Method for utilizing carbon quantum dots to strengthen degradation of wastewater by cladophora and application thereof

Abstract

The application discloses a method for reinforcing degradation of wastewater by using carbon quantum dots and application thereof, and comprises the following steps: pre-culturing of cladophora, pre-contacting of carbon quantum dots and cladophora culture solution, adding of wastewater containing phenol, regulation of a photo-biological reaction system, periodical monitoring, dynamic segmented supplement of carbon quantum dots, end point determination and solid-liquid separation; the regulation mode of pre-contacting of cladophora and carbon quantum dots and dynamic segmented supplement of carbon quantum dots is adopted, the carbon quantum dots have the functions of antioxidation and light gain, can reduce the oxidative damage of hydroquinone to algal cells, stabilize photosynthetic indexes such as chlorophyll a, cell density and Fv / Fm, continuously maintain the anti-toxicity performance of microalgae according to the dynamic feeding of algal physiological recession and the decrease of phenol degradation rate, avoid the defects of excessive carbon quantum dots in the early stage and insufficient reinforcement in the later stage, and greatly improve the operation stability of the wastewater treatment system containing phenol.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, specifically to a method for enhancing the degradation of wastewater by Scenedesmus obliquus using carbon quantum dots and its application. Background Technology

[0002] Hydroquinone is a representative pollutant in phenolic organic wastewater, found in wastewater from developers, dye intermediates, phenolic resins, and coal chemical industries. It exhibits certain water solubility and biotoxicity. Existing methods for treating phenolic wastewater mainly include adsorption, oxidation, and biological treatment. However, adsorption methods suffer from pollutant transfer and material regeneration issues, oxidation methods consume large amounts of reagents, and conventional biological treatment is easily affected by the toxicity of phenolic pollutants, leading to decreased treatment efficiency and operational stability.

[0003] Microalgae treatment of wastewater has advantages such as utilizing light energy, mild operating conditions, and biomass recovery, making it a promising candidate for treating wastewater containing organic pollutants. While green algae like *Scenedesmus obliquus* can participate in the adsorption and biotransformation of organic pollutants, phenolic pollutants in phenolic wastewater inhibit microalgae growth and metabolic activity, reducing their continuous treatment capacity. There is a lack of enhanced control methods to maintain microalgae activity and improve system stability under phenolic toxicity stress. Therefore, it is necessary to provide a microalgae-enhanced treatment technology suitable for phenolic organic wastewater. Summary of the Invention

[0004] The purpose of this invention is to provide a method for enhancing the degradation of wastewater by Scenedesmus obliquus using carbon quantum dots in order to solve the above-mentioned problems and overcome the shortcomings of the prior art, as detailed below.

[0005] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for enhancing the degradation of wastewater by *Scenedesmus obliquus* using carbon quantum dots, comprising the following steps: S1. Microalgae were inoculated into the culture medium and cultured to the logarithmic growth phase. Without the addition of carbon quantum dots, the concentration of *Scenedesmus obliquus* cells in the reaction system was brought to 1.0 × 10⁻⁶. 6 -5.0×10 6 When the cell / mL ratio is reached, the culture medium of *Scenedesmus obliquus* to be enhanced is obtained; S2. Set the target cumulative concentration C of carbon quantum dots in the reaction system. t The concentration is 15-25 mg / L. 12-24 hours before adding phenol-containing organic wastewater, 0.3 Ct-0.5 Ct carbon quantum dots are added to the culture medium of *Scenedesmus obliquus* to be enhanced, so that the carbon quantum dots are in pre-contact with *Scenedesmus obliquus*. S3. Add phenol-containing wastewater to the pre-contact treated microalgae culture medium to make the initial concentration of phenolic pollutants in the reaction system reach 25-75 mg / L. Subsequently, carbon quantum dots are added according to the growth status of Scenedesmus obliquus. The mass concentration ratio of the total amount of carbon quantum dots added to the total amount of phenolic pollutants is 0.2-1.0. S4. Place the reaction system obtained in step S3 in a culture environment with a temperature of 20-30℃, pH of 6.0-8.5 and a light intensity of 2000-6000 lx, and culture it using a light-dark alternation method. The time ratio of the light period to the dark period is 1-2:1. During the reaction, the oscillation culture method is used to maintain the suspended growth of Scenedesmus obliquus and the mixing of the reaction system. S5. Periodically monitor the physiological state indicators of *Scenedesmus obliquus* and the hydroquinone removal rate. When, within two consecutive monitoring periods, the physiological state of *Scenedesmus obliquus* decreases by more than 50% compared to the previous period, and the hydroquinone removal rate decreases by more than 30% compared to the previous monitoring period, add the remaining unadded carbon quantum dots to the reaction system, with each addition amounting to 0.1C. t -0.3C t And the cumulative concentration of carbon quantum dots added does not exceed C t ; S6. When the hydroquinone removal rate in the reaction system reaches 90% or more, or the remaining hydroquinone concentration is not higher than 5 mg / L, stop adding carbon quantum dots, the treatment ends, and further perform solid-liquid separation on the reaction system to recover the Scenedesmus obliquus biomass and obtain the treated effluent.

[0006] Preferably, the culture medium is standard BG11 medium, and the carbon quantum surface contains one or more hydrophilic functional groups of hydroxyl, carboxyl and amino groups. The carbon quantum can be dispersed in BG11 medium for 24 hours without visible sedimentation.

[0007] Preferably, the carbon quantum dots have fluorescence emission properties and a particle size of 1-10 nm.

[0008] Preferably, the target cumulative dosage concentration C of the carbon quantum dots in the reaction system is... t The concentration was 15 mg / L, and the initial concentration of the phenolic pollutant was 50 mg / L.

[0009] Preferably, in step S4, the reaction system is maintained by continuous low-speed oscillation culture to keep the Scenedesmus obliquus suspended and mixed, with an oscillation speed of 60-120 rpm.

[0010] Preferably, in step S5, the physiological state indicators of *Scenedesmus obliquus* include at least one of cell density, chlorophyll a content, and maximum photochemical efficiency Fv / Fm. A detection cycle is set at 48 hours. When the physiological state indicators of *Scenedesmus obliquus* decrease by 50% compared to the previous detection cycle, or when the removal efficiency of phenolic pollutants decreases by 30%-50% compared to the previous detection cycle, dynamic segmented replenishment is performed.

[0011] Preferably, in step S5, the remaining unadded carbon quantum dots are added to the reaction system in a dynamic, segmented manner.

[0012] An application of carbon quantum dots to enhance the treatment of phenol-containing organic wastewater by Scenedesmus obliquus under hydroquinone toxicity stress, wherein the phenol-containing organic wastewater is one of coal chemical wastewater, coking wastewater or phenolic resin production wastewater.

[0013] Preferably, the reaction treatment cycle after mixing the pre-contact treated microalgae culture medium with phenol-containing organic wastewater is 10-15 days.

[0014] The beneficial effects are: 1. The system adopts a control mode of pre-contacting Scenedesmus obliqueis with carbon quantum dots and dynamically adding carbon quantum dots in stages. Carbon quantum dots have both antioxidant and light-enhancing effects, which can reduce the oxidative damage of hydroquinone to algal cells and stabilize photosynthetic indicators such as chlorophyll a, cell density, and Fv / Fm. The system dynamically adds carbon quantum dots according to the physiological decline of algae and the decrease in phenol degradation rate, continuously maintaining the anti-toxic performance of microalgae and avoiding the defects of excessive carbon dots in the early stage and insufficient enhancement in the later stage, thus greatly improving the operational stability of the phenol-containing wastewater treatment system.

[0015] 2. Water-soluble blue carbon quantum dots with surfaces rich in hydroxyl, carboxyl, and amino groups are selected. They exhibit excellent dispersibility and no sedimentation in standard BG11 medium, which can broaden the range of light energy utilization by microalgae, improve photosynthetic metabolism efficiency, and synergistically enhance the adsorption and biotransformation of phenols by microalgae. The process adopts alternating light and dark conditions and continuous low-speed oscillation under mild culture conditions, eliminating the need for high-temperature, strong oxidation, and high-energy-consuming equipment, making the process simple and easy to implement.

[0016] 3. Carbon quantum dots are free of heavy metals and have good biocompatibility, and will not produce secondary toxic residues. After treatment, the biomass of Scenedesmus obliquus can be recovered through simple solid-liquid separation, and the algae can be further utilized as a resource, realizing green biological treatment of phenol-containing organic wastewater. It is suitable for a variety of typical phenol-containing wastewaters from coal chemical, coking, and phenolic resin production, and the treatment cycle can be controlled within 10 to 15 days. It has a wide range of applications and combines environmental benefits with resource value. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of the process of carbon quantum dot-enhanced treatment of phenol-containing organic wastewater by Scenedesmus obliquus according to the present invention; Figure 2 This is the hydroquinone liquid chromatogram on day 10 of Example 2 of the present invention, where a, b, and c are the 15 mg / L CQDs group and d is the CQDs-free group. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0020] This invention provides a method for enhancing the degradation of wastewater by *Scenedesmus obliquus* using carbon quantum dots: Scenedesmus obliquus in its logarithmic growth phase was selected as the experimental algae species and inoculated into standard BG11 medium. Without the addition of carbon quantum dots, the concentration of Scenedesmus obliquus cells in the reaction system was brought to 1.0 × 10⁻⁶. 6 -5.0×10 6 When the cell / mL ratio is reached, the culture medium of *Scenedesmus obliquus* to be enhanced is obtained; Set the target cumulative concentration C of carbon quantum dots in the reaction system. t The concentration of carbon quantum dots is 15-25 mg / L, preferably 15 mg / L. 12-24 hours before adding the phenol-containing organic wastewater, 0.3-0.5 Ct of carbon quantum dots are added to the *Scenedesmus obliquus* culture medium to allow for pre-contact between the carbon quantum dots and the *Scenedesmus obliquus*. The carbon quantum dots contain one or more hydrophilic functional groups, including hydroxyl, carboxyl, and amino groups, and show no visible sedimentation after 24 hours of dispersion in BG11 medium. The carbon quantum dots exhibit fluorescence emission properties and have a particle size of 1-10 nm.

[0021] Add phenol-containing wastewater to the pre-contact treated microalgae culture medium to make the initial concentration of phenolic pollutants in the reaction system reach 25-75 mg / L, preferably 50 mg / L. Subsequently, carbon quantum dots are added according to the growth status of Scenedesmus obliquus. The mass concentration ratio of the total amount of carbon quantum dots added to the total amount of phenolic pollutants is 0.2-1.0. The reaction system obtained in step S3 was placed in a culture environment with a temperature of 20-30℃, pH of 6.0-8.5 and a light intensity of 2000-6000 lx. The system was cultured using an alternating light and dark method, with a light-to-dark ratio of 1-2:1. During the reaction, a continuous low-speed oscillation culture method was used to maintain the suspended growth of Scenedesmus obliquus and the mixing of the reaction system, with an oscillation speed of 60-120 rpm. A 48-hour monitoring cycle was set, and the physiological state indicators of *Scenedesmus obliquus* and the hydroquinone removal rate were monitored periodically. When, within two consecutive monitoring cycles, the physiological state of *Scenedesmus obliquus* decreased by more than 50% compared to the previous cycle, and the hydroquinone removal rate decreased by more than 30% compared to the previous monitoring cycle, the remaining unadded carbon quantum dots were dynamically added to the reaction system in stages, with each addition amounting to 0.1C. t -0.3C t And the cumulative concentration of carbon quantum dots added does not exceed C t Physiological state indicators of *Scenedesmus obliquus* include at least one of cell density, chlorophyll a content, and maximum photochemical efficiency Fv / Fm. A detection cycle of 48 hours is set. When the physiological state indicators of *Scenedesmus obliquus* decrease by 50% compared to the previous detection cycle, or when the removal efficiency of phenolic pollutants decreases by 30%-50% compared to the previous detection cycle, dynamic segmented supplementation is carried out.

[0022] When the hydroquinone removal rate in the reaction system reaches 90% or more, or the remaining hydroquinone concentration is not higher than 5 mg / L, the addition of carbon quantum dots is stopped, the treatment ends, and the reaction system is further subjected to solid-liquid separation to recover the Scenedesmus obliquus biomass and obtain the treated effluent.

[0023] Phenolic organic wastewater is a type of wastewater from coal chemical industry, coking, or phenolic resin production.

[0024] The reaction treatment cycle after mixing the pre-contact treated microalgae culture medium with phenol-containing organic wastewater is 10-15 days.

[0025] Example 1: Effects of carbon quantum dots on the growth of Scenedesmus obliquus under toxic stress Microalgae were inoculated into standard BG11 medium and cultured to the logarithmic growth phase. *Scenedesmus obliquus*, in its logarithmic growth phase, was selected as the experimental species, and the initial cell concentration was adjusted to 1.0 × 10⁻⁶. 6 -5.0×10 6cells / mL. Different concentrations of water-soluble carbon quantum dots were added to the *Scenedesmus obliquus* culture system to achieve final concentrations of 0 mg / L, 5 mg / L, 10 mg / L, 15 mg / L, 20 mg / L, and 25 mg / L. The 0 mg / L group served as a blank control. After a 24-hour pre-contamination, 50 mg / L hydroquinone was added. The culture medium composition, initial algal density, culture volume, temperature, light method, and shaking conditions were kept consistent across all groups.

[0026] Each treatment group was cultured under suitable conditions for the growth of *Scenedesmus obliquus*, at a temperature of 20-30℃, using a light-dark alternating culture method, with a preferred light-dark ratio of 1:1, for 12 hours each time, and subjected to continuous low-speed shaking culture at a rotation speed of 100-120 rpm. After the culture was completed, the effect of different concentrations of carbon quantum dots on the growth of *Scenedesmus obliquus* was evaluated by measuring cell concentration indicators.

[0027]

[0028] The results of the examples show that under 50 mg / L hydroquinone stress, the cell density growth of *Scenedesmus obliquus* in the control group without carbon quantum dots was limited; however, after adding 5–25 mg / L water-soluble carbon quantum dots, the cell density of *Scenedesmus obliquus* increased. Water-soluble carbon quantum dots can alleviate the toxic inhibitory effect of hydroquinone on *Scenedesmus obliquus* and promote microalgal cell proliferation. Among these, the 15 mg / L water-soluble carbon quantum dot treatment group showed the highest cell density, indicating that this concentration is the optimal addition concentration under the conditions of this example.

[0029] This embodiment demonstrates that water-soluble carbon quantum dots can be used in the Scenedesmus obliquus culture system at appropriate dosages, and the optimal carbon quantum dot concentration is selected to provide a basis for algal cell activity under subsequent enhanced treatment under phenol-containing pollutant stress conditions.

[0030] Example 2: The effect of carbon quantum dots on promoting the degradation of hydroquinone by Scenedesmus obliquus Scenedesmus obliqueis, in its logarithmic growth phase, was selected as the experimental algal species and inoculated into standard BG11 medium, with the initial cell concentration adjusted to 1.0 × 10⁻⁶ cells / year. 6 -5.0×10 6 Cells / mL. Water-soluble carbon quantum dots (CQDs) were added to the *Scenedesmus obliquus* culture medium to a final concentration of 10-20 mg / L, preferably 15 mg / L. The CQDs were pre-contaminated with *Scenedesmus obliquus* for 12-24 h. Three experimental groups with a concentration of 15 mg / L and one blank group without CQDs were designed. The culture medium composition, initial algal density, culture volume, temperature, light mode, and shaking conditions were kept consistent for each group.

[0031] After adding hydroquinone-containing wastewater, the initial concentration of hydroquinone in the reaction system was adjusted to 50 mg / L. The reaction system was then placed in a suitable microalgae growth environment at a temperature of 20-30℃ and a pH of 6.0-8.5. A light-dark alternating culture method was used, with a preferred light-dark ratio of 1:1, for 12 hours each time. Continuous low-speed shaking culture was also performed at 100-120 rpm. The preferred treatment cycle was 10 days, with a 48-hour monitoring cycle to detect hydroquinone concentration and TOC, among other indicators.

[0032] After treatment, the biomass of *Scenedesmus obliquus* can be recovered through sedimentation, centrifugation, filtration, or membrane separation. The supernatant can be discharged in compliance with standards, further treated, or reused. Hydroquinone residual concentration and TOC, among other indicators, can be used to evaluate the treatment effect, but are not mandatory process steps in this invention.

[0033] The hydroquinone liquid chromatogram on day 10 is shown below. Figure 2 As shown, a, b, and c are the 15 mg / L CQDs group, and d is the CQD-free group.

[0034]

[0035] The results of Example 2 show that no obvious hydroquinone characteristic peaks were observed in the liquid phase samples of each treatment group, indicating that the residual concentration of hydroquinone parent material has been significantly reduced or is below the detectable level under the current detection conditions. Since no peaks were observed in the experimental group without the addition of CQDs, the possibility of hydroquinone being converted into organic matter cannot be ruled out. Therefore, it is necessary to conduct a combined analysis based on the TOC results.

[0036] The results showed that on day 10, the TOC concentration in each group was significantly lower than the theoretical organic carbon level corresponding to 50 mg / L hydroquinone (including other organic matter), indicating a significant reduction in the organic carbon load of the system. Furthermore, the addition of CQDs significantly promoted the degradation of organic matter by microalgae, increasing the degradation rate by approximately 20%. These results demonstrate that the water-soluble carbon quantum dot synergistic system with *Scenedesmus obliquus* can promote the removal of hydroquinone precursors and reduce the organic carbon load of the system.

[0037] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for enhancing the degradation of wastewater by *Scenedesmus obliquus* using carbon quantum dots, characterized in that: Includes the following steps: S1. Microalgae were inoculated into the culture medium and cultured to the logarithmic growth phase. Without the addition of carbon quantum dots, the concentration of *Scenedesmus obliquus* cells in the reaction system was brought to 1.0 × 10⁻⁶. 6 -5.0×10 6 When the cell / mL ratio is reached, the culture medium of *Scenedesmus obliquus* to be enhanced is obtained; S2. Set the target cumulative concentration C of carbon quantum dots in the reaction system. t The concentration is 15-25 mg / L. 12-24 hours before adding phenol-containing organic wastewater, 0.3 Ct-0.5 Ct carbon quantum dots are added to the culture medium of *Scenedesmus obliquus* to be enhanced, so that the carbon quantum dots are in pre-contact with *Scenedesmus obliquus*. S3. Add phenol-containing wastewater to the pre-contact treated microalgae culture medium to make the initial concentration of phenolic pollutants in the reaction system reach 25-75 mg / L. Subsequently, carbon quantum dots are added according to the growth status of Scenedesmus obliquus. The mass concentration ratio of the total amount of carbon quantum dots added to the total amount of phenolic pollutants is 0.2-1.

0. S4. Place the reaction system obtained in step S3 in a culture environment with a temperature of 20-30℃, pH of 6.0-8.5 and a light intensity of 2000-6000 lx, and culture it using a light-dark alternation method. The time ratio of the light period to the dark period is 1-2:

1. During the reaction, the oscillation culture method is used to maintain the suspended growth of Scenedesmus obliquus and the mixing of the reaction system. S5. Periodically monitor the physiological state indicators of *Scenedesmus obliquus* and the hydroquinone removal rate. When, within two consecutive monitoring periods, the physiological state of *Scenedesmus obliquus* decreases by more than 50% compared to the previous period, and the hydroquinone removal rate decreases by more than 30% compared to the previous monitoring period, add the remaining unadded carbon quantum dots to the reaction system, with each addition amounting to 0.1C. t -0.3C t And the cumulative concentration of carbon quantum dots added does not exceed C t ; S6. When the hydroquinone removal rate in the reaction system reaches 90% or more, or the remaining hydroquinone concentration is not higher than 5 mg / L, stop adding carbon quantum dots, the treatment ends, and further perform solid-liquid separation on the reaction system to recover the Scenedesmus obliquus biomass and obtain the treated effluent.

2. The method for enhancing the degradation of wastewater by *Scenedesmus obliquus* using carbon quantum dots according to claim 1, characterized in that: The culture medium used is standard BG11 medium. The carbon quantum surface contains one or more hydrophilic functional groups, including hydroxyl, carboxyl and amino groups. The carbon quantum can be dispersed in BG11 medium for 24 hours without visible sedimentation.

3. The method for enhancing the degradation of wastewater by *Scenedesmus obliquus* using carbon quantum dots according to claim 1, characterized in that: The carbon quantum dots exhibit fluorescence emission properties and have a particle size of 1-10 nm.

4. The method for enhancing the degradation of wastewater by *Scenedesmus obliquus* using carbon quantum dots according to claim 1, characterized in that: The target cumulative concentration C of the carbon quantum dots in the reaction system t The concentration was 15 mg / L, and the initial concentration of the phenolic pollutant was 50 mg / L.

5. The method for enhancing the degradation of wastewater by *Scenedesmus obliquus* using carbon quantum dots according to claim 1, characterized in that: In step S4, the reaction system is maintained by continuous low-speed oscillation culture to keep the Scenedesmus obliquus suspended and mixed, with an oscillation speed of 60-120 rpm.

6. The method for enhancing the degradation of wastewater by *Scenedesmus obliquus* using carbon quantum dots according to claim 1, characterized in that: In step S5, the physiological state indicators of *Scenedesmus obliquus* include at least one of cell density, chlorophyll a content, and maximum photochemical efficiency Fv / Fm. A detection cycle is set at 48 hours. When the physiological state indicators of *Scenedesmus obliquus* decrease by 50% compared to the previous detection cycle, or when the removal efficiency of phenolic pollutants decreases by 30%-50% compared to the previous detection cycle, dynamic segmented replenishment is performed.

7. The method for enhancing the degradation of wastewater by *Scenedesmus obliquus* using carbon quantum dots according to claim 1, characterized in that: In step S5, the remaining unadded carbon quantum dots are added to the reaction system in a dynamic segmented manner.

8. An application of the method according to any one of claims 1-7 to enhance the treatment of phenol-containing organic wastewater using carbon quantum dots, characterized in that: The phenol-containing organic wastewater is one of the following: coal chemical wastewater, coking wastewater, or phenolic resin production wastewater.

9. The application of carbon quantum dots to enhance the degradation of wastewater by *Scenedesmus obliquus* according to claim 8, characterized in that: The reaction treatment cycle after mixing the pre-contact treated microalgae culture medium with phenol-containing organic wastewater is 10-15 days.

Images