天然铁(氢)氧化矿物对铜离子的吸附特征

研究了天然铁的(氢)氧化矿物对铜离子的吸附行为，结果显示不同种类的铁(氢)氧化矿物的吸附能力明显不同。针铁矿的吸附能力最强，赤铁矿吸附能力变化最大。对于同种矿物，矿物中杂质的种类和含量是影响吸附能力的主要因素，矿物的晶体习性有一定的影响。粒度较大(＞140目)的情况下，天然矿物的粒度对矿物吸附能力的影响没有规律性。pH值对铁氧化物矿物的吸附性能影响最大。随着初始浓度的增加，铁的(氢)氧化矿物的吸附量增加，但吸附率变化复杂且没有明显规律性。铁的(氢)氧化矿物的吸附行为较为符合Freundlich型和Temkin型等温吸附式。     

铁（氢）氧化物悬液中磷酸盐的吸附-解吸特性研究

铁（氢）氧化物对P的吸持和释放在一定程度上决定着P的生物有效性和水体富营养化。以两种环境中常见晶质铁氧化物（针铁矿和赤铁矿）为对照,采用X射线衍射（XRD）、透射电镜（TEM）、热重分析（TGA）和孔径分析以及动力学和吸附-解吸热力学平衡等技术方法,研究了弱晶质水铁矿对P吸附-解吸特性,并探讨了相关机制。实验表明,三种矿物对P的吸附分为起始的快速反应和随后的慢速反应,它们均符合准一级动力学过程,反应中OH释放明显滞后于P吸附,P吸附经历了从外围到内囤配位、单齿到多齿配位过渡的过程,与晶质氧化铁比,水铁矿吸附容量和OH释放量更大、慢速吸附反应更快、存在缓慢扩散反应阶段,吸附容量依次是：水铁矿（436μmol／m^2）〉针铁矿（262μmol／m^2）〉赤铁矿（228μmol／m^2）,针铁矿和赤铁矿吸附P符合L（Langmuir）模型,而水铁矿更符合F（Fremldlictl）模型。中性盐介质（KCl）中在最大吸附量时P的解吸率依次为：水铁矿（85％）〈针铁矿（10％）〈赤铁矿（125％）,柠檬酸通过配体解吸和诱导溶解两种机制促进P的解吸,最大吸附量时解吸率依次是：针铁矿（25％）〈水铁矿（32％）〈赤铁矿（50％）。     

针铁矿及其热活化产物除Sb(Ⅲ)效果对比

以合成针铁矿为原料,通过煅烧法获得比表面积分别为85.74和22.65 m2/g的多孔纳米赤铁矿和磁赤铁矿,通过静态实验探究了针铁矿和煅烧产物的Sb(Ⅲ)吸附性能.结果 表明,Sb(Ⅲ)吸附效率为赤铁矿＞磁赤铁矿＞针铁矿,其前二者效率显著高于针铁矿.Sb(Ⅲ)在3种矿物表面的吸附均为快速的化学吸附,吸附在2h内即可接近平衡,符合准二级动力学反应和Freundlich等温吸附模型,为自发进行的吸热反应,升高温度有利于反应的进行.在45 ℃、pH=7的条件下,针铁矿、赤铁矿和磁赤铁矿的最大吸附量分别可达16.04、50.44和33.53 mg/g.pH对赤铁矿和磁赤铁矿的Sb(Ⅲ)吸附效率影响不大,但pH升高会导致针铁矿的吸附能力降低.CO32-、SiO44-、PO43-和胡敏酸会与Sb(Ⅲ)竞争吸附位,抑制3种矿物对Sb(Ⅲ)的吸附,但这种抑制作用只在阴离子浓度较高的条件下有效.研究认为磁赤铁矿具有更多的表面活性位和较强的磁回收能力,是优于针铁矿和赤铁矿的含Sb(Ⅲ)废水处理材料.     

合成铁氧化物矿物对苯酚的吸附实验研究

采用TEM、X射线衍射分析及BET比表面积测定等手段对合成的针铁矿、赤铁矿两种矿物进行表征,对此两种矿物对苯酚的吸附特性并批处理吸附实验研究结果表明,针铁矿对苯酚的吸附量大于赤铁矿的吸附量;针铁矿吸附苯酚的pH吸收边为峰型曲线,且峰值在pH 7～8。Langmuir方程拟合结果的吻合程度较之Henry线性方程与Freundlich方程高。实验中氧化铁矿物表面对苯酚的吸附则主要表现为表面分子吸附,可能存在表面疏水性作用,而静电离子交换和表面配合吸附模式不明显。     

黄土剖面中赤铁矿和针铁矿的定量分析与气候干湿变化研究

为了了解黄土剖面中赤铁矿和针铁矿的分布特征,文章采用对铁氧化物矿物灵敏的漫反射光谱法(DRS),开展了赤铁矿和针铁矿的鉴定和测定研究,提出了定量分析赤铁矿和针铁矿含量的DRS新方法.选择天然典型黄土和古土壤样品,首先采用柠檬酸盐-重碳酸盐-连二亚硫酸盐(CBD)方法去除其中成壤成因的铁氧化物矿物,以其为基体配制含不同赤铁矿和针铁矿的系列标样,然后进行DRS测试和多元逐步回归分析,分别建立测定赤铁矿和针铁矿含量的校准方程并加以检验.利用DRS方法,分析了多个黄土剖面的赤铁矿和针铁矿含量,发现黄土-古土壤剖面的赤铁矿/针铁矿比值可作为东亚季风干/湿变化的敏感指标.该比值较高时反映了干燥土壤环境,而较低时指示了潮湿土壤环境.对灵台和洛川剖面中赤铁矿/针铁矿比值的分析,初步揭示了2.6Ma年以来黄土高原东亚季风阶段性变强的特征.     

离子吸附型稀土矿床中稀土的富集—分异特征:铁氧化物—黏土矿物复合体的约束

在离子吸附型稀土矿床中,黏土矿物被认为是离子吸附态稀土的主要吸附载体。风化壳中黏土矿物的透射电镜分析发现,黏土矿物普遍与铁氧化物形成复合体,很可能对稀土元素的富集–分异产生重要影响。本研究以广东省梅州市平远县仁居矿区的典型离子吸附型稀土矿床剖面为对象,通过提取富矿层中的细颗粒组分,采用X射线衍射、穆斯堡尔谱和透射电镜,对黏土矿物–铁氧化物复合体进行物相和形貌分析;在此基础上,采用顺序提取法测定不同化学形态的稀土总量,探究黏土矿物和铁氧化物对稀土元素富集–分异的贡献。结果表明,从全风化层到表土层,复合物从长石/伊利石–水铁矿/针铁矿复合体到高岭石/埃洛石–针铁矿/赤铁矿复合体,最终向高岭石–赤铁矿复合体转变。在表土层和全风化层的细颗粒组分中,离子交换态稀土约占25%和80%,而铁氧化物结合态稀土约占75%和20%。在表土层中, Ce的富集导致离子交换态和铁氧化物结合态稀土均富轻稀土。在全风化层中,随着深度的增加,离子交换态稀土由富轻稀土转变为富重稀土,铁氧化物结合态稀土均表现富重稀土的分异特征。离子交换态稀土总量和分异特征主要受黏土矿物–稀土界面作用和淋滤作用控制;而铁氧化物结合态稀土总...     

基于漫反射光谱的铁氧化物定量分析在南方黄土中的适用性研究

基于漫反射光谱(DRS)的铁氧化物定量分析已用于北方黄土的研究,而在南方黄土中的适用性尚不清楚。本文选取江西九江黄土为研究对象,采用柠檬酸钠-重碳酸钠-连二亚硫酸钠(CBD)方法去除其中的游离铁,以此为基体分别加入不同比例的赤铁矿和针铁矿,通过多元线性回归方法建立了标样的铁氧化物含量与光谱反射率之间的校准方程。经检验,校准方程应用于未经CBD处理的剖面天然样品时,获得的赤铁矿含量较准,而针铁矿含量不准。原因可能在于:(1)混合体系中,赤铁矿与针铁矿互为基体,赤铁矿对针铁矿的干扰远大于针铁矿对赤铁矿的影响,这种差异在南方地区尤为明显;(2)CBD处理不能完全清除南方黄土中的针铁矿,可能与样品中含有部分结晶较好、粒度较粗的针铁矿有关,这也增加了针铁矿定量分析的不准确性;(3)铝的同晶替代现象对南方黄土影响更为显著,其中受晶格结构等影响,针铁矿的铝替代量比赤铁矿更大,可能对定量分析结果的准确性影响也更大。因而在南方黄土中,基于DRS的铁氧化物定量分析方法对于赤铁矿是适用和有效的,对于针铁矿的应用则需要慎重。     

广西贵港岩溶地质高背景区富含铁锰结核土壤的矿物学与重金属地球化学特征

岩溶地质高背景区土壤中普遍存在的铁锰结核对重金属的赋存状态和有效性有重要影响。选择广西贵港覃塘岩溶地质高背景区富含铁锰结核的表层土壤(0~20 cm)为研究对象,筛分出不同粒径的铁锰结核(10~120目)和细粒径土壤(<120目)样品进行化学分析,针对以下三个方面开展研究:(1)重金属(As、Cd、Cr、Cu、Hg、Ni、Pb和Zn)在铁锰结核和细粒径土壤中的分布分配规律和铁氧化物矿物的组成;(2)铁氧化物矿物对富含铁锰结核的土壤中Cd等重金属富集的影响;(3)重金属在富含铁锰结核的土壤中的赋存机制。研究发现,铁锰结核中的Fe和Mn以及Cd等重金属含量随着粒径的增大而不断增加,说明Cd等重金属元素更倾向于在大粒径铁锰结核中富集;土壤中Cd等重金属总量的约90%赋存在结核中,表明研究区土壤中重金属主要以结核形式赋存;富含铁锰结核的土壤中赤铁矿和针铁矿的平均含量分别为0.61%和4.94%,且结核粒径越大,针铁矿和赤铁矿含量越高;除Hg外,Cd等重金属含量与针铁矿和赤铁矿的含量均呈现极显著正相关,与赤铁矿的相关性稍优于针铁矿,表明铁氧化物矿物与富含铁锰结核土壤中的Cd等重金属元素富集密切相关。铁锰结核的存在既能促进Cd等重金属在土壤中的富集,又能降低土壤中重金属的生物有效性,研究结果为解释岩溶地质高背景区土壤Cd等重金属元素高含量、低生物有效性提供了理论依据。     

热处理褐铁矿去除水中的Mn2+

本文利用褐铁矿中针铁矿经热脱水相变获得以纳米晶赤铁矿为主要物相的纳米-微米多级孔结构材料,并用于模拟净化富Mn~(2+)地下水。同时考察了热处理温度、初始pH值、初始Mn~(2+)浓度、吸附反应时间等对材料去除溶液中Mn~(2+)的影响。XRD、TEM、BET表征结果表明,300℃热处理产物中赤铁矿孔径最小为2.7 nm,比表面积最大达到107.4 m~2/g。吸附实验结果表明,在pH值5~10的范围内,p H值对煅烧褐铁矿颗粒对Mn~(2+)去除效果影响较小;材料在贫氧条件下对水中低浓度Mn~(2+)的最大吸附量为6.45 mg/g;吸附动力学符合准二级动力学模型;褐铁矿热处理形成的纳米晶赤铁矿对Mn~(2+)具有吸附和催化氧化作用,其中的杂质锰氧化物对Mn~(2+)的吸附和催化氧化具有增强作用。     

石英砂表面铁胶膜的合成——pH和铁摩尔比[Fe(Ⅱ)]/[Fe(Ⅲ)]的影响

本研究以石英砂为载体,在其颗粒表面合成铁胶膜,并探讨合成体系不同初始pH(5,6,7)及铁摩尔比(R=[Fe(Ⅱ)]/[Fe(Ⅲ)])等条件对铁胶膜形成的影响。研究表明,在初始pH相同的情况下,当R为0时,铁胶膜的矿物成分为弱晶质的水铁矿;R为0.01时的矿物成分为赤铁矿;R分别为0.02、0.04、0.06和0.10时为针铁矿,且随着R增加,X射线衍射(XRD)图谱中针铁矿的峰强度逐渐增加,扫描电镜(SEM)可观察到针铁矿晶形逐渐变大,且在R为0.10时晶体形貌最大;当R为0.50和体系只加入Fe(Ⅱ)时合成的铁矿物主要为针铁矿与磁铁矿的混合物。当R一定时,随着合成体系初始pH的增加,胶膜中针铁矿的XRD峰强度逐渐增强,在初始pH为7时其峰最强,且晶形逐渐变大。     

Structural constraints of ferric (hydr)oxides on dissimilatory iron reduction and the fate of Fe(II)

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.     

Molecular-scale mechanisms of distribution and isotopic fractionation of molybdenum between seawater and ferromanganese oxides

The distribution of Mo between seawater and marine ferromanganese oxides has great impacts on concentration and isotopic composition of Mo in modern oxic seawater. To reveal the adsorption chemistry of Mo to ferromanganese oxides, we performed (i) detailed structural analyses of Mo surface complexes on δ-MnO₂, ferrihydrite, and hydrogenetic ferromanganese oxides by L₃- and K-edge XAFS, and (ii) adsorption experiments of Mo to δ-MnO₂ and ferrihydrite over a wide range of pHs, ionic strengths, and Mo concentrations. XAFS analyses revealed that Mo forms distorted octahedral (Oh) inner-sphere complexes on δ-MnO₂ whereas it forms a tetrahedral (Td) outer-sphere complex on ferrihydrite. In the hydrogenetic ferromanganese oxides, the dominant host phase of Mo was revealed to be δ-MnO₂. These structural information are consistent with the macroscopic behaviors of Mo in adsorption experiments, and Mo concentration in modern oxic seawater can be explained by the equilibrium adsorption reaction on δ-MnO₂. In addition, the large isotopic fractionation of Mo between seawater and ferromanganese oxides detected in previous studies can be explained by the structural difference between and adsorbed species on the δ-MnO₂ phase in ferromanganese oxides. In contrast, smaller fractionation of Mo isotopes on ferrihydrite is due to little change in the Mo local structures during its adsorption to ferrihydrite.The structures of Mo species adsorbed on crystalline Fe (oxyhydr)oxides, goethite, and hematite were also investigated at pH 8 and I = 0.70 M (NaNO₃). Our XAFS analyses revealed that Mo forms inner-sphere complexes on both minerals: Td edge-sharing (46%) and Oh double corner-sharing (54%) for goethite, and Td double corner-sharing (14%) and Oh edge-sharing (86%) for hematite. These structural information, combined with those for amorphous ferrihydrite and δ-MnO₂, show the excellent correlation with the magnitude of adsorptive isotopic fractionation of Mo reported in previous studies: the proportion of Oh species or their magnitude of distortion in Mo surface complexes become larger in the order of ferrihydrite < goethite < hematite < δ-MnO₂, a trend identical to the magnitude of isotopic fractionation.Based on the comparison with previous reports for Mo surface species on various oxides, the chemical factors that affect Mo surface complex structures were also discussed. The hydrolysis constant of cation in oxides, log K_OH (or the acidity of the oxide surfaces, PZC) is well correlated with the mode of attachment (inner- or outer-sphere) of Mo surface complexes. Furthermore, the symmetric change in Mo species from Td to Oh is suggested to be driven by the formation of inner-sphere complexes on specific sites of the oxide surfaces.     

Evidence for a simple pathway to maghemite in Earth and Mars soils

Soil magnetism is greatly influenced by maghemite (γ-Fe₂O₃), the presence of which is usually attributed to the following: (1) heating of goethite in the presence of organic matter; (2) oxidation of magnetite (Fe₃O₄); or (3) dehydroxylation of lepidocrocite (γ-FeOOH). Formation of the latter two minerals in turn requires the presence of Fe(II) in the system. No laboratory experiment or soil study to date has shown whether maghemite can form from ferrihydrite, a poorly crystalline Fe(III) oxide [∼Fe_4.5(O,OH,H₂O)_13.5], below 250°C. However, ferrihydrite is the usual precursor of goethite (α-FeOOH) and hematite (α-Fe₂O₃), the most frequently occurring crystalline Fe(III) oxides in soils. Here is presented in vitro evidence that ferryhidrite can partly transform into maghemite at 150°C. This transformation occurs upon aging of ferrihydrite precipitated in the presence of phosphate or other ligands capable of ligand exchange with Fe-OH surface groups. This maghemite coexists with hematite and is a transient phase in the transformation of ferrihydrite to hematite, which is apparently stabilized by the adsorbed ligands. Its particle size is small (10 to 30 nm), and its X-ray diffraction pattern exhibits superstructure reflections. The possible formation of maghemite in Mars and in different Earth soils can partly be explained in the light of this pathway with minimal ad hoc assumptions.     

Sorption and desorption of dissolved organic phosphorus onto iron (oxyhydr)oxides in seawater

Sorption of phosphorus (P) onto particulate surfaces significantly influences dissolved P concentrations in aquatic environments. We present results of a study contrasting the sorption behavior of several dissolved organic phosphorus (DOP) compounds and phosphate onto three commonly occurring iron (oxyhydr)oxides (Fe_ox): ferrihydrite, goethite, and hematite. The DOP compounds were chosen to represent a range of molecular weights and structures, and include: adenosine triphosphate (ATP), adenosine monophosphate (AMP), glucose-6-phosphate (G6P), and aminoethylphosphonic acid (AEP).All P compounds displayed decreasing sorption as a function of crystallinity of the Fe_ox substrate, with ferrihydrite adsorbing the most, hematite the least. In general, maximum sorption density decreased with increasing molecular weight of P compound; sorption of G6P onto goethite and hematite excepted. P compound size and structure, and the nature of the Fe_ox substrate all appear to play a role dictating relative sorption capacity. Failure of a simple, 1-step sorption-desorption model to describe the data suggests that P sorption cannot be explained by a simple balance between sorption and desorption. Instead, the data are consistent with a 2-step sorption model consisting of an initial rapid surface sorption, followed by a slow, solid-state diffusion of P from surface sites into particle interiors. Desorption experiments provide additional support for the 2-step sorption model.Without exception, DOP compounds showed less efficient sorption than did orthophosphate. This suggests that in aquatic systems enriched in reactive Fe_ox, whether as suspended particulates in the water column or in benthic sediments, DOP bioavailability may exceed that of orthophosphate. Since biological uptake of P from DOP requires enzymatic cleavage of orthophosphate, a system enriched in DOP relative to orthophosphate may impact ecosystem community structure.     

泥炭沼泽源酚酸对铁有机复合体的溶解作用及其环境意义

泥炭沼泽是具有全球意义的湿地类型,研究泥炭沼泽源酚酸对铁有机复合体的溶解作用有助于深入了解铁碳耦合地球化学循环过程.以中国东北金川泥炭沼泽为研究对象,提取了泥炭腐殖质,并实验合成了铁有机复合体及一系列的铁氧化物.选择原儿茶酸、咖啡酸和没食子酸等代表性泥炭沼泽源酚酸对铁有机复合体以及铁氧化物等系统开展了不同条件下的溶解试验.结果表明酚酸对无定型的水铁矿和新合成的铁有机复合溶解能力相对较弱,而对结晶态的赤铁矿、针铁矿和老化后的铁有机复合体的溶解能力较强.pH值、酚酸浓度和铁氧化物自身的结构和组成都对铁矿物的溶解作用产生影响.反映了铁有机复合体在酚酸溶液体系中比无机铁氧化物更稳定,这与泥炭沼泽中有机结合态铁比例较高、而普通矿质土壤中结晶态铁氧化物占比更大的事实相吻合.证明了铁有机复合体是泥炭沼泽中影响铁碳循环耦合的关键载体.泥炭沼泽中铁碳作用十分复杂,既能以铁有机络合物形式向海洋等水生生态系统输出大量的溶解性铁,也能通过铁有机复合体的形成促进泥炭沼泽有机碳的保存,进而影响全球铁碳循环耦合,具有重要的生态环境意义.      

Determination of bioavailable Fe(III) content in column experiments with the reactive tracer phosphate

Different methods were compared to evaluate the oxidation capacity of ferric iron in column studies. The specific adsorption of the reactive tracer phosphate on the Fe(III) oxide surface was used as an alternative approach to determine the oxidation capacity utilizing the linear correlation between the long-term extent of Fe(III) reduction and the specific surface area of the oxide. Although a low crystalline form of ferric iron (two-line ferrihydrite) was used as electron acceptor and toluene as a carbon source, only 31 and 24% respectively of the total iron was reduced by Geobacter metallireducens in parallel experiments. The results of the phosphate tracer tests were in good agreement with the Fe(III) that was actually reduced and the microbially oxidized toluene. The oxidation capacity of ferric iron is therefore overestimated by the chemical extraction methods, which completely dissolve the ferrihydrite and neglect surface-dependent limitations.     

A Study of Chromium Adsorption on Natural Goethite Biomineralized with Iron Bacteria

Goethite, especially biogenic goethite, has high specific surface area and great capacity for the adsorption of many contaminants including metal ions and organic chelates. Chromium is a redox actively toxic metal ion that exists as either Cr^Ⅲ or Cr^Ⅵ in nature, and as such it is essential to understand its behavior of adsorption on natural goethite mineralized by iron bacteria, as Gallionella and Leptothrix in water body. The adsorption of Cr^3＋ and Cr^Ⅵ on naturally biomineralized goethite is studied in this paper. The results show that both Langmuir and Freundlich adsorption isothermal models are able to accurately describe the adsorption of these two ions. Investigation of SEM/EDS, TEM/EDS indicates that the two ions do not adsorb homogeneously on goethite owing to the different microstructures of goethite, and that the microspherical goethite has a greater adsorption capacity for chromium ions than the helical one. XPS data show that redox reaction of chromium on the surface of biomineralized goethite takes place in the adsorption of both Cr^3＋ and Cr^Ⅵ. The CrvI adsorbed on biogoethite is much easier to transform into CrIII than the oxidization of Cr^Ⅲ on the bio-goethite.     

Natural ferrihydrites in surface deposits from Finland and their association with silica

Young ochreous precipitations from Fe-bearing spring waters in Finland consist mainly of ferrihydrite. a poorly ordered Fe-oxide with a layer structure and the bulk composition 5 Fe₂O₃ ·9 H₂O Crystallinity ranges from a reasonably well developed structure to a highly disordered one with only two prismatic reflections at 2.5 and 1.5 Å. In contrast to other Fe-oxides. ferrihydrite is highly soluble in oxalate. Electron microscopy shows spherical particles 2–5 nm in diameter forming aggregates of 100–300 nm. The specific surface ranges from 220 to 560 m²/g. During their formation, the ferrihydrites adsorb large quantities of silica, part of which is unpolymerized as indicated by Si-O-Fe bonds (i.r.), and part of which is polymerized. NaOH preferentially extracts polymerized silica causing a shift in the i.r. absorption band. Silica also causes a shift in the temperature at which ferrihydrite converts to hematite. ‘Hydrous Fe(III)-oxides’ with 0–15mol% Si prepared from Si containing Fe(III) salt solutions showed similar properties: Si-O-Fe bonds are shown by i.r. and increasing temperatures of transformation to hematite with increasing amount of Si. Adsorbed Si may also retard the transformation of ferrihydrite to the more stable goethite in nature.     

Effect of adsorbed and substituted Al on Fe(II)-induced mineralization pathways of ferrihydrite

The poorly crystalline Fe(III) hydroxide ferrihydrite is considered one of the most important sinks for (in)organic contaminants and nutrients within soils, sediments, and waters. The ripening of ferrihydrite to more stable and hence less reactive phases such as goethite is catalyzed by surface reaction with aqueous Fe(II). While ferrihydrite within most natural environments contains high concentrations of adsorbed or co-precipitated cations (particularly Al), little is known regarding the impact of these cations on Fe(II)-induced transformation of ferrihydrite to secondary phases. Accordingly, we explored the extent, rates, and pathways of Fe(II)-induced secondary mineralization of Al-ferrihydrites by reacting aqueous Fe(II) (0.2 and 2.0 mM) with 2-line ferrihydrite containing a range of Al levels substituted within (6-24 mol% Al) or adsorbed on the surface (0.1-27% Γ_max). Here, we show that regardless of the Fe(II) concentration, Al substituted within or adsorbed on ferrihydrite results in diminished secondary mineralization and preservation of ferrihydrite. In contrast to pure ferrihydrite, the concentration of Fe(II) may not in fact influence the mineralization products of Al-compromised ferrihydrites. Furthermore, the secondary mineral profiles upon Fe(II) reaction with ferrihydrite are not only a function of Al concentration but also the mode of Al incorporation. While Al substitution impedes lepidocrocite formation and magnetite nucleation, Al adsorption completely inhibits goethite formation and appears to have a lesser impact on magnetite nucleation. When normalized to total Al content associated with ferrihydrite, Al adsorption results in greater degree of ferrihydrite preservation relative to Al substitution. These findings provide insight into mechanisms that may be responsible for ferrihydrite preservation and low levels of secondary magnetite typically found in sedimentary environments. Considering the preponderance of cation substitution within and adsorption on ferrihydrite in soils and sediments, the reactivity of natural (compromised) ferrihydrites and the subsequent impact on mineral evolution needs to be more fully explored.