Heterogeneous Reduction of 2-Chloronitrobenzene by Co-substituted Magnetite Coupled with Aqueous Fe²⁺: Performance, Factors, and Mechanism

Reductive transformation is the predominant degradation pathway for nitrobenzene (NB) derivatives in natural matrices. Magnetite coupled with aqueous Fe²⁺ (Fe²⁺_aq) displays reducing capability toward NB derivatives, but it is still unclear whether the substitution of redox-active metals in magnetite has significant influence on the reducing capability of the coupled system. This study investigated the potential of the heterogeneous reduction of 2-chloronitrobenzene (2-Cl-NB) by co-substituted magnetite (Fe_3-xCo_xO₄, 0.00 ≤ x ≤ 1.00) coupled with Fe²⁺_aq. Both reaction kinetics and extent of electron transfer illustrated that appropriate Co substitution (x ≤ 0.77) significantly promoted the reduction activity of Fe_3-xCo_xO₄/Fe²⁺_aq systems, while excess Co (x > 0.77) retarded the process. A good linear correlation (R² ≥ 0.94) was established between the electrical conductivity of Fe_3-xCo_xO₄ and the rate constant (k_obs), calculated from a three-parameter single-exponential decay model. The improvement of the reduction activity was ascribed to the redox pairs Co(II)/Co(III) and Fe(II)/Fe(III) on the octahedral sites, which accelerated the electron transfer in magnetite. As Co substitution increased up to x = 0.85, however, structural Fe(II) occupying the octahedral sites of magnetite was too low, resulting in a decrease of the reducing capability of the coupled system. During the redox reaction, adsorbed Fe²⁺_aq and structural Fe(II) were oxidized gradually, while the spinel structure of Fe_3-xCo_xO₄ was maintained. These results shed light on the role of magnetite group minerals and their impact on the fate of contaminants in anoxic environments.