Last week's lecture focused on biodegradation and its importance as a natural attenuation process, but this week, we're going to shift gears slightly and start talking about the abiotic degradation reactions. So Dave, as you recall when we talked about the history of MNA during week one, an important step for the acceptance of MNA were these protocols that were published in the 1990s and early 2000s. So these were really important, but as I recall, they really didn't have much about these abiotic reactions at all. >> No, the focus of those early protocols was definitely biodegradation as the primary destructive mechanism for attenuation. So non-biological processes, like dispersion, absorption, they didn't get a lot of attention. Outside of a few reactions, like hydrolysis of 1,1,1-TCA, there wasn't this idea that abiotic degradation was even worth considering. >> And that's pretty interesting because our call at that time, there's a lot of research about how metals can be used to degrade contaminants, particularly these chlorinated solvents. We talked about installing these permeable reactive barriers, containing this zero valent iron to degrade some of these solvents. And we're seeing a lot of success with that technology. >> Yeah, and then you started to see a few studies that documented that even naturally occurring minerals could reduce contaminants. Research groups like Kim Hayes at University of Michigan, Bill Bachelor at Texas A&M, Paul Trackneyack at Oregon, Michelle Shearer at Iowa, Renee Schwarzenbach at AVOG, and even Dave Friedman at Clemson, probably lots more I'm forgetting too. >> It's a lot of great research. And I know that the EPA and the Department of Defense began to get really interested in the potential of these abiotic reactions about the same time. And folks like John Wilson and Richard Wilkin at EPA were doing a lot of the great research and a lot of publishing about these abiotic reactions. And in terms of any milestone, sort of my recollection of how this field was developing was this EPA protocol guidance document published 2009, right, that talked about hey, abiotic degradations needs to be an important part of MNA for certain compounds. >> Yeah, and that EPA protocol in 2009 was definitely important. We'll revisit that later on in this week's lectures. I think to start off, though, let's get an idea of what we'll be talking about then during this series of videos that we're going to do. And so that's shown here on this first slide. Basically, you gotta remember we're dealing with natural attenuation here, so we're talking about abiotic degradation of contaminants in natural conditions. So the reactions that are mediated by reactive mineral species that are already in the formation. So they may be biogenic in origin, but they're natural. And we're also then dealing with hydrolysis, which is mediated by water. So on the other hand, what we're not talking about, then, is basically engineered abiotic reactions, things like permeable reactive barriers, ZVI or nano ZVI, in situ biochemical methods based on adding some sort of a carbon or substrate in order to drive these conditions. And we're also not talking then about sequestration, so we're not specifically dealing with precipitation of metals. >> Okay, so natural minerals down there, like iron and things of this nature, but not zero valent iron is the first key point. And second key point, in my impression, this is mostly about chlorinated solvents, is that right? >> Yeah, yeah, that's definitely where most of the attention has been, and we will deal with that a little bit later. >> Right. >> The one thing to remember about abiotic degradation is it can be a little bit of a complicated process. And so the cartoon that we're going to show here steps you through this a little bit. But just remember that there's a lot of players involved when we're talking about these degradations. So on the right-hand side, there's your contaminant. And this is what we're trying to reduce, and it requires electrons to reduce those. Those electrons come from these reactive mineral species, so they're in a reduced form like an Iron II, and then it's donating those electrons, and that reactive mineral is then changing to a different state. And that doesn't necessarily just happen on its own, there's usually some sort of a biological component to it, and that's shown on these next reactions here. Where those formation of those minerals, formation of those reduced irons, those reduced sulfides are coming biogenically. So you've got the action of sulfate and iron reducing bacteria that create the necessary species in order to sort of drive those reactions. And that happens because there might be organic carbon present in the soil for these bugs to work. So there's all these players that are potentially involved in this. And this is an adaptation of a diagram that Michelle Scherer made. And I think I really like how it lays out the individual players and shows that that can be a relatively complicated process that we're relying on here. >> Yeah, but it doesn't involve zero valent iron. It's all about these electron flow natural minerals. >> Exactly, so we're going to go through a series of sort of the relevant reactions then for abiotic degradation. We'll start off with reductive elimination. So your sort of standard chemical form is shown up there on the top. So you've got this chlorinated hydrocarbon, and you're adding two electrons, transferred from the mineral to the contaminant. So this is a dichloroelimination reaction, where you're removing two chlorides from either the one carbon atom, in which case, you're calling it an alpha-elimination, or from two different carbon atoms, where it would be a beta-elimination. In that case, you'd then form an additional bond between the carbons. So this is an example of a single-bonded carbon compound going to a double-bonded carbon compound. So what does that look like in terms of contaminants that we deal with? Well, TCE is a pretty common example. So if it's going up this reductive elimination reaction, it would be forming chloroacetylene as the product in this case. So that's reductive elimination. It's important to us, in this case, because a by product end of the chloroacetylene can be acetylene. And this is considered sort of a major indicator that this abiotic degradation pathway is happening. And this is one of the main pathways that acetylene itself is subject to further degradation, so while we're trying to look at it, it may not be something that we're readily able to detect. >> And to me, it's sort of this amazing, new compound we've not worked with on the biological world, is this acetylene, the triple bond, but it's basically the same thing they use for welding and things of that nature. >> Yeah, exactly. >> And so, all of a sudden, there's this new marker out there, but it's got some tricks to it that's hard to find, right? >> Yeah, and we'll deal with that a little bit more when we talk about the lines of evidence for abiotic degradation in the future. >> Great. >> So second reaction is hydrogenolysis, and so this should be a familiar reaction if you recall back to what Peter was talking about with biodegradation. This is basically our reductive dechlorination reactions. So, you can see it there in the chemical formula. Again, you're adding two electrons that are donated from the mineral and going to the contaminant. During this process, you're removing one of those chlorides and replacing it with a proton. So an example of this, obviously, PCE to TCE, reductive dechlorination reaction. Chloroacetylene to acetylene, again, a reductive dechlorination reaction. This can be done abiotically, just like we saw biologically, but under the abiotic scenarios, it's thought to be a relatively minor pathway. >> Okay, but acetylene's still the end product, right? >> Exactly. >> Mm-hm. >> Following up with a different reaction here is dehydrohalogenation. So in this case, we're removing one chloride and one proton from adjacent carbon bonds. This is an interim reaction because you don't need electrons from an external source, meaning, in this case, it's abiotic, but it's not mediated by minerals. This can be important reaction for a few chlorinated ethanes, but it's not necessarily universal abiotic reaction. So what chlorinated ethane, a good example that we see at a lot of sites is 1, 1, 1-TCA. So in this case, if you're undergoing dehydrohalogenation, you're forming 1,1-DCE is the product, in this case. >> Pretty important reaction for that chemical. >> Yeah, exactly, and we'll revisit this later as well. The final reaction, then, we'll look at in terms of abiotic reactions is hydrolysis. So in this case, we're dealing with a very straightforward, predictable chemical reaction based on nucleophilic substitution, where there's a hydroxide replacing the chloride. Water is essentially supplying the nucleophilic species, the hydroxide ion in this case. So again, we have no need for an external source of electrons in this case, so it's, again, not mediated by mineral species in this case. And we're going to spend a whole lecture on this in lecture five. So we'll definitely deal more in more detail in this, but let me show at least an example reaction here. Again, we're looking at 1,1,1-TCA, and the steps that you go in order to get to hydrolysis to acetic acid, in this case. >> Okay, and I had one prof talk about how this is almost like water molecules banging into the chemical, and it sort of falls apart? Is that a good explanation? >> If that helps you remember it over nucleophilic substitution, be my guest, that's exactly right. And acetic acid, again, we're talking about a relatively weak acid in these cases, usually produced in fairly low yields, so not necessarily a problem in terms of pH control within the aquifer. >> Good point. >> So let's go over some of the key points, then, from this lecture. We're talking about contaminants being degraded abiotically, via hydrolysis reactions or reactions mediated by reactive mineral species. >> And maybe the second point is these abiotic reaction pathways, they produce different products than these biological reactions. >> And then, final thing to remember is that these reactive mineral species, which are the important contributors to this reaction, they're often generated by biological reduction, so there is a biological component, even though we're talking about abiotic reactions.
