Iron and manganese based nanomaterials for the removal of selenite and selenate from aqueous solution
Selenium, a naturally occurring element in the environment, is an essential nutrient for humans and animals. However, there is a narrow range between selenium deficiency and toxicity. Selenium in natural waters generally has a concentration of < 10 ppb (parts per billion); however, in the San Joaquin Valley of California concentrations of selenium ranging from 140 to 1400 ppb have been observed. Anthropogenic practices such as agricultural irrigation drainage, coal burning power plants, combustion of fossil fuels, and mining operations are increasing selenium concentrations in aqueous environments. Once in aqueous environments, selenium exists primarily as the two inorganic oxoanions selenite (SeO32-) and selenate (SeO 42-). Both of these selenium oxoanions are known to bioaccumulate and can cause embryotoxic and teratogenic effects to waterfowl. There has been a variety of treatment technologies developed for remediation of both selenium oxoanions in water including bacterial reduction, membrane filtration, chemical reduction, reverse osmosis, and solar ponds. However, these treatment technologies are not cost efficient. In this study an alternative treatment technique known as adsorption was used with three synthetic nanomaterials consisting of an iron oxide, manganese oxide, and an iron/manganese oxide. The pH profiles, time dependencies, competitive anion effects, and adsorption isotherms were performed with each of the nanomaterials for both selenite and selenate. In addition, X-ray absorption spectroscopy (XAS) studies were performed to determine both the oxidation state and binding coordination of the selenium oxoanion binding to the nanomaterials. The investigation revealed the non microwave-assisted and microwave-assisted synthetic Fe3O4 and Mn3O4 nanomaterials, as well as the non microwave-assisted synthetic MnFe2O4 nanomaterial had the phases of magnetite, hausmannite, and Jacobsite, while the grain sizes were 27, 25, 25, 34, and 27 nm, respectively. The optimal binding of selenium oxoanions for all nanomaterials examined was reached at pH 4. Both non microwave-assisted and microwave-assisted Fe3O 4 and non-microwave-assisted MnFe2O4 had binding times of 5 min while non microwave-assisted and microwave-assisted Mn 3O4 nanomaterials had a binding time of 10 min. The presence of Cl- ions only significantly decreased selenate binding to Mn3O4 nanomaterials while the NO3-ion significantly decreased selenate binding to microwave-assisted Fe3O 4 and Mn3O4 nanomaterials. The inclusion of the SO42- ion deceased selenite binding to only Mn 3O4 nanomaterials and decreased selenate binding to all nanomaterials. Both selenite and selenate binding to all nanomaterials tested was significantly decreased by the addition of the PO43- ion. Non microwave-assisted and microwave-assisted Fe3O4, non microwave-assisted and microwave-assisted Mn3O4, and non microwave-assisted MnFe2O4 displayed binding capacities of 1893, 2380, 507, 1000, and 6573.76 mg selenite/kg nanomaterial and 1428, 2369, 800, 934.5, and 769.23 mg selenate/kg nanomaterial, respectively. X-ray Absorption Near Edge Structure (XANES) studies revealed all nanomaterials tested do not change the oxidation state of selenite and selenate once binding has occurred. The results of Extended X-ray Absorption Fine Structure (EXAFS) displayed the possible binding modes of selenium oxoanions to all materials tested is, binuclear bidentate.
Analytical chemistry|Environmental science
Gonzalez, Christina Marie, "Iron and manganese based nanomaterials for the removal of selenite and selenate from aqueous solution" (2010). ETD Collection for University of Texas, El Paso. AAI1479556.