Calcium-Modified Spodumene Mining Waste as a Basic Heterogeneous Catalyst for Biodiesel Production

This work investigates Ca-modified α- and β-spodumene mining waste as low-cost heterogeneous catalysts for biodiesel production via vegetable oil transesterification. Calcium incorporation promoted the formation of Ca(OH)₂ and calcium silicates, increasing the basicity of the materials. However, catalytic activity was found to depend more strongly on the nature, dispersion, and accessibility of the basic sites than on their total concentration. The catalyst 10CaαEsp850 showed the highest performance, achieving complete conversion within 2 h, despite not exhibiting the highest basicity. Leaching tests indicated that dissolved calcium species also contribute to the reaction, demonstrating that the catalysts operate through a combined heterogeneous–homogeneous mechanism. These results highlight the potential of Ca-modified spodumene mining waste as an efficient and sustainable catalyst for biodiesel production.