Method for reducing cadmium content of rice in rice field by using nitrogen-containing organic waste hydrothermal pyrolysis liquid of rural sewage and application thereof

Abstract

The present application belongs to the field of agricultural environmental protection, and particularly relates to a method for reducing cadmium content of rice in rice field by using organic waste water hydrolysis liquid containing nitrogen and application thereof. The method comprises the following steps: mixing rural wastewater with crop straw and / or discarded fruits and vegetables, and performing water hydrolysis treatment to obtain an alkaline water hydrolysis liquid with pH of 7.5-9.5, diluting and performing safety detection; 15-20 days before rice mature harvest, diluting the alkaline water hydrolysis liquid with water, performing safety detection, applying to cadmium contaminated rice field for irrigation, and controlling the depth of field water to be 3-5 cm; 10-15 days after irrigation, performing drainage, keeping soil moist during the period from the drainage to the mature harvest of rice, and performing local conventional water management to avoid continuous excessive dryness. The present application performs water hydrolysis on rural wastewater, crop straw and / or discarded fruits and vegetables, reduces cadmium content of rice at mature harvest by increasing the pH of pore water and reducing the concentration of Cd ions in pore water.

Description

Technical Field

[0001] This invention belongs to the field of agricultural environmental protection, specifically relating to a method for reducing the cadmium content of rice in paddy fields using hydrothermal pyrolysis solution of organic waste from nitrogen-containing rural sewage and its application. Background Technology

[0002] Studies have shown that the rate of Cd exceeding the standard in mid-to-late-season rice in southern China is significantly higher than that in early-season rice, with the grain-filling to maturity stage being the critical window for Cd accumulation in rice. During this period, rice roots remain active in absorbing Cd from the soil. 2+ It is transported from the xylem to the aboveground parts, distributed through the stem nodes, and ultimately accumulated in the grains. More importantly, the conventional drainage and desiccation operations (to facilitate mechanical harvesting) during the late grain-filling to maturity stages cause the paddy field soil to shift from a reducing to an oxidizing state: the extremely insoluble CdS formed under flooded reducing conditions is oxidized and decomposed, and Cd... 2+ The re-release of Cd into the soil solution, coupled with a decrease in soil pH, further exacerbates the dissolution and release of Cd. This chain reaction of "drainage-oxidation-Cd activation" is one of the core reasons for the high Cd exceedance rate in late-season rice in southern China. Therefore, the key technical challenge in reducing Cd accumulation in rice lies in maintaining a reducing soil environment while inhibiting secondary Cd activation caused by drainage acidification during the critical grain-filling and ripening period.

[0003] Alongside Cd pollution in paddy fields, another rural environmental problem is the disposal of domestic sewage. Rural areas generate large quantities of nitrogen-containing organic wastewater (commonly known as "wastewater," including toilet waste, kitchen wastewater, and wastewater from free-range livestock), but the collection and treatment rate is low. While direct application to the fields can theoretically recover nitrogen and phosphorus nutrients, it poses two major risks: first, the spread of pathogens (such as E. coli, Salmonella, and roundworm eggs), threatening agricultural product safety and human health; second, the spread of antibiotics and their resistance genes. In recent years, tetracycline, sulfonamide, and fluoroquinolone antibiotic residues have been frequently detected in livestock wastewater, and direct application to the fields can induce the soil microbial community to produce antibiotic resistance genes, causing even greater ecological risks. Therefore, the resource utilization of rural nitrogen-containing organic wastewater must first address the technical bottlenecks in pathogen inactivation and antibiotic degradation. Traditional anaerobic fermentation (biogas slurry) can partially remove pathogens, but its antibiotic removal efficiency is limited and requires significant engineering investment and operation and maintenance costs.

[0004] Hydrothermal carbonization ( Hydrothermal CarbonizationHydrothermal carbonization (HTC) is a thermochemical conversion technology for treating wet biomass at 180-260℃ and autogenous pressure (1.5-5.0 MPa). The high temperature and high pressure conditions can completely kill pathogens (studies show that treatment at above 160℃ for 30 minutes can reduce fecal coliform bacteria by more than 6 log units), while effectively degrading antibiotic molecules through hydrolysis, decarboxylation, and condensation reactions (tetracyclines can achieve a removal rate of over 95% under hydrothermal conditions at 200℃ for 30 minutes). Therefore, hydrothermal carbonization provides a technical pathway for the safe resource utilization of nitrogen-containing organic wastewater in rural areas.

[0005] More noteworthy is that hydrothermal carbonization produces two products: solid-phase hydrothermal carbon (which can be used for soil amendment or carbon sequestration) and liquid-phase hydrothermal carbonization liquid. Traditional research has focused primarily on the solid-phase products, while the liquid phase is typically considered wastewater. However, recent studies have found that liquid-phase hydrothermal carbonization liquid is rich in organic matter, humic precursors, nitrogen-containing heterocyclic compounds, and other active substances. Especially when the raw materials contain proteinaceous organic matter, the Maillard reaction occurs during hydrothermal carbonization, generating nitrogen-containing compounds such as melanoidins, pyrazines, and amides, while simultaneously releasing NH3, making the liquid phase alkaline. This property has particular value for controlling Cd pollution in paddy fields—the alkaline liquid can raise the soil pH during irrigation, inhibiting the acidification and dissociation of Cd, while the humic substances can complex Cd. 2+ This reduces its bioavailability.

[0006] Currently, Cd pollution control technologies in paddy fields (lime passivation, flooding during the grain-filling stage) and rural wastewater treatment technologies (anaerobic fermentation, constructed wetlands) are two independent fields lacking functional coupling. While lime passivation can increase soil pH, it does not provide organic matter and leads to compaction; flooding during the grain-filling stage maintains a reducing environment, but uses clean water and lacks active passivation function; hydrothermal carbonization of rural wastewater allows for safe disposal, but the agricultural value of its liquid phase products has not been fully explored. More importantly, no existing technology has systematically integrated the "natural alkalization characteristics of nitrogen-containing organic wastewater," the "in-situ generation of complexing active substances by hydrothermal carbonization," and the "Cd control window during flooding during the grain-filling stage" for Cd control in mature rice. The conventional understanding in this field regarding hydrothermal carbonization liquid is that it is acidic (pH 3.1~4.7 for carbonization liquids made from cellulose raw materials), thus usually requiring external alkali adjustment or application with biogas slurry. This further hinders the technical approach of directly using HTC liquid phase for alkaline passivation of Cd pollution.

[0007] This invention is proposed based on the aforementioned dual background: on the one hand, Cd activation caused by drainage during the grain-filling and ripening period of mid-to-late-season rice in southern China is a direct cause of excessive Cd levels in rice; on the other hand, there is an urgent need for the safe treatment and resource utilization of nitrogen-containing organic wastewater in rural areas. This invention organically combines these two aspects—using nitrogen-containing organic wastewater from rural areas and straw / fruits and vegetables as raw materials, hydrothermal carbonization is used to kill pathogens and degrade antibiotics, while simultaneously utilizing the natural alkalization properties of proteinaceous organic matter to prepare an alkaline irrigation solution. This solution is then precisely applied during the grain-filling period, simultaneously maintaining a reducing environment, inhibiting soil acidification, and reducing Cd levels. 2+ The four objectives of chelation fixation and waste resource utilization ultimately achieve the dual goals of "pollution reduction and carbon reduction" and "rice safety". Summary of the Invention

[0008] To address the problems existing in the prior art, the purpose of this invention is to design and provide a method for reducing the cadmium content of rice in paddy fields using hydrothermal pyrolysis solution of nitrogen-containing rural sewage organic waste, and the technical solution for its application.

[0009] The present invention is specifically implemented using the following technical solutions: The first aspect of this invention provides a method for reducing the cadmium content of rice from paddy fields, comprising the following steps: (1) Mix rural wastewater with crop straw and / or waste fruits and vegetables and carry out hydrothermal treatment to obtain an alkaline hydrothermal solution with a pH of 7.5-9.5; (2) 15-20 days before the rice matures and is harvested, the alkaline hydrothermal solution is diluted with water and applied to the cadmium-contaminated paddy field for irrigation, and the water depth on the field surface is controlled to be 3-5 cm. (3) Drainage should be carried out 10-15 days after irrigation. After drainage, the soil should be kept moist until the rice matures and is harvested, and local conventional water management should be implemented to avoid continuous excessive drying.

[0010] Furthermore, the rural sewage includes one or more of the following: toilet wastewater, kitchen wastewater, and livestock and poultry breeding wastewater. The rural sewage meets at least two of the following criteria: total nitrogen of 50-300 mg / L, protein organic matter of 80-300 mg / L, COD of 500-2000 mg / L, and ammonia nitrogen of 20-150 mg / L.

[0011] Furthermore, the crop straw is selected from one or more of rice straw, wheat straw, and corn straw; the waste fruits and vegetables are selected from one or more of vegetable residues, fruit processing waste, and leftover vegetables from farmers' markets.

[0012] Furthermore, in step (1), the wet basis mass ratio of rural wastewater to crop straw and / or waste fruits and vegetables is 2:1-10:1, and the solid content of the mixed system is 5%-20%.

[0013] Furthermore, in step (1), the hydrothermal treatment temperature is 180-260℃, the treatment time is 0.5-4 hours, and the preferred hydrothermal treatment temperature is 220℃, the treatment time is 2 hours, and the pressure is 2.8-3.2 MPa.

[0014] In the reaction process of this invention, proteinaceous organic matter, amino acids, ammonia nitrogen and soluble nitrogen-containing organic matter in rural wastewater react with reducing sugars and lignin fragments produced by the degradation of straw or fruits and vegetables to form humic acid substances, amide substances and nitrogen-containing heterocyclic compounds.

[0015] Furthermore, after the hydrothermal treatment in step (1), the process also includes sedimentation, filtration, and solid-liquid separation steps, with the resulting liquid phase serving as the alkaline hydrothermal solution; or the solid-liquid mixture obtained after hydrothermal treatment is diluted, sieved, and stirred evenly before application.

[0016] Furthermore, the pH of the alkaline hydrothermal solution in step (1) is 7.5-9.5, and the alkaline hydrothermal solution contains 2000-3500 mg / L of dissolved organic carbon, 150-250 mg / L of total nitrogen, and 0.5-2.5 g / L of humic acid, and has a wavelength of 1600-1700 cm⁻¹ in the FTIR spectrum. -1 The characteristic peak of amide I within the range.

[0017] Furthermore, the volume ratio of the alkaline hydrothermal solution to water in step (2) is 1:1 to 1:10, preferably 1:5, and the pH of the diluted irrigation solution is 7.2 to 8.8.

[0018] Furthermore, the diluted irrigation solution described in step (2) must undergo safety testing before irrigation. The safety testing includes testing for pH, EC, dissolved organic carbon, total nitrogen, humic acid substances, and heavy metal indicators such as Cd, As, Pb, Cr, and Hg, to ensure that the heavy metal content is lower than the corresponding limit for farmland irrigation water.

[0019] This invention introduces diluted alkaline hydropyrolysis solution into the field surface water and soil pore water system, thereby increasing the pH of the pore water and buffering the acidic rhizosphere environment; humic acids, amide groups, and nitrogen-containing heterocyclic compounds react with Cd 2+ Complexation reduces the concentration of free Cd ions in pore water; short-term shallow water layers promote the formation of a local reducing environment in the rhizosphere and reduce the continued translocation of Cd to grains before maturity; ultimately, it promotes the transformation of soil Cd from exchangeable to carbonate-bound, iron-manganese-bound, organic-bound, or residual states, thereby reducing the Cd content in rice.

[0020] A second aspect of the present invention provides the application of the method described in any of the above methods in reducing the Cd content in brown rice from Cd-contaminated paddy fields.

[0021] The present invention has the following beneficial effects: (1) This invention uses hydrothermal decomposition of rural wastewater with crop straw and / or waste fruits and vegetables to achieve synergistic resource utilization of toilet wastewater, kitchen wastewater, livestock and poultry breeding wastewater and plant residues.

[0022] (2) The present invention obtains alkaline hydrothermal solution without adding external alkaline agents such as lime and sodium hydroxide, thus avoiding the risk of soil compaction caused by long-term application of lime.

[0023] (3) This invention does not directly apply hydrothermal hydrolysate at high concentrations, but irrigates with water 15-20 days before maturity and harvest, and controls the water depth on the field surface at 3-5 cm, which ensures that the effective components enter the rhizosphere and pore water system, and avoids the pressure on field management caused by excessively deep water layers or high concentration liquids.

[0024] (4) This invention specifies that drainage should be carried out 10-15 days after irrigation. The subsequent flooding time after drainage is not specifically limited and can be compatible with local water management and mechanical harvesting needs during the ripening period.

[0025] (5) This invention reduces the Cd content of rice at maturity by increasing the pH of pore water, reducing the Cd ion concentration in pore water, and promoting the transformation of soil Cd from an exchangeable state to a more stable state. Attached Figure Description

[0026] Figure 1 The image shows the Fourier transform infrared (FTIR) spectrum of the hydrothermal carbonization pyrolysis solution.

[0027] Figure 2 This is a three-dimensional fluorescence spectrum (3D-EEM) of hydrothermal carbonization pyrolysis.

[0028] Figure 3 This is the liquid phase mass spectrum (LC-Ms) of hydrothermal carbonization.

[0029] Figure 4 The effects of irrigation on the dynamic changes of pH and Cd in paddy field pore water and the form of Cd in soil. Detailed Implementation

[0030] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features can be combined with each other. Unless otherwise specified, the methods used in the embodiments of the present invention are conventional methods, and the reagents used are commercially available.

[0031] Example 1: Preparation of alkaline hydrothermal solution by co-hydrothermal carbonization of rural sewage and rice straw 1.1 Source and Properties of Raw Materials Rural wastewater was collected from a mixed collection pond in a typical rural residential area in southern China. This residential area has approximately 150 households. The collection pond collects toilet flushing wastewater, kitchen wastewater, and wastewater from free-range livestock and poultry farming. Toilet flushing wastewater accounts for approximately 60%, kitchen wastewater for approximately 25%, and wastewater from free-range livestock and poultry farming for approximately 15%. After simple grit removal and homogenization, representative samples were obtained using a multi-point sampling and mixing method. Basic water quality indicators of the wastewater are shown in Table 1.

[0032] Table 1. Basic water quality indicators of rural wastewater (mean ± standard deviation, n=3) .

[0033] Rice straw is sourced from local indica rice fields. After harvesting, it is naturally sun-dried until its moisture content is approximately 15%, then crushed to a length of 1-3 cm using a straw crusher. Waste fruits and vegetables can be sourced from local farmers' markets, such as cabbage leaves, tomato scraps, and cucumber stems. These are cleaned, washed, crushed, and pulped before being mixed with the straw. This example uses rice straw as a plant-based organic waste.

[0034] 1.2 Co-hydrothermal carbonization reaction Crushed rice straw and rural wastewater were mixed at a wet-basis mass ratio of 1:5, i.e., 1 kg of straw to 5 L of rural wastewater. After thorough mixing, a mixture with a solid content of approximately 12% was obtained. The mixture was then transferred to a 2 L stainless steel hydrothermal reactor. The reactor was equipped with a polytetrafluoroethylene liner, mechanical stirring, and a temperature control system, with the filling degree controlled at approximately 70%.

[0035] Before the reaction, check the airtightness of the reactor. Turn on the stirrer at 150 r / min and heat to 220℃ at a rate of approximately 5℃ / min. After reaching the target temperature, maintain the temperature for 2 hours. During the reaction, the autogenous pressure inside the reactor is approximately 2.8-3.2 MPa. After the reaction is complete, stop heating and allow it to cool naturally to room temperature. Once the pressure inside the reactor has dropped to atmospheric pressure, open the reactor to obtain a solid-liquid mixed hydrothermal product.

[0036] This hydrothermal process causes proteinaceous organic matter, amino acids, ammonia nitrogen, and nitrogen-containing soluble organic matter in rural wastewater to react with reducing sugars, cellulose fragments, hemicellulose fragments, and lignin fragments produced by straw degradation, generating nitrogen-containing heterocyclic compounds, amides, and humic acids, and making the liquid phase alkaline.

[0037] 1.3 Solid-liquid separation and physicochemical properties of hydrothermal solution The solid-liquid mixture after the reaction was separated by vacuum filtration through a 0.45 μm aqueous filter membrane to obtain solid-phase hydrothermal carbon and liquid-phase product. The liquid-phase product is the alkaline hydrothermal solution described in this invention.

[0038] The basic physicochemical properties of the alkaline hydrothermal solution are shown in Table 2.

[0039] Table 2. Basic physicochemical properties of alkaline hydrothermal solutions (mean ± standard deviation, n=3) .

[0040] The above results indicate that after co-hydrothermal carbonization of rural wastewater and rice straw, the pH of the liquid phase increased from approximately 7.3 in the original wastewater to 8.64, and the liquid phase contained high concentrations of DOC, total nitrogen, and humic acid substances, thus possessing the basic qualities to serve as a functional rice irrigation solution.

[0041] 1.4 Spectroscopic and Mass Spectroscopic Characterization To verify the generation of nitrogen-containing organic active components during co-hydrothermal carbonization, the alkaline hydrothermal solution was characterized by FTIR spectroscopy, three-dimensional fluorescence spectroscopy, parallel factor analysis, and liquid chromatography-mass spectrometry.

[0042] The alkaline hydrothermal solution was freeze-dried to obtain lyophilized powder, which was then compressed into tablets using the KBr method at 4000-400 cm⁻¹. -1 Scanning within the range. FTIR spectrum shows: approximately 3420 cm⁻¹ -1 A broad and strong absorption peak appears at approximately 1650 cm⁻¹, corresponding to the stretching vibrations of OH and NH, indicating the presence of phenolic hydroxyl and amide groups; a distinct strong absorption peak appears at approximately 1650 cm⁻¹, corresponding to the C=O stretching vibration of the amide I band, and this peak can serve as key spectral evidence for the Maillard reaction and the formation of nitrogen-containing organic reactive substances; approximately 1400 cm⁻¹… -1 The appearance of a CO stretching vibration peak at approximately 1100 cm⁻¹ indicates the presence of humic acid-like substances. -1 The COC stretching vibration peak appears at approximately 1650 cm⁻¹, originating from cellulose or hemicellulose-derived structures. Compared to the hydrothermal control of pure water and straw, the alkaline hydrothermal solution in this example exhibits a peak at approximately 1650 cm⁻¹. -1 The significantly enhanced amide I band indicates that nitrogen-containing organic matter in rural wastewater participated in the hydrothermal reaction and generated nitrogen-containing organic active components that are difficult to form in a pure water system (see appendix). Figure 1 ).

[0043] The alkaline hydrothermal solution was filtered through a 0.22 μm filter membrane and diluted to approximately 10 mg / L DOC. Three-dimensional fluorescence spectra were measured using a fluorescence spectrophotometer. The excitation wavelength was 200-450 nm, the emission wavelength was 250-550 nm, the excitation and emission slit widths were both 5 nm, and the scan rate was 2400 nm / min. PARAFAC analysis revealed three main fluorescent components: C1, Ex / Em = 230, 300 / 380 nm, a fulvic acid-like component indicating low molecular weight humic substances; C2, Ex / Em = 250, 340 / 440 nm, a humic acid-like component with high aromaticity and strong metal complexing ability; and C3, Ex / Em = 280 / 340 nm, a tryptophan-like component indicating soluble microbial metabolites and nitrogen-containing organic matter. The combined humic components C1 and C2 accounted for 68.2% of the total fluorescence intensity, significantly higher than the 41.3% in the pure water plus straw control, indicating that proteinaceous organic matter in rural wastewater promotes the formation of highly aromatic humic substances. (See appendix) Figure 2 ).

[0044] After solid-phase extraction and concentration, the alkaline hydrothermal lysis solution was analyzed non-targeted by UHPLC-QTOF-MS. The results detected nitrogen-containing organic compounds, including pyrazines, pyridines, and amides, such as 2,5-dimethylpyrazine, methylpyridine, and acetamide derivatives. These nitrogen-containing heterocyclic compounds and amides can provide N and O coordination sites, exhibiting affinity for Cd. 2+ Potential for complexation or coordination. The types and abundance of nitrogen compounds in the pure water plus straw control solution were significantly lower than those in this example (see attached). Figure 3 ).

[0045] 1.5 Safety Evaluation To verify the safety of using alkaline hydrothermal hydrolysate for irrigation of rice before maturity, its heavy metal content, fecal coliforms, and typical antibiotic residues were tested. The test results are shown in Table 3.

[0046] Table 3 Safety Indicators of Alkaline Hydropyrolysis Solution .

[0047] The fecal coliform count in the original wastewater was approximately 1.6 × 10^7 MPN / L, which decreased to <3 MPN / L after hydrothermal treatment at 220℃, indicating that the hydrothermal process can effectively reduce the risk of pathogenic microorganisms. Residues of typical antibiotics such as tetracyclines, sulfonamides, and fluoroquinolones were all below the detection limit by LC-MS / MS. These results demonstrate that hydrothermal treatment can simultaneously reduce the risk of pathogens and antibiotic residues that may result from the direct return of rural wastewater to farmland.

[0048] Example 2: Pot Experiment with Diluted Irrigation Before Harvest 2.1 Test soil, potting equipment and rice materials The pot experiment was conducted under rainproof conditions in a greenhouse. The test soil was collected from the 0-20cm topsoil layer of a typical Cd-contaminated paddy field in northern Zhejiang Province. The soil type was yellow clay paddy field developed from lacustrine sediment parent material. After collection, stones, root stubble, and other debris were removed, and the soil was air-dried, passed through a 5 mm sieve, and then mixed thoroughly for use. Before the experiment, a mixed soil sample was taken to determine the basic soil properties, and the results are shown in Table 4.

[0049] Table 4. Basic physicochemical properties of the tested soils (mean ± standard deviation, n=5) .

[0050] The total Cd concentration in the tested soil was 0.86 mg / kg, which is higher than the screening value for agricultural land soil pollution risk, indicating a moderate to mild Cd pollution level. This soil is suitable for use as the soil for Cd control experiments in rice cultivation. The pots used were round polyethylene pots with an upper diameter of approximately 30 cm, a bottom diameter of approximately 24 cm, and a height of approximately 32 cm. Each pot contained 15 kg of air-dried soil and was pre-cultured for 7 days at 70% of field capacity. Three holes were filled with uniformly growing rice seedlings of the variety "Fengliangyouxiang No. 1" per pot, with two seedlings per hole. No drainage holes were provided at the bottom of the pots. A PVDF pore water sampler was installed on the pot wall approximately 8-10 cm below the soil surface, with a side drain outlet equipped with a valve to simulate the drainage process 10-15 days after irrigation before maturity.

[0051] 2.2 Test Treatment The experiment consisted of four treatments, with three replicates per treatment, for a total of 12 pots. The pots were randomly arranged in the greenhouse and rotated every three days to minimize the impact of variations in light, temperature, and ventilation on the experimental results. Except for the pre-maturation treatment solution, the basal fertilizer, top dressing, pest and disease control, and routine water management remained consistent across all treatments.

[0052] T0, clean water irrigation control. 18 days before maturity and harvest, apply clean irrigation water to the pots to make the water level in the pots reach a depth of 3-5 cm. On the 12th day after irrigation, open the side drainage outlet to drain the water. After draining, keep the soil moist until harvest.

[0053] T1, lime and water irrigation control. Nineteen days before maturity and harvest, slaked lime powder was evenly spread on the surface of the potting soil and gently mixed into the top 0-2 cm of soil. The application rate was 0.11 g / kg dry soil, about 1.6 g per pot. The next day, clean water was poured in to make the water layer in the pot reach a depth of 3-5 cm. Drainage was carried out on the 12th day after irrigation. The rest was the same as T0.

[0054] T2, irrigation with the diluted hydropyrolysis solution of this invention. Eighteen days before harvest, the alkaline hydropyrolysis solution obtained in Example 1 is mixed evenly with clean irrigation water at a volume ratio of 1:5 to obtain a diluted irrigation solution. The diluted irrigation solution has a pH of 7.8, EC of approximately 1.2 mS / cm, and DOC of approximately 470 mg / L. This diluted irrigation solution is added to the basin all at once, maintaining a water depth of 3-5 cm. Drainage is carried out on the 12th day after irrigation, and the rest is the same as in T0.

[0055] T3, undiluted hydrothermal solution was directly irrigated as a control. 18 days before maturity and harvest, the alkaline hydrothermal solution obtained in Example 1 was added directly to the pot without dilution, and the water depth in the pot was controlled at 3-5 cm. The rest was the same as T2.

[0056] When mature, each pot is harvested, threshed, and sampled for testing separately.

[0057] 2.3 Sampling and Detection Methods (1) Pore water collection. Pore water in the 0-20 cm soil layer was collected using a miniature negative pressure clay head sampler pre-buried in the potting soil 0 days before irrigation, 3 days after irrigation, 7 days after irrigation, 10 days after irrigation, 15 days after irrigation, and on the harvest day. Before each sampling, the sampler was evacuated to approximately -60 kPa, and after equilibration for 24 hours, approximately 30-50 mL of pore water was collected. pH and EC were measured on-site. After being brought back to the laboratory, the sample was filtered through a 0.45 μm filter membrane, acidified with analytical grade nitric acid to pH < 2, and stored at 4℃. Cd was determined by ICP-MS. 2+ concentration.

[0058] (2) Rice sample collection. Each batch of rice was harvested separately at maturity, threshed, dried, and cleaned. All rice grains were collected and the weight of each batch was recorded. After ridging and crushing, the rice was digested by microwave with nitric acid-hydrogen peroxide. The Cd content was determined by graphite furnace atomic absorption spectrophotometry or ICP-MS, and the analytical quality was controlled by rice component analysis standard material.

[0059] (3) Soil sample collection. After harvesting, the 0-20 cm soil layer of each pot was thoroughly mixed, and representative soil samples were taken using the quartering method. After air drying, the samples were passed through 2 mm and 0.15 mm nylon sieves. Soil Cd speciation was determined by graded extraction using a continuous extraction method, and the proportions of exchangeable, carbonate-bound, iron-manganese-bound, organic-bound, and residual Cd were determined sequentially.

[0060] (4) Statistical analysis. All data are expressed as mean ± standard deviation. One-way ANOVA was used to test differences between treatments, with a significance level of p < 0.05.

[0061] Example 3: pH and Cd of pore water after irrigation 2+ Concentration dynamics 3.1 Dynamic changes in pore water pH From the appendix Figure 4 It is evident that the pH of the pore water in treatment T2 reached 6.62 on the 7th day after irrigation, an increase of 1.21 units from the initial value; it remained at 6.55 on the 10th day and 6.12 on the harvest day. In contrast, the pH of treatment T0 (clean water irrigation) remained essentially unchanged, while treatment T1 (lime treatment), although increasing the pH, dropped to 5.95 on the harvest day. The pH maintenance effect of treatment T2 indicates that the organic alkaline components and humic substances in the diluted hydropyrolysis solution have a certain buffering and retention effect.

[0062] 3.2 Pore water Cd 2+ Concentration dynamics From the appendix Figure 4 It can be seen that T2 treatment of pore water Cd 2+ The concentration decreased from an initial 42.4 μg / L to 10.9 μg / L on day 10, a reduction of 74.3%. At the same time point, the concentration in the T1 lime treatment was 20.3 μg / L, and the T2 treatment showed a further reduction of approximately 46.3% compared to T1. However, the pH difference between T2 and T1 is insufficient to fully explain the Cd concentration reduction. 2+ The significant differences in concentration suggest that humic acids, amide groups, and nitrogen-containing heterocyclic compounds in the hydrothermal solution have a significant impact on Cd. 2+ It has additional complexing, adsorption, or stabilizing effects. After drainage, T0 and T1 treatments are applied to the pore water Cd. 2+ The concentrations all rebounded significantly. Although the concentration in treatment T2 rebounded, it remained at 18.1 μg / L on the harvest day, which was significantly lower than that in treatments T1 and T3. This indicates that the diluted hydrothermal solution can inhibit the secondary activation of Cd caused by the drainage of the potted system.

[0063] Example 4: Cd content in rice after mature harvest After harvest, the Cd content and yield of brown rice in each treatment are shown in Table 5.

[0064] Table 5. Cd content and yield per pot of brown rice after mature harvest (mean ± standard deviation, n=3) .

[0065] The results showed that the Cd content of brown rice treated with T2 was 0.13 mg / kg, a decrease of 71.7% compared to T0, significantly better than the lime treatment (T1) and the undiluted treatment (T3). The Cd reduction effect of T2 treatment was related to the Cd content in pore water. 2+The consistent decreasing trend in concentration indicates that irrigation with diluted hydropyrolysis solution before harvest can reduce Cd accumulation in rice grains by decreasing Cd activity in pore water. The T2 treatment yielded 146.6 g / pot of rice, approximately 2.2% higher than T0. This yield increase may be related to the nitrogen, phosphorus, potassium, organic carbon, and humic substances introduced after dilution of the hydropyrolysis solution. While the undiluted T3 treatment reduced Cd in brown rice, the yield improvement was not significant, suggesting that moderate dilution helps reduce the adverse effects of locally high EC or high concentrations of organic components on rice roots.

[0066] Example 5: Soil Cd morphology after mature harvest From the appendix Figure 4 It is evident that treatment T2 reduced the proportion of exchangeable Cd from 40.8% in T0 to 17.9%, while simultaneously increasing the proportions of carbonate-bound, iron-manganese-bound, and organically bound Cd. Specifically, the proportion of organically bound Cd in treatment T2 was 13.6%, higher than in T0 and T1, indicating that organic ligands in the hydrothermal solution participated in Cd fixation. Compared to lime treatment T1, T2 not only exhibits precipitation and adsorption effects induced by pH increase but also demonstrates an organic complexation effect, thus showing superior performance in reducing Cd in pore water and brown rice.

[0067] Comparative Example 1: Irrigated with only clean water before maturity Eighteen days before harvest, only clean water was applied to the pots, with the water level controlled at 3-5 cm. Drainage was carried out on the 12th day after irrigation. Results showed that the pore water pH only increased from 5.42 to 5.50 on the 7th day, and reached 5.38 on the harvest day; the pore water Cd... 2+ The concentration briefly dropped to 37.6 μg / L on day 7, then gradually rebounded as the water was drained, reaching 45.0 μg / L on harvest day; the Cd content in brown rice was 0.46 mg / kg. These results indicate that shallow irrigation before maturity alone is insufficient to stably reduce pore water Cd activity and rice Cd content.

[0068] Comparative Example 2: Irrigation with lime and water before maturity Lime was applied 19 days before harvest, followed by irrigation with clean water, maintaining a water depth of 3-5 cm in the basin. Drainage was carried out 12 days after irrigation. Results showed that lime treatment increased pore water pH and decreased Cd in brown rice, but the Cd in pore water decreased after drainage. 2+ The concentration rebounded from 20.3 μg / L on day 10 to 28.5 μg / L on the harvest day, with a Cd content of 0.21 mg / kg in brown rice. These results indicate that lime treatment has a certain Cd-reducing effect, but lacks the stabilizing effect of organic ligands, and Cd still shows a tendency to reactivate after drainage.

[0069] Comparative Example 3: Direct irrigation with undiluted hydropyrolysis solution Eighteen days before harvest, the alkaline hydropyrolysis solution obtained in Example 1 was added directly to the basin without dilution with water. Results showed that the pH increased rapidly in the initial stage of undiluted treatment, but the pore water Cd on harvest day was lower. 2+ The concentration was 29.2 μg / L, higher than the 18.1 μg / L in treatment T2; the Cd content in brown rice was 0.18 mg / kg, higher than the 0.13 mg / kg in treatment T2. These results indicate that undiluted hydrothermal hydrolysate may have reduced stabilization due to higher EC or excessively high local concentrations of organic components; therefore, appropriate dilution is a preferred technical step in this invention.

Claims

1. A method for reducing cadmium content in paddy rice, characterized in that, Includes the following steps: (1) Mix rural wastewater with crop straw and / or waste fruits and vegetables and carry out hydrothermal treatment to obtain an alkaline hydrothermal solution with a pH of 7.5-9.5; (2) 15-20 days before the rice matures and is harvested, the alkaline hydrothermal solution is diluted with water and applied to the cadmium-contaminated paddy field for irrigation, and the water depth on the field surface is controlled to be 3-5 cm. (3) Drainage should be carried out 10-15 days after irrigation. After drainage, the soil should be kept moist until the rice matures and is harvested, and local conventional water management should be implemented to avoid continuous excessive drying.

2. The method according to claim 1, characterized in that, The rural sewage includes one or more of the following: toilet wastewater, kitchen wastewater, and livestock and poultry breeding wastewater. The rural sewage meets at least two of the following criteria: total nitrogen is 50-300 mg / L, protein organic matter is 80-300 mg / L, COD is 500-2000 mg / L, and ammonia nitrogen is 20-150 mg / L.

3. The method according to claim 1, characterized in that, The crop straw is selected from one or more of rice straw, wheat straw, and corn straw; the waste fruits and vegetables are selected from one or more of vegetable residues, fruit processing waste, and leftover vegetables from farmers' markets.

4. The method according to claim 1, characterized in that, In step (1), the wet basis mass ratio of rural wastewater to crop straw and / or waste fruits and vegetables is 2:1-10:1, and the solid content of the mixed system is 5%-20%.

5. The method according to claim 1, characterized in that, In step (1), the hydrothermal treatment temperature is 180-260℃ and the treatment time is 0.5-4 hours.

6. The method according to claim 1, characterized in that, After hydrothermal treatment in step (1), the process also includes sedimentation, filtration, and solid-liquid separation steps, with the resulting liquid phase serving as the alkaline hydrothermal solution; or the solid-liquid mixture obtained from hydrothermal treatment is diluted, sieved, and stirred evenly before application.

7. The method according to claim 1, characterized in that, The alkaline hydrothermal solution described in step (1) has a pH of 7.5-9.5, contains 2000-3500 mg / L dissolved organic carbon, 150-250 mg / L total nitrogen, and 0.5-2.5 g / L humic acid, and exhibits a wavelength of 1600-1700 cm⁻¹ in its FTIR spectrum. -1 The characteristic peak of amide I within the range.

8. The method according to claim 1, characterized in that, The volume ratio of the alkaline hydrothermal solution to water in step (2) is 1:1 to 1:10, and the pH of the diluted irrigation solution is 7.2 to 8.

8.

9. The method according to claim 1, characterized in that, Before irrigation, the diluted irrigation solution described in step (2) needs to undergo safety testing. The safety testing includes testing for pH, EC, dissolved organic carbon, total nitrogen, humic acid substances, and heavy metal indicators such as Cd, As, Pb, Cr, and Hg, to ensure that the heavy metal content is lower than the corresponding limit for farmland irrigation water.

10. An application of the method according to any one of claims 1-9 in reducing the Cd content in brown rice from Cd-polluted paddy fields.

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