| Summary: | 碩士 === 嘉南藥理科技大學 === 環境工程與科學系曁研究所 === 97 === Abstract
The anions and cations from water and soil were easily to form complex reaction at the surface of minerals containing iron oxide which are very effective to consider as adsorbents applied to pollutants control system. In this study, the common soil containing iron oxide minerals, lepidocrocite and ferrihydrite, were selected to test phosphates adsorption, and examined by adsorption models for discussion.
This study evidenced that both lepidocrocite and ferrihydrite illustrated good adsorption ability for phosphates. The adsorption equilibrium time for lepidocrocite and ferrihydrite were 360 and 720 minutes, respectively. The phosphate adsorption capacity decreased with lowering pH value. Moreover, under the experimental conditions of pH 2, 25 ℃, and the initial concentration of phosphate = 2 mM, the equilibrium adsorption capacity of the ferrihydrite (678.96 mmol/kg) was greater than lepidocrocite (505.07 mmol/kg).
The phosphates adsorption kinetics of lepidocrocite and ferrihydrite followed the pseudo second-order reaction. The rate constant (k2) of lepidocrocite was in the range of 4.02 × 10-4 ~ 2.08 × 10-5 kg / mmole • min-1, and it of ferrihydrite was in the range of 2.69 × 10-4 ~ 6.91 × 10-6 kg / mmole • min-1.
Based on the thermodynamic parameters, the negative value of free energy (△G) indicates the reactions of phosphates adsorbed onto iron oxides might be spontaneous. Moreover, the enthalpy change (△H) was positive and less than 10 kcal / mol, which reveals their adsorption might be endothermic and physical reactions.
Desorption study was also conducted to examine the experimental results. Applied 0.1N NaOH to lepidocrocite showed the best desorption result, reached up to 99% desorption efficiency. It is noted that the lepidocrocite was outside complex reacting with phosphates, whereas the ferrihydrite was internal.
Keywords: Phosphate, Lepidocrocite, Ferrihydrite, Adsorption
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