A method for efficiently recovering phosphorus from microalgae to prepare vivianite

Abstract

The present invention provides a method for efficiently recovering phosphorus from microalgae to prepare vivianite, belonging to the field of resource recovery and utilization of solid waste. The method comprises the following steps: (1) preparing microalgae biochar by slow pyrolysis, and guiding the change of phosphorus form by adding a NaHCO3 activator during the pyrolysis process; (2) using water or dilute acid as the leaching solution to leach the soluble phosphorus in the biochar to obtain a phosphorus-rich solution; (3) adjusting the pH of the phosphorus-rich solution to neutral, adding ferrous ions according to the molar ratio of Fe / P = 1.5 - 1.7, and shaking for 40 - 50 hours at room temperature to obtain vivianite precipitate. The present invention includes a three-step method of pyrolysis, leaching and crystallization. The recovered phosphorus is finally prepared into vivianite with economic value, providing a comprehensive solution to alleviate the global phosphorus crisis and the problem of disposal of algal sludge caused by algal blooms. It not only realizes the sustainable utilization of phosphorus resources in microalgae, but also reduces its negative impact on the environment.

Description

Technical Field

[0001] The present invention relates to the field of resource recovery and utilization of solid waste, and particularly to a method for efficiently recovering phosphorus from microalgae to prepare vivianite. Background Art

[0002] Phosphorus is an essential element to support life on Earth and human social production. However, at present, global phosphorus resources are facing a crisis of depletion. At the same time, harmful algal blooms caused by excessive phosphorus input frequently occur in fresh water and coastal waters around the world. In the situation where phosphorus shortage and surplus coexist, algae may play an important role in solving these problems.

[0003] Algae are a kind of biomass rich in phosphorus, and the phosphorus content in their bodies accounts for 0.1 - 5% of the dry weight of algae. The ability to effectively absorb and store phosphorus is a key competitive advantage of algae. They have acquired a complex mechanism called luxury phosphorus uptake during the process of evolution, which is an important determinant of harmful algal bloom. The phosphorus absorbed by algae is stored in the body for cell growth, and during the decline period of algae, it can be released into the overlying water, inevitably providing available phosphorus for the next round of algal blooms. Therefore, the absorption of phosphorus during the development of algal blooms and the release of phosphorus during the decline of algal blooms are important processes in the phosphorus biogeochemical cycle.

[0004] It is estimated that the maximum algal biomass generated in global harmful algal blooms is about 3.75×10 8 t / y. If harvested in time, 1.21×10 6 t of phosphorus will be withdrawn from the global elemental cycle every year in these algal biomasses. Currently, converting algal biomass into functional biochar has been considered an effective way to alleviate the impact of harmful algal crises. In this process, phosphorus is enriched from algae into biochar, providing a potential way for it to re - integrate into the natural circulation system. However, currently, people's attention to algal biochar mainly focuses on its application effects, while ignoring the huge phosphorus resources contained in it. Summary of the Invention

[0005] In view of the above problems existing in the prior art, the present invention provides a method for efficiently recovering phosphorus from microalgae to prepare vivianite. The present invention includes a three - step method of pyrolysis, leaching, and crystallization. The recovered phosphorus is finally prepared into vivianite with economic value, providing a comprehensive solution to alleviate the global phosphorus crisis and the problem of algal sludge disposal caused by algal blooms. It not only realizes the sustainable utilization of phosphorus resources in microalgae but also reduces its negative impact on the environment.

[0006] The technical solution of the present invention is as follows:

[0007] A method for efficiently recovering phosphorus from microalgae to prepare vivianite, comprising the following steps:

[0008] (1) Preparation of microalgae biochar: Microalgae biochar was prepared by slow pyrolysis, and the addition of NaHCO3 activator during pyrolysis was used to guide the change of phosphorus forms;

[0009] (2) Leaching of soluble phosphorus in microalgae biochar: Water or dilute acid was used as the leaching solution to leach the soluble phosphorus in the biochar to obtain a phosphorus-rich solution;

[0010] (3) Preparation of vivianite: The biochar was separated from the leaching solution, the pH of the phosphorus-rich solution was adjusted to neutral, ferrous ions were added according to the Fe / P molar ratio = 1.5 - 1.7, and vivianite precipitate was obtained by shaking at room temperature for 40 - 50 hours.

[0011] Preferably, before the slow pyrolysis in step (1), the microalgae were dehydrated and dried by air drying, steam drying or electrothermal blast drying.

[0012] Preferably, the temperature of the slow pyrolysis in step (1) was 400 - 800 °C; the heating program was: rising from room temperature to the set temperature at a rate of 5 - 15 °C / min and maintaining for 120 - 150 min; the gas atmosphere was N2.

[0013] Preferably, the addition amount of the NaHCO3 activator in step (1) was 20 - 50 wt% of the microalgae mass.

[0014] Preferably, when using water or dilute acid as the leaching solution to leach the soluble phosphorus in the biochar in step (2), the ratio of the leaching solution to the biochar was 100 - 150 mL / g, and the leaching time was 4 - 24 h.

[0015] Preferably, the dilute acid in step (2) included dilute HCl or dilute H2SO4.

[0016] More preferably, the concentrations of the dilute HCl and dilute H2SO4 were 0.1 - 0.4 mol / L respectively.

[0017] Preferably, the NaOH solution with a concentration of 0.8 - 1.2 mol / L was used to adjust the pH of the phosphorus-rich solution to 7 in step (3).

[0018] Preferably, the source of the ferrous ions in step (3) included ferrous sulfate heptahydrate or ferrous chloride tetrahydrate.

[0019] More preferably, the shaking at room temperature in step (3) was in an anaerobic environment.

[0020] The beneficial technical effects of the present invention are as follows:

[0021] 1. The present invention prepares microalgae biochar through slow pyrolysis, promotes the transformation of organic phosphorus, pyrophosphate, and polyphosphate in Chlorella into soluble inorganic phosphorus during the pyrolysis process by adjusting the type and doping amount of co-survival activators, and extracts the soluble phosphorus in the biochar using water or dilute acid as the leaching solution to obtain a phosphorus-rich solution. Further, ferrous ions are added to the phosphorus-rich solution, and high-purity vivianite precipitate is obtained under room temperature oscillation.

[0022] The above steps provide optimal conditions for the transformation and extraction of phosphorus forms in microalgae. Under the combined action of these conditions, various forms of phosphorus in microalgae can be maximally transformed into orthophosphate, which is then dissolved in water and acidic solutions and finally recovered in the form of vivianite, achieving the maximum utilization of resources.

[0023] 2. In the prior art, adding activators during pyrolysis is to obtain porous materials with well-developed pore structures and large specific surface areas, so that the materials have excellent properties in adsorption, catalysis, energy storage, etc. However, the present invention screens activators according to the change of phosphorus forms during pyrolysis to maximally transform various forms of phosphorus in microalgae into inorganic phosphorus for better extraction in the next step.

[0024] Specifically, the type and doping amount of the activator of the present invention have an impact on the transformation of phosphorus forms. After adding sodium bicarbonate as the activator, the organic phosphorus and some insoluble phosphorus in microalgae are transformed into inorganic and soluble phosphorus forms during pyrolysis.

[0025] 3. The effect of recovering phosphorus from algae in the present invention is better than that of recovering phosphorus from other media to form vivianite because the content of heavy metals and impurity elements such as silicon in microalgae is very low, and there is basically no interference from other coexisting ions in the leaching solution of the biochar with microalgae as the raw material. The ionic environment of the leaching solution is relatively pure, and it is not necessary to purify the leaching solution before vivianite precipitation.

[0026] 4. The present invention can synchronously achieve the transformation and recovery of phosphorus in microalgae by coupling slow pyrolysis, leaching, and precipitation crystallization processes. According to the steps of the present invention, up to 98.88% of various forms of phosphorus in microalgae can be transformed into orthophosphate, and the purity of the recovered vivianite is as high as 98.13%. The method in the present invention is simple to operate and has low treatment costs, which is conducive to large-scale application and has excellent application prospects.

[0027] 5. Generally speaking, the present invention proposes a comprehensive solution to alleviate the global phosphorus crisis and the problem of algal sludge disposal caused by algal blooms. It not only realizes the sustainable utilization of phosphorus resources in microalgae and reduces its negative impact on the environment; moreover, the treated biochar has a high surface volume and pore volume, and the pore structure is well-developed, which can further play a good adsorption and catalytic role to achieve the maximum utilization of resources. Description of the Drawings

[0028] Figure 1 Total phosphorus and phosphorus content in the leachate for one hour in biochar samples at different pyrolysis temperatures, where the columns represent total phosphorus and the dots represent phosphorus in water.

[0029] Figure 2 Phosphorus concentration after 8-hour leaching of BC-700-20% sample in 0.1 mol / L HCl solution.

[0030] Figure 3 XRD pattern and SEM surface morphology pattern of the generated vivianite product. Detailed implementation mode

[0031] The present invention will be specifically described below in conjunction with the accompanying drawings and embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0032] Example 1:

[0033] (1) Weigh dry Chlorella powder and NaHCO3 according to a mass ratio of 5:1, stir evenly to obtain a mixed material. The drying method of Chlorella is to dry it in an oven at 105 °C for 3 h.

[0034] Place the mixed material in a corundum crucible and pyrolyze it in a tubular furnace under a N2 atmosphere. Heat it to the set temperature (400 °C, 500 °C, 600 °C, 700 °C, and 800 °C) at a rate of 10 °C / min and hold for 120 min.

[0035] After pyrolysis, grind the obtained biochar sample and sieve it through a 80-mesh sieve and seal it for storage. The biochar sample is named BC-400 / 500 / 600 / 700 / 800-20% according to the pyrolysis temperature.

[0036] (2) Conduct a one-hour leaching experiment on BC-400 / 500 / 600 / 700 / 800-20% using deionized water. The ratio of the leaching solution to the biochar is 100 mL / g. Screen the optimal pyrolysis temperature according to the phosphorus concentration in the solution for subsequent experiments.

[0037] The results are as Figure 1 shown. It can be seen from the figure that the one-hour water phosphorus leaching amounts in the biochars obtained at pyrolysis temperatures of 400 - 800 °C are 13.66 mg / g, 15.33 mg / g, 13.96 mg / g, 18.44 mg / g, and 11.70 mg / g respectively. Therefore, take BC-700-20% for an 8-hour acid leaching experiment to further increase the phosphorus leaching amount.

[0038] Weigh 1 g of the biochar sample prepared at a pyrolysis temperature of 700 °C and place it in 100 mL of 0.1 mol / L HCl solution. Oscillate at 25 °C and 200 r for 8 hours to extract soluble phosphorus. After the oscillation, filter and separate the biochar from the solution to obtain a phosphorus-rich solution. The concentration of phosphorus in the leachate is as Figure 2 shown. After leaching with 0.1 mol / L HCl for 8 hours, 94.37% of the phosphorus in the biochar is transferred to the solution.

[0039] (3) Separate the biochar from the leaching solution, adjust the pH of the phosphorus-rich solution to 7 using 1 mol / L NaOH solution, add ferrous sulfate heptahydrate solution according to the Fe / P molar ratio = 1.5, and perform anaerobic oscillation at room temperature for 48 h to obtain vivianite precipitate.

[0040] Collect the precipitate, freeze-dry it, and perform surface morphology and composition identification of the precipitate. The results are as Figure 3 shown. The composition of the precipitate is determined to be vivianite in the shape of a flower. According to the content of the main elements in vivianite measured by ICP, the results are shown in Table 1.

[0041] Table 1 Content of main elements in vivianite products

[0042] Element Fe P Ca Mg Al ％ 59.26 38.78 0.96 0.87 0.13

[0043] It can be calculated from Table 1 that Fe / P in the vivianite sample is 1.52, which is very close to the theoretical value of 1.50. According to the calculation results of purity, the purity of the recovered vivianite is 98.13%.

[0044] Example 2:

[0045] Biochars with and without NaHCO3 were prepared from Chlorella at 500 °C, 600 °C, and 700 °C respectively according to the method in Example 1. The biochar samples with NaHCO3 added are named BC-500 / 600 / 700-20% according to the pyrolysis temperature, and the biochar samples without NaHCO3 added are named BC-500 / 600 / 700.

[0046] The inorganic phosphorus (IP) and organic phosphorus (OP) in each biochar sample and Chlorella were determined using the SMT phosphorus sequential extraction method used under the framework of the European Committee for Standardization. The results are shown in Table 2.

[0047] Table 2 Content of inorganic phosphorus and organic phosphorus in each sample

[0048] Samples IP(%) TP(%) Chlorella 30.61 69.39 BC-500 28.74 71.26 BC-600 9.69 90.31 BC-700 5.95 94.05 BC-500-20wt% 76.69 23.31 BC-600-20wt% 79.11 20.89 BC-700-20wt% 98.88 1.12

[0049] The results showed that the inorganic phosphorus content in the original Chlorella vulgaris and the biochar samples without NaHCO3 addition was low, while the inorganic phosphorus content in the biochar with NaHCO3 addition gradually increased with the increase of pyrolysis temperature. In the BC-700-20wt% biochar sample, the proportion of inorganic phosphorus was as high as 98.88%, which was exactly the reason for the highest extractable phosphorus in the BC-700-20wt% biochar sample mentioned in Example 1.

[0050] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the specification and embodiments. It can be fully applied to various fields suitable for the present invention. For those skilled in the art, for those of ordinary skill in the art, various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. Therefore, without departing from the general concept defined by the claims and the equivalent scope, the present invention is not limited to specific details.

Claims

1. A method for efficiently recovering phosphorus from microalgae to prepare vivianite, characterized in that, It includes the following steps: (1) Preparation of microalgae biochar: Microalgae biochar is prepared by slow pyrolysis, and the addition of NaHCO3 activator during the pyrolysis process is used to guide the change of phosphorus form; (2) Leaching of soluble phosphorus in microalgae biochar: Water or dilute acid is used as the leaching solution to leach the soluble phosphorus in the biochar to obtain a phosphorus-rich solution; (3) Preparation of vivianite: The biochar is separated from the leaching solution, the pH of the phosphorus-rich solution is adjusted to neutral, ferrous ions are added according to the Fe / P molar ratio = 1.5 - 1.7, and vivianite precipitate is obtained by shaking at room temperature for 40 - 50 hours.

2. The method according to claim 1, wherein Before the slow pyrolysis in step (1), the microalgae is dehydrated and dried by air drying, steam drying or electrothermal blast drying.

3. The method according to claim 1, wherein The temperature of the slow pyrolysis in step (1) is 400 - 800 °C; the heating program is: rising from room temperature to the set temperature at a rate of 5 - 15 °C / min and maintaining for 120 - 150 min; the gas atmosphere is N2.

4. The method according to claim 1, wherein The addition amount of the NaHCO3 activator in step (1) is 20 - 50 wt% of the mass of the microalgae.

5. The method according to claim 1, characterized in that When using water or dilute acid as the leaching solution to leach the soluble phosphorus in the biochar in step (2), the ratio of the leaching solution to the biochar is 100 - 150 mL / g, and the leaching time is 4 - 24 h.

6. The method according to claim 1, wherein The dilute acid in step (2) includes dilute HCl or dilute H2SO4.

7. The method according to claim 6, characterized in that, The concentrations of the dilute HCl and dilute H2SO4 are 0.1 - 0.4 mol / L respectively.

8. The method according to claim 1, wherein In step (3), the pH of the phosphorus-rich solution is adjusted to neutral by using a NaOH solution with a concentration of 0.8 - 1.2 mol / L.

9. The method according to claim 1, wherein The source of the ferrous ions in step (3) includes ferrous sulfate heptahydrate or ferrous chloride tetrahydrate.

10. The method according to claim 1, characterized in that The shaking at room temperature in step (3) is carried out in an anaerobic environment.

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