Gradient core-shell zero-valent iron porous ceramsite as well as preparation method and application thereof

Abstract

The invention discloses a preparation method and application of gradient core-shell zero-valent iron porous ceramsite, and belongs to the technical field of solid waste recycling and water pollution control. According to the invention, steel slag, red mud, fly ash and municipal sludge are taken as main raw materials, and free heavy metals are removed in a targeted manner through composite acid leaching and Fe-Mn modified charcoal adsorption pretreatment; through in-situ curing of a composite curing agent, pre-sintering and three-section type high-temperature sintering, heavy metal is stably dissolved in ceramsite lattices in a solid mode, secondary pollution caused by leaching of heavy metal ions is avoided, a gradient shell structure with a TiO2 inner layer / biochar-fly ash outer layer is constructed, SiO2 coating modification is conducted on zero-valent iron, and a double-barrier and activity protection mechanism is formed. The prepared gradient core-shell zero-valent iron porous ceramsite shows excellent deep purification capacity in arsenic-containing wastewater treatment, safe recycling of multi-source solid waste and efficient deep purification of high-concentration arsenic-containing wastewater are synchronously achieved, and the gradient core-shell zero-valent iron porous ceramsite has remarkable environmental and economic benefits.

Description

Technical Field

[0001] This invention relates to the fields of solid waste resource utilization and water pollution control, specifically to a gradient core-shell zero-valent iron porous ceramsite, its preparation method, and its application. Background Technology

[0002] Arsenic-containing wastewater is a typical highly toxic industrial wastewater discharged from industries such as mining, smelting, and chemical processing. Among them, trivalent arsenic (As(III)) is about 60 times more toxic than pentavalent arsenic (As(V)) and has strong carcinogenicity, bioaccumulation, and mobility, posing a serious threat to the ecological environment and human health. As(III) mainly exists in the form of neutral H3AsO3 in aqueous solution, which is difficult to remove directly by traditional adsorption methods. Therefore, achieving efficient oxidation and simultaneous fixation of As(III) is the core technical bottleneck of the deep treatment of arsenic-containing wastewater.

[0003] Among the relevant treatment technologies, chemical precipitation is prone to generating a large amount of arsenic-containing sludge, causing secondary pollution. Ion exchange resin is costly and difficult to regenerate. Membrane separation is prone to membrane fouling, leading to a decrease in treatment efficiency. Advanced oxidation-adsorption synergistic technology has become a research hotspot because it can simultaneously achieve As(III) oxidation and As(V) fixation. Among them, the zero-valent iron activated permonosulfate (PMS) system has attracted much attention due to its high oxidation efficiency and mild reaction conditions. However, this system generally suffers from problems such as easy agglomeration and deactivation of zero-valent iron, difficulty in separation and recovery after reaction, and high iron dissolution rate. There is an urgent need to develop new composite materials that combine efficient activation and stable loading functions.

[0004] The discharge of industrial solid wastes such as steel slag, red mud, fly ash, and municipal sludge remains at a high level, highlighting their environmental risks and utilization challenges. Steel slag, being hard, is prone to weathering and pulverization during open-air storage, and its heavy metal components may leach out with rainwater, polluting surrounding soil. Red mud is highly alkaline and has a high water content; large-scale storage not only occupies land resources but also poses a risk of dam failure, and its highly alkaline leachate can cause alkaline pollution of groundwater systems. While fly ash is currently used as an admixture in building materials, the overall utilization rate is less than 60%, and its added value is low; its fine particulate matter is easily dispersed, exacerbating air pollution. Municipal sludge is rich in organic matter and heavy metals; direct landfilling will produce highly polluted leachate, threatening groundwater safety; incineration poses a risk of releasing toxic gases such as dioxins.

[0005] Current industrial solid waste resource utilization technologies face systemic limitations: First, the resource utilization model is singular, mainly focusing on low-value utilization, lacking process design for multi-solid waste synergy, making it difficult to achieve component complementarity and maximize environmental benefits; second, pollution control throughout the entire process is lacking, especially during the drying and sintering stages, heavy metals are prone to volatilization into the atmosphere, and the products are prone to leaching excessive heavy metals in acidic and humid environments. Although existing pretreatment methods (such as single strong acid rinsing) can partially remove heavy metals, they severely damage the solid waste skeleton structure and lack subsequent stabilization measures, resulting in the continuous existence of pollution migration risks. The overall technical route is still out of touch with actual environmental needs such as water pollution control, restricting the dual improvement of resource utilization efficiency and ecological benefits.

[0006] Therefore, developing a multifunctional material that can achieve efficient synergy among multiple solid wastes, precise control of process pollution, and high added value is of great and urgent practical significance. Summary of the Invention

[0007] To address the issues of heavy metal pollution migration, resource conversion, and insufficient environmental safety, the present invention aims to provide a method for preparing gradient core-shell zero-valent iron porous ceramsite, specifically including the following steps: S1: Crush steel slag, sieve, and dry it to obtain steel slag powder; crush red mud, sieve, and dry it to obtain red mud powder; sieve fly ash and dry it to obtain fly ash powder; dry municipal sludge, grind it, and sieve it to obtain municipal sludge powder.

[0008] S2: Pre-treat steel slag powder, red mud powder, fly ash powder and municipal sludge powder to obtain pre-treated steel slag powder, red mud powder, fly ash powder and municipal sludge powder.

[0009] S3: Crush rice husks, sieve, and dry to obtain rice husk powder; pyrolyze the rice husk powder at 500~550℃ under a nitrogen atmosphere for 2~2.5h to obtain rice husk biochar; add the rice husk biochar to a Fe / Mn mixed solution, wherein the liquid-solid ratio of the rice husk biochar to the Fe / Mn mixed solution is 8mL:1g, shake the reaction, and then calcine at 500~550℃ under a nitrogen atmosphere for 1~1.5h to obtain Fe-Mn modified rice husk biochar.

[0010] S4: Mix the steel slag powder, red mud powder, fly ash powder and municipal sludge powder after the pretreatment in step S2 in a mass ratio of (15~25):(20~30):(25~35):(10~15) to obtain mixed powder A; add Fe-Mn modified rice husk biochar to mixed powder A, with a mass ratio of mixed powder A to Fe-Mn modified rice husk biochar of (20~12.5):1 to obtain mixed powder B; add water to mixed powder B, with a liquid-solid ratio of mixed powder B to water of 5mL:1g; shake to react, filter, and dry to obtain a metal solid waste mixture.

[0011] S5: Mix the metal solid waste mixture and starch in a mass ratio of (70~85):(2~5), add composite curing agent at the same time, dry mix at 25℃ for 40min to obtain a dry mixed system, add a 10% PVA aqueous solution to the dry mixed system, the liquid-solid ratio of PVA aqueous solution to dry mixed system is 1mL:6g, wet mix at 25℃ for 30~40min to obtain mud.

[0012] S6: Extrude the clay material to form a blank; age the blank at 25℃ for 12~24h to obtain an aged blank.

[0013] S7: Place the aged green body in a forced-air drying oven for three-stage gradient drying: first dry at 50~60℃ for 2~3 hours, then raise the temperature to 80~90℃ for 2~3 hours, and finally raise the temperature to 105~110℃ for 4~6 hours to obtain the dried green body.

[0014] S8: Introduce a mixed gas (5% CO2 volume fraction, the remainder being air) into the dried green body, raise the temperature to 400-450℃ at a rate of 2-5℃ / min, and hold for 2-3 hours; then, without any atmosphere, perform three-stage gradient calcination in the muffle furnace: raise the temperature to 300℃ at a rate of 5-8℃ / min and hold for 1 hour; then raise the temperature from 300℃ to 600℃ at a rate of 3-5℃ / min and hold for 2 hours; then raise the temperature from 600℃ to 900-1000℃ at a rate of 4-5℃ / min and hold for 1.5-2.5 hours to obtain the ceramsite core.

[0015] S9: Immerse the core of the ceramic particle in a water-based TiO2 sol with a mass percentage concentration of 5%, the liquid-solid ratio of TiO2 sol to ceramic particle core is 8mL:1g, let it stand to react, take it out, dry it, and obtain ceramic particles with an inner TiO2 shell.

[0016] S10: Immerse the ceramic particles with inner TiO2 shells into a biochar-fly ash mixed slurry. The liquid-solid ratio of the biochar-fly ash mixed slurry and the ceramic particles with inner TiO2 shells is 10 mL: 1 g. Allow the mixture to stand and react. Remove the particles and dry them to obtain the gradient shell ceramic particle core.

[0017] S11: Immerse the gradient shell ceramic core in a mixed aqueous solution of Na2SiO3 and FeSO4, wherein the liquid-solid ratio of the mixed aqueous solution to the gradient shell ceramic core is 1mL:8~12g, then purge with nitrogen for 40min to remove oxygen, stir and react to obtain a pretreated ceramic system.

[0018] S12: Under nitrogen protection, add a 0.05-0.3 mol / L NaBH4 aqueous solution to the pretreated ceramsite system at a rate of 1-2 mL / min. The NaBH4 reacts with Fe in the pretreated ceramsite system. 2+ The molar ratio is 1~2:1. After the addition is complete, stir for 30~60 minutes to obtain a wet-based ceramsite system.

[0019] S13: The wet-based ceramsite system is filtered and separated, rinsed with nitrogen-saturated deionized water 3-5 times, and then the wet-based ceramsite is immersed in a KMnO4 aqueous solution with a concentration of 0.01-0.05 mol / L. The liquid-solid ratio of the wet-based ceramsite to the KMnO4 aqueous solution is 1 mL: 5-8 g. The reaction is stirred, filtered, and vacuum dried to obtain gradient core-shell zero-valent iron porous ceramsite.

[0020] Preferably, the sieving conditions in steps S1 and S3 of the present invention are both: passing through an 80-120 mesh sieve.

[0021] Preferably, the drying conditions in step S1 of the present invention are: drying at 105~120℃ to constant weight.

[0022] Preferably, the drying conditions in step S9 of the present invention are: drying at 60~80℃ for 3~4 hours.

[0023] Preferably, the drying conditions in step S10 of the present invention are: drying at 80~100℃ for 1~2 hours.

[0024] Preferably, the vacuum drying conditions in step S13 of the present invention are: vacuum drying at 60°C for 2-4 hours.

[0025] Preferably, the conditions for the shaking reaction in step S3 of the present invention are: shaking reaction at 25°C for 20~24h.

[0026] Preferably, the conditions for the shaking reaction in step S4 of the present invention are: shaking reaction at 25°C for 2-4 hours.

[0027] Preferably, the conditions for the static reaction in step S9 of the present invention are: static reaction at 25°C for 1 hour.

[0028] Preferably, in step S10 of the present invention, the conditions for the static reaction are: static reaction at 25°C for 30-40 minutes.

[0029] Preferably, the stirring reaction conditions in step S11 of the present invention are: stirring reaction at 25~40℃ for 1~2 hours.

[0030] Preferably, the stirring reaction conditions in step S13 of the present invention are: stirring reaction at 25°C for 15~30 min.

[0031] Preferably, the preprocessing in step S2 of the present invention specifically includes: ① Add steel slag powder to a 0.5~1.0 mol / L citric acid-tartaric acid composite aqueous solution with a solid-liquid ratio of 10 mg: 1 L. Stir at 25~30℃ for 60~70 min, filter, wash with deionized water until the pH of the washing solution is 6.0~7.0, and dry at 105℃ to constant weight to obtain pretreated steel slag powder.

[0032] ② Add the red mud powder to a 0.5~1.0 mol / L citric acid-tartaric acid composite aqueous solution. The solid-liquid ratio of the red mud powder to the composite acid aqueous solution is 10 mg: 1 L. Stir at 25~30℃ for 60~70 min, filter, wash with deionized water until the pH of the washing solution is 6.0~7.0, and dry at 105℃ to constant weight to obtain the pretreated red mud powder.

[0033] ③ Add fly ash powder to a 0.5~1.0 mol / L citric acid-tartaric acid composite aqueous solution with a solid-liquid ratio of 10 mg: 1 L. Stir at 40~50℃ for 90~120 min, filter, rinse with deionized water until the pH of the washing solution is 6.0~7.0, and dry at 105℃ to constant weight to obtain pretreated fly ash powder.

[0034] ④ Add municipal sludge powder to a 0.5~1.0 mol / L citric acid-tartaric acid composite aqueous solution. The solid-liquid ratio of municipal sludge powder to composite acid aqueous solution is 10 mg: 1 L. Stir at 40~50℃ for 90~120 min, filter, wash with deionized water until the pH of the washing solution is 6.0~7.0, and dry at 105℃ to constant weight to obtain pretreated municipal sludge powder.

[0035] Preferably, the citric acid-tartaric acid composite aqueous solution of the present invention is composed of citric acid and tartaric acid in a molar ratio of 2:1.

[0036] Preferably, in step S3 of the present invention, the Fe / Mn mixed solution is composed of a mixture of Fe(NO3)3 aqueous solution and Mn(NO3)2 aqueous solution, wherein the concentration of Fe(NO3)3 aqueous solution is 0.2~0.25 mol / L and the concentration of Mn(NO3)2 aqueous solution is 0.07~0.075 mol / L.

[0037] Preferably, in step S3 of the present invention, the molar ratio of Fe to Mn in the Fe / Mn mixed solution is 3:1.

[0038] Preferably, in step S5 of the present invention, the mass ratio of the metal solid waste mixture to the composite curing agent is (20~12.5):1.

[0039] Preferably, in step S5 of the present invention, the composite curing agent is composed of NH4H2PO4 and sodium bentonite mixed in a mass ratio of (1~2):(2~3).

[0040] Preferably, the moisture content of the mud material in step S5 of the present invention is 28-32%.

[0041] Preferably, the extrusion blanking conditions in step S6 of the present invention are: blanking is performed under a pressure of 0.8~1.2MPa and an extrusion rate of 5~8cm / s; the blank is a cylinder with a diameter of 3~6mm and a length-to-diameter ratio of 1.5~2.

[0042] Preferably, in step S10 of the present invention, the biochar-fly ash mixed slurry is composed of modified biochar, fly ash and deionized water in a mass ratio of (1~2):(3~5):(10~12).

[0043] Preferably, in step S11 of the present invention, the concentration of FeSO4 in the mixed aqueous solution of Na2SiO3 and FeSO4 is 0.1~0.2 mol / L, and the molar ratio of Na2SiO3 to FeSO4 is 1:5~6.

[0044] Another objective of this invention is to provide an application of gradient core-shell zero-valent iron porous ceramic particles in the adsorption of arsenic in water.

[0045] Mechanism of this invention: This invention achieves safe controlled release of heavy metals from solid waste through pretreatment removal, in-situ solidification, and high-temperature locking, and efficiently removes arsenic pollution from water using a gradient core-shell-zero-valent iron activation synergistic system. Citric acid-tartaric acid leaching and Fe-Mn biochar adsorption target and remove free heavy metals, while a phosphate-bentonite composite agent solidifies residual metals in situ (solidification rate ≥97%). High-temperature sintering then stably dissolves the heavy metals in the ceramic lattice, resulting in leaching concentrations far below national standards. Furthermore, by constructing a TiO2 inner layer / biochar-fly ash outer gradient shell and coating zero-valent iron with SiO2, a double barrier is formed, preventing the leaching of internal heavy metals and increasing the utilization rate of zero-valent iron to 90%. During arsenic removal, the protected zero-valent iron continuously activates PMS, oxidizing As(III) to As(V); the gradient shell enriches arsenic and PMS, and the Fe-O / Al-O groups on the ceramic surface fix As(V) through complexation; simultaneously, steel slag in the solid waste slowly releases OH-. - The pH of the system is automatically adjusted to a suitable range to synergistically ensure the activity and fixation effect of zero-valent iron, and finally form an integrated synergistic mechanism of controlled release-oxidation-adsorption-fixation, so as to simultaneously realize the safe resource utilization of solid waste and the deep purification of arsenic-containing wastewater.

[0046] This invention provides a gradient core-shell zero-valent iron porous ceramsite, its preparation method, and its application, which have the following beneficial effects: (1) This invention reduces the amount of heavy metals in solid waste at the source and permanently locks them in the ceramic lattice through a three-step pretreatment process of rinsing-adsorption-solidification combined with a pre-calcination-sintering process. The leaching concentration is far lower than the national standard, eliminating secondary pollution. The TiO2 / biochar-fly ash gradient shell and the SiO2 coating of zero-valent iron form a double barrier, effectively isolating the locked heavy metals from the external environment and the active sites of zero-valent iron. This not only prevents the leaching of heavy metals but also avoids the deactivation of the zero-valent iron catalyst, making the utilization rate of zero-valent iron exceed 90%, thus ensuring the long-term stability and safety of the material.

[0047] (2) The gradient core-shell zero-valent iron porous ceramsite prepared by the present invention exhibits excellent deep purification capabilities in the treatment of arsenic-containing wastewater. In particular, it can maintain a high arsenic removal rate even under high arsenic concentration (e.g., 200 mg / L), demonstrating excellent pollutant tolerance and structural stability. This ceramsite material is not only suitable for upgrading conventional low-concentration arsenic-containing wastewater, but also effectively addresses high-concentration, highly toxic arsenic-containing wastewater generated by industries such as smelting, chemical, and mining, significantly broadening its industrial application scenarios. The present invention provides an efficient, reliable, and environmentally friendly solution for the treatment of such difficult-to-treat wastewater, demonstrating outstanding technical competitiveness and practical engineering application potential.

[0048] (3) This invention achieves the unity of waste treatment and cost reduction and efficiency improvement. Using steel slag, red mud, fly ash, municipal sludge and rice husk as raw materials, it transforms them into high-value core-shell structure catalysts through an environmentally friendly biodegradable acid pretreatment process. This process not only avoids secondary pollution, but also turns low-value / negative-cost waste into resources, increasing the added value by more than 10 times. It provides an innovative path for the large-scale high-value utilization of solid waste that has both environmental and economic benefits. Detailed Implementation

[0049] The technical solutions of this invention will now be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0050] The citric acid-tartaric acid composite aqueous solution used in the embodiments and comparative examples of this invention is composed of a mixture of citric acid and tartaric acid in a molar ratio of 2:1.

[0051] Example 1 The specific steps for preparing gradient core-shell zero-valent iron porous ceramsite are as follows: S1: Crush steel slag, pass it through an 80-mesh sieve, and dry it at 105℃ to constant weight to obtain steel slag powder; crush red mud, pass it through an 80-mesh sieve, and dry it at 105℃ to constant weight to obtain red mud powder; pass fly ash through an 80-mesh sieve, and dry it at 105℃ to constant weight to obtain fly ash powder; dry municipal sludge at 105℃ to constant weight, grind it, and pass it through an 80-mesh sieve to obtain municipal sludge powder.

[0052] S2: Pretreatment of steel slag powder, red mud powder, fly ash powder, and municipal sludge powder, specifically: ① Add steel slag powder to a 0.5 mol / L citric acid-tartaric acid composite aqueous solution (the solid-liquid ratio of steel slag powder to composite acid aqueous solution is 10 mg: 1 L), stir at 25℃ for 60 min, filter, wash with deionized water until the pH of the washing solution is 7.0, and dry at 105℃ to constant weight to obtain pretreated steel slag powder.

[0053] ② Add the red mud powder to a 0.5 mol / L citric acid-tartaric acid composite aqueous solution (the solid-liquid ratio of red mud powder to composite acid aqueous solution is 10 mg: 1 L), stir at 25 °C for 60 min, filter, wash with deionized water until the pH of the washing solution is 7.0, and dry at 105 °C to constant weight to obtain the pretreated red mud powder.

[0054] ③ Add fly ash powder to a 0.5 mol / L citric acid-tartaric acid composite aqueous solution (the solid-liquid ratio of fly ash powder to composite acid aqueous solution is 10 mg: 1 L), stir at 40℃ for 90 min, filter, wash with deionized water until the pH of the washing solution is 7.0, and dry at 105℃ to constant weight to obtain pretreated fly ash powder.

[0055] ④ Add municipal sludge powder to a 0.5 mol / L citric acid-tartaric acid composite aqueous solution (the solid-liquid ratio of municipal sludge powder to composite acid aqueous solution is 10 mg: 1 L), stir at 40℃ for 90 min, filter, wash with deionized water until the pH of the washing solution is 7.0, and dry at 105℃ to constant weight to obtain pretreated municipal sludge powder.

[0056] S3: Crush rice husks, pass them through an 80-mesh sieve, and dry them at 105℃ to constant weight to obtain rice husk powder; pyrolyze the rice husk powder at 500℃ under a nitrogen atmosphere for 2 hours, and obtain rice husk biochar after cooling; add the rice husk biochar to a Fe / Mn mixed solution (the liquid-solid ratio of rice husk biochar and Fe / Mn mixed solution is 8mL:1g, shake at 25℃ for 24 hours, and then calcine at 500℃ under a nitrogen atmosphere for 1 hour to obtain Fe-Mn modified rice husk biochar; the Fe / Mn mixed solution is composed of Fe(NO3)3 aqueous solution and Mn(NO3)2 aqueous solution, the concentration of Fe(NO3)3 aqueous solution is 0.2mol / L, the concentration of Mn(NO3)2 aqueous solution is 0.07mol / L, and the molar ratio of Fe to Mn in the Fe / Mn mixed solution is 3:1.

[0057] S4: Mix the steel slag powder, red mud powder, fly ash powder and municipal sludge powder pretreated in step S2 at a mass ratio of 15:20:25:10 to obtain mixed powder A; add Fe-Mn modified rice husk biochar to mixed powder A (the mass ratio of mixed powder A to Fe-Mn modified rice husk biochar is 20:1) to obtain mixed powder B; add water to mixed powder B (the liquid-solid ratio of mixed powder B to water is 5mL:1g), shake at 25℃ for 2h, filter, and dry at 105℃ to constant weight to obtain a metal solid waste mixture.

[0058] S5: Mix the metal solid waste mixture and starch at a mass ratio of 70:5, and simultaneously add the composite curing agent (mass ratio of metal solid waste mixture to composite curing agent of 20:1). Put the mixture into a twin-shaft mixer and dry mix for 40 minutes to obtain a dry-mixed system. The composite curing agent is composed of NH4H2PO4 and sodium bentonite mixed at a mass ratio of 1:2. Add a 10wt% PVA aqueous solution to the dry-mixed system (liquid-solid ratio of PVA aqueous solution to dry-mixed system of 1mL:6g), and stir wet-mix for 30 minutes to obtain sludge with a moisture content of 28%.

[0059] S6: Put the mud into a screw extruder and prepare a green body under the conditions of pressure of 1.2MPa and extrusion rate of 8cm / s. The green body is a cylinder with a diameter of 3~6mm and a length-to-diameter ratio of 1.5~2. The green body is aged at 25℃ for 24h to obtain the aged green body.

[0060] S7: Place the aged green body in a forced-air drying oven for three-stage gradient drying: first dry at 50℃ for 2 hours, then heat up to 80℃ for 3 hours, and finally heat up to 105℃ for 4 hours to obtain the dried green body.

[0061] S8: Place the dried green body into a muffle furnace, introduce a mixed gas (5% CO2 volume fraction, the remainder being air), and raise the temperature to 400℃ at a rate of 2℃ / min, holding for 2 hours; then, without any atmosphere in the muffle furnace, raise the temperature to 300℃ at a rate of 5℃ / min, holding for 1 hour; then raise the temperature from 300℃ to 600℃ at a rate of 5℃ / min, holding for 2 hours; finally, raise the temperature from 600℃ to 900℃ at a rate of 4℃ / min, holding for 2 hours to obtain the ceramsite core.

[0062] S9: Immerse the core of the ceramsite in a 5% (w / w) water-based TiO2 sol, with a liquid-to-solid ratio of 8 mL:1 g. Let it stand at 25°C for 1 hour, then remove it and dry it at 60°C for 4 hours to obtain ceramsite with an inner TiO2 shell.

[0063] S10: Immerse the ceramic particles with the inner TiO2 shell into a biochar-fly ash mixed slurry. The biochar-fly ash mixed slurry is composed of modified biochar, fly ash and deionized water in a mass ratio of 1:3:10. Control the liquid-solid ratio to 10mL:1g. Let it stand and soak for 30min at 25℃. Take it out and dry it at 80℃ for 1h to form the outer shell and obtain the gradient shell ceramic particle core.

[0064] S11: Immerse the gradient shell ceramic core into a mixed solution of Na2SiO3 and FeSO4 (the concentration of FeSO4 in the mixed solution of Na2SiO3 and FeSO4 is 0.2mol / L, and the molar ratio of Na2SiO3 to FeSO4 is 1:5). The liquid-solid ratio of the gradient shell ceramic core and the mixed solution is 1mL:8g. Nitrogen gas is introduced for 40min to remove oxygen. The mixture is stirred in a constant temperature water bath at 25℃ for 2h to obtain the pretreated ceramic system.

[0065] S12: Under nitrogen protection, add a 0.2 mol / L NaBH4 solution (NaBH4 to FeSO4 molar ratio of 1:1) dropwise to the pretreated ceramsite system at a rate of 1 mL / min. After the addition is complete, stir at 200 rpm for 30 min to obtain a wet-based ceramsite system.

[0066] S13: The wet-based ceramsite system was filtered and separated, rinsed three times with nitrogen-saturated deionized water, and then the wet-based ceramsite was immersed in a 0.03 mol / L KMnO4 aqueous solution (the liquid-solid ratio of wet-based ceramsite to KMnO4 aqueous solution was 1 mL: 5 g). The mixture was stirred at 25 °C for 15 min (stirring speed 100 rpm), filtered, and vacuum dried at 60 °C for 2 h to obtain gradient core-shell zero-valent iron porous ceramsite.

[0067] Example 2 The specific steps for preparing gradient core-shell zero-valent iron porous ceramsite are as follows: S1: Crush steel slag, pass it through a 100-mesh sieve, and dry it at 110℃ to constant weight to obtain steel slag powder; crush red mud, pass it through a 100-mesh sieve, and dry it at 110℃ to constant weight to obtain red mud powder; pass fly ash through a 100-mesh sieve, and dry it at 110℃ to constant weight to obtain fly ash powder; dry municipal sludge at 110℃ to constant weight, grind it, and pass it through a 100-mesh sieve to obtain municipal sludge powder.

[0068] S2: Pretreatment of steel slag powder, red mud powder, fly ash powder, and municipal sludge powder, specifically: ① Add steel slag powder to a 0.8 mol / L citric acid-tartaric acid composite aqueous solution (the solid-liquid ratio of steel slag powder to composite acid aqueous solution is 10 mg: 1 L), stir at 28℃ for 70 min, filter, wash with deionized water until the pH of the washing solution is 6.5, and dry at 105℃ to constant weight to obtain pretreated steel slag powder.

[0069] ② Add the red mud powder to a 0.8 mol / L citric acid-tartaric acid composite aqueous solution (the solid-liquid ratio of red mud powder to composite acid aqueous solution is 10 mg: 1 L), stir at 28℃ for 70 min, filter, wash with deionized water until the pH of the washing solution is 6.5, and dry at 105℃ to constant weight to obtain the pretreated red mud powder.

[0070] ③ Add fly ash powder to a 0.8 mol / L citric acid-tartaric acid composite aqueous solution (the solid-liquid ratio of fly ash powder to composite acid aqueous solution is 10 mg: 1 L), stir at 45℃ for 120 min, filter, wash with deionized water until the pH of the washing solution is 6.5, and dry at 105℃ to constant weight to obtain pretreated fly ash powder.

[0071] ④ Add municipal sludge powder to a 0.8 mol / L citric acid-tartaric acid composite aqueous solution (the solid-liquid ratio of municipal sludge powder to composite acid aqueous solution is 10 mg: 1 L), stir at 45℃ for 120 min, filter, wash with deionized water until the pH of the washing solution is 6.5, and dry at 105℃ to constant weight to obtain pretreated municipal sludge powder.

[0072] S3: Crush rice husks, pass them through a 100-mesh sieve, and dry them at 110℃ to constant weight to obtain rice husk powder; pyrolyze the rice husk powder at 525℃ under a nitrogen atmosphere for 2.5h, and obtain rice husk biochar after cooling; add the rice husk biochar to a Fe / Mn mixed solution (the liquid-solid ratio of rice husk biochar and Fe / Mn mixed solution is 8mL:1g, shake at 25℃ for 20h, and then calcine at 525℃ under a nitrogen atmosphere for 1.5h to obtain Fe-Mn modified rice husk biochar; the Fe / Mn mixed solution is composed of Fe(NO3)3 aqueous solution and Mn(NO3)2 aqueous solution, the concentration of Fe(NO3)3 aqueous solution is 0.22mol / L, the concentration of Mn(NO3)2 aqueous solution is 0.072mol / L, and the molar ratio of Fe to Mn in the Fe / Mn mixed solution is 3:1.

[0073] S4: Mix the steel slag powder, red mud powder, fly ash powder and municipal sludge powder pretreated in step S2 at a mass ratio of 25:30:35:15 to obtain mixed powder A; add Fe-Mn modified rice husk biochar to mixed powder A (the mass ratio of mixed powder A to Fe-Mn modified rice husk biochar is 15:1) to obtain mixed powder B; add water to mixed powder B (the liquid-solid ratio of mixed powder B to water is 5mL:1g), shake at 25℃ for 3h, filter, and dry at 105℃ to constant weight to obtain a metal solid waste mixture.

[0074] S5: Mix the metal solid waste mixture and starch at a mass ratio of 80:3, and simultaneously add the composite curing agent (the mass ratio of the metal solid waste mixture and the composite curing agent is 15:1). Put the mixture into a twin-shaft mixer and dry mix for 40 minutes to obtain a dry-mixed system. The composite curing agent is composed of NH4H2PO4 and sodium bentonite mixed at a mass ratio of 1:1. Add a 10wt% PVA aqueous solution to the dry-mixed system (the liquid-solid ratio of the PVA aqueous solution to the dry-mixed system is 1mL:6g), and stir and wet mix for 35 minutes to obtain sludge with a moisture content of 30%.

[0075] S6: Put the mud into a screw extruder and prepare a green body under the conditions of pressure of 0.8MPa and extrusion rate of 5cm / s. The green body is a cylinder with a diameter of 3~6mm and an aspect ratio of 1.5~2. The green body is aged at 25℃ for 18h to obtain the aged green body.

[0076] S7: Place the aged green body in a forced-air drying oven for three-stage gradient drying: first dry at 55℃ for 3 hours, then heat up to 85℃ for 2 hours, and finally heat up to 108℃ for 6 hours to obtain the dried green body.

[0077] S8: Place the dried green body into a muffle furnace, introduce a mixed gas (5% CO2 by volume, the remainder being air), and raise the temperature to 420℃ at a rate of 4℃ / min, holding for 3 hours; then, without any atmosphere in the muffle furnace, raise the temperature to 300℃ at a rate of 6℃ / min, holding for 1 hour; then raise the temperature from 300℃ to 600℃ at a rate of 4℃ / min, holding for 2 hours; finally, raise the temperature from 600℃ to 950℃ at a rate of 5℃ / min, holding for 2.5 hours to obtain the ceramsite core.

[0078] S9: Immerse the core of the ceramsite in a 5% (w / w) water-based TiO2 sol, with a liquid-to-solid ratio of 8 mL:1 g between the water-based TiO2 sol and the core of the ceramsite. Let it stand at 25°C for 1 hour, then remove it and dry it at 70°C for 3 hours to obtain ceramsite with an inner TiO2 shell.

[0079] S10: Immerse the ceramic particles with the inner TiO2 shell into a biochar-fly ash mixed slurry. The biochar-fly ash mixed slurry is composed of modified biochar, fly ash and deionized water in a mass ratio of 1:5:12. Control the liquid-solid ratio to 10mL:1g. Let it stand and soak for 35min at 25℃. Take it out and dry it at 90℃ for 2h to form the outer shell and obtain the gradient shell ceramic particle core.

[0080] S11: Immerse the gradient shell ceramic core in a mixed solution of Na2SiO3 and FeSO4 (the concentration of FeSO4 in the mixed solution of Na2SiO3 and FeSO4 is 0.1mol / L, and the molar ratio of Na2SiO3 to FeSO4 is 1:5). The liquid-solid ratio of the gradient shell ceramic core to the mixed solution is 1mL:10g. Nitrogen gas is introduced for 40min to remove oxygen. The mixture is stirred in a constant temperature water bath at 35℃ for 1.5h to obtain the pretreated ceramic system.

[0081] S12: Under nitrogen protection, a 0.05 mol / L NaBH4 solution (NaBH4 to FeSO4 molar ratio of 3:2) was added dropwise to the pretreated ceramsite system at a rate of 1.5 mL / min. After the addition was complete, the mixture was stirred at 200 rpm for 45 min to obtain a wet-based ceramsite system.

[0082] S13: The wet-based ceramsite system was filtered and separated, rinsed 4 times with nitrogen-saturated deionized water, and then the wet-based ceramsite was immersed in a 0.01 mol / L KMnO4 aqueous solution (the liquid-solid ratio of wet-based ceramsite to KMnO4 aqueous solution was 1 mL: 6 g). The mixture was stirred at 25 °C for 20 min (stirring speed 100 rpm), filtered, and vacuum dried at 60 °C for 3 h to obtain gradient core-shell zero-valent iron porous ceramsite.

[0083] The gradient core-shell zero-valent iron porous ceramsite prepared in this embodiment has a similar effect on removing arsenic from arsenic-containing wastewater as in Example 1.

[0084] Example 3 The specific steps for preparing gradient core-shell zero-valent iron porous ceramsite are as follows: S1: Crush steel slag, pass it through a 120-mesh sieve, and dry it at 120℃ to constant weight to obtain steel slag powder; crush red mud, pass it through a 120-mesh sieve, and dry it at 120℃ to constant weight to obtain red mud powder; pass fly ash through a 120-mesh sieve, and dry it at 120℃ to constant weight to obtain fly ash powder; dry municipal sludge at 120℃ to constant weight, grind it, and pass it through a 120-mesh sieve to obtain municipal sludge powder.

[0085] S2: Pretreatment of steel slag powder, red mud powder, fly ash powder, and municipal sludge powder, specifically: ① Add steel slag powder to a 1.0 mol / L citric acid-tartaric acid composite aqueous solution (the solid-liquid ratio of steel slag powder to composite acid aqueous solution is 10 mg: 1 L), stir at 30℃ for 65 min, filter, wash with deionized water until the pH of the washing solution is 6.0, and dry at 105℃ to constant weight to obtain pretreated steel slag powder.

[0086] ② Add the red mud powder to a 1.0 mol / L citric acid-tartaric acid composite aqueous solution (the solid-liquid ratio of red mud powder to composite acid aqueous solution is 10 mg: 1 L), stir at 30℃ for 65 min, filter, wash with deionized water until the pH of the washing solution is 6.0, and dry at 105℃ to constant weight to obtain the pretreated red mud powder.

[0087] ③ Add fly ash powder to a 1.0 mol / L citric acid-tartaric acid composite aqueous solution (the solid-liquid ratio of fly ash powder to composite acid aqueous solution is 10 mg: 1 L), stir at 50 °C for 100 min, filter, wash with deionized water until the pH of the washing solution is 6.0, and dry at 105 °C to constant weight to obtain pretreated fly ash powder.

[0088] ④ Add municipal sludge powder to a 1.0 mol / L citric acid-tartaric acid composite aqueous solution (the solid-liquid ratio of municipal sludge powder to composite acid aqueous solution is 10 mg: 1 L), stir at 50 °C for 100 min, filter, wash with deionized water until the pH of the washing solution is 6.0, and dry at 105 °C to constant weight to obtain pretreated municipal sludge powder.

[0089] S3: Crush rice husks, pass them through a 120-mesh sieve, and dry them at 120℃ to constant weight to obtain rice husk powder; pyrolyze the rice husk powder at 550℃ under a nitrogen atmosphere for 2.2h, and obtain rice husk biochar after cooling; add the rice husk biochar to a Fe / Mn mixed solution (the liquid-solid ratio of rice husk biochar and Fe / Mn mixed solution is 8mL:1g, shake at 25℃ for 22h, and then calcine at 550℃ under a nitrogen atmosphere for 1.2h to obtain Fe-Mn modified rice husk biochar; the Fe / Mn mixed solution is composed of Fe(NO3)3 aqueous solution and Mn(NO3)2 aqueous solution, the concentration of Fe(NO3)3 aqueous solution is 0.25mol / L, the concentration of Mn(NO3)2 aqueous solution is 0.075mol / L, and the molar ratio of Fe to Mn in the Fe / Mn mixed solution is 3:1.

[0090] S4: Mix the steel slag powder, red mud powder, fly ash powder, and municipal sludge powder pretreated in step S2 at a mass ratio of 20:25:30:10 to obtain mixed powder A; add Fe-Mn modified rice husk biochar to mixed powder A (the mass ratio of mixed powder A to Fe-Mn modified rice husk biochar is 12.5:1) to obtain mixed powder B; add water to mixed powder B (the liquid-solid ratio of mixed powder B to water is 5mL:1g); shake at 25℃ for 4h; filter; and dry at 105℃ to constant weight to obtain a metal solid waste mixture.

[0091] S5: Mix the metal solid waste mixture and starch at a mass ratio of 85:5, and simultaneously add the composite curing agent (the mass ratio of the metal solid waste mixture and the composite curing agent is 12.5:1). Put the mixture into a twin-shaft mixer and dry mix for 40 minutes to obtain a dry-mixed system. The composite curing agent is composed of NH4H2PO4 and sodium bentonite mixed at a mass ratio of 1:3. Add a 10wt% PVA aqueous solution to the dry-mixed system (the liquid-solid ratio of the PVA aqueous solution to the dry-mixed system is 1mL:6g), and stir and wet mix for 40 minutes to obtain sludge with a moisture content of 32%.

[0092] S6: Put the mud into a screw extruder and prepare a green body under the conditions of 1.0MPa pressure and 6cm / s extrusion rate. The green body is a cylinder with a diameter of 3~6mm and an aspect ratio of 1.5~2. The green body is aged at 25℃ for 12h to obtain the aged green body.

[0093] S7: Place the aged green body in a forced-air drying oven for three-stage gradient drying: first dry at 60℃ for 2.5h, then heat to 80℃ for 2.5h, and finally heat to 110℃ for 5h to obtain the dried green body.

[0094] S8: Place the dried green body into a muffle furnace, introduce a mixed gas (5% CO2 by volume, the remainder being air), and raise the temperature to 450℃ at a rate of 5℃ / min, holding for 2.5h; then, without any atmosphere in the muffle furnace, raise the temperature to 300℃ at a rate of 8℃ / min, holding for 1h; then raise the temperature from 300℃ to 600℃ at a rate of 3℃ / min, holding for 2h; finally, raise the temperature from 600℃ to 1000℃ at a rate of 5℃ / min, holding for 1.5h to obtain the ceramsite core.

[0095] S9: Immerse the core of the ceramic particle in a water-based TiO2 sol with a mass percentage concentration of 5%, the liquid-solid ratio of the water-based TiO2 sol to the core of the ceramic particle is 8mL:1g, let it stand at 25℃ for 1h, take it out, and dry it at 80℃ for 3.5h to obtain ceramic particles with an inner TiO2 shell.

[0096] S10: Immerse the ceramic particles with the inner TiO2 shell into a biochar-fly ash mixed slurry. The biochar-fly ash mixed slurry is composed of modified biochar, fly ash and deionized water in a mass ratio of 2:5:12. Control the liquid-solid ratio to 10mL:1g. Let it stand and soak for 40min at 25℃. Take it out and dry it at 100℃ for 1.5h to form the outer shell and obtain the gradient shell ceramic particle core.

[0097] S11: Immerse the gradient shell ceramic core into a mixed solution of Na2SiO3 and FeSO4 (the concentration of FeSO4 in the mixed solution of Na2SiO3 and FeSO4 is 0.15mol / L, and the molar ratio of Na2SiO3 to FeSO4 is 1:5). The liquid-solid ratio of the gradient shell ceramic core and the mixed solution is 1mL:12g. Nitrogen gas is introduced for 40min to remove oxygen. The mixture is stirred in a constant temperature water bath at 40℃ for 1h to obtain the pretreated ceramic system.

[0098] S12: Under nitrogen protection, add a 0.3 mol / L NaBH4 solution (NaBH4 to FeSO4 molar ratio of 2:1) dropwise to the pretreated ceramsite system at a rate of 2 mL / min. After the addition is complete, stir at 200 rpm for 60 min to obtain a wet-based ceramsite system.

[0099] S13: The wet-based ceramsite system was filtered and separated, rinsed 5 times with nitrogen-saturated deionized water, and then the wet-based ceramsite was immersed in a 0.05 mol / L KMnO4 aqueous solution (the liquid-solid ratio of wet-based ceramsite to KMnO4 aqueous solution was 1 mL: 8 g). The mixture was stirred at 25 °C for 30 min (stirring speed 100 rpm), filtered, and vacuum dried at 60 °C for 4 h to obtain gradient core-shell zero-valent iron porous ceramsite.

[0100] The gradient core-shell zero-valent iron porous ceramsite prepared in this embodiment has a similar effect on removing arsenic from arsenic-containing wastewater as in Example 1.

[0101] Comparative Example 1 The specific steps for preparing ceramsite are as follows: S1: Crush steel slag, pass it through an 80-mesh sieve, and dry it at 105℃ to constant weight to obtain steel slag powder; crush red mud, pass it through an 80-mesh sieve, and dry it at 105℃ to constant weight to obtain red mud powder; pass fly ash through an 80-mesh sieve, and dry it at 105℃ to constant weight to obtain fly ash powder; dry municipal sludge at 105℃ to constant weight, grind it, and pass it through an 80-mesh sieve to obtain municipal sludge powder.

[0102] S2: Pretreatment of steel slag powder, red mud powder, fly ash powder, and municipal sludge powder, specifically: ① Add steel slag powder to a 0.5 mol / L citric acid-tartaric acid composite aqueous solution (the solid-liquid ratio of steel slag powder to composite acid aqueous solution is 10 mg: 1 L), stir at 25℃ for 60 min, filter, wash with deionized water until the pH of the washing solution is 7.0, and dry at 105℃ to constant weight to obtain pretreated steel slag powder.

[0103] ② Add the red mud powder to a 0.5 mol / L citric acid-tartaric acid composite aqueous solution (the solid-liquid ratio of red mud powder to composite acid aqueous solution is 10 mg: 1 L), stir at 25 °C for 60 min, filter, wash with deionized water until the pH of the washing solution is 7.0, and dry at 105 °C to constant weight to obtain the pretreated red mud powder.

[0104] ③ Add fly ash powder to a 0.5 mol / L citric acid-tartaric acid composite aqueous solution (the solid-liquid ratio of fly ash powder to composite acid aqueous solution is 10 mg: 1 L), stir at 40℃ for 90 min, filter, wash with deionized water until the pH of the washing solution is 7.0, and dry at 105℃ to constant weight to obtain pretreated fly ash powder.

[0105] ④ Add municipal sludge powder to a 0.5 mol / L citric acid-tartaric acid composite aqueous solution (the solid-liquid ratio of municipal sludge powder to composite acid aqueous solution is 10 mg: 1 L), stir at 40℃ for 90 min, filter, wash with deionized water until the pH of the washing solution is 7.0, and dry at 105℃ to constant weight to obtain pretreated municipal sludge powder.

[0106] S3: Crush rice husks, pass them through an 80-mesh sieve, and dry them at 105℃ to constant weight to obtain rice husk powder; pyrolyze the rice husk powder at 500℃ under a nitrogen atmosphere for 2 hours, and obtain rice husk biochar after cooling; add the rice husk biochar to a Fe / Mn mixed solution (the liquid-solid ratio of rice husk biochar and Fe / Mn mixed solution is 8mL:1g, shake at 25℃ for 24 hours, and then calcine at 500℃ under a nitrogen atmosphere for 1 hour to obtain Fe-Mn modified rice husk biochar; the Fe / Mn mixed solution is composed of Fe(NO3)3 aqueous solution and Mn(NO3)2 aqueous solution, the concentration of Fe(NO3)3 aqueous solution is 0.2mol / L, the concentration of Mn(NO3)2 aqueous solution is 0.07mol / L, and the molar ratio of Fe to Mn in the Fe / Mn mixed solution is 3:1.

[0107] S4: Mix the steel slag powder, red mud powder, fly ash powder and municipal sludge powder pretreated in step S2 at a mass ratio of 15:20:25:10 to obtain mixed powder A; add Fe-Mn modified rice husk biochar to mixed powder A (the mass ratio of mixed powder A to Fe-Mn modified rice husk biochar is 20:1) to obtain mixed powder B; add water to mixed powder B (the liquid-solid ratio of mixed powder B to water is 5mL:1g); shake at 25℃ for 120min; filter; and dry at 105℃ to constant weight to obtain a metal solid waste mixture.

[0108] S5: Mix the metal solid waste mixture and starch at a mass ratio of 70:5, and simultaneously add the composite curing agent (the mass ratio of the metal solid waste mixture and the composite curing agent is 20:1). Put the mixture into a twin-shaft mixer and dry mix for 40 minutes to obtain a dry-mixed system. The composite curing agent is composed of NH4H2PO4 and sodium bentonite mixed at a mass ratio of 1:2. Add a 10wt% PVA aqueous solution to the dry-mixed system (the liquid-solid ratio of the PVA aqueous solution to the dry-mixed system is 1mL:6g), and stir and wet mix for 30 minutes to obtain sludge with a moisture content of 28%.

[0109] S6: Put the mud into a screw extruder and prepare a green body under the conditions of pressure of 1.2MPa and extrusion rate of 8cm / s. The green body is a cylinder with a diameter of 5mm and an aspect ratio of 1.5. The green body is aged at 25℃ for 24h to obtain the aged green body.

[0110] S7: Place the aged green body in a forced-air drying oven for three-stage gradient drying: first dry at 50℃ for 2 hours, then heat up to 80℃ for 3 hours, and finally heat up to 105℃ for 4 hours to obtain the dried green body.

[0111] S8: Place the dried green body into a muffle furnace, introduce a mixed gas (5% CO2 volume fraction, the remainder being air), and raise the temperature to 400℃ at a rate of 2℃ / min, holding for 2 hours; then, without any atmosphere in the muffle furnace, raise the temperature to 300℃ at a rate of 5℃ / min, holding for 1 hour; then raise the temperature from 300℃ to 600℃ at a rate of 5℃ / min, holding for 2 hours; finally, raise the temperature from 600℃ to 900℃ at a rate of 4℃ / min, holding for 2 hours to obtain the ceramsite core.

[0112] S9: Immerse the core of the ceramsite in a 5% (w / w) water-based TiO2 sol, with a liquid-to-solid ratio of 8 mL:1 g. Let it stand at 25°C for 1 hour, then remove it and dry it at 60°C for 4 hours to obtain ceramsite with an inner TiO2 shell.

[0113] S10: Immerse the ceramsite with the inner TiO2 shell into a biochar-fly ash mixed slurry. The biochar-fly ash mixed slurry is composed of modified biochar, fly ash and deionized water in a mass ratio of 1:3:10. Control the liquid-solid ratio to 10mL:1g. Let it stand and soak for 30min at 25℃. Take it out and dry it at 80℃ for 1h to form the outer shell and obtain the ceramsite.

[0114] The ceramsite materials prepared in Examples 1-3 and Comparative Example 1 were used to remove arsenic from wastewater. The wastewater came from the sulfuric acid workshop of a zinc smelter in Southwest China, which produced a large amount of arsenic and other impurities after washing smelting flue gas. Ultrapure water was used to adjust the arsenic concentration to 50 mg / L. The main components and their concentrations in the wastewater are shown in Table 1. The specific application steps were as follows: the pH of the arsenic-containing wastewater with an arsenic concentration of 50 mg / L was adjusted to 3 ± 0.5; 2.0 g of ceramsite material was added to 50 mL of arsenic-containing wastewater; then 0.03 g of permonosulfate (PMS) was added; the mixture was stirred at 350 rpm for 24 h at room temperature and pressure. The concentration of metal ions in the filtrate after the reaction was completed was determined by ICP method.

[0115] Table 1 The concentration of metal ions in the filtrate after arsenic removal was determined by ICP method, as shown in Table 2.

[0116] Table 2 As shown in Tables 1-2, this invention uses four types of solid waste—steel slag, red mud, fly ash, and municipal sludge—as raw materials to establish a three-stage synergistic system: pretreatment targeted removal, in-situ solidification preparation, and calcination lattice locking. In the pretreatment stage, a combination of biodegradable acid stepwise rinsing and Fe-Mn modified biochar adsorption technology is employed: the citric acid-tartaric acid composite acid selectively dissolves free heavy metals, avoiding the damage to the solid waste matrix structure caused by traditional strong acids; the Fe-Mn modified biochar further removes residual heavy metals through complexation and electrostatic interaction, reducing the total heavy metal content in the solid waste to ≤10 mg / kg; in the raw material mixing stage, a phosphate-bentonite composite solidifying agent is introduced, whose layered structure enables physical encapsulation of heavy metals, achieving a better solidification effect. During pre-calcination, CO removes Cr... 6+ Reduced to easily curable Cr 3+ Subsequent high-temperature sintering causes SiO2 and Al2O3 in the solid waste to form a glassy melt, which stably dissolves the solidified heavy metals inside the crystal lattice. The final heavy metal leaching concentration of the ceramic particles is significantly lower than the national standard, completely avoiding the risk of secondary pollution.

[0117] This invention designs a TiO2 inner layer-biochar / fly ash outer layer gradient shell structure and modifies zero-valent iron with SiO2 coating to form a dual barrier and active protection mechanism. The arsenic removal performance of the prepared gradient core-shell zero-valent iron porous ceramsite depends on the synergistic effect of zero-valent iron and the gradient core-shell structure: zero-valent iron removes arsenic through Fe... 0 →Fe 2+ →Fe 3+ The valence cycle continuously activates persulfate (PMS), producing SO4• - •OH and 1 O2 efficiently oxidizes the difficult-to-adsorb As(III) into the easily adsorbed As(V); the outer biochar-fly ash shell can enrich As(V) and PMS, while the inner TiO2 shell assists in the adsorption of As(V). At the same time, the Al-O and Fe-O groups on the surface of the ceramsite matrix fix As(V) through electrostatic adsorption and complexation reactions (forming Fe-As-O and Al-As-O complexes); in addition, the CaO contained in the steel slag in the solid waste can slowly release OH-. - The system pH is automatically adjusted to a suitable range of 3-7 without the need for external acid, which avoids the leaching of heavy metals under acidic conditions and ensures the activation efficiency of PMS by zero-valent iron.

[0118] In contrast, the ceramsite without added zero-valent iron (Comparative Example 1) mainly relies on the physical adsorption of its porous structure and the weak hydroxyl exchange on the surface of Fe2O3 in red mud. Its affinity for As(III) is limited, its adsorption is highly reversible and has low selectivity, leading to a significant decrease in arsenic removal efficiency. The gradient core-shell structure zero-valent iron porous ceramsite prepared in this invention, compared to Comparative Example 1, achieves integrated synergistic removal of heavy metals through controlled release, oxidation, adsorption, and fixation. This provides an innovative solution for the deep purification of arsenic-containing wastewater and the high-value-added resource utilization of diverse solid wastes, with broad application prospects.

[0119] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing gradient core-shell zero-valent iron porous ceramsite, characterized in that, Specifically, the following steps are included: S1: Crush steel slag, sieve, and dry it to obtain steel slag powder; crush red mud, sieve, and dry it to obtain red mud powder; sieve fly ash and dry it to obtain fly ash powder; dry municipal sludge, grind it, and sieve it to obtain municipal sludge powder. S2: Pre-treat steel slag powder, red mud powder, fly ash powder and municipal sludge powder to obtain pre-treated steel slag powder, red mud powder, fly ash powder and municipal sludge powder. S3: Crush rice husks, sieve, and dry to obtain rice husk powder; pyrolyze the rice husk powder at 500~550℃ under a nitrogen atmosphere for 2~2.5h to obtain rice husk biochar; add the rice husk biochar to a Fe / Mn mixed solution, wherein the liquid-solid ratio of the rice husk biochar to the Fe / Mn mixed solution is 8mL:1g, shake the reaction, and then calcine at 500~550℃ under a nitrogen atmosphere for 1~1.5h to obtain Fe-Mn modified rice husk biochar; S4: Mix the steel slag powder, red mud powder, fly ash powder and municipal sludge powder after the pretreatment in step S2 in a mass ratio of (15~25):(20~30):(25~35):(10~15) to obtain mixed powder A; add Fe-Mn modified rice husk biochar to mixed powder A, with a mass ratio of mixed powder A to Fe-Mn modified rice husk biochar of (20~12.5):1 to obtain mixed powder B; add water to mixed powder B, with a liquid-solid ratio of mixed powder B to water of 5mL:1g; shake to react, filter, and dry to obtain a metal solid waste mixture; S5: Mix the metal solid waste mixture and starch in a mass ratio of (70~85):(2~5), add composite solidifying agent at the same time, dry mix at 25℃ for 40 min to obtain a dry mixed system, add a 10% PVA aqueous solution to the dry mixed system, the liquid-solid ratio of PVA aqueous solution to dry mixed system is 1mL:6g, wet mix at 25℃ for 30~40 min to obtain mud; S6: Extrude the clay to form blanks; age the blanks at 25℃ for 12~24h to obtain aged blanks; S7: Place the aged green body in a forced-air drying oven for three-stage gradient drying: first dry at 50~60℃ for 2~3 hours, then raise the temperature to 80~90℃ for 2~3 hours, and finally raise the temperature to 105~110℃ for 4~6 hours to obtain the dried green body. S8: Introduce a mixed gas (5% CO2 volume fraction, the remainder being air) into the dried green body, raise the temperature to 400-450℃ at a rate of 2-5℃ / min, and hold for 2-3 hours; then, without any atmosphere, perform three-stage gradient calcination in the muffle furnace: raise the temperature to 300℃ at a rate of 5-8℃ / min and hold for 1 hour; then raise the temperature from 300℃ to 600℃ at a rate of 3-5℃ / min and hold for 2 hours; then raise the temperature from 600℃ to 900-1000℃ at a rate of 4-5℃ / min and hold for 1.5-2.5 hours to obtain the ceramsite core; S9: Immerse the core of the ceramsite in a water-based TiO2 sol with a mass percentage concentration of 5%, the liquid-solid ratio of TiO2 sol to ceramsite core is 8mL:1g, let it stand to react, take it out, dry it, and obtain ceramsite with an inner TiO2 shell; S10: Immerse the ceramic particles with inner TiO2 shells into a biochar-fly ash mixed slurry. The liquid-solid ratio of the biochar-fly ash mixed slurry and the ceramic particles with inner TiO2 shells is 10mL:1g. Allow the mixture to stand and react. Remove the particles and dry them to obtain the gradient shell ceramic particle core. S11: Immerse the gradient shell ceramic core in a mixed aqueous solution of Na2SiO3 and FeSO4, wherein the liquid-solid ratio of the mixed aqueous solution to the gradient shell ceramic core is 1mL:8~12g, then purge with nitrogen for 40min to remove oxygen, stir and react to obtain a pretreated ceramic system. S12: Under nitrogen protection, add a 0.05-0.3 mol / L NaBH4 aqueous solution to the pretreated ceramsite system at a rate of 1-2 mL / min. The NaBH4 reacts with Fe in the pretreated ceramsite system. 2+ The molar ratio is 1~2:

1. After the addition is complete, stir for 30~60 minutes to obtain a wet-based ceramsite system. S13: The wet-based ceramsite system is filtered and separated, rinsed with nitrogen-saturated deionized water 3-5 times, and then the wet-based ceramsite is immersed in a KMnO4 aqueous solution with a concentration of 0.01-0.05 mol / L. The liquid-solid ratio of the wet-based ceramsite to the KMnO4 aqueous solution is 1 mL: 5-8 g. The reaction is stirred, filtered, and vacuum dried to obtain gradient core-shell zero-valent iron porous ceramsite.

2. The method for preparing gradient core-shell zero-valent iron porous ceramsite according to claim 1, characterized in that, The sieving conditions described in steps S1 and S3 are both: passing through an 80-120 mesh sieve; The drying conditions described in step S1 are all: drying at 105~120℃ to constant weight; The drying conditions described in step S9 are: drying at 60~80℃ for 3~4 hours; The drying conditions described in step S10 are: drying at 80~100℃ for 1~2 hours; The vacuum drying conditions described in step S13 are: vacuum drying at 60°C for 2-4 hours.

3. The method for preparing gradient core-shell zero-valent iron porous ceramsite according to claim 1, characterized in that, The conditions for the shaking reaction described in step S3 are: shaking reaction at 25°C for 20~24h; The conditions for the shaking reaction described in step S4 are: shaking reaction at 25°C for 2-4 hours; The conditions for the static reaction in step S9 are: static reaction at 25°C for 1 hour; The conditions for the static reaction in step S10 are: static reaction at 25°C for 30-40 minutes; The conditions for the stirring reaction in step S11 are: stirring at 25~40℃ for 1~2 hours; The conditions for the stirring reaction in step S13 are: stirring at 25°C for 15-30 minutes.

4. The method for preparing gradient core-shell zero-valent iron porous ceramsite according to claim 1, characterized in that, The preprocessing described in step S2 specifically includes: ① Add steel slag powder to a 0.5~1.0 mol / L citric acid-tartaric acid composite aqueous solution with a solid-liquid ratio of 10 mg: 1 L. Stir at 25~30℃ for 60~70 min, filter, wash with deionized water until the pH of the washing solution is 6.0~7.0, and dry at 105℃ to constant weight to obtain pretreated steel slag powder; ② Add red mud powder to a 0.5~1.0 mol / L citric acid-tartaric acid composite aqueous solution. The solid-liquid ratio of red mud powder to composite acid aqueous solution is 10 mg: 1 L. Stir at 25~30℃ for 60~70 min, filter, wash with deionized water until the pH of the washing solution is 6.0~7.0, and dry at 105℃ to constant weight to obtain pretreated red mud powder. ③ Add fly ash powder to a 0.5~1.0 mol / L citric acid-tartaric acid composite aqueous solution. The solid-liquid ratio of fly ash powder to composite acid aqueous solution is 10 mg: 1 L. Stir at 40~50℃ for 90~120 min, filter, wash with deionized water until the pH of the washing solution is 6.0~7.0, and dry at 105℃ to constant weight to obtain pretreated fly ash powder. ④ Add municipal sludge powder to a 0.5~1.0 mol / L citric acid-tartaric acid composite acid aqueous solution. The solid-liquid ratio of municipal sludge powder to composite acid aqueous solution is 10 mg: 1 L. Stir at 40~50℃ for 90~120 min, filter, wash with deionized water until the pH of the washing solution is 6.0~7.0, and dry at 105℃ to constant weight to obtain pretreated municipal sludge powder. The citric acid-tartaric acid composite aqueous solution is composed of citric acid and tartaric acid in a molar ratio of 2:

1.

5. The method for preparing gradient core-shell zero-valent iron porous ceramsite according to claim 1, characterized in that, The Fe / Mn mixed solution in step S3 is composed of a mixture of Fe(NO3)3 aqueous solution and Mn(NO3)2 aqueous solution, with the concentration of Fe(NO3)3 aqueous solution being 0.2~0.25 mol / L and the concentration of Mn(NO3)2 aqueous solution being 0.07~0.075 mol / L; the molar ratio of Fe to Mn in the Fe / Mn mixed solution is 3:

1.

6. The method for preparing gradient core-shell zero-valent iron porous ceramsite according to claim 1, characterized in that, In step S5, the mass ratio of the metal solid waste mixture to the composite curing agent is (20~12.5):1; the composite curing agent is composed of NH4H2PO4 and sodium-based bentonite mixed in a mass ratio of (1~2):(2~3); the moisture content of the sludge is 28~32%.

7. The method for preparing gradient core-shell zero-valent iron porous ceramsite according to claim 1, characterized in that, The extrusion conditions in step S6 are: the blank is formed under a pressure of 0.8~1.2MPa and an extrusion rate of 5~8cm / s; the blank is a cylinder with a diameter of 3~6mm and a length-to-diameter ratio of 1.5~2.

8. The method for preparing gradient core-shell zero-valent iron porous ceramsite according to claim 1, characterized in that, The biochar-fly ash mixed slurry described in step S10 is composed of modified biochar, fly ash and deionized water mixed in a mass ratio of (1~2):(3~5):(10~12).

9. The method for preparing gradient core-shell zero-valent iron porous ceramsite according to claim 1, characterized in that, In step S11, the concentration of FeSO4 in the mixed aqueous solution of Na2SiO3 and FeSO4 is 0.1~0.2 mol / L, and the molar ratio of Na2SiO3 to FeSO4 is 1:5~6.

10. The application of gradient core-shell zero-valent iron porous ceramic particles prepared by the method of any one of claims 1 to 9 in the adsorption of arsenic in water.

Images