Proper management of wastewater contaminated with heavy metals through biosorption supports climate change mitigation, green growth, and the circular bioeconomy by generating innovative, biodegradable biosorbents. In this study, lignocellulosic materials derived from residual fennel seeds (Foeniculum vulgare), in both virgin (FS) and modified (MFS) forms, were employed for the simultaneous adsorption of Cd, Cr, Cu, and Pb ions from simulated and real wastewaters, for the first time. Physicochemical and instrumental characterisations confirmed the presence of diverse functional groups, high cation exchange capacity, and favourable morphology essential for efficient adsorption. The influence of pH, biosorbent mass, initial concentration, and contact time on the adsorption kinetics and capacity was evaluated. Under optimal conditions (solution pH: 5 (FS)/4 (MFS), initial individual analyte concentration: 50 mg/L, contact time: 20 min with 300 mg of biosorbent), maximum adsorption capacities reached 5.1/7.5 (Cr), 6.1/10.2 (Cu), 3.6/6.7 (Cd), and 7.9/14.7 mg/g (Pb) for FS/MFS, with corresponding removal efficiencies of 50-87.5% (FS) and 50-97.5% (MFS). The metal binding affinity followed the order: Cd < Cr < Cu < Pb. Kinetic analysis revealed a strong correlation with the pseudo-second-order and intraparticle diffusion models (average fit of the capacity values: 99.60% (FS) and 99.85% (MFS), with determination coefficients (R2) close to 1). Overall, the developed biosorption process is cost-effective, environmentally benign, and technically applicable, fully aligning with the principles of green chemistry.