Effects of alkaline pretreatment on the anaerobic digestion of polylactic acid and agricultural biocomposite: Physicochemical properties, methane yields, and microbial community

Abstract

This study evaluated the effects of alkaline pretreatment on solubilization, anaerobic digestion (AD), and microbial community dynamics of polylactic acid (PLA) and agricultural biocomposite products (ABPs). Pretreatment with 1, 5, and 10 M NaOH enhanced solubilization, with PLA exhibiting rapid and high chemical oxygen demand (COD) release, while ABPs showed slower, substrate-limited solubilization. Liquid chromatography-organic carbon detection (LC-OCD) and Fourier transform infrared spectroscopy (FTIR) analyses showed structural degradation of broken ester bonds in PLA, while ABPs displayed partial lignocellulosic breakdown and increased humic-like substances. AD of untreated PLA showed limited methane production due to structural recalcitrance. In contrast, pretreatment significantly improved PLA biodegradability, achieving methane yields close to theoretical potential. However, ABPs showed reduced methane yield after pretreatment, attributed to the formation of inhibitory compounds and reduced availability of readily degradable organics. Volatile fatty acids (VFAs) profiles supported these trends, indicating enhanced acidogenesis in pretreated PLA and suppressed VFAs production in pretreated ABPs. Microbial analysis revealed taxonomic and functional shifts driven by pretreatment and substrates, with fermentative and methanogenic taxa aligning with solubilized organics in multiple factor analysis (MFA). These findings underscore the importance of tailoring pretreatment strategies based on substrate type to optimize methane production and microbial function. Alkaline pretreatment of PLA enhanced its solubilization and biodegradability, enabling methane production close to its theoretical potential. In contrast, pretreatment of ABPs requires careful optimization to avoid inhibitory byproducts and reduced digestion efficiency.