LIMITATIONS, CHALLENGES, AND THE ENVIRONMENTAL IMPACTS OF CYANIDE LEACHING

For cyanidation, three main types of cyanides can be considered; (i) free cyanides, (ii) weak acid-dissociable cyanides (WADs), and (iii) strong acid-dissociable cyanides (SADs). However, the German Standard Method only differentiates between releasable cyanides (hydrocyanic acid, their alkali, and alkaline earth salts, including cyanide complexes of Ag, Zn. Cd, and Cu) and strong cyanide complexes (of cobalt, nickel, iron, and gold). Hence, the term total cyanides includes free cyanides, WADs, SADs, cyanate ion, OCN', thiocyanate ions, SCNO, and cyanogen chloride, C1CN (Oelsner et al. 2001). The chemical name for hydrogen cyanide, HCN, includes hydrogen cyanide, hydrocyanic acid, and prussic acid which, depending on the pH and redox potential, can exist in either the free or complex forms. With the increase in the stability of cyanide compounds, the ability to release/ form free cyanide in solution decreases. This is advantageous for halting cyanide mobility, but disadvantageous in the treatment/remediation of effluent as the cyanide ions form stable ferrous [Fe(CN)₆⁴-] and ferric [Fe(CN)₆³i cyano-complexes. Although cyanide losses (either in the atmosphere during handling or via generated effluent) during urban mining of precious metals are far less than in mining practices from native ore sources, the International Cyanide Management Code has imposed a limit of 50 ppm cyanide solution for gold mining in order to control losses (Riani, et al. 2007).

When cyanides are discharged to the environment, their ability to undergo a number of processes—dissolution, adsorption, precipitation, (bio)oxidation, and biodegradation—along with a variety of metal complexes makes it difficult to trace their path in soil, water, or air. These processes often occur simultaneously, and depending on the prevailing physical or chemical conditions, result in degradation or attenuation of cyanides. The toxicity of cyanides is a function of the dissociation of free cyanides into the environment. Dissociation reduces with increasing stability of the cyanide complex, from weak complexes to strong complexes. The stability constants of the cyanide complexes with lead, silver, copper, and nickel are less than for ion complexes, which release HCN more easily and are therefore more toxic (Oelsner et al., 2001). The cyano-complexes of copper and zinc in WADs are insoluble in water but soluble in ammonia solution. The production of ammonia from natural attenuation would, therefore, result in the dissolution of copper and zinc in cyanide solution. This results in an increase in bioavailability and cyanide concentration within a specific environment. Except for the volatilization of the free cyanide, the released cyanides may still be available to drive other chemical processes under suitable conditions. This holds true for photolysis, precipitation, and complexation processes.

Cyanide is a toxic substance that can be inhaled, ingested orally (through contaminated water or food), or diffused through the skin. Cyanide prevents the uptake and subsequent transportation of oxygen to cells (Logsdon et ah, 1999). If iron, which co-ordinates the uptake and transport of oxygen to the cells, is consumed by cyanide, failure of the respiratory system, rapid breathing convulsions, loss of consciousness, and suffocation can occur if there is no medical intervention, although the body can detoxify small concentrations of cyanide to less toxic cyanate, preventing the accumulation of cyanide in the body. Cyanide diffusion through the skin is supported by the small molecular size of HCN and the fact that cyanide dissolves readily in lipids of the human body (Simeonova and Fishbein, 2004). Concentrations of 20-40 ppm HCN in the air are toxic; increasing the concentration up to 250 ppm (1-3 mg CN per kg body weight) causes death within minutes. The LD₅₀ (lethal dose-50) values representing the toxicity of cyanide and its derivatives are given in Table 4.2.

TABLE 4.2

Boiling point, exposure limit, and toxicity of various cyanide derivatives

Source: Lowehein and Moran (1975); Ilyas and Lee (2018).

Cyanide undergoes a number of redox reactions in the open, forming various cyanide species of different toxicity levels, especially at a higher concentration. These compounds include:

Cyanogen Chlorides: toxic compounds, formed as intermediates during the chlorine oxidation of cyanides to cyanates. In the presence of ammonia, another class of toxic compounds, chloramines, are formed.

Cyanogens: produced in acidic environments when free cyanide encounters oxidants like oxidized copper minerals; nevertheless their formation is not expected in alkaline conditions.

Cyanates: usually formed as intermediate products via the reaction of cyanide with oxidants (ozone, hypochlorite, chlorine, hydrogen peroxide) during oxidative degradation of cyanides.

Thiocyanates: formed by the action of cyanide with sulphur or sulphur-containing chemical species that remain present in mineral ores. They persist in acid mine drains for decades after the closure of the mines.

Nitrate and Ammonia: chemical dissociation of cyanides and cyanide derivatives generates a large number of nitrates and ammonia, toxic to aquatic organisms, as degradation products.

For protection of aquatic resources from cyanide toxicity, the contaminated water must be detoxified before its discharge. The US Environmental Protection Agency (USEPA) has set limits of 200 ppb and 50 ppb cyanide in drinking water and for aquatic-biota, respectively (Gurbuz et al., 2004). The US health service has proposed a maximum permitted limit for cyanide in effluent of 0.2 mg/L, with 0.01 mg/L as a guideline. Swiss and German regulatory standards for cyanide are 0.01 mg/L for drinking or surface water and 0.5 mg/L for effluent. For cyanide disposal in Mexico it is 0.2 mg/L. The minimal national standard (MINAS) for the discharge of cyanide set by India’s central pollution control board (CPCB) is 0.2 mg/L, so in order to avoid the toxicity of cyanide, it is important for industrial waste water to be treated by one of following methods: ion exchange, electrowinning, hydrolysis distillation, membrane treatment, electro dialysis, acidification, volatilization, flotation, addition of metal ions, alkali chlorination, hypochlorite oxidation of cyanides, hydrogen peroxide process, ozone treatment of cyanides, photolytic degradation, heterogeneous photo-catalysis, and the INCO process (Ilyas and Lee, 2018; Dash et ah, 2008; IAEA 2002; Ahmaruzzaman, 2011; Sorokin et ah, 2001; Kim et ah, 2003; Wang et ah, 2006; Carrillo-Pedroza and Soria-Aguilar, 2001a, b; Australia Environment, 2003; Ozomax, 2005; Volesky and Naja, 2005; Oelsner et ah, 2001; EPRI Environmental Community Center, 1997; Miller, 2003; Davies et ah, 1998; Barr et ah 2007; Latkowska and Figa, 2007;Bucshet ah. 1980; Young et ah, 1984; Botz, 1999;Durney, 1984; Logsdon et ah 1999; Young and Jordan, 1995; Iordache et ah 2003; EPA 1994; Terry et ah, 2001; Scott, 1984; Lemos et ah 2006; USEPA, 1994; Gogate and Pandit, 2004; Botz and Mudder, 2000; Simovic et al, 1985; Aguado et ah 2002).

Leaching with biogenic cyanide can be a potential alternative to chemical cyani- dation, as biogenic cyanide is destroyed by the same type of micro-organisms, if they remain in excess, through a combination of hydrolytic, oxidation, reduction, and substitution processes. On the hydrolytic pathway, there is direct cleavage of the

C-N bond, eliminating the possibility of further reactivity. The decomposition can be catalyzed by the cyanidase-forming by-products formic acid and ammonia (hydrolysis) or by cyanide hydratase forming the formamide by-product (hydration) (Dumestre et al„ 1997). In the oxidative pathway, monoxygenase enzymes catalyze the cyanides to cyanates that can be further hydrolyzed to ammonia and carbon dioxide dioxygenase enzymes (Knowles, 1976).

Although the anaerobic conditions for destroying cyanide are uncommon, numerous microbes follow this route to form ammonia and methane as the destructive products (Kao et ah, 2003; Ilyas and Lee, 2018), as in the reactions below:

ACKNOWLEDGEMENTS

This work was supported by the Brain Pool Programme through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (Grant No. 2019H1D3A2A02101993) and the Basic Science Research Programme through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Project no. 2020R111 A1 AO 1074249).