Leaching of boron, arsenic and selenium from sedimentary rocks: I. Effects of contact time, mixing speed and liquid-to-solid ratio

Sedimentary rocks of marine origin excavated in tunnel projects were recently identified as potentially hazardous because they could release significant amounts of toxic trace elements when exposed to the environment. This study investigated the leaching characteristics of B, As, Se and the major coexisting ions under various conditions to identify the factors and processes controlling their evolution in the leachate. In addition, we evaluated whether the parameters of the currently used leachability test for excavated rocks were adequate. Although the leachabilities of B, As and Se similarly increased at longer contact times, only those of B and As were influenced by the mixing speed and/or liquid-to-solid ratio (L/S). The majority of trace elements dissolved in the leachate originated from the dissolution of soluble salts formed from seawater of the Cretaceous trapped during the formation of the sedimentary rocks. Moreover, the alkaline pH of the leachates could be attributed to the simultaneous dissolutions at varying degrees of the mineral components of the rocks as well as the precipitation of clay minerals. In the leaching test of excavated rocks for regulatory purposes, the best values of contact time and mixing speed should represent conditions of the highest trace element extractabilities, which in this study were found at longer contact times ( > 48 h) and the fastest mixing speed (200 rpm). The most appropriate L/S for the leaching test is 10 because it was around this L/S that the extractabilities and leaching concentrations of the trace elements were simultaneously observed at their highest values.