Development of an acidized biochar-supported hydrated Fe(III) oxides for highly efficient cadmium and copper...

Biochar-supported metallic oxides are attractive adsorbents for heavy metal cleanup, but the adsorption performance is still unsatisfactory as a result of the self-aggregation of the incorporated metallic oxides. A new hybrid nano-material was prepared through impregnating hydrated ferric oxide (HFO) nanoparticles within biochar bearing high-density charged oxygen-containing groups (e.g., carboxyl and hydroxyl groups) (ABC) derived from HNO₃ treatment. The as-made adsorbent, denoted as HFO-ABC, possesses highly dispersed HFO nanoparticles with typical size lower than 20 nm, and exhibits greater sorption capacity for Cd(II) and Cu(II) than the pristine biochar-supported HFO. It also shows great sorption preference toward Cd(II) and Cu(II) in co-presence of high levels of Ca²⁺, Mg²⁺ and humic acid (HA). Such prominent performance is put down to the high-density charged functional groups on the host ABC, which not only promote the dispersion of the immobilized HFO nanoparticles but also generate the potential Donnan membrane effect, i.e., the pre-concentration and permeation of target metals prior to their preferable adsorption by nano-HFO. The predicted effective coefficients of intra-particle diffusion for Cu(II) and Cd(II) are 3.83 × 10^-9 and 4.33 × 10^-9 cm²/s, respectively. HFO-ABC exhibits excellent performance for fixed-bed column application, and yields 513 and 990 BV effluents for Cd(II) and Cu(II) to achieve their discharge standards, respectively. The spent HFO-ABC could be in situ regenerated using binary HCl-CaCl₂ solution with desorption efficiency higher than 95%. All results manifest that increasing charged functional groups via HNO₃ treatment is an effective measure for boosting sorption performance of biochar-based nanocomposites.