6Kuan Y C, Lee I, Chem J M. Heavy metal extraction from PCB wastewater treatment sludge by sulfuric acid [J]. Journal of Hazardous Materials, 2010,177( 1-3 ): 881-886.
7Tessier A, Campbell P G C, Bisson M. Sequential Extraction Procedure for the Speciation of Particulate Trace Metals [J]. Analytical Chemistry, 1979,51 (7): 844- 850.
8Deng J, Feng X, Qiu X. Extraction of heavy metal from sewage sludge using ultrasound-assisted nitric acid [J]. Chemical Engineering Journal, 2009,152( 1): 177-182.