| Summary: | 碩士 === 國立成功大學 === 環境工程學系 === 104 === Roxarsone (4-Hydroxy-3-nitrobenzenearsonic, ROX) is an aromatic hydrocarbon compound with a pentavalent arsenic acid. It is widely used as feed additives for livestock, such as chickens, pigs, and turkeys to promote growth and control diseases. This arsenic containing pollutant could be discharged into the environment through agricultural application of animal manure and pose potential threats to our environment. ROX in the environment may be transported or degraded into metabolic byproducts with higher toxicity. In this study, the influence of pH and metal ions (Cu2+, Fe3+, Zn2+, Mn2+, Ca2+ and Mg2+) on the adsorption of ROX were investigated in order to understand their mobility in the environment. After adsorption, the competitive anions, phosphate and hydroxide, which are common substances in the environment and potential competing anions for ROX, were added to replace ROX from bayerite/goethite. The results showed the surface of bayerite was positive charged and had very strong affinity to negatively charged ROX in acidic condition. When the ROX’s loading was 50 µM and pH was less than 6, it can be all removed by 1 g/L bayerite. With increasing pH, the ROX adsorption on bayerite decreased and reached the lowest adsorption at pH 9.5 which is the pHpzc of bayerite. Surprisingly, when pH was higher than 10, both ROX and bayerite were negatively charged, but the amount of adsorption rebounded to 30-55%. It was observed that bayerite dissolute into flocs or flakes in the solution at pH 〉 10 which was very different from its solid suspension at lower or neutral pH. In the desorption experiment, phosphate was more efficient than hydroxide to replace ROX from goethite/bayerite. The results demonstrated that the presence of metal ions enhanced ROX adsorption and decreased ROX desorption, and the effect from Fe3+ and Cu2+ was more pronounced. However, the released ROX readsorbed on bayerite surface in a few hours when phosphate was the replacing agent. The re-adsorption of the ROX on bayerite might result from the fact that phosphate caused Al3+ ion dissolution and the Al3+ ions tend to associate with hydroxide ion and decrease the pH. Comparing the results with bayerite and goethite, it suggested that the flocs formation in the reaction with bayerite may influence the pH, and that the colloidal particles of bayerite formed in strong basic condition may play an important role on the fate of ROX in the environment.
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