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This study investigates the removal of Cr(VI) by nanoscale zero-valent iron (nZVI)-multiwalled carbon nanotubes (MWCNTs) composites from aqueous solutions. Various influential environmental factors including temperature, pH, co-existing ions, and natural organic matters (NOM) on Cr(VI) removal were evaluated. Results revealed that temperature variations significantly influenced Cr(VI) removal and the efficiency decreased to 22% with increasing in temperature from 10 to 40°C. Cr(VI) removal efficiency decreased with increasing pH and the final pH increased to 9.1, 10.1, and 10.2 from 5, 7, and 9, respectively, after 2 h of the reaction. Interestingly, most of the co-existing ions except copper presented comparatively lower final removal rates. However, co-existing ions including Pd2+, Ca2+, Zn2+, and Mg2+ facilitated higher Cr(VI) removal. Copper was observed to greatly promote the reaction and 99.5% removal efficiency was achieved within 30 min after adding 20 mg L−1 of Cu2+. In contrast, Cr(VI) removal efficiency drastically dropped to 58.9% from 97.9% in the presence of As(V). At higher concentrations of humic and tannic acid; the two representatives of NOM were observed to exhibit a negative impact on Cr(VI) removal. The changes appeared on nZVI-MWCNTs before and after the reaction with Cr(VI) were investigated by transmission electron microscopy. The reusability of nZVI-MWCNTs showed a recovery rate of 53.4% in the third round and demonstrated a better utility of nZVI-MWCNTs composites in wastewater treatment applications. [ABSTRACT FROM AUTHOR]

In the present study, coupled reduction-sorption of Cr(VI) using three iron containing minerals (magnetite, hematite, and pyrrhotite) in aqueous solutions was investigated as a function of solution pH, contact time, and particle size. Detailed characterizations were performed using Brunauer-Emmett-Teller (BET), environmental scanning electron microscopy, and X-ray diffraction (XRD) techniques. Results demonstrated that over 99% of Cr(VI) removal was achieved just after 1 min of the reaction using pyrrhotite with particle size (100-200 mesh). In comparison, removal efficiency of hematite and magnetite against Cr(VI) was below 40% after 2 h of the reaction. Acidic aqueous medium proved effective for Cr(VI) removal and nearly 100% Cr(VI) removal was achieved at pH 3 using pyrrhotite (30-40 mesh), while the surface efficacy of hematite and magnetite has also improved in acidic solution. In terms of removal efficiency, the three minerals followed the order: pyrrhotite > magnetite > hematite. However, no significant difference in the BET specific surface area was recorded among these minerals, but the mineralogical compositions played an important role for removing Cr(VI) in aqueous solutions. Analysis of XRD pattern revealed that iron contents produce Fe3FeSiO4(OH)5 and FeCr2O4 after the reaction with soluble Cr(VI) and precipitated. This study suggested that high ferrous ions-based minerals can effectively remove Cr(VI) from contaminated water environments. [ABSTRACT FROM AUTHOR]

Sulfate in groundwater has been previously shown to change the reactivity of Fe0 in permeable reactive barriers for reducing chlorinated organics. To better understand the effect and mechanism of SO, the degradation of 1,1,1-trichloroethane (TCA) by Fe0 in unbuffered aqueous solutions with and without SO was investigated. In a Fe0-TCA-H2O system with initial pH of 2.0 to 10.0, the maximum removal rate of TCA was achieved at the initial pH 6.0 with pseudo-first-order constant Kobs 9.0 × 10−3/min. But in a Fe0-TCA-Na2SO4-H2O system, the removal rate of TCA decreased remarkably with a reduction in Kobs to 1.0 × 10−3/min, and the pH varied from 6.0 to 9.6, indicating an inhibition of TCA dehydrochlorination by SO. Sulfate remarkably inhibited TCA degradation via changing the route of Fe0 dissolution. It accelerated the dissolution of Fe0 and transformed the intermediate form Fe(OH)ads to Fe2(SO4)ads, which weakened the affinity between Fe and TCA, and thus depressed the degradation of TCA by Fe0. [ABSTRACT FROM AUTHOR]