Environ. Sci. Technol. 2008, 42, 1922–1927
Adsorption Thermodynamics of p-Arsanilic Acid on Iron (Oxyhydr)Oxides: In-Situ ATR-FTIR Studies SARAH DEPALMA, SCOTT COWEN, TUAN HOANG, AND HIND A. AL-ABADLEH* Chemistry Department, Wilfrid Laurier University, Waterloo, ON N2L 3C5 Canada
Received July 16, 2007. Revised manuscript received December 10, 2007. Accepted December 21, 2007.
The organoarsenical p-arsanilic acid (p-AsA) is used in the U.S. poultry industry as a feed additive and its structure resembles one of the stable biodegradation products of Roxarsone (ROX) in anaerobic environments. With the implementation of recent EPA MCL of total arsenic in drinking water (10 ppb), there are concerns about the fate of organoarsenicals introduced to the environment through the application of arseniccontaminated manure. We report herein, for the first time, the thermodynamics of p-AsA binding to Fe-(oxyhydr)oxides using ATR-FTIR. ATR-FTIR spectra were used to quantify surface coverage of p-AsA, p-AsA(ads), by analyzing the broadband assigned to v(As-O) at 837 cm-1. Adsorption isotherms were measured in situ at 298 K and pH 7 in the concentration range 1 µM to 40 mM. Values of Keq were obtained from Langmuir model fits and they range from 1411 to 3228 M-1. We also determined the maximum adsorption capacities of Fe(oxyhydr)oxides to p-AsA, and they range from 1.9 × 1013 to 2.6 × 1013 molecules/cm2. Our results suggest that p-AsA is more mobile than methylated and inorganic forms of arsenic and that the transport of nanoparticles with p-AsA(ads) might play a role in its mobility in geochemical environments.
Introduction In areas with intense poultry production in the United States (1), p-AsA (4-aminophenylarsonic acid, C6H8NO3As; see Figure 1 for structure) and ROX (3-nitro-4-hydroxyphenylarsonic acid, C6H6NO6As) are the two most commonly used aromatic organoarsenicals as feed additives (2). The current federal regulations permit only one of the aromatic organoarsenicals to be used to promote growth, control coccidial diseases, and improve pigmentation in broiler chickens (2). For example, Roxarsone (28.5% As) is approved at 22.7–45.4 g/ton of feed, and in 2006, it was added to the feed of about 70% of the 9.1 billion broiler chickens (3, 4). The majority of Roxarsone does not get metabolized in the chicken and is excreted chemically unchanged in the manure, which is then applied as a fertilizer to croplands (3). While the FDA-approved practice of adding antimicrobial compounds to poultry feed is over half-a-century old, there are increased concerns about potential arsenic pollution of amended soils and nearby water sources in light of current * Corresponding author phone: (519) 884-0710, ext. 2873; fax: (519) 746-0677; e-mail:
[email protected]. 1922
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 6, 2008
EPA maximum contaminant level (MCL) of As in drinking water (10 ppb, 10 µg/L), which came into effect in February 2006 (5). Several studies have concluded that biogeochemical transformations of organoarsenicals under anaerobic environments result in forming the more toxic inorganic arsenate (iAs(V)) and arsenite (iAs(III)), and hence, aromatic organoarsenicals contribute to the soluble fraction of arsenic in natural systems (see ref 6 and references therein). In general, the fate of contaminants in geochemical environments depends on their surface interactions with the different components of soil particles (7). The mechanism of iAs(V) and iAs(III) adsorption to different minerals was investigated in a number of studies using batch experiments, ex-situ synchrotron X-ray absorption (XAS), and infrared techniques (8). There are fewer studies, however, that report the adsorption/desorption behavior and nature of surface complexes of organoarsenical compounds. Adsorption studies of monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) on Fe-(oxyhydr)oxides (9) and titanium dioxide (10) revealed that the degree of methylation and oxidation state of arsenic in these compounds affect their binding affinities and geometry of surface complexes. Brown et al. investigated ROX adsorption in soil samples collected from a vadose zone of an agricultural area known for intense poultry productions (11). Because soil samples were analyzed for the total percentage of clay and soil organic matter, molecular-level structure–function relationships could not be derived from these measurements. The conclusions that can be drawn from the above studies are (1) due to the high water solubility of aromatic organoarsenicals, irrigation and rain events would mobilize organoarsenicals; (2) depending on the level of microbial activity and redox potential in soils, biotransformation processes eventually lead to the formation of the more toxic inorganic forms of arsenic (iAs(V) and iAs(III)); (3) aromatic arsenicals with amino groups, particularly HAPA (4-hydorxy3-aminophenylarsonic acid, a stable degradation product of ROX in anaerobic environments) and p-AsA, have a longer lifetime in soils; and (4) while weak and strong adsorption processes of ROX on soils have been quantified, little is known about the adsorption behavior of other aromatic organoarsenicals (especially the biodegradation products), and the reactive centers on the surfaces of soils responsible for the adsorption process, whether it is the mineral phase or the natural organic component. Such molecular-level information is needed to increase our understanding of organoarsenicals behavior in natural systems (i.e., transport, bioavailability, and transformations) (12), which ultimately will lead to effective pollution control and waste management strategies (8). Our studies reported herein are the first spectroscopic investigations of p-AsA binding to Fe-(oxyhydr)oxides including R- and γ-Fe2O3 and R-FeOOH particles. p-AsA was chosen because it is one of the most commonly used aromatic organoarsenicals in poultry feed (vide supra), and its structure resembles HAPA. We employed the surface-sensitive attenuated total reflection Fourier transform infrared technique (ATR-FTIR) (13) for in situ spectroscopic identification of surface complexes and measurements of the adsorption isotherms using solution concentrations as low as 1 µM at pH 7. Because of the abundance of Fe-(oxyhydro)oxides in natural systems (14), and the high affinity of arsenic compounds to these materials (8, 11), we studied the adsorption of p-AsA to the two most thermodynamically stable phases that are ubiquitous in the environment: goethite (R-FeOOH) and hematite (R-Fe2O3), in addition to maghemite 10.1021/es071752x CCC: $40.75
2008 American Chemical Society
Published on Web 02/14/2008
These IEP values are consistent with those summarized by Kosmulski (16). ATR-FTIR Experiments. ATR-FTIR spectra were collected using a HATRPlus accessory (Pike Technologies) installed in a Nicolet 8700 FTIR spectrometer (Thermo Instruments) equipped with a purge gas generator and a liquid N2-cooled MCT detector. The HATR flow cell houses a 60° ZnSe crystal as the internal reflection element (IRE) (80 × 10 × 4 mm) on which the Fe-(oxyhydr)oxide films were directly deposited from slurries of a known amount of powder in a water/ethanol mixture. Detailed experimental procedure of film deposition is described in the Supporting Information (SI). Also, the effective angle of incidence and number of reflections were calculated according to the procedure described in the SI. All spectra were collected at 4 cm-1 resolution by averaging 300 scans for the low concentration range (99.9% purity), maghemite (γ-Fe2O3, NanoArc, Alfa Aesar, 99.95% purity), and goethite powder (R-FeOOH, Alfa Aesar, >99.9% purity). The BET surface areas of these materials using N2 as an adsorbent were measured to be 19, 26, and 21 m2g-1, respectively. From the analysis of the scanning electron microscopy (SEM) images, the Fe-oxide particles are spherical in shape and their particle size distribution is 67 ( 45 and 71 ( 17 nm for R- and γ-Fe2O3, respectively. Goethite crystals obtained after Wig-L-Bug (for 60s) are needle-shaped and their sizes along the a axis range from 0.1 to 0.9 µm. Moreover, the isoelectric point (IEP) was measured for each material via a zeta potential titration (ZetaProble Analyzer, Colloidal Dynamics) using 0.2 N HCl as the titrant and found to be 8.6, 7.7, and 8.8, respectively.
Speciation of p-AsA(aq) using ATR-FTIR. Figure 1 shows the ATR-FTIR spectra of 20 mM p-AsA solution at pH 10, 7, and 3 (from top). The pKa values of p-AsA(aq) are 1.9, 4.1, and 9.2 corresponding to the deprotonation of the sNH3+ substituent on the para position and the two arsonic acid (AssOH) groups, respectively (17). Absorption bands assigned to the symmetric (vs) and asymmetric stretching (vas) vibration of AssO bond are located in the 750–950 cm-1 spectral range and the number of resolved components is sensitive to the protonation state (18) and organic substituents (19, 20) on the AsO3 moiety in the molecule. The major bands shown in Figure 1 centered at 818, 870, 906 cm-1 (from top) correspond to the p-AsA species predominant at each pH: sAsO32s, sAs(OH)O2s, and sAsO(OH)2, respectively, reflecting the significant increase in the force constant of AssO bond with increasing protonation. This assignment is consistent with that of the major bands reported for the different protonation states of iAs(V) (18, 21) and methylated arsenate ions in solution (22, 23). Therefore, the v(AssO) is used to identify the structure of surface complexes and quantify the surface coverage of p-AsA(ads) on Fe-(oxyhydr)oxides studied herein. ATR-FTIR Spectra of p-AsA(ads) on Fe-(Oxyhydr)Oxides. The frequencies and intensities of the different spectral components that can be resolved from v(AssO) absorption are sensitive to coordination to mineral surfaces (18). Representative ATR-FTIR absorption spectra of p-AsA(ads) onto Fe-(oxyhydr)oxides are shown in Figure 2 (left panel), which were recorded in situ and at equilibrium using solution concentrations in the range 1 µM-8 mM (pH 7) (see Figure caption for actual concentrations). The spectra of p-AsA(ads) are clearly distinguishable from that of p-AsA(aq) at pH 7 (Figure 1). Increasing solution concentration of p-AsA results in increasing the intensity of the resolved components and indicates an increase in the surface coverage of p-AsA(ads). The right panel of Figure 2 shows the spectra of Fe(oxyhydr)oxides after collecting isotherm data, flowing water over the films several times, and drying overnight. The persistence of absorption bands in v(AssO) spectral range suggests an irreversible binding mechanism. Spectra ii-iv are compared to that of crystalline p-AsA shown in spectrum (i), where the molecules exist in a zwitterionic form (i.e., NH3+C6H4AsO2(OH)-) in which arsonic groups are involved in two H-bonds (24). Bond lengths in the arsonic group increase in this order: AsdO (1.66Å) ≈AssO- (1.67Å)
