2009-05-19_REVISION - M1977416 - Laserfiche WebLink

Home Browse Search

• The fraction of selenium (x) that substitutes into the phase depends on the pH and the concentration of selenium in solution (the ratio of SeO3-2 to OH-). Selenium Adsorption Adsorption of selenium depends on the speciation of selenium, pH, and concentrations of competing ions in the water, and the specific properties of the adsorption media. Selenite is adsorbed to ferric hydroxide to a greater extent than selenate. As mentioned previously, both selenite and selenate are negatively charged ions (anions) and tend to adsorb to positively charged surfaces. Selenite tends to adsorb significantly better than selenate on most solid surfaces. Selenite forms a strong bond called an "inner-sphere complex", while selenate forms a weaker bond with solid surfaces called an "outer-sphere complex". At low pH, the surfaces of minerals tend to be positively charged, while at high pH the surfaces are negative. Therefore, adsorption efficiency and capacity for selenite and selenate increase with decreasing pH. At low pH values, the water and mineral surfaces have higher concentrations of hydronium ion (H30+) which imparts a positive charge to the surface. As the pH increases, the hydronium ion concentration decreases relative to the hydroxide ion (OH-) concentration in both the water and the solid materials within the water. At a specific • threshold pH value called the pH of the zero-point-of-charge (ZPC), the surface charge transitions from positive to neutral to negative. Once the surface charge becomes negative, adsorption of the negatively charged arsenate ions become less prevalent. The pH of the ZPC is different for different materials, as shown in Table 1. Table 1 off of the Zero-Point-of-Charae (oH7PAfor Various Minerals Material -- - - -- ---------- Formula HZPC Magnetite Fe304 6.5 Goethite Fe00H 7.8 Hematite Fe203 6.7 Amorphous Ferric hydroxide Fe(OH)3 8.5 Amorphous Silica Si02 2.0 Montmorillonite Clay Nao_2Cao.iA12Si40Io OH 2.10 H2O 2.5 Kaolinite Clay A12Si2O5(OH)4 4.6 The materials with a higher pHzrc are able to maintain a positive charge at a higher pH than for materials with a lower pHzrc. Of the materials listed in Table 1, amorphous ferric hydroxide is the best anion adsorbent at higher pH values (below 8.5). Negatively charged ions or neutral species, such as phosphate and silicate, respectively, can compete with selenium for adsorption sites on the media. Therefore, selenium adsorption is favored for waters low in phosphate and silicate. References