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Simultaneous elimination of hydrated silica, arsenic and phosphates from real groundwater by electrocoagulation using a cascade-shaped up-flow reactor
Electrochimica Acta 331 (2020) 135365
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Simultaneous elimination of hydrated silica, arsenic and phosphates from real groundwater by electrocoagulation using a cascade-shaped up-flow reactor ~ eda a, Oscar Coren ~ o b, Jose L. Nava c, * Locksley F. Castan tica e Hidra ulica, Av. Jua rez 77, Zona Centro, C.P. 36000, Guanajuato, Mexico CONACYT - Universidad de Guanajuato, Departamento de Ingeniería Geoma rez 77, Zona Centro, C.P. 36000, Guanajuato, Mexico Universidad de Guanajuato, Departamento de Ingeniería Civil, Av. Jua c tica e Hidra ulica, Av. Jua rez 77, Zona Centro, C.P. 36000, Guanajuato, Mexico Universidad de Guanajuato, Departamento de Ingeniería Geoma a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2019 Received in revised form 23 November 2019 Accepted 24 November 2019 Available online 27 November 2019
This paper deals with the elimination of hydrated silica, arsenic, and phosphates from real groundwater collected in central Mexico (arsenic 22 mg L1, hydrated silica 161 mg L1, sulfate 50 mg L1, phosphate 0.41 mg L1, pH 7.6 and conductivity 508 mS cm1) by electrocoagulation (EC), using a cascade-shaped up-flow reactor with a six-cell stack open to the atmosphere at the top. Aluminum plates were used as electrodes. The influence of both current density (4 j 10 mA cm2) and mean linear flow rates (1.2 u 4.8 cm s1) in the EC reactor on the removal efficiency of hydrated silica, arsenic, and phosphates were examined. The best removal of arsenic after EC (reaching a residual concentration, CAs ¼ 1.5 mg L1) was obtained at j ¼ 10 mA cm2 and u ¼ 1.2 cm s1, meeting the World Health Organization (WHO) recommendation (<10 mg L1), while the residual concentrations of hydrated silica, 1 1 and CSO 2 phosphates and sulfates were Chs ¼ 42 mg L1, C 3 PO4 ¼ 0.03 mg L 4 ¼ 35 mg L , respectively. The experimental conditions at j ¼ 10 mA cm2 and u ¼ 1.2 cm s1, produced a large amount of coagulant (85.4 mg L1), so this condition was repeated in a second round of the EC process to further reduce the 1 were obtained with concentration of hydrated silica. This time, Chs ¼ 2.6 mg L1 and CSO 2 4 ¼ 30 mg L the complete abatement of arsenic and phosphates. The total cost of the EC was 1.093 USD m3, which included the electrolytic energy consumption, the price of aluminum, the costs of pumping and confinement of sludge, emphasizing that the calculation was based on Mexican costs. SEM-EDS, XRD, XRF and FTIR analyses on flocs revealed that the coagulant reacted with silica forming aluminum silicates, while arsenic and phosphates were removed by adsorption on flocs. The partial removal of sulfates (40%) is associated with weak adsorption on aggregates. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Arsenic removal Hydrated silica elimination Groundwater treatment Electrocoagulation Aluminosilicates flocs
1. Introduction Mexico is one of many regions in the world that currently face problems of groundwater contamination with arsenic. Specifically, in the north-central area of the state of Guanajuato, groundwater is mainly contaminated with arsenic (20e134 mg L1) and considerable amounts of hydrated silica (50e132 mg L1) [1,2]. The contamination comes from the dissolution of rocks containing minerals with these compounds [3,4]. Other ions, such as sulfates and phosphates are present in concentrations between 50 and
* Corresponding author. ~ eda), [email protected] (O. Coren ~ o), E-mail addresses: [email protected] (L.F. Castan [email protected] (J.L. Nava). https://doi.org/10.1016/j.electacta.2019.135365 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
160 mg L1 and 0.2e1.8 mg L1, respectively. Arsenic contaminated groundwater is a crucial problem since this compound is recognized as toxic, mutagenic, and carcinogenic [5]. The chronic ingestion of arsenic affects human health in various ways, from skin lesions such as keratosis and hyperpigmentation to different types of cancer, such as lung, skin, and prostate cancers, and so on [5]. Because of the highly toxic effects of arsenic on human health, the WHO recommends a maximum value of 10 mg L1 for drinking water [6,7]. On the other hand, hydrated silica is not currently regulated in Mexico or anywhere else in the world, despite causing damages to the lungs and making people susceptible to diseases such as tuberculosis, bronchitis, silicosis, and even cancer [8]. In industry, hydrated silica generates problems of salt embedding in the walls of
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~ eda et al. / Electrochimica Acta 331 (2020) 135365 L.F. Castan
pipes, heat exchangers, cooling towers, and many operation units [2]. For this reason, the elimination of hydrated silica from concentrated solutions is required. Nowadays, different arsenic removal methods are being implemented, such as adsorption [9,10], coagulation-flocculation [11], and membrane technologies [12,13], among others [14]. On the other hand, the removal of hydrated silica has been performed by reverse osmosis [15,16], coagulation-ultrafiltration [17,18], hybrid electrolysis-crystallization [19] and precipitation with magnesium [20]. The EC has proved to be a very versatile technology for the removal of arsenic from different water samples [17,21e27]. The implementation of EC with aluminum electrodes as sacrificial anodes, in filter-press type cells with parallel electrodes, has been investigated by our work group, providing good results in arsenic removal, but showing moderate performance in the removal of hydrated silica from groundwater [1,28]. The modest removal of silica is attributed, on the one hand, to the low coagulant dose released in the filter-press cells, conditioned by the low current densities applied, j < 7 mA cm2; and on the other, at j 7 mA cm2, to the excess of hydrogen bubbles produced at the cathode, which usually break the flocs, inhibiting the removal of hydrated silica [1,28]. To avoid floc breakage at j > 7 mA cm2, a parallel plate flow cell, using vertical aluminum electrodes, open to the atmosphere at the top of the reactor, was employed for the removal of hydrated silica from real groundwater [2]. In that cell, higher current densities (j > 7 mA cm2) were used for the massive production of coagulant (85.4 mg L1), which reacted with silica forming aluminum silicates, and achieving the elimination of hydrated silica [2]. Other coexisting ions found in groundwater, such as phosphates and sulfates, which compete with arsenates for actives sites from flocs, have been moderately removed by adsorption process on the aluminum flocs [1,2,28]. On the other hand, the use of a horizontal continuous flow EC reactor, equipped with iron plates as sacrificial electrodes, permitted 96% removal of arsenic from groundwater [29]; however, that study reports no results about the removal of coexisting ions present in the water sample. Another paper reported the removal of arsenic in the presence of silica and phosphates, in a reactor consisting of a 1 L glass beaker, with two iron rods immersed in the aqueous solution [30]. In that research, 99.9% of arsenic was removed by adsorption on lepidocrocite (g-FeOOH), from an initial arsenic concentration between 100 and 1000 mg L1, using a pair of iron plates with a gap of 0.5 cm, applying a current intensity of 22 mA during 120 min, obtaining around 50 mg L1 of iron during the EC trial, while hydrated silica was not removed [30]. Moreover, these authors reported that phosphates inhibited the elimination of arsenic [30]. A comparison on the simultaneous removal of arsenic and hydrated silica from geothermal waters by electrocoagulation, using iron and aluminum as sacrificial electrodes, was performed by Mroczek and coworkers [31]. They emphasized that the best removal of arsenic was reached with iron electrodes, while hydrated silica was better removed with aluminum electrodes. It is worth to mention that there is no evidence on the mechanism of silica removal with iron as coagulant [30,31]. To our knowledge, the simultaneous abatement of hydrated silica, arsenic, and phosphates from real underground water has not yet been reported. The operation parameters in the EC reactors, such as flow rate, applied current density, retention time and the presence of coexisting ions influence the removal of pollutants [1,2,32,28,29]. This paper deals with the elimination of hydrated silica, arsenic, and phosphates from real groundwater by EC, using a cascadeshaped up-flow reactor with a six-cell stack open to the
atmosphere at the top. Aluminum plates were horizontally assembled in a serpentine array. The influence of current density and mean linear flow rates (retention time) applied to the EC reactor, on the removal efficiency of arsenic, hydrated silica and phosphates were analyzed. SEM-EDS, XRF, XRD, and FTIR analyses on the flocs were also completed to elucidate the removal mechanism of the pollutants. 1.1. Electrocoagulation process The electrocoagulation process consists in the generation of coagulant in situ by anodic electro-dissolution of aluminum, Eq. (1), while the generation of hydrogen bubbles takes place at the cathode, Eq. (2).
AlðsÞ / Al3þ þ 3e
(1)
3H2 O þ 3e /1:5H2 þ 3OH
(2) 3þ
Under neutral pH conditions, aluminum ions (Al ) react with bulk H2O, allowing the formation of Al(OH)3(s) and Al2O3(s) flocs. It is well known that arsenate is the only arsenic species that can be removed by adsorption process on the aluminum flocs. In this context, the addition of a typical concentration of hypochlorite employed for disinfection purposes (1 mg L1), before the EC trials, is necessary to oxidize the arsenite to arsenate [32]. In addition, the hydrated silica reacts with coagulant to form aluminum silicates, which also remove arsenates by adsorption processes [28]. Phosphates are removed by adsorption on flocs, while sulfates are partially adsorbed on the aggregates [1,2,28]. One of the biggest problems of EC process is the anodic passivation, reflected in the anode potential increase, which is caused by Al2O3(s) precipitation on the anode [32,21,28,33]. In order to minimize the passivation of the anodes, electrodes of the same material are commonly used, and if necessary, the polarity of the current is changed [1,33]. 2. Experimental details 2.1. Real groundwater sample The real groundwater sample (arsenic 22 mg L1, hydrated silica 161 mg L1, sulfate 50 mg L1, phosphate 0.41 mg L1, pH 7.6 and conductivity 508 mS cm1) was collected from a deep well (150 m deep) in the north-central area of Guanajuato State, in central Mexico. The addition of 1 mg L1 hypochlorite, before the EC trials, was added to oxidize the arsenite to arsenate; at such pH 7.6, a mixture of hypochlorous acid and hypochlorite anion (free chlorine), is present. 2.2. EC reactor The EC reactor and its components are shown in Fig. 1. The reactor is composed of a six-cell stack using aluminum plate electrodes horizontally placed in a way to allow the electrolyte flow in a serpentine-like flow path. The reactor is open at the top to enable the rapid release of hydrogen bubbles formed on the cathode. The reactor contains eight 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 the electrolyte, respectively), out of which four are used as cathodes and three as anodes. The liquid inlet is situated at the bottom of the cell. At the top of the cell, there is a window followed by an electrolyte collector to direct the solution towards
~ eda et al. / Electrochimica Acta 331 (2020) 135365 L.F. Castan
a)
3
d)
b)
9 cm
Electrolyte outlet
d = 1.27 cm Electrolyte inlet Aluminum plate electrode
c)
e)
z
12 cm
8 cm
3.5 cm
Channel separator
x y Electrolyte inlet Fig. 1. (a) Sketch of the EC reactor, (b) bottom plate, (c) electrode spacer, (d) electrode, and (e) collector to direct the solution towards the outlet.
Table 1 Magnitudes of the EC reactor. Electrode length, L (cm) Electrode height, B (cm) Electrode spacing, S (cm) rea, A (cm2) Electrode a Hydraulic diameter in the rectangular channel, dh ¼ 2BS/(B þ S) (cm) Number of channels Diameter of the inlet pipe, d (cm) Dimensions of the window, electrolyte outlet, in the upper part (cm)
8 3 0.46 24 0.8 8 1.27 3.4 length 1.5 width
CAlðIIIÞN ¼
j $ L $ Mw 1 106 n$F $S$u
(3)
Where CAlðIIIÞN and j are given in mg L1 and A cm2 , respectively; the atomic weight of aluminum is Mw ¼ 26.98 g mol1 , L is the channel length (8 cm), the Faraday constant is F ¼ 96,485 C mol1 ,
Effluent to jar test Exit zone Cell 6
+
Cell 1 Calm zone
Reservoir
Flowmeter
Power supply
2.3. Methodology The EC trials were carried out in the experimental set-up shown in Fig. 2. The volumetric flow rates (q) employed during EC tests were 0.1, 0.2, 0.3 and 0.4 L min1, corresponding to linear flow velocities (u) of 1.2, 2.4, 3.6 and 4.8 cm s1, with retention times in the EC reactor (t) of 55.9, 27.8, 18.5 and 13.9 s, respectively. Before the electrolysis, the electrodes were polished with 600 grit silicon carbide emery paper and then rinsed with plenty of water. The Faraday’s law was used to calculate the theoretical value of the aluminum used as a coagulant, CAl(III) (N):
_
Cell 4 Cell 3 Cell 2
Pump
Influent
Fig. 2. Hydraulic and electrical system for the EC test.
Valve
cathodes
Cell 5 anodes
the outlet. More details on the cell can be consulted elsewhere [34]. Table 1 shows the dimensions of the EC reactor used here. Fig. 2 shows a sketch 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-10 L), a valve and a flowmeter (0.1e1 L min1, White Industries), all of which are connected 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, directly recording the cell potential.
~ eda et al. / Electrochimica Acta 331 (2020) 135365 L.F. Castan
4
n ¼ 3 is the number of electrons, S is the interelectrode gap (0.46 cm), and the factor 1 106 allows obtaining CAlðIIIÞN in mg L1 . Once the groundwater crossed the EC reactor, the resulting solution went straight to a jar test, where the solution was mixed at 30 rpm during 15 min to facilitate the growth of the flocs. Then, stirring was stopped to allow the precipitation of the aggregates for 1 h. Once the clarified solution was obtained, the concentrations of arsenic, hydrated silica, sulfates, and phosphates were analyzed. The experimental aluminum dose, CAlðIIIÞ , formed in the EC trials was determined after the redissolution of the flocs, using sulfuric acid to attain a pH ¼ 2. The results were averaged from three EC tests. The dry flocs were also examined by SEM-EDS, XRF, XRD, and FTIR techniques [35,36]. 2.4. Analytical procedure 2.4.1. Water analysis Atomic absorption spectroscopy at 188.9 nm was used to measure the arsenic concentration in a PerkinElmer PinAAcle 900F coupled to a PerkinElmer MHS-15 manual hydride generator (detection limit of 0.05 mg L1), following the procedure established in the standard method suggested by APHA (1998) [37]. The same atomic absorption spectrometer, with a detection limit of 0.1 mg L1 (309.27 nm wavelength), was used to determine the concentration of aluminum. A HI 83,200 multiparameter bench photometer, from Hanna instruments, was the equipment employed to measure hydrated silica, phosphate, and sulfate. The silica analysis was carried out by heteropoly blue method using the HI 93,705 kit. Phosphate was determined by the amino acid method using the HI 93,706 kit and sulfate by precipitation with barium chloride crystals (light absorbance method) using the HI 93,751 kit. The detection limit of hydrated silica, phosphate, and sulfate was 0.2 mg L1. Conductivity and pH measurements were carried out using a waterproof instrument from Hanna, model HI 991,300. Analytical grade reagents were used. The results were averaged from three analyses.
analyses were carried out on a Rigaku Ultima IV diffractometer, with nickel filter and Cu K a1 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 of the flocs was carried out in a PerkinElmer Spectrum GX FTIR Spectrometer, using an EasiDiff diffuse reflectance accessory. 2.5. Energy consumption and costs of EC The elements required to calculate the overall cost of EC include the cell potential (Ecell) of the electrolysis, the price of aluminum, the costs of pumping, and confinement of sludge. The energy consumption (Econs ), cost of aluminum dose ($AlðIIIÞ ), and overall cost of EC ($OC) were calculated by Eqs. (4)e(6), respectively:
Econs ¼
Ecell $ I ð3:6Þ $S $B $ u
(4)
Where the units of Econs and I are kWh m3, and C s1 , respectively. B is the channel width (3 cm), S is the electrode spacing (0.46 cm), and factor 3.6 is used to obtain Econs in kWh m3.
$AlðIIIÞ ¼ ðCAlðIIIÞ Þ $ 2:008 USD Kg1 ð0:001Þ
(5)
The aluminum price, in Mexico, is $ 2:008 USD Kg1 and 0.001 is a conversion factor to obtain $AlðIIIÞ in $ USD m3 .
OC ¼ $AlðIIIÞ þ aEcons þ aEpump þ bMsludge
(6)
3
OC is expressed in units of USD m , a is the cost of electricity in central Mexico (0.0976 USD (kWh)1), Epump is expressed in units of kWh m3, and b is the sludge confinement cost in Mexico ($ 0.035 USD Kg1 ). 3. Results and discussion
2.4.2. Flocs analysis 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 utilizing a JEOL detector incorporated in the SEM microscope. X-ray diffraction (XRD) 50
50
4 mA cm-2
80
80 6 mA cm-2
60
60
40
40
20
20
20
10
10
0
0 80
80 8 mA cm-2
60
60
0
0 100
100 10 mA cm-2 80
80
60
60
40
40
20
20
Aluminum dose / mg L-1
20
Residual arsenic / μg L-1
30
Aluminum dose / mg L-1
30
Residual arsenic / μg L-1
Fig. 3 shows the residual arsenic concentration, CAs, as a function
Residual arsenic 40 Theoretical aluminum Experimental aluminum
40
40
40
20
20 0
3.1. Removal of arsenic, hydrated silica, and phosphates from groundwater by EC
0
1
2
3
4
5
0
0
0
1
2
3
4
5
0
Mean linear flow velocity, u / cm s-1 Fig. 3. Effect of the flow velocity on the remaining arsenic concentration and coagulant dose, at different current densities.
concentration of phosphates, CPO 3 4 ; exhibited a behavior similar to that observed for hydrated silica, where CPO 3 4 varied between 0.41 and 0.03 mg L1. Other papers have reported that the sulfates and phosphates are adsorbed on the aluminum aggregates [1,2,28,38]. 3.2. Characterization of aluminum flocs Fig. 5 shows SEM micrographs of flocs taken after the EC trial at 10 mA cm2 and 1.2 cm s1. The images allow observing irregular aggregates with sizes up to 600 mm; these flocs are composed of fine particles with a size below 100 nm. The chemical composition of these flocs was determined by EDS and XRF, and the results are reported in Table 2. Si and Al are present in a relatively high percentage in EDS results. This indicates the formation of aluminosilicate compounds. Arsenic was not detected due to the low concentration present in the aggregates. However, the presence of arsenic in the flocs was confirmed by XRF analysis. The difference between the chemical compositions of the flocs obtained by these two techniques could be originated by different sampling areas employed. In SEM-EDS, five areas of about 0.18 mm2 were sampled, and the average of these measurements is reported in Table 2. However, in XRF tests, a sampling area of 8.02 cm2 was analyzed. Fig. 6 (a) displays the X-ray diffraction pattern of flocs, obtained
Residual phosphate / mg L-1
of the mean linear flow rate at current densities of 4, 6, 8 and 10 mA cm2. It also shows the experimental, CAl(III), and theoretical, CAl(III) (N), aluminum doses. The EC performed at 4 mA cm2 shows an increase in CAs from 4.57 to 15.54 mg L1, as u increases. This pattern is attributed to a depletion of the experimental aluminum dose, CAl(III), with mean linear flow rate. The residual concentration of arsenic (4.17 and 9.57 mg L1) at u of 1.2 and 2.4 cm s1, met the WHO guideline (<10 mg L1). It is worth to mention that the experimental aluminum doses (11.9 CAl(III) 48 mg L1) obtained at such a low current density (4 mA cm2), surpass the theoretical values (calculated by Faraday’s Law), which is attributed to the chemical dissolution of aluminum provoked by the presence of hypochlorite in the solution. At 6 mA cm2, CAs also increases with u, and it can be observed that at u 3.6 cm s1, CAs < 10 mL1, which is achieved due to the increased coagulant dose, 19.5 CAl(III) 71.6 mg L1. At this current density, the experimental coagulant dose was moderately higher than the theoretical one, calculated by Eq. (3). However, at current densities of 8 mA cm2 and 10 mA cm2, CAs complied with the WHO recommendation at all flow rates tested. The two last results are due to the high production of aluminum dose, 19.6 CAl(III) 85.4 mg L1, which favors the removal of the pollutant. It is noteworthy that at j of 8 and 10 mA cm2, CAl(III) shows close agreement with CAl(III) (N), as expected, because at such current densities, the electrodissolution of aluminum, Eq. (1), predominates over the chemical dissolution. Fig. 4 presents the simultaneous removal of hydrated silica, sulfate, and phosphate during the experiments shown in Fig. 3. The residual hydrated silica concentration (Chs) exhibits a behavior similar to that obtained during the removal of arsenic, as shown in Fig. 3, and it should be noted that the removal of silica increases with current density and lower flow rates. At 10 mA cm2 and u ¼ 1.2 cm s1, Chs reached a value of 42 mg L1, which represents removal of 74%. The silica removal at such condition consumed a coagulant dose of 85.4 mg L1. It is well known from previous studies that silica reacts with coagulant forming aluminum silicates as flocs [28]. The residual concentration of sulfates, Fig. 4, remains almost constant with u and j, reaching values between 35 and 45 mg L1, which represents a removal of 10e30%. However, the residual
Residual sulfate / mg L-1 Residual hydrated silica / mg L-1
~ eda et al. / Electrochimica Acta 331 (2020) 135365 L.F. Castan
5
120 100 80 60 40 20
4 mA cm-2 6 mA cm-2 8 mA cm-2 10 mA cm-2
0 50 40 30 20 10 0 0.4 0.3 0.2 0.1 0
0
4 5 1 2 3 -1 Mean linear flow velocity, u / cm s
Fig. 4. Effect of the flow velocity on the remaining concentrations of hydrated silica, sulfate, and phosphate, determined from the EC trials showed in Fig. 3.
from the EC trials at 10 mA cm2 and 1.2 cms1. The phases were identified using the software MATCH! Version 1.11j, which employs the American Mineralogist Crystal Structure Data Base (AM-CSD). The narrow peaks of highest intensity correspond to the calcite (CaCO3); this indicates that part of the carbonate ions precipitates as calcite, which coincides with the observed decrease in alkalinity of the solution from 242.5 to 166.4 mg L1. Wide peaks are observed at 10 < 2q < 18, 22 < 2q < 34, 35 < 2q < 42. These broad peaks are the result of the superposition of peaks that could be assigned to dicalcium silicate (Ca2SiO4), albite (NaAlSi3O8), wollastonite-2M (CaSiO3), andesine Ca$24Na$26(Al$735Si1.265)O4, and labradorite Ca$325Na$16(Al$81Si1.19)O4. The XRD spectrum confirms that silica reacts with coagulant forming aluminum silicates and
~ eda et al. / Electrochimica Acta 331 (2020) 135365 L.F. Castan
6
Fig. 5. SEM micrographs of the flocs obtained at j ¼ 10 mA cm2 and u ¼ 1.2 cm s1.
Table 2 Composition of the flocs determined by EDS and XRF analyses. Si
S
Cl
K
Ca
O
Na
As
27.94 14.97
10.50 6.14
0.27 0.42
1.11 0.35
1.10 0.66
7.18 3.3
50.15 71.64
2.41 2.50
ND 0.002
ND ¼ Not detected.
calcium-sodium silicates as flocs. Arsenic phases were not detected by XRD due to the low concentration of this element. Fig. 6 (b) shows FTIR spectrum of the flocs, obtained after EC trial at j ¼ 10 mA cm2 and u ¼ 1.2 cm s1. The wide band at 3435 cm1 is attributed to the OeH stretching vibrations [1,2,28,39,40]. The peaks observed at 1643 and 614 cm1 belong to NaeF and AleF bond, respectively. The peaks located at 1443 and 1004 cm1 correspond to AleOeSi and SieO bond, respectively [1,38,41], while the peak situated at 876 cm1 correspond to the AseO bond [28]. The presence of aluminosilicates is also confirmed by these results. In addition, FTIR spectrum suggests that arsenic is removed by adsorption on the flocs [28]. In the same way, the elimination of traces of fluoride from groundwater (below the detection limit of 0.02 mg L1, measured with a selective ion electrode [1]) was observed. It is important to mention that peaks related to S bonds did not appear in the FTIR spectrum, which can be associated with weak adsorption of sulfates on aggregates [1,2,28,36]. The absence of phosphates in the floc composition determined by EDS and XRF analyzes, and the absence of phosphate phases in XRD analyzes and the non-appearance of PeO bonds in FTIR patterns, lead us to suggest that phosphates are removed by adsorption in the flocs [1,2,32,28,38], or well that these are trapped within the aggregates. 3.3. Performance and overall costs of the EC process Table 3 summarizes the experimental conditions for the EC in terms of current density, mean linear flow velocity, and retention
10
20
30
40
50
60
70
80
90
100
2
100
b)
80
1
Al
EDS XRF
% Transmittance / mg L-
wt. %
Intensity / a.u.
a)
60 40 4000 3600 3200 2800 2400 2000 1600 1200 800 400
Wavenumber / cm-1 Fig. 6. (a) XRD and (b) FTIR spectra of the flocs obtained at j ¼ 10 mA cm2 and u ¼ 1.2 cm s1.
time obtained in the EC reactor. This table also shows the remaining concentrations of arsenic and hydrated silica, the experimental aluminum doses, Ecell, Econs, Epump, Msludge, and the overall cost of EC.
~ eda et al. / Electrochimica Acta 331 (2020) 135365 L.F. Castan
7
Table 3 Remaining arsenic and hydrated silica concentrations after EC tests. The experimental coagulant dose, Ecell, Econs, Epump, Msludge are also shown. Initial composition of deep well water: arsenic 22 mg L1, hydrated silica 161 mg L1, phosphate 0.41 mg L1, sulfate 50 mg L1, alkalinity 242.5 mg L1, hardness 90 mg L1, conductivity 508 mS cm1 and pH 7.6. J (mA cm2)
u (cm s1)
t (s)
CAs (mg L1)
Chs (mg L1)
CAl(III) (mg L1)
Ecell (V)
Econs (KWh m3)
Epumping (kWh m3)
Msludge (Kg m3)
OC (USD m3)
4
1.2 2.4 3.6 4.8 1.2 2.4 3.6 4.8 1.2 2.4 3.6 4.8 1.2a 2.4 3.6 4.8
55.9 27.8 18.5 13.9 55.9 27.8 18.5 13.9 55.9 27.8 18.5 13.9 55.9 27.8 18.5 13.9
4.1 9.5 14.4 15.5 1.0 5.1 9.5 11.2 2.6 5 7.8 8.8 1.5 4.1 7.4 8.3
83 111 118 125 78 94 104 110 65 90 102 104 42 83 100 102
48.0 22.8 16.0 11.9 71.6 34.9 24.3 19.5 65.4 35.0 22.5 22.1 85.4 39.9 24.4 19.6
7.8 7.3 7.05 6.99 11 10.9 9.5 9.4 14.7 14.2 12.6 10.3 14.7 14.2 12.6 10.3
0.75 0.35 0.22 0.16 1.59 0.78 0.45 0.34 2.84 1.37 0.81 0.49 3.55 1.71 1.01 0.62
0.50 0.25 0.16 0.12 0.50 0.25 0.16 0.12 0.50 0.25 0.16 0.12 0.50 0.25 0.16 0.12
0.2 0.09 0.5 0.03 0.28 0.19 0.15 0.07 0.48 0.25 0.15 0.16 0.51 0.26 0.18 0.13
0.217 0.104 0.086 0.051 0.345 0.172 0.110 0.084 0.463 0.231 0.141 0.107 0.570 0.274 0.165 0.113
6
8
10
a
Residual concentrations: phosphate 0.03 mg L1, sulfate 35 mg L1, alkalinity 166.4 mg L1, hardness 20 mg L1, conductivity 440 mS cm1 and pH 8.29.
In Table 3, the overall cost of EC is seen to decrease with the flow rate for all current densities tested. The overall cost of EC varies between 0.051 OC 0.57 USD m3. The remaining concentration of arsenic and hydrated silica ranges between 1.1 CAs 15.5 mg L1, and 42 Chs 125 mg L1, respectively. The best removal of arsenic (CAs ¼ 1.5 mg L1) and hydrated silica (Chs ¼ 42 mg L1) was obtained at j ¼ 10 mA cm2 and u ¼ 1.2 cm s1; at such conditions, the coagulant dose was 85.4 mg L1, which produced dry sludge of 0.51 kg m3, with electrolysis energy consumption of 3.55 kWh m3 and an overall cost of EC of 0.57 USD m3. On the other hand, the remaining concentration of silica (Chs ¼ 42 mg L1) after the EC trials (at j ¼ 10 mA cm2 and u ¼ 1.2 cm s1), suggested performing a second round of EC treatment, with the aim to abate the rest of silica. In this context, we applied the EC process to the treated water once again (using 10 mA cm2 and 1.2 cm s1). The analysis of the solution (after the second round of EC) revealed that hydrated silica and sulfate were removed until 2.6 mg L1, and 30 mg L1, while the arsenic and phosphates were eliminated. The dose of aluminum generated in the second EC was 58.9 mg L1; also, 0.51 kg m3 of sludge was produced, with Econs ¼ 3.55 kWh m3, and an operational cost of 0.523 USD m3. The total cost of the EC process, considering the two EC tests, reached the value of 1.093 USD m3. It is noteworthy that the total cost of EC can vary among different countries depending on the prices of the confinement of sludge and electricity. 4. Conclusions The elimination of hydrated silica, arsenic, and phosphates from real groundwater was successfully achieved after applying two rounds of EC process to groundwater, at 10 mA cm2 and 1.2 cm s1, in each trial. The conclusive EC test results were reached using a cascade-shaped up-flow reactor, with a six-cell stack open to the atmosphere at the top. Aluminum plates were used as sacrificial electrodes. The total cost of the EC was of 1.093 USD m3. XRD, SEM-EDS, XRF, and FTIR analysis on flocs showed that these are mainly composed of aluminosilicates and silicates with divalent cations, which are formed by the reaction between coagulant and hydrated silica. The elimination of arsenic and phosphates is carried out by adsorption on flocs. The modest removal of sulfates (40%) is related to weak adsorption on flocs. The results showed herein evidenced that EC is an affordable
technology that eliminates silica, arsenic and phosphates from groundwater. These results can be applied to other water samples containing silica, arsenic and fluoride ion, recommending the use of a continuous flow reactor, applying current and hydrodynamic conditions like those used here. Author contributions section All the authors have contributed to the paper and have seen and revised the final version of the manuscript. Declaration of Interest Statement The authors declare that there is no conflict of interest regarding the publication of this paper. Acknowledgments The authors thank to SICES (project No. IJ-19-78), CONACYT (project No. 759) and the University of Guanajuato (projects No. 102/2019 and 150/2019) for financial support. Authors acknowledge Dr. Raul Miranda and Daniela Moncada from LICAMM-UG Laboratory for spectroscopy analysis. References ~ o, Y.M. Li, J.H. Herna ndez, [1] M.A. Sandoval, R. Fuentes, J.L. Nava, O. Coren Simultaneous removal of fluoride and arsenic from groundwater by electrocoagulation using a filter-press flow reactor with a three-cell stack, Separ. Purif. Technol. 208 (2019) 208e216. ~ o, J.L. Nava, Removal of hydrated silica, fluoride and [2] M. Rosales, O. Coren arsenic from groundwater by electrocoagulation using a continuous reactor with a twelve-cell stack, Chemosphere 211 (2018) 149e155. ~ ana-Ruiz, J. Vazquez-Arenas, F.S. Sosa-Rodríguez, I. Labastida, [3] Y.A. Sarin n-Pin ~ a, M.A. Escobedo-Bretado, M.A. Armienta, A. Armienta, A. Arago lez-Valdez, P. Ponce-Pen ~ a, H. Ramírez-Aldaba, R.H. Lara, Assessment L.S. Gonza of arsenic and fluorine in surface soil to determine environmental and health risk factors in the Comarca Lagunera, Mexico, Chemosphere 178 (2017) 391e401. [4] J. Marcais, A. Gauvain, T. Labasque, B.W. Abbott, G. Pinay, L. Aquilina, F. Chabaux, D. Viville, J.R. de Dreuzy, Dating groundwater with dissolved silica and CFC concentrations in crystalline aquifers, Sci. Total Environ. 636 (2018) 260e272. [5] M.F. Hughes, B.D. Beck, Y. Chen, A.S. Lewis, D.J. Thomas, Arsenic exposure and toxicology: a historical perspective, Toxicol. Sci. 132 (2011) 305e332. [6] WHO, Guidelines for Drinking Water Quality 1 (2006) 306. [7] USEPA, Drinking Water Standard for Arsenic, U.S. Government Printing Office, Washington DC, 2001, p. 1. EPA-815-F-00-015. [8] R. Merget, T. Bauer, H.U. Küpper, S. Philippou, H.D. Bauer, R. Breitstad, T. Bruening, Health hazards due to the inhalation of amorphous silica, Arch.
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Toxicol. 75 (2002) 625e634. [9] H.J. Shipley, S. Yean, A.T. Kan, M.B. Tomson, A sorption kinetics model for arsenic adsorption to magnetite nanoparticles, Environ. Sci. Pollut. Res. 17 (2010) 1053e1062. [10] Y. Pan, C.T. Chiou, T. Lin, Adsorption of arsenic (V) by iron oxide-coated diatomite (IOCD), Environ. Sci. Pollut. Res. 17 (2010) 1401e1410. [11] C. Hu, H. Liu, G. Chen, J. Qu, Effect of aluminum speciation on arsenic removal during coagulation process, Separ. Purif. Technol. 86 (2012) 35e40. [12] A. Oehmen, R. Valerio, J. Llanos, J. Fradinho, S. Serra, M.A.M. Reis, J.G. Crespo, S. Velizarov, Arsenic removal from drinking water through a hybrid ion exchange membrane-coagulation process, Separ. Purif. Technol. 83 (2011) 137e143. [13] M.C. Shih, An overview of arsenic removal by pressure-driven membrane process, Desalination 172 (2005) 85e97. [14] S.V. Jadhav, E. Bringas, G.D. Yadav, V.K. Rathod, I. Ortiz, K.V. Marathe, Arsenic and fluoride contaminated groundwaters: a review of current technologies for contaminants removal, J. Environ. Manag. 162 (2015) 306e325. [15] A.M. Al-Rehaili, Comparative chemical clarification for silica removal from RO groundwater feed, Desalination 159 (2003) 21e31. [16] W. Den, C.J. Wang, Removal of silica from brackish water by electrocoagulation pretreatment to prevent fouling of reverse osmosis membranes, Separ. Purif. Technol. 59 (2008) 318e325. [17] Y. Chen, J.C. Baygents, J. Farrell, Evaluating electrocoagulation and chemical coagulation for removing dissolved silica from high efficiency reverse osmosis (HERO) concentrate solutions, J. Water. Process. Eng. 16 (2017) 50e55. [18] H.H. Cheng, S.S. Chen, S.R. Yang, In-line coagulation/ultrafiltration for silica removal from brackish water as RO membrane pretreatment, Separ. Purif. Technol. 70 (2009) 112e117. [19] O. Gorni-Pinkesfeld, D. Hasson, R. Semiat, H. Shemer, Hybrid electrolysisecrystallization system for silica removal from aqueous solutions, Desalination 407 (2017) 41e45. [20] I. Latour, R. Miranda, A. Blanco, Silica removal with sparingly soluble magnesium compounds. Part II, Separ. Purif. Technol. 149 (2015) 331e338. [21] M. Kobya, U. Gebologlu, F. Ulu, S. Oncel, E. Demirbas, Removal of arsenic from drinking water by the electrocoagulation using Fe and Al electrodes, Electrochim. Acta 56 (2011) 5060e5070. [22] J.N. Hakizimana, B. Gourich, M. Chafi, Y. Stiriba, C. Vial, P. Drogui, J. Naja, Electrocoagulation process in water treatment: a review of electrocoagulation modeling approaches, Desalination 404 (2017) 1e21. [23] S. Garcia-Segura, M.M.S.G. Eiband, J.V. de Melo, C.A. Martínez-Huitle, Electrocoagulation and advanced electrocoagulation processes: a general review about the fundamentals, emerging applications and its association with other technologies, J. Electroanal. Chem. 801 (2017) 267e299. [24] M.Y.A. Mollah, R. Schennach, J.R. Parga, D.L. Cocke, Electrocoagulation (EC) science and applications, J. Hazard Mater. B84 (2001) 29e41. [25] D.T. Moussa, M.H. El-Naas, M. Nasser, M.J. Al-Marr, A comprehensive review of electrocoagulation for water treatment: potentials and challenges, J. Environ. Manag. 186 (2017) 24e41. [26] O.T. Can, M. Kobya, E. Demirbas, M. Bayramoglu, Treatment of the textile wastewater by combined electrocoagulation, Chemosphere 62 (2006)
181e187. ~ izares, C. Buitro n, C. Sa ez, Electrochemical technologies [27] M.A. Rodrigo, P. Can for the regeneration of urban wastewaters, Electrochim. Acta 55 (2010) 8160e8164. ~ o, I. Rodriguez, S. Gutierrez, Arsenic and fluoride [28] A. Guzm an, J.L. Nava, O. Coren removal from groundwater by electrocoagulation using a continuous filterpress reactor, Chemosphere 144 (2016) 2113e2120. [29] E. Mohora, S. Ron cevi c, J. Agbaba, K. Zrni c, Aleksandra Tubi c, B. Dalmacija, Arsenic removal from groundwater by horizontal-flow continuous electrocoagulation (EC) as a standalone process, J. Environ. Chem. Eng. 6 (2018) 512e519. [30] W. Wan, T.J. Pepping, T. Banerji, S. Chaudhari, D.E. Giammar, Effects of water chemistry on arsenic removal from drinking water by electrocoagulation, Water Res. 45 (2011) 384e392. [31] E.K. Mroczek, D. Graham, L. Bacon, Removal of arsenic and silica from geothermal fluid by electrocoagulation, J. Environ. Chem. Eng. 7 (2019) 103232. ~ o, E. Elorza, F. Martínez, Arsenic removal from [32] O.J. Flores, J.L. Nava, G. Carren groundwater by electrocoagulation in a pre-pilot-scale continuous filter press reactor, Chem. Eng. Sci. 97 (2013) 1e6. [33] E. Mohora, S. Roncevic, B. Dalmacija, J. Agbaba, M. Watson, E. Karlovic, M. Dalmacija, Removal of natural organic matter and arsenic from water by electrocoagulation/flotation continuous flow reactor, J. Hazard Mater. 235e236 (2012) 257e264. ~ eda, J.L. Nava, Simulations of single-phase flow in an up-flow [34] L.F. Castan electrochemical reactor with parallel plate electrodes in a serpentine array, J. Electroanal. Chem. 832 (2019) 31e39. [35] H.J. You, I.S. Han, Effects of dissolved ions and natural organic matter on electrocoagulation of As(III) in groundwater, J. Environ. Chem. Eng. 4 (2016) 1008e1016. [36] M.A. Sandoval, R. Fuentes, J.L. Nava, I. Rodríguez, Fluoride removal from drinking water by electrocoagulation in a continuous filter press reactor coupled to a flocculator and clarifier, Separ. Purif. Technol. 134 (2014) 163e170. [37] American Public Health Association/American Water Works Association/Water Pollution Control Federation, Standard Methods for the Examination of Water and Wastewater, twentieth ed., 1998. Washington DC. [38] D. Ghosh, C.R. Medhi, M.K. Purkait, Treatment of fluoride containing drinking water by electrocoagulation using monopolar and bipolar electrode connections, Chemosphere 73 (2008) 1393e1400. [39] G. Socrates, Infrared and Raman Characteristic Group Frequencies: Tables and Charts, third ed., Wiley, United Kingdom, 2004. [40] U. Gross, S. Rüdiger, E. Kemnitz, K.W. Brzezinka, S. Mukhopadhyay, C. Bailey, A. Wander, N. Harrison, Vibrational analysis study of aluminium trifluoride phases, J. Phys. Chem. A 111 (2007) 5813e5819. [41] N. Drouiche, S. Aoudja, M. Hecinia, N. Ghaffour, H. Lounici, N. Mameri, Study on the treatment of photovoltaic wastewater using electrocoagulation: fluoride removal with aluminum electrodes characteristics of products, J. Hazard Mater. 169 (2009) 65e69.
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Simultaneous elimination of hydrated silica, arsenic and phosphates from real groundwater by electrocoagulation using a cascade-shaped up-flow reactor ~ eda a, Oscar Coren ~ o b, Jose L. Nava c, * Locksley F. Castan tica e Hidra ulica, Av. Jua rez 77, Zona Centro, C.P. 36000, Guanajuato, Mexico CONACYT - Universidad de Guanajuato, Departamento de Ingeniería Geoma rez 77, Zona Centro, C.P. 36000, Guanajuato, Mexico Universidad de Guanajuato, Departamento de Ingeniería Civil, Av. Jua c tica e Hidra ulica, Av. Jua rez 77, Zona Centro, C.P. 36000, Guanajuato, Mexico Universidad de Guanajuato, Departamento de Ingeniería Geoma a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2019 Received in revised form 23 November 2019 Accepted 24 November 2019 Available online 27 November 2019
This paper deals with the elimination of hydrated silica, arsenic, and phosphates from real groundwater collected in central Mexico (arsenic 22 mg L1, hydrated silica 161 mg L1, sulfate 50 mg L1, phosphate 0.41 mg L1, pH 7.6 and conductivity 508 mS cm1) by electrocoagulation (EC), using a cascade-shaped up-flow reactor with a six-cell stack open to the atmosphere at the top. Aluminum plates were used as electrodes. The influence of both current density (4 j 10 mA cm2) and mean linear flow rates (1.2 u 4.8 cm s1) in the EC reactor on the removal efficiency of hydrated silica, arsenic, and phosphates were examined. The best removal of arsenic after EC (reaching a residual concentration, CAs ¼ 1.5 mg L1) was obtained at j ¼ 10 mA cm2 and u ¼ 1.2 cm s1, meeting the World Health Organization (WHO) recommendation (<10 mg L1), while the residual concentrations of hydrated silica, 1 1 and CSO 2 phosphates and sulfates were Chs ¼ 42 mg L1, C 3 PO4 ¼ 0.03 mg L 4 ¼ 35 mg L , respectively. The experimental conditions at j ¼ 10 mA cm2 and u ¼ 1.2 cm s1, produced a large amount of coagulant (85.4 mg L1), so this condition was repeated in a second round of the EC process to further reduce the 1 were obtained with concentration of hydrated silica. This time, Chs ¼ 2.6 mg L1 and CSO 2 4 ¼ 30 mg L the complete abatement of arsenic and phosphates. The total cost of the EC was 1.093 USD m3, which included the electrolytic energy consumption, the price of aluminum, the costs of pumping and confinement of sludge, emphasizing that the calculation was based on Mexican costs. SEM-EDS, XRD, XRF and FTIR analyses on flocs revealed that the coagulant reacted with silica forming aluminum silicates, while arsenic and phosphates were removed by adsorption on flocs. The partial removal of sulfates (40%) is associated with weak adsorption on aggregates. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Arsenic removal Hydrated silica elimination Groundwater treatment Electrocoagulation Aluminosilicates flocs
1. Introduction Mexico is one of many regions in the world that currently face problems of groundwater contamination with arsenic. Specifically, in the north-central area of the state of Guanajuato, groundwater is mainly contaminated with arsenic (20e134 mg L1) and considerable amounts of hydrated silica (50e132 mg L1) [1,2]. The contamination comes from the dissolution of rocks containing minerals with these compounds [3,4]. Other ions, such as sulfates and phosphates are present in concentrations between 50 and
* Corresponding author. ~ eda), [email protected] (O. Coren ~ o), E-mail addresses: [email protected] (L.F. Castan [email protected] (J.L. Nava). https://doi.org/10.1016/j.electacta.2019.135365 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
160 mg L1 and 0.2e1.8 mg L1, respectively. Arsenic contaminated groundwater is a crucial problem since this compound is recognized as toxic, mutagenic, and carcinogenic [5]. The chronic ingestion of arsenic affects human health in various ways, from skin lesions such as keratosis and hyperpigmentation to different types of cancer, such as lung, skin, and prostate cancers, and so on [5]. Because of the highly toxic effects of arsenic on human health, the WHO recommends a maximum value of 10 mg L1 for drinking water [6,7]. On the other hand, hydrated silica is not currently regulated in Mexico or anywhere else in the world, despite causing damages to the lungs and making people susceptible to diseases such as tuberculosis, bronchitis, silicosis, and even cancer [8]. In industry, hydrated silica generates problems of salt embedding in the walls of
2
~ eda et al. / Electrochimica Acta 331 (2020) 135365 L.F. Castan
pipes, heat exchangers, cooling towers, and many operation units [2]. For this reason, the elimination of hydrated silica from concentrated solutions is required. Nowadays, different arsenic removal methods are being implemented, such as adsorption [9,10], coagulation-flocculation [11], and membrane technologies [12,13], among others [14]. On the other hand, the removal of hydrated silica has been performed by reverse osmosis [15,16], coagulation-ultrafiltration [17,18], hybrid electrolysis-crystallization [19] and precipitation with magnesium [20]. The EC has proved to be a very versatile technology for the removal of arsenic from different water samples [17,21e27]. The implementation of EC with aluminum electrodes as sacrificial anodes, in filter-press type cells with parallel electrodes, has been investigated by our work group, providing good results in arsenic removal, but showing moderate performance in the removal of hydrated silica from groundwater [1,28]. The modest removal of silica is attributed, on the one hand, to the low coagulant dose released in the filter-press cells, conditioned by the low current densities applied, j < 7 mA cm2; and on the other, at j 7 mA cm2, to the excess of hydrogen bubbles produced at the cathode, which usually break the flocs, inhibiting the removal of hydrated silica [1,28]. To avoid floc breakage at j > 7 mA cm2, a parallel plate flow cell, using vertical aluminum electrodes, open to the atmosphere at the top of the reactor, was employed for the removal of hydrated silica from real groundwater [2]. In that cell, higher current densities (j > 7 mA cm2) were used for the massive production of coagulant (85.4 mg L1), which reacted with silica forming aluminum silicates, and achieving the elimination of hydrated silica [2]. Other coexisting ions found in groundwater, such as phosphates and sulfates, which compete with arsenates for actives sites from flocs, have been moderately removed by adsorption process on the aluminum flocs [1,2,28]. On the other hand, the use of a horizontal continuous flow EC reactor, equipped with iron plates as sacrificial electrodes, permitted 96% removal of arsenic from groundwater [29]; however, that study reports no results about the removal of coexisting ions present in the water sample. Another paper reported the removal of arsenic in the presence of silica and phosphates, in a reactor consisting of a 1 L glass beaker, with two iron rods immersed in the aqueous solution [30]. In that research, 99.9% of arsenic was removed by adsorption on lepidocrocite (g-FeOOH), from an initial arsenic concentration between 100 and 1000 mg L1, using a pair of iron plates with a gap of 0.5 cm, applying a current intensity of 22 mA during 120 min, obtaining around 50 mg L1 of iron during the EC trial, while hydrated silica was not removed [30]. Moreover, these authors reported that phosphates inhibited the elimination of arsenic [30]. A comparison on the simultaneous removal of arsenic and hydrated silica from geothermal waters by electrocoagulation, using iron and aluminum as sacrificial electrodes, was performed by Mroczek and coworkers [31]. They emphasized that the best removal of arsenic was reached with iron electrodes, while hydrated silica was better removed with aluminum electrodes. It is worth to mention that there is no evidence on the mechanism of silica removal with iron as coagulant [30,31]. To our knowledge, the simultaneous abatement of hydrated silica, arsenic, and phosphates from real underground water has not yet been reported. The operation parameters in the EC reactors, such as flow rate, applied current density, retention time and the presence of coexisting ions influence the removal of pollutants [1,2,32,28,29]. This paper deals with the elimination of hydrated silica, arsenic, and phosphates from real groundwater by EC, using a cascadeshaped up-flow reactor with a six-cell stack open to the
atmosphere at the top. Aluminum plates were horizontally assembled in a serpentine array. The influence of current density and mean linear flow rates (retention time) applied to the EC reactor, on the removal efficiency of arsenic, hydrated silica and phosphates were analyzed. SEM-EDS, XRF, XRD, and FTIR analyses on the flocs were also completed to elucidate the removal mechanism of the pollutants. 1.1. Electrocoagulation process The electrocoagulation process consists in the generation of coagulant in situ by anodic electro-dissolution of aluminum, Eq. (1), while the generation of hydrogen bubbles takes place at the cathode, Eq. (2).
AlðsÞ / Al3þ þ 3e
(1)
3H2 O þ 3e /1:5H2 þ 3OH
(2) 3þ
Under neutral pH conditions, aluminum ions (Al ) react with bulk H2O, allowing the formation of Al(OH)3(s) and Al2O3(s) flocs. It is well known that arsenate is the only arsenic species that can be removed by adsorption process on the aluminum flocs. In this context, the addition of a typical concentration of hypochlorite employed for disinfection purposes (1 mg L1), before the EC trials, is necessary to oxidize the arsenite to arsenate [32]. In addition, the hydrated silica reacts with coagulant to form aluminum silicates, which also remove arsenates by adsorption processes [28]. Phosphates are removed by adsorption on flocs, while sulfates are partially adsorbed on the aggregates [1,2,28]. One of the biggest problems of EC process is the anodic passivation, reflected in the anode potential increase, which is caused by Al2O3(s) precipitation on the anode [32,21,28,33]. In order to minimize the passivation of the anodes, electrodes of the same material are commonly used, and if necessary, the polarity of the current is changed [1,33]. 2. Experimental details 2.1. Real groundwater sample The real groundwater sample (arsenic 22 mg L1, hydrated silica 161 mg L1, sulfate 50 mg L1, phosphate 0.41 mg L1, pH 7.6 and conductivity 508 mS cm1) was collected from a deep well (150 m deep) in the north-central area of Guanajuato State, in central Mexico. The addition of 1 mg L1 hypochlorite, before the EC trials, was added to oxidize the arsenite to arsenate; at such pH 7.6, a mixture of hypochlorous acid and hypochlorite anion (free chlorine), is present. 2.2. EC reactor The EC reactor and its components are shown in Fig. 1. The reactor is composed of a six-cell stack using aluminum plate electrodes horizontally placed in a way to allow the electrolyte flow in a serpentine-like flow path. The reactor is open at the top to enable the rapid release of hydrogen bubbles formed on the cathode. The reactor contains eight 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 the electrolyte, respectively), out of which four are used as cathodes and three as anodes. The liquid inlet is situated at the bottom of the cell. At the top of the cell, there is a window followed by an electrolyte collector to direct the solution towards
~ eda et al. / Electrochimica Acta 331 (2020) 135365 L.F. Castan
a)
3
d)
b)
9 cm
Electrolyte outlet
d = 1.27 cm Electrolyte inlet Aluminum plate electrode
c)
e)
z
12 cm
8 cm
3.5 cm
Channel separator
x y Electrolyte inlet Fig. 1. (a) Sketch of the EC reactor, (b) bottom plate, (c) electrode spacer, (d) electrode, and (e) collector to direct the solution towards the outlet.
Table 1 Magnitudes of the EC reactor. Electrode length, L (cm) Electrode height, B (cm) Electrode spacing, S (cm) rea, A (cm2) Electrode a Hydraulic diameter in the rectangular channel, dh ¼ 2BS/(B þ S) (cm) Number of channels Diameter of the inlet pipe, d (cm) Dimensions of the window, electrolyte outlet, in the upper part (cm)
8 3 0.46 24 0.8 8 1.27 3.4 length 1.5 width
CAlðIIIÞN ¼
j $ L $ Mw 1 106 n$F $S$u
(3)
Where CAlðIIIÞN and j are given in mg L1 and A cm2 , respectively; the atomic weight of aluminum is Mw ¼ 26.98 g mol1 , L is the channel length (8 cm), the Faraday constant is F ¼ 96,485 C mol1 ,
Effluent to jar test Exit zone Cell 6
+
Cell 1 Calm zone
Reservoir
Flowmeter
Power supply
2.3. Methodology The EC trials were carried out in the experimental set-up shown in Fig. 2. The volumetric flow rates (q) employed during EC tests were 0.1, 0.2, 0.3 and 0.4 L min1, corresponding to linear flow velocities (u) of 1.2, 2.4, 3.6 and 4.8 cm s1, with retention times in the EC reactor (t) of 55.9, 27.8, 18.5 and 13.9 s, respectively. Before the electrolysis, the electrodes were polished with 600 grit silicon carbide emery paper and then rinsed with plenty of water. The Faraday’s law was used to calculate the theoretical value of the aluminum used as a coagulant, CAl(III) (N):
_
Cell 4 Cell 3 Cell 2
Pump
Influent
Fig. 2. Hydraulic and electrical system for the EC test.
Valve
cathodes
Cell 5 anodes
the outlet. More details on the cell can be consulted elsewhere [34]. Table 1 shows the dimensions of the EC reactor used here. Fig. 2 shows a sketch 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-10 L), a valve and a flowmeter (0.1e1 L min1, White Industries), all of which are connected 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, directly recording the cell potential.
~ eda et al. / Electrochimica Acta 331 (2020) 135365 L.F. Castan
4
n ¼ 3 is the number of electrons, S is the interelectrode gap (0.46 cm), and the factor 1 106 allows obtaining CAlðIIIÞN in mg L1 . Once the groundwater crossed the EC reactor, the resulting solution went straight to a jar test, where the solution was mixed at 30 rpm during 15 min to facilitate the growth of the flocs. Then, stirring was stopped to allow the precipitation of the aggregates for 1 h. Once the clarified solution was obtained, the concentrations of arsenic, hydrated silica, sulfates, and phosphates were analyzed. The experimental aluminum dose, CAlðIIIÞ , formed in the EC trials was determined after the redissolution of the flocs, using sulfuric acid to attain a pH ¼ 2. The results were averaged from three EC tests. The dry flocs were also examined by SEM-EDS, XRF, XRD, and FTIR techniques [35,36]. 2.4. Analytical procedure 2.4.1. Water analysis Atomic absorption spectroscopy at 188.9 nm was used to measure the arsenic concentration in a PerkinElmer PinAAcle 900F coupled to a PerkinElmer MHS-15 manual hydride generator (detection limit of 0.05 mg L1), following the procedure established in the standard method suggested by APHA (1998) [37]. The same atomic absorption spectrometer, with a detection limit of 0.1 mg L1 (309.27 nm wavelength), was used to determine the concentration of aluminum. A HI 83,200 multiparameter bench photometer, from Hanna instruments, was the equipment employed to measure hydrated silica, phosphate, and sulfate. The silica analysis was carried out by heteropoly blue method using the HI 93,705 kit. Phosphate was determined by the amino acid method using the HI 93,706 kit and sulfate by precipitation with barium chloride crystals (light absorbance method) using the HI 93,751 kit. The detection limit of hydrated silica, phosphate, and sulfate was 0.2 mg L1. Conductivity and pH measurements were carried out using a waterproof instrument from Hanna, model HI 991,300. Analytical grade reagents were used. The results were averaged from three analyses.
analyses were carried out on a Rigaku Ultima IV diffractometer, with nickel filter and Cu K a1 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 of the flocs was carried out in a PerkinElmer Spectrum GX FTIR Spectrometer, using an EasiDiff diffuse reflectance accessory. 2.5. Energy consumption and costs of EC The elements required to calculate the overall cost of EC include the cell potential (Ecell) of the electrolysis, the price of aluminum, the costs of pumping, and confinement of sludge. The energy consumption (Econs ), cost of aluminum dose ($AlðIIIÞ ), and overall cost of EC ($OC) were calculated by Eqs. (4)e(6), respectively:
Econs ¼
Ecell $ I ð3:6Þ $S $B $ u
(4)
Where the units of Econs and I are kWh m3, and C s1 , respectively. B is the channel width (3 cm), S is the electrode spacing (0.46 cm), and factor 3.6 is used to obtain Econs in kWh m3.
$AlðIIIÞ ¼ ðCAlðIIIÞ Þ $ 2:008 USD Kg1 ð0:001Þ
(5)
The aluminum price, in Mexico, is $ 2:008 USD Kg1 and 0.001 is a conversion factor to obtain $AlðIIIÞ in $ USD m3 .
OC ¼ $AlðIIIÞ þ aEcons þ aEpump þ bMsludge
(6)
3
OC is expressed in units of USD m , a is the cost of electricity in central Mexico (0.0976 USD (kWh)1), Epump is expressed in units of kWh m3, and b is the sludge confinement cost in Mexico ($ 0.035 USD Kg1 ). 3. Results and discussion
2.4.2. Flocs analysis 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 utilizing a JEOL detector incorporated in the SEM microscope. X-ray diffraction (XRD) 50
50
4 mA cm-2
80
80 6 mA cm-2
60
60
40
40
20
20
20
10
10
0
0 80
80 8 mA cm-2
60
60
0
0 100
100 10 mA cm-2 80
80
60
60
40
40
20
20
Aluminum dose / mg L-1
20
Residual arsenic / μg L-1
30
Aluminum dose / mg L-1
30
Residual arsenic / μg L-1
Fig. 3 shows the residual arsenic concentration, CAs, as a function
Residual arsenic 40 Theoretical aluminum Experimental aluminum
40
40
40
20
20 0
3.1. Removal of arsenic, hydrated silica, and phosphates from groundwater by EC
0
1
2
3
4
5
0
0
0
1
2
3
4
5
0
Mean linear flow velocity, u / cm s-1 Fig. 3. Effect of the flow velocity on the remaining arsenic concentration and coagulant dose, at different current densities.
concentration of phosphates, CPO 3 4 ; exhibited a behavior similar to that observed for hydrated silica, where CPO 3 4 varied between 0.41 and 0.03 mg L1. Other papers have reported that the sulfates and phosphates are adsorbed on the aluminum aggregates [1,2,28,38]. 3.2. Characterization of aluminum flocs Fig. 5 shows SEM micrographs of flocs taken after the EC trial at 10 mA cm2 and 1.2 cm s1. The images allow observing irregular aggregates with sizes up to 600 mm; these flocs are composed of fine particles with a size below 100 nm. The chemical composition of these flocs was determined by EDS and XRF, and the results are reported in Table 2. Si and Al are present in a relatively high percentage in EDS results. This indicates the formation of aluminosilicate compounds. Arsenic was not detected due to the low concentration present in the aggregates. However, the presence of arsenic in the flocs was confirmed by XRF analysis. The difference between the chemical compositions of the flocs obtained by these two techniques could be originated by different sampling areas employed. In SEM-EDS, five areas of about 0.18 mm2 were sampled, and the average of these measurements is reported in Table 2. However, in XRF tests, a sampling area of 8.02 cm2 was analyzed. Fig. 6 (a) displays the X-ray diffraction pattern of flocs, obtained
Residual phosphate / mg L-1
of the mean linear flow rate at current densities of 4, 6, 8 and 10 mA cm2. It also shows the experimental, CAl(III), and theoretical, CAl(III) (N), aluminum doses. The EC performed at 4 mA cm2 shows an increase in CAs from 4.57 to 15.54 mg L1, as u increases. This pattern is attributed to a depletion of the experimental aluminum dose, CAl(III), with mean linear flow rate. The residual concentration of arsenic (4.17 and 9.57 mg L1) at u of 1.2 and 2.4 cm s1, met the WHO guideline (<10 mg L1). It is worth to mention that the experimental aluminum doses (11.9 CAl(III) 48 mg L1) obtained at such a low current density (4 mA cm2), surpass the theoretical values (calculated by Faraday’s Law), which is attributed to the chemical dissolution of aluminum provoked by the presence of hypochlorite in the solution. At 6 mA cm2, CAs also increases with u, and it can be observed that at u 3.6 cm s1, CAs < 10 mL1, which is achieved due to the increased coagulant dose, 19.5 CAl(III) 71.6 mg L1. At this current density, the experimental coagulant dose was moderately higher than the theoretical one, calculated by Eq. (3). However, at current densities of 8 mA cm2 and 10 mA cm2, CAs complied with the WHO recommendation at all flow rates tested. The two last results are due to the high production of aluminum dose, 19.6 CAl(III) 85.4 mg L1, which favors the removal of the pollutant. It is noteworthy that at j of 8 and 10 mA cm2, CAl(III) shows close agreement with CAl(III) (N), as expected, because at such current densities, the electrodissolution of aluminum, Eq. (1), predominates over the chemical dissolution. Fig. 4 presents the simultaneous removal of hydrated silica, sulfate, and phosphate during the experiments shown in Fig. 3. The residual hydrated silica concentration (Chs) exhibits a behavior similar to that obtained during the removal of arsenic, as shown in Fig. 3, and it should be noted that the removal of silica increases with current density and lower flow rates. At 10 mA cm2 and u ¼ 1.2 cm s1, Chs reached a value of 42 mg L1, which represents removal of 74%. The silica removal at such condition consumed a coagulant dose of 85.4 mg L1. It is well known from previous studies that silica reacts with coagulant forming aluminum silicates as flocs [28]. The residual concentration of sulfates, Fig. 4, remains almost constant with u and j, reaching values between 35 and 45 mg L1, which represents a removal of 10e30%. However, the residual
Residual sulfate / mg L-1 Residual hydrated silica / mg L-1
~ eda et al. / Electrochimica Acta 331 (2020) 135365 L.F. Castan
5
120 100 80 60 40 20
4 mA cm-2 6 mA cm-2 8 mA cm-2 10 mA cm-2
0 50 40 30 20 10 0 0.4 0.3 0.2 0.1 0
0
4 5 1 2 3 -1 Mean linear flow velocity, u / cm s
Fig. 4. Effect of the flow velocity on the remaining concentrations of hydrated silica, sulfate, and phosphate, determined from the EC trials showed in Fig. 3.
from the EC trials at 10 mA cm2 and 1.2 cms1. The phases were identified using the software MATCH! Version 1.11j, which employs the American Mineralogist Crystal Structure Data Base (AM-CSD). The narrow peaks of highest intensity correspond to the calcite (CaCO3); this indicates that part of the carbonate ions precipitates as calcite, which coincides with the observed decrease in alkalinity of the solution from 242.5 to 166.4 mg L1. Wide peaks are observed at 10 < 2q < 18, 22 < 2q < 34, 35 < 2q < 42. These broad peaks are the result of the superposition of peaks that could be assigned to dicalcium silicate (Ca2SiO4), albite (NaAlSi3O8), wollastonite-2M (CaSiO3), andesine Ca$24Na$26(Al$735Si1.265)O4, and labradorite Ca$325Na$16(Al$81Si1.19)O4. The XRD spectrum confirms that silica reacts with coagulant forming aluminum silicates and
~ eda et al. / Electrochimica Acta 331 (2020) 135365 L.F. Castan
6
Fig. 5. SEM micrographs of the flocs obtained at j ¼ 10 mA cm2 and u ¼ 1.2 cm s1.
Table 2 Composition of the flocs determined by EDS and XRF analyses. Si
S
Cl
K
Ca
O
Na
As
27.94 14.97
10.50 6.14
0.27 0.42
1.11 0.35
1.10 0.66
7.18 3.3
50.15 71.64
2.41 2.50
ND 0.002
ND ¼ Not detected.
calcium-sodium silicates as flocs. Arsenic phases were not detected by XRD due to the low concentration of this element. Fig. 6 (b) shows FTIR spectrum of the flocs, obtained after EC trial at j ¼ 10 mA cm2 and u ¼ 1.2 cm s1. The wide band at 3435 cm1 is attributed to the OeH stretching vibrations [1,2,28,39,40]. The peaks observed at 1643 and 614 cm1 belong to NaeF and AleF bond, respectively. The peaks located at 1443 and 1004 cm1 correspond to AleOeSi and SieO bond, respectively [1,38,41], while the peak situated at 876 cm1 correspond to the AseO bond [28]. The presence of aluminosilicates is also confirmed by these results. In addition, FTIR spectrum suggests that arsenic is removed by adsorption on the flocs [28]. In the same way, the elimination of traces of fluoride from groundwater (below the detection limit of 0.02 mg L1, measured with a selective ion electrode [1]) was observed. It is important to mention that peaks related to S bonds did not appear in the FTIR spectrum, which can be associated with weak adsorption of sulfates on aggregates [1,2,28,36]. The absence of phosphates in the floc composition determined by EDS and XRF analyzes, and the absence of phosphate phases in XRD analyzes and the non-appearance of PeO bonds in FTIR patterns, lead us to suggest that phosphates are removed by adsorption in the flocs [1,2,32,28,38], or well that these are trapped within the aggregates. 3.3. Performance and overall costs of the EC process Table 3 summarizes the experimental conditions for the EC in terms of current density, mean linear flow velocity, and retention
10
20
30
40
50
60
70
80
90
100
2
100
b)
80
1
Al
EDS XRF
% Transmittance / mg L-
wt. %
Intensity / a.u.
a)
60 40 4000 3600 3200 2800 2400 2000 1600 1200 800 400
Wavenumber / cm-1 Fig. 6. (a) XRD and (b) FTIR spectra of the flocs obtained at j ¼ 10 mA cm2 and u ¼ 1.2 cm s1.
time obtained in the EC reactor. This table also shows the remaining concentrations of arsenic and hydrated silica, the experimental aluminum doses, Ecell, Econs, Epump, Msludge, and the overall cost of EC.
~ eda et al. / Electrochimica Acta 331 (2020) 135365 L.F. Castan
7
Table 3 Remaining arsenic and hydrated silica concentrations after EC tests. The experimental coagulant dose, Ecell, Econs, Epump, Msludge are also shown. Initial composition of deep well water: arsenic 22 mg L1, hydrated silica 161 mg L1, phosphate 0.41 mg L1, sulfate 50 mg L1, alkalinity 242.5 mg L1, hardness 90 mg L1, conductivity 508 mS cm1 and pH 7.6. J (mA cm2)
u (cm s1)
t (s)
CAs (mg L1)
Chs (mg L1)
CAl(III) (mg L1)
Ecell (V)
Econs (KWh m3)
Epumping (kWh m3)
Msludge (Kg m3)
OC (USD m3)
4
1.2 2.4 3.6 4.8 1.2 2.4 3.6 4.8 1.2 2.4 3.6 4.8 1.2a 2.4 3.6 4.8
55.9 27.8 18.5 13.9 55.9 27.8 18.5 13.9 55.9 27.8 18.5 13.9 55.9 27.8 18.5 13.9
4.1 9.5 14.4 15.5 1.0 5.1 9.5 11.2 2.6 5 7.8 8.8 1.5 4.1 7.4 8.3
83 111 118 125 78 94 104 110 65 90 102 104 42 83 100 102
48.0 22.8 16.0 11.9 71.6 34.9 24.3 19.5 65.4 35.0 22.5 22.1 85.4 39.9 24.4 19.6
7.8 7.3 7.05 6.99 11 10.9 9.5 9.4 14.7 14.2 12.6 10.3 14.7 14.2 12.6 10.3
0.75 0.35 0.22 0.16 1.59 0.78 0.45 0.34 2.84 1.37 0.81 0.49 3.55 1.71 1.01 0.62
0.50 0.25 0.16 0.12 0.50 0.25 0.16 0.12 0.50 0.25 0.16 0.12 0.50 0.25 0.16 0.12
0.2 0.09 0.5 0.03 0.28 0.19 0.15 0.07 0.48 0.25 0.15 0.16 0.51 0.26 0.18 0.13
0.217 0.104 0.086 0.051 0.345 0.172 0.110 0.084 0.463 0.231 0.141 0.107 0.570 0.274 0.165 0.113
6
8
10
a
Residual concentrations: phosphate 0.03 mg L1, sulfate 35 mg L1, alkalinity 166.4 mg L1, hardness 20 mg L1, conductivity 440 mS cm1 and pH 8.29.
In Table 3, the overall cost of EC is seen to decrease with the flow rate for all current densities tested. The overall cost of EC varies between 0.051 OC 0.57 USD m3. The remaining concentration of arsenic and hydrated silica ranges between 1.1 CAs 15.5 mg L1, and 42 Chs 125 mg L1, respectively. The best removal of arsenic (CAs ¼ 1.5 mg L1) and hydrated silica (Chs ¼ 42 mg L1) was obtained at j ¼ 10 mA cm2 and u ¼ 1.2 cm s1; at such conditions, the coagulant dose was 85.4 mg L1, which produced dry sludge of 0.51 kg m3, with electrolysis energy consumption of 3.55 kWh m3 and an overall cost of EC of 0.57 USD m3. On the other hand, the remaining concentration of silica (Chs ¼ 42 mg L1) after the EC trials (at j ¼ 10 mA cm2 and u ¼ 1.2 cm s1), suggested performing a second round of EC treatment, with the aim to abate the rest of silica. In this context, we applied the EC process to the treated water once again (using 10 mA cm2 and 1.2 cm s1). The analysis of the solution (after the second round of EC) revealed that hydrated silica and sulfate were removed until 2.6 mg L1, and 30 mg L1, while the arsenic and phosphates were eliminated. The dose of aluminum generated in the second EC was 58.9 mg L1; also, 0.51 kg m3 of sludge was produced, with Econs ¼ 3.55 kWh m3, and an operational cost of 0.523 USD m3. The total cost of the EC process, considering the two EC tests, reached the value of 1.093 USD m3. It is noteworthy that the total cost of EC can vary among different countries depending on the prices of the confinement of sludge and electricity. 4. Conclusions The elimination of hydrated silica, arsenic, and phosphates from real groundwater was successfully achieved after applying two rounds of EC process to groundwater, at 10 mA cm2 and 1.2 cm s1, in each trial. The conclusive EC test results were reached using a cascade-shaped up-flow reactor, with a six-cell stack open to the atmosphere at the top. Aluminum plates were used as sacrificial electrodes. The total cost of the EC was of 1.093 USD m3. XRD, SEM-EDS, XRF, and FTIR analysis on flocs showed that these are mainly composed of aluminosilicates and silicates with divalent cations, which are formed by the reaction between coagulant and hydrated silica. The elimination of arsenic and phosphates is carried out by adsorption on flocs. The modest removal of sulfates (40%) is related to weak adsorption on flocs. The results showed herein evidenced that EC is an affordable
technology that eliminates silica, arsenic and phosphates from groundwater. These results can be applied to other water samples containing silica, arsenic and fluoride ion, recommending the use of a continuous flow reactor, applying current and hydrodynamic conditions like those used here. Author contributions section All the authors have contributed to the paper and have seen and revised the final version of the manuscript. Declaration of Interest Statement The authors declare that there is no conflict of interest regarding the publication of this paper. Acknowledgments The authors thank to SICES (project No. IJ-19-78), CONACYT (project No. 759) and the University of Guanajuato (projects No. 102/2019 and 150/2019) for financial support. Authors acknowledge Dr. Raul Miranda and Daniela Moncada from LICAMM-UG Laboratory for spectroscopy analysis. References ~ o, Y.M. Li, J.H. Herna ndez, [1] M.A. Sandoval, R. Fuentes, J.L. Nava, O. Coren Simultaneous removal of fluoride and arsenic from groundwater by electrocoagulation using a filter-press flow reactor with a three-cell stack, Separ. Purif. Technol. 208 (2019) 208e216. ~ o, J.L. Nava, Removal of hydrated silica, fluoride and [2] M. Rosales, O. Coren arsenic from groundwater by electrocoagulation using a continuous reactor with a twelve-cell stack, Chemosphere 211 (2018) 149e155. ~ ana-Ruiz, J. Vazquez-Arenas, F.S. Sosa-Rodríguez, I. Labastida, [3] Y.A. Sarin n-Pin ~ a, M.A. Escobedo-Bretado, M.A. Armienta, A. Armienta, A. Arago lez-Valdez, P. Ponce-Pen ~ a, H. Ramírez-Aldaba, R.H. Lara, Assessment L.S. Gonza of arsenic and fluorine in surface soil to determine environmental and health risk factors in the Comarca Lagunera, Mexico, Chemosphere 178 (2017) 391e401. [4] J. Marcais, A. Gauvain, T. Labasque, B.W. Abbott, G. Pinay, L. Aquilina, F. Chabaux, D. Viville, J.R. de Dreuzy, Dating groundwater with dissolved silica and CFC concentrations in crystalline aquifers, Sci. Total Environ. 636 (2018) 260e272. [5] M.F. Hughes, B.D. Beck, Y. Chen, A.S. Lewis, D.J. Thomas, Arsenic exposure and toxicology: a historical perspective, Toxicol. Sci. 132 (2011) 305e332. [6] WHO, Guidelines for Drinking Water Quality 1 (2006) 306. [7] USEPA, Drinking Water Standard for Arsenic, U.S. Government Printing Office, Washington DC, 2001, p. 1. EPA-815-F-00-015. [8] R. Merget, T. Bauer, H.U. Küpper, S. Philippou, H.D. Bauer, R. Breitstad, T. Bruening, Health hazards due to the inhalation of amorphous silica, Arch.
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