Synergistic co-sintering of contaminated dredged sediment and red mud for ceramsite: Concurrently managing toxic metal behavior during co-sintering process and developing a Cd(II) adsorbent

Co-sintering contaminated dredged sediment (DS) with red mud (RM) into ceramsite is an effective route for the integrated management of two large-volume industrial and hazardous waste streams. However, the regulation of toxic metal behavior (e.g., volatilization and immobilization) by sintering parameters remains insufficiently understood, and the value-added utilization of the product is still limited. Therefore, this study developed a synergistic co-sintering strategy to clarify how atmosphere (N 2 vs. air), temperature, time, and DS/RM ratio jointly control the behavior of As, Cd, Cr, Cu, Pb, and Zn along the gas–particle–solid pathway, and concurrently explored the resulting ceramsite as a Cd(II) adsorbent. The results showed that an N₂ atmosphere strongly promoted volatilization removal of Cd and Pb (up to 82.6%–87.7%) and promoted the derivatization of metals into residual fractions (>89.6%), whereas an air atmosphere favored As(V) generation but increased Cr mobility via Cr(VI) formation. The co-sintering of RM and DS greatly promoted the volatilization of Pb and Cd, while enhancing the retention of As and Cr. An operating window of approximately 1150 °C and 10–15 min balanced microstructural densification with suppression of As re-volatilization and excessive Cr(VI) generation. Moderate RM addition (e.g., DS/RM = 60:15) optimized the trade-off between volatile emissions and solid-phase retention by enhancing Fe–Al-bearing stabilizing phases without excessively increasing As and Cr contents or leaching in the ceramsite. The selected ceramsite exhibited a Cd(II) adsorption capacity of 35.6 mg/g (pH = 7) based on precipitation, electrostatic attraction, and complexation. Overall, this work established a mechanistic, parameter-based framework for process-window design in DS/RM co-sintering and demonstrated a waste-to-resource pathway for safely transforming hazardous solid wastes into functional materials for toxic-metal wastewater treatment. • Synergistic ceramsite production from dredged sediment (DS) and red mud (RM) was achieved. • N₂ promoted Cd/Pb volatilization and shifted most metals into residual fractions, whereas air favoured As(V) but increased Cr(VI). • DS/RM mass ratio controlled the trade-off between volatile emissions and solid-phase retention of toxic metals. • Sintering temperature and time dictated microstructural evolution and toxic metal partitioning. • The synthesized ceramsite adsorbed aqueous Cd(II) via coupled precipitation, electrostatic attraction and complexation.