Sorption mechanisms of cadmium onto nano-hydroxyapatite: Comparative uptake studies and correlative solubility analysis

Hydroxyapatite (HAP) nanorod was synthesized by using ordered bicontinuous microemulsion, and was characterized by X-ray diffraction spectrum, scanning electron microscopy and high-resolution transmission electron microscopy. The solubility was investigated using ion chromatogram measurement, and was sensitively dependent on the value of pH. The capabilities to capture cadmium ions of HAP nanorods and micro-particles from aqueous salt solution were studied, and besides, X-ray diffractometry was used in order to characterize the mechanisms of cadmium uptake. As Cd²⁺ adsorption occurred, it has undergone the coprecipitation of Cd₂P₂O₇ crystal to give multiple solids composed of HAP and Cd₂P₂O₇ crystallites, which affected the dissolution of hydroxyapatite. During Cd²⁺ sorption process, solubility variations of the sediments showed a more complex pattern than previously reported. Contribution of surface mechanisms, in association with precipitation of cadmium phosphate phases, could explain this behavior.     

Influence of Calcium Sources on Microbially Induced Calcium Carbonate Precipitation by Bacillus sp. CR2

Stimulation of microbially induced calcium carbonate precipitation (MICCP) is likely to be influenced by calcium sources. In order to study such influences, we performed MICCP using Bacillus sp. CR2 in nutrient broth containing urea, supplemented with different calcium sources (calcium chloride, calcium oxide, calcium acetate and calcium nitrate). The experiment lasted 7 days, during which bacterial growth, urease activity, calcite production and pH were measured. Our results showed that calcium chloride is the better calcium source for MICCP process, since it provides higher urease activity and more calcite production. The influences of calcium sources on MICCP were further studied using Fourier transform-infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses. These analyses confirmed that the precipitate formed was CaCO₃ and composed of predominantly calcite crystals with a little amount of aragonite and vaterite crystals. The maximum yield of calcite precipitation was achievable with calcium chloride followed by calcium nitrate as a calcium source. The results of present study may be applicable to media preparation during efficient MICCP process.     

Thermal decomposition of Cd(CH3COO)2·2H2O studied by a coupled TG-DTA-MS method

The thermal decomposition of cadmium acetate dihydrate in helium and in air atmosphere has been investigated by means of a coupled TG-DTA-MS method combined with X-ray diffraction analysis. Dehydration of Cd(CH₃COO)₂·2H₂O is a two-stage process with Cd(CH₃COO)₂·H₂O as intermediate. The way of Cd(CH₃COO)₂ decomposition strongly depends on the surrounding gas atmosphere and the rate of heating. CdO, acetone and CO₂ are the primary products of decomposition in air. In helium decomposition goes by two parallel and consecutive reactions in which intermediates, Cd and CdCO₃, are formed. Metallic cadmium oxidizes and cadmium carbonate decomposes giving CdO. Some of the metallic cadmium, depending on the heating rate and the concentration of oxygen, evaporates. Acetone is partially oxidized in secondary reactions with oxygen. This revised version was published online in July 2006 with corrections to the Cover Date.     

Adsorption of mercury(II) on macroreticular styrene-divinylbenzene copolymer beads

The adsorption characteristics of mercury(II) on several kinds of styrene-divinylbenzene copolymer beads having different surface properties were studied. It was found that the polymer beads selectively adsorbed mercury(II) from solutions over a wide range of pH with high efficiency. The amount of mercury(II) adsorbed increased with increase in specific surface area of the polymer beads and the adsorption behaviour was found to be of the Langmuir type. The presence of chloride strongly reduced the adsorption, but this interference was not observed with nitrate, sulphate, perchlorate, cadmium(II), cobalt(II), copper(II), nickel(II), silver(I) and zinc(II). More than 95% of the mercury(II) adsorbed on a column of polymer beads could be recovered with dilute hydrochloric acid.     

Kinetics of the thermal decomposition of cadmium carbonate (CdCO3)

Cadmium carbonate used in the study was prepared from cadmium chloride, ammonium carbonate and ammonia. The X-ray powder diffraction, infrared spectral and chemical analysis conducted on the product show that the sample is of analytically acceptable purity. The thermal decomposition kinetics of cadmium carbonate was then studied by using the isothermal thermogravimetric method under a flow of dry nitrogen gas. The decomposition kinetics is best described by a two-dimensional phase boundary reaction mechanism (R ₂). An activation energy (E _a) of 135.006 kJ·mol^?1 and natural logarithm of the frequency factor (lnZ) of 16.754 were obtained in the range of 9 temperatures (400, 390, 380, 370, 360, 350, 340, 330 and 320°C).     

纳米钛酸钙粉体的制备及其对水中铅和镉的吸附行为

采用柠檬酸络合溶胶-凝胶法制备了纳米钛酸钙粉体. 以X射线衍射(XRD)、透射电子显微镜(TEM)和傅立叶变换红外光谱(FTIR)对其进行了表征. 考察了该纳米粉体对水中重金属铅和镉的吸附性能, 并以镉离子为例, 系统地研究了吸附热力学和动力学. 结果表明, 该法合成的钛酸钙为钙钛矿结构的纳米粉体, 粒径大小受灼烧温度的影响, 灼烧温度越高, 粉体平均粒径越大, 600 ℃灼烧2 h条件下, 粉体的平均粒径最小, 约为20 nm. 当介质的pH值为4～8时, 钛酸钙对水中的铅和镉具有很强的吸附能力. 其对镉离子的吸附行为符合Langmuir 吸附等温模型和HO准二级动力学方程式, 吸附过程焓变(ΔH)为39.312 kJ•mol－1, 各温度下的自由能变(ΔG)均小于零, 熵变(ΔS)均为正值, 吸附过程的活化能(Ea)为20.359 kJ•mol－1. 该吸附过程是自发的吸热物理过程. 被吸附的铅和镉均可用1 mol•L－1的硝酸完全洗脱回收. 对铅和镉的富集因子均超过200. 将其应用于水中痕量铅和镉的吸附富集和测定, 回收率分别为96.3%～107.2%和93.5%～104.0%, 与石墨炉原子吸收光谱法测定结果一致.     

Structural characterization of ammonium molybdophosphate with different amount of cesium adsorption

Powdered crystal AMP was synthesized by precipitation method and different Cs-loading AMP samples were prepared in this work. They were studied by modern methods of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA) to gain a better understanding of the ion exchange mechanism of ammonium molybdophosphate (AMP). The XRD patterns combined with the FT-IR analysis indicate that the Keggin structure is retained after Cs adsorption, while the changes of certain characteristic diffraction peaks show crystal lattice distortion caused by the mismatching sizes of Cs⁺ and [NH₄]⁺. The TGA showed that the weight percent of H₂O and [NH₄]⁺ decreases as the amount of Cs adsorbed onto AMP increases, which corresponds with the theory of isomorphous exchange.     

磁性Fe_3O_4@SiO_2@ZrO_2对水中磷酸盐的吸附研究

合成了以Fe3O4为核,以SiO2为壳的磁性纳米微粒(Fe3O4@Si O2),并采用沉淀沉积法将ZrO2包覆到材料表面。通过XRD、TEM、VSM、ζ电位、XPS和N2吸附/脱附等手段对材料进行表征,结果表明材料Fe3O4@SiO2@ZrO2上沉积了氧化锆纳米颗粒,具有超顺磁性,可在外加磁场作用下实现从水中快速分离。同时系统研究了材料对水中磷酸盐的吸附行为,结果表明沉积Zr O2使得材料对磷酸盐表现出良好的吸附性能,并且随着沉积量的增大吸附量增加。吸附等温线符合Freundlich方程。吸附动力学可用拟二级动力学模型描述,吸附速率随磷酸盐初始浓度增加而减小。磷酸盐吸附量随溶液p H值的增大而减小,但几乎不受离子强度影响。     

磁性Fe3O4@SiO2@ZrO2对水中磷酸盐的吸附研究

合成了以Fe₃O₄为核,以SiO₂为壳的磁性纳米微粒（Fe₃O₄@SiO₂）,并采用沉淀沉积法将ZrO₂包覆到材料表面。通过XRD、TEM、XPS和N₂吸附/脱附等手段对材料进行表征,结果表明材料Fe₃O₄@SiO₂@ZrO₂上沉积了氧化锆纳米颗粒,具有超顺磁性,可在外加磁场作用下实现从水中快速分离。同时系统研究了材料对水中磷酸盐的吸附行为,结果表明沉积ZrO₂使得材料对磷酸盐表现出良好的吸附性能,并且随着沉积量的增大吸附量增加。吸附等温线可用Freundlich方程拟合。吸附动力学可用拟二级动力学模型拟合,吸附速率随初始浓度增加而减缓。磷酸盐吸附量随溶液pH值的增大而减小,但几乎不受离子强度影响。     

Adsorption of fluoride ions onto carbonaceous materials

The characteristics of fluoride ion adsorption onto carbonaceous materials were derived as adsorption isotherms at different temperatures and in different pH solutions. The fluoride ion was adsorbed into pores in carbonaceous materials produced from wood; the larger the specific surface area, the more fluoride ions adsorbed. Bone char was the most effective adsorbent. The composition of bone char includes calcium phosphate, calcium carbonate, and so on. This suggests that the phosphate ion in bone char was exchanged with a fluoride ion. Moreover, the mechanism of fluoride ion adsorption onto bone char is clearly chemical in nature because the amount of fluoride ion adsorbed onto bone char increased with increasing temperature and decreasing pH. The amount of fluoride ion adsorbed onto bone char was also shown to depend on the concentration of sodium chloride in solution because of the "salting-out" effect. The adsorption of fluoride ion onto bone char is endothermic. Bone char can be utilized to remove fluoride ions from drinking water.     

Fluoride removal using lanthanum incorporated chitosan beads

Highly selective material based on naturally occurring biomaterial namely chitosan has been designed for the defluoridation of water. Lanthanum incorporated chitosan beads (LCB) were prepared using precipitation method. The synthesis was optimized by varying different synthesis parameters namely lanthanum loading, complexation and precipitation time, strength of ammonia solution used for precipitation, drying time, etc. Lanthanum incorporated chitosan beads were characterized using SEM, FTIR, XRD and EDX. Surface area of LCB was observed to be 2.76 m² g⁻¹. The equilibrium adsorption data fitted well to Langmuir adsorption isotherm and showing maximum fluoride adsorption capacity of 4.7 mg g⁻¹ with negligible lanthanum release. Kinetic study reveals that adsorption of fluoride is fast and follows pseudo-first-order kinetics. The effect of pH was also studied and the best efficiency was observed at pH 5. Presence of sulphate, nitrate and chloride marginally affected the removal efficiency, however drastic reduction in fluoride uptake was observed in the presence of carbonate and bicarbonate. Negative value of change in free energy (ΔG°) and positive value of change in entropy (ΔS°) suggest the adsorption of fluoride by LCB is feasible and spontaneous process. Positive value of change in enthalpy (ΔH°) suggests the process of fluoride adsorption is endothermic in nature. Regeneration study reveals that 1 M ammonium chloride solution appears to be the promising regeneration media showing 81.22% regeneration. The adsorption capacity of LCB was similar in fluoride-contaminated ground water collected from Dhar district of Madhya Pradesh, India, as compared to simulated water.     

Preparation,characterization and application of a CTAB-modified nanoclay for the adsorption of an herbicide from aqueous solutions: Kinetic and equilibrium studies

In this study, surfactant-modified pillared montmorillonites (MMT) were prepared using cetyltrimethylammonium bromide (CTAB) by the intercalation method and used as adsorbent to remove bentazon from aqueous solutions. The main compositions of MMT and CTAB/MMT were characterized by Fourier transform–infrared spectroscopy (FT–IR), X-ray diffraction (XRD), scanning electron micrography (SEM) and energy dispersive X-ray (EDX) spectroscopy. The removal efficiency of bentazon was studied as a function of adsorbent dosage, pH, initial bentazon concentration and ionic strength (sodium carbonate, sodium bicarbonate, sodium sulfate and sodium chloride). The removal efficiency of bentazon by CTAB/MMT was more than that of MMT in similar conditions. By increasing adsorbent dosage and initial bentazon concentration, the removal efficiency was increased and declined, respectively. The results showed that the maximum adsorption of organo-modified montmorillonite was obtained at pH 3. The maximum adsorption capacity was estimated to be 500 mg/g at pH 3 and room temperature. The study of the adsorption kinetic model revealed that the pseudo-second order model was the best applicable one to describe the adsorption of bentazon onto CTAB/MMT. Adsorption data were analyzed by both Langmuir and Freundlich adsorption isotherms and the results showed that it was better described by the Langmuir model. The adsorption capacities of the samples were found to increase with Na₂CO₃ anion saturation, while they decreased in the presence of NaHCO₃, Na₂SO₄ and NaCl.     

Adsorption of cadmium ions from aqueous solutions using nano-montmorillonite: kinetics,isotherm and mechanism evaluations

With increasing industrial development, heavy metal pollution, e.g., cadmium (Cd) pollution, is increasingly serious in soil and water environments. This study investigated the sorption performance of nano-montmorillonite (NMMT) for Cd ions. Adsorption experiments were carried out to examine the effects of the initial metal ion concentration (22.4–224 mg/L), pH (2.5–7.5), contact time (2–180 min) and temperature (15–40 °C). A simulated acid rain solution was prepared to study the desorption of Cd adsorbed on NMMT. After the adsorption or desorption process, the supernatant was analyzed using a flame atomic absorption spectrometry method. The Cd removal rate increased as the pH and contact time increased but decreased as the initial metal ion concentration increased. The maximum adsorption capacity was estimated to be 17.61 mg/g at a Cd²⁺ concentration of 22.4 mg/L. The sorption process can be described by both the Langmuir and Freundlich models, and the kinetic studies revealed that the pseudo-second-order model fit the experimental data. The Cd desorption rate when exposed to simulated acid rain was less than 1%. NMMT possesses a good adsorption capacity for Cd ions. Additionally, ion exchange was the main adsorption mechanism, but some precipitation or surface adsorption also occurred.     

磁性水铁矿的制备及其对二甲基亚胂酸的吸附性能

二甲基亚胂酸会对人体和环境造成严重危害。用水热法合成磁性水铁矿,对产物进行了X射线衍射分析、BET比表面积分析和磁滞回线分析,结果表明磁性水铁矿纯度较高,比表面积较大,具有较强的磁性。用磁性水铁矿作为吸附剂,考察了二甲基亚胂酸在磁性水铁矿上的吸附动力学及吸附等温线。二甲基亚胂酸在磁性水铁矿上的吸附符合准二级动力学模型,吸附速率为0.34g·mg^-1·h^-1;吸附等温线符合Freundlich模型。采用Zeta电位测定、FT-IR、SEM-EDS和XPS对吸附机理进行分析,表明磁性水铁矿通过配位络合作用和静电作用来吸附二甲基亚胂酸,在吸附过程中磁性水铁矿表面形了成Fe-O-As三元络合物。研究结果为含有二甲基亚胂酸污染物的水体处理和净化提供了新方法。     

Selective self-propagating combustion synthesis of hexagonal and orthorhombic nanocrystalline yttrium iron oxide

Macroporous nanocrystalline YFeO₃ was prepared by a self-propagating combustion method using yttrium nitrate and iron nitrate as precursors and glycine as a fuel. The phase structure of the product can be selectively controlled to be hexagonal or orthorhombic by simply adjusting the ratio of glycine to nitrate. The samples were characterized by X-ray diffraction (XRD) analysis, N₂ adsorption, micro-Raman spectroscopy, Fourier transform infrared absorption spectroscopy (FT-IR), thermal analysis (TGA/DSC), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and magnetic property analysis. Photocatalytic activity for the degradation of methylene blue in water under visible light irradiation shows that the orthorhombic YFeO₃ is superior to the hexagonal form. More adsorbed oxygen and ferromagnetism of the orthorhombic sample may explain its high activity.     

Shape evolution of SrCO3 particles in the presence of poly-(styrene-alt-maleic acid)

In this paper, strontium carbonate particles with different morphologies were prepared by a simple precipitation reaction of sodium carbonate with strontium nitrate in the absence and presence of poly-(styrene-alt-maleic acid) (PSMA). The as-prepared products were characterized with scanning electron microscopy (SEM) and X-ray diffraction (XRD). The effects of the concentration of PSMA on the morphologies and phase structures of strontium carbonate particles were investigated and discussed. The results showed that SrCO₃ particles with various shapes, such as bundles, dumbbells, irregular aggregates and spheres could be obtained by varying the concentration of PSMA. A schematic illustration was proposed to account for the shape evolution of the as-prepared SrCO₃ particles.     

Mechanisms and efficiency of the simultaneous removal of metals and cyanides by using ferrate(VI): crucial roles of nanocrystalline iron(III) oxyhydroxides and metal carbonates

The reaction of potassium ferrate(VI), K₂FeO₄, with weak‐acid dissociable cyanides—namely, K₂[Zn(CN)₄], K₂[Cd(CN)₄], K₂[Ni(CN)₄], and K₃[Cu(CN)₄]—results in the formation of iron(III) oxyhydroxide nanoparticles that differ in size, crystal structure, and surface area. During cyanide oxidation and the simultaneous reduction of iron(VI), zinc(II), copper(II), and cadmium(II), metallic ions are almost completely removed from solution due to their coprecipitation with the iron(III) oxyhydroxides including 2‐line ferrihydrite, 7‐line ferrihydrite, and/or goethite. Based on the results of XRD, Mössbauer and IR spectroscopies, as well as TEM, X‐ray photoelectron emission spectroscopy, and Brunauer–Emmett–Teller measurements, we suggest three scavenging mechanisms for the removal of metals including their incorporation into the ferrihydrite crystal structure, the formation of a separate phase, and their adsorption onto the precipitate surface. Zn and Cu are preferentially and almost completely incorporated into the crystal structure of the iron(III) oxyhydroxides; the formation of the Cd‐bearing, X‐ray amorphous phase, together with Cd carbonate is the principal mechanism of Cd removal. Interestingly, Ni remains predominantly in solution due to the key role of nickel(II) carbonate, which exhibits a solubility product constant several orders of magnitude higher than the carbonates of the other metals. Traces of Ni, identified in the iron(III) precipitate, are exclusively adsorbed onto the large surface area of nanoparticles. We discuss the relationship between the crystal structure of iron(III) oxyhydroxides and the mechanism of metal removal, as well as the linear relationship observed between the rate constant and the surface area of precipitates.     

The synthesis of nucleus-shell Cd(OH)<Subscript>2</Subscript>/CdS structures by chemical precipitation from aqueous solutions

The region of stable coexistence of Cd(OH)₂ and CdS as a function of pH and the concentration of the complex-forming agent (ammonia) was determined by thermodynamic analysis with the purpose of the preparation of Cd(OH)₂ cadmium hydroxide particles surrounded by cadmium sulfide CdS shells. In this region, powders were obtained by chemical precipitation during various precipitation times from aqueous solutions. X-ray diffraction was used to track the growth of the CdS nanophase of a disordered structure and hexagonal Cd(OH)₂ phase consumption with time. It was found by complexonometry that part of cadmium formed insoluble structures as a result of the formation of continuous nanosized CdS shells on single crystalline Cd(OH)₂ particles. A comparative analysis of the experimental data was used to determine the kinetics of formation of nucleus-shell Cd(OH)₂/CdS particles in the system.     

Photodissolution of ferrihydrite in the presence of oxalic acid: an in situ ATR-FTIR/DFT study

The photodissolution of the iron oxyhydroxide, ferrihydrite, in the presence of oxalic acid was investigated with vibrational spectroscopy, density functional theory (DFT) calculations, and batch geochemical techniques that determined the composition of the solution phase during the dissolution process. Specifically, in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR- FTIR) was used to determine the structure of the adsorbed layer during the dissolution process at a solution pH of 4.5. DFT based computations were used to interpret the vibrational data associated with the surface monolayer in order to help determine the structure of the adsorbed complexes. Results showed that at pH 4.5, oxalate adsorbed on ferrihydrite adopted a mononuclear bidentate (MNBD) binding geometry. Photodissolution at pH 4.5 exhibited an induction period where the rate of Fe(II) release was limited by a low concentration of adsorbed oxalate due to the site-blocking of carbonate that was intrinsic to the surface of the ferrihydrite starting material. Oxalate displaced this initial carbonate over time, and the dissolution rate showed a corresponding increase. Irradiation of oxalate/ferrihydrite at pH 4.5 also ultimately led to the appearance of carbonate reaction product (distinct from carbonate intrinsic to the starting material) on the surface.     

Epoxidation of the polyisobutene using WO3/ZrO2 catalyst

Tungsten-promoted zirconia (WO₃/ZrO₂) was prepared by surfactant-assisted precipitation. Textural and structural characterizations of the catalysts were performed by means of N₂ adsorption, X-ray diffraction, and BET surface area. Temperature-programmed desorption of ammonia and FTIR spectra of adsorbed pyridine were used to determine the acid properties. The catalytic activities of samples have also been evaluated by the epoxidation reaction of the polyisobutene in a heterogeneous system. It was observed that WO₃/ZrO₂ has a beneficial effect modified by PEG-400.