Advances in Phytoremediation

Phytoremediation has many proponents to treat widespread chemical contaminants in soil, water, and air. It is low-cost, eco-friendly, and doesn’t require complicated technology and infrastructure. As it is a new branch of science, it is necessary to understand the processes involved in the different methods of phytoremediation to advise decision-making for application of this technology.

Understanding Phytoremediation

Pollution of air, water, and soil is increasing worldwide due to mining, industrial production, vehicles, and intensive agriculture. Hazardous waste produced by these human activities needs to be dealt with carefully because it can contain heavy metals, volatile organic compounds, and inorganic elements that are toxic to people, vegetation, and wildlife.

With the problem of hazardous waste increasing, many treatment options are being tried, including phytoremediation.

Phytoremediation is a process where plants and bacteria are used to treat wastes that have accumulated in the air, soil, and water to control their levels. It is a new and emerging field of ecological engineering.

There are various technologies of phytoremediation used to rid the soil of hazardous contaminants (See Figure 1). These technologies include the following:

• Phytostabalization: The plants produce chemicals that bind the pollutants and retain them in the ground. This method is useful for organic and metal contaminants.
• Phytodegradation: The organic pollutants are broken down by enzymes produced by plants in the soil or after absorbing the volatile organic compounds into the plants. The end products are harmless and no longer toxic.
• Phytoaccumulation: This process is also called phytoextraction. The plants absorb the contaminants and accumulate them in their tissues – leaves or stems. This is the method used to manage heavy metals. The whole plant is then burned to smelt the metals and reuse them or the plant is treated as hazardous waste.
• Phytovolatilization: Plants absorb the pollutants, which could either be organic compounds or metals. In the plants, the contaminants are broken down and the components converted into gases and released into the air.

Areas of Research

Phytoremediation is not equally useful for all kinds of waste because it depends on the species of plants used, depth of contamination, site features, and concentration of waste (to mention a few factors). While it has been successfully established as a reliable treatment for some waste, other wastes are currently undergoing experimentation.

Research groups are relying on the use of modern and accurate field tools to help them in the collection and analysis of data on the field. This provides the research groups with more time, as convention analyses are often cumbersome, time-consuming, and require extensive equipment.

Here we discuss some new studies that have recently been conducted in phytoextraction to reduce heavy metal levels with the use of state-of-the-art field instrumentation.

Figure 1: Methods of phytoremediation, Khan and Noor, 2017. (Image credits: DOI: 10.1080/24749508.2017.1332849)

Phytoextraction uses crops with high biomass or those that hyper accumulate metals. A third method is to use chemicals to make the metals more bioavailable in the soil so that plants can absorb them easily. The idea behind all of these methods is to accumulate as much of the metals as possible. Repeated growing and harvesting of crops laden with heavy metals ultimately reduces toxicity in the soil.

Leaf Area Changes Detect Lead Contamination

Lead is a common heavy metal contaminant near urban areas, in both soil and water. When levels of this metal increase in the land, they can be absorbed by plants. Lead impairs the growth and productivity of plants and can affect crop yield.

Rose scented geranium (Pelargonium graveolens) was tested by scientists in India as a potential plant for phytoremediation. Growth hormones called brassinosteroids help plants to withstand the stress. So, geranium was treated with 2 µΜ Ebl of 24-epibrassinolide, a brassinosteroid, to check if it helped the plants tolerate lead toxicity.

Some plants in this greenhouse study were also subjected to 2mM Pb of lead. Growth parameters like plant height, fresh weight, dry weight, and leaf area were measured after sixty days.

The CI-203 Handheld Laser Leaf Area Meter, from CID Inc., was the field instrument used in this case.
The device is useful for rapid and non-destructive measurements. CI-203 is a small instrument that can flatten leaves of different sizes and thicknesses to measure the length, width, area, perimeter and shape factor of the leaf. It can store data, which can be later transferred by USB to a computer.

The scientists found that lead toxicity reduced the growth of geranium and the plant’s final weight. The leaf area was the most severely affected parameter, and leaves also showed signs of chlorosis due to chlorophyll deficiency. Treatment with 24-epibrassinolide helped to mitigate lead toxicity and restore leaf area. It also boosted production of chlorophyll.

Since essential oils are extracted from geranium by distillation, the yield from even polluted sites can be used without fear of contaminating the final product. The whole plant which accumulates lead should be disposed of carefully.

Photosynthesis under Cadmium Toxicity

In Pakistan, scientists experimented with the possibility of phytoextraction of cadmium in raps (Brassica napus L.) fields through the addition of a chelator – citric acid. Chelators bind to metal ions and make them more bioavailable to plants, so phytoextraction is enhanced.

Cadmium is toxic to raps. The experiment wanted to check if citric acid helped raps deal with cadmium toxicity. The scientists found that at lower levels of availability of cadmium, there was little uptake by plants of the metal. On the other hand, plant growth, production of chlorophyll, photosynthesis, and antioxidant levels in raps suffered due to cadmium toxicity.

When citric acid was added, the levels of cadmium absorption by raps plants increased significantly, and there was more cadmium in the plant tissues. When citric acid was added to the soil, chlorophyll levels, photosynthesis, and plant biomass increased, reducing toxic stress on the plants. With an increase in biomass, the concentration of cadmium in the plant tissues decreased. Moreover, citric acid increased antioxidant activity that helps plants to cope with cadmium stress.

So, citric acid improves cadmium extraction by plants but still protects them from heavy metal toxicity.

In this study, one of the field instruments used was the CI-340 Handheld Photosynthesis System, manufactured by CID Inc. The CI 340 is a light-weight tool that can be used in the field or greenhouse. It is suitable for use with open or closed system analysis of photosynthesis. It is a gas analyzer that measures photosynthesis as the amount of carbon dioxide consumed by the plants during the process. It can be used with ten leaf chambers of varying sizes to accommodate the leaves of different species. There are also modules for carbon dioxide, water vapor, light, and temperature control, so the experiment can be regulated as needed.

Technology Aids Science

As research in new avenues increases, there is a need for precision tools that are handy, light, and easy to use. Luckily, some of the new field devices, such as those from CID Inc., provide multifaceted and complex technology at your finger-tips. Data is analyzed as it is collected, simultaneously, saving time and providing valuable insight within seconds. It takes the drudgery out of scientists’ work and allows them to delve deeper into the complex physiological processes within plants.

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Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Feature photo courtesy of Daniela

Sources

Center for Public Environmental Oversight (CPEO). Phytoremediation. Retrieved from http://www.cpeo.org/techtree/ttdescript/phytrem.htm

Ehsan, S., Ali, S., Noureen, S., Mahmood, K., Farid, M., Ishaque, W., Shakoor, M.B., Rizwan, M. (2014). Citric acid assisted phytoremediation of cadmium by Brassica napus L. Ecotoxicology and Environmental Safety, 106:164-172. DOI: https://doi.org/10.1016/j.ecoenv.2014.03.007

Greipsson, S. (2011). Phytoremediation. Nature Education Knowledge 3(10):7. Retrieved from https://www.nature.com/scitable/knowledge/library/phytoremediation-17359669/

Rao, S.S.R., & Raghu, K. (2016). Amelioration of lead toxicity by 24-epibrassinolide in rose-scented geranium [Pelargonium graveolens (L.) Herit], a potential agent for phytoremediation. Journal of Medicinal Plants Studies, 4: 06-08. Retrieved fromhttp://www.plantsjournal.com/archives/2016/vol4issue6/PartA/4-5-4-677.pdf