Synergistic Co-Pyrolysis of Tannery Sludge and Oyster Shells for Enhanced Chromium Speciation Transformation and Recovery Efficiency

Chromium-bearing tannery sludge poses both environmental and resource challenges, but current methods often rely on high-temperature calcination with inorganic Ca sources. To address this gap, this study investigates the synergistic effects of oyster shell (OS), a natural biomineralized calcium material, in chromium recovery from dewatered tannery sludge (TS) through co-pyrolysis, with comparative analysis against inorganic calcium carbonate (CaCO₃). Laboratory-scale experiments were conducted at pyrolysis temperatures ranging up to 900°C, with varying OS concentrations. The results demonstrate that OS incorporation significantly influenced chromium phase transformation, effectively converting Cr₂O₃ (exclusive to TS) into chromate salts during the co-pyrolysis process. The optimized co-pyrolysis conditions achieved remarkable chromium recovery efficiency exceeding 95%, substantially outperforming the CaCO₃-assisted pyrolysis system. Mechanistic analysis revealed that the organic matrix proteins inherent in OS played a crucial role in facilitating chromium adsorption and subsequent phase transformation processes. This work provides a new way to recover chromium using OS powder, which performs better than CaCO₃ and also utilizes marine waste material. The process improves chromium recovery and supports resource reuse in the tannery industry. This innovative co-pyrolysis approach utilizing OS powder offers a sustainable and economically viable solution for chromium recovery, simultaneously addressing environmental concerns and resource utilization challenges in the tannery industry. Furthermore, the oxidation of Cr(III) to Cr(VI) in tannery wastewater not only enhances chromium recyclability but also promotes the conversion of waste streams into valuable resources, thereby advancing circular economy principles within the leather manufacturing sector.