Adsorption Of Phosphate Anions Research Articles

Impact of silt chemical composition on deflocculation and coagulation of clay-rich paste

This paper investigates the impact of the chemical composition of two silts, quartz and limestone, on the deflocculation/coagulation process for poured earth application. The driving mechanisms behind the evolution of the rheological behavior after the addition of sodium hexametaphosphate are highlighted through zeta potential measurementand the chemical changes with ICP-OES and 31P MAS NMR spectroscopy. The first results show that adding sodium hexametaphosphate into the quartz silt modifies its inter-particle forces through electrostatic repulsion forces induced by the adsorption of phosphate anions onto the quartz structure. For limestone paste, the deflocculation mechanism is affected by the dissolution of the calcite. This dissolution leads to a release of Ca2+ cations and an increase in the paste's pH, reducing the paste deflocculation rate. These dissolved Ca2+ cations in the solution conduct to a partial coagulation making the delayed flocculation targeted initially with the addition of magnesium oxide less effective.

Antipoisoning catalysts for the selective oxygen reduction reaction at the interface between metal nanoparticles and the electrolyte

AbstractOne of the primary challenges in relation to phosphoric acid fuel cells is catalyst poisoning by phosphate anions that occurs at the interface between metal nanoparticles and the electrolyte. The strong adsorption of phosphate anions on the catalyst surface limits the active sites for the oxygen reduction reaction (ORR), significantly deteriorating fuel cell performance. Here, antipoisoning catalysts consisting of Pt‐based nanoparticles encapsulated in an ultrathin carbon shell that can be used as a molecular sieve layer are rationally designed. The pore structure of the carbon shells is systematically regulated at the atomic level by high‐temperature gas treatment, allowing O2 molecules to selectively react on the active sites of the metal nanoparticles through the molecular sieves. Besides, the carbon shell, as a protective layer, effectively prevents metal dissolution from the catalyst during a long‐term operation. Consequently, the defect‐controlled carbon shell leads to outstanding ORR activity and durability of the hybrid catalyst even in phosphoric acid electrolytes.

Quaternary amine functionalized chitosan for enhanced adsorption of low concentration phosphate to remediate environmental eutrophication

Phosphate contamination of natural waterways from agricultural runoff is a major global issue due to its widespread occurrence and severe environmental effects, namely accelerated eutrophication, and subsequent negative impacts on aquatic ecosystems. This study reports on the synthesis of a biopolymer capable of remediating environmentally relevant concentrations of phosphate: a glutaraldehyde crosslinked chitosan, quaternised with 2-chloro-N,N-diethylethylamine hydrochloride (DEAE). This modification generated a notable increase in the overall positive surface charge on the chitosan promoting adsorption of phosphate anions. The chemical transformations on the adsorbent’s surface are evidenced by the significant shift in zeta potential, increasing from –12 eV to +27 eV at pH 7, and analysis of the XPS spectra showed a substantial increase in the N1s peak centred at 402 eV, corresponding to the positively charged amine species. Langmuir isotherm modelling indicated the maximum capacity for phosphate to be 20.1 mg P/g and a Langmuir constant of 1.15 L/mg. Other adsorption studies were designed to focus on the identified issue of low concentration phosphate removal and demonstrated the modified-chitosan to be a highly effective adsorbent: using an adsorbent dose of 2 g/L resulted in 97% removal, leaving less than 0.1 ppm P in solution. A higher removal, 98.4%, was achieved at a dose of 6 g/L, however this represented considerable diminishing returns. Furthermore, there was no measurable loss of the intrinsic adsorption ability after 9 adsorption-desorption cycles showing exceptionally effective regeneration using 0.1 M NaCl.

Enhanced phosphate removal with fine activated alumina synthesized from a sodium aluminate solution: performance and mechanism.

Fine activated alumina (FAA) acting as an adsorbent for phosphate was synthesized from an industrial sodium aluminate solution based on phase evolution from Al(OH)3 and NH4Al(OH)2CO3. This material was obtained in the form of γ-Al2O3 with an open mesoporous structure and a specific surface area of 648.02 m2 g−1. The phosphate adsorption capacity of the FAA gradually increased with increases in phosphate concentration or contact time. The maximum adsorption capacity was 261.66 mg g−1 when phosphate was present as H2PO4− at a pH of 5.0. A removal efficiency of over 96% was achieved in a 50 mg L−1 phosphate solution. The adsorption of phosphate anions could be explained using non-linear Langmuir or Freundlich isotherm models and a pseudo-second-order kinetic model. Tetra-coordinate AlO4 sites acting as Lewis acids resulted in some chemisorption, while (O)nAl(OH)2+ (n = 4, 5, 6) Brønsted acid groups generated by the protonation of AlO4 or AlO6 sites in the FAA led to physisorption. Analyses of aluminum-oxygen coordination units using Fourier transform infrared and X-ray photoelectron spectroscopy demonstrated that physisorption was predominant. Minimal chemisorption was also verified by the significant desorption rate observed in dilute NaOH solutions and the high performance of the regenerated FAA. The high specific surface area, many open mesopores and numerous highly active tetra-coordinate AlO4 sites on the FAA all synergistically contributed to its exceptional adsorption capacity.

Evaluation of phosphate adsorption by porous strong base anion exchangers having hydroxyethyl substituents: kinetics, equilibrium, and thermodynamics.

Phosphate anions are recognized as the main responsible for the eutrophication of surface waters. In this work, two strong base anion exchangers having either N,N-dimethyl 2-hydroxyethylammonium (SBAEx.2M) or N,N-diethyl 2-hydroxyethylammonium (SBAEx.2E) functional groups, as highly efficient sorbents in the removal of phosphate anions, are presented. The influence of the main parameters (pH, contact time, initial concentration of phosphate, temperature) on the adsorption performances was investigated in batch mode. Modeling the kinetics data by Lagergren, Ho and McKay, and Elovich kinetic models indicated chemisorption as the main mechanism of sorption. The sorption at equilibrium was modeled with Langmuir, Freundlich, Sips, Dubinin-Radushkevich, and Temkin isotherm models. The experimental isotherms were the best fitted by Langmuir and Sips isotherms, the maximum sorption capacity for phosphate anions being 233.88 mg g-1 SBAEx.2M and 223.5 mg g-1 SBAEx.2E, at pH 3, and 23 °C. Adsorption of phosphate anions in competitive conditions showed that the interference with co-existing anions was low in the case of Cl- ions and much higher with SO42- ions, the ion exchange having an important contribution in the adsorption process. The adsorption was spontaneous and endothermic, the degree of spontaneity increasing with the increase of temperature. The high level of reusability, the adsorption capacity decreasing with only ~ 7% in the case of SBAEx.2E and with ~ 9% in the case of SBAEx.2M, after five sorption/desorption cycles, recommends these SBAEx as promising adsorbents for phosphate removal.

Adsorption of phosphate by halloysite (7 Å) nanotubes (HNTs)

AbstractThe adsorption and retention of phosphates in soil systems is of wide environmental importance, and understanding the surface chemistry of halloysite (a common soil clay mineral) is also of prime importance in many emerging technological applications of halloysite nanotubes (HNTs). The adsorption of phosphate anions on tubular halloysite (7 Å) has been studied to gain a greater understanding of the mechanism and kinetics of adsorption on the surface of HNTs. Two well-characterized tubular halloysites with differing morphologies have been studied: one polygonal prismatic and one cylindrical, where the cylindrical form has a greater surface area and shorter tube length. Greater phosphate adsorption of up to 42 μmol g–1 is observed on the cylindrical halloysite when compared to the polygonal prismatic sample, where adsorption reached a maximum of just 15 μmol g–1 compared to a value for platy kaolinite (KGa-2) of 8 μmol g–1. Phosphate adsorption shows strong pH dependence, and the differences in phosphate sorption between the prismatic and cylindrical morphologies suggest that phosphate absorption does not occur at the same pH-dependent alumina edge sites and that the lumen may have a greater influence on uptake for the cylindrical form.

Anion Resistant Oxygen Reduction Electrocatalyst in Phosphoric Acid Fuel Cell

Phosphoric acid fuel cells are successfully used as energy conversion technologies in stationary power applications. However, decreased proton conductivity and lower oxygen permeability of phosphoric-acid-imbibed membranes require prohibitive loadings of the traditional noble-metal-based electrocatalyst, such as platinum supported on carbon. Additionally, specific adsorption of phosphate anions on the catalyst results in a surface poisoning that further reduces electrocatalytic activity. Here we report a nonplatinum group metal (non-PGM) electrocatalyst as an alternative cathode electrocatalyst for oxygen reduction in phosphoric acid fuel cells. The non-PGM was prepared in a one-pot synthesis using a metal organic framework and iron salt precursor. Phosphate anion poisoning was monitored electrochemically and spectroscopically in reference to the current state-of-the-art Pt-based catalyst at room temperature. Unlike Pt-based catalysts that are prone to phosphate poisoning, the non-PGM electrocatalyst exhi...

Efficient removal of toxic phosphate anions from aqueous environment using pectin based quaternary amino anion exchanger

Pectin based quaternary amino anion exchanger (Pc-QAE) was prepared using simple crosslinking polymerization method. This anion exchanger was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Pc-QAE was applied for the removal of phosphate anion from the aqueous solution. The adsorption process which was pH dependent showed maximum adsorption of phosphate anions at pH 7. Pc-QAE showed good monolayer adsorption capacity for phosphate anions which demonstrated its good capability towards Langmuir isotherm model. Moreover, the adsorption was evaluated thermodynamically and the negative value of Gibbs free energy (−1.791KJ/mol) revealed the spontaneity of adsorption process. The value of ΔH° and ΔS° were found to be 15.28 and 49.48KJ/mol, respectively representing the endothermic nature and enhancement in degree of freedom due to the adsorption process.

Phosphate-mediated electrochemical adsorption of cisplatin on gold electrodes

This manuscript reports the potential-dependent adsorption and deposition of cisplatin on polycrystalline gold electrode. It was found that this process is mediated by the adsorption of phosphate anions on the gold electrode and that the maximum coverage of Pt adsorbed is given by the maximum coverage of phosphate adsorbed at a given potential. The interaction of cisplatin with the phosphate groups was confirmed by in situ FTIR spectroscopy under external reflexion configuration. Quantitative analysis suggests that the stoichiometry of the phosphate species and the cisplatin adsorbed was 1:1. Moreover, the relationship between the charge of the Pt deposited and the charge of the electrochemical surface area of the Pt deposited on the gold electrodes indicates that 3D nanoclusters of a few atoms of Pt were formed over the gold electrode upon the electrochemical reduction of the adsorbed cisplatin.The Pt nanoclusters formed under these conditions were later evaluated for the oxidation of a monolayer of carbon monoxide. The Pt nanoclusters showed a high overpotential for the oxidation of the carbon monoxide monolayer and the high oxidation overpotential was attributed to the absence of adsorption sites for OH species on the Pt clusters: only at potentials where the OH species are adsorbed at the edge between the Pt nanocluster and the gold support, the oxidation of the carbon monoxide on the Pt nanoparticles takes place.

Deciphering the Platinized Surface Reactivity to Improve the Detection of Hydrogen Peroxide in Bioanalyses

AbstractThe reactivity of platinized electrode surface (Pt‐black) towards the oxidation of hydrogen peroxide is analyzed in the most common physiological buffer, namely phosphate buffer saline, used in the development of biosensors. It is shown here that the oxidation of hydrogen peroxide by Pt‐black involves first the adsorption of phosphate anions, which facilitates the oxidation of hydrogen peroxide at lower potentials when compared to classic polycrystalline platinum. This gives insight in the excellent analytical features of platinized electrodes for the detection of H2O2, even at nanomolar concentrations. The reactivity of platinized electrodes can be further improved by an oxygen plasma treatment. We demonstrate herein that such treatment creates an outermost layer of PtO2 oxides on the deposit, which improves the oxidation of hydrogen peroxide thermodynamically and kinetically, as well as of hydroquinone and superoxide radical anion.

Effect of Small Organic Acid Anions on the Adsorption of Phosphate Anions onto Synthetic Goethite from Aqueous Solution

Batch experiments were conducted to study the effect of small organic acids (citric, oxalic and acetic) on the removal of phosphate anions from aqueous systems using synthesized goethite as the adsorbent. The results showed that phosphate anion adsorption was most significantly inhibited by citric among the three organic acids studied. Phosphate anion adsorption was affected at pH values below 6.5. The relationship between citrate anion-promoted goethite dissolution and citrate anion-suppressed phosphate anion adsorption in equilibrated goethite-phosphate anion-citric acid systems with different contact orders was studied. The results indicated that there is an obvious correlation between the suppression of phosphate anion adsorption and the dissolution of goethite. It is proposed that competition for binding sites play a key role in the reduction of phosphate anion adsorption by citric acid.

Hydrolysis of cellobiose by β-glucosidase in the presence of soil minerals – Interactions at solid–liquid interfaces and effects on enzyme activity levels

Abstract Extracellular enzymatic activities in soils are essential for the cycling of organic matter. These activities take place in multiphase environments where solid phases profoundly affect biocatalytic activities. Aspergillus niger is ubiquitous in soils; its β-glucosidase plays an important role in the degradation of cellulose, and therefore in the global carbon cycle and in the turnover of soil organic matter. However, the information on the interactions of this protein with soil minerals is very limited, and even less is known about their consequences for the hydrolysis of the natural substrate cellobiose. We therefore characterised the sorptive interactions of this enzyme with the soil minerals montmorillonite, kaolinite and goethite and quantified the resulting changes in the hydrolysis rate of cellobiose. Fractions of adsorbed protein, and the resulting catalytic activity loss, were lower for montmorillonite than for kaolinite and goethite at given experimental conditions; adsorption was 9.7 ± 7.3% for montmorillonite, 70.3 ± 3.1% for kaolinite and 71.4 ± 1.8% for goethite, respectively. Adsorption of the protein to the minerals caused a total decrease in the catalytic activity of 18.8 ± 3.4% for kaolinite and 17.9 ± 4.7% for goethite whereas it was not significant for montmorillonite. The average catalytic activity lost by the pool of adsorbed molecules was 26.8% for kaolinite and 25.0% for goethite. Both the amount of adsorbed protein and the resulting loss of catalytic activity were found to be independent of the specific surface areas yet were influenced by the electrical properties of the mineral surfaces. Under the experimental conditions, montmorillonite and kaolinite are negatively charged whereas goethite is positively charged. However, because of the adsorption of phosphate anions from the buffer, a charge reversal took place at the surface of goethite. This was confirmed by zeta ( ζ )-potential measurements in phosphate buffer, revealing negative values for all the tested minerals. Indeed goethite interacted with the enzyme as a negatively charged surface: the amount of adsorbed protein and the resulting catalytic activity loss were very similar to those of kaolinite. Our results show that, even if an important fraction of β-glucosidase is adsorbed to the minerals, the catalytic activity is largely retained. We suggest that this strong activity retention in presence of soil minerals results from a selective pressure on A. niger , which benefits from the activity of the adsorbed, and thus stabilized, enzyme pool.

Adsorption of phosphate species on poly-oriented Pt and Pt(1 1 1) electrodes over a wide range of pH

The adsorption of phosphate anions from phosphate solutions at poly-oriented and single-crystal platinum electrodes, primarily Pt(1 1 1), was studied over a wide range of pH by cyclic voltammetry. The features observed at the poly-oriented Pt electrode in phosphate solution may be related to the different crystalline facets, the (1 1 1) orientation presenting the most significant behavior in terms of phosphate adsorption. On the reversible hydrogen electrode (RHE) scale, the phosphate adsorption strength decreases with increasing alkalinity of the solution. Qualitatively, three different pH regions can be distinguished. At pH < 6 only a broad reversible peak is observed, corresponding to the adsorption of H 2PO 4 − and further deprotonation to adsorbed HPO 4 −. For 6 < pH < 11 a butterfly feature followed by one or two anodic peaks (depending on scan rate) is observed, ascribed to the adsorption of HPO 4 − followed by its subsequent deprotonation to adsorbed PO 4 3−. The splitting into two or three voltammetric features, and the irreversibility of the two features at more positive potential, is ascribed to the deprotonation reaction leading to a surface species (i.e. phosphate) which needs to change its surface coordination. At pH > 11 a reversible pre-wave and a sharp spike are observed, ascribed to the co-adsorption of phosphate and hydroxide.

Influence of phosphate anion adsorption on the kinetics of oxygen electroreduction on low index Pt(hkl) single crystals

The detrimental effects of phosphate anion adsorption on the oxygen reduction reactions (ORR) on low index Pt single crystal electrodes were studied in 0.1 M perchloric acid by using a hanging meniscus rotating disk electrode in the presence of varied concentrations of H(3)PO(4). The kinetic current for ORR decreased dramatically on Pt(100), Pt(110), Pt(111), and PtSn(111) even with the addition of a small amount (1 mM) of H(3)PO(4) into the perchloric acid solution, most probably due to the adsorption of phosphate anions onto the Pt active sites that impeded the electroreduction of O(2). Remarkably, the extent of decline was found to vary with the specific single crystal surface, following the order of Pt(111) > PtSn(111) > Pt(110) ∼ Pt(100). Consistent behaviors were also observed in Tafel analysis and in electrochemical impedance spectroscopic measurements. Within the present experimental context, Pt(110) was found to be the optimal crystal surface for ORR in phosphoric acid fuel cells with the smallest charge transfer resistance, whereas the poisoning effects of phosphate anion adsorption were the most pronounced on Pt(111), most likely because the phosphate anions primarily adsorbed on the 3-fold sites on the Pt(111) faces, as manifested in in situ X-ray absorption spectroscopic measurements.

Highly Efficient and Selective Enrichment of Phosphopeptides Using Porous Anodic Alumina Membrane for MALDI-TOF MS Analysis

Because of its good biocompatibility, high surface-to-volume ratio, and distinct surface electrical properties, porous anodic alumina (PAA) membrane has been used to selectively enrich phosphopeptides from a mixture of synthetic peptides and tryptic digest product of beta-casein by a direct MALDI-TOF MS analysis. As we reported previously, PAA membrane has strong incorporation ability to the phosphate anion. Herein, we describe the application of PAA membrane as a selective sampling absorbent for phosphopeptides. The PAA membrane could enrich phosphopeptides with high efficiency and selectivity; for example, the tryptic digest product of beta-casein at a concentration as low as 4 x 10(-9) M can be satisfactorily detected. Compared to that from the nonenriching peptide mixture, the MS signal of the phosphorylated peptides enriched by the PAA membrane is remarkably improved. In addition, acidic peptides have insignificant influence on the enriching process. Results show that the adsorption of phosphate anions on the PAA membrane plays a determining role in achieving highly selective enriching capacity toward phosphopeptides. The feasibility of PAA membranes as specific absorbents for phosphopeptides is also demonstrated.

Formation of cellular films consisting of wurtzite-type zinc oxide nanosheets by mediation of phosphate anions

We successfully prepared a wide variety of wurtzite-type zinc oxide films exhibiting columnar, cellular and densely packed morphologies in a simple aqueous solution system containing phosphate anions. As the phosphate concentration increased, the shape of crystalline units composing the films varied from hexagonal needles into seaweed-like sheets. A novel type of open cellular structures was obtained by assembly of nanoscale zinc oxide sheets covered with phosphate. Specific adsorption of phosphate anions on (001) of the wurtzite-type crystal flattened the crystal grains, and then induced the structural evolution into a cellular form. A blue shift of the absorption edge suggested that the quantum size effect occurred in the nanoscale platy crystals composing the cellular films.

Specific adsorption of anions on a copper (100) single crystal electrode studied by charge displacement by CO adsorption and infrared spectroscopy

Specific adsorption of anions on a Cu(100) electrode was studied with 0.2 mol/dm 3 solutions at 0 °C by voltammetric and FTIR spcetroscopic techniques. Voltammetric redox waves of specific adsorption of sulfate and phosphate anions at pH 3 on the Cu(100) electrode are separated from hydrogen evolution reaction ( her) and is clearly shown by the charge displacement adsorption of CO. Perchlorate anion is not specifically adsorbed on the Cu(100) electrode, and no redox wave is observed. Specific adsorption of phosphate anions on the Cu(100) was studied in more detail in pH ranging from 1.5 to 12.9. The redox potential of the charge displacement CO adsorption–desorption depends on pH with the slope of ca. 30 mV/pH. Under the assumption of electrochemical equilibrium, PO 4 3− and/or HPO 4 2− are concluded as the adsorbed phosphate species on the Cu(100) electrode in the pH range studied. Most of the negative charge of the phosphate anions is withdrawn by the electrode, bringing the p K a value of the anions significantly down in the specific adsorption process.

Surface-Enhanced Raman Spectroscopy of Phosphate Anions: Adsorption on Silver, Gold, and Copper Electrodes

The adsorption of phosphate anions on Ag, Au, and Cu electrodes from H2O and D2O solutions has been studied by means of surface-enhanced Raman spectroscopy (SERS). The interpretation of the spectra based on the solution Raman data and frequency shifts upon solution H2O/D2O exchange are presented. The prominent band at 1070−1100 cm-1, observed from adsorbed PO43- and HPO42- ions, exhibits downshifts of about 10 cm-1 in D2O solutions and has been assigned to the asymmetric P−O stretching mode. The corresponding asymmetric deformation mode has been assigned to the band located at ∼570 cm-1 which shows an upshift of 9−15 cm-1 in D2O solutions. Monodentate surface coordination of the PO43- and HPO42- ions is proposed. The dependence of the relative intensity of the internal modes on electrode potential was interpreted in terms of the migration of P−O groups from the surface as potential became more negative. Spectroscopic evidence was found for chemisorption of H2PO4- ion on the Cu electrodes, but no such evidence was found in the cases of the Au and Ag electrodes. The adsorbed H2PO4- ion on Cu showed an intense band at ∼907 cm-1 which was assigned to the symmetric stretching mode of the P−OH groups, based primarily on the considerable frequency downshift (∼11 cm-1) and peak broadening (∼15 cm-1) in D2O solutions. The formation of a P−O−P bond in the adsorbed state on Cu electrodes in acidic solutions is suggested. The force constants derived from experimental metal−oxygen frequencies are compared for the three electrodes, and the following strength order has been estimated: k(Cu−O‘) > k(Au−O‘) > k(Ag−O‘).

Phosphate Adsorption on Hematite, Kaolinite, and Kaolinite–Hematite (k–h) Systems As Described by a Constant Capacitance Model

The constant capacitance model was used to describe phosphate adsorption on hematite, kaolinite, and a kaolinite–hematite system (k–h). The model assumes a ligand exchange mechanism and considers the charge on both adsorbate and adsorbent. The model is shown to provide a quantitative description of phosphate adsorption on these, including the effect of varying pH values. The computer program Ma-Za 2, a program that fits equilibrium constants to experimental data using an optimization technique, was used to obtain optimal values for the anion surface complexation constants on hematite, kaolinite, and a kaolinite–hematite system, while the PC program Ma-Za 1 in Q-Basic language was used for the application of the constant capacitance model. The model represented adsorption of phosphate anions well over the entire pH range studied (3.8–9.0). The main advantage of the model is its ability to represent changes in anion adsorption occurring with changes in pH. Extension of the model to describe phosphate adsorption in a mixed system, such as the kaolinite–hematite system, using the surface protonation–dissociation constant of hematite was qualitatively successful. In mixed system the model reproduced the shape of the adsorption isotherms well over the pH range 3.8–9.0. However, phosphate adsorption was overestimated. The hematite and the kaolinite–hematite system were synthesized and identified by X-ray, NMR, and FT-IR spectroscopy.

Reduced bovine serum albumin adsorption by prephosphatation of powdered zirconium oxide

The adsorption of phosphate anions to ZrO 2 and its influence on protein adsorption have been studied. Phosphate adsorption experiments revealed that phosphate anions had a high affinity for zirconia: in appropriate conditions, a monomolecular layer was adsorbed. Using the electrophoretic light scattering technique, it was shown that the isoelectric point of the zirconium oxide was shifted from pH 6.4 down to pH 3.8. From this shift, it was concluded that specific interactions were responsible for the adsorption behaviour. In addition, the adsorption of bovine serum albumin (which was selected as a model protein) to both bare and phosphated zirconia was investigated: prephosphatation reduced the BSA adsorption by over 50%. Comparing the adsorption behaviour at pH 4.15 and 5.35, it was concluded that electrostatic interactions were of minor importance. From contact angle measurements, the different adsorption behaviour was shown to be due to the increased hydrophilicity of the phosphated adsorbent. On the basis of the present results, the observed effect of phosphate ions in ultrafiltration of proteins on ZrO 2 ceramic ultrafiltration membranes may be explained.