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 soilproperties such as water holding capacity. The structure of those materials is composed of various organic moleculesstabilized mostly byweak interactions. Therefore, excess ofwater causes only partial swelling, but most ofthe physical structure is destabilized. This accelerates the desiccation and hampers their application as naturalhydrogel-like substances. In order to stabilize the structure of lignite humic acids and improve the water holdingcapacity, we applied formaldehyde cross-linking procedure based on covalent coupling of aromatic humic acidsmoieties. 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 reductionin moisture sorption capacity at lowrelative humidity and an increase at higher relative humidity,whichwas 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-foldhigher water holding capacity in comparison with the parental sample. The distribution of relaxation times ofwater protons in swollen humic acids revealed the unification of pore size distribution upon cross-linking. Althoughthe improved hydration of cross-linked lignite humic acids already resembles the hydration of some hydrophilicpolymers, the water holding capacity is still belowthe capacity of classical hydrogels. Nevertheless, thelowprice of lignite, sorption properties and its overall positive affect on soil quality and productivity give a promisein application of this material both in agriculture and remediation technologies.