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PHYTOREMEDIATION OF SOILS
where K , is the Henry’s Law constant, [A(aq)J is the concentration of the
component in the solution phase, and PA is the partial pressure of A in the gas
phase. Ryan et ul. (1988) compiled a list of Henry’s constants for priority
pollutants. Some attempts have been made to define a value for the Henry’s
constant above which volatilization is an important mechanism for bioavailability, but one must consider other chemical properties such as adsorption
to the soil, microbial decomposition, and solubility in water. For example, the
polyaromatic hydrocarbons (PAHs) are generally considered semivolatile. However, the microbial decomposition of the lower molecular weight compounds
(such as naphthalene) and/or strong adsorption to soil components for higher
molecular weight PAHs (such as pyrene and benzralanthracene) make volatilization from soil a minor pathway for this class of compounds.
2. Water Phase
Lipophilicity is the most important property of a chemical in determining its
movement into and within a plant. Lipophilicity is the “balance between the
affinity of the chemical for aqueous phases and that for lipid-like phases” (Bromilow and Chamberlain, 1995). This property determines the ease of movement
across plant membranes. Lipophilicity is related to the partition coefficient of the
pollutant between I-octanol and water (Kow).The K , , is one of the most widely
available experimental parameters for xenobiotics. The values of K O , cover the
wide range from 10-2 to 10’0. For ease of discussion these numbers are often
reported in log form. A small KO, is indicative of high water solubility and low
lipophilicity, and high values are associated with compounds that have high
lipophilicity and low water solubility. Organic contaminants in soil can transfer
to roots in the soil-pore water by diffusion and by mass flow. In either case, the
transfer is mediated by the aqueous phase. The spot-to-spot variations of soil
texture, organic matter composition, water conditions, and distribution of organic contaminants make the root exposure to soil organic contaminants a highly
heterogeneous and constantly changing phenomenon. This explains why many
experiments have been conducted in hydroponic cultures in which an idealized
condition can be easily created and maintained. For a hydroponic system, the
concentration of the test compound can be kept constant to all parts of the root
during the entire experimental period.
Compounds that are most water soluble will be most available for mass transport and diffusion into the rhizosphere. However, very soluble compounds with
little afYinity for the soil solids also will be subject to leaching out of the root zone
and can become physically unavailable for root uptake. Marginally soluble components will not move with the water and require the root to grow near them
before uptake is possible. Positively charged organics will tend to be retained by
the soil’s cation exchange sites and their availability for assimilation will be
reduced. Negatively charged compounds are excluded from the generally nega-
S. D. C U " G H A M
tively charged soil surfaces, making them more available to the roots but further
subject to leaching.
Research on the plant uptake of organic chemicals from soil has centered on
pesticides and a few high-profile contaminants (such as PCBs and dioxin). Pesticides have been emphasized because of the importance of plant uptake in their
function and their potential for effects on the food chain and nontarget crops.
Also, knowledge of a pesticide's environmental fate is necessary for its registration and sale. Plant uptake models have been developed using experimental data
for those compounds that have been studied and these models allow us to predict
plant uptake of organic compounds for which no experimental data exist.
3. Solid Phase
Conceivably, an organic compound adsorbed to a soil particle can directly
transfer into a plant root via solid-to-solid partitioning. This route of transfer may
allow us to rationalize some putative high root uptakes of very lipophilic compounds with high vapor pressures as well (McMullin, 1993). This concept is
technically feasible and well demonstrated in dermal sorption studies in mammalian systems; however, due to the vast surface area in a soil and the relatively
low surface area of a root in comparison, it is hard to imagine this as a viable
remediation strategy. Nevertheless, we do not wish to rule this out as either a
mechanism of plant uptake or a potential remediation technology in some creative form.
FATEIN A PLANTSYSTEM
1. Transport within the Plant
Physiological factors of plant roots that control uptake of organic chemicals
have been summarized by McFarlane (1995). Briefly, water and dissolved constituents can move easily and relatively unimpeded from the soil solution into the
root's apparent free space, that area of the root outside the endodermis and the
Casparian strip. The apparent free space is characterized by cortex cells with
porous cell walls and many voids. The Casparian strip in the endodermis is a
waxy barrier that inhibits movement of water into the interior of the root. At this
point, all water, solutes, and non-aqueous phase liquids must pass through cell
membrane at least two times. For most xenobiotics (both ionic and nonionic
substances), this movement a p p r s to be a passive process depending upon
retention by the membrane, solubility in water, and diffusion. Many observations
on xenobiotic root uptake and root-to-shoot translocation have been made (Devine and Vanden Borden, 1991). However, most were aimed at describing the
PHYTOREMEDIATION OF SOILS
behavior of specific compounds of interest. Here we only review those studies
that are instrumental in revealing the underlying general principles governing the
Movement of nonionized organic chemicals into the root consists of the equilibration of the solution phase outside the root with the solution in the apparent
free space and sorption of the compounds onto the root surface. Shone and Wood
(1974) examined the uptake of the herbicide sirnazine by barley growing in
solution culture. One of the more interesting observations of their research was
that the xylem translocation of simazine apparently was restricted because the
concentration of simazine in the transpiration stream was less than the concentration in solution. A transpiration stream concentration factor (TSCF) was developed to describe this behavior.
concentration in xylem sap
concentration in external solution
In the case of simazine or any other compound whose xylem translocation is
apparently restricted, TSCF is less than 1. 0. Other triazines also were found to
have TSCF less than 1.0 (Shone el a!., 1974). Shone and Wood (1974) also
defined a root concentration factor (RCF).
external solution concentration
This line of study was extended by Briggs et al. (1982) to a series of other
pesticides (0-methylcarbamoyloximes and substituted phenylureas). They found
no relationship between RCF and TSCF and suggested that root accumulation of
one class (triazines) was mostly physical adsorption to the surfaces of the roots.
For other classes (carbamoxyloxime and phenylureas), an empirical model was
made to relate root uptake by intact barley plants to lipophilicity:
log (RCF - 0.82)
0.77 log KO, - 1.52.
This equation is expected to vary somewhat among plant species depending upon
the composition of lipids in the roots.
They found that the TSCFs were less than 1 .O for all 18 compounds studied,
and that TSCFs showed a bell-shaped relationship with log K , , values.
[(log KO, - 1.78)]*
The maximum TSCF was observed for log KO, of about 1. 8. They also plotted
those literature data with sufficient details and found that despite the diversity of
plant species, compounds, and experimental techniques, they largely conform to
the same relationships they found between log K , , and RCF and TSCF. The
general conclusions of Briggs et al. (1982) were further validated by Hsu et al.
S. D. CUNNINGHAM ET AL.
(1990), albeit with a different maxima, on a series of herbicide (cinmethylin)
analogs using a root pressure chamber technique.
While the linear relationship between RCF and log KO, is immediately intuitive, the log KO, and TSCF relationship requires explanation. It seems to be
intimately related to the pathways from root surface to xylem vessels situated in
the stele. The root pathways for water and solutes have received reviews in the
past (Weatherley, 1975). The salient feature of the main pathway is that a molecule has to cross cell membranes at least twice, once before the Casparian strip
(CS) barrier on the endodermis, and once after that, to gain access to the xylem
in the stele (Weatherley, 1975; Clarkson and Robards, 1975). This is referred to
as the symplastic pathway. The apoplastic pathway, available only near the tip of
the root where the CS has not yet developed well, usually accounts for an
insignificant amount of the total flux unless the root is disturbed (Moon et af.,
1986) or stressed (Hanson et al., 1985; Skinner and Radin, 1994). Compounds
with low log KO, values can move through the intercellular space along with the
mass flow of water until reaching the CS barrier. There, the two membrane
crossing steps are slow due to low lipophilicity. On the other hand, delivery
efficiency of compounds with higher log KO, values through the pre-CS barrier
parts is low due to low water solubility and losses to lipophilic tissue constituents. But the rate of their membrane crossing is more rapid, compensating for the
earlier slower passage.
Once compounds are partitioned into lipophilic membranes interior to the CS
barrier, they need to be desorbed into aqueous solution in order to go into xylem
vessels. Conceptually, the entire root pathway may be simplified as a step of
partitioning into the post-CS membrane and a desorption step off this membrane.
The first step of aqueous-to-lipid partitioning obviously favors more lipophilic
compounds. However, the second lipid-to-aqueous desorption step favors more
hydrophilic compounds. The interplay of these two crucial rate-limiting steps
would produce the observed relationship between log KO, and TSCF (Briggs et
al., 1982; Hsu et al., 1990).
In soil the idealized hydroponic condition is compromised. Sorption of organics to soils can limit their availability to roots. For many organic compounds, one
can assume that the adsorption is a linear function of concentration and organic
carbon content of the soil,
9e = K d f ,
wheref,, is the fraction of organic carbon in the soil, C , is the concentration of
the organic compound in the soil solution, and Kd is the linear adsorption coefficient. This equation suggests that as organic carbon content increases, adsorption
increases and availability for root uptake decreases. It is estimated that in soil the
log KO, for maximum TSCF shifts down by about two units (Hsu et al., 1990).
In the field of phytoremediation, not much attention has yet been paid to the
mechanistic aspects of root uptake and xylem translocation. Most studies use
PHYTOREMEDIATION OF SOILS
endpoint analyses of shoot contents of radiolabeled test compounds. Such studies
are always complicated by shoot metabolism and severely limited by the availability of radiolabeled compounds. Simple techniques are now available to allow
for mechanistic experimentation with nonradiolabeled compounds free of shoot
metabolism complications. This kind of mechanistic approach can yield crucial
information to help design the most optimal phytoextraction scheme for important soil organic contaminants.
Conceptually, the total amount of organic compound phytoextracted into the
(easily harvestable) above ground fraction can be defined as:
Amount delivered to shoot = Conc. in sap
Volume of sap/time unit.
The xylem sap can be obtained from the decapitated stem near the soil line
either by applying pressure to the root of a potted plant in a modified pressure
chamber as that used in Hsu et al. (1990) or by applying negative pressure to the
cut stem (Gil de Carrasco et al., 1994; Ambler et al., 1992). Two cautions need
to be observed to obtain xylem saps matching those of intact plants: ( I ) adjust the
sap expression rate to match that of the sap volume flow in intact plants, and (2)
avoid the initial sap sample which may contain artifacts associated with a given
technique (Else et al., 1994). Due to the generally simple composition of xylem
saps, the presence of a target organic compound and its metabolites can be
readily quantified by analytical techniques. Here the analytical challenge mentioned earlier (Section 1I.A) is quite easily met. This is evidenced by the successful quantification of low concentrations of the natural plant growth regulators
abscisic acid and zeatin in xylem sap (Ambler et al., 1992; Davies and Zhang,
1991). The whole-plant xylem sap flux rate can be measured with a stem-flow
gauge. The accuracy of the xylem mass flow measured with this method has been
validated in many studies (Baker and Van Bavel, 1987; Steinberg et al., 1989;
Dugas, 1990; Devitt et ul., 1993).
The adaptability of the xylem sap expression techniques and the xylem mass
flow measurement technique makes them particularly suitable for phytoextraction research. Both techniques can be used for both laboratory and field experiments with nonradiolabeled compounds. For laboratory studies, different plants
can be grown in soil with the target organic contaminant. The sap flow can be
measured, and xylem sap obtained for analytical quantifications. Different xylem
sap concentration values from different plants multiplied by their respective total
sap volume flow rates will generate the total amounts extracted by test species.
The study can be run at different times of day to cover diurnal variations of xylem
flow rates and sap contaminant concentrations. The method can also detect
whether the build-up of the target compound in shoot causes any self-limitation
of further phytoextraction by an inhibition of sap flow rate. By using these
monitoring techniques, different innovative methods can be tested to see if they
produce any improvements on net phytoextraction. For field studies, sap analysis
and sap flow measurements can be made to existing plants in a contaminated site.
S . D. CUNNINGHAM ET AL.
These measurements can quickly lead to the identification of the most advantageous plant species for phytoremediation.
Although this chapter deals specifically with organic compounds, the same
xylem sap analysis and sap flow measurement can be effectively applied to the
mechanistic study of phytoextraction of other soil contaminants, particularly
inorganics. Knowing their xylem sap contaminant content as well as the chemical identity of the chelating compound(s) would greatly help us in pinpointing the
potential rate-limiting step in phytoextraction.
Despite these general rules, there are distinct variations among plants in their
ability to accumulate organics into the roots. Selecting plant species and varieties
for maximizing this trait, however, has not been approached systematically. The
subject has been studied with certain pesticides, but few data exist for other
organic pollutants. The best-studied cases seem to involve chlorinated organic
insecticides. For example, beets, turnips, potatoes, and radishes tend to accumulate less than do carrots (Lichtenstein and Schulz, 1965). Sugar beet roots accumulated more dieldrin than carrots, potatoes, corn, and alfalfa (Harris and Sans,
1967). Different varieties of carrot can have four-fold differences in endrin uptake (Hermanson et af., 1970). More detailed comments on the influence of plant
properties on root uptake of organic compounds are provided by Bell (1992) and
Shimp and co-workers (1993).
In addition to root uptake, the general microbial stimulation in the root zone
suggests that the most logical starting point in selecting species is to focus on the
plant root. Species with extensive and fine root systems should have the greatest
potential for enhancing bioremediation. These roots and their associated microflora would be more apt to have greater soil/root surface contact and be able to
penetrate small pores than species with a coarse taproot system. Mycorrhizae
may provide additional advantages because of their fine architecture ability to
increase the effective surface area of the root as well as their microbial metabolic
2. Metabolism within the Plant
Humankind was not the first to create biologically disruptive organic compounds and place them in a soil-plant environment. Plants and their associated
microflora evolved in an environment where they were continually assaulted with
a wide array of microbial and plant toxins. Fungal and bacterial toxins are well
known (TeBeest, 1991; Yoder, 1980; Rice, 1974). Certain plants produce “allelopathic” chemicals that suppress the growth of other plants around them
(Putnam, 1985; Durbin, 1981). In addition to plant-produced herbicides, plants
also manufacture a wide range of compounds with adverse pharmacological
effects in herbivores. Rotenone and pyrethroids are plant-produced insecticides.
Coumesterol can alter the mammalian estrus cycles and decrease birth rates.
PHYTOREMEDIATION OF SOILS
Taxol can alter mammalian cell cycle (and cure some forms of cancer). The
genetic induction, enzymatic metabolism, and biological effects of some of these
plant components (e.g., flavonoids, isoflavonoids, and coumarins) are well studied (Stafford and Ibrahim, 1992; Cody e t a / . , 1986). Many of the initial steps in
their production from the PAL (phenyl alanine ammonium lyase) have been
cloned and successfully expressed in other plant tissues. This is an active area of
research and other important enzyme classes, pathways, and genes remain, no
doubt, to be discovered.
In many cases, there is a remarkable structural and chemical similarity between a toxic xenobiotic pollutant and either a natural toxin or a specific natural
enzyme substrate. This is not coincidental. Many acutely toxic xenobiotics gain
their “toxic” classification because they interfere with cell processes in ways
similar to natural products. Some agricultural products are specifically modeled
after a natural analog (e.g., bacterial glutamine synthase inhibitors are equivalent
to a commercial herbicide). It is also not uncommon to seek out plant extracts
and test them for bioactive compounds in the discovery of antimicrobial, antiinsect, and phytotoxic products. All of these natural compounds are synthesized
by biological systems, are tolerated by at least some members of the biological
community, and are finally degraded by organisms in the environment. It is
therefore not surprising to plant biochemists, pharmacologists, and traditional
medicine men that plants and their associated root zones have developed significant capacities to metabolize both natural and xenobiotic toxins.
Most of our knowledge about plant-based metabolism of xenobiotics comes
from the development of agricultural pesticides. This technical basis dramatically skews our knowledge base for two reasons. The first is that most studies
have been carried out on pesticides and not on industrial pollutants. Studies on
the uptake, translocation, tolerance or metabolism of industrial pollutants represent only 3% of the literature base (Nellessen and Fletcher, 1993). Second,
pesticides are generally tested on crop plants, most of which have little chance of
survival in truly contaminated soils. Tabulated data again show that 77% of the
studies were conducted on crop plants, and that the metabolic capacity of most
weeds is unstudied (Nellessen and Fletcher, 1993).
Many people unfamiliar with contaminated soils expect that such sites would
be barren of all vegetation. In some cases this is true, however, on most sites
hardy, tolerant, weed species exist. These “volunteers” spread out over time to
establish a general cover at most sites. Often sites that are heavily polluted are
colonized from the edges inward, with the rate of colonization seemingly dependent on contaminant load, physical soil factors, and general cultural conditions.
Many of these sites spontaneously revegetate, the most common exception being
those sites with active weed control programs. This spontaneous revegetation
phenomenon is probably not unfamiliar to those who have spilled oil or gasoline
on the lawn. In cases where spontaneous revegetation does not occur, fertilizer,
S. D. CUNNINGHAM E T AL.
loosening up the soil, and water may make dramatic improvements. We have
surveyed many contaminated areas and seen this spontaneous revegetation occurring across many soil types and climates. It is also interesting to note that the
species which seem to come into these areas are nearly unanimously the weed
species coincident with that region, These hardy weed species tend to have windor animal-borne seed distribution techniques and can be found growing in many
of the nutrient-poor, disturbed areas (e.g., road cuts, abandoned fields) throughout the region.
Perhaps not so coincidentally, it is many of these species that are problematic
weeds in farmers’ fields and are therefore specifically targeted in the development of new herbicides. In one author’s experience (S.D.C.) at least half of the
top 10 weeds targeted for new herbicides development can readily be found as
volunteers on contaminated soils in that region. As these casual field observations might indicate, and as the best efforts of hundreds of the world’s pesticide
chemists can attest to, certain weed species are difficult to kill and are relatively
insensitive to chemicals that easily kill crop plants. Given the skewed data base
on metabolic capacity of plants, it appears obvious that phytoremediation would
benefit from additional research in the study of weed metabolic capacity.
The results of agricultural product research also suggest there are wide differences in the ability of plants to metabolize xenobiotics. The backbone of the
multibillion dollar selective herbicide business is based on this differential metabolism. Nearly all modem selective herbicides are selective due to plant metabolism. The tolerant plant selectively metabolizes the herbicide to a nontoxic
compound and remains unaffected, while the nontolerant weed species either
cannot metabolize the compound or metabolizes a nontoxic compound into a
toxic one, thereby committing suicide (Hathway, 1989). There are significant
differences between monocots and dicots in this capacity as well as between
individual genera and species that may be exploited in phytoremediation. Differences in plants’ abilities to metabolize environmental pollutants are also increasingly evident from screening at both the whole plant level (Schnoor er al., 1995;
Hughes and Saunders, 1995) and the cell culture level (Groeger and Fletcher,
Plant xenobiotic metabolism is remarkably similar to the types of xenobiotic
metabolism that occur in mammalian livers. Relatively lipophilic materials undergo an enzymatic attack, which results in more water-soluble compounds. In
mammalian systems the final disposition is often through excretory routes which
plants lack. In principle, however, both plant and liver metabolic systems can be
divided into the same three phases: transformation, conjugation, and final disposal. The final stage in plant metabolism consists of transport and compartmentalization of the metabolized products into cellular vacuoles, intracellular spaces, or
various cell wall components (Sandermann, 1992). Contributing to the comparison is the fact that two major enzyme systems responsible for liver detoxification
processes are also found in plants: ( 1 ) cytochrome P450 oxygenases, and (2)
PHYTOREMEDIATION OF SOILS
glutathione S-transferases. It is on the basis of these comparisons that plants have
been dubbed “green livers” (Sandermann el al., 1977).
Some genes necessary for the degradative capacities in plants are constitutively expressed. Many herbicides are metabolized in some plant species almost
immediately and there is no detectable lag phase. For phytoremediation processes that would rely on these processes functioning, concentration thresholds
would be expected to be fairly broad. In other cases, enzymatic activity is not
constitutive but induced. In these cases a low level of a pollutant may have one
fate in a plant and a higher level might have another. Much like certain microbial
systems, the presence of one toxicant (e.g., toluene) can induce metabolic pathways that will then also degrade other pollutants. In plant metabolism, the
clearest example of a parallel phenomena is in the development of compounds
called “herbicide safeners.” These chemicals are applied either prior to the application of the pesticide or coincident with its application. Their purpose is to
trigger the production or activation of degradative enzymes which degrade the
herbicide before it has a lethal effect on the plant. Most herbicides kill plants by
interfering with a specific metabolic pathway or process. Sublethal quantities of
herbicide or analogs may also trigger this inducible metabolic activities in many
cases. Exogenously applied inducers of metabolic activity are an underutilized
laboratory and field tool in phytoremediation. A review of the area of these
metabolic enhancers and other potential manipulations of plant degradative capacity is provided by Hatzios and Hoagland (1989).
It is widely speculated that phytodecontamination systems might be most
appropriately managed by maximizing the various stress conditions on the plant
(chemical, fertilizer, planting densities, etc.). This is an approach considerably
different than managing a crop for conventional yield purposes. Much more
research in this area is needed prior to making informed field decisions.
Despite all the above discussion of plant degradative capacities, plant metabolic systems pale in comparison to their microbial analogs. Plant systems do not
have as broad a substrate range, nor can they act over as wide a concentration
range, as their microbial counterparts. This can easily be illustrated by comparing the respective abilities of plants and microbes to break aromatic and aliphatic
C-Cl bonds. In plants, there are perhaps four well-documented C-Cl bond
breakage reactions (Hathway, 1989). In microbes there are perhaps a dozen
(Neilson. 1990; Chaudry and Chapalamadugu, 1991). This type of comparison
has prompted the current research to exploit plant-microbial associations and/or
engineer plants for better metabolic activities.
3. Sequestration within the Plant
A xenobiotic compound entering the plant need not necessarily be metabolized
for successful phytoremediation. In some cases, it may be possible to use root
crops with high lipid contents to absorb lipophilic organics from the soil. The
S. D. CUNNINGHAM ET AL.
roots would then be harvested and processed. This type of phytoextraction of
certain contaminants from low-organic-matter soils may be possible. One replicated pilot scale experiment used carrots to remediate a DDT-contaminated soil.
Five carrot varieties were grown, harvested, solar-dried, and incinerated. DDT
levels were reported to decrease in a carrot variety dependent manner by 30 to
86% over the controls (McMullin, 1993). The need for harvesting and postharvest processing has economic consequences on the phytodecontamination
scheme; however, it would appear that phytoextraction and root harvesting may
be potentially viable in certain cases. There remain, however, significant questions concerning the general utility of such an approach across soil types and
Beyond phytoextraction, if plant roots could be demonstrated to take up a
pollutant and sequester it into an unavailable fraction, such a process might also
be useful as a basis for phytostabilization. In such a case it would be imperative
that the compound be so tightly sequestered that it would be essentially unavailable even to animals that might feed on the root tissue. This process has been
clearly demonstrated in the case of certain I4C-labeled pesticides that become
irreversibly bound into plant roots. Such residues are resistant to exhaustive
solvent, acid, alkali, and enzymatic extraction protocols. Furthermore, direct
feeding to rats does not result in release of the compound into the animal’s
system and the rat passes the labeled compound through the digestive system
with the other nondigestible fraction of the food (Kahn, 1982). Residues deposited in the lignin fraction of the plant seem to be relatively biologically inert. As
they do not appear in regulatory extraction protocols they are often considered
“degraded.” Additional research is needed to determine the extent to which such
materials are released upon the death and decay of the root.
V. PHYTOREMEDIATION EX PLANTA
A. Ex PLANTAENZYMATIC
As has been previously stated, plant enzymes can metabolize a wide variety of
xenobiotic pollutants. Plant degradative enzymes are not limited, however, to
functioning only internal to the plant root, stems, and leaves. These enzymes can
also be found in their active forms both in soil and in sediments.
The best characterized of these plant enzymes that occur external to the plant
root are certain oxido-reductases and laccases. More. recently, however, a greater
variety of active plant enzymes has been discovered in sediments far from their
plant source. These enzymes include dehalogenases, nitroreductases, and nitrilases (Schnoor et a l . , 1995).
PHYTOREMEDMTION OF SOILS
Some of the best studied oxido-reducatases are the peroxidases. These enzymes have been identified on the external root surfaces of such diverse plants as
cotton (Mueller and Beckman, 1978), wheat (Smith and O’Brien, 1979), cress
(Zaar, 1979), and tomato and water hyacinth (Adler et al., 1994). In tests with
water-borne contaminants the capacity of these plant enzymes to interact with
phenolic, aniline, and certain other aromatic contaminants is well documented
(Adler et al., 1994; Dec and Bollag, 1994). Due to analytical difficulties, similar
reactions are more difficult to clearly demonstrate in soils. Initial research in this
area by a number of labs, however, seems promising, and additional work is
The result of the action of oxido-reductases on pollutants is often the polymerization of the pollutant either onto the root surface or into the soil humic fraction.
Contaminants bound in such a manner are no longer available to most, if not all,
biological processes and ordinary chemical extraction protocols. Of particular
note is the fact that they are not extracted by regulatory mandated extraction
protocols. These polymeric complexes are often referred to as “bound residues.”
The overall enzymatic incorporation into the polymeric humic fraction of soils
has also been referred to as the “humification” process, and is depicted in Fig. 2
as part of phytostabilization. Some researchers in this field would suggest that the
humification process should be listed under the phytodecontamination category,
as the regulatory analytical results suggest. We consider humification a stabilization process, as certain analytical techniques, including some forms of hightemperature thermal extraction and super critical fluid extraction, have been
shown to release some of these bound residues.
There are significant differences between plants at the level of enzyme production. There may also be differences in the release rate and timing (age, season,
stress induced, etc.) although this remains to be tested. Certain plants (e.g.,
horseradish, Armoruciu rusticana) are cultivated for their root enzymatic capacities. Their value as a condiment and in commercial enzyme production is derived
from their peroxidase production. Their potential use in soil remediation is
untested; however, they have shown intriguing possibilities in water decontamination processes (Dec and Bollag, 1994). These investigators are currently
conducting a survey of plant laccase and peroxidase and initial results show
significant variations between plant species.
In addition to humification by enzymes directly derived from the growing plant, fungal symbionts, parasites, and saprophytes, living in conjunction
with the plant and its detritus, produce a wide variety of enzymes which may
be involved in humification (Bollag et a l., 1995). Individually. same of these
fungal species (e.g., white-rot fungi) are specifically targeted to pollutants
(Yadav and Reddy 1993) and are currently being used in some field scale remediations.
Degradation capacities depend not only on the production of these enzymes,