CN108558086A - A kind of group technology removing hexavalent selenium in water removal - Google Patents

Abstract

The invention discloses a combined process for removing hexavalent selenium in water, belonging to the technical field of water treatment. The steps are as follows: (1) adding sodium sulfite; (2) adjusting pH; (3) ultraviolet irradiation; (4) adding ferric salt and alkali solution and stirring for solid-liquid separation; (5) bubbling into the liquid Air or oxygen, or add hydrogen peroxide solution and stir; (6) Add alkali to adjust pH. Compared with the prior art, the combined process proposed by the present invention has the following advantages: (1) the processing speed is fast and the effect is good; (2) the cost of adding reagents is low and the environment is friendly; (3) it is not affected by coexisting ions such as sulfate; (4) The method of controlling the dosage of coagulation reagent is convenient and accurate, which can ensure the coagulation effect.

Description

A combined process for removing hexavalent selenium in water

technical field

The invention belongs to the technical field of water treatment, and more specifically relates to a combined process for efficiently removing hexavalent selenium in water.

Background technique

Selenium is a trace essential element for life activities, but excessive selenium will have negative effects on animal and plant growth and human health. Water is an important source of selenium intake for humans, animals and plants, and the wastewater discharged from human activities, such as coal burning and other fuels, smelting, mining, etc., often contains a large amount of high concentration of selenium, polluting groundwater and surface water. In my country's current comprehensive sewage discharge standards, the first, second, and third-level selenium discharge standards are 0.1, 0.2, and 0.5 mg/L, respectively, and the selenium limits for drinking water, I-III surface water, and I-III groundwater are all the same. 10μg/L. Therefore, selenium-containing wastewater needs to be strictly treated to meet the standard limit.

In the aquatic environment, selenium mainly exists in the form of hexavalent selenium (selenate), tetravalent selenium (selenite), zero-valent selenium and negative divalent selenium. Tetravalent selenium is easily adsorbed on metal oxides, clays, and organic matter to form a firm inner layer coordination, but hexavalent selenium and these adsorbents mainly form outer layer coordination complexes, which have weak binding force and are affected by the coexistence of ions. Greater impact. The current methods for removing hexavalent selenium in water mainly include ion exchange, adsorption, coagulation, and chemical reduction. Although ion exchange, adsorption, and coagulation can effectively remove tetravalent selenium in water, the selectivity to hexavalent selenium is poor. When the water contains a variety of coexisting ions, especially high-concentration sulfate, the removal rate of selenium is greatly reduced, and it is difficult to Realize the deep removal of selenium.

The removal of hexavalent selenium by chemical reduction is considered to be an effective treatment method, but the reduction of hexavalent selenium by common homogeneous reducing agents (such as sodium sulfite, sodium pyrosulfite, hydrazine hydrate, etc.) requires harsh conditions (high temperature, extreme pH, etc.) Or noble metal catalysis is required, as reported in "Chemical Letters" (Chemistry Letters, 2015, Vol. 44, pp. 1563-1565) in aqueous solution, the reductive removal of selenium by hydrazine relies on platinum catalysts loaded with TiO ₂ . For example, "Chemical Engineering Journal" (Chemical Engineering Journal, 2017, volume 308, pages 963-973) reported that in aqueous solution, the reductive removal of selenium by hydrazine hydrate relies on platinum catalysts. Moreover, even in the presence of catalysis, the removal takes a long time (requires 8 hours), the conversion rate is low, and the cost of large-scale water treatment is high.

The conditions for the heterogeneous reduction of hexavalent selenium by zero-valent iron are relatively mild, and hexavalent selenium can be reduced to tetravalent selenium and further reduced to zero-valent selenium, but the method is generally slow. -Environment" (AppliedCatalysisB-Environmental, 2011, volume 104, pages 185-192) reported that 1g/L of zero-valent iron was used to remove hexavalent selenium with an initial concentration of 10mg/L under the condition of pH=8, and it took 10h Just can reach the removal efficiency of more than 90%; And, realize the time that the removal of hexavalent selenium needs especially long under the coexistence condition of a large amount of sulfate radicals, for example document " Environmental Quality Magazine " (Journal of Environmental Quality, 2005, 34 volumes, Pages 487-495) reported that 50g/L 40-60 mesh zero-valent iron removes hexavalent selenium with an initial concentration of 1mg/L, and it can reach more than 99% in 7h when 10mmol/ ^LCl- coexists, while at 10mmol/LCl- When LSO ₄ ^2- coexisted, it took 16 hours to achieve the removal rate of 93%.

Heterogeneous reduction of hexavalent selenium by using new ecological ferrous hydroxide (ferrous hydroxy complex) can reduce tetravalent selenium relatively quickly, but this method must be carried out under strict anaerobic conditions, and the presence of dissolved oxygen makes hydrogen Ferrous oxide quickly fails, so there are certain limitations in the application range of hexavalent selenium using the new ecological ferrous hydroxide reduction method.

Hydrated electrons are among the most active reducing species known so far. Under the excitation of ultraviolet rays, sulfite can undergo photoionization to release hydrated electrons. Using the photochemical properties of sulfite, hydrated electrons can be generated by constructing a UV/sulfite system, as shown in Journal of Environmental Sciences (Journal of Environmental Sciences, 2017, Volume 54, Pages 231-238) reported that the UV/sulfite system can be used for the reductive removal of 4-bromophenol.

In "Reductive Dehalogenation Efficiency of Sulfite/UV System", it is pointed out that the system can degrade and dehalogenate toxic halogenated organic compounds (HOCs), and discuss the related mechanism. The results show that the type of halogen substitution and the degree of halogenation determine the direct photolysis characteristics of HOCs, and the rate increases with the increase of chlorine, bromine, iodine substitution order and halogenation degree. It can be seen that the action mechanism of the sulfite/ultraviolet system is relatively complicated. In addition, the removal of hexavalent selenium is affected by many factors such as the concentration of dissolved oxygen in water and other high-concentration coexisting ions. Can this system be used for the removal of hexavalent selenium? Elimination has not been reported in the literature so far.

Contents of the invention

1. The problem to be solved

Aiming at the problems that existing methods for removing hexavalent selenium in water have poor selectivity, low removal rate and long time consumption when other ions coexist at high concentrations, the present invention aims to provide a method for quickly and efficiently removing hexavalent selenium in water.

2. Technical solution

In order to solve the above problems, the technical scheme adopted in the present invention is as follows:

The invention provides a combined process for removing hexavalent selenium in water, comprising the following steps:

1) adding sodium sulfite to the water containing hexavalent selenium, and mixing to obtain a solution;

2) adjusting the pH of the solution treated in step 1) to 8-11;

3) irradiating the solution treated in step 2) with ultraviolet light;

4) adding ferric salt and alkali solution to the solution treated in step 3) and stirring, solid-liquid separation after coagulation and precipitation to obtain the separated solution; the added alkali solution is sodium hydroxide or calcium hydroxide solution, The purpose is to provide hydroxide ions.

5) Bubble air or oxygen into the separated solution in step 3), or add hydrogen peroxide solution and stir.

As a further improvement of the present invention, in step 1), the molar ratio of sodium sulfite to hexavalent selenium in water is (5-80): 1, and the concentration of sulfite in the mixed solution after adding sodium sulfite is not less than 1mmol/L .

As a further improvement of the present invention, in step 3), the ultraviolet irradiation time is 20-40 minutes.

As a further improvement of the present invention, in step 4), the molar ratio of Fe(III) and hexavalent selenium in the water after adding the ferric salt is (5-30): 1, and the hydroxide and the hexavalent selenium in the water after adding the alkali solution The molar ratio of Fe(III) added is (1-2.8):1.

As a further improvement of the present invention, the added ferric salt is ferric chloride or ferric sulfate.

As a further improvement of the present invention, in step 4), the stirring includes fast stirring and slow stirring in sequence.

As a further improvement of the present invention, the fast stirring speed is 150-250 rpm, and the duration is 1-5 min; the slow stirring speed is 20-60 rpm, and the duration is 10-40 min.

As a further improvement of the present invention, in step 5), the solid-liquid separation method is one or a combination of sedimentation, filtration or centrifugation.

As a further improvement of the present invention, the step 5) further includes a pH adjustment step, adjusting the pH value to 6.5-9.0.

As a further improvement of the present invention, the step 2) adjusts the pH of the solution treated in step 1) to 9-10. 3. Beneficial effects

Compared with the prior art, the beneficial effects of the present invention are:

(1) The combined process for removing hexavalent selenium in water of the present invention, first, adopts the generation of hydrated electrons based on ultraviolet/sulfite, and efficiently reduces hexavalent selenium to tetravalent selenium, and the highest conversion rate can reach 99%; The steps have the advantages of short time and high efficiency, and usually only need 10 to 30 minutes. Compared with the time scale of more than ten hours or even several days for the reduction of hexavalent selenium by zero-valent iron, the processing time is greatly reduced. In addition, ferric salt and alkaline solution are used for coagulation treatment to further remove the tetravalent selenium generated by reduction, and the residual sulfite after coagulation is converted into non-toxic and harmless sulfate through a low-cost removal method, and finally the water The total selenium reached the discharge standard, and sulfite was not detected. The entire treatment process achieved efficient and thorough removal of total selenium.

(2) The combined process for removing hexavalent selenium in water of the present invention can still work efficiently in the presence of high concentrations of other ions, such as sulfate radicals and chloride ions. Existing methods such as adsorption and ion exchange are greatly affected by the suppression of coexisting ions, and it is difficult to meet increasingly stringent emission standards. The zero-valent iron reduction method belongs to heterogeneous reduction, and adsorption is the premise of heterogeneous reduction, so in When high-concentration sulfate coexists, the treatment rate is extremely slow, and the removal rate is also inhibited; therefore, the method of the present invention adopts the sulfite/ultraviolet system to generate hydrated electrons and the reaction of hexavalent selenium is a homogeneous reaction, and there is no influence of competitive adsorption . Moreover, after the hexavalent selenium is reduced to tetravalent selenium, the tetravalent selenium forms a firm inner layer coordination with the iron oxide formed in the coagulation stage, and is not affected by the inhibition of coexisting sulfate radicals, and is further effectively removed by coagulation. Thereby reaching the discharge standard of total selenium.

(3) The combined process of removing hexavalent selenium in water of the present invention strictly controls the pH value of the solution before ultraviolet irradiation, because in the inventive method, in the step of reducing hexavalent selenium by the hydration electrons produced by the ultraviolet/sulfite system, The reduction conversion rate is greatly affected by the pH of the solution, and the reduction conversion rate of hexavalent selenium under the condition of pH<8 is very low, so the pH value is strictly controlled at 8-11 before ultraviolet irradiation to ensure a higher conversion of hexavalent selenium The results showed that 99% of hexavalent selenium was reduced to tetravalent selenium under this condition.

(4) The combined process of removing hexavalent selenium in water of the present invention adopts the total amount method to control the amount of iron and hydroxide radicals added without adopting the method for adjusting pH, which is convenient and accurate; because the effect of ferric salt coagulation is extremely It is easily affected by pH, and the pH itself changes continuously during the coagulation process, so adjusting the pH is very cumbersome. Even if the transient pH is adjusted accurately, the pH will change again due to the subsequent changes in the form of iron such as polymerization, and the coagulation effect cannot be guaranteed. . The present invention adopts the method of controlling the molar ratio of hydroxide and ferric salt to accurately control the zeta potential of the flocs to be positive, and make the generated iron flocs have a suitable degree of polymerization to strengthen the adsorption capacity, which can ensure coagulation effect.

(5) The combined process for removing hexavalent selenium in water of the present invention uses ultraviolet/sulfite reduction and coagulation of ferric iron in combination, realizes the rapid and efficient removal of hexavalent selenium in water step by step, and properly handles residual sulfite and The pH of the effluent is acidic, and the entire combined process is simple to operate, low in cost, and environmentally friendly, which is conducive to popularization.

Detailed ways

The present invention will be further described below in conjunction with specific embodiments.

Example 1

This embodiment is a combined process for removing hexavalent selenium in water under the coexistence of high-concentration sulfate. The combined process includes the following steps:

1) Use an aqueous solution containing 10 mg/L Se(VI), 1000 mg/L SO ₄ ^2- , and an initial pH=7.0 to simulate an aqueous solution containing hexavalent selenium. The total volume of the simulated aqueous solution is 50 mL. Add 0.5 The sodium sulfite solution 0.5mL of mol/L, obtains mixed solution after mixing; The mol ratio of the sodium sulfite added and hexavalent selenium in the aqueous solution is 39.5:1, and the concentration of sulfite in the water after adding theoretically in the present embodiment is 5mmol/L, because dissolved oxygen will consume a small amount of sulfite, the concentration of sulfite in the water after the actual addition is determined to be 4.2mmol/L by iodometric method.

2) It is determined that the pH value of the solution in step 1) is 9.2, which meets the requirement of a pH value of 8-11, and no pH adjustment is performed.

3) After the solution treated in step 2) was irradiated with ultraviolet rays by a medium-pressure mercury lamp for 30 minutes, it was measured by an atomic fluorescence spectrometer, and the results showed that 99% of Se(VI) was reduced to Se(IV).

4) Add 1.5mmol/L FeCl ₃ and 2.7mmol/L NaOH to the solution after step 3), and carry out coagulation precipitation, filtration separation through fast stirring, slow stirring, obtain the solution after separation; In the present embodiment The fast stirring time is 2min, the rotating speed is 200r/min, the slow stirring time is 15min, and the rotating speed is 40r/min.

In this example, the molar ratio of Fe(III) to Se(VI) in the original aqueous solution is 11.9:1, and the molar ratio of hydroxide to Fe(III) in the solution after adding NaOH is 1.8:1. The precise control of the molar ratio of Fe(III) ensures a high removal rate of Se.

5) Pass oxygen into the separated solution in step 3) for 2 minutes, measure the pH to be 3.5, add NaOH solution to adjust to pH 7.0, due to flocculation and stirring, although most of the sulfite in the solution has been oxidized to sulfuric acid However, there are still some residual sulfite ions, which may have an adverse effect on the quality of the effluent. In this step, an added amount of air is used to achieve the purpose of complete removal.

6) Measurement of results: the sulfite content in the effluent was not detected, and the residual Se was 0.18 mg/L.

Example 2

This embodiment is a combined process for removing hexavalent selenium in water under the coexistence of high-concentration chloride ions. The combined process includes the following steps:

1) Use an aqueous solution containing 23.7 mg/L Se(VI), 10 mmol/L Cl ^- , and initial pH = 8.0 to simulate an aqueous solution containing hexavalent selenium. The total volume of the simulated aqueous solution is 50 mL. Add 0.5 mol/L 0.15mL of sodium sulfite solution in L, mix well to obtain a mixed solution; the molar ratio of the added sodium sulfite to the hexavalent selenium in the aqueous solution is 5:1, and the concentration of sulfite in the water after the theoretical addition in this embodiment is 1.5mmol /L, because dissolved oxygen will consume a small amount of sulfite, the concentration of sulfite in the water after the actual addition is determined to be 1.1mmol/L by iodometric method.

2) It is determined that the pH value of the solution in step 1) is 9.2, and no pH adjustment is required.

3) After the solution treated in step 2) was irradiated with ultraviolet rays by a medium-pressure mercury lamp for 20 minutes, it was measured by an atomic fluorescence spectrometer, and the results showed that 99% of Se(VI) was reduced to Se(IV).

4) Add 4.5mmol/L Fe ₂ (SO ₄ ) ₃ and 9mmol/L NaOH to the solution treated in step 3), and carry out coagulation precipitation and centrifugation through rapid stirring and slow stirring to obtain the separated solution Carrying out fast stirring time in the present embodiment is 5min, and rotating speed is 150r/min, and slow stirring time is 40min, and rotating speed is 20r/min.

In this example, the molar ratio of Fe(III) to Se(VI) in the original aqueous solution is 30:1, and the molar ratio of hydroxide to Fe(III) in the solution after adding NaOH is 1:1. The precise control of the molar ratio of Fe(III) ensures a high removal rate of Se.

5) 1 mmol/L H ₂ O ₂ was added to the separated solution in step 3), the measured pH was 3.2, and NaOH solution was added to adjust the pH to 9.0.

6) Measurement of results: the sulfite content in the effluent was not detected, and the residual Se was 0.42mg/L.

Example 3

This embodiment is a combined process for removing hexavalent selenium in water under the coexistence of high-concentration sulfate ions. The combined process includes the following steps:

1) An aqueous solution containing 10mg/L Se(VI), 1000mg/L SO ₄ ^2- , and initial pH=3.0 simulates an aqueous solution containing hexavalent selenium. The total volume of the simulated aqueous solution is 50mL. Add 0.5mol/ 0.5mL of sodium sulfite solution in L, mixed to obtain a mixed solution; the molar ratio of the added sodium sulfite to the hexavalent selenium in the aqueous solution was 39.5:1, and the concentration of sulfite in the water after the theoretical addition in this embodiment was 5mmol/ L, because dissolved oxygen will consume a small amount of sulfite, the concentration of sulfite in the water after the actual addition is determined to be 4.2mmol/L by iodometric method.

2) It is determined that the pH value of the solution in step 1) is 6.9, and the pH value is adjusted to 8.0 with sodium hydroxide.

3) After the solution treated in step 2) was irradiated with ultraviolet rays by a medium-pressure mercury lamp for 30 minutes, it was measured by an atomic fluorescence spectrometer, and the results showed that 99% of Se(VI) was reduced to Se(IV).

4) Add 1.5mmol/L FeCl ₃ and 2.7mmol/L NaOH to the solution after step 3), and carry out coagulation precipitation, filtration separation through fast stirring, slow stirring, obtain the solution after separation; In the present embodiment The fast stirring time is 2min, the rotating speed is 200r/min, the slow stirring time is 15min, and the rotating speed is 40r/min.

In this example, the molar ratio of Fe(III) to Se(VI) in the original aqueous solution is 11.9:1, and the molar ratio of hydroxide to Fe(III) in the solution after adding NaOH is 1.8:1. The precise control of the molar ratio of Fe(III) ensures a high removal rate of Se.

5) Air was passed through the separated solution in step 3) for 2 minutes, the pH was measured to be 3.6, and NaOH solution was added to adjust the pH to 7.0.

6) Determination of results: the content of sulfite in the water was not detected, and the residual Se was 0.18mg/L.

Example 4

This embodiment is a combined process for removing hexavalent selenium in water under the coexistence of multiple ions. The combined process includes the following steps:

1) Use an aqueous solution containing 10mg/L Se(VI), 1000mg/L Cl ^- , 1000mg/L SO ₄ ^2- , 10mg/L PO ₄ ^3- and 5mg/LNO ₃ ^- with an initial pH of 7.0 to simulate hexavalent The aqueous solution of selenium, the volume of the simulated aqueous solution is 50mL, the total volume of the simulated aqueous solution is 50mL, 1.01mL of 0.5mol/L sodium sulfite solution is added to the aqueous solution, and a mixed solution is obtained after mixing; the added sodium sulfite and aqueous solution The molar ratio of hexavalent selenium in the medium is 80:1. In this embodiment, the concentration of sulfite in the water after adding theoretically is 10.1mmol/L. Since dissolved oxygen will consume a small amount of sulfite, the amount of sulfite in the water after actually adding The concentration was determined to be 9.3mmol/L by iodometric method.

2) It is determined that the pH value of the solution in step 1) is 7.0, and the pH value is adjusted to 11.0 with sodium hydroxide.

3) After the solution treated in step 2) was irradiated with ultraviolet light by a medium-pressure mercury lamp for 40 minutes, it was measured by an atomic fluorescence spectrometer, and the results showed that 99% of Se(VI) was reduced to Se(IV).

4) Add 0.63mmol/L FeCl ₃ and 1.77mmol/L NaOH to the solution after step 3), and carry out coagulation precipitation, filtration separation through fast stirring, slow speed stirring, obtain separated solution; In the present embodiment The fast stirring time is 1min, the rotating speed is 250r/min, the slow stirring time is 10min, and the rotating speed is 60r/min.

In this embodiment, the molar ratio of Fe(III) to Se(VI) in the original aqueous solution is 5:1, and the molar ratio of hydroxide to Fe(III) in the solution after adding NaOH is 2.8:1. The precise amount control of the molar ratio of Fe(III) ensures a high removal rate of Se in this step.

5) Air was passed through the separated solution in step 3) for 2 minutes, the pH was measured to be 4.6, and NaOH solution was added to adjust the pH to 6.5.

6) Measurement of results: the sulfite content in the effluent was not detected, and the residual Se was 0.33 mg/L.

Comparative example A

This comparative example A is a comparative example in which pH is not controlled by ultraviolet irradiation.

1) An aqueous solution containing 10mg/L Se(VI), 1000mg/L SO ₄ ^2- , and initial pH=3.0 simulates an aqueous solution containing hexavalent selenium. The total volume of the simulated aqueous solution is 50mL. Add 0.5mol/ 0.5mL of sodium sulfite solution in L, mixed to obtain a mixed solution; the molar ratio of the added sodium sulfite to the hexavalent selenium in the aqueous solution was 39.5:1, and the concentration of sulfite in the water after the theoretical addition in this embodiment was 5mmol/ L, because dissolved oxygen will consume a small amount of sulfite, the concentration of sulfite in the water after the actual addition is determined to be 4.2mmol/L by iodometric method.

2) It is determined that the pH value of the solution in step 1) is 6.9, and no pH adjustment is performed in this step.

3) After the solution treated in step 2) was irradiated with ultraviolet light by a medium-pressure mercury lamp for 30 minutes, it was measured by an atomic fluorescence spectrometer, and the result showed that 43% of Se(VI) was reduced to Se(IV).

4) Add 1.5mmol/L FeCl ₃ and 2.7mmol/L NaOH to the solution after step 3), and carry out coagulation precipitation, filtration separation through fast stirring, slow stirring, obtain the solution after separation; In the present embodiment The fast stirring time is 2min, the rotating speed is 200r/min, the slow stirring time is 15min, and the rotating speed is 40r/min.

In this example, the molar ratio of Fe(III) to Se(VI) in the original aqueous solution is 11.9:1, and the molar ratio of hydroxide to Fe(III) in the solution after adding NaOH is 1.8:1. The precise control of the molar ratio of Fe(III) ensures a high removal rate of Se in this step.

5) Air was passed through the separated solution in step 3) for 2 minutes, the pH was measured to be 3.6, and NaOH solution was added to adjust the pH to 7.0.

6) Results measurement: the sulfite content in the effluent was not detected, but the residual Se was as high as 4.5 mg/L, and the residual Se was not effectively removed.

The reason why only a small amount of hexavalent selenium was reduced to tetravalent selenium in Comparative Example A was that the reduction conversion rate of hexavalent selenium under the condition of pH<8 was very low. Since hexavalent selenium was not effectively reduced, the final residual Se was not effectively removed.

Comparative Example B

This comparative example B is a comparative example in which the molar ratio of hydroxide to Fe(III) is not precisely controlled.

1) Use an aqueous solution containing 10mg/L Se(VI), 1000mg/L Cl ^- , 1000mg/L SO ₄ ^2- , 10mg/L PO ₄ ^3- and 5mg/LNO ₃ ^- with an initial pH of 7.0 to simulate hexavalent An aqueous solution of selenium, the volume of the simulated aqueous solution is 50mL, 1.01mL of sodium sulfite solution of 0.5mol/L is added to the aqueous solution, and a mixed solution is obtained after mixing; the mol ratio of the added sodium sulfite to the aqueous solution is 80 : 1, the concentration of sulphite in the water after adding in theory is 10.1mmol/L in the present embodiment, because dissolved oxygen can consume a small amount of sulphite, the concentration of sulphite in the water after actually adding adopts iodometric method to measure as 9.3 mmol/L.

2) It is determined that the pH value of the solution in step 1) is 7.0, and the pH value is adjusted to 11.0 with sodium hydroxide.

3) After the solution treated in step 2) was irradiated with ultraviolet light by a medium-pressure mercury lamp for 40 minutes, it was measured by an atomic fluorescence spectrometer, and the results showed that 99% of Se(VI) was reduced to Se(IV).

4) Add 0.63mmol/L FeCl ₃ and 1.77mmol/L NaOH to the solution after step 3), and carry out coagulation precipitation, filtration separation through fast stirring, slow speed stirring, obtain separated solution; In the present embodiment The fast stirring time is 1min, the rotating speed is 250r/min, the slow stirring time is 10min, and the rotating speed is 60r/min.

In this comparative example, the molar ratio of Fe(III) to Se(VI) in the original aqueous solution is 5:1, and the molar ratio of added sodium hydroxide to Fe(III) is 3:1.

5) Air was passed through the separated solution in step 3) for 2 minutes, the pH was measured to be 4.6, and NaOH solution was added to adjust the pH to 6.5.

6) Results measurement: the sulfite content in the effluent was not detected, but the residual Se was as high as 1.9 mg/L, and the residual Se was not effectively removed.

Since the molar ratio of hydroxide to Fe(III) in the solution after adding NaOH in this comparative example is 3:1; usually when the molar ratio of added hydroxide to added iron>2.9, floc electrical reaction will occur. However, the removal rate of Se by flocculation is very low, so the residual Se has not been effectively removed.

The above schematically describes the present invention and its implementation, and the description is not restrictive. Therefore, if a person of ordinary skill in the art is inspired by it, without departing from the inventive concept of the present invention, he or she can design Structural modes and embodiments similar to the technical solution shall belong to the protection scope of the present invention.

Claims (10)

1. a combination process for removing hexavalent selenium in water, is characterized in that: comprise the following steps: 1) adding sodium sulfite to the water containing hexavalent selenium, and mixing to obtain a solution; 2) adjusting the pH value of the solution treated in step 1) to 8-11; 3) irradiating the solution treated in step 2) with ultraviolet light; 4) adding ferric salt and alkali solution to the solution treated in step 3), stirring, coagulating and precipitating, and then performing solid-liquid separation to obtain the separated solution; 5) Bubble air or oxygen into the separated solution in step 3), or add hydrogen peroxide solution and stir. 2. The combined process for removing hexavalent selenium in water according to claim 1, characterized in that: in step 3), the ultraviolet irradiation time is 20-40 minutes. 3. according to claim 1 and the combined process of removing hexavalent selenium in water described in 2, it is characterized in that: in step 4), after adding alkali solution, the mol ratio of hydroxide radical in water and the added Fe(III) is ( 1～2.8):1. 4. the combined process of removing hexavalent selenium in water according to claim 3 is characterized in that: in step 4), the mol ratio of Fe(III) and hexavalent selenium in the water after adding the ferric salt is (5～ 30): 1. 5. The combined process for removing hexavalent selenium in water according to claim 1 or 4, characterized in that: in step 1), the molar ratio of sodium sulfite to hexavalent selenium in water is (5-80): 1, After adding sodium sulfite, the concentration of sulfite in the mixed solution should not be lower than 1mmol/L. 6. The combined process for removing hexavalent selenium in water according to claim 5, characterized in that: in step 4), the added ferric salt is ferric chloride or ferric sulfate. 7. The combined process for removing hexavalent selenium in water according to claim 5, characterized in that: in step 4), the stirring includes fast stirring and slow stirring in sequence. 8. The combined process for removing hexavalent selenium in water according to claim 7, characterized in that: the fast stirring speed is 150 to 250 rpm, and the duration is 1 to 5 minutes; the slow stirring speed is 20 to 60 rpm, and the duration is 10 to 5 minutes. 40min. 9. The combined process for removing hexavalent selenium in water according to claim 1 or 2, characterized in that in step 5), the solid-liquid separation method is any one or combination of filtration or centrifugation. 10. The combined process for removing hexavalent selenium in water according to claim 1 or 2, characterized in that: after the step 5), a pH adjustment step is also included to adjust the pH value to 6.5-9.0.