A sulfur element slow-release carrier for biochemical denitrification and a preparation method thereof

Abstract

The application discloses a kind of elemental sulphur slow-release carriers for biochemical denitrification and a preparation method thereof.The carrier uses agricultural waste as base material, forms porous carbon-based material after carbonization treatment, and realizes the smooth slow release of elemental sulphur through gradient structure design.The outer layer is mixed with elemental sulphur and carbon-based substrate, the middle layer is mixed with equal amount, and the inner layer is mixed with elemental sulphur and slow-release carbon source, meeting the denitrification needs of low C / N ratio wastewater.Meanwhile, the carrier introduces a pH response mechanism, which self-adaptively controls through microencapsulated alkali source when pH changes, maintaining the stability of the reaction system.The preparation process uses low-temperature pressing and microwave curing, which is energy-saving and environmentally friendly.The technology has the advantages of low cost, high efficiency and environmental protection, and is suitable for environmental protection field.

Description

Technical Field

[0001] This invention relates to the field of environmental remediation, specifically to a slow-release carrier of elemental sulfur for biochemical denitrification and its preparation method. Background Technology

[0002] With the increasing severity of nitrate pollution in water bodies, denitrification technology has become an effective method for treating nitrogen pollution. Traditional denitrification processes use elemental sulfur as an electron donor, but existing slow-release carriers for elemental sulfur suffer from uneven release rates and poor environmental adaptability in practical applications, affecting denitrification efficiency. Furthermore, pH fluctuations during denitrification inhibit the activity of denitrifying bacteria, and traditional methods typically require the addition of an alkali source to adjust the pH, increasing operating costs and complexity.

[0003] To address the aforementioned issues, this invention proposes a gradient-release sulfur carrier based on agricultural waste, combined with a pH-responsive mechanism, to achieve stable sulfur release and adaptive pH control of the reaction system. This technology not only improves denitrification efficiency but also reduces operating costs, demonstrating strong application potential. Summary of the Invention

[0004] To address the aforementioned problems, this invention proposes a slow-release carrier for biochemical denitrification and its preparation method. This carrier achieves stable release of elemental sulfur through a gradient structure and adaptively regulates the release in response to pH changes using a pH-responsive mechanism, ensuring the stability and efficiency of the denitrification process. Furthermore, it employs a low-energy, green preparation process, reducing production costs and making it suitable for water treatment applications in complex environments.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A slow-release carrier for biochemical denitrification and its preparation method, wherein the carrier is prepared from the following raw materials in parts by weight: 40-55 parts of porous carbon-based substrate, 30-40 parts of elemental sulfur, 8-15 parts of slow-release carbon source, 2-4 parts of first pH-sensitive lignin derivative binder, 1-2 parts of second pH-sensitive lignin derivative binder, and 2-5 parts of pH-sensitive coating material. The carrier is a porous carbon-based substrate formed by the carbonization of agricultural waste, and a three-layer gradient structure of "outer layer-middle layer-inner layer" distributed in the porous carbon-based substrate, with each layer being composited with pH-sensitive functional materials.

[0006] Preferably, the elemental sulfur accounts for 10-15% of the total mass of the outer layer, and the outer layer is compounded with a first pH-sensitive lignin derivative binder accounting for 5-8% of the total mass of the outer layer. The first pH-sensitive lignin derivative binder has a degradation rate of 3-5 times that at pH 6.0-6.5 compared to that at pH 7.0-8.5.

[0007] Preferably, the middle layer is an equal mixture of porous carbon-based substrate and elemental sulfur, wherein the mass ratio of porous carbon-based substrate to elemental sulfur is 1:1, and the middle layer contains a second pH-sensitive lignin derivative binder accounting for 2-3% of the total mass of the middle layer. The degradation rate of the second pH-sensitive lignin derivative binder at pH 6.0-6.5 is 1.5-2 times that at pH 7.0-8.5.

[0008] Preferably, the inner layer is a mixture of elemental sulfur and a slow-release carbon source, wherein the mass ratio of elemental sulfur to slow-release carbon source is 2:1-3:1, the slow-release carbon source is polycaprolactone, and the inner layer is coated with a pH-sensitive coating material, which begins to degrade at a pH of 6.0-6.5 and reaches its maximum degradation rate at a pH of 5.0-5.5.

[0009] Preferably, the agricultural waste is corn cob or rice husk, and the porous carbon-based substrate is made by carbonizing the agricultural waste at a high temperature of 300-400℃ for 2-3 hours.

[0010] Preferably, the pH-sensitive coating material is modified polylactic acid, which is prepared by blending polylactic acid with 5-10% aminocaproic acid by mass fraction.

[0011] Preferably, the modified polylactic acid is prepared by blending polylactic acid with an amino-containing organic acid, wherein the amino-containing organic acid is one or more of aminocaproic acid, aminobutyric acid and aminopropionic acid, and its mass fraction in the modified polylactic acid is 5-10%.

[0012] Preferably, the preparation method of the sulfur-containing slow-release carrier for biochemical denitrification is as follows: S1. Crush corn cobs or rice husks to a particle size of 50-80 mesh, carbonize them at 300-400℃ for 2-3 hours, and obtain porous carbon-based substrates after cooling. S2. Mix the porous carbon-based substrate, elemental sulfur and first pH-sensitive lignin derivative binder at a mass ratio of 80-85:10-15:5-8, add deionized water and stir until a uniform paste is formed to obtain the outer layer material. A porous carbon-based substrate, elemental sulfur, and a second pH-sensitive lignin derivative binder are mixed in a mass ratio of 48-49:48-49:2-3, and deionized water is added and stirred until a uniform paste is formed to obtain the middle layer material. Sulfur and polycaprolactone are mixed and melted in a mass ratio of 2:1-3:1, cooled and then pulverized to a particle size of 100-120 mesh. The inner layer material is then coated with a pH-sensitive coating material with a coating thickness of 5-8 μm. S3. Using a layered pressing mold, first fill in the outer layer material and pre-press for 3-5 minutes at 80-100℃ and 10-15 MPa; then fill in the middle layer material and pre-press for 3-5 minutes under the same conditions; finally, put in the inner layer material and press for 5-8 minutes at 80-100℃ and 15-20 MPa. S4. Place the pressed blank into a microwave curing device and cure it for 5-10 minutes at a power of 500-600 W and a temperature of 80-100℃. After cooling, the sulfur element slow-release carrier is obtained.

[0013] Preferably, the scraps generated in steps S3 and S4 are crushed to a particle size of 50-80 mesh and then returned to step S2 to be mixed and reused with the corresponding layer material.

[0014] The beneficial effects of this invention are: The sulfur-releasing carrier prepared by this invention exhibits stable slow-release performance, meeting the sulfur source requirements during denitrification. It also features adaptive pH-responsive regulation to ensure reaction system stability. The carrier utilizes agricultural waste as a substrate, reducing production costs, and its green preparation process is energy-efficient and environmentally friendly. The gradient structure design enables slow release at different rates, optimizing the denitrification effect. This carrier demonstrates good stability, is suitable for water treatment in complex environments, and has broad application prospects. Attached Figure Description

[0015] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0016] Figure 1 This is a line graph showing the change in nitrate removal rate over time for different samples in this invention; Figure 2 This is a bar chart comparing the porosity and compressive strength of different samples from this invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0018] Example 1: This Example 1 describes a slow-release carrier for elemental sulfur used in biochemical denitrification. The carrier is prepared from the following raw materials in parts by weight: 48 parts of porous carbon-based substrate, 35 parts of elemental sulfur, 10 parts of slow-release carbon source: polycaprolactone, 3 parts of first pH-sensitive lignin derivative binder: carboxymethyl lignin, 1.5 parts of second pH-sensitive lignin derivative binder: sulfonated lignin, and 2.5 parts of pH-sensitive coating material: polylactic acid and 8% aminocaproic acid. This embodiment describes a method for preparing a slow-release carrier of elemental sulfur for biochemical denitrification. The specific preparation steps are as follows: S1. Crush corn cobs to 60 mesh, carbonize them at 350℃ for 2.5 h, and obtain porous carbon-based substrate after cooling; S2. Mix 28 parts of porous carbon-based substrate, 5 parts of elemental sulfur, and 3 parts of pH-sensitive lignin derivative binder, add deionized water and stir at 400 rpm / min for 12 min until a uniform paste is formed to obtain the outer layer material. Mix 20 parts of porous carbon-based substrate, 10 parts of elemental sulfur, and 1.5 parts of second pH-sensitive lignin derivative binder, add deionized water and stir at 400 rpm / min for 12 min until a uniform paste is formed to obtain the middle layer material. Mix 20 parts of elemental sulfur with 10 parts of slow-release carbon source, melt and stir at 130-140℃ for 5-8 minutes until uniform, cool and pulverize to 110 mesh, and coat with 2.5 parts of pH-sensitive coating material by spraying, controlling the coating thickness to 6 μm. S3. Using a layered pressing mold, first fill in the outer layer material and pre-press for 4 minutes at 90℃ and 12 MPa; then fill in the middle layer material and pre-press for 4 minutes under the same conditions; finally, put in the inner layer material and press for 6 minutes at 90℃ and 18 MPa. S4. Place the pressed blank into a microwave curing device and cure it for 8 minutes at a power of 550 W and a temperature of 90℃. After cooling, the sulfur element slow-release carrier is obtained. The exhaust gas is recycled after being adsorbed by activated carbon, and the scraps are crushed and recycled.

[0019] Example 2: This Example 2 describes a slow-release carrier for elemental sulfur used in biochemical denitrification. The carrier is prepared from the following raw materials in parts by weight: 48 parts of porous carbon-based substrate, 35 parts of elemental sulfur, 10 parts of slow-release carbon source: polycaprolactone, 3 parts of first pH-sensitive lignin derivative binder: carboxymethyl lignin, 1.5 parts of second pH-sensitive lignin derivative binder: sulfonated lignin, and 2.5 parts of pH-sensitive coating material: polylactic acid and 8% aminobutyric acid. In this embodiment, the preparation method of a slow-release carrier for biochemical denitrification of elemental sulfur is the same as that in Example 1, except that the pH-sensitive coating material is changed to polylactic acid and 8% aminobutyric acid.

[0020] Example 3: This embodiment 3 provides a slow-release carrier for elemental sulfur in biochemical denitrification. The carrier is prepared from the following raw materials in parts by weight: 48 parts of porous carbon-based substrate, 35 parts of elemental sulfur, 10 parts of slow-release carbon source: polycaprolactone, 3 parts of first pH-sensitive lignin derivative binder: carboxymethyl lignin, 1.5 parts of second pH-sensitive lignin derivative binder: sulfonated lignin, and 2.5 parts of pH-sensitive coating material: polylactic acid and 8% aminocaproic acid. In this embodiment, the preparation method of a slow-release sulfur carrier for biochemical denitrification is the same as in Example 1, except that the mass ratio of the inner sulfur layer to the slow-release carbon source is changed. At this time, the mass ratio of the inner sulfur layer to polycaprolactone is 3:1, with 22.5 parts of the inner sulfur layer and 7.5 parts of polycaprolactone. The outer sulfur layer is adjusted to 5 parts, and the middle sulfur layer is adjusted to 7.5 parts. The total amount of sulfur remains 35 parts. The remaining components and preparation process are the same as in Example 1.

[0021] Comparative Example 1: To investigate the effect of gradient carriers on the pH response mechanism, the carrier in Comparative Example 1 was prepared from the following parts by weight of raw materials: 48 parts of porous carbon-based substrate, 35 parts of elemental sulfur, 10 parts of slow-release carbon source: polycaprolactone, 4.5 parts of ordinary lignin binder: alkali lignin, 0 parts of pH-sensitive coating material: polylactic acid and 8% aminocaproic acid; The carrier in this comparative example was prepared in the same way as in Example 1, using a non-pH-sensitive common lignin binder, and the inner layer had no pH-sensitive coating material.

[0022] Comparative Example 2: The carrier in Comparative Example 2 was prepared from the following parts by weight of raw materials: 48 parts of porous carbon-based substrate, 35 parts of elemental sulfur, 10 parts of slow-release carbon source: polycaprolactone, 3 parts of first pH-sensitive lignin derivative binder: carboxymethyl lignin, 1.5 parts of second pH-sensitive lignin derivative binder: sulfonated lignin, and 2.5 parts of pH-sensitive coating material: polylactic acid and 8% aminocaproic acid. The preparation method of the support in this comparative example is the same as that in Example 1, but the support is a homogeneous structure in which porous carbon-based substrate, elemental sulfur, and slow-release carbon source are mixed in a ratio of 48:35:10, without the gradient division of "outer layer-middle layer-inner layer".

[0023] Comparative Example 3: The carrier in Comparative Example 3 was prepared from the following parts by weight of raw materials: 48 parts of porous carbon-based substrate, 35 parts of elemental sulfur, 10 parts of slow-release carbon source: polycaprolactone, 3 parts of first pH-sensitive lignin derivative binder: carboxymethyl lignin, 1.5 parts of second pH-sensitive lignin derivative binder: sulfonated lignin, and 2.5 parts of pH-sensitive coating material: polylactic acid and 8% aminocaproic acid. The carrier in this comparative example is prepared using the same method as in Example 1, but a high-temperature sintering process of 1200℃ for 2.5 h is used instead of the "low-temperature pressing-microwave curing" process.

[0024] Performance testing 1. Denitrification efficiency and pH stability test (1) A sequencing batch reactor with an effective volume of 5 L was used. 4 L of low C / N simulated wastewater (C / N = 2, nitrate concentration 50 mg / L, COD = 80 mg / L, initial pH 7.2) was added. 50 g of carrier was added. The temperature was controlled at 25℃, dissolved oxygen ≤0.5 mg / L, hydraulic retention time 8 h, and samples were taken every 12 h. The nitrate concentration in the supernatant was determined by ultraviolet spectrophotometry, and the nitrate removal rate was calculated after 72 h.

[0025] (2) pH stability test Simultaneously with the denitrification efficiency test, a pH meter was used to monitor the pH changes in the reactor water in real time, and data was recorded every 6 hours to calculate the pH fluctuation range within 72 hours.

[0026] Table 1. Nitrate removal rate data for different samples (%)

[0027] Table 2. pH stability test results of different samples

[0028] 2. Test on the sustained-release performance of elemental sulfur 20 g of vacuum-dried carrier to constant weight was placed in 2 L of deionized water. The mixture was magnetically stirred at a constant temperature of 25 ± 0.5℃ and a stirring speed of 150 rpm / min to ensure thorough dispersion of the carrier without violent collisions. Samples of 50 mL were taken at 0 h, 6 h, 12 h, 24 h, 48 h, and 72 h, and an equal volume of deionized water was immediately added after each sampling to maintain a constant system volume. The samples were filtered through a 0.4 5 μm microporous membrane to remove suspended impurities. The sulfide ion concentration was determined using an ion chromatograph (DIONEX ICS-1100). The chromatographic conditions were: IonPac AS11-HC column (4 × 25 0 mm), eluent of 3.5 mmol / L sodium carbonate solution, flow rate of 1.0 mL / min, and injection volume of 20 μL. A blank control group was also included, containing only 2 L of deionized water. After subtracting background interference, the cumulative release and average release rate of sulfide ions were calculated.

[0029] Table 3. Test data on the sustained-release performance of elemental sulfur in different samples.

[0030] 3. Carrier physical performance testing (1) Porosity determination Mercury intrusion porosimetry was used. Before testing, the support was prepared into 10 mm × 10 mm × 10 mm cube samples, dried in a vacuum drying oven at 60℃ for 12 h to constant weight, and then cooled to room temperature before being placed in the sample cell of a mercury intrusion porosimeter (model: AutoPore IV 9500). The instrument pressure range was set to 0.001-414 MPa, the mercury contact angle to 140°, and the surface tension to 485 mN / m. The instrument automatically recorded the mercury intrusion volume under different pressures, and the total porosity, open porosity, and pore size distribution of the support were calculated. Each sample was tested in triplicate, and the average value was taken.

[0031] (2) Determination of compressive strength Using a universal testing machine (model: CMT5105), the support was prepared into a standard cylindrical sample with a diameter of 10 mm × 10 mm, ensuring that the upper and lower surfaces of the sample were flat and perpendicular to the axis. During testing, a displacement control mode was used with a loading rate of 2 mm / min. Loading was stopped when the sample showed obvious cracking or the stress dropped to 80% of the peak value. The maximum compressive load was recorded. The compressive strength was calculated using the formula σ = F / S, where F is the maximum load and S is the cross-sectional area of ​​the sample. Five samples were tested for each group, and the average value was taken after removing outliers.

[0032] Table 4. Physical property test data of different samples

[0033] 5. Long-term operational stability test The sequencing batch reactor (SBR) was used for denitrification efficiency testing at a temperature of 25 ± 0.5℃, dissolved oxygen ≤0.5 mg / L, and a hydraulic retention time of 8 h. Simulated wastewater was replaced every Monday morning. Before replacement, 80% of the supernatant was drained from the reactor, while approximately 1 L of the carrier and bottom mixture (20%) was retained to maintain a stable denitrifying bacteria concentration. Fresh simulated wastewater (same as before: C / N = 2, nitrate concentration 50 mg / L, COD = 80 mg / L, initial pH 7.2) was aerated and deoxygenated before being injected into the reactor, bringing the volume to 5 L. The reactor was operated continuously for 30 days. Nitrate removal rate tests were conducted at 72 h on days 5, 10, 15, 20, 25, and 30, using the same testing method as the denitrification efficiency test. Three parallel samples were taken for each test, and the average value was calculated. Meanwhile, each time the medium is changed, 5 carriers are randomly selected and their appearance (color, surface biofilm adhesion) and structural integrity (whether there are cracks, flaking, breakage, etc.) are observed using a stereomicroscope (magnification 10-50 times), and photos are taken and recorded. After the operation is completed, the mass loss rate of the carriers (the ratio of the remaining mass after drying to the initial mass) is measured to evaluate their long-term wear resistance.

[0034] Table 5. Results of Long-Term Operational Stability Tests for Different Samples

[0035] It can be seen from the above table: (1) Examples 1-3 used pH-sensitive binders such as carboxymethyl lignin and sulfonated lignin, and polylactic acid-amino organic acid coating materials. The nitrate removal rate was higher than 89% after 72 h, and remained above 85% after 30 days of long-term operation. The pH fluctuation of the system was controlled between 6.6 and 8.0, which is within the suitable survival range of denitrifying bacteria. Compared with Comparative Example 1, which has no pH response mechanism, it proves that the pH-sensitive material can achieve precise matching between sulfur source release and pH adjustment through controllable degradation under acidic environment, thus avoiding inhibition of bacterial activity.

[0036] (2) The gradient design of the "outer layer low sulfur - middle layer medium sulfur - inner layer high sulfur" in the example stabilized the average sulfur ion release rate at 0.62-0.66 mg / (L·h), which not only met the continuous sulfur source requirements for denitrification but also avoided secondary pollution caused by excessive release. The comparative example 2 without a gradient structure had a lower removal rate at 72 h and lower long-term stability than the example due to uneven sulfur distribution, which verified the regulatory effect of the gradient structure on the release rate.

[0037] (3) The embodiment uses a 90℃ pressing and 550 W microwave curing process, resulting in a carrier porosity of 56.5-58.6% and a compressive strength of 7.8-8.2 MPa. This ensures both the microbial attachment space and structural stability while reducing energy consumption. Although the conventional high-temperature sintering method of Comparative Example 3 has a higher compressive strength, its porosity is only 45.3%, and its energy consumption is significantly increased, demonstrating the energy-saving and structural advantages of the process of this invention.

[0038] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A slow-release carrier for elemental sulfur in biochemical denitrification, characterized in that, The carrier is prepared from the following raw materials in parts by weight: 40-55 parts of porous carbon-based substrate, 30-40 parts of elemental sulfur, 8-15 parts of slow-release carbon source, 2-4 parts of first pH-sensitive lignin derivative binder, 1-2 parts of second pH-sensitive lignin derivative binder, and 2-5 parts of pH-sensitive coating material. The carrier is a porous carbon-based substrate formed by carbonization of agricultural waste, and a three-layer gradient structure of "outer layer-middle layer-inner layer" distributed in the porous carbon-based substrate, each layer of which is composited with pH-sensitive functional materials; The sulfur content accounts for 10-15% of the total mass of the outer layer, and the outer layer is compounded with a first pH-sensitive lignin derivative binder accounting for 5-8% of the total mass of the outer layer. The first pH-sensitive lignin derivative binder has a degradation rate of 3-5 times that at pH 6.0-6.5 compared to that at pH 7.0-8.

5. The middle layer is an equal mixture of porous carbon-based substrate and elemental sulfur, wherein the mass ratio of porous carbon-based substrate to elemental sulfur is 1:1, and the middle layer contains a second pH-sensitive lignin derivative binder accounting for 2-3% of the total mass of the middle layer. The degradation rate of the second pH-sensitive lignin derivative binder at pH 6.0-6.5 is 1.5-2 times that at pH 7.0-8.

5. The inner layer is a mixture of elemental sulfur and a slow-release carbon source, wherein the mass ratio of elemental sulfur to slow-release carbon source is 2:1-3:1, the slow-release carbon source is polycaprolactone, and the inner layer is coated with a pH-sensitive coating material. The pH-sensitive coating material begins to degrade at a pH of 6.0-6.5, and the degradation rate reaches its maximum at a pH of 5.0-5.

5. The agricultural waste is corn cob or rice husk, and the porous carbon-based substrate is made by carbonizing the agricultural waste at a high temperature of 300-400℃ for 2-3 hours. The pH-sensitive coating material is modified polylactic acid, which is made by blending polylactic acid with 5-10% aminocaproic acid by mass.

2. A method for preparing a slow-release carrier of elemental sulfur for biochemical denitrification, wherein the carrier is as described in claim 1, characterized in that, The steps are as follows: S1. Crush corn cobs or rice husks to a particle size of 50-80 mesh, carbonize them at 300-400℃ for 2-3 hours, and obtain porous carbon-based substrates after cooling. S2. Mix the porous carbon-based substrate, elemental sulfur and first pH-sensitive lignin derivative binder at a mass ratio of 80-85:10-15:5-8, add deionized water and stir until a uniform paste is formed to obtain the outer layer material. A porous carbon-based substrate, elemental sulfur, and a second pH-sensitive lignin derivative binder are mixed in a mass ratio of 48-49:48-49:2-3, and deionized water is added and stirred until a uniform paste is formed to obtain the middle layer material. Sulfur and polycaprolactone are mixed and melted in a mass ratio of 2:1-3:1, cooled and then pulverized to a particle size of 100-120 mesh. The inner layer material is then coated with a pH-sensitive coating material with a coating thickness of 5-8 μm. S3. Using a layered pressing mold, first fill in the outer layer material and pre-press for 3-5 minutes at 80-100℃ and 10-15 MPa; then fill in the middle layer material and pre-press for 3-5 minutes under the same conditions; finally, put in the inner layer material and press for 5-8 minutes at 80-100℃ and 15-20 MPa. S4. Place the pressed blank into a microwave curing device and cure it for 5-10 minutes at a power of 500-600 W and a temperature of 80-100℃. After cooling, the sulfur element slow-release carrier is obtained.

3. The method for preparing a slow-release carrier of elemental sulfur for biochemical denitrification according to claim 2, characterized in that, The scraps generated in steps S3 and S4 are crushed to a particle size of 50-80 mesh and then returned to step S2 to be mixed and reused with the corresponding layer of material.

Images