Upgrading of biogas using iron nanoparticles produced by HTC

• Autores: Elena Passalacqua
• Directores de la Tesis: Andrés Fullana Font (tut. tes.), Julia Moltó Berenguer (tut. tes.), Elena María Collina (dir. tes.), Ignacio Aracil Sáez (codir. tes.), Marco Giovanni Malusà (codir. tes.)
• Lectura: En la Universitat d'Alacant / Universidad de Alicante ( España ) en 2026
• Idioma: inglés
• Tribunal Calificador de la Tesis: Elena Ficara (presid.), Elena María Collina (secret.), Ignacio Aracil Sáez (voc.), Julia Moltó Berenguer (voc.)
• Programa de doctorado: Programa de Doctorado en Agua y Desarrollo Sostenible por la Universidad de Alicante
• Calificación: Sobresaliente Cum Laude
• Enlaces
  • Tesis en acceso abierto en: RUA
• Resumen

  • This thesis investigates innovative strategies to improve biogas quality, with a specific focus on removing hydrogen sulphide (H2S), a corrosive and toxic contaminant present in the biogas. To address this challenge, carbon-encapsulated iron nanoparticles (CE-nFe) were synthesized from residual biomasses such as olive mill wastewater and microalgae grown on municipal effluents. This process valorizes carbon-rich biomasses while producing functional nanoparticles, thus connecting technological innovation with sustainability and circularity. After the synthesis, the nanoparticles were tested as possible adsorbents under two different experimental conditions: ex situ, through tests on synthetic biogas in an external system, and in situ, by direct addition into anaerobic digesters. Carbon-encapsulated iron nanoparticles represent an improvement over conventional iron nanoparticles, since the carbon shell derived from biomass acts as a protective layer, enhancing particle stability over time and reducing the ageing effects that typically limit iron reactivity. In addition, the carbon coating increases the specific surface area and facilitates scale-up, addressing common limitations—as the use of chemical solvents—which are avoided with hydrothermal synthesis. To better characterise these nanoparticles and understand the action mechanisms, several analyses were conducted, including electron microscopy to examine morphology, X-ray diffraction to identify crystalline phases, Brunauer–Emmett–Teller surface area measurements, elemental analyses (energy-dispersive X-ray spectroscopy and inductively coupled plasma), and basic magnetic and pH assessments to evaluate behavior in aqueous media. In the ex situ configuration, carbon-encapsulated iron nanoparticles were synthesized from olive mill wastewater and microalgal biomass grown on municipal effluents. For each feedstock, separate syntheses were carried out by varying the iron-to-carbon ratio (Fe/C = 0.02, 0.05, 0.2) to identify the most effective composition. The resulting nanoparticles were suspended in deionized water at 1 g L⁻¹, and a synthetic biogas containing ~60 ppm H₂S was bubbled through the suspension under controlled flow. Two gas compositions were tested: without CO₂ and with CO₂ added to reproduce a realistic biogas matrix. The results without carbon dioxide demonstrated a promising adsorption capacity. Nanoparticles produced from olive mill waste residues at an iron-to-carbon ratio of 0.05 reached a specific adsorption capacity of 9.66 ± 0.07 H2S g−1 CE-nFe. Nanoparticles produced from microalgae at an iron-to-carbon ratio of 0.2 achieved a very similar value of 9.55 ± 0.11 H2S g−1 CE-nFe. Other olive-mill preparations provided lower capacities, around 7.07 ± 0.13 and 8.15 ± 0.23 H2S g−1 CE-nFe, and microalgal preparations with iron-to-carbon ratios of 0.05 and 0.02 performed more modestly, around 2.72 ± 0.25 and 1.46 ± 0.05 H2S g−1 CE-nFe. These differences are consistent with the interplay between total iron, accessible surface area and aggregation behavior, which influence the availability of reactive sites for sulphur capture. The introduction of carbon dioxide into the gas stream, however, caused a strong reduction in performance, probably due to acidification of the water–nanoparticle suspension and the formation of repulsive interactions between nanoparticles and ions. With CO₂, the best olive-mill CE-nFe reached only 3.51 ± 0.34 mg H2S g−1 CE-nFe, while microalgae-based materials fell below 1 mg H2S g−1 CE-nFe in most tests (often ~0.45–0.77 H2S g−1 CE-nFe depending on Fe/C). Since carbon dioxide is a typical component of real biogas, this finding highlights a limitation of the ex situ system and indicates the need for process modification. One possible improvement could involve the use of carbon-encapsulated iron nanoparticles as catalysts in a Fenton-like process, where iron would promote radical formation in the presence of hydrogen peroxide, shifting the treatment from adsorption to chemical oxidation of sulphur species. The study also assessed regeneration by alternating adsorption and reactivation steps. After each adsorption run, the spent nanoparticles were exposed to air (without chemicals) to oxidize retained sulphide species and restore reactive iron sites and were then reused in the next run under the same operating conditions. This use-regeneration sequence was repeated ten times. Over ten consecutive use-regeneration cycles, the cumulative specific capacity reached 53.2 H2S g−1 CE-nFe derived from olive mill waste residues and 34.1 H2S g−1 CE-nFe for those derived from microalgae. These cumulative performances indicate that regeneration is feasible and can extend the useful life of the materials, which is a critical factor for a process that aims to be both effective and sustainable at scale. The in situ configuration provided a complementary perspective by introducing microalgae-based nanoparticles into laboratory-scale anaerobic digesters operating under semi-continuous conditions with a hydraulic retention time of 21 days. Three nanoparticle concentrations—100, 500 and 1000 g L-1—were evaluated in a Test reactor and compared to a parallel Control reactor without nanoparticles. Each semi-continuous trial lasted 3 hydraulic retention times (63 days), and no further additions of nanoparticles were made. The nanoparticles were recovered magnetically during reactor loading and unloading operations and reintroduced into the digesters. Across all doses, the Test reacto outperformed the Control in both methane and total biogas production, and the differences were statistically significant according to paired t-tests. At 100 g L-1, methane production averaged 180 ± 50 NmL gVS⁻¹ in the Test reactor compared to 146 ± 26 NmL gVS⁻¹ in the Control, with an increase of 23.3%. Total biogas increased from 188 ± 36 to 248 ± 58 NmL gVS⁻¹, representing a 32% increase. At 500 g L-1, methane and biogas rose by 16.7% and 18.9%, respectively. At 1000 g L-1, methane reached 253 ± 59 NmL gVS⁻¹ compared to 207 ± 70 NmL gVS⁻¹ in the Control (22%), while biogas rose to 353 ± 82 NmL gVS⁻¹ compared to 279 ± 95 NmL gVS⁻¹ (27% higher). Microbial community analysis revealed dose-dependent structural shifts induced by the nanoparticles, with enrichment of methanogenic families such as Methanoregulaceae, while overall microbial richness remained stable. These results show that the nanoparticles can improve the overall reactor performance under the tested conditions. Hydrogen sulphide behavior in the in situ tests was also favorable. Reactors containing nanoparticles maintained hydrogen sulphide levels in the biogas below 100 ppm, whereas the Control reactors without nanoparticles exceeded 500 ppm. This corresponds to a reduction greater than 80% and confirms the dual benefit of the in situ approach: gas purification and increased energy yield. This doctoral research underscores the promise of carbon-encapsulated iron nanoparticles as an innovative tool for biogas upgrading. However, a few challenges emerged. In the ex situ configuration, the main issue is carbon dioxide: it acidifies the medium and promotes repulsive nanoparticle–ion interactions, decreasing adsorption capacity. This limitation could be mitigated by transitioning from adsorption to catalytic oxidation, by exploiting the iron in the nanoparticles within advanced oxidation processes (e.g., Fenton-like reactions), where hydrogen peroxide can generate radicals to convert sulphur species. In parallel, regeneration strategies deserve deeper investigation. The multi-cycle reuse observed in this work is encouraging, and its optimization could lower operating costs and environmental impacts, improving overall process sustainability. For the in situ approach, results showed clear benefits not only for hydrogen sulphide removal but also for methane yield and overall biogas production, making the method suitable for practical deployment. The open issue remains nanoparticle management inside the digester. Future studies should therefore develop scalable, reliable recovery and reuse strategies to ensure environmental safety. Finally, further developments could investigate the use of carbon-encapsulated iron nanoparticles not only in aqueous suspension but also in filtration systems or fixed-bed columns, which may enhance the contact efficiency between biogas and nanoparticles.