Redox reactions and potentials in natural waters at disequilibrium

The study investigated the redox chemistry of natural non-thermal surface waters and 0 to 90 °C ground waters in North Iceland. The waters are dilute with dissolved solid concentrations between 6 and 431 ppm and a pH ranging from 6.3 to 10.4. The redox potential was calculated for the following redox couples based on measured concentration of a given element in a given oxidation state: Fe3+ / Fe2+, O2 / H2O, H2S / SO4, NO3 / NO2, NO3 / NH4 and NO2 / NH4. The redox potentials differed by up to 1200 mV between couples, indicating redox disequilibrium in the waters. The differences were independent of water age, which ranged from modern to 25,000 years, but was dependent on water type. Based on these findings it is concluded that the redox chemistry of natural open aqueous systems are controlled by steady state between supply of the elements to the system and the kinetics of reactions involving these elements. The concentrations of individual elements like O2 and Fe may be controlled by equilibrium of kinetically fast reactions, whereas an overall redox equilibrium will not be reached until the system is completely closed. The measured in situ electrode potential with a Pt electrode in a disequilibrium system represents a mixed potential. In waters with low concentrations of redox species and low exchange currents the measured potential with the Pt electrode is related to a mixed potential arising from redox reactions on the Pt electrode surface and other redox reactions. We conclude that the measured redox potential with Pt electrodes in dilute natural waters has limited meaning and that it can not be used for quantitative modeling of the transport properties and the aqueous speciation of redox sensitive elements from bulk analysis only. In natural waters at redox disequilibrium, aqueous speciation of redox sensitive elements must be determined by independent measurement of each oxidation state of a given element and by solving separately mass balance and mass action equations for each oxidation state.