Green polymer-based harvesting of microalgae cultivated using nanourea as a nitrogen source and its subsequent usage for bioethanol production

Third-generation biofuel using algal biomass as feedstock has gained importance in recent years. The present study explores an integrated approach for bioethanol production from Micractinium sp., biomass cultivated using nanourea (NU) as a nitrogen source. Moreover, a novel L-arginine-tagged guar gum (AGG) polymer was developed for the eco-friendly harvesting of microalgal cells. A maximum biomass yield of 5.94 ± 0.07 g L⁻¹, productivity of 0.38 ± 0.02 g L⁻¹ d⁻¹, and a carbohydrate content of 44.78 ± 2.09% (w/w) was observed on supplementing 2.66 mM nanourea in a 2-L bubble column reactor. The flocculant AGG polymer showed 97.75 ± 1.12% flocculation efficiency at 100 ppm, further improving to 99.46% with an augmentation FeCl₃ dosage (60 ppm). The harvested microalgal biomass was subjected to microwave-assisted mixed organic acid (acetic acid and formic acid) pretreatment and subsequent enzymatic saccharification with a cocktail of cellulase and amylase (1%, w/w), giving 43.12 ± 1.05% (w/v) reducing sugars yield. These sugars were then converted into bioethanol with a yield of 0.48 ± 0.01 g g⁻¹ of reducing sugars using a 250-mL conical flask. This study presents a novel and eco-friendly strategy for microalgal cultivation, harvesting, and bioethanol production, offering a significant step forward in sustainable biofuel technologies and contributing to resource efficiency.

Graphical Abstract