||Comparison of adsorption and post-adsorption behavior of oxyanions between ferrihydrite and schwertmannite
Water contamination is a serious problem around the world. Many toxic elements such as arse-nic, chromium and selenium are serviouly problem in the surface and ground water because most of them are oxyanions in the the natural water and tend to be less adsorbed as the pH increases to alkaline condition. Removing oxyanions is a problematic task, because in the nat-ural water there are coexisting oxyanions which havesimilar properties with toxicanions, which exist in high conconcentrations in the natural environment. Therefore, it is important to consider a technology and find out the best material to apply for water contamination treatment.
Chapter1 refers to the background and research objectives of this study. This chapter describes iron oxides and hydroxides minerals, which are excellent scavengers both for hazardous cations and oxyanions. In iron oxides and hydroxide minerals, there are several mineral species such as goethite, hematite, and ferrihydrite etc. There also exists a mineral; schwertmannite which is a meta-stable iron oxy-and hydroxy-sulfate found in acidic iron and sulfate rich environments such as sulfide metal mines. Schwertmannite and ferrihydrite are both well known to play an important role in the removal toxic elements from acid mine drainage and in natural attenuation processes of hazardous elements in acid mine water.
Chapter 2 shows the adsoption properties of some/other oxyanions. Schwertmannite and ferri-hydrite are meta stable iron oxide minerals, which are well known as excellent adsorbents for oxyanions such as arsenate, but the comparison behavior of schwertmannite and ferrihydrite with other kinds of oxyanions has not been fully or systematically investigated. With this back-ground, the adsorption properties of oxyanions including arsenate, phosphate, chromate and selenate of schwertmannite and ferrihydrite were investigated. The result of adsorption capaci-ties between schwertmannite and ferrihydrite, under these conditons showed that the schwert-mannite's adsorption capacity is higher than the ferrihydrite's adsorption capacity. However, the adsorption selectivity of oxyanion adsorption on both schwertmannite and ferrihydrite de-creases inthe following order: arsenate ≥ phosphate > chromate >>selenate.
In chapther 3: schwertmannite and ferrihydrite are metastable minerals and both transform to a stable phase as goethite. It was therefore necessary to consider the transformation process of the disposed waste from adsorption process by dissolution and precipitation; after schwertman-nite and ferrihydrite adsorbed the oxyanions.as the the toxic elements may be released to the environment. The post-adsorption behavior of oxyanion onto schwertmannite and ferrihydrite was investigated. To better understand the stabilization of mineral, the solubility of schwert-mannite with different oxaynions was calculated by the solid solution theory. A comparison of the results of the post-adsorption behavior between schwertmannite and ferrihydrite showed that solubiliy of ferrihydrite is lower than schwertmannite's solubility. In case of comparison of other oxyanions adsorption on schwertmannite showed that the degree of retardation in trans-formation to goethite decreased as arsenate=phosphate>chromate>selentate>sulfate. The solu-bility increases in the following order: arsenate>phosphate<chromate<selenate≌sulfate. There-fore, oxyanions with a high selectivity can stabilize schwertmannite by lowering the solubility of schwertmannite after adsorption of the oxyanions.
Chapter 4 shows that the natural of adsorption for oxyanion species for a wide range of minerals and environmental conditions is fundamental to the prediction of migration and long-term fate of oxyanions in the natural environments. The surface complexation modeling which is known as a theoretical method and a tool for prediction of adsorption in the natural system was applied. However, ferric oxide has already been established in many of the adsorption conditions, such as ferrihydrite adsorbing arsenate and phosphate as inner-sphere complexes, was modeled by extended triple layer modeling (ETLM). Meanwhile, information is lacking about schwertman-nite surface complexation modeling. This study characterized schwertmannite by comparation with ferrihydrite because, as explained in chapter 2, schwertmannite and ferrihydrite have sim-ilar adsorption tendencies. Thus, the main propose is to apply surface complexation modeling to schwertmannite by using similar input parameters of ferrihydrite. In this present study, oxy-anion speciation reaction equations with surface of schwertmannite was predicted theoretically following previous work of ferrihydrite. The ETLM result for some oxyanions were shown as a trial to explain the difference in adsorption behavior of oxyanion on schwertmannite from the oxyanions adsorption on the pure ferrihydrite.
Chapter 5 presents the summary and general conclusion of this study. Following from chapter 2 to 4, it was concluded that the adsorption behavior between schwertmannite and ferrihydrite have similar anions selectivity. Arsenate and phosphate were inner-sphere complexes with both schwertmannite and ferrihydrite. Selenate and sulfate were outer-sphere complexes with both schwertmannite and ferrihydrite. Selenate and sulfate were outer-sphere complexes. Chromate is an intermediate with inner-and outer-sphere with schwertmannite and ferrihydrite. The stabi-lization of ferrihydrite is higher than schwertmannite due to ferrihydrite's lower stability com-pared to schwertmannite. Some of oxyanions which made inner sphere complexes with the minerals can change the stabilization of mineral by lowering solubility
Hokkaido University（北海道大学）. 博士(工学)