Sorption of trace metals to bacteria and (hydr)oxide minerals : controls on the chemistry of natural waters

• Main Author: Moon, Ellen Margaret
• Published: University of Southampton 2011
• Subjects: 551.6091
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.580562

Accurately predicting the fate and mobility of trace metals in natural aqueous systems requires a comprehensive, molecular understanding of metal binding to both the organic and mineral components present in terrestrial and oceanic sediments. This information can then be used to help inform and constrain thermodynamic models for trace metal fate and mobility, and provide an insight into important geochemical processes like trace metal stable isotope fractionation, which may reveal information about past climate. Cu is both a micro-nutrient and common contaminant of freshwater and marine systems, where its reactivity and cycling is dependent on sorption processes to the mineral and organic phases present. Iron (hydr)oxides precipitated in the presence of bacteria are increasingly recognised as a significant proportion of the reactive iron mineral phase in natural systems. Molecular-level EXAFS spectroscopy and macroscopic thermodynamic surface complexation modelling were combined to investigate Cu(II) adsorption to ferrihydrite and Bacillus subtilis (a common freshwater and soil bacterium). At environmentally relevant pH and Cu loading, Cu was found to form an inner-sphere, bidentate, edge-sharing surface complex on ferrihydrite. A stability constant of log K(=FeOH2Cu+ = 8.61 was determined. Cu adsorption to B. subtilis was found to occur via inner-sphere, monodentate surface complexation to carboxyl functional groups present on the bacterial cell walls from pH ~2 to 7. A stability constant of log K=RCOOCu+ = 7.13 was derived. These findings were combined to characterise Cu adsorption to ferrihydrite-B. subtilis composites. Adsorption of Cu to the composites is intermediate to adsorption on ferrihydrite and B. subtilis, and total Cu adsorption appears to be independent of composite mass ratio. EXAFS spectroscopy revealed that Cu adsorbs to both fractions in the composites, forming the same surface complexes as for the isolated end members, and that the distribution of the total adsorbed Cu between the two fractions is dependent on both pH and composite mass ratio. Thermodynamic surface complexation modelling showed that for composites dominated by ferrihydrite, Cu adsorption is approximately additive, unlike composites where B. subtilis dominates, for which surface potential effects were considered the likely cause of non-additivity. This work will ultimately help improve our understanding of metal-mineral-microbe interactions and our ability to predict trace metal fate and mobility. Marine ferromanganese precipitates are found throughout the worlds oceans as hydrogenetic precipitates, diagenetic nodules and hydrothermal deposits, each with Mn mineralogy reflecting its Mn content and growth environment. Being largely composed of Mn and Fe (hydr)oxides, these precipitates are potent adsorbents of oceanic trace metals, including TI, whose stable isotope time-series data in hydrogenetic deposits shows great promise as a tracer of past marine and climatic conditions. Using XAS, the mechanism of TI sorption to Mn and Fe (hydr)oxides was deduced. TI(I) is oxidised to TI(III) during sorption to hexagonal birnessite, but not during sorption to todorokite, triclinic birnessite or ferrihydrite. TI(III) forms an inner-sphere complex at the hexagonal birnessite surface, located at vacant octahedral sites in the phyllomanganate sheets. 205TI enrichment has been observed in hydrogenetic deposits (hexagonal birnessite rich) and furthermore, recent theoretical calculations predict a large equilibrium stable isotope fractionation between TI(I) and TI(III), leading to TI(III) species that are enriched in the heavy 205TI isotope. In light of this work, a molecular sorption-oxidation-fractionation mechanism was proposed to provide the first unifying explanation for the observed geochemical behaviour of TI in marine ferromanganese-rich sediments. This novel mechanism will ultimately help interpret TI signatures in marine sedimentary archives to provide new constraints on past oceanic and climatic change.