Thermodynamic Feasibility of Biodegradation

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

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Rice University

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

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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.

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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

Rencontrer les enseignants

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.

Consideration of thermodynamics is important to understand under which

conditions it is feasible to see biodegradation of different types of

pollutants.

In this lecture, we will contrast the thermodynamic feasibility for

biodegradation of chlorinated solvents versus hydrocarbons and

they're both aerobic and anaerobic conditions.

It is important to recognize that the metabolism of halogens often involves

energy yielding redox reactions where electrons are transferred from an electron

donor to an electron acceptor and this generates energy for microbial growth,

for microorganisms to move to communicate and to take up nutrients and pollutants.

Now some pollutants, as you will see soon can serve as

electron donors while others can serve as electron acceptors

provided that the electron transfer is thermodynamically feasible.

So, a convenient way to understand the thermodynamic feasibility of

electron transfer is to consider this falling ball analogy.

Here, we have a ball that can be dropped from a height h.

Before the ball is dropped, it has potential energy,

which would be equal to the mass times gravity times the height, mgh.

And of course, as the ball drops and it hits the bottom,

that potential energy is converted to kinetic energy, one-half of mv squared.

So similarly, we can use the Electron Tower Concept where the ball is now

the electrons that will fall from the electron donor and

will be captured by the electron acceptor.

And the idea is that the larger the drop, in other words,

the greater the difference in potential between the electron donor and

the electron acceptor, then the more chemical energy that can be harvested.

For example, let's consider benzene,

which is commonly the most important contaminant associated with fuel spills.

Benzene as is the case of all hydrocarbons is fairly reduced and

is thus an excellent electron donor, and

the aerobic biodegradation of benzene is thermodynamically highly visible.

Not surprisingly, benzene degrades relatively fast under aerobic conditions.

In contrast, the anaerobic biodegradation of benzene

under methanogenic conditions while it's possible is not as feasible.

Thermodynamically, the Gibbs free energy associated with this reaction

is 24 times smaller.

So not surprisingly, benzine degrades much slower under anaerobic conditions.

Consider now a highly chlorinated compound such as hexachloroethane.

HCE cannot be oxidized by oxygen.

This is thermodynamically unfeasible,

as indicated by a positive views free energy value.

This compound is so

oxidized already that it could never be biodegraded under aerobic conditions.

In contrast, HCE could be reduced by hydrogen resulting in

its dechlorination to perchloroethyelene by a reductive mechanism known as

dehaloelimination and this is a highly visible reaction, thermodynamically.

And as dechlorinated compounds become reductively dechlorinated,

that is their chlorine atoms get replaced by hydrogen atoms the bipolarisis

become increasingly amenable for aerobic biodegradation.

So, the key point is that reduced compounds such as hydrocarbons can yield

significant amounts of energy upon oxidation.

Energy that microorganisms can harvest and

that's why we use hydrocarbons as fuels, and

their biodegradation is thermodynamically highly feasible under aerobic conditions.

In contrast, oxidized compounds such as highly chlorinated solvents

may resist further oxidation under aerobic conditions, but

they may be more rapidly degrading under anaerobic conditions.

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