Surface Interactions of Aromatic Organoarsenical ... - ACS Publications

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Surface Interactions of Aromatic Organoarsenical Compounds with Hematite Nanoparticles Using ATR-FTIR: Kinetic Studies Derek Arts, Md Abdus Sabur, and Hind A. Al-Abadleh* Chemistry Department, Wilfrid Laurier University, Waterloo, ON N2L 3C5 Canada S Supporting Information *

ABSTRACT: Aromatic organoarsenicals p-arsanilic acid (pAsA) and roxarsone (ROX) are used as feed additives in developing countries that allow the use of arsenic-containing compounds in their poultry industry. These compounds are introduced to the environment through the application of contaminated poultry litter. Little is known about the surface chemistry of these organoarsenicals on the molecular level with reactive components in soils. We report herein the first in situ and surface-sensitive rapid kinetic studies on the adsorption and desorption of pAsA to/from hematite nanoparticles at pH 7 using ATR-FTIR. Values for the apparent initial rates of adsorption and desorption were extracted from experimental data as a function of spectral components. Hydrogen phosphate was used as a desorbing agent due to its ubiquitous presence in litter, and its adsorption kinetics was investigated on surfaces with and without surface arsenic. Initial first-order pseudo-adsorption rate constant for pAsA was lower by a factor of 1.6 than that of iAs(V), suggesting an average behavior for the formation of quantitatively more weakly bonded monodentate or hydrogen-bonded complexes for the former relative to strongly bonded bidentate surface complexes for the latter under our experimental conditions. Initial first-order pseudo-adsorption rate constants for hydrogen phosphate decrease in this order: fresh hematite > pAsA/hematite ≈ phenylarsonic acid (PhAs)/hematite > iAs/ hematite by factors 1.5 and 3 relative to fresh films, respectively. Initial desorption kinetics of aromatic organoarsenicals due to flowing hydrogen phosphate proceed with a nonunity overall order, suggesting a complex mechanism, which is consistent with the existence of more than one type of surface complexes. The impact of our studies on the environmental fate and transport of aromatic organoarsenicals in geochemical environments and their overall surface chemistry with iron (oxyhyr)oxides is discussed.

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INTRODUCTION

India. Hence, the health and environmental risk of their usage continues to be of concern to these communities. The majority of ROX and pAsA does not get metabolized in the poultry and is excreted chemically unchanged in poultry litter (estimated at 10−50 mg/kg),15 which is turned into fertilizer pellets for commercial use on cropland, home landscaping, gardening, and golf courses.16 Biogeochemical transformation of these compounds during composting and stockpiling or after application to soils occurs as a result of enhanced microbial activity or exposure to UV radiation.17−21 These processes lead to the formation of other organic species of arsenic such as dimethylarsinic acid (DMA) and inorganic arsenic (iAs).18,22,23 Under certain geochemical conditions, arsenic compounds can accumulate in soils24 and plants25 or be mobilized and liberated into nearby drinking water sources.26−29 For workers in the confined animal feeding operations of the poultry industry, arsenic is regarded as an occupational health hazard because it can make its way to airborne particulate matter.30 Exposure pathways of the general population include

Aromatic organoarsenicals are man-made compounds that found their way to the environment through their extensive use in the poultry growing industry as feed additives in the second half of the 20th century.1−3 These compounds include parsanilic acid (4-aminobenzenearsenic acid, pAsA) and roxarsone (4-hydroxy-3-nitrobenzenearsenic acid, ROX), which were approved by the U.S. Food and Drug Administration as anticoccidials and growth promoters.3,4 The above industrial practice has raised a number of environmental and health concerns5 because arsenic, in various organic and inorganic forms, is a known carcinogen6−10 and more recently was correlated with hypertension and other cardiometabolic diseases.11 As a result, the use of these compounds is rapidly declining in some areas around the world. For example, ROX was banned in the European Union in 1999.12 In 2011, Pfizer voluntarily suspended its sale to U.S. markets indefinitely13 and informed their foreign markets of their decision. In collaboration with Health Canada, Pfizer Canada and Dominion Veterinary Laboratories also voluntarily suspended its sale in Canada effective August 8, 2011.14 However, these drugs are still being sold in other regions around world, particularly in fast-developing countries such as China and © 2013 American Chemical Society

Received: November 23, 2012 Revised: February 18, 2013 Published: February 19, 2013 2195

dx.doi.org/10.1021/jp311569m | J. Phys. Chem. A 2013, 117, 2195−2204

The Journal of Physical Chemistry A

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inhalation,31 consuming grains such as rice32 or vegetables33 grown on land amended with pAsA- or ROX-containing fertilizer, untreated drinking water from wells close to operation sites,5 and high consumption of chicken meat.34 Given that pAsA and ROX are water-soluble compounds, several field samples were analyzed to investigate the fate of arsenic in soils treated with contaminated poultry litter.15,35−38 Brown et al.15 looked at the adsorption and biotransformation of ROX as a function of soil depth using batch experiments. They found that some ROX quickly biotransforms, forming iAs(V), and some leaches to subsurface layers. Also, arsenic was detected in soil water, suggesting that in the presence of phosphate and organic matter, competitive adsorption with arsenic to minerals in soil affects its leachability. Jackson et al.35 conducted a series of experiments aimed at disentangling factors that affect solubility of arsenic in soils amended with poultry litter. They found that ROX and iAs(V) are exchangeable with phosphate to some extent, leaving behind strongly bonded species. They also found that increased levels of dissolved organic matter increase the solubility of arsenic species along with copper through competitive adsorption and complexation. Moreover, Rutherford et al.36 looked at the release of arsenic compounds and other elements due to water and strong acids (HCl and HNO3) and estimated that 75% of arsenic in poultry litter was readily soluble in water. They also showed that weakly bonded arsenic from soil extractions was mobilized by water and correlates positively with C, P, Cu, and Zn. In addition, the same group36 showed that strongly bonded arsenic correlates positively with Fe in amended fields, which highlights the role that adsorption or coprecipitation plays in the soil column. Using synchrotron X-ray fluorescence (SXRF) and microfocused X-ray absorption near-edge structure (XANES) spectroscopies coupled to chemical digestion and batch experiments, Arai et al. investigated solid-state chemical speciation, desorbability, and total levels of arsenic in poultry litter and long-term amended soils, respectively. SXRF images clearly showed the association of arsenic with needle-shaped particles containing Ca, Cu, and Fe. XANES spectra confirmed the presence of iAs(V) due to biotransformation of ROX. The same group38 estimated that 15% of total arsenic was released from litter after 5 days at pH 7, which suggested the presence of strongly bonded species. They also showed no significant accumulation of arsenic in long-term amended soils, which suggested the importance of surface and subsurface transport processes. More recently, Oyewuni and Schreiber37 examined field-scale release of trace elements from poultry litter into the subsurface, in addition to leaching rates using laboratory stepwise extractions of litter. The results show increases in major ion, nutrient, and trace element concentrations in soil water after application, and that calculated leaching rates are fastest for arsenic, followed by copper and zinc. The authors concluded that leaching rate, adsorption, uptake by plants and dilution control the fate and transport of trace elements and that biotransformation of arsenic also affects its overall fate in these environments. The above studies clearly show that mechanistic details of the adsorption and competitive adsorption of organoarsenicals and their degradation products are needed to better understand their environmental fate. Whereas surface chemistry of inorganic and methylated arsenic species on reactive soil components has received attention from researchers including our group,39−52 very few studies were conducted on aromatic arsenical compounds relevant to those released from poultry

litter. Molecular-level understanding of surface interactions demands using surface-sensitive techniques to investigate the kinetics and thermodynamics of chemistry at the liquid/solid interface under environmentally relevant conditions. We previously reported structural characterization and thermodynamics of pAsA adsorption and desorption to/from iron(oxyhydr)oxides using attenuated total internal reflectance spectroscopy (ATR-FTIR).50,53,54 While it is used as a feed additive, the structure of pAsA resembles a stable biodegradation product of ROX, namely, 4-hydroxy-3-aminophenylarsonic acid (HAPA).21 Our results indicate that pAsA forms at least two types of inner-sphere complexes that are most likely monodentate over a wide pH range, desorption of pAsA is more efficient by phosphate in solution than chloride, and binding constants of pAsA are higher than that of DMA. More recently and using batch reactors, Chen and Huang reported results on the adsorption kinetics and thermodynamics of ROX and pAsA on iron and aluminum oxides, FeOOH and Al2O3.55 They showed that both ROX and pAsA exhibit identical adsorption characteristics on both materials with three times lower adsorption efficiency on Al2O3 than FeOOH on a surface site basis and that phosphate and organic matter interfere with the adsorption process, with the former also being an efficient desorption agent to some extent. It is important to highlight that the kinetic studies reported by Chen and Huang55 were over a long time frame (minutes to hours) that is typical of batch work, which does not capture the initial rates of surface exposure to organoarsenicals. We previously demonstrated the usefulness of using ATR-FTIR in conducting rapid adsorption and desorption kinetic studies of DMA, iAs(V), and phosphate to/from goethite and hematite particles.51,52 We utilize this technique to obtain pseudo-initial rates of adsorption of pAsA and iAs(V) on hematite nanoparticles at pH 7. Adsorption kinetics of phosphate was also investigated on hematite surfaces in the absence and presence of aromatic and inorganic surface arsenic, which was complemented with desorption kinetics of arsenic species to gain further insight into the ligand exchange process. The significance of the results is also discussed in light of observations from field studies.

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EXPERIMENTAL METHODS Chemicals. Arsenical compounds used in this study include pAsA (p-arsanilic acid, C6H8AsNO3, 99%, Sigma-Aldrich, used as received), PhAs (phenylarsonic acid, C6H7AsO3, 97%, Acros Organics), and iAs(V) (sodium arsenate, AsO4HNa2·7H2O, ACS reagent, J. T. Baker). Caution: The aforementioned arsenical compounds are highly toxic via inhalation and skin contact and is a carcinogen. Table 1 shows the structure and pKa of these compounds. Solutions at given concentrations were prepared using 18 MΩ-cm Millipore water, adjusted to pH 7, and then flowed over films of hematite for the adsorption part of the experiments. Solutions of 0.01 M KCl and monohydrogen phosphate (HPO42−(aq), 99.99%, Na2HPO4, Sigma-Aldrich) adjusted to pH 7 were used for the desorption part of the experiments. The Fe-(oxyhydr)oxide used herein is hematite nanoparticles (α-Fe2O3, >99.9%, nanostructured and amorphous materials, 19 m2/g surface area, 67 nm average diameter, and 8.6 isoelectric point). This material was previously used for isotherm53 and pH envelope experiments of pAsA.50 Thin hematite films were prepared on the ATR internal reflection element (IRE) by making a slurry of 6 mg sample in a 1.5 mL water/ethanol mixture [1:0.4 (v/v)]. Each slurry was then 2196



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arsenicals by aqueous hydrogen phosphate as a desorption agent was carried out. Collection of desorption spectra started once the solution of the desorbing agent entered the ATRFTIR flow cell for 30 min. Throughout the desorption part of experiments, spectral averaging was five scans for which the collection acquisition time is 6 s. Because desorption of surface arsenicals due to flowing aqueous hydrogen phosphate would occur concurrently with the adsorption of phosphate species, control experiments were carried out for the adsorption of aqueous hydrogen phosphate on freshly prepared films at pH 7. Each single beam spectrum was referenced to the last one recorded for the background solution (KCl) to obtain the absorbance spectra reported herein. To determine the uncertainty in our measurements, we repeated experiments two to four times on freshly prepared films under identical conditions. Generating Kinetic Curves from Spectral Data. To extract adsorption and desorption kinetics of surface species, a relatively accurate peak analysis method has to be employed on spectral data to estimate surface coverage. Spectra collected as a function of adsorption time used to generate adsorption kinetic curves by plotting the time dependence of the baselinecorrected ATR absorbance [i.e., peak height at a given wavenumber, A(ṽ)] of spectral features assigned to surface complexes. We recently outlined the advantages of this method in estimating the surface coverage of adsorbates in contrast with integrating peak areas.52 Using the height tool in OMNIC software that runs the FTIR spectrometer and a custom-written macro, values of A(ṽ) were generated relative to the absorbance at 2000 cm−1, which has no absorptions from any of the species used in our studies. The main assumption in this analysis method is that the uncertainty from intensity contributions of neighboring peaks is lower than that estimated from averaging multiple experiments under the same conditions.

Table 1. Structure and pKa of Some Organoarsenicals Commonly Identified in Environmental Samples from Areas with Potential Arsenic Contamination

ultrasonicated for 1 h (default power, Fisher Scientific Mechanical Ultrasonic Cleaner FS20) and then spread 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 thickness (h) of the dry film was estimated to be ∼5 × 10−4 cm, which is larger than the effective penetration depth of the evanescent wave (∼2 × 10−4 cm/reflection at 837 and 880 cm−1 and 1.5 × 10−4 cm/ reflection at 1003 cm−1 using the same method used previously in ref 53). In brief, a 14 mg thin film with was prepared on a glass slide in place of the ZnSe ATR crystal covering an area, a, that equals 5.0 cm2. Then, using a razor blade, the film was cut at multiple places to create “valleys”. Using a surface profiler (Vecco), the height of the “valleys” was measured; then, the average value was found to be 11(4) × 10−4 cm. Because the 6 mg film used in our experiments was deposited under the same conditions as the 14 mg film used in ref 53, both films will have the same bulk density, ρb. The bulk density is calculated to be 2.45 g/cm3 from the deposited mass (m), area covered (a = 5.0 cm2), and measured thickness of the films (h) according to ρb = m/(a·h). Using ρb (14 mg film), h (6 mg film) is estimated to be 5 × 10−4 cm. 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 detector. The ATR flow cell contains a 60° ZnSe crystal IRE (80 × 10 × 4 mm, 100 μL). The solutions flowed at a rate of 2 mL/min across the film using Tygon tubes (0.8 mm I.D., Masterflex) and a compact pump (Masterflex L/S). Singlebeam ATR-FTIR spectra were collected at 8 cm−1 resolution for adsorption and desorption kinetic experiments. Collection of spectra was automated for scan averages below 100 scans using a custom-written macro run on a PC computer with these specifications: Dell Optiplex GX620, Intel, ACPI Multiprocessor, and Premium 4 CPU 3.20 GHz with 1 GB RAM. At the beginning of every adsorption experiment, 0.01 M KCl at pH 7 was flowed first to record background spectra. Then, an arsenical solution of known concentration at pH 7 was flowed for the adsorption part of the experiment. Throughout the adsorption process, spectral averaging was 60 scans for the first 15 min and 100 scans for up to 30 min. The collection acquisition times were calculated from the time saved by the computer in the file names. For collecting 60 and 100 averaged scans, these times are 30 and 50 s, respectively. Collection of adsorption spectra started as the arsenical solution entered the ATR flow cell for up to 30 min. After that, desorption of surface

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RESULTS AND DISCUSSION This article is organized into three main sections: (A) the adsorption kinetics of pAsA on hematite nanoparticles relative to iAs(V), (B) adsorption kinetics of hydrogen phosphate on surfaces with and without surface arsenic, and (C) concurrent desorption kinetics of aromatic organoarsenicals pAsA(ads) and PhAs(ads) due to aqueous hydrogen phosphate. Experiments were conducted under neutral pH and kinetic data were collected during the initial times of exposure to the surface. (A). Adsorption Kinetics of pAsA(aq). Representative ATR-FTIR spectra of pAsA(ads) on hematite nanoparticles collected as a function of time are shown in Figure 1a. These spectra were collected using 0.5 mM concentration of pAsA(aq), which is below the detection limit of our ATRFTIR accessory (5 mM). Detailed assignment of these spectral features was provided in our previous publications53,54 from isotherm, pH-, and ionic strength-dependent ATR-FTIR studies. In brief, by analyzing spectral features in the 700− 950 cm−1 range containing stretching vibrations of As−O bonds [v(AsO)] for pAsA in the bulk aqueous and solid phases56 and on the surfaces of Fe-(oxyhydr)oxides,32,34 we reported the formation of inner-sphere pAsA(ads) species, which are likely monodentate that become protonated under acidic pH(D) or outersphere complexes. More specifically, the low-frequency component at 768 cm−1 (not labeled) has major contributions from inner-sphere complexes and was assigned to v(As-OFe). The component at 793 cm−1 was assigned to innersphere complexes with uncomplexed As−O bonds involved in 2197



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Herein, we investigate the kinetics of the adsorption process to complement our previous work on thermodynamics and structural characterization. Figure 1b shows the kinetic curves for the spectral components 837 and 793 cm−1. Similar experiments and data analysis were done as a function of pAsA concentration. In all of these experiments, equilibrium was reached after ∼9 min under our experimental conditions. Figure S1 (top) in the Supporting Information (SI) shows adsorption isotherm of pAsA(ads) obtained from the plateau of the 837 cm−1 kinetic curve, which shows that our adsorption experiments result in the formation of a monolayer of pAsA(ads) using these aqueous phase concentrations at pH 7. The surface coverage of pAsA(ads) was estimated from these absorbance values to be 2(0.3) × 1013 molecules/cm2 using the method previously described,50 and details of the calculations are shown in the SI. The number between parentheses is the uncertainty ±σ. For comparison, adsorption kinetic experiments were conducted using iAs(V)(aq), as shown in the right panel of Figure 1. We refer the reader to our previous publication summarizing the literature on the assignment of the spectral features at 875 and 787 cm−1 characteristic of iAs(ads).52 Figure 1e shows the kinetic curves generated from the baseline-corrected heights at 875 and 787 cm−1. Values in the plateau of the former (from concentration-dependent kinetic experiments) were used to construct the adsorption isotherm of iAs(ads), as shown in Figure S1 in the SI (bottom). These results show the formation of a monolayer of iAs(ads) using these aqueous phase concentrations at pH 7 with an estimated surface coverage of 1(0.2) × 1013 molecules/cm2. To extract apparent adsorption rates from these experimental data, we used the simple first-order Langmuir adsorption kinetic model for the adsorption of species A, θ(t) = b(1 − e−robs·t), where θ(t) is the relative surface coverage and b is a collection of constants that equals kads[A(aq)]/robs. The apparent rate of phosphate adsorption, robs, is related to the pseudo adsorption and desorption rate constants: robs = kads[A] + kdes. This model can be written in terms of A(ṽ) because θ(t) is equivalent to A(ṽ)/Amax(ṽ): A(ṽ) = b′(1 − e−robs·t), where b′ = Amax(ṽ)b was obtained by averaging data points in the plateau region of Figure 1b,e. This equation was used to fit the data shown in these Figures. The linear form of the model is: ln(1 − A(ṽ)/b′)= −robs·t. Plotting the experimental data in the linear form of the Langmuir model allows for extracting values for robs by applying a linear least-squares procedure, as shown in Figure 1c,f. The linear fits shown are data collected during the initial times of adsorption (t < 5 min), which is the focus of these studies and in contrast with batch kinetic studies that report kinetic parameters from longer time frames.45,47,55,57 This Langmuir adsorption kinetics model predicts a linear dependency of robs on [As(aq)]. Figure 2 shows the dependency of robs on [pAsA(aq)] and [iAs(V)(aq)] from concentration-dependent adsorption kinetic experiments. Data shown were extracted from analyzing the 837 and 875 cm−1 characteristic of pAsA(ads) and iAs(ads), respectively. Similar trends were observed using data at 793 and 787 cm−1 (Figure S2 in the SI). Because values of robs are sensitive to the flow rate of the aqueous phase, our experiments were conducted at 2 mL/min flow rate, which we find to be high enough to minimize diffusion contribution to robs while maintaining the hematite film intact for the duration of the measurements. The characteristic diffusion time for our system and flow conditions is ∼0.03 s, as calculated in the SI. Lines through the data in Figure 2 and Figure S2 in the SI represent linear least-squares

Figure 1. Adsorption of 0.5 mM pAsA(aq) (left panel, a−c) and 0.5 mM iAs(V)(aq) (right panel, d−f) on hematite nanoparticles (6 mg film) at pH 7, I = 0.01 M KCl, and 2 mL/min flow rate at room temperature. (a,d) ATR-FTIR absorption spectra collected as a function of time. (b,e) Adsorption kinetic curves generated from the baseline-corrected ATR absorbances at 837 and 793 cm−1 for pAsA(ads) and 875 and 787 cm−1 for iAs(ads) (filled markers). Lines through the data represent the Langmuir adsorption kinetic model. (See the text for details.) (c,f) Experimental data plotted in the linearized form of the Langmuir adsorption model. Error bars are ±σ from averaging two to four experiments, each on a freshly prepared film.

strong H-bonding similar to protonated monodentate complexes under acidic conditions. Additionally, the component around 837 (most intense) and 877 (not labeled) was assigned to , suggesting a complex with two bonds in resonance (e.g., a deprotonated monodentate complex). The existence of outer-sphere complexes cannot be ruled out based on this analysis because it is expected that this complex will give rise to spectral features similar to those observed for pAsA(aq) at pH(D) 7.54,56 Reactions 1 and 2 for the formation of two types of surface complexes for pAsA on hematite nanoparticles were previously incorporated in a triple-layer surface complexation model, which resulted in excellent fits to the isotherm and pH-envelope data:50 FeOH + H 2AsO3 C6H4NH 2 ⇌ FeHAsO3C6H4NH 2 + H 2O

(1)

FeOH + H 2AsO3 C6H4NH 2 ⇌ FeAsO3C6H4NH 2− + H 2O + H+

(2) 2198



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associated with them. This desorption parameter arises from the two-way reaction for which the Langmuir adsorption model was derived. For our system, the background electrolyte solutions, Cl−(aq), would cause desorption of surface species. However, the ligand exchange process of oxyanions under dynamic flow conditions of continuously flowing arsenical solutions is very likely a one-way (i.e., forward) reaction in nature rather than a two-way, and weak desorption agents like chloride will have little to no effect. This may explain the negative value reported above for kdes in the case of iAs(ads) (Figure 2). However, for the case of pAsA(ads) reported herein and DMA(ads),51 the higher proportion of weakly bonded surface complexes relative to bindendate as in the case of iAs(ads) increases the importance of the reverse reaction while flowing solutions of pAsA(aq) and DMA(aq). In the following sections, adsorption of hydrogen phosphate is described along with concurrent analysis of desorption kinetics of arsenicals. (B). Adsorption Kinetics of Phosphate on Surfaces with Adsorbed Arsenicals. After each adsorption experiment with a given arsenical, desorption kinetics were measured using aqueous hydrogen phosphate as a desorbing agent under neutral conditions. The goal of these experiments was to obtain values for the pseudo-adsorption rate constants, kads, which provide insight into the kinetics of ligand exchange of aqueous hydrogen phosphate with different surface groups. Figures 3

Figure 2. Concentration dependence of observed adsorption rates, robs, of pAsA(aq) and iAs(V)(aq) on hematite nanoparticles (6 mg film) at pH 7, I = 0.01 M KCl, and 2 mL/min flow rate from the analysis of spectral components at 837 and 875 cm−1 assigned to pAsA(ads) and iAs(ads), respectively.

fits, where the slopes and y intercepts correspond to pseudo kads and kdes, respectively. These values are listed in Table 2. Table 2. Best-Fit Parameters from Linear Least-Squares Fits to the Experimental Data of robs1 versus [As(aq)] shown in Figure 2 and Figure S2 in the SI peak (ṽ)

slope (kads1)/min−1 M−1

837 793

425(42) 571(114)

875 787

661(66) 716(143)

intercept (kdes1)/min−1

R2

pAsA 0.3(0.1) 0.3(0.1)

0.95 0.94

−0.01(0.01) 0.03(0.01)

0.97 0.97

iAs(V)

Moreover, the relationship between mass transfer coefficient, the rate constants, the bulk concentration, and the surface coverage is derived in the SI for pAsA adsorption kinetics using 0.85 mM as an illustration. After taking into account the uncertainties in the values of kads, it is clear that iAs(V) adsorbs faster than pAsA by a factor of 1.6. The value of kads for pAsA (a monosubstituted organoarsenical) is comparable to that we previously reported for DMA (a disubstituted organoarsenical).51 Our previous work on DMA desorption kinetics by phosphate,52 which was complemented with detailed analysis of related studies reported in the literature,43,45,47 revealed that 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. With this in mind, the results reported herein suggest that the rates of formation of monodentate and weakly bonded complexes are lower than those for strongly bonded bidentate complexes, which predominantly characterize arsenate adsorption on iron(oxyhydr)oxides. Because these arsenicals simultaneously form inner- and outer-sphere complexes, we have to emphasize that kads values reported herein represent the average behavior for the formation of surface complexes. Hence, within the time frame of our measurements, relatively higher kads values originate from the formation of a higher proportion of strongly bonded bindentate complexes, as in the case of iAs(ads) compared with weaker ones, as in the case of pAsA(ads) and DMA(ads). Moreover, the y intercepts in Figure 2 and Figure S2 in the SI correspond to pseudo kdes, which have large uncertainties

Figure 3. Representative ATR-FTIR absorption spectra collected as a function of time for the adsorption of 10−3 M HPO42−(aq) on pAsA/ α-Fe2O3 film (6 mg) and 2 mL/min flow rate at pH 7. Under these conditions, the estimated initial surface coverage of pAsA(ads) is 2(0.3) × 1013 molecule/cm2, and that of surface phosphate after 30 min is 1.4(0.3) × 1013 molecule/cm2 using the method described in ref 50.

and 4 show ATR-FTIR absorption spectra collected as a function of time for the desorption of pAsA and PhAs from hematite samples, respectively, and concurrent adsorption of hydrogen phosphate (from a 1 mM solution) at pH 7 and 2 mL/min flow rate. Studies using PhAs were conducted to investigate the role of the amine group present as a substituent in the para position of the benzene ring of pAsA relative to the −AsO3H− group. The spectral range between 700 and 1200 cm−1 contains features characteristic of adsorbed phosphate and arsenical compounds. The detailed assignment of the spectral 2199



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which increases in intensity with time as more phosphate adsorbs to the surface, along with other features around 1045 and 1103 cm−1 observed on fresh hematite surfaces and those with DMA(ads) and iAs(ads).52 The upper figures shown in the left panels of Figures 5 and 6 show kinetic curves for the adsorption of phosphate generated

Figure 4. Representative ATR-FTIR absorption spectra collected as a function of time for the adsorption of 10−3 M HPO42‑(aq) on PhAs/αFe2O3 film (6 mg film, 2 mL/min flow rate) at pH 7.

features of adsorbed phosphate was previously provided for experiments conducted by flowing hydrogen phosphate solutions over surfaces with DMA(ads) and iAs(ads).52 We also refer the reader to a recent paper by Kubicki et al.,58 where the structure and energies of different phosphate surface complexes were modeled theoretically on multiple goethite planes and results were correlated with ATR-FTIR and X-ray absorption measurements. The synthesis of these studies is that under high surface loading of phosphate and near-neutral conditions, similar to those used herein, observed spectral features are characteristic of deprotonated bidentate and monoprotonated monodentate surface phosphate complexes. The bottom spectrum in Figures 3 and 4 shows features due to pAsA(ads) and PhAs(ads) after reaching equilibrium under our experimental conditions (30 min). Given that adsorption of these compounds on hematite nanoparticles takes place through their −AsO3H− moiety at pH 7 (above pKa1 for each compound, as shown in Table 1), they show nearly identical spectral features at 837 and 793 cm−1 assigned to v(As−O) in their respective surface complexes. This suggests that the presence of the amine group in pAsA has little influence on the adsorption process. Also, the spectrum shown in the bottom of Figure 3 shows well-defined features at 1187 and 1097 cm−1, which are assigned to in-plane bending of aromatic C−H groups with a contribution from v(As−C) in the latter.59 The intensity of these vibrational modes is sensitive to the nature and number of the substituents on the benzene ring. In the case of pAsA(ads), the substituents are −AsO3 and −NH2, and hence both features have pronounced intensity. In the case of PhAs(ads), only the 1095 cm−1 shows up (Figure 4) due to the presence of the As−C bond, and the other one is too low in intensity to be observed. The 1095 cm−1 feature is absent from the spectrum of aqueous aniline solution, and only one feature at 1178 cm−1 is observed (Figure S3 in the SI). The upper spectra in Figures 3 and 4 were collected during the first 10 min of flowing hydrogen phosphate solution. The features at 1097 and 1095 cm−1 assigned above contribute to absorbances assigned to surface phosphate. However, given that these features have a narrow bandwidth, ∼11 cm−1 at half height, they have minimum contribution to the feature at 1003 cm−1,

Figure 5. Kinetic curves generated for hydrogen phosphate adsorption (left panel) on pAsA/α-Fe2O3 film (6 mg) and 2 mL/min flow rate at pH 7 as a function of [HPO42−(aq)] from the analysis of the spectral component at 1003 cm−1 assigned to PO4 (ads). The right panel shows kinetic curves for the concurrent desorption of pAsA as a result of flowing hydrogen phosphate solutions, from the analysis of the spectral component at 837 cm−1 assigned to pAsA(ads). The bottom figures show the linearized form of the experimental data (filled markers, average of two experiments) according to the Langmuir adsorption and desorption models. (See the text for details.) The lines represent the linear least-squares fitting to the data. Error bars were removed for clarity. The uncertainty in the measurements is 5% (±σ).

from the baseline-corrected height at 1003 cm−1 as a function of hydrogen phosphate solution concentration on hematite films with pAsA(ads) and PhAs(ads), respectively. Because of the small number of scans averaged for these experiments (5 scans at 8 cm−1 resolution), Figure S4 in the SI shows representative spectra within the first 1 min of flowing 0.2 mM phosphate solution as an example for the quality of spectra collected using these parameters. To extract values of robs for phosphate on these surfaces, the first-order Langmuir adsorption model described above was used. Experimental data were plotted in the linear form of this model, as shown in the bottom left Figures. Linear least-squares fittings were used to obtain values for robs, which are plotted as a function of hydrogen phosphate concentration (0.1 to 2 mM at pH 7) in the top part of Figure 7. For comparison, similar studies were conducted on fresh hematite films (i.e., no adsorbed arsenic) and on surfaces exposed to a near-monolayer of iAs(V). Spectra for these latter surfaces were identical to those we previously reported,52 as shown in Figure S5 in the SI. Because fresh hematite surfaces and those with iAs(ads) do not show any spectral features in the 1200−1000 cm−1, features at 1045 and in the vicinity of 1103 cm−1 assigned to adsorbed phosphate were also analyzed for phosphate adsorption kinetics on these 2200



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Figure 6. Kinetic curves generated for phosphate adsorption (left panel) on PhAs/α-Fe2O3 film (6 mg film, 2 mL/min flow rate) at pH 7 as a function of [HPO42−(aq)] from the analysis of the spectral component at 1003 cm−1 assigned to PO4(ads). The right panel shows kinetic curves for the concurrent desorption of PhAs(ads) as a result of flowing hydrogen phosphate solutions, from the analysis of the spectral component at 837 cm−1 assigned to PhAs(ads). The bottom figures show the linearized form of the experimental data (filled markers, average of two experiments) according to the Langmuir adsorption and desorption models. (See the text for details.) The lines represent the linear least-squares fitting to the data. Error bars were removed for clarity. The uncertainty in the measurements is 5% (±σ).

films. The concentration dependence of the values of robs obtained from analyzing these spectral features is shown in the middle and bottom parts of Figure 7. As mentioned in the previous section, the lines through the data shown in Figure 7 represent linear-least-squares fits, and the slopes of these lines represent pseudo kads. The trend in the values of kads decreases in this order: fresh > pAsA/hematite ≈ PhAs/hematite > iAs/hematite, which is shown graphically in Figure 8 as a function of spectral components. The uncertainties in the values of kads were propagated from the uncertainties in the values of robs (ca. 15%). As we previously observed for phosphate adsorption on fresh, DMA/, and iAs/ hematite, the trend is explained by the nature of surface groups involved in the ligand exchange process with incoming hydrogen phosphate molecules. More specifically, the highest values of kads (and hence the fastest phosphate adsorption kinetics) are observed on freshly prepared hematite films, followed by those with nearly monolayer coverage of aromatic arsenicals, pAsA(ads) and PhAs(ads). The lowest value of kads (and hence slowest phosphate adsorption kinetics) is observed on hematite films with nearly monolayer coverage of iAs(ads). For fresh films and those with iAs(ads), the similarity in the initial phosphate adsorption kinetic behavior of the three spectral components at 1003, 1045, and 1110 cm−1 suggests that they originate from the same type of surface complexes. When the above results are compared with those obtained from initial phosphate adsorption kinetics on DMA/hematite,52 where we reported comparable trends of kads on freshly prepared and DMA/hematite films, it becomes clear that phosphate exchange with DMA surface complexes (a

Figure 7. Dependency of initial phosphate adsorption rate, robs, on [HPO42−(aq)] from analyzing spectral components assigned to PO4(ads) on hematite surfaces in the presence and absence of surface arsenic. The spectral components shown are those assigned to PO4(ads). Lines through the data represent linear least-squares fits. Slopes of linear fits represent pseudo kads and are plotted in Figure 8. Error bars are removed for clarity. The uncertainty is ±15%.

Figure 8. Trends in the values of pseudo kads for phosphate adsorption on hematite surfaces in the presence and absence of surface arsenic as a function of spectral components assigned to PO4(ads) at pH 7 and 2 mL/min flow rate.

disubstituted arsenical) are faster than those with pAsA and PhAs surface complexes (monosubstituted arsenicals), and hence these data suggest that relative amounts of weakly bonded DMA complexes are larger than those for pAsA and PhAs complexes under our experimental conditions (neutral 2201



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pH and in contact with flowing solutions). It is likely that changing the hydration conditions by drying the samples to lower their water content forces a transition from weakly bonded outer-sphere and monodentate to strongly bonded bidentate complexes. Such transitions would explain best fits to EXAFS data with bidentate structures, as observed in the case of DMA complexes.45,47 In addition, data shown in Figure 8 show similar phosphate adsorption kinetics behavior on the surfaces with the two adsorbed aromatic organoarsenicals, pAsA(ads) and PhAs(ads). This is not surprising given that both organoarsenicals are bonded to sites on hematite via their −AsO3 moieties. This observation also suggests that the presence of the amine group on pAsA(ads) does not influence phosphate adsorption kinetics within the initial phase of adsorption at pH 7. (C). Desorption Kinetics of Aromatic Arsenicals. Kinetics of the exchange process between hydrogen phosphate in the aqueous phase and pAsA(ads) and PhAs(ads) can also be examined by analyzing the desorption behavior of surface species. The top figures in the right panel of Figures 5 and 6 show the kinetic curves generated from temporal changes in the baseline-corrected heights at 837 cm−1 assigned to pAsA(ads) and PhAs(ads) due to flowing hydrogen phosphate in the concentration range 0.1 to 2 mM at pH 7 and 2 mL/min flow rate. As previously mentioned, these experiments were run for 30 min, but only the first 10 min are shown to clearly see the effect of hydrogen phosphate concentration on the initial rates of desorption. Similar kinetic curves were generated by analyzing the 793 cm−1 peak (top panel of Figure S6 in the SI). To extract values for the pseudo-desorption rate constant, kdes, experimental data were plotted in the linear form of the Langmuir desorption model, ln(θ(t)/θ0) = −k′des·t, where θ(t) is the absorbance at a given ṽ, θ0 is the maximum absorbance ′ is the initial desorption rate before desorption starts, and kdes constant that equals kdes[HPO42−(aq)]n, where n is the order of desorption. The linear form of this equation is: ln(k′des) = ln(kdes) + n·ln[HPO42−(aq)]. The bottom figures in the right panel of Figures 5 and 6 show experimental data plotted in the linear form of the Langmuir model. Lines through the data represent linear least-squares fits with slopes corresponding to −kdes ′ . Similar trends were obtained from the analysis of the 793 cm−1 spectral components (bottom panel of Figure S6 in the SI). To extract the values of n and kdes from the experimental data, linear least-squares fitting is applied to the experimental ′ ) versus ln[HPO42−(aq)]. data shown in Figure 9 for ln(kdes Best-fit parameters are very close for both systems: y intercepts = −0.9(0.2) and slopes =0.4(0.1). Values of kdes extracted from the y intercepts are around 0.4(0.1) min−1 mM−0.4 (6(1) min−1 M−0.4) from both slopes. We previously reported52 kdes = 973(84) min−1 M−1 for the desorption of DMA and kdes = 12(5) min−1 M−0.6 and 4(2) min−1 M−0.4 for the desorption of iAs(V) depending on the spectral component analyzed. For comparison with values reported herein, each value will be multiplied by the concentration of water, 55.5 M, raised to the proper power to eliminate the molar units. This calculation results in the following kdes numbers: for pAsA and PhAs, kdes = 30(6) min−1; for DMA, kdes = 5.4(0.4) × 104 min−1; and for iAs(V), kdes = 134(54) and 20(10) min−1, respectively. It clear from the trend observed for these numbers that under our experimental conditions the initial desorption kinetics of monosubstituted aromatic organoarsenicals is closer to that of iAs(V) and lower by about three orders of magnitude than that of DMA. Whereas simultaneous formation of inner- and outer-

Figure 9. Dependency of the initial desorption rate constants, kdes ′ , obtained from the linear least-squares fitting to the experimental data shown in Figures 5 and 6 for the desorption of pAsA(ads) and PhAs(ads) as a function of [HPO42−(aq)] at pH 7 and 2 mL/min flow rate. The data are plotted in the natural log form, and best-fit parameters are listed in the text.

sphere complexes was suggested for arsenate and organoarsenicals under equilibrium condition, in light of these kinetic results, we conclude that (1) during initial times of surface interactions, the proportion of outer-sphere complexes on hematite is largest for DMA complexes, (2) pAsA and PhAs form mostly inner-sphere monodentate, and (3) arsenate exists mostly as a mixture of mono- and strongly bonded bidentate complexes. The implications of these results are discussed below.

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CONCLUSIONS AND SIGNIFICANCE Results reported herein are the first to analyze in detail the rapid kinetics of pAsA adsorption and desorption by hydrogen phosphate on hematite nanoparticles under neutral conditions using ATR-FTIR. Initial adsorption kinetics were compared with those of arsenate under the same conditions, and insights into the ligand exchange mechanism were gleaned from analyzing the trend in phosphate adsorption kinetics in the absence and presence of surface arsenic. The main conclusion of these studies is that monosubstituted aromatic arsenicals adsorb slower than iAs(V) on iron oxides by nearly a factor of 2 due to the formation of quantitatively more weakly bonded complexes under our experimental conditions. The presence of these organoarsenical surface complexes lowers the initial adsorption rate for hydrogen phosphate by nearly a factor of 2 relative to fresh iron oxides. Analysis of concurrent desorption kinetics of aromatic organoarsenicals confirms the existence of more than one type of surface complexes. As stated in the Introduction, from summarizing the literature on field studies, adsorption and competition with other species in soil solutions were suggested to be important factors in explaining the release and fate of aromatic organoarsenicals in poultry litter. The results reported herein are significant because they constitute the first systematic and in situ kinetic investigations of pAsA surface interactions with hematite nanoparticles using the surface-sensitive technique ATR-FTIR. On the basis of our results, transport of colloidal nanoparticles rich in iron could be an important factor in understanding the overall fate of aromatic organoarsenicals and their degradation products. Extrapolating our results to the field is not as straightforward because soils are chemically 2202



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Nutrition. http://ec.europa.eu/food/fs/afs/substances/substances01_ en.pdf. (13) U.S. Food and Drug Administration, FDA: Pfizer Will Voluntarily Suspend Sale of Animal Drug 3-Nitro (Fda Press Release June 8, 2011). http://www.fda.gov/NewsEvents/Newsroom/ PressAnnouncements/ucm258342.htm (14) Health Canada. Pfizer and Dominion Veterinary Laboratories Suspending Sales of Veterinary Drugs Containing Roxarsone (Information Note on June 22, 2011). http://www.hc-sc.gc.ca/dhpmps/vet/issues-enjeux/roxarsone-eng.php. (15) Brown, B. L.; Slaughter, A. D.; Schreiber, M. E. Controls on Roxarsone Transport in Agricultural Watersheds. Appl. Geochem. 2005, 20, 123−133. (16) Nachman, K. E.; Graham, J. P.; Price, L. B.; Silbergeld, E. K. Arsenic: A Roadblock to Potential Animal Waste Management Solutions. Environ. Health Perspect. 2005, 113, 1123−1124. (17) Garbarino, J. R.; Bednar, A. J.; Rutherford, D. W.; Beyer, R. S.; Wershaw, R. L. Environmental Fate of Roxarsone in Poultry Litter. I. Degradation of Roxarsone During Composting. Environ. Sci. Technol. 2003, 37, 1509−1514. (18) Cortinas, I.; Filed, J. A.; Kopplin, M.; Garbarino, J. R.; Gandolfi, A. J.; Sierra-Alvarez, R. Anaerobic Biotransformation of Roxarsone and Related N-Substituted Phenylarsonic Acids. Environ. Sci. Technol. 2006, 40, 2951−2957. (19) Arroyo-Abad, U.; Mattusch, J.; Moder, M.; Elizalde-Gonzalez, M. P.; Wennrich, R.; Matysik, F. M. Identification of Roxarsone Metabolites Produced in the System: Soil-Chlorinated Water-Light by Using HPLC-ICP-MS/ESI-MS, HPLC-ESI-MS/MS and High Resolution Mass Spectrometry (ESI-TOF-MS). J. Anal. At. Spectrom. 2011, 26, 171−177. (20) Bednar, A. J.; Garbarino, J. R.; Ferrer, I.; Rutherford, D. W.; Wershaw, R. L.; Ranville, J. F.; Wildeman, T. R. Photodegradation of Roxarsone in Poultry Litter Leachates. Sci. Total Environ. 2003, 302, 237−245. (21) Stolz, J. F.; Perera, E.; Kilonzo, B.; Kail, B.; Crable, B.; Fisher, E.; Ranganthan, M.; Wormer, L.; Basu, P. Biotransformation of 3-Nitro-4hydroxybenzene Arsonic Acid (Roxarsone) and Release of Inorganic Arsenic by Clostridium Species. Environ. Sci. Technol. 2007, 41, 818− 823. (22) Makris, K. C.; Quazi, M.; Punamiya, P.; Sarkar, D.; Datta, R. Fate of Arsenic in Swine Waste from Concentrated Animal Feeding Operations. J. Environ. Qual. 2008, 37, 1626−1633. (23) Jackson, B. P.; Bertsch, P. M. Determination of Arsenic Speciation in Poultry Wastes by IC-ICP-MS. Environ. Sci. Technol. 2001, 35, 4868−4873. (24) Datta, R.; Sarkar, D.; Sharma, S.; Sand, K. Arsenic Biogeochemistry and Human Health Risk Assessment in OrganoArsenical Pesticide-Applied Acidic and Alkaline Soils: An Incubation Study. Sci. Total Environ. 2006, 372, 39−48. (25) Meharg, A. A.; Hartley-Whitaker, J. Arsenic Uptake and Metabolism in Arsenic Resistant and Nonresistant Plant Species. New Phytol. 2002, 154. (26) Smedley, P. L.; Kinniburgh, D. G. A Review of the Source, Behavior and Distribution of Arsenic in Natural Waters. Appl. Geochem. 2002, 17, 517−568. (27) Inskeep, W. P.; McDermott, T. R.; Fendorf, S. Arsenic (V)/(III) Cycling in Soils and Natural Waters: Chemical and Microbiological Processes. In Environmental Chemistry of Arsenic; Frankenberger, W. T., Jr., Ed.; Marcel Dekker, Inc.: New York, 2002; pp 183−215. (28) Ravenscroft, R.; Brammer, H.; Richards, K. Arsenic Pollution: A Global Synthesis; Wiley-Blackwell: Malden, MA, 2009. (29) Huang, J.-H.; Hu, K.-N.; Decker, B. Organic Arsenic in the Soil Environment: Speciation, Occurrence, Transformation, and Adsorption Behavior. Water, Air, Soil Pollut. 2011, 219, 401−415. (30) Carter, S.; Sparks, D. L.; Rule, A. M.; Tappero, R.; Benson, E. In The 22nd V. M. Goldschmidt Conference, Montréal, Canada, 2012. (31) O’Connor, R.; O’Connor, M.; Iroglic, K.; Sabrsula, J.; Gurleyuk, H.; Brunette, R.; Howard, C.; Garcia, J.; Brien, J.; Brien, J.; Brien, J.

heterogeneous and contain mineral, organic matter, and bacteria surfaces with noticeable affinities toward arsenic- and phosphorus-containing compounds. In addition, the presence of competing species in the soil solution further complicates the extrapolation process. With that in mind and because salts, phosphate, and dissolved organic matter exist in abundance on land with applied contaminated litter, adsorption rates reported herein for aromatic organoarsenicals are at best upper limits to actual values. More field and laboratory studies are needed in this area to further confirm this prediction.

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ASSOCIATED CONTENT

* Supporting Information S

Figures showing spectra, isotherm, surface coverage calculations, and detailed kinetic analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (519) 884-0710, ext. 2873. Fax: (519)746-0677. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge partial funding from WLU, NSERC, and Early Researcher Award from Ontario’s Ministry of Research and Innovation. We also acknowledge anonymous reviewers for their insightful comments and helpful suggestions.

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REFERENCES

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