Yang and Davis, 2000, 2001; Emilio et al., 2002; Ghiselli et al., 2004; Abbaspour and Mehrgardi, 2005; Kocot et al., 2006). The mediated electrochemical oxidation (MEO) process is an emerging technology for the destruction of organic compounds working at ambient temperatures (below 373 K) and at atmospheric pressure which uses an electrochemical cell to generate the oxidizing species (ArmentaArmenta and Diaz, 2005; Raju and Basha, 2005; Balaji et al., 2007a,b; Chung et al., 2007; Matheswaran et al.,

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Fig. 1. Scheme of electro-oxidation.

2007). In the MEO process (Fig. 1), the oxidizing species (mediator ions) are produced at the anode in an acidic medium. Several mediators such as Ag(II)/Ag(I), Ce(IV)/ Ce(III), Co(III)/Co(II), etc., have been used in the MEO process for organic mineralization reactions (Khan et al., 2003; Armenta-Armenta and Diaz, 2005; Raju and Basha, 2005). In an acidic medium, these mediators act as powerful oxidants and completely destroy the organic compounds to CO2 and water in a totally enclosed chamber, without any harmful emissions like polychlorinated dibenzo p-dioxins. It is found that cerium, when used as a mediator, has many advantages: (i) it does not form precipitate with chloro-organic compounds, (ii) the rate of water oxidation by Ce(IV) is minimal compared to silver(II) and cobalt(III) ions (Bringmann et al., 1995), (iii) it possesses good oxidizing power with a high redox potential in nitric acid medium (E0 = 1.62 V), and (iv) it can be recovered and reused without much loss and is therefore, preferred to other mediator ions. The high solubility of cerium in nitric acid was the main reason for the choice of nitric acid over other electrolytes particularly sulfuric acid (Paulenova et al., 2002). Since the mediated metal ions have a strong potential to oxidize, a high temperature is not required for organic oxidation and therefore, less volatile and off gases are produced (Raju and Basha, 2005). The MEO process can be made more attractive if it is optimized for electrical energy consumption and the present paper proposes some possible solutions to understand the destruction patterns through mathematical modeling and simulation. The organic destruction in the MEO process can be carried out in either a batch or in a continuous feeding mode. In the case of batch type reaction, the organic is added at one time (zero time) in the reactor, and the process is carried out with or without Ce(IV) regeneration. However, in real applications, the continuous organic addition is used mainly for minimizing the oxidant usage by simultaneous regeneration, and by this way more quantity of the organic materials can be destructed than in the batch process. Usually, in the continuous process, an organic substance is added for a long time (e.g. hours, days etc.) at a particular flow rate and the oxidant concentration is maintained nearly at the same level by in situ electrochemical regener-

ation. However, it is also possible to predict the course of the destruction process by simulation based on simple kinetic models. The predictions based on fundamental and practical kinetic models are less expensive than the actual trial experiments. There is no literature report available on the modeling and simulation of the Ce(IV)–MEO process for the destruction of EDTA. The intention of the present investigation was to explore the possibility of simple modeling study for Ce(IV)-mediated electrochemical oxidation taking EDTA as the model pollutant and to simulate the destruction patterns using the basic kinetic and thermodynamic parameters derived from our experimental results. In this work, we present the experimental and theoretical results for the oxidation of EDTA by electrochemically generated Ce(IV) in nitric acid solutions. A continuous stirred tank reactor (CSTR) model was developed employing the total mass balance, the component mass balance, the energy balance in the reactor, and the energy balance in the jacket. Sensitivity to the key operating conditions such as the initial concentration of EDTA, feeding time, temperature, rate laws corresponding to the zero-, first-, second-, and third-order reaction was analyzed. 2. Model formulation A CSTR model of the MEO process was developed under the following assumptions: The reaction A ! B occurs in a CSTR, the reaction follows first-order reaction kinetics, the reaction is exothermic and the reaction heat is removed by a cooling jacket surrounding the reactor, the reactant solutions are perfectly mixed, and the jacket water is perfectly mixed and the mass of the metal wall is negligible, so the thermal inertia of the metal is not considered. Applying the above assumptions, the governing CSTR model equations may be described by the following equations. Mass balance of reactant A:

  dðVC A Þ E ¼ F in C A;in  F out C A  Vk 0 C A exp  dt RT

ð1Þ

Energy balance in the reactor: qC p

dðVT Þ ¼ qC p ðF in T in  F out T Þ dt 

E  DHVk 0 C A exp  RT

  UAðT  T w Þ

ð2Þ

Energy balance in the jacket: qw C p;w V w

dT w ¼ qw C p;w F w;in ðT w;in  T w Þ þ UAðT  T w Þ dt ð3Þ

A description of the variables and parameter values is given in Table 1. The mass and heat balances (Eqs. (1)–(3)) were numerically solved in a personal computer using the

J.-W. Lee et al. / Chemosphere 68 (2007) 1067–1073

The overall schematic diagram of the MEO system is shown in Fig. 2. The system essentially consisted of an anolyte tank, a catholyte tank and a CSTR, each with a volume capacity of 1.5 l coupled to an electrochemical cell. The electrochemical cell (Moon, 2005) was fabricated in our laboratory, with mesh type DSA anode (IrO2/Ti; 140 cm2) and cathode (Ti; 140 cm2) separated by Nafion 324 membrane, and used for the oxidation of Ce(III) in nitric acid at a constant current of 10 A, before the actual organic destruction experiment was started. The dimensions and characteristics of the electrochemical cell are available in the previous report from our lab (Balaji et al., 2007). Then, during the experiments, it was used for keeping the Ce(IV) concentration nearly at the same level. Constant current was applied to the cell by stabilized power supplies (Korea Switching) and the cell voltage and current were monitored continuously. The anodic compartment was filled with 1 M cerium(III) nitrate in 3 M nitric acid and the catholyte with 4 M HNO3 and the solutions were circulated through the cell using magnet pumps (Pan World Co., USA). The Ce(III) was then oxidized to Ce(IV) at a constant current of 10 A. The catholyte vessel was provided with a gas scrubber, and the atmospheric oxygen was bubbled into the catholyte solution by an aerator to convert the nitrous oxide vapors produced during the cathodic reaction into nitric acid. During Ce(III) oxidation, the concentration of Ce(IV) solution was checked by measuring the redox potential using an Orion pH/ISE meter (Model No. 720 A, Orion Co. Ltd., USA) with a Pt–Ag/AgCl combined electrode. The concentration of Ce(IV) was calculated from the measured redox potential values using the previously obtained calibration graph (Balaji et al., 2007). This value was also cross checked at random by potentiometric titration with ferrous ammonium sulfate (Wei et al., 2005). The optimum conditions for Ce(III) oxidation in 3 M nitric acid solution

Table 1 Variables, nominal operating conditions and parameter values Variable V Fin Fout CA T Tw Fw,in CA,in CA,0 Tin Vw k0 E U A Tw R DH Cp Cp,w q qw

Description

Value 3

Reactor volume (m ) Volumetric flow rate for the inlet stream (m3 h1) Volumetric flow rate for the outlet stream (m3 h1) Reactant concentration for the outlet stream (mM) Reactor temperature (K) Jacket temperature (K) Volumetric flow rate of cooling water (m3 h1) Reactant concentration of inlet stream (mM) Initial reactant concentration (mM) Inlet stream temperature (K) Jacket volume (m3) Pre-exponential factor from Arrhenius law (h1) Activation energy (J mol1) Overall heat transfer in the jacket (J h1 m2 K1) Heat transfer area (m2) Inlet stream cooling water temperature (K) Ideal-gas constant (J mol1 K1) Enthalpy of reaction (J mol1) Heat capacity inlet and out streams (J g1 K1) Heat capacity of cooling water (J g1 K1) Density of the inlet and out streams (g m3) Density of cooling water (g m3)

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1.5 · 103 1.8 · 104 0 – 353 353 1.13 67 0 353.15 1.0 5.0 · 1010 7.0 · 104 1.123 · 104 23.22 353.15 8.314 7.0 · 104 4.19 4.18 1.04 · 106 1.0 · 106

FORTRAN compiler in double precision and with LSODI of the International Mathematics and Science Library. 3. Experimental Ammonium cerium(IV) nitrate was purchased from Sigma–Aldrich Co., cerium(III) nitrate hexahydrate from TERIO Corporation, China, Disodium EDTA (extra pure) from Daejung Chemicals and Metals Co. Ltd., Korea, and nitric acid (60%) was obtained from Sam Chun Chemicals, Korea. All the chemicals were used as received.

CO2 Analyzer

Vent Scrubber Electrochemical Cell

Condenser

Nitrogen Cylinder

Condenser

Flow Meter Pump Feed Organic Pump Feed Tank

CSTR

Anolyte Tank

Pump

Rectifier

Fig. 2. Schematic diagram of the experimental setup.

Catholyte Tank

J.-W. Lee et al. / Chemosphere 68 (2007) 1067–1073

were found to be 10 A applied cell current (current density: 71 mA cm2) and 353 K. The oxidation studies were carried out in a CSTR under the optimum conditions of the electrochemical cell for Ce(IV) production. The EDTA solution (67 268 mM) was continuously fed to the reactor with a particular flow rate (3 or 7.5 ml min1) for 30–50 min using a peristaltic pump (Cole–Parmer Instrument Co., USA). The reaction was carried out at 353 K. The carbon dioxide produced during the organic decomposition was purged by the carrier gas stream (nitrogen) and taken to the infra red CO2 analyzer (Anagas, Model CD 95, Environmental Instruments, USA). The flow rate of the carrier gas was kept constant (2.0 l min1) throughout the experiment. The CO2 evolved was measured on line and the integrated volume at each time interval was used to express the liquid phase carbon concentration calculated using the overall reaction (Eq. (4)).

120 50 mM 100 mM 150 mM 200 mM

80 40

[Organic Carbon]Liq (mM)

1070

0 60

30 min 60 min 120 min 180 min

40 20 0 60

363 K 353 K 343 K 333 K 323 K

40

4. Results and discussion 20

4.1. Sensitivity analysis for model parameters 0

It is very important to check the sensitivity of the model parameters to determine the rate-controlling step and the major design variable. The profiles of calculated liquid phase carbon concentrations during the destruction of EDTA at different initial EDTA concentrations (50– 200 mM), feeding times (30–180 min) and reaction temperatures (323–363 K) are depicted in Fig. 3. The various experimental conditions and the experimentally derived values and model parameters for the destruction of EDTA used in simulation calculations are listed in Tables 1 and 2. The calculated liquid phase carbon concentrations at different initial EDTA concentrations are shown in Fig. 3a. It can be seen that even if the concentration was increased fourfold, nearly 80–90% destruction was observed at 120 min. After that, complete destruction takes a much longer time. The effect of various feeding times (60, 120 and 180 min) under the fixed conditions of EDTA concentration (50 mM) and temperature (353 K) is shown in Fig. 3b. It was observed that after stopping the addition, the liquid phase carbon concentration did not start dropping suddenly, but rather after a time gap. This means that the added EDTA had a residence time in the reactor for the destruction to occur and it can also be seen that during the longer duration of feeding, a steady state region was attained for liquid phase carbon concentration. This observation is in line with the one reported by Turner (2002). It has been shown by Turner that the remaining organic in the silver(II)–MEO process reached a steady state during organic feeding (100 h) and then after stopping feeding to have a complete destruction the system was made to run another 60 h to reach a negligible minimum in the liquid phase carbon content. For the experimental conditions shown for Fig. 3b the steady state region was established at nearly 70 min. The calculated profiles of the liquid phase

0

50

100

150

200

250

300

Time (min) Fig. 3. The calculated concentration profiles of EDTA in the solution for (a) different initial EDTA concentrations and (b) its feeding time, and (c) reaction temperatures (runs 1, 8, 9, 10, and 11).

carbon concentration at different reaction temperatures (323–363 K) are given in Fig. 3c. It was observed that at 363 K, the highest temperature considered in this modeling study, the concentration diminished very quickly compared to the lower temperatures. At 323 K, the decrease in carbon concentration was very slow and hence, a long time is needed for complete mineralization to take place. It was also observed that the reaction temperatures and the feeding time of EDTA, compared to the initial concentration of reactant (EDTA), highly affect the destruction process. The influence of rate laws corresponding to a zero-, first-, second-, and third-order reaction on the calculated liquid phase carbon concentration profiles is depicted in Fig. 4. 4.2. Oxidation kinetics The oxidation reaction of EDTA by the MEO process can be represented by the following expression: C10 H14 O8 N2 Na2 þ 40CeðIVÞ þ 12H2 O ! 10CO2 þ 40CeðIIIÞ þ 38Hþ þ N2 þ 2Naþ dCEDTA 12  ¼ k 1 C40 CeðIVÞ CH2 O CEDTA dt

ð4Þ ð5Þ

where, CEDTA, CCe(IV), CH2 O are the concentrations of organics, Ce(IV) and water, respectively, k1 is the rate constant, and t is the time. When the concentration of Ce(IV)

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Table 2 Experimental conditions for the oxidation reaction of EDTA Run no.

EDTA concentration (mM)

Flow rate (ml min1)

Feeding time (min)

Temperature (K)

Order of the reaction

Run Run Run Run Run Run Run Run Run Run Run Run Run Run Run Run Run

50 100 150 200 50 50 50 50 50 50 50 50 50 50 67 67 268

3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 7.5 3

30 30 30 30 60 120 180 30 30 30 30 30 30 30 30 30 50

353 353 353 353 353 353 353 323 333 343 363 353 353 353 368 353 353

First First First First First First First First First First First Zero Second Third First First First

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Solution components: [Ce(IV)] = 0.95 M, HNO3 = 3 M; [Ce(III) + Ce(IV)]Total = 1 M.

30 Zero First Second Third

30

[Organic Carbon]Liq (mM)

[Organic Carbon]Liq (mM)

40

20

10

0

Data Simulated

20

10

0 0

50

100

150

200

250

300

0

40

80

120

160

Time (min)

Time (min)

Fig. 5. Determination of model parameters (run 15). Fig. 4. The influence of rate laws corresponding to a zero-, first-, second-, and third-order reaction on the calculated concentration profiles of EDTA (runs 1, 12, 13, and 14).

and water are in large excess, pseudo-first-order kinetics can be applied:   dCEDTA E  ¼ k 0 exp  ð6Þ CEDTA RT dt where, k0 is the pre-exponential factor from the Arrhenius law and E is the activation energy. The two reaction parameters (k0, E) were determined by a minimization routine by comparison between the experimental data and the predicted results (Riggs, 1994). In this case, the object function for the routine may be defined as the average routemean square of differences between the calculated and experimental concentrations. The determined values of k0 and E were 5.0 · 1010 h1 and 7.0 · 104 J mol1, respec-

tively. The close agreement between the experimental data and simulated results can be observed as shown in Fig. 5, using 353 K as a typical example. 4.3. Model verification The comparison between experimental and simulated liquid phase carbon concentration profiles during EDTA destruction for the experimental conditions of run 16 and 17 (Table 1) is shown in Fig. 6. It can be observed that the simple model simulates the destruction behavior of EDTA. Although the proposed model in this work is able to simulate the experimental data obtained under our experimental conditions, a more precise model should be developed by taking into account the complex reaction mechanism of EDTA. Ku et al. (1998) reported the decomposition of EDTA in aqueous solution by a UV/H2O2 process. They proposed the following reaction mechanism and

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5. Conclusions

[Organic Carbon]Liq (mM)

80

Data Simulated

To analyze and simulate the MEO process, a simple CSTR model was formulated by employing the total mass balance, the component mass balance, and the energy balance in the reactor and in the jacket. It was found that, from an engineering point of view, the proposed simple model successfully simulated the experimental data obtained under the limited experimental conditions of this work. From this work, however, we found that a more precise model should be developed to understand the Ce(IV)–MEO process in detail by taking into account the reaction mechanism of EDTA. The results will be reported in the near future.

60

40

20

[Organic Carbon]Liq (mM)

0 160

Data Simulated

120

Acknowledgements 80

40

0

0

60

120

180

240

Time (min) Fig. 6. Experimental and simulated results for the experimental conditions of (a) run 16 and (b) run 17.

a simplified two-step kinetic model based on the elemental mass balances of carbon and nitrogen which may be applied here also, to describe the temporal behavior of the reacting species during the reaction. As this involves a detailed investigation of the reaction intermediates and product analysis with reaction time, which was found difficult because of the vigorous oxidizing medium employed, will be considered in the future. k1

k 2c

k1

k 2n

ðEDTAÞc ! ðIntermediateÞc ! ðCO2 Þc ðEDTAÞn ! ðIntermediateÞn ! ðNO 3 Þn

ð7Þ ð8Þ

The mineralization of EDTA to the final products encompasses several intermediates and each step of the reaction was assumed to be irreversible and of first-order with respect to the reactant. The concentrations of EDTA, organic intermediates, carbonates, and nitrates were used by Ku et al. (1998) to establish the elemental mass balance of carbon and nitrogen during the reaction as follows: ðEDTAÞTot;c ¼ ðEDTAÞc þ ðIntermediateÞc þ ðCO2 Þc ðEDTAÞTot;n ¼

ðEDTAÞn þ ðIntermediateÞn þ ðNO 3 Þn

ð9Þ ð10Þ

Although the reaction mechanism for the decomposition of EDTA by UV/H2O2 and Ce(IV)–MEO processes is not the same, a similar trend in mechanism could be expected for MEO process. Therefore, the experimental and theoretical studies are being attempted to unveil the complex reaction mechanism of EDTA by the Ce(IV)–MEO process and will be addressed in the near future.

This work performed in the framework of the project by the Ministry of Commerce, Industry and Energy (MOCIE) through Regional Innovation Centre (RIC). Research performed as part of the Core Environmental Technology Development Project for Next Generation (Eco-Technopia-21) of Korea Institute of Environmental Science and Technology (KIEST). Research partially supported by the Korea Research Foundation Grant funded by MOEHRD (Ref. No.: KRF-2005-210-D00028), Republic of Korea. The financial help from the above funding agencies is gratefully acknowledged here. One of the authors (Dr. S.B.) thanks the management of Sri Chandrasekharendra Saraswathi Viswa Maha Vidyalaya (Deemed University), Kanchipuram, India for granting research leave. References Abbaspour, A., Mehrgardi, M.A., 2005. Electrocatalytic activity of Ce(II)EDTA complex toward the oxidation of nitric ion. Talanta 67, 579– 584. Alder, A.C., Siegrist, H., Gujier, W., Giger, W., 1990. Behaviour of NTA and EDTA in biological wastewater treatment. Water Res. 24, 733– 742. Armenta-Armenta, M.E., Diaz, A.F., 2005. Oxidation of benzoic acid by electrochemically generated Ce(IV). Environ. Sci. Technol. 39, 5872– 5877. Balaji, S., Chung, S.J., Thiruvenkatachari, R., Moon, I.S., 2007a. Mediated electrochemical oxidation process: electro-oxidation of cerium(III) to cerium(IV) in nitric acid medium and a study on phenol degradation by cerium(IV) oxidant. Chem. Eng. J. 126, 51–57. Balaji, S., Kokovkin, V.V., Chung, S.J., Moon, I.S., 2007b. Destruction of EDTA by mediated electrochemical oxidation process: monitoring by continuous CO2 measurements. Water Res., doi:10.1016/ j.waters.2006.12.003. Bringmann, J., Ebert, K., Galla, U., Schmieder, H., 1995. Electrochemical mediators for total oxidation of chlorinated hydrocarbons: formation kinetics of Ag(II), Co(III) and Ce(IV). J. Appl. Electrochem. 25, 846– 851. Chung, S.J., Balaji, S., Matheswaran, M., Ramesh, T., Moon, I.S., 2007. Preliminary studies using hybrid mediated electrochemical oxidation (HMEO) for the removal of persistent organic pollutants (POPs). Water Sci. Technol. 55, 261–266. Emilio, C.A., Jardim, W.F., Litter, M.I., Mansilla, H.D., 2002. EDTA destruction using the solar ferrioxalate advanced oxidation technology

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