Mechanistic and kinetic understanding of Pb-phosphate biomineralization from humic acid-bound Pb under active growth of phosphate solubilizing Enterobacter aerogenes W6.

Humic acid-bound Pb (HA-Pb), as one of the representative solid-associated Pb species, plays important roles in Pb mobility and toxicity in aqueous environments. Stable Pb-phosphate minerals formation mediated by phosphate solubilizing bacteria (PSB) is a promising approach to immobilizing Pb in contaminated waters. However, the underlying processes and kinetics of Pb-phosphate biomineralization from labile HA-Pb species remain unclear. Here, experiments were conducted using dialysis bags to separate PSB Enterobacter Aerogenes W6 cells from HA-Pb and FePO solids, and the time-dependent evolutions of solution conditions, PSB metabolites, Pb species, and Pb-phosphate minerals were systematically analyzed. Acid-soluble Pb species (∼25%) in HA-Pb, mainly complexing with phenolic hydroxyl groups, served as the source for Pb-phosphate mineralization. Extracellular secreted small organic acids contributed to Pb solubilization from HA-Pb, while polysaccharide macromolecules played critical roles in promoting Pb-phosphate nucleation on PSB cell surfaces. By coupling time-dependent bacterial growth, an integrated kinetic model was developed and performed well in describing the underlying processes of Pb-phosphate biomineralization, including Pb solubilization from HA-Pb (RMSE<1%), Pb adsorption onto PSB cells (RMSE<20%), and Pb-phosphate mineral precipitation (RMSE<5%). The findings provide insights into the Pb-phosphate biomineralization of solid-associated Pb and could help to predict the fate of Pb in natural and engineering systems.