Leaching behavior of arsenic from various rocks by controlling geochemical conditions

Hydrothermally altered igneous rock and mudstone formed under the sea are often rich in arsenic (As). Therefore, As leaching from these rocks excavated during construction work may affect the surrounding soils and groundwater. High-volume, low-concentration contaminants contained in the waste rock as muck have been a major concern in Japan since the Soil Contamination Countermeasures Law was enforced in 2003. The aim of this study is to develop ways to reduce As concentration leached from muck by controlling the geochemical conditions. Batch leaching experiments were carried out to determine the leaching behavior of As as a function of mixing period, solution pH, oxygen concentration, or mixing ratio of different rocks. The results showed that the As concentration in the leachate approached almost constant after 1 to 7 days mixing and increased with the As content in the rock, and that the maximum As leaching was observed at around pH 9. The concentration of oxygen gas and the mixing ratio of different rocks also affected the As leaching by changing the leachate pH. This indicates that the solution pH as well as the As content in the rock greatly affects the As leaching from rocks.