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Ask the Expert Question-and-Answer Archive (Wastewater Treatment)

by Mike McGinness, EcoShield Environmental Systems, Inc.
December, 2002

Wastewater From Chemical Etching of Stainless Steel

Q. I regulate an industrial facility that performs etching of stainless steel sheets using a heated ferric chloride / HCl solution. Following etching, the parts are rinsed and the rinse water is treated for nickel and chrome removal. Im trying to figure out if the chrome etched from the stainless steel in this process would be in the hexavalent or trivalent oxidation state. Would you have any idea?

A. It could be some of both. There is only one way to find out for sure and that is to have a lab sample the waste water (Using EPA sampling and testing protocalls) and report total disolved hex and tri-valent chromium as separate quantities. Trivalent chromium will precipitate (mostly....) at an alkaline pH, but hexavalent chromium will not. Hex chromium must be reduced with a reducing agent to the trivalent state at an acidic pH before it can be precipitated at the alkaline pH. Hope this helps.



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