Diffuse Layer Model Research Articles

Rare earth elements sorption to iron oxyhydroxide: Model development and application to groundwater

Iron oxyhydroxides are among the most important colloids that control rare earth elements (REE) concentrations and transport in natural hydrosystems. In this study, REE surface complexation to iron oxyhydroxides (Fe(OH)3(a)) was described by using the Donnan diffuse layer model and a two-site (i.e. ≡FesOH and ≡FewOH) model. The specific surface area and pH of zero charge were fixed as 100000 m2/mol and 8.0, respectively. The surface site density for weak and strong binding sites were fixed at 0.1 moL/mol Fe (≡FewOH) and 0.001 moL/mol Fe (≡FesOH) respectively. The two site types were used with pKa1int = 7.29 and pKa2int = 8.93. Using linear free energy relationship, the estimated equilibrium surface complexation constants (log K) increased from light REE (LREE) to heavy REE (HREE). Results of REE modeling calculation using the determined log K revealed a good fit of experimental data, showing an order of sorption on iron oxyhydroxides: HREE > MREE > LREE and preferential sorption of HREE at a lower pH. However, sorption edges only showed a slight change with ionic strength (0.1–0.7 moL/L) for the whole REE series. The generalized model was subsequently used to evaluate the impact of iron oxyhydroxides on REE speciation in groundwater. Application of the model to “model groundwater” showed that iron oxyhydroxide complexes of REE were significant in near neutral and weakly alkaline pH. This study contributes to putting forward a comprehensive database which would be useful for the application of surface complexation model to describe REE sorption by amorphous ferric hydroxides in nature.

Highly effective Zn(II) and Pb(II) removal from aqueous solutions using Mg-Fe layered double hydroxides: Comprehensive adsorption modeling coupled with solid state analyses

Comprehensive mechanistic and modeling approaches are needed to effectively evaluate sorption of metal ions from aqueous solutions. However, such a complex study using layered double hydroxides has not yet been presented. Therefore, adsorption modeling was performed coupled with solid state analyses describing the mode of zinc and lead removal by magnesium-iron layered double hydroxides, and an excellent removal efficiency for both metal ions was observed. The maximal adsorbed concentration, as established by the Langmuir model, increased with the increasing magnesium/iron molar ratio. The pH-dependent sorption was fitted by the diffuse layer model, which described the formation of monodentate inner-sphere complexes, indicating strong binding between metal ions and the layered double hydroxides surface. Based on the solid state analyses of materials with high surface concentrations of zinc (1.44 mmol g−1) and lead (1.65 mmol g−1), respectively, the whole sorption mechanism was also influenced by other processes, i.e., precipitation (lead) and surface accumulation/precipitation/isomorphic substitution (zinc). Transmission electron microscopy-based elemental mapping showed a heterogeneous distribution of zinc and lead on the surface of particles. Low-temperature Mössbauer spectra were nearly identical for the studied materials before/after zinc and lead sorption indicating no structural changes in incorporated iron. Generally, we suggest that these layered double hydroxides are highly effective sorbents for metal ions from aqueous solutions. Furthermore, we propose a comprehensive mechanistic/modeling approach as a powerful tool for describing the mechanism of metal ions binding on layered double hydroxides in contaminated waters.

Adsorption isotherm, mechanism, and geometry of Pb(II) on magnetites substituted with transition metals

Iron oxides are abundant in natural waters and soils and have high capacities for scavenging Pb(II) by adsorption, which affects the transport and fate of Pb on the earth's surface. We investigated the adsorption of Pb(II) on magnetites substituted with commonly incorporated transition metals such as Cr, Mn, Co, and Ni. The adsorption capacity, mechanism, and local coordination of Pb(II) were investigated by traditional macroscopic studies, i.e., acid-base titration and batch adsorption experiment, complemented with X-ray absorption fine structure (XAFS) spectrum analysis and surface complexation model (SCM). The substitution increased the surface site density, while pHpzc did not vary. Pb(II) adsorption was not suppressed by the presence of background electrolyte and improved as pH increased. The isotherms were well fit to the Langmuir adsorption model. The XAFS analysis demonstrated that Pb(II) ions were adsorbed on magnetite surface predominantly via inner-sphere complexation, where the adsorbed Pb(II) species was in bidentate binuclear corner-sharing geometry, independent of the adsorption capacity. This adsorption geometry can be applied to fit the experimental adsorption data well with the diffuse layer model (DLM). The substitutions improved the adsorption capacity in the following order: Cr>Ni>Mn>Co, and were discussed regarding the measured values of active site density and local coordination of adsorbed Pb(II). This study is the first documentation of Pb(II) adsorption on magnetite with different substitutions. The obtained results are of great significance for the understanding of Pb(II) surface complexation reactions on magnetite surface.

Experimental and modelling approaches to Am(III) and Np(V) adsorption on the Maoming kaolinite

The goal of this paper is to study the adsorption of Am(III) and Np(V) on the Maoming kaolinite and to provide corresponding adsorption models. The surface properties of Maoming kaolinite were evaluated by continuous potentiometric titration. Comparisons with the titration curves of KGa-1, KGa-2, Zettlitz, St Austell and Twiggs County kaolinite samples indicate that the Maoming kaolinite may have different surface acid-base chemistry than these kaolinite samples. These titration curves of the Maoming kaolinite can be interpreted with both the diffuse double layer model (DDLM) and a non-electrostatic model (NEM). The adsorptions of Am(III) and Np(V) were studied as functions of contact time, solid-to-liquid ratios, pH, ionic strength, and concentration using a batch method. The adsorptions of Am(III) and Np(V) on kaolinite were rapid and equilibrated within 20 and 10 h, respectively. The adsorption of 6 × 10−10 mol/L Am(III) was shown to be sensitive to both pH and ionic strength, whereas the adsorption of 1 × 10−6 mol/L Np(V) was sensitive only to pH. The DDLM overestimates the effect of ionic strength on Np(V) adsorption. Finally, the adsorption of Am(III) and Np(V) was quantitatively interpreted using the NEM with the addition of one cation exchange reaction for Am(III). These adsorption data and corresponding models are useful for the safety assessment of an high–level nuclear waste (HLW) repository, which may be built in the clayey formation in China.

Experimental Measurements and Surface Complexation Modeling of U(VI) Adsorption onto Multilayered Graphene Oxide: The Importance of Adsorbate-Adsorbent Ratios.

Surface complexation models use experimental adsorption measurements to calculate stability constants that quantify the thermodynamic stability of adsorbed species. However, these constants are often poorly constrained due to nearly complete removal of the solute from solution and/or because the tested adsorbate:adsorbent ratios are not varied sufficiently. Using data sets that quantify the adsorption of U(VI) to multilayered graphene oxide (MLGO), we tested whether three different U(VI):MLGO ratios (3 ppm U; 20-210 mg L-1 MLGO) affect the ability of nonelectrostatic and diffuse layer models to predict U(VI) adsorption behaviors across a range of ionic strength (1-100 mM) and pH (2-9.5) conditions. Model formulations assumed interactions between discrete MLGO surfaces sites and the most abundant aqueous U(VI) complex(es) within a given pH range. We determined that the observed extents of U(VI) binding require adsorption of more than one U(VI) species (UO22+ and uranyl hydroxide(s) and/or carbonate(s)) and calculated the respective stability constants for the important U(VI)-MLGO surface complexes. The results also unequivocally illustrated that models using adsorption data from treatments with higher U(VI):MLGO ratios provide better fits throughout the tested range of experimental conditions, meaning that the U(VI)-MLGO stability constants calculated herein can be confidently applied to a range of natural or engineered systems.

Overcoming Pitfalls in Boundary Elements Calculations with Computer Simulations of Ion Selective Membrane Electrodes.

Finite difference analysis of ion-selective membranes is a valuable tool for understanding a range of time dependent phenomena such as response times, long and medium term potential drifts, determination of selectivity, and (re)conditioning kinetics. It is here shown that an established approach based on the diffusion layer model applied to an ion-exchange membrane fails to use mass transport to account for concentration changes at the membrane side of the phase boundary. Instead, such concentrations are imposed by the ion-exchange equilibrium condition, without taking into account the source of these ions. The limitation is illustrated with a super-Nernstian potential jump, where a membrane initially void of analyte ion is exposed to incremental concentrations of analyte in the sample. To overcome this limitation, the two boundary elements, one at either side of the sample-membrane interface, are treated here as a combined entity and its total concentration change is dictated by diffusional fluxes into and out of the interface. For each time step, the concentration distribution between the two boundary elements is then computed by ion-exchange theory. The resulting finite difference simulation is much more robust than the earlier model and gives a good correlation to experiments.

Decontamination of Sr(II) on Magnetic Polyaniline/Graphene Oxide Composites: Evidence from Experimental, Spectroscopic, and Modeling Investigation

The interaction of Sr(II) on magnetic polyaniline/graphene oxide (PANI/GO) composites was elucidated by batch, EXAFS, and surface complexation modeling techniques. The batch experiments showed that decreased uptake of Sr(II) on magnetic PANI/GO composites was observed with increasing ionic strength at pH 5.0. The maximum uptake capacity of magnetic PANI/GO composites derived from the Langmuir model at pH 3.0 and 293 K was 37.17 mg/g. The outer-sphere surface complexation controlled the uptake of Sr(II) on magnetic PANI/GO composites at pH 3.0 due to the similarity to the EXAFS spectra of Sr2+ in aqueous solutions, but the Sr(II) uptake at pH 7.0 was inner sphere complexation owing to the occurrence of the Sr–C shell. According to the analysis of surface complexation modeling, uptake of Sr(II) on magnetic PANI/GO composites was well simulated using a diffuse layer model with an outer-sphere complex (SOHSr2+ species) and two inner-s...

Experimental study of turbulent natural-convective condensation on a vertical wall with smooth and wavy film interface

Pressurized water reactor nuclear plants, currently under construction, have been designed with passive containment cooling systems. Turbulent, natural-convective condensation, with high non-condensable mass fraction, on the walls of the containment vessel is a primary heat transfer mechanism in these new plant designs. A number of studies have been completed over the past two decades to justify use of the heat and mass transfer analogy for this scenario. These studies are founded upon natural-convective heat transfer correlations and, in many cases, apply a diffusion layer model to couple heat and mass transfer. Reasonable success in predicting experimental trends has been achieved when correction factors are applied. The corrections are attributed to mass transfer suction, film waviness or mist formation even though little experimental evidence exists to justify these claims. The purpose of this work is to investigate the influence of film waviness and mass transfer suction on the turbulent, natural-convective condensing flow with high non-condensable mass fraction. Testing was conducted using 0.457m×2.13m and a 0.914m×2.13m condensing surfaces suspended in a large pressure vessel. The test facility implements relatively high accuracy calorimetric and condensate mass flow measurements to validate the measured heat and mass transfer rates. Test results show a significant enhancement in heat transfer is caused by disruption of the gas boundary layer due to film waviness. This result has implications for the scalability of existing correlations. A new correlation is proposed and results compared to several datasets.

One-Pot Synthesis of LDH/GO Composites as Highly Effective Adsorbents for Decontamination of U(VI)

The removal mechanism of U(VI) on Mg–Al-layered double hydroxide-supported graphene oxide (LDH/GO) composites was investigated by batch, spectroscopic, and surface complexation modeling. The batch experiments showed that the enhanced removal of U(VI) on LDH and LDH/GO composites in the presence of carbonate was observed at pH < 5.0, whereas the presence of carbonate significantly inhibited U(VI) removal at pH starting from 7.0 to 9.0. It is demonstrated that the oxygenated functional groups (i.e., −OH) were responsible for the high effective removal of U(VI) by XPS analysis. The results of XANES and EXAFS spectra indicated that adsorption of U(VI) on LDH/GO composites was inner-sphere surface complexation. According to surface complexation modeling, the removal of U(VI) on LDH/GO composites can be satisfactorily fitted by a diffuse layer model with an ion exchange (X2UO2) and two inner-sphere surface complexes (SOUO2+ and SOUO2(CO3)23– species). The maximum adsorption capacities of LDH and LDH/GO composit...

Effect of carboxymethyl cellulose on dissolution kinetics of carboxymethyl cellulose-sodium carbonate two-component tablet

Abstract Sodium carbonate and carboxymethyl cellulose powders are compressed into two-component tablets with three mass ratios, 97%:3%, 95%:5% and 93%:7%. The dissolution tests for two-component tablets and reference pure sodium carbonate tablets are carried out at various temperatures. The dissolution process of each tablet is measured by electrical conductivity tracking method and the concentration of dissolved sodium carbonate is quantified with calibrated conductivity-concentration converting equation of sodium carbonate. The quantified dissolution data is fitted with both surface reaction model and diffusion layer model and the results clearly show that surface reaction model is suggested as the appropriate dissolution model for all measured tablets. Therefore, it is determined that carboxymethyl cellulose is a stable element to remain the dissolution mechanism of tablet unchanged. The dissolution rate constant quantified with surface reaction model presents that carboxymethyl cellulose-sodium carbonate two-component tablets obtain significant higher dissolution rate constant than pure sodium carbonate tablet and higher proportion of carboxymethyl cellulose leads to apparent higher dissolution rate constant. The results prove for the usage of carboxymethyl cellulose in most practical applications at a relative low-level, the effect of carboxymethyl cellulose is effective and positive for two-component tablet to enhance the dissolution process and improve dissolution rate constant and this effect is speculated coming from its dynamic physical transforming process in water including dilation and conglutination.

Decontamination of U(VI) from nZVI/CNF composites investigated by batch, spectroscopic and modeling techniques

The nZVI/CNF composites were synthesized by borohydride reduction of ferrous salts in the CNF suspension. The characteristic results indicated that metallic Fe0 was uniformly dispersed the surface of CNFs. The decontamination mechanism of U(VI) from nZVI/CNF composites was investigated by batch, XPS, EXAFS and modeling techniques. The batch experiments indicated that removal amount of U(VI) on nZVI/CNF was significantly higher than that of single nZVI, whereas reduction amount of U(VI) to U(IV) on nZVI at pH>6.0 was significantly higher than that of nZVI/CNF. The EXAFS spectra of U-nZVI at pH7.0 was similar to the U(IV)O2(s), whereas the EXAFS spectra of U-nZVI/CNF at 2.98Å can be satisfactorily fitted by U-U shell, which was further demonstrated by XANES analysis and surface complexation modeling. The diffuse layer model gave better fits to U(VI) removal on nZVI/CNFs composites, whereas underestimation of U(VI) removal on nZVI was observed at pH>5.0 due to the presence of reductive precipitation. These findings indicated that nZVI-based composites presented the higher adsorption performance and the excellent reduction capacity for U(VI) under permeable reactive barrier system.

Speciation and reactivity of lead and zinc in heavily and poorly contaminated soils: Stable isotope dilution, chemical extraction and model views

Correct characterization of metal speciation and reactivity is a prerequisite for the risk assessment and remedial activity management of contaminated soil. To better understand the intrinsic reactivity of Pb and Zn, nine heavily and poorly contaminated soils were investigated using the combined approaches of chemical extractions, multi-element stable isotopic dilution (ID) method, and multi-surface modelling. The ID results show that 0.1–38% of total Pb and 3–45% of total Zn in the studied soils are isotopically exchangeable after a 3-day equilibration. The intercomparison between experimental and modelling results evidences that single extraction with 0.43 M HNO3 solubilizes part of non-isotopically exchangeable fraction of Pb and Zn in the studied soils, and cannot be used as a surrogate for ID to assess labile Pb and Zn pools in soil. Both selective sequential extraction (SSE) and modelling reveal that Mn oxides are the predominant sorption surface for Pb in the studied soils; while Zn is predicted to be mainly associated with soil organic matter in the soil with low pH and Fe/Mn oxides in the soils with high pH. Multi-surface modelling can provide a reasonable prediction of Pb and Zn adsorption onto different soil constituents for the most of the studied soils. The modelling could be a promising tool to decipher the underlying mechanism that controls metal reactivity in soil, but the submodel for Mn oxides should be incorporated and the model parameters, especially for the 2-pK diffuse layer model for Mn oxides, should be updated in the further studies.

Application of a duplex diffusion layer model to pulse reverse plating

The application of Ibl’s duplex diffusion layer model to the analysis of mass transport in pulse reverse plating with bipolar current pulses has been investigated. Although originally proposed to describe normal pulse plating, Yin has recently extended Ibl’s model to include pulse reverse plating. Using the expressions derived by Yin the pulse limiting current density was determined over a wide range of pulse plating conditions, and then compared to values calculated using more accurate numerical solutions. In general, there was good agreement between the two approaches which demonstrated the essential validity of Yin’s extension to Ibl’s original model. The simplified model is most accurate at long duty cycles, small dimensionless pulse times and for low values of the dimensionless pulse reverse current where its underlying assumptions are most likely to be valid. At very long dimensionless pulse times (i.e. T* > 1) the model becomes increasingly inaccurate and its use in these circumstances cannot be justified.

Uptake of Ni(II) from aqueous solution onto graphene oxide: Investigated by batch and modeling techniques

Abstract The contamination of Ni(II) on the apple orchards soil is more serious nowadays. The uptake of Ni(II) from aqueous solutions on graphene oxide (GO) was investigated by batch, XPS and modeling techniques. The batch experiments indicated that the uptake of Ni(II) on GO increased significantly with increasing pH from 2.0 to 6.0, and the high-level uptake of Ni(II) was observed at pH > 6.0. No effect of ionic strength on Ni(II) uptake indicated inner-sphere surface complexation dominated the uptake of Ni(II) on GO. The maximum uptake capacity of GO for Ni(II) calculated from Langmuir model was calculated to be 81.97 mg/g at pH 5.0 and 293 K. The results of XPS spectra indicated that a variety of oxygen-containing functional groups were responsible for the uptake of Ni(II) on GO. The uptake process of Ni(II) on GO can be satisfactorily fitted by surface complexation modeling using diffuse layer model with two inner-surface complexes. These findings are crucial for the potential application of graphene oxide in the uptake of heavy metals in environmental cleanup.

Sorption mechanisms of arsenate on Mg-Fe layered double hydroxides: A combination of adsorption modeling and solid state analysis

Layered double hydroxides have been proposed as effective sorbents for As(V), but studies investigating adsorption mechanisms usually lack a comprehensive mechanistic/modeling approach. In this work, we propose coupling surface complexation modeling with various spectroscopic techniques. To this end, a series of batch experiments at different pH values were performed. Kinetic data were well fitted by a pseudo-second order kinetic model, and the equilibrium data were fitted by the Freundlich model. Moreover, the pH-dependent As(V) sorption data were satisfactorily fitted by a diffuse layer model, which described the formation of >SOAsO3H− monodentate and >(SO)2AsO2− bidentate inner-sphere complexes (“>S” represents a crystallographically-bound group on the surface). Additionally, XPS analyses confirmed the adsorption mechanisms. The sorption mechanisms were affected by anion exchange, which was responsible for the formation of outer sphere complexes, as identified by XRD and FTIR analyses. Furthermore, a homogenous distribution of As(V) was determined by HR-TEM with elemental mapping. Using low-temperature Mössbauer spectroscopy on isotope 57Fe, a slight shift of the hyperfine parameters towards higher values following As(V) sorption was measured, indicating a higher degree of structural disorder. In general, mechanistic adsorption modeling coupled with solid state analyses presents a powerful approach for investigating the adsorption mechanism of As(V) on Mg-Fe LDH or other sorbents.

Generalization of the Nernst layer model to take into account the difference in diffusivity between the components of the system in bromate reduction in steady-state one-dimensional mode: Current limiting by proton transport

The reduction of nonelectroactive bromate anion BrO3- from acidic solutions while limiting the maximum current by diffusion transport of protons was studied by methods of numerical integration of transport equations. The calculation was performed based on a generalization of the Nernst steady diffusion layer model, in which the choice of the layer thickness for each component of the system is made using the Levich formula and takes into account the difference in diffusivity between the components. This difference in layer thickness was shown to have a significant effect on the main characteristics of the system, such as the maximum possible discharge current, and also the concentration profiles of the components.

Model based prediction of nanostructured thin film morphology in an aerosol chemical vapor deposition process

The aerosol chemical vapor deposition (ACVD) process has been demonstrated as a promising approach to the single step synthesis of nanostructured metal oxide thin films. Multiple process parameters control the nanostructure morphology and the growth of thin films. This work focuses on utilizing a simulation based approach to understand the role of these parameters in governing the morphology of the thin film. A finite element based computational fluid dynamics model, coupled with a discrete-sectional aerosol model, and a boundary layer diffusion and sintering model has been formulated to predict the evolution of particle size distribution and the morphology of the synthesized nanostructured film. The morphology predicted by the model was validated by experimental observations. The model enables scale up and wider application of the ACVD process and can be extended to other gas phase deposition systems.

Theoretical and experimental study of the mechanisms of phosphate removal in the system containing Fe(III)-ions.

A process-oriented investigation of phosphate removal by ferric salt was carried out in this study. The kinetics of amorphous ferric phosphate (FePO4(s)) formation has been investigated over the pH range of 6.0-8.0 using sulfosalicylic acid as a competitive ligand. The FePO4(s) formation rate constants varied in a narrow range over the pH range examined in this study. And the maximum of (0.90±0.11)×104Lmol-1s-1 was obtained at pH7.5 and the minimum value of (0.05±0.01)×104Lmol-1s-1 was obtained at pH6.0. These values are two orders of magnitude lower than the rate constants for Fe(III) hydrolysis-precipitation, and hence, the extent of FePO4(s) formation when ferric ions are added to aqueous solution is extremely low. Subsequently, the characteristics of the amorphous ferric oxide (AFO) with different ages were investigated, and it was found that the BET surface area, the average pore width, and the charge capacitance were various for different AFO with various ages. Phosphate adsorption by AFO was significantly affected by AFO aging and the manner of adding Fe(III), which was successfully described by a diffuse layer model. By using surface sites concentration obtained, the kinetics constant of AFO aging could be calculated by a functional equation at a certain pH and time. Graphic abstract Description of the precipitation and aging processes for the phosphate removal by Fe(III)-ions.

Study on the effects of soluble microbial product on phosphate adsorption onto fresh hydrous ferric oxides by surface complexation models.

As one of the major soluble organic compounds in membrane bioreactors, soluble microbial product (SMP) can combine the surface sites on hydrous ferric oxides (HFO), which can compete with HFO for the uptake of phosphate. In this study, SMP was extracted from an MBR plant with long-term stable operation, and the effects of SMP on phosphate adsorption onto fresh HFO (FHFO) were investigated at pH values of 6.0 and 7.8 in a two-phase system composed of FHFO (solid phase) and SMP/phosphate (liquid phase). The phosphate adsorption capacity of FHFO was found to decrease significantly with increase in the pH value and SMP concentration. A diffuse layer model (DLM) and a charge distribution multisite complexation (CD-MUSIC) model, which take these factors into account, were developed to describe the speciation of SMP and phosphate on FHFO surface. Both of the models provided good description of phosphate adsorption behavior in the presence of SMP at different pH. The equilibrium constants of surface complexation reactions between SMP and HFO were obtained by inversion calculation as log10K = 15.1 ± 0.11 in DLM model and log10K = 12.1 ± 0.5 in CD-MUSIC model, which can improve the equilibrium constants database of surface complexation models.

Electrophoretic mobility of carboxyl latex particles in the mixed solution of 1:1 and 2:1 electrolytes or 1:1 and 3:1 electrolytes: Experiments and modeling

•We measured and calculated electrophoretic mobility of carboxyl latex particles in mixed solution of KCl and CaCl2 or KCl and LaCl3.•Used theoretical models consider the deprotonation, the binding of Ca2+/La3+, diffuse double layer model, and the double layer relaxation. Experimental data are reasonably described by the theoretical models except the case with La3+ at high pH.•The surface charge reduces due to the binding of Ca2+/La3+ to deprotonated carboxyl groups.•The model with the binding of La3+ is unable to describe charge reversals in EPM at high pH.