Effects of particle size on marine biodegradation of poly(L-lactic acid) and poly(e-caprolactone)

The rising quantity of non-degradable microplastics in the world's oceans is becoming a major environmental and health issue. In this context, the advent of biodegradable plastics is a key step. However, information about the degradation of these polymers in the marine environment is scarce. In this study, the effect of particle size on laboratory-scale non-enzymatic hydrolysis, enzymatic hydrolysis, and seawater biodegradation of polymers was investigated. Compostable and biodegradable polymers, such as poly(L-lactic acid) (PLA) and poly(e-caprolactone) (PCL), were cryo-milled and segregated into different size fractions. While non-enzymatic hydrolysis remained largely unaffected by the particle size, the enzymatic hydrolysis rate tended to increase with decreasing particle size for both polymers. The results indicated that the enzymatic hydrolysis rate could be accelerated by making the polymer particles finer. Furthermore, the seawater biodegradation rate and logarithm of the specific surface area were positively correlated. Therefore, it was expected that seawater biodegradation could also be accelerated. The hydrolysis rate of PLA in seawater, however, was not accelerated even with the smallest particle size, suggesting that the specific surface area, as well as the number of bacteria and concentration of extracellular enzymes on the particle surface, are essential for its seawater biodegradation.