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Influence of the supporting electrolyte on the electrolyses of dyes with conductive-diamond anodes
Chemical Engineering Journal 184 (2012) 221–227
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Influence of the supporting electrolyte on the electrolyses of dyes with conductive-diamond anodes b ˜ José Mario Aquino a , Manuel A. Rodrigo b , Romeu C. Rocha-Filho a , Cristina Sáez b,∗ , Pablo Canizares a b
Department of Chemistry, Universidade Federal de São Carlos, C.P. 676, 13560-970 São Carlos, SP, Brazil Department of Chemical Engineering, Facultad de Ciencias Químicas, Universidad de Castilla La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain
a r t i c l e
i n f o
Article history: Received 25 July 2011 Received in revised form 18 November 2011 Accepted 8 January 2012 Keywords: BDD anode Electrochemical degradation of dyes Chloride mediated oxidation Mass-transport controlled reactions
a b s t r a c t In this work, electrolyses of solutions containing one anthraquinonic (Acid Blue 62) and three azoic (Reactive Red 141, Direct Black 22, and Disperse Orange 29) synthetic dyes were carried out using conductive-diamond anodes. From the obtained results, clearly the oxidation was largely influenced by the supporting electrolyte used: (i) in the presence of chloride ions, the primary mechanism is mediated electrooxidation in solution with formation of a significant amount of intermediates; (ii) in the presence of sulfate ions, the dyes are directly mineralized by hydroxyl radicals. In every condition, the synthetic dyes were completely removed from the solutions. The kinetics of the electrooxidation process was mass-transport controlled, although the proper treatment of such dye solutions was assured with energy consumptions below 60 kWh m−3 . © 2012 Elsevier B.V. All rights reserved.
1. Introduction In the recent years, several works have been published concerning the application of conductive-diamond electrochemical oxidation (CDEO) in the treatment of aqueous wastes. Among them, it is worth to mention those focused on bench-scale electrolytic studies of the treatability of synthetic wastewaters polluted with different organics such as phenolic compounds, carboxylic acids, cyanides, surfactants, herbicides, etc. The obtained results support that conductive diamond is a material with good properties for the electrochemical treatment of wastewaters polluted with organic compounds, because it allows their almost complete mineralization with high current efficiencies [1]. Consequently, the operation costs reported for this technology are lower than those required by other electrochemical technologies [1,2]. These facts have been related to the generation of significant amounts of hydroxyl radicals on the conductive-diamond surface, which allows considering CDEO as an advanced oxidation process (AOP). However, the main drawback of this technology is the high price of conductive-diamond anodes. The electrochemical treatment of dyes has also demanded attention in recent years, as reported in several papers that have focused mainly on the comparison of different anode materials [3–5] or technologies [6–9], the removal of particular dyes [10–16],
∗ Corresponding author. Tel.: +34 926902204100x6708; fax: +34 926295318. E-mail address: [email protected] (C. Sáez). 1385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2012.01.044
and the feasibility of the electrochemical technology to remove dyes [17–20]. In those works, three different parameters are typically used to follow the progress of the oxidation process: total organic carbon content (TOC), chemical oxygen demand (COD), and absorbance; the information provided by each of these parameters is very different, although always related to the oxidation process. Thus, information about the reactivity of the chromophore groups contained in the dye molecule may be inferred from absorbance measurements, but not necessarily about the pollutant concentration. In principle, this concentration could be obtained by a COD assay; however, this technique strictly applies to dye molecules that are chemically oxidizable by dichromate ions (under the strong and standard conditions of the COD assay). On the other hand, the actual mineralization (transformation into inorganic carbon) of the dye molecule can be assessed by TOC measurements, which is the most important parameter to inform whether the complete removal of the pollutant contained in the wastewater was attained or not [17]. Taking into account the above, the goal of this work is to increase the understanding of CDEO dye degradation by studying the different changes in absorbance, COD, and TOC during the electrolyses of four different dyes in sulfate and sulfate/chloride supporting electrolytes. Thus, one anthraquinonic (Acid Blue 62) and three azoic (Reactive Red 141, Direct Black 22, and Disperse Orange 29) dyes were studied. These dyes present complex chemical structures (many functional groups – see Fig. 1) and high molecular weights. Consequently, they might also be used as model compounds of large-molecule pollutants [21]. Furthermore, these compounds are
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Fig. 1. Chemical structures of the dyes studied in this work: (a) Acid Blue 62, (b) Direct Black 22, (c) Reactive Red 141, and (d) Disperse Orange 29.
also highly soluble in water and persistent, once discharged into a natural environment. Thus, their removal from industrial effluents is also a subject of major importance from the environmental point of view.
2. Experimental 2.1. Chemicals All chemicals, including NaCl (a.r., Panreac), Na2 SO4 (a.r., Panreac), H2 SO4 (a.r., Sigma–Aldrich), NaOH (a.r., Panreac), Reactive Red 141 (RR 141 – Dystar), Acid Blue 62 (AB 62 – Quimanil), Direct Black 22 (DB 22 – Quimanil), and Disperse Orange 29 (DO 29 – Quimanil), were used as received. Doubly deionized water (Millipore Milli-Q system, resistivity ≥ 18.2 M cm) was used for the preparation of all solutions.
2.2. Electrochemical degradation experiments The electrochemical experiments were carried out in a homemade one-compartment filter-press reactor in the batch mode. Boron-doped diamond (BDD) and stainless steel (AISI 304) plates were used as anode and cathode, respectively. Both electrodes were circular (100 mm diameter, geometric area of 78 cm2 ); the inter-electrode distance was 9 mm. The anode was fixed on a circular stainless steel plate using a silver paste for electric contact. The BDD film (Adamant Technology, Switzerland) was prepared through the hot filament chemical vapor deposition (HFCVD) technique on a monocrystalline silicon (p-doped) substrate. The BDD specified boron content was 500 ppm. All the experiments were carried out using 0.6 dm3 of a 100 mg dm−3 aqueous solution of the dye in 100 mM Na2 SO4 , at a flow rate of 400 dm3 h−1 . To check the effect of the presence of chloride ions, some of the experiments
were carried out in solutions containing 20 mM NaCl, in addition to the dye and the sodium sulfate. 2.3. Analyses In order to monitor the absorbance of each dye solution, spectra from 200 nm to 800 nm were obtained at certain time intervals using an UV–vis spectrophotometer (UV-1603) from Shimadzu, until complete decolorization was accomplished. The solution COD was monitored until its total removal by sampling 2 mL of the electrolyzed solution at certain time intervals; these samples, after the measurement of their absorbance, were mixed in a glass tube filled with an oxidizing solution purchased from Merck (Spectroquant® ; [Cl− ] in samples was kept below 2 g L−1 , as recommended). The solution TOC was also monitored by sampling 5 mL of the electrolyzed solution at certain time intervals and using a Shimadzu TOC-5050 analyzer. The production of peroxodisulfate was assessed for electrolyses of sulfuric acid solutions in a double-compartment cell using Si/BDD and stainless steel as anode and cathode, respectively. A cationic exchange membrane (STEREOM L-105) was used to separate the compartments. The applied current density was 30 mA cm−2 and the electrogenerated oxidants were determined using the I2 /I− titration method [22]. 3. Results and discussion Fig. 2 shows the relative absorbance (at 596 nm), COD, and TOC removals as a function of the electric charge per unit volume of solution (Qap ) for the electrolysis of a 100 mg dm−3 AB 62 solution in two different supporting electrolytes: (i) 100 mM Na2 SO4 and (ii) 100 mM Na2 SO4 + 20 mM NaCl. For comparison purposes, lines for 100% and 10% COD removal efficiency are included in Fig. 2a; these efficiencies were calculated as reported by Kapalka et al. [23]. As can be observed, the electrolyses with the conductive-diamond
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electrode can allow depleting the initial concentration of the dye. However, there are significant changes between the trends observed with the different parameters used to quantify the depollution of the solution. In addition, the supporting electrolyte has a strong influence on the results, and the removal efficiency is significantly improved in the chloride medium [24]. Thus, in this medium COD is removed with efficiency close to 100% (see the dotted line for reference), while in the case of the non-chloride medium the efficiency in the removal of COD decreases to around 10%. This change in the behavior of the COD removal is very surprising because in the chloride medium the COD is completely depleted for a Qap value of about 1 Ah dm−3 , while at this Qap the TOC merely decayed by less than 30%. This suggests that recalcitrant intermediate species are being formed in the chloride medium, most probably organochloride species that are not oxidizable by the COD methodology. Hence, false COD values are obtained, as discussed below. Clearly, in this case, the chemical oxidation carried out by the COD methodology is weaker than that carried out in the TOC equipment and, also, than the electrochemical oxidation carried out on the conductive-diamond electrode. Another important observation from Fig. 2a is that the mineralization rate (TOC removal) seems not to be significantly influenced by the presence of chloride in the supporting electrolyte, suggesting that mediated oxidation by chloride-based electrogenerated oxidants (hypochlorous acid or hypochlorite ions) does not lead to the
Fig. 2. Effect of the chloride concentration on the electrolyses of solutions containing the anthraquinonic dye AB 62: (a) relative COD and TOC removals (100 Xrel ) and (b) relative absorbance removal (100 Arel ), at 596 nm, as a function of the applied charge (Qap ). The COD efficiency lines were calculated considering the initial COD values of the dyes based on the dichromate assay. Experimental conditions: j = 30 mA cm−2 , pH 7, = 35 ◦ C, [Na2 SO4 ] = 100 mM, and [AB 62] = 100 mg dm−3 .
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strong oxidation conditions necessary to break the dye molecule. It simply leads to oxidation of the dye molecule, forming organochloride species that cannot be further oxidized by dichromate, leading to a rapid decrease of the measured COD, whose value is no longer real. At this point, it is important to observe that there are no significant differences between the relative TOC and COD changes in the electrolysis of the non-chloride medium, suggesting that in this case the dye molecule is oxidized almost directly to carbon dioxide (without appreciable concentration of intermediates). According to Deborde and Von Gunten [25], hypochlorous acid can react in three different ways with organic compounds: (i) oxidation reactions, (ii) addition reactions in unsaturated bonds, and (iii) substitution reactions. Consequently, these reactions can add chlorine atoms to the organic molecule, making it recalcitrant to the chemical oxidation. Thus, distinct observations will result for the measured and real COD values, as already observed by Baker et al. [26] with chlorinated organic compounds. Changes in the relative color removal (see Fig. 2b) indicate that chlorine-containing oxidants rapidly attack the chromophore groups; thus, color is completely removed with less than half the value of Qap required to remove COD. The color is also removed more efficiently than COD and TOC in the case of the non-chloride containing electrolyte, but in this case the differences are less significant. This indicates that although the electrochemical oxidation in non-chloride medium seems to be a direct mineralization process, its first stage is the attack of the chromophore groups [27]. Fig. 3 shows the relative COD and TOC removal versus Qap during the electrolyses of two different azoic dyes in the same conditions than those shown in Fig. 2 (100 mM sodium sulfate solutions with and without the addition of 20 mM NaCl). The first important observation is the rapid COD decrease during the electrolyses in the presence of chloride ions. For both dyes, COD removal efficiencies are close to the line that indicates a current efficiency of 100%, while at the same time the TOC decay is smaller than about 30%; this observation is similar to the one made for the anthraquinonic dye AB 62. This indicates again that the chemical oxidation carried out by the COD methodology is weaker than the electrochemical oxidation carried out on the conductive-diamond electrode. Hence, the results obtained by the electrochemical technology can be seen as an actual alternative to the treatment of these compounds by chemical technologies. Likewise and except for the smaller efficiency observed for the removal of the DB 22 dye in chloride-free media, it is significant that the COD removal of all the dyes occurs with similar efficiencies. A Qap value of 12 Ah dm−3 is enough in those cases to totally remove the dyes’ organic load. If a cell voltage of 5 V is assumed (typical for an electrochemical full-scale process), this means that an energy consumption below 60 kWh m−3 assures the proper treatment of such dye solutions, even at a high dye concentration, like the one used in these experiments (100 g m−3 ); this means that for those dyes this technology can successfully compete with other AOPs [28]. Another observation that should be highlighted is the specificity of the effect of chloride on the mineralization rate. In the electrolyses of the RR 141 dye there is no effect, a behavior similar to that observed for the anthraquinonic dye. However, in the electrolyses of the DB 22 dye, the chlorine-containing oxidants clearly promoted the mineralization rate, probably due to susceptibility to oxidation by the chloro radicals [29]. This is an important observation, because it demonstrates that for some dyes chlorine-containing oxidants do not only produce oxidation of functional groups, but they can also produce the oxidation of the dye molecules, with the formation of carbon dioxide. A last important observation from Fig. 3 is that in the electrolyses of the non-chloride containing media, changes in the TOC and COD removals are very close, as observed for the AB 62 dye,
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Fig. 3. Effect of the chloride concentration on the electrolyses of solutions containing the azoic dyes DB 22 (a) and RR 141 (b) for the COD and TOC removals (100 Xrel ). The COD efficiency lines were calculated considering the initial COD values of the dyes based on the dichromate assay. Experimental conditions: j = 30 mA cm−2 , pH 7, = 35 ◦ C, [Na2 SO4 ] = 100 mM, and [dye] = 100 mg dm−3 .
suggesting that in this case a rapid mineralization (with very small concentration of intermediates) is promoted. As pointed out by Polcaro et al. [30], the electrooxidation of an organic compound in the presence of high concentrations of chlorine-containing oxidants may be positively affected (higher removal rates) or may lead to organochloride byproducts, depending on the nature of that organic compound. The results presented in Figs. 2a and 3 illustrate this quite well. In the case of the azoic dyes DB 22 and RR 141, the removal of the latter is not affected by the presence of the chlorine-containing oxidants (see Fig. 3b), while the removal of the former is significantly affected (see Fig. 3a). Similarly, the removal of the anthraquinonic dye AB 62 is affected, but not as much as for the DB 22 dye. Results reported by other authors also show that the effect is dependent on the nature of the organic molecule being electrooxidized. For example, Costa et al. [31] reported that the removal of the Acid Black 210 dye was not affected by the presence of 100 mM NaCl, at pH 6.8. On the other hand, Montanaro and Petrucci [32] found that the addition of chloride had a positive effect on the removal of the Remazol Brilliant Blue R dye; however, this effect diminished as the chloride concentration was increased. Fig. 4 shows the relative color removal for the DB 22 and RR 141 azoic dyes in the same electrolyses conditions of Fig. 3. The rapid absorbance decay in the case of the chloride-containing
Fig. 4. Effect of the chloride concentration on the electrolyses of solutions containing the azoic dyes DB 22 (a) and RR 141 (b) for the relative absorbance removal (100 Arel ) at 490 nm and 520 nm, respectively. Experimental conditions: j = 30 mA cm−2 , pH 7, = 35 ◦ C, [Na2 SO4 ] = 100 mM, and [dye] = 100 mg dm−3 .
electrolyses indicates the selectivity of the oxidation of chlorineoxidants for the azoic chromophore groups. In the case of the non-chloride containing solution, the less efficient color removal observed for the DB 22 dye is similar to that observed for the anthraquinonic dye and indicates that, although direct mineralization seems to be the main reaction in the electrolysis of the dyes in non-chloride containing media, decolorization is a first step in this process. At this point, it might be interesting to recall that Muthukumar et al. [33], investigating acid azo dyes, found that the higher the number of sulfonic acid groups in the dye’s molecular structure the faster the decolorization occurred; this behavior was explained as due to the higher solubility brought on by the sulfonic acid groups. Interestingly, the RR 141 dye, whose molecule has eight sulfonic acid groups in its structure compared to only three groups in the DB 22 molecule, presents a much faster decolorization rate than the DB 22 dye, in the absence of chloride ions. Fig. 5 shows the relative COD and color removals during the electrolyses of the disperse azoic dye DO 29 in the presence and absence of chloride ions. The lower efficiency of this dye oxidation process is due to the aggregation of dye molecules into small particles, thus requiring application of greater values of Qap to obtain the same removal degree as that for the other investigated dyes. An interesting observation is that, in chloride-containing medium, the COD was removed much faster (lower Qap ) than the color. This observation might indicate that the DO 29 dye molecules underwent
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Fig. 6. Comparison of the apparent first-order kinetic constants calculated by mathematical fitting of the COD experimental results in the absence of chloride and the theoretical one obtained from a purely mass-transport controlled system.
first-order kinetics and compare (see Fig. 6) the resulting kinetic constants to that for a well-known simple model of electrochemical processes based on mass-transport control [34–37], in which the efficiency () of the electrochemical process is obtained as the ratio (Eq. (1)) between the limiting current density (ilim ), calculated by Eq. (2), and the applied current density (iappl ). The value of iappl is maintained constant throughout the electrolyses by operating in the galvanostatic mode. =
ilim (t) iappl
ilim (t) = nFkm [COD(t)]
Fig. 5. Effect of the chloride concentration on the electrolyses of solutions containing the azoic dye DO 29 for the relative COD (a) and absorbance at 450 nm (b) removals (100 Xrel ). Experimental conditions: j = 30 mA cm−2 , pH 7, = 35 ◦ C, [Na2 SO4 ] = 100 mM, and [DO 29] = 100 mg dm−3 .
structural changes in chloride-containing medium, yielding compounds that are recalcitrant to chemical oxidation by dichromate, once again leading to a rapid decrease of the measured COD. TOC data are not presented because measurements were not possible to be made due to retention of the dye in the equipment filter. All these observations are confirmed by analyses of the UV–vis spectra (see Figs. SM-1 to SM-4 in the supplementary material) obtained during the electrolyses of the different dye solutions. For the oxidation of the dyes in the chloride-free electrolyte (Figs. SM1a, SM-2a, SM-3a, and SM-4a), there are no changes in the spectra as the electrochemical process proceeds; this indicates that direct mineralization is occurring with no appreciable concentration of intermediates, but simply with a consumption of the dye. On the other hand, for the oxidation of the dyes in the chloride-containing electrolyte (Figs. SM-1b, SM-2b, SM-3b, and SM-4b), new peaks appear in the spectra as the electrolyses proceed, clearly indicating the appearance of degradation intermediates. From the data discussed hereinbefore, it can be inferred that the degradation process in sulfate medium occurs with no significant formation of intermediates, while oxidants formed during electrolyses in the presence of chloride ions strongly influence the degradation mechanism (with the appearance of intermediates) and performance. At this point, it is interesting to fit the COD removal data obtained in the absence of chloride ions to a
(1) (2)
where km is the mass-transport coefficient, n the number of electrons involved in the oxidation of the organic substance, F the Faraday constant, and COD(t) the chemical oxygen demand at a given time t. The km value of the electrochemical cell within the fluid dynamic conditions used (400 dm3 h−1 , 298 K) is 1.28 × 10−5 m s−1 , calculated from a standard limiting current test for the [Fe(CN)6 ]3− /[Fe(CN)6 ]4− redox couple [38]. Taking into account the surface/volume ratio of the electrochemical cell [23], this km value corresponds to a kinetic constant of around 0.015 min−1 . The value of this first order kinetic constant is in the range of values obtained by mathematical fitting for the electrolyses of the AB 62 dye (see Fig. 6). From the different values obtained for each dye, it can be assumed that mediated oxidation is happening in the electrolytic reactor, although in a zone very close to the anode surface, according to the theory of reaction cage proposed by Comninellis’ group [39–41]. Thus, once a dye molecule accesses this reaction cage, it is attacked by many oxidants up to the complete degradation of the molecule and consequent transformation into carbon dioxide without releasing intermediates to the bulk volume. This means that, from a macroscopic point of view, the process can be considered as the electrochemical incineration (full conversion into carbon dioxide) of the dye, as it has been previously proposed in the literature [42]. At this point, the smaller value found for the kinetic constant for the degradation of the DO 29 dye can be explained by the particulate nature of this pollutant, which significantly modifies the mass-transport coefficient. In the case of the DB 22 dye, the interaction of the oxidants with the dye molecule could be the rate-determining step, due to the different substituent present in comparison to the one in the RR 141 dye molecule, as these compounds have similar molar mass. The substituent bonded to the organic ring determines the dye’s reactivity (electron donor or electron acceptor) and solubility, which influence the degree of
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Table 1 Efficiency in the production of peroxodisulfate by electrolysis with conductive diamond anodes. Raw concentration of sulfuric acid (M)
Temperature (◦ C)
Efficiency/mmol S2 O8 2− (Ah)−1
0.1 1.0 2.0 1.0 1.0
25 25 25 10 40
0.00 0.75 1.97 3.77 0.61
interaction with the oxidants, as studied by Muthukumar et al. [33]. In fact, the much higher apparent first-order kinetic constant for the RR 141 dye might be due to its significantly higher solubility, compared to the other dyes. Taking into account the supporting electrolyte used, peroxodisulfates should be the primary oxidants [43] to explain this mediated oxidation and the increase in the oxidation kinetic constant over the value proposed by the mass-transport coefficient for the RR 141 and AB 62 dyes; however, at the operation conditions applied, no peroxodisulfates are detected in the bulk solution. Some data concerning the production of peroxodisulfate during electrolyses of sulfuric acid solutions (summarized in Table 1) can be of great interest to interpret the results; from these data it can be inferred that peroxodisulfate anions do not occur in the bulk solution during the electrolyses of the dyes, but just near the electrode surface, such as many other potential oxidants (ozone, hydrogen peroxide, etc.). Consequently, these oxidant compounds led to an increased efficiency of the electrolytic process over the value expected for a purely mass-transport controlled process [31,44,45]. 4. Conclusions Several conclusions can be drawn from this work. First, the anthraquinonic dye AB 62 and the azoic dye RR 141 can be completely mineralized by electrolysis with conductive-diamond anodes. Mass transport controls the process kinetics; nevertheless, the proper treatment of such dye solutions, even at a 100 g m−3 concentration, is attained with energy consumptions below 60 kWh m−3 . Second, the supporting electrolyte plays a key role in the oxidation mechanism of the dyes. The presence of chloride ions leads to the occurrence of oxidants in the bulk solution, which contribute significantly to the oxidation of the dyes and their intermediates. On the other hand, within the concentration range used (100 mM), the presence of sulfate ions does not lead to the occurrence of oxidants in the bulk electrolyte, but just in a zone very close to the anode surface (reaction cage proposed by Comninellis’ group), in which dyes are completely degraded during the electrolyses without net formation of intermediates in the bulk solution. This means that from a macroscopic point of view, the process can be considered as the electrochemical incineration (full conversion into carbon dioxide) of the dye. Finally, the reported data on COD, TOC, and absorbance clearly provide very different information about the progress of the dyes oxidation. This information is not complete but complementary, being fully required to understand the performance of the electrolyses of the dyes; moreover, due to the problems with COD measurements in chloride-containing media, TOC data by themselves might be the best choice. Acknowledgments This work was supported by the JCCM (Junta de Comunidades de Castilla La Mancha, Spain) through the project PEII11-00972026. The Brazilian agencies CNPq and CAPES (for the scholarship awarded to J.M. Aquino) are also acknowledged.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cej.2012.01.044.
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J.M. Aquino et al. / Chemical Engineering Journal 184 (2012) 221–227 ˜ [27] P. Canizares, A. Gadri, J. Lobato, B. Nasr, R. Paz, M.A. Rodrigo, C. Saez, Electrochemical oxidation of azoic dyes with conductive-diamond anodes, Ind. Eng. Chem. Res. 45 (2006) 3468–3473. ˜ [28] M. Faouzi, P. Canizares, A. Gadri, J. Lobato, B. Nasr, R. Paz, M.A. Rodrigo, C. Saez, Advanced oxidation processes for the treatment of wastes polluted with azoic dyes, Electrochim. Acta 52 (2006) 325–331. [29] J.J. Pignatello, E. Oliveros, A. Mackay, Advanced oxidation processes for organic contaminant destruction based on the fenton reaction and related chemistry, Crit. Rev. Environ. Sci. Technol. 36 (2006) 1–84. [30] A.M.V.A. Polcaro, M. Mascia, S. Palmas, J.R. Ruiz, Electrochemical treatment of waters with BDD anodes: kinetics of the reactions involving chlorides, J. Appl. Electrochem. 39 (2009) 2083–2092. [31] C.R. Costa, F. Montilla, E. Morallón, P. Olivi, Electrochemical oxidation of acid black 210 dye on the boron-doped diamond electrode in the presence of phosphate ions: effect of current density, pH, and chloride ions, Electrochim. Acta 54 (2010) 7048–7055. [32] D. Montanaro, E. Petrucci, Electrochemical treatment of Remazol Brilliant Blue on a boron-doped diamond electrode, Chem. Eng. J. 153 (2009) 138–144. [33] M. Muthukumar, D. Sargunamani, N. Selvakumar, Statistical analysis of the effect of aromatic, azo and sulphonic acid groups on decolouration of acid dye effluents using advanced oxidation processes, Dyes Pigm. 65 (2005) 151–158. ˜ J. Garcia-Gómez, J. Lobato, M.A. Rodrigo, Modeling of wastewater [34] P. Canizares, electro-oxidation processes. Part II. Application to active electrodes, Ind. Eng. Chem. Res. 43 (2004) 1923–1931. ˜ [35] P. Canizares, J. García-Gómez, J. Lobato, M.A. Rodrigo, Modeling of wastewater electro-oxidation processes. Part I. General description and application to inactive electrodes, Ind. Eng. Chem. Res. 43 (2004) 1915–1922. ˜ [36] P. Canizares, J. Lobato, R. Paz, M.A. Rodrigo, C. Saez, Electrochemical oxidation of phenolic wastes with boron-doped diamond anodes, Water Res. 39 (2005) 2687–2703.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Influence of the supporting electrolyte on the electrolyses of dyes with conductive-diamond anodes b ˜ José Mario Aquino a , Manuel A. Rodrigo b , Romeu C. Rocha-Filho a , Cristina Sáez b,∗ , Pablo Canizares a b
Department of Chemistry, Universidade Federal de São Carlos, C.P. 676, 13560-970 São Carlos, SP, Brazil Department of Chemical Engineering, Facultad de Ciencias Químicas, Universidad de Castilla La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain
a r t i c l e
i n f o
Article history: Received 25 July 2011 Received in revised form 18 November 2011 Accepted 8 January 2012 Keywords: BDD anode Electrochemical degradation of dyes Chloride mediated oxidation Mass-transport controlled reactions
a b s t r a c t In this work, electrolyses of solutions containing one anthraquinonic (Acid Blue 62) and three azoic (Reactive Red 141, Direct Black 22, and Disperse Orange 29) synthetic dyes were carried out using conductive-diamond anodes. From the obtained results, clearly the oxidation was largely influenced by the supporting electrolyte used: (i) in the presence of chloride ions, the primary mechanism is mediated electrooxidation in solution with formation of a significant amount of intermediates; (ii) in the presence of sulfate ions, the dyes are directly mineralized by hydroxyl radicals. In every condition, the synthetic dyes were completely removed from the solutions. The kinetics of the electrooxidation process was mass-transport controlled, although the proper treatment of such dye solutions was assured with energy consumptions below 60 kWh m−3 . © 2012 Elsevier B.V. All rights reserved.
1. Introduction In the recent years, several works have been published concerning the application of conductive-diamond electrochemical oxidation (CDEO) in the treatment of aqueous wastes. Among them, it is worth to mention those focused on bench-scale electrolytic studies of the treatability of synthetic wastewaters polluted with different organics such as phenolic compounds, carboxylic acids, cyanides, surfactants, herbicides, etc. The obtained results support that conductive diamond is a material with good properties for the electrochemical treatment of wastewaters polluted with organic compounds, because it allows their almost complete mineralization with high current efficiencies [1]. Consequently, the operation costs reported for this technology are lower than those required by other electrochemical technologies [1,2]. These facts have been related to the generation of significant amounts of hydroxyl radicals on the conductive-diamond surface, which allows considering CDEO as an advanced oxidation process (AOP). However, the main drawback of this technology is the high price of conductive-diamond anodes. The electrochemical treatment of dyes has also demanded attention in recent years, as reported in several papers that have focused mainly on the comparison of different anode materials [3–5] or technologies [6–9], the removal of particular dyes [10–16],
∗ Corresponding author. Tel.: +34 926902204100x6708; fax: +34 926295318. E-mail address: [email protected] (C. Sáez). 1385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2012.01.044
and the feasibility of the electrochemical technology to remove dyes [17–20]. In those works, three different parameters are typically used to follow the progress of the oxidation process: total organic carbon content (TOC), chemical oxygen demand (COD), and absorbance; the information provided by each of these parameters is very different, although always related to the oxidation process. Thus, information about the reactivity of the chromophore groups contained in the dye molecule may be inferred from absorbance measurements, but not necessarily about the pollutant concentration. In principle, this concentration could be obtained by a COD assay; however, this technique strictly applies to dye molecules that are chemically oxidizable by dichromate ions (under the strong and standard conditions of the COD assay). On the other hand, the actual mineralization (transformation into inorganic carbon) of the dye molecule can be assessed by TOC measurements, which is the most important parameter to inform whether the complete removal of the pollutant contained in the wastewater was attained or not [17]. Taking into account the above, the goal of this work is to increase the understanding of CDEO dye degradation by studying the different changes in absorbance, COD, and TOC during the electrolyses of four different dyes in sulfate and sulfate/chloride supporting electrolytes. Thus, one anthraquinonic (Acid Blue 62) and three azoic (Reactive Red 141, Direct Black 22, and Disperse Orange 29) dyes were studied. These dyes present complex chemical structures (many functional groups – see Fig. 1) and high molecular weights. Consequently, they might also be used as model compounds of large-molecule pollutants [21]. Furthermore, these compounds are
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Fig. 1. Chemical structures of the dyes studied in this work: (a) Acid Blue 62, (b) Direct Black 22, (c) Reactive Red 141, and (d) Disperse Orange 29.
also highly soluble in water and persistent, once discharged into a natural environment. Thus, their removal from industrial effluents is also a subject of major importance from the environmental point of view.
2. Experimental 2.1. Chemicals All chemicals, including NaCl (a.r., Panreac), Na2 SO4 (a.r., Panreac), H2 SO4 (a.r., Sigma–Aldrich), NaOH (a.r., Panreac), Reactive Red 141 (RR 141 – Dystar), Acid Blue 62 (AB 62 – Quimanil), Direct Black 22 (DB 22 – Quimanil), and Disperse Orange 29 (DO 29 – Quimanil), were used as received. Doubly deionized water (Millipore Milli-Q system, resistivity ≥ 18.2 M cm) was used for the preparation of all solutions.
2.2. Electrochemical degradation experiments The electrochemical experiments were carried out in a homemade one-compartment filter-press reactor in the batch mode. Boron-doped diamond (BDD) and stainless steel (AISI 304) plates were used as anode and cathode, respectively. Both electrodes were circular (100 mm diameter, geometric area of 78 cm2 ); the inter-electrode distance was 9 mm. The anode was fixed on a circular stainless steel plate using a silver paste for electric contact. The BDD film (Adamant Technology, Switzerland) was prepared through the hot filament chemical vapor deposition (HFCVD) technique on a monocrystalline silicon (p-doped) substrate. The BDD specified boron content was 500 ppm. All the experiments were carried out using 0.6 dm3 of a 100 mg dm−3 aqueous solution of the dye in 100 mM Na2 SO4 , at a flow rate of 400 dm3 h−1 . To check the effect of the presence of chloride ions, some of the experiments
were carried out in solutions containing 20 mM NaCl, in addition to the dye and the sodium sulfate. 2.3. Analyses In order to monitor the absorbance of each dye solution, spectra from 200 nm to 800 nm were obtained at certain time intervals using an UV–vis spectrophotometer (UV-1603) from Shimadzu, until complete decolorization was accomplished. The solution COD was monitored until its total removal by sampling 2 mL of the electrolyzed solution at certain time intervals; these samples, after the measurement of their absorbance, were mixed in a glass tube filled with an oxidizing solution purchased from Merck (Spectroquant® ; [Cl− ] in samples was kept below 2 g L−1 , as recommended). The solution TOC was also monitored by sampling 5 mL of the electrolyzed solution at certain time intervals and using a Shimadzu TOC-5050 analyzer. The production of peroxodisulfate was assessed for electrolyses of sulfuric acid solutions in a double-compartment cell using Si/BDD and stainless steel as anode and cathode, respectively. A cationic exchange membrane (STEREOM L-105) was used to separate the compartments. The applied current density was 30 mA cm−2 and the electrogenerated oxidants were determined using the I2 /I− titration method [22]. 3. Results and discussion Fig. 2 shows the relative absorbance (at 596 nm), COD, and TOC removals as a function of the electric charge per unit volume of solution (Qap ) for the electrolysis of a 100 mg dm−3 AB 62 solution in two different supporting electrolytes: (i) 100 mM Na2 SO4 and (ii) 100 mM Na2 SO4 + 20 mM NaCl. For comparison purposes, lines for 100% and 10% COD removal efficiency are included in Fig. 2a; these efficiencies were calculated as reported by Kapalka et al. [23]. As can be observed, the electrolyses with the conductive-diamond
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electrode can allow depleting the initial concentration of the dye. However, there are significant changes between the trends observed with the different parameters used to quantify the depollution of the solution. In addition, the supporting electrolyte has a strong influence on the results, and the removal efficiency is significantly improved in the chloride medium [24]. Thus, in this medium COD is removed with efficiency close to 100% (see the dotted line for reference), while in the case of the non-chloride medium the efficiency in the removal of COD decreases to around 10%. This change in the behavior of the COD removal is very surprising because in the chloride medium the COD is completely depleted for a Qap value of about 1 Ah dm−3 , while at this Qap the TOC merely decayed by less than 30%. This suggests that recalcitrant intermediate species are being formed in the chloride medium, most probably organochloride species that are not oxidizable by the COD methodology. Hence, false COD values are obtained, as discussed below. Clearly, in this case, the chemical oxidation carried out by the COD methodology is weaker than that carried out in the TOC equipment and, also, than the electrochemical oxidation carried out on the conductive-diamond electrode. Another important observation from Fig. 2a is that the mineralization rate (TOC removal) seems not to be significantly influenced by the presence of chloride in the supporting electrolyte, suggesting that mediated oxidation by chloride-based electrogenerated oxidants (hypochlorous acid or hypochlorite ions) does not lead to the
Fig. 2. Effect of the chloride concentration on the electrolyses of solutions containing the anthraquinonic dye AB 62: (a) relative COD and TOC removals (100 Xrel ) and (b) relative absorbance removal (100 Arel ), at 596 nm, as a function of the applied charge (Qap ). The COD efficiency lines were calculated considering the initial COD values of the dyes based on the dichromate assay. Experimental conditions: j = 30 mA cm−2 , pH 7, = 35 ◦ C, [Na2 SO4 ] = 100 mM, and [AB 62] = 100 mg dm−3 .
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strong oxidation conditions necessary to break the dye molecule. It simply leads to oxidation of the dye molecule, forming organochloride species that cannot be further oxidized by dichromate, leading to a rapid decrease of the measured COD, whose value is no longer real. At this point, it is important to observe that there are no significant differences between the relative TOC and COD changes in the electrolysis of the non-chloride medium, suggesting that in this case the dye molecule is oxidized almost directly to carbon dioxide (without appreciable concentration of intermediates). According to Deborde and Von Gunten [25], hypochlorous acid can react in three different ways with organic compounds: (i) oxidation reactions, (ii) addition reactions in unsaturated bonds, and (iii) substitution reactions. Consequently, these reactions can add chlorine atoms to the organic molecule, making it recalcitrant to the chemical oxidation. Thus, distinct observations will result for the measured and real COD values, as already observed by Baker et al. [26] with chlorinated organic compounds. Changes in the relative color removal (see Fig. 2b) indicate that chlorine-containing oxidants rapidly attack the chromophore groups; thus, color is completely removed with less than half the value of Qap required to remove COD. The color is also removed more efficiently than COD and TOC in the case of the non-chloride containing electrolyte, but in this case the differences are less significant. This indicates that although the electrochemical oxidation in non-chloride medium seems to be a direct mineralization process, its first stage is the attack of the chromophore groups [27]. Fig. 3 shows the relative COD and TOC removal versus Qap during the electrolyses of two different azoic dyes in the same conditions than those shown in Fig. 2 (100 mM sodium sulfate solutions with and without the addition of 20 mM NaCl). The first important observation is the rapid COD decrease during the electrolyses in the presence of chloride ions. For both dyes, COD removal efficiencies are close to the line that indicates a current efficiency of 100%, while at the same time the TOC decay is smaller than about 30%; this observation is similar to the one made for the anthraquinonic dye AB 62. This indicates again that the chemical oxidation carried out by the COD methodology is weaker than the electrochemical oxidation carried out on the conductive-diamond electrode. Hence, the results obtained by the electrochemical technology can be seen as an actual alternative to the treatment of these compounds by chemical technologies. Likewise and except for the smaller efficiency observed for the removal of the DB 22 dye in chloride-free media, it is significant that the COD removal of all the dyes occurs with similar efficiencies. A Qap value of 12 Ah dm−3 is enough in those cases to totally remove the dyes’ organic load. If a cell voltage of 5 V is assumed (typical for an electrochemical full-scale process), this means that an energy consumption below 60 kWh m−3 assures the proper treatment of such dye solutions, even at a high dye concentration, like the one used in these experiments (100 g m−3 ); this means that for those dyes this technology can successfully compete with other AOPs [28]. Another observation that should be highlighted is the specificity of the effect of chloride on the mineralization rate. In the electrolyses of the RR 141 dye there is no effect, a behavior similar to that observed for the anthraquinonic dye. However, in the electrolyses of the DB 22 dye, the chlorine-containing oxidants clearly promoted the mineralization rate, probably due to susceptibility to oxidation by the chloro radicals [29]. This is an important observation, because it demonstrates that for some dyes chlorine-containing oxidants do not only produce oxidation of functional groups, but they can also produce the oxidation of the dye molecules, with the formation of carbon dioxide. A last important observation from Fig. 3 is that in the electrolyses of the non-chloride containing media, changes in the TOC and COD removals are very close, as observed for the AB 62 dye,
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Fig. 3. Effect of the chloride concentration on the electrolyses of solutions containing the azoic dyes DB 22 (a) and RR 141 (b) for the COD and TOC removals (100 Xrel ). The COD efficiency lines were calculated considering the initial COD values of the dyes based on the dichromate assay. Experimental conditions: j = 30 mA cm−2 , pH 7, = 35 ◦ C, [Na2 SO4 ] = 100 mM, and [dye] = 100 mg dm−3 .
suggesting that in this case a rapid mineralization (with very small concentration of intermediates) is promoted. As pointed out by Polcaro et al. [30], the electrooxidation of an organic compound in the presence of high concentrations of chlorine-containing oxidants may be positively affected (higher removal rates) or may lead to organochloride byproducts, depending on the nature of that organic compound. The results presented in Figs. 2a and 3 illustrate this quite well. In the case of the azoic dyes DB 22 and RR 141, the removal of the latter is not affected by the presence of the chlorine-containing oxidants (see Fig. 3b), while the removal of the former is significantly affected (see Fig. 3a). Similarly, the removal of the anthraquinonic dye AB 62 is affected, but not as much as for the DB 22 dye. Results reported by other authors also show that the effect is dependent on the nature of the organic molecule being electrooxidized. For example, Costa et al. [31] reported that the removal of the Acid Black 210 dye was not affected by the presence of 100 mM NaCl, at pH 6.8. On the other hand, Montanaro and Petrucci [32] found that the addition of chloride had a positive effect on the removal of the Remazol Brilliant Blue R dye; however, this effect diminished as the chloride concentration was increased. Fig. 4 shows the relative color removal for the DB 22 and RR 141 azoic dyes in the same electrolyses conditions of Fig. 3. The rapid absorbance decay in the case of the chloride-containing
Fig. 4. Effect of the chloride concentration on the electrolyses of solutions containing the azoic dyes DB 22 (a) and RR 141 (b) for the relative absorbance removal (100 Arel ) at 490 nm and 520 nm, respectively. Experimental conditions: j = 30 mA cm−2 , pH 7, = 35 ◦ C, [Na2 SO4 ] = 100 mM, and [dye] = 100 mg dm−3 .
electrolyses indicates the selectivity of the oxidation of chlorineoxidants for the azoic chromophore groups. In the case of the non-chloride containing solution, the less efficient color removal observed for the DB 22 dye is similar to that observed for the anthraquinonic dye and indicates that, although direct mineralization seems to be the main reaction in the electrolysis of the dyes in non-chloride containing media, decolorization is a first step in this process. At this point, it might be interesting to recall that Muthukumar et al. [33], investigating acid azo dyes, found that the higher the number of sulfonic acid groups in the dye’s molecular structure the faster the decolorization occurred; this behavior was explained as due to the higher solubility brought on by the sulfonic acid groups. Interestingly, the RR 141 dye, whose molecule has eight sulfonic acid groups in its structure compared to only three groups in the DB 22 molecule, presents a much faster decolorization rate than the DB 22 dye, in the absence of chloride ions. Fig. 5 shows the relative COD and color removals during the electrolyses of the disperse azoic dye DO 29 in the presence and absence of chloride ions. The lower efficiency of this dye oxidation process is due to the aggregation of dye molecules into small particles, thus requiring application of greater values of Qap to obtain the same removal degree as that for the other investigated dyes. An interesting observation is that, in chloride-containing medium, the COD was removed much faster (lower Qap ) than the color. This observation might indicate that the DO 29 dye molecules underwent
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Fig. 6. Comparison of the apparent first-order kinetic constants calculated by mathematical fitting of the COD experimental results in the absence of chloride and the theoretical one obtained from a purely mass-transport controlled system.
first-order kinetics and compare (see Fig. 6) the resulting kinetic constants to that for a well-known simple model of electrochemical processes based on mass-transport control [34–37], in which the efficiency () of the electrochemical process is obtained as the ratio (Eq. (1)) between the limiting current density (ilim ), calculated by Eq. (2), and the applied current density (iappl ). The value of iappl is maintained constant throughout the electrolyses by operating in the galvanostatic mode. =
ilim (t) iappl
ilim (t) = nFkm [COD(t)]
Fig. 5. Effect of the chloride concentration on the electrolyses of solutions containing the azoic dye DO 29 for the relative COD (a) and absorbance at 450 nm (b) removals (100 Xrel ). Experimental conditions: j = 30 mA cm−2 , pH 7, = 35 ◦ C, [Na2 SO4 ] = 100 mM, and [DO 29] = 100 mg dm−3 .
structural changes in chloride-containing medium, yielding compounds that are recalcitrant to chemical oxidation by dichromate, once again leading to a rapid decrease of the measured COD. TOC data are not presented because measurements were not possible to be made due to retention of the dye in the equipment filter. All these observations are confirmed by analyses of the UV–vis spectra (see Figs. SM-1 to SM-4 in the supplementary material) obtained during the electrolyses of the different dye solutions. For the oxidation of the dyes in the chloride-free electrolyte (Figs. SM1a, SM-2a, SM-3a, and SM-4a), there are no changes in the spectra as the electrochemical process proceeds; this indicates that direct mineralization is occurring with no appreciable concentration of intermediates, but simply with a consumption of the dye. On the other hand, for the oxidation of the dyes in the chloride-containing electrolyte (Figs. SM-1b, SM-2b, SM-3b, and SM-4b), new peaks appear in the spectra as the electrolyses proceed, clearly indicating the appearance of degradation intermediates. From the data discussed hereinbefore, it can be inferred that the degradation process in sulfate medium occurs with no significant formation of intermediates, while oxidants formed during electrolyses in the presence of chloride ions strongly influence the degradation mechanism (with the appearance of intermediates) and performance. At this point, it is interesting to fit the COD removal data obtained in the absence of chloride ions to a
(1) (2)
where km is the mass-transport coefficient, n the number of electrons involved in the oxidation of the organic substance, F the Faraday constant, and COD(t) the chemical oxygen demand at a given time t. The km value of the electrochemical cell within the fluid dynamic conditions used (400 dm3 h−1 , 298 K) is 1.28 × 10−5 m s−1 , calculated from a standard limiting current test for the [Fe(CN)6 ]3− /[Fe(CN)6 ]4− redox couple [38]. Taking into account the surface/volume ratio of the electrochemical cell [23], this km value corresponds to a kinetic constant of around 0.015 min−1 . The value of this first order kinetic constant is in the range of values obtained by mathematical fitting for the electrolyses of the AB 62 dye (see Fig. 6). From the different values obtained for each dye, it can be assumed that mediated oxidation is happening in the electrolytic reactor, although in a zone very close to the anode surface, according to the theory of reaction cage proposed by Comninellis’ group [39–41]. Thus, once a dye molecule accesses this reaction cage, it is attacked by many oxidants up to the complete degradation of the molecule and consequent transformation into carbon dioxide without releasing intermediates to the bulk volume. This means that, from a macroscopic point of view, the process can be considered as the electrochemical incineration (full conversion into carbon dioxide) of the dye, as it has been previously proposed in the literature [42]. At this point, the smaller value found for the kinetic constant for the degradation of the DO 29 dye can be explained by the particulate nature of this pollutant, which significantly modifies the mass-transport coefficient. In the case of the DB 22 dye, the interaction of the oxidants with the dye molecule could be the rate-determining step, due to the different substituent present in comparison to the one in the RR 141 dye molecule, as these compounds have similar molar mass. The substituent bonded to the organic ring determines the dye’s reactivity (electron donor or electron acceptor) and solubility, which influence the degree of
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Table 1 Efficiency in the production of peroxodisulfate by electrolysis with conductive diamond anodes. Raw concentration of sulfuric acid (M)
Temperature (◦ C)
Efficiency/mmol S2 O8 2− (Ah)−1
0.1 1.0 2.0 1.0 1.0
25 25 25 10 40
0.00 0.75 1.97 3.77 0.61
interaction with the oxidants, as studied by Muthukumar et al. [33]. In fact, the much higher apparent first-order kinetic constant for the RR 141 dye might be due to its significantly higher solubility, compared to the other dyes. Taking into account the supporting electrolyte used, peroxodisulfates should be the primary oxidants [43] to explain this mediated oxidation and the increase in the oxidation kinetic constant over the value proposed by the mass-transport coefficient for the RR 141 and AB 62 dyes; however, at the operation conditions applied, no peroxodisulfates are detected in the bulk solution. Some data concerning the production of peroxodisulfate during electrolyses of sulfuric acid solutions (summarized in Table 1) can be of great interest to interpret the results; from these data it can be inferred that peroxodisulfate anions do not occur in the bulk solution during the electrolyses of the dyes, but just near the electrode surface, such as many other potential oxidants (ozone, hydrogen peroxide, etc.). Consequently, these oxidant compounds led to an increased efficiency of the electrolytic process over the value expected for a purely mass-transport controlled process [31,44,45]. 4. Conclusions Several conclusions can be drawn from this work. First, the anthraquinonic dye AB 62 and the azoic dye RR 141 can be completely mineralized by electrolysis with conductive-diamond anodes. Mass transport controls the process kinetics; nevertheless, the proper treatment of such dye solutions, even at a 100 g m−3 concentration, is attained with energy consumptions below 60 kWh m−3 . Second, the supporting electrolyte plays a key role in the oxidation mechanism of the dyes. The presence of chloride ions leads to the occurrence of oxidants in the bulk solution, which contribute significantly to the oxidation of the dyes and their intermediates. On the other hand, within the concentration range used (100 mM), the presence of sulfate ions does not lead to the occurrence of oxidants in the bulk electrolyte, but just in a zone very close to the anode surface (reaction cage proposed by Comninellis’ group), in which dyes are completely degraded during the electrolyses without net formation of intermediates in the bulk solution. This means that from a macroscopic point of view, the process can be considered as the electrochemical incineration (full conversion into carbon dioxide) of the dye. Finally, the reported data on COD, TOC, and absorbance clearly provide very different information about the progress of the dyes oxidation. This information is not complete but complementary, being fully required to understand the performance of the electrolyses of the dyes; moreover, due to the problems with COD measurements in chloride-containing media, TOC data by themselves might be the best choice. Acknowledgments This work was supported by the JCCM (Junta de Comunidades de Castilla La Mancha, Spain) through the project PEII11-00972026. The Brazilian agencies CNPq and CAPES (for the scholarship awarded to J.M. Aquino) are also acknowledged.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cej.2012.01.044.
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