Quantifying mass transport processes in environmental systems using magnetic resonance imaging (MRI)

TA Engineering (General). Civil engineering (General);GE Environmental Sciences

Ramanan, Baheerathan ; Phoenix, Vernon

University:University of Glasgow

Department:Institute of Neuroscience and Psychology

关键词: Mass transport, environmental systems, MRI, reaction-transport models;   

Others  :  http://theses.gla.ac.uk/2974/1/2011ramananphd.pdf

来源: University of Glasgow

【 摘 要 】

Understanding the transport behaviour of pollutants is key to enhance remediation strategies and to inform predictive models of pollutant behaviour in environmental and engineered systems. This work investigates magnetic resonance imaging (MRI) as a methodology for imaging heavy metal, molecular and nanoparticle transport in two different saturated porous systems: biofilms and saturated porous geologic media (gravel).While most renowned for its use in medicine, magnetic resonance imaging (MRI) is enabling us to image the transport of heavy metals, macro-molecules and nanoparticles inside biofilms and porous columns in real time. This is achieved using either ions which are paramagnetic (e.g. Cu2+) or molecules labelled with paramagnetic ions (e.g. Gd3+) or superparamagnetic (e.g. nanomagnetite) nanoparticles. Presence of these tracers causes a concentration dependent shortening of relaxation times (T1 or T2) of the surrounding 1H nuclei and thus creates noticeable changes in the MRI signal. Critically, this enables the transport of (super)paramagnetic ions, molecules or nanoparticles through the biofilm or porous geological media to be imaged. Moreover, the actual concentrations of molecules can be quantified, as changes in relaxation rates have a linear relationship with the concentration of the tracer molecules. Hence, MRI can be used not only to track but also to quantify the transport of (super)paramagnetic molecules inside biofilms and saturated porous columns. The key advantages of MRI over other techniques are its ability to image inside systems opaque to other methods and its ability to collect data non-invasively, hence the system is unperturbed by the analysis. In this study, the transport of Gd-DTPA, a commonly used MRI contrast agent, was successfully imaged through phototrophic biofilms of 10 and 2.5 mm thicknesses. To improve spatial resolution, for the 2.5 mm thickness biofilm, a bespoke 5 mm diameter RF coil was constructed. The comparison of spatially distributed, time-varying concentrations of Gd-DTPA inside the biofilms with diffusion models illustrated that transport was via both diffusion and advection. This work illustrated the potential of using paramagnetically labelled molecules to quantify molecular pollutant transport and fate in biofilms.MRI was also used to image heavy metal trasport in artificial biofilms (composed of agar and bacteria) to test the suitability of an existing adsorption-diffusion model to represent heavy metal transport and fate in biofilms. While the diffusion coefficients and adsorption constants estimated were appropriate, discrepancies between the model and the data illustrates models may need to be developed further to incorporate factors such as concentration dependant diffusion or cell lysis.Finally, the ability to image inside opaque systems was further exploited to image nanoparticle transport inside a coarse-grained packed column. This was undertaken to illustrate the potential for MRI to image nanoparticle pollutant transport in systems relevant to river beds and sustainable urban drainage systems (SUDS). MRI was successfully used to image the nanoparticle transport, with significant transport inhibition was observed in positively charged nanoparticles compared to negatively charged nanoparticles due to permanent attachment.