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Trivalent Cr vs. Hexavalent Cr, difference please

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A client has interest in a high nickel material that is presently regarded as a characteristic haz-waste due to the chromium levels. I have informed the generator that there is an exclusion under the Hazardous Waste standards that if you are able to rule out that the process does not generate hexavalent Cr, that the waste is typically managed in a non-oxidizing environment and that the Cr in the waste is exclusively trivalent then you can exclude the waste from being considered a haz-waste.  My question is what is the difference of trivalent and hexavalent other than understanding that hex leaches much quicker and is considered haz where trivalent does not leach. How does tri become hex? Or hex become tri?

Brandon Smith
- Cleveland, Ohio


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For the record, I need to preface this by saying that I don't know quite what you are talking about, Brandon :-)

"A client has interest in..." unfortunately has multiple meanings, and "a high nickel material" does as well. I don't know if you're talking about buying or already owning, and whether it's stainless steel chips, or a pickling waste, or a sludge from the wastewater treatment of plating effluent. And I don't know what "the process" is.

With that out of the way, trivalent chromium is chrome ions that are in the +3 oxidation state, hexavalent chromium is chrome ions in the +6 state, and chromium metal (such as in stainless steel) is neither. Hexavalent chromium becomes trivalent through a chemical reduction process conducted as a part of the wastewater treatment, where sulfur dioxide or sodium bisulphite is added as a reducing agent. The reason it is reduced like that is so that it can be precipitated; the reason hexavalent chromium leaches is that it is soluble at normal pH. Hexavalent chromium can be produced by an oxidizing process, for example when chrome plated parts are stripped in an alkaline electrolytic cleaning process.

If you can be a little more specific I may be able to answer your questions more directly. Thanks.

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


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In addition to all of the Correct information already supplied, there was an article published about 10+ years ago by Dr. Peter Klos, in which he indicated yet another species of Chrome ion. He mentioned the existence of a Cr+2 ion in Trivalent Passivate Baths. If you can get a copy, it makes good reading.

Ed Budmanbudman signature
Ed Budman
- Pennsylvania


I'm sure that Ed fully realizes his understatement regarding that seminal paper :-)

Many believe it to be the breakthrough that made modern trivalent conversion coatings and the RoHS requirements practical.

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

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