CN115957718A - Fly ash biochar composite material as well as preparation method and application thereof - Google Patents
Fly ash biochar composite material as well as preparation method and application thereof Download PDFInfo
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- CN115957718A CN115957718A CN202211649787.5A CN202211649787A CN115957718A CN 115957718 A CN115957718 A CN 115957718A CN 202211649787 A CN202211649787 A CN 202211649787A CN 115957718 A CN115957718 A CN 115957718A
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Abstract
The invention provides a fly ash biochar composite material as well as a preparation method and application thereof, belonging to the field of heavy metal treatment. The fly ash biochar composite material is prepared from straws and alkali fusion fly ash; the mass ratio of the straw to the alkali fusion fly ash is 4; the alkali fusion fly ash is prepared from fly ash and an alkaline substance, wherein the mass ratio of the fly ash to the alkaline substance is 1. The maximum adsorption capacity of BCs reaches 14.84mgg, 15.56 mgg and 17.85mgg respectively ‑1 . The maximum adsorption capacity of FBCs reaches 68.24, 84.25 and 137.1mgg respectively ‑1 . At the same temperature, fly ash is generatedThe adsorption capacity of the charcoal composite material is increased by 4.6 times, 5.4 times and 7.7 times compared with the original corn straw charcoal. This shows that the adsorption performance of the corn straw biochar after the fly ash is compounded is greatly improved.
Description
Technical Field
The invention relates to the field of heavy metal treatment, in particular to a fly ash biochar composite material as well as a preparation method and application thereof.
Background
Biological charcoal prepared from original lignocellulose (agricultural and forestry waste such as corn stalk and the like) for Cd in solution 2+ The adsorption effect is general, and researchers modify the biochar with acid, alkali, oxidant and metal ions in order to improve the physicochemical property of the biochar to adapt to different environmental processes. The influence of acid modification on the specific surface area of the biochar is different due to different types and concentrations of acid: after 1M hydrochloric acid treatment, the specific surface area of the reed biochar is 58.75M 2 The/g is increased to 88.35m 2 (ii) in terms of/g. However, the straw biochar had a surface area from 71.35m after treatment with sulfuric acid (2%) 2 The/g is reduced to 56.9m 2 (iv) g. The alkali modification generally increases the specific surface area of the biochar as compared to the acid modification. After the straw biochar is treated by 1MKOH, the specific surface area is 71.35m 2 The/g is increased to 143.3m 2 (iv) g. In general, the purpose of modification is to enlarge the specific surface area, change the functional group, and improve the magnetic properties and catalytic ability. The modification of the oxidant can increase the content of oxygen-containing functional groups on the surface of the biochar. H 2 O 2 The modification can increase oxygen-containing functional groups, particularly carboxyl groups, in the peanut shell biochar prepared by hydrothermal carbonization, thereby improving the adsorption capacity of the peanut shell biochar on Pb (the adsorption capacity of the carboxyl and the hydroxyl on Cd is good, but the peanut shell biochar can be oxidized if the temperature is too high), and H 2 O 2 Similarly, KMnO 4 The modification can also add oxygen-containing functional groups and can introduce MnO 2 The biological carbon is loaded to improve the adsorption capacity of the biological carbon.
In order to make the biochar have better adsorption capacity for heavy metal ions, researchers have adopted various methods to prepare the modified biochar. The Primetric uses KOH and FeCl 3 The wheat straw is modified, so that the surface area of the composite material is increased by 19.11 times compared with that of the original biochar, the quantity of aromatic and oxygen-containing functional groups is increased, a new functional group Fe-O is introduced, and the adsorption quantity of Cd is improved. Liu bang yuThe adsorption of the biochar prepared by mixing the sludge and the corn straws in different proportions on Pb is greater than that of biochar prepared by single agricultural waste biomass or sludge reported by literatures; zhang utilizes sludge and straw to pyrolyze together and improve the pore characteristics of the biochar, and increases the adsorption capacity of the biochar. The nanometer zero-valent iron (nZVI) has the characteristics of small particle size, rich active center, high reaction speed and the like, and is widely applied to the treatment of pollutants, but the environmental application of the nZVI faces a plurality of problems: including its thermodynamic instability, easy oxidation, short reaction duration, poor air stability, rapid agglomeration, etc., it is an effective strategy to improve the stability of nZVI to select a suitable porous carrier as the carrier of nZVI. The biochar is rich in various functional groups, has a large specific surface area and a small carbon skeleton pore size, and has good conductivity, and the characteristics can adjust the particle size, corrosion, dispersion and electron transfer capacity of the nZVI. In recent years, research on biochar as a carrier substrate for various catalysts and adsorbents has been increasingly widespread.
Meanwhile, due to the fact that the lignocellulose raw material is compact in structure, researchers can conduct pretreatment, grinding, hydrolysis, irradiation, drying and the like on crop straw raw materials, then further use the raw materials, and efficiency is high. The pretreatment method includes physical, chemical and biological pretreatment methods. Lin firstly carries out steam explosion pretreatment (SE) on 5 crop straws (wheat, rice, corn, rape and cotton), and then carries out pyrolysis for 2 hours at 500 ℃, and the result shows that the hemicellulose and cellulose in the straws are respectively reduced by 47-95% and 5-16% by steam explosion; the SE treated biochar had a lower Specific Surface Area (SSA) and pore volume than the untreated feedstock, with one exception that SE increased the SSA of the canola straw biochar by about 16 times, indicating that it may be a suitable material for SE pretreatment relative to other crops.
The biochar derived from the corn stalks is widely used for removing pollutants in wastewater due to the large specific surface area, the complex pore structure and rich functional groups. Because of the low adsorption performance of the original corn stalk biochar, in previous studies, researchers began to study the modification of corn stalk biochar: boron modified corn stoverThe straw biochar can convert Fe 2+ Has an adsorption amount of 111.56mgg -1 Increased to 132.78mgg -1 (ii) a The alpha-FeOOH is used for modifying the corn straw biochar and finding that the carbon can treat Cu 2+ The maximum adsorption capacity reaches 144.7mgg -1 (ii) a Modification of corn stalk biochar with acrylonitrile, para Cd 2+ The adsorption capacity of the adsorbent reaches 85.65mgg -1 However, the single corn stalk is not beneficial to developing functional biochar with various properties, and the modification effect on the single corn stalk biochar is not great. Co-pyrolysis of two or more feedstocks has been shown to be effective in improving the performance of biochar.
Fly ash is an industrial solid waste from coal-fired power plants, growing at 5 million tons per year, and has been of great interest for its reasonable disposal and disposal. The main chemical component of the fly ash is SiO 2 、Al 2 O 3 、Fe 2 O 3 And CaO. In recent years, fly ash has been increasingly used as an adsorbent for removing pollutants due to its small particle size and large specific surface area. Sun et al investigated the adsorption behavior of fly ash in aqueous solution to two reactive dyes and two acid dyes to determine the colored material removal capacity of this solid waste (1.860-10,937 mg g) -1 ) (ii) a Roshan Prabhakar used fly ash synthetic zeolite to remove cadmium from water and found a maximum single layer absorption capacity of 145.931mgg -1 (ii) a Wang et al found that the composite of fly ash and pig manure biochar greatly improved the adsorption capacity to methylene blue in wastewater to the maximum of 131.58mgg -1 . However, both virgin and modified fly ashes suffer from the disadvantage of high agglomeration, general adsorption properties and high cost when used alone as an adsorbent.
Disclosure of Invention
In order to solve the problems, the invention provides a fly ash biochar composite material, and a preparation method and application thereof.
In order to achieve the above purpose, the invention provides the following technical scheme:
the invention provides a fly ash biochar composite material which is prepared from straws and alkali fusion fly ash;
the mass ratio of the straw to the alkali fusion fly ash is 4;
the alkali fusion fly ash is prepared from fly ash and an alkaline substance, wherein the mass ratio of the fly ash to the alkaline substance is 1.
Preferably, the alkaline substance is sodium hydroxide.
Preferably, the straw comprises corn stover.
The invention also provides a preparation method of the fly ash biochar composite material in the technical scheme, which comprises the following steps:
1) Mixing the fly ash with an alkaline substance and then carrying out heat treatment to obtain alkali fusion fly ash;
2) Mixing the alkali fusion fly ash obtained in the step 1) with straws and water, carrying out oscillation reaction to obtain an oscillation reactant, drying the oscillation reactant, and carbonizing to obtain the fly ash biochar composite material.
Preferably, the heat treatment conditions of step 1) include: heat treatment is carried out for 0.5h at 650 ℃;
step 2) the conditions of the oscillatory reaction include: the temperature is 22 ℃, the time is 12h, and the rotating speed is 180rpm.
Preferably, the volume ratio of the total mass of the straws and the alkali fused fly ash in the step 2) to the water is 1g.
Preferably, the carbonization conditions in step 2) include: carbonizing for 2 hours at 300-700 ℃ in nitrogen atmosphere.
The invention also provides the application of the fly ash biochar composite material in the technical scheme in heavy metal adsorption.
Preferably, the heavy metal comprises cadmium.
The invention has the beneficial effects that:
the maximum adsorption capacity of BCs reaches 14.84mgg -1 、15.56mgg -1 And 17.85mgg -1 . The maximum adsorption capacity of the FBCs reaches 68.24mgg -1 、84.25mgg -1 And 137.1mgg -1 . At the same temperature, the fly ash and the biochar are made ofThe adsorption capacity is increased by 4.6, 5.4 and 7.7 times compared with the original corn stalk biochar. This shows that the adsorption performance of the corn straw biochar after the fly ash is compounded is greatly improved.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required in the embodiments will be briefly described below.
FIG. 1 (a) is an SEM image of various biochar;
FIG. 1 (b) is an EDS diagram for different biochar;
FIG. 2 shows N of different biochar 2 Adsorption-desorption isotherms and pore size profiles;
FIG. 3 shows adsorption of Cd by charcoal 2+ FTIR spectra before and after;
FIG. 4 shows Cd 2+ XPS spectra of biochar before and after adsorption: spectra and full spectra (e) of C1s (a), O1s (b), si2p (C), cd3d (d);
FIG. 5 shows the adsorption of Cd by charcoal 2+ Front and back XRD patterns;
FIG. 6 shows adsorption of Cd by charcoal 2+ Langmuir (a) and Freundlich model (b);
FIG. 7 shows the adsorption of Cd by charcoal 2+ Thomas model (a), yoon-Nelson model (b) and BJP model map (c).
Detailed Description
The invention provides a fly ash biochar composite material which is prepared from straws and alkali fusion fly ash; the mass ratio of the straw to the alkali fusion fly ash is 4; the alkali fusion fly ash is prepared from fly ash and an alkaline substance, wherein the mass ratio of the fly ash to the alkaline substance is 1.
In the present invention, the alkaline substance is preferably sodium hydroxide, and the sodium hydroxide functions as: naOH can react with particles on the surface of fly ash to form holes on the surface and increase the specific surface area of the fly ash, so that the dissolution of an aluminosilicate phase is promoted, easily soluble phases such as sodium silicate and sodium aluminosilicate are generated, and then the biochar is subjected to composite modification.
In the present invention, the straw preferably comprises corn stover. In the present invention, the corn stalks are preferably in a powder form and sieved through a 10-mesh sieve.
In the invention, the alkali fusion fly ash is prepared from fly ash and an alkaline substance, wherein the mass ratio of the fly ash to the alkaline substance is 1. In the invention, the fly ash and an alkaline substance are mixed and then subjected to heat treatment to obtain the alkali fusion fly ash. In the present invention, the conditions of the heat treatment preferably include: heat treatment is carried out for 0.5h at 650 ℃. The source of the fly ash is not particularly limited in the invention, and the fly ash can be used by a person skilled in the art according to the conventional method.
The use method of the fly ash biochar composite material is not particularly limited, and the fly ash biochar composite material can be used by a person skilled in the art according to the conventional method.
The invention also provides a preparation method of the fly ash biochar composite material in the technical scheme, which comprises the following steps:
1) Mixing the fly ash with an alkaline substance and then carrying out heat treatment to obtain alkali fusion fly ash;
2) Mixing the alkali fusion fly ash obtained in the step 1) with straws and water, carrying out oscillation reaction to obtain an oscillation reactant, drying the oscillation reactant, and carbonizing to obtain the fly ash biochar composite material.
The alkali fusion fly ash is obtained by mixing the fly ash with an alkaline substance and then carrying out heat treatment. In the present invention, the conditions of the heat treatment preferably include: heat treatment is carried out for 0.5h at 650 ℃.
The method comprises the steps of mixing the obtained alkali fusion fly ash with straws and water, carrying out oscillation reaction to obtain oscillation reactants, drying the oscillation reactants, and carbonizing the oscillation reactants to obtain the fly ash biochar composite material. In the present invention, the conditions of the shaking reaction preferably include: the temperature is 22 ℃, the time is 12h, and the rotating speed is 180rpm. In the invention, the volume ratio of the total mass of the straw and the alkali fused fly ash to water is preferably 1g. In the present invention, the carbonization conditions preferably include: carbonizing the mixture for 2 hours at the temperature of 300-700 ℃ in a nitrogen atmosphere, wherein the carbonizing temperature is more preferably 700 ℃.
The invention also provides application of the fly ash biochar composite material in the technical scheme in heavy metal adsorption. In the present invention, the heavy metal preferably includes cadmium.
In order to further illustrate the present invention, the following examples are given in detail, but they should not be construed as limiting the scope of the present invention.
Example 1
1. Preparing alkali fusion fly ash:
uniformly mixing 25 g of Fly Ash (FA) dried by an oven (105 ℃) and 25 g of NaOH, putting the mixture into a nickel crucible, and putting the nickel crucible into a muffle furnace at 650 ℃ for 10 ℃ min -1 The temperature is kept for half an hour, and then the furnace is cooled and taken out from the muffle furnace. And obtaining alkali fusion fly ash, which is marked as AFFA.
2. Mixing pretreatment of corn straw and alkali fusion fly ash:
the corn straws are firstly washed for 3 times by tap water (to remove the soil and dust remained on the corn straws), then naturally dried and cut into small sections of 1-2 cm. And (3) putting the sample into a constant-temperature oven at 105 ℃ for drying, and grinding the sample into powder capable of passing through a 10-mesh sieve by using a grinder, wherein the powder is marked as CS. Then, mixing Alkali Fusion Fly Ash (AFFA) and Corn Straw (CS) according to the mass ratio of AFFA to CS of 1:4, mixing. Specifically, 5g of alkali fusion fly ash and 20g of corn straw are mixed with 200mL of deionized water in a conical flask, and in order to mix the materials uniformly, the flask is placed in a shaking table after being covered with a bottle stopper, the conditions are set to 22 ℃, the reaction is carried out for 12 hours at 180rpm, then the mixture is evaporated to be pasty through an electric furnace, the mixture is continuously dried to constant weight at 65 ℃, crushed and sieved by a 100-mesh sieve, and a pretreated mixture is obtained.
3. Pyrolysis of alkali fusion fly ash biochar:
taking 10g of the mixture pretreated in the step 2, drying the mixture at the high temperature of 85 ℃ for 4 hours, paving the mixture in a corundum crucible, placing the corundum crucible in a quartz tube constant-temperature area of a tube furnace, vacuumizing the corundum crucible for 10 minutes, and then filling nitrogen to replace air in the quartz tube, wherein the flow rate of the nitrogen is stabilized at 85mLmin -1 At 5 deg.C for min under nitrogen protection -1 Heating to a temperature of 300 ℃ at a heating rate ofAnd carbonizing for 2 hours, cooling to room temperature, taking out the sample, washing the obtained product with deionized water for multiple times until the filtrate is neutral, finally performing vacuum drying on the washed product, uniformly grinding the carbonized sample, sieving with a 100-mesh sieve again, drying at 80 ℃, filling into a sealing bag, sealing, and storing in a drying dish for later use. Recording the mass of the sample before and after carbonization, wherein the yield of the biochar is the ratio of the mass after carbonization to the mass of the raw material. The fly ash biochar composite material is marked as FBCs and is hereinafter referred to as FBC300.
Example 2
The difference from the example 1 is that the carbonization temperature in the step 3 is 500 ℃, and the rest conditions are the same, so that the fly ash biochar composite material is obtained, and is marked as FBCs, which is hereinafter referred to as FBC500.
Example 3
The difference from the embodiment 1 is that the carbonization temperature in the step 3 is 700 ℃, and the other conditions are the same, so that the fly ash biochar composite material, marked as FBCs, is obtained, and is hereinafter referred to as FBC700.
Comparative example 1
Preparing biochar:
1. pretreatment of corn straw
The corn straws are firstly washed for 3 times by tap water (to remove the soil and dust remained on the corn straws), then naturally dried and cut into small sections of 1-2 cm. And (3) putting the sample into a constant-temperature oven at 105 ℃ for drying, grinding the sample into powder capable of passing through a 10-mesh sieve by using a grinder, and putting the sample into a sealing bag for storage, wherein the sample is marked as CS.
2. Preparation of corn stalk biochar
Taking 10g of pretreated corn straw, drying at 85 ℃ for 4 hours, spreading in a corundum crucible, placing the corundum crucible in a quartz tube constant-temperature area of a tube furnace, vacuumizing for 10min, then filling nitrogen to replace air in the quartz tube, and stabilizing the flow rate of the nitrogen at 85mLmin -1 At 5 deg.C for min under nitrogen protection -1 Heating to 300 deg.C at a heating rate, carbonizing for 2 hr, cooling to room temperature, taking out, grinding, sieving with 100 mesh sieve, oven drying at 80 deg.C, and standingPlacing into a dryer, keeping constant temperature, placing into a sealed bag, sealing, and storing in a drying dish for use. Recording the mass of the sample before and after carbonization, wherein the yield of the biochar is the ratio of the mass after carbonization to the mass of the raw material. The original biochar is labeled as BCs, hereinafter referred to as BC300.
Comparative example 2
The difference from the comparative example 1 is that the carbonization temperature in the step 2 is 500 ℃, and the original biochar is marked as BCs, hereinafter referred to as BC500, under the same conditions.
Comparative example 3
The difference from comparative example 1 is that the carbonization temperature in step 2 is 700 ℃, and the original biochar is marked as BCs, hereinafter referred to as BC700, under the same conditions.
Example 4
The characterization results of the fly ash biochar composite materials obtained in examples 1-3 and the biochar obtained in comparative examples 1-3 are as follows:
the basic physicochemical properties of raw corn stover Biochar (BCs) and fly ash biochar composites (FBCs) pyrolyzed at different temperatures are shown in table 1. The pyrolysis temperature has a significant effect on the yield of biochar, and as the pyrolysis temperature is increased from 300 ℃ to 700 ℃, the yield of the original corn straw biochar and fly ash biochar composite material is reduced. Under the condition of high temperature (more than or equal to 500 ℃), the biomass forms a uniform pore structure, which is beneficial to the release of volatile components and leads to the reduction of the yield of the biochar. However, at the same temperature, the yield of the fly ash biochar composite material is higher than that of the original corn straw biochar, because the fly ash contains less organic matters and more ash, the thermal stability of the ash is stronger, and the degraded specific gravity is small. For raw Corn Stover (BCs), C is the most predominant element (64-74%), followed by O, H and N, consistent with elements from other feedstocks in other studies. After the alkali fused fly ash biochar composite material (FBCs) is prepared, the content of C is reduced, and the content of other elements is not changed greatly. This is probably due to the fact that alkali fusion modified fly ash produces silicates, which may provide weak oxygen and may react with amorphous carbon formed on the surface of corn stover biochar at high pyrolysis temperatures (see equations (1) and (2)), resulting in a reduction in carbon content.
≡Si-O+C→CO↑+≡Si-(1)
≡Si-O+2C→CO↑+≡Si-C(2)
Compared with the original corn stalk biochar, the pH value of the alkali fusion fly ash biochar composite material shows an upward trend, which is probably mainly caused by deprotonation of silicon-containing groups. In addition, the ash contents of FBC300, FBC500 and FBC700 are 5.4, 5.1 and 4.8 times as high as BC300, BC500 and BC700, respectively, and the increase of ash is closely related to the increase of pH, so the increase of ash is also one of the causes of the increase of pH after modification.
In the invention, the appearance difference before and after the alkali fusion fly ash composite corn straw biochar is compared through SEM-EDS. FIG. 1 (a) shows SEM scanning images of different biochar, and it can be seen from the images that original corn stalk Biochar (BCs) have smooth surface, regular structure and small pore size distribution; the fly ash modified composite biochar materials (FBCs) have porous surfaces, uneven pore diameters, irregular distribution and rough surfaces, and particles are attached to the surfaces. The EDS results (FIG. 1 (b)) demonstrated that the surface material of the biochar consisted of C, al, si, O, etc. This material may contain silicates and carbonates, indicating AFFA was successfully loaded on corn stover biochar.
Performing N on original corn straw Biochar (BCs) and fly ash biochar composite materials (FBCs) 2 Adsorption-desorption isotherm experiments. As can be seen from FIG. 2, N of BCs and FBCs 2 Adsorption-desorption isotherms and pore size distributions, showing a typical Langmuir type IV isotherm, an H3 hysteresis line, indicate the presence of mesopores in the biochar. As can be seen in Table 1, the mean pore size of FBCs increases by a factor of 1.46, 1.84 and 3.29, respectively, under pyrolysis conditions of 300, 500 and 700 deg.C, as compared to BCs. This indicates that the higher temperature is more helpful for the compounding of AFFA and corn stalks, and the increase of the pore diameter is probably due to the fact that the volatilization of volatile substances in the biochar is accelerated by metal oxides in the AFFA under the high-temperature condition, and the pore expansion effect is generated on the biochar.
TABLE 1 physicochemical Properties of different biochar
Surface functional groups on BCs/FBCs were analyzed by FTIR (FIG. 3). At 300-700 ℃, BCs have fewer surface functional groups than FBCs. 3448cm -1 The band of (B) is generated by C-OH/Si-OH stretching vibration, 1573cm -1 The functional group at represents C = C/C = O. After the fly ash and the corn straw are compounded, CO 3 2- Appears at about 1438cm -1 And the intensity of this peak increases with increasing pyrolysis temperature. Si-O-Si appears at 860cm -1 And 1020cm -1 Si-O is present at-460 cm -1 To (3). The situation shows that the composition of the oxygen-containing functional groups in the original corn straw biochar is changed by the composition of the alkali fusion fly ash.
XPS studies characterize the electronic structure of BCs/FBCs. FIG. 4 shows photoelectron spectra of C1s, O1s and Si2p, and a spectrum e in FIG. 4 is a full spectrum of BCs/FBCs. The C1s (a in FIG. 4) peak of BC700 is 284.79, 285.39, 287.35 and 293.60eV, representing C-C/C-H, C-OH, C = O and CO respectively 3 2- 284.76, 285.61 and 287.88eV of FBC700 respectively represent C-C/C-H, C-OH, C = O and 290-294 eV represent CO 3 2- A group. CO 2 3 2- Possibly from ash of the biochar after pyrolysis. After AFFA is compounded with biochar, the relative content of C-C/C-H groups is reduced by 7.42%, the relative content of C = O groups is reduced by 4.38%, and the content of C-OH groups is increased from 28.15% to 33.32%, CO 3 2- The content of radical groups increased from 6.54% to 13.17%. The O1s of BC700 is decomposed into three peaks (b in fig. 4), O-Si-O/Si-OH (532.51 eV), C-OH (531.42 eV), C = O (533.37 eV), respectively, while the peak of FBC700 is 531.58, 532.47, 533.36, 537.10eV, representing C-OH, O-Si-O/Si-OH, C = O, C-OOR groups, respectively. <xnotran> , 4 (c) Si2p , Si-O-Si ( 102.3 eV), si-OH/C-Si-O ( 103.0 eV) C-Si ( 103.9 eV), AFFA . </xnotran> XPS spectra (d in FIG. 4) of Cd3d showed efficient adsorption of cadmium ions by BCs/FBCs.
XRD analysis (figure 5) showed that the major reflections in the BCs series biochar pattern were caused by KCl; in the FBCs series, a large amount of CaSiO is present as the pyrolysis temperature increases 3 Occurs at 29.86 °, 34.4 °, 37.6 °, 41.18 °, 46.6 °, 48.14 ° of 2 θ, siO 2 Peaks appear at 21.11 °, 48.14 ° of 2 θ, caCO 3 The peak appears at 29.3 ° of 2 θ. MgCO production on FBC700 at 700 deg.C 3 2 θ =34.33 °, in addition to the above, the AFFA was again confirmed to have been successfully loaded on corn stalk biochar, caCO 3 And CaSiO 3 Is the most important crystalline phase.
Example 5
The adsorption capacity of the fly ash biochar composite materials obtained in examples 1-3 and the biochar obtained in comparative examples 1-3 is examined, and the steps are as follows:
1. static adsorption
In order to obtain the saturated adsorption capacity of the material to cadmium ions, the isothermal adsorption experiment of the biochar is carried out.
Accurately weighing 0.02g of biochar sample, placing the biochar sample into a sample bottle, and adding 20mL of biochar sample with different initial concentrations (0, 20, 50, 80, 100, 120, 150, 200, 300 and 400 mgL) -1 ) Cd (NO) 3 ) 2 Solutions (three replicates per concentration design) were placed in a constant temperature shaker at 150rmin -1 Oscillating at constant temperature of 30 ℃ for 24h at the rotating speed, taking out a sample bottle, centrifuging, filtering, taking supernatant, and measuring the concentration of cadmium ions by ICP-MS.
Experimental data were fitted using Langmuir and Freundlich
(1) Langmuir model:
the Langmuir adsorption isotherm is based on the uniform surface of the adsorbent and Cd on adjacent sites 2+ The assumption that there is no interaction between ions is monolayer adsorption. The calculation is as follows:
in the formula, Q m (mgg -1 ) As adsorbent per unit mass in equilibriumAdsorption of Cd 2+ Theoretical maximum adsorption capacity of ions; q e (mgg -1 ) And C e (mgL -1 ) Respectively adsorbing Cd 2+ Capacity and Cd in equilibrium solution 2+ The concentration of (d); c 0 (mgL -1 ) For Cd in solution 2+ The lowest initial concentration of (c); k L The Langmuir equilibrium constant associated with surface heterogeneity is shown, indicating the adsorption strength.
(2) Freundlich model
Freundlich adsorption isotherms are based on the assumption that the adsorbent surface is heterogeneous. Thus, it can describe the multi-layer adsorption behavior of highly heterogeneous adsorbents. The empirical formula is as follows:
Q e ＝K F ×C e 1/n
in the formula K F (mg 1-1/n g -1 L 1/n ) Adsorption constant for the Freundlich model; n is a constant of Freundlich model, is related to surface heterogeneity and represents adsorption strength.
2. Dynamic adsorption
The adsorption effect of the prepared biochar material on cadmium in a dynamic environment is researched by utilizing a columnar bed adsorption experiment. The adsorption column used in the experiment was made of acrylic resin, and had a height of 15cm and an inner diameter of 1cm. The same mass (0.8 g) of BC700 and FBC700 was weighed into two adsorption columns, respectively. The prepared solution has the cadmium concentration of 80mgL -1 The cadmium solution passes through the adsorption bed from top to bottom with the flow rate set to 3mLmin -1 And diluting the leachate, and measuring the cadmium ion concentration in the leachate by using ICP-AES.
The experimental data were fitted using Thomas model, yoon-Nelson model and BJP model:
(1) Thomas model
In the dynamic experiment, a Thomas model is used for describing a penetration curve of a fixed bed column, and the maximum adsorption capacity of the adsorbent can be calculated, wherein the expression is as follows:
ln(C 0 /C t -1)＝K Th *q m *M/Q-K Th *C 0 *t
with ln (C) 0 /C t -1) plotting the time t on the ordinate and the abscissa, the maximum adsorption q being calculated by fitting the slope, intercept and rate constants m And K Th
C 0 Is Cd in aqueous solution 2+ Concentration of (2), mgL -1 ；C t Indicating Cd in the effluent at time t 2+ Concentration of (2), mgL -1 ；K Th Represents the rate constant mLmin · -1 mg -1 ；q m Mgg as maximum adsorption -1 (ii) a M is the mass of the adsorbent in the adsorption column, g; q represents the flow rate, mLmin -1 。
(2) Yoon-Nelson model
The Yoon-Nelson model is a semi-empirical formula for evaluation of adsorption behavior without regard to adsorbent mass and velocity, and is as follows:
ln[C t /(C 0 -C t )]＝K yn *(t-τ)
K yn is adsorption rate constant, min -1 And tau is the time when the outlet water concentration reaches half of the inlet water concentration.
(3) The BJP model:
zhang et al, combined with previous experience and knowledge of some fixed bed filtration models, adjusted and optimized the location and form of the parameters, and proposed a simpler, more accurate model, called the BJP model, with the following formula:
wherein C is 0 (mgL -1 ) Is the initial Cd 2+ Concentration; c t (mgL -1 ) The Cd in the effluent at the time of t 2+ Concentration; p is a dimensionless constant; k BJP (min -1 ) Is the rate constant of the BJP model.
Fly ash biochar composite material pair Cd 2+ The results of the adsorption behavior study of (a) are as follows:
as shown in FIG. 6 and Table 2, both Langmuir and Freundlich models fit C welld 2+ Adsorption isotherm shows that the biochar is aligned with Cd 2+ Adsorption of (b) is a complex process that may be controlled by a variety of adsorption mechanisms. However, as can be seen for BCs, the Langmuir model (R) 2 = 0.896-0.979) to Freundlich model (R) 2 = 0.733-0.957) can more accurately describe Cd pair by BCs 2+ Adsorption Process, as do FBCs, R of Langmuir model 2 The value (0.974-0.996) is higher than that of Freundlich model (0.835-0.953), which indicates that the biochar pair Cd 2+ The adsorption of (b) is a chemical monolayer adsorption. The adsorption capacity of BCs does not change greatly along with the rise of pyrolysis temperature, and the maximum adsorption capacity reaches 14.84mgg -1 、15.56mgg -1 And 17.85mgg -1 . For FBCs, the adsorption capacity is rapidly improved along with the improvement of the pyrolysis temperature, and the maximum adsorption capacity reaches 68.24mgg respectively -1 、84.25mgg -1 And 137.1mgg -1 . At the same temperature, the adsorption capacity of the fly ash biochar composite material is increased by 4.6 times, 5.4 times and 7.7 times compared with that of the original corn straw biochar. This shows that the adsorption performance of the maize straw biochar after the fly ash is compounded is greatly improved.
TABLE 2 adsorption of Cd by charcoal 2+ Adsorbed Langmuir and Freundlich model isotherm parameters
Compared with batch experiments, the column experiments are more convenient in evaluating the practical application of the adsorbent. The fixed bed column experimental data were fitted using Thomas, yoon-Nelson and BJP models, and the fitted breakthrough curve is shown in FIG. 7. In conclusion, the fitting coefficients of the 3 models are all better (R is more than or equal to 0.8470) 2 ≦ 0.9759, see Table 3), experimental data for the fixed bed column may be described. In contrast, the BJP model fitted R 2 The value is larger than that of Thomas or Yoon-Nelson model, which shows that the BJP model can better describe the biochar column pair Cd 2+ Dynamic adsorption behavior of (2).
Further, the BC700 (Q) calculated by Thomas m ＝5.090mgg -1 ) And FBC700 (Q) m ＝30.79mgg -1 ) The maximum column adsorption (Table 3) is significantly lower than the calculated BC700 (Q) of Langmuir e ＝17.85mgg -1 ) And FBC700 (Q) e ＝137.1mgg -1 ) Batch adsorption (table 2). This is probably due to the short residence contact time and Cd in the fixed bed column adsorption process 2+ Low initial concentration of the solution. However, it was further found that Q of FBC700 m Or Q e The values were about 6 times higher than BC700, respectively, indicating that AFFA effectively increased the dynamic adsorption capacity of raw biochar (BC 700). Furthermore, under the same conditions, the breakthrough time (t) and τ values for the column packed with FBC700 were much longer than those for the BC700 (fig. 7 and table 3). This is probably due to the AFFA-induced mineral precipitation mechanism increasing Cd on the surface of FBC700 2+ Thereby delaying and prolonging the breakthrough point and time. The result shows that the FBC700 is more suitable for practical application than the BC700 and can process more Cd-rich materials 2+ The wastewater of (2).
TABLE 3 adsorption of Cd by charcoal 2+ Adsorption model parameters of the columnar bed
Although the present invention has been described in detail with reference to the above embodiments, it is only a part of the embodiments of the present invention, not all of the embodiments, and other embodiments can be obtained without inventive step according to the embodiments, and the embodiments are within the scope of the present invention.
Claims (9)
1. A fly ash biochar composite is characterized by being prepared from straws and alkali fusion fly ash;
the mass ratio of the straw to the alkali fusion fly ash is 4;
the alkali fusion fly ash is prepared from fly ash and an alkaline substance, wherein the mass ratio of the fly ash to the alkaline substance is 1.
2. The fly ash biochar composite material as claimed in claim 1, wherein the alkaline substance is sodium hydroxide.
3. The fly ash biochar composite of claim 1, wherein the straw comprises corn stover.
4. A method for preparing a fly ash biochar composite material according to any one of claims 1 to 3, which is characterized by comprising the following steps:
1) Mixing the fly ash with an alkaline substance and then carrying out heat treatment to obtain alkali fusion fly ash;
2) Mixing the alkali fusion fly ash obtained in the step 1) with straws and water, carrying out oscillation reaction to obtain an oscillation reactant, drying the oscillation reactant, and carbonizing to obtain the fly ash biochar composite material.
5. The method according to claim 4, wherein the heat treatment conditions of step 1) include: heat treatment is carried out for 0.5h at 650 ℃;
step 2) the conditions of the oscillatory reaction include: the temperature is 22 ℃, the time is 12h, and the rotating speed is 180rpm.
6. The preparation method of claim 4, wherein the volume ratio of the total mass of the straw and the alkali fused fly ash in the step 2) to the water is 1 g.
7. The method according to claim 4, wherein the carbonization conditions of the step 2) include: carbonizing for 2 hours at 300-700 ℃ in nitrogen atmosphere.
8. Use of the fly ash biochar composite of any one of claims 1 to 3 for adsorbing heavy metals.
9. Use according to claim 8, wherein the heavy metal comprises cadmium.
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