Foundation item: Project (2012BAC09B04) supported by National Key Technology Research and Development Program of the Ministry of Science and
Technology of China; Project (2010-277-027) supported by Science and Technology Foundation of Environmental Protection in Hunan Province, China; Project (2011SK3262) supported by Science and Technology Planning of Hunan Province, China
Corresponding author: XIAO Xi-yuan; Tel: 86-731-88836442; E-mail: xiaoxy@csu.edu.cn