Pb remobilization by bacterially mediated dissolution of pyromorphite Pb5(PO4)3Cl in presence of phosphate-solubilizing Pseudomonas putida

Abstract

Remediation of lead (Pb)-contaminated sites with phosphate amendments is one of the best studied and cost-effective methods for in situ immobilization. In this treatment, a very stable mineral, pyromorphite Pb5(PO4)3Cl, is formed. Several studies propose to improve this treatment method with the addition of phosphate-solubilizing bacteria (PSB). The effect of bacteria on solubilization of pyromorphite is unknown. In this study, the effect of the soil microorganisms on the stability of pyromorphite Pb5(PO4)3Cl has been investigated in a set of batch solution experiments. The mineral was reacted with Pseudomonas putida, a common soil microorganism. Dissolution of pyromorphite was enhanced by the presence of P. putida, resulting in an elevated Pb concentration in the solution. This occurred even when the bacteria were provided with an additional source of phosphate in the solution. Pyromorphite has been shown to be a potential source of nutrient phosphorus for common soil bacteria. Thus, the use of PSB in remediation treatments of Pb contaminated sites may have adverse long-term impacts on Pb immobilization. Conscious phosphate management is suggested for long-term sustainability of the in situ Pb immobilization by pyromorphite formation.

Fig. 1

The SEM microphotographs and the EDS spectra of precipitates used in the experiments: a pyromorphite Pb₅(PO₄)₃Cl and b chlorapatite Ca₅(PO₄)₃Cl. Samples were carbon-coated prior to the analysis

Fig. 2

The optical density (OD₆₀₀) of the bacterial suspension as a function of time. Error bars represent the standard deviation of triplicate. a The absorbance readings for reactors with MP + solution with aqueous phosphates; b the absorbance readings for reactors with MP− solution with chlorapatite; c the absorbance readings for reactors with MP− solution with pyromorphite. In P-rich environment, the mineral presence in a solution had no effect on microbial growth. Nutrient shortage resulted in a limited bacterial growth. P. putida could grow if apatite was the only source of P in a solution. In reactors with P-deficient solution, only the beginning of the logarithmical growth was observed. The bacteria grown under nutrient-rich conditions exhibited the full-growth cycle for the same experimental time

Fig. 3

The variation of pH with time in all experimental reactors. Error bars represent the standard deviation of triplicate. In the abiotic experiments, pH remained constant for the whole experimental time, whereas in the microbial experiments, pH increased

Fig. 4

Total concentration of Pb in the experimental reactors with solution amended with pyromorphite as a function of time. Error bars represent the standard deviation of triplicate. Microbially enhanced dissolution of pyromorphite is apparent. Pyromorphite dissolves significantly more in presence of the bacteria than in abiotic environment. The highest concentration of Pb is in the reactor with bacteria and P-deficient solution, whereas in abiotic solutions (P rich and P deficient), pyromorphite dissolution is almost identical within the experimental error

Fig. 5

Total concentration of P in the experimental reactors from the E.II series as a function of time. Error bars represent the standard deviation of triplicate. a P-rich solution. A fluctuation of P concentration in this suspension resembles the inversion of the culture growth cycle; b P-deficient solution. The P concentration in the solution inoculated with bacteria constantly increased with time and was significantly higher than in the corresponding abiotic experiment. Note the scale difference

Fig. 6

Molar Pb/P ratio in the MP− suspension with pyromorphite as a sole source of phosphates for the microbes. Elevated Pb content in comparison to the P in the suspension was observed, indicating enhanced dissolution of PY associated with bacterial P uptake