Rhodamine B Adsorptive Removal and Photocatalytic Degradation on MIL-53-Fe MOF/Magnetic Magnetite/Biochar Composites

MIL-53-Fe metal–organic framework (MOF) was grown using the terephthalic acid linker and FeCl3 into an already prepared, high surface area, magnetic, Douglas fir biochar/Fe3O4 (MBC) adsorbent hybrid. This resulting triphase hybrid, multifunctional, magnetically recoverable, sorptive, photocatalytic and degradative, adsorbent (MOF–MBC) was used both to remove and catalyze the photodegradation of Rhodamine B (Rh B) with or without Cr6+ present. Rh B is a widely used colorant in textile, printing and tanning industries that is also associated with deleterious health effects. Batch aqueous sorption studies were performed at various pHs, Rh B concentrations and temperatures in-order to determine the optimum adsorption pH, kinetics, thermodynamics and sorption capacity. This adsorption followed pseudo-2nd-order kinetics and exhibited a Rh B Langmuir adsorption capacity of ~ 55 mg/g at pH 6, 200 rpm agitation and 25 °C. This MOF–MBC hybrid was characterized by SEM, TEM, EDS, XRD, FT-IR, TGA, BET, Elemental Analysis and XPS. Deethylated and carboxylic compounds were identified as photodegradation intermediates. Electrostatic and π–π stacking interactions are thought to play a significant role in Rh B sorption. Hexavalent chromium (Cr6+) and Rh B often co-exist in tannery and printing waste water. Cr6+ can trigger the photo-degradation of Rh B into CO2 and H2O in the presence of both MIL-53-Fe MOF and MOF–MBC. Hence, adsorbent stripping regeneration can be minimized in real world applications. The biochar phase, aids to disperse the MOF, to minimize particle aggregation, to provide extra stability to the MOF, and serves as secondary adsorption site for heavy metal, oxy anion and organic contaminants. Large biochar particles allow reasonable flow through column beds while supporting other nanophases, which would cause large pressure drops when used alone.