Selectivity of Co and Ni by K-depleted micas

Contamination of the environment with heavy metals, including cationic radionuclides, is a serious problem which has yet to be fully overcome. A class of potentially effective cation exchangers for sequestering heavy metals which has received little attention is K-depleted mica. The purpose of this study was to investigate the heavy-metal cation exchange properties of K-depleted phlogopite and biotite, which were prepared from a natural phlogopite and biotite, respectively, using sodium tetraphenylborate (NaTPB). The X-ray diffraction (XRD) patterns showed that interlayer K⁺ ions were completely replaced with exchangeable Na⁺ ions, resulting in the expansion of the d₀₀₁ spacing of both K-depleted phlogopite and K-depleted biotite. In order to investigate the cation exchange selectivity of K-depleted phlogopite and biotite for Co ²⁺ and Ni²⁺, cation exchange isotherms and Kielland plots were constructed. The isotherms and Kielland plots indicated that both K-depleted phlogopite and biotite are highly selective for Co²⁺ as well as Ni²⁺. The XRD patterns after both 2Na⁺ → Co²⁺ and Ni²⁺ exchange reactions suggest that double sheets of interlayer water are present in the interlayer. These K-depleted micas are potential cation exchange materials for removal of some heavy metals such as Ni and radioactive species such as ⁶⁰Co from solution.