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
A call for help
When the problem’s proportions became known, the World Bank provided funds to the Bangladesh Arsenic Mitigation Water Supply Project (BAMWSP) to help alleviate the emergency. One of the first priorities was to implement a well surveillance program to identify problem wells and prevent their use.
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
One of the biggest challenges in developing a kit to meet needs of the Bangladesh project was to increase the sensitivity of the test without dramatically increasing quantities of reagents needed, thus keeping the price down. This conundrum was solved through use of an innovative new cap design. A 0.5-x-0.5-inch mercuric bromide test pad is used as the indicator in the test, but the exposed area is only 3/16 inch in diameter. This arrangement allows all of the generated gas to come into contact with the mercuric bromide and thus be reacted. The excess paper around the hole provides sufficient reactant to absorb all of the generated arsine (see Figure 1). This increases sensitivity of the test pad so that a good, clear color match is obtained at levels as low as 10 ppb and protects the operator from arsine gas (see Table 1). This combined with the large—50 milliliter (ml)—sample size allows for a calibrated range of 0 to 500 micrograms per liter (ug/L), or ppb. The calibration scale is graduated at 0, 10, 30, 50, 70, 300, and 500 ug/L. Each step is distinct and allows for quantification at levels needed to ensure health and safety standards.
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
After a thorough evaluation, BAMWSP found the test kit to be accurate and safe, but they felt it might still have been too complex for some expected users. BAMWSP also expressed a desire for the kit to go to higher ranges and take less time if possible.
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.
When designing test or treatment equipment for the developing world, it’s important to keep in mind the needs and skill levels of the end-user. The most advanced technology isn’t always the best fit for a given situation. With this in mind, it’s possible to design systems that solve the problem without resorting to overly complex technologies.
- Bruton-Ward, C., “Arsenic,” Water and Wastewater International, Vol. 15, No. 2, pp. 9-14, April 2000.
- World Health Organization, “Arsenic in Drinking Water,” Fact Sheet No. 210, February 1999, website: http://www-p11.who.int/inf-fs/en/fact210.html
- Lepkowski, W., “Arsenic Crisis in Bangladesh,” Chemical & Engineering News, pp.27-28, Nov. 16, 1998.
About the author
Dan Kroll is a senior research and development chemist for Hach Company, a leader in the development of analytical systems for water and wastewater, where he has worked for 12 years. Kroll has bachelor’s degrees in microbiology and genetics and a master’s degree in water resource management and environmental engineering from Iowa State University. He can be reached at (970) 663-1377, ext. 2637, or email: [email protected].