Magnetic resonance imaging of structure, diffusivity, and copper immobilization in a phototrophic biofilm

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Magnetic resonance imaging (MRI) was used to spatially resolve the structure, water diffusion, and copper transport of a phototrophic biofilm and its fate. MRI was able to resolve considerable structural heterogeneity, ranging from classical laminations 500 µm thick to structures with no apparent ordering. Pulsed-field gradient (PFG) analysis spatially resolved water diffusion coefficients which exhibited relatively little or no attenuation (diffusion coefficients ranged from 1.7 x 10–9 m2 s–1 to 2.2 x 10–9 m2 s–1). The biofilm was then reacted with a 10-mg liter–1 Cu2+ solution, and transverse-parameter maps were used to spatially and temporally map copper immobilization within the biofilm. Significantly, a calibration protocol similar to that used in biomedical research successfully quantified copper concentrations throughout the biofilm. Variations in Cu concentrations were controlled by the biofilm structure. Copper immobilization was most rapid (5 mg Cu liter–1 h–1) over the first 20 to 30 h and then much slower for the remaining 60 h of the experiment. The transport of metal within the biofilm is controlled by both diffusion and immobilization. This was explored using a Bartlett and Gardner model which examined both diffusion and adsorption through a hypothetical film exhibiting properties similar to those of the phototrophic biofilm. Higher adsorption constants (K) resulted in longer lag times until the onset of immobilization at depth but higher actual adsorption rates. MRI and reaction transport models are versatile tools which can significantly improve our understanding of heavy metal immobilization in naturally occurring biofilms.

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