AN INVESTIGATION OF THE FORMATION OF ARSENIC-CONTAINING MINERALS ON 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 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. Laboratory water treatment (batch and column) experiments with various types of limestone indicated significant retention of arsenic but showed some regional variations. 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.

Under highly concentrated arsenic solutions, precipitates of arsenic-containing minerals on the limestone were observed with scanning electron microscopy (SEM). The formation of precipitates is likely due to the formation of outer-sphere complexes and is facilitated by the high surface area and high surface pH of the limestone minerals. Surface area measurements are also presented. The influence of trace elements in the limestone is being investigated.