A solar-driven interfacial evaporation method for sludge drying

Abstract

This invention relates to solid waste treatment technology, aiming to provide a solar-driven interfacial evaporation method for sludge drying. The method includes: cutting balsa wood into long cylindrical blocks, impregnating them with a lignin-removing solution, and then performing vacuum freeze-drying; subsequently, adding ammonium persulfate solution and pyrrole dispersion sequentially to the upper surface of the lignin-removed balsa wood blocks, allowing ammonium persulfate to act as an oxidant and undergo an oxidative polymerization reaction with pyrrole to form a polypyrrole layer; vertically inserting the balsa wood blocks into water-containing sludge, and placing moisture sensors at the polypyrrole layer and at the interface with the sludge; utilizing sunlight to create a photothermal interfacial evaporation effect on the polypyrrole layer, and monitoring the voltage differences of the moisture sensors based on the hydroelectric effect caused by evaporation to understand the changes in moisture content and confirm the sludge drying progress. The method of this invention utilizes sunlight to dry sludge, reducing dependence on traditional fossil fuels; it is simple and convenient to operate, low in cost, safe to operate, and the evaporation material can be recycled.

Description

Technical Field

[0001] This invention relates to solid waste treatment technology, and in particular to sludge drying and disposal. It uses photothermal interface evaporation technology to perform low-carbon drying of water-containing sludge and remove heavy metals, while realizing real-time monitoring of sludge moisture content. Background Technology

[0002] Sludge is a waste generated during wastewater treatment, mainly composed of suspended matter removed from sewage, containing large amounts of organic and inorganic substances (such as heavy metals). Proper sludge treatment is crucial for environmental protection and preventing secondary pollution. Traditional sludge treatment technologies mainly include steps such as thickening, conditioning, dewatering, stabilization, drying, or incineration. Since the water content of sludge is typically greater than 90%, sludge drying is a critical step in the treatment process. Especially before sludge is incinerated or otherwise utilized for resource recovery, its water content must be significantly reduced to ensure its calorific value.

[0003] Traditional sludge drying technologies mainly include mechanical dewatering, thermal drying, and solar drying. Mechanical dewatering includes centrifugal dewatering, belt filter presses, and screw presses, which can reduce the moisture content of sludge from over 90% to approximately 70%–80%. Although mechanical dewatering effectively reduces sludge volume, it consumes a large amount of energy and usually cannot reduce the moisture content to a level suitable for incineration. Therefore, mechanical dewatering can only be used as a pre-dewatering treatment, and further, more thorough drying treatment is still required. Thermal drying technology is currently a widely used sludge drying method. By directly or indirectly heating the sludge to evaporate the moisture, the moisture content can be reduced to below 50%, or even lower. This method is highly efficient, but its energy consumption, carbon emissions, and operating costs are all very high. To reduce the energy consumption of sludge drying, the invention patent "A High-Temperature Heat Pump Superheated Steam Drying System" (CN117700068 A) provides a technology that combines a high-temperature heat pump with superheated steam drying. It utilizes a steam circulation device to extract and circulate the steam generated during drying, effectively improving energy efficiency. However, this system still requires a large amount of electricity to operate. To further reduce energy consumption and carbon emissions, the invention patent "Temperature-Controllable Sludge Drying System and Temperature Control Method" (CN117945621 A) provides a method that converts solar energy into electricity to power the sludge drying system. However, this method requires a complex photovoltaic and control system, increasing equipment costs. In addition, the invention patent "Solar Photothermal Coupled Sludge Drying System" (CN 108409095 A) uses a trough-type solar collector and storage system as the heat source for a disc-type indirect sludge drying system. This method is simpler and has lower energy consumption compared to the two invention patents mentioned above, but it does not remove heavy metals from the sludge.

[0004] Therefore, finding a low-cost, low-energy-consumption, low-carbon-emission, clean, environmentally friendly, and efficient sludge drying method is an urgent problem to be solved. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a sludge drying method based on solar-driven interfacial evaporation.

[0006] To solve the technical problem, the solution of the present invention is:

[0007] A method for sludge drying using solar-driven interfacial evaporation is provided, comprising the following steps:

[0008] (1) Dissolve sodium hypochlorite and glacial acetic acid together in water and mix them evenly to prepare a lignin-free solution;

[0009] (2) Cut the balsa wood into long columns, immerse them completely in the lignin-removing solution, and heat them at 100°C for 12 to 16 hours; after taking them out, rinse them with water until the pH of the rinsing solution is neutral, and then perform vacuum freeze-drying to obtain lignin-removed balsa wood blocks.

[0010] (3) Add ammonium persulfate solution and pyrrole dispersion dropwise onto the upper surface of the lignin-free balsa block, so that ammonium persulfate acts as an oxidant to react with pyrrole in an oxidative polymerization reaction; after reacting for 10 to 20 minutes, rinse the residual liquid on the surface with water; and then perform vacuum freeze-drying to obtain a lignin-free balsa block with a polypyrrole layer on the upper surface.

[0011] (4) Take several lignin-free balsa wood blocks with polypyrrole layers and insert them vertically into the water-containing sludge, ensuring that 80% to 90% of their height is exposed outside the sludge; set moisture sensors at the polypyrrole layers of the balsa wood blocks and at the interface with the sludge, and connect each sensor to the detection recorder through a signal line.

[0012] Sunlight is irradiated onto the surfaces of the sludge and balsa wood blocks, creating a photothermal interface evaporation effect on the polypyrrole layer. Based on the hydroelectric effect caused by evaporation, differences in the voltage values ​​of moisture sensors are generated at different locations to monitor the changes in moisture content at different locations. When the moisture content monitoring data meets the expected value, the balsa wood blocks are removed, at which point the sludge has also completed the drying process.

[0013] (5) Soak the balsa wood blocks in an acidic solution and remove heavy metals under ultrasonic conditions; after taking them out, rinse them with water to remove residual acid and heavy metals until the pH of the rinsing solution is between 6 and 7; then dry the balsa wood blocks to obtain reusable balsa wood blocks.

[0014] As a preferred embodiment of the present invention, in step (1), when preparing the lignin-free solution, 4-6 g of sodium hypochlorite and 1-2 mL of glacial acetic acid are added to every 100 mL of water.

[0015] As a preferred embodiment of the present invention, in step (2), the material of the balsa wood block is balsa wood, paulownia wood or pine wood; the long column is a cylinder or square column with a height of 10 to 20 cm and a radial dimension or transverse width of 2 to 5 cm.

[0016] As a preferred embodiment of the present invention, in step (2) or step (3), the freeze-drying process is carried out in a freeze dryer, with the drying temperature set at -40°C, the drying time at 12 to 24 hours, and the vacuum degree below 10Pa; after drying, the temperature is slowly raised to room temperature at a rate of 0.4 to 0.6°C / minute to avoid damage to the internal structure of the wood.

[0017] As a preferred embodiment of the present invention, in step (3), 2-3 g of ammonium persulfate is added to 100 mL of water and mixed to obtain an ammonium persulfate solution; 0.5-1 mL of pyrrole is added to 100 mL of water and mixed to obtain a pyrrole dispersion; the ammonium persulfate solution and the pyrrole dispersion are added dropwise in an equal volume manner, and the amount of addition is controlled so that the thickness of the polypyrrole layer is 50-100 micrometers.

[0018] As a preferred embodiment of the present invention, in step (4), the water content of the sludge is 90% to 98%, the total volume of the balsa wood blocks used accounts for 5% to 10% of the total volume of the sludge, and the balsa wood blocks are evenly arranged in the sludge in an array.

[0019] As a preferred embodiment of the present invention, in step (4), the moisture sensor has a copper electrode; when the voltage difference between the two electrodes is consistently and stably lower than 0.02 to 0.05V, the drying process ends, at which point the moisture content of the sludge is between 30% and 40%.

[0020] As a preferred embodiment of the present invention, in step (5), the cleaning time under ultrasonic conditions is 10 to 30 minutes; the ultrasonic frequency is set to 28 kHz and the ultrasonic power is 500 to 1000 W.

[0021] As a preferred embodiment of the present invention, in step (5), the acidic solution is 0.1 mol / L hydrochloric acid or 0.1 mol / L nitric acid solution; the heavy metals removed by ultrasonication include at least copper, lead, zinc and cadmium.

[0022] Description of the invention principle:

[0023] This invention proposes using modified balsa wood as a photothermal evaporation medium to improve sludge drying efficiency while enhancing the removal of heavy metals from the evaporator, thereby achieving the goal of reducing pollution and carbon emissions. The relevant technical principles are as follows:

[0024] (1) Material selection and preparation:

[0025] Balsa wood (balsa, paulownia, pine) was chosen as the base material primarily because of its low density and high porosity. These balsa woods grow quickly in their natural state, are easy to process, and are relatively inexpensive. However, the raw wood has poor water transport capacity. Specific chemical treatments remove lignin from the balsa wood while preserving its cellulose-rich structure, without damaging its original porous structure, further increasing the material's porosity and providing excellent hydrophilicity, making it an ideal water transport medium for water evaporation. Ammonium persulfate solution and pyrrole dispersion were sequentially added dropwise to the surface of the lignin-removed balsa wood blocks. Ammonium persulfate, acting as an oxidant, reacted with pyrrole in an oxidative polymerization reaction to generate polypyrrole. Due to the extremely high specific surface area and porosity of the balsa wood block surface, the generated fine polypyrrole quickly adsorbed onto the fiber surface and was difficult to desorb. This modification not only did not affect the pore structure of the balsa wood blocks but also improved its ability to absorb and convert light and heat energy. During the freeze-drying process, the temperature is slowly increased to room temperature at a rate of 0.4–0.6 °C / min to avoid damage to the wood structure, which could affect the subsequent evaporation and heavy metal adsorption and removal effects.

[0026] (2) Photothermal interface evaporation:

[0027] The polypyrrole layer significantly improves the material's photothermal efficiency, enabling it to rapidly absorb sunlight and convert it into heat energy. This efficient photothermal conversion promotes the rapid evaporation of moisture in the sludge. The high specific surface area and porous structure of the polypyrrole-derived balsa wood block allow it to quickly transfer heat energy while in contact with the sludge, accelerating moisture evaporation and efficiently and rapidly reducing the sludge's moisture content. Furthermore, by connecting electrodes (made of corrosion-resistant copper) to the surface of the polypyrrole-derived balsa wood block and the sludge interface, the voltage change between the electrodes due to the hydroelectric effect caused by evaporation enables real-time monitoring of the sludge's moisture content. This self-monitoring technology can not only determine the drying end time but also adjust processing parameters based on the sludge's drying status, optimizing the drying process.

[0028] (3) High-efficiency adsorption of heavy metals:

[0029] In addition to its drying effect, polypyrrole-treated lignin-free balsa wood blocks also possess the function of adsorbing heavy metals. After lignin removal treatment, the lignin in the cell walls of the wood is removed, leaving more pores, i.e., increased porosity. These pores act as channels for moisture and pollutants, not only promoting water transport and evaporation but also providing more surface area for adsorbing heavy metals (such as copper, lead, cadmium, and zinc) and organic pollutants in the sludge. During the drying process, as moisture evaporates, heavy metal ions in the sludge increase their contact opportunities with the surface of the polypyrrole-treated lignin-free balsa wood blocks due to the concentration effect, thereby improving adsorption efficiency. Furthermore, the heat generated during evaporation also helps to open more pores and active sites, enhancing the adsorption of pollutants such as heavy metals. After drying, the balsa wood blocks are removed and subjected to ultrasonic acid cleaning to remove heavy metals and impurities adsorbed from the sludge. Due to the strong adsorption capacity of polypyrrole on the fiber surface, no significant desorption occurs, and the pore structure is not affected. The polypyrrole-derived lignin-free balsa wood blocks exhibit extremely strong physicochemical stability and can be repeatedly recycled, reducing the cost of material use.

[0030] Based on the above principles, modified balsa wood blocks (polypyrrole lignin-free balsa wood blocks) were used to achieve efficient evaporation of water and efficient removal of heavy metals in sludge by combining interfacial evaporation and chemical adsorption processes, as well as a clever self-monitoring method for moisture content. The drying end time was determined by real-time monitoring of the moisture content.

[0031] Compared with the prior art, the present invention has the following advantages:

[0032] 1. This invention utilizes sunlight to dry sludge, reducing dependence on traditional fossil fuels and lowering carbon emissions by more than 95% compared to traditional drying methods;

[0033] 2. This invention is simple and convenient to operate, low in cost, safe to run, and the evaporation material can be recycled;

[0034] 3. The specifications of the cork blocks used in this invention are adjustable, making them suitable for drying sludge with various moisture contents, and they have high evaporation efficiency;

[0035] 4. This invention not only achieves efficient evaporation, but also efficiently removes heavy metals from sludge, with a removal rate exceeding 70%.

[0036] 5. Installing a moisture sensor on the cork block enables real-time monitoring of the sludge drying process, allowing for adjustments to the evaporation conditions. Attached Figure Description

[0037] Figure 1 This is a flowchart of the present invention. Detailed Implementation

[0038] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0039] Part One: Implementation Scheme of the Invention

[0040] The sludge drying method using solar-driven interfacial evaporation described in this invention includes the following steps:

[0041] (1) Add 4-6g of sodium hypochlorite and 1-2mL of glacial acetic acid to every 100mL of water, mix well, and prepare a lignin-free solution.

[0042] (2) Cut the balsa wood into long columns, immerse them completely in the lignin-removing solution, and heat them at 100°C for 12 to 16 hours; after taking them out, rinse them with water until the pH of the rinsing solution is neutral, and then perform vacuum freeze-drying to obtain lignin-removed balsa wood blocks.

[0043] The balsa wood blocks can be made of balsa, paulownia, or pine; their cylindrical shape can be cylindrical or square, with a height of 10–20 cm and a radial or transverse width of 2–5 cm. Freeze-drying is carried out in a freeze dryer at a temperature of -40℃ for 12–24 hours, with a vacuum level below 10 Pa. After drying, the temperature is slowly raised to room temperature at a rate of 0.4–0.6℃ / minute to avoid damage to the internal structure of the wood.

[0044] (3) Add 2-3 g of ammonium persulfate to every 100 mL of water, mix well to obtain an ammonium persulfate solution; add 0.5-1 mL of pyrrole to every 100 mL of water, mix well to obtain a pyrrole dispersion; add the ammonium persulfate solution and pyrrole dispersion dropwise to the upper surface of the lignin-free balsa block in equal volumes, so that the ammonium persulfate acts as an oxidant to react with the pyrrole in an oxidative polymerization reaction; control the amount of dropwise addition so that the thickness of the polypyrrole layer is 50-100 micrometers. After reacting for 10-20 minutes, rinse the surface with water to remove residual liquid; then perform vacuum freeze-drying to obtain a lignin-free balsa block with a polypyrrole layer on the upper surface;

[0045] The freeze-drying process is carried out in a freeze dryer, with the drying temperature set at -40℃ and the drying time at 12 to 24 hours, and the vacuum degree set below 10Pa. After drying, the temperature is slowly raised to room temperature at a rate of 0.4 to 0.6℃ / minute to avoid damage to the internal structure of the wood.

[0046] (4) Take several lignin-free balsa wood blocks with polypyrrole layers and insert them vertically into the water-containing sludge, ensuring that 80% to 90% of their height is exposed outside the sludge; set moisture sensors at the polypyrrole layer of the balsa wood blocks and the interface with the sludge respectively, and connect each sensor to a detection recorder through a signal line; let sunlight shine on the surface of the sludge and the balsa wood blocks, and form a photothermal interface evaporation effect in the polypyrrole layer; based on the hydroelectric effect caused by evaporation, form a difference in the monitoring voltage value of the moisture sensor at different positions, and use this voltage difference to monitor the change in moisture content at different positions; when the moisture content monitoring data meets the expected value, take out the balsa wood blocks, at which point the sludge has also been dried;

[0047] As an optional solution, the sludge has a moisture content of 90% to 98%, and the total volume of the balsa wood blocks used accounts for 5% to 10% of the total volume of the sludge. The balsa wood blocks are evenly arranged in the sludge in an array. The moisture sensor has copper electrodes. When the voltage difference between the two electrodes is consistently and stably lower than 0.02 to 0.05V, the drying process ends. At this time, the moisture content of the sludge is between 30% and 40%.

[0048] (5) Soak the balsa wood blocks in an acidic solution and remove heavy metals under ultrasonic conditions; after taking them out, rinse them with water to remove residual acid and heavy metals until the pH of the rinsing solution is between 6 and 7; then dry the balsa wood blocks to obtain reusable balsa wood blocks.

[0049] As an optional option, the cleaning time under ultrasonic conditions is 10 to 30 minutes; the ultrasonic frequency is set to 28 kHz and the ultrasonic power is 500 to 1000 W; the acidic solution is 0.1 mol / L hydrochloric acid or 0.1 mol / L nitric acid solution; the heavy metals removed by ultrasonication include at least copper, lead, zinc, and cadmium.

[0050] Part Two: Specific Embodiments and Comparative Examples Specific Implementation Example 1

[0052] (1) Add 4g of sodium hypochlorite and 1mL of glacial acetic acid to every 100mL of water, mix well, and prepare a lignin-free solution.

[0053] (2) Cut the balsa wood into long cylinders. Balsa wood is selected as the balsa wood. The dimensions of the long cylinder are set to a height of 10cm and a bottom diameter of 2cm. It is then completely immersed in the lignin removal solution and heated at 100℃ for 12 hours. After heating, the balsa wood blocks are rinsed with clean water and the pH of the rinsing solution is tested until the pH of the rinsing solution reaches neutral. The treated balsa wood blocks are then obtained.

[0054] (3) The treated balsa wood blocks were placed in a freeze dryer and dried. The drying temperature was set to -40℃, the drying time was 12 hours, and the vacuum degree was less than 10Pa. After the drying time was completed, the temperature was slowly raised to room temperature at a rate of 0.4℃ / minute to obtain lignin-free balsa wood blocks.

[0055] (4) Add 2g of ammonium persulfate to every 100mL of water and mix well to obtain an ammonium persulfate solution; add 0.5mL of pyrrole to every 100mL of water and mix well to obtain a pyrrole dispersion; prepare for the subsequent polypyrrole layer.

[0056] (5) Ammonium persulfate solution and pyrrole dispersion were sequentially added dropwise to the surface of the lignin-free balsa block, controlling the amount added to ensure that the polypyrrole layer thickness was 50 micrometers. After reacting for 10 minutes, the liquid residue on the surface was rinsed off with water. The wood block was then placed in a freeze dryer and dried at a temperature of -40 degrees Celsius for 12 hours under a vacuum of less than 10 Pa. After the drying time was completed, the temperature was slowly increased to room temperature at a rate of 0.4°C / min to obtain a lignin-free balsa block with a polypyrrole layer on the upper surface.

[0057] (6) Prepare several polypyrrole-free balsa wood blocks according to steps (1-5) above, and place them in sludge. The selected sludge should have a moisture content of around 90%, and the volume of the polypyrrole-free balsa wood blocks should account for 5% of the sludge volume. The sludge and polypyrrole-free balsa wood blocks should be evenly placed on the sludge in an array, ensuring that 10% of the height of the wood blocks is submerged in the sludge and 90% of the height is exposed outside the sludge. Place the sludge under sunlight to dry. Set a moisture sensor at the interface between the polypyrrole-free balsa wood blocks and the sludge. The sensor can detect and record changes in moisture content. The moisture sensor measures the voltage change generated by the hydroelectric effect caused by evaporation by two copper electrodes installed on the upper surface of the polypyrrole-free balsa wood blocks and the sludge. The moisture content in the sludge is determined by real-time monitoring of the voltage. When the monitored voltage is lower than the set threshold of 0.02V and remains stable, remove the wood blocks to end the drying process. The final sludge moisture content is 40%.

[0058] (7) After the sludge is dried, the polypyrrole-de-ligninized balsa wood blocks are taken out, and the heavy metal removal rate of the sludge is 74%. Heavy metal removal is performed on the wood blocks, mainly targeting harmful metals such as copper, lead, zinc, and cadmium. The wood blocks are soaked in 0.1 mol / L hydrochloric acid and ultrasonically cleaned. The ultrasonic cleaning time is set to 10 minutes, the ultrasonic frequency to 28 kHz, and the ultrasonic power to 500 W. After cleaning, the blocks are rinsed with clean water to remove residual hydrochloric acid solution and heavy metals. The rinsing is stopped when the pH of the rinsing solution is between 6 and 6. The rinsed wood blocks are then dried to obtain reusable polypyrrole-de-ligninized balsa wood blocks. Specific Implementation Example 2

[0060] (1) Add 5g of sodium hypochlorite and 1.5mL of glacial acetic acid to every 100mL of water, mix well, and prepare a lignin-free solution.

[0061] (2) Cut the balsa wood into rectangular prisms. The balsa wood is paulownia wood. The dimensions of the rectangular prisms are set to a height of 15cm and a bottom width of 3.5cm. They are then completely immersed in a lignin removal solution and heated at 100°C for 14 hours. After heating, the balsa wood blocks are rinsed with clean water, and the pH of the rinsing solution is tested until the pH of the rinsing solution reaches neutral. The treated balsa wood blocks are then obtained.

[0062] (3) The treated balsa wood blocks were placed in a freeze dryer and dried. The drying temperature was set to -40℃, the drying time was 18 hours, and the vacuum degree was less than 10Pa. After the drying time was completed, the temperature was slowly raised to room temperature at a rate of 0.5℃ / minute to obtain lignin-free balsa wood blocks.

[0063] (4) Add 2.5 g of ammonium persulfate to every 100 mL of water and mix well to obtain an ammonium persulfate solution; add 0.75 mL of pyrrole to every 100 mL of water and mix well to obtain a pyrrole dispersion; prepare for the subsequent polypyrrole layer.

[0064] (5) Ammonium persulfate solution and pyrrole dispersion were sequentially added dropwise to the surface of the lignin-free balsa block, controlling the amount added to ensure that the polypyrrole layer thickness was 75 micrometers. After reacting for 15 minutes, the liquid residue on the surface was rinsed off with water. The wood block was then placed in a freeze dryer and dried at a temperature of -40 degrees Celsius for 18 hours under a vacuum of less than 10 Pa. After the drying time was completed, the temperature was slowly increased to room temperature at a rate of 0.5°C / min to obtain a lignin-free balsa block with a polypyrrole layer on the upper surface.

[0065] (6) Prepare several polypyrrole-free balsa wood blocks according to steps (1-5) above, and place them in sludge. The selected sludge has a moisture content of around 94%, and the volume of the polypyrrole-free balsa wood blocks accounts for 7.5% of the sludge volume. The sludge and polypyrrole-free balsa wood blocks are evenly placed on the sludge in an array, ensuring that 15% of the height of the wood blocks is submerged in the sludge and 85% of the height is exposed outside the sludge. Place the sludge under sunlight to dry. Set a moisture sensor at the interface between the polypyrrole-free balsa wood blocks and the sludge. The sensor can detect and record changes in moisture. The moisture sensor measures the voltage change generated by the hydroelectric effect caused by evaporation by two copper electrodes installed on the upper surface of the polypyrrole-free balsa wood blocks and the sludge. The moisture content in the sludge is determined by real-time monitoring of the voltage. When the monitored voltage is lower than the set threshold of 0.035V and remains stable, the wood blocks are removed to end the drying process. The final moisture content of the sludge is 34%.

[0066] (7) After the sludge is dried, the polypyrrole-de-ligninized balsa wood blocks are taken out, achieving a heavy metal removal rate of 78% for the sludge. The wood blocks are then subjected to heavy metal removal, primarily targeting harmful metals such as copper, lead, zinc, and cadmium. The wood blocks are immersed in a 0.1 mol / L nitric acid solution, supplemented by ultrasonic cleaning. The ultrasonic cleaning time is set to 20 minutes, the ultrasonic frequency to 28 kHz, and the ultrasonic power to 750 W. After cleaning, the blocks are rinsed with clean water to remove residual nitric acid solution and heavy metals. The rinsing is stopped when the pH of the rinsing solution is between 6.5 and 6.5. The rinsed wood blocks are then dried to obtain reusable polypyrrole-de-ligninized balsa wood blocks.

[0067] Specific Implementation Example 3

[0068] (1) Add 6g of sodium hypochlorite and 2mL of glacial acetic acid to every 100mL of water, mix well, and prepare a lignin-free solution.

[0069] (2) Cut the balsa wood into long cylinders. Pine wood is selected as the balsa wood. The dimensions of the long cylinder are set to a height of 20cm and a bottom diameter of 5cm. It is then completely immersed in the lignin removal solution and heated at 100℃ for 16 hours. After heating, the balsa wood blocks are rinsed with clean water, and the pH of the rinsing solution is tested until the pH of the rinsing solution reaches neutral. The treated balsa wood blocks are then obtained.

[0070] (3) The treated balsa wood blocks were placed in a freeze dryer and dried. The drying temperature was set to -40℃, the drying time was 24 hours, and the vacuum degree was less than 10Pa. After the drying time was completed, the temperature was slowly raised to room temperature at a rate of 0.6℃ / minute to obtain lignin-free balsa wood blocks.

[0071] (4) Add 3g of ammonium persulfate to every 100mL of water and mix well to obtain an ammonium persulfate solution; add 1mL of pyrrole to every 100mL of water and mix well to obtain a pyrrole dispersion; prepare for the subsequent polypyrrole layer.

[0072] (5) Ammonium persulfate solution and pyrrole dispersion were sequentially added dropwise to the surface of the lignin-free balsa block, controlling the amount added to ensure that the polypyrrole layer thickness was 100 micrometers. After reacting for 20 minutes, the liquid residue on the surface was rinsed off with water. The wood block was then placed in a freeze dryer and dried at a temperature of -40 degrees Celsius for 24 hours under a vacuum of less than 10 Pa. After the drying time was completed, the temperature was slowly increased to room temperature at a rate of 0.6°C / min to obtain a lignin-free balsa block with a polypyrrole layer on the upper surface.

[0073] (6) Prepare several polypyrrole-free balsa wood blocks according to steps (1-5) above, and place them in sludge. The selected sludge has a moisture content of around 98%, and the volume of the polypyrrole-free balsa wood blocks accounts for 10% of the sludge volume. The sludge and polypyrrole-free balsa wood blocks are evenly placed on the sludge in an array, ensuring that 20% of the height of the wood blocks is submerged in the sludge and 80% of the height is exposed outside the sludge. Place the sludge under sunlight to dry. Set a moisture sensor at the interface between the polypyrrole-free balsa wood blocks and the sludge. The sensor can detect and record changes in moisture. The moisture sensor measures the voltage change generated by the hydroelectric effect caused by evaporation by two copper electrodes installed on the upper surface of the polypyrrole-free balsa wood blocks and the sludge. The moisture content in the sludge is determined by real-time monitoring of the voltage. When the monitored voltage is lower than the set threshold of 0.05V and remains stable, remove the wood blocks to end the drying process. The final moisture content of the sludge is 30%.

[0074] (7) After the sludge is dried, the polypyrrole-de-ligninized balsa wood blocks are taken out, achieving an 80% heavy metal removal rate from the sludge. The wood blocks are then subjected to heavy metal removal, primarily targeting harmful metals such as copper, lead, zinc, and cadmium. The wood blocks are immersed in a 0.1 mol / L hydrochloric acid solution, supplemented by ultrasonic cleaning. The ultrasonic cleaning time is set to 30 minutes, the ultrasonic frequency to 28 kHz, and the ultrasonic power to 1000 W. After cleaning, the blocks are rinsed with clean water to remove residual hydrochloric acid solution and heavy metals. Rinsing is stopped when the pH of the rinsing solution is between 7 and 7. The rinsed wood blocks are then dried to obtain reusable polypyrrole-de-ligninized balsa wood blocks.

[0075] Comparative Example 1

[0076] Referring to the steps of Example 1, except that the lignin removal heating impregnation process is not performed; that is, the balsa wood blocks used for sludge drying still retain the original lignin components inside, and their upper surface also has a polypyrrole layer.

[0077] After undergoing the same drying process and time, the final sludge moisture content was 72%, and the heavy metal removal rate of the sludge was 20%.

[0078] Comparative Example 2

[0079] The steps are the same as in Example 1, except that the operation of adding ammonium persulfate solution and pyrrole dispersion is not performed; that is, the surface of the balsa wood block used for sludge drying does not have a polypyrrole layer.

[0080] After undergoing the same drying process and time, the final sludge moisture content was 89%, and the heavy metal removal rate of the sludge was 15%.

[0081] Comparative Example 3

[0082] Referring to the steps of Example 1, except that the balsa wood block is replaced with absorbent cotton strips used in conventional technology; that is, ordinary absorbent material is used, and its upper surface also has a polypyrrole layer.

[0083] After undergoing the same drying process and time, the final sludge moisture content was 75%, and the heavy metal removal rate of the sludge was 25%.

[0084] Comparative Example 4

[0085] The absorbent material was prepared in accordance with the contents of "A Salt Lake Brine Concentration Device" (CN202222429630.3); that is, other absorbent materials were used to replace the balsa wood blocks of the present invention and placed in the water-containing sludge.

[0086] After undergoing the same drying process and time, the final sludge moisture content was 45%, and the heavy metal removal rate of the sludge was 32%.

[0087] Comparative Example 5

[0088] Commercial absorbent sponges were purchased as absorbent materials; that is, ordinary commercial absorbent materials were used instead of the balsa wood blocks of the present invention and placed in water-containing sludge.

[0089] After undergoing the same drying process and time, the final sludge moisture content was 50%, and the heavy metal removal rate of the sludge was 30%.

[0090] Comparative Example 6

[0091] A hydrophilic modified commercial hydrophobic polyurethane sponge was prepared as a water-absorbing material, referring to the content described in the journal article "Preparation and Performance of Modified PU Sponge Three-Dimensional Porous Interface Evaporator"; that is, a specific modified water-absorbing material was used to replace the balsa wood block of the present invention and placed in water-containing sludge.

[0092] After undergoing the same drying process and time, the final sludge moisture content was 42%, and the heavy metal removal rate of the sludge was 35%.

[0093] Data analysis and conclusions:

[0094] From the specific embodiments of the present invention above, and the changes in the final moisture content and heavy metal removal rate data after parameter changes and element replacements in the comparative examples, the following conclusions can be drawn:

[0095] Compared with traditional absorbent materials or unmodified balsa wood blocks, the specific embodiments of the present invention have significant advantages in reducing the moisture content of sludge and improving the removal rate of heavy metals. This is mainly due to the following points: (1) Material preparation and modification: The modified balsa wood blocks (Py-H-wood) are treated with a polypyrrole coating, which not only improves the photothermal conversion efficiency but also increases the adsorption sites on the surface, effectively adsorbing heavy metals. (2) Enhanced utilization of photothermal effect: The addition of the polypyrrole layer enables the wood blocks to more effectively convert light energy into heat energy under sunlight, accelerating the evaporation process of water and thus reducing the moisture content of sludge. (3) Improved heavy metal adsorption effect: Compared with traditional absorbent materials, Py-H-wood shows better performance in removing heavy metals, which is attributed to its surface being specially treated to increase the adsorption capacity for heavy metals.

[0096] Furthermore, for the specific drying target of sludge, the modified balsa wood block photothermal interface evaporation material prepared in this invention exhibits superior adaptability and compatibility, thus demonstrating higher drying efficiency and heavy metal removal efficiency. The drying principle for sludge characteristics is as follows:

[0097] (1) By removing lignin, the internal pore structure of balsa wood blocks is significantly improved. While preserving the original water migration pathways within the wood, the porosity is increased, further enhancing the ability of water to migrate and diffuse within the wood. Sludge contains a large number of macromolecular impurities (organic matter, pathogens, etc.). This structural optimization not only improves the water absorption and water transport speed of balsa wood blocks but also ensures rapid water rise and evaporation, reducing the possibility of clogging internal channels. In contrast, traditional absorbent materials (such as polyurethane foam) or other synthetic materials, although also possessing high porosity, typically have a disordered, interwoven three-dimensional pore structure that is relatively closed and cannot form smooth migration channels. In long-term use in high-impurity applications, clogging can easily occur, leading to performance degradation.

[0098] (2) As water evaporates, the sludge becomes less fluid and more viscous. This invention uses lignin-free balsa wood blocks as absorbent materials, which have high mechanical properties. They can maintain good structural stability in both low and high moisture sludge, while other absorbent materials (such as hydrophilic sponges) are prone to changing their structure with changes in moisture, thus affecting water transport performance and further affecting drying efficiency.

[0099] (3) Modified balsa wood blocks provide a large number of specific active adsorption sites for heavy metals, which can capture more heavy metals and thus improve the efficiency of heavy metal removal. In contrast, traditional absorbent materials are less effective in removing heavy metals and mainly rely on physical adsorption, which is less efficient.

[0100] (4) Based on the aforementioned principle, the photothermal interface evaporation material of the present invention has excellent recyclability and uses natural balsa wood blocks as raw materials. Compared with traditional organic materials, it reduces disposal costs and environmental impact while treating sludge, which meets the requirements of sustainable development.

[0101] In summary, the modified balsa wood blocks of this invention not only fully adapt to the unique properties of sludge but also improve the efficiency and environmental safety of sludge treatment through structural and functional optimization. Specifically, the solar-driven interfacial evaporation sludge drying method provided by this invention uses low-cost, low-density, high-porosity balsa wood as raw material. By removing lignin to increase porosity and adding a polypyrrole coating to improve photothermal conversion efficiency and water evaporation rate, this system can quickly and efficiently reduce the moisture content in sludge. Simultaneously, enhanced adsorption efficiently removes heavy metals from the sludge, reducing potential environmental pollution risks. Furthermore, an integrated moisture sensor enables real-time monitoring and adjustment of the drying process. Therefore, this method has the effect of efficient evaporation and efficient removal of heavy metals. It is simple to operate, has low maintenance costs, uses non-toxic and harmless materials with stable physicochemical properties, and is recyclable. It is a highly commercially promising, efficient, low-cost, low-energy, low-carbon, and environmentally friendly sludge drying and disposal technology.

[0102] Obviously, those skilled in the art can make various subsequent applications, additions, modifications, and variations to this invention without departing from the spirit and scope of this invention. If various applications, additions, modifications, and variations based on this invention fall within the scope of the claims of this invention and their equivalents, then this invention also intends to include these applications, additions, modifications, and variations.

Claims

1. A method for drying sludge using solar-driven interfacial evaporation, characterized in that, Includes the following steps: (1) Take water, sodium hypochlorite and glacial acetic acid in a ratio of 100mL: 4~6g: 1~2mL; dissolve sodium hypochlorite and glacial acetic acid together in water and mix evenly to prepare a lignin-free solution; (2) Cut the balsa wood into long columns, immerse them completely in the lignin removal solution, and heat them at 100°C for 12-16 hours; after taking them out, rinse them with water until the pH of the rinsing solution is neutral, and then perform vacuum freeze-drying to obtain lignin-removed balsa wood blocks. (3) Prepare pyrrole dispersion according to the following ratio: Add 2-3g of ammonium persulfate to 100mL of water, mix well to obtain ammonium persulfate solution; add 0.5-1mL of pyrrole to 100mL of water, mix well to obtain pyrrole dispersion; Equal volumes of ammonium persulfate solution and pyrrole dispersion were sequentially added dropwise to the upper surface of the lignin-free balsa block, allowing ammonium persulfate to act as an oxidant and undergo an oxidative polymerization reaction with pyrrole. After reacting for 10-20 minutes, the residual liquid on the surface was rinsed off with water. The balsa block was then subjected to vacuum freeze-drying to obtain a lignin-free balsa block with a polypyrrole layer on the upper surface. The amount of dropwise addition was controlled to ensure that the thickness of the polypyrrole layer was 50-100 micrometers. (4) Take several lignin-free balsa wood blocks with polypyrrole layers and insert them vertically into the water-containing sludge, ensuring that 80% to 90% of their height is exposed outside the sludge; set moisture sensors on the polypyrrole layers of the balsa wood blocks and the interface with the sludge respectively, and connect each sensor to the detection recorder through a signal line. Sunlight is irradiated onto the surface of sludge and balsa wood blocks, creating a photothermal interface evaporation effect in the polypyrrole layer. Based on the hydroelectric effect caused by evaporation, a moisture sensor is formed at different locations to monitor the difference in voltage values. This voltage difference is used to monitor the changes in moisture content at different locations. The moisture sensor has copper electrodes; when the voltage difference between the two electrodes is consistently and stably below 0.02~0.05V, the moisture content monitoring data is considered to meet the expected value; the drying process ends and the balsa wood block is removed. At this time, the moisture content of the sludge is between 30% and 40%, and the drying process has been completed. (5) Soak the balsa wood blocks in an acidic solution and remove heavy metals under ultrasonic conditions; after taking them out, rinse them with water to remove residual acid and heavy metals until the pH of the rinsing solution is between 6 and 7; then dry the balsa wood blocks to obtain reusable balsa wood blocks.

2. The method according to claim 1, characterized in that, In step (2), the material of the balsa wood block is balsa wood, paulownia wood or pine wood; the long column is a cylinder or square column with a height of 10~20cm and a radial dimension or transverse width of 2~5cm.

3. The method according to claim 1, characterized in that, In step (2) or step (3), the freeze-drying process is carried out in a freeze dryer, with the drying temperature set at -40℃, the drying time at 12~24 hours, and the vacuum degree below 10Pa. After drying, the temperature is slowly raised to room temperature at a rate of 0.4~0.6℃ / minute to avoid damage to the internal structure of the wood.

4. The method according to claim 1, characterized in that, In step (4), the sludge has a moisture content of 90% to 98%, the total volume of the balsa wood blocks used accounts for 5% to 10% of the total volume of the sludge, and the balsa wood blocks are evenly arranged in the sludge in an array.

5. According to the method of claim 1, in step (5), the cleaning time under ultrasonic conditions is 10 to 30 minutes; the ultrasonic frequency is set to 28 kHz and the ultrasonic power is 500 to 1000 W.

6. According to the method of claim 1, in step (5), the acidic solution is 0.1 mol / L hydrochloric acid or 0.1 mol / L nitric acid solution; the heavy metals removed by ultrasonication include at least copper, lead, zinc and cadmium.

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