Wastewater Balancing Act: Stoichiometry and DTC

A reader sent me the following question about stoichiometry, and it's one I'm asked about sometimes.

Frank Altmayer“I am new to the field of treatment of metal finishing wastewater. We operate what I understand is a conventional treatment system that uses cyanide oxidation, chromium reduction, pH adjustment to render metals insoluble, and polishing with “DTC” before clarification. The manual I was given for the waste treatment system states that DTC should be added at up to 100%–200% over the “stoichiometric” amount. I took chemistry more than 20 years ago and never needed it until now, and I am not sure what this means. Can you elaborate?”

Stoichiometry is a fancy word used in chemistry for describing the “balancing” of reacting compounds or elements. Chemical compounds and elements that react with each other do so by ratios that are based upon molecular/atomic weights. The “stoichiometric” amount of a chemical required for a reaction is that amount which, theoretically, will react if all of the reacting compounds are totally consumed. I suppose this is still confusing, so let’s go over a basic example.

If we wanted to produce water by reacting oxygen and hydrogen, we would need to know how much of each to mix. First, we write a balanced chemical equation:

2H₂ + O₂ ➞ 2H₂O

4 + 32 ➞ 36

Under the equation, I wrote the atomic weights of the reactants. This shows you that hydrogen reacts with oxygen in a 1:8 ratio. In other words, it would take 4 pounds of hydrogen to convert 32 pounds of oxygen to 36 pounds of water under ideal conditions (in the metric system, we would say it takes 4 grams of hydrogen to convert 32 grams of oxygen to 36 grams of water). The stoichiometric amount of hydrogen required per pound of oxygen is therefore 1/8 pound.

Know the Exact Chemical Formula of DTC

For your problem, we first need to know the exact chemical formula of “DTC.” DTC is a commonly used acronym for several organic compounds that fall into a broad class of chemicals called carbamates. I will assume your DTC is actually sodium dimethyldithiocarbamate, which can be written as Na[(CH₃)₂NC=SS]. If this is not what you are using, you will have to get the right chemical formula from the MSDS or from the supplier. The molecular weight of Na[(CH₃)₂NC=SS] is obtained by adding the atomic weights of all the elements. Na[(CH₃)₂NC=SS] contains 1 sodium, 2 sulfur, 3 carbon, 1 nitrogen, and 6 hydrogen atoms, each with atomic weights of 23, 32, 12, 1, and 14, respectively. When you multiply each atom by its atomic weight and add them, the sum is 143.

Let’s assume that the only metal you are precipitating is nickel. Nickel has an atomic weight of 58.7. We now write the chemical reaction of nickel and “DTC” as follows:

Ni⁺² + 2[(CH₃) NC=SS]Na = Ni2[(CH₃)₂NC=SS] + 2Na+ 58.7 + 286

Note that nickel is always a divalent ion (Ni+2). At the same time, sodium is monovalent, so two molecules of sodium-DTC react with one nickel ion (the valences must add up to 0 for the reaction to be complete).

Stoichiometry based upon the above reaction predicts that for every 58.7 pounds (or grams or any other weight unit) of nickel that is in the water, you will need 286 pounds of “DTC.” The ratio is 286/58.7 or 4.9 to 1, so for every gram of nickel you are trying to remove, you will need 4.9 grams of “DTC.” That is the stoichiometry for this reaction. If you want a 100% excess, you would add 9.8 grams, etc.

Also, most DTC-containing products are only about 40% DTC by weight. Based on a specific gravity of 1.2, a gallon of DTC would weigh 8.34 x 1.2 = 10 pounds. At 40%, a gallon of DTC product would contain 4 pounds of DTC.

Calculate the Required Amount of DTC

If you have other metals besides nickel, you will need to calculate the required amount of DTC for each of the other metals that DTC can react with. Common metals precipitated with DTC include copper, zinc, and cadmium. Other metals include lead, mercury, silver, cobalt, divalent manganese, and ferrous iron. Note that DTC reacts only with divalent metals and is therefore not recommended for precipitation of trivalent chromium.

Now that we have covered the theory, most operators use jar testing to determine how much DTC gets the best results. If you are concerned about using too much DTC, Dr. Wing provided a procedure for determining residual DTC in treated wastewater:

1. Take a known volume of wastewater (example: 100 mL for 1–2 mg DTC).
2. Add 50 mL of toluene.
3. Transfer to a 250 mL separatory funnel.
4. Add 5 mL concentrated ammonium hydroxide and 10 mL of 2% copper sulfate solution. Shake thoroughly.
5. Allow for separation; draw the aqueous layer into a beaker.
6. Filter the toluene into a 250 mL graduated cylinder.
7. Add 50 mL of toluene to the aqueous layer, shake, separate, and collect the toluene in the same graduate.
8. Repeat until the toluene extract is colorless.
9. Transfer the colored toluene to a volumetric flask, rinse, and dilute to volume.
10. Place an aliquot in the spectrophotometer using toluene as a reference. Note transmission. Convert percent transmission to optical density.
11. Use optical density to determine the mg/25mL (M value) from a calibration graph prepared with known standards.
12. Calculation: [(V x M x 1,000 x 0.942)] / (25 x S) = mg/L DTC, where V= size of volumetric flask, M = value determined from calibration graph, and S = sample size (mL).

Frank Altmayer is a Master Surface Finisher and an AESF Fellow and the technical education director of the AESF Foundation and NASF. He owned Scientific Control Laboratories from 1986 to 2007 and has over 50 years of experience in metal finishing. He received the AESF Past Presidents Award, NAMF Award of Special Recognition, AESF Leadership Award, AESF Fellowship Award, Chicago Branch AESF Geldzahler Service Award, and NASF Award of Special Recognition.