Migration and conversion of phosphorus in hydrothermal carbonization of municipal sludge with hydrochloric acid

Purpose: Phosphate ore is a non-renewable resource, so �nding a replacement is necessary. Municipal sludge has signi�cant recycling potential because of its high phosphorus content and large discharge characteristics. Methods: The migration and transformation of phosphorus in sewage sludge treated with different concentrations of HCl were studied using the standards, measurements, and testing phosphorus extraction protocol from two aspects: phosphorus complexation and mineral form. Results: The results showed that more than 95% of phosphorus was concentrated in the solid products (hydrochar) after hydrothermal carbonization (HTC) without HCl, and the main form of phosphorus was organophosphorus (OP). With 0.5% to 2.5% HCl added, the phosphorus concentration of the liquid product (process water, PW) ranged from 13.14 to 219.41 mg/L, and the concentration of ammonia nitrogen (NH 4 + -N) increased by 0.32-to 1.88-fold. With the addition of 2.5% HCl, the phosphorus content in the hydrochar was 90% that of the original sludge, with a concentration of 64.17 mg/g, and the proportion of non-apatite inorganic phosphorus was approximately 94%. Conclusion: This study provides technical support for the recovery of phosphorus resources from municipal sludge.


Introduction
Phosphorus is an essential structural element of the cell membrane in all living organisms and is an important agricultural fertilizer [1].The increasing shortage and depletion of phosphorus resources worldwide is concerning because of the large demand for phosphorus fertilizers and non-renewable phosphate ores [2].Such an imbalance between phosphorus demand and resources necessitates the development of different ways to reduce phosphorus mining.Wastewater accounts for more than half of the phosphate consumption in the EU [3], and municipal sludge (MS) is the main phosphorus sink during wastewater treatment, ranging from 1000 mg/kg to several weight percent [4], which is the second largest source of phosphorus after phosphate rocks.However, with the growing global population and increasing urbanization, wastewater sludge poses several challenges, including overproduction, treatment requirements, and limitations in agricultural applications [5][6][7].The Organization for Economic Cooperation and Development predicts that the cost of dealing with the human waste currently produced will be 2% of the global GDP [8].Therefore, it is important to nd alternative options for the valorization of sludge and its transformation into a more valuable product.
Hydrothermal carbonization (HTC) is a promising alternative residue management process that can recover nutrients from biological waste with high moisture [9][10].As a sustainable, economical, and e cient method [11], the HTC process involves heating the biomass in water at subcritical temperatures (180-240°C) and autogenous pressure.To ensure that the pressure consistently exceeds the saturation vapor pressure of water, liquid water is used as the reaction medium.During this process, the raw material undergoes a complex series of reactions such as hydrolysis, decarboxylation, polymerization, and aromatization.Finally, a carbon-rich solid with fuel properties similar to those of lignite [12] is generated, called hydrochar.
Before using hydrothermal carbonization techniques to recover phosphorus, its behavior during this process must be studied.Current ndings include the following aspects [13][14][15][16]: (1) A large amount of phosphorus is distributed in hydrochar after HTC; (2) HTC treatment results in the conversion of organophosphorus (OP) to inorganic phosphorus (IP), and the degree of conversion is related to the type of raw materials and reaction conditions; (3) The conversion of apatite inorganic phosphorus (AP) and non-apatite inorganic phosphorus (NAIP) in hydrochar is signi cantly affected by pH.Acid can affect product properties by promoting hydrolysis and solution precipitation.Thus, acid-supported HTC treatment could be a novel strategy for controlling hydrochar properties.However, current research has focused on analyzing the in uence of process parameters, such as temperature and reaction time, on the physicochemical properties of products [17][18], while the extraction effect of hydrochloric acid on phosphorus in the HTC reaction and its in uence mechanism are poorly understood.
The main objective of this study was to explore the effect of HCl on the behavior of phosphorus.Speci cally, MS was hydrothermally carbonized with various HCl concentrations, and the concentration, morphology, and distribution of phosphorus in the hydrochar/process water (PW) were characterized to determine the mechanism of migration and conversion of phosphorus.
2 Methods And Materials

Municipal sludge
The MS used in this study was collected from a local municipal sewage treatment plant in Karamay, Xinjiang, China, in August 2022.It was then dried under open-air conditions for several days before being ground in a hammer mill to reduce its particle size to approximately 0.15 mm.The MS was stored in a refrigerator (4°C) prior to the HTC experiments.Sludge samples were analyzed according to GB/T 212-2008.The moisture, ash, volatile matter, and xed carbon contents were 26.77%, 40.63%, 30.15%, and 2.45%, respectively, and the phosphorus content was 25.51 mg/g.The hydrochar yield, R C , was calculated using Eq.(1).
1 M C is the mass of the hydrochar, and M MS is the mass of the dry base sludge.

HTC experimental procedure
The hydrothermal carbonization reaction was conducted in a stainless-steel reactor, which was equipped with an automatic temperature controller and pressure gauge.The inner material of the reaction kettle was PTFE.The reactor was lled with a mixture of 4 g of dried sludge and 40 mL of solution (deionized water or acid solution) to maintain a 1:10 biomass-to-water ratio.Generally, the temperature range of the HTC was approximately 170-240°C.Reaction time has little impact on phosphorus allocation [19], and the formation of insoluble phosphorus species is faster at higher temperatures [20]; therefore, the experimental temperature was set at 205°C, and the hold time was 40 min.At the end of the experiment, the reactor was naturally cooled to room temperature (25-30°C).Hydrochar and PW were separated by suction ltration.The samples were stored in a refrigerator at 4°C until further testing.The concentration gradients of hydrochloric acid were 0%, 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% (wt.%).

Morphological characterization of phosphorus
The standards, measurements, and testing (SMT) phosphorus extraction protocol (Fig. 1) divides phosphorus into ve fractions [21]: total phosphorus (TP), OP, IP, AP (the component associated with Ca), and NAIP (related to oxides and hydroxides of Al, Fe, and Mn).The correlations between these ve fractions were as follows: TP = IP + OP and IP = AP + NAIP.Each experiment was repeated three times to obtain the average value.The molybdenum blue colorimetric method was used to detect the IP content in the supernatant of the product.After the samples were digested at 121°C for 30 min with K 2 S 2 O 4 (5.0%,w/w) in an autoclave, TP was determined using the same IP measurement procedure.The OP content was the difference between IP and TP.Absorbance measurements were performed using a HACHDR1900 spectrophotometer.

Distribution of phosphorus
To explore the distribution of phosphorus in the solid-liquid products of sludge after hydrothermal carbonization, the TP concentration in hydrochar with different HCl concentrations was determined, and the TP retention rate (Rp) was used to represent phosphorus enrichment in hydrochar [22]: P C and P MS represent TP concentrations in hydrochar and raw sludge, respectively.
Figure 2 shows the distribution of phosphorus between hydrochar and PW.With the HTC treatment without acid, the Rp was as high as 99%, indicating that nearly all the phosphorus was retained in the hydrochar.After adding HCl co-hydrothermal carbonization, Rp decreased exceptionally slowly.When 2.5% HCl was added, Rp declined to 91.1%, which was 7% less than the previous gradient.This critical phenomenon con rms what was previously reported by Dai et al. [16].The high levels of Rp at 205 ℃ and 40 min acidic conditions may be due to the promoted transformation of metal phosphate.As shown later (Section 3.5), approximately half of the reaction products were apatite, which conditionally binds with Mg and Zn and has a strong adsorption a nity for Fe and Al (hydroxide) [23].
The migration of phosphorus to the PW was mainly attributed to the lower pH [16], which was con rmed by the shift in the hydrothermal pH (Section 3.3): the minimum Rp occurred with the lowest PW pH.The low-pH environment inhibited the formation of stable products in the form of Ca 5 (PO 4 ) 3 (OH) and Fe 7 (PO 4 ) 6 , and promoted the release of phosphorus [14].Similar results have been reported for different phosphorus-rich biomass.By adding HCl and NaOH, Zhang et al. [24] investigated the migration and distribution of phosphorus in pig manure digestive uid after HTC and found that the release e ciency of phosphorus decreased in the order of acidic HTC, natural HTC, and alkaline HTC processes.Ekpo et al. [25] also showed that the HTC process of adding alkali leads to the precipitation of orthophosphate and reduces the concentration of phosphorus in PW.Furthermore, Heilmann et al.
[26] hypothesized a mechanism for the electrostatic capture of released PO 4 3− on dissolved proteins and its action in subsequent aldol condensation and polymerization, leading to the formation of C-spheres.

Morphological transformation of phosphorus in process water
Figure 3 shows the effect of acid addition on the morphological transformation of phosphorus.The positive effect of using HCl additives on improving the morphology and fraction of phosphate was noticeable.The amount of phosphorus in the natural HTC (no acid addition) PW was 7.45 mg/L or 0.3% of TP, and the proportion of IP was 34.77%, indicating that only 0.1% of the phosphorus in the sludge was extracted into the solution as phosphate.The phosphorus content of the HTC PW with 0.5% HCl was 13.14 mg/L, an increase of 1.76-fold compared to that of the acid-free solution, with the maximum amount of phosphate (219.41 mg/L) observed in the aqueous phase at 2.5% HCl.Previous studies have shown low anaerobic biodegradability of PW obtained from HTC treatments due to the formation of refractory compounds [27].Interestingly, the OP increased with the addition of 0.5% HCl in this study, indicating that the addition of a small amount of HCl may inhibit the formation of insoluble compounds to some extent.While an apparent increase in the amount of phosphate in PW was observed with increasing HCl concentration, the most likely explanation is that phosphorus compounds do not have a high priority in the reactions, and that the extraction of phosphorus is gradually apparent after the critical point of 2.5% HCl.The increase in TP concentration was mainly attributed to the rise in IP, as the acid promoted the hydrolysis of OP during the HTC process.Simultaneously, the degradation of polyphosphates at higher temperatures led to a rise in the number of short-chain polyphosphates and provided additional P-O bonds for metal complexation [28], resulting in a substantial increase in the proportion of IP from 31.25-97.17%.When 2% HCl was added, the OP concentration dropped signi cantly to 2.84%.

Changes of process water pH
An important factor recognized as affecting phosphorus release is pH.The adsorption, precipitation, solubilization, and other chemical reactions related to phosphorus transformation are greatly affected by pH, and a decrease in PW pH promotes the release of phosphorus in hydrochar [16].Accordingly, the pH of the liquid was measured before and after the reaction (Fig. 4) and the natural carbonization process enhanced the weakly acidic environment.This is because some sugar polymers are depolymerized and hydrolyzed into oligosaccharides at low temperatures and hydrothermal conditions, producing glucose or fructose; after dehydration at elevated temperatures, small molecules such as carboxylic acids form, and lipids are hydrolyzed into fatty acids [29].The increase in conductivity was facilitated by HCl, which is an acid that undergoes full dissociation, and the mass transfer between PW and hydrochar in the HTC process caused slight uctuations in conductivity before and after the reaction.
Ammonia plays a leading role in the pH of PW [29].A graph of pH/NH 4 + -N and HCl concentration (Fig. 4) was constructed by analyzing the standard solutions and calculating the hydrogen ion concentration variation in the sample before and after HCl addition (absolute value, ).From the PW with added 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% HCl concentrations, the corresponding hydrogen ion changes were 2.58×10 − 6 , 1.08×10 − 4 , 1.18×10 − 3 , 7.03×10 − 3 , and 1.07×10 − 2 mol/L, respectively.With an increase in , the concentration of NH 4 + -N increased simultaneously, and the two had a favorable relation.In the HTC process, the ring-opening and deamination of amino acids contribute to the generation of NH 3 [30], which exists in the form of ammonium ions in the PW and eventually increases the pH.However, owing to the addition of HCl, the absolute value of pH was extremely low, and the PW containing HCl was acidic.

Variation of hydrochar yield
The solid yield of the hydrochar produced under various experimental conditions is shown in Fig. 5.The hydrochar yield without acid addition was 54.24%, indicating that the hydrolysis of polymers and decomposition of organic matter after hydrothermal carbonization of the sludge facilitated the dissolution of many minor soluble substances in the PW, resulting in approximately half of the sludge being reduced.When 0.5% HCl was added for collaborative hydrothermal carbonization, the hydrochar yield decreased by 7.66%.Over the range of HCl concentrations investigated, the hydrochar yield decreased gradually from 50.08-35.48%.This was in line with the ndings of Andres et al. [31] and Dai et al. [16] because acid can increase the solubility of mineral salts and promote the transfer of C to PW [16].
Additionally, with a decrease in the initial pH (Fig. 5(a)), the formation of related aromatic structures such as monocyclic and bicyclic aromatics containing N (pyrazine, pyridine, indole derivatives, pyrrole, and quaternary ammonium salts) is inhibited under acidic conditions [32], hindering the generation of hydrochar.

Transformation of phosphorus in hydrochar
Figure 6 illustrates the SMT results of the phosphorus fractions in the raw sludge and hydrochar samples under different HCl concentrations.After HTC treatment, the concentration of TP in the sludge increased from 25.18 mg/g to 46.29 mg/g, an increase of 83.83% compared to that of the raw sludge (Fig. 6(a)).This is because the HTC process is accompanied by the continuous degradation of organic matter and destruction of microbial cell structure, which leads to the concentration of the remaining material.With the addition of 0.5% HCl, TP increased to 50.02 mg/g and continued to rise to 67.16 mg/g with increasing HCl concentrations before stabilizing.This can be explained by the addition of acid increasing the surface structural porosity of the hydrochar, inducing a phosphorous complexation process, and the associated phosphorous species also exhibited adsorption behavior [33].OP was particularly low in raw sludge, accounting for approximately 11% of the TP.Natural HTC treatment increased the fraction of OP to approximately 25%, and the content doubled compared with that of the original MS.After HTC with acid addition, the proportion of IP gradually increased to 98% (with the addition of 1.5% HCl) and tended to be stable.Phosphorus in hydrochar was consistent with that in the PW because the HTC reaction occurred in a liquid medium.In the HTC process, organic matter is hydrolyzed, OP is dissolved and gradually transformed into IP, and organic functional groups are replaced by metals at higher temperatures [34].
As shown in Fig. 6(b), the AP of the MS was 18.47 mg/g, which accounted for 72.39% of the TP.With HTC processing, the NAIP concentration was 16.04 mg/g, or 222.4% that of MS.After HTC with HCl was added, the NAIP concentration began to increase substantially, from 24.39 mg/g with 0.5% HCl to 60.66 mg/g with 2.5% HCl.The highest NAIP occurred with the addition of 2.5% HCl, which accounted for 94.5% of the TP.
From the perspective of phosphorus speciation (complexation and mineral forms) in the reaction mixture after extraction, elevated amounts of Ca and Fe in MS can induce complex processes of the phosphate anion.This can result in the formation of insoluble calcium-and iron-associated phosphate minerals precipitated by hydrochar during the HTC process, which explains the enrichment of phosphorus in hydrogen coke after hot carbonization without acid addition [15].Furthermore, orthophosphates decomposed from organophosphates may form phosphate precipitates or be adsorbed onto minerals, and unstable IP species may also undergo dissolution and recombination/recrystallization [35].The addition of hydrochloric acid reduces the pH of the reaction mixture, which in turn impedes phosphorus complexation by changing the solubility of calcium-and iron-associated phosphorus precipitation, resulting in an increase in the quantity of phosphorus in the PW [33].
The effect of HCl concentration on the ratio of NAIP to AP (NAIP/AP) was additionally studied by regression analysis (Fig. 7).An increase in NAIP/AP indicates a transition from NAIP to AP.According to the above results, because the addition of 2.5% HCl had a more noticeable shift in the distribution of phosphorus and resulted in a more signi cant loss of phosphorus in hot carbon in water compared with the addition of 0-2% HCl, the data with the addition of 0-2% HCl were selected for modeling analysis.C HCl in the tting expression in Eq. ( 3) below is the mass fraction of hydrochloric acid, and the analysis result yielded R 2 = 0.914, indicating that the model had a decent degree of t within the range of 0-2% HCl addition. 3 Overall, NAIP/AP increased with increasing HCl concentration, indicating a transition from NAIP to AP, as con rmed by the rst derivative in the tted curves.Similar results have been reported by Shi et al. [36].
Wang et al. [37] also conducted hydrothermal carbonization by changing the in uent pH at 200°C and found that alkaline feed water pH was conducive to the dissolution of NAIP and an acidic environment inhibited the stability of AP.
At pH < 4.8, phosphate anions exist primarily in their monobasic form (H 2 PO 4 − ), which facilitates the formation of acid phosphates with cations such as Al 3+ , the bonding of which is favored by the increased number of H + [31] .However, very acidic pH levels are not conducive to the formation of phosphates with cations, such as Ca 2+ or Mg 2+ .The protonation of H 2 PO 4 − , which has a relatively high a nity for calcium (Eq.( 5)), generates H 3 PO 4 (Eq.( 4)), resulting in a reduced oversaturation of the solution [38].

5
According to the experimental results, HCl addition can promote the conversion of OP to IP in hydrochar and PW, and the concentration has a positive effect on NAIP/AP in hydrochar, which has been described in detail in previous sections.Figure 8 summarizes the behavior of phosphorus during the hydrothermal carbonization process with added acid.

Conclusion
In this study, more than 99% of the phosphorus was enriched in hydrochar after hydrothermal carbonization of the sludge, resulting in a yield of approximately 53% and an increase in the TP concentration.The dominant forms of phosphorus in the hydrochar were IP and OP in the PW.
Hydrothermal carbonization supported by HCl promotes the decomposition of phosphorous compounds       Relationship between NAIP/AP and HCl addition Figures

Figure 4 Changes
Figure 4

Figure 5 Changes
Figure 5