Polymerization of lactic acid produced from food waste by metal oxide-assisted dark fermentation

Abstract

Lactic acid (LA) is the precursor monomer used in the production of biodegradable and eco-friendly polylactic acid (PLA) polymer. The purpose of this study was to first determine the role of nano-metal oxides in improving the production of L-LA from food waste via the dark fermentation process and then to compare the conversion of L-LA to PLA using conventional and novel catalysts. Compared to the control run without nanoparticles (NPs) addition, maximum L-LA concentration (38.86 g/L), yield (0.194 g/g- TS), and productivity (3.88 g/L-day) were achieved for iron oxide (FeOx) nanoparticles (NPs)-assisted fermentation at an optimal dosage of 15 mg/L. Nickel oxide (NiOx) and cobalt oxide (CoOx) NPs also showed similar enhancing effects for L-LA production. However, high dosages of the metal oxide NPs decreased the LA production in the medium. Moreover, a new nanocatalyst, iron phosphate (FePO4), was prepared and examined for PLA production from the as-synthesized L-LA monomeric units by ring-opening polymerization. A high yield of lactide (84.71%) was produced in a shorter reaction time (10 h) using FePO4 than the conventional tin chloride (SnCl2) catalyst. PLA polymer with molecular weights in the range of 4,900–5,200 Da was obtained using FePO4 and tin chloride dihydrates (SnCl2) catalysts under the optimized conditions. As such, this work has demonstrated a practical way to improve LA production from organic waste streams and its effective conversion to PLA for potential applications in the biomedical and bioplastic sectors.