JP2014073441A - Insolubilizing method of heavy metal - Google Patents

Abstract

Provided is a method for effectively insolubilizing and removing heavy metals from alkaline treated water having a pH of 11 or more containing heavy metals.
(1) Insolubilization treatment of heavy metals contained in highly alkaline water by adding at least one of phosphoric acid and phosphate and calcium sulfate to treated water containing heavy metals and having a pH of 11 or higher. And (2) a heavy metal insolubilization treatment method in which a chemical that further insolubilizes water-soluble calcium is further added to water to be treated containing water-soluble calcium.
[Selection figure] None

Description

  The present invention relates to an insolubilization method for heavy metals, and more particularly to an insolubilization method for detoxifying heavy metals contained in highly alkaline water containing heavy metals.

Wastewater from a cleaning plant or the like often requires treatment because it contains heavy metals resulting from the components of the incinerated ash. In addition, such wastewater contains a lot of alkali components and generally has a high pH.
When heavy metal ions in water such as industrial wastewater are separated and removed, many heavy metals generate insoluble hydroxides by raising the pH, so heavy metals in wastewater with a high pH are easily precipitated and separated. Since heavy metal ions having amphoteric properties such as zinc and zinc are soluble and form soluble ions at a high pH, it is necessary to adjust the pH to around 9.

On the other hand, it is known that phosphates of heavy metals have very low solubility compared to hydroxides. For example, Patent Document 1 discloses phosphoric acid or acidic phosphates, coagulants and calcium in heavy metal-containing wastewater. A method of insolubilizing and detoxifying by adding a source has been proposed.
In addition, as a method capable of processing lead at a high level even at a high pH, a method of adding sodium sulfide to lead sulfide and a method of chelating with dithiocarbamate are known.
Further, Patent Document 2 proposes a wastewater treatment method in which an alkali metal salt of silicic acid and a flocculant are added to alkaline wastewater having a pH of 10 or more to insolubilize and remove heavy metals.

Japanese Patent Publication No. 1-2556 JP-A-8-103774

In Patent Document 1, it is described that metal phosphate is stable because it is not amphoteric, but lead phosphate is amphoteric and has a disadvantage that it dissolves when the pH increases. Moreover, in patent document 1, although a phosphate is formed and a metal containing material is precipitated, it is described that it is preferable to set it as a sulfate in that case. However, the solubility of lead sulfate cannot satisfy the wastewater standard value regardless of the pH.
On the other hand, sodium sulfide and dithiocarbamate, which can process lead at a high pH, are both sulfur-based compounds and may easily generate harmful gases such as hydrogen sulfide and carbon disulfide depending on the usage environment. Can not.
In addition, in the method of treating alkaline wastewater having a pH of 10 or more described in Patent Document 2, an alkali metal salt of silicic acid and a flocculant are added, but as shown in the examples, the pH needs to be finally 11 or less. There is.

As described above, a treatment method that can easily insolubilize heavy metals even at a high pH has not been established so far.
This invention is made | formed in view of such a situation, and it aims at providing the method of insolubilizing this heavy metal effectively from the alkaline to-be-treated water of 11 or more pH containing heavy metal.

  As a result of intensive studies to achieve the above object, the present inventors have added at least one of phosphoric acid and phosphate and calcium sulfate to highly alkaline treated water containing heavy metals. Thus, it was found that heavy metals in water can be effectively insolubilized even under highly alkaline conditions of pH 11 or higher. The present invention has been completed based on such findings.

That is, the present invention provides the following [1] to [7].
[1] A method for insolubilizing heavy metals contained in highly alkaline water, comprising adding at least one of phosphoric acid and phosphate and calcium sulfate to water to be treated having a heavy metal and having a pH of 11 or more.
[2] The heavy metal insolubilization method according to the above [1], wherein the water to be treated further contains water-soluble calcium, and an agent for insolubilizing the water-soluble calcium is further added to the water to be treated.
[3] The heavy metal insolubilization method according to the above [1] or [2], wherein the calcium sulfate is obtained by smoke desulfurization.
[4] The heavy metal insolubilization method according to the above [2] or [3], wherein the chemical that insolubilizes water-soluble calcium is a chemical that can supply at least one selected from carbonate ions, sulfate ions, and silicate ions. .
[5] Any of the above [2] to [4], wherein the agent for insolubilizing water-soluble calcium is at least one selected from carbonic acid, carbonate and bicarbonate, sulfuric acid and sulfate, and silicate. The method for insolubilizing heavy metals as described in 1.
[6] The above-mentioned [2] to [5], wherein the agent for insolubilizing water-soluble calcium is at least one selected from blast furnace slag, fine coal ash, synthetic zeolite, coal ash artificial zeolite, and natural zeolite. The method for insolubilizing heavy metal according to any one of the above.
[7] The above [2] to [2] to which calcium sulfate and at least one of phosphoric acid and phosphate are added to the water to be treated at the same time as or after the addition of the agent for insolubilizing water-soluble calcium. [6] The method for insolubilizing heavy metal according to any one of [6].

  ADVANTAGE OF THE INVENTION According to this invention, the method of insolubilizing this heavy metal effectively from the alkaline to-be-processed water of pH 11 or more containing a heavy metal can be provided.

It is a graph which shows the peak ratio of the hydroxyapatite / gypsum in the reaction product of gypsum and phosphoric acid. (A) is a SEM photograph of desulfurized gypsum, and (b) is a SEM photograph of hydroxyapatite produced by reacting desulfurized gypsum with phosphoric acid.

  The method for insolubilizing heavy metals contained in highly alkaline water according to the present invention comprises at least one of phosphoric acid and phosphate (hereinafter, referred to as a heavy metal insolubilizing agent) for water to be treated having a heavy metal pH of 11 or higher. (Also referred to as “phosphoric acid”) and calcium sulfate.

<Insolubilization mechanism of heavy metals by insolubilizing agent>
In the method of the present invention, the mechanism of insolubilization of heavy metals is not completely known, but (i) hydroxyapatite is produced by the reaction of calcium sulfate and phosphoric acid, and heavy metals in the water to be treated are contained in the crystal structure of this hydroxyapatite. And (ii) an agent that insolubilizes water-soluble calcium added as needed (hereinafter also referred to as “water-soluble calcium insolubilizer”) insolubilizes calcium in the water to be treated. It is presumed that the reaction between calcium and phosphoric acid is suppressed and the reaction (i) is favorably generated.

(About (i) above)
The insolubilizing agent used in the present invention contains calcium sulfate in addition to phosphoric acid and the like. This calcium sulfate has a property of destabilizing in an alkaline region of pH 11 or higher, and particularly destabilizing in a high alkaline region of pH 12 or higher. For this reason, this calcium sulfate reacts with phosphoric acid or the like to quickly and efficiently become calcium phosphate, thereby forming a hydroxyapatite crystal structure having an excellent heavy metal fixing effect.
In particular, when desulfurized gypsum, which will be described later, is used as calcium sulfate, when calcium sulfate reacts with phosphoric acid or the like, a porous skeleton having a surface area composed of insoluble silica components present in the crystal structure of calcium sulfate remains. Many heavy metals can be fixed on the surface to form a large hydroxyapatite crystal structure. At the same time, heavy metals can be favorably incorporated into the crystal structure in the process of forming the hydroxyapatite crystal structure.
As a result, it is considered that heavy metals can be well insolubilized even under highly alkaline conditions.

FIG. 1 is obtained by adding 83.4 g / L of desulfurized gypsum to a sodium phosphate solution (10 g-P / L), adjusting to a predetermined pH with sulfuric acid and sodium hydroxide, and reacting at 80 ° C. for 6 hours. 3 is a graph showing the peak ratio of hydroxyapatite / gypsum in the reaction product. The vertical axis of the graph represents the ratio of the maximum peak of hydroxyapatite and the maximum peak of gypsum in the XRD measurement result of the reaction product, and the horizontal axis of the graph represents the average value of the pH measurement values at the start and end of the test.
It can be seen that the decomposition of gypsum and the generation of hydroxyapatite are particularly advanced in a high alkaline region at pH 11 or higher. In the reaction in which calcium sulfate and phosphoric acid react to decompose calcium sulfate to produce hydroxyapatite, the reaction rate increases in the region where the pH is 11 or more.

On the other hand, when phosphoric acid or the like is added, heavy metals cannot be insolubilized satisfactorily as in the present invention under highly alkaline conditions. For example, when lead is insolubilized, it is assumed that lead phosphate is generated and lead is insolubilized. However, in view of the solubility equilibrium of lead phosphate in a highly alkaline region of pH 11 or higher, particularly pH 12 or higher, lead drainage It is difficult to satisfy the reference value (0.1 mg / L).
In addition, water-soluble calcium other than calcium sulfate is expected to produce a heavy metal insolubilizing effect by reacting with phosphoric acid or the like to form calcium phosphate to form a hydroxyapatite crystal structure. However, in this case, the production rate of the hydroxyapatite crystal structure is slow, so there is a problem that the effect of fixing heavy metals is low. If a long period of time is used, there is a possibility that the immobilization rate of heavy metals may be improved by crystal growth, but this is not practical.

(About (ii) above)
When water-soluble calcium is contained in the water to be treated, the water-soluble calcium reacts with phosphoric acid and the like, so that phosphoric acid and the like are consumed. Thus, the reaction of phosphoric acid and the like with calcium sulfate according to the method of the present invention. There is a problem that the effect of insolubilizing heavy metals is suppressed. In this case, by further adding a water-soluble calcium insolubilizer, the water-soluble calcium insolubilizer preferentially reacts with the water-soluble calcium in the water to be treated, so that the water-soluble calcium is insolubilized. Reaction with water-soluble calcium is suppressed, and phosphoric acid or the like is not wasted. Thereby, phosphoric acid etc. reacts favorably with calcium sulfate and contributes to the formation of a hydroxyapatite crystal structure, and it is considered that the insolubilizing effect of heavy metals is further improved.
Therefore, in the heavy metal insolubilization method of the present invention, when the water to be treated contains water-soluble calcium, it is preferable to add a water-soluble calcium insolubilizing agent to the water to be treated.
According to the heavy metal insolubilization method of the present invention, fluorine, arsenic and boron can be insolubilized in addition to heavy metals such as lead, cadmium, copper, zinc, selenium, nickel and uranium in water.

<At least one of phosphoric acid and phosphate (phosphoric acid, etc.)>
The heavy metal insolubilizing agent used in the present invention contains at least one of phosphoric acid and phosphate. As described above, it is considered that heavy metals are well insolubilized by incorporating heavy metals into the crystal structure of hydroxyapatite generated by the reaction between phosphoric acid and the like in the insolubilizing agent and calcium sulfate.
Examples of the phosphoric acid include acids such as orthophosphoric acid, polyphosphoric acid, metaphosphoric acid, and pyrophosphoric acid, polycondensates of these acids, salts of these acids, and the like. Examples of the salt of phosphoric acid include salts of lithium, potassium, sodium, ammonium, and the like with phosphoric acid.
The total addition amount of at least one of phosphoric acid and phosphate (phosphoric acid, etc.) with respect to non-treated water is generally preferred as the effect of insolubilizing heavy metals is improved. The total addition amount of acids and the like is preferably 0.1 to 30 g / L, more preferably 0.5 to 10 g / L, still more preferably 1 to 5 g / L in terms of normal phosphoric acid.

<Calcium sulfate>
The heavy metal insolubilizing agent used in the present invention contains calcium sulfate. As described above, it is considered that heavy metals are well insolubilized by incorporating heavy metals into the crystal structure of hydroxyapatite generated by the reaction between phosphoric acid and the like in the insolubilizing agent and calcium sulfate.
The molar ratio of calcium and phosphorus constituting hydroxyapatite (Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ ) is 5: 3. Therefore, from the viewpoint of satisfactorily producing a hydroxyapatite crystal structure, the content ratio of calcium sulfate and phosphoric acid to be added is preferably such that calcium and phosphorus are close to the molar ratio. From this point of view, the mass ratio of the anhydrous calcium sulfate equivalent addition amount of calcium sulfate to the phosphoric acid equivalent addition amount (phosphoric acid equivalent addition amount / phosphoric acid equivalent addition amount of phosphoric acid, etc.) is preferable. Is 1-10, More preferably, it is 1.5-6, More preferably, it is 2-3.

As calcium sulfate, calcium sulfate dihydrate (dihydrate gypsum: CaSO ₄ · 2H ₂ O), calcium sulfate hemihydrate (hemihydrate gypsum: CaSO ₄ · 1 / 2H ₂ O), and calcium sulfate One type or two or more types of anhydride (anhydrous gypsum: CaSO ₄ ) may be mentioned.
As this calcium sulfate, desulfurized gypsum discharged when SOx contained in the flue gas of the combustion boiler is absorbed and removed by lime milk or the like, that is, desulfurized gypsum obtained by smoke desulfurization treatment is suitable. In desulfurization gypsum, the insoluble silica component present in the particles becomes a skeleton during use, and a part of the particles dissolve to produce porous hydroxyapatite while maintaining this skeleton structure, thereby taking in many heavy metals. Can be considered. This desulfurized gypsum usually consists mainly of calcium sulfate dihydrate, a small amount of calcium sulfate hemihydrate and calcium sulfate anhydrate.

FIG. 2A is an SEM photograph of desulfurized gypsum, and FIG. 2B is an SEM photograph of hydroxyapatite produced by reacting desulfurized gypsum with phosphoric acid. As shown in these photographs, it is considered that the gypsum particles became porous because the specific surface area increased despite the change to hydroxyapatite while maintaining the appearance. As a result of measuring the respective specific surface area BET1 point method, the specific surface area of the desulfurization gypsum 2 (a) is a 6.2 m ^2, specific surface area of hydroxyapatite in FIG. 2 (b) 82.5M ² It is.

<A drug that insolubilizes water-soluble calcium (water-soluble calcium insolubilizer)>
The drug (water-soluble calcium insolubilizing agent) for insolubilizing water-soluble calcium according to the present invention insolubilizes water-soluble calcium contained in the water to be treated, so that phosphoric acid and the like and calcium sulfate react well to produce hydroxyapatite. It is considered that the heavy metal is efficiently taken into the crystal structure and the heavy metal is insolubilized well.
Examples of the water-soluble calcium insolubilizing agent include an agent capable of supplying at least one selected from carbonate ions, sulfate ions, and silicate ions.
The carbonate ion reacts with water-soluble calcium to produce calcium carbonate, thereby insolubilizing the calcium.
The sulfate ion reacts with water-soluble calcium to produce calcium sulfate and, at the same time, when elution of aluminum from solid waste is accompanied, ettringite (3CaO.Al ₂ O ₃ .3CaSO ₄ .30 to 32H ₂ O) Water-soluble calcium is insolubilized by producing minerals such as sulfite and monosulfate (3CaO.Al ₂ O ₃ .CaSO ₄ .12H ₂ O).
The silicate ion reacts with calcium to produce minerals such as tobermorite (3CaO.2SiO ₂ .3H ₂ O), thereby insolubilizing water-soluble calcium.

Specific examples of the water-soluble calcium insolubilizer include at least one selected from carbonic acid, carbonates and bicarbonates capable of supplying carbonate ions, sulfuric acid and sulfates capable of supplying sulfuric acid, and silicates.
As said carbonate, bicarbonate, sulfate, and silicate, 1 type, or 2 or more types chosen from an alkali metal salt and alkaline-earth metal salt are preferable.
Preferred examples of the carbonate include sodium carbonate, preferred examples of the bicarbonate include sodium bicarbonate, preferred examples of the sulfate include sodium sulfate, and preferred examples of the silicate. Examples thereof include calcium silicate and aluminum silicate.

As a raw material of calcium silicate, what is necessary is just to contain many calcium silicates, for example, blast furnace slag and blast furnace cement are mentioned preferably. In particular, blast furnace slag fine powder produced by pulverizing blast furnace granulated slag is more preferable in that it does not raise the pH by hydration.
Further, from the viewpoint of suppressing calcium insolubilization and pH increase of the solution, the water-soluble calcium insolubilizer, particularly blast furnace slag, is mainly composed of calcium silicate, and the content of calcium in the insolubilizer in terms of CaO is , Preferably it is 60 mass% or less, More preferably, it is 50 mass% or less.
Examples of the raw material for aluminum silicate include at least one selected from fine coal ash and zeolite. As fine coal ash, it is preferable to use coal ash fly ash having a volume median particle size (D50) of preferably 10 μm or less, more preferably 7 μm or less. Such fine-grained coal ash can be obtained by adjusting the particle size with a classifier, but the fine-grained coal ash collected in the latter part of the dust collector stage of the electric dust collector installed in the thermal power plant is used as it is. You can also Zeolites that fix water-soluble calcium in the crystal structure by ion exchange can also be used as water-soluble calcium insolubilizers. As zeolite, synthetic zeolite (4A zeolite), coal ash artificial zeolite, natural zeolite (clinoptilolite, mordenite) and the like can be used.

  The amount of the water-soluble calcium insolubilizing agent added to the water to be treated is generally preferable as the amount increases, so that the effect of insolubilizing heavy metals is improved. The insolubilizing agent / water to be treated is preferably 1 to 100 g / L, more preferably 1 to 60 g / L, and still more preferably 1 to 30 g / L.

  In the method of the present invention, the order of addition of the insolubilizing agent is not particularly limited, but when the water to be treated contains water-soluble calcium, the water-soluble calcium insolubilizing agent is added at the above-described ratio, or After the addition, it is preferable to add calcium sulfate, phosphoric acid, etc. in the proportions described above.

EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited at all by these examples.
In each example, the Pb and Ca concentrations in the filtrate and the pH of the filtrate were measured by the following methods.

(1) Pb concentration, Ca concentration Sample solution a (solvent pH: 6) in accordance with the dissolution test of the Environmental Agency Notification No. 13 “Method for Testing Metals Contained in Industrial Waste” February 17, 1973 3) was prepared, and the lead content was measured by flame atomic absorption method in JIS K0102: 1998.
(2) pH
Prepared sample liquid a (solvent pH: 6.3) according to the dissolution test of February 13, 1973, Environment Agency Notification No. 13 “Testing Method for Metals in Industrial Waste” The pH of the liquid was measured.

<Example 1>
6% desulfurized gypsum is used as a lead insolubilizer for an aqueous solution in which lead nitrate is added to pure water so that the lead ions are 2, 20, and 100 ppm and the pH is adjusted to 12 to 13 with sodium hydroxide. The filtrate was added at a concentration of .6 g / L, then 75% by mass phosphoric acid solution was added at a concentration of 2.25 g / L, stirred for 20 hours, and then suction filtered using a membrane filter having a diameter of 0.45 μm. The lead concentration and pH were measured. The results are shown in Table 1.

<Comparative Example 1>
It processed like Example 1 except having added only 2.25 g / L of 75 mass% phosphoric acid liquid as a lead insolubilizer. The results are shown in Table 1.
<Comparative example 2>
The treatment was performed in the same manner as in Example 1 except that 5.2 g / L of calcium hydroxide was added as a lead insolubilizing agent, and then 2.25 g / L of 75% by mass phosphoric acid solution was added. The results are shown in Table 1.
<Comparative Example 3>
The treatment was performed in the same manner as in Example 1 except that 4.0 g / L of hydroxyapatite was added as a lead insolubilizer. The results are shown in Table 1.

  From the comparison between Example 1 and Comparative Examples 1 to 3 in Table 1, when phosphoric acid and desulfurized gypsum are added, lead ions are remarkably insolubilized even at pH 12 or higher, whereas only phosphoric acid is added (Comparative Example 1). ) And addition of hydroxyapatite (Comparative Example 3) shows that lead ions are not sufficiently insolubilized. It can also be seen that phosphoric acid and calcium hydroxide (calcium other than gypsum, Comparative Example 2) are not insolubilized as much as desulfurized gypsum.

<Example 2>
14 mL of sodium carbonate as a water-soluble calcium insolubilizing agent is added to an aqueous solution in which lead nitrate is added to pure water so that lead ions become 100 ppm and calcium hydroxide is added at 5.2 g / L to a pH of 12 or more. 0.8 g / L, desulfurized gypsum as a lead insolubilizer at a concentration of 6.6 g / L, and then 75% by weight phosphoric acid solution at a concentration of 2.25 g / L for 6 hours. After stirring, the lead concentration and pH in the filtrate which was suction filtered using a membrane filter having a diameter of 0.45 μm were measured. The results are shown in Table 2.

<Example 3>
The same treatment as in Example 2 was conducted except that sodium sulfate was added as a calcium insolubilizer at a concentration of 20 g / L. The results are shown in Table 2.
<Example 4>
A blast furnace slag fine powder (CaO: 42 mass%, SiO ₂ : 34 mass%) was added as a calcium insolubilizer at a concentration of 2 g / L, and the same treatment as in Example 2 was performed. The results are shown in Table 2.
<Example 5>
Except for adding a fine coal ash (volume median particle size (D50): 4.3 μm) collected as a calcium insolubilizer directly from the latter stage of an electric dust collector of a domestic coal-fired power plant at a concentration of 10 g / L The same treatment as in Example 2 was performed. The results are shown in Table 2.

<Example 6>
The same treatment as in Example 2 was conducted except that artificial zeolite produced from coal ash (manufactured by Nippon Steel Engineering Co., Ltd., maximum particle size 0.075 mm) was added as a calcium insolubilizer at a concentration of 10 g / L. The results are shown in Table 2.
<Example 7>
The same treatment as in Example 2 was performed except that clinoptilolite (produced by Nikicho, Yoichi-gun, Hokkaido, 100 mesh) was added as a calcium insolubilizer at a concentration of 10 g / L. The results are shown in Table 2.
<Example 8>
The same treatment as in Example 2 was performed except that synthetic zeolite 4A (manufactured by Wako Pure Chemical Industries, CAS. NO. 1318-02-1, 75 μm passage) was added as a calcium insolubilizer at a concentration of 10 g / L. The results are shown in Table 2.

<Comparative Example 4>
The treatment was performed in the same manner as in Example 2 except that the calcium insolubilizing agent was not added. The results are shown in Table 2.

  From the comparison between Examples 2 to 8 and Comparative Example 4 in Table 2, even if phosphoric acid and desulfurized gypsum are added to an aqueous solution prepared by adding calcium hydroxide in advance, lead is added as in Example 1. Various water-soluble calcium insolubilizing agents such as sodium carbonate, sodium sulfate, blast furnace slag, fine-grained coal ash, coal ash artificial zeolite, clinoptilolite, and synthetic zeolite are added to this, although it cannot be remarkably insolubilized. This shows that the insolubilizing effect of lead ions can be improved.

<Example 9>
Phosphoric acid (9 gP) and desulfurized gypsum (88.3 g) are added to 1 L of an alkaline (pH: 11.8) solution having the following water quality composition, and potassium hydroxide is further added to adjust the pH to 12.4 to 12.5. Adjusted. This was stirred at 80 ° C. for 24 hours and then separated into solid and liquid. As desulfurization gypsum, desulfurization gypsum from three different coal-fired power plants was used.
The results of measuring the water quality in the liquid phase obtained after solid-liquid separation are shown in Table 3 (pH is the value after treatment).
It was shown that the hydroxyapatite conversion of desulfurized gypsum has the effect of simultaneously reducing heavy metal concentrations of boron (B), arsenic (As), and selenium (Se) in addition to lead. The analysis was performed using an ICP emission analyzer (ICP-8100) manufactured by Shimadzu Corporation.

  The heavy metal insolubilization treatment method of the present invention can industrially advantageously insolubilize heavy metals from alkaline treated water having a pH of 11 or more containing heavy metals.

Claims (7)

  A method for insolubilizing a heavy metal contained in highly alkaline water, comprising adding at least one of phosphoric acid and a phosphate and calcium sulfate to water to be treated having a heavy metal and having a pH of 11 or more.   The heavy metal insolubilization method according to claim 1, wherein the water to be treated further contains water-soluble calcium, and an agent for insolubilizing the water-soluble calcium is further added to the water to be treated.   The heavy metal insolubilization method according to claim 1 or 2, wherein the calcium sulfate is obtained by smoke desulfurization.   The method for insolubilizing heavy metals according to claim 2 or 3, wherein the agent for insolubilizing water-soluble calcium is an agent capable of supplying at least one selected from carbonate ions, sulfate ions and silicate ions.   The insolubilization of heavy metal according to any one of claims 2 to 4, wherein the agent for insolubilizing water-soluble calcium is at least one selected from carbonic acid, carbonate and bicarbonate, sulfuric acid and sulfate, and silicate. Processing method.   The heavy metal according to any one of claims 2 to 5, wherein the agent for insolubilizing water-soluble calcium is at least one selected from blast furnace slag, fine-grained coal ash, synthetic zeolite, coal ash artificial zeolite, and natural zeolite. Insolubilization treatment method.   The calcium sulfate and at least one of phosphoric acid and phosphate are added to the water to be treated at the same time as or after the addition of the agent that insolubilizes the water-soluble calcium. The method for insolubilizing heavy metals as described in 1.