The conversion and migration behavior of phosphorus speciation during pyrolysis of different sludges

1.   Introduction

Sewage sludge is a necessary by-product of sewage disposal process, which will cause serious harm to the environment if not handled properly ^[1]. In China, about 60,000,000 tons of raw sewage sludge was generated in 2019 and the figure is expected to exceed 90,000,000 tons by 2025 (calculated according to 80% water content) ^[2,3]. Among various sewage sludge treatment technologies, pyrolysis is considered a valid alternative method, which can significantly reduce sludge bulk, destroy pathogenic bacterium in sludge, and transform the sludge into biochar, which was the high value-added carbonaceous materials for many purposes, such as soil improvement ^[4], carbon sequestration ^[5], and pollution reduction ^[6]. The application of biochar prepared from sludge as a soil conditioner in improving the properties of urban soil and promoting grassland growth has been reported ^[7]. It was also shown that the use of biochar prepared from sludge as a soil conditioner plays an important role in stabilizing metal ingredients ^[8,9], attenuating phenanthrene/pyrene adsorption ^[10], and reducing the bioavailability of polycyclic aromatic hydrocarbons (PAHs) ^[11].

Sewage sludge usually contains 2-3% dry mass phosphorus ^[12], which makes the biochar derived from it a potential fertilizer in the process of soil application ^[13,14] but also a potential threat to the environment if not handled properly. The pyrolysis of sludge affects the content and speciation of phosphorus in the solid residues ^[15,16]. Pyrolysis can decompose organophosphorus compounds into inorganic phosphorus compounds. Organophosphate compounds are compounds containing carbon-phosphorus bonds, such as organophosphate esters and organophosphate anhydrides. The carbon-phosphorus bonds of these organophosphorus compounds break after pyrolysis, producing inorganic phosphorus compounds such as phosphate and phosphoric acid. These inorganic phosphorus compounds can be absorbed and utilized by plants in the soil, which plays a role in promoting plant growth ^[17]. Relevant studies have shown that the mobility and bioavailability of phosphorus are closely related to the speciation of phosphorus ^[18,19]. In-depth understanding of phosphorus speciation is necessary to understand the fate of phosphorus. On one hand, the utilization of biochar prepared by pyrolysis of sludges as soil conditioner or fertilizer requires plant-available phosphorus, but it is easily released. On the other hand, its use in building materials requires its fixation, because once excessive P is released into the water ^[20,21], it will cause eutrophication. Qin et al. ^[22] also reported that in most large lakes, phosphorus reduction can mitigate eutrophication, but in eutrophication lakes, especially shallow lakes (or bays), both nitrogen and phosphorus reduction may be required. Therefore, when the biochar is completely applied to the soil or building materials and so on, it is important to fully understand the behavior of the transformation and conversion of P in the biochar during thermal treating of the sewage sludge.

The speciation of phosphorus in sewage sludge and sludge biochar has engaged the consideration of researchers and has been reported in some existing studies. Based on sequential fractionation, Qian and Jiang ^[23] reported the transplantation of phosphorus operating form to the middle and long-term plant-absorbable phosphorus pool in sludge biochar provided at high temperature. When sludge biochar is applied to the soil, it is necessary to understand the conversion and transplantation of phosphorus in the sludge during the pyrolysis process. Some research reports the delivery and preservation of phosphorus depends on its chemical speciation and its combined metal ions ^[23,24]. McDowell et al. ^[25] showed that the removable phosphorus speciation are the solvable phosphorus speciation that are infirmly assembled on Fe, Al, and Ca. Sharpley et al. ^[26] found that the conversion of phosphorus combined with Al, Fe, and Ca leads to a reduction in water-soluble phosphorus in the soil.

Understanding the speciation of phosphorus in sludge biochar is essential for understanding the fate of phosphorus, which helps predict the potential pollution of sludge biochar in the environment. However, little is known about the influence of pyrolysis temperature and different sludges on phosphorus chemical speciation conversion and transplantation of sludge biochar during pyrolysis. Moreover, previous studies have only focused on the standards, measurements, and testing (SMT) agreement for P speciation classification into total phosphorus (TP), inorganic phosphorus (IP), organic phosphorus (OP), non-apatite inorganic phosphorus (NAIP), and apatite inorganic phosphorus (AP), ignoring the speciation of inorganic phosphorus combined with metal ions. From the above considerations, this study creatively used the fractionation of soil phosphorus according to ^[27] to differentiate among soluble and loosely bound IP (SL-IP), aluminum-bound IP (Al-IP), Fe-bound IP (Fe-IP), oxide occluded IP (O-IP), and Ca-bound IP (Ca-IP). Only by systematically studying the migration and transformation of phosphorus in sludge at different pyrolysis temperatures can we understand the impact of phosphorus on environmental pollution. Therefore, the objectives of this work were: (1) to compare the phosphorus transformation of three different sludge during pyrolysis (200800 ℃) process and (2) to conclude the bioavailability and chemical properties of phosphorus speciation of sludges and their biochar determined by fractionation of soil phosphorus and X-ray diffraction analysis.

Phosphorus speciation of sewage sludges after microwave-assisted pyrolysis or incineration has been reported in literature ^[28,29], but little research has compared phosphorus speciation of different sludges and their biochar. The research reported here was undertaken to compare and contrast the conversion and migration behavior of phosphorus speciation of different sludges after pyrolysis. Through the in-depth discussion of the experimental results of phosphorus morphology, this study deduced the possible phosphorus phase and provided an important explanation for the phosphorus migration during the application of sludge and biochar prepared from its in soil. In this study, the morphology of phosphorus was explained in depth, which provided a foundation for further study on the conversion and transplantation of phosphorus morphology during sludge pyrolysis.

2.   Materials and methods

2.1.   Materials collection and pre-treatment

Three typical of sewage sludge were selected in this study: municipal sludge (MS), town sludge (TS), and slaughterhouse sludge (SS). MS and SS were gathered in two diverse sewage disposal plants in Changsha, China. They differ from the efficiency of phosphorus removal (chymic phosphorus-sediment and enhanced biological phosphorus-removal) and the subsequent processing of sewage sludge (aerobic and anaerobic steadiness). The municipal sewage disposal plant was in a first step biological precipitation and aerobic stabilization, and the next step is flocculant precipitation. The flocculants are mainly used for sludge dewatering in sewage plants, which can change the physicochemical property of sludge particles, undermine the colloidal structure of sludge, reduce the affinity of sludge particulate and water, and build up the dehydration property of sludge accordingly. The slaughterhouse sludge was contemporaneously precipitated with FeCl₂ and then rapidly fermented to form anaerobic stable sludge. TS was gathered from a rural sewage disposal plant in Dongguan city, China. The sewage disposal plant mainly collects household sewage, some industrial effluents, and overland runoff. Therefore, TS from this plant involved a lot of inorganic matter, containing heavy metals and deposit sediment. The plant adopted circulating activated sludge technology for sewage disposal. The circulating activated sludge system is composed of anoxic selector, pre-reaction aerobic zone, main aerobic reactor, etc. Unlike municipal sludge and slaughter sludge, hydrogen peroxide is added to the TS to form Fenton reagent, oxidation by hydrogen peroxide at acidic conditions dissolve bacterial cells and waste-activated sludge composed of extracellular polymeric substances, promoting the subsequent sludge dewatering. All samples were dried in an oven at 105 ℃ for 24 hours to ensure that the moisture content was less than 1%. Then, the dried sample was crushed and sieved into fractions of particle diameter 80–200 meshes.

2.2.   Pyrolyzer and pyrolysis process

A lab-scale pyrolysis device (SK-G08123K, China) was used in this pyrolysis experiment. The pyrolysis device used in this study is the same as the previous study ^[30]. Municipal sludge, town sludge and slaughterhouse sludge were utilized for laboratory scale pyrolysis experiments. Specific pyrolysis process was refer to Shao et al. ^[30]. The pyrolysis temperatures were 200 ℃, 350 ℃, 500 ℃, 650 ℃, and 800 ℃, the heating rate was 10 ℃ min^-1. Pyrolysis temperatures were sustained for 30 minutes.

The biochars produced from MS, TS, and SS at various temperatures (200 ℃, 350 ℃, 500 ℃, 650 ℃, and 800 ℃) were designated as follows: MS200, MS350, MS500, MS650, MS800; TS200, TS350, TS500, TS650, TS800; and SS200, SS350, SS500, SS650, SS800, respectively. The resulting biochars were stored in a dryer at an indoor temperature for subsequent use.

2.3.   Analytical procedures

2.3.1.   Physicochemical analysis

For the methods of ultimate composition and proximate analysis of sludges and corresponding biochars, refer to Zeng et al. ^[31]. Mineral phases of sludges and corresponding biochars were identified by X-ray diffraction (XRD) measurement.

2.3.2.   Sequential extraction

Total P (TP) and inorganic P (IP) concentrations were analyzed by the ignition approach ^[32]. The TP content was analyzed by igniting a 1.0 g manure or biochar at550 ℃ for 1 hr and then extracting with 25 mL 0.5 mol L^-1 H₂SO₄ for 2 h. The IP concentration was analyzed by the same extraction routine. We centrifuged at 4000 RPM for 20 minutes to separate the liquid from the solid phase. The supernatant liquid was then filtered through a 0.45 μm membrane filter and the P concentration was measured by the molybdenum blue method ^[33]. The organic P (OP) concentration was figured as the difference between TP and IP.

The IP fractions were separated using the fractionation of soil phosphorus ^[27] and soil agrochemical analysis method. Soil inorganic P can be further classified into different fractions, such as soluble and loosely bound IP (SL-IP), aluminum-bound IP (Al-IP), Fe-bound IP (Fe-IP), oxide occluded IP (O-IP), and Ca-bound IP (Ca-IP). Although there are some reports of deficient extraction of Al-P and Fe-IP fractions ^[34], this method can adequately distinguish these fractions. The above-mentioned P-fraction was extracted with different extraction agents.

Step 1: soluble and loosely bound IP (SL-IP) was extracted with 50 ml of 1.0 mol L⁻¹ NH₄Cl. Step 2: aluminum-bound IP (Al-IP) was extracted with 50 ml volume of 0.5 mol L⁻¹ NH₄F (pH8.2). Step 3: Fe-bound IP (Fe-IP) was extracted with 50 ml volume of 0.1 mol L⁻¹ NaOH. Step 4: Oxide occluded IP (O-IP) was extracted with 40 mL 0.3 mol L⁻¹ Na₃C₆H₅O₇ and 1.0 g Na₂S₂O₄. Step 5: Ca-bound IP (Ca-IP) was extracted with 50 mL of 0.25 mol L^-1 H₂SO₄. The specifics of extraction method was reported previously ^[35]. Each experiment was conducted in quadruplicate.

2.4.   Characteristic of raw sludges and their biochar

The contents of volatile matter, fixed carbon, and ash (proximate analysis) of the MS, TS and SS (on dry matter basis) and the ultimate compositions (ultimate analysis) of MS, TS, and SS were revealed in Table 1. The outcomes of proximate analysis and ultimate analysis revealed that the volatile matter and the fixed carbon were as high as 58.74 % and 12.11% in SS and the content of ash was 29.15%. But, for MS, the content of carbon was 3.56% and the ash reached up to 54.12%. Meanwhile, the percentages of ash was the highest and the contents of fixed carbon and volatile matter were the lowest in TS. This illustrates the complexity of sludge and the multiplicity of its components. Moreover, the contents of C, H, and N of MS were the highest and the contents of C, H, O, and N of TS and SS were very similar. The difference in element composition can be illustrated by the difference in the catchment area of the sewage disposal plant.

Table 1.  Properties of MS, TS, and SS.

Items	Proximate analysis a (wt %)			Elemental analysis (wt %)
	VM	Ash	FC	N	C	H	S	O b
MS	42.32	54.12	3.56	5.25	24.73	3.62	0.61	65.79
TS	29.02	68.31	2.67	2.76	15.48	2.98	0.73	78.05
SS	58.74	29.15	12.11	2.98	18.87	2.06	0.27	75.82
Notes: VM: volatile matter; FC: fixed carbon, a Dry basis, b Calculated by difference, (O = 100 - (C + H + N + S)).

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3.   Result and discussion

3.1.   Total concentration of Phosphorus and residual rate of Phosphorus in biochar during pyrolysis

The total concentrations of phosphorus in sludges (MS、TS, and SS) and biochars pyrolyzed by sludges (MS, TS, and SS) were revealed in Table 2. Among the three types of raw sludge, TS has the highest total phosphorus contents, followed by MS, and SS has the lowest total phosphorus concentration. TS has a higher P-concentration compared to MS and SS, which can be illustrated by the differences between the catchment areas of the the sewage disposal plants ^[36]. The sewage disposal plants of MS and SS have a more industrial catchment area, while the sewage disposal plant of TS is in a more rural area.

Table 2.  Contents of total phosphorus in the sludge and the biochar and residual rate of phosphorus in biochar.

Temperature	MS		TS		SS
	contents (mg g−1)	Ratios (%)	contents (mg g−1)	Ratios (%)	contents (mg g−1)	Ratios (%)
Sludges	12.5	/	19.64	/	9.70	/
200	13.86	99.10	21.90	98.70	11.01	99.24
350	19.89	94.84	23.91	97.89	17.07	99.61
500	24.46	93.10	25.24	96.16	19.56	95.87
650	21.93	79.49	22.14	81.65	18.66	87.31
800	22.72	77.11	22.54	81.01	18.15	80.24

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All biochars prepared from sludge showed expressively increased TP content with increasing pyrolysis temperature where TP promoted from 12.5 mg/g in MS to 22.72 mg/g in MS800, 19.64 mg/kg in TS to 22.54 mg/g in TS800, and 9.70 mg/g in SS to 18.15 mg/g in SS800. This may be caused by the decomposition of organic components in the sludge ^[37]. TP recovery showed that a large proportion of the P was reserved in the biochar with recovery percentages as follows: 77.11–99.10% in MS biochar, 81.01–98.70% in TS biochar, and 80.24–99.61% in SS biochar. When the pyrolysis temperature ranges from 200 ℃ to 500 ℃, there is no significant difference in the recovery rate of TP. When the pyrolysis temperature ranges from 650 ℃–800 ℃, the TP recovery rate decreased slightly. The higher phosphorus recovery percent indicates that the phosphorus involved in bio-oil or gas was expressively shallower than that in biochar. The results are identified with fore works, which recovered almost all P speciation from biochar ^[38]. As the pyrolysis process progresses, part of the oxygen and hydrogen, as well as nitrogen and sulfur, are lost in the feedstock, thereby enriching the remaining P into the biochar ^[39,40]. These results indicate that phosphorus can be recycled or reused through the pyrolysis of organic waste. P mainly remains in the solid residues, and its distribution in the liquid and gas phase is restricted. The relative fractions of P in solid phase, liquid phase, and gas phase are in connection with the type of raw material, P speciation, and processing circumstances (mainly temperature). For instance, when the pyrolysis temperature is 250–600 ℃, P recovery in the solid residues is about 80% for wheat straw, about 100% for maize straw and peanut husk, and about 100% for sewage sludges and manures ^[41].

3.2.   Influence of pyrolysis temperature on the phosphorus speciation.

The total concentrations and recovery of different speciation of phosphorus in sludges (MS, TS, and SS) and biochars pyrolyzed by sludges (MS, TS, and SS) were revealed in Table 3. The total phosphorus content in TS was the highest while that in SS was the lowest. All biochars showed significantly decreased organic phosphorus (OP) content with increasing pyrolysis temperature where OP reduced to 6.96 mg/g in MS200 to 0.61 mg/g in MS800, 3.35 mg/g in TS200 to 0.75 mg/g in TS800, and 2.50 mg/g in SS200 to 0.40 mg/g in SS800. As the temperature increases, the concentration of IP extracted is continuously increasing, the O-IP and Ca-IP content in the IP of MS, TS, and SS increased but the SL-IP, Al-IP, and Fe-IP content of MS and SS decreased, slightly differently the Al-IP content of TS increased with increasing temperature. The total of sequential fractions of the TP was 89.78–105.19% for MS, 95.54–108.80% for TS, and 90.61–102.67% for SS. The P recovery expressively decreased with heating temperature, thus demonstrating less available P speciations at high temperatures. Our results obviously indicated a model of P transplantation during pyrolysis treatment, where pyrolysis technique expressively increased O-IP and Ca-IP fractions at the consumption of reduced proportion of SL-IP, Al-IP, and Fe-IP. A similar model was reported in a previous study ^[23].

Table 3.  P-fractionation concentrations and recovery in sludges and their biochar (mg g⁻¹).

Biochar	SL-IP	Al-IP	Fe-IP	O-IP	Ca-IP	OP	Total	Recovery
MS	1.78	2.03	1.83	0.48	0.08	6.96	12.5	105.19
MS200	1.41	2.15	2.18	0.65	0.18	5.88	13.86	89.78
MS350	0.24	0.50	1.18	2.04	8.60	7.98	19.89	103.28
MS500	0.13	0.50	1.27	2.39	10.92	9.84	24.46	102.41
MS650	0.03	0.15	1.04	1.92	17.80	1.98	21.93	104.52
MS800	0.01	0.17	1.58	1.03	19.68	0.61	22.72	101.54
TS	0.05	3.25	12.79	0.82	0.33	3.35	19.64	104.80
TS200	0.03	3.84	13.44	0.90	0.38	3.72	21.90	101.87
TS350	0.03	5.29	9.01	0.91	5.75	2.32	23.91	97.49
TS500	0.03	4.70	8.27	1.30	9.18	2.18	25.24	101.68
TS650	0.05	4.17	6.87	1.33	8.04	1.34	22.14	98.43
TS800	0.05	3.67	5.80	1.38	9.89	0.75	22.54	95.54
SS	0.31	1.43	4.29	0.37	0.09	2.50	9.70	92.67
SS200	0.51	1.89	4.46	0.54	0.18	2.40	11.01	90.61
SS350	0.05	0.79	2.55	1.12	10.73	2.27	17.07	102.67
SS500	0.04	0.73	2.36	1.89	12.96	1.65	19.56	100.32
SS650	0.01	0.06	2.47	2.09	12.80	1.28	18.66	100.28
SS800	0.01	0.08	2.20	1.22	13.77	0.40	18.15	97.33

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Table 4 shows the rate of different speciation of phosphorus in sludges (MS, TS, and SS) and biochars pyrolyzed by sludges (MS, TS, and SS) in the temperature range of 200–800 ℃. Phosphorus speciation in MS, TS, SS, and corresponding biochars from different temperatures are revealed in Figure 1. Among the three raw sludges, the organic phosphorus (OP) content in MS is the highest, and the organic phosphorus content in TS is the lowest. It is showed that MS has a higher amount of OP than TS and SS, as a result of the biological removal of P during sewage disposal ^[42]. When the pyrolysis temperature rises to 650 ℃, the OP content in MS is greatly reduced. When the pyrolysis temperature rises to 800℃, the OP content in all sludge is reduced to a minimum.

Table 4.  The rate of P-fractionation in sludge and biochars (%).

Biochar	SL-IP	Al-IP	Fe-IP	O-IP	Ca-IP	OP
MS	12.71	14.52	13.11	3.41	0.58	55.67
MS200	12.33	18.84	19.11	5.74	1.60	42.38
MS350	1.16	2.37	5.62	9.74	41.00	40.11
MS500	0.51	1.97	4.99	9.38	42.91	40.23
MS650	0.13	0.66	4.51	8.35	77.31	9.04
MS800	0.04	0.74	6.83	4.44	85.27	2.68
TS	0.24	15.65	61.53	3.94	1.58	17.05
TS200	0.15	17.14	60.02	4.03	1.70	16.96
TS350	0.12	22.76	38.76	3.90	24.75	9.71
TS500	0.11	18.28	32.19	5.05	35.73	8.65
TS650	0.23	19.14	31.53	6.11	36.91	6.08
TS800	0.22	17.06	26.96	6.44	45.98	3.35
SS	3.59	16.32	49.08	4.18	1.00	25.83
SS200	5.22	19.49	46.04	5.58	1.88	21.80
SS350	0.31	4.51	14.52	6.36	61.00	13.30
SS500	0.19	3.71	12.04	9.61	66.02	8.42
SS650	0.05	0.33	13.20	11.18	68.39	6.86
SS800	0.03	0.43	12.46	6.93	77.98	2.18

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Figure 1.  Percentages of phosphorus speciation in material and biochars.

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OP accounted for about 55.67% of raw MS. After pyrolysis, OP proportion decreased to 2.68% at 800 ℃. Raw TS contained 17.05% of OP, and when the pyrolysis temperature was elevated from 200 ℃ to 800 ℃, OP content of TS decreased from 16.96% to 3.47%. OP concentration in biochar decreased gradually with heating temperature. OP of SS through slow pyrolysis at temperatures ranging from 250 ℃ to 800 ℃ and observed similar trends. After pyrolysis, TP concentration in biochar increased gradually with heating temperature. IP proportion in TP was enhanced while OP share was decreased. The decrease in the proportion of organic phosphorus may be due to the decomposition of organic matter during the pyrolysis process, resulting in the release of microbial intracellular phosphorus, which further reacts with the metal ions (Fe²⁺, Al³⁺) in the sludge at high temperatures, leading to the formation of more types of IP ^[43].

Raw MS contained 12.71% of SL-IP. After pyrolysis, SL-IP content in biochar decreased gradually with heating temperature. Pyrolysis of TS at ≥ 200 ℃ reduced SL-IP from 0.24% to 0.10%. Thermal treatments of TS at 650–800 ℃ SL-IP increased from 0.10% to 0.22%. Conversion of SS into biochar through slow pyrolysis at ≥ 350 ℃ decreased SL-IP from 3.59% to below 0.03%. The proportion of SL-IP expressively decreased at 350 ℃ and was unchanged at higher temperatures. The SL-IP values were 12.71%, 0.24%, and 3.59% for raw MS, raw TS, and raw SS, respectively. The content of SL-IP of TS is much lower compared to MS and SS, and this may be due to the different of the removal of P and in the subsequent processing of sewage sludge ^[44]. These values were expressively reduced confronted with 0.13% in MS650, 0.23% in TS650, and 0.05% in SS650. These results are similar to those gained in a previous work where pyrolysis at 300 ℃ expressively reduced the s SL-IP fraction of > 25% in wheat straw, maize straw, and peanut husk to < 11% in their biochars ^[38]. Cantrell et al. ^[39] also indicated that the expressively reduced SL-IP in manure biochar was due to pyrolysis temperature. In environmental studies, SL-IP is a prominent indicator of the short-term P loss possible in biosolids, which results in eutrophication of aquatic ecosystem ^[45].

As observed, Al-IP accounted for 14.52% of IP in raw MS, with increasing pyrolysis temperature Al-IP proportion increased to 18.84% at 200 ℃ and then decreased with rising temperature. The amounts of Al-IP of TS were initially increased at 200 ℃–350 ℃ and then decreased to 18.28% at 500 ℃. However, when the pyrolysis temperature was increased to 650 ℃ Al-IP content of TS increased to 19.14%, and then decreased to 17.06% at 800 ℃. Raw SS has Al-IP content about 16.32%, after pyrolysis, AI-IP proportion increased to 19.49% at 200 ℃ and then decreased with rising temperature. Among the three raw sludges, the content of Al-IP is similar, but after pyrolysis, the content of Al-IP in TS does not decrease much compared with MS and SS. This may be because the TS was gathered from a rural sewage disposal plant, the large amount of clay minerals mixed into the TS, and the clay minerals have a higher content of Al.

Fe-IP accounted for 13.11% of IP in raw MS, with increasing pyrolysis temperature Fe-IP proportion increased to 19.11% at 200 ℃ and then decreased with rising temperature but Fe-IP proportion increased from 4.51% to 6.82% at 650–800 ℃. Fe-IP accounted for 61.53% of IP in raw TS, which is the major species of IP in raw TS. When the pyrolysis temperature increases from 200 ℃ to 800 ℃, Fe-IP content of TS decreased from 60.02% to 26.96%. Raw SS has Fe-IP content about 49.08%, when the pyrolysis temperature increases from 200 ℃ to 500 ℃ Fe-IP content of SS decreased from 46.04% to 12.04%, but Fe-IP proportion increased to 13.20% at 650 ℃ and then decreased with rising temperature.

Raw MS has O-IP content of about 3.41%. The amounts of O-IP of MS were initially increased at 200–350 ℃ and then decreased with rising temperature. Raw TS has O-IP content about 3.94%, after pyrolysis, O-IP proportion increased to 4.03% at 200 ℃, but when the temperature increases from 200 ℃ to 350 ℃, O-IP content decreased to 3.9% and then increased with rising temperature. And raw SS has O-IP content about 4.18%. When the pyrolysis temperature increased from 200 ℃ to 650 ℃, O-IP content of SS increased from 5.58% to 11.18%, but O-IP proportion decreased to 6.93% at 800 ℃.

All biochars prepared from sludge revealed expressively increased Ca-IP content with increasing pyrolysis temperature where Ca-IP increased from 0.58% in raw MS to 85.27% in MS800, 1.58% in raw TS to 45.98% in TS800, and 1.00% in raw SS to 77.98% in SS800. Compared with MS and SS, the formation of Ca-IP in TS is less. This may be due to the content of organophosphorus in TS was lower than that in MS and SS. The conversion of organophosphorus during pyrolysis is the main factor for the formation of Ca-IP ^[28]. Second, it could be due to the low conversion of Al-IP and Fe-IP of TS during pyrolysis, as Al-IP and Fe-IP were found to be stable at high temperatures ^[46,47]. Notably, Ca-IP was expressively increased in the sludge biochar than in the feedstock. At lower pyrolysis temperature, carboxyl and carbonyl groups would be reacting with Ca, restraining massive formation of insoluble P forms. These groups, however, evaporate at high temperatures. Therefore, when the pyrolysis temperature was 500 ℃ or higher, the more aromatic structure of the biochars would not inhibit the constitution of Ca-IP compounds ^[48].

The influence of temperature during the pyrolysis is important for the mineral structure of the sludges. XRD measurement showed the presence of many crystallized minerals in sludge and their biochar. The acquired XRD curves of the sludge and their biochar and the peaks of the minerals appeared in the samples have been signaled with number symbols in Figure 2 and Table 5. We found that the aluminum ion (Al³⁺) combined with orthophosphate ions (PO₄³⁻) in raw MS, TS, and SS to constitute AlPO_4, which appeared in the form of non-apatite inorganic phosphorus. When the pyrolysis temperature was 200–500 ℃, some apatite containing hydroxyl group appeared, which was more easily absorbed and utilized by plants. When the temperature reached 650–800 ℃, various phosphorus speciations in biochars were gradually converted to Ca₂Pb₈(PO₄)₆₍OH)₂, Ca_5.5Pb_4.5(PO₄)₆(OH)₂, Ca₉HPO₄(PO₄)₅OH), and other more stable orthophosphates ^[49]. The crystal phases of AlPO₄, Fe₄(PO₄)₃(OH)₃, Fe₃(PO₄)₂(OH)₂, Fe₃(PO₄)₂·H₂O, Ca₂P₂O₇, CaP₂O₆, and Ca_5.5Pb_4.5(PO₄)₆(OH)₂ were found in raw MS. Besides, other new crystal phases such as Fe₅(PO₄)₃(OH)₅, FePO₄·2H₂O, Fe₄(PO₄)₃(OH)₃·5H₂O, Fe₂PO₄(OH), Ca₄P₆O₁₉, Ca₃(PO₄)₂, Ca₂Pb₈(PO₄)₆(OH)₂, and Ca₉HPO₄(PO₄)₅OH were found in MS200, MS350, MS500, MS650, and MS800 and did not appear in MS. In particular, the crystal phase of Ca₉HPO₄(PO₄)₅OH only existed in MS650. The crystal phases of AlPO₄, Fe₄(PO₄)₃(OH)₃, Fe₃(PO₄)₂(OH)₂, Fe₅(PO₄)₃(OH)₅, Fe₄(PO₄)₃(OH)₃·5H₂O, Fe₃(PO₄)₂·H₂O, Fe₂PO₄(OH), and Ca₂P₂O₇ were found in raw TS. New peaks such as FePO₄·2H₂O, Ca₄P₆O₁₉, and Ca₃(PO₄)₂ occurred in the XRD curves produced from the TS200, TS350, TS500, and TS650. It is worth noting that a lot of new peaks such as (Ca₂Pb₈(PO₄)₆(OH)₂, Ca_5.5Pb_4.5(PO₄)₆(OH)₂, and Ca₉HPO₄(PO₄)₅OH) appeared in the XRD curves of TS800. The XRD curves of raw SS almost coincided with raw MS, and an increase (200–650 ℃) and decrease (650–800 ℃) of the species of crystal phases can be found in their biochar. The results are in accordance with a previous study ^[31]. Similarly, the crystal phases of Ca₂Pb₈(PO₄)₆(OH)₂ and Ca₉HPO₄(PO₄)₅OH) appeared in the XRD curves of SS650. It is worth mentioning that the P, which is adsorbed in the lattice of the minerals, is steadier than that adsorbed on raw sludges. Those P- containing minerals are difficult to release and therefore difficult to use in the short term ^[23]. This could be the real reason for the stabilization of P convert to the H₂SO₄-extract (Ca-IP) at high temperatures (e.g., 800 ℃). Lipscombite (Fe₃(PO₄)₂(OH)₂) was detected in all the XRD curves (Figure 2). In addition, the peak position was confronted with patterns of these minerals appeared in other works ^[50,51]. Lipscombite is normally formed P-minerals during sewage disposal and P-removal with Fe-salts. These mineral phases are the base for the probe into P conversion during pyrolysis. The XRD analysis fingered that in addition to the compounds of orthophosphate, other compounds of pyrophosphate could not be spotted, though these compounds accounted for a sizeable proportion of the total phosphorus in the biochars, which may be caused by the combinations between P and biochar in which P could be adsorbed on Al, Fe, or Ca, same as in soil ^[26]. The conclusions from the XRD curves of the MS and SS are compliance with the outcomes of the transplantations of P from Fe-IP and Al-IP to Ca-IP at the stated temperature ranges. Therefore, biochars disposed at low temperature (0–200 ℃) revealed tiny differences in their XRD curves, proving that the types of P compounds in the biochars disposed at low temperatures are similar hence revealing comparable quantities of Fe-IP, Al-IP, and Ca-IP in these biochars. Nevertheless, biochars disposed at high temperature (350−800 ℃) formed a new type of P-containing mineral, a considerable proportion of which would be stabilized in the minerals, resulting in a noteworthy increase of Ca-IP of these biochars.

Figure 2.  X-ray diffraction diagrams of MS, TS, SS, and their biochars.

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Table 5.  The crystal phases of phosphorus at different pyrolysis temperatures.

MS	MS200	MS350	MS500	MS650	MS800
1.AlPO4	1.AlPO4	1.AlPO4	1.AlPO4	1.AlPO4	1.AlPO4
2.Fe4(PO4)3(OH)3	2.Fe4(PO4)3(OH)3	2.Fe4(PO4)3(OH)3	2.Fe4(PO4)3(OH)3	2.Fe4(PO4)3(OH)3	2.Fe4(PO4)3(OH)3
3.Fe3(PO4)2(OH)2	3.Fe3(PO4)2(OH)2	3.Fe3(PO4)2(OH)2	3.Fe3(PO4)2(OH)2	3.Fe3(PO4)2(OH)2	3.Fe3(PO4)2(OH)2
7.Fe3(PO4)2·H2O	4. Fe5(PO4)3(OH)5	4. Fe5(PO4)3(OH)5	4. Fe5(PO4)3(OH)5	4. Fe5(PO4)3(OH)5	4. Fe5(PO4)3(OH)5
9.Ca2P2O7	6. Fe4(PO4)3(OH)3·5H2O	6. Fe4(PO4)3(OH)3·5H2O	5. FePO4·2H2O	7.Fe3(PO4)2·H2O	7.Fe3(PO4)2·H2O
10.CaP2O6	7.Fe3(PO4)2·H2O	7.Fe3(PO4)2·H2O	6. Fe4(PO4)3(OH)3·5H2O	9.Ca2P2O7	9.Ca2P2O7
14.Ca5.5Pb4.5(PO4)6(OH)2	9.Ca2P2O7	9.Ca2P2O7	7.Fe3(PO4)2·H2O	10.CaP2O6	11. Ca4P6O19
	14.Ca5.5Pb4.5(PO4)6(OH)2	12.Ca3(PO4)2	9.Ca2P2O7	12.Ca3(PO4)2	12.Ca3(PO4)2
		14.Ca5.5Pb4.5(PO4)6(OH)2	10.CaP2O6	13.Ca2Pb8(PO4)6(OH)2	13.Ca2Pb8(PO4)6(OH)2
			11. Ca4P6O19	14.Ca5.5Pb4.5(PO4)6(OH)2
			12.Ca3(PO4)2	15. Ca9HPO4(PO4)5OH
			14.Ca5.5Pb4.5(PO4)6(OH)2
TS	TS200	TS350	TS500	TS650	TS800
1.AlPO4	1.AlPO4	1.AlPO4	1.AlPO4	1.AlPO4	1.AlPO4
2.Fe4(PO4)3(OH)3	2.Fe4(PO4)3(OH)3	2.Fe4(PO4)3(OH)3	2.Fe4(PO4)3(OH)3	2.Fe4(PO4)3(OH)3	2.Fe4(PO4)3(OH)3
3.Fe3(PO4)2(OH)2	3.Fe3(PO4)2(OH)2	3.Fe3(PO4)2(OH)2	3.Fe3(PO4)2(OH)2	3.Fe3(PO4)2(OH)2	3.Fe3(PO4)2(OH)2
4. Fe5(PO4)3(OH)5	4. Fe5(PO4)3(OH)5	6. Fe4(PO4)3(OH)3·5H2O	4. Fe5(PO4)3(OH)5	4. Fe5(PO4)3(OH)5	4. Fe5(PO4)3(OH)5
6. Fe4(PO4)3(OH)3·5H2O	6. Fe4(PO4)3(OH)3·5H2O	7.Fe3(PO4)2·H2O	5. FePO4·2H2O	5. FePO4·2H2O	5. FePO4·2H2O
7.Fe3(PO4)2·H2O	7.Fe3(PO4)2·H2O	9.Ca2P2O7	6. Fe4(PO4)3(OH)3·5H2O	6. Fe4(PO4)3(OH)3·5H2O	6. Fe4(PO4)3(OH)3·5H2O
8. Fe2PO4(OH)	9.Ca2P2O7	12.Ca3(PO4)2	7.Fe3(PO4)2·H2O	7.Fe3(PO4)2·H2O	7.Fe3(PO4)2·H2O
9.Ca2P2O7	11. Ca4P6O19		8. Fe2PO4(OH)	8. Fe2PO4(OH)	8. Fe2PO4(OH)
	12.Ca3(PO4)2		9.Ca2P2O7	9.Ca2P2O7	9.Ca2P2O7
			12.Ca3(PO4)2	12.Ca3(PO4)2	10.CaP2O6
					11. Ca4P6O19
					12.Ca3(PO4)2
					13.Ca2Pb8(PO4)6(OH)2
					14.Ca5.5Pb4.5(PO4)6(OH)2
					15. Ca9HPO4(PO4)5OH
SS	SS200	SS350	SS500	SS650	SS800
1.AlPO4	1.AlPO4	1.AlPO4	1.AlPO4	1.AlPO4	1.AlPO4
2.Fe4(PO4)3(OH)3	2.Fe4(PO4)3(OH)3	2.Fe4(PO4)3(OH)3	2.Fe4(PO4)3(OH)3	2.Fe4(PO4)3(OH)3	2.Fe4(PO4)3(OH)3
3.Fe3(PO4)2(OH)2	3.Fe3(PO4)2(OH)2	3.Fe3(PO4)2(OH)2	3.Fe3(PO4)2(OH)2	3.Fe3(PO4)2(OH)2	3.Fe3(PO4)2(OH)2
4. Fe5(PO4)3(OH)5	4. Fe5(PO4)3(OH)5	4. Fe5(PO4)3(OH)5	4. Fe5(PO4)3(OH)5	4. Fe5(PO4)3(OH)5	4. Fe5(PO4)3(OH)5
6. Fe4(PO4)3(OH)3·5H2O	6. Fe4(PO4)3(OH)3·5H2O	6. Fe4(PO4)3(OH)3·5H2O	6. Fe4(PO4)3(OH)3·5H2O	7.Fe3(PO4)2·H2O	7.Fe3(PO4)2·H2O
7.Fe3(PO4)2·H2O	7.Fe3(PO4)2·H2O	9.Ca2P2O7	7.Fe3(PO4)2·H2O	9.Ca2P2O7	12.Ca3(PO4)2
9.Ca2P2O7	9.Ca2P2O7	10.CaP2O6	9.Ca2P2O7	10.CaP2O6	14.Ca5.5Pb4.5(PO4)6(OH)2
10.CaP2O6		11. Ca4P6O19	10.CaP2O6	11. Ca4P6O19
14.Ca5.5Pb4.5(PO4)6(OH)2			12.Ca3(PO4)2	12.Ca3(PO4)2
			14.Ca5.5Pb4.5(PO4)6(OH)2	13.Ca2Pb8(PO4)6(OH)2
			15. Ca9HPO4(PO4)5OH	15. Ca9HPO4(PO4)5OH

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3.3.   Changes of Olsen-P before and after pyrolysis

The contents of Olsen-p of the MS, TS and SS and their biochar were shown in Table 6. Of the three raw sludges, MS has the highest Olsen-p content, while TS has the lowest, as the Olsen-p content decreases with the decrease of pH due to the acidification treatment in the wastewater treatment project, which has also been reported in other study ^[52]. When the pyrolysis temperature reaches 200 ℃, the content of Olsen-p of all samples increased. When the pyrolysis temperature continues to rise, the content of Olsen-p decreased. The rate of Olsen-p in three different sludges and their biochar were shown in Table 7. It revealed that the rate of Olsen-p in biochar decreased as temperature increased. Thermal process reduce the fraction of mobile P phases (Olsen-P) and transplants more P into less mobile phases (O-IP and Ca-IP) ^[41]. Therefore, with the increase of temperature, there was an distinct tendency for P to move towards less available speciations. Our results are consistent with previous works, the reduction of Olsen P was watched during the pyrolysis of sewage sludges ^[53], plant biomass ^[38], and livestock manures ^[54]. For instance, Christel et al. ^[55] determined phosphorus availability in solid fractions of pig manure biochar using diffusive gradients in thinfilm technology. The results showed that the availability of phosphorus decreases greatly with the increase of temperature. The reduced Olsen P in biochar may be because of the formation of stable aromatic or crystalline insoluble compounds during pyrolysis process ^[56,57].

Table 6.  The concentration of Olsen-P in sludge and biochars (mg g⁻¹).

Samples	Sludge	Biochar (℃)
		200	350	500	650	800
MS	1.48	1.89	0.32	0.24	0.25	0.32
TS	0.46	1.18	0.70	0.52	0.43	0.37
SS	0.61	0.65	0.35	0.13	0.26	0.13

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Table 7.  The rate of Olsen-P in sludge and biochars.

Samples	Sludge	Biochar (℃)
		200	350	500	650	800
MS	11.86	13.65	1.61	0.98	1.16	1.43
TS	2.36	5.39	2.92	2.06	1.94	1.64
SS	6.299	5.87	2.05	0.66	1.40	0.74

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4.   Conclusions

Sludge and its derived biochar are widely used in soil amendments and construction materials. Mastering the speciation of phosphorus is of great significance for further understanding of the potential impact of phosphorus in sludge and its derived biochar on the environment. The results of this study showed the phosphorus speciation changes in biochar after pyrolysis of sludge was hinge on both sewage disposal techniques and raw material composition. XRD analysis and sequential extraction revealed the immobilization of P after pyrolysis, because of the forming of relatively stable Ca-associated P speciation as same as underlying physical constraints. The degradation of organic phosphates and the crystallization of calcium phosphate minerals are detected during the pyrolysis process, and were highly dependent on the pyrolysis temperature. As a result, phosphorus is better immobilized when the pyrolysis temperature is higher and extracted primarily by H₂SO₄. Results from the study afforded deeper perceptions into the thermochemical processes occurred during the pyrolysis of sludges, as well as guidance for P conversion and transplantation from these sludges.

Use of AI tools declaration

The authors declare they have not used Artificial Intelligence (AI) tools in the creation of this article.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (42107435), Key Research and Development Programs of Hunan (2020NK2011, 2021NK2012), Key Project Science Foundation of Hunan Education Department, China (21A0143, 21A0132).

Conflict of interest

The authors declare no conflict of interest.