Chemical solidification/stabilization of arsenic sulfide and oxide mixed wastes using elemental sulfur: Efficiencies, mechanisms and long-term stabilization enhancement by dicyclopentadiene

Highlights: • As₂S₃ and As₂O₃ mixed wastes are chemically solidified/stabilized by elemental sulfur. • Solidified body shows high compressive strength and low arsenic leaching concentration. • As₂S₃ transforms to stable copolymers through thermal copolymerization with sulfur. • As₂O₃ transforms to As₂S₃ in the presence of the poly(As₂S₃-r-S) copolymers. • Dicyclopentadiene can greatly increase the long-term stability of the solidified body. Large amounts of hazardous arsenic sulfide (As₂S₃) wastes are generated in many industries. These wastes, which are extremely unstable and can partially transform into highly soluble arsenic oxide (As₂O₃) and then transform into As₂S₃ and As₂O₃ mixed wastes (ASOW), are difficult to be solidified/stabilized using common binders. This study proposed a thermally initiated copolymerization method employing elemental sulfur (S₈) to chemically solidify/stabilize ASOW. Under thermal conditions (140–200 °C), the elemental sulfur rings break and polymerize into diradical polymeric sulfur chains (• S-(S)_m-S• ). The ASOW is solidified/stabilized not only by transforming As₂S₃ into poly(As₂S₃-r-S) copolymers through copolymerization of • S-(S)_m-S• with As₂S₃ but also by transforming As₂O₃ into As₂S₃ in the presence of poly(As₂S₃-r-S) copolymers. However, the sulfur chain in poly(As₂S₃-r-S) copolymers gradually crystallizes into S₈ after long-term aging, resulting in the depolymerization of copolymers. Dicyclopentadiene (DCP) greatly improves the long-term stability of the solidified body through maintaining the sulfur chain form by forming highly stable poly(As₂S₃-r-S-r-DCP) copolymers. The solidified body showed high compressive strength (25.7 MPa) and low leaching concentration of arsenic (−1) even after 732 days of aging. This study provides a theoretical foundation for the S₈-based chemical solidification/stabilization of ASOW as well as other sulfide-containing wastes.