Environmental catalysis using nano-sized bimetallic particles: Selenium remediation

Selenium remediation using nano-sized bimetallic particles was developed and studied. The nano-sized bimetallic particle of nickel and iron (NiFe) provided better selenium reduction than other commercial adsorbents. The process involved reduction of selenium, both selenite (Se (IV)) and selenate (Se (VI)), on the nickel sites. The oxidized form of nickel was regenerated via the transfer of electrons from the iron sites. Since iron oxidation is spontaneous in aqueous solutions, the electronic bridge between iron and nickel helped increase selenium reduction rates. When zerovalent iron (Fe 0) was used in conjunction with second metal such as nickel, the second metal primarily has the following functions: (1) as a catalyst, (2) preventing the formation of the oxide film on the surface of Fe0, and (3) inducing Fe0 to release electrons at a faster rate due to galvanic coupling and thus enhancing the rate of reduction. A comparison of the removal efficiencies of selenium by commercial adsorbents showed that NiFe provided 200 % greater selenium removal as compared to activated carbon, alumina and 50 % greater than Fe0. This process leads to complete catalytic immobilization of selenium species with faster reduction rates and increased longevity of the process. The other advantage of this process was the use of nanosized particles having higher surface area, thereby resulting in higher removal rates. The reduction rates of Se (IV) and Se (VI) was also compared. It was observed that reduction rate of Se (IV) to Se (0) was 3-fold lower than the reduction of Se (VI) to Se (IV). A nickel composition of 50 % (N is Fe = 1:1) was required to achieve maximum removal due to the requirement of a galvanic couple for electron transfer.; Experiments were conducted to determine the effect of initial selenium concentration, catalyst loading, pH, temperature, and dissolved oxygen. Selenium reduction rates decreased with initial concentration and catalyst loading, while pH had not bearing on the removal rates. Higher temperatures and oxidizing conditions provided increased removal efficiencies. The presence of anionic co-solutes such as sulfate inhibited the removal rate via competitive adsorption. Efficacy of NiFe particles in removing selenium from contaminated agricultural drainage water showed that 10 g L-1 was enough for complete immobilization of selenium, even under the presence of high amounts of sulfate. A complex chemical reaction model was developed to describe the kinetics of the process and to predict the reaction profile.