Abstract
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).
HIGHLIGHTS
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
INTRODUCTION
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.
MATERIALS AND METHODS
Composition of simulated groundwater (SGW) used in the present study
As(tot) (ppb) . | Ca2+ (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) . | ![]() | Mg2+ (mg/L) . | pH . |
---|---|---|---|---|
500 [As(III):As(V) = 1:1] | 100 | 500 | 39 | 7 ± 0.2 |
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.
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.
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).
RESULTS AND DISCUSSION
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.
Performance of the continuous granular media filter run, Run-1 and Run- 2.
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.
CONCLUSIONS
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.
ACKNOWLEDGEMENTS
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.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.