Reduction of divalent mercury by humic substances — kinetic and quantitative aspects

The combination of a computer controlled titration device with a cold vapour atomic absorption spectrometer was used to examine kinetic and quantitative aspects of the reaction between humic substances and Hg(II). Subsequent injections of Hg(II) into an excess of humic acid produce data about short- and long-term changes in the reductive capacity of humic substances for Hg(II). The use of a second burette for pH-stat titration excludes the effect of pH changes and buffering electrolytes. The pH dependence of the reaction is similar to results with redox titrations: the amount of elemental mercury produced increases with increasing pH. The amount of Hg(II) reduced did not change with different HS concentrations but is correlated linearly to the amount of Hg(II) added. An interference of chloride ions was not observed. The proportion of elemental mercury produced within the first 10 min of the reaction compared to the 60-min interval increases with increasing pH and with the presence of daylight.