JPS5817677B2 - Method for removing arsenic in aqueous solution - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for effectively removing arsenic contained in an aqueous solution.

Because arsenic is highly toxic, the discharge standard for wastewater containing arsenic is set at 0.5 m9/l or less.

Conventionally, the most common method for removing arsenic from an aqueous solution is the so-called coprecipitation removal method, and by this coprecipitation removal method, arsenic can be removed to a level below the emission standards.

However, this coprecipitation removal method (a method in which ferric ions are added to an aqueous solution and then neutralized to produce ferric hydroxide, in which arsenic is coprecipitated) Unless the weight ratio of ions (this is called the arsenic ratio) is increased to approximately 10 for pentavalent arsenic and 20 to 30 for trivalent arsenic, it will not be possible to meet the emission standards. Arsenic cannot be removed until

The fact that the desired removal rate cannot be obtained unless the arsenic ratio is increased creates an extremely troublesome situation in actual operation.

In other words, a large amount of ferric ions must be added, resulting in the production of a large amount of ferric hydroxide sludge containing arsenic, and the reality is that post-treatment of this sludge is difficult. be.

Moreover, when separating this sludge from the liquid, the equipment becomes too large and inefficient operations are forced.

The present invention provides a highly effective arsenic removal method that can reduce the amount of sludge produced to less than 1/10 of the conventional method, and can also perform arsenic separation and removal using simple equipment. The gist of this is that ferric ions are allowed to coexist in an arsenic-containing aqueous solution at an arsenic ratio of about 0.5 to 3 (if the aqueous solution to be treated does not contain ferric ions, (adding iron ions) and a state in which the ferric ions precipitate as ferric hydroxide (preferably at pH 4 to 5).
), this aqueous solution is neutralized to produce fine precipitates containing arsenic and iron, an anionic surfactant is added to the liquid in which these fine precipitates are formed, and this liquid is then subjected to flotation treatment. A method for removing arsenic from an aqueous solution involves flotation separation of the fine precipitate.

Ferric ions are made to coexist in an aqueous solution containing pentavalent arsenic so that the arsenic ratio is as small as 0.5 to 3, and an alkali neutralizing agent such as sodium hydroxide or slaked lime is added to this to make p.
When H is adjusted to 4 to 6, preferably 4 to 5, a precipitate is formed which is assumed to be a mixture of iron arsenate and ferric hydroxide adsorbing arsenic.

Since this precipitate is very fine and stably dispersed, it is practically difficult to separate it by sedimentation.

The present inventors have discovered that iron arsenate, arsenic, and ferric hydroxide
After adding an anionic surfactant, such as dodecylbenzene sulfonate, preferably sodium dodecylbenzene sulfonate, or alkylbenzene sulfonate, to a dispersion of fine precipitates that are thought to contain iron, They discovered that when the liquid is flotated, the finely dispersed precipitates (solids) float very efficiently, making it easy to perform solid-liquid separation.

To explain in detail according to the examples, FIG. 1 shows the test results of the comparative example (curve a) and the present invention example (curve b). In the case of the comparative example (curve a), the Ferric ions were added in varying amounts to an aqueous solution containing pentavalent arsenic, slaked lime was added to adjust the pH to around 5, and then 1
The supernatant liquid after allowing the precipitate to settle for a period of time was sampled, and the relationship between the residual arsenic concentration in this liquid and the amount of ferric ion added was investigated. ), ferric ions are added in different amounts to the same aqueous solution containing 10 μ/l of pentavalent arsenic, sodium hydroxide is added to adjust the pH to around 5, and then dodecylbenzene is added. After adding 7.5 m9/l of sodium sulfate, this liquid was directly put into a Denver-type flotation machine and subjected to flotation treatment for 3 minutes, and the residual arsenic concentration and ferric iron concentration in the treated tail liquid were determined. This study investigated the relationship with the amount of ions added.

As is clear from the results in Figure 1, in the case of the comparative example (curve a), the amount of ferric ion added was 10 am9/l (
Even with an arsenic ratio of 10), the discharge standard is 0.5m9. Although it is difficult to achieve an arsenic concentration of Even in 2, the emission standard is 0.5T.
The arsenic concentration can be reduced to n9/l or less.

In addition to this sister example, tests were conducted in which the amount of anionic surfactant added was varied, and it was found that the smaller the arsenic ratio, the smaller the amount of surfactant added.

Furthermore, it was found that anionic surfactants contribute sufficiently to improving flotation efficiency, although their effects vary somewhat.

It has been found that when the arsenic dissolved in the aqueous solution to be treated is trivalent arsenic, a high removal rate cannot be obtained unless the amount of ferric ions is increased compared to the case of pentavalent arsenic.

However, if this trivalent arsenic is present, add an oxidizing agent such as bleaching powder to oxidize it to pentavalent arsenic, and then
By carrying out the method of the present invention, arsenic could be removed at a high removal rate under a small arsenic ratio.

As described above, according to the method of the present invention, compared to the conventional coprecipitation removal method, only about 1/10 of the amount of ferric ions is required, and the amount of generated sludge is greatly reduced.

Moreover, no time is required for sedimentation, and a solid-liquid separation tank is also not required.

In addition to these operational advantages, the method of the present invention:
The arsenic concentration in the treatment solution is further reduced than in conventional methods, which is an extremely significant effect in preventing pollution.

□Brief explanation of the drawings Figure 1 is a diagram showing the relationship between the amount of ferric ions coexisting in an arsenic-containing aqueous solution and the residual arsenic concentration in the solution after treatment. Curve a is a comparative example according to the conventional method; 1 shows actual case results according to the method of the present invention.

Claims (1)

[Claims] 1 When separating and removing arsenic dissolved in an aqueous solution, the arsenic iron ratio (Fe
Ferric ions are allowed to coexist so that ``/As) is 03 to 3, and then an alkaline agent is added to this solution to adjust the pH to 4 to 6.
Fine precipitates containing arsenic and iron are generated in the liquid by neutralization treatment, an anionic surfactant is added to the liquid in which these fine precipitates are dispersed, and this liquid is then subjected to flotation treatment. A method for removing arsenic from an aqueous solution, which comprises floating and separating the fine precipitate.