Mechanistic study of fouling and regeneration of palladium-based bimetallic catalysts used for the removal of pollutants from drinking water.

摘要

Nitrate is one of the world's most widespread pollutants in both surface and groundwater and is caused by the over application of fertilizers and leaking septic tanks. The consumption of drinking water containing high levels of nitrate has been directly linked to adverse health effects in humans. Palladium-based bimetallic catalysts hold promise as a potential technology for the removal of nitrate from drinking water. The success of catalytic nitrate reduction however is dependent on the longevity of the catalyst. The primary goals of this work were to assess the effects of non-target water constituents on catalytic nitrate reduction, determine regeneration strategies for fouled-catalysts, and gain insight into the fouling and regeneration mechanisms associated with Pd-based catalysts. A series of batch experiments with Pd-Cu/gamma-Al 2O3 and Pd-In/gamma-Al2O3 catalysts showed that sulfide was the most severe foulant, decreasing the nitrate reduction rate by over two orders of magnitude. Sodium hypochlorite and heated air were effective regenerants for sulfide-fouled catalysts, restoring nitrate reduction rates for a Pd-In/gamma-Al2O3 catalyst from 20% to between 39 and 60% of original levels. Results from ICP-MS revealed that sodium hypochlorite caused dissolution of Cu from the Pd-Cu catalyst but that the Pd-In catalyst was chemically stable during oxidative regenerative conditions. These results indicate that Pd-In catalysts show promise for being robust under fouling and regeneration conditions that may occur when treating natural waters. A subsequent study tested Pd-In/gamma-Al2O3 catalysts for nitrate reduction with hydrogen in a continuous-flow packed-bed reactor. Results showed that the main products of nitrate reduction were ammonia and nitrogen, and the distribution was sensitive to solution conditions. Increases in solution pH, H2, and sulfide concentrations resulted in increases in ammonia production. Regeneration of the sulfide-fouled catalyst bed was able to restore nitrate reduction to near its pre-fouled level, but high ammonia production and low levels of nitrous oxide were observed. Ammonia production from the fresh and regenerated sulfide-fouled catalyst was 32.1+/-0.5 and 82.3+/-1.9% of nitrate reduced, respectively. These results emphasize the need for the removal of reduced sulfur species from nitrate-contaminated source water before they come in contact with Pd-In catalysts.