Origin of heat-induced structural changes in dissolved organic matter.

Humic substances play an important role in many environmental processes such as sequestration and transport of hydrophobic compounds. The supramolecular character of humic substances imparts high flexibility of the aggregates associated with their variable reactivity under different conditions. In this study, heat-induced transitions and character of the hydration shell of sodium salts of humic and fulvic acids originating from various sources were investigated using ultrasonic velocimetry in the temperature interval from 5 to 90 °C. Results clearly showed differences in stability and characteristics of the hydrated states at concentrations above and below 1 g L(-1) with the exception of Pahokee peat fulvic acids. It has been concluded that predominantly the relaxation part of the adiabatic compressibility plays an important role below 1 g L(-1) in contrast to both relaxation and intrinsic parts of the compressibility being important at higher concentrations. Dilution brought several temperature induced transitions which were investigated with respect to composition of all investigated humic substances. Correlation analysis revealed that the transition around 17 °C is associated with disruption of H-interactions whereas the transition around 42 °C depends on the aromaticity. Comparison of cooling and heating records revealed hysteresis in the structural relaxation resembling the behavior of physically stabilized hydrogels. Results indicated a difference in the conformation and therefore reactivity of dissolved humic substances in the dependence on temperature and thermal history. It has been hypothesized that this may play an important role in the transport and sequestration of hydrophobic pollutants by dissolved organic matter.