Enhanced heavy metal removal using nano-engineered biochars synthesized under variable pyrolysis conditions and feedstocks

This study investigates the adsorption efficiency of nano-biochars (n-BC) produced via ball-milling technique from various feedstocks (willow-WL, sewage sludge-SSL, rice husk-RH, oilseed rape-OSR) under different pyrolysis conditions. The most notable differences between n-BC and bulk biochars (b-BC) were observed in specific surface area (S_BET), total pore volume (V_p), and ash content—all of which increased in n-BCs, potentially enhancing their adsorption capacity. Adsorption kinetics, isotherms, and the impact of feedstock, pyrolysis temperature, atmosphere, and particle size on PTEs removal were analyzed. The adsorption performance toward Cd(II), Zn(II), and Pb(II) ions was evaluated separately in individual experiments to avoid competitive effects. The results demonstrated that n-BC significantly enhances the adsorption of Cd(II), Zn(II), and Pb(II) ions, with Pb(II) showing the highest adsorption capacity ranged from 276.4 to 431.9 mg/g. Adsorption followed a pseudo-second-order model, indicating chemisorption as the dominant mechanism. Reducing b-BC to the nanoscale increased adsorption capacity by up to 332 %, with specific feedstocks and pyrolysis conditions optimizing performance. This study provides valuable insights into BC's potential for sustainable PTEs remediation and highlights the critical role of nanostructuring and process optimization in enhancing adsorption efficiency. Moreover, this study offers a unique and systematic comparison of n-BCs derived from a wide range of feedstocks, spanning from lignocellulosic materials to more complex, nutrient- and ash-rich sources such as SSL. This comprehensive approach enables an in-depth exploration of how the intrinsic physicochemical properties of different feedstocks influence the PTEs adsorption performance of n-BCs. In addition, the study highlights the critical role of N₂ as a pyrolysis atmosphere in significantly enhancing Zn(II) adsorption. This distinctive observation emphasizes the importance of tailoring pyrolysis conditions to maximize the functional performance of biochar (BC) materials.

By providing a detailed understanding of the interplay between feedstock characteristics and pyrolysis parameters, this work makes a substantial contribution to the development of next-generation, sustainable technologies for the removal of PTEs.