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Arsenic and hydrated silica removal from groundwater by electrocoagulation using an up-flow reactor in a serpentine array
Journal of Environmental Chemical Engineering 7 (2019) 103353
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Arsenic and hydrated silica removal from groundwater by electrocoagulation using an up-flow reactor in a serpentine array
T
Locksley F. Castañedaa, Oscar Coreñob, José L. Navaa,
⁎
a b
Department of Geomatic and Hydraulic Engineering, University of Guanajuato, Av. Juárez 77, Centro, 36000, Guanajuato, Guanajuato, Mexico Department of Civil Engineering, University of Guanajuato, Av. Juárez 77, Centro, 36000, Guanajuato, Guanajuato, Mexico
ARTICLE INFO
ABSTRACT
Keywords: Arsenic removal Hydrated silica elimination Groundwater treatment Electrocoagulation Aluminosilicates
This paper concerns with the elimination of arsenic and hydrated silica from deep well water, collected form the septentrional area of Guanajuato in Mexico (arsenic 23 μg L−1, hydrated silica 154 mg L−1, sulphate 160 mg L−1, phosphate 0.3 mg L−1, pH 7.5 and conductivity 550 μS cm−1) by electrocoagulation (EC) using an up-flow electrochemical reactor, opened to the atmosphere, with a six-cell stack in a serpentine array. Aluminum plates were used as electrodes. The efficiencies of arsenic and hydrated silica removal at different current densities (4 ≤ j ≤ 7 mA cm-2) and mean linear flow rates (1.2 ≤ u ≤ 4.8 cm s−1) were examined. The best EC was obtained at 7 mA cm-2 and 1.2 cm s−1, which satisfies the WHO guideline for arsenic (< 10 μg L−1) and permits the abatement of hydrated silica, giving values of electrolytic energy consumption and overall cost of EC of 1.64 kW h m-3 and 0.387 USD m-3, respectively. SEM-EDS, FXRD, XRD and FTIR analyses on the flocs revealed that they are structured mainly by aluminosilicates due to the reaction between aluminum and silica. While arsenates, sulfates and phosphates are removed by adsorption on aluminum flocs.
1. Introduction Mexico is one of many regions worldwide with serious problems of groundwater contamination with arsenic [1,2]. Specifically, in the septentrional area of Guanajuato in central Mexico, groundwater is mainly contaminated with arsenic, and in some places considerable amounts of hydrated silica are also found. The arsenic, hydrated silica and coexisting ions present in groundwater come from the dissolution of minerals containing rocks in contact with water [1,3,4]. The concentration of arsenic detected in this area varies in a range of 40–134 μg L−1, while hydrated silica fluctuates between 50–132 mg L−1 [1,5]. The chronic consumption of polluted water with arsenic provokes the apparition of different types of chronic diseases, such as the blackfoot disease, pigmentation, brain injuries, keratosis, bones diseases, nausea, diabetes, arsenicosis and several types of cancer in various organs [2,3,6–12]. In accordance with Mexican regulations, the concentration limit for arsenic in drinking water is 25 μg L−1, while the WHO recommendation establishes a maximum value of 10 μg L−1. On the other hand, the exposure to hydrated silica crystals mainly affects the lungs, increasing the risk of suffering tuberculosis, silicosis, chronic obstructive pulmonary disease, chronic bronchitis and lung cancer [13]. Although hydrated silica is not currently regulated in Mexico in terms of health effects. Hydrated silica not only generates ⁎
health complications, but it also creates problems in industrial pipelines and in several unit operations, such as heat exchangers, where silica salts are impregnated on the equipment walls [2,14]. There are several techniques that have been implemented for the removal of arsenic that lead to diminish the concentration to acceptable levels. Techniques that have commonly been used are precipitation [15], adsorption [16,17], coagulation-flocculation [18,19], membrane technologies [20,21], among others [22,23]. On the other hand, some other techniques have been tested to remove hydrated silica as reverse osmosis [24,25], coagulation-ultrafiltration [26,27], hybrid electrolysis-crystallization [28] and the precipitation with magnesium [29,30]. In recent years, the electrocoagulation process has proven to be a potential technology in the treatment of drinking water [31–38]. The EC has several advantages over other technologies, such as compact treatment installation, the possibility of automating the process and the main one, a high removal efficiency [34,39–41]. Recently, our research group has been using the EC to remove arsenic, fluoride and hydrated silica from groundwater [1,2,5,42]. Two previous papers report the arsenic abatement from groundwater by EC using aluminum as sacrificial anodes, but with modest elimination of concurrent ions, such as hydrated silica, phosphates and sulfates [1,5]. These communications report that aluminum reacts with
Corresponding author. E-mail addresses: [email protected] (L.F. Castañeda), [email protected] (O. Coreño), [email protected] (J.L. Nava).
https://doi.org/10.1016/j.jece.2019.103353 Received 2 May 2019; Received in revised form 4 July 2019; Accepted 9 August 2019 Available online 11 August 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 103353
L.F. Castañeda, et al.
Fig. 1. (a) Sketch of the up-flow EC reactor in a serpentine array opened to the atmosphere and, (b) schematic diagram of the EC reactor coupled to a hydraulic and electrical system.
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hydrated silica to produce aluminosilicates; arsenic is adsorbed on the aluminosilicate aggregates, satisfying the WHO guideline. The modest removal of hydrated silica (49.2% from initial concentration 132 mg L−1) has been due to the low doses of aluminum (72 mg L−1). These low doses have been restricted by the presence of hydrogen bubbles, at high current densities (> 6 mA cm-2) inside the closed filterpress reactors employed, because the bubbly flow breaks the flocs, affecting the removal efficiency of the pollutants [1,5]. In another research, our group employ a continuous EC reactor to remove hydrated silica from groundwater. The peculiarity in that reactor is that the aluminum plates are stacked vertically, and the cell is opened at the top, which allows rapid release of hydrogen bubbles formed on the cathode towards the atmosphere [2]. In that paper, the EC was performed at relatively high current densities, 8 mA cm−2, which permitted the quasi complete removal of hydrated silica (initially having 72 ppm). Other researchers, such as Wan and coworkers [43], have investigated the elimination of arsenic by EC using iron electrodes, finding that lepidocrocite adsorbs arsenic. However, the presence of silica (20 mg L−1) in water inhibits the formation of this iron phase, which also disfavors the removal of arsenic. It is worth mentioning that the removal of hydrated silica using iron as coagulant is negligible [43]. The novelty of this paper deals with the simultaneous abatement of arsenic and hydrated silica from groundwater by EC using a novel EC reactor, where the aluminum plates are horizontally assembled (conforming a six-cell stack in a serpentine array); at the top, the cell is exposed to the atmosphere to allow the rapid hydrogen gas release at high current densities. The influence of current density (aluminum dose) and mean linear flow rate (retention time) on the arsenic and hydrated silica removal performance was examined. SEM-EDS, XRFEDS, XRD and FTIR analyses of the flocs were also performed to elucidate the removal pathway of the pollutants.
3H2 O+ 3e
1.1. Electrocoagulation process
2.2. EC reactor in a serpentine array opened to the atmosphere
The electrocoagulation process involves the production of coagulants in situ by electrodissolution of aluminum sacrificial anodes, Eq. (1); in the cathode, bubbles of the hydrogen gas are generated, Eq. (2), and the overall reaction of the cell is described by Eq. (3).
Fig. 1(a) shows a schematic diagram of the up-flow electrochemical reactor in a serpentine array. The polypropylene was the construction material to the reactor components of the cell. The reactor is similar in dimensions of width, length and inter-electrode thickness of the commercial laboratory cell, known as Ecocell®. However, the arrangement of the electrodes differs from Ecocell®, because in this reactor, the
Al (s)
Al3 + + 3e
Al (s) + 3H2 O
1.5H2 + 3OH Al(OH )3 + 1.5H2
(3)
3+
The aluminum ions (Al ) diffuse away from the anode and react with H2O in the bulk, in neutral conditions of pH, yielding to the formation of Al(OH)3(s) and Al2O3(s). It is well known that both aluminum salts adsorb arsenate. The arsenate is the only specie that is efficiently removed by EC. In this context, the arsenite is oxidized to arsenate, by the addition of 1 mg L−1 hypochlorite, a typical concentration employed for disinfection purposes [5]. Additionally, there is an evidence that Al(III) reacts with hydrated silica contained in groundwater to form aluminosilicates, which also remove arsenates by adsorption processes [5]. It is worth mentioning that the anodic passivation represents the biggest problem of the EC, since the precipitation of the aluminum salts inhibits the EC performance, increases the electrolytic energy consumption and the cost of the EC [5,34,42,44]. Electrodes of the same material are used to attenuate the passivation, varying periodically the polarity of the current [1,44]. Another way to avoid passivation is the use of flow cells, where mean linear flow rate favors the transport of Al3+ from the anode to the bulk [1,5]. 2. Experimental 2.1. Deep well water sample The groundwater (arsenic 23 μg L−1, hydrated silica 154 mg L−1, sulphate 160 mg L−1, phosphate 0.3 mg L−1, pH 7.5 and conductivity 550 μS cm−1) was collected from the septentrional region of Guanajuato in central Mexico. It is important to mention that 1 mg L−1 of hypochlorite was added to the water sample prior to EC process.
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L.F. Castañeda, et al.
EC tests were carried out at mean linear flow velocities (u) applied to the EC reactor of 1.2, 2.4, 3.6 and 4.8 cm s−1, that correspond to volumetric flow rates (q) of 0.1, 0.2, 0.3 and 0.4 L min−1, and retention times in the EC reactor (τ) of 55.9, 27.8, 18.5 and 13.9 s , respectively. Immediately after the electrolyte leaves the EC reactor, the resulting solution is passed to the jar test, where the solution is mixed at 30 rpm during 15 min, to allow the aggregate grow. Once the slow mixing was completed, the aggregates were settled down for 1 h without stirring. The concentration of As, hydrated silica, sulfates and phosphates in the clarified solution is analyzed. The flocs were dried overnight, and then, they were analyzed [46,47].
Table 1 Dimensions of the EC reactor. Electrode length, L / cm Electrode height, B/cm Electrode spacing, S/cm Electrode área, A/cm2 Hydraulic diameter in the rectangular channel, dh = 2BS/(B + 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
2.4. Analytical procedure
parallel aluminum plates are in horizontal mode, leaving a slot in each of the ends of each plate (3 cm width and 1 cm length), allowing that the electrolyte flows as in a serpentine. Eight empty channels stacked form the reactor in a serpentine array, which consist of 3 cm × 8 cm × 0.46 cm, width, length and thickness, respectively, in contact with solution. Seven parallel aluminum plates (3 cm width, 8 cm length and 0.3 cm thickness) were employed as electrodes. At the bottom of the reactor, the electrolyte inlet was located, with a diameter of 1.27 cm. To allow the gas release, generated in the electrodes during the electrolysis, the reactor was designed, so that the top part was opened to the atmosphere, Fig. 1(a). The reactor had a window in the upper channel, where the electrolyte was emptied (with the dimensions of 3.4 cm length and 1.5 cm height) through a liquid collector of 12 cm long to drive the electrolyte to the outlet. The EC reactor was constructed after several trials of CFD simulations; more details about the cell can be consulted elsewhere [45]. The dimensions of the reactor are shown in Table 1. A schematic diagram of the reactor coupled with a hydraulic system is shown in Fig. 1(b), which consists of a reservoir of 15 L capacity for electrolyte sample, a centrifugal pump (1/125 HP, Iwaki, MD-10 L), a valve followed by a flowmeter (0.1-1 L min−1, White Industries), coupled by PVC pipe of 0.5 in. of diameter. The current during the EC trials was applied by a power source BK Precision model 1090.
The concentration of arsenic was measured by atomic absorption spectroscopy at 188.9 nm in a Perkin Elmer PinAAcle 900 F coupled to a Perkin Elmer MHS-15 manual hydride generator (detection limit of 0.05 mg L−1), according to the standard method suggested by APHA (1998) [48]. The hydrated silica, sulfates and phosphates concentrations were measured by photometric techniques described elsewhere [5]. The concentration of aluminum was determined using a Perkin Elmer AAnalyst™ 200 atomic absorption spectrometer (detection limit of 0.1 mg L−1), at a wavelength of 309.27 nm. The reported concentrations were the average of the analysis performed in triplicate. The analyses of SEM, EDAX, EDXRF and FTIR, performed to the flocs, were carried out using the equipment and methodologies described elsewhere [2]. 3. Results and discussion 3.1. Simultaneous arsenic and hydrated silica removal by EC Fig. 2 shows the residual arsenic concentration, CAs, the experimental and theoretical aluminum doses CAl(III) and CAl(III)(N), respectively, as a function of u at 4, 5, 6 and 7 mA cm−2. At 4 mA cm−2, we note that CAs increases linearly with flow rate, highlighting that at 1.2 and 2.4 cm s-1, CAs was 5.1 and 9.4 μg L-1, which fulfills the WHO guideline. Then, at u of 3.6 and 4.8 cm s-1, the remaining concentration was 15.2 and 16.9 μg L-1; this is attributed to the low CAl(III) obtained at such flow rates as 15.4 and 12.1 mg L-1. It is important to note that
2.3. Methodology The experimental system used for EC tests is shown in Fig. 1(b). The
Fig. 2. Influence of the mean linear flow rate on the residual arsenic concentration at different current densities shown inside the Figure. The theoretical and experimental coagulant (aluminum) doses are also graphed. 3
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4 mA cm−2. The results obtained here coincide with that reported in other communications in which As has been satisfactorily eliminated from groundwater, from 45 mg L-1 to 6 [44]. Fig. 3(a) shows the remaining concentration of hydrated silica (Chs) for the same EC trials shown in Fig. 2(a). Chs increases with u due to the depletion of coagulant dose with flow rate. The removal of hydrated silica is favored with current density, highlighting that at j =7 mA cm−2 and u = 1.2 cm s-1, Chs =5.6 mg L-1, which fulfill the Mexican recommendation (Chs < 12 mg L-1). This later is related with the highest concentration of coagulant, CAl(III) =87.4 mg L-1. This achievement is one of the benefits of this innovative EC reactor that allows to apply current densities greater than 6 mA cm−2, without breaking the flocs, what typically occurs in the closed filter-press type EC reactors [1,5,46]. Fig. 3(b) and (c) show the removal of sulfates and phosphates, respectively. The initial concentration of sulfate was 160 mg L−1, and after EC, the remaining concentration of sulfates CSO 42 decreased only close to 130 mg L−1, showing a decreasing trend with the mean linear flow rate. On the other hand, the remaining concentration of phosphates CPO43 presents a similar behavior to that of silica pattern; CPO43 was reduced from 0.2 to 0 mg L−1. According to previous results, the sulfate and phosphate ions are adsorbed on the aluminum aggregates and compete with arsenates for the flocs active sites [5,42]. 3.2. Characterization of the aggregates Fig. 4(a) and (b) show SEM micrographs of the flocs, obtained after the EC trial at 7 mA cm−2 and 1.2 cm s-1. From the analysis of Fig. 4(a), irregular shaped aggregates with sizes up to around 20 μm can be seen; these aggregates are formed by particles with a size below 100 nm, as shown in Fig. 4(b). Table 2 shows the chemical composition of the flocs determined by SEM-EDS and XRF-EDS analyses. In the SEM-EDS results, the presence of Si in a relatively high percentage indicates the generation of aluminosilicate complexes, while the arsenic was not detected due to the low concentration present in the flocs. Thakur et al. reported EDS results performed on the aluminum flocs, after the removal of As and F from groundwater by EC, in which they found arsenic, fluoride and aluminum in their sludge with a higher percentage of O and Al [40]; this groundwater did not contain silica. On the other hand, XRF-EDS analyses confirmed the presence of arsenic in the dry flocs, Table 2. It is important to highlight that there is no agreement between the results obtained using these two techniques, i.e. K and As were not detected by SEM-EDS. Five areas of about 0.16 mm2 were sampled in SEM-EDS (the average of these measurements is reported in Table 2) and a sampling area of 8.02 cm2 for XRF-EDS. The difference between the element percentages is due to the sampling area. The smaller sampling area in SEM-EDS produced high standard deviation. For example, the standard deviations for O and Al were 7.50 wt. % and 5.14 wt. %, respectively. In this context, the FRX-EDS analyses are the most representative of the flocs. Fig. 5(a) shows the X-ray diffraction pattern of flocs, obtained from the EC trial at 7 mA cm−2 and 1.2 cm s-1. The identification of the phases was performed using the software MATCH! Version 1.11 j, which uses the American Mineralogist Crystal Structure Database (AMCSD). From the analysis of this Figure, the calcite (CaCO3, database code AMCSD 0012867) was the only phase, for which peaks could be clearly identified. These results coincide with the decrease of alkalinity from 112 to 89.4, Table 3, which indicates that part of the carbonate ions precipitated as calcite. Wide peaks are observed at 10 < 2θ < 17, 18 < 2θ < 34, 35 < 2θ < 43, 45 < 2θ < 58, 60 < 2θ < 70. The broad peaks are the result of the superposition of peaks that could be assigned to Bytownite ((Ca3.44Na0.56)Al7.76Si8.24O32, AMCSD 0009655), Anorthite (Si2Al2CaO8, AMCSD 0001286), Wallastonite-2 M (CaSiO3, AMCSD 0010870), and Albite (NaAlSi3O8, AMCSD 0003798). It is worth mentioning that arsenic phases were not identified by XRD due to
Fig. 3. Influence of the mean linear flow rate on the remaining concentration of: (a) hydrated silica, (b) sulfates, and (c) phosphates, from the same EC trials shown in Fig. 2.
CAl(III) > CAl(III)(N) is attributed to the redox dissolution of the aluminum, which is provoked by the presence of hypochlorite in the solution. CAl(III)(N) is calculated by Faraday’s law:
CAl (III ) N =
j L MW (1x106) n F S u
(4)
where CAl (III ) N is expressed in mg L−1, j is the current density (A cm-2), L is the length of the channel (8 cm), Mw is the molecular weight of coagulant (aluminum =26.98 g mol−1), n is the number of exchanged electrons ( = 3), F is the Faraday constant (96,485 C mol−1), S is the gap between electrodes (0.46 cm) and the value of 1 × 10-6 is a factor to convert CAl (III ) N in mg L−1. At 5 A cm−2 and 6 mA cm−2, CAs increases linearly with flow rate, highlighting that at u of 1.2, 2.4 and 3.6 cm s-1, CAs < 10 mg L-1. While at 6, CAs fulfills the WHO recommendation for the four mean linear flow rates studied here. These findings are attributable to high doses of coagulant. At such set of EC trials, CAl(III) > CAl(III)(N), as it occurred at 4
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Fig. 4. SEM micrographs of aluminum flocs obtained at the end of the EC process at j =7 mA cm−2 and u = 1.2 cm s-1.
the low concentration of this element. Fig. 5 (b) shows a FTIR spectrum, acquired for wave numbers between 4000 and 400 cm−1 to analyze the chemical bonds of the flocs, obtained after EC trial at j =7 mA cm-2 and u = 1.2 cm s−1. The spectrum shows a wide band at 3468 cm−1 that belongs to the OeH stretching vibrations [1,2,5,49,50]. The peaks, situated at 1641 and 603 cm−1, correspond to the Na-F and Al-F bounding, respectively, while the peaks located at 1530, 1419, 999 cm−1 correspond mainly to the Al-O bending, Al-O-Si bounding and Si-O bond, respectively [1,51,52]. Finally, the peak located at 854 cm−1 corresponds to the AsO bond [5]. The FTIR spectrum confirms the presence of aluminosilicates. Moreover, FTIR also corroborates that the arsenic is removed by adsorption on flocs. It is important to mention that in the FTIR spectrum bands related to S bonds did not appear, which is due to weak adsorption of sulphates on the aggregates [1,2,5,46]. The peaks associated with the F bonds indicate the presence of traces of fluoride in the groundwater, which were not detected by the selective ion electrode technique, having a limit of detection of 0.2 mg L−1 [1,2,5].
Table 2 Chemical composition of the flocs determined by SEM-EDS and XRF-EDS analyses. wt. %
Al
Si
S
Cl
K
Ca
O
Na
As
SEM-EDS XRF-EDS
26.17 15.16
6.87 4.02
0.17 0.22
5.89 2.54
ND 0.18
2.77 1.90
54.76 72.17
3.29 3.80
ND 0.004
ND = Not detected.
3.3. Energy consumption and costs of EC Table 3 abbreviates the experimental conditions of EC (current density and mean linear flow velocity), the remaining concentrations of arsenic and hydrated silica, the experimental aluminum doses, the cell potential (Ecell), electrolytic energy consumption (Econs), pumping energy consumption (Epumping), mass of sludge produced (Msludge) and overall cost of the EC ($OC). The energy consumption used in the electrolysis was determined by:
Econs =
Ecell I (3.6) sBu
(5)
Where Ecell has units of V, I is the current intensity (C s−1), the value 3.6 is a conversion factor to get units of kWh m-3 for Econs, s is the channel width (cm) and B is the channel height (cm). The cost of the aluminum dose was calculated by:
$Al (III ) = (CAl (III ) )($ 1.84 USD Kg 1)(0.001) −1
(6)
where $1.84 USD Kg is the aluminum price and 0.001 is a factor to convert units of $USD m-3 to $Al(III). Finally, the overall cost of EC was calculated by:
Fig. 5. Typical XRD (a) and FTIR (b) spectra of the dried aluminum flocs obtained at the end of the EC test at j =7 mA cm−2 and u = 1.2 cm s-1.
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Table 3 Residual arsenic and hydrated silica concentrations after EC trials. The experimental aluminum dose, cell potential, electrolytic and pumping energy consumption, mass of dried flocs is also shown. Initial composition of groundwater: arsenic 23 μg L−1, hydrated silica 154 mg L−1, phosphate 0.3 mg L−1, sulfate 160 mg L−1, alkalinity 112 mg L−1, hardness 50 mg L−1, conductivity 550 μS cm−1 and pH 7.56. The retention times of flocculation and sedimentation in all jar tests were 15 and 60 min, respectively. j (mA cm−2)
u (cm s−1)
τ (s)
Chs (mg L−1)
CAs
4
1.2 2.4 3.6 4.8 1.2 2.4 3.6 4.8 1.2 2. 3.6 4. 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
28.2 39.8 49.7 95 22 37.4 47 81.5 15.6 29.6 28.4 59 5.6 15.6 19.8 33.5
5.1 9.4 15.2 16.9 1.6 6.5 9.4 15.5 1.4 4.5 6 11.1 1.3 2.4 2.6 4.1
5
6
7
1 (μg L− )
CAl(III) (mg L−1)
Ecell (V)
Econs (kWh m−3)
Epumping (kWh m−3)
Msludge (Kg m−3)
$ OC (USD m−3)
42.8 29.1 15.4 12.1 54.5 36.6 18.6 14.3 62.7 30.7 22.1 17.1 87.4 46.3 35.1 22.6
6.1 6.2 6.8 8.5 7.7 7.6 8.1 8.2 8.4 8.9 9.2 9.4 9.7 9.5 9.0 8.2
0.59 0.30 0.22 0.21 0.93 0.46 0.33 0.25 1.21 0.64 0.44 0.34 1.64 0.80 0.51 0.35
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
0.203 0.078 0.067 0.050 0.254 0.118 0.606 0.056 0.330 0.074 0.053 0.067 0.502 0.283 0.223 0.205
0.192 0.109 0.067 0.056 0.248 0.14 0.103 0.064 0.294 0.146 0.101 0.079 0.387 0.197 0.137 0.094
a Satisfy Mexican guideline for hydrated silica. For this EC test the residual concentrations are: phosphate (not detected), sulfate 140 mg L−1, alkalinity 89.44 mg L−1, hardness 31 mg L−1, conductivity 510 μS cm−1 and pH 8.26.
$OC = $Al (III ) + Econs + Epumping + Msludge
u = 1.2 cm s-1, reaching residual concentration values of arsenic and hydrated silica of 1.3 μg L-1 and 5.6 mg L-1, respectively, with overall EC cost of 0.387 USD m-3. XRD, XRF-EDS, SEM-EDS and FTIR analyses of flocs showed that they are structured mainly by aluminosilicates due to the reaction between aluminum and silica. While arsenates, sulfates and phosphates are removed by adsorption on aluminum flocs. The well-engineered EC reactor allowed the simultaneous removal of arsenic and hydrated silica. The EC has a promising potential in the industry, particularly for preconditioning of silica-containing water, as it is easy to operate, compact and has affordable treatment costs.
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Where the $OC is in units of USD m−3, α is the cost of the electricity in central Mexico ($0.0976 USD (kWh)-1), Epumping is in units of kWh m−3 and β is the sludge confinement cost in Mexico ($0.035 USD kg-1). From the analysis of Table 3, it is visible that the Econs, Msludge and $OC decrease with mean linear flow rate in all current densities tested. The residual concentration of arsenic and hydrated silica after the EC trials varied from 1.3 to 16.9 μg L−1 and 5.6 to 95 mg L−1, respectively. The best EC that fulfills the WHO and Mexican recommendations for As (CAs = 1.3 μg L−1) and hydrated silica (Chs =5.6 mg L−1) was the one obtained at j =7 mA cm-2 and u = 1.2 cm s−1, giving values of electrolytic energy consumption and overall cost of EC of 1.64 kW h m-3 and 0.387 USD m-3. At such conditions, an experimental aluminum dose of 87.4 mg L−1 was required, which produces a dried sludge mass of aluminum flocs of 0.502 kg m-3. Due to the variation of the sludge confinement and the electricity costs between different countries, the overall cost of the EC may vary depending on the country. It is worth mentioning that there are many studies reported in the literature, during the removal of As from groundwater by EC, where the costs were informed, to mention some of them, Omwene and coworkers reported operating costs between 0.026-0.32 USD m−3, noting that these costs only included the costs of electrolysis and coagulant [12]. In the same way, Thakur et al., also reported operating costs of 0.357 USD m−3, during the simultaneous removal of arsenic and fluoride from groundwater by EC in batch mode [7]. In another communication from our group, we reported operating costs of 0.71 USD m−3 during the removal of arsenic and fluoride from groundwater by EC through a continuous EC reactor [1]. The total costs obtained in this research demonstrate the affordable costs of EC treatment.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in 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. References [1] M.A. Sandoval, R. Fuentes, J.L. Nava, O. Coreño, Y.M. Li, J.H. Hernández, Simultaneous removal of fluoride and arsenic from groundwater by electrocoagulation using a filter-press flow reactor with a three-cell stack, Sep. Purif. Technol. 208 (2019) 208–216. [2] M. Rosales, O. Coreño, J.L. Nava, Removal of hydrated silica, fluoride and arsenic from groundwater by electrocoagulation using a continuous reactor with a twelvecell stack, Chemosphere 211 (2018) 149–155. [3] Y.A. Sariñana-Ruiz, J. Vazquez-Arenas, F.S. Sosa-Rodríguez, I. Labastida, M.A. Armienta, A. Armienta, A. Aragón-Piña, M.A. Escobedo-Bretado, L.S. González-Valdez, P. Ponce-Peña, H. Ramírez-Aldaba, R.H. Lara, Assessment of arsenic and fluorine in surface soil to determine environmental and health risk factors in the Comarca Lagunera, Mexico, Chemosphere 178 (2017) 391–401. [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) 260–272. [5] A. Guzmán, J.L. Nava, O. Coreño, I. Rodriguez, S. Gutierrez, Arsenic and fluoride removal from groundwater by electrocoagulation using a continuous filter-press reactor, Chemosphere 144 (2016) 2113–2120. [6] A. Sakar, B. Paul, The global menace of arsenic and its conventional remediation – a critical review, Chemosphere 158 (2016) 37–49. [7] L.S. Thakur, P. Mondal, Simultaneous arsenic and fluoride removal from synthetic and real groundwater by electrocoagulation process: parametric and cost
4. Conclusions The removal of arsenic and hydrated silica from real groundwater with initial concentrations of 23 μg L−1and 154 mg L−1, respectively, was performed successfully by EC, using an innovative up-flow EC reactor with a six-cell stack in form of serpentine, which was also opened to the atmosphere to permit the rapid H2 release from the cathodes to the environment. In this EC reactor, high current densities (> 6 mA cm2 ) can be applied with success without breaking the flocs. The best EC that satisfies the WHO and Mexican recommendations for As and hydrated silica was obtained at j =7 mA cm−2 and 6
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evaluation, J. Environ. Manage. 190 (2017) 102–112. [8] V.L. Dhadge, C.R. Medhi, M. Changmai, M.K. Purkait, House hold unit for the treatment of fluoride, iron, arsenic and microorganism contaminated drinking water, Chemosphere 199 (2018) 728–736. [9] P.V. Nidheesh, T.S. Singh, Arsenic removal by electrocoagulation process: recent trends and removal mechanism, Chemosphere 181 (2017) 418–432. [10] B.Z. Can, R. Boncukcuoglu, A.E. Yılmaz, B.A. Fil, Arsenic and boron removal by electrocoagulation with aluminum electrodes, Arab. J. Sci. Eng. 41 (2016) 2229–2237. [11] T. Banerji, S. Chaudhari, Arsenic removal from drinking water by electrocoagulation using iron electrodes- an understanding of the process parameters, J. Environ. Chem. Eng. 4 (2016) 3990–4000. [12] P.I. Omwene, M. Çelen, M.S. Öncel, M. Kobya, Arsenic removal from naturally arsenic contaminated ground water by packed-bed electrocoagulator using Al and Fe scrap anodes, Process. Saf. Environ. 121 (2019) 20–31. [13] 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. Toxicol. 75 (2002) 625–634. [14] S.L. Gelover, S. Pérez, A. Martín, I.E. Villegas, Electrogeneration of aluminium to remove silica in water, Water Sci. Technol. 65 (2012) 434–439. [15] E.O. Kartinen, C.J. Martin, An overview of arsenic removal process, Desalination 103 (1995) 78–88. [16] I. Rau, A. Gonzalo, M. Valiente, Arsenic (V) adsorption by imobilized iron mediation. Modeling of the adsorption process and influence of interfering anions, React. Funct. Polym. 54 (2003) 85–94. [17] Y. Li, F.S. Zhang, F.R. Xiu, Arsenic (V) removal from aqueous system using adsorbent developed from a high iron-containing fly ash, Sci. Total Environ. 407 (2009) 5780–5786. [18] B. Han, T. Runnels, J. Zimbron, R. Wickamasinghe, Arsenic removal from drinking water by floculation and microfiltration, Desalination 145 (2002) 293–298. [19] S. Song, A. Lopez-Valdivieso, D.J. Hermandez-Campos, C. Peng, M.G. MonroyFernandez, I. Razo-Soto, Arsenic removal from high arsenic water by enhanced coagulation with ferric ion sand coarse calcite, Water Res. 40 (2006) 364–372. [20] Y. Sato, M. Kang, T. Kamei, Y. Magara, Performance of nanofiltration for arsenic removal, Water Res. 36 (2002) 3371–3377. [21] M.C. Shih, An overview of arsenic removal by pressure-driven membrane process, Desalination 172 (2005) 85–97. [22] M. Vaclavikova, G.P. Gallios, S. Hredzak, S. Jakabsky, Removal of arsenic from water streams: an overview of available techniques, Clean Technol. Environ. Policy 10 (2008) 89–95. [23] 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. Manage. 162 (2015) 306–325. [24] A.M. Al-Rehaili, Comparative chemical clarification for silica removal from RO groundwater feed, Desalination 159 (2003) 21–31. [25] W. Den, C.J. Wang, Removal of silica from brackish water by electrocoagulation pretreatment to prevent fouling of reverse osmosis membranes, Sep. Purif. Technol. 59 (2008) 318–325. [26] H.H. Cheng, S.S. Chen, S.R. Yang, In-line coagulation/ultrafiltration for silica removal from brackish water as RO membrane pretreatment, Sep. Purif. Technol. 70 (2009) 112–117. [27] I. Latour, R. Miranda, A. Blanco, Silica removal from newsprint mill effluents with aluminum salts, Chem. Eng. J. 230 (2013) 522–531. [28] O. Gorni-Pinkesfeld, D. Hasson, R. Semiat, H. Shemer, Hybrid electrolysis–crystallization system for silica removal from aqueous solutions, Desalination 407 (2017) 41–45. [29] I. Latour, R. Miranda, A. Blanco, Silica removal with sparingly soluble magnesium compounds, Part I, Sep. Purif. Technol. 138 (2014) 210–218. [30] I. Latour, R. Miranda, A. Blanco, Silica removal with sparingly soluble magnesium compounds. Part II, Sep. Purif. Technol. 149 (2015) 331–338. [31] M. Kobya, O.T. Can, M. Bayramoglu, Treatment of textile wastewaters by electrocoagulation using iron and aluminum electrodes, J. Hazard. Mater. 100 (2003) 163–178.
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Arsenic and hydrated silica removal from groundwater by electrocoagulation using an up-flow reactor in a serpentine array
T
Locksley F. Castañedaa, Oscar Coreñob, José L. Navaa,
⁎
a b
Department of Geomatic and Hydraulic Engineering, University of Guanajuato, Av. Juárez 77, Centro, 36000, Guanajuato, Guanajuato, Mexico Department of Civil Engineering, University of Guanajuato, Av. Juárez 77, Centro, 36000, Guanajuato, Guanajuato, Mexico
ARTICLE INFO
ABSTRACT
Keywords: Arsenic removal Hydrated silica elimination Groundwater treatment Electrocoagulation Aluminosilicates
This paper concerns with the elimination of arsenic and hydrated silica from deep well water, collected form the septentrional area of Guanajuato in Mexico (arsenic 23 μg L−1, hydrated silica 154 mg L−1, sulphate 160 mg L−1, phosphate 0.3 mg L−1, pH 7.5 and conductivity 550 μS cm−1) by electrocoagulation (EC) using an up-flow electrochemical reactor, opened to the atmosphere, with a six-cell stack in a serpentine array. Aluminum plates were used as electrodes. The efficiencies of arsenic and hydrated silica removal at different current densities (4 ≤ j ≤ 7 mA cm-2) and mean linear flow rates (1.2 ≤ u ≤ 4.8 cm s−1) were examined. The best EC was obtained at 7 mA cm-2 and 1.2 cm s−1, which satisfies the WHO guideline for arsenic (< 10 μg L−1) and permits the abatement of hydrated silica, giving values of electrolytic energy consumption and overall cost of EC of 1.64 kW h m-3 and 0.387 USD m-3, respectively. SEM-EDS, FXRD, XRD and FTIR analyses on the flocs revealed that they are structured mainly by aluminosilicates due to the reaction between aluminum and silica. While arsenates, sulfates and phosphates are removed by adsorption on aluminum flocs.
1. Introduction Mexico is one of many regions worldwide with serious problems of groundwater contamination with arsenic [1,2]. Specifically, in the septentrional area of Guanajuato in central Mexico, groundwater is mainly contaminated with arsenic, and in some places considerable amounts of hydrated silica are also found. The arsenic, hydrated silica and coexisting ions present in groundwater come from the dissolution of minerals containing rocks in contact with water [1,3,4]. The concentration of arsenic detected in this area varies in a range of 40–134 μg L−1, while hydrated silica fluctuates between 50–132 mg L−1 [1,5]. The chronic consumption of polluted water with arsenic provokes the apparition of different types of chronic diseases, such as the blackfoot disease, pigmentation, brain injuries, keratosis, bones diseases, nausea, diabetes, arsenicosis and several types of cancer in various organs [2,3,6–12]. In accordance with Mexican regulations, the concentration limit for arsenic in drinking water is 25 μg L−1, while the WHO recommendation establishes a maximum value of 10 μg L−1. On the other hand, the exposure to hydrated silica crystals mainly affects the lungs, increasing the risk of suffering tuberculosis, silicosis, chronic obstructive pulmonary disease, chronic bronchitis and lung cancer [13]. Although hydrated silica is not currently regulated in Mexico in terms of health effects. Hydrated silica not only generates ⁎
health complications, but it also creates problems in industrial pipelines and in several unit operations, such as heat exchangers, where silica salts are impregnated on the equipment walls [2,14]. There are several techniques that have been implemented for the removal of arsenic that lead to diminish the concentration to acceptable levels. Techniques that have commonly been used are precipitation [15], adsorption [16,17], coagulation-flocculation [18,19], membrane technologies [20,21], among others [22,23]. On the other hand, some other techniques have been tested to remove hydrated silica as reverse osmosis [24,25], coagulation-ultrafiltration [26,27], hybrid electrolysis-crystallization [28] and the precipitation with magnesium [29,30]. In recent years, the electrocoagulation process has proven to be a potential technology in the treatment of drinking water [31–38]. The EC has several advantages over other technologies, such as compact treatment installation, the possibility of automating the process and the main one, a high removal efficiency [34,39–41]. Recently, our research group has been using the EC to remove arsenic, fluoride and hydrated silica from groundwater [1,2,5,42]. Two previous papers report the arsenic abatement from groundwater by EC using aluminum as sacrificial anodes, but with modest elimination of concurrent ions, such as hydrated silica, phosphates and sulfates [1,5]. These communications report that aluminum reacts with
Corresponding author. E-mail addresses: [email protected] (L.F. Castañeda), [email protected] (O. Coreño), [email protected] (J.L. Nava).
https://doi.org/10.1016/j.jece.2019.103353 Received 2 May 2019; Received in revised form 4 July 2019; Accepted 9 August 2019 Available online 11 August 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 103353
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Fig. 1. (a) Sketch of the up-flow EC reactor in a serpentine array opened to the atmosphere and, (b) schematic diagram of the EC reactor coupled to a hydraulic and electrical system.
(2)
hydrated silica to produce aluminosilicates; arsenic is adsorbed on the aluminosilicate aggregates, satisfying the WHO guideline. The modest removal of hydrated silica (49.2% from initial concentration 132 mg L−1) has been due to the low doses of aluminum (72 mg L−1). These low doses have been restricted by the presence of hydrogen bubbles, at high current densities (> 6 mA cm-2) inside the closed filterpress reactors employed, because the bubbly flow breaks the flocs, affecting the removal efficiency of the pollutants [1,5]. In another research, our group employ a continuous EC reactor to remove hydrated silica from groundwater. The peculiarity in that reactor is that the aluminum plates are stacked vertically, and the cell is opened at the top, which allows rapid release of hydrogen bubbles formed on the cathode towards the atmosphere [2]. In that paper, the EC was performed at relatively high current densities, 8 mA cm−2, which permitted the quasi complete removal of hydrated silica (initially having 72 ppm). Other researchers, such as Wan and coworkers [43], have investigated the elimination of arsenic by EC using iron electrodes, finding that lepidocrocite adsorbs arsenic. However, the presence of silica (20 mg L−1) in water inhibits the formation of this iron phase, which also disfavors the removal of arsenic. It is worth mentioning that the removal of hydrated silica using iron as coagulant is negligible [43]. The novelty of this paper deals with the simultaneous abatement of arsenic and hydrated silica from groundwater by EC using a novel EC reactor, where the aluminum plates are horizontally assembled (conforming a six-cell stack in a serpentine array); at the top, the cell is exposed to the atmosphere to allow the rapid hydrogen gas release at high current densities. The influence of current density (aluminum dose) and mean linear flow rate (retention time) on the arsenic and hydrated silica removal performance was examined. SEM-EDS, XRFEDS, XRD and FTIR analyses of the flocs were also performed to elucidate the removal pathway of the pollutants.
3H2 O+ 3e
1.1. Electrocoagulation process
2.2. EC reactor in a serpentine array opened to the atmosphere
The electrocoagulation process involves the production of coagulants in situ by electrodissolution of aluminum sacrificial anodes, Eq. (1); in the cathode, bubbles of the hydrogen gas are generated, Eq. (2), and the overall reaction of the cell is described by Eq. (3).
Fig. 1(a) shows a schematic diagram of the up-flow electrochemical reactor in a serpentine array. The polypropylene was the construction material to the reactor components of the cell. The reactor is similar in dimensions of width, length and inter-electrode thickness of the commercial laboratory cell, known as Ecocell®. However, the arrangement of the electrodes differs from Ecocell®, because in this reactor, the
Al (s)
Al3 + + 3e
Al (s) + 3H2 O
1.5H2 + 3OH Al(OH )3 + 1.5H2
(3)
3+
The aluminum ions (Al ) diffuse away from the anode and react with H2O in the bulk, in neutral conditions of pH, yielding to the formation of Al(OH)3(s) and Al2O3(s). It is well known that both aluminum salts adsorb arsenate. The arsenate is the only specie that is efficiently removed by EC. In this context, the arsenite is oxidized to arsenate, by the addition of 1 mg L−1 hypochlorite, a typical concentration employed for disinfection purposes [5]. Additionally, there is an evidence that Al(III) reacts with hydrated silica contained in groundwater to form aluminosilicates, which also remove arsenates by adsorption processes [5]. It is worth mentioning that the anodic passivation represents the biggest problem of the EC, since the precipitation of the aluminum salts inhibits the EC performance, increases the electrolytic energy consumption and the cost of the EC [5,34,42,44]. Electrodes of the same material are used to attenuate the passivation, varying periodically the polarity of the current [1,44]. Another way to avoid passivation is the use of flow cells, where mean linear flow rate favors the transport of Al3+ from the anode to the bulk [1,5]. 2. Experimental 2.1. Deep well water sample The groundwater (arsenic 23 μg L−1, hydrated silica 154 mg L−1, sulphate 160 mg L−1, phosphate 0.3 mg L−1, pH 7.5 and conductivity 550 μS cm−1) was collected from the septentrional region of Guanajuato in central Mexico. It is important to mention that 1 mg L−1 of hypochlorite was added to the water sample prior to EC process.
(1) 2
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EC tests were carried out at mean linear flow velocities (u) applied to the EC reactor of 1.2, 2.4, 3.6 and 4.8 cm s−1, that correspond to volumetric flow rates (q) of 0.1, 0.2, 0.3 and 0.4 L min−1, and retention times in the EC reactor (τ) of 55.9, 27.8, 18.5 and 13.9 s , respectively. Immediately after the electrolyte leaves the EC reactor, the resulting solution is passed to the jar test, where the solution is mixed at 30 rpm during 15 min, to allow the aggregate grow. Once the slow mixing was completed, the aggregates were settled down for 1 h without stirring. The concentration of As, hydrated silica, sulfates and phosphates in the clarified solution is analyzed. The flocs were dried overnight, and then, they were analyzed [46,47].
Table 1 Dimensions of the EC reactor. Electrode length, L / cm Electrode height, B/cm Electrode spacing, S/cm Electrode área, A/cm2 Hydraulic diameter in the rectangular channel, dh = 2BS/(B + 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
2.4. Analytical procedure
parallel aluminum plates are in horizontal mode, leaving a slot in each of the ends of each plate (3 cm width and 1 cm length), allowing that the electrolyte flows as in a serpentine. Eight empty channels stacked form the reactor in a serpentine array, which consist of 3 cm × 8 cm × 0.46 cm, width, length and thickness, respectively, in contact with solution. Seven parallel aluminum plates (3 cm width, 8 cm length and 0.3 cm thickness) were employed as electrodes. At the bottom of the reactor, the electrolyte inlet was located, with a diameter of 1.27 cm. To allow the gas release, generated in the electrodes during the electrolysis, the reactor was designed, so that the top part was opened to the atmosphere, Fig. 1(a). The reactor had a window in the upper channel, where the electrolyte was emptied (with the dimensions of 3.4 cm length and 1.5 cm height) through a liquid collector of 12 cm long to drive the electrolyte to the outlet. The EC reactor was constructed after several trials of CFD simulations; more details about the cell can be consulted elsewhere [45]. The dimensions of the reactor are shown in Table 1. A schematic diagram of the reactor coupled with a hydraulic system is shown in Fig. 1(b), which consists of a reservoir of 15 L capacity for electrolyte sample, a centrifugal pump (1/125 HP, Iwaki, MD-10 L), a valve followed by a flowmeter (0.1-1 L min−1, White Industries), coupled by PVC pipe of 0.5 in. of diameter. The current during the EC trials was applied by a power source BK Precision model 1090.
The concentration of arsenic was measured by atomic absorption spectroscopy at 188.9 nm in a Perkin Elmer PinAAcle 900 F coupled to a Perkin Elmer MHS-15 manual hydride generator (detection limit of 0.05 mg L−1), according to the standard method suggested by APHA (1998) [48]. The hydrated silica, sulfates and phosphates concentrations were measured by photometric techniques described elsewhere [5]. The concentration of aluminum was determined using a Perkin Elmer AAnalyst™ 200 atomic absorption spectrometer (detection limit of 0.1 mg L−1), at a wavelength of 309.27 nm. The reported concentrations were the average of the analysis performed in triplicate. The analyses of SEM, EDAX, EDXRF and FTIR, performed to the flocs, were carried out using the equipment and methodologies described elsewhere [2]. 3. Results and discussion 3.1. Simultaneous arsenic and hydrated silica removal by EC Fig. 2 shows the residual arsenic concentration, CAs, the experimental and theoretical aluminum doses CAl(III) and CAl(III)(N), respectively, as a function of u at 4, 5, 6 and 7 mA cm−2. At 4 mA cm−2, we note that CAs increases linearly with flow rate, highlighting that at 1.2 and 2.4 cm s-1, CAs was 5.1 and 9.4 μg L-1, which fulfills the WHO guideline. Then, at u of 3.6 and 4.8 cm s-1, the remaining concentration was 15.2 and 16.9 μg L-1; this is attributed to the low CAl(III) obtained at such flow rates as 15.4 and 12.1 mg L-1. It is important to note that
2.3. Methodology The experimental system used for EC tests is shown in Fig. 1(b). The
Fig. 2. Influence of the mean linear flow rate on the residual arsenic concentration at different current densities shown inside the Figure. The theoretical and experimental coagulant (aluminum) doses are also graphed. 3
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4 mA cm−2. The results obtained here coincide with that reported in other communications in which As has been satisfactorily eliminated from groundwater, from 45 mg L-1 to 6 [44]. Fig. 3(a) shows the remaining concentration of hydrated silica (Chs) for the same EC trials shown in Fig. 2(a). Chs increases with u due to the depletion of coagulant dose with flow rate. The removal of hydrated silica is favored with current density, highlighting that at j =7 mA cm−2 and u = 1.2 cm s-1, Chs =5.6 mg L-1, which fulfill the Mexican recommendation (Chs < 12 mg L-1). This later is related with the highest concentration of coagulant, CAl(III) =87.4 mg L-1. This achievement is one of the benefits of this innovative EC reactor that allows to apply current densities greater than 6 mA cm−2, without breaking the flocs, what typically occurs in the closed filter-press type EC reactors [1,5,46]. Fig. 3(b) and (c) show the removal of sulfates and phosphates, respectively. The initial concentration of sulfate was 160 mg L−1, and after EC, the remaining concentration of sulfates CSO 42 decreased only close to 130 mg L−1, showing a decreasing trend with the mean linear flow rate. On the other hand, the remaining concentration of phosphates CPO43 presents a similar behavior to that of silica pattern; CPO43 was reduced from 0.2 to 0 mg L−1. According to previous results, the sulfate and phosphate ions are adsorbed on the aluminum aggregates and compete with arsenates for the flocs active sites [5,42]. 3.2. Characterization of the aggregates Fig. 4(a) and (b) show SEM micrographs of the flocs, obtained after the EC trial at 7 mA cm−2 and 1.2 cm s-1. From the analysis of Fig. 4(a), irregular shaped aggregates with sizes up to around 20 μm can be seen; these aggregates are formed by particles with a size below 100 nm, as shown in Fig. 4(b). Table 2 shows the chemical composition of the flocs determined by SEM-EDS and XRF-EDS analyses. In the SEM-EDS results, the presence of Si in a relatively high percentage indicates the generation of aluminosilicate complexes, while the arsenic was not detected due to the low concentration present in the flocs. Thakur et al. reported EDS results performed on the aluminum flocs, after the removal of As and F from groundwater by EC, in which they found arsenic, fluoride and aluminum in their sludge with a higher percentage of O and Al [40]; this groundwater did not contain silica. On the other hand, XRF-EDS analyses confirmed the presence of arsenic in the dry flocs, Table 2. It is important to highlight that there is no agreement between the results obtained using these two techniques, i.e. K and As were not detected by SEM-EDS. Five areas of about 0.16 mm2 were sampled in SEM-EDS (the average of these measurements is reported in Table 2) and a sampling area of 8.02 cm2 for XRF-EDS. The difference between the element percentages is due to the sampling area. The smaller sampling area in SEM-EDS produced high standard deviation. For example, the standard deviations for O and Al were 7.50 wt. % and 5.14 wt. %, respectively. In this context, the FRX-EDS analyses are the most representative of the flocs. Fig. 5(a) shows the X-ray diffraction pattern of flocs, obtained from the EC trial at 7 mA cm−2 and 1.2 cm s-1. The identification of the phases was performed using the software MATCH! Version 1.11 j, which uses the American Mineralogist Crystal Structure Database (AMCSD). From the analysis of this Figure, the calcite (CaCO3, database code AMCSD 0012867) was the only phase, for which peaks could be clearly identified. These results coincide with the decrease of alkalinity from 112 to 89.4, Table 3, which indicates that part of the carbonate ions precipitated as calcite. Wide peaks are observed at 10 < 2θ < 17, 18 < 2θ < 34, 35 < 2θ < 43, 45 < 2θ < 58, 60 < 2θ < 70. The broad peaks are the result of the superposition of peaks that could be assigned to Bytownite ((Ca3.44Na0.56)Al7.76Si8.24O32, AMCSD 0009655), Anorthite (Si2Al2CaO8, AMCSD 0001286), Wallastonite-2 M (CaSiO3, AMCSD 0010870), and Albite (NaAlSi3O8, AMCSD 0003798). It is worth mentioning that arsenic phases were not identified by XRD due to
Fig. 3. Influence of the mean linear flow rate on the remaining concentration of: (a) hydrated silica, (b) sulfates, and (c) phosphates, from the same EC trials shown in Fig. 2.
CAl(III) > CAl(III)(N) is attributed to the redox dissolution of the aluminum, which is provoked by the presence of hypochlorite in the solution. CAl(III)(N) is calculated by Faraday’s law:
CAl (III ) N =
j L MW (1x106) n F S u
(4)
where CAl (III ) N is expressed in mg L−1, j is the current density (A cm-2), L is the length of the channel (8 cm), Mw is the molecular weight of coagulant (aluminum =26.98 g mol−1), n is the number of exchanged electrons ( = 3), F is the Faraday constant (96,485 C mol−1), S is the gap between electrodes (0.46 cm) and the value of 1 × 10-6 is a factor to convert CAl (III ) N in mg L−1. At 5 A cm−2 and 6 mA cm−2, CAs increases linearly with flow rate, highlighting that at u of 1.2, 2.4 and 3.6 cm s-1, CAs < 10 mg L-1. While at 6, CAs fulfills the WHO recommendation for the four mean linear flow rates studied here. These findings are attributable to high doses of coagulant. At such set of EC trials, CAl(III) > CAl(III)(N), as it occurred at 4
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Fig. 4. SEM micrographs of aluminum flocs obtained at the end of the EC process at j =7 mA cm−2 and u = 1.2 cm s-1.
the low concentration of this element. Fig. 5 (b) shows a FTIR spectrum, acquired for wave numbers between 4000 and 400 cm−1 to analyze the chemical bonds of the flocs, obtained after EC trial at j =7 mA cm-2 and u = 1.2 cm s−1. The spectrum shows a wide band at 3468 cm−1 that belongs to the OeH stretching vibrations [1,2,5,49,50]. The peaks, situated at 1641 and 603 cm−1, correspond to the Na-F and Al-F bounding, respectively, while the peaks located at 1530, 1419, 999 cm−1 correspond mainly to the Al-O bending, Al-O-Si bounding and Si-O bond, respectively [1,51,52]. Finally, the peak located at 854 cm−1 corresponds to the AsO bond [5]. The FTIR spectrum confirms the presence of aluminosilicates. Moreover, FTIR also corroborates that the arsenic is removed by adsorption on flocs. It is important to mention that in the FTIR spectrum bands related to S bonds did not appear, which is due to weak adsorption of sulphates on the aggregates [1,2,5,46]. The peaks associated with the F bonds indicate the presence of traces of fluoride in the groundwater, which were not detected by the selective ion electrode technique, having a limit of detection of 0.2 mg L−1 [1,2,5].
Table 2 Chemical composition of the flocs determined by SEM-EDS and XRF-EDS analyses. wt. %
Al
Si
S
Cl
K
Ca
O
Na
As
SEM-EDS XRF-EDS
26.17 15.16
6.87 4.02
0.17 0.22
5.89 2.54
ND 0.18
2.77 1.90
54.76 72.17
3.29 3.80
ND 0.004
ND = Not detected.
3.3. Energy consumption and costs of EC Table 3 abbreviates the experimental conditions of EC (current density and mean linear flow velocity), the remaining concentrations of arsenic and hydrated silica, the experimental aluminum doses, the cell potential (Ecell), electrolytic energy consumption (Econs), pumping energy consumption (Epumping), mass of sludge produced (Msludge) and overall cost of the EC ($OC). The energy consumption used in the electrolysis was determined by:
Econs =
Ecell I (3.6) sBu
(5)
Where Ecell has units of V, I is the current intensity (C s−1), the value 3.6 is a conversion factor to get units of kWh m-3 for Econs, s is the channel width (cm) and B is the channel height (cm). The cost of the aluminum dose was calculated by:
$Al (III ) = (CAl (III ) )($ 1.84 USD Kg 1)(0.001) −1
(6)
where $1.84 USD Kg is the aluminum price and 0.001 is a factor to convert units of $USD m-3 to $Al(III). Finally, the overall cost of EC was calculated by:
Fig. 5. Typical XRD (a) and FTIR (b) spectra of the dried aluminum flocs obtained at the end of the EC test at j =7 mA cm−2 and u = 1.2 cm s-1.
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Table 3 Residual arsenic and hydrated silica concentrations after EC trials. The experimental aluminum dose, cell potential, electrolytic and pumping energy consumption, mass of dried flocs is also shown. Initial composition of groundwater: arsenic 23 μg L−1, hydrated silica 154 mg L−1, phosphate 0.3 mg L−1, sulfate 160 mg L−1, alkalinity 112 mg L−1, hardness 50 mg L−1, conductivity 550 μS cm−1 and pH 7.56. The retention times of flocculation and sedimentation in all jar tests were 15 and 60 min, respectively. j (mA cm−2)
u (cm s−1)
τ (s)
Chs (mg L−1)
CAs
4
1.2 2.4 3.6 4.8 1.2 2.4 3.6 4.8 1.2 2. 3.6 4. 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
28.2 39.8 49.7 95 22 37.4 47 81.5 15.6 29.6 28.4 59 5.6 15.6 19.8 33.5
5.1 9.4 15.2 16.9 1.6 6.5 9.4 15.5 1.4 4.5 6 11.1 1.3 2.4 2.6 4.1
5
6
7
1 (μg L− )
CAl(III) (mg L−1)
Ecell (V)
Econs (kWh m−3)
Epumping (kWh m−3)
Msludge (Kg m−3)
$ OC (USD m−3)
42.8 29.1 15.4 12.1 54.5 36.6 18.6 14.3 62.7 30.7 22.1 17.1 87.4 46.3 35.1 22.6
6.1 6.2 6.8 8.5 7.7 7.6 8.1 8.2 8.4 8.9 9.2 9.4 9.7 9.5 9.0 8.2
0.59 0.30 0.22 0.21 0.93 0.46 0.33 0.25 1.21 0.64 0.44 0.34 1.64 0.80 0.51 0.35
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
0.203 0.078 0.067 0.050 0.254 0.118 0.606 0.056 0.330 0.074 0.053 0.067 0.502 0.283 0.223 0.205
0.192 0.109 0.067 0.056 0.248 0.14 0.103 0.064 0.294 0.146 0.101 0.079 0.387 0.197 0.137 0.094
a Satisfy Mexican guideline for hydrated silica. For this EC test the residual concentrations are: phosphate (not detected), sulfate 140 mg L−1, alkalinity 89.44 mg L−1, hardness 31 mg L−1, conductivity 510 μS cm−1 and pH 8.26.
$OC = $Al (III ) + Econs + Epumping + Msludge
u = 1.2 cm s-1, reaching residual concentration values of arsenic and hydrated silica of 1.3 μg L-1 and 5.6 mg L-1, respectively, with overall EC cost of 0.387 USD m-3. XRD, XRF-EDS, SEM-EDS and FTIR analyses of flocs showed that they are structured mainly by aluminosilicates due to the reaction between aluminum and silica. While arsenates, sulfates and phosphates are removed by adsorption on aluminum flocs. The well-engineered EC reactor allowed the simultaneous removal of arsenic and hydrated silica. The EC has a promising potential in the industry, particularly for preconditioning of silica-containing water, as it is easy to operate, compact and has affordable treatment costs.
(7)
Where the $OC is in units of USD m−3, α is the cost of the electricity in central Mexico ($0.0976 USD (kWh)-1), Epumping is in units of kWh m−3 and β is the sludge confinement cost in Mexico ($0.035 USD kg-1). From the analysis of Table 3, it is visible that the Econs, Msludge and $OC decrease with mean linear flow rate in all current densities tested. The residual concentration of arsenic and hydrated silica after the EC trials varied from 1.3 to 16.9 μg L−1 and 5.6 to 95 mg L−1, respectively. The best EC that fulfills the WHO and Mexican recommendations for As (CAs = 1.3 μg L−1) and hydrated silica (Chs =5.6 mg L−1) was the one obtained at j =7 mA cm-2 and u = 1.2 cm s−1, giving values of electrolytic energy consumption and overall cost of EC of 1.64 kW h m-3 and 0.387 USD m-3. At such conditions, an experimental aluminum dose of 87.4 mg L−1 was required, which produces a dried sludge mass of aluminum flocs of 0.502 kg m-3. Due to the variation of the sludge confinement and the electricity costs between different countries, the overall cost of the EC may vary depending on the country. It is worth mentioning that there are many studies reported in the literature, during the removal of As from groundwater by EC, where the costs were informed, to mention some of them, Omwene and coworkers reported operating costs between 0.026-0.32 USD m−3, noting that these costs only included the costs of electrolysis and coagulant [12]. In the same way, Thakur et al., also reported operating costs of 0.357 USD m−3, during the simultaneous removal of arsenic and fluoride from groundwater by EC in batch mode [7]. In another communication from our group, we reported operating costs of 0.71 USD m−3 during the removal of arsenic and fluoride from groundwater by EC through a continuous EC reactor [1]. The total costs obtained in this research demonstrate the affordable costs of EC treatment.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in 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. References [1] M.A. Sandoval, R. Fuentes, J.L. Nava, O. Coreño, Y.M. Li, J.H. Hernández, Simultaneous removal of fluoride and arsenic from groundwater by electrocoagulation using a filter-press flow reactor with a three-cell stack, Sep. Purif. Technol. 208 (2019) 208–216. [2] M. Rosales, O. Coreño, J.L. Nava, Removal of hydrated silica, fluoride and arsenic from groundwater by electrocoagulation using a continuous reactor with a twelvecell stack, Chemosphere 211 (2018) 149–155. [3] Y.A. Sariñana-Ruiz, J. Vazquez-Arenas, F.S. Sosa-Rodríguez, I. Labastida, M.A. Armienta, A. Armienta, A. Aragón-Piña, M.A. Escobedo-Bretado, L.S. González-Valdez, P. Ponce-Peña, H. Ramírez-Aldaba, R.H. Lara, Assessment of arsenic and fluorine in surface soil to determine environmental and health risk factors in the Comarca Lagunera, Mexico, Chemosphere 178 (2017) 391–401. [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) 260–272. [5] A. Guzmán, J.L. Nava, O. Coreño, I. Rodriguez, S. Gutierrez, Arsenic and fluoride removal from groundwater by electrocoagulation using a continuous filter-press reactor, Chemosphere 144 (2016) 2113–2120. [6] A. Sakar, B. Paul, The global menace of arsenic and its conventional remediation – a critical review, Chemosphere 158 (2016) 37–49. [7] L.S. Thakur, P. Mondal, Simultaneous arsenic and fluoride removal from synthetic and real groundwater by electrocoagulation process: parametric and cost
4. Conclusions The removal of arsenic and hydrated silica from real groundwater with initial concentrations of 23 μg L−1and 154 mg L−1, respectively, was performed successfully by EC, using an innovative up-flow EC reactor with a six-cell stack in form of serpentine, which was also opened to the atmosphere to permit the rapid H2 release from the cathodes to the environment. In this EC reactor, high current densities (> 6 mA cm2 ) can be applied with success without breaking the flocs. The best EC that satisfies the WHO and Mexican recommendations for As and hydrated silica was obtained at j =7 mA cm−2 and 6
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