2.2. Synthesis of mag and H-mag

Sodium magadiite (abbreviated as mag) was hydrothermally synthesized based on our previous report [36]. In a typical synthesis, mixtures of silica gel and NaOH with a molar ratio SiO₂ : NaOH : H₂O = 9 : 3 : 162 were sealed in a Teflon-lined autoclave and hydrothermally treated at 150°C for 48 h. After reaction, suspension was filtered and washed carefully with distilled water to remove excess NaOH, and dried at 80°C for 24 h. The empirical chemical formula of the obtained mag can be expressed as Na₂Si₁₄O₂₉·9H₂O. Cation exchange capacity (CEC) of mag was 200 meq/100 g [25]. H⁺ exchanged mag (abbreviated as H-mag) was synthesized by adding mag to HCl (0.1 M) until pH is about 2 with magnetic stirring for 24 h. When the ion exchange reactions were finished, the products were obtained with a centrifugal separator and washed with deionized water several times until a negative AgNO₃ test was achieved.