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Biodegradation of plastics in soil: The effect of temperature
Polymer Degradation and Stability 170 (2019) 109017
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Biodegradation of plastics in soil: The effect of temperature Alessandro Pischedda, Maurizio Tosin, Francesco Degli-Innocenti* Novamont S.p.A, via Fauser 8, 28100, Novara, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2019 Received in revised form 16 October 2019 Accepted 22 October 2019 Available online 24 October 2019
The assessment of the intrinsic biodegradability of plastic materials is made under optimized environmental conditions in order not to limit the microbial growth and activity and follow the biodegradation process until completion. In particular, biodegradation tests are carried out at constant temperature in the range between 20 and 28 C in order to favour the growth of mesophilic microorganisms. On the other hand, if the purpose is to predict the environmental fate of consumer or professional products made with biodegradable plastics after accidental or deliberate release into the environment, then the biodegradation rate attainable under less optimal conditions should be estimated. In this work pellets of a commercial biodegradable plastic material were tested for soil biodegradation at 28, 20, and 15 C. The CO2 evolution was followed for more than one year using the ASTM D 5988e18 test method. The mineralization rates (mg C/day, i.e. the amount of organic carbon converted into CO2 per day) were determined by applying a linear regression from day 140 onwards on the organic carbon depletion curves, when the biodegradation reaction was constant. The specific mineralization rates, i.e. the rate per surface area unit (mg C/day/cm2) were determined by dividing the mineralization rates by the available surface areas of the pellets tested. A thermal performance curve (TPC) was obtained by plotting the specific mineralization rates against the respective temperatures. The TPC curve was perfectly described by an exponential model that was in agreement with the Arrhenius equation. This suggests that biodegradation is dominated by simple thermodynamic effects in the tested temperature ranges (15e28 C). The apparent activation energy of the biodegradation reaction was 108.7 kJ/mol. Using the TPC, it was possible to estimate the time needed for total mineralization of a product made with the test material with a given surface area when exposed to different temperatures. Clearly, the effective biodegradation rate was affected by other environmental factors (e.g. nutrients, pH, gas exchange, etc.) besides temperature. The current work indicates that temperature, an important environmental factor, affects biodegradation rates, in accordance with the Arrhenius equation. The observation that the apparent activation energy of the biodegradation reaction does not vary with temperature in the tested temperature range indicates a persistency in the metabolic activities of the involved mesophilic microbial communities. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Keywords: Biodegradation Biodegradable Plastics Temperature Environmental fate Arrhenius
1. Introduction Two terms are used as synonyms, even by the experts, namely biodegradability and biodegradation. This is erroneous and of source of misunderstandings [1]. Biodegradability refers to a potentiality, the property of a material to be degraded by a biological agent. It is the general recognition that the chemical bonds of the material can be cleaved by
* Corresponding author. E-mail addresses: [email protected] (A. Pischedda), [email protected] (M. Tosin), [email protected] (F. Degli-Innocenti).
microbes and enzymes in the biosphere. Biodegradation refers to a reaction that happens under certain conditions, in a given time, with results which can be measured. It is the process during which the biodegradable material is effectively reduced to its basic constituents thanks to the action of microbes. A biodegradable material may or may not undergo a biodegradation process, depending on the conditions. To draw a parallel, flammability is a characteristic of paper, while combustion is a reaction. A sheet of paper is flammable but it does not necessarily burn under any condition. The fact that centennial libraries do exist is not evidence that paper is fire-resistant, of course. Likewise, a biodegradable material will not necessarily always
https://doi.org/10.1016/j.polymdegradstab.2019.109017 0141-3910/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
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undergo a fast biodegradation process, and thus a biodegradation process limited by some environmental factor is not an indication of a lack of biodegradability. How is biodegradability proven? In theory, if we knew the catalytic sites of all enzymes and all the possible biochemical mechanisms, we could deduce the biodegradability from the chemical structure of the polymer. This will probably be possible in the future. Today, we must apply a laboratory approach: in order to verify the biodegradability of a material, we must elicit a biodegradation process and measure it. The biodegradability of a polymer is inferred by studying biodegradation processes [2]. The biodegradation of plastic materials under aerobic conditions is routinely assessed by measuring respiration in the laboratory [3]. The test methods used to measure biodegradation differ depending on the state (solid, liquid and sometimes biphasic), the origin of the microorganisms (using microbial inocula sampled from different environments, for example soil, compost, etc.), the temperature, and the type of measurement (disappearance of O2 or evolution of CO2, respectively, the reagent and product of aerobic respiration). It should be noted that all the current test methods measure the degree of mineralization, rather than the degree of biodegradation, because the production of biomass is not accounted for by a lack of proper methodology, as mentioned above. However, for reasons of simplicity, this value is called the degree of biodegradation. Biodegradation in soil is measured using two standard test methods that are very similar in their general approach: the International standard ISO 17556 [4] and the ASTM D5988-18 [5]. The plastic material is exposed to soil samples in closed jars. The soil acts both as a carrier matrix and as a source of soil microorganisms and nutrients. The jars are incubated at a temperature that favours the growth of mesophilic soil microorganisms, and under optimum oxygen and moisture conditions. In particular, the standard EN 17033 [6] on biodegradable mulch films requires a constant testing temperature, in the range between 20 and 28 C, preferably 25 C, i.e. in the central part of the mesophilic range (10 - 45 C). Biodegradation tests are carried out so as not to limit the microbial development. The purpose of the biodegradation tests is to measure the biodegradation under optimal conditions and thus substantiate the intrinsic biodegradability of the plastic material. Sub-optimal conditions (scarce nutrients, insufficient water activity, low temperature, reduced gas exchange, etc.) would limit the proper development of microbes, thereby slowing down the biodegradation reaction. However, this is not desirable, both for practical reasons (longer duration of laboratory tests) and for theoretical reasons (the purpose is to determine the intrinsic biodegradability of a material, not the effects of environmental factors on microbial vitality). Thus, when the interest is in the environmental fate of plastic products rather than in the intrinsic biodegradability of the material, other approaches must be followed. A key environmental factor is temperature. Temperature has huge effects on chemical and biochemical reactions and strongly influences the taxonomic composition and metabolic activities of microbial communities [7]. The simplest way to describe the effects of temperature on the rate of a biochemical, physiological, or behavioural process is to generate a thermal performance curve (TPC) [8]. TPCs tend to have three distinct regions: (1) a rising phase, as the temperature increases; (2) a plateau phase, which encompasses the thermal optimum (Topt) for the trait; and (3) a steep falling phase, at higher temperatures. Above a certain temperature, enzymes start to denature, and the total effect is detrimental. Cellular growth ceases. These boundary values define the maximum and minimum temperatures between which life can
exist (and grow). The testing temperature for soil biodegradation in the laboratory is usually around 25/28 C. However the average temperature in the temperate zones is generally lower than 28 C; thus it is expected that the biodegradation rate will be slower. The build-up of plastics will depend on the input rate and the output rate, i.e. the effective biodegradation rate that occurs in nature. Therefore, important information for determining the environmental fate of plastics is the effect of temperature on the degradation rate. To our knowledge, there are no systematic studies on the effect of temperature on the biodegradation rate of biodegradable plastics. Lotto et al. [9] studied mass loss of poly(ε-caprolactone) (PCL), poly-b-(hydroxybutyrate) (PHB), and poly-b-(hydroxybutyrate-coh-valerate) (PHBV) incubated in mature compost at 46 and 24 C. The biodegradation was greater at 46 C, for the three polymers studied. Nishide et al. [10] monitored the mass loss of different polymers in soil burial experiments carried out at 30 and 52 C. PHBV underwent faster degradation at 30 C, than at 52 C, in soil, under aerobic conditions, but there was no remarkable difference between 30 and 52 C, in the degradation rate of PCL, polybutylene succinate and adipate (PBSA), or polybutylene succinate (PBS). PHB demonstrated the fastest rate of degradation among the four plastics at 30 C, and PBSA, the fastest at 52 C. Mergaert et al. [11] studied the mass loss of PHB and PHBV, in soil incubated at 15, 28, and 40 C. The mass loss was greater at 40 C, than at 15 or 28 C, especially for PHBV. The effect of temperature on the biodegradation of PCL was tested in a bench-scale composting reactor under controlled laboratory composting conditions, by Ohtaki et al. [12]. Temperatures of 40, 50, and 60 C were used, and the optimum temperature for the PCL degradation was found to be 50 C. The purpose of this work was to carry out a systematic study of the effect of temperature on the mineralization rate of a biodegradable plastic material and use this information to estimate the potentiality of environmental build-up, using a simulation approach based on biodegradation constants determined on a laboratory scale. 2. Materials and methods 2.1. Materials The test material used in this study was Mater-Bi HF03V1, a commercial, biodegradable plastic produced by Novamont in the form of pellets. This plastic material is made with biodegradable polyesters (about 65%), starch (about 28%), and a natural plasticizer (about 6%). This material complies with the EN 17033 biodegradability requirements [6]. Polyesters are made with monomers that biodegrade in soil [13]. The plasticizer is a biobased, biodegradable polyol. This substance is completely biodegraded within 28 days at room temperature, under aqueous aerobic conditions (Organic Waste Systems, Belgium, data not shown). The carbon content determined by elemental analysis of the test material was C ¼ 56.4% (analysis carried out by Redox Snc, Monza, Italy). The density was 1.28 kg/dm3. Pure micro-crystalline cellulose in powder (Merck) was used as a reference material. The carbon content determined by elemental analysis (analysis carried out by Redox Snc, Monza, Italy) of the test and reference materials is shown in subsection 2.3 (Tables 1e3). 2.2. Specific surface area determination of pellets The specific surface area (cm2/g) of the pellets was determined considering these as a scalene ellipsoid. The dimensions of the
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Table 1 Biodegradation test set-up. Test carried out at 15 C. Reactor
Test material
Total Amount of test material (mg)
R1-15 R2-15 R3-15 R4-15 R5-15 R6-15
Blank Blank HF03V1 HF03V1 Microcrystalline cellulose Microcrystalline cellulose
e e 1027.5 1003.3 1000.9 1003.6
Available surface area cm2
Soil (g)
56.4 56.4 42.5 42.5
e e 579.6 566.0 425.4 426.5
15.41 15.05
200 200 200 200 200 200
%C
Total carbon of test material (mg)
Available surface area cm2
Soil (g)
56.4 56.4 42.5 42.5
e e 573.6 573.2 429.5 425.8
15.25 15.24
200 200 200 200 200 200
%C
Total carbon of test material (mg)
Available surface area cm2
Soil (g)
56.4 56.4 44.4 44.4
e e 575.4 569.7 436.8 431.5
15.06 15.18
200 200 200 200 200 200
%C
Total carbon of test material (mg)
Table 2 Biodegradation test set-up. Test carried out at 20 C. Reactor
Test material
Total Amount of test material (mg)
R1-20 R2-20 R3-20 R4-20 R5-20 R6-20
Blank Blank HF03V1 HF03V1 Microcrystalline cellulose Microcrystalline cellulose
e e 1016.9 1016.2 1010.5 1001.9
Table 3 Biodegradation test set-up. Test carried out at 28 C. Reactor
Test material
Total Amount of test material (mg)
R1-28 R2-28 R3-28 R4-28 R13-28 R14-28
Blank Blank HF03V1 HF03V1 Microcrystalline cellulose Microcrystalline cellulose
e e 1020.0 1010.0 983.0 971.0
three axes of 50 pellets were measured using a calibre. The approximate formula [Eq. (1)] was used to calculate their surface area:
S ðcm2 Þ ¼ 4p
ap bp þ ap cp þ bp cp 3
1=p (1)
where a, b and c are the semi-axes, and p ¼ 1.6075 (Knud Thomsen correction). Next, the specific surface area (cm2/g) was calculated from the surface and weight averages of the 50 pellets. 2.3. Biodegradation test The biodegradation was determined by means of respirometric tests, in accordance with the ASTM D 5988e18 test method [5], based on the measurement of CO2 production. The tests were performed with two replicates instead of three as required by the standard method. This modification was necessary for technical reasons related to the organization of our laboratory and based on the expectation (later confirmed) of obtaining acceptable deviations between the two replicas on the basis of past experiments. Soil was collected from an agricultural field at the Centro Sperimentazione ed Assistenza Agricola (CeRSAA), in Albenga (Italy). The soil is routinely analysed by CeRSAA, and has a C/N ratio of 10.4. The soil is classified as sandy loam following the Soil Taxonomy USDA. Soil sieved to a 5-mm particle size was enriched with compost (40 g/kg) and a salt solution (0.2 g KH2PO4; 0.1 g MgSO4; 0.4 g NaNO3; 0.2 g Urea; 0.4 g NH4Cl per kg of soil). The water content was measured as weight loss (105 C), and the final soil moisture was adjusted to 14.6% (about 50% of the Water Holding Capacity). The final soil pH was determined in a mixture of soil and deionized
water: ratio of 1:2.5 (w/v) [14]. The pH was 7.9. For each replicate, 1 g of test material, in the form of pellet, was mixed with 200 g of soil in a 1000 ml hermetically-sealed glass jar. The test was set up with blank jars (without material) and with reference material jars (1 g of cellulose). Two replicates were carried out for the test material, for blank and for reference, and incubated in the dark, at 15 ± 2 C, 20 ± 2 C, 28 ± 2 C. The set-up of the three tests is shown in Tables 1e3. A 50 ml beaker filled with 30 ml of 0.5 M KOH, as a CO2 trapping solution, was placed in each jar. The amount of CO2 produced was measured by means of titration of the KOH solution, with 0.3 N HCl [15,16] with a Mettler Toledo (T50) potentiometric titrator. The measurement was made every 2e3 days during the first two weeks, when the mineralization rate was expected to be maximal, and weekly or biweekly thereafter. The moisture content was kept constant by adding deionized water throughout the biodegradation test, whenever the KOH solution was titrated and replaced with a fresh one. The net CO2 production evolved from the test materials was calculated by subtracting the average amount of CO2 produced in the blank soils from the amount of CO2 produced in the test material jars. The biodegradation percentages were calculated from the ratio between the net CO2 production and the theoretical CO2 production (ThCO2) based on the carbon content. The regression analysis and data plotting were done using the statistical functions of Excel (Microsoft) and Statgraphics Centurion XVII. 3. Results and discussion 3.1. Soil biodegradation tests The biodegradation in soil of a biodegradable plastic material
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named Mater-Bi HF03V1 was tested at different temperatures: 15, 20, and 28 C. The plastic material was tested in the form of pellets. The pellets had an average specific surface area of 15 cm2/g (see subsection 2.2). The cumulative CO2 evolutions of the different reactors incubated at 15, 20, and 28 C are shown in Figs. 1e3, respectively. The replicates (same symbol, with a continuous or dotted line in Figs. 1e3) showed courses that were overlapping in most cases. The average mineralization curves are shown in Fig. 4. In all cases, the mineralization of the cellulose was in line with the validity requirements (i.e. mineralization >70% after 6 months) of the standard test method ASTM D 5988e18 [5], demonstrating that the soil was biologically active and the tests valid, also at the lowest temperature. At the end of the test (day 385), the sample tested at 28 C had reached a mineralization level equal to 97% of that of cellulose. This value was above the minimum mineralization value (90%) for a material to be considered suitable for biodegradable mulch films, according to the standard EN 17033 [6]. This result is in agreement with other findings [17]. The other samples tested at lower temperatures were still well below that threshold.
Fig. 2. Carbon dioxide (CO2) trend in the different reactors including blank and reference material (cellulose). The test was carried out at 20 C.
3.2. Mineralization rates The carbon evolved as net carbon dioxide (CeCO2) was subtracted from the amount of carbon originally present in the plastic sample at time 0, to obtain the plot of residual, un-mineralized carbon (Fig. 5). This value included both the “plastic” carbon still bound in the plastic constituents, and the carbon progressively assimilated as biomass. For the sake of simplicity we will call this value “organic carbon” (Corg), that is, carbon that has not been converted into inorganic carbon, i.e. mineralized into CO2 yet. A linear regression analysis was applied to the replicates for each temperature, from day 140 onwards, in order to determine the mineralization rates (k), at the different temperatures (Fig. 6). The k values (mg C/day) were divided by the available surface area to get the specific rate K (mg C/day/cm2). The output is shown in Table 4. The rates are reported as positive values, for convenience, but should be considered as negative, as they are depletion rates.
Fig. 3. Carbon dioxide (CO2) trend in the different reactors including blank and reference material (cellulose). The test was carried out at 28 C.
3.3. Thermal performance curve The specific mineralization rates K were plotted vs. the respective temperatures (T), to obtain the thermal performance curve (TPC; Fig. 7). A regression analysis showed that the best fitting of
Fig. 4. Mineralization curves of test material (HF03V1) and reference material (cellulose), tested at different temperature (15, 20, and 28 C). Each curve is the mean of two replicates.
the data was the exponential model with an R-squared ¼ 1: K ¼ exp (7.13203 þ 0.150532*T) Fig. 1. Carbon dioxide (CO2) trend in the different reactors including blank and reference material (cellulose). The test was carried out at 15 C.
(2)
This behaviour is in agreement with the Arrhenius equation [Eq.
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Fig. 5. Organic carbon, Corg, (determined from the evolved CeCO2), plotted as a function of time, of the replicates incubated at different temperature: R3-15 and R415 at 15 C; R3-20 and R4-20 at 20 C; R3-28 and R4-28 C at 28 C.
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Fig. 7. Thermal Performance Curve (TPC), showing an exponential behaviour.
The natural log transformation of the Arrhenius equation is shown in [Eq. (4)].
ln k ¼
Ea þ ln A RT
(4a)
By plotting ln K (as mol C/s/cm2) against 1/T in kelvin, a straight line is obtained by linear regression: ln K ¼ - 13081 1/T þ 19.75 (R-squared ¼ 0.9997) (5)
Fig. 6. Linear regression analysis of organic carbon, Corg, (determined from the evolved CeCO2), plotted as a function of time, at the different temperatures, from day 140 onwards.
(3)], which predicts that the rising phase of a TPC should be exponential in shape. Ea
k ¼ A eRT
The slope of the line gives eEa/R, from which the apparent activation energy Ea ¼ 108.7 kJ/mol for the biodegradation process being calculated. From the intercept (ln A) the value of A was calculated as 3.79E08. This behaviour suggests that the factor Ea (i.e. the apparent activation energy) does not vary with temperature in the tested temperature range, and this in turn indicates a persistency in the metabolic activities of mesophilic microbial communities. The TPC found in this work only includes the rising phase. The plateau phase, with the thermal optimum (Topt), and the following steep falling phase are very likely positioned at higher temperatures, which were not tested in this work.
3.4. Prediction model of soil biodegradation at different temperatures
(3)
where: k is the rate of reaction; A is the pre-exponential factor (which is constant at biologically relevant temperatures); Ea is the activation energy of the reaction; R is the gas constant (8.31 J mol1K1); T is the temperature in kelvin.
A prediction model on the fate of the plastic material HF03V1 can be built on the basis of these findings. At any given time t, the amount of residual organic carbon, Corg (we have defined this value as the carbon present in the plastic material to be mineralized into CO2 during biodegradation; it also includes biomass, i.e. carbon assimilated but not mineralized yet) can be estimated, using the following Eq. (4). Corg(t) ¼ k t þ Corg
(4b)
(t0)
Table 4 Mineralization rates, k, determined by linear regression analysis of the mineralization curves. The specific rates (K) were obtained by dividing the k by the available surface area. Temperature C
k mg C/day
Intercept
R-squared
Surface area (average) cm2
Specific rate K mg C/day/cm2
Specific rate K mol C/s/cm2
15 20 28
0.1168 0.2459 0.8196
461.22 456.02 441.02
0.98 0.88 0.98
15.23 15.24 15.12
0.00767 0.01613 0.05421
7.397 E12 1.556 E11 5.228 E11
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where: Corg(t) ¼ residual carbon still to be mineralized at time t (expressed in mg) k ¼ mineralization rate, expressed as amount of carbon mineralized per day (mg Corg/day) t ¼ time (expressed in days) Corg(t0) ¼ carbon present in the plastic sample at time t ¼ 0, introduced into the soil (expressed in mg) The mineralization rate (k) depends on the specific mineralization rate (KT, expressed as mg Corg/day/cm2, applicable at the considered temperature T in C) and on the available surface area (S, expressed in cm2). k ¼ KT *S
(5)
Eq. (2) allows estimating the specific mineralization rate (KT) at any temperature (T) in the considered range (15e28 C). The time needed to get complete the mineralization of Corg can be obtained using Eq. (6) t ¼ Corg
(t0)/
k
(6)
where Corg (t0) is expressed in mg. The model is based on some general assumptions. The first assumption is that mineralization is constant and linear. This is a simplification, because at the beginning, the mineralization rates are higher, presumably because of the fast biodegradation of smaller molecules, such as plasticizers [17]. Furthermore, the rates are expected to rise further, when the fragmentation of the plastic samples increases the available surface area, and thus the biodegradation rates. In any case, the constant rate seems to be applicable for a substantial part of the biodegradation process (see Figs. 5 and 6). A further assumption is that mineralization is complete, i.e. all the organic carbon is mineralized at the same rate until completion. This is a simplification, because the times for complete oxidation of the formed biomass can be very long [18,19], and therefore the final mineralization stage is usually lower than 100%, and asymptotic. The constants applied in the model were determined in the laboratory under controlled conditions using only one type of soil. The effective environmental conditions of exposure will normally differ from the laboratory conditions, thus the process can be slowed down or accelerated, or even stopped (for example in the case of extreme dryness). In particular it will be interesting to repeat the experiment using soils with different characteristics. In any case, the model presented here is useful in order to get general indications on the effect of temperature on the environmental fate of products when they are intentionally or accidently released into the environment. 3.5. Examples of application of the model A 1 cm2 piece of film of HF03V1 with a 15 mm thickness has a volume of 0.0015 cm3 and a mass of 1.92 mg, given its density is equal to 1.28 kg/dm3. The amount of carbon is 1.08 mg, given that the %C is equal to 56.4. By applying the model, it is possible to predict the soil biodegradation of the film at any given temperature, within the tested range (15e28 C). At 15 C, the specific mineralization rate was measured, so there is no need to apply the model. At 15 C, K ¼ 0.00767 mg C/day/cm2 (see Table 4). The total surface area of the sample (i.e. considering the two faces of the film) is 2 cm2 (the surface area provided by the
thickness is negligible). With a surface area of 2 cm2, the mineralization rate k ¼ 0.00767 2 ¼ 0.015338 mg C/day (see Eq. (5)). The amount of residual organic carbon Corg(t), at any given time t, is provided by Eq. (4). The theoretical time to achieve total mineralization at 15 C is t ¼ Corg (t0)/k ¼ 1.083072/0.015338 ¼ 70.6 days. The limits of this approach and the meaning of this value have been discussed before. The model is a tool that, by definition, contains several simplifications and indicates a trend in a multifactorial process, and not an exact prediction. For example, we can anticipate that mineralization will not be completed in 70.6 days, because a certain amount of biomass will not be oxidized in this short time. The validity of the model for temperatures outside the tested range (15e28 C) is questionable, but it seems a reasonable hypothesis that the model holds when extending the range of validity by a few degrees centigrade beyond the two extremes. For example, from 13 to 30 C, i.e. well within the mesophilic temperature range (minimum: 10e15 C, maximum 45 C). In Italy the average annual temperature of soils at a depth of 15e20 cm is in the range 12e16 C [20]. Thus, we can consider that the “average” soil temperature in Italy to be 14 C. At 14 C, the value of K predicted by the regression analysis is K14 ¼ exp (7.13203 þ 0.150532 14) ¼ 0.006574 mg C/day/cm2. The mineralization rate is k ¼ KT *S ¼ 0.006574 2 ¼ 0.013148 mg C/day. The time for complete mineralization is t ¼ Corg (t0)/k ¼ 1.083072/0.013148 ¼ 82 days. 4. Conclusions It is likely that environmental parameters, such as temperature, have a direct impact on biodegradable plastic build-up after accidental or deliberate release, as they directly affect plastic biodegradation rates. In particular, temperature is explicitly used as a predictor variable when modelling pollutant degradation half-lives in chemical fate and exposure models used for risk assessment purposes [21]. Until now, research has focused on determining intrinsic biodegradability, which is essential information when classifying the environmental properties of plastic materials. When the interest shifts from the intrinsic characteristics of materials to the behaviour of products when introduced into the environment, it is necessary to understand the influence of the fundamental environmental parameters on the degradation rates. In this article we have addressed the effect of temperature on the speed of biodegradation, showing that it is perfectly described by the Arrhenius equation in the tested temperature range. This is an initial step towards the development of a methodology to simulate field dissipation kinetics taking into account the effects of soil temperature. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] F. Degli-Innocenti, Biodegradability and compostability e the international norms, in: E. Chiellini, R. Solaro (Eds.), Biodegradable Polymers and Plastics, Kluwer Academic Plenum Publishers, New York, 2003, p. 33. https://doi.org/ 10.1007/978-1-4419-9240-6. [2] M. van der Zee, Methods for evaluating the biodegradability of environmentally degradable polymers, in: C. Bastioli (Ed.), Handbook of Biodegradable Polymers, second ed., Smithers Rapra Shawbury, Shrewsbury, 2014, ISBN 9781-84735-526-3, p. 1. [3] B. De Wilde, Biodegradation testing protocols, in: K. Khemani, C. Scholz (Eds.), Degradable Polymers and Materials: Principles and Practice (2nd Edition) ACS
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[12] A. Ohtaki, N. Akakura, K. Nakasaki, Effects of temperature and inoculum on the degradability of poly-ε-caprolactone during composting, Polym. Degrad. Stab. 62 (2) (1998) 279e284, https://doi.org/10.1016/S0141-3910(98)00008-1. [13] M. Siotto, M. Tosin, F. Degli-Innocenti, V. Mezzanotte, Mineralization of monomeric components of biodegradable plastics in preconditioned and enriched sandy loam soil under laboratory conditions, Water Air Soil Pollut. 221 (2011) 245e254, https://doi.org/10.1007/s11270-011-0787-8. [14] P. Violante, P. Adamo, II. Reazione, in: P. Violante (Ed.), Metodi di Analisi Chimica del Suolo, Franco Angeli, Milano, 2000, p. 1. [15] A. Modelli, B. Calcagno, M. Scandola, Kinetics of aerobic polymer degradation in soil by means of the ASTM D 5988-96 standard method, J. Environ. Polym. Degrad. 7 (1999) 109e116, https://doi.org/10.1023/A:1021864402395. [16] ASTM D7991, Standard Test Method for Determining Aerobic Biodegradation of Plastics Buried in Sandy Marine Sediment under Controlled Laboratory Conditions, ASTM International, West Conshohocken, PA, 2015. https://www. astm.org/Standards/D7991.htm. accessed on 22 August 2019. [17] M. Tosin, A. Pischedda, F. Degli-Innocenti, Biodegradation kinetics in soil of a multi-constituent biodegradable plastic, Polym. Degrad. Stab. 166 (2019) 213e218, https://doi.org/10.1016/j.polymdegradstab.2019.05.034. [18] P.W. Hill, J.F. Farrar, D.L. Jones, Decoupling of microbial glucose uptake and mineralization in soil, Soil Biol. Biochem. 40 (2008) 616e624, https://doi.org/ 10.1016/j.soilbio.2007.09.008. [19] E. Oburger, D.L. Jones, Substrate mineralization studies in the laboratory show different microbial C partitioning dynamics than in the field, Soil Biol. Biochem. 41 (2009) 1951e1956, https://doi.org/10.1016/j.soilbio.2009.06.020. [20] E.A.C. Costantini, F. Castelli, M. Iori, S. Magini, P. Lorenzoni, S. Raimondi, Regime termico del suolo in alcuni campi sperimentali del nord, centro e sud Italia. Atti del Convegno SISS “La scienza del suolo in Italia: bilancio di fine secolo” Gressonay Saint Jean, 1999. http://soilmaps.entecra.it/download/pubSiss99_costantini.pdf. accessed on 22 August 2019. [21] M. Matthies, S. Beulke, Considerations of temperature in the context of the persistence classification in the EU, Environ. Sci. Eur. 29 (2017) 15, https:// doi.org/10.1186/s12302-017-0113-1.
Contents lists available at ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Biodegradation of plastics in soil: The effect of temperature Alessandro Pischedda, Maurizio Tosin, Francesco Degli-Innocenti* Novamont S.p.A, via Fauser 8, 28100, Novara, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2019 Received in revised form 16 October 2019 Accepted 22 October 2019 Available online 24 October 2019
The assessment of the intrinsic biodegradability of plastic materials is made under optimized environmental conditions in order not to limit the microbial growth and activity and follow the biodegradation process until completion. In particular, biodegradation tests are carried out at constant temperature in the range between 20 and 28 C in order to favour the growth of mesophilic microorganisms. On the other hand, if the purpose is to predict the environmental fate of consumer or professional products made with biodegradable plastics after accidental or deliberate release into the environment, then the biodegradation rate attainable under less optimal conditions should be estimated. In this work pellets of a commercial biodegradable plastic material were tested for soil biodegradation at 28, 20, and 15 C. The CO2 evolution was followed for more than one year using the ASTM D 5988e18 test method. The mineralization rates (mg C/day, i.e. the amount of organic carbon converted into CO2 per day) were determined by applying a linear regression from day 140 onwards on the organic carbon depletion curves, when the biodegradation reaction was constant. The specific mineralization rates, i.e. the rate per surface area unit (mg C/day/cm2) were determined by dividing the mineralization rates by the available surface areas of the pellets tested. A thermal performance curve (TPC) was obtained by plotting the specific mineralization rates against the respective temperatures. The TPC curve was perfectly described by an exponential model that was in agreement with the Arrhenius equation. This suggests that biodegradation is dominated by simple thermodynamic effects in the tested temperature ranges (15e28 C). The apparent activation energy of the biodegradation reaction was 108.7 kJ/mol. Using the TPC, it was possible to estimate the time needed for total mineralization of a product made with the test material with a given surface area when exposed to different temperatures. Clearly, the effective biodegradation rate was affected by other environmental factors (e.g. nutrients, pH, gas exchange, etc.) besides temperature. The current work indicates that temperature, an important environmental factor, affects biodegradation rates, in accordance with the Arrhenius equation. The observation that the apparent activation energy of the biodegradation reaction does not vary with temperature in the tested temperature range indicates a persistency in the metabolic activities of the involved mesophilic microbial communities. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Keywords: Biodegradation Biodegradable Plastics Temperature Environmental fate Arrhenius
1. Introduction Two terms are used as synonyms, even by the experts, namely biodegradability and biodegradation. This is erroneous and of source of misunderstandings [1]. Biodegradability refers to a potentiality, the property of a material to be degraded by a biological agent. It is the general recognition that the chemical bonds of the material can be cleaved by
* Corresponding author. E-mail addresses: [email protected] (A. Pischedda), [email protected] (M. Tosin), [email protected] (F. Degli-Innocenti).
microbes and enzymes in the biosphere. Biodegradation refers to a reaction that happens under certain conditions, in a given time, with results which can be measured. It is the process during which the biodegradable material is effectively reduced to its basic constituents thanks to the action of microbes. A biodegradable material may or may not undergo a biodegradation process, depending on the conditions. To draw a parallel, flammability is a characteristic of paper, while combustion is a reaction. A sheet of paper is flammable but it does not necessarily burn under any condition. The fact that centennial libraries do exist is not evidence that paper is fire-resistant, of course. Likewise, a biodegradable material will not necessarily always
https://doi.org/10.1016/j.polymdegradstab.2019.109017 0141-3910/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
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undergo a fast biodegradation process, and thus a biodegradation process limited by some environmental factor is not an indication of a lack of biodegradability. How is biodegradability proven? In theory, if we knew the catalytic sites of all enzymes and all the possible biochemical mechanisms, we could deduce the biodegradability from the chemical structure of the polymer. This will probably be possible in the future. Today, we must apply a laboratory approach: in order to verify the biodegradability of a material, we must elicit a biodegradation process and measure it. The biodegradability of a polymer is inferred by studying biodegradation processes [2]. The biodegradation of plastic materials under aerobic conditions is routinely assessed by measuring respiration in the laboratory [3]. The test methods used to measure biodegradation differ depending on the state (solid, liquid and sometimes biphasic), the origin of the microorganisms (using microbial inocula sampled from different environments, for example soil, compost, etc.), the temperature, and the type of measurement (disappearance of O2 or evolution of CO2, respectively, the reagent and product of aerobic respiration). It should be noted that all the current test methods measure the degree of mineralization, rather than the degree of biodegradation, because the production of biomass is not accounted for by a lack of proper methodology, as mentioned above. However, for reasons of simplicity, this value is called the degree of biodegradation. Biodegradation in soil is measured using two standard test methods that are very similar in their general approach: the International standard ISO 17556 [4] and the ASTM D5988-18 [5]. The plastic material is exposed to soil samples in closed jars. The soil acts both as a carrier matrix and as a source of soil microorganisms and nutrients. The jars are incubated at a temperature that favours the growth of mesophilic soil microorganisms, and under optimum oxygen and moisture conditions. In particular, the standard EN 17033 [6] on biodegradable mulch films requires a constant testing temperature, in the range between 20 and 28 C, preferably 25 C, i.e. in the central part of the mesophilic range (10 - 45 C). Biodegradation tests are carried out so as not to limit the microbial development. The purpose of the biodegradation tests is to measure the biodegradation under optimal conditions and thus substantiate the intrinsic biodegradability of the plastic material. Sub-optimal conditions (scarce nutrients, insufficient water activity, low temperature, reduced gas exchange, etc.) would limit the proper development of microbes, thereby slowing down the biodegradation reaction. However, this is not desirable, both for practical reasons (longer duration of laboratory tests) and for theoretical reasons (the purpose is to determine the intrinsic biodegradability of a material, not the effects of environmental factors on microbial vitality). Thus, when the interest is in the environmental fate of plastic products rather than in the intrinsic biodegradability of the material, other approaches must be followed. A key environmental factor is temperature. Temperature has huge effects on chemical and biochemical reactions and strongly influences the taxonomic composition and metabolic activities of microbial communities [7]. The simplest way to describe the effects of temperature on the rate of a biochemical, physiological, or behavioural process is to generate a thermal performance curve (TPC) [8]. TPCs tend to have three distinct regions: (1) a rising phase, as the temperature increases; (2) a plateau phase, which encompasses the thermal optimum (Topt) for the trait; and (3) a steep falling phase, at higher temperatures. Above a certain temperature, enzymes start to denature, and the total effect is detrimental. Cellular growth ceases. These boundary values define the maximum and minimum temperatures between which life can
exist (and grow). The testing temperature for soil biodegradation in the laboratory is usually around 25/28 C. However the average temperature in the temperate zones is generally lower than 28 C; thus it is expected that the biodegradation rate will be slower. The build-up of plastics will depend on the input rate and the output rate, i.e. the effective biodegradation rate that occurs in nature. Therefore, important information for determining the environmental fate of plastics is the effect of temperature on the degradation rate. To our knowledge, there are no systematic studies on the effect of temperature on the biodegradation rate of biodegradable plastics. Lotto et al. [9] studied mass loss of poly(ε-caprolactone) (PCL), poly-b-(hydroxybutyrate) (PHB), and poly-b-(hydroxybutyrate-coh-valerate) (PHBV) incubated in mature compost at 46 and 24 C. The biodegradation was greater at 46 C, for the three polymers studied. Nishide et al. [10] monitored the mass loss of different polymers in soil burial experiments carried out at 30 and 52 C. PHBV underwent faster degradation at 30 C, than at 52 C, in soil, under aerobic conditions, but there was no remarkable difference between 30 and 52 C, in the degradation rate of PCL, polybutylene succinate and adipate (PBSA), or polybutylene succinate (PBS). PHB demonstrated the fastest rate of degradation among the four plastics at 30 C, and PBSA, the fastest at 52 C. Mergaert et al. [11] studied the mass loss of PHB and PHBV, in soil incubated at 15, 28, and 40 C. The mass loss was greater at 40 C, than at 15 or 28 C, especially for PHBV. The effect of temperature on the biodegradation of PCL was tested in a bench-scale composting reactor under controlled laboratory composting conditions, by Ohtaki et al. [12]. Temperatures of 40, 50, and 60 C were used, and the optimum temperature for the PCL degradation was found to be 50 C. The purpose of this work was to carry out a systematic study of the effect of temperature on the mineralization rate of a biodegradable plastic material and use this information to estimate the potentiality of environmental build-up, using a simulation approach based on biodegradation constants determined on a laboratory scale. 2. Materials and methods 2.1. Materials The test material used in this study was Mater-Bi HF03V1, a commercial, biodegradable plastic produced by Novamont in the form of pellets. This plastic material is made with biodegradable polyesters (about 65%), starch (about 28%), and a natural plasticizer (about 6%). This material complies with the EN 17033 biodegradability requirements [6]. Polyesters are made with monomers that biodegrade in soil [13]. The plasticizer is a biobased, biodegradable polyol. This substance is completely biodegraded within 28 days at room temperature, under aqueous aerobic conditions (Organic Waste Systems, Belgium, data not shown). The carbon content determined by elemental analysis of the test material was C ¼ 56.4% (analysis carried out by Redox Snc, Monza, Italy). The density was 1.28 kg/dm3. Pure micro-crystalline cellulose in powder (Merck) was used as a reference material. The carbon content determined by elemental analysis (analysis carried out by Redox Snc, Monza, Italy) of the test and reference materials is shown in subsection 2.3 (Tables 1e3). 2.2. Specific surface area determination of pellets The specific surface area (cm2/g) of the pellets was determined considering these as a scalene ellipsoid. The dimensions of the
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Table 1 Biodegradation test set-up. Test carried out at 15 C. Reactor
Test material
Total Amount of test material (mg)
R1-15 R2-15 R3-15 R4-15 R5-15 R6-15
Blank Blank HF03V1 HF03V1 Microcrystalline cellulose Microcrystalline cellulose
e e 1027.5 1003.3 1000.9 1003.6
Available surface area cm2
Soil (g)
56.4 56.4 42.5 42.5
e e 579.6 566.0 425.4 426.5
15.41 15.05
200 200 200 200 200 200
%C
Total carbon of test material (mg)
Available surface area cm2
Soil (g)
56.4 56.4 42.5 42.5
e e 573.6 573.2 429.5 425.8
15.25 15.24
200 200 200 200 200 200
%C
Total carbon of test material (mg)
Available surface area cm2
Soil (g)
56.4 56.4 44.4 44.4
e e 575.4 569.7 436.8 431.5
15.06 15.18
200 200 200 200 200 200
%C
Total carbon of test material (mg)
Table 2 Biodegradation test set-up. Test carried out at 20 C. Reactor
Test material
Total Amount of test material (mg)
R1-20 R2-20 R3-20 R4-20 R5-20 R6-20
Blank Blank HF03V1 HF03V1 Microcrystalline cellulose Microcrystalline cellulose
e e 1016.9 1016.2 1010.5 1001.9
Table 3 Biodegradation test set-up. Test carried out at 28 C. Reactor
Test material
Total Amount of test material (mg)
R1-28 R2-28 R3-28 R4-28 R13-28 R14-28
Blank Blank HF03V1 HF03V1 Microcrystalline cellulose Microcrystalline cellulose
e e 1020.0 1010.0 983.0 971.0
three axes of 50 pellets were measured using a calibre. The approximate formula [Eq. (1)] was used to calculate their surface area:
S ðcm2 Þ ¼ 4p
ap bp þ ap cp þ bp cp 3
1=p (1)
where a, b and c are the semi-axes, and p ¼ 1.6075 (Knud Thomsen correction). Next, the specific surface area (cm2/g) was calculated from the surface and weight averages of the 50 pellets. 2.3. Biodegradation test The biodegradation was determined by means of respirometric tests, in accordance with the ASTM D 5988e18 test method [5], based on the measurement of CO2 production. The tests were performed with two replicates instead of three as required by the standard method. This modification was necessary for technical reasons related to the organization of our laboratory and based on the expectation (later confirmed) of obtaining acceptable deviations between the two replicas on the basis of past experiments. Soil was collected from an agricultural field at the Centro Sperimentazione ed Assistenza Agricola (CeRSAA), in Albenga (Italy). The soil is routinely analysed by CeRSAA, and has a C/N ratio of 10.4. The soil is classified as sandy loam following the Soil Taxonomy USDA. Soil sieved to a 5-mm particle size was enriched with compost (40 g/kg) and a salt solution (0.2 g KH2PO4; 0.1 g MgSO4; 0.4 g NaNO3; 0.2 g Urea; 0.4 g NH4Cl per kg of soil). The water content was measured as weight loss (105 C), and the final soil moisture was adjusted to 14.6% (about 50% of the Water Holding Capacity). The final soil pH was determined in a mixture of soil and deionized
water: ratio of 1:2.5 (w/v) [14]. The pH was 7.9. For each replicate, 1 g of test material, in the form of pellet, was mixed with 200 g of soil in a 1000 ml hermetically-sealed glass jar. The test was set up with blank jars (without material) and with reference material jars (1 g of cellulose). Two replicates were carried out for the test material, for blank and for reference, and incubated in the dark, at 15 ± 2 C, 20 ± 2 C, 28 ± 2 C. The set-up of the three tests is shown in Tables 1e3. A 50 ml beaker filled with 30 ml of 0.5 M KOH, as a CO2 trapping solution, was placed in each jar. The amount of CO2 produced was measured by means of titration of the KOH solution, with 0.3 N HCl [15,16] with a Mettler Toledo (T50) potentiometric titrator. The measurement was made every 2e3 days during the first two weeks, when the mineralization rate was expected to be maximal, and weekly or biweekly thereafter. The moisture content was kept constant by adding deionized water throughout the biodegradation test, whenever the KOH solution was titrated and replaced with a fresh one. The net CO2 production evolved from the test materials was calculated by subtracting the average amount of CO2 produced in the blank soils from the amount of CO2 produced in the test material jars. The biodegradation percentages were calculated from the ratio between the net CO2 production and the theoretical CO2 production (ThCO2) based on the carbon content. The regression analysis and data plotting were done using the statistical functions of Excel (Microsoft) and Statgraphics Centurion XVII. 3. Results and discussion 3.1. Soil biodegradation tests The biodegradation in soil of a biodegradable plastic material
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named Mater-Bi HF03V1 was tested at different temperatures: 15, 20, and 28 C. The plastic material was tested in the form of pellets. The pellets had an average specific surface area of 15 cm2/g (see subsection 2.2). The cumulative CO2 evolutions of the different reactors incubated at 15, 20, and 28 C are shown in Figs. 1e3, respectively. The replicates (same symbol, with a continuous or dotted line in Figs. 1e3) showed courses that were overlapping in most cases. The average mineralization curves are shown in Fig. 4. In all cases, the mineralization of the cellulose was in line with the validity requirements (i.e. mineralization >70% after 6 months) of the standard test method ASTM D 5988e18 [5], demonstrating that the soil was biologically active and the tests valid, also at the lowest temperature. At the end of the test (day 385), the sample tested at 28 C had reached a mineralization level equal to 97% of that of cellulose. This value was above the minimum mineralization value (90%) for a material to be considered suitable for biodegradable mulch films, according to the standard EN 17033 [6]. This result is in agreement with other findings [17]. The other samples tested at lower temperatures were still well below that threshold.
Fig. 2. Carbon dioxide (CO2) trend in the different reactors including blank and reference material (cellulose). The test was carried out at 20 C.
3.2. Mineralization rates The carbon evolved as net carbon dioxide (CeCO2) was subtracted from the amount of carbon originally present in the plastic sample at time 0, to obtain the plot of residual, un-mineralized carbon (Fig. 5). This value included both the “plastic” carbon still bound in the plastic constituents, and the carbon progressively assimilated as biomass. For the sake of simplicity we will call this value “organic carbon” (Corg), that is, carbon that has not been converted into inorganic carbon, i.e. mineralized into CO2 yet. A linear regression analysis was applied to the replicates for each temperature, from day 140 onwards, in order to determine the mineralization rates (k), at the different temperatures (Fig. 6). The k values (mg C/day) were divided by the available surface area to get the specific rate K (mg C/day/cm2). The output is shown in Table 4. The rates are reported as positive values, for convenience, but should be considered as negative, as they are depletion rates.
Fig. 3. Carbon dioxide (CO2) trend in the different reactors including blank and reference material (cellulose). The test was carried out at 28 C.
3.3. Thermal performance curve The specific mineralization rates K were plotted vs. the respective temperatures (T), to obtain the thermal performance curve (TPC; Fig. 7). A regression analysis showed that the best fitting of
Fig. 4. Mineralization curves of test material (HF03V1) and reference material (cellulose), tested at different temperature (15, 20, and 28 C). Each curve is the mean of two replicates.
the data was the exponential model with an R-squared ¼ 1: K ¼ exp (7.13203 þ 0.150532*T) Fig. 1. Carbon dioxide (CO2) trend in the different reactors including blank and reference material (cellulose). The test was carried out at 15 C.
(2)
This behaviour is in agreement with the Arrhenius equation [Eq.
A. Pischedda et al. / Polymer Degradation and Stability 170 (2019) 109017
Fig. 5. Organic carbon, Corg, (determined from the evolved CeCO2), plotted as a function of time, of the replicates incubated at different temperature: R3-15 and R415 at 15 C; R3-20 and R4-20 at 20 C; R3-28 and R4-28 C at 28 C.
5
Fig. 7. Thermal Performance Curve (TPC), showing an exponential behaviour.
The natural log transformation of the Arrhenius equation is shown in [Eq. (4)].
ln k ¼
Ea þ ln A RT
(4a)
By plotting ln K (as mol C/s/cm2) against 1/T in kelvin, a straight line is obtained by linear regression: ln K ¼ - 13081 1/T þ 19.75 (R-squared ¼ 0.9997) (5)
Fig. 6. Linear regression analysis of organic carbon, Corg, (determined from the evolved CeCO2), plotted as a function of time, at the different temperatures, from day 140 onwards.
(3)], which predicts that the rising phase of a TPC should be exponential in shape. Ea
k ¼ A eRT
The slope of the line gives eEa/R, from which the apparent activation energy Ea ¼ 108.7 kJ/mol for the biodegradation process being calculated. From the intercept (ln A) the value of A was calculated as 3.79E08. This behaviour suggests that the factor Ea (i.e. the apparent activation energy) does not vary with temperature in the tested temperature range, and this in turn indicates a persistency in the metabolic activities of mesophilic microbial communities. The TPC found in this work only includes the rising phase. The plateau phase, with the thermal optimum (Topt), and the following steep falling phase are very likely positioned at higher temperatures, which were not tested in this work.
3.4. Prediction model of soil biodegradation at different temperatures
(3)
where: k is the rate of reaction; A is the pre-exponential factor (which is constant at biologically relevant temperatures); Ea is the activation energy of the reaction; R is the gas constant (8.31 J mol1K1); T is the temperature in kelvin.
A prediction model on the fate of the plastic material HF03V1 can be built on the basis of these findings. At any given time t, the amount of residual organic carbon, Corg (we have defined this value as the carbon present in the plastic material to be mineralized into CO2 during biodegradation; it also includes biomass, i.e. carbon assimilated but not mineralized yet) can be estimated, using the following Eq. (4). Corg(t) ¼ k t þ Corg
(4b)
(t0)
Table 4 Mineralization rates, k, determined by linear regression analysis of the mineralization curves. The specific rates (K) were obtained by dividing the k by the available surface area. Temperature C
k mg C/day
Intercept
R-squared
Surface area (average) cm2
Specific rate K mg C/day/cm2
Specific rate K mol C/s/cm2
15 20 28
0.1168 0.2459 0.8196
461.22 456.02 441.02
0.98 0.88 0.98
15.23 15.24 15.12
0.00767 0.01613 0.05421
7.397 E12 1.556 E11 5.228 E11
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A. Pischedda et al. / Polymer Degradation and Stability 170 (2019) 109017
where: Corg(t) ¼ residual carbon still to be mineralized at time t (expressed in mg) k ¼ mineralization rate, expressed as amount of carbon mineralized per day (mg Corg/day) t ¼ time (expressed in days) Corg(t0) ¼ carbon present in the plastic sample at time t ¼ 0, introduced into the soil (expressed in mg) The mineralization rate (k) depends on the specific mineralization rate (KT, expressed as mg Corg/day/cm2, applicable at the considered temperature T in C) and on the available surface area (S, expressed in cm2). k ¼ KT *S
(5)
Eq. (2) allows estimating the specific mineralization rate (KT) at any temperature (T) in the considered range (15e28 C). The time needed to get complete the mineralization of Corg can be obtained using Eq. (6) t ¼ Corg
(t0)/
k
(6)
where Corg (t0) is expressed in mg. The model is based on some general assumptions. The first assumption is that mineralization is constant and linear. This is a simplification, because at the beginning, the mineralization rates are higher, presumably because of the fast biodegradation of smaller molecules, such as plasticizers [17]. Furthermore, the rates are expected to rise further, when the fragmentation of the plastic samples increases the available surface area, and thus the biodegradation rates. In any case, the constant rate seems to be applicable for a substantial part of the biodegradation process (see Figs. 5 and 6). A further assumption is that mineralization is complete, i.e. all the organic carbon is mineralized at the same rate until completion. This is a simplification, because the times for complete oxidation of the formed biomass can be very long [18,19], and therefore the final mineralization stage is usually lower than 100%, and asymptotic. The constants applied in the model were determined in the laboratory under controlled conditions using only one type of soil. The effective environmental conditions of exposure will normally differ from the laboratory conditions, thus the process can be slowed down or accelerated, or even stopped (for example in the case of extreme dryness). In particular it will be interesting to repeat the experiment using soils with different characteristics. In any case, the model presented here is useful in order to get general indications on the effect of temperature on the environmental fate of products when they are intentionally or accidently released into the environment. 3.5. Examples of application of the model A 1 cm2 piece of film of HF03V1 with a 15 mm thickness has a volume of 0.0015 cm3 and a mass of 1.92 mg, given its density is equal to 1.28 kg/dm3. The amount of carbon is 1.08 mg, given that the %C is equal to 56.4. By applying the model, it is possible to predict the soil biodegradation of the film at any given temperature, within the tested range (15e28 C). At 15 C, the specific mineralization rate was measured, so there is no need to apply the model. At 15 C, K ¼ 0.00767 mg C/day/cm2 (see Table 4). The total surface area of the sample (i.e. considering the two faces of the film) is 2 cm2 (the surface area provided by the
thickness is negligible). With a surface area of 2 cm2, the mineralization rate k ¼ 0.00767 2 ¼ 0.015338 mg C/day (see Eq. (5)). The amount of residual organic carbon Corg(t), at any given time t, is provided by Eq. (4). The theoretical time to achieve total mineralization at 15 C is t ¼ Corg (t0)/k ¼ 1.083072/0.015338 ¼ 70.6 days. The limits of this approach and the meaning of this value have been discussed before. The model is a tool that, by definition, contains several simplifications and indicates a trend in a multifactorial process, and not an exact prediction. For example, we can anticipate that mineralization will not be completed in 70.6 days, because a certain amount of biomass will not be oxidized in this short time. The validity of the model for temperatures outside the tested range (15e28 C) is questionable, but it seems a reasonable hypothesis that the model holds when extending the range of validity by a few degrees centigrade beyond the two extremes. For example, from 13 to 30 C, i.e. well within the mesophilic temperature range (minimum: 10e15 C, maximum 45 C). In Italy the average annual temperature of soils at a depth of 15e20 cm is in the range 12e16 C [20]. Thus, we can consider that the “average” soil temperature in Italy to be 14 C. At 14 C, the value of K predicted by the regression analysis is K14 ¼ exp (7.13203 þ 0.150532 14) ¼ 0.006574 mg C/day/cm2. The mineralization rate is k ¼ KT *S ¼ 0.006574 2 ¼ 0.013148 mg C/day. The time for complete mineralization is t ¼ Corg (t0)/k ¼ 1.083072/0.013148 ¼ 82 days. 4. Conclusions It is likely that environmental parameters, such as temperature, have a direct impact on biodegradable plastic build-up after accidental or deliberate release, as they directly affect plastic biodegradation rates. In particular, temperature is explicitly used as a predictor variable when modelling pollutant degradation half-lives in chemical fate and exposure models used for risk assessment purposes [21]. Until now, research has focused on determining intrinsic biodegradability, which is essential information when classifying the environmental properties of plastic materials. When the interest shifts from the intrinsic characteristics of materials to the behaviour of products when introduced into the environment, it is necessary to understand the influence of the fundamental environmental parameters on the degradation rates. In this article we have addressed the effect of temperature on the speed of biodegradation, showing that it is perfectly described by the Arrhenius equation in the tested temperature range. This is an initial step towards the development of a methodology to simulate field dissipation kinetics taking into account the effects of soil temperature. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] F. Degli-Innocenti, Biodegradability and compostability e the international norms, in: E. Chiellini, R. Solaro (Eds.), Biodegradable Polymers and Plastics, Kluwer Academic Plenum Publishers, New York, 2003, p. 33. https://doi.org/ 10.1007/978-1-4419-9240-6. [2] M. van der Zee, Methods for evaluating the biodegradability of environmentally degradable polymers, in: C. Bastioli (Ed.), Handbook of Biodegradable Polymers, second ed., Smithers Rapra Shawbury, Shrewsbury, 2014, ISBN 9781-84735-526-3, p. 1. [3] B. De Wilde, Biodegradation testing protocols, in: K. Khemani, C. Scholz (Eds.), Degradable Polymers and Materials: Principles and Practice (2nd Edition) ACS
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