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