Microbial Pb(II) precipitation : kinetic modelling of Pb(II) removal and microbial growth

Abstract

The study aimed to propose a preliminary kinetic model for Pb(II) bioremoval by an industrially obtained microbial consortium. The consortium has previously been shown to be extremely effective at precipitating Pb(II) from solution. For data generation, 100 mL batch reactors were set up anaerobically and spiked with either 80 ppm Pb(II) or 500 ppm Pb(II). Each of the concentrations contained either Standard LB broth or Simulated LB broth; Simulated LB broth contained double the amount of nutrients (yeast extract and tryptone) as Standard LB broth. Four datasets where thus used with notation, 80LB, 80Sim, 500LB and 500Sim. The study focused on the initial 33 h of experimentation with all four conditions. It was observed that most of the Pb(II) were removed within the first 3 h (± 50%) in all the reactors, in the absence of visual changes, followed by a slower rate of Pb(II) removal and dark precipitation forming. The Pb(II) removal was found to be independent of the amount of the microbial growth rate or nutrients present. A two-phase exponential decay model was proposed with rapid Pb(II) removal linked to an adsorption mechanism within the initial 3 h, followed by a slower Pb(II) precipitation mechanism. Microbial growth was found to be dependent on the concentration of Pb(II), nitrates, and available nutrients in the system. Growth in the samples in all the samples was modelled in one phase, namely a nitrate dependent exponential growth phase, modelled using Monod type kinetics. The nitrate dependent exponential growth phase was constructed using the Monod kinetic model in conjunction with a non-competitive Pb(II)-inhibition Michaelis-Menten term. The same maximum specific growth rate (28.2 d-1) and Pb(II)-inhibition constant were determined for all fermentation conditions. These results suggest a detoxification mechanism via adsorption of Pb(II) onto biomass present in order to initiate growth, followed by the biological precipitation of the adsorbed Pb(II). The study presents the first model for microbial Pb(II) precipitation and provides a basis for the design of a continuous reaction setup required for future industrial application.