Volume 44 Number 1
World Spotlight: Improving Water Quality in Bangladesh -- Groundwater & Arsenic Monitoring in the Developing World
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Summary: Arsenic concentration monitoring in water is difficult and challenging under the best of circumstances. These challenges are exacerbated when tests must be run under field conditions, an analyst has no formal training, costs must be kept to a minimum, and accurate results are literally a matter of life or death. Development of a field testing kit for arsenic that fulfills these needs is explored here.
Contamination of drinking water supplies with levels of arsenic that can be harmful to public health is a problem that's rapidly becoming recognized as being of major global significance. From 1972 to 1997, the United Nations International Children's Emergency Fund (UNICEF) supported the installation of over a million tube wells in Bangladesh and West Bengal India to provide an alternative to surface water as a source for domestic use. This is in addition to another two million or so wells dug with private funds.
Surface water in the area was highly contaminated with pathogens, and its use for human consumption resulted in a large number of illnesses and deaths each year. After the installation of tube wells, rates of morbidity and mortality related to drinking water quickly fell. Unfortunately, no one thought to test the new supplies of drinking water for chemical contamination. The wells contained high levels of naturally occurring arsenic, which is detrimental to human health.1
The consequence of this has endangered health of the affected population. In Bangladesh alone, there have been over 200,000 cases of documented arsenic poisoning. These areas have a population of 79.9 million people at risk of exposure. In the 42 most exposed districts, 61.9 percent of well water samples aren't suitable for drinking as per the World Health Organization (WHO) recommended value of 10 parts per billion (ppb), and 55.6 percent of the water samples contain arsenic above the WHO maximum permissible limit (50 ppb). 2 As a result of drinking the contaminated water, the inhabitants suffer from a variety of maladies including depigmentation, skin lesions leading to gangrene and, eventually, cancer. 3 The magnitude of this crisis has made testing of an unprecedented number of wells for arsenic imperative.
A call for help
BAMWSP placed a public tender for delivery of kits capable of performing five million tests. Field tests for arsenic usually use a modified Gutzeit methodology where arsenic present in the water is converted to arsine gas. A mercuric bromide test strip is then exposed to the gas causing a white-yellow-brown color change dependent upon the quantity of arsenic in the original sample. The problem was that kits available at the time of the tender were inadequate for required testing. Existing field methods for arsenic suffered from several problems. The existing kits could only quantify down to 100 ppb rather than the needed 10 ppb. The kits were quite complex and difficult to use. They used hazardous reagents such as concentrated hydrochloric acid and exposed users to hazardous levels of arsine gas. They were also prone to interference from sulfide. A new kit was needed that would address these problems.
Designing a new kit
Most tests for arsenic, including this method, rely on conversion of arsenic to arsine gas:
As2O3 + 6 Zn + 12 HCl ® 2 AsH3 + 6 ZnCl2 + 3 H2O
H3AsO4 + 4 Zn + 8 HCl ® AsH3 + 4 ZnCl2 + 4H2O
As the reaction above indicates, arsine gas is commonly generated by reduction with zinc metal and hydrochloric acid. Hydrochloric acid is dangerous and difficult to use. The new method substitutes a solid acid (sulfamic) packaged in a granular powder form. This alleviates hazards and difficulties associated with using liquid hydrochloric acid. Zinc and all other reagents in the method are packaged in unit dose form for convenience and to minimize handling. The mercuric bromide test paper is on the end of a long plastic strip to eliminate the need to come into contact with the mercuric bromide. Sulfide interference in the kit was handled by an oxidation procedure that converts up to 5 parts per million (ppm) sulfide to sulfate. This resulted in a test kit that consisted of five reagents and 12 simple steps that gave results in 30 minutes. The test was accurate and safe and has been used successfully all over the world to detect arsenic in the field.
Ups, downs and ups
A second kit was designed to leave out the sulfide removal steps and substitute optional use of a lead acetate soaked cotton ball to remove sulfide. The range was expanded by having the option of either a 9.6-ml or 50-ml sample. The bottle was filled to the 50-ml mark and the scale is set at 0, 10, 25, 50, 70, 100, 250 and 500 ppb. If a higher range is required, fill the measuring cell to the top (9.6 ml) and pour it into the reaction vessel. The scale then becomes 0, 35, 75, 175, 500, 1,500 and 4,000 ppb providing an overall range of 0-4,000 ppb with 13 distinct cutoffs. The test comes with two reaction vessels so that more than one sample or both ranges can be run simultaneously. The reaction time for this kit is only 20 minutes.
After a second evaluation, the redesigned kit was chosen by BAMWSP to fill the World Bank tender. Production of the five million tests is being expedited and delivery to Bangladesh was to be completed by November.
2. World Health Organization, "Arsenic in Drinking Water," Fact Sheet No. 210, February 1999, website: http://www-p11.who.int/inf-fs/en/fact210.html
3. Lepkowski, W., "Arsenic Crisis in Bangladesh," Chemical & Engineering News, pp.27-28, Nov. 16, 1998.
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