Effect of dewatering conditioners on phosphorus removal efficiency of sludge biochar

ABSTRACT Based on the best dehydration effect, this study compared the adsorption phosphorus effect of sludge biochar after sludge conditioning with FeCl3, KMnO4, and cationic polyacrylamide (CPAM). This provided insights into the effects of chemical conditioning during the sludge dewatering stage on the overall phosphate adsorption of the dewatered sludge biochar. The phosphorus adsorption mechanism of the dewatering sludge biochar was analysed by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Fourier-transform infrared spectroscopy. Under the optimal pyrolysis temperature (300°C), the phosphate adsorption capacity of FeCl3-conditioned sludge biochar (SB-FeCl3) was increased 77 times of the unconditioned sludge biochar. In different solution environments (e.g. pH and coexisting anions), Phosphate adsorption of SB-FeCl3 was srtrongest when the pH of 9 and contained CO3 2-. Through the analysis of surface elements and functional groups, it was explained that the phosphorus removal effect of SB-FeCl3 comes from abundant active sites containing iron. Phosphorus release occurred in sludge biochar (SB) during the study. SB-FeCl3 solved SB the release of phosphorus, and improved the adsorption capacity of phosphorus. GRAPHICAL ABSTRACT


Introduction
Sewage sludge was a by-product produced by sewage treatment plants, and contains a large number of harmful substances and organisms such as heavy metals, parasites, and pathogens [1,2]. Continued urbanization has led to an increase in the output of sewage sludge, where the annual output in China in 2019 was about 39 million tons with an average annual growth rate of nearly 20%. A lack of treatment and disposal of large amounts of sludge could lead to secondary pollution [3], which had become an urgent problem. The moisture content of these sludge was usually above 90%, and the addition of conditioner could improve their dewatering performance [4]. Dewatered sludge was more conducive to subsequent treatment and disposal, and reduces the overall volumes of sludge. The subsequent disposal or utilization of sludge mainly includes sanitary landfills, pyrolysis, building materials preparation, aerobic composting, and agricultural land utilization [5]. In particular, solid sludge biochar obtained from pyrolysis has a good surface structure, and was often used as an adsorbent of environmental pollutants [6,7]. The addition of metal elements in sludge would promote the removal phosphorus effect of sludge biochar [8][9][10].
Water eutrophication was another serious global environmental issue, which was caused by the drainage of the surface runoff of industrial wastewater and agricultural fertilization into water bodies to significantly increase the nutrient density. Phosphorus was a restrictive element of water eutrophication [11], and recent studies had demonstrated that sludge biochar exhibits good adsorption capacity as a phosphorus removal agent to combat water eutrophication.
The study of Wang, Xiao [12] showed that the phosphorus adsorption capacity of the iron-rich sludge biochar reached 1.84 mg/g; Wang, Miao [10] researched that the phosphorus adsorption capacity of the new calcium-containing paper sludge based biochar prepared was 68.49 mg/g; In Li, Zhao's [13] study, the phosphorus adsorption capacity of a novel sludge-based magnetic gel bead prepared could be as high as 87.97 mg/g. Thus, sludge biochar to combat eutrophication caused by excessive phosphorus in water serves as an opportunity to use waste to treat waste, while solving the problem of sludge disposal.
At present, the main research of sludge biochar as a phosphorus removal agent was to consider how to improve the phosphate adsorption capacity through modification after pyrolysis, without considering the influence of conditioning agents often added in sewage treatment on the experiment, especially those dewatering conditioners added to reduce the moisture content of sludge [8,14]. Common conditioning agents used in sewage treatment plants to treat modified sludge are polyacrylamides, iron salts, and aluminium salts [15]. In addition to improving the dewatering performance, these agents change various physical and chemical properties of the sludge. For example, KMnO 4 and other cracking conditioners destroy the cell structure of sludge and improve the dewatering effect [16], while FeCl 3 , cationic polyacrylamide (CPAM), and other flocculation conditioners facilitate charge neutralization to increase the size of sludge flocs and reduce the hydrophilicity of the sludge [17,18]. FeCl 3 -modified sludge biochar exhibits an improved phosphorus removal performance due to the formation of a Fe-rich biochar surface [19]. These conditioner added to the sludge will also eventually exist in the prepared biochar, which will affect its phosphate adsorption. However, there was no study on the effect of dehydration conditioner on phosphate adsorption of sludge biochar.
In this study, the influence mechanism of dehydration conditioner on phosphate adsorption was considered. The surface morphology, surface elements and functional groups of dewatering sludge biochar (DSB) were observed by scanning electron microscopy, energy dispersive X-ray spectroscopy, and Fourier-transform infrared spectroscopy. By analysing the surface morphology and element distribution of sludge biochar prepared with different conditioners, the effects of three common sludge dewatering regulators (FeCl 3 , CPAM and KMnO 4 ) on the phosphorus removal performance of sludge biochar prepared were investigated. This study provides a theoretical basis for the selection of dewatering conditioners for sludge biochar as phosphorus removal agent.

Materials
Sewage sludge was obtained from Xintian Sewage Treatment Plant in Wanzhou Chongqing ( Table 1). The sludge was processed in the sewage treatment plant according to the Obel oxidation ditch process, where 3000 m³ of domestic sewage was processed per day.
The dewatering effect of the three different conditioners was maximized by considering previous reports on the optimal dosages [18,20,21]. Thus, the dosages of FeCl 3 , KMnO 4 , and CPAM were 128.21, 20, and 6.01 g/kg dry sludge (DS). Each of the conditioners was added to the sludge and mixed with a six-unit mixer. The sludge conditioned using KMnO 4 was left to react for 30 min. Vacuum dewatering was performed to obtain a dehydrated cake, which was transferred to a blast oven and dried to a constant weight at 105±5°C. The dewatering cakes were passed through a 40-mesh sieve. Then the pre-treatment sludge particles were pyrolyzed in a vacuum pit electric furnace at pyrolysis temperatures of 300, 400, and 500°C for 2 h. The resulting biochar was cooled to room temperature in the furnace before analysis.

Phosphorus adsorption capacity
The phosphorus adsorption capacity of the DSB samples was evaluated using phosphate. DSB (1 g) was transferred to a 250 mL conical flask with KH 2 PO 4 solution (100 mL; 10 mg P /L). The mixture was oscillated for 12 h at 120 rpm/min in a shaking box at a constant temperature of 25°C for adsorption. The solution was filtered through a 0.45 μm filter membrane, and the phosphate concentration of the filtrate was determined using molybdate spectrophotometry. The experiment was repeated three times to reduce the error. The phosphate adsorption capacity of the sludge biochar was determined as follows: where q e is phosphate adsorption quantity (mg/g, calculated as P), C 0 and C e are the initial and post-adsorption phosphate concentrations of the solution (mg/L), respectively, V is the solution volume (L), and m is the dosage of sludge biochar (g).

Adsorption isotherm
DSB (1 g) was added to KH 2 PO 4 solution (100 mL) with initial concentration of 2, 4, 6, 8, 10 and 12 mg P/L, respectively. The experiment was repeated for three times at 120 rpm/min and 25°C for 240 min. The phosphorus adsorption capacity of the biochar was calculated using Equation (1), and used to determine the adsorption isotherms based on fitting using the Freundlich (Equation (2)) and Langmuir (Equation (3)) models: Langmuir: where K F (mg/g) and n are Freundlich constants, K L is the Langmuir constant, and q max is the maximum adsorption capacity of the adsorbent (mg/g).

Phosphorus adsorption over time
DSB (1 g) was added to 100 mL KH 2 PO 4 solution (10 mg P/L), and the adsorption was oscillated at 120 rpm/min and 25°C. The experiment was repeated for three times. The adsorption time was set as 0, 15, 30, 45, 60, 90, 120, 240, 720, 1440, 2160 and 2700 min, and the adsorption capacity of phosphate was determined according to Equation (1).

Influence of pH and coexisting anions on phosphorus adsorption
In order to explore the effect of different pH on phosphate adsorption of DSB, different pH environment was provided in the biochar adsorption process. DSB (1 g) to KH 2 PO 4 solution (100 mL; 10 mg P/L) at the pH was adjusted by NaOH and HCl to 3, 5, 7, 9, and 11, and the adsorption was oscillated at 120 rpm/min and 25°C. The experiment was repeated for three times. The phosphate adsorption capacity was determined according to Equation (1). The influence of coexisting anions, namely of Cl -, NO 3 -, , on the phosphate adsorption performance of DSB was evaluated by adding it (1 g) to phosphate solutions (100 mL; 10 mg P /L) containing NaCl, NaNO 3 , Na 2 SO 4 , or Na 2 CO 3 (10 mg/L), and the adsorption was oscillated at 120 rpm/min and 25°C. The experiment was repeated for three times. The phosphate adsorption capacity was determined according to Equation (1).

Analytical methods
The determination of phosphate was conducted using molybdate spectrophotometry with a ultraviolet (UV)visible spectrophotometer (T6 New Century, Shanghai Jingxue). The surface morphology and element distribution of DSB before and after phosphate adsorption were observed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) (Zeiss Merlin Compact). The adsorbed sludge biochar was taken from chapter 2.2 phosphorus adsorption experiment. The surface functional groups on the DSB were determined using by Fourier-transform infrared spectroscopy (FTIR) (Nicolet 670, USA).

Effect of pyrolysis temperature on phosphorus adsorption performance
The DSB was prepared as a phosphorus removal agent at different pyrolysis temperatures, which led to different phosphate adsorption effects (Figure 1). The phosphate adsorption capacity of the sludge biochar was greatly improved when conditioning agents were used, where the addition of FeCl 3 led to the highest phosphate adsorption performance. This was consistent with a previous report by [12]. Specifically, the phosphate adsorption capacity of the FeCl 3 -conditioned sludge biochar pyrolyzed at 300°C was increased 77 times compared to that of the unconditioned sludge biochar, while those of the KMnO 4 and CPAM conditioned sludge biochar were increased 24 and 11 times, respectively. Overall, the conditioned sludge biochar pyrolyzed at 300°C exhibited the best phosphate adsorption capacities, while the unconditioned biochar performed best after pyrolysis at 500°C. Thus, the samples considered in the subsequent experiments were unconditioned sludge biochar pyrolyzed at 300°C and 500°C (referred to as SB-300 and SB-500, respectively), and those prepared using FeCl 3 , KMnO 4 , and CPAM conditioners and pyrolyzed at 300°C (referred to as SB-FeCl 3 , SB-KMnO 4 , and SB-CPAM, respectively).

Adsorption isotherm
The phosphate adsorption isotherms of the unconditioned and condition sludge biochars were fitted using the Langmuir and Freundlich models ( Figure A1 and Table 2). SB-FeCl3 was best fitted using the Langmuir model (R 2 = 0.93), which indicated that SB-FeCl 3 underwent single-layer adsorption. Further, the value of 1/n was greater than 1 in the Freundlich model, which revealed that the biochar adsorption process was synergistic [22]. The various dewatered sludge biochars had different levels of release at low phosphate concentrations, where that of SB-500 was the highest. The fitting of the adsorption isotherms of SB-500 using the two models was poor due to phosphate release, so as SB-KMnO 4 and SB-CPAM. Overall, SB-FeCl 3 exhibited the best adsorption capacity for phosphate.

Phosphorus adsorption over time
The phosphate adsorption process of each sludge biochar was observed over 2700 min (Figure 2). The adsorption capacity of SB-300 and SB-500 reached saturation at 1440 min, while SB-FeCl 3 , SB-KMnO 4 , and SB-CPAM exhibited very slow phosphate adsorption. Thus, the phosphate adsorption before 240 min was low, after which it increased sharply. SB-300, SB-KMnO 4 , SB-CPAM, and SB-500 achieved higher phosphate absorption than release capacity after 720, 240, 120, and 2160 min, respectively. Overall, the adsorption effect of SB-FeCl 3 was outstanding throughout the adsorption process, and no phosphorus release phenomenon was observed. This was consistent with the results of Zhang, Deng [9], Zhang, Lin [23].
The final order of adsorption capacities was SB-500 < SB-300 < SB-CPAM < SB-KMnO 4 < SB-FeCl 3 . In previous studies, the species and contents of metal on biochar   were confirmed to be key factors to affect the adsorption of phosphorus [8,24]. For example, the PO 4 3--P could be absorbed by the metal elements via the surface deposition [9]. Fe and Mn metal elements had embedded in SB-FeCl 3 and SB-KMnO 4 respectively after sludge dewatering. There was a certain chemical reaction between Fe and Mn species to phosphorus in SB-FeCl 3 and SB-KMNO 4 , which greatly improves the adsorption capacity of phosphate [12]. SB-CPAM phosphate adsorption capacity was greater than that of raw sludge, because CPAM carried a large number of positive charges in the sludge, resulting in a higher surface charge potential compared with the original sludge. Therefore, the physical adsorption capacity of SB-CPAM was stronger than that of raw sludge biochar [25]. Based on the best dewatering effect, FeCl 3 was the conditioning agent with the largest adsorption capacity for phosphate, and largely reduced the risk of phosphorus being released from the sludge biochar into the solution.
Yang, Wang [19] reported that iron-modified sludge biochar exhibits faster phosphate adsorption in the first 60 min, after which adsorption equilibrium was reached. The sludge biochar prepared in this experiment was produced with an added conditioning agent in dewatering stage, where stirring ensured that the conditioner was evenly distributed throughout the biochar (i.e. on the surface and internally). During adsorption, phosphate initially came into contact with the biochar via surface adsorption sites, while contact with the internal adsorption sites occurred more gradually. This led to the weak phosphate adsorption capacity of the conditioned sludge biochar in the early stages, followed by a gradual increase in the effect after 240 min.

Influence of pH on phosphorus adsorption
Phosphate adsorption was evaluated under different pH conditions (Figure 3). Phosphate release occurred readily under acidic conditions due to the high phosphorus content of the sludge itself [26]. Thus, none of the sludge biochar samples adsorbed phosphate in pH 3 environment, and the phosphorus release rate of SB-300 was as high as 52.4% (0.53 mg/g). Only SB-FeCl 3 maintained an adsorption rate of 16.3% (0.16 mg/g) in an alkaline environment of pH 11, while the other biochar samples released phosphate to varying degrees. The phosphate adsorption rate of some biochars reached a maximum at pH 9, which indicated that the best phosphate adsorption capacity may be achieved under these pH conditions. Compared with several conditioner and raw sludge biochar, the phosphate adsorption capacity of SB-FeCl 3 was significant at pH 5-11. Therefore, SB-FeCl 3 can maintain good adsorption capacity under large pH fluctuation.
The form of phosphate in solution was affected by pH, and exists as H 3 PO 4 below pH 2.2, H 2 PO 4 between pH 2.2 and 7.2, and HPO 4 2between pH 7.2 and 12.3 [27].Under acidic conditions, phosphates exist in the form of H 3 PO 4 and the electrostatic repulsion in the solution was lower [10]. The negatively charged phosphate present in the sludge biochar was more readily dissociated into the solution. This results in an increased risk of phosphorus release from sludge biochar under acidic conditions. With the increase of pH, the electric potential in solution decreases continuously, while the electrostatic repulsion between phosphate in sludge biochar and solution increases continuously [28]. Phosphate in biochar was more difficult to release into the solution environment, and the performance of phosphate adsorption capacity gradually becomes significant. In alkaline environment, the surface of biochar also gradually becomes negative charge, and the electrostatic repulsion with phosphate in solution increases continuously. When pH is higher than 9, the adsorption capacity of sludge organisms begins to decrease.

Influence of coexisting anions on phosphorus adsorption
Phosphate adsorption of the various sludge biochar samples was affected by the presence of coexisting anions, namely Cl -, NO 3 -, SO 4 2-, CO 3 2-, in different ways (Figure 4). The adsorption capacity of SB-500 was Overall, the presence of coexisting anions promoted the phosphate adsorption of the sludge biochar in this study. However, Li, Xie [27] reported that coexisting anions inhibit the phosphate adsorption effect of modified sludge biochar, especially in the case of the competitive adsorption between CO 3 2and PO 4 3-, while Zhu, Huang [29] reported similar findings. Only SB-500 corresponded with the previously reported trends, while SB-300, SB-FeCl 3 , SB-KMnO 4 , and SB-CPAM inhibited the release of phosphate in the presence of coexisting anions, and ultimately showed promote phosphate adsorption. The adsorption capacity of SB-FeCl 3 to phosphate remains the best when there are several other anions in phosphate solution.
3.6. Proposed mechanism 3.6.1 Effect of surface structure on adsorption performance The surface morphology image of the sludge biochar was evaluated using scanning electron microscopy. The sludge biochar had a rough surface structure and large specific surface area, which were beneficial for adsorption ( Figure A2). Conditioning led to higher surface roughness of the sludge biochar, indicating that substances may be attached to the conditioned sludge biochar ( Figure A2 (b), (c), (d)). Further, FeCl 3 and KMnO 4 had a cracking effect on the sludge structure [30,31], which promoted the formation of pores on the surface of the sludge biochar. CPAM had a flocculating effect that facilitated cross-linking on the surface of the sludge biochar [32]. The surface morphology was observed after phosphate adsorption ( Figure A3). The surface of the sludge biochar was covered with a dense material, thereby confirming the adsorption of phosphate. SB-FeCl 3 exhibited a more obvious phosphate adsorption effect than the other sludge biochars. The adsorption capacity of each phosphate adsorption site on the surface of SB-FeCl 3 was stronger, which led to aggregation of phosphate in a spherical shape and attachment to the surface. Although the surface of SB-KMnO 4 was rougher, KMnO 4 has a lower affinity for phosphate than that of FeCl 3 , and aggregate adsorption was not observed. The enhanced phosphate adsorption of SB-CPAM was attributed to the enrichment of cations on the surface of the biochar due to CPAM [33]. These findings corresponded to the earlier adsorption results in Section 3.3.

Effect of surface elements on adsorption performance
The surface elements of the sludge biochar before and after phosphate adsorption were analysed using EDS ( Table 3). The addition of FeCl 3 and KMnO 4 conditioners in the biochar increased the Fe and Mn contents by 1.4% and 0.1%, respectively. Thus, the presence of these two metal elements in the sludge biochar after addition during sludge dewatering and conditioning was confirmed. After phosphate adsorption using SB-CPAM, the P content was decreased by 0.3%. This indicated that phosphorus desorption occurred from the surface of the sludge biochar into the phosphate solution, thus confirming its poor adsorption capacity. SB-300, SB-FeCl 3 , and SB-KMnO 4 exhibited increases in P and O content to varying degrees after phosphate adsorption. Overall, SB-FeCl 3 had the highest P content, thereby confirming it had the best phosphate adsorption. The changes in surface phosphorus before and after sludge biochar adsorption were measured by EDS, and the results are shown in Figure A4. The presence of phosphorus on the surface of the sludge biochar before adsorption was obvious, and the release of phosphorus was inevitable. Phosphorus was enriched on the surface of the SB-FeCl 3 after adsorption, while the surface content of phosphorus on SB-CPAM was greatly reduced. This demonstrated that CPAM was unable to securely fix phosphorus in the sludge biochar, thereby leading to desorption. The adsorption effect of SB-300, SB-500, and SB-KMnO 4 was not obvious.
These findings were consistent with Section 3.3, where SB-FeCl 3 exhibited the best phosphorus removal due to the promoting effect of Fe on the phosphate adsorption behaviour of the sludge biochar [12]. This was visualized in Figure A5 as an enrichment of Fe on the surface of SB-FeCl 3 , which provided more adsorption sites for phosphate.

Effect of functional groups on adsorption performance
The surface functional group structure of the sludge biochar samples was evaluated based on the FTIR spectra before and after phosphate adsorption to analyse the phosphate adsorption mechanism ( Figure  A6). Stretching vibration peaks near 3428 and 1658 cm −1 indicated the presence of -OH and -C = O [34], while the stretching vibration peaks near 2927 and 1454 cm −1 were attributed to -CH 2 . Thus, the FTIR spectra confirmed that the sludge biochar contained organic matter. Further, absorption peaks at 466 and 1067 cm −1 indicated the presence of haematite and Fe-OH in SB-FeCl 3 [35,36]. These Fe-containing functional groups and -OH on the sludge biochar served as adsorption sites for phosphate, where the EDS results confirmed that SB-FeCl 3 contained the most Fe. Overall, these findings confirm that Fe was linked to high phosphate adsorption capacity.

Conclusion
This study mainly researched the effect of three sludge conditioning agents (FeCl 3 , KMnO 4 , CPAM) on the phosphorus removal efficiency of sludge biochar. The results showed that the sludge biochar had the best phosphorus adsorption capacity at pyrolysis temperature of 300°C and under the dewatering conditioner of FeCl 3 . Compared with the raw sludge biochar, the phosphorus adsorption capacity of SB-FeCl 3 was increased 77 times. SB-FeCl 3 had the best adsorption capacity for phosphate, and there was no risk of phosphorus release. Under the influence of acid and alkali conditions, SB-FeCl 3 maintained its good adsorption capacity, and achieved the highest phosphate adsorption at pH 9. Evaluation of various coexisting anions showed that SB-FeCl 3 could maintain good adsorption capacity even in the presence of other anions. Overall, FeCl 3 was the best pre-treatment dewatering conditioner for the preparation of phosphorus adsorption by sludge biochar, because of the presence of Fe element provided active sites for phosphorus adsorption.

Data availability statement
The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This work was supported by the National Natural Science Foundation of China (grant numbers 51808089); the Science and Technology Innovation Project of Wan Zhou (grant numbers wzstc20210308); and Chongqing Three Gorges University Graduate Research Innovation Project (grant numbers YJSKY2005).