KR101879744B1 - Removing method of selenium in wastewater - Google Patents

Abstract

The present invention relates to a method for removing selenium in wastewater and a soluble selenium remover for the same. More particularly, the present invention relates to a method for removing selenium in wastewater by removing soluble selenium containing sodium borohydride, a base and a barium compound.

Description

Technical Field [0001] The present invention relates to a method for removing selenium from wastewater,

The present invention relates to a method for removing selenium in wastewater, and a soluble selenium remover for the same. More particularly, the present invention relates to a method for removing selenium in wastewater by removing soluble selenium containing sodium borohydride and a base and further a barium compound.

Selenium (Se) belongs to group 16 in the periodic table and has chemical properties close to that of sulfur (S) and tellurium (Te), and oxidation number has +6, +4, 0, -2 as shown in Fig.

In the industrial field, the ore is generated as a by-product in the extraction of the ore, and the discharge standard of selenium is set at 1 mg / ℓ (in the Canadian area) in the Law on the Protection of Water Quality and Aquatic Ecosystem. However, it is known that Se ⁴⁺ and Se ⁶⁺ are very stable and can not be removed by a conventional wastewater treatment method. Especially, Se ⁶⁺ is almost impossible.

Currently, Se removal technologies applied or proposed in the water treatment industry are as follows:

(1) adsorption treatment using an adsorbent,

(2) coagulation / coagulation treatment,

(3) Treatment with ion exchange resin,

(4) Treatment with nano-type dissimilar metal particles or zero valence iron.

However, the removal efficiency is very low in cases (1), (2), and (3), and the case (4) is in progress for research purpose.

Korean Patent Laid-Open Publication No. 2001-0031417 (published on Jan. 16, 2001) discloses a technique for adsorbing / precipitating selenium dissolved in various wastewaters by iron salts and potassium permanganate to remove them. However, a series of continuous stirring reactors and centrifuges are essential And it has a disadvantage of low competitiveness in terms of equipment cost and operating cost.

US Patent Publication No. 2012-0241381 discloses a technique of generating HSe gas under strongly acidic conditions after precipitation with iron salts and recovering it in a vacuum condition. However, the operation conditions are severe and the cost of maintaining vacuum is high In addition, there is a weak point in terms of equipment cost and operating cost.

Therefore, there is a great demand in industry for the removal of selenium dissolved in wastewater.

Korean Patent Publication No. 2001-0031417 (published on January 16, 2001) US Open Patent No. 2012-0241381 (released on September 27, 2012)

SUMMARY OF THE INVENTION The present invention has been conceived to solve the above-mentioned problems, and it is an object of the present invention to provide a method for removing selenium in an oxidized state, which is difficult to remove, by effectively reducing the selenium to strong reducing agent sodium borohydride or ion- The present invention provides a method for removing selenium.

It is still another object of the present invention to provide a remover for soluble selenium containing sodium borohydride, a base and a barium compound, which is used in a method for removing selenium in the wastewater.

In order to achieve the above object, the method for removing selenium in wastewater according to the present invention is characterized in that, in wastewater containing selenium, 1 part by weight of sodium borohydride and 2 to 500 parts by weight, preferably 3 to 400 parts by weight, And adding 5 to 300 parts by weight of the resin.

The method for removing selenium in wastewater according to the present invention may further comprise, before, after or simultaneously with the step of adding sodium borohydride, 0.5 to 500 parts by weight, preferably 0.6 to 450 parts by weight, of barium compound per part by weight of sodium borohydride, More preferably 0.7 to 400 parts by weight, based on the total weight of the composition.

In addition, the method for removing selenium in the wastewater according to the present invention may further comprise, before, after or simultaneously with the step of adding sodium borohydride, an aluminum coagulant per 1 part by weight of sodium borohydride in the wastewater; The aluminum condensing agent and polyamine; The aluminum condenser and PolyDADMAC; Or 0.01 to 200 parts by weight, preferably 0.03 to 150 parts by weight, more preferably 0.05 to 100 parts by weight, of the aluminum coagulant, polyamine and polybrominated diphenylmethane.

In addition, the barium compound may be selected from the group consisting of barium chloride, barium hydroxide, and mixtures thereof.

The aluminum condensing agent may be selected from the group consisting of aluminum chloride, aluminum sulfate, sodium aluminate, alum, polyaluminium chloride (PAC), polyaluminium chloride silicate (PACS), polyaluminum silicate sulfate (PASS) PAHCS (polyaluminium hydroxy chlorosulfate), and mixtures thereof.

In addition, the base may be selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, and mixtures thereof.

The sodium borohydride may be added in an amount of 5 to 1,000 parts by weight, preferably 7 to 800 parts by weight, more preferably 10 to 600 parts by weight, per 100 parts by weight of selenium in the wastewater.

On the other hand, the soluble selenium remover of the present invention comprises 1 part by weight of sodium borohydride and 2 to 500 parts by weight, preferably 3 to 400 parts by weight, more preferably 5 to 300 parts by weight of a base in one form of the same formulation .

The soluble selenium remover of the present invention may further comprise 0.5-500 parts by weight, preferably 0.6-450 parts by weight, more preferably 0.7-400 parts by weight of the barium compound per part by weight of the sodium borohydride, separately from the soluble selenium scavenger And may be further included in the formulation.

In addition, the soluble selenium remover of the present invention may further comprise an aluminum coagulant, the aluminum coagulant and the polyamine, the aluminum coagulant and the polyDADMAC, or the aluminum coagulant, the polyamine and the polybrominated dodecane 0.01 to 200 parts by weight per 1 part by weight of the sodium borohydride. Preferably 0.03 to 150 parts by weight, more preferably 0.05 to 100 parts by weight, based on 100 parts by weight of the composition.

The present invention is characterized by using selenium borohydride to convert selenium into a reducing state which is easy to remove, and to use a barium compound together for ionic bonding with barium ions or for adsorption removal on other solid salts. Further, by adding an aluminum flocculant, selenium atoms and ions in a colloidal state can be coagulated / coagulated and removed. As a result, it is possible to overcome the limitations of the prior art by using expensive equipments such as centrifugation or vacuum recovery or reacting under strongly acidic conditions. As a result, the present invention can economically remove selenium in various concentration ranges It has the advantage of being able to.

FIG. 1 is a conceptual diagram showing the presence form of selenium and the change in oxidation number in each form.
2 is a graph showing selenium removal rate when sodium borohydride and a base of the present invention are added.
FIG. 3 is a graph showing selenium removal rates when the sodium borohydride, the base, the barium compound and the aluminum coagulant of the present invention are added.

Hereinafter, preferred embodiments of the present invention will be described in detail. In the following description, numerous specific details, such as specific elements, are set forth in order to provide a thorough understanding of the present invention, and it is to be understood that the present invention may be practiced without these specific details, It will be obvious to those who have knowledge of. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

The method for removing selenium in wastewater according to the present invention comprises the steps of adding 1 part by weight of sodium borohydride and 2 to 500 parts by weight, preferably 3 to 400 parts by weight, more preferably 5 to 300 parts by weight, of sodium borohydride to wastewater containing selenium And a control unit.

As described above, Se ⁴⁺ and Se ⁶⁺ are in a very stable state in an aqueous solution, and thus it is difficult to remove them by a general wastewater treatment method. Especially, Se ⁶⁺ is almost impossible.

Therefore, it is important to convert Se ⁶⁺ into Se ⁺⁴ and Se ⁰ forms and reduce Se ⁴⁺ to Se ⁰ as shown in Reaction Schemes 1 through 3 below, It has been confirmed that selenium can be effectively removed in water by using an aqueous solution of sodium borohydride (NaBH4) as a reducing agent.

[Reaction Scheme 1]

NaBH ₄ + 2SeO ₄ ^2- (selenate) → NaBO ₂ + 2SeO ₃ ^2- (selenite) + 4H ⁺

[Reaction Scheme 2]

2NaBH ₄ + SeO ₃ ^2- (selenite) + H ₂ O → 2NaBO ₂ + Se (s) + 10H ⁺

[Reaction Scheme 3]

2NaBH ₄ + SeO ₄ ^2- (selenate) -> 2NaBO ₂ + Se (s) + 8H ⁺

In the case of sodium borohydride sodium (NaBH ₄ ) solution, it acts not only as a powerful reducing agent but also does not generate secondary pollutants, so it can be used in wastewater treatment plants and treatment plants for unit processing. Above all, it does not require a strong acidic condition as in the prior art, and can easily remove selenium from a relatively mild condition to a solid state rather than a gas, which is advantageous in workability and stability.

The sodium borohydride quickly disintegrates in neutral or acid water and releases toxic diborane gas or flammable / explosive hydrogen gas. Therefore, it is essential to adjust the pH to 13 or more using an appropriate base, and it is therefore preferred that the sodium borohydride and the base are present together in one form of the same formulation.

The base applicable to the present invention may be selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, and mixtures thereof.

When the amount of the base is less than the above range, there is a problem that the stability of sodium borohydride is lowered and a sufficient reducing power can not be exerted. Conversely, if the amount exceeds the above range, the degree of increase in the selenium removal effect is insignificant In addition, there is a problem that it starts to deteriorate.

The sodium borohydride may be added in an amount of 5 to 1,000 parts by weight, preferably 7 to 800 parts by weight, more preferably 10 to 600 parts by weight, per 100 parts by weight of selenium in the wastewater. When the amount of sodium borohydride to be added is within the above range, the selenium-removing function can be sufficiently exhibited, while high economic efficiency in terms of drug cost can be satisfied at the same time.

The method for removing selenium in the wastewater according to the present invention may further comprise, before or after the sodium borohydride addition step, 0.5 to 500 parts by weight, preferably 0.6 to 450 parts by weight, more preferably 0.6 to 450 parts by weight of a barium compound per part by weight of sodium borohydride And further adding from 0.7 to 400 parts by weight of the resin.

The major selenium eliminators of barium compounds are as follows:

(1) ionic bonding of Ba ²⁺ and SeO ₃ ^2- , SeO ₄ ^2-

[Reaction Scheme 4]

Ba ²⁺ + SeO ₃ ^2- ? BaSeO ₃ (s)

[Reaction Scheme 5]

Ba ²⁺ + SeO ₄ ^2- ? BaSeO ₄ (s)

(2) SeO ₃ ^2- and SeO ₄ ^2- ions adsorbed and immobilized on BaSO ₄ (s) formed by the reaction of Ba ²⁺ and SO ₄ ^2- in water

[Reaction Scheme 6]

Ba ²⁺ + SO ₄ ^2- ? BaSO ₄ (s)

(3) macrocyclic floc formation through physicochemical reaction, induction of rapid precipitation.

Therefore, it is possible to remove selenium at a high concentration of 500 to 3,000 ppm when a barium compound is injected and reacted after reducing Se ⁶⁺ and Se ⁴⁺ using sodium borohydride. Furthermore, the combination of sodium borohydride and barium compounds as described above is mainly applied to high-concentration selenium-containing solutions, but also effectively removes selenium from low-concentration selenium solutions.

The barium compounds applicable to the present invention may be selected from the group consisting of barium chloride, barium hydroxide, and mixtures thereof.

It is possible to adsorb and immobilize a sufficient amount of selenium while preventing a large amount of solids (sludge) from being produced due to excessive barium compounds when the amount of the barium compound is within the above range. In this case, it is preferable that the barium compound is composed of a separate formulation different from sodium borohydride, that is, a two-liquid type, unlike the above base.

Also, the method for removing selenium in the wastewater according to the present invention may further comprise, before or after the step of adding sodium borohydride, an aluminum coagulant, an aluminum coagulant and a polyamine, an aluminum coagulant, and a polyDADMAC per one part by weight of sodium borohydride, , Or 0.01 to 200 parts by weight, preferably 0.03 to 150 parts by weight, more preferably 0.05 to 100 parts by weight, of the aluminum flocculant, polyamine and polybrominated dipterol.

When the amount of these substances is within the above range, the effect of condensation is sufficiently exhibited and smooth solid-liquid separation is possible, but the reducing effect of sodium borohydride is sufficiently exhibited, whereby the selenium removal efficiency can be suitably maintained.

However, it is generally preferable that these materials are composed of separate formulations that are different from sodium borohydride such as barium compounds, that is, two-liquid form. However, in the case of sodium aluminate, it is also possible to form them in the same formulation as the sodium borohydride, that is, . In this case, the content of sodium aluminate is more preferably 0.01 to 20 parts by weight per 1 part by weight of sodium borohydride.

In the case of the aluminum coagulant containing aluminum, it is useful for precipitating and removing Se ^{4 +} type selenium through physicochemical adsorption action on selenium on the surface of the flocculant particle in the water. In addition, Se ^{4 +} , Se ⁰ , Se ² -like selenium in colloidal state or very small particle size of less than 1 ㎛ in diameter, natural sedimentation is almost impossible. Therefore, aluminum coagulant is injected and coagulation / It is possible to easily precipitate and remove selenium.

The aluminum flocculant that can be applied to the present invention includes aluminum chloride, aluminum sulfate, sodium aluminate, alum, polyaluminium chloride (PAC), polyaluminium chloride silicate (PACS), polyaluminium silicate sulfate, polyaluminium hydroxy chlorosulfate (PAHCS), and mixtures thereof.

On the other hand, the soluble selenium remover of the present invention comprises 2 to 500 parts by weight of a base, preferably 3 to 400 parts by weight, in the same formulation of one-part type, so that 1 part by weight of sodium borohydride and sodium borohydride can be stably maintained in an aqueous solution , And more preferably 5 to 300 parts by weight. When the amount of the base is less than the above range, there is a problem that the stability of sodium borohydride is lowered and sufficient reducing power can not be exhibited. On the contrary, if the amount exceeds the above range, the degree of increase in the selenium removal effect is insignificant But there is a problem that starts to deteriorate rather.

The soluble selenium remover of the present invention may further comprise 0.5 to 500 parts by weight, preferably 0.6 to 450 parts by weight, more preferably 0.7 to 400 parts by weight of the barium compound per 1 part by weight of the sodium borohydride. It is possible to adsorb and immobilize a sufficient amount of selenium while preventing a large amount of solids (sludge) from being produced due to excessive barium compounds when the amount of the barium compound is within the above range. However, in order to maintain selenium-removing ability of the sodium borohydride and barium compounds, it is preferable that the barium compound is composed of a separate formulation different from sodium borohydride, that is, a two-liquid type, unlike the base.

The soluble selenium remover of the present invention may further comprise an aluminum coagulant, an aluminum coagulant and a polyamine, an aluminum coagulant and a polyDADMAC, or an aluminum coagulant, a polyamine and a polyvinylidene chloride 0.01 to 200 parts by weight per 1 part by weight of the sodium borohydride. Preferably 0.03 to 150 parts by weight, more preferably 0.05 to 100 parts by weight, based on 100 parts by weight of the composition. When the amount of these substances is within the above range, the effect of condensation is sufficiently exhibited and smooth solid-liquid separation is possible, but the reducing effect of sodium borohydride is sufficiently exhibited, whereby the selenium removal efficiency can be suitably maintained.

However, it is generally preferable that these materials are composed of separate formulations that are different from sodium borohydride such as barium compounds, that is, two-liquid form. However, in the case of sodium aluminate, it is also possible to form them in the same formulation as the sodium borohydride, . In this case, the content of sodium aluminate is more preferably 0.01 to 20 parts by weight per 1 part by weight of sodium borohydride.

Hereinafter, embodiments of the present invention will be described.

Example

Example  1: soluble selenium remover

Sodium borohydride was added in an amount of 10% by weight to a 45% by weight aqueous solution of sodium hydroxide to prepare a soluble selenium remover of the present invention.

Test Example 1: Precious metal extraction process waste liquid (1)

The soluble selenium remover of Example 1 was added to the waste solution of the noble metal extraction process of a non-ferrous metal plant or iron and steel making plant in the composition shown in the following Table 1, reacted at 70 ° C for 3 hours, and the concentration of selenium in the filtered solution was measured. Are shown in Table 1.

NaBH ₄ (ppm) 0 200 500 1,000 2,000 Se (ppm) 2,910 2,340 2,130 1,924 1,602

Test Example 2: Precious metal extraction process waste liquid (2)

The procedure of Test Example 1 was followed and barium chloride (BaCl ₂ ) was added with the soluble selenium remover in the composition shown in the following Table 2, and the results are shown in Table 2.

NaBH ₄ (ppm) 0 200 500 1,000 2,000 BaCl ₂ (ppm) 0 30,000 30,000 30,000 30,000 Se (ppm) 2,910 102 89 72.1 25.2

Test Example 3: Precious metal extraction process waste solution (3)

The same procedure as in Test Example 2 was carried out and 17% by weight aqueous solution of PAC (polyaluminium chloride, Korea) was added with the soluble selenium remover and barium chloride in the composition shown in the following Table 3, Respectively.

NaBH ₄ (ppm) 0 200 500 1,000 2,000 BaCl ₂ (ppm) 0 30,000 30,000 30,000 30,000 PAC 17% (ppm) 0 500 1,000 1,200 1,500 Se (ppm) 2,910 96 83 62 20.4

Test Example 4: Rare Metal Extraction Process Waste (1)

The soluble selenium remover of Example 1 was added to the waste solution of a rare metal extraction process of a non-ferrous metal plant or a steel plant in accordance with the composition shown in Table 4 below, reacted at 70 ° C for 4 hours, and the concentration of selenium in the filtered solution was measured. The results are shown in Table 4.

NaBH ₄ (ppm) 0 250 500 750 1,000 Se (ppm) 340 308 263 227 206

Test Example 5: Rare metal extraction process waste solution (2)

The same procedure as in Test Example 4 was carried out and barium chloride (BaCl ₂ ) was added with the soluble selenium remover in the composition shown in the following Table 5, and the results are shown in Table 5.

NaBH ₄ (ppm) 0 250 500 750 1,000 BaCl ₂ (ppm) 0 1,000 1,000 1,000 1,000 Se (ppm) 340 264 210 167 148

Test Example 6: Rare metal extraction process waste solution (3)

The same procedure as in Test Example 5 was carried out and 17% by weight aqueous solution of polyaluminium chloride (PAC) was added with the above-described soluble selenium remover and barium chloride in the composition shown in the following Table 6, and the results are shown in Table 6.

NaBH ₄ (ppm) 0 250 500 750 1,000 BaCl ₂ (ppm) 0 1,000 1,000 1,000 1,000 PAC 17% (ppm) 0 600 800 1,000 1,500 Se (ppm) 340 254 198 154 132

Test Example 7: Desulfurization process wastewater (1)

The soluble selenium remover of Example 1 was added to the desulfurization process wastewater of a non-ferrous metal plant or an iron and steel making plant, and the selenium concentration in the effluent was measured three times, Are shown in Table 7 and Fig.

NaBH ₄ (ppm) 0 150 200 250 300 350 Se (ppm) 9.92 3.8 0.67 0.54 0.55 0.47 Se (ppm) 11.94 4.49 2.85 0.64 0.6 0.56 Se (ppm) 9.9 6.76 1.71 1.24 1.19 1.26

Test Example 8: Desulfurization Process Waste Water (2)

The same procedure as in Test Example 7 was carried out, and an aqueous solution of 17% by weight of barium chloride (BaCl ₂ ) and PAC (polyaluminium chloride) was added to the solution of the above-described soluble selenium remover with the composition shown in Table 8 below. Respectively.

NaBH ₄ (ppm) 0 150 200 250 300 350 BaCl ₂ (ppm) 0 300 300 400 400 400 PAC 17% (ppm) 0 100 100 150 150 200 Se (ppm) 9.92 3 0.55 0.44 0.48 0.4 Se (ppm) 11.94 3.97 2.61 0.53 0.53 0.47 Se (ppm) 9.9 6.1 1.61 0.99 0.97 1.09

Test Example 9 Wastewater Treatment Plant Influent (1)

The soluble selenium remover of Example 1 was added to the influent of the wastewater treatment plant of a non-ferrous metal plant or a steel plant in accordance with the composition shown in Table 9 below, and the concentration of selenium in the effluent was measured three times with different days after the coagulating bath and the settling tank , And the results are shown in Table 9.

NaBH ₄ (ppm) 0 50 100 150 200 Se (ppm) 1.69 1.24 0.97 0.71 0.60 Se (ppm) 2.42 1.87 1.50 1.21 0.95 Se (ppm) 1.89 1.42 1.12 0.87 0.73

Test Example 10 Wastewater Treatment Plant Influent (2)

The same procedure as in Test Example 9 was carried out, and an aqueous solution of 17% by weight of barium chloride (BaCl ₂ ) and PAC (polyaluminium chloride) was added to the above-described soluble selenium remover with the composition shown in Table 10 below.

NaBH ₄ (ppm) 0 50 100 150 200 BaCl ₂ (ppm) 0 300 300 500 500 PAC 17% (ppm) 0 100 100 100 100 Se (ppm) 1.69 1.10 0.86 0.54 0.49 Se (ppm) 2.42 1.52 1.28 1.00 0.77 Se (ppm) 1.89 1.21 0.97 0.71 0.63

Test Example 11: Precious metal extraction process waste liquid (4)

The same procedure as in Test Example 1 was carried out except that sodium thiosulfate (for removing Na ₂ S ₂ O ₃ .Se ⁴⁺ ) and thiourea ((NH ₂ ) ₂ CSO _2. Thiourea, Se ^{6 +} , Se ⁴⁺ removal purpose) was added in the composition shown in Table 11, and the results are shown in Table 11 together with the total amount of the gasses.

Na ₂ S ₂ O ₃ (ppm) 0 5,000 10,000 15,000 20,000 (NH ₂ ) ₂ CSO ₂ (ppm) 0 5,000 10,000 15,000 20,000 Se (ppm) 2,910 938 384 148 52.5 T-N (ppm) 387 2,007 4,177 6,721 9,982

As shown in Table 11, the sodium thiosulfate has a high reducing ability against Se ⁴⁺ but has a very low reducing ability against Se ⁶⁺ . The thiourea has a constant reducing power against Se ⁶⁺ , As can be seen from [(NH ₂ ) ₂ CSO ₂ ], a considerable amount of N remains after the chemical treatment. Therefore, secondary pollution problem due to increase in TN concentration of discharged water frequently occurs.

These results show that the selenium removal efficiency is significantly reduced at pH 7.7 as shown in Table 12 below as a result at pH 1. Therefore, sodium thiosulfate and thiourea are disadvantageous in that they have to have a severe treatment condition of strong acidity.

Na ₂ S ₂ O ₃ (ppm) 0 10,000 15,000 20,000 (NH ₂ ) ₂ CSO ₂ (ppm) 0 10,000 15,000 20,000 Se (ppm) 2,910 2,724 2,684 2,559

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, Of course it is possible. Accordingly, the scope of the present invention should not be construed as being limited to the above-described embodiments, but should be determined by equivalents to the appended claims, as well as the following claims.

Claims (9)

Adding 1 part by weight of sodium borohydride and 2 to 500 parts by weight of base to the wastewater containing selenium,
The base is selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, and mixtures thereof,
Wherein the pH of the wastewater after the addition of the base is 13 or more. The method according to claim 1,
Further comprising adding 0.5 to 500 parts by weight of a barium compound per part by weight of sodium borohydride to the wastewater before, after or simultaneously with the step of adding the sodium borohydride. The method of claim 2,
Before, after, or simultaneously with the sodium borohydride addition step
An aluminum condensing agent per 1 part by weight of sodium borohydride in the wastewater; The aluminum condensing agent and polyamine; The aluminum condenser and PolyDADMAC; Or 0.01 to 200 parts by weight of the aluminum coagulant, polyamine and poly-D-dimer. The method of claim 2,
Wherein the barium compound is selected from the group consisting of barium chloride, barium hydroxide, and mixtures thereof. The method of claim 3,
The aluminum flocculant may be selected from the group consisting of aluminum chloride, aluminum sulfate, sodium aluminate, alum, polyaluminium chloride (PAC), polyaluminium chloride silicate (PACS), polyaluminium silicate sulfate (PASS) polyaluminium hydroxy chlorosulfate, and mixtures thereof. < Desc / Clms Page number 19 > The method according to claim 1 or 2,
Wherein the sodium borohydride is added in an amount of 5 to 1,000 parts by weight per 100 parts by weight of selenium in the wastewater. 1 part by weight of sodium borohydride, and
2 to 500 parts by weight of a base,
The base is selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, and mixtures thereof,
Wherein the pH of the wastewater after the addition of the base is 13 or more. The method of claim 7,
Further comprising 0.5 to 500 parts by weight of a barium compound per part by weight of said sodium borohydride in a separate formulation from said soluble selenium scavenger.

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