| The reduction of Cr(VI) by zero valent iron (ZVI) is a surface mediated reaction,hence, enrichment of Cr(VI) on ZVI surface can facilitate the electron transfer, andthus the reduction efficiency of Cr(VI) by ZVI can be greatly improved. Besides, inthe ZVI treatment, solution pH could increase during the reaction process, and thensome corrosion products such as oxide or hydroxide of iron, or other insolublesubstances would be formed on iron surface. Therefore, the reactivity and longevity ofZVI could greatly decrease.In this paper, the effect of pillared bentonite (OH-Al-bent) on Cr(VI) removal byZVI was investigated in column and batch experiments.In the first part of this paper, the OH-Al-bent was combined with ZVI to investigatethe performance on Cr(VI) removal in continuous column experiments simulating theoperation of Fe0-based permeable reactive barriers (Fe0-PRBs). The effects of mixingmanner, different bentonites, initial pH and OH-Al-bent amounts on Cr(VI) removalin the OH-Al-bent/Fe0-PRBs treatment system were also invesitgated. Finally, X-rayphotoelectron spectroscopy (XPS) was used to analyse to reaction products. Theresults are as following:1) When OH-Al-bent was uniformly mixed with ZVI, the Cr(VI) reduction by ZVIwas distinctly improved and the longevity of ZVI was extended. The enhancedremoval was contributed to the synergetic effect between adsorption by OH-Al-bentand reduction by ZVI. The adsorption by OH-Al-bent facilitated the mass transfer ofCr(VI) from aqueous to iron surface, and thus accelerated the reduction of Cr(VI) byZVI.2) The enhancement of OH-Al-bent for Cr(VI) reduction by ZVI was much higherthan that of the original bentonite (Na-bent), which was ascribed to be higheradsorption and more functional groups of OH-Al-bent.3) These functional groups existing on the surface of OH-Al-bent could showbuffering effect of solution pH.4) The XPS results conformed that OH-Al-bent could reduce the surface passivation of ZVI in reactive column to prolong the longevity of ZVI.In the second part of this paper, OH-Al-bent was used as a carrier, and a novelcomposite of nanoscale zero valent iron (NZVI) supported on pillared bentonite(NZVI/OH-Al-bent) was prepared through the reaction NaBH4with FeSO4aqueoussolution. The NZVI/OH-Al-bent was characterized by X-ray Diffraction (XRD),transmission electron microscope (TEM) and scanning electron microscope (SEM).The enhancement of OH-Al-bent on Cr(VI) reduction by NZVI was investigated bybatch experiments, and the influences of initial pH and initial Cr(VI) concentrationwere also evaluated. The results are as following:1) As a carrier, OH-Al-bent can effectually prevent the aggregation of NZVI.2) Under the same condition, the removal of Cr(VI) by NZVI/OH-Al-bent was notonly much higher than that of NZVI, but also superior to the sum of reduction byNZVI and adsorption by OH-Al-bent. This was attributed to the synergetic effectbetween OH-Al-bent adsorption and NZVI reduction in the novel composite ofNZVI/OH-Al-bent. Due to the adsorption by OH-Al-bent, Cr(VI) concentrationincreased in the vicinity of iron surface, and the mass transfer was facilitated,resulting in the enhanced removal efficiency.3) Compared with NZVI, NZVI/OH-Al-bent showed good pH buffering effect. Inaddition, NZVI/OH-Al-bent exhibited long-term stability and reusability... |
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