Biodiesel Production from Chlorella homosphaera by Two-Step Catalytic Conversion Using Waste Radish Leaves as a...

The economic viability of algal biodiesel can be improved by enhancing the microalgal lipid accumulation and using agricultural waste as a cheap and sustainable source of catalysts. In the current study, the effect of various nitrogen concentrations on the growth and lipid of Chlorella homosphaera were investigated. Furthermore, two-step catalytic conversion was applied to convert the oil of C. homosphaera with high free fatty acids (FFA) to biodiesel using waste radish leaves as a source of a heterogeneous base catalyst. The result revealed that the maximum lipid productivity of 25.0 mg L^-1 day^-1 and lipid content of 30.83% were obtained under nitrogen-depleted and limited nitrogen conditions, respectively. The FFA was reduced from 18.79 to 0.76%, and the acid value was decreased from 37.4 to 1.52 mg KOH g^-1 using a 15:1 methanol to oil molar ratio (MTOR), 1.5 wt.% H₂SO₄, at 60 °C for 150 min. Under the optimized conditions, i.e., MTOR of 10:1, 3 wt.% of catalyst ratio for 120 min at 60 °C, the highest oil conversion of 96.61% was obtained. The physicochemical properties of the produced biodiesel were in the range of the standard specification norms for biodiesel. Hence, the proposed two-step catalytic conversion using calcined radish leaves as a heterogeneous catalyst has thus exhibited good potential for biodiesel production using algal oil with high FFA.