Electrocoagulation process for the removal of co-existent fluoride, arsenic and iron from contaminated drinking water

Electrocoagulation process for the removal of co-existent fluoride, arsenic and iron from contaminated drinking water

Accepted Manuscript Electrocoagulation Process for the Removal of Co-Existent Fluoride, Arsenic and Iron from Contaminated Drinking Water João F.A. Si...

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Accepted Manuscript Electrocoagulation Process for the Removal of Co-Existent Fluoride, Arsenic and Iron from Contaminated Drinking Water João F.A. Silva, Nuno S. Graça, Ana M. Ribeiro, Alírio E. Rodrigues PII: DOI: Reference:

S1383-5866(17)33091-5 https://doi.org/10.1016/j.seppur.2017.12.055 SEPPUR 14288

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

22 September 2017 15 December 2017 28 December 2017

Please cite this article as: J.F.A. Silva, N.S. Graça, A.M. Ribeiro, A.E. Rodrigues, Electrocoagulation Process for the Removal of Co-Existent Fluoride, Arsenic and Iron from Contaminated Drinking Water, Separation and Purification Technology (2017), doi: https://doi.org/10.1016/j.seppur.2017.12.055

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Electrocoagulation Process for the Removal of Co-Existent Fluoride, Arsenic and Iron from Contaminated Drinking Water João F. A. Silva, 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 study of the EC process on the removal of iron, fluoride and arsenic from drinking water is an important step on the development of effective water treatment technologies for some world regions where the presence of those contaminants is a major health concern. Experimental results show that arsenic can be easily removed even in the presence of the other contaminants. Despite the slight reduction on the arsenic removal rate in the presence fluoride, the total removal can be achieved in less than 30 minutes of EC operation. On the other hand, the presence of iron presents a positive effect on the arsenic removal rate. Both fluoride and iron removal seem to benefit from each other presence in the EC operation. Moreover, the removal of both components is not significantly affected by the presence of arsenic. The simultaneous removal of the three pollutants can be achieved in less than 1h of operation.

1

1. Introduction

The water pollution resulting from human activity became a major concern worldwide. The importance of the water natural resources for the adequate development of living activities led some governments to impose more stringent guidelines for water production and disposal. Therefore, there is an increasing interest in finding efficient water treatment systems that alone or in conjunction with the conventional technologies, can deal with different types of water pollutants and allow the fulfillment of water quality restrictions. The water pollution has huge impact specially in the poorest regions of the world. The presence of hazardous contaminants in underground water has been reported from different parts of world [1]. Therefore, there is a need to focus greater attention on the future impact of water resources planning and development, taking into consideration all the related issues. In India, fluoride, iron and arsenic are the major inorganic contaminants of natural origin found in groundwater [1]. The presence of iron is one the most common water problem faced worldwide. Bad taste, discoloration, staining and high turbidity are some of the problems associated with its presence in water. The World Health Organization (WHO) and the European Commission recommend that the iron in water supplies should be less than 0.3 and 0.2 mg/L [2, 3], respectively. Arsenic is a carcinogen and its consumption can negatively affect the gastrointestinal tract, cardiac, vascular and central nervous systems. Exposure to arsenic through drinking water is a great threat to human health. Chronic health effects of arsenic include hyperpigmentation, hypopigmentation, and keratosis of the hands and feet [4]. Considering the high toxicity of arsenic, the World Health Organization (WHO) and USEPA set a maximum acceptable level of arsenic in drinking water at 10 μg/L (USEPA, 2002; WHO, 1993). Fluoride is responsible for affecting the calcium present in mineralized tissues, such as bone, causing diseases like dental and skeletal fluorosis and osteoporosis [5]. Two of the most populated countries in the world, 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 suffer crippling due to fluorosis [2]. Several studies showed that electrochemistry based processes can be an attractive alternative to the traditional methods of water treatment [6-11]. One of these

2

methods is the electrocoagulation (EC) process, which is based on the electrochemical production of destabilization agents that allow the removal of pollutant compounds by charge neutralization and consequently coagulation. Reduced sludge production, no requirement for chemical handling and storage, easy operation, and low space requirement are some of the advantages of the EC process [12, 13]. In its simple form, the EC reactor is constituted by one anode and one cathode. When a potential is applied from an external power source, the anode metal undergoes oxidation while the cathode will be subjected to reduction or reductive deposition of elemental metals. At the anode, the oxidation of aluminum generates aluminum ions (Al3+); the aluminum ions are then transformed into aluminum hydroxides (Al(OH) 3(s)) and aluminum oxides (Al2O3(s)) in the bulk [14]:

(1)

(2)

(3)

At the aluminum cathode, hydrogen gas is released:

(4)

Although the aluminum and iron are the most used electrodes in electrochemical based processes, other materials have been shown good results in different types of water treatment. For instance, the use cooper for removing phosphorus from water [15], zinc to reduce the organic load from liquid effluents [16], titanium and platinum electrodes combinations for treating dye solutions [17]. Several published works report the use of the EC process for fluoride, arsenic and iron removal from water. However, most of those works focused on single component removal [18-23], only few works accessed the co-existence of arsenic and fluoride on the EC process [20, 24], and to the best of our knowledge there are no publication reporting the application of EC process to ternary mixtures of arsenic, 3

fluoride and iron. Therefore, since the present work intends to address the water contamination problems in some poor rural areas of countries such India, the study of the removal of co-existent iron, fluoride and arsenic from water by the electrocoagulation process is an important step to reach that goal. The present work aims the study of the removal efficiency for each component in binary and ternary mixtures at different conditions of initial contaminant concentration using aluminum electrodes. Additionally, it was also performed the determination of the total suspended solids and particle size distribution resulting from the EC operation. The data obtained in the present work will be very useful in the further development of a low-cost electrocoagulation based process suitable to be used in poor rural areas where these contaminants are present in the drinking water supplies, constituting a major health concern for those populations.

2. Experimental 2.1 Equipment The laboratorial experiments were carried out using an acrylic tank, 0.18 x 0.18 x 0.18 m. Aluminum electrodes, 15 x 10 x 0.2 cm, with a total effective area of 0.0152 m2 were used for the EC process. The gap between the electrodes was maintained at 0.008 m. A magnetic stirrer at 200 rpm was used to maintain a homogeneous solution in the reactor. In each experiment, a 3 L solution was prepared with the desired concentration of each contaminant, and placed in the reactor. The iron, fluoride and arsenic solutions were prepared by dissolving FeSO4 7H2O, NaF and HAsNa2O4 7H2O, respectively. The conductivity and pH were measured using a multi-parameter meter (VWR MU 6100 L). The conductivity was adjusted by adding some drops of a sodium chloride solution (0.35 mg L-1) and the pH was adjusted with a sodium hydroxide (1 N) or with a hydrochloric acid solution (1 N), depending on the situation. The electrodes were fixed on the top of the tank and partially dipped on the solution (4 cm), the anode and cathode were connected to a DC power supply (Velleman LABPS3020) and energized at a fixed current. A schematic diagram of the experimental setup is shown in Figure 1. Samples 4

were collected at the desired times and filtered before being analyzed. At the end of the experiment a sample was collected to determine the size distribution of the particles formed during the EC operation. The determination of the total suspended solids present (TSS) at the end of each experiment was also performed.

Figure 1 – Schematic diagram of the experimental setup: 1. DC power supply, 2. digital ammeter, 3. aluminum electrodes, 4. EC reactor, 5. magnetic stirrer bar, 6. magnetic stirrer

2.2 Analytical methods The concentration of each contaminant was determined using a spectrophotometer (Merck Millipore Spectroquant Prove 300) and the respective analysis kits (Merck Millipore iron test 114761, Merck Millipore fluoride test 114598, Merck Millipore arsenic test 101747). The particle size distribution was measured with a laser diffraction particle size analyzer (Beckam Coulter LS 230).

5

3. Results and discussion

A total of sixteen EC combinations of the three contaminants at different concentrations were used in the present work. For each experiment, the initial and final values of both conductivity and pH were measured. Additionally, the determination of total suspended solids (TSS) was performed at the end of each experiment by vacuum filtering a known volume of solution; then the previously weighed filter was dried in an oven for 2 hours at 105ºC and weighed again. Every particle bigger than 2 µm got retained in the filter and this total mass can be determined through the filter mass variation. For the determination of TSS it is also necessary to consider that the experiments were performed with different durations and the amount of aluminum flocs produced is dependent on the amount of dissolved aluminum which is by itself dependent on the time according to the Faraday’s equation:

(5)

where is the applied current density (A∙m-2), is the treatment time (s), mass of the anodic metal (g∙mol-1), (

,

is the molar

is the valence number of the ion of the substance

is Faraday’s constant (94,485 C∙mol-1) and A is the effective area of the

electrodes (m2). Therefore, in order to compare the values of TSS for the different experiments its necessary to take in to account the duration of each experiment and express TSS in mg∙l-1∙min-1. A summary of the initial concentrations and operation parameters measured in each EC experiment is presented in Table 1. The removal efficiency for each EC experiment was calculated from the following equation:

6

(6) where

and

are the initial and final concentration of pollutant, respectively.

Table 1- Experimental initial and final operating conditions for the EC experiments with a current density of 12.5 A∙m-2. Initial concentration (mg∙l-1)

Beginning

[Fe]

[F]

[As(V)]

pH

Conductivity (S cm-1)

25 25 25 25 0 5 10 15 25 25 0 15 0 0 0 20

0 5 10 15 15 15 15 15 0 0 0 0 15 15 5 10

0 0 0 0 0 0 0 0 2 4 4 4 2 4 4 3

6.3 6.0 5.9 6.1 6.3 5.9 5.9 6.0 5.8 6.2 6.3 6.2 7.2 7.0 7.2 6.1

345 344 342 349 370 342 342 344 344 344 370 346 346 344 347 345

End pH

Conductivity (S cm-1)

6.3 5.1 6.7 7.2 8.6 8.1 8.1 8.0 4.6 4.9 8.6 7.5 8.5 8.4 8.3 7.2

335 264 257 266 361 312 314 291 302 292 361 326 341 340 346 285

TSS (mg∙l-1∙min-1) 1.95 1.23 1.73 1.77 0.92 1.55 2.21 3.66 1.47 0.89 0.27 1.39 2.13 1.75 2.19 1.10

3.1. Simultaneous removal of fluoride and iron by EC The influence of the fluoride on the iron removal was accessed by performing experiments at different fluoride initial concentrations keeping the same initial iron concentration (Figure 2a). The experimental results shown that the removal of iron is faster when the concentration of fluoride is higher. To better understand these results, it is important to look at the mechanism of the fluoride removal, which occurs by the substitution of the hydroxide groups present in the Al(OH)3 flocs by the fluoride anions dissolved on the water, according to the following equation [20, 25, 26]: (7)

7

The removal of fluoride involves the release of

to the water, these aspect is

corroborated by the experimental results, since for the experiments with higher initial fluoride concentration (15 mg·l-1) the final pH increased significantly (Table 1). This increase on the pH leads to a higher formation of iron hydroxide precipitates during the EC experiments [27] that can be easily removed by filtration. Therefore, the removal rate of iron improves in the presence of fluoride (Figure 2a). The influence of iron on the removal of Fluoride is presented in Figure 2b. It can be observed that the removal of fluoride is favored by the presence of iron. Besides the aluminum hydroxides formed, the formation of iron hydroxides due to the increase of pH contributes to increase the amount of the flocs inside the EC unit improving the removal rate of fluoride. This increase of the flocs amount can be observed in Table 1, where the higher amount of TSS occurs for the experiments where both components are

100

100

90

90

80

80

70

70

% of F - removal

% of Fe removal

present.

60 50 40 30

60 50 40 30

[F- ] = 0 mg.L -1

20

-

[F ] = 5 mg.L -

[F ] = 15 mg.L

[Fe] = 10 mg.L [Fe] = 25 mg.L

0

-1

[Fe] = 15 mg.L -1

10

-1

-1

[Fe] = 5 mg.L -1

20

-1

[F- ] = 10 mg.L -1

10

[Fe] = 0 mg.L

-1

0 0

10

20

30

40

50

60

70

80

0

10

20

30

Time / min

40

50

60

70

80

Time / min

(a)

(b) -2

Figure 2 – Simultaneous removal of fluoride and iron: i=12.5 A∙m , (a) CFe,0= 25 mg∙L-1, (b) CF,0= 15 mg∙L-1

The results obtained for the particle size distribution are presented in Figure 3. For the EC experiments at high iron concentration (25 mg·l-1) the presence of fluoride has a small effect on the particle size range (Figure 3a). Conversely, the presence of iron, even at the lowest concentration (5 mg·l-1) affects the particle size distribution for the experiments performed at high fluoride concentration (15 mg·l-1). It is important to understand that during the simultaneous removal of iron and fluoride, besides the presence aluminum flocs resulting from the electro-dissolution of the anode there are also precipitates formed by the hydrolyzation of the iron. Therefore, due to the higher 8

concentration of particles available for collision the floc growth rate is also higher, however, the big flocs formed early in the process are more vulnerable to breakage [28]. Moreover, the breakage of the flocs is partially irreversibly [29] which can result in a boarder particle size distribution at the end of the process. The dynamics of formation and breakage combined with complex interactions between the aluminum and iron flocs results in rather irregular particle size distribution patterns (Figure 3(b)) when the concentration of iron is increased. Consequently, it is hard to establish a correlation between the concentration of iron and the shape of the particle size distribution.

8

8 [F- ] = 0 mg.L-1

7

[Fe] = 0 mg.L-1

7

[F- ] = 10 mg.L-1 [F- ] = 15 mg.L-1

[Fe] = 10 mg.L-1

6

Volume fraction / %

Volume fraction / %

6 5 4 3 2 1

[Fe] = 5 mg.L-1

5

[Fe] = 15 mg.L-1 [Fe] = 25 mg.L-1

4 3 2 1

0

0 10

0

10

1

10

Particle diameter /

2

10

3

10

0

m

10

1

10

Particle diameter /

(a)

2

10

3

m

(b)

Figure 3 – Particle size distribution for the simultaneous removal of fluoride and iron: i=12.5 A∙m-2, (a) CFe,0= 25 mg∙L-1, (b) CF,0= 15 mg∙L-1

3.2. Simultaneous removal of arsenic (V) and iron by EC Figure 4 shows the results for the removal of both pollutants during the time. The presence of arsenic affects the initial iron removal rate (Figure 4a), however, this effect seems to decrease for higher operation times. At a pH close to neutral, the As(V) is predominating present in the negative form the

and

[30] which can be adsorbed by

flocs [31]:

(8)

9

(9)

At the beginning of the EC process both arsenic and iron are adsorbed at the aluminum flocs, therefore, both pollutants are removed by a complex competitive adsorptive process which can be observed in the removal of iron in the presence of arsenic (Figure 4a), however, as the experiment proceed the removal of iron by hydroxides formation and precipitation becomes more predominant which makes the iron removal less affected by the presence of arsenic. For the arsenic removal, the presence of iron presents a big impact on the initial removal rate (Figure 4b), in fact the arsenic removal efficiency reaches 100% in the first 10 minutes of operation. The presence of iron and the consequent formation of iron hydroxides precipitates increases the total amount of adsorbent material in which the arsenic can be removed by adsorption. Therefore, the presence of iron enhances the

100

100

90

90

80

80

% of As(V) removal

% of Fe removal

arsenic removal by EC.

70 60 50 40 30 20

[As(V)] = 0 mg.L

10

[As(V)] = 2 mg.L-1

-1

-1

70 60 50 40 30 20

[Fe] = 0 mg.L

10

[Fe] = 15 mg.L -1

[As(V)] = 4 mg.L

-1

[Fe] = 25 mg.L

0

-1

0 0

10

20

30

40

50

60

0

10

20

Time / min

30

40

50

60

Time / min

(a)

(b) -2

Figure 4 – Simultaneous removal of arsenic and iron: i=12.5 A∙m , (a) CFe,0= 25 mg∙L-1, (b) CAs,0= 4 mg∙L-1

The determination of particle size distribution at the end of each electrocoagulation experiment shows that for both cases, in spite of some difference in the particles size distribution shape, the range of particle size does not significantly change (Figure 5).

10

8

8 [As(V)] = 0 mg.L-1

7

[Fe] = 0 mg.L-1

7

[As(V)] = 2 mg.L-1

[Fe] = 15 mg.L-1

[As(V)] = 4 mg.L-1

[Fe] = 25 mg.L-1

6

Volume fraction / %

Volume fraction / %

6 5 4 3 2 1

5 4 3 2 1

0

0 10

0

10

1

10

Particle diameter /

2

10

3

10

0

10

m

1

10

Particle diameter /

(a)

2

10

3

m

(b)

Figure 5 – Particle size distribution for the simultaneous removal of arsenic and iron: i=12.5 A∙m-2, (a) CFe,0= 25 mg∙L-1, (b) CAs,0= 4 mg∙L-1

3.3. Simultaneous removal of arsenic (V) and fluoride by EC The experimental results for the removal of fluoride by EC showed that the presence of arsenic has no significant impact on fluoride removal efficiency (Figure 6a). However, the presence of fluoride clearly reduced the removal rate of arsenic (Figure 6b). These experimental results suggest that the substitution of OH - groups by F- on the

100

100

90

90

80

80

% of As(V) removal

% of F - removal

aluminum flocs (see Eq.(7)) has a negative impact on its capacity to adsorb arsenic.

70 60 50 40 30 20

[As(V)] = 0 mg.L

10

[As(V)] = 2 mg.L-1

-1

-1

70 60 50 40 30 20

[F ] = 0 mg.L

10

[F- ] = 10 mg.L -1

-

-

[As(V)] = 4 mg.L

-1

[F ] = 15 mg.L

0

-1

0 0

10

20

30

40

50

60

0

10

20

Time / min

30

40

50

60

Time / min

(a)

(b) -2

Figure 6 – Simultaneous removal of arsenic and fluoride: i=12.5 A∙m , (a) CF,0= 15 mg∙L-1, (b) CAs,0= 4 mg∙L-1

11

At the end of each experiment the determination of particle size distribution showed that for the removal of arsenic at different fluoride concentrations the range and distribution of particles diameters does not change significantly (Figure 7b). However, when the experiments were performed at different arsenic concentration keeping the fluoride concentration constant (15 mg·l-1) the distribution of the particles diameters was clearly affected by the presence of arsenic. These results suggest that the adsorption of arsenic on the aluminum flocs affects its growth dynamics.

8

8 [As(V)] = 0 mg.L-1

7

[F- ] = 0 mg.L-1

7

[As(V)] = 2 mg.L-1

[F- ] = 5 mg.L-1

[As(V)] = 4 mg.L-1

[F- ] = 15 mg.L-1

6

Volume fraction / %

Volume fraction / %

6 5 4 3 2 1

5 4 3 2 1

0

0 10

0

10

1

10

Particle diameter /

2

10

3

10

0

m

(a)

10

1

10

Particle diameter /

2

10

3

m

(b)

Figure 7 – Particle size distribution for the simultaneous removal of arsenic and fluoride: i=12.5 A∙m-2, (a) CF,0= 15 mg∙L-1, (b) CAs,0= 4 mg∙L-1

3.4. Efficacy of the EC treatment on a ternary mixture

The simultaneous removal of the three pollutants was experimentally performed in the EC unit using a direct electrical current density of 12.5 A∙m-2 (Figure 8). The experimental results shown that the complete removal of the three pollutants can be achieved in less than 1 hour. Similarly to the single component and binary mixtures experiments, the arsenic is the easiest contaminant to remove, however, the removal of both fluoride and iron seems to be enhanced in the ternary mixture. This enhancement of the contaminants removal can be explained by the combined effect of the increase of OH- ions concentration due to the removal of fluoride and the consequent formation of more iron hydroxides.

12

100 90 80

% of removal

70 60 50 40 30 20

Iron Fluoride Arsenic(V)

10 0 0

10

20

30

40

50

Time / min

Figure 8 – Simultaneous removal of iron, arsenic and fluoride: i=12.5 A∙m-2, CFe,0= 20 mg∙L-1, CF,0= 10 mg∙L-1, CAs,0= 3 mg∙L-1

The determination of the particle size distribution at the end of the EC operation for the removal of a ternary mixture (Figure 9) of the contaminants showed that the particles diameters is in the same range as the one determined for the single component and binary mixture EC experiments. Therefore, similar downstream clarification methods can be used with the EC process to treat polluted waters with one, two or the three contaminants.

8 7

Volume fraction / %

6 5 4 3 2 1 0 10

0

10

1

Particle diameter /

10

2

10

3

m

Figure 9 – Particle size distribution for the simultaneous removal of iron, arsenic and fluoride: i=12.5 A∙m-2, CFe,0= 20 mg∙L-1, CF,0= 10 mg∙L-1, CAs,0= 3 mg∙L-1

13

4. Conclusions

The experiments performed with single component, binary and ternary mixtures of the contaminants showed the suitability of the EC process to remove those pollutants from the water. Moreover, the results showed that arsenic is the easier contaminant to be removed mainly in the presence of iron. However, the presence of fluoride slightly reduces the removal rate of arsenic. The fluoride and iron seem to benefit each other removal and are little affected by the presence of arsenic. These experimental results suggest that the simultaneous removal of the three contaminants is feasible by the EC process. The determination of the particle size distribution at the end of each experiment showed that the size range of the flocs does not change significantly for the different operating conditions considered on the present work, this information is useful for the further development of downstream clarification processes.

5. Acknowledgement

14

This work was financially supported by: Project POC -01-0145-FE E -00 984 – Associate La oratory LS E-LC

funded

y FE E

through CO PE E2020 -

Programa O eraciona Com etiti idade e nternaciona i a o (POC ) – and y nationa funds through FC I

- Funda o

ara a Ci ncia e a

ecno ogia; Project Inn-

O/0003/2014 funded y FC - Funda o ara a Ci ncia e a ecno ogia.

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Highlights   

Iron in water has positive effect on the removal of arsenic by electrocoagulation; Fluoride and iron removal benefits from each other presence in the EC operation; The complete removal of the three pollutants can be achieved in less than 1 hour.

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