Fig. 3: Mechanism of PFAS mineralization during the REM process.

a XRD patterns of PFOA/biochar before (red) and after (blue) REM. b C 1s XPS fine spectra of PFOA/biochar before (top) and after (bottom) REM. c F 1s XPS fine spectra of PFOA/biochar before (top) and after (bottom) REM. d IR spectra of PFOA/biochar before (red) and after (blue) REM. e Gibbs free energy change (ΔG) of each PFOA degradation step with (solid line) and without (dash line) Ca²⁺ varied with temperature. The black dash line denotes ΔG = 0 kJ mol⁻¹. f Simulated variation of C-F bond ratio during REM process with calcium (blue line, F105Ca96) and without calcium (red line). g Optimized structure snapshots after simulated heating treatment with calcium (left, F105Ca96) or without calcium (right). h Relationship between C-F bond ratio after REM and the input atomic ratio of calcium and fluorine. Insets are the structures of PFOA (top right) and mineralized CaF₂ (bottom left).