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Continuous multiplication and urbanization of human population have prompted broad progression in the anthropogenic and technogenic processes e g mining metallurgical operations car industries paper and pulp production tanning wood preservation electroplating and novel agronomical works including different agrochemicals Ahemad 2015 Because of these continuous and ceaseless procedures a prolific release of heavy metals as by products has to lead to significant contamination of soils and water Quaggiotto et al 2007 Cd Cr Pb Al Hg etc are some non essential heavy metals that don't have a physiological function but they are very toxic even at very low concentrations Garzón et al 2011 Gill et al 2013 Singh et al 2016 Generally heavy metals of biological importance are divided into two categories of redox active metals such as Cr Mn Cu and Fe and non redox active metals such as Cd Al Hg Ni and Zn according to their physicochemical characteristics Emamverdian 2015 Bucker Neto 2017 Shahid 2014 karimi and Khataee 2012 The redox metals can straightly produce ROS or oxygen free radicals species in plants via Haber Weiss and Fenton reactions Jozefczak et al 2012 A specific example of a heavy metal contaminant is Cr It is one of the nonessential metallic elements which is a part of the first transition series in the periodic table Ahmad and Misra 2014 and exists in two distinct states hexavalent chromium Cr 6 and trivalent chromium Cr 3 Cr 6 which exists as an oxyanion at physiological pH is soluble highly mobile very unstable and more toxic than Cr 3 Eleftheriou et al 2015
