Valorization of chitosan-derived humins for removal of Cd(II), Cr(III) and Cr(VI)

• Chitosan-derived humins were synthesized by catalysis of oxalic acid. • Maximum adsorption of humins for Cr(VI) and Cd(II) reached 358.7 and 152.6 mg/g. • Adsorption of Cd(II)/Cr(III) and Cr(VI) proceeds through different mechanisms. • The structure-performance relationship was disclosed through structural modification. • Humins exhibit remarkable adsorption performance even in complex environments. The generation of humins by-products in biorefinery is often unavoidable. We here report the valorization of chitosan-derived humins for removal of heavy metal ions to enhance the atomic economy of the biomass conversion. Chitosan-derived humins were synthesized by catalysis of oxalic acid (OA), whereby the adsorption performance for Cd(II), Cr(III) and Cr(VI) was explored. The adsorption data were well fitted by the pseudo-second-order kinetic model, complemented by the Freundlich and Temkin isotherm models, revealing maximum adsorption capacities of humins for heavy metal ions as follows: 358.7 mg/g for Cr(VI), 152.6 mg/g for Cd(II), and 40.2 mg/g for Cr(III). The mechanistic studies revealed that the adsorption of Cd(II) and Cr(III) by humins mainly occurs through ion exchange (carboxyl groups) and surface complexation (hydroxyl, carbonyl, and amino groups). In comparison, the adsorption of Cr(VI) by humins involves electrostatic attraction, alongside ion exchange and surface complexation. After carbonization and activation, the specific surface area increased markedly, however the adsorption capacity decreased owing to reduced oxygen- and nitrogen-containing functional groups. Competitive adsorption studies, industrial wastewater experiments, and cyclic adsorption tests demonstrate that humins exhibit remarkable adsorption performance even in complex environments. This work highlights that chitosan-derived humins are promising adsorbents for the remediation of heavy metal ions.