Darmayanti, L., S. Notodarmodjo, and E. Damanhuri. “Removal of Copper (II) Ions in Aqueous Solutions by Sorption onto Fly Ash”. Journal of Engineering and Technological Sciences, vol. 49, no. 4, Oct. 2017, pp. 546-59, doi:10.5614/j.eng.technol.sci.2017.49.4.9.