Enhanced Electrokinetic Remediation Of Polluted Kaolinite With An Azo Dye

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Enhanced electrokinetic remediation of polluted kaolinite with an azo dye M. Pazos, M.T. Ricart, M.A. Sanrom´an, C. Cameselle ∗ Department of Chemical Engineering, University of Vigo, Building Fundicion, 36310 Vigo, Spain Received 18 October 2005; received in revised form 16 March 2006; accepted 24 April 2006

Abstract In this work, the feasibility of the combination of electrokinetic remediation and electrochemical oxidation for the remediation of polluted soil with organic compounds had been development and evaluated in kaolinite spiked with Reactive Black 5 (RB5) an azo dye. The process consists in the use of two combined phenomena to achieve a full remediation of RB5 spiked kaolinite and the degradation of the organic pollutant. Those phenomena were soil electrokinetic treatment combined to liquid electrochemical oxidation. Reactive Black 5 (0.39 g dye kg−1 ) could be effectively removed from the kaolinite matrix by electrokinetics, however the removal results largely depended on the operating conditions. Complete removal of RB5 was achieved using K2 SO4 as processing fluid (which enhanced the desorption of RB5 from the kaolinite matrix) and operating with pH control at 7 on the anode. This favoured the alkalinization of the system and, at high pH values, RB5 was ionized and migrated towards the anode chamber where it was collected and could be oxidized electrochemically. Also, it must be pointed out that in these optimized conditions the electric power consumption (56 kW/mg of removed dye) was ten times lower compared to the unenhanced electrokinetic process (with no pH-control in the electrode solutions). Separate electrochemical decolourization tests of RB5 showed the effectiveness of K2 SO4 in the efficiency of the process. A linear relationship between K2 SO4 concentration and the decolourization rate was found. Thus, nearly complete decolourization was achieved after 2 and 3 h of electrochemical treatment when the electrolyte concentration was 0.1 and 0.01 M of K2 SO4 , respectively. © 2006 Elsevier Ltd. All rights reserved. Keywords: Electrokinetic; Decolourization; Electrochemical treatment; Reactive Black 5; Soil remediation

1. Introduction Around 106 tonnes are produced annually world wide and used extensively in textile dyeing/finishing and also in food, paper and cosmetic industries. It is estimated that about 15% of total world production of wastewater are lost in industrial effluents [1]. The discharge of these highly coloured wastewaters into the ecosystem involves environmental problems. There are more than 10,000 of chemically different synthetic dyes and pigments. Among them, azo dyes are a major class of synthetic, coloured organic compounds and account for about half of the textile dyestuffs used today [2]. Several methods are using to decolourize wastewater (e.g., membrane technologies, coagulation and flocculation technologies) with different colour removal capabilities, costs and might produce large amounts of solid wastes, which become pollutants on its own creating disposal problems [3–10]. Biological,

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Corresponding author. Tel.: +34 986 812318; fax: +34 986 812380. E-mail address: [email protected] (C. Cameselle).

physical and chemical processes to remove organic pollutants have been considered for remediation of contaminated soils [11]. However, few reports about dye removal from contaminated soils were found in the literature. Hence, there is a great interest for developing a soil remediation technology cost-effective and ecofriendly to this kind of organic pollutants. The application of electrokinetic remediation for the decontamination of soil polluted with heavy metals has been very promising, and recently this technique has been employed for remediation of soil polluted with organic compounds [12,13]. The electrokinetic remediation is primarily a separation and removal technique for extracting contaminants from soils. It can be applied in situ, which allows soil to be treated without being excavated and transported, resulting in potentially significant cost savings and in a less destruction of the soil matrix. The principle of electrokinetic remediation is based on the application of a low-intensity direct current through the soil, between two electrodes (anode and cathode). Pollutants are transported towards the electrodes by electromigration (positively and negatively charged organic compounds are

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transported towards the cathode and anode, respectively) and electroosmosis (the movement of a liquid containing ions relative to a stationary charged surface) [14,15]. Moreover, the oxidation/reduction of water at the anode/cathode generates H+ and OH− that move towards the cathode/anode creating an acid and basic front in the soil, respectively [16]. During the electrokinetic treatment of a contaminated soil, charged dye is transported towards one of the electrode chambers where it will be concentrated. Removal of contaminants of this concentrated solution may be accomplished by several means, among which liquid electrochemical oxidation is a promising technology. There are few reports that used the electrochemical technology to degrade wastewater with high dye concentration. Among them, Ciorba et al. [17], Gutierrez et al. [18], Donaldson et al. [19], Kim et al. [20], Sanroman et al. [21,22] have reported the enormous potential of the electrochemical technology to decolourize wastewaters contaminated with several kinds of dyes. They found that electrochemical oxidation is a versatile alternative with a high potential to replace or improve existing processes. In this work, an environment-friendly in situ integral method is presented. Hence, the method proposed in this paper consists in two combined phenomena: soil electrokinetic treatment and liquid electrochemical oxidation on the electrode (anode). This coupling could permit the in situ total dye degradation. In order to test the feasibility of the method purposed, the electrokinetic remediation of kaolin polluted with a representative dye of the azo group, Reactive Black 5 (RB5) and its degradation by electrochemical oxidation have been studied. 2. Materials and methods 2.1. Electrokinetic treatment 2.1.1. Kaolinite sample preparation Kaolinite samples were prepared mixing thoroughly 120 g of kaolinite clay with 120 mL of a solution of RB5 (0.5 g L−1 ). A slurry of 0.5 g dye kg−1 dry kaolinite clay was obtained. RB5 (Fig. 1) was purchased from Sigma. The mixture stood for 24 h to allow the possible sorption of dye to the surface of kaolinite particles. The pH of the mixture was 4. The resulting slurry was compacted to a constant density and pressure in a consolidometer unit [23], which creates homogeneous, near-saturated soil

matrixes. During the consolidation process, a fraction of the initial water and dye was eliminated from the slurry. RB5 concentration in the soil matrix after the consolidation process was 0.39 g dye kg−1 of dry matter, and its pH was 4.2. The moisture content was around 32%, expressed as the weight of water per unit weight of wet kaolinite. In this sample, RB5 was adsorbed on the kaolinite particles since no dye was released in extraction tests with deionised water. Kaolinite was selected as model matrix since it shows much lower buffering capacity, because of its lower cation exchange capacity compared with other clay minerals. The kaolin used has a particle size average of 3 ␮m and a specific surface of 13.5 m2 g−1 . The mineralogy analysis by X-ray diffraction indicated the presence of kaolinite clay 85%, mica 14% and quartz 1%. 2.1.2. Electrokinetic cell The experiments were performed in a cylindrical glass cell depicted in Fig. 2. The sample, 80 g of RB5 spiked kaolinite (dry weight) was introduced in the central tube whose dimensions are 100 mm length and 32 mm inner diameter. The two electrode chambers (with 300 mL working volume) are placed at each end of the sample compartment isolated from this one by a paper filter and porous stones. Graphite electrodes were used for both anode and cathode (P1, P2 and P3). Three auxiliary electrodes allow to measure the electric field distribution along the sample. Electrode chambers were filled with K2 SO4 (0.1 M) as processing fluid. Recirculation of liquid is applied by pumps to avoid concentration gradients. In some experiments pH on the anode or the cathode chambers was controlled using NaOH 0.1 M or H2 SO4 0.1 M, respectively. A DC electric current with maximum values of 30 V or 10 mA was applied in all experiments. Readings of the voltage drop, current intensity and the pH in the electrode compartments were taken periodically. Analytical methods: upon completion of each experiment, samples were taken from cathode and anode solutions and the clay matrix for chemical analysis. The clay sample was divided in five sections of approximately 16 cm3 each, named S1–S5 from anode to cathode. They were analysed for moisture content, pH and RB5 concentration. The pH in the kaolinite sections after the experiment was measured adding KCl 1 M to dry kaolinite in a ratio 2.5 mL g−1 . After 1 h of contact time, pH was measured with a pH-meter Sentron model 1001. The RB5 was extracted from kaolinite sections with a solution of K2 SO4 0.1 M in a ratio 1 g of dry kaolinite: 8 mL. The extraction procedure was done five times to assure a complete recovery of the dye. RB5 concentration was determined from the absorbance at the maximum wavelength in the visible spectrum (597 nm) using a calibration curve. 2.2. Liquid dye decolourization

Fig. 1. Chemical structure of Reactive Black 5.

2.2.1. Electrochemical cell Experiments were carried out in an electrochemical cell with a working volume of 300 mL. Electrodes were made of graphite, with an effective area of 38 cm2 and the electrode gap was 8 cm.

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Fig. 2. Schematic diagram of experimental set-up: (1) soil sample, (2) porous stones, (3) electrodes, (4) electrode chambers, (5) auxiliary electrodes, (6) gas valves, (7) recirculation pumps and (8) power supply.

Magnetic stirring was used to avoid concentration gradients. A constant potential difference (5 V) was applied with a power supply (HP model 3662) and the process was monitored with multimeter (Fluke 175). The electrochemical decolourization of Reactive Black 5 (67 mg L−1 ) was studied using K2 SO4 as electrolyte (10–100 mM). 2.2.2. Decolourization Samples of reaction mixtures were taken from the electrochemical cell to be analysed for pH and dye concentration. pH was measured with a Sentron pH meter (model 1001). The dye concentration was measured from the absorbance at the maximum wavelength in the visible spectrum (597 nm) using a calibration curve. Dye decolourization was associated with the decrease in the concentration and expressed in terms of percentage. The assays were done in duplicate, and the experimental error was less than 3%. 3. Results and discussion 3.1. Electrokinetic treatment of kaolinite polluted with RB5 A saturated kaolin clay sample polluted with RB5 was subjected to the electrokinetic process. The objective of this experiment (exp. 1) was to determine the feasibility of the remediation of kaolin contaminated with RB5, using an electrokinetic technique to mobilize the dye and collect it in the chamber of one of the electrodes. In order to improve this electrokinetic process, it is necessary to add an electrolyte that increases the electric conductivity and favours the desorption of dye from the polluted clay sample. Potassium sulphate was found to have been very effective in the extraction of dye from the mineral matrix; in fact, potassium sulphate was used in the extraction of the residual RB5 from kaolinite in the analytical procedure. It is a strong electrolyte, which is completely dissociated in solution. When a solution of potassium sulphate is used as processing fluid in the electrode chambers, K+ and SO4 2− ions are transported through the kaolinite sample from one electrode chamber to the other by the effect of the electric field. The presence of the potassium sulphate increases the electric conductivity and favours the desorption of RB5 from the clay particles. Thus, the desorbed RB5

can be transported by electroosmosis and/or electromigration towards the electrode chambers. For this reason, in this experiment potassium sulphate was selected as processing fluid, filling the anode and cathode chamber with 0.1 M potassium sulphate. In this experiment, a pH jump between section S4 and S5 (Fig. 3) was developed due to the electrolysis of water upon the electrodes [24]. The formation of this pH jump was the responsible of the decreasing in the current intensity registered from the second day of treatment and the large increase observed in the electrical resistance. As can be seen in Fig. 3, 14% of the initial RB5 was removed from the soil and it is supposed to have reached the electrode chambers by electromigration, electroosmosis and diffusion. RB5 was almost complete removed from section S5, its concentration was reduced by 94%. There was also an important decrease in dye concentration after the treatment in sections S1 (84%) and S2 (54%). However, RB5 was accumulated in section S3 and especially in section S4, where the final concentration was twice the initial one. The electrokinetic treatment of the kaolinite sample polluted with a coloured compound was followed visually. In the beginning, the colour intensity was uniformly distributed along the sample. The application of the electric field mobilized the organic compound. The movement of RB5 results in the appearance of white zones (or pale colour zones) on both sides of the kaolinite sample. The thickness of those white zones

Fig. 3. Electrokinetic treatment of RB5 spiked kaolinite (exp. 1). Normalized concentration (ⵦ) and pH ().

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was slowly widened during the first 5 days. However, no more visual changes were observed during the rest of the experiment. So, it suggests that the reported results in RB5 removal were achieved in 5 days of treatment and the rest of the treatment do not contribute significantly to the decontamination process. The electric power consumption registered at the end of the experiment was 60 kWh (907 kWh mg−1 of removed dye), but considering that the treatment was not effective from the fifth day on, the electric power consumption was only 37 kWh (592 kWh mg−1 of removed dye). In this experiment it is very clear that the pH jump limited the current intensity and increased the electrical resistance of the system. In other words, the transport of ions was limited and therefore it affected the RB5 removal. On the other hand, an important removal of RB5 was found on both sides of the kaolinite sample, where the pH values were complete different. On the anode side, the pH of the kaolinite was very acid (pH 2.4), and pH was alkaline on the cathode side (pH 9.7). So, it is proposed to study the behaviour of the system under pHcontrolled conditions. It suppressed the pH jump and permitted to study the removal of RB5 in alkaline or acid environment, with the objective of revealing the optimum operating conditions for RB5 removal.

with the initial concentration, and a slightly accumulation in sections S1 and S2 was observed. In these sections the final concentration was 20% higher than the initial concentration. These results show that RB5 migrated towards the anode. In fact, during the experiment, the advance of the dye was followed by the change in colour of the soil (from blue to white). The white layer appeared in the soil close to the cathode on the second day and it moved towards the anode at a constant rate of 0.17 cm day−1 , until the day 15, when it reached a thickness of 4 cm. It corresponds with sections S4 and S5, which shows the highest removal. The mass balance showed that 26% of RB5 was removed from the kaolinite and was collected on the anode chamber, where it was electrochemically decolourized. No RB5 was detected on the anode and cathode solution at the end of the experiment. The electric power consumption was 338 kWh mg−1 of removed dye. 3.3. Electrokinetic treatment with pH control on anode (exp. 3)

In this experiment, pH value was controlled at 6.8 ± 0.2 on the cathode chamber with sulphuric acid 0.1 M. It avoided the formation of an alkaline environment at the cathode side and favoured the advance of the acid front. So, the kaolinite sample was acidified reaching an almost flat pH profile (from 2.9 to 3.2) as shown in Fig. 4. The pH control at the cathode avoided the formation of the pH jump, and the electrical resistance of the system was kept low compared to experiment 1. The distribution of RB5 in the kaolinite was very different compared to experiment 1 (Fig. 4). A very important decrease in RB5 concentration was observed in section S4 (78%) and especially in section S5 (92%). The final concentration of RB5 in section S3 (in the middle of the kaolinite sample) coincided

The pH control on the anode depolarized the oxidation of water avoiding the formation of H+ ions and the acid front. NaOH 0.1 M was used in the pH control to keep the pH value at 7.2 ± 0.2 in the anode. In this condition, the basic front generated at the cathode penetrated into the kaolinite sample, increasing the pH of the interstitial fluid. At the end of the experiment, the kaolinite sample reached a pH profile almost flat, from 9.7 to 9.9 (Fig. 5). The electrical resistance of the system was slightly higher than in experiment 2. It is probably due to the lower mobility of the OH− ions (the predominant in experiment 3) than the H+ ions (the predominant in experiment 2). At the end of the experiment, 94% of RB5 was removed from the kaolinite sample demonstrating the high effectiveness of the treatment. The removal degree in the different sections ranged from 91% in S1 to 96% in S5 (Fig. 5). During the experiment, it was observed that RB5 was transported towards the anode side. In the first day of treatment, a white layer 1 cm width was detected on the cathode side. The rest of the kaolinite sample was blue. Both zones were separated

Fig. 4. Electrokinetic treatment of RB5 spiked kaolinite with pH control in the cathode (exp. 2). Normalized concentration (ⵦ) and pH ().

Fig. 5. Electrokinetic treatment of RB5 spiked kaolinite with pH control in the anode (exp. 3). Normalized concentration (ⵦ) and pH ().

3.2. Electrokinetic treatment with pH control on cathode (exp. 2)

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by a sharp line. The thickness of the white layer was increasing during the treatment and after 5 days all the dye was removed from the kaolinite sample and was concentrated in the anodic solution. RB5 is a complex chemical compound that has four sulphonic groups. In alkaline medium, those groups will be ionized forming an anion with four negative charges. In the beginning, the pH of the kaolinite sample is slightly acid (around 4) but, during the electrokinetic treatment, the alkaline front increased the pH of the kaolinite inducing the ionization of the dye molecule. At the same time, potassium sulphate was transported by electromigration and electroosmosis from the electrode chambers to the kaolinite sample. The presence of potassium sulphate favoured the desorption of RB5 from kaolinite solid matrix. Both phenomena desorbed and ionized the dye, which can migrate towards the anode. The first 3 days, the registered migration rate was 1 cm day−1 but the increase of pH and potassium sulphate concentration into the kaolinite sample, accelerated the migration reaching 3.5 cm day−1 on the fourth and fifth days. On the fifth day the dye was colleted in the solution of the anode chamber, where it was electrochemically decolourized upon the surface of the electrode. The electric power consumption was only 56 kWh mg−1 of removed dye. 3.4. Electrochemical decolourization of RB5 In order to obtain the full remediation of polluted kaolin it is necessary to study the electrochemical oxidation of RB5 extracted from the soil by electrokinetic treatment and colleted in the anode chamber. The electrochemical decolourization of RB5 in liquid phase was studied in an electrochemical cell of 300 mL, with a solution similar to that found in the anode chamber. The experiments were conducted at 25 ◦ C and at constant potential difference: 5 V. This value was selected since it was determined as the optimum value for treatment time and electric power consumption [21]. The decolourization experiments were carried out in a solution of RB5 with initial concentration of 67 mg L−1 and different concentrations of potassium sulphate. In the electrochemical decolourization, potassium sulphate acted as inert electrolyte [25]. In order to enhance the decolourization process, the effect of the electrolyte concentration was studied in the range from 0.01 to 0.1 M. In Fig. 6, the influence of the concentration of potassium sulphate in the RB5 decolourization is depicted. In general, high electrolyte concentration increases the electric current densities in the solution and, for this reason, high concentrations of sulphate significantly increased the reaction rate, and the total decolourization of the solution is reached in shorter treatment time. Thus, 180 min are necessary to reach 90% of decolourization with 0.01 M K2 SO4 . If the concentration of sulphate is increased until 0.1 M, the treatment time necessary to obtain the same decolourization was 110 min. This is an important reduction in the treatment time. The concentration profile along treatment time was fitted to a pseudo-first order kinetic equation (Eq. (1)), and the rate constant was calculated using the Sigma Plot 8.00 software. The

5

Fig. 6. Electrochemical decolourization of Reactive Black 5 with potassium sulphate: 0.01 M (♦); 0.025 M (); 0.05 M (); 0.1 M ().

Sigma Plot curve fitter uses an iterative procedure, based on the Marquardt–Levenberg algorithm, which seeks the values of the parameters that minimize the sum of the squared differences between the observed and predicted values of the dependent variable. dC = −kC dt

(1)

where C is the concentration of RB5 (mg L−1 ), t the reaction time (min) and k is the pseudo-kinetic coefficient for the first order reaction (min−1 ). As it can be seen in Fig. 7, the pseudo-kinetic coefficient depends linearly on the potassium sulphate concentration. So, the previous kinetic expression can be rewritten as follows: dC = −0.143 CPS C dt

(2)

where CPS is the concentration of potassium sulphate (M) and t is the reaction time (min).

Fig. 7. Relationship between the potassium sulphate concentration and the pseudo-kinetic parameter obtained with Eq. (1) in the electrochemical decolourization of RB5.

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These results confirm the double utility of potassium sulphate as processing fluid in electrokinetic treatment and as electrolyte, which enhances the electrochemical decolourization of RB5. Hence, its presence in the anode chamber favoured the electrochemical decolourization upon the surface of the electrode of the dye that was transported from the soil to the electrode chamber. Moreover, high concentration of potassium sulphate favoured the electric conductivity and the desorption of RB5 in the soil, and the dye decolourization in the processing fluid. 4. Conclusions In view of the results obtained in the present paper, it can be concluded that the combination of electrokinetic remediation and electrochemical oxidation shows a great potential for the remediation of polluted soils with anionic organic compounds. The operating conditions, especially the pH value into the soil sample, are decisive to achieve a full remediation. Moreover, the effect of several electrolyte concentrations on liquid electrochemical oxidation was assessed. It was found that high concentration of potassium sulphate was able to electrocatalyze efficiently the oxidation of dye shortening significantly the treatment time of the processing fluid obtained from the electrokinetic remediation. The above results suggest the feasibility to apply the two combined phenomena (soil electrokinetics combined to liquid electrochemical oxidation) for remediation of soil environments contaminated with organic pollutants. With the configuration designed and employed in the present report, full remediation of polluted kaolin at low cost without operational problems can be achieved. Acknowledgement This research was funded by Xunta de Galicia (Project PGIDIT04TAM314003PR).

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