Surface Speciation Research Articles

Dirac's dream: Understanding metal sorption by geomedia using density functional theory

Density functional theory (DFT) can provide accurate, computationally-manageable, first-principles characterizations of mineral surface speciation at molecular scales. This is accomplished by solving the Kohn-Sham equation, a tractable method of applying quantum mechanics to multi-electron systems that was inspired by some visionary remarks of P. A. M. Dirac, one of the founders of quantum theory. Fifty years of advances in DFT and the development of massively-parallel computing platforms have paved the way for molecular-scale modeling of the mineral surface that is directly relevant to the study of natural nanoparticles. In fact, DFT has matured sufficiently to be applied to metal sorption by geomedia as a “computer experiment,” yielding quantitative molecular-scale information that fully complements insights gained from surface spectroscopy. This powerful approach is illustrated first through a series of structural simulations of chalcophanite group minerals and hetaerolite–hausmannite solid solutions, then through several recent studies by the authors and their collaborators focusing on transition and heavy metal sorption by layer-type Mn(IV) oxide and Fe(II) sulfide nanoparticles. Our results show how DFT simulations reveal the mechanistic basis of metal partitioning trends observed experimentally while creating useful models for resolving experimental surface speciation conundrums.

Antimony leaching and chemical species analyses in an industrial solid waste: Surface and bulk speciation using ToF-SIMS and XANES

The surface chemistry and bulk chemical speciation of solid industrial wastes containing 8wt-% antimony (Sb) were investigated using synchrotron X-ray Absorption Near Edge Structure (XANES) and Time-of-Flight Ion Secondary Mass Spectrometry (ToF-SIMS). Leaching experiments were conducted in order to better understand the behavior of Sb in waste streams and to inform regulatory management of antimony-containing wastes. The experiments also demonstrate how a combination of XANES and ToF-SIMS adds value to the field of waste investigations. Leaching treatments (acid and base) were performed at a synchrotron over 24h time periods. Surface analyses of the wastes before leaching showed the presence of Sb associated with S and O. Bulk analyses revealed Sb to be present, primarily, as trivalent sulfide species. Both acid and base leaching did not change the antimony speciation on the solid. Leaching transferred about 1% of the total Sb into solution where Sb was found to be present as Sb(V). XANES data showed similarities between leachate and FeSbO4. During base leaching, the Sb content in solution gradually increased over time, and potential desorption mechanisms are discussed.

Nanosilver as a disinfectant in dental unit waterlines: Assessment of the physicochemical transformations of the AgNPs

Dental unit water lines (DUWL) are susceptible to biofilm development and bacterial growth leading to water contamination, causing health and ecological effects. This study monitors the interactions between a commonly used nanosilver disinfectant (ASAP-AGX-32, an antimicrobial cleaner for dental units, 0.0032% Ag) and biofilm development in DUWL. To simulate the disinfection scenario, an in-house DUWL model was assembled and biofilm accumulation was allowed. Subsequent to biofilm development, the disinfection process was performed according to the manufacturer's instructions. The pristine nanosilver particles in the cleaner measured between 3 and 5 nm in diameter and were surrounded by a stabilizing polymer. However, the polymeric stabilizing agent diminished over the disinfection process, initiating partial AgNPs aggregation. Furthermore, surface speciation of the pristine AgNPs were identified as primarily AgO, and after the disinfection process, transformations to AgCl were observed. The physicochemical characteristics of AgNPs are known to govern their fate, transport and environmental implications. Hence, knowledge of the AgNPs characteristics after the disinfection process (usage scenario) is of significance. This study demonstrates the adsorption of AgNPs onto biofilm surfaces and, therefore, will assist in illustration of the toxicity mechanisms of AgNPs to bacteria and biofilms. This work can be an initial step in better understanding how AgNPs transform depending on the conditions they are exposed to during their lifetime. Until this date, most research has been focused on assessing the impacts of pristine (lab synthesized) nanomaterials on various systems. However, it is our belief that nanoparticles may undergo transformations during usage, which must be taken into consideration. Furthermore, this experiment is unique as it was conducted with a commonly used, commercially available nanosilver suspension leading to more realistic and applicable findings.

Catalytic hydrogenation of CO2 to methane over supported Pd, Rh and Ni catalysts

CO2 methanation over Rh/Al2O3, Rh/CeO2 and Ni/CeO2 at 350 °C highlighting the different surface speciation during reaction.

Surface Polymerization of Mo(VI) and W(VI) Anions on Hematite Revealed by in Situ Infrared Spectroscopy and DFT+U Theoretical Study

The study of the interactions between dissolved Mo(VI) or W(VI) species and the surfaces of metal (hydr)oxides is relevant for two main areas: the optimization of the preparation of catalytic supports and the understanding of the environmental fate of these elements. For the latter, iron (hydr)oxides are the most important sinks for pollutants, and recently, their surface reactivity was the focus of many research works. In this work, we develop a joint approach, using in situ infrared spectroscopy and DFT simulations, to characterize the Mo(VI) and W(VI) species adsorbed on hematite (α-Fe2O3). Surface sorbed polymers of tungstate and molybdate on hematite were identified for low pH and at high concentration of these elements, which is similar to the formation of polyoxometalates in solution phase. However, the surface speciation is different from the adsorption of polymolybdate or polytungstate already formed in solution and should be consistent with the growth of a surface polymer. For low concentrations...

Impacts of Surface Site Coordination on Arsenate Adsorption: Macroscopic Uptake and Binding Mechanisms on Aluminum Hydroxide Surfaces.

Aluminum hydroxides play important roles in regulating the fate and transport of contaminants and nutrients in soils and aquatic systems. Like many metal oxides, these minerals display surface functional groups in a series of coordination states, each of which may differ in its affinity for adsorbates. The distribution of functional group types varies among distinct surfaces of aluminum hydroxides, and we thus hypothesize that the adsorption behavior and mechanisms will show a dependence on particle morphology. To test this hypothesis, we investigate arsenate adsorption on two aluminum hydroxide polymorphs with distinct particle morphologies, gibbsite [γ-Al(OH)3] and bayerite [α-Al(OH)3], at pH 4 and 7. Synthetic gibbsite platelets expose large (001) basal surfaces predicted to be terminated by doubly coordinated functional groups (>Al2OH). In contrast, synthetic bayerite microrods display mainly edge surfaces (parallel to the c axis) containing abundant singly coordinated functional groups (>AlOH2). Macroscopic adsorption studies show that gibbsite adsorbs less arsenate per unit surface area than bayerite at both pH values and suggest that two surface complexes form on each material. Similar electrokinetic behavior is displayed at the same relative coverages of arsenate, suggesting that similar reactive surface groups (>AlOH2) control the surface charging on both particles. EXAFS spectroscopy shows that there is no variation in arsenate surface speciation on a given mineral with surface coverage or pH. Whereas bidentate binuclear inner-sphere species are the dominant complexes present, the EXAFS result suggest that outer-sphere species also occur on both minerals, with a greater abundance on gibbsite. This binding mode likely involves adsorption to >Al2OH sites, which have a slow ligand exchange rate that inhibits inner-sphere binding. These results demonstrate that adsorption mechanisms and capacity, even when normalized for specific surface area, vary with metal oxide particle morphology because of the distribution of distinct functional groups.

Catalytic effect of polyethylene glycol on sulfur oxidation in chalcopyrite bioleaching by Acidithiobacillus ferrooxidans

Polyethylene glycol (PEG) was used as a catalyst to enhance chalcopyrite bioleaching with Acidithiobacillus ferrooxidans in shake flasks. The effects of PEG on the sulfur oxidation of A. ferrooxidans and on the process of chalcopyrite bioleaching were investigated. The morphology, main components and sulfur speciation of bioleached chalcopyrite surfaces were evaluated combining with XRD, SEM and XPS. It was demonstrated that addition of PEG could significantly improve the bioleaching of chalcopyrite. Furthermore, the results indicated that elemental sulfur and jarosite were the main components of the passivation layer during the bioleaching of chalcopyrite. PEG could promote the sulfur oxidation by A. ferrooxidans due to an increase in bacterial attachment, and therefore accelerate the biooxidation of elemental sulfur generated in leaching process. It was found that PEG eliminated inhibitory elemental sulfur from the chalcopyrite surface which could be the main reason for enhanced the leaching efficiency.

Predictive model for Pb(II) adsorption on soil minerals (oxides and low-crystalline aluminum silicate) consistent with spectroscopic evidence

Mobility of Pb(II) in surface condition is governed by adsorption processes on soil minerals such as iron oxides and low-crystalline aluminum silicates. The adsorption effectiveness and the surface complex structures of Pb(II) vary sensitively with solution conditions such as pH, ionic strength, Pb(II) loading, and electrolyte anion type. This study was undertaken to construct a quantitative model for Pb(II) on soil minerals. It can predict the adsorption effectiveness and surface complex structures under any solution conditions using the extended triple layer model (ETLM).The Pb(II) adsorption data for goethite, hydrous ferric oxide (HFO), quartz, and low-crystalline aluminum silicate (LCAS) were analyzed with ETLM to retrieve the surface complexation reactions and these equilibrium constants. The adsorption data on goethite, HFO and quartz were referred from reports of earlier studies. Those data for LCAS were measured under a wide range of pH, ionic strength and Pb(II) loadings in NaNO3 and NaCl solutions. All adsorption data can be reasonably regressed using ETLM with the assumptions of inner sphere bidentate complexation and inner sphere monodentate ternary complexation with electrolyte anions, which are consistent with previously reported spectroscopic evidence.Predictions of surface speciation under widely various solution conditions using ETLM revealed that the inner sphere bidentate complex is the predominant species at neutral to high pH conditions. The inner sphere monodentate ternary complex becomes important at low pH, high surface Pb(II) coverage, and high electrolyte concentrations, of which the behavior is consistent with the spectroscopic observation.Comparisons of the obtained adsorption constants on goethite, HFO and quartz exhibited good linear relations between the reciprocals of dielectric constants of solids and adsorption constants. Those linear relations support predictions of the adsorption constants of all oxides based on Born solvation theory. The adsorption constants of LCAS are comparable to those of goethite. The predicted adsorption constants of soil minerals suggest that the LCAS is an important sorbent for Pb(II).

Molybdenum(VI) Adsorption onto Lepidocrocite (γ-FeOOH): In Situ Vibrational Spectroscopy and DFT+U Theoretical Study

In situ polarized ATR-FTIR spectroscopy and the DFT+U calculations were used to investigate the Mo(VI) ions surface speciation on lepidocrocite. Adsorption of molybdate ions is found to be tetrahedral monodentate binuclear (C3v) on the (010) face. DFT+U calculations were performed to help characterizing the geometries of the complexes adsorbed to assign the vibrational frequencies and get insight into the energies of adsorption. The (010) surface was chosen as a model of lepidocrocite surface. Calculations are consistent with the sorption of MoO42– ions to form a monodentate complex at basic pH. This complex is protonated when the surface coverage increases, and the pH decreases. At the full coverage, calculations show alternate monodentate protonated binuclear complexes and surface hydroxyl groups. Moreoever, the formation of hydrogen bonds with the surface hydroxyls and between adsorbed complexes plays an important role in the structure stability.

Antimonate and arsenate speciation on reactive soil minerals studied by differential pair distribution function analysis

The pentavalent oxyanions antimonate (Sb(V)) and arsenate (As(V)) are toxic metalloids in environmental systems. In this study, we compare their sorption and surface speciation on 2-line ferrihydrite (Fh), δ-MnO2 and Na-montmorillonite (Mont) at pH5.5. The Sb(V) and As(V) sorption affinity and the highest measured surface excess (mmol:g) show that mineral reactivity towards Sb(V) or As(V) sorption increases in the order of Mont<δ-MnO2<Fh. In addition, Sb(V) showed greater uptake relative to As(V) by both Fh and δ-MnO2. Using differential pair distribution function (d-PDF) analysis of high-energy X-ray scattering data, we found that the larger SbO6 octahedron (RSb–O=1.95Å) can bind in both single edge-(1E) and double corner-sharing (2C) geometries to Fh and δ-MnO2, whereas the smaller AsO4 tetrahedron (RAs–O=1.69Å) binds only in the 2C geometry. Thus, a greater number of surface sites support Sb(V) adsorption relative to As(V) adsorption on Fh and δ-MnO2. Conversely, Mont supports only 2C adsorption geometries, which is consistent with the similar uptake behaviors of Sb(V) and As(V) on this mineral. Finally, we indexed atomic pairs from adsorbed Sb(V) and As(V) at R>6Å in the d-PDFs. The analysis of the intermediate-ranged surface structure of our samples substantiated our interpretations of oxyanion adsorption geometries and provided information regarding changes in the underlying mineral surface upon ion adsorption. Our results show that the differences between Sb(V) and As(V) interactions with reactive soil minerals, including their sorption geometries, are linked to their distinct first-shell coordination environments.

Aqueous arsenic (As) and antimony (Sb) removal by potassium ferrate

Priority pollutants arsenic (As) and antimony (Sb) were removed from aqueous solution by potassium ferrate. As(III) and Sb(III) were oxidized, then separated from solution by the adsorption of Fe(VI) decay product (DP/Ferrate). XRD and FTIR results revealed the “amorphous” characteristics of DP/Ferrate. Adsorption studies in single-solute and bi-solute systems were conducted. The maximum adsorption capacity (Qmax) of single As was 162.02mg (gK2FeO4)−1 at pH0 6.5 and of single Sb was 129.93mg (gK2FeO4)−1 at pH0 4.0. Regarding bi-solute adsorption, a synergistic effect between As and Sb was demonstrated. Compared to single-solute adsorption, the adsorption rates of both elements were enhanced significantly. The adsorption isotherm of Sb in the presence of As was reproduced well by the Brunauer–Emmett–Teller (BET) model, indicating heterogeneous multilayer adsorption. The FTIR spectra featured the appearances of new peaks compared to the blank spectrum of DP/Ferrate, indicating that the surface speciation contained Fe–O–As and Fe–O–Sb bonds in the corresponding single-solute system; As–O–Sb binding was found in the bi-solute system. Furthermore, high-resolution XPS analysis pointed to the formation of arsenato antimonates of a polymer-like configuration, which was believed to account for the synergistic effect on simultaneous adsorption of As and Sb by DP/Ferrate. This paper demonstrated that using potassium ferrate is a viable water treatment technology to remediate arsenic and antimony combined pollution.

Effect of phosphate on heterogeneous Fenton oxidation of catechol by nano-Fe3O4: Inhibitor or stabilizer?

The effect of phosphate on adsorption and oxidation of catechol, 1,2-dihydroxybenzene, in a heterogeneous Fenton system was investigated. In situ attenuated total reflectance infrared spectroscopy (ATR-FTIR) was used to monitor the surface speciation at the nano-Fe3O4 catalyst surface. The presence of phosphate decreased the removal rate of catechol and the abatement of dissolved organic compounds, as well as the decomposition of H2O2. This effect of phosphate was mainly due to its strong reaction with surface sites on the iron oxide catalyst. At neutral and acid pH, phosphate could displace the adsorbed catechol from the surface of catalyst and also could compete for surface sites with H2O2. In situ IR spectra indicated the formation of iron phosphate precipitation at the catalyst surface. The iron phosphate surface species may affect the amount of iron atoms taking part in the catalytic decomposition of H2O2 and formation of hydroxyl radicals, and inhibit the catalytic ability of Fe3O4 catalyst. Therefore, phosphate ions worked as stabilizer and inhibitor in a heterogeneous Fenton reaction at the same time, in effect leading to an increase in oxidation efficiency in this study. However, before use of phosphate as pH buffer or H2O2 stabilizer in a heterogeneous Fenton system, the possible inhibitory effect of phosphate on the actual removal of organic pollutants should be fully considered.

Surface speciation of myo-inositol hexakisphosphate adsorbed on TiO2 nanoparticles and its impact on their colloidal stability in aqueous suspension: A comparative study with orthophosphate

Despite extensive research demonstrating the influence of organic matter and inorganic phosphate on the stability of TiO2 nanoparticles (NPs), far less research has assessed the impact of myo-inositol hexakisphosphate (IHP), a common organic phosphate widely present in the environment. In this study, the adsorption of IHP on TiO2 NPs and its impact on their colloidal stability were investigated using batch experiments, dynamic light scattering (DLS) techniques, in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) and solid-state 31P nuclear magnetic resonance (NMR) spectroscopy. Inorganic orthophosphate (Pi) adsorption was run for comparison. The ratio of the Pi/IHP adsorption density (1.528: 0.453) at pH5.0 suggested that IHP may complex on the TiO2 surface through three of its six phosphate groups. Zeta potential measurements, ATR-FTIR and NMR spectra indicated that IHP/Pi adsorbed onto TiO2 NPs by forming inner-sphere complexes and modified the surface charge of these NPs, which exerted a great impact on their colloidal stability. Interactions between NPs measured by sedimentation and aggregation size highly depended on the pH, surface phosphorus coverage, and surface phosphorus species. The impact of IHP on the aggregation and dispersion of TiO2 NPs was significantly larger than that of Pi, in agreement with the calculation from the DLVO theory. This study highlighted the impact of IHP relative to Pi on the colloidal stability of TiO2 NPs in phosphorus-enriched environments.

Study on the surface speciation of Fe-pillared montmorillonite and mechanism of its photocatalytic effect on degradation of ionic dye rhodamine-B

The surface protonation constants of Na-montmorillonite (abbreviated as Namt) and Fe-pillared montmorillonite (abbreviated as Femt) were obtained from experimental determination and then fitted with Protfit 2.1 software. The values of pKa1, pKa2 and Nt as well as the iron content of Femt are higher than those of Namt. The surface speciation of the sample presents lagging performance as the pH changes. The adsorption amount and catalytic decolorization rate of Femt for rhodamine-B are higher than that of Namt at the same pH. When the pH value increases, the adsorption amount and catalytic decolorization rate of Femt for rhodamine-B decline. The content of >FeOH2+ on the surface of Femt is positively correlated with the adsorption amount and catalytic decolorization rate for the dye. The mechanism of >FeOH2+ for the photocatalytic degradation of rhodamine-B may be interpreted as follows: after >FeOH2+ effectively captures hydrogen peroxide and photoelectrons in the valence band of >FeOH2+, hydroxyl radicals are produced. Hydroxyl radicals are also produced by electron holes on the valence band of >FeOH2+ absorbing OH⿿. The interrupted electrostatic field produced by >FeOH2+ on the surface of Femt can prevent the electron⿿hole recombination, which improves the catalytic efficiency of the Femt. Rhodamine-B is photocatalytically degraded by hydroxyl radicals.

The oxidation and surface speciation of indium and indium oxides exposed to atmospheric oxidants

Metallic indium and its oxides are useful in electronics applications, in transparent conducting electrodes, as well as in electrocatalytic applications. In order to understand more fully the speciation of the indium and oxygen composition of the indium surface exposed to atmospheric oxidants, XPS, HREELS, and TPD were used to study the indium surface exposed to water, oxygen, and carbon dioxide. Clean In and authentic samples of In2O3 and In(OH)3 were examined with XPS to provide standard spectra. Indium was exposed to O2 and H2O, and the ratio of O2− to OH− in the O1s XPS region was used to monitor oxidation and speciation of the surface. HREELS and TPD indicate that water dissociates on the indium surface even at low temperature, and that In2O3 forms at higher temperatures. Initially, OH− is the major species at the surface. Pure In2O3 is also OH− terminated following water exposure. Ambient pressure XPS studies of water exposure to these surfaces suggest that high water pressures tend to passivate the surface, inhibiting extensive oxide formation.

The role of low-energy electrons in focused electron beam induced deposition: four case studies of representative precursors.

Focused electron beam induced deposition (FEBID) is a single-step, direct-write nanofabrication technique capable of writing three-dimensional metal-containing nanoscale structures on surfaces using electron-induced reactions of organometallic precursors. Currently FEBID is, however, limited in resolution due to deposition outside the area of the primary electron beam and in metal purity due to incomplete precursor decomposition. Both limitations are likely in part caused by reactions of precursor molecules with low-energy (<100 eV) secondary electrons generated by interactions of the primary beam with the substrate. These low-energy electrons are abundant both inside and outside the area of the primary electron beam and are associated with reactions causing incomplete ligand dissociation from FEBID precursors. As it is not possible to directly study the effects of secondary electrons in situ in FEBID, other means must be used to elucidate their role. In this context, gas phase studies can obtain well-resolved information on low-energy electron-induced reactions with FEBID precursors by studying isolated molecules interacting with single electrons of well-defined energy. In contrast, ultra-high vacuum surface studies on adsorbed precursor molecules can provide information on surface speciation and identify species desorbing from a substrate during electron irradiation under conditions more representative of FEBID. Comparing gas phase and surface science studies allows for insight into the primary deposition mechanisms for individual precursors; ideally, this information can be used to design future FEBID precursors and optimize deposition conditions. In this review, we give a summary of different low-energy electron-induced fragmentation processes that can be initiated by the secondary electrons generated in FEBID, specifically, dissociative electron attachment, dissociative ionization, neutral dissociation, and dipolar dissociation, emphasizing the different nature and energy dependence of each process. We then explore the value of studying these processes through comparative gas phase and surface studies for four commonly-used FEBID precursors: MeCpPtMe3, Pt(PF3)4, Co(CO)3NO, and W(CO)6. Through these case studies, it is evident that this combination of studies can provide valuable insight into potential mechanisms governing deposit formation in FEBID. Although further experiments and new approaches are needed, these studies are an important stepping-stone toward better understanding the fundamental physics behind the deposition process and establishing design criteria for optimized FEBID precursors.

Time-Resolved Investigation of Cobalt Oxidation by Mn(III)-Rich δ-MnO2 Using Quick X-ray Absorption Spectroscopy.

Manganese oxides are important environmental oxidants that control the fate of many organic and inorganic species including cobalt. We applied ex situ quick X-ray absorption spectroscopy (QXAS) to determine the time evolution of Co(II) and Co(III) surface loadings and their respective average surface speciation in Mn(III)-rich δ-MnO2 samples at pH 6.5 and loadings of 0.01-0.20 mol Co mol(-1) Mn. In this Mn oxide, which contained few unoccupied vacancies but abundant Mn(III) at edge and interlayer sites, Co(II) sorption and oxidation started at the particle edges. We found no evidence for Co(II) oxidation by interlayer Mn(III) or Mn(III, IV) adjacent to vacancy sites at <10 min. After 10 min, basal surface sites were implicated due to slow Co oxidation by interlayer Mn(III) and reactive sites formed upon removal of interlayer Mn(III), such that 50-60% of the sorbed Co was incorporated into the MnO2 sheets or adsorbed at vacancy sites by 12 h. Our findings indicate that the redox reactivity of surface sites depends on Mn valence and crystallographic location, with Mn(III) at the edges being the most effective oxidant at short reaction times and Mn(III,IV) in the MnO2 sheet contributing at longer reaction times.

Rate equations for sodium catalyzed quartz dissolution

Quartz dissolution rate data were fit to an equation that predicts the dissolution flux (J, mol/m2sec) as a function of temperature (T, K), sodium concentration (mNa+, molal), and hydrogen ion activity (aH+). J=0.64610-748002.303R1T+6.6410-880002.303R1TmNa+0.331aH+0.441The same data fit equally well to an equation that expresses the rate as a function of temperature, sodium concentration, and hydroxide ion activity (aOH-). J=0.64610-748002.303R1T+1120010-716002.303R1TmNa+0.326aOH-0.441These equations are more convenient to use than those given by Bickmore et al. (2008) because rates can be predicted without the implementation of a surface speciation model. They predict that at 25°C quartz dissolves more than 200 times faster in seawater than in pure water.These two equations fit the data just as well as five other equations from Bickmore et al. (2008) that are based on surface species concentrations. All of these rate equations contain information about the reaction mechanism(s) for quartz dissolution but that information is ambiguous because the independent variables used to develop the equations are correlated. This means that rate equations alone cannot be used to infer the dissolution mechanism. Existing surface complexation, surface charge, terrace-ledge-kink, and Lewis acid-base models must be modified and amalgamated in order to develop a reliable model of the reaction mechanism(s).

Speciation of adsorbates on surface of solids by infrared spectroscopy and chemometrics

Speciation, i.e. identification and quantification, of surface species on heterogeneous surfaces by infrared spectroscopy is important in many fields but remains a challenging task when facing strongly overlapped spectra of multiple adspecies. Here, we propose a new methodology, combining state of the art instrumental developments for quantitative infrared spectroscopy of adspecies and chemometrics tools, mainly a novel data processing algorithm, called SORB-MCR (SOft modeling by Recursive Based-Multivariate Curve Resolution) and multivariate calibration. After formal transposition of the general linear mixture model to adsorption spectral data, the main issues, i.e. validity of Beer–Lambert law and rank deficiency problems, are theoretically discussed. Then, the methodology is exposed through application to two case studies, each of them characterized by a specific type of rank deficiency: (i) speciation of physisorbed water species over a hydrated silica surface, and (ii) speciation (chemisorption and physisorption) of a silane probe molecule over a dehydrated silica surface. In both cases, we demonstrate the relevance of this approach which leads to a thorough surface speciation based on comprehensive and fully interpretable multivariate quantitative models. Limitations and drawbacks of the methodology are also underlined.

Insights into tetracycline adsorption onto kaolinite and montmorillonite: experiments and modeling.

Adsorption of tetracycline (TC) on kaolinite and montmorillonite was investigated using batch adsorption experiments with different pH, ionic strength, and surface coverage. As a result, pH and ionic strength-dependent adsorption of TC was observed for the two clay minerals. The adsorption of TC decreased with the increase of pH and ionic strength, and high initial TC concentration had high adsorption. In addition, a triple-layer model was used to predict the adsorption and surface speciation of TC on the two minerals. As a result, four complex species on kaolinite (≡X(-)∙H3TC(+), ≡X(-)∙H2TC(±), ≡SOH(0)∙H2TC(±), and ≡SOH(0)∙HTC(-)) and three species on montmorillonite (≡X(-)∙H3TC(+), ≡X(-)∙H2TC(±), and ≡SOH(0)∙HTC(-)) were structurally constrained by spectroscopy, and these species were also successfully fitted to the adsorption edges of TC. Three functional groups of TC were involved in these adsorption reactions, including the positively charged dimethylamino group, the C=O amide I group, and the C=O group at the C ring. Combining adsorption experiments and model in this study, the adsorption of TC on kaolinite and montmorillonite was mainly attributed to cation exchange on the surface sites (≡X(-)) compared to surface complexation on the edge sites (≡SOH) at natural soil pH condition. Moreover, the surface adsorption species, the corresponding adsorption modes, and the binding constants for the surface reactions were also estimated.