Jianfa Li

Mechanism insights into enhanced Cr(VI) removal using nanoscale zerovalent iron supported on the pillared bentonite by macroscopic and spectroscopic studies.

NZVI was supported on a pillared bentonite (Al-bent) to enhance the reactivity of NZVI and prevent its aggregation. The performance and mechanisms of the combined NZVI/Al-bent on removing hexavalent chromium (Cr(VI)) was investigated by batch and XAFS experiments. The batch investigations indicated that Cr(VI) could be almost completely removed by NZVI/Al-bent after 120 min. The efficiency was not only much higher than that by NZVI (63.0%), but also superior to the sum of NZVI reduction and Al-bent adsorption (12.4%). Besides, NZVI/Al-bent exhibited good stability and reusability, and Al-bent could reduce the amount of iron ions released into the solution. XANES results provided evidence that NZVI/Al-bent could reduce Cr(VI) entirely into Cr(III), while NZVI reduced Cr(VI) partly into Cr(III) with a trace of Cr(VI) adsorbed on the corrosion products. The structure of Cr(VI)-treated NZVI/Al-bent determined with EXAFS revealed the formation of Cr-Al/Si bond, suggesting that some insoluble Cr(III) species might be transferred to the surface of Al-bent, therefore the precipitates on iron surface could be greatly reduced. The results demonstrated that Al-bent plays a significant role in enhanced reactivity and stability of NZVI, and may shed new light on design and fabrication of supported NZVI for environmental remediation.

Enhanced Removal of Uranium(VI) by Nanoscale Zerovalent Iron Supported on Na-Bentonite and an Investigation of Mechanism

The reductive removal of U(VI) by nanoscale zerovalent iron (NZVI) was enhanced by using Na(+)-saturated bentonite (Na-bent) as the support, and the mechanism for the enhanced removal were investigated comprehensively. Under the same experimental conditions, NZVI supported on the negatively charged Na-bent showed much higher removal efficiency (99.2%) of cationic U(VI) than either bare NZVI (48.3%) or NZVI supported on the positively charged bentonite (Al-bent) did. Subsequent experimental investigations revealed the unique roles of bentonite on enhancing the reactivity and reusability of NZVI. First, Na-bent can buffer the pH in reaction media, besides preventing NZVI from aggregation. Second, Na-bent promoted the mass transfer of U(VI) from solution to NZVI surface, leading to the enhanced removal efficiency. Third, the bentonite may transfer some insoluble reduction products away from the iron surface according to X-ray absorption fine structure (XAFS) study. Finally, Na-bent as the adsorbent to Fe(II) makes it more reactive with U(VI), which enhanced stoichiometrically the reduction capacity of NZVI besides accelerating the reaction rate.

Removal of nitrate by zero-valent iron and pillared bentonite.

The pillared bentonite prepared by intercalating poly(hydroxo Al(III)) cations into bentonite interlayers was used together with Fe(0) for removing nitrate in column experiments. The obvious synergetic effect on nitrate removal was exhibited through uniformly mixing the pillared bentonite with Fe(0). In such a mixing manner, the nitrate was 100% removed, and the removal efficiency was much higher than the simple summation of adsorption by the pillared bentonite and reduction by Fe(0). The influencing factors such as bentonite type, amount of the pillared bentonite and initial pH of nitrate solutions were investigated. In this uniform mixture, the pillared bentonite could adsorb nitrate ions, and facilitated the mass transfer of nitrate onto Fe(0) surface, then accelerated the nitrate reduction. The pillared bentonite could also act as the proton-donor, and helped to keep the complete nitrate removal for at least 10h even when the nitrate solution was fed at nearly neutral pH.

Enhanced reduction of chlorophenols by nanoscale zerovalent iron supported on organobentonite.

The reactivity of nanoscale zerovalent iron (NZVI) on removing chlorophenols (2-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol and pentachlorophenol) was remarkably enhanced by using a hydrophobic support of organobentonite (CTMA-Bent), namely the bentonite modified with organic cetyltrimethylammonium (CTMA) cations. The complete dechlorination of chlorophenols and total conversion into phenol using this novel NZVI/CTMA-Bent combination was observed in batch experiments. The kinetic studies suggested that the reduction of chlorophenols by NZVI was accelerated due to the enhanced adsorption onto CTMA-Bent, which facilitated the mass transfer of chlorophenols from aqueous to iron surface. The enhanced reduction rate by NZVI/CTMA-Bent was positively related to the hydrophobicity of chlorophenols, and an increasing linear relationship was obtained between the relative enhancement on reaction rate constants (k2/k1) and logKow values of chlorophenols. XPS results suggested there were fewer precipitates of ferric (hydro)xides formed on the surface of NZVI/CTMA-Bent, which may also lead to the improved reactivity and repetitive usability of NZVI/CTMA-Bent on removing chlorophenols.

The roles of a pillared bentonite on enhancing Se(VI) removal by ZVI and the influence of co-existing solutes in groundwater

The zero-valent iron permeable reactive barrier (ZVI-PRB) is a promising technology for in-situ groundwater remediation. However, its long-term performance often declined due to the blocked reactive sites by corrosion products and by interference of co-existing solutes. In order to address these issues, a pillared bentonite (Al-bent) was homogeneously mixed with ZVI for removing selenate (Se(VI)) from simulated groundwater in column experiments. The Se(VI) removal was enhanced because first Al-bent could facilitate the mass transfer of Se(VI) from solution to iron surface and accelerate Se(VI) reduction. XANES analysis indicated that Se(VI) was almost completely reduced to Se(0) and Se(-II) of less toxicity and solubility by the ZVI/Al-bent mixture, and the buffering effect of Al-bent could maintain the pH at a lower level that favored the Se(VI) removal. Besides, Al-bent could transfer the corrosion products away from iron surface, leading to the enhanced reactivity and longevity of ZVI. The inhibition on reactivity towards Se(VI) in both the single ZVI and the ZVI/Al-bent systems increased in the order of Cl(-)<NO3(-)<HCO3(-)<SO4(2-), and the removal efficiency decreased with the increasing HA concentration. However, the lower decrease of Se(VI) removal in the ZVI/Al-bent system indicates its resistance to the interference of these co-existing solutes in groundwater.