Activated biochars from heavy metal-contaminated biomass for CO2 capture: Adsorption performance and dominant mechanisms

Abstract

This study evaluates the CO₂ adsorption performance of activated biochars derived from heavy metal (HM)-contaminated biomass, presenting a sustainable approach that integrates waste valorization with environmental remediation. HM-enriched biomass was converted into high-performance CO₂ adsorbents through pyrolysis followed by activation via CO₂ and steam gasification under varying conditions. Comprehensive characterization using ICP-OES, SEM, FTIR, Raman spectroscopy, XPS, and N₂ adsorption analyses revealed high microporosity and surface area, with negligible structural differences between biochars from contaminated and uncontaminated biomass. The highest CO₂ uptake (2.10 mmol g⁻¹ at 25 °C) was achieved for pine-based biochar activated under pure CO₂ at 800 °C for 120 min. Adsorption isotherms measured at 25–75 °C were well described by both Langmuir and Freundlich models. Kinetic analysis showed strong agreement with the pseudo-first-order, pseudo-second-order, and Avrami models, with the latter providing the best fit, indicating surface-limited physisorption as the dominant mechanism. Adsorption–desorption cycling over ten consecutive runs confirmed the structural stability and reusability of the optimized biochar. Comparative evaluation demonstrated superior performance of these biochars over commercial activated carbons. Notably, the presence of heavy metals had an insignificant effect on CO₂ adsorption capacity, highlighting the dominant role of physicochemical properties in adsorption performance. These results underscore the potential of HM-contaminated biomass as a low-cost and effective precursor for CO₂ adsorbent production, contributing to both climate change mitigation and environmental sustainability.