Focusing phenomenon caused by soil conductance heterogeneity in the electrokinetic remediation of chromium (VI)-contaminated soil

• Soil conductance heterogeneity causes focusing bands during EKR.
• The focusing band is movable under electric field.
• The initial locations of the focusing bands can be predicted.
• Some high conductance bands prolong the EKR duration.

Focusing bands, as “hot spots”, severely influence the clean-up verification of contaminated soil remediation projects using electrokinetic remediation (EKR) technology. In this experiment, artificial high conductance bands (HCB) were created in the Cr(VI)-contaminated kaolin–gypsum soil to imitate the conductance heterogeneity of nature soils. Potential gradient monitoring was used to trace the behavior of HCBs. Visual observation and X-ray fluorescence analysis were used to trace the generation and moving patterns of the focusing bands caused by HCBs. Results show that the HCBs caused focusing bands in the soil by ion-induced potential gradient trapping effect (IIPGWTE), not chemical precipitation or isoelectric effect, and the heavier the HCB, the heavier the focusing phenomenon. The HCBs gradually disappeared due to the ionic diffusion increase caused by temperature increase during EKR, whereas the focusing bands remained and moved toward the anodes. In addition to causing focusing bands, some HCBs also could prolong the EKR duration resulting in higher energy consumption. In this case, the HCB’s location plays more significant role than its magnitude. Only the HCB that is caught up with by the tail of chromium (VI) plug-flow can slow down the EKR process resulting in lower energy efficiency.

An interim frame of an animation illustrating the generation and movement patterns of CrO42-/CrO4- ions in Cr(VI)-contaminated soils.Figure optionsDownload as PowerPoint slide