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Article

Migration and Conversion of Phosphorus in Hydrothermal Carbonization of Municipal Sludge with Hydrochloric Acid

by
Yang Xue
1,
Zhipu Wang
1,*,
Yue Wu
2,
Ruiqi Wu
3,4 and
Fengtao Zhao
3,4
1
Faculty of Engineering, China University of Petroleum-Beijing at Karamay, Karamay 834000, China
2
Karamay Shuntong Environmental Technology Co., Ltd., Karamay 834000, China
3
Xinjiang Academy of Environmental Protection Science, Urumqi 830011, China
4
Xinjiang Key Laboratory for Environmental Pollution Monitoring and Risk Warning, Urumqi 830011, China
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(8), 6799; https://doi.org/10.3390/su15086799
Submission received: 27 February 2023 / Revised: 5 April 2023 / Accepted: 12 April 2023 / Published: 18 April 2023
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Abstract

:
Phosphate ore is a non-renewable resource, so finding a replacement is necessary. Municipal sludge has significant recycling potential because of its high phosphorus content and large discharge characteristics. The migration and transformation of phosphorus in municipal 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. After the hydrothermal carbonization treatment without HCl, the hydrochar retained 99.7% of phosphorus in the sludge, and the organophosphorus percentage was about 30%. In the hydrothermal carbonization treatment with the addition of 0.5–2.5% HCl, the phosphorus content in the hydrochar decreased gradually from 99.5% (46.18 mg/g) to 91.8% (64.17 mg/g) that of the original sludge, and the proportion of non-apatite inorganic phosphorus increased from 34% to 94%. Hydrochloric acid provides a low-pH environment and promotes the dissolution of calcium-related phosphorus precipitates and enhances the dehydration reaction. This study provides technical support for the recovery of phosphorus resources from municipal sludge.

Graphical Abstract

1. 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 are 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. In the EU, phosphorus loss comes from accumulation in the land (40%) and as waste (51%), and, in this 51%, wastewater accounts for more than half of the phosphate consumption [3]. Municipal sludge 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 Gross Domestic Product [8]. The phosphorus in sludge can be recovered by converting the ash produced by incineration into inorganic phosphate or as fertilizer for agricultural use. However, disposal methods such as incineration can lead to the loss of nutrients [9,10], whereas the direct use of sludge as fertilizer in farmland brings risks due to the availability of nutrients and mobility of potentially harmful elements or substances [11,12]. Therefore, it is important to find 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 [11,12]. As a sustainable, economical, and efficient method [13], 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 [14] is generated, called hydrochar. The liquid product is called process water (PW) and has a high quantity of nutrients.
It is necessary to study the behavior of phosphorus in HTC because the conversion of phosphorus into a stable and single form is conducive to its subsequent recovery (e.g., struvite crystallization and calcium carbonate precipitation [2]). Current findings include the following aspects [15,16,17,18]: (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 significantly affected by pH. Acid can affect the product’s properties by promoting hydrolysis and solution precipitation. Thus, acid-supported HTC treatment could be a novel strategy for controlling hydrochar properties. However, the current application of hydrochloric acid in HTC treatment mainly focuses on soaking hydrochar (phosphorus extraction) [19]. Meanwhile, studies on the combined use of hydrochloric acid and sludge as HTC reactants are limited, and the extraction effect of hydrochloric acid on phosphorus in the HTC reaction and its influence mechanism are poorly understood.
The main objective of this study was to explore the effect of HCl on the behavior of phosphorus. Specifically, the sludge was hydrothermally carbonized with various HCl concentrations, and the concentration, morphology, and distribution of phosphorus in the hydrochar/PW were characterized to determine the mechanism of migration and conversion of phosphorus.

2. Methods and Materials

2.1. Municipal Sludge

The sludge 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 sludge was stored in a refrigerator (4 °C) prior to the HTC experiments. The sludge samples were analyzed according to GB/T 212-2008; the scheme is based on the quality loss after heat treatment to calculate the relevant index. The ultimate analyses were conducted using an elemental analyzer (DHF82, Karamay, Chana). The hydrochar yield was expressed as the mass ratio of hydrochar to sludge (converted to a percentage). The solid mass before and after the HTC was obtained using an analytical balance (XPR106DUHQ/AC, Karamay, China).

2.2. Experimental Methods

2.2.1. HTC Experimental Procedure

The hydrothermal carbonization reaction was conducted in a cylindrical 250 mL autoclave (h = 110 mm, d = 60 mm), which was equipped with an automatic temperature controller and a pressure gauge. The inner material of the reaction kettle was PTFE. The reactor was filled 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 [20], and the formation of insoluble phosphorus species is faster at higher temperatures [21]; therefore, the experimental temperature was set at 205 °C, and the hold time was 40 min. After loading the raw materials, the lid was closed tightly, and the heating was started. When the temperature reached the set value, the timer started. At the end of the experiment, the reactor was naturally cooled to room temperature (25–30 °C). The hydrochar and PW were separated by vacuum filtration. The hydrochar was dried at 105 °C for two hours to remove the moisture. The samples were stored in a refrigerator at 4 °C until further testing. The concentrations of hydrochloric acid were 0%, 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% (wt.%).

2.2.2. Sample Characterization

The standards, measurements, and testing (SMT) phosphorus extraction protocol (Figure 1) divides phosphorus into five fractions [22]: 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 five fractions are 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 K2S2O4 (5 wt.%) in an autoclave, the TP was determined using the same IP measurement procedure. The OP content was the difference between the IP and the TP. Absorbance measurements were performed using a HACHDR1900 spectrophotometer. The ammonium concentration was obtained by measuring the absorbance by adding Nessler reagent. The pH was measured with a digital pH meter (PHS-3C, Karamay, China). The conductivity was measured by a portable conductivity meter, and the calcium ion concentration was determined by ion chromatography. Each experiment was repeated three times. Equation (1) is the method to calculate the change in the hydrogen ion concentration in the solution.
Δ H + = + lg p H P W − lg p H F e e d w a t e r
p H P W  is the pH value of the PW, and  p H F e e d w a t e r  is the pH value of the feedwater.

3. Results

3.1. Variation of Hydrochar Yield

The solid yield of the hydrochar produced under the various experimental conditions is shown in Figure 2a. 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. Over the range of HCl concentrations investigated, the hydrochar yield decreased gradually from 50.08% to 35.48%. This was in line with the findings of Andres et al. [23], because acid can increase the solubility of mineral salts and promote the transfer of C to PW [18]. With a decrease in the initial pH, 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 [24], hindering the generation of hydrochar. This explanation is confirmed by the VK diagram (Figure 2b, calculated from the data in Table 1), which shows that hydrochloric acid promotes dehydration in the HTC process and inhibits aromatization.

3.2. Distribution of Phosphorus

To explore the distribution of phosphorus in the solid–liquid products of the sludge after hydrothermal carbonization, the TP concentration in the hydrochar with different HCl concentrations was determined, combined with the concentration of phosphorus and the volume of the sample to calculate the content of phosphorus (P content is a multiplication of them) and then determine the distribution of the phosphorus.
Figure 3 shows the distribution of phosphorus between the hydrochar and the PW. In the HTC treatment without acid, the proportion of phosphorus in the hydrochar (Retention of phosphorus, RP) was as high as 99%, indicating that nearly all the phosphorus was retained in the hydrochar. After adding HCl, the RP decreased exceptionally slowly. When 2.5% HCl was added, the RP declined to 91.1%, which was 7% less than the previous gradient. This critical phenomenon confirms what was previously reported by Dai et al. [18]. The high levels of Rp at 205 °C 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 affinity for Fe and Al (hydroxide) [25].
The migration of phosphorus to the PW was mainly attributed to the lower pH [18], which was confirmed by the shift in the hydrothermal pH (Section 3.4): the minimum Rp occurred with the lowest PW pH. The low-pH environment inhibited the formation of stable products in the form of Ca5(PO4)3(OH) and Fe7(PO4)6 and promoted the release of phosphorus [16]. Similar results have been reported for different phosphorus-rich biomasses. By adding HCl and NaOH, Zhang et al. [26] investigated the migration and distribution of phosphorus in pig manure digestive fluid after HTC and found that the release efficiency of phosphorus decreased in the order of acidic HTC, natural HTC, and alkaline HTC processes. Ekpo et al. [27] 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. [28] hypothesized a mechanism for the electrostatic capture of released PO43− in dissolved proteins and its action in subsequent aldol condensation and polymerization, leading to the formation of C-spheres.

3.3. Morphological Transformation of Phosphorus in Process Water

Figure 4 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 the 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 [29]. 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 the PW was observed with an increasing HCl concentration, the most likely explanation is that phosphorus compounds do not have a high priority in the reactions, and the extraction of phosphorus is gradually apparent after the critical point of 2.5% HCl. The increase in the TP concentration was mainly attributed to the rise in IP, as the acid promoted the conversion of OP to IP during the HTC process: hydrochloric acid promotes the hydrolysis reaction of the HTC process, resulting in the decomposition of the organophosphorus complex and its conversion into phosphate. The low-pH environment provided by hydrochloric acid will also inhibit the regeneration of organophosphorus. Simultaneously, the degradation of polyphosphoester at higher temperatures led to a rise in the number of short-chain polyphosphates [30], resulting in a substantial increase in the proportion of IP from 31.25% to 97.17%. When 2% HCl was added, the OP concentration dropped significantly to 2.84%.

3.4. Changes in 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 [18]. Accordingly, the pH of the liquid was measured before and after the reaction (Figure 5), 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 [31]. The increase in conductivity was facilitated by HCl, which is an acid that undergoes full dissociation, and the mass transfer between the PW and the hydrochar in the HTC process caused slight fluctuations in conductivity before and after the reaction.
Ammonia plays a leading role in the pH of PW [31]. A graph of pH/NH4+-N and HCl concentration (Figure 5) was constructed by analyzing the standard solutions and calculating the hydrogen ion concentration variation in the sample before and after HCl addition (absolute value,  Δ H + , calculated by Equation (2)). 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  Δ H + , the concentration of NH4+-N increased simultaneously, and the two had a favorable relationship. In the HTC process, the ring-opening and deamination of amino acids contribute to the generation of NH3 [32], 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.

3.5. 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 the 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 (Figure 6a). 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, the TP increased to 50.02 mg/g and continued to rise to 67.16 mg/g with increasing HCl concentrations before stabilizing. OP was particularly low in raw sludge, accounting for approximately 11% of the TP. Natural HTC treatment increased the fraction of the OP to approximately 25%, and the content doubled compared with that of the original sludge. After HTC with the acid addition, the proportion of IP gradually increased to 98% (with the addition of 1.5% HCl) and tended to be stable. The phosphorus in the 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 [33].
As shown in Figure 6c, the AP of the sludge was 18.47 mg/g, which accounted for 72.39% of the TP. With the HTC processing, the NAIP concentration was 16.04 mg/g, or 222.4% of the sludge. After adding HCl 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.
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 [34]. The solubility of calcium-related phosphorus precipitation increases with the increase in calcium and phosphorus content in PW (Figure 7). From the perspective of phosphorus speciation (complexation and mineral forms) in the reaction mixture after extraction, elevated amounts of Ca and Fe in sludge can induce complex processes of the phosphate anion. This can result in the formation of insoluble calcium- and iron-associated phosphate minerals precipitated by the hydrochar during the HTC process, which explains the enrichment of phosphorus in hydrogen coke after hot carbonization without acid addition [17]. At a pH < 4.8, phosphate anions exist primarily in their monobasic form (H2PO4−), which facilitates the formation of acid phosphates with a metal cation, the bonding of which is favored by the increased number of H+ [23]. However, very acidic pH levels are not conducive to the formation of phosphates with cations, such as Ca2+ or Mg2+. The protonation of H2PO4−, which has a relatively high affinity for calcium (Equation (3)), generates H3PO4 (Equation (2)), resulting in a reduced oversaturation of the solution [35]. This mechanism is confirmed by changes in the concentration of calcium ions in the liquid phase, which can be seen in Figure 7; as the concentration of hydrochloric acid increases, the calcium ions continue to dissolve out. 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 [36].
H 2 P O 4 − + H + ⇌ H 3 P O 4
C a 2 + + H P O 4 2 − ⇌ C a H P O 4 ;   K a = 642   M − 1
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.

4. Conclusions

In this study, more than 99% of the phosphorus was enriched in the 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 in the sludge and the conversion of OP to IP, resulting in an increase in the concentration of OP and IP in the hydrothermal carbon and hydrothermal solution. When the HCl concentration reached 1.5%, the IP in the hydrothermal carbon reached 98% and stabilized. With the addition of 2.5% HCl, the IP content in both products exceeded 90%. As the concentration of HCl increased, AP in the hydrochar was converted to NAIP, the hydrochar yield decreased, and a slight amount of phosphorus was dissolved in the liquid phase. In summary, HCl provides a sufficient basis for the interaction of exchangeable forms of elements (micronutrients and macronutrients) during the HTC process, facilitating the conversion of phosphorus to NAIP. The enrichment and stabilization of phosphorus in sludge were carried out in this study. In follow-up studies, process parameters such as temperature should be optimized to save costs, and economical methods for phosphorus recovery should be selected.

Author Contributions

Conceptualization, Z.W.; methodology, Y.W.; software, R.W.; validation, F.Z.; writing, Y.X. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Natural Science Foundation of the Xinjiang Uygur Autonomous Region (2021D01F37), the Karamay Innovative Environment Construction Plan Project (2023hjcxrc0100), Tianshan Youth Program (2020Q021), Karamay School Enterprise Cooperative School-running Project (XQZX20220070), and the Third Xinjiang Comprehensive Scientific Investigation Project (2022xjkk1002).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All the data generated or analyzed during this study are included in this published article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Morphological flow chart of phosphorus extraction by SMT method.
Figure 1. Morphological flow chart of phosphorus extraction by SMT method.
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Figure 2. Changes in hydrochar yield (a) and Van Krevelen diagram (b).
Figure 2. Changes in hydrochar yield (a) and Van Krevelen diagram (b).
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Figure 3. Distribution of phosphorus between the hydrochar and the PW.
Figure 3. Distribution of phosphorus between the hydrochar and the PW.
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Figure 4. Morphological transformation of phosphorus in HTC process water.
Figure 4. Morphological transformation of phosphorus in HTC process water.
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Figure 5. NH4+-N concentration, pH, and conductivity in PW.
Figure 5. NH4+-N concentration, pH, and conductivity in PW.
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Figure 6. The concentration of TP, IP, and OP in hydrochar (a); the proportion of IP and OP in hydrochar (b); the concentration of NAIP and AP in hydrochar (c); the proportion of NAIP and AP in hydrochar (d).
Figure 6. The concentration of TP, IP, and OP in hydrochar (a); the proportion of IP and OP in hydrochar (b); the concentration of NAIP and AP in hydrochar (c); the proportion of NAIP and AP in hydrochar (d).
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Figure 7. The concentration of calcium ions.
Figure 7. The concentration of calcium ions.
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Figure 8. Migration and transformation of phosphorus in sludge HTC process.
Figure 8. Migration and transformation of phosphorus in sludge HTC process.
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Table
Table 1. Properties of tested samples.
Table 1. Properties of tested samples.
Proximately Analysis (wt.%)Ultimately Analysis (wt.%)
SamplesAshVolatileFixed CarbonCHNSO a
Dewatered sludge55.4841.183.3413.38 3.08 1.12 3.61 23.33
HTC-0% HCl57.3136.126.5711.79 2.06 0.44 3.72 24.69
HTC-0.5% HCl61.5430.787.6811.91 2.35 0.43 3.95 19.83
HTC-1% HCl63.4727.878.6612.58 2.41 0.42 4.22 16.89
HTC-1.5% HCl62.6527.489.8713.11 2.43 0.42 4.36 17.04
HTC-2% HCl63.0324.6312.3414.01 2.58 0.41 4.46 15.52
HTC-2.5% HCl64.0121.414.5914.62 2.63 0.40 4.67 13.67
a By difference:100–C (wt.%)–H (wt.%)–N (wt.%)–S (wt.%)–Ash (wt.%).
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Xue, Y.; Wang, Z.; Wu, Y.; Wu, R.; Zhao, F. Migration and Conversion of Phosphorus in Hydrothermal Carbonization of Municipal Sludge with Hydrochloric Acid. Sustainability 2023, 15, 6799. https://doi.org/10.3390/su15086799

AMA Style

Xue Y, Wang Z, Wu Y, Wu R, Zhao F. Migration and Conversion of Phosphorus in Hydrothermal Carbonization of Municipal Sludge with Hydrochloric Acid. Sustainability. 2023; 15(8):6799. https://doi.org/10.3390/su15086799

Chicago/Turabian Style

Xue, Yang, Zhipu Wang, Yue Wu, Ruiqi Wu, and Fengtao Zhao. 2023. "Migration and Conversion of Phosphorus in Hydrothermal Carbonization of Municipal Sludge with Hydrochloric Acid" Sustainability 15, no. 8: 6799. https://doi.org/10.3390/su15086799

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