Cellulosic L-Lactic Acid Production by Engineered Na⁺-Tolerant Pediococcus acidilactici and Bipolar Membrane...

Ultra-pure L-lactic acid is a crucial precursor for the synthesis of L-lactide and polylactic acid (PLA). Regular purification of L-lactic acid results in a low L-lactic acid recovery yield and CaSO₄ solid waste generation. This study used highly simplified bipolar membrane electrodialysis for purification of sodium L-lactic acid using wheat straw feedstock, taking advantage of sharply reduced electricity cost from rapid development of photovoltaic and wind power. An engineered L-lactic acid bacterium, Pediococcus acidilactici HL2 with high Na⁺ tolerance was constructed by co-expression of heterologous Na⁺/H⁺ antiporter and native β-component of ATPase to meet the needs of bipolar membrane electrodialysis for soluble sodium L-lactate. The bipolar membrane electrodialysis for the cellulosic sodium L-lactate purification broths received 94.6% of Na⁺ removal, and the overall recovery yield was 80.2%, considerably higher than regular purification methods. 80% of sodium hydroxide was recovered after electrodialysis and recycled for pH adjustment in fermentation. The electric power consumption was calculated as 1.43 kWh per kg of pure L-lactic acid, which is merely $50 per metric ton of L-lactic acid based on the current price of green power. The L-lactide synthesized using the purified L-lactic acid successfully met the standard of commercial lactide for PLA polymerization. This study provides a green and sustainable procedure for L-lactide synthesis from lignocellulose feedstocks by eliminating sulfuric acid addition, avoiding solid waste generation, and recycling neutralizer for fermentation.