Table of Contents

The catastrophe is spreading downstream to other European waterways. Current remediation efforts have been futile because government officials and environmentalists have not been able to come up with a bona fide strategy to prevent and remediate the cyanide problem. More importantly, they have not explored a chemical solution to eliminating cyanide.

Known chemical reactions offer a simple solution to detoxify waters polluted with cyanide. Intensive mixing of polluted water with air releases hydrogen cyanide from cyanide salts. HCN is evaporated, oxidized to the less-toxic cyanic acid, HOCN, and then further converted to carbon dioxide and ammonia, ultimately forming ammonium carbonate.

The most radical prevention method would be to close the outdated (>100-year-old) extraction facilities that use cyanide. This solution is unlikely to be used, as long as economic considerations prevail over environmental concerns. Similar, though smaller, incidents have occurred in the past—this suggests that cyanide pollution will occur in the future. Consequently, early detection of pollution is of primary importance.

Microbial sensors are recommended for measuring oxygen consumption at cyanide concentrations in the range of 0.3–150 µM (1). The mold Phanerochaete chrysosporium could be used to remove cyanide from contaminated soil (2). However, no chemical approach to remove cyanide pollution from rivers has been established. In this article, I discuss possible chemical strategies for addressing the problem of cyanide pollution in rivers.

Removal of cyanide

Removing cyanide from rivers must be done without further polluting aquatic life. Potential treatments with sulfur, iron salts, or chlorine present the challenge of mixing a huge volume of polluted river water with a chemical agent.

Sulfur and sulfur compounds. Converting cyanide to thiocyanate (rhodanide) with sulfur is not a spontaneous process: It requires energy.

S + NaCN ^→ NaSCN

Moreover, in its elementary form, sulfur does not dissolve in water. It is better to use sodium thiosulfate, which reacts with cyanide spontaneously:

Na₂S₂O₃ + NaCN ^→ Na₂SO₃ + NaSCN

This is one of the reactions used in the treatment of cyanide poisoning. The enzyme rhodanase catalyzes the conversion (3, 4).

Iron salts. Ferrous compounds, with few exceptions (e.g., the double salt ferrous ammonium sulfate, Mohr’s salt), are more or less susceptible to superficial oxidation. In alkaline solution, the oxidation process is still favored. Iron(II) forms complexes, and most are octahedral. Ferrous complexes, including those with cyanide, can be oxidized to ferric complexes. The Fe(II)–Fe(III) aqueous system provides a good example. However, ferricyanide [Fe(CN)₆^3–] complexes are toxic. In an aqueous environment, Fe(II) without a complexing agent tends to associate with water to form an Fe(H₂O)₆²⁺ complex. Secondary reactions and ecological side effects of iron salts are expected; therefore, their use cannot be recommended against cyanide pollution.

Chlorine. Chlorine is toxic because when it is generated, it produces nascent oxygen in the atomic state. Atomic oxygen is an aggressive oxidizing agent that destroys living tissue. Molecular oxygen has no toxic effect on aquatic life. When chlorine is dissolved in water, hydrogen chloride is also produced. HCl, a strong acid, will liberate the weak acid HCN from cyanide salts. In the presence of oxygen, HCN is oxidized to the less toxic HOCN. Because chlorine is toxic, it is not suitable for detoxifying cyanide.

Cl₂ + H₂O ← HOCl + HCl

2HOCl ^→ 2HCl + O₂

2HCN + O₂ ^→ 2HOCN

More nascent oxygen can be released with potassium permanganate. One permanganate molecule produces three oxygen atoms under neutral or alkaline conditions, and five atomic oxygens in an acidic environment. Although the use of permanganate should be avoided as a treatment method, it is useful as an indicator of the progression of cyanide oxidation in water samples taken from polluted rivers, and a permanganate-based titration method will be used to determine cyanide.

Warming the river to the boiling point of HCN (26 ºC) would certainly remediate the cyanide problem; however, the cost of heating the river would be enormous, and the HCN must first be released from cyanide salts by acidification. To the extent that this method is at all practical, the cyanide catastrophe would have been less severe in summer when rivers warm up.

What is the solution?

The simple solution for detoxifying cyanide is to mix polluted water with air. The first reaction is the absorption of CO₂ into polluted water. Carbonic acid is a weak acid, but it is stronger than HCN; consequently, it releases HCN from cyanide salts. HCN either evaporates, is converted to the less toxic HOCN in the presence of molecular oxygen, as shown above, or hydrolyzes to ammonia and CO₂.

HOCN + H₂O ^→ CO₂ + NH₃

Two molecules of NH₃ and one of CO₂ produce (NH₄)₂CO₃, and the excess CO₂ redissolves to convert more cyanide ion to HCN. Thus, detoxification becomes a self-sustaining process.

Removal of cyanide works optimally when both CO₂ and oxygen are present, and air contains both. To start cyanide detoxification, HCN formation has to be initiated. The initial reaction is speeded up by compressing an air–CO₂ mixture into polluted water. The CO₂ content of air is low (0.034 vol%); thus, the initial reaction is slow. The formation of CO₂ is a self-generating process, and air compression is sufficient to maintain continuous reactions. The products of detoxification are NH₃ and CO₂, which ultimately form (NH₄)₂CO₃. NH₃ is measured according to the Bertelot photometric method (5).

Aerating the polluted water liberates HCN from its salts and evaporates it. Aerating water is a simple, nontoxic way to remove cyanide and reduce its toxic concentration to a tolerable level. To achieve the desired degree of detoxification, polluted rivers can be aerated by compressing air into water using turbines, windmills, or water mills. Aeration should take place

• near the site of contamination (but it can be done anywhere along the river),
• away from inhabited areas, and
• under safe conditions, because HCN is released into the air.

Long-term prevention of cyanide pollution can be achieved if windmills and water mills are built in the waterfalls of rivers in the vicinity of cyanide discharge points. These mills will unquestionably have other practical purposes and benefits.

References

1. Nakanishi, K.; Ikebukur, K.; Karube, I. Appl. Biochem. Biotechnol. 1996, 60, 97–106.
2. Shah, M. M.; Grover, T. A.; Aust, S. D. Arch. Biochem. Biophys. 1991, 290, 173–178.
3. Sörbo, B. Acta Chem. Scand. 1953, 7, 1129–1137.
4. Sörbo, B. Acta Chem. Scand. 1954, 8, 694.
5. Comprehensive Inorganic Chemistry; Bailar, J. C., Jr., Emeleus, H. J., Nyholm, R. S., Trotman-Dickinson, A. F., Eds.; Pergamon Press: Oxford, U.K., 1973; pp 199–227.