在地下水砷循環中，鐵(氫)氧化物的還原溶解是將砷釋入水體之重要機制，然而水鐵礦作為眾多鐵(氫)氧化物的前驅物，目前鮮有研究探討水鐵礦對砷循環的貢獻。本研究以共沉澱與吸附實驗，合成含砷水鐵礦，量化水鐵礦擷取水溶液中砷之能力，制約其於環境砷循環中的角色。
合成的水鐵礦分為固定鐵濃度與降低鐵濃度組，固定鐵濃度組是先配製416 mL 砷濃度分別為0、0.125、0.625、1.25、6.25、12.5及62.5 mg/L並含26 g Fe(NO3)3·9H2O的溶液，取16 mL測量砷與鐵濃度，剩餘400 mL溶液利用pH-meter監測並加入約100 mL NaOH溶液，並控制混合後pH值範圍在7–8之間，沉澱前混合液砷濃度([As]i)分別0.1、0.5、1、5、10和50 mg/L。混合溶液放置24小時，水鐵礦沉澱後，經由冷凍乾燥處理成粉末。降低鐵濃度組實驗是減少水鐵礦沉澱前混合溶液中的鐵濃度，以評估其對水鐵礦擷取砷能力的影響，使固定鐵濃度組[As]i = 5 mg/L溶液中Fe(NO3)3·9H2O減少為12.5 g，而[As]i = 10 mg/L溶液中Fe(NO3)3·9H2O減少為5 g，使這兩個混合液之砷/鐵比值與[As]i = 10及50 mg/L的混合液一致。所有合成水鐵礦具有寬且低強度的二線水鐵礦XRD特徵峰。而沉澱後殘留溶液砷濃度低於Q-ICP-MS的偵測極限，代表水鐵礦具有極強的砷擷取能力。
所有的水鐵礦沉澱依照本研究砷序列萃取進行分析並結合XRD結果進行判讀，其中，砷序列萃取以固/液比值(S/L) = 0.01與0.001進行，分別為0.1 及0.01 g水鐵礦與10 mL萃取液反應，以擬合不同水鐵礦含量的樣本進行砷序列萃取的結果。結果表明，(NH4)H2PO4溶液萃取水鐵礦吸附砷，鹽酸羥胺溶液無法萃取任何與鐵氧化物共沉澱砷，草酸溶液能夠全萃取水鐵礦共沉澱砷並且能溶解樣本中所有水鐵礦，而抗壞血酸溶液僅能萃取殘餘的針鐵礦及赤鐵礦共沉澱砷，且S/L = 0.01與S/L = 0.001兩組實驗各萃取步驟的砷與鐵回收率相近，顯示本研究使用的砷序列萃取對於分析含鐵(氫)氧化物樣本有極高的選擇性，且可用於不同水鐵礦含量的樣本。相較於S/L = 0.01，S/L = 0.001的在第一次草酸步驟，即可回收> 80% 水鐵礦共沉澱砷，表明僅需重複兩次草酸萃取即可回收目標砷。所有水鐵礦樣本經過序列萃取分析後確認了，共沉澱相砷/吸附相砷比值約為9–12，因此水鐵礦主要以共沉澱的方式擷取砷。而共沉澱砷與水鐵礦的砷/鐵比值與沉澱前系統的砷/鐵比值相近，表明系統中的砷與鐵濃度同時影響水鐵礦共沉澱砷。因此，利用本研究水鐵礦於砷序列萃取草酸萃取液的砷與鐵濃度，並結合實驗室對於針鐵礦的分析結果作圖，可用於推測鐵(氫)氧化物沉澱前系統的砷/鐵比值。
為獲得腐植質對含砷水鐵礦的砷釋放潛能，進行了腐植質溶液與含砷水鐵礦反應的實驗，腐植質溶液有兩種，一是將2.5 g 腐植酸溶解在500 mL 0.1 M NaOH溶液中，配置成5 g/L 腐植酸溶液(鹼性，pH = 12)。另一個是將45 g由咖啡渣、蔬菜葉和果皮組成堆肥與450 mL去離子水混合並震盪48 h，使用藥材袋過濾後離心分離出上清液，以製成弱酸性(pH值約為5.2)的堆肥溶液。結果表明，含砷水鐵礦在鹼性環境中與OH-反應，釋放大量的吸附砷及共沉澱砷，然而腐植酸能夠與OH-競爭水鐵礦表面的點位，從而抑制砷的釋放。在弱酸性的環境中，腐植質能夠萃取含砷水鐵礦的共沉澱砷，但是萃取效率低，需要花費更長的時間或是更高的濃度，才能溶解水鐵礦並釋放砷。
結合本實驗的結果與相關研究，對於水鐵礦於砷循環有以下認知，在三價鐵離子沉澱為水鐵礦的同時，能夠擷取大量砷，並藉由陰離子競爭釋放吸附相砷，水鐵礦的還原溶解釋放吸附相、共沉澱相砷與二價鐵離子，水鐵礦老化轉換成針鐵礦或赤鐵礦並釋放吸附相及共沉澱相砷，溶解的腐植質藉由羧酸基競爭吸附在水鐵礦表面上並釋放吸附砷，也可藉由陽離子橋聯的形成砷-鐵-腐植質錯合物並擷取砷，也能夠萃取水鐵礦共沉澱砷。

In groundwater arsenic models, the reductive dissolution of iron (hydr)oxides serves as a source of arsenic. However, ferrihydrite, a precursor to many iron (hydr)oxides, is seldom studied for its contribution to the arsenic cycle. This study provides experimental data on arsenic co-precipitation with synthesized ferrihydrite, elucidating ferrihydrite's role in environmental arsenic cycling. Results indicate that ferrihydrite effectively removes arsenate (V) from the environment through adsorption and co-precipitation, with co-precipitated arsenic predominating over adsorbed arsenic. The arsenic sequential extraction procedure combined with XRD results indicates that the (NH4)H2PO4 solution extracted arsenic adsorbed on the surface of ferrihydrite, the NH2OH-HCl solution could not extract any arsenic or iron from arsenic-bearing ferrihydrite, the oxalic acid solution was able to fully extract co-precipitated arsenic from ferrihydrite and dissolve all ferrihydrite in the sample, and the ascorbic acid solution could only extract co-precipitated arsenic from residual goethite and hematite in the sample. This study demonstrates the high selectivity of the arsenic sequential extraction procedure for analyzing iron (hydr)oxide samples. Using arsenic and iron concentrations in oxalic acid extracts, models can predict arsenic/iron ratios in systems before iron (hydr)oxide precipitation. Finally, arsenic-bearing ferrihydrite reacts with OH- in alkaline environments, releasing adsorbed and co-precipitated arsenic, whereas humic acids compete with OH- on ferrihydrite surfaces, inhibiting arsenic release. In a weakly acidic environments, humic substances can extract co-precipitated arsenic from arsenic-bearing ferrihydrite, but with lower efficiency, requiring longer times or higher concentrations to dissolve ferrihydrite and release arsenic.

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