## How to identify reaction (leaching) mechanism?

Discussions on general chemistry and chemical engineering, organic chemistry, analytical chemistry, etc.

### How to identify reaction (leaching) mechanism?

Hi there!
One part of my project is about determining the release mechanism of a toxic elements from a rock. I am into study the kinetic release and identify the key parameters which can affect the leaching reactions. However, I am not sure about how I can study and run my test to reach the mechanisms. I mean, we know that our leaching has an overall reaction but this reaction should not be an elementary reaction and there should be some intermediate elementary reactions which can be also the rate-limiting steps. Do you mind tell me how I should study and run my experiments to reach this information?!
mj.mirazimi
Forum Neophyte

Posts: 15
Joined: 30 Aug 2010

### Re: How to identify reaction (leaching) mechanism?

mj.mirazimi,

Welcome back to the forums! It's been quite a while since your last post - I'm surprised you were able to keep track of your account name and password all these years.

My first thought is to attack the problem like this:
1. Determine what chemical species and elements are involved, especially those for:
1. the toxic elements;
2. the solvent that they're leaching into; and
3. the rock.
2. Create a model. A complete, computer-runnable model would be ideal, but not necessary at this point. Just, what do you expect the rocks to do over time as the solvent encounters them? How do you expect the leaching process to go forward?
• If you were able to form a complete model, run it - either on a digital computer or in your head - and use it to determine your answers, and you're done. If not, continue.
• If you weren't able to form a complete model, try to identify the major unknowns and weak points in the model you did assemble, then fix those weak points.
• Were you unsure about what chemical species are present in the rocks? Analyze the rocks.
• Are you unsure about what sorts of reactions the toxins have with the solvent? Looks up 'em, or if they're truly exotic, fall back on lab experiments or/and ab initio simulations to figure it out.
• Keep going back to Step 3 'til you're done.
So, where are you at in this process? Do you already know what chemical species/elements are involved? What reactions are taking place? What experimental leaching rates are expected?

Anyway, at the end you'll want to validate your model by making-and-confirming its predictions about leaching rates, assuming that you intend to report it as the leaching mechanism.
Natural ChemE
Forum Moderator

Posts: 2744
Joined: 28 Dec 2009
 mj.mirazimi liked this post

### Re: How to identify reaction (leaching) mechanism?

Hi,

Thanks so much for your kind response :)
I am so happy to be here one again.
Actually, I am in the first (and also I think the most important) step; designing my project. However, when I was reading about the differences between the elementary and non-elementary reactions, I got confused. I searched so much, but there was not any specified method, how we can determine the elementary reactions (intermediate reactions) that can make up the overall reactions. I am into study the release mechanism for my target toxic elements and now that after leaching experiments I will have some concentration data over the leaching time. So, I can identify the reaction rate law (for overall reaction) and maybe the overall chemical reaction. But about the intermediate reactions which can be rate limiting steps and they can determine the release mechanism, I am not sure how to identify them!
mj.mirazimi
Forum Neophyte

Posts: 15
Joined: 30 Aug 2010

### Re: How to identify reaction (leaching) mechanism?

mj.mirazimi,

I think that most folks just propose various possible mechanisms, then attempt to fit the data to each 'til they find a mechanism that seems to work.

With regards to leaching, you may have more to consider than just simple chemical kinetics. This is because textbook chemical kinetics assume well-mixed solutions with the same chemical concentrations throughout, though with leaching, you're looking at coupled transport equations. This could potentially be a complex problem.

A classic example of kinetics coupled with transport phenomena is the Deal–Grove model, which was an early tool for helping engineers oxidize computer chips appropriately. In the oxidation process, oxygen both moved through the silicon and reacted with it, so it was insufficient to merely do kinetics or transport alone. More robust (and complicated) models are in use today.

In your case, you'll want to figure out what the rocks are made out of, what the solvent's made out of, what the leaching chemicals are, etc.. Then you'll need to figure out how all of that stuff reacts and moves around.

My suggestion is that you compile everything you know, then start attacking the knowledge gaps 'til you know enough to make whatever predictions you need. We can recommend a lot of tools to attack specific knowledge gaps, but since the overall attack design will require knowledge of the exact problem situation, you'll have to do the hard work laying out the basic foundation.
Natural ChemE
Forum Moderator

Posts: 2744
Joined: 28 Dec 2009
 mj.mirazimi liked this post

### Re: How to identify reaction (leaching) mechanism?

Can you model this using something like Matlab Simulink?

BioWizard

Posts: 12071
Joined: 24 Mar 2005
Location: United States
Blog: View Blog (3)
 mj.mirazimi liked this post

### Re: How to identify reaction (leaching) mechanism?

Thanks for your help and kindness :)
Based on your suggestion, I have to think more and collect all the data include all chemical reagents presented in my leaching system and after that I can draw what kind of possibilities I would have in my system. After that we can assume what kind of intermediate elementary reactions we can have to make an overall reaction

Natural ChemE » June 18th, 2016, 3:47 pm wrote:mj.mirazimi,

I think that most folks just propose various possible mechanisms, then attempt to fit the data to each 'til they find a mechanism that seems to work.

With regards to leaching, you may have more to consider than just simple chemical kinetics. This is because textbook chemical kinetics assume well-mixed solutions with the same chemical concentrations throughout, though with leaching, you're looking at coupled transport equations. This could potentially be a complex problem.

A classic example of kinetics coupled with transport phenomena is the Deal–Grove model, which was an early tool for helping engineers oxidize computer chips appropriately. In the oxidation process, oxygen both moved through the silicon and reacted with it, so it was insufficient to merely do kinetics or transport alone. More robust (and complicated) models are in use today.

In your case, you'll want to figure out what the rocks are made out of, what the solvent's made out of, what the leaching chemicals are, etc.. Then you'll need to figure out how all of that stuff reacts and moves around.

My suggestion is that you compile everything you know, then start attacking the knowledge gaps 'til you know enough to make whatever predictions you need. We can recommend a lot of tools to attack specific knowledge gaps, but since the overall attack design will require knowledge of the exact problem situation, you'll have to do the hard work laying out the basic foundation.
mj.mirazimi
Forum Neophyte

Posts: 15
Joined: 30 Aug 2010

### Re: How to identify reaction (leaching) mechanism?

Thanks for your suggestion! I have not used Matlab in my experiments, is it possible to put some reagents and chemical components in the software and get possible chemical reactions between them?

BioWizard » June 18th, 2016, 4:35 pm wrote:Can you model this using something like Matlab Simulink?
mj.mirazimi
Forum Neophyte

Posts: 15
Joined: 30 Aug 2010

### Re: How to identify reaction (leaching) mechanism?

No it won't predict chemistry for you. But it will let you model reaction and diffusion kinetics relatively easily.

BioWizard

Posts: 12071
Joined: 24 Mar 2005
Location: United States
Blog: View Blog (3)
 mj.mirazimi liked this post

### Re: How to identify reaction (leaching) mechanism?

Oh, I see, it could be so helpful for me to study the leaching kinetic. thanks :)
I was wondering why there is not any specified method for predicting the elementary reactions, when I was reading the text books or even research articles, they just wrote some elementary reactions for their overall reaction, but they did not say how they can say these reactions are the right ones as there are numerous possibilities if we have for example 5 or 6 reagents in our system.
BioWizard » June 18th, 2016, 5:11 pm wrote:No it won't predict chemistry for you. But it will let you model reaction and diffusion kinetics relatively easily.
mj.mirazimi
Forum Neophyte

Posts: 15
Joined: 30 Aug 2010

### Re: How to identify reaction (leaching) mechanism?

mj.mirazimi » June 18th, 2016, 5:22 pm wrote:I was wondering why there is not any specified method for predicting the elementary reactions, when I was reading the text books or even research articles, they just wrote some elementary reactions for their overall reaction, but they did not say how they can say these reactions are the right ones as there are numerous possibilities if we have for example 5 or 6 reagents in our system.

Once you identify the chemical species and elements in the system, you can start to assemble the set of relevant reactions.

For example, because your system probably includes water, you'll probably want to include water's dissociation, either
$\begin{array}{c c c}&{\fs{-1}{{K}_{\text{water}}}}&\\{{\text{H}}_{2}}{\text{O}}&{{\longrightarrow[50]}\\{\longleftarrow[50]}}&{{\text{OH}}^{\text{-}}}+{{\text{H}}^{\text{+}}}\end{array}\hspace{28}$ or $\hspace{28}\begin{array}{c c c}&{\fs{-1}{{K}_{\text{water}}}}&\\2{{\text{H}}_{2}}{\text{O}}&{{\longrightarrow[50]}\\{\longleftarrow[50]}}&{{\text{OH}}^{\text{-}}}+{{{{\text{H}}_{3}}{\text{O}}}^{\text{+}}}\end{array}$
are customary.

If you can find prior work on your chemical system, you might be able to find an existing compilation of known reactions. If not, you'll have to figure out what chemical reactions may be involved, which is something you may've had a lot of classes on back in college. In difficult cases, you may have to use ab initio simulations or/and lab experiments to figure out what the chemicals do with each other.

So, first you want to figure out everything that's in your system, then you can start compiling a list of possible reactions.
Natural ChemE
Forum Moderator

Posts: 2744
Joined: 28 Dec 2009
 mj.mirazimi liked this post

### Re: How to identify reaction (leaching) mechanism?

Exactly, I will prepare a list of my chemical components and the possible reactions can happen between them. Thanks again for your kind help and responses :)

Natural ChemE » June 19th, 2016, 2:07 am wrote:
mj.mirazimi » June 18th, 2016, 5:22 pm wrote:I was wondering why there is not any specified method for predicting the elementary reactions, when I was reading the text books or even research articles, they just wrote some elementary reactions for their overall reaction, but they did not say how they can say these reactions are the right ones as there are numerous possibilities if we have for example 5 or 6 reagents in our system.

Once you identify the chemical species and elements in the system, you can start to assemble the set of relevant reactions.

For example, because your system probably includes water, you'll probably want to include water's dissociation, either
$\begin{array}{c c c}&{\fs{-1}{{K}_{\text{water}}}}&\\{{\text{H}}_{2}}{\text{O}}&{{\longrightarrow[50]}\\{\longleftarrow[50]}}&{{\text{OH}}^{\text{-}}}+{{\text{H}}^{\text{+}}}\end{array}\hspace{28}$ or $\hspace{28}\begin{array}{c c c}&{\fs{-1}{{K}_{\text{water}}}}&\\2{{\text{H}}_{2}}{\text{O}}&{{\longrightarrow[50]}\\{\longleftarrow[50]}}&{{\text{OH}}^{\text{-}}}+{{{{\text{H}}_{3}}{\text{O}}}^{\text{+}}}\end{array}$
are customary.

If you can find prior work on your chemical system, you might be able to find an existing compilation of known reactions. If not, you'll have to figure out what chemical reactions may be involved, which is something you may've had a lot of classes on back in college. In difficult cases, you may have to use ab initio simulations or/and lab experiments to figure out what the chemicals do with each other.

So, first you want to figure out everything that's in your system, then you can start compiling a list of possible reactions.
mj.mirazimi
Forum Neophyte

Posts: 15
Joined: 30 Aug 2010