Journal Pre-proof Removal of fluoride and hydrated silica from underground water by electrocoagulation in a flow channel reactor Locksley F. Castañeda, Oscar Coreño, José L. Nava, Gilberto Carreño PII:
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Received Date: 2 August 2019 Revised Date:
9 November 2019
Accepted Date: 18 November 2019
Please cite this article as: Castañeda, L.F., Coreño, O., Nava, José.L., Carreño, G., Removal of fluoride and hydrated silica from underground water by electrocoagulation in a flow channel reactor, Chemosphere (2019), doi: https://doi.org/10.1016/j.chemosphere.2019.125417. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Ms. Ref. No.: CHEM64794R1
Removal of fluoride and hydrated silica from underground water by
electrocoagulation in a flow channel reactor
Locksley F. Castañedaa, Oscar Coreñob, José L. Navaa,*, Gilberto Carreñoa
Departamento de Ingeniería Geomática e Hidráulica, Universidad de Guanajuato, Av.
Juárez 77, Centro, 36000, Guanajuato, Guanajuato, Mexico. E-mail: [email protected]
; [email protected]
Departamento de Ingeniería Civil, Universidad de Guanajuato, Av. Juárez 77, Centro, 36000, Guanajuato, Guanajuato, Mexico. E-mail: [email protected]
10 11 12 13 14 15 16 17 18 19 20 21 22
author: [email protected]
Tel: + 52-473-1020100 ext. 2289; fax: + 52-473-1020100 ext. 2209
Ms. Ref. No.: CHEM64794R1 25
This paper concerns simultaneous removal of fluoride and hydrated silica from
groundwater (4.08 mg L-1 fluoride, 90 mg L-1 hydrated silica, 50 mg L-1 sulfate, 0.23 mg L-
up-flow EC reactor, with a six-cell stack in a serpentine array, opened at the top of the cell
to favor gas release. Aluminum plates were used as sacrificial electrodes. The effect of
current density (4 ≤ j ≤ 7 mA cm-2) and mean linear flow rate (1.2 ≤ u ≤ 4.8 cm s-1), applied
to the EC reactor, on the elimination of fluoride and hydrated silica was analyzed. The
removal of fluoride followed the WHO guideline (< 1.5 mg L-1), while the hydrated silica
was abated at 7 mA cm-2 and 1.2 cm s-1, with energy consumption of 2.48 kWh m-3 and an
overall operational cost of 0.441 USD m-3. Spectroscopic analyses of the flocs by XRD,
XRF-EDS, SEM-EDS, and FTIR indicated that hydrated silica reacted with the coagulant
forming aluminosilicates, and fluoride replaced a hydroxide from aluminum aggregates,
while sulfates and phosphates were removed by adsorption process onto the flocs. The
well-engineered EC reactor allowed the simultaneous removal of fluoride and hydrated
phosphate, pH 7.38 and 450 µS cm-1conductivity) by electrocoagulation (EC), using an
Keywords: Hydrated silica removal; Electrocoagulation; Aluminum electrodes; Fluoride
44 45 46 47 48
Ms. Ref. No.: CHEM64794R1 49 50
Groundwater is an important source of water supply for human consumption, which in arid
and semiarid regions worldwide is contaminated by inorganic salts, metalloids, and metals,
among other pollutants. Mexico is not the exception and in different areas, it is common to
find groundwater contaminated mainly with fluoride and hydrated silica, whose
concentrations range between 1-9.5 mg L-1, and 50-132 mg L-1, respectively (Guzmán et
al., 2016; Rosales et al., 2018; Sandoval et al., 2014). Other ions such as phosphates and
sulfates are also found in groundwater. The contamination of groundwater by fluorides and
hydrated silica occurs mainly from the dissolution of minerals in contact with water
(Battula et al., 2014; Behbahani et al., 2011; Emamjomeh et al., 2011; Zhu et al., 2007).
Both Mexican standard and international regulations, such as the World Health
Organization (WHO), indicate that the maximum allowable fluoride concentration in water
for human consumption must be <1.5 mg L-1. While at low concentrations fluoride is
beneficial in helping to combat tooth decay (Emamjomeh et al., 2009a; Essadki et al.,
2009), at concentrations > 4 mg L-1 it can cause severe harm to human health, such as
thyroid disorder, neurological damage, mottled teeth, skeletal and dental fluorosis,
osteoporosis and diseases in the kidneys, lungs, liver, muscles, and nerves, among others
(Camargo, 2003; Emamjomeh et al., 2011; Gosh et al., 2008; Hu et al., 2003; Maleki et al.,
Regarding hydrated silica, there is currently no official standard that establishes the
maximum permissible concentrations in water for human consumption. However, it is 3
Ms. Ref. No.: CHEM64794R1 73
known that prolonged exposure to these crystals can cause damage to human health, mainly
in the lungs, generating diseases such as tuberculosis, silicosis, bronchitis, and cancer
(Merget et al., 2002; Rosales et al., 2018; Sakar and Paul, 2016; Sariñana-Ruiz et al., 2017).
In addition to the above, hydrated silica can also seriously affect pipelines and certain unit
operations of industries that use this kind of water in their processes, permeating the walls
of equipment and causing failures (Gelover et al., 2012; Rosales et al., 2018).
One of the most used processes for fluoride removal from water is the precipitation-
flocculation by aluminum and calcium salts (Singh et al., 2016). This process produces too
much sludge because the counter-ion of the above-mentioned salts consumes coagulant
(Singh et al., 2013). Other processes such as adsorption (Gosh et al., 2008; Zhao et al.,
2011), chemical precipitation (He and Cao, 1996), and membrane process (Hu and
Dickson, 2006; Tor, 2007) have been used to remove fluorides from water as well.
86 87 88
In terms of electrochemical processes, electrocoagulation (EC) is a very efficient process
that destabilizes anions and dispersed fine particles from contaminated water by electrolysis
(Emamjomeh et al., 2009b). The EC technology allows the possibility of automating the
process, while the installation of the treatment is compact, does not need the addition of
chemicals, and the generation of sludge is minimal (Hu et al. 2007; Essadki et al., 2009;
Emamjomeh et al., 2009b), It can even be used together with other electrochemical
processes (Medel et al., 2019; Tirado et al., 2018).
Some reported works indicate the use of aluminum electrodes as sacrificial anodes in the
EC process to remove fluoride from contaminated water (Sandoval et al. 2014), using 4
Ms. Ref. No.: CHEM64794R1 98
parallel plate electrodes fitted in continuous flow reactors. These flow cell reactors, at lab-
scale, can be easily scalable at pilot plants (Castañeda and Nava, 2019).
The EC process involves the generation of coagulants in situ by electrodissolution of
aluminum sacrificial anodes, Eq. 1, while in the bulk of the solution, at neutral pH, the
formation of aluminum salts takes place, Eqs. 2, 3. At the cathode, the evolution of
hydrogen gas bubbles occurs, Eq. 4.
Al() → Al + 3e
+ 3 → ( )() + 3
2 + 3 → + 6
3H O + 3e → 1.5H + 3OH
According to the literature, the removal of fluoride occurs via co-precipitation of fluoro-
aluminum complexes and a chemical substitution reaction between a fluoride ion and a
hydroxide from aluminum flocs (Hu et al., 2003; Mohamad et al., 2011). Meanwhile, the
removal of hydrated silica occurs by the formation of aluminosilicates (Guzman et al.,
2016; Rosales et al., 2018). Other coexisting ions such as phosphates and sulfates have
been partially removed by adsorption processes on aluminum aggregates (Rosales et al.,
2018; Sandoval et al., 2019).
A common problem with aluminum electrodes is the anodic passivation. The use of flow
cells with plate electrodes favors the transport of Al3+ ions from the electrode surface to the
Ms. Ref. No.: CHEM64794R1 121
bulk of the solution, diminishing the passivation of the anode (Rosales et al., 2018;
Sandoval et al., 2019).
On the other hand, hydrated silica (72 mg L-1) has been efficiently removed from
groundwater by EC using several aluminum plate electrodes fitted in a stack of a flow EC
reactor opened to the atmosphere (Rosales et al., 2018). It is worth mentioning that this
paper shows for the first time the efficient abatement of hydrated silica by EC, highlighting
that aluminum reacts with silica species to yield aluminosilicates. The removal of pollutants
from groundwater containing silica by EC using Fe as sacrificial anodes has already been
tested, but unfortunately, the silica removal has been very deficient (Wan et al., 2011).
The novelty of this paper consists in the simultaneous removal of fluoride and hydrated
silica from real groundwater, using a filter-press reactor with a novel design, in which the
horizontally located aluminum plate electrodes make up a six-cell stack, while at the top,
the cell is opened to the atmosphere allowing the fast release of hydrogen bubbles produced
at the cathodes. It examines the influence of the coagulant dosage (in terms of current
density employed) and the retention time (dictated by the mean linear flow velocity) on the
efficiency of the simultaneous removal of fluoride and hydrated silica. Spectroscopic
analyses such as XRD, SEM-EDS, XRF-EDS, and FTIR are performed to elucidate the
mechanism of elimination of the pollutants contained in groundwater.
2. Materials and methods
2.1. Deep well water
The real groundwater sample was obtained from the plateau region of Guanajuato in
Mexico (4.08 mg L-1 fluoride, 90 mg L-1 hydrated silica, 50 mg L-1 sulfate, 0.23 mg L-1 6
Ms. Ref. No.: CHEM64794R1 145
phosphate, 263 mg L−1 alkalinity, 50 mg L−1 hardness, pH 7.38 and 450 µS cm-1
conductivity), which exceeds the WHO guideline for fluoride; a high concentration of
hydrated silica was also found.
2.2 Electrocoagulation reactor
The sketch of the EC reactor and its components is shown in Fig. 1. The EC flow reactor is
composed of a stack of aluminum plate electrodes horizontally fitted so that the electrolyte
flows in the form of a serpentine; the cell is opened at the top to facilitate the fast release of
hydrogen bubbles formed on the cathode during the EC process.
Fig. 1. (a) Sketch of the reactor, (b) bottom plate, (c) channel separator, (d) aluminum
electrode, and (e) electrolyte collector at the exit.
Ms. Ref. No.: CHEM64794R1 159
The reactor in a serpentine array consist of a six-cell stack containing 8 empty channels
with 3 cm width, 8 cm length and 0.46 cm thickness, and 7 parallel aluminum plates as
electrodes (3 cm × 8 cm × 0.46 cm, width, length and thickness, in contact with electrolyte,
respectively), out of which four are used as cathodes and three as anodes. The electrolyte
inlet is located at the bottom of the cell, having a diameter of 1.27 cm. The top of the
reactor was designed, after several CFD simulation trials (not shown herein), to allow the
fast release of the gas generated in the cell, and therefore the cell was opened to the
atmosphere. Moreover, at the top of the reactor, there is a window of 3.4 cm length and 1.5
cm height, followed by a liquid collector of 10 cm in length to transport the electrolyte
towards the exit. More details on the cell can be consulted elsewhere (Castañeda and Nava,
2019). The dimensions of the EC reactor are shown in Table SM-1.
Fig. SM-1 shows a schematic diagram of the hydraulic and electric system coupled with the
EC reactor, which contains a 15 L-capacity reservoir for the groundwater sample, a
centrifugal pump (1/125 HP, Iwaki, MD-10L), a valve and a flowmeter (0.1-1 L min-1,
White Industries), all this joined to each other by a 0.5-inch diameter PVC pipe. A B&K
Precision 1090 power source was used to supply the current during the EC trials, which
directly records the cell potential.
The EC tests were carried out in the system shown in Fig. SM-1. Current densities (j) of 4,
5, 6 and 7 mA cm-2 and mean linear flow velocities (u) of 1.2, 2.4, 3.6 and 4.8 cm s-1 were
implemented for EC tests, which matched volumetric flow rates of 0.1, 0.2, 0.3 and 0.4 L
min-1, and retention times () of 55.9, 27.8, 18.5 and 13.9 s, respectively. The Faraday´s 8
Ms. Ref. No.: CHEM64794R1 183
law was used to calculate the theoretical value of the aluminum used as coagulant,
" ∙ % ∙ &' ( ∙ ) ∙ * ∙ +
(1 × 10. )
where () and j are given in mg L 2 and A cm , respectively, the molecular weight
of aluminum is Mw = 26.98 g mol 2 , 5 is the channel length (8 cm), the Faraday constant is
6= 96,485 C mol 2 , 8 = 3 is the number of electrons, 9 is the interelectrode gap (0.46
cm), and the factor 1×106 permits to obtain ()
electrocoagulation tests, 1 mg L-1 of hypochlorite (typical concentration used for
disinfection purposes) was added to groundwater to avoid passivation (Guzmán et al., 2016;
Rosales et al., 2018; Sandoval et al., 2014). It is worth to mention that the EC tests in the
reactor were stabilized for a period of 10 minutes to achieve the steady state. The EC tests
were performed in triplicate, obtaining similar results.
in mg L 2 . Before the
Once the electrolyte left the electrocoagulation reactor, it went to the jar test device, where
the coagulant produced inside the EC cell was slowly mixed (30 rpm) during 15 minutes,
so that the aggregate could grow; then, the flocs were left to rest for 60 minutes until the
aggregates settled. Afterward, the clarified solution, free of flocs, was analyzed to quantify
the residual concentration of fluoride, hydrated silica, and coexisting ions. The
experimental aluminum dose, () , formed in the EC trials was determined after the
redissolution of the flocs, using sulfuric acid to attain a pH = 2. Spectroscopy analyses were 9
Ms. Ref. No.: CHEM64794R1 205
carried out on the dry flocs. Before electrolysis, the electrodes were polished with 600
grade carbon emery paper, and then rinsed with plenty of water. The results were the
average of three EC tests.
2.4 Analytical procedure
2.4.1. Groundwater analysis
A HI 83200 multiparameter bench photometer, from Hanna instruments, was the equipment
used to measure hydrated silica, phosphate and sulfate. The silica analysis was carried out
by heteropoly blue method using the kit HI 93705. Phosphate was determined by amino
acid method using the HI 93706 kit and sulfate was determined by precipitation with
barium chloride crystals (light absorbance method) using the kit HI. The detection limit of
hydrated silica, phosphate, and sulfate was 0.2 mg L-1. The concentration of fluoride was
measured by a fluoride ion selective electrode (27502-19, Cole Palmer) with a detection
limit of 0.02 mg L−1. A Perkin Elmer AAnalyst™ 200 atomic absorption spectrometer, with
a detection limit of 0.1 mg L-1 (309.27 nm wavelength), was used to determine the
concentration of aluminum. Conductivity and pH measurements were carried using a
waterproof instrument from Hanna, model HI 991300. Analytical grade reagents were used.
The results were the average of three analyses.
2.4.2. Flocs characterization
The scanning electron microscopy (SEM) analysis was carried out using a JEOL JSM-6010
PLUS/LA device. The energy dispersive analysis of X-rays (EDS) was performed using a
JEOL detector incorporated in the SEM microscope. X-ray diffraction (XRD) analyzes
Ms. Ref. No.: CHEM64794R1 228
were made on a diffractometer Rigaku Ultima IV, with nickel filter and Cu K:2 radiation.
The elemental compositions of the flocs were determined by energy dispersive X-ray
fluorescence (XRF), using a Rigaku Nex CG X-ray fluorescence spectrometer, equipped
with an X-ray tube with Pd anode. The Fourier transform infrared spectroscopy (FTIR)
examination in the flocs was carried out in a Perkin Elmer Spectrum GX FTIR
Spectrometer, using an EasiDiff diffuse reflectance accessory.
2.5. Energy consumption and costs of EC
The energy consumption (ABCD), cost of aluminum dose ($F() ), and overall cost of EC
($OC) were calculated by Eqs. (6), (7) and (8), respectively:
GHIJJ ∙ K (..) ∙* ∙L ∙ +
where the units of ABCD , cell potential (ABM ), and I are kWh m-3, V, and C s 2 ,
respectively. N is the channel width (3 cm), S is the electrode spacing (0.46 cm), and the
factor 3.6 is used to obtain Econs in kWh m-3.
$() = O() P(2.008 USD Kg 2 )(0.001)
The aluminum price, in Mexico, is 2.008 USD kg 2 and 0.001 is a conversion factor to
obtain $() in USD m .
$ = $() + :ABCD + :EXYZX + βMY\]M
Ms. Ref. No.: CHEM64794R1 248
$OC is expressed in units of USD m-3, α is the cost of the electricity in central Mexico
(0.0976 USD (kWh)-1), Epump is in units of kWh m-3, and β is the sludge confinement cost
in Mexico (0.035 USD Kg 2 ).
3. Results and discussion
3.1 Removal of fluoride and hydrated silica by EC
Both the theoretical, () , and experimental, () , aluminum dosages and the residual
fluoride concentration, ^_ , are shown in Fig. 2. Results were obtained at different mean
linear flow velocities (1.2 < u < 4.8 cm s-1) and current densities of 4, 5, 6 and 7 mA cm-2.
The residual fluoride concentration after all the EC trials meets the WHO guideline (^_ <
1.5 mg L-1), evidencing a decrease with current density owing to the increase in the
experimental aluminum dosage (coagulant). A modest decrease in ^_ was obtained as a
function of mean linear flow velocity. The experimental aluminum dosage is greater than
that theoretically obtained by Faraday's law, Eq. 5, which is attributed to the chemical
oxidation of aluminum plates with the hypochlorite (1 mg L-1) present in the groundwater
Ms. Ref. No.: CHEM64794R1
Fig. 2. Effect of the mean linear flow velocity on the remaining fluoride concentration after
EC trials at different current densities: (a) 4 mA cm-2, (b) 5 mA cm-2, (c) 6 mA cm-2 and (d)
7 mA cm-2.
The residual concentrations of hydrated silica, sulfate, and phosphate are shown in Figs.
3(a), (b) and (c), respectively. This Fig. shows that the residual concentration of hydrated
silica (Chs) increases with flow rate due to the decrease in the coagulant dosage, and Chs
decreases with current density owing to the massive reaction between aluminum and silica
to yield aluminosilicates (Rosales et al., 2018). The best removal of hydrated silica was
obtained at 7 mA cm-2 and at 1.2 cm s-1 giving Chs = 6.2 mg L-1, equivalent to the removal
278 279 280
Ms. Ref. No.: CHEM64794R1
Fig. 3. Effect of the mean linear flow rate on the removal of: (a) hydrated silica, (b) sulfate
and (c) phosphate concentrations, after the same EC trials shown in Fig. 2.
Figure 3 (b) shows the residual concentration of sulfate, *`ab_ , that remains almost
constant as a function of flow rate but decreases with current density, which is attributable
to the coagulant dosage. The best removal of sulfate, *`ab_ = 25 mg L-1 (50% removal), was
obtained at 7 mA cm-2 and 1.2 cm s-1. The residual phosphate concentration, c`ad_ , Figure 14
Ms. Ref. No.: CHEM64794R1 289
3 (c), increases with u, owing to the decrease in the aluminum dosage, but decreases with j
(coagulant dosage). Once again, the best removal was obtained at 7 mA cm-2 and 1.2 cm s-1,
where the phosphate concentration was completely removed. These results agree with what
is reported in the literature, a well-known fact is that sulfate and phosphate ions are
adsorbed in the active sites of aluminum flocs (Guzmán et al., 2016). It is worth to mention
that the pH remains almost constant during the EC trials at a value of around 8.7. This
small variation is attributed to the substitution reaction between fluoride and hydroxide
from flocs during fluoride removal (Guzmán et al., 2016).
3.2 Flocs characterization
Figs. SM-2 (a)-(b) show SEM micrographs of the flocs obtained from EC test at 7 mA cm-2
and 1.2 cm s-1 at different scales. SM-2 (a) shows aggregates with sizes from less than 10
µm up to around 500 µm; whereas the aggregates from SM-2 (b) consist of particles with
sizes below 100 nm. SEM-EDS and XRF-EDS analyses were used to define the chemical
composition of aluminum flocs, and the results are shown in Table 1. According to the high
percentage of silicon obtained by SEM-EDS and XRF-EDS analyses, these revealed the
generation of aluminosilicate complexes (Guzmán et al., 2016; Rosales and Nava, 2018).
However, it was not possible to detect fluorine. The element compositions from both
analyses were similar, with small differences attributed to different sizes of sampled areas,
since the area sampled for XRF-EDS was 8.02 cm2 , compared with five areas of around
0.16 mm2 for SEM-EDS.
Ms. Ref. No.: CHEM64794R1 313
Table 1. Flocs composition obtained by XRF-EDS and SEM-EDS from EC at 7 mA cm-2
and 1.2 cm s-1. wt. %
Fig. 4 (a) shows a typical XRD pattern of flocs obtained by EC at 7 mA cm-2 and 1.2 cm s-
(Al1.52Ca0.52Na0.48O8Si2.48), oligoclase (Na0.723Ca0.277)(Al1.277Si2.723)O8, lisetite (Ca0.98Na
((Na0.98Ca0.02)(Al0.02Si2.97)O8). These aluminosilicate phases adsorb arsenates, sulfates, and
phosphates (Guzmán et al., 2016; Rosales and Nava, 2018). The carbonates (alkalinity)
contained in the groundwater precipitate as calcite after the EC tests.
. The broad peaks are produced by the superposition of peaks that could correspond to (CaCO3),
FTIR spectrum obtained for wave numbers between 4000 and 400 cm-1 is shown in Fig. 4
(b), after EC tests at 7 mA cm-2 and u = 1.2 cm s-1. The peaks were identified using the
reference (Socrates, 2004). The peaks match with the chemical bonds O-H, Na-F, Al-O, Al-
O-Si, Si-O and Al-F, which agree with those reported in the literature for EC tests with
aluminum as a sacrificial anode (Drouiche et al., 2009; Ghosch et al., 2008; Guzmán et al.,
2016). The peaks placed on 599 cm-1 related to the Al-F bounding prove the chemical
substitution reaction between fluoride and hydroxide from aluminum aggregates (Sandoval
eta al., 2014). The Al-O-Si bond confirms the reaction between aluminum coagulant and
silica to yield aluminosilicates. It is worth mentioning that sulfate and phosphate bounds 16
Ms. Ref. No.: CHEM64794R1 334
were not detected in the FTIR spectra, possibly because those were encapsulated inside the
flocs; however, it is well known that both anions are removed by adsorption onto aluminum
aggregates (Guzmán et al., 2016; Thakur and Modal, 2017).
Fig. 4. Characteristic (a) XRD and (b) FTIR spectra of the flocs from EC at 7 mA cm-2 and
1.2 cm s-1.
3.3 Energy consumption and operational EC costs
According to the results condensed in Table 2, Ecell decreases with u for all the j tested,
which is related to the fast removal of the coagulant from the electrode to the bulk solution, 17
Ms. Ref. No.: CHEM64794R1 345
decreasing the resistance on the electrode (preventing passivation), as well as the ohmic
drop in the interelectrode space, to provide the fast release of H2 bubbles from the solution
to the atmosphere. It is noteworthy that the Msludge decreases with u but increases with j, as
expected; however, the sludge generated by EC is minor and varies between 0.037 < Msludge
< 0.425 kg m-3. The residual concentration of fluoride adheres to the WHO guideline (1.5
mg L-1 < CF) for all the EC trials.
The best removal of hydrated silica was obtained at 7 mA cm-2 and of 1.2 cm s-1, giving a
Chs = 6.2 mg L-1, with the operational cost of EC of $OC = 0.441 USD m-3. Finally, it is
important to mention that the operational cost of EC reported here may vary in other
countries, due to the fluctuation of prices of the electricity and sludge confinement.
Ms. Ref. No.: CHEM64794R1
Table 2. Remaining fluoride and hydrated silica concentrations after EC tests, as well as experimental aluminum dose, cost of aluminum dose, Ecell, Econs, Msludge, and overall cost of EC. Initial composition of groundwater: 4.08 mg L−1 fluoride, 90 mg L−1 hydrated silica, 0.23 mg L−1 phosphate, 50 mg L−1 sulfate, 263 mg L−1 alkalinity, 50 mg L−1 hardness, conductivity 450 µS cm−1 and pH 7.4. CAl(III) Chs Econs Epump Msludge j u $ OC Ecell ^_ τ -2 -1 -3 -1 -1 -1 -3 -3 -3 (s) (mg L ) (mg L ) (V) (kWh m ) (kWh m ) (kg m ) (USD m ) (mA cm ) (cm s ) (mg L ) 55.9 1.2 0.995 27.4 7.8 0.754 0.5 0.164 0.208 43.2 27.8 2.4 1.213 64 7.3 0.353 0.25 0.093 0.116 29.3 4 18.5 3.6 16.8 1.345 67.5 7.05 0.227 0.16 0.037 0.070 13.9 4.8 14.6 1.370 72.5 6.99 0.169 0.125 0.075 0.058 55.9 1.2 54.3 0.942 17 8.8 1.063 0.5 0.316 0.264 27.8 2.4 36.1 0.966 57 8.6 0.519 0.25 0.080 0.144 5 18.5 21.4 3.6 1.080 64 8.1 0.326 0.16 0.124 0.091 13.9 4.8 1.360 71 8.5 0.257 0.125 0.695 0.095 18.2 55.9 1.2 58 0.847 13.6 11 1.594 0.5 0.313 0.322 27.8 2.4 0.871 51 10.9 0.790 0.25 0.089 0.180 41.1 6 18.5 3.6 26.1 0.970 60.5 9.5 0.459 0.16 0.095 0.112 13.9 4.8 1.052 69 9.4 0.341 0.125 0.061 0.087 21.4 55.9 1.2* 0.764 6.2 14.7 2.486 0.5 0.425 0.441 73.2 27.8 2.4 43.3 0.804 21 14.2 1.200 0.25 0.247 0.230 7 18.5 3.6 26.5 0.823 57 12.6 0.710 0.16 0.069 0.136 13.9 4.8 20.4 0.858 65 10.3 0.435 0.125 0.050 0.094 -1 -1 -1 -1 *Other residual concentrations are: 0 mg L phosphate, 25 mg L sulfate, 178.8 mg L alkalinity, 24 mg L hardness, 416 µS cm-1 conductivity and pH 8.7.
Ms. Ref. No.: CHEM64794R1 337
The residual concentration of fluoride after the EC treatment adhered to the WHO guideline
(< 1.5 mg L-1), while the hydrated silica was completely removed at 7 mA cm-2 and 1.2 cm
s-1, with energy consumption and overall operational cost of 2.48 kWh m-3 and 0.441 USD
m-3, respectively. XRD, XRF-EDS, SEM-EDS and FTIR analyses on the flocs confirmed
that during the EC process, the aluminum reacted with silica forming aluminosilicates.
Meanwhile, fluoride substituted a hydroxide from aluminum flocs, and the sulfates and
phosphates were removed by adsorption process onto aluminum aggregates.
The well-engineered EC reactor permitted to apply high current densities to generate high
aluminum dosages (14-73 mg L-1) that reacted with hydrated silica allowing its complete
removal. Therefore, the EC process has great potential to be applied in the industry,
particularly for the preconditioning of water containing silica and affordable treatment
The authors thank to SICES (project No. IJ-19-78), CONACYT (project No. 759) and the
University of Guanajuato (projects No. 102/2019, 150/2019) for financial support. Authors
acknowledge Dr. Raul Miranda and Daniela Moncada from LICAMM-UG Laboratory for
Ms. Ref. No.: CHEM64794R1 359
Battula, S.K., Cheukuri, J., Raman, N.V.V.S., Himabindu, V., Bhagawan, D., 2014.
Effective removal of fluoride from ground water using electro-coagulation. Int. J. Eng. Res.
Appl. 4, 439-445.
Behbahani, M., Alavi, M.R., Arami, M., 2011. Techno-economical evaluation of fluoride
methodology. Desalin. 271, 209-218.
Camargo J.A., 2003. Fluoride toxicity to aquatic organisms: a review. Chemosphere 50,
Castañeda, L.F., Nava, J.L., 2019. Simulations of single-phase flow in an up-flow
electrochemical reactor with parallel plate electrodes in a serpentine array. J. Elecroanal.
Chem. 832, 31-39.
Drouiche, N., Aoudja, S., Hecinia, M., Ghaffour, N., Lounici, H., Mameri, N., 2009. Study
on the treatment of photovoltaic wastewater using electrocoagulation: fluoride removal
with aluminum electrodes–characteristics of products. J. Hazard. Mater. 169, 65–69.
Emamjomeh, M.M., Sivakumar, M., 2009a.
Fluoride removal by a continuous flow
electrocoagulation reactor. J. Environ Manage. 90, 1204–1212.
Emamjomeh, M.M., Sivakumar, M., 2009b.
Review of pollutants removed by
electrocoagulation and electrocoagulation/flotation processes. J. Environ Manage. 90,
Emamjomeh, M.M., Sivakumar, M., Varyani, A.S., 2011. Analysis and the understanding
of fluoride removal mechanisms by an electrocoagulation/flotation (ECF) process. Desalin.
Essadki, A.H., Gourich, B., Vial, Ch., Delmas, H., Bennajah, M., 2009. Defluoridation of
drinking water by electrocoagulation/electroflotation in a stirred tank reactor with a 21
Ms. Ref. No.: CHEM64794R1 390
comparative performance to an external-loop airlift reactor. J. Hazard. Mater. 168, 1325-
Gelover, S.L., Pérez, S., Martín, A., Villegas, I.E., 2012. Electrogeneration of aluminium to
remove silica in water. Water Sci. Technol. 65, 434-439.
Ghosh, D., Medhi, C.R., Purkait, M.K., 2008. Treatment of fluoride containing drinking
water by electrocoagulation using monopolar and bipolar electrode connections.
Chemosphere 73, 1393–1400.
Guzmán, A., Nava, J.L., Coreño, O., Rodríguez, I., Gutiérrez, S., 2016. Arsenic and
fluoride removal from groundwater by electrocoagulation using a continuous filter-press
reactor. Chemosphere 144, 2113–2120.
He, G.L., Cao, S.R., 1996. Assessment of fluoride removal from drinking water by calcium
phosphate systems. Fluoride 29, 212–216.
Hu C.Y., Lo S.L., Kuan W.H., 2003. Effects of co-existing anions on fluoride removal in
electrocoagulation (EC) process using aluminum electrodes. Water Res. 37, 4513–4523.
Hu, Ch.Y., Lo, S.L., Kuan, W.H., 2007. Simulation the kinetics of fluoride removal by
electrocoagulation (EC) process using aluminum electrodes. J Hazard Mater. 145, 180-185.
Hu, K., Dickson, J.M. 2006. Nanofiltration membrane performance on fluoride removal
from water. J. Membr. Sci. 279, 529–538.
Maleki, A., Mahvi, A.H., Daraei, H., Rezaei, R., Meihami, N., Mohammadi, K., Zandi S.,
electrocoagulation/electroflotation. Fluoride. 48, 23-47.
Ms. Ref. No.: CHEM64794R1 420
Medel A., Ramírez J.A., Cárdenas J., Sirés I., Meas Y., 2019. Evaluating the
electrochemical and photoelectrochemical production of hydroxyl radical during
electrocoagulation process. Sep. Purif. Technol. 208, 59-67.
Merget, R., Bauer, T., Küpper, H.U., Philippou, S., Bauer, H.D., Breitstad, R., Bruening,
T., 2002. Health hazards due to the inhalation of amorphous silica. Arch. Toxicol. 75, 625-
Mohammad, M., Muttucumaru, S., 2011. Analysis and the understanding of fluoride
removal mechanisms by an electrocoagulation/flotation (ECF) process. Desalin. 275 102–
Rosales, M., Coreño, O., Nava, J.L., 2018. Removal of hydrated silica, fluoride and arsenic
from groundwater by electrocoagulation using a continuous reactor with a twelve-cell
stack. Chemosphere 211, 149-155.
Sakar, A., Paul, B., 2016. The global menace of arsenic and its conventional remediation –
A critical review. Chemosphere 158, 37-49.
Sandoval, M.A, Fuentes, R., Nava, J.L., Rodríguez, I., 2014. Fluoride removal from
drinking water by electrocoagulation in a continuous filter press reactor coupled to a
flocculator and clarifier. Sep. Purif. Technol. 134, 163–170.
Sandoval, M.A, Fuentes, R., Nava, J.L., Coreño, O., Li, Y.M., Hernández, J.H., 2019.
Simultaneous removal of fluoride and arsenic from groundwater by electrocoagulation
using a filter-press flow reactor with a three-cell stack. Sep. Purif. Technol. 208, 208-216.
Sariñana-Ruiz, Y.A., Vazquez-Arenas, J., Sosa-Rodríguez, F.S., Labastida, I., Armienta
M.A., Armienta, A., Aragón-Piña, A., Escobedo-Bretado, M.A., González-Valdez, L.S.,
Ponce-Peña, P., Ramírez-Aldaba, H., Lara R.H., 2017. Assessment of arsenic and fluorine
Ms. Ref. No.: CHEM64794R1 450
in surface soil to determine environmental and health risk factors in the Comarca Lagunera,
Mexico. Chemosphere 178, 391-401.
Singh, J., Singh, P., Singh, A., 2016. Fluoride ions vs removal technologies: a study. Arab.
J. Chem. 9, 815–824.
Singh, K., Lataye, D.H., Wasewar, K.L., Yoo, Ch.K., 2013. Removal of fluoride from
aqueous solution: status and techniques. Desalin. Water Treat. 51, 3233–3247.
Socrates, G., 2004. Infrared and Raman Characteristic Group Frequencies: Tables and
Charts, third ed. Wiley, United Kindom.
Thakur, L.S., Modal, P., 2017. Simultaneous arsenic and fluoride removal from synthetic
and real groundwater by electrocoagulation process: Parametric and cost evaluation, J.
Environ. Manag. 190, 102-112.
Tirado L., Gökkuş Ö., Brillas E. Sirés I., 2018. Treatment of cheese whey wastewater by
combined electrochemical Processes, J. Appl. Electrochem. 48, 1307-1319.
Tor, A., 2007. Removal of fluoride from water using anion-exchange membrane under
Donnan dialysis condition. J. Hazard. Mater. 141, 814–818.
Wan, W., Pepping, T.J., Banerji, T., Chaudhari, S., Giammar, D.E., 2011. Effects of water
chemistry on arsenic removal from drinking water by electrocoagulation. Water Res. 45,
Zhao, X., Zhang, B., Liu, H., Qu, J., 2011. Simultaneous removal of arsenite and fluoride
via an integrated elctro-oxidation and electrocoagulation process. Chemosphere 83, 726-
Ms. Ref. No.: CHEM64794R1 480
Zhu, J., Zhao, H., Ni, J., 2007. Fluoride distribution in electrocoagulation defluoridation
process. Sep. Purif. Technol. 56, 184-191.
Highlights • Abatement of fluoride and silica from underground water by electrocoagulation. • Flow channel cell with aluminum electrodes open to the atmosphere to favor H2 exit. • Significant aluminum dosages were produced at current densities > 6 mA cm-2. • Hydrated silica reacted with the coagulant forming aluminosilicates as flocs. •
The removal of fluoride followed the WHO recommendation, while silica was abated.
Authors contribution section All authors participated in the preparation of the paper contributing ideas to carry out experiments, discussions, and assisted in the final writing of the manuscript.
Declaration of Interest Statement
The authors declare that there is no conflict of interest regarding the publication of this paper.