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23.4Abiotic degradation

23.4.1Hydrolysis


As pH falls, HCN may hydrolyse to form formic acid (HCOOH) and ammonium formate (NH4COOH; Smith and Mudder, 1993). Rates for cyanide hydrolysis of
2%-4% per month have been estimated, which is relatively slow compared with other cyanide degradation processes (Smith and Mudder, 1993).

23.4.2Photolysis


The iron-cyanide complex compounds are generally very stable in the dark and elevated levels of HCN in solution are attained only in aged iron-cyanide complex solutions of high concentration (APHA, 1998). However, these complexes are subject to extensive and rapid photolysis, mobilising HCN on exposure to direct sunlight (APHA, 1998; ANZECC/ARMCANZ, 2000a; Kjeldsen, 1999). The rate of photodecomposition depends on exposure to ultraviolet radiation, and therefore is slow in deep, turbid or shaded waters (APHA, 1998) and negligible in subsurface soils or groundwater. Decomposition is typically complete and continues until ferric or ferrous ions and cyanide ions are released.

Meeussen et al. (1992) reported the results of a flask study where the cyanide present was initially in the form of iron complexes. Exposure to diffuse day sunlight produced a transformation to free cyanide at a decomposition rate of 8% per hour. Faster degradation rates than this were reported for groundwater contaminated with iron complexes exposed to artificial UV-light (Marsman and Appelman, 1995).

Free cyanide may be photolysed to cyanate (OCN-), but this requires the presence of a catalyst (e.g. titanium dioxide, cadmium sulphide, zinc oxide; Frank and Bard, 1977). Cyanide may be converted to cyanate in the soil on the surface of organic and inorganic materials (Chatwin, 1988), and potentially in solutions.

The photochemical dissociation of Fe(CN)x-6 is known to proceed by an aquation mechanism forming the aquopentacyanocomplex, as follows for ferrocyanide (Kuhn and Young, 2005):

Fe(CN)64- + 2H2O + uv  Fe(CN)5H2O3- + HCN + OH-

In alkaline solutions, the hydroxopentacyano complex can form subsequently by the reaction:

Fe(CN)5H2O3- + OH-  Fe(CN)5(OH)4- + H2O

The first reaction can proceed in reverse when illumination is stopped; although during prolonged exposure the reverse reaction is incomplete (Johnson et al., 2002).

Photodegradation rates for strong metallocyanide complexes vary according to sunlight intensity, light absorption within the water column, cyanocomplex concentration (assuming the reaction is first order), the quantum yield for the particular complex, temperature and any catalytic effects (Broderius and Smith, 1980; Kuhn and Young, 2005). Fe(CN)6x- catalysts include mercury, silver and gold. Johnson et al. (2002) reported dissociation rate half-lives for Fe(CN)6x- (or time to release 50% of contained CN-) of 0.05 to 5 hours at 28°C and 0.3 to 2.7 hours in natural waters or deionised waters at 20°C.

23.4.3Formation of cyanates and other products


In the presence of strong oxidisers (e.g. ozone, hydrogen peroxide, hypochlorite), cyanide in solution may oxidise to hydrogen cyanate (HOCN) and the cyanate ion (OCN-). Although unlikely to be significant in the natural environment, this can occur during ore processing, such as where operations add strong oxidisers to degrade free cyanide to cyanate (e.g. alkaline chlorination), or during cyanide spill remediation works where detoxification chemicals are applied.

Alkaline chlorination may also form chloramine (NH2Cl) or similar chlorinated compounds (Moran and Brackett, 1998).

Cyanogen gas (CN)2 may form under acidic conditions in the presence of an oxidant such as an oxidised copper mineral. The formation of cyanogen is unlikely where mining solutions remain alkaline; however, conditions conducive to acid formation (e.g. acid rock drainage) in the presence of cyanide solutions may potentially result in cyanogen formation (Moran and Brackett, 1998).

23.4.4Thiocyanate formation


Cyanide may react with sulphur (e.g. thiosulphate, sulphide ions) to form thiocyanate as follows:

Sx2- + CN-  [S(x-1)]2- + SCN- and S2O33- + CN-  SO32- + SCN-

Thiocyanate may decompose to carbon dioxide, ammonium and sulphate (Kjeldsen, 1999). Reaction of CN- with sulphur may be slow; however, as the water flow rates in most soils (and bedrock) are also slow, if amenable sulphur is present, a significant proportion of the cyanide in solution can be converted to thiocyanate (Chatwin et al., 1987).

23.5Biotic degradation


Cyanide can be produced, degraded or utilised by micro-organisms in both aerobic and anaerobic conditions (Watanabe et al., 1998). Cyanide is also a metabolic inhibitor for some micro-organisms (Chapatwala et al., 1995).

Biodegradation of cyanide in natural waters is dependent on such factors as cyanide concentrations, pH, temperature, availability of nutrients, and acclimatisation of micro-organisms (Hagelstein and Mudder, 1997a). During degradation, cyanide can be a source of nitrogen and carbon (Kao et al., 2002; Kjeldsen, 1999; Raybuck, 1992).


23.5.1Aerobic conditions


In aerobic conditions, cyanide can be converted to formate and then to carbon dioxide by formate dehydrogenase (Knowles, 1976). Conversion is either direct (by nitrilase) or indirect (via formamide by cyanide hydratase and formamidase) to produce formate. Under these conditions, cyanide may break down to ammonia and carbon dioxide (Kjeldsen, 1999). Enzymic reactions can be summarised as substitution/addition reactions, hydrolysis, oxidation, and reduction (Watanabe et al., 1998). Various micro-organisms can apparently also convert cyanide into cyanate and then CO2. Reported biodegradation pathways for cyanide complexes are listed in Table 6.2.

23.5.2Anaerobic conditions


Under anaerobic conditions, biodegradation of cyanide is slower (Smith and Mudder, 1993). In anaerobic methanogenic conditions, HCN may be degraded to ammonia/ammonium and formate (HCOO), with the formate rapidly transformed to bicarbonate ( Table 6.2; Nagle et al., 1995). Ammonia may be denitrified to nitrogen gas (N2; Hagelstein and Mudder, 1997a; Ferguson, 1988). Thiocyanate may be metabolised by micro-organisms to carbonyl sulphide and ammonia (Katayama et al., 1992) and to cyanate, which may be biodegraded further to carbon dioxide and ammonia (Stratford et al., 1994b).


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