JP2007098299A - Method for cleaning heavy metal contaminated soil using chelating agent as cleaning agent - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for inexpensively and efficiently cleaning heavy metal contaminated soil which maintains a cleaning capacity and realizes a high circulating efficiency by re-utilizing a recovered chelating agent for cleaning contaminated soil through highly effectively recovering the chelating agent by acid precipitation directly using hydrochloric acid without performing removal of heavy metals after the contaminated soil is cleaned by using the aqueous solution of the chelating agent and the heavy metals are extracted as complexes. <P>SOLUTION: The method for cleaning heavy metal contaminated soil comprises the following processes: (A) a process of obtaining an extracting solution containing the heavy metals as the complexes by cleaning the contaminated soil with the heavy metals by the chelating agent; (B) a process of recovering the chelating agent by solid-liquid separation and washing by water after precipitating the chelating agent as the complexes formed by acid precipitation using hydrochloric acid of 5-20 wt.% from the extracting solution containing the heavy metals as the complexes; and (C) a process of preparing a chelating agent aqueous solution for cleaning soil by using the recovered chelating agent. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an improved method for purifying soil contaminated with heavy metals using a chelating agent.

In recent years, soil contamination due to heavy metals such as lead and cadmium has been found in various places mainly in factory sites, etc. Also, with the establishment and construction of the soil contamination countermeasure law, research on purification methods for heavy metal contaminated soil has been actively conducted. Can be advanced.
Since heavy metals cannot be decomposed like organic matter, in order to treat heavy metal contaminated soil, excavate the contaminated soil and carry it to the disposal site, generate insoluble heavy metal salts and insolubilize it, cement in the contaminated soil, etc. Methods such as adding a solidifying agent to solidify, or applying electricity to the soil to generate heat and solidifying into a glassy state are taken. However, all of these methods leave heavy metals in the soil, and depend on reliability in terms of long-term stability.

  On the other hand, a method of removing heavy metals from contaminated soil using a chelating agent has been proposed (for example, see Non-Patent Document 1), and various methods such as ethylenediaminetetraacetate (EDTA) and nitrilotriacetate (NTA). Chelating agents have been investigated. However, although the method using a chelating agent is highly effective, since a solution containing 5 to 20% by weight of a chelating agent such as EDTA is used for washing, the consumption of the chelating agent is large and the processing cost is high. there were. In addition, there is a problem with the treatment of waste liquid containing complexes generated by washing.

  In general, as a method of removing heavy metals from a complex solution formed with a chelating agent, the pH of the solution is adjusted, and the heavy metal is adsorbed on the ion effect resin by passing through the ion exchange resin. There is a technique for obtaining an aqueous solution (see, for example, Patent Document 1). In this method, it is necessary to adjust the temperature at the time of treatment, and instead of removing heavy metals by ion exchange, the concentration of metal ions such as sodium or calcium increases, the pH increases significantly, or the H-type ion effect This is significantly reduced by using resin. Since the extraction ability of heavy metals by the chelating agent varies greatly depending on the pH, it is necessary to adjust the pH again after the ion exchange treatment when the chelating agent is reused. Further, the ion exchange resin needs to be periodically replaced or regenerated, and wastewater treatment at the time of regeneration is necessary.

  There is a method in which calcium or sulfide is added under alkaline conditions to remove heavy metals as a precipitate, and then EDTA is precipitated and recovered using a mineral acid such as hydrochloric acid or sulfuric acid. When precipitating heavy metals, it is necessary to adjust the pH to a strongly acidic condition again, and it is difficult to completely precipitate heavy metals if the chelating agent is present at a high concentration. Even when heavy metals are extracted using the prepared EDTA, it is necessary to adjust the pH (see, for example, Patent Document 2).

In addition, a method has also been proposed in which an aqueous EDTA solution containing heavy metals is adjusted to a pH of 9 or more, then heavy metals are precipitated using an insolubilizing agent such as sodium sulfide, and the resulting separated liquid is reused for washing ( For example, see Patent Document 3.) However, in this method, unless the pH is maintained at a predetermined pH before the heavy metal precipitate is formed or after the heavy metal precipitate is removed, a precipitate is not formed and the precipitate is very fine and cannot be completely collected. Furthermore, the separated liquid has a high pH, and when it is used as it is, the desired heavy metal removal rate cannot be achieved, and the cleaning ability is lowered. As a result, pH adjustment is necessary. There is a problem that it is difficult to maintain.
When recovering the chelating agent, if heavy metals are removed as precipitates in advance, the liquid content of the separated precipitate tends to be high, and EDTA contained in the liquid containing the precipitate can be recovered. There was also a problem that it was difficult to achieve a high recovery rate.

Robert W. Peters, Journal of Hazardous Materials, 66, 151-210 (1999) Japanese Unexamined Patent Publication No. 1-111827 Japanese Patent Laid-Open No. 51-10176 JP-A-4-263874

  The purpose of the present invention is to advance the current state of such technology, wash contaminated soil with an aqueous solution of a chelating agent, extract heavy metals as a complex, and then use hydrochloric acid directly without removing heavy metals. By recovering the chelating agent at a high recovery rate by acid precipitation, the recovered chelating agent is reused for the washing operation of contaminated soil, maintaining high washing capacity and at the same time achieving high circulation efficiency and low cost. It is often to provide a method for cleaning heavy metal contaminated soil.

In order to achieve the above object, the present inventors have conducted intensive studies. As a result, the cleaning method for heavy metal-contaminated soil of the present invention comprises the following steps.
A) Washing soil contaminated with heavy metals with a chelating agent aqueous solution to obtain an extract containing heavy metal as a complex B) Acidifying operation using 5 to 20% by weight hydrochloric acid from the extract containing heavy metal as a complex Step of precipitating the chelating agent that has formed a complex by the following steps: solid-liquid separation, recovering the chelating agent by washing with water, and C) preparing the aqueous chelating agent solution for soil washing using the recovered chelating agent At the same time as adjusting the concentration of hydrochloric acid by the operation, by aging the precipitate by setting the time from the start of the precipitation to the start of separation to 0.5 hours or more, by reducing the liquid content of the solid-liquid separation, The metal concentration remaining in the recovered chelating agent can be lowered, and the recovery rate of the chelating agent can be maintained high. By collecting the chelating agent itself, it can be adjusted to a desired concentration as a soil cleaning agent, and the cleaning efficiency of heavy metal-contaminated soil can be maintained high.

  According to the present invention, a contaminated soil is washed with an aqueous solution of a chelating agent, a heavy metal is extracted as a complex, and then a chelating agent containing a small amount of metal is obtained by acid precipitation directly using hydrochloric acid without performing an operation of removing the heavy metal. Recovering at a high recovery rate and reusing the recovered chelating agent for cleaning operations of contaminated soil has enabled the maintenance of high cleaning capacity and at the same time achieving high circulation efficiency.

  Examples of the heavy metal-contaminated soil to be purified in the present invention include urban areas, mountain forests, industrial sites, agricultural land, marshes, and soil containing heavy metal element units, compounds, or ions such as waste soil. The heavy metal elements that can be purified with the aqueous chelating agent solution of the present invention are not particularly limited, such as lead, cadmium, mercury, arsenic, and selenium, but are extremely effective for lead, mercury, and cadmium.

In the aqueous chelating agent solution used in the present invention, the chelating agent used as the cleaning agent component is a cleaning agent selected from aminocarboxylic acid-based chelating agents and forms a complex with heavy metals, such as glutamic acid diacetic acid 4 soda, aspartic acid 2 Examples include acetic acid 4 soda (ASDA), methylglycine diacetate 3 soda (MGDA), N, N-ethylenediamine succinic acid (EDDS4H), N, N-ethylenediamine succinic acid 3 soda (EDDS3Na), and ethylenediamine tetraacetate (EDTA). It is done.
The chelating agent concentration in these chelating agent aqueous solutions is 0.01 to 1M, particularly 0.01 to 0.5M, in order to reduce drug costs, reduce the frequency of washing heavy metal contaminated soil, and reduce the load of washing waste liquid treatment. However, it is preferable to adjust according to the heavy metal concentration in the soil.

  The pH of these chelating agent aqueous solutions is preferably 4-6. Many of the aminocarboxylic acid-based chelating agents have a large number of acid-type functional groups for complex formation, but the pH increases when these functional groups are bonded to an alkali. When there are many bonds with alkalis and the pH is high, the exchange of heavy metals and alkaline components in the soil is slow, resulting in a decrease in the extraction rate of heavy metals. Further, when there is no bond with alkali, most of the chelating agents have extremely low solubility in water, the chelating agent concentration cannot be increased, and the cleaning efficiency is lowered. When heavy metal contaminated soil is washed in-situ, it is necessary to consider the effect on the underground structure. At low pH, there is a concern about the effect on the underground structure. When washing heavy metal contaminated soil When the pH of the washing solution is high, it is generally known that heavy metals become insoluble in the high pH range, and ionic iron and aluminum components contained in the soil are insoluble. When the state is reached, heavy metal components such as arsenic, cadmium, selenium, and mercury are co-precipitated and the purification efficiency is lowered. Furthermore, when the pH is used at 4 to 6, there is little pH change due to the buffer effect, and stable purification ability can be maintained. In order to ensure the buffering effect, a known buffer solution may be used. In addition, pH adjustment of the aqueous solution of the chelating agent used by this invention can be adjusted with general purpose alkalis, such as caustic soda.

  The type of washing heavy metal contaminated soil using such an aqueous solution of a chelating agent is not particularly limited, and is a method of washing with chemical water in situ, a method of excavating and washing the soil, and then refilling the purified soil Any of these may be used.

In addition, in the present invention, washing with an aqueous solution is not only mixing the soil and the aqueous solution directly, but also adding a cleaning agent and water separately to the soil and mixing and washing, and washing the soil containing water by mixing the cleaning agent The method of doing is also included.
By treating in this way, heavy metals in the soil are eluted in the aqueous solution. Cleaning with an aqueous solution varies depending on the cleaning method described above, but is performed at least several times when excavated and cleaned. By performing such a washing operation, a heavy metal and a chelating agent form a complex, and an aqueous solution (extract) containing the complex is obtained.

  In the aqueous solution (extract) separated from the soil after washing, heavy metals extracted together with the chelating agent are present. In the case of excavation washing, solid materials such as commonly used centrifuges and filtration devices are used. The soil and the extract are separated using a liquid separator. When soil purification is performed in the original position by passing chemicals, the extract is drawn from the pumping outlet. An acid is added to the extract containing the heavy metal, and only the chelating agent component is precipitated by an acid precipitation operation. In this case, in order to increase the recovery rate, the pH is preferably set to 2 or less. In order to maintain a high recovery rate, the pH is more preferably 1 or less.

  A mineral acid such as sulfuric acid or nitric acid can be used as the acid used in the acid precipitation operation. However, it is necessary to use hydrochloric acid in terms of maintaining a high recovery rate and preventing heavy metals from remaining in the recovered chelating agent. Although the chelating agent dissolves in water even at a very low concentration but at a low pH, it is important to suppress an increase in the liquid volume in order to achieve a high recovery rate, but 35 weight percent aqueous hydrochloric acid ( The reason for using concentrated hydrochloric acid directly is unknown, but the recovery rate decreases. Further, the metal remaining in the chelating agent component recovered at the same time tends to increase. In order to suppress the recovery rate and residual metal components in the recovered chelating agent, the concentration of hydrochloric acid used needs to be 5 to 20 weight percent. The acid precipitation method is not particularly limited, and any of a batch method, a continuous method, and a semi-continuous method can be applied. The longer the time from the start of the addition of hydrochloric acid to the start of solid-liquid separation, the precipitated chelating agent crystals are sufficiently aged, so that solid-liquid separation becomes easier and the liquid content of the separated chelating agent can be reduced. Although there is a tendency to reduce the amount of residual metal, the time required from the start of hydrochloric acid addition to the start of solid-liquid separation is sufficient if it is 0.5 hours or longer.

  For solid-liquid separation, commonly used centrifuges, filtration devices and the like can be used, and although not particularly limited, those having a water washing function are preferred. Only EDTA can be recovered as a solid from the complex of EDTA and heavy metal precipitated as a solid by this water washing operation, and heavy metal can be obtained as a high-concentration aqueous solution. When there is no water washing function, the separated chelating agent precipitate is washed with water and dehydrated again. There is no limitation on the amount of water used for water washing, but the recovery rate decreases and the residual metal concentration tends to decrease as the amount of water used increases. Generally, the metal concentration remaining in the recovered chelating agent can be reduced to 100 ppm or less by washing with 1 to 5 times the weight of the recovered chelating agent. In addition, when washing heavy metal contaminated soil, a method of use such as increasing the amount of water used and reducing the residual metal concentration with the progress of the washing operation can be applied. When the chelating agent is recovered by the above method, the recovery rate slightly changes depending on the chelating agent concentration, hydrochloric acid concentration, pH, and amount of washing water, but it is possible to stably achieve a recovery rate of 95% or more.

  The recovered chelating agent can be used in the step of preparing a cleaning agent for cleaning heavy metal contaminated soil again without drying. By separating and recovering the chelating agent, it can be adjusted to a desired concentration and pH, and caustic soda, which is a general-purpose alkali, can be used for pH adjustment. When the chelating agent is recovered by the above method, even when an aqueous solution for soil washing using only the recovered chelating agent is prepared and used, the cleaning ability is about the same as that of a new article, and there is almost no decrease in heavy metal removal ability. Along with soil washing, the chelating agent itself is generally lost in the range of 5 to 20% due to sorption and decomposition to the soil. In some cases, it is necessary to compensate for the loss, but it is recovered in the above preparation step. It can be prepared without problems by blending a new product with the product.

  The waste liquid containing the separated heavy metal can be treated by a general heavy metal wastewater treatment method by removing the chelating agent in advance, and in particular, by using a Ca component such as calcium hydroxide, it becomes alkaline. The heavy metal is precipitated by the precipitation method, and the water separated by solid-liquid separation can be directly discharged as sewage.

  As described above, according to the present invention, the amount of chelating agent required for the purification of heavy metal-contaminated soil can be greatly reduced, and wastewater treatment is facilitated, and the heavy metal-contaminated soil is reduced in treatment costs and environmental impacts. Can be cleaned.

EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention does not receive any limitation by this. Heavy metal quantitative analysis was carried out with an ICP analyzer. The extraction rate was calculated by the following formula.
Extraction rate (%) = [Metal concentration in the extract] × [Amount of extract] / [Content] × 100
Content = [Contaminated soil content test result] × [Amount of soil used]

[Example 1]
As a result of testing based on the content test of Ministry of the Environment Notification No. 46, 2 parts by weight of 2% EDTA aqueous solution adjusted to pH 4 was mixed with contaminated soil with a lead content of 1200 mg / kg and an arsenic content of 600 mg / kg. Then, an EDTA-heavy metal complex was formed. Then, as a result of confirming the metal concentration of the extract obtained by filtration separation according to a conventional method, the lead extraction rate was 90% and the arsenic extraction rate was 20%. To this extract, 10 weight percent aqueous hydrochloric acid was added until the pH reached 1, and after stirring for 0.5 hour, the precipitate was filtered, washed with water corresponding to about 2 parts by weight of the precipitate, and dehydrated again. As a result, the recovery rate of EDTA was 95%, and the metal contained in the recovered EDTA was 90 mg / kg (90 ppm). Further, the recovered EDTA was adjusted to a pH of 4 with caustic soda and again into a 2% by weight aqueous solution, and mixed with 2 parts by weight with respect to the same contaminated soil as described above. %, And the extraction rate of arsenic was 21%.

[Example 2]
The same contaminated soil as in Example 1 was filled in the column-type apparatus, and 2 wt% EDTA aqueous solution having a pH adjusted to 4 was passed from the lower part, and the contaminated soil was washed by discharging from the upper part of the column. About 10 parts by weight of an EDTA aqueous solution was passed through the soil packed in the column, and then a heavy metal content evaluation test was performed on the contaminated soil inside the column. As a result, the lead content was 100 mg / kg. To the discharged EDTA aqueous solution, 10% by weight of hydrochloric acid is added until the pH becomes 1, and after stirring for 0.5 hour, the precipitate is filtered and washed with water corresponding to about twice the weight of the precipitate. As a result of dehydration again, the recovery rate of EDTA was 96% by weight, and the metal contained in the recovered EDTA was 75 mg / kg. Further, the recovered EDTA was adjusted again to a 2 wt% aqueous solution while adjusting the pH to 4 with caustic soda, and the contaminated soil was washed while passing water from the bottom of the column filled with the contaminated soil in the same manner as described above. About 10 parts by weight of EDTA aqueous solution was passed through the soil packed in the column, and then a content test was performed on the contaminated soil inside the column. As a result, the lead content was 110 mg / kg.

[Comparative Example 1]
The same procedure as in Example 1 was performed, and precipitation of EDTA was performed with concentrated hydrochloric acid (35% by weight). As a result, the recovery rate of EDTA was 85%, and the metal concentration in the recovered EDTA was 150 mg / kg.

[Comparative Example 2]
The same procedure as in Example 1 was performed, and the precipitation of EDTA was completed in 10 minutes after adding 10% by weight of hydrochloric acid, and solid-liquid separation was performed. As a result, the recovery rate of EDTA was 83% and the recovered EDTA The metal concentration of was 180 mg / kg.

  According to the present invention, a contaminated soil is washed with an aqueous solution of a chelating agent, a heavy metal is extracted as a complex, and then a chelating agent containing a small amount of metal is obtained by acid precipitation directly using hydrochloric acid without performing an operation of removing the heavy metal. Recovering at a high recovery rate and reusing the recovered chelating agent for cleaning operations of contaminated soil has enabled the maintenance of high cleaning capacity and at the same time achieving high circulation efficiency. In addition, wastewater treatment that remains by recovering the chelating agent at a high recovery rate is facilitated, and heavy metal-contaminated soil can be washed with reduced processing costs and environmental impact.

Claims (7)

A method for cleaning heavy metal contaminated soil using a chelating agent as a cleaning agent, comprising the following steps.
A) Washing soil contaminated with heavy metals with a chelating agent aqueous solution to obtain an extract containing heavy metal as a complex B) Acidifying operation using 5 to 20% by weight hydrochloric acid from the extract containing heavy metal as a complex The step of precipitating the chelating agent that has formed a complex by the step of solid-liquid separation, recovering the chelating agent by water washing operation, and C) preparing the aqueous chelating agent solution for soil washing using the recovered chelating agent   The cleaning method according to claim 1, wherein the chelating agent is an aminocarboxylic acid chelating agent.   The cleaning method according to claim 2, wherein the chelating agent is ethylenediaminetetraacetate.   The pH of the chelating agent aqueous solution used for washing | cleaning is 4-6, The washing | cleaning method of any one of Claims 1-3 characterized by the above-mentioned.   The cleaning method according to any one of claims 1 to 4, wherein the pH in the acid precipitation operation is 2 or less.   The cleaning method according to any one of claims 1 to 5, wherein the time required from the addition of hydrochloric acid to the start of solid-liquid separation in an acid precipitation operation is 0.5 hours or more.   The cleaning method according to claim 1, wherein the total metal concentration contained in the recovered chelating agent is 100 ppm or less.