Effect of electrokinetic treatment time on energy consumption and salt ions removal from clayey soils

Abstract

Electrokinetics effectively removes contaminants, but its field-scale applications are limited mainly due to its high energy cost. In previous studies, the energy consumption was determined either by changing the soil's specimens initial salt concentration while keeping the treatment time fixed or by changing the treatment time and keeping the same initial salt concentrations for all the specimens. Since both the initial salt concentration and treatment time are important parameters in determining reclamation cost, therefore, in this study, the soil specimens intentionally contaminated with different concentrations of sodium chloride (NaCl), i.e., varying from 3.7 to 15.5 g/kg, were exposed to a constant DC electric field of 1V/cm for different time durations, i.e., varying from 6 to 72 hrs. The results show that electroosmotic flow (EOF) was directed from the anode to the cathode and higher for specimens contaminated with relatively low salt concentration, i.e., up to 7.6 g/kg. Therefore, for these specimens, due to the combined effect of electroosmosis and electromigration, the removal of Na+ was higher than the Clˉ. However, for the specimen contaminated with a higher salt concentration, i.e., 15.5 g/kg, the Clˉ removal exceeded Na+ due to the marginalization of EOF. Regardless of initial salt concentration, the electroosmotic flow and salt ions removal rates decreased with increasing treatment time, which might be attributed to the development of acidic and alkaline environments in soil. The collision of acidic and alkaline fronts resulted in a large potential gradient in a narrow soil region of pH jump, diminishing it everywhere else. This nonlinearity in the electric potential distribution in soil reduced the EOF and electromigration of salt ions.

Experimental Study of Electroosmosis in Rock Cores Based on the Dual Pressure Sensor Method

“…This indicates that the sinusoidal input signal is detected by the lock-in amplifier with a magnitude of 0.285 Pa and no noise. (2) The output data of the Double + LIA method (red solid line), first increases rapidly from zero, then oscillates between 0.25 and 0.34 Pa, and finally settles at 0.275 Pa. This indicates that the electroosmotic pressure signal has an amplitude of about 0.275 Pa with little noise.…”

“…For this example, the excitation frequency is 0.05 Hz, then the frequency of the electroosmotic pressure is also 0.05 Hz. This is shown as a sine wave with a period of 20 s in the time domain plot and as a peak at 0.05 Hz in the frequency domain (indicated by the vertical In order to eliminate the "burr" on the conversion coefficient curve in Figure 4a, this paper adopts the following methods: (1) As shown in Figure 3, measure the background noise of the two pressure sensors and the two pressure sensors when there is no excitation pressure, and obtain the frequency-domain signals; (2) In the frequency-domain signals, average the two curves with their surrounding points, so that frequency domain curves become relatively smooth; (3) Do point-to-point division on the two averaged smooth frequency domain curves to obtain the averaged conversion coefficients of a single measurement; (4) Repeat steps 1 to 3 to obtain the averaged conversion coefficients of multiple single measurements; and (5) Do averaging on the averaged conversion coefficients of multiple single measurements to obtain the averaged conversion coefficients of multiple measurements, as shown in Figure 4b. From comparison of Figure 4a,b, it can be seen that, through the averaging process, the conversion coefficient curve on the "burr" has been eliminated.…”

“…The output data from the reference group (dashed line) converges quickly to 0 • and then remains constant. This indicates that the lock-in amplifier detects a sinusoidal signal with a phase value of 0 • and no noise (2). The output data (solid red line) of the Double + LIA method oscillates slightly between −60 • and −75 • .…”

“…On the contrary, under the action of an electric field, the net surplus ions hold the nearby solution hostage to flow, a phenomenon known as the electroosmotic effect. These two coupling effects are collectively referred to as the electrokinetic effect [1][2][3]. These two kinds of electrokinetic effects have important applications in the directions of seismic early warning, seismic exploration and electrokinetic logging in the field of geophysics.…”