Growth behavior, phenol removal and lipid productivity of microalgae in mixotrophic and heterotrophic conditions under synergistic effect of phenol and bicarbonate for biodiesel production

The use of microalgae to remediate phenol and subsequent algal biodiesel production is an eco-sustainable and effective process. The growth, phenol removal, and lipid production of Tetradesmus obliquus and Chlorella sp. were investigated under synergistic effect of sodium bicarbonate (2, 4, and 6 g L−1) and phenol (50, 100, and 150 mg L−1) in mixotrophic and heterotrophic conditions. In general, the mixotrophic conditions showed better results compared to heterotrophic mode as reflected by higher biomass and lipid productivities. Under mixotrophic conditions of 4 g L−1 of NaHCO3 and 100 or 150 mg L−1 of phenol, the biomass and lipid productivities of T. obliquus were ~ 2.3- and 4.5-fold higher than the phenol-less controls, respectively. Similarly, the mixotrophically grown Chlorella exhibited a ~ 1.9-fold increase in biomass productivity and threefold increase in lipid productivity at 6 g L−1 of NaHCO3 and 150 mg L−1 of phenol. Furthermore, the algal growth and phenol removal kinetics were fitted using modified logistic model and integrated Gompertz model, respectively. The phenol consumption rate was markedly influenced by bicarbonate concentration as well as the growth conditions. The algal biomass produced under combined phenol and bicarbonate treatment also was investigated as a biodiesel feedstock. Different biodiesel characteristics were estimated such as saponification value, iodine value, and cetane number which agreed with the international standards. The findings of the present study suggested that bicarbonate addition could enhance phenol phycoremediation and the produced biomass could be utilized as a suitable biodiesel feedstock.