Mechanical activation of zero-valent iron (ZVI) in the presence of CaCO3: Improved reactivity of ZVI for enhancing As(III) removal from water

Nanoscale zero-valent iron (nZVI) is highly useful to remediate contaminants such as arsenic but it is currently difficult to be widely applied because of the complicated and costly synthesis process of nZVI. In this work, a novel production method by simple grinding of microscale ZVI with calcium carbonate (CaCO3) is proposed to enhance the reactivity of ZVI for the removal of As(III) from water. The mechanically activated ZVI/CaCO3 by the above grinding process was characterized by XRD, SEM-EDS, XPS, etc. The modified ZVI/CaCO3 by grinding shows excellent As(III) removal performance and good regeneration property. The mechanisms of co-grinding ZVI with CaCO3 leading to the increased reactivity of ZVI and resulting in enhanced As(III) removal are deciphered. The process of grinding helps to strip the original passivation layer from the surface of micro ZVI while the CaCO3 prevents the agglomeration of ZVI during the grinding process and protects the fresh ZVI from forming a passivation layer. During the process of arsenic removal, the CaCO3 covering on the surface of ZVI is continuously removed into the solution to expose fresh surface of ZVI simultaneously. In addition, the presence of CaCO3 facilitates the corrosion of ZVI and the formation of Fe(II) and H2O2 to trigger more highly reactive intermediates by Fenton reaction to promote the oxidation of As(III) to As(V) and Fe(II) to Fe(III), which results in dramatically enhanced As(III) removal. The presently developed simple grinding method using cost-effective CaCO3 remarkably improved the reactivity of ZVI and thus, offers an alternative for using microscale ZVI to deal with environmental pollution of arsenic more effectively and economically.