Research On Trsnsformation Of 2-Line Ferrihydrite And Its Effect On Cadmium Adsorption

Ferrihydrite?F h?, is a common iron oxide in surface environments wide speedy such as young soil and water environment deposits. Fh is precursor of other iron oxide hydroxides. Recently ferrihydrite has received much attention as an effective adsorbent for removing cadmium from water because of its large surface area, high adsorption capacity on heavy metals, controlling and influencing the geochemical cycle of iron element and correlative elements in the environment 2- line ferrihydrite is a poorly crystalline mineral, and is thermodynamically unstable with respect to goethite and hematite, therefore, Fh will ultimately transform to goethite or hematite. However the crystalline iron oxides are likely to have diminished capacity to combine with heavy metals when 2- line ferrihydrite transformed to goethite or hematite. So, in the process of synthesizing Fh2, mixing Fe3+ salts with an inhibitor maybe retard transformation of Fh2 and retain higher adsorption capacity for cadmium. In this study, The Fh2 s were synthesized by mixing Fe3+ salts with an inhibitor, and the influence of inhibitors(Ca²⁺, Mg²⁺)and heat treatment on F h2 transformation and adsorption on Cadmium was investigated. We studied the adsorption on cadmium in the very of p H value and initial concentration of cadmium meanwhile.?1? In the process of synthesizing Fh2, The presence of Ca²⁺ has no effect to synthesize Fh2, and the presence of Mg²⁺ has obvious effect. The Fh2-Mg series consisted of mixed solid phases?2-line ferrihydrite and akaganeite?, and with increasing the Mg/Fe molar ratio, the higher intensity of peaks of iron oxide hydroxide.?2? After heat-treating Fh2 and Fh2-Cas at 200°C for 24 h, the complete decomposition of 2-line ferrihydrite occurred, resulting in the formation of a well ordered hematite as a single solid phase. And after heat-treating Fh2-Mgs at 200°C for 24 h, all of the diffraction peaks in these patterns can be also assigned to hematite. However, Fh2-Mg-1 had a stronger retarding effect on the transformation of amorphous Fe oxide to hematite than was observed with Fh2-Mg-0.1 and Fh2-Mg-0.5, that is, the extent of transformation decreased with an increase in the Mg/Fe molar ratio.?3? Initial concentration of cadmium is one of key factors to Fh2 s and Fh2-200 s absorbing cadmium. Within the same p H value, as the concentration of cadmium begins to increase from 0.2mg/g to 25mg/g, the adsorption capacity of Fh2 s and Fh2-200 s on cadmium increasing, but the removaling rate of Fh2 s and Fh2-200 s on cadmium decreasing.?4? PH value affected the adsorption capacity of Fh2 s and Fh2-200 s on cadmium in significant measure, and influence rule is the same for Fh2 s and Fh2-200 s. While p H value was equal or greater then 10, Its adsorption capacity and removal rate reach climax point and removal rate approaches one hundred percent. So removing Cd²⁺ from water to the benefit of the alkaline environment by Fh2 s and Fh2-200 s.?5? The adsorption density for Cd²⁺ on the Fh2 s decreased in the order: Fh2 ? Fh2-Cas > Fh2-Mgs. And the adsorption density for Cd²⁺ on the Fh2-200 s decreased in the order: Fh2-200-Ca-0.1 ? Fh2-200-Mg-1 > Fh2-200-Ca-0.5 > Fh2-200-Mg-0.5 > Fh2-200-Mg-0.1 ? Fh2-200-Ca-1 > Fh2-200. Maybe it duo to the Fh2-Mg series consisted of mixed solid phases?2- line ferrihydrite and akaganeite?, and Fh2-Mgs had a stronger retarding effect on the transformation of amorphous Fe oxide to hematite, with increasing the Mg/Fe molar ratio, the higher intensity of peaks of akaganeite and the extent of transformation decreased.?6? All the isotherm data of Fh2 s and Fh2-200 s adsorption for Cd²⁺ was well fitted by Freundlich model. ‘Kf', affinity coefficient of Freundlich model, decreased in the order: Fh2 > Fh2-Cas > Fh2-Mgs and Fh2-200-Mgs > Fh2-200-Cas > Fh2-200 s, which indicated a stronger adsorption force of Fh2 and Fh2-200-Mgs than others. Fh2 s and Fh2-200 s were effective adsorbent of Cd²⁺ in the water environment.The results of this study showed that Ca²⁺ and Mg²⁺ had a positive influence on Fh2 transformation during heat treatment and cadmium adsorption, above all the influence of Mg²⁺. Therefore, the inhibitors can play a significant role in both cadmium removal in water treatment and sludge transformation behavior at waste-disposal sites. Compared to the process with Fh2 suspension alone in water treatment, the use of Fh2-Cas or Fh2-Mgs as filter media might be advantageous for cadmium removal and enhance cadmium retention at waste-disposal sites.