A method for efficiently dissolving colorants from colored plastics and simultaneously recycling carbon resources

Abstract

This invention belongs to the field of solid waste disposal technology and discloses a method for efficiently leaching colorants from colored plastics and simultaneously recovering carbon resources. The method includes the following steps: after crushing and sieving the colored plastics, organic acids are added, and a photocatalytic reaction is carried out under irradiation conditions. Under the action of the organic acids, the colorants are dissolved. After the reaction, the product is post-treated to obtain a recycled plastic matrix, thus realizing the leaching of colorants and the recovery of carbon resources. The low-molecular-weight organic acids in this invention all contribute to the release of lead chromate from the plastics. Specifically, under the action of 0.1 mol / L tartaric acid, 100% of the lead chromate phase decomposes and is released into the liquid phase, thereby efficiently recovering carbon-rich polyethylene plastics. This invention uses a simple and green photocatalytic strategy to achieve the complete liquid-phase conversion of highly chemically stable lead chromate and the simultaneous recovery of carbon-rich polyethylene plastics, solving the problems of low extraction rates in traditional wet leaching and high carbon and lead dust emissions from pyrometallurgical processes.

Description

Technical Field

[0001] This invention belongs to the field of solid waste disposal technology, specifically relating to a method for efficiently leaching colorants from colored plastics and simultaneously recovering carbon resources. Background Technology

[0002] Given human demand for the aesthetics and versatility of plastics, a wide variety of inorganic and organic colorants are added during plastic synthesis, resulting in the widespread availability of colored plastics (yellow, red, black, etc.) globally. In 2023, global plastic production reached 413.8 million tons, and is projected to reach 1.1 billion tons annually by 2040. This large-scale use of plastics inevitably generates a massive amount of waste plastic. It has been reported that yellow and brown plastics account for up to 26% of the plastics floating in the ocean, and orange microplastics have been found in human placenta, closely related to lead chromate and its variant colorants. The global market for lead chromate colorants accounts for 3% of global lead consumption annually, ranking second. Alarmingly, only 14% of plastics are currently recycled; the remainder is incinerated, landfilled, or leaked. After weathering, these lead chromate colorants can be ingested, inhaled, and absorbed by humans and marine life through the mouth, posing a serious threat to both. There is an urgent need for the recycling or upgrading of lead chromate-colored plastics.

[0003] Currently, there are few reports on the recycling of lead chromate-colored plastics, mainly focusing on the acid leaching of lead chromates. However, its stable structure (from 10...) -12.60 Up to 10 -10.71 This leads to relatively low efficiency in existing hydrometallurgical leaching processes. For example, Gao et al. revealed in the journal EST that the leaching rate of lead chromate was only 11.28% after 7 hours. Previous studies have shown that lead chromate is photochemically active and is activated by sunlight, exhibiting weak photodissolution behavior. This is attributed to the rapid recombination of photogenerated carriers and the low utilization rate of photogenerated electrons, resulting in low decomposition efficiency of lead chromate. Related studies have shown that the propagation and fragmentation of surface cracks in plastics increase the surface area, thereby accelerating the release of Pb(II) and Cr(VI), with higher release efficiency in acidic environments than in neutral or alkaline conditions. However, the complete degradation of plastics takes 200 to 500 years, making current methods inefficient and greatly limiting the reusability of carbon-rich plastics.

[0004] Given these challenges, developing more “green” strategies—that is, efficiently leaching lead chromate from lead chromate-colored plastics while simultaneously recycling carbon-rich plastics under milder or even more environmentally friendly conditions—has become an urgent research focus. Summary of the Invention

[0005] In order to overcome the shortcomings and deficiencies of the prior art, this invention discloses a method for efficiently leaching colorants from colored plastics and simultaneously recovering carbon resources. It uses low-molecular-weight organic acids to regulate the photochemical activity of lead chromate to achieve efficient leaching of lead and chromium, while simultaneously recovering the plastics efficiently.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A method for efficiently leaching colorants from colored plastics and simultaneously recovering carbon resources includes the following steps:

[0008] After being crushed and sieved, colored plastics are added with organic acids and subjected to photocatalytic reaction under irradiation conditions. Under the action of organic acids, the colorants in the colored plastics are dissolved. After the reaction is completed, the product is post-processed to obtain recycled plastic matrix, thus realizing the dissolution of colorants and the recovery of carbon resources.

[0009] Preferably, the organic acid is at least one of tartaric acid, succinic acid, citric acid, malic acid, 2,3-dihydroxysuccinic acid, or glutaric acid.

[0010] Preferably, the organic acid is at least one of tartaric acid, citric acid, or malic acid.

[0011] Preferably, the concentration of the organic acid is 0.05~0.75 mol / L, and the mass-to-volume ratio of the plastic to the organic acid is 1:5~15 mg / mL.

[0012] Preferably, the photocatalytic reaction takes 1 to 24 hours, more preferably 3 to 12 hours.

[0013] Preferably, the photocatalytic reaction is carried out in a photocatalytic reactor equipped with circulating condensate to maintain a constant reaction temperature.

[0014] Preferably, the particle size of the sieve is 30-250 mesh, more preferably 100-150 mesh.

[0015] Preferably, the irradiation conditions are: power of 200~500W and illuminance of 500~2000lx.

[0016] Preferably, the plastic matrix of the colored plastic is one of polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyester, or polyurethane.

[0017] Preferably, the colorant is PbCrO4 or PbCr 1-x S x O4 or PbCr 1-x Mo x At least one of O4, wherein 0 < x < 1.

[0018] Preferably, the post-treatment is filtration, washing and drying, wherein the drying is performed at 80-100°C for 12-24 hours, more preferably at 80°C for 12 hours.

[0019] Preferably, the crushing method is a crusher or a ball mill.

[0020] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0021] (1) This method is simple to operate, green and sustainable, does not use strong acids or alkalis, is pollution-free, has low energy consumption, and avoids the problems of lead dust and high carbon dioxide emissions from pyrometallurgical processes.

[0022] (2) The present invention can decompose 100% lead chromate phase and release it into the liquid phase under the action of only 0.1 mol / L tartaric acid, thereby efficiently recovering carbon-rich polyethylene plastic.

[0023] (3) The efficient detoxification (reduction to trivalent chromium) of chromate in this invention is achieved by the direct reduction of photogenerated electrons and the deoxygenation coupled electron reduction of chromate by superoxide radicals. Attached Figure Description

[0024] Figure 1 The sample phase (a) and structural composition (b) are shown.

[0025] Figure 2 ICP-OES images of samples with different mesh sizes.

[0026] Figure 3 Images of original samples and samples after crushing, showing samples of different mesh sizes.

[0027] Figure 4 The effects of different mesh sizes (a), different low molecular weight organic acids (b), acid concentrations (c), and different control states (d) on lead leaching rate.

[0028] Figure 5 The structure and composition of the samples before and after the reaction with 0.1 mol / L tartaric acid; the elemental distribution map and elemental composition comparison map (c) of the lead chromate pigment plastic before irradiation (a) and after 12 hours of irradiation (b); and the Fourier transform infrared spectrum (FTIR) (d).

[0029] Figure 6 These are the intrinsic characteristics of the five organic acids.

[0030] Figure 7 Transient photocurrent response of lead chromate pigment plastics under different low-molecular-weight organic acid conditions (a), Nyquist plot (impedance plot) (b), •O2 - EPR plot (c), and the time-dimensional concentration of hexavalent chromium in the reaction system (d). Detailed Implementation

[0031] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings. However, the implementation of the present invention is not limited thereto. For process parameters not specifically noted, conventional techniques can be referred to.

[0032] The photocatalytic reactor is model CEL-HXF300, manufactured by AULTT in China, and is equipped with circulating condensate water. It operates at a power of 300 W and an illuminance of 1000 lx.

[0033] Example 1

[0034] To clarify the specific composition and photochemical activity of the colored plastic samples, XRD, FT-IR, UV-Vis DRS, and ultraviolet photoelectron spectroscopy (UPS) were used for analysis. Figure 1 In Figure 1(a), the sample conforms to the standard XRD pattern card (PDF#73-2059) for PbCrO4 and shows no additional impurity phases, indicating that the main component of the sample is lead chromate. Further, the specific type of plastic was analyzed using FT-IR spectroscopy (Figure 1(b)). 2916 cm⁻¹ -1 and 2846 cm -1 The peak at 1473.3 cm⁻¹ is attributed to the stretching vibration of the CH bond. -1 The peak at 815.9 cm⁻¹ corresponds to the bending vibration of -CH₂, indicating that the plastic in the sample is polyethylene (PE). Furthermore, the peak at 815.9 cm⁻¹... -1 The band at that location corresponds to CrO4. 2- The stretching vibrations observed were consistent with the XRD results. These results indicate that the sample was composed of lead chromate and polyethylene plastic.

[0035] Example 2: Effect of sample size on lead chromate release behavior

[0036] After the colored plastic samples are crushed, they are sieved using sieves of different mesh sizes to obtain different samples. Specifically, the mesh sizes are: 30-50 mesh, 50-100 mesh, 100-150 mesh, 150-200 mesh, and 200-250 mesh. Figure 3 The sieved sample was placed in a photocatalytic reactor with a power of 300W and an irradiance of 1000 lx. 100 mL of 0.1 mol / L tartaric acid was added per 10 mg of sample, and the reaction was carried out under irradiation conditions for 3–12 h. After the reaction, the leaching residue was filtered, washed, and dried at 80–100 °C for 12–24 h to obtain the post-reaction sample, which is the recovered plastic matrix, achieving the leaching of colorant and the recovery of carbon resources.

[0037] Given that the stoichiometry of lead in PbCrO4 is constant, the experiment used lead release as the core evaluation index for heavy metal leaching. The lead leaching rate was calculated using the formula: Lead Leaching Rate = CV / mW, where C is the lead content in the solution, V is the solution volume, m is the mass of lead chromate plastic added in the photocatalytic experiment, and W is the lead content of the lead chromate plastic added in the photocatalytic experiment. The measured lead content in samples with different particle sizes (30-250 mesh) was 43.86 ± 1.17% (mean ± standard deviation). XRD and ICP-OES confirmed that the ball milling process did not significantly change the phase composition of PbCrO4 (*p*> 0.05).

[0038] The irradiation experiment results show (see) Figure 4 In (a) of the paper, there is a non-linear relationship between lead leaching rate and particle size: after 12 hours of reaction, when the particle size increases from 30 mesh to 100-150 mesh, the lead leaching rate increases by 40%; however, when the particle size is further reduced to 250 mesh, the lead leaching rate decreases by 15%. This phenomenon may be due to the dual effect of surface area: the increase in surface area (from 30 mesh to 150 mesh) increases the number of reaction sites, but excessively fine particles hinder the transmission of light / mass due to agglomeration.

[0039] Example 3: Effects of different low-molecular-weight organic acids on lead chromate release behavior

[0040] Low-molecular-weight organic acids (0.1 mol / L) were used, namely citric acid (C6H8O7), tartaric acid (C4H6O6), malic acid (C4H6O5), succinic acid (C4H6O4), and glutaric acid (C5H8O4). The sample particle size was 100-150 mesh. 100 mL of the above low-molecular-weight organic acids was added for every 10 mg of sample. The results showed that under the above five low-molecular-weight organic acid systems, the lead leaching amount increased with increasing reaction time. Tartaric acid showed a higher leaching rate and achieved complete dissolution of lead after 12 hours. Figure 4 In (b) of the study, the average leaching efficiencies of succinic acid, glutaric acid, malic acid and citric acid were 31.72%, 44.85%, 69.10% and 86.86%, respectively.

[0041] Example 4: Effect of low molecular weight organic acid concentration on lead chromate release behavior

[0042] Tartaric acid concentrations of 0.05 mol / L, 0.75 mol / L, and 0.1 mol / L were used, with sample particle sizes of 100–150 mesh. 100 mL of tartaric acid at these concentrations was added per 10 mg of sample. Results showed that a decrease in tartaric acid concentration significantly reduced lead leaching. Figure 4(c)). Considering the lead leaching rate and energy consumption, 0.1 mol / L tartaric acid is the preferred condition.

[0043] Comparative Example 1

[0044] To clarify the key influencing factors or synergistic effects of the system, a series of comparative experiments were conducted.

[0045] The sample particles were 100-150 mesh. Under acid-free, dark conditions (essentially equivalent to a natural aging process), after 12 hours of natural aging, the lead leaching rate was less than 5%. Figure 4 (d) in the middle.

[0046] Comparative Example 2

[0047] The sample particles were 100-150 mesh in size. Without the introduction of low molecular weight organic acids, the lead leaching rate was only 18% after 12 hours of illumination at a power of 300 W and an illuminance of 1000 lx, demonstrating the necessity of low molecular weight organic acids.

[0048] Comparative Example 3

[0049] In a dark environment, with sample particles of 100-150 mesh, 100 mL of 0.1 mol / L tartaric acid was added to every 10 mg of sample. After acid leaching for 12 hours, the lead leaching rate only increased to 52%, indicating the necessity of light leaching.

[0050] The synergistic effect of light and tartaric acid can significantly increase the lead leaching rate to 99%, which confirms the significant synergistic effect of acid and light on the release of heavy metals from lead chromate colored plastics.

[0051] Test Example 1

[0052] Based on the optimized reaction conditions (tartaric acid at a concentration of 0.1 mol / L and 100-150 mesh for 12 hours), the samples before and after the reaction were analyzed by SEM-EDS and infrared spectroscopy. Before the reaction, the lead chromate-colored plastic exhibited a sheet-like structure, and after the reaction, it remained predominantly sheet-like, but with varying degrees of bending at the edges. Figure 5 (a) and (b) in the text). More notably, EDS analysis showed that chromium was not detected in the post-reaction samples. Figure 5 (c) in the figure, which is in high agreement with the FTIR spectrum results. Figure 5 (d)). A small amount of oxygen is still present in the oxygen distribution area, which may be related to plastic oxidation, but it does not change the structure of polyethylene plastic. Figure 5 (b) and (d) in the text.

[0053] Test Example 2

[0054] To clarify the specific effects of acids, the intrinsic characteristics of five acids (succinic acid, glutaric acid, malic acid, citric acid, and tartaric acid) were first analyzed. Figure 6 Structural analysis revealed that the number of hydroxyl groups in each acid was 2 (succinic acid), 2 (glutaric acid), 3 (malic acid), 4 (citric acid), and 4 (tartaric acid), respectively, while the number of carboxyl groups was 2, 2, 2, 3, and 2, respectively. The total number of functional groups in each acid was 4, 4, 5, 7, and 6, respectively. At a concentration of 0.1 mol / L, the pH values ​​of each acid solution exhibited a significant gradient: succinic acid (2.61) > glutaric acid (2.53) > dl-malic acid (2.19) > citric acid (2.05) > tartaric acid (2.00). These results indicate that the leaching rate of lead chromate is related to the total amount and type of functional groups in the acid, as well as pH.

[0055] Furthermore, the effects of five low-molecular-weight organic acids on the photochemical properties of lead chromate were analyzed. The results showed that, under simulated sunlight conditions, all five low-molecular-weight organic acids significantly increased the instantaneous photocurrent of the system, indicating an enhanced intensity of photogenerated electrons compared to the acid-free control group. Figure 7 (a)). Meanwhile, the impedance diagram shows a decrease in charge transfer resistance (Rct) at the lead chromate / electrolyte interface. Figure 7 (b)). The overall enhancement trend is as follows: tartaric acid > citric acid > malic acid > succinic acid > glutaric acid to promote interfacial electron transfer. To clarify the reactive oxygen species components of the system, spin-trapped electron paramagnetic resonance (ESR) analysis was performed. The results showed that organic acids promoted the electron reduction of dissolved oxygen to generate superoxide radicals, and this trend was consistent with that of photocurrent / electrochemical impedance spectroscopy (EIS). Figure 7 (c)). It is well known that hexavalent chromium (Cr(VI)) has a high redox potential of +1.33 V, thus readily undergoing reduction reactions. The reduction potential of the hexavalent chromium / trivalent chromium (Cr(VI) / Cr(III)) redox system decreases with increasing pH; at 25°C, it decreases by 0.138 V for every pH increment, thus photogenerated electrons are sufficient to support its reduction. However, the presence of hexavalent chromium during chromium detection occurs over time. Figure 7 (d) indicates that the system is not simply an electronic reduction process. Combining the above results, the reduction of chromate to trivalent chromium is achieved by a combination of direct reduction by photogenerated electrons and electron reduction coupled with superoxide radical deoxygenation of chromate.

[0056] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for efficiently leaching colorants from colored plastics and simultaneously recovering carbon resources, characterized in that, Includes the following steps: After being crushed and sieved, colored plastics are added with organic acids and subjected to photocatalytic reaction under irradiation conditions. Under the action of organic acids, the colorants in the colored plastics are dissolved. After the reaction is completed, the product is post-processed to obtain recycled plastic matrix, thus realizing the dissolution of colorants and the recovery of carbon resources.

2. The method for efficiently leaching colorants from colored plastics and simultaneously recovering carbon resources according to claim 1, characterized in that, The organic acid is at least one of tartaric acid, succinic acid, citric acid, malic acid, 2,3-dihydroxysuccinic acid, or glutaric acid.

3. The method for efficiently leaching colorants from colored plastics and simultaneously recovering carbon resources according to claim 2, characterized in that, The organic acid is at least one of tartaric acid, citric acid, or malic acid.

4. The method for efficiently leaching colorants from colored plastics and simultaneously recovering carbon resources according to claim 1, characterized in that, The concentration of the organic acid is 0.05~0.75mol / L, and the mass-to-volume ratio of the plastic to the organic acid is 1:5~15 mg / mL.

5. The method for efficiently leaching colorants from colored plastics and simultaneously recovering carbon resources according to claim 1, characterized in that, The photocatalytic reaction takes 1 to 24 hours.

6. The method for efficiently leaching colorants from colored plastics and simultaneously recovering carbon resources according to claim 1, characterized in that, The particle size of the sieve is 30 mesh to 250 mesh.

7. The method for efficiently leaching colorants from colored plastics and simultaneously recovering carbon resources according to claim 1, characterized in that, The irradiation conditions are: power of 200~500W and illuminance of 500~2000lx.

8. The method for efficiently leaching colorants from colored plastics and simultaneously recovering carbon resources according to claim 1, characterized in that, The plastic matrix of the colored plastic is one of polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyester, or polyurethane.

9. The method for efficiently leaching colorants from colored plastics and simultaneously recovering carbon resources according to claim 1, characterized in that, The colorant is PbCrO4, PbCr 1-x S x O4 or PbCr 1-x Mo x At least one of O4, wherein 0 < x < 1.

10. The method for efficiently leaching colorants from colored plastics and simultaneously recovering carbon resources according to claim 1, characterized in that, The post-treatment includes filtration, washing, and drying, with the drying process being 80-100℃ for 12-24 hours.

Images