Biodegradation Kinetics of Polymer Starch Blends (II)

Polymer biodegradation analysis

The carbon balance of the polymer. Biodegradation involves very complex biochemical processes. We assume that there is no polyethylene biodegradation process in the blend.

The design of carbon dioxide measuring instruments has been described in other literatures.

The inoculum standard takes the sludge from the sewage treatment into a microbial inoculum through a two-step pretreatment process. In order to obtain a standard inoculum, the nutrients in the inoculum should be removed in advance so that the production of carbon dioxide completely comes from the sample. The first step was to prepare a standard inoculum. 30 mL of sludge was placed in a 250 mL flask and 21 mL of nutrients and 50 mL of deionized water were added. Mix with electromagnetic stirring at 1000 rpm. The concentration of microorganisms was measured using a spectrophotometer, where an operating curve was obtained by measuring the light intensity of microorganisms containing known concentrations. The light intensity at 660 nm was measured using a light scattering spectrophotometer (Model Spectronic 20).

Once the microbiological concentration is stable, it is transplanted to a common medium and the cell concentration in the medium is cultured to a certain concentration with an average light intensity (OD) OD 660 to 2 . 10 mL of glucose was added to the inoculum and the concentration of microorganisms was measured by spectroscopy. This glucose-adaptable microorganism is used for starch biodegradation studies in PE-S blends.

Biodegradation Analysis of PE-S Blends Glucose-adaptable microorganisms were pre-cultured in a starch environment prior to biodegradation of the PE-S blends. Starch-adapted microbes (250 mL) were transferred to an absorption unit and 1725 mL of deionized water and 525 mL of nutrients were added. After 12 hours of air without carbon dioxide, the starch in the culture solution was completely removed.

For five groups of samples (1) 100% starch; (2) 45% to 55% starch-to-volume ratio of PE-S blends; (3) 65 to 35% volume ratio of PE-S blends; (4) 80 to 20% by volume of the PE-S blend; (5) Blank. Experimental conditions are detailed elsewhere.

Carbon balance experiment

Analysis of CO2 CO2 produced by biodegradation was blown out of the bottle by using CO2-free air and collected using 125 g of 0.1 M NaOH solution. Periodic analysis of the collection device to reduce the amount of alkali, can quantitatively analyze the production of CO2. The procedure was to first add 12.5 g of 1 M NaOH to an absorption device, remove 25 g of it, titrate with hydrochloric acid, and use phenolphthalein as an indicator (pH 7-8). 25 g of 2.5 mL of saturated lanthanum chloride solution can also be added, titrated with hydrochloric acid, using acetochlor green as indicator (pH 3-4).

Analysis of Biomass and Dissolved Carbon The stirring of the experimental apparatus was paused and solid particles were deposited. A certain percentage of the solution was withdrawn from the solution and the optical density at 660 nm was recorded to give the number of microorganisms and the carbon content in the microorganisms. At the end of the experiment, dissolved carbon will also be determined.

The total amount of carbon released from starch includes carbon dioxide, biomass, and dissolved carbon. The proportion of carbon in the starch portion is the biodegradation rate of starch at this time. The total biodegradation rate is plotted against time.

Results and discussion

Microbial intrusion Fig. 2(a) shows an electron micrograph of the PE-S blend before it was buried in the soil. The starch volume ratio was 40%. Figure 2(b) is a higher resolution picture showing different diameters of starch granules (2-20 microns, average 15 microns) and uneven distribution in the PE matrix.

After the soil was buried, the color and weight loss of PE-S occurred, indicating the growth of microorganisms and the degradation of starch. The increase in transparency is due to the degradation of starch. Figure 3 is an electron micrograph of a 240-day soil-buried blend. As the digestion of starch is very serious, the degraded material has been damaged. We can see that the degradation rate of starch is very high and only a very small amount of starch remains because it has not been invaded by microorganisms. We believe that because of the lack of infiltration channels, there is still a portion of starch that is not in contact with microorganisms. Figure 3(b) shows the growth of microorganisms in the channels and pores originally occupied by the starch granules. These microorganisms growing in starch cavities in the polymer matrix appear spherical, dumbbell-shaped and star-shaped. Through testing, it was found that after six months of buried soil degradation, the nitrogen content in the blend increased from 0 to 60% to 016%, indicating that the microorganism used the nitrogen source in the compost. Similarly, similar intrusive processes exist under fresh and salt water conditions. These observations are the main basis for our model simulation.

The computer simulation of microbial intrusion takes into account the number of microorganisms, the activity of microorganisms, the efficiency of the conversion of starch into carbon dioxide, the lifespan of microorganisms, and the reproduction rate of microorganisms in the simulation of the intrusion of starch by microorganisms. Computer simulations of images of bioinvasive (single-sided) monodisperse (1 micron particle) starch-polyethylene blends at a fixed time interval, as shown in Figure 4(a-c), where the starch concentration is equal to, greater than And less than pc. The simulation of the two-sided invasion of microbes for both monodisperse and polydisperse PS-E blends is shown in Figure 5 (a, b). Comparison of these two figures shows the effect of starch granules on starch accessibility and microstructure. For the monodisperse and polydisperse blends, since the starch is surrounded by the non-degradable polyethylene, no permeation channels are present, so there are clusters of starch particles that cannot be easily contacted by the microorganisms.

Figure 6 (a-c) is a representative kinetic result of microbial invasion of the blend when the starch fraction p is 057, 059, and 061. Microbial growth profile is a typical S curve. The curve can be divided into three phases: (1) a delay period; (2) a growth period; and (3) a stable period. The initial stage (1) the concentration of microorganisms is constant. Each microorganism is decomposed into two monomers during the growth period, and the concentration of microorganisms increases. Then there is the stabilization period. There is no first stage in Figure 6, mainly because the starch adaptable microorganisms were selected by simulation. It can be seen from the figure that the final degradation of starch is related to the proportion of starch in the blend. For example, for a constant concentration of microorganism, 52% of the starch is degraded at p=0.57 (predicted by 54% in Equation 3), and 84% of the starch is degraded when p=0.61.

When the lattice size is L d and the critical concentration is pc, the contact fraction can be calculated by Equation 2. For a large lattice contact fraction can also be calculated according to the following formula: (5) For example, when L = 80, b = 1, p = 0.59, and the fractional degree of D = 7/4, we get A =57%. Due to the effect of additional surface contact, this value is lower than the observations in FIGS. 7 and 8 (84%).
The computer simulation of enzyme dispersion simulates the double-sided dispersion of the enzyme in the two-dimensional lattice of monodisperse and polydisperse PE-S blends. The release rate of the enzyme, the dispersion coefficient of the enzyme, and the dispersion coefficient of the hydrolysis product glucose were followed. Amylase was selected for simulation calculations. For a two-sided enzyme dispersion process of monodisperse PE-S blends (starch particles less than 1 micron), the simulations at starch concentrations close to and greater than pc are shown in Figure 7(a,b). Take the case of internal connectivity when the starch content is greater than pc. As time progresses, the enzyme gradually infiltrates the cluster of starch. Due to the infiltration process, there are enzyme gradients and degradation products from the edge of the pattern to the center. Figure 8 shows the results of the dispersion of the enzymes in the blend with a representative starch concentration fraction of p=0.57, 0.59, and 0.61. From 057 to 061, the slope of the curve increases rapidly. For example, for a constant enzyme concentration, such as p=0.57 in the blend, 42% of the starch is degraded (54% in equation (3)), and 82% of the starch in the blend is p=0.61. degradation.
Dynamic Power Exponential Contact Fraction A(t) The data in Figures 6 and 8 can be expressed as a power exponential form A(t) ~ tn, or A(t) = Kt n (6)

t is time, K is a constant, and n is an exponent. n is from 0 to 5/4, which is related to p, and the process is affected by starch degradation. Table 1 summarizes the simulation time index for microbial invasion and enzyme dispersion. The initial slope uses 6% of the initial data, and the main slope uses 80% of data, which is the most representative data.

The index in Table 1 shows that when time-dependent intrusion or dispersion occurs on a split channel with dimension D and concentration x, the contact fraction for a sample of thickness L is: A=x (D- d+1)/L (7) where x<ξ, where ξ is the relevant length. The relationship between the degraded peak length Nf and x is (Fig. 4): N f - XD (8) At this point, D is the fractional dimension of the surface. In a lattice of width L, (L/ξ) d-1 independent units of size xd are contained, and equation (7) is therefore derived. When the contact depth x is greater than the correlation length, a stable state with Nf constant will be obtained. This was discussed by Family et al.

As the diffuser grows, its roughness increases to a steady state. The relationship between this time and x is:
x(t)=α0t β (9)
Where α0 and β are constants. For the pattern intrusion process, we consider β=1; for the dispersion process, β=1/2 when p A(t)=αt n/L (10)
The index n is:
n=β(D-d+1) (11)
When β=1 or 1/2:
α=α0D-d+1 (12)

Table 2 shows the theoretical index based on equation (11) for microbial intrusion (β=1) and two-dimensional, three-dimensional, fractal dimension permeation cluster enzyme diffusion (β=1/2) at equal, greater, or less than critical values. . As p increases, the fractal dimension gradually increases until p = 1 when D = d. Since the initial stage of the split channel has not yet been formed, the initial index is completely independent of p, and the starch is only exposed at the surface. Therefore, the initial slope reflects maximally the inherent behavior of t, A-t β.

The main dynamic index n determines the fractional dimension in the vicinity of pc. For polydisperse systems, the data with a score of 159 better describes the intrusion and dispersion process than D~1.8 for the monodisperse test. The difference in fractional dimensions is caused by the difference in the roughness of the polydisperse channels (average diameter 10 mm) and monodisperse channels (diameter 1 mm). In general terms, the theoretical index and the simulation index have fairly good agreement.

Aerobic Biodegradation Figure 9 (a-c) shows the degradation rate of starch A(t) over time under organic oxygen conditions. The starch fractions in Figure 9 are p=0.2 (a), 0.35 (b), 0.5 (c), respectively. Below the critical permeability value (Fig. 8(a)), the contact fraction f of the starch can be quantitatively calculated by equation (2). For example, consider a blend containing 20% ​​starch, where pc = 0.3117, p = 0.2, d = 3, b = 15 microns, and h = 120 microns. In combination with equation (3), a surface contact score of f=7.5% can be obtained, which is consistent with the final result in FIG. 9 (20 days).

In p A(t)=1.3t 0.6%/min (13)
Since d=3, the cluster's fractional dimension is about D~2.5, and the index from the experimental value is about n~0.6, resulting in beta~1. Therefore, the process of microbial degradation is mainly an invasive process (beta-1) rather than a diffusion process. (β ~ 1/2).

Above the percolation threshold, the degradation becomes more in-depth as the microorganisms and the enzyme make further contact with the permeate channels, as shown in FIGS. 1 and 9 b and c. When p = 0.35, Equation (1) predicts that the final contact score is 75%, which is consistent with the data in Figure 9(b). The initial kinetic slope is n = 0.82, which is 10 during degradation. When the starch content was increased to p=0.5, the final degradation rate reached A to 100%, and the kinetic slope was exactly 1, consistent with the results of the intrusion of microorganisms with an irregular structure of dimension d=3.

During the initial period of degradation, the accessibility of microorganisms to starch is strictly limited by the contact surface. As degradation progresses, the number of microorganisms increases to a maximum. After a stable period of development, it is possible to discover the growth behavior of microorganisms and the production of carbon dioxide. For example, in Figure 9(b,c), the stabilization phase is approximately 10 days. As p increased from 0 to 35 to 0.5, the amount of starch degraded in this 10 days increased from 25% to 50%. When p increases to 1, complete starch degradation takes only about 5 to 6 days under the same conditions.

In the kinetic equation A(t)=Kt n, both K and n are related to p, and the following results can be approximated for biodegradation data.