Use of carbon-encapsulated zero-valent iron nanoparticles from waste biomass to hydrogen sulphide wet removal

This study evaluates the use of carbon-encapsulated zero-valent iron nanoparticles for biogas upgrading in wet systems. The nanoparticles were produced by hydrothermal carbonization, using olive mill waste (OMW) or microalgae as carbon sources. The solids were characterized to investigate the specific surface area, total and zero-valent iron content, pH_PZC and chemical and crystalline composition. Their adsorption performance towards hydrogen sulphide (H₂S) was tested by treating two types of synthetic biogas with and without CO₂. In both cases, the starting H₂S concentration was approximately 60 ppm and the experiments lasted until the complete saturation of the nanoparticles. Optimal Fe/C ratios of 0.05 for OMW nanoparticles and 0.2 for microalgae nanoparticles demonstrated H₂S-specific adsorption capacities of 9.66 and 9.55 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mrow><mml:mi>mg</mml:mi></mml:mrow><mml:mrow><mml:msub><mml:mi>H</mml:mi><mml:mi>2</mml:mi></mml:msub><mml:mi>S</mml:mi></mml:mrow></mml:msub><mml:mspace></mml:mspace><mml:msubsup><mml:mi>g</mml:mi><mml:mrow><mml:mi>CE-nZVI</mml:mi></mml:mrow><mml:mrow><mml:mo>-</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msubsup></mml:mrow></mml:math>, respectively, in a synthetic biogas without CO₂. The addition of CO₂ in biogas reduced adsorption, possibly due to system acidification. X-ray photoelectron spectroscopy analysis revealed surface compounds on the surface of the spent nanoparticles, including disulphides, polysulphides and sulphate. The saturated adsorbents were effectively regenerated with air, leading to the oxidation of sulphur species and desorption. The regeneration allowed a total adsorption capacity of 53.25 and 34.14 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mrow><mml:mi>mg</mml:mi></mml:mrow><mml:mrow><mml:msub><mml:mi>H</mml:mi><mml:mi>2</mml:mi></mml:msub><mml:mi>S</mml:mi></mml:mrow></mml:msub><mml:mspace></mml:mspace><mml:msubsup><mml:mi>g</mml:mi><mml:mrow><mml:mi>CE-nZVI</mml:mi></mml:mrow><mml:mrow><mml:mo>-</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msubsup></mml:mrow></mml:math>, after 10 consecutive cycles of adsorption/regeneration with a single batch of olive mill and microalgae nanoparticles, respectively.