Studies on the electrochemical decontamination of wastewater containing arsenic

Separation and Purification Technology 73 (2010) 114–121

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Studies on the electrochemical decontamination of wastewater containing arsenic P. Lakshmipathiraj, S. Prabhakar, G. Bhaskar Raju ∗ National Metallurgical Laboratory (Madras Centre), CSIR Madras Complex, Taramani, Chennai 600 113, India

a r t i c l e

i n f o

Article history: Received 24 June 2009 Received in revised form 18 March 2010 Accepted 19 March 2010 Keywords: Electrocoagulation Zeta potential Removal Oxidation Arsenic Steel electrode

a b s t r a c t The arsenic removal from aqueous solutions by electrocoagulation (EC) using mild steel electrodes was studied. Effect of electrolytes such as NaCl, NaNO3 and Na2 SO4 on anodic dissolution of iron and in turn the arsenic removal was deliberated. The arsenic removal was observed to be 98% in the presence of NaCl whereas it is around 75% in the presence of Na2 SO4 and NaNO3 . The removal of arsenic by EC process was found to be almost similar irrespective of its oxidation state. Almost 95% of the total arsenic was removed within 5 min from its initial concentration of 10 mg L−1 . The precipitates formed during EC were characterized using FT-IR, SEM-EDAX, XRD, XPS and magnetometer. The iron oxy-hydroxide precipitate formed during EC was identified as maghemite and lepidocrocite. The magnetic property, particle size and surface properties of the iron oxy-hydroxide precipitate were found to be influenced by arsenic adsorption. The oxidation state of arsenic and iron in EC products was ascertained by XPS. It was observed that some amount of As3+ was converted to As5+ during EC. The electrokinetic and FT-IR measurements revealed the co-precipitation of arsenic by specific chemical interaction between arsenic species and iron oxide precipitate. The effect of initial As3+ concentration and current density on arsenic removal was also discussed. The TCLP test (USEPA method-1311) was conducted on EC products to assess their toxicity. It was confirmed that the solid waste generated during EC process is non-hazardous and can be safely disposed. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The contamination of groundwater by arsenic in parts of India, Bangladesh, Argentina, USA, Taiwan and Japan is reported to be alarming and unfit for human consumption [1]. Since continuous exposure to arsenic leads to lung cancer, neurological disorder and hyperkeratosis, the environmental protection agencies throughout the world has suggested that the arsenic content in drinking water should be less than 10 ␮g L−1 . While groundwater contamination of arsenic arises mainly due to the dissolution of arsenic minerals, the surface water is contaminated by arsenical pesticides and wood preservatives. The inorganic arsenic is reported to be more toxic than the organic counterparts and within inorganic species, arsenite (As3+ ) is 60 times more toxic than arsenate (As5+ ) [2]. The distribution of As3+ and As5+ in natural water depends upon the redox condition of the water system. In general, As5+ is more stable species under aerobic condition and exists as arsenic acid whereas As3+ is predominant species in anaerobic environment and exists as arseneous acid.

∗ Corresponding author. Tel.: +91 44 22542077; fax: +91 44 22541027. E-mail address: [email protected] (G.B. Raju). 1383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2010.03.009

Among the various methods explored for arsenic removal from groundwater, coagulation with iron or alum in the presence of cationic or anionic polyelectrolyte and subsequent filtration through bi-layered sand bed is very common [3]. Adsorption of arsenic on hematite [4], goethite [5], synthetic goethite [6], akaganeite-type nanocrystals [7], Ce (IV) doped iron oxide [8], iron oxide coated polymeric materials [9], granular ferric hydroxide [10], nanoparticles of zero-valent iron [11] was extensively studied. Since the adsorption of As3+ on natural minerals was observed to be relatively weak, pre-oxidation of As3+ to As5+ using ozone or chlorine was suggested to improve the removal [12]. However, the presence of excess oxidizing agents will affect the quality of drinking water. Therefore in situ oxidation of As3+ using manganese doped iron oxy-hydroxide [13] and Fe–Mn binary system consisting of iron and manganese ore were tried [14,15]. Though these materials were found to be effective for As3+ removal, very high quantity of adsorbent is required. Electocoagulation (EC) is an enigmatic electrochemical process that involves several unit operations namely flotation, coagulation, co-precipitation, particle entrapment, adsorption and redox reactions [16]. Hence, EC can be tried to remove colloidal/suspended as well as soluble inorganics and organics [17]. Depending on the pH and exposure to oxygen, metal oxides, oxy-hydroxides and hydroxides are formed in the reaction medium. In particular, EC is

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found to be successful for the removal of colloidal size suspended solids from industrial effluents [18] and restaurant wastewaters [19]. Very few researchers have reported the removal of arsenic from aqueous solutions using EC technique [20–22]. The National Metallurgical Laboratory, India has developed and patented an electrochemical gadget that proved very effective for the removal of arsenic in ground water. Though substantial research work has been reported on arsenic removal from aqueous solutions, the mechanism of arsenic removal is not yet well established. Furthermore, the toxicity and stability of the sludge generated during the EC process is to be investigated. In the present study, the EC product was characterized by X-ray diffraction (XRD), Vibrating Sample Magnetometer (VSM), X-ray photoelectron spectroscopy (XPS) and Fourier Transform Infrared (FT-IR) techniques. Also, the efficacy of electrocoagulation process, the mechanism of arsenic removal and the stability of the sludge by TCLP (USEPA method-1311) was assessed. 2. Materials and methods

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The detection limit of arsenic is 2 ␮g L−1 . The filtrate samples were stored in polyethylene containers and the analysis was performed on the same day of the experiment. Freshly prepared solutions were used for arsenic analysis. An average of three replicate readings was reported in the text. 2.5. Zeta potential measurement Zeta potential measurements were conducted using zeta-meter (3.0+ model of Zeta-meter Inc., USA). The zeta potential of iron oxyhydroxide (generated by anodic dissolution of mild steel in 0.01 M NaCl electrolyte) both in the presence and absence of arsenite and arsenate species was measured at different pH values. The iron oxy-hydroxide was equilibrated at different pH values for 30 min. The equilibrated slurry was injected in to the micro-electrophoresis cell using disposable syringes. Minimum of six readings with standard deviation of less than 2.0 was taken and the mean value was reported. Prior to each measurement, the electrophoresis cell was thoroughly washed and rinsed with de-ionized water followed by rinsing with the sample solution.

2.1. Reagents 2.6. Characterization of EC product All the chemicals used in the present study are of analytical grade. The test solutions of As3+ and As5+ were prepared from the standard NaAsO2 and H3 AsO4 (Merck-NIST Certified) using double distilled water. The pH of the aqueous solutions was adjusted using 0.1 M NaOH and 0.1 M HCl.

2.6.1. Fourier transform infrared (FT-IR) characterization The FT-IR spectra of the samples were recorded using PerkinElmer spectrophotometer. 10 mg of the dried sample was dispersed in 200 mg of spectroscopic grade KBr and 40 scans were collected for each spectrum at a resolution of 4 cm−1 .

2.2. Electrochemical cell A batch type electrochemical tank with an effective volume of 3.0 L was fabricated using Plexiglas. Mild steel rods assaying 99% Fe and each rod measuring 0.6 cm diameter and 11.0 cm length were used as electrodes. Six such rods connected to a common rod formed the anode assembly and an equal number of rods with similar arrangement formed the cathode assembly. The anode and cathode were arranged in the same plane. The entire electrode assembly (un-divided electrochemical cell) was seated on a nonconducting wedge fixed to the bottom plate of the cell. In order to avoid short-circuiting, non-conducting spacers were provided between the anode and cathode rods. The gap between anode and cathode was maintained at 2 mm to minimize the ohmic loss. The active anodic surface area of electrodes was estimated to be 198 cm2 . The detail of the electrochemical cell was given elsewhere [23,24]. 2.3. Experimentation Two liters of arsenic solution with known concentration (5–100 mg L−1 ) and pH (≈7.0) was taken in the electrochemical cell. The initial arsenic concentration of the working solution was chosen on the basis of arsenic contaminant present in the industrial wastewater [25]. The anode and cathode leads were connected to the DC rectifier equipped with digital ammeter and voltmeter. The solution was kept in agitation by using magnetic stirrer. After passing the required current (5.2–20.8 mA cm−2 ) for desired duration, sample was collected and filtered through 0.2 ␮m size membrane filter and filtrate was analyzed for arsenic content.

2.6.2. X-ray photoelectron spectroscopy (XPS) XPS of EC products were taken using KARTOS ESCA model AXIS 165 equipped with the monochromatic Al K␣ X-ray at 1487 eV and a facility of ultra high vacuum (10−9 Torr). For energy calibration, the carbon peak at 285 eV was used as an internal standard to shift all photoelectron lines to their correct binding energies. 2.6.3. Scanning electron microscope and energy dispersive X-ray (SEM-EDAX) analysis SEM-EDAX pattern were obtained using Scanning Electron Microscope coupled with Energy Dispersive X-ray Analyzer (HITACHI Model–S3400). The precipitates formed during EC (with and without arsenic species) were collected and dried at 80 ◦ C for 16 h prior to SEM-EDAX analysis. 2.6.4. X-ray diffraction (XRD), particle size and BET surface area analysis The powder XRD of the samples were recorded using SIEMENS X-ray diffractometer (Model: D500) using Co K␣ radiation with Fefilter at scan speed of 1◦ /min. The particle size distribution was measured using laser particle size analyzer (CILAS-1180). The surface area was estimated by nitrogen adsorption method using BET surface area analyzer (MICROMERETICS, ASAP 2020). 2.6.5. Magnetization measurements The magnetic strength of EC products was studied using Vibrating Sample Magnetometer (VSM Lakeshore 7404, USA) at room temperature. The magnetization measurements are recorded at an applied magnetic field of 12 KG.

2.4. Analysis

2.7. Toxicity characteristic leaching procedure (TCLP)

The precipitates generated during EC process were subjected to microwave digestion using 1:1 hydrochloric acid. The arsenic and iron content was analyzed using Atomic Absorption Spectrophotometer (AAS). A hydride generator kit (GBC-Avanta HG-2000) attached to AAS was used to estimate As3+ and total arsenic [26].

TCLP test as per the USEPA was carried out using TCLP apparatus supplied by Millipore Corporation (Model No. Y1320 RAHW). The precipitate generated during EC was dried at 80 ◦ C and the dried sample was gently grounded to obtain the particles with less than 1 mm size. The extraction fluid comprising 5.7 mL of glacial acetic

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acid, 64.3 mL of 1N NaOH and 930 mL distilled water was found to be appropriate for the extraction of arsenic bearing EC precipitate. The pH of the extraction fluid was measured to be 4.93 ± 0.05. A solid/liquid ratio of 1:20 and the extraction duration of 16 ± 2 h were maintained. 3. Results and discussion 3.1. Effect of electrolyte The effect of NaCl, Na2 SO4 and NaNO3 on As3+ removal was studied at a fixed current density of 5.2 mA cm−2 and an initial concentration of 25 mg L−1 . Since the concentration of chloride ion in groundwater vary between 200 and 700 mg L−1 , the working electrolyte concentration was maintained at 0.01 M. During the process of EC, samples were collected at different time intervals and the concentration of arsenic was determined. Removal of As3+ in the presence of different electrolytes and change in pH with time during EC is shown in Fig. 1. It is apparent that 98% of arsenic could be removed in the presence of NaCl whereas the removal is 80% and 75% in the presence of Na2 SO4 and NaNO3 respectively. This may be attributed to the formation of passive film in the presence of SO4 2− and NO3 − that hinders the effective dissolution of iron [27]. It was also observed that the pH was shifted from 7.0 to 10.5 during electrocoagulation. The electrochemical reactions of electrolysis are complex and are not entirely known for quantitative treatment. Nevertheless, the anodic dissolution of iron from steel electrodes can be represented as 2Fe → 2Fe2+ + 4e− In the presence of dissolved oxygen, dized to Fe3+ [28] according to Eq. (2). 2Fe2+ + 12 O2 + H2 O → 2Fe3+ + 2OH−

(1) Fe2+

ions are readily oxi(2)

In addition to the dissolution of iron, generation of oxygen can be expected from the oxidized surface as secondary electrochemical reaction. H2 O →

1 O 2 2

+ 2H+ + 2e−

(3)

Fig. 1. Effect of electrolyte on As3+ removal (current density: 5.2 mA cm−2 and electrolyte concentration: 0.01 M).

The simultaneous evolution of hydrogen from the cathode can be represented as 8H2 O + 8e− → 4H2 + 8OH−

(4)

The hydroxyl ions formed at the vicinity of cathode in turn react with Fe2+ and/or Fe3+ and form monomeric and polymeric iron oxyhydroxides that serve as coagulant. The XRD of iron precipitate produced during electrolysis is shown in Fig. 2a. The d-values of the XRD are found to match with maghemite. The magnetic property of the precipitate was also assessed and shown in Fig. 2b. The hysteresis loop clearly suggests that the iron precipitate is ferrimagnetic and its magnetization value is 68.73 emu g−1 . Thus it is evident that maghemite is formed during electrolysis by utilizing oxygen. 4Fe(OH)2(s) + O2(aq) → 2Fe2 O3(s) + 4H2 O

(5)

Fig. 2. (a) X-ray diffraction pattern of iron oxide precipitate in the absence and presence of arsenic ions (current density: 5.2 mA cm−2 , initial concentration of As3+ and As5+ : 100 mg L−1 , and NaCl: 0.01 M). (b) Magnetic hysteresis curve of iron precipitate in the absence and presence of arsenic ions at room temperature (current density: 5.2 mA cm−2 , initial concentration of As3+ and As5+ : 100 mg L−1 , and NaCl: 0.01 M).

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The shift in pH during the process could be attributed to formation of OH− at the cathode as represented in Eq. (4). At an open circuit potential, it is very difficult to obtain the oxide free surface of steel electrode. It has been reported that water and precursors of oxide film always exist on the electrode surface [29,30]. In the presence of NaCl, the chloride ion catalyzes the dissolution of the iron by diffusing into the oxide film [31,32]. Thus the chloride ion aids in preventing the passive film formation on electrode surface and enhance the effective dissolution of iron and in turn arsenic removal. The effectiveness of chloride ion on the removal of organics in wastewater was reported [33,34]. The anodic discharge of Cl2 as a secondary electrochemical reaction from metal oxide and in turn the formation of hypochlorite (OCl− ) and hypochlorous acid (HOCl) is also possible [34]. Consequently, As3+ and Fe2+ ions are oxidized to As5+ and Fe3+ . The removal of arsenic is comparatively less in the presence of Na2 SO4 and NaNO3 . Though the nitrates accelerate the corrosion of iron in acidic solutions, the effect is very marginal in neutral and alkaline solutions. The anomalous behavior of the nitrate ions in the alkaline solution at an open circuit potential was in good agreement with the earlier studies on anodic dissolution of aluminum by Pyun et al. [35]. It was reported that thin film of Al(NO3 )3 is formed on the electrode surface. This transitory compound would inhibit the anodic dissolution of aluminum. The influence of SO4 2− on corrosion of iron is very marginal compared to chloride and nitrate. The inhibition effect of the sulphate ions on the removal of arsenic was mainly accounted for reducing the surface area of the active electrode by the competitive adsorption of the sulphate ions with hydroxyl ions. Thus the resistance to the corrosion potential of iron electrode is increased, and in turn the effective removal of arsenic is affected. These observations are consistent with the earlier studies on effect of sulphate ions on anodic dissolution of aluminum electrode in 0.01 M of NaOH solution [35]. The similar behavior of the electrolytes was observed while removing hexavalent chromium [24] and humic substance [34] from wastewater using iron electrode system. The active chlorine formation in EC is very marginal [36] compared with that of electrooxidation process using dimensionally stable anodes [37]. Therefore, the possibility of formation of halogenated organic compounds in EC is limited. Hence the present study was not focused on the formation of halogenated organic compounds.

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Fig. 3. (a) Effect of initial concentration on removal of As3+ (current density: 5.2 mA cm−2 and NaCl: 0.01 M). (b) Effect of current density on removal of As3+ (initial concentration of As3+ : 25 mg L−1 and NaCl: 0.01 M).

3.2. Effect of initial concentration and current density

whereas it increased to 0.082 mg s−1 at 20.8 mA cm−2 . This clearly suggests that the removal of As3+ depends on the generation of Fe2+ from anode and the subsequent formation of ferric hydroxide complexes [20,28]. The fast and efficient removal of arsenic in EC process could be attributed to the simultaneous formations of Fe-As complex rather than that of adsorption of arsenic on to pre formed ferric hydroxides [6,7,13]. The following equation describes the relationship between the current density and the amount of

The effect of initial concentration of arsenic on its removal was studied at 5, 10, 25, 50, 75 and 100 mg L−1 of As3+ concentration at a constant current density of 5.2 mA cm−2 and 0.01 M NaCl. The results presented in Fig. 3a clearly suggest that the arsenic could be removed to the extent of 99%. The arsenic removal at the tenth minute of EC was observed to be 9.80, 19.0, 42.48, 50.04, 50.41 and 41.14 mg from its initial concentration of 10, 20, 50, 100, 150 and 200 mg respectively. The iron content in the precipitates collected at the tenth minute was simultaneously estimated and found to be around 115 mg. The ratio of Fe/As in the precipitate is around 2.0 in the initial stage. The partial removal of arsenic when it is present in very high concentration may be attributed to the insufficient generation of iron hydroxide. Thus it is evident that the removal of arsenic is governed by generation of iron hydroxide. The rate of arsenic removal is linear at the initial stage of the experiment and becomes exponential over a period of time. This indicates that the latter part of the adsorption process is controlled by diffusion. The removal of As3+ from its initial concentration of 25 mg L−1 was studied at different current densities ranging from 5.2 to 20.8 mA cm−2 . The results shown in Fig. 3b clearly suggests that the rate of arsenic removal increases by increasing the current density. For example, during the initial 10 min of electrolysis, the rate of arsenic removal at a current density of 5.2 mA cm−2 is 0.070 mg s−1

Fig. 4. Effect of arsenic oxidation state +3 and +5 on removal by electrocoagulation (current density: 5.2 mA cm−2 , initial concentration of As3+ and As5+ : 10 mg L−1 and NaCl: 0.01 M.

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metal ion liberated during the course of electrolysis. m=

Mjt ZF

(6)

where m is the theoretical mass of iron produced per unit surface area by current densities j passed for duration of time t. Z is number of electron involved in the electrochemical reaction. M is atomic weight of anode material and F is Faraday’s constant. At higher current densities more bubbles are generated which would enhance the homogeneous mass transport [38,39] as well as the removal rate of arsenic. By increasing the current density from 5.2 to 20.8 mA cm−2 the power consumption was increased to 0.22 Wh g−1 from its initial value of 0.017 Wh g−1 . The

rate of removal was observed to follow first order kinetics and the rate constant (k) values were proportionally increased from 0.094 to 0.35 min−1 by increasing the current density from 5.2 to 20.8 mA cm−2 . 3.3. Removal of As3+ and As5+ The removal of As3+ and As5+ by electrocoagulation was studied separately at a fixed current density of 5.2 mA cm−2 and 0.01 M NaCl. Separate solutions of As3+ and As5+ with an initial concentration of 10 mg L−1 were taken and their removal by EC was attempted. From the results presented in Fig. 4, it is apparent that most of the arsenic is removed within 5 min. It is also evident that

Fig. 5. SEM-EDAX pattern of EC products in the absence and presence of arsenic ions. (a) Iron precipitate, (b) iron precipitate in the presence of As3+ , and (c) iron precipitate in the presence of As5+ (current density: 5.2 mA cm−2 , initial concentration of As3+ and As5+ : 100 mg L−1 , and NaCl: 0.01 M).

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the removal of both As3+ and As5+ is almost same expect that the As5+ removal is slightly better at the initial stage. The oxidation of As3+ during EC was monitored by measuring the concentration of As3+ and As5+ in aqueous solution and the results are presented in Fig. 4 (inset). The results indicate the presence of As5+ to the extent of 800 ␮g L−1 at the beginning of EC process confirming the in situ oxidation of As3+ to As5+ . The iron oxides synthesized by electrochemical process were tried as adsorbents for the removal of As3+ and As5+ separately [20,40]. The adsorption results revealed that the removal of As5+ was profound, whereas As3+ was very marginal. Thorough removal of As3+ during EC process clearly suggests the in situ oxidation of As3+ to As5+ . The magnetic hysteresis curves for the iron oxy-hydroxide precipitate formed in the presence of As3+ and As5+ solutions at room temperature were recorded and the results are shown in Fig. 2b. The magnetic property of these precipitates was observed to be very low (only 0.49 and 1.17 emu g−1 ) when compared to the iron oxyhydroxide precipitate formed in the absence of arsenic. The weak magnetization of the precipitate exposed to arsenate and arsenite solutions could be explained due to the incorporation of arsenic ions into the interstitial space of the maghemite structure. It could be seen from the hysteresis loop that the magnetization has reached saturation in the case of iron precipitate whereas it has not reached saturation in the case of iron precipitate formed in the presence of arsenic. The XRD of the precipitates (Fig. 2a) formed in the presence of arsenic reflects the amorphous nature of the material. The poor magnetization coupled with amorphous nature of the precipitates could be ascribed to the incorporation of arsenic ions within maghemite. 3.4. SEM-EDAX analysis

Fig. 6. XPS spectrum of EC products. (a) As (3d) spectrum of iron precipitate and (b) Fe (2p) spectrum of iron precipitate (current density: 5.2 mA cm−2 , initial concentration of As3+ and As5+ : 1000 mg L−1 , and NaCl: 0.01 M).

The SEM-EDAX pattern (Fig. 5) and the particle size distribution of iron oxy-hydroxide precipitate in the presence of As3+ and As5+ were recorded. The average grain size and surface area of iron oxyhydroxide precipitate was observed to be 7.13 ␮m and 20 m2 g−1 . It was also observed that 50% of the particles are below 1.0 ␮m. The mean grain size was increased to 35 ␮m in the presence of arsenic. The SEM images clearly suggest the agglomeration of iron precipitate in the presence of arsenic. The presence of arsenic peak in the EDAX pattern of iron precipitate formed in the presence of As3+ and As5+ support the inclusion of arsenic in iron precipitate. Deliyanni et al. [7] reported that the size of the akaganite nanocrystals was increased in the presence of As5+ and the increase in size was attributed to the formation of iron arsenate. Based on the above evidence, it could be concluded that arsenic is co-precipitated with iron either by adsorption or by complex formation.

energy of As3+ and As5+ in As2 O3 [43] and As2 O5 [42]. It indicates that the As3+ is oxidized to As5+ during EC. The oxidation of As3+ to As5+ could be attributed due to oxygen and active chlorine evolved from the anode. XPS spectrum of Fe (2p) region for iron precipitate formed in the presence of As5+ and As3+ solutions (1000 mg L−1 ) is shown in Fig. 6b. Iron oxy-hydroxide precipitate formed in the presence of As5+ and As3+ solutions have exhibited single peak at 710.4 eV. This is attributed to the characteristic binding energy of Fe (III) in Fe–O bond [21,44].

3.5. X-ray photoelectron spectroscopy studies The oxidation state of arsenic and iron in the precipitate formed during EC process was identified using XPS. The concentration of As5+ and As3+ solutions was maintained at 1000 mg L−1 . The XPS spectrum of As (3d) and Fe (2p) is shown in Fig. 6. The binding energies associated with As (3d) spectra of iron precipitate formed in the presence of As5+ and As3+ are shown in Fig. 6a. The precipitate obtained in the presence of As5+ solution has exhibited a peak around 46.4 eV which can be attributed to the characteristic binding energy of As2 O5 [21,41,42]. The corresponding binding energies of As3+ or As0 were not detected. Hence it could be concluded that As5+ was not reduced during the process of adsorption. Conversely, As (3d) broad photoelectron spectrum of the precipitate formed in the presence of As3+ has resulted in two peaks at the binding energies of 45.4 eV and 46.1 eV. These two binding energies identified in the region of As (3d) spectrum could be attributed to the binding

3.6. FT-IR characterization The FT-IR spectra of As2 O3 , As2 O5 , and iron precipitate generated in the absence and presence of As3+ and As5+ are shown in Fig. 7. The prominent band at 800 cm−1 shown in Fig. 7a and 906, 853 and 809 cm−1 presented in Fig. 7b could be assigned to the stretching vibration of As–O bond in As2 O3 and As2 O5 [45]. The spectrum of iron oxy-hydroxide shown in Fig. 7c, the bands at 3779 cm−1 and the broad band at 3435 cm−1 are attributed to the stretching vibration of free hydroxyl group and hydrogen bonded hydroxyl group. The bands at 1026 and 861 cm−1 are the characteristic bands of lepidocrocite. The intense band at 633 and 587 cm−1 could be attributed to the stretching vibration of Fe–O bond. The bands in the frequency range of 831–826 and 814–808 cm−1 could be assigned to As–O–Fe groups and As–O stretching vibration of As O4 −3 (Fig. 7d and e). This indicates the oxidation of arsenite ion prior to the formation of surface complex with Fe3+ generated during the EC process. The spectral data is in close agreement with the data reported earlier [21,46].

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crete mineral phase such as magnetite and lepidocrocite [47]. In this case, the surface would be negatively charged beyond pH 7.8. The iep was found to shift towards acidic region in the presence of arsenic. The shift in iep towards acidic region could be attributed to the specific interaction of arsenic species with iron oxide precipitate. The pKa values of H3 AsO4 (2.3, 7.0, and 11.5) and H3 AsO3 (9.23, 12.13 and 13.4) suggests that the arsenic species exists as anions beyond pH 2.3 and 9.23. The following interactions were proposed between iron oxy-hydroxide and arsenic species. FeOOH + H2 AsO4 − → FeOHAsO4 − + H2 O −

FeOOH + H2 AsO4 → FeOH2 AsO4 + OH

−

3FeOOH + HAsO4 2− → (FeO)3 AsO4 − + H2 O + 2OH−

(7) (8) (9)

Fe(OH)2 + H2 AsO4 − → FeAsO4 + 2H2 O

(10)

FeOOH + H3 AsO3 → FeOH2 AsO3 + H2 O

(11)

−

−

FeOOH + H2 AsO3 → FeOHAsO3 + H2 O FeOOH + HAsO3

2−

→ FeOAsO3

2−

+ H2 O

(12) (13)

The negatively charged arsenic species could interact with iron oxy-hydroxide by ion exchange as depicted in Eqs. (8) and (9). Simultaneous de-protonation of arsenic species and reaction with iron hydroxide by condensation mechanism could be suggested as shown in Eqs. (7), (10)–(13). 3.8. TCLP test (method-1311)

Fig. 7. FT-IR spectra of EC products. (a) As2 O3 , (b) As2 O5 , (c) iron precipitate, (d) iron precipitate in the presence of As3+ , and (e) iron precipitate in the presence of As5+ (current density: 5.2 mA cm−2 , initial concentration As3+ and As5+ : 100 mg L−1 , and NaCl: 0.01 M).

3.7. Zeta potential studies Zeta potential is a promising tool to understand the interaction between liquid and solid surface. Zeta potential of iron oxy-hydroxide both in the absence and presence of As3+ and As5+ solutions are presented in Fig. 8. The isoelectric point (iep) of iron oxy-hydroxide formed in the absence of arsenic was observed around the pH of 7.8. This value is consistent with the iep of dis-

TCLP is a testing procedure prescribed by USEPA (method-1311), which is mainly used to describe whether the waste (liquid, solid and wastes with multi phase) is hazardous or non-hazardous prior to disposal off or land filling. The TCLP tests were carried out on the precipitates formed during EC process to ascertain the hazardous nature of the waste. The ratio of Fe/As in these precipitates is 2.34, 2.02, 1.99 and 2.19. The respective initial concentration of As3+ is 25, 50, 75 and 100 mg L−1 . The concentrations of total arsenic from the leachate collected from each TCLP extraction are 48, 57, 60 and 58 ␮g L−1 which is well below the TCLP limit (The permissible limit of TCLP concentration for arsenic is 5 mg L−1 ). Thus the results of TCLP test confirm that the solid waste generated during EC process is not hazardous to the environment. It may be noted that the amount of solid waste formation is very less in the EC process. 4. Conclusion The study clearly indicates the amenability of electrocoagulation process for the effective removal of arsenic either in the form of arsenite or arsenate. During the EC process, discrete minerals such as magnetite and lepidocrocite are formed due to anodic dissolution of steel. The As3+ was found to be oxidized to As5+ during the process of EC. The complete removal of arsenic in the presence of NaCl as supporting electrolyte could be attributed to the prevention of passive film formation on the electrode surface by pitting corrosion and catalytic dissolution of the iron. The X-ray Photoelectron Spectroscopy and zeta-potential studies indicate specific chemical interaction between arsenic and iron oxy-hydroxide. Condensation mechanism was suggested to describe the interaction between arsenic and iron oxy-hydroxide at neutral pH. The stability of the solid waste generated during EC process is found to be non-hazardous to the environment. Acknowledgements

Fig. 8. Zeta potential of EC product in the absence and presence of As3+ and As5+ .

The authors are thankful to the Director, National Metallurgical Laboratory for his permission to publish this work. One of the

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