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Abstract
In this study, a new strategy of enriching phosphorus by degradative solvent extraction of sludge and phosphorus recovery with low-cost adsorbents was introduced. The phosphorus recovery performance of three unmodified, low-cost adsorbents was compared, and the effects of the adsorbent dosage, phosphorus concentration, and pH of the extraction solution on the adsorption efficiency of Mg/Al-LDH were studied. The adsorption mechanism of phosphorus in acid leaching environment was also discussed. Results show that phosphorus-rich extraction residue is successfully separated from sludge. Compared to other adsorbents, Mg/Al-LDH has stronger adsorption capacity, and its adsorption efficiency is 8 times and 1.7 times higher than that of activated alumina and bentonite, respectively. The adsorption equilibrium time is 90 seconds, with phosphorus concentrations ranging from 528 to 145 mg/l in an acidic environment, achieving a phosphorus recovery rate of up to 99%. The influence of the pH value is not obvious. The interface phenomenon of Mg/Al-LDH for phosphorus adsorption involves many physical and chemical processes, including physical adsorption, anion exchange and surface complexation. The results show that the scheme is economic and feasible, which can obviously reduce environmental pollution and has considerable economic and environmental benefits.
Keywords: Adsorption, Degradative solvent extraction, Ion exchange, Municipal sludge, Phosphate recovery
Graphical Abstract
Keywords: Adsorption, Degradative solvent extraction, Ion exchange, Municipal sludge, Phosphate recovery
1. Introduction
With the progress of sewage treatment technology and the continuous expansion of treatment facilities around the world, the annual production of municipal sludge in China and the United States reaches 7.8 million tons and 12.7 million tons, respectively [1]. Phosphorus in sludge mainly comes from human excreta, cleaning agents and food residues [2]. The average total phosphorus content in municipal sludge ranges from 21 to 40 g/kg, comprising 2% to 4% of the dry sludge [3–4]. Therefore, sludge is regarded as the most prominent “second phosphate rock” at present, and recovering phosphorus from sludge is a promising method to slow down the consumption of phosphate rock [5–6].
Recovering phosphorus from municipal sludge has greater environmental advantages than treating it directly. For example, it is helpful to alleviate the problems of large land occupation, unpleasant volatile gases release and the leakage of high-moisture sludge leachate. In addition, it prevents phosphorus in sludge from polluting water and causing eutrophication. Recovering phosphorus from sludge also reduces the demand for developing phosphorus resources and the long-term management costs of municipal sludge treatment, which conforms to the principles of a circular economy and contributes to the overall environmental sustainability [7–8].
However, municipal sludge contains a lot of organic matter, nitrogen, phosphorus and other nutrients, of which the organic matter content accounts for more than 30%–40% of the dry weight [9]. Direct incineration of sludge cannot make full use of organic matter, resulting in secondary pollution, including harmful substances such as nitrogen oxides, sulfur dioxide and dioxins. Although most phosphorus is concentrated in the ashes generated by incineration, collecting these ashes requires high-performance dust removal equipment, which increases recovery costs. In addition, the recovery of phosphorus through chemical precipitation, such as struvite formation after acid leaching of ash, requires a magnesium source and is costly [10–11].
The effective separation and classified utilization of organic matter and phosphorus resources in sludge is conducive to the recycling, reduction and the safe disposal of sludge. Degradative solvent extraction technology for municipal sludge shows potential to solve these challenges. The content of organic matter in the liquid products can be as high as 93.2%, which is suitable for preparing high-value nitrogen-containing chemicals [12]. Phosphorus in sludge is usually concentrated in the solid residue after extraction, and easily dissolved in hydrochloric acid [13]. However, the residue solution contains many anions, such as Cl−, NO3− and SO42−, which poses challenges to phosphorus absorption.
The adsorption method can effectively remove specific pollutants because of its simple equipment requirements and good stability [14]. However, most adsorbents are usually used to recover low-concentration phosphorus directly from domestic sewage. The extraction efficiency in such cases is often low, necessitating modifications to improve phosphate selectivity. These modifications increase the cost of preparation and operation of the adsorbent [15]. In addition, when adsorbents are used to extract residues at high concentration, they are easily influenced by other coexisting ions in the solution. Efficient and cost-effective methods for recovering phosphorus from complex extraction residues have not been reported yet.
There are various types of adsorbents, including natural minerals such as bentonite, activated alumina and synthetic adsorbents such as bimetallic hydroxide adsorbent materials. Bentonite, activated alumina and Mg/Al-LDH all show strong adsorption performance, which makes them widely used as adsorbents [16–17]. Other modified adsorption materials, such as MCM-41 and SBA-15 [18], have demonstrated excellent performance in removing aquatic pollutants. However, these adsorbents were not used in this study to recover phosphorus from the residue after sludge extraction for several reasons. Firstly, in order to control the costs of the phosphorus recovery, the unmodified adsorbents, including Mg/Al-LDH, are used instead of more expensive modified materials, such as MCM-41 and SBA-15. Secondly, in terms of adsorption performance, this study compared Mg/Al-LDH with bentonite and activated alumina, revealing that Mg/Al-LDH exhibits superior adsorption capabilities, warranting a more detailed examination. Although the modified adsorbent may provide better phosphorus adsorption, it still needs a method to reduce its preparation costs.
In this study, a new strategy of enriching phosphorus by degradative solvent extraction of sludge and phosphorus recovery with low-cost adsorbents was introduced, with the aim of enriching and recovering phosphorus in municipal sludge to the maximum extent. The phosphorus content in the sludge was 56.7 mg/g. The recovery performance of three unmodified, low-cost adsorbents for phosphorus in the extraction residue was compared and analyzed. The effects of the adsorbent dosage, phosphorus concentration, and pH of the extraction solution on the adsorption efficiency of Mg/Al-LDH were studied. Two kinetic models were employed to assess the kinetic characteristics of phosphorus adsorption, and the reaction mechanism for phosphorus recovery in high-concentration phosphorus extracts were proposed using BET, XRD, and FTIR analyses. Finally, the economic benefits and environmental sustainability of this phosphorus recovery method were evaluated.
2. Materials and Methods
2.1. Materials
The dehydrated sludge used in this study was sourced from the sewage treatment plant in Maanshan, Anhui Province, China. This sludge was dried in an oven at 105°C for 48 hours. After grinding and sieving, the average particle size of the sludge was 0.3 mm, which is stored in a sealed bag for later use. In this study, 1-methylnaphthalene was chosen as the organic solvent for the extraction of degradation solvent, because it has excellent thermochemical stability and the ability of effective recovery and recycling, which ensures the environmental sustainability of the experimental process. The pharmaceutical materials used in the experiment, including activated alumina (r-Al2O3, 1–2 mm), anhydrous calcium chloride, bentonite (1–2 mm), Mg/Al-LDH (15μm), and polyaluminum chloride, were all procured from Shanghai Aladdin Bio-Chem Technology Co. Ltd. in Shanghai.
2.2. Degradative Solvent Extraction and Mg/Al-LDH Adsorption Recovery Method
The degradation solvent extraction reaction was carried out in a high-pressure stainless-steel reactor (GSH-500, manufactured by Xinfa Experimental Instrument Factory in Taixing, Jiangsu Province). The specific program steps are shown on Fig. 1. 100 g of dried and screened sludge and 300 ml of 1-methylnaphthalene(1-MN) were added into the reactor. Next, the kettle is heated to a preset temperature, and kept at this temperature for 30 minutes, then cooled to room temperature. After the reaction, the product was filtered, and the liquid product was distilled at 170°C in a rotary evaporator to obtain the products with high viscosity and dark color, which were called extracts. The organic solvent can be recovered and reused after evaporation and concentration. The solid product obtained from filtration was dried in a drying oven to produce the final product which was called residue. Two distinct solid extraction products can be derived from sludge following degradative solvent extraction: extract and residue. Through this study, it has been confirmed that sludge residue contains a lot of inorganic phosphorus. Therefore, after washing and drying, the residue can be used as one of the raw materials for extracting phosphorus.
The residue obtained at a reaction temperature of 300°C was chosen for the phosphorus-rich extract experiment. In this test, 1 mol/l hydrochloric acid solution (solid-liquid ratio is 100 mg/l) was firstly added to 5 g of residue, and the mixture was shaken in a constant temperature oscillation box for 16 hours at room temperature. Secondary, it was filtered using a 0.45 μm filter membrane to achieve solid-liquid separation. Take the extract and put it in a 500 ml extraction solution for later use and analysis. NaOH and HCl solutions were used to adjust the pH value. Thirdly, different adsorbents were added to the solution, and the obtained solid-liquid mixture was transferred to a 50 ml centrifuge tube. The mixture was left to settle and precipitate after agitation. The phosphorus content in the supernatant and flocs formed was determined.
2.3. Data Processing and Analysis
According to the standard of ASTMD 3172–89, the proximate analysis was carried out to obtain the volatile matter, fixed carbon, and ash contents. The sample’s carbon, hydrogen, nitrogen, and sulfur contents were determined using an ultimate analyzer (Macro cube, Vario, North Rhine-Westphalia, Germany). The X-ray fluorescence spectrometer (Thermo Fisher Scientific) is equipped with Unpiquant 5.0 software, enabling approximate quantitative analysis of related substances without the need for standards.
Accelerated Surface Area and Porosimetry System (USA-Mike-ASAP 2460) was used to detect the surface pore structure of the adsorbent. Brunauer-Emmett-Teller (BET) equation was used to analyze the experimental data, and Barrett-owner-calendar (BJH) model was used to evaluate the pore size and volume of materials.
Fourier transform infrared spectroscopy (FTIR) was conducted using a Nicolet 6700 spectrometer from Thermofisher Company, with a scanning range of 400–4000 cm−1 and a spectral resolution of 0.09 cm−1. X-ray diffraction (XRD) analysis was performed using a D8ADVANCE model X-ray diffractometer from Bruker Company, Germany. The XRD system utilized a Cu target with a power of 2.2 kW, voltage not exceeding 40 kV, and current not exceeding 40 mA. Additionally, it incorporated an energy dispersive array LYNXEYE XE with a maximum count of 1 × 109 cps, a linear range of 4 × 107 cps, and an energy resolution better than 680 eV.
The standard measurement and test (SMT) protocol was employed to analyze the phosphorus content in both the sludge and residue [19]. The SMT method finds widespread application in the analysis of soil, sediment, and sludge samples. Through successive extraction via the SMT procedure, phosphorus within the sludge can be categorized into five distinct forms: total phosphorus (TP), organic phosphorus (OP), inorganic phosphorus (IP), non-apatite phosphorus (NAIP), and apatite phosphorus (AP). The relationship among these five phosphorus forms is described by the following formulae: TP = OP + IP, and IP = AP + NAIP. The concentration of phosphate ions in the extract was assessed using the molybdenum blue method. The absorbance of the extract was measured at 660 nm wavelength using an INESA ultraviolet spectrophotometer.
The recovery rate (%) and adsorption capacity (qe, mg/g) of the extract were used as the calculation indices to evaluate the adsorption removal effect. They can be calculated as Eq. (1) to (4):
(1)
(2)
(3)
(4)
where C0 (mg/l) is the initial concentration of the extract; Ce (mg/l) represents the residual concentration of the extractive solution at adsorption equilibrium; V (ml) stands for the volume of the extract; m (g) represents the mass of the adsorbent; α (%) signifies the ratio of phosphorus in the floc to the phosphorus content in the extract, and β (%) denotes the ratio of phosphorus in the supernatant to the phosphorus content in the extract; CFLOC (mg/l) indicates the concentration of the floc at adsorption equilibrium; CSF (mg/l) indicates the concentration of the supernatant at adsorption equilibrium.
3. Results and Discussion
3.1. Properties of Products after the Degradative Solvent Extraction Reaction
Table 1 gives the proximate and ultimate analyses of municipal sludge and the products from the degradative solvent extraction conducted at 300°C. The results show that the extraction process has effectively achieved deoxidation, desulfurization, and carbon enrichment of the sludge. As indicated in the table, the volatile and ash contents of the extract are 93.2% and 1.3% respectively, with a carbon content exceeding 70%. This represents a remarkable improvement over the original municipal sludge. In contrast, the oxygen content has been reduced from 23.4% to 10%, while the oxygen-carbon ratio and hydrogen-carbon ratios are now very similar to coal and peat. These findings highlight the transformed product as a typical high-grade energy source, suitable for high-value applications [20–21].
In Table 1, the volatile content in the residue of degradative solvent extraction decreased significantly from 58.8% to 28.5% compared with the raw sludge, while the fixed carbon content slightly increases from 5.8% to 9.3%, and the ash content significantly increases from 35.4% to 62.2%. This shows that most of the ash in municipal sludge still remains in the residue. The residue itself has the characteristics of high ash content and many impurities [22]. It can be seen from the table that the contents of TP, OP, IP, AP and NAIP in the raw sludge are 56.7, 4.1, 52.5, 25.3 and 27.2 mg/g respectively, and the ratios of AP to NAIP are similar. After degradative solvent extraction, the TP and IP contents in the residue increased to 63.1 and 69.8 mg/g, respectively. Compared with raw sludge, the growth rates are 11.3% and 15.8%, respectively. At the same time, the content of OP decreased to 2.4 mg/g, which indicated that almost all organic phosphorus was transformed into inorganic phosphorus at the extraction temperature of 300°C, proportion of inorganic phosphorus to total phosphorus increased from 92.6% to 96.4%.
Additionally, due to the significantly higher contents of Fe and Al compared to Ca and Mg in the sludge, Diester-P in the OP fraction is first converted into Monoester-P and then into Pyro-P during the extraction process. After extraction, the contents of AP and NAIP were 29.7 mg/g and 31.1 mg/g. As the main metal elements that form NAIP, iron and aluminum promote the transformation of AP into NAIP, resulting in higher NAIP content and a lower AP content in the extraction residue compared with raw sludge [23].
Therefore, the degradative solvent extraction of municipal sludge realizes the efficient separation of organic matter and high concentration of inorganic phosphorus. The residue can be regarded as a phosphorus-rich resource comparable to phosphate rock, and the necessity of developing efficient and cost-effective phosphorus extraction technologies is emphasized.
3.2. Efficient Recovery of Phosphorus from Degradative Solvent Extraction Residue
3.2.1. Comparison of extraction effects using different additives
To obtain the phosphorus extract, the inorganic phosphorus within the extraction residue is dissolved in a 1 mol/l hydrochloric acid solution. Different from raw sewage, the solution has a high phosphorus concentration and contains a lot of chlorine. It also shows an atmosphere of low organic content and high acidity. Therefore, it is very important to select appropriate and cost-effective additives to extract phosphorus effectively.
The standard phosphate solution is analyzed using the ammonium molybdate sulfate method [24]. Fig. 2(a) shows the standard curve for phosphate. Fig. 2(b) shows the macroscopic changes in the extract after adding five different additives to an extract with a pH = 1.4 and 20 ml at room temperature. The solid-to-liquid ratios are all fixed at 16 g/l. The mixture was stirred with a magnetic stirrer and left standing for 30 minutes. In this study, bentonite, hydrotalcite and activated alumina were used as adsorbents to recover phosphorus, and calcium chloride and polyaluminum chloride were used to recover phosphorus through chemical precipitation method. The results showed that there was no obvious change in the extract after adding polyaluminium chloride, anhydrous calcium chloride or activated alumina. Both the extract of bentonite and Mg/Al-LDH have obvious flocculation phenomenon, but the flocculation phenomenon in the Mg/Al-LDH tube was more obvious and rapid. The adsorption and precipitation time of bentonite was as long as 1 h.
When the flocs in the tubes are almost completely precipitated, the inorganic phosphorus in the five tubes was determined, and the results are shown in Fig. 2(c). In the figure, α and β represent the proportions of phosphorus in the flocculating precipitate and the supernatant, respectively. The results showed that phosphorus recovery was ineffective when adding polyaluminum chloride and CaCl2. The former was due to the high acidity of the extract, which made it unsuitable for the required precipitation conditions, necessitating a large volume of sodium hydroxide solution to adjust the pH value. The latter was ineffective because the pH of the extract was lower than 2.16, and phosphate ion precipitation is typically more effective under specific pH conditions. Furthermore, the acid leaching solution may contain numerous impurities, such as iron, calcium, and magnesium ions. These ions can compete with phosphate to form precipitates or complexes, inhibiting phosphorus recovery. Additionally, the reaction between polyaluminum oxide and phosphate may take considerable time to reach precipitation equilibrium [25]. Activated alumina, bentonite, and Mg/Al-LDH demonstrate the ability to recover phosphorus from the extraction solution. The recovery rate of activated alumina is the lowest, about 12%. This deviation could be due to the low pH value of the extract, which may lead to acid dissolution of activated alumina and other anions, thus hindering adsorption. Bentonite and Mg/Al-LDH exhibit a remarkable ability to recover phosphorus from leaching solutions, with recovery rates of 58% and 99%, respectively. This difference can be attributed to the inherent properties of the materials. Bentonite primarily consists of fine mineral particles arranged in a layered structure, which is relatively stable [26]. Phosphate recovery from bentonite mainly occurs through electrostatic adsorption, cationic bridge adsorption, and physical adsorption. In contrast, Mg/Al-LDH features a double-layer structure that can accommodate more phosphate ions between its layers. Its adsorption mechanism involves not only physical adsorption but also chemical interactions, making the adsorption of phosphate by Mg/Al-LDH more selective and efficient [27]. Therefore, it is more effective to choose Mg/Al-LDH to recover phosphorus, because it is especially suitable for recovering phosphorus from residues after degradative solvent extraction.
3.2.2. BET analysis of Mg/Al-LDH
To further study the adsorption capacity of Mg/Al-LDH, its pore characteristics are shown in Fig. 3. Standard Mg/Al-LDH was used in this study, and its adsorption capacity was influenced by several factors, including surface area, pore size, chemical composition, and surface activity. According to IUPAC standards, the pore sizes of Mg/Al-LDH are categorized as macropores (>50 nm), mesopores (2–50 nm), and micropores (<2 nm), with the pore size distribution shown in Figure 3. The results reveal that micropores constitute only 1.94% of the Mg/Al-LDH, while mesopores and macropores are predominant, comprising 55.94% and 42.12%, respectively. The specific surface area is a crucial factor in increasing the adsorption rate, as a higher surface area provides more adsorption sites and facilitates the diffusion and adsorption of ions. Although the BET specific surface area of Mg/Al-LDH is relatively low, which is 18.02 m2/g, its average pore size is 15.02 nm, which is 4.22 times the size of phosphate ions. Phosphate adsorbed in micropores will not impede the subsequent transport of other phosphate molecules through these micropores, nor will the diffusion of phosphate be hindered by the formation of a hydration shell, which can slow down the adsorption rate. Furthermore, Mg/Al-LDH can achieve efficient adsorption of anionic pollutants, such as phosphate, through its unique interlayer structure and anion exchange mechanism [28].
3.2.3. Effects of hydrotalcite addition and phosphorus concentration in extraction solution
Fig. 4(a) illustrates the effect of adding Mg/Al-LDH on phosphorus recovery and adsorption capacity regarding phosphorus concentration of the extract. In Fig. 4(a), where the phosphorus content in the extraction solution is 200 mg/l, the recovery rate at adsorption equilibrium increases with the increasing dosage of the adsorbent. When the dosage was 16 g/l, the recovery of phosphorus from the extract reached a peaks of more than 95%, although the adsorption capacity gradually decreased with the increase of dosage. Fig. 4(b) illustrates the effect of the extract concentrations on the adsorption capacity and phosphorus recovery rate. In this figure, the dosage of Mg/Al-LDH is 16 g/l, and the concentration of extract ranges from 120 to 530 mg/l. The phosphorus recovery rate of Mg/Al-LDH is over 95% across all concentrations. However, with the increase of the concentration of the extract, the adsorption capacity increases from 11.4 mg/g to 27.7 mg/g.
This is because the common Mg/Al-LDH is used in this study. The adsorbent exhibits good adsorption and regeneration performance [29]. When a larger amount of adsorbent is added, or the phosphorus concentration is low—resulting in fewer phosphate molecules than available sites on the hydrotalcite—the phosphate molecules may fail to fill the pores of the adsorbent. Consequently, many sites on the adsorbent surface do not fully engage. However, the large pore size of hydrotalcite still allows a certain quantity of phosphate to diffuse in and be adsorbed, leading to a gradual decrease in adsorption capacity while the phosphate recovery rate remains high [30].
3.2.4. Effect of adsorption time
Fig. 5(a) shows the effect of the adsorption time on the phosphorus recovery. As shown in the figure, Mg/Al-LDH adsorbs phosphorus very quickly, and the adsorption capacity stabilizes in a short time of 90 seconds. Despite the complex reaction environment in this study, which differs significantly from the conditions in Tong et al.’s [31] research where Mg/Al-LDH adsorbs phosphorus from an aqueous solution. However, in this study, the time to reach adsorption equilibrium is shorter than that of Tong et al (about 300 s). This shows that Mg/Al-LDH maintains high adsorption capacity even in a complex environment. When the adsorption time was over 60 seconds, the recovery efficiency reached 97.6%, and the phosphorus recovery rate remained basically stable. The results show that the adsorption process can be roughly divided into two stages including an initial rapid adsorption stage, and a slower adsorption stage. In the first stage, the recovery rate reaches 80.6% within 30 s.
The adsorption kinetics characteristic of Mg/Al-LDH for phosphorus was analyzed, and the data were fitted by quasi-first-order and quasi-second-order models. The results are shown in Fig. 5(b). Comparative analysis shows that compared with the quasi-first-order model (R2 = 0.97), the quasi-second-order model (R2= 0.99) can better describe the kinetics of phosphorus adsorption by Mg/Al-LDH. This indicates that chemical adsorption may be the main reaction mechanism of phosphorus adsorption by Mg/Al-LDH.
To further study the adsorption mechanism of Mg/Al-LDH for phosphorus, XRD was used to characterize the materials before and after adsorption. Fig. 5(c) shows XRD patterns of hydrotalcite before and after adsorption. It is obvious that the first diffraction peak (peak 003) of Mg/Al-LDH perpendicular to the crystal direction of the layer moves to a lower angle, while the intensity of other characteristic peaks is obviously weakened. It is worth noting that there are no discernible new peaks, which indicates that the surface chemical precipitation is the least. Based on the position of peak 003 shown in the figure, Fig. 5(c) reveals that the interlayer distance of Mg/Al-LDH increases from 1.94 to 2.95, with the vertical cell parameter also shifting from 20.22 to 23.25 following phosphorus adsorption. This indicates that phosphorus is adsorbed within the interlayer space of Mg/Al-LDH, causing the interlayer expansion due to the larger size of phosphorus compared to the original intercalated anion [32].
The analysis shows that ion exchange occurs in the process of phosphate adsorption by Mg/Al-LDH, which is composed of double hydroxide layers of Mg and Al and exchangeable anions such as CO32−. In this layered structure, Mg is predominantly present as Mg(OH)2, while Al is incorporated as Al(OH)3. These hydroxyl ions (OH−) coordinate with metal cations to form the fundamental layer structure of Mg/Al-LDH. When Mg/Al-LDH is placed in a solution containing phosphate ions, the strong electrostatic attraction between the phosphate ions (with three negative charges) and the positively charged Mg/Al-LDH layers promotes the exchange of anions between them, thus maintaining the overall charge balance. Due to their trivalent nature and high charge density, phosphate ions typically have a higher priority in the ion exchange process. Consequently, Mg/Al-LDH is more likely to exchange phosphate ions when a variety of anions are present. This process does not involve breaking strong chemical bonds, such as covalent or ionic bonds. Instead, the main changes involve shifts in electrostatic interactions between the original anions and phosphate ions, as well as the rearrangement of hydrogen bonds between water molecules and ions. The ions released are primarily CO32−, which were originally present between the layers. The soluble adsorption rate of intercalated phosphorus in the interlayer spacing can be described by the mass ratio of phosphorus released in solution to phosphorus absorbed by LDH. The embedded ions can be released by CO32−, and it is estimated that phosphorus adsorbed through ion exchange accounts for 36.06% of the total adsorbed phosphorus. After desorption with a 1 mol/l NaOH solution, high purity phosphorus-containing solution was obtained.
3.2.5. Effect of initial pH
Fig. 6(a) shows the effect of initial pH value of the solution on phosphorus recovery. Upon adding Mg/Al-LD Hand adjusting the pH value from 1 to 8, it is obvious that the phosphorus recovery rate is stable above 95%, and the adsorption capacity is stable at 23.6 mg/g, showing good adsorption effect. When pH value is 2, the recovery rate is 1.01% lower than that when pH value is 8, indicating that acidic conditions have little effect on the adsorption.
When compared to current phosphorus recovery methods, such as struvite (Mg(NH4)PO4·6H2O) crystallization and vivianite (Fe3(PO)2·H2O)8) generation with Mg/Al-LDH [33], several advantages emerge. It is worth noting that these methods do not require strict reaction conditions or pH adjustments. For example, in the case of vivianite crystallization, the process needs to adjust the Fe: P ratio to 1.5 and keep the S: P ratio below 1.1, all of which are carried out in an anaerobic environment to realize the phosphorus recovery of vivianite at pH of 6–8 [34].
Under acidic conditions, the surface of hydrotalcite becomes protonated and positively charged, making the charge interaction between Mg/Al-LDH and phosphate ions attractive. Phosphate primarily exists as H2PO4− and HPO42− in these conditions. Although these ions have lower charges, they exhibit high reactivity and strong ion exchange capabilities with the interlayer anions of Mg/Al-LDH. Furthermore, the relatively small ionic radius of H2PO4− under acidic conditions enhances its effective adsorption within the interlayer structure of Mg/Al-LDH, making it the dominant ion, which further promotes the adsorption of phosphate anions [35]. When measuring the pH value at adsorption equilibrium, the initial pH is 1.4, and the final pH of the solution rises to 6, which is consistent with the results of Hameed et al [36]. Mg/Al-LDH has a significant buffering effect on acid solutions, which prevents the structure from being dissolved and degraded by acid and is beneficial to ion exchange [37]. Even if a small amount of Mg and Al ions are leached from Mg/Al-LDH, these metal ions can combine with phosphate to form insoluble magnesium phosphate and aluminum phosphate. These compounds can adhere to the surface of the adsorbent through electrostatic attraction, chemical bonding, or physical adsorption, further enhancing phosphate removal from the aqueous solution and improving the overall adsorption effectiveness [18]. Mg/Al-LDH can maintain a high adsorption capacity even in acidic environment. In neutral and mildly alkaline conditions, the positive charge density on the surface of Mg/Al-LDH decreases, reducing its electrostatic adsorption capacity. Phosphate in solution primarily exists as HPO42− or PO43−, both of which carry a greater negative charge. The interaction between hydrotalcite and phosphate is largely repulsive due to charge effects, limiting electrostatic adsorption. However, under these conditions, Mg/Al-LDH exhibits greater stability, allowing phosphate to still be efficiently adsorbed through ion exchange and surface complexation. As the pH of the solution continues to increase, the rising concentration of OH− ions begin to compete with phosphate, leading to a slight reduction in phosphate adsorption capacity.
FTIR spectra of Mg/Al-LDH before and after phosphorus adsorption is shown in Fig. 6(b). There is a strong peak of asymmetric stretching vibration of C-O at 1340 cm−1. Two diffraction peaks at 443 cm−1 and 623 cm−1 indicate the presence of metal-oxygen bonds. The strong peak at 443 cm−1 corresponds to the vibrational absorption of the layered metal skeleton (Me-O-Me) in hydrotalcite-like compounds. After the reaction, the intensity of this peak diminishes, suggesting a transformation from the original Me-O-Me structure to a Me-O-P structure due to the formation of metal complexes. This change includes both the coordination exchange between hydroxyl groups and phosphate via surface complexation, as well as the exchange of interlayer ions with phosphate. Following the reaction, a broad and intense absorption peak at 1040 cm−1, attributed to the bending vibration of P-O, indicates that phosphate is effectively adsorbed by Mg/Al-LDH and forms complexes with metal ions through surface coordination [38].
However, the absorption peaks observed at 3440 cm−1 and 1580 cm−1 are caused by the vibrations of hydroxyl groups formed through hydrogen bonding between interstitial water molecules and the layered structure of adsorbent or surface hydroxyl groups. After adsorption, the peak intensity of the stretching vibration at 3440 cm−1 increases, while the bending vibration peak at 1580 cm−1 exhibits a redshift. This indicates a stronger interaction between phosphate and surface hydroxyl groups (such as hydrogen bonding), enhancing the interaction between the material and water molecules or polar anions, thereby increasing the hydrophilicity of the material surface. As phosphate is a highly polar anion, it complements the hydrophilic nature of Mg/Al-LDH, which is further enhanced during the phosphate adsorption process. This confirms that the phosphate adsorption by Mg/Al-LDH is hydrophilic [38]. When the adsorbent surface is rough and hydrophilic, liquids spread more easily across its surface. This is due to the rough surface structure, which increases the actual contact area between the liquid and the solid, allowing the liquid to adhere better and penetrate the tiny pores and protrusions on the surface. As a result, the contact angle decreases, indicating improved wettability.
3.3. Analysis of the Mechanism and Economic Environment of the Whole Reaction Cycle
3.3.1. Reaction mechanism
Under the optimum reaction conditions, the ash in municipal sludge is mainly accumulated in the extraction residue. In addition, organic phosphorus within the residue is transformed into inorganic phosphorus, thus increasing the content of inorganic phosphorus and making it easier to extract with acid solution. This transformation process is due to the complexation of small molecular acids with aluminum ions and iron ions, resulting in the transformation of soluble salts of Al-P/Fe-P activated by Al-P/Fe-P. In addition, degradable solvent extraction is helpful to transform macromolecular organic acids existing in municipal sludge into smaller molecular forms. Consequently, during degradative solvent extraction, these small molecular organic acids play a crucial role in activating Al-P and Fe-P in the sludge through complexation. Subsequently, they react, convert into soluble phosphate, and accumulate in the residue, so that they can be extracted in acidic solution [40–41]. This process is illustrated as Eq. (5):
(5)
When Mg/Al-LDH adsorbed phosphate, the interface phenomenon involved many physical and chemical processes, including physical adsorption, anion exchange, surface complexation and so on, as shown in Fig. 7 (a). Mg/Al-LDH has a positively charged surface that can directly interact with negatively charged phosphate ions through electrostatic interactions, forming a stable adsorption state. This surface adsorption is primarily a physical process, where weak electrostatic forces may lead to reversibility. Ion exchange occurs when phosphate in solution replaces intercalated anions in hydrotalcite, allowing phosphate to enter the structure while releasing these anions into the solution. This reaction can alter the unit cell coefficients and interlayer spacing of hydrotalcite [42]. Surface complexation mainly happens on the outer surface of LDH, rather than in the interlayer structure. This chemical adsorption enhances LDH’s ability to immobilize phosphate. During ligand exchange, hydroxyl groups on the Mg/Al-LDH surface are replaced by phosphate, forming covalent bonds between the ligand and metal atoms. The resulting inner sphere complex consists of phosphate molecules bonded to one or two metal atoms via single or double oxygen bonds. In contrast, other anions, such as Cl− and NO3− epically form outer-layer complexes, with water molecules separating metal oxide ions from the surface. The inner sphere complex exhibits stronger interactions than the outer sphere complex, providing a significant driving force for phosphate adsorption and granting Mg/Al-LDH high selectivity for phosphate [43–45].
3.3.2. Economic and environmental analysis of the entire reaction cycle
In theory, 56.7 kg of inorganic phosphorus can be recovered from 1 ton of sludge. However, the system is capable of recovering over 73.5% of phosphorus, which is approximately 41.3 kg, demonstrating the feasibility of effectively separating phosphorus from sludge residue after extraction by degradation solvent and acid leaching. Take the treatment of 1 ton of dry sludge as an example, as shown in Fig. 7(b). The power consumption of the degradative solution extraction process is 116.7 kWh. Given that the unit price of electricity is $0.11 per kWh, the system’s use of 1-MN is sustainable since it is a green and recyclable material. The inactivated 1-MN can be purified through multistage distillation and reused dozens, or even hundreds, of times [46–47]. Assuming a life cycle of 15 cycles, the material cost is about $99.9, and the purification cost is about $11.6. The Mg/Al-LDH used in this study is a low-cost material that can be reused multiple times following adsorption and desorption processes [48], Based on a life cycle of 10 cycles, the material cost is about $ 40. The costs of sodium hydroxide and dehydrated sludge are negligible. For small and medium-sized sludge treatment plants, using two 10-cubic-meter reactors with a daily processing capacity of 80 tons, the straight-line depreciation method yields a depreciation cost of $0.5 per ton of sludge (with a depreciation period of 10 years). Equipment maintenance costs typically range from 5% to 10% of the equipment purchase cost, resulting in an estimated maintenance cost of $0.4 per ton of sludge. To meet operational needs, three operators may be required. Assuming a daily wage of $28 per operator, the labor cost per ton of sludge would be $1.1. Therefore, the total estimated cost per ton of dry sludge is approximately $167.0.
High purity phosphorus recovered from Mg-Al hydrotalcite can be used as a raw material to react with an iron source (ferrous sulfate) to generate iron phosphate. According to the latest market data, the price of iron phosphate, which is a key material in lithium iron phosphate batteries, can reach as high as $2,000 per ton [49]. The phosphorus recovered from 1 ton of sludge can be processed into 63.5 kg of iron phosphate, generating a profit of approximately $127. After accounting for the iron source and other costs, which total $33.6, the net income is $93.4. Additionally, 131.2 kg of bio-oil can be produced from 1 ton of dry sludge, providing an extra income of $103 at the current bio-oil price of around $786 per ton. The heat recovered during the degradation solvent extraction process can be used for steam production, with industrial steam priced between $10 and $50 per ton, resulting in a profit of $76.3. Therefore, the total economic benefit of recovering phosphorus from 1 ton of sludge using this method is approximately $272.7.
Nevertheless, it is essential to assess environmental protection throughout the entire reaction cycle from three perspectives: solvent recovery, waste gas treatment, and residual residue management. The organic solvent 1-MN used in the reaction provides high solubility and facilitates solvent recovery, thus realizing an efficient and environmentally friendly extraction process. Any gas produced during solvent extraction can be absorbed and neutralized using acid or alkali solutions. The acid leaching residue, after phosphorus recovery, can be repurposed as building materials, reducing the amount destined for landfill. Even when these by-products are generated, they are easier to treat and recycle, supporting environmental protection and sustainable development. Additionally, even if the adsorption capacity of Mg/Al-LDH finally weakens to the point of complete failure and produces secondary waste, it can be recovered and reused in several ways. For instance, magnesium and aluminum of the spent can be extracted from used hydrotalcites by acid dissolution. These metals can be recovered through electrolysis or other methods and could also be used as additives for cement or concrete. Therefore, the spent Mg/Al-LDH can be recovered and reused in many ways, which is helpful to reduce the environmental burdens related to waste disposal [50–51]. Recovered phosphorus can be utilized in agriculture, catalysts, electrode materials, and other applications to enhance resource utilization and support the development of a circular economy. Additionally, the heat generated during degradative solvent extraction is used to produce steam, and the energy from the treatment process is maximized.
4. Conclusions
Phosphorus-rich extraction residue is successfully separated from sludge. After degradative solvent extraction, the TP and IP contents in the residue increased to 63.1 and 69.8 mg/g, respectively.
Mg/Al-LDH demonstrated a significantly stronger adsorption capacity compared to other adsorbents, being 8 times more effective than activated alumina and 1.7 times more effective than bentonite.
At a dosage of 16 g/l, the adsorption equilibrium time was 90 seconds, with phosphorus concentrations ranging from 528 to 145 mg/l in an acidic environment, achieving a recovery rate of up to 99%. The influence of the pH value is not obvious.
The interface phenomenon of Mg/Al-LDH for phosphorus adsorption involves many physical and chemical processes, including physical adsorption, anion exchange and surface complexation.
Choosing modified adsorbents with low cost and high cycle durability can effectively reduce recovery costs. In addition, combining phosphorus recovery with the production process of high-value products is another promising future research field.
Acknowledgments
Financial support from the Natural Science Foundation of Anhui Province (no. 2108085ME161), and Anhui Province University Excellent Talents Training Funding Project (no. gxyqZD2021108) are gratefully acknowledged.
Notes
Author Contributions
R.F. (graduate student) collects and analyzes relevant documents and writes manuscripts. L.Z. (professor) reviews and supervises. F.D. (professor) reviewed and edited the manuscript.
Conflict-of-Interest Statement
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Fig. 1
Process diagram of municipal sludge degradative solvent extraction, phosphorus enrichment and recovery system in residue.
Fig. 2
(a) Standard curve of phosphate solution. (b) Changes in the extraction solution inside the tube after adding different additives. (c) The effect of different additives on phosphorus recovery in extraction solutions.
Fig. 3
Pore size distribution diagram of Mg/Al-LDH.
Fig. 4
(a) Changes in adsorption capacity and recovery rate with dosage. (b) Effect of initial concentration on adsorption.
Fig. 5
(a) Changes in adsorption capacity and recovery rate over time. (b) Adsorption kinetics of Mg/Al-LDH. (c) The XRD of the Mg/Al-LDH before and after adsorption.
Fig. 6
(a) Adsorption capacity and recovery rate of phosphorus in extraction solution by Mg/Al-LDH at different initial pH levels. (b) The FTIR of the Mg/Al-LDH before and after adsorption.
Fig. 7
(a) The mechanisms for the adsorption of phosphate onto Mg/Al-LDH. (b) Economic and environmental analysis of the whole reaction period
Table 1
Proximate and ultimate analyses, along with the phosphorus content, of sewage sludge, solutes, and residues