Alteration, adsorption and nucleation processes on clay–water interfaces: Mechanisms for the retention of uranium by altered clay surfaces on the nanometer scale
Alteration, adsorption and nucleation processes on clay–water interfaces: Mechanisms for the retention of uranium by altered clay surfaces on the nanometer scale
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DOI：
10.1016/j.gca.2014.12.020
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发表时间：
2015-03
影响因子：
5
通讯作者：
M. Schindler;Christine Legrand;M. Hochella
中科院分区：
文献类型：
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作者：
M. Schindler;Christine Legrand;M. Hochella
粘土矿物固-水界面上的纳米尺度过程控制着金属在环境中的流动性。这些过程可能发生在粘土缓冲层和屏障的有限孔隙空间以及受污染的沉积物中，涉及多种物质和相的蚀变、吸附和成核过程的组合。这项研究的特点是纳米级的粘土矿物和铀酰溶液之间的界面上的过程接近中性pH值。样品的粘土矿物接触pH值为6.7的收集从一个U磨和矿山尾矿在基湖，萨斯喀彻温省，加拿大。尾矿中含有Cu、As、Co、Mo、Ni、Se等多金属相，并含有过量的Ca（OH）2和Na 2CO 3消石灰。少量的含硫酸和铀的研磨过程溶液偶尔会排放到尾矿表面，排放后通过与熟石灰反应而被中和。透射电子显微镜（TEM）结合聚焦离子束（FIB）技术和其他分析方法（SEM、XRD、XRF和ICP-OES）用于表征粘土矿物蒙脱石和高岭石的受限孔隙空间内以及周围尾矿材料中的相的化学和矿物学组成。粘土矿物周围的蚀变带的特征是不同世代的次生硅酸盐，其中含有不同比例的吸附铀酰和砷酸盐物种，以及硅酸盐与铀酰矿物铜铅矿Cu[（UO 2）2（SiO 3OH）2]（H2O）6和变沸石Cu[（UO 2）（AsO 4）2]（H2O）8的共生。大多数蚀变相，如伊利石、伊利石-蒙皂石、高岭石和蛭石，最有可能形成于铀矿存款的沉积盆地中，并且含有少量的Fe（<5 at.%）。富铁铝硅酸盐或伊利石-蒙脱石（Fe >10 at.%）在高接触pH值（≤ 10.5）下，最有可能在石灰尾矿中形成，其结构特征是长程有序度低。铀的吸附和成核的偏沸石和铜铬榴石强烈控制的存在下，吸附的含氧阴离子物种砷酸盐和二氧化硅上的富铁硅酸盐。铀酰矿物纳米晶体的非均质成核最可能发生在二元铀酰、砷酸盐和二氧化硅络合物的吸附位点上，以及三元铀酰砷酸盐或铀酰硅酸盐络合物上。铀酰矿物作为取向错误的纳米尺寸晶体的聚集体出现，并且是过饱和溶液和大量成核位点的结果，这些成核位点阻止了通过奥斯瓦尔德熟化形成更大的晶体。本研究的结果提供了一个了解的界面纳米级的过程之间的铀酰物种和改变粘土缓冲区在潜在的核废物处置库的粘土类似的蚀变条件可能会发生在一个多屏障系统。
Nano-scale processes on the solid–water interface of clay minerals control the mobility of metals in the environment. These processes can occur in confined pore spaces of clay buffers and barriers as well as in contaminated sediments and involve a combination of alteration, adsorption and nucleation processes of multiple species and phases. This study characterizes nano-scale processes on the interface between clay minerals and uranyl-bearing solution near neutral pH. Samples of clay minerals with a contact pH of ∼6.7 are collected from a U mill and mine tailings at Key Lake, Saskatchewan, Canada. The tailings material contains Cu-, As-, Co-, Mo-, Ni-, Se-bearing polymetallic phases and has been deposited with a surplus of Ca(OH)2and Na2CO3slaked lime. Small volumes of mill-process solutions containing sulfuric acid and U are occasionally discharged onto the surface of the tailings and are neutralized after discharge by reactions with the slaked lime. Transmission electron microscopy (TEM) in combination with the focused ion beam (FIB) technique and other analytical methods (SEM, XRD, XRF and ICP-OES) are used to characterize the chemical and mineralogical composition of phases within confined pore spaces of the clay minerals montmorillonite and kaolinite and in the surrounding tailings material. Alteration zones around the clay minerals are characterized by different generations of secondary silicates containing variable proportions of adsorbed uranyl- and arsenate-species and by the intergrowth of the silicates with the uranyl-minerals cuprosklodowskite, Cu[(UO2)2(SiO3OH)2](H2O)6and metazeunerite, Cu[(UO2)(AsO4)2](H2O)8. The majority of alteration phases such as illite, illite–smectite, kaolinite and vermiculite have been most likely formed in the sedimentary basin of the U-ore deposit and contain low amounts of Fe (<5 at.%). Iron-enriched Al-silicates or illite–smectites (Fe >10 at.%) formed most likely in the limed tailings at high contact pH (∼10.5) and their structure is characterized by a low degree of long-range order. Adsorption of U and nucleation of metazeunerite and cuprosklodowskite are strongly controlled by the presence of the adsorbed oxy-anion species arsenate and silica on the Fe-enriched silicates. Heterogeneous nucleation of nano-crystals of the uranyl minerals occurs most likely on adsorption sites of binary uranyl-, arsenate- and silica-complexes as well as on ternary uranyl–arsenate or uranyl–silicate complexes. The uranyl minerals occur as aggregates of misoriented nano-size crystals and are the result of supersaturated solutions and a high number of nucleation sites that prevented the formation of larger crystals through Oswald ripening. The results of this study provide an understanding of interfacial nano-scale processes between uranyl species and altered clay buffers in a potential Nuclear Waste repository as similar alteration conditions of clays may occur in a multi-barrier system.