Long term metal release risk of lithium slag based mine backfill under different pH conditions

Lithium slag is increasingly considered for mine backfilling, but its long-term environmental safety remains poorly constrained. In this study, a lithium slag-based backfill material (LSBM) was evaluated using 28-day strength screening, a 64-day semi-dynamic tank leaching test under acidic, neutral, and alkaline conditions, and diffusion-based modeling to characterize the release of Be, Tl, and Cr. LSBM met the basic strength requirement for backfill application and exhibited strong hydrochemical buffering capacity, with leachate chemistry remaining relatively stable despite contrasting initial pH conditions. Metal release was generally diffusion-dominated, but showed clear pH dependence: Be was more sensitive to acidic exposure, whereas Tl and especially Cr exhibited higher mobility in alkaline media. Model fitting confirmed that both classical and non-classical diffusion approaches adequately described long-term release behavior, and 100-year predictions indicated very low cumulative leaching fractions of 0.0087% for Be, 0.0145% for Tl, and 0.1252% for Cr. Overall, lithium slag can be reused as a mine backfill constituent with low long-term environmental risk under the tested conditions.