針鐵礦(α-FeOOH)形成時可以透過共沉澱與吸附的方式富集砷，但在不同的氧化還原環境會溶解或相轉變成其他礦物相造成砷在不同宿主的遷徙，地下水的砷問題與含水層的沉積物息息相關，前人針對孟加拉地區的地下水進行研究，提出地下水的砷來源為含砷的氧氫氧化鐵(FeOOH)溶解。針鐵礦是常見且熱力學穩定的氧氫氧化鐵，過去針鐵礦與砷的實驗大多集中在吸附砷的研究，較少實驗透過與砷共沉澱的針鐵礦探討形成時從水體中擷取砷到其結構中的機制，而且這些實驗沒有仔細討論針鐵礦形成時的砷濃度、酸鹼值與氧化還原電位。本次研究合成三種與砷共沉澱的針鐵礦，並比較自然界的含砷針鐵礦結核樣本，驗證針鐵礦形成時能夠擷取砷到其結構當中或是以吸附的方式附著於表面。合成的針鐵礦樣本由X光繞射分析圖譜鑑定礦物相與結晶度，經由連續萃取法(sequential extraction procedure, SEP)分析結構砷與吸附砷的比例。合成三種不同含砷的針鐵礦分別為Gtnitrate、Gtchloride及Gtsulfate，Gtnitrate是由硝酸鐵混合氫氧化鈉溶液加熱共沉澱合成出結晶度佳的針鐵礦；Gtchloride是由氯化亞鐵混合碳酸氫鈉溶液曝氣共沉澱形成結晶度較差的針鐵礦；Gtsulfate是由硫酸亞鐵混合碳酸氫鈉溶液曝氣共沉澱形成結晶度較差的針鐵礦與纖鐵礦的混合物。Gtnitrate、Gtchloride及Gtsulfate在初始砷濃度為0.1、0.5、1、10、50 mg/L的環境共沉澱後，經SEP分析所有含砷樣本，有80%以上的共沉澱砷，與天然的針鐵礦結核擷取砷的方式一致，此結果證實從地下水體共沉澱出的針鐵礦主要以共沉澱的方式擷取砷。
Gtchloride及Gtsulfate與砷共澱後，上清液為中性，兩者對砷的移除率皆為100%；而Gtnitrate共沉澱後上清液呈鹼性，隨著砷濃度越高，砷移除率下降。從系統為中性或鹼性的差異，推論Gtnitrate形成時，氫氧根(OH-)濃度過高，導致Gtnitrate的前驅物硝酸鐵中的三價鐵(Fe3+)與氫氧根快速結合，形成結晶度極佳的針鐵礦，使Gtnitrate擷取砷到結構中的能力有限，造成砷濃度越高移除率下降，殘留在液體當中的砷也因為氫氧根濃度過高，使Gtnitrate無法以吸附的方式移除液體中殘留的砷。Gtchloride與Gtsulfate的前驅物分別為氯化亞鐵以及硫酸亞鐵，鹼液為碳酸氫鈉，碳酸氫根與水反應形成氫氧根，但濃度不高，兩價的亞鐵離子(Fe2+)在此系統中先形成混合價態前驅物綠鏽。綠鏽對於砷的移除能力極佳，綠鏽擷取砷後，完全氧化成Gtchloride及Gtsulfate的過程中仍將砷留在結晶結構當中。Gtchloride為100%針鐵礦，但綠鏽轉變為FeOOH的過程中可能會形成纖鐵礦(γ-FeOOH)，Gtsulfate為纖鐵礦與針鐵礦混合的礦物相。
計算SEP分析Gtnitrate、Gtchloride與Gtsulfate針對鐵氫氧化物目標相萃取液砷與鐵的比例，了解當多少量的含砷針鐵礦還原溶解造成定體積水體砷濃度增加至與地下水體砷濃度相符，進一步由砷與鐵的比例推測天然的含砷針鐵礦結核的形成機制。將地下水砷濃度高區域的沉積物連續萃取實驗針對鐵氫氧化物目標相的砷濃度與砷鐵比作圖，並加入本研究Gtchloride與Gtnitrate的萃取結果，進一步判斷沉積物當中不同結晶度含砷針鐵礦的貢獻。
由過去所量測地下水資料的陰陽離子、酸鹼值、氧化還原電位、溫度，建立出針鐵礦在不同環境中溶解或沉澱，釋放或擷取砷的機制。並比較砷問題嚴重的地區中地下水砷、鐵濃度與氧化還原電位的關係，釐清針鐵礦還原溶解造成地下水砷濃度的增加的機制。

Goethite (α-FeOOH) has been commonly recognized as an arsenic host that contributes to groundwater arsenic budget by reductive dissolution (e.g., Nickson et al., 2000; McArthur et al., 2001). However, goethite can incorporate arsenic during its precipitation and subsequently adsorb arsenic after its formation. Hence, the relative contribution of co-precipitated and adsorbed arsenic remains unsolved because most of the experimental constraints were on the adsorption species. Moreover, the few goethite-arsenic co-precipitation experiments did not explore the effects of the variations of system arsenic concentration and pH condition during the goethite formation. In this study, goethite powders were synthesized from three solutions with initial arsenic concentrations [As]i of 0, 0.1, 0.5, 1, 10, 50 ppm. The three synthesized arsenic-containing goethite samples were labeled as Gtnitrate, Gtsulfate, and Gtchloride, representing well-crystallized goethite from iron(III) nitrate, poor crystalline goethite and lepidocrocite from iron(II) sulfate, and poor crystalline goethite from iron(II) chloride, respectively. These experimentally produced goethite samples were analyzed with the sequential extraction procedure (SEP) modified from Wenzel et al. (2001) and Keon et al. (2001) to evaluate the selectivity of the extraction reagents. Then, the compositions of these samples, mainly the co-precipitated arsenic abundance and As/Fe ratio, were compared to those of natural arsenic-bearing goethite nodules from the Ernjen River in the Chianan Plain and the goethite crust from the Paoli River in Pingtung, southern Taiwan to infer the formation mechanisms of these natural goethite.
The results showed that most of the arsenic (over 80%) was incorporated into the structure of synthetic goethite during its formation, consistent with the results from the natural goethite nodules. The modified sequential extraction procedure (SEP) demonstrated excellent extraction selectivity on goethite. The ammonium dihydrogen phosphate extraction effectively extracted adsorbed arsenic without dissolving iron. The oxalic acid and ascorbic acid extractions were consistent with the XRD results, reflecting the varying crystallinity of goethite. The arsenic solid/liquid partition coefficients of Gtchloride and Gtsulfate were relatively high, ranging from 806 to 1719, and they showed no correlation with the [As]i, implying attainment of arsenic equilibrium between the solid and liquid phases. For Gtnitrate, the arsenic solid/liquid partition coefficients ranged from 26 to 151, and they decreased with increasing initial arsenic concentrations, indicating that higher arsenic concentrations make it more difficult for arsenic to enter the solid phase. The goethite nodules from Ernjen River exhibited higher As/Fe ratios compared to the goethite crust from Paoli River. By comparing these ratios with the [As]i values of the Gtchloride in this study, it can be inferred that the formation of goethite with high As/Fe ratios required elevated local arsenic concentrations, possibly derived from arsenic-bearing minerals such as iron sulfides. The As/Fe ratio provides insights into the formation mechanism of naturally occurring arsenic-bearing goethite. By considering the documented groundwater composition data, we inferred the state of goethite (dissolution or precipitation) to further predict its arsenic release potential in various environmental systems.

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