| Rocks generated from tunnel construction projects for roads and railways throughout Japan have often leached out hazardous trace elements, such as arsenic (As) and selenium (Se). In nature, the oxyanionic species of As and Se have a variety of chemical species, so speciation is one of the crucial factors in their migration through natural geologic media. In this study, column experiments consisting of four types of crushed rock samples containing
As and Se, and a river sediment (RS) as an adsorbent obtained near the tunnel construction site were conducted to evaluate the leaching and adsorption behavior of arsenite (As (III) ), arsenate (As (V) ), selenite (Se (IV) ), and selenate (Se (VI) ). The results showed that the dominant speciation of As and Se in the effluent from the rock layer was As (V) and Se (VI), and that the addition of a bottom RS adsorption layer or the mixing of RS with the rock layer decreased the leaching concentrations of As (III), As (V), Se (IV), and Se (VI). Cumulative leachability (CL) for each speciation through the column experiments was calculated to evaluate the amounts of As and Se retained in RS. The calculated CL showed that the bottom RS layer or mixing of RS with the rock reduced the CL of As (III), As (V), Se (IV), and Se (IV) ranging from 60 to 89%, 73 to 89%, 9 to 75%, and 36 to 60%, respectively; however, mixing of RS with the rock layer was ineffective in decreasing CL of Se (VI). The reduction of CL may be due to adsorption and/or coprecipitation by iron and/or aluminum oxides contained in RS. These results indicated that utilization of RS for the bottom adsorption layer was effective in reducing As and Se concentrations irrespective of their speciation, although that of mixed with rock layer was effective only in reducing As concentrations irrespective of their speciation.
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