基金
This work was supported by the National Natural Science Foundation of China （Grant No. 20877104）
the Key Research Project of Guangdong Province （No. 2009B030802006）
and the National Science and Technology Key Project for Water Protection, China （Grant No. 2009ZX07211-002-3）
The authors are indebted to Kaylin Huang from USA and Roy Luo from MTC Industries, Inc., Edgewood, NY, USA for helpful comments in the preparation of this manuscript.
