Municipal sludge biochar skeleton sodium alginate beads for removing phosphates

Abstract

The present application relates to the technical field of water treatment agent production, in particular to municipal sludge biochar skeleton sodium alginate beads for removing phosphate, the present application uses municipal sludge biochar as filler, SA as natural polymer carrier, Fe 2+ And La 3+ As cross-linking reagents, a new type of Fe / La modified biochar filled SA beads (SA-KBC-Fe / La) is prepared. The structure of the beads is characterized by various methods. The effects of different proportions, adsorbent dosage, solution pH value and ion coexistence on P adsorption are studied. In addition, the adsorption isotherm and adsorption kinetics are also studied. The adsorption mechanism of P is further verified by X-ray photoelectron spectroscopy (XPS). In addition, the potential distribution on the surface of the gel beads is also revealed in detail. The research results are expected to provide a new type of adsorbent for removing or recycling P, and provide a new strategy for resource recycling and disposal of municipal sludge.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of water treatment agent production, in particular to a municipal sludge biochar framework sodium alginate bead for removing phosphates. BACKGROUND

[0002] Phosphorus (P) is an important nutrient for plant growth, but excessive discharge of phosphorus can accelerate water eutrophication. The World Health Organization stipulates that the concentration of P in water should not exceed 0.5 mg / L. Therefore, effective removal of P in aquatic environments is a top priority and a key issue to minimize the impact of natural water quality deterioration.

[0003] Among various P removal methods, adsorption method is an effective method for removing P in aquatic ecosystems due to its simple operation and simple design. Common P removal adsorbents include activated carbon, mineral clay and biochar. Among them, biochar is widely used to remove P in wastewater due to its wide source, simple preparation and low cost.

[0004] Municipal sludge is a solid waste discharged from municipal wastewater treatment plants. According to statistics, the sludge production with a moisture content of 80% has exceeded 65 million tons in 2020, and it is estimated that the annual production of sludge in China will exceed 90 million tons in 2025, which can cause harm to the environment if not properly disposed. The existing treatment methods include sanitary landfill, incineration, building materials, etc. However, these methods have problems such as high cost, narrow application range, and possible secondary pollution. At the same time, municipal sludge contains a large amount of organic matter and microorganisms, which is an ideal raw material for preparing biochar, which can properly dispose of sludge and also recycle resources. Liang et al. studied the characteristics of sludge biochar at a pyrolysis temperature of 700℃ and its effect on P adsorption capacity. The results showed that the P adsorption capacity was 5.469 mg / g. Liu et al. prepared a biochar adsorbent from sludge for removing P and optimized the adsorption process. The results showed that under acidic conditions, the optimal adsorption capacity could reach 8.77 mg / g at a pyrolysis temperature of 400℃. Wu et al. activated and modified sludge-based biochar using HCl, HNO3 and NaOH to study its adsorption performance for dissolved organic matter in wastewater. It was found that the adsorption capacity of modified biochar was significantly higher than that of original biochar. However, the adsorption capacity of municipal sludge biochar is low, which makes it difficult to separate from water, which greatly limits its further application.

[0005] Sodium alginate (SA) is a natural organic substance extracted from brown algae, which is composed of β-D-mannuronic acid (M unit) and α-L-guluronic acid (G unit) monomers. Under acidic conditions, it can be chelated with polyvalent metal cations to form hydrogel through the tight connection of G units with metal cations. It is a promising P adsorbent with the characteristics of low cost, easy gelation and separation. Generally, calcium ions are used for cross-linking, but when the concentration of calcium chloride exceeds 2%, a dense cross-linked structure will quickly form on the surface of the gel, which will reduce its mechanical properties and lead to a decrease in adsorption capacity. To solve the above problems, the following two aspects can be considered: 1) Introducing organic / inorganic fillers. Fu et al. prepared a recyclable biochar microbead for adsorbing P in wastewater by combining biochar from hydrocotyle vulgaris with SA, and the removal rate of P can reach 87%. Wu et al. synthesized a new type of hydrogel bead adsorbent using SA immobilized diatomite. Its removal rate of P can reach more than 90%. 2) Using multiple metal ions for cross-linking. Kong et al. selected five kinds of polyvalent metal cations as cross-linking agents to prepare different SA microspheres, and studied the effect of adding metal cations on their performance. The results show that cross-linking of metal ions with different radii can improve the performance of SA. SUMMARY

[0006] In view of the deficiencies of the prior art, the present application provides a municipal sludge biochar framework sodium alginate bead for removing phosphate to solve the problems raised in the background art.

[0007] To achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0008] The municipal sludge biochar framework sodium alginate bead for removing phosphate is characterized by comprising the following preparation method:

[0009] Step one, first, 20g of municipal sludge passing through a 100-mesh sieve is placed in a quartz boat and pyrolyzed in a tube furnace under N2 conditions, and then kept at 300-400℃ for 1h; then after cooling to room temperature, it is washed with water and ethanol three times to remove impurities, and municipal sludge biochar (BC) is obtained; finally, it is dried at 60℃ for further use;

[0010] Step two, first, 10g of BC and 5g of KOH are thoroughly ground in a mortar for 30min, and then pyrolyzed in an N2 atmosphere furnace at 800-900℃ with a heating rate of 10℃ / min for 2h; then, after natural cooling to room temperature, the sample is neutralized with 1M HCl solution and stirred for 12h to remove soluble ash; finally, the KOH-activated sludge biochar (KBC) is washed with water and ethanol and then dried for use;

[0011] Step three, firstly, 1 g of sodium alginate (SA) was dissolved in 50 mL of water at 60℃, then 1 g of KBC was added to the above solution, ultrasonic for 1 h, magnetic stirring for 2 h to obtain a uniform suspension; then, the mixture was injected dropwise into a 4% (w / w) FeSO4·7H2O:LaCl3·7H2O (mass ratio = 1:2) solution using a 10 mL syringe, the formed gel beads were resolidified in the Fe / La solution for 24 h, then washed thoroughly with water and ethanol, and freeze-dried for 12 h to obtain municipal sludge biochar skeleton sodium alginate beads (SA-KBC-Fe / La).

[0012] Preferably, in step one, the heating rate of pyrolysis is 5℃ / min.

[0013] Compared with the prior art, the beneficial effects of the present application are: the present application uses municipal sludge biochar as filler, SA as natural polymer carrier, Fe 2+ and La 3+ as cross-linking reagents, and a new type of Fe / La modified biochar filled SA beads (SA-KBC-Fe / La) is prepared. The structure of the beads is characterized by various methods. The effects of different ratios, adsorbent dosage, solution pH and ion coexistence on P adsorption are studied. In addition, the adsorption isotherm and adsorption kinetics are also studied. The adsorption mechanism of P is further verified by X-ray photoelectron spectroscopy (XPS). In addition, the potential distribution on the surface of the gel beads is also revealed in detail. The research results are expected to provide a new type of adsorbent for removing or recycling P, and provide a new strategy for the resource recycling disposal of municipal sludge. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 SEM images of SA-Fe / La (a) and SA-KBC-Fe / La (b); N2 adsorption-desorption isotherms (c) and pore size distribution curves (d) of SA-Fe / La and SA-KBC-Fe / La;

[0015] Figure 2 (a) XRD and (b) Fourier transform infrared images of the samples; (c) effect of solution pH on P adsorption by SA-KBC-Fe / La and pHpzc diagram; (d) effect of coexisting ions on P adsorption by SA-KBC-Fe / La;

[0016] Figure 3 P removal capacity of SA-KBC-Fe / La under different Fe / La addition amounts;

[0017] Figure 4 Effect of dosage on the adsorption performance of SA-KBC-Fe / La;

[0018] Figure 5.298K, 308K and 318K, (a) pseudo-first order, (b) pseudo-second order, (c) intra-particle diffusion and (d) film diffusion model plots for P removal by SA-KBC-Fe / La;

[0019] Figure 6 .298K, 308K and 318K, (a) Langmuir, (b) Freundlich and (c) Sips model plots for P removal by SA-KBC-Fe / La;

[0020] Figure 7 Adsorption regeneration of SA-KBC-Fe / La;

[0021] Figure 8 (a) site adsorption energy and (b) site energy distribution for P adsorption by SA-KBC-Fe / La;

[0022] Figure 9 XPS spectra of (a) SA-KBC-Fe / La, (b) C 1s, (c) O 1s, (d) Si 2p, (e) Fe 2p, (f) La 3d and (g) P 2p before and after P adsorption; DETAILED DESCRIPTION

[0023] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present application.

[0024] Embodiment 1

[0025] Materials: Municipal sludge was collected from a wastewater treatment plant in Gansu Province, China. The sample was air-dried and ground to 100 mesh for use. All chemical reagents used in the experiment were of analytical grade and were used without further purification, including KH2PO4, (NH4)2MoO4, C6H8O6, C8H4K2OSb2, H2SO4, KOH, HCl, C6H7NaO6, FeSO4·7H2O, LaCl3·7H2O. Deionized water (Aike-Advanced-Ⅱ, Chengdu, China) was used throughout the experiment.

[0026] Preparation method: 20 g of municipal sludge sieved through 100 mesh was placed in a quartz boat and pyrolyzed in a tube furnace under N2flow (heating rate of 5 °C / min, 1 h at 350 °C), after cooling to room temperature, washed with water and ethanol three times, respectively, to remove impurities, and dried at 60 °C to obtain municipal sludge biochar (BC) for further use.

[0027] 10 g of BC and 5 g of KOH were thoroughly ground in a mortar for 30 min, and then pyrolyzed at 800 °C in a N2atmosphere furnace at a heating rate of 10 °C / min for 2 h. After natural cooling to room temperature, the sample was neutralized using 1 M HC1 solution and stirred for 12 h to remove soluble ash. Finally, the KOH-activated sludge biochar (KBC) was washed with water and ethanol and then dried for use.

[0028] 1 g of sodium alginate (SA) was dissolved in 50 mL of water at 60 °C. Then, 1 g of KBC was added to the above solution, ultrasonicated for 1 h, and magnetically stirred for 2 h to obtain a uniform suspension. The mixture was injected dropwise into a 4% (w / w) FeS04·7H20:LaCl3·7H20 (mass ratio = 1:2) solution using a 10 mL syringe. The formed gel beads were re-solidified in the Fe / La solution for 24 h, and then thoroughly washed with water and ethanol. After 12 h of freeze-drying, municipal sludge biochar skeleton sodium alginate beads (SA-KBC-Fe / La) were obtained.

[0029] Example 2

[0030] Characterization of SA-KBC-Fe / La: The surface morphology was studied using a scanning electron microscope (SEM, Regulus 8100, Hitachi, Japan). The specific surface area was recorded by physical adsorption / desorption at 77 K using a surface area analyzer (BET, ASAP2020, Micromeritics, USA). The crystal structure characteristics of the samples were characterized using an X-ray powder diffractometer (XRD, X’PRO, PANalytical, Holland) at 40 kV and 40 mA. The surface functional groups were observed by Fourier transform infrared spectroscopy (FTIR, NEXUS6700, Thermo, USA). The molecular structure and valence state were analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB Xi250, Thermo, USA).

[0031] Example 3

[0032] Adsorption experiments: The adsorption behavior of SA-KBC-Fe / La was studied by batch adsorption experiments, including solution pH, adsorption time, initial P concentration and temperature. In the adsorption experiments, 50 mg of adsorbent was added to a conical flask containing 25 mL of P solution, which was placed in a constant temperature oscillation bed for dynamic adsorption at a speed of 120 rpm / min. All tests were repeated three times, and the results were expressed as the average value.

[0033] The effect of different solution pH on adsorption was studied. The pH of 100 mg / L KH2PO4 solution was adjusted to 2-12 using 0.1 M HCl and 0.1 M NaOH, respectively. Kinetic adsorption experiments were carried out at temperatures of 298 K, 308 K and 318 K for different time intervals of 5-1440 min; adsorption isotherm experiments were carried out for 24 h for different initial P concentrations (25, 50, 75, 100, 125, 150, 175 and 200 mg / L).

[0034] After the adsorption process, the residual concentration was measured by ultraviolet spectrophotometer (UV1280, SHIMAZU) at a wavelength of 700 nm, and the P content adsorbed by unit mass of adsorbent (q 2 ) and removal rate (η) were calculated according to the calibration curve (y = 0.0943x + 0.019, R e = 0.9991), formula (1) and formula (2):

[0035] q e = (C0-C e )V / m (1)

[0036] η = (C0-C e ) / C0x 100% (2)

[0037] Where q e (mg / g) represents the adsorption amount at equilibrium; C0(mg / L) and C e (mg / L) represent the P concentrations in the solution at the initial and equilibrium times, respectively; V (L) represents the solution volume; m (g) represents the mass of the adsorbent; and η (%) represents the removal rate.

[0038] Adsorption-desorption experiments were carried out in 0.01 M NaOH solution for 4 h: crosslinking in 4% metal salt solution with Fe:La = 1:2 (mass ratio) for 4 h.

[0039] Results

[0040] 1. Characterization of SA-KBC-Fe / La

[0041] The morphology of SA-Fe / La and SA-KBC-Fe / La was characterized by scanning electron microscopy, as shown in Figure 1(a) and (b). The SA-Fe / La surface was dense with coarse wrinkles, which might be due to the stability defect after crosslinking of pure alginate with Fe / La. After the addition of KBC, a large number of nano- to micro-sized layered pores could be clearly seen on the surface of the gel beads, which might be related to the increase in the number of inner-layer adsorption sites induced by the porous structure of the biochar.

[0042] To further detect the porous structure of SA-KBC-Fe / La, N2 adsorption-desorption isotherms were used to measure the specific surface area and pore size distribution. Figure 1 (c) and (d) show the graphs, and the specific data are shown in Table 1. From Figure 1 (c) it can be seen that according to the classification of the International Union of Pure and Applied Chemistry (IUPAC), both isotherms showed type IV isotherms with H3 hysteresis loops, indicating that the material had mesoporous characteristics of irregular pore structure. It was calculated that the BET specific surface area of SA-KBC-Fe / La was 54.22 m 2 / g, which was nearly 3 times that of SA-Fe / La. The mesoporous specific surface area of SA-KBC-Fe / La was 49.69 m 2 / g, which was also much higher than that of SA-Fe / La. This showed that the addition of KBC could greatly increase the specific surface area of the gel beads, which was beneficial to further adsorption of P. In addition, from Figure 1 (d) it can be seen that compared with SA-Fe / La, SA-KBC-Fe / La had similar pore volume and smaller pore size. The ionic radius of PO4 3- was about 0.077 nm, which might be more suitable for the pore size of SA-KBC-Fe / La.

[0043] Table 1 Specific surface area and pore size distribution of SA-KBC-Fe / La

[0044]

[0045] XRD was used to verify the crystal structure of the samples. As shown in Figure 2 (a), both BC and KBC had crystalline and amorphous structures, and the characteristic peak of graphitized carbon (002) could be seen at 2θ = 22°. Due to the influence of high-temperature carbonization and KOH activation, many characteristic peaks of BC became weak or almost disappeared compared with KBC. The disappearance of the SiO2 peak at 26.8° in BC was because SiO2 reacted with KOH. After crosslinking of Fe / La with SA, the peaks of CaCO3 (111) and (103) at 28.6° and 34° in BC were weakened, indicating the successful formation of the gel beads, thus forming an amorphous structure.

[0046] FTIR of KBC, SA-KBC-Fe / La and SA-KBC-Fe / La-P samples are shown in Figure 2 (b). It can be seen that the peak at 3420 cm -1 corresponds to the stretching vibration of -OH, the peak at 1064 cm -1 corresponds to the stretching vibration of Si-O-Si, which is due to the adsorbed water molecules or the hydroxyl structure of organic matter and SiO2 in municipal sludge. After cross-linking of Fe / La with SA, the peaks at 1415 cm -1 and 1632 cm -1 correspond to the symmetric and asymmetric stretching peaks of the -COO- group of SA, respectively. Compared with KBC, the peak of Si-O-Si at 1064 cm -1 is slightly reduced. The weak peak at 2925 cm -1 is due to the combination of cation Fe / La with the mannose-uric acid unit in SA, which restricts the stretching of -CH group. In addition, a new peak appears at 595 cm -1 which is related to the vibration of La-O group, indicating the formation of La-based inorganic compounds. The absorption peak at 458 cm -1 is the Fe-O vibration bond, which confirms the presence of iron oxide in the structure, indicating that Fe / La is successfully cross-linked in the SA-KBC-Fe / La gel beads.

[0047] 2. Batch adsorption experiments

[0048] 2.1 Effect of Fe / La ratio on P removal

[0049] Figure 3 The adsorption capacity of gel beads with different Fe / La ratios for P is shown. It can be seen from Figure 3 that the adsorption capacity of SA is only 20.67 mg / g at the same Fe / La ratio, which is much lower than that of SA-KBC-Fe / La, which shows that the addition of biochar can greatly improve its adsorption capacity for P. The adsorption capacity varies with the different Fe / La ratios. When Fe:La = 1:2, the adsorption capacity of the gel beads is the highest, reaching 43.19 mg / g, which shows that different Fe / La addition amounts may affect the degree of polymerization, leading to a decrease in adsorption capacity. Therefore, the ratio of Fe:La = 1:2 is used in the subsequent experiments.

[0050] 2.2 Effect of pH on P removal

[0051] The existence form of phosphate in water and the surface charge characteristics of hydrogel depend on the pH value of the solution. Figure 2(c) shows the effect of solution pH on the P adsorption capacity of SA-KBC-Fe / La. When the pH value increases from 3 to 6, the P removal efficiency of SA-KBC-Fe / La is up to 93.31%, while when the pH value increases from 7 to 11, the removal efficiency begins to decrease. In particular, at pH = 12, the removal efficiency is only 4.40%. Therefore, the maximum adsorption capacity at pH = 6 is 46.65 mg / g. The obvious effect of pH value is closely related to the variability of phosphate and the surface charge of the adsorbent.

[0052] Phosphate is a salt that is relatively sensitive to pH value. It has three dissociation constants (pK a1 = 2.13, pK a2 = 7.20 and pK a3 = 12.36), hydrolysis and ionization are carried out by equations (3)-(5), which means that phosphate exists in different ionic forms at different pH values, the main forms are as follows: H3PO4 (pH < 2.13), H2PO4 - (pH = 2.13-7.20), HPO4 2- (pH = 7.20-12.36) and PO4 3- (pH > 12.36). Similarly, the surface charge of the adsorbent is also usually affected by the change of the solution pH value, and the surface charge can be preliminarily calculated by the point of zero charge (pHpzc). When the pH value of the solution is less than pH pzc , the surface of the adsorbent is positively charged. If the pH value of the solution is higher than the pH pzc value, the surface of the adsorbent will be negatively charged.

[0053] The pH pzc value of SA-KBC-Fe / La is 6.5, which means that the surface is positively charged when the pH value is less than 6.5, and the pH pzc is negatively charged when the pH value is greater than 6.5. Under strong acidic conditions (pH < 2), the surface of SA-KBC-Fe / La is positively charged. H3PO4 and H2PO4 - are the main components of the phosphate solution. Therefore, it can be considered that when the pH value is less than 2, almost no H3PO4 is involved in the adsorption, but part of the negatively charged H2PO4 - will be removed from the aqueous phase under the electrostatic attraction of the positively charged adsorbent. In the pH value range (2-12) experiment, the main species when the pH value is 2-7 is H2PO4 - , and the main species when the pH value is 7-12 is HPO4 2- . When the pH value is 2-7, the -OH on the surface of the adsorbent is positively charged due to protonation. The positively charged -OH 2+An electrostatic attraction effect between the negatively charged phosphate oxyanions and the adsorbent occurs. However, at higher pH values (pH values greater than 8), the adsorbent will have more negative charges and form an electrostatic repulsion with the phosphate oxyanions. In addition, under alkaline conditions, OH - will also compete with HPO4 2- for the adsorption sites, resulting in a sharp decrease in the phosphate adsorption capacity.

[0054] PO4 3- + H2O = HPO4 2- + OH - (3)

[0055] HPO4 2- + H2O = H2PO4 - + OH - (4)

[0056] H2PO4 - + H2O = H3PO4 + OH - (5)

[0057] 2.3 Effect of adsorbent dosage

[0058] Figure 4 The effect of adsorbent dosage on the adsorption capacity is shown. As the adsorbent dosage increases, the P removal efficiency increases from 36% to 94%, while the adsorption capacity increases from 30 mg / g to 41 mg / g and then gradually decreases to 17.01 mg / g. The reasons can be as follows: 1) as the adsorbent dosage increases, the available adsorption sites increase, thus improving the adsorption efficiency; 2) the total number of available adsorption sites is limited under a certain adsorbent dosage. Therefore, the utilization rate of adsorption sites is higher at low dosage. However, due to the aggregation of adsorbent polymer particles, the actual adsorption capacity decreases with the increase of adsorbent dosage. Therefore, in the subsequent experiments, the dosage of the adsorbent is 50 mg.

[0059] 2.4 Effect of ionic strength

[0060] Under acidic conditions, the coexisting cations (such as Na + and Mg 2+ ) in natural water will produce an electrostatic repulsion with the gel beads. The coexisting organic substances, such as humic acid, have a molecular weight much higher than that of phosphate. Therefore, their competition effect is low, and we studied the four coexisting anions HCO3 - , SO4 2- , Cl - and NO3 - . The coexisting anions in water will compete with phosphate for the surface active sites on the adsorbent, thus reducing the adsorption capacity of phosphate. For example, Figure 2(d) as shown, the four coexisting anions have different effects on the adsorption of SA-KBC-Fe / La. The effect of SO4 2- >NO3 - >Cl - >Cl - >Cl - The presence of HCO3 - and HCO3 - has little effect on the adsorption of P. For NO3 2- , the amount of phosphate adsorbed is only reduced by about 5%. But SO4 2- has a significant negative effect on the adsorption of P. When the concentration of SO4 2- increases from 5 to 100 mg / L, the amount of P adsorbed decreases from 37.59 mg / g to 28.86 mg / g. This is probably due to the strong binding force of SO4 - to the adsorption sites. Studies have shown that the coexistence of SO4 - has a greater effect on the adsorption of P by SA-KBC-Fe / La than HCO3 - , NO3 2- and Cl 2- , because the ionic radius of SO4 - is similar to that of H2PO4 - , while the ionic radius of H2PO4

[0061] 2.5 Adsorption kinetics

[0062] To determine the adsorption process of P by SA-KBC-Fe / La gel beads, the pseudo-first-order (equation (6)), pseudo-second-order (equation (7)), Weber-Morris intra-particle diffusion (equation (8)) and film diffusion (equation (9)) models were applied to study the adsorption kinetics of the gel beads:

[0063] q t = q e [1-exp(-k1t)] (6)

[0064] q t = (k2q e 2 t) / (1+k2q e t) (7)

[0065] q t = k id ·t 0.5 +C i (8)

[0066] ln(1-F) = -k fd ·t (F = q t / q e ) (9)

[0067] where q t (mg / g) and q e (mg / g) are the adsorption capacity at contact time and equilibrium, respectively; k1(min -1 ) and k2(g / (mg-min)) are the pseudo-first-order constant and pseudo-second-order constant, respectively; t is the contact time; k id (mg / (g-min0.5)-1) and k fd are the rate constants of the intra-particle diffusion model and the film diffusion model, respectively; C i (mg / g) is the intra-particle diffusion model constant.

[0068] The effect of SA-KBC-Fe / La on P adsorption capacity was investigated by nonlinear fitting of the pseudo-first-order and pseudo-second-order models at temperature gradients of 298 K, 308 K and 318 K. The results are shown in Figs. Figure 5 (a), Figure 5 (b) and Tables 2 and 3. The diffusion trends at the three temperatures were basically similar, and the adsorption process could be divided into three stages, namely, stage 1, stage 2 and stage 3. Generally, in stage 1, the adsorbate was adsorbed on the outer surface driven by the concentration gradient between the solution and the adsorbent, which promoted the adsorbate to occupy the available sites on the outer surface in a fast and scalable manner, which was called film diffusion, and stages 2 and 3 were generally regarded as a joint process with high continuity, often referred to as intra-particle diffusion. Stage 1 experienced rapid adsorption, stage 2 experienced slow growth, and stage 3 was almost static. Within the initial 2 h, the adsorption speed was very fast. This was because there were a large number of adsorption sites on the surface of the composite material in the initial stage. As the adsorption time was prolonged, the adsorption rate decreased significantly within 2-12 h, indicating that most of the adsorption sites had been occupied. After 12 h, the adsorption capacity changed steadily and reached equilibrium within 24 h. At the three temperatures, the R 2 values (0.9913, 0.9906 and 0.9908) of the pseudo-second-order model were all higher than the R 2 values (0.988, 0.9676 and 0.9605) of the pseudo-first-order model, indicating that the pseudo-second-order model was more suitable for describing the P adsorption behavior of SA-KBC-Fe / La. Therefore, according to the assumption of the pseudo-second-order kinetic equation, the adsorption rate was mainly dominated by chemical adsorption.

[0069] As shown in Fig. Figure 5 (c), at the same temperature, the R 2 values of the three stages increased in turn with the progress of the reaction. Taking 298 K as an example, the R 2 value of stage 3 was 0.99475, which was greater than the R 2(0.98818 and 0.96864), indicating that the inner surface of SA-KBC-Fe / La has abundant adsorption sites. This is probably because the increase in temperature weakens the boundary resistance between SA-KBC-Fe / La and P, allowing more P molecules to pass through the boundary and eventually reside on the adsorption sites on the inner surface of SA-KBC-Fe / La. As the temperature increases, the R 2 from 0.98091 to 0.8901, indicating that membrane diffusion significantly reduces the adsorption rate and further affects the intraparticle diffusion.

[0070] While Figure 5 The adsorption data in (d) show a certain linear relationship, indicating that membrane diffusion plays a certain role in the adsorption process. Taking 298 K as an example, the diffusion rate constant K id,1 is 1.66301, which is higher than K id,2 and K id,3 (0.5922 and 0.2478), proving that the adsorption rate on the surface of SA-KBC-Fe / La is very fast, and the adsorption rate in stage 1 is much higher than that in stages 2 and 3. K id,2 The constant is 0.5922, at which point the adsorbate begins to diffuse into the pores of the inner surface of SA-KBC-Fe / La and adsorb to the active sites, indicating that there is no extra space for other adsorbent molecules to occupy, so these excess target substances penetrate into the internal space of the adsorbent. While the constant K id,3 (0.2478) is close to zero, indicating that P is difficult to diffuse into the micropores of the particles, at which point the adsorption reaches equilibrium.

[0071] Table 2 Kinetic parameters of SA-KBC-Fe / La for P removal at different temperatures

[0072]

[0073] Table 3 Intraparticle diffusion and membrane diffusion parameters of SA-KBC-Fe / La for P removal

[0074]

[0075]

[0076] 2.6 Adsorption isotherm

[0077] Langmuir and Freundlich isotherms are commonly used to describe adsorption behavior in liquid-solid reaction systems. Langmuir model (Equation (10)) is generally more suitable for explaining monolayer adsorption behavior, such as hydrogen bonding and precipitation, while Freundlich model (Equation (11)) considers multilayer adsorption behavior, such as electrostatic attraction and van der Waals adsorption. Sips model, also known as Langmuir-Freundlich model, is a semi-theoretical model that combines the advantages of Langmuir and Freundlich models (Equation (12)):

[0078] q e = q m K L C e / (1+K L C e ) (10)

[0079] q e = K F C e 1 / n (11)

[0080] q e = q m (K S C e ) m / (1+(K S C e ) m ) (12)

[0081] where C0(mg / L) and C e (mg / L) are initial and residual concentrations, respectively; q m (mg / g) is the maximum adsorption capacity; K L (L / g) is the Langmuir constant; K F (mg / g (L / mg)1 / n) and n are Freundlich constants; K S (L / mg) and m are Sips constants.

[0082] As Figure 6As shown in Table 4, most adsorption isotherms are regular and concave towards the horizontal axis. To determine the most suitable correlation model, we calculated the parameters by nonlinear fitting of the adsorption isotherms. During adsorption, the adsorption capacity of P increases with increasing P concentration. However, with increasing P concentration, they compete fiercely for a limited number of adsorption sites, leading to an equilibrium in the adsorption process. In the range of 298 K to 318 K, the equilibrium concentration decreases with increasing temperature, and at the same initial concentration, the adsorption capacity increases, indicating that endothermic adsorption promotes the reaction. The n parameter is very important in the Freundlich model. In Table 4, n>1 indicates favorable adsorption, and 1 / n<0.5 indicates a very favorable adsorption process. According to R... 2 The Langmuir model is better suited for adsorption data than the Freundlich model. In the Sips model, when n=1, the Sips model becomes the Langmuir model; when (K... S C e When 1 / n < 1, the Sips model becomes the Freundlich model. According to the data in the table, the Sips model is closer to the Langmuir model. This result is consistent with the fitting results of the Langmuir model discussed above.

[0083] Table 4. Isothermal parameters for P removal in SA-KBC-Fe / La

[0084]

[0085] 3. Adsorption and regeneration

[0086] like Figure 7 As shown, SA-KBC-Fe / La gel beads were regenerated five times at an initial P concentration of 100 mg / L to evaluate their adsorption and reuse capacity for P. Desorption of the sample was performed using 0.1 M NaOH solution, as the desorption rate of phosphorus-containing biochar is relatively stable at this concentration. Compared to the initial adsorption capacity of 43.19 mg / g, after five regeneration cycles, the adsorption capacity decreased to 35.48 mg / g, maintaining approximately 82% of the initial adsorption. The main reason for the decrease in recovery rate is likely related to the reduction in available adsorption sites, attributed to corrosion or adhesion of other components to the adsorbent and incomplete desorption of the adsorbent. However, after five regeneration cycles, SA-KBC-Fe / La still exhibits superior adsorption capacity and recovery rate compared to other adsorbents.

[0087] 4. Site energy distribution

[0088] Site energy distribution (SED) can provide specific information on the distribution of energy adsorption sites on the adsorbent surface. [57,60] According to the theory of non-uniform surfaces, the SED frequency function can be written as follows:

[0089]

[0090] where q e (C e ) is the total adsorption amount of the adsorbent on the heterogeneous surface; q h (E,Ce) is the homogeneous isotherm on the local adsorption site with adsorption energy E; F(E) is the site energy frequency distribution; E is the difference in adsorption energy of the solute and solvent on the adsorption site, which can be converted to equation (14)

[61] :

[0091] C e =C s exp[-(E-E s ) / RT]=C s exp(-E * / RT) (14)

[0092] where Cs is the maximum solubility of the adsorbent; E* is the difference in adsorption energy at Ce and Cs, respectively

[62] .

[0093] The Sips model is the most suitable for this adsorption process, so the parameters of the Sips model are combined with the SED function to obtain the following function for E*:

[0094] F(E * )=q m (K S C S ) 1 / n exp[-E * / (nRT)](nRT) -1 {1+(K S C S ) 1 / n exp[-E * / (nRT)]} -2 (15)

[0095] The area contained by the curve of F(E*) and the x-axis represents the maximum adsorption amount of the adsorbent. Em* represents the maximum distribution of the adsorbent surface energy points. The adsorbent surface can be divided into two energy points: high energy area and low energy area. The high energy area is distributed on the right side of the energy distribution graph, and vice versa for the low energy area. The calculation method of Em* value is as follows:

[0096] E m * =RTln(K S C S ) (16)

[0097]

[0098] Wherein, P(E*) is the percentage of binding sites with adsorption energies greater than Em*.

[0099] μ(E*) represents the affinity between adsorbents; the higher the value, the stronger the affinity. The weighted average can be expressed as follows:

[0100]

[0101] σe* represents the energy non-uniformity above the adsorbent.

[63] It can be calculated using formulas (19) and (20):

[0102] σ e * =[μ(E *2 )-μ(E * ) 2 ] 0.5 (19)

[0103]

[0104] The fitting results are shown below. Figure 8 The calculated data are shown in Table 5. Adsorption energy (E*) and adsorption capacity (q) e The changes in temperature (E*) show the trend of P adsorption capacity of SA-KBC-Fe / La. Increased temperature leads to increased adsorption energy, indicating that increased temperature promotes P absorption by SA-KBC-Fe / La. Generally, increased temperature reduces water viscosity, thereby reducing the resistance of the solid-liquid boundary layer. Furthermore, the solubility of P also increases with increasing temperature, and the driving force increases with increasing P concentration. Adsorption energy (E*) and adsorption capacity (q) are shown in the figure. e The inverse relationship between P and the energy level indicates that P preferentially occupies high-energy sites and then gradually diffuses to low-energy sites.

[0105] Table 5 shows that the Em* value varies between 13.5 kJ / mol and 15 kJ / mol at different temperatures, indicating that the maximum value of the energy point distribution increases with increasing temperature, and that solution temperature promotes the adsorption of P. The higher the temperature, the greater the adsorption: the reasons are as follows: 1) At temperature gradients of 293 K, 303 K, and 313 K, the solubility of P per 100 mL increases by 22.6 g, 28.0 g, and 33.5 g, respectively, at higher temperatures. Furthermore, the increased P solubility leads to a larger solid-liquid phase concentration difference, and coupled with a decrease in solution viscosity, more P molecules are filled onto the surface of the SA-KBC-Fe / La microspheres; 2) The higher the temperature, the larger the μ(E*) value, and the significantly increased affinity of P for SA-KBC-Fe / La, resulting in an increased P absorption.

[57] .

[0106] Table 5 Site adsorption energies related to SA-KBC-Fe / La adsorption of P

[0107]

[0108]

[0109] In addition, the carboxyl, hydroxyl and oxygen-containing functional groups contained in SA-KBC-Fe / La are also the cause of the uneven site energy. When the temperature rises from 298 K to 318 K, σe* decreases from 4.99 kJ / mol to 3.85 kJ / mol, indicating that the degree of heterogeneity is higher. Similarly, as the temperature rises, the percentage distribution of non-adsorption energy sites is also decreasing, which indicates that the adsorption of P on the surface of SA-KBC-Fe / La is affected by temperature. The higher the temperature, the stronger the activation effect on the adsorbent, and the greater the adsorption capacity.

[0110] 5. Adsorption mechanism

[0111] XPS is a qualitative and semi-quantitative analysis technique for analyzing chemical composition, chemical state and elemental

[64] In order to reveal the adsorption mechanism, XPS was used to characterize the samples before and after adsorption. Figure 9 The total spectrum and characteristic spectrum of C, O, Si, Fe, La and P are shown.

[0112] From Figure 9 As can be seen from (a), the atomic ratio of Si decreases significantly after adsorption, while the atomic ratio of other elements increases (Table 6). After adsorption, the intensity of the energy band at 134.0 eV increases significantly, because P is successfully adsorbed on SA-KBC-Fe / La. Compared with the baseline spectrum of P (133.70 eV), SA-KBC-Fe / La P 2p has a slight shift, suggesting that P forms solid Fe-O-P and La-O-P inner sphere complexes with Fe / La through Lewis acid-base interaction, thereby exhibiting strong ligand adsorption of P. The electron-rich phosphate oxygen anion acts as a Lewis base, coordinating with the empty orbital electron pair present in Fe / La, thereby forming an inner sphere complex.

[0113] From Figure 9 (b), it can be seen that the three independent peaks at 284.80, 286.51 and 288.51 eV belong to C-C, C-O-C and O-C=O, respectively. After adsorption, the position and shape of these peaks change significantly, indicating that there is a hydrogen bond interaction between C-O and HPO4 2- Figure 9 ​As can be seen in (c), the O 1s peaks at 530.82 eV, 531.97 eV, 532.80 eV and 532.5 eV correspond to O-La, C-O, C=O and C-O-Fe, respectively, which further confirms the presence of various oxygen-containing groups. The O-La and C-O-Fe peaks indicate that La and Fe have chelated with the surface functional groups of SA-KBC-Fe / La. From Figure 9 As can be seen in (f), the peaks at 835.97 eV and 852.12 eV correspond to the binding energy of La 3d5 / 2, and the peaks at 839.16 eV and 856.07 eV correspond to the binding energy of La 3d3 / 2, indicating that there is a strong affinity and interaction between P and La. This indicates that La-OH has reacted with H + La-OH2 + La(OH)3, and forms electrostatic attraction with P on the surface of the positive adsorbent.

[0114] From Figure 9 As can be seen in (d), the Si 2p peaks are organic Si, SiO2 and Si4 2- However, after the addition of P, all the peaks have shifted, indicating that Si-O and P have bonded. In addition, from Figure 9 As can be seen in (e), the iron compound is composed of Fe 2p3 / 2(714.58 eV, Fe2O3) and Fe 2p1 / 2(728.85 eV, FeOOH), and there may be complexation between iron(III) on SA-KBC-Fe / La and P groups. From Figure 9 As can be seen in (g), the binding energy of P 2p3 / 2 is 133.35 eV. After adsorption, the area of P 2p increased significantly from 15454.70 to 94634.79 (CPS.eV), indicating the formation of a strong chemical bond. According to the above results, P first migrates from the liquid phase to the solid phase surface through convective diffusion, and then interacts with the beads in the following ways:

[0115] 1) Electrostatic attraction. After the P ions come into contact with the beads, part of the P ions are quickly adsorbed on the surface of the beads under the action of electrostatic attraction, forming an outer sphere complex.

[0116] 2) Ligand exchange. The electron-rich Fe / La forms a chemical coordination bond with the oxygen-containing anion of P through electron donor-acceptor interaction, and P replaces -OH- to form a stable inner sphere complex.

[0117] Table 6 Atomic ratio of SA-KBC-Fe / La before and after adsorption

[0118]

[0119]

[0120] 6. Comparison with adsorbents of the same type

[0121] Table 7 compares the adsorption capacity of several adsorbents. The adsorption capacity of SA-KBC-Fe / La gel beads is higher than that of other natural matrix materials, carbon materials and metal-loaded mesoporous materials, because the Fe / La cross-linked KBC framework SA gel beads have a strong ability to form complexes with P. Therefore, SA-KBC-Fe / La gel beads have great application potential as P adsorbents.

[0122] Table 7 Comparison of P adsorption by adsorbents of the same type

[0123]

[0124] Conclusion A new type of efficient Fe / La cross-linked municipal sludge biochar framework SA gel beads was synthesized by a simple sol-gel method for the removal of P from water. The modification of Fe / La can enhance the mechanism of the gel beads, and the addition of biochar can make the beads form a layered porous structure with a larger specific surface area, thereby increasing the active adsorption sites and improving the adsorption performance. The adsorption process is mainly controlled by chemical adsorption, and the adsorption rate is mainly controlled by intraparticle diffusion, but is also controlled by heterogeneous adsorption. The adsorption mechanism of SA-KBC-Fe / La beads for P mainly includes electrostatic attraction and ligand exchange. The ion coexistence and adsorption regeneration cycle tests show that SA-KBC-Fe / La gel beads have good practical value. SA-KBC-Fe / La gel beads can solve the dual problems of P removal in water and final disposal of municipal sludge resources.

[0125] The embodiments of the application are described above with reference to the accompanying drawings, but the application is not limited to the specific embodiments described above, which are merely illustrative rather than restrictive, and a person of ordinary skill in the art can make many forms under the inspiration of the application without departing from the scope of the application and the scope protected by the claims.

Claims

1. Municipal sludge biochar skeleton sodium alginate beads for removal of phosphates, characterized in that, The preparation method comprises the following steps: Step one, first, 20 g of municipal sludge passing through a 100 mesh sieve was placed in a quartz boat and pyrolyzed in a tube furnace under N2, and was kept at 300-400 ℃ for 1 h; then after cooling to room temperature, it was washed with water and ethanol three times to remove impurities, to obtain municipal sludge biochar (BC); finally, it was dried at 60 ℃ for further use; Step two, first, 10 g of BC and 5 g of KOH were ground in a mortar for 30 min, and then pyrolyzed in an N2 atmosphere furnace at a heating rate of 10 ℃ / min to 800-900 ℃ for 2 h; then, after natural cooling to room temperature, the sample was neutralized with a 1 M HCl solution and stirred for 12 h to remove soluble ash; finally, the obtained KOH-activated sludge biochar (KBC) was washed with water and ethanol, and then dried for use; Step three, first, 1 g of sodium alginate (SA) was dissolved in 50 mL of water at 60 ℃, then 1 g of KBC was added to the above solution, and ultrasonic treatment was performed for 1 h and magnetic stirring was performed for 2 h to obtain a uniform suspension; then, the mixture was dropped into a solution of 4% (w / w) FeSO4·7H2O and LaCl3·7H2O with a 10 mL syringe, the mass ratio of FeSO4·7H2O to LaCl3·7H2O was 1:2, the formed gel beads were solidified in the Fe / La solution for 24 h, and then washed with water and ethanol, and then freeze-dried for 12 h to obtain municipal sludge biochar framework sodium alginate beads (SA-KBC-Fe / La).

2. The biosolids activated carbon framework sodium alginate bead for removing phosphates of claim 1, wherein: In step one, the heating rate of pyrolysis is 5 ℃ / min.

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