Phytoremediation processes, Advantages & Disadvantages (Lecture 23)
Rhizofiltration (‘rhizo’ means ‘root’) is the adsorption or precipitation onto plant roots (or absorption into the roots) of contaminants that are in solution surrounding the root zone. Rhizofiltration is similar to phytoextraction, but the plants are used to clean up contaminated groundwater rather than soil.
Plants with high root biomass, or high absorption surface, with more accumulation capacity (aquatic hyperaccumulators) and tolerance to contaminants achieve the best results. Promising examples include Helianthus annus, Brassica juncea, Phragmites australis (grasses), Fontinalis antipyretica (moss) and several species of Salix (willow), Populus and Lemna (aquatic plants)
Phytostabilisation is the use of certain plant species to immobilize contaminants in the soil and groundwater through absorption and accumulation by roots, or precipitation within the root zone of plants (rhizosphere).
This process reduces the mobility of the contaminant and prevents migration to the groundwater or air, and also reduces bioavailability for entry into the food chain .
The Use of Phytoremediation to Treat Organic Contaminants :
Ø Phytostimulation is the process where root released compounds (Sugars, alcohols, and organic acids etc..) that can enhance the microbial activity in the rhizosphere.
Ø Rhizosphere = soil + root + microbes
Ø Symbiotic relation
- Enhanced rhizosphere biodegradation
- Plant assisted bioremediation
Ø Plants uptake contaminants which are water soluble and release them into the atmosphere as they transpire the water
- The contaminant may become modified along the way, as the water travels along the plant’s vascular system from the roots to the leaves, whereby the contaminants evaporate into the air surrounding the plant
- Poplar trees volatilize up to 90% of the TCE they absorb
Bioaugmentation vs. biostimulation:
Biostimulation involves the modification of the environment to stimulate existing microorganisms capable of bioremediation.
Indigenous populations may not be capable of degrading the xenobiotics or the wide range of potential substrates present in complex pollutant mixtures.
Bioaugmentation is the introduction of a group of natural microbial strains or a genetically engineered variant to treat contaminated soil or water.
Rhizodegradation – Decomposition of organic pollutants or biotransformation of metals by rhizospheric organisms.
Phyto degradation – Uptake of contaminants and the subsequent transformation, & mineralization by the plant itself through various internal enzymatic reactions and metabolic processes
Role of genetics:
Ø Genetic engineering is a powerful method for
enhancing natural Phytoremediation capabilities, or for introducing new capabilities into plants.
- Example, genes encoding a nitroreductase from a bacterium were inserted
into tobacco and showed faster removal of TNT (2,4,6-trinitrotoluene) and enhanced resistance to the toxic effects of TNT
Ø A plant that absorbs toxins, such as heavy metals, to a greater concentration than that in the soil in which it is growing
Ø A number of interactions may be affected by metal hyperaccumulation:
- mutualism (including mycorrhizae)
Ø Phytoscreening”: The Use of Trees for Discovering Subsurface Contamination by VOCs (Volatile organic compounds).
Ø Phytoscreening is a simple, fast, noninvasive, and inexpensive screening method for detecting subsurface contamination, and is particularly useful in urban settings where conventional methods are difficult and expensive to employ.
Ø Plants are able to translocate and accumulate particular types of contaminants:
- plants can be used as biosensorsof subsurface contamination
- Phytoscreening may lead to more optimized site investigations and reduce contaminated site cleanup costs
Pakistan Bioremediation Model for wastewater treatment established:
- The Capital Development Authority (CDA) with the collaboration of Pakistan Agricultural Research Council (PARC) has established Pakistan Bioremediation Model for Wastewater Treatment at Rose & Jasmine Garden and F-9 Park Islamabad to clean the wastewater of streams passing at these places.
- The treated water is used for green belts and other horticultural activities. The bioremediation technology will help to save our drinking water and to protect the environment and beauty of Islamabad city.
- PAKISTAN-US SCIENCE AND TECHNOLOGY COOPERATION PROGRAM
- Bioremediation of Chromium and Arsenic from Industrial Wastewater
Ø Cost effective when compared to other more conventional methods.
Ø the cost of the phytoremediation is lower than that of traditional processes both in situ and ex situ
Ø “natural” method, more aesthetically pleasing.
Ø minimal land disturbance.
Ø reduces potential for transport of contaminants by wind, reduces soil erosion
Ø hyper-accumulators of contaminants mean a much smaller volume of toxic waste.
Ø multiple contaminants can be removed with the same plant.
Ø Possibility of contaminated plants entering the food chain.
Ø Possible spread of contaminant through falling leaves.
Ø Trees and plants require care.
Ø Contaminant might kill the tree.
Ø Degradation product could be worse than original contaminant.
- Cleanup takes several years
Although much remains to be studied,
Ø Phytoremediation will clearly play some role in the stabilization and remediation of many contaminated sites.
Ø The main factor driving the implementation of Phytoremediation projects are low costs with significant improvements in site aesthetics and the potential for ecosystem restoration.
Ø The possibility of using biotechnology to improve the efficiency of phytoremediation processes makes even better than any other existing methods.