DEVELOPMENT OF A SELECTIVE EXTRACTION SCHEME USING COBALT AND CADMIUM DOPED HFO

Heavy metal contamination of near surface systems has been induced by anthropogenic inputs to soils and ground water since the industrial revolution. Better understanding of the speciation of metals such as Co and Cd will improve assessments of their mobility and bioavailability. Reductive dissolution of iron oxides (HFO) by microorganisms such as the iron reducing bacteria Shewanella putrefaciens may release metals co-precipitated with or adsorbed to HFO. Laboratory experiments will be conducted to assess changes in Co and Cd speciation during microbially-mediated reductive dissolution of Co- and Cd- doped HFO. To track changes in Co and Cd speciation, an extraction scheme is being calibrated in this study to distinguish metals bound within the HFO crystal lattice from metals bound to the HFO surface or to microbial biomass. To achieve this, HFO is synthesized by titration of a FeCl₃ solution with NaOH, using a method adopted from Cornell and Schwertmann (2000) to make five types of HFO: undoped or doped with 1 or 10% Co or Cd. The five types of HFO are extracted as a function of time using reagents including: 1M MgCl₂, 1M NaAcetate, .04M NH₂OH∙HCL, 32%H₂O₂ (based on methods from Tessier et al., 1979; 1982) and 2M HCl to develop an optimal scheme for delineating metal speciation. The same reagents will be tested on HFO and Shewanella putrefaciens with a known quantity of surface bound Cd or Co. The 2M HCl completely dissolved the HFO in less than 60 min. This data was used to calculate the percentage metals released in the other extraction experiments. The MgCl₂ released about 50% of the total Co and Cd from 1 or 10% doped ferrihydrite, and no Fe was released. NaAcetate released up to 5% of the Fe from the pure and 1% doped HFO together with ~50% of the Co or Cd after 7 days. However, only 40% of the Co compared to 100% of the Cd was released from the 10% doped HFO samples, and in these Fe release increased linearly with time for 7 days. NH₂OH∙HCL released ~50% of the Fe from all samples, together with up to 40% of the Co from the 1% Co doped HFO and 100% of the Co from the 10% doped HFO in 3 days. In contrast, no detectable Cd was released from the 1% doped HFO and up to 60% of the Cd was released from the 10% doped HFO after 3 days. The results are consistent with formation of a secondary Co or Cd precipitate in the 10% doped HFO synthesis.