Modeling of Wastewater Electro-oxidation Processes Part II

Apr 1, 2004 - Examples of the first kind of electrode include Pt, IrO2, and stainless steel electrodes.8-12 Typical non-active electrodes include diam...
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Ind. Eng. Chem. Res. 2004, 43, 1923-1931

1923

Modeling of Wastewater Electro-oxidation Processes Part II. Application to Active Electrodes Pablo Can ˜ izares, Jesu ´ s Garcı´a-Go´ mez, Justo Lobato, and Manuel A. Rodrigo* Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias Quı´micas, Universidad de Castilla-La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain

A new mathematical approach has been developed to describe the processes occurring in the electrochemical treatment of wastewater polluted with organic materials, and it is applied to cells that incorporate active electrodes. The reactivity of these electrodes causes difficulties in the elaboration of the model, because the electrogenerated species and the processes in which they are involved must be included in the model. A number of adjustable parameters corresponding to the rates of the chemical process are included in the model. The model is applied to the electrochemical treatment of wastes containing only inorganic salts as well as to systems containing several organic compounds, such as carboxylic acids (formic, oxalic, and maleic) or phenol, on stainless steel (SS) electrodes (as an example of typical active behavior). The good agreement obtained between model and experimental results provides evidence for the reliability of the model and validates the simplifications assumed to explain the behavior of the active anodes. Introduction The use of electrochemical technologies for the treatment of organic materials contained in industrial wastewaters has received a great deal of attention in recent years.1-7 Nevertheless, high operating costs still preclude the widespread application of these technologies. In recent years, significant research effort has been focused on obtaining a better understanding of the processes occurring in electrochemical systems. As has been described in the literature, the anodic material has a marked influence on the process performance. Two limiting types of anode behavior have been described. Active electrodes undergo significant changes during the process, and the species on their surfaces participate directly in the reaction mechanisms. Nonactive electrodes simply act as electron sinks, and their components do not take part in the process. Examples of the first kind of electrode include Pt, IrO2, and stainless steel electrodes.8-12 Typical non-active electrodes include diamond thin-film electrodes13-15 and fully oxidized metal oxides16-18 such as PbO2 or SnO2. The changes observed on the surface of active electrodes can be accompanied by the release of certain species into the solution. For example, it has been reported that the use of stainless steel anodes can cause the appearance in solution of several species (usually metal ions) that can induce a number of chemical reactions such as electrocoagulation processes.19-21 These additional mechanisms for the removal of organic materials increase the complexity of modeling these processes, and certain simplifications must be assumed to obtain an useful model. In the work described here, a previously developed model describing the electrochemical treatment of wastes containing organic compounds was applied to describe the processes occurring when wastewaters polluted with * To whom correspondence should be addressed. E-mail: [email protected].

organic materials are treated in cells with active anodes (stainless steel). Mathematical Model In part I of this work,22 a simplified model was reported for describing the electrochemical treatment of wastewater polluted with organic compounds. This model was based on several assumptions concerning the cell description, mass transfer, and kinetics. The model was applied to a batch process in which phenol- and carboxylic-acid-containing wastes were treated using non-active electrodes. Good agreement was obtained with the experimental results, thus supporting the assumptions on which the model was based. Active electrode surfaces undergo significant changes and influence the process performance. For this reason, the particular characteristics of the electrode have to be taken into account during the formulation of the model. In the electrochemical treatment of organic-polluted wastewaters, the main processes in terms of pollutant removal are irreversible oxidative reactions. Consequently, the reductive processes are less important, and it can be assumed that, in the cathodic zone (Figure 1), only hydrogen evolution occurs. (a). However, if a compound can be reduced on the cathode, the masstransfer (b) and the reduction (c) processes must be included. When a typical active material is employed as the anode, a number of additional species generated on the electrode surface must also be considered. These species can influence the process performance, causing additional chemical reactions to occur on the electrode surface if the redox couple remains at the surface (i.e., Pt/PtO) or in the bulk solution if the electrogenerated species are dissolved (i.e., Al/Al3+). A scheme outlining the processes that need to be considered in the anodic electrochemical zone is shown in Figure 2. The first process to be taken into account

10.1021/ie0341303 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/01/2004

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Figure 1. Electrochemical processes considered in the cathodic zone.

Figure 3. Experimental setup: (a) bench-scale plant, (b) section of the electrochemical flow cell.

Figure 2. Electrochemical processes considered in the anodic zone.

is the formation of oxidized species on the electrode surface (a). These species can either remain on the surface or move toward the bulk zone. In the latter case, mass transfer to the bulk zone (b) and possible chemical reactions in this zone (c) must be considered. At the same time, the mass transfer of the oxidizable compounds from the bulk zone to the anodic zone (d) must also be considered. In this anodic zone, the organic

materials can undergo direct oxidation on the electrode surface (process e1)sa process similar to that described for non-active electrodes, in which the anode acts only as an electron sink. At the same time, if an electrogenerated compound remains on the electrode surface, organic oxidation by the electrogenerated oxidants can occur chemically (process e2). These processes can occur in either one stage or multiple stages, and they proceed until the final oxidation product is generated (usually carbon dioxide). Simultaneously, the decomposition of water molecules produces oxidants (f) (ozone, hydrogen peroxide, peroxodisulfates, chlorine, etc.) that can react with the organic matter through mediated oxidation processes (g) or can promote the formation of oxygen (h). If these oxidants arrive in the bulk zone, it is necessary to take into account their mass-transfer processes (i) and the chemical oxidation of the organics in the bulk zone (j). According to part I of this work,22 the position dependence of the model can be simplified by dividing the electrochemical reactor into three zones: two zones (electrochemical zones) close to the electrodes (anode and cathode) and a third zone corresponding to the bulk solution (chemical zone). In these three zones, the concentration of each compound is assumed to depend only on the time and not on the position. Equation 1 can be assumed to model the mass transfer rate of component i (ri) between the electrochemical and chemical reaction zones.

Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 1925

Figure 4. Sketch representing the processes considered in the modeling of wastewaters without organics (SS anode).

ri ) kA(S/i - Si)

(1)

In this expression, S/i and Si are the concentrations (mol m-3) of component i in the electrochemical and chemical zones, respectively; k (m s-1) is the masstransfer coefficient, and A (m2) is the specific interfacial area between the electrochemical and chemical zones. To model the kinetics of the direct oxidation processes (ri), eq 2 was proposed in part I.22

I I (∆Vwork - ∆Vi) ) ri ) Relectrode i F F (∆Vwork - ∆Vi)

∑i

(2)

In this expression, I is the applied current intensity, is the fraction of the applied current intensity Relectrode i involved in the particular electrochemical process i, F is the Faraday constant, ∆Vwork ) Vwork - Vreference, ∆Vi ) Vi - Vreference, and Vi is the oxidation/reduction potential of process i. In all cases, ∆Vwork must be greater than ∆Vi; otherwise, process i cannot develop. In modeling the chemical processes involved in the electrochemical treatment of wastewater (processes c, e2, g, and j), one can consider second-, first-, or zerothorder kinetics depending on the reaction characteristics. Experimental Details Electrochemical Cell. Bulk oxidation of aqueous wastes containing several carboxylic acids (formic, oxalic, and maleic) and phenol was performed in a onecompartment electrolytic flow cell (Figure 3) using stainless steel as the electrode material. Both electrodes were square in shape (100 mm in length) with an interelectrode gap of 9 mm. The electrolyte was stored in a 500-cm3 glass tank and circulated through the

Figure 5. Results obtained [simulation (line) versus experimental data (points)] for two experimental runs (j ) 30 mA cm-2, T ) 25 °C): (a) pH ) 2, (b) pH ) 12.

electrolytic cell by a centrifugal pump. A heat exchanger maintained the temperature at the desired set point. Analytical Procedure. The carbon concentration was monitored using a Shimadzu TOC-5050 analyzer. The organic compounds were identified and quantified

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insoluble compounds were dissolved in sulfuric acid and diluted to the measurement range. Preparation of the Stainless Steel Electrode. Commercial AISI 304 stainless steel sheets were used as the cathode material. Each cathode was sanded (0.3 mm in average diameter), degreased with 2-propanol in a ultrasound bath, and cleaned with deionized water. Experimental Procedures. Galvanostatic electrolyses were carried out under several sets of experimental conditions. The wastewaters tested contained 5000 mg of Na2SO4 dm-3 and were polluted with phenol or a carboxylic acid (formic, oxalic, or maleic) in the concentration range of 0-20 mM. The pH was adjusted by addition of H2SO4 or NaOH. The range of current densities studied was 15-60 mA cm-2. To study the influence of temperature, experiments were carried out in the range from 25 to 60 °C. Results and Discussion

Figure 6. Simplified oxidation mechanism for the carboxylic acids studied (not to scale).

by liquid chromatography (HPLC). The carboxylic acids were monitored using a Supelcogel H column with a mobile phase of 0.15% phosphoric acid solution at a flow rate of 0.15 cm3 min-1. The UV detector was set at 210 nm. Phenol was monitored using a Nucleosil C18 column with a mobile phase of 40% methanol/60% water at a flow rate of 0.50 cm3 min-1. In this case, the UV detector was set at 270 nm. The total amount of iron in solution was quantified by atomic absorption measurements, which were obtained using a SpectrAA 220 FS spectrophotometer. Prior to measurements, samples containing

The model was used to simulate the anodic oxidation of aqueous wastes, containing organic matter or not, on stainless steel (SS) electrodes. Although stainless steel can be defined as a consumable anode material, it can also be classified as an active material because its surface undergoes significant changes during the process and the species formed on its surface participate directly in the reaction mechanisms. Anodic Oxidation of Wastes Not Containing Organic Compounds. When an aqueous solution containing only sodium sulfate is electrolyzed in cells using SS electrodes, only the evolution of oxygen and hydrogen and the release of iron ions are observed. On the basis of these observations and in accordance with the conclusions of previous studies,12,23 the processes

Figure 7. Sketch representing the processes considered in the modeling of wastewater polluted with carboxylic acids (SS anode).

Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 1927 Table 1. Experimental Conditions and Coefficient of Variation (CV) Values for the Electrochemical Treatment of Wastes that Do Not Contain Organicsa experimental conditions run

pH

j (mA cm-2)

T (°C)

CV (%)

1 2

2 12

30 30

25 25

7.8 5.2

a

Supporting medium ) 5000 mg of Na2SO4 dm-3.

that must be considered to model the system are represented in Figure 4. The first processes involve the oxidation of the iron contained in the electrode (1) and simultaneous oxygen evolution (2). Then, iron ions can be transferred to the bulk zone (3) and subsequently to the cathodic zone (4). Reduction of these iron ions (5) and hydrogen evolution (6) at the cathode must be taken into account to complete the model scheme. All of these processes are direct, and eq 2 is considered to represent the kinetics of the system. The results obtained in galvanostatic electrolysis assays and those predicted by the model are presented in Figure 5. It can be seen that good agreement is achieved. Coefficient of variation (CV) values are reported in Table 1, and the mean value is below 7%. This result indicates that the model can satisfactorily explain the dissolution of iron from the anode in the absence of organic matter. This is a key factor in understanding the active behavior of the stainless steel electrode, because the most important process to consider in the treatment of organic-polluted wastewaters with this kind of electrode is the assisted electrochemical coagulation of the organic matter with iron ions.12 Anodic Oxidation of Carboxylic-Acid-Containing Wastes. Several wastewater samples polluted with carboxylic acids (formic, oxalic, and maleic) were oxidized using SS electrodes. In all cases, the products detected were soluble iron, carbon dioxide, and very small quantities of solid particles (iron coagulation). The oxidation of these compounds was mediated by oxidants generated from the anode. This situation was confirmed by a voltammetric study.12 To determine the influence of mediated oxidation reactions, a typical I2/I- test was also performed, but the results were negative. Oxidation products were detected (e.g., carbon dioxide), and so it was assumed that some oxidant compounds were being generated on the electrode surface and that they acted as mediated oxidants. Because these compounds are highly reactive, their effect is so rapid and occurs so close to the anode surface (as direct processes) that they cannot be detected by the I2/I- technique. These oxidants could come from the oxidation of water (e.g., hydroxyl radicals, hydrogen peroxide) or of sulfate (monoperoxosulfate, diperoxosulfate), or the oxidation might even be performed by the electrocatalytic system Fe(II)/Fe(III). A simplified scheme for carboxylic acid oxidation is shown in Figure 6. The processes that must be considered to model the system are illustrated in Figure 7. Carboxylates from the bulk solution are transferred to the anodic electrochemical zone (1). In this zone, two processes must be considered: iron dissolution (2) and oxidant formation (3). The latter species are assumed to be very unstable and to rapidly combine to form oxygen (4) or oxidize carboxylic acids to form carbon dioxide (5). Both reactions are chemical in nature. The mass transport of iron (6) and carbon dioxide (7) to the bulk zone follows the

Figure 8. Results obtained [simulation (lines) versus experimental data (points)] for three experimental runs (j ) 30 mA cm-2, T ) 25 °C, pH ) 2): 9 carboxylic acid, b carbon dioxide, 2 total iron. (a) Formic acid, (b) oxalic acid, (c) maleic acid.

anodic processes. Soluble iron in the bulk zone can be transported to the cathodic electrochemical zone (8), where it can be reduced (9). The reduction of water to form hydrogen should also be considered in the cathodic zone (10). The direct anodic processes (2 and 3) can be modeled according to eq 2, where only iron and water oxidation are considered in the calculation of Relectrode . However, i oxidants formed by water decomposition can later react chemically with organic materials to form carbon dioxide or, alternatively, can promote the evolution of oxygen. The rates of these latter processes (4 and 5) are very high,13,24 and so, the limiting step in the formation of oxygen and the oxidation of organic matter is the direct electrochemical process (3). It can therefore be assumed that all of the oxidants formed in this process

1928 Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 Table 2. Experimental Conditions and Coefficient of Variation (CV) Values for the Electrochemical Treatment of Wastes Containing Carboxylic Acidsa experimental conditions run

compound

3 4 5

formic acid oxalic acid maleic acid a

Figure 9. Simplified oxidation mechanism for phenol (not to scale).

react immediately to form one of the two products. This allows the process model to be simplified by considering only processes 4 and 5 (not process 3) and assuming that the rates of both processes (eqs 3 and 4) are only a fraction of the rate of the direct electrochemical oxidant generation (process 3).

I re ) Ranode φ F c

(3)

I (1 - φ) rd ) Ranode F c

(4)

In these expressions, the adjustable parameter φ quantifies the proportion of the oxidants that attacks carboxylic acids. This parameter substitutes for the chemical rate constants of processes 5 and 4 and provides a more representative physical description of the system.

C0 j T (mmol dm-3) pH (mA cm-2) (°C) 10 10 10

2 2 2

30 30 30

25 25 25

φi

CV (%)

0.04 2.5 0.05 3.8 0.09 1.4

Supporting medium ) 5000 mg of Na2SO4 dm-3.

The results obtained in a series of galvanostatic electrolysis assays are shown in Figure 8. The coefficients of variation between the experimental and modeling results are listed in Table 2 with the values of the parameter φ. Good agreement is observed in all cases, with a mean value for the coefficient of variation of 2.5%. The values for φ indicate that, for the acids studied in this work, the oxidizability of the carboxylic acids increases with increasing molecular weight. Anodic Oxidation of Phenol-Containing Wastes. Aqueous phenol-containing wastes were oxidized using SS electrodes. The main soluble compounds generated were soluble iron, quinonic products, carboxylic acids, and carbon dioxide, although in alkaline media, quinones were not detected. In addition to the soluble compounds, some solid particles formed through the coagulation of iron with phenol were detected in the treatment. A previously proposed12 simplified phenol oxidation mechanism is shown in Figure 9. Aromatic ring opening leads to the formation of maleic and oxalic acids. These acids are subsequently oxidized directly to carbon dioxide. Phenol can react with the ferric species generated from the anode to give coagulated compounds. A voltammetric study revealed that direct oxidation reactions do not occur on the electrode surface. Likewise, inorganic oxidants were not detected by the I2/I- tech-

Figure 10. Sketch representing the processes considered in the modeling of phenol-polluted wastewater (SS anode).

Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 1929

Figure 11. Results obtained [simulation (lines) versus experimental data (points)] for four experimental runs (j ) 30 mA cm-2): [ phenol, 2 maleic acid, 9 oxalic acid, b carbon dioxide. (a) T ) 25 °C, pH ) 2; (b) T ) 25 °C, pH ) 12; (c) T ) 25 °C, pH ) 2; (d) T ) 60 °C, pH ) 12. Table 3. Experimental Conditions and Coefficient of Variation (CV) Values for the Electrochemical Treatment of Aqueous Phenolic Wastesa experimental conditions run

C0 (mmol dm-3)

pH

j (mA cm-2)

T (°C)

φ4b

φ5c

φ6d

φ7e

kcoag (×105 s-1)

CV (%)

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

10 10 10 10 20 5 10 10 10 10 20 5 10 10 10

2 2 2 2 2 2 12 12 12 12 12 12 5 7 9

30 60 15 30 30 30 30 60 15 30 30 30 30 30 30

25 25 25 60 25 25 25 25 25 60 25 25 25 25 25

0.05 0.12 0.09 0.20 0.14 0.03 0.02 0.02 0.03 0.12 0.03 0.03 0.05 0.05 0.08

0.04 0.10 0.07 0.16 0.11 0.02 0.01 0.01 0.02 0.09 0.02 0.01 0.04 0.04 0.06

0.04 0.10 0.07 0.16 0.11 0.02 0.01 0.01 0.02 0.09 0.02 0.01 0.04 0.04 0.06

0.87 0.68 0.77 0.48 0.64 0.93 0.96 0.96 0.93 0.70 0.93 0.95 0.87 0.87 0.80

7 7 3 10 3 7 7 9 3 7 3 10 7 5 7

2.7 5.7 2.8 8.0 3.4 2.6 3.4 7.9 2.9 3.4 2.7 2.5 3.5 1.6 2.9

a

Supporting medium ) 5000 mg of Na2SO4 dm-3. b Phenol oxidation. c Maleic acid oxidation.

nique. Hence, as described previously for the treatment of carboxylic-acid-containing wastes, the oxidation of organics should be carried out by oxidants generated during the decomposition of water. The reaction scheme used in the modeling of this system is shown in Figure 10. First, phenol is transported to the anodic zone (1). At the anode surface, dissolution of iron (2) and water decomposition (3) to form oxidants take place. These compounds react rapidly with organic matter (processes 4-6) or combine to form oxygen (7). The soluble iron and the intermediates are then transferred to the bulk zone following these oxidation processes (8-11). In the bulk zone, the coagulation of phenol with soluble iron can also occur (12). Moreover, iron can be transferred to the cathodic zone (13) to be reduced (14). Cathodic evolution of hydrogen can also take place in this zone (15).

d

Oxalic acid oxidation. e O2 evolution.

In accordance with the assumptions made in the study of the treatment of wastes polluted with carboxylates, the chemical oxidation of organics by oxidants and the evolution of oxygen (processes 4-7) can be understood in terms of a two-stage process. In this process, the electrochemical formation of oxidants is the limiting step (process 3). Thus, the kinetics of the chemical processes can be modeled as a function of the rate of the limiting step (eq 5), using adjustable parameters (φi) that quantify the fraction of oxidants involved in process i.

I ri ) Ranode φi F 3

(5)

By definition, it is clear that ∑i φi ) 1, as the total oxidants formed in the water decomposition process

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oxidation of organics by electrogenerated oxidants, as well as oxygen evolution, can be understood in terms of a two-stage process. In this process, the electrochemical formation of oxidants is the rate-limiting step. Thus, the chemical processes kinetics can be modeled as a function of the rate of this limiting step, using adjustable parameters (φi) that quantify the fraction of oxidants involved in each process i. The good agreement obtained between the model and the experimental results shows the reliability of the model and demonstrates the validity of the simplifications used to explain the behavior of the SS electrode. Acknowledgment Figure 12. Results obtained [simulation (lines) versus experimental data (points)] for two experimental runs (j ) 30 mA cm-2, T ) 25 °C, pH ) 2): b [phenol]0 ) 0 mM; 2 [phenol]0 ) 10 mM.

match the electrons removed to form them. In expresis calculated by assuming sion 7, the value of Ranode 3 that the only direct anodic processes are water decomposition and iron dissolution. On the other hand, coagulation processes are assumed to exhibit first-order kinetics with respect to the phenol concentration. The kinetic constant for these processes is denoted as kcoag. The results obtained in a number of galvanostatic electrolysis assays are shown in Figures 11 and 12. These results are compared with those predicted by the proposed model. Values for the adjustable parameters are presented in Table 3, along with the coefficients of variation. Good agreement is achieved in all cases, with an average coefficient of variation value of 3.7%. Values for the effectiveness coefficient (φi) and the coagulation constant (kcoag) for each assay are also given in Table 3. It can be seen that the fraction of oxidants that promotes oxygen evolution is always greater than the fraction used in the oxidation of the organic matter. The current density does not seem to have a clear influence on the distribution of oxidants, but increasing temperature increases the proportion of oxidants used in the oxidation of the organic compounds. Conversely, alkaline pH favors oxygen evolution. The influence of the experimental conditions on the parameter kcoag is complex, given that several processes such as complexation, adsorption, and sedimentation are involved in the coagulation mechanism. However, despite this complex situation, the values obtained for this parameter under different experimental conditions are similar. Conclusions The description of the processes occurring when organic-polluted wastewaters are treated in an electrochemical cell with active electrodes is very complex because the new species generated on the electrode, as well as the processes in which they are involved, must be included in the model. For this reason, a number of adjustable parameters corresponding to the rates of these chemical process must be included in the model. Electrochemical treatment using SS electrodes (as an example of typical active-electrode behavior) can be modeled assuming that iron dissolution and water decomposition to form inorganic oxidants take place at the anode surface. These oxidants rapidly react with organic matter or combine to form oxygen. The chemical

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Received for review September 16, 2003 Revised manuscript received January 30, 2004 Accepted February 18, 2004 IE0341303