Sorption Density Research Articles

Adsorption kinetics of organic phosphates on goethite and aluminium oxide: The equation used to describe the reaction

AbstractAdsorption kinetics of three organic phosphate compounds (OPs) with varying molecular sizes and structures and inorganic phosphate (Pi) were investigated on α‐Al2O3 and poorly crystalline goethite. The organic phosphates were inositol hexaphosphate (IHP), glycerol phosphate (GlyP) and glucose‐6‐phosphate (G6P), and the inorganic phosphate was KH2PO4. Batch adsorption experiments were performed at 25°C. We tested the sorption kinetic data using various non‐linear models/equations and on their transformed linear forms by applying appropriate statistics. Besides, we also used a modified non‐linear equation having four parameters to this effect. Data were found to fit best with the modified equation and described the whole sorption process satisfactorily. For sorption of compounds on to the surface of these minerals, the equation with four parameters may be used in contrast with many standard equations applied for kinetic studies in soils. Sorption was described to take place in two processes: a fast one that takes place in less than 45 min and a slow one that takes place in several hours or more. The rate of the slow process did not depend directly on the concentration of phosphate compounds in solution, but depended linearly on the amount of phosphate that was adsorbed during the fast process. These initially adsorbed ions carrying some amount of negative charge likely hindered the movement of subsequent adsorbate ions to the solid surface due to decreased surface potential. This caused the variation in fast and slow sorption rate constants. Sorption densities increased in the order, Pi >Gly P >G6P >IHP, which revealed that the sorption density and initial sorption rate of OPs decreased with increasing molecular weights of OPs.

The role of Fe(II)-silicate gel in the generation of Archean and Paleoproterozoic chert

Abstract Chert is abundant in Archean and Paleoproterozoic rocks and is commonly densely packed with authigenic Fe(II)-silicate nanoparticles such as greenalite, indicating a close relationship between iron and silica deposition. We investigate the relationship between Fe(II)-silicate minerals and dissolved silica during precipitation, settling, and diagenesis using anoxic synthesis, sorption, and heating experiments. Excess silica is associated with the solid during precipitation, resulting in high molar Si/Fe ratios (<1.52) that exceed that of stoichiometric greenalite (0.67). At pH 8–8.5, silica sorbs to the surface, reaching sorption densities of 0.68 mmol Si per mmol Fe(II)-silicate. Furthermore, excess Si is released upon heating as the Fe(II)-silicate gel crystallizes. We suggest that Fe(II)-silicate minerals acted as an effective Si shuttle between the water column and the sediments in Archean and Paleoproterozoic marine environments, providing sites for the growth of early diagenetic chert, consistent with observations from the sedimentary record. Our results explain the exceptional preservation of greenalite in early chert and indicate that these minerals could provide a robust archive of marine geochemical data.

Quantitative Rapid Magnetic Immunoassay for Sensitive Toxin Detection in Food: Non-Covalent Functionalization of Nanolabels vs. Covalent Immobilization.

In this study, we present a novel and ultrasensitive magnetic lateral flow immunoassay (LFIA) tailored for the precise detection of zearalenone, a mycotoxin with significant implications for human and animal health. A versatile and straightforward method for creating non-covalent magnetic labels is proposed and comprehensively compared with a covalent immobilization strategy. We employ the magnetic particle quantification (MPQ) technique for precise detection of the labels and characterization of their functionality, including measuring the antibody sorption density on the particle surface. Through kinetic studies using the label-free spectral phase interferometry, the rate and equilibrium constants for the binding of monoclonal antibodies with free (not bound with carrier protein) zearalenone were determined to be kon = 3.42 × 105 M-1s-1, koff = 7.05 × 10-4 s-1, and KD = 2.06 × 10-9 M. The proposed MPQ-LFIA method exhibits detection limits of 2.3 pg/mL and 7.6 pg/mL when employing magnetic labels based on covalent immobilization and non-covalent sorption, with dynamic ranges of 5.5 and 5 orders, correspondingly. We have successfully demonstrated the effective determination of zearalenone in barley flour samples contaminated with Fusarium graminearum. The ease of use and effectiveness of developed test systems further enhances their value as practical tools for addressing mycotoxin contamination challenges.

A two-sorbent system for fast uptake of arsenate from water: Batch and column studies

There is a critical need to use decentralized and/or point-of-use systems to address some challenging water quality issues in society. Sorption-based approaches are uniquely suitable for such applications because of their simplicity in operation; however, the sorbents must possess fast contaminant uptake kinetics to overcome short hydraulic contact times often encountered in small systems. Here we designed a two-sorbent system consisting of Fe2O3-coated mesoporous carbon (FeMC) and nano-Fe2O3-coated activated carbon (FeAC) and demonstrated its ability to remove arsenate with a < 1 min empty bed contact time (EBCT) by a capture-and-storage process. Batch experiments showed rapid capture of arsenate by FeMC, likely occurred on the rod-like structures protruding to the liquid film. The captured arsenate could subsequently be relocated to FeAC for storage, which had a higher apparent sorption capacity. Column studies, operated with a 10 h running time followed by a 14 h pump-off time, showed that with a 102 μg-As/L influent concentration and at 0.85 min EBCT, the column treated 20,022 bed volumes until the 10 μg-As/L breakthrough, corresponding to a sorption density of 2.36 mg-As/g. This capture-and-storage technique resulted in a rapid and high-capacity arsenate removal through a combined effect of facile access to sorption sites on one sorbent and dynamic equilibrium in the two-sorbent system possessing a large total sorption capacity.

Sorption of Per- and Polyfluoroalkyl Substances (PFAS) using Polyethylene (PE) microplastics as adsorbent: Grand Canonical Monte Carlo and Molecular Dynamics (GCMC-MD) studies

ABSTRACT Per- and Polyfluoroalkyl Substances (PFAS) and microplastics (MPs) removal is one of the main technical measures in these emerging pollutant control and MPs can sorb PFAS acting as carriers. In order to explore the sorption process of polyethylene (PE MPs) for different PFAS, Grand Canonical Monte Carlo and molecular dynamics simulation was used. Seven commonly found PFAS compounds including PFNA (Perfluorononanoic acid), PFHxS (Perfluorohexanesulphonic acid), PFHxA (Perfluorohexanoic acid), PFDA (Perfluorodecanoic acid), PFOS (Perfluorooctanesulfonic acid), PFBS (Perfluorobutanesulfonic acid) and Perfluorooctanoic acid (PFOA) were selected. The sorption on PE MPs was based on fixed loading and the sorption (PE MPs+PFAS) average total energy for all simulation steps followed PFHxA (−29.17 kcal/mol) < PFBS (−52.71 kcal/mol) < PFOA (−57.12 kcal/mol) < PFNA (−70.58 kcal/mol) < PFDA (−78.27 kcal/mol) < PFHxS (−80.06 kcal/mol) < PFOS (−102.30 kcal/mol). However, the binding/interaction energies followed PFHxS > PFOS > PFNA > PFDA > PFOA > PFBS > PFHxA. The MD simulation was carried out under NVT ensemble at 298 K, the thermodynamic states and relaxation pretreatment procedures were then examined quantitatively by radial distribution function, X-ray scattering, relative concentration profile, mean square displacement and space time correlation function. The results show that effective sorption of PFAS molecule by PE MPs involves hydrogen or chemical bonding as well non-bond energy such as Van der Waals (VdW), electrostatic and intramolecular energies. The cell volume did not change during single and multicomponent sorption while the sorption density increased about threefold the single component (0.36 g/cm3), suggesting that as the pollutant number increased the sorption capacity increased. This study showed that PE MPs can used for single and multicomponent sorption of PFASs effectively and also provided theoretical support for our knowledge of the sorption mechanism at the molecular level, which will help in better understanding of PFAS removal by MPs.

Metal (hydr)oxide surface precipitates and their effects on potassium sorption.

Surface precipitation has been shown to occur on rapid time scales in clay and metal oxide mineral systems. The formation of surface precipitates is hypothesized to present new potential sorption sites for potassium (K), where K can become incorporated into newly formed interlayer spaces (e.g., between tetrahedral-octahedral-tetrahedral stacked sheets). The objective of this study is to determine the effects of newly formed mineral surface precipitates on K sorption. Potassium adsorption experiments were conducted by utilizing Al2O3 and SiO2 sorbents in the presence of various cations (magnesium, zinc, and nickel) that helped to catalyze the formation of surface precipitates. Dissolved concentrations of elements were monitored via inductively coupled plasma optical emission spectrometry (ICP-OES). Solids were characterized via X-ray diffraction (XRD), and K surface complexation was analyzed via X-ray absorption near edge structure (XANES) spectroscopy. X-ray diffraction analysis indicated bayerite, layered double hydroxides (LDH), and silicated LDH were formed as reaction products, thus creating new surface sites for potential K adsorption. The presence of Si increased K adsorption perhaps due to its role in the formation of LDH surface precipitates. When the differences between observed and theoretical surface area normalized K sorption densities were averaged, a 31% increase in K adsorption was observed in the presence of Si. XANES analysis indicated that the binding mechanism of K to Si is different than that of K to Al, perhaps due to the presence of inner-sphere complexation of K to Al-oxide. Samples reacted for one month versus one week yielded more intense XANES post-edge peaks which indicated that the K sorption complex changes over time. Overall, our findings provide novel insights into the mechanisms of K fixation in soil and has high implication in providing improved K fertilizer recommendation to growers.

EXAFS investigation of Ni(Ⅱ) sorption at the palygorskite-solution interface: New insights into surface-induced precipitation phenomena

Sorption reactions at the mineral/water interface control the fate and transport of trace metals in aqueous geochemical environments. While surface complexation theory is well established, understanding of surface induced precipitation at mineral/water interfaces is still limited. In this research, we employed a combination of macroscopic batch sorption studies, transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS), and extended X-ray absorption fine structure (EXAFS) spectroscopy to elucidate the sorption mechanisms of nickel (Ni) at palygorskite/solution interfaces. Compared to extensive studies focused on oxides and layer-structured clay minerals, research on chain-type clay minerals (e.g., palygorskite and sepiolite) is sparse. Thus, novel investigations using chain-type clay minerals can provide new insights into the metal sequestration mechanisms and improve equilibrium modeling. In this work, temperature-dependent kinetic experiments demonstrated that the activation reaction energy (Ea) for Ni sorption on palygorskite at pH 6.0 is 35.1 kJ mol−1, significantly lower than that of pH 7.5 (Ea = 102.1 kJ mol−1). These values indicated an adsorption reaction at low pH and a precipitation reaction at high pH. EXAFS analysis confirmed that below pH 6, Ni sorption was dominated by outer-sphere surface complexes at low ionic strength (I = 0.01 M) and inner-sphere surface complexation at high ionic strength (I = 0.1 M). Above pH 7.5, EXAFS data suggested the formation of Ni phyllosilicate precipitates, in agreement with its linear sorption isotherm. An important finding is that the precipitates can form at very low Ni concentrations (∼0.07 mM), with the sorption density of Γ = 0.09 μmol m−2, which corresponds to a 0.3% monolayer coverage for palygorskite. In contrast, at this low level of Ni concentration, surface precipitates did not form on γ-Al2O3. We propose that Ni can be directly induced to precipitate on the palygorskite surface in a process we term “continuous nucleation”. This is mechanistically different from the traditional adsorption-to-polymerization process on the γ-Al2O3 surface, which we term “staged nucleation”. Further testing revealed that a linear-shaped isotherm was also observed for Ni sorption on sepiolite, a clay mineral with similar structure and chemical composition to palygorskite, indicating a similar reactivity for both chain-structure phyllosilicates. This is distinct from the Langmuir-type sorption isotherm observed for Ni reaction with Al oxides and montmorillonite. The findings presented in this study improve not only the current understanding of metal sequestration and surface precipitation at mineral/water interfaces but also shed light on the large differences in the surface reactivities of clay minerals with chain structure versus layered structure.

Arsenate removal by resin-supported ferric ions: Mechanism, modeling, and column study

Removal of 10 mg/dm3 of As(V) by resin-supported Fe(III) was investigated in batch and column studies. The best As(V) removal was achieved at pH 3, and a high sorption density of 0.40 mmol-As/mmol-Fe was obtained after 96 h in a batch study. This sorption density could not be explained only by As(V) surface complexation with ferrihydrite. X-ray absorption near edge structure (XANES) analysis at the As K-edge suggested that over 90% of the As(V) was removed by surface precipitation as poorly crystalline ferric arsenate and the remainder was removed by surface complexation with ferrihydrite. XANES analysis at Fe K-edge suggested that 14%–35% of Fe(III) in the resin was used in the ferrihydrite and ferric arsenate. Kinetic modeling of the precipitation of ferrihydrite and ferric arsenate combined with a surface complexation model for As(V) and ferrihydrite successfully reproduced the batch experiment results. The kinetic constant for precipitation of ferrihydrite and ferric arsenate obtained by fitting of the batch experiment results successfully reproduced the column experiment results. The ratio of surface precipitation of ferric arsenate and As(V) surface complexation with ferrihydrite obtained by the constructed model was the same as in the XANES results. In the column study, a slow flow was advantageous for As(V) removal because surface precipitation of ferric arsenate took a long time.

Sorption studies of 4-NP onto goethite: effects of contact time, pH, concentration, ionic strength and temperature

The effects of different state variables (contact time, pH, concentration, ionic strength and temperature) were investigated on the adsorption of 4-nitrophenol (4-NP) onto goethite for its potential utilization in mitigating water pollution. The results suggested that the adsorption of 4-NP reached a quasi-steady state equilibrium at 24 h and followed pseudo-second order kinetics. The computed value of pseudo-second order rate coefficient for adsorption of 4-NP on goethite was 447.57 kg mM−1 h−1. Adsorption of 4-NP on goethite was higher when the solution pH was < pHzpc of goethite and near pKa(7.15)of 4-NP and vice versa. The effect of concentration of 4-NP on adsorption revealed the heterogeneity of sorption sites on goethite surface; highly selective sites of higher bonding energy up to sorption density of 1.42 mM kg−1 and lower selectivity sites of lesser bonding energy for sorption density above 2.07 mM kg−1. An increase in ionic strength, increased the adsorption of 4-NP on goethite but an increase in temperature decreased the adsorption. The changes in thermodynamic parameters indicated that adsorption of 4-NP was a spontaneous and exothermic reaction with an increase in the entropy of the system. Based on these results, the optimal conditions for the maximal removal of 4-NP were 6.7–7.0 pH, 0.015–0.02 ionic strength, 15 °C temperature with 1: 100 goethite (sorbent)-solution ratio.

Macroscopic and microscopic behaviors of Mn(II) (ad)sorption to goethite with the effects of dissolved carbonates under anoxic conditions

Manganese and its compounds have been extensively studied because of their far-reaching roles in a wide range of biogeochemical processes in natural systems. The (ad)sorption behavior of Mn(II), however, is poorly understood despite its important role as the primary reaction step for surface-catalyzed Mn(II) oxidation that is the principal abiotic process forming various Mn (oxyhydr)oxides in nature. Here, we systematically examined Mn(II) (ad)sorption to one of the most common natural sorbents, goethite, in oxygen- and carbonate-free systems. Traditional sorption edge and isotherm experiments were conducted by varying sorbate-to-sorbent ratio (0.027–15 μmol∙m−2) and solution pH (pH 5.0–9.0). The effects of dissolved carbonates on Mn(II) sorption were also assessed in a range of naturally prevalent concentrations (0.5–10 mM NaHCO3). The Mn(II) uptake on goethite followed a typical Langmuir isotherm in the absence of dissolved carbonates, with increasing maximum adsorption capacities (Γmax) from 0.19 at pH 6.5 to 3.4 μmol∙m−2 at pH 9.0. The presence of dissolved carbonates raised the extent of Mn(II) adsorption, which appeared to be directly correspondent to that of the adsorption of dissolved carbonates. Extended X-ray absorption fine structure (EXAFS) analysis indicated that Mn(II) predominantly formed inner-sphere binuclear bidentate surface complexes. Mn(II) uptake became deviated from the Langmuir model and showed a clear indication of surface precipitation when the Mn(II) sorption density (Γ) exceeded a threshold value in a given solution composition. This secondary Mn(II) phase was identified as rhodochrosite using X-ray diffraction (XRD) and transmission electron microscope (TEM). Additionally, it would be plausible that a minor fraction of adsorbed Mn(II) coexisted with the secondary rhodochrosite according to a linear combination fitting (LCF) of the X-ray absorption near edge structure (XANES) spectra of Mn reference and samples. These systematic investigations of the macroscopic and microscopic behaviors of Mn(II) (ad)sorption to goethite provide a critical avenue for disentangling surface-catalytic Mn(II) oxidation processes, which ultimately lead to the formation of diverse Mn (oxyhydr)oxides in the environment.

Sorption capacity of seaweed-like sodium titanate mats for Co2+ removal.

The development of new technologies for securing and recycling water resources are in high demand. A key focus of these technologies is the development of various ion exchangers or adsorbents that are used for the purification of aqueous solutions. Layered sodium titanate is one of the cation exchangers utilised in the removal of heavy metals and radionuclides from wastewater. To enhance the removal efficiency, the precise design of the crystal morphology, structure, and chemical composition is important. Herein, we synthesised a unique seaweed-like sodium titanate mat (SST) using a template-free alkaline hydrothermal process. The Co2+ sorption capacity of SST was investigated by batch testing with cobalt(ii) nitrate. SST, which was synthesised from titanium sulphate in a 10 M NaOH solution at 200 °C, had a seaweed-like structure composed of randomly distributed nanofibres of layered sodium titanate that is approximately 9 nm in diameter. The crystal shape changed from roundish crystals to fibrous crystals as the hydrothermal reaction period increased. The Co2+ sorption isotherm of SST was fitted with the Langmuir isotherm model and the maximum sorption density was 1.85 mmol g−1. The selectivity of the Co2+ sorption on SST was high in comparison to that of Ca2+ and Mg2+. Herein, the Co2+ sorption mechanisms of SST were studied in comparison with commercially available sodium titanate. Results show that controlling the crystal morphology, structure, and Na concentration of the layered titanate that can be ion-exchanged determines the cation sorption properties of sodium titanate.

A novel composite of layered double hydroxide/geopolymer for co-immobilization of Cs+ and SeO42− from aqueous solution

Geopolymers are always considered as promising materials for the treatment of radioactive wastes. In order to extend the application of geopolymer to the immobilization of anionic species, a novel composite of layered double hydroxide/geopolymer (LDH/GEO) was synthesized and applied for cosorption of Cs+ and SeO42−. The ability of LDH/GEO to sorb Cs+ was maintained as that of pure GEO, even though the surface of geopolymer was homogeneously covered by the LDH platelets. The sorption of Cs+ onto LDH/GEO composite occurred via ion exchange, which was controlled by particle diffusion. It is different with Cs+ sorption onto pure GEO governed by film diffusion. Therefore, “Pocket diffusion” was proposed for the particle diffusion as the case of LDH/GEO because this kind of diffusion would be restricted in a certain distance around the ring entrance gate due to the amorphous essence of GEO. For SeO42− sorption by LDH/GEO, ion-exchange with the interlayer NO3− and surface sorption could be the main mechanisms. Importantly, the sorption speed of SeO42− achieved by LDH/GEO composite was much faster than that by pure LDH. In the binary system (Cs++ SeO42−), the sorption of Cs+ was slightly suppressed compared to the single system, which might be due to the formation of ion-pair complex of [CsSeO4]−. However, it did not have negative effect on the SeO42− sorption. In the presence of other cations or anions, the cosorption performances of Cs+ and SeO42− were satisfactorily obtained. Furthermore, the Cs+ and SeO42- sorption densities were superior to the previously reported values. The combined MgAl-LDH/geopolymer composite could be a promising material for the immobilization of Cs+ and SeO42−, and this work would provide guidance for the development of geopolymer-based materials for environmental applications.

Arginine and lysine-functionalized layered double hydroxides as efficient sorbents for radioactive Co2+ removal by chelate-facilitated immobilization

An increase in heavy metal contamination in aquatic environments require an efficient sorbent for the removal and reuse of toxic elements. We attempted to synthesize arginine/lysine-functionalized MgAl LDHs in one-pot without using any hazardous alkaline reagents. The LDHs produced at lower temperatures showed larger numbers of amino acids on their surfaces, while these are exchanged with CO32− at higher temperatures. The arginine/lysine present on the surface of LDH enhanced the adsorption of Co2+ and showed the highest adsorption capacity of 1.159 and 1.170 mmol/g for the LDHs functionalized with lysine and arginine, respectively. Kinetics studies indicated that the adsorption of Co2+ occurred by multiple mechanisms. The Co2+ adsorption on these amino acid functionalized LDHs occurs by the formation chelation complex with amino acid, which provide better vicinity of Co2+ to basic LDH that facilitating the enhanced immobilization. The sorption of other divalent metal ions on these arginine/lysine functionalized LDHs followed the order of Co2+ > Ni2+ > Mn2+ > Fe3+. The Co2+ forms diamine-like coordination that is stable on the surface of LDH and causes higher sorption densities, while other metals form partial glycine-like coordination that detaches the amino acid from the surface of LDH, thereby leading to lesser sorption capacity. The conversion of anionic LDH in to a cationic sorbent was successfully fabricated by these arginine/lysine-assisted methods and explored for the remediation of Co2+ from aqueous solution.

Effects of Myo-inositol Hexakisphosphate on Zn(II) Sorption on γ-Alumina: A Mechanistic Study

Myo-inositol hexakisphosphate (IHP), a most common organic phosphorus in many soils, can strongly interact with aluminum (Al) oxides and influence the fate of metal ions. In this study, the effects of presorbed IHP on γ-Al2O3 (γ-alumina) surfaces on Zn(II) sorption were investigated in batch experiments using a combination of powder X-ray diffraction (XRD), in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), 31P and 27Al solid-state nuclear magnetic resonance spectroscopies (NMR), and Zn K-edge extended X-ray absorption fine structure spectroscopy (EXAFS). The results of the batch experiments show that the presorption of IHP increases the sorption density of Zn(II) on γ-Al2O3 surfaces. The XRD data indicate that the presorption of IHP hinders the formation of Zn–Al layered double hydroxide (LDH) precipitates by raising the critical concentration of Zn(II) required to precipitate the complex. Solid-state NMR spectra further suggest that the chemical environment and speciation of IHP presorbed change, i.e., from inner-sphere surface complexes to ternary surface complexes and to zinc phytate precipitates (Zn-IHP) with the increase in Zn(II) concentration or pH. Linear combination fittings (LCFs) of the EXAFS spectra indicate that the proportion of Zn(II) in binary or ternary surface complexes decreases and that in Zn–Al LDH increases with increasing concentration of Zn(II) at pH 7. Furthermore, the order at which IHP and Zn are added in the reaction can influence the cosorption mechanism. At pH 7, more binary or ternary Zn surface complexes and Zn-IHP form, and less Zn–Al LDH precipitates form if Zn is added first. These results demonstrate that both the timing and concentration of IHP and divalent metals have sweeping influences on their solubility and speciation and these intricacies need to be taken into consideration toward predicting their fates in the environment.

From swollen gels to dried films: Relating the IR spectra of ferrihydrite dried as a film on an ATRIR crystal to aqueous suspensions

In aquatic systems ferrihydrite forms as highly porous aggregates that resemble swollen gels. Much insight into the surface chemistry of ferrihydrite has been gleaned using a technique in which ferrihydrite is dried on to the surface of a crystal that acts as an Attenuated Total Reflectance (ATR) medium for in situ infrared (IR) studies of ligand sorption. In this paper BET N2 adsorption, acid-base titration, H4SiO4 isotherms and the relationship between H4SiO4 sorption and H4SiO4 interfacial oligomerization are used to systematically probe the influence of this drying step on ferrihydrite's surface chemistry. This means that insights into sorption reactions revealed from in situ ATR-IR are can be increasingly related to real environmental systems. Comparing the measured acid-base titration curves to the theoretical curve provide a specific surface area (SSA) estimation of ≈600 and 290 m2 g−1 for the fresh and film-dried ferrihydrite respectively, compared to the BET N2 adsorption values of 240 and 140 m2 g−1 where the fresh ferrihydrite was freeze dried prior to BET measurement. The H4SiO4 isotherms were identical on the two ferrihydrite samples when normalized to the titration derived surface areas. The extent of interfacial H4SiO4 oligomerization provides insights into surface structures. Within experimental uncertainty, the process of drying ferrihydrite into a film did not alter either the general trend in the relationship between sorption and oligomerization, or the extent of oligomerization as a function of surface coverage normalized to the titration derived surface areas. Therefore, in comparable laboratory studies where sorption densities for dried films are compared with fresh suspensions the SSA values of 290 and 600 m2 g−1 respectively should be a reasonable estimate.

Opposite effects of dissolved oxygen on the removal of As(III) and As(V) by carbonate structural Fe(II)

Freshly prepared carbonate structural Fe(II) (CSF) was used to immobilize As(III) and As(V) in wastewater under oxic and anoxic conditions. Dissolved oxygen was found to exert opposite effects on these two arsenic species. The sorption density of As(III) was higher under oxic conditions, whereas that of As(V) was higher under anoxic conditions. X-ray diffraction and infrared spectroscopic analyses indicated that crystalline parasymplesite (Fe(II)3(AsO4)2·8H2O) was formed when As(V) was removed under anoxic conditions, while an amorphous Fe-As-containing precipitate was formed when As(III) was removed under oxic conditions. The distribution of arsenic and iron between the solution and sediments suggested that the oxidation of structural Fe(II) promoted coprecipitation process and inhibited surface complexation. X-ray photoelectron spectroscopic analyses revealed that more As(III) was oxidized under oxic condition, which contributed to a higher sorption capacity for As(III). The formation of parasymplesite through surface complexation/precipitation was proposed to be more effective for the removal of As(V) by CSF, while As(III) was more efficiently removed through coprecipitation. Together, the results suggest that CSF may be an effective material for sequestering both As(III) and As(V). In addition, attention should be paid to the dissolved oxygen content when remediating different arsenic species.

An EXAFS investigation of the mechanism of competitive sorption between Co(II) and Ni(II) at γ-alumina/solution interface

Co(II) and Ni(II) are two common toxic heavy metals, and may simultaneously exist in contaminated water, soil, and sediment systems in Earth’s surface environment. Under this circumstance, competitive adsorption between the two metals may influence their migration, toxicity, and bioavailability. In this research, the competitive sorption of Co(II) and Ni(II) on γ-Al2O3 was studied using both macroscopic sorption experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy. Results suggest that Ni(II) reduced the amount of Co(II) sorption in a binary-solute system at pH 6.0. This is because both Co(II) and Ni(II) form inner-sphere surface complexes during sorption on γ-Al2O3 and compete for the surface reactive sites. However, Co(II) exhibited a negligible influence on sorption amount of Ni(II) under the same conditions, which suggests Ni(II) has a stronger affinity to alumina surface. At pH 7.5, Co(II) and Ni(II) sorption density were much higher than that at pH 6.0, but there no mutual competitive effect was observed. EXAFS analysis further revealed that formation of layered double-hydrated precipitates was the dominant sorption mechanism for both Co(II) and Ni(II) at pH 7.5. Because this type of sorption does not rely on surface reactive sites, there was no competition between Co(II) and Ni(II). This finding sheds light on risk assessment and remediation of Ni/Co pollution.

Quantitative and spectroscopic investigations of the co‐sorption of myo‐inositol hexakisphosphate and cadmium(II) on to haematite

SummaryThe interactions between phosphates, both inorganic and organic ones, and heavy metals on mineral surfaces affect their environmental fate and transport. In this research, we investigated the co‐sorption of myo‐inositol hexakisphosphate (IHP) and cadmium (Cd(II)) on to haematite at various pHs in batch experiments, zeta (ζ) potential measurement and in situ attenuated total reflectance Fourier transform infrared (ATR‐FTIR) spectroscopy. In the haematite–IHP binary system, IHP formed inner‐sphere complexes on haematite in which some phosphate groups of the IHP molecule bonded with the haematite surface, and other dissociated phosphate groups were ionized. In the haematite–IHP–Cd ternary system, the presence of aqueous Cd(II) increases IHP sorption and the ζ potential of haematite relative to that in the haematite–IHP binary system. The enhancement is more obvious at higher pHs, reversing the initial pH dependence of IHP sorption in the binary system. Similarly, the coexisting IHP promotes the retention of Cd(II) on haematite. The ATR‐FTIR spectroscopy indicates the formation of two structurally distinct ternary surface complexes (the Hm–IHP–Cd surface complex and the Hm–Cd–IHP–Cd surface complex). Our results suggest that simultaneous sorption not only increases the sorption density of IHP and Cd(II), but also changes the surface complex configuration of IHP and Cd(II) on haematite.Highlights The interactions between IHP and Cd(II) on haematite were investigated. The presence of Cd(II) increases IHP sorption and vice versa. IHP and Cd(II) co‐sorption is enhanced as pH increases. Two structurally distinct ternary surface complexes form on haematite surfaces.

Sorption mechanisms of chromate with coprecipitated ferrihydrite in aqueous solution

Hexavalent chromium (Cr(VI)) attracted researchers’ interest for its toxicity, natural availability and removal difficulty. Nevertheless, its sorption mechanism is not clearly understood yet. In this work, we elucidated the sorption mechanism of the co-precipitation of chromates with ferrihydrite through quantitative analysis. The influence of Cr/Fe molar ratio on sorption was investigated by zeta potential measurements, X-ray diffraction (XRD) and X-ray adsorption fine-structure analysis (XAFS). Coprecipitation at pH 5 showed almost twice the sorption density of adsorption at pH 5. In co-precipitation, a shift of the XRD peak due to inner-sphere sorption of chromate was observed at Cr/Fe molar ratio 0.5. For adsorption, the same peak shift was confirmed at Cr/Fe molar ratio of 1. Zeta potential at pH 5 suggested that the sorption mechanism changed at Cr/Fe molar ratio 0.25 for coprecipitation and at Cr/Fe molar ratio of 1 for adsorption. Fitting of Cr and Fe K-edge extended X-ray adsorption fine-structure suggested that ferrihydrite immobilized Cr(VI) via outer sphere surface complexation for lower Cr/Fe ratios and via inner-sphere surface complexation for higher molar ratios. At higher molar ratios, bidentate binuclear CrFe bonds were well established, thus resulting in the expansion of the ferrihydrite structure.

Macroscopic and microscopic investigation of adsorption and precipitation of Zn on γ-alumina in the absence and presence of As

Contaminants zinc (Zn) and arsenate (As) often coexist in soils. However, little is known concerning the impacts of coexisting As on Zn adsorption and precipitation on soil minerals. In the present study, adsorption and precipitation of Zn on γ-alumina in the absence and presence of arsenate was investigated employing batch experiments and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Results indicated that Zn formed edge-sharing tetrahedral surface complexes at pH 5.5 and Zn-Al LDH-like (layered double hydroxide) precipitates at pH 7.0 on the surface of γ-alumina. The presence of arsenate significantly enhanced Zn sorption densities, and remarkably changed its bonding environment. At pH 5.5, SR-XRD (Synchrotron Radiation-based X-ray Diffraction) and EXAFS showed that koettigite-like precipitate were formed in the cosorption of Zn and As on γ-alumina regardless of the addition sequence of As and Zn. At pH 7.0, when Zn was preequilibrated with γ-alumina prior to the As introduction, mixed Zn-Al LDH-like and amorphous adamite-like precipitates formed. However, when Zn and As were added simultaneously, only amorphous adamite-like precipitate was observed. Zn inner-sphere complexes and surface ternary complexes γ-alumina-As-Zn were the main outcome when As was preequilibrated firstly. Zn-arsenate precipitates could significantly decrease the concentration of Zn in aqueous solution and decrease the bioavailability and mobilization of Zn in soils.