Poly(acrylic acid) and Poly(acrylic acid-b-acrylamide) via RAFT: Effect of Composition on Ni²+ Binding Capacity

Two poly(acrylic acid-b-acrylamide) copolymers, and a poly(acrylic acid) homopolymer were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization and tested for nickel removal from aqueous media. RAFT was used for better interchain composition homogeneity and to facilitate the synthesis of the block copolymers. Liquid-phase polymer-based retention (LPR) was used to compare the nickel binding capacity. Uptake tests were carried out at pH 3.0, 4.0, and 5.0. Nickel adsorption isotherms in the range of 1 to 7 mmol L⁻¹ were achieved and the Langmuir adsorption model was applied. Maximum binding capacity (Q_m) of Ni²⁺ at 298 K depended on the composition of the copolymers and pH. The polyacid block (PAA) was the major structural feature responsible for high values of nickel binding. All materials presented better uptake at higher pH, probably due to polyacid deprotonation and the increase in the electrostatic interactions. Up to approximately 100 mg of nickel per gram of homopolymer could be retained at pH = 5.0. Even though the presence of a poly(acrylamide) block decreases the binding capacity compared to PAA, that block has a specific contribution to nickel binding and can also provide better features to the material as better solubilization, and others.

Keywords:
hydrophilic block copolymers; RAFT polymerization; nickel removal; liquid-phase polymer-based retention; decontamination