APPRAISAL OF A SIMPLE ARSENIC REMOVAL METHOD FOR GROUNDWATER OF BANGLADESH

A simple three-pitcher (locally known as ‘3-kalshi’) filtration assembly made entirely from readily available local materials is tested for its efficacy in removing arsenic from the groundwater of Bangladesh. In a 3kalshi assembly, the first kalshi has iron chips and coarse sand, the second kalshi has wood charcoal and fine sand, and the third kalshi is the collector for filtered water. About 240 L of arsenic contaminated groundwater and groundwater spiked with high concentrations of both As(III) and As(V) were filtered. Analytical measurements were performed by using anodic stripping voltammetry (ASV) for trace level As(III) and As(total) and redox potential. Atomic absorption spectrometry with graphite furnace and Zeeman background correction (AASGF-Z) and inductively coupled plasma atomic emission spectrometry (ICPAES) were used to validate measurements of arsenic and measure 24 other metals before and after filtration. Total Fe, ionic conductivity, Eh, pH, temperature and flow rates were measured at various stages of the filtration process. It has been shown that more toxic As(III) can be removed from 800 ppb to below the detection limit of 2 ppb. The As(total) can be removed to a concentration below 10 ppb for most samples even at the highest input concentration of 1100 ppb As(total). The dissolved iron concentration decreased from an average 6000 ppb to 200 ppb. Calculations based on compound formation and arsenic adsorption on hydrous ferric oxide show that, with a constant input of dissolved iron the arsenic removal capacity increases linearly with each filtration. Although the role of metallic iron was difficult to quantitate, it provided excess soluble iron in the filtering media of the second kalshi. The wood charcoal was used to remove any organic impurities that may be present in groundwater. The redox potential change shows speciation of iron in agreement with literature data. The decrease in conductivity by 35% of the original value indicates substantial removal of dissolved ions. This is also supported by ICPAES measurement. The filtered water remained crystal clear for months and free from most toxic metal ions. The daily capacity of the 3-kalshi system varies from 42 -148 L/day. The final water quality meets and exceeds the guideline values suggested by USEPA, World Health Organization and Bangladesh. We suggest the use of this simple setup to make potable water. 1 *Corresponding author; ahussam@gmu.edu. First circulation November 11, 1999; Journal submissionacceptance/ Internet news media circulationDecember 23/30, 1999; BEN-2000 presentation: January 14, 2000. A Simple Arsenic Removal Method: Khan et. al 2 J. ENVIRON. SCI. HEALTH, A35(7), 1021-1041 (2000) INTRODUCTION Recently, a high concentration of arsenic was found in the groundwater of Bangladesh and neighboring India (Chatterjee, et. al., 1995; Das et. al., 1995; Samanta, et. al., 1999). The geochemical nature of arsenic contamination of the aquifers of Bengal deltaic region is of serious concern because groundwater is the primary source of drinking water. Hyperkeratosis on the palms and feet is the common symptoms of arsenic poisoning. Long-term exposure to low concentrations of arsenic has been reported to cause cancer of bladder, skin and other internal organs (Tseng, et. al., 1968; International Agency for Research on Cancer, 1980; Smith, 1992). Also, arsenic in its various ionic forms is known to be very toxic to most microorganisms (Cervantes, et. al., 1994). World Health Organization (WHO, 1999) reports that there are about 2.5 million tubewells in Bangladesh and more than 95% of the Bangladesh population of 120 million drinks well water. The present crisis may have affected more than 50 million of people in Bangladesh and the neighboring India. According to WHO, the maximum contamination level (MCL) of arsenic in drinking water is 50 μg/L (or ppb: part-per-billion) and 10 μg/L as a provisional guideline value. Total arsenic measurement shows that more than 60% of water from shallow and deep tubewells (wells with a metal casing) has above 10 ppb guideline value (International Conference, 1998). The primary concern now is the measurement and purification of drinking water from arsenic. Because arsenic is found only in the well water, the obvious choice would be to drink surface water (river, pond etc.). However, this is limited by the extensive contamination of surface water by fecal matters. Although Bangladesh has one of the world’s largest freshwater resources, the lack of appropriate water treatment facilities prevents majority of the population from clean drinking water. An array of methods and technologies are available for the purification of water from arsenic and other trace metal contaminants (EPA, 1999). These are coagulationfiltration, lime softening, activated alumina, ion exchange, reverse osmosis, electrodialysis, nanofiltration and in-situ treatment of well water (Rott, et. al., 1993). The USEPA (United States Environmental Protection Agency) has identified three prospective methods: ion exchange with brine recycles, iron addition-coagulation with direct filtration, and attenuation by naturally occurring iron in water. Although, USEPA technologies are generally developed for large-scale treatment facilities, the basic chemistry for the later method can be utilized in a small scale. This work was motivated by the need to develop a simple, low cost technique for the removal of arsenic from the groundwater of Bangladesh by using locally available materials. Instead of using methods mentioned above, it was decided to test one of the oldest water purification methods known as the three pitcher (locally known as ‘3kalshi’) for its efficacy in removing arsenic and other impurities. This work is a scientific appraisal of the 3-kalshi method for the purification of water and its viability in Bangladesh. An evaluation of the method was conducted by measuring water chemistry parameters such as As(III), As(total), Fe(total), ionic conductivity, Eh, pH, temperature and flow rates at various stages of the filtration process. The water quality was further tested by measuring concentrations of 23 other major, minor and trace metals by ICPAES. A Simple Arsenic Removal Method: Khan et. al 3 J. ENVIRON. SCI. HEALTH, A35(7), 1021-1041 (2000) EXPERIMENTAL SECTION The basic 3-pitcher (hereafter referred as ‘3-kalshi’, the local name) water purification setup is shown in Figure 1. It consists of three kalshis made of fired unglazed clay used as reservoir for drinking water by 80% of the people in Bangladesh. Local artisans make a variety of shapes of these kalshis. We have used the ones with narrow mouth, round bottom, and have a volume of about 18-L. The three kalshis are stacked on top of each other. The bottom of top two kalshis has a small hole plugged with 100% polyester cloth or similar materials. The first kalshi contains 3 kg (approx. 1/6 kalshi volume) of iron chips (filings) and 2 kg of coarse sand. The low carbon iron chips were obtained from a local foundry (Renwick Ironworks, Kushtia). Small iron nails or rods cut into small pieces may also be used. Care should be taken not to use either galvanized (containing Zn) iron or alloy materials. Both coarse and fine sand was obtained from the local Garai River. The river sand has about 0.05% iron oxide. The second kalshi consists of 1-kg wood charcoal and 2 kg of fine sand. The wood charcoal was collected from burned wood for cooking. All precautions were taken to avoid the fine wood ash, which may dissolve in water and produce a basic solution. The function of the wood charcoal is to adsorb organic impurities that may be present in groundwater. If the wood charcoal is lighter than water, it can be placed in between two layers of fine sand. The fine sand is the final filter. The third kalshi was the collector. To prevent bacterial contamination from sand and iron chips, they may be boiled and washed. All data obtained in this work were obtained after discarding first two kalshi volumes of water. The concentrations of As(III), and As(total) were measured by a Model HQ-2040 (Advanced Analytics, Virginia, USA) personal computer controlled electrochemical analyzer. This multifunctional computer controlled potentiostat is capable of performing a variety of electrochemical experiments and has a magnetic stirrer controlled by the computer (Hussam, 1988; Hussam et. al. 1988). For arsenic analysis the instrument uses a staircase anodic stripping voltammetry (ASV) which has been shown to be more sensitive than differential pulse anodic stripping voltammetry (Christie, et. al., 1976; Eisner, et. al. 1976). Glass electrochemical cells with Teflon top and Teflon magnetic stir bar were used throughout the experiment. The cell-top has provisions to insert Teflon purging tube and micropipet tips for standard additions. A thin gold film coated glassy carbon (3.0-mm diameter) was used as a working electrode. A platinum wire directly immersed in test solution (53 mm long) was used as the counter electrode. The reference electrode was an Ag/AgCl in saturated KCl isolated from the test solution by a porous Vycor junction. The working electrode preparation and other analytical protocols are based on published methods (Davis et. al., 1978; Sun, et. al., 1997; Burguera et. al., 1997; Huiliang, et. al., 1988) and that of a modified EPA method 7063: Arsenic in Aqueous Samples and Extracts by Anodic Stripping Voltammetry (ASV) (Pyles, et. al., 1999). Briefly, As(III) is deposited from a 6M HCl acidic solution on a preplated gold coated glassy carbon electrode at -150 mV vs. Ag/AgCl, saturated KCl reference electrode for 60 s. The deposited arsenic is stripped off the electrode by a linear potential ramp from -150 to 500 mV in the same solution while the oxidation current is recorded as a function of potential. The resulting linear scan anodic stripping voltammogram (LSASV) was used to measure the concentr