Evaluation of Biodegradation-Promoting Additives for Plastics

Biodegradation-promoting additives for polymers are increasingly being used around the world with the claim that they effectively render commercial po...
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Evaluation of Biodegradation-Promoting Additives for Plastics Susan Selke,*,† Rafael Auras,*,† Tuan Anh Nguyen,† Edgar Castro Aguirre,† Rijosh Cheruvathur,† and Yan Liu‡ †

School of Packaging, Michigan State University, East Lansing, Michigan 48824, United States Biosystems and Agricultural Engineering, Michigan State University, East Lansing, Michigan 48824, United States

‡

S Supporting Information *

ABSTRACT: Biodegradation-promoting additives for polymers are increasingly being used around the world with the claim that they effectively render commercial polymers biodegradable. However, there is a lot of uncertainty about their effectiveness in degrading polymers in different environments. In this study, we evaluated the effect of biodegradationpromoting additives on the biodegradation of polyethylene (PE) and polyethylene terephthalate (PET). Biodegradation was evaluated in compost, anaerobic digestion, and soil burial environments. None of the five different additives tested significantly increased biodegradation in any of these environments. Thus, no evidence was found that these additives promote and/or enhance biodegradation of PE or PET polymers. So, anaerobic and aerobic biodegradation are not recommended as feasible disposal routes for nonbiodegradable plastics containing any of the five tested biodegradation-promoting additives.

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INTRODUCTION Earth’s ecological systems are approaching a planetary shift due to human influences.1 Increasing marine pollution due to plastic debris and increasing generation of municipal solid waste (MSW) due to plastics disposal are becoming an escalating public and governmental concern.2−5 Legislation has been introduced in many countries to deal with the plastic MSW problem and plastic debris, although the approaches have not been systematic. The best-known legislation bans plastic bags. Ireland, Australia, Israel, Bangladesh, Mali, Mauritania, China, and some locations in the U.S., such as San Francisco, Washington D.C., and Maine, to name a few, have adopted legislation taxing or banning plastic bags in retail stores.6−11 There is increasing interest in adopting biodegradable plastics in order to use biodegradation as a disposal route. However, most of the commercial polymers from the polyolefin and polyester families are not biodegradable. Polyolefins are the most-used nonbiodegradable polymers in the U.S.12 and the world, and are responsible for the largest amount of plastics in MSW, followed by PET.13 A number of countries have adopted legislation promoting the use of biodegradation-promoting additives in polyolefins and PET. For example, in 2012, the United Arab Emirates (U.A.E.) banned all disposable plastic bags with the exception of those made from oxo-biodegradable and compostable plastic.14 Similar legislation is being adopted or considered in a number of countries, such as Pakistan and Argentina.15 The underlying goal behind using biodegradable plastics is the assimilation of these materials back into the environment. © XXXX American Chemical Society

Biodegradation of plastics is described as taking place when microorganisms use the plastics as a source of energy for metabolic processes as well as a source of carbon for growth or reproduction.16 Most of the carbon in the metabolized substrates is used to generate energy, typically through chemical transformation to carbon dioxide in aerobic environments, and to a mix of CO2 and methane in anaerobic environments. Conversion to growth of the organisms themselves is usually a relatively small percentage of the total although larger in aerobic than anaerobic degradation. Therefore, measurements of the generation of carbon dioxide and methane (compared to the initial carbon content) are considered the most appropriate measures of biodegradation in most circumstances. Other measures of changed performance can provide insight into the degradation process, but are not in themselves indicators of biodegradation. Change in physical properties such as decrease in tensile strength and embrittlement obviously affect performance of the plastic article, and fragmentation can result in the plastic no longer being visible in the environment, but they do not necessarily indicate true biodegradation. Similarly, measurement of changes in molecular weight can provide insight into the degradation process, but decreased molecular weight is not itself an indicator of aerobic or anaerobic biodegradation.17 Complete assimilation of polymers into the environment Received: September 2, 2014 Revised: February 19, 2015 Accepted: February 27, 2015

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h to ensure an even exposure. Calibration of the irradiance was done every 100 h using a CR-10 calibration radiometer from Qlab. For the PE films without additives, the exposure time was 276 h, whereas for the films containing 1% Symphony it was 152 h.26 Elongation Percentage at Break to Determine Degradation End Point. UV-aged polyethylene samples were periodically removed from the exposure apparatus and conditioned at 23 ± 2 °C and 50 ± 5% RH for 48 h before they were tested to determine the % elongation at break in accordance with ASTM D3826, using a Universal Testing machine (Instron 5585, Instron, Norwood, MA) equipped with Bluehill software for data analysis. At least 5 samples of width 2.54 cm and guage length of 10.2 cm were tested at a strain rate of 1 cm per min.27 Carbonyl Index (CI). Fourier Transform Infrared Spectroscopy (FTIR) analysis was performed on the aged PE and PE S1 films using a Shimadzu IR Prestige-21 FTIR equipment with IR Solution software for data analysis (Shimadzu, Canby, OR). The IR spectrum was obtained from 400 to 4000 cm−1 in transmission mode with 40 scans at a resolution of 2 cm−1. The peaks of interest were the methylene scissoring band at 1460 cm−1 and the carbonyl band at 1715 cm−1. The CI value was determined as the ratio of the peak intensity at 1715 cm−1 to the peak intensity at 1460 cm−1.22 Anaerobic Biodegradation. Anaerobic digestion of the samples was done in general accordance with ASTM D5526-12. This test method is intended to simulate biologically active landfills and is operated under accelerated anaerobic landfill conditions, where moisture and temperature are controlled and gas recovery is promoted. PE films and PET sheets were exposed to anaerobic digestion environments at 35 ± 1 and 50 ± 1 °C. The anaerobic inoculum was obtained directly from a properly operating anaerobic digester at Michigan State University (MSU). As permitted by ASTM D5526-12, pretreated household waste was replaced by fresh dairy manure obtained from the MSU dairy farm and added with water to create a 5% (w/v) total solids mixture. The lower solid content was used as opposed to the standard because lower solid content generally leads to greater biodegradation.28 Manure was the only nitrogen source, whereas both manure and the plastics were carbon sources. Manure also acts as a buffer to help maintain the pH within the optimal range (6.8−7.2) so that microorganisms can grow and thrive. The treatments and controls were mounted on orbital shakers model 361 at 95 ± 5 rpm, and placed in an incubator model 11-690D at 35 and 50 °C all from Fisher Scientific, Hampton, NH. In the test, the powder form of cellulose and cornstarch was used rather than the analytical grade cellulose for thin-layer chromatography suggested by the standard. Sample Preparation. PE films and PET sheets with and without additives were cut into 0.635 × 0.635 cm2 pieces and weighed before insertion into 125 mL glass serum bottles. These bottles were airtight and fitted with septa for measuring gas production. The weight of each reagent was calculated to yield a C/N ratio within the optimal 20−30 range. Initially, samples with 4.337 g of starch and cellulose evolved large amounts of gas resulting in an uncontrollable drop in pH level to less than 5.29 Therefore, a new experiment with only the blank and two positive controls was conducted, using 0.55 and 1.1 g of cellulose for the positive controls with the same manure and inoculum. Anaerobic biodegradation of these control samples was continued for 250 d; testing of the plastic samples

requires eventual total breakdown of the molecular structure of the polymer. A key factor in such assimilation is the time period that is required. For there to be substantive environmental benefit, such assimilation must occur within a reasonable time frame.18 While there is disagreement on the exact length of time that is “reasonable,” there is general consensus that time frames must be on the order of months to a few years, rather than decades to centuries.16−19 Extensive work has been conducted to understand the degradation of polyolefins,20,21 and the effect of using biodegradation-promoting additives in these polymers.22−24 However, there remains considerable controversy regarding the biodegradation of these polymers.21,25 Making improper or unsubstantiated claims can produce consumer backlash, fill the environment with unwanted polymer debris, and expose companies to legal penalties.6−8,25 Here, we investigate the effect of 3 different types of biodegradation-promoting additives on the biodegradation of a blend of linear low and low-density polyethylene (commonly used for bread, supermarket, and trash bags) and PET sheets under active anaerobic digestion, aerobic degradation (compost), and soil burial environments. According to the public information at the time of additive selection, one of these was of the oxo-biodegradable type, one was a nonoxo additive, and the third was a combination. The base experiments evaluated all 3 types in all 3 environments for PE, and both available additives in all 3 environments for PET, to provide further understanding of the effect of disposing these polymers in the environment.

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MATERIALS AND METHODS Film Production. LDPE/LLDPE (PE) Films. Low density polyethylene (LDPE 501I) and linear low density polyethylene (DOWLEX 2045G) resins were obtained from the Dow Chemical Company (Houston, TX). LDPE and LLDPE were blended at a ratio of 70:30 by weight. One and five percent by weight of the biodegradation-promoting additive masterbatches were added to this blend for extrusion. Additives for PE films were obtained from 3 different companies: Wells Plastics Ltd. (Reverte for PE) (Staffordshire, U.K.), Ecologic (Eco-one EL 10) (Oshkosh, WI), and Symphony (d2w) (Shropshire, U.K.). Samples were labeled, for example, PE W1, PE E1, and PE S1 indicating PE films added with 1% wt. of Wells, Ecologic, and Symphony additives, respectively. Detailed information on the PE extrusion process is provided in the Supporting Information (SI). The overall thickness of the PE control film was 22.9 ± 5.1 μm. Poly(ethylene terephthalate)PET Sheets. PET resin was mixed with 1 and 5 wt % of different biodegradation-promoting additive masterbatches from 2 companies made for PET, Wells Plastics (Reverte for PET) and Ecologic (Eco-one EC 80). Detailed information on the PET sheet production is provided in the SI. The overall thickness of the PET sheet was 234 ± 15 μm. Ultraviolet Degradation. UV degradation was carried out in a QUV Accelerated Weathering Tester from Q- Lab Corporation, Cleveland, OH equipped with a Solar Eye UV Irradiance Controller. Samples were continuously exposed with no dark or wet cycles at 60 ± 2 °C at an irradiance of 0.80 W/ m2 using UVA lamps (340 nm). The samples were mounted onto the aluminum sample holders at a distance of 5 cm from the lamps. The positions of the samples were rotated from the center to the edges every 24 B

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Figure 1. Anaerobic biodegradation of PE films and PET sheets. Total accumulated gas in mL vs time in d for PE films (a @ 35 °C and b @ 50 °C) and PET sheets (c @ 35 °C and d @ 50 °C) exposed to liquid anaerobic digestion environments created from fresh dairy manure obtained from the MSU dairy farm, inoculum from the MSU anaerobic digestor and water added to create a 5% (w/v) total solids mixture. Total accumulated gas for PE films and PET sheets were not significantly different than the blank (M1 & M2) samples (at P = 0.05). The total accumulated gas for cellulose 1.10 and 0.55 g was significantly higher than the blanks (M1 & M2) and PE films and PET sheets at both temperatures. Inserts show selective spiking of bioreactors at 464 d with 0.55 g of cornstarch.

to conduct this determination. The headspace gas (85% N2, 10% H2, and 5% CO2) was supplied by Airgas Inc. (Radnor Township, PA). The bottles were shaken before being opened. A pH meter model Accumet AB15 (Fisher Scientific, Hampton, NJ) with Ag/AgCl electrode was inserted into the opening. If the pH was lower than 6.7, then NaOH 10% solution was added to bring it close to a pH of 6.9. At low concentration, Na+ can stimulate growth of anaerobic bacteria. However, at higher concentrations, Na+ slows down and even inhibits bacterial growth by disrupting their metabolisms.29 The half maximal inhibitory concentration of Na+, the amount of Na+ needed to inhibit the growth of anaerobic bacteria by half, was 5.6 to 53 g/L. For the reactor containing plastics only, the maximum amount of NaOH 10% solution added was less than 1 mL (or 0.0638 g Na+). For the positive control experiment, the maximum amount of 10% NaOH added was less than 2 mL (or 0.1276 g Na+). The total amount of NaOH added to each bioreactor was less than 4 g/L.

continued for 464 d. Table S1 in the SI shows the treatment compositions. Biogas Measurement. Total accumulated gas in mL (approximately 2/3 CH4 and 1/3 CO2) from all treatments was quantified, and compared to positive and negative controls. The gas production was measured using the water displacement method. A detailed description of the method is provided in the SI. Initially, measurements were taken every 3 d (for the first 100 d) and then after every 7 d. Spiking of the Bioreactors. After 464 d of running the initial experiment, 0.55 g of corn starch was added to one replicate of PE, PE E5, PE S5, PE W5, PET E5, PET W5, and the blank in both the 35 and 50 °C incubators. The biogas production as well as the pH level was monitored for the following 50 d. pH Determination. The pH of each serum bottle was checked several times during the study to ensure that it was close to 6.9. A controlled environment anaerobic chamber model 855 from Plas Laboratories, Inc. (Lansing, MI) was used C

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sheets were also produced containing one and five percent by weight of biodegradation promoting additive masterbatches obtained from Wells PlasticsW1 & W5and EcologicE1 & E5. Controls without additives were also produced. Samples were exposed to anaerobic and aerobic biodegradation as discussed below. Anaerobic Biodegradation. Figure 1 shows the evolution of total gas produced from PE films and PET sheets exposed to anaerobic digestion environments at 35 and 50 °C representing a modified simulated biological active landfill for 464 d. Individual plots for each treatment are provided in SI Figures S3−S6. The total accumulated gas for PE and PET films at 35 and 50 °C were not significantly different than the blank samples at days 252 and 464 (at P = 0.05). Optical microscopy confirmed that negligible amounts of biofilm were formed on the surfaces of the plastic samples retrieved (Figure not shown). After 464 d selected bioreactorsPE, PE E5, PE S5, PE W5, PET E5, PET W5, and the blank (M1)were spiked with corn starch; the increase in gas production indicated that the microorganisms present in the bioreactors did become active when a food source was provided. The additional total accumulated gas for the PE films and PET sheets in the spiked bioreactors at 35 and 50 °C were not significantly different from the blank samples at day 514 (at P = 0.05). Evaluating degradation of polymers under anaerobic digestion conditions is important for a number of end-of-life scenarios, such as marine disposal, anaerobic digestion units designed for energy recovery, and landfill. Most of these scenarios can be operated at low mesophilic conditions (∼35 °C) to high thermophilic conditions (∼55 °C and more). Most degradable materials degrading under anaerobic digestion go through four main stages: (1) hydrolysis of complex organic materials such as proteins, carbohydrates, and lipids into amino acids, carbohydrates, fatty acids, and alcohols; (2) fermentation of amino acids and carbohydrates producing short-chain fatty acids, succinate, aminovalerate, and H2 and resulting in ethanol, acetate, H2, and CO2; (3) anaerobic oxidation of long-chain fatty acids and alcohols with end products of acetate and propionate; and (4) anaerobic oxidation of short-chain fatty acids such as propionate and butyrate to acetate and H2 called methanogenesis, producing the end products of CH4 and CO2.32 As we can observe in Figure 1, no additional hydrolysis and/or anaerobic oxidation of the PE films and PET sheets was observed in order to provide additional intermediate products to increase the final methanogenesis of these reactors. Therefore, we can safely conclude that this long-term gas evolution test suggested that PE films and PET sheets without and with biodegradation promoting additives disposed in an active anaerobic environment with temperature up to 50 °C would not appreciably degrade at least for 500 d, and no evidence was found to suggest that additional exposure for a moderate period of time would change the result. Most studies of the anaerobic digestion of polymers reported in the literature examine biodegradable polyesters such as poly(lactic acid) (PLA),33,34 poly(β-hydroxybutyrate) (PHB),35 and poly(ε-caprolactone) (PCL),35 which have ester groups that can be consumed by microorganims. Kolstad et al. concluded that amorphous PLA generated a small amount of gas when tested under anaerobic conditions at 35 °C, but that semicrystalline PLA did not produce significant generation of gas.33 However, Yagi et al. found that PLA powder (10 g, 125− 250 μm) degraded 91% in 75 d when tested under anaerobic conditions at 55 °C in undiluted sludge.34 The large difference

Aerobic Biodegradation. Materials. Two types of compost were used: Earthgro organic humus and manure from Scotts Miracle-Gro (Marysville, OH) for test 1; and 12-monthold mature compost obtained from the MSU Composting Facility (East Lansing, MI) for test 2. In both cases, the compost was sieved on a 10 mm screen and preconditioned at 58 °C for a period of 3 d. Deionized water was added to increase the moisture content to about 50%. Saturated vermiculite premium grade (Sun Gro Horticulture Distribution Inc., Bellevue, WA) was added in both cases to the compost 1:4 parts (dry weight compost). The pH of the compost was found to be 7.6 and 8.3 for tests 1 and 2, respectively. The C/N ratio of the compost in both cases was found to be 12.5. A summary of physical-chemical parameters of the compost is shown in SI Table S2. The list of the materials used for the aerobic biodegradation test is shown in SI Table S3, along with the carbon and nitrogen content determined using a PerkinElmer 2400 Series II CHNS/O Elemental Analyzer (Shelton, CT). All films were cut to 1 × 1 cm2 pieces. Additionally, cellulose powder (particle size ∼20 μm) was obtained from SigmaAldrich (St. Louis, MO). Aerobic Biodegradation Experiments. Conditioned compost (550 and 400 g for test 1 and 2, respectively) was weighed and mixed with 8 g of the test material in a container to get a homogeneous mixture. Then, the mixture was transferred to the bioreactor, which was tightly closed to prevent leakage. Bioreactors were filled to about three-quarters of their volume with the mixture, leaving sufficient headspace to allow further manual shaking. Subsequently, bioreactors were placed into the environmental chamber which was preconditioned at a constant temperature of 58 ± 2 °C. Aeration was initiated using watersaturated carbon-dioxide-free air, and the flow rate through each bioreactor was set at 40 sccm. The tests were carried out in the dark for a period of 140 and 60 d for test 1 and 2, respectively. Throughout the testing period, water was added to avoid dryness of the compost. Further details of the apparatus and testing procedure can be found in references.30,31 The calculation method of the evolved CO2 and the mineralization of the samples is presented in the SI. Ecotoxicity. After the biodegradation tests under composting conditions were completed, the contents of selected bioreactors (i.e., Blank, Cellulose, PE, PE S5, and PET E5) were carefully removed and thoroughly mixed and used to evaluate plant germination and growth in accordance with ASTM D6954-04 and OECD/OCDE 208 (2006) as explained in the SI. Soil Burial. The biodegradation of PE films and PET sheets of 0.20 × 0.18 m2 buried in soil under ambient conditions was determined. A plot of 30 × 30 m2 was secured at the MSU Horticulture Research Facility (SI Figure S1) to conduct the study for 1095 d. A number of optical, physical, and structural properties of the films were evaluated after they were removed from the soil. Detailed description of the experimental setup is provided in the SI.

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RESULTS AND DISCUSSION Films were produced from a blend of LDPE/LLDPE at a ratio of 70:30 wt % (PE) containing one and five percent by weight of biodegradation-promoting additive masterbatches obtained from Wells Plastics (Reverte for PE)W1 & W5a combined oxo-degradable and nonoxo-degradable additive, Ecologic (Ecoone EL 10)E1 & E5a nonoxo-degradable additive, and Symphony (d2w)S1 & S5an oxo-degradable additive. PET D

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Figure 2. Aerobic biodegradation under simulated compost environment of PE films. Aerobic biodegradation of cellulose, control, and PE films (a and b), and UV aged PE films (c and d), and PE powder (e and f) under standard laboratory composting conditions (58 ± 2 °C and 60% RH). a, c and e CO2 evolution in g vs time in d for PE films, UV aged PE films, PE powder respectively; (b, d, and f) % mineralization vs time in d for PE films, UV aged PE films, PE powder, respectively. Vertical bars (a, c, and e) represent standard errors (n = 3). E

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Environmental Science & Technology in the degradation between these two studies may be due to the high temperature testing by Yagi et al., which accelerates PLA hydrolysis.36 Abou-Zied et al. tested PHB and PCL and found that PHB degraded 100% in 14 d and PCL 20% in 42 d in anaerobic “methane sludge” from sugar industry wastewater at 37 °C.35 Yagi et al. found that PCL powder (10 g, 125−250 μm) degraded 92% in 40 d in an anaerobic environment at 55 °C.34 To the best of the authors’ knowledge, only Mohee et al. compared the degradation of biodegradable starch blends and PE containing an oxo-degradable additive under anaerobic digestion conditions.37 They showed that starch blends biodegraded to a large extent after 42 d. However, they concluded that the total gas evolved from the PE with additives and the controls were similar at 32 d, and no biodegradation was observed. These findings coincide with our work, so there is no reason and evidence to expect that PE films with biodegradable-promoting additives will substantially degrade in an anaerobic digestion environment. Aerobic Biodegradation. Aerobic composting tests were conducted under laboratory conditions at 58 ± 2 °C and 60% RH (Figure 2). CO2 evolution from the blank and PE films (Figure 2a) did not differ significantly (at P = 0.05) through 140 d. Figure 2b shows that mineralization values for cellulose reached 70%, but no mineralization was observed for samples with the additives. A second biodegradation test was run as shown in SI Figure S7a,b. PE films again did not show appreciable mineralization; moreover, for PE films loaded with 5 wt % master batch additives CO2 evolution was noticeably inhibited. Similar behavior was observed for PET films (SI Figure S8a,b) So, PE films and PET sheets containing any of these three additives would be mostly unaffected after an industrial composting process is finished, between 90 and 180 days,17 leaving behind plastic debris and contaminants. Many of these additive producers claim to promote aerobic biodegradation if an initial large reduction of the molecular weight (Mw) of the films is achieved through photodegradation. Figure 3a,b shows carbonyl index and elongation at break in % for PE and PE S1 films exposed to UV radiation for 276 and 152 h until they reached 9.1 ± 2.9% and 7.0 ± 2.6% elongation at break, respectively, which is equivalent to an average outdoor exposure of 104 and 58 d, respectively, in Miami, FL.26 CO2 evolution from these samples in compost conditions did not differ significantly from the blank (at P = 0.05)Figure 2c,d indicating that a significant reduction of Mw was not sufficient to promote aerobic biodegradation of the samples under compost conditions. Previous researchers suggested that the degradation of PE is a two stage process, first abiotic oxidation followed by mineralization of the oxidized products.22 A detailed proposed mechanism for PE degradation can be found elsewhere.21,22 Albertsson and co-workers identified a number of degradation products by chromatographic analysis formed during oxidization of PE such as alkanes, alkenes, aldehydes, ketones, alcohols, carboxylic acids, lactones, and esters, with dicarboxylic acids the most abundant in oxidized samples.21,38 Chiellini et al. and other researchers suggested that it is these components produced during the degradation process that are first consumed by the microbial community, followed by the ethylene units.22,24,39,40 Chiellini et al. reported mineralization values around 60% for thermally fragmented LDPE film samples (Mw = 6.72 kDa) added with pro-oxidants exposed to mature compost for 600 d at 55 °C.39 Jakubowicz et al. reported 91% and 43% mineralization for UV treated LDPE films added with pro-oxidants when exposed to soil at 23 °C

Figure 3. Carbonyl index vs time and elongation at break in percentage of PE and PE S1 films. Vertical bars represent standard errors (n = 3). (a) Carbonyl index vs time of exposure to UV radiation for PE and PE S1 films and (b) elongation at break vs time of exposure to UV radiation for PE and PE S1 films. PE S1 films exposed for 152 h had a final Mw = 31.4 kDa, Mn = 5.52, and polydispersity index = 5.7. Fitted lines are included as visual guidelines.

and compost at 55 °C, respectively.23 Yashchuk et al. reported a maximum of ∼25% mineralization at 90 d for high molecular weight PE film exposed to UV radiation and degraded in mature compost at 55 °C,41 and Esmaeili et al. reported 30% mineralization at 126 d for LDPE films exposed to selected microorganisms at 55 °C.24 However, early studies assessing the biological oxygen demand during aerobic degradation of aliphatic paraffins with different molecular weight concluded that only alkenes with molecular weight below 618 Da consumed additional oxygen than the soil incubated control.42 Figure 2c,d does not show considerable degradation of these degraded samples (i.e., PE and PE S1). So, if these biodegradation promoting additives are added to commercial nonbiodegradable polyolefins expecting that they will biodegrade in a compost environment after exposure to solar radiation such as is the case for mulch films, then we will expect that these polymers will not be used as a food source by the microorganism cosmos present in these environments. F

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Environmental Science & Technology In order to further understand whether low molecular weight PE and/or the ethylene units of PE will potentially be degraded under simulated compost conditions, PE powder of Mn = 2.9 kDa; Mw = 22 kDa; and polydispersity = 7.63, obtained from a polyethylene polymerization plant, was exposed to the compost environment. After 60 d of biodegradation, the PE powder did not exhibit mineralization (Figure 2e,f). CO2 evolution from these samples was lower than from the blank (P < 0.01). The PE powder used in this work had three times the Mn of the one reported by Haines and Alexander for aliphatic paraffins as the limit for being considered as a food source for microorganisms.42 So, PE films, photodegraded PE films, and PET sheets with and without biodegradation-promoting additives, as well as PE powder, did not biodegrade in a compost environment, and were not used as a food/energy source by the compost microorganisms. Thus, PE films and PET sheets containing biodegradation-promoting additives would visually contaminate compost and end up accumulating in the environment. So, this route of disposal is not recommended for PE films and PET sheet without or with biodegradation promoting additives as tested here. Plant growth and germination tests were performed to observe whether inhibition was due to toxic residues that might be killing microorganisms responsible for biodegradation, and those toxic residues, if present, would affect the proper growth of plants as well. Plant growth and germination tests did not significantly differ among the samples (SI Tables S8−S11 detail the % of germinated seeds and their growth). Soil Burial. PE films and PET sheets were buried at a depth of 0.45 m in sandy soil for 1095 d, and the temperature, water content, pH, and soil properties were monitored. SI Figure S9 shows the air temperature, soil temperature, electrical conductivity, and volumetric water content vs time for the samples buried in the field. Most of the samples met the U.S., Canada, Japan, and European limits on heavy metal concentrations, but the concentration of PE-E5 for selenium was above the required limit in the European standards (SI Tables S5−S7). Optical, mechanical, and thermal properties were periodically evaluated. Figure 4 shows the PE films before burial and at 1095 d. SI Figure S10 shows the PET films. No major visual disintegration of the films was observed. The material became slightly more fragile, but the presence of additives did not accelerate this behavior (SI Tables S12 and S13). We recognize that 3 years testing for PE films and PET sheets are very short degradation times for these polymers. However, considering littering in the environment, this time frame provides a good indication of the short-term consequence of disposing these polymers in the environment. Medium term studies were conducted by Albertsson and Karlsson studing the aerobic degradation of LDPE films labeled with C14 (Mn = 18.2 and Mw = 84 kDa) with thicknesses of 0.02 and 0.16 mm with and without a photochemical degradation additive for 10 years in dark soil cultivation at 25 °C.43 The evolved 14CO2 was trapped in KOH solutions and measured with a liquid scintillation counter. PE films without oxidation additives only evolved 0.2% wt. in 10 years of CO2, and PE films with UV sensitizer irradiated for 42 d lost around 5% wt. in 10 years. In this work, the particular degraded compound was not discussed, but it was also observed that even samples exposed to high irradiation for 42 d did not significantly biodegrade. Therefore, it is safe to conclude that PE films and PET sheets disposed in sandy soil will litter the environment,

Figure 4. Image of the PE films buried in sandy soil for 1095 d: (a and b) PE film @ 0 and 1095 d, respectively; (c and d) PE E5 film @ 0 and 1095 d, respectively; (e and f) PE W5 film @ 0 and 1095 d, respectively; and (g and h) PE S5 film @ 0 and 1095 d, respectively. Sample dimensions were 0.2 m width and 0.18 m height.

since they do not fragment or biodegrade for at least 1095 d, and there is no reason to suggest that they will undergo considerable biodegradation when exposed to this environment for longer periods of time (Figure 4). Thus, anaerobic and aerobic biodegradation routes are not recommended as a feasible disposal route for nonbiodegradable plastics containing any of the five tested biodegradationpromoting additives.

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ASSOCIATED CONTENT

S Supporting Information *

Most of the raw data summarized in this paper. This material is available free of charge via the Internet at http://pubs.acs.org. G

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AUTHOR INFORMATION

Corresponding Authors

*Phone: 517 353-4891; fax: 517 353-8999; e-mail: sselke@msu. edu. *Phone: 517 432-3254; fax: 517 353-8999; e-mail: aurasraf@ msu.edu. Author Contributions

S.S. and R.A. designed the work. All the authors have conducted and written the manuscript. All authors have given approval to the final version of the manuscript. Funding

This work was financially supported by the Center for Packaging Innovation and Sustainability at the School of Packaging, Michigan State University. R.A. thanks partial support of the USDA National Institute of Food and Agriculture and Michigan AgBioResearch, Hatch project R. Auras. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the three additives companies for providing the master batch samples. The authors thank all the SoP undergraduate and graduate students that helped in this project.

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DOI: 10.1021/es504258u Environ. Sci. Technol. XXXX, XXX, XXX−XXX