Abstract:

The sorption of selenite ions onto Fe₃O₄ and Fe/Fe₃C nanoparticles (NPs) was studied in aqueous solutions under anoxic conditions using γ spectrometry and X-ray absorption spectrometry (XAS) techniques. This is the first study related to the remedial applications of Fe/Fe₃C NPs. Fe₃O₄ NPs have been prepared by conventional coprecipitation of Fe(II) and Fe(III) in basic solutions. Stable Fe/Fe₃C NPs have been prepared by Fe(CO)₅ sonicating in diphenylmethane solutions and subsequently annealing the as-prepared product. Kinetic study demonstrated that Se(IV) sorption is extremely rapid: the equilibrium is reached in approximately 10 and 30 min for Fe₃O₄ and Fe/Fe₃C NPs, respectively, at pH = 4.9−5.1 in solutions of 0.1 M NaCl. The distribution coefficients are also very high for both kinds of NPs (K_d > 3000). Increasing the pH to 10.3 or adsorption of organic ligands, like L-lysine or dodecanoate, at the surface of NPs causes the decrease in K_d values. However, even in these cases K_d values exceed 150. Magnetic NPs loaded with selenium can be easily and completely removed from solution with a 0.4 T permanent magnet. XAS study revealed the absence of Se(IV) reduction during the sorption onto Fe₃O₄ NPs in the pH range of 4.8–8.0. By contrast, the removal of Se(IV) with Fe/Fe₃C NPs in anaerobic conditions occurs via Se(IV) reduction to Se(−II) and subsequent formation of iron selenide at the particle surface. Thus, the Fe/Fe₃C NPs are superior to Fe₃O₄ NPs due to their ability to immobilize rapidly and irreversibly Se(IV) via reductive mechanism. Presumably these particles could be also effective for the removal of other contaminants such as hexavalent chromium, actinides, technetium, and toxic organic compounds.