Well Dave, I know you said at the end of the last lecture that we're going to revisit a different type of a biotic degradation process and we skipped over, and what is that? >> Well let me ask you and see if you can remember, what reaction we're talking about here. >> Okay, I think this is hydrolysis, is that right? >> Right! >> Okay, [LAUGH] well, I did sort of try to brush up on some of the material this morning before I showed up, drinking my coffee. But anyway, I remember back in the early days of groundwater cleanups, when I first got into this business in the late 1980's, 1990's. We thought many of these contaminants, which really were the solvents, were pretty recalcitrant. But even then, we recognized that hydrolysis was an important reaction for a couple of these organics, a couple of these chlorinated solvents. >> Yeah, and hydrolysis is a pretty basic chemical reaction. So we knew that it caused transformation of a few of these man-made compounds long before we started to clean them up out of groundwater. >> So how was hydrolysis different than other abiotic reactions and biotic reactions for that matter? >> Well let's take a look at this again then on this first slide, we sort of take a look at a compound like 1,1,1-TCA. >> Very common and degreasers and things of that nature so- >> Yeah. >> A lot of that, lot of plumes out there >> And the biotic pathway for this, it needs specific geochemical conditions to happen, you need to have the right microbes present. You need and not have a lot of competition or inhibition from other things that might be out there. Abiotic by reactive minerals, we've spent the last few lectures talking about that, in terms of what's needed. You need to have specific geochemical conditions as well as the right minerals and then again, that microbial contribution as well. But what we're talking about today then, is basically hydrolysis, another abiotic reaction, so what's needed for that? >> Nothing and isn't it this idea that it's just water molecules bang into that contaminant. And just because of the structure of the water molecule and the structure of the contaminant, it will break apart, it will fly apart with enough of these collisions. >> Yeah. >> Is that one way to think about it? >> That's one way to think about it. So, again, really nothing that we need to have this reaction proceed. It's got distinct byproducts that we can sort of look for and a portion of those byproducts are innocuous. So sort of a very favorable reaction if it's relevant for these compounds. >> Great. >> So let's take this reaction again, take a look at it. We're talking about nucleophilic substitution in this case. So there's our water molecule, and we're going to have that react with 1,1,1-TCA. So on this case, we're replacing one of the chlorides in there with this hydroxide iron in the intermediate in this case is this 2.2-Dichloroethanol. And after that we then get acetic acid after a few more steps. >> So you could make vinegar with this reaction? >> If you had enough maybe you could, most times you're talking about pretty low yields in these cases. So maybe not enough to really turn your aquifer acidic in these cases. >> I think I should stay away from the vinegar business, take it from 1,1,1-TCA? >> Yeah. >> Okay. >> Look at this in a little bit more detail. Hydrolysis we're talking about an SN2 reaction in these cases so some of the steps are showing again, here. But what I'd like to point out then is there's this carbocation intermediate that's formed. So that's the second one here on that case. So eventually, if you go through that hydrolysis reaction you're going to get acetic acid out of that. That intermediate is also subject to dehydrohalogenation reaction, an E1 reaction. So in this case, out of that you get an alternative end product, you're looking at 1,1 DCE. 1,1-Dichloroethene, which is shown there. >> A key sort of daughter product of these things. >> Mm-hm. >> But both those reactions, the dehydrohalogenation and the hydrolysis is sort of lumped under this hydrolysis umbrella, is that right? >> Yeah, exactly. And so looking at those again, we can kind of take a look at those products. And what we see when we do these in sort of a controlled situation, is you see a product yield where 80% of that initial 1,1,1-TCA shows up as acetic acid. >> A non-toxic compound. >> A non-toxic compound. 20% of it ends up as 1,1-DCE. >> And why is that, why do you see this difference? >> Well it's pretty simple. In this case it's kinetics is the reason that faster, the top reaction there is basically 5X faster so you see a higher yield of that acetic acid. >> So you got the 1,1,1-trichlorethane, the water molecule hits it, and then it just turns out that more often than not it's going to form this acetic acid. But one out of every four or one out of every five is going to then turn into this 1,1-dichlorethane. >> Yeah. So what are the really the key contaminants in terms of being relevant for hydrolysis? Well there's quite a few of them listed under here. You see a lot of ethane typed compounds and also chlorinated methane, carbon tetrachloride. So all of these undergo hydrolysis reactions. And the various products that you might expect to see with those are there. And these might not be the end products. They might be subject to further degradation. But they're sort of these things that you might see if you were measuring for them. And they occur as sort of a result of this hydrolysis type reaction. >> Okay, well how about some other contaminants that I care about? How about the Petroleum hydrocarbons, how about TCE, how about these other guys? >> Well, I know you care a lot about a lot of these things, and unfortunately I hate to break it to you, we can't really rely on hydrolysis for these types of things. So again, yeah, we're talking about a lot of the chlorinated ethanes, the things that might form from PCE or TCE. 1,1-DCE itself is not subject to further hydrolysis. Chloroform, a lot of the petroleum hydrocarbons, and again sort of the list of emergent containment, probably not of relevant for hydrolysis. >> So chloroethanes, you've got thumbs up for some of those important ones but then it's this desert maybe for these other compounds. >> Yeah. Looking at rates that might be associated with these reactions, we can rely on a great paper that came out in water research in 2011, where they compiled a lot of these things and again, we're talking primarily about a fairly limited number of compounds. So they were looking at trichloroethane as well as chloroethane. They did a great job of putting sort of what they saw for sort of rate constants. They even converted them into half life for those half life people. So they listed them here in some of the products, and temperatures that are associated with those. >> So thumbs up for me. It has a lot of information. Some of those papers from 1959, but where are some of the ranges that we're seeing in that table? >> Yeah, so TCA in this case, we're looking at half-life somewhere around the order of one to 10 years in this case. So pretty reasonable half-life in this case. The one thing that you might want to be aware of, though, in this case is that TCA is actually relatively biodegradable. So, if you want to do a comparison between this abiotic hydrolysis rate and biodegradation, maybe about an order magnitude or even two orders of magnitude which you see in difference of this rates. The biodegradation is generally faster in these cases. >> But the biodegradation only occurs under that certain sites or some sites where there's no biodegradation, right? >> Yeah. And then a lot of these cases one to ten years in terms of half life might be relevant for what your needs are in terms of the viability of natural attenuation. So, not to dismiss that the hydrolysis rates are actually relevant here. >> So that's cool, but now let's talk about temperature. How does it affect it? >> Well, these are chemical reactions. So if you have a higher temperature, you're going to proceed at a faster rate. And this follows the Arrhenius equation, and just take a look at this. If you see your term there with the T, the temperature term. If you increase that, you're going to increase the k associated with that reaction. >> And I'm a sort of a fan of history of science and I like reading up about some of this stuff. And Arrhenius Equation is a key thing that we use a lot in our business thinking about how temperature might affect these rates. Turns out this guy is a Swedish chemist around the turn, around the 1900 time period. He was one of the first guys besides this equation, thinking about how enzymes worked, how reactions worked, identified the potential for global warming from the burning of coal. But he's been on it in a 1900 time frame was that he's in Sweden and global warming would be a great thing. >> No, well we're in a little different place now, but good to know. >> But we still use this formula for the future. >> Yeah, and in this case, just remember if the temperature increases, k is also going to increase. So, one way to look at this sort of temperate dependence of these rates is to plot a bunch of data. So, this is a pretty standard [INAUDIBLE] plot in this nice paper, looking at hydrolysis of TCA. And so in this plot here, we've got the natural log of the rate coefficient the hydrolysis rate coefficient associated with these various temperatures. So as you're moving front the bottom right to the upper left, you're seeing an increase in temperature associated with an increase in rate. >> Okay, now this is one of my least favorite types of way to express temperature. This is one over temperature in degrees kelvin. What does this mean in real life in terms of how much temperature change will increase the rates or change the rates? Well, one way to look at it is sort of, you might see a cold weather site, a cold run water might be something like 10 degrees Celsius and a hot maybe in a warmer climate you might see something like 25 degree Celsius. The rate increase associated with that sort of the very matter temperature changes is 16 time increased in the hydrolysis rate coefficient. So pretty significant. >> That's a lot and that's the increase in the rate of insinuation. I think some folks are actually gone to TCA sites and try to heat them up just a little bit to get it to 25 degrees increase that hydrolysis rate. >> Mm-hm. >> Mm-hm. So is this process relevant at you site, at person's site? >> Well let's take a look at some of the. Some of the key factors here. Basically if you're at a TCA site you see it disappearing. You look for 1,1-DCE and acetic acid as products. And you can use the ratio of those things maybe even to help you estimate the age of the release. Particularly at sites where the conditions might not be favorable for biodegradation. >> A little tough to do in some cases, a lot of other factors in there that whole dating thing can be tough but there are papers about how to try to do it. >> Yeah, and it's relying on that it's a predictable reaction, that you may be able to actually know what those rate coefficients are with some >> Some degree of uncertainty. >> It's a lot of unknowns in that dating stuff. >> Yeah. Rates are significant, and in a lot of cases from natural attenuation, because we're talking about releases that may have occurred decades ago, so if you half-lives in the one to ten year range, you might have seen quite a bit of degradation of related, just the hydrolysis. And then again, these types of reactions are must less dependent on the geo-chemical conditions or other things that might be present. One thing that we like to highlight is the positive impact of a warmer ground water temperature. >> Okay well, let's wrap up. >> So hydrolysis again, a key abiotic degradation pathway. Doesn't rely on the presence of other minerals, microbes or even geochemical conditions to a large part. And these hydrolysis rates are pretty predictable. If you know the chemical and you know the temperature, you can get these rates from the literature, a lot of good, sort of basic information about it. >> Yeah, and you can look for these very specific products of these reactions in order to sort of do some forensic information about whether the process is relevant at your site. >> But it's only limited to a couple of these key chemicals where that water will bang into that chemical and it will >> It will fall apart, 1,1,1-trichloroethane is sort of the big guy in this world.
