A method for adsorbing short-chain perfluorinated compounds

Abstract

The present invention discloses a method for adsorbing short-chain perfluoro compounds, comprising the following steps: preparing restricted water-modified activated carbon with different micropore distribution ratios by water vapor diffusion according to the concentration of dissolved organic matter DOC in the target water body, and using the restricted water-modified activated carbon to adsorb the short-chain perfluoro compounds in the target water body. The method constructs a restricted water structure in the activated carbon pores by a reagent-free, low-cost water vapor diffusion method, and drives the mass transfer process of short-chain perfluoro compounds in the activated carbon nanopores with a highly broken restricted water hydrogen bond network, thereby significantly improving the adsorption capacity of short-chain perfluoro compounds. The restricted water-modified activated carbon prepared by the method can upgrade the existing activated carbon adsorption process in situ, realize the efficient removal of short-chain perfluoro compounds in micro-polluted water bodies, drinking water and sewage, which is of great significance to ensuring water quality safety.

Description

Technical Field

[0001] The invention relates to a water treatment method, in particular to a method for adsorbing short-chain perfluorinated compounds. Background Art

[0002] There is a worldwide consensus on the persistence, accumulation and health hazards of perfluorinated compounds in the ecological environment. Among them, the most widely used long-chain perfluorinated compounds (chain length > 7), such as perfluorooctane sulfonic acid PFOS and perfluorooctanoic acid PFOA, have been restricted in production and use by the Stockholm Convention. In order to make up for the lack of long-chain perfluorinated compounds, short-chain perfluorinated compounds (chain length ≤ 7) that are not banned are produced and discharged in large quantities, and gradually replace long-chain perfluorinated compounds to become the main category of perfluorinated compounds that pollute various environmental water bodies (for example, rivers, drinking water and municipal sewage, etc.). However, relevant studies have shown that short-chain perfluorinated compounds are more likely to migrate through water than longer chains to cause human exposure and have a longer half-life in the blood, resulting in more serious human health problems such as developmental toxicity, endocrine disruption and immunodeficiency. Therefore, maximizing the removal of short-chain perfluorinated compounds in polluted water bodies and ensuring the health of water ecology and the safety of drinking water are the key to responding to the current global ecological crisis of perfluorinated compounds.

[0003] Since the CF bond energy is as high as 536 kJ / mol, perfluorinated compounds can resist most chemical and biological enzymatic reactions, resulting in the failure of traditional water treatment processes. Adsorption is currently the most commonly used method for removing perfluorinated compounds from water bodies, especially activated carbon adsorption technology, which is believed to be able to break the "permanent cycle" of perfluorinated compounds in water environments. However, as the chain length decreases, the enhanced hydrophilicity of short-chain perfluorinated compounds makes it impossible to match the hydrophobic mass transfer mode of activated carbon, which leads to a rapid breakthrough in the practical application of activated carbon adsorption devices. In response to this problem, current research has been devoted to developing new adsorption materials to enhance the removal of short-chain perfluorinated compounds. For example, the Chinese invention patent with application number 201610104217.6 discloses a method for preparing an adsorbent for removing typical perfluorinated compounds from water environments. The invention uses molecular imprinting technology to synthesize a bifunctional monomer molecular imprinting polymer TFMAA / 4-Vpy-MIP using 2-(trifluoromethyl)acrylic acid and 4-vinylpyridine as bifunctional monomers, PFOA as a template molecule, and EDMA as a crosslinker. The results show that the adsorbent can remove multiple perfluorinated compounds in the water environment at the same time, and the adsorption efficiency can still be maintained at 74.9% to 93.5% after repeated use for 5 times. However, the preparation process of the adsorbent used in this method is complicated, many chemicals are required, and the removal rate of short-chain perfluorinated compounds is only 13.2% to 47.6%, which is difficult to be applied in practice. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a method for adsorbing short-chain perfluorinated compounds which can be driven efficiently and is convenient for practical application.

[0005] Technical solution: The method for adsorbing short-chain perfluorinated compounds of the present invention comprises the following steps:

[0006] (1) Constructing a confined water structure in the activated carbon pores by water vapor diffusion to obtain confined water-modified activated carbon;

[0007] (2) Using the restricted water-modified activated carbon to adsorb short-chain perfluorinated compounds in the target water body.

[0008] Preferably, in step (1), the concentration C of dissolved organic matter DOC in the target water body is DOC Select different micropore distribution ratios D micro of activated carbon.

[0009] Preferably, the micropore distribution ratio D micro Calculated according to the following formula:

[0010]

[0011] Among them, S micro is the specific surface area of ​​pores with pore diameter ≤ 2 nm, S meso It is the specific surface area of ​​pores with pore diameter greater than 2nm and less than or equal to 50nm.

[0012] Preferably, the concentration of the dissolved organic matter DOC is C DOC The target water is filtered through a 0.45μm filter membrane and then measured.

[0013] Preferably, the C DOC >10mg / L, choose D micro <70% activated carbon; said C DOC When the concentration is 3-10 mg / L, select D micro is 70% to 90% of activated carbon; DOC When <3mg / L, choose D micro >90% activated carbon.

[0014] Preferably, in step (1), the activated carbon is placed in a closed environment with a humidity of 90% to 100% to allow water vapor to diffuse for 12 to 24 hours, then placed in 2 to 4 times the volume of pure water for equilibration for 6 to 10 hours, and finally a water-carbon liquid with a water / carbon volume ratio of 1 to 3 is prepared to obtain restricted water modified activated carbon.

[0015] Preferably, in step (2), the restricted water modified activated carbon is loaded into an adsorption column to adsorb short-chain perfluorinated compounds in the target water body.

[0016] Preferably, in the step (2), the water-limited modified activated carbon is loaded into the adsorption column, the pretreated target water body is introduced, the water body is discharged after passing through the adsorption layer, and the empty bed residence time is controlled to be 10 to 30 minutes; when the concentration of short-chain perfluorinated compounds in the effluent reaches 80% of that in the inlet water, the adsorption-saturated water-limited modified activated carbon is regenerated.

[0017] Preferably, the target water body after the pretreatment has a dissolved organic matter concentration of less than 15 mg / L, a suspended particulate matter concentration of less than 20 mg / L, and a single short-chain perfluorinated compound concentration of less than 1000 ng / L.

[0018] Preferably, the target water body is at least one of slightly polluted water, drinking water, and secondary effluent from municipal sewage.

[0019] Preferably, when the target water body is slightly polluted water and drinking water, the empty bed residence time is controlled to be 10 to 15 minutes, and when the target water body is secondary effluent of municipal sewage, the empty bed residence time is controlled to be 20 to 30 minutes.

[0020] From the perspective of chemical principles, the present invention constructs a restricted water structure in the activated carbon nanopores by means of water vapor diffusion, and drives the mass transfer of short-chain perfluorinated compounds in the activated carbon with a highly broken restricted water hydrogen bond network, thereby improving the adsorption performance of the restricted water modified activated carbon for short-chain perfluorinated compounds. The generation of this mechanism relies on both the properties of the activated carbon pores and the structural characteristics of the short-chain perfluorinated compounds. Through a large number of experiments and theoretical analyses, the key nodes of the influence of the activated carbon micropore distribution ratio and the dissolved organic matter in the water body on the adsorption performance of the restricted water modified activated carbon were determined, thereby formulating an activated carbon pore distribution selection strategy based mainly on dissolved organic matter.

[0021] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0022] 1. The present invention utilizes the confined water structure to provide mass transfer power in the mass transfer process of short-chain perfluorinated compounds to improve the original mass transfer process hindered by bulk water, significantly enhances the adsorption performance of confined water-modified activated carbon for short-chain perfluorinated compounds, and realizes the in-situ improvement of the treatment performance of existing adsorption facilities.

[0023] 2. The activated carbon pore distribution selection strategy formulated by the present invention, which is mainly based on soluble organic matter, ensures the maximization of the adsorption capacity of short-chain perfluoro compounds while minimizing the negative effects of soluble organic matter on pore blockage. Therefore, it has the ability to adapt to the purification treatment of different types of water bodies and has a wider range of applications.

[0024] 3. The method of the present invention uses the most widely used activated carbon as the main raw material, and adopts a reagent-free, low-cost water vapor diffusion method to completely maintain its basic physical characteristics such as size and strength after modification. Compared with the hydrophilic functional group grafting scheme commonly used in the prior art, it has significant advantages such as simplicity, low cost, and no secondary pollution. It can be directly promoted and applied in existing adsorption facilities, solving the problem of incompatibility between the performance of new adsorption materials and adsorption facilities in the prior art, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The present invention is a flow chart of the method for adsorbing short-chain perfluorinated compounds. DETAILED DESCRIPTION

[0026] The technical solution of the present invention is further described below in conjunction with specific embodiments and drawings.

[0027] Example 1

[0028] The target water body of this embodiment is the effluent from a water plant's horizontal flow sedimentation tank, and its basic water quality characteristics are: an average pH value of 7.2, an average COD value of 4.53 mg / L, an average DOC value of 1.92 mg / L, an average total nitrogen value of 1.72 mg / L, and an average suspended particulate matter value of 3 mg / L; four typical short-chain perfluorinated compounds are added to the inlet water of the adsorption column device to simulate contaminated drinking water, and the concentrations after addition are: an average value of 203 ng / L for perfluorohexane sulfonic acid, an average value of 232 ng / L for perfluorohexanoic acid, an average value of 184 ng / L for perfluorobutane sulfonic acid, and an average value of 168 ng / L for perfluorobutyric acid.

[0029] The drinking water is treated by the method of the present invention. For the steps, please refer to Figure 1 First, the effluent sample from the water plant advection sedimentation tank was filtered with a 0.45 μm filter membrane, and the concentration of dissolved organic matter DOC was determined to be 1.92 mg / L, denoted as C DOC ; According to the concentration of dissolved organic matter C DOC =1.92mg / L<3mg / L, select micropore distribution ratio D micro >90% activated carbon. Specifically, D micro The activated carbon with a volume ratio of 93% was placed in a closed environment with a humidity of 90% to 100% to allow water vapor to diffuse for 24 hours to complete the self-assembly of the confined water structure in the activated carbon nanopores, and then placed in 2 times the volume of pure water for 8 hours. After the confined water structure was stable, a water-carbon liquid with a water / carbon volume ratio of 3 was prepared. The activated carbon pore structure was characterized by ASAP2020 specific surface area and pore structure tester, D micro Calculated according to the following formula:

[0030] Among them, Smicro is the specific surface area of ​​pores with pore diameter ≤ 2 nm, S meso The specific surface area of ​​the pores with pore diameters > 2 nm and ≤ 50 nm is shown in Table 1.

[0031] Table 1 Pore structure distribution and micropore distribution ratio of activated carbon used in different embodiments

[0032]

[0033] The corresponding restricted water modified activated carbon is loaded into the adsorption column to adsorb the short-chain perfluorinated compounds in the effluent of the water plant's horizontal sedimentation tank. Because the water quality indicators of the effluent of the water plant's horizontal sedimentation tank in this embodiment meet the dissolved organic matter concentration of 1.92mg / L<15mg / L, the suspended particulate matter concentration of 3mg / L<20mg / L, and the concentrations of the four short-chain perfluorinated compounds are all less than 1000ng / L, they are directly introduced into the adsorption column without pretreatment, and the empty bed residence time of drinking water is controlled to be 10min, so that the water body flows through the adsorption layer and then is discharged. When the concentration of short-chain perfluorinated compounds in the effluent reaches 80% of the influent, the saturated restricted water modified activated carbon is regenerated.

[0034] The water volume treated by the limited water modified activated carbon adsorption column, the average concentrations of effluent COD and four typical short-chain perfluorinated compounds are shown in Table 2. The original activated carbon (control group) adsorption column can treat 4000BV of water, the average concentration of effluent COD is 2.65mg / L, the average concentration of PFHxS is 112ng / L, the average concentration of PFHxA is 142ng / L, the average concentration of PFBS is 109ng / L, and the average concentration of PFBA is 92ng / L. After upgrading and transformation by this method, the water treatment capacity of the limited water modified activated carbon adsorption column can be increased by 8.8 times to 35,000 BV, and the average effluent COD concentration is reduced to 2.01 mg / L, the average PFHxS concentration is reduced to 85 ng / L, the average PFHxA concentration is reduced to 96 ng / L, the average PFBS concentration is reduced to 88 ng / L, and the average PFBA concentration is reduced to 71 ng / L, which meets the World Health Organization's human health standards for short-chain perfluorinated compounds in drinking water (single short-chain perfluorinated compound ≤ 100 ng / L).

[0035] Table 2 Treatment effect of adsorption column in Example 1

[0036]

[0037] Comparative Example 1

[0038] Except for the different simulated compounds added to the influent of the adsorption column device, the other steps of this comparative example are the same as those of Example 1. The specific difference is that in this comparative example, acetaminophen, phenol and diethyl phthalate are added to the influent of the adsorption column device to simulate drinking water contaminated by non-perfluorinated compounds. The concentrations after addition are: the average value of acetaminophen is 215 ng / L, the average value of phenol is 192 ng / L, and the average value of diethyl phthalate is 231 ng / L.

[0039] After operating in the same manner as Example 1, the water volume treated by the restricted water modified activated carbon adsorption column and the average concentration of pollutants in the effluent are shown in Table 3. For non-perfluorinated drinking water contaminated by perfluorinated compounds, the water volume treated by the restricted water modified activated carbon and the average concentration of pollutants in the effluent are basically consistent with those of the original activated carbon (control group), indicating that the enhanced adsorption effect of the method of the present invention has a higher selectivity for short-chain perfluorinated compounds, that is, the potent mechanism of restricted water modification is significantly affected by the structural characteristics of short-chain perfluorinated compounds.

[0040] Table 3 Comparative Example 1 Adsorption Column Treatment Effect

[0041]

[0042] Comparative Example 2

[0043] In this comparative example, the adsorption column device is filled with different adsorption materials, and the other steps are the same as those in Example 1. The specific difference is that: in this comparative example, the adsorption column device is filled with a micropore distribution ratio D micro 100% clinoptilolite and micropore distribution ratio D micro The limited water-modified clinoptilolite and limited water-modified carbon xerogel prepared with 95% carbon xerogel.

[0044] After operating in the same manner as Example 1, the water volume treated by the restricted water modified clinoptilolite and restricted water modified carbon xerogel adsorption columns, the effluent COD and the average concentrations of four typical short-chain perfluorinated compounds are shown in Table 4. After restricted water modification, there was no significant change in the water volume treated by clinoptilolite and carbon xerogel and the effluent short-chain perfluorinated compound concentrations, indicating that the enhanced effect of the method of the present invention relies on the unique hydrophobic nanopores of activated carbon, and the improvement of the mass transfer energy state of short-chain perfluorinated compounds cannot be achieved in other types of nanopores.

[0045] Table 4 Treatment effect of adsorption column in Example 2

[0046]

[0047] Example 2

[0048] The target water body in this embodiment is a river sample, and its basic water quality characteristics are: average pH value is 6.8, average COD value is 25.53 mg / L, average DOC value is 8.14 mg / L, average total nitrogen value is 7.72 mg / L, and average suspended particulate matter value is 15 mg / L; four typical short-chain perfluorinated compounds are added to the inlet water of the adsorption column device to simulate the high exposure risk of micro-polluted water body, and the concentrations are: average value of perfluorohexane sulfonic acid is 273 ng / L, average value of perfluorohexanoic acid is 356 ng / L, average value of perfluorobutane sulfonic acid is 296 ng / L, and average value of perfluorobutyric acid is 314 ng / L.

[0049] The method of the present invention is used to treat the slightly polluted water body. For the steps, please refer to Figure 1 First, the collected river sample was filtered with a 0.45 μm filter membrane, and the concentration of dissolved organic matter DOC was determined to be 8.14 mg / L, recorded as C DOC ; Dissolved organic matter concentration C DOC =8.14mg / L Between 3 and 10mg / L, select micropore distribution ratio D micro Activated carbon between 70% and 90%. micro The activated carbon with a volume ratio of 84% was placed in a closed environment with a humidity of 90% to 100% to allow water vapor to diffuse for 18 hours to complete the self-assembly of the confined water structure in the activated carbon nanopores. It was then placed in 3 times the volume of pure water and stirred for 7 hours to stabilize the confined water structure. A water-carbon liquid with a water / carbon volume ratio of 2 was prepared. The activated carbon pore structure was characterized using an ASAP2020 specific surface area and pore structure tester. micro Calculated according to the following formula:

[0050] Among them, S micro is the specific surface area of ​​pores with pore diameter ≤ 2 nm, S meso The specific surface area of ​​the pores with pore diameters > 2 nm and ≤ 50 nm is shown in Table 1.

[0051] The restricted water modified activated carbon is loaded into the adsorption column to adsorb the short-chain perfluorinated compounds in the river sample. Because the water quality indicators of the river sample in this embodiment meet the dissolved organic matter concentration of 8.14mg / L<15mg / L, the suspended particulate matter concentration of 15mg / L<20mg / L, and the concentrations of the four short-chain perfluorinated compounds are all less than 1000ng / L, they are directly introduced into the adsorption column without pretreatment, and the empty bed residence time of the slightly polluted water body is controlled to be 15min, so that the water body flows through the adsorption layer and then is discharged. When the concentration of short-chain perfluorinated compounds in the effluent reaches 80% of the influent, the saturated restricted water modified activated carbon is regenerated.

[0052] The water volume treated by the limited water modified activated carbon adsorption column, the average concentrations of effluent COD and four typical short-chain perfluorinated compounds are shown in Table 5. The original activated carbon (control group) adsorption column can treat 2800BV of water, the average concentration of effluent COD is 10.25mg / L, the average concentration of PFHxS is 154ng / L, the average concentration of PFHxA is 233ng / L, the average concentration of PFBS is 194ng / L, and the average concentration of PFBA is 265ng / L. After upgrading and transformation by this method, the water volume that can be treated by the limited water modified activated carbon adsorption column is increased by 6.4 times to 18000BV, and the average concentration of effluent COD is reduced to 9.72mg / L, the average concentration of PFHxS is reduced to 94ng / L, the average concentration of PFHxA is reduced to 105ng / L, the average concentration of PFBS is reduced to 98ng / L, and the average concentration of PFBA is reduced to 142ng / L.

[0053] Table 5 Treatment effect of adsorption column in Example 2

[0054]

[0055] Example 3

[0056] The target water body of this embodiment is the biochemical tail water of a municipal sewage plant, and its basic water quality characteristics are as follows: the average pH value is 7.9, the average COD value is 42.53 mg / L, the average DOC value is 17.25 mg / L, the average total nitrogen value is 15.31 mg / L, and the average suspended particulate matter value is 23 mg / L; the concentrations of four typical short-chain perfluorinated compounds are: the average perfluorohexane sulfonic acid value is 322 ng / L, the average perfluorohexanoic acid value is 531 ng / L, the average perfluorobutane sulfonic acid value is 552 ng / L, and the average perfluorobutyric acid value is 648 ng / L.

[0057] The method of the present invention is used to treat the secondary effluent of the municipal sewage. The steps are as follows: Figure 1 First, the biochemical tailwater sample of the municipal sewage plant was filtered through a 0.45 μm filter membrane, and the concentration of dissolved organic matter DOC was determined to be 17.25 mg / L, recorded as C DOC ; Organic matter concentration C DOC =17.25mg / L>10mg / L, select micropore distribution ratio D micro <70% activated carbon. Specifically, activated carbon with a micropore distribution ratio of 68% was placed in a closed environment with a humidity of 90% to 100% for water vapor diffusion for 12 hours to complete the self-assembly of the restricted water structure in the activated carbon nanopores, and then placed in 4 times the volume of pure water for 6 hours of stirring and balancing. After the restricted water structure is stabilized, a water-carbon liquid with a water / carbon volume ratio of 1 is prepared. The activated carbon pore structure is characterized by ASAP2020 specific surface area and pore structure tester, D micro Calculated according to the following formula:

[0058] Among them, S micro is the specific surface area of ​​pores with pore diameter ≤ 2 nm, S meso The specific surface area of ​​the pores with pore diameters > 2 nm and ≤ 50 nm is shown in Table 1.

[0059] The restricted water modified activated carbon is loaded into the adsorption column to adsorb the short-chain perfluorinated compounds in the biochemical tail water of the municipal sewage plant. The secondary effluent of the municipal sewage is pre-treated in the high-efficiency sedimentation tank and then introduced into the adsorption column. The empty bed retention time of the secondary effluent of the municipal sewage is controlled to be 25 minutes, so that the water flows through the adsorption layer and is discharged. When the concentration of short-chain perfluorinated compounds in the effluent reaches 80% of the influent, the saturated restricted water modified activated carbon is regenerated.

[0060] The water volume treated by the limited water modified activated carbon adsorption column, the average concentrations of effluent COD and four typical short-chain perfluorinated compounds are shown in Table 6. The original activated carbon (control group) adsorption column can treat 1000BV of water, the average concentration of effluent COD is 21.56mg / L, the average concentration of PFHxS is 209ng / L, the average concentration of PFHxA is 415ng / L, the average concentration of PFBS is 398ng / L, and the average concentration of PFBA is 432ng / L. After upgrading and transformation by this method, the water volume that can be treated by the limited water modified activated carbon adsorption column is increased by 5.6 times to 5600BV, and the average concentration of effluent COD is reduced to 19.14mg / L, the average concentration of PFHxS is reduced to 168ng / L, the average concentration of PFHxA is reduced to 231ng / L, the average concentration of PFBS is reduced to 262ng / L, and the average concentration of PFBA is reduced to 301ng / L.

[0061] Table 6 Treatment effect of adsorption column in Example 3

[0062]

Claims

1. A method for adsorbing short-chain perfluorinated compounds, characterized in that: The following steps are involved: (1) Constructing a confined water structure in the activated carbon pores by water vapor diffusion to obtain confined water-modified activated carbon; (2) using the restricted water-modified activated carbon to adsorb short-chain perfluorinated compounds in the target water body; In step (1), the concentration C of dissolved organic matter DOC in the target water body is DOC Select different micropore distribution ratios D micro of activated carbon; The micropore distribution ratio D micro Calculated according to the following formula: Among them, S micro is the specific surface area of ​​pores with pore diameter ≤ 2 nm, S meso The specific surface area of ​​pores with pore diameters greater than 2 nm and less than or equal to 50 nm; The C DOC >10mg / L, choose D micro <70% activated carbon; said C DOC When the concentration is 3-10 mg / L, select D micro is 70% to 90% of activated carbon; DOC When <3mg / L, choose D micro >90% activated carbon; In the step (1), the activated carbon is placed in a closed environment with a humidity of 90% to 100% to allow water vapor to diffuse for 12 to 24 hours, then placed in 2 to 4 times the volume of pure water for equilibration for 6 to 10 hours, and finally a water-carbon liquid with a water / carbon volume ratio of 1 to 3 is prepared to obtain limited water modified activated carbon.

2. The method for adsorbing short-chain perfluorinated compounds according to claim 1, characterized in that: In the step (2), the restricted water modified activated carbon is loaded into the adsorption column to adsorb the short-chain perfluorinated compounds in the target water body.

3. The method for adsorbing short-chain perfluorinated compounds according to claim 1, characterized in that: In the step (2), the water-restricted modified activated carbon is loaded into the adsorption column, the pretreated target water body is introduced, the water body is discharged after passing through the adsorption layer, and the empty bed residence time is controlled to be 10 to 30 minutes; when the concentration of short-chain perfluorinated compounds in the effluent reaches 80% of the influent, the adsorption saturated water-restricted modified activated carbon is regenerated.

4. The method for adsorbing short-chain perfluorinated compounds according to claim 3, characterized in that: The target water body after the pretreatment has a dissolved organic matter concentration of less than 15 mg / L, a suspended particle concentration of less than 20 mg / L, and a single short-chain perfluorinated compound concentration of less than 1000 ng / L.

5. The method for adsorbing short-chain perfluorinated compounds according to claim 1, characterized in that: The target water body is at least one of slightly polluted water, drinking water, and secondary effluent from municipal sewage.

6. The method for adsorbing short-chain perfluorinated compounds according to claim 5, characterized in that: When the target water body is slightly polluted water body and drinking water, the empty bed residence time is controlled to be 10 to 15 minutes. When the target water body is secondary effluent of municipal sewage, the empty bed residence time is controlled to be 20 to 30 minutes.

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