Specific effects of background electrolytes on the kinetics of step propagation during calcite growth

The mechanisms by which background electrolytes modify the kinetics of non-equivalent step propagation during calcite growth were investigated using Atomic Force Microscopy (AFM), at constant driving force and solution stoichiometry. Our results suggest that the acute step spreading rate is controlled by kink-site nucleation and, ultimately, by the dehydration of surface sites, while the velocity of obtuse step advancement is mainly determined by hydration of calcium ions in solution. According to our results, kink nucleation at acute steps could be promoted by carbonate-assisted calcium attachment. The different sensitivity of obtuse and acute step propagation kinetics to cation and surface hydration could be the origin of the reversed geometries of calcite growth hillocks (i.e., rate of obtuse step spreading < rate of acute step spreading) observed in concentrated (ionic strength, IS = 0.1) KCl and CsCl solutions. At low IS (0.02), ion-specific effects seem to be mainly associated with changes in the solvation environment of calcium ions in solution. With increasing electrolyte concentration, the stabilization of surface water by weakly paired salts appears to become increasingly important in determining step spreading rate. At high ionic strength (IS = 0.1), overall calcite growth rates increased with increasing hydration of calcium in solution (i.e., decreasing ion pairing of background electrolytes for sodium-bearing salts) and with decreasing hydration of the carbonate surface site (i.e., increasing ion pairing for chloride-bearing salts). Changes in growth hillock morphology were observed in the presence of Li⁺, F⁻ and , and can be interpreted as the result of the stabilization of polar surfaces due to increased ion hydration. These results increase our ability to predict crystal reactivity in natural fluids which contain significant amounts of solutes.     

Influence of ion exchange processes on salt transport and distribution in historic sandstone buildings

Crystallisation of salts in the pores of stone is a major concern in the preservation of heritage buildings, monuments and sculptures, but the mechanism of transport and distribution of salts is still not properly understood. The fractionation and distribution of salts in the porous matrix has, in building material research, conventionally been attributed to the solubility and concentration of salts present in the groundwater. We propose another mechanism contributing to the control of the salt distribution based on the interaction of ions in the salt solution with the charged mineral phases within the stone. The transport of mixed salt solutions was studied in laboratory simulated flow-through experiments on two fluvial sandstones – a Permo-Triassic red bed sandstone and a Carboniferous sandstone, both from the UK. The experiments were carried out under non-evaporative conditions, eliminating the possibility of solubility-dependent crystallisation. The results indicate that the process of ion exchange significantly controls the transport of ions in the pores and leads to fractionation of solutes in the course of transport even in the absence of evaporation and crystallisation. The sandstones behave like a chromatographic column and retention of various ionic species is significantly controlled by ion exchange processes. A quantitative estimation of cation exchange capacity (CEC) indicates that sandstones with higher CEC have greater influence on retention and fractionation of salts in the course of capillary transport than those with lower CEC. Simple scoping calculations using a geochemical modelling code and the ion exchange properties based on those determined in the laboratory experiments, demonstrate that ion exchange can have a significant effect on mineral precipitation during evaporation.     

X-ray absorption spectroscopy investigation of aqueous Co(II) and Sr(II) sorption at clay–water interfaces

Sorption processes typically control trace metal concentrations in aquatic systems. To illustrate the impact of various types of surface sites on metal ion sorption behavior, Co(II) and Sr(II) sorption by several clay minerals under a range pH and background electrolyte conditions was studied. X-ray absorption spectroscopy (XAS) was used to characterize the surface complexes formed to explain the basis for the sorption trends. At low pH, Co(II) could be displaced from the surface by increasing the Na ion concentration. XAS analysis of these samples showed that sorbed Co(II) retained the coordination structure of aqueous phase Co(II), suggesting the formation of weakly associated, outer-sphere, mononuclear Co complexes at permanent charge sites. At high pH, sorbed Co could not be displaced by increasing the Na ion concentration. The XAS analyses of these samples indicated the formation of Co coprecipitates. The results of the Sr(II) sorption experiments suggested weaker bonding between sorbed Sr and the solid surfaces, regardless of solution conditions and adsorbent. XAS analysis of Sr sorption samples revealed the formation of mononuclear, outer-sphere complexes of Sr at clay–water interfaces, similar to the outer-sphere Co sorption samples observed only at low pH.     

聚合羟基铁铝蒙脱石复合体对磷的吸附行为及其动力学

在一定条件下利用钠基化蒙脱石合成了聚合羟基铝-蒙脱石复合体(HyA1-Mt)、2个不同铁含量的聚合羟基铁-蒙脱石复合体(HyFe-Mt)和3个不同Fe:Al摩尔比的聚合羟基铁铝-蒙脱石复合体(HyFeAl-Mt),研究了酸性和弱酸性条件下(pH=3.0～6.5)以上蒙脱石复合体对磷的吸附行为和动力学特征。结果表明,HyFeAl-Mt对磷的吸附容量大于HyA1-Mt和HyFe-Mt复合体,且随着Fe:Al摩尔比的增大,对磷的吸附容量依次增加。随pH升高,蒙脱石复合体对磷的吸附量容量变小。但随Fe:Al摩尔比增大,HyFeAl-Mt复合体零电荷点(pHZPC)升高,pH对磷的吸附的影响越来越小,HyFe-Mt对磷的吸附几乎不受pH的影响。随Fe:A1摩尔比的增大,HyFeAl-Mt对磷的吸附能力增强,铁含量是磷吸附量的重要控制因素。磷在各蒙脱石复合体上的吸附实验数据可很好地用Langmuir吸附等温方程拟合。磷在各蒙脱石复合体上的吸附动力学过程可分为快速和慢速两个过程,快速过程中的动力学受铁含量影响明显。动力学数据可同时用准二级动力学方程和Elovich方程拟合。     

Kinetics of crystal nucleation in ionic solutions: Electrostatics and hydration forces

The heat of precipitation, the mean crystal size and the broadness of crystal size distribution of barium sulfate precipitating in aqueous solutions of different background electrolytes (KCl, NaCl, LiCl, NaBr or NaF), was shown to vary at constant thermodynamic driving force (supersaturation) and constant ionic strength depending on the salt present in solution. The relative inversion in the effect of respective background ions on the characteristics of barite precipitate was observed between two studied supersaturation (Ω) and ionic strength (IS) conditions. The crystal size variance (β²) increased in the presence of background electrolytes in the order LiCl < NaCl < KCl at Ω = 10^3.33 and IS = 0.03 M and KCl < NaCl < LiCl at Ω = 10^3.77 and IS = 0.09 M. At a given Ω and IS the respective size of barite crystals decreased with increasing β² in chloride salts of different cations and remained constant in sodium salts of different anions.We suggest that ionic salts affect the kinetics of barite nucleation and growth due to their influence on water of solvation and bulk solvent structure. This idea is consistent with the hypothesis that the kinetic barrier for barium sulfate nucleation depends on the frequency of water exchange around respective building units that can be modified by additives present in solution. In electrolyte solution the relative switchover between long range electrostatic interactions and short range hydration forces, which influence the dynamics of solvent exchange between an ion solvation shell and bulk fluid, results in the observed inversion in the effect of differently hydrated salts on nucleation rates and the resulting precipitate characteristics.     

Sorption of uranyl and arsenate on SiO2, Al2O3, TiO2 and FeOOH

Migration of uranium and arsenic in aquatic environments is often controlled by sorption on minerals present along the water flow path. To investigate the sorption behaviour, batch experiments were conducted for uranium and arsenic as single components and also solutions containing both uranium and arsenic in the presence of SiO₂, Al₂O₃, TiO₂ and FeOOH at a pH ranging from 3 to 9. In solutions containing only U(VI) or As(V) with the minerals, the sorption of U(VI) was low at acidic pH range and increases with increasing pH, whereas As(V) showed opposite sorption behaviour to Al₂O₃, TiO₂ and FeOOH from acidic pH range to alkaline condition. For the As(V)–SiO₂ system, the sorption was low for almost all pH. Sorption of U(VI) and As(V) on SiO₂ and FeOOH is almost similar in solutions containing either U(VI) or As(V) separately, or both together. In the U(VI)–As(V)–Al₂O₃ system, a significant retardation in uranyl sorption and an enhancement in arsenate sorption on Al₂O₃ were observed for a wide range of pH. The sorption behaviour of U(VI) and As(V) was changed when Al₂O₃ was replaced by TiO₂, where an increase in sorption was observed for both elements. The sorption behaviour of uranyl and arsenate in the U(VI)–As(V)–TiO₂ system gives evidence for the formation of uranyl–arsenate complexes. The change in sorption retardation/enhancement of U(VI) and As(V) could be explained by the formation of uranyl–arsenate complexes or due to the competitive sorption between uranyl and arsenate species.     

Clarification of nonlinear retardation factors for colloid-enhanced transport in porous media

There have been a couple of contaminant retardation factors reported for the three-phase (aqueous, solid, and colloid) groundwater system. However, the retardation factor has often been presented by itself and not incorporated into the relevant transport equation, particularly when derived from the mass fraction approach. This may cause a misunderstanding of the retardation factor especially for the systems where multi-phases exist due to the presence of colloids and/or nonlinear sorption processes are involved. It is, therefore, necessary to clarify the form of the nonlinear retardation factor along with the relevant transport equation in the multi-phase system. Alternative forms of the retardation factor and relevant transport equation for specific conditions are presented in various combinations of the nonlinearity of involved sorption mechanisms. The retardation factors for specific conditions are compared with the ones available in the literature. The results indicate that more caution should be given in applying the retardation factor in order to explore contaminant transport in the multi-phase system where any nonlinear sorption is involved. Finally, presentation of the retardation factor along with the relevant transport equation in this study would help prevent possible misuse of the retardation factor in investigating contaminant transport in the multi-phase system.     

花果树磷矿重介质选矿研究与应用综述

条带结构是宜昌地区磷矿资源的赋存特点,是在较大颗粒下采用重介质分选磷质条带和脉石的有利条件。通过对花果树矿的技术改造,引进重介质三产品旋流器,以全粒级不分级入选,在较高分选密度(2.88～2.90)下,对微差比重矿物分离获得成功,旋流器内置高耐磨材料,极大减少运行成本。完善了介质控制、回收与净化系统,有效减少介质污染、密度变化、矿介分离的不稳定性,保障工业化正常生产。     

Sorption of Cu and Pb to kaolinite-fulvic acid colloids: Assessment of sorbent interactions

The sorption of Cu(II) and Pb(II) to kaolinite-fulvic acid colloids was investigated by potentiometric titrations. To assess the possible interactions between kaolinite and fulvic acid during metal sorption, experimental sorption isotherms were compared with predictions based on a linear additivity model (LAM). Suspensions of 5 g L⁻¹ kaolinite and 0.03 g L⁻¹ fulvic acid in 0.01 M NaNO₃ were titrated with Cu and Pb solutions, respectively. The suspension pH was kept constant at pH 4, 6, or 8. The free ion activities of Cu²⁺ and Pb²⁺ were monitored in the titration vessel using ion selective electrodes. Total dissolved concentrations of metals (by ICP-MS) and fulvic acid (by UV-absorption) were determined in samples taken after each titration step. The amounts of metals sorbed to the solid phase, comprised of kaolinite plus surface-bound fulvic acid, were calculated by difference. Compared to pure kaolinite, addition of fulvic acid to the clay strongly increased metal sorption to the solid phase. This effect was more pronounced at pH 4 and 6 than at pH 8, because more fulvic acid was sorbed to the kaolinite surface under acidic conditions. Addition of Pb enhanced the sorption of fulvic acid onto kaolinite at pH 6 and 8, but not at pH 4. Addition of Cu had no effect on the sorption of fulvic acid onto kaolinite. In the LAM, metal sorption to the kaolinite surface was predicted by a two-site, 1-pK basic Stern model and metal sorption to the fulvic acid was calculated with the NICA-Donnan model, respectively. The LAM provided good predictions of Cu sorption to the kaolinite-fulvic acid colloids over the entire range in pH and free Cu²⁺ ion activity (10⁻¹² to 10⁻⁵). The sorption of Pb was slightly underestimated by the LAM under most conditions. A fractionation of the fulvic acid during sorption to kaolinite was observed, but this could not explain the observed deviations of the LAM predictions from the experimental Pb sorption isotherms.     

金属同位素质谱中分析样品处理的基本原则与方法

二十年来,国内外相继建立了多种金属（铁铜锌镁钙锂钼硒汞铬镉矾钡钛等）同位素的分析方法。金属同位素分析中的样品处理包括两个过程：样品的消解和样品中待测元素的分离纯化。为了获得真实、准确的金属同位素数据,样品处理过程必须遵守两个基本原则：①不引入待测元素以及可能会对待测元素同位素分析产生干扰的元素;②待测元素不发生损失。金属同位素分析常用的样品消解方法是酸溶法（包括高压闷罐法和微波消解法）。待测元素的分离纯化主要使用离子交换分离法。相同的树脂可以用于不同元素的化学分离,同一种元素也可以使用不同的树脂进行化学分离。不同类型样品的基质差异较大,需要不同的流程对待测元素进行分离。研究人员可以通过改变前人的分离流程,包括改变树脂的用量、变换淋洗液或用量、增加分离步骤等方法来满足不同样品的分离要求。本文提出了金属同位素样品处理中需要注意的一些细节：①如果消解样品时使用了高氯酸,必须将高氯酸在高温下彻底去除,因为残余的高氯酸具有强氧化性会使后续化学分离中使用的离子交换树脂失效,影响分离效果;②同一体积的树脂放入不同内径的交换柱中,树脂柱越细越长,淋洗液流速越慢、洗脱时间越长,并且待测元素洗脱出来越滞后;③离子交换过程中,每次加入的试剂体积越小,淋洗出来的元素越集中,分离效果越好。     

Fate and transport of zinc in a sand tank model: monitoring of 2-D plume using TDR method

Sorption is a well-known phenomenon that may cause the retardation effect of zinc in the subsurface environment. In this study, the governing process for zinc sorption during transport was investigated by conducting 2-D plume tests in a laboratory scale sand tank model using the time domain reflectometry (TDR) method. Tracer solutions of NaNO₃ and ZnSO₄ were applied at a constant flow rate as a pulse type to capture the plumes of both solutes based on TDR-measured resistance. It was revealed that the observed zinc sulfate plume showed no retardation relative to sodium nitrate with a retardation factor of R ≈ 1. Instead of retardation, a prominent reduction of zinc sulfate mass occurred during transport through the tank model due to the irreversible sorption as well as longitudinal dispersion. This indicates that the controlling factor for the sorption process of zinc sulfate in the sand tank model is kinetic rather than equilibrium. These hydrogeological parameters would provide valuable information on the prediction of the fate of zinc in sandy aquifer materials.     

Sorption behavior of the sandy residual unconsolidated materials from the sandstones of the Botucatu Formation,the main aquifer of Brazil

Pollutant transport through porous geological materials depends on the intrinsic characteristics of the materials that define the sorption behavior. This is the main environmental aspect that must be evaluated in terms of natural attenuation and retardation factor of the pollutants. Sorption is directly related to the electrostatic charge of the mineral, the organic matter, and the oxide and hydroxide contents. We assessed the sorption characteristics of the sandy residual unconsolidated material of the Botucatu Formation, which is part of the main aquifer of Brazil, using Batch Equilibrium Tests. The tests used multicomponent solutions of NaCl, KCl, ZnCl₂, and CuCl₂·H₂O with a total concentration that varied from 20 to 1,000 ppm. Different plotting systems were applied so that the isotherms better reflected the sorption behavior of the studied cations onto the unconsolidated materials. The cation Na⁺ was not sorbed. The Langmuir I and Freundlich equations adequately represent the behavior of Cu⁺⁺, the Langmuir II approximation better represented K⁺, and the Langmuir I and Freundlich equations were reasonably fitted Zn⁺⁺.     

Removal of Cadmium Ions from Aqueous Solution by Silicate-incorporated Hydroxyapatite

This article reports a preliminary research on silicate-incorporated hydroxyapatite as a new environmental mineral used to remove cadmium ions from aqueous solutions. The silicate-incorporated hydroxyapatite was prepared by coprecipitation and calcining, and silicate was incorporated into the crystal lattice of hydroxyapatite by partial substitution of phosphate. The amount of cadmium ions removed by silicate-incorporated hydroxyapatite was significantly elevated, which was 76% higher than that of pure hydroxyapatite. But the sorption behavior of cadmium ions on silicate-incorporated hydroxyapatite was similar to that of pure hydroxyapatite. Morphological study revealed that silicate incorporation confined the crystal growth and increased the specific surface area of hydroxyapatite, which were in favor of enhancing the cadmium ion sorption capacity of the samples. Incorporation of silicate into hydroxyapatite seems to be an effective approach to improve the environmental property of hydroxyapatite on removal of aqueous cadmium ions.     

The Composition of Phosphatized Bones in Recent Sediments

Mineral and chemical compositions of bone phosphate were studied in two samples from the outer Namibian shelf sediments composed of fish skull fragments and whale ribbon. Fossilization of bones is accompanied by the accumulation of lithogenic components, iron, sulfur, rare earth and other trace elements (Ni, Cu, Co, Cd, Mo, La, Ce, Th, U, and others), whereas the organic and mineral carbon content decreases. The evolution of bone phosphate during fossilization consists in transition from primary hydroxylapatite to a gel-type material, which subsequently becomes globular and crystallizes as fluorcarbonate-apatite crystallites. Additionally, some authigenic minerals, including both relatively widespread minerals (pyrite, uraninite, and coffinite) and rare minerals (graphite and calcium and germanium oxides) are formed in the bones. A considerable proportion of uranium in bones consists of uranium minerals, which also contain rare earth elements.     

Numerical evaluation of multicomponent cation exchange reactive transport in physically and geochemically heterogeneous porous media

Experimental evidence and stochastic studies strongly show that the transport of reactive solutes in porous media is significantly influenced by heterogeneities in hydraulic conductivity, porosity, and sorption parameters. In this paper, we present Monte Carlo numerical simulations of multicomponent reactive transport involving competitive cation exchange reactions in a two-dimensional vertical physically and geochemically heterogeneous medium. Log hydraulic conductivity, log K, and log cation exchange capacity (log CEC) are assumed to be random Gaussian functions with spherical semivariograms. Random realizations of log K and log CEC are used as input data for the numerical simulation of multicomponent reactive transport with CORE^2D, a general purpose reactive transport code. Longitudinal features of the fronts of reactive and conservative species are computed from the temporal and spatial moments of depth-averaged concentrations. Monte Carlo simulations show that: (1) the displacement of reactive fronts increases with increasing variance of log K, while it decreases with the variance of log CEC; (2) second-order spatial moments increase with increasing variances of log K and log CEC; (3) uncertainties in the mean arrival time are largest (smallest) for negatively (positively) correlated log K and Log CEC; (4) cations undergoing competitive cation exchange exhibit different apparent velocities and retardation factors due to both physical and geochemical heterogeneities; and (5) the correlation between log K and log CEC affects significantly apparent cation retardation factors in heterogeneous aquifers.     

苯和甲苯在土壤中的吸附行为研究

选取某采油厂不同有机碳含量的表层土壤作为吸附剂,采用批实验方法对原油中代表性组分苯和甲苯的单组分吸附和双组分竞争吸附行为进行了研究。研究表明,单组分溶液中,苯和甲苯在土壤中的吸附符合线性规律,吸附能力与土壤有机碳含量成正比;双组分溶液中,苯和甲苯共存时存在竞争吸附,土壤对它们的吸附小于单组分时的情况。竞争吸附的结果说明,除分配吸附以外,介质表面点位对苯和甲苯的吸附也起着重要的作用。     

An electron paramagnetic resonance (EPR) study of the adsorption of copper complexes on montmorillonite and imogolite

The adsorption of three copper species, the hydrated cupric ion, bisglycine Cu(II) and a Cu(II)-humic acid complex, on montmorillonite and imogolite at pH 7 was investigated by electron paramagnetic resonance (EPR) spectroscopy. The spectra of the supernatant solutions indicated that adsorption of the glycine complexes was very much less than that of the uncomplexed ion for both minerals and that montmorillonite adsorbed significantly more Cu from the humic acid solution than did imogolite. In every case the adsorbed Cu was characterized by more than one type of chemical environment and the spectral parameters show differences between the adsorbed species depending on their freedom of movement and their mode and site of adsorption. Qualitatively similar spectra were obtained when the uncomplexed ion was adsorbed on either of the mineral species and it is suggested that simple Cu(II) ions were involved and that their coordination environments comprised water molecules and hydroxyl groups. With the bisglycine Cu(II) complexes, the spectra were characterized by two components which may be accounted for by adsorption at two different types of site in the structures. In each case one component had parameters that are similar to those of the bisglycine Cu(II) in the solid state, but the second component in the spectra of the montmorillonite sample was quite different to that obtained with imogolite. The Cu(II)-humic acid complex with montmorillonite gave spectra that were similar to that from copper humate, but with imogolite the spectra from the Cu(II)-humic acid system were similar to those obtained with the uncomplexed Cu(II) ion, indicating that imogolite is able to extract copper from humic acid.     

Models of natural organic matter and interactions with organic contaminants

Adsorption of organic contaminants onto soils, sediments and other particulates has the potential to be a major controlling factor in their bioavailability, fate and behavior in the environment. Models for estimating the amount and stability of sorbed organic contaminants based on the fraction of organic carbon in a soil or sediment can oversimplify the process of sorption in the environment. In order to help understand sorption of organic contaminants in soils and sediments, we modeled various components of natural organic matter (NOM) that are possible substrates for sorption. These substrates include soot particles, lignin, humic and fulvic acids. The molecular scale interactions of selected aromatic hydrocarbons with different substrates were also simulated. Results of the simulations include the 3-D structures of the NOM components, changes in structure with protonation state and solvation and the sorption energy between PAH and substrate. This last parameter is an indicator of the amount of contaminant that will sorb and the energy required to free the contaminant from the substrate. Although the simulation results presented in this paper represent a first-order examination of NOM and contaminant interactions, the findings highlight a number of essential features that should be included in future molecular models of NOM and contaminant sorption.     

Removal of cadmium and humic acid from aqueous solutions using surface modified nanozeolite A

The sorption of cadmium and humic acids from aqueous solutions using surface-modified nanozeolite A has been investigated under various examination conditions. The morphology of untreated and treated nanozeolite was studied under scanning electron microscope and transmission electron microscope. Isotherms of cadmium adsorption onto surface-modified nanozeolite A were studied at different pH, solid to liquid ratio, adsorbate concentration and interaction time. Kinetic and equilibrium studies were conducted and the equilibrium data have been analyzed using Langmuir and Freundlich isotherm models. The study revealed that experimental results were in agreement with the Freundlich model. The Langmuir monolayer adsorption capacity was found to be 1666.67 g cadmium and 6.75 g humic acid per gram of modified nanozeolite A, which is higher than that of reported value for other zeolites. The sorption ability was enhanced by surface modification and reduction in size and enabled the zeolite to adsorb cadmium. The adsorption of cadmium and humic acid on nanozeolite was found to be the highest at pH 6 and 3, respectively. Results showed that solid to liquid ratio and pH are the most important factors for cadmium and humic acid removal, respectively. Effect of competitive ions was studied and results showed that there is no competition between cadmium and humic acid sorption and presence of these ions.     

Lithium sorption to Yucca Mountain tuffs

The Li ion has been used as a reactive tracer in field tests performed in the saturated and unsaturated-zone in volcanic tuffs at Yucca Mountain, Nevada. Lithium sorbs weakly by cation exchange and permits field-scale testing of laboratory-based predictions of reactive-solute transport. A series of laboratory studies show that Li sorption is nonlinear and varies with lithology in the different horizons of the tuff. In particular, both Li sorption and Li-specific cation-exchange capacity vary as functions of tuff mineralogy, and can be predicted given quantitative X-ray diffraction data. These results indicate that Li sorption is dominated by clay and zeolite minerals, and that sorption coefficients can be predicted given mineralogic analysis results.