The role of nanoscale aggregation of ferrihydrite and amorphous silica in the natural attenuation of contaminant metals at mill tailings sites
The role of nanoscale aggregation of ferrihydrite and amorphous silica in the natural attenuation of contaminant metals at mill tailings sites
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DOI：
10.1016/j.gca.2021.02.004
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发表时间：
2021-04
影响因子：
5
通讯作者：
Keisuke Kawamoto;H. Yokoo;Asumi Ochiai;Yuriko Nakano;Ayaka Takeda;Takumi Oki;M. Takehara;M. Uehara;K. Fukuyama;Y. Ohara;T. Ohnuki;M. Hochella;S. Utsunomiya
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文献类型：
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作者：
Keisuke Kawamoto;H. Yokoo;Asumi Ochiai;Yuriko Nakano;Ayaka Takeda;Takumi Oki;M. Takehara;M. Uehara;K. Fukuyama;Y. Ohara;T. Ohnuki;M. Hochella;S. Utsunomiya
水铁矿是一种含水氧化铁，在富含二氧化硅的矿山排水场中普遍存在，尽管关于其纳米级纹理和胶体性质（如其与无定形二氧化硅的关联以及与污染金属的反应）仍存在一些不确定性。为了了解由水铁矿和无定形二氧化硅聚集体控制的污染物自然衰减的机制，利用许多传统的溶液和质谱方法，以及同步加速器为基础的方法，FIB和分析TEM为基础的技术，详细研究了来自日本Ningyo-toge铀存款的尾矿材料。在该地点，过去的采矿活动导致微酸性矿井排水和具有相对高浓度的有毒元素（包括U、As和Ra）的尾矿池污染。我们系统地研究了水、胶体和沉积物的地球化学，这些水来自于从前矿场（现已修复）排水的地下水，向下倾斜至水pH值保持在6.2左右的尾矿池。溶解氧（DO）为0.18 mg/L的地下水暴露于大气中后，迅速氧化至DO为1.65 mg/L，溶解的Fe 2+瞬间氧化形成直径大于200 nm的水铁矿纳米颗粒聚集体，吸附溶液中总As（3.36 µmol/L）的约67%。对于通过200 nm过滤器的溶液和颗粒/聚集体，1.17 µmol/L As的约17%吸附在这些较小的水铁矿聚集体上。同时，观察到不同数量的无定形二氧化硅与粒径小到几个纳米吸附到水铁矿聚集体。砷主要以砷酸盐As（V）的形式存在，并优先吸附在这些铁铁矿-无定形二氧化硅聚集体中的水铁矿上。当Si/Fe = 0.25时，SiO_2吸附对砷的吸附没有抑制作用。铀也被吸附在胶体的聚集体上。在静态暴露于环境大气24小时后，在含氧水中形成的聚集体的zeta电位从-16 mV降至-24 mV，接近纯二氧化硅的zeta电位，可能导致胶体稳定性增加。一些大尺寸的聚集体沉积在收集井的底部并缓慢地吸附少量的Mn（1.45wt.%）;其它聚集体被输送到下游的研磨尾矿池，在那里水被进一步氧化（DO = 3.09 mg/L），并与水钠锰矿一起沉积在沉积物上，水钠锰矿是一种水合锰氧化物，在这种情况下似乎是生物源的。在沉积过程中，聚集体内的无定形二氧化硅的协会在保留吸附有毒元素如As的水铁矿结构中发挥关键作用。因此，在纳米级的自然衰减机制的限制的铁氧体-无定形二氧化硅聚集体的化学和物理性质是至关重要的，在Ningyo-toge厂尾矿网站。它表明，它也是重要的，在其他矿山排水和磨尾场地，由于无处不在的和占主导地位的发生在这些类型的设置中的Si和Fe。
Ferrihydrite, a hydrous ferric oxide, is ubiquitous in silica-rich mine drainage sites, although some uncertaity remains concerning its nanoscale texture and colloidal properties such as its association with amorphous silica and reactions with contaminant metals. To understand the mechanisms of natural attenuation of contaminants governed by ferrihydrite and amorphous silica aggregations, mine tailings material from the Ningyo-toge uranium deposit, Japan, has been investigated in detail utilizing many traditional solution and mass spectroscopy methods, as well as synchrotron-based methods the latest in FIB- and analytic TEM-based technologies. At this site, the past mining activity results in slightly acidic mine drainage and the contamination of a mill tailings pond with relatively high concentrations of toxic elements, including U, As, and Ra. We have systematically investigated the geochemistry of water, colloids, and sediments from the groundwater draining the former mine site (now remediated), down gradient to the mill tailings pond where the water pH remains ∼6.2. Groundwater with 0.18 mg/L dissolved oxygen (DO) is oxygenated rapidly to levels of 1.65 mg/L DO when exposed to the atmosphere along this flow path. Dissolved Fe2+is oxidized instantly to form ferrihydrite nanoparticle aggregates > 200 nm in diameter, adsorbing ∼67% of the total As (3.36 µmol/L) in solution. For solutions and particles/aggregates passing through 200-nm filters, ∼17% of the 1.17 µmol/L As is sorbed on these smaller ferrihydrite aggregates. Simultaneously, varying amounts of amorphous silica with particle sizes as small as several nanometers were observed to adsorb onto ferrihydrite aggregates. Arsenic is present predominantly as arsenate, As(V), and preferentially adsorbs to ferrihydrite within these ferrihydrite–amorphous silica aggregates. Arsenate adsorption is not inhibited by silica adsoption on ferrihydrite when the Si/Fe composition is ∼0.25. Uranium is also adsorbed onto aggregates of colloids. After 24 h of static exposure to the ambient atmosphere, the zeta potential of the aggregate that formed in oxygenated water decreases from −16 to −24 mV, near the zeta potential of pure silica, potentially leading to increased colloidal stability. Some large-sized aggregates are deposited at the bottom of the collecting well and slowly adsorb minor amounts of Mn (1.45 wt.%); other aggregates are transported to the mill tailings pond downsteam, where the water is further oxidized (DO = 3.09 mg/L), and deposited onto the sediment together with birnessite, a hydrated Mn-oxide that appears to be biogenic in this case. During sedimentation, associations of amorphous silica within the aggregate play a key role in retaining the ferrihydrite structure that adsorbs toxic elements such as As. Consequently, the mechanism of natural attenuation at the nanoscale constrained by the chemical and physical properties of ferrihydrite–amorphous silica aggregates is of critical importance in the Ningyo-toge mill tailing site. It is shown that it is also important in other mine drainage and mill tailings sites due to the ubiquitous and dominant occurrence of both Si and Fe in these types of settings.