Kinetic ATR-FTIR Studies on Phosphate Adsorption on Iron (Oxyhydr

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Kinetic ATR-FTIR Studies on Phosphate Adsorption on Iron (Oxyhydr)oxides in the Absence and Presence of Surface Arsenic: Molecular-Level Insights into the Ligand Exchange Mechanism Julia Tofan-Lazar and Hind A. Al-Abadleh* Chemistry Department, Wilfrid Laurier University, Waterloo, ON N2L 3C5, Canada S Supporting Information *

ABSTRACT: The surface chemistry of phosphorus and arsenic compounds in their organic and inorganic forms is of great interest to the scientific and industrial communities due to its role in controlling their transport, bioaccessibility and speciation. We report herein in situ and surface-sensitive rapid kinetic studies on the adsorption of phosphate to Fe (oxyhydr)oxides in the presence and absence of dimethylarsinic acid (DMA) and arsenate. These studies were conducted at pH 7 using ATR-FTIR in the flow mode, which were complemented with detailed kinetic analysis of the desorption behavior of DMA and arsenate over the same range of aqueous [phosphate]. Values for apparent rates of adsorption and desorption were extracted from the time dependence of given spectral components characteristic of surface phosphate, DMA, or arsenate. Results show that pseudo adsorption rate constants of phosphate on Fe (oxyhydr)oxide films increase in this order: arsenate-covered < DMA-covered ≤ freshly prepared. Also, pseudo desorption rate constants of DMA complexes are 7−12 times higher than arsenate using phosphate as a desorbing agent. When these results are combined with earlier work on the thermodynamics, kinetics, and structure of surface complexes, they suggest that, during initial times of surface interactions, increasing organic substitution on arsenate increases the proportion of relatively weakly bonded complexes (monodentate and outer-sphere).

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INTRODUCTION The surface chemistry of arsenic compounds with adsorbents in soils, catalytic media, or materials used for remediation of contaminated soils and waters under various conditions is of great interest. To minimize natural and anthropogenic sources of arsenic contamination, researching for effective and relatively cheap arsenic removal technologies at multiple scales, from small household scales to large industrial scales, is increasingly becoming an issue of urgency and high priority to meet strict governmental regulations. In a recent review, Mudhoo et al.1 concluded that arsenic removal efficiencies is affected by arsenic concentration and speciation, pH, and the presence of other dissolved constituents. At the molecular level, the majority of these technologies are based on the adsorption of arsenic species from the liquid phase to the active sites of the treatment media. Studies reporting the efficiency of arsenic removal use only inorganic forms of arsenic and assume that organic forms will behave the same.2 Motivated by the above, a number of research groups, including ours, investigated the effect of organic substitution on the structure of surface complexes and binding strengths of © 2012 American Chemical Society

arsenic compounds to Fe and Al (oxyhydr)oxides (see ref 3 for detailed literature review). However, fewer studies were published on the adsorption and desorption kinetics. Shimizu et al. reported macroscopic batch sorption kinetics of arsenate, monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) on amorphous aluminum oxide and on soils in the time range 5 min to 90 h.4,5 They found that sorption rates increased in the order DMA < MMA < arsenate, which was explained on the basis of the molecular structural differences among these molecules. Batch studies on the desorption efficiency of arsenic compounds using a given desorbing agent often report the amount desorbed within a given time and use that as an indicator for apparent desorption rates.4−7 All of these studies showed that monohydrogen phosphate, HPO42− (aq), is a more efficient desorbing agent than chloride, Cl−(aq), and that increasing the degree of methylation on arsenate increases the percent desorbed arsenic. Received: August 2, 2012 Revised: September 24, 2012 Published: September 25, 2012 10143

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As stated by Parikh et al.,8 initial rates of adsorption are difficult to obtain from batch kinetic data because of the few number of data points within the first few minutes of adsorption. To address this issue, they reported a methodology for studying rapid kinetic reactions using ATR-FTIR for the oxidation of As(III) via Mn oxide as an illustration. Kinetic curves were constructed from the absorbances assigned to surface species. More recently, Carabante et al.9 utilized ATRFTIR spectroscopy to study the competitive adsorption of arsenate and phosphate on ferrihydrite as a function of time (5−300 min). Kinetic curves were generated from the integrated area of bands assigned to adsorbed phosphate and arsenate, and conclusions were made about the strength of binding of each oxyanion and their relative amounts at given times. In their studies, no kinetic model was used to extract pseudo rate constants of adsorption and desorption. We recently reported3 the first in situ and surface-sensitive rapid kinetic studies on the adsorption and desorption of DMA to/ from hematite and goethite at pH 7 and I = 0.01 M KCl using ATR-FTIR. The adsorption kinetic data showed fast and slow rates, consistent with the formation of more than one type of adsorbed DMA. Pseudo desorption rate constants were also extracted from desorption experiments using hydrogen phosphate and chloride solutions, and were found to be higher by 1−2 orders of magnitude than those using chloride. In this paper, we expand on this work and report detailed analysis of the ligand exchange mechanism between phosphate, DMA and arsenate on hematite and goethite under neutral conditions during the initial times of surface interactions. We find that although the formation of inner- and outer-sphere complexes is thermodynamically favorable for both DMA and arsenate, increasing organic substitution on arsenate has direct consequences on their adsorption and desorption kinetics due to an increase in the proportion of relatively weakly bonded complexes (monodentate and outer-sphere) for the former.

over a clean and dry ZnSe ATR crystal and allowed to dry overnight in air at room temperature. A new freshly deposited film was prepared for each experiment. The estimated thickness of the dry hematite film is ca. 5 × 10−4 cm, which is larger than the effective penetration depth of the evanescent wave (around 2 × 10−4 cm/reflection at 837 and 880 cm−1, and 1.3 × 10−4 cm/reflection at 1110 cm−1 using the same method used earlier in ref 10). ATR-FTIR Kinetic Experiments. ATR-FTIR spectra were collected as a function of time on a freshly prepared film using a HATRPlus accessory (Pike Technologies) installed in a Nicolet 8700 FTIR spectrometer (Thermo Instruments) equipped with an MCT-A detector and flow through system described previously.10 At the beginning of every adsorption experiment, 0.01 M KCl at pH 7 is flowed first to record background spectra. Three sets of experiments were performed: one set was carried out for the adsorption of HPO42−(aq) on freshly prepared films at pH 7, and the other sets were for the adsorption of HPO42−(aq) on films previously exposed to DMA and arsenate, respectively. For the latter set of experiments, aqueous DMA and arsenate solutions, 0.5 mM, were flowed first on freshly prepared films at a rate of 1 or 2 mL/min and single beam ATR-FTIR spectra were collected at 8 cm−1 resolution for up to 80 min. After that, HPO42−(aq) adsorption was carried out (which also desorbed surface DMA and arsenate) by collecting spectra once the solution enters the ATR-FTIR flow cell for 30 min (with Fe2O3 films) and 40 min (for FeOOH films) by averaging 15 scans. Experiments were repeated 4−8 times on freshly prepared films under identical conditions to determine the uncertainty in our measurements. The uncertainty in the raw data (error bars in the kinetic curves) is relatively higher using the lowest phosphate concentration, 0.1 mM, and using goethite as a film (larger particles than hematite). Also, larger error bars are associated with data points extracted from spectra collected at times longer than 5 min.

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EXPERIMENTAL METHODS Chemicals. Monohydrogen phosphate solutions (HPO42−(aq), 99.99%, Na2HPO4, Sigma-Aldrich) were prepared using 18 MΩ cm Millipore water, adjusted to pH 7 and flowed over films of hematite and goethite. Some experiments were conducted on films previously exposed to DMA and arsenate solutions at the same pH (sodium cacodylate trihydrate, C2H6AsO2Na·3H2O, Sigma-Aldrich, and sodium arsenate, AsO4HNa2·7H2O, ACS reagent, J. T. Baker, used as received) Caution: DMA and arsenate are highly toxic via inhalation and skin contact and are carcinogens. The Fe (oxyhydr)oxides films were prepared using hematite (αFe2O3, >99.9%, Nanostructured and Amorphous Materials) and goethite (α-FeOOH, >99.9%, Alfa Aesar), and their characterization of BET surface area, particles’ shape and size, and isoelectric points was reported earlier:10 19 m2/g, 67 nm average diameter, and 8.6 for spherical α-Fe2O3 particles, and 21 m2/g, 0.1−0.9 μm (average length along a axis), and 8.8 for needle-shaped α-FeOOH particles, respectively. Details on the experimental procedure for preparing thin Fe (oxyhydr)oxide films on the ATR internal reflection element (IRE) were described in the Supporting Information of ref 10. Briefly, αFe2O3 films were prepared by making a slurry of a 6 mg sample in a 1.5 mL water/ethanol mixture (1:0.4 (v/v)). Slurries of αFeOOH particles were prepared by mixing a 16 mg sample of ground goethite (Wig-L-Bug, 1 min) in a 0.75 mL ethanol. Each slurry was then ultrasonicated for 1 h and then spread

RESULTS AND DISCUSSION Adsorption Kinetics of HPO42−(aq). The focus of this paper is on the adsorption kinetics of phosphate on Fe (oxyhydr)oxide films in the absence and presence of nearly monolayer coverage of surface DMA and arsenate [referred to later as DMA(ads) and iAs(ads)]. As shown in the following paragraphs, kinetic data were obtained from in situ measurements of temporal changes in the spectral features assigned to surface phosphate [PO4(ads)], DMA(ads) and iAs(ads) during the initial times of exposure to the surface. These studies are in contrast to kinetic batch studies that analyze the concentration of aqueous phase species before and after surface interactions for a given time. Kinetic data obtained from direct measurements of surface species provide unique insights into the ligand exchange mechanism of oxyanions.3,8,9 In our studies, freshly prepared α-Fe2O3 were exposed to either aqueous DMA or arsenate (0.5 mM) for 30 min and spectra were collected as a function of time till equilibrium was established. Figure S1 (Supporting Information) shows adsorption isotherms of (a) DMA(ads) and (b) iAs(ads) obtained from the plateau of kinetic curves generated from the most intense spectral components 840 and 875 cm−1, respectively, after reaching equilibrium under our experimental conditions (30 min). As detailed in next section on the desorption kinetics of DMA(ads) and iAs(ads), these spectral components are characteristic of DMA and arsenate surface complexes. The 10144

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left panel of Figure 1 shows representative time-dependent ATR-FTIR spectra of PO4(ads) on DMA(ads)/α-Fe2O3 and

Figure 2. ATR-FTIR absorption spectra collected as a function of time for the adsorption of 10−3 M HPO42−(aq) on iAs(V)/α-Fe2O3 film (6 mg film, 2 mL/min flow rate) at pH 7.

frequencies12 with excellent correlation with experimental data on hematite,11 goethite,13 and two-line ferrihydrite.14 We have to emphasize though that given the structural differences in the nature of sites of hydrated iron oxides and hydroxides, the intensities of these spectral components will vary even if surface loading and solution pH is the same. For example, spectra shown for PO4(ads) on hematite in Figures 1, 2, and S2 (Supporting Information) (near natural pH and monolayer of surface coverage) resemble those shown in Figure 5 reported by Elzinga and Sparks11 at pH 6.1 and pD 6.2 for high phosphate loading. More specifically, the most intense feature under these conditions is around 1100 cm−1, followed by two overlapping features between 1000 and 1050 cm−1.11 The weakest feature in intensity is located between 900 and 950 cm−1.11 Although goethite and ferrihydrite are structurally different, the spectra shown for PO4(ads) on goethite in Figures 1 and S2 (Supporting Information) resemble that shown in Figure 5 of Arai and Sparks.14 Again, these spectra were collected in situ under high surface phosphate loading and near neutral pH. In their study, the most intense feature is between 1025 and 1050 cm−1 followed by a feature at 1075 and 1100 cm−1 and then a third one between 950 and 975 cm−1.14 Our data show an intense feature at 1041 cm−1 followed by ones at 1084 and ∼945 cm−1 (not labeled). Spectra reported by Persson et al.15 were collected using solid samples of goethite with PO4(ads) and hence cannot be used for comparison with our spectra due to differences in the degree of sample hydration. As shown below, the growth of the labeled features in Figures 1, 2, and S2 (Supporting Information) as a function of time was used in quantifying the kinetics of phosphate adsorption as a function of spectral components. The right panel of Figure 1 shows adsorption kinetic curves of PO4(ads) generated from the spectra shown in the left panel by plotting the baseline-corrected ATR absorbance [i.e., peak height at a given wavenumber, A(ν̃)] at 1049 and 1041 cm−1, for hematite and goethite films, respectively. Values of A(ν̃) were obtained using the height tool in OMNIC software that runs the FTIR spectrometer used in our experiments. The baseline correction was relative to the absorbance at 2000 cm−1,

Figure 1. ATR-FTIR absorption spectra and adsorption kinetic curves collected as a function of time for the adsorption of 10−3 M HPO42−(aq) on α-Fe2O3 (6 mg film, upper panel) and FeOOH (16 mg film, lower panel), respectively. These films were previously exposed to 0.5 mM DMA(aq) at pH 7 and 2 mL/min flow rate. For kinetic curves, markers and lines represent experimental data and the least-squares fit using the Langmuir adsorption kinetics model, respectively. Apparent adsorption rates (shown in Figure 2 for a given [HPO42‑(aq)]) are obtained from the slopes of linear form of this model applied to the normalized experimental data.

DMA(ads)/α-FeOOH. For comparison, representative spectra of PO4(ads) on iAs(ads)/α-Fe2O3 and freshly prepared films are shown in Figures 2 and S2 (Supporting Information), respectively. The spectral range shown in Figures 1, 2, and S2 (Supporting Information) (1300−900 cm−1) contains infrared absorbances due to the stretching vibrations (v) of the uncomplexed PO, PO, POH, and PO−Fe bonds in PO4(ads). The surface structure of PO4(ads) on Fe (oxyhydr)oxide using ATR-FTIR was reviewed by Elzinga and Sparks,11 where it was stated that the location and intensity of the observed spectral features vary with pH, degree of protonation of surface complexes, degree and strength of hydrogen bonding to neighboring sites, surface loading, data acquisition technique (i.e., ex situ versus in situ IR), and substrate characteristics. Experimental infrared frequencies were correlated with theoretical ones calculated using density functional theory (DFT) to further aid in the interpretation of experimental data.12 Cluster models for surface sites used were a realistic compromise to mimic hydrated surfaces of iron oxides and hydroxide (i.e., octahedral arrangement of OH and OH2 around Fe3+ and a −OH link between the two Fe3+ cations). On the basis of these earlier studies, it was concluded that under high surface loading of phosphate and near neutral conditions, similar to those used herein, deprotonated bidentate and monoprotonated monodentate surface phosphate complexes result in calculated IR 10145

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which has no absorptions from any of the species used in our studies. Similar kinetic curves were generated for the other labeled spectral components (not shown here). Sperline et al.16 proposed a model that showed that the surface coverage of adsorbates on porous materials is best determined from peak heights rather than integrated area from data collected using ATR-FTIR. This is because the penetration depth of the evanescent wave and molar absorptivity are wavenumberdependent. We derived a modified version of this model earlier and used it to quantify the surface coverage of organoarsenicals for experiments conducted at equilibrium in units of molecules/cm2 .10,17 Hence, the baseline-corrected ATR absorbance used herein at a given wavenumber assigned to surface species is proportional to the surface coverage of that species. The main assumption in using this peak analysis method on a system like ours is that uncertainty obtained from averaging multiple experiments is larger than that obtained from accounting for the intensity contributions of neighboring peaks. This quantification method is more accurate than integrating peak areas over 200 cm−1 spectral range or performing arbitrary peak deconvolutions followed by integration. If the latter method is to be employed, peak widths have to be set to less than 40 cm−1 to ensure minimum change in penetration depth of evanescent wave. This means an arbitrary number of peaks with frequencies that does not accurately represent the P−O or As−O vibrations arising from the surface complexes. Integrating over large or small spectral range still introduces large uncertainty to the values of surface coverage of adsorbates (and therefore rates) because molar absorptivity are determined at a given wavenumber. Also, similar curves were generated from experiments conducted at 1 mL/min flow rate of HPO42−(aq) (not shown here). To extract apparent (and initial) adsorption rates from these experimental data, we used the simple firstorder Langmuir adsorption kinetic model, θ(t) = b(1 − e−robs·t), where θ(t) is the relative surface coverage of PO4(ads), and b is a collection of constants that equal kads[HPO42−(aq)]/robs. The apparent rate of phosphate adsorption, robs, is related to the pseudo adsorption and desorption rate constants: robs = kads[HPO42−(aq)] + kdes. Because θ(t) is equivalent to A(ν̃)/ Amax(ν̃), the time dependence of θ(t) can be rewritten in terms of A(ν̃): A(ν̃) = b′(1 − e−robs·t), where b′ = Amax(ν̃)b. The latter equation is used to fit the experimental data shown in the right panel of Figure 1. The linear form of this model is ln(1 − A(ν̃)/ b′) = −robs·t. Values of b′ were obtained by averaging data points in the plateau region of data in the right panel of Figure 1 using the criteria of minimum change in the absorbance values as a function of time indicating that equilibrium was established under our experimental conditions. Values of robs were obtained from linear-least-squares fits to the experimental data plotted in the linear form for the initial times of adsorption (t < 5 min), which is the focus of these studies. As stated above, the first-order Langmuir adsorption kinetic model predicts a linear dependency of robs on [HPO42−(aq)]. Figure 3 shows these linear trends for phosphate adsorption on hematite films in the presence and absence of DMA(ads) as a function of wavenumber of the spectral components assigned to PO4(ads) (at 1003, 1049, and 1110 cm−1). Similar figures were generated from experiments done on goethite films and are shown in Figure S3 (Supporting Information). For comparison, results from phosphate adsorption kinetic experiments were conducted on hematite films with a monolayer of iAs(ads). The lines through the experimental data are linear-

Figure 3. Dependency of apparent phosphate adsorption rate, robs, on [HPO42−(aq)] from analyzing spectral components assigned to PO4(ads) on hematite surfaces in the presence and absence of DMA(ads) and iAs(ads). Lines through the data represent linear leastsquares fits. Slopes of linear fits represent pseudo kads and are plotted in Figure 4. Error bars are removed for clarity. The uncertainty is ±15%.

least-squares fit to the experimental data, and the slopes of these lines represent pseudo kads. Trends in the values of kads are plotted in Figure 4 as a function of wavenumber. Values of kads from studies conducted using 1 mL/min flow rate and hematite films are not shown and found to be relatively lower by about a factor of 1.5 than those obtained from the data collected at 2 mL/min flow rate. This suggests more diffusion contribution to robs at the slower flow rate. Experiments at higher flow rates (>2 mL/min) would result in mechanically removing the film from the ATR-FTIR crystal during the data collection time. From this observation, experiments on goethite were conducted using 2 mL/min flow rate only. The uncertainties in the values of kads (slopes) were propagated from the uncertainties in the values of robs. As detailed below, the trend observed from the values of kads shows that phosphate adsorption kinetics through ligand exchange depends on the type of the leaving groups. From the kinetic behavior of spectral components at 1003 and 1110 cm−1 for hematite, and 1041 and 1084 cm−1 for goethite, the following conclusion can be inferred taking into account the uncertainties in the values of kads: the highest values of kads (and hence the fastest phosphate adsorption kinetics) are 10146

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these rates are not corrected for geometrical differences in the shape of the particles (spherical versus needle-shaped) or the density of deposited films, which are factors that impact kinetics of surface interactions. In addition, the y-intercept in Figures 3 and S3 (Supporting Information) represent the pseudo kdes according to the Langmuir adsorption kinetic model (vide supra). Values of pseudo rate constants obtained from these surface-sensitive kinetic measurements can be used to calculate the binding constant at equilibrium, Keq, according to this relation: Keq = kads/kdes. Performing such a calculation on our system would not yield an accurate estimate of Keq because of the large uncertainty associated with kdes. This uncertainty stems from the nature of the ligand exchange process of oxyanions under dynamic flow conditions, which is very likely a one-way (i.e., forward) reaction in nature rather than a two-way as often written in surface complexation models. In other words, one cannot estimate phosphate desorption kinetics accurately from data collected during adsorption processes. Values of pseudo kdes are best estimated from experiments designed to collect data as desorption is taking place. For our system, this means flowing DMA(aq) or iAs(aq) over Fe (oxyhydr)oxide films with PO4(ads), which are experiments currently underway in our lab. In summary, the results above clearly show that values of kads of phosphate are lower by a factor of 1−2 and 6−8 on DMA(ads)/hematite and iAs(ads)/hematite surfaces at 2 mL/ min, respectively, relative to freshly prepared films, with similar trends observed on goethite films. This observation shows that the fastest ligand exchange process takes place between HPO42−(aq) and surface water followed by DMA(ads) and iAs(ads). Although both DMA(ads) and iAs(ads) form simultaneously inner- and outer-sphere complexes, the trend in the values of phosphate kads clearly suggests that DMA(ads) exists mostly in outer-sphere or monodentate complexes, whereas iAs(ads) exists mostly in strongly bonded bidentate complexes. Similar conclusions are made below from analyzing the desorption kinetics of DMA(ads) and iAs(ads) due to flowing the same concentration range of HPO42−(aq). This suggests that fast kinetic studies that probe the adsorption behavior during initial times of surface interactions are better than those conducted at equilibrium in delineating molecular level differences among different surfaces. Desorption Kinetics of DMA(ads) and iAs(ads) Due to Flowing HPO42−(aq). Kinetics of ligand exchange of HPO42−(aq) with different surface groups can be examined from trends in the pseudo desorption rate constants, kdes, of adsorbed species. We reported earlier analysis of the desorption behavior of DMA(ads) due to flowing chloride and hydrogen phosphate from ATR-FTIR studies.3 In that reference, desorption kinetic curves were generated from features assigned to DMA(ads) in the spectral range 720−920 cm−1. Herein, we expand on these previous studies and show results on the desorption kinetics of DMA(ads) and iAs(ads) due to flowing hydrogen phosphate in the concentration range 10−4 to 5 × 10−3 M at pH 7 and 2 mL/min flow rate. Assignment of spectral features characteristic of DMA(ads) used to generate desorption kinetic curves can be found elsewhere.3,20 For the case of iAs(ads), the bottom spectrum in Figure 2 shows a broad absorption feature, with the most intense at 875 cm−1 due to arsenate surface complexes on hematite. Upon flowing HPO42−(aq), a reduction in the overall intensity of this band is observed, and a feature at 787 cm −1 becomes more

Figure 4. Trends in the values of pseudo kads for phosphate adsorption on hematite (top) and goethite (bottom) surfaces in the presence and absence of DMA(ads) and iAs(ads) as a function of spectral components assigned to PO4(ads) at pH 7 and 2 mL/min flow rate.

observed on freshly prepared Fe (oxyhydr)oxide films and those with nearly monolayer coverage of DMA(ads). The lowest value of kads (and hence slowest phosphate adsorption kinetics) are observed on hematite films with nearly monolayer coverage of iAs(ads). The kinetic behavior of the spectral component at 1049 cm−1 on hematite yielded kads values that increase in this order: arsenate-covered < DMA-covered < freshly prepared. As stated above, the spectral features observed in our experiments for PO4(ads) were assigned to deprotonated bidentate and monoprotonated monodentate complexes. For a given arsenic-covered hematite film [i.e., in the presence of DMA(ads) or iAs(ads)], the similarity in the kinetic behavior of the two spectral components at 1003 and 1110 cm−1 suggest that they originate from the same type of surface complex. In comparison, the kinetic behavior of the spectral component at 1049 cm−1 shows values of kads that are lower by about a factor of 2 on these arsenic-covered hematite films. This suggests that the phosphate complex characterized by the 1049 cm−1 forms as a result of a ligand exchange process with a strongly bonded DMA(ads) or iAs(ads). However, on a freshly prepared hematite film, the opposite trend is observed, where the kinetic behavior of the spectral component at 1049 cm−1 shows values of kads that are slightly higher by a factor of 1.3, suggesting a preference for phosphate to access sites similar to those accessed by arsenic compounds. Despite being conducted on different time scales, this latter result is similar to earlier interpretations made in reports on competitive adsorption of phosphate and arsenate on Fe (oxyhydr)oxides.9,18,19 The above observation suggest that the 1049 cm−1 might be arising mostly from the deprotonated bidentate phosphate complex that exists at neutral pH (vide supra). As for observed trends in kads from the data on goethite films, the similarities in the kinetic behavior of the spectral components analyzed for phosphate adsorption on films with DMA(ads) and freshly prepared films suggest that they originate from the same type of surface complexes, predominately the deprotonated bidentate as reported by Luengo et al.13 Comparing values of kads between hematite and goethite is not straightforward because 10147

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pronounced. The IR signature of iAs(ads) has been studied extensively at equilibrium on a number of metal (oxyhydr)oxides that include hematite and goethite.21−26 These studies coupled with results from X-ray absorption studies reported that arsenate forms predominantly inner-sphere bidentate binuclear complexes, which gives rise to v(As−O−Fe) in the range 824−800 cm−1.21−25 The formation of inner-sphere monodentate complexes involved in hydrogen bonding with neighboring sites was also reported as a function of pH by Loring et al.26 Values of v(As−O) in these complexes range from 860 to 780 cm−1 for singly protonated monodentate complexes around neutral pH, and from 885 to 810 cm−1 for doubly protonated monodentate complexes under acidic pH (less than 3). The lowest frequency is assigned to v(As−OH) in these protonated complexes, which is in general weaker in intensity than features located ≥800 cm−1. Simultaneous formation of inner- and outer-sphere complexes of adsorbed arsenate was reported by Catalano et al.25 at pH 5, where the latter can give rise to v(As−O) in the range 861−854 cm−1 due to weak interactions with −FeOH sites.23 In light of these studies, and given the data shown in Figure 2 under neutral pH (lower than PZC of hematite,10 with a higher aqueous concentration of HAsO42− than H2AsO4−), we believe that over the time frame of our measurements, arsenate forms a mixture of mono- and bidentate inner-sphere protonated complexes giving rise to v(As−OH) at 787 cm−1 (too low in frequency to be assigned to v(As−OFe) according to Myneni et al.21), and a broad absorption in the range 800−900 cm−1 composed on a number of overlapping bands. The latter feature contains absorptions assigned to v(As−OFe), uncomplexed v(As−O) with a bond order of 1.5 due to resonance, or v(As− O) weakly H-bonded to neighboring −FeOH sites. The spectra in Figure 2 also shows that upon flowing hydrogen phosphate, the feature at 787 cm−1 persists suggesting that it is very likely due to the presence of a protonated bidendate complex. The formation of outer-sphere complexes cannot be excluded, and the reduction in the intensity of the 875 cm−1 might be interpreted as evidence for their presence on the surface under our experimental conditions as suggested by Roddick-Lanzilotta et al.23 Kinetic curves were generated by plotting the baselinecorrected heights of the absorbances at 840 and 875 cm−1 characteristic of DMA(ads) and iAs(ads), respectively (Figure S4, Supporting Information). To extract kdes, experimental data were plotted in the linear form of the desorption Langmuir model, ln(θ(t)/θ0) = −k′des·t, where θ(t) is the absorbance at a given ν̃, θ0 is the maximum absorbance before desorption starts, and k′des is the initial desorption rate constant that equals kdes[HPO42−(aq)]n, where n is the order of desorption. Figure S5 (Supporting Information) shows an example of the linear form of the kinetic curves shown in Figure S4 (Supporting Information) using 1 mM HPO42−(aq) solution. Linear-leastsquares fits of the data during the initial desorption times is also shown from which values of k′des are obtained. The dependency of k′des on [HPO42−(aq)] is shown in Figure 5 for the desorption of DMA(ads) and iAs(ads) from hematite surfaces. The results show that under our experimental conditions using [HPO42−(aq)] in the range 10−4 to 5 × 10−3 M, an order of n = 1 produces an excellent fit to the DMA data. For the arsenate desorption case, values of n = 0.4 and 0.6 produce a better fit than n = 1 for data obtained from analyzing the 787 and 875 cm−1 spectral features, respectively. The kinetics from analyzing the 787 cm−1 spectral feature are slower by nearly a factor of 2

Figure 5. Dependency of initial desorption rates, k′des, of surface DMA (top) and arsenate (bottom) on the concentration of [HPO42−(aq)] from studies conducted using α-Fe2O3 films (6 mg) at pH 7 and 2 mL/min flow rate. Both adsorbed DMA and arsenate were introduced by flowing 0.5 mM aqueous solutions at pH and I = 0.01 M KCl. Lines through the data represent least-squares fits. Best fit parameters are (top) n = 1 and kdes = 973(84) min−1 M−1; (bottom) n = 1 and kdes = 138(20) min−1 M−1 (solid line) and n = 0.6 and kdes = 12(5) min−1 M−0.6 (dashed line) for the 875 cm−1 data, and n = 1 and kdes = 79(16) min−1 M−1 (solid line) and n = 0.4(2) and kdes = 4(2) min−1 M−0.4 (dashed line) for the 787 cm−1 data.

relative to that from the 875 cm−1 feature, clearly showing that they arise from at least two different types of iAs(ads). In light of the above literature review on the structure of iAs(ads), outer-sphere complexes contribute to the feature at 875 cm−1, whereas bidentate binuclear give rise mostly to the feature at 787 cm−1, and monodentate can contribute to both features. Overall, it is clear that DMA(ads) desorbs faster by nearly a factor of 7 than iAs(ads) using hydrogen phosphate as a desorbing agent. These results support our interpretation above that the proportion of outer-sphere and/or monodentate DMA complexes on hematite is larger than bidentate, and that arsenate exists mostly as strongly bonded bidentate. To contrast our results with previous surface-senstive work, EXAFS studies reported As−Fe interatomic distances in bidentate DMA(ads) and iAs(ads) to be 3.3 and in the range 3.23−3.37 Å, respectively,4 which were in close agreement with the values we reported from DFT calculations on hydrated bidentate DMA- and iAs-Fe(oxhydr)oxide clusters.27 It is important to highlight here that samples prepared for EXAFS allow for at least 24 h equilibrium time with the aqueous phase before a paste-like sample with unknown amount of water is prepared for measurements. Also, the coordination number (CN) used in fitting the raw EXAFS data is one of the fitting parameters along with interatomic distances. In most cases, least-squares fitting using a CN of 2 produces the best leastsquares fit to the data, which is not surprising given that fitting procedures in EXAFS data analysis is based on the crystallo10148

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Notes

graphic data reported for arsenate salts. It is rarely reported how a CN of 1 for monodentate arsenic-containing complexes would fit raw EXAFS data. The above analysis suggests that the presence of two methyl groups on arsenate instead of oxygens does not dramatically lengthen the As−Fe interatomic distances. However, it is reported that increasing organic substitution on arsenate results in a lower binding affinity of disubstituted DMA than arsenate to Fe- and Al-(oxyhdr)oxides by 1−2 orders of magnitude at pH 7.5,6 This observation was explained by the decrease in the overall negative charge on organoarsenicals relative to arsenate, which impacts the electrostatic attraction with positively charged surface sites, and hence the ligand exchange mechanism. In light of the data analysis presented in this Results and Discussion, integrated analysis of structural, thermodynamic, and kinetic analysis strongly suggests that increasing organic substitution (two methyl groups on DMA versus none on arsenate) reduces the proportion of strongly bonded bidentate to relatively weaker monodentate and outersphere complexes. In other words, increasing organic substitution increases the importance of weaker hydrogen bonding relative to stronger electrostatic interaction in driving the ligand exchange process, and hence the number of weakly bonded surface complexes.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge partial funding from WLU, NSERC, and an Early Researcher Award from Ontario’s Ministry of Research and Innovation. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research.

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CONCLUSIONS AND SIGNIFICANCE Results reported herein are the first to analyze in detail the rapid kinetics of phosphate binding to Fe (oxyhydr)oxides using ATR-FTIR in the presence and absence of DMA(ads) and iAs(ads). Our data suggest that under neutral solution conditions, the kinetics of ligand exchange reactions between aqueous phosphate is fastest on freshly prepared films dominated by water surface groups. When DMA or arsenate is present on the surface near saturation coverage, the kinetics of the ligand exchange of aqueous hydrogen phosphate is faster with DMA(ads) than with iAs(ads). This observation, combined with analysis of the adsorption kinetics and structural data, strongly suggests that the proportion of weakly bonded monodentate and/or outer-sphere DMA(ads) complexes is higher than those formed by iAs(ads), which exist predominantly in the bidentate configuration. Hence, under neutral conditions with relatively high Fe and P conditions, DMA becomes mobilized, and readily bioaccessible for uptake and recycling to other forms of arsenic. In technologies aimed at lowering the arsenic content in organic-rich fuels or industrial wastewater, introducing Fe (oxyhdr)oxides in a form that maximizes contact with the contaminated media would be an efficient procedure. However, careful analysis has to be done to the type of stable species that coexist with arsenic compounds, particularly those such as phosphorus that have the same or higher affinities to compete for sites on the Fe-containing removal media.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing spectra and detailed kinetic analysis for phosphate, DMA, and arsenate. This material is available free of charge via the Internet at http://pubs.acs.org.

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dx.doi.org/10.1021/jp308913j | J. Phys. Chem. A 2012, 116, 10143−10149