Formation Of Ternary Surface Complexes Research Articles

Nalidixic Acid and Fe(II)/Cu(II) Coadsorption at Goethite and Akaganéite Surfaces.

Interactions between aqueous Fe(II) and solid Fe(III) oxy(hydr)oxide surfaces play determining roles in the fate of organic contaminants in nature. In this study, the adsorption of nalidixic acid (NA), a representative redox-inactive quinolone antibiotic, on synthetic goethite (α-FeOOH) and akaganéite (β-FeOOH) was examined under varying conditions of pH and cation type and concentration, by means of adsorption experiments, attenuated total reflectance-Fourier transform infrared spectroscopy, surface complexation modeling (SCM), and powder X-ray diffraction. Batch adsorption experiments showed that Fe(II) had marginal effects on NA adsorption onto akaganéite but enhanced NA adsorption on goethite. This enhancement is attributed to the formation of goethite-Fe(II)-NA ternary complexes, without the need for heterogeneous Fe(II)-Fe(III) electron transfer at low Fe(II) loadings (2 Fe/nm2), as confirmed by SCM. However, higher Fe(II) loadings required a goethite-magnetite composite in the SCM to explain Fe(II)-driven recrystallization and its impact on NA binding. The use of a surface ternary complex by SCM was supported further in experiments involving Cu(II), a prevalent environmental metal incapable of transforming Fe(III) oxy(hydr)oxides, which was observed to enhance NA loadings on goethite. However, Cu(II)-NA aqueous complexation and potential Cu(OH)2 precipitates counteracted the formation of ternary surface complexes, leading to decreased NA loadings on akaganéite. These results have direct implications for the fate of organic contaminants, especially those at oxic-anoxic boundaries.

Synergistic mechanism of iron manganese supported biochar for arsenic remediation and enzyme activity in contaminated soil

This study prepared and characterized bamboo-derived biochar loaded with different ratios of iron and manganese; evaluated its remediation performance in arsenic-contaminated soil by studying the changes in various environmental factors, arsenic speciation, and arsenic leaching amount in the soil after adding different materials; proposed the optimal ratio and mechanism of iron–manganese removal of arsenic; and explained the multivariate relationship between enzyme activity and soil environmental factors based on biological information. Treatment with Fe–Mn-modified biochar increased the organic matter, cation exchange capacity, and N, P, K, and other nutrient contents. During the remediation process, O-containing functional groups such as Mn–O/As and Fe–O/As were formed on the surface of the biochar, promoting the transformation of As from the mobile fraction to the residual fraction and reducing the phytotoxicity of As, and the remediation ability for As was superior to that of Fe-modified biochar. Mn is indispensable in the FeMn-BC synergistic remediation of As, as it can increase the adsorption sites and the number of functional groups for trace metals on the surface of biochar. In addition to electrostatic attraction, the synergistic mechanism of ferromanganese-modified biochar for arsenic mainly involves redox and complexation. Mn oxidizes As(Ⅲ) to more inert As(V). In this reaction process, Mn(Ⅳ) is reduced to Mn(Ⅲ) and Mn(II), promoting the formation of Fe(Ⅲ) and the conversion of As into Fe–As complexes, while As is fixed due to the formation of ternary surface complexes. Moreover, the effect of adding Fe–Mn-modified biochar on soil enzyme activity was correlated with changes in soil environmental factors; catalase was correlated with soil pH; neutral phosphatase was correlated with soil organic matter; urease was correlated with ammonia nitrogen, and sucrase activity was not significant. This study highlights the potential value of FM1:3-BC as a remediation agent in arsenic-contaminated neutral soils.

Co-benefits of biochar-supported nanoscale zero-valent iron in simultaneously stabilizing soil heavy metals and reducing their bioaccessibility

We investigated the performance and encapsulation mechanisms of novel biochar-supported nanoscale zero-valent iron (nZVI-BC) used for the remediation of soil co-contaminated with arsenic (As), cadmium (Cd), and lead (Pb) via incubation and column experiments. Compared with the control, 0.50% of nZVI-BC significantly decreased the leakage of As, Cd, and Pb by 97.94–98.45%, 42.86–81.12%, and 82.14–92.49%, respectively. In addition, 0.50% of nZVI-BC could transform the fraction of unstable heavy metals into a stable form, which substantially decreased the availability, leachability, and bioaccessibility of the heavy metals and hence greatly reduced the human health exposure risk. Column experiments showed that 0.50% of nZVI-BC effectively restrained the leaching of As, Cd, and Pb by 95.60–99.84%, 70.82–84.18%, and 91.68–99.81%, respectively. The predominant encapsulation mechanisms of nZVI-BC included complexation, precipitation/co-precipitation, reduction, and the formation of ternary surface complexes. Based on these insights, we can devise new strategies for the remediation of soil co-contaminated with As, Cd, and Pb.

Competitive arsenate and phosphate adsorption on α-FeOOH, LaOOH, and nano-TiO2: Two-dimensional correlation spectroscopy study

Competitive adsorption of arsenate (AsO43−) and phosphate (PO43−) on α-FeOOH, LaOOH, and nano-TiO2 was studied using batch adsorption experiments and in-situ flow cell ATR-FTIR coupled with two-dimensional correlation spectroscopy (2D-COS) for the first time. With a higher temporal resolution, our results found a highly dynamic adsorption sequence for AsO43− and PO43−. When AsO43− and PO43− were simultaneously exposed to the adsorbents at the same concentrations, AsO43− was preferentially adsorbed by α-FeOOH and TiO2, but PO43− adsorption was dominant on LaOOH. The results implied that the PO43− adsorbed on LaOOH had to be remobilized to allow for AsO43− adsorption, but that PO43− adsorption on α-FeOOH and TiO2 was hindered by faster AsO43− adsorption. Crystal orbital Hamilton population (COHP) analysis revealed that AsO43− complexes bonded more strongly on α-FeOOH and TiO2, whereas PO43− complexes were more stable on LaOOH. Different adsorption sequences and the stability of the complexes were attributed to the diverse geometric configurations of AsO43− and PO43− on metal oxides surfaces with specific bonding chemistry. The presence of Ca2+ did not affect AsO43− and PO43− adsorption sequence on α-FeOOH or LaOOH, but it reversed the adsorption sequence on TiO2 due to the formation of ternary surface complexes on TiO2 surfaces.

Influence of metal cation and surface iron oxide on the transport of sulfadiazine in saturated porous media

Sulfadiazine (SDZ) is an antibiotic frequently detected in soil and groundwater. The transport of SDZ in subsurface environment is a critical process affecting its retention in soil. To date, the effects of iron oxide and metal cation on the transport of SDZ remain largely unknown, so we investigated the transport properties of SDZ in cleaned and iron oxides coated quartz sand, as affected by the presence of conventional cations (Ca2+, Mg2+, K+, and Na+) and Cd2+ through column experiments and simulation. We found that iron oxide coating on sand surface inhibited the transport of SDZ, mainly due to hydrophobic effect, complexation, and electrostatic attraction. The inhibitory effect became more marked with increasing concentration of Cd2+. It favors the transport of Cd2+ due to the electrostatic repulsion between positively charged iron oxide and Cd2+. Ca2+ promoted the transport of SDZ in coated sand, while all the conventional cations had no effect on the transport of SDZ in cleaned sand. The increase in the concentration of Cd2+ favors the transport of SDZ in cleaned sand. However, in iron oxide coated sand, the influence of Cd2+ on the transport of SDZ was dependent on the concentration of Cd2+. At lower concentration of Cd2+ and by competition, the transport is favored. At high concentration, the transport is inhibited mainly due to the formation of ternary surface complexes. A convective–dispersive transport model was applied to simulate and interpret experimental data. Breakthrough curves fitted well with a one-site model (OSM), indicating that SDZ adsorption on the sand experiences reversible kinetic. A low level of KF values with nearly linear sorption isotherm shows high mobility of SDZ and a high potential risk of surface and groundwater contamination. However, such high mobility can be reduced by increasing the content of iron oxides in porous media.

Ternary Complex Formation of Phosphate with Ca and Mg Ions Binding to Ferrihydrite: Experiments and Mechanisms

Calcium (Ca) and magnesium (Mg) are the most abundant alkaline-earth metal ions in nature, and their interaction with ferrihydrite (Fh) affects the geochemical cycling of relevant ions, including phosphate (PO4). The interfacial interactions of Ca and Mg (M2+) with PO4 have not been analyzed yet for freshly precipitated Fh. Here, we studied experimentally this interaction in binary M2+-PO4 systems over a wide range of pH, M2+/PO4 ratios, and ion loadings. The primary adsorption data were scaled to the surface area of Fh using a recent ion-probing methodology that accounts for the size-dependent chemical composition of this nanomaterial (FeO1.4(OH)0.2·nH2O). The results have been interpreted with the charge distribution (CD) model, combined with a state-of-the-art structural surface model for Fh. The CD coefficients have been derived independently using MO/DFT/B3LYP/6-31+G** optimized geometries. M2+ and PO4 mutually enhance their adsorption to Fh. This synergy results from the combined effect of ternary surface complex formation and increased electrostatic interactions. The type of ternary complex formed (anion- vs cation-bridged) depends on the relative binding affinities of the co-adsorbing ions. For our Ca-PO4 systems, modeling suggests the formation of two anion-bridged ternary complexes, i.e., ≡(FeO)2PO2Ca and ≡FeOPO3Ca. The latter is most prominently present, leading to a relative increase in the fraction of monodentate PO4 complexes. In Mg-PO4 systems, only the formation of the ternary ≡FeOPO3Mg complex has been resolved. In the absence of Ca, the pH dependency of PO4 adsorption is stronger for Fh than for goethite, but this difference is largely, although not entirely, compensated in the presence of Ca. This study enables the use of Fh as a proxy for the natural oxide fraction, which will contribute to improved understanding of the mutual interactions of PO4 and M2+ in natural systems.

A novel multifunctional adsorbent synthesized by modifying acidified organo-montmorillonite with iron hydroxides

Multifunctional adsorbents based on montmorillonite (Mt) have been of highly concerns due to their unique 2D layer structures and impressive co-adsorption performances. To further enhance the support ability and reactivity, a strategy was developed in this study to enable the co-existence of porous nanosilica (PNS) and hydrophobic interlayer spaces in Mt structure, and a novel multifunctional adsorbent (iron modified-acidified organo-Mt, Fe-AOMt) was then synthesized, aiming to simultaneously remove hydrophobic contaminants, oxyanion contaminants and heavy metals in water. The combination of XRD, FTIR and TEM verified the co-existence of amorphous PNS and layered structure (containing intercalated surfactant) in the acidified organo-Mt (AOMt), and results of STEM-EDX revealed uniform distribution of Fe after the further introduction of Fe hydroxides to AOMt. Additionally, pore structure analyses suggested that the acid activation can obviously increase the pore volume and specific surface area of OMt. Batch adsorption experiments revealed that Fe-AOMt as a promising multifunctional adsorbent was able to effectively and simultaneously remove nitrobenzene, phosphate, and Cd(II) in the multi-contaminant system. The synergetic adsorption of phosphate and Cd(II) on Fe-AOMt suggested the formation of ternary surface complexes (≡Fe–P–Cd), while the pH-independent uptake of nitrobenzene was hardly influenced by adsorption of phosphate and Cd(II), implying dissimilar adsorption mechanisms. The co-adsorption mechanism on Fe-AOMt was further revealed via XPS. Our work provided a novel and efficient Mt-based multifunctional adsorbent with a newly-proposed structure, which was a preliminary exploration of diversifying locations and types of active sites in Mt structure.

Molecular dynamics simulation of the interaction of uranium (VI) with the C–S–H phase of cement in the presence of gluconate

Due to their high durability and immobilization properties, cementitious materials have found a considerable application in the design and construction of radioactive waste repositories in the last decades. During cement paste production, organic additives are introduced to modify various properties of cement. The presence of such organic complexants may negatively affect the immobilizing properties of cement with respect to radionuclides. For better understanding and prediction of the effects of interactions between organic molecules and cementitious materials with radionuclides, we have developed several representative models consisting of three principal components: (i) calcium silicate hydrate (C–S–H) phase - the main binding phase of cement; (ii) gluconate, a simple well-described molecule, as a representative of organic additives; (iii) U(VI), as one of the most studied radionuclides of the actinide series. The C–S–H phase with low Ca/Si ratio (~0.83) typical for “low-pH” and degraded cement pastes has been selected for this modelling study. Structural, and energetic aspects of the sorption processes of uranyl, gluconate, and their mutual correlations on the surface of cement were quantitatively modeled by classical molecular dynamics (MD) and potential of mean force (PMF) calculations. The ternary surface complex formation between uranyl hydroxides and Ca2+ cations at the C–S–H aqueous interfaces is shown to have an important role in the overall sorption process. In the presence of gluconate, U(VI) sorption on C–S–H is facilitated by weakening the Ca2+ binding with the surface. Additionally, Na+ is proven to be an important competitor for certain surface sorption sites and can potentially affect the equilibrium properties of the interface.

Pronounced Antagonism of Zinc and Arsenate on Toxicity to Barley Root Elongation in Soil

Zinc (Zn) and arsenic (As) occur as mixed contaminants in soil and the interactions between them remain unclear. Here, we investigated a Zn2+ and H2AsO4− mixture interaction and their effects on plant growth. Three different soils were spiked with ZnCl2 and NaH2AsO4, each dosed singly or in combination. The soils were leached to remove excessive salt and were aged (>7 days), before toxicity testing using a 5-day root elongation of barley (Hordeum vulgare L.). In the single treatments, the 50% inhibitory effect concentrations in the soil (EC50, total measured concentration) were 2000–3800 mg Zn kg−1 and 96–620 mg As kg−1, depending on the soils. The mixture analyses based on the total concentrations showed overall and significant Zn–As antagonism in two soils, either based on the concentration addition (CA) or independent action (IA) model, whereas no significant interactions (either CA or IA) were found in one soil, which had the lowest content of Fe-oxides. The soil solution composition showed a decreased As concentration upon the addition of ZnCl2 at an equal soil As total concentration; however, the reverse was not found, in line with the cation–anion electrostatic interaction or formation of ternary surface complexes on Fe-oxides. The data revealed that the Zn–As antagonisms (total concentrations) are partially related to the increased Zn immobilizing As in soil.

Preparation of a Phosphate-Modified Flower-Like α-FeOOH Composite and Its Application for Aqueous U(VI) Removal

Goethite is a stable and widespread mineral in soil, which affects the transportation and immobilization of heavy metals in soil. Here, the three-dimensional flower-like goethite (TDFLG) was synthesized by refluxing precipitation method. The modified three-dimensional flower-like goethite (MTDFLG) was prepared by NaH2PO4 with dipping method. The obtained samples were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, N2 adsorption–desorption (BET), and X-ray diffraction (XRD). SEM images showed that the modification of phosphate had no major changes on the morphology of the original sample and the morphology of MTDFLG after adsorbed U(VI) had clearly change. For the goethite and modified goethite, the BET-specific surface area was 229.96 and 203.17 m2/g, respectively. Moreover, the effects of adsorption time, sorbent dose, solution pH, and initial uranium concentration on the uranium adsorption behaviors were investigated using the two materials as adsorbent for the treatment of uranium-containing wastewater. The results showed that MTDFLG had better adsorption capacity than TDFLG on uranium. The increase in uranium removal on MTDFLG was due to the formation of ternary surface complexes (≡FePO4UO2). TDFLG and MTDFLG followed the pseudo-second-order kinetic model and the Langmuir adsorption isotherm model, which indicated that uranium adsorption on TDFLG or MTDFLG is mainly based on chemisorption, and the maximum adsorption capacity of two adsorbents is 48.24 and 112.36 mg/g, respectively.

In Situ Cobalt Speciation on γ-Al2O3 in the Presence of Carboxylate Ligands in Supported Catalyst Preparation

Molecular-scale understanding of metal impregnation on a porous oxide support is a key step toward the development of optimized (and cost-effective) catalytic materials with enhanced activity, selectivity and time on stream stability. In the present contribution in situ ATR-IR and ex situ spectroscopic measurements (XPS, UV–vis DRS) have been performed in order to describe the influence of three different oxygenated organic molecules (acetate, citrate, and nitrilotriacetate) on cobalt sorption on a conventional γ-alumina support. Variation of the preparation conditions has been done by changing the pH of adsorption, and the nature of additives (organic molecules) as well as their concentrations. Different sorption mechanisms and cobalt speciation have been evidenced during in situ ATR-IR measurement through comparison with reference spectra of pure Co and organic additives solutions. More specifically, citrate and nitriloacetate additives leads to the formation of ternary surface complexes (alumina–ligand...

Retention of Eu(III) in muscovite environment: Batch and spectroscopic studies

Retention of trace Eu(III) in the medium enriched with muscovite was investigated under aerobic conditions combining batch experiments and spectroscopic techniques including time resolved laser-induced fluorescence spectroscopy (TRLFS), extended X-ray absorption fine structure (EXAFS), and X-ray photoelectron spectroscopy (XPS). The underlying retention mechanisms were explored by identifying the coordination structures of adsorbed Eu(III) on muscovite. Batch experiments showed that the sorption of Eu(III) on muscovite was strongly dependent on pH and ionic strength. TRLFS showed that the H2O/OH− molecules in the first coordination shell of adsorbed Eu(III) reduced over pH ∼7.0, indicating that inner-sphere complexation was the dominant mechanism for Eu(III) sorption on muscovite. In the presence of inorganic ligands, Eu(III) retention was enhanced by phosphate while suppressed by sulfate, and TRLFS showed that the enhanced Eu(III) retention in the presence of phosphate was mainly attributed to the formation of ternary surface complex. EXAFS and XPS results further confirmed that the P-bridge type ternary surface complex was the main sorption species in the observed system. The proposed Eu(III) retention mechanism in present work can provide an important experimental reference for predicting the migration behaviors of Ln/An(III) in mica-rich terrains at a molecular scale.

Controlled synthesized natroalunite microtubes applied for cadmium(II) and phosphate co–removal

Treatment of wastewater containing several kinds of contaminants poses great challenges, because heavy metal and inorganic anion contaminants possess different fate and transport mechanisms. Individual adsorption of Cd(II)/phosphate on clay or metallic oxides has been extensively investigated, but the mutual effects of these two species in co–existing systems have received little attention. In this study, five natroalunite samples with different morphologies were synthesized by a simple hydrothermal method with appropriate volume ratio of ethylene glycol (EG) to water. The volume ratio of EG to water plays a key role in the formation of natroalunite samples, and dramatically affects their adsorption capacities. The mutual effects of Cd(II) and phosphate on their interaction with natroalunite microtubes (NMs) were investigated by varying experimental conditions, such as pH, temperature and addition sequences. The results demonstrate that highly efficient co–removal of Cd(II) and phosphate can be accomplished using NMs, and the process is strongly dependent on solution pH and temperature via the formation of ternary surface complexes. This study implies that the hydrothermally synthesized NMs can be regarded as a potential promising material for the co–removal of Cd(II) and phosphate from large volumes of aqueous solutions in pollution management.

Cu(II) removal by Anoxybacillus flavithermus–iron oxide composites during the addition of Fe(II)aq

There is currently poor understanding of metal removal by composites of bacteria and iron oxide minerals, even though they commonly co-occur and are among the most important sorbents in near-surface fluid–rock environments. This study evaluated Cu removal by composites of Anoxybacillus flavithermus and iron oxide over time during the addition, oxidation, and hydrolysis of Fe(II)aq and precipitation of the mineral, in comparison to Cu removal in the two single-sorbent end-member systems. In the absence of iron oxide, Cu removal by A. flavithermus was well described by a previously published surface complexation model, after inclusion of additional reactions describing aqueous complexation by exudate ligands released by the bacteria. In the absence of bacterial cells, Cu removal by iron oxide synthesized in the presence of the bacterial exudate ligands demonstrated the formation of ternary surface complexes. Removal of Cu by the A. flavithermus–iron oxide composites was ca. 20% greater than the prediction based on assumption of additivity in the two end-member systems. This non-additive behavior was attributed to (1) progressive physical blockage of bacterial surface sites by the iron oxide particles, (2) physical blockage of adsorption sites as a result of self-aggregation of the iron oxide particles, and (3) the reduction of Cu(II) to Cu(I) at the bacterial cell surface, as demonstrated by X-ray absorption spectroscopy. The extent of reduction of Cu(II) to Cu(I) was proportional to the concentration of solid phase Fe(II), suggesting that iron oxidation and copper reduction are linked. This study has shown that Cu removal by bacteria–iron oxide composites is greatly affected by redox processes such as Cu(II) reduction on the cell surface both by other bacterial surface ligands and the oxidation of sorbed Fe(II), as well as Fe(II) redox interactions, and aging effects of the mineral (i.e. surface site masking).

Surface complexation modeling of Eu(III) and phosphate on Na-bentonite: Binary and ternary adsorption systems

Abstract This study aims to investigate and model the adsorption of Eu(III) on bentonite in the presence of phosphate. Binary (phosphate/bentonite) and ternary (Eu(III)/phosphate/bentonite) systems were studied as a function of contact time, pH, solid-to-liquid ratio and Eu(III)/phosphate concentration by using a batch experimental method. The adsorption of phosphate on bentonite slightly increased in the pH range of 2.5–6.5, and decreased in the pH range of 6.5–9.4. This adsorption can be quantitatively interpreted by a model considering the formation of three monodentate surface complexes. In the ternary system, a synergistic adsorption was observed in the presence of both phosphate and Eu(III). In addition to the two sub-models describing Eu(III) and phosphate adsorption, the formation of ternary surface complexes had to be considered in order to explain the synergistic effect experimentally observed. The experimental data could be quantitatively explained when Eu(III) ( SOEuH2PO4+ and SOEuHPO4) or phosphate ( SPO4Eu+) are the bridged atoms. Complementary experiments carried out by X-ray photoelectron spectroscopy suggested that the second case is the most probable. The proposed model can be used in order to predict Eu(III) adsorption on buffer/backfilling material in the presence of phosphate.

Surface complexation modeling of Eu(III) adsorption on silica in the presence of fulvic acid

Humic substances (HS) substantially affect heavy metal (M) adsorption on mineral surfaces. However, quantitative descriptions of ternary systems involving M, HS and mineral surfaces remain unclear. This study examines adsorption in a model ternary system including Eu(III), fulvic acid (FA) and silica, and describes the adsorption of Eu(III) and FA by combining a double-layer model (DLM) and the Stockholm humic model (SHM). SHM explains the binding of H+ and Eu3+ to FA and the DLM for FA and Eu(III) adsorption on silica. Experimental results showed that the presence of FA promotes Eu(III) adsorption at acidic pH values, but decreases it at basic pH values, which indicates the formation of ternary surface complexes. Modeling calculations have shown that two ternary surface complexes are required to describe the experimental results in which Eu3+ acts as a bridge between the surface site and FA. The present study suggests that the discrete-site approach to HS is a promising method for interpreting the adsorption data for M, HS and mineral ternary systems.

Zn(II) and Cu(II) adsorption and retention onto iron oxyhydroxide nanoparticles: effects of particle aggregation and salinity.

BackgroundIron oxyhydroxides are commonly found in natural aqueous systems as nanoscale particles, where they can act as effective sorbents for dissolved metals due to their natural surface reactivity, small size and high surface area. These properties make nanoscale iron oxyhydroxides a relevant option for the remediation of water supplies contaminated with dissolved metals. However, natural geochemical processes, such as changes in ionic strength, pH, and temperature, can cause these particles to aggregate, thus affecting their sorption capabilities and remediation potential. Other environmental parameters such as increasing salinity may also impact metal retention, e.g. when particles are transported from freshwater to seawater.ResultsAfter using synthetic iron oxyhydroxide nanoparticles and nanoparticle aggregates in batch Zn(II) adsorption experiments, the addition of increasing concentrations of chloride (from 0.1 M to 0.6 M) appears to initially reduce Zn(II) retention, likely due to the desorption of outer-sphere zinc surface complexes and subsequent formation of aqueous Zn-Cl complexes, before then promoting Zn(II) retention, possibly through the formation of ternary surface complexes (supported by EXAFS spectroscopy) which stabilize zinc on the surface of the nanoparticles/aggregates. In batch Cu(II) adsorption experiments, Cu(II) retention reaches a maximum at 0.4 M chloride. Copper-chloride surface complexes are not indicated by EXAFS spectroscopy, but there is an increase in the formation of stable aqueous copper-chloride complexes as chloride concentration rises (with CuCl+ becoming dominant in solution at ~0.5 M chloride) that would potentially inhibit further sorption or encourage desorption. Instead, the presence of bidentate edge-sharing and monodentate corner-sharing complexes is supported by EXAFS spectroscopy. Increasing chloride concentration has more of an impact on zinc retention than the mechanism of nanoparticle aggregation, whereas aggregation condition is a stronger determinant of copper retention.ConclusionsBased on these model uptake/retention studies, iron oxyhydroxide nanoparticles show potential as a strategy to remediate zinc-contaminated waters that migrate towards the ocean. Copper retention, in contrast, appears to be optimized at an intermediate salinity consistent with brackish water, and therefore may release considerable fractions of retained copper at higher (e.g. seawater) salinity levels.

Ternary surface complexes in silica-Ni2+ ions-2,2′-dipyridyl systems

The adsorption of complexes of nickel ions with an organic base (2,2′-dipyridyl) at an interface between silica and an aqueous electrolyte solution has been considered in terms of the theory of complexation. It has been shown that, on the silica surface, ternary complexes are formed, in which nickel ions are bonded to silanol groups. The equilibrium constants of the reactions of ternary surface complex formation have been calculated from the adsorption curves describing the pH dependences of nickel ion and dipyridyl adsorption.

Macroscopic and molecular approaches of enrofloxacin retention in soils in presence of Cu(II)

The co-adsorption of copper and the fluoroquinolone antibiotic enrofloxacin (ENR) at the water-soil interface was studied by means of batch adsorption experiments, and extended X-ray absorption fine structure (EXAFS) spectroscopy. The system was investigated over a pH range between 6 and 10, at different contact times, ionic strengths, and ENR concentrations. Adsorption coefficient – Kd – was determined at relevant environmental concentrations and the value obtained in water at a ionic strength imposed by the soil and at soil natural pH was equal to 0.66Lg−1. ENR adsorption onto the soil showed strong pH dependence illustrating the influence of the electrostatic interactions in the sorption processes. The simultaneous co-adsorption of ENR and Cu(II) on the soil was also investigated. The presence of Cu(II) strongly influenced the retention of the antibiotic, leading to an increase up to 35% of adsorbed ENR amount. The combined quantitative and spectroscopic results showed that Cu(II) and ENR directly interacted at the water-soil interface to form ternary surface complexes. Cu K-edge EXAFS data indicated a molecular structure where the carboxylate and carbonyl groups of ENR coordinate to Cu(II) to form a 6-membered chelate ring and where Cu(II) bridges between ENR and the soil surface sites. Cu(II) bonds bidentately to the surface in an inner-sphere mode. Thus, the spectroscopic data allowed us to propose the formation of ternary surface complexes with the molecular architecture soil-Cu(II)-ENR.

Microscopic level investigation of Ni(II) sorption on Na-rectorite by EXAFS technique combined with statistical F-tests

Extended X-ray absorption fine structure (EXAFS) spectroscopy combined with statistical F-tests is used to investigate the local atomic structures of Ni(II) adsorbed on Na-rectorite. The EXAFS analysis results of Ni(II) sorption samples indicate that the first coordination shell consists of ∼6 O at the Ni–O interatomic distance (R) of ∼2.04Å. The presence of Ni backscattering at RNi–Ni=∼3.06Å in the second coordination shell suggests the formation of Ni(II) precipitate. The results of F-tests show that the Ni(II) precipitate is Ni–Al layered double hydroxide (LDH). Our results demonstrate that Ni(II) ions are retained via different mechanisms depending on solution conditions. At low pH, Ni retention is controlled mainly by the outer-sphere surface complexation. With increasing pH, outer-sphere and inner-sphere surface complexation dominate Ni uptake. Furthermore, Ni surface loading increases with temperature increasing at pH 6.5 due to the formation of inner-sphere surface complexes and Ni–Al LDH. The formation of Ni–Al LDH becomes the dominate mechanism at the elevated pH and temperature. In the presence of humic substances, the sorption of Ni(II) on Na-rectorite is dominated by the formation of ternary surface complexes. These results are important to understand the physicochemical behavior of Ni(II) in the natural environment.