Effects of co-existing ions and natural organic matter on removal of chromium (VI) from aqueous solution by nanoscale zero valent iron (nZVI)-Fe3O4 nanocomposites

This study describes the chromium (VI) removal in aqueous solution by nanoscale zero-valent iron (nZVI)-Fe 3 O 4 nanocomposites, which were prepared by a novel in situ reduction method. The effects of several factors, including ionic strength, temperature, co-existing anions and cations and natural organic matters were evaluated. The ionic strength was found to be insignificant to the removal efficiency, while temperature was critical. With the increase in temperature from 10–40   °C, 32% removal efficiency increased within 2   h from 49.96%. Most anions hindered the reaction, especially in high concentration, such as HCO 3 - and PO 4 3 - , causing a drop of the removal efficiency from 66.48% to 57.47% and 59.93% respectively. In contrast, both cations and moderate natural organic matter facilitated the removal of Cr(VI). It is noteworthy that calcium hardness can greatly promote the reaction, 100% removal efficiency was achieved within 90   min and the rate constant k 2 reached 0.528   g   mg −1   min −1 after adding 40   mg   L −1 Ca 2+ , which was ten times more than that in other cases. Possible mechanism was presented, and TEM, SEM and FITR analyses were used to verify these speculations. Hopefully, these results can contribute to a better development of the water treatment facilitates for nZVI-Fe 3 O 4 composites, maintaining its high efficiency in real aqueous environment.