Heavy metal remediation: Defusing the ticking chemical bomb

Conflict” Picture made by San Jose

Think about these questions:

1. Do you understand what is meant by heavy metal pollution?
2. Do you know which heavy metals cause pollution?
3. Do you know which methods can control and reduce heavy metal pollution?
4. Have you ever had a heavy metal pollution incident where you live?
5. Do you think it is worthwhile to degrade the environment to develop the economy?

OK, you know for sure that we are not talking about heavy metal music! If you cannot answer the first three questions, or haven’t experienced the situation described in question 4, or not quite sure about the answer to question 5, please continue reading.

Exhaust emissions in Guangzhou, China
Photographed by Mitchell Masilun

Heavy metal contamination refers to environmental pollution caused by heavy metals or their compounds. Heavy metal contamination is primarily caused by human factors, such as mining, exhaust emissions, sewage irrigation and the use of heavy metal over-standard products.

Most heavy metal contamination is with cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), lead (Pb), zinc (Zn), arsenic (As), boron(B), and the platinum group metals. Unlike many organic pollutants, which eventually degrade to carbon dioxide and water, heavy metals are non-destructible. Heavy metals, therefore, accumulate in the environment, especially in lakes, estuaries or marine sediments, and soils. Metals can be transported from one environmental compartment to another. This is why heavy metal pollution is such a concern.

Kokkinopezoula lake polluted by mining in Cyprus
Photographed by Dimitris Vetsikas

Most people would expect that heavy metal pollution is quite far from them, but in truth, it surrounds all of us. The first pollution of heavy metal was discovered in Minamata City in Kumamoto prefecture, Japan in 1956. It was caused by the release of methyl-mercury in the industrial wastewater from the Chisso Corporation’s chemical factory, from 1932 to 1968. As of March 2001, 2265 victims had been officially recognised as having Minamata disease (1,784 of whom had died). According to the data from the China Food Safety High-Level Forum report, 1/6 of China’s cultivated land is polluted by heavy metals, and the area of heavy metal contaminated soil is at least 20 million hectares. The Chinese Ministry of Land and Resources has also publicly stated that 12 million tons of grain are polluted by heavy metals every year in China, and direct economic losses exceed 20 billion yuan (2009). Industrial and commercial activities, as well as treatment and disposal of wastes, account for the main sources of heavy metals. The world’s heavily affected areas from this pollution have been proven as health risks to more than 10 million people in various countries. Although its toxicity is well-documented, managing its exposure and related risks is a challenge around the world as explained by the World Health Organisation in 2011.

Aerial view of the site contaminated with petroleum hydrocarbons at the beginning of the phytoremediation trial (Trecate, Italy) Photographed by Plata Chebbah

Heavy metal remediation methods are divided into three approaches: physical, chemical and biological. Among green technologies addressing at containing metal pollution, phytoremediation has received increasing attention in recent years as an alternative to physical and chemical methods of decontamination. It is a method for absorbing toxic and harmful substances in contaminated soil by its roots and transporting it to the upper part of the plant and taking away the pollutants in the soil by harvesting the aboveground material. As biological technology, it requires extensive periods of time to remove the metal fraction.

Professor Nicholas Dickinson from Lincoln University and three other scientists from Italy have produced a 10-year study of metal-polluted soils. Two experiments were established in pot trials with agricultural soil and pyrite waste. Brassicaceae species were cultivated in rhizoboxes with the polluted soil, focusing on root growth and metal accumulation at an early stage. Another pot trial used pyrite cinders with an application of humic acids, indolebutyric acid (IBA) and ethylene diamine disuccinic acid (EDDS), in order to improve plant growth and metal uptake. Different agronomic practices (such as ploughing vs. ripping) and fertiliser application in soil management were also tested. In total, 18 species were used as test species in these trials.

The results
showed that root length was positively correlated with shoot metal concentrations, highlighting the importance of promoting root growth through agronomic management. A second chelator test (Humic acids, IBA and EDDS), did successfully improve Cu, Co (Cobalt), Zn and Pb concentrations in the above-ground biomass of both species, but reduced plant biomass. In terms of agronomic practices, roots growth improved with ripping compared with ploughing, as roots can extract more heavy metals. They found after fertiliser application that the phytoremediation potential increased with respect to the soil tillage experiment. For phytoremediation of poor fertile wastes, sufficient supply of nutrients is important.

The beauty of nature
Photographed by Sathish Kumar Periyasamy

Phytoremediation efficiency with crop species may be improved through increased productivity by suitable soil management, involving mineral and organic fertilisers, contaminant dilution, soil capping, and metal immobilization with inorganics and biochar. This provides a lot of scientific references for people to use phytoremediation of heavy metals in the future. Phytoremediation is an Eco-friendly way to reduce the impacts of heavy metals.

Well, I hope you can at least answer one or two of the questions above now. For additional details, please visit the link to the research paper below:
Advances in agronomic management of phytoremediation: methods and results from a 10-year study of metal-polluted soils

Authors: Teofilo Vamerali, Luca Marchiol, Marianna Bandiera, Guido Fellet, Nicholas M. Dickinson, Paola Lucchini, Giuliano Mosca, Giuseppe Zerbi

Wei Zhang did this article as part of his ECOL 608 Research Methods in Ecology course as part of his postgraduate Certificate of Proficiency at Lincoln University.

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