A bimetallic composite coagulant and its application in simultaneous removal of arsenic and fluorine pollutants

Abstract

The application discloses a bimetallic composite coagulant and application thereof in simultaneous removal of arsenic and fluorine pollutants. The composite coagulant is a bimetallic composite coagulant of titanium salt and zirconium salt. The bimetallic composite coagulant is prepared by a sol-gel method with organic titanium salt, organic zirconium salt, triethanolamine, ethanol and hydrochloric acid as raw materials. The obtained bimetallic composite coagulant can simultaneously remove trivalent arsenic and fluoride pollutants in water under the conditions of pH value of 3.5-5.0 of the water body and dosage of 0.4-1.5 mM, and can simultaneously reduce the concentrations of the two pollutants to below the safety limit values of drinking water, and the residual metal in the coagulation effluent can be ignored.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of drinking water treatment, in particular to a bimetallic composite coagulant and its application in simultaneous removal of arsenic and fluorine pollutants. BACKGROUND

[0002] Groundwater arsenic and fluorine composite pollution seriously affects the health of the Chinese people, especially threatening the drinking water safety of rural residents. In the "Drinking Water Health Standards" (GB5749-2022) of China, it is clearly stipulated that the mass concentration of arsenic and fluorine should be less than 0.01 mg / L and 1.0 mg / L, respectively.

[0003] Titanium salt and zirconium salt have been widely studied as a substitute coagulant for aluminum salt and iron salt, which can avoid the disadvantages brought by the use of aluminum salt and iron salt alone. Compared with traditional coagulants, titanium salt and zirconium salt have the following advantages: they can work in a wide pH range, avoiding the cumbersome adjustment of acid and alkalinity; the floc grows fast, the volume is large, and the settling time is short, which is conducive to the design of more compact settling tanks and the combination with subsequent treatment units; the residual metal concentration in the coagulation effluent is much lower than that of aluminum and iron salt coagulation.

[0004] Titanium and zirconium salts show excellent coagulation performance in arsenic and fluoride pollution control, respectively. Titanium salt coagulants can effectively remove As(III) without pre-oxidation to As(V). In the pH range of 4.0-10.0, the As(III) removal rate of Ti(SO4)2 is higher than that of Fe2(SO4)3(Lakshmanan D, Clifford D A, Samanta G. Arsenic coagulation with iron, aluminum, titanium, and zirconium salts. Water Research Foundation, 2009; Sun Y, Zhou G, Xiong X, et al. Enhanced arsenite removal from water by Ti(SO4)2 coagulation. Water Research, 2013, 47(13): 4340-4348). However, titanium salt coagulants perform poorly in fluoride removal(Zhang J, Brutus T E, Cheng J, et al. Fluoride removal by Al, Ti, and Fe hydroxides and coexisting ion effect. Journal of environmental sciences, 2017, 57: 190-195). Zirconium, which is in the same main group as titanium, can effectively remove As(III) and As(V) and exhibits better fluoride removal performance in the acidic pH range, with very low residual metal concentrations in the coagulation effluent(Gan Y, Wang X, Zhang L, et al. Coagulation removal of fluoride by zirconium tetrachloride: performance evaluation and mechanism analysis. Chemosphere, 2019, 218: 860-868).

[0005] Chinese patent application CN202311400712.8 discloses a defluorination agent for fluoride-containing wastewater treatment and a preparation method thereof. The defluorination agent is prepared by using chitosan as the biomolecular skeleton and magnesium salt, titanium salt, zirconium salt, aluminum salt, iron salt, rare earth material, and organic material as the blending raw materials. However, the material obtained by simply mixing multiple metal salts cannot fully exert the maximum effect of titanium salt and zirconium salt as coagulants. It is necessary to reasonably control the hydrolysis of titanium salt and zirconium salt to fully exert their respective potential in deep pollutant removal.

[0006] Chinese patent application CN202210444894.8 discloses a method for removing fluorine in a neutral or weakly acidic solution, which prepares a titanium-zirconium composite adsorbent for removing fluorine in a neutral or weakly acidic solution, and for the coagulation process, controlling the hydrolysis of titanium-zirconium is the key to obtaining a soluble composite coagulant. SUMMARY

[0007] To solve the above problems, the present application provides a dual-metal composite coagulant and its application in simultaneously removing arsenic and fluoride pollutants, which can reduce the concentrations of trivalent arsenic and fluoride pollutants to below the drinking water standard limit, and the residual metal concentration in the coagulated water is negligible.

[0008] The dual-metal composite coagulant and its application in simultaneously removing arsenic and fluoride pollutants are realized as follows:

[0009] Firstly, the present application provides a dual-metal composite coagulant, and the preparation method thereof includes the following steps:

[0010] (1) Triethanolamine is added to an ethanol solution and stirred until completely dissolved, denoted as solution A;

[0011] (2) Organic titanium salt and organic zirconium salt are added to solution A in a certain molar ratio, and solution B is obtained after stirring;

[0012] (3) A mixture of deionized water and hydrochloric acid is added to solution B and stirred to obtain solution C;

[0013] (4) Solution C is dried at a certain temperature by rotary evaporation to obtain a solid composite coagulant.

[0014] The organic titanium salt is one or both of isopropyl titanate and tetrabutyl titanate.

[0015] The organic zirconium salt can be one or both of zirconium n-butylate and zirconium n-propylate.

[0016] The molar ratio of the organic titanium salt to the organic zirconium salt is (0.25-4.0):1.

[0017] The molar ratio of triethanolamine to the total amount of metal is 0.125:1.

[0018] The molar ratio of the total amount of water added to the total amount of metal is 4.0:1.

[0019] The molar ratio of hydrochloric acid to the total amount of metal is 1:1.

[0020] The temperature of rotary evaporation is 60℃.

[0021] Secondly, the application also provides application of the above-mentioned double-metal composite coagulant in simultaneous removal of arsenic and fluorine pollutants. The specific treatment method is as follows: the pH value of the water body is adjusted to 3.5-5.0, and the double-metal composite coagulant is added in an amount of 0.4-1.5 mM to simultaneously remove trivalent arsenic and fluoride in the water body.

[0022] The application is based on development of the titanium-zirconium double-metal composite coagulant. Compared with the conventional direct alkali polymerization, the double-metal titanium-zirconium composite coagulant with high polymerization degree is prepared by simultaneously controlling the hydrolysis polymerization process of titanium salt and zirconium salt through the sol-gel method, so that the removal capacity of the titanium salt and the zirconium salt on trivalent arsenic and fluoride can be fully exerted. The double-metal titanium-zirconium composite coagulant is applied to simultaneous removal of arsenic and fluorine pollutants. Compared with the existing treatment method, the application can simultaneously reduce the concentration of trivalent arsenic and fluoride in the water body to below the standard limit value of drinking water under the optimized pH value and dosage, and the residual concentration of metal after coagulation can be ignored. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a scanning electron microscope (SEM) image and an XPS full spectrum image of the double-metal composite coagulant in Example 1;

[0024] Figure 2 is a schematic diagram of arsenic and fluorine pollutant co-removal performance evaluation of the double-metal composite coagulant (ZTXC) and the traditional coagulant (polyaluminum chloride PAC, polymeric ferric sulfate PFS) in Example 2, (a) fluoride removal condition; (b) trivalent arsenic removal condition;

[0025] Figure 3 is a schematic diagram of residual metal concentration in the coagulation effluent after arsenic and fluorine pollutant co-removal of the double-metal composite coagulant (ZTXC) and the traditional coagulant (polyaluminum chloride PAC, polymeric ferric sulfate PFS) in Example 2. DETAILED DESCRIPTION

[0026] The application will be further described below in conjunction with examples, and it should be noted that the scope of the application is not limited to the contents disclosed in the following examples.

[0027] The reagents involved in the examples are commercially available unless otherwise specified.

[0028] Preparation and characterization of the double-metal composite coagulant in Example 1

[0029] The organic titanium salt in the application is one or both of isopropyl titanate and tetrabutyl titanate; the organic zirconium salt can be one or both of zirconium n-butanolate and zirconium n-propanolate. Since the butoxy group and the propoxy group have little effect on the hydrolysis polymerization process, different metal sources have no effect on the synthesis process. In the following, the commonly used zirconium n-butanolate and isopropyl titanate are used as raw materials, and the experimental results are equivalent to those of zirconium n-propanolate and tetrabutyl titanate. The double-metal composite coagulant is prepared as follows:

[0030] (1) 1.49-5.95 mmol of triethanolamine was added to 20.0 mL of ethanol solution, and stirred until completely dissolved, and was recorded as solution A;

[0031] (2) 9.52 mmol of zirconium n-butyl alcohol and 2.38-38.08 mmol of isopropyl titanate were added to solution A, and stirred to obtain solution B;

[0032] (3) A mixture of 0.12-0.48 mL of deionized water, 0.99-3.97 mL of 37% (mass concentration) hydrochloric acid and 10 mL of ethanol was added to solution B and stirred to obtain solution C, and the specific amount of addition was shown in Table 1;

[0033] In (1)-(3), the total amount of triethanolamine, water (including water in hydrochloric acid), and the total molar ratio of hydrochloric acid to metal were 0.125, 4.0, and 1.0, respectively.

[0034] (4) Solution C was dried at 60°C to obtain a series of solid composite coagulants by rotary evaporation.

[0035] The mass fraction of titanium / zirconium in the bimetallic composite coagulant prepared with different metal ratios was analyzed, and the results were shown in Table 1. A series of bimetallic composite coagulants (ZTXC) were synthesized by different molar ratios of zirconium and titanium (Zr:Ti = 4:1-1:4).

[0036] A certain amount of ZTXC coagulant was dissolved, and the metal content of Zr and Ti was determined by inductively coupled plasma spectrometer (iCAP7000). The Zr content was about 12-36%, and the Ti content was about 4-26%. The pH value of the solution of the ZTXC coagulant with a concentration of 1.0 g / L was about 2.30-2.65, indicating that the prepared ZTXC had a high degree of polymerization (Table 2). As shown in Table 2, the SEM image showed that Ti, Zr, O, Cl and C were distributed in the bimetallic composite coagulant, and C was mainly distributed on the surface of the coagulant solid, while Ti and Zr elements were uniformly distributed in the coagulant particles through O and Cl, indicating that the Ti and Zr composite copolymerization degree was high. Figure 1

[0037] Table 1 Content of various substances in the synthesis process of the bimetallic composite coagulant of the present application

[0038]

[0039] Table 2 Mass fraction of titanium / zirconium in the bimetallic composite coagulant and pH value of the 1.0 g / L stock solution

[0040]

[0041] ​Example 2 Evaluation of the co-removal performance of bimetallic composite coagulants for arsenic and fluorine pollutants

[0042] Using simulated water samples with a fluoride concentration of 2.0 mg / L and an As(III) concentration of 50.0 μg / L as experimental water samples, the co-removal performance of bimetallic composite coagulants at different dosages (0.1-1.5 mM) was evaluated under the condition that the initial pH of the solution was controlled at 5.0.

[0043] from Figure 2 The evaluation results show that the bimetallic composite coagulant (ZTXC) has a significantly better performance in removing arsenic and fluoride pollutants than traditional coagulants. Polyferric sulfate (PFS) and polyaluminum chloride (PAC) are insufficient to simultaneously reduce the residual levels of fluoride and As(III) pollutants to the drinking water standards of 1.0 mg / L and 10 μg / L, respectively. However, the bimetallic composite coagulant requires only a dosage of 0.4 mM to simultaneously control the residual levels of fluoride and As(III) pollutants below the drinking water standards, reducing the concentrations of fluoride and As(III) to 0.45 mg / L and 9.78 μg / L, respectively.

[0044] With increasing dosage, the residual levels of coexisting pollutants such as fluoride and As(III) are further reduced, reaching 0.06 mg / L and 2.55 μg / L respectively at a dosage of 1.5 mM.

[0045] In addition, such as Figure 3 As shown, no residual metals were detected after ZTXC coagulation, while PAC and PFS coagulated water still contained some residual metals, with PAC coagulation having a residual aluminum concentration as high as 0.5 mg / L.

[0046] The embodiments described above are merely illustrative of implementation methods of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications without departing from the concept of the present invention, and all such modifications fall within the protection scope of the present invention.

Claims

1. A method for preparing a bimetallic composite coagulant, characterized by, The method comprises the following steps: (1) adding triethanolamine into an ethanol solution and stirring until completely dissolved, denoted as solution A; (2) adding an organic titanium salt and an organic zirconium salt into solution A and stirring to obtain solution B; the organic titanium salt is one or both of isopropyl titanate and tetrabutyl titanate; the organic zirconium salt is one or both of zirconium n-butylate and zirconium n-propylate; (3) adding a mixed solution of deionized water, hydrochloric acid and ethanol into solution B and stirring to obtain solution C; (4) drying solution C by a rotary evaporation method to obtain a solid composite coagulant.

2. The method for preparing the bimetallic composite coagulant according to claim 1, characterized in that: The molar ratio of the organic titanium salt to the organic zirconium salt is (0.25-4.0):

1.

3. The method for preparing the bimetallic composite coagulant according to claim 1, characterized in that: The molar ratio of the triethanolamine to the total amount of metals is 0.125:

1.

4. The method for preparing the bimetallic composite coagulant according to claim 1, characterized in that: The molar ratio of the total amount of added water to the total amount of metals is 4.0:

1.

5. The method for preparing the bimetallic composite coagulant according to claim 1, characterized in that: The molar ratio of the hydrochloric acid to the total amount of metals is 1:

1.

6. The method for preparing the bimetallic composite coagulant according to claim 1, characterized in that: The temperature of the rotary evaporation is 60℃.

7. A bimetallic composite coagulant prepared by the method of any one of claims 1-6.

8. Use of the double metal composite coagulant according to claim 7 for simultaneous removal of arsenic and fluorine contaminants, characterized in that, The pH value of the water body is adjusted to 3.5-5.0, and the dosage of the bimetallic composite coagulant is 0.4-1.5 mM, so as to simultaneously remove trivalent arsenic and fluoride in the water body.

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