*5.2.2 Nano zero-valent iron*

Due to the reducing capacity of iron (0) and the high reactivity of ferric oxide, nZVI composite was extensively investigated as a novel adsorbent to remove heavy metals from wastewater [53]. Several methods have been developed to produce nZVI including abrasion, grinding, reduction with sodium borohydride, carbothermal reduction, ultrasound assisted method, electrolysis or biosynthesis [54]. In spite of their effectiveness in water treatment and remediation, some disadvantages mainly due to aggregation, difficult separation and lack of stability are encountered with the use of nZVI. To overcome these technical difficulties, modifications of nZVI could be applied such as doping the composite with noble metals, surface coating with polymers or anionic surfactants, emulsification or immobilization with silica, activated carbons or zeolites. Huang et al. (2015) applied modified nZVI with sodium dodecyl sulfate anionic surfactant for chromium (VI) removal [55]. They found an enhanced removal capacity and a great stability with nZVI modification. The adsorption followed the pseudo-second order kinetic model and the Freundlich Isotherm. The highest removal efficiency of chromium (VI) was obtained at pH 3.0 and 25°C, at the value of 98.919% after 120 min of contact time.
