Lowering Wastestream pH

Q: We use an alkaline etch containing sodium hydroxide (NaOH) and sodium nitrite (NaNO2 ) for aluminum parts. Prior to discharge, we need to neutralize the waste solution and precipitate out the metals. Currently, we add sulfuric acid to lower the pH into the 8–9 range. This process generates nitrogen dioxide gas, and the air quality is very poor in the treatment area. Is there a way to lower the pH of this highly alkaline wastestream without generating the NO2 ?

Thanks. Anyadviceyou can give wouldbe appreciated. D.Z.

A: D.Z., I have to admit that I don’t have any direct experience with your specific situation. I even asked my crack staff for assistance, and they are somewhat stumped; one has a chemical engineering background. But here goes.

It is likely that both nitrogen oxide (NO) and nitrogen dioxide (NO2 ) are produced by the reaction of sodium nitrite and sulfuric acid. Nitrogen dioxide is mostly soluble in the alkaline solution, while the NO is not and would come out of solution as a gas and enter the air, where it would react with oxygen and form nitrogen dioxide.
One thought is to use a “weaker” acid, that is, an acid that does not react as vigorously with caustic, creating heat that increases the formation of nitrogen oxide gas. There are three possible candidates: phosphoric acid, acetic acid and carbon dioxide.

Phosphoric acid (H3 PO4 ) and glacial acetic acid (CH3 CO2 H) will more slowly disassociate and have longer reaction times. Phosphoric acid will add phosphorus to your wastewater, so you need to check you permit to verify whether or not this is an issue.

Glacial acetic acid is fairly viscous and somewhat difficult to handle in your situation as well as being quite expensive. In both cases, you will use significantly more acid than sulfuric acid.

Another “weaker” acid is carbonic acid, H2 CO3 , formed by dissolving carbon dioxide, CO2 , into water. The best way to dissolve CO2 into a wastestream such as yours is to pump the waste through a pipe, inject the carbon dioxide through a diffuser in the pipe under pressure, and recirculate the wastewater back to the treatment tank. The carbon dioxide can be supplied from 50-lb high-pressure cylinders or 250-lb low-pressure liquefied cylinders. Your highly alkaline bath will react quite quickly with carbon dioxide, although slower than sulfuric acid, until pH drops to below 10.

While carbon dioxide is capable of driving pH to between 8 and 9, you may want to “polish” the neutralized etch solution with sulfuric or phosphoric acid. The cost of setting up this scheme is likely to be quite expensive.
Another thought is to first remove the sodium nitrite, a reducing chemical, with an oxidizer, such as hydrogen peroxide. The addition of hydrogen peroxide may not only convert sodium nitrite to sodium nitrate (NaNO3 ), but may also cause a side reaction with the formation of nitric acid (HNO3 ), which will neutralize the sodium hydroxide. Theoretically, 1 gallon of 35% hydrogen peroxide will oxidize 7½ lb of sodium nitrite. Once all the sodium nitrite has been converted to sodium nitrate, add acid to bring the pH down to 8 or 9.

Trying phosphoric acid, glacial acetic acid and hydrogen peroxide on a bench-scale basis under an exhausted lab hood is relatively easy, while doing so with carbon dioxide would be more difficult.

I have two final thoughts. When performing this treatment activity, make sure you have vigorous agitation. The reaction between caustic and any acid is exothermic or hot. Since this heat may be contributing to the generation of the noxious gas, by conducting the treatment with vigorous agitation the acid will be quickly dispersed throughout the solution so as to minimize “hot spots.”

Finally, you may have to resort to adding more ventilation to this area and exhausting to the outside when conducting this treatment. I would look into this cost, and you may likely find that its capital and operating costs less than changes in treatment procedures.
If any of you have had experience with this waste, please feel free to contact me.