Processing of Clay Minerals with an Electric Field.

Clay minerals are important sources of raw materials used in many fields. Because of their electric properties, clay minerals control greatly the physic, chemical, and biological processes in soils and sediments. Clays can carry negative charges, derived mainly from the isomorphic substitutions; however, the positive charges may also be developed on the edge or broken structures of clay minerals upon pH changes. The excess charges are counterbalanced by the ions or masked by other soil components, which may not be efficiently removed during clay processing. Thus, the electric properties exhibited on the clay surfaces might not reflect the true charges developed during clay formations. In the study, a direct current electric field was generated by applying 20 V voltage, and the pristine clays were placed between parallel stainless steel bipolar nets. The clays collected at both outfalls were then examined by XRD and potential/charge analyses. Preliminary results indicated that, upon electro-processing, the clays collected at cathode (denoted as cathode-clay) had significant differences in the structures, surface functional groups, and the charged properties as compared with those of pristine and anode-clay samples. There is evidence that the formation of hydroxy layer in the interlayers of clays after electro-processing even if the changes in the characteristic peaks of cathode-clay were not very clear. This may be caused by the accumulation of protons on cathode, leading to the replacement of the structural cations into the interlayers under the force of electric field. The cathode-clay exhibited not only a higher negative potential (-52.7 mV) than pristine clay (-21.3 mV) but also a greater influence on the movement of ionic compounds. For instance, in the presence of cathode-clay, the diffusion coefficients of cationic methyl viologen and hexaammineruthenium decreased 65.72% and 32.47%, respectively, but anionic hydroquinone and ferricyanide increased 111.45% and 144.86%, respectively. Accordingly, the applications of electric field could change both the structures and the charge properties of clay particles, induced probably by the ion exchange and movement reactions. This newly developed technique is simple and shows the benefits of rapid modifications of clay minerals for future uses in the environments as adsorbents or catalysts.