全球的砷循環受制於多項因素，包含多種礦物相與有機質，至今仍未釐清各參與相的影響力，鐵氫氧化物的還原溶解為目前最主要用來闡述砷自沉積物釋放於地下水的機制。錳氧化物與鐵氫氧化物為常見之共生礦物，具備相似的化學特性，如高砷吸附能力與還原溶解特性。然而，目前卻少有文獻探討錳氧化物對於環境中砷循環之影響。現今常用於瞭解沉積物中砷元素分布的連續萃取法亦缺乏研究詳細檢視其萃取溶劑對目標相的選擇性 (selectivity)。本研究利用含砷溶液合成四種錳氧化物，並採用砷連續萃取法分析，探討萃取方法對於錳氧化物的選擇性，以期釐清錳氧化物擷取砷之機制與釋放砷之潛能；此外，亦進行砷吸附實驗，瞭解該礦物對於砷的吸附行為與最大吸附量。
連續萃取分析顯示萃取溶劑的選擇性並不如預期，指定用於萃取錳氧化物的鹽酸僅能溶解小於2%的錳氧化物，除了黑錳礦 (hausmannite)被溶解10-35%，但仍不足以完全萃取錳氧化物，顯示萃取方法無法如實反應錳氧化物對於砷之影響。針對此疑慮本研究採用更具還原能力的草酸/草酸銨將所有錳氧化物溶解，雖能有效將錳氧化物的砷釋放，卻大幅降低草酸/草酸銨萃取液對於針鐵礦的選擇性；透過結合磷酸氫鈉與草酸/草酸銨萃取，能有效分離與量化錳氧化物所具之吸附與結構砷。將萃取方法應用於四種錳氧化物，結果顯示結晶度差水鈉錳礦 (low crystallinity birnessite)具有最高的砷吸收能力，當合成溶液中初始砷濃度從 0.1 ppm增加到50 ppm時，其吸收能力從3.6 μg As/1 g Mn 上升至2140 μg As/1 g Mn。黑錳礦 (hausmannite)與硬錳礦 (hollandite)的砷吸收能力皆約為結晶度差水鈉錳礦的一半，結晶度佳水鈉錳礦對於砷的富集量則幾近於零。透過萃取方法的分離與量化，顯示當砷濃度上升，硬錳礦所擷取之砷，其吸附與共沉澱比例保持在約40:60；而結晶度差水鈉錳礦的兩相砷比例則從0:100增加到40:60，指示結晶度差水鈉錳礦會優先擷取結構砷，當結構砷趨於飽和，則轉以吸附的形式富集水溶液中砷。藉由量測共沉澱前後水溶液的砷濃度指出低結晶度的水鈉錳礦於沉澱後已將水體中所有的砷移除，故可能低估水鈉錳礦的吸附砷富集能力。為了獲得低結晶度水鈉錳礦對砷的最大吸附量，我們利用吸附動力學實驗評估砷吸附於水鈉錳礦的平衡時間，指出反應平衡需長達1-2個星期，將其結果應用於等溫吸附實驗取得低結晶度水鈉錳礦對砷的最大吸附量為12.94 mg/g (pH 7的中性環境)，其值大於共沉澱實驗所呈現的吸附能力(0.83 mg/g)，證實共沉澱實驗低估水鈉錳礦對砷之吸附能力。等溫吸附實驗採用高砷濃度的條件 (50-250 ppm)，促使水鈉錳礦的吸附砷快速飽和；然而，嘉南平原地下水的砷濃度遠低於等溫實驗的條件，普遍低於0.5 ppm，因此我們亦於近似於地下水的砷濃度條件進行低砷吸附實驗 (0.5 ppm)，發現低結晶度水鈉錳礦富集之吸附砷在兩周內快速上升1.42 mg/g，三個月後則緩慢增加至1.52 mg/g，顯示其吸附能力並未如高砷吸附實驗所呈現於短時間內達到飽和 (12.94 mg/g)，暗示水鈉錳礦的吸附砷在地下水條件可能未飽和，因此在評估該礦物對吸附砷之影響時，應同時考量吸附砷未飽和之情形。
水鈉錳礦的砷吸收能力與As/Mn比值將應用於嘉義新生地區沉積物，以量化水鈉錳礦對於沉積物中砷之影響。根據新生沉積物所提供的萃取資料，討論的重點將分為吸附砷與沉澱相砷。水鈉錳礦對於吸附砷的貢獻將透過水鈉錳礦的豐度與砷吸附能力來進行評估，若沉積物中的錳氧化物皆為水鈉錳礦且吸附砷飽和 (12.94 mg/g)，其所能供給的最大吸附砷含量為0.0022-0.0168 mg，與沉積物既有之吸附砷相比，其佔比為3-121%。然而，若考慮現今地下水的砷濃度，假設水鈉錳礦的吸附砷未飽和 (1.52 mg/g)，其所能貢獻的吸附砷佔比至多為11%；水鈉錳礦對於沉澱砷的貢獻則藉由水鈉錳礦的含量與As/Mn比值 (0.000095)進行評估，本研究已證實錳氧化物於草酸/草酸銨溶解，因此水鈉錳礦所能富集的沉澱砷將與草酸/草酸銨萃取的沉澱砷含量相比，藉由計算結果顯示水鈉錳礦所能富集的沉澱砷含量為0.00002-0.00012 mg，其佔比小於7%，顯示草酸/草酸銨溶劑萃取的沉澱砷更可能源於非晶質的鐵氫氧化物。若水鈉錳礦自嘉南平原地下水富集砷，不論是吸附或是沉澱砷，其貢獻量皆不足以代表沉積物既有之砷，凸顯出錳氧化物並非沉積物中砷之主要富集者。然而，錳氧化物的貢獻不能僅依據沉積物評斷，雖水鈉錳礦並非為砷的主要富集者，但透過Eh-pH圖顯示該礦物於地下水系統並非穩定相含錳礦物相，揭示該礦物易溶解，顯示錳氧化物更可能為釋放者的角色。

Models for arsenic cycling in surface environments generally invoked goethite (FeOOH) as a major arsenic host with its reductive dissolution contributing to the arsenic budget in groundwater. Although other arsenic hosts and processes were also considered in arsenic cycling, their roles have not been as intensively addressed as that of goethite. Among them, Mn oxides are of particular important for their abundance in sediments and soils, high adsorption capacity and dissolution potential. To improve our understanding on the role of Mn oxides on arsenic cycling, we synthesized four Mn oxides from solutions containing 0.1-50 ppm arsenic. They are hausmannite (Mn₃O₄), hollandite (α-MnO₂), hexagonal H-birnessite (low crystallinity δ-MnO₂), and triclinic Na-birnessite (high crystallinity δ-MnO₂). The products were analyzed with an arsenic sequential extraction procedure (SEP) to quantify arsenic sorption as a whole and the proportions of adsorption and co-precipitation as well as to reveal their arsenic release potentials. In addition, adsorption experiments on hexagonal H-birnessite were carried out to determine its maximum arsenic adsorption capacity.
Inconsistent with the designation of most arsenic SEP protocols that have Mn oxides extracted mainly by 1 N HCl solution, all the four synthesized Mn oxides nearly remained intact upon reacting with HCl, except of the 10-50% dissolution from hausmannite. Although the subsequent oxalic acid extraction completely dissolved the synthesized hollandite after three repetitions, it only recovered 60–65% and 40–45% of the bulk Mn from the hexagonal H-birnessite and triclinic Na-birnessite, respectively. Stronger ascorbic acid extraction designated for FeOOH dissolution was required to complete the SEP for birnessites. Apparently, the current arsenic SEPs cannot resolve the contributions from FeOOH and Mn oxides. Arsenic release rate was not in proportion to the dissolution rate of the MnO₂ hosts. About one-third of the co-precipitated arsenic in the hollandite persisted until the third repetition of the oxalic acid extraction that dissolved the final 10–15% Mn. In contrast, over 90% of co-precipitated arsenic in the low crystallinity birnessite was released during the first and second repetitions of the oxalic acid extraction that recovered only ~60% of Mn, implying a higher arsenic release potential.
Among the four synthesized MnO₂, hexagonal H-birnessite (low crystallinity δ-MnO₂) has the highest arsenic sorption capacity that increased from 3.6 to 2140 μg As/1 g Mn with the arsenic concentration in the initial solutions increasing from 0.1 to 50 ppm. The arsenic sorption capacity of the synthesized hollandite (α-MnO₂) and hausmannite (Mn₃O₄) was half of that of the hexagonal H-birnessite, for all the five corresponding initial arsenic concentrations. However, as the initial arsenic concentration increased from 0.1 to 50 ppm, the arsenic adsorption:co-precipitation proportion of the synthesized hollandite remained at a nearly constant ratio of 40:60, whereas that of the hexagonal H-birnessite increased from 0:100 to 25:75 to 30:70, and to 40:60 for < 1 ppm, 1 ppm, 10 ppm, and 50 ppm initial arsenic, respectively. In contrast, the high crystallinity birnessite was characterized by a low adsorption capacity with nearly nil co-precipitated arsenic. Since hausmannite partially dissolved in NaH₂PO₄ extraction, the adsorbed and precipitated arsenic cannot be further separated.
The birnessite adsorption experiment used initial arsenic concentrations up to 250 ppm and revealed a maximum adsorption capacity of 12.9 mg/g at pH 7. However, the inference based on the maximum adsorption capacity might overemphasize the role of adsorption/desorption on arsenic cycling. This is particularly pertinent to groundwater that contains arsenic abundance lower than those in the isotherm experiments. To better simulate the natural systems, we performed adsorption experiments using low arsenic concentrations of 0.5 ppm and a low solid/liquid ratio of 0.01–0.001. The resulting adsorption capacity increased rapidly to 1.42 mg/g in two weeks and then gradually rose to 1.52 mg/g three months later. Compared to those from high arsenic experiment, the relatively low adsorption capacity from low arsenic experiments reflected under-saturation of adsorption sites.
Finally, the model calculations using the experimental and natural As/Mn ratios and arsenic adsorption capacity indicated that birnessite was not the major arsenic sink in the Hsin-Sheng core sediments for their low Mn content. However, birnessite still has a high arsenic release potential, given its high susceptibility and reactivity in the natural pH-Eh conditions.

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