Immobilization of thallium and fluoride in lithium slag using low-cost cement-based stabilization systems

Lithium slag (LS), a by-product of lithium-ion battery recycling, contains hazardous contaminants that pose potential environmental risks during disposal. The LS used in this study contained 7.46 mg kg^-1 thallium (Tl) and 2.56 wt% fluorine (F) based on initial material characterization. This study investigates low-dosage cement-based stabilization systems using ordinary Portland cement (OPC) or composite Portland cement (CPC) combined with CaO for the simultaneous immobilization of Tl and F in lithium slag. Four formulations with LS:binder:CaO ratios of 20:(0.30-0.75):0.10 were evaluated over 28 days of curing. Leaching behavior was assessed using the toxicity characteristic leaching procedure (TCLP), and microstructural evolution was examined using X-ray diffraction (XRD) and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS). Untreated LS exhibited significant contaminant mobility, with TCLP leachate concentrations of 13.24 mg L^-1 F and 89 μg L^-1 Tl. Cement-CaO stabilization substantially reduced contaminant release. The optimized low-OPC formulation (20:0.30:0.10) reduced Tl concentration to 0.17 μg L^-1 (>99 % reduction) and F concentration to 3.39 mg L^-1 (∼74 % reduction). Microstructural observations indicated progressive matrix densification associated with the formation of C-S-H gel and Ca-rich hydration products. XRD and SEM-EDS results suggest that Tl retention is mainly associated with adsorption or incorporation within C-S-H phases, while fluoride stabilization is related to the formation of Ca-F phases (e.g., CaF₂) under alkaline conditions. The optimized formulation achieved regulatory compliance with an estimated material cost of approximately 11 RMB t^-1 (∼1.5 USD t^-1) and a carbon footprint of 16.5 kg CO₂ t^-1, representing a 54 % reduction compared with higher-binder systems. These results indicate that low-dosage cement stabilization provides a potentially cost-effective strategy for reducing contaminant mobility in lithium slag, although further long-term durability assessments are required.