Interaction of metal ions with clays: I. A case study with Pb(II)

Pb(II) adsorption was studied under different conditions (pH, time, metal ion concentration, clay amount, temperature) on kaolinite, montmorillonite, and their poly(hydroxo)zirconium (ZrO–kaolinite, ZrO–montmorillonite) and tetrabutylammonium (TBA–kaolinite, TBA–montmorillonite) derivatives. All samples were calcined (ZrO-derivatives at 773 K, TBA-derivatives at 973 K) before using as adsorbents. The data were interpreted assuming first- and second-order kinetics. The rate constants including the pore diffusion rate constant are reported. The adsorption data could be fitted with Freundlich and Langmuir isotherms, and the coefficients indicated favorable adsorption of Pb(II) on the clays. Determination of the thermodynamic parameters, ΔH, ΔS, and ΔG showed the adsorption to be exothermic accompanied by decrease in entropy and Gibbs energy.     

A molecular mechanics study of the adsorption of ethane and propane on V2O5(001) surfaces with oxygen vacancies

Adsorption geometries and energies of ethane and propane physisorbed on the (001) surface of vanadium pentoxide with oxygen vacancies were determined by a molecular mechanics simulation. Three types of oxygen vacancies, built up by removal of vanadyl oxygen, two-fold and three-fold coordinated oxygen, respectively, have been modeled as defects. The energetically most favorable adsorption site is on top of a vacancy of two-fold coordinated oxygen for ethane and propane, respectively. The next favorable site for both alkanes is on top of a vacancy of vanadyl oxygen. Due to the generation of a “van der Waals cage” which traps the hydrocarbon the adsorption on the defect site is favored in comparison with the ideal surface for these two defect types. A vacancy of three-fold coordinated oxygen does not lead to an enhancement of adsorption and pushes the reactant towards the unperturbed surface areas due to the fact, that no energy minimum can be obtained in the vicinity of the defect. This revised version was published online in August 2006 with corrections to the Cover Date.     

Optimization of kinetic data for two‐stage batch adsorption of Pb(II) ions onto tripolyphosphate‐modified kaolinite clay

BACKGROUND: Most adsorption studies consider only the adsorption of pollutants onto low cost adsorbents without considering how equilibrium and kinetic data can be optimized for the proper design of adsorption systems. This study considers the optimization of kinetic data obtained for the removal of Pb(II) from aqueous solution by a tripolyphosphate modified kaolinite clay adsorbent. RESULTS: Modification of kaolinite clay with pentasodium tripolyphosphate increases its cation adsorption capacity (CEC) and specific surface area (SSA) from 7.81 to 78.9 meq (100 g)^?1 and 10.56 to 13.2 m² g^?1 respectively. X‐ray diffraction patterns for both unmodified and tripolyphosphate‐modified kaolinite clay suggest the modification is effective on the surface of the clay mineral. Kinetic data from the batch adsorption of Pb(II) onto the tripolyphosphate‐modified kaolinite clay adsorbent were optimized to a two‐stage batch adsorption of Pb(II) using the pseudo‐second‐order kinetic model. Mathematical model equations were developed to predict the minimum operating time for the adsorption of Pb(II). Results obtained suggest that increasing temperature and decreasing percentage Pb(II) removal by the adsorbent enhanced operating time of the adsorption process. The use of two‐stage batch adsorption reduces contact time to 6.7 min from 300 min in the single‐stage batch adsorption process for the adsorption of 2.5 m³ of 500 mg L^?1 Pb(II) under the same operating conditions. CONCLUSION: Results show the potential of a tripolyphosphate‐modified kaolinite clay for the adsorption of Pb(II) from aqueous solution and the improved efficiency of a two‐stage batch adsorption process for the adsorption of Pb(II) even at increased temperature. Copyright © 2009 Society of Chemical Industry     

CH3Cl在ZnO(001)和ZnO(100)表面吸附的第一性原理计算

基于密度泛函理论的第一性原理方法,建立ZnO(100)和ZnO(001)表面的吸附模型,计算了吸附能、电荷密度、态密度以及过渡态等参数,研究了CH3Cl在ZnO不同表面、不同位点、不同吸附方式的吸附情况。结果表明,CH3Cl在ZnO(100)和ZnO(001)表面的吸附过程均为化学吸附。当CH3Cl整体吸附时,CH3Cl分子中的Cl原子可以与ZnO表面的Zn(2a)原子生成Zn-Cl键,CH3Cl在ZnO(100)表面的吸附能比在ZnO(001)表面的吸附能更低(-0.57 eV vs. -0.42 eV),体系更稳定;并且CH3Cl在ZnO(100)面吸附后,Cl原子的3p轨道态密度峰向左移动,且靠近费米能级处的峰值降低,表明Cl原子在吸附过程中提供电子,与Zn形成更稳定的相互作用。当CH3Cl解离吸附时,甲基自由基中的C原子可以分别与ZnO(100)表面的O(2a)和O(3a)吸附,CH3Cl解离吸附在Zn(2a)和O(2a)原子处的吸附能为-1.09 eV,在费米能级左侧O 2p轨道和C 2p轨道存在3个共振峰,证明C原子和O原子有较强的相互作用,而在Zn(2a)和O(3a)原子处的吸附能为-1.02 eV,且费米能级右侧O 2p轨道和C 2p轨道存在1个共振峰,表明C和O原子存在反键作用。过渡态的计算结果表明,CH3Cl解离吸附在Zn(2a)和O(2a)位点的过渡态能垒比在Zn(2a)和O(3a)位点更低(1.69 eV vs. 2.06 eV),因此CH3Cl解离吸附反应倾向于在ZnO(100)表面上相邻的Zn和O原子之间发生。     

Adsorption of Co(II) from Aqueous Medium on Natural and Acid Activated Kaolinite and Montmorillonite

Abstract

Hazardous metal cations enter water through the natural geochemical route or from the industrial wastes. Their separation and removal can be achieved by adsorptive accumulation of the cations on a suitable adsorbent. In the present work, toxic Co(II) ions are removed from water by accumulating them on the surface of clay minerals. Clay adsorbents are obtained from kaolinite, montmorillonite, and their acid activated forms, and are characterized with the measurement of XRD patterns, specific surface area, and cation exchange capacity. The adsorption experiments are carried out in a batch process in environments of different pH, initial Co(II) concentration, amount of clay, interaction time, and temperature. Adsorption of Co(II) on the clays increases continuously from pH 1.0 to 8.0 after which adsorption could not be carried out due to the decreasing solubility of Co(II). Under appropriate conditions, the adsorption of Co(II) is very fast at low coverage approaching equilibrium within 240 min and the interactions are best described by second order kinetics. Langmuir monolayer capacity has been computed in the range of 11.2 to 29.7 mg/g and Co(II) accumulation has the order of acid‐activated montmorillonite>montmorillonite>acid activated kaolinite>kaolinite. Adsorption of Co(II) on kaolinite and acid‐activated kaolinite is endothermic driven by entropy increase but the same process follows exothermically on montmorillonite and acid‐activated montmorillonite supported by entropy decrease. In both cases, spontaneous adsorptive accumulation is ensured by favorable Gibbs energy decrease. It is found that acid activation enhances the adsorption capacity of kaolinite and montmorillonite.     

Understanding the adsorption behavior of oxygen on the 3C–SiC(1 1 0) surface: A first-principles study

The adsorption behavior of atomic oxygen and molecular O₂ on the 3C–SiC(1 1 0) surface is investigated by first-principles calculations. The atomic O prefers to be adsorbed at the C top site (C–O) with adsorption energy of −1.95 eV after zero-point energy correction, followed by the C–O–Si bridge site, Si–O–Si bridge site, and the Si top site (Si–O) with adsorption energies of −1.46, −1.36, and −1.13 eV, respectively. The molecular O₂ separately trapped by the second nearest neighboring C and Si atoms (C–O–O–Si, M4 type) is the most stable configuration with the adsorption energy of −2.46 eV, which is followed by the Si–O–O–Si (M5 type), C–O–O–Si (M3 type), O–Si–O (M2 type), and Si–O=O (M1 type) configurations with the adsorption energies of −2.24, −1.87, −1.07, and −0.75 eV, respectively. All these molecular O₂ adsorption configurations exhibit high tendency to dissociate with the dissociation barriers range of 0.09–0.19 eV. The adsorbed atomic O seems to be easily trapped at the C–O site due to the extremely low diffusion barrier. In addition, the infrared spectra of all the atomic O and molecular O₂ adsorption configurations are predicted and compared with available experimental observations.     

Structural Understanding of Self-Assembled Rare Earth Disilicide Nanostructures Via Scanning Probe Microscopy and First Principles Studies

We performed systematic experimental and computational studies to investigate the adsorption geometries of Y atoms on the Si(001) surface. This paves a way for understanding and eventually controlling the growth of rare earth disilicide wires on the Si(001) substrate that are promising for various applications. For a single Y atom, the interrow_dn site was found to be at least 400 meV lower in energy than other possible binding sites. The emulated STM images are in good agreement with experimental results of Er on Si(001). The strong bias and coverage dependence indicates the need for theoretical guidance for the correct interpretation of experimental data. We elucidate the Y-Si binding mechanism and provide insights toward the onset of formation of hexagonal rare earth disilicide wires.     

Theoretical Study of the Acetonitrile Flip-Flop with the Electric Field Orientation: Adsorption on a Pt(111) Electrode Surface

The interaction of acetonitrile on Pt(111) electrode has been studied using density functional theory methods. Periodic calculations for a coverage of 0.25 show that the most stable adsorption mode of acetonitrile in UHV conditions on Pt(111) is the side-on η²(C,N) state. In this state, the short-bridge surface site is energetically preferred to the long-bridge site. The nitrogen end-on on top of a surface platinum atom is also likely to occur. An external uniform electric field introduced in the cluster models perpendicular to the surface serves to mimic the electrode potential. It makes the adsorbed molecule move and orient perpendicular to the surface. Depending on the direction of the electric field, the acetonitrile is oriented with the nitrogen atom or the methyl group toward the surface. Our results are in agreement with recent SFG experiments performed in electrochemical environments.     

The Interaction of Noble Metal With La1–xSrxMnO3 (001) Surface and Catalytic Role for Oxygen Adsorption: A Density Functional Theory Study

Adsorption mechanisms of noble metals (Ag, Pd, Pt) on MnO₂‐terminated (001) surface and their catalytic role for oxygen adsorption have been investigated using the first‐principles density functional theory calculations. The analysis of the adsorption energies reveals that the energetically favorable configuration for Ag and Pd adsorption is at the O site, whereas one for Pt adsorption is at the Mn site. Pt atom exhibits the largest adsorption energy, followed by Pd and Ag atoms. Both bond population and PDOS (partial density of states) analysis confirm the formation of adatom–O–Mn bonds. Adsorption is accompanied by a charge transfer between adatoms and surface atoms. Significantly, we predict that the order on the increase of O₂ adsorption energy follows the Pd > Ag > Pt due to pre‐adsorbed noble metal atoms. The calculated bond length and bond population of O₂ molecule demonstrate that pre‐adsorbed noble metal atoms facilitates O₂ molecule dissociate to O atoms, thus contributing to the surface oxygen diffusion process. Our calculations identify an important catalytic role of noble metal in LSM‐based catalysts, which may improve electrochemical performance for SOFCs cathodes.     

Pb(II) sorption on molecular sieve analogues of MCM-41 synthesized from kaolinite and montmorillonite

Mesoporous molecular sieves were prepared with montmorillonite and kaolinite as silica sources, respectively, (denoted as P-M and P-K) by a hydrothermal method. The two prepared materials were characterized by XRD, FTIR and N₂ adsorption–desorption. The results indicated that both P-M and P-K are typical mesoporous molecular sieves with high specific surface areas. Sorption of Pb(II) from aqueous solution to P-M and P-K was studied by using a batch technique. The effect of contact time, pH and temperature on the sorption of Pb(II) to P-M and P-K was investigated. A simplified surface complexation model was used to simulate the complexation of Pb(II) ions onto molecular sieves. The results suggested that sorption of Pb(II) was strongly dependent on pH values. Kinetics of sorption showed that the pseudo-second-order kinetic model held for the sorption process. Equilibrium modeling showed that the sorption of Pb(II) was fitted well by the Langmuir isotherm model. The thermodynamic parameters (ΔH°, ΔS°, and ΔG°) were calculated from the sorption of Pb(II) at three different temperatures of 283 K, 303 K and 333 K. The sorption reaction was endothermic and the process was favored at high temperature. The results indicated that both P-M and P-K are suitable materials for removal of Pb(II) from large volumes of aqueous solutions.     

Uptake of Ni(II) Ions from Aqueous Solution by Kaolinite and Montmorillonite: Influence of Acid Activation of the Clays

Abstract

Kaolinite and montmorillonite were treated with 0.25 M H₂SO₄ and the acid activated clays along with the parent clays were tested for their uptake capacity for Ni(II) ions from aqueous solution. The batch adsorption experiments were conducted under a set of variables (concentration of Ni(II) ion, amount of clay, pH, time and temperature of interaction). Increasing pH favored Ni(II) uptake till the ions were precipitated as the insoluble hydroxides at pH > 8.0. The uptake was rapid up to 40 min and equilibrium was obtained within 180 min. The kinetics of the process was evaluated by subjecting the results to a number of models like the pseudo-first order, second order, Elovich equation, liquid film diffusion, and intra-particle diffusion and it was found that the data more closely resembled a second order process. The experimental data conformed to both Langmuir and Freundlich isotherms showing that the interactions were mostly chemical in nature. The clays had reasonable monolayer adsorption capacity of 10.4, 11.9, 28.4, and 29.5 mg g^?1 for kaolinite, acid activated kaolinite, montmorillonite, and acid-activated montmorillonite respectively. Montmorillonite had much better adsorption capacity than kaolinite and the acid activation boosted the adsorption capacity of both kaolinite and montmorillonite. The interactions were exothermic in nature, accompanied by decrease in both entropy and Gibbs energy. The results have established good potentiality for kaolinite, montmorillonite and their acid-activated forms to take up and separate Ni(II) from aqueous medium through adsorption-mediated immobilization.     

Mechanism of methanol decomposition on the Pd/WC(0001) surface unveiled by first-principles calculations

In this study, the decomposition of methanol into the CO and H species on the Pd/tungsten carbide (WC)(0001) surface is systematically investigated using periodic density functional theory (DFT) calculations. The possible reaction pathways and intermediates are determined. The results reveal that saturated molecules, i.e., methanol and formaldehyde, adsorb weakly on the Pd/ WC(0001) surface. Both CO and H prefer three-fold sites, with adsorption energies of −1.51 and −2.67 eV, respectively. On the other hand, CH₃O stably binds at three-fold and bridge sites, with an adsorption energy of −2.58 eV. However, most of the other intermediates tend to adsorb to the surface with the carbon and oxygen atoms in their sp³ and hydroxyl-like configurations, respectively. Hence, the C atom of CH₂OH preferentially attaches to the top sites, CHOH and CH₂O adsorb at the bridge sites, while COH and CHO occupy the three-fold sites. The DFT calculations indicate that the rupture of the initial C–H bond promotes the decomposition of CH₃OH and CH₂OH, whereas in the case of CHOH, O–H bond scission is favored over the C–H bond rupture. Thus, the most probable methanol decomposition pathway on the Pd/WC(0001) surface is CH₃OH → CH₂OH → trans-CHOH → CHO → CO. The present study demonstrates that the synergistic effect of WC (as carrier) and Pd (as catalyst) alters the CH₃OH decomposition pathway and reduces the noble metal utilization.     

Adsorption characterization of lead(II) and cadmium(II) on crosslinked carboxymethyl starch

The adsorption of Pb(II) and Cd(II) ions with crosslinked carboxymethyl starch (CCS) was investigated as function of the solution pH, contact time, initial metal‐ion concentration, and temperature. Isotherm studies revealed that the adsorption of metal ions onto CCS better followed the Langmuir isotherm and the Dubinin–Radushkevich isotherm with adsorption maximum capacities of about 80.0 and 47.0 mg/g for Pb(II) and Cd(II) ions, respectively. The mean free energies of adsorption were found to be between 8 and 16 kJ/mol for Pb(II) and Cd(II) ions; this suggested that the adsorption of Pb(II) and Cd(II) ions onto CCS occurred with an ion‐exchange process. For two‐target heavy‐metal ion adsorption, a pseudo‐second‐order model and intraparticle diffusion seem significant in the rate‐controlling step, but the pseudo‐second‐order chemical reaction kinetics provide the best correlation for the experimental data. The enthalpy change for the process was found to be exothermic, and the ΔS^θ values were calculated to be negative for the adsorption of Pb(II) and Cd(II) ions onto CCS. Negative free enthalpy change values indicated that the adsorption process was feasible. The studies of the kinetics, isotherm, and thermodynamics indicated that the adsorption of CCS was more effective for Pb(II) ions than for Cd(II) ions. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Pb(II) ion adsorption by biomass-based carbonaceous fiber modified by the integrated oxidation and vulcanization

Biomass-based activated carbonaceous fiber (ACF) was modified by nitric-acid oxidation under microwave heating (ACF-O) and then further treated by thioglycolic acid (ACF-S) to prepare carbon materials with high capability for the removal of Pb(II) ions. The physico-chemical properties of the original and modified ACF samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Zeta potential, Boehm titration, BET, Raman spectrum and X-ray photoelectron spectroscopy (XPS). It was found that modification treatments damage the pore and graphite crystalline structure of ACF, while the micropore structure is protected and extra oxygen-containing surface functional groups are grafted on its surface. The adsorption performance of the original and the modified ACF samples affected by adsorption conditions regarding to Pb(II) ion strength (10 mg/L–105 mg/L), contact time (10 min–120 min), pH value (2.5–6.5), and solvent temperature (15 °C–45 °C) was investigated through batch experiments. Compared to the maximum Pb(II) ion adsorption capacity of 75.24 mg/g by ACF sample, the value was substantially improved by the integrated modification method (193.42 mg/g for ACF-O and 209.21 mg/g for ACF-S sample). The Biot number determined from the homogeneous surface diffusion model (HSDM) was between 1 and 100 for the original and modified ACF samples, suggesting that the adsorption process of Pb(II) ions is limited by both the surface diffusion and film mass transfer.     

Magnetic Pb(II) Ion-Imprinted Polymer Prepared by Surface Imprinting Technique and its Adsorption Properties

A novel magnetic Pb(II) ion-imprinted polymer was prepared via surface ion-imprinting technique by using magnetic Fe₃O₄@SiO₂ microspheres as supporter, Pb(II) as template ion, methacrylic acid and salicylaldoxime as monomers, and ethylene glycol dimethacrylate as crosslinker. The product was characterized by FT-IR, VSM, XRD, and SEM. The adsorption experiments showed that the imprinted polymer was employed successfully in comparison with non-imprinted polymer. When the temperature was in a range of 277 K to 291 K the maximum adsorption was about 81.83 mg/g with an optimal pH 6.0. Its relative selectivity coefficient values of Pb(II)/Cu(II), Pb(II)/Zn(II), and Pb(II)/Cd(II) were 2.60, 6.38, and 7.89 times greater than the ones of the magnetic non-imprinted polymer. The Langmuir adsorption model was more favorable for M-IIP than Freundlich or Temkin adsorption models. The Scatchard analysis suggested that M-IIP was processed with two kinds of binding sites with different affinity. Thermodynamic experiment showed that the adsorption was a spontaneous and endothermic process for Pb(II). The mechanism for Pb(II) adsorption on the imprinted polymer was also investigated.     

Kinetics and Thermodynamics of Lead (II) Adsorption on Vermiculite

Abstract

The kinetics and thermodynamics of Pb(II) adsorption on vermiculite have been studied by the sets of experiments at various conditions (temperature, initial lead concentration and adsorption time). The structures of the vermiculite before and after Pb(II) adsorption were measured using X‐ray diffraction (XRD), thermogravimetric analysis (TA), and X‐ray photoelectron spectroscopy (XPS). Adsorption of Pb(II) was strongly affected by pH. First order kinetics model best described the reaction rate, and the adsorption capacity calculated by the model was consistent with that actual measurement. Isotherms for the adsorption of Pb(II) on vermiculite were developed and the equilibrium data fitted well to the Langmuir and Freundlich models. Thermodynamic parameters such as enthalpy, entropy, and free energy were calculated using the Van't Hoff equations. The thermodynamics of Pb(II) on vermiculite indicates the spontaneous and endothermic nature of adsorption. Quantitative desorption of Pb(II) from vermiculite was found to be more than 40% which facilitates the sorption of metal by ion exchange     

Adsorption of Cd(II), Pb(II), and Ag(I) in aqueous solution on hollow chitosan microspheres

Cross‐linked chitosans synthesized by the inverse emulsion cross‐link method were used to investigate adsorption of three metal ions [Cd(II), Pb(II), and Ag(I)] in an aqueous solution. The chitosan microsphere, was characterized by FTIR and SEM, and adsorption of Cd(II), Pb(II), and Ag(I) ions onto a cross‐linked chitosan was examined through analysis of pH, agitation time, temperature, and initial concentration of the metal. The order of adsorption capacity for the three metal ions was Cd²⁺ > Pb²⁺ > Ag⁺. This method showed that adsorption of the three metal ions in an aqueous solution followed the monolayer coverage of the adsorbents through physical adsorption phenomena and coordination because the amino (? NH₂) and/or hydroxy (? OH) groups on chitosan chains serve as coordination sites. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The different characteristics of boron and nitrogen atoms/ions on silicon (001) substrate and their effect on boron nitride growth

The interactions of boron and nitrogen atoms and their ions with a silicon (001) surface have been studied by means of molecular orbital and density functional theories with a Si₁₇H₂₀ model simulating Si(001) surface. Based on a frontier orbital theory, the sticking of boron ions on the substrate was found to be superior to that of nitrogen ions and their neutral atoms. Potential energy surfaces calculated for a boron atom and a nitrogen atom on the substrate surfaces show that the nitrogen adsorption favors the formation of more ordered bonding than the boron adsorption. This study reveals that a surface nitride of substrate as the first step in the boron nitride growth would facilitate ordered interfacial bonding so that a good adhesion of the grown film with the substrate would be achieved.     

Calcined tetrabutylammonium kaolinite and montmorillonite and adsorption of Fe(II), Co(II) and Ni(II) from solution

Kaolinite and montmorillonite were modified with tetrabutylammonium (TBA) bromide, followed by calcination. The structural changes were monitored with XRD, FTIR, surface area and cation exchange capacity measurements. The modified clay minerals were used for adsorption of Fe(III), Co(II) and Ni(II) ions from aqueous solution under different conditions of pH, time and temperature. The uptake of the metal ions took place by a second order kinetics. The modified montmorillonite had a higher adsorption capacity than the corresponding kaolinite. The Langmuir monolayer capacities for the modified kaolinite and montmorillonite were Fe(III): 9.3 mg g^− 1 and 22.6 mg g^− 1; Co(II): 9.0 mg g^− 1 and 22.3 mg g^− 1; and Ni(II): 8.4 mg g^− 1 and 19.7 mg g^− 1. The modified kaolinite interacted with Co(II) in an endothermic manner, but all the other interactions were exothermic. The decrease of the Gibbs energy in all the cases indicated spontaneous adsorption.     

Behaviour and mechanism of Zn(II) adsorption on Chinese loess at dilute slurry concentrations

BACKGROUND: Zn(II) is commonly present in mining drainage in developing countries. Since loess is abundant and always located near mining sites in China, it would be useful to investigate the possibility and efficiency of using loess to remove Zn(II) from aqueous solution. RESULTS: The Zn(II) adsorption capacity of Chinese loess was determined as 215.9 mg g^?1. The adsorption followed pseudo‐second‐order kinetics and took place mainly by surface diffusion. Generally, higher initial pH and solute concentration resulted in higher % Zn(II) removal, while higher temperature and slurry concentration led to lower % Zn(II) removal. A thermodynamic study revealed that the adsorption process was exothermic, with the predicted enthalpy change ranging from ?20.87 to ?4.06 kJ mol^?1. With the assistance of X‐ray photoelectron spectroscopy and X‐ray diffraction, the high adsorption capacity was ascribed to the growth of micro‐organisms and mineral constituents such as kaolinite and goethite. CONCLUSION: Chinese loess proved effective for Zn(II) adsorption in this study. The optimal adsorption conditions included pH > 3.0, temperature ?15 °C and contact time ≈ 400 min. As an abundant natural soil in arid areas with very low population density, it would be appropriate to develop this material into a wastewater‐purifying agent. Copyright © 2008 Society of Chemical Industry