Waste bone China as a reactive calcium phosphate matrix for cobalt immobilization: Concentration-dependent ion exchange and cobalt phosphate formation

Abstract

Ceramic wastes are commonly landfilled as inert materials; however, phosphate-bearing ceramics may remain chemically reactive and influence the mobility of dissolved metals in leachate environments. Here, we demonstrate for the first time that waste Bone China (BC) functions as a reactive calcium-phosphate ceramic for Co(II) immobilization in aqueous systems, offering a waste-to-resource framework. Batch experiments conducted at a fixed solid-to-liquid ratio of 5.0 g/L with varying contact time (0.5–72 h), initial pH (4–7), and initial Co2+ concentration (10–700 mg/L) revealed a concentration-dependent, two-stage immobilization mechanism, with a maximum Co2+ uptake capacity of 0.413 mmol/g (at pH 4). At low Co2+ concentrations (< 50 mg/L), removal was dominated by Ca–Co ion exchange within the β-tricalcium phosphate (β-TCP) phase. With increasing Co2+ concentration (≥ 50 mg/L), the dominant mechanism transitions to a dissolution–precipitation process, resulting in the formation of stable cobalt phosphate octahydrate (CPO, Co3(PO4)2·8 H2O). This study provides direct atomic-scale evidence for Co2+ removal by BC, supported by XRD, FTIR, SEM–EDS, and XAS analyses, confirming both ion exchange and CPO formation. These findings clarify the transition from reversible exchange to mineralization-driven fixation and highlight the potential of phosphate-bearing ceramic waste as a reactive medium for metal sequestration and environmentally safer waste management.