Politics Unfortunately,
Politics Unfortunately, all the focus is on demonstrating the break down or degradation of the carbon product (like weight loss, or oxidation levels) but no data on how much and in what time frame did the microorganisms present in the disposal environment consume the carbon food. This is how it gets misused and abused – by focusing only on the degradation but no data showing the utilization of the fragments by the microorganisms present in the disposal environment. Break down (decomposition) by non-biological processes or even biological processes, generates fragments that is utilized by the microorganisms, but also leaves behind fragments (and in some cases 50- 80% of the original weight) which in many cases has been shown to be detrimental and toxic to the ecosystem. This constitutes only degradation/fragmentation, and not biodegradation. As will be shown later, hydrophobic polymer fragments pose great risk to the environment, unless the degraded fragments are completely consumed as food and energy source by the microorganisms present in the disposal system in a very short period (one year) that is the degraded fragments must be completely removed from the environment by safely entering into the food chain of the microorganisms. Measurement of Biodegradability Microorganisms use the carbon substrates to extract chemical energy that drives their life processes by aerobic oxidation of glucose and other readily utilizable C- substrates: C - substrate + 6O 2 → 6CO 2 + 6H 2 O, ∆G 0 = - 686 kcal/mol (CH 2 O) x ; x = 6 Thus, a measure of the rate and amount of CO 2 evolved in the process is a direct measure of the amount and rate of microbial utilization (biodegradation) of the C-polymer. This forms the basis for various international standards for measuring biodegradability or microbial utilization of the test polymer/plastics. Thus, one can measure the rate and extent of biodegradation or microbial utilization of the test plastic material by using it as the sole added carbon source in a test system containing a microbially rich matrix like compost in the presence of air and under optimal temperature conditions (preferably at 58°C – representing the thermophilic phase). Figure 2 shows a typical graphical output that would be obtained if one were to plot the percent carbon from the plastic that is converted to CO 2 as a function of time in days. First, a lag phase during which the microbial population adapts to the available test C-substrate. Then, the biodegradation phase during which the adapted microbial population begins to utilize the carbon substrate for its cellular life processes, as measured by the conversion of the carbon in the test material to CO 2 . Finally, the output reaches a plateau when utilization of the substrate is largely complete. Standards such as ASTM D 6400 (see also D 6868), EN 13432, ISO 17088 etc. are based on this principle. The fundamental requirements of these world-wide standards discussed above for complete biodegradation under composting conditions are: 1. Conversion to CO 2 , water & biomass via microbial assimilation of the test polymer material in powder, film, or granule form. 2. 90% conversion of the carbon in the test polymer to CO 2 . The 90% level set for biodegradation in the test accounts for a +/- 10% statistical variability of the experimental measurement; in other words, there is an expectation for demonstration of virtually complete biodegradation in the composting environment of the test. 3. Same rate of biodegradation as natural materials – leaves, paper, grass & food scraps 4. Time – 180 days or less; (ASTM D6400 also has the requirement that if radiolabeled polymer is used and the radiolabeled evolved CO 2 is measured then the time can be extended to 365 days). Two further requirements are also of importance : Disintegration -
Politics % C conversion to CO 2 (% biodegradation) 100 90 80 70 60 50 40 30 20 10 lag phase biodegradation degree biodegradation phase plateau phase Polymer chains with susceptible linkages Environment - soil, compost,waste water plant, marine Hydrolytic oxidative Enzymatic Oligomers & polymer fragments Complete microbial assimilation defined time frame, no residues!!! 0 0 20 40 60 80 100 120 140 160 180 Time (days) Figure 2: Test method to measure the rate and extent of microbial utilization (biodegradation) of biodegradable plastics Figure 3: Complete biodegradation CO 2 + H 2 O + Cell biomass concentrate these chemicals, resulting in a toxic legacy in a form that may pose risks in the environment. Japanese researchers (Mato et al., 2001) have similarly reported that PCBs, DDE, and nonylphenols (NP) can be detected in high concentrations in degraded polypropylene (PP) resin pellets collected from four Japanese coasts. This work indicates that plastic residues may act as a transport medium for toxic chemicals in the marine environment. Therefore, designing hydrophobic polyolefin plastics, like polyethylene (PE) to be degradable, without ensuing that the degraded fragments are completely assimilated by the microbial populations in the disposal infrastructure in a short time period, has the potential to harm the environment more than if it was not made degradable. These concepts are illustrated in Figure 3 which shows that heat, moisture, sunlight and/or enzymes shorten and weaken polymer chains, resulting in fragmentation of the plastic and some cross-linking creating more intractable persistent residues. It is even possible to accelerate the breakdown of the plastics in a controlled fashion to generate these fragments, some of which could be microscopic and invisible to the naked eye. However, this degradation/fragmentation is not biodegradation per see and these degraded, hydrophobic polymer fragments pose potential risks in the environment unless they are completely assimilated by the microbial populations present in the disposal system in a relatively short period. Summary The take home message is very simple -- Biodegradability is an end-of-life option for single use disposable, packaging, and consumer plastics that harnesses microbes to completely utilize the carbon substrate and remove it from the environmental compartment -- entering into the microbial food chain. However, biodegradability must be defined and constrained by the following elements: • The disposal system – composting, anaerobic digestor, soil, marine. • Time required for complete microbial utilization in the selected disposal environment – short defined time frame, and in the case of composting the time frame is defined as 180 days or less. • Complete utilization of the substrate carbon by the microorganisms as measured by the evolved CO 2 (aerobic) and CO 2 + CH 4 (anaerobic) leaving no residues. • Degradability, partial biodegradability, or will eventually biodegrade is not an option! – Serious health and environmental consequences can occur as documented in literature. • Measured quantitatively by established International, and National Standard Specifications -- ASTM D6400 for composting environment, ASTM D6868 for coatings on paper substrates in composting environment, ASTM D7081 marine environment, European specification, EN13432 for compostable packaging, and International ISO 17088 for composting environment. • If other disposal environments like landfills, anaerobic digestor, soil, and marine are specified, then data must be provided showing time required for complete biodegradation using established standardized ASTM, ISO, EN, OECD methods. • All stakeholders should review biodegradability claims against ‘data’ and if necessary use a third party independent laboratory to verify and validate the data using established standardized test methods and specifications, and based on the fundamental principles and concepts outlined in this paper. narayan@msu.edu bioplastics MAGAZINE [01/09] Vol. 4 31
