Performance and mechanism of Pb²⁺ adsorption by modified biocharcalcium alginate aerogel balls from solution

Abstract

Targeting at enhancing the adsorption performance of biochartowards heavy metals, in this study, composite aerogel balls were prepared using KMnO₄-modified cotton stalk biochar, sodium alginate, and CaCl₂ (crosslinking agent) as precursors by sol-gel and freeze-drying methods, and their performance and mechanism of Pb²⁺ adsorption from solution were investigated. The results showed that the specific surface area (SSA) of the aerogel balls was 28.23 m²/g, the zero point of charge was 1.6, the diameter was approximately 3 mm, the single particle weight was about 1.3 mg, and they had interconnected developed three-dimensional pore structure, rich functional groups, and exchangeable cations.  The pseudo-second-order and Langmuir equations fitted best; separation and exponential factors both fell within 0–0.5, signalling facile, predominantly monolayer chemisorption.  Elevated temperature accelerated uptake and raised capacity; at 35 °C the Langmuir maximum for Pb²⁺ on the aerogel beads was 665.2 mg g⁻¹. After 8 cycles of reuse, the Pb²⁺ removal rate of the aerogel balls remained stable at 56%, indicating good regenerative performance. The combination of calcium alginate gel and modified biochar had a synergistic effect on Pb²⁺ adsorption; the aerogel balls could simultaneously adsorb Pb²⁺ through complexation, ion exchange, precipitation, pores, π bonds, electrostatic attraction, and other effects. The research results provide theoretical data for the performance of biochar aerogel materials in adsorbing heavy metals and also provide references for the development of green, low-cost biochar aerogel materials.