Surface Site Density Research Articles

Effect of ionic sizes on the stability ratio of a dispersion of particles with a charge-regulated surface

The influence of the ionic sizes on the stability of a dispersion of particles, which have an amphoteric, charged-regulated surface, is discussed. A modified Poisson–Boltzmann equation, which takes into account the sizes of ionic species, is adopted to describe the electrical field. An extended DLVO theory, which takes into account the electrical, the van der Waals, and the hydration energies, is used to estimate the stability of a colloidal dispersion. The effects of the key parameters, including ionic strength, pH, and density of surface sites, on the behavior of problem under consideration are examined. The results obtained are qualitatively consistent with experimental findings in the literature.

Numerical investigation on combustion characteristics of methane in a hybrid catalytic combustor

Combustion characteristics of a hybrid catalytic combustor, which consists of a catalyst bed and a thermal combustor section, were investigated using two-dimensional boundary layer approximations with detailed homogeneous and heterogeneous chemistries. Lean methane-air mixture was supplied to the inlet of a catalyst bed section. In order to validate our numerical analyses, the surface site density of platinum-coated monolith was estimated through the comparisons with the experimental data reported in literature. Discussed in detail were the effects of homogeneous chemistry and heterogeneous chemistry on catalytic combustion characteristics. The extensive numerical calculations performed with the selected surface site density revealed that homogeneous reactions in the monolith had little effects on the spatial distributions of temperature and CH 4 conversion and the location of heterogeneous ignition, while they showed an important role to the initial reaction processes in the thermal combustor section through productions of the significant amount of intermediate species, such as OH and CO, near the catalyst bed exit. The parametric study for various operating conditions, such as equivalence ratio, temperature, velocity, entrance diameter of the monolith channel and inlet pressure, were also performed to investigate their effects on catalytic combustion. In the thermal combustor, N 2O was a dominant component in emissions that are mainly formed under the reaction mechanism, N 2+O(+M)→N 2O(+M), in case no additional fuel is injected. Furthermore, the formations of CO and NO x emission were also characterized with controlling the amount of additional methane injection to the thermal combustor.

Surface Chemical Heterogeneity of Bacteriogenic Iron Oxides from a Subterranean Environment

This study quantifies the surface chemical heterogeneity of bacteriogenic iron oxides (BIOS) and its end-members (2-line ferrihydrite and intermixed intact and fragmented bacteria). On a dry weight basis, BIOS consisted of 64.5 +/- 1.8% ferrihydrite and 34.5 +/- 1.8% organic matter. Enrichment of Al, Cu, Cr, Mn, Sr, and Zn was shown in the solid versus the aqueous phase (1.9 < log Kd < 4.2). Within the solid-phase Al (69.5%), Cu (78.7%), and Zn (77.9%) were associated with the bacteria, whereas Cr (59.8%), Mn (99.8%), and Sr (79.4%) preferred ferrihydrite. Acid-base titration data from the BIOS and bacteria were fitted using FOCUS pKa spectroscopy. The bacteria spectrum with pKa's of 4.18 +/- 0.37, 4.80 +/- 0.54, 6.98 +/- 0.45, and 9.75 +/- 0.68 was similar to discrete and continuous spectra for intact and fragmented bacteria. The BIOS spectrum recorded pKa's of 4.27 +/- 0.51, 6.61 +/- 0.51, 7.89 +/- 1.10, and 9.65 +/- 0.66 and was deconvoluted to remove overlapping binding site contributions from the bacteria. The resulting residual iron oxide spectrum coincided with discrete MUSIC spectra for goethite and lepidocrocite with pKa values of 4.10 +/- 0.43, 6.53 +/- 0.45, 7.81 +/- 0.76, and 9.51 +/- 0.68. Surface site density analysis showed that acidic sites (pKa < 6) were contributed by the bacteria (37%), whereas neutral sites (6 < pKa < 8) were characteristic of the iron oxide fraction (35%). Basic sites (8 < pKa) were higher in the bacteria (57%), than in the BIOS (44%) or iron oxide fractions (47%). This analysis suggested a high degree of bacterial group masking and a similarity between the BIOS and goethite surface reactivity. An understanding of the BIOS surface chemical heterogeneity and inherent proton and metal binding capacity was obtained through the use of FOCUS apparent pKa spectroscopy.

Variability in goethite surface site density: evidence from proton and carbonate sorption

Goethite is a representative iron oxide in natural environments due to its abundance and thermodynamic stability and may be responsible for many surface-mediated processes, including ion retention and mobility in aqueous settings. A large variability in morphologies and specific surface areas of goethite crystals exists but little work has been done to compare surface reactivity between them. The present work offers experimental evidence for the existence of an inverse relationship between sorption capacity for protons and carbonate ions and specific surface area of goethite for three synthetic goethite preparations spanning surface area differences by a factor of 2. An explanation for this was found by assuming a variable reactive site density between preparations in direct relationship to their sorption capacity based on congruency of carbonate sorption computed on a per-site basis. Previous evidence of maximum sorption capacities supports this explanation, and site density ratios between the goethites studied here were obtained. Triple layer surface complexation modeling was successful in describing adsorption data for all goethite preparations using equal stoichiometries. A new formulation of standard state for activities of surface species based on a 1.0 mole fraction of sites on the solid allowed transformation of the conventional molar concentration-based affinity constants to values based on site occupancy. In this fashion, by applying the appropriate site density ratios, a single set of affinity constant values was found that described accurately the adsorption data for all preparations.

Effect of solubility on the determination of the protonable surface site density of oxyhydroxides

The impact of the dissolved quantity of aluminum on the calculation of surface charge from titration experiments of hydrated γ-alumina was investigated. Two methods were developed to correct this effect: direct determination of aluminum in solution in batch titration experiments and application of a dissolution rate model in continuous titration experiments. There is a large effect of dissolution on surface charge determination for pH>10 and pH < 4.5. Application of these correction methods to these pH ranges lead to apparent saturation surface charges of 1.3 and 2.3 at nm −2, respectively. The last value seems to be the real proton-active site density, while the hydroxyl-active site density cannot be reached for easily achievable basic pH values.

Electrochemical Oxidation of 8‐Oxo‐2′‐Deoxyguanosine on Glassy Carbon, Gold, Platinum and Tin(IV) Oxide Electrodes

AbstractCyclic voltammetry was used to investigate the oxidation of 8‐oxo‐2′‐deoxyguanosine (8‐oxo‐dG) on the glassy carbon (GC), platinum, gold and SnO2 electrodes over a range of the sweep rate, 8‐oxo‐dG concentration and the solution pH. Reaction mechanism that is common to all these electrodes involves the two‐electron two‐proton charge transfer step followed by the irreversible chemical reaction(s). Rate of the charge transfer reaction decreases with the increasing solution pH (GC, Pt, Au), and depends on the nature of the electrode material following the sequence GC&gt;Pt, Au&gt;&gt;SnO2. These effects can be related to the degree of oxidation of the electrode surface (Pt, Au, SnO2), or to the density of the active surface sites (GC). Any of these electrodes can be used for the fabrication of an amperometric detector for 8‐oxo‐dG .

Modeling sorption of divalent metal cations on hydrous manganese oxide using the diffuse double layer model

Manganese oxides are important scavengers of trace metals and other contaminants in the environment. The inclusion of Mn oxides in predictive models, however, has been difficult due to the lack of a comprehensive set of sorption reactions consistent with a given surface complexation model (SCM), and the discrepancies between published sorption data and predictions using the available models. The authors have compiled a set of surface complexation reactions for synthetic hydrous Mn oxide (HMO) using a two surface site model and the diffuse double layer SCM which complements databases developed for hydrous Fe (III) oxide, goethite and crystalline Al oxide. This compilation encompasses a range of data observed in the literature for the complex HMO surface and provides an error envelope for predictions not well defined by fitting parameters for single or limited data sets. Data describing surface characteristics and cation sorption were compiled from the literature for the synthetic HMO phases birnessite, vernadite and δ-MnO 2. A specific surface area of 746 m 2g −1 and a surface site density of 2.1 mmol g −1 were determined from crystallographic data and considered fixed parameters in the model. Potentiometric titration data sets were adjusted to a pH IEP value of 2.2. Two site types (≡XOH and ≡YOH) were used. The fraction of total sites attributed to ≡XOH ( α) and p K a2 were optimized for each of 7 published potentiometric titration data sets using the computer program FITEQL3.2. p K a2 values of 2.35±0.077 (≡XOH) and 6.06±0.040 (≡YOH) were determined at the 95% confidence level. The calculated average α value was 0.64, with high and low values ranging from 1.0 to 0.24, respectively. p K a2 and α values and published cation sorption data were used subsequently to determine equilibrium surface complexation constants for Ba 2+, Ca 2+, Cd 2+, Co 2+, Cu 2+, Mg 2+, Mn 2+, Ni 2+, Pb 2+, Sr 2+ and Zn 2+. In addition, average model parameters were used to predict additional sorption data for which complementary titration data were not available. The two-site model accounts for variability in the titration data and most metal sorption data are fit well using the p K a2 and α values reported above. A linear free energy relationship (LFER) appears to exist for some of the metals; however, redox and cation exchange reactions may limit the prediction of surface complexation constants for additional metals using the LFER.

Speciation of uranium(VI) at the solid/solution interface: sorption modeling on zirconium silicate and zirconium oxide

SummaryIn the field of nuclear waste storage, knowledge of the sorption behavior of radionuclides onto mineral surfaces is of fundamental importance in order to evaluate environmental impacts. We have studied uranyl ion sorption mechanisms onto zirconium silicate and zirconium oxide with a double approach, coupling both macroscopic and structural investigations. The spectroscopic results, reported in a previous paper, allowed a complete determination of the different species involved in the studied sorption reactions. This paper presents the results of the macroscopic part of the study. Surface characterizations of the solids were first carried out: determination of the pH of the point of zero charge, of the surface site densities and of the surface acidity constants. The experimental retention data were then interpreted in terms of a constant capacitance model using the FITEQL code. Results from structural investigation were used to constrain the modeling. This coupled approach led to accurate reactions stoichiometry and associated sorption constants values, since they were determined with respect to different experimental results, both macroscopic and microscopic.

황토로부터 합성한 제올라이트에 의한 Cd(II) 및 Cu(II) 흡착특성

Various kinds of zeolites, such as analcime (ANA), cancrinite (CAN), Na-Pl and sodalite octahydrate (SOD) could be synthesized from Hwangto by hydrothermal reaction in a high-pressure vessel. The adsorption characteristics of Cu(II) and Cd(II) by Hwangto and its synthetic zeolites were investigated using the chemical and electrochemical surface parameters of these adsorbents. The heavy metal adsorptivity among the adsorbents decreased in the following sequences: Na-Pl > SOD > ANA > CAN > Hwangto. This sequence was the same with the values of surface site density (Ns) of these adsorbents and was correlated inversely with the values of pHpzc (pH of the point of zero charge) and the va]ues of K<TEX>$\_$</TEX>a2/(int) (intrinsic surface deprotonation constant) of the adsorbents for synthetic zeolites, i.e., the adsorbents with higher values of Ns and with lower values of pHpzc and K<TEX>$\_$</TEX>a2/(int) for synthetic zeolites showed higher heavy metal adsorptivity. With increasing pH, the heavy metal adsorptivity increased greatly between pHpzc and pH 6 or 7 because of the steep increase of negatively charged sites for synthetic zeolites, but for Hwangto, it increased broadly because of slow increase of negatively charged sites based on its lower surface sites.

Gallium adsorption on (0001) GaN surfaces

We study the adsorption behavior of Ga on (0001) GaN surfaces combining experimental specular reflection high-energy electron diffraction with theoretical investigations in the framework of a kinetic model for adsorption and ab initio calculations of energy parameters. Based on the experimental results we find that for substrate temperatures and Ga fluxes typically used in molecular-beam epitaxy of GaN, finite equilibrium Ga surface coverages can be obtained. The measurement of a Ga/GaN adsorption isotherm allows the quantification of the equilibrium Ga surface coverage as a function of the impinging Ga flux. In particular, we show that a large range of Ga fluxes exists, where $2.5\ifmmode\pm\else\textpm\fi{}0.2$ monolayers (in terms of the GaN surface site density) of Ga are adsorbed on the GaN surface. We further demonstrate that the structure of this adsorbed Ga film is in good agreement with the laterally contracted Ga bilayer model predicted to be most stable for strongly Ga-rich surfaces [Northrup et al., Phys. Rev. B $61,$ 9932 (2000)]. For lower Ga fluxes, a discontinuous transition to Ga monolayer equilibrium coverage is found, followed by a continuous decrease towards zero coverage; for higher Ga fluxes, Ga droplet formation is found, similar to what has been observed during Ga-rich GaN growth. The boundary fluxes limiting the region of 2.5 monolayers equilibrium Ga adsorption have been measured as a function of the GaN substrate temperature giving rise to a Ga/GaN adsorption phase diagram. The temperature dependence is discussed within an ab initio based growth model for adsorption taking into account the nucleation of Ga clusters. This model consistently explains recent contradictory results of the activation energy describing the critical Ga flux for the onset of Ga droplet formation during Ga-rich GaN growth [Heying et al., J. Appl. Phys. $88,$ 1855 (2000); Adelmann et al., J. Appl. Phys. $91,$ 9638 (2002).].

Microcalorimetric and IR spectroscopic studies of CO adsorption on molybdenum nitride catalysts

The adsorption of CO on both nitrided and reduced passivated Mo2N catalysts in either alumina supported or unsupported forms was studied by adsorption microcalorimetry and infrared (IR) spectroscopy. The CO is adsorbed on nitrided Mo2N catalysts on three different surface sites: 4-fold vacancies, Moδ+ (0 < δ < 2) and N sites, with differential heats of CO adsorption decreasing in the same order. The presence of the alumina-support affects the energetic distribution of the adsorption sites on the nitrided Mo2N, i.e. weakens the CO adsorption strength on the different sites and changes the fraction of sites adsorbing CO in a specific form, revealing that the alumina supported Mo2N phase shows lower electron density than pure Mo2N. On reduced passivated Mo2N catalysts the CO was found to adsorb mainly on Mo4+ sites, although some slightly different surface Moδ+ (0 < δ < 2) sites are also detected. The nature, density and distribution of surface sites of reduced passivated Mo2N/γ-Al2O3 were similar to those on reduced MoO3/γ-Al2O3.

Fe(III) Oxide Reactivity Toward Biological versus Chemical Reduction

Initial rates of biological (Shewanella putrefaciens strain CN32, pH 6.8) and chemical (ascorbate, pH 3.0) reduction of synthetic Fe(III) oxides with a broad range of crystallinity and specific surface area were examined to assess how variations in these properties are likely to influence the kinetics of bacterial Fe(III) oxide reduction in heterogeneous natural Fe(III) oxide assemblages. The results indicate that bacterial Fe(III) oxide reduction does not respond strongly to oxide crystal thermodynamic properties (ΔGf) which exert a significant impact on the kinetics of abiotic reductive dissolution. These findings suggest that oxide mineral heterogeneity in natural soils and sediments is likely to affect initial rates of bacterial reduction (e.g. during the early stages of anaerobic metabolism following the onset of anoxic conditions) mainly via an influence on reactive surface site density and that inferences regarding the competitiveness of bacterial Fe(III) oxide reduction as a pathway for organic matter oxidation in anoxic environments cannot be based on assumed thermodynamic properties of the dominant oxide phase(s) in the soil or sediment.

Oxidation and reduction of the metal surface in supported Pt using dissociative N2O adsorption coupled with H2 and CO titration

Not only was the surface site density in a 0.78% Pt/SiO2 catalyst determined by using selective chemisorption techniques, but the surface chemistry related to decompositive N2O adsorption on the Pt surface was also described byin situ DRIFTS techniques. The“O”, coverage established by N2O decomposition at 363 K on a clean Pt surface was equal to thatvia hydrogen adsorption at 300 K; however, both the coverage of chemisorbed oxygenvia O2 chemisorption at 300 K and the COirr coverage were somewhat lower than the“O” monolayer coverage. Surface titration of the“O”-covered Pt crystallites after N2O decomposition at 363 K gave a consistent Pts density with the hydrogen chemisorption.In situ DRIFTS spectra of CO adsorbed at 300 K on both clean and H-covered Pt surfaces exhibited a strong peak at 2,076 cm−1 for linearly adsorbed CO with a small extent of multi-coordinated CO near 1,803 cm−1. The adsorption of CO at 300 K on an“O”,-covered Pt surfacevia dissociative N2O adsorption at 363 K appeared subsequently a band at 2,186 cm−1 due to a tiny amount of PtsO crystallites, which could be completely reduced to H-covered ones, when titrated with H2 at 300 K. The adequate description for these CO adsorption behaviors on different surfaces is PtsO+2CO(g)→PtsCO+CO2(g), although to very small extent, the addition onto PtsO occurs. Spectra of CO adsorbed on the oxidized Ptsvia N2O decomposition gave consistent surface chemistry within situ gravimetric measurements. The surface reactions acquired by DRIFTS spectra potentially offer an approach to remove N2O from emission sources by combining its catalytic dissociation with titration of the chemisorbed“O” atoms using either H2 or CO, particularly H2 because of complete recovery to a clean Pts.

Surface Site Densities of Uranium Oxides: UO2, U3O8

ABSTRACTThe estimation of the surface site density for UO2 (main component of the SF matrix) is an important aspect to take into account in the development of radiolytical models for the dissolution of the spent nuclear fuel (SF). Also, other oxides can be formed on the SF surface due to the effect of radiolytically-formed oxidizing species. Due to the lack or reliable data in literature we have studied the surface site densities of two uranium oxides: UO2 and U3O8.For this determination, the knowledge of both the reactive surface area and the surface charge of the solid are necessary. In this work the reactive surface area of the two solids was determined by the BET method while the surface charge was determined by potentiometric acid-base titrations of suspensions of the solids at different ionic strengths (0.001, 0.01, and 0.1 mol·dm−3) under N2 atmosphere.The amount of adsorbed protons was calculated by subtracting blank titration from thesolid suspension titration. The single-site nonelectrostatic model (NEM) was used to describe titration data. Uranium speciation in solution was included in the model as well.The surface area values obtained were 0.15 ± 0.01 m2·g−1 for UO2 and 0.77 ± 0.02 m2·g−1 for U3O8, while the surface acidic site densities were determined to be 165 ± 10 sites·nm−2 and 48 ± 3 sites·nm−2 for UO2 and U3O8, respectively.

Limitations of the potentiometric titration technique in determining the proton active site density of goethite surfaces

Density of proton active surface sites at mineral surfaces is a property of fundamental importance in equilibrium modeling of surface complexation reactions. In this article, methods for an experimental determination of these sites at the surface of α-FeOOH (goethite) are explored. It is shown that previously obtained saturation data of goethite with respect to protons do not yield a site density that can be considered as an intrinsic sorbent property: the results are below crystallographically expected values and values for different ionic media in terms of composition and concentration yield different numbers—for example, chloride would yield higher values than nitrate at the same concentration, and higher electrolyte concentration would favor higher apparent maxima. Although site saturation might be explained by electrostatic repulsion, which is more efficient at high electrolyte concentration or for certain ions, further independent experimental results show that no saturation occurs on goethite down to ph ≡ −log[H +] = 2.2 and possibly to ph = 1.0 in 0.6 M NaCl. For those very low pH values, the experimental charging curve was obtained by coulometric back titration (using the Gran plot) or titrations with tris (hydroxymethyl)-aminomethane of the supernatant of acidified goethite suspension. These experimental data are to our knowledge the first high quality data at such low pHs. However, small errors in the determination of proton concentrations (1%) are shown to strongly affect the shape of the charging curve for ph < 2. Furthermore, goethite dissolution (proton consumption and iron reduction in coulometric titrations) and liquid junction effects interfere at low ph, hampering the straightforward application of coulometric Gran titrations over the whole pH range. From these experiments, it can nonetheless be ascertained that a minimum of 2.5 protons/nm 2 can be adsorbed at the goethite surface from the point of zero charge (ph 9.4) to pH 0.9. Although these studies are restricted to goethite, those studies in which titrations with excess acid and base have been used for the determination of proton active site concentrations of sorbents should be reconsidered.

Adsorption of Water in Activated Carbons: Effects of Pore Blocking and Connectivity

We present a simulation study of the adsorption mechanism of water in a realistic carbon structure. Water molecules are modeled using a recently developed fixed-point charge water model optimized to the vapor−liquid coexistence properties.1 Reverse Monte Carlo techniques2 are used to generate a realistic porous carbon model composed of graphitic microcrystals consisting of rigid basal plates. Arrangements of the carbon plates are driven by a systematic refinement of simulated carbon−carbon radial distribution functions to match experimentally measured radial distribution functions. The adsorption of water in activated (having oxygenated surface groups) and nonactivated (graphitic) carbon is investigated using the grand canonical Monte Carlo simulation method. The adsorption behavior is found to be strongly dependent on the presence of activated sites. No appreciable adsorption occurs in the graphitic carbon until the pressure approaches the bulk gas saturation pressure. Effects of the surface site density, surface site position, and the magnitude of the surface site electrostatic charge on the adsorption behavior are investigated. Water adsorption is found to increase as the activated site density increases. The presence of water vapor in porous carbons is shown to dramatically affect the connectivity of the available pore space. Finally, cluster calculations are performed to investigate the growth mechanism of water clusters in activated carbons.

FT-IR Spectroscopic Studies of Thiophene Adsorption and Reactions on Mo2N/γ-Al2O3 Catalysts

The adsorption and reactions behaviors of thiophene on reduced passivated and nitrided Mo2N/gamma-Al2O3 catalysts have been studied by in situ FT-IR spectroscopy at temperatures from room temperature to 673 K. Thiophene mainly adsorbs on the cus Mo sites of the two catalysts surfaces, and its adsorption mode is determined to be eta(1)(S). Thiophene alone shows no reaction on the two catalysts even at 673 K, while in the presence of H-2, it becomes reactive initially at 573 and 373 K on reduced passivated and nitrided Mo catalysts, respectively, producing a surface a-bonded butene species that is confirmed by the adsorption of 1-butene, cis- and trans-2-butenes, and 1,3-butadiene on the catalysts. Adsorption of CO clearly shows that the catalyst surfaces become sulfided completely with a treatment of thiophene/H-2 at temperatures higher than 573 K. The surface sulfidation together with the sigma-bonded C-4 species may play a role in the deactivation of the nitride catalysts for the HDS of thiophene. The high reactivity of thiophene toward nitrided Mo catalyst in the initial stage of thiophene HDS is ascribed to the high density of surface sites and unique electronic property of nitrided Mo for the activation of both H-2 and thiophene.

Iron oxide surface-catalyzed oxidation of ferrous iron by monochloramine: implications of oxide type and carbonate on reactivity.

The maintenance of monochloramine residuals in drinking water distribution systems is one technique often used to minimize microbial outbreaks and thereby maintain the safety of the water. Reactions between oxidizable species and monochloramine can however lead to undesirable losses in the disinfectant residual. Previous work has illustrated that the Fe(II) present within distribution systems is one type of oxidizable species that can exert a monochloramine demand. This paper extends this prior work by examining the kinetics of the reactions between Fe(II) and monochloramine in the presence of a variety of iron oxide surfaces. The identity of the iron oxide plays a significant role in the rate of these reactions. Surface area-normalized initial rate coefficients (k(init)) obtained in the presence of each oxide at pH approximately 6.9 exhibit the following trend in catalytic activity: magnetite > goethite > hematite approximately = lepidocrocite > ferrihydrite. The differences in the activity of these oxides are hypothesized to result from variations in the amount of Fe(II) sorbed to each of the oxides and to dissimilarities in the surface site densities of the oxides. The implications of carbonate on Fe(II) sorption to iron oxides are also examined. Comparing Fe(II) sorption isotherms for goethite obtained under differential carbonate concentrations, it is apparent that as the carbonate concentration (C(T,CO3)) increased from 0 to 11.7 mM that the Fe(II) sorption edge (50% sorption) shifts from a pH of approximately 5.8 to a pH of 7.8. This shift is hypothesized to be the result of the formation of aqueous and surface carbonate-Fe(II) complexes and to competition between carbonate and Fe(II) for surface sites. The implications of these changes are then discussed in light of the variable oxide studies.

Surface Charge of Variable Porosity Al2O3(s) and SiO2(s) Adsorbents

The surface charge properties of two SiO2 and three Al2O3 mineral adsorbents with varying degrees of framework porosity were investigated using discontinuous titration and ion adsorption methodologies. Points of zero net charge (p.z.n.c) for porous and non-porous SiO2 were 7. Variation of surface charge density with aqueous proton concentration was nearly identical within a mineral type (i.e., SiO2 or Al2O3) irrespective of the degree of framework porosity, indicating that the densities of dissociable surface sites are equivalent, when normalized to surface area. The results suggest that the use of titration methods alone may be insufficient for thorough surface charge characterization, particularly at low and high pH. Proton titrations should be coupled with concurrent ion adsorption measurements to confirm surface charge development. Discontinuous proton titration and ion adsorption data, which were in agreement in the slightly acidic through slightly basic pH range, both indicated that p.z.n.c. was equal to the point of zero net proton charge (p.z.n.p.c.) for the variable charge minerals investigated.

Copper Sorption on a Straw Lignin: Experiments and EPR Characterization

Spectroscopic and physicochemical data, X-ray photoelectron spectroscopy (XPS), solid-state 13C cross-polarization magic-angle-spinning NMR, GC/MS, specific surface area, site density, and surface acidity constants have been recorded or determined for a ligno-cellulosic substrate (LS) extracted from straw. Its copper(II)-bound ability has also been studied. The LS solid that exhibits two types of binding sites, carboxylic and phenolic groups, has a great affinity for copper(II), with a maximum of adsorption at around 4 mg g−1 as early as the pH reaches 6. The structural characterization of the Cu(II) surface complexes has been investigated using EPR spectroscopy. The CuO4 chromophore of the inner-sphere surface complexes is a slightly distorted square. A surface oxidation occurred simultaneously, which leads to semiquinonic radicals. Furthermore, our empirical approach, which used a correlation between thermodynamics data and EPR parameters, has led to a surface complexation constant log β equal to 12.6. This indicates that copper(II) surface complexes are relatively stable.