Abstract
Changes in the absorption intensity (absorbance) at 254 nm of a fulvic acid solution in the presence of goethite powder were monitored in situ in a liquid cell at a temperature of 80 °C in a UV-visible spectrometer. The absorbance at 254 nm decreased rapidly with time in the initial stage in the presence of goethite with a first-order rate constant of k1UV = 3 × 10-4 s-1, while in the absence of goethite, the absorbance at 254 nm decreased slowly with time, with a first-order rate constant of k0UV = 6 × 10-7 s-1. A hydrothermal vessel was connected to a long-path gas cell infrared spectrometer to monitor the volatile compounds formed on heating the fulvic acid at 80 °C. Carbon dioxide formed gradually over time from the fulvic acid solution in the absence of goethite (k0CO2 = 1.5 × 10-6 s-1). The formation of CO2 in the presence of goethite occurred via two first-order reactions. The initial first-order rate constant of CO2 formation from fulvic acid in the presence of goethite (k1CO2 = 3.2 × 10-4 s-1) had the same magnitude as the decrease in absorbance at 254 nm. The second rate constant of CO2 formation (k2CO2 = 1.8 × 10-6 s-1) had the same magnitude as CO2 formation in the absence of goethite. These results suggest that the formation of CO2 from the fulvic acid is greatly accelerated by the presence of goethite, possibly through the adsorption of the fulvic acid on the goethite surface with subsequent redox reactions occurring.
