Simultaneous redox transformation and removal of Cr(Ⅵ) and As(Ⅲ) by polyethyleneimine modified magnetic mesoporous polydopamine nanocomposite: Insights into synergistic effects and mechanisms

Chromium(Ⅵ) and arsenic(Ⅲ) as typical anionic heavy metal pollutants normally coexist in the environment, greatly aggravating their environmental risks and elevating the difficulty of remediation. Here, a novel polyethyleneimine modified magnetic mesoporous polydopamine nanocomposite (Fe3O4 @mesoPDA/PEI) with abundant active functional groups was exploited as the synchronous adsorbent of Cr(Ⅵ) and As(Ⅲ). The results showed that Cr(Ⅵ) and As(Ⅲ) could mutually promote their conversions and adsorptions on Fe3O4 @mesoPDA/PEI. The adsorption mechanisms of Fe3O4 @mesoPDA/PEI were primarily redox chemistry and also involved electrostatic interactions and coordination. Cr(Ⅵ) was mainly reduced by reductive catechol, while As(Ⅲ) was oxidized to As(Ⅴ) by oxidative active substances (e.g., H2O2, •OH, and quinone). Meanwhile, active intermediate (semiquinone radicals) generated during the Cr(Ⅵ) reduction and As(Ⅲ) oxidation could constitute redox microcirculation with Cr(Ⅵ) and As(Ⅲ) to further accelerate redox reactions of Cr(Ⅵ) and As(Ⅲ) on Fe3O4 @mesoPDA/PEI, thereby exhibiting a synergistic effect. Moreover, newly immobilized Cr(Ⅲ) onto Fe3O4 @mesoPDA/PEI became extra active sites for As adsorption through cation bridges and then recovered by magnetic separation in favor of diminishing the environmental hazards of Cr and As. These findings also provide new inspirations for the roles of redox-active functional groups in the remediation of multiple redox-sensitive heavy metals including Cr(Ⅵ) and As(Ⅲ).