JP2022036515A - Treatment method for water containing selenic acid ions - Google Patents

Abstract

To provide a method for treating water containing selenic acid ions in which the reaction for selenic acid ions removal proceeds quickly at room temperature, selenic acid ions and several other heavy metals can be efficiently removed even from water in which sulfate ions coexist at much higher concentrations than selenic acid ions, and the amount of various agents added can be flexibly changed on-site according to the water quality of target water by using general-purpose agents.SOLUTION: A method for treating water containing selenic acid ions, characterized in that calcium and aluminum compounds are added to the water to be treated containing selenic acid ions, and the selenic acid ions are removed as a solid solution of ettringite, together with sulfuric acid ions that coexist in the water to be treated from the beginning or are added later.SELECTED DRAWING: None

Description

The present invention relates to a method for treating water containing selenate ion, and more specifically, removes selenate ion from groundwater containing selenate ion, spring water, industrial wastewater, etc., and keeps their concentration below the wastewater standard or the environmental standard. Regarding the method.

Selenium is a trace element and its average content in the crust is only 0.05 mg / kg.
However, it is known that the average content in the sediment is as high as about 0.4 mg / kg.
It exists in the crust and sediments as various selenium minerals and selenite minerals, and by substituting sulfur, a sulfide mineral.

Therefore, the selenium concentration of groundwater and springs that have passed through the sediment layer with high selenium content may be high. In addition, if excavated selenium consisting of deposits and sedimentary rocks with high selenium content is left on the ground surface, selenium contained in selenium minerals, selenite minerals, selenium-containing sulfide minerals, etc. will be oxidized by oxygen in the atmosphere. It may elute as a selenate ion.

In addition, waste liquid containing selenium is discharged in industrial activities such as metal refining from metal sulfide and sulfuric acid production.

Although selenium is an essential element for animals, its required amount is very small, and it is known that when it is ingested excessively, it exhibits an adverse effect similar to that of arsenic.
For this reason, among the environmental standards for water pollution, the environmental standards for the protection of human health stipulate that the selenium concentration is 0.01 mg / L or less.
In addition, according to the uniform wastewater standard, the concentration of selenium and its compounds is 0.1 mg / L or less in terms of selenium.

Therefore, in order to remove selenium from groundwater containing selenium having a concentration higher than the above standard, spring water, seepage water, and wastewater from industrial activities, and to reduce the selenium concentration to 0.1 mg / L or less or 0.01 mg / L or less. Technology is required.

The main forms of selenium dissolved in groundwater, spring water, seepage water, wastewater, etc. are selenate ion and selenate ion. The former is often called tetravalent selenium and the latter is also called hexavalent selenium.

Of these, the selenite ion has high reactivity with metal oxides and hydroxides, for example, iron oxide hydroxide minerals, and is strongly bonded to the surface hydroxy group portion of these minerals as a surface complex. Therefore, by adding minerals with such properties to water containing selenite ions or by adding an appropriate metal salt to water and hydrolyzing them in water to form hydroxides. , Can be removed relatively easily.

On the other hand, selenate ion has similar chemical properties to sulfate ion, and it is difficult to form a stable surface complex on metal oxides and hydroxides. Is difficult.

Therefore, a method of reducing selenate ion to convert it to selenite ion and applying a removal technique for selenite ion has been developed.
Patent Document 1 and Patent Document 2 describe a reduction method using an iron (II) salt, Patent Document 3 describes a reduction method using metallic iron, and Patent Document 4 uses metallic titanium. The reduction method used is described.

Further, in order to remove the selenate ion without reducing it to the selenite ion, a method using an adsorbent having a relatively high selectivity for the selenate ion has also been developed.
For example, Patent Document 5 describes an oxide or hydroxide of yttrium, lanthanum, cerium or ytterbium, a material having a methylglucamine group, and the like. Patent Document 6 describes a method using Schwertmanite, which is one of iron oxide hydroxide minerals.

In addition, a method for precipitating and removing a selenic acid compound having low solubility without reducing the selenate ion to a selenite ion has also been developed.
For example, Patent Document 7 describes a method of using a water-soluble barium salt as a precipitate, and Patent Document 8 describes a method of using calcium aluminate, aluminum sulfate, lime and an alkali metal phosphate in combination.

Japanese Unexamined Patent Publication No. 10-99874 Japanese Unexamined Patent Publication No. 2004-275903 Japanese Unexamined Patent Publication No. 2009-220102 Japanese Unexamined Patent Publication No. 2009-11914 Japanese Unexamined Patent Publication No. 2013-244450 Japanese Unexamined Patent Publication No. 2005-95732 Japanese Unexamined Patent Publication No. 2018-158307 Japanese Unexamined Patent Publication No. 2013-119056

As mentioned above, various techniques have been developed to remove selenium from water containing selenate ion at a concentration exceeding the standard, but all of them are widely used for the treatment of selenate ion-containing water. It is not the situation that has been done. It is thought that there are the following reasons.

When reducing selenate ion to selenite ion, the reduction reaction is a relatively slow reaction, and a reaction time of at least several hours, preferably 24 hours or more is required to achieve the treatment target. The reaction rate increases as the temperature of the treated water increases, but for that purpose, it is necessary to heat the temperature to at least 35 ° C, preferably several tens of ° C or higher. It requires a lot of energy for that.

Adsorbents with high selectivity for selenic acid are not easily available or expensive when using rare elements as raw materials. In addition, the adsorbents that have been developed so far do not completely selectively adsorb only selenate ion, but selenate ion and sulfate ion having similar chemical properties compete with each other. Therefore, when the sulfate ion concentration is 10000 times (substance amount ratio) or more of the selenate ion concentration, the efficiency of removing the selenate ion decreases due to competition with the sulfate ion.

The precipitation removal method using a water-soluble barium salt or calcium aluminates as a main component is excellent in that the reaction rate is high and the removal reaction proceeds within about 20 minutes. However, the method using a barium salt has a problem that the barium salt is relatively expensive, and it is necessary to perform additional treatment so that barium ions do not remain in the treated solution. There is also a problem that the removal efficiency is lowered due to competition with sulfate ions.

The precipitation removal method using calcium aluminates does not require any post-treatment other than alkaline correction for the treated water, but the calcium aluminates used for this treatment is specially produced for that purpose. It is a thing, and its composition cannot be changed on the spot according to the water quality of the target water. Nor is it always readily available on the market.

The present invention is intended to solve the above-mentioned problems of the prior art, in which the reaction for removing selenate ion proceeds rapidly at room temperature, and the concentration of sulfate ion is much higher than that of selenate ion. Selenate ion and some other heavy metals can be efficiently removed from the water in which selenate is coexisting, and the amount of various chemicals added can be flexibly changed at the site according to the water quality of the target water using a general-purpose chemical. It is an object of the present invention to provide a method for treating water containing such selenate ion.

Further, other problems of the present invention will be clarified by the following description.

The above problems are solved by the following inventions.
(Claim 1)
A calcium compound and an aluminum compound are added to the water to be treated containing selenate ion, and the selenate ion is removed as a solid solution of ettringite together with the sulfate ion coexisting with the water to be treated from the beginning or the sulfate ion to be added later. A method for treating water containing a selenate ion, which comprises the above.
(Claim 2)
Claim 1 is characterized in that the calcium compound is at least one selected from calcium oxide, calcium hydroxide or calcium chloride, and the aluminum compound is at least one selected from aluminum chloride and sodium aluminate. The method for treating water containing a selenate ion according to the above.
(Claim 3)
Claim 1 is characterized in that when a solid solution of ettringite is formed from the sulfate ion and the selenate ion, a larger amount of sulfate ion than the selenate ion coexists in the water to be treated containing the selenate ion. Alternatively, the method for treating water containing selenate ion according to 2.
(Claim 4)
To add 4 or more parts of the calcium compound and 4/3 or more of the aluminum compound to 1 substance amount of the total amount of sulfate ion and selenate ion contained in the water to be treated. The method for treating water containing a selenate ion according to any one of claims 1 to 3.
(Claim 5)
Any of claims 1 to 4, wherein when a solid solution of ettringite is formed from the sulfate ion and the selenic acid ion, the calcium compound and the aluminum compound are added and then stirred for 10 minutes or more to react. The method for treating water containing a selenate ion according to.

According to the present invention, selenate ion can be removed in a shorter time than the method of reducing selenate ion to selenite ion to remove it.
Further, according to the present invention, since calcium compounds and aluminum compounds that are simply and easily available are used, they can be applied to water of various water qualities by independently adjusting their input amounts.

Powder X-ray diffraction pattern of the precipitate generated when well water containing selenium is treated with calcium hydroxide and sodium aluminate.

Hereinafter, embodiments of the method for treating water containing selenate ion of the present invention will be described.
In the treatment method of the present invention, a calcium compound and an aluminum compound are added to the water to be treated containing selenic acid ion (hereinafter, also referred to as raw water), and the sulfate ion coexisting with the water to be treated from the beginning or sulfuric acid to be added later. Along with the ions, the selenate ion is removed as a solid solution of ettringite.

Examples of the solid solution of ettringite include compounds represented by the following formulas.

Here, the number m of water of crystallization is exemplified as 26, but the number of water of crystallization is not limited. Further, in the formula, x is a number of 3 or less.

In order to remove the selenate ion as a solid solution of ettringite, the coexistence of sulfate ion is necessary.
In the case of coexistence of sulfate ions, the sulfate ions coexisting may be present in the water to be treated from the beginning, or the sulfate ions may be added later to allow the coexisting sulfate ions to exist.
The concentration of coexisting sulfate ion should be appropriately adjusted so that the amount of calcium compound and aluminum compound required for preparing a solid solution of ettringite is not excessive and the composition is suitable for removing selenate ion. Is preferable.

In the present invention, when forming a solid solution of ettringite from sulfate ion and selenate ion, it is preferable that a larger amount of sulfate ion than selenate ion coexists in the water to be treated containing selenate ion.
According to the experiment of the present inventor, it has been obtained that selenate ion is less likely to be incorporated into the tringeite structure than sulfate ion or selenate ion. If conditions are created such that a much larger amount of sulfate ion coexists in water containing selenate ion, it becomes possible to precipitate all the sulfate ion and selenate ion as ettringite.

The concentration of sulfate ion coexisting with selenate ion is preferably in the range of 0.5 mmol / L to 15 mmol / L in the water to be treated.
When the water to be treated does not exist in the coexisting sulfate ion or is less than 0.5 mmol / L, it is preferable to add a water-soluble sulfate so as to fall within the above range. This is to increase the removal rate of selenate ion.
When the concentration of sulfate ion in the water to be treated exceeds 15 mmol / L, the sulfate ion is removed. For example, a substance that forms a sparingly soluble compound with sulfate ions can be added, and a part of the sulfate ions can be precipitated and removed as a sparingly soluble salt. Specifically, a water-soluble calcium salt, a chloride salt of a polynuclear hydroxoaluminum ion having a kegin-type structure, chitosan and the like are preferable. At this time, a polymer flocculant can be added to promote the sedimentation of the precipitate. Such removal of sulfate ions is preferable because the amount of the calcium compound and the aluminum compound to be added in the subsequent stage can be reduced.

When the sulfate ion reaches a predetermined concentration range, the calcium compound and the aluminum compound are added to the water to be treated. Then, if necessary, alkali is added.
The calcium compound, the aluminum compound, and the alkali are mixed separately, simultaneously, or even in advance, and then added and reacted to obtain a solid solution of ettringite represented by the above formula.

As the calcium compound, at least one selected from calcium chloride, calcium oxide and calcium hydroxide can be used, and as the aluminum compound, at least one selected from aluminum chloride and sodium aluminate can be used. can.

In the present invention, as the aluminum compound, aluminum sulfate should not be used when sulfate ions are contained in the water to be treated. The reason is described below. If aluminum sulfate is present in the water to be treated, adding aluminum sulfate requires precipitation of both the sulfate ion in the water to be treated and the sulfate ion added later as aluminum sulfate. It is inefficient.
Furthermore, if the raw water contains a considerable concentration of sulfate ions, adding aluminum as aluminum sulfate results in a molar ratio of (sulfate ion + selenate ion) / aluminum ion greater than 2/3 regardless of the amount added. Even if it grows larger and calcium hydroxide is added, the total amount of sulfate ions cannot be converted to ettringite. Here, 2/3 is the molar ratio of sulfate ion / aluminum ion of ettringite.
Since selenate ion is less likely to be incorporated into a solid solution than sulfate ion, selenate ion tends to remain when the concentration of residual sulfate ion in water is high.
In the present invention, by using an aluminum compound containing no sulfate ion, the molar ratio of (sulfate ion + selenate ion) / aluminum ion can be reduced to 2/3 or less, and the sulfate ion concentration can be sufficiently lowered. ..

As the alkali, at least one selected from sodium hydroxide, calcium oxide and calcium hydroxide can be used.

That is, when the raw water contains sulfate ions at a convenient concentration (approximately 2 to 8 mmol / L), an aluminum compound (aluminum chloride or sodium aluminate) containing no sulfate ions and calcium oxide or calcium hydroxide are added.

When the raw water contains almost no sulfate ion, (1) To add sulfate ion, add sulfate such as sodium sulfate, and then add aluminum compound containing no sulfate ion and calcium oxide or calcium hydroxide. (2) Alternatively, a mixture of aluminum chloride and aluminum sulfate is used as the aluminum compound. The mixing ratio depends on the sulfate ion concentration.

If the sulfate ion concentration of the raw water is very high, the method of adding an aluminum compound (aluminum chloride or sodium aluminate) that does not contain sulfate ion and calcium oxide or calcium hydroxide can be applied as it is, but it is a drug. Since the amount of input is large, and as a result, the amount of precipitation is also large and solid-liquid separation may be difficult, sulfate ions are removed as a pretreatment.

In the present invention, the calcium compound having a substance amount of 4 or more and the aluminum compound having a substance amount of 4/3 or more is provided with respect to 1 substance amount of the total amount of sulfate ion and selenate ion contained in the water to be treated. It is preferable to add it.

In the present invention, the amount of the calcium compound and the aluminum compound to be added is determined by the total concentration of sulfate ion and selenate ion contained in the water to be treated and the treatment time (stirring time).
As a preferred embodiment, 4 or more parts of a calcium compound and 4/3 or more parts of an aluminum compound are added to 1 part by substance of the total amount.

In the present invention, the above-mentioned calcium compound, the above-mentioned aluminum compound, and the above-mentioned alkali are added to the water to be treated (raw water), and the mixture is stirred and reacted for a treatment time (stirring) of 10 minutes or more. It is preferable to use selenate ion together with sulfate ion as a solid solution of ettringite. In the present invention, the treatment time is preferably 10 minutes or more and 30 minutes or less, and more preferably 15 minutes or more and 25 minutes or less.

The calcium compound, aluminum compound and sodium hydroxide added here may be powder, or may be a solution or a suspension suspended in water.

Except when the water to be treated (raw water) is neutral to weakly acidic and calcium chloride is used as the calcium compound, the amount of two calcium compounds is relative to the total amount of one substance of sulfate ion and selenate ion. By adding 2/3 of the amount of the aluminum compound, the pH of the water to be treated becomes 11 or more, which is a suitable condition for the formation of a solid solution of ettringite.

When the calcium compound and the aluminum compound are added as described above and reacted until almost equilibrium is reached, the selenate ion concentration can be at least uniformly below the wastewater standard. When the water temperature is 10 ° C to 30 ° C, it takes several hours or more to reach an equilibrium.
By increasing the amount of the calcium compound and the aluminum compound added to be larger than the amounts described above, the treatment time can be shortened.

When the acidity of the raw water (water to be treated) is strong, and when calcium chloride is used as the calcium compound, the pH after the addition of the calcium compound and the aluminum compound does not reach a value suitable for forming a solid solution of ettringite. There is. In this case, an additional alkali, preferably at least one alkali selected from calcium oxide, calcium hydroxide, sodium hydroxide, and potassium hydroxide, is added so that the pH of the target water is at least 11 or higher. Added.

Next, in the present invention, a polymer flocculant or a substance capable of adsorbing selenate ions and other coexisting harmful substances is added in addition to the polymer flocculant, or such ability is exhibited in water. A substance that can be changed is added to the substance to be produced, and the solid solution of the produced ettringite and other suspended substances are aggregated and precipitated for solid-liquid separation.
If the settling is sufficiently fast without using the flocculant, the addition of the flocculant can be omitted.
The settling portion is removed from the container, dehydrated, and treated as sludge. The supernatant is transferred to another container and discharged as a treatment liquid.

In the above, a substance capable of adsorbing selenate ions and other coexisting harmful substances is added, or a substance capable of changing into a substance exhibiting such ability in water is added, and the substance is aggregated together with the ettringite produced. It can also be settled and removed.
Such substances include oxides and hydroxides of iron or aluminum, and water-soluble salts of iron and aluminum.

When a water-soluble salt of iron or aluminum is added, the amount of the addition should be kept so that the pH of the water after the addition and dissolution does not fall below 11.

Next, in the present invention, the pH of water after solid-liquid separation is adjusted and discharged or reused. That is, an acid is added to the treated water after solid-liquid separation, the pH is adjusted to be within the wastewater standard, and the water is discharged, or reused for some purpose.
The type of acid used for pH adjustment is generally not limited, but an acid of such a type that the water quality after pH adjustment meets the wastewater standard is used.

The treatment method of the present invention has been described above, but the treatment method of the present invention includes selenium and lead such as groundwater containing selenium ion, spring water, exudate from excavated earth and sand, soil cleaning plant wastewater, and factory wastewater. , Can be widely used to remove mercury, selenium, boron and fluorine.

Hereinafter, the effects of the present invention will be illustrated by examples.

Example 1
1. 1. Water to be treated Well water with a selenate ion concentration of 0.91 mg / L as selenium and a coexisting sulfate ion concentration of 58 mg / L was used.

2. 2. Test and evaluation results Calcium hydroxide was added as a calcium compound and sodium aluminate was added as an aluminum compound to the water to be treated to remove selenate ions.
Since the sulfate ion concentration of the water to be treated was in the range of about 0.5 mmol / L to about 15 mmol / L, the concentration was not adjusted.
Take 100 mL of raw water in a plastic container, add calcium hydroxide powder and 5.87 mol / L sodium aluminate solution while stirring with a magnetic stirrer, and stir for another 15 minutes to obtain a nonionic polymer flocculant. After adding and coagulating and precipitating, a part of the supernatant was filtered through a 0.45 μm membrane filter, and then the sulfate ion concentration and the selenium concentration of the filtrate were measured.

Table 1 shows the addition amounts of calcium hydroxide and sodium aluminate, and the sulfate ion concentration and selenium concentration of the treated water.
Addition of 75 mg of calcium hydroxide to 100 mL of raw water is about 17 parts of substance for a total of 1 substance of sulfate ion and selenate ion, and addition of 0.06 mL of sodium aluminate solution is with sulfate ion. It corresponds to about 11 parts of substance with respect to a total of 1 part of substance of sulfate ion.
The addition of 150 mg of calcium hydroxide is about 34 parts of substance with respect to the total amount of substance of sulfate ion and selenate ion, and the addition of 0.12 mL of sodium aluminate solution is of sulfate ion and selenate ion. It corresponds to about 22 parts of substance for a total of 1 part of substance.

Even when the coexisting sulfate ion concentration is low, the selenate ion concentration is uniformly drained even if the reaction time is shortened to 20 minutes by increasing the addition amount ratio of the calcium compound and the aluminum compound to the total of the sulfate ion and the selenate ion. I was able to make it below the standard.

FIG. 1 shows a powder X-ray diffraction pattern of the precipitate produced in the process according to Example 1.
The diffraction peak with E is derived from ettringite, and the peak with C is derived from calcium carbonate.
The precipitate was shown to consist mostly of ettringite and calcium carbonate.
Ethringite is produced by the reaction of sulfate ion and selenate ion contained in the water to be treated, and added calcium hydroxide and sodium aluminate.

It is considered that the coexisting calcium carbonate was produced by the calcium hydroxide remaining unreacted by reacting with carbon dioxide derived from the atmosphere during the reaction and the preparation of the sample for X-ray diffraction.

The above problems are solved by the following inventions.
(Claim 1)
After adjusting the concentration range of sulfate ion coexisting in the water to be treated containing selenate ion to 0.5 to 15 mmol / L, a calcium compound and an aluminum compound were added to the water to be treated.
When forming a solid solution of ettringite from the sulfate ion and the selenate ion, the sulfate ion having a substance amount larger than the substance amount of the selenate ion coexists in the water to be treated containing the selenate ion.
A method for treating water containing selenate ion, which comprises removing the selenate ion together with sulfate ion as a solid solution of ettringite to a concentration at least equal to or less than a uniform emission standard .
(Claim 2)
Claim 1 is characterized in that the calcium compound is at least one selected from calcium oxide, calcium hydroxide or calcium chloride, and the aluminum compound is at least one selected from aluminum chloride and sodium aluminate. The method for treating water containing a selenate ion according to the above.
(Claim 3)
To add 4 or more parts of the calcium compound and 4/3 or more of the aluminum compound to 1 substance amount of the total amount of sulfate ion and selenate ion contained in the water to be treated. The method for treating water containing a selenate ion according to claim 1 or 2 .
(Claim 4)
Any of claims 1 to 3 , wherein when a solid solution of ettringite is formed from the sulfate ion and the selenic acid ion, the calcium compound and the aluminum compound are added and then stirred for 10 minutes or more to react. The method for treating water containing a selenate ion according to.

The above problems are solved by the following inventions.
(Claim 1)
After adjusting the concentration range of sulfate ion coexisting in the water to be treated containing selenate ion to 0.5 to 15 mmol / L, a calcium compound and an aluminum compound were added to the water to be treated.
The calcium compound of 4 substance parts or more and the aluminum compound of 4/3 substance amount or more are added to 1 substance amount part of the total amount of sulfate ion and selenate ion contained in the water to be treated.
When forming a solid solution of ettringite from the sulfate ion and the selenate ion, the sulfate ion having a substance amount larger than the substance amount of the selenate ion coexists in the water to be treated containing the selenate ion.
A method for treating water containing selenate ion, which comprises removing the selenate ion together with sulfate ion as a solid solution of ettringite to a concentration of at least 0.1 mg / L or less.
(Claim 2)
Claim 1 is characterized in that the calcium compound is at least one selected from calcium oxide, calcium hydroxide or calcium chloride, and the aluminum compound is at least one selected from aluminum chloride and sodium aluminate. The method for treating water containing a selenate ion according to the above.
(Claim 3)
The selenium according to claim 1 or 2 , wherein when a solid solution of ettringite is formed from the sulfate ion and the selenic acid ion, the calcium compound and the aluminum compound are added and then stirred for 10 minutes or more to react. A method for treating water containing acid ions.

Table 1 shows the addition amounts of calcium hydroxide and sodium aluminate, and the sulfate ion concentration and selenium concentration of the treated water.
Addition of 75 mg of calcium hydroxide to 100 mL of raw water is about 17 parts of substance for a total of 1 substance of sulfate ion and selenate ion, and addition of 0.06 mL of sodium aluminate solution is with sulfate ion. It corresponds to about 5.8 parts of substance with respect to the total amount of substance of sulfate ion (the above is a reference example) .
The addition of 150 mg of calcium hydroxide is about 34 parts of substance with respect to the total amount of substance of sulfate ion and selenate ion, and the addition of 0.12 mL of sodium aluminate solution is of sulfate ion and selenate ion. It corresponds to about 12 parts of substance for a total of 1 part of substance (the above are examples) .

Claims (5)

A calcium compound and an aluminum compound are added to the water to be treated containing selenate ion, and the selenate ion is removed as a solid solution of ettringite together with the sulfate ion coexisting with the water to be treated from the beginning or the sulfate ion to be added later. A method for treating water containing a selenate ion, which comprises the above. Claim 1 is characterized in that the calcium compound is at least one selected from calcium oxide, calcium hydroxide or calcium chloride, and the aluminum compound is at least one selected from aluminum chloride and sodium aluminate. The method for treating water containing a selenate ion according to the above. Claim 1 is characterized in that when a solid solution of ettringite is formed from the sulfate ion and the selenate ion, a larger amount of sulfate ion than the selenate ion coexists in the water to be treated containing the selenate ion. Alternatively, the method for treating water containing selenate ion according to 2. To add 4 or more parts of the calcium compound and 4/3 or more of the aluminum compound to 1 substance amount of the total amount of sulfate ion and selenate ion contained in the water to be treated. The method for treating water containing a selenate ion according to any one of claims 1 to 3. Any of claims 1 to 4, wherein when a solid solution of ettringite is formed from the sulfate ion and the selenic acid ion, the calcium compound and the aluminum compound are added and then stirred for 10 minutes or more to react. The method for treating water containing a selenate ion according to.

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