Synthesis and Application of Positively Charged and Magnetically Separable Magnetite/Silica-Ammonium as an Effective Platinum(IV) Adsorbent por Ngatijo, Ngatijo, Lenny, Lenny, Bemis, Restina, Heriyanti, Heriyanti, Gusti, Diah Riski, Lazuardi, Ahmad Fatwa, Ishartono, Bayu, Basuki, Rahmat