Extraction of calcium cation and production of lactic acid from calcium lactate fermentation broth using di-(2-ethylhexyl) phosphoric acid: Process optimization and mechanism analysis

Lactic acid (LA) is a crucial platform chemical for biodegradable plastics, primarily produced via microbial fermentation. However, the conventional purification process via calcium salt precipitation generates substantial calcium sulfate waste. This study proposes a novel solvent extraction method using di-(2-ethylhexyl) phosphoric acid (P204) to extract calcium ions from calcium lactate, thereby releasing LA into the aqueous phase. Under multistage counter-current conditions, calcium extraction and stripping efficiencies exceeded 99.7 % and 98.5 %, respectively, yielding a high-purity CaCl₂·2H₂O by-product instead of calcium sulfate. The process demonstrated broad applicability to calcium salts of weak organic acids (pKa > ∼3.40). Mechanistic studies revealed a 2:1:1 (P204:Ca:LA) coordination complex. Green chemistry assessment confirmed the process’s environmental sustainability and practical viability. This work presents an efficient strategy for LA production that avoids calcium sulfate waste, contributing to greener biorefinery processes.