Synergistic Nano–Bio–Phytochemical Treatment for Microbial Enrichment, Wastewater Remediation, and Enhanced Hydrogen Yield

Enhancing microbial enrichment while maintaining microbial compatibility is essential for efficient biological wastewater treatment and sustainable resource recovery. In the present investigation, an integrated nano–bio–phytochemical strategy using low-dimensional ZnO nanofluid and Croton bonplandianum (Bantulsi) extract was developed to enrich pollutant-degrading microorganisms and improve municipal wastewater treatment. Activated sludge was enriched under four treatment conditions (S1–S4), and the combined ZnO nanofluid–Bantulsi system (S4) exhibited the highest microbial proliferation and wastewater treatment efficiency, with significant reductions in turbidity, total dissolved solids, total suspended solids, electrical conductivity and organic contaminants, together with improved dissolved oxygen. 16S rRNA sequencing, BLAST analysis and phylogenetic characterization confirmed the successful enrichment of Enterobacter and Pseudomonas species, exhibiting 97.3–98.8% sequence identity, 95.93–99.80% BLAST similarity, and an E-value of zero. Toxicity assessment revealed a 0 mm zone of inhibition, demonstrating excellent microbial compatibility of the optimized ZnO nanofluid. The treated wastewater was subsequently utilized for alkaline hydrogen generation, wherein the optimized treatment achieved ~ 98% hydrogen production, corresponding to a 112% enhancement over untreated wastewater. Furthermore, optimization studies identified smaller ZnO NPs, 1 wt% ZnO concentration, and higher Bantulsi extract concentration as the optimum conditions for maximizing hydrogen production. Overall, the developed nano–bio–phytochemical platform provides an environmentally sustainable approach for simultaneous microbial enrichment, wastewater remediation, and green hydrogen production.