Harnessing microalgae for a synergistic approach to CO₂ capture, wastewater treatment, and biodiesel production: A review

This review explores microalgae as a sustainable solution for CO₂ reduction and wastewater treatment, particularly in resource-limited GCC regions. The study highlights recent advances in microalgal biodiesel production and promotes integrated systems combining CO₂ capture, nutrient recovery, and biofuel generation to support circular biorefinery models. Microalgae, particularly Chlorella vulgaris exhibit high photosynthetic efficiency with CO₂ fixation rates up to 1.5 g CO₂/L/day while removing > 90 % of nitrogen and phosphorus from wastewater. Under optimized mixotrophic conditions, C. vulgaris achieves biomass concentrations of 4.0 g/L with 20–40 % lipid content, supporting viable biodiesel yields. These characteristics make it particularly suitable for integrated environmental remediation and biofuel production. Innovative cultivation strategies including high-rate algal ponds, membrane bioreactors, and CO₂-enriched flue gas significantly enhance productivity and resource efficiency. Advanced harvesting techniques like flocculation, dissolved air flotation, and decanter centrifugation have improved biomass recovery (> 93 %) while reducing energy consumption. Extraction methods using ionic liquids, bio-based solvents (e.g., 2-MeTHF), and supercritical CO₂ achieve yields up to 97 %, while optimized transesterification processes using microwave or ultrasound-assisted catalysis report fatty acid methyl ester yields up to 88 %. Despite promising developments, commercial implementation faces challenges including high operational costs, energy-intensive processing, and biomass quality variability. Hybrid systems coupling microalgal cultivation with wastewater treatment and industrial effluents offer scalable pathways to enhance economic feasibility. This review summarizes advancements in MABS, identifies knowledge gaps, and outlines future directions for integrating CO₂ capture, wastewater treatment, and biofuel production, while emphasizing the importance of policy support and innovation for sustainable environmental and energy systems.