Efficient recovery of phosphorus in sewage sludge through hydroxylapatite enhancement formation aided by calcium-based additives

Journal Pre-proof Efficient recovery of phosphorus in sewage sludge through hydroxylapatite enhancement formation aided by calcium-based additives Jingjing Chen, Siqi Tang, Feng Yan, Zuotai Zhang PII:

S0043-1354(19)31227-8

DOI:

https://doi.org/10.1016/j.watres.2019.115450

Reference:

WR 115450

To appear in:

Water Research

Received Date: 25 September 2019 Revised Date:

26 December 2019

Accepted Date: 27 December 2019

Please cite this article as: Chen, J., Tang, S., Yan, F., Zhang, Z., Efficient recovery of phosphorus in sewage sludge through hydroxylapatite enhancement formation aided by calcium-based additives, Water Research (2020), doi: https://doi.org/10.1016/j.watres.2019.115450. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Graphical abstract

1

Efficient recovery of phosphorus in sewage sludge through hydroxylapatite

2

enhancement formation aided by calcium-based additives

3

Jingjing Chen1,2, Siqi Tang3, Feng Yan1,4 and Zuotai Zhang1,4*

4

1

5

Groundwater Pollution Control, Southern University of Science and Technology, Shenzhen, 518055, P.R.

6

China

7

2

School of Environmental, Harbin Institute of Technology, Harbin 150091, P.R. China

8

3

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

9

100871, People’s Republic of China

School of Environmental Science and Engineering, Guangdong Provincial Key Laboratory of Soil and

10

4

11

City, Southern University of Science and Technology, Shenzhen, 518055, P.R. China

12

*Corresponding author: [email protected]

The Key Laboratory of Municipal Solid Waste Recycling Technology and Management of Shenzhen

1

13

ABSTRACT: Reclaiming abundant phosphorus from sewage sludge (SS) via pyrolysis for use as a

14

fertilizer has gained increasing attention owing to the rapid depletion of global P reserves. In this study,

15

the enhancement effect of Ca-based additives on sludge P transformation to hydroxylapatite through

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pyrolysis was systematically investigated. Three Ca-based additives were added in the pyrolysis of SS,

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and they were found to promote the conversion of sludge P to hydroxylapatite, which is bioavailable to

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plants. The characterization of the sludge-derived pyrochars indicated that the addition of 10% CaO, 5%

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Ca(OH)2, or 10% Ca3(PO4)2 facilitated peak hydroxylapatite production. The thermodynamic simulation

20

of the production of hydroxylapatite during pyrolysis showed that these additives increased the enthalpy

21

of the pyrolysis system. Furthermore, the pyrolysis with CaO addition had the lowest enthalpy, thereby

22

suggesting that the addition of CaO during sludge pyrolysis was preferable for recovering sludge P in

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the form of hydroxylapatite. Moreover, the hydroxylapatite produced with these additives was unstable

24

when the pyrolysis temperature was above 900 °C. The pot experiment demonstrated the feasibility

25

using the sludge-derived pyrochars as P fertilizer for plant growth. Therefore, changing the Ca form

26

and/or Ca/P ratio with the addition of Ca-based additives could be an effective strategy for reclaiming P

27

from SS in the form of hydroxylapatite, and this demonstrates a pathway for global P sustainability by

28

recycling P from P-abundant wastes.

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Keywords: Sewage sludge pyrolysis; Phosphorus transformation; Ca-based additives; Hydroxylapatite;

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P sustainability

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1. Introduction

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Phosphorus

is a non-renewable, but essential element for all living organisms in the synthesis of

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nucleic acid and the transfer of energy at the cell level. It is estimated that the current worldwide P

34

reserveswill be exhausted within the next 50 – 100 y (Atienza–Martínez et al. 2014), or even sooner at

35

the current peak rate of production. With the increasing population and strong demand for P-based

36

products, such as fertilizers, detergents, and insecticides, the depletion of P reserves threatens the future

37

supply of P needed for agriculture and human life (Rahaman et al. 2014, Scholz et al. 2013). 2

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Considering that phosphate rocks are the exclusive sources of P and are non-renewable, there is a

39

pressing need to find alternative P resources to ensure its sustainability. It is well known that a large

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portion of P consumed by human activities eventually accumulates in municipal wastewater treatment

41

plants (WWTPs) through urban wastewater pipelines (Cieślik and Konieczka 2017). For example, in

42

China, over 28 million metric tons of dehydrated sewage sludge (SS) (with a moisture content of 85%)

43

was generated in 2015, which needed to be urgently disposed of with fast volume reduction and in an

44

environmentally benign manner (2018). As the wastewater in WWTPs is processed, soluble P is bound

45

to or precipitated in SS. The content of P in the SS can be up to 2–4% by weight depending on the

46

source of the sewage and the sewage treatment process (Günther 1997, Rappaport et al. 1987).

47

Consequently, SS rich in P is generated with the biological P removal technologies widely used in

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WWTPs. Considering the continuous production of SS in WWTPs, it appears that P-enriched sludge

49

will be a substantial source of P (Huang et al. 2015b, Shiba and Ntuli 2017).

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Previous studies focused on the recycling of P from the incinerated sewage sludge ash (ISSA)

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generated by combusting SS in the ambient atmosphere at 850 ℃ (Biswas et al. 2009, Ottosen et al.

52

2013, Petzet et al. 2012, Wzorek et al. 2006). Compared with that of the raw sludge matrix, the mass

53

content of P enriched in ISSA reached approximately 8 wt.% (Biswas et al. 2009); however, the P in

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ISSA cannot be directly utilized by plants as a fertilizer owing to its inert bioavailability (Kleemann et al.

55

2017). Currently, the research on recovering P from ISSA mainly includes acidic leaching, alkaline

56

leaching, thermal treatment, and electrodialysis (Guedes et al. 2014, Petzet et al. 2012). In addition, a

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direct recovery method of P from SS has been explored by Meyer et al. (2018), who found that P

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fertilizer derived from SS through sequential extraction by resin and NaHCO3 was almost as effective as

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the reference water-soluble P fertilizer. A key issue with these methods is achieving efficient separation

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of P and heavy metals concentrated in ISSA after combustion (Herzel et al. 2016). Furthermore, even if

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these methods could separate P and the heavy metals in ISSA to some extent, other concerns still persist,

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such as the complexity of the process, significant consumption of chemicals, and the large number of

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by-products that are produced (Donatello and Cheeseman 2013, Ebbers et al. 2015, Fang et al. 2018, 3

64

Guedes et al. 2014). Therefore, a satisfactory approach to recover P from SS should not only reconcile

65

the presence of heavy metals, but also enhance the biocompatibility and bioavailability of P so that it

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can be directly used. The bioavailability of heavy metals in SS is rapidly reduced after pyrolysis

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involving the occurrence of various thermochemical reactions in an oxygen-limited or absent

68

atmosphere (Devi and Saroha 2014, Jin et al. 2016, Li et al. 2018).

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Natural P minerals are mainly composed of three morphologies of apatite with a general formula of

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Ca5(PO4)3X (X=OH, F, or Cl) (Kazin et al. 2014). Among these three forms, the apatite with X=OH, i.e.,

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hydroxylapatite, is the most abundant morphology in natural P rocks, and is more stable and easier to

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generate (Lower Steven et al. 1998). Furthermore, it has been recognized that hydroxylapatite is

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biocompatible with soil and can enhance plant growth (Jiang et al. 2012). Therefore, specifically

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transforming the P in SS to hydroxylapatite during sludge pyrolysis seems to be the most reasonable

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method to realize the optimal recycling of P. As shown in Equation 1 (Joris and Amberg 1971, Kreidler

76

and Hummel 1970), the production of hydroxylapatite depends strongly on the value of x ranging from

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0 to 1; x=0 and x=1 indicated the maximum and minimum content, respectively, where the

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corresponding Ca/P molar ratio falls within 1.67 (x=0) to 1.50 (x=1). Accordingly, the enrichment of

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hydroxylapatite can be achieved by adjusting the Ca/P ratio. Furthermore, the forms of Ca and P

80

determine the evolution of the generated hydroxylapatite. Thus, efficiently converting the P in SS to the

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species of hydroxylapatite during pyrolysis is crucial to reclaiming sludge P resources. Therefore,

82

simultaneously adjusting the Ca/P ratio and the forms of Ca and P is expected to promote the formation

83

of the desired P mineral. Furthermore, it is necessary to study the effects of different chemical forms of

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Ca on hydroxylapatite formation to achieve an optimal method of enrichment. This, in turn, provide a

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solution for the disposal of SS and recycling of P as a fertilizer. Ca10-x (PO4 ) (HPO4 ) OH 6-x

x

Sintering 2-x

1-x Ca10 (PO4 ) OH 2 +3xCa3 (PO4 ) 1 6

2

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In this work, we investigated the production of hydroxylapatite during SS pyrolysis by

87

simultaneously adjusting the molar ratios of Ca/P and changing the forms of Ca and P using common

88

chemicals. Three Ca/P-based chemicals, namely CaO, Ca(OH)2, and Ca3(PO4)2, were used, and the 4

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addition ratios were changed with reference to CaO addition (5–50%). In addition, to understand the

90

regulation mechanism of the Ca/P ratio on the evolution of hydroxylapatite, thermodynamic simulation

91

using the principle of Gibbs free energy minimization was performed for the SS pyrolysis process. The

92

results are expected to guide the recovery of P from SS with the addition of feasible Ca-based additives

93

during sludge pyrolysis, thereby further facilitating P recovery and reclamation in practice.

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2. Materials and methods

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2.1 Sludge sample preparation

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The SS with a moisture content of 85–90% was sampled from a WWTP located in Shenzhen,

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China. In this WWTP, the municipal sewage was treated by biological (active sludge) treatment units.

98

The dewatered sludge was heated in an oven at 105 °C for 2 days. Then oven-dried sludge samples were

99

sequentially ground, screened and sieved with a 100-mesh screen to collect the sludge sample for further

100

use. The composition characteristics of the sludge feedstock to be used in the experiments are listed in

101

Table 1.

102

2.2 Pyrolysis of sewage sludge

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The pyrolysis of SS was conducted in a lab-scale horizontal electric-heated tube furnace (NBD

104

1200, NOBODY, China). The sludge samples were placed in a quartz combustion chamber with a length

105

of 1000 mm and inner diameter of 80 mm. The pyrolysis temperatures of the raw SS were 300, 500, 700,

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and 900 °C, which were within the commonly used temperature range of SS pyrolysis (Chen et al. 2014,

107

Yu et al. 2018). Heating was conducted at a constant heating rate of 10 °C/min with N2 as the carrier gas

108

with a flow rate of 100 mL/min. The weighed raw sludge, which was approximately 10.00 g, was held

109

in an alumina crucible. The crucible was immediately pushed to the center of the combustion chamber

110

and held for 60 min upon the furnace reaching the desired pyrolysis temperature. When the heating

111

program ended, the alumina crucible was immediately moved to the end of the chamber and then

112

quenched by means of purging with N2. The sludge-derived pyrochars were collected and labeled as per 5

113

the pyrolysis temperature.

114

To explore the enhancement effect of different ratios of Ca to P and the chemical form of Ca on P

115

speciation during the SS pyrolysis, three types of additives, namely CaO (99.95%, Alfa Aesar, USA),

116

Ca(OH)2 (95.00%, Macklin, China), and Ca3(PO4)2 (96.00%, Aladdin, China), were employed. These

117

additives were adequately mixed with the raw sludge at different mass ratios of 5%, 10%, 15%, 20%,

118

and 50% by a ball mill. The amount of Ca(OH)2 and Ca3(PO4)2 added was based on the amount of Ca in

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CaO to maintain the same ratio of Ca/P with the three different additives. Since the temperature

120

dominated the production of hydroxylapatite as revealed by Equation 1, a lower temperature (500 °C)

121

and a higher temperature (900 °C) were selected among the above four temperatures to pyrolyze the

122

mixed sludge samples under the same operating conditions as those of the raw sludge. The mixed sludge

123

samples were pyrolyzed at 500 °C and 900 °C under the same operating conditions as those of the raw

124

sludge. The sludge-derived pyrochars were labeled according to the mass ratio and pyrolysis

125

temperature. For example, the label 15% Ca3(PO4)2-900 °C represented pyrochar generated from the

126

pyrolysis of the sludge with the addition of 15% Ca3(PO4)2 at 900 °C.

127

2.3 The determination of total phosphorus content

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The content of total phosphorus in the sludge and derived sludge pyrochars was determined by

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UV-vis spectrophotometry (Cary 60, Agilent, USA). The method for measuring the quantity of total

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phosphorus was based on the method reported by Huang and Tang (2015) and Tang et al. (2018). Prior

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to the measurement, the sludge feedstock and its derived pyrochars were first combusted in a muffle

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furnace at 600 °C for 2h, and the obtained residues were dissolved in 1M HCl solution, followed by a

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vibration at a rotational speed of 180 rpm in a shaker. Next, the solution was centrifuged at 10000 rpm

134

in a centrifuge (ST40, Thermo Fisher, USA). The suspension was filtered through a 0.45-um membrane,

135

and the filtrate was analyzed at 700 nm using the UV-vis spectrophotometer. The standard curve of the

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total phosphorus using KH2PO4 as standard (shown in Fig. S1) was made in accordance with the

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national standard method (HJ 631-2011). 6

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2.4 Characterization of phosphorus speciation

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Quantitative 31P solid-state nuclear magnetic resonance (NMR) spectra were recorded on a Bruker

140

600 MHz Avance III HD wide bore spectrometer with magic angle spinning (MAS). The solid samples

141

were packed into a zirconia rotor and rotated at a spinning speed of 12 kHz. The parameters of data

142

acquisition using direct polarization mode were 2048 data points over an acquisition time of 20.9 ms, a

143

recycle delay of 30 s, and 400 scans. Chemical shift corrections were externally referenced to

144

(NH4)2HPO4 at 1.0 ppm. The processing of the NMR spectra was accomplished by using MestReNova

145

software (version 8.1.4).

146

Additionally, X-ray photoelectron spectra (XPS) were obtained using a Physical Electronic PHI

147

5000 VersaProbe III spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV), and

148

the most frequently used X-ray setting was 200um/50W/15kV. All spectra were corrected to the main

149

line of the C 1s spectrum (adventitious C) and set to a binding energy of 284.8 eV. The acquired data

150

were analyzed using PHI MultiPak software (version 9.8). The Shirley background subtraction method

151

was used to fit the curve line shape with the Gaussian-Lorentzian function.

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The X-ray powder diffraction (XRD) patterns of the mixed sludge and derived pyrochars were

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obtained using a Rigaku Smartlab powder diffractometer equipped with Cu Kα radiation, operating at

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45 kV and 200 mA. The scanning was conducted in the 2θ range between 10° and 80° with a scan speed

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corresponding to 2°/min. In addition, the pyrochar samples were measured by Fourier transformation

156

infrared spectroscopy (FTIR, Nicolet iS50, Thermal Scientific, USA). The samples blended with KBr at

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a ratio of 1:200 were pelletized, and the spectra were recorded in the range of 4000–400 cm-1 with a

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resolution of 2 cm-1.

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2.5 Thermodynamic simulation for phosphorus speciation during sludge pyrolysis

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After determining the chemical composition of the raw sludge and the blending ratio of the

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additives, P transformation towards equilibrium conditions of the pyrolysis reaction could be predicted

7

162

using FactSage software (Bale et al. 2016). This software is specialized in describing equilibrium ash

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properties, the behavior of P, and thermodynamic parameters. In the FactSage platform (version 7.2),

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three main principles namely, mass balance, energy balance, and minimum Gibbs free energy, are

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involved in the simulation of sludge pyrolysis (Bale et al. 2016). The criterion directed in this

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calculation is to minimize the Gibbs free energy based on two hypotheses (Li et al. 2012), that is, (1) all

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the input reactants were considered ideal substances and were assumed to be completely mixed when

168

two or more substances were defined as being in the same phase and (2) the initial system would

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eventually reach steady-state equilibrium in thermodynamics. In the thermodynamic simulation, we

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chose the Equilib module, which is the main force for Gibbs energy minimization in FactSage. It

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calculated the concentrations of chemical substance when specified elements or compounds reached

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chemical equilibrium (Bale et al. 2009). We input the sludge composition with different mass ratios of

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additives based in 1 g of SS and chose the FToxid database (mineral) and FTmisc database

174

(miscellaneous) for possible products (pure solid and gas) determination. The pyrolysis temperature was

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selected in the range of 300–1200 °C with an increment of 100 °C, and the total pressure of the sludge

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pyrolysis system (closed system) was assumed to be 1 atm. The total Gibbs energy of the pyrolysis

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system can be calculated using Equation 2. By minimizing the total Gibbs energy of the system, the

178

combinations of ni, Pi, and Xi could be determined.

G = ∑ ni ( gio + RT ln Pi ) + ideal gas

179

+

∑

solution−1

180

∑

ni gi o

pure condensed phases

ni ( gi + RT ln X i + RT ln γ i ) + o

(2)

∑

solution −2

ni ( gi + RT ln X i + RT ln γ i ) + ...... o

2.6 Pot experiment

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To confirm the mian effect of the sludge-derived pyrochars when Ca-based additives were added

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during the SS pyrolysis, a pot experiment was performed using these pyrochars as a fertilizer. The 8

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chemical characteristics of the soil feedstock are listed in Table S1. In this study, a soil sample was,

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which free from heavy metal contamination in accordance with the Chinese standard for soil quality was

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collected in the SUSTech campus (Shenzhen, China) (Table S2). The obtained soil was air-dried and

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then sequentially ground, screened and passed through a 2 mm sieve for further use. Five hundred grams

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of the soil was adequately mixed with the sludge-derived pyrochars at two different mass ratios, namely

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1% and 4%. The pyrochars were harvested from the pyrolyzed sludge samples with the addition of 15%

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Ca-based additives at 500 °C and 900 °C. Ryegrass, which is sensitive to the abundance or paucity of P

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in fertilizer during growth, was selected. A duplicate experiment was conducted, and a control

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experiment was also conducted as a blank run without the addition of sludge pyrochar to serve as a

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reference. After the ryegrass was grown for 30 d, it was harvested and photographs were taken, which

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are shown in Fig. S2. The height and chlorophyll content of the obtained ryegrass were determined to

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evaluate its growth (Olorunfemi et al. 2008). The determination of chlorophyll a, chlorophyll b, and

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total chlorophyll in fresh leaves was based on the standard method of Arnon (1949).

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3. Results and discussion

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3.1 Total P content in the sludge samples and sludge-derived pyrochars

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Figure 1 presents the pyrochar yields and their total P (TP) content. The calculation of the TP mass

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balance showed that the P in the SS was almost retained in the pyrochars obtain from the pyrolysis of

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either the raw sludge or the mixed sludge samples (Table S3). As shown in Fig. 1(a), the pyrochar yield

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increased with the increase in the addition of Ca-based additives. The three additives showed different

202

effects on the production of pyrochar depending on the pyrolysis temperature. An increase in the

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pyrolysis temperature led to a decrease in the pyrochar yield. Because the increased pyrolysis

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temperature induced the yield of pyrolysis gas, the yield of pyrochar tended to decline (Liu et al. 2015).

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These results indicated that the form of Ca or the ratio of Ca/P influenced the pyrolytic reactions of the

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SS, which was reflected in the amount of produced pyrochar (Chanaka Udayanga et al. 2019).

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The change in the TP in the sludge-derived pyrochar when Ca-based additives were added during 9

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the pyrolysis was determined. As shown in Fig. 1(b), the TP contents in the raw sludge pyrochar at

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500 °C and 900 °C were 2.87× 104 mg/kg and 3.83× 104 mg/kg of dry sludge, respectively. When the

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Ca-based additives were mixed with the raw sludge, the sludge-derived pyrochars showed lowered TP

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contents compared with those of the pyrochars derived from the raw sludge, except in the case of

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Ca3(PO4)2. This might have been attributed to the decomposition of organic components in the sludge

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feedstock, which could be observed from the decreased intensities of the organic functional bonds in the

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derived sludge (see in Fig. S3). Furthermore, an increase in the addition of CaO and Ca(OH)2 appeared

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to decrease the content of TP in the sludge-derived pyrochars, while an increase in the addition of

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Ca3(PO4)2 increased the TP content, which could be attributed to the introduction of P. Considering the

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results of the sludge biochar yield, the change in the TP content in the sludge biochar varied with the

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form of Ca and/or the ratio of Ca/P.

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3.2 Characteristics of phosphorus speciation during sludge pyrolysis with calcium-based additives

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Precisely characterizing the distribution behaviors of P in pyrochar is crucial to understand the

221

effects of Ca-based additives or the ratio of Ca/P on the recovery of P. The cutting-edge technique of

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solid state

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was supplemented by P 2p XPS and XRD. Figures 2(a, c, e) show the obtained solid-state

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NMR of the sludge-derived pyrochars when the sludge samples with Ca-based additives were pyrolyzed

225

at 900 °C. The resonance centered at a chemical shift of -3.8 ppm was considered to be the center band

226

reflecting the isotropic

227

center band were spinning sidebands (represented by the spades) (Bleam et al. 1989). Deconvolution for

228

the spectra in the range of -20 ppm to 20 ppm was performed to quantify the species of P (Cade-Menun

229

2005). A Lorentzian (80%) – Gaussian (20%) function was employed in the spectral deconvolution, and

230

the resulting deconvolution is shown in Figs. S4–6.

31

P MAS NMR was used to measure the species of P in the sludge-derived pyrochars, and

31

31

P MAS

P chemical shift, while the other resonances symmetrically spaced from the

231

Figures 2(b, d, f) show the fractions of the P species identified in the pyrochars according to the

232

particular chemical shift characteristics of each P species. Detailed data for the fractions of these 10

233

identified species are presented in Table S4. For the pyrochar derived from the pyrolysis of raw sludge,

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the Al-P, i.e., Al2(OH)3PO4 (76.8%) and AlPO4 (12.4%), were the main P subcomponents (Fig. S7). This

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could be ascribed to the use of Al-containing flocculants for the enhancement of SS dewatering in the

236

sewage treatment process (Golob et al. 2005). When these three Ca-based additives were added during

237

the SS pyrolysis, the speciation of P changed dramatically with the ratio of addition. We found that the

238

addition of Ca-based additives transformed the P present mostly in the form of inorganic P species

239

including Al-P and hydroxylapatite (Ca5(PO4)3OH). The new species of hydroxylapatite with a low

240

dissociation constant and a high bioavailability for plant growth (Misra and Chaturvedi 2007, Suhartono

241

et al. 2015), strengthened the feasibility of the use of sludge-derived pyrochars as slow-release P

242

fertilizer. Meyer et al. (2018) also observed that the presence of P in the form of hydroxylapatite in rock

243

phosphate can promote plant growth. Thus, the addition of Ca-based additives was beneficial to the

244

formation of hydroxylapatite; however, the benefit differed based on the additive and ratio.

245

As shown in Fig. 2(b), the P subcomponents identified in the sludge-derived pyrochars from the

246

pyrolysis of sludge with CaO addition included orthophosphate, orthophosphate monoesters,

247

orthophosphate diesters, polyphosphate with a terminal P group, and hydroxylapatite. The addition of

248

CaO promoted the transformation of organic P to inorganic P species (orthophosphate, polyphosphate

249

with terminal P group and hydroxylapatite), which was evident from P the increased fraction of

250

inorganic P. Orthophosphate monoesters were the main forms of organic P, while orthophosphate and

251

hydroxylapatite were the main forms of inorganic P. As the addition of CaO increased, the fraction of

252

organic P decreased sharply while the fraction of inorganic P increased. Particularly, the fraction of

253

hydroxylapatite increased with the increase in CaO addition when the addition was below 10%;

254

however, it decreased when the addition was increased further. Therefore, the optimal fraction (37.53%)

255

of hydroxylapatite was achieved with the addition of 10% CaO (Table S4). In the case of Ca(OH)2, the

256

same P subcomponents were identified in the sludge-derived pyrochars. The trends of the fractions of

257

inorganic P and organic P were similar to those in the case of CaO addition. Furthermore, the change in

258

the hydroxylapatite fraction was consistent with that observed in the pyrochars derived from the 11

259

pyrolysis of sludge with CaO addition. However, the promotion effect of the transformation of organic

260

P to inorganic P induced by the addition of Ca(OH)2 was inferior to that induced by the addition of CaO.

261

This effect was also reflected in the change in the hydroxylapatite fraction when the two additions were

262

compared. We found that the addition of 5% Ca(OH)2 results in the peak hydroxylapatite fraction. This

263

result indicated that the Ca form influenced the formation of hydroxylapatite in the process of SS

264

pyrolysis. The fraction of orthophosphate in the pyrochars derived from the pyrolysis of sludge with

265

Ca(OH)2 addition tended to decrease with the increase in Ca(OH)2 addition over 5% compared with that

266

of CaO addition, which supported the previous statement. As shown in Fig. 2(f), the enhancement of the

267

transformation of organic P to inorganic P induced by Ca3(PO4)2 became weaker compared with that in

268

the case of the former two Ca-based additives. The case with 10% Ca3(PO4)2 addition appeared to reach

269

the peak hydroxylapatite fraction. In addition, the decreasing trend of the orthophosphate fraction in the

270

pyrochars derived from the pyrolysis of sludge with Ca3(PO4)2 addition occurred with the increase in

271

addition. These results verified that the ratio of Ca/P as well as the form influenced the formation of

272

hydroxylapatite in the process of SS pyrolysis, and this forms the basis for a strategy for the recovery of

273

P from the SS.

274

To further support the analysis of P speciation in the derived sludge pyrochars in accordance with

275

the obtained 31P NMR spectra, the P 2p XPS and XRD techniques were specifically used. Figures 3(a,

276

b, c) show the P 2p XPS spectra of the sludge-derived pyrochars after pyrolysis, and the spectral

277

deconvolution was performed on these spectra. Figures. 3(d, e, f) show the resulting fraction of

278

P-associated species from the spectral deconvolution. We found that inorganic P bound to the Ca related

279

minerals (Ca(HPO4)2 and Ca2P2O7) as well as the Al related minerals (AlPO4) was identified in the

280

pyrochar derived from the pyrolysis of raw sludge. When these Ca-based additives were added to the

281

raw sludge, the new P species of hydroxylapatite was identified in all the sludge-derived pyrochars, and

282

the dehydration of AlPO4 produced anhydrous berlinite. This indicated the evolution of P-bound

283

minerals related to the pyrolytic reactions during sludge pyrolysis, as also revealed by the C 1s XPS

284

spectra (Fig. S8). As for the change in the hydroxylapatite fraction with the increasing addition of 12

285

Ca-based additives, the maximum fraction of hydroxylapatite was obtained with 10% CaO addition, 5%

286

Ca(OH)2 addition, or 10% Ca3(PO4)2 addition. This was in good agreement with the results found in the

287

aforementioned analysis of

288

berlinite with the increasing addition of Ca-based additives showed that the dehydration of AlPO4 was

289

enhanced to form berlinite, whose fraction was negligible. Because AlPO4 with bound water cannot be

290

detected using the 31P NMR technique, the trend of the Al-P fraction with the addition observed in the

291

analysis of P 2p XPS spectra was consistent with that in the analysis of 31P NMR spectra. The difference

292

between the NMR and XPS results for the number of P species might have been due to the different

293

principles of the NMR and XPS techniques. The NMR technique detects the spin-orbit difference of the

294

nucleus, and the XPS technique does not consider this difference when detecting the photoelectron

295

energy (Andrade 1985, Wolff and Ziegler 1998). Additional information revealed by the XRD patterns

296

of all the sludge-derived pyrochars was obtained for the evolution of the mineral phase, and is presented

297

in Fig. 4.

31

P NMR spectra. The fractional change in Al-P between AlPO4 and

298

In the XRD patterns of the raw sludge-derived pyrochar, SiO2 and CaO were identified as the main

299

phase (Fig. S9). After adding Ca-based additives, the main phases identified in the sludge-derived

300

pyrochars included CaCO3, Ca5(PO4)3(OH), CaO, Ca(OH)2, Ca3(PO4)2, and CaHx(PO4). Moreover, their

301

intensity changed with the increase in the pyrolysis temperature, as shown in Fig. 4. We found that the

302

intensity of these additives was strengthened when the mass ratio of Ca-based additives exceeded 15%,

303

thereby suggesting their excess after pyrolysis. The major phase of CaCO3 formed in the pyrochars

304

derived from the pyrolysis of sludges with CaO addition, and Ca(OH)2 addition appeared at 500 °C but

305

disappeared at 900 °C, which revealed that the increasing temperature could enhance the reactions of

306

CaO and Ca(OH)2 with the sludge matrix in P transformation during pyrolysis. However, the intensities

307

of these detected major phases were changed slightly with the increase in the addition of Ca-based

308

additives. The addition of Ca-based additives appeared to enhance the formation of hydroxylapatite, as

309

indicated by the increased intensities. Importantly, the intensity of hydroxylapatite in the sludge-derived

310

pyrochars was notably larger at 900 °C compared with that at 500 °C, which revealed that the 13

311

production of hydroxylapatite could be thermodynamics-dependent. The Al-associated minerals were

312

not identified in the obtained XRD patterns, possibly due to the dilution effect caused by the

313

introduction of Ca-based additives. In addition, the CaHx(PO4) phase present in the derived pyrochars

314

pyrolyzed with the addition of Ca-based additives at 900 °C revealed that, curbing the development of

315

CaHx(PO4) at a high temperature during sludge pyrolysis could increase hydroxylapatite production.

316

Therefore, the supplementary characterizations of P 2p XPS and XRD further supported that the change

317

in Ca form and/or Ca/P ratio can be regarded as an effective strategy to recover P resources from SS in

318

the form of hydroxylapatite.

319

3.3 Thermodynamic simulation of hydroxylapatite production during sludge pyrolysis with

320

calcium-based additives

321

As stated above (section 3.2), the formation of hydroxylapatite strongly depend on the pyrolysis

322

temperature. Thus, performing thermodynamic simulation of hydroxylapatite production during sludge

323

pyrolyzed with the addition of Ca-based additives can improve the understanding of the transformation

324

of sludge P. Figure 5 shows the resulting concentration of hydroxylapatite generated when the process

325

of sludge pyrolysis reached equilibrium at the determined temperatures. The largest concentration

326

(0.052 g/g SS) of hydroxylapatite in the raw sludge-derived pyrochar was achieved at a pyrolysis

327

temperature between 700 °C and 900 °C, and as the temperature increased beyond 900 °C, its

328

concentration approached near zero (Fig. S10). After adding the Ca-based additives, the concentration

329

of hydroxylapatite increased by 180% (0.1460 g/g SS), 165% (0.1379 g/g SS) and 165% (0.1379 g/g SS)

330

in the pyrochars derived from sludge with CaO addition, Ca(OH)2 addition, and Ca3(PO4)2 addition at

331

900 °C, respectively, compared with that (0.0520 g/g SS) in the raw sludge-derived pyrochar. This

332

indicated that the addition of Ca-based additives could enhance the conversion of P to hydroxylapatite

333

during sludge pyrolysis, which was consistent with the experimental results. More specifically, when the

334

pyrolysis temperature was higher than 1000 °C, the concentration of hydroxyapatite in the

335

sludge-derived pyrochars decreased by 16–58%. 14

336

As shown in Figs. 5(a, b), when the addition of CaO and Ca(OH)2 was below 15%, the trends of

337

hydroxylapatite production between pyrolysis of sludge with Ca addition and sludge with Ca(OH)2

338

addition at equilibrium were similar as long as the pyrolysis temperature was below 900 °C (Table S5),

339

while the difference in hydroxylapatite production appeared to become pronounced when the

340

temperature approached 900 °C (at which the concentrations of hydroxylapatite were 0.1460 g/g SS and

341

0.1379 g/g SS in the pyrochar derived from sludge with CaO addition and Ca(OH)2 addition,

342

respectively). With the further increase in the additives (i.e., beyond 15%), the equilibrium content of

343

hydroxylapatite (0.3312 g/g) remained constant as long as the pyrolysis temperature was below 900 °C.

344

The content of hydroxylapatite decreased notably with the increase in the pyrolysis temperature beyond

345

900 °C, and this revealed that the hydroxylapatite phase was not stable above 900 °C. On the other hand,

346

the hydroxylapatite production at equilibrium during the pyrolysis of sludge with Ca3(PO4)2 addition

347

remained nearly constant with the increase in pyrolysis temperature, while an increase in the addition

348

markedly increased the equilibrium content of the hydroxylapatite, as shown in Fig. 5(c). It was also

349

observed that, when the pyrolysis temperature was over 900 °C, a clear decrease in the hydroxylapatite

350

production occurred, which confirmed the stability of hydroxylapatite shown in the cases of sludge with

351

CaO and Ca(OH)2 addition. The differences in the production of hydroxylapatite with the addition of

352

different Ca-based additives indicated that the form of Ca and/or the change in the Ca/P ratio influenced

353

the transformation of P in the sludge to hydroxylapatite. In addition, the enthalpy increased with the

354

increase in the amount of Ca-based additives or the increase in the pyrolysis temperature (Fig. 5d).

355

However, the enthalpy during the pyrolysis of sludge with CaO showed the lowest values, which

356

suggested that CaO was preferable for the conversion of sludge P to hydroxylapatite in terms of the

357

energy input to the pyrolysis system.

358

A difference in the hydroxylapatite production between the experimental and simulated results was

359

observed, as shown in Fig. 5. The difference could have been attributed to the hypothesis used in the

360

simulation (Huang et al. 2015a) and the limitations in the experimental determination of the

361

hydroxylapatite content (Jäger et al. 2006). In the simulation, the method of Gibbs free energy 15

362

minimization was adopted to calculate the pyrolysis system reaching the thermodynamic steady-state

363

based on two hypotheses (Li et al. 2012). Specifically, all input reactants were considered ideal

364

substances, and they were assumed to be completely mixed when two or more substances were allocated

365

as being in the same phase. The initial pyrolysis system would eventually reach steady-state equilibrium

366

in thermodynamics. However, this discrepancy did not affect the simulation of hydroxylapatite during

367

pyrolysis with the addition of Ca-based additives, to gain a deep insight into the transformation of

368

sludge P during sludge pyrolysis for efficient recovery. Therefore, it could be inferred that the

369

introduction of Ca-based additives enhanced the transformation of sludge P to hydroxylapatite, which is

370

preferred by plants, and the addition of CaO appeared to be an optimal solution for the enhancement of

371

hydroxylapatite during sludge pyrolysis in terms of energy savings.

372

3.4 Environmental application

373

Figure 6 shows the growth of ryegrass in terms of height and chlorophyll a/b contents with the

374

utilization of the sludge-derived pyrochars as a prospective P fertilizer. As shown in Fig. 6(a), the

375

ryegrass in the control group had an average plant height (20 randomly selected ryegrasses plants were

376

measured) of 9.3 cm, whereas the average plant heights of ryegrass in the experimental groups were

377

increased. This indicated that the derived pyrochars from the pyrolysis of sludge with the addition of

378

Ca-based additives could be beneficial to the growth of ryegrass. Furthermore, by comparing the heights

379

of the ryegrass in the experimental runs, it was noticed that the plants that utilized the pyrochars

380

produced at 900 °C were taller than those that utilized the pyrochars produced at 500 °C. This suggested

381

that P enriched in the pyrochars produced at 900 °C was more amenable to be adsorbed by ryegrass

382

during growth. In other words, the enhancement effect of the sludge-derived pyrochars on plant growth

383

was strengthened with the increase in the pyrolysis temperature. The chlorophyll content in the blank

384

(control) run was 1.15 mg/g, and the ratio of chlorophyll a to chlorophyll b was 2.2. Except for the

385

experimental group with the addition of pyrochar labeled 15%Ca3(PO4)2-900 °C at a blending ratio of

386

1%, the contents of chlorophyll in the other experimental groups were higher than that of the control 16

387

group. Furthermore, the content of chlorophyll varied with the blending ratio, and the pyrolysis

388

temperature. We found that the content of chlorophyll in the ryegrass plants that utilized the pyrochars

389

labeled 15%CaO-500 °C, 15%Ca(OH)2-900 °C and 15%Ca3(PO4)2-900 °C reached their local peaks

390

when the blending ratios were 4%, 1%, and 4%, respectively. This result was in agreement with that

391

observed in the resulting ratio of chlorophyll a to chlorophyll b. In addition, Meyer et al. (2018) found

392

that hydroxylapatite from rock phosphate can be employed as a P fertilizer for ryegrass growth. It could

393

be inferred that the bioavailability of P in the derived sludge pyrochars was related to (i) the difference

394

in the Ca form and/or Ca/P ratio with the addition of Ca-based additives, and (ii) the increase in the

395

blending ratio which tended to enhance the uptake of P during plant growth. In terms of the energy

396

savings for the pyrochar preparation, the addition of CaO appeared to be advantageous in sludge P

397

transformation. This agreed well with the findings in the theoretical simulation for sludge P

398

transformation to hydroxylapatite through pyrolysis. Therefore, the results from this bioassay

399

established that pyrochars derived from the addition of Ca-based additives can be employed as a novel P

400

fertilizer to promote plant growth.

401

Based on the aforementioned results, the introduction of Ca-based additives during sludge

402

pyrolysis can promote the conversion of P to hydroxylapatite in produced pyrochars. A scheme was

403

proposed for the enhancement of the addition of Ca-based additives in sludge P transformation to

404

hydroxylapatite during sludge pyrolysis, as shown in Fig. 7. Previous studies confirmed that sludge P is

405

composed of orthophosphate monoesters, orthophosphate diester, orthophosphate, polyphosphate, and

406

P-bound minerals (Ca-P and Al-P) (Huang and Tang 2015, Tang et al. 2018). Without the addition of

407

Ca-based additives, the sludge P species were mainly comprised of Al-P minerals (Al2(OH)3PO4) and a

408

portion of Ca2P2O7 after pyrolysis, which cannot be easily absorbed by the plants. However, when

409

Ca-based additives were added during sludge pyrolysis, the transformed sludge P was mainly present in

410

Ca-P, particularly hydroxylapatite. The addition of CaO, Ca(OH)2, and Ca3(PO4)2 enhanced the

411

transformation of sludge P to hydroxylapatite (Jin et al. 2016). We found that the addition of 10% CaO,

412

5% Ca(OH)2, or 10% Ca3(PO4)2 facilitated peak hydroxylapatite production. Simultaneously, the 17

413

addition of Ca-based additives to some degree decreased the apparent content of Al-P minerals which

414

are harmful for plant growth, thereby facilitating the agricultural utilization of pyrochar rich in P in the

415

form of hydroxylapatite. Therefore, reclaiming P from SS via pyrolysis with the addition of Ca-based

416

additives could be an effective strategy for enriching P in pyrochar in the form of hydroxylapatite to be

417

utilized in agriculture.

418

4. Conclusion

419

The transformation of sludge P during pyrolysis with the addition of Ca-based additives was

420

investigated in this study. The Ca-based additives were beneficial to the conversion of P in the SS to

421

hydroxylapatite, which is more amenable to absorption by plants than other P-associated phases such as

422

Al-P. The addition of 10% CaO, 5% Ca(OH)2, or 10% Ca3(PO4)2 facilitated the peak production of

423

hydroxylapatite. The thermodynamic simulation of hydroxylapatite production during pyrolysis

424

demonstrated that these three additives increased the enthalpy of the total pyrolysis system, and the

425

addition of CaO resulted in the lowest enthalpy of sludge pyrolysis. This implied that the addition of

426

CaO during SS pyrolysis may be considered preferentially over the other two Ca-based additive to

427

recover sludge P in the form of hydroxylapatite in terms of energy savings. The pot experiment verified

428

that the pyrochar derived from the pyrolysis of SS with the addition of Ca-based additives can be

429

regarded as P fertilizer to promote plant growth.

430

Author contributions

431

The experiment was conducted by Jingjing Chen, and the manuscript was written through the

432

contributions of all authors. All authors have given approval to the final version of the manuscript.

433

Declaration of interest statement

434

The authors declare that they have no known competing financial interests or personal relationships

435

that could have appeared to influence the work reported in this paper.

436

Acknowledgements 18

We

437

gratefully

acknowledge

supports

from

National

Key R&D

Program

of

China

438

[2018YFC1902904] and the National Natural Science Foundation of China [51772141]. This work was

439

also

440

[KQJSCX2018032215150778,

441

[KQTD20160226195840229]. Additional support was Guangdong Province Universities and Colleges

442

Pearl River Scholar Funded Scheme 2018.

supported

financially by Shenzhen

Science

and

JCYJ20170412154335393]

443

19

Technology and

Innovation

Shenzhen

Committee

Peacock

Plan

444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488

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545

22

1

Table 1 The compositional characteristic of the sludge feedstock. Sample

Proximate analyses (dry basis, wt.%)a

Ultimate analyses (dry basis, wt.%)b

SS

Moisture 2.33

Ash 41.03

SS

SiO2

Volatile Fixed carbon C 52.03 4.61 37.03 Mineral component (oxide, wt.%)c Al2O3 Fe2O3 CaO SO3

17.17

14.07

5.65

2.91

3.21

H 2.72

Od 9.65

N 5.51

S 4.06

K2O

MgO

Na2O

1.03

0.35

0.64

2

a

Measured according to the Chinese Standard GB/T 17664-1999; b: Measured using a Thermo Scientific element

3

analyzer (Flash 2000, USA); c: Measured using an Thermo Scientific X-ray fluorescence spectrometer (ARL ADVANT

4

XP+, USA); d Calculated by O = 100 - (C +H + N + S + Ash).

1

1

2 3

Fig. 1. The char yield of different samples at different temperatures and the total P in the sludge-derived chars at

4

different addition ratios. (a) char yield; (b) the content of total P in the derived chars. The dashed lines denote the

5

derived sludge pyrochars from the pyrolysis of raw sludge at different temperatures; char yield = mass of the pyrochar

6

produced (g)/ mass of the sludge feedstocks (g)× 100%.

1

7 8

Fig. 2. Solid state 31P MAS NMR spectra (a, c, e) and the identified P-related species (b, d, f) of the pyrochars derived

9

from the pyrolysis of the mixed sludge with Ca-based additives at 900 °C (spades indicate the spinning side bands).

2

10 11

Fig. 3. P 2p XPS spectra (a, c, e) and the identified P-related species (a, b, c) of the sludge-derived pyrochars from the

12

pyrolysis of the mixed sludge with the addition of Ca-based additives at 900 °C.

3

13 14

Fig. 4. XRD patterns of the sludge-derived pyrochars from the pyrolysis of the mixed sludge with Ca-based additives

15

at 500 °C and 900 °C.

16 4

17 18

Fig. 5. Comparison of the production of the hydroxylapatite between the experimental and the simulating during the

19

pyrolysis of the mixed sludge with the addition of Ca-based additives: (a) CaO, (b) Ca(OH)2, and (c) Ca3(PO4);

20

experimental results (the concentration of hydroxylapatite) = the total P content determined in pyrochars × the fraction

21

of hydroxylapatite (based on NMR results). (d) the calculated Gibbs free energy.

22

5

23

24 25

Fig. 6. The ryegrass height (a) and chlorophyll content (b) in pot experiment.

26

6

27 28

Fig. 7. A scheme of sludge P transformation during pyrolysis with the addition of Ca-based additives.

7

Highlights Ca-based additives enhanced the conversion of P to hydroxylapatite Optimal fraction of hydroxylapatite (37.53%) was achieved at the addition of 10%CaO The hydroxylapatite content decrease sharply over 900 Sludge-derived pyrochars are an effective P-based fertilizer An effective strategy for the recovery of P in sewage sludge is proposed

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: