Chitin-dominated molecular mechanism for hexavalent chromium biosorption by Aspergillus niger spores: Spectroscopic elucidation and DFT validation

Abstract

Addressing the critical environmental challenge of hexavalent chromium [Cr(VI)] contamination, understanding the underlying biosorption mechanisms is essential for developing effective and sustainable biosorbents for heavy metal remediation. This study systematically elucidated the chitin-dominated molecular mechanism of Cr(VI) biosorption by Aspergillus niger spores (AS) through comprehensive characterization and mechanistic analysis. Ion chromatography and gas chromatography-mass spectrometry revealed that AS comprises 91.7 % polysaccharides, with chitin presence confirmed via D-glucosamine hydrochloride (13.2 %), establishing the molecular foundation for metal binding. The echinulate morphology provides abundant contact sites, while the chitin-rich composition creates multifunctional binding sites including amide groups from chitin, hydroxyl groups from glucans, and carboxyl groups from proteins. Batch adsorption experiments demonstrated exceptional performance with a maximum adsorption capacity of 106.29 mg g^-1 at pH 2.0, following pseudo-second-order kinetics that confirmed chemisorption as the predominant mechanism. DFT calculations validated chitin's dominant role, showing strong binding affinity with a binding energy of 2.47 eV through coordination interactions. Density functional theory calculations validated chitin's dominant role with a binding energy of 2.47 eV, while Fukui function analysis precisely localized reactive sites to amino and hydroxyl functional groups. Based on comprehensive experimental and theoretical evidence, a chitin-dominated three-stage molecular mechanism was proposed: pH-dependent electrostatic interactions facilitate initial contact, chitin mediates Cr(VI) reduction to less toxic Cr(III), and coordination complex formation ensures stable immobilization. This mechanistic understanding provides theoretical guidance for developing chitin-based functional materials and advancing sustainable heavy metal remediation strategies.

Keywords: Aspergillus niger spores; Biosorption; Chitin; Cr(VI); Molecular mechanism.