Synthesis and phase transformations involving scorodite, ferric arsenate and arsenical ferrihydrite: Implications for arsenic mobility
Synthesis and phase transformations involving scorodite, ferric arsenate and arsenical ferrihydrite: Implications for arsenic mobility
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DOI：
10.1016/j.gca.2008.03.012
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发表时间：
2008-06
影响因子：
5
通讯作者：
D. Paktunc;J. Dutrizac;V. Gertsman
中科院分区：
文献类型：
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作者：
D. Paktunc;J. Dutrizac;V. Gertsman
臭葱石、砷酸铁和含砷水铁矿是广泛存在于环境中的重要砷载体，也是冶金工业中用于控制废水中砷的常见沉淀物。这些化合物的溶解度和稳定性是有争议的，因为它们在非均相介质中的识别和表征的复杂性。为了深入了解臭葱石、砷酸铁和水铁矿的形成，在70°C和pH 1、2、3和4.5下，从还含有0.02-0.2M Na 2 HAsO 4的0.2M Fe（SO 4）1.5溶液中进行了一系列合成实验。用透射电子显微镜、X射线衍射和X射线吸收精细结构技术对沉淀物进行了表征。砷酸铁是臭葱石形成的前驱体，其特征在于XRD图谱上有两个宽的弥散峰，结构式为FeAsO 4·4- 7 H2O。正如As XAFS和Fe XAFS所定义的那样，砷酸铁的局部结构与臭葱石的结构有很大的不同。据推测，砷酸铁的结构是由单链的角共享的Fe（O，OH）6八面体与桥砷酸盐四面体交替沿链沿着。臭葱石从Fe/As摩尔比为1的溶液中在1-4.5的pH范围内沉淀。pH强烈地控制臭葱石的形成及其从砷酸铁转化的动力学。通过熟化和聚集，臭葱石的晶粒尺寸从7 nm增加到33 nm。在pH 4.5下由Fe/As摩尔比为1至4的溶液连续合成得到的砷酸盐，类似于文献中称为砷酸铁、含砷水铁矿和富As水合氧化铁的化合物，代表砷酸铁和水铁矿的可变混合物。当Fe/As比增加时，水铁矿的比例以砷酸铁为代价增加。砷酸盐吸附似乎阻碍铁的沉淀物中的水铁矿生长的摩尔Fe/As比为1-4，而增加反应逐渐转变为六线铁的Fe/As比为5和更大的两线水铁矿。
Scorodite, ferric arsenate and arsenical ferrihydrite are important arsenic carriers occurring in a wide range of environments and are also common precipitates used by metallurgical industries to control arsenic in effluents. Solubility and stability of these compounds are controversial because of the complexities in their identification and characterization in heterogeneous media. To provide insights into the formation of scorodite, ferric arsenate and ferrihydrite, series of synthesis experiments were carried out at 70°C and pH 1, 2, 3 and 4.5 from 0.2M Fe(SO4)1.5solutions also containing 0.02–0.2M Na2HAsO4. The precipitates were characterized by transmission electron microscopy, X-ray diffraction and X-ray absorption fine structure techniques. Ferric arsenate, characterized by two broad diffuse peaks on the XRD pattern and having the structural formula of FeAsO4·4–7H2O, is a precursor to scorodite formation. As defined by As XAFS and Fe XAFS, the local structure of ferric arsenate is profoundly different than that of scorodite. It is postulated that the ferric arsenate structure is made of single chains of corner-sharing Fe(O,OH)6octahedra with bridging arsenate tetrahedra alternating along the chains. Scorodite was precipitated from solutions with Fe/As molar ratios of 1 over the pH range of 1–4.5. The pH strongly controls the kinetics of scorodite formation and its transformation from ferric arsenate. The scorodite crystallite size increased from 7 to 33nm by ripening and aggregation. Precipitates, resulting from continuous synthesis at pH 4.5 from solutions having Fe/As molar ratios ranging from 1 to 4 and resembling the compounds referred to as ferric arsenate, arsenical ferrihydrite and As-rich hydrous ferric oxide in the literature, represent variable mixtures of ferric arsenate and ferrihydrite. When the Fe/As ratio increases, the proportion of ferrihydrite increases at the expense of ferric arsenate. Arsenate adsorption appears to retard ferrihydrite growth in the precipitates with molar Fe/As ratios of 1–4, whereas increased reaction gradually transforms two-line ferrihydrite to six-line ferrihydrite at Fe/As ratios of 5 and greater.