PEDRO ALVAREZ: Determining on moral and biodegradation kinetics is very important to assess the feasibility of natural attenuation. For example, this enables us to compare removal rates to migration rates, and therefore decide whether the plume is likely to expand or contract. Let me begin with some basic principles by considering Monod's nodes equation, which has some mechanistic and symbological bases, and is quite similar in its hyperbolic form to the Michaelis-Menten equation that is often used to predict enzyme kinetics. This equation predicts that as the limiting substrate concentration increases, microorganisms will achieve a maximum specific degradation rate that reflects their metabolic capabilities. This asymptote shown here as k, is known as the maximum specific substrate utilization rate. The other hyperbolic coefficient that defines this rate law is k of s the half velocity coefficient. K of s is defined as the substrate concentration at which the specific degradation rate will be one half of the maximum value. And it is inversely proportional to the affinity of the enzyme or the microorganism for the substrate. So that high k s values represent low affinity for the substrate, possibly due to reduced bioavailability. Monod's equation also predicts faster rate higher with anti microbial concentrations. Of course, if you have more microorganisms working for you, you would expect more contaminants to be biodegradable. But let us focus on the region where the decontaminate concentration is relatively low, with regards to k s, and we observe a linear relationship between pollutant concentration and biodegradation rate. Here, the rate approaches what is known as first order kinetics, as predicated by collision theory. In this range we can ignore the term c in the denominator. In Monod's equation then reduces to a first order equation, with respect to the contaminant concentration c, here where lambda would be first order reaction coefficient. It is important by comparing these two equations, the first order expression and Monod's equation, that this tells us what influences lambda. We can see here that lambda depends on k, which in turn depends on the types of microorganisms that you have present. It depends on k s, which is dependent on the type of enzymes that are acting, and their affinity towards the pollutant and the bioavailability of the compound. And it also depends on x, which is the active microbial population, which is not constant. And it depends, of course, on environmental conditions and aquifer chemistry, including available substrate. The bottom line is that this lambda is not constant. It's actually a coefficient. And it can vary in time and in space due to microbial population shifts and to metabolic shifts resulting from changes in aquifer chemistry. An important factor affecting rates is the type and concentration of prevailing microbial species. When mass transfer processes are not rate limiting, the rate should be proportional to the active microbial concentration as predicted by Monod's kinetics. In this example, we have a culture with 10 million cells per milliliter that can degrade Toluene 80 milligrams per liter of Toluene in two weeks, and there denitrifying condition. But if we use a different culture with a lower microbial concentration, in this case only 20 milligrams per liter, it takes it 20 weeks. So what this suggests is that the lag period that we often observe in biodegradation, often reflects the time required for the specific degraders to grow to a critical mass capable of measuring degradation rates. And by the way, the solid lines in this graph are not data feeding. This is Monod's equation, predicted with bottles with the parameters that were independently determined. Now what this implies is that the rate of degradation of a target pollutant could be significantly enhanced by the presence of structural analogs. That is nontoxic compounds that have similar chemical structure. Such as benzoate in this example, whose present is significantly enhancing or attenuating the migration of benzene through this one dimensional column that resembles a one dimensional barrier. Here, the blue line shows that it takes some time for microorganisms to acclimate and begin to grow and degrade total benzene. And when this happens benzene degradation rates increase, and the effluent concentration decreases. But the red line shows that the acclimation is enhanced when benzoate at relatively low concentrations-- only one kilograms per liter-- is present. Because this enhanced the growth of organisms that fortuitously can degrade benzene. Apparently the aromatic nature of benzoate enhances the proliferation of BTX degraders, which results in enhanced natural attenuation. Another factor that can have important effects on biodegradation rates but is often overlooked by models, is the nature of the contaminant mixture. The presence of some other compounds may enhance or inhibit the rate of biodegradation, depending on which mechanism is displayed. You can observe positive effects when an alternative substrate serves us a fortuitous enzyme inducer. Or as a primary substrate as we just saw for enhanced growth of specific degraders, or even for cometabolism of the target compound. But you could also have negative effects when the alternative contaminants are consumed by aerobic organisms, and this depletes the available oxygen during the biodegradation, or when they are preferentially utilized, or when they repress enzyme induction or exert competitive innovation of the necessary enzymes. Causing again, unfavorable also microbial population shifts. So the key points of these lectures are that at relatively low concentrations, contaminant concentrations will decrease exponentially following first order kinetics as a result of biodegradation. And biodegradation rates will increase as the population of active microorganisms grow on the pollutants. The rate and extent of biodegradation can be affected by environmental conditions, such as the pH, temperature, and redox potential of the groundwater, as well as the bioavailability of the contaminant, but also by the presence of alternative substrates or co-contaminates that may accelerate or slow down biodegradation.
