Metal finishing Q&As since 1989

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Charge of a nickel dithiocarbamate complex molecule

When dithiocarbamates are used to remove nickel ions from wastewater; what is the charge on the nickel carbamate complex? Is it negative or positive?

Ron Vidra
- Middletown, Pennsylvania, USA
 

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I'm not sure if I understand the question, but my answer is that if you are wondering what polyelectrolyte to use, it's anionic :-)

Also note that the floc that is formed can be so tiny that you may need proprietary coprecipitants to get it to settle.

DTC chemistry is basically sulfide chemistry, and is similar to hydroxide chemistry except that the solubility of sulfides is much lower than hydroxides, and the added strength of the bond allows complex-breaking that hydroxide precipitation can't accomplish.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
 

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Ted,

Really interested in your answer. Would be interested in refs as to how to decide on poly electrolyte type and as to your ref as to need of other cation or coprecipitants to help pptn. Is ppt from Dithiocarbamate any diff to just poly sulfide?

Geoffrey Whitelaw
- Port Melbourne, Australia

I don't have any specific refs as to why to use anionic polyelectrolytes on metal finishing wastewater, Geoff, but I have read it a few places and have never seen cationic or nonionic polyelectrolyte used in a metal finishing installation. I have worked with a good number, and anionic polyelectrolytes worked well in the lab in every case as well as in practice, whether hydroxide, calcium polysulfide, DTC, or Degussa TMT15 was the precipitant.

I have personally never observed a clear and convincing difference in the effectiveness of calcium polysulfide vs. DTC, although DTC seemed to be a smidgeon more powerful on ammoniated zinc.

The need for proprietary co-precipitants is due to the way DTC is usually used (as a polishing treatment) rather than anything special about DTC as far as I know. If you remove, say, 99 percent of the metal with hydroxide and then use DTC on the filtrate, there is so very little metal precipitating that it is virtually colloidal instead of agglomerating. Even with a polyelectrolyte, you basically get just a cloud in the wastewater. The special co-precipitants provide body so that there is something of substance to precipitate and settle. What exactly they are, I don't know--they may be nothing more complex than an aluminum or iron salt--but I just used the proprietaries that the client bought, I didn't try to reverse engineer them.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
 

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I believe: Zero. Each DTC ion is univalent (the charge is distributed over the two sulfur atoms) and I would guess that two DTC ions come in, on the same plane, on either side of the nickel ion. The vacant d orbital of the Ni2+ is then occupied - I gather in a square planar configuration.

I, too have had the problem of colloidal suspension of the Ni-DTC complex. I've found it helps to: 1) Use the absolute minimum of DTC and 2)Add a bit of alum or sodium aluminate solution to aid coagulation.

Dave Wichern
Consultant - The Bronx, New York