2 Action to Remediation of Soil contamination(1)?Heavy metals


1. Features of contamination
The Soil Contamination Countermeasures Law specifies ten items of inorganic matter, for instance heavy metals as Second Class Specific Toxic Substance. Among these particular substances, cadmium, lead, chromium hexavalent, arsenic, mercury (including alkyl mercury), and selenium are examples of heavy metals. Fluoride, boron, and cyanogen are also considered “heavy metals” and are classified under the same group since the investigation technique applies to the remediation of soil polluted with such metals.
When taking into account the contamination due to heavy metals, the migration of contaminants is a slow process as compared with volatile organic compounds. The heavy metals exist near the surface of the soil because they are easily adsorbed into the soil, as well. Moreover, in the case of a highly concentrated contamination, especially with relatively soluble substances, such as chromium hexavalent and cyanogen, the occurrence of rainfall is likely to expand the contamination into groundwater.

2. Technology for the measures against soil contamination due to heavy metals
The measures against soil contaminated with heavy metals are classified into two general processes. 1) Purification is the process of separating polluted substances from the contaminated soil and allowing subsequent decomposition of the heavy metals. 2) Containment is a manner of isolating the contaminated soil from the environment and preventing the spread of the contamination from diffusion into nearby soil. (Table) In this paper, the soil purification process, which is classified as a purification measure, the insolubilization treatment, which is often performed prior to containment, and the cyanogen bioremediation, which has been studied recently, are outlined.

Table Classification of Measure Methods for Soil Contamination by Heavy
Table Classification of Measure Methods for Soil Contamination by Heavy

2. 1 Soil washing
The soil washing method separates the contaminants absorbed in the soil particles or soil pore by effecting the liquid phase of the soil using water or agents. The method is divided into two processes. 1) Soil flushing collects the pollutants by letting rinse water circulate in the ground. 2) Plant washing cleanses the excavated pollutants at the Washing Plant. The basic flow chart of plant washing is shown in Fig. 1. The excavated contaminated soil is mixed with rinse water and then placed in the washing equipment. After washing, the soil is separated into fine soil and coarse-grained soil, which contains the contaminants. The washed soil is utilized as in-site backfill soil. Meanwhile, the turbid water is dehydrated at the turbid water treatment plant. The effluent is recycled as washing water. The dehydrated sewage is then disposed as waste off-site. The advantages of soil washing are the following. 1) The treatment of polluted soil takes comparatively a short time to complete. 2) The purification cost can be reduced because volume reduction of soil becomes possible. However, a difference in the treatment outcome may appear depending on the kind and form of contaminants, property of objective soil, or gradation distribution.

Fig. 1 Flow chart of Soil Washing
Fig. 1 Flow chart of Soil Washing

2. 2 Insolubilization treatment
Insolubilization treatment method chemically changes the contaminants into hardly-soluble substances by mixing with agents, and consequently stabilizes them. There are three different types of reaction agents for insolubilization as follows:
 1) Sulfuration treatment agent forms a sulfide.
 2) Reduction treatment agent creates insoluble salt by using a reduction reaction.
 3) Treatment agent, which reacts with iron ion, develops insoluble matter, such as chlorination of iron.
Recently, insolubilization of fluorine or boron has also been studied and applied to hardly-soluble substances. The insolubilization of fluorine causes immobilization of fluoride ion by using magnesia oxide fixation agents or Hydrotalcite-like adsorbent. To date, there are applied cases in the field concerning groundwater contamination. As regards the application to fluorine polluted soil, an insolubilization effect can be obtained with 5% combination ratio according to the verification test, which employed real polluted soil. Future utilization of the mentioned technique is expected.

2. 3 Bioremediation of the soil contaminated by cyanogen
Cyanogen compounds are generally degraded by microbes. Therefore, application of certain microorganisms to polluted soil can be beneficial. This method incorporates nutrition salt and produces aeration into the excavated soil. The soil is agitated to allow biological decomposition to advance. In case of in-situ polluted soil, some microbes are capable of activating native cyanogen-decomposing bacillus by supplying oxygen with air parsing and adding nutrition salt, and afterward promoting decomposition of cyanogen. (Fig. 2)

Fig. 2 Bioremediation of cyanide contamination
Fig. 2 Bioremediation of cyanide contamination

3. Future purification of contaminated soil due to heavy metals
In Japan, the presence of soil contamination due to heavy metals is often revealed when conducting assessment of real estate or redevelopment of land areas. Therefore, in most cases, prompt measures are required. Consequently, what usually happens is that contaminated soil is excavated and at a different place without rendering the appropriate measure to treat and prevent further spread of the contamination. Disposal at a different place is a reliable action, which requires only a short time. However, this only means the replacement of the contaminated soil and is not a fundamental solution to the problem. Hereafter, it is important to select the methods of purification and containment of the pollution according to the extent of application or the degree of adverse effects of the method based on the assessment of the risk of the contaminated land.
The eradication of soil contamination due to heavy metals will likely be a big contribution to the advancement in the field of purification technology for soil contamination.

1 Remediation technologies applied to contaminated sites in Japan
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3 Action to Remediation of Soil contamination(2) - Volatile organic compound