A segmented high-efficiency enrichment foam separation method based on PFAS chain length difference

Abstract

The present application relates to a kind of based on the segment efficient enrichment foam separation method of PFAS chain length difference, the method includes: the water body to be handled containing different chain length, different types PFAS is carried out first stage nanobubble pretreatment and foam separation, forms and collects first foam liquid, so that long-chain PFAS or strong hydrophobic PFAS is preferentially enriched in first foam liquid;Cationic surfactant is added to the water phase after first stage foam separation, and second stage water body to be handled is obtained;The second stage water body to be handled is carried out second stage nanobubble pretreatment and foam separation, forms and collects second foam liquid, so that residual short-chain PFAS and / or medium-chain PFAS is enriched in second foam liquid.The method is by chain length segmented processing, adding cationic surfactant and two-stage nanobubble auxiliary foam separation, realize the overall efficient removal and enrichment of different chain length, different types PFAS, suitable for the separation processing of multiple types PFAS contaminated water body.

Description

Technical Field

[0001] This invention relates to the field of water treatment technology, specifically to a segmented, efficient foam enrichment and separation method based on the difference in PFAS chain length. Background Technology

[0002] Per- and polyfluoroalkyl substances (PFAS) are a class of fluorinated organic pollutants with high stability, high mobility, and environmental persistence, and are widely present in various aquatic media such as groundwater, surface water, wastewater treatment plant effluent, and landfill leachate. Since PFAS are difficult to degrade effectively using conventional biological treatment processes, developing efficient PFAS separation and removal technologies has become an important research direction in the field of water treatment.

[0003] Foam separation is a pollutant separation technology based on gas-liquid interface adsorption. This technology involves blowing gas into a water body to form bubbles, causing interfacially active PFAS (phosphorus alkoxides) to be adsorbed at the gas-liquid interface and float to the liquid surface with the bubbles, forming a foam layer. The foam liquid is then collected by overflow or skimming, thus achieving the enrichment and separation of PFAS. Due to its simple process, low operating cost, and ability to simultaneously enrich pollutants, foam separation is considered one of the important technical routes for PFAS removal.

[0004] However, different PFAS exhibit significantly different enrichment behaviors during foam separation. Long-chain PFAS (such as PFNA and PFDA) are relatively easy to separate and enrich due to their strong hydrophobicity and interfacial activity; while short-chain PFAS (such as PFBA and PFPeA) have higher water solubility and weaker adsorption capacity at the gas-liquid interface, resulting in a significantly lower removal rate than long-chain PFAS, making it difficult to achieve simultaneous and efficient separation of PFAS of different chain lengths. To improve the removal efficiency of short-chain PFAS, existing foam separation systems typically add cationic surfactants. Existing research shows that although cationic surfactants can improve the enrichment and removal of short-chain PFAS, they also undergo ion association and hydrophobic aggregation with some long-chain or strongly hydrophobic PFAS, limiting the migration of these PFAS into the foam phase and making it difficult to simultaneously achieve the removal and enrichment of PFAS of different chain lengths.

[0005] Furthermore, traditional foam separation typically uses microporous aerators to generate millimeter-sized bubbles, which have a relatively limited gas-liquid interface area. The migration of PFAS from the aqueous bulk to the gas-liquid interface mainly relies on molecular diffusion. Therefore, in actual operation, to achieve a high PFAS removal rate, the system usually requires a gas-liquid contact time of about 20-30 minutes, and some systems even require an operating time of more than 60 minutes, significantly reducing treatment efficiency and increasing operating energy consumption. From an engineering perspective, long operating times will further lead to a chain reaction of effects, such as increased foam liquid (concentrated phase) yield, decreased enrichment ratio, and increased load on subsequent destruction / deep treatment.

[0006] The aforementioned problems indicate that while existing PFAS foam separation technologies have improved treatment effects by increasing aeration intensity and adding surfactants, they still struggle to simultaneously achieve efficient removal and high-enrichment of mixed PFAS systems. Existing research has achieved PFAS separation through the synergistic injection of nanobubbles and microbubbles, but this typically involves a one-time treatment of the mixed PFAS system. In this mixed PFAS system, PFAS of different chain lengths compete simultaneously for the gas-liquid interface and the foam film. Long-chain PFAS, due to their stronger hydrophobicity and interfacial activity, preferentially occupy the interface, while short-chain PFAS, due to their higher water solubility and weaker interfacial adsorption capacity, not only have difficulty migrating but may also reduce the effective PFAS loading in the overall foam liquid, making efficient separation of PFAS of different chain lengths difficult. Furthermore, simply increasing aeration intensity, extending operating time, or directly introducing nanobubbles can easily lead to an increase in foam liquid volume or fail to effectively improve the PFAS loading capacity per unit volume of foam liquid. Therefore, this application proposes a segmented, highly efficient enrichment foam separation method based on PFAS chain length differences, fully utilizing the differences in interfacial adsorption behavior of PFAS of different chain lengths to achieve hierarchical separation. Summary of the Invention

[0007] To address the problems of insufficient separation of short-chain PFAS in existing PFAS foam separation technologies, the susceptibility of some long-chain PFAS or strongly hydrophobic PFAS to the influence of cationic surfactants, which restricts their migration to the foam phase, and the large volume and insufficient enrichment ratio of foam liquid, this invention provides a segmented and efficient enrichment foam separation method based on the difference in PFAS chain length.

[0008] The technical solution is as follows: This invention provides a segmented and efficient foam enrichment and separation method based on PFAS chain length differences, comprising the following steps:

[0009] S1: First-stage nanobubble pretreatment: The first-stage nanobubble pretreatment is carried out on the first water body containing PFAS with different chain lengths to disperse the nanobubbles in the aqueous phase.

[0010] S2: First-stage foam separation: Foam separation and collection are performed on the water that has undergone the first-stage nanobubble pretreatment to form the first foam liquid and the first-stage residual liquid;

[0011] S3: Add cationic surfactant: Add cationic surfactant to the remaining liquid of the first stage to obtain the water to be treated in the second stage;

[0012] S4: Second-stage nanobubble pretreatment: The water to be treated in the second stage is subjected to second-stage nanobubble pretreatment, so that the nanobubbles are dispersed in the aqueous phase containing cationic surfactants and residual PFAS.

[0013] S5: Second-stage foam separation: The foam in the pretreated water body with second-stage nanobubbles is separated and collected to form the second foam liquid and the second-stage residual liquid;

[0014] S6: Foam liquid treatment: The first foam liquid and the second foam liquid are respectively processed in the post-processing unit.

[0015] Preferably, the first stage nanobubble pretreatment, the second stage nanobubble pretreatment, the first stage foam separation, and the second stage foam separation all employ a micro / nanobubble generator to generate bubbles;

[0016] Among them, the first stage nanobubble pretreatment and the second stage nanobubble pretreatment control the gas input intensity to generate or introduce nanobubbles in the aqueous phase.

[0017] The first and second stages of foam separation enhance the gas input intensity, continuously generating or introducing nanobubbles in the aqueous phase, while simultaneously forming micron-sized and / or millimeter-sized bubbles capable of carrying nanobubbles to the surface, and forming a continuously collectable foam layer on the liquid surface.

[0018] Preferably, the average particle size of the nanobubbles is less than 1000 nm, more preferably less than 500 nm, and even more preferably less than 200 nm.

[0019] Preferably, the micron-sized bubbles and / or millimeter-sized bubbles have a particle size greater than 1 μm and less than 2 mm.

[0020] Preferably, the termination conditions for the first-stage nanobubble pretreatment and the second-stage nanobubble pretreatment are one or more of the following conditions:

[0021] The concentration of nanobubbles in the aqueous phase is not less than 1×10 7 cells / mL;

[0022] The water's transparency decreases or its turbidity increases.

[0023] No continuously collectable foam layer forms on the liquid surface;

[0024] The nanobubble pretreatment time has reached the set value.

[0025] Preferably, the termination conditions for the first stage of foam separation and the second stage of foam separation are one or more of the following conditions:

[0026] The mass recovery rate of the target PFAS in the foam solution at the corresponding stage is lower than the set threshold.

[0027] The target PFAS concentration or the mass ratio of target PFAS to total PFAS in the aqueous phase after the corresponding stage of treatment is lower than the set threshold.

[0028] The foam liquid generation rate is lower than the set threshold during the corresponding stage;

[0029] The running time of the corresponding stage reaches the set value;

[0030] The foam layer ceases to grow during the corresponding stage.

[0031] Preferably, the target PFAS corresponding to the first foam liquid is a long-chain PFAS or a strongly hydrophobic PFAS;

[0032] The target PFAS of the second foam liquid is short-chain PFAS and / or medium-chain PFAS.

[0033] Preferably, the cationic surfactant includes one or more of hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, quaternary ammonium salt cationic surfactants, or other cationic surfactants that can promote the enrichment of PFAS interfaces.

[0034] Preferably, the first foam liquid and the second foam liquid enter the subsequent processing unit separately or are mixed.

[0035] The subsequent processing unit includes one or more of the following: oxidation, reduction, incineration, adsorption, concentration, mineralization, or surfactant recovery units.

[0036] Preferably, the method is capable of treating polluted water bodies containing one or more of the following: perfluorocarboxylic acid PFAS, perfluorosulfonic acid PFAS, fluoropolymer PFAS, sulfonamide PFAS, or PFAS precursors.

[0037] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0038] (1) The present invention sets up a segmented foam separation process based on the difference in PFAS chain length, so that long-chain PFAS or strongly hydrophobic PFAS preferentially enter the first foam liquid before the cationic surfactant acts, which can reduce the risk that long-chain PFAS will be restricted from migrating to the foam phase due to ion association, hydrophobic aggregation or micelle encapsulation.

[0039] (2) In this invention, the cationic surfactant is added later so that it mainly acts on the short-chain and / or medium-chain PFAS remaining after the first stage of treatment, thereby enhancing the foam separation effect of short-chain and / or medium-chain PFAS and avoiding its premature action on the multi-chain long PFAS mixed system.

[0040] (3) In the first and second stages, the present invention sets up nanobubble pretreatment and continuously generates or introduces nanobubbles in the subsequent foam separation process, so that a dispersed nanobubble system is formed before each stage of foam separation, which is beneficial to improve the loading capacity of unit foam liquid for PFAS and reduce the volume of foam liquid required to achieve the target removal effect.

[0041] (4) In the foam separation stage, the present invention increases the gas input intensity so that the system can form floating micron-sized bubbles and / or millimeter-sized bubbles while continuously generating or introducing nanobubbles, thereby realizing the continuous connection between the nanobubble dispersion interface and the collectable foam layer and improving the transfer efficiency of PFAS from the aqueous phase to the foam liquid.

[0042] (5) The present invention can improve the enrichment ratio of PFAS in foam liquid while taking into account the PFAS removal effect, reduce the volume of subsequent concentrated liquid treatment, and reduce the scale and operating load of subsequent treatment units such as oxidation, reduction, incineration, adsorption, concentration, mineralization or surfactant recovery.

[0043] (6) This invention is applicable to one or more polluted water bodies containing perfluorocarboxylic acid PFAS, perfluorosulfonic acid PFAS, fluorinated polymer PFAS, sulfonamide PFAS or PFAS precursors, and is especially applicable to the treatment of complex polluted water bodies containing PFAS with different chain lengths. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of a segmented and efficient foam enrichment and separation method based on the difference in PFAS chain length;

[0045] Figure 2 This is a comparison chart of the removal rates of long-chain PFAS in Example 1 and Comparative Example 1;

[0046] Figure 3 This is a comparison chart of the removal rates of short-chain and / or medium-chain PFAS in Example 1 and Comparative Example 1.

[0047] Figure 4This is a comparison chart of the enrichment rates of long-chain PFAS in Example 1 and Comparative Example 1;

[0048] Figure 5 This is a comparison chart of the enrichment rates of short-chain and / or medium-chain PFAS in Example 1 and Comparative Example 1.

[0049] Figure 6 This is a comparison chart of the removal rates of long-chain PFAS in Example 1 and Comparative Example 2;

[0050] Figure 7 This is a comparison chart of the removal rates of short-chain and / or medium-chain PFAS in Example 1 and Comparative Example 2;

[0051] Figure 8 This is a comparison chart of the enrichment rates of long-chain PFAS in Example 1 and Comparative Example 2;

[0052] Figure 9 This is a comparison chart of the enrichment rates of short-chain and / or medium-chain PFAS in Example 1 and Comparative Example 2;

[0053] Figure 10 This is a comparison chart of the removal rates of long-chain PFAS in Example 1 and Comparative Example 3;

[0054] Figure 11 This is a comparison chart of the removal rates of short-chain and / or medium-chain PFAS in Example 1 and Comparative Example 3;

[0055] Figure 12 This is a comparison chart of the enrichment rates of long-chain PFAS in Example 1 and Comparative Example 3;

[0056] Figure 13 This is a comparison chart of the enrichment rates of short-chain and / or medium-chain PFAS in Example 1 and Comparative Example 3. Detailed Implementation

[0057] The technical solution of the present invention will be further described below with reference to the accompanying drawings. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Equivalent substitutions made by those skilled in the art to specific equipment forms, operating parameters, composition of treated water, or foam liquid collection methods without departing from the technical concept of the present invention should all fall within the scope of protection of the present invention.

[0058] like Figure 1As shown, this invention provides a segmented, high-efficiency foam separation method based on PFAS chain length differences. The method includes a first-stage nanobubble pretreatment, a first-stage foam separation, post-addition of a cationic surfactant, a second-stage nanobubble pretreatment, a second-stage foam separation, and foam liquid treatment. The first-stage nanobubble pretreatment, first-stage foam separation, second-stage nanobubble pretreatment, and second-stage foam separation all employ a micro / nanobubble generator. The generation of nanobubbles, micrometer bubbles, and / or millimeter bubbles is achieved by controlling the gas input intensity, and the gas input intensity is represented by the gas flow rate Q.

[0059] The water body to be treated is a polluted water body containing one or more PFAS. The PFAS may include one or more of the following: perfluorocarboxylic acid PFAS, perfluorosulfonic acid PFAS, fluoropolymer PFAS, sulfonamide PFAS, or PFAS precursors.

[0060] PFAS of different chain lengths are classified according to the number of carbon atoms in the fluorinated carbon chain, the type of functional group, and the interfacial activity. PFAS with fluorinated carbon chain carbon number of not less than 7 are classified as long-chain PFAS, PFAS with fluorinated carbon chain carbon number of not more than 4 are classified as short-chain PFAS, and PFAS with fluorinated carbon chain carbon number of 5 to 6 are classified as medium-chain PFAS.

[0061] Furthermore, PFAS with different chain lengths exhibit varying enrichment capabilities at the gas-liquid interface. This difference not only affects foam separation efficiency but also forms the basis for achieving selective separation. Long-chain PFAS, due to their longer fluorocarbon chains, exhibit enhanced hydrophobicity, reducing their tendency to dissolve in the aqueous phase. They also possess higher interfacial adsorption driving forces, thus preferentially enriching at the gas-liquid interface without the introduction of cationic surfactants, making them suitable as the first-stage separation target.

[0062] For PFAS that are not long-chain PFAS but have strong hydrophobicity, high interfacial activity or precursor structure, they can also be included as strong hydrophobic PFAS in the first stage of separation. Among them, strong hydrophobicity and high interfacial activity mean that PFAS have a higher tendency to adsorb at the gas-liquid interface than short-chain PFAS without the addition of cationic surfactants, which is manifested as higher foam enrichment capacity, interfacial adsorption capacity or lower tendency to migrate in the aqueous phase.

[0063] Short-chain and / or medium-chain PFAS, due to their shorter fluorocarbon chains and higher water solubility, cannot achieve effective migration solely through gas-liquid interface adsorption. They require the assistance of cationic surfactants to form ion-associated structures with negatively charged PFAS via electrostatic interactions, thereby enhancing their interfacial activity. These surfactants can then be used as the separation targets in the second stage. The specific processing steps are as follows:

[0064] S1. First-stage nanobubble pretreatment

[0065] Water bodies containing PFAS of different chain lengths are sent to the first-stage treatment unit. The water bodies are pretreated with nanobubbles using a micro-nano bubble generator. The gas input intensity is controlled so that nanobubbles are generated and dispersed in the aqueous phase, forming an aqueous phase system containing dispersed nanobubbles. The average particle size of the nanobubbles is less than 1000 nm, preferably less than 500 nm, and more preferably less than 200 nm.

[0066] In the first stage of nanobubble pretreatment, no cationic surfactants or surfactants that can form strong ionic associations with anionic PFAS are added. No additives may be added in the first stage, or nonionic foaming agents, neutral foaming agents, or foaming agents that do not form strong ionic associations with anionic PFAS may be added as needed for foam formation.

[0067] The first stage of nanobubble pretreatment is controlled by the formation of a dispersed nanobubble system in the aqueous phase. During this stage, the gas input intensity is controlled to prevent the formation of a continuously collectable foam layer on the liquid surface. The completion of the nanobubble pretreatment can be determined based on one or more of the following conditions: the nanobubble concentration in the aqueous phase reaches a preset value, such as not less than 1 × 10⁻⁶. 7 The specific value of nanoparticles / mL can be measured by a nanoparticle tracking analysis (NTA) instrument; the water body shows a decrease in transparency or an increase in turbidity; no continuously collectable foam layer forms on the liquid surface; or the nanoparticle pretreatment time reaches a set value, which can be obtained through experimental verification.

[0068] S2. First-stage foam separation

[0069] After the first-stage nanobubble pretreatment, the gas input intensity is increased to initiate the first-stage foam separation process. During this process, nanobubbles are continuously generated or introduced, simultaneously forming floatable micron-sized and / or millimeter-sized bubbles, which then form a continuously collectable first foam layer on the liquid surface. The micron-sized and / or millimeter-sized bubbles have a particle size greater than 1 μm and less than 2 mm.

[0070] The first-stage foam separation is used to preferentially migrate long-chain PFAS or strongly hydrophobic PFAS into the foam phase. After the first foam layer is formed, the first foam liquid is collected by overflow, skimming, foam scraping, or negative pressure suction. The remaining liquid is the first-stage residual liquid. The first foam liquid, as the first-stage PFAS enriched phase, can be stored separately or sent to subsequent processing units.

[0071] The end of the first stage of foam separation can be determined based on one or more of the following conditions: the unit time mass recovery rate of long-chain PFAS or strongly hydrophobic PFAS in the first foam solution is lower than a set threshold; the mass ratio of long-chain PFAS or strongly hydrophobic PFAS to total PFAS in the aqueous phase after the first stage treatment is lower than a set threshold; the first foam solution generation rate is lower than a set threshold; the first stage running time reaches a set value; or the first foam layer no longer continues to grow, wherein the set threshold is obtained through experimental verification.

[0072] S3. Post-addition of cationic surfactant

[0073] After the first stage of foam separation is completed, the aqueous phase after the first stage of foam separation is retained as the first stage residual liquid, and a cationic surfactant is added to the first stage residual liquid to obtain the second stage water body to be treated.

[0074] The cationic surfactant may include one or more of the following: quaternary ammonium salt cationic surfactants such as hexadecyltrimethylammonium bromide (CTAB), hexadecyltrimethylammonium chloride (CTAC), dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), or other types of cationic surfactants. The cationic surfactant is added after the first-stage foam separation unit and before the second-stage nanobubble pretreatment unit; alternatively, it may be added simultaneously upon entering the second-stage nanobubble pretreatment unit.

[0075] The dosage of cationic surfactant can be determined based on the concentration and composition of residual anionic PFAS in the water to be treated in the second stage, the target removal requirements, and the formation state of the second foam layer. After addition, the cationic surfactant can be uniformly dispersed in the water to be treated in the second stage through stirring, circulation, pumping, pipeline mixing, or gas-liquid mixing.

[0076] S4. Second-stage nanobubble pretreatment

[0077] The second stage of nanobubble pretreatment was carried out on the water body to be treated after the addition of cationic surfactant, so that the nanobubbles were dispersed in the aqueous phase containing cationic surfactant and residual PFAS.

[0078] The second-stage nanobubble pretreatment is also controlled by the formation of a dispersed nanobubble system. In this stage, the gas input intensity is controlled to prevent the formation of a continuously collectable foam layer on the liquid surface. The second-stage nanobubble pretreatment can be terminated based on one or more of the following conditions: the nanobubble concentration in the aqueous phase reaches a set value; the water's appearance transparency decreases or its turbidity increases; a continuously collectable foam layer does not form on the liquid surface; or the nanobubble pretreatment time reaches a set value.

[0079] S5. Second-stage foam separation

[0080] After the second-stage nanobubble pretreatment, the gas input intensity is increased to initiate the second-stage foam separation process. During this process, nanobubbles are continuously generated or introduced, forming floating micron-sized and / or millimeter-sized bubbles, and a continuously collectable second foam layer is formed on the liquid surface.

[0081] The second-stage foam separation is used to allow the short-chain and / or medium-chain PFAS remaining after the first-stage treatment to enter the second foam liquid. After the second foam layer is formed, the second foam liquid is collected by overflow, skimming, foam scraping, or negative pressure suction. The remaining liquid is the second-stage residual liquid. The second foam liquid, as the PFAS-enriched phase of the second stage, can be stored separately from the first foam liquid or sent separately to subsequent processing units.

[0082] The end of the second-stage foam separation can be determined based on one or more of the following conditions: the unit time mass recovery rate of short-chain PFAS and / or medium-chain PFAS in the second foam solution is lower than a set threshold; the target PFAS concentration in the aqueous phase after the second-stage treatment is lower than a set threshold; the second foam solution generation rate is lower than a set threshold; the second-stage running time reaches a set value; or the second foam layer no longer continues to grow.

[0083] S6. Foaming liquid treatment

[0084] The first foaming liquid and the second foaming liquid are collected separately. The first foaming liquid and the second foaming liquid can be collected through different foaming liquid collection pipelines, collection tanks or storage units, or they can be sent to different processing units according to subsequent processing needs.

[0085] The subsequent processing unit may include one or more of the following: oxidation, reduction, incineration, adsorption, concentration, mineralization, or surfactant recovery units. The first foam liquid has a low or no cationic surfactant content, while the second foam liquid may contain cationic surfactants. Therefore, different subsequent processing methods can be selected based on the PFAS composition, surfactant content, and concentration level of the two foam liquids.

[0086] Simultaneously, samples were taken and analyzed from the first body of water to be treated, the first foam liquid, the first stage residual liquid, the second body of water to be treated, the second foam liquid, and the second stage residual liquid to calculate the PFAS removal rate and enrichment ratio, and to verify the separation effect.

[0087] This invention does not simply achieve PFAS foam separation by increasing the number of foam separation cycles or introducing nanobubbles. Instead, it fully utilizes the differences in interfacial adsorption behavior of PFAS with different chain lengths, dividing the separation process into two stages. This achieves segmented PFAS treatment by "first separating easily enriched components and then enhancing difficult-to-enrich components," reducing interfacial competition between different PFAS components. This ensures that the foam liquid produced in each stage has a high target PFAS mass concentration, increases the ratio of PFAS mass to foam liquid volume, and ultimately achieves a higher PFAS loading capacity per unit foam liquid and an overall enrichment rate.

[0088] Specifically, in the first stage, without the addition of cationic surfactants, the long-chain PFAS and strongly hydrophobic PFAS selectively enter the first foam liquid by utilizing their own gas-liquid interface adsorption properties. In this stage, no surfactant is introduced, avoiding the increase in foam liquid volume and PFAS dilution caused by the surfactant entering the foam phase, thus increasing the concentration of PFAS in the first foam liquid. By adding cationic surfactants in the later stage, the short-chain and medium-chain PFAS that are difficult to migrate in the remaining liquid of the first stage form a PFAS-cationic surfactant association structure, improving their hydrophobicity and interfacial adsorption capacity. Then, in the second stage, the PFAS that were originally difficult to separate are efficiently migrated to the second foam liquid, effectively improving the enrichment rate and separation efficiency of short-chain and medium-chain PFAS.

[0089] Among them, the cationic surfactant only acts on the residual PFAS after the first stage of screening, rather than the entire mixed system. This can reduce the excessive addition of surfactant, reduce the proportion of non-target substances entering the foam liquid, and avoid strong interfacial competition between PFAS of different chain lengths in the same stage.

[0090] Both stages involve the continuous generation or introduction of nanobubbles during nanobubble pretreatment and foam separation, increasing the contact opportunities between PFAS or PFAS-cationic surfactant complex and the dispersed gas-liquid interface, thereby improving the PFAS loading capacity of the unit foam liquid. This reduces the amount of foam liquid generated, increases the PFAS enrichment rate in the foam liquid, and reduces the scale and load of subsequent concentrate treatment, surfactant recovery, or PFAS destruction treatment while achieving the target removal effect.

[0091] Furthermore, the processing order of the first and second stages of this invention cannot be simply interchanged. Long-chain PFAS preferentially accumulate in the first foam solution under cationic surfactant-free conditions, achieving separation in the first stage and helping to maintain the interfacial behavior differences between PFAS of different chain lengths. While adding cationic surfactants in advance can improve the interfacial migration ability of some short-chain PFAS, it also alters the interfacial adsorption state of long-chain PFAS, causing PFAS of different chain lengths to form surfactant-associated structures, reducing chain length selectivity, and affecting the enrichment rate of the first-stage foam solution.

[0092] The following will provide further explanation in conjunction with examples, comparative examples, and test cases.

[0093] Example 1

[0094] The segmented foam separation method (NBs-pre + NBs-FF) described in this invention is used to treat PFAS-containing water bodies. This embodiment includes a first stage of removing and enriching long-chain PFAS or strongly hydrophobic PFAS using a nonionic foaming agent system without cationic surfactants, and a second stage of removing and enriching residual short-chain PFAS and / or medium-chain PFAS using a cationic surfactant system.

[0095] In this embodiment, the water to be treated is a simulated water body containing PFAS, inorganic salts, and nonionic foaming agents. The inorganic salt is sodium chloride, i.e., NaCl, with a concentration of 10 mM; the nonionic foaming agent is hexadecyl polyoxyethylene (10) ether (abbreviated as Brij 56). The cationic surfactant is CTAB. The PFAS include PFBA, PFPeA, PFHxA, PFHpA, PFOA, PFNA, PFDA, PFBS, PFHxS, PFOS, FOSA, N-EtFOSAA, 5:3 FTCA, 6:2 FTS, GenX, and 4:2 FTS, with an initial concentration of 50 μg / L for each PFAS.

[0096] (I) First stage: Removal and enrichment of long-chain PFAS by the Brij 56-containing system

[0097] The water to be treated is placed in a foam separation container. In the first stage, no CTAB or other cationic surfactants are added; Brij 56 is used as a nonionic foaming agent; the Brij 56 concentration is approximately 9.4 mg / L.

[0098] The first stage is nanobubble pretreatment: The micro / nanobubble generator is started and operated at a low gas flow rate to generate and disperse nanobubbles in the aqueous phase. This stage focuses on forming a dispersed nanobubble system, without forming a continuously growing and collectable liquid surface foam layer. In this embodiment, the gas flow rate Q1 is 25 mL / min, the pretreatment time is set to 5 min, and the equivalent diameter of the generated nanobubbles is less than 250 nm.

[0099] First-stage foam separation: After pretreatment, the gas flow rate is increased, causing the system to continuously generate or introduce nanobubbles while simultaneously forming floatable micron-sized and / or millimeter-sized bubbles, thus forming a first foam layer on the liquid surface. In this embodiment, the gas flow rate Q2 is 0.5 L / min, and the equivalent diameter of the micron-sized and / or millimeter-sized bubbles is 390 μm to 1.58 mm. Skimming and collection are performed simultaneously during foam layer formation, with a foam separation and collection time of 5 min, yielding the first foam liquid and the first-stage residual liquid.

[0100] The first foaming solution was used to evaluate the removal and enrichment effects of long-chain PFAS or strongly hydrophobic PFAS under conditions unaffected by cationic surfactants.

[0101] (ii) Second stage: Removal and enrichment of short-chain PFAS and / or medium-chain PFAS using a CTAB-containing system

[0102] The cationic surfactant CTAB was added to the aqueous phase after the first-stage foam separation, with a concentration of 10 mg / L. After addition, CTAB was uniformly dispersed in the water to be treated in the second stage by stirring, circulating, or pumping.

[0103] The second stage involves nanobubble pretreatment: The water body to be treated in the second stage after CTAB addition undergoes nanobubble pretreatment, dispersing nanobubbles in the aqueous phase containing CTAB and residual PFAS. This stage prevents the formation of a continuously growing and collectable surface foam layer. In this embodiment, the gas flow rate Q1 is 25 mL / min, the pretreatment time is set to 5 min, and the generated nanobubbles have an equivalent diameter of less than 200 nm.

[0104] Second-stage foam separation: After pretreatment, the gas flow rate is increased, allowing the system to continuously generate or introduce nanobubbles while simultaneously forming floatable micron-sized and / or millimeter-sized bubbles, creating a second foam layer on the liquid surface. In this embodiment, the gas flow rate Q2 is 0.5 L / min, and the equivalent diameter of the micron-sized and / or millimeter-sized bubbles is 500 μm to 2 mm. Skimming and collection are performed simultaneously during foam layer formation, with a foam separation and collection time of 5 min, yielding the second foam liquid and the remaining liquid from the second stage.

[0105] The second foaming solution was used to evaluate the removal and enrichment effect of residual short-chain PFAS and / or medium-chain PFAS after the first stage treatment under the action of CTAB.

[0106] Comparative Example 1

[0107] The PFAS-containing water body in Example 1 was treated using a conventional foam separation method (FF). This comparative example also includes a first stage of removal and enrichment of long-chain or strongly hydrophobic PFAS using a nonionic foaming agent system, and a second stage of removal and enrichment of residual short-chain and / or medium-chain PFAS using a cationic surfactant system. Unlike Example 1, this comparative example does not involve nanobubble pretreatment, and no micro / nanobubble generator is used to continuously generate or introduce nanobubbles during the foam separation process.

[0108] (I) First stage: Traditional foam separation of Brij 56 system

[0109] The same water body as in Example 1 was placed in a foam separation container. In the first stage, no CTAB or other cationic surfactants were added; Brij 56 was used as a nonionic foaming agent at a concentration of approximately 9.4 mg / L. Aeration separation was performed using a conventional microporous aerator. Micron-sized and / or millimeter-sized bubbles generated during aeration rose to the liquid surface to form the first foam layer, which was continuously collected by skimming.

[0110] In this comparative example, the gas flow rate in the first stage was 0.5 L / min, and the foam separation and collection time was 5 min. After the first stage, the first foam liquid and the remaining liquid from the first stage were obtained.

[0111] (II) Second Stage: Traditional Foam Separation with CTAB System

[0112] The residual liquid after the first stage of traditional foam separation is taken as the water to be treated in the second stage. CTAB is added to it to make the CTAB concentration in the water to be treated in the second stage 10 mg / L. After addition, CTAB is uniformly dispersed in the water to be treated in the second stage by stirring, circulation or pumping.

[0113] The second stage of foam separation is then performed using a conventional microporous aeration head. Micron-sized and / or millimeter-sized bubbles generated during aeration rise to the liquid surface to form a second foam layer, which is continuously collected by skimming.

[0114] In this comparative example, the gas flow rate in the second stage was 0.5 L / min, and the foam separation and collection time was 5 min. After the second stage, the second foam liquid and the remaining liquid from the second stage were obtained.

[0115] Comparative Example 2

[0116] This comparative example employs a nanobubble-based PFAS foam separation method (NBs-pre + NBs-FF) and a simultaneous PFAS treatment approach. Water containing PFAS of different chain lengths, NaCl, and CTAB are placed in the same system for treatment, without a first-stage pre-separation step without cationic surfactants. The concentration of CTAB is 10 mg / L, the concentration of NaCl is 10 mM, and the initial concentration of each PFAS is 50 μg / L.

[0117] Pretreatment stage: The water to be treated is placed in a foam separation container, and the micro / nano bubble generator is started at a low gas flow rate to generate a large number of nanobubbles in the aqueous phase. This stage mainly focuses on forming a dispersed nanobubble system, without forming a continuously growing and collectable foam layer. In this comparative example, the gas flow rate Q1 is 25 mL / min, and the pretreatment time is 5 min.

[0118] Foam separation stage: After pretreatment, the gas flow rate is increased to allow the system to continuously generate nanobubbles while simultaneously forming floatable micron- and millimeter-sized bubbles, resulting in a foam layer on the liquid surface. In this comparative example, the gas flow rate Q2 is 0.5 L / min. Skimming and collection are performed simultaneously during the foam layer formation process, with a foam separation and collection time of 5 min.

[0119] Foam collection stage: The foam layer formed on the liquid surface is continuously collected to obtain foam liquid; the liquid below the foam layer is the residual liquid after treatment.

[0120] Comparative Example 3

[0121] This comparative example uses a traditional foam separation method (FF) and a simultaneous PFAS treatment method. The water containing PFAS of different chain lengths, NaCl and CTAB are placed in the same system for treatment, without a first-stage pre-separation step without cationic surfactants. The concentration of CTAB is 10 mg / L, the concentration of NaCl is 10 mM, and the initial concentration of each PFAS is 50 μg / L.

[0122] In this comparative example, no nanobubble pretreatment was performed. The water to be treated was placed in a foam separation container, and aeration separation was carried out using a traditional microporous aerator. The bubbles generated during aeration rose to the surface to form a foam layer, and the foam liquid was continuously collected by skimming.

[0123] In this comparative example, the gas flow rate was 0.5 L / min, and the foam separation and collection time was 5 min. The collected foam liquid was used as the PFAS enriched phase, and the liquid below the foam layer was used as the residual liquid after treatment.

[0124] Test Example 1

[0125] To evaluate the treatment effects of Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3, samples of the influent, foam liquid and residual liquid after treatment of each treatment system were taken and analyzed, and the PFAS removal rate and enrichment ratio were calculated.

[0126] For Example 1 and Comparative Example 1, tests were conducted in the first and second stages, respectively. In the first stage, the PFAS concentrations in the first-stage influent, the first foam solution, and the first-stage residue were measured to evaluate the removal and enrichment effects of long-chain PFAS or strongly hydrophobic PFAS in systems containing nonionic surfactants and systems without cationic surfactants. In the second stage, the PFAS concentrations in the second-stage influent, the second foam solution, and the second-stage residue were measured to evaluate the removal and enrichment effects of cationic surfactant systems on residual short-chain PFAS and / or medium-chain PFAS.

[0127] For Comparative Examples 2 and 3, the concentrations of PFAS in the influent, foam solution, and residual solution of the simultaneous treatment system were measured to evaluate the removal and enrichment effects of long-chain PFAS, medium-chain PFAS, and short-chain PFAS when chain length segmentation treatment was not performed.

[0128] In each test, foam liquid was collected after 5 minutes of foam separation, and the corresponding residual liquid sample after treatment was obtained. PFAS concentration was determined by liquid chromatography-tandem mass spectrometry.

[0129] In the corresponding stage or treatment system, the PFAS removal rate is calculated using the following formula:

[0130] Removal rate = (m in – m res ) / m in × 100%;

[0131] The enrichment ratio of PFAS in foaming liquid is calculated using the following formula:

[0132] Enrichment ratio = C foam / C in ;

[0133] In the formula, C in The concentration of PFAS (μg / L) in the influent of the corresponding stage or treatment system, m in The mass of PFAS in the influent (μg); C foam The concentration of PFAS (μg / L) in the foam solution at the corresponding stage or in the corresponding treatment system; m res The mass (μg) of PFAS in the residual liquid after treatment in the corresponding stage or treatment system.

[0134] The comparison curves of PFAS removal rate and enrichment rate of the above-mentioned Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3 are as follows: Figures 2 to 13 As shown.

[0135] Based on the comparison results between Example 1 and Comparative Example 1, under the same segmented treatment conditions, after using the NBs-pre + NBs-FF method in Example 1, the overall removal rate of long-chain PFAS or strongly hydrophobic PFAS in the first stage was approximately 95%–97%, higher than that of Comparative Example 1 (approximately 69%–81%); the PFAS enrichment ratio in the first foam solution was approximately 21–29, also higher than that of Comparative Example 1 (approximately 12–22). In the second stage system containing CTAB, the overall removal rate of short-chain PFAS and / or medium-chain PFAS in Example 1 was approximately 79%–97%, higher than that of Comparative Example 1 (approximately 49%–83%); the PFAS enrichment ratio in the second foam solution was approximately 10–13, higher than that of Comparative Example 1 (approximately 5–11).

[0136] Based on the comparison results of Example 1 and Comparative Example 2, it can be seen that when PFAS of different chain lengths are placed in the same system with CTAB and treated simultaneously using NBs-pre + NBs-FF without chain length segmentation, short-chain PFAS and / or medium-chain PFAS can still achieve high removal rates and enrichment effects, with an overall removal rate of approximately 78%–97% and an enrichment ratio of approximately 9–13, similar to the results of the second stage of Example 1. However, for long-chain PFAS or strongly hydrophobic PFAS, the effect of simultaneous treatment is significantly lower than that of segmented treatment in Example 1. Specifically, in Comparative Example 2, the removal rates of PFOA, PFNA, PFDA, PFOS, FOSA, and N-EtFOSAA are approximately 77%–91%, lower than the removal rate of PFAS in the first stage of Example 1; its enrichment ratio is approximately 6–9, also significantly lower than the enrichment ratio of PFAS in the first stage of Example 1.

[0137] The comparison results between Example 1 and Comparative Example 3 show that when PFAS of different chain lengths are placed in the same system with CTAB and treated simultaneously using conventional FF without chain length segmentation or nanobubble pretreatment, the removal rate and enrichment factor of long-chain PFAS or strongly hydrophobic PFAS are significantly lower. In Comparative Example 3, the overall removal rate of PFAS is approximately 8%–59%, and the enrichment factor is approximately 2–5, both lower than the removal rate and enrichment factor of the corresponding substances in Example 1. In Comparative Example 3, the removal rate of short-chain PFAS and / or medium-chain PFAS is approximately 43%–79%, far lower than the removal rate of the corresponding substances in Example 1; the enrichment factor is approximately 5–11, comparable to the enrichment factor of the corresponding PFAS in Example 1.

[0138] In summary, Example 1, compared to Comparative Example 1, demonstrates that under the same segmented treatment conditions, NBs-pre + NBs-FF can improve the removal rate of long-chain PFAS or strongly hydrophobic PFAS in the first stage, and the removal rate of residual short-chain PFAS and / or medium-chain PFAS in the second stage, as well as the enrichment rate of the foam liquid. Example 1, compared to Comparative Example 2, demonstrates that although introducing nanobubbles into the synchronous CTAB system can maintain a certain separation effect of short-chain PFAS and / or medium-chain PFAS, it is difficult to achieve high enrichment of long-chain PFAS or strongly hydrophobic PFAS. Example 1, compared to Comparative Example 3, further demonstrates that the traditional synchronous FF system is insufficient in separating long-chain PFAS or strongly hydrophobic PFAS, as well as some short-chain / medium-chain PFAS.

[0139] The above results demonstrate that the staged selective foam separation strategy based on PFAS chain length differences in this invention can improve the enrichment efficiency of PFAS. Firstly, through a first-stage nanobubble-assisted foam separation under cationic surfactant-free conditions, long-chain PFAS or strongly hydrophobic PFAS are preferentially enriched, reducing interfacial competition between PFAS of different chain lengths. Then, by adding cationic surfactants and through a second-stage nanobubble-assisted foam separation, the removal and enrichment of residual short-chain PFAS and / or medium-chain PFAS are enhanced. This improves the overall separation effect of PFAS of different chain lengths and the unit foam liquid loading capacity, and helps reduce the scale and operational load of subsequent concentrate treatment, surfactant recovery, or PFAS destruction.

[0140] Finally, it should be noted that the above embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Equivalent substitutions, modifications, or improvements made to the present invention by those skilled in the art without departing from the concept of the present invention should all fall within the scope of protection of the present invention.

Claims

1. A segmented, efficient foam enrichment and separation method based on PFAS chain length differences, characterized in that: Includes the following steps: S1: First-stage nanobubble pretreatment: The first-stage nanobubble pretreatment is carried out on the first water body containing PFAS with different chain lengths to disperse the nanobubbles in the aqueous phase. S2: First-stage foam separation: Foam separation and collection are performed on the water that has undergone the first-stage nanobubble pretreatment to form the first foam liquid and the first-stage residual liquid; S3: Add cationic surfactant: Add cationic surfactant to the remaining liquid of the first stage to obtain the water to be treated in the second stage; S4: Second-stage nanobubble pretreatment: The water to be treated in the second stage is subjected to second-stage nanobubble pretreatment, so that the nanobubbles are dispersed in the aqueous phase containing cationic surfactants and residual PFAS. S5: Second-stage foam separation: The foam in the pretreated water body with second-stage nanobubbles is separated and collected to form the second foam liquid and the second-stage residual liquid; S6: Foam liquid treatment: The first foam liquid and the second foam liquid are respectively processed in the post-processing unit.

2. The foam separation method according to claim 1, characterized in that: The first stage nanobubble pretreatment, the second stage nanobubble pretreatment, the first stage foam separation, and the second stage foam separation all use a micro-nanobubble generator to generate bubbles. Among them, the first stage nanobubble pretreatment and the second stage nanobubble pretreatment control the gas input intensity to generate or introduce nanobubbles in the aqueous phase. The first and second stages of foam separation increase the gas input intensity, continuously generating or introducing nanobubbles in the aqueous phase, while forming micron-sized and / or millimeter-sized bubbles that can carry the nanobubbles to the surface, and forming a continuously collectable foam layer on the liquid surface.

3. The foam separation method according to claim 2, characterized in that: The average particle size of the nanobubbles is less than 1000 nm, preferably less than 500 nm, and more preferably less than 200 nm.

4. The foam separation method according to claim 2, characterized in that: The micron-sized and / or millimeter-sized bubbles have a particle size greater than 1 μm and less than 2 mm.

5. The foam separation method according to claim 1 or 2, characterized in that: The termination conditions for the first-stage nanobubble pretreatment and the second-stage nanobubble pretreatment are one or more of the following conditions: The concentration of nanobubbles in the aqueous phase is not less than 1×10 7 cells / mL; The water's transparency decreases or its turbidity increases. No continuously collectable foam layer forms on the liquid surface; The nanobubble pretreatment time has reached the set value.

6. The foam separation method according to claim 1 or 2, characterized in that: The termination conditions for the first stage of foam separation and the second stage of foam separation are one or more of the following conditions: The mass recovery rate of the target PFAS in the foam solution at the corresponding stage is lower than the set threshold. The target PFAS concentration or the mass ratio of target PFAS to total PFAS in the aqueous phase after the corresponding stage of treatment is lower than the set threshold. The foam liquid generation rate is lower than the set threshold during the corresponding stage; The running time of the corresponding stage reaches the set value; The foam layer ceases to grow during the corresponding stage.

7. The foam separation method according to claim 6, characterized in that: The target PFAS corresponding to the first foam liquid is a long-chain PFAS or a strongly hydrophobic PFAS; The target PFAS of the second foam liquid is short-chain PFAS and / or medium-chain PFAS.

8. The foam separation method according to claim 7, characterized in that: The cationic surfactant includes one or more of hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, quaternary ammonium salt cationic surfactants, or other cationic surfactants that can promote the enrichment of PFAS interfaces.

9. The foam separation method according to claim 1, characterized in that: The first foam liquid and the second foam liquid are either separately or mixed and then enter the subsequent processing unit. The subsequent processing unit includes one or more of the following: oxidation, reduction, incineration, adsorption, concentration, mineralization, or surfactant recovery units.

10. The foam separation method according to claim 9, characterized in that: The method is capable of treating polluted water bodies containing one or more of the following: perfluorocarboxylic acid PFAS, perfluorosulfonic acid PFAS, fluorinated polymer PFAS, sulfonamide PFAS, or PFAS precursors.

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