Chemical Stability of Framboidal Pyrite Containing Geogenic Arsenic in Alluvial Sediments

The number of underground construction projects is increasing worldwide, resulting in the production of a huge amount of waste excavated material. Excavated soils from alluvial plains are rich in pyrite. When these soils are exposed to air, the pyrite oxidizes, resulting in the generation of H2SO4, which acidifies the soil and releases potentially harmful trace metals and metalloids such as arsenic into the environment. Polarized light microscopy and field‐emission scanning electron microscopy (FE‐SEM) revealed the presence of framboidal pyrite within the shells of microorganisms in alluvial soil samples collected in Japan. Analysis with FE‐SEM/energy‐dispersive X‐ray spectrometry and electron probe microanalysis (EPMA) revealed that this framboidal pyrite contained As. When soaked in distilled water for 6 h or in 1 M HCl for 2 h, the framboidal pyrite remained unchanged; however, when it was soaked in 10% H2O2 for 6 h, it was oxidized and swelling was observed on the surface of pyrite microcrystals. A 1 M HCl extraction test indicated that arsenic release from framboidal pyrite would be difficult under acidic conditions. Framboidal pyrite in alluvial sediment is stable at low pH; however, it is unstable under oxidative conditions, suggesting that waste alluvial soil should be kept under reducing conditions. The chemical stability of geogenic arsenic in pyrite in soils and sediments provides a useful way to determine a correct handling of the soils for reclamation or as a covering material. Core Ideas Chemical stability of framboidal pyrite in acidic or oxidative conditions was examined. Geogenic As was accumulated mostly in framboidal pyrite. Framboidal pyrite was stable in acidic conditions but was unstable in oxidative conditions.