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M. Phytoremediation for the removal of organic contaminants

Prior to the 20^th century, naturally occurring biodegradation processes were largely adequate for recycling organic materials on the surface of the Earth. A number of different biochemical processes were able to cope with the organic substances that rarely accumulated in sufficient quantity and concentration to cause environmental pollution problems. However, during the 20^th Century, humankind achieved the ability to synthesise and disseminate large quantities of industrial chemicals. Simultaneously, the use of chemicals per capita and the human population increased, causing a dramatic increase in the over-all production and discharge of industrial chemicals. The consequent environmental pollution problems have become particularly prominent due to a concomitant improvement in analytical chemistry, epidemiology, and toxicology that allowed an understanding of the link between the chemicals the pollution problems. The rates of production and dispersal of these, often toxic, industrial substances have completely out paced naturally occurring biodegradation, and an unsteady state has resulted. This unsteady state threatens both human health and ecosystem function. Thus, the development of phytoremediation technologies becomes a pivotal ecotechnological issue because the failure of ecosystem functions jeopardises not only the ecosystem integrity, but also human health.

The typical process utilised in evaluating and implementing phytoremediation technologies involves six steps:

1. Mapping the geographical distribution of the contaminant concentrations using standard analytical chemistry, and the conduct of an environmental risk assessment based upon analyses of the soil pore water.
2. Laboratory testing to verify the applicability of bioremediation technologies.
3. Assessing, often by appropriate modelling, the feasibility of the method in situ.
4. Identifying and producing in sufficient amounts the plants to be used for the phytoremediation.
5. Implementing the phytotechnologies in situ, managing ground water levels as required to prevent flooding and other potentially negative impacts.
6. Monitoring the results using a broad spectrum of analytical methods, including conservative tracers, detection of intermediary metabolites, and respiration rate measurements.

A particularly important application of phytoremediation is in the protection of ground water against toxic compound contamination. Contamination of ground water is a serious concern for public health and environmental quality. The problem is commonly manifested as a contaminant plume migrating in the direction of ground water flow from a point source. In such cases, containment of the contaminant plume is important in preventing further migration. Plants, in some cases, can be used to contain these plumes and remove toxic organic matter. Vegetation can efficiently take up even moderately hydrophobic organic substances, with K_ow values of about 0.5 to 3.0. Hydrophobic substances with a K_ow value exceeding 3.0 bind so strongly to the soil and the roots that they are not easily taken up by plants.

Plants can also enhance microbiological decomposition rates because the roots pump oxygen into the root zone, thereby creating a good environment for the microbiological oxidation of organic matter. It may be necessary to add fertilisers to increase biomass of the plants and, thereby, promote biological activity in the root zone, as illustrated by Lin and Mendelssohn (1998). They described the successful removal of crude oil from a marsh by fertilisation of Spartina alterniflora and Spartina patens communities. The rate of oil degradation within the soil was significantly enhanced by the application of fertiliser in conjunction with the presence of plants.

The productivity of the vegetation may also be enhanced by the addition of ion exchange substrates to completely depleted soils and barren sands (Soldatov et al. 1997 suggest that a 1% addition is sufficient).

Soils contaminated by both heavy metals and toxic organics are most difficult to remediate, because the presence of heavy metals causes a distinct decline in the rate of organic matter decomposition (see Zwolinski 1994). Thus, it generally is necessary to first remove the heavy metals, and then to remove the toxic organic components.

N. Phytoremediation for the removal of heavy metals

Two, different phytoremediation techniques can be applied for the removal of heavy metals from the environment; namely, the use of algae, and the use of heavy metal-tolerant plants. Both possibilities may be applied to reclaim contaminated soils or treat polluted waters. Indeed, the techniques may be applied to waters polluted by contaminated soils.

Both techniques are attractive alternatives to the traditional environmental technologies based on physico-chemical processes, such as ion exchange, liquid extraction, precipitation, and crystallization. Phytoremediation, in most cases, is considerably less expensive than these physico-chemical techniques, particularly if high concentrations of heavy metals can be achieved within the vegetation. In addition, the recovery and reuse of the metals is possible using phytoremediation methods. For example, acid extraction could be used to recover heavy metals from plants. However, the costs of the recovery processes often exceed the value of the metals. Notwithsanding, all of the processes currently available for removing heavy metals from water and soils, including those processes based on traditional environmental technologies, only transfer the heavy metals into a more concentrated and more accessible form that can facilitate disposal or recovery. However, recovery or disposal is generally satisfactory.

Marine algae possess large quantities of biopolymers - including polysaccharides, uronic acids, and sulphated polysaccharides - that can bind heavy metals. Their metal uptake capacity is consequently quite high. The marine alga, Sargassum spp., can take up as much as 40% of the algal dry weight in gold (Kuyucak and Volesky 1989). One advantage of using such seaweeds as phytoremediation technologies is that these plants are large enough to facilitate the immobilisation of the metals. These plants can, for instance, be applied in packed columns without any pretreatment of the effluent introduced into the columns for treatment.

Biosorption in algae has been attributed primarily to the cell wall, where both electrostatic attraction and the formation of complexes play a role. It is, therefore, important to know the cell wall characteristics of the different groups of algae proposed to be used for bioremediation projects in order to select the most promising species (Schiewer and Volesky 2000). The ion exchange properties of the sulphated polysaccharides in seaweeds are also of importance (Schiewer and Volesky 1995), and may explain a significant part of the observed biosorption.

As with terrestrial plants, Langmuir adsorption isotherms can be used to describe the uptake dynamics exhibited by algae (Holan and Volesky 1994):

M = B × K(M)/( 1 + K(M))

Soils and wastewaters may contain more than one type of heavy metal, and various metals may compete for the same binding sites within the algae. A multi-Langmuir adsorption isotherm can be applied in these cases. If a two-component competition scenario is considered (metals M1 and M2), the multi-Langmuir adsorption isotherm will take the following form:

M = B × K(M1)/( 1 + K(M1) + K(M2))

The binding capacity of the algae at low pH values is usually lower than at higher pH values, as demonstrated in Figure 5.16. This property may be utilised in the recovery of heavy metals removed from the environment by marine algae. Desorption of the metals can be achieved by application of acid extraction.

The binding of metals to cell surfaces is a rapid process. A large percentage of the binding is achieved within a few minutes, while a complete equilibrium can be attained within a few hours. The rate is generally proportional to the displacement from the equilibrium condition, and to the external surface area of the alga. A more detailed model of the kinetics of metal binding includes consideration of film diffusion from the solution to the surface, pore diffusion through the particles, and chemical binding processes. However, these processes are beyond the scope of this application.

Fig. 5.16. Experimental data on the copper binding capacity of marine algae at pH = 2.5 and pH = 4.5. Notice the high binding capacity of the algae at pH = 4.5, which equates to 2 meq/g or 64 mg copper/g

The practical application of marine algae in heavy metal removal may be realised in situ by using either a batch process or a packed bed column. In both cases, it is advantageous to pretreat (reinforce and immobilise) the biosorbent particles. Reinforcement can be achieved by chemical cross-linking with formaldehyde or gluralaldehyde, which successfully reduces the swelling and leaching of biomolecules (Holan et al. 1993). Immobilisation is possible by treatment of the biosorbent with alginate, silica, and polyacrylamide (Holbein 1990, Bedell and Darnell 1990).

Freshwater aquatic plants, like their marine counterparts, have been used to remove heavy metals from water. For example, Sharma and Gaur (1995) used duckweed in the removal of zinc, lead, and nickel. Other aquatic plants, such as Salvinia and Spriodela, have been used to remove chromium and nickel from wastewaters. A significant removal efficiency, in the concentration range of 1 ppm to 8 ppm, although with fluctuations, has been reported by Srivastav et al. (1994). Fungi, likewise, have recently been proposed for use in the removal of heavy metals. The amounts of metal adsorbed per unit biomass are on the order of several mg/g, for cadmium, copper, lead, and nickel (Kapoor and Viraraghavan 1998).

Microbial biotraps, a variation of algal-based phytotechnologies utilising both dead and living biomass in adsorption and ion exchange processes (White et al. 1995), have also been used to remove heavy metals from water. This process has been used in wastewater treatment systems, where more than 50% removal of heavy metals has been shown to occur. The so-called Meander system, which passes drainage water containing heavy metals through various channels containing cyanobacteria and algae, can achieve a metals removal efficiency of greater than 99% (Erlich and Brierley 1990). This process most likely utilises precipitation and entrapment of particulate matter within the meanders, in addition to the algal-mediated adsorption and ion exchange processes, since the metals tend to be concentrated in the channel sediments.

Many microorganisms, including algae, produce extracellular polymers composed mostly of polysaccharides and sulphated-polysaccharides. These commonly occur in the form of a "slime" layer surrounding the cells. This layer binds metal ions tightly in an ion exchange-like process. Hydrogen sulphide produced by sulphate-reducing bacteria, when utilised in the removal of metals from water by biota under anaerobic conditions, results in the formation of metal sulphides having extremely low solubilities. This feature can be utilised in the removal of toxic metals from water in the form of sulphides (Brierly 1995). Other precipitation processes, which may be utilised as phytotechnologies, include the formation of insoluble uranium phosphate complexes (Macaskie 1991), and copper phosphates and copper oxalates (Crusberg et al. 1994).

It has been shown, recently, that a few plant species, such as the alpine pennycress, can be used as bioaccumulators (also known as hyperaccumulators). Hyperaccumulators are usually plants that are slow growing and slowly remove heavy metals over a period of years. While fast growing poplar trees are good phytoremediators because of their rapid growth, they generally do not make good hyperaccumulators. When growing in contaminated soils, bioaccumulators are able to concentrate metals up to 100 times the heavy metal concentrations in normal plant species. When these plants are harvested, the corresponding amounts of heavy metals are removed from the soils. If the soils are only slightly contaminated with heavy metals, this method may be able to remediate the soils within just a few years, making this method very attractive due to its low cost. For very contaminated soils, however, the period needed for complete recovery of the soil may be too long. Nevertheless, by watering the plants carefully with a diluted EDTA solution, it may be possible to enhance the biological uptake of heavy metals. Several plant species are known to naturally release chelating ligands and enzymes into the soil which accelerate metal removal rates and reduce the toxicity of heavy metal ions. Jørgensen (1993) reported removal rates (from an initial lead concentration of 380 mg Pb/kg soil (dry matter)) of up to 11.5% lead per harvest of plants watered with a 0.02 M EDTA solution, because the increased solubility of the metal ions.

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