Synergistic microalgal acclimatization and growth for sustainable wastewater detoxification and quality biodiesel production.

The escalating contamination of water resources by heavy metals, bicarbonates, seawater, and drain-wastewater intrusion is a multifaceted challenge, necessitating sustainable remediation strategies. A multi-objective bioremediation approach that acclimatizes and utilizes microalgae to mitigate pollutants concurrently is adopted. Twelve microalgae were acclimatized to heavy metals (copper, cadmium, and chromium) up to 100 mg/L, salinity similar to seawater up to 50% v/v, elevated bicarbonate levels up to 40.78 g/L, and drain-wastewater up to 50% v/v, simultaneously (sample not from an actual plant). This was carried out in batches using modified-Zarrouk's medium added with heavy metals, seawater, bicarbonate, and wastewater, with a four-step increment to reach the above concentrations and slowly acclimatize during microalgae cultivation and bioremediation. Bicarbonate in the medium was intermittently added with 3 g/L increments, gradually escalating to 13.5 g/L over a 15-day batch. Other operational conditions for mixotrophic microalgae cultivation were: illumination- red (680 nm, 45000 lx); initial sugar- 5.5 g/L; initial pH- 9.2; and temperature- 28 ± 2 °C. The microalgae demonstrated a synergistic capacity for multi-contaminant removal and production of value-added byproducts, achieving removal efficiencies exceeding 90% for heavy metals, a 100% reduction in bicarbonate levels, and a marked decrease in salinity. Lipids from Nannochloropsis sp., Arthrospira platensis, Scenedesmus obliquus, Chlorella vulgaris, Chlorococcum sp., and Navicula sp. 2 produced 3rd-generation quality biodiesel via transesterification, with high fatty acid methyl ester content and viscosity as per ASTM-D6751 standard. Eco-friendly wastewater treatment technology developed showed high bioremediation efficiency and also provided alternative energy as a sustainable solution, supporting UN-SDGs 6 and 7.