A surface complexation and ion exchange model of Pb and Cd competitive sorption on natural soils

The bioavailability and fate of heavy metals in the environment are often controlled by sorption reactions on the reactive surfaces of soil minerals. We have developed a non-electrostatic equilibrium model (NEM) with both surface complexation and ion exchange reactions to describe the sorption of Pb and Cd in single- and binary-metal systems over a range of pH and metal concentration. Mineralogical and exchange properties of three different acidic soils were used to constrain surface reactions in the model and to estimate surface densities for sorption sites, rather than treating them as adjustable parameters. Soil heterogeneity was modeled with >FeOH and >SOH functional groups, representing Fe- and Al-oxyhydroxide minerals and phyllosilicate clay mineral edge sites, and two ion exchange sites (X⁻ and Y⁻), representing clay mineral exchange. An optimization process was carried out using the entire experimental sorption data set to determine the binding constants for Pb and Cd surface complexation and ion exchange reactions.Modeling results showed that the adsorption of Pb and Cd was distributed between ion exchange sites at low pH values and specific adsorption sites at higher pH values, mainly associated with >FeOH sites. Modeling results confirmed the greater tendency of Cd to be retained on exchange sites compared to Pb, which had a higher affinity than Cd for specific adsorption on >FeOH sites. Lead retention on >FeOH occurred at lower pH than for Cd, suggesting that Pb sorbs to surface hydroxyl groups at pH values at which Cd interacts only with exchange sites. The results from the binary system (both Pb and Cd present) showed that Cd retained in >FeOH sites decreased significantly in the presence of Pb, while the occupancy of Pb in these sites did not change in the presence of Cd. As a consequence of this competition, Cd was shifted to ion exchange sites, where it competes with Pb and possibly Ca (from the background electrolyte). Sorption on >SOH functional groups increased with increasing pH but was small compared to >FeOH sites, with little difference between single- and binary-metal systems. Model reactions and conditional sorption constants for Pb and Cd sorption were tested on a fourth soil that was not used for model optimization. The same reactions and constants were used successfully without adjustment by estimating surface site concentrations from soil mineralogy. The model formulation developed in this study is applicable to acidic mineral soils with low organic matter content. Extension of the model to soils of different composition may require selection of surface reactions that account for differences in clay and oxide mineral composition and organic matter content.     

Solution thermochemistry of humic substances—I. Acid-base equilibria of humic acid

The technique of titration calorimetry has been used to characterize the acidic functional groups of humic acid. Both the concentrations of acidic functional groups and the thermodynamic parameters (pK_a, ΔH_a, etc.) for ionization of these functional groups were determined. The calculated values confirm the generally accepted opinion that humic acid contains both carboxyl and phenolic hydroxyl groups. Furthermore, at least one-third of the phenolic hydroxyl groups are not ortho to carboxyl groups and thus cannot participate in chelation of metal ions via salicylate-like complexing sites. The nature of the remaining two-thirds of the phenolic hydroxyl groups is still unknown.     

Surface chemistry of disordered mackinawite (FeS)

Disordered mackinawite, FeS, is the first formed iron sulfide in ambient sulfidic environments and has a highly reactive surface. In this study, the solubility and surface chemistry of FeS is described. Its solubility in the neutral pH range can be described by K_s^app = {Fe²⁺} · {H₂S(aq)} · {H⁺}⁻² = 10^+4.87±0.27. Acid-base titrations show that the point of zero charge (PZC) of disordered mackinawite lies at pH ∼7.5. The hydrated disordered mackinawite surface can be best described by strongly acidic mono-coordinated and weakly acidic tricoordinated sulfurs. The mono-coordinated sulfur site determines the acid-base properties at pH < PZC and has a concentration of 1.2 × 10⁻³ mol/g FeS. At higher pH, the tricoordinated sulfur, which has a concentration of 1.2 × 10⁻³ mol/g FeS, determines surface charge changes. Total site density is 4 sites nm⁻². The acid-base titration data are used to develop a surface complexation model for the surface chemistry of FeS.     

硅藻土表面酸位及其来源探讨

用红外、拉曼谱学方法探讨了硅藻土表面酸位，并分析了其来源。结果表明，硅藻土表面有限的Lewis酸位来源于矿样中的少量粘土矿物。其表面的Brφnsted酸位也很稀少，这是由于大部分表面羟基是非质子酸性。     

高岭石表面的酸碱性质

采用双位模式（即假定高岭石表面存在＞AIOH和＞SiOH基团）拟合高岭石表面的酸碱滴定数据并描述表面上发生的质子化反应，Al位和Si位的表观常数拟合值分别为pKal,Al=1.78、pKa2,Al=8.47和pKa2,Si=5.12，它们的酸性比对应的（氢）氧化物表面位的更强。高岭石的总表面位密度远大于氧化铝和二氧化硅，其原因很可能是溶液中的质子或羟离子能够渗入高岭石的层间，与层间的羟基发生反应。此外，Al位密度也比Si位大近一个数量级，这种与理论化学式偏离的现象可受多种因素的影响。高岭石表面总体在pH低于4.0时带正电荷，在pH高于4.0时带负电荷。正电荷仅由＞AlOH基团通过质子化作用形成＞AlOH2^ 表面化合态来提供，而负电荷则由＞AlOH和＞SiOH基团的去质子化作用产生，分别形成＞AlO^-和＞SiO^-表面化合态。     

Surface chemistry and relative Ni sorptive capacities of synthetic hydrous Mn oxyhydroxides under variable wetting and drying regimes

The surface binding site characteristics and Ni sorptive capacities of synthesized hydrous Mn oxyhydroxides experimentally conditioned to represent three hydrological conditions—MnO_XW, freshly precipitated; MnO_XD, dried at 37°C for 8 d; and MnO_XC, cyclically hydrated and dehydrated (at 37°C) over a 24-h cycle for 7 d—were examined through particle size analysis, surface acid-base titrations and subsequent modelling of the pK_a spectrum, and batch Ni sorption experiments at two pH values (2 and 5). Mineralogical bulk analyses by XRD indicate that all three treatments resulted in amorphous Mn oxyhydroxides; i.e., no substantial bulk crystalline phases were produced through drying. However, drying and repeated wetting and drying resulted in a non-reversible decrease in particle size. In contrast, total proton binding capacities determined by acid-base titrations were reversibly altered with drying and cyclically re-wetting and drying from 82 ± 5 μmol/m² for the MnO_XW to 21 ± 1 μmol/m² for the MnO_XD and 37 ± 5 μmol/m² for the MnO_XC. Total proton binding sites measured decreased by ≈75% with drying from the MnO_XW and then increased to ≈50% of the MnO_XW value in the MnO_XC. Thus, despite a trend of higher surface area for the MnO_XD, a lower total number of sites was observed, suggesting a coordinational change in the hydroxyl sites. Surface site characterization identified that changes also occurred in the types and densities of surface sites for each hydrologically conditioned Mn oxyhydroxide treatment (pH titration range of 2-10). Drying decreased the total number of sites as well as shifted the remaining sites to more acidic pK_a values. Experimentally determined apparent pH_zpc values decreased with drying, from 6.82 ± 0.06 for the MnO_XW to 3.2 ± 0.3 for the MnO_XD and increased again with rewetting to 5.05 ± 0.05 for the MnO_XC. Higher Ni sorption was observed at pH 5 for all three Mn oxyhydroxide treatments compared to pH 2. However, changes in relative sorptive capacities among the three treatments were observed for pH 2 that are not explainable simply as a function of total binding site density or apparent pH_zpc values. These results are the first to our knowledge, to quantitatively link the changes induced by hydrologic variability for surface acid base characteristics and metal sorption patterns. Further, these results likely extend to other amorphous minerals, such as Fe oxyhydroxides, which are commonly important geochemical solids for metal scavenging in natural environments.     

The speciation of water in silicate melts

Previous models of water solubility in silicate melts generally assume essentially complete reaction of water molecules to hydroxyl groups. In this paper a new model is proposed that is based on the hypothesis that the observed concentrations of molecular water and hydroxyl groups in hydrous silicate glasses reflect those of the melts from which they were quenched. The new model relates the proportions of molecular water and hydroxyl groups in melts via the following reaction describing the homogeneous equilibrium between melt species: H₂O_molecular (melt) + oxygen (melt) = 2OH (melt). An equilibrium constant has been formulated for this reaction and species are assumed to mix ideally. Given an equilibrium constant for this reaction of 0.1–0.3, the proposed model can account for variations in the concentrations of molecular water and hydroxyl groups in melts as functions of the total dissolved water content that are similar to those observed in glasses. The solubility of molecular water in melt is described by the following reaction: H₂O (vapor) = H₂O_molecular (melt).These reactions describing the homogeneous and heterogeneous equilibria of hydrous silicate melts can account for the following observations: the linearity between f_H2O and the square of the mole fraction of dissolved water at low total water contents and deviations from linearity at high total water contents; the difference between the partial molar volume of water in melts at low total water contents and at high total water contents; the similarity between water contents of vapor-saturated melts of significantly different compositions at high pressures versus the dependence on melt composition of water solubility in silicate melts at low pressures; and the variations of viscosity, electrical conductivity, the diffusivity of “water,” the diffusivity of cesium, and phase relationships with the total dissolved water contents of melts.This model is thus consistent with available observations on hydrous melt systems and available data on the species concentrations of hydrous glasses and is easily tested, since measurements of the concentrations of molecular water and hydroxyl groups in silicate glasses quenched from melts equilibrated over a range of conditions and total dissolved water contents are readily obtainable.     

Multisite surface reaction versus transport control during the hydrolysis of a complex oxide

Dissolution experiments of a tholeiite basalt glass carried out at different pH and T (up to 300°C) using a rotatingdisc apparatus show that, depending on pH and T, dissolution can be controlled by one of the following steps: (1) surface reaction; (2) transport of reactants in solution; and (3) mixed reaction. The activation energies of these different processes were found to be 60, 9 and 15–50 kJ mol⁻¹, respectively. Taking account of these results, it appears likely that surface reactions are not rate limiting for the hydrolysis of most crystalline silicate minerals in hydrothermal and metamorphic processes, and that caution should be exercised when predicting rate of reactions at high temperatures solely on the basis of activation energies measured at low temperatures.

Comparison of experimental and theoretical potentiometric titrations of the basalt glass and its constituent oxides indicates that the adsorption of H⁺ and OH⁻ ions at the basalt surface is metal cation specific and that the net adsorption can be predicted from the sole knowledge of the acidity constants of the network-forming constituent oxides. We found that in the acidic pH region dissolution is promoted by the adsorption of H⁺ on al and Fe surface sites while in the basic region, dissolution is promoted by the adsorption of OH⁻ on Si sites. The combination of the two distinct types of surface sites, Al and Fe on the one hand, and Si on the other hand, results in a dissolution rate minimum at a pH-value between the pH_zpc of the two groups of oxide components. Linear regressions with a slope n=3.8 are observed both in acid and alkaline solutions in logarithmic plots of the rate of dissolution vs. the surface charge. The value of n, which represents the number of protonation or hydroxylation steps prior to metal detachment, has been found equal to the mean valence of the network-forming metals.

Combining concepts of surface coordination chemistry with transition state theory afforded characterisation of the activated complexes involved in basalt dissolution processes. From the values obtained for the thermodynamic properties of activation for basalt dissolution it is assumed that the activated complexes formed during the H₂O-promoted dissolution of the basalt glass are more tightly bonded than those formed during H⁺- or OH⁻-promoted dissolution.     

Water in Albitic Glasses

Infrared spectroscopy has been calibrated to provide a preciseand accurate method for determining the concentrations of molecularwater and hydroxyl groups in hydrous albitic glasses. At totalwater contents less than 4 wt.%, most of the water is dissolvedas hydroxyl groups; at higher total water contents, molecularwater becomes the dominant species. For total water contentsabove 4 wt.%, the amount of water dissolved as hydroxyl groupsis nearly constant at about 2 wt.% and additional water is incorporatedas molecular water. These trends in the concentrations of theH-bearing species are similar to those observed in other silicateglasses using infrared and NMR spectroscopies. The ratio ofmolecular water to hydroxyl groups at a given total water contentis independent of the pressure and only weakly dependent onthe temperature of equilibration. Molecular water and hydroxyl group concentrations in glassesprovide constraints on the dissolution mechanisms of water insilicate liquids. Several mixing models involving homogeneousequilibria of the form H₂O+O^2– = 2OH^– among meltspecies in albitic melts have been developed. These models canaccount for the measured species concentrations if the effectsof non-ideal behavior or mixing of polymerized units are included,or by allowing for several different anhydrous species. We used two thermodynamic models of hydrous albitic melts tocalculate phase equilibria. The first assumes that Henry's lawholds for molecular water in albitic liquids; i.e. that theactivity of molecular water in the melt is proportional to itsmole fraction as determined by infrared spectroscopy. The seconddescribes the speciation and thermodynamics of hydrous albiticmelts using the formalism of a strictly regular solution. Thesemodels can account reasonably well for the position of the vapor-saturatedsolidus of high albite and the pressure and temperature dependenceof the solubility of water in albitic melts. To the extent thatthese models are successful, our approach provides a directlink between measured species concentrations in hydrous albiticglasses and the macroscopic thermodyn amic properties of theNaAlSi₃O₈-H₂O system.     

Study of diatoms/aqueous solution interface. I. Acid-base equilibria and spectroscopic observation of freshwater and marine species

This work reports on a concerted study of diatom-water interfaces for two marine planktonic (Thalassiosira weissflogii= TW, Skeletonema costatum= SC) and two freshwater periphytic species (Achnanthidium minutissimum= AMIN, Navicula minima= NMIN). Proton surface adsorption was measured at 25°C, pH of 3 to 11 and ionic strength of 0.001 to 1.0 M via potentiometric titration using a limited residence time reactor. Electrophoretic mobility of living cells and their frustules was measured as a function of pH and ionic strength. Information on the chemical composition and molecular structure of diatoms surfaces was obtained using FT-IR (in situ attenuated total reflectance) and X-ray Photoelectron Spectroscopy (XPS). The surface area of living cells and their frustules in aqueous solutions was quantified using Small Angle X-ray Scattering Spectroscopy (SAXS).These observations allowed us to identify the nature and to determine the concentration of the major surface functional groups (carboxyl, amine and silanol) responsible for the amphoteric behavior of cell surfaces in aqueous solutions. Taking into account the relative proportion of surface sites inferred from XPS and FT-IR measurements, a surface complexation model of diatom-solution interfaces was generated on the basis of surface titration results. The cell-normalized ratios of the three major surface sites {>COOH}: {>NH₃}: {>SiOH} are 1:1:0.1, 1:10:0, 1:1:0.4 and 1:1:0.3 for TW, SC, AMIN and NMIN, respectively. The total amount of proton/hydroxyl active surface sites for investigated species ranges from 1 (NMIN) to 9 (SC) mmol/g dry weight. Normalization of these site densities to the area of siliceous skeleton yields values between 0.3 (NMIN) and 0.9 mmol/m² (SC) which are an order of magnitude higher than corresponding values for organic-free frustules or amorphous silica. This suggests that the amphoteric properties and possibly the affinity for metal adsorption of diatom cultures are essentially controlled by the 3-D organic layers covering the silica frustule.     

Comparison of the acid-base behaviour and metal adsorption characteristics of a gram-negative bacterium with other strains

Thermodynamic parameters for proton and metal adsorption onto a gram-negative bacterium from the genus Enterobacteriaceae have been determined and compared with parameters for other strains of bacteria. Potentiometric titrations were used to determine the different types of sites present on bacterial cell walls. Stability constants for adsorption of Pb, Cu and Zn to specific sites were determined from batch adsorption experiments at varying pH with constant metal concentration. Titrations revealed 3 distinct acidic surface sites on the bacterial surface, with pK values of 4.3±0.2, 6.9±0.5 and 8.9±0.5, corresponding to carboxyl, phosphate and hydroxyl/amine groups, with surface densities of 5.0±0.7×10⁻⁴, 2.2±0.6×10⁻⁴ and 5.5±2.2×10⁻⁴ mol/g of dry bacteria. Only carboxyl and phosphate sites are involved in metal uptake, yielding the following intrinsic stability constants: Log K_carboxyl: Zn=3.3±0.1, Pb=3.9±0.8, and Cu=4.4±0.2, Log K_phosphoryl: Zn=5.1±0.1 and Pb=5.0±0.9. The deprotonation constants are similar to those of other strains of bacteria, while site densities are also within an order of magnitude of other strains. The similarities in surface chemistry and metal stability constants suggest that bacteria may be represented by a simple generic thermodynamic model for the purposes of modelling metal transport in natural environments. Comparison with oxide-coated sand shows that bacteria can attenuate some metals to much lower pH values.     

天然含钒金红石加热、淬火及电子辐射改性实验研究

天然含钒金红石样品经过1273K加热、淬火和高能电子辐照处理后性质发生不同程度的变化。XRD谱峰显示处理前后样品没有发生物相变化，显微IR图谱证明原样、加热、淬火样品的表面有少量吸附水存在，而辐射样品的显微IR图谱上没有检测到吸附水的吸收峰。用XPS分析比较了处理前后样品表面主要元素Ti、O和V的价态及含量，结果表明，原样经过处理后表面钒和氧的原子百分含量增加，其中淬火样品表面吸附水和钒的含量相对于原样增加最为明显，光催化活性相应较高；加热样品表面的吸附水和钒的含量比原样稍有增加，光催化活性与原样相当；电子辐射样品表面钒的原子百分含量相对原样增加不明显，而对水的吸附能力相对原样较低，光催化效率也略低。     

高岭石表面酸碱反应的电位滴定实验研究

采用表面酸碱电位滴定法探讨高岭石表面酸碱性质,基于多位模式(即假定高岭石表面存在3种基团Al2 OH 、AlOH 和SiOH ) ,根据实验所得数据对高岭石表面的质子化和去质子化过程的相关参数进行拟合,讨论各个位点所发生的反应,并探讨了支持电解质浓度、高岭石溶解过程对表面酸碱电位滴定结果的影响。高岭石的表面零净质子电荷点(pHPZNPC,5 .2 )不等同于零电荷点,当pH <5 .2时,高岭石表面荷正电荷,主要由于表面富硅贫铝层的形成和Al位的质子化所致;当pH >5 .2时,高岭石表面荷负电荷,以Si位和Al的去质子化反应为主。     

矿物-水界面的表面离子化和络合反应模式

表面络合模式已广泛应用于研究矿物表面吸附和溶解反应,尤其是(氢)氧化物和层状铝硅酸盐矿物。表面官能团是矿物参与表面络合反应的基本单元,(氢)氧化物矿物只具有表面羟基,而层状铝硅酸盐矿物除表面羟基外还具有离子交换位。矿物的表面电荷由永久结构电荷、配位表面电荷和离解表面电荷组成,各种零表面电荷状态可由不同的零电荷点来表征。恒电容模式、双层模式和三层模式是3种最常用的表面络合模式,它们在双电层结构、表面反应(质子化反应和络合反应)、表面电荷与质量平衡方程及应用范围上存在着一些差异。     

The flux of oxygen from the basal surface of gibbsite (α-Al(OH)3) at equilibrium

Experiments were conducted on gibbsite to determine whether oxygen-isotope exchange rates at hydroxyl bridges (μ₂-OH) on the basal sheet exhibit similar reactivity trends as in large aluminum polyoxocations, for which high-quality kinetic data exist. We followed the exchange of ¹⁸O from the mineral surface to solution by using a high-surface-area solid that had been enriched to tens of percent in ¹⁸O. To establish this high enrichment, we initially react the solid hydrothermally with highly enriched H₂¹⁸O in order to tag all oxygens near the mineral surface, and then back exchange the most reactive oxygens with isotopically normal water. This enrichment procedure isolates ¹⁸O into the least-reactive sites, which are presumably μ₂-OH on the basal surface. By analogy with aqueous aluminum complexes, including large multimers, the η-OH₂ sites exchange within fractions of a second and should be isotopically normal using this procedure.When suspended in isotopically normal electrolyte solutions, we find that the rates of release of ¹⁸O from the mineral fall close to the rates of dissolution. The lack of steady isotopic exchange of μ₂-OH on gibbsite surfaces contrasts with the aluminum polyoxocations, where the μ₂-OH exchange many hundreds of times with bulk water molecules before the molecule dissociates. Additional experiments were conducted in solutions at near-neutral pH to determine the flux of oxygens at conditions near thermodynamic equilibrium. As in more acidic solutions, rates are close to values expected from dissolution of the mineral and there is no evidence for steady exchange of hydroxyl bridges with water molecules in the bulk solution.     

X-ray absorption fine structure determination of pH-dependent U-bacterial cell wall interactions

X-ray absorption fine structure (XAFS) measurements was used at the U L3-edge to directly determine the pH dependence of the cell wall functional groups responsible for the absorption of aqueous UO₂²⁺ to Bacillus subtilis from pH 1.67 to 4.80. Surface complexation modeling can be used to predict metal distributions in water-rock systems, and it has been used to quantify bacterial adsorption of metal cations. However, successful application of these models requires a detailed knowledge not only of the type of bacterial surface site involved in metal adsorption/desorption, but also of the binding geometry. Previous acid-base titrations of B. subtilis cells suggested that three surface functional group types are important on the cell wall; these groups have been postulated to correspond to carboxyl, phosphoryl, and hydroxyl sites. When the U(VI) adsorption to B. subtilis is measured, observed is a significant pH-independent absorption at low pH values (<3.0), ascribed to an interaction between the uranyl cation and a neutrally charged phosphoryl group on the cell wall. The present study provides independent quantitative constraints on the types of sites involved in uranyl binding to B. subtilis from pH 1.67 to 4.80. The XAFS results indicate that at extremely low pH (pH 1.67) UO₂²⁺ binds exclusively to phosphoryl functional groups on the cell wall, with an average distance between the U atom and the P atom of 3.64 ± 0.01 Å. This U-P distance indicates an inner-sphere complex with an oxygen atom shared between the UO₂²⁺ and the phosphoryl ligand. The P signal at extremely low pH value is consistent with the UO₂²⁺ binding to a protonated phosphoryl group, as previously ascribed. With increasing pH (3.22 and 4.80), UO₂²⁺ binds increasingly to bacterial surface carboxyl functional groups, with an average distance between the U atom and the C atom of 2.89 ± 0.02 Å. This U-C distance indicates an inner-sphere complex with two oxygen atoms shared between the UO₂²⁺ and the carboxyl ligand. The results of this XAFS study confirm the uranyl-bacterial surface speciation model.     

Determination of point of zero charge of PILCS with single and mixed oxide pillars prepared from Tunisian-smectite

Different pillared interlayer clays (PILCs) with single oxide pillars of Cr and Al and mixed oxide pillars of these metals have been prepared from a Tunisian purified smectite (Hp): Several Al/metal, OH/metal and metal/clay ratios were used in order to investigate the effect on the chemical and physical properties, specifically the point of zero charge of the synthesized pillared clays. These chromium pillared clays compounds at n meq (Nitrate) and at n meq (Chlorite) are noted pillared clay and pillared clays \(Hp(20,40,60)_{NO_3 } \) and Hp(20,40,60)_Cl. The structure of the pillared materials are studied by XRD and cationic exchange capacity, the textural property are investigated by the specific surface area S_BET. The acid-base chemistry “surface acidity” of these products was analysed by using acid-base potentiometric in order to determine the PZC of each sample. The resulting materials exhibited basal spacings in the range of 13.96–21.13 Å, with high surface areas (10.58–198 m² g^?1). Pillared clays prepared from Tunisian purified smectite showed an increase of basal spacings and surface areas. A relatively strong interaction between metal and aluminium in the pillars was observed. The samples are studied by acid-base potentiometric. This experimental method is used to determine the point of zero charge (PZC). The PZC values of the edge sites are 8.2 for Tunisian purified smectite and 6.8; 5.9; 5.19; 6.84; 5.86; 5.73; 6.78; 7.56 for the differents samples respectively: \(Hp(20,40,60meq)_{NO_3 } \) Hp(20,40,60meq)_Cl; Hp-Cr/Al; Hp-Al.     

Hydrated goethite (α-FeOOH) (1 0 0) interface structure: Ordered water and surface functional groups

Goethite(α-FeOOH), an abundant and highly reactive iron oxyhydroxide mineral, has been the subject of numerous studies of environmental interface reactivity. However, such studies have been hampered by the lack of experimental constraints on aqueous interface structure, and especially of the surface water molecular arrangements. Structural information of this type is crucial because reactivity is dictated by the nature of the surface functional groups and the structure or distribution of water and electrolyte at the solid-solution interface. In this study we have investigated the goethite (1 0 0) surface using surface diffraction techniques, and have determined the relaxed surface structure, the surface functional groups, and the three dimensional nature of two distinct sorbed water layers. The crystal truncation rod (CTR) results show that the interface structure consists of a double hydroxyl, double water terminated interface with significant atom relaxations. Further, the double hydroxyl terminated surface dominates with an 89% contribution having a chiral subdomain structure on the (1 0 0) cleavage faces. The proposed interface stoichiometry is ((H₂O)(H₂O)OH₂OHFeOOFeR) with two types of terminal hydroxyls; a bidentate (B-type) hydroxo group and a monodentate (A-type) aquo group. Using the bond-valence approach the protonation states of the terminal hydroxyls are predicted to be OH type (bidentate hydroxyl with oxygen coupled to two Fe³⁺ ions) and OH₂ type (monodentate hydroxyl with oxygen tied to only one Fe³⁺). A double layer three dimensional ordered water structure at the interface was determined from refinement of fits to the experimental data. Application of bond-valence constraints to the terminal hydroxyls with appropriate rotation of the water dipole moments allowed a plausible dipole orientation model as predicted. The structural results are discussed in terms of protonation and H-bonding at the interface, and the results provide an ideal basis for testing theoretical predictions of characteristic surface properties such as pK_a , sorption equilibria, and surface water permittivity.     

Adsorption of natural dissolved organic matter at the oxide/water interface

Natural organic matter is readily adsorbed by alumina and kaolinite in the pH range of natural waters. Adsorption occurs by complex formation between surface hydroxyls and the acidic functional groups of the organic matter. Oxides with relatively acidic surface hydroxyls, e.g. silica, do not react strongly with the organic matter. Under conditions typical for natural waters, almost complete surface coverage by adsorbed organic matter may be expected for alumina, hydrous iron oxides and the edge sites of aluminosilicates. Potentiometric titration and electrophoresis indicate that most of the acidic functional groups of the adsorbed organic matter are neutralized by protons from solution. The organic coating is expected to have a great influence on subsequent adsorption of inorganic cations and anions.     

DOM不同分子质量组分对石英砂吸附卡马西平的影响研究

为探究溶解性有机质(DOM)不同相对分子质量组分对药物和个人护理品(PPCPs)类污染物吸附过程的影响,以卡马西平(CBZ)为目标污染物,以石英砂代表无机矿物,用商用腐殖酸(HA)制备DOM并进行超滤分级,然后开展吸附实验,并采用荧光光谱和红外光谱表征等手段,研究了DOM不同分子质量组分对石英砂吸附CBZ的影响及机制。结果表明,以3 000和10 000的超滤膜进行分级后的不同相对分子质量HA结构有明显差别,大分子质量HA中苯环等疏水性结构以及羟基的含量较多,芳香构造化程度高,多为分子量较高、稳定性较好的疏水性有机物,但中、小分子质量HA结构差异不大,含有更多羰基、羧基等含氧官能团,主要是一些分子质量较小的亲水物质; DOM不同分子质量组分对CBZ在石英砂上的吸附有明显影响,其中大分子质量组分促进吸附,而中、小分子质量组分抑制吸附,原因在于大分子质量组分可疏水结合于矿物表面,增加介质表面的活性吸附位点,同时其芳香性或脂肪性结构可与CBZ的疏水基团发生疏水作用共吸附或累积吸附于介质表面,从而促进CBZ的吸附;而中、小分子质量DOM的极性官能团可与CBZ酰胺部分通过极性作用结合,对CBZ分子有增溶作用,从而抑制吸附。