This study reports the development, optimization and mechanistic evaluation of bio-based cellulose/lignin–tannin membrane adsorbents for the removal of representative inorganic and organic water pollutants: Cr(VI), Pb²⁺, difenoconazole (DFC) and bisphenol A (BPA). Waste cellulose fibers (wCF) from cigarette-package paperboard were converted into reactive membrane constituents by amino/ester and epoxy functionalization and subsequently cross-linked with epoxy-modified kraft lignin (EpKL), with or without tannic acid (TA), yielding wCF/EpKL and wCF/EpKL/TA membranes. Response surface methodology was used to optimize formulation parameters, while structural, morphological, and physicochemical analyses supported successful functionalization, cross-linking, and formation of porous, structurally modified surfaces. At 298 K, the Langmuir adsorption capacities of wCF/EpKL/wCF/EpKL/TA reached 85.37/88.78 mg g⁻¹ for Cr(VI), 155.6/163.9 mg g⁻¹ for Pb²⁺, 51.80/84.49 mg g⁻¹ for DFC, and 194.4/195.4 mg g⁻¹ for BPA, respectively. Isotherm, kinetic, and thermodynamic analyses indicated favorable, heterogeneous, and spontaneous adsorption involving specific surface interactions and diffusion-related contributions. Spectroscopy-supported mechanistic interpretation suggested that metal species are retained mainly through complexation with amino, phenolic, and oxygen-containing groups, together with electrostatic interactions, whereas DFC and BPA adsorption involves hydrogen bonding, hydrophobic interactions, and π–π interactions with lignin/tannin aromatic domains. Fixed-bed experiments confirmed the applicability of the membranes under continuous-flow conditions, while regeneration, stabilization/degradation, and soil-burial tests supported their potential within a more sustainable water treatment cycle. Overall, the results demonstrate that waste-derived cellulose/lignin–tannin membranes are promising regenerable sorbents for the adsorption of chemically diverse inorganic and organic pollutants.