Easing Vietnam’s arsenic crisis
Simple sand filters are surprisingly effective at removing arsenic from iron-rich groundwater in Vietnam.
Symptoms of severe arsenic poisoning have been cropping up in Vietnam. In 1998, researchers discovered that for drinking water, people there rely on groundwater that is heavily contaminated with naturally occurring arsenic at levels comparable to those found in Bangladesh. Now, a simple, cheap, low-tech solution could offer relief to a large portion of the ~10 million people in Vietnam at risk of chronic arsenic exposure.
In research published today on ES&T’s Research ASAP website (DOI: 10.1021/es060144z), scientists show that household sand filters, which use locally available materials and operate without chemicals, can achieve average arsenic removal rates of 80% in groundwater with high iron and low phosphate concentrations. Additionally, analyses of hair samples indicate that people consuming this sand-filtered water can significantly lower their arsenic body burden.
An increasing number of households in rural villages in the Red River Delta region of northern Vietnam are using the filters to remove iron from their water because of its fishy taste. Moreover, “iron precipitates fairly quickly, so it makes a bit of a mess if you use it for cooking by turning rice brown, and it stains clothes if you use it for washing,” explains Michael Berg, an environmental chemist at the Swiss Federal Institute of Aquatic Science and Technology (Eawag) and the paper’s lead author. People often store water in open settling tanks until the iron precipitates out, but the easier-to-operate filters get the job done in minutes rather than hours, he notes. An added benefit is efficient arsenic removal.
Berg and his colleagues from Eawag, Hanoi University of Science (Vietnam), and the University of Karlsruhe (Germany) studied 43 sand filters currently in use. Raw groundwater pumped from household wells contained 10–382 µg/L of arsenic. They found that 90% of the filters reduced arsenic concentrations to <50 µg/L, and 40% to <10 µg/L, which is the World Health Organization’s current drinking-water guideline and Vietnam’s drinking-water standard. Berg and his colleagues attribute the 10% of households still exceeding 50 µg/L even after filtration to low iron concentrations (<3.7 mg/L), high phosphate levels (>2.5 mg/L), or a combination of the two in the groundwater. The filters simultaneously removed 99% of the iron, 90% of the phosphate, and 71% of the manganese in the water.
These results are surprising, says Alexander van Geen, a geochemist at Columbia University. “People have known for quite awhile that arsenic sticks to iron oxides,” he says. Because of this, researchers, primarily in Bangladesh, have been looking at ways to clean water naturally without adding reagents, he points out. However, “this Vietnam study is really the first where it seems to work a lot better than in most of the other places where it has been tried.”
Why the method works better in Vietnam is still a bit of a mystery. Berg and his colleagues ran parallel coprecipitation experiments with the same groundwater to try to shed some light on the mechanisms involved in arsenic removal. They simulated arsenic and iron removal in open settling tanks, where, in contrast to the sand filters, iron precipitates from the water without coming into contact with sand particles. They saw an average arsenic removal rate of 76%, nearly identical to that of the filters. This, Berg says, indicates that the same mechanisms—namely, arsenic oxidation and coprecipitation with iron, and possibly manganese—are responsible.
However, Flemming Larsen of the Technical University of Denmark points out that this speculation doesn’t fully explain the oxidation mechanism. He notes that most of the groundwater in Vietnam is As(III), which others have shown isn’t as easy to remove as As(V). Measuring As(III) and As(V) at the filter inlet and outlet, instead of total arsenic only, he adds, “would’ve strengthened the paper a lot.”
For now, the results are reassuring enough that the Vietnamese government is encouraging the use of sand filters for domestic water uses such as washing and bathing, says Nguyen Khac Hai, director of the National Institute of Occupational and Environmental Health (Vietnam). For drinking-water purposes, the government is trying to improve arsenic removal efficiencies with smaller filters to treat the water after sand filtration.
