According to Appenroth (2010) heavy metals include three elemental sub-groups that are as follows:• Whole transition elements• Rare earth elements that are subdivided into actinides series and lanthanides series• Elements from heterogeneous groups including Pb, Ta, Po, Te, Bi, Sn, As, In, Sb, Ga, Al and GeAccording to Dufus (2002) “heavy metal” is a poor and loosely defined term that is used for “bad metals” for environment.According to Barakat (2011) metals with greater than 5 g/cm3 density are called heavy metals.
Walker et al (2012) have proposed that a metal is considered heavy if its density larger than 5 compared to water.Barakat, M. A. 2011. “New Trends in Removing Heavy Metals from Industrial Wastewater.” Arabian Journal of Chemistry 4 (4): 361-377.
Walker, C. H., R. M. Sibly, S. P. Hopkin, and D.
B. Peakall. 2012. Principles of Ecotoxicology. 4th ed.
Boca Raton: CRC Press.Appenroth, K.-J. 2010. “What Are “Heavy Metals” in Plant Sciences?” Acta Physiologiae Plantarum 32 (4): 615-619. doi: 10.
1007/s11738-009-0455-4.Duffus, J. H. 2002. “”Heavy Metals”—a Meaningless Term? (Iupac Technical Report).” Pure and Applied Chemistry 74 (5): 793-807. doi: 10.1351/pac200274050793.