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Electrocoagulation using perforated electrodes: An increase in metalworking fluid removal from wastewater
Chemical Engineering & Processing: Process Intensification 139 (2019) 113–120
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Chemical Engineering & Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Electrocoagulation using perforated electrodes: An increase in metalworking fluid removal from wastewater
T
Oriane Avancini Diasa, Eduardo Perini Muniza,b, , Paulo Sérgio da Silva Portoa,c ⁎
a
Universidade Federal do Espírito Santo, Programa de Pós-graduação em Energia, Rodovia Governador Mário Covas, km 60, Bairro Litorâneo, CEP 29932-540, São Mateus, ES, Brazil b Universidade Federal do Espírito Santo, Departamento de Ciências Naturais, Rodovia Governador Mário Covas, Bairro Litorâneo, CEP 29932-540, São Mateus, ES, Brazil c Universidade Federal do Espírito Santo, Departamento de Engenharias e Tecnologia, Rodovia Governador Mário Covas, km 60, Bairro Litorâneo, CEP 29932-540, São Mateus, ES, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Effluent remediation Electroflotation Metalworking fluid Perforated electrodes Flow rate
A continuous electrocoagulation reactor with electrode polarity switch was used for removal of a metalworking fluid from synthetic oily water. The effects of perforating the aluminum electrodes, changing the number of holes, flow rate and the distance between electrodes were studied. Higher values of flow rate reduced final pH. Perforating the electrodes led to faster convergence of pH to its maximum value with no measurable increase in mass loss. If the distance between electrodes is also increased, there is an improvement in efficiency. With 10 holes, adjusting flow rate and inter-electrode distance, 90.2 ± 0.3% oil removal was achieved with a final pH of 8.83, which is within limits allowed by legislation.
1. Introduction
electrodes help in the separation process [6]. The reactors used in EC are usually small with a high throughput [7]. Chen and coworkers [8] for instance built a 0.30 L reactor to remove oil and grease from restaurant wastewaters and achieved more than 95% of removal efficiency with a flow rate of 9 Lh−1 (2.5 mLs−1). The critical components in EC reactors are coagulants, contaminants and generated bubbles [7]. Material and geometry of the electrode are crucial in the EC reactor project since they alter the coagulants, size and amount of bubbles produced [9]. There is a significant discussion on the appropriate electrode material for each contaminant. For oily wastewater aluminum (Al) electrodes tend to result in a better treatment performance than iron electrodes [10]. Increasing electrical current leads to a high aluminum dissolution rate [11,12], thus higher rate of coagulant production. Tito and coworkers [13] state that current intensity is the critical performance-limiting parameter in EC while current density is secondary. However, the authors that obtained an increase in efficiency by punching holes in the electrodes often justified their results with arguments based on current density. According to Khandengar and Saroha [14], electrodes with punched holes are expected to result in higher removal efficiency compared to
The heavy manufacturing industry uses metal cutting fluids (MCF) to reduce heat during the processing of metals, to lubricate between workpiece and tool [1] and to carry the shards that must be removed to improve finishing. These metalworking fluids contain emulsified oil and surfactants which allow for the formation of a stable emulsion when mixed with water [2] and thus reduce corrosion and bacterial growth [3]. MCF suffer thermal degradation and accumulate impurities with use and thus need to be replaced periodically. The organic waste generated must be taken away and treated. These effluents contain surface-active agents and other organic matters; they also tend to penetrate in the soil being a danger to the purity of groundwater [3]. Electrocoagulation (EC), is a technique often used to remediate oily wastewater. It combines coagulation, flocculation, and flotation; the electrical current that goes through sacrificial electrodes releases positive metal ions in the solution, also forming negative hydroxyl ions and hydrogen gas [4]. The metal ions combine with hydroxyl forming flocculants that may react with the contaminants in the suspension [5]. EC is also called electroflotation since the air bubbles generated in the
Abbreviations: EC, electrocoagulation; DE, distance between electrodes; N, number of holes drilled in the electrodes; Sc, spearman correlation; Q, flux rate; ORf, percent of metal cutting fluid removed from the water after 30 min of EC; OR%, percent of metal cutting fluid removed from the water ⁎ Corresponding author at: Universidade Federal do Espírito Santo, Programa de Pós-graduação em Energia, Rodovia Governador Mário Covas, km 60, Bairro Litorâneo, CEP 29932-540, São Mateus, ES, Brazil. E-mail address: [email protected] (E. Perini Muniz). https://doi.org/10.1016/j.cep.2019.03.021 Received 29 November 2018; Received in revised form 27 March 2019; Accepted 31 March 2019 Available online 01 April 2019 0255-2701/ © 2019 Elsevier B.V. All rights reserved.
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plane electrodes. They studied the effect of the number of holes in aluminum electrodes in the color removal efficiency to remove acid 131 dye from distilled water [15]. Efficiency increases with the number of holes from one to four and is not altered from four to eight holes. There is no apparent change in efficiency with hole distribution. Removal efficiency increases with hole diameter from 2 to 4 mm but not from 4 to 5 mm. The results were attributed to the higher current discharge from the punched electrode. Hussin and co-workers [16] propose the use of perforated zinc electrodes for lead (Pb) separation from water. Removal efficiency was higher for perforated electrodes with a hole diameter of 0.5 cm than for those with a hole diameter of 0.2 cm, and both were more efficient than the plane, non-perforated electrodes. The presence and size of the holes altered the electrical current between the electrodes and the authors associated the higher efficiency with higher current density. In the present work, electrical current was kept constant while the distance between electrodes, the number of holes drilled on them and flow rate were varied. The objective was to verify if other factors besides current intensity or current density interfere in the change of efficacy in oil removal (efficacy is defined as the amount of contaminant removed expressed in % [13]) and in the final pH when electrodes are perforated.
perforated and perforated plates, of same material and dimensions, to evaluate the effect of the presence of holes when the electrical current modulus is kept constant. The position of the drilled holes in the perforated plates is represented in Fig. 2. Holes with 6 mm diameter were drilled in symmetrical patterns around the center of the plates, to guarantee a uniform flux of oily water, oxygen and hydrogen bubbles. The central hole and the hole at the top of the upper corner were filled, used to realign the connection between the plates with the help of a threaded rod. A schematic drawing detailing the shape, layout, and assembly of the electrode plates is shown in Fig. 3. Two sets of three plates were aligned in the form of a beehive, each set interspersed and arranged in parallel (monopolar mode), the plates being separated by screws covered with a polymer for electrical insulation. Distances between electrodes (DE) were set as demanded by experimental design (next section). From the literature, there is a tendency of improvement in the process of separation of oil from water when DE is increased from 5 to 10 mm in a reactor with polarity switch [17], justifying the use of 10 mm as a minimum value in the initial experimental plan.
2. Methodology
The transport of emulsified water from the storage tank to the reactor occurs by pumping. The inlet and the exit of the effluent from the rectangular reactor are 2.0 and 14.0 cm above the bottom, respectively. When the effluent reached the exit height, a useful volume of 3 L inside the reactor was obtained. At that time the flow rate was adjusted using a test tube, i.e., the volume of effluent collected in one minute was measured. After an amount of water equal to the useful volume of the reactor passed through it, the system was considered to be in a steady state. For each experiment, an aliquot was collected before turning on the power supply (zero time). Then the power was turned on, and the chronometer started. Each 5, 10, 20, and 30 min of the process, a sample of 350 mL was collected, stored in an amber bottle and maintained in a refrigerator at 4 °C for subsequent analysis. The values used for flow rate (Q), the number of holes (N) and the distance between electrodes (DE) were as shown in Table 2. To determine the experimental variables, initially a 23 factorial experimental design was used (experiments 1–8), then a central point was added in triplicate (experiments 9, 10 and 11). After the initial analysis, since it showed some improvement in efficacy with increasing N, it was decided to do a second set of experiments with maximum N (fixed value), using a central composite design with Q and DE as variables (experiments 12,13 and 14). By design one of the values tried for Q should be 1.18 mL.min−1 but this value was discarded since it is too low to be of practical use. The central point of this new set of experiments was made in triplicate (experiments 15, 16 and 17). Once all was finished, the system with nonperforated electrodes had to be evaluated as a reference (experiments 18 and 19). The electrical current tends to pass only in the metallic parts of the electrodes. If holes are drilled the area of the plates will be reduced and current density increased. The current density is estimated to be 493.83 A. m−2 for a flat plate without the presence of holes. It grows to 508.01 and 515.42 A. m−2 for 6 and 10 holes, respectively.
2.3. EC procedure
2.1. Preparation of synthetic cutting oil effluent To prepare 1 L of synthetic effluent, 0.2000 g of Exxon Mobil MOBILCUT 102 soluble cutting fluid (density = 0.89 at 15 °C and viscosity =35 cSt at 40 °C) and 10.0 g of sodium chloride were dissolved in distilled water. Sodium chloride was added to guarantee a higher and near-constant electrical conductivity, independent of the amount of aluminum dissolved [17]. This higher current with lower voltage drop tends to decrease power consumption [18]. The solution was stabilized using a rotor-stator mixing device where three blades rotated at 3000 rpm for 15 min, shearing the larger oil surfaces. Preliminary tests indicated that oil content was stable after 10 h from preparation. The solution was then kept at rest and watched for 24 h to check for stability before EC was performed. 2.2. Electrocoagulation system As described in a previous work [17], a monopolar arrangement of three pairs of aluminum electrodes, polarity switch to preserve the electrodes and increase efficiency was used. Polarity reversal turns the cathode into anode and vice versa, which contributes to reducing the passivation effect on the cathode and increases the lifespan of the electrode [17]. While the modulus of the electrical current was kept constant at 4 A, polarity was changed each 30 s forming an input sign step function (polarity reversal). Voltage modulus oscillated around 12 ± 3 V due to electrical resistance changes created by electrode passivation during the process. The voltage switch time was adjusted for electrical stability, resulting in a higher value than the 10 s found in the literature [17,19]. The continuous flow EC pilot unit consisted of a rectangular electrolytic cell, a set of aluminum electrodes, and an electronic device (plate) to switch the polarity of the current from a DC power source (Fig. 1). The monopolar mode electrodes were connected to the polarity switch and then to the DC power supply brand MINIPA model MPC3005. The electrodes are aluminum plates with the following dimensions: 130 mm height, 65 mm width and 0.50 mm thick, located vertically in the central part of the interior of the plexiglass reactor, whose electrical wiring passes through holes located in the lid (Fig. 1). The dimensions of the feed tank, reactor and receiver tank are shown in Table 1. Tests were performed by measuring MCF removal efficacy with non-
2.4. Analytical methodology The characterization of the samples collected was carried out according to methodologies adapted from the Standard Methods for the Examination of Water and Wastewater [20]. 2.5. Determination of pH The pH of all samples was measured at 0, 5, 10, 20 and 30 min of 114
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Fig. 1. Schematic design of continuous flow electrolytic system. Where: (a) current power supply; (b) Polarity inverter; (c) Feed tank for raw fluent storage; (d) submersible pump; (e) flow regulating valve; (f) electrode; (g) reactor input; (h) reactor output and (i) receiver tank for storing effluent after treatment.
procedure were based on Method 4500 from the Standard Methods for the Examination of Water and Wastewater [20].
Table 1 Dimensions of the EC apparatus. Components of the system
Dimensions (cm)
Effective Volume (mL)
Feed tank EC reactor (retangular) Receiver tank
33.0 × 22.0 × 22.0 30.2 × 15.2 × 15.0 25.2 × 18.0 × 18.0
15000 3000 8000
2.6. Determination of electrical conductivity Electrical conductivity was obtained with the aid of the conductivity meter BEL Engineering W12D, resolution of 0.1 μS. cm−1 and accuracy of ± 1%. For each test the conductivity was verified, using a standard NaCl solution of 1000 μS. cm−1. The calibration of the conductometer and the measurement procedure were based on Method 2510 from the Standard Methods for the Examination of Water and Wastewater [20].
electrocoagulation using a pH meter, Mark MS TECNOPON mPA210 with automatic temperature compensation, resolution 0.01 and accuracy ± 0.01%. The calibration of the pH meter and the measurement
Fig. 2. Aluminum plates used to compose the electrode: (a) perforated plates with 6 holes (b) perforated plates with 8 holes (c) plates with 10 holes. 115
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Fig. 3. Representation of the electrode assembly showing the mechanical connections between plates.
function. From an analysis of the UV–vis spectra, the wavelength of 263 nm was ideal; a calibration curve was plotted and fit by a linear model (adjusted R-square of 0.9996), Eq. (1):
Table 2 Operational conditions used for the experiments. Experiment
N
Q (mL.s−1)
DE (mm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
6 10 6 10 6 10 6 10 8 8 8 10 10 10 10 10 10 0 0
2 2 6 6 2 2 6 6 4 4 4 6.8 4 4 4 4 4 2 6
10 10 10 10 20 20 20 20 15 15 15 15 7.95 22.05 15 15 15 10 20
OC (gL 1) = (336 ± 2)(gL 1)* Absorbance
(1)
Where OC is oil concentration. Oil removal efficacy of the process was evaluated by percent of oil removal (OR%) as calculated using Eq. (2):
OR% =
OC0
OCf
OCf
× 100
(2)
where: OC0 is the initial oil concentration (time zero), OCf is the oil concentration at the end of the process (30 min). 2.8. Determination of electrode consumption The mass of the electrodes was measured before and after each experiment to estimate the mass loss occurring during the process (gravimetric measurement). Electrodes were washed and dried before mass measurement to avoid the influence of eventual flakes or other impurities attached during the EC process. The measurements were carried out in an analytical balance Bioprecise brand FA-2104 N with a precision of 0.1 mg.
2.7. Determination of the content of oils and greases A spectral method was used, based on method 5520 from the Standard Methods for the Examination of Water and Wastewater [20]. The oil was extracted from 50 mL samples using hexane as a solvent. Then the UV–vis spectrum was measured, and the absorbance at a given wavelength was used in a calibration curve to obtain oil concentration. The spectrometer used was a QUIMIS 4802. Spectra were measured from 190 to 700 nm with a precision of 1 nm. The value of the wavelength was chosen as the one where the relation between concentration and absorbance was closer to a linear
2.9. Microscopy The surface of the electrodes was observed with an optical USB linked Microscope with magnification up to 1,000X, connected to a laptop. The microscope lens was placed about 10 mm above, perpendicular to the plate and with the same illumination for all samples. 116
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2.10. Statistical analysis
hydroxides that are responsible for oil removal. The relation between kpH and the input variables will be discussed ahead, using Spearman correlation. For all experiments, adjusted Rsquare is higher than 0.94, with pHf in the range from 8.60 to 9.75. As listed in the methodology, the p-value test was considered statistically significant for p-value < 0.05.
To look for monotonic relations between input and output variables Spearman’s coefficient was calculated for the raw data using Origin™ software. Spearman’s is a non-parametric rank statistic that can be used as a measure of the strength of an association between two variables [21] even when one of these variables is noncontinuous as is the case for the number of holes drilled in a plate. The p-value test (p-value < 0.05 was considered statistically significant) was used to verify if a Spearman correlation was significant. Only correlations that passed this test are discussed in this work. Since Spearman`s method finds only monotonic correlations, Q2, N2, DE2, N x Q, N x DE, Q x DE and Q x N x DE were used as variables to search for more complex relationships between input and output data. The parameters of the kinetic models for pH and OR% were estimated by nonlinear regression using Origin™ software. To evaluate the quality of fit, the adjusted R-square, the standard error, the p-value test (p-value < 0.05 was considered statistically significant) and the F-test were used. All R-square were higher than 0.93.
3.2. Variation of oil concentration with time Typical results for variation of OR% with time are represented in Fig. 5. All results could be adjusted, by non-linear fit, to an exponential model, with the form of Eq. (4):
OR% = A (1
3.1. Variation of pH with time Typical results for pH variation with time are represented in Fig. 4. The data can be fit by an exponential model (Eq. (3)) as described in a previous work [17]:
Be
kpH t
kOR t )
(4)
In this model, kOR (min−1) is the reaction rate of oil with aluminum hydroxide [17] and gives a measurement of reaction speed. The other fitting parameter, A (%) is related to the amount of oil that might be removed in an infinite time. The fit parameters A and kOR are given in Table 4 together with the final (t =30 min) percentage of oil removal (ORf). For all experiments, the adjusted R-square of the exponential fit was higher than 0.93. The exponential fit (Eq. (4)) is a characteristic equation for change of concentration due to a pseudo-first-order chemical reaction. This model is also used in the literature to describe EC, for the treatment of water contaminated with synthetic effluents of acid blue 113 dye [23] arsenic [24], phenol [25] and basic dye rhodamine [26]. The pseudofirst-order behavior can be explained if the amount of aluminum hydroxide is constant with time.
3. Results and discussion
pH = Y 0
e
(3)
where: Y0 is the adjusted value for the final pH, B is a constant that subtracted from Y0 gives the estimated initial value, kpH (min−1) is a reaction rate constant, a measurement of the number of dissolved ions responsible for the rise in the pH value [17], t (min) is the time since the beginning of the EC. The variation of pH with time for the other experiments also can be adjusted by Eq. (3), with the fit parameters shown in Table 3. Measured pH value at the end of the EC process (pHf) is also displayed in the table. High kpH does not mean maximum pHf (Fig. 4), but a fast convergence to a final value as happened in experiment 8 where pH stabilized after 10 min of treatment. It is usual for pH to rise during the EC process. One known reason for this behavior is the formation of an excess of OH− on the cathode [22]. Excess meaning OH− that does not combine with the Al+3 released by the anode to form the aluminum
3.3. Electrical conductivity The electrical conductivity decreased by 0.9 ± 0.3 mScm−1 in all experiments independent of the process variables (including the presence of holes in electrodes). The average value of the conductivity calculated for all experiments was 14.02 ± 0.07 mScm−1. Electrical conductivity is related to the salinity of the medium, and the same amount of NaCl was used in all experiments. The slight tendency of conductivity decrease may indicate a small reduction in salt concentration with time. 3.4. Electrodes loss of mass Percent loss of mass by the electrodes during the process was within the range of 3.4 ± 0.8% for all experiments. These results agree with Faraday`s equation, where the ratio of mass released to the fluid is dependent on the electrical current modulus. Loss of mass is a critical component of the cost associated with EC, and it can be altered by other variables besides electrical current, including DE [27] or possibly N. If it had increased with N, drilling holes would not be an economically viable way of raising efficacy. Despite the similar total loss of mass, the corrosion of the electrodes took a different form for each experimental set. Optical micrography with 1000x magnification shows that the diameter of the corrosion pits (black areas in Fig. 6) and the apparent roughness of the plate increase when 10 holes are used, with Q = 2 mLs−1 (Fig. 6A and B). The opposite happens when Q and DE are higher, and the electrode is perforated by 10 holes, there is a visible reduction in corrosion pit diameter and in roughness (Fig. 6C and D). The surface texture of the aluminum electrode is determinant in the production of large bubbles in EC [28]. Rough surfaces may make it more difficult for EC to generate small bubbles that tend to increase separation efficiency. However, independent on the observed variations in roughness or pit diameter, ORf increased when holes were drilled if the other variables were kept constant.
Fig. 4. pH as a function of time. The lines represent the exponential fit as described in the text. ■ = experiment 3, = experiment 6, experiment 8. 117
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Table 3 Fit parameters that describe the variation of pH with time. Y0
B
kpH
Exp.
Value
Standard Error
Value
Standard Error
Value
Standard Error
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
9.5917 9.5908 9.0477 8.5516 9.5982 9.6142 8.8992 8.5598 9.3733 9.4109 9.2053 8.5723 8.7326 9.2761 9.1833 9.2851 9.3683 9.4540 9.0071
0.1708 0.1530 0.0672 0.0365 0.1472 0.1392 0.0590 0.0526 0.1010 0.1133 0.0412 0.0288 0.0761 0.1303 0.0456 0.0214 0.0648 0.0518 0.1226
1.7529 1.8942 1.1815 0.8186 1.8518 2.1959 1.1872 1.1360 2.4986 1.9637 2.2284 1.6969 1.3046 2.2802 2.1786 2.2637 2.0618 1.8135 1.1657
0.2180 0.2122 0.0743 0.0606 0.1980 0.2063 0.0754 0.0931 0.1517 0.1514 0.0665 0.0439 0.1283 0.2081 0.0750 0.0328 0.0885 0.0800 0.1389
0.1302 0.1503 0.1034 0.2304 0.1423 0.1711 0.1308 0.3000 0.1765 0.1406 0.2117 0.1827 0.2443 0.2055 0.2243 0.1847 0.1460 0.1876 0.1076
0.0414 0.0422 0.0172 0.0443 0.0384 0.0401 0.0212 0.0721 0.0267 0.0274 0.0161 0.0118 0.0637 0.0475 0.0199 0.0067 0.0158 0.0207 0.0337
Adjusted R-Square
pHf
0.9452 0.9537 0.9875 0.9790 0.9582 0.9667 0.9854 0.9743 0.9859 0.9781 0.9965 0.9974 0.9631 0.9682 0.9954 0.9992 0.9931 0.9925 0.9545
9.69 9.71 9.03 8.60 9.70 9.75 8.92 8.63 9.47 9.48 9.23 8.57 8.83 9.43 9.24 9.30 9.40 9.50 9.03
Table 4 Parameters that describe the variation of oil removal with time. A
Fig. 5. OR% as a function of time. The lines represent the exponential fit as experiment 8. described in the text. ■ = experiment 3, = experiment 6,
3.5. Statistical analysis
kOR
Exp.
Value
Standard Error
Value
Standard Error
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
109.1354 96.3177 83.2771 83.4168 103.7627 106.7409 89.5475 92.8345 92.6149 92.9350 91.5662 82.7170 92.2164 92.9200 96.2243 95.4284 96.0944 106.8671 88.5883
20.4490 13.9830 3.0336 2.5992 13.6654 16.5129 8.1523 7.7904 1.6914 3.0087 1.5952 2.7875 1.8067 7.5894 4.4661 5.0410 4.9490 19.8072 8.0054
0.0625 0.0925 0.2009 0.3012 0.0757 0.0697 0.1312 0.1284 0.1177 0.1130 0.1161 0.3077 0.1136 0.1282 0.0975 0.0996 0.0972 0.0645 0.1324
0.0257 0.0378 0.0292 0.0489 0.0246 0.0253 0.0385 0.0343 0.0067 0.0112 0.0063 0.0550 0.0068 0.0333 0.0129 0.0152 0.0142 0.0267 0.0388
Adjusted RSquare
ORf (%)
0.9619 0.9352 0.9872 0.9883 0.9680 0.9646 0.9529 0.9614 0.9983 0.9951 0.9985 0.9862 0.9982 0.9635 0.9926 0.9899 0.9909 0.9593 0.9529
89.3 89.5 86.5 87.3 90.5 91.4 88.5 89.6 90.1 90.1 90.1 87.0 90.1 89.7 90.7 90.2 90.2 88.6 87.9
equal to 2 mL.s−1 and 6 mL.s−1 can be understood from Fig. 7. For Q equal to 4 mL.s−1 the value of ORf was practically constant at 90.2 ± 0.3%, independent of N x DE. For Q of 2 mL.s−1, ORf has a linear relationship with N x DE (Fig. 7). While for Q of 6 mL.s−1 a linear fit of ORf with N x DE is possible, with an exception for the non-perforated electrode. Nonetheless, Fig. 7 shows a tendency of growth in ORf if holes are made in non-perforated electrodes. For any value of Q, if DE is kept constant and N is increased, there is a tendency of growth in ORf, but if DE is reduced and N increased, ORf will be reduced. The ideal behavior to improve process efficacy is to compensate the turbulence created by drilling holes in the electrode by increasing DE. The question posed in this work is answered, if increasing current density were enough to improve efficacy, N would interfere in ORf independent of DE. Since the improvement depends on a combination of the two variables, it must be related to the passage of fluid through the holes. Power consumption (related to electrical current modulus) and
Spearman's correlation coefficients between the variables have been given in Table 5. N has a significant positive correlation with kpH (Table 5). Since kpH may be associated with the excess of OH− [22,17], drilling holes may be helping in the production of this ion. Possible reasons are the increase in electrical current density and the flux of gas through the holes. Q has a significant negative correlation with pHf and ORf, as expected. With higher Q, the molecules of the fluid have less time to interact with the aluminum, resulting in smaller total variation in pH. The positive correlation with kOR shows that the faster removal of ions of the electrode surfaces with higher Q leads to an increase in corrosion, as was observed by Krystynik and Tito [29]. DE does not have a significant correlation with any other variable, and this is unusual in EC; thus quadratic correlations were searched. Spearman`s correlations were calculated with Q2, N2, DE2, N x Q, N x DE, Q x DE and Q x N x DE as variables. The only combination that resulted in correlations not seen for the component variables was between N x DE and ORf. The change in ORf when N x DE increases for Q 118
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Fig. 6. Optical micrographs of the electrode plates after EC. A – Experiment 18 (N = 0, DE = 10, Q = 2), B- Experiment 2 (N = 10, DE = 10, Q = 2), C- Experiment 19 (N = 0, DE = 20, Q = 6), D - Experiment 8 (N = 10, DE = 20, Q = 6).
electrode replacement (related to mass loss) are the main responsible for costs in EC and were not altered by drilling holes in the electrodes. On the other hand, when electrodes are perforated, pHf is reduced, and ORf increased. The data for the two linear fits shown in Fig. 7 are in Table 6. If the linear behavior from Fig. 7 is kept for higher values of DE, the two lines, due to Q = 2 and Q = 6 mL.s−1 will meet when DE is 40 mm, and N is 10. By this theoretical model, ORf will then be close to 94%. For a good design, the values for Q, N, and DE must correspond to the higher values of ORf, the higher kOR and to pHf closer to neutral. By Brazilian law, the maximum concentration of oily effluents based on mineral oil that can be discharged in the environment is 20 mg.L−1, and the pH range must be between 5 and 9 [30]. The higher value of ORf obtained with pHf smaller than 9 was 90.1% in Experiment 13 (Q =4 mL.s-1, DE =7.95 mm, N = 10). This experiment generates an effluent compatible with the limits imposed by the legislation. 4. Conclusions Effluents containing metalworking fluid can be treated by electrocoagulation in a continuous flow, with more than 90% of removal efficiency and a final pH within the range allowed by legislation. Drilling
Fig. 7. Evolution of ORf with N x DE for Q = 2 mL.s−1 ( ) and 6 mL.s−1 ( ). The label 10 means DE = 10 mm and 20 means DE = 20 mm.
Table 5 Spearman’s correlation data. ORf
N Q DE N x DE
kOR
pHf
kpH
Loss of Mass
Sc
p-value
Sc
p-value
Sc
p-value
Sc
p-value
Sc
p-value
0.3652 −0.5640 0.2530 0.5059
0.1242 0.0119 0.2960 0.0271
0.0822 0.9441 0.1485 0.0428
0.7379 < 0.0001 0.5440 0.8621
−0.1855 −0.9082 0.0550 −0.0116
0.4470 < 0.0001 0.8230 0.9625
0.6213 0.0660 −0.0880 0.3776
0.0045 0.7884 0.7202 0.1110
0.1921 −0.2723 −0.0715 0.2414
0.4307 0.2595 0.7712 0.3195
Where: Sc means Spearman coefficient, N is the number of holes, Q is flow rate, DE distance between electrodes. A two-tailed test of significance was used, bold values indicate significant correlations (p-value < 0.05). 119
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Table 6 Linear fit of ORf as a function of N x DE. Q (mL.s−1)
Intercept Value
Standard Error
Value
Standard Error
2 6
85.3176 88.4667
0.5591 0.2310
0.0224 0.0145
0.0043 0.0020
Slope
[9] N.K. Khosla, S. Venkatachalam, P. Somasundaran, Pulsed electrogeneration of bubbles for electroflotation, J. Appl. Electrochem. 21 (1991) 986–990, https://doi. org/10.1007/BF01077584. [10] M.M. Emamjomeh, M. Sivakumar, Review of pollutants removed by electrocoagulation and electrocoagulation/flotation processes, J. Environ. Manage. 90 (2009) 1663–1679, https://doi.org/10.1016/J.JENVMAN.2008.12.011. [11] N. Boudjema, N. Drouiche, N. Abdi, H. Grib, H. Lounici, A. Pauss, N. Mameri, Treatment of Oued El Harrach river water by electrocoagulation noting the effect of the electric field on microorganisms, J. Taiwan Inst. Chem. Eng. 45 (2014) 1564–1570, https://doi.org/10.1016/J.JTICE.2013.10.006. [12] T. Ouslimane, S. Aoudj, M. Amara, N. Drouiche, Removal of copper and fluoride from mixed Cu-CMP and fluoride-bearing wastewaters by electrocoagulation, Int. J. Environ. Res. 11 (2017) 677–684, https://doi.org/10.1007/s41742-017-0058-7. [13] D.N. Tito, P. Krystynik, P. Kluson, Notes on process and data analysis in electrocoagulation—the importance of standardization and clarity, Chem. Eng. Process. Process Intensif. 104 (2016) 22–28, https://doi.org/10.1016/J.CEP.2016.02.011. [14] V. Khandegar, A.K. Saroha, Electrocoagulation for the treatment of textile industry effluent - a review, J. Environ. Manage. 128 (2013) 949–963, https://doi.org/10. 1016/j.jenvman.2013.06.043. [15] V. Khandegar, A.K. Saroha, Effect of electrode geometry on the performance of electrocoagulation, Int. J. Adv. Res. Sci. Eng. 5 (2016) 379–376. [16] F. Hussin, F. Abnisa, G. Issabayeva, M.K. Aroua, Removal of lead by solar-photovoltaic electrocoagulation using novel perforated zinc electrode, J. Clean. Prod. 147 (2017) 206–216, https://doi.org/10.1016/J.JCLEPRO.2017.01.096. [17] L.C.A. Gobbi, I.L. Nascimento, E.P. Muniz, S.M.S. Rocha, P.S.S. Porto, Electrocoagulation with polarity switch for fast oil removal from oil in water emulsions, J. Environ. Manage. 213 (2018) 119–125, https://doi.org/10.1016/J. JENVMAN.2018.01.069. [18] H.K. Hansen, P. Nuñez, D. Raboy, I. Schippacasse, R. Grandon, Electrocoagulation in wastewater containing arsenic: Comparing different process designs, Electrochim. Acta 52 (2007) 3464–3470, https://doi.org/10.1016/j.electacta.2006. 01.090. [19] Z.B. Gönder, G. Balcıoğlu, I. Vergili, Y. Kaya, Electrochemical treatment of carwash wastewater using Fe and Al electrode: techno-economic analysis and sludge characterization, J. Environ. Manage. 200 (2017) 380–390, https://doi.org/10.1016/J. JENVMAN.2017.06.005. [20] APHA, Standard Methods For Examination of Water And Wastewater. American Public Health and Association/American Water Works 1 Association/Water Environmental Federation, Washington, DC, 2005. [21] J. Hauke, T. Kossowski, Comparison of values of Pearson’s and Spearman’s correlation coefficients on the same sets of data, Quaest. Geogr. 30 (2011) 87–93, https://doi.org/10.2478/v10117-011-0021-1. [22] T. Harif, M. Khai, A. Adin, Electrocoagulation versus chemical coagulation: coagulation/flocculation mechanisms and resulting floc characteristics, Water Res. 46 (2012) 3177–3188, https://doi.org/10.1016/j.watres.2012.03.034. [23] N. Mohan, N. Balasubramanian, V. Subramanian, Electrochemical treatment of simulated textile effluent, Chem. Eng. Technol. Ind. Chem. Equip. Process Eng. 24 (2001) 749–753. [24] N. Balasubramanian, T. Kojima, C. Srinivasakannan, Arsenic removal through electrocoagulation: kinetic and statistical modeling, Chem. Eng. J. 155 (2009) 76–82, https://doi.org/10.1016/j.cej.2009.06.038. [25] Y. Yavuz, A.S. Koparal, Ü.B. Öğütveren, Treatment of petroleum refinery wastewater by electrochemical methods, Desalination 258 (2010) 201–205, https://doi. org/10.1016/j.desal.2010.03.013. [26] A.I. Adeogun, R.B. Balakrishnan, Kinetics, isothermal and thermodynamics studies of electrocoagulation removal of basic dye rhodamine B from aqueous solution using steel electrodes, Appl. Water Sci. 7 (2017) 1711–1723, https://doi.org/10. 1007/s13201-015-0337-4. [27] M. Changmai, M. Pasawan, M.K. Purkait, Treatment of oily wastewater from drilling site using electrocoagulation followed by microfiltration, Sep. Purif. Technol. 210 (2019) 463–472, https://doi.org/10.1016/J.SEPPUR.2018.08.007. [28] G. Chen, X. Chen, P.L. Yue, Electrocoagulation and electroflotation of restaurant wastewater, J. Environ. Eng. 126 (2000) 858–863, https://doi.org/10.1061/(ASCE) 0733-9372(2000)126:9(858). [29] P. Krystynik, D.N. Tito, Key process parameters affecting performance of electrocoagulation, Chem. Eng. Process. Process Intensif. 117 (2017) 106–112, https://doi. org/10.1016/j.cep.2017.03.022. [30] Brasil, Resolução CONAMA no 430, de 13 de maio de 2011, (2011) (Accessed 22 November 2017), www.mma.gov.br.
Adjusted Rsquare
0.9295 0.8978
holes in the electrodes led to an increase in the ratio of change of pH, resulting in faster convergence to its final value. Removal efficiency depends on a combination of the number of holes and distance between electrodes. If new holes are added and distance increased, efficiency improves. These facts indicate that current density is not the only factor acting when holes are drilled. There was no noticeable increase in mass loss of the electrodes due to the presence of holes, and there is no visual evidence of alteration in corrosion around the holes. No evidence of alterations in corrosion due to the non-uniform current density. There is nonetheless a change in the average size and distribution of the corrosion pits. Acknowledgments This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) Finance Code 001. The authors are also grateful for brazilian financial support: CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) and FAPES (Fundação de Amparo à Pesquisa e Inovação do Espírito Santo). References [1] M. Kobya, E. Demirbas, M. Bayramoglu, M.T. Sensoy, Optimization of electrocoagulation process for the treatment of metal cutting wastewaters with response surface methodology, Water Air Soil Pollut. 215 (2011) 399–410, https://doi.org/ 10.1007/s11270-010-0486-x. [2] E. Demirbas, M. Kobya, Operating cost and treatment of metalworking fluid wastewater by chemical coagulation and electrocoagulation processes, Process Saf. Environ. Prot. (2016), https://doi.org/10.1016/j.psep.2016.10.013. [3] K. Bensadok, S. Benammar, F. Lapicque, G. Nezzal, Electrocoagulation of cutting oil emulsions using aluminum plate electrodes, J. Hazard. Mater. 152 (2008) 423–430, https://doi.org/10.1016/j.jhazmat.2007.06.121. [4] P. Asaithambi, M. Susree, R. Saravanathamizhan, M. Matheswaran, Ozone assisted electrocoagulation for the treatment of distillery effluent, Desalination 297 (2012) 1–7, https://doi.org/10.1016/j.desal.2012.04.011. [5] A. Fernandes, M.J. Pacheco, L. Ciríaco, A. Lopes, Review on the electrochemical processes for the treatment of sanitary landfill leachates: present and future, Appl. Catal. B Environ. 176 (2015) 183–200, https://doi.org/10.1016/j.apcatb.2015.03. 052. [6] Y.O. Fouad, Separation of cottonseed oil from oil-water emulsions using electrocoagulation technique, Alexandria Eng. J. 53 (2014) 199–204, https://doi.org/10. 1016/j.aej.2013.10.005. [7] I.L. Mickova, Advanced electrochemical technologies in wastewater treatment. Part II: electro-flocculation and electro-flotation, Am. Sci. Res. J. Eng. Technol. Sci. 14 (2015) 273–294 (accessed March 23, 2018), http://asrjetsjournal.org/index.php/ American_Scientific_Journal/article/view/1017. [8] X. Chen, G. Chen, P.L. Yue, Separation of pollutants from restaurant wastewater by electrocoagulation, Sep. Purif. Technol. 19 (2000) 65–76, https://doi.org/10.1016/ S1383-5866(99)00072-6.
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Electrocoagulation using perforated electrodes: An increase in metalworking fluid removal from wastewater
T
Oriane Avancini Diasa, Eduardo Perini Muniza,b, , Paulo Sérgio da Silva Portoa,c ⁎
a
Universidade Federal do Espírito Santo, Programa de Pós-graduação em Energia, Rodovia Governador Mário Covas, km 60, Bairro Litorâneo, CEP 29932-540, São Mateus, ES, Brazil b Universidade Federal do Espírito Santo, Departamento de Ciências Naturais, Rodovia Governador Mário Covas, Bairro Litorâneo, CEP 29932-540, São Mateus, ES, Brazil c Universidade Federal do Espírito Santo, Departamento de Engenharias e Tecnologia, Rodovia Governador Mário Covas, km 60, Bairro Litorâneo, CEP 29932-540, São Mateus, ES, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Effluent remediation Electroflotation Metalworking fluid Perforated electrodes Flow rate
A continuous electrocoagulation reactor with electrode polarity switch was used for removal of a metalworking fluid from synthetic oily water. The effects of perforating the aluminum electrodes, changing the number of holes, flow rate and the distance between electrodes were studied. Higher values of flow rate reduced final pH. Perforating the electrodes led to faster convergence of pH to its maximum value with no measurable increase in mass loss. If the distance between electrodes is also increased, there is an improvement in efficiency. With 10 holes, adjusting flow rate and inter-electrode distance, 90.2 ± 0.3% oil removal was achieved with a final pH of 8.83, which is within limits allowed by legislation.
1. Introduction
electrodes help in the separation process [6]. The reactors used in EC are usually small with a high throughput [7]. Chen and coworkers [8] for instance built a 0.30 L reactor to remove oil and grease from restaurant wastewaters and achieved more than 95% of removal efficiency with a flow rate of 9 Lh−1 (2.5 mLs−1). The critical components in EC reactors are coagulants, contaminants and generated bubbles [7]. Material and geometry of the electrode are crucial in the EC reactor project since they alter the coagulants, size and amount of bubbles produced [9]. There is a significant discussion on the appropriate electrode material for each contaminant. For oily wastewater aluminum (Al) electrodes tend to result in a better treatment performance than iron electrodes [10]. Increasing electrical current leads to a high aluminum dissolution rate [11,12], thus higher rate of coagulant production. Tito and coworkers [13] state that current intensity is the critical performance-limiting parameter in EC while current density is secondary. However, the authors that obtained an increase in efficiency by punching holes in the electrodes often justified their results with arguments based on current density. According to Khandengar and Saroha [14], electrodes with punched holes are expected to result in higher removal efficiency compared to
The heavy manufacturing industry uses metal cutting fluids (MCF) to reduce heat during the processing of metals, to lubricate between workpiece and tool [1] and to carry the shards that must be removed to improve finishing. These metalworking fluids contain emulsified oil and surfactants which allow for the formation of a stable emulsion when mixed with water [2] and thus reduce corrosion and bacterial growth [3]. MCF suffer thermal degradation and accumulate impurities with use and thus need to be replaced periodically. The organic waste generated must be taken away and treated. These effluents contain surface-active agents and other organic matters; they also tend to penetrate in the soil being a danger to the purity of groundwater [3]. Electrocoagulation (EC), is a technique often used to remediate oily wastewater. It combines coagulation, flocculation, and flotation; the electrical current that goes through sacrificial electrodes releases positive metal ions in the solution, also forming negative hydroxyl ions and hydrogen gas [4]. The metal ions combine with hydroxyl forming flocculants that may react with the contaminants in the suspension [5]. EC is also called electroflotation since the air bubbles generated in the
Abbreviations: EC, electrocoagulation; DE, distance between electrodes; N, number of holes drilled in the electrodes; Sc, spearman correlation; Q, flux rate; ORf, percent of metal cutting fluid removed from the water after 30 min of EC; OR%, percent of metal cutting fluid removed from the water ⁎ Corresponding author at: Universidade Federal do Espírito Santo, Programa de Pós-graduação em Energia, Rodovia Governador Mário Covas, km 60, Bairro Litorâneo, CEP 29932-540, São Mateus, ES, Brazil. E-mail address: [email protected] (E. Perini Muniz). https://doi.org/10.1016/j.cep.2019.03.021 Received 29 November 2018; Received in revised form 27 March 2019; Accepted 31 March 2019 Available online 01 April 2019 0255-2701/ © 2019 Elsevier B.V. All rights reserved.
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plane electrodes. They studied the effect of the number of holes in aluminum electrodes in the color removal efficiency to remove acid 131 dye from distilled water [15]. Efficiency increases with the number of holes from one to four and is not altered from four to eight holes. There is no apparent change in efficiency with hole distribution. Removal efficiency increases with hole diameter from 2 to 4 mm but not from 4 to 5 mm. The results were attributed to the higher current discharge from the punched electrode. Hussin and co-workers [16] propose the use of perforated zinc electrodes for lead (Pb) separation from water. Removal efficiency was higher for perforated electrodes with a hole diameter of 0.5 cm than for those with a hole diameter of 0.2 cm, and both were more efficient than the plane, non-perforated electrodes. The presence and size of the holes altered the electrical current between the electrodes and the authors associated the higher efficiency with higher current density. In the present work, electrical current was kept constant while the distance between electrodes, the number of holes drilled on them and flow rate were varied. The objective was to verify if other factors besides current intensity or current density interfere in the change of efficacy in oil removal (efficacy is defined as the amount of contaminant removed expressed in % [13]) and in the final pH when electrodes are perforated.
perforated and perforated plates, of same material and dimensions, to evaluate the effect of the presence of holes when the electrical current modulus is kept constant. The position of the drilled holes in the perforated plates is represented in Fig. 2. Holes with 6 mm diameter were drilled in symmetrical patterns around the center of the plates, to guarantee a uniform flux of oily water, oxygen and hydrogen bubbles. The central hole and the hole at the top of the upper corner were filled, used to realign the connection between the plates with the help of a threaded rod. A schematic drawing detailing the shape, layout, and assembly of the electrode plates is shown in Fig. 3. Two sets of three plates were aligned in the form of a beehive, each set interspersed and arranged in parallel (monopolar mode), the plates being separated by screws covered with a polymer for electrical insulation. Distances between electrodes (DE) were set as demanded by experimental design (next section). From the literature, there is a tendency of improvement in the process of separation of oil from water when DE is increased from 5 to 10 mm in a reactor with polarity switch [17], justifying the use of 10 mm as a minimum value in the initial experimental plan.
2. Methodology
The transport of emulsified water from the storage tank to the reactor occurs by pumping. The inlet and the exit of the effluent from the rectangular reactor are 2.0 and 14.0 cm above the bottom, respectively. When the effluent reached the exit height, a useful volume of 3 L inside the reactor was obtained. At that time the flow rate was adjusted using a test tube, i.e., the volume of effluent collected in one minute was measured. After an amount of water equal to the useful volume of the reactor passed through it, the system was considered to be in a steady state. For each experiment, an aliquot was collected before turning on the power supply (zero time). Then the power was turned on, and the chronometer started. Each 5, 10, 20, and 30 min of the process, a sample of 350 mL was collected, stored in an amber bottle and maintained in a refrigerator at 4 °C for subsequent analysis. The values used for flow rate (Q), the number of holes (N) and the distance between electrodes (DE) were as shown in Table 2. To determine the experimental variables, initially a 23 factorial experimental design was used (experiments 1–8), then a central point was added in triplicate (experiments 9, 10 and 11). After the initial analysis, since it showed some improvement in efficacy with increasing N, it was decided to do a second set of experiments with maximum N (fixed value), using a central composite design with Q and DE as variables (experiments 12,13 and 14). By design one of the values tried for Q should be 1.18 mL.min−1 but this value was discarded since it is too low to be of practical use. The central point of this new set of experiments was made in triplicate (experiments 15, 16 and 17). Once all was finished, the system with nonperforated electrodes had to be evaluated as a reference (experiments 18 and 19). The electrical current tends to pass only in the metallic parts of the electrodes. If holes are drilled the area of the plates will be reduced and current density increased. The current density is estimated to be 493.83 A. m−2 for a flat plate without the presence of holes. It grows to 508.01 and 515.42 A. m−2 for 6 and 10 holes, respectively.
2.3. EC procedure
2.1. Preparation of synthetic cutting oil effluent To prepare 1 L of synthetic effluent, 0.2000 g of Exxon Mobil MOBILCUT 102 soluble cutting fluid (density = 0.89 at 15 °C and viscosity =35 cSt at 40 °C) and 10.0 g of sodium chloride were dissolved in distilled water. Sodium chloride was added to guarantee a higher and near-constant electrical conductivity, independent of the amount of aluminum dissolved [17]. This higher current with lower voltage drop tends to decrease power consumption [18]. The solution was stabilized using a rotor-stator mixing device where three blades rotated at 3000 rpm for 15 min, shearing the larger oil surfaces. Preliminary tests indicated that oil content was stable after 10 h from preparation. The solution was then kept at rest and watched for 24 h to check for stability before EC was performed. 2.2. Electrocoagulation system As described in a previous work [17], a monopolar arrangement of three pairs of aluminum electrodes, polarity switch to preserve the electrodes and increase efficiency was used. Polarity reversal turns the cathode into anode and vice versa, which contributes to reducing the passivation effect on the cathode and increases the lifespan of the electrode [17]. While the modulus of the electrical current was kept constant at 4 A, polarity was changed each 30 s forming an input sign step function (polarity reversal). Voltage modulus oscillated around 12 ± 3 V due to electrical resistance changes created by electrode passivation during the process. The voltage switch time was adjusted for electrical stability, resulting in a higher value than the 10 s found in the literature [17,19]. The continuous flow EC pilot unit consisted of a rectangular electrolytic cell, a set of aluminum electrodes, and an electronic device (plate) to switch the polarity of the current from a DC power source (Fig. 1). The monopolar mode electrodes were connected to the polarity switch and then to the DC power supply brand MINIPA model MPC3005. The electrodes are aluminum plates with the following dimensions: 130 mm height, 65 mm width and 0.50 mm thick, located vertically in the central part of the interior of the plexiglass reactor, whose electrical wiring passes through holes located in the lid (Fig. 1). The dimensions of the feed tank, reactor and receiver tank are shown in Table 1. Tests were performed by measuring MCF removal efficacy with non-
2.4. Analytical methodology The characterization of the samples collected was carried out according to methodologies adapted from the Standard Methods for the Examination of Water and Wastewater [20]. 2.5. Determination of pH The pH of all samples was measured at 0, 5, 10, 20 and 30 min of 114
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Fig. 1. Schematic design of continuous flow electrolytic system. Where: (a) current power supply; (b) Polarity inverter; (c) Feed tank for raw fluent storage; (d) submersible pump; (e) flow regulating valve; (f) electrode; (g) reactor input; (h) reactor output and (i) receiver tank for storing effluent after treatment.
procedure were based on Method 4500 from the Standard Methods for the Examination of Water and Wastewater [20].
Table 1 Dimensions of the EC apparatus. Components of the system
Dimensions (cm)
Effective Volume (mL)
Feed tank EC reactor (retangular) Receiver tank
33.0 × 22.0 × 22.0 30.2 × 15.2 × 15.0 25.2 × 18.0 × 18.0
15000 3000 8000
2.6. Determination of electrical conductivity Electrical conductivity was obtained with the aid of the conductivity meter BEL Engineering W12D, resolution of 0.1 μS. cm−1 and accuracy of ± 1%. For each test the conductivity was verified, using a standard NaCl solution of 1000 μS. cm−1. The calibration of the conductometer and the measurement procedure were based on Method 2510 from the Standard Methods for the Examination of Water and Wastewater [20].
electrocoagulation using a pH meter, Mark MS TECNOPON mPA210 with automatic temperature compensation, resolution 0.01 and accuracy ± 0.01%. The calibration of the pH meter and the measurement
Fig. 2. Aluminum plates used to compose the electrode: (a) perforated plates with 6 holes (b) perforated plates with 8 holes (c) plates with 10 holes. 115
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Fig. 3. Representation of the electrode assembly showing the mechanical connections between plates.
function. From an analysis of the UV–vis spectra, the wavelength of 263 nm was ideal; a calibration curve was plotted and fit by a linear model (adjusted R-square of 0.9996), Eq. (1):
Table 2 Operational conditions used for the experiments. Experiment
N
Q (mL.s−1)
DE (mm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
6 10 6 10 6 10 6 10 8 8 8 10 10 10 10 10 10 0 0
2 2 6 6 2 2 6 6 4 4 4 6.8 4 4 4 4 4 2 6
10 10 10 10 20 20 20 20 15 15 15 15 7.95 22.05 15 15 15 10 20
OC (gL 1) = (336 ± 2)(gL 1)* Absorbance
(1)
Where OC is oil concentration. Oil removal efficacy of the process was evaluated by percent of oil removal (OR%) as calculated using Eq. (2):
OR% =
OC0
OCf
OCf
× 100
(2)
where: OC0 is the initial oil concentration (time zero), OCf is the oil concentration at the end of the process (30 min). 2.8. Determination of electrode consumption The mass of the electrodes was measured before and after each experiment to estimate the mass loss occurring during the process (gravimetric measurement). Electrodes were washed and dried before mass measurement to avoid the influence of eventual flakes or other impurities attached during the EC process. The measurements were carried out in an analytical balance Bioprecise brand FA-2104 N with a precision of 0.1 mg.
2.7. Determination of the content of oils and greases A spectral method was used, based on method 5520 from the Standard Methods for the Examination of Water and Wastewater [20]. The oil was extracted from 50 mL samples using hexane as a solvent. Then the UV–vis spectrum was measured, and the absorbance at a given wavelength was used in a calibration curve to obtain oil concentration. The spectrometer used was a QUIMIS 4802. Spectra were measured from 190 to 700 nm with a precision of 1 nm. The value of the wavelength was chosen as the one where the relation between concentration and absorbance was closer to a linear
2.9. Microscopy The surface of the electrodes was observed with an optical USB linked Microscope with magnification up to 1,000X, connected to a laptop. The microscope lens was placed about 10 mm above, perpendicular to the plate and with the same illumination for all samples. 116
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2.10. Statistical analysis
hydroxides that are responsible for oil removal. The relation between kpH and the input variables will be discussed ahead, using Spearman correlation. For all experiments, adjusted Rsquare is higher than 0.94, with pHf in the range from 8.60 to 9.75. As listed in the methodology, the p-value test was considered statistically significant for p-value < 0.05.
To look for monotonic relations between input and output variables Spearman’s coefficient was calculated for the raw data using Origin™ software. Spearman’s is a non-parametric rank statistic that can be used as a measure of the strength of an association between two variables [21] even when one of these variables is noncontinuous as is the case for the number of holes drilled in a plate. The p-value test (p-value < 0.05 was considered statistically significant) was used to verify if a Spearman correlation was significant. Only correlations that passed this test are discussed in this work. Since Spearman`s method finds only monotonic correlations, Q2, N2, DE2, N x Q, N x DE, Q x DE and Q x N x DE were used as variables to search for more complex relationships between input and output data. The parameters of the kinetic models for pH and OR% were estimated by nonlinear regression using Origin™ software. To evaluate the quality of fit, the adjusted R-square, the standard error, the p-value test (p-value < 0.05 was considered statistically significant) and the F-test were used. All R-square were higher than 0.93.
3.2. Variation of oil concentration with time Typical results for variation of OR% with time are represented in Fig. 5. All results could be adjusted, by non-linear fit, to an exponential model, with the form of Eq. (4):
OR% = A (1
3.1. Variation of pH with time Typical results for pH variation with time are represented in Fig. 4. The data can be fit by an exponential model (Eq. (3)) as described in a previous work [17]:
Be
kpH t
kOR t )
(4)
In this model, kOR (min−1) is the reaction rate of oil with aluminum hydroxide [17] and gives a measurement of reaction speed. The other fitting parameter, A (%) is related to the amount of oil that might be removed in an infinite time. The fit parameters A and kOR are given in Table 4 together with the final (t =30 min) percentage of oil removal (ORf). For all experiments, the adjusted R-square of the exponential fit was higher than 0.93. The exponential fit (Eq. (4)) is a characteristic equation for change of concentration due to a pseudo-first-order chemical reaction. This model is also used in the literature to describe EC, for the treatment of water contaminated with synthetic effluents of acid blue 113 dye [23] arsenic [24], phenol [25] and basic dye rhodamine [26]. The pseudofirst-order behavior can be explained if the amount of aluminum hydroxide is constant with time.
3. Results and discussion
pH = Y 0
e
(3)
where: Y0 is the adjusted value for the final pH, B is a constant that subtracted from Y0 gives the estimated initial value, kpH (min−1) is a reaction rate constant, a measurement of the number of dissolved ions responsible for the rise in the pH value [17], t (min) is the time since the beginning of the EC. The variation of pH with time for the other experiments also can be adjusted by Eq. (3), with the fit parameters shown in Table 3. Measured pH value at the end of the EC process (pHf) is also displayed in the table. High kpH does not mean maximum pHf (Fig. 4), but a fast convergence to a final value as happened in experiment 8 where pH stabilized after 10 min of treatment. It is usual for pH to rise during the EC process. One known reason for this behavior is the formation of an excess of OH− on the cathode [22]. Excess meaning OH− that does not combine with the Al+3 released by the anode to form the aluminum
3.3. Electrical conductivity The electrical conductivity decreased by 0.9 ± 0.3 mScm−1 in all experiments independent of the process variables (including the presence of holes in electrodes). The average value of the conductivity calculated for all experiments was 14.02 ± 0.07 mScm−1. Electrical conductivity is related to the salinity of the medium, and the same amount of NaCl was used in all experiments. The slight tendency of conductivity decrease may indicate a small reduction in salt concentration with time. 3.4. Electrodes loss of mass Percent loss of mass by the electrodes during the process was within the range of 3.4 ± 0.8% for all experiments. These results agree with Faraday`s equation, where the ratio of mass released to the fluid is dependent on the electrical current modulus. Loss of mass is a critical component of the cost associated with EC, and it can be altered by other variables besides electrical current, including DE [27] or possibly N. If it had increased with N, drilling holes would not be an economically viable way of raising efficacy. Despite the similar total loss of mass, the corrosion of the electrodes took a different form for each experimental set. Optical micrography with 1000x magnification shows that the diameter of the corrosion pits (black areas in Fig. 6) and the apparent roughness of the plate increase when 10 holes are used, with Q = 2 mLs−1 (Fig. 6A and B). The opposite happens when Q and DE are higher, and the electrode is perforated by 10 holes, there is a visible reduction in corrosion pit diameter and in roughness (Fig. 6C and D). The surface texture of the aluminum electrode is determinant in the production of large bubbles in EC [28]. Rough surfaces may make it more difficult for EC to generate small bubbles that tend to increase separation efficiency. However, independent on the observed variations in roughness or pit diameter, ORf increased when holes were drilled if the other variables were kept constant.
Fig. 4. pH as a function of time. The lines represent the exponential fit as described in the text. ■ = experiment 3, = experiment 6, experiment 8. 117
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Table 3 Fit parameters that describe the variation of pH with time. Y0
B
kpH
Exp.
Value
Standard Error
Value
Standard Error
Value
Standard Error
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
9.5917 9.5908 9.0477 8.5516 9.5982 9.6142 8.8992 8.5598 9.3733 9.4109 9.2053 8.5723 8.7326 9.2761 9.1833 9.2851 9.3683 9.4540 9.0071
0.1708 0.1530 0.0672 0.0365 0.1472 0.1392 0.0590 0.0526 0.1010 0.1133 0.0412 0.0288 0.0761 0.1303 0.0456 0.0214 0.0648 0.0518 0.1226
1.7529 1.8942 1.1815 0.8186 1.8518 2.1959 1.1872 1.1360 2.4986 1.9637 2.2284 1.6969 1.3046 2.2802 2.1786 2.2637 2.0618 1.8135 1.1657
0.2180 0.2122 0.0743 0.0606 0.1980 0.2063 0.0754 0.0931 0.1517 0.1514 0.0665 0.0439 0.1283 0.2081 0.0750 0.0328 0.0885 0.0800 0.1389
0.1302 0.1503 0.1034 0.2304 0.1423 0.1711 0.1308 0.3000 0.1765 0.1406 0.2117 0.1827 0.2443 0.2055 0.2243 0.1847 0.1460 0.1876 0.1076
0.0414 0.0422 0.0172 0.0443 0.0384 0.0401 0.0212 0.0721 0.0267 0.0274 0.0161 0.0118 0.0637 0.0475 0.0199 0.0067 0.0158 0.0207 0.0337
Adjusted R-Square
pHf
0.9452 0.9537 0.9875 0.9790 0.9582 0.9667 0.9854 0.9743 0.9859 0.9781 0.9965 0.9974 0.9631 0.9682 0.9954 0.9992 0.9931 0.9925 0.9545
9.69 9.71 9.03 8.60 9.70 9.75 8.92 8.63 9.47 9.48 9.23 8.57 8.83 9.43 9.24 9.30 9.40 9.50 9.03
Table 4 Parameters that describe the variation of oil removal with time. A
Fig. 5. OR% as a function of time. The lines represent the exponential fit as experiment 8. described in the text. ■ = experiment 3, = experiment 6,
3.5. Statistical analysis
kOR
Exp.
Value
Standard Error
Value
Standard Error
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
109.1354 96.3177 83.2771 83.4168 103.7627 106.7409 89.5475 92.8345 92.6149 92.9350 91.5662 82.7170 92.2164 92.9200 96.2243 95.4284 96.0944 106.8671 88.5883
20.4490 13.9830 3.0336 2.5992 13.6654 16.5129 8.1523 7.7904 1.6914 3.0087 1.5952 2.7875 1.8067 7.5894 4.4661 5.0410 4.9490 19.8072 8.0054
0.0625 0.0925 0.2009 0.3012 0.0757 0.0697 0.1312 0.1284 0.1177 0.1130 0.1161 0.3077 0.1136 0.1282 0.0975 0.0996 0.0972 0.0645 0.1324
0.0257 0.0378 0.0292 0.0489 0.0246 0.0253 0.0385 0.0343 0.0067 0.0112 0.0063 0.0550 0.0068 0.0333 0.0129 0.0152 0.0142 0.0267 0.0388
Adjusted RSquare
ORf (%)
0.9619 0.9352 0.9872 0.9883 0.9680 0.9646 0.9529 0.9614 0.9983 0.9951 0.9985 0.9862 0.9982 0.9635 0.9926 0.9899 0.9909 0.9593 0.9529
89.3 89.5 86.5 87.3 90.5 91.4 88.5 89.6 90.1 90.1 90.1 87.0 90.1 89.7 90.7 90.2 90.2 88.6 87.9
equal to 2 mL.s−1 and 6 mL.s−1 can be understood from Fig. 7. For Q equal to 4 mL.s−1 the value of ORf was practically constant at 90.2 ± 0.3%, independent of N x DE. For Q of 2 mL.s−1, ORf has a linear relationship with N x DE (Fig. 7). While for Q of 6 mL.s−1 a linear fit of ORf with N x DE is possible, with an exception for the non-perforated electrode. Nonetheless, Fig. 7 shows a tendency of growth in ORf if holes are made in non-perforated electrodes. For any value of Q, if DE is kept constant and N is increased, there is a tendency of growth in ORf, but if DE is reduced and N increased, ORf will be reduced. The ideal behavior to improve process efficacy is to compensate the turbulence created by drilling holes in the electrode by increasing DE. The question posed in this work is answered, if increasing current density were enough to improve efficacy, N would interfere in ORf independent of DE. Since the improvement depends on a combination of the two variables, it must be related to the passage of fluid through the holes. Power consumption (related to electrical current modulus) and
Spearman's correlation coefficients between the variables have been given in Table 5. N has a significant positive correlation with kpH (Table 5). Since kpH may be associated with the excess of OH− [22,17], drilling holes may be helping in the production of this ion. Possible reasons are the increase in electrical current density and the flux of gas through the holes. Q has a significant negative correlation with pHf and ORf, as expected. With higher Q, the molecules of the fluid have less time to interact with the aluminum, resulting in smaller total variation in pH. The positive correlation with kOR shows that the faster removal of ions of the electrode surfaces with higher Q leads to an increase in corrosion, as was observed by Krystynik and Tito [29]. DE does not have a significant correlation with any other variable, and this is unusual in EC; thus quadratic correlations were searched. Spearman`s correlations were calculated with Q2, N2, DE2, N x Q, N x DE, Q x DE and Q x N x DE as variables. The only combination that resulted in correlations not seen for the component variables was between N x DE and ORf. The change in ORf when N x DE increases for Q 118
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Fig. 6. Optical micrographs of the electrode plates after EC. A – Experiment 18 (N = 0, DE = 10, Q = 2), B- Experiment 2 (N = 10, DE = 10, Q = 2), C- Experiment 19 (N = 0, DE = 20, Q = 6), D - Experiment 8 (N = 10, DE = 20, Q = 6).
electrode replacement (related to mass loss) are the main responsible for costs in EC and were not altered by drilling holes in the electrodes. On the other hand, when electrodes are perforated, pHf is reduced, and ORf increased. The data for the two linear fits shown in Fig. 7 are in Table 6. If the linear behavior from Fig. 7 is kept for higher values of DE, the two lines, due to Q = 2 and Q = 6 mL.s−1 will meet when DE is 40 mm, and N is 10. By this theoretical model, ORf will then be close to 94%. For a good design, the values for Q, N, and DE must correspond to the higher values of ORf, the higher kOR and to pHf closer to neutral. By Brazilian law, the maximum concentration of oily effluents based on mineral oil that can be discharged in the environment is 20 mg.L−1, and the pH range must be between 5 and 9 [30]. The higher value of ORf obtained with pHf smaller than 9 was 90.1% in Experiment 13 (Q =4 mL.s-1, DE =7.95 mm, N = 10). This experiment generates an effluent compatible with the limits imposed by the legislation. 4. Conclusions Effluents containing metalworking fluid can be treated by electrocoagulation in a continuous flow, with more than 90% of removal efficiency and a final pH within the range allowed by legislation. Drilling
Fig. 7. Evolution of ORf with N x DE for Q = 2 mL.s−1 ( ) and 6 mL.s−1 ( ). The label 10 means DE = 10 mm and 20 means DE = 20 mm.
Table 5 Spearman’s correlation data. ORf
N Q DE N x DE
kOR
pHf
kpH
Loss of Mass
Sc
p-value
Sc
p-value
Sc
p-value
Sc
p-value
Sc
p-value
0.3652 −0.5640 0.2530 0.5059
0.1242 0.0119 0.2960 0.0271
0.0822 0.9441 0.1485 0.0428
0.7379 < 0.0001 0.5440 0.8621
−0.1855 −0.9082 0.0550 −0.0116
0.4470 < 0.0001 0.8230 0.9625
0.6213 0.0660 −0.0880 0.3776
0.0045 0.7884 0.7202 0.1110
0.1921 −0.2723 −0.0715 0.2414
0.4307 0.2595 0.7712 0.3195
Where: Sc means Spearman coefficient, N is the number of holes, Q is flow rate, DE distance between electrodes. A two-tailed test of significance was used, bold values indicate significant correlations (p-value < 0.05). 119
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Table 6 Linear fit of ORf as a function of N x DE. Q (mL.s−1)
Intercept Value
Standard Error
Value
Standard Error
2 6
85.3176 88.4667
0.5591 0.2310
0.0224 0.0145
0.0043 0.0020
Slope
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Adjusted Rsquare
0.9295 0.8978
holes in the electrodes led to an increase in the ratio of change of pH, resulting in faster convergence to its final value. Removal efficiency depends on a combination of the number of holes and distance between electrodes. If new holes are added and distance increased, efficiency improves. These facts indicate that current density is not the only factor acting when holes are drilled. There was no noticeable increase in mass loss of the electrodes due to the presence of holes, and there is no visual evidence of alteration in corrosion around the holes. No evidence of alterations in corrosion due to the non-uniform current density. There is nonetheless a change in the average size and distribution of the corrosion pits. Acknowledgments This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) Finance Code 001. The authors are also grateful for brazilian financial support: CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) and FAPES (Fundação de Amparo à Pesquisa e Inovação do Espírito Santo). References [1] M. Kobya, E. Demirbas, M. Bayramoglu, M.T. Sensoy, Optimization of electrocoagulation process for the treatment of metal cutting wastewaters with response surface methodology, Water Air Soil Pollut. 215 (2011) 399–410, https://doi.org/ 10.1007/s11270-010-0486-x. [2] E. Demirbas, M. Kobya, Operating cost and treatment of metalworking fluid wastewater by chemical coagulation and electrocoagulation processes, Process Saf. Environ. Prot. (2016), https://doi.org/10.1016/j.psep.2016.10.013. [3] K. Bensadok, S. Benammar, F. Lapicque, G. Nezzal, Electrocoagulation of cutting oil emulsions using aluminum plate electrodes, J. Hazard. Mater. 152 (2008) 423–430, https://doi.org/10.1016/j.jhazmat.2007.06.121. [4] P. Asaithambi, M. Susree, R. Saravanathamizhan, M. Matheswaran, Ozone assisted electrocoagulation for the treatment of distillery effluent, Desalination 297 (2012) 1–7, https://doi.org/10.1016/j.desal.2012.04.011. [5] A. Fernandes, M.J. Pacheco, L. Ciríaco, A. Lopes, Review on the electrochemical processes for the treatment of sanitary landfill leachates: present and future, Appl. Catal. B Environ. 176 (2015) 183–200, https://doi.org/10.1016/j.apcatb.2015.03. 052. [6] Y.O. Fouad, Separation of cottonseed oil from oil-water emulsions using electrocoagulation technique, Alexandria Eng. J. 53 (2014) 199–204, https://doi.org/10. 1016/j.aej.2013.10.005. [7] I.L. Mickova, Advanced electrochemical technologies in wastewater treatment. Part II: electro-flocculation and electro-flotation, Am. Sci. Res. J. Eng. Technol. Sci. 14 (2015) 273–294 (accessed March 23, 2018), http://asrjetsjournal.org/index.php/ American_Scientific_Journal/article/view/1017. [8] X. Chen, G. Chen, P.L. Yue, Separation of pollutants from restaurant wastewater by electrocoagulation, Sep. Purif. Technol. 19 (2000) 65–76, https://doi.org/10.1016/ S1383-5866(99)00072-6.
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