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Novel calcium oxide-enhancement phosphorous recycling technique through sewage sludge pyrolysis Siqi Tang, Feng Yan, Chunmiao Zheng, and Zuotai Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01492 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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ACS Sustainable Chemistry & Engineering

Novel

calcium

oxide-enhancement

phosphorous

recycling

technique through sewage sludge pyrolysis Siqi Tanga, Feng Yanb,d, Chunmiao Zhenga,b and Zuotai Zhangb,c* a

Department of Energy and Resources Engineering, College of Engineering, Peking

University, No.5 Yiheyuan Road, Haidian District, Beijing 100871, People’s Republic of China b

School of Environmental Science and Engineering, Southern University of Science and

Technology, No.1088 Xueyuan Road, Nanshan District, Shenzhen 518055, People’s Republic of China c

Key Laboratory of Municipal Solid Waste Recycling Technology and Management of

Shenzhen City, No.1088 Xueyuan Road, Nanshan District, Shenzhen 518055, People’s Republic of China d

School of Environment, Tsinghua University, No.30 Shuangqing Road, Haidian District,

Beijing 100084, People’s Republic of China

*Corresponding author: Prof. Dr. Zuotai Zhang (School of Environmental Science and Engineering, Southern University of Science and Technology, No.1088 Xueyuan Road, Nanshan District, Shenzhen 518055, People’s Republic of China) Tel./Fax: +86-755-88018019; E-mail address: [email protected]

ABSTRACT: Municipal sewage sludge is abundant and rich in phosphorus, making it a promising alternative phosphorus reserve. A good knowledge of the phosphorus transformation during pyrolysis will underlie the industrial phosphorus recycling and 1 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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reclamation of sewage sludge. This work first advanced a novel calcium oxide (CaO)-enhancement phosphorus-recycling technique based on pyrolysis of sewage sludge, by regulating the transformation of phosphorus in sewage sludge during pyrolysis through the CaO addition. The obtained results indicated that CaO addition promoted the formation of hydroxylapatite, which is a desirable phosphorus species for plant growth. The factors of pyrolysis temperature and inherent composition of sewage sludge both influenced the fraction of hydroxylapatite during pyrolysis. An increase in pyrolysis temperature and a sludge with a high content of ash and a low content of volatile matters potentially promoted the transformation of P from organic phosphorus to the inorganic species during pyrolysis with the addition of CaO, particularly for the formation of hydroxylapatite. Increasing CaO addition significantly increased the fraction of hydroxylapatite in the obtained char, and the maximum content of 25 wt.% hydroxylapatite over total phosphorus was attained. This enhanced transformation of hydroxylapatite may be potentially attributed to the interaction between CaO and the polyphosphate with the aid of the inherent minerals that appeared to benefit the immobilization of phosphorus during sludge pyrolysis. As the formation of hydroxylapatite was enhanced, this facile technology of CaO-enhancement sewage sludge pyrolysis could be used for the direct recycling of P as well as the disposal of sewage sludge. KEYWORDS: Municipal sewage sludge; Phosphorus transformation; Pyrolysis; Calcium oxide; Hydroxylapatite

INTRODUCTION Phosphorus (P) is an essential and non-sustainable element that is essential to the life of all organisms, as it enables the synthesis of DNA and RNA and the transfer of energy 1. In the earth, the pristine deposits of P from mined rocks are limited

2, 3

. Due to the substantial

production of plant-needed phosphorus fertilizer and P-derived articles used in human daily 2 ACS Paragon Plus Environment

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life, the amount of P reserves is being depleted, posing threats to the P supply for future agriculture and human life 4. Hence, seeking alternative sustainable P sources is pressing to alleviate the P supply shortage. Actually, a significant portion of P consumed by human activities is finally conveyed into municipal wastewater treatment plants (WWTPs), carried by sewage discharged to the civil wastewater pipeline 5. As a consequence, a considerable amount of P-enriched sewage sludge is generated during the phosphorus removal process from wastewater 6. In China, the generation of sewage sludge (with moisture of 78%) increased to ~28 million tonnes by 2015, as the vastly increasing volume of municipal wastewater 7. Therefore, the P-enriched sewage sludge has been identified as a most promising alternative source for P recycling and reclamation 8, 9. Generally, P is recycled from the incineration sewage sludge ash (ISSA) acidic or alkaline leaching process

10-12

13

, through

, high temperature thermochemical treatment

14

and

electrodialytic process 15. Although the P content in ISSA increased by many folds with regard to the raw sludge, the ISSA could not be directly used as phosphorus fertilizer because the bioavailability of P is significantly decreased

16, 17

. Furthermore, the heavy metals are also

concentrated in the ISSA after incineration at 850 °C 10-12, leading to a technological difficulty in separating P and heavy metals during the recycling process. Since the direct recycling of P from ISAA faces many challenges, it is still an important research question to realize the secure disposal of sewage sludge simultaneously with the efficient recycling of P. The pyrolysis technology has increasingly attracted much attention in sewage sludge disposal 18. The pyrolysis of sewage sludge is generally performed under an oxygen absent or deficient atmosphere at ~700 °C 18, resulting in the production of gas 19, liquid 18 and char 20. The resultant char rich in carbonaceous materials could achieve the CO2 sequestration 21, and the resultant char has a porous structure and an available surface

22

, and importantly, the

heavy metals contained in the raw sludge were severely immobilized in the resultant char 3 ACS Paragon Plus Environment


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during the pyrolysis process 23, 24. Those features of sewage sludge-derived char can therefore make it used to remedy the soils contaminated with heavy metals

22, 25, 26

. It should also be

noted that the resultant char obtained from pyrolysis contains abundant nutrients such as phosphorus that are benefit to plant growth

27

, which indicated that the P contained in the

resultant char can be utilized in soil environment 28. However, the risk of secondary pollution induced by the leaching of P element into soil should be concerned. Therefore, it is of great importance to regulate the progress of sludge pyrolysis and thus, simultaneously control P chemical state during sludge pyrolysis. To the best knowledge of author, there are few studies accentuating the P transformation along with the progress of sludge pyrolysis, which motivated this work partially. It has been proved that the chemical state of P in nature dominates the plant utilization for P and soil environment conditioning 27, 28, as well as the global recycling of P in ecosystem 29

. Qian, et al. 30 reported that a relatively lower temperature promoted the P transformation to

a medium-term P available to plant after sludge pyrolysis. Therefore, it is hypothesized that the regulation of P during pyrolysis process may be crucial for P efficient recycling. It is well known that the pyrolysis temperature and the composition of sewage sludge are the key factors for pyrolysis product quality 24, 31. Nevertheless, few current studies have clarified for the transformation mechanism of sewage sludge P during pyrolysis, particularly with the key factors. In the other hand, the external additives such as facile CaO are added to mitigate the pollution emission during sludge pyrolysis, because the presence of nitrogen (N) and/or sulfur (S) in sewage sludge contributing to the formation of pollutants deteriorates the quality of the pyrolytic products 32, 33. It was found that the addition of CaO not only inhibited the pollutant emission 34 but also improved the quality of the pyrolytic products

35, 36

. This effect could be

attributed to the enhancement effect of CaO on the transformation of sludge N and S during pyrolysis. Based on the function of CaO during pyrolysis, it is anticipated that the addition of 4 ACS Paragon Plus Environment

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CaO may potentially enhance the transformation of sewage sludge P during pyrolysis to the desirable species (such as hydroxylapatite (Ca5(PO4)3OH)

37

) for plant growth, which is

meaningful to achieve the efficient co-processing of sludge-derived char and the P utilization. In this work, a novel CaO-enhancement P-recycling technique was first proposed based on the pyrolysis of sewage sludge. The key factors, e.g., pyrolysis temperature and the composition of sewage sludge on sludge P transformation during pyrolysis were investigated systematically. The enhancement effect of CaO on the transformation of phosphorus into hydroxylapatite, which could be directly used as phosphorus fertilizer, was subsequently investigated with the addition of 10-50 wt. % CaO. Owing to the high content of bioavailable P in the resultant char, this facile approach could be used for the direct recycling of P, as well as the disposal of sewage sludge.

MATERIALS AND METHODS Material of sewage sludge Two typical sewage sludge samples both in the form of a dewatered cake were separately collected from the WWTPs in Beijing and Shenzhen, China. The obtained sludges were heated in an oven at 105 °C for two days. The dried sludges were sequentially crushed, screened, and sieved to 0.075-0.15 mm, which were named BS and SS for Beijing and Shenzhen, respectively. Prior to the pyrolysis experiment, the sieved sludges were preserved in an air-proof plastic bag for further use. Table 1 shows the proximate analysis, the ultimate analysis, the composition analysis of sludge ash and the heating value analysis for BS and SS. The difference in chemical composition between BS and SS could be attributed to the different influent quality and wastewater treatment process, as summarized in Table S1.

Table 1. The physicochemical properties of sludge feedstocks used in the present study Item Unit Proximate analysis (dry basis)a

BS

5 ACS Paragon Plus Environment

SS


Moisture Volatile matter Ash Fixed carbona

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wt.% wt.% wt.% wt.%

1.32 40.87 52.64 5.17

2.33 52.03 41.03 4.61

Ultimate analysis (dry basis)b C wt.% H wt.% Oc wt.% N wt.% S wt.%

24.43 4.12 14.66 3.94 1.21

37.03 2.72 9.65 5.51 4.06

Mineral composition (oxides, dry basis)d SiO2 wt.% Al2O3 wt.% P2O5 wt.% Fe2O3 wt.% CaO wt.% SO3 wt.% K2O wt.% MgO wt.% Na2O wt.%

27.50 16.06 10.25 3.83 3.70 1.68 1.08 0.86 0.68

17.17 14.07 13.19 5.65 2.91 3.21 1.03 0.35 0.64

Heating value (dry basis)e MJ/kg 9.8 14.1 measured according to the Chinese Standard GB/T 17664-1999. Fixed carbon (FC) was

a

calculated according to the formula: FC(%) = 100% - Volatile matter (%) - Ash(%) - Moisture (%) (1); b measured using the EA 3000 element analyzer (Euro Vector, Italy); c calculated by difference (O(%) = 100% - C (%) - H (%) - N (%) - S (%) - Ash(%) (2)); d measured using the ARL ADVANT XP+ X-Ray Fluorescence spectrometer (ThermoFisher, USA); e: measured using the 6400 automatic isoperibol calorimeter (Parr Instrument Company, USA).

Pyrolysis experiment of sewage sludge The pyrolysis of sewage sludge was performed in a lab-scale horizontal furnace with temperature-programmed control (NBD1200, NOBODY, China). The pyrolysis temperatures were selected as 300, 400, 500, 600 and 700 °C, spanning the common range used in sludge pyrolysis. In each run, the furnace was heated from room temperature (30 °C) to the pyrolysis temperature with a constant heating rate (10 °C /min) and then held for 60 min. Before heating the furnace, 50-g sludge was held in an alumina crucible and put away in the bare side 6 ACS Paragon Plus Environment

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near the outlet of the furnace. Once the furnace arrived at the pyrolysis temperature, the crucible containing sludge sample was instantly pushed into the center part of the furnace. A constant flow of 99.99 vol. % N2 (200 mL/min) was purged into the furnace as carrier gas throughout the heating. When the isothermal phase ended, the crucible was pulled out immediately to stay at the original position and then quenched under N2 sweeping. The derived chars were recovered and preserved for further characterization and were designated based on their pyrolysis conditions, e.g., BS700 and SS700 for the chars generated from the pyrolysis of BS and SS at 700 °C, respectively. To investigate the enhancement effect of CaO on phosphorus transformation, the sieved sludges were mixed with different mass ratios (10-50%) of CaO (99.95%, Alfa Aesar, USA) through mechanical dry mixing. This mass ratio range was designed to highlight the effect of CaO on P transformation during sludge pyrolysis and further to reveal possible relevant law. The mixed sludge samples were isothermally pyrolyzed at 700 °C for 60 min under the same operations mentioned above. The derived chars were named based on the mass ratios of CaO, e.g., 20%CaO-BS700 and 20%CaO-SS700 for the chars generated from the pyrolysis of BS and SS with 20% CaO added at 700 °C, respectively. Characterization The elemental composition was characterized, using an element analyzer (Euro Vector, EA 3000, Italy). The chemical functional group was characterized using a Fourier transform infrared (FTIR) spectrometer (ThermoFisher, Nicolet iS50, USA). The specific surface area and the pore size distribution were determined at 77K using a gas adsorption analyzer (Micrometrics, ASAP 2020, USA). The element chemical state (C 1s and P 2p) was measured using an X-ray photoelectron spectrometer (Thermo Scientific, Escalab 250Xi, USA). The phase composition of the chars was determined using a MiniFlex 600 X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation (40kV, 15 mA). The images from transmission electron 7 ACS Paragon Plus Environment


microscope

equipped

with

high-angle

annular

dark

field

Page 8 of 30

(HAADF-STEM)

and

energy-dispersive X-ray (EDX) were taken on an FEI Tecnai F30 microscope (FEI, USA). Determination of total P content in sludge char The total phosphorus contents in terms of orthophosphate were measured using UV-vis spectrophotometry. The samples were first combusted at 600 °C in air for 10 hours, and the obtained ashes were dissolved with 1 M HCl and later vigorously vibrated in a shaker at 180 rpm. Next, the solution was centrifuged in a Thermo Fisher centrifuge (ST40, USA) with a rotating frequency of 10000 rpm. The supernatant was filtered using a 0.45 μm membrane to recover the filtrate. An aliquot of the filtrate was taken to measure the absorbance at 700 nm using an Agilent UV-vis spectrophotometer (Carey 60, USA). The standard curve of KH2PO4 as the standard substance was made and found to be acceptable (Figure S2), in accordance with the national standard method 38. Solid state 31P nuclear magnetic resonance for sludge char The

31

P NMR spectra were acquired with a magic angle spinning (MAS) and proton

decoupling method on a Bruker Avance 400 spectrometer operated at a

31

P resonance

frequency of 161.9 MHz. Solid samples were packed into a zirconia rotor and spun at 5 kHz. Direct polarization (DP) data collection mode was used to acquire the data, compared to cross polarization (CP) data collection mode (Figure S1). The parameters of DP mode were 2048 data points over an acquisition time of 12.6 ms, a recycle delay of 180 s and 128 scans. Chemical shift corrections were carried out referencing to NH4H2PO4 at 0.72 ppm. The data processing work for the obtained

31

P NMR spectra included phase correction, baseline

correction, reference correction and pick picking, integration and multiplet analysis, all accomplished with MestReNova software (Version 8.1.4).

RESULTS AND DISCUSSION Total phosphorus in the sludge feedstocks and their derived chars 8 ACS Paragon Plus Environment

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Figure 1 shows the content and recovery rate of P in the chars obtained at different pyrolysis temperatures. After pyrolysis, the total P content in the derived char was increased as shown in Figure 1(a), compared to the raw feedstocks (BS and SS). It is observed that the total P content in sludge-derived char increased with increasing pyrolysis temperature. This result could be attributed to the decomposition of sludge organic components, as indicated by the decreased intensities of FTIR spectra in Figure S3 and the increased aromaticity and hydrophobicity in Figure S4. As a consequence, the thermal degradation of sludge organics resulted in the decrease in sludge char yield. As shown in Figure 1(b), the P recovery rate in sludge chars produced at different temperatures was close to 1, regardless of the source of sludge feedstock, suggesting that the P all in sludge feedstocks remained in the sludge matrix in the process of sludge pyrolysis (under 700 °C). This result agreed well with that of Huang, et al.

39

but was contrary to that of Qian, et al. 30. The reason may potentially be attributed to

the differences in the composition as well as pyrolysis temperature used. The comparison of total P content between BS and SS indicated that, the fractions of volatile matters and ash in sludge feedstocks affected the P concentration in the derived chars, although the total P contents in their feedstocks were close (12743.21 mg/kg dry sludge for BS and 12488.68 mg/kg dry sludge for SS). The high contents of total P in both sludge feedstocks are due to the adoption of activated sludge processes with enhanced removal of P in WWTPs. Regarding the results mentioned above, the presence of ash with a higher content and volatile with a lower content may potentially be beneficial to immobilize P in sludge char during pyrolysis under 700 °C.



Page 10 of 30

Figure 1. Total P changes in sludge-derived chars at different pyrolysis temperatures: (a) the content of total phosphorus in sludge chars derived from BS and SS (dash lines denote the sludge feedstocks); (b) the recovery rate of phosphorus for those derived sludge chars with respect

to

the

feedstock

and

char

yield

(P

recovery

rate

39

=

(3))

Phosphorus speciation in the sludge feedstocks and their derived chars Solid state 31P MAS NMR spectra of sludge feedstocks and the derived chars produced at different pyrolysis temperatures are shown in Figure 2(a, b). The side-band effect caused by the spinning of the sample was observed on those obtained spectra, presented using the spades. This effect was also observed in the study of Huang, et al. 39. The chemical shifts of various phosphorus-containing matters in nature fall into the range of -40 to 20 ppm in which the spinning side effect is ruled out 40. Thus, the present spectra corresponding to this range were used for the identification of the P-related subcomponents by means of spectral deconvolution using a Lorentzian (80%)-Gaussian (20%) function. The deconvoluted spectra for the NMR spectra are provided in Figure S5 and S6. The fraction that each subcomponent accounted for was determined, using the ratio of the corresponding integral area to the total area. Figure 2(c, d) presents the resulting fractions of each phosphorus species characteristic of the particular chemical shift in the sludge feedstocks and the obtained chars produced at 10 ACS Paragon Plus Environment

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different pyrolysis temperatures. It was found that the P subcomponents identified in the sludge feedstocks (BS and SS) included orthophosphate, orthophosphate monoesters, orthophosphate

diesters,

pyrophosphate,

ATP

or

ADP

α -phosphate,

wavellite

(Al3(OH)3(PO4)2∙5H2O), senegalite (Al2(OH)3(PO4)∙H2O) and polyphosphate. Those subcomponents were also found in previous study

39

. Among them, besides the

orthophosphate monoesters, orthophosphate diesters and ATP or ADP α-phosphate which belong to the organic P species, the rest belongs to the inorganic P species. The organic P species came from the microbial bodies, dead or aged 41, which accounted for almost 19% and 29% in BS and SS, respectively. In contrast, the inorganic P species are the predominant form of P in sewage sludge, probably attributed to the precipitation of effluent P in the primary settling tank and the degradation of organic P during sludge dewatering at the WWTP 5. In addition, the difference of P fraction between BS and SS, in combination with the composition (see Table 1), indicated that the source of municipal wastewater and the treatment process affects the transformation of P in a WWTP. Once in pyrolysis, the fraction of P in sludge was changed with increasing pyrolysis temperature. The subcomponent of ATP or ADP α-phosphate disappeared. Considering the chemical structure of ATP and/or ADP, it is reasonable to conclude that the P subcomponent of polyphosphate terminal P group was formed, due to the decomposition of ATP or ADP during sludge pyrolysis. It was observed that the fraction of this new P species seemed to increase with the increase of pyrolysis temperature, especially under high temperature (such as 700 °C). The transformation of orthophosphate diesters to orthophosphate monoesters was enhanced as pyrolysis temperature increased. It is reasoned that under high temperature, the cleavage of O-R bond (R denotes organic group such as alkyl and aryl) in orthophosphate diesters was preferable, resulting in the formation of orthophosphate monoesters. Due to the evolution of organic P species, the fraction of inorganic P species varied with the progress of 11 ACS Paragon Plus Environment


Page 12 of 30

sludge pyrolysis as pyrolysis temperature increased. After pyrolysis, the disappearance of pyrophosphate (inorganic P) in the obtained chars indicated the breaking of P-O-P, resulting in the formation of a monomer which is the basic unit of orthophosphate. Nevertheless, the fraction of orthophosphate was never increased, instead remaining closing to zero, which revealed that the secondary combinations of the derived monomer probably contributed to the fraction of polyphosphate terminal P group. The fraction of wavellite termed mineral-bound P was increased in the derived chars after pyrolysis, which could be attributed to the loss of the sludge organic fractions, while it almost kept constant as the pyrolysis temperature increased, suggesting that the wavellite was thermally stable and only slightly impacted by the progress of sludge pyrolysis (~700 °C). Opposite observations were found in the fractions of variscite and senegalite. The bound water of both was removed gradually with the increase of pyrolysis temperature, leading to the formation of the dehydrated phases, i.e., AlPO4 and Al2(OH)3(PO4). Both are common phases reported in combustion ash of sewage sludge rich in Al

42, 43

. The fraction of

Al2(OH)3(PO4) was improved with the increase of pyrolysis temperature, accompanying with the elimination of senegalite. It is reasoned that the thermal stability of senegalite was poor during sludge pyrolysis. However, the fraction of AlPO4 was maximized in the chars produced at 400 °C (BS400 and SS400), and later tended to decrease when the pyrolysis temperature further increased. The change in the AlPO4 fraction as well as the variscite fraction implied that the water derived due to the dehydroxylation of sludge matrix at temperature over 400 °C was confined in the crystallized structure of variscite, when it passed through the porous structure of the char (see Figure S7, S8 and Table S2). Additional data on the P chemistry in the obtained chars were provided using P 2p XPS and HAADF-STEM combined with EDX, as shown in Figure 3 and Figure 4, respectively. As shown in Figure 3(a, b) (the survey spectra are offered in Figure S9), the P bound to 12 ACS Paragon Plus Environment

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Al-involved minerals (Al-P) was identified in the BS and SS. In addition, the P bound to Ca-involved minerals (Ca-P) with orthophosphates was identified in the sludge feedstocks 44. The presence of pyrophosphate (Pyro-P) was indicated by the bonding energy of 133.3 eV, which agreed well with the aforementioned NMR spectral analysis. Once in pyrolysis, the chemical reactions relevant to the decomposition of sludge components resulted in the change of P chemistry, as indicated by the obtained XPS spectra (see C 1s XPS spectra in Figure S10, S11). As shown in Figure 3(c, d), the fraction of Al-P in the obtained chars varied with the increase of pyrolysis temperature, which was in accordance with that found in NMR spectra. The Ca-P fraction involving the species of Ca3(PO4)2 and Ca(HPO4)2 in the derived chars was increased when pyrolysis temperature increased, while the fraction of Pyro-P was decreased. The changes of these P fractions in the obtained chars with the increase of pyrolysis temperature revealed that, the chemical state of P was transformed from the organic forms to the inorganic forms with the progress of sludge pyrolysis. Again, compared to the sludge feedstock (as illustrated in Figure 4(a)), the HAADF-STEM and EDX mapping images of P, Si, Al, Fe, Ca indicated that the P in sludge matrix was distributed uniformly especially evolving with the presence of Al, throughout the progress of sludge pyrolysis (see Figure 4(b)). Overall, it is concluded that both the pyrolysis temperature and the inherent composition of sewage sludge influenced the transformation of P during pyrolysis, while the desirable P species of hydroxylapatite was hard to form.



Figure 2. Solid state 31P MAS NMR spectra (spades indicate the spinning side bands) of the sludge feedstocks and the derived chars at different pyrolysis temperatures: (a) BS and its chars; (b) SS and its chars; (c) the content of P subcomponents identified in (a); (d) the content of P subcomponents identified in (b). (Note: orthophosphate monoesters (a) (b) (c) differentiate with an alkyl or aryl bound)


Page 14 of 30

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Figure 3. P 2p XPS spectra of the sludge feedstocks and the derived chars at different pyrolysis temperatures: (a) BS and its chars; (b) SS and its chars; (c) the content of P chemical states identified in (a); (d) the content of P chemical states identified in (b).

Figure 4. HAADF-STEM images for mapping and concerned elements mapping images of the sludge feedstock and the derived chars at 700 °C: (a) BS, (b) BS700 (b) and (c) 30%CaO-BS700. 15 ACS Paragon Plus Environment


Page 16 of 30

Effect of CaO on the speciation of phosphorus in the sludge-derived chars Figure 5 (a, b) presents the acquired NMR spectra, when CaO was added with different mass ratios. Spectral deconvolutions were performed on the spectra ranging from -40 ppm to 20 ppm to quantify the P subcomponents. The resulting fractions of P species in the obtained chars are shown in Figure 5 (c, d). It was found that the addition of CaO changed the fraction of P species, varying with the increase of the amount added. The variations of side-band effect on the obtained spectra indirectly indicated the effect of CaO on P transformation during sludge pyrolysis. For the evolution of the organic P species, the addition of CaO increased the fraction of orthophosphate monoesters in the chars derived from BS. Furthermore, increasing the addition appeared to increase the fraction of orthophosphate monoesters when the addition mass ratio arrived at 30%, but slightly decreased at over 30%. In contrast, the fraction of orthophosphate monoesters in the chars derived from SS was maximized when the addition arrived at 20% and then decreased when the addition was increased further. For the fraction of orthophosphate diesters species, it increased with the increasing addition of CaO (up to 20%) and then decreased when the addition was further increased. For these organic subcomponents, the addition of CaO more largely influenced their fractions in the chars from SS than the chars from BS. This was probably due to the lesser amount of ash and the higher volatile matter content for SS, both of which facilitated the reactions between sludge organic components and CaO such as catalysis, with regard to BS. The addition of CaO also changed the fraction of inorganic P subcomponents in the obtained chars. The desirable species of hydroxylapatite, also found in previous study

37, 42

,

showed an opposite trend in the chars from BS and SS as the addition of CaO increased. For the chars derived from BS, the fraction of the hydroxylapatite appeared to decrease with increasing CaO addition up to 20%, followed by an increase with further increasing addition. 16 ACS Paragon Plus Environment

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Nevertheless, the fraction of the hydroxylapatite in the chars from SS appeared to decrease significantly and even disappear with increasing CaO addition. It is noted that the difference in the fraction of orthophosphate was significant between the chars from BS and from SS with the increasing addition of CaO, as well as in the fraction of polyphosphate terminal P group. Those results proved that the inherent composition of sewage sludge significantly influenced the transformation of P among the different species, especially for the formation of hydroxylapatite. Additional information about the phase of the derived chars was further revealed by XPS spectra (see the survey spectra in Figure S12) and X-ray diffractograms. Figure 6 (a, b) show the P 2p XPS spectra, and the resulting fractions of P chemical states are shown in Figure 6 (c, d). It can be seen that the fraction of organic P species was decreased significantly, and the P in the chars derived from CaO-added sludges was predominantly in the form of inorganic species. The changes of P in the inorganic subcomponents revealed the participation of CaO in the transformation of P. The changes of the fractions in Al-P (variscite and berlinite) supported the dehydration of CaO stated above. The disappearance of chemical state at 134.0 eV indicated that the H2PO42- in Ca-P was converted to the other Ca-P such as hydroxylapatite and the P2O74-. The P in hydroxylapatite is desirable, as it is available for plants

30

. As

revealed by NMR spectra and XPS spectra, an increasing addition of CaO seemed to promote the formation of hydroxylapatite, and the comparison of hydroxylation fraction between both indicated that the formation of hydroxylapatite was related to the composition of sludge feedstock. It should be pointed out that the contradiction between NMR and XPS results for hydroxylapatite was observed in Figure 5 and Figure 6. Since the heterogeneity of sludge matrix resulted in the inhomogeneous distribution of P after sludge pyrolysis

20

, the XPS

measurements for P chemistry are limited to semi-quantitative results, which were used to assist the analysis of NMR spectra. 17 ACS Paragon Plus Environment


The XRD patterns of the derived chars, illustrated in Figure 7, also evinced the changes in the mineral-bound P found above. Al-P and Ca-P were found to be the main P species in the obtained chars. With the addition of CaO increasing, the intensity of Ca-P (hydroxylapatite) tended to exceed that of Al-P (wavellite, senegalite and muscovite). Those Al-P minerals can be used as long-term P fertilizers for the growth of plants

45

. Furthermore, the presence of

Al-P consolidated the results coming from the NMR spectra. The HAADF-STEM and EDX mapping images of P and Ca, presented in Figure 4(c), supported the function of CaO to P transform during sludge pyrolysis, addressed on the results of NMR and XPS spectra. Based on these changes in the organic and inorganic P subcomponents, it can be reasoned that CaO participated in the transformation of those P subcomponents during sludge pyrolysis. It is observed that CaO could catalyze the progress of sludge pyrolysis under high temperature (such as at 700 °C)

35

. Hence, the sludge organic P was released, leading to the

formation of orthophosphate monoesters and diesters. Since the derived gaseous products such as CO2, H2O and the tar vapor highly reacted with CaO at high temperature (such as 700 °C) 35, the amount of CaO added affected the function posed on the transformation of P, and further resulted in the variations in the fractions of these P subcomponents, particularly for the phase of hydroxylapatite. In addition, the sludge inherent minerals interfered with the effect of CaO on P transformation during pyrolysis, especially in the formation of hydroxylapatite, as indicated by the difference of these P subcomponents in the chars from BS and SS. As listed in Table 1, the amount of the ash and its composition between them was different significantly. Thus, to a degree, it was speculated that pyrolysis temperature and the inherent composition of sewage sludge (particularly for the inherent minerals) influenced the transformation of P during pyrolysis, specifically for the desirable P species of hydroxylapatite.


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Figure 5. Solid state 31P MAS NMR spectra (spades indicate the spinning side bands) of the chars derived from the sludge mixed with CaO at different mass ratios at 700 °C: (a) the chars from BS with CaO added; (b) the chars from SS with CaO added; (c) the content of P subcomponents identified in (a); (d) the content of P subcomponents identified in (b).



Figure 6. P 2p XPS spectra of the chars derived from the sludge mixed with CaO at different mass ratios at 700 °C: (a) the chars from BS with CaO added; (b) the chars from SS with CaO added; (c) the content of P chemical states identified in (a); (d) the content of P chemical states identified in (b).

Figure 7. XRD pattern of the sludge chars derived from CB and CS with different amounts of CaO added at 700 °C.

Environmental applications 20 ACS Paragon Plus Environment

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The global sustainability of phosphorus has increasingly been one of the core issues relevant to human’s life and development, due to the rapid depletion of phosphorus reserves in the past decades and the shortage of sustainable P resources. Human-induced activities that utilize phosphorus mainly include the manufacture of agricultural P fertilizer and life necessity. It is estimated that the annual consumption of P in agriculture amounts to 22-26 million tonnes 46. The increasing depletion of P is associated with the fertilization of carbon and nitrogen, which results in a strong imbalance of global P availability 47. The imbalance of P, in turn, affects the carbon sequestration and the stability of earth’s ecological system. For the global P cycle, most of the P consumed by humans’ activities is conveyed into WWTPs; therefore, recycling the P from sewage sludge will most significantly contribute to the global P availability and balance. Recently, the pyrolysis technique has been widely used for the disposal of sewage sludge due to the lower treatment temperature recycling of char for soil remediation

18

, the sequestration effect of CO2

21, 25

, and the

22, 25

. After pyrolysis, the P in sewage sludge is almost

entirely transferred into the resultant char, mainly existing as P-containing iron and/or aluminum compounds 30. Moreover, the inherent heavy metals in sewage sludge were tightly immobilized in the sludge char matrix, resulting in a low leaching bioavailability

23, 48

.

Although the total amount of P and the bioavailability of P in the resultant char are highly enhanced over those in the ISSA, the resultant char still contains limited amount of bioavailable P to act as the phosphorus fertilizer for plant growth. More seriously, the bio-unavailable P cannot enter the P cycle through plant uptake, and even causes eutrophication of water body with continuous rainwater seepage 29. Thus, we are attempting to clarify the transformation mechanism of P and enhance the bioavailable amount of P in the resultant char through the addition of CaO. The enhancement effect of CaO during sewage sludge pyrolysis was proposed, as 21 ACS Paragon Plus Environment


illustrated in Figure 8. It is found that the P in sewage sludge consists of the organic fraction and inorganic fraction, including orthophosphate monoesters, orthophosphate diester, orthophosphate, pyrophosphate, polyphosphate and P-bound minerals (Ca-P and Al-P). Without the addition of CaO, the increase in pyrolysis temperature appears to transform the orthophosphate diesters to the orthophosphate monoesters. The formed monomer PO43- is likely to combine mutually to develop the phase of polyphosphate terminal P group, while it is likely to be transformed to the pyrophosphate with the presence of CaO during pyrolysis. The composition of inorganic P species was influenced at high temperature (such as 700 °C), particularly with the CaO addition. When CaO was added to the raw sludge, the transformation of P in organic species was enhanced during pyrolysis. The enhancement was related to the sludge composition. The high content of inherent minerals and low content of volatile matters seems to pose a significant enhancement. It is noteworthy that the P in the inorganic species was changed due to the addition of CaO. The presence of CaO not only enhanced the dehydration but also promoted the formation of hydroxylapatite, which is desirable as a plant-preferable P. Moreover, the increase in the addition of CaO is beneficial to the formation of hydroxylapatite, the maximum content of which could reached 25 wt.% of total phosphorus in the obtained char. However, if the CaO-added sludge char obtained after pyrolysis is used to ameliorate the acid-degraded soil, the practical addition of CaO should take into account the chemical composition of sludge feedstock due to a concern of soil salinization. Simultaneously, the relatively high content of Al should be concerned due to the potential harmfulness for plant growth, although aluminum phosphate minerals can act as long-term fertilizers 45, and a systematical investigation on the leaching behavior of Al as well as P will have to be performed, if the obtained CaO-added sludge char is applied for plant growth in practice. Therefore, the novel technique of CaO-enhancement sewage sludge pyrolysis will be meaningful in practice, both for the P recycling and reclamation from 22 ACS Paragon Plus Environment

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sewage sludge.

Figure 8. Illustration for P transformation during sewage sludge pyrolysis with CaO added.

CONCLUSIONS The transformation of P during municipal sewage sludge pyrolysis and the effect of CaO were investigated in this work. The addition of CaO not only promoted the transformation of orthophosphate diesters to orthophosphate monoester for the formation of polyphosphate terminal P group but also changed the phases of inorganic P species and formed the phase of hydroxylapaptite which is a plant-desirable P mineral. The increase in the addition of CaO appeared to enhance the formation of hydroxylapatite, probably due to the reaction of CaO with polyphosphate on the support of the inherent minerals that was beneficial to immobilize P in sludge matrix. Based on the attained results, a novel CaO-enhancement sewage sludge pyrolysis technique was proposed, which is beneficial to the subsequent utilization of P recycling and reclamation in practice. 23 ACS Paragon Plus Environment


ASSOCIATED CONTENT Supporting Information Determination of solid-state

31

P NMR data collection modes (CP and DP); Standard

curve of KH2PO4 for total phosphorus determination; FTIR spectra, H/C and (N+S)/C ratios, deconvolution of solid state

31

P NMR spectra, N2 adsorption-desorption isotherms, XPS

survey spectra, deconvolution of C 1s spectra, structural parameters of the sludge feedstocks and sewage sludge-derived chars; XPS survey spectra of the chars derived from CaO-added sludge.

AUTHOR INFORMATION Corresponding Author *Tel./Fax: +86-755-88018019; E-mail address: [email protected] Present Address *School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, People’s Republic of China Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This study was supported by the National Science Fund for Distinguished Young Scholars (51522401) and the National Natural Science Foundation of China (51472007, 51772141). This work was also supported financially by the Shenzhen Science and Technology Innovation Committee (ZDSYS201602261932201, JCYJ20170412154335393, and KQTD2016022619584022). Additional support was also provided by the Southern University of Science and Technology (Grant No. G01296001) and the Guangdong Provincial Key Laboratory of Soil and Groundwater Pollution Control (Grant No. 2017B030301012).

REFERENCES 24 ACS Paragon Plus Environment

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Synopsis: The enhanced formation of hydroxylapatite desirable for plants as P fertilizer was achieved through regulating sewage sludge P transformation during pyrolysis with CaO addition.


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