Membrane recovery method of heavy metal catalyst in medicine wastewater

Abstract

The invention provides a membrane recovery method of a heavy metal catalyst in medicine wastewater. Relates to the technical field of environmental engineering and membrane separation, and comprises the following steps: S1, adding a biodegradable chelating agent into the pharmaceutical wastewater, so that an ionic heavy metal catalyst is complexed and fused with an original micro-colloid to form a catalyst micelle; s2, enabling the wastewater treated in the step S1 to pass through a hydrophilic-oleophylic synergistically modified ultrafiltration membrane assembly in a shear cross-flow manner, and intercepting the catalyst micelles to obtain a pre-concentrated solution. The membrane recovery method of the heavy metal catalyst for the pharmaceutical wastewater meets the industrial application requirements of low energy consumption, low maintenance cost and long-period stable operation, and the economical efficiency and sustainability of a membrane separation recovery process are remarkably improved.

Description

Technical Field

[0001] The invention relates to the technical field of environmental engineering and membrane separation, and in particular to a membrane method for recovering heavy metal catalysts from pharmaceutical wastewater. Background Art

[0002] Currently, the common technologies for recovering heavy metal catalysts from pharmaceutical industry wastewater include chemical precipitation, adsorption / ion exchange, electrochemical recovery, and traditional membrane separation technology. Chemical precipitation converts soluble heavy metal ions into insoluble solids by adding precipitants such as lime milk and sulfide, but this produces a large amount of sludge. Adsorption / ion exchange uses activated carbon, ion exchange resins, or modified biomaterials to capture heavy metal ions, which is suitable for low-concentration wastewater, but the adsorbent is easily saturated and the regeneration cost is high. Electrochemical recovery reduces and precipitates heavy metals through electrolytic deposition, resulting in high purity, but the equipment cost and energy consumption are high. Microfiltration and ultrafiltration are effective at retaining suspended particles but have difficulty removing dissolved ions. Nanofiltration can retain multivalent ions but is prone to contamination and has rapid flux decay. Reverse osmosis has a comprehensive retention effect but is expensive and energy-intensive, making it unsuitable for the economical recovery of precious metal catalysts.

[0003] Although the above technologies each have their own advantages, there are still several shortcomings in practical applications: the comprehensive retention efficiency of catalyst microcolloids and ionic states is low, resulting in serious waste of resources; traditional membrane separation processes are prone to membrane fouling and clogging during long-term continuous operation, with high maintenance costs and difficulty in regeneration, making it difficult to meet the needs of efficient, stable and low-consumption industrial recovery. Summary of the Invention

[0004] In response to the shortcomings of the existing technology, the present invention provides a membrane recovery method for heavy metal catalysts in pharmaceutical wastewater, which solves the problem of how to achieve efficient and stable retention and recovery of ionic and microcolloidal heavy metal catalysts in pharmaceutical wastewater by combining chelating agent pre-complexation, shear cross-flow ultrafiltration pre-concentration and Au / SiO2 nanorod composite nanofiltration online photothermal self-cleaning.

[0005] To achieve the above objectives, the present invention is implemented through the following technical solutions: A membrane method for recovering heavy metal catalysts from pharmaceutical wastewater, comprising:

[0006] S1. Adding a biodegradable chelating agent to the pharmaceutical wastewater to complex the ionic heavy metal catalyst and fuse it with the original microcolloids to form catalyst micelles;

[0007] S2. The wastewater treated in step S1 is passed through a hydrophilic-lipophilic synergistically modified ultrafiltration membrane assembly in a shear cross-flow manner to intercept the catalyst micelles and obtain a pre-concentrated solution;

[0008] S3. Using a composite nanofiltration membrane assembly having Au / SiO2 core-shell nanorods uniformly embedded in an upstream filtration layer, filtering the pre-concentrated liquid to obtain a filtered concentrate, and periodically irradiating the surface of the composite nanofiltration membrane assembly with near-infrared light to achieve an online photothermal-self-cleaning operation, thereby maintaining a stable flux and achieving high retention of heavy metal catalysts;

[0009] S4. Adding a desorption liquid of a renewable component to the filtered concentrate to desorb the heavy metal catalyst and recover it as a solid catalyst through subsequent treatment;

[0010] S5. Perform oxidation cleaning and physical cleaning on the ultrafiltration membrane assembly and the composite nanofiltration membrane assembly in sequence to restore membrane flux and achieve long-term continuous operation.

[0011] Preferably, the biodegradable chelating agent is carboxymethyl chitosan, with a degree of substitution of 0.2-0.8, a number average molecular weight of 5kDa-30kDa, and an addition amount of 0.5-2.0g·L-1.

[0012] Preferably, step S1 is completed at pH 4.0-5.0 and stirring for 10-20 min, and the catalyst micelles have an average particle size of 60-120 nm by dynamic light scattering and a surface potential of -30 mV to -15 mV.

[0013] Preferably, the shear cross-flow velocity is 1-3 m·s -1 , the flux attenuation rate is not higher than 5% when running continuously for 8 hours.

[0014] Preferably, the ultrafiltration membrane assembly comprises a polyvinylidene fluoride-grafted polyethylene glycol methacrylate hollow fiber membrane having an average pore size of 15-30 nm, a water contact angle of no more than 60°, and a catalyst micelle rejection rate of no less than 90%.

[0015] Preferably, the Au / SiO2 core-shell nanorods have an aspect ratio of 3-4, a longitudinal plasmon resonance peak at 800-820 nm, and a mass fraction of 0.05-0.20 wt%.

[0016] Preferably, the near-infrared light irradiation power density is 0.4-0.7 W·cm -2 , Continuous irradiation for 3-8 minutes every 2-3 hours, the instantaneous temperature rise ΔT on the surface of the composite nanofiltration membrane assembly satisfies the following thermal balance model formula:

[0017]

[0018] Where P is the light power density, η is the photothermal conversion efficiency of nanorods, τ is the single irradiation time, ρ is the density of the slurry, cp is the specific heat capacity of the liquid, and δ is the light penetration depth.

[0019] Preferably, the desorption liquid is composed of 0.5-1.5 mol·L-1 thiourea and 1-3 wt% ammonia water, and the metal catalyst recovery rate R and the desorption liquid volume fraction L conform to the following model formula:

[0020] R=R max [1-exp(-βL)],

[0021] Where R max is the theoretical maximum recovery rate, β is the desorption kinetic coefficient, which ranges from 0.8 to 1.6, and L is the desorption liquid volume fraction.

[0022] The present invention provides a membrane method for recovering heavy metal catalysts from pharmaceutical wastewater.

[0023] Beneficial effects:

[0024] This membrane-based recovery method for heavy metal catalysts in pharmaceutical wastewater achieves efficient and synergistic retention of both microcolloidal and ionic heavy metal catalysts in pharmaceutical wastewater through a coupled process of pre-complexation, shear ultrafiltration, and photothermal self-cleaning nanofiltration. First, a biodegradable chelating agent complexes with heavy metal ions to form stable micelles. Combined with shear cross-flow ultrafiltration, this significantly enriches the catalyst particles, which are then further refined through a nanofiltration membrane embedded with Au / SiO2 core-shell nanorods, ensuring a comprehensive catalyst retention rate of ≥95% and a recovery rate of ≥95%. At the same time, the combination of a composite membrane with adjustable passivation parameters and a chelating agent avoids the selective loss of ionic and microparticle-type catalysts, minimizing resource waste and achieving high-purity regeneration of precious metal catalysts.

[0025] The present invention also significantly suppresses membrane fouling and flux attenuation by combining online near-infrared photothermal self-cleaning with in-situ oxidation / ultrasonic physical cleaning. Without interrupting production, photothermal pulses remove the adsorbed fouling layer; regular ozone / hydrogen peroxide cleaning and ultrasonic treatment restore the membrane flux to ≥90% of the initial value, effectively extending the membrane module's operating cycle by more than 60 days and reducing the frequency of chemical cleaning by more than 70%. Thus, this method takes into account the industrial application requirements of low energy consumption, low maintenance costs, and long-term stable operation, significantly improving the economy and sustainability of the membrane separation and recovery process. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a flowchart for implementing the invention. DETAILED DESCRIPTION

[0027] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0028] like Figure 1 As shown, an embodiment of the present invention provides a membrane method for recovering heavy metal catalysts from pharmaceutical wastewater, comprising: S1. Adding a biodegradable chelating agent to the pharmaceutical wastewater to complex the ionic heavy metal catalyst and fuse it with the original microcolloids to form catalyst micelles. The biodegradable chelating agent is carboxymethyl chitosan, with a degree of substitution of 0.2-0.8, a number average molecular weight of 5kDa-30kDa, and an addition amount of 0.5-2.0g·L-1. Step S1 is completed under pH 4.0-5.0 and stirring for 10-20 minutes. The dynamic light scattering average particle size of the catalyst micelles is 60-120nm, and the surface potential is -30mV to -15mV.

[0029] The specific implementation is as follows:

[0030] Reaction device preparation:

[0031] A 5 L glass reactor was equipped with a mechanical stirring paddle and a pH meter, a thermometer, and a sampling interface for a dynamic light scattering particle size analyzer.

[0032] Wastewater pretreatment:

[0033] The wastewater containing heavy metal catalysts such as palladium and platinum generated in the drug synthesis process is placed in a reactor and replaced with nitrogen for 5 minutes to eliminate the influence of dissolved oxygen on the subsequent chelation process.

[0034] pH adjustment:

[0035] Slowly add 1M sulfuric acid or sodium hydroxide solution to the reactor to adjust the pH of the wastewater to the range of 4.0–5.0 and maintain the temperature at 20–25°C.

[0036] Chelating agent configuration and addition:

[0037] Weigh the appropriate amount of carboxymethyl chitosan powder in a ratio of 0.5–2.0 g·L⁻¹ carboxymethyl chitosan to wastewater volume, dissolve it in an appropriate amount of deionized water, and pre-wet it with ultrasound for 5 minutes. Slowly add the prepared chelating agent solution dropwise to the reactor while stirring at 350 rpm.

[0038] Chelation reaction and micelle formation:

[0039] The reaction was carried out at a pH of 4.0–5.0 and stirred at 350 rpm for 10–20 minutes. During this process, the carboxyl groups on the carboxymethyl chitosan formed stable coordination bonds with the ionic heavy metal ions and interacted with the existing microcolloids in the wastewater, driving the micelles to self-aggregate and form a catalyst-chelating agent composite colloid with a negatively charged surface.

[0040] Micellar property detection:

[0041] After the reaction, 2 mL of reaction liquid was extracted through the sampling interface, and the average particle size of the micelles was measured by DLS to be 60–120 nm. The surface potential was tested using a ζ-potential meter and was between –30 mV and –15 mV, indicating that the micelles had good dispersion stability and sufficient charge repulsion, making them suitable for subsequent ultrafiltration retention.

[0042] Subsequent transfer:

[0043] After completing the above steps, the obtained micelle-enriched reaction solution is directly transported to the ultrafiltration unit and enters the high shear cross-flow pre-concentration in step S2.

[0044] S2. The wastewater treated in step S1 is passed through a hydrophilic-lipophilic synergistically modified ultrafiltration membrane assembly in a shear cross-flow manner to intercept the catalyst micelles and obtain a pre-concentrated solution. The shear cross-flow flow rate is 1-3 m·s -1 The flux attenuation rate is no more than 5% after 8 hours of continuous operation. The ultrafiltration membrane assembly includes a polyvinylidene fluoride-grafted polyethylene glycol methacrylate hollow fiber membrane. The polyvinylidene fluoride-grafted polyethylene glycol methacrylate hollow fiber membrane has an average pore size of 15-30nm, a water contact angle of no more than 60°, and a catalyst micelle retention rate of no less than 90%.

[0045] The specific implementation is as follows:

[0046] Equipment and materials:

[0047] Liquid storage tank: capacity 10L, polypropylene material.

[0048] Circulation system: peristaltic pump, flow meter, pressure sensor and piping.

[0049] Ultrafiltration membrane components:

[0050] The substrate is polyvinylidene fluoride hollow fiber, and polyethylene glycol methacrylate is grafted on the surface.

[0051] The average pore size is 15–30 nm, and the outer surface of the membrane is treated with plasma activation.

[0052] Water contact angle ≤60°, effective membrane area about 1.5m 2 .

[0053] Pre-concentration process:

[0054] The micelle-enriched solution obtained in step S1 was placed in a liquid storage tank and replaced with nitrogen for 3 minutes.

[0055] Start the peristaltic pump to circulate the reaction solution through the ultrafiltration membrane assembly at 20–25°C.

[0056] Adjust the flow rate so that the shear velocity on the membrane surface reaches 1.0–3.0 m·s -1 , maintaining the transmembrane pressure difference at 0.1–0.2 MPa.

[0057] Run continuously for 8 hours and monitor the permeate production and system pressure difference in real time.

[0058] The permeate and concentrate samples were collected and recorded, and the heavy metal concentrations were determined by inductively coupled plasma spectrometry.

[0059] Performance indicators:

[0060] Flux stability: The flux decrease does not exceed 5% after 8 hours of operation.

[0061] Retention efficiency: The concentration of target metals such as palladium and platinum in the permeate is reduced, and the retention rate is not less than 90%.

[0062] Concentration ratio: The liquid volume ratio before and after circulation can reach 3-8 times.

[0063] Pre-concentrate collection:

[0064] After the operation is completed, the residual concentrated liquid in the membrane side plate is recovered to the liquid storage tank as feed for subsequent nanofiltration separation.

[0065] Membrane module cleaning and regeneration:

[0066] First backwash with raw water for 30 minutes to remove loose impurities.

[0067] Then, 0.1 wt % sodium hydroxide solution was circulated for cleaning for 20 minutes to dissolve the adsorbed pollutants.

[0068] Finally, the membrane was rinsed with deionized water until the pH of the effluent was neutral and the membrane flux recovered to more than 90% of the initial value.

[0069] S3. A composite nanofiltration membrane assembly with Au / SiO2 core-shell nanorods exhibiting localized surface plasmon resonance (LSPR) properties uniformly embedded in the upstream filtration layer was used to filter the preconcentrate to obtain a filtered concentrate. The composite nanofiltration membrane assembly surface was periodically illuminated with near-infrared light to achieve online photothermal self-cleaning, thereby maintaining stable flux and achieving high retention of heavy metal catalysts. The Au / SiO2 core-shell nanorods had an aspect ratio of 3-4, a longitudinal plasmon resonance peak located at 800-820 nm, and a mass fraction of 0.05-0.20 wt%.

[0070] Near-infrared light irradiation power density 0.4-0.7W·cm -2 , Continuous irradiation for 3-8 minutes every 2-3 hours, the instantaneous temperature rise ΔT on the surface of the composite nanofiltration membrane component satisfies the following thermal balance model formula:

[0071]

[0072] Where P is the light power density, η is the photothermal conversion efficiency of nanorods, τ is the single irradiation time, ρ is the density of the slurry, c p is the specific heat capacity of the liquid, and δ is the light penetration depth.

[0073] The specific implementation is as follows:

[0074] Preparation of composite nanofiltration membrane components:

[0075] Nanorod dispersion:

[0076] Au / SiO2 core-shell nanorods were weighed to a mass fraction of 0.10 wt% and dissolved in an ethanol / water mixed solution.

[0077] Ultrasonic dispersion was performed for 15 min to obtain a uniform dispersion.

[0078] Membrane surface coating:

[0079] The PES nanofiltration membrane was immersed in the nanorod dispersion for 2 minutes, taken out and allowed to stand for 5 minutes.

[0080] The nanorods were dried at room temperature for 2 h to allow them to be firmly adsorbed on the surface of the upstream layer of the membrane.

[0081] Oven cure at 60°C for 1 h to enhance bonding strength.

[0082] Membrane module assembly:

[0083] The above membrane sheets are wound into hollow fiber modules and placed in a stainless steel membrane shell.

[0084] Connect to the water inlet and outlet pipes and pressure and temperature measurement interfaces. The overall effective membrane area is about 0.8m 2 .

[0085] Filter device and light source configuration:

[0086] Nanofiltration circulation system:

[0087] High-pressure pump, the liquid supply pressure can be adjusted to 0.1-0.6MPa.

[0088] Flow meter, pressure sensor, and temperature probe for online monitoring.

[0089] Near-infrared light source:

[0090] 808nm LED array light, adjustable power density 0.4–0.7W·cm -2 .

[0091] The light board is about 10 cm away from the film surface and illuminates evenly through the light-transmitting window.

[0092] Equipped with a timing controller to support periodic irradiation.

[0093] Operation process:

[0094] System pre-flush:

[0095] First, flush the membrane assembly with 0.2MPa raw water for 20min to remove residual impurities;

[0096] Nanofiltration separation

[0097] The S2 pre-concentrate was pumped into the membrane shell, and the transmembrane pressure difference was controlled to be 0.3 MPa. The circulation rate corresponded to a shear rate of about 1000 s. -1 .

[0098] The inlet and outlet flow ratio was maintained at 1:0.1, and the recovery rate was approximately 10%.

[0099] Photothermal self-cleaning:

[0100] Start near-infrared light irradiation every 2.5 hours and continue irradiation for 5 minutes.

[0101] During the instantaneous irradiation, the membrane surface temperature rises from room temperature to about 50°C, which promotes the desorption of the adsorbed dirt layer.

[0102] After irradiation, it automatically switches to conventional nanofiltration mode.

[0103] Online monitoring:

[0104] The permeate flow rate is recorded in real time, and the flux attenuation rate is kept ≤3% / d.

[0105] The permeate and feed samples were taken regularly and the metal concentration was determined by ICP-OES, and the rejection rate was maintained at ≥95%.

[0106] Filter concentrate collection:

[0107] When the system recovery volume ratio reaches 10%, water production is stopped and the concentrated liquid is collected for subsequent S4 desorption and recovery.

[0108] Membrane maintenance after the run:

[0109] Water washing: Backwash with low-pressure raw water for 30 minutes.

[0110] Enhanced photothermal cleaning: irradiate with near-infrared light for an additional 5 minutes under pressure-free conditions to remove stubborn dirt.

[0111] Chemical cleaning: circulate 0.05wt% H2O2 solution for 20 minutes.

[0112] Final rinse: rinse with deionized water to pH 7, and the membrane flux is restored to ≥90% of the initial value.

[0113] S4. Adding a desorption liquid containing renewable components to the filtered concentrated liquid to desorb the heavy metal catalyst and recovering it as a solid catalyst through subsequent treatment.

[0114] The desorption liquid consists of 0.5-1.5 mol·L-1 thiourea and 1-3 wt% ammonia water. The metal catalyst recovery rate R and the desorption liquid volume fraction L conform to the following model formula:

[0115] R=R max [1-exp(-βL)].

[0116] Where R max is the theoretical maximum recovery rate, β is the desorption kinetic coefficient, which ranges from 0.8 to 1.6, and L is the desorption liquid volume fraction.

[0117] The specific implementation is as follows:

[0118] Equipment and materials:

[0119] Desorption reactor: Made of stainless steel, with a four-paddle stirrer and a constant temperature jacket.

[0120] Separation equipment: high-speed centrifuge, Buchner funnel and vacuum filtration device.

[0121] Desorbent:

[0122] Thiourea, prepared at 0.5–1.5 mol·L-1.

[0123] Ammonia water, prepared at 1–3 wt%.

[0124] Desorption solution preparation:

[0125] Thiourea was dissolved in deionized water at 25°C to prepare a stock solution within the target concentration range.

[0126] Aqueous ammonia was added dropwise to a concentration of 1–3 wt%, and the pH of the solution was adjusted to 9–10.

[0127] After stirring evenly, the desorption liquid of the renewable component is obtained.

[0128] Desorption reaction:

[0129] The ratio is determined according to the desorption liquid volume fraction L, and L≥3 is usually used.

[0130] The filtered concentrate was added to the reactor, heated to 40–60°C and kept at a constant temperature.

[0131] Stir at 300–500 rpm and add the pre-made desorption liquid into the reactor in batches until the total volume reaches the set ratio.

[0132] Continue stirring the reaction for 30–90 min to ensure that thiourea is fully complexed with and desorbed from the metal ions.

[0133] Solid-liquid separation:

[0134] After the reaction was completed, the mixture was centrifuged at 5000 rpm for 10 min to separate the supernatant and the precipitate.

[0135] The precipitate was gently washed once with dilute ammonia in a vacuum funnel to remove residual complexes.

[0136] Catalyst recovery and regeneration:

[0137] The washed metal thiourea complex precipitate is dispersed in a 0.1-0.5 mol·L-1 sodium borohydride aqueous solution with a pH of 9-10, and stirred at room temperature of 25-30°C for 30-60 minutes to reduce and precipitate heavy metal ions; then, the mixture is centrifuged at 5000 rpm for 10 minutes to separate the obtained metal precipitate, and the mixture is repeatedly washed with deionized water until it is neutral. Finally, the washed precipitate is placed in a vacuum drying oven and dried at 50-80°C for 2 hours to obtain a heavy metal catalyst solid.

[0138] S5. Perform oxidation cleaning and physical cleaning on the ultrafiltration membrane assembly and composite nanofiltration membrane assembly in sequence to restore the membrane flux and achieve long-term continuous operation.

[0139] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A membrane method for recovering heavy metal catalysts from pharmaceutical wastewater, characterized in that: include: S1. Adding a biodegradable chelating agent to the pharmaceutical wastewater to complex the ionic heavy metal catalyst and fuse it with the original microcolloids to form catalyst micelles; S2. The wastewater treated in step S1 is passed through a hydrophilic-lipophilic synergistically modified ultrafiltration membrane assembly in a shear cross-flow manner to intercept the catalyst micelles and obtain a pre-concentrated solution; S3. Using a composite nanofiltration membrane assembly having Au / SiO2 core-shell nanorods uniformly embedded in an upstream filtration layer, filtering the pre-concentrated liquid to obtain a filtered concentrate, and periodically irradiating the surface of the composite nanofiltration membrane assembly with near-infrared light to achieve an online photothermal-self-cleaning operation; S4. Adding a desorption liquid of a renewable component to the filtered concentrate to desorb the heavy metal catalyst and recover it as a solid catalyst through subsequent treatment; S5. Perform oxidation cleaning and physical cleaning on the ultrafiltration membrane assembly and the composite nanofiltration membrane assembly in sequence to restore membrane flux and achieve long-term continuous operation.

2. The membrane method for recovering heavy metal catalysts from pharmaceutical wastewater according to claim 1, characterized in that: The biodegradable chelating agent is carboxymethyl chitosan, with a substitution degree of 0.2-0.8, a number average molecular weight of 5kDa-30kDa, and an addition amount of 0.5-2.0g·L-1.

3. The membrane method for recovering heavy metal catalysts from pharmaceutical wastewater according to claim 1, characterized in that: Step S1 is completed under the conditions of pH 4.0-5.0 and stirring for 10-20 minutes. The catalyst micelles have a dynamic light scattering average particle size of 60-120 nm and a surface potential of -30 mV to -15 mV.

4. The membrane method for recovering heavy metal catalysts from pharmaceutical wastewater according to claim 1, characterized in that: The shear cross-flow velocity is 1-3 m·s -1 , the flux attenuation rate is not higher than 5% when running continuously for 8 hours.

5. The membrane method for recovering heavy metal catalysts from pharmaceutical wastewater according to claim 1, characterized in that: The ultrafiltration membrane assembly comprises a polyvinylidene fluoride-grafted polyethylene glycol methacrylate hollow fiber membrane having an average pore size of 15-30 nm, a water contact angle of no more than 60°, and a catalyst micelle interception rate of no less than 90%.

6. The membrane method for recovering heavy metal catalysts from pharmaceutical wastewater according to claim 1, characterized in that: The Au / SiO2 core-shell nanorods have an aspect ratio of 3-4, a longitudinal plasma resonance peak located at 800-820 nm, and a mass fraction of 0.05-0.20 wt%.

7. The membrane method for recovering heavy metal catalysts from pharmaceutical wastewater according to claim 1, characterized in that: The near-infrared light irradiation power density is 0.4-0.7 W·cm -2 , Continuous irradiation for 3-8 minutes every 2-3 hours, the instantaneous temperature rise ΔT on the surface of the composite nanofiltration membrane assembly satisfies the following thermal balance model formula: Where P is the light power density, η is the photothermal conversion efficiency of nanorods, τ is the single irradiation time, ρ is the density of the slurry, c p is the specific heat capacity of the liquid, and δ is the light penetration depth.

8. The membrane method for recovering heavy metal catalysts from pharmaceutical wastewater according to claim 1, characterized in that: The desorption liquid is composed of 0.5-1.5 mol·L-1 thiourea and 1-3 wt% ammonia water. The metal catalyst recovery rate R and the desorption liquid volume fraction L conform to the following model formula: R=R max [1-exp(-βL)], Where R max is the theoretical maximum recovery rate, β is the desorption kinetic coefficient, which ranges from 0.8 to 1.6, and L is the desorption liquid volume fraction.

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