珍珠表面微形貌的AFM和SEM研究

利用原子力显微镜(AFM)结合传统的扫描电镜(SEM)观察不同质量的珍珠表面，在微米级至纳米级的范围内对珍珠表面进行了全面的超显微结构特征分析，探讨SEM下无法涉及的珍珠纳米级表面结构与珍珠物理特性(光泽度、粗糙度、伴色)之间的相关性。研究显示，珍珠等级与文石层、文石微层、文石板块及其内部文石集合体的有序度、致密度等均呈正相关关系。本文还提出在珍珠养殖与交易的珍珠分级中可利用微形貌数据如平均粗糙度(Ra)增加定量化标准。     

In situ imaging of μm and sub-μm-sized grains in a cometary environment by atomic force microscopy

The micro-imaging dust analysis system (MIDAS) is an essential element among the scientific payload on the international Rosetta mission to comet 46P/Wirtanen. The MIDAS instrument based on an atomic force microscope (AFM) collects small particles drifting outwards from the nucleus surface. AFM is able to image small structures in 3D at nanometer-scale resolution. These images provide morphological and statistical information like grain size distribution on the dust population. In order to support the development of the flight hardware, optimisation of the control functions and consolidation of a proper scheme of data interpretation, laboratory studies with similar instruments were carried out. The obtained data demonstrate the capabilities of this technique. For the first time an instrument is able to observe the smallest (nm-sized) grains which are predicted by models and were to a certain extent deduced from previous measurements on the Giotto and Vega missions to comet 1P/Halley. On larger (μm-sized) particles the complex morphology will be visualised with high precision in 3D, and if present, within these aggregates crystalline materials with defined crystal faces can be identified.     

Adsorption and flocculation behaviors of polydiallyldimethylammonium (PDADMA) salts: Influence of counterion

Two modified polymer flocculants of PDADMA salts, PDADMA nitrate (PDADMAN) and PDADMA sulfate (PDADMAS), were prepared from the most widely used commercial product of polydiallyldimethylammonium chloride (PDADMAC). Solution properties such as conductivity and viscosity were investigated, showing that the conductivity follows the order of PDADMAS > PDADMAC > PDADMAN, while the reduced viscosity increases in the order of PDADMAS < PDADMAC < PDADMAN. The results indicate that, compared with PDADMAC, PDADMAN has a more cationic density and a more extended polymer chain, whereas PDADMAS exhibits a smaller coiled polymer size because of the stronger charge affinity of sulfate ion to the polycation of PDADMA. Flocculation experiments were performed on kaolin suspensions. It was found that PDADMAN is the most efficient in turbidity removal, while PDADMAS has wider optimum dosages, larger floc size and more rapid settling rate. Atomic force microscopy (AFM) was employed for imaging the adsorption structures of the three polymers on the negatively charged mica surface. Remarkable differences in molecular conformation were observed. Compared with the pearl necklace-like aggregation of hemispheroids of PDADMAC polymer, PDADMAN takes a sponge-like thin layer structure and PDADMAS exhibits a uniform hemispherical structure.     

原子力显微镜在粘土矿物学研究中的应用

研究粘土矿物表面组成与表面结构的手段和方法多种多样,但由于粘土矿物特殊的晶体结构,这些手段和方法都有一定的局限性和不足。原子力显微镜是近年来发展起来的一种能达到原子分辨能力的表面结构分析仪器,能够分辨粘土矿物硅氧四面体片上的六方环结构及八面体片的羟基团,是研究粘土矿物表面反应、表面改性及表面溶蚀作用的重要手段。     

方解石表面Cd2+与Pb2+类质同象的AFM研究

 借助原位液槽原子力显微镜（in situ AFM）的观察,通过Cd2+,Pb2+替代方解石最外层晶格Ca2+生长模式的实验研究, 探讨了Cd2+与Pb2+作用下方解石表面溶解与结晶行为。在液体反应槽中,分别将含不饱和Cd2+与Pb2+溶液流经方解石{101 _ 4}解理面,结果发现:（1）Cd2+的存在不影响方解石沿<4_41> 晶向台阶的溶解,而Pb2+的存在则强烈阻碍了方解石沿<441>+晶向台阶的溶解;（2）停止输入溶液含Cd2+,Pb2+溶液后,随着方解石表面与溶液达到平衡,溶解过程逐渐转变为结晶过程。结果显示在Cd2+存在时,单分子生长层具有方解石原有的定向性,而在Pb2+存在时的生长则不具任何定向性。尽管有此差异, 但（Ca,Cd）CO3 和（Ca,Pb）CO3 固溶体都受控于单分子层外延生长这一结晶机理。 　　含Cd2+和Pb2+溶液对方解石溶解动力学的作用与选择性吸附的阳离子半径大小、吸附复合体的几何形状及其结晶学取 向有关。Cd2+离子倾向于优先进入更狭小的<4_41>- 晶向的微台阶上,而Pb2+则倾向于形成扭曲的八面体络合物吸附在更开 阔的<4_41>+ 晶向台阶上。因此,Pb2+存在下方解石表面生长方向无序可认为是白铅矿和方解石结构差异的原因。     

Muscovite surface structure imaged by fluid contact mode AFM

The surface structure of muscovite was imaged using an atomic force microscope (AFM) in contact mode in water. The following three types of AFM images were observed: (1) those showing clearly the arrangement of hexagonal rings of SiO₄ tetrahedra; (2) those showing a hexagonal array of bright spots separated by a distance of about 5.3 Å; and (3) those changing gradually from image (2) to image (1). Image (1) successfully provides information on the tetrahedral tilt and basal surface corrugation that are particularly characteristic of dioctahedral micas. The mean unit cell dimensions for the muscovite surface measured from Image (1) were slightly longer than those of the bulk structure, due to the rehydration of the tetrahedral sheet and/or surface relaxation. Image (2) was made by varying the scan angles, even on the same surface in which Image (1) was obtained. Image (3) has information on a single plane rather than on two or more planes involving steps, kinks and so on. Therefore, what is depicted in Images (2) and (3) is not the arrangement of interlayer K ions but the basal plane of the tetrahedral sheet. Some structural relaxation of the tetrahedral sheet surface was also observed. Gradual expansion and contraction of hexagonal rings were randomly found on the muscovite surface. The surface relaxation results from a tetrahedral rotation and/or tilting after cleaving, since significant variations of both distances and bond angles between adjacent SiO₄ tetrahedra on the surface were found.     

Dissolution Kinetics of Calcite in 0.1 M NaCl Solution at Room Temperature: An Atomic Force Microscopic (AFM) Study

Atomic force microscopy (AFM) was used to study the rates of migration of the (10¯1 4) plane of a single-crystal of calcite dissolving in 0.1 M NaCl aqueous solutions at room temperature. The solution pH and P_{CO 2} controlled in the ranges 4.4 < pH < 12.2 and 0 < P_{CO 2} < 10^-3.5 atm (ambient), respectively. Measured step velocities were compared with the mineral dissolution rates determined from the calcium fluxes. The step velocity is defined as the average of the velocities of the obtuse and acute steps. Rates of step motion increased gradually from 1.4(±0.2) at pH 5.3 to 2.4(±0.3) nm s^-1 at pH 8.2, whereas the rates inverted and decreased to the minimum value of 0.69(±0.18) nm s^-1 at pH 10.8. For pH > 10.8, only the velocity of the obtuse steps increased as pH increased, whereas that of acute steps gradually decreased.The dissolution rate of the mineral can be calculated from the measured step velocities and average slope, which is proportional to the concentration of exposed monomolecular steps on the surface. The average slope of the dissolving mineral, measured at pH 5.6 and 9.7, was 0.026 (±0.015). Using this slope, we calculate bulk dissolution rates for 5.3 < pH < 12.2 of 4.9(±3.0) × 10^-11 to 1.8(±1.0) × 10^-10 mol cm^-2 s^-1. The obtained dissolution rate can be expressed by the following empirical equation:R_dss = 10^{-4.66(±0.13)}[H⁺] + 10^{-3.87(±0.06)}[HCO₃ ^-] + 10^{-7.99(plusmn; 0.08)}[OH^-]We propose that calcite dissolution in these solutions is controlled by elementary reactions that are similar to those that control the dissolution of other amphoteric solids, such as oxides. The mechanisms include the proton-enhanced hydration and detachment of calcium-carbonate ion pairs. The detachments are enhanced by the presence of adsorbed nucleophiles, such as hydroxyl and bicarbonate ions, and by protons adsorbed to key oxygens. A molecular model is proposed that illustrates these processes.     

微斜长石（010）解理面的原子力显微镜研究

用原子力显微镜研究了微斜长石的解理面，获得了高分辨率的表面结构图象。从几何结晶学的角度对结构图象进行了讨论，认为图象显示了微斜长石解理面的钾原子和硅氧四面体原子基因。     

Behaviour of Fe-oxides relevant to contaminant uptake in the environment

The behaviour of Fe-oxides was investigated during precipitation and co-precipitation, phase transformation and dissolution, while their ability to adsorb and incorporate trace components was examined. Some samples were synthesised and studied under controlled laboratory conditions and other samples were taken from experiments designed to test the effectiveness of waste treatment strategies using iron. Surface-sensitive and high-resolution techniques were used to complement information gathered from classical, macroscopic methods.

Adsorption isotherms for Ni²⁺ uptake on synthetic ferrihydrite (Fe₅HO₈·4H₂O, often written simply Fe(OH)₃), goethite (-FeOOH), hematite (-Fe₂O₃) and magnetite (Fe₃O₄) were all similar, increasing as expected at higher pH. Desorption behaviour was also similar, but one third or more of the Ni²⁺ failed to return to solution. In the past, “irreversible sorption” has been blamed on uptake into micro-fractures or pores, but during examination (using Atomic force microscopy, AFM) of hundreds of Fe-oxide particles, no evidence for such features could be found, leading to the conclusion that Ni²⁺ must become incorporated onto or into the solids. When solutions of Fe(II) are oxidised in controlled laboratory conditions or during treatment of ash from municipal waste incinerators, two-line ferrihydrite forms rapidly and on never-dried samples, AFM shows abundant individual particles with diameter ranging from 0.5 to several tens of nanometers. Aging in solution at 70°C promotes growth of the particles into hematite and goethite and their identification (by X-ray powder diffraction, XRPD, with Rietveld refinement) becomes possible at the same aging stage as mineral morphology becomes recognisable by AFM. In other experiments that were designed to mimic natural attack by organic acids, colloidal lepidocrocite (γ-FeOOH) was observed in situ by AFM, while reductive dissolution removed material on specific crystal faces. Lath ends are eroded fastest while basal planes are more stable.

In order to help elucidate mechanisms of contaminant immobilisation by Fe-oxides, we examined samples from a reactive barrier made with 90% quartz sand, 5% bentonite and 5% zero-valent iron filings that had reacted with a solution typical of leachate from coal-burning fly ash using time-of-flight secondary ion mass spectroscopy (TOF-SIMS). Fe(0) oxidised to Fe(III), while soluble and toxic Cr(VI) was reduced to insoluble Cr(III). Chemical maps show Fe-oxide coatings on bentonite; Cr is associated with Fe-oxides to some extent but its association with Ca in a previously undescribed phase is much stronger. Other samples taken from municipal waste incinerator ash that had been treated by aeration in Fe(II) solutions were examined with transmission electron microscopy (TEM), selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDS). Pb and some Zn are seen to be dispersed throughout two-line ferrihydrite aggregates, whereas Sn and some Zn are incorporated simply as a result of entrainment of individual ZnSn-oxide crystallites.

Geochemical speciation models that fail to account for contaminant uptake in solid solutions within major phases or as thin coatings or entrained crystals of uncommon phases such as those described here risk to underestimate contaminant retardation or immobilisation.     

Natural attenuation of TCE, As, Hg linked to the heterogeneous oxidation of Fe(II): an AFM study

Hydrous ferric oxide (HFO) colloids formed, in strictly anoxic conditions upon oxidation of Fe²⁺ ions adsorbed on mineral surface, were investigated under in situ conditions by contact mode atomic force microscopy (AFM). Freshly cleaved and acid-etched large single crystals of near endmember phlogopite were pre-equilibrated with dissolved Fe(II) and then reacted with Hg(II), As(V) and trichlorethene (TCE)-bearing solutions at 25 °C and 1 atm. HFO structures are found to be of nanometer scale. The As(V)–Fe(II) and Hg(II)–Fe(II) reaction products are round (25 nm) microcrystallites located predominantly on the layer edges and are indicative of an accelerated Fe(II) oxidation rate upon formation of Fe(II) inner sphere surface complexes with the phyllosilicate edge surface sites. On the other hand, TCE–Fe(II)–phlogopite reaction products are needle-shaped (45 nm long) particles located on the basal plane along the Periodic Bond Chains (PCBs) directions. Experiments with additions of sodium chloride confirm the importance of the Fe(II) adsorption step in the control of the overall heterogeneous Fe(II)–TCE electron transfer reaction.

Kinetic measurements at the nanomolar level of Hg° formed upon reduction of Hg(II) by Fe(II) in presence of phlogopite particles provide further convincing evidence for reduction of Hg(II)_aq coupled to the oxidation of Fe(II) adsorbed at the phlogopite–fluid interface, and indicate that sorption of Fe(II) to mineral surfaces enhances the reduction rate of Hg(II) species. The Hg(II) reduction reaction follows a first-order kinetic law. Under our experimental conditions, which were representative of many natural systems, 80% of the mercury is transferred to the atmosphere as Hg° in less than 2 h.

The reduction of a heavy metal (Hg), a toxic oxyanion (arsenate ion) and a chlorinated solvent (TCE) thus appear to be driven by the high reactivity of adsorbed Fe(II). This is of environmental relevance since these three priority pollutants are that way reductively transformed to a volatile, an immobilizable and a biodegradable species, respectively. Such kinetic data and reaction pathways are important in the evaluation of natural evaluation scenarios, in the optimization of Fe(II)/mineral mixtures as reductants in technical systems, and in general, in predicting the fate and transport of pollutants in natural systems.