Phytotechnology and Phytoremediation Techniques

PHYTOTECHNOLOGY

PHYTOREMEDIATION The processes of bioaccumulation, phytoextraction, phytostabilisation and rhizofiltration are collectively often referred to as phytoremediation.

PHYTOEXTRACTION The process of phytoextraction involves the uptake of metal contaminants from within the soil by the roots and their translocation into the above-ground regions of the plants involved. Currently, the best candidates for removal by phytoextraction are copper, nickel and zinc, since these are the metals most readily taken up by the majority hyperaccumulator plants of the varieties of

There have been some recent early successes in attempts to find suitable phytoextractors for cadmium, nickel and even arsenic Arsenic behaves quite differently from other metal pollutants, typically being found dissolved in the groundwater in the form of arsenite or arsenate,and does not readily precipitate.

BIPOLAR ELECTROLYSIS Bipolar arsenate, which reacts with ferric ions from an introduced iron anode, but generally conventional remediation techniques aim to produce insoluble forms of the metal s salts, which, though still problematic, are easier to remove. electrolysis to oxidise arsenite into

All phytotechnology centers on the removal and accumulation of unwanted substances within the plant tissues themselves, their removal and subsequent volatisation to atmosphere or the facilitation of in-soil treatment. Plant-based treatments make use of natural cycles within the plant and its environment and, clearly, to be effective, the right plant must be chosen The species selected must be appropriate for the climate, and it must, obviously, be able to survive in contact with the contamination to be able to accomplish its goal.

TERRESTRIAL PHYTO-SYSTEMS (TPS) Phytoremediation methods offer significant potential for certain applications and, additionally,permit much larger sites to be restored than would generally be possible using more traditional remediation technologies There are over 400 different species considered suitable for use as phytoremediators Amongst these, some hyperaccumulate contaminants within the plant biomass itself, which can subsequently be harvested, others act as pumps or siphons, removing contaminants from the soil before venting them into the atmosphere, while others enable the biodegradation of relatively large organic molecules, like hydrocarbons derived from crude oil

Chinese ladder brake fern,Pteris vittata,has been found to accumulate arsenic in concentrations of 5 grams per kilogramme of dry biomass. Certain strains of naturally occurring alpine pennycress (Thlaspi caerulescens) can bioaccumulate around 1.5% cadmium,on the same dry weight basis

HYPERACCUMULATION Hyperaccumulators are defined as the plant species that are capable of accumulating metal(loid)s above the threshold concentrations of 10,000mgkg 1 dry weight of shoots for Zn and Mn, 1000mgkg 1 for Co, Cu, Ni, As, and Se, and 100mgkg 1 for Cd (Baker and Brooks,1989;Brown et al.,1994).

RHIZOFILTRATION Rhizofiltration is the absorption into, or the adsorption or precipitation onto, plant roots of contaminants present in the soil water Rhizofiltration is typically used to deal with contamination in the groundwater, rather than within the soil itself The plants destined to be used in this way are normally brought on hydroponically and gradually acclimatised to the specific character of the water which requires to be treated

PHYTOSTABILISATION Phytostabilisation does employ both extractive and filtrative techniques Harvesting the grown plants is not a feature of the process It does not remove immobilises them, deliberately concentrating and containing them within a living system, where they subsequently remain. The idea behind this is to accumulate soil or groundwater the pollutants, but

A second benefit of this method is that on sites where elevated concentrations of metals in the soil inhibits natural plant growth, the use of species which have a high tolerance to the contaminants present vegetation to be re-established This can be of particular exposed sites, min- imising the effects of wind erosion, wash off or enables a cover of importance for soil leaching, which

ORGANIC PHYTOREMEDIATION Variety of organic chemicals are commonly encountered as environmental including many types of pesticides, solvents and lubricants pollutants These hydrocarbons are not especially mobile, tend to adhere closely to the soil particles themselves and are generally localised within 2 metres of the surface.

PHYTODEGRADATION Phytodegradation, which is sometimes known by the alternative name of phyto transformation, involves the biological breakdown of contaminants, either inter- nally, having first been taken up by the plants, or externally, using enzymes secreted by them The complex organic molecules of the pollutants

RHIZODEGRADATION Rhizodegradation, which is also variously described as phytostimulation or enhanced rhizospheric biodegradation, refers to the biodegradation of contaminants in the soil by edaphic microbes enhanced by the inherent character of the rhizosphere itself This region generally supports high microbial biomass and consequently a high level of microbiological activity, which tends to increase the speed and efficiency of the biodegradation of organic rhizosphere compared with substances other within regions the and soil

It has been estimated that the release of sugars, amino acids and other exudates from the plant and the net root oxygen contribution can account for photosynthetic activity per year (Foth 1990), of which denitrifying bacteria, Pseudomonas heterotrophs are the principal beneficiaries. up to 20% of plant spp., and general Mycorrhizae fungi associated with the roots also play a part in metabolising organic contaminants they have unique enzymatic pathways that enable the biodegradation of organic substances that could be otherwise transformed solely by bacterial

PHYTOVOLATILISATION Phytovolatilisation involves the uptake of the contaminants by plants and their release into the atmosphere,typically in a modified form This phytoremediation biotechnology generally relies on the transpiration pull of fast-growing trees, which accelerates the uptake of the pollutants in groundwater solution,which are then released through the leaves The contaminants are removed from the soil, often being transformed within the plant before being voided to the atmo- sphere.

Trichloroethylene (TCE), an organic compound used in engineering and other industries for degreasing, is a particularly mobile pollutant, typically forming plumes which move beneath the soil s surface. In a number of studies, poplars have been shown to be able to volatilise around 90% of theTCE they take up.

HYDRAULIC CONTAINMENT Large plants can act as living pumps, pulling large amounts of water out of the ground which can be a useful property for some environmental applications, since the drawing of water upwards through the soil into the roots and out through the plant decreases the movement of soluble contaminants downwards, deeper into the site and into the groundwater.

Trees are particularly useful in this respect because of their enormous transpiration pull and large root mass. Poplars, for example, once established, have very deep tap roots and they take up large quantities of water, transpiring between 200 1100 litres daily

The contamination scenario is to create a functional water table depression, to which pollutants will tend to be drawn and from which they may additionally be treatment aim of applying this to a taken up for

MACROPHYTETREATMENT SYSTEMS (MATS) The primary processes within the MaTS as the uptake and transformation of contaminants by micro-organisms and plants and their subsequent biodegradation and biotransformation; absorption, adsorption and ion exchange on the surfaces of plants and the sediment; the filtration and chemical precipitation sediment contact; the settlement of suspended the of pollutants via

ALGAL TREATMENT SYSTEMS (ATS)

ALGAL CULTIVA-TION SYSTEM AND CARBON SINK (ACSACS)