Utilization of the Finer Particle Fraction of Arsenic-Bearing Excavated Rock Mixed with Iron-Based Adsorbent as Sorption Layer

Excavated rocks generated during tunnel construction may pose an environmental hazard due to the release of acidic leachate containing potentially toxic elements (PTEs). Addressing this concern requires strategic countermeasures against mitigating the release of PTEs. This study investigated the efficacy of a novel approach for managing altered excavated rocks that generate acidic leachates with elevated arsenic (As) by utilizing the finer altered rock as a base material for the sorption layer. The proposed method involves classifying the altered excavated rocks into coarse (9.5–37.5 mm) and finer (<9.5 mm) fractions, with the finer fractions incorporated with iron (Fe)-based adsorbent to form a bottom sorption layer for the disposal of coarser rock samples. Leaching behavior and As immobilization efficiency were assessed through shaking, stirring leaching tests, batch sorption tests, and column tests under varying particle size fractions of the rock samples. Results indicate that altered finer rock fractions exhibit increased As leaching under shaking conditions due to enhanced dissolution. The addition of >1% of Fe-based adsorbent to the finer rock in the sorption layer effectively suppressed As leaching concentration, meeting the management criterion of <0.3 mg/L for specially controlled contaminated soils in Japan. Batch sorption tests using the finer rock samples with the Fe-based adsorbent confirmed their efficacy as effective adsorbents. This efficacy was further elucidated in column experiments consisting of the coarse rock samples and fine altered rock samples mixed with the Fe based adsorbent at the bottom as a sorption layer. Results showed that the sorption layer effectively decreased the As leached from the rock layer, utilizing the altered excavated fine rock as a base material in the sorption layer. This approach highlights the potential for repurposing excavated rocks as sorption media, enabling sustainable management strategies for As-contaminated rocks. This study provides an innovative framework for integrating adsorption-based remediation, contributing to sustainable countermeasure strategies for excavated rocks.