1. School of Environment, Nanjing Normal University, Nanjing, Jiangsu 210023, P. R. China
2. Department of Applied Chemical Engineering, Hamhung University of Chemical Industry, Hamhung, Democratic People's Republic of Korea
3. Institute of Chemical Engineering, Hamhung University of Chemical Industry, Hamhung, Democratic People's Republic of Korea
4. Department of Physics, Kim Il Sung University, Pyongyang, Democratic People's Republic of Korea