The Treatment of Cyanides in Wastewater

The cyanide ion, CN~, binds strongly to many metals, especially those of the transition series, and is often used to extract them from mixtures. Consequently, cyanide is widely used in mining, refining, and electroplating metals such as gold, cadmium, and nickel. Unfortunately, cyanide ion is very poisonous to animal life since it binds strongly to metal ions in living matter, e.g., to the iron in proteins that are necessary for molecular oxygen to be utilized by cells.

Cyanide is a very stable species and does not quickly decompose on its own or in the environment. Thus it is an important water pollutant and should be destroyed chemically rather than simply disposed of in a waterway.

We can deduce the type of treatment that will be effective for cyanide by considering its acid-base and redox characteristics. Cyanide ion is the conjugate base of the weak acid hydrocyanic acid, HCN, which has limited solubility in water. Thus, acidification of cyanide solutions will result in the release of poisonous HCN gas from it and therefore is not a good solution to the problem of cyanide contamination.

The redox chemistry of cyanide ion can be predicted by considering the oxidation numbers of the two atoms involved. If nitrogen, the more electronegative atom, is considered to be in its fully reduced —3 oxidation form, then the carbon must be +2, Thus one way to destroy cyanide ion is to oxidize the carbon more fully, to +4 as it is in C02 and HCO3. This oxidation can be accomplished by dissolved molecular oxygen if high temperatures and elevated air pressures are used:

2 CN~ + 02 + 4 H20-> 2 HCO3- + 2 NH3

The use of stronger oxidizing agents, such as Cl2 or CIO-, not only oxidizes the carbon from +2 to +4, but can also oxidize the nitrogen from the -3

state to the zero oxidation number of molecular nitrogen:

2 CN ■ + 5 Cl2 + 8 OH"-» 2 C02 + N, + lOCh + 4 H,0

(Four of the ten electrons gained collectively by the chlorines are used by the carbons and six by the nitrogens in this overall process.) Other oxidizing agents that are used in cyanide treatment include hydrogen peroxide, H202, and/or molecular oxygen, in both cases with a copper salt added as a catalyst.

The process can also be carried out electrochemically for high cyanide concentrations; the remaining low concentration can be subsequently oxidized by CIO".

Sodium cyanide is now used in some shallow tropical waters such as those in Indonesia to stun reef fish so that they can be captured and sold live as seafood or pets. Unfortunately, the cyanide kills smaller fish and destroys the coral.

PROBLEM 14-13

If an oxidizing agent even more powerful than chlorine or hypochlorite were to be used in the treatment of cyanide, what other possibilities for the nitrogen-containing product would there be?

PROBLEM 14-14

For HCN in water, Ka = 6.0 X 10~10. Calculate the fraction of cyanide that exists as the anion rather than in the molecular form at pH values of 4, 7, and 10.

Continue reading here: The Disposal of Sewage Sludge

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