Medical brain Notes 9

  Phytotechnology and Photosynthesis From a practical standpoint, phytotechnology is the use of plants in environmental biotechnology applications, and draws on many of the characteristics which have already been described. In this respect, it does not represent a single unified technology, or even application, but rather is a wider topic, defined solely by the effector organisms used.  Plants of one kind or another can be instrumental in the biological treatment of a large number of substances which present many different types of environmen-tal challenges. Accordingly, they may be used to remediate industrial pollution, treat effluents and wastewaters or solve problems of poor drainage or noise nui-sance. The processes of bioaccumulation, phytoextraction, phytostabilisation and rhizofiltration are collectively often referred to as phytoremediation. Although it is sometimes useful to consider them separately, in most functional respects, they are all aspects of the same funda...

  Terrestrial Phyto-Systems (TPS) The importance of pollution, contaminated land and the increasing relevance of bioremediation have been discussed in previouly. 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. The processes of photosynthesis described earlier are fundamental in driving what is effectively a solar-energy driven, passive and unengineered system and hence may be said to contribute directly to the low cost of the approach.  A large range of species from different plant groups can be used, ranging from pteridophyte ferns, to angiosperms like sunflowers, and poplar trees, which employ a number of mechanisms to remove pollutants. There are over 400 differ-ent species considered suitable for use as phytoremediators. Amongst these, some hyperaccumulate contaminants within the plant biomass itself, whi...

  Metal Phytoremediation The remediation of sites contaminated with metals typically makes use of the natural abilities of certain plant species to remove or stabilise these chemicals by means of bioaccumulation, phytoextraction, rhizofiltration or phytostabilisation.   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. Certain species, termed hyperaccumulators, have an innate ability to absorb exceptionally large amounts of metals compared to most ordinary plants, typically 50 – 100 times as much (Chaney  et al . 1997, Brooks  et al . 1998) and occasionally considerably more. The original wild forms are often found in naturally metal-rich regions of the globe where their unusual ability is an evolutionary selective advantage. Currently, the best candidates for removal by phytoextraction are copper, nickel and zinc, sin...

  Hyperaccumulation Hyperaccumulation itself is a curious phenomenon and raises a number of fun-damental questions. While the previously mentioned pteridophyte,  Pteris vittata , tolerates tissue levels of 0.5% arsenic, certain strains of naturally occurring alpine pennycress ( Thlaspi caerulescens  ) can bioaccumulate around 1.5% cadmium, on the same dry weight basis. This is a wholly exceptional concentration. Quite how the uptake and the subsequent accumulation is achieved are interesting enough issues in their own right. However, more intriguing is why so much should be taken up in the first place. The hyperaccumulation of copper or zinc, for which there is an underlying certain metabolic requirement can, to some extent, be viewed as the outcome of an over-efficient natural mechanism. The biological basis of the uptake of a completely nonessential metal, however, particularly in such amounts, remains open to speculation at this point. Nevertheless, with plants like...

  Rhizofiltration Rhizofiltration is the absorption into, or the adsorption or precipitation onto, plant roots of contaminants present in the soil water. The principal difference between this and the previous approach is that rhizofiltration is typically used to deal with contamination in the groundwater, rather than within the soil itself, though the distinction is not always an easy one to draw. 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. Once this process has been completed, they are planted on the site, where they begin taking up the solution of pollutants. Harvesting takes place once the plants have become saturated with contaminants and, as with the phytoextraction, the collected biomass requires some form of final treatment. The system is less widely appreciated than the previous technology, but it does have some very important potential appli...

  Phytostabilisation In many respects, phytostabilisation has close similarities with both phytoextraction and rhizofiltration in that it too makes use of the uptake and accumulation by, adsorption onto, or precipitation around, the roots of plants. On first inspection, the difference between these approaches is difficult to see, since in effect, phy-tostabilisation does employ both extractive and filtrative techniques. However, what distinguishes this particular phytoremediation strategy is that, unlike the preceding regimes, harvesting the grown plants is not a feature of the process. In this sense, it does not remove the pollutants, but immobilises them, delib-erately concentrating and containing them within a living system, where they subsequently remain. The idea behind this is to accumulate soil or groundwater contaminants, locking them up within the plant biomass or within the rhizosphere, thus reducing their bio-availability and preventing their migration off site. Metals d...

  Organic Phytoremediation A wide variety of organic chemicals are commonly encountered as environ-mental pollutants including many types of pesticides, solvents and lubricants. Probably the most ubiquitous of these across the world, for obvious reasons, are petrol and diesel oil. 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. Accordingly, since they are effectively in direct contact with the rhizosphere, they are a good example of ideal candidates for phytoreme-diation. The mechanisms of action in this respect are typically phytodegradation, rhizodegradation, and phytovolatilisation. 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. Hence, the complex o...

  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 substances within the rhizosphere compared with other soil regions and microfloral communities. Part of the reason for this is the tendency for plant roots to increase the soil oxygenation in their vicinity and exude metabolites into the rhizosphere. 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 up to 20% of plant photosynthetic activity per year (Foth 1990), of which denitrifying bacteria,  Pseudomonas spp ., and gene...

  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. Thus the contaminants are removed from the soil, often being transformed within the plant before being voided to the atmo-sphere. One attempt which has been explored experimentally uses a genetically modified variety of the Yellow Poplar,  Liriodendron tulipifera , which has been engineered by the introduction of mercuric reductase gene ( mer A ) as discussed. This confers the ability to tolerate higher mercury concentrations and to convert the metal’s ionic form to the elemental and allows the plant to withstand contaminated conditions, remove the pollutant from the soil and volatilise it. The advantages...

  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 quanti-ties of water, transpiring between 200 – 1100 litres daily. In situations where grass-land would normally support a water table at around 1.5 metres, this action can lead to it being up to 10 times lower. The aim of applying this to a 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 taken up for treatment...

  Plant Selection It should be obvious that the major criteria for plant selection are the particular requirements for the method to be employed and the nature of the contaminants involved. For example, in the case of organic phytotransformation this means species of vegetation which are hardy and fast growing, easy to maintain, have a high transpiration pull and transform the pollutants present to nontoxic or less toxic products. In addition, for many such applications, deep rooting plants are particularly valuable.  On some sites, the planting of grass varieties in conjunction with trees, often in between rows of trees to stabilise and protect the soil, may be the best route since they generate a tremendous amount of fine roots near to the sur-face. This particularly suits them to transforming hydrophobic contaminants such as benzene, toluene, ethylbenzene, xylenes (collectively known as BTEX) and polycyclic aromatic hydrocarbons (PAHs). They can also be very helpful in con-...

  Applications Phytotechnology has many potentially beneficial land uses, though for the most part the applications are still in the development stage. Several have been tested for the treatment of contamination, and in some cases successfully tried in the field, but generally they remain in the ‘novel and innovative’ category, lacking well-documented data on their performance under a variety of typical operating conditions. As a result, some researchers have voiced doubts, suggesting that the beneficial effects of plant utilisation, particularly in respect of phytoremediation, have been overstated. Some have argued that the reality may range from genuine enhancement to no effect, or even to a negative contribution under certain cir-cumstances and that the deciding factors have more to do with the nature of the site than the plants themselves. In addition, some technologies which have been successfully used on some sites may simply serve to complicate matters on others. One such ap...

  Aquatic Phyto-Systems (APS) Aquatic phytosystems are principally used to process effluents of one form or another, though manufactured wetlands have been used successfully to remediate some quite surprising soil contaminants, including TNT residues. Though the latter type of application will be discussed in this section, it is probably best considered as an intergrade between the other APS described hereafter and the TPS of the previous. Many of the aspects of the biotreatment of sewage and other wastewaters have already been covered in the previouly and so will not be restated here. The major difference between conventional approaches to deal with effluents and phytotechnological methods is that the former tend to rely on a faster, more intensively managed and high energy regime, while in general, the stabilisation phase of wastewaters in aquatic systems is relatively slow. The influx and exit of effluent into and out of the created wetland must be controlled to ensure an adequa...

  Macrophyte Treatment Systems (MaTS) The discharge of wastewaters into natural watercourses, ponds and wetlands is an ancient and long-established practice, though rising urbanisation led to the development of more engineered solutions, initially for domestic sewage and then later, industrial effluents, which in turn for a time lessened the importance of the earlier approach. However, there has been a resurgence of interest in simpler, more natural methods for wastewater treatment and MaTS systems, in particular, have received much attention as a result. While there has, undoubtedly, been a strong upsurge in public understanding of the potential for environmentally har-monious water cleaning  per se , a large part of the driving force behind the newly found interest in these constructed habitats comes from biodiversity concerns. With widespread awareness of the dwindling number of natural wetlands, often a legacy of deliberate land drainage for development and agricultural pu...

  Nutrient Film Techniques (NFT) An alternative approach to the use of aquatic macrophytes, which was tried exper-imentally, involved growing plants on an impermeable containment layer, in a thin film of water. In this system, the wastewater flowed directly over the root mass, thereby avoiding some of the mass transfer problems sometimes encoun-tered by other aquatic phytotreatment regimes. Though the early work indicated that it had considerable potential for use in the biological treatment of sewage and other nutrient-rich effluents, it does not appear to have been developed further and little is known as to the conditions which govern its successful practical applica-tion. One interesting aspect which did, however, emerge was that the cultivation system could also be extended to most terrestrial plants, which may yet be of possible relevance to the future development of land-based phytotreatments.

  Algal Treatment Systems (ATS) Algae have principally been employed to remove nitrogen and phosphorus from wastewaters, though some organic chemicals can also be treated and a relatively new application has emerged which makes use of their efficient carbon seques-tration potential.