Recovery of lactic acid from biodegradable straw waste through a CO₂-assisted thermochemical process

Although biodegradable straws are widely used, they are often unsuitable for disposal in open environments and cannot be recycled in the same manner as conventional plastic straws. Hence, an effective disposal method needs to be developed. Herein, the thermochemical conversion of biodegradable straws made mostly of polylactic acid (PLA) was considered, and the effects of CO2 on the characteristics of thermochemical products produced from biodegradable straws were explored. The thermochemical conversion of the biodegradable straw in CO2 atmosphere yielded more non-condensable gases than the thermochemical conversion in N2 owing to an enhanced thermal cracking of volatiles at 800 °C. At temperatures > 600 °C in CO2, the reverse water-gas shift reaction considerably increased the CO selectivity. Using CO2 in the thermochemical conversion decreased yields of char and wax, which was attributed to CO2 enhancing thermal cracking of volatiles evolved during the thermochemical conversion and C-H and O-H bonds present in the feedstock. The CO2 thermal agent promoted the cleavage of the polymeric bond of PLA to produce up to 14-fold more lactic acid (the monomer of PLA) than that produced with N2 agent, while it suppressed radical reactions to produce fewer phenyls and polycyclic aromatic hydrocarbons. Thermochemical treatment in a CO2 environment has a significant feature of upcycling biodegradable plastic waste, such as biodegradable straws.