Water dynamics and its role in structural hysteresis of dissolved organic matter

Knowledge of structural dynamics of dissolved organic matter (DOM) is of paramount importance for understanding DOM stability and role in the fate of solubilized organic and inorganic compounds (e.g., nutrients and pollutants), either in soils or aquatic systems. In this study, fast field cycling (FFC) 1H NMR relaxometry was applied to elucidate structural dynamics of terrestrial DOM, represented by two structurally contrasting DOM models such as Suwanee River (SRFA) and Pahokee peat (PPFA) fulvic acids purchased by the International Humic Substance Society. Measurement of NMR relaxation rate of water protons in heating–cooling cycles revealed structural hysteresis in both fulvic acids. In particular, structural hysteresis was related to the delay in re-establishing water network around fulvic molecules as a result of temperature fluctuations. The experiments revealed that the structural temperature dependency and hysteresis were more pronounced in SRFA than in PPFA. This was attributed to the larger content of hydrogel-like structure in SRFA stabilized, at a larger extent, by H-bonds between carboxylic and phenolic groups. Moreover, results supported the view that terrestrial DOM consist of a hydrophobic rigid core surrounded by progressively assembling amphiphilic and polar molecules, which form an elastic structure that can mediate reactivity of the whole DOM

Influence of water content and drying on the physical structure of native hyaluronan

Hydration properties of semi-diluted hyaluronan were studied by means of time domain nuclear magnetic resonance. Based on the transverse proton relaxation times T2, the plasticization of hyaluronan which was precipitated by isopropylalcohol and dried in the oven have been determined at water content 0.4 g of water per g of hyaluronan. Above this water content, the relaxation times increased and levelled off around 0.8 g of water per g of hyaluronan which agrees well with values determined earlier by differential scanning calorimetry and dielectric relaxometry. The freeze dried and oven dried samples showed differences in their physical structure such as glass transition, plasticization concentration and sample topography which influenced their kinetics and mechanisms of hydration. Results confirmed earlier hypothesis that some native biopolymer structures can be easily modified by manipulation of preparation conditions, e.g. drying, giving fractions with specific physicochemical properties without necessity of their chemical modification

Dynamics of hyaluronan aqueous solutions as assessed by fast field cycling NMR relaxometry

Fast field cycling (FFC) NMR relaxometry has been used to study the conformational properties of aqueous solutions of hyaluronan (HYA) at three concentrations in the range 10 to 25 mg mL–1. Results revealed that, irrespective of the solution concentration, three different hydration layers surround hyaluronan. The inner layer consists of water molecules strongly retained in the proximity of the HYA surface. Because of their strong interactions with HYA, water molecules in this inner hydration layer are subject to very slow dynamics and have the largest correlation times. The other two hydration layers are made of water molecules which are located progressively further from the HYA surface. As a result, decreasing correlation times caused by faster molecular motion were measured. The NMRD profiles obtained by FFC-NMR relaxometry also showed peaks attributable to 1H–14N quadrupole interactions. Changes in intensity and position of the quadrupolar peaks in the NMRD profiles suggested that with increasing concentration the amido group is progressively involved in the formation of weak and transient intramolecular water bridging adjacent hyaluronan chains. In this work, FFC-NMR was used for the first time to obtain deeper insight into HYA–water interactions and proved itself a powerful and promising tool in hyaluronan chemistry

Factors influencing structural heat-induced structural relaxation of dissolved organic matter

Physical and chemical structure affect properties of dissolved organic matter (DOM). Recent observations revealed that heating and cooling cycles at higher temperature amplitude lead to a change in DOM physical conformation assumingly followed by a slow structural relaxation. In this study, changes at lower temperature amplitudes and their relation to DOM composition were investigated using simultaneous measurements of density and ultrasonic velocity in order to evaluate the adiabatic compressibility, which is sensitive indicator of DOM structural microelasticity. Six fulvic acids (FAs) having various origins were analyzed at concentrations of 0.12, 0.6 and 1.2 g L−1 and at different temperature amplitudes. First, we validated that the used technique is sensitive to distinguish conclusively the structural changes upon heating and cooling of DOM with heating/cooling amplitude of ± 3 °C and higher. This amplitude was then applied to observe the relationship between change in adiabatic compressibility and chemical composition of FA. No correlation was observed with elemental composition and aromatic structures. Positive correlations were observed with content of alkyl moieties, carboxylic and carbonyl carbons and biological activity. Based on literature data, it was concluded that alkyl moieties undergo (re)crystalization during thermal fluctuation and their structural relaxation back is very slow (if occurs). The polar moieties form a flexible hydrogel responding to thermal fluctuation by moderate dissolution and re-aggregation. Negative correlation was observed in relation to the amount of peptide and O-alkyl systems, which can be attributed to very fast structural relaxation of proteinaceous materials, i.e. their larger content leads to lower difference between original and heat-induced compressibility. Last, the increase of the heating/cooling amplitude from ± 3 to ± 15 °C resulted in an increase of the change of the adiabatic compressibility and in the extension of the relaxation time needed for DOM structure to return to the equilibrium. We conclude that this increase is caused by the increase in inner energy, and DOM conformation can reach a cascade of energy minima, which may influence DOM reactivity and biodegradability

Solid state nuclear magnetic resonance spectroscopy in the evaluation of soil organic matter changes following thermal variations

Soil organic matter (SOM) is an ubiquitous, complex material which is produced by the degradation of plant tissues and animal bodies. It is the major indicator of soil quality since it is directly involved in the maintenance of soil fertility, prevention of erosion and desert encroachment and provision of suitable environment for biological activity. Organic matter is an important driving force in environmental global change as it acts as both a source and sink of atmospheric carbon. However, SOM is subjected to rapid changes due to environmental transformations such as massive deforestations, fires, intensive land uses, temperature increases and so on. In the present work, a characterization of humic substances was done in order to obtain information about the transformation occurring to SOM as affected by temperature increases. For the first time variable temperature cross polarization magic angle spinning (CPMAS) 13C NMR spectroscopy was applied in combination with thermal analyses (TG and DSC) on environmentally relevant soil organic matter. The results show that the conformational changes occurring in humic substances as temperature is raised can be associated to melting of alkyl components connected with sublimation of some organic compounds. The simultaneous application of solid phase micro extraction GC-MS also allowed the identification of the components which were released by sublimation processes

Biochar production from the pyrolysis of food waste: Characterization and implications for its use

Food waste (FW) represents a large group of wastes that impose several issues on their management, especially in terms of microbiological and leaching pollution, and greenhouse gas emissions. According to the EU circular economy vision, finding a new way for FW valorisation to obtain reusable materials or compounds represents a priority. Thermal treatment represents one of the suitable ways for FW processing, and pyrolysis in particular presents many advantages in producing solid carbonaceous biochar, reusable oil and gas. This paper analyses biochar that was produced via thermal pyrolysis of FW. The influence of an organic additive (wooden sawdust) and a catalyst (zeolite) on the pyrolytic process at 600 °C was investigated. The results highlight how the initial composition of the feedstock (FS) influenced the characteristics of the obtained biochar. The addition of organic additives and catalyst did not change significantly the Brunauer–Emmett–Teller surface area and the calorific value. For all the analysed parameters, all tested FS respected the guidelines proposed by International Biochar Initiative (IBI) and the European Biochar Certificate (EBC) for possible reuse in agriculture and urban areas. The results suggest that biochar from FW could be potentially used in agriculture and urban green infrastructure, but the authors suggest further studies, especially on the effect of high electrical conductivity due to the typical high concentration of salts in FW