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Cr(VI) is a toxic pollutant and its reduction to relatively less toxic Cr(III) can solve this problem to a greater extent. In the present study, coupled reduction–sorption of Cr(VI) in aqueous solutions using limonite was investigated as a function of pH, limonite dose and particle size. Results demonstrated that the smaller limonite particle size and low pH aqueous medium favored high Cr(VI) removal. Significant Cr(VI) removal (>55±1%) was achieved with 100–200mesh, whereas only 25±0.7% and 10±0.5% removals were attained using 30–40mesh and 20–30mesh, respectively, after 2h reaction. Acid pH proved beneficial and the complete Cr(VI) removal was observed at pH <4.0 compared to that of 10±0.5% at pH9.0. Moreover, above 53±2.5% of Cr(VI) removal rates were maintained in the first three batch experiment runs and then drastically decreased to below 5±0.3% in experiment Run-7. The Brunauer–Emmett–Teller (BET) surface area and sorption capacity of limonite were 35.22m2 g−1 and 10.03mgg−1, respectively. The reaction mechanism demonstrated that under acidic aqueous medium, the dissolved Fe(II) and S(II) on limonite surface reacted with Cr(VI) and formed Cr3S4 and Cr5Si3 precipitates. But the resultant precipitates on limonite surface hindered further Cr(VI) removal and passivated, affirmed in XRD and ESEM analyses. This study suggested that limonite can be used for the effective removal of Cr(VI) from contaminated water environments. [ABSTRACT FROM AUTHOR]

Abstract: This study describes the chromium (VI) removal in aqueous solution by nanoscale zero-valent iron (nZVI)-Fe3O4 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 2h from 49.96%. Most anions hindered the reaction, especially in high concentration, such as and , 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 90min and the rate constant k 2 reached 0.528gmg−1 min−1 after adding 40mgL−1 Ca2+, 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-Fe3O4 composites, maintaining its high efficiency in real aqueous environment. [Copyright &y& Elsevier]