Ikhsan, J., S. Sulastri, and E. Priyambodo. “The Modification of Silica Surfaceseparated from Overflowing Mud in Lapindo Indonesia as Cation Exchange Adsorbent”. YRU Journal of Science and Technology, vol. 2, no. 2, May 2018, pp. 1-7, https://li01.tci-thaijo.org/index.php/yru_jst/article/view/124546.