Cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater and preparation method of cerium-loaded tannin-based hydrogel adsorbent

Abstract

The invention discloses a cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater and a preparation method of the cerium-loaded tannin-based hydrogel adsorbent, vegetable tannin and polyvinyl alcohol react under a heating condition, tannin is coated in a hydrogel structure, the vegetable tannin is blended with cerium ions through a phenolic hydroxyl structure of the vegetable tannin to realize loading of cerium sites, and the cerium-loaded tannin-based hydrogel adsorbent is prepared under an alkaline condition. The method comprises the following steps: grafting triethylene tetramine containing rich amino groups to PVA (Polyvinyl Alcohol) by utilizing epichlorohydrin to realize ammonification of the adsorbent so as to improve an isoelectric point, thereby obtaining the vegetable tannin-based adsorbent. According to the method, leaching of metal ions is not involved, the method has the advantages of being environmentally friendly and low in cost, and the obtained adsorbent is good in adsorption effect, high in anti-interference capacity and capable of rapidly adsorbing and removing phosphorus in the presence of multiple ions.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparation of water treatment adsorbent materials, and particularly relates to a cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater and a preparation method thereof. Background Art

[0002] The removal of phosphorus in water by the adsorption method uses solid materials with many active sites, a large specific surface area, and a high affinity for phosphorus as phosphorus adsorbents, and combines phosphates on the surface of the adsorbent through physical or chemical adsorption to achieve the process of phosphorus removal. There are six adsorption mechanisms for phosphorus removal by the adsorption method: electrostatic attraction, ligand exchange (inner-sphere surface complexation), ion exchange (outer-sphere surface complexation), surface deposition, hydrogen bond interaction, and internal diffusion of the adsorbent. According to the adsorption mechanism, when designing an adsorbent, not only the chemical and physical properties of the adsorbent itself (such as specific surface area, porosity, surface functional groups) but also the influence of reaction conditions (such as pH value, coexisting anions) should be considered to improve the phosphorus removal effect.

[0003] The development of adsorbents is the core of the adsorption method. Currently, most adsorbents are iron-based and aluminum-based materials, and these adsorbents generally have problems such as insufficient adsorption capacity for phosphates and leaching of metal ions. Therefore, the prepared or used adsorbent should be biodegradable, easily available, environmentally friendly, and have a high adsorption capacity. Due to the advantages of rich resources, low cost, and low risk of secondary pollution, biopolymers have great application prospects in phosphorus removal. Based on this, the preparation of adsorbents from natural biomass materials for phosphorus removal has attracted more and more attention. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater and a preparation method thereof in view of the above-mentioned disadvantages existing in the prior art.

[0005] The technical solution for the present invention to solve the above technical problems is as follows: A cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater, the adsorbent comprising the following components: plant tannin, polyvinyl alcohol, and cerium ions.

[0006] The preparation method of the above-mentioned cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater comprises the following specific steps: S1. Disperse polyvinyl alcohol and plant tannin in deionized water, and stir well until completely dissolved under water bath heating conditions to obtain a first solution; S2. Slowly drop the first solution into the cross-linking agent solution with a syringe barrel to prepare a hydrogel, and cure and cross-link at a low temperature to obtain a hydrogel precursor; S3. After rinsing the hydrogel precursor obtained in step S2 several times with distilled water and soaking it, place it in a cerium salt solution, stir well and then let it stand, wash it with distilled water, and freeze-dry to obtain a cerium-loaded hydrogel; S4. Immerse the cerium-loaded hydrogel obtained in step S3 in a triethylenetetramine solution, adjust the pH of the solution to alkaline with NaOH solution, then add epichlorohydrin and stir well to mix. Finally, heat the mixed solution in a water bath to react and freeze-dry to obtain a cerium-loaded tannin-based hydrogel adsorbent.

[0007] The further limited solutions of the present invention are as follows: As an optional implementation mode, the plant tannin is one of gallotannin, mimosa tannin or ellagic tannin.

[0008] As an optional implementation mode, the mass ratio of polyvinyl alcohol to plant tannin is (2:1)~(6:1), and after mixing, stir for at least 30 min.

[0009] As an optional implementation mode, in step S2, the cross-linking agent is boric acid, and the volume fraction is 2%~5%.

[0010] As an optional implementation mode, in step S3, the cerium salt is one of water-soluble cerium chloride or cerium nitrate, and the cerium ion concentration is 4 g / L~6 g / L.

[0011] As an optional implementation mode, in step S4, the volume ratio of triethylenetetramine to epichlorohydrin is (1:1)~(3:1).

[0012] The beneficial effects of the present invention are as follows: In the present invention, plant tannin and polyvinyl alcohol are reacted under heating conditions, and the tannin is coated in the hydrogel structure to enhance the mechanical strength of the adsorbent. Through the phenolic hydroxyl structure of the plant tannin itself and the cerium ions are blended to achieve the loading of cerium sites ( Figure 1 A), strengthen the specific adsorption of phosphate. Under alkaline conditions, epichlorohydrin is used to graft triethylenetetramine rich in amino groups onto PVA to realize the ammoniation of the adsorbent, thereby increasing the isoelectric point ( Figure 1 B), and obtain the cerium-loaded tannin-based hydrogel adsorbent, which does not involve the leaching of metal ions, has the advantages of environmental friendliness and low cost, and the obtained adsorbent has good adsorption effect and strong anti-interference ability, and can quickly adsorb and remove phosphorus in the presence of various ions.

[0013] Different from traditional adsorbents, the cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater in the present invention anchors Ce in the adsorbent structure through the coordination and chelation of plant tannins with cerium ions, which can effectively avoid the migration and dissolution of metal ions during the adsorption process while achieving efficient adsorption; by using the ammonium positive potential points to provide a large number of positive charges, the isoelectric point of the adsorbent is significantly increased, and the pH application range is broadened; by utilizing the three-dimensional structure and water absorption and retention characteristics of the hydrogel, the mechanical strength of the adsorbent is enhanced while the adsorption capacity of the adsorbent is synergistically enhanced. The present invention is simple, fast and easy to operate.

[0014] Compared with traditional adsorbents, the cerium-loaded tannin-based hydrogel adsorbent prepared by the preparation method of the present invention has a specific surface area of 4.832 m 2 / g, and the cerium sites have only Ce 3+ One valence state. After ammoniation, the isoelectric point increases from 5.36 to 7.23, and the maximum adsorption capacity is 64.6 mg / g. The adsorbent has a relatively high isoelectric point, and can maintain a high adsorption efficiency for phosphorus within the pH range of 2 - 12. When added to the actual water body, the pH of the water body does not need to be adjusted directly for use, which reduces the cost. It has strong adaptability to pH and can maintain a relatively high adsorption capacity under alkaline conditions. The adsorption process conforms to the pseudo-second-order kinetics and the Langmuir isotherm model; through the exploration of the adsorption mechanism, it is found that the phosphorus removal mechanism of the adsorbent includes electrostatic attraction, coordination exchange, and internal diffusion of the adsorbent. Description of the Drawings

[0015] Attached Figure 1 is a reaction schematic diagram of a preparation method of a cerium-loaded tannin-based hydrogel adsorbent of the present invention; Attached Figure 2 is the SEM image of the cerium-loaded tannin-based hydrogel adsorbent before and after adsorbing phosphate in Example 1; Attached Figure 3 is the FTIR image of the cerium-loaded tannin-based hydrogel adsorbent before and after adsorbing phosphate in Example 2; Attached Figure 4 is the XPS image of the cerium-loaded tannin-based hydrogel adsorbent before and after adsorbing phosphate in Example 3. Detailed Embodiments Example 1

[0016] This example provides a cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater, and the adsorbent includes the following components: plant tannin, polyvinyl alcohol, and cerium ions.

[0017] The preparation method of the above-mentioned cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater includes the following steps: S1. Disperse polyvinyl alcohol and plant tannin in deionized water, and stir thoroughly under water bath heating conditions until completely dissolved to obtain the first solution. Among them, the mass ratio of polyvinyl alcohol to plant tannin is 6:1, and after mixing, stir for at least 30 min; S2. Slowly drop the first solution into the crosslinking agent solution with a syringe barrel to prepare a hydrogel, and cure and crosslink it at low temperature to obtain a hydrogel precursor. Among them, the crosslinking agent is boric acid, and the volume fraction is 5%; S3. After rinsing the hydrogel precursor obtained in step S2 several times with distilled water and soaking it, place it in a cerium salt solution, stir thoroughly and then let it stand. After washing with distilled water, freeze-dry to obtain a cerium-loaded hydrogel. Among them, the cerium salt is one of water-soluble cerium chloride or cerium nitrate, and the cerium ion concentration is 5 g / L; S4. Immerse the cerium-loaded hydrogel obtained in step S3 in a triethylenetetramine solution, adjust the pH of the solution to alkaline with NaOH solution, then add epichlorohydrin and stir to mix thoroughly. Finally, heat the mixed solution in a water bath to react, and freeze-dry to obtain a cerium-loaded tannin-based hydrogel adsorbent. Among them, the volume ratio of triethylenetetramine to epichlorohydrin is 3:1.

[0018] The adsorbent dosage is 0.04 g, and the adsorption amount of the adsorbent for phosphate is measured at oscillation times of 0, 3, 5, 30, 60, 120, 480, 720, and 1440 min respectively. Add 50 mL of phosphate solution (50 mg / L) to a 100 mL conical flask, and adjust the pH value of the solution to 3 with 1 mol / L HCl and NaOH. After oscillating the conical flask at 25 °C for 24 h, take a sample and measure the phosphate concentration in the solution.

[0019] Analyzing the experimental results shows that from Figure 2 it can be seen that as the contact time increases, the adsorption amount also increases until the adsorption sites are gradually occupied and the adsorption equilibrium is reached. The adsorption of phosphate by the adsorbent is mainly divided into a rapid stage, a slow stage, and an adsorption equilibrium stage. The rapid stage occurs within the first 30 min. The phosphate adsorption rate in this stage is very high because there are a large number of adsorption sites on the surface of the adsorbent, and at the beginning of adsorption, the concentration of phosphate is relatively high, so it is easy for the adsorbate and the adsorbent to collide. It is controlled by outer-sphere surface complexation (electrostatic attraction) within the first 30 min. The slow stage mainly occurs in the range of 30 - 120 min, mainly because the active sites on the surface of the adsorbent are gradually occupied, and due to the decrease in the phosphate concentration, the chance of collision and contact between the adsorbent and phosphate is reduced, thus reducing the adsorption rate. Adsorption gradually reaches equilibrium after 120 min. Example 2

[0020] This embodiment provides a preparation method of a cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater, including the following steps: S1. Disperse polyvinyl alcohol and vegetable tannin in deionized water, and stir well under water bath heating conditions until completely dissolved to obtain a first solution. Among them, the mass ratio of polyvinyl alcohol to vegetable tannin is 2:1, and stir for at least 30 min after mixing; S2. Slowly drip the first solution into the crosslinking agent solution with a syringe barrel to prepare a hydrogel, and cure and crosslink it at low temperature to obtain a hydrogel precursor. Among them, the crosslinking agent is boric acid, and the volume fraction is 5%; S3. After rinsing and soaking the hydrogel precursor obtained in step S2 several times with distilled water, place it in a cerium salt solution, stir well and then let it stand. After washing with distilled water, freeze-dry to obtain a cerium-loaded hydrogel. Among them, the cerium salt is one of cerium chloride or cerium nitrate that is easily soluble in water, and the cerium ion concentration is 4 g / L; S4. Immerse the cerium-loaded hydrogel obtained in step S3 in a triethylenetetramine solution, adjust the pH of the solution to alkaline with NaOH solution, then add epichlorohydrin and stir well to mix. Finally, heat the mixture in a water bath to react, and freeze-dry to obtain a cerium-loaded tannin-based hydrogel adsorbent. Among them, the volume ratio of triethylenetetramine to epichlorohydrin is 1:1.

[0021] Add 50 mL of a phosphate solution with a concentration of 50 mg / L to a 100 mL conical flask, and adjust the pH value to 3 with 1 mol / L HCl and NaOH solutions. The dosage of the adsorbent is set to 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 g / L. After shaking the conical flask at 25°C for 24 h, take samples and measure the phosphate concentration in the filtrate. Finally, obtain the adsorption capacity and phosphate removal rate of the adsorbent.

[0022] Analysis of the experimental results shows that from Figure 3 As can be seen from (a), the adsorption amount of the adsorbent is significantly increased after TETA loading, but the adsorption amounts of both decrease with the increase of the dosage. This is because when the phosphate content in the solution is certain, with the gradual increase of the dosage, the adsorption amount of phosphate per unit mass of the adsorbent gradually decreases. When the dosage is 0.2 g / L, the adsorption amount of the adsorbent for phosphate is 104.1 mg / g. When the dosage increases to 2.0 g / L, the adsorption amount of the adsorbent for phosphate is 27.5 mg / g. From Figure 3(b)It can be observed that as the dosage gradually increases, the removal rates of phosphate by both adsorbents show an increasing trend. However, the removal rate of phosphorus before TETA loading is always less than that after loading. As the dosage increases, the phosphorus removal rate gradually increases from 16.3% at the beginning to 95.8%, indicating that the adsorbent has a good removal effect on phosphate. In addition, the optimal dosage of the adsorbent is 0.8 g / L, while the optimal dosage before loading TETA is 1.4 g / L. This shows that after ammoniation, the dosage of the adsorbent decreases, which is beneficial to cost savings. Example 3

[0023] This example provides a preparation method of a cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater, including the following steps: S1. Polyvinyl alcohol and plant tannin are dispersed in deionized water and stirred thoroughly until completely dissolved under water bath heating conditions to obtain a first solution. Among them, the mass ratio of polyvinyl alcohol to plant tannin is 4:1, and after mixing, it is stirred for at least 30 minutes. S2. The first solution is slowly dropped into the cross-linking agent solution with a syringe barrel to prepare a hydrogel, and it is cured and cross-linked at low temperature to obtain a hydrogel precursor. Among them, the cross-linking agent is boric acid, and the volume fraction is 5%. S3. After the hydrogel precursor obtained in step S2 is rinsed several times with distilled water and soaked, it is placed in a cerium salt solution, stirred thoroughly, and then left standing. After washing with distilled water, it is freeze-dried to obtain a cerium-loaded hydrogel. Among them, the cerium salt is one of cerium chloride or cerium nitrate that is easily soluble in water, and the cerium ion concentration is 6 g / L. S4. The cerium-loaded hydrogel obtained in step S3 is added to a triethylenetetramine solution and soaked. The pH of the solution is adjusted to alkaline with NaOH solution, and then epichlorohydrin is added and stirred thoroughly to mix. Finally, the mixed solution is heated and reacted in a water bath and freeze-dried to obtain a cerium-loaded tannin-based hydrogel adsorbent. Among them, the volume ratio of triethylenetetramine to epichlorohydrin is 3:1.

[0024] Add 50 mL of phosphate solution (50 mg / L) to a 100 mL conical flask, and adjust the pH value of the solution to 2 - 12 with 1 mol / L HCl and NaOH. Add 0.04 g of the adsorbent. After shaking at 25°C for 24 h, take a sample and measure the phosphate concentration in the filtrate.

[0025] Analysis of the experimental results shows that Figure 4 as can be seen from (a), when pH = 3.0, the phosphate adsorption capacity is the largest, and the adsorbents reach 46 and 64.6 mg / g respectively. The change trends of the adsorption capacities of the two adsorbents with pH are basically the same, and a relatively high adsorption capacity is maintained within a wide pH range. Generally speaking, within the pH range of 2 - 12, the adsorbents maintain stable and efficient phosphorus removal performance. From Figure 4As can be seen from (b), the zero point charges of the adsorbent are 5.36 and 7.23 respectively, indicating that the isoelectric point of the adsorbent is increased after TETA modification, which is beneficial to the application of the adsorbent in natural water bodies. When the solution pH < 7.23, -OH and -NH2 on the surface of the adsorbent are protonated to obtain -OH2 + and -NH3 + , which act as adsorption positive points and promote the adsorption of negatively charged phosphate; when pH > 7.23, the surface of the adsorbent is negatively charged, which is not conducive to the removal of phosphate.

[0026] In addition to the above embodiments, the present invention may have other embodiments. All technical solutions formed by equivalent replacement or equivalent transformation fall within the protection scope required by the present invention.

Claims

1. A cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater, characterized in that: The adsorbent comprises the following components: plant tannin, polyvinyl alcohol and cerium ions.

2. The cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater according to claim 1, characterized in that: The plant tannin is a kind of gallotannin, black wattle tannin or ellagitannin.

3. A method for preparing a cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater according to any one of claims 1 to 2, characterized in that: The specific steps include: S1. Dispersing polyvinyl alcohol and plant tannins in deionized water, stirring thoroughly in a water bath until completely dissolved, to obtain a first solution; S2. slowly adding the first solution to the crosslinker solution using a syringe to prepare a hydrogel, and crosslinking and curing at low temperature to obtain a hydrogel precursor; S3. The hydrogel precursor obtained in step S2 is rinsed with distilled water several times and soaked, placed in a cerium salt solution, stirred thoroughly, and allowed to stand, washed with distilled water, and freeze-dried to obtain a cerium-loaded hydrogel; S4. The cerium-loaded hydrogel obtained in step S3 is added to a triethylenetetramine solution and soaked, and the pH of the solution is adjusted to alkaline with a NaOH solution, and then epichlorohydrin is added and stirred to mix thoroughly. Finally, the mixture is heated in a water bath for reaction, and freeze-dried to obtain a cerium-loaded tannin-based hydrogel adsorbent.

4. The method for preparing a cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater according to claim 3, characterized in that: In the step S1, the mass ratio of polyvinyl alcohol to plant tannin is (2:1) to (6:1), and the mixture is stirred for at least 30 minutes after mixing.

5. The method for preparing a cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater according to claim 3, characterized in that: In step S2, the cross-linking agent is boric acid, and the volume fraction is 2% to 5%.

6. The method for preparing a cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater according to claim 3, characterized in that: In the step S3, the cerium salt is a kind of cerium chloride or cerium nitrate which is easily soluble in water, and the cerium ion concentration is 4 g / L to 6 g / L.

7. The method for preparing a cerium-loaded tannin-based hydrogel adsorbent for treating phosphorus-containing wastewater according to claim 3, characterized in that: In step S4, the volume ratio of triethylenetetramine to epichlorohydrin is (1:1) to (3:1).

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