硝酸根对零价铁去除地下水中六价铬的影响

Effect of nitrate on removal of six valence chromium in groundwater by zero valence iron

通过批实验和连续通流的柱模拟实验对比研究了存在NO_3^-和不存在NO_3^-条件下零价铁(Fe^0)去除Cr(Ⅵ)的反应特征,并对比分析了NO_3^-对零价铁(Fe^0)去除Cr(Ⅵ)的影响。批实验结果表明,在不同初始pH条件下,NO_3^-的加入均使得零价铁(Fe^0)去除Cr(Ⅵ)的速率升高;当初始pH=7时,NO_3^-的促进作用最强,反应速率是无NO_3^-条件下的2.1倍。柱实验结果表明,NO_3^-的加入使得Fe^0去除Cr(Ⅵ)的反应分成了3个不同的阶段:高效期(<198孔隙体积数(PV)),快速钝化-平稳钝化期(198-1025 PV)以及彻底钝化期(>1025 PV)。在高效期,反应中的NO_3^-表现为对Fe^0反应位点的竞争,抑制了Cr(Ⅵ)还原。在后期的2个阶段中,反应中NO_3^-则呈现明显的促进作用。NO_3^-的存在使得零价铁(Fe^0)柱趋于整体性同步钝化,延长了Fe^0柱的运行寿命,并且增加了零价铁(Fe^0)柱对Cr(Ⅵ)的去除效率。NO_3^-对Fe^0去除Cr(Ⅵ)反应的促进作用原因是由于NO_3^-与Fe^0反应产生Fe3O4,Fe3O4可以还原Cr(Ⅵ),并且具有阻滞反应的产物在Fe^0表面堆积和促进电子传导作用。

The characteristics of Cr （Ⅵ） removal by zero valence iron （Fe^0） in the presence or absence of NO3- were studied using batch experiments and continuous-flow column experiments. The effect of NO3- on the removal of Cr（ Ⅵ ） by Fe0 was studied. Batch experiment results show that the rate of removal of Cr（ Ⅵ ） by Fe0 （zero valent iron） was increased under different pH conditions when NO3- was present. The strongest role of NO3- in promoting chromium removal appeared when pH was 7, and the reaction rate was 2.1 times that of the reaction without NO3- . Column experiment results indicated that the process of removal of Cr（Ⅵ） by Fe0 was di- vided into three different stages in the presence of NO3- ：a high efficiency phase （ 〈 198 pore volumes （PV））, fast deactivation-stable passivation stage （198-1 025 PV）,and the completely passivated period （ 〉 1 025 PV）. During the period of high efficiency, NO3- competed for the reaction sites of Fe0 with Cr（ Ⅵ ） on the surface of Fe0 ,which inhibited the reduction of Cr（ Ⅵ ）. In the last two stages,the NO3- showed a significant role in promo- ting the reduction of Cr（ Ⅵ ） by Fe0. The existence of NO3- made the Fe0 column tend toward overall synchronous passivation, extended the longevity of the Fe0 column, and increased the removal efficiency of Cr（Ⅵ） in the Fe0 （ zero value iron） column. It was speculated that the reason for the promotion effect of NO3- in the removal reaction of Cr（ Ⅵ ） was due to the production of Fe3O4 during the reaction of NO3-and Fe0. Fe3O4 not only reduced the Cr（Ⅵ） directly,but also acted as an inhibitor of the formation of passive film on the Fe0 surface, as well as an electron mediator that facilitated electron transport from Fe0 to the adsorbed Cr（ Ⅵ ）.