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1.
<正>水铁矿(Fh)是一种铁的羟基氧化物,其结晶程度较弱,具有颗粒粒径小、比表面积大、分布较广等的特点。水铁矿广泛分布于含Fe2+或Fe3+的河流、泉水、淡水湖泊等地表水中,中性或偏碱性的水体更有利于水铁矿的形成。水铁矿在水系沉积物、土壤、岩层中也大量存在[1]。水铁矿表面带有大量的可变电荷,具有吸附环境中污染物的能力,再加上其较大的比表面积和较高的表面活性,因此,可以通过人工合成纳米水铁矿来吸附环境中的重金属污染物,同时利用重金属-铁共沉淀法来去除水体中的重金属,其效果  相似文献   

2.
从氧原子最紧密堆积以及阳离子充填四面体和八面体空隙原理出发 ,以简单氧化物矿物最紧密堆积结构类型金绿宝石、尖晶石、铁钒矿为基础 ,深入讨论了复杂氧化物矿物最紧密堆积结构类型彭志忠石、尼日利亚石、塔菲石、黑铝镁铁 (钛 )矿等晶体结构构筑原理。以O表示全部为阳离子八面体配位的层 ;以T层表示阳离子八面体配位与阳离子四面体配位的混合层 ,其中T1表示阳离子八面体配位与一种方向阳离子四面体配位的混合层 ,T2 表示阳离子八面体配位与两种方向阳离子四面体配位的混合层。这类矿物晶体结构可用O、T1、T2 堆积方式表征 ,O层与T层交替排列。如 :彭志忠石 ( 6H)的晶体结构表示为…OT2 OT1OT1… ,塔菲石 ( 8H)的晶体结构表示为…OT2 OT1OT2 OT1… ,尼日利亚石 ( 2 4R)的晶体结构表示为…OT1OT2 OT2 OT1…× 3 ,等等 ;它们的晶体结构中既有尖晶石的…OT2 OT2 …晶体结构单位 ,又有铁钒矿的…OT1OT1…晶体结构单位。  相似文献   

3.
叶绿矾是具有岛状基本结构的硫酸盐类矿物,对产于新疆东天山北部红山矿区硫化物矿床氧化带中的叶绿矾进行单晶衍射实验发现,该叶绿矾具有明显的超结构弱衍射点,利用包括弱衍射点在内的全部可观察点求得的超晶胞参数为:a=1.49441(7)nm,b=1.83429(9)nm,c=1.14507(6)nm,α=93.2390(10)°,β=112.0330(10)°,γ=98.2800(10)°,V=2.8583(2)nm3,空间群为P1,Z=3。去掉弱衍射点后利用强衍射点得到的最小亚晶胞参数是a=0.737121(4)nm,b=1.83079(11)nm,c=0.73106(4)nm,α=93.9250(10)°,β=102.7252(10)°,γ=98.9463(10)°,V=0.947321(2)nm3,空间群为P1,Z=1。超晶胞与亚晶胞相比体积增加了2倍,其b轴的方向和大小基本不变,a和c轴产生了差异。该叶绿矾的晶体化学式为Fe0.656Fe4(SO4)6(OH)2·19.4H2O,属高铁叶绿矾。结构中,部分Fe-O八面体和SO4四面体联合形成的复合链,并沿[102]方向分布,形成一种稳定的骨干结构;部分孤立Fe-O八面体,位于链间,阳离子相当于晶体化学式中A位置的离子,通常有缺失。链和孤立八面体的分布均平行(010)面网,分别构成链层和八面体层,并按2∶1的比例堆垛,结构单元之间全部由氢键相连。超结构产生的原因是电价补偿机制造成的孔道中Fe的位置分离和占位差异造成。  相似文献   

4.
<正>铁(氢)氧化物在土壤、沉积物和水相颗粒物中分布广泛,表面氧化还原活性高,以及电子输运能力强,对环境污染物(重金属和有机污染物)的地球化学过程起着重要的控制作用。相比于常见的针铁矿和赤铁矿等含铁矿物,磁铁矿在吸附-转化污染物方面具有一些独特的结构优势:1)磁铁矿表面的Fe2+具有强还原性,能通过矿物表面或内部结构向有机物、重金属传递电子,使污染物还原;2)磁铁矿具有反尖晶石结构,八面体位同时被Fe2+和Fe3+占据,电子在这两种氧化态之间迅速转移,赋予磁铁矿良好  相似文献   

5.
针铁矿-四方纤铁矿-水体系氧同位素分馏的实验研究   总被引：3，自引：0，他引：3  
针铁矿是非常重要的三价铁氧化物之一,其氧同位素组成对于古环境再造具有很大的价值。以4种不同的铁化合物作为Fe3+离子的源物质,于30～120℃范围内,采用强迫水解方法,在不同同位素组成的水中分别实验合成针铁矿和四方纤铁矿。结果表明,以Fe(NO3)3·9H2O、NH4Fe(SO4)2·12H2O、Fe(SO4)3·7H2O为Fe3+源物质合成的是纯针铁矿,而以FeCl3·6H2O为Fe3+源物质合成的是四方纤铁矿。氧同位素分析显示,在30～120℃范围内实验测定的针铁矿-水体系和四方纤铁矿-水体系氧同位素分馏几乎不可区分,并且满足下列分馏关系:103lnα针铁矿-水=9.59×103/T-26.39103lnα四方纤铁矿-水=8.85×103/T-24.44实验测定的针铁矿-水体系氧同位素分馏不仅与前人实验结果一致,而且与增量方法理论计算相近。由于实验采用不同反应途径得到了一致的分馏结果,因此所测定的针铁矿-水体系氧同位素分馏代表了热力学平衡。  相似文献   

6.
作为表生土壤环境中易生成且分布广泛的氧化锰矿物,水锰矿(γ-MnOOH)能参与铁氧化物的生成过程,影响Fe_(2+)的迁移、转化和归趋。本文考察了pH值为3.0~7.0的模拟水溶液体系中水锰矿与Fe_(2+)的相互作用及其生成铁氧化物的过程,分析了Fe_(2+)浓度、pH值和空气(氧气)对Fe(Ⅲ)氧化物晶体结构类型、化学组成和反应速率的影响。研究结果表明,水锰矿氧化Fe_(2+)产物主要为针铁矿和纤铁矿;pH值为3.0~5.0时产物为针铁矿,而pH值为7.0时产物为针铁矿与纤铁矿的混合物,且高浓度Fe_(2+)会促使纤铁矿生成;引入空气利于针铁矿生成;反应速率随着pH值升高、氧气分压的增大而加快。本工作丰富了对铁氧化物在常见锰氧化物表面形成和转化过程的认识。  相似文献   

7.
白云母的富铁变种已知有两个:铁白云母(Fonimuscouit)和铁硅白云母(Feniphonght)铁白云母为富含Fe~(3 )的白云母,结晶化学式为KAlFe~(3 )(AlSi_3O_10)(OH)2、铁硅白云母为富含铁的多硅白云母结晶化学式为KAl(Fe~(3 ),Fe(2 ),Mg)(Al_3Si_3O_19)。我们于1981年在浙江昌化含钨被石英脉晶  相似文献   

8.
砷是土壤中重要的(类)重金属污染物,其毒性主要取决于在环境中的形态及氧化还原状态。游离态Fe(Ⅱ)(Fe(Ⅱ)_(aq))驱动铁(氢)氧化物晶相重组过程是土壤铁循环的重要组成,对土壤中重金属的吸附、固定、钝化等环境行为有重要影响。本研究采用~(57)Fe稳定同位素示踪方法研究厌氧条件下Fe(Ⅱ)_(aq)驱动针铁矿晶相重组过程中砷的氧化还原及形态变化过程。结果显示,在只有针铁矿存在的对照处理中,针铁矿本身对As(Ⅲ)没有氧化作用,但83%的As(Ⅲ)被吸附到针铁矿表面。在Fe(Ⅱ)_(aq)和针铁矿共存体系中,Fe(Ⅱ)_(aq)可与针铁矿中结构态Fe(Ⅲ)发生铁原子交换,As(Ⅲ)的存在降低了铁原子交换速率。同时,在Fe(Ⅱ)_(aq)驱动针铁矿晶相转化过程中,77%的As(Ⅲ)被氧化成As(Ⅴ),As活性降低。另外,部分吸附在针铁矿表面的As(Ⅲ)和氧化转化后的As(Ⅴ)通过针铁矿的晶格单元包裹或取代Fe结构位的形式被针铁矿结构化固定,从而进一步降低了As的活性。  相似文献   

9.
通过对湖北省富水、南河水库坝基渗漏水及其析出物的研究 ,探讨了不同水库沉积环境中显微针铁矿的成因差异及微生物在铁的生物矿化中的作用和意义。采用化学分析、X射线衍射、红外光谱、穆斯堡尔谱、透射电镜和扫描电镜分析等手段 ,研究了两个水库坝基渗漏水析出物的组成、物相和形貌特征。两个水库坝基渗漏水的成分相似 ,阳离子均以Ca2 + 、Mg2 + 、K+ 、Na+ 为主 ,Fe2 + 和Fe3 + 含量极少 ,阴离子以HCO-3 为主。渗漏水析出物的化学成分以铁的氧化物为主 ,析出物的主要矿物相为针铁矿、石英、伊利石、蒙脱石及微量方解石 ,呈弱结晶状态。穆斯堡尔谱图和SEM显微形貌特征证实了南河水库中针铁矿以化学成因为主 ,系菱铁矿风化而成 ;富水水库中针铁矿的形成受铁细菌的影响 ,以生物成因为主。  相似文献   

10.
安徽姑山矿浆型铁矿床Fe同位素初步研究   总被引：2，自引：0，他引：2  
文章报道了宁芜矿集区内姑山矿浆型铁矿床中的铁氧化物、辉石闪长玢岩和赋矿围岩的Fe同位素组成,其δ57Fe的总体分布范围为-0.05‰~0.79‰。结果显示,姑山铁矿床的铁氧化物赤铁矿和镜铁矿均比硅酸盐岩浆结晶产物(辉石闪长岩)富集重的Fe同位素,并且硅酸盐岩浆的Fe同位素组成比已报道的火山岩的平均Fe同位素组成更富集轻的Fe同位素,表明在岩浆不混溶的过程中Fe同位素发生了分馏,富铁熔体相对富集重的Fe同位素,而硅酸盐熔体相对富集轻的Fe同位素;相对于赋矿地层(黄马青组石英砂岩)和辉石闪长玢岩,赤铁矿和镜铁矿更富集重的Fe同位素,围岩地层和闪长岩岩体则富集轻的Fe同位素。因此,姑山铁矿床的铁质不大可能来自于地层或闪长玢岩岩体,而主要来源于深部岩浆房。  相似文献   

11.
Ferrihydrite (2.5 Fe2O2-4.5 H2O) is an unstable colloidal mineral. It dissolves in highly alkaline solutions and is precipitated from them in the form of goethite. Jarosite is stable at very low pH but is decomposed at higher values of pH with separation of iron oxides. Experiments show that in rapid decomposition of jarosite a protohematite substance, ferrihydrite, is formed. This transformation occurs at moderate pH values when solutions percolate through the aggregates of jarosite. Ferrihydrite, an unstable colloidal hydrated oxide of ferric iron, changes spontaneously to stable hematite with time. Very slow decomposition of jarosite results in its replacement by iron hydroxide, goethite. Under laboratory conditions in alkaline solutions lepidocrocite may be obtained from jarosite. The synthesis of this iron hydroxide passes through a stage of intermediate products: ferrihydrite and hydrated ferric oxide - ferriprotolepidocrocite, formed by solution of ferrihydrite in strongly alkaline solutions. The transformation of ferriprotolepidocrocite into lepidocrocite may be regarded as a topotactic reaction. —Authors.  相似文献   

12.
Due to the strong reducing capacity of ferrous Fe, the fate of Fe(II) following dissimilatory iron reduction will have a profound bearing on biogeochemical cycles. We have previously observed the rapid and near complete conversion of 2-line ferrihydrite to goethite (minor phase) and magnetite (major phase) under advective flow in an organic carbon-rich artificial groundwater medium. Yet, in many mineralogically mature environments, well-ordered iron (hydr)oxide phases dominate and may therefore control the extent and rate of Fe(III) reduction. Accordingly, here we compare the reducing capacity and Fe(II) sequestration mechanisms of goethite and hematite to 2-line ferrihydrite under advective flow within a medium mimicking that of natural groundwater supplemented with organic carbon. Introduction of dissolved organic carbon upon flow initiation results in the onset of dissimilatory iron reduction of all three Fe phases (2-line ferrihydrite, goethite, and hematite). While the initial surface area normalized rates are similar (∼10−11 mol Fe(II) m−2 g−1), the total amount of Fe(III) reduced over time along with the mechanisms and extent of Fe(II) sequestration differ among the three iron (hydr)oxide substrates. Following 16 d of reaction, the amount of Fe(III) reduced within the ferrihydrite, goethite, and hematite columns is 25, 5, and 1%, respectively. While 83% of the Fe(II) produced in the ferrihydrite system is retained within the solid-phase, merely 17% is retained within both the goethite and hematite columns. Magnetite precipitation is responsible for the majority of Fe(II) sequestration within ferrihydrite, yet magnetite was not detected in either the goethite or hematite systems. Instead, Fe(II) may be sequestered as localized spinel-like (magnetite) domains within surface hydrated layers (ca. 1 nm thick) on goethite and hematite or by electron delocalization within the bulk phase. The decreased solubility of goethite and hematite relative to ferrihydrite, resulting in lower Fe(III)aq and bacterially-generated Fe(II)aq concentrations, may hinder magnetite precipitation beyond mere surface reorganization into nanometer-sized, spinel-like domains. Nevertheless, following an initial, more rapid reduction period, the three Fe (hydr)oxides support similar aqueous ferrous iron concentrations, bacterial populations, and microbial Fe(III) reduction rates. A decline in microbial reduction rates and further Fe(II) retention in the solid-phase correlates with the initial degree of phase disorder (high energy sites). As such, sustained microbial reduction of 2-line ferrihydrite, goethite, and hematite appears to be controlled, in large part, by changes in surface reactivity (energy), which is influenced by microbial reduction and secondary Fe(II) sequestration processes regardless of structural order (crystallinity) and surface area.  相似文献   

13.
Characterization of Fe(III) (hydr)oxides in soils near the Ichinokawa mine was conducted using X-ray absorption fine structure (XAFS) and Mössbauer spectroscopies, and the structural changes were correlated with the release of As into pore-water. The Eh values decreased monotonically with depth. Iron is mainly present as poorly-ordered Fe(III) (hydr)oxides, such as ferrihydrite, over a wide redox range (from Eh = 360 to −140 mV). Structural details of the short-range order of these Fe(III) (hydr)oxides were examined using Mössbauer spectroscopy by comparing the soil phases with synthesized ferrihydrite samples having varying crystallinities. The crystallinity of the soil Fe (hydr)oxides decreased slightly with depth and Eh. Thus, within the redox range of this soil profile, ferrihydrite dominated, even under very reducing conditions, but the crystalline domain size, and, potentially, particle size, changed with the variation in Eh. In the soil–water system examined here, where As concentration and the As(III)/As(V) ratio in soil water increased with depth, ferrihydrite persisted and maintained or even enhanced its capacity for As retention with increased reducing conditions. Therefore, it is concluded that As release from these soils largely depends on the transformation of As(V) to As(III) rather than reductive dissolution of Fe(III) (hydr)oxide.  相似文献   

14.
Iron (hydr)oxides not only serve as potent sorbents and repositories for nutrients and contaminants but also provide a terminal electron acceptor for microbial respiration. The microbial reduction of Fe (hydr)oxides and the subsequent secondary solid-phase transformations will, therefore, have a profound influence on the biogeochemical cycling of Fe as well as associated metals. Here we elucidate the pathways and mechanisms of secondary mineralization during dissimilatory iron reduction by a common iron-reducing bacterium, Shewanella putrefaciens (strain CN32), of 2-line ferrihydrite under advective flow conditions. Secondary mineralization of ferrihydrite occurs via a coupled, biotic-abiotic pathway primarily resulting in the production of magnetite and goethite with minor amounts of green rust. Operating mineralization pathways are driven by competing abiotic reactions of bacterially generated ferrous iron with the ferrihydrite surface. Subsequent to the initial sorption of ferrous iron on ferrihydrite, goethite (via dissolution/reprecipitation) and/or magnetite (via solid-state conversion) precipitation ensues resulting in the spatial coupling of both goethite and magnetite with the ferrihydrite surface. The distribution of goethite and magnetite within the column is dictated, in large part, by flow-induced ferrous Fe profiles. While goethite precipitation occurs over a large Fe(II) concentration range, magnetite accumulation is only observed at concentrations exceeding 0.3 mmol/L (equivalent to 0.5 mmol Fe[II]/g ferrihydrite) following 16 d of reaction. Consequently, transport-regulated ferrous Fe profiles result in a progression of magnetite levels downgradient within the column. Declining microbial reduction over time results in lower Fe(II) concentrations and a subsequent shift in magnetite precipitation mechanisms from nucleation to crystal growth. While the initial precipitation rate of goethite exceeds that of magnetite, continued growth is inhibited by magnetite formation, potentially a result of lower Fe(III) activity. Conversely, the presence of lower initial Fe(II) concentrations followed by higher concentrations promotes goethite accumulation and inhibits magnetite precipitation even when Fe(II) concentrations later increase, thus revealing the importance of both the rate of Fe(II) generation and flow-induced Fe(II) profiles. As such, the operating secondary mineralization pathways following reductive dissolution of ferrihydrite at a given pH are governed principally by flow-regulated Fe(II) concentration, which drives mineral precipitation kinetics and selection of competing mineral pathways.  相似文献   

15.
Reduction of As(V) and reductive dissolution and transformation of Fe (hydr)oxides are two dominant processes controlling As retention in soils and sediments. When developed within soils and sediments, Fe (hydr)oxides typically contain various impurities—Al being one of the most prominent—but little is known about how structural Al within Fe (hydr)oxides alters its biotransformation and subsequent As retention. Using a combination of batch and advective flow column studies with Fe(II) and Shewanella sp. ANA-3, we examined (1) the extent to which structural Al influences reductive dissolution and transformations of ferrihydrite, a highly reactive Fe hydroxide, and (2) the impact of adsorbed As on dissolution and transformation of (Al-substituted) ferrihydrite and subsequent As retention. Structural Al diminishes the extent of ferrihydrite reductive transformation; nearly three-orders of magnitude greater concentration of Fe(II) is required to induce Al-ferrihydrite transformation compared to pure two-line ferrihydrite. Structural Al decreases Fe(II) retention/incorporation on/into ferrihydrite and impedes Fe(II)-catalyzed transformation of ferrihydrite. Moreover, owing to cessation of Fe(II)-induced transformation to secondary products, Al-ferrihydrite dissolves (incongruently) to a greater extent compared to pure ferrihydrite during reaction with Shewanella sp. ANA-3. Additionally, adsorption of As(V) to Al-ferrihydrite completely arrests Fe(II)-catalyzed transformation of ferrihydrite, and it diminishes the difference in the rate and extent of ferrihydrite and Al-ferrihydrite reduction by Shewanella sp. ANA-3. Our study further shows that reductive dissolution of Al-ferrihydrite results in enrichment of Al sites, and As(V) reduction accelerates As release due to the low affinity of As(III) on these non-ferric sites.  相似文献   

16.
Pyridine-2,6-bis(monothiocarboxylate) (pdtc), a metabolic product of microorganisms, including Pseudomonas putida and Pseudomonas stutzeri was investigated for its ability of dissolve Fe(III)(hydr)oxides at pH 7.5. Concentration dependent dissolution of ferrihydrite under anaerobic environment showed saturation of the dissolution rate at the higher concentration of pdtc. The surface controlled ferrihydrite dissolution rate was determined to be 1.2 × 10−6 mol m−2 h−1. Anaerobic dissolution of ferrihydrite by pyridine-2,6-dicarboxylic acid or dipicolinic acid (dpa), a hydrolysis product of pdtc, was investigated to study the mechanism(s) involved in the pdtc facilitated ferrihydrite dissolution. These studies suggest that pdtc dissolved ferrihydrite using a reduction step, where dpa chelates the Fe reduced by a second hydrolysis product, H2S. Dpa facilitated dissolution of ferrihydrite showed very small increase in the Fe dissolution when the concentration of external reductant, ascorbate, was doubled, suggesting the surface dynamics being dominated by the interactions between dpa and ferrihydrite. Greater than stoichiometric amounts of Fe were mobilized during dpa dissolution of ferrihydrite assisted by ascorbate and cysteine. This is attributed to the catalytic dissolution of Fe(III)(hydr)oxides by the in situ generated Fe(II) in the presence of a complex former, dpa.  相似文献   

17.
Subterranean estuaries are characterized by the mixing of terrestrially derived groundwater and seawater in a coastal aquifer. Subterranean estuaries, like their river water-seawater counterparts on the surface of the earth, represent a major, but less visible, hydrological and geochemical interface between the continents and the ocean. This article is the first in a two-part series on the biogeochemistry of the subterranean estuary at the head of Waquoit Bay (Cape Cod, MA, USA). The pore-water distributions of salinity, Fe and Mn establish the salt and redox framework of this subterranean estuary. The biogeochemistry of Fe, Mn, P, Ba, U and Th will be addressed from the perspective of the sediment composition. A second article will focus on the groundwater and pore-water chemistries of Fe, Mn, U and Ba.Three sediment cores were collected from the head of Waquoit Bay where the coastal aquifer consists of permeable sandy sediment. A selective dissolution method was used to measure the concentrations of P, Ba, U and Th that are associated with “amorphous (hydr)oxides of iron and manganese” and “crystalline Fe and Mn (hydr)oxides.” The deeper sections of the cores are characterized by large amounts of iron (hydr)oxides that are precipitated onto organic C-poor quartz sand from high-salinity pore waters rich in dissolved ferrous iron. Unlike Fe (hydr)oxides, which increase with depth, the Mn (hydr)oxides display midcore maxima. This type of vertical stratification is consistent with redox-controlled diagenesis in which Mn (hydr)oxides are formed at shallower depths than iron (hydr)oxides. P and Th are enriched in the deep sections of the cores, consistent with their well-documented affinity for Fe (hydr)oxides. In contrast, the downcore distribution of Ba, especially in core 3, more closely tracks the concentration of Mn (hydr)oxides. Even though Mn (hydr)oxides are 200-300 times less abundant than Fe (hydr)oxides in the cores, Mn (hydr)oxides are known to have an affinity for Ba which is many orders of magnitude greater than iron (hydr)oxides. Hence, the downcore distribution of Ba in Fe (hydr)oxide rich sediments is most probably controlled by the presence of Mn (hydr)oxides. U is enriched in the upper zones of the cores, consistent with the formation of highly reducing near-surface sediments in the intertidal zone at the head of the Bay. Hence, the recirculation of seawater through this type of subterranean estuary, coupled with the abiotic and/or biotic reduction of soluble U(VI) to insoluble U(IV), leads to the sediments acting as a oceanic net sink of U. These results highlight the importance of permeable sediments as hosts to a wide range of biogeochemical reactions, which may be impacting geochemical budgets on scales ranging from coastal aquifers to the continental shelf.  相似文献   

18.
Iron oxides may undergo structural transformations when entering an anoxic environment. These transformations were investigated using the isotopic exchange between aqueous Fe(II) and iron oxides in experiments with 55Fe-labelled iron oxides. 55Fe was incorporated congruently into a ferrihydrite, two lepidocrocites (#1 and #2), synthesised at 10°C and 25°C, respectively, a goethite and a hematite. The iron oxides were then submerged in Fe2+ solutions (0-1.0 mM) with a pH of 6.5. In the presence of aqueous Fe2+, an immediate and very rapid release of 55Fe was observed from ferrihydrite, the two lepidocrocites and goethite, whereas in the absence of Fe2+ no release was observed. 55Fe was not released from hematite, even at the higher Fe2+ concentration. The release rate is mainly controlled by characteristics of the iron oxides, whereas the concentration of Fe2+ only has minor influence. Ferrihydrite and 5-nm-sized lepidocrocite crystals attained complete isotopic equilibration with aqueous Fe(II) within days. Within this timeframe ferrihydrite transformed completely into new and more stable phases such as lepidocrocite and goethite. Lepidocrocite #2 and goethite, having larger particles, did not reach isotopic equilibrium within the timeframe of the experiment; however, the continuous slow release of 55Fe suggests that isotopic equilibrium will ultimately be attained.Our results imply a recrystallization of solid Fe(III) phases induced by the catalytic action of aqueous Fe(II). Accordingly, iron oxides should properly be considered as dynamic phases that change composition when exposed to variable redox conditions. These results necessitate a reevaluation of current models for the release of trace metals under reducing conditions, the sequestration of heavy metals by iron oxides, and the significance of stable iron isotope signatures.  相似文献   

19.
Iron (hydr)oxides are common in natural environments and typically contain large amounts of impurities, presumably the result of coprecipitation processes. Coprecipitation of Al with Fe (hydr)oxides occurs, for example, during alternating reduction-oxidation cycles that promote dissolution of Fe from Fe-containing phases and its re-precipitation as Fe-Al (hydr)oxides. We used chemical and spectroscopic analyses to study the formation and transformation of Al coprecipitates with Fe (hydr)oxides. In addition, periodic density functional theory (DFT) computations were performed to assess the structural and energetic effects of isolated or clustered Al atoms at 8 and 25 mol% Al substitution in the goethite structure. Coprecipitates were synthesized by raising the pH of dilute homogeneous solutions containing a range of Fe and Al concentrations (100% Fe to 100% Al) to 5. The formation of ferrihydrite in initial suspensions with ?20 mol% Al, and of ferrihydrite and gibbsite in initial suspensions with ?25 mol% Al was confirmed by infrared spectroscopic and synchrotron-based X-ray diffraction analyses. While base titrations showed a buffer region that corresponded to the hydrolysis of Fe in initial solutions with ?25 mol% Al, all of the Al present in these solutions was retained by the solid phases at pH 5, thus indicating Al coprecipitation with the primary Fe hydroxide precipitate. In contrast, two buffer regions were observed in solutions with ?30 mol% Al (at pH ∼2.25 for Fe3+ and at pH ∼4 for Al3+), suggesting the formation of Fe and Al (hydr)oxides as two separate phases. The Al content of initial coprecipitates influenced the extent of ferrihydrite transformation and of its transformation products as indicated by the presence of goethite, hematite and/or ferrihydrite in aged suspensions. DFT experiments showed that: (i) optimized unit cell parameters for Al-substituted goethites (8 and 25 mol% Al) in clustered arrangement (i.e., the formation of diaspore-like clusters) were in good agreement with available experimental data whereas optimized unit cell parameters for isolated Al atoms were not, and (ii) Al-substituted goethites with Al in diaspore-like clusters resulted in more energetically favored structures. Combined experimental and DFT results are consistent with the coprecipitation of Al with Fe (hydr)oxides and with the formation of diaspore-like clusters, whereas DFT results suggest isomorphous Al for Fe substitution within goethite is unlikely at ?8 mol% Al substitution.  相似文献   

20.
Quartz and iron (hydr)oxide are reactive surface phases that are often associated with one another in soils and sediments. Despite the several studies on the coating of quartz with iron oxides, the reactivity of dissolved species (Si) leached from quartz with iron (hydr)oxides has received limited attention. In this study, goethite synthesized on quartz substrates were characterized using field emission scanning electron microscopy, X-ray diffraction (XRD), transmission electron microscopy, and Fourier-transform infrared (FT-IR) spectroscopy. The SEM characterization revealed that bundles of thin parallel aligned goethite rods were formed at pH?>?10, while large pseudohexagonal crystals of twinned goethite needles were synthesized at pH?≤?10 after dehydration and hydration in the alkaline media. TEM analysis showed expanded and distorted lattice spacing of the crystal structure of iron (hydr)oxide due to silica incorporation. The characterization showed that silica increased the crystallite size of the goethite and transformed its acicular texture to a larger, twinned needle structure. FT-IR and XRD analyses revealed band shifts in crystal bonds as well as new bond formations, which indicate the presence of changes in the chemical environment of Fe–O and Si–O bonds. Thus, the presence of sorbed silicates modifies the crystal and lattice structure of goethite.  相似文献   

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