Constant Capacitance Surface Complexation Model Research Articles

Surface complexation model of rare earth element adsorption onto bacterial surfaces with lanthanide binding tags

Lanthanide binding tags (LBTs) have been engineered onto the cell surface of E. coli to enhance biosorption and recovery of rare earth elements (REEs). The protonation behavior of the bacterial surfaces before and after LBT-display was compared by modeling acid-base titration data. A multiple discrete site, constant capacitance surface complexation model was constructed to examine rare earth (Tb) binding to cell surface functional groups, comparing wild type and LBT-engineered surfaces. Our acid-base titrations show similar pKa values between the two strains, suggesting induction of LBTs does not significantly alter cell surface protonation behavior. Tb sorption onto the wild type cell surface can be captured by a one-site carboxyl model. The LBT strain exhibited a higher metal loading that can be explained by the increase of sorption sites in the form of lanthanide binding tags. Furthermore, carboxyl site concentrations between wild type and LBT-induced cells were statistically indistinguishable. We thus attribute the engineered strains’ increase in Tb adsorption capacity and affinity to the addition of lanthanide binding tags to the cell surface. The Tb stability constant with the LBT site is two orders of magnitude higher than that with the carboxyl functional group. As a result, at low metal loading <10 μM, the Tb binding to the cell surface of the LBT-strain is controlled by the presence of high-affinity, but lower capacity, LBT sites. At higher metal loadings >10 μM, a more abundant but low affinity functional group becomes the main source of adsorption that results in an overall higher sorption capacity. This work demonstrates how surface complexation modeling can be implemented for bacterial surfaces engineered with a known protein tag to optimize REE recovery from fluids with variable pH and metal loadings.

The adsorption of myo-inositol hexaphosphate onto kaolinite and its effect on cadmium retention

Anions are known to affect the adsorption of metal ions to soil minerals, with the change in adsorption depending on the nature of the anion. Organic phosphates, like inositol hexaphosphate (IP6), are present in significant amounts in most soil systems but little is known about their effect on metal ion adsorption. Since strong complexes are formed between organic phosphates and metal ions, their presence in soils should significantly affect metal ion adsorption. Adsorption edge and isotherm experiments were performed for binary IP6–kaolinite, Cd(II)–kaolinite and IP6–Cd(II) systems, and the ternary IP6–Cd(II)–kaolinite system. All results were modeled with Extended Constant Capacitance surface complexation models. The presence of IP6 significantly increased the adsorption of Cd(II) to kaolinite in the pH range from 4 to 8.5 with a small decrease in adsorption at higher pH values. There was evidence of Cd(II)–IP6 precipitate formation at higher concentrations. Modeling indicated that IP6 was bound to the surface as both inner- and outer-sphere complexes while two extra ternary complexes were required to fit the IP6–Cd(II)–kaolinite data. Suppression of Cd(II) adsorption at higher pH most probably resulted from the formation of soluble Cd(II)–IP6 complexes.

The effect of inositol hexaphosphate on cadmium sorption to gibbsite

HypothesisOxides, hydrous oxides and hydroxides of aluminium and iron are important in determining the availability of trace and heavy metals in soil systems. The presence of complexing anions is also known to affect the binding of these metals in soils. Since organophosphates, such as inositol hexaphosphate (IP6), are present in most soil systems they are expected to affect the nature of the interaction between metal ions and metal (hyr)oxides. ExperimentsBoth adsorption edge and isotherm experiments were conducted on Cd(II)–gibbsite and Cd(II)–IP6–gibbsite systems. In addition, solid-state 31P MAS NMR measurements were performed on the ternary system. All results were used to develop Extended Constant Capacitance surface complexation models of both the Cd(II)–gibbsite and IP6–Cd(II)–gibbsite sorption systems. FindingsThe presence of IP6 significantly increased sorption of Cd(II) to gibbsite below pH 8 especially at higher concentrations of Cd(II) and IP6. The 31P MAS NMR spectra, together with surface complexation modeling, indicated the presence of two outer-sphere ternary complexes with the first, [(SOH2)33+(LHCd)9−]6−, important at relatively low concentrations, while the second, [SLH38−Cd2+]6−, dominated sorption at higher sorbate concentrations. Thus the presence of organophosphates in soil systems increases sorption and may therefore decrease the availability of trace and heavy metals to plants.

Chemical Modeling of Boron Adsorption by Humic Materials Using the Constant Capacitance Model

The constant capacitance surface complexation model was used to describe B adsorption behavior on reference Aldrich humic acid, humic acids from various soil environments, and dissolved organic matter extracted from sewage effluents. The reactive surface functional groups on thehumicmaterialswere assumedto be a carboxylsite, XOH, and a phenol site, YOH. Initially, total concentrations of the sites, XOHTand YOHTand the proton dissociation constants for the carboxyl site, LogKX- ,a nd the phenol site, LogKY-, were optimized by fitting the constant capacitance model to potentiometric titration data on reference Aldrich humic acid ob- tained from the literature. Subsequently, the model was fit to experimental B adsorption data obtained from the literature by optimizing two tetrahe- dral B surface complexation constants: LogKXB- for a carboxyl site and LogKYB- for a phenol site. The model was well able to describe the exper- imental B adsorption data both as a function of solution B concentration (isotherm data) and solution pH (envelope data) for all humic materials. The ability to represent changes in B adsorption as a function of solution pH is the main advantage of the constant capacitance model over adsorp- tion isotherm equations. Results from the current study can be used to de- scribe B adsorption behavior on diverse humic materials of interest in environmental and agricultural situations.

A Review of Molybdenum Adsorption in Soils/Bed Sediments: Speciation, Mechanism, and Model Applications

Mo is an essential trace element for both plants and animals in low concentrations (<5 ppm). However, provoked by uncontrolled industrial waste releases in freshwater or seawater, it is plausible that excessive availability of soluble Mo(VI) would be potentially toxic. In the environment, soluble Mo(VI) is mainly present in anionic forms of molybdate (MoO4 2−) and/or tetrathiomolybdate (MoS4 2−). The fate and transport of soluble Mo(VI) anions in surface and subsurface aquatic environments is typically controlled by adsorption in acidic soils and sediment. As such, the ability of soils/bed sediments to retain Mo(VI) is a key to determine its general mobility in the aquatic environment. This article reviews the sources and distribution of Mo speciation in solution and Mo(VI) anions adsorption mechanisms in soils and bed sediments, and evaluates the surface adsorption complexation models at the solid-water interface to estimate Mo(VI) anions adsorption in these chemical systems. Mo(VI) anions adsorption mechanisms included MoO4 2− and MoS4 2− adsorption by several prevailing adsorbent contents (including clay, Fe, Al oxides, iron sulfide, manganese oxides, and organic matter) of soils and bed sediments, and the influence of the competitive adsorption of other anions (e.g., sulfate, selenate, phosphate, arsenate, silicate, or tungstate). Models to estimate Mo(VI) anions adsorption include the triple layer model (TLM), the diffuse layer model (DLM), the constant capacitance surface complexation model (CCM), and charge distribution multisite complexation model (CD-MUSIC).

Predicting Arsenate Adsorption on Iron-Coated Sand Based on a Surface Complexation Model

Equations were developed to predict arsenate sorption on iron oxide coated sand, on the basis of the constant capacitance surface complexation model, assuming formation of bidentate surface complexes between arsenate and iron oxyhydroxide surface sites. The developed equations can predict arsenate adsorption when initial arsenate concentration, adsorbent concentration, and solution pH are known. The average discrepancy between experimental data and modeling results was less than 5%. The maximum arsenate sorption capacity is the only parameter required that needs to be estimated from experimental data. The developed equations were validated by modeling arsenate adsorption on iron oxide sand over the pH range 5–8, under various initial arsenate concentrations and iron oxide coated sand solid concentrations. The developed predictive equation can also be used for calculating arsenate adsorption performance on iron impregnated activated carbon. Finally, the developed equation can be used to calculate sorption media dose requirements by specifying initial arsenate concentration, arsenate removal goal, and solution pH.

Arsenate adsorption at the sediment–water interface: sorption experiments and modelling

Arsenate adsorption was studied in three clastic sediments, as a function of solution pH (4.0–9.0) and arsenate concentration. Using known mineral values, protolytic constants obtained from the literature and Kads values (obtained by fitting experimental adsorption data with empirical adsorption model), the constant capacitance surface complexation model was used to explain the adsorption behavior. The experimental and modelling approaches indicate that arsenate adsorption increases with increased pH, exhibiting a maximum adsorption value before decreasing at higher pH. Per unit mass, sample S3 (smectite–quartz/muscovite–illite sample) adsorbs more arsenate in the pH range 5–8.5, with 98% of sites occupied at pH 6. S1 and S2 have less adsorption capacity with maxima adsorption in the pH ranges of 6–8.5 and 4–6, respectively. The calculation of saturation indices by PHREEQC at different pH reveals that the solution was undersaturated with respect to aluminum arsenate (AlAsO42H2O), scorodite (FeAsO42H2O), brucite and silica, and supersaturated with respect to gibbsite, kaolinite, illite and montmorillonite (for S3 sample). Increased arsenate concentration (in isotherm experiments) may not produce new solid phases, such as AlAsO42H2O and/or FeAsO42H2O.

Acid−Base Behavior of the Gaspeite (NiCO3(s)) Surface in NaCl Solutions

Gaspeite is a low reactivity, rhombohedral carbonate mineral and a suitable surrogate to investigate the surface properties of other more ubiquitous carbonate minerals, such as calcite, in aqueous solutions. In this study, the acid-base properties of the gaspeite surface were investigated over a pH range of 5 to 10 in NaCl solutions (0.001, 0.01, and 0.1 M) at near ambient conditions (25 +/- 3 degrees C and 1 atm) by means of conventional acidimetric and alkalimetric titration techniques and microelectrophoresis. Over the entire experimental pH range, surface protonation and electrokinetic mobility are strongly affected by the background electrolyte, leading to a significant decrease of the pH of zero net proton charge (PZNPC) and the pH of isoelectric point (pH(iep)) at increasing NaCl concentrations. This challenges the conventional idea that carbonate mineral surfaces are chemically inert to background electrolyte ions. Multiple sets of surface complexation reactions (i.e., ionization and ion adsorption) were formulated within the framework of three electrostatic models (CCM, BSM, and TLM) and their ability to simulate proton adsorption and electrokinetic data was evaluated. A one-site, 3-pK, constant capacitance surface complexation model (SCM) reproduces the proton adsorption data at all ionic strengths and qualitatively predicts the electrokinetic behavior of gaspeite suspensions. Nevertheless, the strong ionic strength dependence exhibited by the optimized SCM parameters reveals that the influence of the background electrolyte on the surface reactivity of gaspeite is not fully accounted for by conventional electrostatic and surface complexation models and suggests that future refinements to the underlying theories are warranted.

Competitive adsorption of copper(II), cadmium(II), lead(II) and zinc(II) onto basic oxygen furnace slag

Polluted and contaminated water can often contain more than one heavy metal species. It is possible that the behavior of a particular metal species in a solution system will be affected by the presence of other metals. In this study, we have investigated the adsorption of Cd(II), Cu(II), Pb(II), and Zn(II) onto basic oxygen furnace slag (BOF slag) in single- and multi-element solution systems as a function of pH and concentration, in a background solution of 0.01 M NaNO 3. In adsorption edge experiments, the pH was varied from 2.0 to 13.0 with total metal concentration 0.84 mM in the single element system and 0.21 mM each of Cd(II), Cu(II), Pb(II), and Zn(II) in the multi-element system. The value of pH 50 (the pH at which 50% adsorption occurs) was found to follow the sequence Zn > Cu > Pb > Cd in single-element systems, but Pb > Cu > Zn > Cd in the multi-element system. Adsorption isotherms at pH 6.0 in the multi-element systems showed that there is competition among various metals for adsorption sites on BOF slag. The adsorption and potentiometric titrations data for various slag–metal systems were modeled using an extended constant-capacitance surface complexation model that assumed an ion-exchange process below pH 6.5 and the formation of inner-sphere surface complexes at higher pH. Inner-sphere complexation was more dominant for the Cu(II), Pb(II) and Zn(II) systems.

Adsorption of arsenite and arsenate onto muscovite and biotite mica

Arsenite and arsenate sorption was studied on two silt-sized phyllosilicates, namely muscovite and biotite, as a function of solution pH (pH 3–8 for muscovite, and 3–11 for biotite) at an initial As concentration of 13 μM. The amount of arsenic adsorbed increases with increasing pH, exhibiting a maximum value, before decreasing at higher pH values. Maxima correspond to 3.22 ± 0.06 mmol kg −1 As(V) at pH 4.6–5.6 and 2.86 ± 0.05 mmol kg −1 As(III) at pH 4.1–6.2 for biotite, and 3.08 ± 0.06 mmol kg −1 As(III) and 3.13 ± 0.05 mmol kg −1 As(V) at pH 4.2–5.5 for muscovite. The constant capacitance surface complexation model was used to explain the adsorption behavior. Biotite provides greater reactivity than muscovite toward arsenic adsorption. Isotherm data obeyed the Freundlich or Langmuir equation for the arsenic concentration range 10 −7–10 −4 M. Released total Fe, Si, K, Al, and Mg in solution were analyzed. Calculation of saturation indices by PHREEQC indicated that the solution was undersaturated with respect to aluminum arsenate (AlAsO 4⋅2H 2O), scorodite (FeAsO 4⋅2H 2O), and claudetite/arsenolite (As 4O 6).

Modeling the Adsorption of Organic Dye Molecules to Kaolinite

AbstractSimple extended constant capacitance surface complexation models have been developed to represent the adsorption of polyaromatic dyes (9-aminoacridine, 3,6-diaminoacridine, azure A and safranin O) to kaolinite, and the competitive adsorption of the dyes with Cd. The formulation of the models was based on data from recent publications, including quantitative adsorption measurements over a range of conditions (varying pH and concentration), acid-base titrations and attenuated total reflectance-Fourier transform infrared spectroscopic data. In the models the dye molecules adsorb as aggregates of three or four, forming outer-sphere complexes with sites on the silica face of kaolinite. Both electrostatic and hydrophobic interactions are implicated in the adsorption processes. Despite their simplicity, the models fit a wide range of experimental data, thereby supporting the underlying hypothesis that the flat, hydrophobic, but slightly charged silica faces of kaolinite facilitate the aggregation and adsorption of the flat, aromatic, cationic dye molecules.

Influence of Temperature on the Adsorption of Mellitic Acid onto Kaolinite

The adsorption of mellitic acid (benzene-1,2,3,4,5,6-hexacarboxylic acid) onto kaolinite was investigated at five temperatures between 10 and 70 degrees C. Mellitic acid adsorption increased with increasing temperature at low pH (below pH 5.5), but at higher pH, the effect of increasing temperature was to reduce the amount adsorbed. Potentiometric titrations were conducted, adsorption isotherms were measured over the same temperature range, and the data obtained were used in conjunction with adsorption edge and ATR-FTIR spectroscopic data to develop an extended constant capacitance surface complexation model of mellitic acid adsorption. A single set of reactions was used to model all data at the five temperatures studied. The model indicates that mellitic acid sorbs via outer-sphere complexation to surface hydroxyl (SOH) groups on the kaolinite surface rather than to permanent charge sites. The reactions proposed are SOH + L6- + 2H+ <-->[(SOH2)+(LH)5-]4- and SOH + L(6-) <--> [(SOH)(L)6-]6-. Thermodynamic parameters calculated from the temperature dependence of the equilibrium constants for these reactions indicate that the adsorption of mellitic acid onto kaolinite is accompanied by a large entropy increase.

The influence of temperature on the adsorption of mellitic acid onto goethite

The adsorption of mellitic acid (benzene-1,2,3,4,5,6-hexacarboxylic acid) onto goethite was investigated at five temperatures between 10 and 70 °C. Mellitic acid adsorption increased with increasing temperature below pH 7.5, but at higher pH the effect of increasing temperature was to reduce the amount adsorbed. Potentiometric titrations were conducted and adsorption isotherms were measured over the same temperature range, and the data obtained were used in conjunction with adsorption edge data to develop an Extended Constant Capacitance Surface Complexation Model of mellitic acid adsorption. A single set of reactions was used to model the adsorption for the three different experiment types at the five temperatures studied. The adsorption reactions proposed for mellitate ion (L 6−) adsorption at the goethite surface (SOH) involved the formation of two outer-sphere complexes: SOH + L 6 − + 3H + ⇌ [ ( SOH 2 ) + ( LH 2 ) 4 − ] 3 − , 2SOH + L 6 − + 2H + ⇌ [ ( SOH 2 ) 2 2 + ( L ) 6 − ] 4 − . This mechanism is consistent with recent ATR-FTIR spectroscopic measurements of the mellitate-goethite system. Thermodynamic parameters calculated from the temperature dependence of the equilibrium constants for these reactions indicate that the adsorption of mellitic acid onto goethite is accompanied by a large entropy increase.

Redox potential measurements and Mössbauer spectrometry of Fe II adsorbed onto Fe III (oxyhydr)oxides

The redox properties of Fe II adsorbed onto a series of Fe III (oxyhydr)oxides (goethite, lepidocrocite, nano-sized ferric oxide hydrate (nano-FOH), and hydrous ferric oxide (HFO)) have been investigated by rest potential measurements at a platinum electrode, as a function of pH (−log 10[H +]) and surface coverage. Using the constant capacitance surface complexation model to describe Fe II adsorption onto these substrates, theoretical values of the suspension redox potential ( E H) have been computed, under the assumption that Fe II adsorption occurs at crystal growth sites of the substrate surface. Good agreement between calculated and experimental E H values is observed for nano-FOH and HFO, however the redox potentials measured for lepidocrocite and goethite are significantly more oxidizing than predicted. Mössbauer spectroscopic analysis of 57Fe II adsorbed onto HFO and goethite shows that in both cases the adsorbed 57Fe II is incorporated into the crystal structure of the substrate, in broad agreement with the thermodynamic model, but is almost completely oxidized to 57Fe III. The mechanism by which the adsorbed 57Fe II is oxidized is not resolved in this work, but is thought to be due to electron transfer to the substrate, rather than a net oxidation of the suspension. The disagreement between experimental and calculated rest potential measurements in the goethite and lepidocrocite systems is thought to be due to the poor electrochemical equilibration of these suspensions with the platinum electrode, rather than a failure of the thermodynamic model. The model developed for the redox potential of adsorbed Fe II allows direct assessment of the reactivity of this species towards oxidized pollutants.

Competitive adsorption behavior of heavy metals on kaolinite

Polluted and contaminated soils can often contain more than one heavy metal species. It is possible that the behavior of a particular metal species in a soil system will be affected by the presence of other metals. In this study we have investigated the adsorption of Cd(II), Cu(II), Pb(II), and Zn(II) onto kaolinite in single- and multi-element systems as a function of pH and concentration, in a background solution of 0.01 M NaNO 3. In adsorption edge experiments, the pH was varied from 3.5 to 10.0 with total metal concentration 133.3 μM in the single-element system and 33.3 μM each of Cd(II), Cu(II), Pb(II), and Zn(II) in the multi-element system. The value of pH 50 (the pH at which 50% adsorption occurs) was found to follow the sequence Cu < Zn < Pb < Cd in single-element systems, but Pb < Cu < Zn < Cd in the multi-element system. Adsorption isotherms at pH 6.0 in the multi-element systems showed that there is competition among various metals for adsorption sites on kaolinite. The adsorption and potentiometric titrations data for various kaolinite–metal systems were modeled using an extended constant-capacitance surface complexation model that assumed an ion-exchange process below pH 7.0 and the formation of inner-sphere surface complexes at higher pH. Inner-sphere complexation was more dominant for the Cu(II) and Pb(II) systems.

Surface complexation modeling of the sorption of 2-, 3-, and 4-aminopyridine by montmorillonite

The sorption of 2-, 3-, and 4-aminopyridine on K-saturated Wyoming (SWy-K) and Texas (STx-K) and Ca-enriched Texas (STx-Ca) montmorillonite was measured at 25 °C with 10 mM KNO 3 or 3.3 mM Ca(NO 3) 2 as the background electrolyte. The aminopyridines adsorbed to montmorillonite at low pH, but not at high pH. Extended constant capacitance surface complexation models (ECCMs) and attenuated total reflectance-FTIR data indicate that aminopyridines sorb to the silica-like faces by cation exchange, forming outer-sphere complexes between aminopyridinium ions and permanent negatively charged surface sites (X −). X-ray diffraction data and sorption kinetics suggest that sorption occurs not only at external X − sites but also at those in the interlayer spaces. Differences in the sorption behaviors of 2-, 3-, and 4-aminopyridine result from differences in their p K a s. The extent of sorption of aminopyridines by the montmorillonite samples (SWy-K > STx-K > STx-Ca) results from the higher cation-exchange capacity of SWy-K, and from the fact that Ca 2+ is much more effective than K + in competing with protonated aminopyridines for the X − sites.

Surface complexation modeling of the sorption of Zn(II) by montmorillonite

Adsorption of Zn(II) to montmorillonite samples K +-Wyoming (SWy-K), Ca 2+-Texas (STx-Ca) and K +-Texas (STx-K) was studied at 25 °C in the presence of 10 mM KNO 3 for SWy-K and STx-K or 3.3 mM Ca(NO 3) 2 for STx-Ca. X-ray diffraction measurements showed no change in the interlayer spacing of the montmorillonite samples on adsorption of zinc. Data from adsorption edges, adsorption isotherms, and potentiometric titrations were fitted by an extended constant capacitance surface complexation model (ECCM) which included two dominant sorption reactions: cation exchange at permanent negatively charged sites on the siloxane faces including interlayer regions of montmorillonite, and inner-sphere surface complex formation at variable-charge surface hydroxyl groups on the edges. At pH values above 9 the fit of the model to the data was less satisfactory, most probably because of surface precipitation. Generally the best fit parameters of the ECCM for the three samples agreed well. The exception was the concentration of permanent-charge sites, which was about four times greater for the Wyoming sample.

Sorption Behaviour of Atrazine onto Natural Sediments under Various Solution Conditions

Potentiometric titrations, XRD analysis and batch adsorption experiments were conducted under various solution chemistry conditions to study the uptake of atrazine (AT) from sediment/water suspensions. A constant capacitance surface complexation model was then applied to interpret the reaction mechanism at the aqueous sediment surfaces. The results obtained showed that the sediment sample was negatively charged over a large range of pH values, with the model calculations matching well with the experimental results.Increasing pH value and low ionic strength, as well as high solid concentrations, led to a decrease in the adsorption of AT. The existence of dissolved organic matter (DOM) in the sediment/water system also influenced the uptake of AT by the sediment. A large increase in AT adsorption occurred for sediment pre-incubated with DOM, with a decrease in AT uptake occurring when herbicide was pre-incubated with DOM. In addition, the results suggested that long aliphatic chains must play an important r...

Surface Properties of Cleaved Mica

The structural characteristics of cleaved mica are studied. Cleavage of phlogopite and muscovite is shown to be accompanied by the deformation of their crystal lattices. The structural charge emerging as a result amounts to 90–95% of the total charge of the mica surface. The acid–base properties of the surfaces of these minerals are studied by the method of potentiometric titration. As is shown using constant capacitance surface complexation model, only deprotonation reactions are characteristic for muscovite, whereas phlogopite undergoes also ion-exchange reactions. The corresponding constants of acid–base equilibrium are determined.

Adsorption of aspartic acid on kaolinite

The interaction of aspartic acid with kaolinite was studied by potentiometric titrations and by adsorption measurements both at constant aspartate concentration (but varying pH) and at a constant pH of 5.5. The temperature was 25 °C, and the ionic medium 5 mM KNO 3. Aspartic acid dissociation constants estimated from titrations agreed with those from the literature. The adsorption of aspartic acid to kaolinite was weak and varied only slightly with pH; 10–18% of 100 μM aspartic acid adsorbed to kaolinite at 100 m 2 L −1 between pH 3 and 10. Data from the titrations and adsorption experiments were fitted closely by an extended constant-capacitance surface complexation model, in which monodentate outer-sphere complexes formed between deprotonated aspartic acid molecules and protonated sites on the variable-charge edges of the kaolinite crystals. There appeared to be no adsorption to the permanently charged crystal faces.