It has been synthesized, characterized and tested a new biomaterial AlgS (sodium alginate functionalized with cysteine) to remove Pb(II) in aqueous media. The maximum Pb(II)-sorption capacity of AlgS (Qmax = 770 mg·g−1) is between almost two and nine times higher than other alginate-materials reported in the literature. Techniques, such as TGA/DSC, SEM/EDS, BET, FTIR, UV–Vis, XRD and 13C solid state-NMR have been used to study the chemical-modification of alginate at oxidation and aminofication stages. The formation of the imine intermediate (–C=N–), after 24 h of reaction was identified by a UV band at 348 nm. Typical IR-bands of AlgS were identified at 2970, 955, 949 and 1253 cm−1 which are associated to C–H, S–Pb, S–H and C–N stretching vibrations, respectively. 13C solid state-NMR spectra of AlgS, show peaks at 33–38 ppm and 55–60 ppm associate to δ (HS-CH2-) of cysteine and δ (C–N) respectively. The ΔH° and ΔG° negative values for Pb(II) sorption indicate that it is an exothermic process and occur spontaneously. Finally, it was found that the Pb(II) sorption on AlgS is significantly affected by the presence of cationic (Na+, Mg2+ and Al3+) and anionic (Cl−, NO3−) co-ions.