Metal oxide/biochar hybrid nanocomposites for adsorption and photocatalytic degradation of textile dye effluents: A review

The emerging organic pollutants from industrial wastewater have led to growing environmental concerns. The health hazards associated with these pollutants bring an urgency to adopt and develop an effective and sustainable method for their removal before disposal into water resources. In that regard, photocatalytic degradation using metal oxides has emerged as one of the most promising approaches for the remediation of industrial pollutants. However, the limitations of metal oxide in selective removal of pollutants, coupled with their wide band gap are the challenges that need to be addressed. Biochar is a cost-effective, environmentally friendly, and sustainable material derived from biomass obtained through agricultural and forestry residues as well as municipal waste. However, its limited adsorption capacity, difficulties in surface activation, and post-treatment sludge formation are major challenges in remediation of dye pollutants. The uniform dispersion of metal oxide nanoparticles over biochar surface significantly increases the specific surface area, resulting in higher number of active sites that enhances adsorption capacity as well as photocatalytic degradation efficiency. As a result, employing metal oxide/biochar nanocomposites (MOBNCs) for the adsorption and photocatalytic degradation of toxic dyes has emerged as a promising approach due to their tunable structural and morphological properties, environmental sustainability, and excellent stability. This review article summarizes recent advancements in MOBNCs synthesis using co-precipitation, sol-gel, hydrothermal/solvothermal and ball-milling methods. It further discusses various properties and characterizations of synthesized MOBNCs. The formation of MOBNCs produces synergistic effect in adsorption and photocatalytic degradation of dye pollutants from wastewater. This review article also highlights various operational parameters affecting adsorption and photocatalytic degradation of dyes and their mechanisms associated with different MOBNCs.