JP3081910B2 - Arsenic (V) ion removal method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a process for efficiently removing arsenic (V) ions dissolved in wastewater or geothermal water from, for example, wood preservative treatment, glass production, electronic component production processes, etc. The present invention relates to a method for reusing an agent.

[0002]

2. Description of the Related Art In order to remove arsenic dissolved in water, an optimum method should be selected in consideration of the properties of raw water, the amount of treatment, the use of treated water, the form of arsenic, the economic efficiency, and the like. . The coprecipitation method, which removes arsenic dissolved in water as a precipitate of arsenic compounds with low solubility or captures and removes flocs of hardly soluble metal hydroxides generated in a certain pH range, is simple and easy to operate. Widely implemented because of its suitability for volumetric water treatment. Among these, iron (III) salts, polyaluminum chloride (PAC), and calcium salts have been used as arsenic (V) treating agents by the precipitation method.
FIGS. 7A, 7B, and 7C show examples of removing 0.25 mM arsenic (V) ions by the treating agent. Iron (III) (sulfate, chloride) is most widely used for water treatment and restoration of contaminated groundwater in coal-fired power plants, water purification plants and semiconductor manufacturing plants. As shown in FIG.
The addition of an equimolar concentration of iron (III) reduces the arsenic (V) concentration to about 0.06 mM in a narrow range of pH about 3-4, but the arsenic (V) removal is incomplete. Arsenic (V) is also removed by PAC and aluminum sulfate used in many water purification plants. As shown in FIG. 7 (B), the addition of equimolar PAC only reduces the arsenic (V) concentration to about 0.12 mM in a narrow range of about pH 5 to 6. In addition, calcium salts are also considered to be effective as precipitants, but as shown in the example of FIG.
It is noted that arsenic (V) is hardly removed by the calcium salt in H2-12. Actually 2.6m
M or higher concentration of calcium salt, and adjust the pH to 1
It is said that it is necessary to adjust to 0 to 11. FIG.
As can be seen from (A), (B) and (C), it can be said that iron (III) salt is the most effective in removing arsenic (V) ions among the conventionally used precipitating agents. However, 0.2
Removal equimolar iron (III) arsenic in ion amount (V) relative to 5mM of arsenic (V) ions is incomplete, the disadvantage of not satisfy the discharge standards (0 .1ppm ≒ 0.0013mM) is there. Therefore, it is necessary to increase the amount of iron (III) salt added. FIGS. 8A and 8B show the case where the initial iron (III) salt concentration was increased to 0.75 and 2.5 mM. By increasing the initial iron (III) salt concentration to 0.75 mM and 2.5 mM, it was confirmed that the arsenic (V) ion residual concentration was reduced to 0.001 mM or less in the pH range of 5 to 6 and 5 to 7 respectively. Can be However, as described above, the iron (III) concentration needs to be three times or more the molar concentration in order to satisfy the drainage standard, and there is a disadvantage that an excessive amount of the chemical is used. Also,
The optimal pH range is too narrow. Similarly, for aluminum salts and calcium salts, the effective pH range is narrowed, requiring the use of larger amounts of drugs.

[0003]

SUMMARY OF THE INVENTION The present invention overcomes the drawbacks of the conventional method in which the effective pH range is narrow and requires a large amount of a processing agent, and reduces arsenic ions with a small amount of the processing agent in a wider pH range. It is intended to provide a method that can be removed. That is, an object of the present invention is to provide a water treatment method capable of efficiently removing arsenic ions in water to be treated with a small amount of a treating agent and easily achieving a residual concentration of arsenic ions below the wastewater standard. Another object of the present invention is to provide a method for recycling a treating agent used for water treatment.

[0004]

The present inventors have made intensive studies on the precipitation reaction of arsenic ions and found that arsenic (V)
It has been found that the use of a lanthanum compound which forms a compound which is hardly soluble with ions can reduce the arsenic (V) ion concentration in a wide pH range with a small amount of a treating agent. Reached. That is, the present invention relates to the treatment of water containing (1) arsenic (V) ions.
Lanthanum arsenate produced by adding an aqueous solution of a lanthanum compound
Precipitation removal, residual concentration of arsenic (V) ions in the water to be treated
Arsenate from which sediment is reduced to below the drainage standard
Treatment with alkaline aqueous solution higher than pH 9
That more lanterns can be collected and reused.
And a water treatment method characterized by the following.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, arsenic (V) in water can be more effectively removed as lanthanum arsenate by using the least expensive lanthanum compound among the rare earth elements as a precipitant. As the lanthanum compound used in the present invention, a water-soluble lanthanum compound is used, and specifically, lanthanum chloride, lanthanum nitrate and the like are used. The addition amount may be a minimum amount according to the arsenic ion concentration. That is, by adding a lanthanum compound in an equimolar concentration with respect to the concentration of arsenic ions, arsenic ions can be efficiently precipitated and removed, and if the lanthanum compound is used in excess, the residual concentration of arsenic ions can be reduced by drainage. Even lower levels, below the standard, can be used.
The pH of the water to be treated when adding the treating agent is preferably 5
-12, more preferably 5-9. By separating and removing the precipitated lanthanum arsenate after the addition of the lanthanum compound, the residual concentration of arsenic ions in the water to be treated is reduced to 0.1%.
ppm or less. The reaction between the arsenic (V) ion and the lanthanum ion is represented by equations (1) and (2). H ₂ AsO ₄ ⁻ + La ³⁺ → LaAsO ₄ + 2H ⁺ (1) HAsO ₄ ²⁻ + La ³⁺ → LaAsO ₄ + H ⁺ (2) By treating the separated lanthanum arsenate at a pH of about 13, lanthanum and arsenic can be separated and lanthanum can be reused. That is, at a pH of about 13, only arsenic (V) elutes and lanthanum remains in the solid phase. Therefore, by separating arsenic (V) in a concentrated form, lanthanum can be recovered and reused.

[0006]

Next, the present invention will be described in more detail with reference to examples. Example 1 FIG. 1 shows the arsenic (V) and lanthanum mixed solutions prepared so that the initial concentrations were all 0.25 mM.
The arsenic (V) and lanthanum residual concentrations after time are shown. pH
In regions 5 to 8, the arsenic (V) residual concentration was reduced to 0.01 mM or less. Compared with the result of iron (III) (FIG. 7A), a lower arsenic (V) residual concentration was obtained in a wider pH range. When only the initial concentration of lanthanum was increased to 0.75 mM, as shown in FIG. 2, at pH 5 to 9, the residual concentration of arsenic (V) decreased to 0.001 mM or less, which was below the drainage standard (FIG. 2 (A)). ). Furthermore, the initial concentration of lantern is 2.5m
M, the arsenic (V) residual concentration decreased to 0.001 mM or less over a wide range of pH 5 to 12 (FIG. 2).
(B)). The reaction between lanthanum and arsenic (V) proceeds more efficiently as the concentration of both is higher. As shown in FIGS. 3 and 4, the initial concentration of arsenic (V) ions is as high as 0.75 and 2.5 mM. By adding an equimolar concentration of lanthanum to the solution, it was possible to remove arsenic (V) ions in a wide pH range. In the treatment of arsenic (V) -containing wastewater, it is important to know in what form arsenic (V) is removed. From the value of the acid dissociation constant of arsenic acid (H ₃ AsO ₄ ), the arsenic (V) ions in the water were adjusted to pH 4-5 and 9-10.
It is known that H ₂ AsO ₄ ⁻ and HAsO ₄ ²⁻ are present as dominant chemical species in the vicinity, respectively. In order to analyze the composition of the precipitate formed by the reaction between arsenic (V) ions and lanthanum ions, the pH of the mixed solution of arsenic (V) and lanthanum having an initial concentration of 35.7 mM was adjusted to 5 by a pH stat.
X-ray diffraction patterns of the precipitates obtained by holding in FIGS.
(A) and (B) show. All were identified as LaAsO ₄ (15-176) by comparison with JCPDS data. The obtained precipitate was dissolved in (1 + 1) hydrochloric acid, and As and La were quantified. As: La ≒ 1:
Since it was 1, it was confirmed that arsenic (V) ions form LaAsO ₄ with lanthanum ions and are precipitated and removed.

Example 2 The lanthanum arsenate separated by precipitation removal in Example 1 was adjusted to pH
Were dispersed in different aqueous solutions and the solubility was examined. FIG. 6 shows the result. It can be seen that at pH 5 to 9, lanthanum arsenate is stable and does not dissolve. In the acidic region below pH 5, lanthanum arsenate dissolves and lanthanum and arsenic (V) elute. However, in an alkaline pH range higher than pH 9, only arsenic (V) elutes and lanthanum remains in the solid phase. From this, it is possible to separate arsenic (V) in a concentrated form by treating lanthanum arsenate with, for example, an alkaline solution having a pH of about 13, and to collect and reuse lanthanum.

[0008]

By using lanthanum chloride or lanthanum nitrate as a precipitant, arsenic (V) ions can be removed to a concentration below the wastewater standard with a smaller addition amount and in a wider pH range. In particular, since it is effective in a neutral pH range, it can be discharged without adjusting the pH of the treated water after removing arsenic (V) ions. Further, by treating the removed lanthanum arsenate with an alkaline solution, it is possible to separate arsenic (V) in a concentrated form and to collect and reuse lanthanum.

[Brief description of the drawings]

FIG. 1 is a graph showing arsenic (V) ion removal characteristics of lanthanum.

FIG. 2 is a graph showing arsenic (V) ion removal characteristics of lanthanum. (A): arsenic (V) initial concentration, 0.25 mM; lanthanum initial concentration, 0.75 mM (B): arsenic (V) initial concentration, 0.25 mM; lanthanum initial concentration, 2.5 mM

FIG. 3 is a graph showing arsenic (V) ion removal characteristics of lanthanum.

FIG. 4 is a graph showing arsenic (V) ion removal characteristics of lanthanum.

FIG. 5 is an X-ray diffraction diagram of a precipitate obtained by the method of the present invention. (A): Precipitation obtained at pH 5 (B): Precipitation obtained at pH 9 (C): Lanthanum arsenate (LaAsO ₄ )

FIG. 6 is a graph showing the dissolution characteristics of lanthanum arsenate.

FIG. 7 is a graph showing arsenic (V) ion removal characteristics of iron (III) salt, PAC, and calcium salt. (Arsenic (V) initial concentration, 0.25 mM) (A): Iron (III) initial concentration, 0.25 mM; (B): PA
C initial concentration, 0.25 mM; (C): calcium initial concentration,
0.25 mM

FIG. 8 is a graph showing arsenic (V) ion removal characteristics by iron (III) salt. (A): arsenic (V) initial concentration, 0.25 mM; iron (III) initial concentration, 0.75 mM (B): arsenic (V) initial concentration, 0.25 mM; iron (III) initial concentration, 2.5 mM

Continuation of front page (56) References JP-A-9-66284 (JP, A) JP-A-54-113952 (JP, A) JP-A-54-22953 (JP, A) Shuzo Tokunaga et al., “Arsenic and lead Research on Advanced Treatment Technology for Wastewater Contained in Materials ”, Report of the National Institute of Materials Science and Technology, National Institute of Materials Technology, published on February 28, 1997, Volume 5, No. 1, p. 21-38 (58) Field surveyed (Int. Cl. ⁷ , DB name) C02F 1/58

Claims (1)

(57) [Claims] 1. A method for treating treated water containing arsenic (V) ions.
Lanthanum arsenate produced by adding an aqueous solution of a lanthanum compound
Precipitation removal, residual concentration of arsenic (V) ions in the water to be treated
Arsenate from which sediment is reduced to below the drainage standard
Treatment with alkaline aqueous solution higher than pH 9
That more lanterns can be collected and reused.
Characterized water treatment method.