IN-SITU KINETIC SPECTROSCOPIC MONITORING FOR HYDROTHERMAL TRANSFORMATION OF HUMIC SUBSTANCES

Dissolved humic substances in aquatic environments are considered to transform in long geological time scales influencing material cycling at the earth's surface. Previous studies have only reported global decrease trends of these humic substances without the details on functional group changes. In order to determine reaction mechanisms and kinetics of dissolved organic compounds particularly with respect to their functional groups, in-situ hydrothermal spectroscopic methods were developed here for monitoring directly hydrothermal organic reactions. An in-situ heating UV/Vis cell up to 200 oC and 4 MPa was developed for studying hydrothermal transformation of organic compounds in low concentrations. As a case study, it was applied to an aromatic carboxylic acid (salicylic acid), and accurate kinetic and thermodynamic parameters for its decarboxylation could be obtained. This method was then applied to a standard Nordic humic acid. The absorbance at 254 nm, reflecting the existence of substituted functional groups mainly to aromatic rings, were continuously monitored at 120-200 °C. By the present in-situ measurement, it was discovered for the first time that hydrothermal transformation of a humic acid show multiple (at least three) reaction stages at 180 and 200 oC. These stages were considered to correspond to concurrent decarboxylation and oxidation, and subsequent decomposition of refractory aromatic skeletons. The decarboxylation from aromatic carboxyls is considered to be dominant in the early stage of the reaction profile and its rate constants were determined at 120 to 200 °C. By extrapolating the decrease rates of aromatic carboxyls in the humic acid to lower temperatures at the earth's surface (e.g. 25 °C), the half life is estimated to be about 90 days. This lifetime is much shorter than those reported by a global decrease of humic substances by the conventional batch methods (e.g. the half life is several tens of years). Since the aromatic carboxyls are considered to be the most reactive sites against environmentally important chemicals such as heavy metals, the conventional global long-term behavior of humic substances has been largely underestimated the lifetime of the reactive sites in the aquatic environments.