Method for removing nano plastic in water based on in-situ oxidation coagulation process

Abstract

The invention provides a method for removing nano plastics in water based on an in-situ oxidation coagulation process, which comprises the following steps: preparing a ferrous coagulant: adding alkali liquor into a ferrous solution to obtain the ferrous coagulant; preparing an oxidant solution and activating the oxidant solution; in-situ oxidation coagulation: under a stirring condition, simultaneously adding the activated oxidant solution and a ferrous coagulant into the to-be-treated water body containing the nano plastic, controlling the feeding ratio of the ferrous coagulant to the oxidant solution so that the mass concentration ratio of Fe to the oxidant is in a range of 1: 0.05 to 1: 0.3, and carrying out in-situ oxidation coagulation; according to the method, the ferrous coagulant is added into the nano plastic particles, so that ferrous ions in the ferrous coagulant are converted into iron ions, and the iron ions are adsorbed on the surfaces of the nano plastic particles and are rearranged and crystallized to form flocculate with a sheet-shaped or flower-shaped flexible nanosheet structure, so that the nano plastic monomer particles are wrapped and efficiently removed.

Description

Technical Field

[0001] The present disclosure belongs to the technical field of water treatment, and particularly relates to a method for removing nanoplastics in water based on an in-situ oxidation coagulation process. Background Art

[0002] Plastic materials are widely used due to their characteristics such as light weight, durability, and low cost. However, the nanoplastics (NPs) generated by their degradation have become new environmental pollutants due to their small particle size (usually <100 nm), high chemical stability, negatively charged surface, and easy penetration of biological barriers. NPs are widely present in drinking water systems, natural waters, and groundwater, and can enter the human body through the drinking water route, inducing damage to the oxidative stress, inflammatory response, and neuroendocrine system, posing a severe threat to the ecosystem and public health.

[0003] Coagulation is the most widely used pollutant removal technology in water treatment, mainly relying on the hydrolysis of aluminum salts (such as aluminum sulfate) or iron salts (such as ferric chloride) to form charged flocs, and settling pollutants through adsorption-entrapment. However, the traditional coagulation process faces many key bottlenecks when treating NPs. For example: NPs are difficult to be effectively captured by conventional flocs due to their extremely small particle size, strong dispersibility, and density close to water (1.02 - 1.2 g / cm 3 ); at the same time, the treatment efficiency of common flocculants is low and they are prone to produce toxic side effects; and in related technologies, the microscopic morphology of flocs is not regulated, and the spherical flocs have insufficient adsorption sites for NPs, making it difficult to overcome the colloidal stability of NPs.

[0004] In summary, due to the structural defects, chemical risks, and lack of morphological control of flocs, the traditional coagulation process is difficult to efficiently remove nanoplastics in water. Therefore, developing an efficient, safe, and applicable nanoplastics removal technology for actual water bodies has become an urgent topic in the field of water treatment. Summary of the Invention

[0005] In view of this, to solve at least one technical problem in the related art and other aspects, the present disclosure proposes a method for removing nanoplastics in water based on an in-situ oxidation coagulation process, including:

[0006] Preparing a ferrous coagulant: adding an alkali solution to a ferrous solution to obtain a ferrous coagulant;

[0007] Preparing an oxidant solution and activating the oxidant solution;

[0008] In-situ oxidation coagulation: Under stirring conditions, the activated oxidant solution and ferrous coagulant are simultaneously added to the water body to be treated containing nanoplastics. Among them, the feeding ratio of the ferrous coagulant to the oxidant solution is controlled so that the mass concentration ratio of Fe to the oxidant is in the range of 1:0.05 to 1:0.3, thereby converting ferrous ions in the ferrous coagulant into ferric ions, and enabling the ferric ions to adsorb on the surface of the nanoplastic particles and rearrange and crystallize to form flocculants with a flaky or flower-shaped flexible nanosheet structure.

[0009] According to an embodiment of the present disclosure, in the ferrous solution, the dissolved oxygen concentration is less than 1 mg / L, and the concentration of ferrous ions is 1 - 5 mol / L.

[0010] According to an embodiment of the present disclosure, the dissolved oxygen concentration of the alkali solution is less than 1 mg / L.

[0011] According to an embodiment of the present disclosure, in the ferrous coagulant, the ratio of the [OH - concentration to the Fe ion concentration is (0.5:1) - (2:1).

[0012] According to an embodiment of the present disclosure, the operation of adding the alkali solution to the ferrous solution is carried out under a nitrogen atmosphere and under stirring conditions with a rotation speed of 400 - 600 rpm.

[0013] According to an embodiment of the present disclosure, the oxidant includes any one of sodium percarbonate, persulfate, or hydrogen peroxide;

[0014] The concentration of the oxidant solution is 1 - 5 g / L.

[0015] According to an embodiment of the present disclosure, the activated oxidant solution includes:

[0016] An acid solution is added to the oxidant solution to make the pH of the oxidant solution less than 4.

[0017] According to an embodiment of the present disclosure, the dosing concentration range of the ferrous coagulant is 0.05 - 0.4 mmol / L.

[0018] According to an embodiment of the present disclosure, the stirring includes: first stirring at a rotation speed of 100 - 300 rpm for 30 s - 1 min, and then stirring at a rotation speed of 20 - 70 rpm for 30 min.

[0019] According to an embodiment of the present disclosure, the addition rates of the activated oxidant solution and the ferrous coagulant are the same, and the addition rate is 0.1 - 0.3 mL / min.

[0020] According to an embodiment of the present disclosure, the present disclosure is based on a charge transfer mechanism and, through synergistic regulation of oxidation and coagulation, in-situ generates flexible flocs with a flaky or flower-like structure on the surface of nanoplastics, and attempts to improve the coagulation performance by regulating the type and oxidation state of the iron salt coagulant, especially to improve its performance in removing pollutants led by nanoplastics. Specifically, the directional activation of the oxidant and the synchronous dosing of the ferrous coagulant, on the one hand, inhibit the damage of the strong oxidant to the morphology of the flocs and maintain the growth of flaky crystals, and on the other hand, promote the in-situ conversion of Fe 2+ to Fe 3+ , enabling the direct hydrolysis and rearrangement of iron ions on the surface of nanoplastics to form a tightly coated floc-pollutant complex, strengthening the separation effect. Among them, the flexible floc structure with a flaky or flower-like structure has a high specific surface area and flexible coating characteristics, significantly enhancing the adsorption sites and encapsulation ability for nanoplastics, and overcoming the problem of low capture efficiency caused by the compact structure of traditional spherical flocs. Description of the Drawings

[0021] Figure 1 is a flowchart of the method for removing nanoplastics in water based on the in-situ oxidation coagulation process in the embodiments of the present disclosure;

[0022] Figure 2 is a microscopic electron micrograph in Test Example 1 of the present disclosure. Among them, a is the microscopic electron micrograph of the flocs after treatment with Fe(III) solution in Comparative Example 1, b is the microscopic electron micrograph of the flocs after treatment with Fe(II) solution in Comparative Example 2, c is the microscopic electron micrograph of the flocs after treatment with a coagulant with Fe(III) solution:Fe(II):SPC = 1:0.05 in Example 1, and d is the microscopic electron micrograph of the flocs after treatment with a coagulant with Fe(II):SPC = 1:0.5 in Example 1;

[0023] Figure 3 is the floc growth curve showing the change in the macroscopic size of the flaky flocs over time in Test Example 1 of the present disclosure;

[0024] Figure 4 is a comparison chart of the overall turbidity of the supernatant in the water body, the turbidity contributed by nanoplastics, and the zeta potential of the flocs in Test Example 1 of the present disclosure;

[0025] Figure 5 is the floc growth curve graph showing the change in the macroscopic size of the reaction flocs over time under different dosing amounts in Example 2 of the present disclosure;

[0026] Figure 6 is the floc growth curve graph showing the change in the macroscopic size of the reaction flocs over time under different dosing amounts in Comparative Example 3 of the present disclosure;

[0027] Figure 7It is a comparison chart of the overall turbidity of the supernatant in the treated water body, the turbidity contributed by nanoplastics, and the zeta potential of the flocs in Example 2 of the present disclosure;

[0028] Figure 8 It is a comparison chart of the overall turbidity of the supernatant in the water body, the turbidity contributed by nanoplastics, and the zeta potential of the flocs after treating water bodies with different pH values in Test Example 3 of the present disclosure;

[0029] Figure 9 It is a comparison chart of the overall turbidity of the supernatant in the water body, the turbidity contributed by nanoplastics, and the zeta potential of the flocs when treating polluted water bodies containing fulvic acid in Test Example 4 of the present disclosure;

[0030] Figure 10 It is a comparison chart of the overall turbidity of the supernatant in the water body, the turbidity contributed by nanoplastics, and the zeta potential of the flocs when treating polluted water bodies containing humic acid in Test Example 4 of the present disclosure;

[0031] Figure 11 It is a comparison chart of the overall turbidity of the supernatant in the water body, the turbidity contributed by nanoplastics, and the zeta potential of the flocs when treating polluted water bodies containing dissolved organic matter in Test Example 4 of the present disclosure. Detailed Implementation Modes

[0032] To make the objectives, technical solutions, and advantages of the present disclosure clearer and more understandable, the following further elaborates on the present disclosure in detail with reference to specific embodiments and the accompanying drawings.

[0033] In the ranges disclosed in the present disclosure, the endpoints and any values are not limited to the precise ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in the present disclosure.

[0034] The terms used herein are merely for describing specific embodiments and are not intended to limit the present disclosure. The terms "including", "comprising", etc. used herein indicate the presence of the described features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0035] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification and should not be interpreted in an idealized or overly rigid manner.

[0036] It should be noted that, unless otherwise defined, the technical terms or scientific terms used in this disclosure should have the ordinary meaning understood by those with ordinary skills in the field to which this disclosure belongs. If descriptions such as "first", "second", etc. are involved throughout the text, these "first", "second", etc. descriptions are only used to distinguish similar objects, and cannot be understood as indicating or implying their relative importance, sequence, or implicitly indicating the quantity of the technical features indicated. It should be understood that the data described as "first", "second", etc. can be interchanged under appropriate circumstances.

[0037] Similarly, in order to streamline this disclosure and help understand one or more of the various disclosed aspects, in the above description of the exemplary embodiments of this disclosure, the various features of this disclosure are sometimes grouped together into a single embodiment, figure, or description thereof. Descriptions referring to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples", etc. mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of this disclosure. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0038] In addition, the technical solutions between various embodiments can be combined with each other, but it must be based on the ability of those of ordinary skill in the art to implement. When the combination of technical solutions results in contradictions or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection required by this disclosure.

[0039] Among the existing flocculants, the spherical-like nanoparticles formed by typical trivalent iron salts have a compact structure and limited specific surface area, with insufficient coating ability for NPs. The removal rate under low-concentration conditions is usually lower than 50%. For example, when treating sewage with trivalent iron salts at a concentration of 0.2 mmol / L, the removal rate of nanoplastics is only about 37.5%. Residual aluminum ions (Al 3+ ) in aluminum salts have the risk of neurotoxicity, while organic polymer coagulants (such as polyacrylamide and chitosan) are prone to cause microbial proliferation and the formation of disinfection by-products. Traditional coagulation strategies face significant limitations in the treatment of nano-pollutants, and there is an urgent need for innovation in mechanisms and material structures.

[0040] During the implementation of this disclosure, it was found that: Fe 2+ exhibits different behaviors during the hydrolysis precipitation process and can form hydroxide particles with various shapes, including flaky, needle-like, and spherical. This phenomenon is attributed to the geochemical mechanism called the charge transfer process: Fe 2+ ions can transfer electrons to Fe 3+ which has been oxidized.In primary particles, induced atomic rearrangement transforms amorphous particles into a more ordered crystal structure. As the particle morphology changes significantly, from tiny spherical to flaky or needle-like structures, the coagulation behavior of Fe salts also changes significantly. Factors such as the presence of organic matter and redox potential also affect the morphology of Fe particles through this process. Oxidants such as potassium permanganate and sodium hypochlorite, due to their strong oxidizing properties, will destroy the subsequent growth of nanosheets while accelerating the hydrolysis and precipitation of Fe 2+ While accelerating the hydrolysis and precipitation, they will destroy the subsequent growth of nanosheets; on the contrary, weak oxidizing agents can maintain the flaky characteristics while accelerating the in-situ formation of flocs. Therefore, a reasonable choice of weak oxidizing agents is of great significance for controlling crystal growth.

[0041] Figure 1 It is a flowchart of the method for removing nanoplastics in water based on the in-situ oxidation coagulation process in the embodiments of the present disclosure.

[0042] The present disclosure proposes a method for removing nanoplastics in water based on the in-situ oxidation coagulation process, as Figure 1 shown, including the following steps S101 - step S103:

[0043] Step S101: Prepare a ferrous coagulant: Add an alkali solution to a ferrous solution to obtain a ferrous coagulant;

[0044] Step S102: Prepare an oxidant solution and activate the oxidant solution;

[0045] Step S103: In-situ oxidation coagulation: Under stirring conditions, simultaneously add the activated oxidant solution and the ferrous coagulant to the water body to be treated containing nanoplastics, wherein the feeding ratio of the ferrous coagulant to the oxidant solution is controlled so that the mass concentration ratio of Fe:oxidant is in the range of 1:0.05 to 1:0.3, thereby converting ferrous ions in the ferrous coagulant into ferric ions, and making the ferric ions adsorb on the surface of nanoplastics particles and rearrange and crystallize to form flocs with a flaky or flower-like flexible nanosheet structure.

[0046] According to the embodiments of the present disclosure, based on the charge transfer mechanism, through the synergistic regulation of oxidation - coagulation, flexible flocs with a flaky or flower-like structure are in-situ generated on the surface of nanoplastics, and an attempt is made to improve the coagulation performance by regulating the type and oxidation state of the ferric coagulant, especially to improve its performance in removing pollutants led by nanoplastics. Specifically, the directional activation of the oxidant and the synchronous addition of the ferrous coagulant, on the one hand, inhibit the destruction of the floc morphology by strong oxidants and maintain the growth of flaky crystals, and on the other hand, promote the conversion of Fe 2+ to Fe 3+In-situ transformation enables iron ions to be directly adsorbed on the surface of nanoplastics and hydrolyze and rearrange, forming a tightly coated floc-pollutant complex to enhance the separation effect. Among them, the flexible floc structure with a sheet-like or flower-like structure has a high specific surface area and flexible coating characteristics, significantly enhancing the adsorption sites and encapsulation ability for nanoplastics, and overcoming the problem of low capture efficiency caused by the compact structure of traditional spherical flocs.

[0047] According to an embodiment of the present disclosure, the method proposed by the present disclosure abandons the traditional idea of adding additional organic or inorganic coagulants to achieve the improvement of the required NP removal rate, but instead selects to overcome the limitations of traditional coagulation methods in NP removal by regulating the crystal structure of the precipitated particles in the flocs. Compared with traditional Fe(III) coagulants, this method shows better nanoplastics removal effect in actual water bodies (such as surface water, drinking water sources).

[0048] According to an embodiment of the present disclosure, in the ferrous solution, the dissolved oxygen concentration is less than 1 mg / L, and the concentration of ferrous ions is 1-5 mol / L.

[0049] According to an embodiment of the present disclosure, dissolved oxygen (DO) is a natural oxidant. If DO≥1 mg / L, it will cause uncontrolled oxidation of ferrous ions (Fe 2+ ) before dosing, consuming effective ferrous and generating ineffective precipitates. Strictly controlling DO<1 mg / L can ensure that the activity of Fe 2 + is retained until the dosing stage of actual use, ensuring the starting reactant concentration of in-situ oxidation coagulation. And a high DO environment will compete with the oxidant solution to consume Fe 2+ , resulting in a decrease in the utilization rate of the oxidant; while a low DO environment can make the oxidant act centrally on the target reaction (Fe 2+ →Fe 3+ transformation), improving the oxidation efficiency. A high-concentration ferrous solution of 1-5 mol / L can provide an adequate iron source to ensure the in-situ generation of a sufficient amount of sheet-like FeOOH flocs on the surface of nanoplastics, overcoming the defect of incomplete floc coating at low concentrations.

[0050] According to an embodiment of the present disclosure, the dissolved oxygen concentration of the lye is less than 1 mg / L.

[0051] According to an embodiment of the present disclosure, if the lye contains oxygen, the generated Fe(OH)3 colloid will become a heterogeneous nucleation site, prompting the subsequent rapid deposition of Fe 2+ oxidation products on its surface, forming aggregates with uneven particle sizes. Low DO lye ensures that the coagulant is a homogeneous ferrous solution, blocking the pre-oxidation failure chain of ferrous, and enabling subsequent oxidation-hydrolysis to start synchronously on the surface of nanoplastics, blocking the pre-oxidation failure chain of ferrous.

[0052] According to an embodiment of the present disclosure, in the ferrous coagulant, [OH- The ratio of the [OH⁻] concentration to the Fe ion concentration is (0.5:1) to (2:1).

[0053] According to an embodiment of the present disclosure, when [OH - / Fe < 0.5, due to insufficient OH - , incomplete hydrolysis of ferrous ions occurs, generating unstable Fe(OH) + intermediates, which are prone to form amorphous Fe(OH)₃ colloids during subsequent oxidation, with weak adsorption ability; when [OH - / Fe > 2, excessive OH - will trigger forced precipitation, directly generating large-sized Fe(OH)₂ precipitates (e.g., particle size > 1 μm), losing the reaction activity. Therefore, controlling the ratio of the two to 0.5 - 2 can maintain ferrous in a soluble hydrolytic complex state (such as Fe(OH) + , Fe₂(OH)₂ 4+ ), providing an ideal precursor for subsequent oxidation-rearrangement.

[0054] According to an embodiment of the present disclosure, the operation of adding an alkali solution to the ferrous solution is carried out under stirring conditions with a rotation speed of 400 - 600 rpm in a nitrogen atmosphere.

[0055] According to an embodiment of the present disclosure, nitrogen coverage forms an inert gas curtain, making the oxygen partial pressure at the gas-liquid interface approach zero, stably suppressing the dissolved oxygen (DO) concentration, eliminating the non-controlled oxidation of Fe 2+ , and preserving the ferrous form. When the rotation speed < 400 rpm, due to insufficient mixing, local pH jump occurs, triggering the explosive nucleation of Fe(OH)₂; when the rotation speed > 600 rpm, the high shear force destroys the stability of the ferrous complex, inducing colloid aggregation. The stirring speed of 400 - 600 rpm precisely balances mixing and shear, ensuring the stability of the complex size.

[0056] According to an embodiment of the present disclosure, the oxidant includes any one of percarbonate, persulfate, or hydrogen peroxide; the concentration of the oxidant solution is 1 - 5 g / L.

[0057] According to an embodiment of the present disclosure, the standard oxidation potentials of the selected oxidants in the present disclosure are all in the mild range of 0.38 - 1.44 V: the standard oxidation potential of hydrogen peroxide is 0.38 V (acidic), the standard oxidation potential of persulfate (generating sulfate radicals after activation) is 1.44 V, and the standard oxidation potential of percarbonate (slowly releasing hydrogen peroxide) is 0.38 V. This potential range can not only drive the conversion of Fe 2+ →Fe 3+ , but also avoid excessive oxidation leading to Fe 3+Octahedral structure collapse (the standard oxidation potential of strong oxidants such as potassium permanganate is 1.51V, which will destroy lattice rearrangement). When the oxidant concentration < 1g / L, Fe 2+ has a low oxidation rate, and the residual Fe 2+ hinders the charge transfer mechanism, resulting in insufficient crystallinity of the flocs; when the concentration > 5g / L, excessive free radicals initiate secondary oxidation (Fe 3+ →Fe2O3), destroying the flake structure. 1-5g / L precisely matches the mass ratio of Fe:oxidant = 1:0.05~1:0.3 (based on complete generation of Fe 3+ ) to ensure the oxidation rate.

[0058] According to an embodiment of the present disclosure, the activated oxidant solution includes: adding an acid solution to the oxidant solution to make the pH of the oxidant solution less than 4.

[0059] According to an embodiment of the present disclosure, acidification activation enhances the reaction activity of the oxidant, enables free radicals to precisely act on the surface modification of nanoplastics, and drives the in-situ directional growth of flake flocs.

[0060] According to an embodiment of the present disclosure, the dosage concentration range of the ferrous coagulant is 0.05 - 0.4mmol / L.

[0061] According to an embodiment of the present disclosure, stirring includes: first stirring at a rotation speed of 100 - 300rpm for 30s - 1min, and then stirring at a rotation speed of 20 - 70rpm for 30min.

[0062] According to an embodiment of the present disclosure, the high-speed shear in the high-speed stage (100 - 300rpm) homogenizes the oxidant and ferrous ions quickly, eliminating the Fe 3+ concentration gradient and avoiding homogeneous nucleation to generate free flocs; the low-shear field in the low-speed stage (20 - 70rpm) is conducive to promoting the edge-face orientation connection of FeOOH nanosheets to form a flower-like multi-level structure, increasing the specific surface area and avoiding the random packing of spherical aggregates. That is, the high-speed stirring stage can achieve in-situ nucleation on the surface of pollutants, and the low-speed stirring stage can construct an open flower-like network through epitaxial attachment. Compared with the dense pellets formed by traditional single-speed stirring, the pollutant interception capacity is improved.

[0063] According to an embodiment of the present disclosure, the addition speeds of the activated oxidant solution and the ferrous coagulant are the same, and the addition speed is 0.1~0.3mL / min.

[0064] According to an embodiment of the present disclosure, the synchronous addition at 0.1~0.3mL / min dynamically locks the stoichiometric ratio of the reactants, thereby matching the diffusion rate of the iron salt and the crystal growth rate, realizing the near-field reaction on the surface of nanoplastic pollutants, and enabling the flake flocs to have atomic-level coating accuracy and single-crystal-level structural uniformity.

[0065] According to the embodiments of the present disclosure, the present disclosure realizes the structural coating and stable removal of nanoplastics in the coagulation stage for the first time by synergistically utilizing the controlled hydrolysis of Fe(II) and the oxidation induction reaction of sodium percarbonate, providing a new, green and stable solution for the efficient removal of micro-pollutants, and having important environmental application prospects.

[0066] It should be noted that the described embodiments are only a part of the embodiments of the present disclosure, rather than all the embodiments. Based on the embodiments in the present disclosure, other embodiments obtained by those of ordinary skill in the art without creative efforts all fall within the scope of protection of the present disclosure.

[0067] Example 1

[0068] Step 1: Prepare ferrous coagulant

[0069] Provide two bottles of 500 mL deionized water, and introduce nitrogen into them for deoxygenation treatment for 30 minutes until the dissolved oxygen concentration drops below 1 mg / L.

[0070] Add ferrous sulfate heptahydrate powder to one of the above-mentioned deoxygenated deionized water bottles, and stir it with a magnetic stirrer at a speed of 500 rpm to fully dissolve it, and prepare a 2 mol / L ferrous sulfate solution. Continuously introduce nitrogen during the stirring process to prevent the oxidation of Fe(II).

[0071] Add sodium hydroxide pellets to the other bottle of deoxygenated deionized water, and also stir it with a magnetic stirrer at a speed of 500 rpm to prepare a 2 mol / L sodium hydroxide solution, and continuously introduce nitrogen to maintain anaerobic conditions. Load the prepared sodium hydroxide solution into an injection pump, and slowly drop it into the ferrous sulfate solution at a speed of 10 mL / min. During this period, keep the stirring rate at 500 rpm to prevent precipitation caused by excessive hydrolysis. At the same time, continuously introduce nitrogen into the ferrous sulfate solution during the whole dropping process.

[0072] After the dropping is completed, a ferrous coagulant (Fe(II) coagulant) with an iron ion concentration of 1 mo / L and a basicity of [OH-]:[Fe] of 1 is obtained.

[0073] Step 2: Prepare oxidant solution and activate it

[0074] Add a quantitative amount of sodium percarbonate powder to 500 mL deionized water, stir and dissolve it, and perform ultrasonic treatment for 10 minutes to prepare a 2.5 g / L sodium percarbonate solution (SPC).

[0075] Adjust the pH of the sodium percarbonate solution to 4 using dilute sulfuric acid to obtain the activated oxidant solution, which is then transferred to a brown glass bottle and stored in the dark for later use.

[0076] Step 3: In-situ oxidation coagulation

[0077] Use a coagulation mixer to rapidly stir 1 L of the contaminated water to be treated at a rotational speed of 200 rpm.

[0078] Synchronously add the Fe(II) solution and the activated sodium percarbonate solution prepared in Step 1 and Step 2 into the water body from the left and right sides of the mixer respectively through syringes at a speed of 0.2 mL / min, so that the final iron ion concentration in the system is 0.2 mmol / L.

[0079] Furthermore, in Example 1, adjust the injection volume of the sodium percarbonate syringe to obtain a series of concentration ratio gradients of Fe: sodium percarbonate (Fe(II):SPC), namely systems of 1:0; 1:0.05; 1:0.1; 1:0.133; 1:0.2; 1:0.3.

[0080] After maintaining rapid stirring at 200 rpm for 30 s, switch to slow stirring at 50 rpm and continue for 30 minutes. After the coagulation is completed, let it stand and settle for 15 minutes to form flocs.

[0081] Comparative Example 1

[0082] Use a coagulation mixer to rapidly stir 1 L of the contaminated water to be treated at a rotational speed of 200 rpm.

[0083] Directly add an iron coagulant (Fe(III) solution) to the contaminated water to be treated at a speed of 0.2 mL / min, so that the final iron ion concentration in the system is 0.2 mmol / L.

[0084] After maintaining rapid stirring at 200 rpm for 30 s, switch to slow stirring at 50 rpm and continue for 30 minutes. After the coagulation is completed, let it stand and settle for 15 minutes to form flocs.

[0085] Comparative Example 2

[0086] Use a coagulation mixer to rapidly stir 1 L of the contaminated water to be treated at a rotational speed of 200 rpm.

[0087] Directly add the ferrous coagulant (Fe(II) solution or Fe(II):SPC = 1:0) prepared in Example 1 to the contaminated water to be treated at a speed of 0.2 mL / min, so that the final iron ion concentration in the system is 0.2 mmol / L.

[0088] After maintaining rapid stirring at 200 rpm for 30 s, switch to slow stirring at 50 rpm and continue for 30 minutes. After the coagulation is completed, let it stand and settle for 15 minutes to form flocs.

[0089] Test Example 1

[0090] The microscopic morphologies of the flocs obtained by removing nano-plastics in water in Example 1 (Fe(II):SPC = 1:0.05, Fe(II):SPC = 1:0.5), Comparative Example 1, and Comparative Example 2 were observed by transmission electron microscopy.

[0091] Figure 2 is the microscopic electron micrograph of the flocs in Test Example 1 of the present disclosure. Among them, a is the microscopic electron micrograph of the flocs after treatment with Fe(III) solution in Comparative Example 1, b is the microscopic electron micrograph of the flocs after treatment with Fe(II) solution in Comparative Example 2, c is the microscopic electron micrograph of the flocs after treatment with the flocculant of Fe(II):SPC = 1:0.05 in Fe(III) solution in Example 1, and d is the microscopic electron micrograph of the flocs after treatment with the flocculant of Fe(II):SPC = 1:0.5 in Example 1.

[0092] As Figure 2 shown in c and d, it can be clearly observed under the transmission electron microscope that by using the in-situ oxidation coagulation process and changing the feeding ratio between the oxidant and the coagulant, the morphology of the formed nanosheets can be controlled. The formed flower-like nanosheet structure can completely wrap the nano-plastic monomers with a particle size of about 200 nm. In contrast, as Figure 2 shown in a and b, only a small amount of spherical particles adhere to the surface of the nano-plastics in the flocs in the control groups using traditional trivalent iron coagulant in Comparative Example 1 and single use of divalent iron coagulant in Comparative Example 2, and effective wrapping and stable binding cannot be achieved.

[0093] Furthermore, the macroscopic sizes of the flocs in Example 1, Comparative Example 1, and Comparative Example 2 were analyzed over time.

[0094] Figure 3 is the floc growth curve of the macroscopic size of the flaky flocs in Test Example 1 of the present disclosure over time.

[0095] As Figure 3 shown, the flocs formed in Example 1 (Fe(II):SPC = 1:0.05 respectively) are significantly superior to the flocs formed by traditional trivalent iron coagulant in terms of macroscopic size. The floc particle size is larger, which is beneficial for sedimentation and efficient adsorption of nano-plastics in water. After being broken by external force disturbance, the in-situ oxidation-generated flocs in Example 1 show stronger elastic recovery ability, can re-aggregate and grow to a larger size in a short time, and exhibit good structural stability, which is suitable for the fluid disturbance conditions that may occur in the actual water treatment process.

[0096] The water bodies after water treatment in Example 1, Comparative Example 1, and Comparative Example 2 were detected.

[0097] Figure 4 It is a comparative graph of the total turbidity of the supernatant, the turbidity contributed by nanoplastics, and the zeta potential of flocs in the water body after treatment in Test Example 1 of the present disclosure.

[0098] As Figure 4 shown, under different dosages of the oxidant solution (sodium percarbonate), the performance of Example 1, Comparative Example 1, and Comparative Example 2 in removing water turbidity and nanoplastics was investigated. The initial turbidity of the simulated wastewater used in the experiment was 11.8 NTU. Under the same dosage of iron salt, the traditional ferric coagulant in Comparative Example 1 could only reduce the water turbidity to 7.08 NTU, while in Example 1, the turbidity could be reduced to 1.64 NTU at most. More importantly, after external force crushing, some of the removed nanoplastics would be released again by the traditional flocs, resulting in a rise in turbidity. However, the flocs formed in Example 1 could still maintain the stable coating of nanoplastics after crushing, achieving efficient and persistent removal of nanoplastics.

[0099] Example 2

[0100] In Example 2, the preparation and activation of the Fe(II) coagulant solution and sodium percarbonate oxidant, and the in-situ oxidation coagulation process were the same as those in Example 1. Different from Example 1, the dosage of the Fe(II) coagulant was changed so that the final iron ion concentrations added to the system were 0.05, 0.1, 0.2, 0.3, and 0.4 mmol / L respectively. At the same time, the injection amount of the sodium percarbonate syringe was adjusted to keep the concentration ratio of Fe: sodium percarbonate in the coagulation system at 1:0.2 all the time.

[0101] Comparative Example 3

[0102] In Comparative Example 3, the Fe(III) coagulant solution and the coagulation process were the same as those in Comparative Example 1. Different from Comparative Example 1, the dosage of the Fe(II) coagulant was changed so that the final iron ion concentrations added to the system were 0.05, 0.1, 0.2, 0.3, and 0.4 mmol / L respectively.

[0103] Test Example 2

[0104] The water bodies after water treatment in Example 2 and Comparative Example 3 were detected.

[0105] Figure 5 、 Figure 6 They are the floc growth curves showing the change of the macroscopic size of the reaction flocs with time under different dosage conditions in Example 2 and Comparative Example 3 of the present disclosure respectively.

[0106] As Figure 5, Figure 6 As shown, the flocs formed in Example 2 are significantly superior to those formed by traditional trivalent iron coagulants in terms of macroscopic size. The floc particle size is larger, which is beneficial for sedimentation and efficient adsorption of nanoplastics in water. And in Example 2, it is verified that by increasing the dosage of the coagulant, the overall size of the flocs can be significantly increased.

[0107] Figure 7 It is a comparison chart of the total turbidity of the supernatant in the treated water body, the turbidity contributed by nanoplastics, and the zeta potential of the flocs in Example 2 of the present disclosure.

[0108] As Figure 7 shown, under different coagulant dosage conditions, the performance of Example 2 in removing water turbidity and nanoplastics is much better than that of traditional trivalent iron coagulants. The initial turbidity of the simulated wastewater used in the experiment is 11.8 NTU. In Example 2, efficient removal of nanoplastics can be achieved at a lower coagulant dosage, effectively alleviating the treatment pressure on flocculant sludge in the later stage.

[0109] Example 3

[0110] In Example 2, the preparation and activation of the Fe(II) coagulant solution and sodium percarbonate oxidant, and the in-situ oxidation coagulation process are the same as those in Example 1. The difference from Example 1 is that the pH of the polluted water body to be treated is 5, 6, 7, 8, and 9 respectively.

[0111] Comparative Example 4

[0112] In Comparative Example 4, the Fe(III) coagulant solution and the coagulation process are the same as those in Comparative Example 1. The difference from Comparative Example 1 is that the pH of the polluted water body to be treated is 5, 6, 7, 8, and 9 respectively.

[0113] Test Example 3

[0114] The water bodies treated in Example 3 and Comparative Example 4 are detected.

[0115] Figure 8 It is a comparison chart of the total turbidity of the supernatant in the water body, the turbidity contributed by nanoplastics, and the zeta potential of the flocs in Test Example 3 of the present disclosure after treating water bodies with different pH values.

[0116] As Figure 8 shown, under the conditions of water bodies with different pH values, Example 3 can achieve efficient removal of water turbidity and nanoplastics, overcoming the disadvantages of poor coagulation effect of traditional trivalent iron coagulants under acidic conditions and many particles under alkaline conditions. Its performance is much better than that of traditional trivalent iron coagulants. The initial turbidity of the simulated wastewater used in the experiment is 11.8 NTU. Example 3 can achieve efficient removal of nanoplastics in a wide pH range.

[0117] Example 4

[0118] In Example 2, the preparation and activation of the Fe(II) coagulant solution and sodium percarbonate oxidant, and the in-situ oxidation coagulation process were the same as those in Example 1. The difference from Example 1 was that a coagulation stirrer was used to rapidly stir 1 L of polluted water to be treated with different types of organic matter (fulvic acid, humic acid, and dissolved organic matter), and the rotation speed was set at 200 rpm.

[0119] Comparative Example 5

[0120] In Comparative Example 4, the Fe(III) coagulant solution and the coagulation process were the same as those in Comparative Example 1. The difference from Comparative Example 1 was that a coagulation stirrer was used to rapidly stir 1 L of polluted water to be treated with different types of organic matter (fulvic acid, humic acid, and dissolved organic matter), and the rotation speed was set at 200 rpm.

[0121] Test Example 4

[0122] The water bodies treated in Example 4 and Comparative Example 5 were detected.

[0123] Figure 9 , Figure 10 , Figure 11 They are respectively the comparative diagrams of the total turbidity of the supernatant in the water body, the turbidity contributed by nanoplastics, and the zeta potential of the flocs when treating polluted water bodies containing fulvic acid, humic acid, and dissolved organic matter in Test Example 4 of the present disclosure.

[0124] As Figure 9 , Figure 10 , Figure 11 shown, in actual water bodies with different types of organic matter present, Example 4 can effectively overcome the hindrance to coagulation caused by organic matter in water and achieve efficient removal of the turbidity and nanoplastics in the water body. In water bodies with various types of organic matter, it shows better treatment performance than the traditional trivalent iron coagulant. It shows that Example 4 can achieve efficient removal of nanoplastics in various actual water bodies.

[0125] In summary, the method for removing nanoplastics from water proposed in this disclosure introduces an activated oxidant (such as percarbonate) during the coagulation process, inducing an in-situ oxidation reaction of Fe(II) and promoting the in-situ generation of a flower-like nanosheet structure, thereby achieving the complete encapsulation of nanoplastics particles. Compared with the single spherical particles formed by traditional ferric flocculants, the multi-layered structure flocs generated by the method proposed in this disclosure have stronger interfacial adsorption and physical embedding capabilities. This in-situ oxidation coagulation method can significantly reduce the turbidity of water bodies. At the same iron dosage, the turbidity can be reduced from 11.8 NTU to 1.64 NTU, far superior to traditional methods. In addition, the flocs formed by the method of this disclosure will not release nanoplastics again after being broken, solving the problem of pollutant re-release caused by the breaking of flocs in traditional coagulation methods, and achieving the efficient, persistent, and irreversible removal of nanoplastics. Further, the nanosheet structure generated by this method makes the macroscopic size of the flocs larger, with better hydrodynamic properties, including elastic recovery and sedimentation. After being disturbed by external forces, the flocs can still maintain their integrity or quickly return to their original state, showing excellent elastic recovery, which is significantly better than the brittle flocs generated by traditional ferric coagulants. At the same time, this method has a wider application scenario than traditional methods and is not easily interfered by the external water body pH and other dissolved organic matters, ensuring the stable removal of nanoplastics during actual application. The method proposed in this disclosure uses normal temperature and pressure conditions, the preparation of the coagulant and oxidant is simple, and the equipment has strong versatility, facilitating integration and application in existing water treatment systems, and having good engineering promotion value.

[0126] The specific embodiments described above further elaborate on the purpose, technical solutions, and beneficial effects of this disclosure. It should be understood that the above are only specific embodiments of this disclosure and are not used to limit this disclosure. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of this disclosure shall be included within the protection scope of this disclosure.

Claims

1. A method for removing nanoplastics in water based on in-situ oxidation coagulation process, comprising: Preparing ferrous coagulant: adding an alkali solution to a ferrous solution to obtain a ferrous coagulant; Preparing an oxidant solution and activating the oxidant solution; In-situ oxidation coagulation: under stirring conditions, simultaneously adding the activated oxidant solution and the ferrous coagulant into the water body to be treated containing nanoplastics, wherein the feeding ratio of the ferrous coagulant to the oxidant solution is controlled so that the mass concentration ratio of Fe to the oxidant is in the range of 1:0.05 to 1:0.3, thereby converting ferrous ions in the ferrous coagulant into ferric ions, and enabling the ferric ions to adsorb on the surface of nanoplastics particles and rearrange and crystallize to form flocculants with a flaky or flower-like flexible nanosheet structure.

2. The method according to claim 1, wherein, In the ferrous solution, the dissolved oxygen concentration is less than 1 mg / L, and the concentration of ferrous ions is 1-5 mol / L.

3. The method according to claim 1, wherein The dissolved oxygen concentration of the alkali solution is less than 1 mg / L.

4. The method according to claim 1, wherein In the ferrous coagulant, the ratio of the concentration of [OH - to the concentration of Fe ions is (0.5:1) to (2:1).

5. The method according to claim 1, wherein The operation of adding the alkali solution to the ferrous solution is carried out under a nitrogen atmosphere and under stirring conditions with a rotation speed of 400-600 rpm.

6. The method according to claim 1, wherein The oxidant includes any one of sodium percarbonate, persulfate or hydrogen peroxide; The concentration of the oxidant solution is 1-5 g / L.

7. The method according to claim 1, wherein Activating the oxidant solution includes: Adding an acid solution to the oxidant solution to make the pH of the oxidant solution less than 4.

8. The method according to claim 6, wherein The dosing concentration range of the ferrous coagulant is 0.05-0.4 mmol / L.

9. The method according to claim 1, wherein The stirring includes: first stirring at a rotation speed of 100-300 rpm for 30 s-1 min, and then stirring at a rotation speed of 20-70 rpm for 30 min.

10. The method according to claim 1, wherein The adding speeds of the activated oxidant solution and the ferrous coagulant are the same, and the adding speed is 0.1-0.3 mL / min.

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