Selecting correct
soil remediation method is important to prevent further environmental damage.
In Japan, unpolluted soil dressing/ soil replacement is used for soil
remediation but this practice is costly and there is difficulty obtaining
uncontaminated soil. The science journal article written by Makino has outlined
a series of soil remediation methods and did a cost-method analysis on its
applicability.
(i) Water
Management
Flooding paddy fields as late
as possible till harvest time is recommended as the amount of bioavailable
cadmium in paddy soils will be reduced. When the paddy field is flooded,
sulfate ion in soils is reduced to sulfide ion, which can then react with
cadmium to form cadmium sulfide. As cadmium sulfide is precipitated out of the
soil, cadmium concentration in the soil is lowered and there is lesser uptake
of cadmium by rice plants. Though this method is effective, it is not
applicable, as the flooding would impede operation of machineries for harvest.
(ii) Soil
Dressing
This is the most widely
adopted method in Japan as it leaves sites in a relatively pristine condition.
Polluted soils is removed, depth to be removed is based on degree of soil
pollution and plant root elongation. Thereafter, the subsurface soil is
replaced with unpolluted soils. Though this method is very effective, it is
getting costly with increased scarcity of uncontaminated soils.
I. (iii) Soil
Washing
Metal
chelating agents, natural salts and strong acid is used to effectively wash away
cadmium in soil. One common chemical is EDTA (ehtylenediamine tetraacetic acid)
but this chemical is persistent and stays in the environment for a long time.
Also, wastewater drained in the process can pollute surrounding areas such as
agricultural canal, neighboring agricultural field and groundwater. In
addition, usage of strong acids can result in soil acidification. Thus despite
its high efficiency, soil washing can bring more harm to the environment
instead.
In
view of this, the author proposed using ferric chloride instead. Ferric
chloride was selected as iron is a major soil constituent and thus less
environmentally damaging. Furthermore, the chemical is less expensive and thus
economically viable. The proposed soil washing procedure is to first conduct a
chemical wash using ferric chloride solution, then a water wash to eliminate
remaining chemicals, and finally treat the wastewater on-site before water is
discharged.
Image
1: Proposed Diagram for Chemical Washing using Ferric Chloride Solution (Source:
Makino, 2010)
An
on-site experiment was conducted and it was documented that the pH of soil
decreased sharply after the initial chemical wash with ferric chloride. Under
such acidic condition, cadmium is soluble and highly mobile, resulting in 55%
of cadmium to be effectively removed. To prevent soil acidification, lime is
added to restore the initial pH of the soil. Wastewater from chemical washing
was also treated to below Japanese environmental quality standard, with only
0.01 mg of Cd per litre in the water. Since all byproducts from chemical
washing using ferric chloride has been treated to meet environmental standards,
this method is highly applicable.
I. (iv) Phytoremediation
Phytoextraction
using plants (e.g. okra, tall goldenrod, indian mustard, kenaf and sugar beet) is a good alternative too. Rice plants are considered to be one of
the most effective in removing cadmium in soils. This method requires two to
three cycle of phytoextraction before the soil is fully remedied. Besides
phytoextraction, phytostabalization, rhizofiltration and phytovolatilization
can be adopted too (refer to Table 1 below).
Table
1: Types of Phytoremediation methods for soil remediation (Source: Suthersan,
2002)
In
conclusion, Makino recommend the use of chemical washing using ferric chloride to
remove cadmium in soils. Balancing the cost and efficiency of cadmium removal
in soils is usually tough and even if both are met, time is another factor to
be considered. This is seen in phytoremediation method as at least two rounds
of harvesting is needed before the paddy field is remedied. In my next post, I will
be exploring the use of a specific cadmium hyper-accumulator plant to enhance the
phytoremediation process. This will be done with reference to another
scientific journal.
References
Makino,
T. (2007). Heavy metal pollution of soil and a new approach to its
remediation: research experiences in Japan. Food and Fertilizer Technology
Center.
MakinoA,
T., KamiyaB, T., SekiyaC, N., MaejimaA, Y., AkahaneA, I., & TakanoB, H.
(2010, August). Chemical remediation of cadmium-contaminated paddy soils by
washing with ferric chloride: Cd extraction mechanism and on-site verification.
In Proceedings of the 19th World Congress of Soil Science: Soil solutions
for a changing world, Brisbane, Australia, 1-6 August 2010. Symposium 3.5. 1
Heavy metal contaminated soils (pp. 35-38). International Union of Soil
Sciences (IUSS), c/o Institut für Bodenforschung, Universität für Bodenkultur.
Suthersan,
S.S., 2002.Chapter 5: Phytoremiediateion, In: Natural and enhanced remediation
systems. CRC Press Inc., Florida, pp. 239-267.
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