Adsorption of crystal violet dye by biochar made from coffee ground Le Hong Phuc, Nguyen Quan Hien, Bui Thi Hoa, Nguyen Ngoc Truc My, Thai Thi Kim Ngan and Bui Xuan Vuong E3S Web Conf., 626 (2025) 03001 Published online: 15 April 2025 DOI: 10.1051/e3sconf/202562603001