Moly Removal From Wastewater

Q. We are a metal finishing job shop with a wide variety of processes as well as a wastewater pretreatment system. Over recent years, our compliance with our wastewater discharge permit has been excellent—until recently. When our new permit was issued several months ago, we noticed that its molybdenum limit had been dramatically reduced from 2.2–0.5 mg/liter. We then went back to our wastewater monitoring reports to the city to see how our historical molybdenum compared against the new limit. We found that up until a year ago, the molybdenum concentration in the wastewater discharge was consistently below 0.5 mg/liter. However, during the past year, we found that a number of our samples were over the new limit but under the old limit. Then, just last week, the city informed us that the molybdenum concentration in samples they took last month was over the new limit, averaging about 1.5 mg/liter. After carefully reviewing our material safety data sheets, we found that one of our new trivalent chromates does contain a molybdenum compound. As an experiment, we did not run that chromate bath for two days and sampled our wastewater effluent on the second day. The result was 0.3 mg/liter. We then ran the bath for two days and sampled on the second day. The result was 1.3 mg/liter. So it looks like we have found the source of the molybdenum. We cannot change this chromate since it is mandated by one of our key customers and none of the alternatives we have tested perform as well.

In discussing the possible solution with our wastewater chemical supplier, he says that he, nor anyone in his company, has ever encountered this issue and is unsure how to proceed. One possible option they think might work is to add a sulfide precipitant after our clarifier and install a new sand filter. Needless to say, this is quite expensive, and we do not want to commit this kind of capital into something that “might work.” Furthermore, we are not sure that the zinc plating work that uses this chromate can even support this additional expenditure. We have conducted our own search for treatment of molybdenum in metal finishing wastewater and have found very little.

Do you have any more insight on this pesky issue? Since the permit limitation for molybdenum has significantly changed, the city is giving us some time to address the issue before they start issuing notices of violation. R.T.

A. I have been working with metal finishing wastewater for more than 25 years, and this, too, is the first time that I have encountered this issue. Two of our clients do have molybdenum in their wastewater discharge permits, but their concentrations are well above your new limit and pose no compliance issue.

Unfortunately, as you discovered, there is very little information out in the public domain regarding molybdenum (aka moly) removal from wastewater, particularly metal finishing wastewater. The reasons are two fold. First, molybdenum is regulated mainly through local limits and very few cities or sewer districts regulate molybdenum. The likely driver for your city to reduce your moly limitation is the application of their wastewater treatment solids onto land; while moly is a plant trace nutrient, too much moly can be detrimental to plants. Second, because of the pressures to eliminate or reduce hexavalent chrome compounds in metal finishing processes, molybdate (MoO4-2) compounds are being used to replace hexavalent chromates as you have discovered.

Also, I am not aware of any solubility versus pH curves for moly like we have for chrome, copper, zinc, nickel, etc. This makes me think that one needs to precipitate out a moly compound rather than a moly ion.

Based upon my research, I have found the following treatment schemes that claim to remove moly:

Electrocoagulation, which uses an electrical current to dissolve a sacrificial anode of iron or aluminum and thereby introduces these positive ions into the wastestream. At the same time, gases formed by hydrolysis form very fine bubble that attach to the coagulated contaminants and carry them to the surface by flotation where they are removed.
Addition of calcium chloride at several times its theoretical dosage and a very long (up to 12 hours) processing time at a pH of 7.5.
Reduction with sodium metabisulfite and precipitate at a pH <4.0.
Reduction with sodium metabisulfite and precipitate at a pH <6.0.
Reduction with sodium metabisulfite at a low pH, addition of polythiocarbamate (an organic sulfide), and precipitate at pH of 9.0.
Lower pH to 4 then add a ferrous iron salt; raise pH to between 7.0 and 9.0 then add a sulfide precipitant; filtration after the addition of the ferrous iron AND sulfide precipitant may be necessary.
Ion exchange.
Reverse osmosis.

As you will note, a common theme in the literature is that the molybdate ion needs to be reduced, in a similar fashion as hexavalent chromate needs to be reduced, in order to prepare it for precipitation. Also, several references noted that too vigorous mixing tear the molybdate precipitate apart so that it would be too small for settling or filtration.

Given that you have isolated the problem to one of your chromate baths, why not treat the problem at the source instead of at your main wastewater treatment system? If you find a chemical precipitation procedure that works, consider installing a batch treatment system for the offending chromate process. This system could consist of a holding tank, treatment tank and very small filter press. However, if the process time becomes too excessive, the size of the holding and treatment tanks may become too large and costly.

Another option to consider is to “close loop” the chromate rinsewaters. While reverse osmosis can remove molybdate, it generates a reject stream of between 5–15 % of its flow, and this reject stream still has to be treated.

For a “closed loop” option, ion exchange could be a feasible option. Strong base anion resins are capable of removing the molybdate ion. In this alternative, the rinsewaters would be pumped through a carbon filter in order to protect the ion exchange resin from organic fouling, then the ion exchange columns and then back to the rinse tanks. Periodically, the ion exchange resin will need to be regenerated or disposed. Regeneration on your site is not an option since you still would need to treat for molybdenum. Our preference would be to find a supplier that can provide exchangeable resin columns that are regenerated off-site. If this proves to be too expensive or unavailable, another option is to purchase bagged resin to be used to replace the exhausted resin. The exhausted resin, likely to be RCRA hazardous waste due to the presence of chrome, can either be placed with your filter press cake if it, too, is a RCRA hazardous waste or disposed separately. The advantages of this option are its low startup capital costs, high degree of confidence that it will work, and very high rinsewater quality. Its disadvantages include relatively high costs of resin regeneration or purchase/disposal and that the waste chromate solution will need to be disposed as a hazardous waste off-site.

Let’s take advantage of our knowledge and experience Product Finishing readers. If you know of a success story for the consistent removal of molybdenum from metal finishing or similar wastewaters, I invite you to share your experience by responding to www.pfonline.com/environmental/clinics.html.