1. Introduction

2. Methodology

3. Results and discussion

3.2. Application of Acidithiobacillus ferrooxidans in PGMs processing

The application of A. ferrooxidans in different PGMs processes is presented in Figure 4, this is based on the reviewed articles. The use of these bacteria in hydrometallurgy was categorized into bio-oxidation and bioleaching. Bio-oxidation was mainly used in gold processing and in some instances, it was done prior flotation (also referred to as biomodification). Bioleaching was applied to preconcentrate PGMs by leaching the metals from PGM-bearing base metal sulphides. A. ferrooxidans was further applied in flotation where selective flotation was desired. Bio-oxidation has the most application followed by bioleaching; bio-flotation has received the least attention. The application of A. ferrooxidans in biohydrometallurgy and bioflotation is further discussed in turn:

Table 1. Methodological table showing the summarized data for A. ferrooxidans efficiency on sulphide processing.

Table 1. Methodological table showing the summarized data for A. ferrooxidans efficiency on sulphide processing.

References	Category	Approach and Objective	Main findings
[28,33,34,35,36,37,38,39]	Biooxidation of Refractory gold	Quantitative. The main objective was to improve gold recovery by oxidizing the gold-bearing sulfidic minerals, mainly pyrite and arsenopyrite using A. ferrooxidans thereby liberating gold from the sulphide matrix prior cyanidation.	The gold recovery was strongly related to the extent of sulphide biooxidation, with the highest recoveries ranging from 85 to 98% depending on the operational parameters.
[40,41,42,43,44,45,46,47,48,49]	Bioleaching of Cu, Ni, Co from base metal sulphides	Quantitative. The aim was to investigate the efficacy of bioleaching of copper, Ni, Co using A. ferrooxidans (mesophile and moderately thermophile).	Together with the results from chemical leaching, the data indicated that Cu was mainly leached by sulphuric acid (bio-generated and traditional acid leaching) while a high Co extraction required Fe (II)-oxidizing microbial activity (bioleaching). Adding sulphuric acid reduces the needed time to reach the possible maximal recovery of metals. As it is obvious, the maximum recovery of Cu and Ni in both bioleaching and hydrometallurgy processes is competitive and excellent
[50,51,52,53,54,55,56,57,58]	Bioflotation of base metal sulphides	Quantitative. Most research on bioflotation investigated the use of A. ferrooxidans as a pyrite depressant. This was applied in selective flotation of chalcopyrite and pyrite. It was also used in coal processing. The other objective was to ascertain the role of the phenomena in the biomodification of sulphides by Acidithiobacillus ferrooxidans culture (cells and growth media) and their impact in bioflotation.	The main results showed that A. ferrooxidans can facilitate the depression of pyrite while promoting the flotation of other base metal sulphides. It was further concluded that elemental sulphur -concentration increased because of the oxidation generated by bacterial cells, the effect is intensified by the Fe (III) left in the culture and by galvanic contact.
[42,59,60,61,62,63]	Bioprocessing/pre- concentration of PGMs from secondary sources /waste	Quantitative. The aim was to investigate the use of A. ferrooxidans in metal extraction from spent catalytic converters.	It was concluded that the bacteria may concentrate Cu, Cd, Zn and Pb in that way pre-concentrating PGMs prior conventional hydrometallurgy methods and reducing reagents consumption.

3.2.1. Application of A. ferrooxidans in Biohydrometallurgy: Biooxidation and Bioleaching

Biohydrometallurgy makes use of the bacteria's natural-oxidating ability to dissolve metal sulphides, this process allows the extraction and recovery of precious and base metals from primary ores and concentrates [26,64]. A. ferrooxidans have been successfully applied in gold processing as a bio-oxidant which serves to liberate gold prior to leaching [28,65,66,67]. Ore pre-treatment is required prior to gold cyanidation leaching to get high gold recovery from refractory ores such as arsenopyrite and pyrite, these are gold carriers in most deposits. Conventionally, pre-treatment before cyanidation is achieved through roasting or pressure oxidation or chemical oxidation, are employed to improve the gold recovery during cyanidation, bacterial oxidation is an alternative method. The bio-oxidation process employs microorganisms such as A. ferrooxidans to oxidize sulphide minerals, liberating gold particles from the sulphide matrix and making them readily available for cyanidation [67].

Although the conventional pre-treatment methods can effectively extract gold from refractory ores, these have drawbacks, including high energy requirements, a need for a variety of chemicals in large quantities, and, most importantly, a high level of environmental pollution [68,69]. In contrast, bio-oxidation pre-treatment is a straightforward, low-cost, and environmentally friendly method that has been investigated to extract metals from a variety of minerals/ores and waste materials with significantly reduced environmental impacts [39,70,71]. Strict environmental regulations imposed on metallurgical industries has enhanced substantial interest in bioleaching technology as an option for extraction of metals, as opposed to conventional methods using toxic chemical [70,72].

The oxidation mechanisms can occur through direct or indirect mechanism [47]. Direct mechanism involves physical contact between the bacteria and the sulphide minerals, e.g. pyrite (FeS₂), pyrrhotite (FeS), arsenopyrite (FeAsS) and chalcopyrite (CuFeS₂), which then react with dissolved oxygen to convert sulphide-sulphur to sulfate or elemental sulphur according to reactions (1) through (4) [28,65]:

$${2FeS}_2+{8O}_2+{2H}_2 \stackrel{A. ferrooxidans}{\to } {2Fe}^{2+}+{4SO}_4^{2-}+{4H}^{+}$$

(1)

$$4FeS+{8O}_2+{2H}_2{SO}_4 \stackrel{A, ferrooxidans}{\to } {4Fe}^{2+}+6{SO}_4^{2-}+{4H}^{+}$$

(2)

$$FeAsS+{4O}_2+{\frac{3}{2}H}_2 \stackrel{A. ferrooxidans}{\to } {Fe}^{2+}+{SO}_4^{2-}+3H+{AsO}_4$$

(3)

$$CuFe{S}_2+O_2+{2H}_2{SO}_4 \stackrel{A. ferrooxidans}{\to } {Fe}^{2+}+{Cu}^{2+}+{2SO}_4^{2-}+2S+{2H}_{2}0$$

(4)

Indirect mechanism involves oxidation-reduction cycle of ferrous and ferric ions in mineral-solution interface during bio-oxidation process according to the following reaction [28,65]:

$${Fe}^{2+}+\frac{1}{4} O_2+H^{+} \stackrel{A. ferrooxidans}{\to } {Fe}^{3+}+\frac{1}{2} H_{2}O$$

(5)

The ferric ion generated by reaction (5) further plays a role in subsequent oxidation of metal (II) sulphide (MS) into its divalent ions and elemental sulphur according to the following reaction [28]:

$$MS+{2Fe}^{3+} \to M^{2+}+{2Fe}^{2+}+S^0$$

(6)

Bio-oxidation of refractory-gold serves as a pretreatment step where the gold-disseminated in sulphides is liberated and made readily available for direct cyanidation. From the research conducted by Ciftci and Akcil [28], it was stated that the recovery of gold is directly proportional to the rate of bio-oxidation. It was further stated that when A. ferrooxidans were used for oxidation, reagents consumption decrease, in this case, cyanide consumption was related to the extent at which the mineral surfaces were oxidized

3.2.2. The Application of A. ferrooxidans in Bio-Flotation

Beneficiation route for PGMs ores depends on the grade of the ore that is being treated [2]. The conventional methods were optimized for high grades ores which had predictable mineralogy, due to the depletion of high grades ores, it is imperative to investigate innovative processing methods for low grade ores and secondary sources. The aim is to optimize mineral beneficiation processes in an economical and environmentally friendly way such that the by product and tailings produced are not harmful to the environment [3,47,64,73,74]. Usage of micro-organisms is a viable alternative to current methods since the residues from micro-organisms are recyclable, economically viable, environmentally friendly.

A. ferrooxidans was first investigated in bio-flotation research as a less dangerous substitute for cyanide as a pyrite depressant in the desulphurization of coal [75,76,77]. The bacterium's attachment to the pyrite surface caused substantial chemical surface modifications that made the material hydrophilic. Following these preliminary investigations, additional base mineral sulphides were also looked at, including chalcopyrite (CuFeS₂), galena (PbS), pyrrhotite (Fe_(1-x)S) and sphalerite (ZnS) [77]. As aforementioned, PGMs are associated with base metal sulphides and their recovery during the concentration process (flotation) is directly linked to the recovery of these base metal sulphides. In this paper, the application of A. ferrooxidans in flotation is discussed in terms of base metal sulphides which are the carriers of PGMs. Bio-beneficiation of PGMs- bearing sulphides such as arsenic platinum (As-Pt), laurite (RuS₂), braggite (Pt, Pd, Ni) S and a dimorph of braggite known as cooperite (Pt, Pd, Ni) S using A. ferrooxidans has not been reported in open literature.

a) 

Fe- sulphides

Typically, PGM ores are beneficiated through flotation where collectors, depressants, frothers and pH modifier are part reagent suites. Commonly, Xanthates are used as collectors while silicates are used a depressant at alkaline pH of around 9. Activators are added to condition the sulphides prior to floating [2]. Micro-organisms can either be used as collectors, depressants or activators based on the compound of interest and the gangue to be depressed. A. ferrooxidans is mainly applied in flotation where selective separation of pyrite from other base metal sulphides like chalcopyrite is desired. The selective ability of A. ferrooxidans is based on the following:

a)

The presence of aporusticyanin on the surface of the bacterial cell which causes A. ferrooxidans to preferentially adhere on pyrite over other sulphides [22,50],

b)

The formation of hydrophilic jarosite on the surface of pyrite [3],

c)

The development of oxidized layers on pyrite surface because of protracted bacterial interaction [46], and

d)

The rise in bacterial attachment density on pyrite increases in A. ferrooxidans depressant capacity [3].

The presence of aporusticyanin on the bacterial cells is ranked as one of the reasons A. ferrooxidans can be used for pyrite depression replacing the use of sodium cyanide which is well known as one of the depressants of sulphides. The efficiency of A. ferrooxidans is known to be at a pH of less than 2, because that is the favorable condition for its growth. However, for efficient separation, A. ferrooxidans depress pyrite under mildly alkaline conditions (which favors xanthate-copper sulphides interaction) because the production of extracellular polymeric substances (EPS) is not dependent on pH [3,56,78] meaning the bacteria does not lose its depressing ability even in alkaline conditions. The separation of copper sulphides and pyrite occurs at a pH ranging from 10-12 when lime is used, which brings about an advantage the bacteria has over conventional pyrite depressants. A. ferrooxidans has similar impact on pyrrhotite [3], because of A. ferrooxidans' preference for pyrrhotite, the mineral become hydrophilic, and this induced hydrophilic nature enhances depression of pyrrhotite, which would be favorable where selective flotation of PGMs-bearing sulphides is desired. The high density of the hydrophilic cells produced after microbial attachment mitigates the expected increase in floatability of pyrrhotite caused by the creation of S⁰ [3]. Furthermore, it was outlined that if the right conditioning process is followed, A. ferrooxidans can coexist with a collector like potassium isopropyl xanthate [3]. Researchers [23,79] showed that preconditioning the minerals with a collector followed by a brief conditioning period with cells of A. ferrooxidans can provide selective flotation of chalcopyrite from a pyrite/chalcopyrite combination. After preconditioning with the collector, the researchers demonstrated that bacterial cells preferred to adhere onto the pyrite surface, functioning as a depressant during flotation.

b) 

Cu-Fe-sulphides

A. ferrooxidans has high affinity for iron sulphide minerals as iron (II) and oxidized sulphur is their main source of energy. In a case where sulphides are bearing copper, Acidithiobacillus family preferably adheres slowly onto Cu-sulphides because copper ions are considered toxic to this bacteria group [3,50]. The biomodification carried out by A. ferrooxidans culture combines the action of bacteria and ferric ions. In the case of Cu-sulphides, biomodification (partial oxidation) is an indirect mechanism wherein the A. ferrooxidans do not adhere to the minerals surfaces meaning biomodification is imposed by Fe (III) ions produced after iron (II) oxidation. This form of Cu-sulphides oxidation is by means of polysulphides which is a combination of microbially induced H⁺ and Fe (III) from the bacterium conditioning solution. Regardless of ever-existent controversies over the oxidation products of sulphide minerals, it is accepted that the formation of S⁰, in moderately oxidizing potentials, is the most obvious mechanism for explaining the increase in hydrophobicity of sulphides [50].

From the articles [3,23,79,80,81], it was discovered that A. ferrooxidans may be good secondary collectors of chalcopyrite, depending on the solution pH conditions (acidic or alkaline). In acidic conditions, it was noticed that pyrite and chalcopyrite were collected, however, in alkaline condition chalcopyrite is collected, while for pyrite A. ferrooxidans renders hydrophilic nature on the mineral, reducing its floatability [54,55].

selective separation of quartz from pyrite and chalcopyrite can be realized with the intervention of A. ferrooxidans. Chandrapraba et al.; Sandhiya et al. [23,82] compared the settling rates of quartz, pyrite, and chalcopyrite with those of cells that had been cultivated on medium containing ferrous iron or elemental sulphur. According to the researchers [79], cells that had been pre-cultured on solid substrates like elemental sulphur were less effective at suppressing pyrite and chalcopyrite than cells that had been grown on a medium with soluble ferrous iron. Preconditioned bacterial cells made the cell surface more hydrophobic, making them fewer effective depressants for the tested mineral sulphides when grown on a solid substrate like elemental sulphur[3].

c) 

Other sulphides

With regards to pentlandite, its mechanism is extrapolated from interaction of A. ferrooxidans with pyrite. The preferential suppression of pyrite by A. ferrooxidans over non-ferrous galena due to the development of hydrophilic jarosite suggests that ferrous pentlandite can be suppressed by A. ferrooxidans since it is likely to generate jarosite [3]. Additionally, if the proper pH can be produced, any sulphide mineral has reportedly been observed to be flotation friendly. By direct microbial adherence to the mineral surface, A. ferrooxidans have shown to affect the surface chemistry and flotation behavior of the non-iron-sulphides minerals ZnS and PbS [82]. Before interacting with a collector, lead and zinc-sulphides could be selectively floated by conditioning with A. ferrooxidans in an acidic environment (pH 2). At low pH levels, the zinc sulfate that had formed on the ZnS was soluble, while the lead sulfate that had formed on the PbS remained insoluble. The interaction of the collector could be hindered by the development of insoluble lead sulphate, which decreases the PbS' capacity to float [77].

To obtain energy for growth, Acidithiobacillus species adhere to mineral sulphides; mineral oxides would not serve as a source of energy. However, there are several additional factors that contribute to bacterial adherence and biofilm development. It is believed that the development of biofilms helps bacteria survive in stressful situations and hold onto their position in an environment with favorable niche characteristics [83]. It has also been suggested that biofilms are the default mode of growth in nature, and that planktonic growth, which is frequently observed in lab settings, is the result of contrived growth circumstances. From these findings, it was concluded that A. ferrooxidans could be used where selective flotation is desired due to the different responses shown by different sulphides.

4. Conclusion

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