Biodegradation Behaviour of Polymers in the Soil
3.3 The Soil Environment
3.3.2 Underground Factors
65 rodent repellents [16]. Macro-organism degradation occurs in three stages: (a) mastication (chewing) (b) digestion (c) exocorporeal degradation. Mastication results in considerable deterioration of the physical and chemical structure of the polymers. Digestion by macro-organisms removes the digestible components by enzymic, mechanical and chemical action.
Exocorporeal degradation involves the fate of non-digested faecal material and orally contacted pieces of polymer [17]. It has been reported that insects, attracted by some of the constituents of biodegradable polymers, (i.e., starch), have caused deterioration and brittleness by chewing fi lms and producing small holes [18, 19]. For agricultural applications, insects or small animals could cause problems and this should be verifi ed and, if needed, controlled to avoid early damage of the product. Even non-biodegradable polymers can show signs of insect damage [20]. It has been shown that the common soil isopod Armadilidium vulgare could ingest tritium labelled PE (3HPE) and 3HPE + starch blend disks. However, while the disks containing 10% starch were completely consumed, the 100% 3HPE disks were only partially consumed [21]. It has also been thought that macro-biodegradation could be enhanced by increasing the degree of attraction to the woodlouse thus elevating this ubiquitous creature to the status of potential plastic litter scavenger [22]. According to this viewpoint the macro-biological attack can be considered a benefi cial component of the natural cycle, but it needs to be properly controlled. Also mites, collembolas and nematodes have been found in biodegradable plastic sheets buried in soil [23].
66
Table 3.3 Environmental factors active in soil and their possible effects on polymer degradation
Soil factor Main effect Direct consequence on polymer
Biotic effects Texture and
soil structure
Determine porosity
Harsh texture can increase abrasion (mechanical degradation).
Porosity controls water and air circulation (see below).
Heat Temperature
change
Temperature controls rate of abiotic degradation, (i.e., hydrolysis), and mobility of polymeric chain (bio-availability).
Temperature controls the microbial population (living and active species in soil), growth rate of each single species, and enzymic activity.
Soil
composition (mineral)
Determines the cation exchange capacity (CEC)
Contact between polymer and clayey soils can be diffi cult.
Clay could have a catalytic role in polymer degradation.
High CEC assures higher levels of mineral nutrients (NH4+, K+, Mg++, Ca++) which can otherwise become limiting factors.
Soil organic matter (SOM)
Source of nutrients CEC Better soil structure
A good soil structure allows a better contact between soil and polymer and higher gas diffusion.
SOM assures a healthy and active microbial population.
Water Water activity (aw)
Water induces hydrolysis (→MW reduction). Leaching of plasticisers (brittleness).
aw controls microbial growth and thus biodegradation.
Too much water can cause anaerobic conditions and be negative.
Acid/ alkaline compounds
pH Can induce hydrolysis (→MW reduction).
The pH controls the microbial population (living and active species in soil), growth rate of each single species, and enzymic activity.
Air Determines the
O2 and CO2 content
Oxygen is needed for abiotic oxidation reactions leading to decrease of MW.
Air (O2 - CO2) controls the microbial population (living and active species in soil) growth rate of each single species.
HB Biodeg.indb 66
HB Biodeg.indb 66 11/2/05 1:58:52 pm11/2/05 1:58:52 pm
67 3.3.2.1.1 Texture
Soil texture is defi ned by the particle size distribution, which is the most important physical property of soil. The mineral part of soil is classifi ed as ‘sand’, ‘silt’ or ‘clay’
according to the particle size. The proportions of sand, silt, and clay determine the soil texture class. Clays are the smallest particles in soil (diameter < 2 µm); silts are larger (from 2 µm to 50 µm); sands are coarse (diameter from 0.05 mm to 2 mm). The term clay when applied to the texture refers to size; it should not be confused with the term clays used in paragraph 3.3.2.2.2.
3.3.2.1.2 Soil Structure
The soil particles, held together by chemical and physical forces in stable aggregates, form the soil structure. The aggregates may be characterised by their size, shape and surface roughness, even though the size has the most relevance. It is important to note the difference between soil texture and soil structure. The fi rst cannot be easily subjected to modifi cation by agricultural practices. On the other hand, physical changes due to agricultural practices, such as ploughing, cultivating, draining and fertilising (mainly organic fertilisation) as well as compression of soil due to transit on the land of agricultural machines, wetting or drying can strongly affect the structure. The size distribution of aggregates infl uences the amount of water that enters a soil, gas diffusion at the soil surface, heat transfer and soil porosity.
All these factors are very important for growth of microorganisms and biodegradation.
A sandy, granular soil will have a relatively free gas diffusion. On the other hand, a clay, blocky (hard, diffi cult to plough)soil will be poorly aerated.
As a consequence, in the former soil, strictly aerobic microorganisms such as fungi (very active in biodegradation) can develop, while in the latter soil facultative or microaerofi lic aerobes will develop. The microbial population found in a given soil will in turn, infl uence the biodegradation activity.
3.3.2.2 Physical Chemistry of Soil
3.3.2.2.1 Soil Temperature
Soil temperature is a relevant physical factor and has important effects on the biological and chemical processes taking place in the soil. Microbial growth and enzymic processes, in particular, will be strongly affected by the temperature as a consequence of the Arrhenius equation.
HB Biodeg.indb 67
HB Biodeg.indb 67 11/2/05 1:58:52 pm11/2/05 1:58:52 pm
68
Temperature can also directly affect the polymer. For example, the rate of abiotic degradation processes such as hydrolysis, is controlled by temperature [15]. Furthermore, the mobility of the polymeric chains is related to the environmental temperature. This in turn affects the bio-availability of the polymer because an higher mobility will facilitate the contact between the susceptible chemical bonds and the enzymic active sites [24].
3.3.2.2.2 Soil Minerals and Cation Exchange Capacity
Clay minerals are soil secondary minerals derived from the weathering of rocks. Clays have a net negative charge at the surface. Cations are attracted by clay particles. This feature is referred to as the cation exchange capacity (CEC). The CEC of a soil is a measure of the quantity of cations that can be held by a given soil, against the forces of leaching.
The more clay (and organic matter; see next paragraph) a soil contains, the higher the CEC. How does clay content affect biodegradation? The nature and content of clays determine the physical state (texture) of soil under different water regimes. The physical state determines the degree of polymer-soil contact and, therefore, the biodegradation process. For example, clayey soils form clumps which make it diffi cult to mix plastic items and soil together; furthermore air diffusion within the clumps is very limited. This in turn makes degradation diffi cult (unpublished results). Degradation in very clayey soils is therefore impaired by physical constraints.
On the other hand, the CEC of a soil, a factor controlled by clay and organic matter content, is important because it affects the availability of nutrients needed for a balanced microbial growth and a fast biodegradation process. A high CEC is associated with fertile soils, because many cations such as NH4+, K+, Mg++, Ca++ are important nutrients for living organisms and for the effi ciency of the biodegradation processes.
It has been postulated that the presence of clay in soil promotes degradation of polymers.
The hydrolysis would be catalysed by surface Bronsted and Lewis acidities associated with clay minerals [25]. This intriguing hypothesis, which has been developed to explain the behaviour of a specifi c class of polymers (silicone polymers), could also be extended to other classes of carbon-based polymers.
3.3.2.2.3 Soil Organic Matter
Soil organic matter (SOM) is formed by partially decomposed and partially re-synthesised plant and animal residues (lignin). SOM is important for two main reasons: as a nutrient reservoir and as a soil structure improver. Generally the majority of soils (including most agricultural soils), have a relatively poor SOM content, ranging from 0.5-10%. Despite the minor contribution to the total mass of minerals, SOM has a crucial role for soil
HB Biodeg.indb 68
HB Biodeg.indb 68 11/2/05 1:58:52 pm11/2/05 1:58:52 pm
69 fertility and exerts a profound infl uence on biodegradability. It contains all the essential nutrients, released during the process of decomposition (mineralisation): organic carbon compounds, nitrogen, phosphorus and sulfur. The availability of macronutrients is essential to get a fast biodegradation rate since the macronutrients can become limited.
SOM, together with microorganisms (especially fungi), is involved in binding small soil particles into larger particles, with good air diffusion. Furthermore, SOM can directly affect water retention because of its ability to absorb up to 20 times its mass of water.
In slightly acidic to alkaline soils, organic matter can act as a buffer in the maintenance of acceptable soil pH conditions. The high charge characteristics of a SOM (due to the humic fraction) enhance the CEC of a soil.
The addition of organic matter (10% of compost) has been shown to accelerate the rate of degradation although not changing the pattern of degradation [23].
3.3.2.2.4 Water
Water is essential for micro-organism growth, it is the solvent of soil solutions, and it occupies pore spaces competitively with soil gases. Water in a soil can be measured as water content, i.e., the amount of water present in a defi ned soil mass and it is expressed on a percentage basis (grams of water in 100 grams of soil). The water content can be measured by drying a soil sample at 105 °C and measuring the mass loss, which is then ascribed to evaporated water. An important parameter is water activity because it controls microbial growth. Water activity (aw) is the ratio of the water vapour pressure in the soil system to the water vapour pressure of pure water.
aw = Psoil / Pwater
Microbial growth is possible in the range of aw between 1 and 0.6, depending on the species.
Most bacteria need an aw higher than 0.98. Fungi are less sensitive and are able to grow at lower aw, (i.e., 0.8). Osmotolerant fungi are able to grow down to an aw of 0.6 [26].
3.3.2.2.5 pH
Microorganisms are markedly affected by the environmental pH. An increasing soil acidity generally reduces the development of bacteria and on the other hand favours the development of fungi. Due to this there will be less nitrogen fi xation and therefore the rate of soil mineralisation could, as a consequence, decrease. A study performed in our laboratory has shown that degradation of biodegradable polymers in acidic forest soils is rather slowed down. The same soils, if the pH is brought to neutrality with the addition of CaCO3 become very active (Guerrini, Tosin, Degli Innocenti, unpublished results).
HB Biodeg.indb 69
HB Biodeg.indb 69 11/2/05 1:58:53 pm11/2/05 1:58:53 pm
70
3.3.2.2.6 Gas Content
As has been discussed before, the gas content of a soil is proportional to the water content, because these two phases compete for the same pores. Therefore, the O2 content of a soil decreases (and CO2 increases) with increasing water content, as a consequence of soil respiration. The smaller the grains of a soil and therefore the fi ner its porosity, the slower will be the gas exchange within the soil. Anaerobic conditions are established under fl ooded conditions while a lower water content is conducive to aerobic conditions.
Hardly any aerobic degradation of substances can be found in a water saturated soil [27].
Aerobic conditions are generally preferable for a fast biodegradation of plastics, even if exceptions do exist. The most notable example is the faster biodegradation of the poly hydroxy-butyrate-valerate under fl ooded anaerobic conditions [23].
3.3.2.3 Biological Properties of Soil
The living organisms of the soil are in the main, responsible for the continuous synthesis and degradation processes of SOM: they carry out essential environmental functions and they contribute to soil fertility through several biochemical reactions that improve soil structure and transform organic matter into nutrients necessary for life.
The specifi c populations inhabiting soils are dependent upon many factors [28]. The climate and the resulting vegetation signifi cantly infl uence which organisms prevail. The soil factors, discussed in the previous paragraphs, such as temperature, acidity and moisture are also factors that govern the activity of organisms living in the soil. For these reasons, it is not easy to predict the number, kinds, and activities of organisms that one might expect to fi nd in a given soil. But there are few generalisations that might be made. For example, compared to virgin areas, cultivated fi elds generally have lower numbers and weight of soil organisms. This is a consequence of the low SOM present in agricultural soils.
There are over 200 identifi ed bacterial genera and a single soil sample may have over 4,000 genetically distinct bacteria [28]. The greatest population is located in the topsoil, a few millimetres below surface, since conditions of temperature, moisture, aeration, and food are more favourable. The solar radiation reduces the distribution of bacteria on the surface. Deeper in the soil the bacteria are then controlled by the nutrient availability, water content, pH, O2 and CO2 content, and temperature.
Fungi are the dominant organisms in soil, both in terms of processes and biomass. Fungi are active in the decomposition and mineralisation of several complex compounds such as cellulose, lignin and chitin [29, 30]. Fungi are mainly active in acid forest soils, but also play an important role in the other soils [31]. They are not able to oxidise and fi x nitrogen.
HB Biodeg.indb 70
HB Biodeg.indb 70 11/2/05 1:58:53 pm11/2/05 1:58:53 pm
71 Actinomycetes are fungus-like fi lamentous bacteria (Eubacteria) and they are especially numerous in soils high in humus, where the acidity is not too high. They have some characteristics typical of fungi such as hyphal growth form and production of extracellular enzymes. Actinomycetes have a very important role as soil decomposers; they are able to metabolise the SOM, such as cellulose, chitin, and phospholipids, transforming them into nutrients.
From a practical viewpoint, an important parameter is the soil metabolic activity. A simple method to assess the overall activity is by measuring the rate of endogenous respiration of a soil. The specifi c activity, namely the ability of a soil to degrade a specifi c polymer or substance, is also of great interest for practical reasons. Using agar plates containing the polymer of interest as the only carbon source, it is possible to isolate colonies that grow on the polymer. Nishida and Tokiwa found that polyhydroxybutyrate (PHB) and poly(ε-caprolactone) (PCL) degrading (depolymerising) microorganisms are distributed in many kinds of sources, including landfi ll leachate, compost, sewage sludge, forest soil, farm soil, paddy fi eld soil, weed fi eld soil, roadside sand and pond sediment [32].
This type of analysis can be performed for any polymer of interest, when an emulsion [32] or a fi ne powder [33] of the polymer can be used to prepare selective agar plates.
This approach can be of great help to determine the microbial activity of a specifi c fi eld and predict the biodegradation of the polymer.