Method for preparing carbon dots from phosphoric acid industrial waste harmless treatment agent

Abstract

The invention discloses a method for preparing carbon dots by using a phosphoric acid industrial waste harmless treatment agent, which comprises the following steps: S1, dissolving soluble phosphorus in ardealite by using a citric acid aqueous solution as an extracting agent, converting insoluble calcium phosphate into soluble phosphorus through the reaction of citric acid and phosphate ore, extracting, and filtering to obtain filter residues and filtrate; s2, calcining the filter residues in the step S1 to obtain a calcined product; s3, adding the calcined product into the filtrate obtained in S1, extracting lattice phosphorus in filter residues, and filtering after extraction to obtain filtrate and filter residues; and S4, mixing the filtrate in S3 with straw powder for reaction, filtering to obtain filtrate containing the carbon dots, filtering the filtrate with a micro-filtration membrane, and freeze-drying to obtain the carbon dots. According to the method, closed-loop coupling of phosphorus recovery-carbon dot synthesis is completed, and seamless connection of almost complete harmless treatment of industrial wastes and recycling synthesis of the wastes is realized.

Description

Technical Field

[0001] The present invention relates to the fields of harmless treatment of industrial phosphorus and preparation of carbon dots, and particularly to a method for preparing carbon dots with a harmless treatment agent for industrial phosphoric acid waste. Background Art

[0002] Carbon quantum dots (CQDs) may have antibacterial effects on bacteria by disrupting the cell membranes of bacteria, interacting with DNA or proteins, generating reactive oxygen species (ROS), etc. As an environmentally friendly nanomaterial, the application of carbon quantum dots in the antibacterial field helps to reduce the use of traditional antibiotics and heavy metal antibacterial agents, thereby reducing the impact on the environment. The preparation methods of carbon quantum dots are simple and include two categories: top-down and bottom-up. Currently, the carbon dots prepared by conventional methods have the disadvantages of low yield and low antibacterial activity.

[0003] Phosphogypsum is a by-product in the wet process for preparing phosphoric acid, and its main component is CaSO 4 ·2H 2 O, which is an important renewable gypsum resource. It is estimated that the global cumulative emissions of phosphogypsum are about 6 billion tons and are increasing at a rate of 150 million t / year. The phosphorus impurities contained in phosphogypsum are soluble phosphorus, eutectic phosphorus and insoluble phosphorus, and their contents are relatively large and will have a greater impact on the properties of the recycled gypsum. If the phosphorus flows into the natural environment, it will cause excessive reproduction of aerobic microorganisms in the water body, resulting in a large number of aquatic organisms dying due to lack of oxygen, the water quality becoming turbid and deteriorating, and serious eutrophication of the water body. Currently, the harmless treatment methods of phosphogypsum include water washing method, neutralization method, acid leaching method, flotation method and roasting method. Their mechanisms are either to remove soluble phosphorus or to convert it into insoluble substances to prevent harm. First of all, these methods cannot completely and thoroughly treat all the phosphorus in phosphogypsum. In addition, these methods generally have defects. For example, water washing and acid leaching consume a large amount of water and produce a large amount of acid-containing wastewater, the flotation and roasting methods have high energy consumption, and the treatment methods such as the neutralization method that convert into insoluble substances still have certain hazards due to the slow hydrolysis reaction of the insoluble substances. Therefore, the harmless treatment of phosphogypsum without generating three wastes is an urgent problem to be solved in the industry. In addition, mineral resources such as phosphate rock are usually considered non-renewable resources, and the reserve of mineral resources in China is weak, and the growth of reserves of major mineral resources is weak. Therefore, reprocessing and recycling industrial and agricultural wastes such as phosphogypsum by innovative means meet the national carbon neutral development goal and green economic development goal. Summary of the Invention

[0004] To solve the above problems, the object of the present invention is to provide a method for preparing carbon dots with a harmless treatment agent for phosphoric acid industrial waste. After reacting citric acid with phosphogypsum, the mixture is subjected to calcination - extraction - carbon dot preparation treatment, realizing the closed - loop coupling of "phosphogypsum treatment - carbon dot generation", solving the harm and difficult - to - treat problems brought by phosphogypsum, a phosphoric acid industrial waste, and capable of preparing carbon dots with high antibacterial properties.

[0005] The present invention is realized through the following technical solutions:

[0006] A method for preparing carbon dots with a harmless treatment agent for phosphoric acid industrial waste, comprising the following steps: S1. Using an aqueous citric acid solution as an extractant to dissolve the soluble phosphorus in phosphogypsum, and at the same time, through the reaction of citric acid with phosphate rock, converting insoluble calcium phosphate into soluble phosphorus. After extraction, filter to obtain a filter residue and a filtrate; S2. Calcining the filter residue in S1 to obtain a calcined product; S3. Adding the calcined product to the filtrate obtained in S1 to extract the lattice phosphorus in the filter residue. After extraction, filter to obtain a filtrate and a filter residue; S4. Mixing and reacting the filtrate in S3 with straw powder, filter to obtain a filtrate containing carbon dots, and freeze - dry the filtrate after filtering with a microfiltration membrane to obtain carbon dots.

[0007] The present invention innovatively organically applies the phosphorus recovery and carbon dot preparation processes together, completing the closed - loop coupling of "phosphorus recovery - carbon dot synthesis", realizing the seamless connection of almost completely harmless treatment of industrial waste and waste reuse synthesis, with a clever concept. During the phosphorus extraction and production process, phosphogypsum, a phosphoric acid - generated waste with a huge cumulative emission, is used as a material to extract phosphorus for the preparation of highly antibacterial carbon dots with straw as the raw material. The utilization of waste phosphorus further improves the utilization rate of natural phosphorus resources in limited mineral resources. At the same time, this method realizes the harmless treatment of phosphogypsum (i.e., eliminating phosphorus pollution) and the improvement of the properties of regenerated gypsum. The extractant containing soluble phosphorus does not need to be separated, recovered, or subjected to any other treatment, and is directly used for the preparation of target products. The phosphorus extraction process in the present invention fully considers phosphorus with different properties. First, an aqueous citric acid solution is used to extract soluble phosphorus, and through a metathesis reaction, the phosphorus in phosphate is dissolved and extracted. Then, high temperature is used to open the channel to lattice phosphorus, enabling the aqueous citric acid solution to reach the phosphate on the lattice and undergo a metathesis reaction with it to extract lattice phosphorus. Through the above operations, almost all of the phosphorus elements in phosphogypsum are extracted and harmlessly treated.

[0008] In S1, the mass concentration of the aqueous citric acid solution is 10% - 50%, and the mass ratio range of the aqueous citric acid solution to phosphogypsum is 2 - 50.

[0009] In S2, the calcination is carried out in stages: in the first stage, nitrogen is introduced, the calcination temperature is 100 - 400 °C, and the calcination time is 30 - 300 minutes; in the second stage, air is introduced, the temperature is 100 - 700 °C, and the calcination time is 30 - 300 minutes.

[0010] The calcined tail gas is introduced into the filtrate in S1. The purpose of staged calcination is to remove organic matter and prevent phosphorus oxidation, then promote lattice reconstruction to obtain porous properties, increase the specific surface area, and significantly improve the adsorption capacity of the calcined product for the extraction solution. Introducing the calcined tail gas into the filtrate in S1 forms a carbonic acid-citric acid solution mixed system, which further promotes the dissolution of phosphorus and can also improve the yield of carbon dots when manufacturing carbon dots subsequently.

[0011] The extraction temperature in both S1 and S3 is from room temperature to 90 °C, and the extraction time is 30 - 180 minutes.

[0012] The mass ratio of the filtrate to the straw powder in S4 ranges from 2 to 20, the reaction temperature is 120 - 200 °C, and the reaction time is 2 - 8 hours.

[0013] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0014] The present invention completes the closed-loop coupling of phosphorus recovery - carbon dot synthesis, realizes the seamless connection of almost completely harmless treatment of industrial waste and waste reuse synthesis. Using the filtrate after extracting phosphogypsum as a modifier for carbon dots, and doping phosphorus into the carbon dots with the extraction solution mainly containing phosphorus to improve the antibacterial property of the carbon dots. The total recovery rate of phosphorus is as high as over 90%, and the antibacterial rate of the carbon dots against Escherichia coli and Staphylococcus aureus is over 99%. Specific embodiments

[0015] To make the objectives, technical solutions, and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the embodiments. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0016] Example 1:

[0017] A 5% mass fraction aqueous solution of citric acid is mixed with a phosphogypsum powder sample (phosphorus content 0.41%) at a mixing ratio of 100 ml:5 g, stirred at 40 °C for 2 h, and then the mixture is filtered to obtain filter residue 1# and filtrate 1#.

[0018] Filter residue 1# is placed in a tube furnace at 120 °C for 2 h of calcination. After cooling, filtrate 1# is added, and it is stirred at 40 °C for 2 h and filtered again to obtain filter residue 2# and filtrate 2#. Filter residue 2# is the harmless phosphogypsum and can be used as a building material and a raw material for potassium fertilizer production. The total phosphorus recovery rate is 90%.

[0019] Weigh 5 g of self-made wheat straw powder with a mesh size of 50, mix it with 150 ml of filtrate 2#, place it in the inner liner of a high-pressure reactor with a capacity of 250 ml, stir for several minutes, seal it, and place it in an oven at 140 °C for 4 hours. After the reaction is completed and cooled to room temperature, filter the mixture. The filtrate is passed through a 0.2-μm water-based microfiltration membrane and then freeze-dried to obtain 125 mg of carbon dots, and the carbon dot yield is 2.5%.

[0020] Dissolve the carbon dots prepared in Example 1 in water to obtain a carbon dot solution with a concentration of 1.25 mg / mL. Test its antibacterial rate against Escherichia coli as 99.5% and against Staphylococcus aureus as 99.2% according to the "WS / T 650—2019 Hygiene Industry Standard of the People's Republic of China Evaluation Methods for Antibacterial and Bacteriostatic Effects".

[0021] Example 2

[0022] Mix an aqueous solution of citric acid with a mass fraction of 10% and a phosphogypsum powder sample (phosphorus content 0.41%) in a ratio of 100 ml:10 g, stir at 60 °C for 3 h, and then filter the mixture to obtain filter residue 3# and filtrate 3#.

[0023] Place filter residue 3# in a tube furnace at 150 °C, and pass the outlet gas of the tube furnace into filtrate 3#. Calcinate with nitrogen for 2 h and then with air at 650 °C for 2 h. After the calcination product is cooled, add filtrate 3#, stir at 60 °C for 3 h, and filter again to obtain filter residue 4# and filtrate 4#. The total phosphorus recovery rate is 96.2%. The calcination tail gas is passed into filtrate 3# to form tail gas recycling.

[0024] Weigh 5 g of self-made wheat straw powder with a mesh size of 50, mix it with 120 ml of filtrate 4#, place it in the inner liner of a high-pressure reactor with a capacity of 250 ml, stir for several minutes, seal it, and place it in an oven at 160 °C for 4 hours. After the reaction is completed and cooled to room temperature, filter the mixture. The filtrate is passed through a 0.2-μm water-based microfiltration membrane and then freeze-dried to obtain 210 mg of carbon dots, and the carbon dot yield is 4.2%.

[0025] Dissolve the carbon dots prepared in Example 2 in water to obtain a carbon dot solution with a concentration of 0.315 mg / mL. Test its antibacterial rate against Escherichia coli as 99.9% and against Staphylococcus aureus as 99.7% according to the WS / T 650—2019 industry standard.

[0026] Comparative Example 1

[0027] Weigh 5 g of self-made wheat straw powder with a mesh size of 50, mix it with 150 ml of water, place it in the inner liner of a high-pressure reactor with a capacity of 250 ml, stir for several minutes, seal it, and place it in an oven at 140 °C for 8 hours. After the reaction is completed and cooled to room temperature, filter the mixture, filter the filtrate through a 0.2-μm water-based microfiltration membrane, and then freeze-dry it to obtain 58 mg of carbon dots, and the yield of carbon dots is 1.16%.

[0028] Dissolve the carbon dots prepared by the method used in Comparative Example 1 in water to obtain a carbon dot solution with a concentration of 1.25 mg / mL. Test its antibacterial rate against Escherichia coli as 71% and its antibacterial rate against Staphylococcus aureus as 45% according to the "Evaluation Methods for Antibacterial and Bacteriostatic Effects - WS / T 650—2019 National Health Industry Standard of the People's Republic of China".

[0029] Compared with Comparative Example 1, the phosphorus-containing extraction solution in Example 2 has a complex composition, contains metal ions such as trivalent iron ions, which will affect the formation of the structure of the carbon dot reaction, and contains a carbonic acid-citric acid mixed solution, which has good catalytic performance. Therefore, the antibacterial property and yield of the carbon dots in Example 2 are significantly higher than those in Comparative Example 1.

[0030] The specific embodiments described above further elaborate on the purpose, technical solutions, and beneficial effects of the present invention. It should be understood that the above description is only the specific embodiments of the present invention and is not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing carbon dots using a phosphoric acid industrial waste harmless treatment agent, characterized in that: The following steps are involved: S1, using citric acid aqueous solution as an extractant to dissolve the soluble phosphorus in the phosphogypsum, and at the same time, through the reaction of citric acid and phosphate rock, converting insoluble calcium phosphate into soluble phosphorus, and filtering after extraction to obtain filter residue and filtrate; S2, calcining the filter residue in S1 to obtain a calcined product; S3, adding the calcined product to the filtrate obtained in S1, extracting the lattice phosphorus in the filter residue, and filtering to obtain the filtrate and the filter residue after extraction; S4, mixing the filtrate in S3 with straw powder for reaction, filtering to obtain a filtrate containing carbon dots, filtering the filtrate with a microfiltration membrane, and freeze-drying the filtrate to obtain carbon dots.

2. The method for preparing carbon dots using a phosphoric acid industrial waste harmless treatment agent according to claim 1, characterized in that: The mass concentration of the citric acid aqueous solution in S1 is 10% to 50%, and the mass ratio of the citric acid aqueous solution to phosphogypsum is in the range of 2 to 50.

3. The method for preparing carbon dots using a phosphoric acid industrial waste harmless treatment agent according to claim 1, characterized in that: The calcination in S2 is staged calcination: in the first stage, nitrogen is introduced, the calcination temperature is 100-400°C, and the calcination time is 30-300 minutes; in the second stage, air is introduced, the temperature is 100-700°C, and the calcination time is 30-300 minutes.

4. The method for preparing carbon dots using a phosphoric acid industrial waste harmless treatment agent according to claim 3, characterized in that: The tail gas after calcination is introduced into the filtrate in S1.

5. The method for preparing carbon dots using a phosphoric acid industrial waste harmless treatment agent according to claim 1, characterized in that: The extraction temperatures in S1 and S3 were both room temperature to 90°C, and the extraction time was 30 to 180 minutes.

6. The method for preparing carbon dots using a phosphoric acid industrial waste harmless treatment agent according to claim 1, characterized in that: The mass ratio of the filtrate to the straw powder in S4 is in the range of 2 to 20, the reaction temperature is in the range of 120 to 200° C., and the reaction time is in the range of 2 to 8 hours.