Lead distribution in a simulated aquatic environment: Effects of bacterial biofilms and iron oxide

Biofilms influence the transport and fate of heavy metals in aquatic environments both directly by adsorption and complexation reactions and indirectly via interactions with oxides of iron and manganese. These reactions were investigated by introducing lead into a continuous-flow biofilm reactor that was designed to simulate conditions in a flowing freshwater aquatic environment. The reactor provided controlled conditions, and use of a chemically-defined growth medium allowed calculation of lead speciation with a chemical equilibrium program (MINEQL). Pseudomonas cepacia was employed as a test cell strain because of its ability to grow and form biofilms in the defined medium. This bacterium affected lead distribution in the reactor by adsorbing lead both to adherent and suspended cells. When the aqueous bulk lead concentration was 1.4 ± 0.1 μM and biofilm coverage (measured as chemical oxygen demand, COD) was 50 mequiv COD/m2, lead adsorption was increased by about a factor of five relative to bare glass. Of the total lead in solution, only 1% was adsorbed to suspended cells (5 × 107cells/ml). Lead adsorption to biofilms followed a Langmuir isotherm with a maximum adsorption (Γmax) of 56 μmol Pb/equiv COD and an adsorption equilibrium constant (K) of 0.64 liter/μmol Pb. Lead complexed with dissolved bacterial expopolymer was below detection limits. Pretreatment of glass slides with colloidal iron also significantly increased lead adsorption relative to bare glass. Lead adsorption to adsorbed iron fit a Langmuir isotherm with Γmax= 50 μmol Pb/mol fe, and K = 1.3 liter/μmol Pb. Lead binding to glass coated with both cells and iron was additive, and could be predicted by summing adsorption predicted using isotherms for each constituent. The presence of iron surface coatings increased initial biofilm formation rates, but after reaching steady state conditions, biofilm coverage was similar for slides treated with iron and untreated slides. A concentration of 1 μM lead produced a transient reduction in suspended cell counts. Cell counts recovered to the original cell density over the course of five to ten reactor retention times. With iron present, the magnitude of the reduction in cell concentration in response to the addition of lead was greatly reduced, suggesting that toxic effects of lead may be reduced by iron.