Heat-induced changes in soil water-extractable organic matter characterized using fluorescence and FTIR spectroscopies coupled with dimensionality reduction methods

[Abstract:] Water extractable organic matter (WEOM) is a very mobile and reactive soil OM fraction, critical in the translocation of carbon (C) from soils to other environmental compartments. Transformations of WEOM due to soil heating can have implications not only at a local scale, but in places far away from the location of the event. However, their accurate characterization is costly when analyzing a large number of samples. The objectives of this work were to identify common patterns for the changes in WEOM caused by the heating of various soil types using a combination of spectroscopic and dimensionality reduction techniques. Six soils from Spain, Kenya and Israel were collected at depths 0–10 cm and analysed before and after heating in air to temperatures of 300 and 600 °C. Fluorescence EEMs were measured in soil–water extracts containing WEOM, and decomposed using parallel factor (PARAFAC) analysis. The FTIR spectra were measured in freeze-dried extracts and further analysed using non-negative matrix factorization (NMF). Total organic C and SUVA254 values of the extracts experienced changes with the heating treatments that were soil dependant. Four PARAFAC and three NMF components were sufficient to characterize WEOM changes in all soils, which showed common thermal transformation patterns irrespective of their origin and properties. Thermal transformation of fluorescent WEOM led to the increase in the proportion of a component with an emission maximum at Ex 300/Em 392 nm, and to a lower extent one with the emission maximum at Ex 300/Em 426 nm. Concomitantly, the proportion of components with emission maxima at longer excitation wavelengths was reduced. These changes occurred at the lowest heating temperature and were maintained at 600 °C, and they seem to indicate a depletion of fluorescent components more conjugated, bigger in size, and an enrichment in smaller ones. The NMF components obtained from FTIR spectra showed an increase of the proportion of compounds with Csingle bondO bonds, more oxidized. No correlations were found between the components obtained with each method, thus indicating that the information obtained from the fluorescence EEMs-PARAFAC analysis and the NMF decomposition of FTIR spectra is complementary. It can be concluded that there is a common pattern of WEOM changes induced by thermal soil transformations irrespective of the origin and properties of the soils studied, and that the combination of different spectroscopic techniques coupled with dimensionality reduction methods can be used as a simple and low-cost method to fingerprint changes in WEOM composition, in general, and those caused by soil heating, in particular

Effect of halogen substitution on the enthalpies of solvation and hydrogen bonding of organic solutes in chlorobenzene and 1,2-dichlorobenzene derived using multi-parameter correlations

Article on the effect of halogen substitution on the enthalpies of solvation and hydrogen bonding of organic solutes in chlorobenzene and 1,2-dichlorobenzene derived using multi-parameter correlations

Time-independent desorption hysteresis in liquid phase sorption experiments: the concept and the models based on gate-sorption site coupling

Sorption-desorption hysteresis (SDH) is often observed in liquid phase (solution) sorption experiments with various chemicals on complex natural materials, including soils and sediments. Sorption-desorption interactions with soils and sediments are of significant fundamental and applied interest since they control the transport and fate of chemicals in environmental systems. SDH expressed as a difference between sorption and desorption isotherms determined in solutions may demonstrate time-independent behavior. This work aims to propose a concept that could mechanistically explain and allow predictions of time-independent SDH in three different scenarios: (1) sorbed molecules are entrapped and physically blocked from their exchange with the environment; (2) sorbed molecules are irreversibly bound to sorbent matrix such that the sorption sites capable of irreversible binding are not fully occupied in the presence of non-zero concentrations of solutes; (3) SDH is associated with forming of a non-relaxed sorbent state where the free exchange of sorbate molecules with the environment occurs. The proposed concept introduces the gates present in a sorbent matrix and capable of concentration-dependent cooperative opening/closure, thus acting as a switch: sorbate interactions with sorption sites are allowed at increased solute concentrations but not the opposite. Coupling the gates distribution with the distribution of sorption sites allows addressing each scenario of interest and explaining time-independent SDH. The models developed within the concept can represent and even predict desorption data using a minimal number of adjustable parameters. This predictive potential may be improved by accounting for the assumptions introduced while developing the models

Sorption Hysteresis on Soils and Sediments: Obtaining Characteristic Free Energies Using "Single-Point Desorption Isotherms"

Sorption-desorption hysteresis (SDH) may control distributions of chemicals between diverse environmental phases, including soils and sediments. Formation of metastable states caused by pore deformation or inelastic swelling of a sorbent and their persistence during desorption were considered in the literature as one reason for "true" SDH. Such metastable states persisting during desorption lead to the lack of closure of sorption-desorption loop at non-zero sorbate concentrations, which is often observed in soil and environmental literature. Also, SDH was often characterized using single-point desorption isotherms (DIs) combining sorbed states reached during single desorption steps started from different points along a sorption isotherm (SI). The objective of this contribution is to demonstrate how the single-point DIs could be used to characterize SDH in liquid phase sorption experiments in terms of Gibbs free energy. This free energy is accumulated in some non-relaxed sorbed states belonging to DI as compared with the states of the same composition (sorbed concentration) belonging to SI. Using the literature data on SIs and single-point DIs of some polycyclic aromatic hydrocarbons and pesticides on soils and sediments, it is shown how these extra free energies could be obtained and how they could change in the selected sorbate-sorbent systems. When the extent of SDH decreases with increasing solute concentration, these additional free energies decline. They may remain constant or even increase, suggesting in the latter case that a larger work is needed to perturb a sorbent structure at higher sorbed concentrations. This paper proposes a novel approach for quantifying and understanding liquid phase SDH in the cases when a thermodynamic justification is sought, and, therefore, it advances the ability to predict the fate and activity of multiple chemicals in typical soil/sediment environments. </p