Release of potentially harmful elements from excavated rocks with surface alteration under dry-wet cycles.

Abstract

In construction projects, rocks containing potentially harmful elements (PHEs) may be excavated and treated for reuse. Thus, understanding the mechanisms of PHE release from excavated rocks that are to be reused is crucial. Embankments constructed using excavated rocks are primarily affected by exposure to dry-wet cycles. Herein, we aimed to 1) understand the causal relation between the surface alteration of excavated rocks caused by processes such as pyrite oxidation, fine granulation, and calcite and gypsum formation and changes in the PHE release from excavated rocks exposed to dry-wet cycles and 2) identify the changes causing variations in the amounts of different PHE species released from the involved excavated rocks. In the first month of the dry-wet cycles, pyrite oxidation, fine granulation, and gypsum formation were maximal. Progressive fine granulation with continuing dry-wet cycles led to further pyrite oxidation; however, no gypsum was formed after the first month. Fine granulation and gypsum formation did not change the amounts of PHE released. Arsenic, selenium, and cadmium were released from the excavated rocks as pyrite oxidation and fine granulation continued during dry-wet cycles. The amounts of arsenic and lead released during pyrite oxidation were minimized by iron-(hydr)oxides that precipitated, whereas the amounts of released selenium and cadmium, present in soluble forms, were not reduced. The continuous pyrite oxidation and fine granulation mainly controlled the phases of PHE and their release. Overall, the findings suggest that the changes in the amount of PHEs released due to surface alteration after the reuse of excavated rocks must be considered when evaluating their environmental risks.