Simultaneous removal of arsenic and fluoride from synthetic solution through continuous electrocoagulation: Operating cost and sludge utilization

Simultaneous removal of arsenic and fluoride from synthetic solution through continuous electrocoagulation: Operating cost and sludge utilization

Accepted Manuscript Title: Simultaneous removal of arsenic and fluoride from synthetic solution through continuous electrocoagulation: Operating cost ...

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Accepted Manuscript Title: Simultaneous removal of arsenic and fluoride from synthetic solution through continuous electrocoagulation: Operating cost and sludge utilization Authors: Lokendra Singh Thakur, Hemant Goyal, Prasenjit Mondal PII: DOI: Reference:

S2213-3437(18)30752-8 https://doi.org/10.1016/j.jece.2018.102829 JECE 102829

To appear in: Received date: Revised date: Accepted date:

2 July 2018 10 December 2018 12 December 2018

Please cite this article as: Singh Thakur L, Goyal H, Mondal P, Simultaneous removal of arsenic and fluoride from synthetic solution through continuous electrocoagulation: Operating cost and sludge utilization, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.102829 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Simultaneous removal of arsenic and fluoride from synthetic solution through continuous electrocoagulation: Operating cost and sludge utilization

Lokendra Singh Thakur, Hemant Goyal, Prasenjit Mondal* Department of Chemical Engineering, Indian Institute of Technology Roorkee, Uttarakhand, India

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*Corresponding author. Tel.: +91-1332- 285181; fax: +91-1332-276535, E-mail address: [email protected]

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Highlights

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Graphical abstract



Simultaneous removal of arsenic and fluoride from synthetic solution by continuous

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electrocoagulation process was carried out.



The present process is adequate to reduce the arsenic and fluoride concentration in treated water to below their permissible limits as per WHO guidelines.



Sludge characterization confirms that arsenic and fluoride are linked with aluminum hydroxide complexes.

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Brick characteristics and leaching test performance suggest that the produced sludge can be considered as inert and environmentally sustainable and it can be used for construction purposes through brick formation.

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Abstract

The present study deals with simultaneous removal of arsenic and fluoride by continuous electrocoagulation (EC) process using aluminum electrodes. Effects of flow rates: 0.48–1.40 L/h and residence time (τ): 60–173 min have been studied on the removal of arsenic and fluoride. The percentage removals of arsenic and fluoride are found greater than 98.83 % and 87.5 % respectively. The concentrations of arsenic and fluoride in treated water are also found below the

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permissible limits as per World Health Organization (WHO) guidelines (As: 10 µg/ L, F: 1.5 mg/

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L). The optimum cost of continuous EC is found to be 0.358 USD/m3 at a flow rate of 0.88 L/h

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with 95 min residence time. Produced sludge has been analyzed by FESEM, XRD and FTIR. Sludge characterization confirms that arsenic and fluoride are linked with aluminum hydroxide

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complexes. Solidification technique has been used for the management of the produced sludge by its immobilization in the form of clay brick. Leaching test results show that leaching of arsenic

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and fluoride from the brick is much below the permissible limits assigned according to the US

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Environmental Protection Agency (USEPA) norms. The brick quality parameters are found above the respective permissible limits as per Indian standard indicating it suitability for use in

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Keywords

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construction purposes.

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Continuous electrocoagulation; Arsenic; Fluoride; Operating cost; Sludge management

1. Introduction

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Co-existence of arsenic and fluoride in groundwater has become one of the most serious environmental issues worldwide in recent years due to their toxic effects to living beings and ecological environment. Prolonged consumption of arsenic containing water (>10 µg/L) leads to various health effects such as cancer of skin, lungs and bladder (Karim, 2000), While in case of fluoride, low intake of fluoride (1.5 mg/L) is essential for skeleton and dental health. Nonetheless,

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the excess intake leads to mottling of teeth and skeleton fluorosis (Nell and Livanos 1988; Brown et al., 1977, Tian et al., 2011). Epidemiologic studies performed by Rocha-Amador (RochaAmador et al., 2007) revealed that the combined exposure of arsenic and fluoride increased the risk of reducing intelligence quotient (IQ) scores in children. It is also reported that combined exposure of these may lead to both endemic fluorosis and arsenicosis (Alarcón-Herrera et al., 2013). Several natural sources such as oxidative weathering and geochemical reaction (Banerjee

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et al., 2008) and anthropogenic sources such as industries of semiconductor, glass, pesticide,

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mining, metal are accountable to introduce these contaminants in terrestrial and aquatic ecosystems (Lacasa et al., 2011; Meenakshi and Maheshwari, 2006; Ayoob and Gupta 2006). In natural water,

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both arsenic and fluoride are present in the form of anions. Arsenite and arsenate are the

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predominantly present forms of arsenic in natural water in which arsenite is much more toxic than arsenate (Kobya et al., 2011). In 2015, high concentration of arsenic and fluoride has been recorded

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in the groundwater of Rajnandgaon district of Chhattisgarh in India, in the range of 148 - 985 µg/L

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and 3.7 – 27 mg/L. respectively, in which most of the sample contains arsenic concentration < 575 µg/L and fluoride concentration >10 mg/L (Patel et al., 2015). Millions of people of Mexico, Pakistan, Argentina, Bangladesh, Mongolia and India across the globe are also affected due to both

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arsenic and fluoride hazards (Farooqi et al., 2007a; Farooqi et al., 2007b; He et al., 2009; Kumar et al., 2010; Chaurasia et al., 2012; Dutta, 2013; Reyes-Gómez et al., 2015; Gomez et al., 2009).

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Thus, there is a vital need to find a solution which can efficiently remove arsenite and fluoride both from groundwater.

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It is revealed from literature review that there are several technologies such as adsorption,

membrane filtration, coagulation, oxidation, biological etc. are available for treatment of arsenic and fluoride contaminated water (Liu et al., 2012; Mohan and Pittman, 2007; Amor et al., 2001; Ali, 2012). All these technologies have their own limitations. The adsorption process is pH dependent; it requires high treatment time and pretreatment of adsorbent before use (Maheshwari, 2006). Membrane filtration process requires high capital investment and operating cost, skilled 3

labour and disposal of high concentrated discharge generated during operation, which make the process uneconomical (Maheshwari, 2006; Hu et al., 2008; Vasudevan et al., 2011). Large amount of sludge generation and their disposal are the major flaws of coagulation process (Maheshwari, 2006). While, biological process is mainly used for wastewater treatment. The process requires specific type of pathogenic and/or nonpathogenic microorganism, and their selection depends on

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the type of contaminant which has to be treated (Veglio and Beolchini, 1997). Thus, it is not suitable for groundwater treatment.

In context to the above discussed technologies, electrocoagulation (EC) process is one of the most promising technology, gaining enough attention in recent years, due to its high removal efficiency and easy operation (Mollah et al., 2004, Emamjomeh and Sivakumar, 2009; Drouiche et al., 2011). In EC process in situ metal hydroxide coagulant is generated in aqueous solution by

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dissolving the electrodes, which provides the active sites for coagulation and adsorption of

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polluting species. It does not require any external addition of chemicals. The basic steps involved in EC process are (i) electrolytic dissolution of anode material due to oxidation (Eq. 1), (ii)

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formation of hydroxide ions and hydrogen gas at the cathode (Eq. 2), (iii) formation of metal

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hydroxide in solution (Eq. 3), (iv) adsorption of contaminants at metal hydroxide surface and charge neutralization, and (v) Contaminants removal by settling (Drouiche et al., 2011; Ulucan

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and Kurt, 2015). Apart from adsorption, co-precipitation, bridge coagulation, sweep coagulation

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etc. also play important role for the removal of contaminants in EC process (Thakur and Mondal, 2017). The main electrochemical reactions with aluminum electrodes are as follows (Kobya et al., 2011);

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Reaction at anode Al → Al3+ + 3e−

(1)

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Reaction at cathode

2H2 O + 2e− → H2 + 2OH −

(2)

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Reaction in aqueous solution Al3+ + 3OH − → Al(OH)3(s)

(3)

In electrocoagulation, aluminum and hydroxyl ion produced by reactions (1) and (2), undergo hydrolysis and form various monomeric and polymeric species, which are further transformed into amorphous aluminum hydroxide according to complex precipitation kinetics (Mollah et al., 2001;

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Behbahani et al., 2011). This aluminum hydroxide complex is supposed to adsorb arsenic (Kobya et al., 2011; Flores et al., 2013) and fluoride (Hu et al., 2003). Several studies are available in literature for the removal of arsenic and fluoride separately from aqueous solution by applying electrocoagulation process in batch mode (Lacasa et al., 2011; Flores et al., 2013; Song et al., 2014a, Ghosh et al., 2008; Zhu et al., 2007; Zuo et al., 2008; Zhao

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et al., 2009; Un et al., 2013) as well as in continuous mode (García-Lara et al., 2014; Hansen et al., 2006). However, studies on simultaneous removal of arsenic and fluoride from aqueous solution by batch electrocoagulation are limited (Thakur and Mondal, 2017; Zhao et al., 2011; Thakur and Mondal, 2016) and no report is available on simultaneous removal of arsenic and fluoride through continuous electrocoagulation. In our earlier study, simultaneous removal of arsenic and fluoride was achieved in electrocoagulation batch reactor and optimum conditions

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were obtained (Thakur and Mondal, 2017; Thakur and Mondal, 2016). However, for large scale

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application of EC process, continuous reactor study is essential. Thus, suitability in continuous operation is very important for the success of any electrocoagulation process. Moreover, cost

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estimation of continuous electrocoagulation process is also a new approach, which is essential for

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economic feasibility studies.

The main aim of the present study is to examine the performance of continuous

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electrocoagulation process for simultaneous removal of arsenic and fluoride from synthetic

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solution. The effects of variation in flow rates and residence time have been studied along with treatment cost estimation. In order to investigate the removal mechanism, sludge obtained after

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electrocoagulation process has been analyzed through different sophisticated instruments such as FESEM, XRD and FTIR. Point of zero charge has also been determined. Moreover, for sustainable

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utilization of produced sludge, brick has been made. Physical properties of brick such as density and comprehensive strength have been determined. Waste extraction test (WET) of brick has also

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been conducted to find out the leaching of both arsenic and fluoride from it.

2. Materials and methods 2.1. Materials All chemicals used in the present study were of analytical reagent grade. Stock solutions of arsenite and fluoride were prepared from NaAsO2 and NaF salt, respectively. Required 5

concentration of arsenite and fluoride were made from the dilution of stock solution in distilled water. Sodium hydroxide (1N) and hydrochloric acid (1N) were used to maintain the initial pH of solution. 2.2. Experimental setup and procedure

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2.2.1. Continuous electrocoagulation The schematic diagram of the experimental set-up used in the present continuous electrocoagulation study is illustrated in Fig. 1. Details of the reactor and electrode configuration are listed in Table 1. Continuous electrocoagulation experiments were conducted at optimum conditions (Table 1) obtained in batch experimental study (Thakur and Mondal, 2017) with various flow rates (0.48 – 1.40 L/h) and residence times (τ) (60 – 173 min). Peristaltic pump (ENERTECH

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India, ENPD 300) was used to maintain the constant flow rate of feed to reactor at its bottom

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position as shown in Fig.1. DC power supply (0-30 V and 0-10 A) was used to conduct continuous electrocoagulation experiment. Samples were taken out from the continuous electrocoagulation

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reactor outlet at regular time intervals, filtered and analyzed for arsenic and fluoride. Each

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experiment was conducted thrice and the average value is recorded. In addition, after the electrocoagulation experiment, produced sludge was dried at temperature of 105 oC for sludge

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analysis.

Removal (%) =

Ci −C0 Ci

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The removal (%) of contaminant was calculated from Eq. (4). × 100

(4)

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Where, Ci and Co are initial and final contaminants concentration in solution. For operating cost evaluation of the present continuous electrocoagulation process, cost of

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electricity, electrode material (Daneshvar et al., 2006), as well as pumping were considered. Eq. 5 was used for computing operating cost.

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Operating cost = X × CElectrode + Y × (CEnergy for Electrocoagulation + CEnergy for pump ) (5)

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7

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7 Magnetic stirrer

2 Aluminum anode

8 D.C. power supply

3 Aluminum cathode

9 Peristaltic pump

4 Magnetic bar stirrer

10 Feeding tank

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11 Collection tank

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6 Support

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5 Electrode separator

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1 Electrocoagulation reactor

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Fig 1. Schematic diagram of the experimental set-up of continuous electrocoagulation. Where, CElectrode (kg Al/m3), CEnergy for electrocoagulation (kwh/m3) and CEnergy for pumping (kwh/m3) are amount of electrode material, energy for electrocoagulation and pumping consumed

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during electrocoagulation for decontaminating 1 m3 of water, respectively. Whereas, X and Y are coefficients of aluminum electrode material and industrial electricity price for Indian market in year 2015, respectively. Coefficient X: Wholesale aluminum electrode material price = 1.77 USD/kg (Web link-1) Coefficient Y: Industrial electricity price = 0.08 USD/kwh (Web link-2)

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Energy consumption for pump was consider as ~ 0.0027 kwh/m3 (Foley, 2015) Energy consumption for electrocoagulation was computed by Eq. (6). kwh

Energy consumption ( m3 ) =

voltage×current×runtime

(6)

volume

In every experiment, current was kept constant by controlling the voltage. The minute

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change in voltage with time was also counted in cost computation by considering the average value. Cost of electrode material was measured in terms of anode material consumed during electrocoagulation.

Visual MINTEQ version 3.1 simulation program was used for single component speciation of arsenite, fluoride and aluminum at different initial pH (1 - 14) putting the fixed temperature value of 25 oC (Web link-3).

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Table 1 Continuous electrocoagulation reactor, electrode characteristics, and optimized operating

Material

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Electrode characteristic

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Continuous electrocoagulation reactor specification Material Perspex

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conditions

Aluminum

0.15 m × 0.10 Shape m × 0.12 m 1.4 Total effective surface area of electrode (m2)

Rectangular

Type

Up flow

Parallel

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Dimensions (m) Working volume (L)

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Plate arrangement

04 Nos, 0.08 m x 0.08 m Magnetic bar (rpm – 400)

Inter electrode distance (m) NaCl concentration (g/L) Current density (A/m2)

0.01 0.71 10

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Number and Size of each electrode Stirring mechanism

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Optimized operating conditions (Thakur and Mondal, 2017) Initial arsenic 550 concentration (µg/L) Initial fluoride 12 concentration (mg/L) pH 7

2.2.2. Preparation of bricks For studying the final utilization of produced sludge, it was immobilized in the clay brick. Clay was purchased from local bricks manufacturer which contains mainly silica (65 %), alumina (15%), iron oxide (7%). Pure clay or composite containing produced sludge (10 % sludge w/w) and clay were used to make an evenly pressed compacted brick specimen of 27 x 20 x 16 mm size 8

after adding 20 % (w/w) of water to the dry sample. Both brick types were placed at room temperature for air drying (48 h) and then oven dried at 105 °C (24 h) for removing moisture. After that these two brick types were sintered at 800 °C for 60 min at 10 °C/min increase rate of temperature. Total six bricks, three of each type were prepared. 2.2.3. Determination of density, compressive strength and leaching of arsenic and fluoride from

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bricks by Waste Extraction Test (WET)

Density of the brick was determined using dry mass to volume ratio. Volume was determined using Archimedes method (Chiang et al., 2008). Compressive strength of the bricks was tested as per Indian standard (IS 3495-1992) to check their suitability for construction works. Experiments to determine arsenic and fluoride leaching from bricks were performed using California WET test (Ghosh et al., 2004). Bricks were crushed to a size small enough to pass

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through a No. 10 standard sieve (particle size: 2 mm). WET solution of 0.2 M sodium citrate at

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pH 5.0 ± 0.1 (prepared by titrating analytical grade citric acid in Milli-Q water with 4N NaOH) was prepared. Leaching test was conducted in a glass container, having 50 ml of WET solution

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and 5 g of crushed brick, which was placed in rotary incubator shaker for 48 h at 23 °C. Before

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shaking, nitrogen purging was done and the container was sealed. Leaching solution was filtered

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type of bricks are reported.

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with Whatman filter (pore size: 0.45 µ) for arsenic and fluoride detection. Average results for both

2.2.4. Determination of point of zero charge (PZC)

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Point of the zero charge is a specific characteristic of the absorption properties of sludge material. It is a pH value at which the surface charge of material is zero. Solid addition method was used to determine the PZC of sludge material. In this method 40 ml of 0.1 M NaNO3 and 0.2

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g of sludge material were mixed at different initial pH value and constantly stirred for 24 h at 25 °C. The ΔpH (difference between initial and final pH of suspension) value was then plotted against

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the initial pH value. The initial pH at which Δ pH on the resulting curve is zero was taken to be the PZC (Tan et al., 2008). 2.3. Analysis 2.3.1. Inductively coupled plasma mass spectrometry and ion selective electrode

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Arsenic analysis was accomplished by Inductively Coupled Plasma Mass Spectrometer (ICP-MS, Perkin Elmer, model ELAN-DRC-e) as per the standard method USEPA 200.8 (Creed et al., 1994). While, fluoride analysis was accomplished by Ion meter (Orlab India, model-OR930) coupled with compatible fluoride ion selective electrode. In order to prevent the interference of other ions in fluoride detection, TISAB (total ionic strength adjustment buffer) solution was added

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to each aliquot with the volume ratio of 1:1 (Thakur and Mondal, 2017). This is a solution containing 1,2-cyclohexylene diamine tetra acetic acid, sodium hydroxide, acetic acid and sodium chloride (Thakur and Mondal, 2016). 2.3.2. Field emission scanning electron microscopy

Microstructural study of the produced sludge was carried out by a Field Emission Scanning

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Electron Microscopy (FE-SEM, FEI, model- Quanta 200 FEG) equipped with Energy dispersive

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2.3.3. Fourier transform infrared spectroscopy

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X-ray spectroscopy. Mapping of As and F over the sludge sample was also done.

Fourier Transform Infrared spectrometer (Thermo FTIR, model- Nicolet 6700) was used

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to obtain an infrared spectrum of produced sludge in the wavenumber range of 500-4000 cm-1.

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2.3.4. X-ray diffraction

Crystal structure of the produced sludge was examined by X-ray diffractometer (XRD,

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BRUKER D8 advance, with Cu Kα, λ = 1.54 Å, scanning rate is 1° min-1) and the data was analyzed through PANalytical X’Pert HighScore software version 1.0e using database of X-ray

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powder diffraction patterns maintained by the International Centre for Diffraction Data (ICDD) (Web link-4) as well as published literature.

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2.3.5. Statistical analysis

Experimental data were subjected to statistical analysis.

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3. Results and discussion The effects of flow rate and residence time on percentage removal of arsenic and fluoride as well as operating cost are discussed below. The standard deviations of experimental data are found in range of 0 to 1.4. 3.1. Eff ect of flow rate and residence time 10

The percentage removal of arsenic and fluoride as a function of run time at the different residence time τ (60–173 min) and flow rate (0.48–1.40 L/h) is shown in Fig. 2a and 2b, respectively. It is evident from Fig 2a and 2b that percentage removal of both arsenic and fluoride initially increases with the increase in residence time τ and reaches to a constant value. After the run time of 250 min and 150 min for arsenic and fluoride, respectively, the percentage removals

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for all the τ values are similar for both the cases. However, at the initial stage of operation, the rate of change of percentage removals, for all the τ values are not same in both the cases. In the first 15 min of operation, arsenic removal is observed as 28.65 %, 47.55 % and 67.22 % at τ = 60, 95 and 173 min, respectively. Further, arsenic concentration comes below 10 µg/l at run time of 95 min for both residence time of 95 and 173 min. In case of τ = 60 min, arsenic removal reaches to below 10 µg/L at run time of 180 min. Similarly, fluoride concentration attains below 1.5 mg/L at

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run time of 95 min for both τ = 95 and 173 min. With the increase of run time more metals come

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into the solution and coagulant concentration increases, which remove more contaminants and

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results high percentage removal of arsenic and fluoride.

Since, higher the τ value higher is the time the solution gets to be in contact with the

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coagulant, the increase in τ value results higher removal of arsenic and fluoride as shown in Fig. 2a and 2b. It is attributed due to high anode dissolution at high τ and low flow rates (Eq. (7)).

MIt ZFV

(7)

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Celec thero =

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According to Faraday’s law, electrode dissolution in solution is can be quantified by Eq. (7).

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Where, Celect thero (kg Al/m3) is theoretical amount of ions produced by current I (A) passed for a period of time t (s), Z is number of electron transferred (Z = 3 for Al), M is molecular mass of Al

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(26.98 g/mol), F is Faradays constant (96485 C/mol), V is volume (m3) of treated water. After sufficient run time, large amount of coagulants are produced and hence, higher percentage removal of arsenic and fluoride is achieved even with lower values of τ. From Fig 2a it seems that almost

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100 % removal of arsenic is achieved at 250 min run time for all the cases. In case of τ = 60, high run time of 180 and 120 min require to achieve the arsenic and fluoride concentration of 10 µg/L and 1.5 mg/L in treated water, respectively. This behavior is related to the fact that, by increasing the flow rate, the contact time of coagulant with the solution decreases resulting in an increase of run time requirement for removal. Consequently, the rate of formation of aluminumarsenic/fluoride complexes reduces. 11

120 100 80 60

0.48 L/h, τ = 173 min 0.88 L/h. τ = 95 min 1.40 L/h, τ = 60 min

40 20 0 0

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100

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200

250

300

350

400

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Run time (min) 100

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(b)

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80

M

60

0.48 L/h, τ = 173 min 0.88 L/h, τ = 95 min 1.40 L/h, τ = 60 min

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40 20 0

50

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Fluoride removal (%)

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Aresenic removal (%)

(a)

100

150

200

250

300

350

400

Run time (min)

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Fig 2. Percentage removal of a) arsenic, b) fluoride as a function of run time (min) at the different flow rates (0.48–1.40 L/h) and residence time τ (60–173 min). 3.2. Change in pH of solution, cell voltage and operating cost on percentage removal of arsenic

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and fluoride

The pH of solution plays a vital role that governs the removal of both arsenic and fluoride

in electrocoagulation. It is an established fact that pH of the solution increases during electrocoagulation (Fig. 3) due to the formation of OH − ions and H2 gas at the cathode according to Eq. 2 (Vik et al., 1984; Chen et al., 2000). Fig. 3 illustrates the variation in the pH of the solution under continuous electrocoagulation as a function of run time at diff erent flow rates and 12

residence times. At all the flow rates (up to ~ 30 to 60 min of run time), a rapid increase in pH is noticed initially, after that the change in pH is marginal. It is ascribed to the generation of hydroxyl ions in the solution. Shortened residence time ends up with lower final pH value. Long residence time leads to increase in the final pH values due to the availability of more hydroxyl ion concentration due to dissolution of more metal per unit volume of solution and time as well. It is

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also ascertained from Fig. 3, that pH (initial pH = 7) of solution increases with the run time and reaches to maximum pH value of 10, 9.4 and 9.2 at τ = 173. 95 and 60 min, respectively. Elevation of this pH is also involved in the variation of the mechanism of ions removal. Speciation of arsenic, fluoride and aluminum also changes with the solution pH and the redox potential.

Within the pH range of 7 to 8, aluminum exists predominantly as positive charged species (Al(OH)2+ and Al(OH)+ 2 ) along with neutral species Al(OH)3 , and above pH 8, it exists

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predominantly as negatively charged species Al(OH)− 4 as shown in Fig. 4a. Whereas, in the pH

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range of 7 to 8 arsenic predominantly exists as neutral species along with some negatively charged

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species (H2 AsO− 3 ) as shown in Fig. 4b. The negatively charged arsenite species is chemisorbed by the positive charged coagulants. The solubility of mononuclear aluminum hydroxide coagulant is

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very less at solution pH near to neutral (Holt et al., 2002) and more bulkier species Al(OH)− 4 is formed as the pH increases (Rebhun and Lurie, 1993; Song et al., 2014b). Above the pH value of

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8, the Al(OH)− 4 species along with several monomeric and polymeric species of aluminum,

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produced during electrocoagulation, also helps the removal of neutral as well as negatively charged arsenic species through entrapment also known as sweep coagulation (Rebhun and Lurie, 1993).

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These species may further transform into amorphous Al(OH)3(s) according to complex precipitation kinetics (Behbahani et al., 2011). This freshly formed amorphous Al(OH)3(s) has

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minimum solubility and is finally polymerized to Aln (OH)3n, which results into dense flocs formation (Kobya et al., 2011; Behbahani et al., 2011; Kobya et al., 2006). Large surface area of dense floc, delivers entrapment of colloidal arsenic particles and hence, the arsenic removal arises

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(Ouaissa et al., 2014; Bilgili et al., 2016). Similarly, fluoride exists predominantly as F- species at the pH value of ≥ 7 (Fig. 4c). Hence, chemisorption of negatively charged fluoride on the positively charged surface of the coagulants predominates within the pH range of 7 to 8. Above the pH 8, both fluoride and coagulant are negatively charged. Thus, sweep coagulation along with coprecipitation is the predominating route for fluoride removal (Vasudevan et al., 2011; Hu et al., 2003). It is also interesting to note that most of the arsenic and fluoride are removed within run 13

time of ~ 60 min where, solution pH is below 8.8. The XRD (Fig. 7g) and FTIR spectra (Fig. 7f) of produced sludge also confirm the presence of As(III) and fluoride in it. 10.5 10

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9.5

pH

9 8.5 8

0.48 L/h, τ = 173 min

7.5 7

0.88 L/h, τ = 95 min

6.5

1.40 L/h, τ = 60 min

6 50

100

150

200

250

300

350

400

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Run time (min)

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0

L/h) and residence time τ (60–173 min).

(b)

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(a) 100

100

Al 2+ AlOH 4+ Al2(OH)2

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80 60

+

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Al(OH)2 5+

Al3OH4

40

Al(OH)3 (aq) Al(OH)

0

4

6

8

4-

H3AsO3

60

H2AsO-3 HAsO23

40 20 0

10

12

14

0

2

4

6

8

pH

pH

A

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2

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20

80

% As(III) species

3+

% Al species

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Fig. 3 Change in the pH of the solution as a function of run time at different flow rates (0.48–1.40

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10

12

14

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(c) 100

60 -

F HF

40 20 0 0

2

4

6

8

10

12

pH

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% F species

80

14

Fig. 4 Distribution of species as a function of pH, a) Aluminum, b) Arsenite, c) Fluoride in solution

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simulated by Visual MINTEQ version 3.1 (Song et al., 2014b)

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Fig. 5 shows the variation in cell voltage during electrocoagulation for different flow rates. At any flow rate, the gradual decrease in voltage is observed, which attains the minimum value at

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the end of electrolysis. With a starting cell voltage of ~ 0.3–0.4 V and run time up to 100–150 min,

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a drop in cell voltage is noticed irrespective of the flow rate, although, the total drop in voltage is marginal. At a low flow rate (≤ 0.88 L/h, τ ≥ 95 min) with a starting cell voltage of 0.4 V, high

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removal of arsenic and fluoride are observed in less electrolysis time. While, at high flow rate

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(1.40 L/h, τ = 60 min) with a starting cell voltage of 0.3 V, high electrolysis time is required to achieve the same removal. From this observations, it can be concluded that the flow rate has only

A

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limited effect on cell voltage reduction.

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0.5

0.3

0.48 L/h, τ = 173 min 0.88 L/h, τ = 95 min

0.2

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Voltage (v)

0.4

1.40 L/h, τ = 60 min

0.1 0 0

50

100

150

200

250

Run time (min)

300

350

400

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Fig. 5 Variation in cell voltage during electrocoagulation for different flow rates (0.48–1.40 L/h)

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and residence time τ (60–173 min).

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The operating cost of the continuous electrocoagulation process, as well as actual and theoretical aluminum mass loss at different flow rate and residence time is shown in Fig. 6. The

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operating cost of continuous electrocoagulation was computed according to Eq. 5, which is the sum of cost of electrode and energy consumed. The cost of energy is also have two part; cost of

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energy required for electrocoagulation as well as pumping. The pumping cost was consider as ~

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0.0027 USD/m3 for all flow rates (Foley, 2015). It is also found in the present study the voltage loss is marginal due to the variation of low rate. Thus, the major contributing factor of operating

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cost estimation is the cost of electrode. It can be perceived from Fig. 6 that the operating cost reduces with the increase in flow rate, and is found as 0.728, 0.358 and 0.216 USD/m3 at different

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flow rates of 0.48, 0.88 and 1.40 L/h, respectively. According to Faraday’s law (Eq. 7), the amount of electrode dissolution in electrocoagulation is inversely proportion to the volume of treated water, thus, the decrease in operating cost is noticed with the increase in flow rate. In present work,

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the actual loss of anode electrode (Fig. 6) is observed as 0.402, 0.198 and 0.120 kg/m3 at the flow rate of 0.48, 0.88 and 1.4 L/h, respectively. These mass losses are found maximum 16.27 % higher than the theoretical loss computed by Faraday’s law (Eq. 7). Excess dissolution occurs due to the super-faradic dissolution of aluminum electrode (Mechelhoff et al., 2013). It is suggested that the presence of Cl¯ ions in solution stimulate the localized attack of the (hydr)oxide passivation layer, thus causing the chemical dissolution of aluminium electrode (Mouedhen et al., 2008; Silva et al., 16

2004). Therefore, for operating cost computation actual mass loss was considered. Based on the above results, the optimum cost for the flow rate of 0.88 L/h is found as 0.358 USD/m3 at which removal of both arsenic and fluoride are found below the WHO guidelines (As: 10 µg/L, F: 1.5 mg/L) in 95 min run time. It is observed that higher the flow rate lower the operating cost, however the 0.88 L/h is considered as optimum because it brings down the concentration of arsenic and

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fluoride in treated water below permissible limits with run time of 95 min, which was optimized in batch study (Thakur and Mondal, 2017). Optimum run time is also required to have sufficient yield of treated water.

0.4

Operating cost Actual Theoretical

0.35

U

0.3

0.15 0.1 0.05 0

0.2 0.1 0

0.88

1.4

Flow rate (l/h)

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D

0.48

0.5

0.3

A

0.2

0.6

0.4

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0.25

0.7

Operating cost (USD/m3)

0.8

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Aluminum mass loss (kg/m3)

0.45

Fig. 6 Operating cost, actual and theoretical aluminum mass loss at different flow rates (0.48–1.40

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L/h)

3.3. Characteristics of sludge

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Produced sludge characterization has been performed in order to gain an insight on its

properties required to ensure its further management and to examine the removal mechanism of

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arsenic and fluoride as well. The point of the zero charge of the produced sludge is found as 8.2 (Fig. 7a). The surface texture of produced sludge is presented through FE-SEM image (Fig. 7b). Existence of adsorbed arsenic and fluoride in the sludge as shown in FE-SEM images in Fig S1a and S1b, respectively, represents the mapping of arsenic and fluoride over the sludge. For the mapping of a particular element over the sample through FE-SEM, we select only that particular element which is to be mapped, while keeping other all elements in background. This particular 17

element shows white colour in image. Thus, the white points in Fig. S1a and S1b show the presence of arsenic and fluoride, respectively over the sludge. The EDX analysis as demonstrated in Fig. S1c, exhibits the characteristic peaks related to arsenic and fluoride in the sludge along with other elements such as aluminium and oxygen. The peak height of both aluminum and oxygen are higher than the arsenic and fluoride. Also, the surface element content (as shown in inset of Fig. S1c) of

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both aluminum and oxygen are observed higher as compared to arsenic and fluoride. The presence of arsenic and fluoride in sludge also confirms the entrapment of these in sludge. The FTIR spectrum is displayed in Fig. S1d. The peaks at 3550 and 3468 cm-1 are likely ascribed to the presence of O-H stretching (Gomes et al., 2007). The peaks at 1020 and 1640 cm-1 are due to the presence of Al-O bond stretching and O-H bending, respectively (Ghosh et al., 2008; Gomes et al., 2007). The 604 and 646 cm-1 peaks are the characteristics of As(III)-O and Al-F stretching,

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respectively (Gomes et al., 2007; Gross et al., 2007). The X-ray diffraction pattern of aluminum

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electrode coagulant as presented in Fig. S1e shows shallow diffraction peaks, which indicate that the sludge is amorphous or at best very poorly crystalline in nature. The peaks at 2θ = 26, 28, 36,

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40 and 45 are assigned to the presence of Al(OH)3 whereas, the peaks located at 2θ = 22, 60, 68,

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and 73 are due to the presence of AlF3 (3H2 O) (ICDD data base). The presence of AlAsO4 can be anticipated by the peak at 2θ = 67, 76 and 81 (ICDD data base). The above FTIR and XRD results

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of sludge confirm once again that arsenic and fluoride are linked with aluminum hydroxide

2

pHpzc = 8.2

0 3

4

5

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2

EP

(a)

1

Δ pH

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complexes.

-1

6

7

8

(b)

9 10 11 12

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-2 -3 -4

Initial pH

Fig. 7a) Point of zero charge, b) SEM image of produced sludge. 3.4. Sludge management and leaching test 18

Bricks made from pure clay and composite (produced sludge and clay) (Fig. 8) were subjected to analysis for determination of density, compressive strength and leaching property. The density and compressive strength of the bricks produced from pure clay and composite are provided in Table 2. According to the American Society for Testing and Materials (ASTM C6217) for building bricks, compressive strength of brick should be in the range of 175 to 87.69

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kg/cm2. However, as per the Indian standard, compressive strength of bricks for the application in construction purpose should be in the range of 35–350 kg/cm2 [IS 1077:1992]. In present study, the compressive strength of pure bricks and composite are observed 55.14 and 51.29 kg/cm2, respectively, which is less than the ASTM standard, but comes in the acceptable range set by Indian Standard. The addition of sludge in the clay reduces the compressive strength of the brick slightly (55.14 to 51.29 kg/cm2). The released arsenic and fluoride concentration in leaching

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solution of the brick made of composite is found almost similar to that of pure clay brick (As:

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180.31 µg/L and F: 3.2 mg/L), and much lower than the guidelines according to US EPA, EHSO standard of 5000 µg/L for arsenic (The EPA TCLP, 2015) and 48 mg/L according to US EPA

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standard for fluoride (Dou et al., 2011). These results suggest that the produced sludge can be

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considered as inert and environmentally sustainable. Furthermore, it can be used for various types

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EP

TE

D

of construction purposes.

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Fig. 8 Brick produced using the sludge and clay (10 % w/w) and sintered at 800 °C. Table 2 Brick density and compressive strength made from pure clay and composite Brick

Density (g/cm3) Pure clay 1.83 Composite (10 sludge % w/w) 1.71

Compressive strength (kg/cm2) 55.14 51.29

19

4. Conclusion The findings of the present study reveal that the continuous electrocoagulation process is able to reduce the arsenic and fluoride concentration of treated water to below the WHO guidelines (As: 10 µg/L, F: 1.5 mg/L). During continuous electrocoagulation process using aluminum electrode, arsenic and fluoride removal of 98.83 and 87.5 %, respectively are found at a flow rate

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of 0.88 L/h and residence time of τ = 95 min along with the other parameters; initial pH: 7, inter electrode distance: 1 cm, current density: 10 A/m2 and NaCl concentration: 0.71 g/L. The optimum operating cost at this flow rate is computed as 0.358 USD/m3. Analyzing the characteristics of the sludge, it implies that arsenic is removed as arsenite. The leaching test (WET) result also shows negligible concentration of arsenic and fluoride in the leaching solution, which implicates the environmental sustainability of the sludge management through brick formation. Overall, the

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continuous electrocoagulation process can be effectively used for simultaneous treatment of

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arsenic and fluoride from contaminated water.

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Conflict of interest

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The authors declare that there are no conflicts of interest.

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Appendix A. Supplementary material

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