Scientific Journal Review: Enhancing Phytoremediation of cadmium-contaminated agricultural soils using Solanum nigrum L

In this post, I will discuss the use of a cadmium-hyperaccumulator plant to remove cadmium in soils. I have found a scientific journal article (see Reference) by (Ji et. al., 2010) discussing this and it would be interesting to share with you readers.

The research article recommends the use of Solanum nigrum L., or black nightshade (refer to Image 1 below), for phytoremediation of cadmium-contaminated soil. The plant was selected as it has a relatively higher biomass as compared to other hyperaccumulators, such as T. caerulescens and A. halleri. Assessing cadmium uptake of nightshade is applicable for Japan soil countermeasures as the plant grows in Japan too. 

Image 1: Solanum nigrum L (Source: Lee, n.d.)

An experiment using black nightshade was conducted in China at Shenyang Zhangshi Irrigation Area. The farmland at the area has been contaminated by cadmium, with 10 mg Cd/ kg in surface soil. Various planting and harvesting methods was tested to see which is most effective method to enhance phytoremediation.

Results showed that plant biomass was the most critical determinant of cadmium accumulation. The larger the plant roots, the greater its biomass. Planting density will have no effect as the plant biomass depends on its genetic predisposition and the contamination level in soils. One way to increase the biomass is then to conduct double harvesting (harvesting twice per year). This can only be achieved when the plant is cut correctly. Mild cutting, cutting the plant at 50cm above the soil during harvest, was found to achieve the highest biomass. It is advised that harvest is done during the flowering period as cadmium accumulates most in the stems and leaves. Understanding the cutting position and period to harvest the plant is thus beneficial in enhancing the phytoremediation efficiency.

Another conclusive finding is that fertilization is unnecessary, as it did not increase the plant biomass. In fact, phosphate fertilizers reduce the solubility and mobility of cadmium due to formation of metal-phosphates. Cadmium accumulation in black nightshade plant is thus inhibited.

From the article, we understand more about phytoremediation and its relevance to farming. This reminds me of my classmate, Heng Sulli, blog about farming techniques. Maybe this would be a good topic for her to write about, on the impact of hyperaccumulators in heavy metals uptake in plants. When correct farming techniques is adopted, phytoremediation will be enhanced and polluted soils can recover faster. In my opinion, I would urge the Japan government to pump in more funding for such research and the experiment results shared with small-scale farmers. Involvement of small-scale farmers is important in my opinion as they are major stakeholders and regulators of the environment. If farmers are able to preserve the health of their soils, heavy metal concentration in food crops will be reduced and heavy metal poisoning in humans can be prevented. 

References

Ji, P., Sun, T., Song, Y., Ackland, M. L., & Liu, Y. (2011). Strategies for enhancing the phytoremediation of cadmium-contaminated agricultural soils by Solanum nigrum L. Environmental pollution, 159(3), 762-768.

Lee Yu S. (n.d.). Photo taken from http://www.agroatlas.ru/en/content/weeds/Solanum_nigrum/. Web. 

Going Ahead (Part II): Itai-itai Disease Remediation Efforts [from scientific journal articles]

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.