The grafting of urea and biuret on alginate backbone (confirmed by MR spectrometry analysis) increases by 53-84% the sorption efficiency of the biopolymer for Cd(II), Cu(II) and Pb(II) recovery from mild acidic solutions (i.e., pH in the range 4.5-5.5). Sorption isotherms are successfully described by the Langmuir and the Sips equations: maximum sorption capacities reach up to 3.7 mmol Cd g(-1), 4.7 mmol Cu g(-1) and 4.8 mmol Pb g(-1) for alginate-urea beads. The affinity of the sorbent for metal ions is correlated to the electronic properties (softness) and configuration of hydrated ions and to the HSAB properties (Hard and Soft Acid-Bases rules) of the ligands present on the sorbent. The diffusion properties are playing a significant role in the control of the uptake kinetic profiles; however, the pseudo-second order rate equation also allows a good fit of experimental sorption kinetics. In multi-component solutions the sorbents maintain a higher affinity for Pb(II) over the other two divalent cations. (C) 2017 Elsevier B.V. All rights reserved.