**2. Types of heavy metals**

Based on the survey of literature the metals that are considered as heavy metals are chromium, lead, cadmium, iron arsenic, cobalt, mercury, copper and zinc are the Heavy metal. According to Kim et al. [9] studies heavy metals have been classified in to two types as essential and non- essential (**Table 1**). Essential Heavy metals are less toxic at low concentrations and they act as coenzyme in biological process. For example Hemoglobin and Myoglobin consist of Iron, Vitamin B12 consist of cobalt. Non-essential heavy metals are highly toxic even at very low concentrations, they are non -biodegradable and cause severe toxic effects to living organisms.


**Table 1.** *Classification of heavy metals.*
