Pyrolytic study of compost and waste organic matter

Organic Geochemistry Organic Geochemistry 36 (2005) 1054–1071 www.elsevier.com/locate/orggeochem

Pyrolytic study of compost and waste organic matter M.-F. Dignac a

a,*

, S. Houot

b,c

, C. Francou b, S. Derenne

d

Laboratoire de Bioge´ochime des Milieux Continentaux (BioMCo), INRA CNRS Universite´ Paris VI, 78850 Thiverval-Grignon, France b Environnement et Grandes Cultures, UMR INRA, INAP-G, 78850 Thiverval-Grignon, France c CREED, Zone portuaire de Limay, 291, Avenue Dreyfous Ducas, 78520 Limay, France d Laboratoire de Chimie Bioorganique et Organique Physique, ENSCP, 11 rue Pierre et Marie Curie, 75005 Paris, France Received 20 April 2004; accepted 23 February 2005 (returned to author for revision 15 September 2004) Available online 4 May 2005

Abstract The chemical composition of organic matter (OM) in various fresh and composted wastes was characterized using pyrolysis-GC/MS. Composts made from source-separated biowaste, green waste, sewage sludge co-composted with green waste and municipal solid waste were collected from several French plants after four months of composting. Fresh non-composted wastes were analyzed in order to specify the origin of the pyrolysis products. The composition of the pyrolysates varied with the composted waste. The main pyrolysis products of bio- and green waste composts originated from lignin and polysaccharides. In biowaste and municipal solid waste compost, most cellulose-related structures originating from paper and cardboard were degraded after four months of composting. The presence of styrene di- and trimers in the pyrolysate of municipal solid waste compost was in agreement with the occurrence of plastics in this compost. Fatty acids were abundant in the pyrolysis products of sewage sludge and municipal solid waste compost, where they probably originated from greasy waste. Specific ratios of peak areas from pyrolysis products were calculated in order to describe the humification of OM, to confirm the presence of synthetic polymers, to evaluate the contribution of cellulose amongst polysaccharides and to assess the origin and degradation degree of lignin. Ó 2005 Elsevier Ltd. All rights reserved.

1. Introduction Soil organic matter (SOM) plays a major role in the chemical, microbiological and physical aspects of soil fertility. In France, up to 2.7 millions ha of loamy soils are poor in organic matter (OM) and are at risk of erosion and hence of loss of fertility (Le Villio et al., 2001). Organic wastes transformed into humified organic mate-

* Corresponding author. Tel.: +33 130 815 281; fax: +33 130 815 497. E-mail address: [email protected] (M.-F. Dignac).

rial by composting could be used as organic amendments to increase SOM content and improve soil fertility. Only 7% of the 47 million of tons of urban wastes are composted in France at present, although 50% are organic waste that could be composted. New policy and political decisions could favour the development of composting for treating biodegradable waste. The quality of compost OM varies with the nature of the composted waste (Ayuso et al., 1996) and composting process (Serra-Wittling et al., 1996). The main types of urban waste that are composted include biowaste, green waste, municipal solid waste and an increasing contribution of sewage sludge from wastewater

0146-6380/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2005.02.007

M.-F. Dignac et al. / Organic Geochemistry 36 (2005) 1054–1071

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The aim of this study was to improve knowledge of the chemical composition of compost OM, to relate the properties to the composted waste and ultimately to define chemical indicators for compost OM stability. We used pyrolysis-GC/MS, since this technique has been shown suitable for ascertaining the chemical composition of organic waste and the changes resulting from the composting process (Sere´s-Aspax et al., 1985; Garcı´a et al., 1992; Gonza´lez-Vila et al., 2001). Compost samples with contrasting features were collected after four months of composting, during the maturation phase, from several composting plants in France. Non-composted waste, including vegetables and fruits, paper and cardboard, and fresh green waste were also analyzed in order to assign more precisely the biochemical sources of the pyrolysis products observed for the composted waste using pyrolysis-GC/ MS, and to determine precisely the changes in the chemical composition of OM from composting by comparing the relative amounts of some characteristic pyrolysis products in waste and compost.

treatment plants, often co-composted with green waste. Like SOM, compost OM is highly heterogeneous. Depending on its nature, composted waste is made up of different organic materials (cellulose and non-cellulose polysaccharides, proteins, lipids, lignin and plastics) that are partly degraded during composting (Golueke, 1981), which may be divided into four phases (De Bertoldi et al., 1983). In the first mesophilic phase, a high concentration of easily biodegradable OM leads to intensive bacterial activity and an increase in the compost temperature. The temperature in the composting windrow remains above 60 °C during the thermophilic phase. Both the high temperature and the consumption of easily biodegradable OM result in a decrease in microbial activity and temperature during the cooling phase. During the last maturation phase, the humification of compost OM occurs at ambient temperature. The duration of the composting phases depends on the nature of the OM being composted and the efficiency of the process, which is controlled by the aeration, turning and wetting frequency of the compost windrows (Golueke, 1981). The effects of compost on soil properties depend on its OM stability which is defined as the resistance to biodegradation. Stabilized compost may be of high value for soil fertilization (Inbar et al., 1993), while nonstabilized compost may have a negative effect on N availability to crops and on compost storage and use (Mathur et al., 1993). This stability can be used as an indicator of compost maturity in addition to other tests proposed to control compost maturity (Mathur et al., 1993;Francou et al., 2005). Most studies on compost characterization focus on the quantitative variation in the organic and inorganic constituents. The chemical nature of compost OM changes throughout the composting process and can be a useful indication of compost stabilization (Chen, 2003). Only a few studies describe the chemical composition of composts using molecular (Miikki et al., 1994; Gonza´lez-Vila et al., 1999; Pichler and Ko¨gel-Knabner, 2000; Dinel et al., 2001) or spectroscopic techniques (Miikki et al., 1997; Chefetz et al., 1998; Siebert et al., 1998; Pichler et al., 2000).

2. Materials and methods 2.1. Sample collection and preparation Compost samples were collected from industrial composting plants, after four months of composting, during the maturation phase (Table 1). Biowaste (BW) compost was obtained from the composting of the biodegradable fraction of municipal waste, collected separately, and mixed before composting with 9–21% of paper and cardboard, and 37–59% of fresh green waste (GW;Table 1). Paper and green waste are added before composting of biowaste as bulking agents to retain the high moisture present in the biowaste and to allow enough porosity for satisfactory aeration during composting. One BW compost (BW2) also contained a small amount (5%) of plastic. Green waste (GW) compost was made from green waste collected selectively from

Table 1 Description of samples: composting process (slow or enforced aeration) and composition of composted waste

BW1 BW2 BW3 GW1 GW2 GWSS1 GWSS2 MSW

Composting process

GW

BW

Paper %

Plastics

slow ea ea ea slow ea slow ea

37 59 58 100 100 70 71 17

54 19 14

9 14 21

5

slow: slow composting. ea: enforced aeration.

Sewage sludge

30 29 25

25

6

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M.-F. Dignac et al. / Organic Geochemistry 36 (2005) 1054–1071

private and municipal gardens, including grasses, branches and leaves of trees. Two composts of sewage sludge co-composted with green waste (GWSS), a developing approach for sludge treatment, were included. They contained about 70% green waste and 30% sludge (Table 1). GW2 and GWSS2 were sampled from the same plant and were obtained using the same composting process and the same green waste, composted alone for GW2 and used as a co-substrate for composting sewage sludge for GWSS2. The municipal solid waste (MSW) compost comprised residual municipal solid waste after selective collection of recyclable material (paper and cardboard, glass, plastic and metal containers). This compost contained 17% green waste, equal proportion of biowaste and paper (25%) and 6% of plastics (Table 1). BW1, GW2 and GWSS2 were composted by way of a slow process, while BW2, BW3, GWSS1 and MSW underwent accelerated composting (enforced aeration of the windrow during the degradation phase). After sampling, fractions <10 mm were sieved and air dried. Samples were ground to <1 mm before analysis. Samples of fresh vegetables and fruit (salad, zucchini, carrots, oranges) were prepared by cutting the products into pieces, freeze drying and grounding. A mixture of non-composted green wastes was also dried and ground. Additionally, a mixture of paper and cardboard was prepared and ground. 2.2. Elemental composition The C and N contents of the composts were determined by the Analysis Department of INRA (Arras, France) using an elemental analyzer. The organic carbon content was measured using the dry combustion method (ISO 10694). The CO2 released by oxidation of total carbon was quantified by way of catharometric detection. Treatment of the sample with hydrochloric acid was carried out prior to analysis in order to remove carbonate, so that the measured CO2 corresponded to organic carbon. Total nitrogen was measured using the dry combustion method (ISO 13878), after oxidation of nitrogen at 900 °C and quantification using thermal conductivity. 2.3. Soil-compost incubation The kinetics of waste and compost organic matter mineralization were followed during laboratory incubations of soil-waste/compost mixtures. Each compost was incubated separately. For fresh waste, paper and green waste were incubated separately, while a mixture of salad, zucchini, carrots and oranges was used to study the mineralization of vegetables. The soil was sampled from the upper layer (0–28 cm) of a loamy soil with the following characteristics: 187 g kg 1 clay, 756 g kg 1 silt,

57 g kg 1 sand, 11.6 g kg 1 organic carbon, 1.19 g kg 1 total N and had a pH of 6.9. The soil sample was sieved to 5 mm. Four replicates of soil-waste/compost mixtures were incubated for 28 days in the dark, at 28 ± 1 °C, in 0.5 L jars which were hermetically closed; 25 g of dry soil was mixed with dried and ground waste or compost. The doses of incorporated waste or compost were calculated to bring the same proportion of organic carbon to the mixture (4 g of organic carbon per kg of dry soil). The water content of the mixtures was adjusted to 80% of the soil water-holding capacity (19.1% on wet basis) with Milli-Q water and controlled during the incubation period. Control incubations were run with soil alone. In each jar, CO2 released from carbon mineralization was trapped in 10 ml of 1 M NaOH replaced after 1, 3, 7, 10, 14, 21 and 28 days of incubation and analyzed using a colorimetric method with a continuous flow analyzer (Skalar, The Netherlands). The opening of the jar when replacing the CO2 traps made it possible to run the incubations under aerobic conditions.

2.4. Pyrolysis-GC/MS Approximately 0.5–1 mg of sample was loaded on to tubular ferromagnetic wires which were inductively heated to their Curie temperature of 650 °C in 0.15 s (10 s hold) with a pyrolysis unit (GSG Curie-Point Pyrolyser 1040 PSC). Pyrolysis products were separated using a Hewlett Packard HP-5890 gas chromatograph (GC) on a 60 m fused silica capillary column SolGelWax (SGE, 0.32 mm i.d., film thickness 0.5lm), with helium as carrier gas. A polar capillary column was chosen for GC separation to allow a better separation of polar compounds originating from sugars, proteins and lignin. The less polar compounds such as alkanes/alkenes are less easily observed using this column, but on an apolar column they represented relatively small peaks compared to the products originating from fresh organic matter (data not shown). The temperature programme of the GC oven was set with an increase from 30 to 280 °C at 2 °C/min, with the temperature maintained at 280 °C for 15 min. The GC was coupled to a Hewlett Packard HP-5889 mass spectrometer (electron energy 70 eV). Compounds were identified on the basis of their mass spectra, GC retention times, and comparison with library mass spectra. A number of pyrolysis products were identified for each sample, some of them being specific for a macromolecular source. Peaks were integrated using the HP MS Chemstation (Version C.01.05) and the total ion current trace. All pyrograms are displayed with toluene (compound No1) having the same peak height, except the pyrograms for paper and cardboard, where the toluene peak was smaller and was thus not used as reference peak in the figure.

M.-F. Dignac et al. / Organic Geochemistry 36 (2005) 1054–1071

3. Results and discussion 3.1. C and N contents The composts had organic C contents ranging from 15.4% to 28.8% dry weight and N contents from 1.3% to 2.5% dry weight (Table 2). Fresh wastes had higher C contents (25.1–41.4%) than the composted wastes and N contents varying from 0% to 4.0% (Table 2). Vegetables had the highest N contents amongst the fresh wastes, with C/N ratios of 9.4 and 12.3 for salad and zucchini, respectively. A C/N ratio of 30 for the initial mixture is considered optimal for ensuring that the composting process takes place satisfactorily (Golueke, 1981). Then, during composting, C/N ratios decrease to 10–15, as previously reported for fully stabilized compost (Golueke, 1981). As a result, the C/N ratio has been frequently used to assess compost maturity (Mathur et al., 1993). Among the composts, the C/N ratios varied from 11.3 to 15.2, without any clear relationship between the compost type and process. Enforced aeration should accelerate the C/N decrease and such a composting process would be expected to produce compost with lower C/N ratios, which is not the case here. Although the evolution of the C/N ratio probably makes it a possible parameter for following the process, iso-

Table 2 C and N contents (g kg and composted waste

1

C g kg Fresh wastes Salad Zucchinis Carrot Orange Paper Green wastes

dry weight) and C to N ratios of fresh N

1

C/N

Cmin/C (%)

DW

383.1 381.6 397.7 414.2 405.5 251.5

40.6 31.0 10.6 11.8 <0.1 16.1

9.4 12.3 37.4 35.1 nd 15.6

58a

Composted wastes BW1 243.0 BW2 154.9 BW3 174.6

18.2 12.6 12.8

13.3 12.3 13.6

5 2 5

GW1 GW2

207.1 195.6

13.6 16.4

15.2 11.9

5 7

GWSS1 GWSS2

288.9 203.3

24.9 17.7

11.6 11.5

7 4

MSW

243.4

21.6

11.3

14

15 22

nd: not determined. The proportion of C mineralized after 28 days of incubation with a soil (Cmin/C, %) is an indication of the maturity of compost OM. a Mean value for a mixture of salad, zucchini, carrots and orange.

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lated values measured on the final products cannot provide real information of the humification stage of OM of given composts. The stability of fresh and composted waste was estimated from the proportion of C mineralized after 28 days incubation with a soil sample (Table 2). This ranged from 15% to 58% for the fresh wastes and from 2% to 7% for the BW, GW and GWSS composts. The proportion of mineralized C was significantly larger for MSW compost (14%) than for the other composts, indicating the higher degradability of this compost. 3.2. Py-GC/MS 3.2.1. Fresh wastes In order to better relate the pyrolysis products of compost OM with their origin, some fresh wastes were analyzed first using Py-GC/MS. The wastes were representative of those found during the waste characterization of the composting plants (Table 1). Fig. 1 shows the pyrograms from some vegetables and fruits (salad, zucchini, carrots and oranges) that may enter the composition of BW and MSW composts. Fig. 2 shows the pyrograms for paper and cardboard (Fig. 2a), which are part of the composition of BW and MSW composts, and of fresh green waste (Fig. 2b), found in various amounts in all the composts. The main compounds identified in the pyrolysates were grouped into five major classes (Table 3): compounds originating from the pyrolysis of polysaccharides (PS), N-containing compounds (N), lignin-derived products (LIG), compounds with an isoprenoid structure and compounds derived from plastics. Compounds that could be formed from pyrolysis of several distinct biological sources were considered unspecific. 3.2.2. Fresh vegetables and fruits The main pyrolysis products from fresh vegetables and fruits (Fig. 1) were 1-hydroxypropan-2-one (PS1) and furanmethanol (PS12) along with toluene (1), acetic acid (4), phenol (5) and methylphenols (6; 7). The latter are ubiquitous pyrolysis products and hence are of unspecific origin. Indeed, toluene (1) and other alkylbenzenes (mainly C2-benzenes) as well as phenol (5) and methylphenols (6; 7) may originate from tyrosine-containing peptides and proteins (Tsuge and Matsubara, 1985; Chiavari and Galletti, 1992), as well as from lignins (Saiz-Jimenez and De Leeuw, 1986), tannins (Galletti and Reeves, 1992) and polysaccharides (Pouwels et al., 1987). Acetic acid (4) also has a ubiquitous origin, as it can be formed for instance by cleavage of acetyl groups of hemicelluloses (Pouwels et al., 1987), or from pyrolysis of synthetic polymers such as poly(vinyl acetate; Fabbri, 2001). In contrast, 1-hydroxypropan-2-one (PS1) and furanmethanol (PS12) are known pyrolysis products of

M.-F. Dignac et al. / Organic Geochemistry 36 (2005) 1054–1071 PS1

1058

PS12

8

C16

10

C14

N17

7

PS22 N14

N15

CHL2 N12

6 N13

2

5

PS14 N11 CHL1 PS16

N4 N5 N

PS2 PS3

PS5 PS6 N7 PS7 N8 N9 PS9 PS10 N10 PS11

1

PS20

N1

4

PS1

PS12

(a)

8

10

N17

7

C16

N15 PS22 N14

N13

PS16

2

6

N12 CHL2

PS6

PS3

N11

PS2 N4 N5

N3

PS7 N8 PS9 PS10 PS11

N7

PS5

5

1

N1

PS20

4

PS12

PS1

(b)

5

PS23

PS16

C16

2

8

10

PS20

PS14

PS11

6

PS18

N5

PS9

PS5

PS6

PS2 PS3

+ N4

N1

7

N7 PS8

1

PS7

4

PS23

PS20

LIG1-G

9 6

C18

C16

C14

8

PS22

LIG3-G

7

PS18

2

5

PS16

LIM

PS14

4

PS3

1

PS5 PS6 PS7 PS8 PS9 PS10 PS11 PS12

PS1

(c)

10

(d) 20

40

60

80

100

120

Retention Time (min)

Fig. 1. Pyrograms of fresh non-composted vegetables and fruit: salad (a), zucchini (b), carrots (c), oranges (d). Compounds with name or number are listed in Table 3. Other compounds: e, acids;., alkyl benzenes.

9

PS24 PS23

PS19 LIG5-G LIG6-S LIG7-G PS21

LIG3-G

7 6

LIG4-G

PS16 LIG1-G PS17 LIG2-G PS18

PS14

5

PS15

PS13

PS11 PS12

PS3

4

PS4

1

PS6 PS7 + PS8 PS9

PS1 PS2

2

1059

PS5

M.-F. Dignac et al. / Organic Geochemistry 36 (2005) 1054–1071

LIG13-S

LIG12-S

LIG9-S

N14+LIG8-S

LIG7-G

N18

8

N17 LIG

LIG3-G

LIG4-G

9 7

PS22 N15

6

LIG6-S

5

N12 CHL2 LIG2-G

PS14 N11 CHL1 PS16

PS12

PS7 N8 PS6

PS2 PS3 N6

N4

2

N5

N3 LIM

4

PS5

N1

PS1

N7

1

LIG5-G

LIG1-G

(a)

10

(b) 20

40

60

80

100

120

Retention Time (min)

Fig. 2. Pyrograms of fresh waste: (a) paper and cardboard, (b) fresh green waste. Compounds with name or number are listed in Table 3. Other compounds: e, acids; ., alkyl benzenes.

polysaccharides (Helleur et al., 1985; Pouwels et al., 1987) as are a number of other compounds detected in the pyrograms. Indeed, compounds based on furan moieties, such as furancarboxaldehyde (PS5) and derivatives, PS9 and PS23, or compounds based on cyclopentenone and cyclopentenedione structures (PS8, PS10, PS16) and compounds which contain a pyranone unit (PS20) have been shown to originate from polysaccharides (Pouwels et al., 1987; Helleur et al., 1985). In addition to these polysaccharide-derived products, nitrogen-containing compounds occur in the pyrolysates from fresh vegetables and fruit. The most abundant is acetonitrile (N1), known to be formed during the pyrolysis of peptides and proteins (Tsuge and Matsubara, 1985). In the pyrolysates from fresh vegetables, acetonitrile occurs along with other N-containing products, which possibly also originate from proteins, such as pyridine (N4), acetamide (N11), benzeneacetonitrile (N12), benzenepropanenitrile (N13), indole (N15), methyl indole (N17). Indeed, pyridine derivatives can be attributed to the pyrolysis of alanine-containing proteins and peptides (Bracewell and Robertson, 1984; Chiavari and Galletti, 1992). Acetamide can be found in the pyrolysis of glycine-containing proteins (Tsuge and Matsubara, 1985; Chiavari and Galletti, 1992), but may also originate from peptidoglycan (Eudy et al., 1985) and chitin (Stankiewicz et al., 1996). Benzonitrile (N10) and benzenenitriles (N12, N13) are formed upon pyrolysis of phenylalanine-containing peptides, while indole

(N15) and methylindole (N17) are indicators of tryptophan-containing peptides (Tsuge and Matsubara, 1985). Pyrrole and derivatives (N7, N8, N9) are formed by cyclization during pyrolysis of proteins containing proline, hydroxyproline, glycine and glutamic acid (Tsuge and Matsubara, 1985; Chiavari and Galletti, 1992), but are also pyrolysis products of pigments such as chlorophyll (Bracewell et al., 1987; Sinninghe-Damste´ et al., 1992). The alkyl pyrroles observed in the pyrolysates of fresh waste (Fig. 1) did not show the typical distribution (Sinninghe-Damste´ et al., 1992) observed for the thermal cleavage of tetrapyrrole units derived from chlorophyll pigments. Indeed, they were dominated by methyl pyrroles, while C2 pyrroles were only minor products, and C3 and C4 pyrroles were not detected. Pyrrole and derivatives in our pyrolysates are thus more likely to have a protein origin. However, in some of the fresh waste samples, the presence of chlorophyll was indicated by pristene (CHL1) and phytadiene (CHL2) in the pyrolysates of fresh green waste (Fig. 2b) and green vegetable salad and zucchini (Figs. 1a and b). Phytadiene (CHL2) is an acyclic isoprenoid hydrocarbon originating from the phytyl chain of chlorophyll (Ishiwatari et al., 1991) or tocopherol (Goossens et al., 1984). However, in the pyrolysates of plants, it most probably has a chlorophyll origin (Rosenberg et al., 2003). The N-containing products, most of which probably originate from proteins, are significantly more abundant in the pyrolysates from salad and zucchini than in those

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Table 3 Pyrolysis products in fresh and composted waste by Py-GC/MS No.

Name

Compounds derived from polysaccharides PS1 1-Hydroxy- propan-2-one PS2 2-Methyl-furan PS3 2-Methyl-2-cyclopenten-1-one PS4 3-Furancarboxaldehyde PS5 2-Furancarboxaldehyde PS6 1-(2-Furanyl)-ethanone PS7 3-Methyl-cyclopent-2-en-1-one PS8 2,3-Dimethyl-cyclopent-2-en-1-one PS9 5-Methyl-2-furancarboxaldehyde PS10 Cyclopent-2-en-1,4-dione PS11 Dihydro-2(3H)-furanone PS12 2-Furanmethanol PS13 3-Methyl-2(5H)-furanone PS14 Cyclopentane-1,3-dione PS15 3,6-Dihydro-(2H)-pyran-2-one PS16 3-Methyl-cyclopentane-1,2-dione PS17 5-Methyl-2(5H)-furanone PS18 Levoglucosenone PS19 b-L-arabinopyranoside, methyl PS20 2,3-dihydro-3,5-dihydroxy-6-methyl-4HPyran-4-one PS21 1,4:3,6-dianhydro-a-D-glucopyranose PS22 2,3-dihydro-benzofuran PS23 5-(Hydroxymethyl)-2-furancarboxaldehyde PS24 Levoglucosan Compounds derived from lignin LIG1-G 2-Methoxy-phenol (guaiacol) LIG2-G 2-Methoxy-4-methyl-phenol (Me-guaiacol) LIG3-G 4-Ethyl-2-Methoxy-phenol (Et-guaiacol) LIG4-G 2-Methoxy-4-(2-propenyl)-phenol (eugenol) LIG5-G 2-Methoxy-4-(1-propenyl)-phenol (isoeugenol) LIG6-S 2,6-Dimethoxy-phenol LIG7-G 2-methoxy-4-(1-propenyl)-phenol (isoeugenol) LIG8-S 4-Ethyl 2,6-dimethoxy-phenol LIG9-S 2,6 Dimethoxy 4-vinyl-phenol LIG10-G 1-(4-Hydroxy-3-methoxyphenyl)-ethanone LIG11-G 1-(4-Hydroxy-3-methoxyphenyl)-2-propanone LIG12-S 2,6-Dimethoxy-4-(2-propenyl)-phenol LIG13-S 1-(4-hydroxy-3,5-dimethoxyphenyl)-ethanone N-containing compounds N1 Acetonitrile N2 Propanenitrile N3 1-Methyl-1H-pyrrole N4 Pyridine N5 3-Methyl-pyridine N6 2-Methyl-pyridine N7 1H-Pyrrole N8 2-Methyl-1H-pyrrole N9 1-Methyl-1H-pyrrole N10 Benzonitrile N11 Acetamide N12 Benzeneacetonitrile N13 Benzenepropanenitrile N14 2-Hydroxy-pyridine

Table 3 (continued) No.

Name

N15 N16 N17 N18

1H-Indole 2,5-Pyrrolidinedione 3-Methyl-1H-indole diketodipyrrole

Isoprenic compounds LIM Limonene CHL1 Prist-1-ene CHL2 Phytadiene Compounds of unspecific origin 1 Benzene, methyl- = toluene 2 Styrene 3 Benzene, (1-methylethenyl)- = Me styrene 4 Acetic acid 5 Phenol 6 Phenol, 4-methyl7 Phenol, 3-methyl8 Phenol, 4-ethyl9 Phenol, 5-methyl-2-(1-methylethyl)10 1,2-Benzenediol (catechol) Compounds derived from synthetic polymers 11 Benzene ethenyl, dimer 12 Benzene ethenyl, trimer

from carrots, and were not found in the pyrolysate from oranges. This is in agreement with the larger protein to polysaccharide ratio in salad and zucchini, 0.4:1 and 0.3:1, respectively (Atwater and Woods, 1896) than in carrots (0.2:1) and oranges (0.1:1). This is also in accordance with the C/N ratios, which were lower in salad and zucchini than in oranges and carrots (Table 2). The presence of proteins in these samples is further indicated by the occurrence of diketopiperazine derivatives in the pyrolysates. Diketopiperazine derivatives are characteristic of proteins and have a prominent m/z 154 ion when they derive from Pro-Gly, Pro-Lys, ProVal and Pro-Arg moieties and a prominent m/z 168 ion when they derive from the dipeptide Pro-Ala (Chiavari and Galletti, 1992; Stankiewicz et al., 1996). Several diketopiperazine derivatives were detected using selective ion monitoring at the end of the salad pyrogram. However, the peak intensities were too low to be labelled on total ion current traces. The high N content of the salad sample (Table 2) is thus partly explained by the presence of peptides. Diketopiperazine was also detected in zucchini and carrot pyrolysates, although in lower abundance than in salad, which is in agreement with the lower N content of these samples (Table 2) and their lower protein to polysaccharide ratio (Atwater and Woods, 1896). The pyrolysate from carrots (Fig. 1c) contain a limited number of N-containing compounds (N4, N7) which also occurred in low abundance. While the orange sample has a N content similar to that of carrots (Table 2), no N-containing product was detected in the orange

M.-F. Dignac et al. / Organic Geochemistry 36 (2005) 1054–1071

pyrolysate (Fig. 1d). N in the orange sample may be in structures which do not lead to detectable pyrolysis products, while the presence of pigments in carrots may explain the N-containing products observed in the carrot pyrolysate. It must be noted that neither indoles (N15, N17, characteristic of tryptophan), nor benzenenitriles (N12, N13, characteristic of phenylalanine) were detected in the carrot and orange pyrolysates. This is again in agreement with a low contribution of proteins to these vegetables. As a result, a protein origin can be ruled out for the phenol (5) and methylphenols (6; 7) that contribute significantly to the pyrolysates from carrots and oranges. Moreover, no lignin-derived compounds were present in the pyrolysates from salad, zucchini and carrots, which also allowed us to eliminate these macromolecules as potential precursors of phenols in these samples. A polysaccharide origin is thus the most likely for the phenols in the fresh vegetables and fruit, although a tannin contribution cannot be excluded. Styrene (2) was detected in low amounts in all the pyrolysates from vegetables and fruit. It is a ubiquitous pyrolysis product since it can originate not only from phenylalanine-containing proteins and peptides (Tsuge and Matsubara, 1985) but also from degraded lignins (van Smeerdijk and Boon, 1987) and non-hydrolyzable tannins (van Bergen et al., 1997). Apart from these natural origins, styrene may also derive from plastics like polystyrene (Fabbri, 2001). However, based on the lack of N-containing compounds and lignin-derived products, styrene should be derived from tannins in fresh vegetables and fruit. Catechol (1,2-benzenediol, compound no. 10), present in the pyrolysates of vegetables and fruits, may also originate from non-hydrolyzable tannins (catechins). Catechol is also of ambiguous origin, possibly derived from degraded lignin (GonzalezVila et al., 1996) or cellulose (Pouwels et al., 1989). However, catechols in plant pyrolysates have been attributed to tannins (van Bergen et al., 1996) and such an origin can be proposed for this compound in the pyrolysates of fresh vegetables and fruits. Limonene (LIM) is present in the pyrolysates from fresh oranges (Fig. 1d) and fresh green waste (Fig. 2b). It is a monocyclic monoterpene present in the fresh peel of grapefruit, lemons and oranges (Lota et al., 1999) and in various higher plants (Nguyen et al., 2003). Some low molecular weight acids, such as acetic acid (unspecific compounds No 4), butenoic acid or methylbutanoic acid, were detected in the pyrolysis products from fruit (Fig. 1d) and vegetables (Figs. 1b and c). A few ubiquitous even numbered fatty acids were also detected in the pyrolysates from salad and oranges. 3.2.3. Paper and cardbooard The pyrolysates from paper and cardboard (Fig. 2a) were again dominated by polysaccharide-derived prod-

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ucts such as hydroxypropanone (PS1) and furancarboxaldehyde (PS5) and products which could originate specifically from undegraded cellulose structures, such as dihydropyranone (PS15), methylfuranones (PS13 and PS17), pyranose (PS21) and levoglucosan (PS24; Helleur et al., 1985; Pouwels et al., 1987). In addition to these compounds, numerous phenols and an especially large amount of isopropylmethyl phenol (9) were observed in the pyrolysate from paper and cardboard. Several phenolic products derived from lignin were present in the pyrolysate from paper and cardboard, as already found by Galletti et al. (1997) for paperboard. Lignin is a constituent biopolymer of higher plants. Its structure is based on phenolic compounds of the guaiacyl and syringyl types, with contrasting distribution in gymnosperm and angiosperm plants. Gymnosperms mainly contain lignin units of the guaiacyl-type, while angiosperms contain similar proportions of guaiacyl and syringyl-type units (Hedges and Mann, 1979). Pyrolysis products derived from lignins have either a guaiacyl (labeled LIG-G) or a syringyl origin (LIG-S) (Galletti and Reeves, 1992; van der Heijden and Boon, 1994). The ratio of guaiacyl to syringyl-type units may be used as an indicator of the vegetation type and also reflects the degree of microbial reworking of lignin (Ertel and Hedges, 1984). Lignin of the guaiacyl-type has a higher degree of cross linking compared to that of the syringyltype and is thus more resistant to biodegradation. Hence a progressive increase in the guaiacyl-to-syringyl (G/S) ratio has been reported during humification (Ertel and Hedges, 1984). Upon pyrolysis of paper and cardboard (Fig. 2a), the lignin-derived products were only of guaiacyl origin, suggesting that the lignin in this sample originated from lignocellulose material derived from gymnosperm plants, which was also the case for the paperboard samples analyzed by Galletti et al. (1997). This is consistent with the industrial process of paper and cardboard production, which generally uses gymnosperm tree material. Only one product of syringyl origin (dimethoxy phenol, LIG6-S) was detected in the paper and cardboard pyrolysate (Fig. 2a). Styrene (2) was detected in relatively high abundance in the pyrolysate. As stressed above, it is a ubiquitous compound, but the lack of N-containing compounds in the paper and cardboard pyrolysate points to a lignin origin for styrene. Similarly, a protein origin should be ruled out for phenol and methylphenols in the pyrolysate. 3.2.4. Fresh green waste As expected, the pyrogram from fresh green waste (Fig. 2b) was dominated by lignin-derived compounds. These phenolic compounds included a number of syringyl-type compounds (LIG6-S, LIG8-S, LIG9-S, LIG12-S, LIG13-S), along with guaiacyl-type phenols, suggesting that the lignin was mainly derived from

1.5 0.3 3.4 0.3 0.4 2.7 1.5 0.2 3.0 2.3 0.8 4.1 2.2 0.2 2.4 1.2 0.9 3.4 1.5 0.3 3.2 4.0 0.9 2.1 1.6 0.5 5.9 5.5 1.1 3.4 1.5 0.2 3.0 0.6 1.1 4.1 1.1 0.3 3.3 3.9 1.6 2.0 nc: not calculated. The accuracy for the first 5 ratios was better than 15%, and was around 20% for the LIG-G/LIG-S ratio. a Ratio of sum of peak areas of pyrolysis products originating from lignin to those from polysaccharides.

2.0 0.3 2.4 0.9 1.5 4.0 4.7 0.5 2.5 2.4 0.9 1.5 nc nc 6.8 1.0 0.1 8.3 nc nc 1.9 6.0 <0.1 nc 8.7 0.7 1.8 7.4 0 nc

Paper and cardboard Orange Carrot Zucchinis

7.6 1.0 1.3 4.2 0 nc 7.6 0.7 2.0 5.3 0 nc Acetic acid/pyrrole Furancarboxaldehyde/pyrrole Hydroxypropanone/furanmethanol Toluene/styrene Lignins/polysaccharidesa LIG-G/LIG-S

GWS2 GWS1 GW2 GW1 BW3 BW1

BW2

Composts

Salad

Fresh GW Fresh wastes

angiosperm plants. This is consistent with the sources of green waste from municipal and private gardens, which, according to the season, may include different proportions of grasses and deciduous tree leaves and branches, both of the angiosperm type. As for paper and cardboard, a substantial contribution of styrene was observed in the non-composted green waste. Based on the important lignin input to these pyrolysates, styrene should also originate from lignin. A few polysaccharide-derived pyrolysis products and N-containing ones were detected in addition to the lignin-derived products. Indole and methylindole (N15 and N17), derived from tryptophan-containing peptides and proteins (Tsuge and Matsubara, 1985), suggested the presence of proteins in this sample. Remarkable differences were observed between the various pyrograms from the fresh waste. Dihydrodihydroxymethylpyranone (PS20) was only detected in vegetable and fruit pyrolysates, and mainly in the zucchini and oranges ones (Figs. 1b and d). The vegetables did not contain lignin-derived compounds. Methyl b-Larabinopyranoside (PS19) and hydroxymethylfurancarboxaldehyde (PS23) were present in the paper and cardboard pyrolysate (Fig. 2a), as well as in the carrot and orange pyrolysates (Figs. 1c and d). Acetonitrile (N1), pyrrole (N7) and methylpyrroles (N8 and N9) were detected in all the fresh waste pyrolysates except for the oranges and paper and cardboard ones (Figs. 1d and 2a, respectively), that contained only polysaccharide and lignin-derived products, and no N-containing products. Pyrrole (N7) was the dominant N-containing product in the pyrolysates from salad and zucchini (Figs. 1a and b) and non-composted green waste (Fig. 2b). Chlorophyll–derived pyrolysis products were also present, and chlorophyll may also be the origin of pyrrole in these samples. Phenol and methylphenols were detected in both the pyrolysate from oranges (Fig. 1(d)) and that from paper and cardboard (Fig. 2(a)), but, as shown above, different origins must be considered for these compounds. In order to highlight differences in composition amongst fresh non-composted wastes, ratios of some specific pyrolysis products have been calculated as indicators of OM properties (Table 4). Two humification indexes were calculated: (1) the ratio of furancarboxaldehyde, a pyrolytic product originating from polysaccharides, to pyrrole, which is derived from nitrogenous compounds, humified organic matter and microbial cells (Ceccanti et al., 1986), and (2) the ratio of acetic acid, derived from biodegradable products, to pyrrole (Garcı´a et al., 1993). These ratios have been shown to decrease with increasing biodegradation and humification of OM during composting, when polysaccharides and biodegradable compounds are degraded, respectively (Ceccanti et al., 1986; Garcı´a et al., 1993). Although the significance of these ratios is questionable since acetic

MSW

M.-F. Dignac et al. / Organic Geochemistry 36 (2005) 1054–1071

Table 4 Ratio of peak area of some compounds in pyrolysates of fresh non-composted waste and compost samples

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acid and pyrrole are non-specific compounds, they were chosen in this study only in order to compare our dataset with previously published work on the pyrolysis of compost and waste (Diaz-Burgos et al., 1994; Garcı´a et al., 1995; Ayuso et al., 1996). These ratios calculated for fresh waste represent the values for undegraded constituents (Table 4). They were higher for vegetables (7.6–8.7 for acetic acid to pyrrole; 0.7–1.0 for furancarboxaldehyde to pyrrole) than for fresh GW (4.7 and 0.5, respectively). They could not be calculated for oranges and paper and cardboard, which did not display any pyrrole peaks (Table 4). The ratio of hydroxypropanone to furanmethanol was used as an indicator of the relative quantities of cellulose and hemicellulose structures (Table 4), since hydroxypropanone mainly originates from cellulose, while furanmethanol more probably has a hemicellulose origin (Helleur et al., 1985; Pouwels et al., 1987). It was significantly larger for the pyrolysis of paper and cardboard (6.8) than for the pyrolysis of fresh vegetables (between 1.3 and 2.0) and fresh green waste (2.5). Toluene and styrene are released upon pyrolysis of both natural OM and synthetic polymers (Fabbri, 2001). The ratio of their contribution is greater than one for natural samples and lower than one for synthetic polymers (Fabbri et al., 1998). As a result, the toluene/ styrene ratio can be used as an indicator of the presence of synthetic polymers. As expected for natural products, the ratio of toluene to styrene was larger than 1 for vegetables and fruit (4.2–7.4) and fresh green waste (2.4). In the pyrolysate from paper and cardboard, toluene and styrene peaks had the same area (ratio of 1.0). This lower ratio may be due to the presence of styrene copolymers, which are also paper additives (Ishida et al., 1994). Additionally, the ratio of guaiacyl-derived products to syringyl-derived products varies with the origin and degradation degree of the plant material (Ertel and Hedges, 1984). It was 1.5 for the pyrolysis of fresh green waste, and 8.3 for the pyrolysis of paper and cardboard, mainly reflecting the angiosperm and gymnosperm sources of lignin in each case. 3.2.5. Biowaste compost The pyrograms from the biowaste compost (Fig. 3) mainly contain pyrolysis products originating from lignin (LIG1-G, LIG7-G). This is in accord with the high proportion of green waste and papers in these composted wastes (Table 1), which are possible sources of lignin. In the BW1 and BW3 composts (Figs. 3a and c), lignin-derived products were abundant relative to other pyrolysis products and were dominated by the guaiacyl-type units LIG1-G and LIG7-G, the syringylderived units being of low relative abundance. The ratio of guaiacyl to syringyl-type products was 4.0 and 4.1 for the pyrolysis of BW1 and BW3, respectively (Table 4). This ratio suggests that lignin was actually degraded in

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these samples or that the lignin input was mainly of gymnosperm origin, i.e., from paper and cardboard. The ratios LIG-G/LIG-S of BW1 and BW3 were very close, although they contained rather different proportions of green waste and paper (Table 1). This suggests that lignin degradation influences this ratio, as well as its origin. In BW2 (Fig. 3b), lignin-derived compounds originating from syringyl structures were relatively more abundant than in BW1 and BW3, although the ligninderived products were also dominated by LIG1-G and LIG7-G of guaiacyl origin. The ratio of guaiacyl to syringyl units of 2.0 in BW2 (Table 4) may indicate that these structures were less degraded than in BW3 compost, since this compost initially contained waste with the same contribution of green waste as BW3 (ca. 60%, Table 1). Products originating from polysaccharides (hydroxypropanone PS1 and cyclopentanedione PS14) were also relatively abundant in the pyrolysates from BW compost and, according to the pyrograms from fresh waste, they may originate from green waste, vegetables, fruit and/or paper. Pyrrole and acetonitrile were the dominant N-containing products but their origin is ambiguous. Furanmethanol, a compound rather abundant in the pyrolysates from fresh vegetables (Fig. 1) and specific for hemicellulose was only a minor compound in the pyrolysates from BW composts. The ratio of hydroxypropanone to furanmethanol (Table 4) varied from 2.4 to 3.3 for BW composts and was intermediate between that for paper and cardboard (7.2) and that for fresh green waste or vegetables (between 1.3 and 2.5). Some of the polysaccharide-derived products identified in the pyrolysates from fresh vegetables are missing in the pyrolysates of BW composts, indicating that most vegetable constituents were degraded after four months of composting. Dihydro-dihydroxy-methyl-pyranone (PS20), present in all pyrolysates of vegetables and fruits and especially in those of zucchini and oranges (Figs. 1b and d) and hydroxymethyl-furancarboxaldehyde (PS23), found in carrot and orange pyrolysates, were not detected for BW composts. Similarly, some cellulose-derived products observed in the pyrolysate of paper and cardboard and originating from undegraded cellulose structures (PS13, PS15, PS17, PS19 PS21, PS24) were not found in the pyrolysates from BW compost, indicating that most of the cellulose structures in the added paper and cardboard had been degraded after four months of composting. This lack of anhydrosugars and pyranones in the polysaccharides of BW compost indicates that this fraction mainly consisted of biodegraded polysaccharides, probably originating from plants or related to the intense microbial activity occurring during composting. Pristene (CHL1) and phytadiene (CHL2), indicating the presence of chlorophyll (Ishiwatari et al., 1991),

M.-F. Dignac et al. / Organic Geochemistry 36 (2005) 1054–1071 LIG1-G

1064

5

LIG10-G

LIG12-S N18

LIG9-S

8

LIG6-S

7

LIG4-G

N12 CHL2 LIG2-G PS18 LIG3-G N13

CHL1 PS16

PS14 PS12

PS5

PS7 N8 N9

N5

PS2 PS3

N6

N3 N4

N2

3

6

N11

N1

4

PS22 N15 N17

PS1

2

N7

1

LIG7-G

9

N18

LIG12-S

8

7

LIG9-S LIG10-G

PS22 N15 N17

LIG4-G

LIG3-G

PS18

CHL2 LIG2-G N12

PS14 N11 CHL1 PS16

PS12

PS5

PS2 PS3

N6

N5

PS7 N8 N9

PS1 N4 N3 N2

2

4

6

LIG7-G

9

1

LIG6-S

5

N7

N1

LIG1-G

(a)

LIG7-G

LIG12-S N18

LIG10-G

11

LIG9-S

12

PS22 N15 N17

8

LIG6-S

7

LIG4-G

LIG3-G N13

CHL2 PS18LIG2-G

9

6

N12

N11 CHL1 PS16

PS12

PS5

PS2 PS3

N5

N6

N4 N3

N2

4

PS7 N8 N9

PS14

2

3

5

LIG1-G

1

PS1

N1

N7

(b)

(c) 20

40

60

80

100

120

Retention Time (min)

Fig. 3. Pyrograms of BW composts from different industrial plants composted over four months: (a) BW1; (b) BW2; (c) BW3. Compounds with name or number are listed in Table 3. Other compounds: e, acids; ., alkyl benzenes.

and present in the pyrolysates from the green vegetable salad and zucchini (Figs. 1a and b) and fresh green waste (Fig. 2b), were also identified in the three BW composts (Fig. 3). The occurrence of these compounds in the BW compost pyrolysates shows that the skeleton of the chlorophyll phytyl chain remained unaltered after four months of composting. Moreover, these compounds were present in larger amounts in BW2 and BW3 than in BW1. This should reflect the higher proportion of green waste in BW2 and BW3 composts compared to BW1 (Table 1). Styrene was especially abundant in the pyrograms for BW1 and BW3, while it was a minor pyrolysis product for BW2. Although a lignin origin can be invoked, its high abundance may reflect the presence of some plastics (polystyrene) in these BW composts. The ratio toluene/

styrene (Table 4) had values of 0.9 and 0.6 for BW1 and BW3, respectively, possibly indicating the presence of styrene polymers (Fabbri et al., 1998). Compounds 11 and 12, identified as styrene dimer and trimer respectively, are characteristic for the pyrolysis of styrene polymers (Fabbri, 2001) and were found in the pyrolysate of BW3 (Fig. 3c). Their mass spectra are displayed in Fig. 4. They were also identified for BW1 thanks to the selective detection of the molecular ions (m/z 208 and 312, respectively). They were not detected in the BW2 compost pyrolysate, and the ratio toluene/styrene (3.9) of this sample was in the range found for natural OM. Surprisingly, the presence of 5% plastics was reported in the the initial wastes for BW2 (Table 1), while no plastics were indicated for BW1 and BW3. The lack of pyrolysis products indicative of styrene polymers for BW2 may be

M.-F. Dignac et al. / Organic Geochemistry 36 (2005) 1054–1071

Compound 11

91

65 39

m/z--> 20

51 60

Compound 12

1065

77

104 130 115

100

140

208 193 179 180

220

260

300

340

91

117 51 65 77 103 129 m/z--> 20

60

100

140

207 194 221 180

220

312 260

300

340

Fig. 4. Mass spectra of styrene dimer (compound 11) and trimer (compound 12).

due to the lack of styrene polymers in the plastics in BW2, which may be made of a different material. The occurrence of polystyrene derivatives in the pyrolysates for BW1 and BW3, that should not contain plastic, may be due to the presence of copolymer additives in the paper in this waste. It should be noted that the paper content was especially high in BW3 (21%, Table 1). As already mentioned, styrene copolymers are known paper additives (Ishida et al., 1994). The ratios of acetic acid and furancarboxaldehyde to pyrrole (Table 4) were much lower in the BW compost pyrolysates (1.1–2.0 and 0.2–0.3, respectively) than fresh wastes (4.7–8.7 and 0.5–1.0, respectively). This provides evidence that the OM in BW compost was well humified after four months of composting, as already suggested for these composts by maturity tests (Francou et al., 2005). 3.2.6. Green waste compost The main pyrolysis products from GW compost (Fig. 5) originate from lignin (methoxyphenols) and polysaccharides (PS1, PS5, PS12, PS14), along with phenols (5–9). The lignin units in GW compost derive both from guaiacyl and syringyl structures. The ratio of guaiacyl- to syringyl-derived units (Table 4) increased from fresh green waste (1.5) to composted green waste GW1 and GW2 (3.4 and 2.1, respectively), which should be related to lignin degradation upon composting. Based on this ratio, this degradation was

more intense for GW1 than for GW2. When compared to fresh green waste, the main difference observed in the pyrograms was the larger amount of lignin derivatives in the fresh green waste than in the composted green waste. Lignin, although one of the less biodegradable natural compounds, is known to be degraded upon composting (Tuomela et al., 2000). Hydroxypropanone (PS1) was found in large amounts in the pyrolysis products of GW composts (Figs. 5a and b). The contribution of PS1 was larger for GW1 than GW2 and the hydroxypropanone to furanmethanol ratio was 5.9 for GW1 and 3.2 for GW2 (Table 4), likely indicating, amongst polysaccharides, a higher contribution of cellulose to the former. It must be noted that a value of 2.5 was observed for this ratio for fresh green waste. The increase in the hydroxypropanone to furanmethanol ratio should reflect the preferential degradation of hemicellulose with respect to cellulose. This was expected because of the higher resistance of the cellulose component of the ligno-cellulose complex. The higher ratio for GW1 than for GW2 may be related to more extensive degradation of polysaccharides in GW1 due to the use of enforced aeration for composting. This is in accordance with the higher LIG-G/LIG-S ratio for GW1, which is also an indication of the more extensive degradation of lignin. N-containing pyrolysis products were also present in GW compost pyrolysates. These compounds were relatively more abundant for GW2 than for GW1. The

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M.-F. Dignac et al. / Organic Geochemistry 36 (2005) 1054–1071

LIG12-S N18

LIG9-S

8

LIG10-G

PS22 N15 N17

LIG6-S

LIG4-G

7 LIG3-G

PS18

6

N12

PS12

PS7

PS16

PS5 PS4

PS2 PS3

N3 LIM N4 N5

N1

4

PS14 N11 CHL1

N7

LIG2-G

9

1

2

LIG7-G

PS1

LIG1-G

5

(a)

8

LIG12-S

LIG9-S LIG10-G

LIG4-G

7

LIG6-S

9

6

PS18 LIG3-G

CHL2 LIG2-G N12

LIG1-G PS14 N11 CHL1 PS16

PS12

PS7 N8 N9

PS5

PS2 PS3

N5

2

N6

4

N4

N3

PS1

N7

N1

1

LIG7-G PS22 N14+LIG8-S N15 N16 N17

5

(b) 20

40

60

80

100

120

Retention Time (min)

Fig. 5. Pyrograms of GW composts after four months of composting: (a) GW1; (b) GW2. Compounds with name or number are listed in Table 3. Other compounds: e, acids; ., alkyl benzenes.

toluene/styrene ratios in the pyrolysates of GW composts (5.5 and 4.0) were characteristic of natural OM and no synthetic polymers were evidenced for these composts by pyrolysis. Limonene (LIM), present in the pyrolysate from fresh green waste (Fig. 2b), was also detected for the GW composts (Fig. 5), showing the stability of this compound upon composting. The presence of chlorophyllderived compounds in both GW composts was indicated by the CHL1 and CHL2 pyrolysis products, and pyrrole, although this compound may also have derived from proteins. 3.2.7. Sewage sludge compost In the pyrolysates from sewage sludge composted with green waste (Figs. 5a and b), the main products were lignin-derived and N-containing compounds and, to a lesser extent, polysaccharide-derived compounds. Limonene (LIM) and chlorophyll products (CHL1 and CHL2) were detected for both the GWSS composts (Fig. 6). Some low molecular weight acids, such as acetic acid (unspecific compounds no. 4), butenoic acid or methyl-butanoic acid, were also detected and may have originated from bacterial material. Both pyrograms were quite similar, the main difference between them being the presence, in the case of GWSS1, of C14–C18 fatty acids. Since these acids were

not detected upon GW compost pyrolysis, they must have been introduced by the sludge. Since the same relative amount of sludge (ca. 30%) was added in GWSS1 and GWSS2, the occurrence of fatty acids in the GWSS1 pyrolysate likely reflected the higher content of fatty acids in the sludge for GWSS1. A high variability in fatty acid content has been reported for non-composted sludges (Re´veille´ et al., 2003). Such a difference is of importance when the compost is spread on soil since lipids are known to promote the resistance of soil aggregates against water and hence to have a positive effect on soil physical stability (Dinel et al., 1990). Although OM in GWSS1 appears less humified, with large amounts of lipids, the ratios indicating the mineralization of OM were substantially decreased compared to those of fresh waste and were comparable to those for BW and GW composts (Table 4). When compared to GW compost, GWSS compost appears to be less enriched in polysaccharides. Indeed, hydroxypropanone (PS1) was a minor product in the pyrolysates from GWSS (Figs. 5a and b). A direct comparison is possible between GWSS2 and GW2 composts because they came from the same industrial plant. When the lignin input was considered, a lower contribution of syringyl units with respect to the guaiacyl ones was observed for GWSS2 than for GW2 (2.1 and 4.1 for GW2 and GWSS2, respectively). Since

M.-F. Dignac et al. / Organic Geochemistry 36 (2005) 1054–1071

1067

5

C18

C16

C14 LIG12-S

LIG9-S

8

LIG6-S

N12

7

LIG4-G

9

LIG7-G PS22 N15 N17

6 LIG2-G PS18 LIG3-G N13

PS14

PS12

PS7 N8 N9

PS5

PS2 PS3 PS4

N4

N6 PS1

4

N11 CHL1 PS16

PS11

LIM

2

N5

N3

LIG1-G

N7

N1

1

(a)

8

3

LIG9-S LIG10-G LIG11-G LIG12-S N18

9

LIG7-G PS22 N15 N17

7

LIG6-S

LIG4-G

LIG2-G +PS18 LIG3-G N13

6

N12

N11 CHL1 PS16

PS14

PS12

4

PS7 N8 N9

PS2 PS3

PS1 N6

LIM N4 N5

2

LIG1-G

N7

1

PS5

N1

5

(b) 20

40

60

80

100

120

Retention Time (min)

Fig. 6. Pyrograms of sewage sludge co-composted with GW during four months: (a) GWSS1; (b) GWSS2. Compounds with name or number are listed in Table 3. Other compounds: e, acids; d, n-alkane/n-alk-1-enes; ., alkyl benzenes.

acids were observed for GWSS2, but more aliphatic structures were present than in the GW compost, as indicated by the alkanes/alkenes series.

no gymnosperm-derived lignin was expected to have been added with the sewage sludge, this difference likely reflected more efficient degradation of lignin upon composting of GWSS2. It must be noted that no lignin degradation was reported by Garcı´a et al. (1992) after composting of sewage sludge and city refuse. Another major difference between GWSS2 and GW2 was the presence of n-alkane/n-alk-1-enes doublets (from C10 to C26) in the GWSS2 pyrolysate (Fig. 6b). Such doublets are formed by homolytic cleavage of long alkyl chains and are commonly observed in the pyrolysates of plant tissues where they indicate an input of aliphatic biopolymers such as cutan or suberan (Nip et al., 1986; Tegelaar et al., 1989). Alkanes and alkenes were observed in very low amounts in the fresh wastes and in the other composts. Contrary to GWSS1, no fatty

3.2.8. Municipal solid waste compost The strong predominance of styrene in the pyrograms for the MSW compost (Fig. 7) likely reflects the presence of polystyrene from plastic wastes in this compost (6% of initial waste, Table 1). The input of styrene polymers was confirmed by the presence of styrene dimer (11) and trimer (12) in the pyrolysate (Fig. 7). Methylstyrene (3) was also more abundant for MSW than for the other composts. The toluene/styrene ratio was very low for MSW (0.3, Table 4), which is a further indication of the presence of styrene polymers. Beside plastics, these styrene polymers may also originate from

20

40

60

C18

80

100

C14 LIG12-S

89

LIG9-S LIG10-G

7

LIG7-G PS22 N15 N17

N12 LIG2-G

N13

11

LIG6-S

CHL1 PS16 LIG1-G

PS14 N11

PS12

PS7 N8 N9

4

PS5

3

PS2 PS3

N6

N4 N5

N3

N2

PS1

6

12

C16

5

N7

N1

2 1

120

Retention Time (min)

Fig. 7. Pyrogram of MSW composts after four months of composting. Compounds with name or number are listed in Table 3. Other compounds: e, acids; d, n-alkane/n-alk-1-enes; ., alkyl benzenes.

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paper present in large quantities in the initial waste (25%, Table 1). Apart from styrene, pyrolysis products originating from cellulose (PS1, PS5) were found in relatively large amounts for MSW compost (Fig. 7), along with pyrrole and a few lignin-derived compounds, mainly of guaiacyl origin. As already noted for BW compost, the pyrolysis products originating from undegraded cellulose (PS13, PS15, PS17, PS19 PS21, PS24) were not detected in the pyrolysate from MSW compost, indicating that cellulose was mainly degraded after four months of composting. The ratio of the sum of the areas for pyrolysis products originating from lignin to those originating from polysaccharides was significantly lower (0.4) for the MSW compost than for other composts (0.8–1.5). This is in agreement with the higher degradability of MSW compost (mineralized C after 28 days 14% of total organic C, Table 2), indicating that this compost was not yet fully stabilized. The polysaccharide-derived compounds could originate from paper or from biowaste, which both represent 25% of the initial waste (Table 1). The lower amount of green waste (17%) in the composition of the initial waste probably explains how few lignin-derived compounds originating from syringyl units were observed in the pyrolysate of the MSW compost. Some N-containing compounds, dominated by the ubiquitous pyrrole (N7) were also identified. The ratio of acetic acid to pyrrole for MSW compost (1.5, Table 4) was in the same range as found by Ayuso et al. (1996) for MSW compost (1.3). Some n-alkane/n-alk-1-ene doublets in the C10–C26 range were observed in the pyrolysate of MSW compost (Fig. 7). Some fatty acids, C14, C16 and C18, were present in relatively large amounts. Veeken et al. (2000) also found C12–C18 fatty acids in the NaOH-extracted humic acid from the organic fraction of MSW. The C16 and C18 fatty acids were the major peaks in the pyrograms and were only slightly affected by composting. These acids are ubiquitous but, in our pyrolysates, due to the lack of branched compounds, they more likely originate from greasy waste than from bacteria.

4. Conclusions In this study pyrolysis was used to reveal differences in the composition of compost OM with respect to the nature of composted wastes. The pyrolysis of non-composted wastes characteristic of those found in BW, GW, GWSS and MSW composts allowed the determination of the origin of some of the pyrolysis products found in the composted wastes. The comparison of pyrolysates of fresh and composted waste also made it possible to use ratios of peak areas for specific pyrolysis products

to describe some properties of compost OM. These ratios were used to (1) ascertain the origin of styrene in the pyrolysates (natural OM or synthetic polymers); (2) evaluate the relative contribution of cellulose amongst polysaccharides; (3) compare the humification of OM in the different composts; (4) assess the origin and degradation level of lignin input. The polysaccharide-derived pyrolysis products were mainly hydroxypropanone and furancarboxaldehyde for paper and cardboard and non-composted green waste, while they were mainly hydroxypropanone and furanmethanol in the pyrolysates of vegetables and fruit. The lignin-derived pyrolysis products were mainly of guaiacyl type in the pyrolysate of paper and cardboard, while they were a mixture of guaiacyl and syringyl type in fresh green waste, as well as in most composts. Comparison of the pyrolysates from BW compost with those from paper and cardboard indicated that most cellulose structures were degraded after four months of composting. The pyrolysates from two of the BW composts contained styrene di- and trimers, which are evidence for the presence of styrene polymers, which in some cases may originate from paper additives and were not found upon pyrolysis of fresh or composted waste other than the MSW compost. The low ratios of toluene to styrene peaks were an additional indication of the presence of synthetic polymers in these samples. By comparing the pyrolysates from composted and fresh GW, it was found that the relative abundance of N-containing and polysaccharide-derived pyrolysis products increased compared to that of lignin, indicating that lignin was degraded upon composting. In the pyrolysate from sewage sludge co-composted with green waste, more aliphatic structures were observed than for GW composts. The distribution of the other pyrolysis products was mainly unchanged, except for a change in lignin distribution, likely indicating a higher degradation level of lignin. Large amounts of lipids were present in one GWSS compost. This characteristic of compost OM could be used to explain the effects of compost addition on soil properties. The differences observed between the pyrolysates from two different GWSS samples were tentatively attributed to the quality of the added sludge. Pyrolysis products derived from synthetic styrene polymers (styrene and styrene di- and trimers) were found in rather large amounts for the MSW compost obtained from the residual fraction of the selectively collected municipal wastes. MSW also contained large amounts of fatty acids, C14, C16 and C18. Lower abundances of lignin-derived compounds were detected in the MSW pyrograms than for the other composts, which could be explained by their readier biodegradability. As for BW compost, pyrolysis provided evidence that nondegraded cellulose structures, as found in paper, were removed upon composting of MSW.

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Acknowledgments We thank M. Le Villio-Poitrenaud (CREED) for her useful expertise on industrial composts, G. Bardoux and N. Pechot for elemental analysis of fresh waste and C. Girardin for technical assistance with mass spectrometry. We are also grateful to two anonymous reviewers for constructive comments. Associate Editor—C.E. Snape

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