Biodegradability of Photodegraded Polymers I. Development of Experimental Procedures Philip H. Jones and Durga Prasad
Institute of Environmental Sciences and Engineering a n d Department of Civil Engineering, University of Toronto, Toronto, Canada Michael Heskins
EcoPlastics Limited, 35 Steele Valley Road, Thornhill, Ont., Canada Marvin H. Morgan and James E. Guillet”
Department of Chemistry, University of Toronto, Toronto, Canada
Methods have been developed to study the biological oxidation of plastic fragments in natural soils and sewage sludge using a modification of the conventional Warburg apparatus. Rates of biodegradation are estimated from the consumption of oxygen under carefully controlled conditions. Whereas high-molecular-weight plastic particles are shown to be resistant to biodegradation, degraded branched polyethylene and polypropylene show significant rates of oxidation in both environments. Degraded polystyrene appears to be more resistant to microbial attack.
Recently a method has been developed for making many types of plastics photodegradable [Guillet ( I ) ] . Several of these are being commercialized under the trade name Ecolyte to provide a solution to the serious problem of littering in a throwaway society. These plastics disintegrate (lose their structural strength and thus are subject to mechanical breakdown) under the influence of ultraviolet irradiation. It is clear, however, that a breakdown which is purely physical is not sufficient to return the synthetic materials to the natural carbon cycle. It is known, however, that when a substance is broken down to a fine powder (by photodegradation or mechanical grinding), it will normally be more readily available for biological recycling to carbon dioxide and water by microbial cells under aerobic conditions. Furthermore, the photodegradation results in a substantial reduction in the molecular weight of the plastic which should improve the possibility of microbial attack. This study was undertaken to determine generally if photodegraded plastic fragments are subsequently available for further biological breakdown through the action of the natural microbial flora of soil and activated sludge from a municipal sewage treatment plant. This paper describes methods suitable for the evaluation of the biodegradability of such materials. Experimental Methods
Respiration of the microbial flora in soil was taken as the most satisfactory and readily measurable parameter to determine the activity of the organisms capable of metabolizing the breakdown of fragments of the plastic. Two methods were used for these studies: The first was designed to test biodegradability in natural soil; the second was designed to determine the biodegradability of the plastic residues under conditions similar to those occurring in a sewage treatment plant. Procedure for Soil Tests. The first tests were conducted in “Hach” respirometers where the bottles were charged with different soils and quantities of various plas-
tics residues, sealed, and the oxygen consumption was measured by the change in pressure in the bottles, the evolved COS being absorbed by a KOH solution. A control sample was run to determine the “natural” oxygen uptake of the soil with no added carbon source. By difference it was possible to determine (a) if the plastic increased the oxygen uptake rate, indicating biodegradation of the plastic; or (b) if no change in oxygen uptake rate occurred, then the plastic was not being degraded microbiologically; or finally (c) if a reduction in oxygen uptake rate was observed, it might be deduced that the plastic was having an inhibiting effect of some kind. Under the conditions of the tests, the plastic will show virtually no oxygen uptake due to autooxidation. Garden soil was collected in April 1972, and forest and land-fill soils were collected in June 1972. The soils were air-dried in the laboratory, screened to less than 2-mm diameter, and were then stored-in glass bottles at 5°C. At the beginning of the experiments, the substrates (plastic powders) were added directly to the soil which was then wetted to 60% of its maximum water-holding capacity. In a later experiment the moisture loss was determined every two weeks and was compensated for by adding sterile distilled water. In this experiment, the KOH solution was also replaced each week. The pH was determined on suspensions with soil-water ratio of 1:2.5 and water-holding capacity was determined by the method of Coutts (2). Total nitrogen was determined on 2.0 grams of garden soil and forest soil, and on 5.0 grams of land-fill soil, using semimicro Kjeldahl method with CuS04.HzO as catalyst and modified with salicylic acid to include nitrate as described by Cole and Parks (3). Organic matter and organic carbon were determined by the wet combustion method. The characteristics of the different soils used in these experiments are shown in Table I. Activated Sludge Procedure. This series of experiments was conducted to determine the rate of biodegradability of the plastics in an aqueous environment such as a sewage treatment plant or a natural water body. Mixed liquor was collected from the Humber Sewage Treatment Plant, Toronto. A Warburg Respirometer was used to measure 0 2 uptake. A 2.0-ml sample of mixed liTable I. Properties of Soils Used Organic matter,
Gardensoil Forest soil Land-fill soil
Total nitrogen,
%
%
pH
3.47 11.04 2.16
0.1036 0.1365
7.8 6.5 8.2
0.0658
Waterholding capacity
ratio
35.71 56.53
47
30.0
19
C:N
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quor was placed in the main chamber of a conventional Warburg flask. Accordion pleated pieces of filter paper were inserted in the central well and 2.0 ml of 20% potassium hydroxide was added. The flasks were then attached to their respective manometers and were placed in a water bath a t 20°C. Plastic powders (1% w/v) were added directly to the mixed liquor. Manometric determinations of 0 2 uptake were carried out by the “direct method” [Umbreit et al. ( 4 ) ] .
Materials Four materials were used in these experiments. Polypropylene. A sample of Ecolyte PP polypropylene film (1 mil) was allowed to photodegrade under artificial ultraviolet radiation until it became extremely fragile and could be broken u p under finger pressure into small fragments. This was then ground to a fine powder for soil tests. The number average molecular weight of the photodegraded plastic was 2200 (by ebulliometry) . The infrared spectrum (Figure l a ) shows evidence of substantial photooxidation as indicated by carbonyl bands a t 5.83 p. Polyethylene. To obtain a large sample of material suitable for testing, the polymer used was a polyethylene resin prepared by thermal degradation of low-density polyethylene, followed by air oxidation to introduce carbonyl groups. The number average molecular weight of this polymer was 2300 by ebulliometry. The infrared spectrum is shown in Figure l b . This sample is expected to be similar in structure and molecular weight to fragments produced in the photodegradation of plastics such as Ecolyte PE. The sample was ground to a fine powder for soil tests (
