A Comparative Study on the Adsorption Performance and Mechanisms of Different Biochar Materials for Heavy Metal Ions in Water

Imperial College London, UK

^* Corresponding author: tianyepeng2026@163.com

Abstract

The escalating issue of heavy metal contamination in aqueous environments poses a significant threat to ecological balance and human health. Biochar, a carbon-rich material derived from biomass pyrolysis, has emerged as a cost-effective and environmentally benign adsorbent for water remediation. However, the adsorption efficacy of biochar is profoundly dependent on its original feedstock. This study provides a comprehensive comparison of the adsorption performance and underlying mechanisms of three distinct biochars, derived from rice husk (RHB), pine wood (PWB), and chicken manure (CMB)，for the removal of lead (Pb (II)) and cadmium (Cd (II)) from water. The materials were prepared via slow pyrolysis at 500°C and systematically characterized by means of BET, SEM, FTIR, and XRD. Batch adsorption experiments were conducted to evaluate the effects of pH, contact time, and initial ion concentration. The results indicated significant differences in both performance and mechanisms. CMB, rich in minerals and exhibiting a high pH in aqueous solution, exhibited the highest adsorption capacity for Pb (II) (148.5 mg/g) primarily through precipitation and ion exchange. Conversely, RHB, with its high specific surface area (289.4 m²/g) and siliceous structure, showed a superior performance for Cd (II) (89.2 mg/g), which was driven by surface complexation and physical adsorption. PWB demonstrated moderate, non-specific adsorption. Kinetic studies revealed that the pseudo-second-order model best described the adsorption process for all materials, which indicates chemisorption is a key rate-limiting step. The isotherm data of RHB and PWB fitted well with the Langmuir model, while that of CMB was more suitable for the Freundlich model due to its heterogeneous surface. This research underscores the critical importance of feedstock selection in designing biochar adsorbents tailored to remove specific heavy metal pollutants.