Commonwealth of Australia 2010
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- 23.4.2Photolysis
- 23.4.3Formation of cyanates and other products
- 23.4.4Thiocyanate formation
- 23.5Biotic degradation
- 23.5.1Aerobic conditions
- 23.5.2Anaerobic conditions
23.4Abiotic degradation23.4.1HydrolysisAs 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). 23.4.2PhotolysisThe 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)5H2O3- + 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.
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).
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).
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 conditionsIn 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 conditionsUnder 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). Directory: data -> assets -> word doc -> 0017 0017 -> National Industrial Chemicals Notification and Assessment Scheme word doc -> Priority Existing Chemical word doc -> Rail safety news issue 6 – October 2011 word doc -> Review of Multiple Chemical Sensitivity: Identifying 0017 -> Rail Safety News Edition 8, December 2012 0017 -> Real-Time License Plate Localisation on fpga 0017 -> Careers network bulletin issue 6 July 16 2012 0017 -> Wiser science: Finding References inspec for Physics and Engineering 0017 -> Part A: Depth Study: The globalising world Popular culture and public policy Download 2.29 Mb. Share with your friends:
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