PHYTOREMEDIATION

Introduction:

Phytoremediation is the technical term used to describe the treatment of environmental problems through the use of plants.

Phytoremediation uses living plants for in situ and ex situ remediation of contaminated soil, sludges, sediments and ground water through contaminant removal, degradation or stabilization.

Phytoremediation can be used to remediate various contaminants including metals, pesticides, solvents, explosives, petroleum hydrocarbons, polycyclic aromatic hydrocarbons and landfill leachates.

Phytoremediation has been used for point and nonpoint source hazardous waste.

Risk Reduction & Technology Specially selected or engineered plants are used in the process. Risk reduction can be through a process of removal,

degradation of, or containment of a contaminant or a combination of any of these factors.

Green technology that uses plants systems for remediation and restoration.

Encompasses microbial degradation in rhizosphere as well as uptake, accumulation and transformation in the plant.

How does it work?

Plants in conjunction with bacteria and fungi in the rhizosphere:

transform transport store harmful chemicals.Root system surface area to absorb substances and efficient

mechanisms to accumulate water, nutrients and minerals.Selectively take up ionsDeveloped diversity and adaptivity to tolerate high levels of

metals and other pollutants.

Cont… Plants may break down or degrade organic

pollutants, or remove and stabilize metal contaminants. This may be done through one of or a combination of the methods.

The methods used to phytoremediate metal contaminants are slightly different to those used to remediate sites polluted with organic contaminants.

Metal Organic

Phytoextraction Phytodegradation

Rhizofiltration Rhizodegradation

Phytostabilisation Phytovolatilisation

Mechanisms:Phytoremediation uses one basic concept:

the plant takes the pollutant through the roots.

The pollutant can be stored in the plant (phytoextraction),

volatized by the plant (phytovolatization),metabolized by the plant

(phytodegradation).

Types Of Phytoremediation

1. Phytoextraction (Phytoaccumulation2. Rhizofiltration3. Phytostabilisation4. Phytodegradation (Phytotransformation)5. Rhizodegradation6. Phytovolatilization7. Hydraulic control of Pollutants

Phytoextraction The process where plant roots uptake metal contaminants

from the soil and translocate them to their above soil tissues.

This is of particular importance on sites that have been polluted with more than one type of metal contaminant.

Hyperaccumulator plant species are used on may sites due to their tolerance of relatively extreme levels of pollution.

Once the plants have grown and absorbed the metal pollutants they are harvested and disposed of safely.

This process is repeated several times to reduce contamination to acceptable levels.

Metal compounds that have been successfully phytoextracted include zinc, copper and nickel.

Phytoextraction

RhizofiltrationRhizofiltration is the remediation of polluted soils. The contaminants are either adsorbed onto the root

surface or are absorbed by the plant roots. Plants used for rhizoliltration are not planted directly in

situ but are acclimated to the pollutant first. Plants are hydroponically grown in clean water rather

than soil, until a large root system has developed. Once a large root system is in place the water supply is

substituted for a polluted water supply to acclimatise the plant.

Afer the plants become acclimatised they are planted in the polluted area where the roots uptake the polluted water and the contaminants along with it.

Rhizofiltration

Phytostabilisation Phytostabilisation is the use of certain plants to

immobilise soil and water contaminants. Contaminant are absorbed and accumulated by roots,

adsorbed onto the roots, or precipitated in the rhizosphere.

This reduces or even prevents the mobility of the contaminants preventing migration into the groundwater or air, and also reduces the bioavailibility of the contaminant thus preventing spread through the food chain.

This technique can also be used to re-establish a plant community on sites that have been denuded due to the high levels of metal contamination.

Phytostabilization

breakdown of organic contaminants by internal and external metabolic processes driven by the plant. 

Plants metabolic processes hydrolyse organic compounds into smaller units that can be absorbed by the plant.

Some contaminants can be absorbed by the plant and are then broken down by plant enzymes.

These smaller pollutant molecules may then be used as metabolites by the plant as it grows, thus becoming incorporated into the plant tissues.

Plant enzymes have been identified that breakdown ammunition wastes, chlorinated solvents such as TCE (Trichloroethane), and others which degrade organic herbicides.

Phytodegradation

Rhizodegradation Breakdown of organic contaminants in the soil by soil dwelling

microbes which is enhanced by the rhizosphere's presence.

Certain soil dwelling microbes digest organic pollutants such as fuels and solvents, producing harmless products.

Plant root exudates such as sugars, alcohols, and organic acids act as carbohydrate sources for the soil microflora and enhance microbial growth and activity.

The plant roots also loosen the soil and transport water to the rhizosphere thus additionaly enhancing microbial activity. 

Rhizodegradation:

Phytovolatilization: Process where plants uptake contaminaints 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 or volatilize into the air surrounding the plant.

There are varying degrees of success with plants as phytovolatilizers with one study showing poplar trees to volatilize up to 90% of the TCE they absorb. 

Phytovolatalization:

Application Pollutant Medium plant(s)

Phytoextraction & Rhizodegradation

Petroleum & Hydrocarbons

Soil & Groundwater

Alfalfa, poplar, juniper, fescue

Phytostabilisation Heavy Metals Soil Hybrid poplar, grasses

Rhizofiltration Radionuclides Groundwater Sunflowers

Phytoextraction Radionuclides Soil Indian mustard, cabbage

Phytodegradation Expolsives waste Groundwater Duckweed,

parrotfeather

Riparian corridor, phytodegradation Nitrates Groundwater Hybrid poplar

Examples:

Advantages: Phytoremediation is less expensive than the old "pump and

treat" method for the treatment of contaminated water.

Phytoremediation is also much less expensive than digging out the contaminated site.

Up to 95% of TCE present in water could be removed by simply planting trees and letting them grow.

Phytoremediation takes no maintenance once instituted.

Since phytoremediation uses plants, it is aesthetically pleasing.

After plants are introduced, wildlife is able to flourish at the once uninhabitable site.

Solar energy is used to drive the cleansing activity.

Disadvantages: Phytoremediation is limited to sites with lower contaminant

concentrations.

Phytoremediation is restricted to sites with contamination as deep as the roots of the plants being used.

The food chain could be adversely affected by the degradation of chemicals.

The air could be contaminated by the burning of leaves or limbs of plants containing dangerous chemicals.

Risk Assessment:The use of phytoremediation in the field is subject to many

environmental concerns: It is unknown what ecological effects hyperaccumulator

plants may have if ingested by animals Fallout from senescing tissues in autumn may also re-enter

the food chain Do volatilized contaminants remain at 'safe' levels in the

atmosphere Exposure of the ecosystem to contaminants is prolonged as

phytoremediation is a relatively slow process

Mycoremediation One of the primary roles of fungi in

the ecosystem is decomposition, which is performed by the mycelium.

The mycelium secretes extracellular enzymes and acids that break down lignin and cellulose, the two main building blocks of plant fiber.

These are organic compounds composed of long chains of carbon and hydrogen, structurally similar to many organic pollutants.

The key to mycoremediation is determining the right fungal species to target a specific pollutant. 

Mechanism Fungi have ligninase and cellulase enzymes

that break down woody materials. This allows fungi to get the needed carbon and energy they require for growth.

These enzymes are non-specific, meaning they can act on substrates like environmental pollutants.

Hyphae allow fungi to expand their surface area, making it easier to contact the pollutant.

Extracellular enzymes can then go to work.

Organo pollutant degradation white rot fungi, acts to break down pollutants by

adding an -OH group. Fungi used in conjunction with the plant to

remediate atrazine PCB volatility and xenobiotic extraction method

were factors affecting remediation success. Wood and litter decaying fungi have been shown

to metabolize TNT.

MycofiltrationMycofiltration is a similar or same process,

using fungal mycelia to filter toxic waste and microorganisms from water in soil.