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Removal of fluoride and hydrated silica from underground water by electrocoagulation in a flow channel reactor
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:
S0045-6535(19)32657-8
DOI:
https://doi.org/10.1016/j.chemosphere.2019.125417
Reference:
CHEM 125417
To appear in:
ECSN
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
1
electrocoagulation in a flow channel reactor
2 3
Locksley F. Castañedaa, Oscar Coreñob, José L. Navaa,*, Gilberto Carreñoa
4 5
a
Departamento de Ingeniería Geomática e Hidráulica, Universidad de Guanajuato, Av.
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Juárez 77, Centro, 36000, Guanajuato, Guanajuato, Mexico. E-mail: [email protected];
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[email protected]; [email protected]
8 9
b
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
*Corresponding
author: [email protected]
23
Tel: + 52-473-1020100 ext. 2289; fax: + 52-473-1020100 ext. 2209
24
1
Ms. Ref. No.: CHEM64794R1 25
Abstract
26
This paper concerns simultaneous removal of fluoride and hydrated silica from
27
groundwater (4.08 mg L-1 fluoride, 90 mg L-1 hydrated silica, 50 mg L-1 sulfate, 0.23 mg L-
28
1
29
up-flow EC reactor, with a six-cell stack in a serpentine array, opened at the top of the cell
30
to favor gas release. Aluminum plates were used as sacrificial electrodes. The effect of
31
current density (4 ≤ j ≤ 7 mA cm-2) and mean linear flow rate (1.2 ≤ u ≤ 4.8 cm s-1), applied
32
to the EC reactor, on the elimination of fluoride and hydrated silica was analyzed. The
33
removal of fluoride followed the WHO guideline (< 1.5 mg L-1), while the hydrated silica
34
was abated at 7 mA cm-2 and 1.2 cm s-1, with energy consumption of 2.48 kWh m-3 and an
35
overall operational cost of 0.441 USD m-3. Spectroscopic analyses of the flocs by XRD,
36
XRF-EDS, SEM-EDS, and FTIR indicated that hydrated silica reacted with the coagulant
37
forming aluminosilicates, and fluoride replaced a hydroxide from aluminum aggregates,
38
while sulfates and phosphates were removed by adsorption process onto the flocs. The
39
well-engineered EC reactor allowed the simultaneous removal of fluoride and hydrated
40
silica.
phosphate, pH 7.38 and 450 µS cm-1conductivity) by electrocoagulation (EC), using an
41 42
Keywords: Hydrated silica removal; Electrocoagulation; Aluminum electrodes; Fluoride
43
removal; Groundwater.
44 45 46 47 48
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Ms. Ref. No.: CHEM64794R1 49 50
1. Introduction
51
Groundwater is an important source of water supply for human consumption, which in arid
52
and semiarid regions worldwide is contaminated by inorganic salts, metalloids, and metals,
53
among other pollutants. Mexico is not the exception and in different areas, it is common to
54
find groundwater contaminated mainly with fluoride and hydrated silica, whose
55
concentrations range between 1-9.5 mg L-1, and 50-132 mg L-1, respectively (Guzmán et
56
al., 2016; Rosales et al., 2018; Sandoval et al., 2014). Other ions such as phosphates and
57
sulfates are also found in groundwater. The contamination of groundwater by fluorides and
58
hydrated silica occurs mainly from the dissolution of minerals in contact with water
59
(Battula et al., 2014; Behbahani et al., 2011; Emamjomeh et al., 2011; Zhu et al., 2007).
60 61
Both Mexican standard and international regulations, such as the World Health
62
Organization (WHO), indicate that the maximum allowable fluoride concentration in water
63
for human consumption must be <1.5 mg L-1. While at low concentrations fluoride is
64
beneficial in helping to combat tooth decay (Emamjomeh et al., 2009a; Essadki et al.,
65
2009), at concentrations > 4 mg L-1 it can cause severe harm to human health, such as
66
thyroid disorder, neurological damage, mottled teeth, skeletal and dental fluorosis,
67
osteoporosis and diseases in the kidneys, lungs, liver, muscles, and nerves, among others
68
(Camargo, 2003; Emamjomeh et al., 2011; Gosh et al., 2008; Hu et al., 2003; Maleki et al.,
69
2015).
70 71
Regarding hydrated silica, there is currently no official standard that establishes the
72
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
74
in the lungs, generating diseases such as tuberculosis, silicosis, bronchitis, and cancer
75
(Merget et al., 2002; Rosales et al., 2018; Sakar and Paul, 2016; Sariñana-Ruiz et al., 2017).
76
In addition to the above, hydrated silica can also seriously affect pipelines and certain unit
77
operations of industries that use this kind of water in their processes, permeating the walls
78
of equipment and causing failures (Gelover et al., 2012; Rosales et al., 2018).
79 80
One of the most used processes for fluoride removal from water is the precipitation-
81
flocculation by aluminum and calcium salts (Singh et al., 2016). This process produces too
82
much sludge because the counter-ion of the above-mentioned salts consumes coagulant
83
(Singh et al., 2013). Other processes such as adsorption (Gosh et al., 2008; Zhao et al.,
84
2011), chemical precipitation (He and Cao, 1996), and membrane process (Hu and
85
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
89
that destabilizes anions and dispersed fine particles from contaminated water by electrolysis
90
(Emamjomeh et al., 2009b). The EC technology allows the possibility of automating the
91
process, while the installation of the treatment is compact, does not need the addition of
92
chemicals, and the generation of sludge is minimal (Hu et al. 2007; Essadki et al., 2009;
93
Emamjomeh et al., 2009b), It can even be used together with other electrochemical
94
processes (Medel et al., 2019; Tirado et al., 2018).
95 96
Some reported works indicate the use of aluminum electrodes as sacrificial anodes in the
97
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-
99
scale, can be easily scalable at pilot plants (Castañeda and Nava, 2019).
100 101
The EC process involves the generation of coagulants in situ by electrodissolution of
102
aluminum sacrificial anodes, Eq. 1, while in the bulk of the solution, at neutral pH, the
103
formation of aluminum salts takes place, Eqs. 2, 3. At the cathode, the evolution of
104
hydrogen gas bubbles occurs, Eq. 4.
105 106
Al() → Al + 3e
(1)
107
+ 3 → ( )() + 3
(2)
108
2 + 3 → + 6
(3)
109
3H O + 3e → 1.5H + 3OH
(4)
110 111
According to the literature, the removal of fluoride occurs via co-precipitation of fluoro-
112
aluminum complexes and a chemical substitution reaction between a fluoride ion and a
113
hydroxide from aluminum flocs (Hu et al., 2003; Mohamad et al., 2011). Meanwhile, the
114
removal of hydrated silica occurs by the formation of aluminosilicates (Guzman et al.,
115
2016; Rosales et al., 2018). Other coexisting ions such as phosphates and sulfates have
116
been partially removed by adsorption processes on aluminum aggregates (Rosales et al.,
117
2018; Sandoval et al., 2019).
118 119
A common problem with aluminum electrodes is the anodic passivation. The use of flow
120
cells with plate electrodes favors the transport of Al3+ ions from the electrode surface to the
5
Ms. Ref. No.: CHEM64794R1 121
bulk of the solution, diminishing the passivation of the anode (Rosales et al., 2018;
122
Sandoval et al., 2019).
123
On the other hand, hydrated silica (72 mg L-1) has been efficiently removed from
124
groundwater by EC using several aluminum plate electrodes fitted in a stack of a flow EC
125
reactor opened to the atmosphere (Rosales et al., 2018). It is worth mentioning that this
126
paper shows for the first time the efficient abatement of hydrated silica by EC, highlighting
127
that aluminum reacts with silica species to yield aluminosilicates. The removal of pollutants
128
from groundwater containing silica by EC using Fe as sacrificial anodes has already been
129
tested, but unfortunately, the silica removal has been very deficient (Wan et al., 2011).
130 131
The novelty of this paper consists in the simultaneous removal of fluoride and hydrated
132
silica from real groundwater, using a filter-press reactor with a novel design, in which the
133
horizontally located aluminum plate electrodes make up a six-cell stack, while at the top,
134
the cell is opened to the atmosphere allowing the fast release of hydrogen bubbles produced
135
at the cathodes. It examines the influence of the coagulant dosage (in terms of current
136
density employed) and the retention time (dictated by the mean linear flow velocity) on the
137
efficiency of the simultaneous removal of fluoride and hydrated silica. Spectroscopic
138
analyses such as XRD, SEM-EDS, XRF-EDS, and FTIR are performed to elucidate the
139
mechanism of elimination of the pollutants contained in groundwater.
140 141
2. Materials and methods
142
2.1. Deep well water
143
The real groundwater sample was obtained from the plateau region of Guanajuato in
144
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
146
conductivity), which exceeds the WHO guideline for fluoride; a high concentration of
147
hydrated silica was also found.
148 149
2.2 Electrocoagulation reactor
150
The sketch of the EC reactor and its components is shown in Fig. 1. The EC flow reactor is
151
composed of a stack of aluminum plate electrodes horizontally fitted so that the electrolyte
152
flows in the form of a serpentine; the cell is opened at the top to facilitate the fast release of
153
hydrogen bubbles formed on the cathode during the EC process.
154
155 156
Fig. 1. (a) Sketch of the reactor, (b) bottom plate, (c) channel separator, (d) aluminum
157
electrode, and (e) electrolyte collector at the exit.
158
7
Ms. Ref. No.: CHEM64794R1 159
The reactor in a serpentine array consist of a six-cell stack containing 8 empty channels
160
with 3 cm width, 8 cm length and 0.46 cm thickness, and 7 parallel aluminum plates as
161
electrodes (3 cm × 8 cm × 0.46 cm, width, length and thickness, in contact with electrolyte,
162
respectively), out of which four are used as cathodes and three as anodes. The electrolyte
163
inlet is located at the bottom of the cell, having a diameter of 1.27 cm. The top of the
164
reactor was designed, after several CFD simulation trials (not shown herein), to allow the
165
fast release of the gas generated in the cell, and therefore the cell was opened to the
166
atmosphere. Moreover, at the top of the reactor, there is a window of 3.4 cm length and 1.5
167
cm height, followed by a liquid collector of 10 cm in length to transport the electrolyte
168
towards the exit. More details on the cell can be consulted elsewhere (Castañeda and Nava,
169
2019). The dimensions of the EC reactor are shown in Table SM-1.
170 171
Fig. SM-1 shows a schematic diagram of the hydraulic and electric system coupled with the
172
EC reactor, which contains a 15 L-capacity reservoir for the groundwater sample, a
173
centrifugal pump (1/125 HP, Iwaki, MD-10L), a valve and a flowmeter (0.1-1 L min-1,
174
White Industries), all this joined to each other by a 0.5-inch diameter PVC pipe. A B&K
175
Precision 1090 power source was used to supply the current during the EC trials, which
176
directly records the cell potential.
177 178
2.3 Methodology
179
The EC tests were carried out in the system shown in Fig. SM-1. Current densities (j) of 4,
180
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
181
implemented for EC tests, which matched volumetric flow rates of 0.1, 0.2, 0.3 and 0.4 L
182
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,
184
CAl(III)(N):
185
186
( ) =
" ∙ % ∙ &' ( ∙ ) ∙ * ∙ +
(1 × 10. )
(5)
187
188
where ( ) and j are given in mg L 2 and A cm , respectively, the molecular weight
189
of aluminum is Mw = 26.98 g mol 2 , 5 is the channel length (8 cm), the Faraday constant is
190
6= 96,485 C mol 2 , 8 = 3 is the number of electrons, 9 is the interelectrode gap (0.46
191
cm), and the factor 1×106 permits to obtain ( )
192
electrocoagulation tests, 1 mg L-1 of hypochlorite (typical concentration used for
193
disinfection purposes) was added to groundwater to avoid passivation (Guzmán et al., 2016;
194
Rosales et al., 2018; Sandoval et al., 2014). It is worth to mention that the EC tests in the
195
reactor were stabilized for a period of 10 minutes to achieve the steady state. The EC tests
196
were performed in triplicate, obtaining similar results.
in mg L 2 . Before the
197 198
Once the electrolyte left the electrocoagulation reactor, it went to the jar test device, where
199
the coagulant produced inside the EC cell was slowly mixed (30 rpm) during 15 minutes,
200
so that the aggregate could grow; then, the flocs were left to rest for 60 minutes until the
201
aggregates settled. Afterward, the clarified solution, free of flocs, was analyzed to quantify
202
the residual concentration of fluoride, hydrated silica, and coexisting ions. The
203
experimental aluminum dose, ( ) , formed in the EC trials was determined after the
204
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
206
grade carbon emery paper, and then rinsed with plenty of water. The results were the
207
average of three EC tests.
208 209
2.4 Analytical procedure
210
2.4.1. Groundwater analysis
211
A HI 83200 multiparameter bench photometer, from Hanna instruments, was the equipment
212
used to measure hydrated silica, phosphate and sulfate. The silica analysis was carried out
213
by heteropoly blue method using the kit HI 93705. Phosphate was determined by amino
214
acid method using the HI 93706 kit and sulfate was determined by precipitation with
215
barium chloride crystals (light absorbance method) using the kit HI. The detection limit of
216
hydrated silica, phosphate, and sulfate was 0.2 mg L-1. The concentration of fluoride was
217
measured by a fluoride ion selective electrode (27502-19, Cole Palmer) with a detection
218
limit of 0.02 mg L−1. A Perkin Elmer AAnalyst™ 200 atomic absorption spectrometer, with
219
a detection limit of 0.1 mg L-1 (309.27 nm wavelength), was used to determine the
220
concentration of aluminum. Conductivity and pH measurements were carried using a
221
waterproof instrument from Hanna, model HI 991300. Analytical grade reagents were used.
222
The results were the average of three analyses.
223 224
2.4.2. Flocs characterization
225
The scanning electron microscopy (SEM) analysis was carried out using a JEOL JSM-6010
226
PLUS/LA device. The energy dispersive analysis of X-rays (EDS) was performed using a
227
JEOL detector incorporated in the SEM microscope. X-ray diffraction (XRD) analyzes
10
Ms. Ref. No.: CHEM64794R1 228
were made on a diffractometer Rigaku Ultima IV, with nickel filter and Cu K:2 radiation.
229
The elemental compositions of the flocs were determined by energy dispersive X-ray
230
fluorescence (XRF), using a Rigaku Nex CG X-ray fluorescence spectrometer, equipped
231
with an X-ray tube with Pd anode. The Fourier transform infrared spectroscopy (FTIR)
232
examination in the flocs was carried out in a Perkin Elmer Spectrum GX FTIR
233
Spectrometer, using an EasiDiff diffuse reflectance accessory.
234
2.5. Energy consumption and costs of EC
235
The energy consumption (ABCD), cost of aluminum dose ($F ( ) ), and overall cost of EC
236
($OC) were calculated by Eqs. (6), (7) and (8), respectively:
237
238
ABCD =
GHIJJ ∙ K (..) ∙* ∙L ∙ +
(6)
239
240
where the units of ABCD , cell potential (ABM ), and I are kWh m-3, V, and C s 2 ,
241
respectively. N is the channel width (3 cm), S is the electrode spacing (0.46 cm), and the
242
factor 3.6 is used to obtain Econs in kWh m-3.
243
$ ( ) = O ( ) P(2.008 USD Kg 2 )(0.001)
(7)
244
245
The aluminum price, in Mexico, is 2.008 USD kg 2 and 0.001 is a conversion factor to
246
obtain $ ( ) in USD m .
247
$ = $ ( ) + :ABCD + :EXYZX + βM Y\]M
(8) 11
Ms. Ref. No.: CHEM64794R1 248
249
$OC is expressed in units of USD m-3, α is the cost of the electricity in central Mexico
250
(0.0976 USD (kWh)-1), Epump is in units of kWh m-3, and β is the sludge confinement cost
251
in Mexico (0.035 USD Kg 2 ).
252
253
3. Results and discussion
254
3.1 Removal of fluoride and hydrated silica by EC
255
Both the theoretical, ( ) , and experimental, ( ) , aluminum dosages and the residual
256
fluoride concentration, ^_ , are shown in Fig. 2. Results were obtained at different mean
257
linear flow velocities (1.2 < u < 4.8 cm s-1) and current densities of 4, 5, 6 and 7 mA cm-2.
258
The residual fluoride concentration after all the EC trials meets the WHO guideline ( ^_ <
259
1.5 mg L-1), evidencing a decrease with current density owing to the increase in the
260
experimental aluminum dosage (coagulant). A modest decrease in ^_ was obtained as a
261
function of mean linear flow velocity. The experimental aluminum dosage is greater than
262
that theoretically obtained by Faraday's law, Eq. 5, which is attributed to the chemical
263
oxidation of aluminum plates with the hypochlorite (1 mg L-1) present in the groundwater
264
sample.
265
12
Ms. Ref. No.: CHEM64794R1
266 267
Fig. 2. Effect of the mean linear flow velocity on the remaining fluoride concentration after
268
EC trials at different current densities: (a) 4 mA cm-2, (b) 5 mA cm-2, (c) 6 mA cm-2 and (d)
269
7 mA cm-2.
270 271
The residual concentrations of hydrated silica, sulfate, and phosphate are shown in Figs.
272
3(a), (b) and (c), respectively. This Fig. shows that the residual concentration of hydrated
273
silica (Chs) increases with flow rate due to the decrease in the coagulant dosage, and Chs
274
decreases with current density owing to the massive reaction between aluminum and silica
275
to yield aluminosilicates (Rosales et al., 2018). The best removal of hydrated silica was
276
obtained at 7 mA cm-2 and at 1.2 cm s-1 giving Chs = 6.2 mg L-1, equivalent to the removal
277
of 96%.
278 279 280
13
Ms. Ref. No.: CHEM64794R1
281 282
Fig. 3. Effect of the mean linear flow rate on the removal of: (a) hydrated silica, (b) sulfate
283
and (c) phosphate concentrations, after the same EC trials shown in Fig. 2.
284 285
Figure 3 (b) shows the residual concentration of sulfate, *`ab_ , that remains almost
286
constant as a function of flow rate but decreases with current density, which is attributable
287
to the coagulant dosage. The best removal of sulfate, *`ab_ = 25 mg L-1 (50% removal), was
288
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
290
(coagulant dosage). Once again, the best removal was obtained at 7 mA cm-2 and 1.2 cm s-1,
291
where the phosphate concentration was completely removed. These results agree with what
292
is reported in the literature, a well-known fact is that sulfate and phosphate ions are
293
adsorbed in the active sites of aluminum flocs (Guzmán et al., 2016). It is worth to mention
294
that the pH remains almost constant during the EC trials at a value of around 8.7. This
295
small variation is attributed to the substitution reaction between fluoride and hydroxide
296
from flocs during fluoride removal (Guzmán et al., 2016).
297 298
3.2 Flocs characterization
299 300
Figs. SM-2 (a)-(b) show SEM micrographs of the flocs obtained from EC test at 7 mA cm-2
301
and 1.2 cm s-1 at different scales. SM-2 (a) shows aggregates with sizes from less than 10
302
µm up to around 500 µm; whereas the aggregates from SM-2 (b) consist of particles with
303
sizes below 100 nm. SEM-EDS and XRF-EDS analyses were used to define the chemical
304
composition of aluminum flocs, and the results are shown in Table 1. According to the high
305
percentage of silicon obtained by SEM-EDS and XRF-EDS analyses, these revealed the
306
generation of aluminosilicate complexes (Guzmán et al., 2016; Rosales and Nava, 2018).
307
However, it was not possible to detect fluorine. The element compositions from both
308
analyses were similar, with small differences attributed to different sizes of sampled areas,
309
since the area sampled for XRF-EDS was 8.02 cm2 , compared with five areas of around
310
0.16 mm2 for SEM-EDS.
311 312
15
Ms. Ref. No.: CHEM64794R1 313
Table 1. Flocs composition obtained by XRF-EDS and SEM-EDS from EC at 7 mA cm-2
314
and 1.2 cm s-1. wt. %
Al
Si
S
Cl
K
Ca
O
Na
SEM-EDS
29.02
8.79
0.15
0.68
ND
4.49
54.92
2.01
XRF-EDS
16.45
4.90
0.27
0.45
0.22
1.87
73.24
2.60
315 316
Fig. 4 (a) shows a typical XRD pattern of flocs obtained by EC at 7 mA cm-2 and 1.2 cm s-
317
1
318
calcite
319
(Al1.52Ca0.52Na0.48O8Si2.48), oligoclase (Na0.723Ca0.277)(Al1.277Si2.723)O8, lisetite (Ca0.98Na
320
(Al3.96Si4.04)O16),
321
((Na0.98Ca0.02)(Al0.02Si2.97)O8). These aluminosilicate phases adsorb arsenates, sulfates, and
322
phosphates (Guzmán et al., 2016; Rosales and Nava, 2018). The carbonates (alkalinity)
323
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),
bytownite
labradorite
(Ca0.43Na0.07(Al0.92Si1.0O)4),
(Ca0.325Na0.16
(Al0.81Si1.19)O4,
anorthite
and
1.97
albite
324 325
FTIR spectrum obtained for wave numbers between 4000 and 400 cm-1 is shown in Fig. 4
326
(b), after EC tests at 7 mA cm-2 and u = 1.2 cm s-1. The peaks were identified using the
327
reference (Socrates, 2004). The peaks match with the chemical bonds O-H, Na-F, Al-O, Al-
328
O-Si, Si-O and Al-F, which agree with those reported in the literature for EC tests with
329
aluminum as a sacrificial anode (Drouiche et al., 2009; Ghosch et al., 2008; Guzmán et al.,
330
2016). The peaks placed on 599 cm-1 related to the Al-F bounding prove the chemical
331
substitution reaction between fluoride and hydroxide from aluminum aggregates (Sandoval
332
eta al., 2014). The Al-O-Si bond confirms the reaction between aluminum coagulant and
333
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
335
flocs; however, it is well known that both anions are removed by adsorption onto aluminum
336
aggregates (Guzmán et al., 2016; Thakur and Modal, 2017).
337
338 339
Fig. 4. Characteristic (a) XRD and (b) FTIR spectra of the flocs from EC at 7 mA cm-2 and
340
1.2 cm s-1.
341 342
3.3 Energy consumption and operational EC costs
343
According to the results condensed in Table 2, Ecell decreases with u for all the j tested,
344
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
346
drop in the interelectrode space, to provide the fast release of H2 bubbles from the solution
347
to the atmosphere. It is noteworthy that the Msludge decreases with u but increases with j, as
348
expected; however, the sludge generated by EC is minor and varies between 0.037 < Msludge
349
< 0.425 kg m-3. The residual concentration of fluoride adheres to the WHO guideline (1.5
350
mg L-1 < CF) for all the EC trials.
351 352
The best removal of hydrated silica was obtained at 7 mA cm-2 and of 1.2 cm s-1, giving a
353
Chs = 6.2 mg L-1, with the operational cost of EC of $OC = 0.441 USD m-3. Finally, it is
354
important to mention that the operational cost of EC reported here may vary in other
355
countries, due to the fluctuation of prices of the electricity and sludge confinement.
18
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.
19
Ms. Ref. No.: CHEM64794R1 337
4. Conclusions
338
The residual concentration of fluoride after the EC treatment adhered to the WHO guideline
339
(< 1.5 mg L-1), while the hydrated silica was completely removed at 7 mA cm-2 and 1.2 cm
340
s-1, with energy consumption and overall operational cost of 2.48 kWh m-3 and 0.441 USD
341
m-3, respectively. XRD, XRF-EDS, SEM-EDS and FTIR analyses on the flocs confirmed
342
that during the EC process, the aluminum reacted with silica forming aluminosilicates.
343
Meanwhile, fluoride substituted a hydroxide from aluminum flocs, and the sulfates and
344
phosphates were removed by adsorption process onto aluminum aggregates.
345 346
The well-engineered EC reactor permitted to apply high current densities to generate high
347
aluminum dosages (14-73 mg L-1) that reacted with hydrated silica allowing its complete
348
removal. Therefore, the EC process has great potential to be applied in the industry,
349
particularly for the preconditioning of water containing silica and affordable treatment
350
costs.
351 352
Acknowledgments
353
The authors thank to SICES (project No. IJ-19-78), CONACYT (project No. 759) and the
354
University of Guanajuato (projects No. 102/2019, 150/2019) for financial support. Authors
355
acknowledge Dr. Raul Miranda and Daniela Moncada from LICAMM-UG Laboratory for
356
spectroscopy analysis.
357 358
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25
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.
S0045-6535(19)32657-8
DOI:
https://doi.org/10.1016/j.chemosphere.2019.125417
Reference:
CHEM 125417
To appear in:
ECSN
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
1
electrocoagulation in a flow channel reactor
2 3
Locksley F. Castañedaa, Oscar Coreñob, José L. Navaa,*, Gilberto Carreñoa
4 5
a
Departamento de Ingeniería Geomática e Hidráulica, Universidad de Guanajuato, Av.
6
Juárez 77, Centro, 36000, Guanajuato, Guanajuato, Mexico. E-mail: [email protected];
7
[email protected]; [email protected]
8 9
b
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
*Corresponding
author: [email protected]
23
Tel: + 52-473-1020100 ext. 2289; fax: + 52-473-1020100 ext. 2209
24
1
Ms. Ref. No.: CHEM64794R1 25
Abstract
26
This paper concerns simultaneous removal of fluoride and hydrated silica from
27
groundwater (4.08 mg L-1 fluoride, 90 mg L-1 hydrated silica, 50 mg L-1 sulfate, 0.23 mg L-
28
1
29
up-flow EC reactor, with a six-cell stack in a serpentine array, opened at the top of the cell
30
to favor gas release. Aluminum plates were used as sacrificial electrodes. The effect of
31
current density (4 ≤ j ≤ 7 mA cm-2) and mean linear flow rate (1.2 ≤ u ≤ 4.8 cm s-1), applied
32
to the EC reactor, on the elimination of fluoride and hydrated silica was analyzed. The
33
removal of fluoride followed the WHO guideline (< 1.5 mg L-1), while the hydrated silica
34
was abated at 7 mA cm-2 and 1.2 cm s-1, with energy consumption of 2.48 kWh m-3 and an
35
overall operational cost of 0.441 USD m-3. Spectroscopic analyses of the flocs by XRD,
36
XRF-EDS, SEM-EDS, and FTIR indicated that hydrated silica reacted with the coagulant
37
forming aluminosilicates, and fluoride replaced a hydroxide from aluminum aggregates,
38
while sulfates and phosphates were removed by adsorption process onto the flocs. The
39
well-engineered EC reactor allowed the simultaneous removal of fluoride and hydrated
40
silica.
phosphate, pH 7.38 and 450 µS cm-1conductivity) by electrocoagulation (EC), using an
41 42
Keywords: Hydrated silica removal; Electrocoagulation; Aluminum electrodes; Fluoride
43
removal; Groundwater.
44 45 46 47 48
2
Ms. Ref. No.: CHEM64794R1 49 50
1. Introduction
51
Groundwater is an important source of water supply for human consumption, which in arid
52
and semiarid regions worldwide is contaminated by inorganic salts, metalloids, and metals,
53
among other pollutants. Mexico is not the exception and in different areas, it is common to
54
find groundwater contaminated mainly with fluoride and hydrated silica, whose
55
concentrations range between 1-9.5 mg L-1, and 50-132 mg L-1, respectively (Guzmán et
56
al., 2016; Rosales et al., 2018; Sandoval et al., 2014). Other ions such as phosphates and
57
sulfates are also found in groundwater. The contamination of groundwater by fluorides and
58
hydrated silica occurs mainly from the dissolution of minerals in contact with water
59
(Battula et al., 2014; Behbahani et al., 2011; Emamjomeh et al., 2011; Zhu et al., 2007).
60 61
Both Mexican standard and international regulations, such as the World Health
62
Organization (WHO), indicate that the maximum allowable fluoride concentration in water
63
for human consumption must be <1.5 mg L-1. While at low concentrations fluoride is
64
beneficial in helping to combat tooth decay (Emamjomeh et al., 2009a; Essadki et al.,
65
2009), at concentrations > 4 mg L-1 it can cause severe harm to human health, such as
66
thyroid disorder, neurological damage, mottled teeth, skeletal and dental fluorosis,
67
osteoporosis and diseases in the kidneys, lungs, liver, muscles, and nerves, among others
68
(Camargo, 2003; Emamjomeh et al., 2011; Gosh et al., 2008; Hu et al., 2003; Maleki et al.,
69
2015).
70 71
Regarding hydrated silica, there is currently no official standard that establishes the
72
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
74
in the lungs, generating diseases such as tuberculosis, silicosis, bronchitis, and cancer
75
(Merget et al., 2002; Rosales et al., 2018; Sakar and Paul, 2016; Sariñana-Ruiz et al., 2017).
76
In addition to the above, hydrated silica can also seriously affect pipelines and certain unit
77
operations of industries that use this kind of water in their processes, permeating the walls
78
of equipment and causing failures (Gelover et al., 2012; Rosales et al., 2018).
79 80
One of the most used processes for fluoride removal from water is the precipitation-
81
flocculation by aluminum and calcium salts (Singh et al., 2016). This process produces too
82
much sludge because the counter-ion of the above-mentioned salts consumes coagulant
83
(Singh et al., 2013). Other processes such as adsorption (Gosh et al., 2008; Zhao et al.,
84
2011), chemical precipitation (He and Cao, 1996), and membrane process (Hu and
85
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
89
that destabilizes anions and dispersed fine particles from contaminated water by electrolysis
90
(Emamjomeh et al., 2009b). The EC technology allows the possibility of automating the
91
process, while the installation of the treatment is compact, does not need the addition of
92
chemicals, and the generation of sludge is minimal (Hu et al. 2007; Essadki et al., 2009;
93
Emamjomeh et al., 2009b), It can even be used together with other electrochemical
94
processes (Medel et al., 2019; Tirado et al., 2018).
95 96
Some reported works indicate the use of aluminum electrodes as sacrificial anodes in the
97
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-
99
scale, can be easily scalable at pilot plants (Castañeda and Nava, 2019).
100 101
The EC process involves the generation of coagulants in situ by electrodissolution of
102
aluminum sacrificial anodes, Eq. 1, while in the bulk of the solution, at neutral pH, the
103
formation of aluminum salts takes place, Eqs. 2, 3. At the cathode, the evolution of
104
hydrogen gas bubbles occurs, Eq. 4.
105 106
Al() → Al + 3e
(1)
107
+ 3 → ( )() + 3
(2)
108
2 + 3 → + 6
(3)
109
3H O + 3e → 1.5H + 3OH
(4)
110 111
According to the literature, the removal of fluoride occurs via co-precipitation of fluoro-
112
aluminum complexes and a chemical substitution reaction between a fluoride ion and a
113
hydroxide from aluminum flocs (Hu et al., 2003; Mohamad et al., 2011). Meanwhile, the
114
removal of hydrated silica occurs by the formation of aluminosilicates (Guzman et al.,
115
2016; Rosales et al., 2018). Other coexisting ions such as phosphates and sulfates have
116
been partially removed by adsorption processes on aluminum aggregates (Rosales et al.,
117
2018; Sandoval et al., 2019).
118 119
A common problem with aluminum electrodes is the anodic passivation. The use of flow
120
cells with plate electrodes favors the transport of Al3+ ions from the electrode surface to the
5
Ms. Ref. No.: CHEM64794R1 121
bulk of the solution, diminishing the passivation of the anode (Rosales et al., 2018;
122
Sandoval et al., 2019).
123
On the other hand, hydrated silica (72 mg L-1) has been efficiently removed from
124
groundwater by EC using several aluminum plate electrodes fitted in a stack of a flow EC
125
reactor opened to the atmosphere (Rosales et al., 2018). It is worth mentioning that this
126
paper shows for the first time the efficient abatement of hydrated silica by EC, highlighting
127
that aluminum reacts with silica species to yield aluminosilicates. The removal of pollutants
128
from groundwater containing silica by EC using Fe as sacrificial anodes has already been
129
tested, but unfortunately, the silica removal has been very deficient (Wan et al., 2011).
130 131
The novelty of this paper consists in the simultaneous removal of fluoride and hydrated
132
silica from real groundwater, using a filter-press reactor with a novel design, in which the
133
horizontally located aluminum plate electrodes make up a six-cell stack, while at the top,
134
the cell is opened to the atmosphere allowing the fast release of hydrogen bubbles produced
135
at the cathodes. It examines the influence of the coagulant dosage (in terms of current
136
density employed) and the retention time (dictated by the mean linear flow velocity) on the
137
efficiency of the simultaneous removal of fluoride and hydrated silica. Spectroscopic
138
analyses such as XRD, SEM-EDS, XRF-EDS, and FTIR are performed to elucidate the
139
mechanism of elimination of the pollutants contained in groundwater.
140 141
2. Materials and methods
142
2.1. Deep well water
143
The real groundwater sample was obtained from the plateau region of Guanajuato in
144
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
146
conductivity), which exceeds the WHO guideline for fluoride; a high concentration of
147
hydrated silica was also found.
148 149
2.2 Electrocoagulation reactor
150
The sketch of the EC reactor and its components is shown in Fig. 1. The EC flow reactor is
151
composed of a stack of aluminum plate electrodes horizontally fitted so that the electrolyte
152
flows in the form of a serpentine; the cell is opened at the top to facilitate the fast release of
153
hydrogen bubbles formed on the cathode during the EC process.
154
155 156
Fig. 1. (a) Sketch of the reactor, (b) bottom plate, (c) channel separator, (d) aluminum
157
electrode, and (e) electrolyte collector at the exit.
158
7
Ms. Ref. No.: CHEM64794R1 159
The reactor in a serpentine array consist of a six-cell stack containing 8 empty channels
160
with 3 cm width, 8 cm length and 0.46 cm thickness, and 7 parallel aluminum plates as
161
electrodes (3 cm × 8 cm × 0.46 cm, width, length and thickness, in contact with electrolyte,
162
respectively), out of which four are used as cathodes and three as anodes. The electrolyte
163
inlet is located at the bottom of the cell, having a diameter of 1.27 cm. The top of the
164
reactor was designed, after several CFD simulation trials (not shown herein), to allow the
165
fast release of the gas generated in the cell, and therefore the cell was opened to the
166
atmosphere. Moreover, at the top of the reactor, there is a window of 3.4 cm length and 1.5
167
cm height, followed by a liquid collector of 10 cm in length to transport the electrolyte
168
towards the exit. More details on the cell can be consulted elsewhere (Castañeda and Nava,
169
2019). The dimensions of the EC reactor are shown in Table SM-1.
170 171
Fig. SM-1 shows a schematic diagram of the hydraulic and electric system coupled with the
172
EC reactor, which contains a 15 L-capacity reservoir for the groundwater sample, a
173
centrifugal pump (1/125 HP, Iwaki, MD-10L), a valve and a flowmeter (0.1-1 L min-1,
174
White Industries), all this joined to each other by a 0.5-inch diameter PVC pipe. A B&K
175
Precision 1090 power source was used to supply the current during the EC trials, which
176
directly records the cell potential.
177 178
2.3 Methodology
179
The EC tests were carried out in the system shown in Fig. SM-1. Current densities (j) of 4,
180
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
181
implemented for EC tests, which matched volumetric flow rates of 0.1, 0.2, 0.3 and 0.4 L
182
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,
184
CAl(III)(N):
185
186
( ) =
" ∙ % ∙ &' ( ∙ ) ∙ * ∙ +
(1 × 10. )
(5)
187
188
where ( ) and j are given in mg L 2 and A cm , respectively, the molecular weight
189
of aluminum is Mw = 26.98 g mol 2 , 5 is the channel length (8 cm), the Faraday constant is
190
6= 96,485 C mol 2 , 8 = 3 is the number of electrons, 9 is the interelectrode gap (0.46
191
cm), and the factor 1×106 permits to obtain ( )
192
electrocoagulation tests, 1 mg L-1 of hypochlorite (typical concentration used for
193
disinfection purposes) was added to groundwater to avoid passivation (Guzmán et al., 2016;
194
Rosales et al., 2018; Sandoval et al., 2014). It is worth to mention that the EC tests in the
195
reactor were stabilized for a period of 10 minutes to achieve the steady state. The EC tests
196
were performed in triplicate, obtaining similar results.
in mg L 2 . Before the
197 198
Once the electrolyte left the electrocoagulation reactor, it went to the jar test device, where
199
the coagulant produced inside the EC cell was slowly mixed (30 rpm) during 15 minutes,
200
so that the aggregate could grow; then, the flocs were left to rest for 60 minutes until the
201
aggregates settled. Afterward, the clarified solution, free of flocs, was analyzed to quantify
202
the residual concentration of fluoride, hydrated silica, and coexisting ions. The
203
experimental aluminum dose, ( ) , formed in the EC trials was determined after the
204
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
206
grade carbon emery paper, and then rinsed with plenty of water. The results were the
207
average of three EC tests.
208 209
2.4 Analytical procedure
210
2.4.1. Groundwater analysis
211
A HI 83200 multiparameter bench photometer, from Hanna instruments, was the equipment
212
used to measure hydrated silica, phosphate and sulfate. The silica analysis was carried out
213
by heteropoly blue method using the kit HI 93705. Phosphate was determined by amino
214
acid method using the HI 93706 kit and sulfate was determined by precipitation with
215
barium chloride crystals (light absorbance method) using the kit HI. The detection limit of
216
hydrated silica, phosphate, and sulfate was 0.2 mg L-1. The concentration of fluoride was
217
measured by a fluoride ion selective electrode (27502-19, Cole Palmer) with a detection
218
limit of 0.02 mg L−1. A Perkin Elmer AAnalyst™ 200 atomic absorption spectrometer, with
219
a detection limit of 0.1 mg L-1 (309.27 nm wavelength), was used to determine the
220
concentration of aluminum. Conductivity and pH measurements were carried using a
221
waterproof instrument from Hanna, model HI 991300. Analytical grade reagents were used.
222
The results were the average of three analyses.
223 224
2.4.2. Flocs characterization
225
The scanning electron microscopy (SEM) analysis was carried out using a JEOL JSM-6010
226
PLUS/LA device. The energy dispersive analysis of X-rays (EDS) was performed using a
227
JEOL detector incorporated in the SEM microscope. X-ray diffraction (XRD) analyzes
10
Ms. Ref. No.: CHEM64794R1 228
were made on a diffractometer Rigaku Ultima IV, with nickel filter and Cu K:2 radiation.
229
The elemental compositions of the flocs were determined by energy dispersive X-ray
230
fluorescence (XRF), using a Rigaku Nex CG X-ray fluorescence spectrometer, equipped
231
with an X-ray tube with Pd anode. The Fourier transform infrared spectroscopy (FTIR)
232
examination in the flocs was carried out in a Perkin Elmer Spectrum GX FTIR
233
Spectrometer, using an EasiDiff diffuse reflectance accessory.
234
2.5. Energy consumption and costs of EC
235
The energy consumption (ABCD), cost of aluminum dose ($F ( ) ), and overall cost of EC
236
($OC) were calculated by Eqs. (6), (7) and (8), respectively:
237
238
ABCD =
GHIJJ ∙ K (..) ∙* ∙L ∙ +
(6)
239
240
where the units of ABCD , cell potential (ABM ), and I are kWh m-3, V, and C s 2 ,
241
respectively. N is the channel width (3 cm), S is the electrode spacing (0.46 cm), and the
242
factor 3.6 is used to obtain Econs in kWh m-3.
243
$ ( ) = O ( ) P(2.008 USD Kg 2 )(0.001)
(7)
244
245
The aluminum price, in Mexico, is 2.008 USD kg 2 and 0.001 is a conversion factor to
246
obtain $ ( ) in USD m .
247
$ = $ ( ) + :ABCD + :EXYZX + βM Y\]M
(8) 11
Ms. Ref. No.: CHEM64794R1 248
249
$OC is expressed in units of USD m-3, α is the cost of the electricity in central Mexico
250
(0.0976 USD (kWh)-1), Epump is in units of kWh m-3, and β is the sludge confinement cost
251
in Mexico (0.035 USD Kg 2 ).
252
253
3. Results and discussion
254
3.1 Removal of fluoride and hydrated silica by EC
255
Both the theoretical, ( ) , and experimental, ( ) , aluminum dosages and the residual
256
fluoride concentration, ^_ , are shown in Fig. 2. Results were obtained at different mean
257
linear flow velocities (1.2 < u < 4.8 cm s-1) and current densities of 4, 5, 6 and 7 mA cm-2.
258
The residual fluoride concentration after all the EC trials meets the WHO guideline ( ^_ <
259
1.5 mg L-1), evidencing a decrease with current density owing to the increase in the
260
experimental aluminum dosage (coagulant). A modest decrease in ^_ was obtained as a
261
function of mean linear flow velocity. The experimental aluminum dosage is greater than
262
that theoretically obtained by Faraday's law, Eq. 5, which is attributed to the chemical
263
oxidation of aluminum plates with the hypochlorite (1 mg L-1) present in the groundwater
264
sample.
265
12
Ms. Ref. No.: CHEM64794R1
266 267
Fig. 2. Effect of the mean linear flow velocity on the remaining fluoride concentration after
268
EC trials at different current densities: (a) 4 mA cm-2, (b) 5 mA cm-2, (c) 6 mA cm-2 and (d)
269
7 mA cm-2.
270 271
The residual concentrations of hydrated silica, sulfate, and phosphate are shown in Figs.
272
3(a), (b) and (c), respectively. This Fig. shows that the residual concentration of hydrated
273
silica (Chs) increases with flow rate due to the decrease in the coagulant dosage, and Chs
274
decreases with current density owing to the massive reaction between aluminum and silica
275
to yield aluminosilicates (Rosales et al., 2018). The best removal of hydrated silica was
276
obtained at 7 mA cm-2 and at 1.2 cm s-1 giving Chs = 6.2 mg L-1, equivalent to the removal
277
of 96%.
278 279 280
13
Ms. Ref. No.: CHEM64794R1
281 282
Fig. 3. Effect of the mean linear flow rate on the removal of: (a) hydrated silica, (b) sulfate
283
and (c) phosphate concentrations, after the same EC trials shown in Fig. 2.
284 285
Figure 3 (b) shows the residual concentration of sulfate, *`ab_ , that remains almost
286
constant as a function of flow rate but decreases with current density, which is attributable
287
to the coagulant dosage. The best removal of sulfate, *`ab_ = 25 mg L-1 (50% removal), was
288
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
290
(coagulant dosage). Once again, the best removal was obtained at 7 mA cm-2 and 1.2 cm s-1,
291
where the phosphate concentration was completely removed. These results agree with what
292
is reported in the literature, a well-known fact is that sulfate and phosphate ions are
293
adsorbed in the active sites of aluminum flocs (Guzmán et al., 2016). It is worth to mention
294
that the pH remains almost constant during the EC trials at a value of around 8.7. This
295
small variation is attributed to the substitution reaction between fluoride and hydroxide
296
from flocs during fluoride removal (Guzmán et al., 2016).
297 298
3.2 Flocs characterization
299 300
Figs. SM-2 (a)-(b) show SEM micrographs of the flocs obtained from EC test at 7 mA cm-2
301
and 1.2 cm s-1 at different scales. SM-2 (a) shows aggregates with sizes from less than 10
302
µm up to around 500 µm; whereas the aggregates from SM-2 (b) consist of particles with
303
sizes below 100 nm. SEM-EDS and XRF-EDS analyses were used to define the chemical
304
composition of aluminum flocs, and the results are shown in Table 1. According to the high
305
percentage of silicon obtained by SEM-EDS and XRF-EDS analyses, these revealed the
306
generation of aluminosilicate complexes (Guzmán et al., 2016; Rosales and Nava, 2018).
307
However, it was not possible to detect fluorine. The element compositions from both
308
analyses were similar, with small differences attributed to different sizes of sampled areas,
309
since the area sampled for XRF-EDS was 8.02 cm2 , compared with five areas of around
310
0.16 mm2 for SEM-EDS.
311 312
15
Ms. Ref. No.: CHEM64794R1 313
Table 1. Flocs composition obtained by XRF-EDS and SEM-EDS from EC at 7 mA cm-2
314
and 1.2 cm s-1. wt. %
Al
Si
S
Cl
K
Ca
O
Na
SEM-EDS
29.02
8.79
0.15
0.68
ND
4.49
54.92
2.01
XRF-EDS
16.45
4.90
0.27
0.45
0.22
1.87
73.24
2.60
315 316
Fig. 4 (a) shows a typical XRD pattern of flocs obtained by EC at 7 mA cm-2 and 1.2 cm s-
317
1
318
calcite
319
(Al1.52Ca0.52Na0.48O8Si2.48), oligoclase (Na0.723Ca0.277)(Al1.277Si2.723)O8, lisetite (Ca0.98Na
320
(Al3.96Si4.04)O16),
321
((Na0.98Ca0.02)(Al0.02Si2.97)O8). These aluminosilicate phases adsorb arsenates, sulfates, and
322
phosphates (Guzmán et al., 2016; Rosales and Nava, 2018). The carbonates (alkalinity)
323
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),
bytownite
labradorite
(Ca0.43Na0.07(Al0.92Si1.0O)4),
(Ca0.325Na0.16
(Al0.81Si1.19)O4,
anorthite
and
1.97
albite
324 325
FTIR spectrum obtained for wave numbers between 4000 and 400 cm-1 is shown in Fig. 4
326
(b), after EC tests at 7 mA cm-2 and u = 1.2 cm s-1. The peaks were identified using the
327
reference (Socrates, 2004). The peaks match with the chemical bonds O-H, Na-F, Al-O, Al-
328
O-Si, Si-O and Al-F, which agree with those reported in the literature for EC tests with
329
aluminum as a sacrificial anode (Drouiche et al., 2009; Ghosch et al., 2008; Guzmán et al.,
330
2016). The peaks placed on 599 cm-1 related to the Al-F bounding prove the chemical
331
substitution reaction between fluoride and hydroxide from aluminum aggregates (Sandoval
332
eta al., 2014). The Al-O-Si bond confirms the reaction between aluminum coagulant and
333
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
335
flocs; however, it is well known that both anions are removed by adsorption onto aluminum
336
aggregates (Guzmán et al., 2016; Thakur and Modal, 2017).
337
338 339
Fig. 4. Characteristic (a) XRD and (b) FTIR spectra of the flocs from EC at 7 mA cm-2 and
340
1.2 cm s-1.
341 342
3.3 Energy consumption and operational EC costs
343
According to the results condensed in Table 2, Ecell decreases with u for all the j tested,
344
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
346
drop in the interelectrode space, to provide the fast release of H2 bubbles from the solution
347
to the atmosphere. It is noteworthy that the Msludge decreases with u but increases with j, as
348
expected; however, the sludge generated by EC is minor and varies between 0.037 < Msludge
349
< 0.425 kg m-3. The residual concentration of fluoride adheres to the WHO guideline (1.5
350
mg L-1 < CF) for all the EC trials.
351 352
The best removal of hydrated silica was obtained at 7 mA cm-2 and of 1.2 cm s-1, giving a
353
Chs = 6.2 mg L-1, with the operational cost of EC of $OC = 0.441 USD m-3. Finally, it is
354
important to mention that the operational cost of EC reported here may vary in other
355
countries, due to the fluctuation of prices of the electricity and sludge confinement.
18
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.
19
Ms. Ref. No.: CHEM64794R1 337
4. Conclusions
338
The residual concentration of fluoride after the EC treatment adhered to the WHO guideline
339
(< 1.5 mg L-1), while the hydrated silica was completely removed at 7 mA cm-2 and 1.2 cm
340
s-1, with energy consumption and overall operational cost of 2.48 kWh m-3 and 0.441 USD
341
m-3, respectively. XRD, XRF-EDS, SEM-EDS and FTIR analyses on the flocs confirmed
342
that during the EC process, the aluminum reacted with silica forming aluminosilicates.
343
Meanwhile, fluoride substituted a hydroxide from aluminum flocs, and the sulfates and
344
phosphates were removed by adsorption process onto aluminum aggregates.
345 346
The well-engineered EC reactor permitted to apply high current densities to generate high
347
aluminum dosages (14-73 mg L-1) that reacted with hydrated silica allowing its complete
348
removal. Therefore, the EC process has great potential to be applied in the industry,
349
particularly for the preconditioning of water containing silica and affordable treatment
350
costs.
351 352
Acknowledgments
353
The authors thank to SICES (project No. IJ-19-78), CONACYT (project No. 759) and the
354
University of Guanajuato (projects No. 102/2019, 150/2019) for financial support. Authors
355
acknowledge Dr. Raul Miranda and Daniela Moncada from LICAMM-UG Laboratory for
356
spectroscopy analysis.
357 358
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25
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.