The adsorption of heavy metals onto biochar is governed by complex interactions involving surface chemistry, pore structure, and diffusion dynamics. In this study, the mechanisms underlying the removal of Ni(II), Cu(II), Cd(II), and Pb(II) from aqueous solutions were systematically investigated using biochars derived from paper mill sludge (PMS) and rice straw (RS), produced under N₂ and CO₂ purging environments. Adsorption equilibrium data fitted best to the Langmuir model for Cd(II), Cu(II), and Pb(II), indicating monolayer adsorption on homogeneous surfaces, while Ni(II) followed the Freundlich model, suggesting multilayer adsorption on heterogeneous sites. The maximum adsorption capacities varied significantly with feedstock and gas type, with PMS-N₂ biochar showing the highest capacity for Pb(II) at 256.4 mg g⁻¹, highlighting its superior performance.
Kinetic analysis revealed that adsorption rates were rapid during the first hour, with pseudo-second-order kinetics providing the best fit across all systems. This indicates that chemisorption—likely involving electron sharing or exchange between metal ions and functional groups—is the rate-determining step. Intraparticle diffusion modeling further distinguished two phases: film diffusion dominated initially, contributing 30–62% of total uptake within one hour, while pore diffusion controlled the later stages. The intercept values (C) in the intraparticle diffusion model were non-zero, confirming the presence of boundary layer resistance and confirming that both external mass transfer and internal diffusion influence overall kinetics.
Surface characterization via XPS and Raman spectroscopy identified key functional groups responsible for metal binding.Hck Antibody Epigenetics The C1s spectra showed distinct peaks corresponding to aromatic C=C, hydroxyl C–O, and carboxyl O=C–O bonds, with higher oxygen content observed in RS biochars.TBK1 Antibody Epigenetics However, despite greater oxidation levels, RS biochars exhibited lower metal adsorption, suggesting that functional group abundance alone does not dictate performance. Instead, the synergistic effect of mineral content and surface morphology plays a more critical role. For instance, PMS biochars contained abundant Ca and Fe species, which enhanced ion exchange capacity and facilitated precipitation of metal hydroxides at near-neutral pH (equilibrium pH ≈ 8.5). Visual MINTEQ simulations confirmed positive saturation indices for all four metals at pH 8.5, supporting the formation of insoluble hydroxides as a significant removal mechanism.
Additionally, the interaction between CO₂ and organic matter during pyrolysis led to suppression of aliphatic and oxygen-containing functional groups in RS-CO₂ biochar, reducing its affinity for Cu(II) and Cd(II). Conversely, the catalytic action of Fe and Ca in PMS promoted porosity development and preserved functional groups, resulting in higher surface area and better metal retention.PMID:35133008 These findings underscore that while surface functional groups are essential, their effectiveness depends on the underlying matrix and mineral composition.
Overall, the dominant mechanisms include electrostatic attraction, ion exchange, surface complexation, and precipitation. Among these, ion exchange and complexation with hydroxyl and carbonyl groups were particularly effective for Pb(II) and Cu(II). The interplay between physical (surface area, pore size) and chemical (functional groups, mineral content) properties determines the efficiency and stability of metal adsorption. Therefore, optimizing biochar for wastewater treatment requires balancing these factors through strategic feedstock selection and controlled pyrolysis conditions. Future work should explore real effluent matrices to assess the impact of competing ions and organic matter on adsorption performance under dynamic environmental conditions.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com