Kinetics of polylactic acid biodegradation based on multi-parameter carbon, nitrogen and moisture dynamics in windrow composting systems

Abstract

Polylactic acid (PLA) is among the most commercially significant bioplastics; however, its degradation kinetics under real-world solid waste management conditions, specifically windrow composting, remain incompletely characterized. This study presents a rigorous first-order kinetic analysis of PLA-based biopolymer degradation across six heap configurations over a 56-day composting trial, tracking four interdependent parameters: total organic carbon (TOC), volatile organic carbon (VOC), carbon-to-nitrogen (C/N) ratio, and gravimetric moisture content (MC). Raw parameter values were transformed to normalized concentration ratios (Cs/C0) and their natural logarithms [ln(Cs/C0)] to facilitate linear regression against composting duration (0 to 56 days, sampled at 7-day intervals). First-order rate constants (k) and coefficients of determination (R²) were derived for all heap parameter combinations. The TOC degradation rate constants ranged from 0.0135 to 0.0192 d ⁻¹ with R² values of 0.963 to 0.992, confirming strong first-order conformity and active microbial mineralization of the carbonaceous fraction. The VOC rate constants varied from 0.0105 to 0.0211 d ⁻¹, with R² values ranging from 0.913 to 0.991, reflecting differential volatilization and microbial oxidation rates among heap formulations. C/N ratio kinetics yielded k values of 0.0092 to 0.0129 d ⁻¹ (R² = 0.807 to 0.962, with one anomalous outlier at R² = 0.092 in Heap 2), indicating that nitrogen immobilization and carbon depletion are kinetically coupled but subject to heap-specific microbial community composition. Moisture attenuation followed first-order decay with k values ranging from 0.0063 to 0.0130 d ⁻¹ and R² values ranging from 0.962 to 0.989 across the five heaps. Terminal TOC reductions of 64.9–71.6% and final C/N ratios of 10.41–11.39 at Day 56 confirmed maturation consistent with stabilized compost quality standards. These findings establish heap-specific kinetic fingerprints for PLA composting performance, contributing quantitative benchmarks applicable to industrial solid waste valorization, biodegradable polymer certification, and landfill diversion policies.