Nutrient optimization in bioleaching: are we overdosing?

Abstract

The general trend in biomining (i.e., bioleaching and biooxidation) is the use of media with high concentrations of the nutrients (nitrogen as ammonium, phosphorous as phosphate, and K), which are considered to be essential for microbial growth. The depletion of any of the nutrients would affect negatively the bioleaching (and biooxidation) capacity of the microorganisms, so the formulation of the different media ensures that there is a surplus of nutrients. However, some of these nutrients (e.g., phosphate, K) may be already present in the ore and are made available to the microorganisms when the ore is exposed to the low-pH media used during bioleaching. The effect of phosphate addition (109 mg/L) and depletion on the bioleaching of low-grade sulfidic ore alongside the determination of ammonium (i.e., 25 mg/L, 50 mg/L, 109 mg/L, 409 mg/L, and 874 g/L) requirements were studied. The results of the experiments presented showed that the addition of phosphate did not have any effect on the bioleaching of the low-grade sulfidic ore while the addition of ammonium was necessary to obtain higher redox potentials (>650 mV vs. Ag/AgCl) and higher metal (Co, Cu, Ni, and Zn) dissolutions. Temperature was the factor that shaped the microbial communities, at 30°C, the microbial community at the end of all the experiments was dominated by Acidithiobacillus sp. as well as at 42°C, except when nutrients were not added and Sulfobacillus sp. was the dominant microorganism. At 55°C, DNA recovery was unsuccessful, and at 60°C, the microbial communities were dominated by Sulfolobus sp. In conclusion, the amount of nutrients in bioleaching could be reduced significantly to achieve the redox potentials and metal dissolution desired in bioleaching without affecting the microbial communities and bioleaching efficiencies.

Keywords: ammonium; bioleaching; media composition; microbial community; nitrogen; nutrients; phosphate; phosphorous.

Figure 1

Redox potential of shake flask experiments at 30°C, 42°C, 48°C, 55°C, and 60°C under different concentrations of ammonium and phosphate. Error bars indicate standard deviations for 3 biological samples. Media formulations can be found in Table 1.

Figure 2

Metal (Co, Cu, Fe, Ni, and Zn) concentrations (mg/L) at the end of the shake flask experiments at 30°C, 42°C, 48°C, 55°C, and 60°C under different concentrations of ammonium and phosphate. The line on the Fe graph represents metal concentration at the beginning of the experiments. For other metals (Co, Cu, Ni, and Zn), the metal dissolutions at the beginning of the experiments was below the lowest values depicted on the axis. Error bars indicate standard deviations for 3 biological samples. Media formulations can be found in Table 1.

Figure 3

Relative abundance at genus level of microbial communities in the inocula and at the end of the experiments at 30°C, 42°C, 48°C, and 60°C under different concentrations of ammonium and phosphate. Blank spaces correspond to samples that did not yield any DNA. Media formulations can be found in Table 1.

Figure 4

Principal Component Analysis ordination illustrates the compositional similarity of microbial communities at the end of the experiments at 30°C, 42°C, 48°C, and 60°C under different concentrations of ammonium and phosphate. The top five genera driving groups ordination are plotted on top of the PCoA. Media formulations can be found in Table 1.

Figure 5

Non-metric multidimensional scaling (NMDS) analysis plot of microbial communities in inocula (start) and at the end of the experiments at 30°C, 42°C, 48°C, and 60°C under different concentrations of ammonium and phosphate. Media formulations can be found in Table 1.

Figure 6

Upset plots show intersecting sets of samples (exclusive intersection), regarding medium composition (A) and temperature (B). The set size indicates the number of genera present for each experiment and the intersection size indicates the number of genera that are shared among the members of an intersection group. Abbreviations for the genera shared by each group are indicated in each count unit and its phylum is color coded. Key for the abbreviations used in the intersection bar chart are explained in the box on the left. Media formulations can be found in Table 1.