In situ synthesis and characterization of bimetallic MIL-100(Fe, Mn)/chitosan composite: A synergistic adsorbent for arsenite

Arsenic contamination in drinking water is a global issue, prompting the development of various adsorbents, including metal-organic frameworks (MOFs) and their composites, for arsenic removal. Among these, bimetallic MOFs have garnered significant attention due to their superior adsorption properties compared to monometallic analogues. In this study, composite adsorbents have been developed by in situ syntheses of MIL-100(Fe) MOF (trimesic acid + Fe) in a chitosan (CS) matrix with partial isomorphic substitution of iron by manganese, cobalt, nickel, or copper metals. The incorporation of dopants was elucidated using robust techniques such as powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and inductively coupled plasma-optical emission spectrometry (ICP-OES). All doped adsorbents demonstrated enhanced arsenic removal efficiency, with the Mn-doped adsorbent (CS-MIL-100 (Fe, Mn)) showing the best performance. The CS-MIL-100(Fe, Mn) exhibited faster adsorption kinetics compared to monometallic CS-MIL-100(Fe) for both arsenic species, particularly for As(Ⅲ), where the equilibrium time was reduced from 6 h to 3 h. CS-MIL-100(Fe, Mn) achieved comparable maximum uptake capacities of 65.4 and 66.8 mg/g for As(Ⅲ) and As(Ⅴ), respectively. Spectroscopic analyses suggested that the efficient and rapid removal of As(Ⅲ) was due to the presence of Mn sites, which enhance the oxidation of As(Ⅲ) to As(Ⅴ), thereby increasing the overall adsorption of As(Ⅴ) on the Fe nodes. Additionally, CS-MIL-100(Fe, Mn) demonstrated 95 % selectivity towards arsenic species in the presence of various anions, including arsenate mimicking phosphate. • Isomorphic substitution of iron in MIL-100(Fe)/chitosan composite. • The substitution of iron metal nodes with a secondary metal is proved using PXRD. • All CS-MIL-100(Fe, M) exhibited exceptional As(Ⅴ) and As(Ⅲ) adsorption. • CS-MIL-100(Fe, Mn) improved the adsorption capacity by 85 % for As(Ⅲ). • An overall improvement in adsorption kinetics is observed for As(Ⅴ) and As(Ⅲ).