Application of Mechanochemical Procedure on Aqueous Cr(VI) Removal with Additives of Activated Carbon and Fe0/Fe2O3

Abstract

The green technique of mechanical ball milling has been extensively employed in the fabrication of environmentally functional materials. The improved specific surface area and modified surface properties of the resulting materials contribute to the high performance in pollutant removal. In this work, to improve the performance of low- cost activated carbon and sponge iron powder under neutral conditions. Ball milling was used to pretreat activated carbon and treat the mixture of surface oxidized sponge iron powder and contaminant solution, wherein the strong oxidant and toxic Cr(VI) was chosen as the target pollutant. The reduction coupling with precipitation was dominantly attributed to the removal of Cr(VI), wherein the surface enhanced surface functional groups and hydrophilicity within ball milling were the main mechanisms subject to the elimination of Cr(VI) which was substantiated by Boehm’s titration. Furthermore, surface precipitated Cr(III) oxides have been shown to impede Cr(VI) removal, and acidic washing experiments can rejuvenate the used activated carbon by dissolving the Cr(III) oxide layer. Moreover, the reduced Cr (III) and adsorbed Cr (VI) can be recovered by acidic and alkaline elution, respectively. The inherent demerits of zerovalent iron, such as surface passivation in solution and low electron efficiency, could be mitigated perpetually by ball milling. Removal efficiency of Cr(VI) maintained over 60 % over a wide pH of 4-10 in presence of ball milling, while negligible Cr(VI) decrease was noticed in absence of ball milling. XPS spectra analysis supported that reduction of Cr(VI) to Cr(III) followed co-complexed with Fe(III) as Fe0.33Cr0.67(OH)3(s) was the foremost elimination pathway of Cr(VI). The effect of dissolved oxygen on Cr(VI) removal can be divided into two segments as per the pH; the generated Fe(II) that originated from the Fe0 oxidation by dissolved oxygen facilitated to the reduction of Cr(VI) at acidic conditions, whereas the produced Fe(II) ions were oxidized at alkaline conditions and the electron efficiency of Fe was alleviated likewise. Uncovered fresh core Fe0 to the aqueous Cr(VI) by the motion of ball milling which was the main mechanism that diminished the surface passivation layer of Cr(III)/Fe(III) hydro(oxides). Furthermore, the depletion curve of Cr(VI) as function of time under different initial concentration, dosage, and rotational speed was consistent with zero order kinetic model.