砷是臺灣西南部和東北部地下水中常見的污染物，其濃度可達mg/L的等級。研究指出，飲用含砷地下水會對人體影極大的危害，因此我國環保署已將砷的飲用水水質標準由50μg/L降至10μg/L，故需提高淨水處理的技術。粒狀氫氧化鐵是一項有效去除砷的技術，本研究乃利用粒狀氫氧化鐵(GEH)進行吸附砷之試驗，包括吸附動力與平衡、電解質與水中干擾質的競爭吸附等，並以孔隙擴散模式(PDM)來模擬吸附動力，且利用三層模式(Triple Layer Model)來模擬吸附平衡，以瞭解砷吸附於氫氧化鐵之吸附特性。
研究首先對粒狀氫氧化鐵做表面分析，由X光繞射儀(XRD)的鑑定可發現GEH主要是由非結晶的β-FeOOH (Akaganéite)組成，而其等電位點(pHpzc)約為6.3左右，與文獻值相比在類似範圍。
GEH吸附實驗結果顯示，三價及五價砷平衡所需的時間皆需250小時(約10天)左右，GEH對三價及五價砷吸附平衡式均可用Freundlich及Langmuir等溫吸附線來描述。而在中性pH值情況下，五價砷與三價砷初始濃度為5 mg/L時，GEH對五價砷的吸附量約為20 mg/g，對三價砷的吸附量約為27 mg/g。若加入背景電解質NaNO3，在pH大於6時，電解質中的鈉離子會吸附於GEH內層帶負電的位址，降低GEH表面負電性，使砷的去除率隨著離子強度的增高而增加。
研究中並以硫酸鹽及磷酸鹽作為競爭吸附質，由於GEH對硫酸鹽的吸附鍵結力遠小於對砷的吸附鍵結力，以致硫酸鹽對砷吸附的影響力不大，故僅在pH＜5時使五價砷吸附量降低約25%左右，而在pH＞5時則無顯著影響；另一方面，由於磷酸鹽與砷酸鹽是性質類似的物質，在GEH表面上會有類似的吸附行為，故磷酸鹽在pH值越低時對五價砷的競爭吸附越明顯，pH=6時，砷的吸附量降低約50%，且隨著磷酸鹽對砷酸鹽的最初莫耳比升高，吸附量降低的程度越高。
模式方面，本研究利用孔隙擴散模式來描述GEH吸附五價砷與三價砷的動力行為，並推估孔隙擴散係數。在中性pH值，模擬五價砷所得之最佳化的孔隙擴散係數值約為7.0×10-11m2/sec，曲折度為14左右；模擬三價砷所得之最佳化孔隙擴散係數值為4.0～8.0×10-11m2/sec，曲折度為13～25左右。而表面錯合模式用於模擬五價砷在GEH及活性氧化鋁上之吸附平衡，分別利用三個及單一反應式來得到與實驗值相近的模擬結果，以模擬所得之最佳化平衡常數預測其他不同初始濃度的吸附平衡時，除預測GEH低pH值的吸附平衡與預測活性氧化鋁高pH值的吸附平衡時有較大的誤差外，其餘pH範圍中皆有不錯的預測結果，顯示可以利用一組吸附平衡實驗結果，預測在不同條件下，砷在兩種金屬氧化物之吸附結果，且誤差在可接受範圍內。

Arsenic is commonly present in the groundwater of southwestern and northeastern parts of Taiwan. Although the arsenic concentration in the groundwater may be as high as a few hundred g/L, some of the groundwater is still used as drinking water sources. To comply with national drinking water standard of Taiwan for arsenic, which is 10 g/L, advanced treatment techniques, such as granular ferric hydroxide (GEH), are needed. Granular ferric hydroxide is a new adsorbent that is designed for the removal of arsenic from water. Therefore, the kinetic and equilibrium adsorption of arsenic onto GEH is still not clear. In this study, transport and equilibrium of arsenate and arsenite onto GEH is examined.
A batch reactor with temperature control was used to determine the adsorption kinetics and adsorption capacity for arsenate and arsenite onto GEH. The kinetic experiments showed that equilibrium is established for both arsenate and arsenite after about 250 hours. Equilibrium experiment revealed that the uptake of arsenate decreased as pH increased, while the uptake of arsenite increased until pH 7.5, and then dropped as pH increased. Both Freundlich and Langmuir isotherm equations can successfully describe the equilibrium data.
The competition of phosphate and sulfate with arsenate for the adsorption on GFH was also studied. The presence of sulfate did not affect the uptake of arsenate for most pH condition. As pH is less than 4, arsenic uptake is suppressed by sulfate. For the effect of phosphate, the arsenate uptake reduced significantly due to the presence of phosphate, especially under low pH condition.
A pore diffusion model, combined with isotherm parameters, was used to simulate the adsorption kinetic data. In fitting the models to the experimental data, only one parameter, pore diffusivity (Dp), is adjusted. The models conform closely to the experimental data, and the extracted pore diffusion coefficient was 7.0×10-11m2/sec for arsenate and 4.0×10-11 to 8.0×10-11m2/sec for arsenite. These diffusivities are very close to those for arsenate and arsenite in activated alumina grains reported in the literature.
A Triple Layer Model (TLM) was employed to simulate the equilibrium data of arsenate onto GEH and another metal oxide, activated alumina. In the model, three complexation reactions were needed to simulate the adsorption data for GEH, while only one reaction is needed for activated alumina. The models fit the equilibrium uptake data for both oxides fairly well. The extracted equilibrium constants from the models were then used to predict the observed data under different conditions. The model predictions conform closely to the data, substantiating the appropriateness of the model.

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