Microbial Pb(II)-bioprecipitation : characterising responsible biotransformation mechanisms

Abstract

The research aimed to facilitate the scalability of a lead bioremediation process through improved understanding of the dominant mechanisms present. The process utilises a local microbial consortium which previously demonstrated highly efficient bioremoval and -recovery of aqueous lead. To this end the project investigated lead bioremoval, microbial growth, and the effects of two different complementary anions (nitrates from lead nitrate and perchlorate from lead perchlorate) on the resulting precipitate composition and structure. The experiments were conducted anaerobically to facilitate dissimilatory metal reduction with Luria Bertani broth in the presence of different lead concentrations. Analyses included growth, lead removal, nitrate, perchlorate and sulphate measurements, transmission and scanning electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy. A maximum lead removal rate of 4.71 ppm/h and a maximum specific growth rate of 0.76 /h were observed. Both the presence of nitrate, perchlorate, as well as different lead concentrations had significant effects on the precipitate. This contrasted with the growth that was solely affected by the lead concentrations present. Lead sulphide, elemental lead, elemental sulphur, and pyromorphite were present in the precipitate at varying ratios dependent on the conditions. The anoxic biotransformation of lead sulphide to pyromorphite was established, with the production of elemental sulphur in solid-state. A consistent amount of elemental lead was detected throughout, indicating dissimilatory reduction. A symbiotic relationship between sulphate reducing, lead reducing, sulphur oxidizing, and denitrifying bacteria was found to be responsible for biotransformation and bioprecipitation. The improved understanding of the Pb bioremoval and bioprecipitation mechanisms presented in the current study provides much needed insights crucial to future scaling of the technology and consequent cleaner production of lead. To further elucidate the bioremediation processes involved in the system, it is recommended to conduct genomic and transcriptomic research into the microbiome to assess its adaptability under continuous operation.