Effect of temperature and pH on uranium and arsenic adsorption in calcite-rich soils

Arsenic and uranium are known human carcinogens and have been reported in dangerously high levels in groundwater in certain parts of the world. It is believed that the mobility of these heavy metals is influenced by the presence of aqueous carbonate and mineral calcite. Calcite has atypical solubility, meaning it is more soluble at cooler temperatures and therefore dissociates to produce calcium and carbonate ions. In this research, batch adsorption of arsenic and uranium on soil at two different temperatures (4°C and 22°C) was measured. The effect of temperature on the release of carbonate and its effect on arsenic and uranium adsorption were determined. Adsorption edges of arsenate and uranyl nitrate were determined over a pH range from 6.5 to 9. The arsenate adsorption data suggest that the pH shift caused by lower temperature and calcite dissolution is significant enough to cause measurable changes in adsorption of arsenate onto mineral surfaces. The uranium adsorption data indicate that the pH increase, or possibly the release of carbonate and formation of soluble uranyl carbonate species, can increase the mobility of uranium in some soils. The results of this study illustrate the importance of using laboratory conditions that most closely approximate groundwater temperatures whenever possible.