Synthesis of Zirconium-Based MOF–Biochar Composites for Efficient Congo Red Removal from Industrial Wastewater

Organic dye pollution in industrial wastewater is severe and difficult to degrade, posing a significant challenge to environmental management and water resource security. To meet the demand for the efficient elimination of Congo Red (CR) dye from industrial wastewater, this work prepared two zirconium-based metal–organic framework (MOF)–biochar composites, UIO-66@BY and UIO-67@BY, by in situ loading zirconium-based MOFs (UIO-66 and UIO-67) onto biochar (BY) via a solvothermal method. The composite material was comprehensively characterized using transmission electron microscopy (TEM), BET surface area analysis, and X-ray photoelectron spectroscopy (XPS). The adsorption results indicate that UIO-67@BY exhibits a significantly higher maximum adsorption capacity for CR dye compared to pristine biochar (BY), while UIO-66@BY also shows enhanced adsorption performance, but one that is slightly lower than that of UIO-67@BY. Further investigations reveal that the adsorption behavior conformed to a pseudo-second-order kinetic model and was well described by the Langmuir isotherm, suggesting that the adsorbent exhibited a homogeneous adsorption surface, and that chemical adsorption played a dominant role in the process. The primary mechanisms responsible for CR dye uptake by the composite include pore structure characteristics, coordination with functional groups, π–π stacking interactions, and electrostatic forces. The composite material developed herein provides an environmentally sustainable and economically efficient strategy for mitigating wastewater contamination.