ELECTROLYTIC REDUCTION OF SOIL SUSPENSIONS

A three-electrode system and a precision potentiostat were used in the electrolytic reduction of suspensions of two soils. A saturated calomel reference electrode served as a potentiometric probe of the interfacial potential at the surface of the working electrode. Solution resistance between these two electrodes was minimized by suspending the soils in 1 M KCl. In one set of experiments electrolysis was carried out for 4 hours at a range of applied potentials, while in another set of experiments electrolysis at one of two applied potentials was continued until an apparent equilibrium had been reached. Faradic current, suspension redox potential (Eh) and pH, and solution concentrations of Fe were measured as functions of time and applied potential. As evidenced by the appearance of Fe in solution, reduction of Fe (III) compounds began to occur at an applied potential of -48 mV vs. the standard H electrode. This is more negative than the potentials at which Fe first appears in flooded soils. The pH values of the suspensions increased as the applied potentials became more negative and as the duration of electrolysis increased. Measured suspension Ehs were initially lower than the applied potentials, but after 20 h of electrolysis these two quantities had similar values, suspension pH and solution concentrations of Fe had reached relatively constant values, and the faradic current had declined to background levels. Approximately 75% of the citrate-bicarbonate-dithionite-extractable Fe of both soils had been reduced after 20 h of electrolysis at an applied potential of -359 mV. This electrolytic reduction technique potentially offers an alternative in the study of soil redox process under controlled Eh condition.