Polymer Durability and Radiation Effects - American Chemical Society

Chapter 9

Characterization of Biodegradable Polymers: (I) Biodégradation of Poly(vinyl alcohol) under Aerobic and Aqueous Conditions

Downloaded by PURDUE UNIV on July 8, 2016 | http://pubs.acs.org Publication Date: December 21, 2007 | doi: 10.1021/bk-2007-0978.ch009

Jianzhong Lou, Keith Schimmel, Pfumai Kuzviwanza, Dana Warren, and Jizhong Yan Department of Chemical and Mechanical Engineering, North Carolina Agricultural and Technical State University, 618 McNair, Greensboro, NC 27411

Biodegradation experiments were conducted according to ASTM D5271 to determine the rate of biodegradation of poly(vinyl alcohol), under aerobic and aqueous condition, in an activated sludge, using an oxygen respirometer. The percent of polymer biodegraded was estimated from the measured biochemical oxygen demand (BOD) divided by the theoretical oxygen demand (ThOD). The impact of temperature and polymer concentration on the total oxygen uptake, ultimate biochemical oxygen demand (UBOD), and percent degradation of polymer was evaluated. The experiments showed that the increase in the polymer concentration, not the temperature, caused the increase in the rate and percent of polymer biodegradation.

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© 2008 American Chemical Society

Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.


93 Polymers (plastics, rubbers, fibers, coatings, adhesives, and composites, etc.) continue to play an important role in the nation's economy as one of the largest sectors of the chemical industry (1). Polymers have displaced metals, glasses, ceramics and wood in many products, especially in the area of packaging. Commodity plastics like polyethylene (PE), polypropylene (PP), polystyrene (PS) and polyvinyl chloride (PVC) in variety of forms such as films, flexible bags and rigid containers have revolutionized the packaging industry. However, once these materials are discarded, they persist in the environment without being degraded thus giving rise to many ecological and environmental problems. The 1996 U . S. EPA Report showed that over 19 million tons of plastics alone, not to include other types of polymers, which may remain in the environment for decades, were dumped annually as municipal solid waste(2). Of particular concern are polymers used in a single-use, disposable applications such as baby diapers and food packagings. The high molecular weight, covalently bonded polymer molecules are not readily broken down by the waste management infrastructures such as composting. This results in an irreversible buildup of "die-hard" trash that may cause scarring of landscapes, fouling of beaches, and a serious hazard to marine species. With increasing environmental awareness, the shortage of landfill space and emission concerns, the plastic industry is pushing to design and engineer products that are biodegradable. The plastics of today are manufactured with minimal consideration for their ultimate disposability or recycylability. Hence, authorities are pressuring the industry to review its product design and engineering due to mounting concerns over their environmental impact. No longer will it be sufficient to simply specify the required strength, dimensions, cost and preferred appearance of a plastic product or package. Today's engineer designing such a product must take into account where the package will end up after its useful existence (5). This requires that recyclability or biodegradability features be incorporated into the materials, while still retaining performance characteristics. In addition, after the material's intended use it must end up in appropriate infrastructures that use these recyclability or biodegradability attributes (4). Biodegradable polymers offer one of the potential solutions to the environmental problem of polymers. They constitute a loosely defined family of polymers that are designed to degrade through action of living organisms and offer a possible alternative to traditional non-biodegradable polymers where recycling is unpractical or not economical. Interest in biodegradable plastics is being revived by new technologies developed by major companies, such as Bayer, DuPont, and Dow Cargill. Recently, Dow Chemical and Cargill (Dow Chemical is out now) created a joint venture in Blair, Nebraska to become the largest biodegradable polylactide (PLA) producer with annual capacity of 140,000 metric tons. Biodegradable polymers have a wide range of potential applications in markets currently dominated by traditional petroleum-derived plastics. These markets include the packaging industry, farming and also in



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specialized bio-medical applications. However, the main constraint on the use of biodegradable polymers is the difference in the price of these polymers compared to bulk produced petroleum based plastics (despite the recent run-up in oil price, petroleum-derived polymers are still less costly than agriculturallyderived biodegradable polymers). The challenge lies in finding applications that would consume large quantities of these materials to lead to price reductions and allowing biodegradable polymers to compete economically in the market. In addition, engineers are faced with an enormous challenge to design materials that exhibit structural and functional stability during storage and use, yet are susceptible to microbial and environmental degradation upon disposal, without severe consequences to the environment. Biodegradable polymers may be classified as natural and synthetic biodegradable polymers. Cellulose and starch are examples of natural polymers that are produced by all living organisms. They are readily biodegradable through hydrolysis followed by oxidation with the aid of enzymes. Among the synthetic polymers, aliphatic polyesters are generally known to be susceptible to biological attack. Important examples of synthetic biodegradable polymers of industrial scale include poly(vinyl alcohol) (PVOH) and poly(lactic acid) (PLA). The biodegradation of polymeric material follows four-stage kinetics of slow enzyme-catalytic oxidation of organic materials. The first stage of attack by the microorganisms is attachment to the polymer sample. Attachment creates a tiny ecosystem for the microbe to act on the material, usually through enzymes, to break the material down into nutritional requirements for the microbes. The second stage of the biodegradation process isfragmentationof the material. The third stage is disintegration. The material is reduced to powders. Finally, the polymer is reduced to carbon dioxide, water, and minerals. Knowledge of the biodegradability of a polymer is an important aspect of their environmental behavior because a biodegradable substance is expected to cause less ecological problems in the long term than a persistent one. Our research program on biodegradable polymers consists of the respirometric characterization of biodegradation of polymers, as reported in this paper, and mechanical, thermal-analytical and chromatographic characterization which will be reported later. For respirometric characterization, the first step is the determination of ready biodegradability. According OECD (Organisation for Economic Cooperation and Development: Paris, France) Test 301F guidelines (5), a substance is considered readily biodegradable if it reaches 60% ThOD in a 10-day window within the 28-day test period. The window begins when 10% of ThOD is obtained and must end before day 28 of the test. In this paper, we report the biodegradation of poly(vinyl alcohol) (PVOH) (6) in an aqueous environment and investigate the influence of temperature and polymer concentration on the total oxygen uptake, ultimate BOD and percent degradation of PVOH. A series of test were conducted using Test Method D5271 of the American Society of


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Testing and Materials (7). This method indicates the extent and rate of biodegradation of plastic materials by aerobic microorganisms in an aqueous environment and is performed using a respirometer. The test method is used to determine the ultimate biodegradation which is the complete utilization of a test material by microorganisms resulting in the production of carbon dioxide, water, mineral salts and new microbial cellular constituents (biomass) in contrast to a primary biodegradation which is only the change of the identity of the test material (8). The percent of polymer that is biodegraded is calculated by comparing the biochemical oxygen demand (BOD) with the theoretical oxygen demand (ThOD).

Experimental PVOH Sample. The PVOH utilized in the activated sludge tests was a research grade from Aldrich Chemical Company, Inc. of Milwaukee, Wisconsin (6). The PVOH has an average molecular weight of 9,000-10,000 and a degree of hydrolysis of 80%. The PVOH has a low viscosity of about 5.8 cps in a four percent aqueous solution at 20 °C as stated by the supplier. The test material was added to designated reactors at mass loadings of 100 mg and 200 mg PVOH on a dry weight basis. Sludge. As a source of inoculum, activated sludge microorganisms were obtained from the North Buffalo Wastewater Treatment Facility. This wastewater treatment plant was fed with predominantly municipal sewage and industrial wastewater. Sludge Preparation. The sludge was settled and the total suspsended solids were determined using the glass fiber filter method. The inoculum concentration was determined to be 2,532 mg/L of mixed liquid suspended solids (MLSS). However, due to the large biomass concentration and high oxygen uptake rate of the sample, the sludge was diluted to the inoculum concentrations given in the different OECD and ISO protocols (5). The sample was diluted in a two-liter conical flask using the prepared nutrient-mineral-buffer (NMB) solution to give a final test medium inoculum concentration of 500 mg/L of mixed liquid suspended solids (MLSS). Nutrient-Mineral-Buffer (NMB) Solution. Table 1 lists the composition of the nutrient-mineral-buffer (NMB) stock solution used as recommend by ASTM 5271. A trace element solution was also added in order to support biological growth. These solutions were used for all biodégradation tests. Each compound in column one was dissolved in distilled water and diluted to 1L at the concentrations listed in column two. The test medium was formed by adding the volumes listed in column three to 1 L of high-quality reagent water and adjusting the pH to pH 7.0 ± 0.1 with HC1 and NaOH. Approximately 400 mL of the test


96 medium/buffer stock solution was added to each reactor at the beginning of each test.

Table 1. Nutrient-Mineral-Buffer (NMB) Stock Solution Compound(l) Concentration^) 20.00 g/L Ammonium Sulfate, (NH ) S0 27.50 g/L Calcium Chloride, CaCl 0.25 g/L Ferric Chloride, FeCl .6H 0 22.50 g/L Magnesium Sulfate, MgS0 .7H 0 2.00 g/L Allylthiourea, CH CHCH NHCSNH 4

2

4

2

3

2

4


2

Phosphate Buffer: KH HP0 K HP0 Na HP0 .7H 0 NH C1 2

4

2

4

2

4

2

4

Trace Element Solution: FeS0 .7H 0 ZnS0 .7H 0 4

2

4

2

H3BO3

CoCl .6H 0 MnS0 .H 0 Na Mo0 .2H 0 NiCl .6H 0 2

2

4

2

2

4

2

2

2

2

2

2

in 1L Test Medium(3) 10 mL 1 mL 1 mL 1 mL 20 mL 20 mL

8.5 g/L 21.75 g/L 33.4 g/L 1.7 g/L 2mL 200 mg/L 10 mg/L 10 mg/L 10 mg/L 4 mg/L 3 mg/L 2 mg/L

Respirometric Setup. All tests were performed with two 8-cell Model AER200 respirometer systems manufactured by Challenge Environmental Systems, Inc.(Fayettville, AR). The AER-200 respirometer uses the WINDOWS™ 2000 operating system and CHALLENGE™ data acquisition software.

Results and Discussion The results of the respirometric tests are presented in Figures 1 and 2 for 35 °C and 25 °C experiment, respectively. Oxygen uptake data registered by the respirometer was used to monitor biological activity in the reactors. The control represents the oxygen consumption due to endogenous respiration of the microbial cells at the absence of the polymer substrate. The effects of the


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polymer substrate concentration and temperature on the total oxygen uptake are readily seen. At 35°C, the cumulative oxygen uptake over the test period was 545mg 0 / L for the 0.4g PVOH sample and 381 mg 0 / L for the 0.2g PVOH sample. At 25°C, the associated total oxygen uptake ranged from 510mg 0 IL to 790mg 0 / L for the 0.2g and 0.4g sample respectively. The result suggests that increasing the concentration of PVOH will provide more carbon for microbial growth consequently increasing the rate of reaction and biological activity of the microorganisms. Hence, the total oxygen uptake increases with increasing polymer substrate concentration. Respirometric curves for the samples examined at different temperatures have different shapes and differing intensity of biodégradation. At 35°C, the action of the microorganisms causing the PVOH degradation is initially faster for the first 10 days then biological activity slows down. The lower oxygen uptakes show that the microorganisms had not acclimated to PVOH biodégradation. At 25°C, the total oxygen uptake is greatest. This increase in biological activity is mainly attributed to the fact that the microorganisms were better acclimated to the degradation of PVOH at 25°C. This suggests that the biodégradation of PVOH is optimum at 25°C. When interpreting the BOD curves (Figures 3 and 4), it is essential to know the respiration due to the biomass itself, called the endogenous respiration. In biodégradation tests, it is important to determine endogenous respiration because the measured respiration data from the test assays with the test substance have to be corrected by these blank values. The concept is then to subtract this 2

2

2


2

Time (days)

Figure 1. Cumulative Oxygen uptake for OJgPVOH, 0.2g PVOH and Control Sample at35°C


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900

0

5

10

15 Time (days)

20

25

30

Figure 2. Cumulative Oxygen Uptake for 0.4g PVOH, 0.2g PVOH and Control Sample at25°C

350

Time (days)

Figure 3. BOD Curves at 35 °C


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700

Time (days)

Figure 4. BOD Curves at 25 °C

respiration from the measured respiration in order to determine the true substrate respiration. Respirometric results to show the effects of temperature and substrate concentration on the ultimate BOD and percent degradation of PVOH are summarized in Table 2.

Table 2. Summary of Respirometric Results Temp. (oC) 25 25 35 35

Test Duration (days) 28 28 28 28

Amount (g) 0.2 0.4 0.2 0.4

UBOD (mg/L) 301.62 581.11 160.54 323.86

Percent Degraded 83.1 79.9 44.2 44.6

As seen from Table 2 and Figures 3 and 4, the ultimate BOD increases with increasing polymer concentration at 35°C and 25°C. The tests confirm that the greater the amount of carbon available for the microorganisms, the greater the biological activity in the reactors, consequently, the higher the ultimate BOD.



100 The low ultimate BOD values of 160.54 mg/L and 323.86 mg/L at 35°C were probably attributed to the microorganisms not being acclimated to the aqueous environment. As a consequence, the conditions present in the reactors are not good for optimal microbial activity. In addition, the presence of volatile organics in the activated sludge may affect the oxygen uptake rates at higher temperatures. The degrees of biodégradation of PVOH were 44.2-44.6% based on ThOD at 35°C (Table 2, Figure 5). The course of the degradation of the curves at different concentrations is comparable. The curves indicate a lag phase (day 0 to 6), where the adaptation of the microorganisms to PVOH occurs, a degradation phase (day 6 to 19), in which the microorganisms use the PVOH as food and grow, and a plateau phase (day 19 to the end of the test at day 28), in which degradation has ended. However, the degrees of degradation at the end of the test do not exceed the limit value of 60% that is required by the standard to prove the validity of the test. The reason for the low degradation was probably that the microorganisms and enzymes responsible for the degradation of PVOH are ineffective at 35°C. On the other hand, the degrees of biodégradation of PVOH were 79.9-83.1% based on ThOD at 25°C (Figure 6). The curves indicate a lag phase (day 0 to 6), a degradation phase (day 6 to 15) and a plateau phase (day 15 to 28). The test exceeds the limit value of 60% thus proving the ready biodegradability of PVOH in an aqueous environment at an optimum temperature of 25°C.

50

Time (days) Figure 5. Extent of biodégradation at 35 °C


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100

Time (days)

Figure 6. Extentofbiodegradation at 25 °C

Conclusion This paper presents evidence that PVOH can be degraded by cultures derived from activated sludge. The biodegradation testing simulates environmentally realistic conditions and thus we can extrapolate results to natural environments. Results indicate that increasing the concentration of PVOH will increase its bioconversion thus increasing the total oxygen uptake, UBOD, and percent degradation. This may also indicate that PVOH is nontoxic and noninhibitory to activated sludge microorganisms. However no marked improvement in the percent degradation was observed when temperature was increased. This clearly indicates the influence of temperature on biological reactions. Hence, it shows that it is important to adjust the inoculum to the test temperature carefully by an adaptation period.

References 1. SPI Report: Overview of Plastics Indusrty; Society of the Plastics Industry, Inc.: Washington, DC, 2001. 2. Franklin Associates, Ltd. Characterization of municipal solid waste in United States, U.S. Environmental Protection Agency Municipal and


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3. 4.

5.


6. 7. 8.

Industrial Solid Waste Division Office of Solid Waste: Washington, DC, 1996, Report No. EPA530-R-97-015, p. 8. Mohanty, A. K.; Misra, M.; Hinrichsen, G. Macromol Mater. Eng. 2000, 276/277, 1-24. Narayan, R.; Barenberg, S. Α.; Barash, J. L.; Redpath, A. E. Degradable Materials: Perspectives, Issues, and Opportunities, CRC Press, New York, New York, 1990. OECD Guidelines for testing of chemicals: Paris 301 F Manometric respirometry test. Organisation for Economic Co-operation and Development: Paris, France, 1993. Aldrich Catalog: Polyvinyl alcohol, 80% hydrolyzed. Aldrich Chemical Company, Inc.: Milwaukee, Wisconsin, 2005. Annual Book of ASTM Standards; Standard D5271-02; American Society for Testing and Materials. Villanova, PA, 1993; Vol. 08.03. Pagga, U.; Reuschenbach P.; Strotman, U. Water Research 2003, 37, pp 1571-1582.


Polymer Durability and Radiation Effects - American Chemical Society

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