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topic 5040

Chemistry of nitric acid solutions - formation of nitrates?



I am posting this letter for two reasons: 1) to ask a question and 2) to notify the readers of this issue. My company provides environmental consulting services to several clients, a number of which perform metal finishing operations including pickling, acidic or alkaline pre-cleaning, etching, galvanizing, chromate coating, manganate coating, painting, powder coating, etc. The bulk of our services are in performing Toxic Release Inventory (TRI) Reporting (aka SARA 313 or EPCRA reporting) under Community-Right-to-Know. TRI reporting is based on a specific list of chemicals and chemical compounds deemed to be toxic and which regulated facilities must consider (and report) if thresholds are met.

For the 1995 reporting year, the chemical compound category Water Dissociable Nitrates was added to the list of chemicals to be considered (note: only reportable when in aqueous solution). In June 1999, the USEPA published guidance to assist with determining if water dissociable nitrates must be reported. In March 2000, the USEPA announced an enforcement action to ensure that water dissociable nitrates are being reported as appropriate. The enforcement action is initially predicated on any facility that reported treating nitric acid on site in excess of 18,000 lbs/yr (In essence, the USEPA believes such a facility "coincidentally manufactured" sufficient water dissociable nitrates to meet or exceed the threshold for reporting.) One of our clients actually received a letter under this enforcement action asking them to "show cause" why they did not report water dissociable nitrates and/or to admit that they should have. This letter also states the USEPA believes that a facility which submits a report for nitric acid showing minimal releases (including on site treatment) may be in violation (should have releases, treatment, or recycling).

The USEPA's guidance does provide some information on this issue, however, we do not believe the examples adequately portray the chemistry involved in nitric acid streams in metal finishing operations, as the examples basically assume a nitric acid and water only solution treated with sodium hydroxide (i.e., the examples do not account for metal ions, other acids such as HF or sulfuric, or the use of polymers as flocculants).

So much for the dissemination of information part...especially if this issue seems familiar to your facility.

My question, probably pretty obvious by now, is whether there is any solid information available concerning the chemistry of nitric acid and/or nitrates in metal finishing operations and/or the wastewater treatment thereof. What we have used in the past (prior to the USEPA's publishing of this guidance) is a qualitative statement from a wastewater treatment text that when a nitric acid stream which contains metals is neutralized and subject to polymer addition, the bulk of the nitrates will form metal complexes and precipitate out of solution (water dissociable nitrates are only reportable in aqueous solution).

Given the USEPA's decision to concentrate on this issue, we would like to use something more substantial if possible. For some of our clients, the application of the estimation calculations in the USEPA's examples is sufficient to demonstrate that any nitrates formed would not meet the applicable threshold (note: the USEPA's guidance does not consider solubility, although in the case of sodium nitrate it should be considered because it is only partially soluble).

Another piece of information we have come across is that it is possible that a significant portion of the nitric acid is consumed in the metal finishing operation and forms NO, water and the appropriate metal oxide (iron, zinc, chromium, etc. depending on the surface chemistry of the metal being finished), thus reducing the amount of nitric acid available to form nitrates.

Of course, if the available information suggests that nitrates are in fact formed, then we will advise our clients to report them...but the biggest mental stumbling block that I keep coming back to is that none of our clients are required to monitor for nitrogen and/or nitrates is their discharge streams (some are NPDES regulated and some are POTW regulated). It would seem to me that if any of these facilities are discharging 2,000 lb/month of nitrates, they would already be required to monitor for and stay within certain limits of nitrogen and/or nitrates?

Lastly (if you have read this far), I would like to say Kudos to your organization for providing this forum. I scanned through quite a bit of your site before deciding that my particular issue was not previously addressed and found this site very informative. Some of the material discussed I have previously picked up through discussions with our clients and some information was new to me. Thank you.

Patricia L. Steffan
- Pittsburgh, Pennsylvania, USA

Most of the nitric acid would end up as water soluble nitrates, so, yes, I would recommend that the plater admit that it should have been reported.

You are opening a Pandora's box to ask why a plater might not be reporting Nitrous oxides to other regulatory agencies. It is NOT because no one else is concerned with Nitrous oxides. You are supposed to report them, as I read in any air pollution permit I have ever seen. We know that nitrous oxide is a major pollutant causing smog and respiratory problems.

I wish I had a dollar for every plater who is discharging unreported NOx from their general ventilation fans and fume hood scrubbers, especially those who bright dip aluminum. How about the plater or finisher who passivates stainless steel in chromic/nitric solutions? Some NOx is being formed in that reaction.

Estimating the release of NOx: If someone is dipping metal into a solution of nitric acid, especially when you can see the brown NOx fumes leaving a tank, you know that some NOx is being formed. I believe that NOx should then be estimated and reported to the governing agency. The amount of nitrates converted to NOx is not 100%, it is probably not 10%; How about estimating the amount of NOx by estimating the amount of metal being removed during a bright dip,(measured in tenths of a mil), multiplying by an estimate of the surface area of the part, and calculating the NOx from the reaction occurring during the bright dip. The reaction is somewhere in the plating literature.

I used this method when writing an air permit for a finishing company, located in New Jersey, and the application was accepted as written.

The key is to estimate the amount with a chemical equation, and some production based unit. It is quite reasonable to an engineer if you tell her that I process 1500 square feet of aluminum, I remove 0.00001" from every square foot, and the metal removed weighs so many grams, and so many grams will react with so many grams of nitric acid, and the volume from so many grams of NO, at STP, is so many liters, etc.

To this very day, our electroplating community, with the blessings of the American Electroplaters Society and their Government Relations department (committee, Lobby, whatever), instead of trying to estimate the Pollutant of the Month for the EPA, so the EPA can look at it and forget it, starts a campaign to discredit the EPA for even daring to look in our direction for that pollutant. Why do we platers think that we know more about epidemiology than the people who are in that business? If we are so good at it, we should quit our jobs and go into it full time, and show everybody how to do it. It was said best at a Garden State Electroplaters Society, when we complain that the cost of pollution control is too much, we just become "The Greedy Plater" who is not happy to add the cost of control to his product. Why else would be so afraid to estimate the amount of the pollution for the EPA?

Thank you, tom

tom pullizzi monitor
Tom Pullizzi
Falls Township,


You should be aware that nitrates are only slightly consumed during use (those nasty brown fumes coming off nitric strippers, etc.). Nitrates are probably also the most soluble of all salts encountered in finishing operations, and it is extremely unlikely that they are removed during conventional precipitation treatment.

Given these facts, the nitric acid added to any process will most likely show up in the effluent from that process with little net loss.

bill vins
Bill Vins
microwave & cable assemblies
Mesa (what a place-a), Arizona 


As for the technical elements of the question, I suspect that Bill is right on target and your question is answered, Patricia.

Tom is not entirely wrong, but since the questions he asked were rhetorical, I'll respond to them with logic mode off, vent mode on . . .

Think about what makes a man or woman break into a cold sweat: the possible loss of their livelihood, and you'll have to agree that staying employed is every person's "main objective". So, yes, the main objective of a plater is to stay in business and make money; his secondary objective is to deliver useful products to a needing world. And the main objective of the EPA is self-perpetuation and enlargement; improving the environment is only a secondary objective. The main objective of the Government Relations lobbying group is also self-perpetuation and enlargement -- which requires that they earn their keep by fighting on behalf of the platers to minimize unreasonable regulations and reporting requirements. And the primary objective of is to provide its owners and employees with an income by providing a place of camaraderie and technical help.

I'll give Tom two reasons a plater "would be so afraid to estimate the amount of the pollution for the EPA":

The problem is not that platers think that the experts they hire "know more about epidemiology" than the experts that the EPA hires. The problem is the blindness of some people in failing to see that the EPA is not all goodness and light -- it too is staffed with humans whose principal interest is regulation as a means to protect their job.

In the end it may well be that reporting of water soluble nitrates is useful and epidemiologically correct. Who knows? But I tend to doubt it because the EPA has been regulating platers element by element, one after the other, since 1974 -- and after well over 25 years of it, it is only now getting to nitrates; it is completely appropriate to consider the possibility that EPA may be regulating some areas beyond the point of diminishing returns, and that their main incentive may be concern about keeping themselves employed more than real concern about environmental damage to POTWs from the nitrates from plating shops.

Vent mode off, smiley mode on . . .smiley. Cheers!

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

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