<p>Bio-based materials for selective metal cation capture are increasingly sought after as economic and sustainable alternatives to conventional polymeric/ceramic membranes. Here, spruce wood membranes were carboxylated via anhydride esterification with succinic anhydride (SA) and maleic anhydride (MA). Said membranes were used in cation-exchange filtration processes to capture/recover lithium (Li⁺) and ferric (Fe³⁺) ions from aqueous solutions. Structural and chemical analyses, e.g., FTIR, SEM-EDX, WAXS, TGA and DVS experiments, confirmed formation of Na-carboxylate exchange sites, following anhydride esterification and Na-charging with NaHCO₃, while preserving the aligned wood microchannel architecture. Gravity-driven filtration experiments demonstrated significant differences as a result of these two modification routes. MA-modified membranes achieved near-quantitative Li⁺ removal (≈99.9%) with excellent regeneration stability over three cycles, whereas SA-modified membranes showed greater variability and partial performance decline. For Fe³⁺, MA-modified membranes exhibited significantly higher, stable removal efficiencies (≈72%) than succinic-modified membranes. Equilibrium ion-exchange experiments showed Langmuir-type monolayer adsorption on chemically homogeneous carboxylate sites, with higher affinity for Fe³⁺ (K = 0.017–0.020 L·mmol^-1) than for Li⁺ (K = 0.0063-0.0078 L·mmol^-1), reflecting the influence of the cation valence and coordination. Overall, MA modification provides a balanced combination of ion-exchange efficiency, structural robustness, and regeneration compatibility, establishing chemically modified wood membranes as promising, sustainable platforms for metal cation capture and recovery in water treatment applications.</p>