Vaccari D.A., Strom P.F., Alleman J.E. Environmental Biology for Engineers and Scientists

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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 releas ed into soils by the senescence (aging) and

decay of plant tissue, particularly that of ﬁne-root biomass. There is also a beneﬁcial phy-

sical 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 aera-

tion. 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 sub-

sequently 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 n ewly 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 avail-

ability 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 colo-

nized by various forms of bacteria and fungi, and the adjacent rhizospheric soils will com-

monly experience considerable increases in their microbial density. Population densities

of 5  10

total bacteria, 9  10

actinomycetes bacteria, and 2  10

fungi per gram of

air-dried soil have been observed.

This symbiotic relationship between plants and their adjacent microbial consortia sti-

mulates microbes, which in return assist the plants in securing nutrients and essential vita-

mins. 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., micro-

bial degradation). Compared to readily biodegradable compounds, recalcitrant organic

contaminants found in soils may not be directly oxidized by these root-zone microorgan-

isms 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 conver-

sion. 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.

678 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

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 (K

). K

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. K

tends to be inver-

sely related to the polarity and therefore the aqueous solubility of a compound. If a com-

pound has a K

value of 1, it has the same afﬁnity for water as it has for octanol (and by

implication, for lipids). If the K

value were 100, it has 100 times greater afﬁnity for

octanol (and lipids) than for water. Because of the wide range of K

values, it is

often reported as log K

, so the range of K

from 1 to 100 corresponds to log K

from 0 to 2. Compounds amenable to plant uptake reportedly have logK

values in the

range 0.5 to 3. Those materials having higher values, extending beyond the range of mod-

erately 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 q uite polar) that they would tend to remain withi n 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 pathwa ys that involve subsequent conjugation and compartmentation reac-

tions 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 accumu-

lated compounds are passed unchanged into these deposits; in other instances the material

being accumulated is that of degradation fragments produced through preceding biochem-

ical conversions that transformed the contaminants into nonphytotoxic metabolites. How-

ever, it is also possible that some plants may accumulate contaminants internally to a level

where an ecotoxicological hazard develops that would severely restrict subsequent con-

sumption 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 exam-

ple, 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.

SOIL AND GROUNDWATER TREATMENT 679

The next process, phytovolatilization, theoretically involves the uptake and transloca-

tion of contaminants into leaves; plants may then release these compounds into the atmo-

sphere through a volatilization mechanism. One particular plant, arabidopsis (in the

mustard family) has been found to produce a speciﬁc enzyme, mercury reduc tase,

which reduces mercury to elemental mercury, which is then amenable to volatilization

and release. Yet another known volatilization seque nce 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 disp osed 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., evapotranspiration). A single, mature willow tree, for example,

can transpire more than 19 m

of water each day (5000 gallons, or about 3.5 gal/min),

and 1 ha (10,000 m

) 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 environ-

mental preferences. Some plants tend to have shallow roots (i.e., cottonwoods and willows

roots), whereas phreatophyte 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 phytoremediat ion 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 conce ntrations, and the relative depth of the existing

residuals.

Concerns regarding soil type stem from the fact that various plants have different pre-

ferences 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.

680 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

TABLE 16.16 Phytoremediation Agents Relative to Contaminant Species

Organic

Contaminants

Phytoremediation

Agent

Inorganic

Contaminants

Phytoremediation

Agent

Atrazine Alfalfa Arsenic Bluebells

Maple tree

Cattail

Water lily

DDT Pine tree Lead Alpine pennycress

Mangrove Indian mustard

Alyssum

Duckweed

Brass buttons

Total petroleum

hydrocarbons

Alfalfa

Sorghum

Rye grass

St. Augustine grass

Hybrid poplars

Cadmium Duckweed

Brass buttons

Alyssum

Cabbage

Sunﬂower

Turnips

BTEX Alfalfa

Sorghum

Rye grass

St. Augustine grass

Hybrid poplars

Cesium Sunﬂower

Gasoline Daisy Chromium Alyssum

Alfalfa Cabbage

Rye grass Turnips

St. Augustine grass

Hybrid poplars

Chlorinated solvents

(TCE, PCA, etc.)

Hybrid poplars

Horsetail

Cobalt Sunﬂower

Polychlorinated Crabapple Copper Duckweed

biphenols Osage orange Brass buttons

Mulberry Alyssum

Cabbage

Sunﬂower

Turnips

Polycycloaromatic Mulberry Lead Sunﬂower

hydrocarbons Hackberry

Rye grass

Mulberry

Pentachlorophenol Rye grass Mercury Arabidobsis

Mulberry Duckweed

Brass buttons

Nitroaromatics

(TNT, etc.)

(via nitroreductase)

Hornwart

Parrot feather

Eurasian water milfoil

Selenium Duckweed

Brass buttons

Brassica

Rice

Broccoli

Cabbage

Ammonia Alfalfa Strontium Sunﬂower

Nitrate Alfalfa Uranium Sunﬂower

Zinc Sunﬂower

SOIL AND GROUNDWATER TREATMENT 681

The majority of phytoremediation plant systems, such as those depicted schematically

in Figure 16.61, apply to soil depths extending to the ﬁrst 2 to 3 m. In turn, most of these

plants probably draw their water either from roots closely aligned to the surface or water

drawn from vadose-zone pore moisture, as depicted in Figure 16.62. Most poplars, for

instance, tend to have shallower root systems, and as a result, these types of plants

have an inherent level of reliance on water being precipitated into, and then passed

through, the surface soils.

However, there are also deep-rooting plants that maintain a more water-loving life-

style, in which their root systems extend into the capillary soil region or underlying satu-

rated ground water zone (see Figure 16.63). These phreatophytic plants tend to have

remarkable high summertime water uptake rates, possibly as a competitive means of try-

ing to restrict the growth of their fellow plants. These deep-rooting plants generally have

higher levels of plant biomass as well as higher overall growth rates.

With plants such as poplars, although they may not normally pursue this sort of phrea-

tophytic growth mode, it is possible to induce these plants to form deep root systems using

special drip-irrigation practices that effectively train them to pursue progress ively deeper

levels of water uptake. Aside from extending the potential reach of these phytoremediat-

ing plants to lower levels, this induction of a deep-rooting preference helps the plan t with

its future water demand. However, at these deeper strata the matter of oxygen availability

subsequently becomes a concern, as does the presence of nutrients. Furthermore, deeper

soils usually tend to be more tightly packed, such that root impedance may also be an

important factor.

When motivated to adopt this phreatophytic mode, though, alders, ash, aspen, river

birch, and poplar have proven to be fast growers and rapid water user s, with daily uptake

rates during peak summert ime periods ranging from 100 to as much as 1000 L/day per

tree. The resulting uptake of water from the deep, saturated soil zone may actually pro-

duce a sizable depression of 5 to 10 cm in the water table within the capture zone where

water is being used by the trees. Field studies of this sort have been able experimentally to

Figure 16.61 Representative phytoremediation plant variations.

682

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

develop hydraulic barrier strips usin g deeply rooted trees planted in rows aligned perpen-

dicular to the direction of travel for the contaminated plume (as in Figure 16.64).

Of course, the actual process of evapotranspiration depends not only on the location

and depth of the roots, but also on the numb er of plant and atmospheric parameters.

The rate of water use by plants depends on the conductance of water through the plant

stoma as well as the cumulative surface area of the leaves through which the water

will ﬁnally be released into the atmosphere. The air temperature, wind speed, humidity,

and radiation intensity will also play a part in the ﬁnal rate of this release.

For those phytoremediation systems that employ trees, the spacing provided with the

original plants can also prove to be an important factor. Of course, trees spaced too far

apart will not be able to provide nearly as much of a remediating impact, but plants spaced

too tightly together can also experience problems, partic ularly as they reach maturity. In

particular, tightly bunched trees block each other’s light, and they will probably end up

contending for limiting nutrients. Recommended estimates for seedling plantings are in

the neighborhood of several thousand seedlings per hectare, which will drop to a level

of a few thousand after natural thinning takes place during the ﬁrst few years of growth.

Clearly, there are instances in which phytoremediation systems can play a signiﬁcant role

in site restoration, but it is not a universal solution for every contaminated soil. In some

Capillary Fringe

Groundwater Zone

Phytoremediation

Activity

Phytoremediation

Activity

Contaminated Soil Zone

Figure 16.62 Surface and vadose-zone contaminant remediation via phytoremediation.

SOIL AND GROUNDWATER TREATMENT 683

cases, the contaminants may simply be toxic to the plants, and in others the soils will not

be amenable to plant growth. Many strongly sorbed contaminants, such as PCBs or PAHs,

may be too tightly adhered onto soils to be amenable to rhizospheric degradation or

uptake. There will also be instances involving contaminants that would be bioaccumulated

into the plants (either directly or as metabolic intermediates) in forms and at concentra-

tions that are unsafe and unacceptable.

16.7.2 Bioremediation

The process of removing, or at least signiﬁcantly degrading, the presence of contaminants

in soils and groundwater using biology at the microbial level alone is known as bioreme-

diation (Anderson et al., 1993). Bacteri a and fungi play the dominant role in these sys-

tems. They metabolically degrade a variety of industrial, and in some instances even

hazardous, wastes into less toxic, or perhaps nontoxic, products. There are two basic

options to the circumstances under which a bioremediating activity might develop: the

intrinsic and the engineered options. The underlying differences between them are related

to the source of the responsible microbial agents and whether their presence stems from

Groundwater Table

Depression Induced with

Evapotranspiration by

Groundwater Table

Depression Induced with

Evapotranspiration by

Deep-Rooted Phytoremediation Plants

Groundwater Flow

Drawn Toward

Phytoremediation Site

Figure 16.63 Groundwater contaminant remediation via phytoremediation.

684

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

naturally occurring organisms or a manipulated, and possibly even externally seeded,

colonization.

At those sites where intrinsic bioremediation has been found to occur, the environmen-

tal conditions and contaminan t characteristics of the site (i.e., types of contaminants, rela-

tive concentrations, etc.) are such that microbes already present in the soil and/or

groundwater will naturally begin the remediating process of chemical degradation. In

fact, it is likely that many contaminated sites could experience some degree of intrinsic

bioremediation, given the biochemical diversity and metabolic ﬂexibility of the microbial

world. However, the unfortunate reality of intrinsic bioremediation is that it often fails to

develop at a rate or degree that is sufﬁciently robust to fully remediate a given site within a

time frame that is acceptably short. There are many reasons for these shortcomings. The

chemistry of the soils might not have been conducive to microbial growth, as would be

the case with pH extremes or nutrient limitations. There may also be physical limitations

or constraints with these soils, created perhaps by extreme (whether high or low) moisture

contents or unduly low soil permeabilities that hinder the movement of the bacteria or

the necessary substrates and nutrients. Yet another set of problems could develop due

to the contaminants themselves, which might either be recalcitrant in form or present

at levels sufﬁciently high to inhibit, or possi bly even kill, the existing microbes.

The strategy of invoking engineering methods was developed to enhance the attainable

rate and efﬁcacy of bioremediation. A wide variety of engineered bioremediation proce-

dures have now been developed, including plans that attempt to optimize the environmen-

tal conditions under which these microbial reactions might occur and also to instigate and

promote the growth of suitabl e microorganisms. These engineered bioremediation sys-

tems can largely be subdivided into two primary options (see Figure 16.65) In situ treat-

ment means, literally, ‘‘in place,’’ as opposed to ex situ treatment, in which soil is

excavated for treatment aboveground.

Phytoremediation

- Contaminant degradation

- Plume containment

Original Contaminant

PlumeMigration

Phytoremediation

Contaminant degradation

Plume containment

Original Contaminant

PlumeMigration

Original Contaminan

Plume Migra

ion

Figure 16.64 Groundwater uptake leading to contaminant plume containment.

SOIL AND GROUNDWATER TREATMENT 685

The synopsis given in le 16.17 provides an overview of the basic contaminant cate-

gories with which bioremediation has already proven to be a potentially suitable proce-

dure, as well as information regarding the relative characteristics of the various chemical

forms and the appropriate mode (aerobic or anaerobic) of their biochemical transforma-

tion. Several factors play an important role in this regard, including that of the

Soil and

Groundwater

Removal

Ex-Situ

Bioremediation

Above-Ground

Treatment

In-Situ

Bioremediation

Groundwater

Table

Figure 16.65 Engineered bioremediation options.

TABLE 16.17 Contaminant Forms Relative to Potential Bioremediation Treatment

Biodegradability

In Situ

Volatility

Solubility

——————————— Treatment

Contaminant Compounds ð/ K

) ð/ K

) Aerobic Anaerobic Potential

Hydrocarbons

Gasoline (C

–C

) þþþ Yes

Kerosene (C

–C

) þ/þþ Ye s

Domestic fuel (C

–C

) 

þ/ Yes

Lubricants (C

–C

)  No

Aromatic hydrocarbons (BTEX) þþþþ/ Yes

Polycyclic aromati chydrocarbons

Light (2-3 rings) þ/þ/

þ Yes

Heavy (4-5 rings) 

 No

Chlorinated hydrocarbons

Aliphatics þþþ/ Yes

Chlorobenzene þþþ Yes

PCB  No

Pesticides þ/ No

Heavy metals þ/

 Yes

Contaminants that tend to be volatile (with high K

values) are listed as þ.

Contaminants that tend to be soluble in water (with low to moderate K

values) are listed as þ.

Contaminant solubility can be increased by detergents (for hydrocarbons) or acidiﬁcation (for heavy metals).

Source: Adapted from Staps (1990).

686 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

compound’s molecular weight and conﬁguration, solubility, volatility, and degree of

halogenation. For example, the list of suitable hydrocarbon contaminants extends from

lighter fuels such as gasoline well into the heavier fuel range of 10 to 20 carbon atoms

(Kostecki and Calabrese, 1991). However, beyond this level, the heavy carbon compounds

(e.g., lubricants) are considerably less amenable to successful biodegradation. This pattern

extends to the family of aromatic hydrocarbons, where smaller two- to three-ring forms

may be biodegradable, whereas larger, four- and ﬁve-ring compounds are likely to be

highly recalcitrant.

The biochemically resistant nature of the larger aliphatic and aromatic hydrocarbons

stems not just from their size but also from their corresponding shift toward lower levels

of water solub ilities, as quantiﬁed by a determination of their octanol-to-water partition-

ing coefﬁcient (K

). This variable qualiﬁes the extent to which any given compound is

attracted to (and miscible within) water as compared to its corresponding solubility in a

nonpolar solvent, octanol. Contaminants regarded as water-loving or hydrophilic will

have negative log K

values, by which their solubility naturally ensures a maximal

opportunity for metabolic uptake and degradation. However, as the K

increases, the

prospects for successful bioremediation begin to drop.

Moderately hydrophobic compounds with log K

values extending up to 1 to 2 will

still be suitably available. The data given in le 16.18 for the BTEX (benzene, toluene,

ethylbenzene, and xylene) compounds exemplify the upper end of these permissible

values. Even though their log K

ﬁgures range from 2.12 (benzene) to 3.26 (xylene),

these contaminants are all amenable to full aerobic catabol ism.

Nonpolar organic liquids that have low aqueous solubility, and thus form immiscible

phases, are commonly referred to as nonaqueous phase liquids (NAPLs), of which there

are two versions, dense NAPL (DNAPL) and light NAPL (LNAPL), depending on

whether they are more or less dense than water, respectively. Most hydrocarbon solvents

are LNAPLs (e.g., benzene or mixtures such as gasoline), whereas most chlorinated

hydrocarbon solvents (e.g., trichloroethylene, perchloroethylene) are DNAPLs. The sorp-

tive preference of these NAPL forms to coalesce on subsurface soils substantially limits

their available surface area and correspondingly constrains their effective bioavailability.

There are engineering strategies to counter this behavior, however, by which surfactant

(e.g., soaplike agents) chemicals can be introduced to increase the relative solubility of

low-solubility contaminants (e.g., NAPLs) in a fashion which then enhances their pro-

spective availability for metabolic degradation.

The volatility of a contaminant may also affect its rate of biodegradation, particula rly

given the fact that many bioremediation systems require aeration to enhance the

TABLE 16.18 Representative Aromatic Hydrocarbon (BTEX) Properties

Property Units Benzene Ethylbenzene Toluene Xylene

Empirical formula C

Molecular weight 78.12 106.18 92.15 106.18

Density at 20



C g/cm

0.88 0.87 0.87 0.87

Water solubility mg/L 1750 152 535 198

Henry’s constant (K

) atm m

/mol 5:59  10

3

6:43 10

3

6:37 10

3

7:04 10

3

Octanol–water (log K

) Log K

2.12 3.14 2.73 3.26

Partitioning coefﬁcient

Source: Adapted from Verschueren (1983).

SOIL AND GROUNDWATER TREATMENT

687