Opinion Missing the
Opinion Missing the point about biodegradability Do microorganisms care whether the molecule is called oxo or hydro or chemo or bio? In a recent article on bioplasticsnews.com the author Axel Barrett compared “Oxo-biodegradable vs compostable plastics” [1]. Similar comparisons can be found on company web sites touting one technology or one polymer over the other. This is unfortunate, because the fundamental scientific question to be answered is straight forward. Can natural microorganisms present in the disposal environment (compost, soil, land, oceans, anaerobic digestors) completely assimilate/utilize the carbon of the plastic/polymer molecule as food/fuel for its life process in a short time frame based on the disposal environment. The following statement in the article addresses the root of this question “Compostable plastics degrade in a time frame of 2 to 6 months in an industrial compost. OXO will degrade in a timeframe of 1 to 3 years in the open environment” Degradation and accelerated degradation (by the addition of additives) cannot be accepted as an environmentally responsible end-of-life in any environment. Degradability suggests fragmentation and breaking up of the molecule to smaller and smaller pieces that may not be even visible – formation of micro and nano fragments leaking into soil and oceans. However, they will not disappear (this assumption violates the laws of nature / the first law of thermodynamics which is “Matter can neither be created or destroyed”). So, oxo, hydro, chemo, photo degradable abiotic (nonbiological process) mechanisms of degradation in itself is environmentally irresponsible and can cause harm to environment and human health. The abiotic degradations may facilitate the biodegradation step, however, the critical requirement is for the degraded carbon molecules to be efficiently, and efficaciously utilized by microorganisms in that disposal environment in a short time period. Making hydrophobic polyolefin plastics such as PE unstable and degradable, and releasing them into the environment without ensuring that the degraded fragments are completely assimilated by the microbial populations in a short time period has the potential to harm the environment and create human health risks. The fragments, some of which could be microscopic, can be transported through the ecosystem and could have serious environmental and health consequences. In fact, stringent European community ‘Registration, Evaluation, Authorization, and Restriction of Chemical’ substances are subject to laws governing the release of almost all chemicals (small molecules) are becoming the norm in Europe and other countries. It requires the chemical to be completely assimilated by microorganisms in the ecosystem if it is to be released into the environment. Thompson [2] reported that plastic debris around the globe can erode (degrade) and end up as microscopic granularor fibre-like fragments, and these fragments have been steadily accumulating in the oceans. The experiments show that marine animals consume microscopic bits of plastic, as seen in the digestive tract of an amphipod. The US-based Algalita Marine Research Foundation [3] reported that degraded plastic residues can attract and hold hydrophobic elements, such as polychlorinated biphenyls (PCB) and dichlorodiphenyltrichloroethane (DDT), up to one million times higher than the background levels. PCB and DDT are at background levels in soil and are diluted out, so as to not pose significant risk. However, degradable plastic residues with high surface areas concentrate these chemicals, resulting in a toxic legacy in a form that may pose risks in the environment. Japanese researchers [4] have similarly reported that PCB, DDT and nonylphenols can be detected in high concentrations in degraded PP resin pellets collected from four Japanese coastal areas. This work indicates that plastic residues may act as a transport medium for toxic chemicals in the marine environment. More recently, issues were reported surrounding microscopic plastics being released into the environment and causing environmental and human health problems [5, 6]. In summary degradability or partial biodegradability or biodegradability requiring years or tens of years is an irresponsible end-of-life option and cause more harm to the environment. To-date, the oxo plastics which are essentially carbon-carbon backbone polymers like PE, PP containing oxidation catalyst systems have not proven to completely biodegrade in a reasonable time frame of 1-2 years [7-9]. Note on biodegradability and compostability The science of biodegradability and the associated standards for measuring and reporting biodegradability was published in bioblastics MAGAZINE, and other publications [10-12]. To summarize: • Biodegradability is a measure of the utilization of the test substrate carbon by the microorganisms present in the disposal environment through microbial metabolism as measured by the evolved CO 2 . • Certified/ Verified Compostable Plastics is the “enabling technology” to efficiently and efficaciously divert food and other organic wastes from landfills to environmentally responsible end-of-life solutions like composting and anaerobic digestion. 38 bioplastics MAGAZINE [04/20] Vol. 15
Opinion By: Ramani Narayan University Distinguished Professor Department of Chemical Engineering & Materials Science Michigan State University East Lansing, MI, USA • “Compostable” defines the boundary conditions under which complete biodegradation (microbial utilization) needs to be validated using ASTM/ISO International Standards • Enabler for the Ellen MacArthur Foundation “Circularity model” • Enabler for “Organics Recycling” • Design for biodegradablility or compostability may be the best end-of-life option in cases where leakage into the environment is likely or where light, highly-contaminated, and difficult to collect plastic streams exist • The @EnvSciTech viewpoint in [13] summarizes necessary requirements for assessing and reporting plastic biodegradation. It has about 5000+ downloads since publication in August 2019 in American Chemical Society (ACS) Environmental Science &Technology Journal narayan@egr.msu.edu References: [1] Barret, A.: OXO Biodegradable Vs Compostable Plastics; www.bioplasticsnews.com, June 25, 2020; last accessed July 23, 2020 [2] R.C. Thompson, Y. Olsen, R.P. Mitchell, A. Davis, S.J. Rowland, A.W.G. John, D. McGonigle and A.E. Russell, Science, 2004, 304, 838. [3] Algalita Marine Research Foundation, Long Beach, CA, USA. http://www. algalita.org/pelagic_plastic.html. [4] Y. Mato, T. Isobe, H. Takada, H. Kahnehiro, C. Ohtake and T. Kaminuma, Environmental Science & Technology, 2001, 35, 318. [5] E.L. Teuten, J.M. Saquing, D.R.U. Knappe, M.A. Barlaz, S. Jonsson, A. Björn, S.J. Rowland, R.C. Thompson, T.S. Galloway and R. Yamashita, Philosophical Transactions of the Royal Society B, 2009, 364, 2027. [6] R.C. Thompson, C.J. Moore, F.S. vom Saal and S.H. Swan, Philosophical Transactions of the Royal Society B, 2009, 364, 1526. [7] EU Commission Report The Impact of the use of oxo-degradable plastics on the environment For the European Commission DG Environment. Project conducted under Framework Contract No ENV.A.2/FRA/2015/0008 [8] Susan Selke, Rafael Auras, Tuan Anh Nguyen, Edgar Castro Aguirre, Rijosh Cheruvathur, and Yan Liu; Evaluation of Biodegradation-Promoting Additives for Plastics, Environmental Science & Technology, 2015, 49, 6, 3769–3777 [9] Eddie F. Gómez, Frederick C. Michel Jr. Biodegradability of conventional and bio-based plastics and natural fiber composites during composting, anaerobic digestion and long-term soil incubation, Polymer Degradation and Stability 98 (2013) 2583e2591 [10] Ramani Narayan, Fundamental Principles and Concepts of Biodegradability – Sorting through the facts, hypes, and claims of biodegradable plastics in the marketplace, BioPlastics magazine vol 4, Jan 01, 2009; www.bioplasticsmagazine.com [11] Narayan, R.; Biodegradable and Biobased Plastics; In soil degradable BioPlastics for a sustainable modern agriculture, Ed., M. Malinconico; Springer-Verlag GmbH Germany; Ch 2, pg 23-34, 2017 [12] Narayan, R., Principles, Drivers, and Analysis of Biodegradable and Biobased Plastics, Handbook of Biodegradable Polymers, 2nd Edition, Editor: Catia Bastioli; Smithers Rapra Technology, November 2014;Ch 16 pg 561 ISBN-13: 978-1847355270 ISBN-10: 1847355277 [13] Zumstein, M.; Narayay, R. et.al.: Dos and Do Nots When Assessing the Biodegradation of Plastics; https://pubs.acs.org/doi/10.1021/acs. est.9b04513 Figure 1: What does “Biodegradable” mean? % C conversion to CO2 (% biodegradation) 100 90 80 70 60 50 40 30 20 10 0 lag phase What does “Biodegradable” Mean? Can the microorganisms in the target disposal system (composting, soil, anaerobic digestor) assimilate/utilize the carbon substrate as food source completely and in a short definedtime period? Polymer chains with susceptible linkages Environment –soil, compost, waste waterplant, marine biodegradation degree level of biodegradation needed to claim safe and efficacious removal of the plastic carbon from the environmental compartment Hydrolytic STEP 1 Oxidative Enzymatic Oligomers & polymer fragments STEP 2 biodegradation phase Time (days) Complete microbial assimilaton Biodegradation(Step 2): Only if all fragmented residues consumed by microorganisms as a food & energy source as measured by evolved CO 2 in CO 2 + H 2 O + Cell biomass defined time and disposal environment plateau phase 0 20 40 60 80 100 120 140 160 180 200 Figure 2: Test method to measure the rate and extent of microbial utilization {biodegradation} of biodegradable plastics O 2 CO 2 Soil & Test Materials ASTM D5988; ISO 17556 -- Soil biodegradability test method EN 17033 –soil biodegradability specifications for biodegradable mulch film COMPOST -- ASTM D5338; ISO 14855; ISO 18606; EN 13432 AS 4736 & 5810 Specification – ASTM D6400, D6868 ; ISO 17088; EN13432 Figure 3: Dos and Do Nots When Assessing the Biodegradation of Plastics defined time frame, no residues bioplastics MAGAZINE [04/20] Vol. 15 39
