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

This study presents a technical and environmental assessment of H 2 S removal using four rice husk (RH)–derived biochars produced by slow pyrolysis. Biochars were obtained under nitrogen (N 2 ) or carbon dioxide (CO 2 ) atmospheres, with and without potassium hydroxide (KOH) activation. The investigated materials included unmodified biochars (RH_N 2 and RH_CO 2 ) and biochars activated before (KOH_RH_CO 2 ) or after (RH_CO 2 _KOH) CO 2 -assisted-pyrolysis. Commercial activated carbon (CAC) was used as a reference. Dynamic adsorption experiments were conducted at H 2 S concentrations between 35 and 700 ppm and gas hourly space velocities (GHSV) from 3822 to 30,573 h⁻¹ . Compared to N 2 , CO 2 -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_CO 2 and CAC showed the highest H 2 S adsorption capacities. Although KOH_RH_CO 2 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_CO 2 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 CO 2 -assisted pyrolysis. Overall, CO 2 -pyrolyzed and KOH-activated RH biochars emerge as environmentally advantageous adsorbents for H 2 S removal in gas streams.