If the webinar or audio quality degrades, please disconnect and repeat the original login steps to rejoin the webcast. If you have a question, we encourage you to ask it using the question feature located on the webinar panel. We’ll collect your questions and do our best to answer them at the end of the presentation. If we don’t address your question within the time permitting, we’ll make an effort to follow up with you after the webinar.
We are recording this webinar and a link to the recording will be emailed to you once it is available. In order to continue to sponsor events that are of value and worthy of your time, we will be sending out a brief survey following the webinar to get your feedback. Today’s presentation will focus on “in situ Chemical Reduction Core Concepts and their Engineering Implications.” With that, I’d like to introduce our presenters for today. We are pleased to have with us Dr. Paul Tratnyek, professor in the Division of Environmental and Biomolecular Systems and Institute of Environmental Health at the Oregon Health & Science University. Dr. Tratnyek research concerns the physicochemical processes that control the fate and effects of environmental substances, including minerals, metals, organic contaminants and nanoparticles for remediation, as contaminants, and in biomedical applications. Dr. Tratnyek is best known for his work on the degradation of groundwater contaminants with zerovalent metals.
We’re also pleased to have with us today Dr. John Freim. Dr. Freim is well-known throughout the environmental industry for his work in developing breakthrough in situ chemical reduction technologies. He has over 30 years of experience in materials processing, and 15 years in the environmental remediation industry. And currently leads the first REGENESIS state-of-the-art colloidal product manufacturing facility. In his role with REGENESIS, Dr. Freim is responsible for the manufacture of colloidal materials, including PlumeStop Liquid Activated Carbon, the colloidal zerovalent iron technologies AquaZVI and MicroZVI. As well as other new products used in environmental remediation.
All right, that concludes our introduction, so now I’ll hand things over to Paul Tratnyek to get us started.
So the series of books that this came from was assertive sponsored series. And this volume here is the first one amongst I think seven or so other volumes that sort of posted. This particular volume was on “Chlorinated Solvents Source Zone Remediation.” But the chapter that I was asked to write was on in situ chemical reduction. And so we got together with Rick Johnson, and Greg Lowry, and Dick Brown. And wrote a chapter that was an effort to try to give an overall perspective or a comprehensive perspective on what in situ chemical reduction was. For the purposes of the book, it was in the context of source zone Remediation, but my interest was really broader than that was anything any application of in situ chemical reduction. So the first part of that chapter, the technical background portion of that chapter was framed in a general way. And that’s the material that I’m gonna use for the first few slides in this presentation. You can click on these QR codes if you’re prepared, if not they will come back again at the end of my presentation.
All right, so the first part of this is the question of where does this terminology come from? Who gets credit and exactly how much? The term has been used for a little while since about the 1990s and it came up more or less simultaneously in a couple different sources. One significant one was some work by Jan Dolfing, Alan Seech, John Vogan, and others. That was presented at the 5th ChlorCon meeting and is written up in an article in this “Soil Sediment Contamination” Journal. And their perspective on ISCR at the time was really to provide a context for marketing a few specific products that they had which you have mentioned here. They’re not currently using those product names anymore as such. And so that particular version of ISCR has been essentially absorbed into the broader definition that is initially was really articulated by Dick Brown. And he was talking about this in the 5th ChlorCon meeting, in the 6th ChlorCon meeting. A book chapter and one in volume 5 sort of book, and then subsequently in the chapter that I co-authored with him which we will be using as we go.
I got involved a little bit later. My first presentation on this was in the 7th ChlorCon meeting and then this book chapter. And so my mark initial mark on this was to, among other things, try to give a more precise and inclusive definition to what ISCR should be. My perspective is non-proprietary in the sense that I don’t vend any products in this realm. And I was very attuned to trying to give a consensus perspective on this problem, so I took input from a lot of people in articulating that book chapter. It’s interesting to look and see how things have developed since then. In preparing this talk, I took these snapshots of what’s currently on Wikipedia for in situ chemical reduction. And what’s currently on the syrup EnviroWiki for in situ chemical reduction. The latter was actually written by my ex-student, Dimin Fan, now Geosyntec. And it still reflects our perspective as it’s in the book chapter. But the Wikipedia page, I think we probably started this page but it’s since been significantly rewritten by other people. So it’s kind of interesting to just to read and see how that…how the wording has changed too due to the influence of other players in the area.
So now for a definition, I remember I was concerned with trying to give a broad inclusive definition and so this is we came up with. The first bullet here says, “In general, ISCR refers to a category of in situ groundwater remediation technologies where treatment occurs primarily by chemical reduction of contaminants. And there is a strong sense of trying to build on the analogy to in situ chemical oxidation here. The notion that ISCR is the emphasis is on abiotic processes is fundamental. But contaminant reduction by biogenic reducing minerals is included if the role of microbial activity in the contaminant reduction is indirect.”
And so I should have italicized the word indirect there. Because that’s really the key element that defines the boundary between biodegradation biotic…direct microbiological degradation of contaminants. And what we are concerned with in ISCR which is abiotic degradation by reducing materials, say mineral phases. But the second bullet here is to acknowledge the fact that most of these reducing mineral phases are generated by in situ microbiology so biogeochemistry. And so when we have contaminant degradation by ISCR represented by this line here in almost most many cases the source of this is microbiology over here. The only thing we’re excluding is direct biodegradation of the contaminants that would be a different topic.
We also tried to include in the scope of ISCR the idea that there’s both natural so-called intrinsic biogeochemical processes. There’s also scenarios by which you might stimulate in situ microbial activity and wind up with ISCR favorable conditions. Or there’s scenarios where you might engineer ISCR conditions by adding strong chemical reductants, and we deliberately designed this to try to include all of that in our scope.
So when we were working on the chapter, we made a laundry list of technologies that we thought we would try to fit into the ISCR rubric. And we came up with this list and so there’s a bunch of acronyms here some of what you all will all know PRBs, BiRD, MNA. There’s a few that you’ll get quickly if you are familiar with them like this trademark version of ISCR, in situ redox manipulation. A couple of them were neologism of my own. This CRD stands for catalytic reductive dechlorination. I needed a name for palladium-catalyzed hydrogen in situ. I also invented SZTI for source zone targeted injection basically to give a word to how nZVI was primarily used in that context.
So to put all these into a framework for the purposes of the chapter, we came up with this matrix here. That’s got reductant strength on the x-axis and treatment volume on the y-axis with source own treatment at the top here. Plume treatment down here natural reductants so the intrinsic ones here and engineered reductants over here. And the idea is that we can take everything here on the left and we can populate it onto this matrix and we end up with something that looks like this. And I was a little hesitant to do this initially because you could quibble about exactly where things belong.
But for the most part, people have been pretty happy with this. Obviously, abiotic MNA belongs down here under plume treatment with natural reductants. Obviously, source zone targeted injection with nZVI belongs up here under source zone treatment with engineered reductants. And these other guys here could be placed in a variety of positions approximately where we currently have them. This figure has been fairly successful. I’ve seen a number of people reproduce it and modify it to meet their own particular needs and that was part of our intent from the beginning.
So it’s part of the roadmap that I used in developing this talk where I extended that list a little bit. So this red column of names down here on the left is based on what was on the previous slide with a few additions. So I’ve added for example a placeholder for some source of additional type of iron(II). I’ll elaborate on that in a second. I added a placeholder for polysulphide injection, iron sulfides, bimetallic, Sulfidated ZVI. ZVI plus pyrogenic carbonaceous material, this would be a kind of an umbrella term for activated carbon. So this list is a little bit longer than the one in the previous slide but and it will presumably, will continue, will evolve and get longer so it’ll be a working progress.
But this is…this part here is not really what I’m gonna talk about John will talk a little bit about that in the last part of this webinar. Obviously, the hat that I wear in this context is as an academic. And so what I’m supposed to do is help articulate define the core concepts, and this is just sort of the textbook material if you will. And so this is my current outline of core concepts that are most relevant here and this is also gonna be an evolving list. But it was optimized for today’s purpose and you’ll see we’ll follow most of it more or less.
When I originally started drawing this, my thought was that I was gonna draw connecting lines between each technology and the most relevant core concepts. But then I got bogged down deciding which lines to draw so I threw up my hands and drew everything connected. Because the more I thought about it, in almost all cases, the core concepts apply to all the technologies there’s just a few exceptions. Where, for example, say in the case of abiotic monitored natural attenuation. We don’t really need a line connecting that to mobility and deliverability because you’re not injecting any kind of engineered reductant. But those are the minority. In most cases there’s a connection to be drawn, and so we have this kind of disorienting looking pattern here which is gonna be…which we will cherry pick a few lines as we go.
So I’m gonna first start going back. I’m gonna start here at the top. I’m gonna start with mechanism of removal reduction versus sequestration. So the ISCR can remove contaminants from groundwater by two, basically, distinct approaches. One of them is where the contaminants are moved by reduction. And so this would apply to organic chemicals where there are functional groups that are amenable to reduction. So the chlorinated functional groups are certainly the most familiar, nitro groups in explosive type compounds will be another example and that there are others, but so the main category here is chlorinated solvents.
There are also metal oxyanions that are important groundwater contaminants that are subject to reduction. And so I listed chromate is probably the best known to you here, but also pertechnetate, selenate, arsenate, etc. There’s also some non-metal inorganics that are subject to reduction that are sort of relevant here. Nitrate is certainly one of them and perchlorate is kinda not so much, but it’s an interesting exception that sort of proved the rule.
In contrast to reduction, there’s also the scenario that we’re calling sequestration. So sequestration is an umbrella term for any kind of physical removal by adsorption or co-precipitation etc. And so there are metal oxyanions that may not undergo reduction but still are effectively sequestered by ISCR type agents and uranyl uranium would be a pretty good example of that. There’s also all the metal heavy metal cations like cadmium, copper, mercury, nickel, palladium. I’m sorry, lead. Which may be removed from groundwater by a process that’s sometimes called cementation. And in that case, that process is mostly best thought of as sequestration. Although in all cases there’s or many cases there is and some overlap between reduction and sequestration.
And so that overlap is illustrated in a lot of cartoons that are scattered around in the literature for kind of diversity. I picked this one to decorate this slide here it’s from a review paper by Irene Lo and Tang in water research. And you see here she’s got iron zero she’s got magnetite and she’s got iron(III) oxide and she’s got adsorption and reduction and reduction and co-precipitation. And so the point just being here that you can have both reductions and you can have sequestration contributing to the contaminant removal.
So now a few slides on what the reductants are that we’re talking about here?And so the first distinction I want to make or between two branches. The first branch is the relatively mild intrinsic reductants that occur naturally. They are mostly based on iron(II) and sometimes referred to as DVI for divalent iron. And or they’re composed of reduced sulfur here and here. And there’s also I got in here as a sort of a placeholder for natural organic matter, although it’s not the most important in the context of what we’re talking about today. There’s in contrast or in addition to that we have the relatively strong engineered reductants and dithionite is certainly an example that you should all be familiar with. Zero valent iron is the one that everybody’s familiar with and there are other zerovalent metals like zinc that’s that occasionally can be of interest.
I would point out that most of what applies to one here, also applies to two, so there’s some overlap and some blurring between the lines. I had a bunch of space on this slide to fill up and so I wanted a graphic. And so I was thinking, “Well, this notion of branches,” I originally I really got this from Dick Brown, and some of his early talks around 2010 on ISCR. And so I went looking for the image that I remember from his PowerPoints that was I think a tree which is presumably what the branches referred to. But this is the only thing I found I’m not really sure what this is supposed to be. But it clearly illustrates or it does still illustrate the idea that ZVI and DVI are major branches of ISCR, so DVI is up here and ZVI is down here.
Okay, so more on the reductant. So let’s elaborate a little bit on the intrinsic reductants. So these are these are naturally occurring they’re mostly mineral phases that are derived from iron(II), so again DVI. So the particular phases that are candidates for this we include magnetite, green rust, ferruginous clays and iron oxides with adsorbed iron(II).
There’s also minerals that contain reduced sulfur. And so there are a number of them mackinawite is likely the most important. Pyrites often talked about, greigite, marcasite an amorphous iron sulfides are others mineral phases that might be important in some context. And again I have a placeholder down here for natural organic matter, but I won’t delve that much in this talk.
I have a redox ladder over here just to illustrate the how strong these reductants are in a thermodynamic sense. This is Eh on what left and PE on the Y, negative at the bottom is more strongly reducing, positive at the top is less strongly reducing. So you see here the aqueous iron(II), iron(III) couple is not really strongly reducing at all, ZVI is down here. But for the intrinsic natural biogeochemical minerals it’s these guys here. And so those are the ones that are moderately strong reductants, and in theory could be responsible for some contaminant degradation reduction.
There’s an important aspect of this that I wanted to elaborate on that has to do with the relationship between these intrinsic reducing phases and the in situ biogeochemistry. And so this is illustrated by this very nice graphic that was done by the folks that were involved in this AFCEE/ESTCP workshop report back in 2008. And so this diagram has got four parts in each case the… it uses blue for microbes and the microbes are respiring some sort of organic carbon. And when they do that they will reduce what their terminal electron acceptors to make iron(II) and/or sulfides.
So here we have organisms respiring and creating iron(II). Or down here we have a sulfate-reducing organism reducing sulfate and producing sulfide. And the iron(II) and the sulfide then goes on to deposit and either by adsorption and/or co-precipitation to form a new phase. And so that in these figures this is represented here and here and over here, here and there. And it’s there that the contaminant degradation represented here say by TCE, it’s reduced here to produce products. And here’s TCE it’s reduced here to produce products.
So the key thing that’s the distinction that’s being demonstrated here is that the contaminant degradation is remote from the microbiology. The microbiology is producing the iron and the iron is producing a new reducing phase and it’s that reducing phase that’s degrading the TCE. If the microbes were degrading the TC directly that would be biodegradation, it wouldn’t be ISCR. The way this figure is set up the top row represents two cases where we have iron(II) here and here sorbed to a mineral phase. In this case, greigite, and in this case, magnetite to form a new material which is a better reductant of contaminants.
In the bottom row here we’re precipitating two new phases, iron(II) and sulfide are producing iron sulfides. And in this case, iron [II] is precipitating green rust, with both in those two phases are pretty strong reductants. The other thing to note here we’ll come back to in just a second is that the way this diagram is drawn TCE is always going to acetylene or something. And it’s not going to chlorinated intermediates that are the so-called stall products that are problematic. And that’s consistent with the idea that this is an abiotic pathway.
All right, I wanted to reinforce that this idea that iron [II] sorbing on to minerals. So I’m going back one slide now that these top two cases here and here that this is really quite important. And I can’t find a real good sort of metadata summary figure for this. But so I used two raw primary data figures from the literature. This one on the left is carbon tet this one on the right is RDX. And what you see here is that carbon tet is not degraded significantly by Aqueous iron [II] nor is it degraded significantly by suspended greigite which is just iron(III). But if you put the two together carbon tet is quickly degraded reduced in this case to chloroform. And then over here if you take RDX and put it in a suspension of magnetite and you add a little or no iron(II), you get little reaction. But as the amount of iron(II) goes up the reaction rate gets faster. And so clearly this idea of adding iron(II) and having it interact with the mineral surface to supercharge the reactivity of the phase is a strong effect and it’s very important. And it’s an area that there’s a lot of interest in figuring out now how to better characterize it because it’s a little tricky.
Okay, so now on to core concepts the abiotic reductant, so these are the engineered reductants. So ZVI is clearly the one that’s first on the list. I already introduced the idea that DVI is iron(II), would be the other analog. This is referred to I think by Dick Brown as soluble iron. I’m still waiting to find a vendor that’s selling some sort of chelated stabilized version of DVI for in situ reduction. But that presumably will be available soon, but off top of my head, I couldn’t think of one.
Sulfide you could add sulfide people do add polysulfide there’s a lot of applications along those lines. Dithionite we’ve talked about, it'[s something that’s been used in the field. And then in the context of palladium-catalyzed hydrogenation, you would normally people would…what people would do is actually add hydrogen directly. There’s a few other possible chemical reductants, but they’re not well developed at this point.
So looking over here now at this version of a redox ladder, this is similar to the previous one except slightly expanded. The one thing I did here at the bottom is I let the scale extend down here to make enough room for zerovalent zinc. You need to notice zerovalent zinc is a lot stronger reductant than iron, which in turn is a lot stronger than all of these intrinsic biogeochemical mineral phases. And also notice that all of these guys here are pretty low on this ladder relative to these chlorinated organics, reducible organics and/or these reducible metal oxyanion. And so as a general rule, the thermodynamics are fairly favorable certainly for the engineered reductants iron(0), it can reduce any of these things. And certainly, zinc can do that too. Notice that the gray zone here if you can see that on the resolution of your monitor here and here is the boundary for the for water stability. So down here this would be reducing water to hydrogen, and only iron and zinc do that, these other phases don’t do that.
Okay, so now I’m going to adjust this a little bit here. And I’m gonna continue going through some of these core concepts but I’m going to do them from the perspective of sulfonated ZVI. So we’ve been working on sulfonated ZVI a lot recently, a lot of people are interested in it. And so I thought it would be attractive for many of you to hear about these core concepts presented using…specifically using sulfidation as kind of the framework for that. And so again I’m gonna be building off of a recent publication of ours, in this case, it’s a review paper that we did it for ES&T last year. And the lead author on this is Dimin Fan, Ying Lan contributed greatly and a number of other authors also contributed significantly to the applied side of that. Again, those QR code that will be available again at the end of my presentation.
So first of the core concepts, to introduce the kinetics aspect of that I have this generic version of what we call the standard kinetic model. So if we can it contaminant removal by any mineral phase but particularly ZVI. We assume that the reaction order is first-order with respect to the contaminant. And also the metal and so that’s quantified as surface area concentration or mass concentration and that’s what those two symbols represent. And those two things are related to one another via the specific surface area. So the density of the reactive surface sides is proportional to the specific surface area.
So now you take these things and they’re related to one another by this intimidating equation Km equals Ksa times, specific surface area. And if you take the log of that you get parallel contours that are shown on this graph here, where the contour intercept is equal to the specific surface area. So this is log Ksa on the x-axis or y-axis and log Km on the x-axis. And then again these contours are represent the specific surface area. So this plot is very useful because it’s quite flexible we can put any kind of most data can be put onto this graph. And so for the sulfidation review Ying Lan, collected compiled off the whole bunch of data. This is essentially every…all the data that was suitable for this kind of analysis and we have it here showing for carbon tet and for TCE.
And so now the first thing to notice is that the carbon tet numbers as a whole, just to look at the whole cluster of colored dots, lie further up in the upper right corner than the TCE numbers which lie a little bit lower towards the lower left. The X and Y scales are the same, so what that tells you is it tells you that is a general rule carbon tet reacts faster than is reduced more easily than TCE. This is actually more significant insight than it might seem at first blush. Because note that there’s plenty of overlap between this cluster here and that cluster there. So you can cherry-pick data and find cases where it would look like TCE reacts faster than carbon tet, but as a general rule that’s not right. As a general rule, carbon tet reacts about two times faster by reduction than TCE.
Okay, so now you can go further and look a little bit more carefully at the colors. So here the micron-sized ZVI is represented in blue, iron oxides are represented in orange here and nZVI is represented in purple. And blue, oh, sorry, green here is what we’re calling impure ZVI that would be construction-grade scrap iron type materials. And you can see, for example, that the iron oxides and the construction-grade materials generally fall lower on this curve. So their reaction rates are slower and the high purity irons, the nano-ZVI is plot higher, the reaction rates are higher. And that’s true actually in both cases.
The point of all this for the sulfidation review was the new data that are annotated on the slide that are in these on non-circle symbols here. So that purple here, the triangles here, the diamonds here etc. And what you should take from this is that, in all cases, the symbols lie on the upper right direction relative to the corresponding colored symbols that are transparent circles. These are the sulfidated data and so it clearly shows that as a general rule, sulfidation makes the dechlorination go faster. Although, again, there’s quite a lot of overlap, so it’s not always the case at any particular comparison will show a dramatic improvement in the rate of dechlorination due to sulfidation. There’s a general rule, it’s probably fair to say that it’s favorable for dechlorination.
There’s an aspect of this that I wanna emphasize that’s shown really conveniently with this series of photographs that are shown here. And there’s a corresponding YouTube video that goes with this that you might be interested in watching that’s sort of posted for us. And here what we’ve done is we have taken a color dye, which is hit it now and basically a surrogate for a contaminant. But such that you can actually see the reaction when the color changes and also so that it reacts faster. And what we do is we have this colored dye in a syringe that’s connected to a port in a column that contains aquifer material that has been conditioned to be fairly strongly reducing condition. And when we withdraw the water from the column, it dilutes the dye like this and you end up with this lighter blue looking color.
Now, the important and potentially surprising result here is that poor water which by any other metric is quite strongly reducing conditions in that aquifer, model aquifer there, is not reducing this dye. But then what we do is we push the volume of this water into the column so it goes into the pore space in the column. And then pull it back out again and when we do that it comes out pink which is the color of the reduced form. So the message, what this visualizes and I think very compelling way is that the contaminant needs to contact the mineral phase to be reduced. So it isn’t iron(II) in the poor water that’s responsible for this or anything else in the poor water that’s responsible for contaminant reduction under these ISCR conditions. It’s contact between the contaminant and the reducing mineral surface.
Okay, so now changing to the pathways selectivity core concept. I’ve already briefly alluded to this concept of stall intermediate you should all be familiar with this. With, for example, TCE, if you do hydrogenolysis on TCE represented by the solid arrows, you can get DCE or vinyl chloride. And they tend to be pretty recalcitrant to further reduction. And so you end up accumulating them in situ conditions where biodegradation dominates usually this has been classic…historically has been a big problem. The same thing actually applies to carbon tet, so if you start with carbon tet here and you do hydrogenolysis, you tend to get chloroform dichloromethane, chloromethane. And these also are effectively stall intermediates and they tend to be hard to further degrade.
In principle, you can push the reaction all the way to completion by following these arrows all the way through, but that often doesn’t happen. Instead in practice, it’s more effective to make an end-run around those stall intermediates by going around this way. And so this is the reductive elimination pathway for the chlorinated ethenes and then this is the analogous a reductive elimination pathway for carbon tet. In both cases, it gets you to completely dechlorinated products without forming any of these stall intermediates.
So what is sulfidation do in this case? Well, it turns out that … the data for that is not…there’s not a ton of it. And so I had to can use the kind of raw data from a couple different journal articles. And so here I picked one from Han, and Yan, and from He, and Li. And in this case, this product yield here distinguishes acetylene, ethane, ethane. Over here, they have different palladized irons and then sulfidated iron. And in neither case does this directly quantify the branching ratio to the stall intermediates. But in both cases, you note that they’re not reporting chlorinated products here and here they’re basically, this is the fully dechlorinated product. That little tiny little bit of yellow there is the only chlorinated byproducts that seen. So the bottom line is that sulfidation, if anything, helps with respect to the reductive elimination pathway. But the data is not really that clear at this point and it certainly doesn’t hurt it.
All right, so the last topic here will be this question of efficiency and demand. We have spent quite a lot of time on this in my lab. This is from papers by Dimin Fan when he was in our group. And in this particular case what we’re doing is we’re measuring the production of hydrogen which is utilizing the iron metal and it’s reducing water to make hydrogen. And you can see we’re accumulating considerable amounts of hydrogen here inn an experiment where we have untreated nano-ZVI. If we treat that sample by sulfidation we get essentially no hydrogen production. But over here, we’ve measured the degradation of a contaminant in this system, in this case, another probe contaminant I2S.
And in that case, the rate of the contaminant reduction is about the same. And that’s a really important bottom line conclusion about this is that the…is that sulfidation effectively shuts down the hydrogen evolution reaction and doesn’t do any real harm or significant harm. And then, as I said earlier, it might even help a little bit with the contaminant reduction reaction. So that was summarized in this figure here in one Dimin Fan’s papers. So here we have nano-ZVI electron deficiency, so most of the electrons from the ZVI are going to water and only a little bit actually going to TCE. And when we sulfidated it, it was the reverse, almost none went to water and almost all that went to TCE.
And I have a brand new figure here from a new paper by Fan, He’s group that is illustrates the same thing. So on the left here they have sulfidated ZVI, on the right here they have unsalfidated ZVI. Here they show 92% of their electrons are going to reducing water. And over here, most of the electrons are going to reducing the contaminants.
Okay, so very briefly at the very end here. The field-scale implications of this efficiency issue has to do with the suitability of using fine particulate reducing materials like nZVI in situ. Because there’s this question of what’s the reductant demand? And we did an experiment some years ago where we did a pilot-scale test in the ground which is represented here. These are little sample wells along this gradient here and those correspond to these colored symbols here. And this is the direction of groundwater flow which was from back to front here. And we did an nZVI injection and then pulled water from the sample wells here. This is the nearest one, 0.2 meters, 0.5 meters, and 1 meter. And then you can just visualize as the nZVI injection breaks through. So here’s the water that’s unimpacted, here’s the water where nZVI is coming straight through. Further down gradient, that breakthrough happens later. Even further down gradient, that breakthrough happens even later.
The really interesting result here is the yellow bottles. So the yellow bottles are our nZVI that’s been fully oxidized to iron(III). And that’s because the aquifer had a reductant demand that basically used up the nZVI and produced basically only iron(III) oxides. And so this is really an important question for sulfidation, we haven’t done an experiment like this for sulfidated nZVI, but one would expect that would help protect the reducing capacity of the nZVI and the result would be you get more stability of the reductant and in principle a more effective treatment scenario.
All right, so with that, I’ll be wrapping up here. This is a just to remind you of the scope of what we did. We at the beginning, we talked about definitions in such of in situ chemical reduction. We looked at the technology perspective on this and then since then, I’ve followed through us about half of these core concepts. And there’s a whole bunch of other core concepts that we didn’t get to like the role of impurities, the role of passivation, oxidative pathways, delivery etc. Which you’ll have to wait for another time.
Just to acknowledge the funding for this, it’s almost all come from SERDP/ESTCP. I do also have funding from NSF. We had some funding from NESDI which is a Navy environmental technology deployment program for us a little bit of our work. I’ve had some funding from the DOE Subsurface Biogeochemical Research Program that’s relevant to this. I’ve worked on some projects that have it they utilize some of these technologies most specifically which is Geosyntec and Jacobs. And I get materials from just about everybody that’s a vendor in this business. And so I have made a kind of a laundry list of the companies that have supplied bottles that are sitting on our lab bench. And we continue to work with all of you guys and get considerable value from that.
Just to finally acknowledge the people that did the key work Dimin Fan, who’s, again, now at Geosyntec, is the person that’s responsible for the most of what I talked about today. Ying Lan, was probably responsible for the next most of the material that I talked about. He Ji-quin [SP] is a visitor from Tong Chi University, who’s the author on a couple of papers that it should be coming out in the next few months that are significant in the area of sulfidation. And these guys over here were the co-authors on the sulfidation review. And with that, I’ll leave this slide up with the QR codes for those of you that want to try to look at that and then we’ll turn it over to John.
Dr. Freim: Okay, thank you, Paul. I appreciate your perspective. We’re gonna try to segue here a little bit more into some of the commercial applications. And talk about the different ZVI products that are available, contrast and compare them and try to give you an idea what actually might work when you’re trying to do a true in situ remediation project. So can you click through Paul, please?
Okay, so what I’m gonna show you here if I can get back to the right slide. Is this is an example of the different types of zerovalent iron or zerovalent iron materials that are commercially available? And I’m actually gonna try to classify these from small to large. So that the top here are soluble materials which are divalent iron or ferrous iron species, such as CRS that’s sold by REGENESIS, or some other products that are also available. Get a little bit larger, there’s nZVI which is nanocrystalline zerovalent iron. If you look up here there’s a TEM picture showing the nZVI particles. You can see they’ve aggregated these small primary particles but actually it’s kind of a cluster of larger particles. Get a little bit larger, we’re talking about the colloidal products which are what REGENESIS cells which are AquaZVI and MicroZVI. These have discrete 1 to 3 micron particles on average. You get a little bit bigger there’s a super sac over there on the right showing you commodity iron products, which are typically somewhere about 50 to 100 microns in size. They don’t look real big you can actually kind of feel like flour if you put me between your fingers. And the really big stuff will be cast or scrap iron such as the car that’s being crushed right there.
So there’s different ways to put these things into the ground and these include for the smaller products you have some options. You can either do…put these things in the ground through screened wells which is the really enabling of technology if you wanna go into a site that’s deep. You wanna go through existing wells. You can also put the smaller particles, which are the, you know, the nZVI colloidal particles into using the ground using DPT. And if you wanna use the larger particles such as the Commodity Iron you’re more or less relegated to DPT or soil mixing. Paul can you advance a slide, please.
Okay, there you go there’s some pictures there’s on the right there’s a screen well. On the top where the nano iron can be injected into, they can fill right through the slots in the screen and then on the bottom there’s at the DPT-type application.
So let’s expand upon this a little bit further and talk about the different particle sizes that are available. We’ll start here on the left with the soluble iron which is really not technically a ZVI. But it’s similar so we’re included in the discussion. And as we go to the right we’re going to progressively larger size particle materials.
So let’s talk about kinetics. As Paul talked about earlier, there’s Ksa philosophy where things that have more surface area generally reacting more rapidly and that’s generally true. So for the ZVI products, NZVI will probably react the most quickly and the powdered microscale iron will react more slowly. There’s another flip side to that is its longevity or persistence. Stuff that’s more reactive obviously is gonna be reacted away more quickly so it’s not gonna last as long in the ground. So yeah, it might actually have a pretty short-lived remediation program.
If you look right here in the middle there’s kind of a sweet spot where the colloidal ZVI is located somewhere on 1 to 4 microns. That kind of gives you a… we found over the years of experimentation have the best balance between longevity and reactivity. You actually get material that’s suitable for both. We’re gonna talk about here is transport. This line’s a little bit messier but something that’s higher up is easier to inject and something that’s towards the bottom is more difficult to inject. And obviously, a soluble iron such as CRS is easy to inject, it’s fully soluble. Basically, it can be injected anyplace where you can inject water.
But if you notice there’s a little dip here at NZVI and you think, “Well, these are small particles, they’re gonna be really easy to inject in the ground.” But the science is improved since the early days of what the problem with nZVI particle is that they’re so small that they tend to agglomerate. So you instead of injecting discrete individual particles of nanoparticles that are two microns, you’re actually injecting a cluster. I like to add my analogy, I like to use instead of injecting a single grape, you’re injecting a bunch of grapes. And because of that, you have problems with distribution and generally, your ROIs aren’t as good as they are if you actually make the particles a little bit larger. Somewhere around a micron or two, that appears to be the best size will be able to get a good suspension of the materials. And I’ll actually be able to inject them at low pressure through the pore throats of the sand to get a good plug for distribution.
Once you get bigger up than I would say maybe 6, 7 to 10 microns. You run into that problem where your particles are actually larger than the interconnected porosity in your sand. And you end up having to do fracturing or some sort of iodization of your sand or some disruptive process for your soil and your distribution goes down. And if you have the larger material such as scrap iron materials have to be soil mixed or trenched, which are viable methods in some cases.
So I wanna talk about the REGENESIS product line very briefly. There’s three ZVI products that are currently available. One of them is AquaZVI which is aqueous base suspension of sulfidated iron. So we take iron, we process it, we add proprietary dispersants and/organic additives and we also sulfidated in situ. So when you buy this product you get a 40% ZVI suspension in water that’s sulfidated. That is very, very, very reactive with chlorinated ethenes and some other run water contaminants. This can be co-applied with PlumeStop colloidal activated carbon. You wanna have a mixture of absorption and abiotic degradation.
MicroZVI is more of a legacy product. It’s not Sulfidated and the way we prevent the reaction with water is to use glycerol as our medium. This for all is a food-grade biodegradable donor that’s non-reactive with iron and that allows us to keep a stable suspension. Also at 40%, 2 to 3 microns in the glycerol. These both have a viscosity as shift of about 2,000 to 3,000 centipoise and the consistency of it. Generally, Micro ZVI is co-applied with three DME or other sorts of organic amendments and dechlorinating microbes.
CRS is a ferrous iron product it is an organic compound that has a divalent iron molecule or atom in it. When now…what that does is you co-apply that with your organic donor. And you get in situ reactions that form reduced sulfides, reduced oxides, oxyhydroxides, the species that Paul talked about earlier. And that will also stimulate both abiotic degradation and possibly some of the enhanced bioremediation.
So just briefly, how are these ZVI used for in situ remediation? You know, there’s a lot of different ways to do it. Generally, three different options. One is ZVI Alone should be straight abiotic and maybe a little bit of biogeochemical reduction. The other is to co-apply it with organic amendments. Our products can be it can be co-applied they put in the exact same mix tank, part of the same pressure so they get delivered together and so you have a true synergistic effect. The other option is a colloidal ZVI that is similar particle size to colloidal activated carbon. IT can also be injected together or you’re gonna have a great center just a combination of absorption and abiotic degradation. So with that, I am gonna pass this back to Dane, and he is going to take some questions and we will do our best to answer them.
Dane: All right, thank you, very much, John. Yes, as John said, that it’s gonna conclude the formal section of our presentation. And at this point, we would like to shift into the question and answer a portion of the webcast. Before we do this, just a couple of quick reminders. First, you’ll receive a follow-up e-mail with a brief survey. We really appreciate your feedback so please do take a minute to let us know how we did. Also, you will receive a link to the webinar recording as soon as it is available.
All right, so let’s go ahead and circle back to the questions. The first question here is for Paul. And the question is, “What is a stronger biogeochemical reductant?” And they give three options here, ferrous sulfides, ferrous hydroxides or ferrous oxides.
Dr. Tratnyek: So you can hear me, yeah, correct?
Dane: Yeah, we can hear.
Dr. Tratnyek: Okay. That’s actually a difficult question ferrous sulfide, ferrous hydroxide and ferrous I believe the other one was.
Dr. Freim: He said magnetite.
Dr. Tratnyek: So I’m hesitating because we’re actually working on this right now and we’re… and the more we work on this the more subtle the problem becomes. I guess a couple things I can say that as generalizations that are certainly gonna be true. One of them is that when you compare the numbers the iron sulfides almost always are the fastest they reduced dechlorinate say TCE faster than anything else. The one thing that’s surprising is that magnetite tends to be slow. And an iron(II) added to an iron(III) oxide like greigite is surprisingly actually faster than magnetite. Maybe beyond that, I can’t give more simple set of rules.
Dane: Okay, sounds good thanks, Paul. Let’s see next question here also for you Paul. The question is, “Does ZVI reduce sulfate in groundwater? ”
Dr. Tratnyek: So that’s an interesting question it’s actually a very good question. And when we first started working on ZVI a long time ago I got quite invested a fair amount of time into that problem. Because thermodynamically, that is definitely a favorable reaction and so we thought, “Oh, wouldn’t that be cool if we could do abiotic reduction of sulfate to sulfide.” But there is no evidence that that reaction occurs. So thermodynamically, it’s favorable, but kinetically it doesn’t happen.
Dr. Freim: And this is John here. I’m going to put in my two cents from a practical perspective. It’s actually a good thing that doesn’t happen because there’s a lot of sulfate in groundwater. And that actually could be a pretty high reductive demand and you’d be using your iron to reduce you know, 500 milligrams per liter of sulfate. Instead of them you know, 500 ppb of TCE what you’re actually trying to address. So from a practitioners standpoint, it’s actually good that it doesn’t reduce sulfate.
Dane: Okay, all right, great thank you, guys. Let’s see here next question is another question for Paul. And the question is how much water is reduced to hydrogen by… this is an acronym but I think what they’re saying is sulfidated nano zerovalent iron. So how much water is reduced to hydrogen by sulfidated nano zerovalent iron?
Dr. Tratnyek: Well, the general generalization here is that unsulfidated iron is fairly inefficient and that the majority of the electrons actually go to reducing water and not reducing contaminants. When you sulfidated ZVI, that balance shifts and so it’s actually more favorable to reduce the contaminants than to reduce the water? The putting the exert numbers to that is a little bit TBD that data are just sort of coming out now from different groups about what that amounts to. But I think from what I can recall the sulfidated ZVI the amount of water reduction goes down to like 2010, 1% so really pretty low.
Dr. Freim: And from a practical perspective that’s great too is when you’re spending money on a reductant you wanted to go to your contaminant, not to water.
Dane: Okay, all right, sounds good. Thank you, guys. Let’s see here this will probably our final question. This question is for John and it’s regarding AquaZVI. It’s, “What is the viscosity of AquaZVI when it is injected into the ground?”
Dr. Freim: Okay, that’s a great question, Dane. So the way this works is that when the material ship it comes as a fairly viscous suspension like I said earlier 2,000 to 3,000 centipoise. But when it is diluted in water in field applications it’s typically somewhere about 1% to 2% of the material in your mix tank. And when it’s mixed it’s mixed and suspended the viscosity is essentially the same as water. So injecting an AquaZVI or MicroZVI product or most loyal remediation amendments. The real benefit you get out of that is that you’re basically injecting water. Which means that you can use you know, simple pump such as pneumatic diaphragm pumps and you don’t have to get into the realms of multihundred psi injections.
Dane: Okay, great, maybe time for one more here. This is also another question for you John and the question is, “Does ZVI reduce the stall COCs like DCEs and VC in ground water?”
Dr. Freim: I will defer this to Paul. I’m trying to give basically the answer is yes, but slowly. You can do a treatability study of sulfonated ZVI with this and it will go away. I would say with the half-life’s are probably two to three times as what they would be with TCE. But there is certainly abiotic degradation there are some compounds that are basically recalcitrant dichloromethane is basically recalcitrant and 1,2-DCA is also recalcitrant. Those are the two compounds that if you have them in your groundwater it’s strong related… strongly recommended that you use some sort of biological process using the dehalobacters or some other sort of engineered anaerobic bacteria. You have a comment on that Paul?
Dr. Tratnyek: Well, yeah, just to reinforce that. So the advantage of ZVI is not so much that it degrades the stall intermediates, like DCE like John said. It does sort of but that’s very slow the advantage is it doesn’t make them in the first place because it goes by the other pathway so it avoids making those intermediates. So you have a site where there’s by in situ biodegradation that’s already produced a bunch of DCE and dichloride. ZVI in principle could be useful but that’s not an ideal application.
Dr. Freim: Yeah, that’s correct what you need the ZVI, in that case, is to actually get the groundwater more reducing more oxygenated and make this the environment more favorable for the microbes. But in that case, the primary degradation pathway would be biological.
Dane: Okay, all right, got it. Let’s see here may be time for one last question and that is, I’m not sure which. I think this might be a question for John, but maybe for both of you. It’s, “Can you address the feasibility of injecting chemical reductants into fractured bedrock any success with treating contaminants that have been sorbed in the rock matrix?”
Dr. Freim: I’ll answer that question there’s been colloidal products that have been injected into bedrock oh say I’d say at least the number of the injections at least in the tens. The material, the advantage of a colloidal material is that it doesn’t sink they’re buoyant or they sink very slowly. So they’re not going to fall down to the bottom of your injection while you’re tooling so you can get them out into a formation. I mean, obviously fractured bedrock has got large channels so it’s not that difficult to get it there but you got to keep it suspended before it goes in there. Sometimes the challenges with bedrock would be you know, using Packers or some other engineered injection techniques to make sure that you get good vertical distribution and also horizontal distribution. If it’s saturated you’re gonna be injecting to a saturated zone which is incompressible and doesn’t work too well. But you can give me a call or the RRS crew could also help you with that if you have a project related to bedrock.
Dane: Okay, great well, thank you, both very much. That is going to be the end of our chat questions. If we did not get to your questions someone will make an effort to follow up with you. To learn more about Dr. Paul Tratnyek’s research please visit the website of the Tratnyek research group or you can access the resources from the QR codes in the recording of this presentation. If you need immediate assistance with a remediation solution from REGENESIS, please visit regenesis.com to find your local technical representative and they’ll be happy to speak with you. Thanks, again, very much to our presenters Dr. Paul Tratnyek and Dr. John Freim. And thanks, to everyone who could join us. Have a great day.