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Incubation time (days)

FIGURE 5.16 Time profiles of mineralization of PVA samples having different degree of hydrolysis in an aqueous medium in the presence of acclimated PVA-degrading microorganisms.

No detectable amount of PVA was released from montmorillonite samples when suspended in water, suggesting that the adsorption process is almost irreversible.101 The influence of the adsorption by montmorillonite on the biodegradation of PVA was investigated in the presence of an acclimated microbial inoculum90 in liquid cultures containing either soluble PVA or PVA adsorbed on montmorillonite. The recorded mineralization profiles clearly showed that PVA in solution was extensively mineralized (34%), whereas when adsorbed on montmorillonite, only 4% mineralization was achieved within 1 month of incubation (Figure 5.17), thus indicating that PVA adsorption on inorganic substrates effectively inhibits the biodegradation processes.101

Based on these results, the dependence of mineralization rate on the HD and DPn has been further evaluated in soil-burial respirometric experiments by using three different commercial PVA samples having 72, 88, and 98% HD, respectively, and corresponding molecular weight of 25.0, 7.2, and 93.5 kDa. Very limited mineralization was observed in the case of the PVA sample having the highest molecular weight (MW) and HD, whereas lower HD and in particular DPn have been shown to promote the biodegradation propensity of PVA in soil (Figure 5.18).102,103 The results achieved with these studies indicate different biodegradation behaviors of PVA in aqueous and in solid-state tests reproducing different environmental conditions, as well as the role exerted by PVA structural parameters such as molecular weight and degree of hydrolysis.

To clarify this point, different PVA samples having similar degree of polymerization and noticeably different hydrolysis degrees (ranging between 11 and 75%) were prepared by controlled acetylation of a commercial-grade sample (HD = 98%)

Incubation time (days)

FIGURE 5.17 Time profiles of mineralization of soluble and clay-adsorbed PVA in an aqueous medium in the presence of acclimated PVA-degrading microorganisms.

Incubation time (days)

FIGURE 5.17 Time profiles of mineralization of soluble and clay-adsorbed PVA in an aqueous medium in the presence of acclimated PVA-degrading microorganisms.

Predicting Height From Bone Age
FIGURE 5.18 Time profiles of mineralization of PVA samples having different degree of hydrolysis in an aqueous medium in the presence of acclimated PVA-degrading microorganisms.

Incubation time (days)

FIGURE 5.19 Time profiles of mineralization of re-acetylated PVA samples in a soil burial respirometric test.

Incubation time (days)

FIGURE 5.19 Time profiles of mineralization of re-acetylated PVA samples in a soil burial respirometric test.

and submitted to biodegradation respirometric experiments under aqueous, mature compost, as well as soil incubation media.103

In a soil-burial respirometric test, the biodegradation extents of four different PVA (PVA11, PVA30, PVA50, PVA75) samples, obtained by the controlled reacetylation of PVA99, in comparison with the biodegradation behavior of PVA99 (representing the polymeric substrate utilized in the reacetylation reactions) were assessed. Reacety-lated PVA samples having a molar content of vinyl acetate units ranging between 24 and 73%, PVA75, PVA50, and PVA30 showed comparable biodegradation profiles, reaching fairly high mineralization extents (53 to 61%) at the end of the test (Figure 5.19). On the contrary, the almost completely hydrolyzed sample (PVA99), as well as the sample having the highest content of vinyl acetate units (PVA11) (Figure 5.19) did not undergo any significant microbial degradation after approximately 2 years of incubation time.103 A similar behavior, characterized however by lower mineralization extents, was observed in the presence of the same PVA samples when tested in a series of respirometric trials in mature compost (Figure 5.20).

For comparison, reacetylated PVA samples were also submitted to a biodegradation test in aqueous medium in the presence of selected microorganisms. As expected, they dissolved in the aqueous medium at a rate depending upon the degree of hydrolysis. Accordingly, PVA75 was found to be readily soluble in cool water. PVA50 was shown to disintegrate and partially solubilize, whereas PVA30 proved to be swellable and PVA11 was almost completely insoluble.

For comparison PVA99 was also tested both as an insoluble film and as a water solution (attained after heating at 90°C). Under these conditions, soluble PVA75 and PVA99 samples were promptly mineralized to a similar extent by the selected microbial inoculum (Figure 5.21). Lower but appreciable biodegradation degrees

FIGURE 5.20 Time profiles of mineralization of re-acetylated PVA samples in a mature compost respirometric test.

-

-■- Soluble PVA99 insoluble PVA99 PVA75 PVA50 PVA30 --PVA11

4

-

; / ■■ /

r

i

■y

^1 ■ *" ~

, 1<1<1<1<1<1<

10 20 30 40 50 60 70 Incubation time (days)

10 20 30 40 50 60 70 Incubation time (days)

FIGURE 5.21 Time profiles of mineralization of re-acetylated PVA samples in an aqueous medium in the presence of acclimated PVA-degrading microorganisms.

were recorded in the cultures fed with PVA30 and PVA50, but only after a prolonged lag phase, whereas the insoluble reacetylated PVA sample having the highest content of acetyl residues (PVA11) and insoluble PVA99 sample were almost completely recalcitrant to the microbial attack (Figure 5.21).

These data suggest that a certain grade of hydrophobicity achieved by a dominant content of vinyl acetate units in PVA samples may be taken as a practical parameter indicating the propensity of a PVA to be biodegraded in solid media. A similar influence was recognized for the degree of polymerization, resulting, as expected, in a higher mineralization degree for the sample with lower molecular weight.

An opposite trend was instead observed in the biodegradation tests carried out in aqueous medium in the presence of PVA-acclimated microorganisms. In these conditions, the driving force in the biodegradation of PVA appear to be its solubility, which makes the solvated chains vulnerable to the endocleavage of the polymer chains by specific enzymatic systems.

5.2.5 Oxo-Biodegradation of Polyethylene

Free-radical oxidation, as induced by thermal or photolytic preabiotic treatment, constitutes the first step for promoting the eventual biodegradation of both LDPE and LDPE-containing pro-oxidant additives (Figure 5.22). This can be accomplished by monitoring the initial variation of sample weight, molecular weight, and other structural parameters (tensile strength, degree of crystallinity, spectroscopic characteristics). Biodegradation is then observed when degraded oxidized polymer fragments are exposed to biotic environments.104-106

Additionally, the required degree of macromolecular breakdown for the microbial assimilation of polyethylene to occur is substantiated by the observation of the propensity to biodegradation of lower-molecular-weight hydrocarbon molecules. It has been demonstrated that linear hydrocarbon having molecular weight below 500107 or n-alkanes up to tetratetracontane (C44H90, MW = 618)108 can be utilized as carbon source by microorganisms. The degradation of higher-molecular-weight, untreated high-density poly(ethylene) with molecular weights up to 28,000 by a Penicillium simplicissimum isolate has also been reported,109 although the extent of fungal attack of polyethylene matrix was monitored only by physicochemical tools (FT-IR, HT-GPC) as well as by growth-proliferation assays in agar plates containing the polyolefin sample. No respirometric data have been reported.

Therefore, it has been generally accepted that the initial abiotic degradation step represents the major prerequisite in the induction of potential microbial assimilation of poly(ethylene). Many studies have been undertaken to obtain an insight into the mechanisms of radical oxidation of (PE).57,58,62,63 On the other hand, relatively little information on the influence of physical parameters such as humidity, oxygen pressure, as well as the whole biological environmental conditions on the propensity of thermal oxidation of degradable PE films are available.110

Accordingly, the oxidation propensity of PE samples from EPI Inc. (Canada) containing TDPATM pro-oxidant additives, as induced by heat, has been assayed in

O2 OH

Oxo-Biodegradable Fragments

Parameters to be monitored: 1. Weight increase;

2. Carbonyl index;

3. Wettability;

4. Molecular weight;

5. Fractionation by solvent extraction

HO CH2

FIGURE 5.22 Mechanism of radical oxidation of polyethylene.

oven at different temperatures (55 and 70°C) mimicking the thermophylic conditions of the composting process. The influence of the vapor pressure of water has been also investigated by comparing the thermal aging under dry conditions and in atmosphere conditioned at approximately 75% RH.

As the carbonyl group is representative of most of the oxidation products (carboxylic acids, ketones, aldehydes, and lactones)105 of the oxidative degradation of polyethylene, the concentration of carbonyl groups, as determined by the carbonyl index (COO, can be used to monitor the progress of oxygen uptake and degradation.111 COi determinations have been therefore utilized to compare the thermal oxidation behavior of test samples under dry and 75% RH conditions at 55 and 70°C.

The oxidation of the poly(ethylene) matrix in the thermally aged samples was clearly confirmed by FT-IR (Fourier-transform infrared) spectroscopy. The increasing absorption and broadening within time of the band in the carbonyl region was recorded in all samples aged at 70°C (Figure 5.23). Overlapping bands corresponding to acids (1712 cm-1), ketones (1723 cm-1), aldehydes (1730 cm-1), and lactones (1780 cm-1)112 were also observed, thus indicating the presence of different oxidized species (Figure 5.23). Among these, carboxylic acids and ester groups have been

LDPE-DCP540, T 70°C

7 10

1900

1800 1700

Wave number (cm-1)

1600

FIGURE 5.23 Time variation of carbonyl absorption band of LDPE sample containing pro-oxidant additives thermally treated in air in oven at 70°C.

shown to be produced in the early and later stages of polymer matrix oxidation, respectively.113 The increase of specimen weight up to 4 to 5% of the original value, as well as the increase of wettability of the thermally treated films, as determined by contact-angle measurements, further demonstrated the polymer oxidation (Figure 5.24 and Figure 5.25).

COi profiles recorded at 55 and 70°C under dry and 75% RH condition showed that the predominant effect on the oxidation kinetics is the test temperature (Figure 5.26 and Figure 5.27). The high humidity level, comparable with that occurring under real environmental conditions (e.g., composting), was only influencing, in some cases, the rate but not the overall extent of oxidation (Figure 5.26 and Figure 5.27).114

Thermally treated samples were also characterized by high-temperature gel permeation chromatography (HT-GPC) , and the recorded molecular weight and molecular weight distribution (ID) were compared with the COi values detectable at the same time of thermal aging (Table 5.12, Table 5.13, and Table 5.14).114 A drastic decrease of molecular weight below 5 kDa and significant reduction of the ID were recorded after a few days of thermal degradation under both dry and 75% RH conditions. In HT-GPC chromatograms relevant to LDPE samples retrieved at the longest thermal degradation period, and thus having the highest level of oxidation, the elution peaks showed a bimodal shape, and very low (1.7 kDa) molecular-weight fractions (Figure 5.28) were detected.

In addition, the relationship between the molecular weight and COi has been found to fit a mono-exponential trend (Figure 5.29). Accordingly, COi values can be used to predict the rate of MW decrease as a function of the oxidation extent. Moreover, the recorded trend is in agreement with a statistical chain-scission mechanism repeatedly suggested in the photophysical and thermal degradation of polyolefin.57, 58, 62

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Time (days)

FIGURE 5.24 Weight variation profile of LDPE samples containing pro-oxidant additives thermally treated in air in oven at 70°C.

LDPE-DCP540 FCB-ZSK10 FCB-ZSK15

□ Day 0 DDay4 DDay7 DDay 10 DDay 15

FIGURE 5.25 Contact angle determinations with distilled water as wetting agent of LDPE samples containing different EPI-TDPA pro-oxidant additives thermally treated in air in oven at 70°C.

FIGURE 5.26 Carbonyl index (COi) variation of LDPE-DCP54O film sample aged in oven at 55 (a) and 70°C (b), under both dry and 75% RH atmosphere.
Time (days)
FIGURE 5.27 Carbonyl index (COi) variation of FCB-ZSK film sample aged in oven at 55 (a) and 70°C (b), under both dry and 75% RH atmosphere.

TABLE 5.12

Molecular Weight Analysis by HT-GPC of LDPE-DCP540 Sample Thermally Treated at 70°C

Test Condition

TABLE 5.12

Molecular Weight Analysis by HT-GPC of LDPE-DCP540 Sample Thermally Treated at 70°C

Time

MW

Time

MW

(days)

COi a

(kDa)

ID b

(days)

COi a

(kDa)

ID b

0

0.61

39.4

4.24

2

1.60

10.7

2.92

1

1.14

19.5

2.96

3

2.23

10.4

2.88

2

2.32

9.7

2.59

9

5.37

4.8

1.37

9

5.44

4.5

1.27

N/A

N/A

N/A

N/A

a Carbonyl index as DB 1720/DB 1435. b Dispersity index.

TABLE 5.13

Molecular Weight Analysis by HT-GPC of FCB-ZSK10 Sample Thermally Treated at 70°C

Test Condition

TABLE 5.13

Molecular Weight Analysis by HT-GPC of FCB-ZSK10 Sample Thermally Treated at 70°C

Test Condition

Dry (open air)

RH

= 75%

Time

MW

Time

MW

(days)

COi a

(kDa)

ID b

(days)

COi a

(kDa)

ID b

1

0.63

45.7

3.94

2

0.57

28.6

3.63

2

1.40

16.4

2.75

6

2.54

9.9

2.59

3

2.33

10.1

2.47

11

6.20

4.9

1.33

9

5.35

4.4

1.25

N/A

N/A

N/A

N/A

Carbonyl index as DB

1720/DB 1435.

b Dispersity index.

TABLE 5.14

Molecular Weight Analysis by HT-GPC of FCB-ZSK15 Sample Thermally Treated at 70°C

Test Condition

TABLE 5.14

Molecular Weight Analysis by HT-GPC of FCB-ZSK15 Sample Thermally Treated at 70°C

Time

MW

Time

MW

(days)

COi a

(kDa)

ID b

(days)

COi a

(kDa)

ID b

1

0.67

34.4

3.66

3

1.49

18.1

3.53

3

1.59

14.9

2.91

5

2.89

8.1

2.68

5

2.83

7.6

2.44

11

6.20

4.2

1.44

6

4.53

5.1

1.32

N/A

N/A

N/A

N/A

a Carbonyl index as DB 1720/DB 1435. b Dispersity index.

FIGURE 5.28 HT-GPC chromatograms of LDPE samples containing pro-oxidant additives thermally treated in air in oven at 70°C.
Weight Fraction Molecular Weight
FIGURE 5.29 Molecular weight vs Carbonyl index (COi) relationship in LDPE-DCP54O film sample thermally treated in air in oven at 70°C.

The amount of oxidized degradation intermediates extractable by acetone, which can be considered to be more likely to be diffused in the environment, has also been shown to be positively correlated to the level of oxidation. The amount of degradation intermediates is fairly high, corresponding to 25 to 30% of specimen weight (Table 5.15).

TABLE 5.15

Acetone-Extractable Fractions from Original and Thermally Treated LDPE Samples

TABLE 5.15

Acetone-Extractable Fractions from Original and Thermally Treated LDPE Samples

Aging Time (days)

Extract

Residue

Sample

70°C

55°C

COi a

(%)

(%)

FCB-ZSK10

0 b

0 b

0.534

7.7

91.9

4

0.453

6.5

92.6

42

3.583

17.9

82.2

24

6.816

27.1

72.7

FCB-ZSK15

0 b

0 b

0.212

5.9

93.9

5

2.864

9.2

88.4

42

5.193

23.8

75.6

24

7.256

22.6

74.4

LDPE-DCP540

0 c

0 c

0.627

5.5

94.2

4

2.243

11.3

88.5

42

4.818

21.1

78.2

24

5.441

27.7

71.9

a Carbonyl index as DB 1720/DB 1435. b Six months storage at room temperature. c Seven months storage at room temperature.

TABLE 5.16

Molecular-Weight Analysis of Acetone-Extractable Fractions from Original and Thermally Aged LDPE Samples

TABLE 5.16

Molecular-Weight Analysis of Acetone-Extractable Fractions from Original and Thermally Aged LDPE Samples

Extract

MW

Sample

Treatment

COi a

(% weight)

(kDa)

ID

FCB-ZSK10

None b

0.534

7.7

1.52

1.49

4 days at 70°C

0.453

6.5

1.47

1.46

42 days at 55°C

3.583

17.9

1.30

1.39

24 days at 70°C

6.816

27.1

0.92

1.32

FCB-ZSK15

None b

0.212

5.9

1.58

1.46

5 days at 70°C

2.864

9.2

1.67

1.52

42 days at 55°C

5.193

23.8

1.27

1.43

24 days at 70°C

7.256

22.6

1.03

1.36

LDPE-DCP540

None c

0.627

5.5

1.08

1.27

4 days at 70°C

2.243

11.3

1.49

1.41

42 days at 55°C

4.818

21.1

1.08

1.37

24 days at 70°C

5.441

27.7

0.89

1.33

a Carbonyl index as DB

1720/DB 1435.

b Six months storage at room temperature. c Seven months storage at room temperature.

b Six months storage at room temperature. c Seven months storage at room temperature.

It is also presumed that the acetone extraction selected almost exclusively for polar

(e.g., oxidized) degradation intermediates, whereas hydrophobic low-molecular-weight compounds might not be extracted by this polar solvent. Therefore the recorded data have to be considered as partially representative of the overall amount of low-molecular-weight fractions produced during the thermal oxidation of the test material.

Molecular weights as low as 0.8 to 1.6 kDa for the acetone extracts of thermally treated samples were recorded by SEC (Table 5.16). It is also worth noting that at higher levels of oxidation, increasing amounts of the extractable fractions were recorded, and these fractions are characterized by very low molecular weight (Table

5.16).114 Therefore, the recorded data suggest that cross-linking reactions do not notably affect the oxo-degradation behavior of the analyzed samples, which seems to proceed with cleavage of the macromolecules up to low-molecular-weight fractions capable of being assimilated by microorganisms.104 107 115 Indeed, earlier studies have demonstrated the microbial utilization, as a carbon source, of the oxidation products formed during the thermal and photo-oxidation of polyethylene.116117 Accordingly, in a respirometric experiment, designed to evaluate the biodegradation behavior of low-molar-mass aliphatic hydrocarbons in soil, including docosane (C22), branched hexamethyltetracosane (C30), and a,ra docosandioic acid (C22), a fairly high rate and extent of mineralization (70%) of the acetone extractable fraction from a thermally oxidized LDPE-TDPA sample was recorded (Figure 5.30).118 This suggests that, similarly to low-molar-mass hydrocarbons, the oxidized fragments from LDPE can be rapidly biodegraded in natural soil.

Nevertheless, the ultimate biodegradation propensity of the whole thermally treated LDPE-TDPA samples is a fundamental issue that has to be assessed.

FIGURE 5.30 Mineralization profile of carbon substrates in soil burial respirometric tests. Cellulose = pure Whatman cellulose; DOC = docosane; SQUA = squalane; DAD = c§w docosandioic acid; QAE = acetone extractable fraction from a thermally oxidized LDPE-TDPA sample.

Thermally treated LDPE samples containing TDPA pro-oxidant additives from EPI Environmental Products Inc. (Canada) were therefore assayed in respirometric tests aimed at simulating soil and mature compost incubation media.

Several studies that have evaluated the biodegradation of LDPE samples containing pro-oxidant and natural fillers have shown only limited and slow conversion to carbon dioxide. The mineralization rate from long-term biodegradation experiments of UV-irradiated samples,119 nonpretreated samples, and additive-free LDPE samples in natural soils indicates more than 100 years for the ultimate mineralization of polyethylene.120

In a soil-burial test, carried out in the presence of forest soil as incubation media, the biodegradation profile of a thermally pretreated LDPE-TDPA sample was monitored during 830 days of incubation.118 The recorded mineralization kinetics (Figure 5.31) were characterized by the presence of a first exponential step that approached a 4% degree of biodegradation within 30 days of incubation. Afterward, a prolonged (120 days) stasis in the microbial respiration was recorded. At approximately 5 months of incubation it was noted that a further and marked exponential increase in the biodegradation profile took place, thus reaching the highest extent of biodegradation (63.0%) after 85 weeks of incubation (Figure 5.31). This two-step mineralization profile of thermally fragmented LDPE-TDPA samples has been repeatedly observed in both soil and mature compost biodegradation tests (Figure 5.32). In contrast to previous studies,119, 120 very large degrees of mineralization (70 to 80%) have been then recorded, although these were obtained over a relatively long time frame (more than 800 days).114

FIGURE 5.31 Mineralization profile LDPE-TDPA samples in soil burial respirometric tests. Q = LDPE sample containing a pro-oxidant additive, aged in an air oven at 70°C for 14 days prior to testing; Q-RE = residue of the aged Q-LDPE sample after extraction of the fraction (ca 25 wt%) soluble in refluxing acetone.

Incubation time (days)

FIGURE 5.31 Mineralization profile LDPE-TDPA samples in soil burial respirometric tests. Q = LDPE sample containing a pro-oxidant additive, aged in an air oven at 70°C for 14 days prior to testing; Q-RE = residue of the aged Q-LDPE sample after extraction of the fraction (ca 25 wt%) soluble in refluxing acetone.

FIGURE 5.32 Mineralization profile LDPE-TDPA thermally oxidized sample in mature compost respirometric tests.

Incubation time (days)

FIGURE 5.32 Mineralization profile LDPE-TDPA thermally oxidized sample in mature compost respirometric tests.

The first step in the biodegradation kinetics of thermally degraded LDPE in soil could be attributed to the preferential microbial assimilation of low-molecular-weight oxidized fragments present on the surface of the LDPE specimens, as previously suggested.117121-123 Albertsson and coworkers105 reported that abiotically aged pure LDPE, LDPE/starch, and LDPE-additives promoting degradation were characterized by the presence of several degradation products such as mono- and dicarboxylic acids and ketoacids. These almost completely disappeared after the incubation of the polymer samples in the presence of Arthrobacter paraffineus as a consequence of the assimilation of the degradation products by the bacterial strain.

This hypothesis appears to be confirmed by the FT-IR characterization of soil-buried specimens retrieved after a few months of incubation. Indeed, a significant reduction of the intensity of the carbonyl absorption band with respect to the original value has been repeatedly recorded. This result has also been validated by SEM microanalysis that revealed an appreciable reduction of the oxygen from 20.6 to 16.9% by weight in the surface elemental composition of a soil-incubated sample.118

In contrast, a substantial increase in the carbon-carbon unsaturation, as revealed by the increase of the double-bond index from 0.55 at the beginning to 0.88 after 62 weeks of incubation, was recorded. This can be attributed to enzymatic dehydrogenation.105,120 Hence it is likely that the macromolecular cleavage of thermally oxidized LDPE-TDPA can be achieved concomitantly by abiotic oxidation and biotic, enzymatic scission. Furthermore, a dramatic change in the fingerprint region of the IR spectrum between 1300 and 950 cm-1 was observed with increasing incubation time, probably attributable to the presence of lower-molecular-weight fragments.

Therefore, it seems that the ongoing abiotic and biotic degradation of thermally degraded LDPE-TDPA polymer bulk occurs during the incubation in forest soil, with the production of a large amount of degradation intermediates capable of being assimilated by the soil microflora. This might explain the prolonged period of stasis as well as the second, more pronounced exponential phase repeatedly observed in the biodegradation kinetics of these materials in soil-burial respirometric tests.

5.2.6 Microbial Cell Biomass Production during the Biodegradation of Polyvinylics

The increasing development of biodegradable plastic items introduced in the late 1980s has prompted government authorities and decision makers to issue — through technical standardization bodies — standard norms and laboratory test methods to assess the ultimate environmental behavior of plastics. All of these indicated procedures are stating that the biodegradation of a test material, under aerobic conditions, has to be determined as the extent of carbon substrate evolved to carbon dioxide as a consequence of microbial attack. The amount of carbon dioxide evolved is quantitatively compared with the corresponding value achieved by cellulose taken as positive control, under the same operative conditions and time frame. It must be taken into consideration, however, that under aerobic conditions, heterotrophic soil microbes metabolize carbon substrates both as carbon dioxide and for the production of new biomass, while some of it is converted, through chemo-enzymatic reactions, to humic substances. It is generally accepted that over a relatively short term, 50% of the carbon content of most organic substrates is converted to CO2, the remaining part being assimilated as biomass or humified.

The relationship between the structural features of low-molar-mass carbon substrates and the growth efficiency of microorganisms incubated on them has also been thoroughly investigated by thermodynamic studies. In particular, relatively simple, closed systems, such as those represented by liquid cultures of certain microorganisms fed with specific carbon sources, have been assessed. In aqueous media, heterotrophic bacteria utilize the organic substrate both as carbon and energy source, i.e., the input carbon is driven into the biosynthetic pathway by anabolism and can as well be utilized for energetic demand through catabolism.

Metabolic efficiency is therefore strictly correlated to the cellular metabolic network that includes interrelated catabolic and anabolic pathways, especially under substrate-limited conditions,124-126 with the carbon being converted to biomass or the growth yield proportional to the anabolic activity. For instance, it has been repeatedly established that anabolic processes, and consequently growth efficiency, are positively correlated with the standard free energy of oxidation (AG°) of a particular carbon substrate.127-131 This correlation has recently been confirmed by studying the ratio between organic carbon channeled into biomass and that converted to CO2 under substrate-limited cultures when fed with different types of organic substrates.132

In the presence of carbon substrate characterized by a relatively low free-energy content, a large portion of carbon can be converted to CO2, and the energy liberated from catabolic reactions is not substantially utilized for cell growth and proliferation, but it is rather used to sustain the living activities of bacterial cells. Therefore, in the presence of low-free-energy (e.g., oxidized substrates) carbon sources, large amounts of carbon appear to be utilized for the maintenance functions, thus providing great production of carbon dioxide (e.g., high SCO2/SB ratio) rather than being consumed for growth purposes (e.g., low Ys and hence limited cell growth) (see Table 5.17).

The role of free-energy content of carbon substrate, which is repeatedly evidenced in the metabolic efficiency of heterotrophic microorganisms in aqueous media and under substrate-limiting conditions, could also be reasonably applied to the transformation of organic substances by microbes in soil. The substrate carbon channeling between the anabolic and catabolic pathways was evaluated by determining the gross cell growth yield (GY) in the aqueous medium, as well as the net soil microbial biomass (SMB-C), as a function of the substrate addition in soil-burial respirometric tests. The collected data were then analyzed against the amounts of carbon converted to CO2 (mineralization).

The GY was determined by gravimetric analysis of dry cell biomass divided by the amount of organic carbon in the substrate submitted to biodegradation test. Similarly, when the liquid cultures approached the plateau phase, they were submitted to centrifugation, and the resulting pellets washed with distilled water and dried at 105°C up to constant weight. In soil burial respirometric tests, the organic C fixed in soil microbial biomass (SMB-C) was estimated at the end of the tests by applying chloro-fumigation, followed by a K2SO4 extraction procedure reported by Turner et al.132

TABLE 5.17

Growth Yield (Ys) and SCO2/SB Ratio in Substrate-Limited Cultures of Activated Sludge

TABLE 5.17

Growth Yield (Ys) and SCO2/SB Ratio in Substrate-Limited Cultures of Activated Sludge

Substrate

Formal C-Oxidation Number

(kJg-1)

Ys b

SCO2/S,

Carbohydrates

Glucose

-0.16

-15.6

0.80

0.25

Fructose

-0.16

-15.6

0.71

0.41

Sucrose

-0.16

-16.5

0.72

0.40

Lactose

-0.16

-16.5

0.71

0.41

Xylose

-0.16

-15.6

0.63

0.59

Amino acids

Glycine

+1

-12.9

0.38

1.67

Alanine

0

-18.2

0.53

0.88

Glutamic acid

+0.4

-13.5 d

0.48

1.08

Phenylalanine

-0.25

-28.1

0.55

0.82

Fatty acids

Acetic acid

0

-14.6

0.53

0.90

Propionic acid

-0.6

-20.6

0.63

0.58

Butyric acid

-1

-24.8

0.84

0.19

Miscellaneous

Butanol

-1.75

-36.1

0.78

0.29

Benzoic acid

+0.3

-26.4

0.46

1.19

a Values taken from Handbook of Chemistry and Physics, 85th ed., CRC Press, Boca Raton, FL, 2004.

b Determined as weight of carbon incorporated in cell biomass/overall weight carbon in substrate

c Ratio of carbon evolved as CO2 (SCO2) to that converted to biomass (SB) (mg C/mg C). d AG0 of oxidation.

a Values taken from Handbook of Chemistry and Physics, 85th ed., CRC Press, Boca Raton, FL, 2004.

b Determined as weight of carbon incorporated in cell biomass/overall weight carbon in substrate

c Ratio of carbon evolved as CO2 (SCO2) to that converted to biomass (SB) (mg C/mg C). d AG0 of oxidation.

The net CO2 emission (e.g., mineralization) and the GY associated with the metabolization of a water-soluble polymeric material, such as PVA grade 99 and various low-molecular-weight compounds, were evaluated in respirometric biodegradation experiments carried out in a mineral liquid medium (0.4 to 0.6 g substrate/l) under substrate-limited condition in the presence of a selected micro-bial inoculum.133 In particular, the metabolization patterns of aliphatic alcohols such as 1-decanol and 2-octanol were compared with the carbon utilization profiles of compounds with lower free-energy content, i.e., with carbon in an average higher oxidation state, such as 3-hydroxybutyric acid and 2,4-pentanediol. The percentage of theoretical cumulative net CO2 emissions of tested substances are reported in Figure 5.33. 3-Hydroxybutyric acid, with a 0.5 formal average oxidation number of carbon, was shown to be easily metabolized, without a significant lag-phase, throughout an extensive catabolic conversion to CO2. In contrast, the other substrates required a longer time (6 to 10 days) before the onset of the mineralization process.

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