Biodegradation Mechanisms: Chlorinated Solvents vs Hydrocarbons

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Из курса от партнера Rice University

Natural Attenuation of Groundwater Contaminants: New Paradigms, Technologies, and Applications

129 оценки

Rice University

Natural Attenuation of Groundwater Contaminants: New Paradigms, Technologies, and Applications

129 оценки

Cleaning up the large number of groundwater contamination sites is a significant and complex environmental challenge. The environmental industry is continuously looking for remediation methods that are both effective and cost-efficient. Over the past 10 years there have been amazing, important developments in our understanding of key attenuation processes and technologies for evaluating natural attenuation processes, and a changing institutional perspective on when and where Monitored Natural Attenuation (MNA) may be applied. Despite these advances, restoring groundwater contaminated by anthropogenic sources to allow for unrestricted use continues to be a challenge. Because of a complex mix of physical, chemical, and biological constraints associated with active in-situ cleanup technologies, there has been a long standing focus on understanding natural processes that attenuate groundwater contaminant plumes. We will build upon basic environmental science and environmental engineering principles to discover how to best implement MNA as a viable treatment for groundwater contamination plumes. Additionally we will delve into the history behind and make predictions about the new directions for this technology. Any professional working in the environmental remediation industry will benefit from this in-depth study of MNA. We will use lectures, readings, and computational exercises to enhance our understanding and implementation of MNA. At the completion of this course, students will have updated understanding and practical tools that can be applied to all possible MNA sites.

Из урока

Biodegradation Principles

In this series of lectures, we will learn when biodegradation can or cannot occur and what conditions are conducive to faster degradation.

Requirements for Biodegradation11:22

Thermodynamic Feasibility of Biodegradation4:30

All About Electron Donors and Acceptors6:26

Biodegradation Mechanisms: Chlorinated Solvents vs Hydrocarbons8:46

Reactions at Interfaces and Sustained Treatment: Emerging Concepts13:36

Biodegradation Kinetics: from Monod to First-Order Models8:09

Mass Balance Type Calculations11:44

Познакомьтесь с преподавателями

Pedro Alvarez
George R. Brown Professor of Civil and Environmental Engineering
Department of Civil and Environmental Engineering
Charles Newell
Vice President
GSI Environmental Inc.
David Adamson
Senior Environmental Engineer
GSI Environmental Inc.

PEDRO J. ALVAREZ: The most common classes of groundwater contaminants

for which natural attenuation is considered

are chlorinated solvents and hydrocarbons.

So in this lecture, we will consider the mechanisms associated

with their biodegradation to enhance our understanding of what

environmental conditions and factors enhance or hinder their biodegradation.

In lectures 2 and 3, we learned that highly chlorinated compounds,

such as perchloroethylene and TCE can be reductively dechlorinated

under anaerobic conditions.

And this process can be through fortuitous cometabolism,

meaning that it is not coupled to energy harvesting or cell growth.

Or it could be metabolic, meaning that some bacteria can

use dechlorinated compounds as terminal electron acceptors

in a respiratory process that yields energy to support their growth.

And this is known as halorespiration.

And in this case, microbial growth results

in more bacteria capable of attacking these compounds, which

increases degradation rates.

And the rates of halorespiration are consequently orders of magnitude

faster than those for cometabolic degradation.

Let us consider in greater chemical detail, the dechlorination mechanisms,

using here perchloroethylene as an example.

In the Lewis chemical structure, each covalent bond line

represents two shared electrons, shown here in red.

The dechlorination process starts when one electron

is donated by a microbial enzyme or electron carrier.

Also, the electron donor could also be surface-associated ferrous iron,

or even zerovalent iron when the reaction is abiotic.

The chlorine atom accepts this electron and leaves the molecule

as a chloride ion.

And since the first step involves the transfer of one electron,

because elements such as iron or cobalt that transfer one electron at a time

are involved, the dechlorination propensity of a contaminant

depends often on the one electron reduction potential.

Now, this dechlorinated intermediate, which is sort of an organic free

radical, very short lived, is reduced farther.

Specifically, another electron is transferred in a similar manner.

And then a water-derived proton is incorporated,

resulting in the net replacement of the chlorine atom by a hydrogen atom.

And overall, this reaction is known as hydrogenolysis.

It requires two electrons to knock off each chlorine atom

and replace it with a hydrogen atom, and in this case,

trichloroethylene or TCE, is formed.

And this process can be repeated so that sequentially more chlorine

atoms are replaced one at a time by hydrogen atoms.

Now one challenge associated with sequential hydrogenolysis

is that cometabolic dechlorinators can produce

vinyl chloride, which is more toxic than the original trichloroethylene.

In contrast, the halorespiring organisms can perform dechlorination all the way

to ethene, which results in complete detoxification.

Now the thermodynamic feasibility of aerobic biodegradation

of the dechlorinated byproducts increases

as the chlorination progresses, as indicated by the more positive Delta E

values.

So it is quite possible that these compounds

can be degraded by sequential anaerobic-aerobic processes.

For example, as they dechlorinate in an anaerobic zone, and then the byproducts

migrate into the aerobic zone.

Now the key points are that reductive dechlorination

requires appropriate electron donors to first induce anaerobic conditions

and stimulate dechlorination or, that is,

the removal of chlorine atoms, which will

decrease the toxicity with the noted exception of the formation of vinyl

chloride and enhance the solubility of these compounds.

Some strains can respire these chlorinated solvents

as electron acceptors.

And this results in faster and greater potential for complete detoxification.

These halorespiring organisms include those of the genus Dehalobacter

restrictus, Dehalococcoides ethenogenes, and Desulfomonile tiedje, among many.

It's interesting to point out that these bacteria are

pretty widespread among solvent-contaminated sites.

Although it is not always the case that they will be present.

And sometimes they're added in bioaugmentation schemes

to stimulate dechlorination.

Now in contrast to chlorinated solvents, hydrocarbons

are very reduced chemically.

So their oxidation is very favorable thermodynamically.

And that's why we use them as fuel in combustion engines.

Now hydroxylation, which refers to the addition of hydroxyl groups,

is often the first step.

This is mediated by oxygenase enzymes.

This transformation increases the solubility of the compound

and makes it more susceptible to subsequent metabolism.

Now this reaction requires molecular oxygen,

and often it is the diffusion and replenishment of molecular oxygen

what limits the rate of aerobic biodegradation.

One condition for the ring fission for aromatic compounds

is that they have to be di-hydroxylated.

That is, they need two hydroxyl groups inserted by these oxygenase enzymes

before the ring can be opened up.

Then once the ring is opened up, the byproducts

can be easily funneled into central metabolic pathways,

such as Krebs cycle, where complete mineralization to carbon dioxide

occurs.

Anaerobic degradation of BTEX compounds is

very important as a natural attenuation mechanism,

even if it proceeds at much slower rates due to the weaker electron acceptors

that are utilized in this process.

Benzene, which is the most toxic of the BTEX compounds,

is unfortunately relatively recalcitrant under anaerobic conditions,

where it degrades very slowly, if at all,

usually after the other alkylbenzenes are degraded.

And benzoyl-CoA is a common intermediate in these reactions.

It's reduced prior to ring fission by hydrolysis,

and the oxygen that evolves in that CO2 is usually originating from water.

A survey of plume dimensions that was conducted by GSI--

they considered hundreds of sites-- they showed

that BTEX plumes are relatively small, compared to chlorinated plumes.

And this reflects that BTEX compounds are relatively easy to degrade due

perhaps to their natural pyrolytic origin,

compared to these synthetic chlorinated solvents that

are relatively recalcitrant in nature.

So the key points are that chlorinated solvents like TCE and PCE

degrade relatively fast under anaerobic conditions, provided

that suitable electron donors, such as hydrogen and acetate, are present.

Some bacteria, known as "dehalorespirers,"

can obtain metabolic energy from this process

and grow, thus increasing the dechlorination rates

and dechlorinate TCE all the way to ethene,

resulting in complete detoxification.

And hydrocarbons degrade faster under aerobic conditions,

and their biodegradation rate are often limited by oxygen replenishment rates.

Anaerobic degradation of hydrocarbons, although slower,

is also an important mechanism for monitored natural attenuation.

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