Gasification of heavy metal–contaminated biochar: Experimental investigation and thermodynamic analysis

Abstract

The thermochemical conversion of heavy-metal (HM)–contaminated biomass offers a pathway to produce syngas and functional biochar while simultaneously remediating the environment. Here, Zn/Pb-contaminated birch biomass from a phytoremediation site was carbonized and the resulting char was gasified under two oxidizing atmospheres (100 vol% CO₂ and 50/50 vol% H₂O/CO₂) at 700–900 °C. Gas composition, char conversion, biochar properties (SEM, Raman, N₂ adsorption), and solid-phase metal retention (ICP-OES) were evaluated together with thermodynamic equilibrium calculations (FactSage 8.4/FactFlow) based on measured elemental inputs (C, H, O, N, S, ash-forming elements, and trace metals). Gasification in an H₂O/CO₂ atmosphere markedly increased reactivity, achieving > 60 % conversion at 700 °C compared with < 20 % under CO₂ alone. The product gas was dominated by CO and CO₂, with enhanced H₂ under H₂O/CO₂. Zn retention decreased from 52.1 % at 700 °C to < 2 % at 900 °C, while Pb retention decreased from 86.1 % to 13.1 % under H₂O/CO₂. Activation produced biochars with BET surface areas up to ∼ 673 m2 g⁻¹ and average pore diameters up to ∼ 1.50 nm. Equilibrium calculations indicated increased Zn volatilization above ∼ 800 °C and predicted Pb stabilization as condensed PbO/PbS at lower temperatures, while K, Ca and Al were predicted to form stable condensed silicates/oxides. Overall, the combined experimental and equilibrium analysis quantifies trade-offs between conversion/activation performance and HM retention during CO₂ and H₂O/CO₂ gasification of contaminated biomass.