Co-adsorption and selective-adsorption of heavy metals and dyes from aqueous solution by bio-based humus/chitosan hydrogels

An eco-friendly adsorbent was prepared by reverse suspension crosslinking method to remove multiple pollutants from aqueous solution. Both raw materials, derived from humus (HS) and chitosan (CS), are biodegradable and low-cost natural biopolymers. After combining HS with CS, the adsorption capacity was significantly improved due to compensation effects between the two components. HS/CS exhibited the features of amphoteric adsorption through pH adjustment, enabling it to adsorb not only anionic pollutants (methyl orange(MO) and Cr(VI)), but also cationic ones (methylene blue(MB) and Pb(II)). The adsorption capacities were approximately 242 mg/g, 69 mg/g, 188 mg/g and 57 mg/g for MO, Cr(VI), MB and Pb(II), respectively. HS/CS showed a slight preference for MO in MO/Cr(VI) co-adsorption system, whereas strong selectivity for MB over Pb(II) in MB/Pb(II) system under acidic condition (pH<3.0). This selective behavior would allow for potential applications in separating MB/Pb(II) effluents and selectively recycling Pb(II) in acidic environment. The isothermal and kinetic adsorption behaviors followed Langmuir model and pseudo-second-order model, respectively. The density functional theory (DFT) confirmed that the interaction between metal ions and adsorbents were primarily attributed to chelation and electrostatic adsorption, owing to nitric and oxygenic functional groups. Whereas, the adsorption mechanisms for dyes were involved in electrostatic attraction, H-bond and π–π bond, due to available hydrogen, oxygen, nitrogen atoms and aromatic groups on the surface of adsorbent and adsorbates. The adsorbent could be efficiently regenerated and retained over 90% of its adsorption capacity after five cycles, which has a potential for practical applications in water treatment.