Biochar Production with concomitant biopolymer accumulation of Microalgae-Based Wastewater treatment for carbon neutrality

Environmental Pollution caused by plastic and wastewater discharge demands sustainable solution. To address this, microalgal species Spirulina effectively utilized for domestic wastewater (DWW) valorization and sustainable bioproduct development. During a 16-day cultivation period in DWW, biomass productivity increased steadily reaching a maximum of 0.83 g L -1 on day 12. Meanwhile, the methanol-water solvent combination produced the highest amount of pigment phycocyanin of about 22.5 mg/L from the Spirulina biomass, this shows that it is possible to recover valuable co-products. The residual algal biomass further utilized for the production of PHAs and biochar. FTIR analysis of Spirulina -derived PHA bioplastic showed characteristic peaks at 3270.1 cm -1 (O–H), 2979.2 cm -1 (C–H), and a strong ester carbonyl band at 1626 cm -1 . Additional peaks at 1387 cm -1 and 1241.6 cm -1 correspond to methylene, methyl, and methine groups, while bands in the 1527–1050 cm -1 range further confirm the polymer backbone structure ,indicating functional groups associated with PHA polymers. The PHA-bioplastic film shows a uniform thickness of 0.12±1 mm, water absorption 16%, water solubility 33.21% and balanced chemical resistance, indicating the biodegradability of the material. In addition, torrefaction of the residual biomass produces biochar yield of 31%, demonstrating effective biomass valoårization. Thermogravimetric analysis shows that the torrefied biochar contained lower volatile matter, higher fixed carbon content and improved thermal stability. This integrated approach shows the potential of DWW as an efficient medium for sustainable Spirulina cultivation, enabling pigment recovery and valorization of residual biomass into bioplastic and biochar. However, PHA yield optimization, the variability in wastewater composition and detailed product characterization was not yet investigated and could be addressed in future research. • Spirulina biomass in domestic water rose gradually for 16 days, peaking at 0.83 g/L on day 12. • Phycocyanin concentration methanol-water combination solvent was highest 22.5 mg/L. • The residues comprised PHAs with spectra similar to PHB and PHBV biopolymers, according to FTIR. • Spirulina showed a consistent thickness of 0.12±1 mm, 16% water absorption and 32% solubility. • The residues turned into sustainable biochar for various industrial application