Speciation evolution and DFT-assisted coordination interpretation of Li, V, Co, and Ga during supercritical water...

Supercritical water gasification (SCWG) offers a promising route for cleaner coal conversion and hydrogen-rich gas production. However, the speciation evolution and stabilization mechanisms of Li, V, Co, and Ga during SCWG remain unclear. This study investigates their redistribution using the European Community Bureau of Reference (BCR) sequential extraction, reactive force field molecular dynamics (ReaxFF-MD), and density functional theory (DFT). Experiments covered 500-660 °C, 1-45 min, 10-30 wt% coal, and Na₂CO₃/K₂CO₃ addition. Higher temperature induced element-specific redistribution, with Co showing the strongest immobilization and reaching a 55.3% residual fraction at 660 °C. V remained mainly in F3/F4, while Li was governed by mineral redistribution and Ga remained largely in F3/F4 without catalysts. Reaction time caused rapid early transformation followed by redistribution, with Ga showing maximum solid-residue enrichment at 30 min. Coal concentration affected water-solid interactions and mineral exposure, with Co showing the highest residual fraction at 10 wt%. Alkali carbonates enhanced Li, V, and Co stabilization but increased Ga mobility, with its acid-soluble/exchangeable fraction reaching 57.50 μg/g under K₂CO₃. DFT results suggest element-dependent coordination with O, N, S, and mineral oxygen sites. These findings provide insights for regulating trace metal behavior in cleaner coal utilization.