JP2005254127A - Method of treating soil complexedly polluted with heavy metals - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of chemically treating a polluted soil which enables a partial curtailment of stepwise treating process of the reduction treatment of a complexedly polluted soil. <P>SOLUTION: In the case of chemically treating a soil complexedly polluted with arsenic, lead, cadmium, and the like, a reducing agent formed by blending sodium thiosulfate, sodium carbonate and thiourea, and a carbonated compound and a solidifying material are added to the soil at the same time and mixed, to make each of the agents preferentially react with a substance having affinity with it to insolubilize it at once, the re-elution of insolubilized materials in the soil is prevented or suppressed even if the soil becomes alkaline by the solidifying treatment. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  In the present invention, insolubilization treatment of contaminated soil, especially in-situ (local) treatment of complex contaminated soil with heavy metals, by adding to the contaminated soil and performing agitation and mixing treatment to produce insoluble salt of the pollutant source material, The present invention relates to a method for treating heavy metal contaminated soil for detoxification.

As chemical treatment methods for contaminated soil, conventionally, there are known cases of performing on-site (local) treatment and off-site (unloading) treatment.
For example, the contamination site is located in a layer between 2.0 and 5.0 meters underground, and adopting a shallow agitation method and using a treatment agent and moisture-adjusting solidifying material suppresses the confusion and diffusion of the contamination source material. If you think you can do it, the on-site processing will be dominant.

In the case of off-site treatment, the destination of the treated soil is specified, and those that meet the acceptance criteria will be removed after excavation, and the treated soil will be delivered, but for contaminants that exceed the acceptance criteria It must be taken out after processing in the hall.
At present, there are many cases where on-site processing is performed from the viewpoint of avoiding the complexity of work procedures or from an economic point of view.

  As a chemical treatment method for contamination, usually, a reducing agent such as ferrous sulfate is used to reduce soluble heavy metals dissolved in moisture, and these are converted into sulfides or hydroxides to be insolubilized and precipitated. Is known (see Figure 2 for an explanation of the reaction mechanism), but since this method is a reaction that exists only under acidic conditions, then the soil becomes acidic and if left as it is, It will eventually dissociate and return to the oxidized form again.

Therefore, in order to stabilize the treatment after insolubilization, it is necessary to neutralize with lime water or the like to suppress dissociation as a hydroxide. However, neutralization of acidic soil with liquid is quite difficult to achieve uniformity, and is practically impossible.
If the above neutralization treatment is neglected or an incomplete state continues, re-dissociation occurs due to acid rain or air oxidation, and re-treatment is required.

Moreover, in order to give predetermined | prescribed mechanical strength to the ground after insolubilization processing, if a required quantity of solidification material (generally cement type, calcium type) is added, alkaline conditions will end too much.
For example, if it is desired to obtain a uniaxial compressive strength of 2 kg / cm ² or more, a solidifying material of 5% or more with respect to the amount of soil is required. As a matter of course, the pH value is strongly alkaline (pH 13 or more) when mixed.

In general, soil contamination is rarely caused by a single substance and often involves multiple contaminants.
In the contaminated soil, when amphoteric compounds such as lead and arsenic coexist, when the soil becomes excessively alkaline (generally pH 10 or more), it will be dissolved again. Requires extremely complicated and precise control.
Even if an insolubilization treatment such as reduction is performed, there is a possibility of re-elution (release of metals) in the next solidification step as described above.
That is, the possibility of re-dissociation cannot be denied by any method only by reduction treatment.

In addition, the conventional treatment has to be performed by reacting each drug stepwise for each target substance and each reaction process.
JP 2003-290759 A

  Therefore, an object of the present invention is to provide a chemical insolubilization treatment method that solves all of the above-mentioned problems (inconveniences / detriments) that occur in the treatment process of complex heavy metal contaminated soil.

  The method of the present invention preferentially reacts any of a plurality of drugs with a substance having a strong affinity even in the case where a sulfide and a heavy metal that cannot be formed coexist in a pollution source. The carbonic acid compound is formed at the same time to achieve insolubilization, and it is possible to prevent or suppress the resolubilization of the insolubilized material even if the soil becomes alkaline after the treatment. The structure is as described below. is there.

(1) An insolubilizing treatment solution composed of an aqueous solution containing sodium thiosulfate, sodium carbonate and thiourea, and a treatment agent containing a cement-based solidifying material and a chelating agent such as thiourea, are applied to contaminated soil at once. A method for treating complex heavy metal-contaminated soil that can be added, mixed and insolubilized to maintain stabilization even under alkaline conditions during the solidification and aging process of the treated soil.

(A) When the treated soil is in the state of complex contamination, any treatment agent works in place, which is effective.
After the reduction treatment, insolubilized materials may be dissociated when the soil becomes alkaline due to cement solidification treatment, but if sodium carbonate is added, the oxidized chromium that has been re-dissociated by alkali is reduced in chromium carbonate. Can be insolubilized and precipitated, so that re-elution (release) can be prevented or suppressed.

(B) Stabilize so that the insoluble salt produced by the detoxification treatment with the reducing agent is not re-eluted.
Sodium thiosulfate, which reacts in the neutral range for the reduction treatment, is used to produce chromium hydroxide and the like. Since neutralization after reduction is not required, the complexity of the process can be eliminated.
Compared with the conventional method using ferrous sulfate or the like as an insolubilizing agent, neutralization after treatment is not required.
In short, the processing can be achieved with fewer stepped processing steps as compared with the conventional method.

(C) Since both the reducing agent and the insolubilized material stabilizer are neutral salts, there is no mutual reaction and no trouble occurs even if both are blended.
(D) The compounding ratio of the drug can be compounded in an equivalent amount depending on the soil contamination status.
The blending is adjusted in the range of about 100 times the theoretical addition amount.

  A reducing agent and an insolubilized material stabilizer are simultaneously added to the soil and mixed to insolubilize the source of heavy metals at once, thereby shortening the treatment process.

FIG. 1 shows an explanatory view of an insolubilization treatment method in a neutral region of the present invention heavy metal composite contamination.
The components of the treating agent used in the method for treating complex heavy metal contaminated soil are specifically as follows.
(1) Formula (a) Insolubilizing agent Reducing agent (Sodium thiosulfate)
[Effect] Lead Precipitated as sulfide
Mercury Precipitated as mercury sulfide
Arsenic Precipitated as arsenic sulfide
Hexavalent chromium Precipitated as chromium hydroxide

Arsenic that does not react with sodium carbonate can be reduced with sodium thiosulfate.
When an appropriate amount of sodium thiosulfate is blended, it reacts with mercury, lead, cadmium and the like to produce an insoluble substance, so that it can be further stabilized.

Carbonate (sodium carbonate, bicarbonate)
[Effect] Cadmium Precipitated as cadmium carbonate
Precipitate as lead carbonate
Mercury Precipitated as mercury carbonate
Hexavalent chromium Precipitated as chromium carbonate

(B) Insolubilized stabilizer Masking agent (EDTA, thiourea, etc.)
[Effects] By masking dissociated ions such as complex salts generated under excessive drug conditions during treatment as a chelate compound, an insolubilized state is created (can be removed as a precipitate).

Solidifying agent (lime, gypsum, reducing slag, bicarbonate) Especially, it is characterized by blending gypsum.
[Effect] This suppresses hydrolysis of hydroxide that occurs under alkaline conditions due to lime which is the main component of the solidifying agent. Moreover, gypsum (calcium sulfate) is a neutral insoluble substance, and when mixed with a solidifying agent, it absorbs moisture as crystal water and improves the solidification performance.
It is known that the solidification strength is improved by blending carbonate, but here, bicarbonate was employed.

This is because lead, such as lead, produces part of lead hydroxide if it is a carbonate, but if it is bicarbonate, a carbonate is directly produced, so a more reliable insolubilization process is realized.
In addition, when heavy metal carbonate is affected by pH or the like in water, it has an effect of suppressing the influence of alkalinity because it promotes buffering properties.

(2) Amount of drug added to the substance to be treated (a) Theoretical amount of drug to be insolubilized [Substance to be treated] + [Treatment] → [Insoluble substance]
In the case of 1 g / m ³ of lead, 0.52 g of sodium carbonate corresponds.
Similarly, the coexisting substances are calculated as shown in the table below.







┌────────┬──────────┬────────────┐
│ Substances to be treated │ Sodium carbonate (g) │ Sodium thiosulfate (g) │
├────────┼──────────┼────────────┤
│ Lead │ 0.52 │ 3.62 │
│ Lead │ 1.65 │ − │
│ Water Silver │ 0.54 │ 5.00 │
│ Arsenic │-│ 5.00 │
│ Cadmium │ 0.96 │ − │
│ Hexavalent chromium │ 9.10 │ 7.21 │
└────────┴──────────┴────────────┘
Based on the measured value of the pollutant, the theoretically corresponding amount of the treatment agent was calculated. In actual operation, the amount was increased due to various factors, and the estimated addition amount was calculated.

(B) Amount of addition of reducing agent in actual process (α) Sodium carbonate has an estimated value 100 times the theoretical calculation value.
That is, since the theoretical amount is very small and it is considered difficult to mix uniformly, the ratio is increased.
Sodium carbonate dissolves in water and generates carbonate ions, which react with lead. However, the increase in water temperature is expected to reduce the saturation solubility and impair the effectiveness. To do.

(Β) The estimated value of sodium thiosulfate is 12 times.
Empirically, 1.2 times the amount seems to be good, but the theoretical amount is very small and it seems difficult to mix uniformly.
There should be no substances that are affected when the above agent is overloaded.
However, when lead is placed in a position where the pH value fluctuates extremely (overdose addition), it may be re-eluted, but when sodium carbonate is used in combination, the hydroxyl group generated by sodium produces hydroxide. It is thought that it can be suppressed by doing.

(Γ) As a result of insolubilization with sodium carbonate, the amount of carbon dioxide generated is 0.51 liters per gram of sodium carbonate.
Carbon dioxide generated during the treatment is neutralized with calcium in the cement and at the same time is removed from the site, so it is unlikely that all of it will remain.

(3) Treatment effect of the pollution treatment countermeasures (a) All the supernatant waters after the muddy water separation satisfied the public sewer discharge standard value.
Method of measurement: Official method Addition of some alkali (solidifying agent, etc.) during muddy water separation to make it alkaline improves sedimentation (complete in about 30-60 minutes).
Since the precipitation of sulfides is very fine, it is advantageous to perform a coagulation sedimentation treatment with a polymer coagulant before discharging the supernatant water.

(B) Regarding the dissolution test after curing for 7 days on solidified soil (improved soil), the environmental standards for arsenic, lead and cadmium were all satisfied.

(4) Results In the insolubilization treatment of contaminants such as arsenic, lead, cadmium, etc. contained in contaminated soil with a reducing agent, an appropriate amount of masking agent is added so that even if an excessive amount of reducing agent is added, the elution suppression effect It was possible to prevent problems such as re-elution.
In alkaline conditions that cause re-elution of arsenic and lead, re-elution of sulfides such as arsenic and lead could be suppressed.

  In actual operation, it should be planned to administer about 10 times the reducing agent and about 20 times the coagulant aid, and the upper limit of the range of addition of insoluble agents such as reducing agents should be about 100 times the amount. Was confirmed.

Although the above examples have been described on the premise of in-situ treatment, it is clear that the treatment agent of this example has the same effect even when applied in the case of off-site treatment.
Recently, factory sites located in or around cities have been extensively reused, but in order to divert them to residential land, public land, etc., detoxification of contaminated site soil is an essential condition.
According to the method of the present invention, there is no need for a stepwise treatment step for detoxification, and the spread of an economical method for insolubilizing complex contaminated soil is desired.

It is reaction mechanism explanatory drawing of this invention insolubilization processing method in the neutral region in complex heavy metal contaminated soil processing. (Example) The reaction mechanism figure at the time of using ferrous sulfate for the composite heavy metal contamination soil processing of the conventional method is shown.

Explanation of symbols

Pb Lead As Arsenic Hg Mercury Cr Chromium

Claims (1)

An insolubilization treatment adjustment solution consisting of an aqueous solution containing sodium thiosulfate, sodium carbonate and thiourea, and a treatment agent containing a cement-based solidifying material containing a chelating agent such as thiourea are added to the contaminated soil at once. A method of treating complex heavy metal-contaminated soil that can be mixed and insolubilized with metal ions to maintain the treatment stability even under alkaline conditions during the solidification and aging of the treated soil.

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