Bioremediation and natural attenuation

Many natural resources show some degree of anthropogenic impact, includingthe widespread contamination of groundwater aquifers by hazardous wastes.. Table 1.1 provides some statistics on

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dedicated to the memory of my father, Pedro Jose´ A´ lvarez-Chamorro, for hisunconditional support and outstanding example of how to live and how to die.

Pedro Jose´ J AlvarezSiempre habra´ nieve altanera

que vista al monte de armin˜o

y agua humilde que trabaje

en la presa del molino

Y siempre habra´ un sol tambie´n-un sol verdugo y amigo-que trueque en llanto la nieve

y en nube el agua del rı´o

Versos y oraciones del caminante,

Leo´n Felipe

This book is dedicated to my family, Carla Marie, Walter Ricardo, and VictorArthur, for their unfailing love and support They are the ones who endured me notbeing around on nights and weekends to work on this book It is also dedicated to

my parents, Hatsue Yamamoto Illman and William Richard Illman, for nurturingand educating me to this date Without them, I would not be where I am now.The following classical Japanese poem signifies how we need to continue changingand to strive to develop new technologies because life is impermanent

Walter Arthur Illman (Yamamoto Sho)

Heike Monogatari

6 Numerical Modeling of Contaminant Transport, Transformation,

7 Field and Laboratory Methods to Determine Parameters for

Modeling Contaminant Fate and Transport in Groundwater 283

9 Performance Assessment and Demonstration of Bioremediation

Appendix A Chemical Properties of Various Organic Compounds 527Appendix B Free Energy and Thermodynamic Feasibility

Appendix C Commonly Used Numerical Groundwater Flow and

vii

Appendix D Nonparametric Statistical Tests for Determining the

Appendix E Critical Values of the Student t-Distribution 557

Groundwater represents about 98% of the available fresh water of the planet Thus,protecting and restoring groundwater quality is of global strategic importance Onecommon threat to groundwater resources is soil and aquifer contamination byhazardous wastes This widespread problem represents a significant technical andeconomical challenge because underground contamination is difficult to locateand remove by traditional extraction and excavation methods Consequently,there is a need for wider application of cost-effective, in situ remediation approachesthat take advantage of natural phenomena, such as bioremediation and naturalattenuation

This book aims to provide fundamental biological, chemical, mathematical, andphysical principles related to the fate and transport of hazardous wastes in aquifersystems, and their connection to natural attenuation and bioremediation engineer-ing The book is based on the authors’ extensive experience as educators, research-ers, and consultants in environmental biotechnology and hydrogeology and ismeant to serve as a textbook for advanced undergraduate or graduate students inenvironmental engineering and related sciences who are interested in the selection,design, and operation of groundwater treatment systems This work is also intended

to serve as a reference book for practitioners, regulators, and researchers dealingwith contaminant hydrogeology and corrective action Thus, recent advances andnew concepts in bioremediation are emphasized throughout the book

It is difficult to write a book about emerging technologies that have not reachedpedagogical maturity, as is the case with bioremediation and natural attenuation.The implementation of these technologies has experienced a relatively rapid growth

in the past ten years, from negligible levels to more than 15% of all hazardous wastesite remediation approaches Furthermore, monitored natural attenuation has beenselected for managing more than 50% of all sites contaminated by petroleum pro-duct releases from leaking underground storage tanks This recent increase indemand suggests the need for a textbook that organizes, synthesizes, and analyzesscientific and technological information to rapidly discern the applicability, merits,and limitations of these innovative technologies Such a text is useful not only forprocess selection and performance evaluation, but also to enhance the acceptance ofbioremediation and natural attenuation as bona fide mainstream technologies Manydecision makers have historically considered the application of biological processes

to treat natural systems as unpredictable and unreliable Such erroneous perception

ix

dates back to the origins of bioremediation, when failures to meet the expectationsraised by technology salespeople occurred sporadically due to inadequate under-standing of process fundamentals and inappropriate process design and implemen-tation We hope that this book will address such deficiencies and stimulate theoptimization and innovation of environmental biotechnologies.

We are very grateful for the encouragement and stimulating discussion andreviews provided by our friends and colleagues, Richard Heathcote, Joe Hughes,John McCray, Larry Nies, Gene Parkin, Michelle Scherer, Jerry Schnoor, RichardValentine, Timothy Vogel, Herb Ward, and You-Kuan Zhang We thank Andy Craigfor making many of the illustrations in the book We also would like to thank ourstudents in environmental engineering and earth sciences for their editorial contri-butions and intellectual sparring As engineering educators, we strive to buildbridges for students to link theory and sustainable practice and hope that some ofour students will maintain these bridges and build better ones for future generations

PEDROJ J ALVAREZ

WALTERA ILLMAN

Rice University

University of Iowa

INTRODUCTION TO

BIOREMEDIATION

‘‘Water, water everywhere, nor any drop to drink.’’

—The Rime of the Ancient Mariner, Samuel Taylor Coleridge

‘‘All the water that will ever be is, right now.’’

—National Geographic, October 1993

1.1 ENVIRONMENTAL CONTAMINATION BY HAZARDOUS

SUBSTANCES: MAGNITUDE OF THE CONTAMINATION

PROBLEM

Throughout human history, many societies have developed and thrived at theexpense of inefficient and unsustainable exploitation of the environment In thetwentieth century, the tension that existed between civilization and nature grew dis-proportionately For example, about 39–50% of the land surface of our planet wasmodified due to human activities such as agriculture and urbanization, and theatmospheric CO2concentration increased by 40% over the past 140 years (mainlydue to hydrocarbon combustion and deforestation), which raises serious concernsabout global warming (Intergovernmental Panel on Climate Change 2001) Biodi-versity has also been significantly impacted, and more than 20% of bird specieshave become extinct in the last 200 years (Wilson 2002)

Many natural resources show some degree of anthropogenic impact, includingthe widespread contamination of groundwater aquifers by hazardous wastes This

is particularly significant because groundwater represents about 98% of the able fresh water of the planet (Figure 1.1) Table 1.1 provides some statistics on themagnitude of the environmental contamination problem in the United States, andTable 1.2 summarizes the main sources of groundwater contamination The factthat we are already using about 50% of readily available fresh water makes ground-water protection and cleanup of paramount importance

avail-Bioremediation and Natural Attenuation: Process Fundamentals and Mathematical Models

1

97.41%

Ice caps and glaciers 1.984%

Groundwater 0.592%

Lakes 0.007%

Soil moisture 0.005%

Atmospheric water vapor 0.001%

Rivers 0.0001%

Biota 0.0001%

Oceans

97.41%

Ice caps and glaciers 1.984%

Groundwater 0.592%

Lakes 0.007%

Soil moisture 0.005%

fresh water accounts for less than 3% of the total amount of water available (including ice caps) Less than 1% of the world’s fresh water (or 0.01% of all water) is usable in a renewable fashion If ice caps and glaciers are ignored, groundwater represents about 98% of the available fresh water (From Speidel and Agnew 1988.)

Table 1.1 Magnitude of the Hazardous Waste Contamination Problem in theUnited States

United States generates 100 million tons of hazardous wastes per year

In 1987, 4 million tons of toxic chemicals was released to streams, 0.87 million tons wasdischarged to wastewater treatment plants, 1.2 million tons was disposed in landfills, 1.5million tons was injected into deep wells for disposal

There are 300,000–400,000 sites in the United States that are highly contaminated bytoxic chemicals and require remedial action

As of 1997, cleanup activities had not began at 217,000 of these sites

Forty million U.S citizens live within 4 miles of a Superfund site (there are thousands ofSuperfund sites)

About 440,000 out of 2 million underground tanks storing gasoline in the United Stateshave leaked (11 million gallons of gasoline leaked every year in the U.S., and BTEXaccount for 60% of soluble components)

Fifty percent of U.S population drinks groundwater, and 1–2% of readily availablegroundwater is contaminated (1.2 trillion gal of contaminated groundwater infiltratesevery day)

According to a 1982 EPA survey, 20% of drinking water wells showed contamination bysynthetic organic chemicals Approximately 30% of 48,000 public drinking water systemsserving populations in excess of 10,000 were contaminated, and 3% of these systems hadgroundwater contamination in excess of EPA standards

The estimated cost of environmental cleanup and management for the United States is onthe order of $1 trillion

Sources: LaGrega et al (1994) and NRC (1994, 1997).

Remediation costs for sites contaminated with hazardous wastes in Europe areexpected to exceed $1.5 trillion in the near future (ENTEC 1993) In the UnitedStates, the Office of Technological Assessment (OTA) of the U.S Congress esti-mates that the cost of cleaning up more than 300,000 highly contaminated siteswill exceed $500 billion (National Research Council (NRC) 1994) This does notinclude costs associated with about 440,000 sites impacted by gasoline releasesfrom leaking underground storage tanks (U.S Environmental Protection Agency(USEPA) 2003) or about 19,000 landfill sites used for disposal of municipal andindustrial wastes (USEPA 1989) Thus, there is an urgent need for cost-effectivetreatment approaches.

Bioremediation—which will be broadly defined here as a managed or neous process in which biological catalysis acts on pollutants, thereby remedying

sponta-or eliminating environmental contamination present in water, wastewater, sludge,soil, aquifer material, or gas streams—holds great potential as a practical andcost-effective approach to solve a wide variety of contamination problems There-fore, it is expected that bioremediation will play an increasingly important role inthe cleanup of soils, sediments, and groundwater contaminated with hazardousorganic chemicals

1.1.1 Common Groundwater Pollutants

Groundwater contamination by hazardous substances is commonly the result ofaccidental spills that occur during production, storage, or transportation activities.Table 1.3 lists the top 25 hazardous groundwater contaminants in North Americaand Europe Appendix A includes a more exhaustive list of organic pollutantsthat contaminate groundwater aquifers, including their physicochemical properties

As will be discussed in the following chapters, many of these ‘‘traditional’’hazardous wastes can be degraded by a wide variety of chemical (Chapter 2) andbiological mechanisms (Chapter 3), and bioremediation can be utilized to enhancetheir removal to different degrees of efficacy

Table 1.2 Principal Sources of Groundwater Contamination in the

United States

Leaking underground storage tanks (450,000)

Sources: LaGrega et al (1994) and NRC (1994, 1997).

The most common classes of organic groundwater pollutants includearomatic hydrocarbons (Figure 1.2), chlorinated solvents (Figure 1.3) and pesticides(Figure 1.4) Common inorganic groundwater pollutants include nitrate (NO3),arsenic (As), selenium (Se), and toxic heavy metals such as lead (Pb), cadmium(Cd), and chromium (Cr6þ) Such metals and radionuclides (e.g., uranium, techne-tium, and strontium) cannot be destroyed by microorganisms Thus, their removaldoes not commonly rely on bioremediation However, as discussed in Chapter 3,many microorganisms can mediate oxidation–reduction reactions that decreasethe toxicity and/or mobility of these inorganic species, resulting in enhanced riskreduction The following discussion will focus on common priority pollutantsthat are treated using bioremediation.

Petroleum Hydrocarbons

The extensive use of petroleum hydrocarbons as fuels and industrial stock hasresulted in widespread soil and groundwater contamination by this group of con-taminants Common sources of contamination include leaking underground storagetanks, pipelines, oil exploration activities, holding pits near production oil wells,and refinery wastes

Petroleum hydrocarbons comprise a diverse group of compounds, includingalkanes, alkenes, and heterocyclic and aromatic constituents Jet fuel, for example,typically contains more than 300 different hydrocarbons Gasoline, which is a verycommon groundwater pollutant that is amenable to bioremediation, is also a com-plex mixture The most abundant gasoline constituents are generally isopentane,p-xylene, n-propylbenzene, 2,3-dimethylbutane, n-butane, n-pentane, and toluene,which together make up over 50% of the mixture However, the most important

Table 1.3 The 25 Most Frequently Detected Priority Pollutants at HazardousWaste Sites in North America and Europe

13 1,2-Dichloroethene (1,2-DCE)

Source: NRC (1994).

constituent from a public health risk perspective is probably benzene, which ishighly soluble (and thus highly mobile in aquifers) and is a known human carcino-gen Indeed, the drinking water standard for benzene (5 mg=L) is much more strin-gent than that of other monoaromatic hydrocarbons such as toluene (1000 mg=L)and xylenes (10; 000 mg=L), and the presence of benzene is often the drivingforce for the bioremediation of gasoline-contaminated sites.

Another important group of pollutants are the polynuclear aromatic bons (PAHs), which are commonly found near coal conversion facilities and petro-leum plants These hydrophobic pollutants are of concern to both public andenvironmental health because of their tendency to concentrate in food chains(McElroy et al 1989) and acute toxicity (Heitkamp and Cerniglia 1988), andsome PAHs (e.g., benzo[a]pyrene) are recognized mutagens and carcinogens(Mortelmans et al 1986) Consequently, the U.S Environmental Protection Agency(EPA) has listed 16 PAH compounds as priority pollutants (Keith and Telliard 1979)(Figure 1.2b) PAHs are the principal constituents of creosote, which is a complexmixture of about 200 compounds also containing phenolic and heterocyclicpollutants

hydrocar-Hydrocarbons are generally lighter than water and tend to float on top of thewater table if present in a separate organic phase—the so-called light nonaqu-eous-phase liquid (LNAPL) (Figure 1.5) However, different hydrocarbons in themixture exhibit different physicochemical properties that affect their transport,fate, and principal removal mechanism For example, short-chain alkanes tend to

be volatile and are readily stripped from groundwater whereas monoaromatichydrocarbons such as benzene, toluene, ethylbenzene, and xylenes (which are col-lectively known as BTEX) (Figure 1.2a) tend to be relatively soluble and are trans-ported over longer distances by groundwater In fact, BTEX typically representabout 60% of the water-soluble hydrocarbons in gasoline Furthermore, dissimilarhydrocarbons exhibit different levels of resistance to biodegradation Thus, biode-gradation and abiotic weathering processes (e.g., volatilization, sorption and dilu-tion) result in differential removal of specific hydrocarbons, which changes therelative composition of the hydrocarbon mixture over time

Chlorinated Compounds

Chlorinated aliphatic and aromatic compounds make up an important group oforganic pollutants that are both ubiquitous and relatively persistent in aquifers.Chlorinated ethenes fall into a class of chemically stable compounds commonlyknown as ‘‘safety solvents.’’ Because they are resistant to combustion and explo-sion, these compounds were widely used as industrial solvents and degreasers formost of the twentieth century The combination of extensive use, volatility, andchemical stability has led to widespread contamination of groundwater and soil

by such ubiquitous and recalcitrant pollutants Common volatile organic pounds (VOCs) in the chlorinated solvents group include tetrachloroethylene(PCE, CCl2CCl2), trichloroethylene (TCE, CCl2CHCl), dichloroethylene(DCE, CHClCHCl), and vinyl chloride or chloroethylene (VC, CH CHCl)

com-(Figure 1.3) All of these VOCs are potential carcinogens Groundwater tion by 1,1,1-trichloroethane (TCA) and chlorinated methanes, such as carbontetrachloride (CCl4) and chloroform (CHCl3), is also common.

contamina-Chlorinated solvents generally have a higher specific gravity than water and tend

to sink to the bottom of the aquifer if present in a separate organic phase—the called dense nonaqueous-phase liquid (DNAPL) (Figure 1.5) These DNAPLsrepresent a major challenge to site remediation due to their persistence and relativeinaccessibility

so-Unlike the chlorinated solvents, chlorinated aromatic compounds such as chlorobenzene and pentachlorophenol (which are common fungicides used as woodpreservers) or polychlorinated biphenyls (PCBs, which were common dielectricfluids in transformer oil) are similar to PAHs in terms of their potential carcinogeni-city and lipophilic nature (i.e., high affinity for fatty tissue), which is conducive to

alluvial aquifer Oil has a lower specific gravity than water and floats on the water table, forming a light nonaqueous-phase liquid (LNAPL) Trichloroethylene (TCE), on the other hand, is heavier than water and sinks, forming a dense nonaqueous-phase liquid (DNAPL) The dissolved phase travels with flowing groundwater (Adapted from Pankow and Cherry 1996.)

bioaccumulation These compounds also have a strong tendency to sorb to soil andaquifer sediments, and their dispersal is often due to cotransport with sorbents such

as colloidal matter or eroded sediments

Pesticides

Agricultural applications of pesticide represent an important source of soil andgroundwater pollution Pesticides that contaminate aquifers are often insecticidesand herbicides, although some fungicides and rodenticides are also found Pesti-cides are problematic because they are designed to be persistent (for long-lastingaction), and many are lipophilic—often accumulating in animal’s fatty tissuethrough food webs

According to their chemical structure, pesticides can be classified as nochlorides, organophosphates, and carbamates (Figure 1.4) Organochloridessuch as DDT, Aldrin, Chlordane, Endrin, Dieldrin, Heptachlor, Mirex, and Toxa-phene represent an early generation of pesticides that are characterized by theirenvironmental persistence and high toxicity Modern pesticides tend to be organo-phosphates and carbamates Organophosphates such as Malathion, Methyl para-thion, and Diazinon are not as persistent in the environment but are more toxic

orga-to humans and can be absorbed through the skin, lungs, and intestines Carbamatessuch as Carbaryl and Baigon have also some side effects at acute exposure.Pesticides account for eight of the top-twelve list of persistent organic pollutantsidentified by the United Nations Environment Programme (i.e., the UNEP ‘‘dirtydozen’’) (Table 1.4), which are compounds that were recently banned worldwidefor production due to their ubiquitous distribution by atmospheric deposition,high bioaccumulation potential, and propensity to affect reproduction by disruptingthe endocrine system (Figure 1.6) However, no pesticides are included in the list

of top 25 priority pollutants frequently found at hazardous waste sites in North

Table 1.4 The United Nations Environment Programme

(UNEP) List of Top Twelve Persistent Organic Pollutants

(POPs) Recently Banned from Production Worldwide

are not produced for industrial purposes: these are toxic by-products that

may be formed during the chlorine bleaching process at pulp and paper

mills and are also released into the air in emissions from municipal solid

waste and industrial incinerators.

Source: UNEP (1999).

America and Europe (Table 1.3), which are predominantly contaminated withindustrial chemicals.

1.1.2 Emerging Pollutants

During the past decade, a wide variety of environmental contaminants have becomerecognized as potentially important in the fields of environmental science and engi-neering These include endocrine disrupting compounds produced by the pharma-ceutical and personal care products industries, oxidized energetic and rocketpropellants, and small ethers (Figure 1.7) In addition, there is mounting evidencethat pharmaceuticals, hormones, and other organic wastewater contaminants arebeing detected in water resources throughout the United States (Kolpin et al.2002) Little is known about the potential interactive effects of these compounds

on the biosphere

Endocrine disrupting compounds (EDCs) are of concern to environmental healthbecause of their potential to bioaccumulate through food chains and affect repro-duction (Sedlak and Alvarez-Cohen 2003) Although groundwater contamination

by EDCs is not yet a priority issue, it is important to recognize that surfacewater contamination by these compounds is widespread (possibly due to atmo-spheric transport and deposition) and that surface and groundwater resources areinterconnected Therefore, a brief review of this emerging class of pollutants isappropriate

Perfluorinated octanes (PFOs) are an important class of EDCs that constitute thebuilding blocks of 3M’s perfluorinated surfactants, such as Scotchgard1 The per-sistence of PFOs is partly due to the fact that the carbon–fluorine bond is one of thestrongest bonds in nature (110 kcal/mol) Potential adverse effects of PFOs includebioaccumulation, interference with mitochondrial bioenergetics and biogenesis,

pesticides

atrazine, simazine, diuron, bentazone, glyphosate human/

natural hormones estradiol, ß-sitosterol Endocrine disruptors

DDT, lindane, amitrole, vinclozolin synthetic

hormones ethinyl- estradiol analgesics,

veterinary

drugs

Industrial chemicals NTA, EDTA

bisphenol A, nonylphenol phthalates

DDT, lindane, amitrole, vinclozolin synthetic

hormones ethinyl- estradiol analgesics,

increase in liver size, neuroendocrine disruption, and acute aquatic toxicity Due tothese concerns, the 3M Company recently decided to remove this product line frommarket production.

Synthetic musk fragrances are another important class of EDCs These are volatile compounds used for perfumes, cosmetics, soaps, shampoos, and detergentsand are commonly released by down-the-drain disposal of consumer products.Similar to PFOs, these EDCs are subject to atmospheric distribution and deposition,which results in widespread and worldwide contamination at trace levels

semi-Polybrominated diphenyl ethers (PBDEs) are also an important class of EDCsthat are often referred to as the new PCBs These compounds are used as fire retar-dants in electronics, plastics, car upholstery, furniture, and cables PBDEs are alsovery persistent and tend to bioaccumulate in wildlife and human tissue An expo-nential increase in the concentration of PBDEs has been observed in fish tissue inthe Great Lakes and in marine wildlife, and levels on the order of pg/m3have beendetected in human blood and milk (Snyder et al 2003)

Oxidized rocket propellants and energetics represent another important class ofemerging pollutants and include the rocket propellants NDMA and perchlorate andthe military explosive RDX (Figure 1.7) These compounds are increasingly being

OCl OO -

N N

NO 2

4 −

O

OCl OO - O

OCl OO -

NO 2

O 2

NO 2

2

NO 2 2

found in groundwater aquifers, which is of concern due to their potential genicity NDMA, which can also be a by-product of wastewater chlorination, is anextremely potent carcinogen For example, the USEPA has proposed an action level

carcino-of 0:0007 mg=L (for 106 risk), which is orders of magnitude more stringent thanthe drinking water standard for common carcinogens such as benzene or trichlor-oethylene (5 mg=L)

Groundwater contamination by small ethers is also a widespread problem Themost common pollutants in this category are 1,4-dioxane, which is used as a chlori-nated solvent stabilizer, and MTBE, which is a common gasoline oxygenate that isadded to minimize air pollution by hydrocarbon and carbon monoxide emissionsduring combustion These compounds are suspected carcinogens and tend to impactlarge volumes of groundwater due to their high solubility, low tendency tovolatilize, and low tendency to be retarded by sorption The recent finding that1,4-dioxane is commonly found as a trace contaminant in chlorinated solventplumes has earned it the nickname of ‘‘the MTBE of the chlorinated solvent sites.’’The remediation of sites contaminated with some emerging pollutants can be achallenge, not only because of the persistent nature of the contaminants but alsobecause of logistic issues Specifically, the chemical structure of many new pharma-ceuticals and cosmetics are intellectual property, and standards for their analysismay be difficult to obtain Thus, it is very difficult to manage and control contam-ination that cannot be accurately measured Furthermore, there are multiple uptakepathways for compounds such as EDCs that are probably more important thandrinking contaminated groundwater For example, we ingest pharmaceuticals,spray insecticides in the air, apply fragrances, and sit on couches that have bothbrominated flame retardant and perfluorinated surfactant protectants Therefore,groundwater contamination by some of these emerging pollutants is viewed bymany decision makers as relatively low risk in relation to the groundwaterpollutants listed in Table 1.4 Consequently, this book will focus on the bioremedia-tion of common groundwater pollutants such as hydrocarbons and chlorinatedsolvents

1.2 HISTORICAL DEVELOPMET OF ENVIRONMENTAL

BIOTECHNOLOGY AND BIOREMEDIATION

The term ‘‘biotechnology’’ is analogous to ‘‘applied biology’’ and refers to theapplication of biological knowledge and techniques to develop products or to pro-vide beneficial services This term was coined in 1919 by the Hungarian engineerKarl Ereky, although the use of biological processes for the production of foodproducts dates back to the beginning of human history (Table 1.5) Common bio-technology products that date to ancient times include yogurt, cheese, wine, beer,and spirits Microorganisms are also used currently to produce large quantities ofindustrial products such as organic acids, solvents, polymers, enzymes, antibiotics,steroids, hormones, and fine chemicals for the pharmaceutical and cosmetic indus-tries The use of microorganisms to produce such compounds is motivated mainly

Table 1.5 Selected Biotechnology Landmarks

fermentation processes were established in the ancient world, notably inChina The preservation of milk by lactic acid bacteria resulted in yogurt.Molds were used to produce cheese, and vinegar and wine were manufactured

by fermentation

heredity is carried in the semen By analogy, he guessed there is a similar fluid

in women, since children clearly receive traits from each in approximatelyequal proportion

his students that all inheritance comes from the father The male semen, heasserted, determines the baby’s form, while the mother merely provides thematerial from which the baby is made He suggested that female babies arecaused by ‘‘interference’’ from the mother’s blood

nonliving matter Maggots, for example, were supposed to arise from horsehair

Egypt and Persia largely gave up brewing as a result of the influence of Islam.Fermented breads and cereals still maintained their hold in the African diet

to explain why maggots arise on rotting meat He observed that meat covered

to exclude flies did not develop maggots, while similar uncovered meat did.This is regarded as the first disproof of spontaneous generation and wasamong the first uses of a controlled experiment

ground glass lenses as a hobby, used his microscopes to make discoveries inmicrobiology He was the first scientist to describe protozoa and bacteria and

to recognize that such microorganisms might play a role in fermentation

are responsible for fermentation His experiments in the ensuing years provedthat fermentation is the result of activity of yeasts and bacteria andconclusively disproved the theory of spontaneous generation

time to best exploit the environment, a process he referred to as ‘‘naturalselection.’’ He theorized that only the creatures best suited to their

environment survive to reproduce

to inactivate microbes (that would otherwise turn the ‘‘vin’’ to ‘‘vin aigre’’ or

‘‘sour wine’’) while at the same time not ruining the flavor of the wine

by economic factors, since biotechnology is often less costly and environmentallyfriendlier than chemical synthesis Furthermore, biological processes can be used toproduce specific stereochemicals or complex organic molecules such as vitamin B12and riboflavin, which could not be feasibly produced by a chemical synthesisprocess.

Similar to many industrial processes, the main motivation to use biotechnologyfor environmental cleanup is also economics For example, bioremediation is gen-erally more cost-effective than conventional physical and chemical processes totreat excavated soil (Table 1.6) In addition, bioremediation can be a more practical

Table 1.5 (Continued)

Natural Science Society in Brunn, Austria Mendel proposed that invisibleinternal units of information account for observable traits, and that these

‘‘factors’’—which later became known as genes—are passed from onegeneration to the next

causal agent of a disease (i.e., (1) the suspected pathogenic organism should

be present in all cases of the disease and absent from healthy animals; (2) thesuspected organism should be grown in pure culture away from the animalbody; (3) such a culture, when inoculated into susceptible animals, shouldcause disease; and (4) the organism should be reisolated and shown to be thesame as the original)

Pasteur also concurrently developed a rabies vaccine

butanol) could be generated using bacteria

heredity, ‘‘genotype’’ to describe the genetic constitution of an organism, and

‘‘phenotype’’ to describe the actual organism, which results from a

combination of the genotype and the various environmental factors

wastewater, and the process is first used in Salford, England

structures he has discovered that contain extrachromosomal genetic material

complementary, antiparallel model for DNA

William Hayes discovered that plasmids can be used to transfer introducedgenetic markers from one bacterium to another

pipeline spill in Ambler, Pennsylvania

nutrient-amended groundwater

Sources: Alleman and Prakasam (1983), Brock (1961, 1990), Bunch and Hellemans (1993), and Hellemans & Bunch, (1988).

approach to clean up contaminated aquifers than traditional technologies such aspump and treat or incineration of contaminated soil Unlike many physicochemicaltreatment processes that mainly transfer the pollutants from one phase (or location)

to another, bioremediation offers a terminal solution Indeed, bioremediation oftendestroys organic pollutants, thereby eliminating future liability costs Therefore, it

is not surprising that bioremediation has recently gained an important place amongalternatives to clean up sites contaminated with a wide variety of hazardous wastes.The use of biological processes for waste treatment dates back to the RomanEmpire, although the systematic design and application of microorganisms totreat wastewater only began about one century ago The need to treat water andwastewater was traditionally driven by public health protection Prior to theAsian cholera epidemic of London (1848), deaths due to environmental contamina-tion received little attention In 1849, Dr John Snow, the father of modern epide-miology, demonstrated the connection between fecal contamination of a drinkingwater source (i.e., the Broad Street water pump in London) and deaths due to cho-lera (Stainer et al 1986) Around that time, the discoveries of Louis Pasteur and theetiological postulates of Robert Koch (Table 1.5) were also beginning to transformmicrobiology from an observational hobby to a hypothesis-driven experimentalscience This provided the basis for the rational development of environmental bio-technology and bioremediation

In 1860, Pasteur demonstrated the connection between chemical changes andmicrobial activities Pasteur also proved that fermentation was a microbial process,and in related experiments, he also conclusively disproved the theory of sponta-neous generation (advocated by Aristotle), which held the notion that life (e.g.,maggots) could originate from inanimate matter (e.g., horse hair)

In 1882, Dr Angus Smith demonstrated that municipal wastewater could be

‘‘stabilized’’ by aeration, and Ardern and Lockett advanced this concept to developthe activated sludge treatment process, which was first used in Salford, England, in

1914 (Alleman and Prakasam 1983) This process was named activated sludgebecause it relied on an activated mass of microorganisms to treat the wastewater.The development of biological treatment processes in the United States wasinitiated at the Lawrence Experimental Station in Massachusetts (established

Table 1.6 Costs of Alternative Methods to Treat Soil Contaminated with

in 1886), which was a unique facility aimed at experimental verification of differentpossible wastewater treatment procedures In fact, one of the greatest impulses forthe invention of the activated sludge process in England was the trip of Dr GilbertFowler to the Lawrence Experimental Station in 1912 Dr Fowler, from theUniversity of Manchester, undertook this journey as a consultant chemist to theManchester Corporation, and upon his return to England, he encouraged Ardernand Lockett to repeat the experiments with wastewater aeration he observed atthe Lawrence Experimental Station The activated sludge process soon found appli-cation outside the United Kingdom The first experimental activated sludge plant inthe United States was built in Milwaukee in 1915 with the help of Dr Fowler as aconsultant Imhoff performed the first tests with the activated sludge process in

1924 and the first full-scale plant was built in 1926 in Essen-Rellinghausen,Germany Other key engineers and scientists that contributed to the development

of the activated sludge process include Allen Hazen, chemical engineer at theLawrence station; George C Whipple, civil engineer at the Massachusetts Institute

of Technology (MIT); Harrison P Eddy, consultant from Boston; and William R.Nichols, chemistry professor at MIT

The history of in situ bioremediation is considerably shorter, and it reflects manyupturns and downturns as a result of political and economic forces (Table 1.7).Interest in the use of microorganisms to degrade specific hazardous organic chemi-cals probably dates back to Gayle (1952), who proposed the microbial infallibilityhypothesis Gayle postulated that for any conceivable organic compound, thereexists a microorganism that can degrade it under the right conditions If not, evolu-tion and adaptation would produce such a strain (Alexander 1965) This hypothesiscannot be proved wrong, because failure to degrade a contaminant can be attributed

to the researcher’s failure to use the right strain under the right conditions In otherwords, using a popular adage, ‘‘the absence of evidence is not in itself evidence ofabsence.’’

In the 1970s, environmental statutes of unprecedented scope passed, such as theOccupational Safety and Health Act (OSHA) of 1970, the Clean Air Act (CAA) of

1970, the Clean Water Act (CWA) of 1972, the Safe Drinking Water Act (SWA) of

1974, and the Toxic Substance Control Act (TSCA) of 1976 This regulatory sure stimulated interest in site remediation technologies, including bioremediation.However, bioremediation failed to meet the expectations raised by many technol-ogy salespeople who, for example, commonly advocated the addition of specializedbacteria to contaminated sites (i.e., bioaugmentation) Early proponents of thisapproach generally did not recognize that indigenous bacteria already present at

pres-a contpres-aminpres-ated site were probpres-ably better predisposed physiologicpres-ally pres-and cally to mediate the degradation of the target pollutants, but were not accomplish-ing this task because of a number of limiting factors discussed in Chapter 3 Theseinclude a lack of essential nutrients (including oxygen for aerobic processes), insuf-ficient access of the microorganisms to sorbed contaminants (i.e., lack of bioavail-ability), or unfeasible thermodynamics (e.g., attempts to oxidize highly chlorinatedpollutants that resist further oxidation and require reducing (anaerobic) conditions

geneti-to undergo biotransformation) The failure geneti-to meet high performance expectations

created a bandwagon effect and prompted a major downturn in bioremediationapplications.

In the 1980s, it became clear that a fundamental understanding of microbiology,microbial ecology, hydrogeology, and geochemistry was needed to successfully

Table 1.7 Historical Perspective of the Development of Bioremediation

sludge

Microbial infallibility hypothesis proposed by Gayle (1952), borne out ofaerobic lab studies

‘‘micropollutants’’ in wastewaters

(1976 RCRA and TSCA, 1980 CERCLA-Superfund) stimulates development

of remediation technologies Bioremediation successes are reported for thecleanup of aquifers contaminated by gasoline releases, and the firstbioremediation patent is granted to Richard Raymond (1974) Nevertheless,adding acclimated microorganisms to contaminated sites becomes commonpractice Earlier proponents of bioaugmentation often fail to recognize thatindigenous bacteria already present might be better suited genetically andphysiologically to degrade the pollutants, but biodegradation might be limited

by contaminant bioavailability or unfavorable redox conditions rather than by

a lack of catabolic potential Failure to meet expectations creates a bandwagoneffect and prompts a major downturn in bioremediation

processes inherent to bioremediation need to be understood before asuccessful technology can be designed This realization, along with the fear ofliability and federal funding (e.g., Superfund), stimulates the blending ofscience and engineering to tackle environmental problems and improvebioremediation practice

(microbial/chemical) approaches are developed (often reflecting that tion is part of innovation) However, many decision makers continue to regardbioremediation as a risky technology and continue to select (relativelyineffective) pump-and-treat (P&T) technologies for remediation purposes A

adapta-1994 NRC study reports that conventional P&T technologies restoredcontaminated groundwater to regulatory standards at only 8 of 77 sites.Superfund is depleted Poor cleanup record and resource allocation problemsstimulate paradigm shift toward natural attenuation and risk-based correctiveaction (RBCA)

as cost-effective cleanup alternatives for sites contaminated with a widevariety of organic pollutants, and the interest in bioaugmentation grows forenhancing the removal of recalcitrant compounds

design and implement bioremediation systems Bioremediation research and theblending of science and engineering to tackle environmental problems boomed inthe 1980s, partly stimulated by federal funding through initiatives such as the Com-prehensive Environmental Response, Compensation, and Liability Act of 1980(CERCLA) and its Superfund program Numerous bioremediation trials were suc-cessful—primarily those involving the clean up of petroleum product releases—and several hybrid technologies were developed However, many decision makersinsisted on pump-and-treat technologies that were relatively expensive and largelyineffective due to the hydrophobic nature of many pollutants that sorbed to the aqui-fer material This led to a poor cleanup record, which stimulated a recent paradigmshift toward monitored natural attenuation (Chapter 8) and risk-based correctiveaction (RBCA) (NRC 1997, 2000).

RBCA is a decision-making process for the assessment of and response to surface contamination, based on the protection of human health and environmentalresources The objectives of implementing risk-based corrective action are to (1)reduce the risk of adverse human or environmental impacts to appropriate levels(e.g., the maximum contaminant level or MCL) at the point where the receptor is

sub-or may be potentially located, (2) ensure that site assessment activities are focused

on collecting only information that is necessary to make risk-based correctiveaction decisions, (3) ensure that limited resources are focused toward those con-taminated sites that pose the greatest risk to human health and environmentalresources at any time, (4) ensure that the preferred remedial option is the most eco-nomically favorable one that has a high probability of achieving the negotiateddegree of exposure and risk reduction, and (5) evaluate the compliance relative

to site-specific standards (American Society for Testing and Materials (ASTM)1994)

RBCA utilizes a three-tiered approach in assessing petroleum-contaminatedsites In Tier 1 (which involves screening the contamination data with highly con-servative corrective action goals), sites are classified by the urgency of need forinitial corrective action Tier 1 assumes, for example, that the receptor will drinkthe groundwater with the highest contaminant concentrations within the site IfTier 1 analysis concludes that a site needs cleanup, the responsible party has twooptions: meet Tier 1 (most stringent) cleanup goals or conduct a Tier 2 evaluation

At the Tier 2 level (which considers site-specific conditions), the user is providedwith an option for determining site-specific cleanup levels and appropriate points ofcompliance (e.g., the receptor may be located offsite) The Tier 2 approach gener-ally involves the use of analytical models (Chapter 5) to estimate the risk at a point

of exposure This analysis may conclude that a site does not pose a significant risk

to existing receptors, and that monitoring rather than aggressive cleanup is all that isneeded However, similar to the previous step, if this analysis concludes that a siteneeds cleanup, the responsible party has two options: meet Tier 2 goals or conduct aTier 3 evaluation At Tier 3 (which also considers site-specific conditions), theassessment is similar to Tier 2 except that less conservative but more rigorous(and often more costly) numerical modeling (Chapter 6) and analysis isconducted to assess risk (ASTM 1994)

Overall, RBCA is intended to be a solution to a resource allocation problem, butits misuse or abuse could lead to an excuse to do nothing RBCA contains the man-dates that allow for passive bioremediation (natural attenuation), which involvesthorough monitoring and modeling of the contamination However, there is notreatment required if the contamination occurs at a sufficient rate that the plumedoes not spread Natural attenuation, by no means, is a do- nothing option Onehas to monitor the site and prove that natural attenuation is actually happening.Soil samples have to be taken and analyzed to determine the existence of microbes

at the site and the disappearance of contaminants Such analyses and monitoringcan be quite expensive and time consuming Nevertheless, these RBCA provisionsare not as stringent as those mandated by CERCLA and the Superfund Amendmentand Reauthorization Act of 1986 (SARA), where responsible parties are required toclean up everything to meet the compliance requirements (Bedient et al 1994).Thus, for RBCA at the Tier 2 and Tier 3 levels of analysis, it is necessary tohave models to predict the behavior of contaminants at a site (Chapters 5 and 6)

as well as evidence that the implemented remediation strategies are working(Chapter 9)

1.3 MERITS AND LIMITATIONS OF BIOREMEDIATION

Bioremediation is an emerging technology that holds great promise for the effective removal of a wide variety of environmental pollutants Successful applica-tions of bioremediation have been well documented for many sites contaminatedwith three major classes of hazardous wastes that are amenable to bioremediation:petroleum hydrocarbons (33% of all applications), creosotes (22%), and chlorinatedsolvents (9%) Bioremediation has also been applied, to a lesser degree, to cleanupsites contaminated with pesticides, munitions wastes, and other chemical mixtures.Note that these percentages include applications involving aboveground bioreactors

cost-to treat contaminated soil

Bioremediation offers several advantages and limitations compared to traditionalsite remediation approaches such as pump-and-treat or soil excavation followed byincineration (Table 1.8) The principal advantages generally include lower cost andthe ability to eliminate pollutants in situ, often transforming them into innocuousby-products such as CO2and water This eliminates potential liability costs asso-ciated with hazardous waste transportation and storage However, bioremediation isnot universally applicable and it may be marginally effective for recalcitrant pollu-tants or toxic heavy metals if the necessary catabolic capacity is not present orexpressed For example, adverse environmental conditions such as extreme pH,temperature, or the presence of heavy metals at toxic concentrations may hinderspecific microbial activities (Chapter 3)

Many hazardous substances are persistent in the environment when tal conditions are not conducive to the proliferation and activity of specific micro-organisms that can metabolize them Thus, successful bioremediation requires anunderstanding of site-specific factors that limit desirable biotransformations or

environmen-that result in unintended consequences such as the production of toxic metabolites(Chapter 3) However, many site-specific limitations to natural biodegradation pro-cesses can be overcome through engineered manipulations of the environment(Chapter 8).

The requirements for bioremediation are depicted in pyramidal fashion inFigure 1.8 In order of importance, first, we need the presence of microorganismswith the capacity to synthesize enzymes that can degrade the target pollutants.These enzymes catalyze metabolic reactions that often produce cellular energyand building blocks for the synthesis of new cell material Many contaminantdegradation processes involve oxidation–reduction reactions that are discussed inChapters 2 and 3 Briefly, a contaminant or a substrate may serve as an energysource (i.e., cellular fuel), and when it is oxidized, the substrate-derived electronsare transferred to an electron acceptor such as oxygen Therefore, the second level

of the pyramid shows that appropriate energy sources (i.e., electron donors) andelectron acceptors must be present The third level shows the need for sufficientmoisture and acceptable pH, and the fourth level reminds us of the importance

of avoiding extreme temperatures and ensuring the availability of inorganic

Table 1.8 Advantages and Disadvantages of Bioremediation Relative to

Traditional Remediation Approaches Such as Pump-and-Treat

Contaminated Groundwater and Incineration of Source Soils

Advantages

Cleanup occurs in situ, which eliminates hazardous waste transportation and liabilitycosts

salts) rather than transferred from one phase to another, thus eliminating long-termliability

Relies on natural biodegradation processes that can be faster and cheaper (at least 10 lessexpensive than removal and incineration, or pump and treat)

Minimum land and environmental disturbance

Can attack hard-to-withdraw hydrophobic pollutants

Environmentally sound with public acceptance

Does not dewater the aquifer due to pumping

Can be used in conjunction with (or as a follow up to) other treatment technologies.Disadvantages

Certain wastes, such as heavy metals, are not eliminated by biological processes (althoughmany metals can be bioreduced or biooxidized to less toxic and less mobile forms)

It may require extensive monitoring

Requirements for success and removal efficiency may vary considerably from one site toanother

Some contaminants can be present at high concentrations that inhibit microorganisms

Can be a scientifically intensive technique

There is a risk for accumulation of toxic biodegradation products

nutrients such as nitrogen, phosphorus, and trace metals Finally, at the base of thepyramid, we have three environmental requirements that are important for the sus-tainability of bioremediation: the absence of high concentrations of substances thatare toxic to the microorganisms, the removal of metabolites that may inhibit speci-fic microbial activities (perhaps by other members of the microbial community),and the absence of high concentrations of protozoa that act as predators on the bac-teria responsible for contaminant degradation.

In summary, bioremediation engineers and scientists need to satisfy the logical and nutritional requirements of specific degraders and ensure that a compe-titive advantage is provided to desirable (but not undesirable) biotransformationpathways Achieving this can be a very complex task, especially when mass transferlimitations and difficulties in distributing stimulatory materials are considered.Therefore, bioremediation is often implemented as an art rather than as an empiricalscience However, recent advances in our fundamental understanding of geochem-ical (Chapter 2), microbial (Chapter 3), and transport processes (Chapter 4) haveimproved our ability to control and improve the efficacy of bioremediation.Despite many significant technological and scientific advances in the past tenyears, bioremediation is currently an underutilized technology According to theOrganization for Economic Cooperation & Development (OECD), the global mar-ket potential for environmental biotechnology doubled during the 1990s to about

physio-$75 billion in the year 2000 In the United States, the cost associated with thecleanup of about 400,000 highly contaminated sites has been estimated to befrom $500 billion to $1 trillion (NRC 1994) However, the current bioremediationmarket is only about $0.5 billion (Glass 2000) This relatively small market shareprobably reflects an often erroneous perception that bioremediation is a risky tech-nology with uncertain results, cleanup time, and remediation costs Whereas this

MICROORGANISMS

ENERGY SOURCE

ELECTRON ACCEPTORS

ABSENCE OF TOXICITY

REMOVAL OF METABOLITES

ABSENCE OF PREDATORS

BIOREMEDIATION

may have been the case for some of the earlier empirical applications, the poration of sound scientific and engineering principles into the design of modernbioremediation systems has significantly improved the reliability and robustness

incor-of the process As discussed in Chapter 8, many different bioremediation strategiesexist that can be tailored to specific contamination scenarios Therefore, as thebandwagon effect caused by earlier shortcomings fades, bioremediation is likely

to achieve a more prominent role in hazardous waste treatment and site cleanup

We hope that this book will contribute to a broader acceptance of bioremediationand stimulate further optimization and innovation efforts

REFERENCES

American Society for Testing and Materials (1994) 1994 Annual Book of ASTM Standards:Emergency Standard Guide for Risk-Based Corrective Action Applied at PetroleumRelease Sites (Designation: ES 38-94) ASTM, West Conshohocken, PA

Alexander, M (1965) Biodegradation: problems of molecular recalcitrance and microbialfallibility Adv Appl Microbiol 7:35–80

Alexander, M (1999) Biodegradation and Bioremediation, 2nd ed Academic Press, SanDiego, CA

Alleman J and T.B.S Prakasam (1983) On seven decades of activated sludge history JournalWPCF 55(5):436–443

Bedient, P.B., H.S Rifai, and C.J Newell (1994) Groundwater Contamination: Transport andRemediation, PTR Prentice-Hall, Inc., Englewood Cliffs, NJ

Brock, T.D (1961) Milestones in Microbiology Science Tech Publishers, Madison, WI.Brock, T.D (1990) The Emergence of Bacterial Genetics Cold Spring Harbor LaboratoryPress, Cold Spring Harbor, NY

Bunch, B and A Hellemans (1993) The Timetables of Technology Simon & Schuster,New York

Hellemans, A and B Bunch (1988) The Timetables of Science Simon & Schuster, New York.Bioremediation Report (1993) King Publishing Group, Washington, DC

Cookson, J.T (1995) Bioremediation Engineering Design and Application McGraw Hill,New York

ENTEC (1993) Directory of Environmental Technology Earthscan Publications and LewisPublishers/CRC Press, Ann Arbor, MI

Gayle, E.F (1952) The Chemical Activities of Bacteria Academic Press, London.Glass, D.J (2000) The United States Remediation Market Report from D Glass Associates,Inc (http://www.channel1.com/dglassassoc/BIO/usrem.htm)

Heitkamp, M.A and C.E Cerniglia (1988) Mineralization of polycyclic aromatic carbons by a bacterium isolated from sediments below an oil field Appl Environ.Microbiol 54:1612–1614

hydro-Intergovernmental Panel on Climate Change (2001) The Third Assessment of ClimateChange United Nations

Keith, L.H and W.A Telliard (1979) Priority pollutants I—a perspective view Environ Sci.Technol 13:416–423

Kolpin, D.W., E.T Furlong, M.T Meyer, E.M Thurman, S.D Zaugg, L.B Barber, and H.T.Buxton (2002) Pharmaceuticals, hormones, and other organic wastewater contaminants

in U.S streams, 1999–2000: a national reconnaissance Environ Sci Technol 36:1202–1211

La Grega, M.D., P.L Buckingham, and J.C Evans (1994) Hazardous Waste Management.McGraw Hill, New York

McElroy, A.E., J.W Farrington, and J.M Teal (1989) Bioavailability of polycyclic aromatichydrocarbons in the aquatic environment In Metabolism of Polycyclic Aromatic Hydro-carbons in the Aquatic Environment, U Varanasi (Ed.) CRC Press, Boca Raton, FL.Mortelmans K., S Harworth, T Lawlor, W Speck, B Tainerand, and E Zeiger (1986).Salmonella mutagenicity tests II Results from the testing of 270 chemicals Environ.Mutagen 8(Suppl 7):1–119

National Research Council (1994) Alternatives for Ground Water Cleanup National emy Press, Washington, DC

Acad-National Research Council (1997) Innovations in Ground Water and Soil Cleanup: FromConcept to Commercialization National Academy Press, Washington, DC

National Research Council (2000) Natural Attenuation for Ground Water Remediation.National Academy Press, Washington, DC

Pankow, J.F and J.A Cherry (1996) Dense Chlorinated Solvents and Other DNAPLs inGroundwater: History, Behavior, and Remediation Waterloo Press, Guelph, Ontario.Raymond, R.L (1974) Reclamation of hydrocarbon contaminated groundwater U.S Patent3,846,290

Sedlak, D and L Alvarez-Cohen (2003) Emerging contaminants in water Environ Eng Sci.20(5):387–388

Snyder, S.A., P Westerhoff, Y Yoon, and D.L Sedlak (2003) Pharmaceuticals, personal careproducts, and endocrine disruptors in water: implications for the water industry Environ.Eng Sci 20(5):449–470

Speidel, D.H and A.F Agnew (1988) The Wold budger In Perspectives in Water Uses andAbuses, D.H Speidel, L.C Ruedisili, and A.F Agnew (Eds.) Oxford University Press,New York

Stainer, R.Y., J.L Ingraham, M.L Wheelis, and P.R Painter (1986) The Microbial World,5th ed Prentice Hall, Englewood Cliffs, NJ

United Nations Environmental Protection Programme (1999) Inventory of InformationSources on Chemicals: Persistent Organic Pollutants UNEP Chemicals, Geneva,Switzerland

U.S Environmental Protection Agency (1989) Toxic Release Inventory Magnetic TapeNumber P.B 89–186-118, NTIS or TRI Data Base, National Library of Medicine,Bethesda, MD

U.S Environmental Protection Agency (2003) Washington, DC (www.epa.gov/swerust1/pubs/ustfacts.pdf.)

Wilson, E.O (2002) The Future of Life Little Brown & Company, Warner Books,Lebanon, IN

GEOCHEMICAL ATTENUATION MECHANISMS

Ulises: ‘‘One touch of nature makes the whole world kin.’’

—Troilus and Cressida, Shakespeare

‘‘In time and with water, everything changes.’’

1993, 1995) For organizational purposes, however, it is convenient to consider biological (abiotic) and biological attenuation processes separately

non-This chapter will discuss several abiotic processes that can contribute to (or bit) the natural attenuation of groundwater pollutants These will be grouped into(1) processes that transform contaminants to less harmful compounds and (2) pro-cesses that immobilize some contaminants within the aquifer matrix Abiotic trans-formation processes that could be important in groundwater systems includehydrolysis, oxidation–reduction reactions at mineral interfaces, elimination reac-tions where molecules undergo spontaneous rearrangements, and radioactivedecay Photolysis, which involves chemical reactions initiated by the absorption

inhi-of photons, are negligible in aquifers due to the lack inhi-of light penetration and willnot be discussed in this book Immobilization processes that will be consideredinclude sorption, humification, ion exchange, and precipitation reactions Emphasiswill be placed on the geochemical principles responsible for such abiotic naturalattenuation mechanisms

Bioremediation and Natural Attenuation: Process Fundamentals and Mathematical Models

25

2.1 CHEMICAL TRANSFORMATION PROCESSES

2.1.1 Hydrolysis

Hydrolysis can be defined as the addition of the hydrogen and hydroxyl ions ofwater to a molecule, with its consequent splitting into two or more simpler mole-cules that are commonly easier to biodegrade This is illustrated below for a genericmolecule RX, which reacts with water to form a new R—O bond with the oxygenatom from water, and displaces the electron-withdrawing X group (e.g., an attachedhalogen, sulfur, phosphorus, or nitrogen) with OH Therefore, hydrolysis is theresult of a nucleophilic substitution in which water or a hydroxide (a nucleophile)attacks electrophilic carbon or phosphorus atoms:

Hydrolytic degradation provides a baseline loss rate for organic pollutants in eous environments and has been shown to be an important attenuation mechanismfor some common groundwater pollutants that can be hydrolyzed within the one- totwo-decade time span of general interest to site remediation (e.g., organophosphatepesticides and 1,1,1-trichloroethane) (NRC 2000)

aqu-Some classes of pollutants are generally resistant to hydrolysis, such as alkanes,aromatic hydrocarbons, alcohols, ketones, glycols, phenols, ethers, carboxylic acids,sulfonic acids, and polycyclic and heterocyclic hydrocarbons (Neely 1985) Theclasses of pollutants that are generally susceptible to hydrolysis include some alkyl-halides, amides, amines, carbamates, carboxylic acid esters, epoxides, lactones,nitriles, phosphonic acid esters, phosphoric acid esters, sulfonic acid esters, and sul-furic acid esters (Mabey and Mill 1978; Harris 1982) Nevertheless, the hydrolyticrates of such compounds can be highly variable For example, triesters of phosphoricacid hydrolyze in pH-neutral water at ambient temperatures with half-lives rangingfrom several days to several years (Wolfe 1980), and the chlorinated alkanes penta-chloroethane, carbon tetrachloride, and hexachloroethane exhibit hydrolytic half-lives(pH 7, 25C) of about 2 hours, 50 years, and 1000 years, respectively (Mabey andMill 1978; Jeffers et al 1989) The rate of hydrolysis also depends on the pollutantconcentration, the pH, and the groundwater temperature For example, the half-life of1,1,1-trichloroethane is about 12 years at 12C and decreases to about 2.5 years at

20C (Rittman et al 1994) Sorption can also affect the rate of hydrolysis, whichgenerally decreases with increasing ‘‘protection’’ by sorption (Burkhard and Guth1981) There are, however, exceptions to this trend, and some organophosphateinsecticides have been observed to hydrolyze faster when sorbed, presumably due

to the catalytic role of some mineral surfaces (Konrad and Chesters 1969) Examples

of environmentally important hydrolytic reactions are provided below

Alkyl Halides

Halogenated alkanes with a single halogen substituent are hydrolyzed to alcohols,which are easy to biodegrade This is illustrated below for the conversion of methyl

bromide (CH3Br) to methanol (CH3OH):

CH3Brþ H2O! CH3OHþ Hþþ Br ½2:2The hydrolysis of polyhalogenated alkanes can be relatively complex, involvingboth elimination and substitution reactions and yielding different products This

is illustrated below for 1,1,1-trichloroethane (1,1,1-TCA), 80% of which is verted by sequential hydrolysis via 1,1-dichloroethanol to acetic acid (vinegar),and the remaining 20% undergoes dehydration of the alcohol to form 1,1-dichlor-oethylene (1,1-DCE)

1,2-H 2 C CH 2 + OH − Cl

Cl

H 2 C CHCl + Cl − + H

Carboxylic Acid Esters

These compounds are hydrolyzed to carboxylic acids and alcohols The reactionmechanism involves nucleophilic attack by a hydroxide group at the carbonylgroup Consequently, these compounds hydrolyze faster at higher pH values (i.e.,

½2:5

Amides

Although amides are less susceptible to hydrolysis than esters, measurable rates can

be observed under highly acidic or basic conditions In such cases, carboxylic acids

and amines are produced:

R1 C N R3 + H2O

O R2

R1 C OH + HN O

½2:7

Epoxides

Epoxides are functional groups characterized by a triangular arrangement betweentwo carbon atoms and one oxygen atom Some epoxides can be hydrolyzed even atneutral pH, yielding the corresponding diol and sometimes rearranged productsalso:

C C R1 R2

R3 O

R1 R2 R4 R3

OH OH

C C R1 R2

R3 O R4 +

pH 7, with half-lives on the order of a few hours (Wolfe et al 1978):

Ar N C O R + H 2 O

O

H

Ar N H + CO 2 + R H

of magnitude slower under the same conditions:

to carboxylic acid esters, and electron-withdrawing substituents accelerate the tion Conversely, the reaction is much slower when electron-donating substituentsare present, as is the case with the methyl group in chlorpropham above

organopho-O R 2 P O

O O R1

R1

O R2 P O

O S R1

R1

S R 2 P O

O O R1

R1

S R2 P O

O S R1

R1 organophosphate organophosphorothioate

organophosphorothionate organophosphorodithioate

½2:12

Hydrolysis usually occurs by direct nucleophilic attack by OH at the P atom:

O P O

O O

Nucleophilic attack with water can also occur at one of the methyl or ethyl tuents (Lacorte et al 1995):

H

2.1.2 Radioactive Decay

Radioactive decay can be an important attenuation mechanism for elements with 83

or more protons, which are unstable or radioactive and whose atomic nuclei taneously change form into so-called daughter products These by-products are ele-mentally different and can therefore behave differently in the environment.Common radioactive environmental problems include radon, which is a naturallyoccurring gas that leaks into houses and poses an inhalation hazard; plutonium,which is a military waste product; uranium and decay products; radioactiveheavy metals excavated in mines; and radioactive elements used as tracers in med-ical applications, such as cesium and iodine Figure 2.1 illustrates the relationshipbetween common radioactive materials that are likely to pose a threat to residentialhousing

The main concern with the radioactive decay process is the emission of radiation(Table 2.1) Three types of radiation could be emitted:

Alpha Radiation This consists of massive particles (i.e.,4He) that have difficultypenetrating the skin, although such particles are of concern if one breathesthem into the lungs

Beta Radiation These are electrons that can penetrate the skin a few centimetersand pose a greater health risk than alpha particles

Gamma Radiation This consists of very damaging electromagnetic energy ofshort wavelength that has virtually no mass This radiation causes ionization,making biological molecules unstable

Depending on the intensity and duration of exposure, radioactivity can be verydamaging to organisms Detrimental effects range from somatic effects such as can-cer, sterility, and cataracts to genetic effects such as mutation of chromosomes.Nevertheless, subsurface solids absorb radioactive emissions and radionuclidesthat remain in the subsurface pose little or no risk provided that exposure to con-taminated groundwater does not occur (NRC 2000)

The half-lifeðt1=2Þ of a radioactive element is an important parameter related toits decay rate and refers to the time required for half of the atoms to decay to otherelements Radioactive decay rates are independent of temperature and follow first-order kinetics Thus, the decay rateðdC=dtÞ is proportional to the radioactive mate-rial concentrationðCÞ:

ele-Table 2.1 Several Units of Radiation

absorption of 100 ergs of energy

radiation type; for example, if 10 rads of a has same rem as 1 rad of

b, you get the same rem