Acidophilic bacteria are capable of catalysing the oxidation of soluble ferrous ions in solution and insoluble mineral sulfides, such as pyrite (FeS2) and arsenopyrites (FeAsS).
The influence of nickel (Ni2+), cobalt (Co2+) and copper (Cu2+) ranging from (8–160mM) was investigated during the oxidation of a ferrous iron solution (100mM) and a pyritic-gold concentrate with an unadapted mixed mesophilic culture of Thiobacillus ferrooxidans and Leptospirillum ferrooxidans. The influence of these cations was also investigated with an arsenopyrite-gold concentrate with an unadapted moderately thermophilic mixed culture of Sulfobacillus thermosulphidoxidans and sulphobacillus acidophilus.
In the case of the mesophiles during the oxidation of the ferrous substrate the bacteria's ability to oxidise the Fe2+ was unaffected at 8mM of each of nickel, cobalt or copper individually and as the concentration of each base metal increased the oxidising ability of the mixed culture became inhibited. Copper was found to cause the largest inhibition followed by nickel and cobalt. However, during Fe2+ oxidation with the addition of cobalt and a combination of cobalt and nickel respectively, at concentrations >80mM the rate of oxidation was found to increase from that observed at 40mM and showed the addition of cobalt had a positive enhancing effect on the oxidation of ferrous iron.
During the biooxidation of the pyritic-gold concentrate, the addition of 40mM and 80mM Cu2+ respectively deceased the oxidising ability of the mixed culture by ∼30% and ∼50% respectively. However, the addition of either nickel or cobalt ions at either concentration did not significantly inhibit the catalytic ability of the bacteria.
In the case of the moderate thermophiles and oxidation of Fe2+, initially, when the pH increased from pH 1.8 to ∼pH 2.0 the mixed culture displayed incomplete oxidation of the substrate and was only able to oxidise 60–70% of the ferrous iron. The addition of each of Ni2+, Co2+ or Cu2+ at 8, 40 or 80mM respectively led to further decreases in the amount of substrate oxidation to between 20–30% at 80mM of each metal ion respectively. The incomplete oxidation was suggested by Johnson et al., 2000 to be due inhibition of the mixed culture by a ferric iron complex. Subsequently, during Fe2+ oxidation, the pH was maintained ≤pH 1.8 and complete oxidation was observed. The addition of 8mM of each of nickel, cobalt or copper individually inhibited the Fe2+ oxidation in the order Cu>Co>Ni. However, at 40mM and 80mM of each, the order of inhibition was Cu>Ni>Co respectively. Incomplete Fe2+ oxidation was observed with copper (at 40 & 80mM) and Nickel (at 80mM) whereas complete oxidation was still observed with the addition of 80mM Co2+.
The effect of base metals on the oxidising ability of the mixed culture indicates the importance of adapting bacteria to environments containing high concentrations of valuable metal ions.
The oxidation of the arsenopyritic-gold concentrate by the mixed culture was significantly inhibited by the addition of each of nickel (80mM), cobalt (80mM) and copper (40 and 80mM) respectively.
