Environmental Engineering Reference
In-Depth Information
Total cyanide - the sum of all forms of cyanide present in the sample, including iron,
cobalt, and gold complexes. 'Total cyanide' is a toxicologically meaningless term since
its measurement requires harsh treatment to disintegrate intractable complex cyanides
before free cyanide can be liberated and measured.
Weak acid dissociable (WAD) cyanide - cyanide that is readily released from cyanide-
containing complexes when the pH is lowered. Any free cyanide already present plus
cyanide released from nickel, zinc, copper, and cadmium complexes (but not iron or
cobalt complexes) is measured. WAD cyanide (CN WAD ) is measured by treating the
sample with a weak acid buffer solution such as sodium acetate/acetic acid mixture at
pH 4.5 to 6.
Cyanide Amenable to Chlorination (CATC) which refers to the cyanide that is
destroyed by chlorination. CATC is an analytical quantity that requires similar sample
treatment to WAD, but is much less reliable. CATC is commonly used at water treat-
ment plants.
Typically, residual cyanide concentrations of total cyanide in tailings will be in the range of
80 to 400 mg/L, of which about 50% may be CN WAD . As cyanide solutions at such concen-
trations are toxic to many organisms, treatment is required before tailings solutions can be
safely released to the environment.
Unfortunately some jurisdictions do not explain which form of cyanide is being reg-
ulated. Total cyanide is not particularly relevant from an environmental perspective and
free cyanide is difficult to measure and does not include all potentially damaging forms.
Accordingly, WAD cyanide is considered to be the best measure for assessing human and
animal toxicity.
Many different processes and procedures have been used to remove cyanide from proc-
ess wastes. These include both destruction processes and recovery processes and are listed
in Table 6.3 .
The most common means of reducing cyanide levels to achieve safe disposal include:
WAD cyanide is considered to
be the best measure for assessing
human and animal toxicity.
Natural degradation by volatilization, enhanced by exposure to sunlight;
Dilution of cyanide-bearing solutions by mixing with non-cyanide-bearing water;
Oxidation, which can be accomplished using a variety of oxidants such as hydrogen
peroxide (H 2 O 2 ), chlorine (in the form of calcium or sodium hypochlorite), Caro's acid
(H 2 SO 5 ), or sulphur dioxide (in the form of sodium metabisulphite);
Cyanide recovery and reuse, which may be achieved by lowering pH to form HCN,
which is then volatilized and re-dissolved for addition to the leach solution.
Natural degradation was widely used in the past and can be effective and reliable, particu-
larly in the flat arid goldfields of Australia where large, shallow tailings ponds facilitate
rapid degradation of cyanide to form HCN gas. Natural oxidation of cyanide ions from
alkaline solutions may also generate ammonia (NH 3 ), cyanate ion (CNO ) and cyanogen
gas (CN) 2 according to the following equations:
CN
½ O 2
2H 2 O
NH 3
HCO 3 ;
CN
2OH
CNO
H 2 O
2e ;
2CN
(6.15)
(CN) 2
2e .
Degradation of certain dissolved cyanide complexes may also result from precipitation of
stable metallo-cyanides, which in some cases involve the release of HCN gas.
 
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