Biodegradation Behaviour of Polymers in the Soil
3.4 Degradability of Polymers in Soil
3.4.2 Test Methods and Criteria
Two main standardisation issues can be identifi ed. The two missions are different and they should be developed separately in order to avoid misunderstanding.
1. Biodegradability and Environmental Compatibility of Polymers for Soil Applications.
Focus is on the environmental effects of biodegradable polymers in soil. In order to prevent accumulation of non-biodegradable polymeric residues in soil, the inherent biodegradability must be assessed using standard test methods. Agricultural productivity and the environment should not be disturbed by eco-toxic substances generated by the biodegradation of the plastic material.
2. Durability of Products. Standard test methods are also necessary to predict the
‘durability’ of plastic products made with biodegradable polymers when in use, in order to verify if they can resist the severe environmental factors found during life cycle. Durability is of commercial interest and test methods are required to classify the products’ performances.
It is important to note that biodegradability and durability are two different properties.
The fi rst is a property of polymers while the second is a property of a product. A product can be optimal for agricultural applications, offering the required commercial life and then a fast ‘disappearance’ and still not be environmentally compatible because it is not biodegradable or is unsafe. Conversley, a polymer shown to be compatible with the soil environment, could turn out not to be suitable for a given application because it is not stable under environmental conditions, or too persistent (because, for example, it is converted into mulch fi lms that are too thick).
3.4.2.1 Biodegradability and Environmental Compatibility of Polymers for Soil Applications
First, the meaning of two terms frequently used erroneously as synonyms (even by the experts) must be clarifi ed: biodegradability and biodegradation.
Biodegradability refers to a potentiality, (i.e., the ability to be degraded by biological agents).
Biodegradation refers to a process, happening under certain conditions, in a given time, with results which can be measured.
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The inherent biodegradability of a polymer is inferred by studying a real biodegradation process under specifi c laboratory conditions and, from the test results, the conclusion that the polymer is biodegradable, (i.e., it can be biodegraded) can be drawn.
It must be noted that a fully biodegradable polymer can show a very limited biodegradation if environmental conditions are not suitable. In the previous paragraphs it has been clarifi ed how soil can be affected by several parameters. A dry season, a cold temperature, an acidic soil, a limitation in nitrogen, etc., can affect the degradation rate of a polymer in a manner which is diffi cult to predict for each fi eld, or region, or season. Only through repeated fi eld trials performed in the area of interest, can one get suffi cient knowledge about the specifi c behaviour of a given material in that area.
Biodegradability, as a general property (inherent biodegradability), is determined in the laboratory, by measuring the degree of biodegradation of the polymer when exposed to a microbial population. The CO2 evolution or the O2 consumption are measured and the level of conversion of the organic carbon into inorganic carbon is determined. Strictly speaking, this is a measure of mineralisation, which is the oxidation of the organic carbon of the polymer into CO2 as a consequence of the microbial respiration. Several respirometric test methods are available nowadays to measure the inherent biodegradability of plastics.
In principle, it is preferable to adopt a test method which reproduces the conditions of the environment of interest. So, for example, the evaluation of biodegradability under composting conditions is measured preferably in test systems devised to simulate the composting environment such as ISO 14855 [34]. Accordingly, in order to assess the biodegradability of plastic materials in soil, it is preferable to use a test system where the following conditions are met: temperature in the mesophilic range, a mesophilic microbial inoculum, aerobic conditions, and solid state.
A simple system for monitoring the consumption of oxygen by soil is the one described by Miles and Doucette [35]. The system was devised to follow the persistence and the biological effects of hydrocarbons in soil. It can nevertheless be used for testing polymers. Anderson [36] described several methods: a simple system for determination of oxygen consumption; an automated system for determination of oxygen consumption (the Sapromat); a simple system for determination of carbon dioxide production and a system based on radiolabelled substrates. Another interesting respirometric test apparatus which seems very appealing for its simplicity was described by Bartha and Pramer [37].
Nowadays, an International Standard test method is available, ISO 17556 [38]. The test material is mixed with soil to determine the mineralisation rate by measuring the biochemical oxygen demand or the amount of CO2 evolved. A natural soil, collected from the surface layer of fi elds and/or forest, is used. A further standard test method based on soil is described in the American Standard ASTM-D5988 [39]. The test is performed using desiccators, available in most laboratories. A mixture of soil and test material (or compost
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75 containing test material after composting) is placed at the bottom of vessels, on the top of soil a perforated plate is laid and onto it a beaker containing KOH or Ba(OH)2 is placed to trap the CO2 evolved during the biodegradation process. A report indicates that the use of Ba(OH)2 should be avoided because it is unsuitable for trapping CO2 under static conditions [40]. The test soil can be a laboratory mixture of equal parts of sandy top soil, composted manure or natural soil. It can be also a mix of a natural soil and mature compost in the ratio 25:1. An interesting test system has been proposed to increase the reliability of the respirometric test methods. In order to decrease the amount of soil to a minimum it is proposed to use Perlite [41]. This is a chemically inert aluminosilicate largely used in horticultural applications as a component of growing substrates. The purpose is to reduce the amount of CO2 produced by the soil itself compared to the investigated samples and therefore to maximise the signal-to-noise ratio. Compost has been also used as a solid matrix instead of soil, at room temperature [42].
Aquatic tests, such as the ISO 14851 [43] and ISO 14852 [44] can also be applied for demonstrating the inherent biodegradability of a polymer. The test temperature should be restricted to the mesophilic range (room temperature). The aquatic tests are considered the only reliable methods for performing carbon balance and characterisation to show complete degradation and also for the detection of potential metabolites (J. Fritz, personal communication). Albertsson [45] used soil as an inoculum of the aquatic test: 10 grams of garden soil (wet weight) were used to inoculate 250 cm3 of a liquid culture medium applied in a radiolabelling respirometric technique. Radiolabelling respirometric techniques have also been applied in a soil-based test method [46]. Soil-water suspensions have also been used as media to test biodegradability by Suvorova and co-workers [47] and by Calmon-Decriaud and co-workers [48]. Sawada [49] found that the rate of degradation of biodegradable polymers in fi eld tests is consistent with the results found in a laboratory test method based on the OECD Modifi ed MITI Test [50] using activated sludge and measuring oxygen under aerobic conditions. In this very comprehensive study, soil burial tests were performed in 18 different locations in Japan and in one in the USA.
A terrarium for biodegradation of 14C-labelled polymers was described by Guillet and co-workers [51, 52].
In order to perform a fi nal mass balance, recovery from soil of undigested polymeric residues with an organic solvent extraction procedure can be performed [53]. This approach is based on the measurement of the polymer disappearance from soil. A polymer-specifi c solvent has to be used and a specifi c analytical method has to be set up [54]. Solvent extraction procedures and manual retrieval were used by Yabannavar and Bartha [55]. The manual retrieval was necessary because of unsatisfactory results obtained with extractions.
When recovering samples from soil, especially if outdoors, great care must be taken to withdraw a statistically representative sample [56].
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3.4.2.2 Evaluation of Durability
A mulch fi lm is subjected to strong environmental stresses. The possibility of predicting the effect of the combined environmental factors is extremely relevant for commercial success of plastic products. Needless to say, a mulch fi lm destroyed before the end of the cultivation cycle can seriously impair the commercial yield of a crop. Any negative effect on the commercial yield will not be accepted by farmers. It is therefore important to have reliable test methods to predict durability. Furthermore, a plastic fi lm which after use remains intact on the fi eld for too long, can also be a practical problem for farmers, by preventing the use of the fi eld for a second crop cycle. Durability is therefore a double performance issue: durability can be a problem during plant growth if it is scarce while it is a problem after harvest if too prolonged.
The environmental factors which infl uence the mechanical properties of the plastic products are typically due to sunlight (UV irradiation and heat), and/or to biodegradation of the buried parts. A typical example is the mulch fi lm which is in part exposed to the sunlight while the lateral parts are buried to fi x the whole fi lm to the soil (Figure 3.2).
A possible test scheme for the assessment of durability of plastic products in soil is the following. The product is exposed to the surface factors (UV and heat from sun irradiation) to check durability at surface. In parallel the product is directly buried in soil to simulate the behaviour of the parts not exposed to sun (Figure 3.2). The fi lms exposed to UV can then be buried to complete characterisation. The test results can be used to estimate the durability of products. Obviously substantiation of the laboratory results with fi eld trials is needed. The same test approach can be used to defi ne the corresponding problem of product durability after commercial life. The product, after crop harvesting is discarded in the fi eld, generally buried, and it is supposed to disintegrate in a relatively short time.
It is important to know that a given plastic product will disappear, and not cause visual pollution and impair root development or agricultural practices. The assessment of durability can be useful for predicting both performance in use and ‘disappearance’ of plastics after use in soil.
3.4.2.2.1 Soil Burial Test Methods
Soil burial test methods have been established and standardised for testing resistance of plastics to micro-organisms. The methods were originally used on plastics coming in contact with the ground, for example, construction materials and coated tents. The aim was to assess their resistance in soil, rather than their degradability. However, resistance and degradability are two complementary aspects of the same problem and a method devised for testing resistance can be applied for testing degradability as well. The test material is
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77 buried under laboratory or fi eld conditions. Visual assessment of exhumed materials is carried out and mass loss and tensile strength measurements are also performed.
Soil burial tests are used to give an indication of the duration of the test material in a given soil under given conditions. They can be performed outdoors or indoors.
Outdoor Soil Burial
In theory the outdoor testing is expected to give the most faithful indications of ‘real world’
performance. However, fi eld experiments are more diffi cult to perform than laboratory experiments and must be carefully designed. The exposure conditions are not controlled:
temperature, rainfall, humidity and sunlight vary from day to day throughout the year and from year to year. The soil burial locations can also be disturbed by wildlife or even human activities, if the area is not restricted.
The choice of location can affect the test results. Characterisation and use of an habitual testing site is important in order to improve reproducibility and compare different test materials. It is also important to keep records of the environmental conditions during all the testing.
Generally speaking, outdoor experiments are advisable whenever the fate of a polymer in a given fi eld or a region has to be predicted with precision. They are less suitable for general statements because of the diffi culty to easily reproduce the experimental conditions.
Typical analysis performed after burial is the evaluation of the mass loss [49]. An analysis methodology based on numerical vision has been also developed [57]. Mechanical properties [49], molecular weight evaluation [54], IR spectroscopy [18] and electron microscopy [58] have been applied to characterise polymeric samples after degradation in soil.
A problem which can be encountered in outdoors testing is the interference of animals, which can damage the samples. To solve this problem, a fence of slatted plastic can be constructed about one meter beyond the plot boundary to keep out wildlife [56].
An example of equipment used to perform outdoor burial experiment is described by Goheen and Wool [18].
The plastic samples can be buried in perforated boxes which are then buried in soil. The perforation allows the samples to be attacked by microorganisms and keeps the soil moist [59].
Another possibility is to fasten the specimens on the surface of the ground, to cover it lightly with soil, and fi nally to protect the area with a net [60]. The following method
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was used in a very comprehensive test in Japan [49]. A mass of soil is removed from the surface down to approximately 10 cm and then is screened to remove stones, etc. Half of the resulting soil is put back into the hole and its surface is mildly levelled. The area of burying is divided according to the scheme of assessment periods and the test specimens are arranged according to a randomised block design. The space between the test specimens is about 5 cm between the rows and 10 cm between the columns. The remaining soil is then put back to cover the specimen (at about 5 cm in the soil).
It is also possible to run tests outdoors using containers fi lled up with soil. This makes the recovery of the samples easier. A possible example of this approach is to perform the soil burial test in plastic fl ower pots (60 x 20 x 20 cm) placed outdoors [61].
A typical method used for outdoor soil burial tests consists of closing the specimens in pockets prepared using a polypropylene (PP) net (Figure 3.3). The pocket (A) has the purpose of protecting the specimen (B) during recovery to avoid loss of fragments due to mechanical stress. Furthermore, a string (C) tied to the pocket and left unburied above the surface, will help to identify the burial site and to retrieve the sample. The mesh of the net should be large enough to allow contact of the specimen with soil but, at the same time, small enough to decrease the risk of loosing pieces during exhumation. A suitable mesh is about 4-6 mm. The pocket with the specimens should be inclined, as shown in Figure 3.3, to decrease the load caused by water in case of rainfall. A mark, such as a coloured label (D), should be used to identify, after recovery, the specimen. The specimen in the soil is subjected to a gradient of different local environmental conditions (oxygen, carbon dioxide, water content) by the different depth of burial. In the case of non-homogeneous degradation of the specimen, it is important to know the original orientation of the specimen in the soil [48].
Figure 3.3 Simple device for outdoor soil burial test. A = protective net; B = plastic specimen; C = wire D = label for retrieval and identifi cation
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79 Indoor Soil Burial Tests
Practical reasons, together with the need to assure reproducibility have forced scientists to develop and use mainly laboratory soil burial tests. At laboratory level the environmental conditions are controlled and the management of the experiment very simple. Therefore the statements drawn are more general, reproducible and reliable than the results obtained outdoors. On the other hand, storage of moist soils at room temperature causes a loss of microbial biomass and a decrease in the general degradation potential of soil [27, 36].
An international test method applied to perform soil burial test in the laboratory is EN ISO 846:1997 [62]. An example of an ‘Indoor Soil Box’ is described by Goheen and Wool [18]. In order to maintain suitable moisture, plastic trays containing soil were covered with a mesh net and then with a thick paper moistened with tap water [63].
Abiotic control can be used as a negative control using sterile soil. This was obtained by heating up to 500 g of soil in an oven at 125 °C for six hours. Then the water lost during sterilisation was restored by using a 0.02 wt% aqueous solution of sodium azide (NaN3) and thoroughly mixing [63].
Nylon meshes have been used to enclose separate test fi lms before burial [64].
A soil burial test was developed by the American Association of Textile Chemists and Colorists (AATCC 30-1999) for testing fabric specimens [65]. The test method requires a viability control. The soil bed used as a matrix should be considered satisfactory if an untreated fabric loses its mechanical properties after seven days exposure. Recommended types of soil are garden and naturally fertile topsoils, composts and non-sterile greenhouse potting soils. An equal blend of good topsoils, well rotted and shredded manure, and coarse sand should be used. It is considered that these matrices usually have proper physical characteristics, along with an organic content suffi cient to ensure a high degree of microbial activity and the presence of active organisms. The optimum moisture content is fi xed at about 30% moisture of the dry weight. The air-dried soil bed is placed in trays, boxes or suitable containers and brought to the optimum moisture content by gradual addition of water accompanied by mixing to avoid water stagnation. After 24 h, the soil is sieved through a 6.4 mm mesh screen. The soil moisture content must be kept constant and the temperature maintained at 28 °C.
Experiments were performed using poly hydroxy-butyrate-valerate fi lms as a test polymer to determine the optimum ratio of composted cow manure, topsoil and sand as well as moisture contents to maximise degradation rates. The results indicated that the organisms from the composted cow manure were more active in degradation than micro-organisms from the topsoil with 25-50% manure being optimal. The optimal degradation rate in a 1:1:1 sand:topsoil:composted manure mixture was obtained using the 93.75%
of the maximum moisture held [66].
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3.4.2.2.2 Methods to Determine Environmental Ageing
The products for agriculture made with biodegradable polymers are exposed in a fi rst phase to surface factors: UV radiation, heat (from sun irradiation), water (rainfall and irrigation), and mechanical stresses (wind, trampling, blown sand, rain, wave action, vehicular traffi c, etc.). It is important to know how the products resist these factors because the fi rst phase is the functional phase. Any premature damage could negatively affect their functionality and cause decreases in crop yield, (e.g., mulch fi lm). It is known that exposure to physical-chemical factors can lead to signifi cant degradation processes. For instance, the presence of water at a certain pH range can cause hydrolytic degradation of the polymeric chain [67, 68]. Therefore, to better simulate the life cycle of the plastic products, the products could be subjected to a weathering phase [69, 70]. A simulation of weathering effects can be performed alternating cycles of sunlight, humidity, and condensation with an accelerated weathering tester [71]. As an alternative, the samples can be exposed outdoors to sunlight and rainfall. The specimen should be kept in contact with soil to allow the colonisation by the microbial populations. Doing this the specimen is under the action of UV-radiation, humidity and biodegradation. A practical approach is, for example, to lay a fi lm on soil or on grass. To prevent it from fl ying away under the wind action, a large mesh net should be placed on top of the fi lm and tightly fi xed as a cover. This procedure allows testing of the degradation time of a plastic item after littering. Alternatively, the fi lm can be exposed to atmospheric conditions for a given time and then buried. This can be done to test the degradability of mulching fi lm and takes into account all the different factors active outdoors, such as: chemical-physical degradation occurring during application and biodegradation after tillage. Yabannavar and Bartha exposed fi lms to sunlight for periods of 6 or 12 weeks before burying in soil, following ASTM D1436-97 Standard [72] recommended practice for outdoor weathering of plastics using 45°-angle wooden racks, facing south [55]. Similarly in Thailand, plastic sheets were mounted on racks and exposed to natural solar radiation. Temperature, humidity, radiation, and rainfall were recorded during the experiment [59]. Similarly outdoor ageing tests were performed by fi xing the polymer samples to a wooden board which is then placed at 45° facing south on the roof of a two-storey building. Samples for comparison were also placed on the ground where temperature and humidity are the same as on the roof. In this way it was possible to see what effects were due to environmental conditions (sunlight, temperature and humidity) and which were induced by soil-driven biodegradation [73].
A metallic frame rack used to expose samples attached to row to weathering agents is described by Ho and co-workers [69].
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