Suppression of arsenic release from alkaline excavated rock by calcium dissolved from steel slag

Massive quantities of alkaline rocks are excavated from urban coastal and mountain areas to make underground spaces available for infrastructure projects; however, such excavated rock often releases arsenic. In the present study, arsenic release from the excavated rocks with steel slag was investigated using dialysis and batch leaching tests to understand where arsenic is immobilized and which components in the steel slag suppress arsenic release from the excavated rock. Dialysis test indicated that the addition of steel slag at 10 wt% could suppress arsenic release at a level greater than 66%. The total arsenic content in the steel slag did not increase as compared with that before the test. Sequential extraction analysis indicated that the arsenic released during the dialysis test is mainly derived from arsenic fraction 1 (nonspecifically bound) due to the higher amount of this arsenic fraction in the excavated rock with the steel slag. Moreover, the steel slag extract could suppress arsenic release from the excavated rock and remove the arsenic from aqueous solution. The pH dependence test further indicated that the arsenic immobilized by the steel slag extract was stable under alkaline pH conditions. The levels of arsenic release decreased with increasing calcium release from the steel slag regardless of the type of excavated rock with an alkaline pH and were particularly seen at calcium released > 500 mg kg^-1. These results indicate that the arsenic immobilization could be occurred not on the surface of steel slag, but on the excavated rock, and the calcium dissolved from the steel slag regulates the behavior of arsenic release from the surface of excavated rock. The findings of the present study suggest that the steel slag could be utilized to enable the reuse of excavated sedimentary and metamorphic rock of alkaline pH for the control of arsenic release.