Inhibition of calcite precipitation by natural organic material: kinetics, mechanism, and thermodynamics

The inhibition of calcite precipitation by natural organic material (NOM) in solutions seeded with calcite was investigated using a pH-stat system. Experiments were carried out using three NOMs with different physical/chemical properties. For each of the materials, inhibition was found to be more effective at lower carbonate/calcium ratios and lower pH values. The reduction in the precipitation rate could be explained by a Langmuir adsorption model using a conditional equilibrium constant. By identification of the type of site on the NOM molecules that is involved in the adsorption reaction, the "conditional" equilibrium constants obtained at different solution compositions converged to a single "nonconditional" value. The thermodynamic data determined at 25 degrees C and 1 atm suggest that the interaction between NOM molecules and the calcite surface is chemisorptive in nature and that adsorption is an endothermic reaction driven by the entropy change. The greatest degree of inhibition was observed for the NOM with the highest molecular weight and aromatic carbon content. For a given type of NOM, the degree of inhibition of calcite precipitation was dictated bythe balance between the enthalpy change and the entropy change of the adsorption reaction.     

Organic coprecipitates with calcite: NMR spectroscopic evidence

Dissolved organic ligands are well known to interact strongly with the calcite surface, altering precipitation and dissolution rates, crystal morphology, and possibly the ability of calcite to sequester metal contaminants. We show, using NMR spectroscopic techniques, that some of the citrate molecules present in a solution of precipitating calcite are incorporated structurally into the calcite crystal. Calcite grown by a seeded constant-addition method contains approximately 1 wt % coprecipitated citrate and yields 13C{1H} cross-polarization magic-angle spinning NMR spectra that contain broad peaks for citrate plus a signal from carbonate. Results from 13C{1H} heteronuclear correlation NMR experiments show that citrate is located in close spatial proximity to carbonate groups. In addition, calcite/citrate coprecipitates contain about 0.4 wt % excess water, which is not present as fluid inclusions, and some of which occurs as rigid structural water. These results suggestthat water and hydrogen-bonding interactions play a role in the interface between included organic molecules and the calcite host. Additional NMR data obtained for calcite coprecipitates of aspartic and glutamic acids suggest they are also incorporated structurally but at concentrations much lower than for citrate, whereas no evidence was found for phthalate incorporation.     

TRLFS evidence for precipitation of uranyl phosphate on the surface of alumina: environmental implications

We studied the ligand-enhanced sorption of uranyl ions (1-12 μM) on α-alumina colloids suspended in (and pre-equilibrated with) solutions at various concentrations of phosphate ions (P(T) = 0-900 μM). A highly sensitive technique, time resolved laser-induced fluorescence spectroscopy (TRLFS), was used to examine the chemical speciation of uranyl sorbed at trace concentrations (0.4-4 μmol U·g?1). The suspensions with P(T) ≥ 100 μM exhibited high uranyl adsorption, and a very high intensity of fluorescence that increased with the sorbed amounts of phosphate and uranyl. These samples exhibited similar spectral and temporal characteristics of fluorescence emission, evidencing a uniform speciation pattern and a single coordination environment for sorbed U, despite large variation in parameters such as aqueous uranyl speciation, U loading, and extent of coverage of alumina by secondary Al phosphates precipitating on the surface. The results pointed formation of surface precipitates of uranyl phosphates, which are characterized by high quantum yield, peak maxima at positions similar to those of U(VI) phosphate minerals and four lifetimes indicating distortions, in-homogeneities or varying number of water molecules in the lattice. The findings have major implications for our understanding of the mechanisms of immobilization of U at trace levels on surfaces of oxides submitted to phosphated solutions in soils with low pH.     

Potential for carbon adsorption on concrete: surface XPS analyses

The concrete industry is a contributor to the global carbon cycle particularly with respect to the contribution of carbon dioxide in the manufacturing of cement (calcination). The reverse reaction of carbonation is known to occur in concrete, but is usually limited to exterior surfaces exposed to carbon dioxide and humidity in the air. As alternate concrete uses expand which have more surface area, such as crushed concrete for recycling, it is important to understand surface adsorption of carbon dioxide and the positive impacts it might have on the carbon cycle. X-ray photoelectron spectroscopy (XPS) is used in this study to evaluate carbon species on hydrated cement mortar surfaces. Initial estimates for carbon absorption in concrete using othertechniques predictthe potential for carbonate species to be a fraction of the calcination stoichiometric equivalent The XPS results indicate that there is a rapid and substantial uptake of carbon dioxide on the surfaces of these mortars, sometimes exceeding the calcination stoichiometric equivalents, indicative of carbon dioxide surface complexation species. On pure calcite, the excess is on the order of 30%. This accelerated carbon dioxide surface adsorption phenomenon may be importantfor determining novel and effective carbon sequestration processes using recycled concrete.     

Evidence for surface precipitation of phosphate on goethite

Recent studies have suggested that the interaction between phosphate and goethite includes ternary adsorption/ surface precipitation as well as surface complexformation. The ternary adsorption/surface precipitation process envisioned involves the dissolution of the goethite crystal and subsequent adsorption of iron on the surface-bound phosphate. Further evidence to support the suggested process is needed. The process was investigated using two approaches. First, the sorption of iron spiked into a slurry of phosphated goethite and the effect of the iron sorption on phosphate uptake kinetics were investigated to determine whether iron would be adsorbed on the phosphated surface and whether it would enhance phosphate adsorption. Lead was also spiked into solution for comparison. Second, changes in the xi-potential of phosphated goethite were monitored with time. Adsorption of iron on the surface of phosphated goethite should increase the xi-potential of the goethite. Iron spiked into a phosphated goethite slurry was adsorbed on the solid with a concurrent adsorption of phosphate. The iron adsorption did not change the slow phosphate adsorption kinetics. Lead spiked into the solution was also sorbed but to a lesser extent than iron and with a lower apparent P:Pb mole ratio. Lead addition also changed the phosphate adsorption kinetics. With time, the xi-potential of phosphated goethite became more positive, returning almost to the potential of unphosphated goethite at low surface coverages. The slow increase in xi-potential over time indicates that long-term reactions are occurring on the goethite surface, most likely involving the dissolution of goethite to release iron and the subsequent reaction between the iron and surface-bound phosphate. These results provide strong support for the surface precipitation model, and are inconsistent with models envisioning the reaction between phosphate and the goethite surface as involving only monolayer surface complex formation.     

NOM removal by adsorption and membrane filtration using heated aluminum oxide particles

Heated aluminum oxide particles (HAOPs) are a newly synthesized adsorbent with attractive properties for use in hybrid adsorption/membrane filtration systems. This study compared removal of natural organic matter (NOM) from water by adsorption onto HAOPs with that by adsorption onto powdered activated carbon (PAC) or coagulation with alum or ferric chloride (FeCl3); explored the overlap between the NOM molecules that preferentially adsorb to HAOPs and those that are removed by the more conventional approaches; and evaluated NOM removal and fouling in hybrid adsorbent/membrane systems. For equivalent molar doses of the trivalent metals, HAOPs remove more NOM, and NOM with higher SUVA254, than alum or FeCl3. Most of the HAOPs-nonadsorbable fraction of the NOM can be adsorbed by PAC; in fact, that fraction appears to be preferentially adsorbed compared to the average NOM in untreated water. Predeposition of the adsorbents on a microfiltration membrane improves system performance. For the water tested, at a flux of 100 L/m2-hr, predeposition of 11 mg/L PAC and 5 mg/L HAOPs (as Al3+) allowed the system to operate 5 times as long before the transmembrane pressure increased by 1 psi and to remove 10-20 times as much NOM as when no adsorbents were added.     

Simultaneous application of dissolution/precipitation and surface complexation/surface precipitation modeling to contaminant leaching

This paper discusses the modeling of anion and cation leaching from complex matrixes such as weathered steel slag. The novelty of the method is its simultaneous application of the theoretical models for solubility, competitive sorption, and surface precipitation phenomena to a complex system. Selective chemical extractions, pH dependent leaching experiments, and geochemical modeling were used to investigate the thermodynamic equilibrium of 12 ions (As, Ca, Cr, Ba, SO4, Mg, Cd, Cu, Mo, Pb, V, and Zn) with aqueous complexes, soluble solids, and sorptive surfaces in the presence of 12 background analytes (Al, Cl, Co, Fe, K, Mn, Na, Ni, Hg, NO3, CO3, and Ba). Modeling results show that surface complexation and surface precipitation reactions limit the aqueous concentrations of Cd, Zn, and Pb in an environment where Ca, Mg, Si, and CO3 dissolve from soluble solids and compete for sorption sites. The leaching of SO4, Cr, As, Si, Ca, and Mg appears to be controlled by corresponding soluble solids.     

Time scales for sorption-desorption and surface precipitation of uranyl on goethite

The sorption of uranium on mineral surfaces can significantly influence the fate and transport of uranium contamination in soils and groundwater. The rates of uranium adsorption and desorption on a synthetic goethite have been evaluated in batch experiments conducted at constant pH of 6 and ionic strength of 0.1 M. Adsorption and desorption reactions following the perturbation of initial states were complete within minutes to hours. Surface-solution exchange rates as measured by an isotope exchange method occur on an even shorter time scale. Although the uranium desorption rate was unaffected by the aging of uranium-goethite suspensions, the aging process appears to remove a portion of adsorbed uranium from a readily exchangeable pool. The distinction between sorption control and precipitation control of the dissolved uranium concentration was also investigated. In heterogeneous nucleation experiments, the dissolved uranium concentration was ultimately controlled by the solubility of a precipitated uranyl oxide hydrate. The X-ray diffraction pattern of the precipitate is characteristic of the mineral schoepite. Precipitation is kinetically hindered at low degrees of supersaturation. In one experiment, metastable sorption controlled dissolved uranium concentrations in excess of the solubility limit for more than 30 d.     

响应曲面法优化大孔树脂吸附甜菊苷工艺

以甜菊叶为原料提取甜菊苷,通过大孔吸附树脂的筛选,以及探讨大孔吸附树脂AB-8的溶液浓度、pH、温度、吸附时间对树脂吸附条件的影响.利用Box-Behnken中心组合设计原理,应用响应曲面分析方法,确定了大孔吸附树脂对甜菊苷最佳吸附条件为:温度为38.4℃、pH为6.0、浓度为2.45mg/mL,响应曲面优化后树脂在最佳吸附条件下吸附量为662mg/g.     

Atomistic simulations of calcium uranyl(VI) carbonate adsorption on calcite and stepped-calcite surfaces

Adsorption of actinyl ions onto mineral surfaces is one of the main mechanisms that control the migration of these ions in environmental systems. Here, we present computational classical molecular dynamics (MD) simulations to investigate the behavior of U(VI) in contact with different calcite surfaces. The calcium-uranyl-carbonate [Ca(2)UO(2)(CO(3))(3)] species is shown to display both inner- and outer-sphere adsorption to the flat {101?4} and the stepped {314?8} and {31?2?16} planes of calcite. Free energy calculations, using the umbrella sampling method, are employed to simulate adsorption paths of the same uranyl species on the different calcite surfaces under aqueous condition. Outer-sphere adsorption is found to dominate over inner-sphere adsorption because of the high free energy barrier of removing a uranyl-carbonate interaction and replacing it with a new uranyl-surface interaction. An important binding mode is proposed involving a single vicinal water monolayer between the surface and the sorbed complex. From the free energy profiles of the different calcite surfaces, the uranyl complex was also found to adsorb preferentially on the acute-stepped {314?8} face of calcite, in agreement with experiment.     

响应面法优化淀粉微球吸附姜黄素工艺研究

采用反相悬浮法合成淀粉微球作用于姜黄素吸附量的最佳工艺条件。通过单因素实验和Plackett–Burman试验确定了交联剂用量、油水比例、合成温度和乳化剂用量对姜黄素吸附量的影响,根据中心组合设计原理采用四因素三水平的响应面分析法,得到淀粉微球吸附姜黄素的最佳工艺条件。结果表明,淀粉微球吸附姜黄素的最佳条件:交联剂用量为5.5 mL,油水比例为4.01∶1,反应温度为45.1℃,乳化剂用量为0.5 g,姜黄素吸附量预测值为1.228 mg/g,验证值为1.259 mg/g,与预测值的相对误差为0.031 mg/g。     

Spectral induced polarization signatures of hydroxide adsorption and mineral precipitation in porous media

The spectral induced polarization (SIP) technique is a promising approach for delineating subsurface physical and chemical property changes in a minimally invasive manner. To facilitate the understanding of position and chemical properties of reaction fronts that involve mineral precipitation in porous media, we investigated spatiotemporal variations in complex conductivity during evolution of urea hydrolysis and calcite precipitation reaction fronts within a silica gel column. The real and imaginary parts of complex conductivity were shown to be sensitive to changes in both solution chemistry and calcium carbonate precipitation. Distinct changes in imaginary conductivity coincided with increased hydroxide ion concentration during urea hydrolysis. In a separate experiment focused on the effect of hydroxide concentration on interfacial polarization of silica gel and well-sorted sand, we found a significant dependence of the polarization response on pH changes of the solution. We propose a conceptual model describing hydroxide ion adsorption behavior in silica gel and its control on interfacial polarizability. Our results demonstrate the utility of SIP for noninvasive monitoring of reaction fronts, and indicate its potential for quantifying geochemical processes that control the polarization responses of porous media at larger spatial scales in the natural environment.     

桔梗多糖脱蛋白方法的研究

目的：探讨桔梗多糖脱蛋白的方法。方法：以蛋白去除率和多糖损失率为指标,比较Sevag法、三氯乙酸-正丁醇法、正丁醇法、AB-8大孔吸附树脂法去除桔梗多糖中蛋白质的效果。结果：AB-8大孔吸附树脂脱蛋白的效果最好,蛋白去除率为90.1%,多糖损失率为17.4%。结论：AB-8大孔吸附树脂蛋白去除率高,且多糖损失率低,是一种有效的脱蛋白方法。     

Phosphate adsorption on aluminum-impregnated mesoporous silicates: surface structure and behavior of adsorbents

Phosphorus from excess fertilizers and detergents ends up washing into lakes, creeks, and rivers. This overabundance of phosphorus causes excessive aquatic plant and algae growth and depletes the dissolved oxygen supply in the water. In this study, aluminum-impregnated mesoporous adsorbents were tested for their ability to remove phosphate from water. The surface structure of the materials was investigated with X-ray diffraction (XRD), a N2 adsorption-desorption technique, Fourier transform-infrared (FT-IR), and X-ray photoelectron spectroscopy (XPS) to understand the effect of surface properties on the adsorption behavior of phosphate. The mesoporous materials were loaded with Al components by reaction with surface silanol groups. In the adsorption test, the Al-impregnated mesoporous materials showed fast adsorption kinetics as well as high adsorption capacities, compared with activated alumina. The uniform mesopores of the Al-impregnated mesoporous materials caused the diffusion rate in the adsorption process to increase, which in turn caused the fast adsorption kinetics. High phosphate adsorption capacities of the Al-impregnated mesoporous materials were attributed to not only the increase of surface hydroxyl density on Al oxide due to well-dispersed impregnation of Al components but also the decrease in stoichiometry of surface hydroxyl ions to phosphate by the formation of monodentate surface complexes.     

Influence of Bacillus subtilis cell walls and EDTA on calcite dissolution rates and crystal surface features

This study investigates the influence of EDTA and the Gram-positive cell walls of Bacillus subtilis on the dissolution rates and development of morphological features on the calcite [1014] surface. The calcite dissolution rates are compared at equivalent saturation indicies (SI) and relative to its dissolution behavior in distilled water (DW). Results indicate that the presence of metabolically inactive B. subtilis does not affect the dissolution rates significantly. Apparent increases in dissolution rates in the presence of the dead bacterial cells can be accounted for by a decrease of the saturation state of the solution with respect to calcite resulting from bonding of dissolved Ca2+ by functional groups on the cell walls. In contrast, the addition of EDTA to the experimental solutions results in a distinct increase in dissolution rates relative to those measured in DW and the bacterial cell suspensions. These results are partly explained by the 6.5-8 orders of magnitude greater stability of the Ca-EDTA complex relative to the Ca-B. subtilis complexes as well as its free diffusion to and direct attack of the calcite surface. Atomic force microscopy images of the [1014] surface of calcite crystals exposed to our experimental solutions reveal the development of dissolution pits with different morphologies according to the nature and concentration of the ligand. Highly anisotropic dissolution pits develop in the early stages of the dissolution reaction at low B. subtilis concentrations (0.004 mM functional group sites) and in DW. In contrast, at high functional group concentrations (4.0 mM EDTA or equivalent B. subtilis functional group sites), dissolution pits are more isotropic. These results suggest that the mechanism of calcite dissolution is modified by the presence of high concentrations of organic ligands. Since all the pits that developed on the calcite surfaces display some degree of anisotropy and dissolution rates are strongly SI dependent, the rate-limiting step is most likely a surface reaction for all systems investigated in this study. Results of this study emphasize the importance of solution chemistry and speciation in determining calcite reaction rates and give a more accurate and thermodynamically sound representation of dead bacterial cell wall-mineral interactions. In studies of natural aquatic systems, the presence of organic ligands is most often ignored in speciation calculations. This study clearly demonstrates that this oversight may lead to an overestimation of the saturation state of the solutions with respect to calcite and thermodynamic inconsistencies.     

Protease susceptibility of beta-lactoglobulin adsorbed on stainless steel surface as evidence of contribution of its specific segment to adsorption

beta-Lactoglobulin (beta-Lg) is a major constituent of fouling deposits in the dairy industry. To determine the interaction between beta-Lg and stainless steel surfaces, beta-Lg irreversibly adsorbed on stainless steel particles was subjected to lysyl endopeptidase treatment and the course of fragmentation was compared with that observed for beta-Lg in solution. The results showed a distinct difference between the courses of fragmentation: a fragment (residues 102-135) was liberated readily from beta-Lg in solution but scarcely from beta-Lg irreversibly adsorbed on stainless steel particles. This result strongly suggests that residues 102-135 include a segment primarily responsible for the interaction of beta-Lg with stainless steel surfaces. This supports our previous results [Sakiyama et al., J. Biosci. Bioeng., 88, 536-541 (1999)] that showed that residues 125-135 of beta-Lg have a strong affinity toward stainless steel surfaces and probably a major contribution to the adsorption of beta-Lg.     

涤纶织物吸湿排汗整理工艺研究

采用吸湿快干整理剂Hipom A-790对涤纶织物进行吸湿排汗整理,系统研究了整理液质量分数、整理温度和整理时间对涤纶织物吸湿排汗性能的影响,并得到最佳整理工艺条件为:整理液质量分数为3%,整理温度为80℃,整理时间为40 min,经整理后的织物水洗20次后仍保持较好的吸湿排汗性能。