Transport and deposition of colloidal Fe, Mn and Al oxides play key roles in the cycling of toxic transition metals in aquatic environments because these colloids strongly bind transition metals. Further, attachment of biological cells and biofilm growth on surfaces can indirectly affect toxic metal distribution by influencing the deposition of colloidal oxides to surfaces. To elucidate the mechanisms governing these processes, deposition of colloidal oxides onto surfaces must be evaluated in the presence of suspended and adherent bacterial cells. Both particle size and concentration are expected to influence deposition. An experimental protocol was developed to determine the size distribution of iron colloids in mixtures with suspended cells. A Ti(III) reagent was used to reduce and dissolve colloidal Fe(III) from mixtures containing both suspended cells and Fe colloids. The size distribution of Fe(III) colloids in the original solution was then determined from the difference between size distributions before and after dissolution of Fe with Ti(III). The Ti(III) reagent dissolved over 95% of the Fe colloids without altering the size distribution of suspended bacterial cells, and the method accurately determined the size distribution of Fe colloids added to cell suspensions. The applicability of this protocol was tested by applying it to a study of the deposition of Fe(III) oxide particles onto glass surfaces with and without biofilms of the bacterium Burkholdaria cepacia 17616. Experimental results using a laboratory biofilm reactor indicated that the deposition rate of Fe(III) colloids was not significantly affected by the presence of B. cepacia biofilms or by the presence of previously deposited Fe. However, deposition of Fe to reactor surfaces other than the glass surfaces may have interfered with the analyses, and atomic absorption measurements showed a slight increase in Fe deposition onto glass surfaces with biofilms present. Fe deposition to the composite of all reactor surfaces increased with increasing colloidal particle size, indicating a dominance of interception and/or sedimentation in controlling Fe deposition on surfaces in the biofilm reactor.
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