The results observed in Fig. 2a in comparison with Fig. 2b can be
explained as follows: The greatest distance between two clusters
represents the two most different groups. Fig. 2b represents Hg,
Cr, Cd, Ba, F− and V as Group II whereas other heavy metals along
with CN− forms Group I. Group I in Fig. 2b is further subdivided
into group Ia (Cu and Zn) which are placed in PC3 (supplementary
material Table S3) with high negative values. Cu and Zn both have
got good alloying properties and used as a protective finish for most
of the metal products [34]. The usage of Cu and Zn has good contribution
in metallurgical processes. Thus these two metals were
observed as close relationship among all the groups.