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.