Analytical Investigation of Biodegradation Kinetics of Selected Bioplastics Under Nigerian Tropical Soil and Aquatic Conditions

The commercial introduction of bioplastic packaging materials in Nigerian markets requires rigorous characterisation of their biodegradation kinetics under conditions representative of local disposal environments. This study analytically investigates the biodegradation kinetics of polylactic acid, polyhydroxyalkanoate, and thermoplastic starch films under tropical soil burial conditions at two sites in Ogun State, South West Nigeria, and in simulated freshwater conditions representative of Lagos lagoon chemistry. Biodegradation was monitored over 360 days through measurement of mass loss, molecular weight reduction by gel permeation chromatography, mineralisation as evolved carbon dioxide, and surface morphological changes by scanning electron microscopy. First-order and Michaelis-Menten kinetic models were fitted to mineralisation data to characterise degradation rate constants. Soil physicochemical parameters, temperature, moisture, microbial population, and pH, were monitored and correlated with degradation rates. Results show polyhydroxyalkanoate films degraded most rapidly, achieving 89 percent mass loss after 180 days in soil, compared to 54 percent for thermoplastic starch and 22 percent for polylactic acid. Aquatic degradation was significantly slower for all materials, with polyhydroxyalkanoate achieving only 61 percent mineralisation after 360 days in freshwater conditions. Degradation rate constants were positively correlated with soil temperature and microbial count, confirming that Nigerian tropical conditions generally accelerate biodegradation relative to temperate climate data. Molecular weight reduction preceded mass loss in all materials, confirming hydrolytic chain scission as the primary initial mechanism. The study provides biodegradation kinetic data for regulatory characterisation of compostable packaging materials under Nigerian environmental conditions. Keywords: bioplastics, biodegradation kinetics, tropical soil, polylactic acid, polyhydroxyalkanoate