Tracking the Molecular-Scale Mechanistic Pathway of Trapping-Bonding CTAB/Fe3O4-AS for High-Performance Cr(VI) Adsorption

Abstract

Cr(VI) contamination threatens the environment and health, necessitating urgent, sustainable removal solutions. Composite biological adsorbents are promising, yet the molecular-level adsorption mechanisms remain unclear. This gap in understanding hinders the development and application of more efficient biological adsorbent materials. Here, we propose a magnetic composite adsorbent based on Aspergillus niger spores (CTAB/Fe3O4-AS) to track the trapping-bonding mechanisms of Cr(VI) on its surface. Characterization analysis showed that Fe3O4 nanoparticles and CTAB form a stable magnetic shell and an outermost quaternary ammonium layer on the spore surface. This synergistic structure enhanced the Cr(VI) adsorption capacity to 186 mg·g−1 at pH 2.0 in solution and achieved a Cr(VI) removal rate exceeding 99 % in electroplating wastewater, with dosage of 1.8 g·L−1 after 180 min. CTAB/Fe3O4-AS selectively removes Cr(VI) from acidic wastewater, with selectivity coefficients of KCr/Zn = 1334 and KCr/Cu = 2661. Cr(VI) removal occurs mainly through chemical adsorption via heterogeneous processes. DFT calculations and MD simulations visualize reaction pathways and interaction strengths at the molecular scale. The quaternary ammonium cation group ((CH3)4-N+) in CTAB traps Cr(VI) ions electrostatically, transferring them to the spore surface. Electron-donating groups (–COO, –NH-) on the spores reduce Cr(VI) to Cr(III). Amide groups in chitin create optimal coordination sites for Cr(III) by twisting, with nitrogen atoms forming stable complexes through coordination bonds with Cr(III)’s vacant orbitals. This study not only developed an efficient microbial-based composite adsorbent but also proposed a novel synergistic adsorption mechanism: electrostatic trapping–reduction–coordination bonding. This provides valuable insights for developing more efficient and applicable heavy metal ion adsorbent materials in the future.