Laboratory Evaluation of Spontaneous Potential (SP) Electrodes for the Detection of Redox Zones

Abstract

The goal of this thesis was to evaluate the relationship between the measurement of potentials using SP electrodes and redox zones in a series of laboratory experiments. The experiments tested three hypotheses from the literature to explain field detection of redox zones around ore bodies and contaminant plumes. The first hypothesis is that SP electrodes detect current from ion flow driven by redox reactions. The second hypothesis is that SP electrodes detect Eh differences directly. The third hypothesis is that the electric field detected by SP is created by charge separation across the boundary between two different redox zones. Experiments were performed in beakers, glass vessels with flitted plate separating two solutions, and in a sand tank with Pt-Pt, SP-Pt and SP-SP electrode pairs. Solutions with various Eh values were used in the three set ups in different combinations to create contrasting Eh. Small-scale beaker experiments were used to design the sand tank experiments. Results from the two solutions separated by flitted glass tested the hypotheses that electrodes could not measure Eh differences between solutions. Control tank measurements were used as a reference for a tank with a buried plume. The tank with the buried plume tested the third hypothesis. An experiment was done in a tank containing sand saturated with salt solution and battery-induced voltage of 6000 mV to test the first hypothesis. The beaker experiment showed that SP electrodes do not respond to the Eh of the surrounding solution. Voltages between electrode pairs in a single solution should be zero because there is no Eh contrast. However, when a combination of SP-Pt electrodes was used, voltages in the hundreds of m V were measured. Thus, Pt and SP responded differently to the single solution. Results in the fritted glass vessel with two solutions of contrasting Eh confirmed that the SP electrode does not respond to the Eh of the surrounding solution. When Pt and SP electrodes were placed in KMnO4/FeCl2 and KMnO4/H2O2 solution pairs respectively, the recorded voltages were around four hundred m Vs. Similar values were obtained in two experiments with the SP electrode placed in FeCl2 first and H2O2 afterwards. Thus, the SP-Pt pair measured the Eh of the solution where the Pt electrode was inserted but not an Eh difference. Both the Eh and SP-Pt electrode pairs were able to read potential variations in the tank with a buried plume, but the SP-SP pair did not. In the tank with a buried plume, the Eh was 20-40 mV higher near the plume, and SP-Pt read 25-35 mV higher near the plume. SP-SP readings were 5-7 mV throughout this tank showing no difference near the plume. Voltages read by SP-SP in the tank with a battery varied from 1100 mV to 82 mV. The voltage readings decreased inversely proportional to the distances of the wires from the SP electrodes, and confirmed that the SP electrodes could detect current in the sand tank. In all experiments, SP-SP measured neither the Eh of independent solutions nor Eh differences between solutions, as proposed in the literature. The lack of response from the SP electrode may be due to Eh buffering of the copper wire suspended in the copper sulfate solution within the SP electrode. A new explanation must be found for the relationship between the SP and redox zones.