Drinking water contaminated with arsenic is a threat to human health. The present study attempts to remove arsenic using electrocoagulation with iron electrodes (ECFe) in continuous flow mode. Two experimental runs were separately conducted using untreated and treated (acid treatment followed by pH neutralization step) hematite as granular bed. The treatment of the hematite formed ferric oxy-hydroxides on hematite surface which was beneficial for arsenic removal. Total arsenic concentration reduced below 10 ppb from initial concentration of 500 ppb [As(III): As(V) = 1:1] with Fe dose of 5 mg/L after 48 h and 2 h of run conducted with untreated and treated hematite granular bed, respectively. The required Fe/As ratio of 10 was much lesser than the reported requirement of 250 in conventional systems. In the filter prototype using market-available filter cartridge, arsenic concentration reduced below 10 ppb from an initial concentration of 500 ppb [As(III): As(V) of 1:1] in both the absence and presence of phosphate (2 ppm) and silicate (30 ppm).

  • The present study attempts to develop a household arsenic removal unit based on electrical system and natural hematite.

  • The designed prototype filter in the current study reduced As(tot) below 10 ppb from initial concentration of 500 ppb in the presence of phosphate (2 ppm) and silicate (30 ppm).

Graphical Abstract

Graphical Abstract

Arsenic in groundwater is one of the major geogenic contaminants causing worldwide concern for decades. Arsenite [As(III)] and arsenate [As(V)] are the two species of arsenic commonly found in the natural groundwater. Arsenic is a proven carcinogen and has several adverse health effects (Yadav et al. 2021). World Health Organization (WHO) has recommended the permissible limit of arsenic in drinking water as 10 ppb (WHO 2004). As(III) is more toxic and exists as H3AsO3, a neutral species in natural groundwater pH range and is more difficult to remove as compared to As(V). Availability of household filters to consistently remove arsenic below 10 ppb are scarce. The available household arsenic removal units mainly use ferrous iron [Fe(II)] which is only available in groundwater or, zero valent iron (ZVI) as a source of Fe(II). Iron dissolution from ZVI is uncontrolled, inconsistent and gets reduced with time due to passivation of the iron nails or other forms of ZVI used. On the other hand, Fe(II) present in the groundwater may not be sufficient to consistently remove arsenic concentration below 10 ppb. The household arsenic removal column filter developed by Maiti et al. (2010) using raw and treated laterite as granular media, was reported to reduce total arsenic [As(tot)] concentration below 10 ppb from initial concentration of 1,000 ppb [50–60% As(III)]. However, the filtration rate of the laterite filter was quite low (0.12–0.16 m2/m3·h), which further decreased by 30% in 1.5 years of operation. The filter only provided 7.2 L of water after an operation period of 4.8 h per day. The arsenic removal in the laterite filter was also assisted by the iron concentration present in the groundwater, which was in the range of 2.5–4.5 mg/L. SONO (Hussam & Munir 2007) and Kanchan arsenic filter (Ngai et al. 2007) are also two household filters developed to remove arsenic using composite iron matrix and iron nails, respectively. It was reported that the flow rate of SONO filter may decrease by 20–30% per year, if the groundwater contains >5 mg/L of iron. In Kanchan filter, only the dose of iron added from the iron nails (10–20 mg/L) was reported. However, the Fe(II) present in the groundwater of the studied areas, participating in the arsenic removal was not discussed. As(tot) concentration below 10 ppb was inconsistently achieved in Kanchan filter in the places having initial As(tot) concentration above 200 ppb. Therefore, it is important to develop a household arsenic removal technology to consistently provide potable water with arsenic concentration below 10 ppb. Among various arsenic removal technologies, electrocoagulation with iron electrodes (ECFe) exhibits distinctive advantages, such as oxidation of As(III) to As(V) without addition of any external oxidant, controlled addition of iron dose by controlling applied current, ease and flexibility of operation, and less sludge production. In ECFe, a direct current (DC) is applied to a circuit of two submerged mild steel plates to dissolve Fe(II) from the anode. Oxidation of Fe(II) to Fe(III) by dissolved oxygen (DO) produces a reactive intermediate species, ferrate [Fe (IV)], which co-oxidizes As(III) to As(V) (Li et al. 2012). As(V) is present during the hydrolysis and precipitation of iron oxy-hydroxide and Fe(III)-As precipitates are formed, which is similar to co-precipitation (Banerji & Chaudhari 2016). While planning an arsenic removal system that can operate in continuous flow mode, selection of a granular filter media is a key parameter. Various researchers have used different natural iron minerals such as laterite (Maiti et al. 2010), siderite and hematite (Guo et al. 2008), etc. for continuous flow column studies. Since most of the iron minerals are crystalline in structure, their affinity for arsenic adsorption is relatively low (Guo et al. 2007) which might require different chemical treatment for better arsenic removal. In a study conducted by Maiti et al. (2010), the acid treatment followed by base treatment of laterite has been suggested. They have reported the leaching of iron ions into the aqueous phase from solid mineral during the acid treatment and formation of iron-oxyhydroxide (FeOOH) precipitates on the mineral surface in the following base treatment step of laterite. They have also reported that this type of treatment offers advantage of loading iron oxyhydroxides on to the solid particles of iron minerals that leads to better separation of particles from the aqueous phase.

The present study attempts to develop a household arsenic removal unit based on electrical system, where the arsenic containing simulated groundwater was passed through the ECFe cell containing mild steel electrodes followed by a granular filter bed made of hematite. A controlled and consistent Fe(II) dose was added by applying current to the electrode plates. The controlled oxidation Fe(II) (coming from the ECFe cell) to Fe(III) was obtained on the granular bed. The formed Fe(III) precipitates separated along the filter bed; which may promote improved arsenic removal. Efforts were made to develop a prototype for arsenic removal system that can consistently remove arsenic in continuous flow mode with electrocoagulation with iron electrodes (ECFe), followed by granular filter bed of untreated and treated natural hematite.

Laboratory grade reagents were used in the present study. Arsenite and arsenate stock solutions (1,000 ppm) were prepared from sodium arsenite and sodium arsenate, respectively, and diluted to prepare working standards. As(tot) and As(V) were analysed by molybdenum blue method (Dhar et al. 2004). Both total iron [Fe(tot)] and ferrous iron [Fe(II)] and phosphate-phosphorous (PO4-P) were analysed by 1,10 phenanthroline and stannous chloride method, respectively, as described in Standards Methods for Examination of Water and Waste Water (APHA 1998). Hematite particles in the size range of 2.0–3.3 mm, were filled in an acrylic column having inner diameter of 1.4 cm [schematic shown in Figure 1(a)]. Simulated groundwater (SGW) was used for in the present study was based on Roberts et al. (2004) and the composition is shown in Table 1. The DO concentration of the inflow was around 2–3 mg/L. During the treatment of the hematite particles, 20 g of washed and dried natural hematite was taken and 80 mL, 3% HCl was added to it, and the mixture was shaken in an end-to-end shaker for 24 h. Then, 16% NaOH was added with continuous stirring to the mixture of acid and hematite to raise the pH to 7 ± 0.2. Then the whole mixture was washed twice, followed by centrifugation to separate the mixture and the supernatant liquid. Finally, the slurry with hematite particles were dried at 80 °C and sieved to get the desired size range. The natural and treated hematite particles were characterized using X-ray diffraction (XRD) analysis (PANalytical, X'Pert Pro with Cu-Kα source) and X-ray photoelectron spectroscopy (XPS) analysis (Kratos Analytical, AXIS Supra spectrophotometer).
Table 1

Composition of simulated groundwater (SGW) used in the present study

As(tot) (ppb)Ca2+ (mg/L) (mg/L)Mg2+ (mg/L)pH
500 [As(III):As(V) = 1:1] 100 500 39 7 ± 0.2 
As(tot) (ppb)Ca2+ (mg/L) (mg/L)Mg2+ (mg/L)pH
500 [As(III):As(V) = 1:1] 100 500 39 7 ± 0.2 
Figure 1

Schematic of the (a) continuous flow mode (column) study (Run-1 and Run-2) and (b) laboratory set-up of the prototype filter with market available cartridge.

Figure 1

Schematic of the (a) continuous flow mode (column) study (Run-1 and Run-2) and (b) laboratory set-up of the prototype filter with market available cartridge.

Close modal

In experimental Run-1, arsenic containing water was continuously passed through the ECFe cell followed by only untreated hematite bed. If the Fe(III) precipitate is present in the upper half of the granular filter, the incoming Fe(II) would be readily adsorbed in the upper regions of the filter only, which may lead to unavailability of Fe(II) for the rest of the filter length, that could be detrimental for the arsenic removal efficiency. Therefore, Run-2 was performed separately, by filling the upper and lower half (27.5 cm) of the granular filter with untreated and treated (3% HCl, followed by pH neutralization with 16% NaOH) hematite, respectively. The filtration rate was 1 m3/m2.h in both the experimental runs. The lab-based prototype was designed by filling the lower 7 cm and upper 9 cm of a market-available filter cartridge (inner diameter 6.4 cm and length 21 cm) with untreated and treated hematite, respectively, and shown in Figure 1(b). For the lab-based prototype the filtration rate was 0.5 m3/m2.h. Arsenic removal efficiency of the prototype was also observed in the presence of 2 ppm phosphate (PO4-P) and 30 ppm silicate (Si).

Material characterisation

The XPS spectra obtained for untreated hematite in the present study were matched with literature (Ferretto & Glisenti 2002), and the peak present at 709 and 529 eV, for Fe 2p and O 1 s, respectively, suggested that the material was hematite. Similarly, the characteristic peaks obtained during the XRD analysis of the untreated natural hematite was compared with available literature (Das et al. 2011) and peaks present at 25°, 39°, and 42° further confirmed that the material was hematite. For treated (acid treatment followed by pH neutralisation) hematite, presence of characteristic peaks at 34° and 61° suggested the formation of 2-line ferrihydrite or hydrous ferric oxides (Das et al. 2011) during the treatment.

Continuous flow column Run

Experiments were conducted in two different continuous column runs (Run-1 and Run-2). In Run-1, granular filter bed of only untreated hematite was used. Whereas, in Run-2, upper half and lower half of the granular filter bed were filled with untreated and treated hematite, respectively.

In both the experimental Run-1 and Run-2, after 32 h of continuous run, the inflow to the columns was stopped for overnight. After first overnight stoppage, the column run was continued for 8 h every day. The stoppage of the inflow to the column has been shown in Figure 2 using solid black vertical arrows. The black horizontal line shows the WHO guideline value of 10 ppb As(tot) concentration in drinking water. Figure 2 shows that, experimental Run- 1 reduced initial As(tot) concentration of 500 ppb to below 10 ppb in the treated water after 48 h of column run (2nd stoppage in the continuous column run) with an Fe dose of 5 mg/L. It was also observed that the oxidation of As(III) improved from 94% after 2 h, to 100% after the first break at 32 h. Whereas, in Run- 2, As(tot) concentration in the treated water decreased below 10 ppb after 2 h of run, for the same iron dose of 5 mg/L. In both the experimental runs the total iron concentration was below detectable limit in the treated water, which was possibly due to the efficient Fe(II) adsorption on the granular bed of hematite. It was reported that hematite can adsorb Fe(II) (Larese-Casanova & Scherer 2007). The controlled oxidation of Fe(II) (coming from the ECFe cell) by dissolved oxygen (DO) produced reactive intermediate species, possibly ferrate (Fe(IV)) (Li et al. 2012). Now, all the three reactants, DO, Fe(II), and As(III), are now in close proximity at the interface of hematite, which can possibly catalyse the As(III) oxidation reactions through efficient electron transfer, resulting in efficient arsenic removal. It was discussed by Banerji & Chaudhari (2016), that Fe(III) precipitates can adsorb DO, Fe(II) and As(III), therefore, presence of interface can catalyse As(III) oxidation reactions at the interface. Most of the oxidation of Fe(II) was occurring on the hematite granular bed and the Fe(III) precipitates were retained on the bed, resulting in efficient utilization of Fe(IV) for oxidation of As(III) to As(V), without any external chemical oxidant. The stoppage of inflow to the columns allowed the time for the oxidation of the retained Fe(II) on the filter bed and formation of Fe(III) deposits with fresh reactive sites for further arsenic removal. Hence, as the column run progressed, arsenic removal improved in both Run-1 and Run-2. It is evident that arsenic removal efficiency of the granular bed used in experimental Run-2 was better as compared to Run-1, which is possibly due to the formation of hydrous ferric oxides (HFO) on the hematite during the pH neutralization step after the acid treatment, which was also confirmed using the XRD analysis. In the case of Run-2, HFO was already present onto the hematite bed from the starting of the run, which reduced the initial time required for formation of fresh Fe(III) precipitates on the hematite granular bed to achieve As(tot) concentration below 10 ppb at the outlet, as observed in Run-1.
Figure 2

Performance of the continuous granular media filter run, Run-1 and Run- 2.

Figure 2

Performance of the continuous granular media filter run, Run-1 and Run- 2.

Close modal
Further efforts were made to design a prototype filter which can remove arsenic in continuous flow mode using both untreated and treated (acid treatment followed by pH neutralization) hematite as granular bed. The results are shown in Figure 3. In Figure 3, black dotted vertical arrows show addition of 2 ppm PO4-P and 30 ppm Si and solid black arrow shows the change in the Fe dose from 5 mg/L to 6 mg/L. It was observed that in the lab-based prototype, As(tot) in the treated water was consistently below 10 ppb with Fe addition of 5 mg/L from ECFe cell. It is well-reported that phosphate (PO4-P) and silicate (Si) are abundantly present along with arsenic in the groundwater of the Ganga-Meghna-Brahmaputra (GMB) plain (BGS & DPHE 2001) and they adversely affect arsenic removal in iron-based systems (Roberts et al. 2004; Wan et al. 2011). Therefore, arsenic removal efficiency of the lab-based prototype was also evaluated in the presence of phosphate and silicate. After passing 61.5 L of arsenic containing simulated groundwater, PO4-P (2 ppm) and Si (30 ppm) were added to the influent simulated groundwater. The results are presented in Figure 3. In the presence of PO4-P (2 ppm) and Si (30 ppm), with addition of 5 mg/L Fe dose, As(tot) concentration did not reduce below 100 ppb in the treated water. The results suggested that 5 mg/L iron dose was not sufficient for removing arsenic in the presence of both phosphate and silicate. Phosphate has similar structure and affinity as As(V) to bind with Fe(III) precipitates, thus compete with arsenate for potential complexation sites (Jain & Loeppert 2000). Silica physically blocks access to the adsorption sites due to its polymerization with mineral surfaces, thus reduces arsenic removal (Davis et al. 2001; Zeng et al. 2008). Therefore, a higher Fe dose of 6 mg/L was provided after passing 77.7 L of simulated groundwater. It was observed that As(tot) concentration at the outlet started reducing with the increased dose of iron. After passing 83 L of water, As(tot) concentration in the treated water was consistently below 10 ppb until the end of the experimental run.
Figure 3

Performance of the lab-based prototype filter.

Figure 3

Performance of the lab-based prototype filter.

Close modal

In the present study, the lab-based prototype filter was operational until the stoppage of the experiment. The amount of Fe required to remove per unit weight of arsenic (Fe/As ratio) in order to achieve below 10 ppb arsenic concentration in the treated water, indicates the arsenic removal efficiency of any technology. Small Fe/As ratio indicates requirement of low dosages of iron to remove arsenic concentration below 10 ppb in the treated water, which will in turn produce less amount of arsenic laden sludge in the system. The designed prototype filter in the current study reduced As(tot) below 10 ppb from initial concentration of 500 ppb in the presence of phosphate (2 ppm) and silicate (30 ppm) with Fe/As ratio of 12, which was much lesser than the reported requirement of 250 mg Fe/mg As to achieve below 10 ppb arsenic concentration in the treated water (Berg et al. 2006). The Fe/As ratio required in the present study was even lesser than the Fe/As of 45–101 as reported in the literature (Kobya et al. 2011; Amrose et al. 2014; Banerji & Chaudhari 2016) to achieve below 10 ppb arsenic concentration by batch electrocoagulation studies using iron electrodes.

Continuous flow operation with ECFe followed by granular media filter of natural hematite is a novel approach to remove arsenic below 10 ppb from an initial concentration of 500 ppb with Fe/As ratio of 10, which is less than required Fe/As ratio reported in literature. Continuous column run with both untreated and treated hematite in the upper and lower half of the column, respectively, performed better than the column run conducted with only untreated hematite. The designed lab-based prototype consistently removed arsenic concentration below 10 ppb from an initial concentration of 500 ppb with Fe addition of 5 mg/L from the ECFe cell. In the presence of phosphate (2 ppm) and silicate (30 ppm) Fe/As ratio of 12 was needed to achieve below 10 ppb arsenic concentration in the treated water. Efforts can be made to scale-up the lab-based prototype to remove arsenic in actual field conditions.

Authors are thankful to the Ministry of Human Resource Development (MHRD) and Department of Science and Technology (DST), India. We are also grateful for the facilities provided by the Environmental Science and Engineering Department (ESED) and Sophisticated Analytical Instrument Facility (SAIF), Indian Institute of Technology Bombay.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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