Charge Distribution Multisite Complexation Model Research Articles

Arsenate and phosphate adsorption onto Mg-Fe layered double hydroxides: The charge-distribution multisite complexation (CD-MUSIC) modeling as a tool to predict competitive oxyanion behavior

The adsorption of arsenate and phosphate onto natural or synthetic layered double hydroxides (LDHs) has emerged as a promising solution to minimize the environmental effects of these compounds in soils and waters. In the present work, the adsorption of these oxyanions onto Mg-Fe LDHs in both single-ion and competitive systems was studied through wet chemistry experiments, solid-state analyses, and advanced surface complexation modeling (SCM). The Mg-Fe LDH showed a good efficiency to remove both As and P, being the pH of the system a major driver for the adsorption levels observed. Adsorption results for competitive systems showed a decrease for both oxyanions compared to the single-ion adsorption systems. The adsorption behavior is mechanistically described using two types of SCM, namely the diffusion layer model (DLM) and the charge-distribution multisite complexation model (CD-MUSIC). The later model captures better the influence of pH and ionic strength on the adsorption of arsenate and phosphate onto LDH in both single-ion and multicomponent systems. The best description of the experimental results was obtained when combining two types of surface complexes, namely monodentate inner-sphere complex at lower pH values and monodentate outer-sphere complex at higher pH values. This model approach provides more detailed information about the surface properties of LDHs, which may be beneficial for future studies dealing with LDHs as reactive phases influencing metal(loid) mobility in water or soil systems.

Antimony immobilization mechanism on schwertmannite: Insights from the microstructure of schwertmannite

Schwertmannite affects the mobility of antimony (Sb) in acid mine drainage, yet the Sb sorption mechanism remains elusive. In this study, the molecular-level mechanism and the driving force of Sb(V) immobilized on schwertmannite were investigated by a combination of wet chemistry, extended X-ray absorption fine structure (EXAFS) analysis, density functional theory (DFT) calculations, and a surface complexation model (SCM). The observed sorption capacity for Sb(V) was as high as 235.63 mg/g. Such remarkable sorption capacity is primarily achieved by the anion exchange reaction and surface complexation reaction of Sb(V) with sulfate and surface/tunnel inner-surface hydroxyl groups, respectively, and the contribution of the anion exchange reaction to Sb(V) immobilization decreases markedly with increasing Sb(V) loading or pH. The loading of Sb(V) appears to be the key parameter controlling its immobilization mechanism on schwertmannite. Low-loaded Sb(V) primarily generates bidentate-mononuclear and bidentate-binuclear surface complexes (adsorption products) and is concurrently partly incorporated into the Fe-O framework of schwertmannite to form a hexadentate-hexanuclear complex (incorporation product). With increasing Sb(V) loading, Sb is primarily adsorbed on the surface and in the tunnel of schwertmannite with the formation of bidentate-mononuclear (dominant) and bidentate-binuclear complexes without incorporation. The inconsistency of the immobilization mechanism is controlled by the amount of released sulfate induced by Sb(V). The substantial system energy reduction caused by the formation of Sb(V) adsorption and incorporation products on the surface and in the tunnel of schwertmannite is the driving force for the stable immobilization of Sb(V) by schwertmannite, and the energy reduction caused by tunnel complexes is far more obvious than that caused by surface complexes. A charge distribution multisite complexation model was established based on bidentate-mononuclear and bidentate-binuclear surface complexes with fitted Log K values of 21.99 ± 0.18 and 12.84 ± 0.92, respectively, which consistently predicted Sb(V) sorption across pH 3.0–9.0 for all involved Sb(V) loadings. In addition, Sb(V) has a strong stabilizing effect on the schwertmannite structure and broadens its pH stability window (pH 3.0–9.0). This study facilitates elucidation of the fate and geochemical cycling of Sb in the mining environment and helps to promote the application of the microstructure of schwertmannite in other studies of microscopic interface chemistry.

Eu(III) and Am(III) adsorption on aluminum (hydr)oxide minerals: surface complexation modeling

Americium is a highly radioactive actinide element found in used nuclear fuel. Its adsorption on aluminum (hydr)oxide minerals is important to study for at least two reasons: (i) aluminum (hydr)oxide minerals are ubiquitous in the subsurface environment and (ii) bentonite clays, which are proposed engineered barriers for the geologic disposal of used nuclear fuel, have the same ≡AlOH sites as aluminum (hydr)oxide minerals. Surface complexation modeling is widely used to interpret the adsorption behavior of heavy metals on mineral surfaces. While americium sorption is understudied, multiple adsorption studies for europium, a chemical analog, are available. In this study we compiled data describing Eu(III) adsorption on three aluminum (hydr)oxide minerals—corundum (α-Al2O3), γ-alumina (γ-Al2O3) and gibbsite (γ-Al(OH)3)—and developed surface complexation models for Eu(III) adsorption on these minerals by employing diffuse double layer (DDL) and charge distribution multisite complexation (CD-MUSIC) electrostatic frameworks. We also developed surface complexation models for Am(III) adsorption on corundum (α-Al2O3) and γ-alumina (γ-Al2O3) by employing a limited number of Am(III) adsorption data sourced from literature. For corundum and γ-alumina, two different adsorbed Eu(III) species, one each for strong and weak sites, were found to be important regardless of which electrostatic framework was used. The formation constant of the weak site species was almost 10,000 times weaker than the formation constant for the corresponding strong site species. For gibbsite, two different adsorbed Eu(III) species formed on the single available site type and were important for the DDL model, whereas the best-fit CD-MUSIC model for Eu(III)-gibbsite system required only one Eu(III) surface species. The Am(III)-corundum model based on the CD-MUSIC framework had the same set of surface species as the Eu(III)-corundum model. However, the log K values of the surface reactions were different. The best-fit Am(III)-corundum model based on the DDL framework had only one site type. Both the CD-MUSIC and the DDL model developed for Am(III)-γ-alumina system only comprised of one site type and the formation constant of the corresponding surface species was ~ 500 times stronger and ~ 700 times weaker than the corresponding Eu(III) species on the weak and the strong sites, respectively. The CD-MUSIC model for corundum and both the DDL and the CD-MUSIC models for γ-alumina predicted the Am(III) adsorption data very well, whereas the DDL model for corundum overpredicted the Am(III) adsorption data. The root mean square of errors of the DDL and CD-MUSIC models developed in this study were smaller than those of two previously-published models describing Am(III)-γ-alumina system, indicating the better predictive capacity of our models. Overall, our results suggest that using Eu(III) as an analog for Am(III) is practical approach for predicting Am(III) adsorption onto well-characterized minerals.Graphical

Prediction of alkaline earth metal ion adsorption on goethite for various background electrolytes with the CD-MUSIC model

As water scarcity drives the use of more saline water sources, contaminant fate and transport models must capture the impact of high concentrations of alkaline earth metal ions (AEMs) and background electrolytes in these more complex waters. By utilizing macroscopic adsorption data from various electrolyte systems, a Charge Distribution – Multisite Complexation (CD-MUSIC) model, capable of incorporating electrolyte adsorption, was able to accurately simulate the adsorption behavior of alkaline earth metal ions onto goethite. The modeling effort was guided by previous spectroscopic and surface complexation modeling of alkaline earth metal adsorption and built on previous CD-MUSIC modeling that accounted for changes in crystal face contributions to the surface site density as a function of specific surface area. The model was constrained to consider only two dominant surface complex species for each metal ion adsorption reaction. These two species were selected from 44 possible species through objective curve fitting of single-solute macroscopic adsorption data. While most of the alkaline earth metal surface complexes formed outer-sphere complexes at the goethite surface, an inner-sphere species was utilized for Mg2+. With the surface complex species and equilibrium constants obtained from this study, the calibrated model successfully predicted alkaline earth metal ion adsorption over a wide range of solution and surface conditions; the model predictions encompassed a wide range of pH (5–11), solute/solid ratio (1.37 × 10−5– 8.33 × 10−4 mol-solute/g-solid), ionic strengths (0.01 M – 0.7 M), and background electrolytes (Na+, Cs+, Rb+, Cl−, and NO3−) using the same crystal face contribution methodology for site density, capacitance values, and surface acidity constants adopted for proton and cadmium adsorption in previous work (Han and Katz, 2019). Model simulations for a range of background water chemistries demonstrated the potential for Mg2+ to reduce Cd2+ adsorption to goethite in model seawater and oil- and gas-produced waters.

Influencing factors and environmental effects of interactions between goethite and organic matter: A critical review

This paper reviews progresses in the interactions between goethite and organic matter (OM) and their environmental effects in recent decades. The interactions mainly include the effect of organic matter on the surface properties and the crystallization of goethite, molecular changes of OM caused by goethite, and their interaction mechanisms, which can be depicted by the commonly used Langmuir model, the charge distribution multi-site complexation model (CD-MUSIC model), ligand charge distribution model (LCD model), and natural organic matter charge distribution model (NOM-CD model). The influencing factors of the interactions are summarized with emphasis on the external, including pH, ionic strength, carbon dioxide (CO2), and the internal, including the structure of OM and iron species. The goethite—OM complexes caused by the interactions will affect migration and transformation of conventional heavy metals and emerging antibiotics. The complexes, as the carrier of carbon and iron, are also the critical parts of the carbon and iron cycles, which are associated with climate change. This review provides a basis for future mechanism studies of formation, transformation, and effects of goethite—OM complexes (particulate OM or carbon-contained minerals) in different environmental systems at a molecular level.

Surface complexation modeling of arsenic mobilization from goethite: Interpretation of an in-situ experiment

Abstract Sorption has been recognized as a predominant process for the mobility and transport of arsenic (As) in groundwater. However, the model-based description of the chemical and physical mechanisms controlling As interaction with mineral surfaces under natural hydrochemical conditions remains a formidable challenge. In this study we assess and compare the ability of existing surface complexation models (SCMs), the diffuse double layer model (DDL) and the charge distribution multisite complexation model (CD-MUSIC), to simulate As desorption from goethite in groundwater. We consider the outcomes of an in-situ experiment recently performed in an arsenic-contaminated aquifer of Northern China where As-loaded goethite-coated sand was deployed in 7 monitoring wells. Determination and measurements of As-surface species were carried out over a time period of 80 days in all monitoring wells. Simultaneous calibration of the models with the measurements in the 7 wells was performed to obtain single sets of surface complexation parameters for the DDL and the CD-MUSIC models, respectively. Although the general dynamic pattern of the As release at the site is captured by both models, the approach based on the CD-MUSIC agrees best with the field experimental data without modifications of the surface complexation database compiled from previous studies on goethite. Conversely, calibration of the DDL affinity constants led to substantial improvement in the agreement between model simulations and the considered field dataset. The model outcomes and the exploration of the sensitivity of the goethite surface composition to changes in hydrochemical conditions provide insights into the mechanisms controlling arsenic sorption and their different description in the DDL and the CD-MUSIC models. Both SCMs indicate that PO43− acts as the main competitor for As(V) sorption sites, and that Fe(II) does not have a significant effect on the As(V) release despite its strong affinity for the goethite surface. Neither SCM suggests direct binding of As(III) to goethite. However, the CD-MUSIC model predicts significant formation of a goethite-Fe(II)-As(III) complex under the mildly alkaline conditions of the groundwater at the field site and that such complex is insensitive to phosphate competition. The CD-MUSIC implementation also allows capturing the non-linear charge effects of the major ions, including C a 2+ and M g 2+ , which appear to have important implication in the mechanisms of As sorption at the field site.

Adsorption of Quinolone Antibiotics to Goethite under Seawater Conditions: Application of a Surface Complexation Model.

The assessment of antibiotics mobility under seawater conditions has been rarely studied, as an accurate description of such multicomponent systems is quite challenging. In this study, the adsorption of a widely used quinolone antibiotic in aquaculture, Oxolinic acid (OA), to a synthetic goethite (α-FeOOH) was examined in the presence of major (e.g., Mg2+, SO42-) and trace (e.g., Cu2+) ions naturally occurring in seawater. The OA adsorption can be successfully predicted using a charge distribution multisite complexation model (CD-MUSIC) coupled with the three plane model (TPM). This modeling approach allowed a quantification of the competitive and synergetic effects of different ions in seawater over a large range of environmentally relevant conditions. In addition, the transport of OA in flow-through columns can be well predicted through coupling hydrodynamic parameters and surface complexation constants obtained under seawater conditions. These results may have strong implications for assessment and prediction of the fate of quinolones in sediment-seawater interface systems.

Competitive adsorption of arsenic and fluoride on {2 0 1} TiO2

Elevated arsenic (As) and fluoride (F) in natural water present an urgent environmental concern. The demand for their effective removal underscores the fundamental understanding of their solid-liquid interface chemistry. Herein, the efficiency of {2 0 1} TiO2 in As(III/V) and F was explored using macroscopic and molecular-level techniques. Their adsorption isotherms followed the Langmuir equation, and the maximum adsorption capacity was 50.5, 29.3, and 5.0 mg/g for As(III), As(V) and F, respectively. Their adsorption kinetics of As(III), As(V) and F conformed to the pseudo-second-order model. The XPS and in situ ATR-FTIR results identified that the active adsorption sites on {2 0 1} TiO2 included surface hydroxyl groups, but not oxygen vacancies. As(III/V) and F form bidentate binuclear and monodentate mononuclear structures, respectively, regardless of exposed facets. Integrated with the molecular-level mechanism, the charge distribution multisite complexation model well predicted the pH edge behaviors in mono- and co-component systems. The shift of pHPZC of {2 0 1} TiO2 in competitive adsorption systems signified the formation of inner-sphere complexes. The results of this study shed new lights on the adsorption of coexisting ions using high-index faceted TiO2.

Capturing the variable reactivity of goethites in surface complexation modeling by correlating model parameters with specific surface area

The development of predictive tools capable of simulating adsorption behavior on goethite (α-FeOOH) surfaces has evolved over the past several decades. Recent research has focused on capturing the variation of surface reactivity and adsorption behavior for different preparations of synthetic goethites. In this research, an approach to estimate modeling parameters based on the specific surface area (SSA) of goethites was established and combined with a Charge Distribution – Multi-site Complexation (CD-MUSIC) surface complexation model. It was assumed that differences in crystal face contributions (CFC), inner-Helmholtz capacitances (C1), and protonation constants of surface oxygens (pKa’s) among different preparations of goethite contribute to the variation of surface reactivity. Thus, correlations of these modeling parameters with the SSA of a range of goethites were identified by comprehensively combining microscopic, macroscopic, and modeling data reported in literature along with adsorption data collected from macroscopic experiments conducted as part of this study. A CD-MUSIC model shown to be capable of accurately predicting adsorption for several metal ions and oxyanion selenite on a 62.9 m2/g SSA goethite served as the basis for the development of the adsorption modeling approach described in this paper. The predictive capability of the newly developed modeling approach was verified by comparing predictions of protons, SeO32−, and Cd2+ adsorption with experimental data from goethites with SSAs ranging between 21.3 and 105 m2/g; in total, 40 adsorption data sets from 13 different goethites were used for model verification. The results from this study indicate that the variation of surface reactivity of goethites can be successfully simulated by changing CFC, C1, and pK2,singly (i.e. second protonation constant of singly-coordinated oxygens on the goethite surface) based on the SSA, which implies that self-consistent adsorption parameters can be applied to predict adsorption behavior over a range of goethite preparations.

Adsorption of Antimonate, Sulfate, and Phosphate by Goethite: Reversibility and Competitive Effects

Core Ideas Sb(V) adsorption by goethite decreases with increasing pH and is unaffected by ionic strength. Sb(V) adsorption is hysteretic. Adsorbed Sb(V) generates a downward shift in the goethite isoelectric point. The adsorption of Sb(V) decreases in the presence of phosphate, but is unaffected by sulfate. Inner sphere mono‐ and bidentate Sb(V) complexation by goethite is predicted. Goethite is a sink for antimonate Sb(OH)6– [Sb(V)] in soil and sediments. The pH‐ and ionic strength (I)‐dependent mass distribution of Sb(V) between adsorbed and solution phases at equilibrium may be used to indirectly infer an adsorption mechanism (inner vs. outer sphere surface complexation) and to develop predictive surface complexation models. The objectives of this study were to characterize the adsorption of Sb(V) by goethite as a function of pH, I, and in the absence or presence of competing ligands, SO4 or PO4. Ligand adsorption was at a maximum in strongly acidic suspensions and decreased with increasing pH. Adsorption envelopes of Sb(V) where not influenced by I, but increasing I decreased SO4 and increased PO4 retention. In strongly acidic suspensions, SO4 and PO4 retention was not influenced by I. Ligand adsorption decreased the isoelectric point (IEP) of goethite from 8.5 to 6.3 [Sb(V)] and 4 (PO4). The adsorption of SO4 did not affect the IEP, although SO4 reduced the zeta potential of goethite when pH < IEP. Antimonate and PO4 adsorption was hysteretic; SO4 adsorption was reversible. Sulfate did not impact Sb(V) retention; adsorbed Sb(V) generated greater negative surface charge, decreasing SO4 adsorption. Both Sb(V) and PO4 competed for adsorption sites, resulted in reduced retention of both in competitive systems. The triple layer formulation of the charge distribution multisite complexation model, coupled with inner sphere surface complexation, was used to describe the adsorption envelopes for Sb(V), SO4, and PO4, and to predict adsorption in competitive systems.

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

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

Modeling of levofloxacin adsorption to goethite and the competition with phosphate

Interaction between various compounds in natural systems may influence the adsorption of these species and their environmental fate. In this work, we studied the interactions between a widely used antibiotic levofloxacin (LEV) and phosphate at the surface of goethite (α-FeOOH), which was important to better understand the competitive adsorption of antibiotics and oxyanions in natural systems. The presence of phosphate decreased LEV adsorption to goethite significantly over the whole pH range. The other way around, LEV had a little influence on phosphate adsorption. Eight types of LEV–goethite complexes were proposed and modeled in our study. Electrostatic competition was the main reason for the competition of binary components (LEV and phosphate) to goethite surface. Adsorption of single component (LEV or phosphate) to goethite was well predicted using the CD-MUSIC (Charge Distribution Multi-Site Complexation) model. In competition experiments, phosphate adsorption was still predicted well, but LEV adsorption was overestimated in model calculations. This is because less negative charge of LEV is located at outer electrostatic plane in our study, which decreases their electrostatic competition to goethite surface.

Adsorption of zwitterionic fluoroquinolone antibacterials to goethite: A charge distribution-multisite complexation model

Fluoroquinolone (FQ) antibacterials are aquatic contaminants of emerging concern (CEC), and adsorption to mineral surfaces is expected to play an important role in the fate, transport, and treatment of FQs. This study characterizes and models the adsorption of a zwitterionic FQ, ofloxacin (OFX), to goethite (α-FeOOH) over a wide range of pH (3–11), OFX concentration (20–500μM), and electrolyte compositions (0.001–0.1M NaCl and NaClO4). Comparing OFX adsorption to structural analogues demonstrates that the carboxylate group is essential for binding to goethite. ATR-FTIR measurements indicate that FQs complex to goethite surfaces through carboxylate and carbonyl oxygen atoms, and that ClO4− co-adsorbs with OFX. Adsorption of the zwitterionic OFX increases with increasing ionic strength and is enhanced in NaClO4 relative to NaCl electrolyte, whereas adsorption of a non-zwitterionic analogue is insensitive to ionic strength. A CD-MUSIC (charge distribution-multisite complexation) model, incorporating multiple modes of surface complexation constrained by spectroscopic measurements and the crystallographic distribution of goethite surface sites, yields accurate predictions over wide-ranging solution conditions. According to the model, OFX adsorbs predominantly by inner-sphere complexation on terminal surfaces of the rod-shaped goethite crystals in NaCl electrolyte, and OFX-ClO4− ion pairing in NaClO4 induces formation of additional inner- and outer-sphere surface complexes on multiple crystal faces of goethite.

Removal of arsenate with hydrous ferric oxide coprecipitation: Effect of humic acid

Insights from the adverse effect of humic acid (HA) on arsenate removal with hydrous ferric oxide (HFO) coprecipitation can further our understanding of the fate of As(V) in water treatment process. The motivation of our study is to explore the competitive adsorption mechanisms of humic acid and As(V) on HFO on the molecular scale. Multiple complementary techniques were used including macroscopic adsorption experiments, surface enhanced Raman scattering (SERS), extended X-ray absorption fine structure (EXAFS) spectroscopy, flow-cell attenuated total reflectance Fourier transform infrared (ATR-FTIR) measurement, and charge distribution multisite complexation (CD-MUSIC) modeling. The As(V) removal efficiency was reduced from over 95% to about 10% with the increasing HA concentration to 25 times of As(V) mass concentration. The SERS analysis excluded the HA-As(V) complex formation. The EXAFS results indicate that As(V) formed bidentate binuclear surface complexes in the presence of HA as evidenced by an As-Fe distance of 3.26–3.31 Å. The in situ ATR-FTIR measurements show that As(V) replaces surface hydroxyl groups and forms inner-sphere complex. High concentrations of HA may physically block the surface sites and inhibit the As(V) access. The adsorption of As(V) and HA decreased the point of zero charge of HFO from 7.8 to 5.8 and 6.3, respectively. The CD-MUSIC model described the zeta potential curves and adsorption edges of As(V) and HA reasonably well.

Surface complexation modeling of Hg(II) adsorption at the goethite/water interface using the Charge Distribution Multi-Site Complexation (CD-MUSIC) model

HypothesisPrevious attempts to describe Hg(II) adsorption onto mineral surfaces using surface complexation models (SCMs) have proven unsuccessful and/or require the use of hypothetical surface species. Given that metal ion adsorption at the mineral–water interface is greatly influenced by mineral surface heterogeneity and the presence of competing adsorbates in solution, it stands to reason that estimating the crystal face composition (CFC) of the mineral surface and the extent of carbonate contamination in the experimental system will improve SCM predictions. ExperimentsThe Charge Distribution Multi-Site Complexation (CD-MUSIC) model was used to simulate experimental Hg(II) adsorption data, collected on the iron hydroxide mineral goethite, in the presence and absence of competing adsorbates and complexing ligands as a function of pH and ionic strength. The CFC of each goethite sample studied was predicted using a newly discovered relationship between goethite’s proton reactive site density (NH) and specific surface area (SSA). Carbonate’s presence in the experimental systems was determined utilizing a novel methodology developed in this work. FindingsThe CD-MUSIC model developed in this study accurately predicted Hg(II) adsorption onto goethite over the entire range of experimental conditions investigated while only employing surface species consistent with spectroscopic evidence.

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).

Fabrication, Characterization, and Application of a Composite Adsorbent for Simultaneous Removal of Arsenic and Fluoride

Coexisting arsenic (As) and fluoride (F) in groundwater poses severe health risks worldwide. Highly efficient simultaneous removal of As and F is therefore of great urgency and high priority. The purpose of this study was to fabricate a novel composite adsorbent and explore the mechanism for concurrent removal of As(V) and F at the molecular level. This bifunctional adsorbent with titanium and lanthanum oxides impregnated on granular activated carbon (TLAC) exhibits a pronounced As(V) and F adsorption capacity over commercially available iron- and aluminum-based adsorbents for synthetic and real contaminated groundwater samples. Synchrotron-based X-ray microfluorescence analysis demonstrates that La and Ti were homogeneously distributed on TLAC. Extended X-ray absorption fine structure spectroscopic results suggest that As(V) formed bidentate binuclear surface complex as evidenced by an averaged Ti-As bond distance of 3.34 Å in the presence of F. Adsorption tests and Fourier transform infrared spectroscopy analysis indicate that F was selectively adsorbed on lanthanum oxides. The surface configurations constrained with the spectroscopic results were formulated in the charge distribution multisite complexation model to describe the competitive adsorption behaviors of As(V) and F. The results of this study indicate that TLAC could be used as an effective adsorbent for simultaneous removal of As(V) and F.

Adsorption of paraquat on goethite and humic acid-coated goethite

Adsorption of cationic pesticides in soils is generally attributed to mineral clays and organic matter components. However, iron oxides may also contribute to such adsorption or affect it by associating with other components. Using goethite and humic acid as models for iron oxides and organic matter respectively, we studied the adsorption of the cationic pesticide paraquat on goethite and humic acid-coated goethite. At pH 4.0 the adsorption on goethite was not significant, and at pH 10.0, although the surface of the oxide was negatively charged, much less pesticide was adsorbed than on mineral clays. At this pH the adsorption of paraquat decreased as the ionic strength increased, and application of the charge distribution multisite complexation model (CD-MUSIC model) enabled interpretation of the results. At pH 4, the adsorption of paraquat on the humic acid-coated goethite was similar to the adsorption on mineral clays, but was considerably less than the adsorption on humic acid in solution. The lower adsorption on solid organic matter is attributed to a decrease in the number of “active” binding sites on the humic acid as a result of the binding to iron oxide.

Structures and stabilities of Cd(II) and Cd(II)-phthalate complexes at the goethite/water interface

The complexation of Cd(II) and Cd(II)-phthalate at the goethite/water interface were investigated by EXAFS and IR spectroscopy, by batch adsorption experiments and by potentiometric titrations at 298.15 K. The EXAFS spectra showed Cd(II) to form only inner-sphere corner-sharing complexes with the goethite surface sites in the presence and absence of phthalate. EXAFS spectra also showed the presence of Cd(II)-chloride complexes in 0.1 mol/L NaCl. IR spectra also showed phthalate to form (1) an inner-sphere complex with adsorbed corner-sharing Cd(II) surface complexes in the pH 3.5 to 9.5 and (2) an outer-sphere complex with the same type of corner-sharing Cd(II) complex however at pH > 6, in addition to the inner- and outer-sphere complexes of phthalate reported in a previous study. The potentiometric titration and the batch adsorption data were used to constrain the formation constants of the different Cd(II)-phthalate surface complexes on the dominant {110} and the {001} planes of the goethite. The models were carried out with the Charge Distribution Multisite Complexation model coupled to the Three Plane Model and can predict the molecular-scale speciation of cadmium and phthalate in the presence of goethite. Cd(II) adsorption models calibrated on a 90 m 2/g goethite also could accurately predict experimental data for a 37 m 2/g goethite of slightly different basic charging properties.

Arsenic Leachability in Water Treatment Adsorbents

Arsenic leachability in water treatment adsorbents was studied using batch leaching tests, surface complexation modeling and extended X-ray absorption fine structure (EXAFS) spectroscopy. Spent adsorbents were collected from five pilot-scale filters that were tested for removal of arsenic from groundwater in Southern New Jersey. The spent media included granular ferric hydroxide (GFH), granular ferric oxide, titanium dioxide, activated alumina, and modified activated alumina. The As leachability determined with the Toxicity Characteristic Leaching Procedure (TCLP, 0.1 M acetate solution) was below 180 microg L(-1) for all spent media. The leachate As concentration in the California Waste Extraction Test (0.2 M citrate solution) was more than 10 times higher than that in the TCLP and reached as high as 6650 microg L(-1) in the spent GFH sample. The EXAFS results indicate that As forms inner-sphere bidentate binuclear surface complexes on all five adsorbent surfaces. The As adsorption/desorption behaviors in each media were described with the charge distribution multisite complexation model. This study improved the understanding of As bonding structures on adsorptive media surfaces and As leaching behavior for different adsorbents.