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Spectroscopic characterization of water extractable organic matter during composting of municipal solid waste Xiaosong He a,b,, Beidou Xi b , Zimin Wei c , Xujing Guo a,b , Mingxiao Li b,d , Da An a,b , Hongliang Liu b a School of Environment, Beijing Normal University, Beijing 100875, China b Laboratory of Water Environmental System Engineering, Chinese Research Academy of Environmental Science, Beijing 100012, China c Life Science College, Northeast Agricultural University, Harbin 150030, China d College of Agriculture, Northeast Agricultural University, Harbin 150030, China article info Article history: Received 19 June 2010 Received in revised form 20 October 2010 Accepted 20 October 2010 Available online 13 December 2010 Keywords: Water extractable organic matter (WEOM) Municipal solid waste (MSW) FTIR spectra UV–Vis spectra Fluorescence spectra abstract This paper aims to characterize the evolution of water extractable organic matter (WEOM) during the composting of municipal solid waste (MSW), and investigate the correlation between maturity and WEOM characteristics. WEOM was extracted at different stages of MSW composting (0, 7, 14, 21, and 51 d) and characterized by FTIR, UV–Vis, and fluorescence spectroscopy. The results obtained show that the composting process decreased aliphatics, alcohols, polysaccharides, as well as protein-like materials, and increased aromatic polycondensation, humification, oxygen-containing functional groups, molecular weight, and humic-like materials. The maturity of MSW during composting was characterized by the presence of the peak with an excitation/emission wavelength pair of 289/421 nm in excitation–emission matrix spectra. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction A growing population and increasing urbanization have led to rising levels of municipal solid waste (MSW) production in China in the past 20 years. Most of the waste is either burned or buried, posing a serious threat to the environment and causing loss of use- ful nutrients (Hong et al., 2010). Composting is an appropriate waste management alternative because it reduces volume and weight by approximately 50% and results in a stable product that can be used in agriculture (Fialho et al., 2010). The agronomical quality of compost is limited mainly by the stability and maturity of organic matter. Mature compost is a source of nutrients and can be used as soil conditioner in the field. However, the application of unstable or immature compost to soil may inhibit seed germination, reduce plant growth, and damage crops (Cooperband et al., 2003; Plazaa et al., 2007; Said-Pullicino et al., 2007). Therefore, assessing the degree of stability during composting is essential. Composting is the humification of organic matter. The increase in aromatic and alkyl structures plays a signif- icant role in organic matters resistance against biodegradation dur- ing composting. Resistance against biodegradation is one of the stabilizing mechanisms of composting organic matter. Thus, the degree of humification of organic matter can be considered as an index for compost stability. Various spectroscopic techniques, such as carbon 13 NMR, FTIR, UV–Vis, and fluorescence spectroscopy, have been used to assess the organic matter humification (Fuentes et al., 2006; Milori et al., 2006). However, using only one method to investigate organic matter composition is insufficient. The integra- tion of various techniques is the best way to characterize the de- gree of humification and stability of organic matter. Composting is the biochemical transformation of waste organic matter by microorganisms that metabolize in the water-soluble phase. A study of the changes occurring in compost water extract- able organic matter (WEOM), the active organic matter fraction, can be useful for assessing compost stability and maturity (Said- Pullicino and Gigliotti, 2007; Shao et al., 2009). Although the trans- formation of organic matter during composting has been widely studied, most of the reports have focused on water-insoluble hu- mic substances (Huang et al., 2006; Droussi et al., 2009; Fialho et al., 2010). Information on WEOM during composting remains limited, and the correlation between stability and WEOM charac- teristics is unclear. The aims of this study are to observe the evolution that occurs in the WEOM fraction through spectroscopic techniques, such as FTIR, UV–Vis and fluorescence spectroscopy; as well as to investi- gate the correlation between stability and WEOM characteristics. 0045-6535/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2010.10.057 Corresponding author. Tel.: +86 15810441270; fax: +86 10 84913133. E-mail address: [email protected] (X. He). Chemosphere 82 (2011) 541–548 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere

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Page 1: Spectroscopic characterization of water extractable organic matter during composting of municipal solid waste

Chemosphere 82 (2011) 541–548

Contents lists available at ScienceDirect

Chemosphere

journal homepage: www.elsevier .com/locate /chemosphere

Spectroscopic characterization of water extractable organic matter duringcomposting of municipal solid waste

Xiaosong He a,b,⇑, Beidou Xi b, Zimin Wei c, Xujing Guo a,b, Mingxiao Li b,d, Da An a,b, Hongliang Liu b

a School of Environment, Beijing Normal University, Beijing 100875, Chinab Laboratory of Water Environmental System Engineering, Chinese Research Academy of Environmental Science, Beijing 100012, Chinac Life Science College, Northeast Agricultural University, Harbin 150030, Chinad College of Agriculture, Northeast Agricultural University, Harbin 150030, China

a r t i c l e i n f o

Article history:Received 19 June 2010Received in revised form 20 October 2010Accepted 20 October 2010Available online 13 December 2010

Keywords:Water extractable organic matter (WEOM)Municipal solid waste (MSW)FTIR spectraUV–Vis spectraFluorescence spectra

0045-6535/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.chemosphere.2010.10.057

⇑ Corresponding author. Tel.: +86 15810441270; faE-mail address: [email protected] (X. He).

a b s t r a c t

This paper aims to characterize the evolution of water extractable organic matter (WEOM) during thecomposting of municipal solid waste (MSW), and investigate the correlation between maturity andWEOM characteristics. WEOM was extracted at different stages of MSW composting (0, 7, 14, 21, and51 d) and characterized by FTIR, UV–Vis, and fluorescence spectroscopy. The results obtained show thatthe composting process decreased aliphatics, alcohols, polysaccharides, as well as protein-like materials,and increased aromatic polycondensation, humification, oxygen-containing functional groups, molecularweight, and humic-like materials. The maturity of MSW during composting was characterized by thepresence of the peak with an excitation/emission wavelength pair of 289/421 nm in excitation–emissionmatrix spectra.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

A growing population and increasing urbanization have led torising levels of municipal solid waste (MSW) production in Chinain the past 20 years. Most of the waste is either burned or buried,posing a serious threat to the environment and causing loss of use-ful nutrients (Hong et al., 2010). Composting is an appropriatewaste management alternative because it reduces volume andweight by approximately 50% and results in a stable product thatcan be used in agriculture (Fialho et al., 2010).

The agronomical quality of compost is limited mainly by thestability and maturity of organic matter. Mature compost is asource of nutrients and can be used as soil conditioner in the field.However, the application of unstable or immature compost to soilmay inhibit seed germination, reduce plant growth, and damagecrops (Cooperband et al., 2003; Plazaa et al., 2007; Said-Pullicinoet al., 2007). Therefore, assessing the degree of stability duringcomposting is essential. Composting is the humification of organicmatter. The increase in aromatic and alkyl structures plays a signif-icant role in organic matters resistance against biodegradation dur-ing composting. Resistance against biodegradation is one of the

ll rights reserved.

x: +86 10 84913133.

stabilizing mechanisms of composting organic matter. Thus, thedegree of humification of organic matter can be considered as anindex for compost stability. Various spectroscopic techniques, suchas carbon 13NMR, FTIR, UV–Vis, and fluorescence spectroscopy,have been used to assess the organic matter humification (Fuenteset al., 2006; Milori et al., 2006). However, using only one method toinvestigate organic matter composition is insufficient. The integra-tion of various techniques is the best way to characterize the de-gree of humification and stability of organic matter.

Composting is the biochemical transformation of waste organicmatter by microorganisms that metabolize in the water-solublephase. A study of the changes occurring in compost water extract-able organic matter (WEOM), the active organic matter fraction,can be useful for assessing compost stability and maturity (Said-Pullicino and Gigliotti, 2007; Shao et al., 2009). Although the trans-formation of organic matter during composting has been widelystudied, most of the reports have focused on water-insoluble hu-mic substances (Huang et al., 2006; Droussi et al., 2009; Fialhoet al., 2010). Information on WEOM during composting remainslimited, and the correlation between stability and WEOM charac-teristics is unclear.

The aims of this study are to observe the evolution that occursin the WEOM fraction through spectroscopic techniques, such asFTIR, UV–Vis and fluorescence spectroscopy; as well as to investi-gate the correlation between stability and WEOM characteristics.

Page 2: Spectroscopic characterization of water extractable organic matter during composting of municipal solid waste

542 X. He et al. / Chemosphere 82 (2011) 541–548

2. Materials and methods

2.1. Composting process and sampling

The experiment was conducted at the Asuwei Composting Plantin Beijing, China. The capacity of the plant is 800–1000 t d�1. Thewaste collected from various sites in Beijing was sorted manuallyand mechanically to remove metals, plastics and glass. The remain-ing MSW was used for composting. Each composting pile con-tained more than 2 t of the residual MSW. The entire compostingprocess was divided into two stages, i.e., the active and curingstages. The active stage was carried out for 21 d during whichthe piles were turned every 2 d by forklift, and the humidity wasmaintained at around 50–65%. The curing stage took 30 d to com-plete, and the piles were turned mechanically every 7 d. The tem-perature of the composting piles was measured daily at a depth of0.3 m. The temperature increased in the first 4 d from 25 to 69 �C,and oscillated thereafter between 53 and 72 �C in the following17 d because of the turning and mixing of the piles every 2 d.The temperature began to decrease on day 21 and reached a con-stant level with ambient temperature after 51 d. To observe theevolution of the WEOM fraction, triplicate composite samples werecollected at different points from the top to the bottom of the pliesafter 0, 7, 14, 21, and 51 d. Subsamples were air-dried, ground topass through a 0.25 mm sieve, and stored in a desiccator for lateranalyses.

2.2. Extraction of WEOM

WEOM was obtained as described by Said-Pullicino et al.(2007). Briefly, the compost samples were extracted with distilledwater (solid to water ratio of 1:10, w/v) for 24 h in a horizontalshaker at room temperature. The suspensions were then centri-fuged at 10 000 rpm for 10 min, filtered through a 0.45 lm mem-brane filter, and freeze-dried.

2.3. FTIR spectra

The FTIR spectra were recorded on pellets obtained by pressinga mixture of 1 mg of freeze-dried WEOM and 300 mg of dried spec-trometry grade KBr under reduced pressure using a Nicolet NexusFTIR spectrophotometer. Spectra were recorded in the range 4000–400 cm�1 with a 2 cm�1 resolution; 64 scans were performed oneach sample.

2.4. Water extractable organic carbon (WEOC) analysis andadjustment

Prior to UV–Vis and fluorescence analysis, the WEOC of all thesamples was measured with an Analytik Jena model Multi N/C2100 TOC analyzer. The concentrations of all the samples werestandardized to make them comparable to each other and avoid in-ner filter effects for UV-absorbance and fluorescence analysis. NopH adjustment was performed prior to spectroscopic analysisbased on a previous report (Henderson et al., 2009), because thepH of the WEOM solution used for spectroscopic analysis did notchange significantly (pH 6.1–7.8).

2.5. UV–Vis spectroscopy

UV–Vis spectroscopy was performed with a Shimadzu modelUV-1700 PC spectrophotometer in the wavelength range of 200–400 nm, and the integral area was calculated from 240 to400 nm, designated as A240–400. Specific ultraviolet absorbance at254 nm (SUVA254) was calculated as the absorbance divided by

the WEOC concentration. The E2/E3 ratio was calculated as the ratioof absorbance at 250 and 365 nm. In addition, the slope of the 275–295 nm region (S275–295) and that of the 350–400 nm region (S350–

400) were obtained as described by Helms et al. (2008).

2.6. Fluorescence spectroscopy

Fluorescence spectroscopy was recorded using a Perkin–Elmermodel LS50B fluorescence spectrophotometer in a clear quartzcuvette. The slit widths were 10 nm for both excitation and emis-sion monochromators, and the scan speed was set at500 nm min�1. Emission spectra were detected at an excitationwavelength of 351 nm, designated as A351, and the humificationdegree of compost WEOM was calculated according to the reportof Milori et al. (2006). Excitation spectra were obtained over arange of 300–500 nm using an emission wavelength of 520 nm.Synchronous-scan excitation spectra were obtained over a rangeof 250–600 nm with a constant offset (Dk = 30 nm), as proposedby Hur et al. (2009). Excitation–emission matrix (EEM) spectrawere obtained by subsequently scanning the emission spectrafrom 280 to 520 nm by increasing the excitation wavelength by5 nm increments from 200 to 440 nm. After regulating the scatter-ing using interpolation in the areas affected by first- and second-order Rayleigh and Raman scatter (Bahram et al., 2006), the fluo-rescence regional integration (FRI) technique was adopted for anal-ysis (Chen et al., 2003).

2.7. Multivariate statistical analysis

SPSS version 16.0 was used for multivariate statistical analysis,i.e., correlation analyses and hierarchical cluster analysis (HCA).HCA was performed on all data obtained for the entire compostsamples with Ward’s method, which uses the squared Euclideandistance as a similarity measure.

3. Results and discussion

3.1. FTIR spectra

The FTIR spectra of WEOM extracted from uncomposted andcomposted MSW are presented in Fig. 1. The main absorptionbands and corresponding assignments are the following: (a) a com-mon, intense broad band at about 3372–3381 cm�1, usually attrib-uted to the OAH stretching of phenols, alcohols, and carboxylicgroups, and as well as to the NAH stretching of amides andamines; (b) an absorption at about 2981 cm�1, preferentially as-cribed to aromatic CAH stretching; (c) two bands at about 2931–2936 and 2875 cm�1 caused by the CAH stretching vibration of ali-phatic structures, such as fatty acids, waxes and various other ali-phatics; (d) an absorption at 1645–1654 cm�1 attributed to theC@C stretching of aromatic rings; (e) a sharp band at about1517 cm�1 ascribed to the NAH deformation and C@N stretchingof amides (amide II band); (f) a sharp band at about 1408–1419 cm�1, generally attributed to the CAO asymmetric stretching,OAH deformation, and CAOAH deformation of the carboxyl groupsand symmetric stretching of the COO� ions; (g) a weak broad bandcentered at about 1371 cm�1 possibly due to OAH deformationand CAO stretching of phenolic OH and/or to antisymmetricstretching of COO� groups; (h) a weak broad band at about1270 cm�1 generated by the CAO stretching and OAH deformationof COOH and/or CAO stretching of aryl ethers and phenols; (i) apeak at about 1125–1145 cm�1 stemming from the CAOH stretch-ing of aliphatic OH; (k) a sharp band centered at about 1114 cm�1

possibly due to the CAO stretching of secondary alcohols and/orethers; and (j) an absorption at about 1044–1048 cm�1 generally

Page 3: Spectroscopic characterization of water extractable organic matter during composting of municipal solid waste

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Fig. 1. FTIR spectra of WEOM at different stages of composting.

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Fig. 2. UV–Vis spectra of WEOM at different stages of composting.

X. He et al. / Chemosphere 82 (2011) 541–548 543

attributed to CAO stretching of polysaccharides or polysaccharide-like substances (Chefetz et al., 1998; Plazaa et al., 2007; Abouel-wafa et al., 2008; García-Gil et al., 2008; Droussi et al., 2009).

The FTIR spectra of WEOM extracted from uncomposted MSWdiffered markedly from that of composted MSW. The bands at1517, 1371, and 1270 cm�1 disappeared after 7 d of composting,indicating a rapid degradation of amides, aryl ethers, and phenols

during the active stage. The FTIR spectra of WEOM extracted fromcomposted MSW were similar and varied only in the relative inten-sity of absorption bands. During composting, a decrease was ob-served in the relative intensity of absorption bands at 2934,2875, 1115, and 1044 cm�1, whereas an increasing trend was de-tected in the relative intensity at about 1654 cm�1. This can beattributed to the preferential degradation of aliphatics, alcohols,ethers, and polysaccharides used for the energy requirements ofthe microorganisms and an increase in aromaticity during com-posting (Abouelwafa et al., 2008). Thus, we can conclude that thecomposting process is characterized by the disappearance of easilybiodegradable compounds and corresponding increase in aromaticcharacter.

3.2. UV–Vis spectroscopy

The UV–Vis spectra of WEOM extracted from the compost atdifferent stages are shown in Fig. 2. In all cases, the absorptionspectra of the WEOM samples were similar and consisted of anexponential decrease with a shoulder around 280 nm, as typicallyobserved in previous studies (Korshin et al., 1997; Domeizel et al.,2004; Fuentes et al., 2006; Vieyra et al., 2009). The absorbance in-creased with increasing composting time, and the 7-, 14-, 21- and51-d-old composts showed absorbance about fourfold more in-tense than the 0-d-old compost.

Although some authors consider WEOM absorption spectra oflittle usefulness because of its apparent featureless character, anumber of studies have shown that an adequate analysis of thesespectra may provide highly important information on the chemicalstructure of WEOM (Fuentes et al., 2006). To investigate thesecharacteristics in detail, SUVA254, E2/E3, S275–295, S350–400, andA240–400 were measured (Table 1).

The SUVA254 value, which has been widely used as an index forthe abundance of aromatic carbon in the WEOM (Shao et al., 2009),increased with composting time. This means that the aromaticityof the compost WEOM increases over time.

The absorbance ratio at 465 and 665 nm (namely E4/E6) hasbeen widely used for the characterization of humic substances(Kang et al., 2002; Fuentes et al., 2006; Fialho et al., 2010). How-ever, the E4/E6 value was not valid in the case of non-humifiedmaterials (Fuentes et al., 2006). In many cases, little or no measur-able absorption at 665 nm was observed (Helms et al., 2008). Forthis study, therefore, the E2/E3 ratio was chosen to monitor thechanges in WEOM composition during composting. The values ofE2/E3, inversely proportional to humification and molecular weight(Wang et al., 2009), decreased with increasing composting time,suggesting that the degree of humification and molecular weightof the WEOM increases during the composting process.

Page 4: Spectroscopic characterization of water extractable organic matter during composting of municipal solid waste

Table 1UV–Vis spectra of the evolution of SUVA254, E2/E3, S275–295, S350–400 and A240–400 at different stages of composting.

Composting time (d) SUVA254 E2/E3 S275–295 S350–400 A240–400

0 0.19 ± 0.02 7.00 ± 0.26 3.60 ± 0.11 5.52 ± 0.27 0.59 ± 0.077 0.93 ± 0.05 6.80 ± 0.21 3.30 ± 0.23 5.23 ± 0.32 5.18 ± 0.15

14 0.96 ± 0.07 7.78 ± 0.11 3.36 ± 0.16 5.45 ± 0.16 5.16 ± 0.2221 1.19 ± 0.03 6.2 ± 0.16 3.34 ± 0.21 5.12 ± 0.33 6.63 ± 0.3451 1.54 ± 0.11 4.75 ± 0.13 3.25 ± 0.13 4.71 ± 0.18 9.25 ± 0.33

544 X. He et al. / Chemosphere 82 (2011) 541–548

Spectral slopes provide additional insights into the averagecharacteristics of WEOM than absorption values alone, and havebeen widely used as indicators of molecular weight, composition,and source. For example, Helms et al. (2008) successfully describedthe changes in chromophoric dissolved organic matter composi-tion brought by photodegradation using the S275–295 and S350–400

value. Hur et al. (2009) distinguished between the humic acidsfrom soils and lake sediments using the S350–400 value. In thisstudy, the S275–295 values of compost WEOM showed a good corre-lation with the SUVA254 values (R = �0.939, p < 0.05), whereas norelation was found between the S350–400 values and the SUVA254

values. These results suggest that the spectral slope at lower wave-lengths may be a better index for the aromaticity of compostWEOM. This is consistent with the findings of Helms et al.(2008), but contradicts those of Hur et al. (2009). The different re-sults may be explained by the structural complexity and the differ-ence in source of organic matter.

WEOM absorbs light at wavelengths between 200 and 400 nm.According to the report of Korshin et al. (1997), the absorptionband at wavelengths between 200 and 230 nm is referred to asthe benzenoid (Bz) band, whereas the absorption band at wave-lengths between 240 and 400 nm is referred to as the electron-transfer (ET) band. The intensity of the ET band is largely affectedby the presence of polar functional groups, whereas the Bz band isalmost unaffected. The presence of the aromatic rings of polarfunctional groups, such as hydroxyl, carbonyl, carboxyl, and estergroups, increases the intensity of the ET band. By contrast, non-po-lar aliphatic groups attached to the ring do not increase the inten-sity of the ET band (Korshin et al., 1997; Fuentes et al., 2006). Inthis study, the area of the ET band increased from 0.59 in the 0-d-old compost to 9.25 in the 51-d-old compost, and the 7-, 14-,21- and 51-d-old composts showed an area 8-fold larger than the0-d-old compost. These results indicate that the aromatic struc-tures in WEOM present a higher degree of substitution with oxy-gen-containing functional groups with increasing compostingtime—a phenomenon that happens to a significant extent at theinitial stage of composting. This result is consistent with that re-ported by Vieyra et al. (2009).

3.3. Fluorescence spectroscopy

The fluorescence of organic matter is mainly related to the pres-ence of condensed aromatic rings and/or unsaturated aliphatic car-bon chains. According to Senesi et al. (2003), organic substanceswith fluorescence signals at short wavelengths are associated withthe presence of simple structural components with a low degree ofaromatic polycondensation, whereas fluorescence signals at longwavelengths are related to the presence of complex structuralcomponents with a high degree of conjugation. Therefore, a redshift in the maximum fluorescence intensity can be attributed toan increase in aromatic group condensation in these molecules(Kalbitz et al., 1999). Conversely, organic substances with a higherdegree of aromatic polycondensation generally have higher chem-ical stability, increasing the residence time of organic matter in theenvironment and consequently improving soil structure and fertil-ity (Santos et al., 2010).

In this work, the emission spectra of the compost WEOM werecharacterized by a broad band, and the maximum fluorescenceintensity shifted from 360 to 430 nm after 51 d of composting(Fig. 3a), indicating an increase in aromatic group condensationduring the process. The A351 value, which is proportional to the de-gree of humification organic matter, showed a strong increase dur-ing composting (Table 2).This means the degree of humification ofcomposting materials increases over time, which is consistent witha previous report (Marhuenda-Egea et al., 2007).

The excitation spectra of the compost WEOM exhibited morepeaks compared with the emission spectra (Fig. 3b). The excitationspectra of the compost WEOM were characterized by four majorpeaks. According to Provenzano et al. (2004), peaks at around436 nm are possibly attributed to aromatic rings bearing electrondonor groups; peaks at about 383 nm are possibly due to fluoro-phores originated from the polycondensation of carbonyl groupsand lignin-derived phenolic structures, while the other two peaks,centered at around 335 and 350 nm, have not been reported in pre-vious literature (Provenzano et al., 2004). The ratio of fluorescenceintensities at 436 and 383 nm (I436/I383) showed a clear negativecorrelation with the SUVA254 value (R = �0.890, p < 0.05) and A351

value (R = �0.887, p < 0.05), suggesting that the I436/I383 ratio inexcitation spectra can be used to investigate the degree of humifi-cation of compost organic materials.

In general, synchronous-scan excitation spectra represent thesummation of the spectra of several different fluorophores presentin organic matter, and they appear better resolved compared withexcitation and emission spectra. Synchronous fluorescence spectraof WEOM observed from the compost are shown in Fig. 3c. Threemain regions were identified in all of them: Region A (250–308 nm), referred to as the protein-like region (PLR), is associatedwith the presence of proteinaceous materials and monoaromaticcompounds (Santos et al., 2008). Region B (308–363 nm), referredto as the fulvic-like region (FLR), indicates the presence of polycy-clic aromatics with three to four fused benzene rings and two tothree conjugated systems in unsaturated aliphatic structures (Peu-ravuori et al., 2002; Santos et al., 2008), which are usually found infulvic acids. Peak C (363–595 nm) corresponds to polycyclic aro-matics with approximately 5–7 fused benzene rings (Peuravuoriet al., 2002). This region, called humic-like region (HLR), is relatedto the presence of humic substances. For this study, the three rel-ative fluorescence regions assigned to PLR, FLR, and HLR were cal-culated, each of which corresponds to the relative percentage ofthe area of fluorescence at 250–308, 308–363, 363–595 nm,respectively. With an increase in composting time, a decrease inthe PLF value (from 0.70 in the 0-d-old compost to 0.19 in the51-d-old compost) was observed, whereas an increase both inthe FLR (from 0.16 to 0.35) and HLR values (from 0.14 to 0.46)was found. The results of the comparative analysis indicate thatthe composting process is characterized by a decrease in the pro-tein-like materials and an increase in the humic and fulvic-likematerials in the WEOM fraction. This is consistent with the reportof Marhuenda-Egea et al. (2007).

The EEM spectra (Fig. 4) qualitatively showed the WEOM com-position, characterized by excitation/emission (Ex/Em) wavelengthpairs and the specific fluorescence intensity. Three peaks were de-

Page 5: Spectroscopic characterization of water extractable organic matter during composting of municipal solid waste

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51 d

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c

b

Fig. 3. Conventional fluorescence spectra of WEOM at different stages of composting; (a) emission spectra, (b) excitation spectra and (c) synchronous-scan excitation spectra.

Table 2Fluorescence spectra of the changes in humification parameters at different stages of composting.

Compostingtime (d)

A351 I436/I383 PLR FLR HLR Region Ia Region IIb Region IIIc Region IVd Region Ve

0 6565 ± 318 1.11 ± 0.08 0.70 ± 0.05 0.16 ± 0.03 0.14 ± 0.02 0.222 ± 0.007 0.390 ± 0.017 0.195 ± 0.011 0.112 ± 0.009 0.082 ± 0.0037 40 162 ± 1185 0.49 ± 0.05 0.30 ± 0.02 0.30 ± 0.03 0.40 ± 0.02 0.053 ± 0.004 0.209 ± 0.011 0.408 ± 0.023 0.102 ± 0.007 0.229 ± 0.021

14 46 174 ± 2066 0.47 ± 0.04 0.29 ± 0.03 0.32 ± 0.05 0.39 ± 0.02 0.056 ± 0.002 0.190 ± 0.012 0.414 ± 0.013 0.102 ± 0.008 0.238 ± 0.01821 54 843 ± 980 0.43 ± 0.02 0.24 ± 0. 01 0.33 ± 0.04 0.43 ± 0.03 0.028 ± 0.004 0.129 ± 0.009 0.472 ± 0.020 0.094 ± 0.005 0.276 ± 0.01151 72 474 ± 1077 0.45 ± 0.03 0.19 ± 0.04 0.35 ± 0.03 0.46 ± 0.05 0.020 ± 0.002 0.134 ± 0.013 0.449 ± 0.019 0.098 ± 0.009 0.298 ± 0.017

a Region I: tyrosine-like organic compounds.b Region II: tryptophan-like organic compounds.c Region III: fulvic acid-like materials.d Region IV: soluble microbial byproduct-like materials; region.e Region V: humic acid-like materials.

X. He et al. / Chemosphere 82 (2011) 541–548 545

tected in the WEOM extracted from the 0-d-old compost. The firstpeak was characterized by an Ex/Em wavelength pair of 219/348 nm. According to previous studies (Coble, 1996; Chen et al.,2003), this peak is associated with protein-derived compounds,such as tyrosine and tryptophan. The second peak was centeredat an Ex/Em wavelength pair of 278/354 nm and related to solublemicrobial byproduct-like materials (Chen et al., 2003). As the com-posting process proceeded, the aforementioned two peaks disap-peared because of the degradation of organic matters as a resultof microbial activity. The third peak had its own Ex/Em wavelengthpairs of 313/406 nm, and its intensity increased gradually duringcomposting. According to the report of Marhuenda-Egea et al.(2007), the presence of this peak is associated with humic acid-likesubstances, and the increase in peak intensity during the compost-ing process is related to a high degree of humification or matura-tion (Marhuenda-Egea et al., 2007). At day 7, the fourth peaksuddenly emerged with an Ex/Em wavelength pair of 220/429 nm. This peak is related to fulvic acid-like compounds and

can be used to monitor changes in fulvic acid-like materials duringthe composting of MSW (Marhuenda-Egea et al., 2007; Shao et al.,2009). The last peak was observed at an Ex/Em wavelength pair of289/421 nm, which fully emerged in the 51-d-old compost sample.According to Shao et al. (2009), the presence of this peak is associ-ated with humic acid-like matter and indicates the stabilization ofcompost organic matter.

To better understand the EEM fluorescence characteristics ofWEOM samples, we employed the FRI method to analyze the fiveexcitation–emission regions, as described by Chen et al. (2003).In general, peaks at shorter excitation wavelengths (<250 nm)and shorter emission wavelengths (<380 nm) are related to simplearomatic proteins, such as tyrosine and tryptophan (Regions I andII) (Ahmad and Reynolds, 1999). Peaks at shorter excitation wave-lengths (<250 nm) and longer emission wavelengths (>380 nm) arerelated to fulvic acid-like substances (Region III) (Mounier et al.,1999). Peaks located at intermediate excitation wavelengths(250–280 nm) and shorter emission wavelengths (<380 nm) are

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200

250

300

350

400

Exci

tatio

n (n

m)

0 d 7 d

350 400 450 500Emission (nm)

21 d

300 350 400 450 500200

250

300

350

400

Emission (nm)

Exci

tatio

n (n

m)

0

103.4

206.9

310.3

413.8

517.2

620.7

724.1

827.6

931.01000

51 d

200

250

300

350

400

Exci

tatio

n (n

m)

14 d

Fig. 4. EEM spectra of WEOM at different stages of composting.

546 X. He et al. / Chemosphere 82 (2011) 541–548

associated with soluble microbial byproduct-like materials (RegionIV) (Coble, 1996; Reynolds and Ahmad, 1997). Peaks at longer exci-tation wavelengths (>280 nm) and longer emission wavelengths(>380 nm) represent humic acid-like organics (Region V) (Artingeret al., 2000). Normalizing the cumulative excitation–emission areavolumes to relative regional areas resulted in the percent fluores-cence responses (Pi,n), which are shown in Table 2.

Although showing a little variation, the Pi,n of the Regions I, IIand IV exhibit a decreases during the composting process. Thismeans that the simple aromatic proteins and soluble microbialbyproduct-like materials decreased during composting. On the

Fig. 5. Hierarchical cluster analysis of the spectroscop

other hand, the Pi,n of Region V strongly increased during the com-posting process, indicating an increase in humic-like materials. ThePi,n of Region III increased during the active stage, but decreasedduring the curing stage. The increase in the Pi,n of Region III duringthe active stage could be related to the formation of fulvic acids,whereas the decrease in the Pi,n of Region III during the curingstage may be associated with biological bio-oxidation. The resultsreported by Jouraiphy et al. (2008) also suggest that the fulvic acidsare easily broken down and transformed into higher levels of stablestructures through bio-oxidation.

ic parameters at different stages of composting.

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3.4. Multivariate statistical analysis

HCA was performed on based on the data collected in Tables 1and 2. The dendrogram on Fig. 5 shows the similarities and dissim-ilarities between the compost samples during the composting pro-cess. According to the study of Zbytniewski and Buszewski (2005),the lower the distances between the samples, the more similarthey are. The WEOM samples after 7 and 14 d of compostingformed the first cluster, indicating that the two samples are similarand organic matter transformations are slow during the period.The most similar WEOM sample to this cluster was the sample ma-tured for 21 d, whereas the most dissimilar WEOM sample to thefirst cluster was the uncomposted sample. The aforementioned re-sult suggests that the degradation of organic matter is under a slowspeed from day 14 to day 21, whereas the process occurs rapidlyduring the first 8 d. This conclusion is consistent with those drawnfrom the FTIR and UV–Vis spectra analysis. The slow biodegrada-tion of composting mixtures may be related to the thermophilicphase, when microbial activity was restrained.

4. Conclusions

WEOM extracted from MSW during composting was character-ized by spectroscopic techniques. The FTIR spectra showed thateasily degradable organic constituents, such as aliphatics, alcohols,and polysaccharides, were decomposed and as a result, the maturecompost contained more aromatic structures. The UV–Vis spectraof WEOM also showed an increase in aromaticity, degree of humi-fication, oxygen-containing functional groups, and molecularweight after composting. In addition, the UV–Vis spectra suggestthat the spectral slope at lower wavelengths (275–295 nm) maybe a good index for the aromaticity of WEOM from MSW. Fluores-cence spectra in this research demonstrated that the protein-likematerials decreased, whereas the humic-like materials increasedduring the composting process. The fluorescence spectra alsoshowed that the fulvic-like substances increased during the activestage and decreased during the curing stage. The maturity of MSWduring composting was characterized by the presence of the peakwith an Ex/Em wavelength pair of 289/421 nm.

Acknowledgments

This work was financially supported by the Basic Research Pro-gram of China (No. 2005CB724203), the National Public Benefit(Environmental) Research Foundation of China (No. 200909079),and the National Natural Science Foundation of China (No.51078340).

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