JP2007268429A - Method of suppressing elution of arsenic from soil contaminated by arsenic - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a simple and economically superior method of suppressing the elution of arsenic from soil, which is capable of suppressing arsenic in both of trivalent and pentavalent forms from a soil contaminated by arsenic and satisfying the soil elution standard by elution test conforming to the soil contamination countermeasure law. <P>SOLUTION: By the method, an arsenic concentration of 0.01 mg/L or lower being a standard soil elution can be achieved by a small consumption of rare earth salts, by controlling the pH of a soil to be 6-9 by adding a rare earth salt solution containing cerium as a main component and a slaked lime water-dispersed liquid at the same time or separately to the soil contaminated by trivalent or pentavalent arsenic. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a method for suppressing arsenic elution from soil contaminated with arsenic that may occur as a result of improper handling of hazardous chemical substances, illegal dumping, development and utilization of mineral resources, and the like. Furthermore, the present invention relates to a method for suppressing arsenic elution from arsenic-contaminated soil, which satisfies soil elution standards by converting arsenic present in soil into an insoluble substance.

Since arsenic and its compounds are markedly harmful to living organisms, they are designated as specific hazardous substances in the soil environment standards, and the “Soil Contamination Countermeasures Law Enforcement Regulations” based on the “Soil Contamination Countermeasures Law” (December 26, 2002) According to the enforcement), the arsenic content in the soil is 150 mg or less per 1 kg of soil, and the arsenic elution amount by the soil elution test is 0.01 mg or less per liter.

As a method for treating soil contaminated with arsenic, measures such as excavation and removal, containment, prevention of scattering, solidification / insolubilization with cement, and landfill disposal at the final disposal site have been taken. In addition, as a technique for insolubilizing arsenic in soil, a soil treatment method using a chelating agent, a soil treatment method using a calcium compound or an iron compound, a soil restoration technique by electrophoresis, a vitrification method, and a washing method are known. Yes. Further, a treatment method using a rare earth compound such as cerium or lanthanum as an insolubilizing agent has been proposed.

As a method of using the rare earth compound described above as an arsenic insolubilizer, arsenic-contaminated soil is washed with an acid aqueous solution such as phosphoric acid, sulfuric acid, hydrochloric acid, etc., and then lanthanum, cerium, iron salt solution or oxidation thereof. Proposed is a method for insolubilizing arsenic by mixing materials with soil (Patent Document 1), and a method for insolubilizing arsenic by mixing rare earth metal hydrated oxides such as cerium and lanthanum with soil (Patent Documents 2 and 3). Has been. The method of Patent Document 1 performs two-stage treatment of soil washing and insolubilization, so that the treatment effect is very good, but the operation process is complicated, and there is a disadvantage that it is excessive in terms of time and cost. . Moreover, it is not clear because specific usage conditions such as the amount of chemicals used for insolubilization treatment and the optimum pH are not shown due to soil contamination, but it is not clear if 1 L of salt solution is used per 1 kg of soil. Since there is a description that it is sufficient, it is presumed that the amount used is considerably large. On the other hand, in Patent Document 2, the waste abrasive used for the abrasive is used, and cerium hydroxide dissolved in acid and precipitated with alkali is mixed with the soil. The amount used is excessive, for example, 0.1 g is added to 5 g of contaminated soil on the basis of waste abrasive, and there are difficulties in economic efficiency and operability. Also, Patent Document 3 has the same problem.

In addition, arsenic forms present in soil include trivalent arsenous acid (H ₃ AsO ₃ , HAsO ₂ ) and pentavalent arsenic acid (H ₃ AsO ₄ ), and their toxicity is 3 Although it is known that the valence state is stronger, in the above-mentioned patent documents and the like, the treatment method of pentavalent arsenic is described, but the effect on the trivalent arsenic is not clear. .

JP 2002-18421 A JP 2001-200366 A JP 2006-36995 A

The present invention improves the problems of the insolubilization treatment using the above-mentioned rare earth compounds, suppresses the elution of trivalent and pentavalent arsenic from soil contaminated with arsenic, and elution of the soil contamination countermeasure method An object of the present invention is to provide a simple and economical method for suppressing arsenic elution from soil that satisfies the soil elution standard by the test.

In order to solve the above-mentioned problems, the present inventors have studied in detail a method for fixing arsenic in soil using a rare earth salt and suppressing elution of arsenic. As a result, cerium is a trivalent state in soil, Found that all pentavalent arsenic can be immobilized efficiently, corresponding to the arsenic concentration and pH conditions in the soil, the concentration and addition amount of rare earth salts containing cerium as the main component, and the type and addition of pH adjusters By optimizing the method, a simple method that can suppress elution by using a very small amount of drug was developed, and the present invention was achieved.

That is, the present invention is (1) a method for suppressing arsenic elution from arsenic-contaminated soil, comprising adding a rare earth salt solution containing cerium as a main component and slaked lime to soil contaminated with arsenic. (2) A rare earth salt solution containing cerium as a main component is used in an aqueous solution of 0.1 to 3.0% by mass in terms of oxidized rare earth, and the slaked lime is used as an aqueous dispersion of 0.1 to 2.0% by mass. A method for suppressing arsenic elution from arsenic-contaminated soil. (3) The amount of slaked lime is adjusted so that the soil pH after adding the rare earth salt solution and slaked lime containing cerium as main components is 6 to 9, and (4) cerium as the main component After the addition of the rare earth salt solution, or simultaneously with the rare earth salt solution, slaked lime is added.

According to the present invention, by adding a rare earth salt mainly composed of cerium and slaked lime to contaminate soil in any of the trivalent and pentavalent arsenic, the soil elution by the “Soil Contamination Countermeasures” elution test It is possible to provide a method for treating contaminated soil that satisfies the standards and suppresses arsenic elution and is simple and excellent in economic efficiency, and can contribute to the reuse of soil.

Hereinafter, the present invention will be described in detail.
The rare earth salt and slaked lime containing cerium as a main component added to the soil contaminated with arsenic of the present invention is added to the soil as a rare earth salt solution and as a water dispersion. Each concentration depends on the dry state of the soil and the addition method, but the rare earth salt is 0.1 to 3.0% by mass, preferably 0.5 to 2.0% by mass in terms of rare earth oxide. Moreover, slaked lime is 0.1-2.0 mass%. Each concentration added to the soil is closely related to the amount added, and the lower the concentration, the greater the effect with a small amount added per active ingredient, but considering the moisture content, water retention and soil quality of the above, It is preferable to set an appropriate concentration within the concentration range.

The rare earth salt used in the present invention may be any salt selected from hydrochloride, sulfate or nitrate, and the rare earth salt mainly composed of cerium is 90% by mass or more, preferably 92%. A salt of a rare earth element mixture containing at least mass%. In particular, cerium salt is the main component because cerium has both trivalent and tetravalent forms, and trivalent arsenic reacts with the cerium compound by oxidation-reduction reaction with arsenic trivalent. Based on the knowledge that arsenic can be immobilized, and when a large amount of rare earth elements other than cerium is contained, the solubility of metal compounds bound to arsenic ions increases the amount of arsenic dissolved during soil dissolution tests. Depending on the case. Elements other than cerium that may be mixed include scandium, yttrium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, selected from rare earth elements other than cerium and IVb elements. Examples include lutetium, titanium, zirconium, hafnium, and compounds thereof.

The method for adding rare earth salts and slaked lime according to the present invention is either by spraying and mixing an appropriate amount directly on the treated soil surface, or adding an appropriate amount to the soil taken out by means such as excavation, stirring and mixing, and returning to the soil. Do. When pollutants such as heavy metals other than arsenic or organic volatile substances coexist in the contaminated soil, a pretreatment such as a known cleaning method, heat incineration method, extraction method or the like is performed in advance. The method can be carried out. The concentration and addition amount of the rare earth salt and slaked lime of the present invention are determined in advance by measuring the moisture content of the soil, the pH, and the arsenic elution concentration from the soil elution test.

The addition order of the rare earth salt and slaked lime in the method of the present invention is to spray the aqueous solution of the rare earth salt on the soil, and then spray the aqueous dispersion of slaked lime, and then stir and mix the two solutions simultaneously. Alternatively, any order of mixing and spraying immediately after spraying and then stirring and mixing may be used.

According to the method of the present invention, since the rare earth salt is acidic, it is mixed in a dilutely diffused state as rare earth ions when dispersed in the soil, and is insoluble by binding to free arsenate ions in the soil. However, since excess rare earth ions are soluble, adding alkali to them changes the rare earth elements into hydroxides, and both arsenic ions and rare earth ions become insoluble and stable compounds. Can be prevented. On the other hand, when an alkali is added to a rare earth salt solution in advance to form a rare earth hydroxide and then added to the soil, arsenic is immobilized by the adsorption reaction of arsenic ions to the hydroxide particles. The efficiency of insolubilizing is inferior to that of a salt solution.

The slaked lime as an alkali source of the present invention is suitable for preventing harmful effects caused by the excessive elution of the added rare earth salt and for neutralizing the pH in the soil more slowly than a strong base such as caustic soda. The addition amount is determined by investigating in advance such an amount that the pH after adding the rare earth salt and slaked lime to the soil is in the range of 6-9. When the pH is 6 or less, the arsenic elution suppression effect is insufficient and the added rare earth salt is eluted, which is not preferable. When the pH is 9 or more, not only the arsenic elution suppression effect is lowered, but also the soil Since the alkalinity is strong, there is an adverse effect on the natural world such as effects on plants, which is not preferable.

Here, the method for measuring the soil elution concentration of arsenic was performed by adding 500 ml of pure water adjusted to pH 5.8 to 6.3 to 50 g of treated soil in accordance with the test method of Environment Agency Notification No. 46. This is stirred for 6 hours with shaking. The eluate obtained by shaking and stirring is allowed to stand for 20 minutes, and then centrifuged, filtered through a 0.45 μm membrane filter, and the concentration of arsenic (As) contained in the filtrate is reduced to hydride generation— Measured by ICP emission spectrometry. The pH of the soil is determined by a method in which 10 g of soil is stirred in 100 ml of pure water and directly measured with a pH meter.

As an example of a preferred embodiment of the present invention, simulated arsenic-contaminated soil having an arsenic soil content of 40 mg / kg and an arsenic soil elution concentration of 1.29 mg / L was prepared, and 1 gram of cerium oxide was added to 50 g of this soil. From the soil elution test of the soil that was added to the soil to which 10 mL of the cerium chloride solution of 1% by weight was added with varying amounts of slaked lime aqueous dispersion to change the soil pH, and mixed with stirring. The results of examining the relationship between soil pH and arsenic elution amount are shown in FIG. From this result, it is clear that 0.01 mg / L or less, which is the soil elution standard for arsenic, is achieved when the pH of the soil is in the range of 6 to 8.5. (See Example 4)

The present invention will be described in detail below, but the examples are for facilitating the understanding of the present invention and are not intended to limit the present invention. In the examples, “%” means “% by weight”.

Preparation examples and comparative examples of simulated arsenic-contaminated soil samples The soil samples used for comparison are air-dried mountain soil collected from the neighborhood, and screened to a particle size of 2 mm or less as arsenic-contaminated soil of pentavalent arsenic An aqueous solution of disodium hydrogen arsenate (Na ₂ HAsO ₄ · 7H ₂ O), a predetermined amount of an aqueous solution of potassium ortho arsenite (K ₂ AsO ₃ ) added as trivalent arsenic arsenic contaminated soil, and air-dried again Was used. Table 1 shows the results of the arsenic concentration in the soil of the prepared simulated arsenic-contaminated soil sample, the dissolved arsenic concentration by the dissolution test, and the soil pH.

Example 1
Using the simulated arsenic-contaminated soil sample prepared above, 50 g of each soil sample was collected in a plastic container, and a predetermined amount of cerium chloride with 98% cerium purity was diluted to 1% in terms of cerium oxide (1 % -CeO2) and 1% dispersion of slaked lime (1% slaked lime) in a given amount, mixed uniformly, left to stand for one day, and soil dissolution test according to the test method of the Environment Agency Notification No. 46 described above Table 2 shows the results of measuring the elution concentration of cerium and the soil pH. As a result, by adding a predetermined amount of a cerium salt solution corresponding to the arsenic elution concentration of the contaminated soil and a slaked lime dispersion for adjusting the soil pH to around 7-8, an elution concentration of 0.01 mg based on the soil environment standard / L or less can be achieved, and it can be seen that there is no elution of the added cerium. The amount of cerium used should be about 0.02 to 0.2% per soil in terms of cerium oxide with respect to the arsenic elution amount 0.05 to 5.29 mg / L of the contaminated soil. it is obvious.

Example 2 and Comparative Examples 1 and 2
In the same manner as in Example 1, pentavalent arsenic no. The soil sample with an As elution concentration of 1.37 mg / L prepared in 5 was used, and 10 ml of 1% cerium chloride in terms of cerium oxide and 10 ml of 1% slaked lime were added as an alkaline solution. On the other hand, as Comparative Example 1, when no alkali solution was added, and as Comparative Example 2, pH was adjusted using 1N caustic soda solution. The results of measuring the As elution concentration, Ce elution concentration and soil pH of these treated soils are shown in Table 3 in comparison with the blank. As a result, the soil pH is low when the alkaline solution is not added, so the elution concentration of As does not meet the elution standard, but the elution standard is met if the pH is adjusted by adding the alkaline solution, but in the case of caustic soda, Ce It can be seen that the elution concentration is a problem.

Example 3
As in Example 1, trivalent arsenic No. 3 was used as Arsenic Example 3. 1 and no. Using the trivalent arsenic-contaminated soil sample prepared in 2, a predetermined amount of 1% cerium chloride in terms of cerium oxide and 1% slaked lime solution were added. Table 4 shows the results of measuring the As elution concentration, Ce elution concentration, and soil pH of these treated soils. As a result, it can be seen that the elution concentration of As can also satisfy the elution standard even in trivalent arsenic.

As in Example 4 and Comparative Example 1, pentavalent arsenic No. Using the soil sample with an As elution concentration of 1.29 mg / L prepared in Step 4, add 10 ml of 1% cerium chloride in terms of cerium oxide, and change the amount of 1% slaked lime to change the soil pH. Went. The results are shown in Table 5 and FIG. As a result, it is clear that the soil elution standard is satisfied if the amount of slaked lime added is set so as to adjust the soil pH to a range of pH 6.2 to 7.6.

Examples 5 and 6 and Comparative Example 3
As in Example 1, pentavalent arsenic No. In Example 5, 10 mL of 1% cerium chloride equivalent to cerium oxide was added in advance and the soil was stirred and mixed, and 1 hour afterwards, 1% slaked lime using the As elution concentration 1.37 mg / L prepared in 5 7 mL of the liquid was added and mixed well with stirring. In Example 6, cerium chloride and slaked lime were simultaneously added to the soil and mixed well. On the other hand, in Comparative Example 3, 7 mL of 1% slaked lime solution was previously added to 10 mL of 1% cerium chloride in terms of cerium oxide, and after stirring, left to stand at room temperature for 3 days to prepare a cerium hydroxide precipitate. Add and mix well with stirring. Table 6 shows the analysis results of each treated soil. As a result, when the cerium chloride solution and the slaked lime solution are added separately and simultaneously, the soil elution standard is satisfied. Obviously it cannot be satisfied.

  According to the above examples, a 1 wt% aqueous solution of cerium salt and 1 wt% diluted solution of slaked lime are added simultaneously or separately to soil contaminated with trivalent or pentavalent arsenic. Then, by adjusting the soil pH to 6-9, it is possible to achieve a soil arsenic elution concentration of 0.01 mg / L or less, which is a soil elution standard, and cerium salt has an arsenic soil elution concentration of 0.05- It turns out that it can process with the small usage-amount about 0.02-0.2 weight% per soil in conversion of cerium oxide with respect to 5.29 mg / L.

The method for suppressing arsenic elution from arsenic-contaminated soil according to the present invention can suppress elution of arsenic in both trivalent and pentavalent forms in soil contaminated with arsenic, and can be used to repair and reuse contaminated soil. Useful.

It is a graph which shows the relationship between the adjusted pH of a treated soil, and the arsenic elution density | concentration in a soil elution test by adding a cerium salt solution and slaked lime for pH adjustment to the arsenic contaminated soil shown in Example 4.

Claims (4)

A method for suppressing arsenic elution from arsenic-contaminated soil, comprising adding a rare earth salt solution containing cerium as a main component and slaked lime to soil contaminated with arsenic. A rare earth salt solution containing cerium as a main component is used as an aqueous solution of 0.1 to 3.0% by mass in terms of oxidized rare earth, and slaked lime is used as an aqueous dispersion of 0.1 to 2.0% by mass. The method for suppressing arsenic elution from arsenic-contaminated soil according to claim 1. The amount of slaked lime is adjusted so that the soil pH after adding a rare earth salt solution containing cerium as a main component and slaked lime is 6 to 9, and the arsenic-contaminated soil according to claim 1 or 2, Elution suppression method. 4. The method for suppressing arsenic elution from arsenic-contaminated soil according to claim 1, wherein slaked lime is added after the rare earth salt solution containing cerium as a main component is added or simultaneously with the rare earth salt solution.

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