Chemical compatibility of fine and coarse Ca/Mg composite to mitigate natural contamination

Utilization of excavated surplus soil, in which arsenic (As) and lead (Pb) occur naturally, as a geomaterial is of increasing significance over the past decades in Japan because of limited remaining landfill space. For minimizing the release of hazardous heavy metals and metalloids to the surrounding environment from such naturally contaminated geomaterials in an embankment, installing a permeable soil layer with sorption capacity beneath the embankment should be an effective and sustainable way without excessive investments. The containment function of the soil layer can be enhanced when the sorption capacity of the clean parent soil for targeted contaminants is increased. In this regard, the applicability of calcium/magnesium (Ca/Mg) composite, as an immobilizing agent, to enhance the sorption capacity for As was evaluated via batch equilibrium and saturated column sorption testing. Non-linear sorption of As onto soil amended with agent was observed over the concentration ranges of interest. Adding Ca/Mg composite to clean parent soil can enhance the containment of As via an increase in sorption capacity, but the enhanced sorption capacity will be a function of both the amount of agent added and agent particle size. Addition of 5% agent increased the sorption capacity relative to that of soil amended with 3% agent by 50%, whereas the amount of As sorbed by soil amended with fine (under 2-mm) agent was higher relative to that amended with coarse (above 2-mm) agent by a factor of 2. Ca-As compounds which are less mobile and toxic, and stable in the alkaline condition, are likely to be precipitated from the soil-agent sorption layer.