Bioresource Technology 161 (2014) 179–185
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Fractions and biodegradability of dissolved organic matter derived from different composts Zimin Wei a, Xu Zhang a, Yuquan Wei a, Xin Wen a, Jianhong Shi a, Junqiu Wu a, Yue Zhao a,⇑, Beidou Xi b a b
College of Life Science, Northeast Agricultural University, Harbin 150030, China Laboratory of Water Environmental System Engineering, Chinese Research Academy of Environmental Science, Beijing 100012, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Nine mature composts from different
sources in China were investigated. Analyze the distribution of MW
fractions and BOD5 of DOM among the samples. Identify the correlation of BOD5 with the concentration of MW fractions. Describe the feasibility of BOD5 as a biodegradable indicator for DOM from compost.
a r t i c l e
i n f o
Article history: Received 14 December 2013 Received in revised form 4 March 2014 Accepted 6 March 2014 Available online 20 March 2014 Keywords: Dissolved organic matter (DOM) Mature composts Molecular weight Biodegradability
a b s t r a c t An experiment was conducted to determine the fractions of molecular weights (MW) and the biodegradability of dissolved organic matter (DOM) in mature composts derived from dairy cattle manure (DCM), kitchen waste (KW), cabbage waste (CW), tomato stem waste (TSW), municipal solid waste (MSW), green waste (GW), chicken manure (CM), sludge (S), and mushroom culture waste (MCW). There were distinct differences in the concentration and MW fractions of DOM, and the two measures were correlated. Fraction MW > 5 kDa was the major component of DOM in all mature composts. Determined 5 day biochemical oxygen demand (BOD5) of DOM was correlated to the concentration of DOM and all MW fractions except MW > 5 kDa, indicating that the biodegradability of DOM was a function of the content and proportion of fraction MW < 5 kDa. This study suggests that the amount and distribution of low MW fractions affect DOM biodegradability. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Dissolved organic matters (DOM) are an organic continuum of mixed organisms that can pass through 0.45-lm membranes with a series of different molecular sizes and structures, and is a ubiquitous component in aquatic and terrestrial ecosystem (McDowell et al., 2006; Yue et al., 2006). The structure and composition of ⇑ Corresponding author. Tel./fax: +86 451 55190413. E-mail addresses: [email protected] (Z. Wei), [email protected] (Y. Zhao). http://dx.doi.org/10.1016/j.biortech.2014.03.032 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
DOM, however, are complex and difficult to determine, due to its wide range of chemical compounds and variety of decomposed and synthesized products. The DOM may contain low molecular weight (MW) substances (free amino acids, sugars, etc.) as well as various types of macromolecular components (enzymes, amino sugars, polyphenols, humic acid and other mixtures) (Chefetz et al., 1998). The MW of these substances may range from several hundred to several ten thousand Dalton (Da) (Candler et al., 1988). Usually, DOM contains a large number of organic functional groups with high active sites, such as phenol, hydroxyl, carboxyl, thio and amino subunits. These compounds can bind with heavy
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Z. Wei et al. / Bioresource Technology 161 (2014) 179–185
metals to form coordination complexes that are utilizable for crops (Cano-Aguilera et al., 2005). Analysis of DOM usually includes composition, functionalities, and structural, chemical and spectroscopic characteristics (Chefetz et al., 1998; Provenzano et al., 2001; Santos et al., 2010; Caricasole et al., 2010; He et al., 2011a; Wei et al., 2014). Resin adsorption and ultrafiltration are the main methods for analyzing the composition of DOM derived from various sources (Young et al., 2005; Lou and Xie, 2006; Seo et al., 2007; Lu et al., 2009). Using XAD-8 resin adsorption allows DOM to be fractionated into hydrophobic (acid, base, neutral) and hydrophilic compounds, while ultrafiltration using a tangential flow filtration (TFF) system or a high-pressure size exclusion chromatography separates DOM into different MW fractions. Numerous studies have been conducted to evaluate changes of MW fractions in DOM derived from soil, landfill leachate and aquatic ecosystems (Young et al., 2005; Lu et al., 2009; Jones et al., 2012), but few have focused on the MW fractions of DOM derived from composts. Composting is a biological process, through which a stabilized material is formed. The material can be used as a source of nutrients and a soil conditioner. Understanding the characteristic of the DOM formed during composting is important, because most of the biologic activities occur in solid–liquid interface, changes in DOM during composting reflect transformation progress and compost stability (Said-Pullicino et al., 2007). Humic acid-like matter is the major component of DOM in mature composts, and can be transformed into humic acid in soils quickly after land application. The movement of DOM from composts through the soil profile enhances the cycling of C and other nutrients and microbial activities (Cronan et al., 1992), thus improving soil quality and its agronomic/biomass productivity. Although numerous studies have been conducted to evaluate changes of these characteristics during composting (Chefetz et al., 1998; Caricasole et al., 2010; He et al., 2011b), few studies have examined the biodegradability of DOM derived from different composts. Biodegradability is an important factor that influences the utilization of DOM in soils, but varies, depending on the chemical composition, MW, and size of DOM. While it is known that DOM in an ecosystem is typically composed of a labile pool, consisting of simple organic molecules, and a refractory pool that is composed of higher MW humic acids (Geller, 1986; Young et al., 2005), there is no standard method that has been developed for measuring the biodegradable fraction of DOM in terrestrial or aquatic ecosystems (McDowell et al., 2006). The degradability of DOM has been typically determined based on production of CO2 or consumption of dissolved organic carbon using batch materials and flow-through or static bioreactors (Trulleyová and Rulík, 2004; McDowell et al., 2006). However, long terms (several months), large amounts of test materials and costly equipment are needed for the process (Yano et al., 1998; Trulleyová and Rulík, 2004). Alternatively, the amount of dissolved oxygen consumed in a 5-day period by bacteria (5-day biochemical oxygen demand, BOD5) in wastewater is usually used as an indicator of organic loading (Jung et al., 2008). Relatively, measuring BOD5 is less time-consuming, and demands less materials and other consumables. However, the use of BOD5 could be as a biodegradable indicator for DOM derived from different composts has not been studied. The objective of this study was to determine the fractions of DOM derived from different composts based on MW by ultrafiltration and to evaluate the biodegradability of DOM based on the amount and distribution of MW fractions. Furthermore, the feasibility of using BOD5 as a biodegradable indicator for DOM derived from different composts was also demonstrated.
2. Methods 2.1. Sample collection and storage Nine trapezoidal piles of dairy cattle manure (DCM), kitchen waste (KW), cabbage waste (CW), tomato stem waste (TSW), municipal solid waste (MSW), green waste (GW), chicken manure (CM), mushroom culture waste (MCW), and sludge (S) were prepared by Shanghai Songjiang Composting Plant. Composting was considered finished when the temperature of the pile became stable and the germination index approached 80%. Approximately 2 kg of each of the mature composts were collected and stored at 4 °C for analysis of DOM. The concentration of microorganism of different mature compost was from 6.8 106 to 2.3 107 CFU g 1, details of composting and other DOM properties were described in a separate paper (Wei et al., 2014). 2.2. Extraction of DOM DOM was obtained as described by Said-Pullicino et al. (2007). Briefly, samples of composts were extracted with distilled water (solid to water at 1:10, w/v) for 24 h in a horizontal shaker at room temperature. The suspensions were centrifuged at 10,000 rpm for 10 min and filtered through a 0.45-lm membrane. The MW of DOM was fractionated within 24 h. 2.3. Ultrafiltration A tangential flow filtration (TFF) system equipped with membrane packages of 65 Da, 1 kDa, and 5 kDa (Pall Corporation) was used to separate fractions of MW < 65 Da, 65 Da < MW < 1 kDa, 1 kDa < MW < 5 kDa, and MW > 5 kDa. 2.4. Measurements DOM (C, mg kg 1) was analyzed using an aurora combustion total organic carbon analyzer 1030C (OI, America). Total organic matter (TOM) was measured based on potassium dichromate oxidation (Nelson and Sommers, 1982). The dilution method BOD5 test was carried out (Fulazzaky, 2013), and the dissolved oxygen (DO) was determined using the Winkler’s iodometric method (APHA, 1992). Briefly, 25 ml of DOM were transferred into a dissolved oxygen bottle, and diluted inoculated water was added through a siphon to bring the volume to 250 ml. The DO was measured and the bottle was sealed to prevent oxygen from further dissolving in. A blank (no DOM) was prepared following the same procedure. All bottles were then placed into a constant temperature incubator (20 °C) and kept for 5 d ± 4 h in the dark to prevent photosynthesis. The DO was then measured again. The difference between the final DO and initial DO measured was used as the BOD5. 2.5. Multivariate statistical analysis Multivariate analysis was conducted using SPSS, version 17.0, to analyze one-way ANNOVA, correlations, and hierarchical clusters (HCA). 3. Results and discussion 3.1. DOM and its fractions Fig. 1 illustrates the concentrations of DOM and its fractions in different composts. The concentration ranged from 211.8 to
Z. Wei et al. / Bioresource Technology 161 (2014) 179–185
181
Fig. 1. Concentration of DOM (a), MW < 65 Da (b), 65 Da < MW < 1 kDa (c), 1 kDa < MW < 5 kDa (d), and MW > 5 kDa (e) in composts of different sources.
3012.7 mg kg 1 for DOM (Fig. 1a), from 34.4 to 461.5 mg kg 1 for MW < 65 Da (Fig. 1b), from 19.9 to 448.7 mg kg 1 for 65 Da < MW < 1 kDa (Fig. 1c), from 65.9 to 991.3 mg kg 1 for 1 kDa < MW < 5 kDa (Fig. 1d), and from 91.6 to 1453.6 mg kg 1 for MW > 5 kDa (Fig. 1e). There were distinct differences in concentration of DOM and its fractions among the compost sources. The highest concentration of DOM and MW fraction 5 kDa occurred in DCM and KW. The lowest concentration was in MSW for DOM and all of its MW fractions. The ratio of the maximum to minimum concentrations was 14.22, 13.42, 22.55, 15.04, and 15.87 for DOM,
MW < 65 Da, 65 Da < MW < 1 kDa, 1 kDa < MW < 5 kDa, and MW > 5 kDa, respectively. The large variation was a result of the presence of various organic components in these composts (Raber and Kögel-Knabner, 1997). Correlations and 95% confidence intervals between DOM and its MW fractions are presented in Fig. 2. There was a significant correlation (R2 = 0.696, P = 0.005) between DOM and TOM (Fig. 2a), indicating that a high organic matter content would be associated with a high DOM concentration in mature composts. DOM was also significantly correlated with MW < 65 Da (R2 = 0.756, P = 0.002) (Fig. 2b), 65 Da < MW < 1 kDa (R2 = 0.743, P = 0.003) (Fig. 2c),
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Z. Wei et al. / Bioresource Technology 161 (2014) 179–185
Fig. 2. Correlation and 95% confidence intervals of DOM with TOM (a), MW < 65 Da (b), 65 Da < MW < 1 kDa (c), 1 kDa < MW < 5 kDa (d), and MW > 5 kDa (e), MW < 65 Da with 65 Da < MW < 1 kDa (f), and 65 Da < MW < 1 kDa (g), 65 Da < MW < 1 kDa with 1 kDa < MW < 5 kDa (h).
1 kDa < MW < 5 kDa (R2 = 0.653, P = 0.008) (Fig. 2d), and MW > 5 kDa (R2 = 0.653, P = 0.008) (Fig. 2e). Meanwhile, MW < 65 Da, 65 Da < MW < 1 kDa and 1 kDa < MW < 5 kDa were correlated with each other (Fig. 2f–h), however, there was no statistically significant correlation between MW > 5 kDa and other fractions. Furthermore, one-way ANNOVA demonstrated significant differences in concentration among MW fractions of DOM derived from different composts (F = 12.28, P < 0.001). Post hoc test
between MW fractions showed that the low MW fractions (MW < 65 Da, 65 Da < MW < 1 kDa, and 1 kDa < MW < 5 kDa) did not differ from each other, but were significantly different from MW > 5 kDa (F = 890.17, P < 0.05), suggesting that MW = 5 kDa maybe a line of low and high MW in DOM. The MW distribution of DOM was similar among all composts (Fig. 3). The similar distribution was a result of high degrees of humification and stabilization in DOM derived from mature
Z. Wei et al. / Bioresource Technology 161 (2014) 179–185
183
Fig. 3. Distribution of MW fractions in DOM derived from different mature composts.
composts (Wei et al., 2014). In general, composting is associated with increasing aromatization in DOM (Chefetz et al.,1998; SaidPullicino et al., 2007; Caricasole et al., 2010), resulting in enriched aromatic fractions as well as high MW in DOM at the final stage of composting, despite of differences in raw materials. Fractions MW < 65 Da and 65 Da < MW < 1 kDa had the lowest proportions, averaging 10.58% and 9.66%, respectively, while fraction MW > 5 kDa had the highest proportion, averaging 61.53%. The proportion of DOM as MW > 5 kDa was above 50% for all composts except TSW (36.26%), CM (36.88%) and MSW (43.25%). The distribution indicated that higher MW (>5 kDa) substances were the major component of DOM in all mature composts. In recent studies (Wei et al., 2007; He et al., 2011b) compositing was characterized as a process of simultaneous degradation of simple protein-liked materials and formation of humic acid-like materials in DOM. Accordingly, the MW > 5 kDa fraction that accounted for the largest proportion of mature composts was likely humic substances that have a high aromatization degree.
Fig. 4. BOD5 in DOM derived from different mature composts.
4 kDa in landfill leachate were readily biodegradable (He et al., 2006).
3.2. Biodegradability of DOM 3.3. Relationship between BOD5 and DOM and its MW fractions BOD5 is an important index for water pollution (Jung et al., 2008) because it indicates the degree of biological degradation of organic matter. Therefore, a higher BOD5 indicates a higher biodegradability for DOM. The concentration of BOD5 in DOM ranged from 5.7 to 42.9 mg l 1 among different composts (Fig. 4), with the maximum observed in CM, being 7.50 times that in MSW. BOD5 was significantly correlated with DOM (R2 = 0.640, P = 0.010) (Fig. 5a), meaning that the higher the concentration of DOM in mature composts was, the more degradable the material would be. BOD5 was also correlated with MW < 65 Da (R2 = 0.771, P = 0.002) (Fig. 5b), 65 Da < MW < 1 kDa (R2 = 0.798, P = 0.001) (Fig. 5c), and 1 kDa < MW < 5 kDa (R2 = 0.600, P = 0.014) (Fig. 5d), but not with MW > 5 kDa. These data suggested that the lower MW and simpler structural components in DOM were the biodegradable organic compounds, and the higher MW components were less degradable by microorganism. Results were consistent with the report that compounds with a molecular weight less than
We previously described the degree of humification in DOM using HCA that considered correlations between different variables (Wei et al., 2014), but fluorescence spectroscopic analyses failed to reveal differences in DOM concentration between different composts. In order to describe the relationship between biodegradability and concentration of DOM and its MW fractions in composts derived from different sources, HCA was performed based on the concentration of DOM, MW < 65 Da, 65 Da < MW < 1 kDa, and 1 kDa < MW < 5 kDa (Fig. 6). The DOM of the composts were clustered into three groups. The first group included DCM and CM, DOM from these composts contained high concentration and contributed from biodegradable low MW compounds (
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Fractions and biodegradability of dissolved organic matter derived from different composts Zimin Wei a, Xu Zhang a, Yuquan Wei a, Xin Wen a, Jianhong Shi a, Junqiu Wu a, Yue Zhao a,⇑, Beidou Xi b a b
College of Life Science, Northeast Agricultural University, Harbin 150030, China Laboratory of Water Environmental System Engineering, Chinese Research Academy of Environmental Science, Beijing 100012, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Nine mature composts from different
sources in China were investigated. Analyze the distribution of MW
fractions and BOD5 of DOM among the samples. Identify the correlation of BOD5 with the concentration of MW fractions. Describe the feasibility of BOD5 as a biodegradable indicator for DOM from compost.
a r t i c l e
i n f o
Article history: Received 14 December 2013 Received in revised form 4 March 2014 Accepted 6 March 2014 Available online 20 March 2014 Keywords: Dissolved organic matter (DOM) Mature composts Molecular weight Biodegradability
a b s t r a c t An experiment was conducted to determine the fractions of molecular weights (MW) and the biodegradability of dissolved organic matter (DOM) in mature composts derived from dairy cattle manure (DCM), kitchen waste (KW), cabbage waste (CW), tomato stem waste (TSW), municipal solid waste (MSW), green waste (GW), chicken manure (CM), sludge (S), and mushroom culture waste (MCW). There were distinct differences in the concentration and MW fractions of DOM, and the two measures were correlated. Fraction MW > 5 kDa was the major component of DOM in all mature composts. Determined 5 day biochemical oxygen demand (BOD5) of DOM was correlated to the concentration of DOM and all MW fractions except MW > 5 kDa, indicating that the biodegradability of DOM was a function of the content and proportion of fraction MW < 5 kDa. This study suggests that the amount and distribution of low MW fractions affect DOM biodegradability. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Dissolved organic matters (DOM) are an organic continuum of mixed organisms that can pass through 0.45-lm membranes with a series of different molecular sizes and structures, and is a ubiquitous component in aquatic and terrestrial ecosystem (McDowell et al., 2006; Yue et al., 2006). The structure and composition of ⇑ Corresponding author. Tel./fax: +86 451 55190413. E-mail addresses: [email protected] (Z. Wei), [email protected] (Y. Zhao). http://dx.doi.org/10.1016/j.biortech.2014.03.032 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
DOM, however, are complex and difficult to determine, due to its wide range of chemical compounds and variety of decomposed and synthesized products. The DOM may contain low molecular weight (MW) substances (free amino acids, sugars, etc.) as well as various types of macromolecular components (enzymes, amino sugars, polyphenols, humic acid and other mixtures) (Chefetz et al., 1998). The MW of these substances may range from several hundred to several ten thousand Dalton (Da) (Candler et al., 1988). Usually, DOM contains a large number of organic functional groups with high active sites, such as phenol, hydroxyl, carboxyl, thio and amino subunits. These compounds can bind with heavy
180
Z. Wei et al. / Bioresource Technology 161 (2014) 179–185
metals to form coordination complexes that are utilizable for crops (Cano-Aguilera et al., 2005). Analysis of DOM usually includes composition, functionalities, and structural, chemical and spectroscopic characteristics (Chefetz et al., 1998; Provenzano et al., 2001; Santos et al., 2010; Caricasole et al., 2010; He et al., 2011a; Wei et al., 2014). Resin adsorption and ultrafiltration are the main methods for analyzing the composition of DOM derived from various sources (Young et al., 2005; Lou and Xie, 2006; Seo et al., 2007; Lu et al., 2009). Using XAD-8 resin adsorption allows DOM to be fractionated into hydrophobic (acid, base, neutral) and hydrophilic compounds, while ultrafiltration using a tangential flow filtration (TFF) system or a high-pressure size exclusion chromatography separates DOM into different MW fractions. Numerous studies have been conducted to evaluate changes of MW fractions in DOM derived from soil, landfill leachate and aquatic ecosystems (Young et al., 2005; Lu et al., 2009; Jones et al., 2012), but few have focused on the MW fractions of DOM derived from composts. Composting is a biological process, through which a stabilized material is formed. The material can be used as a source of nutrients and a soil conditioner. Understanding the characteristic of the DOM formed during composting is important, because most of the biologic activities occur in solid–liquid interface, changes in DOM during composting reflect transformation progress and compost stability (Said-Pullicino et al., 2007). Humic acid-like matter is the major component of DOM in mature composts, and can be transformed into humic acid in soils quickly after land application. The movement of DOM from composts through the soil profile enhances the cycling of C and other nutrients and microbial activities (Cronan et al., 1992), thus improving soil quality and its agronomic/biomass productivity. Although numerous studies have been conducted to evaluate changes of these characteristics during composting (Chefetz et al., 1998; Caricasole et al., 2010; He et al., 2011b), few studies have examined the biodegradability of DOM derived from different composts. Biodegradability is an important factor that influences the utilization of DOM in soils, but varies, depending on the chemical composition, MW, and size of DOM. While it is known that DOM in an ecosystem is typically composed of a labile pool, consisting of simple organic molecules, and a refractory pool that is composed of higher MW humic acids (Geller, 1986; Young et al., 2005), there is no standard method that has been developed for measuring the biodegradable fraction of DOM in terrestrial or aquatic ecosystems (McDowell et al., 2006). The degradability of DOM has been typically determined based on production of CO2 or consumption of dissolved organic carbon using batch materials and flow-through or static bioreactors (Trulleyová and Rulík, 2004; McDowell et al., 2006). However, long terms (several months), large amounts of test materials and costly equipment are needed for the process (Yano et al., 1998; Trulleyová and Rulík, 2004). Alternatively, the amount of dissolved oxygen consumed in a 5-day period by bacteria (5-day biochemical oxygen demand, BOD5) in wastewater is usually used as an indicator of organic loading (Jung et al., 2008). Relatively, measuring BOD5 is less time-consuming, and demands less materials and other consumables. However, the use of BOD5 could be as a biodegradable indicator for DOM derived from different composts has not been studied. The objective of this study was to determine the fractions of DOM derived from different composts based on MW by ultrafiltration and to evaluate the biodegradability of DOM based on the amount and distribution of MW fractions. Furthermore, the feasibility of using BOD5 as a biodegradable indicator for DOM derived from different composts was also demonstrated.
2. Methods 2.1. Sample collection and storage Nine trapezoidal piles of dairy cattle manure (DCM), kitchen waste (KW), cabbage waste (CW), tomato stem waste (TSW), municipal solid waste (MSW), green waste (GW), chicken manure (CM), mushroom culture waste (MCW), and sludge (S) were prepared by Shanghai Songjiang Composting Plant. Composting was considered finished when the temperature of the pile became stable and the germination index approached 80%. Approximately 2 kg of each of the mature composts were collected and stored at 4 °C for analysis of DOM. The concentration of microorganism of different mature compost was from 6.8 106 to 2.3 107 CFU g 1, details of composting and other DOM properties were described in a separate paper (Wei et al., 2014). 2.2. Extraction of DOM DOM was obtained as described by Said-Pullicino et al. (2007). Briefly, samples of composts were extracted with distilled water (solid to water at 1:10, w/v) for 24 h in a horizontal shaker at room temperature. The suspensions were centrifuged at 10,000 rpm for 10 min and filtered through a 0.45-lm membrane. The MW of DOM was fractionated within 24 h. 2.3. Ultrafiltration A tangential flow filtration (TFF) system equipped with membrane packages of 65 Da, 1 kDa, and 5 kDa (Pall Corporation) was used to separate fractions of MW < 65 Da, 65 Da < MW < 1 kDa, 1 kDa < MW < 5 kDa, and MW > 5 kDa. 2.4. Measurements DOM (C, mg kg 1) was analyzed using an aurora combustion total organic carbon analyzer 1030C (OI, America). Total organic matter (TOM) was measured based on potassium dichromate oxidation (Nelson and Sommers, 1982). The dilution method BOD5 test was carried out (Fulazzaky, 2013), and the dissolved oxygen (DO) was determined using the Winkler’s iodometric method (APHA, 1992). Briefly, 25 ml of DOM were transferred into a dissolved oxygen bottle, and diluted inoculated water was added through a siphon to bring the volume to 250 ml. The DO was measured and the bottle was sealed to prevent oxygen from further dissolving in. A blank (no DOM) was prepared following the same procedure. All bottles were then placed into a constant temperature incubator (20 °C) and kept for 5 d ± 4 h in the dark to prevent photosynthesis. The DO was then measured again. The difference between the final DO and initial DO measured was used as the BOD5. 2.5. Multivariate statistical analysis Multivariate analysis was conducted using SPSS, version 17.0, to analyze one-way ANNOVA, correlations, and hierarchical clusters (HCA). 3. Results and discussion 3.1. DOM and its fractions Fig. 1 illustrates the concentrations of DOM and its fractions in different composts. The concentration ranged from 211.8 to
Z. Wei et al. / Bioresource Technology 161 (2014) 179–185
181
Fig. 1. Concentration of DOM (a), MW < 65 Da (b), 65 Da < MW < 1 kDa (c), 1 kDa < MW < 5 kDa (d), and MW > 5 kDa (e) in composts of different sources.
3012.7 mg kg 1 for DOM (Fig. 1a), from 34.4 to 461.5 mg kg 1 for MW < 65 Da (Fig. 1b), from 19.9 to 448.7 mg kg 1 for 65 Da < MW < 1 kDa (Fig. 1c), from 65.9 to 991.3 mg kg 1 for 1 kDa < MW < 5 kDa (Fig. 1d), and from 91.6 to 1453.6 mg kg 1 for MW > 5 kDa (Fig. 1e). There were distinct differences in concentration of DOM and its fractions among the compost sources. The highest concentration of DOM and MW fraction 5 kDa occurred in DCM and KW. The lowest concentration was in MSW for DOM and all of its MW fractions. The ratio of the maximum to minimum concentrations was 14.22, 13.42, 22.55, 15.04, and 15.87 for DOM,
MW < 65 Da, 65 Da < MW < 1 kDa, 1 kDa < MW < 5 kDa, and MW > 5 kDa, respectively. The large variation was a result of the presence of various organic components in these composts (Raber and Kögel-Knabner, 1997). Correlations and 95% confidence intervals between DOM and its MW fractions are presented in Fig. 2. There was a significant correlation (R2 = 0.696, P = 0.005) between DOM and TOM (Fig. 2a), indicating that a high organic matter content would be associated with a high DOM concentration in mature composts. DOM was also significantly correlated with MW < 65 Da (R2 = 0.756, P = 0.002) (Fig. 2b), 65 Da < MW < 1 kDa (R2 = 0.743, P = 0.003) (Fig. 2c),
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Z. Wei et al. / Bioresource Technology 161 (2014) 179–185
Fig. 2. Correlation and 95% confidence intervals of DOM with TOM (a), MW < 65 Da (b), 65 Da < MW < 1 kDa (c), 1 kDa < MW < 5 kDa (d), and MW > 5 kDa (e), MW < 65 Da with 65 Da < MW < 1 kDa (f), and 65 Da < MW < 1 kDa (g), 65 Da < MW < 1 kDa with 1 kDa < MW < 5 kDa (h).
1 kDa < MW < 5 kDa (R2 = 0.653, P = 0.008) (Fig. 2d), and MW > 5 kDa (R2 = 0.653, P = 0.008) (Fig. 2e). Meanwhile, MW < 65 Da, 65 Da < MW < 1 kDa and 1 kDa < MW < 5 kDa were correlated with each other (Fig. 2f–h), however, there was no statistically significant correlation between MW > 5 kDa and other fractions. Furthermore, one-way ANNOVA demonstrated significant differences in concentration among MW fractions of DOM derived from different composts (F = 12.28, P < 0.001). Post hoc test
between MW fractions showed that the low MW fractions (MW < 65 Da, 65 Da < MW < 1 kDa, and 1 kDa < MW < 5 kDa) did not differ from each other, but were significantly different from MW > 5 kDa (F = 890.17, P < 0.05), suggesting that MW = 5 kDa maybe a line of low and high MW in DOM. The MW distribution of DOM was similar among all composts (Fig. 3). The similar distribution was a result of high degrees of humification and stabilization in DOM derived from mature
Z. Wei et al. / Bioresource Technology 161 (2014) 179–185
183
Fig. 3. Distribution of MW fractions in DOM derived from different mature composts.
composts (Wei et al., 2014). In general, composting is associated with increasing aromatization in DOM (Chefetz et al.,1998; SaidPullicino et al., 2007; Caricasole et al., 2010), resulting in enriched aromatic fractions as well as high MW in DOM at the final stage of composting, despite of differences in raw materials. Fractions MW < 65 Da and 65 Da < MW < 1 kDa had the lowest proportions, averaging 10.58% and 9.66%, respectively, while fraction MW > 5 kDa had the highest proportion, averaging 61.53%. The proportion of DOM as MW > 5 kDa was above 50% for all composts except TSW (36.26%), CM (36.88%) and MSW (43.25%). The distribution indicated that higher MW (>5 kDa) substances were the major component of DOM in all mature composts. In recent studies (Wei et al., 2007; He et al., 2011b) compositing was characterized as a process of simultaneous degradation of simple protein-liked materials and formation of humic acid-like materials in DOM. Accordingly, the MW > 5 kDa fraction that accounted for the largest proportion of mature composts was likely humic substances that have a high aromatization degree.
Fig. 4. BOD5 in DOM derived from different mature composts.
4 kDa in landfill leachate were readily biodegradable (He et al., 2006).
3.2. Biodegradability of DOM 3.3. Relationship between BOD5 and DOM and its MW fractions BOD5 is an important index for water pollution (Jung et al., 2008) because it indicates the degree of biological degradation of organic matter. Therefore, a higher BOD5 indicates a higher biodegradability for DOM. The concentration of BOD5 in DOM ranged from 5.7 to 42.9 mg l 1 among different composts (Fig. 4), with the maximum observed in CM, being 7.50 times that in MSW. BOD5 was significantly correlated with DOM (R2 = 0.640, P = 0.010) (Fig. 5a), meaning that the higher the concentration of DOM in mature composts was, the more degradable the material would be. BOD5 was also correlated with MW < 65 Da (R2 = 0.771, P = 0.002) (Fig. 5b), 65 Da < MW < 1 kDa (R2 = 0.798, P = 0.001) (Fig. 5c), and 1 kDa < MW < 5 kDa (R2 = 0.600, P = 0.014) (Fig. 5d), but not with MW > 5 kDa. These data suggested that the lower MW and simpler structural components in DOM were the biodegradable organic compounds, and the higher MW components were less degradable by microorganism. Results were consistent with the report that compounds with a molecular weight less than
We previously described the degree of humification in DOM using HCA that considered correlations between different variables (Wei et al., 2014), but fluorescence spectroscopic analyses failed to reveal differences in DOM concentration between different composts. In order to describe the relationship between biodegradability and concentration of DOM and its MW fractions in composts derived from different sources, HCA was performed based on the concentration of DOM, MW < 65 Da, 65 Da < MW < 1 kDa, and 1 kDa < MW < 5 kDa (Fig. 6). The DOM of the composts were clustered into three groups. The first group included DCM and CM, DOM from these composts contained high concentration and contributed from biodegradable low MW compounds (
