The Heavy Metals Transformation During The Pyrolysis And Hydrothermal Carbonation of Municipal Sewage Sludge

. The eco-risk of heavy metal has been a constraint to the resource utilization of sludge and sludge biochar, which depends mainly on its chemical form. In recent years, the researches on the speciation and distribution of the heavy metals in sludge and its biochar have received extensive attention and have become a research hotspot in environmental science. In this paper, the technical characteristics of pyrolysis and hydrothermal carbonation (HTC) are discussed. Secondly, the contents and speciations of heavy metals in municipal sewage sludge and biochar were analyzed. Finally, the reaction mechanisms of heavy metals during pyrolysis and HTC were summarized. This paper comprehensively compares the differences in the form transformation of heavy metals in these two processes, which offer an important references for further researches on the immobilization of heavy metals in pyrolysis and HTC.


Introduction
With rapid urbanization and economic development, the number of sewage treatment plants has gradually increased. In 2021, China's sewage treatment capacity increased to 180 million m3/day, and the number of sewage treatment plants reached 2500. It is estimated that by 2025, the sewage discharge will exceed 60 million tons [1][2]. Sludge is the material produced during the sewage treatment process, which contains 70-85% water, and contains a variety of heavy metals and refractory organics. In 2021, China's municipal sludge production was 55.52 million tons, an increase of 8.23% over the same period in 2020. The sludge treatment methods mainly included landfill, incineration and marine dumping [3][4]. However, the landfill method brings secondary pollution to the soil and atmosphere, while incineration of sludge produces a large amount of dioxin, sulfur dioxide and other harmful gases. For coastal cities along the river, the cost of dumping sludge at sea is relatively low. However, sludge carries a large number of toxic and harmful substances into the water body, resulting in eutrophication, excessive heavy metal content and other problems, affecting the aquatic ecological environment. Therefore, the safe and effective treatment of sludge has become a growing concern of researchers [5]. According to the agricultural sludge quality standard [6], the content of one or more heavy metals in municipal sludge exceeds the standard. Improper treatment of these heavy metals causes serious secondary pollution to groundwater and soil. Meanwhile, if the heavy metals in sludge enter the food chain with crops, it will cause great harm to humans, such as bone pain, stunted growth and other diseases. Therefore, there is an urgent need for effective methods to treat sludge.
In addition, sludge has a complex composition and contains a lot of organic compounds, phosphorus, potassium, etc. If the sludge is used reasonably, it can be turned into treasure. Currently, anaerobic fermentation is commoned used for the sludge resource utilization, but this technology has disadvantages such as long treatment cycles.
In recent years, in addition to traditional disposal methods, thermochemical conversion technology has attracted widespread attention. Its advantage is that it can convert sludge into energy while realizing harmless treatment of sludge. Thermochemical conversion includes pyrolysis and hydrothermal carbonization (HTC). Diao et al. [7] discovered that the yield of pyrolysis biochar (pyrochar) decreased from 62.16% to 55.83%, as the pyrolysis temperature was increased from 550℃ to 850℃. The pyrolysis temperature of 300-600℃ facilitates the conversion of heavy metals (Cu, Zn, Cd and Pb) from unstable to relatively stable forms in sludge, which reduces the bioavailability of heavy metals in pyrochar and further reduces the potential environmental risk of sludge biochar [8][9][10].
In addition to pyrolysis, HTC is also a commonly used technology. With the increase of reaction temperature, the carbon content in the HTC biochar (hydrochar) decreased, and the yield showed a downward trend [11][12]. After HTC, the content of oxidized and residual fractions of heavy metals gradually increased [13][14]. To sum up, considering the economic cost, operability, time and universality, thermochemical transformation has been widely used in sludge treatment to meet the growing demand for efficient, eco-friendly and benign sludge disposal.
Currently, numerous of researches have been carried out on the pyrolysis of sludge and the migration and transformation of heavy metals in HTC. For instance, the factors that influence the speciation of heavy metals during pyrolysis are the physical and chemical properties of the sludge itself, the final pyrolysis temperature and heating rate, the reaction time, and the effect of additives on different types of the heavy metals [15][16]. According to the authors' knowledge, there is no literature review comparing the transfer and transformation of heavy metals in sludge between pyrolysis and HTC. Therefore, this paper compares the differences in the contents and forms of heavy metals in the two processes, and discusses the transformation mechanism.
It provides appropriate guidance for future studies of the heavy metals transformation in sludge and provides a better reference for environmental risk assessment of the heavy metals after thermochemical treatment.

Comparison of pyrolysis and hydrothermal processes
Sludge pyrolysis is the decomposition of sludge at high temperature (100℃-600℃) under normal pressure, oxygen free or anoxic environment. However, the dewatered sludge from a wastewater treatment plant typically usually contains a moisture content of 80% and cannot be directly pyrolyzed. It is necessary to reduce the water content of the sludge to 20%-25% through the drying system. HTC refers to the process of generating solid biochar through various chemical reactions, like hydrolysis, decarboxylation and polycondensation in a certain temperature (180-260℃) and a sealed pressure vessel, using biomass or its components as raw materials, water as solvent and reaction medium. As the author knows, HTC has the following advantages: (1) Mild reaction conditions and low temperature; (2) Low energy consumption, dehydration and decarboxylation occur in the reaction, accompanied by the release of heat, which provides part of the energy for the reaction process; (3) The reaction is carried out in aqueous solution without drying raw materials. The whole system is sealed without harmful substances. However, the hydrothermal reaction process is complex, there are many side reactions, and the reaction pressure is relatively high. Therefore, the pressure resistance and airtightness of the reaction equipment are strictly required. Compared with HTC, pyrolysis operation cost is too high and reaction equipment is complex [17].
The application of sludge pyrolysis in practical projects is shown in Figure 1. A complete sludge pyrolysis process includes storage and transportation system, drying system, pyrolysis system, combustion system, energy recovery system and tail gas purification system. As the beginning of the whole process, sludge storage and transportation play a role in sludge storage and transportation into the drying unit.
1-feeding part 2-crane 3-pyrolysis furnace 4-output cooling system 5-pyrolysis gas treatment 6-fan 7-combustion chamber Figure 1. The process flowchart of pyrolysis The process flowchart of HTC is shown in Figure 2. Firstly, municipal sludge enters the heat exchanger through the feed pump for sludge preheating. Secondly, municipal sludge is input into the reactor through the mixing tank constant speed pump, and appropriate catalyst is added under the heating and pressure state, so that the sludge in the reactor can be cracked, and carbonized. The liquid material after carbonization is carbonaceous liquid, which is sent to the cooling tank farm for cooling, and the remaining energy is recycled by the heat exchanger. The carbonaceous carbon liquid after cooling is mechanically dehydrated, and the carbon moisture content generated is about 30%. The water generated in the reaction process can be safely discharged into the water treatment pipe network after treatment. Finally, the sludge after the reaction is dewatered and deeply dried to obtain dry pyrochar.

Heavy metal concentration in sludge
The heavy metals in raw sludge are primarily in the speciation of oxides, mineral salts, sulfides, hydroxides, etc., and rarely in the form of free ions. Due to different factors such as sludge source, season, temperature and sewage treatment technology, the composition and content of sludge will be very different. Table 1 provides a summary of the total heavy metal contents in different municipal sludge. We see that the sludge has the highest content of heavy metals Zn with 497-2387 mg/kg, followed by the higher content of Cu, 141-4673mg/kg, and Cd had the lowest content (0.6-73mg/kg). water-soluble fraction, carbonate fraction, organic bound fraction, manganese oxide bound fraction, amorphous iron oxide bound fraction, crystalline iron oxide bound fraction and residue fraction [26]. Forstne proposed a seven-step continuous extraction method, which divided the forms of heavy metals into exchange fraction, carbonate fraction, amorphous manganese oxide bound fraction, organic fraction, amorphous iron oxide bound fraction, crystalline iron oxide bound fraction and residue fraction [27]. The existing research mainly adopts the technology of continuous extraction of heavy metals. Due to the different extraction reagents and operating methods, it is difficult to compare the experimental data. In order to integrate different classification and operation methods, the European Community Bureau of reference materials proposed a three-step BCR extraction method. At present, Tessier and BCR methods are widely used for speciation analysis of heavy metals in soil, sludge and other media. As shown in Figure 3. Components F1 (exchange and acid-soluble fraction) and F2 (reducible fraction) showed high bioavailability due to ion exchange, adsorption desorption and pH change. Thus, it is readily absorbed and utilized by soil, plants and water systems. Compared with F1 and F2, F3 (oxidizable fraction) is of lower toxicity due to the adsorption of organic substances and the release of soluble heavy metals under oxidation conditions. F4 (residual fraction) is considered the most stable and non-toxic chemical form, because heavy metal ions consist of primary and secondary solids in the crystal structure. To summarize, the transformation of heavy metals to stable forms play an important role in environmental protection.

Chemical forms of different heavy metals in sludge.
The mobility, bioavailability and ecotoxicity of the heavy metals are greatly determined by their chemical forms. Therefore, there is an urgent need to study the chemical forms of heavy metals in sludge biochar. Table 2 shows the distribution of heavy metals in municipal sludge from various sources. The speciation analysis of heavy metals in sludge showed that Cu was mainly present in F3 and F4. The order of Zn in sludge was F2 > F3 > F1 > F4. For municipal sludge, Pb and Cd were mainly in F4. While Cr mainly existed in F3 and F4. These results indicated that the bioavailability of Zn and Cd in sludge was high, followed by Cu and Cr, and the bioavailability of Pb was weak. The speciation distribution of heavy metals in sludge in different literatures was slightly different, which might be related to different sludge types.  Table 3 displays the content of heavy metals Cd, Cr, Cu, Pb and Zn in the raw sludge and in the pyrochar. All heavy metals were present in the pyrochar at higher levels than in the raw sludge, which reflected that heavy metals were enriched in the pyrochar, and the enrichment was intensified by increasing the pyrolysis temperature. This enrichment might be due to the superior thermal stability of heavy metals compared to other components of the sludge. Heavy metals might exist in sewage sludge in the form of various mineral salts, sulfide, hydroxide, oxide and inclusion compound. Among them, mineral salts and hydroxides were usually converted to more thermally stable oxides or sulfides under reducing pyrolysis conditions, so that most of the heavy metals were retained in the pyrochar. The total content of different heavy metals in the order from high to low was Zn>Cr>Cu>Pb>Cd, which was likely caused by large-scale use of Zn plated drainage pipe networks in urban areas [29]. The migration and transformation characteristics of heavy metals were mainly determined by their respective boiling points and corresponding forms. Metals with relatively low boiling points (such as Pb, Zn and Cd) were more easily removed from the pyrolysis reaction process than metals with higher boiling points (such as Cr and Cu).

Heavy metal content of hydrochar
The contents of Cu, Zn and Cr in raw sludge and hydrochar were relatively high, while the contents of Pb and Cd were relatively low (Table 4). This was probably because cadmium mainly presented in the sludge in the form of carbonates, which was easy to occur degassing and transfer reaction. As a result, the contents of Cd in hydrochar was lower than other heavy metals. The amount of heavy metals released from the sludge particles or dissolved into the liquid phase varies with the HTC temperature. When the HTC reaction temperature was 240-280℃, the contents of Zn, Cr and Cu in hydrochar reached the highest value. The highest contents of Cd and Pb were found when the HTC reaction temperature was 280℃. This was caused by the decomposition of organics in sludge, which resulted in the release of heavy metals in combination with organics and co-precipitation with hydrothermal matrix. The weight of the hydrochar was significantly reduced, a small amount of heavy metals were introduced into the bio-oil and gases, and most of them accumulated in the hydrochar.

Speciation analysis of the heavy metals in pyrochar and hydrochar
The speciation of four heavy metals (Cr, Cu, Pb and Zn) in pyrochar is illustrated in Figure 5. Cu was mainly composed of F3 and F4 components, while its F1 and F2 components only accounted for a small proportion in the sludge. The forms of Cr were similar to those of Cu in sludge, but different in pyrochar. Although pyrolysis reduced the content of F1 component, 1.5%-11.0% Zn still existed in the form of F1 in the pyrochar, so Zn had direct ecological toxicity and bioavailability in pyrochar. In addition, F2 component decreased and F3 component increased, so Zn also had potential ecotoxicity and bioavailability in pyrochar, especially in oxidation environment. The sequence of Zn in sludge sample was F2>F3>F1>F4. With the increase of pyrolysis temperature, F1 component in pyrochar gradually decreased, while F4 component gradually increased. Pb and Cd were almost all present in sludge and pyrochar as F4, which indicated low ecotoxicity of Pb and Cd in sludge and pyrochar. However, the ecotoxicity of heavy metals in pyrochar had been alleviated, because the heavy metals were converted to relatively stable forms during pyrolysis. The BCR continuous extraction method was used to extract heavy metals from various forms of solid products obtained from HTC of sludge. The forms of heavy metal in hydrochar at reaction temperature of 180℃, 240℃ and 300℃ are shown in Figure 6. Among the raw sludge, Cu, Pb, Zn with the direct available state (F1+F2) accounted for 76.66%, 65.77% and 63.30%, respectively, indicating that the heavy metals in the raw sludge were highly mobile. If the sludge was directly exposed to the environment without treatment, it would cause harm to the ecological environment. After HTC, the speciation of all metals changed significantly with the change of temperature, and the F4 fraction increased significantly. The heavy metals Cu, Pb and Zn were well passivated with the increase of reaction temperature, and the increase of temperature could promote their transformation to stable state. Figure 5. The speciation of heavy metals in pyrochar [32] r -Sludge sample s-300℃ o-400℃ a-500℃ n-600℃ c-700℃ Figure 6. The speciation of heavy metals in hydrochar [38] r -Sludge sample t-180℃ m-240℃ w-300℃

Heavy metal conversion mechanism during pyrolysis of sludge
The analysis of the total contents and speciation of Cu, Zn, Mn and Pb showed that the transformation characteristics of different heavy metals in biochar were different due to the different properties of the metals. The macroscopic analysis of the migration patterns of various heavy metals in the pyrolysis process can be used to obtain the general laws of their migration and transformation. The specific process is shown in Figure 7. When the pyrolysis temperature was 100-250℃, the sludge was in the stage of water and volatile matter removal, and the heavy metals in the sludge were transformed from F1 form to more stable form due to the dehydration effect. This was probably due to the elimination of interstitial water, capillary water and adsorbed water during pyrolysis, which promoted the transformation of heavy metals from amorphous minerals to crystal stable forms. When the temperature rose to 250-550℃, Fe-Mn nodules would undergo dehydration or decomposition reaction, making the form of heavy metals tend to be stable; When the pyrolysis temperature was greater than 600℃, a large number of inorganic substances were decomposed, and the release of heavy metals in Fe Mn compounds and organic compounds/sulfides was converted into crystal stable form. However, various reactions of fluorine containing compounds at this time led to the formation of metal halides, which might convert F2 and F3 components of Mn, Zn and Cu into F1 components [39].

Heavy metal conversion mechanism during of HTC of sludge
In the HTC process, macro-molecular substances such as protein and extracellular polymer in the sludge were firstly hydrolyzed, which destroyed the organic membrane and extracellular polymer structure in the original sludge. And further released the heavy metal ions originally adsorbed on the surface of the macro-molecule into the liquid phase. Changing the conditions promoted the release of these heavy metal ions, and the specific surface area of the damaged organic membrane structure increased, thus generated more metal adsorption sites. In addition, the destroyed structure is fully expanded in the aqueous solution. On the one hand, this process enhanced the entry of heavy metal ions into the cell, on the other hand, it promoted the easier combination of heavy metal ions and active groups in the cell, forming a stable bond. Through the above process, the heavy metal ions adsorbed on the surface of macro-molecular substances were converted from the previous extracellular adsorption into intracellular adsorption and active group adsorption.

M Z → MZ
(3) Where, P refers to small molecule substances, R refers to organic groups such as (M-S, M-O), Z refers to inorganic anion groups such as SO42 -, OH-and H2PO4.

Conclusion
In this paper, the content and speciation of heavy metals in sludge and biochar were reviewed. It was found that the content of heavy metals Cu and Zn in the sludge raw materials was high, while the content of Pb and Cd was relatively low. During the pyrolysis and HTC processes, the heavy metals in F1 and F2 forms in sludge biochar could be reduced, so as to initially stabilize heavy metals. In general, increasing the pyrolysis temperature ( ≤ 600°C) and HTC temperature (≤320°C) showed an increasing trend of heavy metal content, which could promote the transformation of heavy metals in biochar to more stable forms.
In recent years, the researches on the speciation and distribution of heavy metals in pyrochar and hydrochar has made continuous progress, but the current research is still limited, and the following aspects need further improvement: (1) At present, the transformation mechanism is still unclear, and further researches are needed in the future. (2) The environmental impact assessment of heavy metals in pyrochar and hydrochar can be explored.