**5. Removal of hazardous heavy metals**

#### **5.1 Methods of treatment**

Due to their toxicity, non-biodegradability and persistency, heavy metals can exert adverse effects on the environment and other ecological receptors. Therefore, their removal from soil and aqueous environments has drawn tremendous attention. Various methods have been developed and used to decrease heavy metals concentrations in the ecosystems. These technologies can be categorized in physico-chemical processes such as ion exchange, reverse osmosis, membrane filtration, adsorption, precipitation, electrolytic removal and biological processes involving activated sludge and phytoremediation [12, 47, 48]. Adsorption is one of the most extensively used methods due to its low cost and simple preparation. It is based on mass transfer between the liquid phase and the solid phase called adsorbent. This process can run in reversible mode and the adsorbents will be regenerated by desorption. Some widely used adsorbents for removal of metal ions include clay minerals, activated carbon,, biomaterials, industrial solid wastes and zeolites [48]. Low cost adsorbents include natural material or certain waste from industrial or agricultural operation.

#### **5.2 Nanomaterials for heavy metals removal**

Over the past decades, nanomaterials have gained a lot of attention due to their high specific surface area, catalytic potential and chemical reactivity [49]. Various cost-effective and safe nanomaterials have been developed in treating wastewater solutions. Among them, nano metal oxides (NMO), nano zero-valent iron (nZVI) and hybrid magnetic nanoparticles (MNP) are particularly efficient in the improvement of water quality.

#### *5.2.1 Nano metal oxides*

Metal based nanomaterials are commonly oxides of iron, manganese, titanium, magnesium, copper and cerium. These metal oxides are low-cost materials and provide a high adsorption capacity and selectivity. However, NMO are prone to agglomeration due to their poor stability. Different techniques could be used for NMO synthesis such as hydrothermal techniques, chemical co-precipitation, thermal decomposition or chemical vapor condensation [50, 51]. Generally, highly stable, monodisperse and shape-controlled NMO are the result of efficient synthetic techniques. Luther et al. (2012) synthetized iron oxides nanoparticles for the removal of arsenic (III) or (V) from aqueous solutions. The adsorption followed the Langmuir Isotherm and capacities were determined for each nanomaterial and

arsenic ions [52]. Optimum binding capacities reaching 20000 μg/g for arsenic (III) and 4904 μg/g for arsenic (V) were observed at pH =6, after 24 h of contact time at room temperature.
