Removal of Fluoride from Water by a Continuous Electrocoagulation

Subscriber access provided by UNIV OF CAMBRIDGE

General Research

Removal of Fluoride from Water by a Continuous Electrocoagulation Process Nuno Santos Graça, Ana Mafalda Ribeiro, and Alirio Egidio Rodrigues Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00019 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Removal of Fluoride from Water by a Continuous Electrocoagulation Process Nuno S. Graça*, Ana M. Ribeiro, Alírio E. Rodrigues

Laboratory of Separation and Reaction Engineering - Laboratory of Catalysis and Materials (LSRELCM), Department of Chemical Engineering, University of Porto, Rua Dr. Roberto Frias, s/n, 4200-465 Porto, Portugal.

Abstract The present work proposes a continuous electrocoagulation experimental set-up for the removal of fluoride from water using aluminum electrodes. The experimental results showed that the proposed process can remove 97% of fluoride from 5 L of water with a concentration of 15 mg F-/L. Additionally, was verified that the applied voltage during the operation remains almost constant, indicating that this experimental set-up overcomes the increase of the electrical resistance due to the electrode passivation. The statistical analysis of the results showed that for the fluoride removal all the operating variables have significant effects whereas for the applied voltage only the current intensity and electrodes configuration are significant. A quadratic model showed to be suitable to relate the responses with the operating variables. The experimental set-up was tested for the treatment of 20 L of water. The results showed that for 20 L of water the total removal of fluoride could be achieved with an energy consumption of 506 kWh m-3.

Corresponding author. E-mail address: [email protected] (N. S. Graça) *

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 22

1. Introduction Depending on its concentration, fluoride in water can have both beneficial and harmful effects on the environment and human health. The concentration of fluoride around 1 mg/L in drinking water can help in teeth decay prevention. However, the long-term consumption of water containing an excess of fluoride can lead to fluorosis of teeth and bones.1 The world’s most populated countries, China and India, have a severe health problem associated with fluorosis. In 2012, the World Health Organization (WHO) estimated that 2.7 million people in China have the crippling form fluorosis.1 The contamination of water with fluoride can occur naturally at the foot of volcanic mountains and in regions with geological deposits of marine origin.2 Moreover, fluoride contamination can result from the discharge to the environment of untreated industrial wastewaters, such as glass manufacturing3 and semiconductor industries.4 Several techniques are employed for fluoride removal from water, such as adsorption,5 crystallization,6 and electrochemical processes.7-9 The electrocoagulation process has been suggested as a good alternative to conventional chemical coagulation.10, 11 Several studies showed the effectiveness of electrocoagulation on the defluoridation of water supplies and industrial wastewaters.7, 12-21 The application of electrochemical processes to remove pollutants from water has been gaining increasing interest due to its low associated costs, high efficiency, and relatively simple operation and control.22 More specifically, the electrocoagulation process has been showing its suitability for the treatment of different kinds of contaminated water, namely wastewater from hospitals,23 restaurants,24 textile industry, 25, 26

livestock27 and domestic usage (grey water).28 Moreover, the possibility of

integrating photovoltaic cells makes the electrochemical processes very attractive from the energetic sustainability point of view.29, 30 During the electrocoagulation process, the electrochemical dissolution of the sacrificial anode into soluble and insoluble metal hydroxide species promotes the coagulation and/or adsorption of different pollutant species.31, 32 Different aspects such as the nature of the pollutant and its concentration, current intensity, solution pH, electrode material and configuration affect the performance of the electrocoagulation process.33, 34 2 ACS Paragon Plus Environment

Page 3 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Some strategies have been employed for the continuous removal of fluoride from water by electrocoagulation, namely using an electrocoagulation unit followed by a flotation tank35 or a sedimentation box;36 or using a filter press electrocoagulation unit followed by a flocculator and a clarifier.21 In the present work is proposed a continuous electrocoagulation compact set-up for the removal of fluoride from water using aluminum electrodes. The influence of different operating variables on the process performance was assessed by employing the Box-Behnken design for the experimental plan. Additionally, the suitability of the continuous electrocoagulation process to treat higher volumes of contaminated water was also tested.

2. Materials and methods 2.1.

Analytical method

The concentration of fluoride was measured using a spectrophotometer (Merck Millipore Spectroquant Prove 300) and the respective analysis kit (Merck Millipore fluoride test 114598) allowing the determination in the concentration range of 0.1 to 20 mg/L. The removal efficiency was calculated using the following equation:

𝑅𝑒𝑚𝑜𝑣𝑎𝑙 (%) = 100 ×

𝐶0 ― 𝐶 𝐶0

(1)

where 𝐶0 is the initial concentration of the contaminated water, and 𝐶 is the concentration after the treatment.

2.2.

Experimental set-up and procedure

Figure 1 shows a schematic representation of the continuous electrocoagulation reactor. The plexiglass structure of the reactor, 13 x 26.6 x 12.5 cm, consists of a first compartment directly connected to the reactor inlet containing the electrodes, a second compartment, which receives the water from the first compartment is connected to a third compartment by a 1.5 x 12.5 cm gap on the bottom of the reactor, this separation avoids that the floating solids from the second compartment pass directly to the reactor outlet. The electrodes are fixed to the reactor through a groove in the wall and bottom of the first 3 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 (a) 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 22

compartment. The four aluminum electrodes, 15 x 10 x 0.2 cm, were connected to a DC power supply (Velleman LABPS3020) operating under galvanostatic conditions. The alternated placement of the electrodes (Figure 1(b)) makes the solution flowing in a serpentine pattern. In this configuration, the water flows through the different polarities of the electrodes in a single passage.31 The gap between the electrodes was maintained at 1 cm. Before each experiment, the electrodes were polished with sandpaper and dipped in a 0.1 N HCl solution for 5 min and then cleaned with deionized water.

(b)

Figure 1- Schematic representation of the continuous electrocoagulation reactor: (a) side view; (b) top view (1- inlet, 2- electrodes, 3- outlet)

A peristaltic pump (Watson-Marlow 505U) was used to pump the feed solution through the reactor (Figure 2). Each experiment begins by filling the reactor with an electrolyte solution (deionized water + NaCl) with a conductivity of 250 µS/cm. After that, the current was turned on, and the feed solution was pumped through the reactor inlet. Samples were collected at the reactor outlet during the treatment time and analyzed. The conductivity of the solutions was measured using a multi-parameter meter (VWR MU 6100 L). The feed solution was prepared by dissolving the appropriated amount of sodium fluoride (NaF) (Sigma-Andrich; assay ≥ 99%) in deionized water and then adjusting the conductivity with a saturated solution of NaCl.


Page 5 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2- Schematic representation of the continuous electrocoagulation experimental set-up.

2.3.

Experimental design and statistical analysis

The development of a mathematical model that satisfactorily represents the electrocoagulation processes is not a trivial task. Due to the complexity of the phenomena involved in the electrocoagulation, namely aluminum speciation, polymerization reactions, different solubility of the aluminum species, different removal mechanisms, corrosion and electrodes passivation, many researchers opt for statistical methods to study the electrocoagulation processes.37-40 The experimental design and the statistical analysis were performed using the Minitab Software (version 18). Box-Behnken (BBD) design was used to assess the effect of three continuous factors (applied current (XA), flow rate (XB) and feed concentration (XC)) and one categoric factor (electrode configuration (XD) (Figure S1)) on both fluoride removal (Y1) and applied voltage (Y2) using a continuous electrocoagulation reactor. The values for the continuous factors levels were selected based on preliminary continuous electrocoagulation experiments (section 3.1). Due to its ability to reduce the number of experiments, BBD provides an efficient and economical way to perform this kind of statistical analysis.41 The application of BBD



Page 6 of 22

results in a set of 15 experiments correspondent to the three continuous factors presented in Table 1 and considering three repetitions of the central point, according to:

(2)

𝑁 = 2𝑘(𝑘 ― 1) + 𝐶𝑝

where k is the number of independent factors, and 𝐶𝑝 is the number of the central point repetitions. Each set of 15 experiments was repeated for each level of the categoric factor (series monopolar (SM), series bipolar (SB) and parallel (P)), which results on a total of 45 experiments.

Table 1- Factor and levels used for the Box-Behnken design.

Level

Factor

-1

0

1

XA: Current (mA)

40

100

160

XB: Flow rate (mL/min)

50

100

150

XC: Concentration (mg/L)

5

10

15

Experimental design results were adjusted to the following second-order model:

𝑌𝑖 = 𝛽0 +

∑

𝑛

𝛽𝑖𝑋𝑖 + 𝑖=1

∑

𝑛

𝛽𝑖𝑖𝑋2𝑖𝑖 𝑖=1

+

𝑛

𝑛

𝑖=1

𝑗=1

∑ ∑

𝛽𝑖𝑗𝑋𝑖𝑗

(3)

where 𝛽0, 𝛽𝑖, 𝛽𝑖𝑖 and 𝛽𝑖𝑗 are the regression coefficients for the intercept, linear, square and interaction terms, respectively; and 𝑋𝑖 and 𝑋𝑗 are the independent variables.


Page 7 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3. Results and discussion 3.1.

Preliminary continuous electrocoagulation reactor experiments The treatment of 5 L of water containing 10 mg/L of fluoride was experimentally

performed on the continuous electrocoagulation reactor described in section 2.2. Figure 3 shows the concentration of fluoride on the reactor outlet, at different times, and different current intensities. These results show that increasing the current intensity reduces the outlet fluoride concentration. This effect is associated with the increase of anodes dissolution with the increase of the current intensity; consequently, there will be more soluble and insoluble metal hydroxide species able to participate on the adsorption of the fluoride.31 Also, it can be noticed that when there is no applied current (I=0 mA), after the initial dilution of the feed stream with the electrolyte solution (deionized water + NaCl) inside the reactor at the beginning of the experiment, the outlet concentration of fluoride increases until reach the feed concentration (C/C0=1), therefore there is no fluoride removal. The mass of aluminum dissolved from the anode can be determined based on the Faraday’s law (Eq.(4)). 𝑚𝐴𝑙 =

𝑀𝐴𝑙𝐼𝑡 𝐹𝑍

(4)

where 𝑚𝐴𝑙 is the mass of dissolved aluminum, 𝐼 is the applied current intensity (A), 𝑧 is the valence number of the metal (𝑧𝐴𝑙 = 3), 𝐹 is Faraday’s constant (94,485 C∙mol-1), 𝑀𝐴𝑙 is the aluminum molar mass (26.98 g mol-1) and 𝑡 is the operation time. The formation of precipitates can be attributed mainly to the formation of 𝐴𝑙(𝑂𝐻)3(𝑠). 42, 43



Page 8 of 22

Figure 3- Continuous EC reactor outlet concentration for different current intensities. (Q=100 mL/min, CFeed= 10 mg F-/min)

Figure 4 presents the results regarding both cumulative concentration of fluoride and its removal. The cumulative concentration (Figure 4(a)) shows the concentration of fluoride present in a given amount of collected volume of solution. The results show that for the higher current intensity (190 mA) it is possible to obtain 5 L of solution without fluoride, which corresponds to a removal of 100% (Figure 4(b)).


Page 9 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


( (b)

(a)

Figure 4- (a) Cumulative fluoride concentration and (b) removal for different current intensities. (Q=100 mL/min, CFeed= 10 mg F-/min)

Another studied parameter was the flow rate used during the continuous electrocoagulation reactor operation. Figure 5 shows the effect of the flow rate on the fluoride removal performance. The results show that the increase in flow rate has an adverse effect on the fluoride removal. To understand these results it is important to introduce the concept of charge loading, which is defined as the number of charges transferred by the electrochemical reactions for a given amount of water,44 and for the continuous electrocoagulation reactor is given by: 𝑞=

𝐼𝜏 𝑉𝑅

(5)

where q is the charge loading (C/L), 𝐼 is the applied current intensity (A), 𝜏 is the hydraulic residence time (s) and

𝑉𝑅 is the reactor volume. The charge loading is directly

proportionally to the amount of aluminum released during the electrocoagulation process. Therefore, the decrease of the charge loading by decreasing the hydraulic residence time (increasing the flow rate) will correspond to lower amounts of aluminum flocs able to adsorb the fluoride and, consequently, to a lower removal efficiency.



(a)

Page 10 of 22

(b)

Figure 5- (a) Cumulative fluoride concentration and (b) removal for different flow rates. (I=100 mA, CFeed= 10 mg F-/min)

Despite the reduction on the fluoride removal when the flow rate increases, is necessary to consider that to treat the same amount of solution, the increase of flow rate corresponds to a decrease in the operation time, which has an impact on the process energy consumption (Eq. (6)). 𝐸=

𝐼×𝑈×𝑡 𝑉

(6)

where I is the applied current intensity, U is the applied voltage, t is the operation time, and V is the volume of treated water. Table 2 shows that despite the decrease in the fluoride removal, the energy consumption decreases for the higher flow rates. Moreover, it is important to refer that for the continuous operation, the applied voltage is almost constant during the time of the experiment, which indicates that the liquid flow through the electrodes seems to reduce the effect of electrodes passivation.


Page 11 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2- Effect of flow rate on the removal and energy consumption.

Energy

Q

Operation time

Voltage

(mL/min)

(min)

(V)

80

62.5

14.2

296

80

100

50

13.5

226

78

120

41.7

14.0

194

75

consumption

Removal (%)

(kWh/m3)

Another feature of the reactor is due to its design, the aluminum flocs formed from the Al3+ released by the electro-dissolution of the anodes are retained mainly in the first and second compartments of the reactor, therefore, the amount of aluminum flocs is very small at the reactor outlet.

3.2.

Regression and statistical analysis

The effect of the current intensity (XA), flow rate (XB), feed concentration (XC) and electrodes configuration (XD) on both fluoride removal (Y1) and applied current (Y2) using a continuous electrocoagulation reactor for the treatment of 5 L of water, was assessed by performing a set of experiments designed using the BBD method. Table S1 present the experimental results for the 45 experiments 3. Table S2 and Table S3 present the analysis of variance (ANOVA) of the results. F and P values for the removal model (Table S2) are 77.52 and 0.000, respectively; for the voltage model (Table S3) the F and P values are 1274.58 and 0.000, respectively. In both cases, these values indicate that the models are significant. Additionally, for both models, the P value for the lack-of-fit is higher than 0.05 indicating that the lack-of-fit is not significant relative to pure error.45 The quadratic model fit quality can be assessed by the R-squared coefficient, which represents the total variation of the predicted response. The values obtained of R-squared for removal and voltage models are 0.980 and 0.999, respectively, being very close to one, indicating that the models can explain almost the totality of the response variation (Y1 and Y2).46 The adjusted R-squared obtained is 0.967 for the removal model and 0.998 for the voltage model, these values are close to the 11 ACS Paragon Plus Environment


Page 12 of 22

respective R-squared, indicating a high significance of the model. These results suggest the suitability of these empirical models to relate the independent variables with the responses. From the general quadratic response surface model (Eq. (3)), the response regarding fluoride removal (Y1) is given by the following equations: Series Monopolar 𝑌1 = 91.63 + 0.5225𝑋𝐴 ― 0.3635𝑋𝐵 ― 2.978𝑋𝐶 ― 0.001241𝑋2𝐴 + 0.001505𝑋2𝐵 + 0.1305𝑋2𝐶 (7) ― 0.000129𝑋𝐴𝑋𝐵 ― 0.00160𝑋𝐴𝑋𝐶 ― 0.01545𝑋𝐵𝑋𝐶

Series Bipolar 𝑌1 = 91.09 + 0.5020𝑋𝐴 ― 0.3428𝑋𝐵 ― 2.427𝑋𝐶 ― 0.001241𝑋2𝐴 + 0.001505𝑋2𝐵 + 0.1305𝑋2𝐶 (8) ― 0.000129𝑋𝐴𝑋𝐵 ― 0.00160𝑋𝐴𝑋𝐶 ― 0.01545𝑋𝐵𝑋𝐶

Parallel 𝑌1 = 50.36 + 0.5252𝑋𝐴 ― 0.2682𝑋𝐵 ― 1.807𝑋𝐶 ― 0.001241𝑋2𝐴 + 0.001505𝑋2𝐵 + 0.1305𝑋2𝐶 (9) ― 0.000129𝑋𝐴𝑋𝐵 ― 0.00160𝑋𝐴𝑋𝐶 ― 0.01545𝑋𝐵𝑋𝐶

The response in terms of voltage (Y2) is given by the following equations: Series Monopolar 𝑌2 = 3.13 + 0.12562𝑋𝐴 ― 0.0244𝑋𝐵 ― 0.0001𝑋𝐶 + 0.00002𝑋2𝐴 + 0.000141𝑋2𝐵 ― 0.00016𝑋2𝐶 (10) ― 0.000022𝑋𝐴𝑋𝐵 ― 0.000134𝑋𝐴𝑋𝐶 ― 0.000111𝑋𝐵𝑋𝐶

Series Bipolar 𝑌2 = 3.02 + 0.17699𝑋𝐴 ― 0.0291𝑋𝐵 + 0.092𝑋𝐶 + 0.00002𝑋2𝐴 + 0.000141𝑋2𝐵 ― 0.00016𝑋2𝐶 (11) ― 0.000022𝑋𝐴𝑋𝐵 ― 0.000134𝑋𝐴𝑋𝐶 ― 0.000111𝑋𝐵𝑋𝐶

Parallel 𝑌2 = 1.49 + 0.17699𝑋𝐴 ― 0.0291𝑋𝐵 + 0.092𝑋𝐶 + 0.00002𝑋2𝐴 + 0.000141𝑋2𝐵 ― 0.00016𝑋2𝐶 (12) ― 0.000022𝑋𝐴𝑋𝐵 ― 0.000134𝑋𝐴𝑋𝐶 ― 0.000111𝑋𝐵𝑋𝐶

According to the ANOVA tables, in both models the linear terms have the highest significance on the response, the F values for the linear terms are 246.2 and 3903.9 for 12 ACS Paragon Plus Environment

Page 13 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the removal model and voltage model, respectively. A more detailed analysis of the effect and significance of the different terms of the models can be made by looking at the standardized effect plots (Figure S2 and Figure S3). For the removal model, the four individual factors are significant and produce the highest effect on the response. In the case of the voltage model the current intensity (XA) is by far the factor with higher effect on the response, which was expected due to the direct relation between current and voltage given by the Ohm’s law, however, both the electrodes configuration (XD) and its interaction with the current intensity, also present a significant effect. Figure 6 and Figure 7 show the contour plots for the effect of both current intensity and flow rate on the treatment of 5 L of a 10 mg/L fluoride solution. The analysis of the removal contour plots shows that for the three electrodes configurations the current intensity has a positive effect whereas the flow rate has an adverse effect, being the highest removal efficiency obtained for the higher current intensity and low flow rate. Regarding the electrodes configuration effect, the best removal efficiency is obtained for the series bipolar configuration, while the worst results are obtained for the parallel configuration, which the removal efficiency is way worse than the other two configurations. The analysis of the voltage contour plots shows that, as expected, for the three configurations the applied voltage increases with the increase of the current intensity, on the other hand, the flow rate as a negligible effect on the applied voltage. Analyzing the effect of the electrodes configuration, it can be observed that the series bipolar configuration requires the higher voltage during the electrocoagulation operation whereas the parallel configuration operates at the lowest voltage. This fact can be explained by the electrical resistance associated with each kind of electrodes configuration. The series electrodes configurations present higher resistance since the electrical current must flow through all the electrodes, this resistance is even higher for the bipolar configuration since there is no electrical connection between the electrodes. In the parallel configuration the current is divided between the electrodes; consequently, the resistance is divided for each pair of electrodes.31, 47 The selection of the best operation conditions will depend on the importance of the energy consumption on the overall process operating costs. Therefore, the series monopolar electrodes configuration seems to provide the best compromise between fluoride removal and energy consumption, since it requires less voltage than the bipolar 13 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 (a) 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 (a) 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 22

configuration and presents better removal than the parallel configuration. On the other hand, if the energy consumption is not a critical aspect of the process, the series bipolar configuration provides a slight better performance in terms of fluoride removal.

(b)

(c)

Figure 6- Contour plots for the effect of current intensity and flow rate on the fluoride removal: (a) series monopolar; (b) series bipolar; (c) parallel.

(b)

(c)

Figure 7- Contour plots for the effect of current intensity and flow rate on the applied voltage: (a) series monopolar; (b) series bipolar; (c) parallel.


Page 15 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.3.

Application of CEC reactor to the treatment of high volumes

The continuous electrocoagulation reactor was used to treat 20 L of a fluoride solution with a concentration of 10 mg/L. The electrodes were connected in series monopolar configuration, and the experiment was performed at different current intensities (40, 70 and 100 mA) and flow rates (50, 80 and 100 mL/min). The effect of the current intensity on the accumulated concentration at the reactor outlet is presented in Figure 8. The results show that it is possible to obtain a total removal of fluoride for the higher current intensity, however, the removal efficiency decreases by reducing the current intensity, these results confirm the conclusions drawn from the statistical analysis performed in section 3.2, that the maximum removal efficiency is obtained by increasing the current intensity and decreasing the flow rate.

Figure 8- Effect of current intensity on the outlet cumulative fluoride concentration.

The effect of the flow rate on the accumulated concentration at the reactor outlet is presented in Figure 9. The results show the negative effect of increasing the flow rate 15 ACS Paragon Plus Environment


Page 16 of 22

on the removal efficiency. However, is important to notice that a change in the flow rate corresponds to a change in the operation time; the experiments performed at the lowest flow rate (50 mL/min) have an operation time of 6 hours and 40 minutes; the experiments performed at the highest flow rate (100 mL/min) have an operation time of 3 hours and 20 minutes.

Figure 9- Effect of flow rate on the outlet cumulative fluoride concentration.

The applied voltage value during the experiments is presented in Figure 10. As was observed in section 3.2, the current intensity is the main factor that affects the applied voltage whereas the flow rate has a negligible effect. Moreover, the value of applied voltage remains almost constant during the experiments, indicating that even for higher operation times the electrode passivation effects seem to be absent. Table 3 shows the results of the fluoride removal from 20 L of water and the respective energy consumption. The results show that higher removal is obtained for the higher current and low flow rate, however, at these conditions the energy consumption is maximized, since the increase both operation time (by decreasing the flow rate) and increasing the current intensity (that also increases the voltage) affect the energy consumption positively (Eq. (6)).


Page 17 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10- Effect of current intensity and flow rate on the applied voltage.

Table 3- Effect of current intensity and flow rate on the removal and energy consumption.

Current (mA)

Flow rare (mL/min)

Removal (%)

EC (kWh/m3)

40

50

59.9

80.5

70

50

78.1

264.5

100

50

100.0

506

100

80

83.4

321

100

100

74.1

255.5



Page 18 of 22

4. Conclusions The installation and operation of a continuous electrocoagulation reactor were performed. The reactor was tested for the removal of fluoride using aluminum electrodes. The preliminary results showed that the removal of fluoride increases with the current intensity. The study of the effect of the flow rate showed that the removal decreases with the increase of this parameter; however, the determination of energy consumption showed that the increase of flow rate decreases the energy consumption of the process. Moreover, the applied voltage is almost constant during the operation time, indicating that the liquid flow through the electrodes may eliminate the effect of electrodes passivation. The effect of current intensity, flow rate, feed concentration and electrodes configuration on the fluoride removal efficiency and applied voltage using a continuous electrocoagulation reactor was assessed employing the BBD for the experimental plan. The results showed that for the fluoride removal all the operating variables have significant effects whereas for the applied voltage only the current intensity and electrodes configuration are significant. The quadratic model showed to be suitable to relate the responses with the operating variables. The suitability of the continuous electrocoagulation reactor to treat higher volumes of water was assessed by running experiments using 20 L of fluoride solution. The results showed that the response regarding both removal and the applied voltage is similar to the observed for lower volumes. Also, the almost constant voltage during the operation was also observed. The experiments showed that total removal of fluoride could be achieved with an energy consumption of 506 kWh m-3.

5. Supporting Information Electrodes configuration, Box-Behnken experimental design, ANOVA tables, Standardized effects plots. This information is available free of charge via the Internet at http://pubs.acs.org/


Page 19 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6. Acknowledgements This work was financially supported by: Associate Laboratory LSRE-LCM UID/EQU/50020/2019 - funded by national funds through FCT/MCTES (PIDDAC); Project Inn-INDIGO/0003/2014 funded by FCT - Fundação para a Ciência e a Tecnologia.

7. References (1) World Health Organization & International Programme on Chemical Safety.(1996) Guidelines for drinking-water quality. Vol. 2, Health criteria and other supporting information, 2nd ed Geneva: World Health Organization,1996 (2) Zohoori, F.V.; Duckworth, R.M. Fluoride: Intake and Metabolism, Therapeutic and Toxicological Consequences. in Molecular, Genetic, and Nutritional Aspects of Major and Trace Minerals; J.F. Collins, Ed.; Academic Press: Boston, 2017; pp 539-550. (3) Sujana, M.G.; Thakur, R.S.; Rao, S.B. Removal of fluoride from aqueous solution by using alum sludge. J. Colloid Interface Sci. 1998, 206 (1), 94-101. (4) Toyoda, A.; Taira, T. A new method for treating fluorine wastewater to reduce sludge and running costs. IEEE Trans. Semicond. Manuf. 2000, 13 (3), 305-309. (5) Kanrar, S.; Debnath, S.; De, P.; Parashar, K.; Pillay, K.; Sasikumar, P.; Ghosh, U.C. Preparation, characterization and evaluation of fluoride adsorption efficiency from water of iron-aluminium oxide-graphene oxide composite material. Chem. Eng. J. 2016, 306, 269-279. (6) Deng, L.; Liu, Y.; Huang, T.; Sun, T. Fluoride removal by induced crystallization using fluorapatite/calcite seed crystals. Chem. Eng. J. 2016, 287, 83-91. (7) Mameri, N.; Yeddou, A.R.; Lounici, H.; Belhocine, D.; Grib, H.; Bariou, B. Defluoridation of septentrional Sahara water of North Africa by electrocoagulation process using bipolar aluminium electrodes. Water Res. 1998, 32 (5), 1604-1612. (8) Guzmán, A.; Nava, J.L.; Coreño, O.; Rodríguez, I.; Gutiérrez, S. Arsenic and fluoride removal from groundwater by electrocoagulation using a continuous filter-press reactor. Chemosphere 2016, 144, 2113-2120. (9) Amor, Z.; Bariou, B.; Mameri, N.; Taky, M.; Nicolas, S.; Elmidaoui, A. Fluoride removal from brackish water by electrodialysis. Desalination 2001, 133 (3), 215-223. (10) Mills, D. New process for electrocoagulation. J. Am. Water Works Assoc. 2000, 92 (6), 34-43. (11) Miwa, D.W.; Malpass, G.R.P.; Machado, S.A.S.; Motheo, A.J. Electrochemical degradation of carbaryl on oxide electrodes. Water Res. 2006, 40 (17), 3281-3289. (12) Drondina, R.V.; Drako, I.V. Electrochemical technology of fluorine removal from underground and waste waters. J. Hazard. Mater. 1994, 37 (1), 91-100. (13) Yang, C.L.; Dluhy, R. Electrochemical generation of aluminum sorbent for fluoride adsorption. J. Hazard. Mater. 2002, 94 (3), 239-252. 19 ACS Paragon Plus Environment


Page 20 of 22

(14) Shen, F.; Chen, X.; Gao, P.; Chen, C. Electrochemical removal of fluoride ions from industrial wastewater. Chem. Eng. Sci. 2003, 58 (3-6), 987-993. (15) Emamjomeh, M.M.; Sivakumar, M. An empirical model for defluoridation by batch monopolar electrocoagulation/flotation (ECF) process. J. Hazard. Mater. 2006, 131 (13), 118-125. (16) Ma, W.; Ya, F.Q.; Han, M.; Wang, R. Characteristics of equilibrium, kinetics studies for adsorption of fluoride on magnetic-chitosan particle. J. Hazard. Mater. 2007, 143 (12), 296-302. (17) Zhu, J.; Zhao, H.; Ni, J. Fluoride distribution in electrocoagulation defluoridation process. Sep. Purif. Technol. 2007, 56 (2), 184-191. (18) Changmai, M.; Pasawan, M.; Purkait, M.K. A hybrid method for the removal of fluoride from drinking water: Parametric study and cost estimation. Sep. Purif. Technol. 2018, 206, 140-148. (19) Dhadge, V.L.; Medhi, C.R.; Changmai, M.; Purkait, M.K. House hold unit for the treatment of fluoride, iron, arsenic and microorganism contaminated drinking water. Chemosphere 2018, 199, 728-736. (20) Un, U.T.; Koparal, A.S.; Ogutveren, U.B. Fluoride removal from water and wastewater with a bach cylindrical electrode using electrocoagulation. Chem. Eng. J. 2013, 223, 110-115. (21) Sandoval, M.A.; Fuentes, R.; Nava, J.L.; Rodríguez, I. Fluoride removal from drinking water by electrocoagulation in a continuous filter press reactor coupled to a flocculator and clarifier. Sep. Purif. Technol. 2014, 134, 163-170. (22) Rajeshwar, K.; Ibanez, J.G.; Swain, G.M. Electrochemistry and the environment. J. Appl. Electrochem. 1994, 24 (11), 1077-1091. (23) Murdani; Jakfar; Ekawati, D.; Nadira, R.; Darmadi Application of Response Surface Methodology (RSM) for wastewater of hospital by using electrocoagulation. IOP Conference Series: Materials Science and Engineering 2018, 345 (1), 012011. (24) Qin, X.; Yang, B.; Gao, F.; Chen, G. Treatment of restaurant wastewater by pilotscale electrocoagulation- electroflotation: Optimization of operating conditions. J. Environ. Eng. 2013, 139 (7), 1004-1016. (25) Fajardo, A.S.; Martins, R.C.; Silva, D.R.; Martínez-Huitle, C.A.; Quinta-Ferreira, R.M. Dye wastewaters treatment using batch and recirculation flow electrocoagulation systems. J. Electroanal. Chem. 2017, 801, 30-37. (26) Bilińska, L.; Blus, K.; Gmurek, M.; Ledakowicz, S. Coupling of electrocoagulation and ozone treatment for textile wastewater reuse. Chem. Eng. J. 2019, 358, 992-1001. (27) Wang, Y.; Lin, H.; Hu, B. Electrochemical removal of hydrogen sulfide from swine manure. Chem. Eng. J. 2019, 356, 210-218. (28) Karichappan, T.; Venkatachalam, S.; Jeganathan, P.M. Optimization of electrocoagulation process to treat grey wastewater in batch mode using response surface methodology. J. environ. health sci. eng. 2014, 12 (1). (29) Valero, D.; Ortiz, J.M.; Expósito, E.; Montiel, V.; Aldaz, A. Electrochemical Wastewater Treatment Directly Powered by Photovoltaic Panels: Electrooxidation of a Dye-Containing Wastewater. Environ. Sci. Technol. 2010, 44 (13), 5182-5187. (30) Marmanis, D.; Dermentzis, K.; Christoforidis, A.; Ouzounis, K.; Moumtzakis, A. Electrochemical treatment of actual dye house effluents using electrocoagulation process directly powered by photovoltaic energy. Desalin. Water Treat. 2015, 56 (11), 29882993.


Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31) Mollah, M.Y.A.; Morkovsky, P.; Gomes, J.A.G.; Kesmez, M.; Parga, J.; Cocke, D.L. Fundamentals, present and future perspectives of electrocoagulation. J. Hazard. Mater. 2004, 114 (1), 199-210. (32) Hakizimana, J.N.; Gourich, B.; Chafi, M.; Stiriba, Y.; Vial, C.; Drogui, P.; Naja, J. Electrocoagulation process in water treatment: A review of electrocoagulation modeling approaches. Desalination 2017, 404, 1-21. (33) Silva, J.F.A.; Graça, N.S.; Ribeiro, A.M.; Rodrigues, A.E. Electrocoagulation process for the removal of co-existent fluoride, arsenic and iron from contaminated drinking water. Sep. Purif. Technol. 2018, 197, 237-243. (34) Önder, E.; Koparal, A.S.; Öğütveren, Ü.B. An alternative method for the removal of surfactants from water: Electrochemical coagulation. Separation and Purification Technology 2007, 52 (3), 527-532. (35) Hu, C.-Y.; Lo, S.-L.; Kuan, W.-H.; Lee, Y.-D. Treatment of high fluoride-content wastewater by continuous electrocoagulation–flotation system with bipolar aluminum electrodes. Sep. Purif. Technol. 2008, 60 (1), 1-5. (36) Emamjomeh, M.M.; Sivakumar, M. Fluoride removal by a continuous flow electrocoagulation reactor. J. Environ. Manage. 2009, 90 (2), 1204-1212. (37) Khorram, A.G.; Fallah, N. Treatment of textile dyeing factory wastewater by electrocoagulation with low sludge settling time: Optimization of operating parameters by RSM. Journal of Environmental Chemical Engineering 2018, 6 (1), 635-642. (38) Erkan, H.S.; Guvenc, S.Y.; Varank, G.; Engin, G.O. The investigation of chemical coagulation and electrocoagulation processes for tannery wastewater treatment using response surface methodology. Desalination and Water Treatment 2018, 113, 57-73. (39) Demirbas, E.; Kobya, M.; Oncel, M.S.; Şık, E.; Goren, A.Y. Arsenite removal from groundwater in a batch electrocoagulation process: Optimization through response surface methodology. Separation Science and Technology (Philadelphia) 2018. (40) Mook, W.T.; Aroua, M.K.; Szlachta, M.; Lee, C.S. Optimisation of Reactive Black 5 dye removal by electrocoagulation process using response surface methodology. Water Science and Technology 2017, 75 (4), 952-962. (41) Ferreira, S.L.C.; Bruns, R.E.; Ferreira, H.S.; Matos, G.D.; David, J.M.; Brandão, G.C.; da Silva, E.G.P.; Portugal, L.A.; dos Reis, P.S.; Souza, A.S.; dos Santos, W.N.L. Box-Behnken design: An alternative for the optimization of analytical methods. Anal. Chim. Acta 2007, 597 (2), 179-186. (42) Duan, J.; Gregory, J. Coagulation by hydrolysing metal salts. Adv. Colloid Interface Sci. 2003, 100-102 (SUPPL), 475-502. (43) Flores, O.J.; Nava, J.L.; Carren; tild; o, G. Arsenic removal from groundwater by electrocoagulation process in a filter-press-type FM01-LC reactor. Int. J. Electrochem. Sci. 2014, 9 (11), 6658-6667. (44) Kobya, M.; Demirbas, E.; Ulu, F. Evaluation of operating parameters with respect to charge loading on the removal efficiency of arsenic from potable water by electrocoagulation. J. Environ. Chem. Eng. 2016, 4 (2), 1484-1494. (45) Nair, A.T.; Makwana, A.R.; Ahammed, M.M. The use of response surface methodology for modelling and analysis of water and wastewater treatment processes: a review. Water Sci. Technol. 2013, 69 (3), 464-478. (46) Moradi, M.; Ghanbari, F. Application of response surface method for coagulation process in leachate treatment as pretreatment for Fenton process: Biodegradability improvement. J. Water Process Eng. 2014, 4, 67-73.



Page 22 of 22

(47) Pretorius, W.A.; Johannes, W.G.; Lempert, G.G. Electrolytic iron flocculant production with a bipolar electrode in series arrangement. Water SA 1991, 17 (2), 133138.

For Table of Contents Only


Removal of Fluoride from Water by a Continuous Electrocoagulation

Recommend Documents