[curiouscat]

I'm trying to understand this observation: We had organics-contaminated-effluent-water with 3.5% organics (rest NaCl  16% & NaOH 1%). Extracted it with toluene & the organics extracted out poorly. Final organics content 3.3%.

Next, we treated original contaminated water with H2SO4 & re-extracted with toluene (pH~1). Magically organics are down to less than 1%.

Is this sort of behavior known? Does pH have a huge effect on extractibility of organics?

*Organics are being assayed using Chemical Oxygen Demand as a surrogate.

[discodermolide]

May be that some of the organics were present as the Na salt(s). Acidification re-protonated them and they extracted.

[curiouscat]

May be that some of the organics were present as the Na salt(s). Acidification re-protonated them and they extracted.

Interesting. Is there any easy way to test this hypothesis. Out of academic interest purely. To the process it doesn't matter. But deeper understanding never hurts.

Another motivation to know what's happening is that we are still stuck around 1% organics. Ideally we'd like to go lower than that.

Any tricks up your sleeve?

[discodermolide]

Well remembering some of the processes you have described here finding out what is going on may be difficult assuming the waste comes from them!
Give it two extractions or try extractions at a higher temperature.
Chlorinated solvents may be a better option, but that may be too expensive and may generate more waste than you are treating.

[Arkcon]

As I understand it, the partitioning coefficient will have to be determined experimentally.  So under controlled conditions, determine how much is extracted out of a defined (i.e. prepared in the lab) sample of contaminated water, or a certain volume into a certain volume of toluene.  Once you know that, it should scale, percentage-wise, with a real world sample of much great volume, proportionally.

That's a lot of caveats, but when you get to planning it out, you should see how it all fits.

N.B.  your treated effluent still has organic in it.  You've compensated for the toluene that has now dissolved in your treated effluent after extraction, right?

[DrCMS]

From memory toluene has a solubility in water of ~0.05% which I think is about 50% more than the solubility of water in toluene.