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

by Mike McGinness, EcoShield Environmental Systems, Inc.
March, 2005

Trouble Precipitating Molybdenum

Q. We are having trouble precipitating Molybdenum out of our wastewater. We are using typical pH schemes, but we can not find a solubility curve for molybdenum, do you have one? Also is there any other treatment schemes in which we could try or that you are familiar with to drop out molybdenum?

A. I could use some more details on the specifics of your problem, such as the source and concentration of the molybdenum, the permit or discharge limits, and what other compounds are in the waste water stream, especially reactive compounds such as cyanides that would limit the pH range of treatment steps and any chelating or complexing agents such as EDTA or ammonia.

I am not aware of any pH versus solubility curves for Molybdenum. There is very little data or information in the major references on dealing with molybdenum removal from waste water partly due to its only recently becoming widely used (as a replacement for Hex Chromium), and partly due to the very limited amount of regulation placed on its discharge to date.

I do know that Molybdenum is difficult to remove and similar to Hex Chromium and Arsenic in that pH has little effect on their solubility, unlike metals such as copper and zinc. Also you are probably dealing with a molybdate compound, and not elemental molybdenum. I believe it will require a reducing compound and probably a low pH with the reducing compound to modify the molybdenum compound into a more suitable valence state for precipitation. The common practice for precipitating Hex Chromium is to reduce the pH of the Hex Chromium solution (to an acidic pH) in combination with a reducing agent such as sodium metabisulfite to convert the Hex Chromium into Trivalent Chromium, then raising the pH back to an alkaline pH which will precipitate the Trivalent Chromium. This might work with molybdates if there are not any chelating agents in the solution.

Others options may include electro-coagulation technology, nano-filtration or Reverse osmosis (with some pretreatment first).

If you need help developing, designing and testing a workable treatment system I would be glad to discuss it further with you. I am sure we can develop and design a workable, cost effective solution for you.