Long-term performance of the adsorption layer system for the recycling and repurposing of arsenic-bearing mudstone as road embankment

Abstract

The adsorption layer system has shown great potential as a cost-effective and practical strategy for the recycling and management of excavated rocks containing potentially toxic elements (PTEs). Although this system has been employed in various civil engineering projects throughout Japan, its long-term performance to immobilize PTEs has rarely been investigated. This study aims to evaluate the effectiveness of the adsorption layer system applied in an actual road embankment approximately 11 years after construction. The embankment system is comprised of a layer of excavated arsenic (As)-bearing mudstone built on top of a bottom adsorption layer mixed with an iron (Fe)-based adsorbent. Collection of undisturbed sample was carried out by implementing borehole drilling surveys on the embankment. Batch leaching experiments using deionized water and hydrochloric acid were conducted to evaluate the water-soluble and acid-leachable concentrations of As, Fe, and other coexisting ions. The leaching of As from the mudstone layer was likely induced by As desorption from Fe-oxides/oxyhydroxides naturally present under alkaline conditions, including the oxidation of framboidal pyrite, which was identified as a potential source of As. This was supported by electron probe microanalyzer (EPMA) observations showing the presence of trace amounts of As in framboidal pyrite crystals. Arsenic leached from the mudstone layer was then immobilized by Fe oxyhydroxides found in the adsorption layer. Based on geochemical modeling and X-ray photoelectron spectroscopy (XPS) results, leached As predominantly existed as the negatively charged HAsO42− oxyanion, which is readily sequestered by Fe oxyhydroxides. Moreover, the effectiveness of the adsorption layer was assessed and its lifetime was estimated, and the results revealed it still possessed enough capacity to adsorb As released from mudstone in the foreseeable future. This prediction utilized the maximum potential amount of As that could leach from the excavated rock layer with time.