3.1. Characteristics of mine water samples
Water samples from the Sambarata Mine Site contained high colloidal clay with TSS concentrations of 5.400 mg/L. The Sambarata Mine Site is an active mine owned and operated by PT Berau Coal. The mine, located in the Berau Regency, East Kalimantan, has issues with mine water containing high colloidal clay. Rock overburden samples from different lithologies were tested and analyzed using X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) to determine their mineral compositions. Mineralogical tests on the two overburden samples showed clay mineral contents, including the vermiculite and illite mineral groups. Clay minerals can be classified into several different groups, i.e., kaolinite, smectite, vermiculite, illite, and chlorites. Vermiculite is a clay mineral similar to montmorillonite and belongs to the smectite group. Vermiculite minerals have expanding properties, but are not larger than smectite (Akisanmi 2022). Vermiculite does not form through crystallization from solution, but through alteration or selective replacement of ions in the structure without damaging the structure. Vermiculite and montmorillonite contain highly hydrated exchangeable cations, mainly Ca and Mg, in the interlayer (Abollino et al. 2008; dos Anjos et al. 2014), with a large surface area per unit mass and fine particle size.
Another group of clay minerals detected in the mineralogical tests was Illite, also known as "clay micas," with the general formula of KyAl4(Si8-y, Aly)O20(OH)4. This mineral has a layered structure characterized by swelling and dispersible clay (Hamza et al. 2023). However, illite has a lower swelling behavior than montmorillonite owing to its different charge densities and interlayer cation types (Chen et al. 2020). Montmorillonite can bind to a variety of interlayer cations, including Ca2+, Na+, Mg2+, K+, and Sr2+, whereas illite only has K+ (Kahr and Madsen 1995; Marsh et al. 2018). Therefore, illite has a significantly lower cation exchange capacity than montmorillonite. In soils influenced by high rainfall, illite minerals transform into montmorillonite, whereas, under the influence of temperate or high-temperature climates, illite structures can transform into kaolinite structures.
Table 2
X-Ray Diffraction (XRD) test results.
Phase name
|
Formula
|
Figure of merit
|
Content (%)
|
Mineral Type
|
Sample #1
|
Quartz
|
Si O2
|
0.955
|
49(17)
|
Main (Felsic)
|
Vermiculite
|
(Mg2.36 Fe.48 Al.16) (Al1.28 Si2.72) O10 (OH)2 (H2O)6 Mg
|
1.237
|
0.83(13)
|
Clay
|
Muscovite
|
KAl3Si3O10(OH)2
|
1.681
|
37(5)
|
Main (Felsic)
|
Titanomagnetite
|
Fe2TiO4
|
1.579
|
2.2(14)
|
Magnetic
|
Pyroxene
|
CaMgSi2O6
|
1.834
|
0.8(7)
|
Main (Mafic)
|
Albite
|
Na(AlSi3O8)
|
1.509
|
1.8(6)
|
Na-Plagioclase
|
Posnjakite
|
Cu4(SO4)(OH)6 (H2O)
|
1.206
|
8.1(10)
|
Sulfide
|
Sample #2
Formula
Figure of merit
Content (%)
Jenis Mineral
|
Augite
|
(Ca.818 Mg.792 Fe.183 Fe.086 Al.151 Al.269 Si1.751) O6
|
1.406
|
2.5(8)
|
Main (Mafic)
|
Hollandite
|
K1.54 Ti7.23 Mg0.77 O16
|
1.709
|
17.8(9)
|
Oxides
|
Todorokite
|
NaMn6O12 3H2O
|
1.462
|
6.6(7)
|
Manganese (metals)
|
Vermiculite
|
Mg3.41Si2.86 Al1.14 O10 (O H)2 (H2 O)3.72
|
1.482
|
0.54(7)
|
Clay
|
Quartz
|
SiO2
|
0.331
|
66(5)
|
Main (Felsic)
|
Illite-2M#2
|
KAl2(Si3Al)O10(OH)
|
1.123
|
1.4(6)
|
Clay (Main-Ilit)
|
Muscovite 2M1 - from Effingham township, Ontario, Canada
|
KAl2(AlSi3O10)(OH)2
|
1.523
|
3.5(8)
|
Main (Felsic)
|
Microcline
|
KAlSi3O8
|
1.783
|
0.15(14)
|
Alkali Feldspar (Felsic)
|
Pyroxene
|
Mg2Si2O6
|
1.803
|
1.3(5)
|
Main (Mafic)
|
Table 3
X-Ray Fluorescence (XRF) test results.
Sample #1
|
Sample #2
|
No.
|
Component
|
Result mass %
|
Intensity
|
No.
|
Component
|
Result mass %
|
Intensity
|
1
|
Na
|
0.0986
|
0.0158
|
1
|
Na
|
0.0153
|
0.0084
|
2
|
Mg
|
0.636
|
0.2512
|
2
|
Mg
|
0.484
|
0.1971
|
3
|
Al
|
17.0
|
27.5725
|
3
|
Al
|
18.5
|
30.5696
|
4
|
Si
|
61.5
|
57.8588
|
4
|
Si
|
63.6
|
57.5078
|
5
|
P
|
0.0611
|
0.0338
|
5
|
P
|
0.0335
|
0.0173
|
6
|
Si
|
1.05
|
1.0535
|
6
|
S
|
0.957
|
0.9016
|
7
|
Cl
|
0.0237
|
0.0352
|
7
|
Cl
|
0.0179
|
0.0249
|
8
|
K
|
5.56
|
3.8297
|
8
|
K
|
5.89
|
3.7871
|
9
|
Ca
|
0.957
|
0.9744
|
9
|
Ca
|
0.325
|
0.3076
|
10
|
Ti
|
1.88
|
0.5572
|
10
|
Ti
|
2.33
|
0.6485
|
11
|
Mn
|
0.123
|
0.1284
|
11
|
Mn
|
0.0838
|
0.0833
|
12
|
Fe
|
10.9
|
18.1993
|
12
|
Fe
|
7.58
|
12.1378
|
13
|
Ni
|
0.0387
|
0.0938
|
13
|
Zn
|
0.0600
|
0.2545
|
14
|
Cu
|
0.0300
|
0.0910
|
14
|
Rb
|
0.0535
|
0.6181
|
15
|
Zn
|
0.0397
|
0.1619
|
15
|
Sr
|
0.0243
|
0.3118
|
16
|
Rb
|
0.0521
|
0.5735
|
16
|
Zr
|
0.0965
|
1.6962
|
17
|
Sr
|
0.0475
|
0.5803
|
|
|
|
|
18
|
Y
|
0.0052
|
0.3807
|
|
|
|
|
19
|
Zr
|
0.0840
|
1.5198
|
|
|
|
|
Clay minerals, as non-toxic inorganic minerals, have been recognized for their benefits in wastewater treatment owing to their environmental friendliness and low cost (Hao et al., 2023). Clay minerals have the potential to interact with metal cations often contained in wastewater, including adsorption through ion exchange, precipitation as hydroxides or oxide hydrates on clay surfaces, and adsorption as complex species (Lagaly, 2006; Węgrzyn et al., 2022). However, the colloidal clay content in water bodies has the potential to impact the environment, particularly aquatic life. Colloidal particles potentially affect the geochemical behavior of the environment because they can interact with heavy metals and carry them over long distances along river flows (Chen et al., 2018; Cao et al., 2022). Table 4 presents the characteristics of the mine water samples sourced from overburdened areas containing clay minerals. The mine water sample had a TSS concentration of 5,400 mg/L. Several metals were present at relatively high concentrations, including Fe (52 mg/L), Al (26.63 mg/L), Na (127.9 mg/L), K (11.9 mg/L), Ca (7.4 mg/L), and Mg (4.6 mg/L). The metal content in the mine water is consistent with the characteristics of the rock samples obtained from the XRF test results (Table 3).
Table 4
Characteristics of the raw mine water.
No
|
Parameters
|
Unit
|
Value
|
No
|
Parameters
|
Unit
|
Value
|
1
|
pH
|
-
|
8
|
28
|
Se
|
mg/L
|
0.005941
|
2
|
Conductivity
|
µS/cm
|
538
|
29
|
Rb
|
mg/L
|
0.005022
|
3
|
Temperature
|
℃
|
26.7
|
30
|
Sr
|
mg/L
|
0.088175
|
4
|
TDS
|
mg/L
|
320
|
31
|
Y
|
mg/L
|
0.001277
|
5
|
TSS
|
mg/L
|
5,400
|
32
|
Mo (97)
|
mg/L
|
0.001622
|
6
|
Sulfat
|
mg/L
|
49.4
|
33
|
Mo (98)
|
mg/L
|
0.00168
|
7
|
Fe (total)
|
mg/L
|
52
|
34
|
Ag
|
mg/L
|
< 0.0000001
|
8
|
Mn (total)
|
mg/L
|
0.33
|
35
|
Cd
|
mg/L
|
< 0.0000001
|
9
|
Al (total)
|
mg/L
|
2,663
|
36
|
Cd
|
mg/L
|
< 0.0000001
|
10
|
Fe (dissolved)
|
mg/L
|
13,214
|
37
|
Te
|
mg/L
|
0.000041
|
11
|
Mn (dissolved)
|
mg/L
|
0.0039
|
38
|
Ba
|
mg/L
|
0.030416
|
12
|
Al (dissolved)
|
mg/L
|
0.3141
|
39
|
La
|
mg/L
|
0.001972
|
13
|
Li
|
mg/L
|
< 0.0000001
|
40
|
Ce
|
mg/L
|
0.004371
|
14
|
Be
|
mg/L
|
0.000467
|
41
|
Pr
|
mg/L
|
0.000659
|
15
|
B
|
mg/L
|
0.050446
|
42
|
Nd
|
mg/L
|
0.002548
|
16
|
Na
|
mg/L
|
127.936154
|
43
|
Sm
|
mg/L
|
0.000379
|
17
|
Mg
|
mg/L
|
4.603097
|
44
|
Eu
|
mg/L
|
0.000114
|
18
|
K
|
mg/L
|
11.916106
|
45
|
Gd
|
mg/L
|
0.000378
|
19
|
Ca
|
mg/L
|
7.440835
|
46
|
Dy
|
mg/L
|
0.000307
|
20
|
V
|
mg/L
|
0.002728
|
47
|
Ho
|
mg/L
|
0.000059
|
21
|
Cr
|
mg/L
|
0.001536
|
48
|
Er
|
mg/L
|
0.000128
|
22
|
Co
|
mg/L
|
0.000414
|
49
|
Tm
|
mg/L
|
0.000017
|
23
|
Ni
|
mg/L
|
0.002715
|
50
|
Yb
|
mg/L
|
0.000116
|
24
|
Cu
|
mg/L
|
0.011213
|
51
|
TI
|
mg/L
|
< 0.0000001
|
25
|
Zn
|
mg/L
|
0.007103
|
52
|
Pb (206)
|
mg/L
|
0.000277
|
26
|
Ga
|
mg/L
|
0.000332
|
53
|
Pb (207)
|
mg/L
|
0.000022
|
27
|
As
|
mg/L
|
0.004055
|
54
|
Pb (208)
|
mg/L
|
0.000078
|
|
|
|
|
55
|
U
|
mg/L
|
0.00047
|
3.2. Effect of pH
The pH of the solution contributes to the solubility of metal hydroxides, which can affect the electrocoagulation efficiency. Changes in the pH (Fig. 3) occurred throughout the electrocoagulation experiment. The experimental results show that mine water treated using aluminum and iron electrodes experienced an increase in pH. Iron electrodes produce a higher pH with a pH range of 10.4–11.3, as compared to aluminum electrodes with a pH range of 9.7–10.9. The increase in pH is due to the accumulation of hydroxide ions (OH–) during electrocoagulation (Ni’am et al., 2007; Syaichurrozi et al., 2020). An increase in the pH occurred with an increase in the current and contact time during the experiment. Additionally, dissolved carbonate significantly influences pH changes because CO2 forms during electrocoagulation when H2 microbubbles form at the cathode (Weiss et al. 2021). This shows that an increase in the pH is one of the effects of the electrocoagulation process. Mouedhen et al. (2008) reported that wastewater with an initially acidic pH increased when treated with electrocoagulation. In this study, an increase in the pH to > 9 had the potential to exceed the applicable environmental quality standards of pH values of 6–9.
3.3. Effect of TSS
The water sample in the electro-coagulation experiment had a Total Dissolved Solid (TDS) concentration of 320 mg/L. The TDS in wastewater can result from dissolved organic matter and inorganic salts, including sodium, potassium, calcium, magnesium, chloride, bicarbonate, and sulfate (Chen et al., 2021). The 23rd edition of the American Public Health Association Standard Methods for the Examination of Water and Wastewater defines TDS as the constituents of total solids in a water sample that pass through a nominal pore size of 2.0 µm or less under specified conditions. Figure 4 shows the change in the TDS concentration during the electrocoagulation experiment with the two different electrodes. The TDS concentration on both the aluminum and iron electrodes increased with the magnitude of the current and contact time. The electrocoagulation process using iron electrodes caused a higher increase in the TDS concentration compared to that using aluminum electrodes. At a current variation of 2 A and a contact time of 30 min, the TDS value for the aluminum electrode reached 332 mg/L while that for the iron electrode was 447 mg/L. The Al3+ and Fe2+ ions generated from the electrode acted as charge neutralization agents for the negative ions in the pollutant particles. The presence of these ions reduced the electrical double layer, thus allowing for the occurrence of coagulation. The release of Al3+ and Fe2+ ions from the electrode plate (anode) formed Al(OH)3 and Fe(OH)2 flocs, which could bind to contaminants and particles in the wastewater (Hermaningsih, 2016). The main reactions occurring on the iron electrodes were as follows:
Anode : Fe(s) → Fe2+(aq) + 2e–
Fe2+(aq) + 2OH–(aq) → Fe(OH)2(s)
Cathode : 2H2O(l) + 2e– → H2(g) + 2OH–(aq)
Overall : Fe(s) + 2H2O(l) → Fe(OH)2(s) + H2(g)
The reaction that occurred on the aluminum electrode was as follows:
Anode : Al(s) → Al3+(aq) + 3e–
Cathode : 3H2O(l) + 3e– → 3/2 H2(g) + 3OH–(aq)
Overall : Al+ 3(aq) + 3H2O(l) → Al(OH)3(s) + 3H+(aq)
Additionally, the formation of metal hydroxide precipitates used in the sweep coagulation process also plays a role in increasing the TDS levels in wastewater. At higher current densities and contact times, there was an increase in ions with different charges on the dissolved particles in wastewater, which resulted in the formation of flocs that then agglomerated into larger flocs (Nur and Effendi, 2014).
3.4. Effect of Temperature
Figure 5 shows that the increase in temperature was directly proportional to the contact time and current during the experiment using the aluminum and iron electrodes. With a current variation of 2 A and a contact time of 30 min, the aluminum electrode produced a higher temperature than the iron electrode. The temperature at the aluminum electrode reached 49.5°C and the iron electrode was 31.5°C. Increased temperatures promote the formation of metal hydroxides and lead to greater particle collisions and mobility (El-Ashtoukhy et al., 2009). The release of metal hydroxides during the electrocoagulation process triggers the release of heat, which increases the reaction rate (Holt, 2002). The temperature changed linearly with the contact time, current, and electrolyte concentration. Additionally, higher temperatures allow for the formation of larger hydrogen bubbles, which can increase flotation speed (Koren and Syversen, 1995). The increase in temperature during the electrocoagulation process was due to the release of Al3+ and Fe2+ ions. The greater the current and time applied to the system, the greater the collision between particles owing to the release of metal hydroxides in the system.
3.5. Total Suspended Solids and Metal Concentrations
Figure 6 shows the changes in the TSS concentration during the electrocoagulation process. The experimental results show that in the smallest variable scenario, i.e., a current of 0.5 A and contact time of 15 min, the TSS concentration was still relatively high with the aluminum electrode (341 mg/L) and 2,113 g/L for the iron electrode. This occurred owing to the formation of metal hydroxides, which function as coagulants; this was not sufficient to bind solid particles in mine water samples. The 2 A current and contact time of 15 min at the aluminum electrode achieved the highest TSS removal of 99.58% from an initial concentration of 5,400 to 22.84 mg/L. Iron electrodes yielded different results, achieving the greatest efficiency at 2 A and a contact time of 30 min with a TSS removal of 98.72% from the initial concentration of 5,400 to 65.66 mg/L. The solubility of metal hydroxide precipitates is an important factor in TSS removal and is largely determined by pH. At lower pH values from (3.3 to 10), the solubility of metal hydroxide precipitates produces positive charges, Al3+; Al(OH)3(s) was the dominant aluminum species. At higher pH values, > 10, negative charges, Al(OH)4– forms (Linares-Hernández et al., 2009). This is also consistent with Hudori (2008), who found that the best level of pollutant removal in electrocoagulation using aluminum electrodes occurred in the pH range of 4–10 due to the formation of a coagulant (Al(OH)3).
The TSS concentration also has the potential to increase owing to excessive bubble production at the cathode. Theoretically, bubbles in the electrolyte solution of a sample have a stirring function owing to the upward momentum. With a sufficient number of bubbles, there may be an increase in the contact between the coagulant and solid particles. However, excessive bubbles can cause poor floc formation; therefore, particles cannot agglomerate. Operation at small currents produces relatively few bubbles, causing slow stirring and incorrect flocculation. If the current density increases, the production of bubbles increases and the upward momentum movement accelerates. This increases the number of mixing conditions. The shear force from mixing in the solution can damage and break up the flocs, reducing the effectiveness of pollutant removal.
We examined the metal concentration in the batch electrocoagulation process with the greatest efficiency variation for each electrode (Al and Fe). The dissolution of the aluminum electrode initially produced monomer cations, such as Al3+, which changed into Al(OH)3 at appropriate pH values and eventually polymerized into aluminum complexes (Aln(OH)3n). Aluminum complexes, such as Al(OH)2+, Al2(OH)24+, and Al(OH)4–, could form depending on the pH of the solution (Mollah et al. 2001). In this study, the results showed that variations using aluminum electrodes produced pH values of up to 10.9. At this pH, the dominant aluminum complex was Al(OH)4–, which was soluble/dissolved in the solution. This is consistent with the results in Table 5, which indicate that the concentration of dissolved aluminum in the effluent water increased with the variation in the aluminum electrodes. Although experiments using iron electrodes produce a greater pH than that of aluminum electrodes (up to 11.3), highly insoluble compounds (Fe(OH)2(s)) form predominantly at pH values above 7.5 (Linares-Hernández et al., 2009). Therefore, Table 5 indicates that the dissolved iron in the solution tended to show a small concentration variation using iron electrodes. Metal removal is also possible owing to the presence of a gelatinous suspension (Al(OH)3 and Fe(OH)n) via adsorption, which produces charge neutralization and enmeshment in the precipitate (Mollah et al., 2001).
Table 5
Dissolved metal concentrations of the effluent at the aluminum and iron electrodes (greatest efficiency variation) using ICP-MS.
No
|
Elements
|
Unit
|
Raw Mine Water
|
Aluminum Electrodes
(2 A and 15 min)
|
Iron Electrodes
(2 A and 30 min)
|
1
|
Be
|
mg/L
|
0.000467
|
0.000816
|
0
|
2
|
B
|
mg/L
|
0.050446
|
0.072454
|
0.382319
|
3
|
Na
|
mg/L
|
127.936154
|
144.058524
|
141.4721298
|
4
|
Mg
|
mg/L
|
4.603097
|
0.014466
|
0.3316142
|
5
|
Al
|
mg/L
|
0.314135
|
8.138194
|
0.470262
|
6
|
K
|
mg/L
|
11.916106
|
12.920189
|
9.2447549
|
7
|
Ca
|
mg/L
|
7.440835
|
0.18392
|
1.1705439
|
8
|
V
|
mg/L
|
0.002728
|
0.002317
|
0.0009601
|
9
|
Cr
|
mg/L
|
0.001536
|
0
|
0.019735
|
10
|
Mn
|
mg/L
|
0.003984
|
0
|
0.0136028
|
11
|
Fe
|
mg/L
|
1.321454
|
0.022536
|
0.6417095
|
12
|
Co
|
mg/L
|
0.000414
|
0.00006
|
0.0014958
|
13
|
Ni
|
mg/L
|
0.002715
|
0.000195
|
0.055271
|
14
|
Cu
|
mg/L
|
0.011213
|
0.000927
|
0.0034677
|
15
|
Zn
|
mg/L
|
0.007103
|
0.004118
|
0.0184237
|
16
|
Ga
|
mg/L
|
0.000332
|
0.008757
|
0.0003103
|
17
|
As
|
mg/L
|
0.004055
|
0.005261
|
0.0004057
|
3.5. Effect of Electrode Material: Al vs Fe
The type of electrode used in electrocoagulation has a significant effect on the amount and type of metal ions contained in the solution, coagulant efficiency, and cost (Tiaiba et al., 2017; Sadik, 2019; Ebba et al., 2021; Moneer et al., 2023). The electrode type affects the voltage generated during the electrocoagulation process according to the reactivity series of the metals. In Fig. 7, the more that the metal is to the left of its position in the reactivity series of metals, the more reactive the metal is, or the easier it is to release electrons (i.e., a strong reductant) and is easily oxidized. Meanwhile, the more to the right, the less reactive the metal or the more difficult it is to release electrons and the stronger the oxidizer (i.e., more easily reduced) (Dogra and Dogra, 1990). Iron metal is located to the right of aluminum metal, such that the iron electrode is less reactive; therefore, the metal ions produced are less reactive than the aluminum electrode. This can be observed for the same current variation of 2 A with a contact time of 30 min. The iron electrode produced a voltage of 10.96 V and the aluminum electrode produced a voltage of 45.93 V.
After 18 experiments, changes in the shapes of the aluminum and iron electrodes were observed. The surface of the anode exhibited small holes owing to corrosion. The anode, often referred to as the "sacrificial electrode," experienced corrosion with the removal of the coagulant agent into the solution. The greater the current applied to the system, the faster the corrosion of the anode plate, owing to the faster ion release rate. This statement supports one of the drawbacks of electrocoagulation technology: the anode must be replaced periodically (Mollah et al., 2004). No corrosion pits were detected on the cathode plates. This is because the cathode is inert. However, on the cathode plate, the white spots indicated the release of hydrogen ions in that section (Nur and Effendi 2014). Related to this, Syafila et al. (2023) discussed electrode selection using a statistical approach.
3.6. Effect of Current and Contact Time
In this study, iron and aluminum electrodes were used at a distance of 1.5 cm. The smaller the distance between the electrodes, the smaller the voltage required. The iron and aluminum electrodes produced different voltages at the same current and contact time. At the same current strength of 2 A with a contact time of 30 min, the aluminum electrode produced a voltage of 45.93 V while the aluminum electrode produced 10.96 V (Fig. 9).
The amount of current and contact time determined coagulant production. An optimal combination was required to achieve the highest TSS removal rate. Current density determines the coagulant dosage, floc production rate, and size of the bubbles released from the electrode process (Hidayanti et al. 2021). Current and time variations that are smaller than the optimal conditions cannot remove TSS owing to the lack of coagulant produced, whereas larger and excessively fast currents and times can break the flocs that have been formed due to increased hydrogen bubble production, reducing the effectiveness of pollutant removal (Setyawati et al., 2021). An increase in the voltage causes an increase in the electric current, increasing the temperature. An increase in temperature can cause an increase in the solubility of Al(OH)3 precipitates and the formation of unstable flocs. This causes a decrease in the electrocoagulation efficiency.
3.7. Weight loss of electrode and sludge generation
During the electro-coagulation process, several phenomena occur, including coagulant formation from anode oxidation, destabilization of contaminant particles, and particle aggregation for floc formation (Lakshmanan et al. 2009). Figures 10 and 11 show the formation of larger flocs that allowed solid particles to settle by gravity. Large flocs settled during the sedimentation process, as shown in Fig. 10(c,f). Floc settling can also occur during the electrocoagulation process, as shown in Fig. 10(b,e). Figure 11 shows the electrocoagulation experiment with the aluminum electrode, 30 min contact time, and 2 A current, which was characterized by floc deposition at a thickness of 2 cm from the bottom of the reactor, whereas the floc thickness at the iron electrode was only 1 cm from the bottom of the reactor. The amount of coagulant agent formed affected the number of flocs formed and deposited at the bottom of the reactor. Larger currents and longer contact times tended to result in higher floc formation at the bottom of the reactor. This is because the production of Al3+ and Fe2+ ions from the electrode was sufficient to form the core of the floc. The flocs bound together to form larger flocs. In the coagulation-flocculation process, flocs with heavier masses form when coagulant chemicals or polymers are added. In the electro-coagulation process, the amount of aluminum and iron dissolved in the electrode is proportional to the amount of current and contact time given to the system, in accordance with Faraday's Law 1 (Ciobanu et al., 2007). Thus, the greater the applied current and the longer the contact time, the more Al3+ and Fe2+ ions, which act as coagulant agents, are released from the anode and bind strongly to OH–. The particles are destabilized and form flocs rapidly and in large quantities.
The flocs formed in the electrocoagulation process not only settled at the bottom of the reactor but also floated on the surface of the water. This occurred owing to the formation of gas bubbles during the electrolysis process, which pushed the particles to the surface (flotation). The bubbles were H2 gas that formed at the cathode. The greater the current applied to the system, the faster the occurrence of hydrogen bubble production (Attour et al., 2014). The H2 gas bubbles were small and varied in size from 15 to 80 µm and served to move pollutant particles to the liquid surface (Murugananthan et al., 2004). Electrolyte bubbles cause mixing in the solution through an upward momentum flux. Therefore, bubbles can increase the effectiveness of the contact between the coagulant particles and pollutants (Holt, 2002).