Upcycling pyrolyzed biowaste into functional alkali-activated biochar for hydrogen sulphide capture

Hydrogen sulphide (H₂S) is a hazardous gas that requires effective removal. In this study, biochar derived from vegetable-tanned, chrome-free leather industry residues was produced via pyrolysis at controlled temperatures 750°C and chemically activated using KOH and NaOH (ratios 0.36:1-3:1) to evaluate porosity development and surface chemistry modifications. Additionally, different washing methods (acid, alkaline, and hot water) were tested, with hot water washing proving effective in preserving oxygen functionalities in the optimal sample. The obtained activated biochar, with Brunauer-Emmett-Teller surface areas of up to 2660 m²/g, were evaluated as a sustainable alternative to commercial adsorbents for H₂S removal. Adsorption experiments revealed that KOH-activated biochar exhibited the highest H₂S uptake, reaching 288 mg/g, outperforming conventional coal-based activated carbons (ACs; 150 mg/g). The activation process significantly influenced material properties and adsorption efficiency, with KOH activation leading to enhanced microporosity, increased surface basicity, and improved gas-solid interactions. The study also examined adsorption mechanisms, highlighting the contributions of physisorption, chemisorption, and catalytic oxidation in sulphide retention. These findings demonstrate the potential of biochar-based adsorbents for gas purification applications. In addition, a techno-economic assessment supports the feasibility of utilizing biochar as a cost-effective and sustainable alternative to fossil-based ACs, with an estimated removal cost of $4.2 per kilogram of H₂S, reinforcing its role in waste valorization and circular economy strategies.