| Synthesizing new and high efficiency materials to reduce nitrogenous contaminants has become the focus of water research. Studies on how to convert nitrogen compounds selectively to nontoxic dinitrogen would be an effective method to reduce nitrate or total nitrogen pollution.Aluminum is the most abundant metal element in crust and more than doubled at iron. Aluminum metal has a lower reduction potential (Eo=-1.67 V) compared with zero-valent iron (ZVI) (Eo=-0.43 V). This indicates that aluminum can be used as a strong reductant for effectively reducing contaminants. Therefore, the development of aluminum-based reactive materials for nitrate reduction may be a promising strategy.Liquid-phase synthesized method was applied to facilitate Al/Cu, Al/Ni, Al/Cu/Pd and Al/Ni/Pd compound materials, and these materials were first used to reduce nitrate contamination in acidic and neutral pH aqueous system. The prepared materials were characterized by XRD, SEM-EDX and XPS to disclose the morphology and structure, depicting that zerovalent Cu and Pd was formed on the surface of Al. Attention was also given to the reaction mechanisms, with both the identification of reaction products and their intermediates in our work.In Al/Cu reaction system at pH=4, the presence of Cu on Al significantly enhance the reactivity of Al and the optimal Cu content is 4.0%. The sequence of the reaction rate for four ternary-metal compounds follows the order of Al/Cu/Pd>Al/Cu/Au>Al/Cu/Pt>Al/Cu/Ru, it is clearly that Pd is the optimal noble metal. The introduction of Pd improves N2 selectivity obviously with the value of 25%.In Al/Cu/Pd reaction system at pH=4, the denitrification reaction follows pseudo-first-order kinetic. Low pH is beneficial to the reaction reactivity, and there is no evident degradation rate for nitrate in the neutral and alkaline solutions. The influence of the Pd/Cu ratio on the N2 selectivity exhibits a volcano curve and reaches a maximum value over Pd loading of 10% followed by a drop with a further increase in Pd loading. Based on the experimental results, the reaction course of nitrate reduction for Al/Cu/Pd undergoes two steps. Al deposited Cu and Pd is suggested to promote the abstraction of oxygen from NOx by Hads on the Cu surface, and enhance N2 formation on the Pd surface. The N2 selectivity is function of the concentration of Hads, and abstraction of oxygen from by Hadx is crucial to selectively reduce aqueous nitrate to N2.In Al/Cu reaction system under neutral solution pH, when Cu content is 4.0%. Al/Cu particles show the highest reactivity and 93.3% removal rate is obtained within 30 h. For Al/Cu and Al/Cu/Pd materials, the introduction of Pd significantly improves the N2 selectivity with 12.5% N2 formation, which is consistent with the acidic system.In Al/Ni and Al/Ni/Pd reaction system at pH=4, with the exception of Pd, where the nitrate removal rate for Al/Ni is improved slightly when deposited with Pd, for all the other noble metals (Pt, Au, Ru), the reaction reactivity is significantly inhibited. NH4+-N formation amount increases monotonically with the extension of reaction time, however, no accumulation of nitrite ions and no N2 selectivity in the liquid phase are observed. The reaction course of nitrate reduction for Al/Ni is similar to Al/Cu, and it also suffers two steps. But the difference between Al/Ni and Al/Cu is that metallic catalyst Ni has better selectivity to NH44+ than Cu, which can reduce NO2-(ads) into NH4+ quickly. Accordingly, the detectable nitrite and Pd noble metal are two key in determining the N2 selectivity. |
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