Preparation method and application of iron oxyhydroxide loaded biochar

Abstract

The present application relates to a preparation method and application of iron oxyhydroxide supported biochar. The method comprises: pyrolyzing rice straw to obtain biochar; using non-ferrous grinding medium to perform pre-mechanical force chemical activation on the biochar to obtain ball-milled activated biochar rich in edge defects and oxygen-containing functional groups; after ultrasonic dispersion, slowly adding a ferric salt solution at 35-45 DEG C to enable trivalent iron ions to be positioned and pre-adsorbed on the surface and pores of the carbon; then, adding ammonia water to adjust the pH to 9-10 to enable iron oxyhydroxide to be inhomogeneously nucleated and in-situ precipitated on the surface and pores of the carbon with the pre-adsorbed iron as a nucleation center; and after washing and low-temperature drying at 55-65 DEG C, a product is obtained. Through time sequence control of activation positioning first and phase induction by alkali later, the present application realizes regulation of iron species loading position and binding mode, and the obtained material can be used for arsenic pollution passivation and repair of sandy soil and silty clay.

Description

Technical Field

[0001] This invention belongs to the technical field of contaminated soil remediation materials, and specifically relates to a method for preparing iron hydroxyl oxide-supported biochar, and the application of the material prepared by this method in the passivation remediation of arsenic-contaminated soil. Background Technology

[0002] Arsenic is one of the major pollutants in my country's soil. Mining, smelting, chemical and pesticide production activities result in a large amount of arsenic-containing waste entering the soil environment. Arsenic in soil mainly exists in the form of anions such as arsenite and arsenate. It is highly mobile and bioavailable, and can be absorbed by crops and enter the food chain, posing a threat to ecological security and human health.

[0003] Biochar has a high carbon content and good chemical stability, allowing it to remain in soil for a relatively long time without easily becoming mineralized, making it a suitable carrier for passivating soil arsenic. However, unmodified biochar often carries a negative charge on its surface, resulting in a weak adsorption capacity for arsenate ions in anionic form. Therefore, its application alone is insufficient to effectively reduce the availability of arsenic in the soil.

[0004] Existing technologies mainly employ the following methods to modify biochar for iron-based applications, but all of them have shortcomings:

[0005] (1) The biomass or biochar is impregnated in an iron salt solution and then subjected to high-temperature carbonization or secondary heat treatment. This method requires a second heating to above 600°C, which is energy-intensive and has a long process. Furthermore, iron species are prone to agglomeration and sintering at high temperatures, forming large iron oxide particles that block the biochar pores and reduce the effective specific surface area.

[0006] (2) Iron salts are mixed with biochar and then directly alkali is added for co-precipitation. In this method, iron hydroxyl oxides first undergo homogeneous nucleation and aggregation in the liquid phase, and then attach to the surface of biochar. The active sites inside the aggregates are masked, and there is a lack of interfacial coordination between the iron species and the carbon matrix. The loaded phase is easily detached and lost under soil disturbance and leaching conditions.

[0007] (3) First prepare iron hydroxyl oxide and then mechanically mix it with biochar. In this method, the two are only physically mixed, there is no chemical bonding at the interface, the iron species are unevenly distributed, and the long-term stability is poor.

[0008] (4) Iron salts and biochar are ball-milled together in one step, or a second metal such as manganese or rare earth elements is introduced to improve adsorption performance. The former disperses iron species in a mechanical interlocking manner, and the morphology and distribution are difficult to control; the latter poses an environmental risk of secondary release of the second metal, and the raw material cost is high.

[0009] The common problem with the above-mentioned approaches is that there is a lack of active control over the loading location and binding mode of iron species, and the introduction of iron and the construction of active sites on the carrier surface are not differentiated in terms of timing. Therefore, how to achieve the localized loading and firm binding of iron hydroxyl oxides on the surface and pores of biochar with low energy consumption and without introducing a second metal is a problem that needs to be solved in this field.

[0010] Invention content

[0011] The purpose of this invention is to provide a method for preparing iron hydroxyl oxide-supported biochar, overcoming the shortcomings of existing methods.

[0012] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing iron hydroxyl oxide-supported biochar includes the following steps,

[0013] Step S1, preparation of biochar: Wash, dry, crush and pass rice straw through an 80-mesh sieve. Under nitrogen protection, heat the straw to 380-420℃ at a rate of 8-12℃ / min and pyrolyze it at a constant temperature for 1.5-2.5h. After cooling, biochar is obtained.

[0014] Step S2, Mechanochemical activation: The biochar obtained in step S1 and non-ferrous grinding media are loaded into a ball mill jar at a mass ratio of 1:(8~12). The mixture is dry-milled at 200~300 r / min for 10~14 h without the addition of any iron salts. The direction of rotation is changed every 5~7 h during the ball milling process. The ball milling product is washed with deionized water and dried at 55~65℃ to obtain ball-milled activated biochar.

[0015] Step S3, Pre-adsorption of ferric iron: Disperse the ball-milled activated biochar obtained in step S2 in deionized water to prepare a suspension with an initial mass concentration of 3~5 g / L, and ultrasonically disperse for 15~25 min; Separately, dissolve ferric salt in deionized water to prepare an iron salt solution, and slowly add the iron salt solution dropwise to the above suspension under constant temperature stirring at 35~45℃. After the addition is complete, continue stirring for 8~15 min to pre-adsorb ferric ions on the surface and in the pores of the ball-milled activated biochar.

[0016] Step S4, in-situ alkali-induced phase formation: Under stirring conditions of 500~700 r / min, add 8~12% ammonia water by mass to the system obtained in step S3, adjust the pH of the system to 9~10 and continue stirring for 20~40 min, so that iron hydroxyl oxides precipitate in situ on the surface and in the pores of biochar.

[0017] Step S5, solid-liquid separation and low-temperature drying: After solid-liquid separation, the solid obtained in step S4 is washed three times with deionized water until the washing liquid is neutral, and dried at 55~65℃ to constant weight to obtain iron hydroxyl oxide supported biochar.

[0018] Preferably, the pyrolysis temperature in step S1 is 400℃, the heating rate is 10℃ / min, and the isothermal time is 2h.

[0019] Preferably, the non-ferrous grinding media in step S2 is zirconium oxide beads or zirconium silicate beads; the grinding media is a three-stage gradation combination with particle sizes of 15mm, 5mm and 3mm.

[0020] Preferably, in step S2, the material-to-ball mass ratio is 1:10, the rotation speed is 250 r / min, the ball milling time is 12h, and the rotation direction is changed every 6h.

[0021] Preferably, the trivalent iron salt in step S3 is ferric chloride hexahydrate; the mass ratio of ferric chloride hexahydrate to ball-milled activated biochar is (4~5):1.

[0022] Preferably, in step S3, 0.80g of ball-milled activated biochar is dispersed in 200mL of deionized water, and 3.60g of ferric chloride hexahydrate is dissolved in 80mL of deionized water, and the mixture is slowly added dropwise at 40°C.

[0023] Application of iron hydroxyl oxide-supported biochar obtained by the preparation method according to any one of claims 1 to 6 in the passivation and remediation of arsenic-contaminated soil.

[0024] Preferably, the iron hydroxyl oxide-supported biochar is mixed evenly with arsenic-contaminated soil at a dosage of 1-5% of the dry weight of the soil, the soil moisture content is adjusted to 20-30%, and the soil is cured in the dark at room temperature for 7-28 days.

[0025] Preferably, the dosage is 3% of the dry weight of the soil; the arsenic-contaminated soil is sandy loam or silty clay.

[0026] This invention provides a method for preparing and applying iron hydroxyl oxide-supported biochar. Compared with the prior art, the advantages of this invention are:

[0027] 1. This invention places mechanochemical activation before iron loading and does not introduce exogenous iron during the ball milling stage. Ball milling generates edge defects, bond breakage sites, newly formed oxygen-containing functional groups, and through-pores on the biochar, providing anchoring points for the subsequent targeted adsorption of ferric iron. Compared with the one-step ball milling route of iron salt and biochar, the iron species of this invention form a phase at the liquid interface with chemically adsorbed iron as a precursor, forming an interfacial bond with the carbon matrix, resulting in a firm load and uniform dispersion.

[0028] 2. This invention first completes the pre-adsorption of ferric iron without adding alkali, then adds alkali to push the system into the supersaturated region, transforming the nucleation process from homogeneous nucleation in the liquid phase to heterogeneous nucleation on the carbon surface. Compared with the route of direct co-precipitation after mixing iron salts and biochar, this invention uses pre-adsorbed iron on the surface as the nucleation center. The growth of crystal nuclei is constrained by surface anchoring points and pore space, resulting in higher iron species dispersion and accessibility of active sites. Compared with the route of precipitation followed by mixing, the two in this invention are chemically bonded at the interface rather than physically mixed, making the supported phase less prone to detachment under soil disturbance and leaching conditions. Therefore, the temporal relationship of steps S2, S3, and S4 is a necessary technical feature that distinguishes this invention from existing conventional routes.

[0029] 3. This invention does not involve secondary carbonization, hydrothermal treatment, strong alkaline activators such as KOH, or the introduction of manganese and rare earth elements. It only requires low-temperature drying at 55~65℃, resulting in a short process, low energy consumption, and no risk of secondary release of a second metal.

[0030] 4. The iron hydroxyl oxide formed in this invention is mainly in a low-crystallinity morphology with a high surface hydroxyl density. It can undergo coordination exchange with arsenic oxygen-containing anions to form an inner-layer complex, and has a strong adsorption and co-precipitation fixation ability for arsenic.

[0031] 5. The material of this invention is effective on real arsenic-contaminated soils of different textures. In both sandy loam and silty clay soils, the material can simultaneously and significantly reduce water-soluble arsenic, available arsenic, sulfuric acid-nitric acid extractable arsenic, and TCLP extractable arsenic, indicating that it not only reduces the immediate dissolution of arsenic but also reduces the release risk of arsenic under acidification leaching and leaching scenarios, making it suitable for safe utilization of soil and stabilization before the treatment of contaminated soil.

[0032] 6. This invention uses agricultural waste such as rice straw as raw materials to achieve waste treatment. The raw materials are widely available and inexpensive, making it suitable for large-scale preparation and engineering applications. Attached Figure Description

[0033] Figure 1 This is a process flow diagram of the material preparation method of the present invention;

[0034] Figure 2 This is a graph showing the leaching concentration of water-soluble arsenic in sandy loam soil at different dosages.

[0035] Figure 3 This is a graph showing the leaching concentration of available arsenic in sandy loam soil at different dosages.

[0036] Figure 4 This is a graph showing the arsenic leaching concentration in sandy loam soil at different dosages of sulfuric acid-nitric acid extract.

[0037] Figure 5 This is a graph showing the leaching concentration of arsenic extracted by TCLP in sandy loam soil at different dosages.

[0038] Figure 6 The graph shows the leaching concentrations of various forms of arsenic in silty clay before and after adding 3% of the material of this invention. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of the present invention clearer, the method of the present invention will be described in detail below with reference to embodiments and accompanying drawings.

[0040] Please see Figure 1 The method for preparing iron hydroxyl oxide-supported biochar of the present invention includes the following steps:

[0041] Step S1: Take rice straw, wash it with tap water and deionized water in turn, dry it at 60℃ to constant weight, crush it and pass it through an 80-mesh sieve; place it in a tubular furnace quartz tube, purge it with nitrogen gas, heat it to 400℃ at 10℃ / min, keep it at a constant temperature for 2 hours, and cool it naturally under a nitrogen atmosphere to obtain biochar BCO.

[0042] Step S2: Take BC0 and put it into a 100mL zirconia ball mill jar. Add zirconia beads (15mm, 5mm, 3mm three-stage distribution) at a material-to-ball mass ratio of 1:10. After sealing, dry ball mill the BC0 at 250r / min for 12h, changing the rotation direction every 6h. Wash the ball milling product with deionized water 3 times and dry it at 60℃ to constant weight to obtain ball-milled activated biochar BC.

[0043] Step S3: Weigh 0.80g of BC and disperse it in 200mL of deionized water, and sonicate it for 20min; separately weigh 3.60g of FeCl3·6H2O and dissolve it in 80mL of deionized water, and slowly add it dropwise to the BC suspension under the conditions of 40℃ water bath and 300 r / min stirring. After the addition is complete, continue stirring for 10min.

[0044] Step S4: Under high-speed stirring at 600 r / min, slowly add 10% ammonia water by mass to the above system to adjust the pH of the system to 9.5, and continue stirring for 30 min.

[0045] Step S5: After standing, centrifuge to separate the solid. Wash the solid three times with deionized water until the washing liquid is neutral. Dry at 60°C to constant weight. Grind and pass through a 100-mesh sieve to obtain iron hydroxyl oxide supported biochar, denoted as FeOOH / BC.

[0046] The following section presents the application of iron hydroxyl oxide-supported biochar in the passivation and remediation of arsenic-contaminated soil.

[0047] Example 1: Passivation of arsenic in sandy loam.

[0048] The tested soil was collected from an arsenic-contaminated site in Hubei Province. The soil pH was 8.26, the cation exchange capacity was 33.90 cmol / kg, the total arsenic content was 184 mg / kg, and the soil texture was sandy loam.

[0049] S1. Divide the above soil into 4 groups, each group weighing 1.0 kg (dry weight).

[0050] S2, Group 1 was not added to the blank control CK; Groups 2, 3 and 4 were added to the FeOOH / BC obtained in Example 1 at 1%, 3% and 5% of the dry weight of the soil, respectively, and were denoted as T1, T3 and T5.

[0051] S3. Mix each group thoroughly, add deionized water to adjust the soil moisture content to 25%, and maintain it at 25℃ in the dark for 7 days, replenishing water daily to keep the moisture content constant.

[0052] S4. After curing, air dry and sieve. Water-soluble arsenic, available arsenic, TCLP-extractable arsenic, and sulfuric acid-nitric acid-extractable arsenic are each determined independently, with at least three parallel determinations. After filtering the leachate through a 0.45 μm microporous membrane, the arsenic concentration is determined by inductively coupled plasma mass spectrometry (ICP-MS), and the results are expressed as the mass concentration of arsenic in the leachate (μg / L).

[0053] (1) Water-soluble arsenic: determined according to the "Leaching Toxicity of Solid Waste - Horizontal Oscillation Method" (HJ 557-2010), with a liquid-to-solid ratio of 10:1.

[0054] (2) Available arsenic: 0.5 mol / L NH4H2PO4 was used as the extractant, the soil-to-liquid ratio was 1:25, and the mixture was shaken for 16 hours at room temperature.

[0055] (3) Extraction of arsenic by sulfuric acid-nitric acid: The extraction was carried out in accordance with the "Solid Waste Leaching Toxicity Leaching Method Sulfuric Acid-Nitric Acid Method" (HJ / T299-2007), with a liquid-to-solid ratio of 10:1 and oscillation for 18±2h.

[0056] (4) TCLP extraction of arsenic: The extraction was carried out in accordance with the EPA Method 1311 (TCLP) of the U.S. Environmental Protection Agency. The soil tested in this example was alkaline, and glacial acetic acid-NaOH was used for extraction. The pH value was 4.93±0.05, the liquid-solid ratio was 20:1, and the mixture was shaken for 18±2 hours before measurement.

[0057] Detection results of various forms of arsenic are as follows Figures 2 to 5 As shown in the image.

[0058] Depend on Figure 2It can be seen that the concentration of water-soluble arsenic in the CK group was about 20~21 μg / L; it decreased significantly after the addition of FeOOH / BC, with T1 being about 7 μg / L and T3 and T5 decreasing to about 1~2 μg / L.

[0059] Depend on Figure 3 It can be seen that the effective arsenic concentration in the CK group was approximately 1600 μg / L, decreasing to approximately 930 μg / L at T1, and to approximately 780 μg / L at T3 and T5. The decrease in effective arsenic concentration was less than that in water-soluble arsenic, which is consistent with the general rule that the effective arsenic extraction intensity is higher than that in water-soluble arsenic, while also including weakly bound arsenic. When the dosage increased from 3% to 5%, the decrease in this indicator tended to level off, so 3% was chosen as the optimal dosage.

[0060] Depend on Figure 4 It can be seen that the arsenic extractable by sulfuric acid-nitric acid in the CK group is about 20~21 μg / L, T1 decreases to about 8 μg / L, T3 and T5 decrease to about 1~2 μg / L and below 1 μg / L, respectively, indicating that the material can reduce the risk of arsenic release under acid precipitation leaching scenario.

[0061] Depend on Figure 5 It can be seen that the arsenic extracted by TCLP in the CK group was about 18 μg / L, T1 decreased to about 6 μg / L, and T3 and T5 decreased to about 1 μg / L and below 1 μg / L, respectively, indicating that the material can reduce the migration of arsenic under weak acid leaching conditions in landfills.

[0062] Example 2: Passivation of arsenic in silty clay.

[0063] The basic physicochemical properties of the tested soil are as follows: pH 7.26, cation exchange capacity 35.57 cmol / kg, total arsenic content 133 mg / kg, and texture type silty clay.

[0064] S1. Divide the above soil into 2 groups, each group weighing 1.0 kg (dry weight).

[0065] S2. Group 1 was not added to any material and served as a blank control (CK). Group 2 was added to the FeOOH / BC obtained in Example 1 at 3% of the dry weight of the soil and was denoted as T3.

[0066] S3. Mix each group thoroughly, add deionized water to adjust the moisture content to 25%, and maintain equilibrium at 25℃ in the dark for 7 days, replenishing water daily to maintain constant weight.

[0067] S4. After curing, air dry and sieve. Determine the contents of water-soluble arsenic, available arsenic, sulfuric acid-nitric acid-extractable arsenic, and TCLP-extractable arsenic, respectively. The determination method is the same as in Example 2. The results are as follows: Figure 6 As shown in the image.

[0068] The results showed that the contents of water-soluble arsenic, available arsenic, sulfuric acid-nitric acid-extractable arsenic, and TCLP-extractable arsenic in the CK group were approximately 56 μg / L, 967 μg / L, 26 μg / L, and 23 μg / L, respectively. After adding 3% of the material, these contents decreased to 1 μg / L, 460 μg / L, 0.5 μg / L, and 0.3 μg / L, respectively. Combined with Example 1, it can be seen that the material of the present invention can simultaneously reduce the leaching concentrations of water-soluble arsenic, available arsenic, TCLP-extractable arsenic, and sulfuric acid-nitric acid-extractable arsenic in both sandy loam and silty clay.

Claims

1. A method for preparing iron hydroxyl oxide-supported biochar, characterized in that: Includes the following steps, Step S1, preparation of biochar: Wash, dry, crush and pass rice straw through an 80-mesh sieve. Under nitrogen protection, heat the straw to 380-420℃ at a rate of 8-12℃ / min and pyrolyze it at a constant temperature for 1.5-2.5 h. After cooling, biochar is obtained. Step S2, Mechanochemical activation: The biochar obtained in step S1 and non-ferrous grinding media are loaded into a ball mill jar at a mass ratio of 1:(8~12). The mixture is dry-milled at 200~300 r / min for 10~14 h without the addition of any iron salts. The direction of rotation is changed every 5~7 h during the ball milling process. The ball milling product is washed with deionized water and dried at 55~65℃ to obtain ball-milled activated biochar. Step S3, Pre-adsorption of ferric iron: The ball-milled activated biochar obtained in step S2 is dispersed in deionized water to prepare a suspension with an initial mass concentration of 3-5 g / L, and ultrasonically dispersed for 15-25 min; ferric salt is dissolved in deionized water to prepare an iron salt solution, and the iron salt solution is slowly added dropwise to the above suspension under constant temperature stirring at 35-45℃. After the addition is complete, stirring is continued for 8-15 min to pre-adsorb ferric ions on the surface and in the pores of the ball-milled activated biochar. Step S4, in-situ alkali-induced phase formation: Under stirring conditions of 500~700 r / min, add 8~12% ammonia water by mass to the system obtained in step S3, adjust the pH of the system to 9~10 and continue stirring for 20~40 min, so that iron hydroxyl oxides precipitate in situ on the surface and in the pores of biochar. Step S5, solid-liquid separation and low-temperature drying: After solid-liquid separation, the solid obtained in step S4 is washed three times with deionized water until the washing liquid is neutral, and dried at 55~65℃ to constant weight to obtain iron hydroxyl oxide supported biochar.

2. The method for preparing iron hydroxyl oxide-supported biochar according to claim 1, characterized in that: The pyrolysis temperature in step S1 is 400℃, the heating rate is 10℃ / min, and the isothermal time is 2h.

3. The method for preparing iron hydroxyl oxide-supported biochar according to claim 2, characterized in that: The non-ferrous grinding media in step S2 are zirconium oxide beads or zirconium silicate beads; the grinding media are a three-stage gradation combination with particle sizes of 15mm, 5mm and 3mm.

4. The method for preparing iron hydroxyl oxide-supported biochar according to claim 3, characterized in that: In step S2, the material-to-ball mass ratio is 1:10, the rotation speed is 250 r / min, the ball milling time is 12h, and the rotation direction is changed every 6h.

5. The method for preparing iron hydroxyl oxide-supported biochar according to claim 4, characterized in that: The trivalent iron salt mentioned in step S3 is ferric chloride hexahydrate; the mass ratio of ferric chloride hexahydrate to ball-milled activated biochar is (4~5):

1.

6. The method for preparing iron hydroxyl oxide-supported biochar according to claim 5, characterized in that: In step S3, 0.80 g of ball-milled activated biochar was dispersed in 200 mL of deionized water, and 3.60 g of ferric chloride hexahydrate was dissolved in 80 mL of deionized water. The mixture was then slowly added dropwise at 40 °C.

7. The application of iron hydroxyl oxide-supported biochar obtained by any one of claims 1 to 6 in the passivation and remediation of arsenic-contaminated soil.

8. The application according to claim 7, characterized in that: Mix the iron hydroxyl oxide-supported biochar with arsenic-contaminated soil evenly, adding 1-5% of the soil dry weight, adjust the soil moisture content to 20-30%, and cure at room temperature in the dark for 7-28 days.

9. The application according to claim 8, characterized in that: The dosage is 3% of the dry weight of the soil; the arsenic-contaminated soil is sandy loam or silty clay.

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