Selenium Removal by Nano-Magnetite Impregnated Diatomaceous Earth

The purpose of this study was to remove selenium from aqueous solution using nano-magnetite impregnated diatomaceous earth (DE) as an adsorbent. The adsorbent was prepared at the laboratory with a ratio of magnetite/DE of 10%. Batch adsorption experiments were conducted to evaluate the effects of pH, temperature, adsorbent concentration, selenium speciation, contact time and presence of competing anions on selenium removal efficiency in conjunction with studies on adsorption kinetics, isotherms, and adsorptive thermodynamics. Rapid adsorption occurred within 30 min and selenium uptake decreased with increase in temperature. Low pH values (i.e. 2--4) favored selenium adsorption as expected for anion adsorption but the adsorption of selenate (Se(VI)) was more strongly dependent on pH than that of selenite (Se(IV)). The adsorption isothermal and kinetic data of selenium were well fitted to the Freundlich isotherm and pseudo-second-order kinetic models. The nano-magnetite impregnated DE showed an adsorption capacity of 0.5 mg-Se/g for selenite and 0.25 mg-Se/g for selenate at pH 3.0. At an initial selenium concentration of 250 microg-Se/L, more than 96% removal of selenite and 50% removal of selenate were achieved. It was observed that selenate adsorption was significantly affected by the presence of competing anions such as chloride, nitrate and sulfate, whereas selenite removal was mostly affected by the presence of sulfate in solution. Column tests demonstrated that diatomaceous earth by itself was not able to remove selenite or selenate anions from water; therefore all the adsorption capacity could only be attributed to the nano-magnetite coating. Column adsorption tests using nano-magnetite impregnated DE showed higher adsorption efficiency for selenite anions compared to selenate. However, the adsorption capacity decreased when a natural water sample from Monongahela River was used. In conclusion, it was found that nano-magnetite impregnated diatomaceous earth is a promising low-cost adsorbent to treat aqueous solutions containing low levels of selenium. Besides, this new adsorbent is porous and stable, making it suitable adsorptive filtration in column applications.