Hydration and water holding properties of cross-linked lignite humic acids

Lignite and lignite humic acids, which are used as soil amendments sometimes, are supposed to improve soil properties such as water holding capacity. The structure of those materials is composed of various organic molecules stabilized mostly byweak interactions. Therefore, excess ofwater causes only partial swelling, but most of the physical structure is destabilized. This accelerates the desiccation and hampers their application as natural hydrogel-like substances. In order to stabilize the structure of lignite humic acids and improve the water holding capacity, we applied formaldehyde cross-linking procedure based on covalent coupling of aromatic humic acids moieties. By combining the 1H NMR relaxometry and methods of thermal analysis, the kinetics and degree of hydration, water distribution and moisture uptake were investigated. It was found that cross-linking induced a reduction in moisture sorption capacity at lowrelative humidity and an increase at higher relative humidity,which was attributed to the separation of functional groups and decreasing of structural compactness after crosslinking. As a result, the cross-linked humic acids, exhibited faster water uptake and approximately three-fold higher water holding capacity in comparison with the parental sample. The distribution of relaxation times of water protons in swollen humic acids revealed the unification of pore size distribution upon cross-linking. Although the improved hydration of cross-linked lignite humic acids already resembles the hydration of some hydrophilic polymers, the water holding capacity is still belowthe capacity of classical hydrogels. Nevertheless, the lowprice of lignite, sorption properties and its overall positive affect on soil quality and productivity give a promise in application of this material both in agriculture and remediation technologies

Assessment of soil organic matter supply: Challenges and opportunities

Soil organic matter (SOM) is recognized as an important factor for sustainable land use. Several analyzing techniques were focused on fractionation of soil organic carbon (SOC), on carbon sequestration, soil functions, or other approaches. We combined SOC fractionation with studies on carbon sequestration. Thermogravimetry (TG, recording of mass losses during heating up of soil samples) was selected as a supplemental method to standard analyzing techniques for soils. TG provides recording of thermal mass losses in dependency on temperature what facilitate fractionation together with SOM content determination via mass losses on ignition (MLI). Autocorrelation analyses of TG data enable to assess the carbon sequestration processes. After a gentle sample preparation, more than 370 soil samples in eight sample sets were analyzed from different types of soils and regions of origin. The results extend literature data by revealing quantifiable interrelations between content of SOM, SOC and clay with a coefficient of determination around 0.98. Deviations from the relationship become lower during incubation experiments, with increasing sampling depth, and with decreasing organic fertilization in plots of long term agriculture field experiments etc. We explained these results with changing quantities of extraneous (mostly fresh) organic residues not affected by soil carbon content regulation. These organic residues seems to be quantifiable via difference between measured MLI and the MLI calculated from content of SOC and clay both determined by standard methods. The practical use of found interrelation implies an acceptance of traditional definition of soil and SOM as products of long term ecosystem succession with content regulation as a unifying over regions soil feature. In contrast, the more common and simplified understanding of soil as carbon containing mineral substrates supports public recognition of soils. However, it does not facilitate the comparison of results from different regions and studies about soils. We conclude from these considerations about obligatory distinction between following types of organic carbon as an essential precondition for assessment of SOM supply: 1. SOC (or humus) as a product of long term carbon regulation processes, 2. fresh organic residues, and carbon of 3. geologic (turf, coal, graphite, diamond, …) or 4. anthropogenic origin (black carbon in ashes, cinder, soot, asphalt)

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