Composite sorbent based on Fe$_3$O$_4$ with Fe(N$_2$H$_4$)$_x$Cl$_y$ for the removal of Chromium(VI) from wastewater

The paper presents a methodology for the synthesis of iron complex with hydrazine hydrate Fe(N$_2$H$_4$)$_x$Cl$_y$. The Fe(N$_2$H$_4$)$_x$Cl$_y$ complex was investigated by X-ray phase analysis and scanning electron microscopy. Upon hydrolysis, the Fe(N$_2$H$_4$)$_x$Cl$_y$ complex forms a composite sorbent, which is Fe$_3$O$_4$ in a shell of Fe(N$_2$H$_4$)$_x$Cl$_y$ complex. The composite sorbent can be used to treat wastewater from Cr(VI) ions and is effective in the pH range of 2 to 12. Based on the adsorption and electrokinetic potential data, a conclusion about the nature of the terminal groups of the adsorbent was made, a scheme of the structure of its electrical double layer and the adsorption mechanism were proposed. Depending on the conditions, Cr(VI) can be adsorbed on the composite sorbent or reduced to Cr(III). The efficiency of the composite sorbent in the removal of Cr(VI) ions was tested on a sample of real wastewater.