The strong impact of pH on the reduction of Hg(II) and the subsequent desorption of Hg0 to the exhaust stripping air becomes
obvious by the results presented in Fig. 5. The experiments evaluated are carried out under identical conditions, with the exception
of Cl concentration of the waste water samples. The pH-value is
gradually increased from its initial value after filtration to pH 9.
Both samples show elevated Hg0 desorption for increasing
pH-value. However, Hg0 concentration in the exhaust stripping
air is generally lower for the sample with higher Cl concentration
at the same pH-value. Interestingly, the shapes of both resulting
curves can be qualitatively separated in two pH-ranges (lower
and higher than pH 8). This observation indicates different predominant reaction mechanisms. As illustrated by the dashed line
in Fig. 5, the actual transitional pH-value is shifted towards higher
OH concentration for increased Cl content of the solution. This is
in-line with the theoretical considerations of the transitional
pH-value for the predominant Hg(II) compound in chapter 3.1.
Using the Nernst equation, electrode potentials of the OH/O2
and Hg2+/Hg0 half-cell for actual experimental concentrations
and temperatures are determined. Calculation results indicate that
the redox reaction is thermodynamically favored for pH > 7.5,
which further supports the observed transitional pH-value of
pH 8 in Fig. 5.