Phytoremediation of heavy metals
By: HaYoung Baik
Description and Rationale
The Philippines is a beautiful tropical country with fascinating landscapes and forests with more than 3500 species of plants and animals. However, there are places where the environment is badly contaminated by trash dumps. For example, the fresh water in some rivers have turned into sewage water as a result of careless dumping of litter and trash. Many squatter families are living on the banks of these rivers where the soil and water is polluted. Is there a way to make a cleaner environment?
Can plants detoxify the soil into a more nutritious source? How will certain plants be able to pick up soil toxins? Will the presence of these plants help the livelihood of the people living nearby?
Is it okay to water the plants/ seeds with dirty water? What kind of environment is needed for them
(the plants) to function properly? Will the pollutants in the soil affect the food source that the plants produce?
Scientists’ research shows that the Brassica juncea is a great source to collect and store heavy metals in the soil such as, aluminum, metal, cadmium, zinc, etc…(“Phytoremediation: Using plants to clean up soils”). Are there any other types of plants that act as metal hyper-accumulators? In recent observations by nearby polluted rivers, there are many wastes rotting and the places are all trashed up. What are the reasons for this contamination? How will the neighboring people be able to help in cleaning the environment into a much better place?
The initial purpose of this project will be to research the biology and ecology of the toxin-accumulating plants that can help detoxify the soil. Through the search of literature and some observations, these initial findings will help guide the experimental phase, where key variables in the usefulness of these plants will be further explored.
It is hoped that the new uses of the banks of polluted rivers might help improve the livelihood of the people living along these places through a more informed understanding of a biological resource.
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Biology
Common Names and Synonyms
The Indian Mustard is also called Brassica juncea. This plant is well known to botanists as a plant that is used to remove heavy metals from the soil in hazardous waste sites. The Indian Mustard has high tolerance to store harmful wastes, like heavy metals, in its cells. Other synonyms include Mustard greens, leaf mustard, and brown mustard.
Classification
Kingdom: Plantae (plants)
Phylum: Tracheobionta (vascular plants)
Class: Magnoliopsida (Dicotyledons)
Order: Capparales (Flowering plants)
Family: Brassicaceae/ cruciferae (mustard family)
Genus: Brassica (mustard)
Species: B. juncea (Indian mustard)
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Morphology and Physical Description
External Anatomy
The average size of the plant is between 30 cm – 1 meter tall. The lower leaves are naturally divided and are usually between 10 cm – 20cm long. Towards the upper part of the main stalk, the leaves grow alternately and are smaller than the lower leaves. On the top are the flowers, branched in yellow clusters, 1-1.4cm wide with normally 4 petals. The color of the leaves are usually dark green.
Internal Anatomy
Internally, the Brassica juncea consists of pith, which is known as a large central area for storage and support. This allows the stalk to stay upright and not fall over. The cambium also gives support as it is found as a circle around the inner stem and outer surface. A woody secondary tissue forms for a stronger base.
The cortex is a storage area between the cambium and epidermis- which is a thin layer of skin cell that is made for protection. The xylem does the work of pumping water into the cells and the phloem brings down sap (organic molecule) into the roots.
Other internal structures that help in absorbing heavy metals and the roles of the following are: mycorrhiza, which binds the extracellular sap to the cell wall, the plasma membrane helps limit the uptake of metals and reduce the leakage when pumping it up. The repair of stress-damaged proteins is done by peptides in the cytosol, and the metal is stored in the vacuole by tonoplast-located transporters.
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Getting Food
The best type of soil for the Indian mustard is moist medium-loamy and well-drained soil. It also required semi-shade or no shade at all. When planting Indian mustards, seeds have to be about 1 inch apart and 1/2 inch deep. After germination, water is regularly needed to keep the soil moist. They do best in full sun while receiving water regularly. The Indian mustard can withstand temperatures as low as 20’F (-4’C) without damage; although, it cannot stand temperatures above 85’F (29’C).
Depending on the weather, the plants are grown for 40 – 60 days. At the time of harvesting, entire plants are pulled by hand or cut a few centimeters above ground with sickles. Then, it is dried for 4 – 10 days. In India and other places, seeds are collected and oil is extracted by rotary mills, expeller and through a hydraulic process.
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Reproduction
Sexual reproduction is used to reproduce. Involving the female structures (stigma, style, and ovary) and male structures (filament and anther), the process of pollination transfers the pollen to a stigma. This can be done by wind, insects, birds, and/or small mammals. One nuclei of the pollen grain goes down through the tube into the ovary. Following, a second nuclei travels down the tube and splits into two sperm nuclei that fertilizes the egg to form the endosperm (stored fruit).
The brown mustard seed pods form close to the plant’s stem. It forms a plump, round-shape when they are ripe. The pods are cut carefully and dried in the sun for 7 days and are used either for different types of oil or to plant again.
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Environmental Factors
There are several disease and pest organisms that may use this plant as a host. These include the bacterial black rot, bacterial soft rot, Xanthomonas leaf spot and warmer weather may bring on the aphids and cabbage worms.
To control these pests, it is important to choose fields with soil that have good drainage (raised beds). Also, the use of boom sprayers will reduce the likeliness of bacteria spreading
than when air blast sprayers are used. Farmers say that bacteria can be more scattered onto other plants when air blast sprayers are used. Most importantly, provide the seedlings with regular water supply and fertilization for healthy growth.
Besides these pests and diseases, there are factors in the environment that are helpful. Flying insects like bees, are the pollinators for the flowers. Creating a symbiotic relationship, the flower gets pollinated and the bees get the sweet nectar.
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Origin and Distribution
The Brassica juncea originated in Asia by hybridization between B. rapa and B. nigra. The cultivation has been in Asia and Europe for thousands of years. Now, many of them are grown world-wide (India, China, Japan and North America) for the oil extracted from the seeds. Also, from cultivation, the leaf mustard became established as weed disturbed sites.
As the industry developed, seeds were distributed to other countries in Asia, including the Philippines. Many types of plants in the Brassica family were planted for observation and soon, the plants were spread across many farms. Chinese cabbage, cabbage, pechay, and mustard greens were cultivated.
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Importance to People
Leaves and seeds of the mustard family are used in many different ways. The seeds of Brassica juncea in particular, can be used as mustard oil, and erucic acid, which is used as an ingredient to make margarine and cooking oil.
Many countries around the world use this plant just for food and resources. However, the Brassica juncea can also be used to remove hazardous wastes from the soil. The results from a laboratory test show that the Indian mustard appeared to be the best at removing large quantities of heavy metals, such as, chromium, lead, copper and nickel. As described in the ‘Internal Anatomy’ section, the internal structures of the plant have its own job to absorb the harmful heavy metals. This plant not only provides food for us, but also provides us with clean soil.
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Survivability and Endangered Status
The amount of plants and seeds of the mustard family are abundant around the world. If they are grown in the right environment, seeds can constantly be produced.
About 12,500 hectares of land in the Philippines are farms that grow plants of the Brassica family. With the majority of it being cabbage, they are sold in local markets or sent to factories to be packed and sold in the supermarkets. All year round, these plants are produced and cultivated.
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Potential Solutions
Is the affect of Brassica plants in the soil of garbage dumps good or bad? In several countries around the world, people have used this plant as a source of food and method of Phytoremediation. However, in poor communities of the Philippines, the mustard plants have not been introduced as a source to clean the soil of harmful chemicals. Can this method also be applied to the local families? According to observers, the land used as garbage dumps is seriously contaminated with many toxins. Therefore, it decreases the amount of space people can use and cause skin diseases as a result of long exposure to the contaminated environment. There seem to be several promising livelihood possibilities that may be able to benefit the poor communities (and/or squatter villages) living next to and on top of garbage dumps. Below are 3 possibilities with an analysis of the advantages and disadvantages for each. Along with each possibility is a current status report of progress made to date on each of the possibilities.
Possibility 1 - PHYTOREMEDIATION
A large amount of land area in this Filipino community is severely polluted with harmful chemicals and pollution. Apparently, it is an extremely hard and costly process to purity all the soil. In addition, the poor squatter families do not have enough of the cost and it takes a long period of time to finish the whole process. Through research, it is found that the plants of the mustard family (or of the cabbage family), especially Brassica juncea, clean up the soil as they grow, through a process called phytoremediation. (More details are stated in the biology section – Internal anatomy).
Advantages:
1. Instead of laborious and expensive methods of cleaning up the soil of heavy metals, people can use brown mustard plants as a replacing method. If those plants are carefully looked after and grown with the required needs, it would help both reduce the manual labor and naturally develop the soil into a more environmental-friendly component.
2. For the villagers living within the polluted area, the plants could provide more fresh air and reduce the causes of skin diseases (infections). As the plants grow, it may cover up the gruesome smell of the trash and reduce the amount of dirty pests – like flies and mosquitoes, etc…
3. Growing mustard or cabbage plants could provide shelter to insects and animals that are unharmful to humans.
Disadvantages:
1. If the plants are not maintained, there will be no use of them and will not fulfill their job of cleaning the soil. It would just look more messy.
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Possibility 2 - FOOD SOURCE FOR PEOPLE
There are some plants of the mustard family that grows well in the tropical areas of the world. Then, why not use them as a practical food source here in the Philippines? If they are well maintained, the plants can easily be grown.
Advantages:
1. Many Filipinos enjoy eating meat along with some vegetables. Since many of them enjoy it, they could grow the plants and use it as a food source for their families. If the whole village or community participates, then the people can easily get into the routine of maintaining the plants and soon, they will be able to grow their own vegetables that they need.
2. Rather than buying expensive vegetables at the market, villagers can grow their own. Children might also enjoy watching the process of growing plants and they could apply it to their lives when they grow up.
Disadvantages:
1. The participation of many people is needed. If not many agree to grow their own plants (vegetables), then it will be hard for individuals to maintain the growth and do all the work. Also, individuals would not be able to produce as much.
2. The brown mustard plants grow best in cooler weather temperatures. The temperatures in the months between October and February are fit for plants to grow well, however, what if the temperature gets too hot for them to grow abundantly? Most of the growing will only happen during the cool months.
3. The plants/vegetables are edible only when they are grown in clean soil, and not among wastes. When metal-accumulating plants are grown in polluted soil, the plant stores the harmful heavy metals in the cells of their leaves. Therefore, it is researched that eating those plants is a dangerous thing.
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Possibility 3 - NICER-LOOKING ENVIRONMENT
Areas of and around the garbage dump area does not look pleasant. Because of all the trash and dirty water, it appears gray and black. Why not try to use plants to make one’s living area a bit nicer? Since brown mustard plants can grow well in harmful environments, they can act as metal hyper-accumulators and, at the same time, give a fresh look to the village.
Advantages:
1. Families living in that area could breathe more fresh air than the bad-smelling smell of rotting trash.
2. Piles of trash can produce much carbon dioxide. By planting these plants, they can produce oxygen for the people.
Disadvantages:
1. Good maintenance is required and plants have to be watered regularly
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Bibliography
“A Review on Phytoremediation of Heavy Metals and Utilization of its Byproducts” Biomass and Waste Management Laboratory. 2005. Devi Ahilya University. 16 Apr 2009 <http://www.ecology.kee.hu/pdf/0301_001018.pdf>.
“Brassica juncea” Floridata. 2008. 14 Apr 2009 http://www.floridata.com/index.cfm.
“Brassica juncea” Prime Facts. 2008. Profitable and Sustainable Primary Industries. 16 Apr 2009 http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0018/222246/Brassica-juncea-in-north-west-NSW.pdf .
“Brassica juncea strumata” Plants for a future .2008. Plant database search page. 13 Apr 2009 http://www.pfaf.org/database/index.php.
“Brown Mustard” Herbs 2000.com. 2002. 15 Apr 2009 http://www.herbs2000/com/herbs/herbs_Mustard_brown.htm.
“Cellular Mechanisms for Heavy Metal Detoxification and Tolerance” Oxford Journals. 2005 Oxford University Press. 16Apr 2009 <http://jxb.oxfordjournals.org/cgi/content/full/53/366/1>.
“Mustard” Plant care.com 2002 11 Apr 2009 http://www.plantcare.com/encyclopedia/mustard-2333.aspx.
“Phytoremediation: Using Plant to Clean Soil” Botany. 2000. Global Issue Map. 15 Apr 2009 http://www.mhhe.com/biosci/pae/botany/botany_map/articles/.article_10.html
Tapino, Jerry. C. Personal interview. 19, Apr 2009
“Vegetable Crops” Vegetable MD Online. 1994. Cornell University. 26 Apr 2009 <http://vegetablemdonline.ppath.cornell.edu/factsheets/Crucifers_BR.htm>.
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