Inhibition of Iron and Sulphur-Oxidizing Microorganisms
The solubilisation of metals due to the action of microorganisms has been extensively studied (Rawlings, 2005; Simate and Ndlovu, 2008; Simate et al., 2009; Simate et al., 2010). One shared characteristic amongst the microorganisms particularly the acidophilic microorganisms is their ability to produce the ferric iron and sulphuric acid required to degrade the sulphide-bearing minerals and facilitate metal recovery (Rawlings, 2005). Indeed, many other researchers also argue that microorganisms like acidophilic iron- and sulphur-oxidizing microorganisms (e.g., Thiobacillus ferrooxidans and Thiobacillus thiooxidans) are known to accelerate sulphide oxidation (Sasaki et al., 1998; Bacelar-Nicolau and Johnson, 1999) once the system is in a highly acid state (Kleinmann, 1990). This means that the absence of microorganisms in sulphidic mine wastes could slow down and limit the formation of AMD (Sahoo et al., 2013; Park et al., 2019). It is on the basis of this notion that bactericides such as anionic surfactants, cleaning detergents, organic acids and food preservatives have been used to inhibit the growth of these microorganisms (Kleinmann and Erickson, 1981; Kleinmann, 1982; Kleinmann and Erickson, 1983; Backes et al., 1987; Evangelou, 1995; Lottermoser, 2003; Zhang and Wang, 2017; Park et al., 2019).
According to a number of researchers, in general, bactericides change the protective and greasy coating that allow the internal enzymes in the microorganisms to maintain a circumneutral pH in order to function normally in an acid environment, and/or bactericides may also disrupt the contact between the microorganism and the sulphide mineral surface (Langworthy 1978; Ingledew 1982; Kleinmann 1998; Sahoo et al., 2013). More specifically, anionic surfactants allow protons to penetrate into the microorganism's cell membranes freely thus causing disruptions to its enzymatic functions at low concentrations and eventual death of the cell at high concentrations occurs (Evangelou, 1995; Lottermoser, 2003; Zhang and Wang, 2017; Park et al., 2019). Similarly, organic acids are harmful to acidophiles because they uncouple the respiratory chain of the microorganisms under acidic conditions via the penetration of their protonated forms through the cell membrane which then deprotonates while inside the cells releasing harmful H+ ions (Baker-Austin and Dopson, 2007; Park et al., 2019).
Despite bactericides being cost effective compared to dry or wet cover techniques, they have a number of limitations. Bactericides cannot permanently inhibit microbial activity because they are water-soluble and thus are easily washed away from the sulphidic wastes-microorganism interface (Park et al., 2019); and, therefore, repetitive applications of the bactericide are required (Kleinmann, 1998) so as to maintain their effectiveness in controlling AMD generation (Kuyucak, 2002; Skousen et al., 2000; RoyChowdhury et al., 2015). One critical disadvantage of the water-soluble bactericides such as anionic surfactants is that once they are carried away from the sulphidic wastes-microorganism interface and find themselves into waterbodies, they become toxic to aquatic species (Liwarska et al. 2005; Hodges et al. 2006; Sahoo et al., 2013). Some studies have also found that anionic surfactants work best on fresh and unoxidised sulphides (Johnson and Hallberg, 2005; Sahoo et al., 2013).