Simatupang, Lisnawaty, and Devi Devi. “The Preparation and Characterization of Sinabung Volcanic Ash As Silica Based Adsorbent”. Jurnal Pendidikan Kimia, vol. 8, no. 3, Dec. 2016, pp. 159-63, doi:10.24114/jpkim.v8i3.4478.