Advances in Arsenic Research - American Chemical Society

Chapter 26

Removal of Arsenic from Bangladesh Groundwater with Zero-Valent Iron 1

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Downloaded by FUDAN UNIV on February 20, 2017 | http://pubs.acs.org Publication Date: October 3, 2005 | doi: 10.1021/bk-2005-0915.ch026

Nikolaos P. Nikolaidis , Zhongqi Cheng, and Alexander van Geen 1

Department of Environmental Engineering, Technical University of Crete, Chania, Greece Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964 2

Cartridges containing zero-valent iron filings mixed with sand were tested in Bangladesh to evaluate their effectiveness in removing arsenic from groundwater. The experiments were conducted by pumping, respectively, 1830 and 1140 L of groundwater over the course of a week from two wells containing 560 and 290 μg/L As. The set-up included two cartridges connected in series for As removal, and a downstream aeration vessel followed by a nylon filter to reduce Fe concentrations in the effluent. After conditioning the cartridges for 3-4 days, As concentrations in the effluent consistently remained at or below the W H O guideline of 10 μg/L. Groundwater treatment with iron filings may therefore be a viable technology for arsenic mitigation in Bangladesh.

Dissolved arsenic concentrations in groundwater that are higher than the recommended W H O 10 μg/L guideline for drinking water have been reported in many parts of the world, notably Bangladesh, West Bengal of India, Taiwan, Argentina, Mexico as well as the United Staes (1-8). Poorly-understood natural processes contribute to elevated groundwater As levels across a wide range of geological settings, for example, alluvial floodplains and deltas in Bangladesh

© 2005 American Chemical Society

O'Day et al.; Advances in Arsenic Research ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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362 and West Bengal, loess aquifers in Central Argentina, glacial tills in the northeastern and midwestern US, and hydrothermal systems in California and New Mexico. The reliance on groundwater for human water supply in many parts of the world requires the development of reliable and affordable methods for removing As. One method, coined "Arsenic Remediation Technology" (AsRT, US Patent 6,387,276 B l ) , which is based on iron filings mixed with sand has been developed at the University of Connecticut (9). Field tests conducted in the US (Maine, and New Jersey) (10,11), Greece (12), Argentina and Switzerland (unpublished data) have demonstrated that iron filings can reduce As concentrations to below 10 μg/L for groundwaters of very different composition. The mechanisms of As removal by iron-filings are only partially understood; likely processes include surface precipitation, coprecipitation, and surface complexation (9,10,13). The present study describes the performance of two household-scale As removal units that were deployed in Bangladesh. A significant potential advantage of this approach over other methods for removing As from well water at the household-level (14-16) is that little labor would be required for routine use. The experiments were conducted as part of an interdisciplinary program to determine the causes for elevated As concentrations in Bangladesh groundwater, evaluate various mitigation options, and quantify the health impact of past exposure (http://superfiind.ciesin.columbia.edu).

Methodology Two units were deployed for about a week each at two different wells in January 2001. Each unit consisted of two cartridges connected in series, followed by an aeration vessel and a filtration cartridge (Figure 1). The first two cartridges each contained 1.9 kg of sand and 2.4 kg of iron filings (1:1 by volume); the third cartridge contained a nylon filter. The total volume of the iron-filing/sand media in each cartridge was 2.7 L ; the estimated porosity -0.45. Iron filings were purchased from Connelly-GPM, Inc. of Chicago, IL (stock number CC-1004). The silica sand was a #4 Q-Rok mined by U.S. Silica in Berkley Springs, W V . Well water was pumped through each system at 130-150 mL/min (residence times of 8-9 minutes in the media) by a portable peristaltic pump with three heads (Masterflex). One head pumped water from the well through the first two cartridges and into the aeration vessel. The second head continuously bubbled air into the aeration vessel through a glass frit to oxidize and precipitate ferrous iron released by the filings. The third head pumped the water containing high levels of Fe-oxyhydroxide particles floes, from the aeration vessel through the last cartridge containing a standard 5-μπι nylon filter.



363 Samples were collected daily at the inlet of the system, the outlet of the second iron filing column (i.e. before the aeration vessel), and the outlet of the nylon filter. Temperature, pH, conductivity, the oxidation-reduction potential (ORP), and dissolved oxygen were also measured daily at the inlet and outlet with an ORION meter. A l l samples were acidified to 1% HC1 (OPTIMA) on site. Phosphate concentrations for all samples were measured by colorimetry in the field using colorimetric molybdate-blue method (77). Samples from the inlet and outlet were analyzed for both Ρ and Fe by inductively coupled plasma atomic emission spectrometry with coupled plasma Perkin Elmer Optima 3300XL instrument two months later at the University of Connecticut (UConn). Arsenic was measured at Uconn by hydride generation flame atomic adsorption spectrometry (HG-FAA) using a Perkin-Elmer Model 5100 atomic adsorption spectrometer equipped with an electrodeless discharge lamp (EPA method 6010), with detection limit at 4 μg/L. Inlet and outlet samples collected on January 11 and 18 were also analyzed by high resolution inductively coupled plasma-mass spectrometry (HR ICP-MS) in December 2001 at Lamont Doherty Earth Observatory for a broad suite of inorganic constituents (Na, Mg, Si, K , Ca, Mn, M o , Se, A l , S, Zn, Sr, Sb, Ba, Cd, Cr, Cu, Pb, Zn, N i and Cu), some of which are of potential health concern. The detection limit of this method for As is -0.1 μg/L (18). The composition of the mixed iron-filing/sand mixture was also determined with H R ICP-MS after disgestion with mixed H F - H N 0 acid. 3

Figure 1. AsRT unit setup.


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Results The inlet As, Fe and Ρ concentrations of the two AsRT units did not vary significantly during the deployment period (Figures 2, 3). Arsenic inflow concentrations of 560 and 290 μg/L for AsRT units A and B , respectively, were also not significantly different from concentrations obtained for the same wells (IDs 4133 and 4146) as part of a large survey conducted a year earlier (18). ORP measurements suggest that unit Β was tapping a more reducing aquifer; dissolved Fe and Ρ concentrations were significantly higher and S levels were lower compared to water at the inlet of unit A (Table I). O f all the inorganic constituents other than As that were analyzed for the inflow of both units, only M n concentrations exceeded the corresponding W H O guideline value for drinking water of 0.5 mg/L (Table I). A s R T units A and Β treated 1,830 L and 1,140 L of well water over 8 and 6 days, respectively. During the first few days of the deployment, H G F A A and H R ICP-MS measurements indicated detectable (>4 μg/L) As in the outflow from unit A and concentrations of-100 μg/L in the outflow from unit Β (Figures 2, 3). However, both units became very effective during the second half of the deployment when As concentrations in the effluent remained consistently below 10 μg/L (Table I). Although the dissolution of iron-filing/sand media could have released many elements of potential health concern, their levels at the outlet were all below respective W H O guideline limits (Table II), with the exception of M n (Tables I and II). M n concentrations were reduced by about one third for both units relative to the inflow, but remained above 0.5 mg/L. The corrosion of iron filing material releases significant quantities of Fe, which was confirmed by the orange-colored precipitate formed in the aeration vessel. Despite its simplicity, the combination of aeration and filtration effectively removed excess Fe in the solutions. Iron concentrations in the effluent were maintained below 1 mg/L, the secondary W H O criterion for Fe in drinking water which is based on taste and esthetics rather than health (Figures 2, 3). The time series data indicate that Ρ was also removed by the AsRT units, although the removal was not as effective as for As and declined over the course of the deployments. In the case of unit A , which treated a larger volume of water than unit Β, Ρ concentrations at the intermediate sampling point (just before the aeration vessel) gradually increased to reach the level of the inflow over the course of the deployment (Figure 2). For both AsRT units, the precipitation and removal of Fe oxides contributed to the removal of As and Ρ from the effluent.


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Table I. Performance of A s R T at Two Wells in Bangladesh (January 2001) Unit A-4133 Inflow Outflow

Unit B-4146 Inflow Outflow

pH Temp (°C) Cond. (μΞ/αυ) DO (mg/L) Eh (mV)

6.8 23.8 653 2.1 -20.9

7.1 21.2 612 7.3 109.8

7.1 24.7 724 1.1 -130.4

7.4 22.3 650 7.6 67.1

As (Mg/L) Fe (mg/L) Ρ (Mg/L)

560 1.2 280

Advances in Arsenic Research - American Chemical Society

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