Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: mach747 on 30/10/2004 00:26:37

Title: Phosphate adsorption by clays
Post by: mach747 on 30/10/2004 00:26:37

Hi all,

I'm just dying to get this out of my system !!!!!!! [:D][:D]

Based on the following quote, why do clays end up with positively charged surfaces and why is phosphate adsorption by clays favored by low pH levels?

“Sorption of phosphates and polyphosphates on surfaces is well known, particularly onto clay minerals, by chemical bonding of the anions to positively charged surfaces on the clays. In general, high phosphate adsorption by clays is favored by low pH levels.”

Title: Re: Phosphate adsorption by clays
Post by: chris on 30/10/2004 11:09:04

I'm not too sure about this. Phosphate groups e.g. PO4 are quite negative, whilst clays are alumninium and silica rich. The electronic attraction of the negatively charged phosphate groups for the aluminium ions could underlie this, but I'd be guessing...where's Ylide, he knows the answer to this sort of thing.

Welcome to the forum, by the way.

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

Title: Re: Phosphate adsorption by clays
Post by: mach747 on 30/10/2004 16:05:47

Thanks for the note, Chris. [8D] I'm still trying to figure that one out. And who's Ylide ? Hopefully, he will pick it up soon ...

Title: Re: Phosphate adsorption by clays
Post by: Ylide on 01/11/2004 11:35:30

You have a lot of faith in me, considering my main focus is organic chemistry.  =)  

But, you're basically on the right track, Chris.  The reason that this happens better at low pH is twofold.  Firstly, because the phosphate ions will be protonated, helping polarize the phosphate.  Normally, unprotonated phosphate isn't very polar due to delocalization of the negative charge.  (or resonance, whatever you feel like calling it)  If that negative charge is occupied in sigma bonds with hydrogen, you get an overall increase in polarity.  

Secondly, the phosphate-bound hydrogens can undergo hydrogen bonding with the oxides present in the minerals that make up the clay.  Aluminum and silica minerals also usually contain a large amount of oxygen.  This is secondary to the polar attractive forces, however.







This message brought to you by The Council of People Who Are Sick of Seeing More People

Title: Re: Phosphate adsorption by clays
Post by: lab001 on 12/01/2005 11:54:50

I'm suppossed to do some reading about this stuff. Does anyone know good literature?

Thanks

Wake me up when it's over!