On the removal efficiency of copper ions in wastewater using calcined waste eggshells as natural adsorbents oleh Ming-Yu Chou, Tan-Ang Lee, Ying-Shen Lin, Shan-Yin Hsu, Ming-Fu Wang, Po-Hsien Li, Ping-Hsiu Huang, Wen-Chien Lu, Jou-Hsuan Ho