Decontamination of Soil Using Electro-Osmosis.

Electrokinetic soil processing is an innovative insitu technique to remove contaminants from soils and groundwater. The process is an alternative to conventional processes, with significant economic and technical advantages. A theory is presented for pH gradient development by diffusion and convection in electrokinetic processing of soils. The premises and consequences of the theory are discussed. Based upon this theory, a mathematical model is developed to predict the pH gradient development in electro-osmosis. Electro-osmosis tests are conducted on saturated kaolinite specimens loaded with Pb(II), Cd(II), and Cr(III) to investigate the feasibility, efficiency, and energy requirements of the process in removal of heavy metals. The test results pertaining to the flow and the associated electrochemistry (voltage, current, pH gradients, and conductivity) are presented. Tests indicated that the flow in electro-osmosis with open electrodes is time-dependent and it is strongly influenced by electrochemistry generated as a result of the prevailing pH gradients. High removal efficiency, up to 95%, was achieved on kaolinite specimens loaded with Pb(II) up to 1000 $\mu$g/g of soil. Also the removing efficiency was very high (92 to 100%) for kaolinite specimens loaded with Cd(II) up to 120 mg/g of dry soil. However, tests conducted on kaolinite loaded with Cr(III) (120 $\mu$g/g of dry soil) indicated that only 60 to 70% of absorbed Cr(III) was removed from the anode zone. The influence of fundamental variables affecting the chemistry and efficiency of the process such as the current density, the voltage gradient and contaminant concentration are also investigated and presented. The results indicated that the process can be used efficiently to remove ions from saturated soil deposits. The cost of the process will be a function of the current density and the processing time. In the tests conducted, the energy expenditure varied between 11 to 306 kW-hr/m$\sp3$. The study demonstrates the feasibility of using this process in decontamination and provides a fundamental understanding to the underlying mechanisms controlling the process