Technical-environmental assessment of rice husk biochars as renewable adsorbents for hydrogen sulphide removal

This study presents a technical and environmental assessment of H2S removal using four rice husk (RH)–derived biochars produced by slow pyrolysis. Biochars were obtained under nitrogen (N2) or carbon dioxide (CO2) atmospheres, with and without potassium hydroxide (KOH) activation. The investigated materials included unmodified biochars (RH_N2 and RH_CO2) and biochars activated before (KOH_RH_CO2) or after (RH_CO2_KOH) CO2-assisted-pyrolysis. Commercial activated carbon (CAC) was used as a reference. Dynamic adsorption experiments were conducted at H2S concentrations between 35 and 700 ppm and gas hourly space velocities (GHSV) from 3822 to 30,573 h⁻¹ . Compared to N2, CO2-assisted-pyrolysis enhanced carbon content (74.0 vs. 72.0%), reduced H/C and O/C ratios, and improved specific surface area (SSA) (141.0 vs. 55.7 m²/g), and micropore volume (0.08 vs. 0.02 cm³/g). KOH-activation further enhanced SSA (178.4 m²/g) and micropore volume (0.60 cm³/g) in pre-activation, while decreasing carbon content due to potassium effects. Among the tested materials, KOH_RH_CO2 and CAC showed the highest H2S adsorption capacities. Although KOH_RH_CO2 exhibited a lower adsorption capacity than CAC at 700 ppm and GHSV of 3822 h⁻¹ , it demonstrated superior environmental performance. Life Cycle Assessment results showed that KOH_RH_CO2 achieved a negative global warming impact, corresponding to approximately 108% of the footprint of CAC but with an opposite sign, effectively shifting the process from carbon-intensive to carbon-negative. This outcome is attributed to the use of agricultural waste as feedstock and to carbon sequestration during CO2-assisted pyrolysis. Overall, CO2-pyrolyzed and KOH-activated RH biochars emerge as environmentally advantageous adsorbents for H2S removal in gas streams.