Microalgae-based phosphogypsum low-carbon phosphorus extraction method

Abstract

The invention provides a low-carbon phosphorus extraction method for phosphogypsum based on microalgae. Wastewater which is high in phosphorus concentration content and harmless to microalgae growth is obtained by optimizing phosphogypsum pretreatment, a microalgae culture system is constructed by controlling the concentration of the phosphorus-containing wastewater and the ratio of the phosphorus-containing wastewater to a culture medium BG-11, the purposes of low carbon and phosphorus extraction are achieved by controlling culture conditions, CO2 fixation and high-added-value biomass production are achieved at the same time, and the method is green, efficient and suitable for industrial production. The method has a wide application prospect in treating phosphogypsum solid waste and efficiently recovering phosphorus elements.

Description

Technical Field

[0001] The present invention relates to the field of solid waste treatment, and particularly to a low-carbon phosphorus extraction method for phosphogypsum based on microalgae. Background Art

[0002] The construction of ecological civilization in China has entered a new stage, with carbon reduction as the strategic focus, aiming to achieve the synergistic effect of pollution reduction and carbon reduction. The severe challenges brought about by global climate change have made reducing carbon emissions a task that all countries in the world must actively address. China has continuously strengthened and improved its carbon emission reduction policy system, and explored and implemented a series of innovative technologies and methods. As a clean and efficient carbon sequestration means, microalgae carbon sequestration technology converts carbon dioxide in the atmosphere into organic matter through the photosynthesis of microalgae, which not only improves the economy of the carbon sequestration process, but also realizes the recycling and utilization of carbon. Compared with traditional carbon sequestration methods, microalgae carbon sequestration technology has significant ecological and economic benefits and broad development prospects.

[0003] Phosphogypsum (PG) is a large amount of solid waste generated in the production process of wet-process phosphoric acid. Therefore, improving the resource utilization rate of phosphogypsum is one of the important ways to achieve the "dual carbon" goal. At present, there are still some studies on the treatment of wastewater by microalgae, but the research on directly using microalgae to recover phosphorus from phosphogypsum is still blank. There are significant drawbacks in using other treatment methods to recover phosphorus from phosphogypsum. For example, the chemical treatment method has the disadvantages of high cost, large resource consumption, and easy to cause secondary environmental pollution; the physical treatment method has low efficiency and high energy consumption; other biological treatment methods have a long treatment cycle and are greatly affected by the environment. Therefore, there is an urgent need for a low-carbon and efficient method for treating phosphorus in phosphogypsum. Summary of the Invention

[0004] In view of this, the present invention proposes a low-carbon phosphorus extraction method for phosphogypsum based on microalgae. By optimizing the pretreatment method of phosphogypsum and constructing a microalgae system, microalgae are used for low-carbon phosphorus extraction from the leaching waste liquid of phosphogypsum, so as to achieve the efficient recovery of phosphorus in phosphogypsum, CO2 fixation and the production of high-value biomass, and reduce the greenhouse gas emissions generated during the life cycle of microalgae, and finally achieve carbon-neutral operation.

[0005] The technical solution of the present invention is realized as follows:

[0006] In the first aspect, a low-carbon phosphorus extraction method for phosphogypsum based on microalgae is provided, which includes the following steps: S1, perform leaching pretreatment on phosphogypsum, filter, and the obtained supernatant is the leaching waste liquid of phosphogypsum, and measure the total phosphorus content in the leaching waste liquid;

[0007] S2, prepare the supernatant in step S1 into phosphogypsum washing liquid PG-W, mix it with the culture medium, and carry out microalgae culture;

[0008] S3. Harvest microalgae biomass and perform extraction of additional components.

[0009] Based on the above technical solutions, preferably, in S1, the pretreatment includes using a leaching agent and ultrasound, and the leaching agent includes one of HCl, H2SO4, and oxalic acid. Further preferably, the leaching agent is oxalic acid.

[0010] Based on the above technical solutions, preferably, in S1, the solid-liquid ratio of the phosphogypsum to the leaching agent is (0.8 - 1.2):20, the pretreatment time is 60 - 90 min, and the temperature is 37 - 42 °C. Further preferably, the solid-liquid ratio of the phosphogypsum to the leaching agent is 1:20, the pretreatment time is 90 min, and the temperature is 40 °C.

[0011] Based on the above technical solutions, preferably, in S1, the total phosphorus content in the leaching waste liquid is determined by the molybdenum antimony anti-colorimetric method. Further preferably, the absolute contents of available phosphorus, Al-P, Fe-P, Ca-P, and organic phosphorus in different phosphorus forms in the leaching waste liquid are determined by the chemical sequential extraction method.

[0012] The content of available phosphorus is determined by the molybdenum antimony anti-colorimetric method on a spectrophotometer; for the extraction of Al-P and Fe-P, the pH values of the leaching waste liquid need to be adjusted to 4.2 and 3.0 respectively. After fine-tuning with dilute hydrochloric acid or sodium hydroxide solution, 0.1 mol NaOH and 1 mol NH4Cl are added as leaching agents, and shaken in a constant temperature shaker at 25 °C for 2 hours with a rotation speed of 150 rpm to ensure the full release of metal-bound phosphorus. Subsequently, centrifuge at 4000 rpm for 10 minutes in a centrifuge to separate the solid residue and the phosphorus-containing supernatant, and the phosphorus content in the supernatant is determined by the molybdenum antimony anti-colorimetric method respectively; for the extraction of Ca-P, the pH value of the leaching waste liquid is adjusted to 8.0, 0.5 mol EDTA is added as a chelating agent, shaken at the same temperature and rotation speed for 1 hour to promote the dissolution of Ca-P. After the same centrifugation separation, the supernatant is taken and the phosphorus content is determined by the molybdenum antimony anti-colorimetric method; for the extraction of organic phosphorus, potassium persulfate is added to the leaching waste liquid, oxidized and decomposed in a high-pressure sterilizer at 121 °C for 30 minutes to ensure the complete conversion of organic phosphorus to inorganic phosphorus. After decomposition, cool to room temperature, transfer the solution to a volumetric flask for constant volume, and take the solution after constant volume to determine the inorganic phosphorus content by the molybdenum antimony anti-colorimetric method, which is the content of organic phosphorus.

[0013] Based on the above technical solutions, preferably, in S2, the phosphorus concentration of the PG-W ≤ 30 mg / L.

[0014] Based on the above technical solutions, preferably, in S2, the volume ratio of the PG-W to the culture medium BG-11 is (2 - 0):(2 - 4). Further preferably, the volume ratio of the PG-W to the BG-11 is 2:2. The components of the BG-11 culture medium are shown in Table 1:

[0015] Table 1 Composition of BG-11 medium

[0016]

[0017] Mix PG-W and BG-11 in a volume ratio of 2:2 as the culture medium for microalgae. Without additional carbon source, place it in an illumination incubator at 25±1°C, light intensity 40 μmol / (m 2 ×s), light-dark ratio 12h:12h, and initial pH = 7 for cultivation. Use the organic matter in the leaching solution for the autotrophic cultivation of microalgae. Regularly monitor the biomass, chlorophyll content, photosynthetic rate, and phosphorus absorption efficiency of microalgae, and adjust the cultivation conditions such as light intensity, temperature, and pH according to the monitoring results. Monitor the phosphorus absorption efficiency by analyzing the adsorption and transformation of phosphorus by extracellular polymeric substances (EPS), intracellular polymeric substances (IPS), and soluble microbial products (SMP).

[0018] Based on the above technical solutions, preferably, in S2, the microalgae cultivation further includes an aeration device, and the aeration intensity of the aeration device is 0.1 - 0.3 L / min. More preferably, the aeration intensity is 0.2 L / min to provide more air volume, and the CO2 fixation amount and O2 release rate are monitored in real time. Evaluate the carbon fixation efficiency by measuring the microalgae biomass production and the CO2 fixation efficiency. Adjust the parameters of the aeration device according to the carbon fixation effect to optimize the photosynthesis efficiency and biomass production of microalgae.

[0019] Based on the above technical solutions, preferably, in S2, the carbon fixation efficiency of the microalgae cultivation is to collect biomass up to 0.6 g / L / d, fix CO2 in the air at a rate of 0.9 g / d, and produce O2 at a rate of 6.3 g / d.

[0020] Based on the above technical solutions, preferably, in S3, the additional components include one or more of proteins, lipids, and polysaccharides. The extracted lipids are transformed by an enzymatic transesterification process to produce high-value products such as biodiesel. During the lipid transformation process, avoid using high-energy-consuming drying steps to reduce costs and improve production efficiency.

[0021] In the second aspect, there is provided the application of the above-mentioned microalgae-based low-carbon phosphorus extraction method from phosphogypsum in treating phosphogypsum solid waste.

[0022] The microalgae-based low-carbon phosphorus extraction method of the present invention has the following beneficial effects compared with the prior art:

[0023] (1) The present invention uses microalgae for low-carbon phosphorus extraction from phosphogypsum, which is green and efficient. While achieving phosphorus extraction and carbon fixation, high-value products of microalgae can also be obtained;

[0024] (2) The present invention optimizes the pretreatment of phosphogypsum, using the synergistic treatment of oxalic acid and ultrasound, with high phosphorus extraction efficiency and low fluorine content. The extraction efficiency of oxalic acid for phosphorus is significantly higher than that of other acids, and the phosphorus concentration in the resulting waste liquid is relatively high, facilitating subsequent resource recovery. Moreover, ultrasound improves the elution efficiency of phosphorus; oxalic acid can reduce the dissolution of fluorine (such as fluorapatite) in phosphogypsum within a limited leaching time, thus avoiding excessive fluorine concentration in the waste liquid from inhibiting the growth of microalgae. And compared with ordinary phosphorus-containing wastewater, the main components of the waste liquid obtained after leaching with oxalic acid are clear, the fluorine content is low, and the phosphorus concentration is stable, which can directly enter the resource utilization link;

[0025] (3) The present invention optimizes the ratio of PG-W and BG-11 to form an optimal scheme for cultivating microalgae for green carbon fixation. At the same time, by adding an aeration device, the carbon fixation ability of microalgae is further improved, significantly enhancing the efficiency of microalgae in phosphorus absorption, carbon fixation, and accumulation of biological algal protein, enabling microalgae to carry out low-carbon and efficient recovery of phosphorus resources in the phosphogypsum leaching solution, not only avoiding the problem of sludge treatment, but also fixing carbon dioxide, harvesting algal biomass, and bringing extremely high economic and social value. Description of the Drawings

[0026] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0027] Figure 1 It is the technical roadmap of the present invention. Detailed Embodiments

[0028] The following will combine the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0029] Example 1: Optimization and Screening of Leaching Solution

[0030] Water, HCl, H2SO4, citric acid, and oxalic acid were used as eluents respectively, with a solution concentration of 0.1 M. Phosphogypsum and the eluent were mixed at a solid-liquid ratio of 1:20 (g / mL), and under the conditions of 40 °C and ultrasonic treatment for 90 min, the eluent was allowed to fully contact with phosphogypsum. After the elution, the eluate was separated from the phosphogypsum by filtration. The total phosphorus (TP) content in the separated eluate was detected by ammonium molybdate spectrophotometry. The total phosphorus content in the eluate after elution with different eluents was recorded, and the results are shown in Table 2:

[0031] Table 2 Total phosphorus content in the eluate after elution with different eluents

[0032] Water HCl <![CDATA[H2SO4]]> <![CDATA[H3BO3]]> Citric acid Oxalic acid Total phosphorus content (mg / L) 3 84 86 4 1.5 118

[0033] The comparison of the above experimental results proves that under the same concentration, the extraction rate of phosphorus by oxalic acid is significantly higher than that of water and inorganic acids (HCl, H2SO4, H3BO3), which is convenient for subsequent resource recovery;

[0034] In addition, the fluoride ion content in the eluate when water and oxalic acid were used as eluents was also measured, and the measured results were 92.47 mg / L and 23.85 mg / L respectively. It can be seen that the fluoride ion content in the eluate after elution with oxalic acid decreased significantly by 74%, which is harmless to subsequent microalgae cultivation. Therefore, compared with ordinary phosphorus-containing wastewater, the waste liquid obtained after elution with oxalic acid has a stable phosphorus concentration (reaching 118 mg / L), a low fluoride ion content, a low impurity content, and clear main components (Ca 2+ , PO4 3- , CO4 2- , etc.), and can directly enter the resource recovery link.

[0035] Example 2: Low-carbon phosphorus extraction by microalgae

[0036] On the basis of high extraction rate, fluorine in phosphogypsum (such as fluorapatite) dissolves less within a limited oxalic acid leaching time because the dissolution of fluorapatite is a slow process that requires a long time to reach equilibrium, and oxalic acid leaching mainly targets metal-bound phosphorus in phosphogypsum (such as Al-P and Fe-P). The complexation of oxalic acid with metal ions takes precedence over the direct reaction with fluorapatite. Therefore, using oxalic acid as a leaching solution to treat phosphogypsum can avoid excessive fluorine concentration in the waste liquid from inhibiting the growth of microalgae. The realization of low-carbon phosphorus extraction by microalgae is mainly achieved through culturing microalgae with the leaching waste liquid of phosphogypsum. The main evaluation indicators are as follows: the water purification effect during the microalgae culture process, the changes in the physiological and biochemical indicators of microalgae, and the differences in the phosphorus content of SMP (soluble microbial products), EPS (extracellular polymeric substances), and IPS (intracellular polymeric substances) in the leaching solutions of low and high concentrations of phosphorus under the condition that microalgae can self-sufficiently utilize the organic carbon within the system without additional organic carbon sources. The main phosphorus storage spaces for microalgae to treat phosphorus-containing wastewater are EPS and IPS. IPS and EPS are polymer-rich matrices, including proteins, carbohydrates, and other organic components, located inside and outside the cells. Soluble microbial products (SMP) are soluble organic compounds released during normal biomass metabolism. By monitoring the changes in the phosphorus content in the phosphorus-containing wastewater and microalgae, a dynamic balance is formed.

[0037] Scenedesmus was used as the target strain, and the initial strain was pre-cultured in BG-11 liquid medium until the logarithmic growth phase. The phosphogypsum solid was mixed with 0.1 mol / L oxalic acid solution at a solid-liquid ratio of 1:20 (g / mL), ultrasonically pretreated at 40 °C for 90 minutes (ultrasonic power 200 W, frequency 40 kHz), centrifuged after standing, and the supernatant was taken as PG-W for standby. The following phosphorus-concentration wastewaters were prepared by diluting or concentrating PG-W: Group 1: 78.9 mg / L; Group 2: 47 mg / L; Group 3: 27.7 mg / L; Group 4: 3.5 mg / L.

[0038] The Scenedesmus in the logarithmic growth phase was inoculated into conical flasks with different concentrations of PG-W at an initial biomass concentration of 0.2 g / L. In this experiment, 2.0 L Erlenmeyer flasks were used as photobioreactors, equipped with a light incubator (light intensity 40 μmol / (m 2 ·s), light-dark cycle 12 h:12 h, temperature 25 ± 1 °C), and the pH was adjusted to 7.0. The experimental period was 25 days. The Scenedesmus was cultured under the above conditions, and the phosphorus uptake efficiency was as follows: Group 1 56%; Group 2 94%; Group 3 99%; Group 4 99%.

[0039] When cultivating high-phosphorus-concentration wastewater (concentration of 78.9 mg / L) without additional organic carbon source, the main existing form of phosphorus is IP (inorganic phosphorus), and the main phosphorus storage space is EPS. At this time, the phosphorus uptake efficiency is 56%. When reducing the content of phosphogypsum leaching waste liquid (concentrations of 47 mg / L, 27.7 mg / L, and 3.5 mg / L respectively), the phosphorus form in the microalgae system gradually transforms into OP (organic phosphorus), and the phosphorus storage spaces are mainly EPS and IPS. At this time, the phosphorus absorption efficiencies are 94%, 99%, and 99% respectively. The comparison of the above experimental results proves that reducing the content of phosphogypsum leaching waste liquid helps to improve the phosphorus extraction efficiency, and when controlling the phosphorus concentration ≤ 30 mg / L, the highest efficiency of microalgae assimilating phosphorus can be achieved, reaching the best phosphorus extraction performance.

[0040] Example 3: Optimization of the ratio of PG-W and BG-11

[0041] Using Scenedesmus as the target strain, the initial strain was pre-cultured in BG-11 liquid medium until the logarithmic growth phase. Mix the phosphogypsum solid with 0.1 mol / L oxalic acid solution at a solid-liquid ratio of 1:20 (g / mL), perform ultrasonic pretreatment at 40 °C for 90 minutes (ultrasonic power 200 W, frequency 40 kHz), let it stand and then centrifuge, take the supernatant, and prepare wastewater with a phosphorus concentration (≤ 30 mg / L) as PG-W by diluting or concentrating the supernatant, and set the following experimental groups and control groups:

[0042] Experimental group 1: The volume ratio of PG-W to BG-11 is 0:4;

[0043] Experimental group 2: The volume ratio of PG-W to BG-11 is 2:2;

[0044] Experimental group 3: The volume ratio of PG-W to BG-11 is 1:3;

[0045] Control group 4: The volume ratio of PG-W to BG-11 is 4:0.

[0046] Inoculate the Scenedesmus in the logarithmic growth phase into a conical flask containing a mixed solution of BG-11 and PG-W with different ratios at an initial biomass concentration of 0.2 g / L. In this experiment, a 2.0 L Erlenmeyer flask was used as a photobioreactor, equipped with a light incubator (light intensity 40 μmol / (m 2 ·s), light-dark cycle 12 h:12 h, temperature 25 ± 1 °C), and the pH was adjusted to 7.0. The conical flasks of each group were placed in the light incubator for continuous culture for 25 days, and shaken regularly 3 times a day, 5 minutes each time. The Scenedesmus was cultured under the above conditions, and the biomass productivity is shown in Table 3:

[0047] Table 3 Biomass productivity of each experimental group

[0048] Experimental group 1 Experimental group 2 Experimental group 3 Control group 4 Biomass productivity (g / L / d) 0.25 0.4 0.23 0.19

[0049] The comparison of the above experimental results proves that when the volume ratio of PG-W to BG-11 is set at (2 - 0):(2 - 4), the effective conversion of microalgae biomass productivity can be achieved; when the volume ratio of PG-W to BG-11 is 2:2, the effect is the best, and the generated biomass productivity can reach up to 0.4 g / L / d. In addition, at this time, the efficiency of fixing CO2 in the air can reach 0.6 g / d, and the oxygen production rate is 4.2 g / d, which can ensure the normal operation of the life activities within the microalgae system cycle.

[0050] Example 4

[0051] Under the culture conditions of Example 3, an aeration device was added to the culture system of experimental group 2. An air pump was connected to a microporous aeration pipe, and the aeration intensity was set at 0.2 L / min and accurately controlled by a flow meter. At this time, the optimal biomass collected could reach 0.6 g / L / d, the efficiency of fixing CO2 in the air was about 0.9 g / d, and the oxygen production rate was 6.3 g / d. The above experimental results show that compared with the group without an aeration device, the carbon fixation ability of microalgae is improved, and it has a strong potential for absorbing CO2.

[0052] The high-value products for phosphorus extraction from microalgae mainly refer to microalgae protein. It was found in the experiment that the protein content of microalgae showed a significant concentration-dependent relationship with the content of BG-11 in the culture system. With the increase of the content of BG-11 in the system, the soluble protein content was higher than that in the single PG-W culture group. The synthesis of microalgae protein may be related to the nitrogen content in the system. The total nitrogen (TN) content in PG-W was only 6.8 mg / L, under the environmental condition of N element limitation, and the TN content in BG-11 was about 35 mg / L. Therefore, with the increase of the content of BG-11, it is more conducive to the accumulation of protein. The results show that in the culture groups with BG11 added, the protein accumulation gradually increases, while the protein content decreases after 15 days of single PG-W culture; after 25 days of culture, compared with BG-11, the protein content in the single PG-W aqueous solution decreased by 62.41%, while the culture systems with the volume ratio of PG-W to BG-11 of 2:2 and 1:3 promoted the accumulation of protein content, increasing by 36.08% and 44.18% respectively, and the best protein content could reach 70 mg / gFW. In addition, substances such as lipids with a content of 36% and polysaccharides with 100 mg / gFW could also be obtained.

[0053] It can be seen that when microalgae are exposed to PG-W for a short time, the impact on protein production is relatively small. Under nitrogen-deficient limiting conditions, there is no obvious difference in the protein content in microalgae organisms. However, when microalgae are exposed to a single PG-W environment for a long time, the microalgae cells are severely damaged, and their life metabolic activities are severely restricted. The accumulation of protein is significantly inhibited compared with the control group, which further verifies the feasibility of the microalgae treatment of phosphogypsum wastewater in this invention. By optimizing the volume ratio of BG-11 to PG-W, the efficiency of phosphorus absorption, carbon fixation, and accumulation of biological algal protein by microalgae can be significantly improved, enabling microalgae to efficiently recover phosphorus resources with low carbon in phosphogypsum leachate. This not only avoids the problem of sludge treatment but also fixes carbon dioxide and harvests algal biomass, bringing extremely high economic and social value.

[0054] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A low-carbon phosphorus extraction method from phosphogypsum based on microalgae, characterized in that, It includes the following steps: S1. Perform leaching pretreatment on phosphogypsum, filter, and obtain the supernatant; S2. Prepare the phosphogypsum washing solution from the supernatant in step S1, mix it with the culture medium, and conduct microalgae cultivation; S3. Harvest microalgae biomass and perform extraction of additional components.

2. The microalgae-based low-carbon phosphorus extraction method from phosphogypsum according to claim 1, wherein In S1, the leaching agent for leaching includes one of HCl, H2SO4, and oxalic acid.

3. The method for low-carbon phosphorus extraction from phosphogypsum based on microalgae according to claim 2, characterized in that, The leaching agent is oxalic acid.

4. The method for low-carbon phosphorus extraction from phosphogypsum based on microalgae according to claim 2, wherein, In S1, the solid-liquid ratio of the phosphogypsum to the leaching agent is (0.8 - 1.2):20, the leaching time is 60 - 90 min, and the temperature is 37 - 42 °C.

5. The microalgae-based low-carbon phosphorus extraction method from phosphogypsum according to claim 1, wherein, In S2, the phosphorus concentration of the phosphogypsum washing solution is ≤ 30 mg / L.

6. The microalgae-based low-carbon phosphorus extraction method from phosphogypsum according to claim 1, wherein In S2, the culture medium includes BG-11, and the volume ratio of the phosphogypsum washing solution to BG-11 after mixing is (2 - 0):(2 - 4).

7. The microalgae-based low-carbon phosphorus extraction method from phosphogypsum according to claim 1, wherein, In S2, the microalgae cultivation also includes an aeration device, and the aeration intensity of the aeration device is 0.1 - 0.3 L / min.

8. The method for low-carbon phosphorus extraction from phosphogypsum based on microalgae according to claim 1, characterized in that, In S2, the carbon sequestration efficiency of the microalgae cultivation is that the biomass collection reaches 0.6 g / L / d, the efficiency of fixing CO2 in the air reaches 0.9 g / d, and the rate of generating O2 reaches 6.3 g / d.

9. The method for low-carbon phosphorus extraction from phosphogypsum based on microalgae according to claim 1, characterized in that, In S3, the additional components include one or more of protein, lipid, and polysaccharide.

10. Application of the microalgae-based low-carbon phosphorus extraction method from phosphogypsum according to any one of claims 1 - 9 in the treatment of phosphogypsum solid waste.

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