Method for disposing perfluorooctanoic acid contaminated soil by solid waste and hydrothermal curing in cooperation

Abstract

The present application relates to the technical field of pollution prevention and treatment, and discloses a method for treating PFOS-contaminated soil by solid waste and hydrothermal solidification, which comprises the following steps: pretreating PFOS-contaminated soil, carbide slag and biochar respectively; mixing and molding; and hydrothermal solidification treatment. The carbide slag provides an alkaline environment to make the PFOS undergo decarboxylation and defluorination reaction, and the fluoride ions and calcium ions generate insoluble calcium fluoride to realize the fixation of fluorine. At the same time, the alkaline excitation of the active silicate in the contaminated soil generates hydrated calcium silicate and other cementation products to cement the soil particles. The biochar adsorbs and enriches the PFOS to accelerate the degradation and promote the generation of hydration products, and fills large pores to make the microstructure dense. By using the above technical scheme, the present application can realize the efficient defluorination and degradation of PFOS and the synchronous formation of solidified body strength, and the combination of carbide slag and biochar realizes the resource utilization of solid waste, thereby providing a complete disposal path for the harmless, stable and resourceful utilization of PFOS-contaminated soil.

Description

Technical Field

[0001] This invention relates to the field of pollution prevention and treatment technology, and discloses a method for the co-treatment of perfluorooctanoic acid contaminated soil by solid waste and hydrothermal solidification. Background Technology

[0002] With the acceleration of industrialization and the widespread use of fluorine-containing compounds, perfluorooctanoic acid (PFOA), as a typical perfluoroalkyl compound, is widely used in industrial and civilian production fields due to its excellent thermal stability, good hydrophobicity and oleophobicity, and high surface activity. However, the carbon-fluorine bond of PFOA is extremely stable and difficult to degrade through natural pathways. It has been widely detected in environmental media such as water bodies, soil, and sediments worldwide, and continues to accumulate in soil, posing a serious threat to the ecological environment and human health.

[0003] Remediation technologies for perfluorooctanoic acid (PFOA) contaminated soil mainly include thermal desorption, chemical oxidation, adsorption fixation, microbial degradation, and cement solidification stabilization. Thermal desorption requires high temperatures, resulting in extremely high energy consumption and significant equipment investment. The treated soil's organic matter and clay mineral structure are damaged, losing their cementing capacity and rendering it unusable for direct engineering applications. Chemical oxidation relies on high temperatures or strong oxidizing conditions, which can damage soil cementing and mechanical properties and potentially introduce soil acidification and secondary pollution. Adsorption fixation transfers PFOA to the adsorbent phase but fails to destroy its molecular structure, leading to a long-term environmental risk of pollutant desorption and release after adsorption saturation. Microbial remediation suffers from the difficulty of utilizing PFOA as a carbon source, low survival rate of functional bacteria, and difficulty in colonization, resulting in long remediation cycles and unstable effects. While cement solidification stabilization can impart some mechanical strength to contaminated soil, its fixation effect on perfluorinated compounds is extremely limited. Relying solely on physical encapsulation to seal pollutants, the encapsulated PFOA may still be released back into the environment during long-term service when the solidified body weathers and cracks or is eroded by water.

[0004] However, existing technologies generally face the technical contradiction of simultaneously achieving efficient pollutant removal and ensuring the mechanical strength of the solidified body. Technologies capable of efficiently degrading perfluorooctanoic acid (PFOA) often involve high temperatures or strong oxidizing conditions, which can damage the cementation structure and mechanical properties of the soil. The treated soil becomes loose and lacks strength, requiring secondary solidification treatment, leading to complex procedures and significantly increased costs. On the other hand, technologies that can impart a certain strength to contaminated soil and facilitate engineering applications cannot effectively degrade PFOA; the pollutant is merely physically encapsulated in the solidified body, and long-term environmental risks remain. Therefore, there is an urgent need in this field for a method that can simultaneously achieve efficient PFOA degradation and ensure the strength of the solidified body in the same hydrothermal solidification system, in order to overcome the technical contradictions in existing technologies and provide a practical technical path for the harmless treatment and resource utilization of PFOA-contaminated soil. Summary of the Invention

[0005] The purpose of this invention is to provide a method for the co-treatment of perfluorooctanoic acid contaminated soil by solid waste and hydrothermal solidification, which can effectively solve the problems in the background art.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for co-processing solid waste with hydrothermal solidification to treat perfluorooctanoic acid (PFOA) contaminated soil includes the following steps:

[0008] S1. The perfluorooctanoic acid contaminated soil is pretreated by crushing and screening to obtain pretreated contaminated soil. The carbide slag is dried to constant weight and then crushed to obtain carbide slag powder. The biomass raw material is pyrolyzed, cooled and crushed to obtain biochar powder.

[0009] S2. Ingredient mixing: The pretreated contaminated soil, carbide slag powder and biochar powder are mixed according to the mass ratio, water is added and stirred evenly to obtain a mixture.

[0010] S3. Sample molding: Weigh the mixture according to the designed dry density, put the weighed mixture into the mold and press it into shape to obtain the green body;

[0011] S4. Hydrothermal curing treatment: The blank is placed in a hydrothermal reactor and hydrothermally cured under set temperature and time conditions, followed by drying treatment to obtain a hydrothermal cured test block.

[0012] S5. Strength and pollutant leaching tests: The hydrothermally cured test block is subjected to unconfined compressive strength testing and perfluorooctanoic acid (PFOA) leaching concentration testing.

[0013] Furthermore, the carbide slag in S1 is a gray-black dry powder, the main component of which is calcium hydroxide, with a mass fraction of not less than 90%.

[0014] Furthermore, the biomass raw material in S1 is one or more of crop straw, rice husks, sawdust, fruit shells, or sludge.

[0015] Further, in step S2, mixing the pretreated contaminated soil, carbide slag powder, and biochar powder in a specific mass ratio includes:

[0016] One hundred parts of perfluorooctanoic acid contaminated soil, fifteen to twenty parts of calcium carbide slag powder, and two to six parts of biochar powder.

[0017] Furthermore, the amount of water added in S2 is 15% to 20% of the mass of the other pretreated raw materials after mixing.

[0018] Furthermore, the dry density described in S3 is 1.7 to 1.9 grams per cubic centimeter.

[0019] Furthermore, the temperature of the hydrothermal curing treatment in S4 is 180 to 200 degrees Celsius, and the treatment time is 8 to 16 hours.

[0020] Furthermore, the drying process in step S4 is carried out at a temperature of 80 degrees Celsius for 12 hours.

[0021] Furthermore, during the hydrothermal curing process in S4, the reactor is in an alkaline environment, where hydroxide ions are released by the hydrolysis of carbide slag, and the pH value is 12.5 to 13.

[0022] Furthermore, the perfluorooctanoic acid leaching concentration in S5 meets the limit requirement of not more than 40 nanograms per liter.

[0023] Perfluorooctanoic acid (PFOA) contaminated soil is the object of treatment in this invention and also the main aggregate of the hydrothermal solidification test block. The contaminated soil contains abundant active silica and alumina, which originate from clay minerals and quartz particles in the soil. Under hydrothermal conditions, these active aluminosilicates can dissolve in an alkaline environment and participate in hydration reactions, serving as the silicon and aluminum sources for the formation of cementitious products such as hydrated calcium silicate and hydrated calcium aluminate.

[0024] When calcium carbide slag dissolves in water, it releases hydroxide ions, making the system strongly alkaline. Under alkaline hydrothermal conditions, these hydroxide ions attack the carboxyl carbon in perfluorooctanoic acid (PFOA) molecules, initiating a decarboxylation reaction. Subsequently, through a stepwise defluorination process, PFOA is degraded into short-chain perfluorinated compounds, ultimately mineralized as fluoride ions and carbon dioxide. Simultaneously, calcium ions in the calcium carbide slag are a key component in fixing the degradation product fluoride ions. The fluoride ions generated from the defluorination of PFOA under hydrothermal conditions react with calcium ions in the system to form insoluble calcium fluoride. Calcium fluoride has extremely low solubility under varying environmental pH conditions, enabling the permanent fixation of fluoride and completely eliminating the risk of secondary pollution. Furthermore, under hydrothermal conditions, the calcium hydroxide in the calcium carbide slag alkaline-activated active silica and alumina in the contaminated soil, generating products such as hydrated calcium silicate, hydrated calcium aluminate, and tobermorite. These products can bind dispersed soil particles into a cohesive whole, filling the gaps between soil particles, gradually densifying the microstructure, and continuously increasing the unconfined compressive strength.

[0025] Biochar enriches perfluorooctanoic acid (PFOA) molecules dispersed in soil onto its surface and within its pores, providing a high-concentration local microenvironment for subsequent hydrothermal degradation reactions, thereby accelerating the degradation process. Simultaneously, biochar enhances the reaction kinetics of PFOA hydrothermal degradation; its porous structure acts as a mass transfer channel, promoting the diffusion and contact between hydroxide ions and PFOA molecules. Furthermore, the functional groups on the biochar surface can serve as nucleation sites for hydration products, promoting the formation of cementing products such as hydrated calcium silicate. The biochar particles themselves can act as micro-aggregates, filling the large pores between soil particles, resulting in a denser microstructure of the solidified body. Moreover, the biochar surface can form a good interfacial bond with surrounding hydration products, improving the overall integrity of the solidified body.

[0026] Water is the basic reaction medium in this invention, providing a liquid environment for the dissolution, diffusion and reaction of each component, and also directly participating in the reaction. The proportion of water is a key process parameter that affects the molding quality and curing effect.

[0027] Compared with existing technologies, this invention has the following beneficial effects: Through the synergistic effect of biochar adsorption and enrichment and alkaline hydrothermal degradation of carbide slag, this invention simultaneously achieves efficient defluorination and degradation of perfluorooctanoic acid (PFOA) and the formation of solidified body strength in the same hydrothermal reaction system, achieving both pollution removal and strength assurance, overcoming long-standing problems in existing technologies. The solidification materials used in this invention are all industrial by-products or agricultural waste. The addition of carbide slag avoids the use of traditional high-carbon-emission cementitious materials such as cement. The use of biochar not only realizes the resource utilization of agricultural waste but also has a certain carbon sequestration potential by stably sealing carbon in the solidified body. The combined use of both significantly reduces treatment costs. The hydrothermally solidified test blocks obtained by this invention meet the specifications for unconfined compressive strength and leaching concentration, showing great potential for use as roadbed filler, slope protection blocks, or non-load-bearing materials, providing a complete disposal path for the harmless, stable, and resource-based utilization of PFOA-contaminated soil. Attached Figure Description

[0028] Figure 1 This is a flowchart illustrating a method for the co-treatment of perfluorooctanoic acid contaminated soil by hydrothermal solidification of solid waste, as claimed in an embodiment of the present invention. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0030] The terms "first," "second," and "third" used in this invention are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first," "second," or "third" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified. All directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of this invention are only used to explain the relative positional relationships and movements between components in a specific orientation (as shown in the accompanying drawings). If the specific orientation changes, the directional indications also change accordingly. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or devices.

[0031] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0032] According to Embodiment 1 of the present invention, referring to Figure 1 This invention claims protection for a method for the co-treatment of perfluorooctanoic acid (PFOA) contaminated soil by hydrothermal solidification of solid waste, comprising the following steps:

[0033] S1. The perfluorooctanoic acid contaminated soil is pretreated by crushing and screening to obtain pretreated contaminated soil. The carbide slag is dried to constant weight and then crushed to obtain carbide slag powder. The biomass raw material is pyrolyzed, cooled and crushed to obtain biochar powder.

[0034] S2. Ingredient mixing: The pretreated contaminated soil, carbide slag powder and biochar powder are mixed according to the mass ratio, water is added and stirred evenly to obtain a mixture.

[0035] S3. Sample molding: Weigh the mixture according to the designed dry density, put the weighed mixture into the mold and press it into shape to obtain the green body;

[0036] S4. Hydrothermal curing treatment: The blank is placed in a hydrothermal reactor and hydrothermally cured under set temperature and time conditions, followed by drying treatment to obtain a hydrothermal cured test block.

[0037] S5. Strength and pollutant leaching tests: The hydrothermally cured test block is subjected to unconfined compressive strength testing and perfluorooctanoic acid (PFOA) leaching concentration testing.

[0038] Furthermore, the carbide slag in S1 is a gray-black dry powder, the main component of which is calcium hydroxide, with a mass fraction of not less than 90%.

[0039] Furthermore, the biomass raw material in S1 is one or more of crop straw, rice husks, sawdust, fruit shells, or sludge.

[0040] Further, in step S2, mixing the pretreated contaminated soil, carbide slag powder, and biochar powder in a specific mass ratio includes:

[0041] One hundred parts of perfluorooctanoic acid contaminated soil, fifteen to twenty parts of calcium carbide slag powder, and two to six parts of biochar powder.

[0042] Furthermore, the amount of water added in S2 is 15% to 20% of the mass of the other pretreated raw materials after mixing.

[0043] Furthermore, the dry density described in S3 is 1.7 to 1.9 grams per cubic centimeter.

[0044] Furthermore, the temperature of the hydrothermal curing treatment in S4 is 180 to 200 degrees Celsius, and the treatment time is 8 to 16 hours.

[0045] Furthermore, the drying process in step S4 is carried out at a temperature of 80 degrees Celsius for 12 hours.

[0046] Furthermore, during the hydrothermal curing process in S4, the reactor is in an alkaline environment, where hydroxide ions are released by the hydrolysis of carbide slag, and the pH value is 12.5 to 13.

[0047] Furthermore, the perfluorooctanoic acid leaching concentration in S5 meets the limit requirement of not more than 40 nanograms per liter.

[0048] Specifically, in Example 1, the raw material pretreatment stage includes three independent and parallel material processing operations, which are standardized pretreatments for perfluorooctanoic acid contaminated soil, carbide slag, and biomass raw materials, respectively.

[0049] In the pretreatment of perfluorooctanoic acid (PFOA) contaminated soil, the collected PFOA-contaminated soil samples are first placed in a standard soil crusher for coarse crushing. The coarse crushing process utilizes the jaw crusher principle, where the periodic reciprocating motion between the moving and fixed jaws applies compression and splitting action to the soil sample, breaking down large contaminated soil particles into smaller aggregates. After coarse crushing, the crushed material is graded by particle size using a vibrating screen. The screen uses a double-layer screen structure, with the upper screen having a 2mm aperture and the lower screen having a 0.5mm aperture. A variable frequency vibrating motor drives the screen frame to generate mechanical vibration at a specified frequency and amplitude, causing the material to undergo continuous throwing motion on the screen surface, achieving precise particle size separation. The undersize material with a particle size smaller than 2mm is collected from the screening process as feed for subsequent processing steps, while the gravel and debris with a particle size larger than 2mm are used for construction fill or waste disposal. The pretreated contaminated soil obtained through the above crushing and screening operations has the following particle size distribution characteristics: fine particles with a particle size in the range of 0 to 0.5 mm account for 35%, medium particles with a particle size in the range of 0.5 to 1 mm account for 45%, and coarser particles with a particle size in the range of 1 to 2 mm account for 20%. This particle size distribution characteristic can ensure the uniform dispersion of each component during the subsequent batching and mixing process and the effective interlocking of particles during the pressing and molding process.

[0050] In the pretreatment of calcium carbide slag, the purchased industrial by-product calcium carbide slag is placed in a constant temperature drying oven for drying. The chemical composition of calcium carbide slag is mainly calcium hydroxide, and it is a gray-black powder with a low density and a large number of microporous structures. The main chemical components are shown in Table 1.

[0051] Table 1. Main chemical components and contents of carbide slag

[0052] chemical composition CaO / % <![CDATA[SiO2 / %]]> MgO / % <![CDATA[Al2O3 / %]]> <![CDATA[Fe2O3 / %]]> <![CDATA[K2O / %]]> <![CDATA[Na2O / %]]> Loss / % Quality percentage (%) 65.82 5.27 0.25 2.11 0.41 0.27 0.24 24.45

[0053] The drying process employs a forced-air drying mode, with the drying chamber set at 105 degrees Celsius and temperature control accuracy within ±5 degrees Celsius. During the drying process, samples are taken from different locations within the drying chamber every 2 hours and weighed using an electronic balance with an accuracy of 0.01g. When the mass change rate of three consecutive weighings is less than 0.1%, the calcium carbide slag is considered to have reached a constant weight state. The dried calcium carbide slag is then finely crushed using a high-speed pulverizer. The pulverizer rotor speed is set to 15,000 rpm. The pulverizing chamber is equipped with symmetrically distributed pulverizing blades and impact liners. The material gains significant kinetic energy under the high-speed rotating rotor and pulverizes through high-speed collisions with the impact liners. The pulverized material then enters a classifier via an airflow conveying system. The classifier uses a centrifugal classification principle, precisely controlling the particle size of the material passing through the screen by adjusting the rotor speed. In this embodiment, the screen mesh size of the classifier is set to 100 mesh, i.e., the nominal aperture of the screen is 0.15mm. After powder selection, the particle size distribution of the collected calcium carbide slag powder is as follows: d10 = 0.03 mm, d50 = 0.08 mm, d90 = 0.14 mm, and the specific surface area, determined by nitrogen adsorption, is 25 square meters per gram. The main chemical components of the calcium carbide slag powder, quantitatively analyzed by X-ray fluorescence spectrometry, are: calcium oxide 65.82%, silicon dioxide 5.27%, magnesium oxide 0.25%, aluminum oxide 2.11%, ferric oxide 0.41%, potassium oxide 0.27%, sodium oxide 0.24%, and loss on ignition 24.45%. The mass fraction of calcium hydroxide, determined by a combination of thermogravimetric analysis and X-ray diffraction, is 92.3%, meeting the technical requirement of not less than 90% as specified in this invention.

[0054] In the preparation of biochar powder, biomass raw materials are first selected as the source material for pyrolysis and carbonization. Corn stalks are chosen as the biomass raw material, derived from agricultural waste straw resources, and have the advantages of wide availability, low cost, and high carbon content. Pretreatment of the corn stalks includes removing roots, leaves, and impurities, retaining only the stalks, then cutting them into 3-5 cm long pieces, and air-drying them naturally until the moisture content is below 10%. The pyrolysis and carbonization of the biomass raw material is carried out in a tubular electric resistance furnace. The heating element of the pyrolysis furnace is a silicon carbide rod, with a maximum operating temperature of 1200 degrees Celsius. The pyrolysis process employs a programmed temperature control strategy: First, the temperature is increased from room temperature to 200°C at a rate of 10°C per minute. This temperature range is used for a low-temperature preheating and dehydration stage, maintained for 30 minutes to ensure the complete removal of free and bound water from the material. Then, the temperature is increased to 500°C at a rate of 5°C per minute, entering the main pyrolysis and carbonization stage. This temperature range is maintained for 120 minutes, allowing the biomass to undergo a thermal decomposition reaction, generating biochar and gaseous products. After pyrolysis, the heating switch is turned off, allowing the furnace to cool naturally to below 200°C. Nitrogen gas is then introduced for atmosphere replacement, and the pyrolysis products are further cooled to room temperature under nitrogen protection. The heating and cooling rates, holding time, and atmosphere control parameters throughout the entire pyrolysis process are precisely set and automatically executed by a programmable logic controller (PLC), ensuring the stability of the biochar product quality. The biochar prepared by the above pyrolysis process is a black powder. Its particle size distribution, determined by a laser particle size analyzer, is as follows: d10 = 0.015 mm, d50 = 0.045 mm, d90 = 0.12 mm. The specific surface area, determined by carbon dioxide adsorption, is 368 square meters per gram, and the pore volume is 0.18 cubic centimeters per gram. The cooled biochar is then finely pulverized and particle size-classified using a high-speed pulverizer and a 100-mesh classifier of the same specifications to obtain biochar powder with the required particle size.

[0055] The ingredient mixing stage is a key process in which the three pretreated raw materials prepared by S1 are accurately measured and uniformly mixed according to the specified mass ratio.

[0056] According to the design requirements of the technical solution of this invention, the proportion of the pretreatment raw materials is set as follows by mass: 100 parts of perfluorooctanoic acid contaminated soil, 20 parts of carbide slag powder, and 4 parts of biochar powder. In the laboratory-scale preparation of this embodiment, the raw materials are weighed based on the preparation of a set of standard specimens. The mass of perfluorooctanoic acid contaminated soil is 500 grams, the mass of carbide slag powder is 100 grams, and the mass of biochar powder is 20 grams, for a total mass of 620 grams.

[0057] The mixing process was carried out using a planetary mixer. The planetary mixer works by rotating its arm while simultaneously revolving around the central axis, covering the entire inner wall of the mixing pot and ensuring thorough tumbling and uniform mixing of the materials. The mixer's technical parameters were set as follows: 1200 rpm for a mixing speed of 4 minutes. The specific procedure for preparing the mixture was as follows: First, the weighed perfluorooctanoic acid (PFOA) contaminated soil was poured into the mixing pot, and the mixer was started for dry mixing, allowing the soil particles to form a uniformly dispersed fluidized state under the influence of the stirring arms. Then, during continuous mixing, carbide slag powder and biochar powder were slowly added, and mixing continued until the three solid materials were completely and uniformly mixed. Finally, water was added. The addition of water is a crucial step in controlling the mixture's performance. The amount of water added was 15% of the total mass of the solid materials, i.e., 93 grams. Water was precisely measured using volumetric titration, with the addition rate controlled using a separatory funnel. Stirring was performed while adding water to ensure that water molecules could fully penetrate the surface of each solid particle and form a uniform wetting layer. After the entire ingredient mixing process is completed, a uniform mixture with consistent color and texture is obtained. The performance indicators of the mixture are as follows: moisture content 15%, wet density 1.85 g / cm³, appearance is a uniform gray-black paste, slightly sticky to the touch but does not clump together.

[0058] The sample forming stage is the process of pressing the mixture into a preform of specified size and density. The forming quality directly determines the effectiveness of subsequent hydrothermal curing treatment and the mechanical properties of the final product.

[0059] The sample prepared in this embodiment is a cylindrical specimen with a diameter of 50 mm and a height of 50 mm. Based on the designed dry density of 1.7 g / cm³, the required mass of the mixture for each specimen is 191.83 g. The calculation process is as follows: The volume of the cylinder equals π multiplied by the square of the radius and then multiplied by the height. π is taken as 3.14, the radius is 2.5 cm, and the height is 5 cm, resulting in a volume of 98.125 cm³. The dry density multiplied by the volume equals the dry mass, i.e., 1.7 multiplied by 98.125 equals 166.81 g. Considering the moisture content of the mixture is 15%, the wet mass equals the dry mass divided by one minus the moisture content, i.e., 166.81 divided by 0.85 equals 196.25 g. The above calculations are theoretical values. In actual operation, considering material loss and weighing errors, 191.83 g is taken as the actual weighed mass.

[0060] Compression molding is performed using a laboratory-grade electro-hydraulic press. The press consists of three parts: a hydraulic system, a control system, and a molding die. The hydraulic system uses a gear pump for oil supply, with a maximum working pressure of 60 MPa. Precise pressure control is achieved through pressure sensors and proportional valves. The control system uses a programmable logic controller (PLC) to set the loading rate, holding time, and unloading procedure. The molding die is made of high-quality alloy steel, with internal dimensions strictly machined according to the specifications of cylindrical specimens, and its surface is polished to reduce friction. The specific steps of the molding operation are as follows: First, the weighed mixture is evenly loaded into the mold cavity. During the loading process, a funnel is used to guide the material to fall freely to avoid segregation. After loading, the mold is placed in the center of the press table, and the pressing program is started. The pressing process is divided into two stages: pre-pressing and main pressing. In the pre-pressing stage, the pressure is increased to 20 kN at a rate of 5 kN per minute and maintained at a constant speed for 1 minute to allow the air in the mixture to be initially expelled. In the main pressing stage, the pressure is further increased to the set value at a rate of 10 kN per minute (60 kN in this embodiment) and maintained at a constant speed for 3 minutes to fully densify the mixture. After pressing, the pressure is unloaded to zero at a rate of 20 kN per minute, the mold is opened, and the molded blank is removed. The molded blank should meet the following quality requirements: complete appearance without cracks, smooth surface without obvious defects, dimensional deviation controlled within ±0.5 mm, and density verified by weighing in water to be 1.68 to 1.72 g per cubic centimeter.

[0061] The hydrothermal curing stage is the core process in which the molded preform is placed in a high-temperature, high-pressure hydrothermal environment for chemical reaction and structural densification. The alkaline environment under hydrothermal conditions not only achieves the degradation and mineralization of perfluorooctanoic acid (PFOA), but also promotes the formation of hydration gel products, enabling the cured body to acquire mechanical strength.

[0062] The hydrothermal curing process is carried out in a hydrothermal reactor equipped with heating and pressure control functions. The main body of the hydrothermal reactor is made of titanium alloy, which has excellent corrosion resistance and can withstand high temperatures of 300 degrees Celsius and high pressures of 30 MPa. The reactor is equipped with an electric heating jacket and a cooling water jacket, and precise temperature program control is achieved through an intelligent temperature control system. The reactor lid is equipped with a pressure gauge, a safety valve, and a thermocouple socket, which can monitor the pressure and temperature changes inside the reactor in real time.

[0063] Carefully place the preforms prepared in S3 into the polytetrafluoroethylene liner of the reactor, arranging them neatly in numerical order. Six preforms can be placed in each batch simultaneously. Add an appropriate amount of deionized water as the reaction medium, enough to cover all the preforms; in this example, 150 ml of deionized water is added. Close the reactor lid and tighten the bolts, then start the heating program.

[0064] The process parameters for hydrothermal curing were set as follows: reaction temperature 200°C, reaction time 12 hours, heating rate 2°C / min, and natural cooling within the furnace. The specific procedure for the temperature control curve was as follows: heating from room temperature to 200°C at a rate of 2°C / min, which took approximately 90 minutes; maintaining the temperature at 200°C for 12 hours, during which the pressure inside the reactor automatically increased to approximately 1.55 MPa, forming a saturated vapor pressure environment; after the holding period, heating was stopped, and the reactor was allowed to cool naturally to below 100°C before opening the reactor lid and removing the samples. Throughout the hydrothermal curing reaction, the alkaline environment inside the reactor was maintained by hydroxide ions released from the hydrolysis of calcium carbide slag, and the pH value was measured on-site to be between 12.5 and 13.0.

[0065] After hydrothermal curing, the test block is removed from the reactor and placed in an 80°C constant temperature drying oven for subsequent drying. The drying time is set to 12 hours to allow the free water in the cured body to evaporate fully and form a stable microstructure. The dried test block is the hydrothermally cured test block, which can be used for subsequent performance testing.

[0066] The strength and contaminant leaching testing stages are crucial for the quality acceptance and performance evaluation of hydrothermally cured test blocks. The test results determine whether the method of this invention has achieved the expected technical effect.

[0067] Unconfined compressive strength testing was conducted using a universal testing machine. The machine consists of a loading system, a force measuring system, and a data acquisition system, with a maximum loading capacity of 100 kN and a force measuring accuracy of 0.1 grade. Before testing, both ends of the specimen were sanded smooth with fine sandpaper to ensure the loading surface was perpendicular to the axis. The specimen was placed at the center of the lower pressure plate of the testing machine and loaded uniformly at a rate of 0.5 MPa per second until the specimen failed. The peak load was recorded, and the unconfined compressive strength value was calculated. In this embodiment, three parallel samples were used as a group, and their arithmetic mean was taken as the final test result.

[0068] The perfluorooctanoic acid (PFOA) leaching concentration was determined according to the solid waste leaching toxicity leaching method. First, the hydrothermally cured sample block was crushed into particles smaller than 5 mm. 100 g of the crushed sample was weighed and placed in a leaching bottle, along with deionized water at a liquid-to-solid ratio of 10:1. The bottle was sealed and placed on a horizontal shaker for leaching. The shaking frequency was 110 times per minute, and the shaking time was 8 hours. After shaking, the sample was allowed to stand for 30 minutes. The leachate was then filtered through a 0.45-micron pore size filter membrane, and the filtrate was collected for analysis.

[0069] The concentration of perfluorooctanoic acid (PFOA) in the filtrate was determined using liquid chromatography-mass spectrometry (LC-MS). The analytical conditions were as follows: a C18 reversed-phase column was used at 40°C; the mobile phase was a methanol-water mixture at a flow rate of 0.3 mL / min; and quantitative analysis was performed using an electrospray ionization source and multiple reaction monitoring (MRM) mode. External standard method was used for quantification, and the linear correlation coefficient of the standard curve was greater than 0.999. Referring to the requirements of the drinking water hygiene standards, the PFOA concentration in the leachate should meet the limit requirement of not exceeding 40 nanograms per liter.

[0070] The test results of the cured body obtained in this embodiment are as follows: the average unconfined compressive strength is 34.50 MPa, and the average perfluorooctanoic acid leaching concentration is 33 nanograms per liter, which meets the specification requirements.

[0071] According to Embodiment 1 of the present invention, the required mass of each raw material component is calculated proportionally as follows:

[0072] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 0 parts (0 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (520 g × 15% = 78 g).

[0073] The hydrothermal curing reaction was carried out at 200 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0074] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0075] Example 2

[0076] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0077] Based on the proportional conversion, the required mass of each raw material portion is:

[0078] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 5 parts (25 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (545 g × 20% = 81.75 g).

[0079] The hydrothermal curing reaction was carried out at 200 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0080] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0081] Example 3

[0082] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0083] Based on the proportional conversion, the required mass of each raw material portion is:

[0084] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 10 parts (50 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (570 g × 20% = 85.5 g).

[0085] The hydrothermal curing reaction was carried out at 200 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0086] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0087] Example 4

[0088] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0089] Based on the proportional conversion, the required mass of each raw material portion is:

[0090] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 15 parts (75 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (595 g × 20% = 89.25 g).

[0091] The hydrothermal curing reaction was carried out at 200 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0092] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0093] Example 5

[0094] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0095] Based on the proportional conversion, the required mass of each raw material portion is:

[0096] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (620 g × 15% = 93 g).

[0097] The hydrothermal curing reaction was carried out at 200 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0098] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0099] Example 6

[0100] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0101] Based on the proportional conversion, the required mass of each raw material portion is:

[0102] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 25 parts (125 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (645 g × 15% = 96.75 g).

[0103] The hydrothermal curing reaction was carried out at 200 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0104] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0105] Example 7

[0106] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0107] Based on the proportional conversion, the required mass of each raw material portion is:

[0108] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 0 parts (0 g) of biochar powder, and 15% water (600 g × 15% = 90 g).

[0109] The hydrothermal curing reaction was carried out at 200 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0110] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0111] Example 8

[0112] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0113] Based on the proportional conversion, the required mass of each raw material portion is:

[0114] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 2 parts (10 g) of biochar powder, and 15% water (610 g × 15% = 91.5 g).

[0115] The hydrothermal curing reaction was carried out at 200 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0116] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0117] Example 9

[0118] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0119] Based on the proportional conversion, the required mass of each raw material portion is:

[0120] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 6 parts (30 g) of biochar powder, and 15% water (630 g × 15% = 94.5 g).

[0121] The hydrothermal curing reaction was carried out at 200 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0122] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0123] Example 10

[0124] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0125] Based on the proportional conversion, the required mass of each raw material portion is:

[0126] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 0% water (620 g × 0% = 0 g).

[0127] The hydrothermal curing reaction was carried out at 200 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0128] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+0%)=166.81 g

[0129] Example 11

[0130] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0131] Based on the proportional conversion, the required mass of each raw material portion is:

[0132] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 5% water (620 g × 5% = 31 g).

[0133] The hydrothermal curing reaction was carried out at 200 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0134] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+5%)=175.15 g

[0135] Example 12

[0136] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0137] Based on the proportional conversion, the required mass of each raw material portion is:

[0138] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 10% water (620 g × 10% = 62 g).

[0139] The hydrothermal curing reaction was carried out at 200 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0140] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+10%)=183.49 g

[0141] Example 13

[0142] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0143] Based on the proportional conversion, the required mass of each raw material portion is:

[0144] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 20% water (620 g × 20% = 124 g).

[0145] The hydrothermal curing reaction was carried out at 200 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0146] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+20%)=200.18 g

[0147] Example 14

[0148] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0149] Based on the proportional conversion, the required mass of each raw material portion is:

[0150] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (620 g × 15% = 93 g).

[0151] The hydrothermal curing reaction was carried out at 200 °C for 12 hours, and the dry density of the sample at the time of molding was 1.5 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0152] 3.14×2.5 cm×2.5 cm×5 cm×1.5 g / cm 3×(1+15%)=169.27 g

[0153] Example 15

[0154] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0155] Based on the proportional conversion, the required mass of each raw material portion is:

[0156] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (620 g × 15% = 93 g).

[0157] The hydrothermal curing reaction was carried out at 200 °C for 12 hours, and the dry density of the sample at the time of molding was 1.9 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0158] 3.14×2.5 cm×2.5 cm×5 cm×1.9 g / cm 3 ×(1+15%)=214.4 g

[0159] Example 16

[0160] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0161] Based on the proportional conversion, the required mass of each raw material portion is:

[0162] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (620 g × 15% = 93 g).

[0163] The hydrothermal curing reaction was carried out at 200 °C for 12 hours, and the dry density of the sample at the time of molding was 2.1 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0164] 3.14×2.5 cm×2.5 cm×5 cm×2.1 g / cm 3 ×(1+15%)=236.97 g

[0165] Example 17

[0166] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0167] Based on the proportional conversion, the required mass of each raw material portion is:

[0168] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (620 g × 15% = 93 g).

[0169] The hydrothermal curing reaction was carried out at 140 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0170] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0171] Example 18

[0172] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0173] Based on the proportional conversion, the required mass of each raw material portion is:

[0174] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (620 g × 15% = 93 g).

[0175] The hydrothermal curing reaction was carried out at 160 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0176] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0177] Example 19

[0178] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0179] Based on the proportional conversion, the required mass of each raw material portion is:

[0180] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (620 g × 15% = 93 g).

[0181] The hydrothermal curing reaction was carried out at 180 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3The mass of the mixture weighed for each sample is:

[0182] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0183] Example 20

[0184] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0185] Based on the proportional conversion, the required mass of each raw material portion is:

[0186] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (620 g × 15% = 93 g).

[0187] The hydrothermal curing reaction was carried out at 220 °C for 12 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0188] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0189] Example 21

[0190] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0191] Based on the proportional conversion, the required mass of each raw material portion is:

[0192] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (620 g × 15% = 93 g).

[0193] The hydrothermal curing reaction conditions were 200 °C for 0 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0194] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0195] Example 22

[0196] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0197] Based on the proportional conversion, the required mass of each raw material portion is:

[0198] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (620 g × 15% = 93 g).

[0199] The hydrothermal curing reaction was carried out at 200 °C for 4 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0200] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0201] Example 23

[0202] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0203] Based on the proportional conversion, the required mass of each raw material portion is:

[0204] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (620 g × 15% = 93 g).

[0205] The hydrothermal curing reaction was carried out at 200 °C for 8 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0206] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0207] Example 24

[0208] The method used in this embodiment for treating perfluorooctanoic acid-contaminated soil is basically the same as the steps in Example 1, except that:

[0209] Based on the proportional conversion, the required mass of each raw material portion is:

[0210] 100 parts (500 g) of perfluorooctanoic acid contaminated soil, 20 parts (100 g) of calcium carbide slag powder, 4 parts (20 g) of biochar powder, and 15% water (620 g × 15% = 93 g).

[0211] The hydrothermal curing reaction was carried out at 200 °C for 16 hours, and the dry density of the sample at the time of molding was 1.7 g / cm³. 3 The mass of the mixture weighed for each sample is:

[0212] 3.14×2.5 cm×2.5 cm×5 cm×1.7 g / cm 3 ×(1+15%)=191.83 g

[0213] Comparative Example

[0214] The comparative sample did not undergo any treatment on the soil contaminated with perfluorooctanoic acid (PFOA), and the unconfined compressive strength and PFOA leaching concentration were tested.

[0215] The above examples demonstrate the effects of carbide slag powder, biochar powder, water, dry density, temperature, and time on the unconfined compressive strength and perfluorooctanoic acid leaching concentration of the solidified body. For easy comparison, the proportions of all examples are summarized in Table 2.

[0216] Table 2. Proportion of each component and reaction conditions in the examples

[0217] 1 100 0 4 15 1.7 200 12 2 100 5 4 15 1.7 200 12 3 100 10 4 15 1.7 200 12 4 100 15 4 15 1.7 200 12 5 100 20 4 15 1.7 200 12 6 100 25 4 15 1.7 200 12 7 100 20 0 15 1.7 200 12 8 100 20 2 15 1.7 200 12 9 100 20 6 15 1.7 200 12 10 100 20 4 0 1.7 200 12 11 100 20 4 5 1.7 200 12 12 100 20 4 10 1.7 200 12 13 100 20 4 20 1.7 200 12 14 100 20 4 15 1.5 200 12 15 100 20 4 15 1.9 200 12 16 100 20 4 15 2.1 200 12 17 100 20 4 15 1.7 140 12 18 100 20 4 15 1.7 160 12 19 100 20 4 15 1.7 180 12 20 100 20 4 15 1.7 220 12 21 100 20 4 15 1.7 200 0 22 100 20 4 15 1.7 200 4 23 100 20 4 15 1.7 200 8 24 100 20 4 15 1.7 200 16

[0218] Triple copies were prepared for each of the above examples for unconfined compressive strength testing. The crushed samples were then used for perfluorooctanoic acid (PFOA) leaching concentration experiments. The unconfined compressive strength and PFOA leaching concentration measured in each example were the average of the three parallel samples. The unconfined compressive strength of the solidified body was determined according to the method described in industry standard JTG 3441-2024 "Test Procedure for Inorganic Binder Stabilized Materials in Highway Engineering". The PFOA leaching concentration of the solidified body was determined according to the methods described in industry standard HJ 557-2010 "Leaching Toxicity of Solid Waste - Horizontal Oscillation Method" and international standard ISO 25101:2009 "Determination of Perfluorooctane Sulfonic Acid (PFOS) and Perfluorooctanoic Acid (PFOA) in Water - Solid Phase Extraction and Liquid Chromatography / Mass Spectrometry". Referring to the requirements of GB 5749-2022 "Standards for Drinking Water Quality", the PFOA concentration in the leachate should meet the limit requirement of ≤40 ng / L. The experimental test results are shown in Table 3.

[0219] Table 3 Comparison of Experimental Results

[0220] Example 1 2.56 56 99.81 Example 2 8.11 43 99.85 Example 3 13.25 38 99.87 Example 4 27.33 36 99.88 Example 5 34.50 33 99.89 Example 6 26.74 51 99.83 Example 7 28.73 41 99.86 Example 8 30.72 37 99.88 Example 9 31.14 31 99.90 Example 10 0.00 1563 94.73 Example 11 12.74 557 98.12 Example 12 26.71 68 99.77 Example 13 28.70 33 99.89 Example 14 14.69 88 99.70 Example 15 36.67 39 99.87 Example 16 28.94 57 99.81 Example 17 16.63 1751 94.10 Example 18 19.93 846 97.15 Example 19 28.71 38 99.87 Example 20 26.57 34 99.89 Example 21 2.43 18572 37.37 Example 22 13.27 237 99.20 Example 23 28.77 37 99.88 Example 24 27.93 31 99.90 Comparative Example 0.00 29653 0.00

[0221] Comparing Examples 1-6, it can be seen that the unconfined compressive strength of the examples first increases and then decreases with the increase of calcium carbide slag powder. Example 5 shows the highest unconfined compressive strength. Examples 4 and 6 show a slight decrease in strength compared to Example 5, while Examples 1, 2, and 3 show a significant decrease in strength compared to Example 5. In Examples 1-6, the perfluorooctanoic acid (PFOA) leaching concentration in Examples 1, 2, and 6 did not meet the specification requirements; therefore, the recommended dosage of calcium carbide slag powder is 15-20 parts. Comparing Examples 5 and Examples 7-9, it can be seen that the unconfined compressive strength of the examples first increases and then decreases with the increase of biochar powder. Example 5 shows the highest unconfined compressive strength. Examples 8 and 9 show only a slight decrease in strength compared to Example 5, while Example 7 shows a significant decrease in strength compared to Example 5. The PFOA leaching concentration in the examples decreases with the increase of biochar powder. Except for Example 7, all examples meet the specification requirements; therefore, the recommended dosage of biochar powder is 2-6 parts. Comparing Examples 5 and 10-13, it can be seen that the unconfined compressive strength of the examples first increases and then decreases with increasing moisture content, with Example 5 reaching the maximum. Examples 12 and 13 show a slight decrease in strength compared to Example 5, while Examples 10 and 11 show a significant decrease. The perfluorooctanoic acid (PFOA) leaching concentration in Examples 10-12 did not meet the specification requirements; therefore, the recommended moisture content range is 15%-20%. Comparing Examples 5 and 14-16, it can be seen that the unconfined compressive strength of the examples first increases and then decreases with increasing dry density, with Example 15 reaching the maximum. Except for Example 14, which shows a significant decrease in unconfined compressive strength, the other three examples show little difference in unconfined compressive strength. The PFOA leaching concentration in Examples 14 and 16 did not meet the specification requirements; therefore, the recommended dry density range is 1.7-1.9 g / cm³. 3 .

[0222] Comparing Examples 5 and 17-20, it can be seen that the unconfined compressive strength of the examples first increases and then decreases with increasing treatment temperature. Examples 5, 19, and 20 have relatively high unconfined compressive strength and all meet the specifications for perfluorooctanoic acid (PFOA) leaching concentration. Example 20, compared to Example 5, increased treatment temperature but decreased unconfined compressive strength and increased PFOA leaching concentration. Therefore, the recommended treatment temperature range is 180-200 °C. Comparing Examples 5 and 21-24, it can be seen that the unconfined compressive strength of the examples first increases and then decreases with increasing treatment time. Examples 5, 23, and 24 have relatively high unconfined compressive strength and all meet the specifications for PFOA leaching concentration. Although Example 24 has a lower unconfined compressive strength compared to Example 5, its PFOA leaching concentration also decreases. Therefore, the recommended treatment time range is 8-16 hours.

[0223] In the several embodiments provided by this invention, it should be understood that the disclosed systems, apparatuses, and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be an indirect coupling or communication connection between apparatuses or units through some interfaces, and may be electrical, mechanical, or other forms.

[0224] Furthermore, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated units described above can be implemented in hardware or as software functional units. The above are merely embodiments of the present invention and do not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the description and drawings of the present invention, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

[0225] The specific embodiments of the invention have been described in detail above, but these are merely examples, and the invention is not limited to the specific embodiments described above. For those skilled in the art, any equivalent modifications or substitutions to the invention are also within the scope of this invention. Therefore, all equivalent transformations, modifications, and improvements made without departing from the spirit and principles of this invention should be included within the scope of this invention.

Claims

1. A method for co-processing solid waste with hydrothermal solidification to treat perfluorooctanoic acid (PFOA) contaminated soil, characterized in that, Includes the following steps: S1. The perfluorooctanoic acid contaminated soil is pretreated by crushing and screening to obtain pretreated contaminated soil. The carbide slag is dried to constant weight and then crushed to obtain carbide slag powder. The biomass raw material is pyrolyzed, cooled and crushed to obtain biochar powder. S2. Ingredient mixing: The pretreated contaminated soil, carbide slag powder and biochar powder are mixed according to the mass ratio, water is added and stirred evenly to obtain a mixture. S3. Sample molding: Weigh the mixture according to the designed dry density, put the weighed mixture into the mold and press it into shape to obtain the green body; S4. Hydrothermal curing treatment: The blank is placed in a hydrothermal reactor and hydrothermally cured under set temperature and time conditions, followed by drying treatment to obtain a hydrothermal cured test block. S5. Strength and pollutant leaching tests: The hydrothermally cured test block is subjected to unconfined compressive strength testing and perfluorooctanoic acid (PFOA) leaching concentration testing.

2. The method according to claim 1, characterized in that, The carbide slag mentioned in S1 is a gray-black dry powder, the main component of which is calcium hydroxide, with a mass fraction of not less than 90%.

3. The method according to claim 1, characterized in that, The biomass raw material mentioned in S1 is one or more of the following: crop straw, rice husks, sawdust, fruit shells, or sludge.

4. The method according to claim 1, characterized in that, In step S2, the mixing of the pretreated contaminated soil, carbide slag powder, and biochar powder in a specific mass ratio includes: One hundred parts of perfluorooctanoic acid contaminated soil, fifteen to twenty parts of calcium carbide slag powder, and two to six parts of biochar powder.

5. The method according to claim 1, characterized in that, The amount of water added in S2 is 15% to 20% of the mass of the other pretreated raw materials after mixing.

6. The method according to claim 1, characterized in that, The dry density described in S3 is 1.7 to 1.9 grams per cubic centimeter.

7. The method according to claim 1, characterized in that, The hydrothermal curing treatment in S4 is performed at a temperature of 180 to 200 degrees Celsius for 8 to 16 hours.

8. The method according to claim 1, characterized in that, The drying process in step S4 is carried out at a temperature of 80 degrees Celsius for 12 hours.

9. The method according to any one of claims 1 to 8, characterized in that, During the hydrothermal curing process described in S4, the reactor is in an alkaline environment, where hydroxide ions are released by the hydrolysis of carbide slag, and the pH value is between 12.5 and 13.

10. The method according to any one of claims 1 to 8, characterized in that, The perfluorooctanoic acid leaching concentration in S5 meets the limit requirement of not more than 40 nanograms per liter.

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