3.3 Example 3.1

Li Li: All right. Let's get started on Lesson 3. I suppose everyone has read through the online material. So at this point, you are ready to start the example. Hopefully, you have appreciated the importance of surface complexation and how common they occur and what are the applications as well as what governs surface complexation on clays and different type of material.

So in this video, we are going to go through the example 3.1, which essentially is about setting up a batch reactor for chromium absorption on illite. And we know chromium is a very common contaminant. It can occur in terms of natural occurrence. There are processes in natural system that actually would generate chromium.

It's also why they use it in industry for different type of applications. So chromium can be a tremendous risk to both human and ecosystem health. So clay in natural system usually has the capacity to absorb chromium. So what we are going through here in this example, I'm giving you Table 4 which is the initial conditions for the system.

And essentially, we are really kind of looking at a batch reactor at 25 degrees C. And let's say you have a beaker, 250 milliliters as the volume. And essentially we're adding. So you have this amount of water. This beaker is filled with 250 milliliters of water. And then imagine in that beaker, you have some amount of illite grains.

And I put here illite volume fraction is 0.003, which is 0.3% of the total volume. You can actually calculate what it is. Because we have a 250 milliliters as the total volume. And you also are given the illite-specific surface area, which is 15.36 meters squared per gram.

Now illite is a very common clay material. Some other common clay material like smectite, chlorite, the various type of clay minerals. Clay tend to be very complex its chemical compositions and all kinds of different type of So we pick illite as a representative one, essentially.

Now, so imagine you have all these. And then in the water, you are putting a pH 8.0 and this chromium. What I'm putting here is this concentration, it's before the speciation. So really, it's a total concentration of chromium 6. And then you have sodium chloride, potassium in the background.

So these are what's happened, what's the solution. And that means the second half of the table I'm telling you that there's this surface site SiOH. And this surface site can go through four different surface reactions. One is complexity with H plus to becomes this SiOH2 plus. And then it's log K, its apparent equivalent constant is listed in the right hand side.

Anyway, you have these four different reactions for the surface reaction part. But imagine you would also have all kinds of aqueous complexation reaction happening at the same time. You are not going to just have these surface reactions. So keep that in mind.

So what I'm asking, we went through this Table 4. And what I'm asking here is that, first of all, at the pH of 8.0, I'm asking you to calculate the concentration of the different surface complexes on the surface sites. And what is the pH value of the system which is equivalent?

And these are all equivalent reactions, so you would get a direct number after running the simulation. And if you have initial pH of 4.0, you would do the same thing. And how much difference do you see? So essentially, it's really trying to look at-- by comparing Question 1 and Question 2, you're looking at how much difference does pH make in terms of how much chromium absorbed on the surface.

So let's go through this. There's two things that you actually would need to set up. I talk briefly in the online material texts. One is the database. In order to set up the reactions, you would need the surface reaction. You would need to go through the database.

And in the database, let's see if we start from the beginning. And before that, from previous lessons, you have all the different blocks. We have touched through primary species block, second species block. And then mineral reaction kinetics in the previous lessons.

Here, let's go through the surface complexation. So there's a block listing all these minerals as a surface complexation block. So let's do Control-F to search for surface complexation. And it directly come to me with the beginning surface complexation reaction.

So I already, what I did different in this lecture, a little bit different from previous. Previous one, I tend to start from beginning and we go kind of one-by-one step. Here, I'm already putting in these reactions. Because that will save us a little bit of time. I don't want the video to be too long. Because at some point, it gets boring.

So in this, so what you will need to do in the beginnings of the complexation part you will be putting all the different reactions listed in table. You have four reactions. So you would need four different surface reactions included here.

Now the way I load it is using the products, the absorbed species. So if you observe, for example, the first one here, it's SiOH2 plus. It's actually this one, SiOH2 plus.

Whether you use this like three line equal or you use this larger It doesn't really matter. Because the code we're reading essentially is a text file. And as long as you have consistent representation in the input file and in the database, it can recognize it. It doesn't matter if you use this larger than or if you use this triple equal sign. But the problem will come if you are not consistent between input file and database.

So anyway here, in the input file, I write this way. So essentially, you can see it's this species, similar to the aqueous complexation. So you have this species and equals to 1.0 H plus press 1.0 this SiOH species.

Now it's opposite way of writing as in this table. And this apparent K is written in terms of the reaction in this form. So when we do this form, since it needs to be negative now, so it's minus 0.95. Again, we are looking at 25 degrees C.

This, as a whole form, it is very similar. It's essentially the same as what you see before for the aqueous complexation. So you can see think about surface mixing as really almost like aqueous complexation, except that you are having a reaction with species on the surface. And so, all these different reactions are written similarly.

So this first item is for the first reaction. And then the second one, you have SiO minus as on the surface. And this is equal to essentially minus 1 from this SiOH. So this is another reaction. Now again, in the table, you have minus 6.59. And it's because the reaction is in opposite directions. This needs to be 6.59.

And all these other 500.0's as I mentioned, these are for-- you could have a constant at a different temperature. And you can really ignore them. But you do need to have eight items to represent the eight temperature points. And similarly, you have SiO sodium and SiOH and H2 chromium species. So as long as you pay attention to the sign of the log K values, be consistent with how these terms in terms of reaction are written, it's in the opposite way that is listed in the table, you should be fine.

So this is for the surface complexation block in the database. And then a second item in the database for surface complexation is giving the charge for each of these surface complexes. What is the charge of each of these species? So for example, SiOH is zero. SiOH2 plus is 1.0. Minus is minus 1.0. And you have all these other species. So your list is there. So essentially, two items in the database block.

And then in the input file block, let's look at this. Again, I already put everything in. But we walk through and you kind of need to know. So all these ones we talked about before, these are kind of computation long time

And then, the discretization is for, essentially, total volume. So we have 250 milliliters. I'm putting centimeters as units. This is essentially OK. We only input 250 for x zone as one cell. But essentially, the default is the y zones. And z zones is 1.

So it's like one group block in each direction, so giving you a well-mixed one group block, essentially. If you want to specifically put in y zones, 1 1 and then z zones 1, that's fine too. You can look up the discretization in the Crunch menu.

And then you do need to put in illite. The reason is that because in CrunchFlow, all the surface sites, you need to specify which mineral is this on. So here, this SiOH site is on illite. So you have to have illite there. So in the mineral, you would need to put illite as a mineral.

For systems that you have, for example, you have mineral dissolution precipitation and you also have surface complex. So you might have multiple minerals there. And you might have multiple minerals that have surface complexation sites. And you can put more than these when you have more complex systems.

So this is for surface complexation. And then we go to the condition. Now in the condition, we set up the initial. Because this unit should be really mole per liter. I think in the table, we put mole per liter. So this is mole per liter. Temperature, 25. And then you have pH 8.0. Chromium, sodium, chloride, potassium, they are all listed in the Table 4 in the online material.

Now I do have a few more primary species than what you have in what I have in the table. And the reason, why do I need to do that? The reasoning is that we have illite. So as long as you have-- let's go through what is illite into mineral dissolution reaction. We don't really put mineral dissolution reaction. But as long as you put illite, then you need all the building blocks in aqueous phase, essentially.

So let's search in the database, how does illite-- what is the composition of illite of terms of different chemical species. So I do F for illite. So you have illite. Look at here. So this illite, imagine you will be writing illite as a solid phase dissolving at-- actually plus H plus, 8H plus, and then dissolving to become 0.25 magnesium, 0.6 potassium, 2.3 aluminum, 3.5 silica, and water. And then the rest of these are the eight equivalent constants in different temperature.

And then you have the molecular weight or whatever. And in any case, you can see is that in the illite block, illite is composed of these different cations. So although in the table, I only give you chromium, sodium, chloride, and other species, because illite as a mineral contains magnesium, silica, aluminum as additional species. So we do need to put in the input of five for this species as part of the primary species. So that's what you actually would do.

So these species are already given, essentially, right in the table. And these species are additional because of the inclusion of illite as a mineral. And you need all the building blocks for illite. Otherwise, the code is not going to recognize it.

So all these species are, for example, pH chromium, sodium, chloride according to the concentrations that are given to you in the table. And then SiO2 aluminum magnesium, let's assume that, for example, illite dissolution would be very slow. Let's say you have this. You are doing an absorption experiment. Usually, you want to look at how much does chromium absorb on the solid phase. Usually you do it using a very short period of time. And illite dissolution is relatively slow. So it wouldn't dissolve that much in the solution.

So let's imagine your solution doesn't have much of these species. I could put zero. But usually, I prefer to put this small number instead of zero. A lot of time, computers have some problem with zeros. Try to avoid zero if you can.

So these are the primary species. And then you have the site name. This is a name of site that you put in the database. Now, as I mentioned, if you use this three line equals sign in here, then when you put in database, you also need to put three line equal sign.

So here, this is a site density, which the unit is mole per liter squared. It's 1.0 times 10 to minus 6, which is given in the table. And then you have this illite. 0.003 with a specific surface area of 15.36 meters squared per gram.

So that's a condition block. You not only should put in all the species that are given to you in the table, but also the species that are part of building block of the illite, the mineral that the surface site is on. And then, actually, this probably should be in earlier. But anyway, the order doesn't really matter in the input file.

But just look at the secondary species. You have these. I put these secondary species. But some of them may not be important. We can buy it and go through it in the output file. So once you have this, let's look at this folder that we are going to run the simulation for this example 3.1.

So I click on this one, I get see our example 3.1 dot [INAUDIBLE]. So it went through. It just spit all the output file. But the key ones that you will be looking at would be-- so see our example output file. This one. Maybe this one. So let's look through this example.

Again, so at the early part of this, it'll be walking through your input file and database. Look for everything, for example, number of components, number of secondary species, number of kinetic minerals, and goes through all the K value matrix, essentially.

And if you want, some time when you debug, for example, if something went wrong and you don't know what was wrong with it, it's also hard to look through the early part of this. For example, you might be not putting the right log K values in these two problems when you do the calculations.

So again, these are the condition input, condition initial. We didn't really put a charge balance here. But right now it's a total charge of negative. So probably we should, if we want charge balance, it should be really, the sodium species would need to be put charge balanced.

Now, here. Let's look through. So this, essentially, is a table that is giving you all the concentration of different species, log molality, log activity, and everything. Let's look in log molalities concentration you get.

So the first several line for all the surface species. And then you have H plus, sodium, all the aqueous species here. So let's look at chromium. We are looking at chromium absorption. So it'll be interesting to see what are the dominant species. In the table, you're given a total concentration of chromium 9.6 times 10 to minus 5 mole per liter.

When you look at this, it's H chromium O4 minus H2 chromium O4 and then chromium O4 These are in orders 10 to minus 4, 10 to the minus 5, 10 to the minus 15. And then on the surface, we are supposed to have the surface species of this. But it's very, very small. It's 10 to minus 10, essentially. Very small numbers.

So that means-- what does that mean? You have this 10 to minus 10. And then you also have chromium CrO4 2 minus is 10 to minus 4. 10 to minus 4. And then these are 10 to the minus 5, 10 to minus 15. So what does that mean?

Do you think chromium absorbed on a solid phase or not? If you think about it, comparing the different species, on the solid phase, you are supposed to have this surface species, you have very low concentration of 10 minus 10. And chromium is-- chromium O4 is 2 minus.

So then if you compare this, you know this is, essentially, actually most of chromium is still in the aqueous phase. It's not absorbing much at all at this pH. Solution pH is still 8.0. So it's not really reacting much. Because the absorption reaction doesn't at these high pH values.

All right. So you can pull out these numbers and answer the question, get a table, answer the question. What's the concentrations of different surface complexes on this and the pH value.

And then second question is pH is 4.0. So let's change a condition here. If I change this condition to 4.0, just change the pH to 4, what happens? So I changed it to 4.0. And let's rerun it.

So what happens, I might have mentioned this before, when you run this you can delete all the odd profiles if you want. But you can also just leave it there. Or you can build another folder, if you want, to keep all the odd profiles.

But if you run it in the same folder, the new simulation would overwrite the old file. So you will get the new file with pH at 4. So let's just try this. I need to save this so it will actually reflect. So let's run it again.

OK, because I changed the concentration. This is a solution that is relatively-- I guess it doesn't take-- you would still need to take mole per kilogram. It doesn't really matter when you have relatively dilute solution like we have here. So we have mole per kilogram. Let's re-write. It doesn't recognize mole per liter. Let's do it again.

It's done. Hopefully, it's giving me-- I'm just trying to update it like go to our folder and then we open, make sure all these files are the new updated files. You can look at the time to estimation. It needs to be the latest time.

Anyway, so you have things-- So the output file should be this one. Looks like it's still organizing. It's still not green yet. It's actually ready. So it's full. Every thing else is the same. We can just go down and look at the last block that lists all the concentrations. So now it's pH 4.0.

So we can look at the same, for example, different species for chromium. Chromium, so HCrO4 is still pretty high, minus 4. And then this is H2. It's minus 9. Now in chromium, CrO4 is 10 to minus-- so the aqueous species dominant one is HClO4 now because you have much more hydrogen now. It's a much acidic condition.

Now here you also observe that SiOH, this surface species, before it was in the order of 10 to the minus 10. Now it's 10 minus to minus 4.8 essentially. So increased by about five orders of magnitude, which means you absorbed much more chromium on solid than what do you had before. So pH has a huge impact on how much you can absorb on solid phase.

Let's see if I try pH equal to 3.0, what happens? Let's try it again. Let's close this. And change this to 3.0. So it's updating. Going back. Let's look at the output file.

It's here. Now here with pH 3, we increased more on the solid phase. So essentially, you would have absorbed species this much. So it's in the order of 10 to minus 4.5. So between pH 4 and 3, they all already have a lot of different hydrogen ions in the system. So it doesn't change much as when you change from pH 8 to 4.

Now the reason, here's why. Chromium 6 with a pH 8.0, the dominant species, it's an active charge species. So it tends to absorb when the system, when the surface site, has a lot of positive sites.

Now at high pH condition, the aqueous solution have a lot of OH minus instead of H plus. So the illite surface tend to be negative. And because it's negative, it wouldn't attract much of the negative chromium on the solid surface.

Now when you increase acidity or decrease pH to 3 or to 4, you have a lot of H plus in the system. And this H plus will go through these reactions with the surface site to form SiOH2 plus. So you would have a lot of positive charges on the surface. Now, in that case, chromium would be attracted to the positive charges surface. So that's why under low pH condition, you can absorb much more chromium than the situation of high pH.

So now imagine, this is for chromium and it's a negative charged species. What if you have, for example, cadmium or zinc or other species that these cations that are positively charged, they tend to be attracted by negative charged surfaces.

So think about what condition, under what condition you tend to have more surface complexation for these cations. The answer would be opposite to what you have here. So for example, these heavy metal cations, you tend to see more absorption under high pH conditions.

I think we can end now, I believe. I talked about everything I need to discuss. And just as a reminder, if you're not clear about what you are going to do and you want a bit more detailed information from the manual or everything, I listed in the online materials that for surface complexation examples, the menu, CrunchFlow menu page 6-3, 6-4, and 6-9, they are going through different blocks in Crunch.

And we uploaded a set of exercises with input file and documents that explain what each exercise do. So these are good resources again, good example files for you if you need something to look up. So in that exercise folder, let me just pull it up so you can see it.

In CrunchFlow related, I added a CrunchFlow example exercises. So in that photo, we have photos for different exercises. And in Exercise 4 is for a surface complexation example, if you would like to look up.

Let's stop here. And I hope you had fun working on this and the rest of the homework. For question one homework, which is an extension of example 3.1, you have total chromium. This is essentially the same. But essentially, I added one more surface species, which is aluminum.

And with that, you would need to set up in your database with this species like the site density would be in the input file. And also, all the reactions that are associated with the database, you need these reactions. You also need to put in the beginning surface complexation block to define it. And then you can run the simulation.

So remember, you need to update database. You need to update that input file to run the whole suite of thing. All right. I will stop here and have fun.