Chemical -v- physical reactions.

[Elegance Coral]

"Most organics bind to GAC in a physical way. That is a physical process that results in removing chemicals from the water. They do not chemically react with the GAC."

"GFO and aluminum oxide and lanthanum binding phosphate are all chemical processes as new chemical bonds are formed."

Hello Randy.
The above are quotes from you in a different thread. I honestly do not understand some of these reactions, and was hoping you could elaborate on them.

I think I understand the physical process with GAC, but please correct me if I'm wrong. GAC is simply a very porous material. Organic molecules get trapped in these tiny pores and therefor removed from the liquid in a physical manner. Is that even close to accurate? LOL

I think I pretty much got the lanthanum process too. Lanthanum (in a liquid) binds with phosphate to form a new molecule, lanthanum phosphate, which is virtually insoluble. A chemical reaction.

What I don't understand is how a solid like GFO and Aluminum oxide remove PO4 through a chemical reaction. I have always, and obviously incorrectly, assumed that GFO and aluminum oxide worked through a physical process. Similar to GAC.

I'm not trying to be a Troll, and I'm not trying to debate anything here. I honestly do not understand, and was hoping you could take a moment to explain these processes.

Thanks in advance for any help.
Peace
EC

 

[Randy Holmes-Farley Verified Community Expert]

Sure.

So to get past the definitions aspect, and noting that some people have different definitions, my definition of a chemical process (as opposed to a physical process) is that it involves chemical reactions. So anything not involving chemical reactions, even if it involves complex chemistry issues, is not, by this definition, a chemical process.

This definition is how the terms chemisorption and physisorption relate to adsorption into surfaces (bear with me, this relates to GAC binding organics, for example).

Chemisorption (short for chemical adsorption) is defined as adsorption onto a surface where a chemical bond forms to the surface. Oxygen landing on a bare iron metal surface and forming iron oxide is an example.

Physisorption (short for physical adsorption) is defined as adsorption onto a surface where no chemical bonds form. Water adsorbing on gold, or an organic adsorbing on GAC are examples. It is weakly attached to the surface through things like van der waals interactions. [van der waals interactions come about by the electrons of one molecule interacting with the permanent and transient electric fields of other molecules; it is a very weak interaction, but also very important when stronger interactions do not dominate the interactions.]

In the case of GFO and aluminum oxide you are essentially forming a thin layer of ferric phosphate or aluminum phosphate on the surface. So in that sense it is not appreciably different than lanthanum precipitation except it is a layer in two dimensions along the surface rather than all through the lanthanum phosphate crystal. In both cases, new bonds are formed, and these are all ionic bonds.That said, I think lanthanum actually forms mostly lanthanum carbonate with some phosphate bound to it and in it, so it is not pure lanthanum phosphate either (as best I currently understand what is precipitated from seawater). So these would all be chemisorption and I'd call them chemical processes.

In the case of GAC, the pores are important for providing a very high surface area, but the organics do attach directly to the carbon surface. There isn't any covalent bond formed, but mostly van der walls interactions and perhaps some hydrogen bonds. So that would be physisorption and I'd call it a physical process.

 

[Elegance Coral]

Thanks for the reply.
I read it once. I'll need to read it again when I get home from work.

Keep up the good work.
Peace
EC

 

[Randy Holmes-Farley Verified Community Expert]

You're welcome.

I'm more than happy to more carefully discuss anything that wasn't clear or seems wrong.

 

[Cory]

I understand it visually like this:

Chemical reactions:

Two atoms bind together and exchange something. They are sort of attracted magnetically because they are usually somewhat opposite. And opposites attract. Thats the reaction. An exchange of something for something. When they exchange they get stuck together and form something new. Lol

Gac:

The pore size in gac lets pretend on the surface is 1 micron. While an organic is .8microns. So its small enough to get through the surface of gac. As it travels passed the outer layer of gac, the tunnel gets smaller and it gets stuck. However water is still passing through because its smaller than all of them, pretend .01 microns. So the water is pushing on the organic and keeping it inside the gac or it simply gets stuck.

Good video!

 

[Cory]

This is the simplest video about reactions ive found:


This is a pretty good one too, although more complex:

 

[Elegance Coral]

Thanks again.
So this brings up another question. When phosphate is adsorbed onto the surface of calcium carbonate, is this a chemical reaction? Is calcium phosphate, or some other new molecule formed?
Peace
EC

 

[Randy Holmes-Farley Verified Community Expert]

Yes, phosphate binding to the surface of calcium carbonate is a chemical reaction because it is forming calcium phosphate in the near surface region. A new calcium-phosphate ionic bond forms between the calcium ion with its positive charge and an oxygen on the phosphate with its negative charge. It is not necessarily a complete layer, but a single phosphate replaces a single carbonate at the surface. As more and more phosphate binds, it looks more and more like calcium phosphate, but may never have the exact crystal form of calcium phosphate.

Same for magnesium getting onto and into the near surface region of calcium carbonate, messing up the calcium carbonate crystal structure and reducing the precipitation of further calcium carbonate. This magnesium binding is a big part of the reason why calcium and carbonate can be supersaturated in seawater.

I describe that here:

http://reefkeeping.com/issues/2006-06/rhf/index.php

from it:

Figure 9. A diagram of solid calcium carbonate in a solution containing calcium (white), carbonate (red) and magnesium (black). Magnesium ions replace calcium ions in the structure, and change it chemically so that it no longer looks like calcium carbonate on the surface. The magnesium penetrates a short distance into the surface, but cannot penetrate the entire structure.