Proton Adsorption Research Articles

Thermodynamic analysis of Bacillus subtilis endospore protonation using isothermal titration calorimetry

Bacterial proton and metal adsorption reactions have the capacity to affect metal speciation and transport in aqueous environments. We coupled potentiometric titration and isothermal titration calorimetry (ITC) analyses to study Bacillus subtilis spore-proton adsorption. We modeled the potentiometric data using a four and five-site non-electrostatic surface complexation model (NE-SCM). Heats of spore surface protonation from coupled ITC analyses were used to determine site specific enthalpies of protonation based on NE-SCMs. The five-site model resulted in a substantially better model fit for the heats of protonation but did not significantly improve the potentiometric titration model fit. The improvement observed in the five-site protonation heat model suggests the presence of a highly exothermic protonation reaction circa pH 7 that cannot be resolved in the less sensitive potentiometric data. From the log Ks and enthalpies we calculated corresponding site specific entropies. Log Ks and site concentrations describing spore surface protonation are statistically equivalent to B. subtilis cell surface protonation constants. Spore surface protonation enthalpies, however, are more exothermic relative to cell based adsorption suggesting a different bonding environment. The thermodynamic parameters defined in this study provide insight on molecular scale spore-surface protonation reactions. Coupled ITC and potentiometric titrations can reveal highly exothermic, and possibly endothermic, adsorption reactions that are overshadowed in potentiometric models alone. Spore-proton adsorption NE-SCMs derived in this study provide a framework for future metal adsorption studies.

Differentiate the pseudocapacitance and double-layer capacitance contributions for nitrogen-doped reduced graphene oxide in acidic and alkaline electrolytes

Nitrogen-doped reduced graphene oxide (N-RGO) and reduced graphene oxide (RGO) have been synthesized by microwave-assisted hydrothermal method to discern the actual contribution of nitrogen-containing functional groups on the specific capacitance (CS,T) in acidic and alkaline electrolytes. Material characterization reveals similar porosity, electrolyte-accessible surface area, element composition, and graphitic crystallinity between N-RGO and RGO except the difference in the nitrogen content. In 1 M H2SO4, additional pseudocapacitance provided by pyridinic-N and pyrrolic-N/pyridone-N is clearly observed at the potential negative to 0.6 V (vs. RHE) while this contribution in pseudocapacitance diminishes in 1 M KOH due to the lack of proton in the electrolyte for these basic functional groups to undergo redox reactions. The double-layer capacitance of N-RGO in 1 M H2SO4 in higher than that in 1 M KOH owing to the presence of N-containing functional groups which increase the electronic charge density of graphene and favor proton adsorption in the acidic electrolyte. The contribution of nitrogen-containing functional groups on CS,T in acidic media is more pronounced than that in the alkaline electrolyte. This finding is crucial for the future application of N-doped carbons in supercapacitors to achieve full utilization.

The effect of culture condition and ionic strength on proton adsorption at the surface of the extreme thermophile Acidianus manzaensis

Some previously published studies have used surface complexation theory to model proton adsorption in mesophilic bacteria. However, few experiments, to date, have investigated the effects of cultivation conditions and ionic strength on proton interactions among extreme thermophile archaea. In this study, we characterize proton adsorption of the extreme thermophile Acidianus manzaensis by performing acid–base titrations and electrophoretic mobility measurements in NaNO3 solution (0.001–0.1M), as well as Attenuated Total Reflection Fourier Transformed Infrared Spectroscopy (ATR-FTIR) with pH 2–10. Equilibrium thermodynamics (Donnan model) were applied to predict trends of ion adsorption at the archaea–water interface. We observed that the acidic properties of the A. manzaensis surface were significantly related to culture conditions and ionic strength. The buffering capacity of A. manzaensis cultivated with pyrite was much less than that of A. manzaensis cultured with sulfur. Furthermore, with increasing pH value, the buffering capacity increased and the surface charge became increasingly negative. To our knowledge, A. manzaensis has the highest buffering capacity of per unit wet biomass among all microbes studied. In conclusion, based on our model, the acid–base properties of the cell wall of A. manzaensis can be characterized by invoking three distinct types of cell wall functional groups: Carboxyl, phosphoryl, and amide groups, which were also verified by ATR-FTIR.

How to achieve maximum charge carrier loading on heteroatom-substituted graphene nanoribbon edges: density functional theory study

The practical number of charge carriers loaded is crucial to the evaluation of the capacity performance of carbon-based electrodes in service, and cannot be easily addressed experimentally. In this paper, we report a density functional theory study of charge carrier adsorption onto zigzag edge-shaped graphene nanoribbons (ZGNRs), both pristine and incorporating edge substitution with boron, nitrogen or oxygen atoms. All edge substitutions are found to be energetically favorable, especially in oxidized environments. The maximal loading of protons onto the substituted ZGNR edges obeys a rule of [8-n-1], where n is the number of valence electrons of the edge-site atom constituting the adsorption site. Hence, a maximum charge loading is achieved with boron substitution. This result correlates in a transparent manner with the electronic structure characteristics of the edge atom. The boron edge atom, characterized by the most empty p band, facilitates more than the other substitutional cases the accommodation of valence electrons transferred from the ribbon, induced by adsorption of protons. This result not only further confirms the possibility of enhancing charge storage performance of carbon-based electrochemical devices through chemical functionalization but also, more importantly, provides the physical rationale for further design strategies.

Adsorptive Behavior of Phosphate onto Activated Carbons Varying Surface Physicochemical Properties

This study demonstrated the modification of commercially available activated carbons (CS and R1) by de-ashing (DA), oxidation (Ox) and outgassing (OG) treatment to possess different surface area, pore volume, and the amount of surface functional groups, and these parameters were related to the efficiency of phosphate removal. The results showed that positively charged carbon surface due to the adsorption of protons to the basic sites resulted in the promotion of phosphate adsorption, that is, the extent of the surface area with the basic sites such as Cπ electrons and/or basic oxygen groups would enable to enhance the efficiency of phosphate adsorption.

Modeling and mechanism of the adsorption of proton onto natural bamboo sawdust

Natural bamboo sawdust with cellulose–lignin polymeric structure was used as a raw adsorbent to remove heavy metal and dyes in water. The analysis of surface properties showed that high proton affinity sites were mainly composed of phenolic and alcohol hydroxyl, while low proton affinity sites mainly consisted of carboxylic acid, silicon hydroxyl, aluminum hydroxyl and some low densities of sulfhydryl and phosphoryl groups. The results fitted by NEM surface complexation model showed: three-sites model fitted acid–base titration data better than two-sites and one-site model. Acid–base titration experiments showed the amount of acid and base consumed was not uniform between acid and alkali side. This was probably due to the swelling of cellulose in acid side, but the possibility that different reactions occurred in acid and alkali side cannot be completely excluded. Pznpc of sawdust was between pH 5.2 and 5.5.

A SURROGATE APPROACH TO STUDYING THE CHEMICAL REACTIVITY OF BURROW MUCOUS LININGS IN MARINE SEDIMENTS

Many infaunal marine invertebrates produce mucous excretions, composed primarily of the glycoprotein mucin, that play important roles in burrow stabilization. As with other biopolymers, the ionization of mucin provides highly reactive organic ligands that enable the sorption of metal cations from seawater. Owing to the difficulties in its isolation, however, the specific role of mucin in the adsorptive properties of animal secretions in marine environments is poorly understood. Here we apply a surface complexation approach to model proton and Cd adsorption behavior of partially purified Type III porcine gastric mucin (PGM), a commercially available analog to natural infaunal mucus. FTIR, proton and cadmium adsorption experiments indicate that Type III PGM mimics the acid-base and metal complexation behavior of natural mucous gels excreted by terebellid polychaete worms. At marine pH, nearly two-thirds of the total ligands in mucin-type glycoproteins are deprotonated and thus available to participate in metal cation adsorption reactions. Importantly, the concentration of available organic ligands in mucin exceeds (by up to 5 times) that of a variety of other metal-reactive organic compounds comprising the organic fraction of marine sediments. A substantial fraction of the dissolved organic matter in the bioturbated zone of marine sediments occurs in the form of mucin-associated glycoproteins; the availability of such organic materials may strongly influence the distribution of cations at the burrow margin.

Surface Hydroxyl Identity and Reactivity in Akaganéite

Hydroxyl groups on surfaces of well-defined akaganéite (β-FeOOH) particles were identified by Fourier transform infrared spectroscopy. These efforts, assisted by molecular dynamics simulations, enabled the extraction of spectral signatures for these groups of the dominant (100), (001), and (010) crystallographic planes. Band assignments were supported by spectral variations induced by proton and chloride adsorption as well as temperature-programmed desorption. Molecular dynamics simulations were used to determine patterns and free energies of formation of hydrogen bonds. Surface Fe–O distances as well as hydrogen-bond numbers were also used to predict proton affinities. All spectral component concentrations display highly comparable responses to proton loadings with those of other FeOOH minerals previously studied with our coupled experimental–theoretical approach. These similarities underpin common thermodynamic stabilities for hydroxyls of a given Fe nuclearity on different planes of different minerals.

Adsorption of Lead(II) Ions from Aqueous Solution on Low-Temperature Exfoliated Graphene Nanosheets

Graphene nanosheets (GNSs) that were obtained by vacuum-promoted low-temperature exfoliation were used to adsorb lead ions from an aqueous system. The pristine and thermally modified GNSs were characterized with scanning electron microscopy observation and X-ray photoelectron spectroscopy analysis. It was interestingly found that the adsorption against lead ions was enhanced by heat treatment, although the oxygen complexes of GNSs showed a significant decrease. In addition, lead ion uptake resulted in an increase in the pH value of the solution. It is supposed that the Lewis basicity of GNSs is improved by heat treatment under a high vacuum, in favor of simultaneous adsorption of lead ions and protons onto GNSs.

Cell Wall Reactivity of Acidophilic and Alkaliphilic Bacteria Determined by Potentiometric Titrations and Cd Adsorption Experiments

In this study, we used potentiometric titrations and Cd adsorption experiments to determine the binding capacities of two acidophilic (A. cryptum and A. acidophilum) and two alkaliphilic (B. pseudofirmus and B. circulans) bacterial species in order to determine if any consistent trends could be observed relating bacterial growth environment to proton and Cd binding properties and to compare those binding behaviors to those of neutrophilic bacteria. All of the bacterial species studied exhibited significant proton buffering over the pH range in this study, with the alkaliphiles exhibiting significantly higher acidity constants than the acidophiles as well as the neutrophilic bacterial consortia. The calculated average site concentrations for each of the bacteria in this study are within 2σ experimental error of each other, with the exception of A. cryptum, which has a significantly higher Site 2 concentration than the other species. Despite differing acidity constants between the acidophiles and alkaliphiles, all bacteria except A. cryptum exhibited remarkably similar Cd adsorption behavior to each other, and the observed extent of adsorption was also similar to that predicted from a generalized model derived using neutrophilic bacterial consortia. This study demonstrates that bacteria that grow under extreme conditions exhibit similar proton and metal adsorption behavior to that of previously studied neutrophilic species and that a single set of proton and metal binding constants can be used to model the behavior of bacterial adsorption under a wide range of environmental conditions.

Assessing the Importance of Organic Matrix Materials in Biofilm Chemical Reactivity: Insights from Proton and Cadmium Adsorption onto the Commercially Available Biopolymer Alginate

Biofilms coat the exterior of most water-exposed interfaces, from the surfaces of sediments and rocks to the interior walls of fluid transport systems and even medical and dental apparatus. Composed of a diverse assemblage of microbial species growing in a matrix of extracellular polymeric substances (EPS), biofilms are well-known for their ability to sorb metals and nucleate mineral phases. In this study, purified alginate, a major polysaccharide component of some algal and bacterial EPS, was studied to ascertain its chemical reactivity towards dissolved cadmium and protons, and thus better constrain its role in overall EPS reactivity. FTIR analysis and compositional constraints based on known molecular structure indicate that alginate’s geochemical behaviour is dominated by a single carboxyl functional group. Correspondingly, potentiometric titration data were best fit using a single functional group acidity constant (pKa) and site concentration of 3.98 ± 0.01 and 1.728 ± 0.02 mol/kg, respectively, which are in agreement with typical carboxyl acidity (pKa 3–6) and carboxyl functional group concentration based on alginate polymer composition. The logarithm of the Cd-carboxyl complexation constant (log K) was determined to be −0.52 ± 0.22, lower than carboxyl-Cd stability constants reported from independent studies of isolated microbes. Together, these results place important constraints on organic matrix contributions to overall biofilm reactivity.

CO2 electrochemical reduction via adsorbed halide anions

The electrochemical reduction of CO2 was studied utilizing halide ions as electrolytes, specifically, aqueous solutions of KCl, KBr, KI. Electrochemical experiments were carried out in a laboratory-made, divided H-type cell. The working electrode was a copper mesh, while the counter and reference electrodes were a Pt wire and an Ag/AgCl electrode, respectively. The results of our work suggest a reaction mechanism for the electrochemical reduction of CO2 where the presence of Cu-X as the catalytic layer facilitates the electron transfer from the electrode to CO2. Electron-transfer to CO2 may occur via the X−ad(Br−, Cl−, I−)-C bond, which is formed by the electron flow from the specifically adsorbed halide anion to the vacant orbital of CO2. The stronger the adsorption of the halide anion to the electrode, the more strongly CO2 is restrained, resulting in higher CO2 reduction current. Furthermore, it is suggested that specifically adsorbed halide anions could suppress the adsorption of protons; leading to a higher hydrogen overvoltage. These effects may synergistically mitigate the over potential necessary for CO2 reduction, and thus increase the rate of electrochemical CO2 reduction.

Enthalpies of proton adsorption onto Bacillus licheniformis at 25, 37, 50, and 75 °C

Understanding bacterial surface reactivity requires many different lines of investigation. Toward this end, we used isothermal titration calorimetry to measure heats of proton adsorption onto a Gram positive thermophile Bacillus licheniformis at 25, 37, 50, and 75 °C. Proton adsorption under all conditions exhibited exothermic heat production. Below pH 4.5, exothermic heats decreased as temperature increased above 37 °C; above pH 4.5, there was no significant difference in heats evolved at the temperatures investigated. Total proton uptake did not vary significantly with temperature. Site-specific enthalpies and entropies were calculated by applying a 4-site, non-electrostatic surface complexation model to the calorimetric data. Interpretation of site-specific enthalpies and entropies of proton adsorption for site L 1, L 2, and L 4 are consistent with previous interpretations of phosphoryl, carboxyl, and hydroxyl/amine site-identities, respectively, and with previous calorimetric measurements of proton adsorption onto mesophilic species. Enthalpies and entropies for surface site L 3 are not consistent with the commonly inferred phosphoryl site-identity and are more consistent with sulfhydryl functional groups. These results reveal intricacies of surface reactivity that are not detectable by other methods.

Characterization by electrochemical impedance spectroscopy of magnetite nanoparticles supported on carbon paste electrode

Magnetite nanoparticles were supported on carbon paste electrode and characterized by low scan rate voltammetry and electrochemical impedance spectroscopy (EIS) to obtain mechanistic information related to its oxidation and reduction in acid media. The voltammograms showed only one reduction and one oxidation peak for the supported magnetite, which were attributed to formation of ferrous ion and ferric oxide, respectively. Both peaks are fairly wide, indicating complex mechanisms. Using EIS, a mechanism showing up to three time constants, capacitive all of them, was evidenced, both in anodic and cathodic domain. These were attributed to charge transfer at the highest frequencies, adsorption of generated species at intermediate frequencies, and proton adsorption at low frequencies. Discussion about the nature of the adsorbed species and the concerned mechanism for each domain is developed.

CO 2 attraction by specifically adsorbed anions and subsequent accelerated electrochemical reduction

The electrochemical reduction of CO 2 was studied on a copper mesh electrode in aqueous solutions containing 3 M solutions of KCl, KBr and KI as the electrolytes in a two and three phase configurations. Electrochemical experiments were carried out in a laboratory-made, divided H-type cell. The working electrode was a copper mesh, while the counter and reference electrodes were Pt wire and Ag/AgCl electrode, respectively. Results of our work suggest a reaction mechanism for the electrochemical reduction of CO 2 in the two phase configuration where the presence of Cu-X as the catalytic layer facilitates the electron transfer from the electrode to CO 2. Electron-transfer to CO 2 may occur via the X ad −(Br −, Cl −, I −)–C bond, which is formed by the electron flow from the specifically adsorbed halide anion to the vacant orbital of CO 2. The stronger the adsorption of the halide anion to the electrode, the more strongly CO 2 is restrained, resulting in higher CO 2 reduction current. Furthermore, it is suggested that specifically adsorbed halide anions could suppress the adsorption of protons, leading to a higher hydrogen overvoltage. These effects may synergistically mitigate the overpotential necessary for CO 2 reduction, and thus increase the rate of electrochemical CO 2 reduction.

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.

Colloidal stability of bentonite clay considering surface charge properties as a function of pH and ionic strength

Surface charge properties of bentonite colloids were investigated to study their colloidal stability in a solution as a function of the pH and ionic strength. Potentiometric titrations and zeta potential measurements for the bentonite colloids depending upon the pH and ionic strength were performed to investigate the surface charge properties. It was observed from the potentiometric titrations that a zero net proton adsorption occurred at about pH 8.2 (pH PNZPC ∼ 8.2). Surface charges of the bentonite colloids mainly carrying structural negative charges revealed a very small dependency on the pH. The same behavior was also observed in the zeta potential measurements. The zeta potential measurements for the bentonite colloid showed that the bentonite colloids were stable at lower ionic strengths of 0.01 and 0.001 M NaClO 4 but unstable at a higher ionic strength of 0.1 M NaClO 4 within the whole pH range studied. Therefore, it can be concluded that bentonite colloids would be stable in most of the considered fresh groundwater conditions, i.e., pH from 6 to 10 and an ionic strength from 0.001 to 0.01 M, and then they can be mobilized along a flowing groundwater.

High- and low-affinity binding sites for Cd on the bacterial cell walls of Bacillus subtilis and Shewanella oneidensis

Bulk Cd adsorption isotherm experiments, thermodynamic equilibrium modeling, and Cd K edge EXAFS were used to constrain the mechanisms of proton and Cd adsorption to bacterial cells of the commonly occurring Gram-positive and Gram-negative bacteria, Bacillus subtilis and Shewanella oneidensis, respectively. Potentiometric titrations were used to characterize the functional group reactivity of the S. oneidensis cells, and we model the titration data using the same type of non-electrostatic surface complexation approach as was applied to titrations of B. subtilis suspensions by Fein et al. (2005). Similar to the results for B. subtilis, the S. oneidensis cells exhibit buffering behavior from approximately pH 3–9 that requires the presence of four distinct sites, with p K a values of 3.3 ± 0.2, 4.8 ± 0.2, 6.7 ± 0.4, and 9.4 ± 0.5, and site concentrations of 8.9(±2.6) × 10 −5, 1.3(±0.2) × 10 −4, 5.9(±3.3) × 10 −5, and 1.1(±0.6) × 10 −4 moles/g bacteria (wet mass), respectively. The bulk Cd isotherm adsorption data for both species, conducted at pH 5.9 as a function of Cd concentration at a fixed biomass concentration, were best modeled by reactions with a Cd:site stoichiometry of 1:1. EXAFS data were collected for both bacterial species as a function of Cd concentration at pH 5.9 and 10 g/L bacteria. The EXAFS results show that the same types of binding sites are responsible for Cd sorption to both bacterial species at all Cd loadings tested (1–200 ppm). Carboxyl sites are responsible for the binding at intermediate Cd loadings. Phosphoryl ligands are more important than carboxyl ligands for Cd binding at high Cd loadings. For the lowest Cd loadings studied here, a sulfhydryl site was found to dominate the bound Cd budgets for both species, in addition to the carboxyl and phosphoryl sites that dominate the higher loadings. The EXAFS results suggest that both Gram-positive and Gram-negative bacterial cell walls have a low concentration of very high-affinity sulfhydryl sites which become masked by the more abundant carboxyl and phosphoryl sites at higher metal:bacteria ratios. This study demonstrates that metal loading plays a vital role in determining the important metal-binding reactions that occur on bacterial cell walls, and that high affinity, low-density sites can be revealed by spectroscopy of biomass samples. Such sites may control the fate and transport of metals in realistic geologic settings, where metal concentrations are low.

Proton and cadmium adsorption by the archaeon Thermococcus zilligii: Generalising the contrast between thermophiles and mesophiles as sorbents

Adsorption by microorganisms can play a significant role in the fate and transport of metals in natural systems. Surface complexation models (SCMs) have been applied extensively to describe metal adsorption by mesophilic bacteria, and several recent studies have extended this framework to thermophilic bacteria. We conduct acid–base titrations and batch experiments to characterise proton and Cd adsorption onto the thermophilic archaeon Thermococcus zilligii. The experimental data and the derived SCMs indicate that the archaeon displays significantly lower overall sorption site density compared to previously studied thermophilic bacteria such Anoxybacillus flavithermus, Geobacillus stearothermophilus, G. thermocatenulatus, and Thermus thermophilus. The thermophilic bacteria and archaea display lower sorption site densities than the mesophilic microorganisms that have been studied to date, which points to a general pattern of total concentration of cell wall adsorption sites per unit biomass being inversely correlated to growth temperature.

Goethite surface reactivity: III. Unifying arsenate adsorption behavior through a variable crystal face – Site density model

We developed a model that describes quantitatively the arsenate adsorption behavior for any goethite preparation as a function of pH and ionic strength, by using one basic surface arsenate stoichiometry, with two affinity constants. The model combines a face distribution-crystallographic site density model for goethite with tenets of the Triple Layer and CD-MUSIC surface complexation models, and is self-consistent with its adsorption behavior towards protons, electrolytes, and other ions investigated previously. Five different systems of published arsenate adsorption data were used to calibrate the model spanning a wide range of chemical conditions, which included adsorption isotherms at different pH values, and adsorption pH-edges at different As(V) loadings, both at different ionic strengths and background electrolytes. Four additional goethite–arsenate systems reported with limited characterization and adsorption data were accurately described by the model developed. The adsorption reaction proposed is: FeOH + SOH + AsO 4 3 - + H + → FeOAsO 3 [ 2 - ] … SOH + H 2 O where SOH is an adjacent surface site to FeOH; with log K = 21.6 ± 0.7 when SOH is another FeOH, and log K = 18.75 ± 0.9, when SOH is Fe 2OH. An additional small contribution of a protonated complex was required to describe data at low pH and very high arsenate loadings. The model considered goethites above 80 m 2/g as ideally composed of 70% face (1 0 1) and 30% face (0 0 1), resulting in a site density for FeOH and for Fe 3OH of 3.125/nm 2 each. Below 80 m 2/g surface capacity increases progressively with decreasing area, which was modeled by considering a progressively increasing proportion of faces (0 1 0)/(1 0 1), because face (0 1 0) shows a much higher site density of FeOH groups. Computation of the specific proportion of faces, and thus of the site densities for the three types of crystallographic surface groups present in goethite, may be performed for each preparation either by experimental determination of site saturation by an index ion (e.g., chromate), or by achieving congruency of proton adsorption data with those of ideal goethites when plotted as percentage of proton-reactive ( FeOH + Fe 3OH) sites occupied. The surface arsenate complexes proposed additionally explained: (1) the higher affinity of goethite for As(V) than for Cr(VI) at high pH, and thus the gentle slope of the arsenate pH adsorption edges; and (2) the lower adsorption capacity for As(V) than for Cr(VI) at low pH on low-surface area goethites, through incomplete FeOH site occupancy of As(V). The model is very promising as a practical means of predicting the adsorption behavior of arsenate on any goethite preparation, and may extend to predictive capabilities for adsorption behavior of many other relevant oxyanions, as well as for explaining differences in ligand-promoted surface transformation processes on goethite as a function of particle size.