Andreas Pohlmeier

Characterization of unsaturated porous media by high‐field and low‐field NMR relaxometry

[1] A comparison study of nuclear magnetic resonance relaxometry at high and low magnetic field (7 and 0.1 T) has been initiated for investigating the influence of the magnetic field strength, variable clay content, and different degrees of saturation on the relaxometric properties of four ideal porous media. The samples consisted of medium sand with increasing fractions of kaolin clay ranging from 0 to 15%. Six different volumetric water contents between saturation and θ = 0.05 were used. Changes in water content of the samples were achieved by slow evaporation. T 2 relaxation curves were monitored by the Carr-Purcell-Meiboom-Gill sequence and were further analyzed by inverse Laplace transformation, yielding T 2 distribution functions. Sand shows a slight continuous shift with decreasing water content of a bimodal distribution function of T 2 to faster relaxation at high and low magnetic field. Sand-clay mixtures show broad, bimodal distribution functions for both magnetic field intensities which shift slightly with decreasing water content. Signal amplitude behavior with variation of saturation degree was also monitored. An expected proportionality of the total signal amplitude with water content was observed for all samples at 0.1 T, whereas at 7 T deviations occurred for samples with a clay content higher than 5%, which are assigned to loss of signal in the first echo periods. The relaxivity in unsaturated clay-based porous media is mostly surface dominated, as the weak and comparable dependence of 1/T 2 on T E at both field strengths shows. Nevertheless, for a reliable determination of water content in mixed systems with varying texture and saturation the employment of multiecho sequences at low magnetic field strength are preferable.

Imaging water fluxes in porous media by magnetic resonance imaging using D2O as a tracer

In this study, we investigate the usefulness of D(2)O as a conservative tracer for monitoring water flux by MRI in a heterogeneous sand column. The column consisted of a cylindrical 3x9-cm packing of fine sand in which an 8-mm diameter cylindrical obstacle was placed. Constant steady-state flux densities between J(w)=0.07 and 0.28 cm min(-1) corresponding to mean pore flow velocities between 0.20 and 0.79 cm min(-1) were imposed at the top of the sand column, and a constant hydraulic head of -39 cm was maintained at the lower boundary. We injected pulses of 0.01 M NiCl(2) and 55% D(2)O and monitored the motion of the tracer plumes by MRI using a fast spin echo sequence over a period of 20 min. We observed that the center of gravity of all plumes moved with the mean pore flow velocity, which showed that D(2)O behaves as a conservative tracer. The motion of the tracer plume at J(w)=0.14 cm min(-1) was validated by a numerical simulation using HYDRUS2D, which reproduced the experimentally observed behavior very satisfactorily.

Moisture profiles of the upper soil layer during evaporation monitored by NMR

Near-surface soil moisture profiles contain important information about the evaporation process from a bare soil. In this study, we demonstrated that such profiles could be monitored noninvasively and with high spatial resolution using Nuclear Magnetic Resonance (NMR). Soil moisture profiles were measured in a column exposed to evaporation for a period of 67 days using a stationary Magnetic Resonance Imaging (MRI) high field scanner and a unilateral NMR sensor. The column was packed with medium sand and initially saturated. Two distinct shapes of soil moisture profiles that are characteristic for stage I (evaporation rate is controlled by atmospheric demand) and stage II (evaporation rate is controlled by the porous medium) of the evaporation process were followed by both MRI and unilateral NMR. During stage I, an approximately uniform decrease of soil moisture over time was monitored, whereas during stage II, S-shaped moisture profiles developed which receded progressively into the soil column. These promising results and the specific design of the unilateral NMR system make it very well suited for determining soil moisture profiles in the field.

NMR velocimetry with 13-interval stimulated echo multi-slice imaging in natural porous media under low flow rates.

Characterization and quantification of root water uptake processes play a key role in understanding and managing the effects of global climate change on agricultural production and ecosystem dynamics. Part of this understanding is related to the flow of water towards plant roots in soils. In this study we demonstrate for the first time, to our knowledge, that fluid flow in the voids of the pore space of a model soil system (natural sand) can be detected and mapped to an NMR image for mean flows as low as 0.06 mm/s even under the influence of internal magnetic field gradients. To accomplish this we combined multi-slice imaging with a 13-interval pulse sequence to the NMR pulse sequence 13-interval stimulated echo multi-slice imaging (13-interval STEMSI). The result is a largely reduced influence of the internal magnetic field gradients, leading to an improved signal-to-noise ratio which in turn enables one to acquire velocity maps where conventional stimulated echo methods fail.

Relaxation in a Natural Soil: Comparison of Relaxometric Imaging, T1 – T2 Correlation and Fast-Field Cycling NMR

Longitudinal and transverse relaxation times are used to characterise the pore system of a natural Kalden- kirchen sandy loam. Here we present new results obtained by relaxometric imaging (MEMS) and two-dimensional T1-T2 correlation relaxometry, and compare these with available T1- relaxation time distributions of water obtained by the analy- sis of fast field cycling relaxometry (FFC) data. The soil shows relatively broad bimodal distribution functions P(T1) and P(T2) with a T1/T2 ratio of about 2:1. The average T1 as well as the spatial distribution, which are obtained from the re- laxometric imaging corresponds well to the relaxometric results. From the analysis of the field dependent FFC data at low field including T1 data obtained at high field the basic locally averaged relaxation mechanism is derived from the disper- sion curve, i.e. the dependence of the relaxation rate from the magnetic field strength over five orders of magnitudes. From this we conclude that two-dimensional diffusion at locally flat surfaces controls the relaxation, i.e. the shapes of the distribution functions are controlled by surface relaxation.

Mechanism of molybdenum sorption to iron oxides using pressure-jump relaxation

It is widely accepted that the fixation of oxyanions is due to diffusion of the ions into the pores and interdomains of iron oxides. Most studies have used batch techniques, which do not allow to clearly differentiate chemisorption from mass transport phenomena. Thus, it is not yet clear, whether strengthening of chemical Mo bonding occurs along with residence time, in addition to diffusion processes. In this study we used pressure jump relaxation (p-jump), a very fast kinetic technique, to (1) elucidate the Mo/goethite interaction and to (2) analyze the effects of aging the Mo/goethite complex on Mo chemisorption. A synthetic goethite was incubated with Mo solution (1 mM Mo) for 12, 24, and 72 hours at pH 4. At the end of the incubations p-jump experiments were performed on the suspensions at temperatures ranging from 283 to 303 K. Relaxation kinetics were modelled using a combination of two first order terms. In addition, the amount of Mo sorbed to the goethite after different incubation times was determined by graphite furnace atomic absorption spectroscopy. The MoO4/goethite systems revealed a fast relaxation time (= reciprocal of rate constant, about 4 ms), that decreased with increasing temperature and a slow one (about 60 ms) that did not depend on temperature. Activation energy of the fast process was 76 kJ mol—1. We did not observe any effects of incubation time on the fast process. However, the amount of Mo sorbed to the iron oxide increased with increasing incubation time. We conclude that the fast relaxation represents Mo chemisorption to the goethite. Slow relaxation seems to be due to Mo transport within the suspension. The pressure jump results indicate, that the dominant surface species of Mo sorbed to goethite do not change along with residence time. Mechanismen der Molybdansorption an Eisenoxide Die Fixierung von Oxyanionen durch Eisenoxide wird im allgemeinen auf die Diffusion von Ionen in die Interdomanenraume von Eisenoxiden zuruckgefuhrt. Weil es mit Hilfe herkommlicher Batch-Experimente nicht moglich ist, zwischen Chemisorptions- und Massentransport-Prozessen zu unterscheiden, ist bislang unklar, ob zusatzlich Veranderungen der chemischen Bindung bei langeren Sorptionszeiten fur die Festlegung von Oxyanionen verantwortlich sind. Ziel dieser Untersuchung ist es, mittels Druck-Sprung (p-jump), einer sehr schnellen kinetischen Methode, die Mechanismen der Mo-Fixierung an Eisenoxide aufzuklaren. Ein synthetischer Goethit wurde bei pH 4 mit molybdathaltiger Losung (1 mM Mo) 12, 24 und 72 h inkubiert. Nach dieser Vorinkubation wurden p-jump-Versuche mit den verschieden lange gealterten Suspensionen bei Temperaturen zwischen 283 und 303 K durchgefuhrt. Die Relaxationskinetik wurde dabei durch die Kombination zweier Terme erster Ordnung beschrieben. Zusatzlich wurde die Menge an sorbiertem Mo nach den unterschiedlichen Inkubationszeiten bestimmt. Fur das Mo/Goethit-System ergaben sich eine schnelle (ca. 4 ms) und eine langsame (ca. 60 ms) Relaxationszeit (= Kehr-wert der Geschwindigkeitskonstante). Wahrend die schnelle Relaxation sehr stark temperaturabhangig war, zeigte die langsame Relaxation keinen Temperatureinfluss. Die Aktivierungsenergie fur den schnellen Prozess betrug 76 kJ mol—1. Obwohl die sorbierte Mo-Menge mit der Inkubationszeit deutlich zunahm, war die Relaxationszeit unabhangig von der Dauer der Vorinkubation. Die Ergebnisse deuten darauf hin, dass die schnelle Relaxation durch die Chemisorption von Mo an den Goethit hervorgerufen wird. Die langsame Relaxation ist vermutlich auf Durchmischungsphanomene innerhalb der Suspensionen zuruckzufuhren. Die Untersuchung deutet darauf hin, dass Mo-Fixierung allein auf Transportprozesse, nicht aber auf die Veranderung der chemischen Bindung zuruckzufuhren ist.

MRI in Soils: Determination of Water Content Changes Due to Root Water Uptake by Means of a Multi-Slice-Multi-Echo Sequence (MSME)

Root water uptake by ricinus communis (castor bean) in fine sand was investigated using MRI with multiecho sampling. Before starting the experiments the plants germinated and grew for 3 weeks in a cylindrical container with a di- ameter of 9 cm. Immediately before the MRI experiments started, the containers were water-saturated and sealed, so water content changes were only caused by root water uptake. In continuation of a preceding work, where we applied SPRITE we tested a multi-echo multi-slice sequence (MSME). In this approach, the water content was imaged by setting TE = 6.76 ms and nE = 128 with an isotropic resolution of 3.1mm. We calculated the water content maps by biexponential fitting of the multi-slice echo train data and normalisation on reference cuvettes filled with glass beads and 1 mM NiCl2 solution. The water content determination was validated by comparing to mean gravimetric water content measurements. By co- registration with the root architecture, visualised by a 3D fast spin echo sequence (RARE), we conclude that the largest water content changes occurred in the neighbourhood of the roots and in the upper layers of the soil.

In situ determination of surface relaxivities for unconsolidated sediments

NMR relaxometry has developed into a method for rapid pore size determination of natural porous media. Nevertheless, it is prone to uncertainties because of unknown surface relaxivities which depend mainly on the chemical composition of the pore walls as well as on the interfacial dynamics of the pore fluid. The classical approach for the determination of surface relaxivities is the scaling of NMR relaxation times by surface to volume ratios measured by gas adsorption or mercury intrusion. However, it is preferable that a method for the determination of average pore sizes uses the same substance, water, as probe molecule for both relaxometry and surface to volume measurements. One should also ensure that in both experiments the dynamics of the probe molecule takes place on similar length scales, which are in the order of some microns. Therefore, we employed NMR diffusion measurements with different observation times using bipolar pulsed field gradients and applied them to unconsolidated sediments (two purified sands, two natural sands, and one soil). The evaluation by Mitras short time model for diffusion in restricted environments yielded information about the surface to volume ratios which is independent of relaxation mechanisms. We point out that methods based on NMR diffusometry yield pore dimensions and surface relaxivities consistent with a pore space as sampled by native pore fluids via the diffusion process. This opens a way to calibrate NMR relaxation measurements with other NMR techniques, providing information about the pore size distribution of natural porous media directly from relaxometry. This article is protected by copyright. All rights reserved.

Effect of magnetic pore surface coating on the NMR relaxation and diffusion signal in quartz sand.

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.

Determination of Soil Hydraulic Properties Using Magnetic Resonance Techniques and Classical Soil Physics Measurements

Water and solute movement as any other transport processes through soil are influenced by the hydraulic properties of the soils. The heterogeneities of the soils imply heterogeneous spatial distribution of the hydraulic properties leading to heterogeneous distribution of soil water content. This may affects the water availability for plant growth, the groundwater contamination and nutrients losses within the root zone. The measurement techniques available today for the estimation of soil hydraulic parameters do not account for the heterogeneity of the sample and treat each measurement sample as a homogeneous representative volume. On the other side natural soils contain large heterogeneities mostly in terms of inclusions of different materials. Therefore the purpose of this study is to estimate soil hydraulic properties of a heterogeneous sample by combining classical multi‐step‐outflow (MSO) with magnetic resonance imaging (MRI) experiments. MSO experiments were performed on a sample filled with sand and...