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Abatement of hydrated silica and simultaneous removal of coexisting ions from deep well water by electrocoagulation using an up-flow reactor
Journal of Water Process Engineering 32 (2019) 100923
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Abatement of hydrated silica and simultaneous removal of coexisting ions from deep well water by electrocoagulation using an up-flow reactor Rosa L. Lópeza, Oscar Coreñob, José L. Navaa, a b
T
⁎
Department of Geomatic and Hydraulic Engineering, University of Guanajuato, Av. Juárez 77, Centro, 36000, Guanajuato, Mexico Department of Civil Engineering, University of Guanajuato, Av. Juárez 77, Centro, 36000, Guanajuato, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrated silica elimination Arsenic removal Fluoride removal Electrocoagulation Drinking water treatment
The removal of hydrated silica and coexisting ions from groundwater (hydrated silica 42 mg L−1, fluoride 7.3 mg L−1, arsenic 40 μg L−1, sulfate 57 mg L−1, phosphate 0.26 mg L−1, pH = 8.02 and conductivity 605 μS cm−1) by electrocoagulation (EC) was examined. The EC was carried out in a novel up-flow reactor with a six-cell stack, in a serpentine array opened to the atmosphere, using aluminum plates as electrodes. The influence of current density (10 ≤ j ≤ 16 mA cm−2) and mean linear flow rate in the EC reactor (1.16 ≤ u ≤ 4.67 cm s−1), corresponding to retention times between 13.9 ≤ τ ≤ 55.9 s, on the hydrated silica, fluoride, arsenic, sulfate and phosphate removal was investigated. The best removal of hydrated silica based on energy consumption (0.98 KWh m−3) and overall cost of EC (0.274 USD m−3) was obtained at 12 mA cm−2 and u= 2.33 cm s−1, giving a remaining concentration of silica of 7 mg L−1, while the residual concentrations of fluoride (1.4 mg L−1) and arsenic (1.88 μg L−1) met the WHO guidelines in human drinking water. The characterization of the flocs by SEM-EDS, XRF, XRD and FTIR indicated that the coagulant reacts with silica to yield aluminum silicates, fluoride substitutes a hydroxyl group from flocs, while arsenates, sulfates, and phosphates are adsorbed on aluminum aggregates.
1. Introduction In Central Mexico, around 75% of the water consumed comes from the subsoil [1]. In particular, in the Bajio region, there are reports indicating that groundwater contains hydrated silica, fluoride (F−), and total arsenic, which is a mixture of As(III) and As(V), in concentrations between 50-132 mg L−1, 2.5–5.5 mg L−1, and 40–134 μg L−1, respectively [1,2]. In this zone of Mexico, people consume groundwater for drinking purposes. Fluoride and total arsenic concentrations in some places are above the World Health Organization (WHO) guidelines, CF− < 1.5 mg L−1 and CAs < 10 mg L−1. It is worth mentioning that there are currently no recommendation or regulation for hydrated silica in terms of health effects. When Fluoride in drinking water is above 1.5 mg L−1, it produces dental fluorosis, and higher contents may produce diseases in lungs, kidneys, liver, thyroid, brain, among others [3]. The ingestion of arsenic at concentrations higher than 10 μg L−1 produces diabetes, brain injuries, black foot disease, pigmentation, bone diseases, nausea, keratosis, and several types of cancer [4]. On the other hand, silica crystals cause lung cancer, chronic bronchitis, silicosis, when they are inhaled [5]; however, in Mexico this substance is not controlled, neither in
⁎
drinking water. Several industries in Central Mexico use groundwater in their production processes. If hydrated silica is present in high concentrations, it can create several problems due to the adhesion of silica salts to the walls of pipelines and of many unit operations [6]. Thus, the removal of hydrated silica from groundwater is necessary. To reduce the concentration of hydrated silica, arsenic, or fluoride from water to tolerable quality levels, different techniques have been used. Some of these methods are: coagulation-precipitation [7,8], anion exchange [9], adsorption [7], ultrafiltration [10], chemical softening [11], sorption on different materials such as: iron oxide nanoparticles [12], natural and chemically modified water melon rind [13], peel of Cucumis pubescents [14], Magnetic Fe3O4 and CuO nanocomposites [15], among others. In our research group, the EC has been employed to eliminate arsenic and fluoride from groundwater [1,2]. For As removal, the materials most tested as anodes are aluminum and iron (with similar performance 99-97%), and for fluoride removal, aluminum is typically used [16–19]. Our research group has informed the simultaneous removal of As and F− from deep well water by EC, using filter-press type reactors equipped with aluminum electrodes [1,2], but the content of other ions
Corresponding author. E-mail addresses: [email protected] (R.L. López), [email protected] (O. Coreño), [email protected] (J.L. Nava).
https://doi.org/10.1016/j.jwpe.2019.100923 Received 5 April 2019; Received in revised form 16 August 2019; Accepted 18 August 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Water Process Engineering 32 (2019) 100923
R.L. López, et al.
as hydrated silica, phosphates, and sulfates, has not been significantly reduced. Arsenic, sulfates, and phosphates are adsorbed on aluminumsilicates produced by the reaction of hydrated silica with aluminum. Fluoride substitutes a hydroxyl from aluminum flocs. In these studies, the residual concentration of fluoride and arsenic, after EC, complies with WHO recommendations. However, a limitation on the dose of coagulant used, < 72 mg Al L−1, has only allowed the partial removal of hydrated silica. The low dose of coagulant was originated by the low applied current densities (j < 7 mA cm−2) in the closed filter-press reactors, employed in those investigations. The huge hydrogen bubbles, produced at j > 7 mA cm−2, broke the flocs, minimizing pollutants elimination. Recently, we have reported the removal of these contaminants from groundwater by EC [20], applying up to 8 mA cm−2, using an EC reactor different from the one used in [1,2]. The reactor used by Rosales and coworkers [20] consists of a twelve-cell stack with aluminum plates in a vertical array. This paper concerns the abatement of hydrated silica, with the simultaneous elimination of arsenic and fluoride from groundwater by EC, using a novel up-flow electrochemical reactor with a six-cell stack, in a serpentine array. This cell is opened to the atmosphere, to permit the rapid release of hydrogen bubbles from the electrodes towards the atmosphere. The sacrificial electrodes used were aluminum plates. The effect of the mean linear flow rate (retention time) and the current density (coagulant concentration) on the removal of hydrated silica, arsenic, and fluoride was examined. The influence of sulfates and phosphates on the performance of EC was also analyzed. The obtained flocs were characterized by SEM-EDS, XRF, XRD and FTIR.
Table 1 Composition of the real groundwater collected from a deep well (320 m). Hydrated silica, mg L−1 Total arsenic, μg L−1 Fluoride, mg L−1 Sulfate, mg L−1 Phosphate, mg L−1 pH Conductivity, mS cm−1
3.2. EC reactor Fig. 1(a) displays a 3D sketch of the reactor. The components of the cell, such as bottom plate, Fig. 1(b), channel separator, Fig. 1(c), and electrolyte collector at the exit, Fig. 1(e), were made of polypropylene. The reactor has equivalent dimensions of length, width and electrode gap to the commercial lab cell, Ecocell®, which is a versatile filter-press type electrolyzer, that uses parallel plate as electrodes. It can be assembled for a single cell, or well with a multi-electrode stack. For our purposes, we placed aluminum plates horizontally, leaving a hole at the end of each electrode (with same sizes of the interelectrode space), Fig. 3(d), to permit the fluid flow as in a serpentine. The reactor is composed of a stack of eight channels (8.1 cm × 3.0 cm 0.46 cm, length, width, and thickness, in contact with solution) in a serpentine array (cascade reactor). The solution inlet is situated at the bottom of the cell, with a diameter of 1.27 cm. At the top of the reactor, the channel is opened to the atmosphere to allow out the hydrogen release during the electrolysis. The solution is spilled out through the window (with sizes of 1.5 cm height and 3.5 cm length) of the highest channel, which contains an electrolyte collector to drive the solution to the exit, Fig. 3(e). It is worth mentioning that the reactor was constructed after having done many CFD simulations, which were performed to design some components of cell, such as the size of the hole at the end of each electrode, Fig. 1(d), and the last channel opened to the atmosphere, Fig. 1(e). The EC reactor avoids stagnant zones and electrolyte recirculation, which allows the cell to behave like a plug flow reactor [24]. More details of the characteristics of this reactor can be consulted elsewhere [24]. The dimensions of the reactor are shown in Table 2. The EC reactor was fixed to a hydraulic and electric system, Fig. 2, which contains a 20 L reservoir for groundwater sample. The system was linked by PVC pipe of 0.5 in. diameter. A centrifugal pump (1/125 HP Iwaki model MD-10), a valve to adjust the flow, and a flowmeter (0.1–1 L min−1, WHITES Industries) were also adapted. For EC tests, a BK Precision® power supply model 1090 was used; it directly records the cell potential.
2. EC process The coagulant dose is produced by the electro-dissolution of aluminum, Eq. (1), while H2 bubbles are produced at the cathode, Eq. (2):
Al (s) → Al3 + + 3e−
(1)
3H2 O+ 3e− → 1.5H2+OH−
(2)
3+
The Al ions form aluminum hydroxide and aluminum oxide in neutral aqueous media, Eqs. (3)–(4).
Al3 + + 3H2 O→ Al(OH)3(s) + 3H+
(3)
2Al3 + + 3H2 O→ Al2O3(s) + 6H+
(4)
42 40 7.3 57 0.26 8.02 605
Al(OH)3(s) or Al2O3(s) may precipitate on the anode causing passivation. In most of the papers consulted, and in the research performed by our group, it is a common practice to work with DC, if passivation occurs, the redissolution of aluminum precipitates can be performed by changing the polarity of the electrodes intermittently [21,22]. It is worth mentioning that the bubbling of H2 usually favors the transport of Al3+ from the anode to the bulk, which also inhibits passivation. It is important to highlight that during the electrolysis performed in this research, there was no need to reverse the current, since there was no passivation. The use of AC current was out of the scope of this paper.
3.3. Methodology The EC trials were carried out using the system shown in Fig. 2, at mean linear flow rates in the EC reactor (u ) of 1.16, 2.33, 3.5, and 4.7 cm s−1 , which correspond to volumetric flow rates (q ) of 0.1, 0.2, 0.3 and 0.4 L min−1, and retention times (τ ) of 55.9, 27.8, 18.5 and 13.9 s , respectively. The current densities tested here were 10, 12, 14, and 16 mA cm−2. Before each electrolysis, the electrodes were polished with 600 grade carbon emery paper, and then rinsed with plenty of water. The results were the average of three EC tests. The Faraday´s law was used to calculate the theoretical value of the aluminum used as coagulant, CAl(III)(N):
3. Experimental 3.1. Deep well water The real groundwater employed here was collected from a deep well (320 m), situated in the Bajio region in central Mexico. The sample was collected in May 2018, during the dry season. This collection was done three days a week, then the water was mixed to have a homogeneous composition, Table 1. The water analysis procedure is described in section 3.4.1. Before EC trials, 1 mg L −1 hypochlorite, typical concentration used for disinfection, was added to groundwater for oxidizing arsenite to arsenate, and to prevent anodic passivation [23]. The arsenate is the only specie that can be eliminated by EC [23].
CAl(III)N =
j ∙ L ∙ Mw (1 × 106) n ∙F ∙S ∙u
(5) −1
−2
Where CAl(III)N and j are given in mg L and A cm , respectively, the molecular weight of aluminum is Mw = 26.98 g mol−1, L is the channel length (8 cm), the Faraday constant is F = 96,485 C mol−1, n = 3 is the number of electrons, S is the interelectrode gap (0.46 cm), and the 2
Journal of Water Process Engineering 32 (2019) 100923
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Fig. 1. (a) Exploded view of the EC reactor with a six-cell stack opened to the atmosphere, (b) bottom plate, (c) channel separator, (d) aluminum electrode, and (e) electrolyte collector at exit.
Table 2 Parameters of the EC reactor with a six-cell stack. Electrode length, L/cm Electrode height, B/cm Electrode spacing, S/cm Electrode area, 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 of the electrolyte outlet in the upper part,/cm
8 3 0.46 24 0.8 8 1.27 3.4 length × 1.5 width
factor 1 × 106 permits to obtain CAl(III)N in mg L−1. The exit solution of the EC reactor (containing the coagulant) is transported to a jar test equipment, where the agglomerates grow, under moderate mixing at 30 rpm for 15 min. Then, the flocs settle for 3 h, and the clear water is analyzed. Hydrated silica, arsenic, fluoride, sulfate, and phosphate ions were determined from the clarified solution. Spectroscopy analyses were carried out on the dry flocs. The experimental aluminum dose, CAl(III) , formed in the EC trials was determined after the redissolution of the flocs, using sulfuric acid to attain a pH = 2. 3.4. Analytical procedure Fig. 2. Electrical and hydraulic circuit for the EC process.
3.4.1. Water analysis Fluoride concentration was determined with a selective ion electrode supplied by Hanna model HI4110, with a detection limit of 0.02 mg L−1. Aluminum ion concentration was determined by atomic absorption spectroscopy at 188.9 nm, with a Perkin Elmer PinAAcle™ 900 F spectroscope (detection limit of 0.1 mg L−1). Arsenic measurements were made using the same AA spectroscope coupled to a Perkin Elmer MHS-15 manual hydride generator (detection limit of 0.05 μg L−1), following the American Public Health Association Method. The concentrations of hydrated silica, sulfate, and phosphate were measured with a photometer supplied by Hanna model HI 83,200.
The silica analysis was carried out by Heteropoly Blue Method using the kit HI 93,705. Sulfate was determined by precipitation with barium chloride crystals (light absorbance method) using the kit HI 93751, and phosphate was determined by amino acid method using the HI 93,706 kit. Conductivity and pH measurements were performed using a waterproof instrument from Hanna Instruments, model HI 991,300. Analytical grade reagents were used. The results were the average of three analyses.
3
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Fig. 3. Effect of the flow velocity on the remaining hydrated silica content and coagulant dose (aluminum), at different current densities.
4. Results and discussion
3.4.2. Flocs characterization The scanning electron microscopy (SEM) analysis was carried out using a JEOL JSM-6010 PLUS/LA device. The energy dispersive analysis of X-rays (EDS) was performed using a JEOL detector incorporated in the SEM microscope. X-ray diffraction (XRD) analyzes were made on a diffractometer Rigaku Ultima IV, with nickel filter and Cu Kα1 radiation . The elemental compositions of the flocs were determined by energy dispersive X-ray fluorescence (XRF), using a Rigaku Nex CG X-ray fluorescence spectrometer, equipped with an X-ray tube with Pd anode. The Fourier transform infrared spectroscopy (FTIR) examination in the flocs was carried out in a Perkin Elmer Spectrum GX FTIR Spectrometer, using an EasiDiff diffuse reflectance accessory.
4.1. Removal of hydrated silica and coexisting ions by EC Fig. 3 shows the remaining concentration of silica (Chs ) versus the mean linear flow velocity, at current densities of 10, 12, 14, and 16 mA cm−2. The experimental, CAl(III), and theoretical, CAl(III)(N), aluminum doses are also shown. At 10 mA cm−2, the Chs presents values from 3.8 to 33 mg L−1, as a function of mean linear flow rate. The Chs increase is due to the decrease of CAl(III) from 84.5 to 21.1 mg L−1. The CAl(III) and CAl(III)(N) are similar at 1.16 ≤ u≤ 2.33 cm s−1. At u of 3.5 and 4.67 cm s−1, CAl(III)(N) > CAl(III) ; these values can be a consequence of oxygen evolution reaction (OER), Eq. (9), which typically occurs simultaneously with Eq. (1), and on the other hand, the Al2O3 precipitation on the anode. The current efficiencies of the coagulant (determined by the ratio, (CAl(III) / CAl(III)(N ) )×100) were comprised between (73.8–100)%; the lower values are a result of the OER and Al2O3 precipitation on the anode.
3.5. Energy consumption and costs of EC The energy consumption (Econs ), cost of aluminum dose ($Al(III) ), and overall cost of EC ($OC) were calculated by Eqs. (6)–(8), respectively:
Econs =
Ecell ∙ I (3.6) ∙S ∙B ∙ u
H2O → O2 + 2H+ +2e−
(6)
(7)
The aluminum price, in Mexico, is $2.008USDkg−1 and 0.001 is a conversion factor to obtain $Al(III) in $ USD m−3.
$OC = $Al(III) + αEcons + αEpump + β Msludge
−1
At 12, 14, and 16 mA cm , Chs ranged between 2.3–27.5 mg L , 1.7–14.5 mg L−1 and 1.16–8.4 mg L−1, respectively. The increase of current density permits the best removal of hydrated silica due to the increase in the dose of coagulant produced, as shown in Fig. 3. The CAl(III) /CAl(III)(N) ratio show similar patterns at 12, 14, and 16 mA cm−2 to those obtained at 10 mA cm−2; whereas, the current efficiencies of the coagulant were (76.8–100) %, (77–100) %, and (79–98) %, for j of 12, 14 and 16 mA cm−2, respectively. Fig. 4 shows remaining concentrations of arsenic (CAs ), fluoride (CF−), and sulphate (C SO24−) versus u, determined during the EC trials of Fig. 3. The remaining concentration of arsenic (0.56–3.37 μg L−1) shows a slightly decrease with current density at 10 ≤ j ≤ 16 mA cm−2, due to the higher aluminum dose produced. The curves CAs − u show similar patterns to those obtained from the removal of hydrated silica, except for the tests obtained at j of 10 and 12 mA cm−2 (for u of 3.5 and 4.67 cm s−1). The remaining concentration of F− (0.7―3.72 mg L−1) did not exhibit a pattern with current density. Moreover, CF− tends to decrease with u, contrary to what was obtained with silica and arsenic. This trend shows that the best removal of silica and arsenic inhibits the substitution reaction between the fluoride and the hydroxyl group from flocs [1,2], but the slight removal of silica and arsenic favors the removal of fluoride. This shows that there is a competition between fluoride, silica, and arsenic for active sites in flocs. The residual
Where the units of Econs , cell potential (Ecell ), and I are kWh m−3, V, and C s−1, respectively. B is the channel width (3 cm), S is the electrode spacing (0.46 cm), and the factor 3.6 is used to obtain Econs in kWh m−3.
$Al(III) = (CAl(III) )($2.008USDkg−1)(0.001)
(9) −2
(8)
$OC is expressed in units of USD m−3, α is the cost of the electricity in central Mexico (0.0976 USD (kWh)-1), Epump is in units of kWh m−3, and β is the sludge confinement cost in Mexico ($ 0.035 USD Kg−1). The electrolyzes were performed under galvanostatic conditions, at current densities of 10, 12, 14, and 16 mA cm−2. These tests were performed using the power supply described in the section 3.2, which directly records the cell potential. The duration of the electrolysis within the EC reactor (retention times, τ ) were 55.9, 27.8, 18.5, and 13.9 s , which correspond to mean linear flow rates in the EC reactor (u ) of 1.16, 2.33, 3.5, and 4.7 cm s−1, respectively. Meanwhile, the flocculation and sedimentation processes last 15 min and 3 h, respectively, for each test, as described in section 3.3. 4
Journal of Water Process Engineering 32 (2019) 100923
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Fig. 5. SEM micrographs of the aluminum flocs obtained at u = 0.23 cm s−1 (retention time in the EC reactor of 27.8 s) and: (a) 12 mA cm−2, and (b) 14 mA cm−2.
Fig. 4. Effect of the flow velocity on the remaining concentrations of fluoride, arsenic, and sulfates, evaluated from the EC tests showed in Fig. 3.
Table 3 Composition of the flocs determined by XRF obtained at 2.33 cm s−1 and different current densities. Only elements with concentration above 50 ppm are reported.
concentration of sulfates (C SO24− ) did not vary with u and j (approx. 50 mg L−1); while the phosphates were completely removed after all EC tests, data not plotted. The partial and complete removal of sulfates and phosphates, respectively, is by adsorption on the flocs [1,2,25]. At the end of all EC tests, the pH was around of 8.9, from the initial value of 8.02; this increase corresponds to the substitution of a hydroxyl of the floc with a fluoride [25]. The abatement of hydrated silica is attributed to the massive formation of aluminum silicates during the EC, at the high current densities tested here 10 ≤ j ≤ 16 mA cm−2, resulting in high concentrations of coagulant up to 139.1 mg L−1. Moreover, the residual arsenic and fluoride concentrations met the WHO guidelines. The elimination of hydrated silica achieved here, contrasts with the poor removal of this pollutant found in previous papers [1,2], but coincides with the removal efficiency of arsenic and fluoride; in these investigations we employed two different EC reactors (filter-press type cells, closed at the top), where hydrogen bubbles, cathodically produced, broke the flocs at j > 6 mA cm−2. The novel EC reactor opened to the atmosphere, employed here, allowed the release of gas at j > 10 mA cm−2, and avoided the breaking of the flocs, which permitted the elimination of hydrated silica.
10 mA cm−2
12 mA cm−2
Element
Concentration (ppm)×103
Al Si As O Na Mg Ca K S Cl
55.3 22.5 0.051 592 155 2.82 32.8 11.3 9.56 10.5
14 mA cm−2
125 47.1 0.075 547 115 4.12 42.7 7.24 7.56 6.31
116 36.9 0.054 566 130 3.54 33.7 6.75 9.49 5.61
mA cm−2. The flocs are formed by very fine particles with dimensions less than 100 nm. Table 3 reports the elemental composition of flocs determined by XRF analyses in areas of 650 μm × 490 μm. The three samples show similar compositions. Arsenic was determined in the samples, between 51 to 75 μg g−1, while Fluor was not detected. The existence of silicon confirms the formation of aluminosilicate complexes. Table 4 summarizes typical composition of flocs obtained by EDS analyses. The presence of Si also confirms the formation of aluminosilicates. Arsenic and Fluor were not detected by EDS due to their low concentrations. All the elements detected by EDS in concentrations higher than 1.0 wt. % were also detected by XRF (Table 3). The
4.2. Flocs characterization Fig. 5 shows typical SEM micrographs of the flocs attained from the EC at 12 and 14 mA cm−2 (at u = 2.33 cm s−1). Both micrographs exhibit flocs with irregular forms and dimensions > 10 μm. No significant size differences were observed between the particles at 12 and 14 5
Journal of Water Process Engineering 32 (2019) 100923
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and u= 2.33 cm s−1. The wide peaks located at 10 < 2θ < 19, and 22 < 2θ < 42 are the result of the superposition of many narrow peaks. Peaks corresponding to Bytownite ((Ca3.44 Na .56 ) Al7.76Si8.24 O32 ), Magakalsilite (SiAlKO4 ), Hatrurite (Ca27Si9O45 ), and Melilite (Na2.05Ca5.95 Si 4 O15 ), were identified. These phases caused the removal of arsenic, fluoride, sulfates and phosphates. The FTIR spectrum of the flocs ranged between 4000–500 cm−1 was used to identify the chemical bonds in the samples at 12 mA cm−2 and u= 2.33 cm s−1, Fig. 6(b). Wide bands are distinguished at 587, 854, 999, 1412, 1524, 1634, and 3440 cm−1, very probably attributed to the following bonds: Al-F, As-O, Si-O, Al-O-Si, Al-O, Na-F, and O–H. It is worth noting that the FTIR spectrum is very similar to that reported in previous papers [1,2]. This spectrum confirms that arsenic is adsorbed on flocs, and fluoride substitutes a hydroxyl group from flocs. The EDS, XRF, XRD and FTIR analyzes carried out on the flocs confirm that the coagulant reacts with silica to yield aluminosilicates. On the other hand, Fluoride replaces a hydroxyl group from flocs, whereas arsenates, phosphates and sulfates are adsorbed on aggregates.
Table 4 Composition of the flocs determined by EDS obtained at 2.33 cm s−1 and different current densities. Only elements with concentration above 0.1% are reported. 10 mA cm−2
12 mA cm−2
14 mA cm−2
Element
% Masa
% Atom.
% Masa
% Atom.
% Masa
% Atom.
Na Al Si S Cl K Ca Cu
42.50 29.69 17.41 1.88 3.15 4.17 4.20
48.43 25.92 16.25 1.54 2.32 2.80 2.74
35.74 35.36 19.75 2.22 2.11 1.16 3.66
____
52.42 25.07 12.41 3.50 1.55 1.31 2.21 1.53
30.79 35.75 20.78 2.66 2.80 1.70 5.50
____
45.75 25.68 13.23 4.27 2.08 1.94 3.36 2.21
____
____
4.3. Performance and costs of EC Table 5 shows the residual concentrations of hydrated silica, arsenic, and fluoride. This table also shows the experimental aluminum doses, cell potential (Ecell ), electrolytic energy consumption (Econs ), cost of aluminum dose ($Al(III) ), pumping energy consumption (Epump), mass of sludge generated by EC (Msludge), the overall cost of EC (OC), and the retention time, τ, (electrolysis time), in the EC reactor, for each trial. We observe a decrease of Ecell, Econs, Msludge and $OC as u increases, at 10 mA cm−2; a similar behavior was obtained for the other current densities. It is worth to mention that the value of the Ecell remains constant during the electrolysis time, τ, of each EC trial. Another important aspect is that all the EC trials complied the WHO guidelines for arsenic and fluoride. The residual concentration of hydrated silica after the EC trials varied from 0.5 to 27.5 mg L−1, giving values of overall costs between 0.115 ≤ $OC ≤ 0.747 USD m−3, with experimental aluminum doses ranged between 28 < CAl(III) < 138 mg L−1. It is important to note that the overall cost of the EC process may vary in different countries due to the prices of electricity and sludge confinement.
Fig. 6. Typical XRD (a) and FTIR (b) patterns of the dry aggregates obtained at 12 mA cm−2 and u= 0.23 cm s−1 (retention time in the EC reactor of 27.8 s).
differences in composition determined by these two techniques, resulted from the large sampling area used in the XRF analysis, 8.04 cm2, which is more representative than the reduced scanned area used in EDS, around 0.005 μm2 . Fig. 6(a) shows the XRD pattern of the flocs attained at 12 mA cm−2
5. Conclusions Excellent removal of hydrated silica, arsenic, and fluoride from deep well water (with initial values of 42 mg L−1, 40 μg L−1, and 7.3 mg L−1, respectively) by EC, using aluminum anodes in an up-flow reactor with
Table 5 Remaining hydrated silica, arsenic, and fluoride concentrations, also experimental aluminum dose, Ecell, Epump, Msludge, and overall cost of the EC. j (mA cm−2)
u (cm s−1)
τ (s)
Chs (mg L−1)
CAs (μg L−1)
CF− (mg L−1)
CAl (III) (mg L−1)
Ecell (V)
Econs (kWh m−3)
Epump (kWh m−3)
Msludge (kg m−3)
$ OC (USD m−3)
10
1.16 2.33 3.50 4.67 1.16 2.33a 3.50 4.67 1.16 2.33 3.50 4.67 1.16 2.33 3.50 4.67
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
3.8 8 27.5 33 2.3 7.0 22.5 27.5 1.7 4.0 7.5 14.5 0.5 3.0 5.5 8.4
1.44 3.14 3.20 2.55 1.32 1.88 3.05 3.02 0.65 1.70 2.38 3.37 0.56 1.55 2.02 2.46
1.60 1.50 0.64 0.70 1.50 1.40 0.64 0.67 1.50 1.20 0.68 0.65 1.40 1.30 1.30 0.62
83.48 46.97 38.15 28.04 102.37 59.77 46.95 25.30 118.88 68.82 49.93 38.69 139.11 75.9 55.92 42.92
7.0 6.5 4.5 4.6 7.1 6.5 5.2 5.3 8.4 8.1 5.8 6.1 9.0 9.0 6.6 6.6
1.77 0.82 0.38 0.29 2.16 0.98 0.52 0.40 2.97 1.43 0.68 0.54 3.64 1.81 0.89 0.66
0.994 0.497 0.331 0.248 0.994 0.497 0.331 0.248 0.994 0.497 0.331 0.248 0.994 0.497 0.331 0.248
0.322 0.288 0.227 0.181 0.400 0.285 0.241 0.228 0.418 0.284 0.260 0.288 0.442 0.315 0.320 0.280
0.448 0.232 0.153 0.115 0.526 0.274 0.186 0.148 0.640 0.336 0.208 0.164 0.747 0.388 0.242 0.185
12
14
16
a
Remaining concentrations for sulfate and phosphate were 55 mg L−1 and ˜ 0 mg L−1, pH 8.9 and conductivity 1160 μS cm−1. 6
Journal of Water Process Engineering 32 (2019) 100923
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six-cell stack, opened to the atmosphere, was obtained. The operational conditions in terms of current density and mean linear flow rate were stablished at 10 ≤ j ≤ 16 mA cm−2 and 1.16 ≤ u ≤ 4.67 cm s−1. The best removal of hydrated silica based on energy consumption (0.98 KWh m−3) and overall cost of EC (0.274 USD m−3) was obtained at 12 mA cm−2 and u= 2.33 cm s−1, giving a remaining concentration of silica of 7 mg L−1, while the residual concentrations of fluoride (1.4 mg L−1) and arsenic (1.88 μg L−1) met the WHO guidelines in human drinking water. High concentrations of coagulant, achieved at j ≥ 10 mA cm−2, react with silica to yield aluminosilicates, which was confirmed by EDS, XRD, XRF, and FTIR analyzes on the flocs. On the other hand, fluoride replaces a hydroxyl group from flocs, whereas arsenates, phosphates, and sulfates are adsorbed on aggregates.
cations and anions on prevention of silica fouling, Desalination 139 (2001) 83–95. [9] M.B.S. Ali, B. Hamrouni, S. Bouguecha, M. Dhahbi, Silica removal using ion-exchange resins, Desalination 167 (2004) 273–279. [10] 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. [11] A. Rahardianto, J. Gao, C.J. Gabelich, M.D. Williams, Y. Cohen, High recovery membrane desalting of low-salinity brackish water: integration of accelerated precipitation softening with membrane RO, J. Membrane Sci. 289 (2007) 123–137. [12] T.C. Prathna, D.N. Sitompul, S.K. Sharma, M. Kennedy, Synthesis, characterization and performance of iron oxide/alumina-based nanoadsorbents for simultaneous arsenic and fluoride removal, Desalin. Water Treat. 104 (2018) 121–134. [13] S.M. Bilal, N.K. Niazi, I. Bibi, M. Shahid, F. Sharif, S. Bashir, S.M. Shaheen, H. Wang, D.C.W. Tsang, Y.S. Ok, J. Rinklebe, Arsenic removal by natural and chemically modified water melon rind in aqueous solutions and groundwater, Sci. Total Environ. 645 (2018) 1444–1455. [14] K.T. Gul, K.D. Brahman, J.A. Baig, H.I. Afridi, A new efficient indigenous material for simultaneous removal of fluoride and inorganic arsenic species from groundwater, J. Hazard. Mat. 357 (2018) 159–167. [15] W. Kun, C. Jing, J. Zhang, T. Liu, S. Yang, W. Wang, Magnetic Fe3O4@CuO nanocomposite assembled on graphene oxide sheets for the enhanced removal of arsenic (III/V) from water, App. Surf. Sci. 466 (2019) 746–756. [16] 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) 1–21. [17] D.T. Moussa, M.H. El-Naas, M. Nasser, M.J. Al-Marri, A comprehensive review of electrocoagulation for water treatment: potentials and challenges, J. Environ. Manag. 186 (2017) 24–41. [18] V. Gilhotra, L. Das, A. Sharma, T.S. Kang, P. Singh, R.S. Dhuria, M.S. Bhatti, Electrocoagulation technology for high arsenic wastewater: process optimization and mechanistic study, J. Clean. Product. 198 (2018) 693–703. [19] E. Şik, E. Demirbas, A.Y. Goren, M.S. Oncel, M. Kobya, Arsenite and arsenate removals from groundwater by electrocoagulationusing iron ball anodes: influence of operating parameters, J. Water Process Eng. 18 (2017) 83–91. [20] 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. [21] E. Mohora, S. Rončević, B. Dalmacija, J. Agbaba, M. Watson, E. Karlović, M. Dalmacija, Removal of natural organic matter and arsenic from water by electrocoagulation/flotation continuous flow reactor, J. Hazard. Mat. 235 (2012) 257–264. [22] S. García Segura, C. Martínez-Huitle, Electrocoagulation and advanced electrocoagulation processes: A general review about the fundamentals, emerging applications and its association with other technologies, J. Electroanal. Chem. Lausanne (Lausanne) 801 (2017) 267–299. [23] O.J. Flores, J.L. Nava, G. Carreño, E. Elorza, F. Martínez, Arsenic removal from groundwater by electrocoagulation in a pre-pilot-scale continuous filter press reactor, Chem. Eng. Sci. 97 (2013) 1–6. [24] L.F. Castañeda, J.L. Nava, Simulations of single-phase flow in an up-flow electrochemical reactor with parallel plate electrodes in a serpentine array, J. Electroanal. Chem. Lausanne (Lausanne) 832 (2019) 31–39. [25] 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, Sep. Purif. Technol. 134 (2014) 163–170.
Acknowledgements J.L. Nava thanks to Universidad de Guanajuato (project No. 102/ 2019) for financial support. R.L. López acknowledges CONACYT for the scholarship No. 511394 granted. Authors acknowledge Dr. Raul Miranda and Daniela Moncada from LICAMM-UG Laboratory for spectroscopy analysis. References [1] M.A. Sandoval, R. Fuentes, J.L. Nava, O. Coreño, Y. 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] A. Guzmán, J.L. Nava, O. Coreño, I. Rodríguez, S. Gutiérrez, Arsenic and fluoride removal from groundwater by electrocoagulation using a continuous filter-press reactor, Chemosphere 144 (2016) 2113–2120. [3] S. Vasudevan, B.S. Kannan, J. Lakshmi, S. Mohanraj, G. Sozhan, Effects of alternating and direct current in electrocoagulation process on the removal of fluoride from water, J. Chem. Technol. Biotechnol. 86 (2011) 428–436. [4] Y.A. Sariñana, J. Vazquez, F.S. Sosa, I. Labastida, M.A. Armienta, A. Aragón, M. Escobedo, L.S. González, P. Ponce, H. Ramírez, 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. [5] R. Merget, T. Bauer, H. Küpper, S. Philippou, H.D. Bauer, R. Breitstadt, Health hazards due to the inhalation of amorphous silica, Arch. Toxicol. 75 (2002) 625–634. [6] 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. [7] R.C. Maheshwari, Fluoride in drinking water and its removal, J. Hazard. Mat. 137 (2006) 456–463. [8] R. Sheikholeslami, I.S. Al-Mutaz, T. Koo, A. Young, Pretreatment and the effect of
7
Contents lists available at ScienceDirect
Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe
Abatement of hydrated silica and simultaneous removal of coexisting ions from deep well water by electrocoagulation using an up-flow reactor Rosa L. Lópeza, Oscar Coreñob, José L. Navaa, a b
T
⁎
Department of Geomatic and Hydraulic Engineering, University of Guanajuato, Av. Juárez 77, Centro, 36000, Guanajuato, Mexico Department of Civil Engineering, University of Guanajuato, Av. Juárez 77, Centro, 36000, Guanajuato, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrated silica elimination Arsenic removal Fluoride removal Electrocoagulation Drinking water treatment
The removal of hydrated silica and coexisting ions from groundwater (hydrated silica 42 mg L−1, fluoride 7.3 mg L−1, arsenic 40 μg L−1, sulfate 57 mg L−1, phosphate 0.26 mg L−1, pH = 8.02 and conductivity 605 μS cm−1) by electrocoagulation (EC) was examined. The EC was carried out in a novel up-flow reactor with a six-cell stack, in a serpentine array opened to the atmosphere, using aluminum plates as electrodes. The influence of current density (10 ≤ j ≤ 16 mA cm−2) and mean linear flow rate in the EC reactor (1.16 ≤ u ≤ 4.67 cm s−1), corresponding to retention times between 13.9 ≤ τ ≤ 55.9 s, on the hydrated silica, fluoride, arsenic, sulfate and phosphate removal was investigated. The best removal of hydrated silica based on energy consumption (0.98 KWh m−3) and overall cost of EC (0.274 USD m−3) was obtained at 12 mA cm−2 and u= 2.33 cm s−1, giving a remaining concentration of silica of 7 mg L−1, while the residual concentrations of fluoride (1.4 mg L−1) and arsenic (1.88 μg L−1) met the WHO guidelines in human drinking water. The characterization of the flocs by SEM-EDS, XRF, XRD and FTIR indicated that the coagulant reacts with silica to yield aluminum silicates, fluoride substitutes a hydroxyl group from flocs, while arsenates, sulfates, and phosphates are adsorbed on aluminum aggregates.
1. Introduction In Central Mexico, around 75% of the water consumed comes from the subsoil [1]. In particular, in the Bajio region, there are reports indicating that groundwater contains hydrated silica, fluoride (F−), and total arsenic, which is a mixture of As(III) and As(V), in concentrations between 50-132 mg L−1, 2.5–5.5 mg L−1, and 40–134 μg L−1, respectively [1,2]. In this zone of Mexico, people consume groundwater for drinking purposes. Fluoride and total arsenic concentrations in some places are above the World Health Organization (WHO) guidelines, CF− < 1.5 mg L−1 and CAs < 10 mg L−1. It is worth mentioning that there are currently no recommendation or regulation for hydrated silica in terms of health effects. When Fluoride in drinking water is above 1.5 mg L−1, it produces dental fluorosis, and higher contents may produce diseases in lungs, kidneys, liver, thyroid, brain, among others [3]. The ingestion of arsenic at concentrations higher than 10 μg L−1 produces diabetes, brain injuries, black foot disease, pigmentation, bone diseases, nausea, keratosis, and several types of cancer [4]. On the other hand, silica crystals cause lung cancer, chronic bronchitis, silicosis, when they are inhaled [5]; however, in Mexico this substance is not controlled, neither in
⁎
drinking water. Several industries in Central Mexico use groundwater in their production processes. If hydrated silica is present in high concentrations, it can create several problems due to the adhesion of silica salts to the walls of pipelines and of many unit operations [6]. Thus, the removal of hydrated silica from groundwater is necessary. To reduce the concentration of hydrated silica, arsenic, or fluoride from water to tolerable quality levels, different techniques have been used. Some of these methods are: coagulation-precipitation [7,8], anion exchange [9], adsorption [7], ultrafiltration [10], chemical softening [11], sorption on different materials such as: iron oxide nanoparticles [12], natural and chemically modified water melon rind [13], peel of Cucumis pubescents [14], Magnetic Fe3O4 and CuO nanocomposites [15], among others. In our research group, the EC has been employed to eliminate arsenic and fluoride from groundwater [1,2]. For As removal, the materials most tested as anodes are aluminum and iron (with similar performance 99-97%), and for fluoride removal, aluminum is typically used [16–19]. Our research group has informed the simultaneous removal of As and F− from deep well water by EC, using filter-press type reactors equipped with aluminum electrodes [1,2], but the content of other ions
Corresponding author. E-mail addresses: [email protected] (R.L. López), [email protected] (O. Coreño), [email protected] (J.L. Nava).
https://doi.org/10.1016/j.jwpe.2019.100923 Received 5 April 2019; Received in revised form 16 August 2019; Accepted 18 August 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Water Process Engineering 32 (2019) 100923
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as hydrated silica, phosphates, and sulfates, has not been significantly reduced. Arsenic, sulfates, and phosphates are adsorbed on aluminumsilicates produced by the reaction of hydrated silica with aluminum. Fluoride substitutes a hydroxyl from aluminum flocs. In these studies, the residual concentration of fluoride and arsenic, after EC, complies with WHO recommendations. However, a limitation on the dose of coagulant used, < 72 mg Al L−1, has only allowed the partial removal of hydrated silica. The low dose of coagulant was originated by the low applied current densities (j < 7 mA cm−2) in the closed filter-press reactors, employed in those investigations. The huge hydrogen bubbles, produced at j > 7 mA cm−2, broke the flocs, minimizing pollutants elimination. Recently, we have reported the removal of these contaminants from groundwater by EC [20], applying up to 8 mA cm−2, using an EC reactor different from the one used in [1,2]. The reactor used by Rosales and coworkers [20] consists of a twelve-cell stack with aluminum plates in a vertical array. This paper concerns the abatement of hydrated silica, with the simultaneous elimination of arsenic and fluoride from groundwater by EC, using a novel up-flow electrochemical reactor with a six-cell stack, in a serpentine array. This cell is opened to the atmosphere, to permit the rapid release of hydrogen bubbles from the electrodes towards the atmosphere. The sacrificial electrodes used were aluminum plates. The effect of the mean linear flow rate (retention time) and the current density (coagulant concentration) on the removal of hydrated silica, arsenic, and fluoride was examined. The influence of sulfates and phosphates on the performance of EC was also analyzed. The obtained flocs were characterized by SEM-EDS, XRF, XRD and FTIR.
Table 1 Composition of the real groundwater collected from a deep well (320 m). Hydrated silica, mg L−1 Total arsenic, μg L−1 Fluoride, mg L−1 Sulfate, mg L−1 Phosphate, mg L−1 pH Conductivity, mS cm−1
3.2. EC reactor Fig. 1(a) displays a 3D sketch of the reactor. The components of the cell, such as bottom plate, Fig. 1(b), channel separator, Fig. 1(c), and electrolyte collector at the exit, Fig. 1(e), were made of polypropylene. The reactor has equivalent dimensions of length, width and electrode gap to the commercial lab cell, Ecocell®, which is a versatile filter-press type electrolyzer, that uses parallel plate as electrodes. It can be assembled for a single cell, or well with a multi-electrode stack. For our purposes, we placed aluminum plates horizontally, leaving a hole at the end of each electrode (with same sizes of the interelectrode space), Fig. 3(d), to permit the fluid flow as in a serpentine. The reactor is composed of a stack of eight channels (8.1 cm × 3.0 cm 0.46 cm, length, width, and thickness, in contact with solution) in a serpentine array (cascade reactor). The solution inlet is situated at the bottom of the cell, with a diameter of 1.27 cm. At the top of the reactor, the channel is opened to the atmosphere to allow out the hydrogen release during the electrolysis. The solution is spilled out through the window (with sizes of 1.5 cm height and 3.5 cm length) of the highest channel, which contains an electrolyte collector to drive the solution to the exit, Fig. 3(e). It is worth mentioning that the reactor was constructed after having done many CFD simulations, which were performed to design some components of cell, such as the size of the hole at the end of each electrode, Fig. 1(d), and the last channel opened to the atmosphere, Fig. 1(e). The EC reactor avoids stagnant zones and electrolyte recirculation, which allows the cell to behave like a plug flow reactor [24]. More details of the characteristics of this reactor can be consulted elsewhere [24]. The dimensions of the reactor are shown in Table 2. The EC reactor was fixed to a hydraulic and electric system, Fig. 2, which contains a 20 L reservoir for groundwater sample. The system was linked by PVC pipe of 0.5 in. diameter. A centrifugal pump (1/125 HP Iwaki model MD-10), a valve to adjust the flow, and a flowmeter (0.1–1 L min−1, WHITES Industries) were also adapted. For EC tests, a BK Precision® power supply model 1090 was used; it directly records the cell potential.
2. EC process The coagulant dose is produced by the electro-dissolution of aluminum, Eq. (1), while H2 bubbles are produced at the cathode, Eq. (2):
Al (s) → Al3 + + 3e−
(1)
3H2 O+ 3e− → 1.5H2+OH−
(2)
3+
The Al ions form aluminum hydroxide and aluminum oxide in neutral aqueous media, Eqs. (3)–(4).
Al3 + + 3H2 O→ Al(OH)3(s) + 3H+
(3)
2Al3 + + 3H2 O→ Al2O3(s) + 6H+
(4)
42 40 7.3 57 0.26 8.02 605
Al(OH)3(s) or Al2O3(s) may precipitate on the anode causing passivation. In most of the papers consulted, and in the research performed by our group, it is a common practice to work with DC, if passivation occurs, the redissolution of aluminum precipitates can be performed by changing the polarity of the electrodes intermittently [21,22]. It is worth mentioning that the bubbling of H2 usually favors the transport of Al3+ from the anode to the bulk, which also inhibits passivation. It is important to highlight that during the electrolysis performed in this research, there was no need to reverse the current, since there was no passivation. The use of AC current was out of the scope of this paper.
3.3. Methodology The EC trials were carried out using the system shown in Fig. 2, at mean linear flow rates in the EC reactor (u ) of 1.16, 2.33, 3.5, and 4.7 cm s−1 , which correspond to volumetric flow rates (q ) of 0.1, 0.2, 0.3 and 0.4 L min−1, and retention times (τ ) of 55.9, 27.8, 18.5 and 13.9 s , respectively. The current densities tested here were 10, 12, 14, and 16 mA cm−2. Before each electrolysis, the electrodes were polished with 600 grade carbon emery paper, and then rinsed with plenty of water. The results were the average of three EC tests. The Faraday´s law was used to calculate the theoretical value of the aluminum used as coagulant, CAl(III)(N):
3. Experimental 3.1. Deep well water The real groundwater employed here was collected from a deep well (320 m), situated in the Bajio region in central Mexico. The sample was collected in May 2018, during the dry season. This collection was done three days a week, then the water was mixed to have a homogeneous composition, Table 1. The water analysis procedure is described in section 3.4.1. Before EC trials, 1 mg L −1 hypochlorite, typical concentration used for disinfection, was added to groundwater for oxidizing arsenite to arsenate, and to prevent anodic passivation [23]. The arsenate is the only specie that can be eliminated by EC [23].
CAl(III)N =
j ∙ L ∙ Mw (1 × 106) n ∙F ∙S ∙u
(5) −1
−2
Where CAl(III)N and j are given in mg L and A cm , respectively, the molecular weight of aluminum is Mw = 26.98 g mol−1, L is the channel length (8 cm), the Faraday constant is F = 96,485 C mol−1, n = 3 is the number of electrons, S is the interelectrode gap (0.46 cm), and the 2
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Fig. 1. (a) Exploded view of the EC reactor with a six-cell stack opened to the atmosphere, (b) bottom plate, (c) channel separator, (d) aluminum electrode, and (e) electrolyte collector at exit.
Table 2 Parameters of the EC reactor with a six-cell stack. Electrode length, L/cm Electrode height, B/cm Electrode spacing, S/cm Electrode area, 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 of the electrolyte outlet in the upper part,/cm
8 3 0.46 24 0.8 8 1.27 3.4 length × 1.5 width
factor 1 × 106 permits to obtain CAl(III)N in mg L−1. The exit solution of the EC reactor (containing the coagulant) is transported to a jar test equipment, where the agglomerates grow, under moderate mixing at 30 rpm for 15 min. Then, the flocs settle for 3 h, and the clear water is analyzed. Hydrated silica, arsenic, fluoride, sulfate, and phosphate ions were determined from the clarified solution. Spectroscopy analyses were carried out on the dry flocs. The experimental aluminum dose, CAl(III) , formed in the EC trials was determined after the redissolution of the flocs, using sulfuric acid to attain a pH = 2. 3.4. Analytical procedure Fig. 2. Electrical and hydraulic circuit for the EC process.
3.4.1. Water analysis Fluoride concentration was determined with a selective ion electrode supplied by Hanna model HI4110, with a detection limit of 0.02 mg L−1. Aluminum ion concentration was determined by atomic absorption spectroscopy at 188.9 nm, with a Perkin Elmer PinAAcle™ 900 F spectroscope (detection limit of 0.1 mg L−1). Arsenic measurements were made using the same AA spectroscope coupled to a Perkin Elmer MHS-15 manual hydride generator (detection limit of 0.05 μg L−1), following the American Public Health Association Method. The concentrations of hydrated silica, sulfate, and phosphate were measured with a photometer supplied by Hanna model HI 83,200.
The silica analysis was carried out by Heteropoly Blue Method using the kit HI 93,705. Sulfate was determined by precipitation with barium chloride crystals (light absorbance method) using the kit HI 93751, and phosphate was determined by amino acid method using the HI 93,706 kit. Conductivity and pH measurements were performed using a waterproof instrument from Hanna Instruments, model HI 991,300. Analytical grade reagents were used. The results were the average of three analyses.
3
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Fig. 3. Effect of the flow velocity on the remaining hydrated silica content and coagulant dose (aluminum), at different current densities.
4. Results and discussion
3.4.2. Flocs characterization The scanning electron microscopy (SEM) analysis was carried out using a JEOL JSM-6010 PLUS/LA device. The energy dispersive analysis of X-rays (EDS) was performed using a JEOL detector incorporated in the SEM microscope. X-ray diffraction (XRD) analyzes were made on a diffractometer Rigaku Ultima IV, with nickel filter and Cu Kα1 radiation . The elemental compositions of the flocs were determined by energy dispersive X-ray fluorescence (XRF), using a Rigaku Nex CG X-ray fluorescence spectrometer, equipped with an X-ray tube with Pd anode. The Fourier transform infrared spectroscopy (FTIR) examination in the flocs was carried out in a Perkin Elmer Spectrum GX FTIR Spectrometer, using an EasiDiff diffuse reflectance accessory.
4.1. Removal of hydrated silica and coexisting ions by EC Fig. 3 shows the remaining concentration of silica (Chs ) versus the mean linear flow velocity, at current densities of 10, 12, 14, and 16 mA cm−2. The experimental, CAl(III), and theoretical, CAl(III)(N), aluminum doses are also shown. At 10 mA cm−2, the Chs presents values from 3.8 to 33 mg L−1, as a function of mean linear flow rate. The Chs increase is due to the decrease of CAl(III) from 84.5 to 21.1 mg L−1. The CAl(III) and CAl(III)(N) are similar at 1.16 ≤ u≤ 2.33 cm s−1. At u of 3.5 and 4.67 cm s−1, CAl(III)(N) > CAl(III) ; these values can be a consequence of oxygen evolution reaction (OER), Eq. (9), which typically occurs simultaneously with Eq. (1), and on the other hand, the Al2O3 precipitation on the anode. The current efficiencies of the coagulant (determined by the ratio, (CAl(III) / CAl(III)(N ) )×100) were comprised between (73.8–100)%; the lower values are a result of the OER and Al2O3 precipitation on the anode.
3.5. Energy consumption and costs of EC The energy consumption (Econs ), cost of aluminum dose ($Al(III) ), and overall cost of EC ($OC) were calculated by Eqs. (6)–(8), respectively:
Econs =
Ecell ∙ I (3.6) ∙S ∙B ∙ u
H2O → O2 + 2H+ +2e−
(6)
(7)
The aluminum price, in Mexico, is $2.008USDkg−1 and 0.001 is a conversion factor to obtain $Al(III) in $ USD m−3.
$OC = $Al(III) + αEcons + αEpump + β Msludge
−1
At 12, 14, and 16 mA cm , Chs ranged between 2.3–27.5 mg L , 1.7–14.5 mg L−1 and 1.16–8.4 mg L−1, respectively. The increase of current density permits the best removal of hydrated silica due to the increase in the dose of coagulant produced, as shown in Fig. 3. The CAl(III) /CAl(III)(N) ratio show similar patterns at 12, 14, and 16 mA cm−2 to those obtained at 10 mA cm−2; whereas, the current efficiencies of the coagulant were (76.8–100) %, (77–100) %, and (79–98) %, for j of 12, 14 and 16 mA cm−2, respectively. Fig. 4 shows remaining concentrations of arsenic (CAs ), fluoride (CF−), and sulphate (C SO24−) versus u, determined during the EC trials of Fig. 3. The remaining concentration of arsenic (0.56–3.37 μg L−1) shows a slightly decrease with current density at 10 ≤ j ≤ 16 mA cm−2, due to the higher aluminum dose produced. The curves CAs − u show similar patterns to those obtained from the removal of hydrated silica, except for the tests obtained at j of 10 and 12 mA cm−2 (for u of 3.5 and 4.67 cm s−1). The remaining concentration of F− (0.7―3.72 mg L−1) did not exhibit a pattern with current density. Moreover, CF− tends to decrease with u, contrary to what was obtained with silica and arsenic. This trend shows that the best removal of silica and arsenic inhibits the substitution reaction between the fluoride and the hydroxyl group from flocs [1,2], but the slight removal of silica and arsenic favors the removal of fluoride. This shows that there is a competition between fluoride, silica, and arsenic for active sites in flocs. The residual
Where the units of Econs , cell potential (Ecell ), and I are kWh m−3, V, and C s−1, respectively. B is the channel width (3 cm), S is the electrode spacing (0.46 cm), and the factor 3.6 is used to obtain Econs in kWh m−3.
$Al(III) = (CAl(III) )($2.008USDkg−1)(0.001)
(9) −2
(8)
$OC is expressed in units of USD m−3, α is the cost of the electricity in central Mexico (0.0976 USD (kWh)-1), Epump is in units of kWh m−3, and β is the sludge confinement cost in Mexico ($ 0.035 USD Kg−1). The electrolyzes were performed under galvanostatic conditions, at current densities of 10, 12, 14, and 16 mA cm−2. These tests were performed using the power supply described in the section 3.2, which directly records the cell potential. The duration of the electrolysis within the EC reactor (retention times, τ ) were 55.9, 27.8, 18.5, and 13.9 s , which correspond to mean linear flow rates in the EC reactor (u ) of 1.16, 2.33, 3.5, and 4.7 cm s−1, respectively. Meanwhile, the flocculation and sedimentation processes last 15 min and 3 h, respectively, for each test, as described in section 3.3. 4
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Fig. 5. SEM micrographs of the aluminum flocs obtained at u = 0.23 cm s−1 (retention time in the EC reactor of 27.8 s) and: (a) 12 mA cm−2, and (b) 14 mA cm−2.
Fig. 4. Effect of the flow velocity on the remaining concentrations of fluoride, arsenic, and sulfates, evaluated from the EC tests showed in Fig. 3.
Table 3 Composition of the flocs determined by XRF obtained at 2.33 cm s−1 and different current densities. Only elements with concentration above 50 ppm are reported.
concentration of sulfates (C SO24− ) did not vary with u and j (approx. 50 mg L−1); while the phosphates were completely removed after all EC tests, data not plotted. The partial and complete removal of sulfates and phosphates, respectively, is by adsorption on the flocs [1,2,25]. At the end of all EC tests, the pH was around of 8.9, from the initial value of 8.02; this increase corresponds to the substitution of a hydroxyl of the floc with a fluoride [25]. The abatement of hydrated silica is attributed to the massive formation of aluminum silicates during the EC, at the high current densities tested here 10 ≤ j ≤ 16 mA cm−2, resulting in high concentrations of coagulant up to 139.1 mg L−1. Moreover, the residual arsenic and fluoride concentrations met the WHO guidelines. The elimination of hydrated silica achieved here, contrasts with the poor removal of this pollutant found in previous papers [1,2], but coincides with the removal efficiency of arsenic and fluoride; in these investigations we employed two different EC reactors (filter-press type cells, closed at the top), where hydrogen bubbles, cathodically produced, broke the flocs at j > 6 mA cm−2. The novel EC reactor opened to the atmosphere, employed here, allowed the release of gas at j > 10 mA cm−2, and avoided the breaking of the flocs, which permitted the elimination of hydrated silica.
10 mA cm−2
12 mA cm−2
Element
Concentration (ppm)×103
Al Si As O Na Mg Ca K S Cl
55.3 22.5 0.051 592 155 2.82 32.8 11.3 9.56 10.5
14 mA cm−2
125 47.1 0.075 547 115 4.12 42.7 7.24 7.56 6.31
116 36.9 0.054 566 130 3.54 33.7 6.75 9.49 5.61
mA cm−2. The flocs are formed by very fine particles with dimensions less than 100 nm. Table 3 reports the elemental composition of flocs determined by XRF analyses in areas of 650 μm × 490 μm. The three samples show similar compositions. Arsenic was determined in the samples, between 51 to 75 μg g−1, while Fluor was not detected. The existence of silicon confirms the formation of aluminosilicate complexes. Table 4 summarizes typical composition of flocs obtained by EDS analyses. The presence of Si also confirms the formation of aluminosilicates. Arsenic and Fluor were not detected by EDS due to their low concentrations. All the elements detected by EDS in concentrations higher than 1.0 wt. % were also detected by XRF (Table 3). The
4.2. Flocs characterization Fig. 5 shows typical SEM micrographs of the flocs attained from the EC at 12 and 14 mA cm−2 (at u = 2.33 cm s−1). Both micrographs exhibit flocs with irregular forms and dimensions > 10 μm. No significant size differences were observed between the particles at 12 and 14 5
Journal of Water Process Engineering 32 (2019) 100923
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and u= 2.33 cm s−1. The wide peaks located at 10 < 2θ < 19, and 22 < 2θ < 42 are the result of the superposition of many narrow peaks. Peaks corresponding to Bytownite ((Ca3.44 Na .56 ) Al7.76Si8.24 O32 ), Magakalsilite (SiAlKO4 ), Hatrurite (Ca27Si9O45 ), and Melilite (Na2.05Ca5.95 Si 4 O15 ), were identified. These phases caused the removal of arsenic, fluoride, sulfates and phosphates. The FTIR spectrum of the flocs ranged between 4000–500 cm−1 was used to identify the chemical bonds in the samples at 12 mA cm−2 and u= 2.33 cm s−1, Fig. 6(b). Wide bands are distinguished at 587, 854, 999, 1412, 1524, 1634, and 3440 cm−1, very probably attributed to the following bonds: Al-F, As-O, Si-O, Al-O-Si, Al-O, Na-F, and O–H. It is worth noting that the FTIR spectrum is very similar to that reported in previous papers [1,2]. This spectrum confirms that arsenic is adsorbed on flocs, and fluoride substitutes a hydroxyl group from flocs. The EDS, XRF, XRD and FTIR analyzes carried out on the flocs confirm that the coagulant reacts with silica to yield aluminosilicates. On the other hand, Fluoride replaces a hydroxyl group from flocs, whereas arsenates, phosphates and sulfates are adsorbed on aggregates.
Table 4 Composition of the flocs determined by EDS obtained at 2.33 cm s−1 and different current densities. Only elements with concentration above 0.1% are reported. 10 mA cm−2
12 mA cm−2
14 mA cm−2
Element
% Masa
% Atom.
% Masa
% Atom.
% Masa
% Atom.
Na Al Si S Cl K Ca Cu
42.50 29.69 17.41 1.88 3.15 4.17 4.20
48.43 25.92 16.25 1.54 2.32 2.80 2.74
35.74 35.36 19.75 2.22 2.11 1.16 3.66
____
52.42 25.07 12.41 3.50 1.55 1.31 2.21 1.53
30.79 35.75 20.78 2.66 2.80 1.70 5.50
____
45.75 25.68 13.23 4.27 2.08 1.94 3.36 2.21
____
____
4.3. Performance and costs of EC Table 5 shows the residual concentrations of hydrated silica, arsenic, and fluoride. This table also shows the experimental aluminum doses, cell potential (Ecell ), electrolytic energy consumption (Econs ), cost of aluminum dose ($Al(III) ), pumping energy consumption (Epump), mass of sludge generated by EC (Msludge), the overall cost of EC (OC), and the retention time, τ, (electrolysis time), in the EC reactor, for each trial. We observe a decrease of Ecell, Econs, Msludge and $OC as u increases, at 10 mA cm−2; a similar behavior was obtained for the other current densities. It is worth to mention that the value of the Ecell remains constant during the electrolysis time, τ, of each EC trial. Another important aspect is that all the EC trials complied the WHO guidelines for arsenic and fluoride. The residual concentration of hydrated silica after the EC trials varied from 0.5 to 27.5 mg L−1, giving values of overall costs between 0.115 ≤ $OC ≤ 0.747 USD m−3, with experimental aluminum doses ranged between 28 < CAl(III) < 138 mg L−1. It is important to note that the overall cost of the EC process may vary in different countries due to the prices of electricity and sludge confinement.
Fig. 6. Typical XRD (a) and FTIR (b) patterns of the dry aggregates obtained at 12 mA cm−2 and u= 0.23 cm s−1 (retention time in the EC reactor of 27.8 s).
differences in composition determined by these two techniques, resulted from the large sampling area used in the XRF analysis, 8.04 cm2, which is more representative than the reduced scanned area used in EDS, around 0.005 μm2 . Fig. 6(a) shows the XRD pattern of the flocs attained at 12 mA cm−2
5. Conclusions Excellent removal of hydrated silica, arsenic, and fluoride from deep well water (with initial values of 42 mg L−1, 40 μg L−1, and 7.3 mg L−1, respectively) by EC, using aluminum anodes in an up-flow reactor with
Table 5 Remaining hydrated silica, arsenic, and fluoride concentrations, also experimental aluminum dose, Ecell, Epump, Msludge, and overall cost of the EC. j (mA cm−2)
u (cm s−1)
τ (s)
Chs (mg L−1)
CAs (μg L−1)
CF− (mg L−1)
CAl (III) (mg L−1)
Ecell (V)
Econs (kWh m−3)
Epump (kWh m−3)
Msludge (kg m−3)
$ OC (USD m−3)
10
1.16 2.33 3.50 4.67 1.16 2.33a 3.50 4.67 1.16 2.33 3.50 4.67 1.16 2.33 3.50 4.67
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
3.8 8 27.5 33 2.3 7.0 22.5 27.5 1.7 4.0 7.5 14.5 0.5 3.0 5.5 8.4
1.44 3.14 3.20 2.55 1.32 1.88 3.05 3.02 0.65 1.70 2.38 3.37 0.56 1.55 2.02 2.46
1.60 1.50 0.64 0.70 1.50 1.40 0.64 0.67 1.50 1.20 0.68 0.65 1.40 1.30 1.30 0.62
83.48 46.97 38.15 28.04 102.37 59.77 46.95 25.30 118.88 68.82 49.93 38.69 139.11 75.9 55.92 42.92
7.0 6.5 4.5 4.6 7.1 6.5 5.2 5.3 8.4 8.1 5.8 6.1 9.0 9.0 6.6 6.6
1.77 0.82 0.38 0.29 2.16 0.98 0.52 0.40 2.97 1.43 0.68 0.54 3.64 1.81 0.89 0.66
0.994 0.497 0.331 0.248 0.994 0.497 0.331 0.248 0.994 0.497 0.331 0.248 0.994 0.497 0.331 0.248
0.322 0.288 0.227 0.181 0.400 0.285 0.241 0.228 0.418 0.284 0.260 0.288 0.442 0.315 0.320 0.280
0.448 0.232 0.153 0.115 0.526 0.274 0.186 0.148 0.640 0.336 0.208 0.164 0.747 0.388 0.242 0.185
12
14
16
a
Remaining concentrations for sulfate and phosphate were 55 mg L−1 and ˜ 0 mg L−1, pH 8.9 and conductivity 1160 μS cm−1. 6
Journal of Water Process Engineering 32 (2019) 100923
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six-cell stack, opened to the atmosphere, was obtained. The operational conditions in terms of current density and mean linear flow rate were stablished at 10 ≤ j ≤ 16 mA cm−2 and 1.16 ≤ u ≤ 4.67 cm s−1. The best removal of hydrated silica based on energy consumption (0.98 KWh m−3) and overall cost of EC (0.274 USD m−3) was obtained at 12 mA cm−2 and u= 2.33 cm s−1, giving a remaining concentration of silica of 7 mg L−1, while the residual concentrations of fluoride (1.4 mg L−1) and arsenic (1.88 μg L−1) met the WHO guidelines in human drinking water. High concentrations of coagulant, achieved at j ≥ 10 mA cm−2, react with silica to yield aluminosilicates, which was confirmed by EDS, XRD, XRF, and FTIR analyzes on the flocs. On the other hand, fluoride replaces a hydroxyl group from flocs, whereas arsenates, phosphates, and sulfates are adsorbed on aggregates.
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