According to Appenroth

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 Ge
According 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. doi: 10.1016/j.arabjc.2010.07.019.
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.