Mechanisms of lithium adsorption on Fe/Mn-modified biochar: A ToF-SIMS and surface complexation modeling study

Lithium contamination from the expanding battery industry is an emerging environmental concern, yet Li adsorption mechanisms on heterogeneous sorbents remain poorly understood. Here, Fe/Mn-modified wheat straw biochars were synthesized at three Fe:Mn ratios. The optimal material, F₅M₁BC, achieved a Li⁺ adsorption capacity of 145.2 mg/g, retained 90% uptake over 10 regeneration cycles, and maintained above 85% removal under 20-fold excess competing cations. ToF-SIMS was applied for the first time to lithium adsorption on biochar, coupled with an automated Pearson co-localization protocol screening 130 fragment ions across lateral and depth dimensions. The analysis identified 17 Li-bearing fragments in five binding categories, namely Li-silicate, Li-Mn, Li-Fe, Li-Fe-Mn ternary, and Li-chloride, partitioned between metal oxide and silicate microdomains. Depth profiling resolved a three-layer architecture consisting of a surface metal oxide-hydroxyl domain, an intermediate organic-metal composite domain, and a deep silica-rich interior. A three-site non-electrostatic surface complexation model, with each site defined by a ToF-SIMS depth domain, reproduced pH-edge and isotherm data with fitted pK_a values of 2.41, 6.05, and 9.19, linking spatially resolved binding architecture to macroscopic adsorption. Combining automated ToF-SIMS co-localization screening with site-specific surface complexation modeling provides a generalizable strategy for examining adsorption mechanisms on chemically heterogeneous materials.