Soil And Groundwater Treatment 1671 Phytoremediation

Phytoremediation is the use of plants to remove and/or biotransform contaminants. The process of phytoremediation is comparable to that of constructed wetlands. Both applications make use of macro- and microscale biology, and both concepts have captured considerable attention, even though they each still qualify as evolving technologies given their relatively short histories. However, phytoremediation also has several important distinctions (Schnoor et al., 1995). First, the focus of phytoremediation is that of biologically remediating contaminated soils, sediments, and waters (both surface and ground water) as opposed to wetland applications focused solely on wastewater treatment. Second, phytor-emediation systems may employ trees as well as smaller plants as the primary biological agent. Indeed, for those applications in which they are suitable, phytoremediation systems may offer a highly attractive "green" means of decontaminating lands and waters.

Successful applications have been achieved experimentally with a wide range of inorganic (e.g., metals, ammonia) and organic [e.g., petroleum hydrocarbon, BTEX, creosote wood preservative, polycyclic aromatic hydrocarbon (PAH), refinery waste, organophosphate insecticide, chlorinated pesticide, chlorinated solvent, explosive, cyanate] contaminants (Jackson, 1997).

The potential benefits are again much the same as those of constructed wetlands, including that of an apparently simple, solar-driven, aesthetically pleasing, in situ "green" technology with few, if any, complex or energy-intensive hardware or operational requirements (i.e., as compared to conventional treatment operations that employ pumps, mixers, aerators, etc. that routinely use energy and require careful operator attention). These systems can also be self-sustaining in terms of procuring nutrients, they can make a beneficial contribution to the balance of water in their soils, they can establish a highly evolved complement of degradative enzymes, and they tend to be inexpensive both in their initial startup and in subsequent maintenance. Granted, this remediation approach will not work in all situations, and in even when it is successful, the remediation process will operate on a time-scale measured in years rather than in hours or days. The public perception of phytoremediation is extremely high, though, as a natural means of promoting the restoration of chemically contaminated sites.

However, the seemingly simplistic notion of using plants and trees to clean up these contaminated sites actually involves a far more sophisticated process than what is apparent to the eye. As was the case with constructed wetlands, the visibly "green" above ground portions of these systems are but a part, and in some cases perhaps even a lesser part, of an integrated remediation scheme that encompasses a complex array of physical, chemical, and certainly biological treatment factors. The type, density, and nurturing of the plants and trees is important as well as the nature of the soils (e.g., soil type, conductivity, depth to groundwater, nutrient availability) and climate (e.g., rainfall frequency and duration, radiation, seasonal climate, windspeed, humidity) in which they are grown. Finally, the character, concentration, location, and form (e.g., whether it is sorbed, soluble, solid) of the contaminating materials are also important factors.

Before delving into the underlying sophistication of these phytoremediation systems, though, one must develop a background understanding and appreciation of the vertical layering of the soils in which these bioremediating plants and trees grow, and their corresponding physical and chemical characteristics. Figure 15.2 provides a schematic overview of a representative soil-plant system and its associated horizontal layers. There are two major regions shown in this schematic, situated above and below the groundwater table, respectively, and known as the unsaturated and saturated zones. Within the unsaturated zone the soil water volume does not entirely fill the pore space in the soil. The unsaturated zone is also called the vadose zone. The remaining (partially or fully) open void space, however, will also facilitate better levels of aeration and oxygen transfer, which will then promote more aerobic microbial activity. The saturated zone is where the pores are completely filled with water. The groundwater table is the point in the saturated zone where the hydraulic head is equal to zero. Water is drawn by capillary action into the capillary fringe slightly above the groundwater table. The top of the capillary fringe marks the division between the saturated and unsaturated zones.

The level of the groundwater table may vary considerably from one location to another, and also according to temporal changes in precipitation and climate, but in most instances it lies many meters below the surface and at a level not usually reached by plant root systems. As a result, phytoremediation was developed with systems whose remediating activity took place almost totally within the uppermost layer of the unsaturated zone (i.e., at shallow depths). However, subsequent developments with the nurturing and use of deep-rooting plants and trees have now expanded this technology down to the saturated region, at which point phytoremediation could then deal with contaminants extending fully down to the groundwater table.

With most soil columns there will also be considerable variation in the horizontal and vertical homogeneity of the top and bottom layers. The top, unsaturated zone may be further divided into another series of layers vertically from top to bottom:

• O horizon: consisting mostly of leaf litter and other dead organic matter

• A horizon : the topsoil, with a high content of mostly degraded organic matter, well populated by microorganisms and invertebrates, often coincident with the root zone

• B horizon: less weathered mineral matter plus organic and inorganic matter leached from the A horizon

• C horizon: partially weathered material from the bedrock below, very low in organic matter and living organisms

Once the remediating plants and/or trees have been introduced successfully into a site, the means by which they can attempt to degrade or remove a group of involved contaminants can be quite diverse. An important, yet all too easily overlooked aspect of phyto-remediation is that the plants are generally not the sole means of contaminant treatment. Granted, the plants themselves may contribute many different and important remediating effects (e.g., phytovolatilization), as discussed below, but in most instances their remediation role is metabolically complemented, perhaps even dominated by that of the microbes that are motivated correspondingly to live within the same soils. Figure 16.60 provides an overall synopsis of these prospective plant and microbial mechanisms, and in the following synopsis we examine each of these metabolic contributions relative to the overall process of phytoremediation.

Contaminant release via phytovolatilization

Contaminant degradation via phytodegradation

Contaminant release via phytovolatilization

Water release via evapotranspiration

Contaminant degradation via phytodegradation

Contaminant accumulation via phytoaccumulation

-Organic enrichment Warhizoenrichment -Microbial stimulation

- Microbial degradation Figure 16.60 Phytoremediation mechanisms.

Water release via evapotranspiration

Contaminant accumulation via phytoaccumulation

Contaminant filtration via rhizofiltration

-Organic enrichment Warhizoenrichment -Microbial stimulation

- Microbial degradation Figure 16.60 Phytoremediation mechanisms.

The first such mechanism, rhizoenrichment, stems from the fact that plants release into soils a number of exudates that are rich in organic carbon and that, in turn, effectively nurture the growth of many soil microorganisms. A sizable fraction of the carbon fixed through photosynthesis is released into soils, with estimates ranging from 10 to 30%. This material includes a range of readily biodegradable materials with small to moderately sized molecular weights, including sugar, protein, alcohol, and acids. Yet another group of organic carbon residuals are also released into soils by the senescence (aging) and decay of plant tissue, particularly that of fine-root biomass. There is also a beneficial physical impact with the growth and aging of plant roots, in that they tend to loosen the soil during both their growth and death, forming new paths for transporting water and aeration. This process subsequently tends to pull water to the topsoil surface while drying the lower saturated zones.

The latter enrichment of soils with organic carbon compounds exuded by plants subsequently promotes and maintains a significant enhancement in the growth of microbes within the immediate vicinity of the roots (i.e., microbial stimulation). There are actually two mechanisms by which plants provide this stimulation: by feeding the microorganisms with their exudates and by promoting the availability of oxygen. Here again, the photo-synthetic activity of the plant is important, with at least some of its newly created oxygen being effectively pumped through the roots into the soil. Channeling created by roots, both alive and dead, also provides a means of physically opening the soil matrix and improving its porosity. The net effect of the added substrates and improved oxygen availability leads to levels of microbial activity and density that are considerably higher than those of barren, unvegetated soils, by several orders of magnitude. In a fashion analogous to that shown in Figure 16.39, between 5 and 10% of root surfaces will tend to the colonized by various forms of bacteria and fungi, and the adjacent rhizospheric soils will commonly experience considerable increases in their microbial density. Population densities of 5 x 106 total bacteria, 9 x 105 actinomycetes bacteria, and 2 x 103 fungi per gram of air-dried soil have been observed.

This symbiotic relationship between plants and their adjacent microbial consortia stimulates microbes, which in return assist the plants in securing nutrients and essential vitamins. Given the diversity of the substrate forms available to these microbes and the variable nature of the rhizospheric environment (i.e., with dynamic changes in oxygen content, soil water presence, pH, etc.), a wide range of microbial types is found in these soils. In turn, the metabolic breadth of these bacterial and fungal forms encompasses a considerable range of enzymatic mechanisms and pathways. The resulting, collective effect of these microbes is that they can be expected, either directly or indirectly, to play a significant role in degrading organic contaminants present in the soils (i.e., microbial degradation). Compared to readily biodegradable compounds, recalcitrant organic contaminants found in soils may not be directly oxidized by these root-zone microorganisms as an energy source, but they may nonetheless be converted. Indeed, in the presence of other biodegradable root exudates, the catabolic enzymes generated to catalyze these reactions sometimes cooxidize the recalcitrant materials through a cometabolic conversion. The relative contribution of plants vs. microbes to degradation no doubt varies from one situation to the other, and there are those who would argue that one or the other typically plays a more dominant role. However, irrespective of which contribution might be dominant, the fact remains that the efficacy of phytoremediation commonly involves a coordinated and harmonious set of biological mechanisms that span the micro- to macroscale of life.

Contaminants not degraded by rhizospheric microbes are available for plant uptake by roots, where they may then either be retained or translocated farther upward into a plant's shoots and leaves (i.e., phyto uptake). Some plants simply uptake contaminants and store them in their roots, whereas others both uptake and translocate contaminants. There is, admittedly, a degree of uncertainty about the nature of these combined processes and the conditions under which they may each take place, but it appears that the polarity of the contaminants is an important factor. One of the most frequently mentioned criteria in this regard is that of the octanol-to-water partition coefficient (KOW). KOW is used as an indicator to represent the relative ability of a compound to concentrate in lipids vs. water and is therefore more likely to concentrate in biological materials. KOW tends to be inversely related to the polarity and therefore the aqueous solubility of a compound. If a compound has a KOW value of 1, it has the same affinity for water as it has for octanol (and by implication, for lipids). If the KOW value were 100, it has 100 times greater affinity for octanol (and lipids) than for water. Because of the wide range of KOW values, it is often reported as log KOW, so the range of KOW from 1 to 100 corresponds to log KOW from 0 to 2. Compounds amenable to plant uptake reportedly have logKOW values in the range 0.5 to 3. Those materials having higher values, extending beyond the range of moderately nonpolar would simply be too tightly sorbed onto root surfaces to be translocated any farther within a plant. On the other hand, lower values (under 0.5) have such a high preference for water (i.e., being quite polar) that they would tend to remain within soil water rather than be taken in by roots in the first place.

Some plants not only transport contaminants across their cell membranes (i.e., through phyto uptake) but also move these materials internally beyond their roots (i.e., phytotran-slocation). Here again, the polar vs. nonpolar nature of the contaminant is an issue, as well as the rate of transpiration being maintained by the plant. This rate of transpiration is, in fact, a key variable for translocation, with an apparent direct correlation between these two factors. Once a contaminant has been taken into a plant through these sequential processes of phyto-uptake and phytotranslocation, this contaminant could theoretically then be removed from the site by harvesting and subsequent disposal of the plant's above-ground biomass.

Following uptake and translocation, many plants have evolved compound-specific detoxification pathways that involve subsequent conjugation and compartmentation reactions that effectively bind contaminants into their structural makeup (i.e., phytoaccumu-lation). These biotransformed and phytoaccumulated compounds can either be deposited into vacuoles or converted into insoluble (and frequently covalent) complexes within cell wall components through a process known as lignification. In some cases, the accumulated compounds are passed unchanged into these deposits; in other instances the material being accumulated is that of degradation fragments produced through preceding biochemical conversions that transformed the contaminants into nonphytotoxic metabolites. However, it is also possible that some plants may accumulate contaminants internally to a level where an ecotoxicological hazard develops that would severely restrict subsequent consumption or disposal of the plants.

Phytoremediation plants can also produce a number of enzymes that may promote the internal metabolism and degradation of contaminants (i.e., phytodegradation). For example, nitroreductase enzymes can initiate the breakdown of nitroaromatic munitions; deha-logenase enzymes will promote the degradation of chlorinated compounds; nitrilase will contribute to the degradation of herbicides; phosphatases will facilitate the catalysis of organophosphates; and peroxidases will promote the destruction of phenols.

The next process, phytovolatilization, theoretically involves the uptake and translocation of contaminants into leaves; plants may then release these compounds into the atmosphere through a volatilization mechanism. One particular plant, arabidopsis (in the mustard family) has been found to produce a specific enzyme, mercury reductase, which reduces mercury to elemental mercury, which is then amenable to volatilization and release. Yet another known volatilization sequence involves the treatment of selenium-contaminated soils by rice, broccoli, and cabbage through the production of volatile dimethylselenide and dimethyldiselinide. In addition, there are a number of low-molecular-weight VOC-type organic molecules that appear to be easily translocated and volatilized by various plants. The extent to which the latter reactions actually take place under real-world conditions, however, is not well established.

Although roots generally cannot be harvested in a natural environment, another phy-toremediation process, rhizofiltration, can be used where plants are raised in greenhouses and transplanted to sites to filter metals from wastewaters biochemically. As the roots become saturated with metal contaminants, they can be harvested and disposed of. Phy-toremediation plants have also been used in this fashion to concentrate radionuclides via rhizofiltration in the Ukraine and Ashtabula, Ohio.

Extensive water uptake and release rates can also be maintained by a number of phy-toremediation plants, including poplars, cottonwoods, and willows, in a fashion that will effectively pull contaminated groundwater plumes toward and through these phytoreme-diating tree roots (i.e., évapotranspiration). A single, mature willow tree, for example, can transpire more than 19 m3 of water each day (^5000 gallons, or about 3.5 gal/min), and 1 ha (10,000 m2) of a herbaceous plant such as saltwater cord grass has been found to evapotranspire even four times as much. There are several interrelated issues, including plant type, leaf area, nutrient availability, soil moisture, wind conditions, and relative humidity.

le 16.16 provides a general correlation of the plant types that have been tested for the various contaminant forms believed to be amenable to phytoremediation treatment. Extending beyond the matter of a plant's potential suitability for any given contaminant, there are also a wide range of characteristics for plants in terms of their relative environmental preferences. Some plants tend to have shallow roots (i.e., cottonwoods and willows roots), whereas phréatophyté plants have produced far deeper roots (i.e., aspens and alders). The family of salicaceae trees (including poplars) tends to have very high water uptake rates and is usually able to tolerate high organics. Some plants are rather salt-intolerant (e.g., hybrid poplars), whereas others have a high tolerance for salts (e.g., mesquite, salt cedar). Some plants prefer hot humid climates (e.g., bald cypress), whereas others prefer cold and dry climates (e.g., greasewood). Alfalfa plants, are often used due to their high nitrogen uptake rates and ability to maintain nitrogen fixation in the absence of available nitrogen.

The principles and practice of phytoremediation systems involve several important engineering aspects, but in reality the procedures still qualify as an emerging technology. The issues that must be considered include those of the involved soil characteristics, the targeted contaminants and current concentrations, and the relative depth of the existing residuals.

Concerns regarding soil type stem from the fact that various plants have different preferences for either fine- or coarse-grained soils, which probably reflects the ability of the soils to hold and transfer varying amounts of moisture, air, and nutrients. The site-specific and perhaps seasonally fluctuating depth to the groundwater table is also important, as it affects the means by which a plant can draw water.

TABLE 16.16 Phytoremediation Agents Relative to Contaminant Species

Organic

Phytoremediation

Inorganic

Phytoremediation

Contaminants

Agent

Contaminants

Agent

Atrazine

Alfalfa

Arsenic

Bluebells Maple tree Cattail Water lily

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