Functionalized chitosan adsorbents allow recovery of palladium and platinum from acidic aqueous solutions

Platinum (Pt) and palladium (Pd) are precious metals considered critical in our society and are needed in a variety of sustainable technologies. Their scarcity urges the increase of recycling from secondary waste streams through new and efficient recovery techniques. Adsorption is an established recovery method for liquid streams, where chitosan shows promising results as a low-cost adsorbent, derived from biomass. This biopolymer is able to capture metals, but suffers from a low stability under acidic conditions and poor adsorbing properties. In this study, three new chitosan derivatives were synthesized and employed for Pd(II) and Pt(IV) recovery from acidic solutions. Specific and simple modifications were selected based on their known affinities for these metal ions and taking into account the principles of green chemistry. The prepared derivatives consist of 1,10-phenanthroline-2,9-dicarbaldehyde cross-linked chitosan (Ch-PDC), [2,2'-bipyridine]-5,5'-dicarbaldehyde cross-linked chitosan (Ch-BPDC) and glutaraldehyde cross-linked chitosan grafted with 8-hydroxyquinoline-2-carbaldehyde (Ch-GA-HQC). For all derivatives, the adsorption occurred fast and equilibrium reached within 30 min. The Langmuir isotherms revealed a maximum adsorption capacity for Pd(II) and Pt(IV) of respectively 262.6 mg g(-1) and 119.5 mg g(-1) for Ch-PDC, 154.7 mg g(-1) and 98.3 mg g(-1) for Ch-BPDC and 340.3 mg g(-1) and 203.9 mg g(-1) for Ch-GA-HQC. Such adsorption capacities are considerably higher compared to the biosorbents reported in the literature. Excellent physical properties in homo-and heterogeneous systems and high regeneration performances demonstrate that chitosan-based adsorbents are very promising for Pd(II) and Pt(IV) recovery from acidic solutions.