A method for removing arsenic and mercury from wastewater

Abstract

This application proposes a method for removing arsenic and mercury from wastewater, relating to the field of wastewater treatment technology. The method includes the following steps: adding hydrogen peroxide to the wastewater, allowing it to react fully, then adding calcium hydroxide to adjust the pH to a strongly alkaline state, and stirring to obtain mixture I; adding modified iron salt to mixture I, and stirring to obtain mixture II; performing a first-stage pressure filtration on mixture II, collecting the filtrate, and adding ferrous sulfate to adjust the pH to a weakly alkaline state to obtain mixture III; subjecting mixture III to precipitation and a second-stage pressure filtration, collecting the filtrate to obtain water free of arsenic and mercury. This method can effectively remove arsenic and mercury from wastewater, and in practical applications, it can be automated with defined parameters and standard operating procedures, solving the technical problems of arbitrary operation, lack of accurate parameters, and unstable treatment results in existing technologies.

Description

Technical Field

[0001] This application relates to the field of wastewater treatment technology, and more specifically, to a method for removing arsenic and mercury from wastewater. Background Technology

[0002] Arsenic is a nonmetal, and almost all arsenic and its compounds are highly toxic. Arsenic is stable in nature, but trivalent arsenic in the environment is 60 times more toxic than pentavalent arsenic. Therefore, the prevention, control, and treatment of pollution from arsenic-containing solid waste and wastewater is a very serious issue.

[0003] Currently, common methods for removing harmful substances such as arsenic and mercury from industrial wastewater include precipitation, flocculation, ion exchange, membrane separation, and electrolysis. Precipitation is a traditional and effective method, but its operation is highly variable and time-consuming, and it cannot reliably remove heavy metals and other harmful elements. Flocculation is also effective, but the anionic flocculants used are biologically toxic and expensive, making it uneconomical. Ion exchange and membrane separation require highly sophisticated equipment, hindering widespread adoption. Electrolysis utilizes high-frequency direct current to generate redox reactions, causing heavy metals to precipitate, but the equipment is expensive, consumes a lot of electricity, and has high operating costs. None of these methods can guarantee that the arsenic and mercury levels in the final effluent will consistently meet standards. Summary of the Invention

[0004] The purpose of this application is to provide a method for removing arsenic and mercury from wastewater, which has the advantage of effectively reducing the content of arsenic and mercury in wastewater.

[0005] To solve the above-mentioned technical problems, the technical solution adopted in this application is as follows:

[0006] This application provides a method for removing arsenic and mercury from wastewater, characterized by comprising the following steps:

[0007] A. Add hydrogen peroxide to the wastewater, allow it to react fully, then add calcium hydroxide to adjust the pH to a strongly alkaline state. After stirring and mixing, obtain mixture I.

[0008] B. Add modified iron salt to mixture I, stir and mix to obtain mixture II;

[0009] C. Perform a single pressure filtration on mixture II, collect the filtrate, add ferrous sulfate to adjust the pH to weakly alkaline, and obtain mixture III;

[0010] D. The mixture III is subjected to precipitation and secondary pressure filtration, and the filtrate is collected to obtain water after the removal of arsenic and mercury.

[0011] Compared with the prior art, the embodiments of this application have at least the following advantages or beneficial effects:

[0012] This application first adds hydrogen peroxide to the wastewater. Hydrogen peroxide can oxidize trivalent arsenic in the wastewater to pentavalent arsenic, which is less toxic. Moreover, the decomposition products of hydrogen peroxide in the reaction are water and oxygen, which will not introduce impurities into the entire wastewater treatment process, thus embodying an environmental protection concept. Then, calcium oxide is added to adjust the pH of the wastewater to a strongly alkaline state. Under this strongly alkaline environment, arsenic and mercury in the wastewater begin to react with calcium hydroxide to form complexes that are insoluble in water and accumulate and precipitate in the wastewater. After a certain period of reaction and precipitation, this application uses iron salt particles modified from tea residue biochar. Ferric chloride forms an iron oxide layer on the surface of the tea residue biochar. At the same time, the hydroxyl and carboxyl functional groups on the acidic activated carbon generated electrostatic attraction and complexation on the previously precipitated flocs under alkaline conditions, enhancing the adsorption of heavy metals and forming stable, non-agglomerated arsenic and mercury fixatives, which can be separated from water in the subsequent pressure filtration step.

[0013] After the first pressure filtration, this application also adds ferrous sulfate to the wastewater. The addition of ferrous sulfate neutralizes calcium hydroxide, and the resulting calcium sulfate is insoluble in water. On the other hand, ferrous ions can form water-insoluble complexes with arsenic and mercury in an alkaline environment. After precipitation and secondary pressure filtration, arsenic and mercury in the water are further removed, and the pH value of the final water is adjusted to meet the standard of weak alkalinity, which has a good environmental protection awareness.

[0014] This application standardizes the operation, and the process path can be automated in practical applications. It has parameters and standard operating procedures, which solves the technical problems of arbitrary operation, lack of accurate parameters, and unstable processing results in the prior art. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0016] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present application will now be described in detail with reference to specific embodiments.

[0017] A method for removing arsenic and mercury from wastewater includes the following steps:

[0018] A. Add hydrogen peroxide to the wastewater, allow it to react fully, then add calcium hydroxide to adjust the pH to a strongly alkaline state. After stirring and mixing, obtain mixture I.

[0019] B. Add modified iron salt to mixture I, stir and mix to obtain mixture II;

[0020] C. Perform a single pressure filtration on mixture II, collect the filtrate, add ferrous sulfate to adjust the pH to weakly alkaline, and obtain mixture III;

[0021] D. The mixture III is subjected to precipitation and secondary pressure filtration, and the filtrate is collected to obtain water after the removal of arsenic and mercury.

[0022] This application first adds hydrogen peroxide to the wastewater. Hydrogen peroxide can oxidize trivalent arsenic in the wastewater to pentavalent arsenic, which is less toxic. Moreover, the decomposition products of hydrogen peroxide in the reaction are water and oxygen, which will not introduce impurities into the entire wastewater treatment process, thus embodying an environmental protection concept. Then, calcium oxide is added to adjust the pH of the wastewater to a strongly alkaline state. Under this strongly alkaline environment, arsenic and mercury in the wastewater begin to react with calcium hydroxide to form complexes that are insoluble in water and accumulate and precipitate in the wastewater. After a certain period of reaction and precipitation, this application uses iron salt particles modified from tea residue biochar. Ferric chloride forms an iron oxide layer on the surface of the tea residue biochar. At the same time, the hydroxyl and carboxyl functional groups on the acidic activated carbon generated electrostatic attraction and complexation on the previously precipitated flocs under alkaline conditions, enhancing the adsorption of heavy metals and forming stable, non-agglomerated arsenic and mercury fixatives, which can be separated from water in the subsequent pressure filtration step.

[0023] After the first pressure filtration, this application also adds ferrous sulfate to the wastewater. The addition of ferrous sulfate neutralizes calcium hydroxide, and the resulting calcium sulfate is insoluble in water. On the other hand, ferrous ions can form water-insoluble complexes with arsenic and mercury in an alkaline environment. After precipitation and secondary pressure filtration, arsenic and mercury in the water are further removed, and the pH value of the final water is adjusted to meet the standard of weak alkalinity, which has a good environmental protection awareness.

[0024] This application standardizes the operation, and the process path can be automated in practical applications. It has parameters and standard operating procedures, which solves the technical problems of arbitrary operation, lack of accurate parameters, and unstable processing results in the prior art.

[0025] In some embodiments of this application, the volume ratio of wastewater to hydrogen peroxide in step A above is 1000:(0.5-1), and the reaction time is 1-3 hours.

[0026] In some embodiments of this application, the pH of the strong base in step A above is 11-13, the concentration of the calcium hydroxide is 35-45%, and the stirring and mixing time is 2-5 hours.

[0027] In some embodiments of this application, the modified iron salt in step B above is a mixture of ferric chloride and tea biochar, and the amount of modified iron salt added is 10-30 g / m³. 3 Mixture I, the mixing time is 1-5 hours.

[0028] In some embodiments of this application, the modified iron salt is prepared by the following method: under an inert gas atmosphere, dried tea leaves are heated to 250-280°C and pyrolyzed into char, and then cooled to obtain tea biochar; the tea biochar is ground and then impregnated in ferric chloride solution, reacted at 60-80°C for 10-18 hours, and then dried to obtain the modified iron salt.

[0029] In some embodiments of this application, the drying temperature of the tea residue is 60-70°C, and the drying time is 10-15 hours. At this temperature, the tea residue can be dried stably without damaging the active ingredients within it.

[0030] In some embodiments of this application, the concentration of the ferric chloride solution is 0.2-0.3 mol / L.

[0031] In some embodiments of this application, the weakly alkaline pH in step C above is 8-9, and the mass percentage of the ferrous sulfate aqueous solution is 25-35%.

[0032] In some embodiments of this application, the sedimentation time in step D is 12-48 hours, and the filtration area of ​​the single-pressure filtration is 40 m². 2 The filtration area of ​​the secondary pressure filter is 20m². 2 The standard for wastewater pH discharge is 7-9. Therefore, this application involves sedimentation for 12-48 hours, and the final filtered water will have a pH of 7-8, resulting in a weakly alkaline discharge that complies with environmental protection principles.

[0033] The features and performance of this application will be further described in detail below with reference to the embodiments.

[0034] Example 1

[0035] A method for removing arsenic and mercury from wastewater includes the following steps:

[0036] A. Wastewater containing arsenic and mercury is fed into the reaction device, and the flow rate of the wastewater is controlled at 5t / h. Hydrogen peroxide is added to the wastewater before the reaction, and the flow rate of the hydrogen peroxide is 50mL / min. After reacting for 1 hour, a calcium hydroxide solution with a mass fraction of 42% is added. The pH of the solution is adjusted to 12 by stirring. After stirring and mixing for 3 hours, mixed solution I is obtained.

[0037] B. After brewing tea, the tea leaves are dried at 70℃ for 12 hours, then transferred to a pyrolysis reactor. Under a nitrogen atmosphere, the temperature is increased to 260℃ at a rate of 20℃ / min for 10 minutes. After cooling to room temperature, the tea leaves are removed to obtain tea residue biochar. The tea residue biochar is ground into powder and impregnated in a 0.3mol / L ferric chloride solution. It is then soaked in a water bath at 70℃ for 10 hours to obtain modified iron salt for later use. Mixing solution I is continuously stirred, and the modified iron salt is added to the mixture at a rate of 20g / m³. 3 Mixture I was stirred and mixed for 2 hours to obtain mixture II;

[0038] C. Perform a single pressure filtration on mixture II, with a filtration area of ​​40 m². 2 After collecting the filtrate, 30% ferrous sulfate was added to adjust the pH to 9, resulting in mixture III.

[0039] D. After treating the mixture III with precipitation for 24 hours, perform a second pressure filtration. The filtration area of ​​the second pressure filtration is 20m². 2 The resulting filtrate was purified water after the removal of arsenic and mercury.

[0040] Example 2

[0041] A method for removing arsenic and mercury from wastewater includes the following steps:

[0042] A. Wastewater containing arsenic and mercury is fed into the reaction device, and the flow rate of the wastewater is controlled at 5t / h. Hydrogen peroxide is added to the wastewater before the reaction, and the flow rate of the hydrogen peroxide is 60mL / min. After reacting for 1.5h, a calcium hydroxide solution with a mass fraction of 42% is added. The pH of the solution is adjusted to 12 by stirring. After stirring and mixing for 3h, mixed solution I is obtained.

[0043] B. After brewing tea, the tea leaves are dried at 65℃ for 15 hours, then transferred to a pyrolysis reactor. Under a nitrogen atmosphere, the temperature is increased to 280℃ at a rate of 20℃ / min for 10 minutes. After cooling to room temperature, the tea leaves are removed to obtain tea residue biochar. The tea residue biochar is ground into powder and impregnated in a 0.2 mol / L ferric chloride solution. It is then soaked in a water bath at 70℃ for 10 hours to obtain modified iron salt for later use. Mixing solution I is continuously stirred, and the modified iron salt is added to the mixture at a rate of 20 g / m³. 3 Mixture I was stirred and mixed for 4 hours to obtain mixture II;

[0044] C. Perform a single pressure filtration on mixture II, with a filtration area of ​​40 m². 2 After collecting the filtrate, 30% ferrous sulfate was added to adjust the pH to 8.5, resulting in mixture III.

[0045] D. After treating the mixture III with precipitation for 30 hours, perform a second pressure filtration. The filtration area of ​​the second pressure filtration is 20m². 2 The resulting filtrate was purified water after the removal of arsenic and mercury.

[0046] Example 3

[0047] A method for removing arsenic and mercury from wastewater includes the following steps:

[0048] A. Wastewater containing arsenic and mercury is fed into the reaction device, and the flow rate of the wastewater is controlled at 5t / h. Hydrogen peroxide is added to the wastewater before the reaction, and the flow rate of the hydrogen peroxide is 60mL / min. After reacting for 1 hour, a 40% calcium hydroxide solution is added. The pH of the solution is adjusted to 11.5 by stirring. After stirring and mixing for 3 hours, mixed solution I is obtained.

[0049] B. After brewing tea, the tea leaves are dried at 65℃ for 15 hours, then transferred to a pyrolysis reactor. Under a nitrogen atmosphere, the temperature is increased to 280℃ at a rate of 20℃ / min for 12 minutes. After cooling to room temperature, the tea leaves are removed to obtain tea residue biochar. The tea residue biochar is ground into powder and impregnated in a 0.2 mol / L ferric chloride solution. It is then soaked in a water bath at 70℃ for 12 hours to obtain modified iron salt for later use. Mixing solution I is continuously stirred, and the modified iron salt is added to the mixture at a rate of 20 g / m³. 3 Mixture I was stirred and mixed for 3 hours to obtain mixture II;

[0050] C. Perform a single pressure filtration on mixture II, with a filtration area of ​​40 m². 2 After collecting the filtrate, ferrous sulfate (28% by mass) was added to adjust the pH to 8.5, resulting in mixture III.

[0051] D. After treating the mixture III with precipitation for 42 hours, perform a second pressure filtration. The filtration area of ​​the second pressure filtration is 20 m². 2 The resulting filtrate was purified water after the removal of arsenic and mercury.

[0052] Example 4

[0053] A method for removing arsenic and mercury from wastewater includes the following steps:

[0054] A. Wastewater containing arsenic and mercury is fed into the reaction device, and the flow rate of the wastewater is controlled at 5t / h. Hydrogen peroxide is added to the wastewater before the reaction, and the flow rate of the hydrogen peroxide is 60mL / min. After reacting for 1 hour, a 35% calcium hydroxide solution is added. The pH of the solution is adjusted to 25 by stirring. After stirring and mixing for 2 hours, mixed solution I is obtained.

[0055] B. After brewing tea, the tea leaves are dried at 65℃ for 15 hours, then transferred to a pyrolysis reactor. Under a nitrogen atmosphere, the temperature is increased to 270℃ at a rate of 20℃ / min for 12 minutes. After cooling to room temperature, the tea leaves are removed to obtain tea residue biochar. The tea residue biochar is ground into powder and then impregnated in a 0.25mol / L ferric chloride solution. The solution is soaked in a water bath at 65℃ for 12 hours to obtain modified iron salt for later use. Mixing solution I is continuously stirred, and the modified iron salt is added to the mixture at a rate of 20g / m³. 3 Mixture I was stirred and mixed for 3 hours to obtain mixture II;

[0056] C. Perform a single pressure filtration on mixture II, with a filtration area of ​​40 m². 2 After collecting the filtrate, ferrous sulfate (25% by mass) was added to adjust the pH to 9, resulting in mixture III.

[0057] D. After treating the mixture III with precipitation for 48 hours, perform a second pressure filtration. The filtration area of ​​the second pressure filtration is 20m². 2 The resulting filtrate was purified water after the removal of arsenic and mercury.

[0058] Comparative Example 1

[0059] The processing method of this comparative example is basically the same as that of Example 1. The similarities will not be repeated here. The difference is that step B is omitted in this comparative example, that is, the modified iron salt is not used to treat the mixture I.

[0060] Comparative Example 2

[0061] The processing method of this comparative example is basically the same as that of Example 1, and the similarities will not be repeated. The difference is that in step B of this comparative example, an equal mass of ferric chloride solution is used instead of modified iron salt.

[0062] Comparative Example 3

[0063] The processing method of this comparative example is basically the same as that of Example 1, and the similarities will not be repeated. The difference is that in step B of this comparative example, the same mass of activated carbon is used instead of modified iron salt, and the mass of activated carbon added is the same as that of modified iron salt.

[0064] Experimental Example

[0065] In this experiment, the wastewater was divided into 7 experimental groups. Before the experiment, the contents of arsenic and mercury in the wastewater were measured. Then, experimental groups 1-7 were treated using the methods of Examples 1-4 and Comparative Examples 1-3, respectively. The contents of arsenic and mercury in the treated water were then measured, and the results are shown in Table 1.

[0066] Table 1

[0067]

[0068]

[0069] As can be seen from Table 1, the methods of Examples 1-4 and Comparative Examples 1-3 of this application are all effective in removing arsenic and mercury from wastewater. Examples 1-4, in particular, show better and more stable results. A comparison with Examples 1 and Comparative Example 1 shows that the use of untreated or unmodified activated carbon and iron salts has limited effectiveness.

[0070] In summary, the method for removing arsenic and mercury from wastewater according to the embodiments of this application has the following advantages:

[0071] This application first adds hydrogen peroxide to the wastewater. Hydrogen peroxide can oxidize trivalent arsenic in the wastewater to pentavalent arsenic, which is less toxic. Moreover, the decomposition products of hydrogen peroxide in the reaction are water and oxygen, which will not introduce impurities into the entire wastewater treatment process, thus embodying an environmental protection concept. Then, calcium oxide is added to adjust the pH of the wastewater to a strongly alkaline state. Under this strongly alkaline environment, arsenic and mercury in the wastewater begin to react with calcium hydroxide to form complexes that are insoluble in water and accumulate and precipitate in the wastewater. After a certain period of reaction and precipitation, this application uses iron salt particles modified from tea residue biochar. Ferric chloride forms an iron oxide layer on the surface of the tea residue biochar. At the same time, the hydroxyl and carboxyl functional groups on the acidic activated carbon generated electrostatic attraction and complexation on the previously precipitated flocs under alkaline conditions, enhancing the adsorption of heavy metals and forming stable, non-agglomerated arsenic and mercury fixatives, which can be separated from water in the subsequent pressure filtration step.

[0072] After the first pressure filtration, this application also adds ferrous sulfate to the wastewater. The addition of ferrous sulfate neutralizes calcium hydroxide, and the resulting calcium sulfate is insoluble in water. On the other hand, ferrous ions can form water-insoluble complexes with arsenic and mercury in an alkaline environment. After precipitation and secondary pressure filtration, arsenic and mercury in the water are further removed, and the pH value of the final water is adjusted to meet the standard of weak alkalinity, which has a good environmental protection awareness.

[0073] This application standardizes the operation, and the process path can be automated in practical applications. It has parameters and standard operating procedures, which solves the technical problems of arbitrary operation, lack of accurate parameters, and unstable processing results in the prior art.

[0074] The embodiments described above are some, but not all, of the embodiments of this application. The detailed description of the embodiments of this application is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

Claims

1. A method for removing arsenic and mercury from wastewater, characterized in that, Includes the following steps: A. Add hydrogen peroxide to the wastewater, allow it to react fully, then add calcium hydroxide to adjust the pH to a strongly alkaline state. After stirring and mixing, obtain mixture I. B. Add modified iron salt to mixture I, stir and mix to obtain mixture II; the modified iron salt is a mixture of ferric chloride and tea biochar, and the amount of modified iron salt added is 10-30 g / m3 of mixture I; the stirring time is 1-5 h; C. Perform a single pressure filtration on mixture II, collect the filtrate, add ferrous sulfate to adjust the pH to weakly alkaline, and obtain mixture III; D. The mixture III is subjected to precipitation and secondary pressure filtration, and the filtrate is collected to obtain water after the removal of arsenic and mercury.

2. The method for removing arsenic and mercury from wastewater according to claim 1, characterized in that, In step A, the volume ratio of wastewater to hydrogen peroxide is 1000:(0.5-1), and the reaction time is 1-3 hours.

3. The method for removing arsenic and mercury from wastewater according to claim 1, characterized in that, In step A, the pH of the strong alkali is 11-13, the concentration of the calcium hydroxide is 35-45%, and the stirring and mixing time is 2-5 hours.

4. The method for removing arsenic and mercury from wastewater according to claim 1, characterized in that, The modified iron salt is prepared by the following method: under an inert gas atmosphere, dried tea leaves are heated to 250-280℃ and pyrolyzed into char, and then cooled to obtain tea biochar; the tea biochar is ground and then impregnated in ferric chloride solution, reacted at 60-80℃ for 10-18 hours, and then dried to obtain the modified iron salt.

5. The method for removing arsenic and mercury from wastewater according to claim 4, characterized in that, The drying temperature of the tea residue is 60-70℃, and the drying time is 10-15h.

6. The method for removing arsenic and mercury from wastewater according to claim 4, characterized in that, The concentration of the ferric chloride solution is 0.2-0.3 mol / L.

7. The method for removing arsenic and mercury from wastewater according to claim 1, characterized in that, In step C, the weakly alkaline pH is 8-9, and the ferrous sulfate aqueous solution has a mass percentage of 25-35%.

8. The method for removing arsenic and mercury from wastewater according to claim 1, characterized in that, The time of precipitation in said step D is 12-48 h, the filtration area of the first pressure filtration is 40 m 2 , and the filtration area of the second pressure filtration is 20 m 2 .

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