Application of soluble alkali metal sulfide secondary activated PMS in degradation of norfloxacin

Abstract

The present application relates to the technical field of catalyst, especially to application of soluble alkali metal sulfide secondary activation PMS in degradation of norfloxacin, and provides a reducing agent-soluble alkali metal sulfide for improving reaction efficiency of Fe(II) / PMS homogeneous catalytic system. The reducing agent and the Fe(II) / PMS homogeneous catalytic system can efficiently degrade norfloxacin, no iron mud is generated in the reaction process, it is green and environment-friendly, the pH use range is relatively wide, the initial required Fe(II) dosage in the system is small, the pollutant treatment cost is reduced, and it is beneficial to large-scale popularization and application.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of catalysts, in particular to application of soluble alkali metal sulfide secondary activation PMS in degradation of norfloxacin. BACKGROUND

[0002] Norfloxacin (NOR) has been widely used in medical treatment, animal husbandry and aquaculture. Like other antibiotics, norfloxacin is difficult to be completely absorbed during use, and most of it is discharged into the environment through feces or urine, and finally exists in various water bodies, causing serious harm, so it is particularly important to efficiently degrade norfloxacin in various water environments.

[0003] The ferrous ion activated persulfate (Fe(II) / PMS) homogeneous catalytic system is simple to operate and low in cost, and compared with the heterogeneous catalytic system, it has lower interfacial mass transfer resistance and higher reaction rate, and is therefore often used for degrading pollutants. Fe(II) / PMS has two pathways, as shown in formulas (1) and (2), and in the process, Fe(II) and PMS generate Fe(III), sulfate radicals (SO4 ·- ), and hydroxyl radicals (HO · ) through electron transfer. Generally, Fe(II) / PMS has a two-stage reaction process, which includes a fast stage first and then a slow stage. In the initial stage, free radicals are generated quickly due to sufficient reactants, and with the reaction proceeding, the generation of free radicals slows down due to the consumption of Fe(II).

[0004]

[0005]

[0006] In order to improve the reaction efficiency of Fe(II) / PMS, some researchers use the method of batch adding Fe(II). For example, Ayoub et al. found that the removal rate of sulfamethoxazole under the condition of batch adding Fe(II) is twice that under the condition of once adding Fe(II) after 2 hours of reaction. Jiang et al. found that batch adding Fe(II) can not only improve the degradation rate of bisphenol A, but also improve the removal rate of total organic carbon in the system.

[0007] Some researchers also use organic chelating agents such as ethylenediaminetetraacetic acid and citric acid to improve the reaction efficiency, which has been widely used to accelerate the degradation of pollutants such as sulfamethoxazole, trichloroethylene and 2,4,6-trichlorobenzene in the Fe(II) / PMS system. Among them, the chelating agent and Fe(II) generate a chelate, which on the one hand makes Fe(II) slowly release into the solution, effectively controls the concentration of Fe(II) in the solution, and inhibits the generation of SO4 ·-Side reactions quenched by excess Fe(II) are generated; on the other hand, the generated chelates can overcome the limitation that Fe(II) can only exist stably under acidic conditions, thus broadening the applicable pH range of the system.

[0008] Furthermore, the addition of a reducing agent can effectively promote the cycling of Fe(III) and Fe(II), thereby improving the reaction efficiency of Fe(II) / PMS. Hydroxylamine is currently a widely used reducing agent. Zou et al. investigated the effect of hydroxylamine on the degradation of benzoic acid in the Fe(II) / PMS system. The results showed that the Fe(II) / PMS system could hardly degrade benzoic acid within 15 minutes, while under the same conditions, the addition of hydroxylamine could degrade approximately 80% of the benzoic acid. A decrease in Fe(III) concentration was detected during the process, indirectly indicating that hydroxylamine achieves effective degradation of benzoic acid by promoting the regeneration of Fe(II).

[0009] The Fe(II) / PMS process generates Fe(III) which reacts with PMS to produce SO5. ·- The reactivity is much lower than that of SO4. ·- and HO · Simultaneously, the reaction of Fe(III) with PMS can regenerate Fe(II), but the rate constant of this reaction is extremely low, leading to the accumulation of Fe(III) in the solution. Fe(III) has low solubility in the medium to high pH range, easily precipitating to form a large amount of iron sludge, greatly inhibiting the reaction activity and causing water pollution. Adding Fe(II) in batches still results in low Fe(II) utilization efficiency, and the added Fe(II) will eventually form Fe(III), thus failing to overcome the problem of large amounts of iron sludge. The introduced chelating agent is an organic substance, which not only increases the chemical oxygen demand of the water body but also changes the form of heavy metals in the water, posing a potential risk. Currently, there are few proven effective reducing agents, and the types of reducing agents applicable to different scenarios vary greatly. The most widely used reducing agent—hydroxylamine—is itself toxic and has a high reaction rate constant with free radicals, competing with pollutants for free radicals.

[0010] How to provide a green, environmentally friendly, and efficient method for degrading norfloxacin has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0011] In view of this, the present invention is proposed.

[0012] In a first aspect, the present invention provides the application of soluble alkali metal sulfides in Fe(II) / PMS homogeneous catalytic systems.

[0013] The present application is found through screening a large number of reducing agents that the soluble alkali metal sulfide can realize the conversion of Fe(III) to Fe(II) by electron transfer as a reducing agent, and the remaining PMS in the regenerated Fe(II) secondary activation solution produces SO4 ·- and HO · , which can improve the reaction efficiency; and when the soluble alkali metal sulfide is added as a reducing agent, it can promote recycling, so that the initial required Fe(II) dose in the system is small, and the Fe(III) in the system is "secondarily activated" without additional Fe(II), which can improve the utilization efficiency of Fe(II) on the one hand, and effectively limit the generation of iron sludge on the other hand, so that no iron sludge is generated in the reaction process, which is green and environmentally friendly.

[0014] In addition, the soluble alkali metal sulfide is an inorganic substance, which does not increase the chemical oxygen demand of the system and does not change the existence form of heavy metals in water, and the dosage of the soluble alkali metal sulfide is small, which does not cause other potential risks.

[0015] Preferably, the soluble alkali metal sulfide is Na2S or K2S.

[0016] Preferably, the Fe(II) / PMS homogeneous catalytic system is used for degrading pollutants.

[0017] Preferably, the pollutants exist in natural water, domestic wastewater or industrial wastewater.

[0018] Preferably, the pollutants are norfloxacin.

[0019] Preferably, when the Fe(II) in the Fe(II) / PMS homogeneous catalytic system is consumed, the soluble alkali metal sulfide is added to secondarily activate the PMS.

[0020] The source of Fe(II) in the present application includes but is not limited to FeSO4 and corresponding hydrates.

[0021] In the specific implementation process, the soluble alkali metal sulfide can be derived from other forms such as corresponding hydrates.

[0022] Further, the present application provides a method for degrading norfloxacin, comprising:

[0023] Fe(II), PMS and soluble alkali metal sulfide are simultaneously added to a solution containing norfloxacin for reaction;

[0024] Preferably, Fe(II) and PMS are first added to a solution containing norfloxacin for reaction, and then the soluble alkali metal sulfide is added after the Fe(II) is consumed.

[0025] The present application finds that the NOR degradation rate is more significantly improved by adding the two reagents of soluble alkali metal sulfide and Fe(II) into the system in batches under the condition of secondary activation of Fe(II).

[0026] Preferably, the amount of the added soluble alkali metal sulfide satisfies that the molar ratio of the sulfur ions to Fe(II) is (1-2):1, in terms of the molar ratio of the sulfur ions in the soluble alkali metal sulfide to Fe(II).

[0027] The present application further finds that the higher the dosage of the added soluble alkali metal sulfide is, the better the utilization efficiency of PMS is not, when the dosage is high, not all the consumed PMS is used for the degradation of NOR, and the utilization efficiency of PMS is actually reduced. When the dosage of the soluble alkali metal sulfide is controlled within the above range, the utilization efficiency of PMS is best at a lower dosage, and the utilization efficiency of PMS and the use cost are both considered.

[0028] More preferably, the molar ratio of the sulfur ions to Fe(II) is (1-1.2):1.

[0029] When the above range is satisfied, the better utilization efficiency of PMS can be obtained at a smaller dosage.

[0030] Preferably, the initial pH of the solution is 10 or less.

[0031] Preferably, Fe(II) is added only once initially.

[0032] Compared with the prior art, the present application has the beneficial effects that:

[0033] The present application provides a reducing agent for improving the reaction efficiency of a Fe(II) / PMS homogeneous catalytic system. The reducing agent and the Fe(II) / PMS homogeneous catalytic system can efficiently degrade norfloxacin, no iron sludge is generated in the reaction process, it is green and environmentally friendly, the pH use range is relatively wide, the initial required Fe(II) dosage in the system is small, the pollutant treatment cost is reduced, and it is conducive to large-scale popularization and application. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 is the degradation process of NOR under different Na2S addition conditions.

[0035] Figure 2 is the process of degrading NOR by secondary activation of Fe(II) / PMS under different Na2S dosages.

[0036] Figure 3 is the utilization efficiency of PMS under different Na2S dosages.

[0037] Figure 4are the degradation processes of NOR at different initial pH.

[0038] Figure 5 are the plots of pH (a) and ORP (b) of the solution as a function of time at different initial pH.

[0039] Figure 6 are the species distribution in the presence of Fe(II) and Na2S at pH = 1-12.

[0040] Figure 7 are the statistical graphs of the degradation rate (a) and the degradation contribution rate (b) of NOR by various free radicals.

[0041] Figure 8 are the electron paramagnetic resonance spectra of each process.

[0042] Figure 9 are the plots of the degradation rate of NOR as a function of time.

[0043] Figure 10 are the plots of the concentration of PMS (a) and Fe(II) (b) as a function of time.

[0044] Figure 11 are the degradation mechanism diagrams of Na2S promoting Fe(II) / PMS degradation of NOR. DETAILED DESCRIPTION

[0045] In order to make the objects, technical solutions and advantages of the present application clearer, the technical solutions in the present application will be described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present application.

[0046] If the specific technology or condition is not specified in the embodiments, it is carried out according to the conventional method or the technology or condition described in the literature in the art, or according to the product instruction. If the manufacturer of the reagent and instrument used is not specified, it is a conventional product that can be purchased through a regular channel.

[0047] The following embodiments take Na2S as a representative to illustrate the application of soluble alkali metal sulfide in the Fe(II) / PMS homogeneous catalytic system.

[0048] Embodiment 1

[0049] The embodiment provides a method for degrading norfloxacin, which comprises the following steps:

[0050] Take 20 mL of 200 mg / L NOR stock solution in a 500 mL transparent glass bottle, and take 369.2 mL of ultrapure water, 0.1 M NaOH, and 0.1 M H2SO4 solution to obtain a mixed solution with a pH of 6±0.1. Fix the transparent glass bottle on a shaker with a constant rate. Dissolve 0.0158 g of FeSO4·7H2O and 0.0137 g of Na2S·9H2O in 25 mL of ultrapure water, respectively, and take 5 mL of each to add to the mixed solution. Finally, add 0.8 mL of 0.1 M PMS stock solution to start the reaction.

[0051] Example 2

[0052] This example provides a method for degrading norfloxacin, comprising:

[0053] Take 20 mL of 200 mg / L NOR stock solution in a 500 mL transparent glass bottle, and take 369.2 mL of ultrapure water, 0.1 M NaOH, and 0.1 M H2SO4 solution to obtain a mixed solution with a pH of 6±0.1. Fix the transparent glass bottle on a shaker with a constant rate. Dissolve 0.0158 g of FeSO4·7H2O and 0.0137 g of Na2S·9H2O in 25 mL of ultrapure water, respectively, and take 5 mL of each to add to the mixed solution. Finally, add 0.8 mL of 0.1 M PMS stock solution to start the reaction.

[0054] Example 3

[0055] This example provides a method for degrading norfloxacin, which is only different from Example 2 in that after adding the PMS stock solution to start the reaction, 0.0189 g of Na2S·9H2O is dissolved in 50 mL of ultrapure water, and 10 mL of the solution is added to the mixed solution after 16 minutes of reaction.

[0056] Example 4

[0057] This example provides a method for degrading norfloxacin, which is only different from Example 2 in that after adding the PMS stock solution to start the reaction, 0.0252 g of Na2S·9H2O is dissolved in 50 mL of ultrapure water, and 10 mL of the solution is added to the mixed solution after 16 minutes of reaction.

[0058] Example 5

[0059] This example provides a method for degrading norfloxacin, which is only different from Example 2 in that the pH of the mixed solution is 3.

[0060] Example 6

[0061] This example provides a method for degrading norfloxacin, only different from example 2 is that the pH of the mixed solution is 9.

[0062] Example 7

[0063] This example provides a method for degrading norfloxacin, only different from example 2 is that the pH of the mixed solution is 12.

[0064] Test example

[0065] The process of degrading norfloxacin in the above examples is monitored.

[0066] Figure 1 is the degradation process of NOR under different Na2S addition conditions, wherein Fe(II) / PMS represents no Na2S addition; S(II) / Fe(II) / PMS represents simultaneous addition of Na2S at the beginning of the experiment (example 1); Fe(II) / PMS, S(II) represents addition of Na2S for secondary activation after 16 minutes of reaction (example 2).

[0067] The results show that the NOR degradation rates after 30 minutes of reaction are 72.45% and 82.80% respectively for simultaneous addition at the beginning of the experiment and addition of Na2S after 16 minutes of reaction, which are increased by 12.65% and 23.00% respectively compared with no addition of Na2S. This shows that the addition of Na2S can significantly improve the NOR degradation rate, and the batch addition of Na2S and Fe(II) makes the NOR degradation rate under the condition of Fe(II) secondary activation more significant.

[0068] Figure 2 is the degradation process of NOR by Fe(II) / PMS secondary activation under different Na2S doses, wherein Fe(II):S(II)=1:1 corresponds to example 2, Fe(II):S(II)=1:1.5 corresponds to example 3, Fe(II):S(II)=1:2 corresponds to example 4, and the control group is Fe(II):S(II)=1:0. Figure 3 is the utilization efficiency diagram of PMS under different Na2S doses.

[0069] The results show that as the S(II) dose increases, the NOR degradation rate after 30 minutes of reaction gradually increases; but when the S(II) dose increases, the ratio of the amount of oxidized NOR to the amount of consumed PMS, i.e. the utilization efficiency of PMS, within 0-30 and 15-30 minutes decreases, which shows that when the S(II) dose increases, not all of the consumed PMS is used for NOR degradation.

[0070] Figure 4 is the degradation process of NOR under different initial pH. Figure 5are the pH (a) and ORP (b) of the solution as a function of time at different initial pH. Figure 6 is the species distribution diagram in the range of pH = 1-12 in the presence of Fe(II) and Na2S.

[0071] The results show that when the initial pH of the solution is 3, 6, 9, and 12, the NOR removal rate is 70.23%, 82.80%, 66.05%, and 77.64% respectively after 30 minutes of reaction. This indicates that the pH range of this method is relatively wide. However, it is worth noting that when the initial pH of the solution is 12, the NOR degradation rate shows a trend of first increasing, then decreasing, and then increasing again during the reaction process. Combined with the fact that the pH of the solution is greater than 10.5 and the oxidation-reduction potential (ORP) is at a relatively low value during the reaction process when the initial pH of the solution is 12, as shown in Figure 5 , it is speculated that PMS is not activated. Furthermore, according to the fact that Fe(OH)2 precipitate is generated when the pH of the solution is greater than 10.5, as shown in Figure 6 , it is indicated that NOR is removed through the adsorption of Fe(OH)2 rather than oxidation when the initial pH of the solution is 12.

[0072] Example 8

[0073] The purpose of this example is to reveal the mechanism of action. The quenching experiment was carried out under the experimental conditions of Example 2 described above. Before adjusting the initial pH of the solution, 5.731 mL of tert-butyl alcohol (TBA) and 2.540 mL of methanol (MeOH) were added as quenching agents.

[0074] In the electron paramagnetic resonance experiment, 100 mM of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was used as a spin trapping agent. A certain volume of ultrapure water, DMPO reagent, PMS solution, Fe(II) solution, and S(II) solution were added in sequence. After oscillation for 15 s, 30 μL of sample was taken with a quartz standard sampling tube and placed in an EPR special sample tube for in-situ free radical generation and detection. The EPR temperature was maintained at 20°C by a water circulation system, and the X-band frequency was operated at 9.84 GHz. The parameters of the spectrometer were set as follows: scan width 100 G, center field 3505 G, scan time 60 s, modulation amplitude 1 G, modulation frequency 100 kHz, receiver gain 30 dB, microwave attention 25 dB, conventional time 12.64 ms, and time constant 5.12 ms.

[0075] To determine the relationship between NOR degradation rate, PMS concentration, and Fe(II) concentration in the process, 120 mL of 200 mg / L NOR stock solution was taken in a 500 mL transparent glass bottle, and 268.4 mL of a mixture solution with pH of 6±0.1 was obtained by taking ultrapure water, 0.1M NaOH, and 0.1M H2SO4 solution. The transparent glass bottle was fixed on a shaker with a constant speed. 10 mL of 0.1005 g FeSO4·7H2O dissolved in 50 mL ultrapure water was added to the mixture solution, and 1.6 mL of 0.1M PMS stock solution was added to start the reaction. 10 mL of 0.0795 g Na2S·9H2O dissolved in 50 mL ultrapure water was added to the mixture solution at 16 minutes.

[0076] Each experiment took 3 mL of reaction solution at a predetermined time interval (0.5, 1.5, 2.5, 5, 8, 10, 15, 16.5, 17.5, 18.5, 20, 22, 25, 30 minutes) and added an equal volume of methanol to terminate the reaction. The mixed solution was filtered through a 0.22 μm filter membrane, and the NOR concentration in the sample was determined by high performance liquid chromatography. 1 mL of sample was taken at a predetermined time interval (0.5, 8, 15, 16.5, 19, 22, 30 minutes) into a sample bottle, and the PMS concentration and Fe(II) concentration were determined.

[0077] The NOR degradation rate over time with no quenching agent and MeOH, TBA as quenching agent respectively was obtained, and the degradation rate and degradation contribution rate of each radical to NOR were as shown in Figure 7 It can be seen that "secondary activation" does not change the type of generated radicals, which are SO5 ·- , HO · , and SO4 ·- , but changes the dominant radical from SO5 ·- with a contribution rate of 42.37% to SO4 ·- with a contribution rate of 44.96%.

[0078] The electron paramagnetic resonance spectra of each process are shown in Figure 8 PMS alone cannot generate radicals, and both Fe(II) / PMS and S(II) / Fe(II) / PMS can generate HO · and SO4 ·- . However, compared with Fe(II) / PMS, the HO · and SO4 ·- signals in the electron paramagnetic resonance spectrum of S(II) / Fe(II) / PMS are stronger, indicating that the addition of Na2S can promote the generation of HO · and SO4 ·-The production of SO4 ·- The signal ratio HO · is stronger, which indicates that the contribution rate of SO4 ·- is greater than that of HO · .

[0079] The changes of NOR degradation rate, PMS concentration and Fe(II) concentration with time are shown in Figure 9 and 10 . When the reaction proceeds to 16 minutes, Na2S is added, and it is measured that the Fe(II) concentration suddenly increases to about 6 mg / L, the PMS concentration suddenly decreases to about 0 mM, and the NOR degradation rate increases by about 8% after 30 seconds, which indicates that the added Na2S realizes the "secondary activation" of PMS by reducing Fe(II) to further degrade NOR.

[0080] In summary, Na2S promotes the conversion of Fe(III) to Fe(II), and secondarily activates PMS to produce SO5 ·- , HO · and SO4 ·- radicals, especially SO4 ·- , so as to increase the oxidative degradation of NOR, as shown in Figure 11 .

[0081] Finally, it should be pointed out that the above examples are only used to illustrate the technical solutions of the present application, but not to limit it; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for some technical features thereof; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for degrading norfloxacin, characterized in that, include: First, Fe(II) and PMS are added to a solution containing norfloxacin to react. After Fe(II) is consumed, a soluble alkali metal sulfide is added. Based on the molar ratio of sulfide ions to Fe(II) in soluble alkali metal sulfides, the amount of soluble alkali metal sulfide added satisfies the following: the molar ratio of sulfide ions to Fe(II) is (1~2):

1.

2. The method according to claim 1, characterized in that, The soluble alkali metal sulfide is Na2S or K2S.

3. The method according to claim 1, characterized in that, The initial pH of the solution is below 10.

4. The method according to any one of claims 1 to 3, characterized in that, Fe(II) was added only once initially.

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