Effect of clay mineralogy on the feasibility of electrokinetic soil decontamination technology

To evaluate the effect of clay mineralogy on the feasibility of electrokinetic soil remediation technology, we contaminated six soils with Cu(II), Zn(II) and Pb(II) and performed electroremediation for 570 h. Cation exchange resin saturated with H⁺ was placed between soil and cathode to prevent soil alkalinization and trap the migrated heavy metal cations. After the treatment, the heavy metal cations were sequentially extracted with water, 1 M MgCl₂ and hot 6 M HCl. In soils dominated by crystalline clay minerals, Cu(II) and Zn(II) significantly migrated from anode end and accumulated at the cathode end forming sparingly soluble hydroxides. Removal rates of Cu(II) and Zn(II) were highest in a soil dominated with kaolinite and crystalline hematite. In humic-allophanic and allophanic soils, the high pH-buffering capacity of allophane kept the soil pH above 5, even at the anode end, and Cu(II) and Zn(II) did not migrate significantly. In all soils, the migration of Pb(II) was infinitesimal due to the formation of insoluble PbSO₄ and very strong surface complexation at the mineral surfaces. These results show that the reactivity of component clay minerals to H⁺ and heavy metal cations has a crucial effect on the efficiency of the electrokinetic remediation technology and it is not effective for remediation of allophanic soils. The results also indicate that allophanic soils may be useful as a barrier material in landfill sites.