Nuclear Magnetic Resonance with Fast Field-Cycling Setup: A Valid Tool for Soil Quality Investigation

Abstract

Nuclear magnetic resonance (NMR) techniques are largely employed in several fields. As an example, NMR spectroscopy is used to provide structural and conformational information on pure systems, while affording quantitative evaluation on the number of nuclei in a given chemical environment. When dealing with relaxation, NMR allows understanding of molecular dynamics, i.e., the time evolution of molecular motions. The analysis of relaxation times conducted on complex liquid–liquid and solid–liquid mixtures is directly related to the nature of the interactions among the components of the mixture. In the present review paper, the peculiarities of low resolution fast field-cycling (FFC) NMR relaxometry in soil science are reported. In particular, the general aspects of the typical FFC NMR relaxometry experiment are firstly provided. Afterwards, a discussion on the main mathematical models to be used to “read” and interpret experimental data on soils is given. Following this, an overview on the main results in soil science is supplied. Finally, new FFC NMR-based hypotheses on nutrient dynamics in soils are described

Recent Developments in Understanding Biochar’s Physical–Chemistry

“…-Cavity effects (cav): these refer to the structural organization of solvent molecules (e.g., water) at the solid-liquid interface, thereby accounting also for the transport of the solvated solute towards the point of the surface where it will be adsorbed [163,164] For this reason, they re-elaborated the Freundlich isotherm by accounting for all those effects and calculated the Gibbs free energy involved in the sorption process in Equation ( 10…”

“…However, a quite innovative dynamic approach can be used to understand the nature of the adsorbate-biochar interface interactions. This is based on the evaluation of fast field cycling (FFC) NMR relaxometry investigations [163,164].…”

“…Fast field cycling NMR relaxometry has been specifically designed for the measurement of the longitudinal relaxation times (T 1 ) of NMR observable spin nuclei [163,164,191,192]. T 1 is a measure of the time needed for the recovery of the longitudinal component of the magnetization along the z-axis [193].…”

“…It is widely recognized that the molecular dynamics investigated by the T 1 value measurements of liquid systems, such as water, in biochar pores is affected by the interactions with the porous boundaries [163,164,[191][192][193][194][195][196]. Namely, the smaller the pore size, the more constrained water molecules are.…”

“…Conversely, when pore sizes increase, water molecules become more mobile. For this reason, according to pore size distribution, three different water types can be recognized: (i) strongly bound water in micropores; (ii) weakly bound water in mesopores; (iii) bulk water in macropores [163].…”