Introduction
It is well known that heavy metals such as cadmium and zinc
in environments pose a serious threat to plants, animals and even
human beings because of their bioaccumulation, nonbiodegradable
property and toxicity even at low concentrations [1]. In general,
the pollution caused by heavy metals has detrimental effect on
the environment all over the world. For example, a variety of
toxic effects on aquatic organisms can be produced by endangering
ecosystems; human health can be directly or indirectly influenced
by multiple channels such as touching with skin, drinking water,
and food chain. In addition, in agro-ecological environment, especially
in soils, the phenomenon of heavy metals pollution is now
quite common. Heavy metals cause great harm to the crop growth,
yield and quality. So the removal of heavy metals, such as mercury,
lead, zinc, copper, cadmium, and arsenic, from natural waters
or soils has attracted considerable attention [2]. The conventional
technologies for the removal of heavy metal ions from aqueous
solution include chemical precipitation, ion exchange, reverse