A critical review on green synthesis and modification techniques of biochar: Comparison of efficacies towards adsorption capacities

The green synthesis and modification of biochar are two technological advances in bio-adsorbent-mediated wastewater treatment. The environmental sustainability, circular economy, waste valorization, and efficient management of pollutants are reviewed. "Green synthesis" is the use of biological extracts in the synthesis of metal and metal oxide nanoparticles. The green-synthesized nanoparticles are amalgamated with biochar modified by chemical, mechanical, or thermal means, both in situ and ex situ. The methods of synthesis, types and agents of modification are compared for improved adsorption capacities of biochar. The highest absorption capacity (∼400 mg g⁻¹) is achieved by chemical activation followed by thermal (∼240 mg g⁻¹) and mechanical (∼70 mg g⁻¹) ones. Meanwhile, amines and quaternary ammonium compounds are found as the best chemical agents for the functionalization of biochar. The quantitative analyses of the relationship between biochar modification and its adsorptive capacity are proposed by recent developments in chemistry, such as synchrotron X-ray, gradient boosting regression (GBR) model. Artificial neural networks (ANN), AI, and ML predictive models are also proposed for more accurate assessment of activation mechanisms and adsorption capacities.