Waste Management xxx (2014) xxx–xxx
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Transformation of phosphorus during drying and roasting of sewage sludge Rundong Li ⇑, Jing Yin, Weiyun Wang, Yanlong Li, Ziheng Zhang College of Energy and Environment, Shenyang Aerospace University, The Key Laboratory of Clean Energy in Liaoning Province, Shenyang, China
a r t i c l e
i n f o
Article history: Received 4 August 2013 Accepted 24 March 2014 Available online xxxx Keywords: Sewage sludge Phosphorus Drying and roasting 31 P NMR Transformation
a b s t r a c t Sewage sludge (SS), a by-product of wastewater treatment, consists of highly concentrated organic and inorganic pollutants, including phosphorus (P). In this study, P with different chemical fractions in SS under different drying and roasting temperatures was investigated with the use of appropriate standards, measurements, and testing protocol. The drying and roasting treatment of SS was conducted in a laboratory-scale furnace. Two types of SS samples under different treatment temperatures were analyzed by 31P NMR spectroscopy. These samples were dried by a vacuum freeze dryer at 50 °C and a thermoelectric thermostat drying box at 105 °C. Results show that the inorganic P (IP) content increased as the organic P content decreased, and the bio-availability of P increased because IP is a form of phosphorous that can be directly absorbed by plants. 31P NMR analysis results indicate the change in P fractions at different temperatures. Non-apatite P was the dominant form of P under low-temperature drying and roasting, whereas apatite P was the major one under high-temperature drying and roasting. Results indicate that temperature affects the transformation of P. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Phosphorus (P) is an important and irreplaceable nutrient for all living organisms. However, the extensive use of P may result in eutrophication in aquatic systems. Sewage sludge (SS), a byproduct of wastewater treatment, is rich in P. Sludge generation in different regions of China is uneven. East China produces 61% of the total SS output of China. The transportation and disposal of SS often account for 25–65% of the total operation costs of wastewater treatment plants. The average content of P in dried SS in China is 2.2% (Guo et al., 2009). In some European countries, the maximum P content of SS is as high as 15% (Hoffmann et al., 2010). The known world reserves of phosphate rock may be exhausted in 90–130 years (McNutt, 2010). Therefore, P recycling from SS can contribute to reducing pressure on P resource supply and controlling pollution. Thermal treatment can result in a highly concentrated P content in SS (Franz, 2008; Cyr et al., 2007) and reduce the quantity of SS. Recently, studies have clearly shown that thermal treatment can enhance the anaerobic digestion of excess sludge. However, these studies focused on the thermal hydrolysis of SS and on organic solids that undergo dissolution and hydrolysis. Several studies have investigated the forms of P in SS at high temperatures (Franz, 2008; Cyr et al., 2007; Adam et al., 2009). Concentrated P ⇑ Corresponding author. Tel.: +86 24 8972 8889; fax: +86 24 8972 4558. E-mail address: [email protected] (R. Li).
has also been found in the studied heavy metals in SS during thermo-chemical treatment. Research in this field has therefore mainly focused on the removal of heavy metals in SS ash, the relationship of P and heavy metals, and the bio-availability of P when heavy metals are removed (Nowak et al., 2011; Tang et al., 2008; Kidd et al., 2007; Khan and Jones, 2009). García-Albacete et al. (2012) investigated the transformation of P, particularly non-apatite inorganic phosphorus (NAIP), into different forms by applying appropriate standards, measurements, and testing protocol. Meanwhile, the effect of temperature on the transformation of the P fraction has not been systematically studied. P with different chemical fractions in SS was investigated with the use of the standards, measurements, and testing (SMT) protocol (Pardo et al., 2003; Xie et al., 2011; García-Albacete et al.; 2012). This protocol, which was developed by the European Commission, classified P in SS into five forms, such as total phosphorus (TP), which includes IP and organic phosphorus (OP). IP was further classified into non-apatite IP and apatite P (AP). IP and non-apatite IP are the predominant fractions in SS (Pardo et al., 2003; Xie et al., 2011). The relationships of the five fractions of P are expressed by the equations TP = IP + OP and IP = NAIP + AP (Pardo et al., 2003). AP is bound to Ca and is obtained in P mining. 31 P NMR spectroscopy has been used in numerous studies to determine the distribution of OP and IP. The transformation of P forms in soil, sediments, and SS has been reported (Cade-Menun and Preston, 1996; Peng et al., 2009). 31P NMR spectra were
http://dx.doi.org/10.1016/j.wasman.2014.03.022 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Li, R., et al. Transformation of phosphorus during drying and roasting of sewage sludge. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.03.022
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R. Li et al. / Waste Management xxx (2014) xxx–xxx
obtained for P fractions from SS at different treatment temperatures. The results of this study can be beneficial to increasing the bioavailability of P in SS and the further recycling of P as apatite from SS. This research aims to investigate the relationship between temperature and P fractions. It also studies the transformation of OP as a source of plant-available P, non-apatite IP, and apatite IP.
1996). Spectrophotometric determination of phosphate in all extracts was conducted with molybdenum blue method (Pardo et al., 2003). Phytic acid is a primary component in OP (De Groot et al., 1993). However, several studies indicate that adenosintriphosphate (ATP) is the primary constituent of OP (Jens and Sven, 2007). In this study, phytic acid was added to SS, which was dried at 80 °C and 90 °C to determine the transformation of OP and IP.
2. Material and methods
2.4.
2.1. Sample collection and pre-treatment
Three grams of the freeze-dried sample and 3.0 g of the dried sample at 105 °C from Dalian were placed in 100 mL centrifuge tubes. Then, 1:20 (v/v) of EDTA (0.25 mol L 1 NaOH + 0.05 mol L 1 EDTA, 60 mL) was added to the centrifuge tubes, which were shaken for 24 h at a temperature of 25 °C. After centrifugation of the extracts at 10,000 gravities for 30 min, the supernatants were filtered through a 0.45 lm filter membrane and dried with a vacuum freeze dryer. The freeze-dried sample was re-dissolved in 1 mL of 0.25 mol L 1 NaOH and then centrifuged at 8000 gravities for 5 min at 4 °C. The supernatants (0.6 mL) were added in 0.5 mL of D2O and then transferred to a 5 mm NMR tube. Solution 31P NMR spectra were obtained with a 600 MHz spectrometer that operates at 242.739 MHz at 25 °C. A 7.0 ls observe pulse and a total acquisition time of 2.32 s (acquisition time: 0.32 s, pulse delay) were used. Spectra were collected with 30,000 scans. Chemical shifts were recorded relative to an external 85% H3PO4 standard (d = 0 ppm).
2.2. Instrumentation and reagent Colorimetric determination of phosphate was conducted with molybdenum blue method (Kidd et al., 2007) at 882 nm with a WFJ2100 spectrophotometer (UNICO, Shanghai). A SHA-BA water bath oscillator (RONGHUA, China) was used in the extractions. A TG-18 M centrifuge (PINGFAN, China) was utilized to extract P phases and facilitate complete separation of the extract from the solid phases at 4000 g for 10 min. A FD-1A-50 vacuum freeze dryer (BOYIKANG, Beijing) was used to dry the SS to obtain the freeze-dried samples. X-ray fluorescence (XRF) (ZSX100e, MINIFLEX, Japan) measurements were used to determine the major components of raw SS, such as C, H, S and N contents. An SSX550 scanning electron microscope and an energy dispersive spectrometer (SEM-EDS) (SHIMADZU, Japan) were used to observe morphology and element content. An Advance-600 MHz nuclear magnetic resonance spectrometer (BRUKER, Switzerland) was utilized to observe changes in P fractions. All reagents used in this study were of analytical reagent grade. In the SMT protocol and 31P NMR experiment, hydrochloric acid (1 M and 3.5 M), sodium hydroxide (0.25 M and 1 M), and EDTA (0.05 M) were used as extracting reagents. Glassware and plasticware were soaked in 10% HNO3 for 24 h and then rinsed with distilled water. 2.3. P fractions in SS The SMT program was used in this study to determine P in all sludge samples. Each experiment was replicated three times. NMR measurements were used to observe changes in P fractions. The operating conditions of the SMT protocol and NMR experiment were described in a number of studies (Pardo et al., 2003; Xie et al., 2011; García-Albacete et al., 2012; Cade-Menun and Preston, Table 1 Properties of sewage sludge tested in this study, wt%. Ash Shenyang SS Dalian SS
48.0 21.6
Volatile matter
Fixed carbon
C
44.9 66.7
3.2 11.7
21.48 42.49
P NMR spectroscopy
3. Results and discussion 3.1. P fractions in SS Table 2 summarizes the results of the SMT protocol for the Shenyang SS samples. The results are given as mean values Table 2 Analytical results obtained in Shenyang SS (expressed in mg g extraction protocol.
TP IP OP NAIP AP
1
) using the SMT
60 °C
70 °C
80 °C
90 °C
105 °C
10.2 ± 0.3 7.6 ± 0.2 1.6 ± 0.1 3.4 ± 0.1 3.9 ± 0.2
10.4 ± 0.4 7.5 ± 0.1 1.8 ± 0.2 3.4 ± 0.2 3.9 ± 0.2
10.4 ± 0.3 7.5 ± 0.2 1.9 ± 0.1 3.5 ± 0.3 4.0 ± 0.2
10.5 ± 0.2 8.0 ± 0.1 1.7 ± 0.1 3.7 ± 0.3 4.2 ± 0.2
10.6 ± 0.2 8.2 ± 0.1 1.4 ± 0.1 4.0 ± 0.1 4.2 ± 0.1
Concentrations are expressed as mean value ± standard deviation.
90
Content of IP and OP in TP (%)
Two types of mechanical dehydration sludge were collected from Shenyang North Wastewater Treatment Plant and Dalian Wastewater Treatment Plant in China. Table 1 shows the properties of SS. The Shenyang SS samples were dried and roasted to constant weight at different temperatures for 48 h. The temperature of thermal treatment ranged from 60 °C to 240 °C. The drying and roasting treatment of SS was conducted in a laboratory-scale furnace in an open system. Some Dalian SS samples were dried with a vacuum freeze dryer for 72 h, and others were dried to constant weight in a furnace at a temperature that ranged from 60 °C to 105 °C. All treated samples were ground to less than 150 lm with a QM-QX ball grinding mill (Nanjing).
31
OP in SYSS OP in DLSS
IP in SYSS IP in DLSS
80 70 60
20
10
0
H
N
S
60
70
80
90
100
110
o
T ( C) 2.86 6.79
2.53 6.63
1.57 1.22
Fig. 1. Content of inorganic phosphorus and organic phosphorus in total phosphorus (%) from Shenyang sewage sludge (SYSS) and Dalian sewage sludge (DLSS).
Please cite this article in press as: Li, R., et al. Transformation of phosphorus during drying and roasting of sewage sludge. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.03.022
R. Li et al. / Waste Management xxx (2014) xxx–xxx
Content of IP and OP (mg g-1)
(expressed in mg g 1) and standard deviations of three independent replicates. The content of P fractions do not satisfy the equations TP = IP + OP and IP = NAIP + AP. The sum of IP and OP is lower than the TP. Furthermore, IP as the sum of non-apatite and apatite fractions is lower than the IP concentration. Some studies that investigated the relationship of P fractions obtained similar results in sediment (Pardo et al., 2003). This finding can be attributed to the limitation of the SMT protocol. Table 2 shows that IP was the main fraction in the Shenyang SS samples and that the contents of P fractions distinctively varied at different temperatures. Fig. 1 depicts the analytical results of the
Fig. 2. Content of inorganic phosphorus and organic phosphorus (mg g Shenyang sewage sludge with phytate.
1
) from
3
Shenyang SS. The findings indicate that the IP fraction increased from 72.24% to 75.83% as the OP fraction decreased in the temperature range of 80–90 °C, possibly because of the destruction in cell construction or protein denaturation (Hamer et al., 1994). OP is categorized into three forms: labile OP, moderately labile OP, and nonlabile OP (Pierzynski, 2000). Labile OP consists of nucleic acid, phosphatide, and phosphoglucoprotein; moderately labile OP consists of phytin; and nonlabile OP consists of aluminum-phytate, iron-phytate, and chelate. A total of 5.88 mg g 1 phytic acid was added to the Shenyang SS. The results in Fig. 2 show that the content of OP to the IP fraction in SS with phytic acid additives increased by 6.23 mg g 1 at the temperature range of 80–90 °C compared with that in raw SS. This increase in OP content corresponded to the increase in phytic acid content. Phytic acid was destroyed, and it transformed IP at 90 °C. Further studies indicate that OP was transformed to IP at 80 °C, as shown in Fig. 1. The figure also indicates that the Dalian SS samples dried in the same condition as that of the Shenyang SS samples demonstrated results similar to those on OP, in which OP was transformed to IP at the temperature range of 80–90 °C. OP, an important source of plant-available P, can be mineralized into IP under the action of enzymes, and the produced P can be absorbed by plants (McGill and Cole, 1981; Scott and Condron, 2003). Fig. 3(a) and (b) shows the morphology of SS in the freeze-dried Dalian SS samples and the dried Dalian SS samples at 105 °C. The difference in morphology between the two types of SS samples is attributed to the destruction of the SS structure at 105 °C. During drying and roasting, most of the OP was converted to IP, but the residual organically bound P was still present in the SS; the P here can be separated at a temperature that ranged from 400 °C to 600 °C (Zhang et al., 2012).
(a) Morphology of SS from freeze dried DLSS samples
(b) Morphology of SS from dried DLSS at 105°C Fig. 3. Morphology of Dalian sewage sludge (DLSS) at different drying temperature.
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Fig. 4.
31
P NMR spectra from Dalian sewage sludge samples at different temperature.
3.2.
31
p NMR
The assignment of peaks in 31P NMR spectra in soil was based on a previous study (Cade-Menun and Preston, 1996). The P forms in soil were the same as those in SS (Peng et al., 2009). The solution
Content of IP and OP (mg g -1)
The increase in IP content and the availability of P as a result of changes in temperature is important. Furthermore, a temperature increase can kill some pathogens. These findings indicate that SS that undergoes thermal treatment possesses high bio-availability and can be used for plant growth.
Fig. 5. Content of inorganic phosphorus and organic phosphorus (mg g Shenyang sewage sludge samples.
) from
(a) Detected at 220°C
90
60
NAIP
AP
80 8 6
40
4
Content of AP in IP (%)
Content of NAIP in IP (%)
1
2 0
30 120
140
160
180
200
220
240
o
T ( C) Fig. 6. Content of non-apatite phosphorus and apatite phosphorus in IP (%) from Shenyang sewage sludge (SYSS) samples.
(b) Detected at 240°C Fig. 7. Some of the mineral phases detected by XRD in the Shenyang sewage sludge SYSS sample roasted at 220 °C and 240 °C.
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R. Li et al. / Waste Management xxx (2014) xxx–xxx 31
P NMR spectrum of sediments was provided in some studies (Carman et al., 2000; Ahlgren et al., 2007). 31P spectra were dominated by signals in the range of 30 ppm to 30 ppm and were attributed to phosphonate, orthophosphate, orthophosphate monoesters, orthophosphate diesters, pyrophosphate, and polyphosphate. Fig. 4 presents the 31P NMR spectra of the extracts from the SS. The observed signals of the freeze-dried samples in the 31P spectra are similar to those in a previous report (Peng et al., 2009). The 31P NMR spectra of the SS samples confirmed that the P fractions in the freeze-dried samples were similar to those in soil. Fig. 4 shows that in the two types of samples, the main signal belonged to orthophosphate; the NMR signals of the sample dried at 105 °C decreased in the range of 22 ppm to 0 ppm. The compositions of pyrophosphate, orthophosphate diesters, and orthophosphate monoesters, which belonged to OP, were destroyed and transformed to orthophosphate monoesters. This result indicates that temperature affects the transformation of P fractions.
5
4. Conclusion SS as a P carrier has a high IP content. The content of P fractions differs during drying and roasting. The OP fraction is converted to IP as the temperature increases, and the bio-availability of P is enhanced. NMR analysis confirms the presence of P fractions and the effects of temperature on these fractions. The transformation of the NAIP and AP fractions at 220 °C results from the conversion of P forms associated with aluminum oxides and hydroxides converted to other forms. The common transformation of different P forms occurs during drying and roasting. The P in SS can be concentrated as apatite with an increase in temperature.
Acknowledgements This research was supported by the National Basic Research Program of China (2011CB201500) and the National Natural Science Foundation of China (No. 51276119).
3.3. NAIP and AP in SS References Figs. 5 and 6 show the content of four forms of P at temperatures that ranged from 120 °C to 240 °C. The present experimental study also indicates that the content of the IP fraction in SS gradually increased with the temperature, particularly when it exceeded 120 °C, whereas the content of the OP fraction decreased (see Fig. 5). IP considerably increased as the temperature exceeded 180 °C. This result can be attributed to the release of interstitial water in SS. IP is mainly distributed in the NAIP fraction, which accounts for more than 49.48% of the IP in SS. The AP fraction ranged from 36.52% to 42.62% in IP. The result indicated that NAIP, the most labile P form, can be released from SS as the temperature changed. The contents of the NAIP and AP fractions in SS increased during drying and roasting. The content of the NAIP fraction reached a maximum value of 55.97% (the proportion of NAIP in IP) at 200 °C. Furthermore, the content of the AP fraction clearly increased from 36.52% (the proportion of AP in IP) to 89.03% as the NAIP fraction decreased at temperatures that ranged from 220 °C to 240 °C, as shown in Fig. 6. The contents of P forms associated with aluminum oxides and hydroxides, such as the form of NaAl(P2O7), were detected by XRD at 220 °C, as shown in Fig. 7(a). Those forms that were not detected by XRD at 240 °C are shown in Fig. 7(b). The atomic ratio in the samples was identified with SEM–EDS, as shown in Table 3, and the composition was detected by XRD, as shown in Fig. 7(a). The results demonstrate that sodium aluminum phosphorus (NaAl(P2O7)) and magnesium silicon phosphorus (MgSiP2) were the main existing forms at 220 °C. The content of NaAl(P2O7) in all compositions containing P was approximately 4.5%. However, the main existing forms were silicon phosphorus oxide (SiP2O7) and aluminum silicon phosphorus (AlSi2P3O12) in the Shenyang SS samples roasted at 240 °C. Moreover, the content of NaAl(P2O7) in all compositions containing P almost disappeared at 240 °C. These results indicate that a large amount of apatite, as a phosphate mineral, existed in SS when the temperature was higher than 240 °C and can be recycled. NAIP is the most labile P form and possesses high bio-availability at a temperature below 220 °C.
Table 3 The atomic (%) and weight (%) of element at 220 °C. Element
O
Mg
Al
Si
P
S
K
Ca
Fe
Atomic % Weight %
59.54 42.13
1.98 2.12
1.56 9.13
14.58 18.11
1.77 2.42
4.23 6.00
1.17 2.03
6.29 11.15
2.8 6.91
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Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Transformation of phosphorus during drying and roasting of sewage sludge Rundong Li ⇑, Jing Yin, Weiyun Wang, Yanlong Li, Ziheng Zhang College of Energy and Environment, Shenyang Aerospace University, The Key Laboratory of Clean Energy in Liaoning Province, Shenyang, China
a r t i c l e
i n f o
Article history: Received 4 August 2013 Accepted 24 March 2014 Available online xxxx Keywords: Sewage sludge Phosphorus Drying and roasting 31 P NMR Transformation
a b s t r a c t Sewage sludge (SS), a by-product of wastewater treatment, consists of highly concentrated organic and inorganic pollutants, including phosphorus (P). In this study, P with different chemical fractions in SS under different drying and roasting temperatures was investigated with the use of appropriate standards, measurements, and testing protocol. The drying and roasting treatment of SS was conducted in a laboratory-scale furnace. Two types of SS samples under different treatment temperatures were analyzed by 31P NMR spectroscopy. These samples were dried by a vacuum freeze dryer at 50 °C and a thermoelectric thermostat drying box at 105 °C. Results show that the inorganic P (IP) content increased as the organic P content decreased, and the bio-availability of P increased because IP is a form of phosphorous that can be directly absorbed by plants. 31P NMR analysis results indicate the change in P fractions at different temperatures. Non-apatite P was the dominant form of P under low-temperature drying and roasting, whereas apatite P was the major one under high-temperature drying and roasting. Results indicate that temperature affects the transformation of P. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Phosphorus (P) is an important and irreplaceable nutrient for all living organisms. However, the extensive use of P may result in eutrophication in aquatic systems. Sewage sludge (SS), a byproduct of wastewater treatment, is rich in P. Sludge generation in different regions of China is uneven. East China produces 61% of the total SS output of China. The transportation and disposal of SS often account for 25–65% of the total operation costs of wastewater treatment plants. The average content of P in dried SS in China is 2.2% (Guo et al., 2009). In some European countries, the maximum P content of SS is as high as 15% (Hoffmann et al., 2010). The known world reserves of phosphate rock may be exhausted in 90–130 years (McNutt, 2010). Therefore, P recycling from SS can contribute to reducing pressure on P resource supply and controlling pollution. Thermal treatment can result in a highly concentrated P content in SS (Franz, 2008; Cyr et al., 2007) and reduce the quantity of SS. Recently, studies have clearly shown that thermal treatment can enhance the anaerobic digestion of excess sludge. However, these studies focused on the thermal hydrolysis of SS and on organic solids that undergo dissolution and hydrolysis. Several studies have investigated the forms of P in SS at high temperatures (Franz, 2008; Cyr et al., 2007; Adam et al., 2009). Concentrated P ⇑ Corresponding author. Tel.: +86 24 8972 8889; fax: +86 24 8972 4558. E-mail address: [email protected] (R. Li).
has also been found in the studied heavy metals in SS during thermo-chemical treatment. Research in this field has therefore mainly focused on the removal of heavy metals in SS ash, the relationship of P and heavy metals, and the bio-availability of P when heavy metals are removed (Nowak et al., 2011; Tang et al., 2008; Kidd et al., 2007; Khan and Jones, 2009). García-Albacete et al. (2012) investigated the transformation of P, particularly non-apatite inorganic phosphorus (NAIP), into different forms by applying appropriate standards, measurements, and testing protocol. Meanwhile, the effect of temperature on the transformation of the P fraction has not been systematically studied. P with different chemical fractions in SS was investigated with the use of the standards, measurements, and testing (SMT) protocol (Pardo et al., 2003; Xie et al., 2011; García-Albacete et al.; 2012). This protocol, which was developed by the European Commission, classified P in SS into five forms, such as total phosphorus (TP), which includes IP and organic phosphorus (OP). IP was further classified into non-apatite IP and apatite P (AP). IP and non-apatite IP are the predominant fractions in SS (Pardo et al., 2003; Xie et al., 2011). The relationships of the five fractions of P are expressed by the equations TP = IP + OP and IP = NAIP + AP (Pardo et al., 2003). AP is bound to Ca and is obtained in P mining. 31 P NMR spectroscopy has been used in numerous studies to determine the distribution of OP and IP. The transformation of P forms in soil, sediments, and SS has been reported (Cade-Menun and Preston, 1996; Peng et al., 2009). 31P NMR spectra were
http://dx.doi.org/10.1016/j.wasman.2014.03.022 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Li, R., et al. Transformation of phosphorus during drying and roasting of sewage sludge. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.03.022
2
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obtained for P fractions from SS at different treatment temperatures. The results of this study can be beneficial to increasing the bioavailability of P in SS and the further recycling of P as apatite from SS. This research aims to investigate the relationship between temperature and P fractions. It also studies the transformation of OP as a source of plant-available P, non-apatite IP, and apatite IP.
1996). Spectrophotometric determination of phosphate in all extracts was conducted with molybdenum blue method (Pardo et al., 2003). Phytic acid is a primary component in OP (De Groot et al., 1993). However, several studies indicate that adenosintriphosphate (ATP) is the primary constituent of OP (Jens and Sven, 2007). In this study, phytic acid was added to SS, which was dried at 80 °C and 90 °C to determine the transformation of OP and IP.
2. Material and methods
2.4.
2.1. Sample collection and pre-treatment
Three grams of the freeze-dried sample and 3.0 g of the dried sample at 105 °C from Dalian were placed in 100 mL centrifuge tubes. Then, 1:20 (v/v) of EDTA (0.25 mol L 1 NaOH + 0.05 mol L 1 EDTA, 60 mL) was added to the centrifuge tubes, which were shaken for 24 h at a temperature of 25 °C. After centrifugation of the extracts at 10,000 gravities for 30 min, the supernatants were filtered through a 0.45 lm filter membrane and dried with a vacuum freeze dryer. The freeze-dried sample was re-dissolved in 1 mL of 0.25 mol L 1 NaOH and then centrifuged at 8000 gravities for 5 min at 4 °C. The supernatants (0.6 mL) were added in 0.5 mL of D2O and then transferred to a 5 mm NMR tube. Solution 31P NMR spectra were obtained with a 600 MHz spectrometer that operates at 242.739 MHz at 25 °C. A 7.0 ls observe pulse and a total acquisition time of 2.32 s (acquisition time: 0.32 s, pulse delay) were used. Spectra were collected with 30,000 scans. Chemical shifts were recorded relative to an external 85% H3PO4 standard (d = 0 ppm).
2.2. Instrumentation and reagent Colorimetric determination of phosphate was conducted with molybdenum blue method (Kidd et al., 2007) at 882 nm with a WFJ2100 spectrophotometer (UNICO, Shanghai). A SHA-BA water bath oscillator (RONGHUA, China) was used in the extractions. A TG-18 M centrifuge (PINGFAN, China) was utilized to extract P phases and facilitate complete separation of the extract from the solid phases at 4000 g for 10 min. A FD-1A-50 vacuum freeze dryer (BOYIKANG, Beijing) was used to dry the SS to obtain the freeze-dried samples. X-ray fluorescence (XRF) (ZSX100e, MINIFLEX, Japan) measurements were used to determine the major components of raw SS, such as C, H, S and N contents. An SSX550 scanning electron microscope and an energy dispersive spectrometer (SEM-EDS) (SHIMADZU, Japan) were used to observe morphology and element content. An Advance-600 MHz nuclear magnetic resonance spectrometer (BRUKER, Switzerland) was utilized to observe changes in P fractions. All reagents used in this study were of analytical reagent grade. In the SMT protocol and 31P NMR experiment, hydrochloric acid (1 M and 3.5 M), sodium hydroxide (0.25 M and 1 M), and EDTA (0.05 M) were used as extracting reagents. Glassware and plasticware were soaked in 10% HNO3 for 24 h and then rinsed with distilled water. 2.3. P fractions in SS The SMT program was used in this study to determine P in all sludge samples. Each experiment was replicated three times. NMR measurements were used to observe changes in P fractions. The operating conditions of the SMT protocol and NMR experiment were described in a number of studies (Pardo et al., 2003; Xie et al., 2011; García-Albacete et al., 2012; Cade-Menun and Preston, Table 1 Properties of sewage sludge tested in this study, wt%. Ash Shenyang SS Dalian SS
48.0 21.6
Volatile matter
Fixed carbon
C
44.9 66.7
3.2 11.7
21.48 42.49
P NMR spectroscopy
3. Results and discussion 3.1. P fractions in SS Table 2 summarizes the results of the SMT protocol for the Shenyang SS samples. The results are given as mean values Table 2 Analytical results obtained in Shenyang SS (expressed in mg g extraction protocol.
TP IP OP NAIP AP
1
) using the SMT
60 °C
70 °C
80 °C
90 °C
105 °C
10.2 ± 0.3 7.6 ± 0.2 1.6 ± 0.1 3.4 ± 0.1 3.9 ± 0.2
10.4 ± 0.4 7.5 ± 0.1 1.8 ± 0.2 3.4 ± 0.2 3.9 ± 0.2
10.4 ± 0.3 7.5 ± 0.2 1.9 ± 0.1 3.5 ± 0.3 4.0 ± 0.2
10.5 ± 0.2 8.0 ± 0.1 1.7 ± 0.1 3.7 ± 0.3 4.2 ± 0.2
10.6 ± 0.2 8.2 ± 0.1 1.4 ± 0.1 4.0 ± 0.1 4.2 ± 0.1
Concentrations are expressed as mean value ± standard deviation.
90
Content of IP and OP in TP (%)
Two types of mechanical dehydration sludge were collected from Shenyang North Wastewater Treatment Plant and Dalian Wastewater Treatment Plant in China. Table 1 shows the properties of SS. The Shenyang SS samples were dried and roasted to constant weight at different temperatures for 48 h. The temperature of thermal treatment ranged from 60 °C to 240 °C. The drying and roasting treatment of SS was conducted in a laboratory-scale furnace in an open system. Some Dalian SS samples were dried with a vacuum freeze dryer for 72 h, and others were dried to constant weight in a furnace at a temperature that ranged from 60 °C to 105 °C. All treated samples were ground to less than 150 lm with a QM-QX ball grinding mill (Nanjing).
31
OP in SYSS OP in DLSS
IP in SYSS IP in DLSS
80 70 60
20
10
0
H
N
S
60
70
80
90
100
110
o
T ( C) 2.86 6.79
2.53 6.63
1.57 1.22
Fig. 1. Content of inorganic phosphorus and organic phosphorus in total phosphorus (%) from Shenyang sewage sludge (SYSS) and Dalian sewage sludge (DLSS).
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Content of IP and OP (mg g-1)
(expressed in mg g 1) and standard deviations of three independent replicates. The content of P fractions do not satisfy the equations TP = IP + OP and IP = NAIP + AP. The sum of IP and OP is lower than the TP. Furthermore, IP as the sum of non-apatite and apatite fractions is lower than the IP concentration. Some studies that investigated the relationship of P fractions obtained similar results in sediment (Pardo et al., 2003). This finding can be attributed to the limitation of the SMT protocol. Table 2 shows that IP was the main fraction in the Shenyang SS samples and that the contents of P fractions distinctively varied at different temperatures. Fig. 1 depicts the analytical results of the
Fig. 2. Content of inorganic phosphorus and organic phosphorus (mg g Shenyang sewage sludge with phytate.
1
) from
3
Shenyang SS. The findings indicate that the IP fraction increased from 72.24% to 75.83% as the OP fraction decreased in the temperature range of 80–90 °C, possibly because of the destruction in cell construction or protein denaturation (Hamer et al., 1994). OP is categorized into three forms: labile OP, moderately labile OP, and nonlabile OP (Pierzynski, 2000). Labile OP consists of nucleic acid, phosphatide, and phosphoglucoprotein; moderately labile OP consists of phytin; and nonlabile OP consists of aluminum-phytate, iron-phytate, and chelate. A total of 5.88 mg g 1 phytic acid was added to the Shenyang SS. The results in Fig. 2 show that the content of OP to the IP fraction in SS with phytic acid additives increased by 6.23 mg g 1 at the temperature range of 80–90 °C compared with that in raw SS. This increase in OP content corresponded to the increase in phytic acid content. Phytic acid was destroyed, and it transformed IP at 90 °C. Further studies indicate that OP was transformed to IP at 80 °C, as shown in Fig. 1. The figure also indicates that the Dalian SS samples dried in the same condition as that of the Shenyang SS samples demonstrated results similar to those on OP, in which OP was transformed to IP at the temperature range of 80–90 °C. OP, an important source of plant-available P, can be mineralized into IP under the action of enzymes, and the produced P can be absorbed by plants (McGill and Cole, 1981; Scott and Condron, 2003). Fig. 3(a) and (b) shows the morphology of SS in the freeze-dried Dalian SS samples and the dried Dalian SS samples at 105 °C. The difference in morphology between the two types of SS samples is attributed to the destruction of the SS structure at 105 °C. During drying and roasting, most of the OP was converted to IP, but the residual organically bound P was still present in the SS; the P here can be separated at a temperature that ranged from 400 °C to 600 °C (Zhang et al., 2012).
(a) Morphology of SS from freeze dried DLSS samples
(b) Morphology of SS from dried DLSS at 105°C Fig. 3. Morphology of Dalian sewage sludge (DLSS) at different drying temperature.
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R. Li et al. / Waste Management xxx (2014) xxx–xxx
Fig. 4.
31
P NMR spectra from Dalian sewage sludge samples at different temperature.
3.2.
31
p NMR
The assignment of peaks in 31P NMR spectra in soil was based on a previous study (Cade-Menun and Preston, 1996). The P forms in soil were the same as those in SS (Peng et al., 2009). The solution
Content of IP and OP (mg g -1)
The increase in IP content and the availability of P as a result of changes in temperature is important. Furthermore, a temperature increase can kill some pathogens. These findings indicate that SS that undergoes thermal treatment possesses high bio-availability and can be used for plant growth.
Fig. 5. Content of inorganic phosphorus and organic phosphorus (mg g Shenyang sewage sludge samples.
) from
(a) Detected at 220°C
90
60
NAIP
AP
80 8 6
40
4
Content of AP in IP (%)
Content of NAIP in IP (%)
1
2 0
30 120
140
160
180
200
220
240
o
T ( C) Fig. 6. Content of non-apatite phosphorus and apatite phosphorus in IP (%) from Shenyang sewage sludge (SYSS) samples.
(b) Detected at 240°C Fig. 7. Some of the mineral phases detected by XRD in the Shenyang sewage sludge SYSS sample roasted at 220 °C and 240 °C.
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R. Li et al. / Waste Management xxx (2014) xxx–xxx 31
P NMR spectrum of sediments was provided in some studies (Carman et al., 2000; Ahlgren et al., 2007). 31P spectra were dominated by signals in the range of 30 ppm to 30 ppm and were attributed to phosphonate, orthophosphate, orthophosphate monoesters, orthophosphate diesters, pyrophosphate, and polyphosphate. Fig. 4 presents the 31P NMR spectra of the extracts from the SS. The observed signals of the freeze-dried samples in the 31P spectra are similar to those in a previous report (Peng et al., 2009). The 31P NMR spectra of the SS samples confirmed that the P fractions in the freeze-dried samples were similar to those in soil. Fig. 4 shows that in the two types of samples, the main signal belonged to orthophosphate; the NMR signals of the sample dried at 105 °C decreased in the range of 22 ppm to 0 ppm. The compositions of pyrophosphate, orthophosphate diesters, and orthophosphate monoesters, which belonged to OP, were destroyed and transformed to orthophosphate monoesters. This result indicates that temperature affects the transformation of P fractions.
5
4. Conclusion SS as a P carrier has a high IP content. The content of P fractions differs during drying and roasting. The OP fraction is converted to IP as the temperature increases, and the bio-availability of P is enhanced. NMR analysis confirms the presence of P fractions and the effects of temperature on these fractions. The transformation of the NAIP and AP fractions at 220 °C results from the conversion of P forms associated with aluminum oxides and hydroxides converted to other forms. The common transformation of different P forms occurs during drying and roasting. The P in SS can be concentrated as apatite with an increase in temperature.
Acknowledgements This research was supported by the National Basic Research Program of China (2011CB201500) and the National Natural Science Foundation of China (No. 51276119).
3.3. NAIP and AP in SS References Figs. 5 and 6 show the content of four forms of P at temperatures that ranged from 120 °C to 240 °C. The present experimental study also indicates that the content of the IP fraction in SS gradually increased with the temperature, particularly when it exceeded 120 °C, whereas the content of the OP fraction decreased (see Fig. 5). IP considerably increased as the temperature exceeded 180 °C. This result can be attributed to the release of interstitial water in SS. IP is mainly distributed in the NAIP fraction, which accounts for more than 49.48% of the IP in SS. The AP fraction ranged from 36.52% to 42.62% in IP. The result indicated that NAIP, the most labile P form, can be released from SS as the temperature changed. The contents of the NAIP and AP fractions in SS increased during drying and roasting. The content of the NAIP fraction reached a maximum value of 55.97% (the proportion of NAIP in IP) at 200 °C. Furthermore, the content of the AP fraction clearly increased from 36.52% (the proportion of AP in IP) to 89.03% as the NAIP fraction decreased at temperatures that ranged from 220 °C to 240 °C, as shown in Fig. 6. The contents of P forms associated with aluminum oxides and hydroxides, such as the form of NaAl(P2O7), were detected by XRD at 220 °C, as shown in Fig. 7(a). Those forms that were not detected by XRD at 240 °C are shown in Fig. 7(b). The atomic ratio in the samples was identified with SEM–EDS, as shown in Table 3, and the composition was detected by XRD, as shown in Fig. 7(a). The results demonstrate that sodium aluminum phosphorus (NaAl(P2O7)) and magnesium silicon phosphorus (MgSiP2) were the main existing forms at 220 °C. The content of NaAl(P2O7) in all compositions containing P was approximately 4.5%. However, the main existing forms were silicon phosphorus oxide (SiP2O7) and aluminum silicon phosphorus (AlSi2P3O12) in the Shenyang SS samples roasted at 240 °C. Moreover, the content of NaAl(P2O7) in all compositions containing P almost disappeared at 240 °C. These results indicate that a large amount of apatite, as a phosphate mineral, existed in SS when the temperature was higher than 240 °C and can be recycled. NAIP is the most labile P form and possesses high bio-availability at a temperature below 220 °C.
Table 3 The atomic (%) and weight (%) of element at 220 °C. Element
O
Mg
Al
Si
P
S
K
Ca
Fe
Atomic % Weight %
59.54 42.13
1.98 2.12
1.56 9.13
14.58 18.11
1.77 2.42
4.23 6.00
1.17 2.03
6.29 11.15
2.8 6.91
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