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 e nvironments 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 condi tions, and use of a chemically-defined growth medium allowed calculati on of lead speciation with a chemical equilibrium program (MINEQL). Ps eudomonas cepacia was employed as a test cell strain because of its ab ility to grow and form biofilms in the defined medium. This bacterium affected lead distribution in the reactor by adsorbing lead both to ad herent and suspended cells. When the aqueous bulk lead concentration w as 1.4 +/- 0.1 mu M and biofilm coverage (measured as chemical oxygen demand, COD) was 50 mequiv COD/m(2), lead adsorption was increased by about a factor of five relative to bare glass. Of the total lead in so lution, only 1% was adsorbed to suspended cells (5 x 10(7) cells/ml). Lead adsorption to biofilms followed a Langmuir isotherm with a maximu m adsorption (Gamma(max)) of 56 mu mol Pb/equiv COD and an adsorption equilibrium constant (IC) of 0.64 liter/mu mol Pb. Lead complexed with dissolved bacterial exopolymer was below detection limits. Pretreatme nt of glass slides with colloidal iron also significantly increased le ad adsorption relative to bare glass. Lead adsorption to adsorbed iron fit a Langmuir isotherm with Gamma(max) = 50 mu mol Pb/mol Fe, and K = 1.3 liter/mu mol Pb. Lead binding to glass coated with both cells an d iron was additive, and could be predicted by summing adsorption pred icted using isotherms for each constituent. The presence of iron surfa ce coatings increased initial biofilm formation rates, but after reach ing steady state conditions, biofilm coverage was similar for slides t reated with iron and untreated slides. A concentration of 1 mu M lead produced a transient reduction in suspended cell counts. Cell counts r ecovered to the original cell density over the course of five to ten r eactor retention times. With iron present, the magnitude of the reduct ion in cell concentration in response to the addition of lead was grea tly reduced, suggesting that toxic effects of lead may be reduced by i ron.