Surface Complex Formation Model Research Articles

Analyzing (3-Aminopropyl)triethoxysilane-Functionalized Porous Silica for Aqueous Uranium Removal: A Study on the Adsorption Behavior.

This study synthesized (3-aminopropyl)triethoxysilane-functionalized porous silica (AP@MPS) to adsorb aqueous uranium (U(VI)). To comprehensively analyze the surface properties of the AP@MPS materials, a combination of SEM, BET, XPS, NMR, and zeta potential tests were conducted. The adsorption experiments for U(VI) revealed the rapid and efficient adsorption capacity of AP@MPS, with the solution condition of a constant solution pH = 6.5, an initial U(VI) concentration of 600 mg × L-1, a maximum U(VI) capacity of AP@MPS reaching 381.44 mg-U per gram of adsorbent, and a removal rate = 63.6%. Among the four types of AP@MPS with different average pore sizes tested, the one with an average pore size of 2.7 nm exhibited the highest U(VI) capacity, particularly at a pH of 6.5. The adsorption data exhibited a strong fit with the Langmuir model, and the calculated adsorption energy aligned closely with the findings from the Potential of Mean Force (PMF) analysis. The outcomes obtained using the Surface Complex Formation Model (SCFM) highlight the dominance of the coulombic force ΔG0coul as the principal component of the adsorption energy (ΔG0ads). This work garnered insights into the adsorption mechanism by meticulously examining the ΔG0ads across a pH ranging from 4 to 8. In essence, this study's findings furnish crucial insights for the future design of analogous adsorbents, thereby advancing the realm of uranium(VI) removal methodologies.

The distribution of adsorption energy of U(VI) onto AEPTES-functionalized porous silica with multiple average pore sizes

Amino-functionalized porous silica proved a powerful [U(VI)] adsorbent, yet the adsorption mechanism remains unclear. This study first produces porous silica with different surface areas and average pore sizes. A grafting agent, 3-[2-(2-Aminoethylamino)ethylamino]propyl-trimethoxysilane (AEPTES), is then used to modify the surface of the obtained porous silica. The mesoporous silica adsorbents are thoroughly characterized with SEM, BET, XPS, NMR, and zeta potentials. The adsorbents are used to adsorb [U(VI)], and experimental results are fitted by the Langmuir adsorption isotherm, Potential of Mean Force (PMF) model, and Surface Complex Formation Model (SCFM), as to estimate the adsorption energy ΔG0ads, which consists of specific chemical energy, ΔG0chem, coulombic energy, ΔG0coul, solvation energy, ΔG0solv, and lateral interaction energy, ΔG0lat. From the result of SCFM, the values of ΔG0chem and ΔG0coul are close, and both are the main contributor to ΔG0ads. The result also indicates that AE@MPS has the lowest surface activate site density in terms of U(VI) adsorption at pore size of 4.1 nm. This work provides a fundamental understanding of how amino groups bond to U(VI) cation in solution. It is a potential reference for the molecular design of uranium collectors.

Systematic Investigation on the Adsorption Performance and Mechanism of MnO2/TA Nanoflowers for Cu(II) Removal from Aqueous Solution

AbstractIn this work, a nanoflower‐type manganese dioxide composite with tannic acid (MnO2/TA) was prepared using a facile hydrothermal method. The maximum adsorption capacity of nanoflower MnO2/TA for Cu(II) removal from aqueous solution was 194.3 mg⋅g−1, and this illustrated the excellent adsorptive activity of nanoflower MnO2/TA. Adsorption kinetics and isotherm investigations showed that pseudo second order kinetics and the Langmuir isotherm model fit the experimental data well. The surface complex formation model (SCFM) was used to study the adsorptive process and mechanism of Cu(II) adsorption on the surface of MnO2/TA. Further investigation indicated that the adsorption energies for various forms of Cu(II) were negative which indicated that Cu(II) adsorption on MnO2/TA was a spontaneous process. Electrostatic interactions and chemical complexation were involved in Cu(II) removal by MnO2/TA from aqueous solution. The decline of adsorption capacity was about 10% for MnO2/TA after being reused five times, and this showed that the adsorbent might be reused in Cu(II) removal. The MnO2/TA nanocomposite has been proven to be a low‐cost and efficient adsorbent that could be a potential material for use in environmental treatment. Understanding the adsorption mechanism might provide a technical basis for the processual design of heavy metal adsorbents.

Adsorption of Nickel(II) and Cobalt(II) Ions and Application of Surface Complex Formation Model to Ion Exchange Equilibria

Process effluents containing Co2+ and Ni2+ are often encountered in chemical process industries. Their major sources are hydrometallurgy, surface treatment, and galvanizing industries, as well as wastes from the production of batteries and sludge/wastes/scrap/dust of spent catalysts. Release of large quantities of these heavy metals into the natural environment has resulted in a number of environmental problems. Therefore, removal of these metals with some cation exchange separation has been investigated. Investigations were carried out using the strongly acidic cation exchange resins Amberlite IR 120, Amberjet 1200, Amberlite 200, and Amberlite 252 ZU. Experiments focused on the measurement of exchange equilibria of Co2+ and Ni2+ for H+. Since the state of equilibrium is mainly dominated by the pH of the liquid phase, the initial pH value was varied in each series of samples. For evaluation of equilibrium parameters the surface complexation theory was applied and the generalized separation factors determ...

Adsorption effect of aqueous bichlorobiphenyls onto UV-illuminated titanium dioxide

Bichlorobiphenyls were adsorbed onto TiO 2 (Degussa P25) in batch equilibrium experiments. The results demonstrated that a triple-layer model (TLM) surface complex formation model described the adsorption of chlorobiphenyls onto the surface of the TiO 2 solid. The surface complex configuration and the adsorption reaction may have followed the equation derived from the TLM, involving the loss of a proton during the adsorption process. This dependence indicates that the oxidation reaction between surface-adsorbed substrates and photogenerated oxidants dominates. Both sorbed and dissolved components contribute to the observed degradation rate, that is, the 4,4′-CBP degradation rates might be stated as linear functions of the sorbed and dissolved concentrations. The contribution of the concentration of solution phase in degradation pathways is not negligible.

Modeling Pb(II) adsorption onto sandy loam soil

The adsorption of Pb(II) onto hydrous sandy loam soil was investigated with batch equilibrium adsorption experiments. Results show that the amount of Pb(II) adsorbed increases with increasing pH and surface loading. It was demonstrated that the surface acidity of the soil could be determined using electrophoretic mobility measurements. The surface acidity constants, p K a1 int and p K a2 int, were 1.57 and 3.43, respectively. A surface complex formation model (SCFM) was employed to describe the adsorption. The intrinsic stability constants, p K i s , for the surface reaction between the Pb species and the ionized soil surface hydroxyl groups were determined from SCFM fitting. The adsorption free energy of Pb 2+ and Pb(OH) + ions ranges from −5.74 to −6.48 kcal/mol and from −9.68 to −10.00 kcal/mol, respectively, for surface loadings between 1.21×10 −5 and 2.41×10 −4 mol/g. The adsorption binding calculation indicated that the specific chemical interaction is the major mechanism responsible for the adsorption process.

Cr(VI) Adsorption onto Hydrous Concrete Particles from Groundwater

Adsorption of Cr(VI) onto hydrous concrete particles in synthetic groundwater was investigated with batch equilibrium adsorption experiments. Results show marked adsorption under acidic conditions, and the amount of Cr(VI) adsorbed decreases with increasing solution pH and surface loading. A surface complex formation model was employed to describe the adsorption of Cr(VI) onto concrete particles. The adsorption-free energy of HCrO-4 and CrO2-4 species are −6.16 and −8.53 kcal/mol, respectively. Results also indicate that specific chemical interaction is the major mechanism responsible for the adsorption process.

Application of the surface complex formation model to ion exchange equilibria: Part V. Adsorption of heavy metal salts onto weakly basic anion exchangers

The surface complexation model which so far has been applied to the sorption of ions onto charge-bearing surfaces has been extended to the competitive adsorption of heavy metal salts and strong acids onto weakly basic anion exchanger. Considering local equilibria, logarithmic equilibrium parameters (generalized separation factors) can be deduced by measuring pH values and equilibrium concentrations. The generalized separation factors can be plotted as a function of the dimensionless loadings of the resin phase with metal salts and acids. By this means a set of two equilibrium constants is obtained for the uptake of each component. As found in previous publications about true ion exchange equilibria, it is the major advantage of the theoretical approach that these constants remain unchanged in multicomponent systems. Evaluation of experiments with two commercially available weakly basic anion exchanger products demonstrate the validity of the approach. By means of the parameters derived from simple systems the sorption equilibrium for systems with two heavy metals and systems with additional amounts of neutral salt can be well predicted.

Surface charge and metal ion adsorption on an H-type activated carbon: experimental observation and modeling simulation by the surface complex formation approach

Experimental and modeling studies on both surface charge and adsorption of metals on an H-type activated carbon were conducted. The surface charge density decreased with an increase in pH and a decrease in ionic strength. Higher copper adsorption percentage was achieved with higher pH and ionic strength and lower initial concentration. The metal uptake was in the descending order of Cu 2+>Zn 2+≈Co 2+. Zinc and cobalt did not affect the copper removal; however, adsorption of both reduced in the presence of copper. The two-pK triple layer surface complex formation model successfully elucidated the surface charge density and metal adsorption in various conditions (pH, ionic strength, anions, single- and multiple-metal ions, and initial concentration). Modeling revealed that the copper removal was due to adsorption of Cu 2+, CuOH + and CuCl + on the activated carbon. Zinc and cobalt uptake resulted from the formation of the surface–metal complexes (SO −M 2+ and SO −MOH +, M=Zn, Co). Based on the parameters from the modeling of surface charge density and single-metal-ion removal (i.e. Cu, Zn and Co), modeling multiple-metal-ion adsorption (Cu–Zn, Cu–Co and Cu–Zn–Co) was performed. It was demonstrated that the model described well the competitive effects on the metal adsorption. Finally, sensitivity analysis of model parameters was conducted.

Equilibrium and Kinetics of Metal Ion Adsorption onto A Commercial H-Type Granular Activated Carbon: Experimental and Modeling Studies

Systematic studies on metal ion adsorption equilibrium and kinetics by a commercial H-type granular activated carbon were carried out. Titration of the carbon showed that the surface charge density decreased with an increasing pH. Higher copper adsorption was obtained with increasing solution pH and ionic strength. Metal removal was in the descending order: Cu 2+>Zn 2+≈ Co 2+. Copper removal was not affected by addition of zinc or cobalt, while copper can reduce both zinc and cobalt removal. Kinetic experiments demonstrated that the copper adsorption rapidly occurred in the first 30–60 min and reached the complete removal in 3–5 h. Removal of zinc and cobalt was slightly slower than that of copper. It was found that the mass transfer is important in the metal adsorption rate. The surface complex formation model was used successfully to describe the surface change density, as well as the single- and multi-species metal adsorption equilibrium. The copper removal was due to adsorption of Cu 2+, CuOH +, and CuCl +, while the zinc and cobalt uptake was due to the formation of surface–metal complexes of SOM 2+ and SOMOH + (M=Zn and Co). It was found that the diffusion-control model well described the adsorption kinetics with various metal ions and pH values. Finally sensitivity analysis on the kinetic model's parameters was carried out.

A review of heavy metal adsorption by marine algae

Accumulation of heavy metals by algae had been studied extensively for biomonitoring or bioremediation purposes. Having the advantages of low cost raw material, big adsorbing capacity, no secondary pollution, etc., algae may be used to treat industrial water containing heavy metals. The adsorption processes were carried out in two steps: rapid physical adsorption first, and then slow chemical adsorption. pH is the major factor influencing the adsorption. The Freundlich equation fitted very well the adsorption isotherms. The uptake decreased with increasing ionic strength. The principal mechanism of metallic cation sequestration involves the formation of complexes between a metal ion and functional groups on the surface or inside the porous structure of the biological material. The carboxyl groups of alginate play a major role in the complexation. Different species of algae and the algae of the same species may have different adsorption capacity. Their selection affinity for heavy metals was the major criterion for the screening of a biologic adsorbent to be used in water treatment. The surface complex formation model (SCFM) can solve the equilibrium and kinetic problems in the biosorption.

Determination of thermodynamic equilibrium constants of H +–Zn(II), H +–Cd(II) and H +–Fe(III) exchanges on strongly acidic exchangers

The ion-exchange equilibrium constants of the H +–Zn(II), H +–Cd(II) and H +–Fe(III) systems for the strongly acidic cationic resins Dowex CM-15 and Dowex C650 have been determined. The results have been interpreted by means of the surface complex formation model, the three-parameter model and the Wilson model. A methodology for the prediction of the equilibrium constant values of the three-parameter and Wilson models from the values obtained using the surface complex formation model is given and discussed.

Application of the Surface Complex Formation Model to Ion Exchange Equilibria

The surface complexation model as previously applied to pure anion and pure cation exchange equilibria has been extended to the description of the amphotheric sorption of inorganic ions onto activated alumina. Logarithmic equilibrium parameters (generalized separation factors) can be calculated from equilibrium concentrations in simple systems for the sorption of anions and cations. Both separation factors are functions of the oxide loading composition. Plotting the separation factors vs the loading compositions yields a set of two sorption constants for each kind of ion. The major advantage of the theoretical approach is that these constants can be applied without any adaptation to the prediction of multicomponent systems. Evaluation of experiments with two commercially available activated alumina products demonstrate the validity of the approach. The equilibrium of multicomponent systems can be predicted with excellent accuracy by means of the parameters for the uptake of chloride, nitrate, sulfate, sodium, and potassium ions.

Adsorption of Cr(VI) onto TiO2 from dilute aqueous solutions

Cr(VI) adsorption onto TiO2 (anatase) particles was performed in batch equilibrium experiments. Results showed that pH was the key factor affecting the adsorption characteristics. The marked adsorption was observed under acidic condition and the adsorption capacities decreased with increasing pH, temperature, and ionic strength. The adsorption obeyed the Langmuir adsorption isotherm and the maximum adsorption capacities were determined at specific pH values. A surface complex formation model was proposed to describe the adsorption reaction between aqueous chromates and TiO2 solid surface. Because the magnitude of electrostatic energy, the adverse solvation, and lateral interaction energy which contribute to the total adsorption are relatively small in comparison with the specific chemical energy, we conclude that the specific chemical interaction is the major mechanism responsible for the adsorption process.

Adsorption of cr(vi) onto tio from dilute aqueous solutions

Cr(VI) adsorption onto TiO2 (anatase) particles was performed in batch equilibrium experiments. Results showed that pH was the key factor affecting the adsorption characteristics. The marked adsorption was observed under acidic condition and the adsorption capacities decreased with increasing pH, temperature, and ionic strength. The adsorption obeyed the Langmuir adsorption isotherm and the maximum adsorption capacities were determined at specific pH values. A surface complex formation model was proposed to describe the adsorption reaction between aqueous chromates and TiO2 solid surface. Because the magnitude of electrostatic energy, the adverse solvation, and lateral interaction energy which contribute to the total adsorption are relatively small in comparison with the specific chemical energy, we conclude that the specific chemical interaction is the major mechanism responsible for the adsorption process.

Adsorption of Heavy Metals from Aqueous Solutions onto Activated Carbon in Single Cu and Ni Systems and in Binary Cu-Ni, Cu-Cd and Cu-Zn Systems

Single copper and nickel adsorption from aqueous solutions onto a granular activated carbon is reported. Metal removals increase on raising pH and temperature, and decrease on raising the initial metal concentration at constant carbon dose. The adsorption processes are modelled using the surface complex formation (SCF) Triple Layer Model (TLM) with an overall surface bidentate species. A dependence of the SCF constant on pH, initial molar metal/carbon ratio and temperature is observed, and a correlation for log Kads is determined. The SCF model successfully predicts copper and nickel removals in single metal solutions. Adsorption in the binary metal systems copper–nickel, copper–cadmium and copper–zinc is also reported, showing competitive adsorption effects. © 1997 SCI.

Cadmium and Zinc Adsorption onto Activated Carbon: Influence of Temperature, pH and Metal/Carbon Ratio

The adsorption characteristics of cadmium and zinc onto a granular activated carbon were studied. Cadmium and zinc removals increased on raising pH and temperature, and decreased on raising the molar metal/carbon ratio. The adsorption processes were modelled using the surface complex formation (SCF) Triple Layer Model (TLM) with an overall bidentate species. A dependence of the SCF constant on pH, the molar metal/carbon ratio and temperature was observed, and a correlation for log K ads was determined. The SCF model successfully predicted cadmium and zinc removals.

Cadmium and Zinc Adsorption onto Activated Carbon: Influence of Temperature, pH and Metal/Carbon Ratio

The adsorption characteristics of cadmium and zinc onto a granular activated carbon were studied. Cadmium and zinc removals increased on raising pH and temperature, and decreased on raising the molar metal/carbon ratio. The adsorption processes were modelled using the surface complex formation (SCF) Triple Layer Model (TLM) with an overall bidentate species. A dependence of the SCF constant on pH, the molar metal/carbon ratio and temperature was observed, and a correlation for log Kads was determined. The SCF model successfully predicted cadmium and zinc removals.

Application of the surface complex formation model to exchange equilibria on ion exchange resins Part III: Anion exchangers

The surface complexation theory considers the sorption of ions as a local equilibrium reaction, which is caused by the amphoteric behaviour of the surface. Application of this theory to exchange equilibria with ordinary and chelating weak-acid ion exchange resins which can release protons has opened the way for a comprehensive description. Binary equilibria are described by a logarithmic equilibrium parameter, which is a linear function of the composition of the resin phase. Multicomponent equilibria are considered as a superposition of several binary equilibria. The approach considers the pH-dependent dissociation of functional groups. Furthermore, chemical reactions in the liquid phase in which exchangeable species are involved, can readily be taken into account. In contrast to cation exchange resins, the behaviour of anion exchangers is characterized by the uptake of protons enabling them to also adsorb anions. As a consequence, anion exchange equilibria can be described in similar way. The evaluation of data demonstrates that similar relationships exist between a logarithmic equilibrium parameter and the resin phase composition. As for cation exchangers, multicomponent equilibria can be predicted with excellent accuracy from binary parameters.

Acid‐Base Characteristics of Powdered‐Activated‐Carbon Surfaces

Adsorption of heavy metals onto activated carbon has been described using the surface‐complex‐formation (SCF) model, a chemical equilibrium model. The SCF model requires a knowledge of the amphoteric nature of activated carbon prior to metal adsorption modeling. In the past, a single‐diprotic‐acid‐site model had been employed to describe the amphoteric nature of activated‐carbon surfaces. During this study, the amphoteric nature of two powdered activated carbons were investigated, and a three‐monoprotic site surface model was found to be a plausible alternative. The single‐diprotic‐acid‐site and two‐monoprotic‐site models did not describe the acid‐base behavior of the two carbons studied adequately. The two‐diprotic site was acceptable for only one of the study carbons. The acid‐base behavior of activated carbon surfaces seem to be best modeled as a series of weak monoprotic acids.