Degradation of starch-poly(beta-hydroxybutyrate-co-beta-hydroxyvalerate) bioplastic in tropical coastal waters

Extruded bioplastic was prepared from cornstarch or poly(beta-hydroxybutyra te-co-beta-hydroxyvalerate) (PHBV) or blends of cornstarch and PHBV. The bl ended formulations contained 30 or 50% starch in the presence or absence of polyethylene oxide (PEO), which enhances adherence of starch granules to P HBV. Degradation of these formulations was monitored for 1 y ear at four st ations in coastal water southwest of Puerto Rico. Two stations were within a mangrove stand. The other two were offshore; one of these stations was on a shallow shoulder of a reef, and the other was at a location in deeper wa ter. Microbial enumeration at the four stations revealed considerable flux in the populations over the course of the year. However, in general, the ov erall population densities were 1 order of magnitude less at the deeper-wat er station than at the other stations. Starch degraders were 10- to 50-fold more prevalent than PHBV degraders at all of the stations. Accordingly, de gradation of the bioplastic, as determined by weight loss and deterioration of tensile properties, correlated with the amount of starch present (100% starch >50% starch > 30% starch > 100% PHBV). Incorporation of PEO into ble nds slightly retarded the rate of degradation. The rate of loss of starch f rom the 100% starch samples was about 2%/day, while the rate of loss of PHB V from the 100% PHBV samples was about 0.1%/day. Biphasic weight loss was o bserved for the starch-PHBV blends at all of the stations. A predictive mat hematical model for loss of individual polymers from a 30% starch-70% PHBV formulation was developed and experimentally validated. The model showed th at PHBV degradation was delayed 50 days until more than 80% of the starch w as consumed and predicted that starch and PHBV in the blend had half-lives of 19 and 158 days, respectively. Consistent with the relatively low microb ial populations, bioplastic degradation at the deeper-water station exhibit ed an initial lag period, after which degradation rates comparable to the d egradation rates at the other stations were observed. Presumably, significa nt biodegradation occurred only after colonization of the plastic, a parame ter that was dependent on the resident microbial populations. Therefore, it can be reasonably inferred that extended degradation lags would occur in o pen ocean water where microbes are sparse.