Electrochemical Wastewater Treatment Directly Powered by

Electrooxidation powered by photovoltaic panels, considering their correct number and configuration, is a technique that achieves the elimination of t...
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Environ. Sci. Technol. 2010, 44, 5182–5187

Electrochemical Wastewater Treatment Directly Powered by Photovoltaic Panels: Electrooxidation of a Dye-Containing Wastewater DAVID VALERO, JUAN M. ORTIZ, ´ SITO, EDUARDO EXPO VICENTE MONTIEL,* AND ANTONIO ALDAZ Grupo de Electroquı´mica Aplicada y Electrocata´lisis, Departamento de Quı´mica Fı´sica, Instituto Universitario de Electroquı´mica, Universidad de Alicante, Ap 99, Alicante 03080, Spain

Received February 18, 2010. Revised manuscript received May 28, 2010. Accepted June 4, 2010.

Electrochemical technologies have proved to be useful for the treatment of wastewater, but to enhance their green characteristics it seems interesting to use a green electric energy such as that provided by photovoltaic (PV) cells, which are actually under active research to decrease the economic cost of solar kW. The aim of this work is to demonstrate the feasibility and utility of using an electrooxidation system directly powered by a photovoltaic array for the treatment of a wastewater. The experimental system used was an industrial electrochemical filter press reactor and a 40-module PV array. The influence on the degradation of a dye-containing solution (Remazol RB 133) of different experimental parameters such as the PV array and electrochemical reactor configurations has been studied. It has been demonstrated that the electrical configuration of the PV array has a strong influence on the optimal use of the electric energy generated. The optimum PV array configuration changes with the intensity of the solar irradiation, the conductivity of the solution, and the concentration of pollutant in the wastewater. A useful and effective methodology to adjust the EO-PV system operation conditions to the wastewater treatment is proposed.

1. Introduction Social concern about the environmental impact caused by industry is growing, and new laws demanding more strict environmental protection are being approved. For this reason the search for “greener” and more efficient methods for wastewater treatment is increasing (1, 2). Among the different techniques for wastewater treatment (3, 4), electrochemical methods have achieved a relevant place. In particular, the oxidation of organic pollutants either by anodic oxidation using different electrodes (5-8) or by cathodic generation of hydrogen peroxide (9-11). The aforementioned social concern about the environment has also led to the search and development of new forms of renewable electric energy. One of the most * Corresponding author phone: +34 965903536; fax: +34965903537; e-mail: [email protected]. 5182

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widespread and studied is the photovoltaic (PV) generation of electricity. PV generators are non-polluting, silent, decentralized, and have a long life. The low maintenance cost of these systems is also another positive factor (12). The use of PV modules as a power supply for chemical and electrochemical systems has previously been reported in the literature, i.e., brackish water desalination by reverse osmosis (13, 14), electrodialysis of brackish water (15-17), electrocoagulation of textile effluents (18), and coupled proccesses of cathodic generation of hydrogen and the anodic electrooxidation (EO) of organic compounds (19-21). It is interesting to note that PV modules produce direct current, which can directly be used by the EO system. Furthermore, most of the conventional PV applications in environmental treatment use PV modules connected to a storage battery system (22, 23) sgenerally lead-acid types, to avoid energy fluctuations and for working during night hours. However, the use of these battery banks has several problems: (i) the charge-discharge efficiency is around 70-80% (13, 24), (ii) their lifetime is substantially shorter than the lifetime of PV modules, (iii) a strict control of the state of charge of each battery cell is needed, and (iv) both the economic and environmental costs of battery disposal must also be taken into account. The direct use of the electrical energy supplied by the PV array to the electrochemical reactor could decrease both the cost of the investment and the maintenance cost of the wastewater treatment system. Moreover, the pumps used in these systems can also be powered by PV modules (25); commercial DC-PV pumps are available in the market making the global treatment process self-sufficient. However, there are very few studies about the behavior of an electrolysis reactor fed by a non-constant energy source. So we are particularly interested in verifying and studying how the process is affected by changes in solar irradiation and what will be the behavior of the global process along the day. Thus, the aims of this paper are the following: (i) to demonstrate the feasibility of using an EO system directly powered by a PV array (EO-PV) for the decolorization of a dye-containing solution; the first step of the work was to compare the degradation of the dye using both a conventional electric power source and an EO system directly powered by a PV array; (ii) to determine and comprehend the behavior of the coupled EO-PV system when it works in changing meteorological conditionssto optimize the use of the incident solar energy to carry out the decolorization of the anolyte. So, the influence of the configuration of both the PV generator and the electrochemical reactor on the RB removal has been studied. The pollutant used in this study was a dye employed in the textile industry: Remazol Red RB 133 (RB), the degradation mechanisms of which are reported in ref 26.

2. Experimental Section The experimental system was formed by either 40 PV cells or a conventional power supply as power source, an electrochemical reactor described later, two 100-L tanks for catholyte and anolyte solutions, pumps, and flow meters. The electrochemical system was designed for carrying out the experiments in a batch mode of operation. Temperature, pH, and conductivity were measured online for both solutions. Figure 1 shows a scheme of the EO-PV system and the electrochemical reactor employed. The electrochemical reactor was a divided filter-press electrochemical reactor (27). In this work both, experiments with one single cell (Figure 1b, configuration A) and with 10.1021/es100555z

 2010 American Chemical Society

Published on Web 06/11/2010

FIGURE 1. (a) Scheme of the experimental EO-PV system. (b) Scheme of the electrochemical reactor: unitary cell (configuration A), two cells in bipolar configuration (configuration B). 1. End plates; 2. insulator gaskets; 3. cathode; 4. compartment frame with rubber gaskets; 5. cationic exchange membrane; 6. anode; 7. metal collector. two cells in bipolar electrical configuration (Figure 1b, configuration B) were carried out. DSA-O2 anodes composed of an electrochemically active coating of iridium oxide (IrO2) deposited on a titanium mesh (ID Electroquimica), and threedimensional carbon felt cathodes (Carbone Lorraine) were employed. The geometric area per electrode was 3300 cm2. As separator, Nafion 450 cationic exchange membrane (Du Pont) was used. The conventional electric power used was a KRAUSER 1000A-30 V power source. The PQ10/40/01-02 (AEG) photovoltaic panels used were of polycrystalline silicon, with a peak power of 38.48 W, an open circuit voltage of 20 V, and area of 0.5 m2 (1 m × 0.5 m) each. The experiments were carried out at the University of Alicante (latitude 38°24′05′′ N, length 0°31′ W, altitude 109 m above sea level). The tilt of the photovoltaic panels was 55° and the PV array was south-facing (0.4° W). Two configurations of the PV generator were used: (i) 40 panels connected in parallel and (ii) 40 panels distributed in two stacks connected in series, each being formed by 20 panels in parallel. The incident solar radiation was measured using a pyranometer 80 SPC (Soldata Instruments). For data acquisition of solar irradiation, ambient temperature, and PV array voltage and current, a data acquisition system connected to a PC computer was used.

The volumes of catholyte and anolyte were 50 L. The flow rate of both solutions was 400 L h1-. Electrolyses were carried out at room temperature. The initial anolyte solution was prepared by dissolving Remazol RB 133 dye (DyStar S.A.) in deionized water. For all the experiments, the initial concentration of RB was 50 mg/L and conductivity was 0.6 mS/ cm. pH was adjusted to the required value 2.8 by adding the necessary amount of H2SO4 96% wt. Panreac PRS. Catholyte was a H2SO4 solution at pH ) 3. RB concentration was determined spectrophotometrically at 518 nm (λmax) with a HACH DR 2000 spectrophotometer. The % decolorization, E, was calculated as E)

Ci - Ct × 100 Ct

where Ci is the initial dye concentration (mg/L) and Ct is the dye concentration at time t (mg/L).

3. Results and Discussion 3.1. Electrooxidation Using a Conventional Power Source. First, to know the efficiency of the RB electrooxidation process, a series of experiments using a conventional power source was carried out. The experiments were carried out using the single filter press cell, and the influences of VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Curve % decolorization, E, vs time. pHan ) 2.8, I ) 20 A.

FIGURE 3. Characteristic curve of a PV panel. G ) 800 W/m2 and Tmodule ) 298 K. parameters such as pH and conductivity of the anolyte and the current intensity were established. The best conditions for decolorization in this system were anolyte pH 2.8 and a current intensity of 20 A. Figure 2 shows the variation of E versus time during the electrooxidation experiment at the chosen conditions. The rate of decolorization decreases with time, showing an asymptotic behavior at the end of the experiment. 3.2. Electrooxidation Using a PV Array As Power Source. 3.2.a. Characteristic I-V Curves of a PV Array. To understand the experimental behavior of the coupled EO-PV system, it should be interesting to briefly indicate the most important parameters of the characteristic I-V curve of the PV modules. Figure 3 shows the I-V curve for a single PV panel and for a given irradiation, temperature, and load. The shadow area is the power delivery by the panel for different external loads. The parameters that define a PV panel are the short circuit current (Isc) and the open circuit voltage (Voc). Isc is the current given by the PV panel when the voltage between terminals is zero (Rload ) 0). Voc is the voltage measured in absence of connected load (or Rload ) ∞). The maximum power (Pm) is the electrical maximum power that a PV panel can give for a given solar irradiation (G) and panel temperature (Tmodule). It is defined by the point of the I-V curve where the product of the current (Imp) and voltage (Vmp) is maximum. Figure 3 shows two clearly differentiated zones. First, a plateau is observed where the values of current are approximately equal to Isc for a wide range of voltages. The second region is characterized by a sudden decrease of the current being the voltage approximately equal to Voc. Figure 3 also shows that for a load with an electrical resistance R0, the intersection of the characteristic I-V curve of the PV panel with the straight line I ) (1/R0)V defined the working point of the panel. 5184

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FIGURE 4. Experimental results for exp 1 (data in Table 1). Plot of E, G, Iarray, and Vcell vs t. Experiments were carried out at ambient temperature. Both catholyte and anolyte flows were 400 L h-1. Anolyte was a solution of 50 mg/L Remazol RB at pH ) 2.8. PV panels can be connected in series or in parallel. If a group of identical PV modules are connected in series forming a PV array, the open circuit voltage (Voc,array) of the array increases with the number of panels connected in series (n)sapproximately Voc,array ) nVoc. However, the short circuit current (Isc,array) remains approximately equal to that of a single PV panel, Isc. Similarly, when a group of identical PV panels are connected in parallel, Isc,array increases with the number of panels connected in parallel (m)sapproximately Isc,array ) mIsc, and Voc,array is the same as that of a single PV panel, Voc. 3.2.b. Feasibility and Optimization of the Electrooxidation of Remazol RB Directly Powered by a PV Array. The aim of this study was to demonstrate the feasibility of using an EO system directly powered by a PV array for different meteorological and solar irradiation conditions. In an EO system powered directly by a PV array, both the number of PV modules and their configuration will be imposed by the electric I-V requirements of the electrochemical reactor. However, the current intensity provided by panels will depend on both the solar irradiation, G (W/m2), and the temperature of the PV modules, Tmodule (K). These parameters cannot be controlled and will change during the time of reaction either in a continuous way (i.e., through the hours of the day) or suddenly (i.e., clouds passing over). Figure 4 shows the experimental results obtained during the electrooxidation of Remazol RB (exp 1). Relevant experimental conditions are listed in Table 1. The rest of the experimental conditions were as shown in the Experimental Section. It is important to note that the experiments carried out with the EO-PV system finished when E was higher than 90%. Figure 4 shows a quadruple representation of the solar irradiation (G), current intensity (Iarray), cell voltage (Vcell), and % decolorization (E) as function of time (t) for exp 1. The experiment started at 9:50 a.m. and lasted approximately 7 h. It was carried out on a day with cloudy intervals. It is important to note that in this experiment the solar irradiation was very irregular due to the meteorological variable conditions. Thus, the representation of G vs t shows three different situations: (i) During the first 150 min of the experiment the day was very cloudy with short sunny intervals, which hindered the normal increase of G as time went by. In this stage the value of G was low and remained almost constant. (ii) In the intervals 150-200 and 350-420 min, the day became sunny with short cloudy intervals. In this stage, the crossing of clouds in front of the solar disk caused a sharp decrease of G. (iii) The weather was sunny during the interval time between 200 and 350 min. In this stage the value of G

TABLE 1. Experimental Conditions for the EO Experiments Directly Powered by a PV System experiment 1

2

3

electrochemical reactor number of PV modules and PV array configuration

1 divided cell 40 modules connected in parallel

2 divided cells, bipolar connection 40 (2 stacks connected in series, 20 modules in parallel per stack)

date and initial hour of the experiments (local time) atmospheric conditions

06/18/2009; 9 h 50 min

1 divided cell 40 (2 stacks connected in series, 20 modules in parallel per stack) 06/25/2009; 9 h 45 min

cloudy-sunny

sunny

cloudy-sunny

increased at a constant rate. Also, it is interesting to note that, due to the long duration of the experiment, the curve G vs t displays the “hill shape” characteristic of a progressive evolution of the solar irradiation as the day goes by, corresponding the highest values of solar irradiation around midday. Figure 5 also clearly shows how both Iarray and Vcell were nearly independent of G. This behavior can be seen clearly at approximately 150 min, when a sudden increase of G did not affect either Iarray or Vcell. The final value of E was 89.7%, being the rate of decolorization higher for the first stage of the experiment, with an asymptotic behavior at its end. The shape of the curve E vs t shown in Figure 5 is very similar to that obtained when a conventional power supply was used (see Figure 2). The results obtained in exp 1 clearly point out that decolorization of Remazol RB using an EO system directly powered by PV modules can be successfully carried out even for variable atmospheric conditions. The next step was to explain the electric behavior of the EO-PV systems employing both the I-V curve of the PV array and the electrical resistance of the electrochemical reactor. Figure 5 shows the behavior of the system PV generatorelectrochemical reactor during exp 1. Figure 5 shows the characteristic I-V curves of the PV array at several times along the experiments (t ) A, B, C, and D). For each time the PV array output current, Iarray (A′, B′, C′, and D′), the cell voltage, Vcell (A′′, B′′, C′′, and D′′) and the power generated by the PV array (shadow zones) are indicated. The characteristic I-V curves of the PV array were calculated according to the following procedure: the short

07/03/2009; 9 h 00 min

circuit current is proportional to the incident solar irradiation on the PV array and can be calculated using “the five parameters mathematical model” (28). At point A in Figure 5 (t ) 62 min, G ) 214 W/m2) the value of G is low and the working point of the system is placed at the region of maximum power of the I-V curve of the PV generator. In this case, the value of Iarray is very close to Isc. However, at points B, C, and D (t ) 94, 165, and 270 min, respectively) the values of G are higher (in the two latter cases the solar irradiation is placed in the zone of the highest values of the day), giving rise to an increase in Isc. However, the working point of the EO-PV system (given by the intersection of the characteristic I-V curve of the PV array with the operational line of the EO system) is placed at the zone of the I-V curve characterized by a sudden decrease of the current intensity (points B′, C′, and D′). The behavior of the EO-PV system during exp 1 is determined by the low conductivity of the anolyte, which increases the resistance of the electrochemical reactor and fixes the working point of the EO-PV system during all the experiment, at voltage values very close to Voc. Usually, when an EO process is carried out at industrial scale, the volume and flow rate of effluent treated per batch define the anodic area of the electrochemical reactor. In an EO system directly powered by a PV array, the number of PV panels of the PV stack and its configuration must supply the necessary current to the electrochemical reactor in less favorable atmospheric conditions. Furthermore, from an economic point of view the number of PV panels must also be the lowest possible. In this way, the PV array configuration

FIGURE 5. Experimental behavior of the system PV generator-electrochemical reactor during exp 1. VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Characteristic I-V curves for different solar irradiation values and experimental working points of the electrochemical reactor. Experimental details for exp 2 are provided in Table 1. must be versatile and easily reshaped according to the solar irradiation and the meteorological conditions. Once the feasibility of using an EO system directly powered by a PV array in the electrooxidation of RB was proven, the next step was to optimize the mode of operation of the coupled EO-PV system. For this purpose, a new experiment (exp 2 in Table 1), where the PV generator consisted of 2 stacks connected in series with 20 PV modules in parallel per stack, was carried out. A one-cell electrochemical reactor was used. Figure 6 shows the characteristic I-V curves for exp 2 at three moments of the experiment (A: 90 min, B: 250 min, and C: 425 min), where the instantaneous values of solar irradiation, G, were different. Figure 6 also shows the I-V values corresponding to the working points of the electrochemical reactor (marked A′, B′, and C′). Due to the high anolyte resistance, when the 40 panels of the PV array were connected in parallel configuration (exp 1, see Figure 5), the working points were close to the open circuit voltage, Voc, and the current intensity was very low. For the same experimental conditions, during exp 2 where the PV array was connected in a mixed configuration (2 stacks connected in series, 20 modules in parallel per stack), the working point was very close to the region of maximum power for a broad range of solar irradiation values. Thus, it is clearly established that a mixed configuration is more suitable for the experiments carried out at pHan ) 2.8. It is important to note that the value of E was higher than 95% at the end of the experiment, with the shape of the curve E vs t very similar to those shown in the previous experiments. The previous experiment proves that the configuration of the PV generator has a strong influence on the use of the generated power. The optimum PV array configuration (series, parallel, or mixed) must be determined for each effluent and it must be reshaped during the electrochemical treatment depending on the solar irradiation and both I and V requirements of the electrochemical reactor. As exp 3 shows, we have carried out electrooxidation of a solution of Remazol RB using an electrochemical filter press reactor made up of two cells with bipolar connection and a PV power source made up by 2 stacks connected in series with 20 modules each connected in parallel. A value of E higher than 95% was obtained at the end of the experiment. Figure 7 shows three characteristic I-V curves for exp 3 (points A: 60 min, B: 200 min, and C: 340 min), for which the instantaneous values of G were different. Figure 7 also shows the I-V pairs of values corresponding to the working points of the electrochemical reactor (A′, B′, and C′). Due to the bipolar electrode connection, the electrical resistance of the electrochemical reactor is twice the electrical resistance of 5186

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FIGURE 7. Characteristic I-V curves for different solar irradiation values and experimental working points of the electrochemical reactor. Experimental details for exp 3 are provided in Table 1. one electrochemical cell. For this reason, when G was higher than 400 W/m2 the working point of the electrochemical reactor was placed in the region of the I-V curve characterized by a fast decrease of the current intensity (B′ and C′)seven though a mixed parallel-series configuration of the PV generator was used. In this work, the feasibility of coupling two green technologiessphotovoltaic electric generation and electrochemical treatment of wastewaters has been proved. To do so, a synthetic effluent containing Remazol RB 133 dye has been treated by electrooxidation powered by PV panels and decolorization percentages higher than 95% have been achieved. During the electrooxidation experiments, the position of the working point of the electrochemical system in the characteristic I-V curve of the PV generator determines the relation among solar irradiation, current intensity, and cell voltage. Therefore, to design and optimize an EO-PV system, it is necessary to take into account parameters such as the internal resistance of the electrochemical reactor and its variation during the time of electrolyses, the flow and volume of wastewater to be treated together with its chemical oxygen demand, the meteorological conditions (latitude, temperature, etc.), the number and configuration of PV modules, and the possibility to reshape the stack configuration of the solar panels during the treatment. In addition, the results obtained in this paper could be of interest for studies on direct connection of photovoltaic energy to other electrochemical systems such as electrochemical syntheses powered by photovoltaic energy, electrochemical removal of heavy metals, etc. Future research must include studies on the energy efficiency of the coupled process and operating costs.

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