title: Effect of magnetic pore surface coating on the NMR relaxation and diffusion signal in quartz sand.
abstract: Magnetic impurities are ubiquitous in natural porous media such as sand and soil. They generate internal magnetic field gradients because of increased magnetic susceptibility differences between solid and liquid phase in the pore space and because of the presence of magnetic centers. These internal gradients accelerate NMR relaxation rates and thus might limit the possibility of pore space characterization using NMR. In this study, we investigate the effects of coating the surface of natural sands by the antiferromagnetic iron oxyhydroxide goethite on NMR relaxation and diffusion properties. We found a non-quadratic dependence of the relaxation time distributions on the echo time indicating that the relaxation experiments were not performed in the fast diffusion limit, while the weak dependence on the external magnetic field strength is explained by the preponderance of the surface relaxation over the effect of diffusion in internal gradients. The surface to volume ratio of the pore space, determined by NMR diffusimetry ((S/V)NMR) remains approximately constant, whereas the same quantity, determined from gas adsorption ((S/V)BET) increases proportional to the coating density. This is because gas adsorption measures surface roughness on sub-nanometer scale, whereas NMR diffusimetry averages over structures smaller than few microns. This has consequences for the calculation of the surface relaxivities. The usage of the (S/V)NMRleads to constant values, whereas the usage of (S/V)BETleads to apparently decreasing relaxivities with increasing coating, which is unrealistic. Copyright © 2016 John Wiley & Sons, Ltd.
authors.affiliation: Agrosphere (IBG-3), Forschungszentrum Jülich GmbH, Jülich, 52425, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Duschl
authors.affiliation: Agrosphere (IBG-3), Forschungszentrum Jülich GmbH, Jülich, 52425, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Pohlmeier
authors.affiliation: MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, 6012, New Zealand.
authors.forename: Timothy I
authors.initials: TI
authors.lastname: Brox
authors.affiliation: MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, 6012, New Zealand.
authors.forename: Petrik
authors.initials: P
authors.lastname: Galvosas
authors.affiliation: Agrosphere (IBG-3), Forschungszentrum Jülich GmbH, Jülich, 52425, Germany.
authors.forename: Harry
authors.initials: H
authors.lastname: Vereecken
mesh_terms: D000465:Algorithms; D004058:Diffusion; D004574:Electromagnetic Fields; D005740:Gases; D019015:Geologic Sediments; D058085:Iron Compounds; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008903:Minerals; D016062:Porosity; D011791:Quartz / Q000737:chemistry*; D012822:Silicon Dioxide / Q000737:chemistry*
chemical_list: D005740:Gases; D058085:Iron Compounds; D008903:Minerals; C094886:goethite; D011791:Quartz; D012822:Silicon Dioxide
keywords: 1H; NMR; NMR relaxometry; PFG NMR; fast diffusion limit; magnetic coating; porous media; surface relaxivity