CN114433024A

CN114433024A - Modified biochar and preparation method and application thereof

Info

Publication number: CN114433024A
Application number: CN202210376247.8A
Authority: CN
Inventors: 王风; 冯梦晗; 罗元; 张克强
Original assignee: Agro Environmental Protection Institute Ministry of Agriculture
Current assignee: Agro Environmental Protection Institute Ministry of Agriculture
Priority date: 2022-04-12
Filing date: 2022-04-12
Publication date: 2022-05-06

Abstract

The invention provides modified biochar and a preparation method and application thereof, and relates to the technical field of adsorption materials. The preparation method of the modified biochar provided by the invention comprises the following steps: under the condition of air isolation, performing first calcination on agricultural wastes to obtain a biochar substrate; performing secondary calcination on the Ca-containing waste, and mixing with water to obtain Ca (OH)₂Suspending liquid; placing the biochar substrate in the Ca (OH)₂Immersing in the suspension to obtain the modifiedAnd (4) biochar. The modified biochar prepared by the invention has Ca (OH) on the surface under the initial pH acidic condition₂Preferentially with H in solution⁺And acid-base neutralization reaction is carried out, so that the modified biochar and phosphate are subjected to chemical precipitation under the alkaline condition, and compared with other Ca-loaded biochar, the adsorption capacity is improved by 3-15 times.

Description

Modified biochar and preparation method and application thereof

Technical Field

The invention relates to the technical field of adsorption materials, and particularly relates to modified biochar and a preparation method and application thereof.

Background

Phosphorus (P) is one of 3 major nutrient elements (N, P, K) necessary for plant growth, and plays a significant role in maintaining crop growth and improving grain yield. On the other hand, excessive phosphorus flowing into water body can cause eutrophication, cause water body to lack of oxygen, aquatic animals and plants to die, water quality to be damaged, cause regional economic loss and even threaten human health.

Among various methods for separating aqueous phase phosphate, the adsorption method is considered to be a promising method due to advantages of simple operation, environmental friendliness, high removal rate, and the like. Development of inexpensive and excellent adsorption materials has been the focus of research. Phosphate ions can act as a lewis acid, are generally negatively charged in solution, can undergo coordination complexation with most metals (lewis bases), and various metal-based adsorption materials have been developed and used to recover phosphorus resources in the aqueous phase. Wherein, Ca is a metal element with abundant reserves in the earth crust, has low price and environmental protection, and can efficiently remove phosphate radical in the solution. Therefore, the Ca is loaded on other carrier materials, so that the phosphorus resource in the wastewater can be effectively recovered.

The biochar is a substance which is rich in carbon and is obtained by pyrolysis and conversion of biomass under the anoxic condition, has wide source, simple preparation and large specific surface area, and is a good carrier. Ca is often expressed as Ca²⁺Is loaded on biochar, however Ca²⁺The loaded charcoal is greatly influenced by the pH value of water when adsorbing phosphate, generally has stronger adsorption capacity under the condition of high pH value and 70-80 mg P/g of adsorption capacity, and has poor adsorption capacity under the condition of low pH value. Feng and the like prepare sheep manure biochar loaded Ca²⁺At pH<In the phosphate solution of 7, the adsorption capacity is only 6mg P/g at most (Y. Feng, Y. Luo, Q. He, D. Zhao, K. Zhang, S. Shen, F. Wang, Performance and mechanism of a biochar-based Ca-La composite for the adsorption of phosphate from water, J. environ. chem. Eng. 9 (2021) 105267. https:// doi. org/10.1016/j. jec. 2021.105267.). Ramirez-Mu ñ oz and Anthones prepared Ca-loaded, respectively²⁺The pH value of the eichhornia crassipes biochar and the sludge biochar<Phosphoric acid of 7In the salt solution, the adsorption capacity is 17mg P/g and 36mg P/g respectively (A. Ramirez-Mu ñ oz, S. P rez, E. Fl Louerez, N. Acelas, Recovering phosphor from aqueous solutions using water absorption, J. environ. chem. Eng. 9 (2021) https: doi.org/10.1016/j. je. 2021.225. E. Anchannels, M.V. Jacob. Brodie, P.A. Schneider, Isers, kinetics medium analysis of 1067. calcium channel 1061. calcium chloride, calcium chloride 106calcium chloride, calcium chloride, magnesium chloride, calcium chloride, magnesium chloride, calcium chloride, magnesium chloride, calcium chloride, magnesium chloride, calcium chloride, magnesium chloride, calcium chloride, magnesium. Ca due to low pH²⁺And PO₄ ^3-Binding is inhibited, so the adsorption capacity is poor, limiting the range of applications. And usually the source of Ca is Ca²⁺The form of (b) is supported on biochar, a large amount of strong acid is required to dissolve the Ca source, the strong acid is very corrosive and expensive, and in actual production, it corrodes machinery and increases the production cost.

Disclosure of Invention

The invention aims to provide modified biochar and a preparation method and application thereof, and the modified biochar prepared by the invention can efficiently remove phosphorus under an acidic condition; and the use of strong acid is avoided in the preparation process, so that the production cost is saved.

In order to achieve the above object, the present invention provides the following technical solutions:

the invention provides a preparation method of modified biochar, which comprises the following steps:

under the condition of air isolation, performing first calcination on agricultural wastes to obtain a biochar substrate;

performing secondary calcination on the Ca-containing waste, and mixing with water to obtain Ca (OH)₂Suspending liquid;

placing the biochar substrate in the Ca (OH)₂And (4) dipping in the suspension to obtain the modified biochar.

Preferably, the agricultural waste comprises at least one of tobacco straw, straw and walnut shells.

Preferably, the temperature of the first calcination is 500-700 ℃; the heat preservation time is 1-3 h.

Preferably, the temperature rising rate from room temperature to the temperature of the first calcination is 5-15 ℃/min.

Preferably, the Ca-containing source waste comprises at least one of shellfish waste, egg shells, and crab shells.

Preferably, the temperature of the second calcination is 1000-1500 ℃; the heat preservation time is 1-3 h.

Preferably, the heating rate of the temperature rise from room temperature to the temperature of the second calcination is 5-15 ℃/min.

Preferably, the impregnation is carried out under stirring conditions; the stirring speed is 600-1000 rpm; the stirring time is 5-9 h.

The invention provides the modified biochar prepared by the preparation method in the technical scheme.

The invention provides application of the modified biochar in the technical scheme in removing phosphate in wastewater.

The invention provides a preparation method of modified biochar, which comprises the following steps: under the condition of air isolation, performing first calcination on agricultural wastes to obtain a biochar substrate; performing secondary calcination on the waste containing the Ca source, and mixing the waste with water to obtain Ca (OH)₂Suspending liquid; placing the biochar substrate in the Ca (OH)₂And (4) dipping in the suspension to obtain the modified biochar. In the present invention, the Ca source-containing waste is calcined to produce CaO, which is converted into Ca (OH) during the mixing process with water₂The modified biochar prepared by the invention has Ca (OH) on the surface under the initial pH acidic condition₂Preferentially with H in solution⁺And acid-base neutralization reaction is carried out, so that the modified biochar and phosphate are subjected to chemical precipitation under the alkaline condition, and compared with other Ca-loaded biochar, the adsorption capacity is improved by 3-15 times. The Langmuir fitting maximum adsorption capacity is 88.64mg P/g, the adsorption process conforms to the quasi-second order kinetics, and the adsorption behavior is monolayer chemisorption. The modified charcoal prepared by the invention has good effect on treating four actual waste waters, and the removal rate is over 90 percent. The present invention is Ca-modified organismsThe high-efficiency phosphorus removal under the carbon acid condition and the practical wastewater application provide theoretical basis.

Drawings

FIG. 1 shows the results of isotherm fitting of Ca-BC to phosphate adsorption and the adsorption kinetics curve; wherein, FIG. 1 (a) shows the fitting result of the isotherm of Ca-BC to phosphate adsorption; FIG. 1 (b) is a graph showing the adsorption kinetics of Ca-BC on phosphate;

FIG. 2 is a graph showing the effect of initial pH of a phosphate solution on the amount of Ca-BC adsorbed; wherein, FIG. 2 (a) is the effect of initial pH of phosphate solution on Ca-BC adsorption amount; FIG. 2 (b) shows Ca-BC Zeta potential;

FIG. 3 is a graph showing the effect of several common coexisting ions on Ca-BC phosphate adsorption;

FIG. 4 is a graph showing the results of treatment of 50mL of four actual wastewaters with different masses of Ca-BC added; wherein, FIG. 4 (a) shows the treatment results of wastewater from cattle and pig farms; FIG. 4 (b) shows the results of pond and domestic sewage treatment;

FIG. 5 is a topographic map of biochar BC; wherein, fig. 5 (a) is a BC topography magnified 1000 times; FIG. 5 (b) is a BC topography magnified 5000 times; FIG. 5 (c) is a BC plot magnified 50000 times; FIG. 5 (d) is a diagram of the element distribution of BC;

FIG. 6 is a morphology chart of biochar Ca-BC; wherein FIG. 6 (a) is a 500-fold magnified Ca-BC profile; FIG. 6 (b) is a 5000-fold magnified Ca-BC topography; FIG. 6 (c) is a 50000 times magnified topographical map of Ca-BC; FIG. 6 (d) is a diagram showing the distribution of elements of Ca-BC;

FIG. 7 is a Ca-BC-P topography; wherein, FIG. 7 (a) is a 500-fold enlarged Ca-BC-P topography; FIG. 7 (b) is a 10000 times magnified Ca-BC-P topography; FIG. 7 (c) is a 100000-fold magnified Ca-BC-P topography; FIG. 7 (d) is a diagram showing the elemental distribution of Ca-BC-P;

FIG. 8 is a graph of FTIR of BC, Ca-BC, and Ca-BC-P; wherein FIG. 8 (a) is an FTIR overview of BC, Ca-BC, and Ca-BC-P; FIG. 8 (b) is an FTIR partial magnification of BC, Ca-BC, and Ca-BC-P;

FIG. 9 is an XRD pattern of BC, Ca-BC and Ca-BC-P.

Detailed Description

The method comprises the step of carrying out primary calcination on agricultural wastes under the condition of air isolation to obtain the biochar substrate. In the present invention, the agricultural waste preferably includes at least one of tobacco stalks, straws and walnut shells, and more preferably tobacco stalks. In the present invention, the agricultural waste preferably further comprises a pretreatment before the first calcination; the pretreatment preferably comprises: the agricultural wastes are sequentially cleaned, air-dried and crushed. In the present invention, the particle size of the agricultural waste is preferably 60 to 100 mesh.

In the invention, the temperature of the first calcination is preferably 500-700 ℃, and more preferably 500-600 ℃; the heat preservation time is preferably 1-3 h, and more preferably 2 h. In the present invention, the rate of temperature increase from room temperature to the temperature of the first calcination is preferably 5 to 15 ℃/min, and more preferably 5 ℃/min.

In the present invention, the first calcination is preferably carried out in a crucible which is covered.

The invention carries out secondary calcination on the waste containing Ca source, and the Ca source and the water are mixed to obtain Ca (OH)₂And (4) suspending the solution. In the present invention, the Ca-containing source waste preferably includes at least one of shellfish waste, egg shells, and crab shells; the shellfish waste preferably comprises at least one of oyster shell, scallop shell, mussel shell and clam shell. In the present invention, the Ca-source-containing waste preferably further includes a pretreatment before the second calcination; the pretreatment preferably comprises: and sequentially cleaning, air-drying and grinding the wastes containing the Ca source. In the present invention, the particle size of the Ca source-containing waste is preferably 100 mesh.

In the invention, the temperature of the second calcination is preferably 1000-1500 ℃, and more preferably 1000-1200 ℃; the heat preservation time is preferably 1-3 h, and more preferably 1 h. In the present invention, the temperature increase rate from room temperature to the second calcination temperature is preferably 5 to 15 ℃/min, and more preferably 5 ℃/min.

In the present invention, the second calcination is preferably in N₂Is carried out in an atmosphere.

In the invention, after the calcination, Ca (OH) is obtained after mixing the substance obtained by the calcination with water₂And (4) suspending the solution. In the present invention, the ratio of the amount of the substance obtained by the calcination to water is preferably 10 g: 500-1500 mL. In the present invention, the mixing is preferably carried out under ultrasonic conditions; the power of the ultrasound is preferably 600W; the ultrasonic frequency is preferably 40 KHz; the time of the ultrasound is preferably 1.5h or more.

Obtaining a biochar substrate and Ca (OH)₂After suspension, the biochar substrate is placed in the Ca (OH)₂And (4) dipping in the suspension to obtain the modified biochar. In the present invention, the impregnation is preferably carried out under stirring conditions; the stirring speed is preferably 600-1000 rpm, and more preferably 800 rpm; the stirring time is preferably 5-9 h, and more preferably 6 h. In the present invention, the temperature of the impregnation is preferably room temperature.

In the invention, preferably, after the impregnation, the obtained solid substance is dried to obtain the modified biochar. In the present invention, the temperature of the drying is preferably 105 ℃. The invention has no special requirement on the drying time, and takes the removal of water in the biochar as the standard.

The invention also provides the modified biochar prepared by the preparation method in the technical scheme. In the invention, the modified biochar is loaded with Ca (OH) on the surface₂. In the invention, the specific surface area of the modified biochar is preferably 3.6462-6.5792 m²(iv)/g, more preferably 6.5792m²(ii)/g; the pore volume is preferably 0.000975-0.34251 cm³G, more preferably 0.033904cm³(iv) g; the average pore diameter is preferably 1.0281nm to 36.5167nm, and more preferably 25.0623 nm.

The invention also provides application of the modified biochar in the technical scheme in removing phosphate in wastewaterAnd is especially suitable for removing phosphate from acid waste water. In the invention, the pH value of the wastewater is preferably 3-6. In the invention, the concentration of the wastewater is preferably 5-150 mg P/L in terms of phosphorus content. In the present invention, the phosphate comprises KH₂PO₄、K₂HPO₄And K₃PO₄。

In the present invention, the application preferably includes: mixing the modified biochar with wastewater; the mixing temperature is preferably room temperature, and is specifically 25 +/-0.5 ℃; the mixing is preferably carried out under stirring conditions; the stirring speed is preferably 180 r/min; the mixing time is preferably 5-9 h, and more preferably 6 h. In the mixing process, phosphate in the wastewater and Ca ions on the modified biochar are subjected to chemical reaction to generate Ca₃(PO₄)₂And (4) precipitating.

In the specific embodiment of the invention, 115mg P/L is taken as a concentration standard, and the addition amount of the modified biochar is 20 g/L; the concentration standard is 78mg P/L, and the adding amount of the modified biochar is 10 g/L; 0.15-0.317 mg P/L is taken as a concentration standard, and the addition amount of the modified biochar is 3.5 g/L.

The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. It is to be understood that the described embodiments are merely exemplary of the invention, and not restrictive of the full scope of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Examples and application examples materials used:

cleaning, air-drying and crushing tobacco stalks, sieving the tobacco stalks by a sieve of 60 to 100 meshes, and bagging the tobacco stalks for later use; cleaning, air drying and grinding oyster shells, sieving the oyster shells with a 100-mesh sieve, and bagging the oyster shells for later use;

the monopotassium phosphate is superior pure;

potassium sulfate, potassium nitrate, potassium chloride, crystalline potassium carbonate, ascorbic acid, antimony potassium tartrate, ammonium molybdate and potassium persulfate are all analytically pure;

the test water is deionized water, the resistivityIs 18.2 M.OMEGA.cm^-1。

Example 1

Putting 70g of sieved tobacco stalks in a crucible, covering the crucible to isolate air, heating the crucible to 500 ℃ in a muffle furnace at the heating rate of 5 ℃/min, and preserving the heat for 2h to obtain a biochar substrate;

placing 20g of sieved oyster shell in a crucible, heating to 1000 ℃ in a tubular furnace at a heating rate of 5 ℃/min, and keeping the temperature for 1 h; dissolving calcined oyster shell 10g in 500mL water, and performing ultrasonic treatment for 1.5h to obtain Ca (OH)₂Suspending liquid;

adding 20g of the biochar substrate to the Ca (OH)₂Stirring the suspension evenly, stirring the suspension in a constant-temperature magnetic stirrer at room temperature for 6 hours at 800rpm, transferring the suspension to a 105 ℃ oven, and drying the suspension to remove water to obtain Ca (OH) load₂The modified biochar of (1) is named Ca-BC.

Comparative example 1

The biochar substrate prepared in example 1 was designated BC as comparative example 1.

Application example 1

0.05g Ca-BC was weighed into a series of 100mL Erlenmeyer flasks and 50mL KH was added at different concentration gradients (5, 10, 15, 25, 50, 75, 100, 150, 200mg P/L)₂PO₄And (3) solution. Placing the mixture in a constant temperature shaking box with the temperature of 25 +/-0.5 ℃ and the rotating speed of 180r/min for reaction for 24 hours. After the reaction was completed, the filtrate was filtered through a 0.45 μm microporous membrane and the concentration of phosphorus in the filtrate was determined by ammonium molybdate spectrophotometry, and the experiment was repeated 3 times. Experimental data were fitted using Langmuir and Freundlich. The isothermal adsorption equations are shown in (equation 1) and (equation 2), respectively.

Langmuir isothermal adsorption equation:

(formula 1)

Freundlich isothermal adsorption equation:

(formula 2)

Wherein, K_L(L/mg) is Langmuir adsorptionAn equilibrium constant; k_F（mg^(1-1/n)L^1/n(ii)/g) and n are the adsorption capacity and adsorption strength constants, respectively, of Freundlich; q. q.s_eRepresents the equilibrium adsorption capacity (mg/g); q. q.s_maxRepresents the maximum adsorption capacity (mg/g); c_eIndicates the equilibrium concentration of phosphate in the solution (mg/L).

The results of the isotherm fitting of Ca-BC to phosphate adsorption are shown in fig. 1 (a), and as the equilibrium concentration increases, the adsorption amount rapidly increases first, then increases slowly, and finally reaches equilibrium. The isothermal adsorption curve fitting effect of Langmuir is better than Freundlich (R)²0.98 is more than 0.78), the maximum adsorption capacity obtained by Langmuir fitting is 88.64mg P/g, the result is similar to the actually measured maximum adsorption capacity of 87mg P/g, and the adsorption of phosphate belongs to monolayer adsorption.

Application example 2

0.05g Ca-BC was weighed into a 100mL Erlenmeyer flask, and 50mL KH 150mg P/L was added₂PO₄The solution is then reacted in a constant temperature shaking box with the rotation speed of 180r/min at 25 +/-0.5 ℃, and samples are taken for specific time (10 min, 30min, 60min, 120min, 180min, 240min, 480min, 720min, 1440 min), and filtered through a 0.45 mu m microporous filter membrane, and the concentration of phosphorus in the filtrate is determined by ammonium molybdate spectrophotometry, and the test is repeated for 3 times. The test data were fitted using quasi-first order kinetics and quasi-second order kinetics. The kinetic equations are shown in (equation 3) and (equation 4), respectively.

Quasi first order kinetic equation:

(formula 3)

Quasi-second order kinetic equation:

(formula 4)

Wherein q is_t(mg/g) represents the phosphorus adsorption amount at time t; q. q.s_e(mg/g) is the amount of phosphorus adsorbed at equilibrium; k is a radical of₁（min^-1) Is a quasi first-order equation adsorption rate constant; k is a radical of₂(g/(mg. min)) as the standardSecond order equation adsorption rate constant.

The adsorption kinetics curve of Ca-BC to phosphate is shown in (b) in figure 1, the adsorption rate is fastest in the first 2h, gradually slows down in 2-5 h, and reaches the equilibrium after 5 h. The fitting effect of the quasi-second order kinetic equation is better than that of quasi-first order (R)²0.99 > 0.92), indicating that the adsorption of phosphate by Ca-BC is mainly controlled by the chemisorption process. The method mainly comprises the step of enabling a large number of active adsorption sites to exist on the surface of the adsorbent within the first 2 hours, so that the adsorption rate is high, the adsorption sites are gradually reduced within 2-5 hours, the adsorption sites tend to be saturated after 5 hours, adsorbed phosphate and free phosphate in a solution repel each other, and the adsorption reaches a saturated state at the moment.

Application example 3

0.05g Ca-BC was weighed into a 100mL Erlenmeyer flask, and 50mL KH at different pH were added₂PO₄Solution (concentration 150mg P/L). KH was adjusted with 1mol/L HCl and NaOH₂PO₄The solution is adjusted to different pH values (3, 4, 5, 6, 7, 8, 9, 10 and 11), then the solution is reacted in a constant temperature shaking box with the rotation speed of 180r/min at 25 +/-0.5 ℃ for 24 hours, then the sample is sampled, the sample is filtered through a 0.45 mu m microporous filter membrane, the concentration of phosphorus in the filtrate is measured by using ammonium molybdate spectrophotometry, and the test is repeated for 3 times.

The influence of the initial pH of the phosphate solution on the amount of Ca-BC adsorbed is shown in FIG. 2 (a). As can be seen from the figure, when the initial pH of the phosphate solution fluctuates within the range of 3-11, the Ca-BC has higher adsorption capacity (within the range of 75-96 mg P/g) for the phosphate solution, the adsorption capacity reaches 93-96 mg P/g under acidic conditions, and the adsorption capacity is reduced to 75-88.5 mg P/g under alkaline conditions along with the continuous increase of the pH value. Ca²⁺The adsorption capacity of the loaded biochar is poor under the acidic condition, and Ca is commonly loaded²⁺Biochar at pH<In the phosphate solution of 7, the adsorption capacity is only 6-36 mg P/g. With Ca²⁺Compared with the loaded biochar, the adsorption capacity of Ca-BC under an acidic condition is improved by 3-15 times. Supposing that the adsorption capacity of the Ca-BC under the acidic condition is obviously stronger than that of the Ca²⁺The reason for loading the biochar is that a large amount of Ca (OH) exists on the surface of the Ca-BC material₂After addition to the phosphate solution, Ca (OH)₂OH (C) of^-With H in solution⁺Generating acidAnd (3) carrying out alkali neutralization reaction to ensure that the pH value of the neutralized solution is within 8-9. It is thus seen that the difference is from pH<Ca-loaded in 7-phosphate solutions²⁺Biochar, Ca-BC at initial pH<The chemical precipitation in phosphate solution of 7 is carried out in a more alkaline environment.

Further, the pH of Ca-BC_PZCThe value is 3.81 (fig. 2 (b)), and the electrostatic inhibition effect is exhibited on all phosphate solutions having an initial pH in the range of 3 to 11, and it is assumed that when the initial pH is slightly acidic, the chemical binding energy is dominant, the phosphate adsorption amount is high, the electrostatic inhibition effect is increased with the increase of the initial pH, the electrostatic repulsion becomes a main action force, the phosphate adsorption amount is relatively low, and the phosphate adsorption amount under acidic conditions is slightly higher than that under alkaline conditions due to the change of the main action force under acidic and alkaline conditions.

Application example 4

0.05g of Ca-BC was weighed into a 100mL Erlenmeyer flask, and 50mL of KH, which contains four kinds of coexisting ions, was added to each of the resulting solution₂PO₄And (3) solution. Mixing 150mg P/L KH₂PO₄With a certain mass of Cl^- (KCl)、NO^3- (KNO₃)、CO₃ ^2- (K₂CO₃)、SO₄ ^2- (K₂SO₄) Mixing to make the concentration of coexisting ions in the solution the same as that of phosphorus, reacting in a constant temperature shaking box with the rotation speed of 180r/min at 25 +/-0.5 ℃ for 24h, sampling, filtering with a 0.45 mu m microporous filter membrane, measuring the concentration of phosphorus in the filtrate by using ammonium molybdate spectrophotometry, and repeating the test for 3 times.

The effect of several common coexisting ions on Ca-BC phosphate adsorption is shown in fig. 3. The adsorption capacity of Ca-BC for adsorbing phosphate is hardly influenced by Cl^-、NO^3-And SO₄ ^2-Influence. CO compared to blank control₃ ^2-Has obvious influence on the adsorption capacity, and the adsorption capacity is reduced from 87.5mg P/g to 74.5mg P/g by 15.8 percent. The reason why this phenomenon occurs is presumed to be CO₃ ^2-And Ca (OH)₂Ca in (1)²⁺Formation of CaCO₃Precipitation, the precipitate sticking to the surface, thereby blocking the phosphate nucleation sites. The Ca-BC still has good absorption in the presence of several common ionsHas the advantages of good effect and great potential in the aspect of practical wastewater application.

Application example 5

And selecting cattle farm and pig farm breeding wastewater, pond sewage and domestic sewage to carry out an actual wastewater adsorption experiment. The cattle farm aquaculture wastewater, the pond wastewater and the domestic wastewater are taken from a certain test station in the city of Dali, Yunnan province, the pH values are respectively 7.53 +/-0.1, 8.45 +/-0.1 and 8.13 +/-0.1, the total phosphorus concentrations are respectively 115 +/-2.5, 0.325 +/-0.025 and 0.15 +/-0.025 mg P/L, the pig farm aquaculture wastewater is taken from a certain farm in the city of Dali, Yunnan province, the pH value is 7.15 +/-0.1, and the total phosphorus concentration is 78.75 +/-3.75 mg P/L. Weighing 0.1g, 0.2 g, 0.5g, 1.0 g and 2.0g of Ca-BC into a 100mL conical flask, respectively adding 50mL of cattle farm culture wastewater and pig farm culture wastewater, weighing 0.01 g, 0.02 g, 0.05g, 0.07g and 0.1g of Ca-BC into a 100mL conical flask, respectively adding 50mL of pond sewage and domestic sewage, culturing in a constant temperature shaking box at 25 +/-0.5 ℃ and the rotating speed of 180r/min for 24h, sampling, measuring the residual phosphorus concentration, and calculating the removal rate.

FIG. 4 shows the results of 50mL of four actual wastewaters with different masses of Ca-BC added. The removal rate was significantly improved with the increase in the amount added. The results of the treatment of the wastewater from cattle and pig farms show that (a) in FIG. 4, when the amount of Ca-BC added is 2g, the phosphorus content in the wastewater from cattle farms is reduced from 115mg P/L to 10mg P/L, and the removal rate reaches 91%; the phosphorus content in the pig farm breeding wastewater is reduced from 78mg P/L to 5mg P/L, and the removal rate reaches 95%. For the cattle farm breeding wastewater, when the adding amount is increased from 0.1g to 1g, the removal rate is increased from 73% to 90%, and when the adding amount is increased from 1g to 2g, the removal rate has no significant change, which shows that the removal rate can reach the maximum level by adding 1g of Ca-BC, and waste can be caused by continuously increasing the adding amount. Therefore, in view of cost, the reference value of the addition amount of Ca-BC in practical application is 20g/L by taking 115mg P/L as a concentration standard. For the pig farm breeding wastewater, when the addition amount reaches 0.5g, the removal rate can reach 91 percent, so 78mg P/L is taken as the concentration standard, and the reference value of the addition amount of Ca-BC is 10 g/L. The results of pond sewage and domestic sewage treatment (fig. 4 (b)) show that after 0.1g of Ca-BC is added, the phosphorus content in the pond sewage and the domestic sewage is respectively reduced to 0mg of P/L from the initial 0.317mg of P/L and 0.15mg of P/L, and the removal rate reaches 100%. Compared with the addition of 0.1g, the removal rate of 0.07g of addition does not obviously decrease, so that in practical application, 0.15-0.317 mg of P/L is taken as a concentration standard, and the reference value of the addition amount of Ca-BC is 3.5 g/L.

Application example 6

The specific surface area, pore volume and mean pore diameter were obtained by means of a fully automated physical adsorption apparatus (BET, Micromeritics ASAP 2020, USA), the surface topography of the adsorption material was observed by means of a field emission scanning electron microscope (SEM, Zeiss Sigma 500, Germany), and the elemental distribution on the surface of the adsorbent was observed by means of X-ray spectroscopy (EDS, Zeiss Sigma 500, Germany). Changes in the adsorbent surface functionality and the diffraction patterns were observed using Fourier transform Infrared Spectroscopy (FTIR, Thermo Fisher Nicolet Is5, USA) and X-ray diffractometer (XRD, Bruker D8 Advance X-ray diffractometer, Germany), respectively.

The specific surface area and porosity of the biochar determine to a large extent its adsorption properties. The larger the specific surface area is, the higher the porosity is, the more surface adsorption sites are, and the stronger the adsorption performance is. BC specific surface area of 3.5251m²Per g, pore volume 0.006084cm³(ii)/g, average pore diameter 10.6364 nm; the specific surface area of Ca-BC was 6.5792m²Per g, pore volume 0.033904cm³In terms of/g, the mean pore diameter is 25.0623 nm. The specific surface area, the pore volume and the average pore diameter of the Ca-BC are respectively improved by 86.7 percent, 457.3 percent and 135.6 percent compared with the BC. Therefore, the specific surface area, the pore volume and the average pore diameter of the modified biochar prepared by the invention are remarkably increased because Ca generated after oyster shell high-temperature calcination is converted into Ca (OH) in a water phase₂，Ca(OH)₂Is alkaline and has the function of activating and reaming the biochar.

FIG. 5 is a graph of the morphology and elemental distribution of biochar BC; wherein FIG. 5 (a) is a BC diagram magnified 1000 times; FIG. 5 (b) is a BC topography magnified 5000 times; FIG. 5 (c) is a plot of BC morphology at 50000 times magnification; FIG. 5 (d) is a diagram of the element distribution of BC; FIG. 6 is a morphology chart and an element distribution chart of biochar Ca-BC; wherein FIG. 6 (a) is a 500-fold magnified Ca-BC profile; FIG. 6 (b) is a 5000-fold magnified Ca-BC topography; FIG. 6 (c) is a 50000 times magnified topographical map of Ca-BC; FIG. 6 (d) shows the elemental composition of Ca-BCLaying out a layout; FIG. 7 is a graph of the morphology and elemental distribution of Ca-BC-P; wherein FIG. 7 (a) is a 500-fold magnified topographical view of Ca-BC-P; FIG. 7 (b) is a 10000 times magnified morphological plot of Ca-BC-P; FIG. 7 (c) is a 100000-fold magnified topographical view of Ca-BC-P; FIG. 7 (d) is a diagram showing the elemental distribution of Ca-BC-P. Before Ca is loaded, the biochar BC is integrally in a rod-shaped structure, and the biochar is observed to be smooth in surface and has partial wrinkles after being magnified and possibly related to the appearance of the biomass (figure 5). After Ca is loaded, the whole Ca-BC of the biochar is in a long-strip square lamellar structure, and after the Ca-BC is amplified, the biochar is observed to have rough surface and enlarged pores, and a large amount of Ca (OH)₂(according to FTIR and XRD analysis) is loaded on the surface of the biochar, and granular or filamentous structures are presented on the surface of the biochar (figure 6). After adsorbing phosphate, compared with Ca-BC, Ca-BC-P still has a long-strip square block structure but has more obvious lamella, and after the Ca-BC-P is amplified, the aggregation phenomenon of substances on the surface of the biochar is observed, and pores and the surface are covered by flocculent sediments (figure 7). The results of EDS analysis and element distribution are shown in FIGS. 5 to 7 and Table 2. Before loading, C, O is the main chemical element on the surface of BC and a small amount of Ca exists, which is possibly related to tobacco stalks; after loading Ca, the main element of the Ca-BC surface was C, O, Ca, and the loading of Ca was smaller than that in ICP-OES analysis (table 1), indicating that Ca was loaded in both surface and pore structures. After adsorbing phosphorus, the main elements of the surface of Ca-BC-P are C, O, Ca and P, and the loading amount of P is smaller than that in ICP-OES analysis, which shows that P is combined with Ca on the surface and in the pore structure.

From SEM-EDS analysis, the overall morphology of Ca-BC was greatly changed and the volume was significantly increased compared to BC, presumably due to the large amount of Ca (OH)₂The particles are deposited on the surface of the biochar; besides, the Ca-BC surface porosity is increased, the pore diameter is increased, and the cause of the change is probably Ca (OH) deposited on the surface of the biochar₂And plays a role in activating hole expansion in the loading process. After adsorbing phosphate, a large amount of nano-scale floc can be observed, the P element can be observed to be bonded to the biochar through an EDS chart, and the Ca-BC and the P can be known to pass through by combining thermodynamic and kinetic analysisThe complex chemical reactions produce new species.

TABLE 1 basic LN-WB parameters before and after modification

TABLE 2 energy spectrum analysis results of Ca-BC and Ca-BC-P

To investigate the change in the biochar functional groups before and after loading and adsorption, FTIR analysis was performed on BC, Ca-BC, and Ca-BC-P. As shown in FIG. 8, the results of BC, Ca-BC and Ca-BC-P were found to be 1420 to 1442cm^-1The Duton peak is attributed to C = O stretching vibration, which is probably an organic functional group contained in the biochar itself and is at 3423cm^-1At a distance of 1581cm^-1The peak is marked and is attributed to the stretching vibration and the bending vibration of-OH. At 873cm^-1The peak of the C-O out-of-plane bending characteristic appears. Compared with BC, the absorption peak of Ca-BC is significantly changed and is 3641cm^-1The obvious strong and narrow-OH stretching vibration band appears, and a new characteristic peak is presumed to be derived from Ca (OH)₂，Ca(OH)₂Is successfully loaded onto BC. As shown by comparison before and after adsorbing phosphate, Ca-BC-P is 3641cm^-1the-OH absorption peak at position disappeared and was 1034cm^-1Is exposed to HPO₄ ^2-Characteristic peak of stretching vibration of 959cm^-1、602cm^-1And 564cm^-1PO appears at₄ ^3-The stretching vibration characteristic peak of the method is combined with XRD to show that Ca-BC of the biochar reacts with phosphate to generate Ca₃(PO₄)₂And (4) precipitating. At 1130cm^-1Due to the C-N tensile vibration in addition to the C-H and N-H bending vibration, these abundant organic functional groups on the charcoal surface provide hydrogen bonds and electrostatic adsorption sites for phosphate adsorption.

XRD analysis was performed on BC, Ca-BC, and Ca-BC-P in order to study the biochar phase composition before and after loading and adsorption. Analyzing and processing data through jade 6.0The results are shown in FIG. 9. The characteristic peak of the main phase of BC before loading is CaCO₃(PDF # 99-0022). The main phase characteristic peak of Ca-BC is Ca (OH)₂(PDF #04-0733) and CaCO₃(PDF # 99-0022). Comparison of biochar before and after loading, indicating Ca (OH)₂Has been successfully loaded on the biochar. The main phase characteristic peak of the biochar Ca-BC-P after adsorbing the phosphate is Ca₃(PO₄)₂It is shown that phosphate reacts with Ca to form Ca₃(PO₄)₂And precipitating, thereby achieving the effect of removing the phosphorus in the water body, and being consistent with the conclusion of dynamics, SEM and FTIR.

The results of the examples and application examples show that the invention takes tobacco stalks and oyster shells as raw materials to prepare Ca (OH) -loaded₂The flue gas stalk biochar Ca-BC is subjected to a batch adsorption test, the adsorption characteristics of Ca-BC to phosphate are explored, the dephosphorization effect of Ca-BC in actual wastewater is verified, and the main conclusion is as follows:

the results of Ca-BC thermodynamic and kinetic model fitting show that Langmuir (R)²=0.98) the fitting result is better than Freundlich (R)²=0.78), maximum phosphorus adsorption capacity of 88.64mg P/g, quasi-secondary kinetics (R)²=0.99) better fitting result than quasi-first-order (R)²=0.92), the adsorption reaches equilibrium about 5h, and the adsorption behavior belongs to monolayer chemical adsorption. A comprehensive analysis of the characterization of SEM-EDS, FTIR, XRD, BET and Zeta potentials revealed Ca (OH)₂Successfully loaded on the surface of the tobacco stalk biochar, the specific surface area of the biochar is increased to 2 times of the original specific surface area, the porosity is increased by 4 times, and the adsorption capacity is obviously improved. Ca-BC forms Ca with phosphate in water body through chemical precipitation and electrostatic attraction₃(PO₄)₂To achieve the removal.

The Ca-BC can keep good adsorption capacity to phosphate in a wide pH range, and the adsorption capacity is maintained to be 75-96 mg P/g. Under the condition that the initial pH is slightly acidic (pH = 3-6), the Ca-BC adsorption capacity reaches 93-96 mg P/g, and Ca is mixed with the Ca²⁺Compared with the biochar loaded, the adsorption capacity of Ca-BC is improved by 3-15 times, mainly because Ca (OH) is arranged on the surface of Ca-BC₂Preferentially with H in solution⁺Reacting to make Ca-BC absorb phosphate in alkaline environment; at the beginning of pUnder the condition of H alkalescence (pH = 7-11), the adsorption capacity is 75-88.5 mg P/g and Ca²⁺The adsorption capacity of the loaded biochar is kept equal.

The Ca-BC has good effect when treating actual wastewater, and the removal rate is over 90 percent. For four different actual effluents, the phosphorus removal effect was related to the effluent itself and the amount of Ca-BC added. The optimal addition amount of the cattle farm breeding wastewater in the research is 20g/L, the optimal addition amount of the pig farm breeding wastewater is 10g/L, and the optimal addition amount of pond sewage and domestic sewage is 3.5 g/L.

The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.

Claims

1. A preparation method of modified biochar comprises the following steps:

performing secondary calcination on the waste containing the Ca source, and mixing the waste with water to obtain Ca (OH)₂Suspending liquid;

2. The method of claim 1, wherein the agricultural waste comprises at least one of tobacco straw, and walnut shells.

3. The method according to claim 1, wherein the temperature of the first calcination is 500 to 700 ℃; the heat preservation time is 1-3 h.

4. The method according to claim 3, wherein a temperature increase rate from room temperature to the temperature of the first calcination is5 to 15 ℃/min.

5. The method of claim 1, wherein the Ca-containing source waste comprises at least one of shellfish waste, egg shells, and crab shells.

6. The method according to claim 1, wherein the temperature of the second calcination is 1000 to 1500 ℃; the heat preservation time is 1-3 h.

7. The method according to claim 6, wherein a temperature increase rate from room temperature to the temperature of the second calcination is5 to 15 ℃/min.

8. The production method according to claim 1, wherein the impregnation is performed under stirring conditions; the stirring speed is 600-1000 rpm; the stirring time is 5-9 h.

9. The modified biochar prepared by the preparation method of any one of claims 1-8.

10. Use of the modified biochar of claim 9 for removing phosphate from wastewater.