Enhancing microalgal biohydrogen production: Unlocking higher yields with hydrothermal pretreatment with niobium phosphate

Microalgae cultivated in wastewater hold promise as a substrate for biohydrogen (bioH2) production. However, their rigid cell walls pose a challenge to fermentability. In this context, this study evaluated hydrothermal pretreatment with niobium phosphate (NbP) at 100–180 °C for 0–70 min, using up to 75 % NbP (relative to the dry weight of microalgal biomass). The hydrothermal pretreatment at 180 °C for 10 min with 75 % NbP released 7431 mg total carbohydrates (CHt) L−1, increasing the availability of fermentable substrates in subsequent dark fermentation (DF). When this pretreated biomass was subsequently fermented at pH 5.0 (sample PB5), bioH2 production reached 1.03 mmol H2 mol−1 CHt, with a maximum cumulative output of 0.17 mmol H2 and a CHt conversion efficiency of 83.6 %. In contrast, pH 5.5 and 6.0 reduced bioH2 yields and promoted methanogenic activity, while no pH control resulted in negligible bioH2 evolution. In conclusion, hydrothermal pretreatment with niobium phosphate and pH improvement synergize to enhance hydrogenogenesis, integrating wastewater treatment and renewable biohydrogen production.