Microbial lead(II) precipitation : the influence of aqueous Zn(II) and Cu(II)

Abstract

The current study focused on the impact of heavy metals Zn(II) and Cu(II), regularly found in conjunction with lead in industrially polluted areas, on the Pb(II) bioprecipitation capabilities and metabolic activity of an industrially isolated microbial consortium. The experiments were performed with Pb(II) concentrations of 80 ppm and varying concentrations of Zn(II) (40 ppm and 80 ppm) and Cu(II) (40 ppm, 100 ppm, and 150 ppm). Different conditions were tested namely; Pb(II)&Zn(II), Pb(II)&Cu(II), Zn(II) only, and Cu(II) only. The experiments were run for a period of 7 days, where the residual aqueous Pb(II), Zn(II), and Cu(II) measured the degree of removal of each metal. The growth activities for each combination of metals were quantified using CFU plate count analysis by plating the final sample of the 40 ppm Zn(II) and Cu(II) runs (including and excluding Pb(II)) on agar plates spiked with a constant 80 ppm Pb(II). The results were compared to previous experiments conducted with samples containing only 80ppm Pb(II). Pb(II) concentrations decreased by 69% and 25% in the presence of 40 ppm and 80 ppm Zn(II), respectively. A grey precipitate was only observed in the presence of 40 ppm Zn, with no precipitate observed with 80 ppm Zn(II). Additionally, a limited attenuation in the Zn(II) concentrations of 12 % and 7% were measured for the 40 ppm Zn(II)&Pb(II) and 80 ppm Zn(II)&Pb(II) runs respectively. The results suggest different removal mechanisms present in the 40 ppm and 80 ppm Zn(II) runs, with a precipitation mechanism at 40 ppm Zn and a biosorption mechanism at 80 ppm of Zn(II). Pb(II) concentrations decreased by 0%, 32%, and 26% for the 40 ppm, 100 ppm, and 150 ppm Cu(II)&Pb(II) runs respectively. The corresponding Cu(II) concentrations decreased by 50%, 63%, and 71% respectively, indicating a competitive removal mechanism with no observed production of coloured precipitate, such as biosorption. The Cu(II) only runs exhibited removal percentages of 64%, 63%, and 53% for the 40 ppm, 100 ppm, and 150 ppm runs. During growth activity analysis it was observed that the samples containing Pb(II)&Zn(II) and Pb(II)&Cu(II) showed significantly less growth than that of the Pb(II) only plates previously tested at 5.47±0.83 × 108 CFU/mL, compared to 1.131±0.065 × 107 CFU/mL and 5.98±1.86 × 106 CFU/mL respectively. It can be concluded that the bioprecipitation mechanism of Pb(II) as previously observed are severely inhibited by elevated concentrations of Zn(II) and Cu(II), resulting in an adsorption mechanism dominating. Additionally, it was found that Pb(II) promotes metabolic activity while Zn(II) and Cu(II) inhibits metabolic activity. This is possibly as a result of inhibition of the Pb(II) precipitation mechanism. These results indicate that Zn(II) and Cu(II) ions need to be removed prior to bioprecipitation and recovery of Pb(II) using the specific industrial consortium.