Competitive Adsorption Of Arsenate Research Articles

Dissolved phosphate decreases the stability of amorphous ferric arsenate and nano-crystalline yukonite

Extensive research has been conducted on the competitive adsorption of arsenate (AsO43-) and phosphate (PO43-) to mineral surfaces, but the stability of ferric arsenate mineral(oid)s under elevated phosphate levels remains poorly understood. Therefore, we investigated the impact of dissolved phosphate (0, 0.5, 50 mM) on the stability of amorphous ferric arsenate (AFA; FeAsO4·nH2O) and nano-crystalline yukonite [Ca2Fe3(AsO4)3(OH)4·4H2O], both synthetic and contained in natural As-contaminated soil (∼16 g/kg As) and mine-waste material (∼39 g/kg As) for up to one year. Substantial amounts of As (∼45% of total As) were released into solution from AFA and yukonite at high phosphate concentrations due to incongruent dissolution of the solids and substitution of arsenate by phosphate in both mineral(oids). After one year, both solids sequestered ∼8 wt% P with approximately 20–30% accounting for adsorbed and precipitated species. This P increase was also observed in the soil and mine-waste samples, where AFA and yukonite comprised up to 4.3 and 4.9 wt% P, respectively. The high reactivity of ferric arsenates with aqueous phosphate may lead to a substantial overestimation of adsorbed As determined by sequential As extractions of materials containing these phases and requires increased caution when applying phosphate to stabilize polymetallic mine wastes. Furthermore, long-term phosphate additions via fertilization of As-contaminated soil or renaturalized mine tailings containing amorphous or nano-crystalline ferric arsenates should be reduced to limit the export of As(V) into surface streams and groundwater.

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.

Arsenate and antimonate adsorption competition on 6-line ferrihydrite monitored by infrared spectroscopy

Arsenate and antimonate are water-soluble toxic mining waste species which often occur together and can be sequestered with varying success by a hydrous ferric oxide known as ferrihydrite. The competitive adsorption of arsenate and antimonate to thin films of 6-line ferrihydrite has been investigated using primarily adsorption/desorption kinetics monitored by in situ attenuated total reflectance infrared (ATR-IR) spectroscopy on flowed solutions containing 10−3 and 10−5molL−1 of both species at pH 3, 5, and 7. ICP-MS analysis of arsenate and antimonate adsorbed to 6-line ferrihydrite from 10−3molL−1 mixtures in batch adsorption experiments at pH 3 and 7 was carried out to calibrate the relative surface concentrations giving rise to the IR spectral absorptions. The kinetic data from 10−3 and 10−5molL−1 mixtures showed that at pH 3 antimonate achieved a greater surface concentration than arsenate after 60min adsorption on 6-line ferrihydrite. However, at pH 7, the adsorbed arsenate surface concentration remained relatively high while that of adsorbed antimonate was much reduced compared with pH 3 conditions. Both species desorbed slowly into pH 3 solution while at pH 7 most adsorbed arsenate showed little desorption and adsorbed antimonate concentration was too low to register its desorption behaviour. The nature of arsenate which is almost irreversibly adsorbed to 6-line ferrihydrite remains to be clarified.

Effect of silicic acid on arsenate and arsenite retention mechanisms on 6-L ferrihydrite: A spectroscopic and batch adsorption approach

The competitive adsorption of arsenate and arsenite with silicic acid at the ferrihydrite–water interface was investigated over a wide pH range using batch sorption experiments, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) modeling. Batch sorption results indicate that the adsorption of arsenate and arsenite on the 6-L ferrihydrite surface exhibits a strong pH-dependence, and the effect of pH on arsenic sorption differs between arsenate and arsenite. Arsenate adsorption decreases consistently with increasing pH; whereas arsenite adsorption initially increases with pH to a sorption maximum at pH 7–9, where after sorption decreases with further increases in pH. Results indicate that competitive adsorption between silicic acid and arsenate is negligible under the experimental conditions; whereas strong competitive adsorption was observed between silicic acid and arsenite, particularly at low and high pH. In situ, flow-through ATR-FTIR data reveal that in the absence of silicic acid, arsenate forms inner-sphere, binuclear bidentate, complexes at the ferrihydrite surface across the entire pH range. Silicic acid also forms inner-sphere complexes at ferrihydrite surfaces throughout the entire pH range probed by this study (pH 2.8–9.0). The ATR-FTIR data also reveal that silicic acid undergoes polymerization at the ferrihydrite surface under the environmentally-relevant concentrations studied (e.g., 1.0mM). According to ATR-FTIR data, arsenate complexation mode was not affected by the presence of silicic acid. EXAFS analyses and DFT modeling confirmed that arsenate tetrahedra were bonded to Fe metal centers via binuclear bidentate complexation with average As(V)-Fe bond distance of 3.27Å. The EXAFS data indicate that arsenite forms both mononuclear bidentate and binuclear bidentate complexes with 6-L ferrihydrite as indicated by two As(III)–Fe bond distances of ∼2.92–2.94 and 3.41–3.44Å, respectively. The As–Fe bond distances in both arsenate and arsenite EXAFS spectra remained unchanged in the presence of Si, suggesting that whereas Si diminishes arsenite adsorption preferentially, it has a negligible effect on As–Fe bonding mechanisms.

Competitive adsorption of arsenate and phosphate onto calcite; experimental results and modeling with CCM and CD-MUSIC

The competitive adsorption of arsenate and phosphate onto calcite was studied in batch experiments using calcite-equilibrated solutions. The solutions had circum-neutral pH (7–8.3) and covered a wide span in the activity of Ca2+ and CO32-. The results show that the adsorption of arsenate onto calcite is strongly reduced by the presence of phosphate, whereas phosphate adsorption is only slightly reduced by arsenate addition. Simultaneous and sequential addition (3h apart) yields the same reduction in adsorption, underlining the high reversibility of the system. The reduction in adsorption of both arsenate and phosphate is most likely due to competition for the same sorption sites at the calcite surface, considering the similarity in sorption edges, pKa’s and geometry of the two anions. The strong reduction in arsenate adsorption by competition with phosphate suggests that adsorption of arsenate onto calcite is of minor importance in most groundwater aquifers, as phosphate is often present at concentration levels sufficient to significantly reduce arsenate adsorption. The CD-MUSIC model for calcite was used successfully to model adsorption of arsenate and phosphate separately. By combining the models for single sorbate systems the competitive adsorption of phosphate and arsenate onto calcite in the binary system could be predicted. This is in contrast to the constant capacitance model (CCM) which under-predicted the competition when combining the models for single sorbate systems. This study clearly shows the importance of performing competitive adsorption studies for validation of multi-component models and for estimating the mobility of an ion in the environment.

In situ ATR–FTIR studies on the competitive adsorption of arsenate and phosphate on ferrihydrite

In the present study, in situ ATR-FTIR spectroscopy was used for the first time to study the competitive adsorption of phosphate and arsenate on ferrihydrite. Deuterium oxide was used as solvent to facilitate the interpretations of recorded infrared spectra. It was found that arsenate and phosphate adsorbed more strongly at lower pD-values, showing similarities in the adsorption behavior as a function of pD. However, arsenate complexes were found to be more strongly adsorbed than phosphate complexes in the pD range studied. About five times higher concentration of phosphate in solution was needed to reduce the absorbance due to pre-adsorbed arsenate to the same relative level as for pre-adsorbed phosphate, which was desorbed using a solution containing equal (molar) concentrations in arsenate and phosphate. At pD 4, two phosphate complexes were adsorbed on the iron oxide, one deuterated and one de-deuterated. When phosphate was pre-adsorbed and arsenate subsequently added to the system, the deuterated phosphate complex desorbed rapidly while the de-deuterated phosphate complex was quite stable. At pD 8.5, only the de-deuterated phosphate complex was adsorbed on the iron oxide. Moreover, the arsenate adsorbed was also predominantly de-deuterated as opposite to the arsenate adsorbed at pD 4. During the substitution experiments the configuration of these complexes on the iron oxide surface did not change. To the best of our knowledge, this is the first time this difference in stability of the different phosphate complexes is reported and shows the power of employing in situ spectroscopy for this kind of studies.

Individual and Competitive Adsorption of Arsenate and Phosphate To a High-Surface-Area Iron Oxide-Based Sorbent

Individual and competitive adsorption of arsenate and phosphate were studied on a high-surface-area Fe/Mn-(hydr)oxide sorbent with surface and bulk properties similar to those of two-line ferrihydrite. It has maximum adsorption densities of 0.42 micromol As m(-2) at neutral pH and 1.24 micromol As m(-2) at pH 3. A surface complexation model (SCM) that used the diffuse double layer model was developed that could simulate single and binary sorbate adsorption over pH 4-9. The predominant adsorbed arsenate and phosphate species were modeled as bidentate binuclear surface complexes at low pH and as monodentate complexes at high pH. The model initially overpredicted the inhibition of arsenate adsorption by the presence of phosphate. The overprediction was resolved by separating surface sites into two types: ones to which both arsenate and phosphate bind and a smaller number to which only phosphate binds. The modified model predicted the competitive adsorption of arsenate and phosphate over pH 4-9 at total As concentrations of 6.67 and 80.1 microM and a total P concentration of 129 and 323 microM. The model may be used to predict arsenic adsorption to the sorbent for a given water source based on solution chemistry.

Competitive Adsorption of Arsenate and Arsenite on Oxides and Clay Minerals

Arsenic adsorption on amorphous Al and Fe oxides and the clay minerals, kaolinite, montmorillonite, and illite was investigated as a function of solution pH and As redox state, i.e., arsenite [As(III)] and arsenate [As(V)]. Arsenic adsorption experiments were carried out in batch systems to determine adsorption envelopes, amount of As(III), As(V), or both adsorbed as a function of solution pH per fixed total As concentration of 20 μM As. Arsenate adsorption on oxides and clays was maximal at low pH and decreased with increasing pH above pH 9 for Al oxide, pH 7 for Fe oxide and pH 5 for clays. Arsenite adsorption exhibited parabolic behavior with an adsorption maximum around pH 8.5 for all materials. There was no competitive effect of the presence of equimolar arsenite on arsenate adsorption. The competitive effect of equimolar arsenate on arsenite adsorption was small and apparent only on kaolinite and illite in the pH range 6.5 to 9. The constant capacitance model was able to fit the arsenate and arsenite adsorption envelopes to obtain values of the intrinsic As surface complexation constants. These intrinsic surface complexation constants were then used in the model to predict competitive arsenate and arsenite adsorption from solutions containing equimolar As(III) and As(V) concentrations. The constant capacitance model was able to predict As adsorption from mixed As(III)‐As(V) solutions in systems where there was no competitive effect.

Competitive Adsorption of Arsenate and Arsenite on Oxides and Clay Minerals

Arsenic adsorption on amorphous Al and Fe oxides and the clay minerals, kaolinite, montmorillonite, and illite was investigated as a function of solution pH and As redox state, i.e., arsenite [As(III)] and arsenate [As(V)]. Arsenic adsorption experiments were carried out in batch systems to determine adsorption envelopes, amount of As(III), As(V), or both adsorbed as a function of solution pH per fixed total As concentration of 20 μM As. Arsenate adsorption on oxides and clays was maximal at low pH and decreased with increasing pH above pH 9 for Al oxide, pH 7 for Fe oxide and pH 5 for clays. Arsenite adsorption exhibited parabolic behavior with an adsorption maximum around pH 8.5 for all materials. There was no competitive effect of the presence of equimolar arsenite on arsenate adsorption. The competitive effect of equimolar arsenate on arsenite adsorption was small and apparent only on kaolinite and illite in the pH range 6.5 to 9. The constant capacitance model was able to fit the arsenate and arsenite adsorption envelopes to obtain values of the intrinsic As surface complexation constants. These intrinsic surface complexation constants were then used in the model to predict competitive arsenate and arsenite adsorption from solutions containing equimolar As(III) and As(V) concentrations. The constant capacitance model was able to predict As adsorption from mixed As(III)-As(V) solutions in systems where there was no competitive effect.

EFFECT OF pH, PHOSPHATE AND OXALATE ON THE ADSORPTION/DESORPTION OF ARSENATE ON/FROM GOETHITE

We studied (i) the competitive adsorption of arsenate and phosphate on goethite as affected by pH, anion concentration, and order of anion addition; (ii) the effect of increasing amounts of arsenate or phosphate on the desorption of phosphate or arsenate from the surfaces of goethite; and (iii) the influence of oxalate on the desorption of arsenate and phosphate from goethite. Similar amounts of phosphate and arsenate were adsorbed on goethite samples at pH 3.0-8.5. By adding arsenate and phosphate as a mixture (As + P systems; arsenate/phosphate molar ratio of 1), the amounts of arsenate and phosphate adsorbed on goethite were similar at pH > 6.0, but at pH < 6.0 slightly more arsenate than phosphate was adsorbed. When arsenate was added before phosphate (As before P systems), the efficiency of arsenate in inhibiting the adsorption of phosphate on goethite was higher than that of phosphate in inhibiting the adsorption of arsenate, when phosphate was added before arsenate (P before As systems). Furthermore, we found that the decrease in adsorption of phosphate in the presence of increasing concentrations of arsenate was greater than that of arsenate in the presence of increasing concentrations of phosphate. There is much evidence indicating that arsenate and phosphate compete primarily for a similar set of surface sites on goethite, but they are adsorbed on some sites uniquely specific for arsenate and phosphate. In the pH range of 3.0 to 8.5, much more phosphate previously adsorbed on goethite was desorbed by arsenate (mainly at pH < 6.0) than arsenate by phosphate. The desorption of both the ligands was influenced by time. Finally, the presence of oxalate ions in solution desorbed higher amounts of phosphate than arsenate ions, adsorbed previously on goethite, mainly in acidic systems.

Effect of Competing Anions on the Adsorption of Arsenate and Arsenite by Ferrihydrite

AbstractThe competitive adsorption of arsenate and arsenite and the effect of phosphate and sulfate on adsorption of arsenate and arsenite by ferrihydrite were investigated in the pH range of 3 to 10 and at varying initial ligand concentrations. In dual anion systems, arsenate retention was greater at low pH compared with greater arsenite retention at high pH. In systems with arsenate and arsenite concentrations ≤2.08 molAs kg−1fer each, the effect of arsenate on arsenite sorption was more pronounced than vice versa. On the contrary, at arsenate and arsenite concentrations of 3.47 molAs kg−1fer each, arsenate did not influence arsenite sorption but arsenite substantially reduced arsenate adsorption. The different sorption behavior of arsenite at low and high arsenite concentrations might be due to surface polymerization of adsorbed arsenite at high concentrations. The presence of phosphate resulted in a significant reduction in arsenate and arsenite adsorption by ferrihydrite, with strong dependence on pH and phosphate concentration. The effect of phosphate on arsenate adsorption was greater at high pH than at low pH, whereas the opposite trend was observed for arsenite. Results indicated that arsenate and phosphate compete for the same surface sites, with a moderate preference for arsenate adsorption. There was evidence of the presence of some surface sites that exhibited much higher affinity for arsenite than phosphate. The presence of sulfate did not influence arsenate adsorption but resulted in a considerable reduction in arsenite adsorption below pH 7.0, with the largest reduction at the lowest pH.