Preparation of sludge-based hydrochar at different temperatures and adsorption of BPA

Due to rapid urban growth in recent years, the need for the treatment of urban domestic sewage is paramount. Research has revealed that per year, a large amount of sludge, ∼30 million tons, is generated from urban sewage treatment plants, with only about 25% of the sludge being properly treated (Wang et al. 2017). In the environmental protection industry, handling such a large amount of sludge often poses technical difficulties. Traditionally, sludge is disposed of in landfills or by incineration. These disposal methods have a series of drawbacks and often lead to pollution problems in the environment (Pilli et al. 2011). As a result, sludge recycling is gaining attention.

Urban sludge in most areas can be expressed as C₅H₇NO₂, which is biomass with a carbon content of about 53% (Gil et al. 2019). The rich carbon content provides a prerequisite for the preparation of sludge as adsorbent materials. Rivera-Utrilla et al. (2013) investigated this and successfully made sludge activated carbon. The adsorption capacity of the produced material for antibiotic tetracycline was observed to be in the range 512.1–672.0 mg/g, demonstrating unique abilities for this material to be used in the removal of pollutants contained in water. In general, sludge can be prepared into an adsorbent by two methods, pyrolysis and hydrothermal carbonization (Garlapalli et al. 2016). In pyrolysis, the organic substance is allowed to stay at high temperatures (500–1,000 °C) under an oxygen-free or anoxic condition for a few hours to several days to enable the formation of carbon substance. This is followed by dehydration and drying of organic matter for a pretreatment process. On the other hand, hydrothermal carbonization (HTC) is the process, by which organic matter is converted into hydrochar at a certain temperature range (180–260 °C) and autogenous pressure (1–5 MPa) in a closed system of water (Garlapalli et al. 2016). Hence, HTC treatment of sludge offers the advantages of being unconstrained by moisture content of material, simple to prepare, and requiring mild reaction conditions. In the HTC process, temperature is a key parameter as it provides the energy for the breaking of biomass bonds, which often affects carbon yield and adsorption quality of hydrochar.

Studies have shown little or no research in the area of sludge, thereby prompting the need for more studies and findings. This paper mainly studies the effects of different hydrothermal temperatures (160, 190, and 250 °C) on sludge adsorbents and their removal effects on organic pollutants. Bisphenol A (BPA) with chemical formula C₁₅H₁₆O₂ was chosen as the target pollutant in this study. The global production of BPA is about 8 million tons per year. Its solubility in water is in the range 120–300 mg/L making it a non-volatile and hydrophobic organic pollutant. BPA is a known endocrine disruptor with estrogen activity. Study has shown that even a trace concentration of BPA can adversely affect animal physiological status, reproductive system, and fetal development (Bhatnagar & Anastopoulos 2017).

The aim of this study is to prepare a low cost and efficient adsorbent from sludge by the HTC process. The influence of hydrothermal temperatures on hydrochar was studied. The unique chemical and textural properties of carbon materials synthesized at different temperatures were determined using different analysis techniques to highlight their influence on the performance of the adsorbent in BPA adsorption.

The raw material for this experiment was sludge. It was sourced from the mechanically dewatered sludge of a sewage treatment plant in Suzhou, China. The water content of the sludge was 87 ± 0.27%, the content of total suspended solid was 70 ± 0.54%, and the pH was between 7 and 8.

• m_SBC (g) = the dry mass of the hydrochar,

• m_sludge (g) = the dry mass of the sludge sawdust.

The highest available grade of BPA (purity 99.8%, molecular weight = 228.29, λmax = 278 nm) was purchased from Dr. Ehrenstorfer GmbH (Augsburg, Germany). The stock solution of 1 g/L BPA was prepared by dissolving 0.05 g of BPA in alkaline distilled water. Distilled water was used to dilute the stock solution of BPA to the desired concentrations.

The bulk carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) contents of sludge and hydrochar were characterized by an elemental analyzer (Vario MACRO cube, Elementar, Germany). The ash contents were calculated by the difference in mass of samples before and after heating at 750 °C for 4 h. On the other hand, the bulk content of oxygen (O) was estimated after deduction of ash in samples.

The Brunauer–Emmett–Teller (BET) surface areas and monolayer pore volumes of the hydrochar were determined by using the specific surface area and pore size analyzer (V-Sorb 2800, China) by nitrogen and helium adsorption–desorption.

Surface texture and morphology of sludge samples (before and after HTC) were analyzed using scanning electron microscopy (SEM; Hitachi S-4800, Japan).

Fourier transform infrared spectroscopy (FT-IR) (Nicolet IS 10, Thermo Fisher Scientific, USA) was performed on samples by using the KBr tablet method. The functional groups on the surface sludge (before and after HTC) were scanned with the spectral range varied from 4,000 to 400 cm⁻¹.

The functional groups were also determined by X-ray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250XI, USA).

The crystallographic structures of carbon materials were examined by X-ray diffraction (XRD) (D8, BRUKER AXS, Germany) with a Cu-Kα radiation in the range of 10–90° (2θ).

For the determination of the point of zero charge (pH_pzc) for hydrochar, 0.01 mol/L of NaCl solution was prepared with 50 mL of NaCl solution poured into 11 flasks. The pH values were adjusted from 2 to 12 with HCl and NaOH respectively and then 0.25 g of hydrochar added to the flasks. After passing N₂, the mixture was placed in a shaker for 48 h. The initial pH of the solution was plotted against ΔpH before and after the reaction and the pH_pzc was determined at ΔpH = 0.

For this experiment, 1 g/L of BPA solution was initially diluted to give different concentrations. Adsorption experiments were performed by adding 100 mL of 20 mg/L BPA solution into a 200 mL conical flask containing an appropriate amount of pre-prepared hydrochars. The pH value was adjusted by NaOH or HCl solution. The mixtures were sealed and shaken at a rate of 160 rpm for 24 h. The solutions were then filtrated by using a 0.22 μm filter and BPA concentration determined using a high-performance liquid chromatography system (Agilent 1260 Infinity, USA). An Agilent Zorbax SB-C18 (250 mm × 4.6 mm, 5 μm particle size) was used as the analytical column. The eluent was a mixture of methanol (63%) and water (37%) at a flow rate of 1 mL/min. Detection wavelength was set at 278 nm. The lower detection limit of BPA was 0.5 mg/L. All experiments were performed in triplicate to enhance reliability.

The adsorption isotherm of the BPA adsorption was examined by performing equilibrium experiments. The samples were shaken for 24 h, with initial BPA concentrations of 10–100 mg/L.

• Q (mg/g) = BPA adsorption capacity,

• C₀ (mg/L) = initial BPA concentration in solution,

• C_t (mg/L) = equilibrium BPA concentration in solution,

• v (L) = solution volume, and

• M (g) = mass of adsorbent.

The adsorbed BPA can be converted into hydrochar by hydrothermal treatment and the adsorbent returned to sewage for reuse.

In this experiment, 0.5 g of hydrochars was weighed to adsorb BPA (50 mL, 50 mg/L) for 24 h to the equilibrium of adsorption. The supernatant was filtered to measure the remaining concentration of BPA. The adsorbed hydrochars were then dried and 50 mL distilled water added. This was followed by HTC of the mixture at the corresponding hydrothermal temperature for 4 h. Subsequently, the adsorbent was filtered and dried to adsorb BPA continuously. This process was repeated three times for cyclic adsorption of hydrochar.

Temperature has a significant effect on the process of sludge HTC. Table 1 shows the hydrothermal parameters of hydrochars prepared at different temperatures. At the reaction time of 4 h, as the hydrothermal temperature increased from 160 °C to 190 °C, the solid yield of hydrochar decreased from 84.73% to 57.29% with a drop of about 27.4%. When the reaction temperature was further increased to 250 °C, the yield of hydrochar still reduced but the amplitude was small with only 2.1%. This is mainly because when the temperature is lower than 190 °C, the organic components in the sludge have yet to be completely hydrothermally carbonized, hence the presence of many solid products (Liu et al. 2013). Therefore, a lower reaction temperature is more favorable to improve the yield of hydrothermal charcoal, which is consistent with other research conclusions (Gao et al. 2016; Chen et al. 2017). Furthermore, the gas yield increased from 720 mL to 3,571 mL with an increase in hydrothermal temperature, which indicated that increasing the process temperature leads to more gaseous products. Studies have also shown that decarboxylation is the main reaction leading to gas formation during the HTC process. In addition to the usual concentration of 70–90% CO₂, other gases such as CH₄, CO, and H₂ also can be found (Ramke et al. 2009). The HTC process leads to a decrease in the pH of the hydrothermal fluid, which is due to the decomposition of lignin and cellulose contained in the sludge to form a large amount of volatile fatty acids and thus show acidity. Moreover, as the hydrothermal temperature increases, the further breakdown of protein in sewage sludge results in an increase in pH (Usman et al. 2019). After the completion of the HTC process, the conductivity, chemical oxygen demand (COD), and total dissolved solids (TDS) of hydrothermal fluid all changed greatly. This may be due to a series of reactions such as dehydration, decarboxylation, polycondensation, and aromatization of HTC. It is worth noting that these chemical reactions do not occur continuously, and their order, interaction, and severity are yet to be established (Sevilla & Fuertes 2009; Reza et al. 2014).

The distribution of elements in the hydrochar under different temperatures was analyzed (Table 2).

Compared to sludge, no matter the hydrothermal temperature, the C content of hydrothermal charcoal only decreased by about 3% (Table 2). Most of the C in the sludge was retained in the hydrochar indicating that HTC is beneficial for C sequestration (Libra et al. 2011). The enrichment of carbon in hydrochars means that a greater degree of aromatization and condensation may occur in HTC, while the H and O content are reduced by 0.84–1.57% and 8.5–14.53%, respectively. As a result, the process removes H and O from the solids in the form of H₂O and CO₂. The N content in the hydrothermal charcoal was lowered from 1.71 to 2.47%, indicating that the hydrolysis of protein during the HTC period of sludge is being enhanced as N mainly comes from the protein contained in the sludge. The lower N content in hydrothermal charcoal also indicates that during HTC, a certain amount of N in the sludge has been transferred to the water phase. The content of S in hydrothermal charcoal was also lowered because the sulfur oxides formed during HTC are dissolved in the hydrothermal liquid. In addition, the type of carbon found in hydrochars is also different from the original sludge. The decrease in H/C and O/C values indicates that hydrochars contain more aromatic carbon than sludge, because dehydration and decarboxylation during HTC often lead to the formation of aromatic carbon in biomass and increase the hydrophobicity of hydrochars. The decrease of H/C and O/C with the increase of hydrothermal temperature indicates that the higher the hydrothermal temperature, the more obvious the degree of aromatization.

Table 2 also shows the specific surface area (S_BET) of sludge and hydrochars. The original S_BET of the sludge was only 0.213 m²/g. After HTC, the S_BET of the hydrochars gradually increased. When the hydrothermal temperature is 190 °C, the maximum S_BET increases to 11.92 m²/g, which means that the specific surface area has increased by more than 560%, and the S_BET is greater than other hydrochars prepared from biomass such as lignin (2.59 m²/g) and sawdust (4.41 m²/g) and orange peel (6.96 and 8.65 m²/g) (Nogueira et al. 2019). When the hydrothermal temperature continues to rise to 250 °C, the S_BET of the hydrothermal carbon decreases to 2.92 m²/g, which can be attributed to the decrease in the fiber structure on the surface of the hydrothermal carbon as the temperature rises, resulting in a smoother material surface.

As shown by the XRD patterns of sludge and hydrochars (Figure 1), hydrochar is an amorphous carbon without obvious C-characteristic diffraction peaks. The sharp and strong peaks of 2θ between 20 and 30° indicate that various inorganic components are mainly composed of quartz, and their peak strength increases with increase of hydrothermal temperature. They also indicate that the content of organic matter decreases and the content of inorganic matter increases. The results are consistent with those of elemental analysis (Table 2). Unlike hydrochars prepared by plants, which contain most of the crystals (Keiluweit et al. 2010), the aliphatic component of the sludge-based hydrochar of this study is mainly composed of poorly crystalline C.

Figure 2 shows SEM micrographs of the ‘before’ and ‘after’ sludge HTC. It can be seen that the surface of the dewatered sludge without hydrothermal treatment is smooth, firm, and has an underdeveloped pore structure. After HTC, the surface morphology changed. The SEM images of hydrochars clearly prove that the microstructure of the sludge after hydrothermal treatment will change to different degrees depending on the hydrothermal temperature. SBC160 retained its original sludge structure with only the formation of a small number of microspheres (<2 nm). When the temperature increased to 190 °C, a rich pore structure appeared on the surface of SBC190. This was consistent with the obtained BET result. Obviously, the high temperature accelerated the process of nanosphere formation and ultrafine fiber breaking. The formation of nanospheres and the appearance of pore structures could be attributed to the morphological changes observed on the surface of hydrothermal carbon. Some studies have shown that these microspheres are cellulose-transformed carbon spheres that occur during HTC (Sevilla & Fuertes 2009; García-Bordejé et al. 2017). For sludge, the formation of hydrochars may occur in the following ways: (1) hydrolysis of cellulose, hemicellulose or lignin chains, (2) dehydration and fragmentation, and (3) destruction of original microfibers of sludge. The polymerization and condensation of these microfiber fragments occur in soluble materials and, with aromatization, hydrothermal carbon increases, eventually forming carbon spheres and pore structure on the surface.

Figure 3 shows the FT-IR measurements of sludge and hydrochars. The types of functional groups on the surface of the sludge and hydrochars are similar, indicating that HTC can retain the original functional groups of the carbon material to a large extent, which may be beneficial to the removal of pollutants (Spataru et al. 2016). Near the wavenumber of 3,400 cm⁻¹ is O-H stretching vibration, and the band at about the wavenumber of 2,800–3,000 cm⁻¹ is attributed to aliphatic C-H vibration. The peak at around the wavenumber of 1,600 cm⁻¹ is caused by the stretching vibration of aromatic C = C. The results indicate that an aromatization process occurred during the HTC process, in which the polysaccharide was converted into hydrochars. Another possibility is the presence of aromatic compounds in the protein in charcoal. At the wavenumber of 1,300–1,000 cm⁻¹, there are vibration peaks of C-O bonds in ethanol, phenol, and hydroxyl. Another possible cause is bond stretching of -Si-O-, as SiO₂ is a component of ash in the sludge. Compared with the sludge, the C=O absorption peak of carboxylic acid near the wavenumber of 1,700 cm⁻¹ disappeared in hydrochars due to the decarboxylation reaction.

Figure 4 shows the C-spectrum analysis of the XPS of the sludge and hydrochars. In Figure 4(a)–4(c), the surface functional group of the carbon group (CH_x) generated by the aromatization is measured near 284 eV and the hydroxyl group or ether (-C-OH) from the dehydration reaction is measured near 285 eV. The acid anhydride and ester (-COOR) were measured at about 288.5 eV (Gong et al. 2014). Among these, the carbon group of hydrothermal carbon occupies a certain proportion which indicates that it has formed an aromatic structure. Specifically, these groups of hydrothermal carbon indicate a higher degree of aromatization. In terms of relative strength, the main carbon functional groups are C–(C,H) in the original sludge followed by C–(O,N), C=O, and C–H respectively. After 190 °C of water heating, the intensity of C–(C,H) showed a significant decrease while the C–(O,N) and C–H peaks increased significantly. From the perspective of functional group content (Figure 4(e)), the -COOR and C-OH content of SBC190 is significantly larger than that of SBC160 and SBC250, which may be due to the hydrothermal decomposition of soluble polysaccharides and proteins contained in sludge when the hydrothermal temperature is greater than 180 °C. Some studies have proven that -OH and -COOH play a key role in adsorbing pollutants through adsorbents through surface complexation and ion exchange (Bogusz et al. 2015). The reduction of aromatic C–(C,H) and the enhancement of C–H also indicates that C–(C,H) depolymerizes to C–H at about 190 °C. Furthermore, C–(O,N) was observed to remain at high levels. This may be due to the internal aromatic C conversion of C–(C,H) and the presence of hydroxyl groups from low concentrations of linear alcohols, ethers or phenols, or due to formation of polycyclic nitrogen-containing compound. Interestingly, when C–(O,N) peak is basically unchanged, compared with sludge, the C=O peaks in hydrochars all disappear. It can be inferred that the reduction of O/C is mainly caused by the decrease of C=O not C–O.

Figure 5(a) shows the removal rate of BPA by hydrochars prepared at different hydrothermal temperatures. It can be seen that with the increase of hydrothermal temperature from 160 °C to 190 °C, the removal rate of BPA increased sharply from 91.44% to 96.17% during adsorption equilibrium. However, when the hydrothermal temperature was increased to 250 °C, the rate increased only slightly, by 0.58%. When the adsorption reaction was performed for 6 hours, the removal rate of BPA by SBC190 was more than 95% implying that temperature affects the properties of hydrochars and adsorption capacity of hydrochars for BPA. The change trend is different from the trend of the specific surface area of hydrochars with temperatures, so the adsorption of BPA by hydrochars is not solely affected by physical adsorption and may be related to other adsorption methods such as hydrophobic interaction. In addition, BPA has two benzene rings with two phenol groups, so it has the strongest ability to provide donors, and because of its smallest molecular size, it can enter the adsorption site more easily. It has also been reported that the polarity and structure of polar aromatic compounds may be more suitable for exposed polar aromatic nuclei in biochars (Zhu et al. 2004). In order to further study the isothermal adsorption mechanism, the Langmuir isothermal adsorption model was used for fitting. The fitting curve is shown in Figure 5(b) and fitting parameters shown in Table 3. The Langmuir equation R² values of SBC160, SBC190, and SBC250 are 0.932, 0.968, and 0.956, respectively. This shows that the Langmuir model can well describe the process of BPA adsorption by hydrochars, i.e., following a single layer uniform adsorption. In addition, with the increase of hydrothermal temperature, the adsorption amount increased from 10.859 mg/g to 18.373 mg/g indicating that increasing the hydrothermal temperature in a certain range is beneficial to improve the adsorption effect of hydrochars.

The adsorbent materials prepared from other raw materials are shown in Table 4. The adsorption effect of SBC for BPA proves to be better than that of most carbon materials (Lazim et al. 2015; Sudhakar et al. 2016; Zhou et al. 2018), indicating that HTC of sludge has great potential applications. However, compared with the carbon materials added with activator for pyrolysis reaction, the adsorption capacity of hydrochar is not as good as that of carbon materials (Arampatzidou & Deliyanni 2016). This may be due to the sludge ash not being removed during the HTC process and the specific surface area of hydrochar not being large enough. As a result, activation modification further improves the quality of the adsorbent. In later studies, the performance of hydrochar can be improved by adding acid or alkali for activation in the hydrothermal process.

Hydrochar can be used as an organic-pollutant adsorbent in water; it has a good adsorption effect for BPA and the removal rate can reach 96% in a short time. From the removal effect, SBC250 has the highest removal rate. However, considering the energy consumption and economics, SBC190 offers the best result making the adsorption characteristics of SBC190 worth investigating.

Figure 6 shows the study of the influence factors of SBC190 on BPA adsorption. At a reaction temperature of 15–35 °C, the removal rate of BPA by SBC190 was between 95.25% and 96.10% with the temperature showing no effect on BPA adsorption and indicating that the sludge-based hydrochar can be used for BPA adsorption in a wide temperature range (Figure 6(a)), whereas the pH of the solution has a greater impact on the adsorption process, which is related to the nature of BPA and surface chemical properties of the hydrochars (Figure 6(b)). The point of zero charge (pH_pzc) for the hydrochar was found to be 6.07. For pH < pH_pzc, the surface of the adsorbent is charged positively and the adsorption of anionic molecules increases due to electrostatic attraction. In solutions with higher pH values (pH > pH_pzc), the adsorbent surface becomes predominantly negative and the adsorption of cationic molecules increases. With the increase of pH from 3 to 11, the removal rate of BPA by SBC190 increased from 94.59% to 96.38%, and then decreased to 89.52%. This can be attributed to the pKa of bisphenol A being in the range 9.6–10.2. When pH > pKa, BPA is negatively charged; when pH < pKa, BPA is positively charged. The pH_pzc of hydrochar is 6.07, so alkaline conditions greatly weaken the electrostatic effect, thus inhibiting the adsorption of BPA. Furthermore, the pH of the solution was adjusted to 7, and the influence of the dosage of hydrochars on the adsorption of BPA investigated (Figure 6(c)). It can be observed that when the dosage is gradually increased from 1 g/L to 5 g/L, the removal rate increases sharply from 4.37% to 89.24%. However, when the dosage was more than 10 g/L, the removal rate of BPA was observed to be more than 96%. Further increase in the dosage did not significantly improve the removal rate of BPA, instead wasting some hydrochars. Therefore, 5 g/L is a more economical dosage after comprehensive consideration and its BPA adsorption can reach more than 78% within 1 h.

In the recycle experiment of hydrochars, the sludge-based hydrochar after the adsorption of BPA was hydrothermally treated again (190 °C, 4 h). After three cycles as depicted in Figure 7, its adsorption capacity was being maintained at about 5 mg/g, with an increase of 0.44 mg/g and 0.26 mg/g in the second and third cycles respectively, indicating that the hydrochars obtained after subsequent HTC treatments still has strong adsorption capacity and can be recycled and reused. However, the removal rate of BPA decreased slightly. This may be associated to the decrease in solid yield as a result of the formation of gas during the re-HTC process. In addition, the remaining BPA concentration in the hydrothermal fluid in each cycle was found to be below 0.5 mg/L, indicating that BPA had been decomposed and converted during the HTC process. Therefore, the re-HTC process can decompose BPA into CO₂, H₂O, CH₄, and other gases, which reduces the difficulty and cost of subsequent treatment of BPA. At the same time, the adsorbed BPA can be converted into hydrochar and reused. As a result, the BPA adsorbed on the hydrochars is processed with a reduction in cost through the regeneration process of sludge adsorbent. Therefore, sludge-based hydrochar has the potential for recycling.

The hydrochar production from sludge via HTC under different temperatures is investigated. Results reveal that a higher temperature causes a lower solid yield. Hydrothermal temperatures also affect the apparent morphology and physicochemical properties of hydrochars. The hydrochars could effectively adsorb BPA due to their diverse structures and functional groups. In addition, the maximum adsorption on BPA is over 10 mg/g under different hydrothermal temperatures. The hydrochar still has better adsorption on BPA under the wide pH in the solutions. Furthermore, results show that the economical dosage of the hydrochar under 190 °C preparation is about 5 g/L. After three cycles, the removal of BPA by the hydrochar (190 °C) is still about 80%. It is therefore suggested that hydrochars serve as effective environmental adsorbents to prevent leaching of organic contaminants as well as having potential for recycling.

This study was supported by National Nature Science Foundation of China (No. 51938010).