Comparative environmental assessment of rice straw valorization for integrated biohydrogen and green silica production.

Abstract

Rice straw is a widely available agricultural residue with great potential for biofuels and bioproducts. However, the sustainability of its conversion remains uncertain due to the energy- and resource-intensive processing requirements. This study conducts a consequential life cycle assessment of four promising scenarios for the co-production of hydrogen and silica, each using different pretreatment strategies: (1) alkaline, (2) organosolv-alkaline, (3) acidic-alkaline, and (4) organosolv-alkaline coupled with nano-silica addition during fermentation. In all scenarios, silica was recovered from the alkaline pretreatment liquor, while cellulose-rich residues were subjected to enzymatic hydrolysis and dark fermentation to produce hydrogen. The results demonstrated that the environmental impacts, particularly from the pretreatment stage, outweigh the benefits of avoided products across all scenarios. While scenario 2 produced high experimental hydrogen yields (56.2 mL/g), it performed poorly in all damage categories. Despite the enhanced fermentation efficiency in scenario 4 (63.8 mL/g), the avoided products were still too low to offset the damages associated with organosolv pretreatment, and further impacts were created from adding nano-silica. In contrast, the least-burdensome pathway (scenario 3) with net impacts of 1.6 × 10-3 DALY, 3.3 × 10-6 species.yr, and 22.7 USD2013 per functional unit for the human health, ecosystem, and resource categories, respectively, did not show considerable differences compared to its corresponding single-hydrogen configuration. Sensitivity analysis identified opportunities to reduce the overall environmental impacts by enhancing the solid-to-liquid ratio and replacing fossil-based energy sources with renewable ones. These findings suggest that reducing pretreatment intensity and improving energy efficiency via process optimization and heat recovery may offer greater environmental benefits.