Landfill Biodegradation of Foam Compositions Based on Polymers Not Inherently Biodegradable

By R. F. Grossman, Ph.D.

Abstract

A variety of anaerobic landfill microbes are shown to be able to metabolize expanded polystyrene and polyvinyl chloride foam compositions containing organotitanate or organozirconate additives that provide hydrophilic points of attack, but do not catalyze degradation during service in an aerobic environment.

Background

The interior of a landfill is dark, warmer than ambient and low in available oxygen [1]. Moisture content varies from about 15 to 45% [2]. At the lower level, not even food waste will biodegrade. The most prevalent ingredient, often 40-50%, comprises paper products; plastics tend to run 1-5% (1). Cellulosics such as paper degrade poorly at moisture levels below 50-60%, which are rarely reached in commercial landfills [3].

A variety of the aerobic, Gram-negative bacterium Pseudomonas putida, strain KT2442, has been bred to consume petroleum spills [4]. It can also consume expanded polystyrene (EPS), but Pseudomonad bacteria are (to date) obligate aerobes and have no anaerobic capability [5]. Aerobic microbes typically favor sexual reproduction; if food levels become inadequate, they form spores to provide the next generation and die. They are, therefore, modern. A number of bacteria can function either aerobically or anaerobically; examples include Staphylococcus and E. coli.

Anaerobic microbes predate the Paleozoic oxygen explosion [6]. The emergence of cyanobacteria capable of photosynthesis may have been a significant factor in the development of the oxygen-rich atmosphere. In addition to archaeans and bacteria, microbes capable of anaerobic metabolism include many algae and molds. In the absence of atmospheric oxygen, food is metabolized using oxidizing species such as phosphates, sulfates, nitrates [7] or even metal oxides [8]. The free energy available from fermentation or other anaerobic metabolic paths is low compared to aerobic oxidation [1]. The combination of lack of specialization, low energy diet and reproduction through division leads these species to have an appetite for almost anything organic.

Experimental

Landfills used were based on the guidelines of ASTM D5526 and comprised 90% sterilized sewage (available commercially as Milorganite ), plus 10% actively fermenting compost. Such compost is likely to contain a number of methanogenic bacteria. A number of species of Methanobacterium have been identified, as well as Eubacterium and Cellulomonas [9]. No “standard” compost is available and the extent of microbial variation is largely unknown. This factor introduces a similarly

unknown potential for inconsistency. The above mixture was adjusted to 40-45% moisture. Landfills of this type have been shown to consume plasticized PVC film and sheet [10]. Although ASTM D5526 calls for use of distilled water, water from a local pond was used. This eliminates the delay before microbial attack begins [3]. The use of distilled water in an actual landfill is very unlikely.

An important factor in the utility of common plastics is their water resistance, that is, their hydrophobic character. Part of that utility is the defeat of attack by microbes under ordinary conditions. It was discovered that a class of additives can be employed to produce hydrophilic attachments to points on hydrophobic polymers which enable anaerobic but not aerobic microbial attack [10].

In the above example, BIOchem C-3™, a pyrophosphato titanate chelate quat is shown wherein R = methyl, R’ = propyl. More specifically, the additive can be described as a Di(dioctyl)pyrophosphato ethylene titanate (adduct) N-substituted methacrylamide or a Titanium IV bis(dioctyl) pyrophosphato-O (adduct) 2 moles N, N-dimethylamino-alkyl propenoamide or Titanate (2-), bis [P,P-dioctyl diphosphato (2-)-кO΄΄, кO΄΄΄΄][1, 2-ethanediolato (2-)-кO, кO΄]-, dihydrogen, branched and linear, compound with N-[3-(dimethylamino) propyl] -2-methyl -2-propenamide (1:2) bearing CAS # 198840-66-3. Analogous neoalkoxy organotitanates and organozirconates are also effective [11].

In the following experiments, 5 g of EPS (Joy Sports & Leisure, China) or vinyl foam (3M) were dissolved in 25 ml MEK at room temperature and 50 mg of the above catalyst added – see Figure 1. The solution was allowed to evaporate in an aluminum pan and 2 g added to 50 g of the above landfill in a Petri dish, which was then sealed with several wraps of 3M #33 electrical tape.

Figure 1 – ASTM D5526 Simulated Landfill: Landfill = 90/10 sterilized sewage/active compost, 40-50% moisture, 30-35°C, dark incubator, sample ~ 5%

of landfill mass.

Sample of an EPS article containing 1% organotitanate catalyst

Gas evolution was measured using micro-manometers supplied by Carolina Biological. Gas evolved from 10 g landfill, with or without foam samples, was measured versus time. Landfills included controls and those to which cultures had been added. The latter included Protococcus, Spirulina, Spyrogyra and Cyathus algae; Chlamydomonas, Anabaena, Fischerella and Eucapsis cyanobacteria and the archaean Halobacterium sp. NRC-3. Gas evolution was also measured using a landfill that had previously consumed PVC plastisol based on Geon 121A, with 60 phr DINA and 1 phr titanate catalyst [3]. Here the sample was either the same PVC compound, PVC foam or EPS.

Experiments using 10 g micro landfills as microbial fuel cells were carried out with a University of Reading (UK) kit. Landfills were kept at 30-35C using a Boekel Scientific Model 132000 Incubator.

Results and Discussion

After 21 days in an anaerobic landfill, samples of EPS containing 1% of either the above or similar organotitanates showed flourishing microbial colonies and loss of sample mass – see Figure 2.

Figure 2 – Catalyzed EPS Is Attacked Rapidly After Several Weeks Into Test (at 10x). Microbial Colonies

Are Doing Well, Notable Loss of Mass - Dense Samples Much Slower

After 90 days, these samples had almost completely vanished into the biomass. Control samples were unaffected – see Figure 3.

Figure 3 – After 3 MonthsVery Little EPS Left, Colonies Are Dying Back

Vinyl foam samples behaved similarly, except for leaving small quantities of filler and pigment. Again, control samples showed no effect other than slight microbial colonization at the sample edges – see Figure 4.

Note: The foam formulation used 1phr the BIOchem C-3™ coupling agent in a typical AZO recipe, urea activation employing a tin carboxylate stabilizer. Since the additive functions by enabling microbes to consume plastics, biocides will inhibit effectiveness. For example, zinc-based stabilizers inhibit landfill biodegradation because they are known biocides. Tin carboxylate stabilizers will not interfere with the biodegradation mechanism.

Phthalocyanine pigments will also inhibit landfill biodegradation and should be avoided. In polyolefins, color forming antioxidants, such as BHT and Bisphenol, should be avoided in favor of high efficiency stabilizers, such as Irganox 1010.

Figure 4 – Vinyl Foam, 3 Weeks In Landfill

Gas evolution began within a few hours – see Figure 5. A 10 g ASTM D5526 type landfill yielded 0.2 ml gas in 24 hours and 0.7-0.8 ml in 72 hours. If the landfill contained a culture of Spirulina, Spyrogyra, Anabaena or Fischerella, gas evolution was reduced to 0.1-0.3 ml after 72 hours, increasing slowly to 0.5-0.7 ml after 21 days.

Cyathus and Eucapsis had no such effect. On the other hand, landfills containing Protococcus or Halobacterium did not evolve gas. In these cases, the product of anaerobic metabolism may be bicarbonate ion. Those landfills that did not evolve gas had become slightly alkaline; those evolving methane and carbon dioxide remained at their original pH, about 6.5.

Figure 5 – Gas Evolution From The Landfill

Addition of 0.4 g EPS containing organotitanate catalyst increased the gas yield of the landfill slightly. In the presence of the above cultures, with those that lowered gas yield, addition of EPS appeared to lower gas yield slightly more. Addition of 0.4 g plasticized PVC foam containing catalyst had a similar effect.

Use of a landfill that had previously consumed PVC plastisol increased the rate of gas evolution when a second sample was added. A third experiment did not provide a further increase. A landfill that had consumed PVC plastisol also increased the rate of gas evolution when a PVC foam sample was added, but had no effect on the gas evolution during EPS consumption. The converse was also found: a landfill that had consumed EPS did not increase the rate of gas evolution from degrading vinyl foam. It is likely, therefore, that microbial modifications required to metabolize plastics in an anaerobic environment are, at least in some cases, heritable.

The above observations suggest that the protocol of ASTM D5526 and related standards where gas evolution is taken as the measure of biodegradation may be thoroughly misleading. The observations that are significant are that an object placed in a landfill supports microbial colonization and ultimately vanishes.

Addition of 10 g of the above landfill to the cathode compartment of the University of Reading microbial fuel cell (MFC) with 5% Fe (II)/Fe (III) ammonium sulfate solution in the anode compartment to mediate air oxidation generated 240-250 mV output – see Figure 6. This is reasonable in view of methanogenesis half cell reports [12].

Figure 6 - Landfill Battery: Landfill + Sample Supplying 321 mV vs. Fe(II)/Fe(III) Mediated

Reduction of O2

With a sample comprising 9 g landfill and 1 g EPS, the output, tested daily, rose over 21 days to about 320 mV, then slowly retreated to the original level. It seems likely, therefore, that the sample provided a higher energy food source to the anaerobic feeders in this particular landfill.

The current output of 10 g of the above landfill was about 0.05 mA. A unit of several tons would be required to power a useful circuit, for example, to monitor or operate methane recovery from the landfill.

Proprietary PVC formulations have been developed for commercial signage called BIOflex using subject additive. Figure 7 shows the landfill decomposition of BIOflex Vinyl under ASTM D5526 landfill conditions of 30°C at 50% moisture.

Figure 7

References

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9. A.C. Palmisano & M.A. Barlaz, Microbiology of Solid Waste, CRC Press, 1996, p 49-72.10. R.F. Grossman, J.E. Schleicher, Jr. & L. D’Alessio, J. Vinyl & Additive Tech., 13(3), 132-135 (2007).11. R.F. Grossman, US 7,390,84112. R.A. Alberty, Thermodynamics of Biochemical Reactions, Wiley, 2003, p 162.