Modified Biochar for Dye Removal from Wastewater: Recent Advances in Functionalization Techniques and Adsorption Mechanisms: A Review

Abstract

The presence of synthetic dyes in industrial wastewater presents a serious environmental threat due to their high chemical stability, toxicity, and resistance to biodegradation. This review provides a comprehensive analysis of recent advancements in the use of modified biochar as a sustainable and cost-effective adsorbent for dye removal from wastewater. Various thermal conversion methods, such as pyrolysis, hydrothermal carbonization, gasification, and torrefaction, are discussed, as they affect biochar properties, including porosity, carbon content, and active functional groups. Furthermore, the review systematically examines physical (e.g., heat treatment and grinding), chemical (e.g., modification with acids, bases, and metals), and biological (e.g., using algae or microorganisms) modification methods to enhance the adsorption capacity and selectivity of biochar. Special attention is given to chemical modifications, such as chitosan and metal impregnation, which have been reported to significantly improve dye removal efficiencies, often exceeding 80%. Various adsorption mechanisms were also analyzed, including physical adsorption, ion exchange, and electrostatic interactions. Moreover, the review highlights the role of surface area, porosity, and functional group composition in improving sorption capacities, alongside recent efforts to model and optimize these processes using artificial intelligence and machine learning. Finally, the modified biochar is an environmentally sustainable option for treating industrial wastewater and achieving circular economy goals.