Green synthesis of lactic acid and carbon dots using food waste and seashell waste

Abstract

Waste valorisation plays a crucial role in the sustainable production of valuable chemicals and materials. This study investigates the feasibility of green technology utilising food waste as a renewable substrate for lactic acid and the resulting residue for subsequent carbon dot production through microbial fermentation and hydrothermal reaction, respectively, while evaluating seashell waste as a replacement for commercial neutralisation reagents. The results demonstrated that seashell waste exhibited effective neutralisation performance during lactic acid fermentation. Within the five types of seashell waste studied, apart from abalone seashells, which led to a significant decrease in lactic acid productivity, all resulted in similar lactic acid fermentations. Additionally, fine powders of seashells were found to be optimal when combined with food waste hydrolysate for lactic acid fermentation. The highest lactic acid productivity of 1.48 g L⁻¹ h⁻¹ obtained in 2 L bioreactor batch fermentation using fine shell powder was 1.64-fold and 0.41-fold higher than those obtained using shell pieces and shell powder, respectively. The results of cell immobilisation fermentation exhibited a superior performance with 2.90 g L⁻¹ h⁻¹ glucose consumption rate and 1.89 g L⁻¹ h⁻¹ lactic acid productivity, which were 1.23-fold and 0.97-fold higher compared to those obtained using NaOH as the neutraliser, respectively. The results of life-cycle assessment also revealed lower environmental impacts associated with lactic acid production using food waste and seashell waste. Cell biomass derived from this study was further utilised to synthesise biomass-derived carbon quantum dots (Bio-CQDs), which demonstrated excellent water solubility, photophysical properties, and potential application as an antibiotic sensor. Overall, the study highlights the potential of seashell waste as an acid neutraliser in lactic acid fermentation and showcases the promising properties of fluorescent Bio-CQDs synthesised from cell biomass, providing valuable insights into the development and implementation of green and sustainable production from waste sources.