洛克沙砷(roxarsone，ROX)為一芳香烴有機砷化合物(aromatic organoarsenical)，常被應用於畜牧動物飼料添加劑。其中含有之砷污染物可能隨著動物糞便或是經由動物糞便所製成的有機肥料傳輸至環境當中，不只威脅到人類的健康，也增加微生物產生抗藥性的潛在風險。本研究主要針對洛克沙砷在環境中的傳輸現象進行探討，陰離子方面以氫氧根(OH-)與磷酸根(HPO42-)當作競爭離子，而陽離子則使用六種常見或是存在動物糞便中含量較高的金屬離子，如Zn2+、Cu2+、Fe3+、Mn2+、Mg2+、Ca2+等，並使用針鐵礦(goethite)作為吸附劑進行研究。
在酸性環境當中，洛克沙砷易被針鐵礦表面吸附形成錯合物，而隨著pH值由6增加至10會使得負電荷在針鐵礦表面上累積，產生電性斥力造成洛克沙砷的吸附量大幅下降，另外一方面磷酸根也能造成洛克沙砷從針鐵礦表面上大量脫附，由此可知表面電荷與磷酸根為針鐵礦吸附洛克沙砷的反應的重要因素，因此本研究進行此兩個陰離子之脫附效果的比較。在低pH值條件下以不同順序加入相同濃度的磷酸根與氫氧根，結果顯示磷酸根造成脫附量顯著高於氫氧根之脫附量，但隨著pH提高兩者之間的脫附量會越來越接近，磷酸根的效果越不明顯，但整體來說磷酸根的脫附效果還是大於氫氧根，表示在酸性環境中存在磷酸根會增加洛克沙砷的移動性。環境中除陰離子之外也存在著豐富的金屬離子，當加入不同金屬離子，會提高洛克沙砷的吸附量，並且減低磷酸根與氫氧根的脫附能力。在這些金屬離子當中，Cu2+ 與 Fe3+ 的效果最為明顯，如洛克沙砷在初始濃度為250 μM 的條件下，Cu2+ 與 Fe3+ 各增加10 %與5 %的吸附量，然而要探討個別金屬離子造成不同吸附量的原因則需要更進一步的實驗。此研究的結果能更加了解洛克沙砷在環境中傳輸，並且能幫助預測以及控制其它結構類似的污染物在環境中移動。

Aromatic organoarsenical roxarsone (ROX) is a common feeding additive found in livestock. This arsenic containing pollutant could be discharged into the environment through agricultural application of animal manure and pose potential threats to the population and our environment. In this study, the influence of pH, competing anions and metal ions on the adsorption/desorption of ROX on goethite were examined in order to understand their mobility in the environment. The adsorption of ROX on goethite is favorable at acidic pH. In contrast, adsorption reaction decreases dramatically with increasing pH from 6 to 10. This can be explained by that the negative charge density of the goethite surface increases with the high concentration of hydroxide which leads to a stronger electrostatic repulsion force between ROX and goethite surface. The result suggests that surface charge greatly affects the surface adsorption sites on goethite. Significant desorption of ROX from goethite surface occurs at high pH level as well as the presence of phosphate. Both hydroxide ion and phosphate are common substances in the environment and are both potential competing anions for ROX. The competition experiment was thus conducted to study which ion is more pronounced on ROX desorption. The addition of phosphate desorbed more ROX than the addition of hydroxide ion at low pH. As pH increases, phosphate does not show much different effects from the addition of hydroxide ion. The result provides a strong support that the presence of phosphate will greatly increase the mobility of ROX, especially at low pH. In addition to hydroxide ion and phosphate, many metal ions were present in soils and animal excrements. Six common metal ions, including Zn2+, Cu2+, Fe3+, Mn2+, Mg2+, and Ca2+, were tested. All spiked metal ions enhanced the stability of ROX adsorption on goethite surface and lead to less desorption when hydroxide was added. The results demonstrate that metal ions may form complex or surface precipitate with ROX to enhance its adsorption. The effects from Fe3+ and Cu2+ are more pronounced than other metal ions. For example, the adsorbed amounts of ROX were increased about 10 % and 5 % in the presence of Cu2+ and Fe3+, respectively. However, further studies are necessary to better understand the effects from different metal ions. Although the addition of metal ions can enhance the affinity between ROX and goethite surface, the effect caused by the pH change is more important. Result of this research may shed lights on the environmental fate and transportation of aromatic organoarsenicals in soil and help predict and control the mobility of arsenic contaminants in geochemical environments.

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