1. School of Energy and Environmental Engineering, University of Science and Technology Beijing 1 , 30 Xueyuan Rd., Haidian District, Beijing 100083, China
2. School of Geography, Earth and Environmental Sciences, University of Birmingham 2 , Edgbaston, Birmingham B15 2TT, United Kingdom
3. Natural Sciences and Environmental Research Center, Faculty of Sciences, University of Burundi 3 , Avenue UNESCO No 2 B.P 1550, Bujumbura, Burundi