Arsenic and fluoride removal from groundwater by electrocoagulation using a continuous filter-press reactor

Arsenic and fluoride removal from groundwater by electrocoagulation using a continuous filter-press reactor

Chemosphere 144 (2016) 2113e2120 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere Arseni...

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Chemosphere 144 (2016) 2113e2120

Contents lists available at ScienceDirect

Chemosphere journal homepage: www.elsevier.com/locate/chemosphere

Arsenic and fluoride removal from groundwater by electrocoagulation using a continuous filter-press reactor n a, Jose  L. Nava b, *, Oscar Coren ~ o c, Israel Rodríguez d, Silvia Gutie rrez a Athziri Guzma a

Universidad de Guanajuato, Departamento de Química, Cerro de la Venada s/n, Pueblito de Rocha, 36040 Guanajuato, Guanajuato, Mexico tica e Hidra ulica, Av. Jua rez 77, Zona Centro, 36000 Guanajuato, Guanajuato, Mexico Universidad de Guanajuato, Departamento de Ingeniería Geoma c rez 77, Zona Centro, 36000 Guanajuato, Guanajuato, Mexico Universidad de Guanajuato, Departamento de Ingeniería Civil, Av. Jua d noma de San Luis Potosí, Facultad de Ingeniería-Instituto de Metalurgia, Av. Sierra Leona 550, 78210 San Luis Potosí, San Luis Potosí, Universidad Auto Mexico b

h i g h l i g h t s  Arsenic and fluoride removal from real groundwater by electrocoagulation.  Aluminum as sacrificial anodes.  Electrolyzes at different flow velocities and current densities.  Electrocoagulation led to 100% arsenic removal (43 mg L1) with 0.34 KWh m3.  Fluoride depletion from 2.5 to 0.15 mg L1 meet the WHO standard (<1.5 mg L1).

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 May 2015 Received in revised form 17 October 2015 Accepted 25 October 2015 Available online xxx

We investigated simultaneous arsenic and fluoride removal from ground water by electrocoagulation (EC) using aluminum as the sacrificial anode in a continuous filter-press reactor. The groundwater was collected at a depth of 320 m in the Bajío region in Guanajuato Mexico (arsenic 43 mg L1, fluoride 2.5 mg L1, sulfate 89.6 mg L1, phosphate 1.8 mg L1, hydrated silica 112.4 mg L1, hardness 9.8 mg L1, alkalinity 31.3 mg L1, pH 7.6 and conductivity 993 mS cm1). EC was performed after arsenite was oxidized to arsenate by addition of 1 mg L1 hypochlorite. The EC tests revealed that at current densities of 4, 5 and 6 mA cm2 and flow velocities of 0.91 and 1.82 cm s1, arsenate was abated and residual fluoride concentration satisfies the WHO standard (CF < 1.5 mg L1). Spectrometric analyses performed on aluminum flocs indicated that these are mainly composed of aluminum-silicates of calcium and magnesium. Arsenate removal by EC involves adsorption on aluminum flocs, while fluoride replaces a hydroxyl group from aluminum aggregates. The best EC was obtained at 4 mA cm2 and 1.82 cm s1 with electrolytic energy consumption of 0.34 KWh m3. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Arsenic removal Fluoride removal Sulfate removal Aluminum sacrificial anode Electrocoagulation Groundwater treatment

1. Introduction Groundwater represents one of the mean drinking water sources for most people around the world (Basu et al., 2014). However, uncontrolled consumption of drinking water has led to the drilling of deeper wells (>150 m). At these depths, elements such as arsenic (As) and fluoride (F) are typically found in the Bajío area in Guanajuato, Mexico at concentrations that exceeds the Mexican standard (CAs < 25 mg L1, CF < 1.5 mg L1). Long-term exposure to

* Corresponding author. E-mail address: [email protected] (J.L. Nava). http://dx.doi.org/10.1016/j.chemosphere.2015.10.108 0045-6535/© 2015 Elsevier Ltd. All rights reserved.

polluted water with arsenic and fluoride creates chronic health problems such as hyperpigmentation, and keratosis of hands and feet. It also causes bladder, lung, skin, kidney, liver, and prostate cancer (Smedley and Kinniburgh, 2013); furthermore, it can lead to fluorosis of the teeth and may stunt child growth (Brahman et al., 2013). Considering the toxicity of arsenic, the World Health Organization (WHO) and the U.S. Environmental Protection Agency have set the maximum acceptable level of arsenic in drinking water at 10 mg L1. For fluoride, the maximum acceptable level is 1.5 mg L1 for both the WHO and Mexican standard. Different technologies have been developed to decrease the arsenic and fluoride concentrations in groundwater, such as inverse

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lgora et al., 2013; Cui osmosis, electrodialysis and distillation (Mo et al., 2014). For arsenic removal, adsorption (Kumar et al., 2014) and coagulation-flocculation procedures are the most common. They involve the addition of aluminum sulfate, ferric sulfate, calcium hydroxide or polymeric flocculants. However, due to low concentrations of dissolved As, these processes yield large amounts of sludge, because SO2 4 anions consume 50% of the coagulant (Tang et al., 2014). The arsenic species in water from deep wells (at pH ~ 7.5) are innocuous arsenite, HAsO2 and H3AsO3 (oxidation state III) and arsenate anion, HAsO2 4 (oxidation state V) (Pourbaix, 1974). The second species at neutral pH is more susceptible to removal by EC (Hansen et al., 2006; Balasubramanian et al., 2009; Wan et al., 2011). We have previously discussed that arsenic removal is more efficient when arsenite is oxidized to arsenate, after the addition of a typical amount of sodium hypochlorite used for disinfection purposes (1 mg L1) (Flores et al., 2013). Moreover, it is well known that arsenite oxidizes to arsenate on the anode during the EC process (Wan et al., 2011). For removing fluoride from groundwater, the most widespread method is the coagulation-flocculation procedure by calcium and aluminum salt. Nevertheless, this process causes an increase in pH, which provokes the appearance of soluble aluminum species, which are toxic to human health. Besides, for large volumes of water this process yields large amounts of sludge, resulting in waste management problems (Singh et al., 2013). For this reason, EC was selected in this work, where aluminum, when dissolved electrolytically, supports the coagulation process and diminishes the amount of sludge produced. EC is an effective process to destabilize dispersed fine particles and anions contained in water (Hu et al., 2007; Essadki et al., 2009). Electro-dissolution of aluminum generates bulk aluminum hydroxides (Al(OH)3(s)) and aluminum oxides (Al2O3(s)), which are believed to adsorb arsenic, and remove fluoride by substitution reaction on the surface of the aluminum hydroxides (Flores et al., 2013; Sandoval et al., 2014). These findings have been observed in solutions that only contain As or F separately, and in some cases, each of them are studied showing the influence of other ions such as phosphates, sulfates, hydrated silica, carbonates, humic acids, and calcium, magnesium, among others (Wan et al., 2011; Kumar et al., 2014). Few studies have examined simultaneous removal of arsenic and fluoride (Ingallinella et al., 2011), where iron and aluminum binary oxide (FeAlOxHy) (Zhao et al., 2011; Liu et al., 2015) and freshly-prepared aluminum hydroxide (Liu et al., 2015) are employed as adsorbent materials. It is important to mention that the main interference observed during the simultaneous removal of arsenic and fluorine on freshly-prepared aluminum hydroxide is the formation of soluble complexes of AleF, which solubilize aluminum oxyhydroxides (AlOxHy), increasing the concentration of residual aluminum (Liu et al., 2015). Zhao et al. (2011) reports the efficient removal of both arsenic and fluoride simultaneously (from synthetic solutions) using a combination of electro-oxidation and EC processes. However, the simultaneous removal of arsenic and fluoride from actual samples of groundwater and the possible interferences of other ions by EC have not yet been published. Groundwater may also contain co-existing anions such as SO2 4 , 2þ 2þ PO3 and hydrated species of silica 4 , cations as Mg , Ca (SiO2,xH2O). These co-existing anions compete with F and HAsO2 4 with aluminum flocs (Sandolval et al., 2014). The presence 1 3 of SO2 and 4 and PO4 in concentrations higher than 50 mg L 1 4 mg L , respectively, inhibits arsenic and fluoride removal (Wan et al., 2011). Dissolved hydrated species of silica, contained in natural water generally between 20 and 60 mg L1, or in higher concentrations, also obstruct the removal of As and F. These species have a similar function to hydrated acids such as H2SiO3, H4SiO4,

H6SiO7, H8Si3O and H10SiO13 (Pourbaix, 1974). According to Lakshmanan et al. (2010) the monovalent state of H3SiO4 , which sparingly appears at hydrated silica concentrations greater than 20 mg L1 and at pH > 8, competes with arsenate for adsorption sites. Lartiges et al. (1997) investigated the flocculation of colloidal silica with hydrolyzed aluminum. These authors discuss the formation of aluminum complexes with silica anions (aluminum-silicates). These have potential active sites for aluminum aggregate growing. Likewise, Tokoro et al. (2014) suggest that hydrated silica could be removed during kaolinite (amorphous aluminum silicate) formation by co-precipitation and adsorption processes. On the other hand, the presence of Ca2þ enhances defluoridation owing to the ability of fluoride to form CaF2(s) (Zuo et al., 2008). In the same way, the ion Mg2þ improved fluoride removal, because Mg2þ is a good coagulant (MgF2) and is frequently used as ~ e, 2010). co-coagulant with aluminum salt (Montero and Villafan The aim of this study was to decrease the arsenic and fluoride concentrations in groundwater from the Bajío region by EC using aluminum as the sacrificial anode in a continuous filter-press reactor. We analyzed the influence of current density and mean linear flow velocity on the efficiency of arsenic and fluoride removal. Aluminum flocs were characterized by SEM-EDAX, XRD and FTIR. The energy consumption for electrolysis was also estimated. 2. Electrocoagulation process EC consists of an in situ generation of coagulants by electrodissolution of aluminum electrodes. At the anode, aluminum ions are produced first (Al3þ); afterwards, the aluminum ions are transformed to aluminum hydroxides (Al(OH)3(s)) and aluminum oxides (Al2O3(s)) in volume: Al(s) / Al3þ þ 3e

(1)

Al3þ þ 3H2O / Al(OH)3(s) þ 3Hþ

(2)

2Al3þ þ 3H2O / Al2O3(s) þ 6Hþ

(3)

At the aluminum cathode, hydrogen gas is released: 3H2O þ 3e / 1.5H2 þ 3OH

(4)

Typically, at the cathode interface the solution becomes alkaline with time. The OH migrates and diffuses away from the cathode to the anode, thus favoring water formation, hydroxyl ions reacts with protons of Eqs. (2) and (3), making the remaining solution neutral: OH þ Hþ / H2O

(5)

The major problem of the aluminum anode is its passivation, due to Al2O3(s) precipitation, giving high anode and cell potential, increasing the energy consumption and cost of EC (Kobya et al., 2011; Mohora et al., 2012). Passivation can be controlled at low current densities in combination with turbulent flow conditions, which favors Al3þ ions transport away from the surface to the bulk solution. In addition, the use of cathodes of the same material are recommended to electro-dissolve Al2O3(s) by periodic current reversal (Mohora et al., 2012), which allows an even consumption of aluminum electrodes during the process. As we mentioned above, our group had previously discussed that arsenic removal is more efficient when arsenite is oxidized to arsenate, after the addition of a typical amount of sodium hypochlorite used for disinfection purposes (1 mg L1) (Flores et al., 2013). The Al(OH)3(s) and Al2O3(s) flocs are believed to adsorb

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HAsO2 4 (Kobya et al., 2011):

AlðOHÞ3ðsÞ þ HAsO2 4

i h s / AlðOHÞ3 HAsO2 4

i h 2 Al2 ðOHÞ3ðsÞ þ HAsO2 s 4 / Al2 ðOHÞ3 HAsO4

(6) (7)

The mechanism of fluoride removal by EC is carried out using a chemical substitution, in which F replaces OH group from Al(OH)3 flocs according to equation (8), (Zhu et al., 2007). Al(OH)3 þ xF / Al(OH)3xFx þ xOH

(8)

Groundwater contains high concentrations of co-existing ions which affect arsenic and fluoride removal (Bennajah et al., 2010); hence, the influence of co-existing ions on the As and F removal efficiency are examined. 3. Materials and methods 3.1. Solutions Groundwater samples were collected from deep wells (320 m) located in the Bajío region of Guanajuato, Mexico (arsenic 43 mg L1, fluoride 2.5 mg L1, sulfate 89.6 mg L1, phosphate 1.8 mg L1, hydrated silica 112.4 mg L1, hardness 9.8 mg L1, alkalinity 31.3 mg L1, pH 7.6 and conductivity 993 mS cm1). Water samples were collected from the wells after the addition of 1.0 mg L1 sodium hypochlorite used for disinfection. 3.2. Filter press reactor The experimental setup and the dimensions of the filter-press electrochemical reactor were described in Flores et al. (2013). The system consists of a continuous pre-pilot scale filter-press-type cell, where the coagulant is produced. The resulting solution (a mixture of water and coagulant) is passed to a jar test to induce the flocculation and adsorption of arsenic on the aluminum flocs and reaction between fluoride and aluminum aggregates. The arsenic and fluoride are then precipitated, and the clarified solution is analyzed. 3.3. Methodology EC was performed at different mean linear flow rates between 0.91  u  3.82 cm s1 and current densities of 4, 5 and 6 mA cm2. Immediately, each resulting solution was passed to test jar equipment, mixing at slow speed (45 rpm), for a duration of 15 min, causing the growth of aggregates; then, aggregates are precipitated in the static solution, for approximately three hours. The arsenic, fluoride, phosphate, sulfate, hydrated silica, calcium and magnesium in the clarified solution were analyzed. After dissolution of the flocs, the aluminum contained was also analyzed.

HI 93717 Phosphate High Range, ISM, while silica determination was performed by Heteropoly Blue Method using the kit HI 93705. Both analyses employed a multi-parameter photometer C99 by Hanna Instruments. Sulfate was quantified using the turbidimetry technique (APHA, 1998) using the spectrophotometer Perkin Elmer Lambda 35 UV/VIS. The detection limit of sulfates, phosphates and hydrated silica was 0.2 mg L1. Carbonates and hardness tests were carried out by titration according to standard methods (APHA, 1998). Ca and Mg (which give hardness) were also determined by AA with detection limits of 0.02 and 0.03 mg L1, respectively. Conductivity and pH measurements were carried out using a water proof instrument, model HI 991300 from Hanna Instruments. All chemical reagents were of analytical grade. Each individual experiment was performed at least three times, and then the results were averaged. 3.4.2. Scanning electron microscopy and energy dispersive X-ray analysis The Scanning Electron Microscopy (SEM) analysis was carried out in using a Jeol JSM-6610LV. The Energy Dispersive Analysis of Xrays (EDAX) was carried out in an Oxford X-Max detector integrated in the microscope. 3.4.3. X-ray diffraction analysis (XRD) XRD were performed on a diffractometer Rigaku, model DMAX 220, with nickel filter and radiation of CuK a. 3.4.4. Fourier transform infrared spectroscopy (FTIR) The FTIR analysis in the flocs was carried out in a Perkin Elmer Spectrum GX FTIR Spectrometer using an EasiDiff diffuse reflectance accessory. The flocs samples were prepared using potassium bromide (sample:KBr, 1:14). The diffuse reflectance accessory allowed conducting the FTIR analysis. 3.4.5. Zeta potential The zeta potential was measured using an electrokinetic solids analyzer (Anton Paar SurPass) with the remaining water from EC. 4. Results and discussion 4.1. Simultaneous As and F removal by EC Fig. 1aec shows the influence of mean linear flow velocity on the residual concentration of arsenic (CAs) at constant current density of 4, 5 and 6 mA cm2, respectively. The experimental CAl(III) and theoretical CAl(III) (N) aluminum doses (Eq. (9)) are also included. At 4 mA cm2 the CAs displayed an abatement at 0.91  u  1.82 cm s1, with experimental aluminum doses between 20.6  CAl(III)  12.7 mg L1. At u > 1.82 cm s1 CAs increases up to 37.9 mg L1 with CAl(III) ¼ 7.9 mg L1, indicating that arsenic removal is not favored with mean linear flow velocity owing to depletion of CAl(III). It is important to mention that the CAl(III), is lower than the theoretical value.

3.4. Analytical procedure 3.4.1. Groundwater characterization Based on the standard method suggested by APHA (1998), an atomic absorption spectrometer (Perkin Elmer AAnalist 200), equipped with a manual hydride generator at 188.9 nm wavelength was employed to determine the arsenic concentrations (detection limit of 0.1 mg L1). Fluoride concentrations were measured using an ion selective electrode of fluoride, model 27502-19, from Cole Palmer (detection limit of 0.02 mg L1). The aluminum concentration was determined by Atomic Absorption (AA) with a detection limit of 0.15 mg L1. The analysis of phosphate was carried out by kit

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CAlðIIIÞðNÞ ¼

   jLMW  1  106 ðNÞ zFSu

(9)

where CAl(III)(N) is the theoretical concentration of aluminum dose (mg L1), j is the current density (A cm2), L is the length of one channel (cm), MW is the molecular weight of aluminum (26.98 g mol1), n is the number of exchanged electrons, 3, F is the Faraday constant (96485C mol1), S is the channel width (cm), u is the mean linear flow velocity (cm s1), N is the number of channels, and 1  106 is a conversion factor to obtain units of concentration of aluminum in mg L1.

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Fig. 1. Influence of the mean linear flow rate on the residual arsenic concentration and aluminum dose for groundwater. Applied current density: (a) 4 mA cm2, (b) 5 mA cm2 and (c) 6 mA cm2.

the current efficiency values are close to 100%. Even when the aluminum dose is imposed by j and u, the arsenic abatement is independent of j at 0.91  u  1.82 cm s1. It can be associated on the one hand, with the concentration of aluminum CAl(III) (between 12.72 and 72.46 mg L1), which might favor the EC, and on the other hand, with the effective flocculation performed at such low flow rates. At u > 1.82 cm s1 CAs increases owing to hydrodynamics might promote the breaking of the flocs. Microfilm analysis of the flocs growing during EC processes can be helpful to elucidate the influence of current density and mean linear flow rate on the floc size; although this analysis was beyond of the scope of this paper. During the same EC trials the fluoride residual concentration (CF) was followed, Fig. 2. For the experiment assessed at 4 mA cm2 and at u ¼ 0.91 cm s1, CF decreases to 0.6 mg L1 from the initial concentration (2.5 mg L1), with a CAl(III) of 20.6 mg L1. The major defluoridation, CF ¼ 0.39 mg L1, was obtained at u ¼ 1.82 cm s1, with CAl(III) ¼ 12.70 mg L1. Finally, u > 1.82 cm s1 CF achieves an increased concentration of 0.91 mg L1 at 3.64 cm s1 with CAl(III) ¼ 7.9 mg L1; CF increased due to CAl(III) depletion in addition to the breaking of the flocs. EC tests at 5 and 6 mA cm2 presented a similar behavior to that obtained at 4 mA cm2. The final pH of the groundwater after EC tests slightly increased to 8.5, likely due to F interchanges with OH. It is important to point out that all EC experiments meet Mexican standard (CF  1.5 mg L1). During the same electrolyzes the residual concentration of SO2 4 and PO3 4 (Fig. 3aeb) was determined to analyze their interaction during the EC process. The average residual concentrations of sulfate and phosphate did not present a trend with j and u, possibly due to interferences from the analytical method employed here. However, the average residual concentration after EC tests were 1 around 30 mg L1 SO2 SO2 4 from the initial value of 89.6 mg L 4 , 3 3 1 1 and 1.1 mg L PO4 from initial value of 1.8 mg L PO4 . The 3 depletion of SO2 4 and PO4 suggests that these anions are adsorbed on the flocs and compete with arsenic and fluoride for the flocs active sites (Wan et al., 2011; Flores et al., 2013; Sandoval et al., 2014). Fig. 4aeb showed a slightly residual concentration decrease of Ca2þ (initially 2 mg L1 Ca2þ) and Mg2þ (initially 1.5 mg L1 Mg2þ) with j and remained almost constant with u. The slightly removal of Ca2þ and Mg2þ suggests that they might

The current efficiencies of EC (assessed by the ratio between experimental and theoretical aluminum doses) were comprised between 63.6 and 85.2%. These values can be a consequence of oxygen evolution reaction Eq. (10), which typically occurs simultaneously with Eq. (1), and on the other hand, the passivation of the anode due to Al2O3(s) precipitation on the anode. 2H2O / O2 þ 4Hþ þ 4e

(10)

At 5 mA cm2 CAs is similar to that obtained at 4 mA cm2. It shows an abatement of CAs at 0.91  u  1.82 cm s1 with aluminum doses of 41.4  CAl(III)  18.5 mg L1. Then, at u > 1.82 cm s1 a linear increase of CAs from 0 to 14.2 mg L1 took place with an CAl(III) ¼ 8.3 mg L1. The current efficiency approached 100%. At 6 mA cm2 a similar behavior to that obtained at 4 and 5 mA cm2 was obtained at 0.91  u  1.82 cm s1. However, at u > 1.82 cm s1 the CAs surpasses the obtained at 5 mA cm2 even when CAl(III) increases to 6 mA cm2. The current efficiency at this current density is higher than 100% at 0.91 cm s1, likely a consequence of the aluminum oxides dissolution on the anode that was accumulated in earlier electrolysis. Afterwards, at u > 1.82 cm s1

Fig. 2. Influence of the mean linear flow rate on the residual fluoride concentration, evaluated from the EC tests similar to those from Fig. 1.

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Fig. 3. Influence of the mean linear flow rate on the residual: (a) sulfate concentration, (b) phosphate concentration, evaluated from the EC tests similar to those from Fig. 1.

precipitate in the flocs. On the other hand, the hydrated silica depletion was 0.15 mg L1, from its initial value of 112.4 mg L1, which suggests that silica species were removed by coprecipitation and/or by the formation of aluminum silicates with divalent cations such as Ca2þ and Mg2þ; the latter is validated by XRD analysis shown below. 4.2. Flocs characterization 4.2.1. SEM/EDAX Fig. 5a shows a SEM image of aluminum flocs obtained either from the EC process at 4 mA cm2 and 0.91 cm s1, and Fig. 5b at 5 mA cm2 and 0.91 cm s1. These micrographs show particles with irregular shape and size of up to around 150 mm, Fig. 5a, and up to around 200 mm in Fig. 5b. Fig. 5ced shows a SEM image of aluminum flocs obtained from the EC process at 6 mA cm2 and 0.91 cm s1 and 1.82 cm s1, respectively. Particles are also of irregular shape, whit size of up to around 500 mm. From Fig. 5 it can be observed that maximum particle size increases with current density. Table 1 shows the composition of the flocs, determined by EDAX. Areas of around 800 mm  800 mm were analyzed. The low standard deviations imply that the samples are homogeneous. Six areas were analyzed for flocs produced using currents of 4 mA cm2 and 5 mA cm2, and twelve areas for flocs produced using 6 mA cm2. In flocs corresponding to 4 mA cm2, Mg was not

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Fig. 4. Influence of the mean linear flow rate on the residual: (a) calcium concentration, (b) magnesium concentration, evaluated from the EC tests similar to those from Fig. 1.

detected. At 5 mA cm2 and 6 mA cm2, the six elements were present. Oxygen and aluminum were always present. The presence of Si on the aluminum flocs can produce hydro-aluminum-silicates (Tokoro et al., 2014); this last was confirmed by FTIR as shown below. It has been reported that arsenic is removed by an adsorption  lgora et al., 2013; process on the flocs followed by precipitation (Mo Basu et al., 2014; Cui et al., 2014; Kumar et al., 2014), and it is well known that fluoride replaces the hydroxide ion of the aluminum flocs (through Eq. (8)). Although, these elements were not detected in area analysis, contents of up to 1.23 at. % As and 4.61 at. % F were measured by point analyses. On the other hand, zeta potential measurements on the flocs gave negative values (Table 2), except for those obtained at 5 mA cm2 and 1.82 cm s1, which indicates that fluoride, sulfates, carbonates, and phosphates are removed and form a coordinate complex with hydrated silica species. The above may be performed by precipitation with aluminum flocs, likely through an adsorption process and/or ionic interchange reaction. In addition, the negative zeta potential is also provoked by the slight increase in pH after EC tests (see Table 2). This change in pH also increases the electrical conductivity. 4.2.2. X-ray diffraction analysis (XRD) In Fig. 6a XRD spectrum shows very broad and shallow diffraction peaks, the first from 5 to 18 and the second from 25 to 38 . Bragg reflections possessing very broad humps and low intensities indicate that the analyzed sample has a short range order,

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Fig. 5. SEM images of aluminum flocs obtained from the EC process at 0.91 cm s1 and 4 mA cm2 (a), and 5 mA cm2 (b); SEM images of aluminum flocs obtained from the EC process at 6 mA cm2 and 0.91 cm s1 (c), and 1.82 cm s1 (d). Table 1 Composition of the flocs determined by EDAX. 4 mA cm2

5 mA cm2

6 mA cm2

Element

Average at. %

Standard deviation

Average at. %

Standard deviation

Average at. %

Standard deviation

O Na Mg Al Si Ca

54.60 7.11 0.00 29.29 7.19 1.83

0.76 0.26 0.00 0.42 0.21 0.11

53.31 4.99 1.74 32.16 6.68 1.13

0.65 0.26 0.13 0.26 0.27 0.14

55.22 3.12 1.97 33.02 5.70 0.98

0.71 0.28 0.17 0.21 0.31 0.10

(Abis et al., 2001; Musi c et al., 2011) or amorphous alumina (Wu, 2001; Li and Huang, 2007; Wang et al., 2009). After 38 the XRD pattern shows a continuous series of narrow peaks. Then, it is possible that the two broad peaks are the result of the

characteristic of amorphous phases. From the EDAX analyses, it could be expected that the broad peaks could be assigned to amorphous silica or amorphous alumina. However, these peaks do not match with those previously reported for amorphous silica

Table 2 Residual arsenic and fluoride concentrations after hypochlorite addition satisfying the WHO standard (CAs10 mg L1, CF  1.5 mg L1), as well as experimental aluminum dose, experimental aluminum flow rate, cost of aluminum dosed, Z potential, pH, conductivity, cell potential and energy consumption. Initial composition of groundwater: arsenic 43 mg L1, fluoride 2.5 mg L1, sulfate 89.6 mg L1, phosphate 1.8 mg L1, hydrated silica 112.4 mg L1, hardness 9.8 mg L1 (calcium 2 mg L1, magnesium 1.5 mg L1), alkalinity 31.3 mg L1, pH 7.6 and conductivity 993 mS cm1. J (mA cm2) U (cm s1) CAs (mg L1) CF (mg L1) CAl(III) (mg L1) FAl(III) (mg s1) $Al (USD m3) Z Potential (mV) pH 4 5 6

0.91 1.82* 0.91 1.82 0.91 1.82

0.0 0.0 0.0 0.0 0.0 0.0

0.60 0.39 0.44 0.16 0.17 0.15

20.63 12.72 41.38 18.51 72.46 20.45

0.034 0.042 0.068 0.062 0.12 0.068

0.032 0.020 0.064 0.029 0.112 0.031

10.29 15.08 10.38 0.176 2.436 16.99

8.65 8.77 8.60 8.64 8.87 8.78

Conductivity (mS cm1) Ecell (V) Econs (kWh m3) 1324 1272 1273 1351 1228 1191

6.1 7.0 7.7 8.6 9.8 10.5

0.6 0.34 0.95 0.53 1.45 0.78

*Residual concentrations for the best EC in terms of Econs: sulfate 33.1 mg L1, phosphate 1.6 mg L1, calcium 1.2 mg L1, magnesium 1.4 mg L1, hydrated silica 112.25 mg L1.

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again that arsenic is removed by adsorption according to equations (6) and (7) and F might replace OH group from the flocs (Zhu et al., 2007). In addition, the presence of AleOeSi bounding confirms the formation of hydrated aluminum silicates as a product of EC process. It is important to mention that FTIR spectra did not show bands corresponding to any Selement bounding, which can be associated to a weak SO2 4 adsorption on aluminum flocs (Sandoval et al., 2014). Zuo et al. in (2008) performed an X-ray photoelectron spectroscopy study of aluminum flocs in the presence of SO2 and 4 proposed that SO2 4 has a negative effect on defluoridation, which is associated with the ion exchange competition between SO2 4 and F , that is:  AlðOHÞ3x Fx þ ySO2 4 4AlðOHÞ3x Fx2y ðSO4 Þy þ 2yF

(11)

However, in this paper the AlðOHÞ3x Fx2y ðSO4 Þy phase was not detected by the spectroscopy techniques employed here.

4.3. Energy consumption and costs of EC

Fig. 6. Typical XRD (a) and FTIR (b) spectra of the dried flocs obtained at 4 mA cm2 and 1.82 cm s1.

superposition of a high number of narrow peaks. Peaks corresponding to merwinite (Ca3Mg(SiO4)2), anorthite (CaAl2Si2O8), albite (NaAlSi3O8), boggsite (NaCa2(Al5Si19O48),H2O), cordierite (Mg2Al4Si5O18), tremolite (Ca2Mg5(Si8O22)(OH)2) and rosenhahnite (Ca3SiO8(OH)2) were identified. If a-Al2O3 and g- Al2O3 phases are present in the samples their content might be below 5 wt. %, owing to their XRD peaks were not distinguished. In the same way, phases containing As or F were not detected. This can be explained by the low content of these elements in the groundwater, Arsenic, 43 mg L1 and fluoride 2.5 mg L1. 4.2.3. FT-IR characterization FTIR analysis ranged from wave numbers between 4000 and 450 cm1 was performed to analyze the chemical bonds of the elements present in the flocs (obtained at j of 4 mA cm2 and u of 1.82 cm s1), Fig. 6b. The infrared spectrum analysis showed a wide band at 3550e3200 cm1, which most likely corresponds to OeH stretching vibrations (Socrates, 2004). The peak located at 1750 cm1 is likely attributed to the NaeF bounding. AleO bending is represented by the band at 1500 cm1. The peak at 1300 is likely attributed to AleOeSi. SieO bond is represented at 1100 cm1. The band at 705e690 cm1 may be ascribed to the stretching of AleF (Lartiges et al., 1997; Kobya et al., 2011). The band at 850e800 cm1 is characteristic of AseO bending. The FTIR analysis confirms once

Table 2 reveals that EC at 4, 5 and 6 mA cm2 satisfied the WHO standard for arsenic and fluoride at flow rates between 0.91 and 1.82 cm s1 (volumetric flow rates, q, 1.6  103 and 3.3  103 L s1), corresponding to experimental aluminum dose of 12.7 and 72.5 mg L1 and experimental aluminum flow rate of FAl(III) ¼ 0.042 and 0.12 mg s1, evaluated as FAl(III)]CAl(III)  q. EC at 4 mA cm2 satisfied the WHO standard at flow rates between 0.91 and 1.82 cm s1, corresponding to aluminum doses of 20.63 and12.72 mg L1and aluminum flow rates of 0.034e0.042 mg s1. EC at 5 mA cm2 satisfied the WHO standard at flow rates of 0.91e1.82 cm s1, corresponding to aluminum doses of 41.38 and 18.51 mg L1 and aluminum flow rates of 0.068 and 0.062 mg s1. Finally, EC at 6 mA cm2 satisfied the WHO standard at flow rates between 0.91 and 1.82 cm s1, corresponding to aluminum doses of 72.4 and 20.45 mg L1and aluminum flow rates of 0.12e0.068 mg s1. It should be noted that the cell potential, Ecell, increases with current density according to Ohm law and decreases with mean linear flow rate owing to aluminum ions are transported away from the surface to the bulk solution under such turbulent flow conditions, decreasing Al2O3 precipitation on the anode. The energy consumption (Econs) was evaluated for the EC test shown in Table 2. The energy consumption during electrolysis was determined by:

Econs ¼

Ecell I ð3:6ÞSBu

(12)

where Ecell is given in units of V, I is the current intensity during electrolysis (C s1), B is the channel height (cm), S is the channel width (cm) and the value 3.6, is a conversion factor that allows to obtain Econs in units of kWh m3. The cost of aluminum doses evaluated as $Al(III)](CAl(III)  ($1.55 USD kg1)  0.001) is also included in Table 2. The cost of aluminum considered for this calculations was $1.55 USD kg1 and the value of 0.001 is a conversion factor to obtain $Al(III) in units of $ USD m3. The best EC in terms of energy consumption (Econs) and cost of aluminum dosed was obtained at 4 mA cm2 with u ¼ 1.82 cm s1, giving Econs of 0.34 KWh m3, and $Al(III) of 0.020 USD m3. It is important to mention that in this paper we did not included an estimate of the overall EC process due to electricity fee variance between countries.

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