ARSENIC REMEDIATION OF DRINKING WATER USING A LIMESTONE BASE

Arsenic in surface water and ground water is of great concern because of potential toxic effects in drinking water supplies. The drinking water standard for arsenic, currently set at 50 ppb, is likely to be lowered in because of links to cancer. Current remediation technologies are quite expensive and are designed for large water treatment facilities. Rural and individual water users are in immediate danger of being out of compliance when the new lower standards are put into effect. This will place increased socio-economic pressure on rural America, primarily because of the lack of inexpensive point-of-source treatment technology.

Arsenic is readily soluble and transports easily through ground water. Observations of arsenic contamination from mining areas in the Black Hills of South Dakota indicate arsenic is retained by native limestone. Preliminary laboratory water treatment (batch and column) experiments with Minnekahta limestone indicated significant retention of arsenic. In batch experiments, a 100-mL volume of ~100 ppb arsenic solution (at varying pH levels) was mixed with between 1 and 100 grams of limestone. Typically, 5-10 grams were sufficient to bring the arsenic value to below 10 ppb. Generally, batch experiments were conducted for 48 hours. However, batch tests conducted as a function of time show that over 70% of the arsenic was removed within 2 hrs. Analyses clearly indicate that limestone reduced arsenic concentrations from > 100 ppb to less than 5 ppb. Four limestone grain sizes were used, ranging from 0.5 to 7 mm in diameter. The greatest efficiency of arsenic removal was achieved with limestone of the smallest grain size and greatest surface area. Minnekahta Limestone was successful in effectively removing arsenic from standard solutions, regardless of the water's initial pH, within a wide range of water sources (pH 4 through 10). Efficiency of removal was independent of the initial pH of the solution.