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Toxic Grounds

Imagine standing in the same place for your entire life. Your food is no more than an arms reach away, and you don’t get to be picky on what it looks or tastes like, just aiming for edible. To keep things simple, different species of plants have longer or shorter "arms" (roots), and some species will grab anything in their vicinity, not limiting themselves to just nutrients, but also heavy metals, chemical toxins, and pollutants. Of course it depends on the species, however there is reason behind the caution to picking wild vegetables and herbs. If the ground has a history of toxic materials, you may be picking up those toxins with your plants.



Researchers however have found a positive use of this property of plants for a field called phytoremediation. Plants are used to remove dangerous compounds out of soil and water, cleaning it, and making the land fit for the generations ahead of us. In this article I would like to highlight three different types of phytoremediation that have allowed scientists to clean our environment for a better, healthier, tomorrow.


Wastewater Management


Waste water is categorized into two types: grey and black water. Grey water originates from sinks, tubs, and washers. It is constantly being cycled and reused as it shouldn’t carry any harmful materials. Black water, however, is not reused as it is toilet water. To remediate the water requires a collaborative effort of various field of science including soil scientists, engineers, ecologists, and botanists. Because of this, these projects are very sensitive but effective.


One of the most biologically active systems are wetlands, having been used for a number of things such as cleaning out compounds like trichloroethylene, oxygenating the water, and increasing bioactivity. Microwetlands serve as an excellent method of treating household wastes. Floating wetlands are a larger scale system for treating bigger jobs, one has been installed in Belgium and is currently operating as a safe substitute for treating waste water.


Plant use for remediation of water requires the alignment of many different variables and collaboration but it is showing itself to be a worthy alternative to other means of remediation. Limitations are area size, as a wetland needs to be big enough to have an effect and work as a healthy system, and cost. Otherwise wetlands may reduce water toxicity, unify different fields of science, and provide aesthetic enchantment to a community.


Rhizosphere Remediation

Rhizosphere remediation looks at how plant roots interact with contaminants. These interactions can include several different applications. Plant roots have great influence on the numbers and types of microorganisms around the plant, primarily focusing on microorganisms with degradative properties, both on the root and around it. The plant promotes certain forms of degradation through these microbes by secreting chemicals to them, these chemicals will enhance microbial communities in the soil which in turn will degrade pollutants within that area, cleaning it. This is called rhizosphere enhancement. The role of these plants is to increase the total number of selective microbes for use in degradative processes.


The science behind rhizosphere remediation is a delicate one. Firstly you must confirm that the correct microbes are being selected for by the plants, this would entail much research and experimentation. The plant must survive in these contaminated soils in order to promote the growth of the microbes on site. Microbes must also grow to a high enough density to have an effect on the contamination.

To do this plants must trick the microbes, as described vaguely when mentioning rhizosphere enhancement, to utilize the plant as a food source while also removing the contaminants as a source of energy/by-reaction. This in itself can pose difficulties depending on the contaminant of focus. These are non-water soluble contaminants such as pesticides, PCP, and petroleum. Petroleum is especially toxic to plants and more resistant to degradation the older it is. For this reason an incredible amount of money has been invested into this form of phytoremediation by the EPA, the contaminants of focus are difficult and part of areas that have been affected for a long time.


Metal Extraction

Phytoextraction is a method of utilizing certain plant species for their ability to take up metals from soil substrates. High concentrations of metal in the environment would most often result in the death or inhibition of growth to a plant. Certain plant species have evolved to utilize these high concentrations to grow in areas that otherwise are difficult to live in. Hyperaccumulators (species that can retain a very large amount of material) have a special affinity for collecting the metals, accumulating up to 20 times the metal inside the plant as was present in the soil, without presenting any signs of struggle. This ability makes them ideal for remediation purposes. It was in the 1970s that botanists began to apply these plants towards phytoremediation after metal miners had affiliated clumps of these plants to the indication of metals below the surface.


Interesting applications of this technology include more than correcting and improving land, phytoextraction may also be used in harvesting valuable metals from the ground. One such incident occurred in New Zealand, a heavy presence of gold mines resulted in the soil becoming highly acidic and toxic. The Maori communities resided near these mines but didn’t have the money to restore the area for agricultural use. Through the use of these rare and capable plants, the indigenous people of New Zealand were able to collect the gold out of the ground via plants, they ash the plant material to extract the gold and later used the profits to build their communities.



Phytoremediation has provided us with the opportunity to clean our land naturally. Downsides include species rarity and a longer growth time in comparison to using machinery and other technologies. Before all else, we should plan and find solutions to prevent toxic compounds from building up in our environment.


How do you help your environment?





References


Wastewater management

Ajayi, T. O., & Ogunbayo, A. O. (2012). Achieving environmental sustainability in wastewater treatment by phytoremediation with water hyacinth (Eichhornia crassipes). Journal of Sustainable Development, 5(7), 80. Ceschin, S., Sgambato, V., Ellwood, N. T. W., & Zuccarello, V. (2019). Phytoremediation performance of Lemna communities in a constructed wetland system for wastewater treatment. Environmental and experimental botany, 162, 67-71. Mustafa, H. M., & Hayder, G. (2020). Recent studies on applications of aquatic weed plants in phytoremediation of wastewater: A review article. Ain Shams Engineering Journal. Rezania, S., Ponraj, M., Talaiekhozani, A., Mohamad, S. E., Din, M. F. M., Taib, S. M., ... & Sairan, F. M. (2015). Perspectives of phytoremediation using water hyacinth for removal of heavy metals, organic and inorganic pollutants in wastewater. Journal of environmental management, 163, 125-133. Schröder, P., Navarro-Aviñó, J., Azaizeh, H., Goldhirsh, A. G., DiGregorio, S., Komives, T., ... & Wissing, F. (2007). Using phytoremediation technologies to upgrade waste water treatment in Europe. Environmental Science and Pollution Research-International, 14(7), 490-497. Singh, D., Tiwari, A., & Gupta, R. (2012). Phytoremediation of lead from wastewater using aquatic plants. J Agric Technol, 8(1), 1-11.

Rhizosphere Remediation

Guo, D., Fan, Z., Lu, S., Ma, Y., Nie, X., Tong, F., & Peng, X. (2019). Changes in rhizosphere bacterial communities during remediation of heavy metal-accumulating plants around the Xikuangshan mine in southern China. Scientific reports, 9(1), 1-11. Khan, A. G. (2005). Role of soil microbes in the rhizospheres of plants growing on trace metal contaminated soils in phytoremediation. Journal of trace elements in medicine and biology, 18(4), 355-364. Kirk, J. L., Klironomos, J. N., Lee, H., & Trevors, J. T. (2005). The effects of perennial ryegrass and alfalfa on microbial abundance and diversity in petroleum contaminated soil. Environmental pollution, 133(3), 455-465. Lal, S., Ratna, S., Said, O. B., & Kumar, R. (2018). Biosurfactant and exopolysaccharide-assisted rhizobacterial technique for the remediation of heavy metal contaminated soil: an advancement in metal phytoremediation technology. Environmental Technology & Innovation, 10, 243-263. Mokarram-Kashtiban, S., Hosseini, S. M., Kouchaksaraei, M. T., & Younesi, H. (2019). The impact of nanoparticles zero-valent iron (nZVI) and rhizosphere microorganisms on the phytoremediation ability of white willow and its response. Environmental science and pollution research, 26(11), 10776-10789.

Metal Extraction

Ali, H., Khan, E., & Sajad, M. A. (2013). Phytoremediation of heavy metals—concepts and applications. Chemosphere, 91(7), 869-881. Gardea-Torresdey, J. L., Peralta-Videa, J. R., De La Rosa, G., & Parsons, J. G. (2005). Phytoremediation of heavy metals and study of the metal coordination by X-ray absorption spectroscopy. Coordination chemistry reviews, 249(17-18), 1797-1810. McGrath, S. P., Zhao, F. J., & Lombi, E. (2001). Plant and rhizosphere processes involved in phytoremediation of metal-contaminated soils. Plant and soil, 232(1), 207-214. Muthusaravanan, S., Sivarajasekar, N., Vivek, J. S., Paramasivan, T., Naushad, M., Prakashmaran, J., ... & Al-Duaij, O. K. (2018). Phytoremediation of heavy metals: mechanisms, methods and enhancements. Environmental chemistry letters, 16(4), 1339-1359. Robinson, B., Green, S., Mills, T., Clothier, B., van der Velde, M., Laplane, R., ... & van den Dijssel, C. (2003). Phytoremediation: using plants as biopumps to improve degraded environments. Soil Research, 41(3), 599-611.




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