CHUCK NEWELL: Our last lecture described how we're finally starting to recognize that these abiotic body degradation reactions should be part of the discussion about natural attenuation and contaminated groundwater sites. There's been a lot of research in this area, a lot of information to sort through in terms of understanding how these abiotic reactions work. So a lot going on in this area. DAVE ADAMSON: Yeah, and it could be a little challenging if what you're used to is attenuation by biological pathways, the abiotic reactions, as we saw last time. And they're definitely little bit different. CHUCK NEWELL: And there's also the fact that these abiotic processes, although, their natural reactions, they rely on several different players to proceed. And if there's something missing at the site, then you're not going to see this type of attenuation from these abiotic reactions. DAVE ADAMSON: I think this slide that we showed during our last lecture sort of highlights the different players that are involved in this. We're talking about biogeochemical reaction. So moving from left to right on here, we've got the biological component, the geological component, and then sort of a chemical reaction that we're dealing with. So the key point, though, in all these reactions, remember, that we've got a mineral usually involved and that minerals in sort of reduced form. So you need to have the conditions that support those sorts of reactions to happen. CHUCK NEWELL: And it's different from this whole valent iron process, right? DAVE ADAMSON: Exactly. So one of the questions that we want to introduce here is, which minerals are we actually talking about here? I think the one that most people are familiar with then are these group of minerals called iron sulfide. So we're talking about iron sulfide, simple FES, or mackinawite, or pyrite, in these cases. And we've got a picture here then of iron sulfide. This is a scanning electron microscope image of that from Michelle Scherer at University of Iowa's lab. CHUCK NEWELL: Neat. And sort of how big is that particle? Do you have any sense of the scale? DAVE ADAMSON: Well, I'd say it's definitely falls in the teeny-weeny category. But just looking at it, I think we're talking, you know, the individual crystals on the order of a few microns up to 20 microns, something like that. CHUCK NEWELL: Great, great. DAVE ADAMSON: So those are the iron sulfides, but there's a few other ones that are definitely of interest and definitely been established as reactive minerals. So we're also talking about iron oxides and iron hydroxides, things like magnetite and hematite, as well as mixed iron hydroxides. So most people seeing these referred to as green rust-- which can be a lot of different types of compounds that fall under that category-- as well as iron carbonates, like siderite. And then there's these phyillosilicate clays, biotite, vermiculite, those sorts of things. CHUCK NEWELL: And so this is really from the world of soil science. Those soil scientists know about all this stuff. This can be unfamiliar to some of us, coming from the ground waterfield, or chemical engineering, or environmental engineering, some great names there. Just a quick one, one of the metal, under iron hydroxides, I see magnetite. I see hematite. What's the one that starts with L? How do you pronounce that one? DAVE ADAMSON: I would throw that back to you. I'm not really sure. CHUCK NEWELL: More research is needed on that particular one. DAVE ADAMSON: More research is needed. So those are the minerals that are important, but what geochemical conditions then are we talking about that need to be present in order for these reactions to occur? So remember, since they're being mediated by these mineral species, we need to have those mineral species present. So we need to have iron rich minerals or some sort of source of iron within the ground water. We need to have natural sources of sulfate in order to have sulfide be formed. We also then need to have generally anaerobic conditions. The biological reductions here are occurring through the actions of iron reduction and sulfate reductions. So these occur in an anaerobic conditions. And then we need to have organic carbon present. We need to have enough organic carbon to supply those bugs in order for them to do those reduction reactions. CHUCK NEWELL: Now the anaerobic conditions can they be natural, or they can then be sort of man-made? DAVE ADAMSON: They can be in both cases. So you could have a man-made condition that supports a natural reaction. CHUCK NEWELL: Great. DAVE ADAMSON: And then we ask ourselves, well, which minerals are important for which contaminants? And so we're going to go through a couple slides here, where we talk about the reactivity of iron sulfides, magnetite, and green rust-- which are sort of three are the most important minerals out there-- and which contaminants then can be degraded by these minerals. So if you see, chlorinated solvents, we've got a yes in each one of those categories. And these are probably the most widely studied, definitely reactive with a wide range of minerals. Pesticides, definitely evidence for iron sulfides, but not the other ones. Munitions, magnetite, yes, maybe iron sulfides. And then metals, definitely there's redox sensitive metals. So iron sulfides can mediate reactions in those. But then we come to maybe a little bit less positive news. We've got those same three reactive minerals, iron sulfides, magnetite, and green rust. But we don't see any yeses then when we look at this set of compounds. So petroleum hydrocarbons a big concern on a lot of different sites. So dealing with BTEX or the oxygenates, no evidence so far that abiotic degradation reactions are relevant. 1,4-dioxane, 1,2,3-trichloropropane, some of the more emerging contaminants, not a lot evidence, maybe minor for one 1,2,3-TCP. PFAS, these are per and polyfluorinated alkylated substances, a big concern as an emerging contaminant, no evidence thus far, as well as NDMA. We're still sort of investigating that. There's maybe some indication that it might be relevant, but nothing yet to really substantiate that. CHUCK NEWELL: Let's just back to those PFAS, a lot of concern about that right now, a lot of concern about these things aren't really degrading in the subsurface. But there's a whole series of them, maybe 50 or 60 different types of components of that. Any of those have any instances of abiotic degradation? DAVE ADAMSON: Not anything really yet. It's one of those things that's definitely a subject of a research and maybe something will be uncovered. But those compounds definitely have an issue in terms of whether they're subject to really any natural attenuation mechanisms. That's just sort of being sorted out at this point. CHUCK NEWELL: Very good. DAVE ADAMSON: So let's look at a few examples. So not to get too deep into the chemistry, but we talked about some of the reactions here. So let's look at some of the reaction pathways. So this is an example of a chlorinated ethene. So in this case, we're starting off with PCE. And these are good reaction schemes that John Wilson's research group has published. And just looking at that how you go through these, a lot of these chlorinated ethenes are definitely degraded by these reactive minerals. So iron sulfides, mackinawite, green rust, magnetite are all active for these sorts of substances. CHUCK NEWELL: But this reaction can take it all the way down tom at the very bottom, to ethane? DAVE ADAMSON: Yes. So ethene and ethane are reaction end products or buy products in these cases of these reactions. Chlorinated methane, so these are compounds like carbon tetrachloride, which shown here on the top. Definitely, again, another case where a variety of these reactive minerals are effective for degrading these compounds. There are some intermediates in these cases, things like chloroform that might be of a concern. But in the end, the hydrolysis products or the final products of the hydrogenolysis are not really of concern. CHUCK NEWELL: I think a lot of research for at the Hanford site about carbon tet and abiotic reaction. DAVE ADAMSON: Exactly, exactly. And then chlorinated ethane. So in this case, we're talking about 1,1,1-TCA trichloroethane. And there's been a lot of emphasis on hydrolysis of this compound. And so that's shown there on the top. In terms of the other abiotic reactions, they're definitely relevant for this compound and lead to deformations of things like ethane. There is a lot of evidence in the various reactive minerals also, in this case, in addition to hydrolysis. The one thing that hasn't really been established yet as whether magnetite, for example, is relevant for this compound. And then again, go a little deeper into this question about which minerals are important, you're sort of interested in the trend of reactivity. So which chlorinated solvents might be more reactive to these sorts of minerals. And He and Wilson and that research group at EPA publish a lot of information about this in terms of which ones are the most reactive. And so we sort of sort of go to this reaction change of seeing from the greatest reactivity down to less reactivity through these various different types of mineral species. And this is one study. And they were compiling a lot of different information. And in the caveat here, I guess, is that some individual studies show different patterns. So you may not necessarily be able to distinguish at a field site which of these might be particularly more relevant. CHUCK NEWELL: I notice that magnetite is pretty far down on the list. But still a lot of papers that talk about magnetite. It's still an important reactions, even though it's down there on the list. DAVE ADAMSON: Yes. It's still an important reaction, because it's a fairly prevalent mineral and also something that's relatively easy to measure. So definitely still relevant. CHUCK NEWELL: Have something more about that in upcoming lectures? DAVE ADAMSON: Yeah. And we'll definitely deep dive deeper into that. So in terms of our key points, we're talking about reactive mineral species degrading many different types of chlorinated solvents, pesticides, and metals. CHUCK NEWELL: And we're talking about these different reactive mineral species that are important. We're talking about iron sulfides, the magnetite. We're talking about green rust and other species. It's really a lot of portions of metals that are in their reduced form, like this FE2. DAVE ADAMSON: And the other thing to remember that is that while it's important for chlorinated solvents and some of these other compounds, there's not a lot of evidence yet that it is a relevant reaction mechanism for things like petroleum hydrocarbons or many of the emerging class of contaminants.
