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Controls on roxarsone transport in agricultural watersheds
Applied Geochemistry Applied Geochemistry 20 (2005) 123–133 www.elsevier.com/locate/apgeochem
Controls on roxarsone transport in agricultural watersheds B.L. Brown, A.D. Slaughter, M.E. Schreiber
*
Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg VA 24061-0420, USA Received 20 August 2003; accepted 5 June 2004 Editorial handling by A. H. Welch
Abstract The use of the organoarsenical roxarsone, added to poultry feed to increase weight gain, results in elevated As concentrations (10–50 mg/kg) in poultry litter. This litter is used extensively as fertilizer in agricultural regions. The authors investigated the sources and sinks of As within the vadose zone of an agricultural watershed in the Shenandoah Valley of Virginia, USA, an area of intense poultry production. Batch experiments were constructed to examine adsorption and biotransformation characteristics of roxarsone within the Ap and Bt soil horizons of Frederick series soils, common in the Shenandoah Valley. Roxarsone exhibits weak adsorption to the Ap soils; however, it is rapidly biotransformed to As(V) in this soil horizon. Although the Bt horizon demonstrated strong adsorption of roxarsone and thus may act as a sink for As species, soil water data collected from lysimeters at an agricultural field site suggest that As, as As(V), is mobile in the Bt soil water. It is unclear if this mobilization is due to competitive reactions with phosphate or organic acids, also present in litter. These results have implications for As cycling within poultry-dominated watersheds. For watersheds that have experienced years of litter application, As and other litter-associated species will be attenuated in soils through adsorption to mineral surfaces, but a variety of geochemical processes, such as competitive adsorption, may allow for enhanced transport of As through the vadose zone and into aquifer systems. 2004 Elsevier Ltd. All rights reserved.
1. Introduction The organoarsenical roxarsone (3-nitro-4-hydroxyphenylarsonic acid) is added to poultry feed at a concentration of 22.7–45.5 g/ton (20.6–41.3 mg/kg) (Anderson, 1983). Originally used to aid in the control of coccidiosis, roxarsone is now used for growth stimulation, improved feed conversion, better feathering, increased egg production, and pigmentation (Anderson,
* Corresponding author. Tel.: +1 540 231 3377; fax: +1 540 231 3386. E-mail address: [email protected] (M.E. Schreiber).
1983). Arsenic is detected at low levels in tissues of animals fed arsenicals (Calvert, 1975; Wershaw et al., 1999). However, studies have shown that the As level rapidly decreases to below the US Food and Drug Administration limit of 0.5 mg/kg when the arsenicals are removed from the feed for the required 5-day period before slaughter (Anderson and Chamblee, 2001). During the 42-day growth period, each broiler excretes about 150 mg of roxarsone (Anderson and Chamblee, 2001), resulting in a total As concentration of 10–50 mg/kg in poultry litter (Garbarino et al., 2003). Poultry litter, which is a mixture of poultry manure and bedding material, has widespread use as a fertilizer. In the Shenandoah Valley of Virginia, for example,
0883-2927/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2004.06.001
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there are approximately 33 · 107 kg of poultry litter produced each year, with most of the litter being applied as fertilizer on cropland (Mullins, 2000). Wershaw et al. (1999) estimate that approximately 106 kg/a of roxarsone and its degradation products are added annually to the environment worldwide from the use of poultry litter as fertilizer. In an early study of the distribution of As from poultry litter, Morrison (1969) compared fields that had poultry litter containing 15–30 mg/kg As applied over a 20-year period to fields that had no history of litter application. His results suggest that the roxarsone did not provide a significant input of As to the soil or crops, but the drainage water from litter-treated fields contained 0.29 mg/L As. This concentration is significantly higher than the recently lowered EPA As drinking water standard of 10 lg/L (0.01 mg/L). More recent investigations have documented that roxarsone can be leached easily from poultry litter. Studies by Jackson and Miller (1999) and Rutherford et al. (2003) found that 70–75% of total As from poultry litter is water soluble. Hancock et al. (2002) found concentrations of As up to 27 mg/kg in fresh litter; yet older, composted manure contained less than 2 mg/kg, suggesting that leaching of As had occurred during the composting process. Clearly, the cycling of roxarsone and its potential biotransformation products, including As(V), depends heavily on the extent to which these compounds interact with minerals and organic matter in soils and aquifer sediments. In a study on adsorption equilibria of monomethylarsenate and dimethylarsenate on hydrous Fe(III) oxide, Cox and Ghosh (1994) observed that adsorption of these methylated As(V) compounds was weakly dependent on ionic strength, suggesting inner-sphere surface complexation reactions, as has been suggested for As(V) adsorption to Fe oxides (e.g., Stollenwerk, 2003). Using results of extraction experiments, Rutherford et al. (2003) found a strong correlation between As and acid-extractable Fe in soils from litter-treated fields, suggesting adsorption to or co-precipitation of the As with Fe oxides. To date, however, no quantitative studies on roxarsone adsorption have been conducted. The main goal was to conduct a quantitative study of roxarsone adsorption to the Frederick series soils, the main soil type in the Shenandoah Valley of Virginia, with a broader objective of identifying sources and sinks of As within agricultural watersheds. A secondary objective was to examine the potential for roxarsone biotransformation by microorganisms present in the Frederick soils. The study site is located within the Chesapeake Bay watershed, within which 98% of Virginias concentrated poultry operations are located (VADEQ, 1998). Many other poultry-producing sectors of the US including regions of Kentucky, Arkansas, Missouri and Oklahoma, are located in similar geologic (carbonate)
environments. Thus, results from this research will yield critical information for examining As cycling in watersheds where poultry litter is applied.
2. Field study area The study area is located in a subcatchment (1.2 km2) of the Muddy Creek watershed, approximately 20 km NE of Harrisonburg in Rockingham Co., Virginia, USA. The subcatchment is underlain by the Lower Ordovician-Upper Cambrian Conococheague Limestone, consisting of interbedded limestone, dolostone, and sandstone. The soils of the Muddy Creek subcatchment are of the well-drained silt loam of the Frederick series (Hockman et al., 1982). The vadose zone at the site is highly variable, with thicknesses ranging from 0 to 20 m below surface. The depth to groundwater is also variable, and ranges from 2 to 20 m below surface (Hyer et al., 2001). The study area contains four small monitoring sites, which are instrumented with a total of seven monitoring wells, four drive point samplers, two stream gauging stations, and a tipping-bucket precipitation gauge. Three zero-tension (pan) lysimeters, which collect up to 5 L of soil water at 15, 45, and 90 cm depths, are installed at each of the 4 monitoring sites. Details on lysimeter construction can be found in Kaufmann (1998). The study area includes two farms, which contain cornfields, cattle pastures, and a poultry barn. Dry poultry manure is applied on the cornfields at the annual rate of about 4500 kg/ha, as a fertilizer and method of waste disposal (Hyer et al., 2001). A study of poultry production in the larger Muddy Creek watershed documented close to 1 · 106 animal units, which contribute approximately 2 · 107 kg of poultry litter to the watershed annually (Muddy Creek TMDL Establishment Workgroup, 1999).
3. Methods 3.1. Experimental methods 3.1.1. Soil collection Soil samples for the experiments were collected from a litter-treated cornfield at the Muddy Creek site and from a control site (no poultry litter applied) in Rockingham Co., Virginia. At both sites, samples were collected from the top two horizons (Ap, Bt1) of the Frederick soil profile. At the treated site, soils were collected from four locations within the field and were homogenized. At the control site, soils were collected from one dug trench. Poultry litter was also collected from one of the poultry houses at the field site. Selected characteristics of the soils are presented in Table 1. The
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Table 1 Characteristics of treated and control Frederick series soils Soil type
pH
SOM (%)
Sand (%)
Silt (%)
Clay (%)
Textural class
Specific surface area (m2/g)
Ap-control Ap-treated Bt1-control Bt1-treated
6.1 6.8 5.9 6.3
1.4 2.8 0.7 0.9
27.5 52.2 15.7 22.0
64.6 40.9 68.0 44.4
7.9 6.9 16.3 33.6
Silt loam Loam/Sandy loam Silt loam Clay loam
5.17 6.33 NA NA
Notes: NA, not analyzed; SOM, soil organic matter.
differences in physical properties between the Ap-control and Ap-treated soils are largely due to different management at each site. Management techniques can affect erosion, and thus soil physical properties. For the B horizon, the differences, which are smaller than for the A horizon, can be attributed to the variable nature of the parent materials. 3.1.2. Soil extractions The Ap and Bt soils were extracted for soil organic matter (SOM), soil test P, and selected components found in poultry litter (Cu, Zn, Fe, As) (Table 2). Extractions for soluble components were conducted over a 3-day period on a wrist shaker using 6.25 g soil with 25 mL 0.01 M NaCl adjusted to pH 5. SOM was determined using a Walkey Black chromic acid oxidation method (Jackson, 1960). Soil test P, which includes Al- and Fe-phosphates and P adsorbed on colloidal surfaces, was determined using a Mehlich 1 extraction (Mehlich and Tucker, 1983). ‘‘Total’’ concentrations of As, Cu, Zn, and Fe were extracted from the soils and
poultry litter using an aqua regia digestion. Filtered (0.2 lm) extracts were collected and preserved as necessary (see Section 3.3). The Ap soils were also subjected to sequential extraction for As using a modified method of Tessier et al. (1979), as detailed by Turpeinen et al. (1999). This procedure is used to delineate the As sequestered in solid phases as exchangeable, bound to easily reducible metal-oxide, bound to organic matter, bound to crystalline Fe- and Al-oxide, and residual. The extraction steps are outlined in Table 3. Prior to extraction, soils were homogenized, dried and crushed. Soils were washed with Nanopure water and centrifuged between the extraction steps. The extractions included two modifications of the Turpeinen et al. (1999) procedure. For the organic matter extraction, a procedure utilizing potassium perchlorate, outlined by Breit et al. (2002), was used. For extraction of As from crystalline Fe oxides, an oxalic-dithionite extraction procedure, outlined by Rutherford et al. (2001), was used.
Table 2 Solublea and totalb concentrations of selected components in soils and poultry litter
Soluble Ap-control Ap-treated Bt1-control Bt1-treated Poultry litter
Total Ap-control Ap-treated Bt1-control Bt1-treated Poultry litter
Asa (mg/kg)
DOCa (mg/kg)
Fea (mg/kg)
Pa (mg/kg)
Cua (mg/kg)
Zna (mg/kg)
bdl 0.052 bdl bdl 0.568
24.8 63.2 13.2 21.2 92.4
0.266 4.956 bdl bdl 54.40
0.428 29.32 0.074 0.134 866.0
0.030 0.192 bdl 0.006 44.60
0.567 1.811 0.022 0.032 22.96
Asb (mg/kg)
SOMc
Feb (%)
Soil test Pd (mg/kg)
Cub (mg/kg)
Znb (mg/kg)
6.58 7.03 5.10 10.58 15.5
1.4 2.8 0.7 0.9 NM
1.04 1.56 1.63 3.11 4.09
9 298 1 30 NM
11.9 86.7 7.9 52.0 213
28.3 108 27.7 46.7 124
Notes: DOC, dissolved organic C; SOM, soil organic matter; NM, not measured; bdl, below detection limit; Values represent averages of triplicate samples. a Extracted with 0.01 M NaCl. b Aqua regia digestion. c Walkley–Black extraction. d Mehlich 1 extraction.
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3.1.3. Adsorption experiments All adsorption experiments were conducted using ASTM Method D 4319-93 (ASTM, 1993). Prior to use, soils were dried and sieved to <2 mm. Two series of experiments were conducted: (1) kinetic experiments, to determine the time required for roxarsone adsorption to reach equilibrium with the Ap and Bt soils; (2) isotherm experiments, to determine distribution coefficients for roxarsone adsorption to the two soils. For each experiment, 6.25 g of unsterilized homogenized soil were placed in an acid-washed centrifuge tube with 25 mL of 0.01 M NaCl. The pH was adjusted to five by addition of NaOH or HCl. Kinetic trials were spiked with 100 lg/L of roxarsone as As. Isotherm trials were spiked with 0, 100, 200, 500, and 1000 lg/L of roxarsone as As. Measurements of pH were made frequently during the experiments and pH was adjusted if it deviated more than ±0.2 pH units. After spiking with roxarsone, the soil/solution mixture was placed on a wrist shaker for up to 12 days (kinetic experiments) or three days (isotherm experiments). Both experiments were sampled destructively. During collection, samples were first centrifuged for 30 min (3500 · g) at 25 C and were then filtered (0.2 lm) and preserved as necessary (see Section 3.3). Experiments were conducted either in duplicate (kinetic) or in triplicate (isotherm). 3.1.4. Biotransformation experiments Because the soils were not sterilized prior to use in the adsorption experiments, additional batch experiments were conducted to determine the potential for biotransformation of roxarsone. Experiments were conducted using 6.25 g soil and 25 mL of 0.01 M NaCl solution, adjusted to pH 5. Two series of experiments were conducted: (1) initial experiments, to examine the potential for biotransformation in Ap and Bt soils over a 3-day period; (2) a kinetic experiment, to evaluate roxarsone transformation in the Ap soil over a 12-day period. For both experiments, live and sterilized (1% sodium azide) trials were spiked with 100 lg/L roxarsone as As and were incubated on a wrist shaker in the dark (to prevent photooxidation) at 25 C. Destructive samples were collected at three days for the initial experiment and at 3, 6, 9, and 12 days for the kinetic experiment. During collection, samples were first centrifuged for 30 min (3500 · g) at 25 C and were then filtered (0.2 lm) and preserved as necessary (see Section 3.3). 3.2. Field methods The zero-tension (pan) lysimeters were sampled in May 2003, after litter application. Samples were collected under low-flow (<10 mL/min) pumping conditions. Prior to sample collection, field measurements, including pH, temperature, specific conductance, and
dissolved O2, were taken. Samples for chemical analysis were filtered (0.2 lm), preserved (see Section 3.3), and kept at 4 C until analysis. 3.3. Analytical methods 3.3.1. Soil samples Grain size analysis on soil samples was conducted using pipette and hydrometer methods. Specific surface area was determined using N2-BET methods. As mentioned above, SOM was analyzed using the Walkley– Black chromic acid oxidation method. The Mehlich 1 extraction (Mehlich and Tucker, 1983) of soil test P was analyzed using colorimetric analysis ‘‘Total’’ As, Cu, Zn and Fe (aqua regia extraction), as well as soluble As, Cu, Zn, Fe, and P (0.01 M NaCl extraction) were analyzed using ICP-AES. Arsenic concentrations in samples collected from sequential extraction procedures were analyzed using graphite furnace atomic absorption spectrometry (GFAAS) with Zeeman background correction. 3.3.2. Aqueous (experimental and field) samples Samples for total As analysis were preserved with HNO3 and analyzed by GFAAS. During initial method development, it was determined that analysis by GFAAS yielded full recovery of As from roxarsone (Brown, 2003). Samples for As speciation were preserved with EDTA and analyzed using HPLC separation combined with hydride generation (HG) and detection by ICP-AES, following the method of Garbarino et al. (2002). Samples were analyzed for DOC by combustion after preservation with HCl. Samples for Fe, Cu and Zn were preserved with HNO3 and analyzed by ICP-AES. Samples for PO4 were analyzed by ion chromatography.
4. Results 4.1. Soil extractions Results of soluble extractions of the soils and poultry litter are shown in Table 2. Although there were not enough data collected to make statistically valid interpretations, detectable soluble (0.01 M NaCl) As was extracted from poultry litter and Ap-treated soils, but not from Ap-control or Bt soils. This soluble As is also associated with higher concentrations of DOC, and soluble Fe, P, Cu and Zn. Also shown in Table 2 are ‘‘total’’ (as determined by aqua regia digestion) concentrations of As, Fe, Cu and Zn, soil test P, and SOM. Similar to the results of the soluble extractions, the treated soils have higher concentrations of P, Cu and Zn than do the control soils. It is interesting to note that although all soils contained at least 1 wt% of total
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Table 3 Arsenic sequential extraction results Extraction step
As (mg/kg), Ap-control
As (mg/kg), Ap-treated
Extraction procedurea
1. Exchangeable
bdl
0.067
2. Easily reducible metal oxide
bdl
0.086
4. Organic Matter
5.642
7.242
4. Crystalline Fe oxide
0.256
0.212
1 M MgCl2 (25 mL, pH 7); 4 h (25 C); shaker 0.1 M NH2 OH Æ HCl (10 mL) + 0.01 M HNO3 (15 mL); 30 min (25 C); shaker Add 0.5 g KClO4 and 10 mL of concentrated HCl; 45 min (25 C) intermittent shaking 0.11 M (NH4)2 C 2O4 (10 mL) + 0.09 M C2 H2O4 (10 mL); 45 min (80 C), add 0.4 g Na2 S2 O4 ; 45 min (80 C), cool, rotate overnight, centrifuge
Sequential Extraction Total Aqua Regia Digestion Total
5.898 6.58
7.607 7.03
Addition of Steps 1–4 2 h digestion with aqua regia
Notes: bdl, below detection limit of 0.0056 mg/kg.Values represent averages of triplicate measurements. a Method outlined by Turpeinen et al. (1999), modified by Brown (2003).
Fe, soluble Fe was extracted from the Ap soils but not the Bt soils. This is perhaps due to the higher concentrations of DOC in the Ap soils, which can complex Fe and increase its solubility in water. Total As concentrations do not appear to be significantly higher in the treated soils than in the control soils, but additional sampling would be needed to confirm or deny this. 4.2. Sequential extractions Results of the sequential extraction of the Ap soils are presented in Table 3. Summation of the As concentrations from the sequential extraction procedure is within 10% of the ‘‘total’’ (aqua regia digestion) concentration, suggesting that the procedure was successful in extracting As from all solid phases within these soils. Due to the multiple steps required in this procedure, only one sample each of the Ap-control and Ap-treated samples was extracted, and thus, results cannot be used to show statistically significant differences between the two. However, the results can be used to qualitatively show that although the total As in these two soils are similar (6.58 for control and 7.03 mg/kg for treated), the distribution of As within solid phases in which As is sequestered differs. Arsenic is associated within organic matter and crystalline Fe-oxide in both soils. In the Ap-treated soil, however, additional As is found in more mobile forms, including an exchangeable fraction and a fraction bound to easily reducible metal oxides. 4.3. Adsorption kinetics The first adsorption experiment was designed to evaluate the time required for roxarsone adsorption on
Frederick series soils to reach steady state. Ap- and Bt-treated soils were spiked with 100 lg/L of roxarsone as As and were allowed to equilibrate at pH 5 for up to 12 days (Fig. 1). Results show several interesting trends: (1) roxarsone adsorption is greater to Bt soils than to Ap soils; (2) roxarsone adsorption to the Bt-treated soils does not change significantly with time; (3) in the live Ap-treated soils, there was considerably more variability in concentration within each sampling period, and the overall trend shows decreasing solution concentrations, or increasing adsorption, over time. 4.4. Initial biotransformation experiments Results of the adsorption kinetics experiment (Fig. 1) demonstrated a kinetic control on adsorption of what was thought to be roxarsone on the Ap soils. An initial biotransformation experiment, utilizing live and killed Ap- and Bt-treated soils, was conducted to determine if the kinetic control was impacted by microbial activity. Arsenic speciation analysis (Fig. 2) indicated that in the live Ap-treated soils, approximately 5% of the roxarsone was transformed to As(V) over a three day period. Transformation was not observed in killed Ap-treated soils nor was it observed in live or killed Bt-treated soils. Although these results demonstrate that roxarsone biotransformation can occur in the live Ap soils, the adsorption isotherm experiments were conducted using live (instead of killed) soils for the following reasons: (1) the biotransformation was relatively minor (5%) over the 3-day period chosen for the adsorption isotherm experiments; (2) sterilization would alter soil physicochemical properties, which could significantly impact adsorption characteristics; and (3) live soils are more representative of field conditions.
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Fig. 1. Results of kinetic adsorption experiments at pH 5, showing adsorbed total As (C*, lg As/g soil) vs time for live treated Ap and Bt soils after 100 lg/L roxarsone-As spike. Results represent averaged concentrations from duplicate experiments.
4.5. Bt adsorption isotherms The isotherms for the Bt soils show strong adsorption of roxarsone at pH 5 as compared with the Ap soils (Fig. 3). Because the relationship between solution and sorbed roxarsone is linear at the concentrations used in these experiments, partition coefficients (Kd, in L/g) were calculated using the equation C* = KdC, where C* = mass of solute sorbed per dry weight of solid (lg/g) and C = concentration of solute in solution in equilibrium with the mass of solute sorbed onto the solid (lg/L). The calculated Kd values for Bt1-treated and Bt1-control soils at pH 5 are 0.165 and 0.259 L/g, respectively (Table 4). 4.6. Ap adsorption isotherms It should be noted that the isotherm data were collected assuming that all As was present as roxarsone. However, as described above, minor biotransformation of roxarsone occurred in the Ap soils over the threeday experimental period, so the isotherms represent adsorption characteristics of a roxarsone–As(V) mixture. Even with this limitation, the data show interesting trends. First, similar to the Bt soils, the adsorption isotherms for the Ap soils at pH 5 show a linear response over the range in roxarsone concentrations used; however there is much more variation than for the Bt soils (Fig. 3). Adsorption of roxarsone was stronger to the Ap-control soils than to the Ap-treated soils; this difference is shown in the calculated Kd values (pH 5), which are 0.001 and 0.005 L/g for Ap-treated and Ap-control soils, respectively (Table 4). 4.7. Biotransformation kinetics To further examine the potential for roxarsone biotransformation, a kinetic experiment using Ap soils
was conducted over a 12-day period; results are shown in Figs. 4 and 5. The live Ap-control soil shows an increase in adsorption of total As over time (Fig. 4). In contrast, adsorption does not change in killed Ap-treated soil. Analysis of As speciation in these experiments (Fig. 5) shows that in the live soils, roxarsone concentrations are declining over time, corresponding with an increase in As(V). In the killed soils, however, roxarsone concentrations do not change over time, and very little, if any, As(V) is detected.
5. Discussion 5.1. Impact of poultry litter application on soil and soil water chemistry Although the sample size is small, the extraction results for total and soluble components of the Ap soils (Fig. 2) suggest that litter-treated soils have higher concentrations of organic matter, P, Cu and Zn than the control soils. Similar results have been reported from a variety of surface soils to which poultry litter has been applied (e.g., Kingery et al., 1994; Gupta and Charles, 1999; Jackson et al., 2003; Rutherford et al., 2003). The increased P loading in watersheds where poultry litter is applied is a concern for water quality, as P increases have been linked to eutrophication (Sims and Wolf, 1994; Mullins, 2000). Phosphorus loading is also a concern from the standpoint of As transport, as between 21% and 58% of total P in animal manure is inorganic (Barnett, 1994); inorganic phosphate can compete with As oxyanions for adsorption sites on mineral surfaces. The presence of higher concentrations of soluble Cu, Fe, Zn, P, and DOC in the Ap-treated soils than in the Ap-control soils suggests these components are leaching from poultry litter. DOC can enhance the
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Fig. 3. Adsorption isotherms for roxarsone to live Ap and Bt soils at pH 5. Error bars represent standard deviation of triplicate experiments. Solid lines represent linear regressions (not through origin) of averaged data. Regression is not forced through zero because initial concentration of As in soils is not zero in all cases (see Table 2). Table 4 Calculated Kda values for roxarsone at pH 5 Soil type Ap-control Ap-treated Bt1-control Bt1-treated
Kd (L/g)
Kd (L/g) (SOM removed) b
0.005 ± 0.002 0.001 ± 0.001b 0.260 ± 0.077 0.165 ± 0.048
0.003 ± 0.0003 0.004 ± 0.0003 NM NM
Notes: SOM, soil organic matter; NM, not measured. Values based on the averaged data of triplicate experiments. ± = standard error. a Calculated using a linear regression method where the regression line was forced through zero. Statistical analysis demonstrated no significant difference between this method and one where the regression line was not fit through zero, indicating that leaching of low concentrations of soluble As in the Ap-treated soils (see Table 2) does not affect the calculated Kd value. b Because minor biotransformation of roxarsone to As(V) occurred in the Ap soils, these values represent the adsorption characteristics of a mixture of roxarsone and As(V). Fig. 2. Results of initial biotransformation experiments, showing As speciation chromatographs (area counts vs. step time) of HPLC-HG-ICP-AES analysis after 3-day reaction of roxarsone with: (a) live Ap-treated soil; (b) killed Ap-treated soil; (c) live Bt1-treated soil.
transport of litter-associated metals, such as Cu and Zn, through the soil column, increasing the potential for leaching into ground and surface water (e.g., Li and Shuman, 1997; Gupta and Charles, 1999; Aldrich et al., 2002; Jackson et al., 2003). Soluble (0.01 M NaCl) As was only detected in Aptreated soils and poultry litter (Table 2). Results of sequential extraction (Table 3) of the Ap soils show similar patterns; the treated soil has a higher concentration
of exchangeable As than does the control. These results are consistent with those of Jackson et al. (2003) and Rutherford et al. (2003), who found that 70–75% of As in poultry litter is water soluble. Extractions of soluble components from Bt soils (Table 2) show fewer differences between treated and control soils. It is interesting to note that despite the presence of Fe and As (total digested) in Bt soils, the soluble fractions of these elements are low (Table 3). 5.2. Roxarsone adsorption to Frederick series soils Roxarsone adsorbs strongly to Bt soils (Kd at pH 5 of 0.165 and 0.259 L/g, for control and treated soils,
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Fig. 4. Adsorbed total As (C*, lg As/g soil) at pH 5 for live and killed Ap-control soils after 100 lg/L roxarsone-As spike. Error bars represent standard deviation of triplicate experiments.
Fig. 5. Concentration of As species (arsenate and roxarsone; arsenite was not detected) in 12-day biotransformation kinetic experiments for: (a) live Ap-control soils; (b) killed Ap-control soils.
respectively). Several lines of evidence suggest that the adsorption patterns of roxarsone to Bt soils are controlled by adsorption of the As(V) functional group on
roxarsone to Fe oxides. Results of the total and soluble extractions show that although As is present in the Bt soils (control: 5.1 mg/kg; treated: 10.58 mg/kg), the As is not soluble. In addition, total As in the Bt soils appears to be associated with higher total Fe concentrations (Table 2). As has been discussed in other studies, As(V) forms inner-sphere complexes with Fe oxides; removal of this strongly-bound As requires dissolution of the Fe oxides or addition of a competitive anion such as PO4. Additional support for As(V) bonding to Fe oxides is provided by Jackson and Miller (2000), who found similar desorption patterns of roxarsone and p-arsanilic acid from Fe oxides. Because both of these compounds have As(V) functional groups bound to benzene rings through As–C bonds, but differ with respect to other functional groups (p-arsanilic acid has an amino group, while roxarsone contains nitro and hydroxyl groups), the results suggest that the As(V) oxyanion is the functional group that is actively interacting with the Fe oxide surface. The adsorption patterns of roxarsone to the Ap soils display two important features: weaker adsorption in comparison with Bt soils, and weaker adsorption to the treated than to the control soils. These patterns are likely controlled by a number of factors, including low availability of strong sorbents (e.g., Fe oxides) in the Ap soils. It is well documented that arsenate anions have a strong adsorption affinity for Fe oxides and clays (see Stollenwerk, 2003, for review). The Ap soils have significantly lower concentrations of clays (Table 1) and solid phase Fe (proxy for Fe oxides; Table 2) than do the Bt soils, which is the most likely explanation for the weaker adsorption characteristics. The differences in behavior of roxarsone adsorption to the Ap-treated and Ap-control soils could be caused by a number of factors, including differences in grain size, concentrations of SOM, or concentrations of competing anions. Although there are differences in grain size distribution between the Ap-treated and Ap-control soils, which is most likely caused by differences in agricultural management practices, the specific surface areas for these soils are similar (5.17 and 6.33 m2/g for Ap-control and treated, respectively), suggesting that differences in grain size is not the main control on the adsorption differences. SOM concentrations are higher in the treated (2.8%) than in the control soils (1.4%); because As(V) adsorbs more strongly to mineral surfaces than to organic matter, the weaker adsorption of roxarsone to the treated soils could be impacted by differences in SOM. Phosphate and phytic acid, which are present in poultry litter (Barnett, 1994), have been shown to be strong competitors with arsenate for adsorption sites (e.g., Manning and Goldberg, 1996; Jackson and Miller, 2000; Seaman et al., 2001). The Ap-treated soils have significantly higher soil test P than the Ap-control soils (298 vs. 9 mg/kg; Table 2). Further evidence for the pres-
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Fig. 6. Concentrations of selected chemical parameters (As, Fe, DOC, PO4) of soil water collected from lysimeters at Muddy Creek site in May 2003. The pH of soil water ranges from 5.5 to 6.0: (a) Data from upper site, treated with poultry litter; (b) Data from lower site, not treated with poultry litter. Arsenic and Fe at lower site are below detection (detection limits: 2 lg/L (As) and 10 lg/L (Fe)). Note break in depth on y-axis. Delineation of Ap and Bt horizons is approximate.
ence of competing anions in the treated soils is from the soil water data collected from the Muddy Creek site (Fig. 6). Both PO4 and DOC are present in higher concentrations in soil water from the litter-treated field than in a control field to which litter was not applied. 5.3. Roxarsone biotransformation to As(V)
6, the lysimeter results are not directly comparable to experimental data (pH 5), but they do demonstrate that As can be leached from solid surfaces through the upper soil column. These results have implications for As cycling within agricultural watersheds in the Shenandoah Valley and other poultry-dominated agricultural regions, outlined in a conceptual model in Fig. 7. In surface soils (and
Results of the study demonstrate that roxarsone is biotransformed to As(V) in Ap soils, which is the most probable cause of the kinetic behavior of adsorption of As to these soils. As(V) is adsorbed much more strongly than roxarsone to the Ap soils; thus, as roxarsone is biotransformed to As(V) over time, the apparent adsorption increases. The initial three-day experiment did not show buildup of As(V) in Bt-treated soils; however, additional experiments are being conducted to examine the potential for roxarsone biotransformation over a longer time period. 5.4. Implications for roxarsone transport in agricultural watersheds Although the laboratory experiments demonstrate strong adsorption of low concentrations of roxarsone to Bt soils (0.4–4 lg roxarsone-As/g soil) at pH 5, field monitoring of lysimeters show As present in soil water underlying fields to which poultry litter has been applied (Fig. 6). Because soil water pH values range from 5.5 to
Fig. 7. Conceptual model of roxarsone and As(V) transport in vadose zone of agricultural watersheds in which poultry litter is applied.
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likely within poultry litter), roxarsone can be biotransformed to As(V), which is a more toxic form of As. Roxarsone adsorbs weakly to the Ap soils and can thus leach into the Bt soils. In the Bt horizon, roxarsone can either undergo further biotransformation (which was not observed in the experiments over a 3-day period), or be adsorbed. Although the low concentrations of As(V) produced from biotransformation should adsorb strongly to mineral surfaces, as has been shown in many other studies, the lysimeter data demonstrate that As(V) is detected in soil water in the Ap ands Bt horizons underlying a field to which poultry litter has been applied, suggesting that other biogeochemical processes, such as competitive adsorption from PO4 or organic acids, may allow for enhanced transport through the vadose zone and into ground water systems. Additional experiments are being conducted to address these questions. Acknowledgements This work was supported by funding from the USDA Watershed Processes Program and the Virginia Water Resources Research Center. The authors thank Greg Mullins, Mike Brosius, Jody Smiley, Nancy Frank, John Garbarino, and Tracy Hancock for assistance with sampling and analysis. Comments by Alan Welch, Robert Wershaw and Donald Sparks greatly improved the manuscript.
References Aldrich, A., Kistler, D., Sigg, L., 2002. Speciation of Cu and Zn in drainage water from agricultural soils. Environ. Sci. Technol. 36, 4824–4830. Anderson, B., Chamblee, T., 2001. The effect of dietary 3-nitro4-hydroxyphenylarsonic acid (Roxarsone) on the total arsenic level in broiler excreta and broiler litter. J. Appl. Poultry Res. 10, 323–328. Anderson, C., 1983. Arsenicals as feed additives for poultry and swine. In: Lederer, W., Fensterheim, R. (Eds.), Arsenic – Industrial, Biomedical, Environmental Perspectives. Van Nostrand Reinhold Co., New York. ASTM, 1993. Standard Test Method for Distribution Ratios by the Short-Term Batch Method. ASTM D4319-93 (Revised 2001), ASTM International. Barnett, G., 1994. Phosphorous forms in animal manure. Bioresource Technol. 49, 139–147. Breit, G. N., Whitney, J., Foster, A., Welch, A. H., Yount, J., Sanzolone, R., 2002. Preliminary evaluation of arsenic cycling in the sediments of Bangladesh. In: Arsenic in the Environment Workshop. US Geological Survey, Denver, CO. Brown, B., 2003. The Sorption of Roxarsone, an Organoarsenic Feed Additive. M.S. Virginia Polytechnic Institute and State University, Blacksburg, VA.
Calvert, C., 1975. Arsenicals in animal feed and wastes. In: Woolson, E. (Ed.), Arsenical Pesticides, ACS Symposium Series 7. American Chemical Society, Washington, DC. Cox, C., Ghosh, M., 1994. Surface complexation of methylated arsenates by hydrous oxides. Water Res. 28, 1181–1188. Garbarino, J., Bednar, A., Burkhardt, M., 2002. Methods of analysis by the US Geological Survey National Water Quality Laboratory – Arsenic Speciation in Natural Water Samples. US Geological Survey Water-Resource Investigation Reports, WRI 02-4144, Denver. Garbarino, J.R., Bednar, A.J., Rutherford, D.W., Beyer, R.S., Wershaw, R.L., 2003. Environmental fate of roxarsone in poultry litter. I. Degradation of roxarsone during composting. Environ. Sci. Technol. 37, 1509–1514. Gupta, G., Charles, S., 1999. Trace elements in soils fertilized with poultry litter. Poultry Sci. 78, 1695–1698. Hancock, T., Denver, J., Riedel, G., Miller, C., 2002. Source, transport, and fate of arsenic in the Pocomoke River Basin, a poultry dominated Chesapeake Bay watershed. In: Arsenic in the Environment. US Geological Survey, Denver, CO. Hockman, J.R., Neal, C.F., Racey, D.L., Wagner, D.F., 1982. Soil Survey of Rockingham County, Virginia. US Department of Agriculture, Washington, DC. Hyer, K., Hornberger, G., Herman, J., 2001. Processes controlling the episodic streamwater transport of atrazine and other agrichemicals in an agricultural watershed. J. Hydrol. 254, 47–66. Jackson, B., Miller, W., 1999. Soluble arsenic and selenium species in fly ash/organic waste-amended soils using ion chromatography-inductively coupled plasma spectrometry. Environ. Sci. Technol. 33, 270–275. Jackson, B., Miller, W., 2000. Effectiveness of phosphate and hydroxide for desorption of arsenic and selenium species from iron oxides. Soil Sci. Soc. Am. J. 64, 1616–1622. Jackson, B., Bertsch, P., Cabrera, M., Camberato, J., Seaman, C., Wood, C., 2003. Trace element speciation in poultry litter. J. Environ. Qual. 32, 535–540. Jackson, M., 1960. Soil Chemical Analysis. Prentice-Hall, Inc., Englewood Cliffs, NJ. Kaufmann, S., 1998. Colloid and contaminant transport through an unsaturated soil. PhD. University of Virginia, Charlottesville, VA. Kingery, W.L., Wood, C.W., Delaney, D.P, Williams, J.C., Mullins, G.L., 1994. Impact of long-term land application of broiler litter on environmentally related soil properties. J. Environ. Qual. 23, 139–147. Li, Z.B., Shuman, L.M., 1997. Mobility of Zn, Cd and Pb in soils as affected by poultry litter extract. 1. Leaching in soil columns. Environ. Poll. 95, 219–226. Manning, B.A., Goldberg, S., 1996. Modeling competitive adsorption of arsenate with phosphate and molybdate on oxide minerals. Soil Sci. Soc. Am. J. 60, 121–131. Mehlich, A., Tucker, M., 1983. Reference soil test methods for the southern region of the United States. Southern Cooperative Ser. Bull., Athens, GA. Morrison, J., 1969. Distribution of arsenic in poultry litter in broiler chicken, soils, and crops. J. Agr. Food Chem. 17, 1288–1290. Muddy Creek TMDL Establishment Workgroup. 1999. Fecal coliform TMDL development for Muddy Creek, VA.
B.L. Brown et al. / Applied Geochemistry 20 (2005) 123–133 VADEQ, Richmond, VA. Available from: http://www. deq.state.va.us/pdf/rivers/mdycrk.pdf (as of August, 2003). Mullins, G. 2000. Nutrient management plans – poultry. In: NRAES, (Ed.), Conference for Nutrient Management Consultants, Extension Educators, and Producer Advisors, Camp Hill, PA, 421–433. Rutherford, D.W., Bednar, A.J., Garbarino, J.R., Needham, K.W., Staver, K.W., Wershaw, R.L., 2003. Environmental fate of roxarsone in poultry litter. part II. Mobility of arsenic in soils amended with poultry litter. Environ. Sci. Technol. 37, 1515–1520. Rutherford, D. W., Garbarino, J. R., Kennedy, K. R., Wershaw, R. L., 2001. The sorption and extraction of arsenic in soils that have been amended with chicken manure. In: USGS Workshop on Arsenic in the Environment, February 21–22, 2001, Denver, CO. Seaman, J.C., Arey, J.S., Bertsch, P.M., 2001. Immobilization of nickel and other metals in contaminated sediments by hydroxyapatite addition. J. Environ. Qual. 30, 460–469. Sims, J., Wolf, D., 1994. Poultry waste management: agricultural and environmental issues. In: Sparks, D. (Ed.),
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Advances in Agronomy. Academic Press, San Diego, pp. 1–83. Stollenwerk, K., 2003. Geochemical processes controlling transport of arsenic in groundwater: a review of adsorption. In: Welch, A., Stollenwerk, K. (Eds.), Arsenic in Groundwater. Kluwer Academic Publishers, Boston, pp. 67–100. Tessier, A., Campbell, P., Bisson, M., 1979. Sequential extraction procedure for the speciation of particulate trace metals. Anal. Chem. 51, 844–851. Turpeinen, R., Pantsar-Kallio, M., Haggblom, M., Kairesalo, T., 1999. Influence of microbes on the mobilization, toxixity, and biomethylation of arsenic in soil. Sci. Total Environ. 236, 173–180. VADEQ. 1998. Poultry Waste Management. Richmond, VA. Available from:http://www.deq.state.va.us/pdf/general/ poultry (as of August, 2003). Wershaw, R., Garbarino, J., Burkhardt, M., 1999. Roxarsone in natural water systems. In: Effects of Animal Feeding Operations on Water Resources and the Environment. Proceedings of Technical Meeting. US Geological Survey, Fort Collins, CO.
Controls on roxarsone transport in agricultural watersheds B.L. Brown, A.D. Slaughter, M.E. Schreiber
*
Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg VA 24061-0420, USA Received 20 August 2003; accepted 5 June 2004 Editorial handling by A. H. Welch
Abstract The use of the organoarsenical roxarsone, added to poultry feed to increase weight gain, results in elevated As concentrations (10–50 mg/kg) in poultry litter. This litter is used extensively as fertilizer in agricultural regions. The authors investigated the sources and sinks of As within the vadose zone of an agricultural watershed in the Shenandoah Valley of Virginia, USA, an area of intense poultry production. Batch experiments were constructed to examine adsorption and biotransformation characteristics of roxarsone within the Ap and Bt soil horizons of Frederick series soils, common in the Shenandoah Valley. Roxarsone exhibits weak adsorption to the Ap soils; however, it is rapidly biotransformed to As(V) in this soil horizon. Although the Bt horizon demonstrated strong adsorption of roxarsone and thus may act as a sink for As species, soil water data collected from lysimeters at an agricultural field site suggest that As, as As(V), is mobile in the Bt soil water. It is unclear if this mobilization is due to competitive reactions with phosphate or organic acids, also present in litter. These results have implications for As cycling within poultry-dominated watersheds. For watersheds that have experienced years of litter application, As and other litter-associated species will be attenuated in soils through adsorption to mineral surfaces, but a variety of geochemical processes, such as competitive adsorption, may allow for enhanced transport of As through the vadose zone and into aquifer systems. 2004 Elsevier Ltd. All rights reserved.
1. Introduction The organoarsenical roxarsone (3-nitro-4-hydroxyphenylarsonic acid) is added to poultry feed at a concentration of 22.7–45.5 g/ton (20.6–41.3 mg/kg) (Anderson, 1983). Originally used to aid in the control of coccidiosis, roxarsone is now used for growth stimulation, improved feed conversion, better feathering, increased egg production, and pigmentation (Anderson,
* Corresponding author. Tel.: +1 540 231 3377; fax: +1 540 231 3386. E-mail address: [email protected] (M.E. Schreiber).
1983). Arsenic is detected at low levels in tissues of animals fed arsenicals (Calvert, 1975; Wershaw et al., 1999). However, studies have shown that the As level rapidly decreases to below the US Food and Drug Administration limit of 0.5 mg/kg when the arsenicals are removed from the feed for the required 5-day period before slaughter (Anderson and Chamblee, 2001). During the 42-day growth period, each broiler excretes about 150 mg of roxarsone (Anderson and Chamblee, 2001), resulting in a total As concentration of 10–50 mg/kg in poultry litter (Garbarino et al., 2003). Poultry litter, which is a mixture of poultry manure and bedding material, has widespread use as a fertilizer. In the Shenandoah Valley of Virginia, for example,
0883-2927/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2004.06.001
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there are approximately 33 · 107 kg of poultry litter produced each year, with most of the litter being applied as fertilizer on cropland (Mullins, 2000). Wershaw et al. (1999) estimate that approximately 106 kg/a of roxarsone and its degradation products are added annually to the environment worldwide from the use of poultry litter as fertilizer. In an early study of the distribution of As from poultry litter, Morrison (1969) compared fields that had poultry litter containing 15–30 mg/kg As applied over a 20-year period to fields that had no history of litter application. His results suggest that the roxarsone did not provide a significant input of As to the soil or crops, but the drainage water from litter-treated fields contained 0.29 mg/L As. This concentration is significantly higher than the recently lowered EPA As drinking water standard of 10 lg/L (0.01 mg/L). More recent investigations have documented that roxarsone can be leached easily from poultry litter. Studies by Jackson and Miller (1999) and Rutherford et al. (2003) found that 70–75% of total As from poultry litter is water soluble. Hancock et al. (2002) found concentrations of As up to 27 mg/kg in fresh litter; yet older, composted manure contained less than 2 mg/kg, suggesting that leaching of As had occurred during the composting process. Clearly, the cycling of roxarsone and its potential biotransformation products, including As(V), depends heavily on the extent to which these compounds interact with minerals and organic matter in soils and aquifer sediments. In a study on adsorption equilibria of monomethylarsenate and dimethylarsenate on hydrous Fe(III) oxide, Cox and Ghosh (1994) observed that adsorption of these methylated As(V) compounds was weakly dependent on ionic strength, suggesting inner-sphere surface complexation reactions, as has been suggested for As(V) adsorption to Fe oxides (e.g., Stollenwerk, 2003). Using results of extraction experiments, Rutherford et al. (2003) found a strong correlation between As and acid-extractable Fe in soils from litter-treated fields, suggesting adsorption to or co-precipitation of the As with Fe oxides. To date, however, no quantitative studies on roxarsone adsorption have been conducted. The main goal was to conduct a quantitative study of roxarsone adsorption to the Frederick series soils, the main soil type in the Shenandoah Valley of Virginia, with a broader objective of identifying sources and sinks of As within agricultural watersheds. A secondary objective was to examine the potential for roxarsone biotransformation by microorganisms present in the Frederick soils. The study site is located within the Chesapeake Bay watershed, within which 98% of Virginias concentrated poultry operations are located (VADEQ, 1998). Many other poultry-producing sectors of the US including regions of Kentucky, Arkansas, Missouri and Oklahoma, are located in similar geologic (carbonate)
environments. Thus, results from this research will yield critical information for examining As cycling in watersheds where poultry litter is applied.
2. Field study area The study area is located in a subcatchment (1.2 km2) of the Muddy Creek watershed, approximately 20 km NE of Harrisonburg in Rockingham Co., Virginia, USA. The subcatchment is underlain by the Lower Ordovician-Upper Cambrian Conococheague Limestone, consisting of interbedded limestone, dolostone, and sandstone. The soils of the Muddy Creek subcatchment are of the well-drained silt loam of the Frederick series (Hockman et al., 1982). The vadose zone at the site is highly variable, with thicknesses ranging from 0 to 20 m below surface. The depth to groundwater is also variable, and ranges from 2 to 20 m below surface (Hyer et al., 2001). The study area contains four small monitoring sites, which are instrumented with a total of seven monitoring wells, four drive point samplers, two stream gauging stations, and a tipping-bucket precipitation gauge. Three zero-tension (pan) lysimeters, which collect up to 5 L of soil water at 15, 45, and 90 cm depths, are installed at each of the 4 monitoring sites. Details on lysimeter construction can be found in Kaufmann (1998). The study area includes two farms, which contain cornfields, cattle pastures, and a poultry barn. Dry poultry manure is applied on the cornfields at the annual rate of about 4500 kg/ha, as a fertilizer and method of waste disposal (Hyer et al., 2001). A study of poultry production in the larger Muddy Creek watershed documented close to 1 · 106 animal units, which contribute approximately 2 · 107 kg of poultry litter to the watershed annually (Muddy Creek TMDL Establishment Workgroup, 1999).
3. Methods 3.1. Experimental methods 3.1.1. Soil collection Soil samples for the experiments were collected from a litter-treated cornfield at the Muddy Creek site and from a control site (no poultry litter applied) in Rockingham Co., Virginia. At both sites, samples were collected from the top two horizons (Ap, Bt1) of the Frederick soil profile. At the treated site, soils were collected from four locations within the field and were homogenized. At the control site, soils were collected from one dug trench. Poultry litter was also collected from one of the poultry houses at the field site. Selected characteristics of the soils are presented in Table 1. The
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Table 1 Characteristics of treated and control Frederick series soils Soil type
pH
SOM (%)
Sand (%)
Silt (%)
Clay (%)
Textural class
Specific surface area (m2/g)
Ap-control Ap-treated Bt1-control Bt1-treated
6.1 6.8 5.9 6.3
1.4 2.8 0.7 0.9
27.5 52.2 15.7 22.0
64.6 40.9 68.0 44.4
7.9 6.9 16.3 33.6
Silt loam Loam/Sandy loam Silt loam Clay loam
5.17 6.33 NA NA
Notes: NA, not analyzed; SOM, soil organic matter.
differences in physical properties between the Ap-control and Ap-treated soils are largely due to different management at each site. Management techniques can affect erosion, and thus soil physical properties. For the B horizon, the differences, which are smaller than for the A horizon, can be attributed to the variable nature of the parent materials. 3.1.2. Soil extractions The Ap and Bt soils were extracted for soil organic matter (SOM), soil test P, and selected components found in poultry litter (Cu, Zn, Fe, As) (Table 2). Extractions for soluble components were conducted over a 3-day period on a wrist shaker using 6.25 g soil with 25 mL 0.01 M NaCl adjusted to pH 5. SOM was determined using a Walkey Black chromic acid oxidation method (Jackson, 1960). Soil test P, which includes Al- and Fe-phosphates and P adsorbed on colloidal surfaces, was determined using a Mehlich 1 extraction (Mehlich and Tucker, 1983). ‘‘Total’’ concentrations of As, Cu, Zn, and Fe were extracted from the soils and
poultry litter using an aqua regia digestion. Filtered (0.2 lm) extracts were collected and preserved as necessary (see Section 3.3). The Ap soils were also subjected to sequential extraction for As using a modified method of Tessier et al. (1979), as detailed by Turpeinen et al. (1999). This procedure is used to delineate the As sequestered in solid phases as exchangeable, bound to easily reducible metal-oxide, bound to organic matter, bound to crystalline Fe- and Al-oxide, and residual. The extraction steps are outlined in Table 3. Prior to extraction, soils were homogenized, dried and crushed. Soils were washed with Nanopure water and centrifuged between the extraction steps. The extractions included two modifications of the Turpeinen et al. (1999) procedure. For the organic matter extraction, a procedure utilizing potassium perchlorate, outlined by Breit et al. (2002), was used. For extraction of As from crystalline Fe oxides, an oxalic-dithionite extraction procedure, outlined by Rutherford et al. (2001), was used.
Table 2 Solublea and totalb concentrations of selected components in soils and poultry litter
Soluble Ap-control Ap-treated Bt1-control Bt1-treated Poultry litter
Total Ap-control Ap-treated Bt1-control Bt1-treated Poultry litter
Asa (mg/kg)
DOCa (mg/kg)
Fea (mg/kg)
Pa (mg/kg)
Cua (mg/kg)
Zna (mg/kg)
bdl 0.052 bdl bdl 0.568
24.8 63.2 13.2 21.2 92.4
0.266 4.956 bdl bdl 54.40
0.428 29.32 0.074 0.134 866.0
0.030 0.192 bdl 0.006 44.60
0.567 1.811 0.022 0.032 22.96
Asb (mg/kg)
SOMc
Feb (%)
Soil test Pd (mg/kg)
Cub (mg/kg)
Znb (mg/kg)
6.58 7.03 5.10 10.58 15.5
1.4 2.8 0.7 0.9 NM
1.04 1.56 1.63 3.11 4.09
9 298 1 30 NM
11.9 86.7 7.9 52.0 213
28.3 108 27.7 46.7 124
Notes: DOC, dissolved organic C; SOM, soil organic matter; NM, not measured; bdl, below detection limit; Values represent averages of triplicate samples. a Extracted with 0.01 M NaCl. b Aqua regia digestion. c Walkley–Black extraction. d Mehlich 1 extraction.
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3.1.3. Adsorption experiments All adsorption experiments were conducted using ASTM Method D 4319-93 (ASTM, 1993). Prior to use, soils were dried and sieved to <2 mm. Two series of experiments were conducted: (1) kinetic experiments, to determine the time required for roxarsone adsorption to reach equilibrium with the Ap and Bt soils; (2) isotherm experiments, to determine distribution coefficients for roxarsone adsorption to the two soils. For each experiment, 6.25 g of unsterilized homogenized soil were placed in an acid-washed centrifuge tube with 25 mL of 0.01 M NaCl. The pH was adjusted to five by addition of NaOH or HCl. Kinetic trials were spiked with 100 lg/L of roxarsone as As. Isotherm trials were spiked with 0, 100, 200, 500, and 1000 lg/L of roxarsone as As. Measurements of pH were made frequently during the experiments and pH was adjusted if it deviated more than ±0.2 pH units. After spiking with roxarsone, the soil/solution mixture was placed on a wrist shaker for up to 12 days (kinetic experiments) or three days (isotherm experiments). Both experiments were sampled destructively. During collection, samples were first centrifuged for 30 min (3500 · g) at 25 C and were then filtered (0.2 lm) and preserved as necessary (see Section 3.3). Experiments were conducted either in duplicate (kinetic) or in triplicate (isotherm). 3.1.4. Biotransformation experiments Because the soils were not sterilized prior to use in the adsorption experiments, additional batch experiments were conducted to determine the potential for biotransformation of roxarsone. Experiments were conducted using 6.25 g soil and 25 mL of 0.01 M NaCl solution, adjusted to pH 5. Two series of experiments were conducted: (1) initial experiments, to examine the potential for biotransformation in Ap and Bt soils over a 3-day period; (2) a kinetic experiment, to evaluate roxarsone transformation in the Ap soil over a 12-day period. For both experiments, live and sterilized (1% sodium azide) trials were spiked with 100 lg/L roxarsone as As and were incubated on a wrist shaker in the dark (to prevent photooxidation) at 25 C. Destructive samples were collected at three days for the initial experiment and at 3, 6, 9, and 12 days for the kinetic experiment. During collection, samples were first centrifuged for 30 min (3500 · g) at 25 C and were then filtered (0.2 lm) and preserved as necessary (see Section 3.3). 3.2. Field methods The zero-tension (pan) lysimeters were sampled in May 2003, after litter application. Samples were collected under low-flow (<10 mL/min) pumping conditions. Prior to sample collection, field measurements, including pH, temperature, specific conductance, and
dissolved O2, were taken. Samples for chemical analysis were filtered (0.2 lm), preserved (see Section 3.3), and kept at 4 C until analysis. 3.3. Analytical methods 3.3.1. Soil samples Grain size analysis on soil samples was conducted using pipette and hydrometer methods. Specific surface area was determined using N2-BET methods. As mentioned above, SOM was analyzed using the Walkley– Black chromic acid oxidation method. The Mehlich 1 extraction (Mehlich and Tucker, 1983) of soil test P was analyzed using colorimetric analysis ‘‘Total’’ As, Cu, Zn and Fe (aqua regia extraction), as well as soluble As, Cu, Zn, Fe, and P (0.01 M NaCl extraction) were analyzed using ICP-AES. Arsenic concentrations in samples collected from sequential extraction procedures were analyzed using graphite furnace atomic absorption spectrometry (GFAAS) with Zeeman background correction. 3.3.2. Aqueous (experimental and field) samples Samples for total As analysis were preserved with HNO3 and analyzed by GFAAS. During initial method development, it was determined that analysis by GFAAS yielded full recovery of As from roxarsone (Brown, 2003). Samples for As speciation were preserved with EDTA and analyzed using HPLC separation combined with hydride generation (HG) and detection by ICP-AES, following the method of Garbarino et al. (2002). Samples were analyzed for DOC by combustion after preservation with HCl. Samples for Fe, Cu and Zn were preserved with HNO3 and analyzed by ICP-AES. Samples for PO4 were analyzed by ion chromatography.
4. Results 4.1. Soil extractions Results of soluble extractions of the soils and poultry litter are shown in Table 2. Although there were not enough data collected to make statistically valid interpretations, detectable soluble (0.01 M NaCl) As was extracted from poultry litter and Ap-treated soils, but not from Ap-control or Bt soils. This soluble As is also associated with higher concentrations of DOC, and soluble Fe, P, Cu and Zn. Also shown in Table 2 are ‘‘total’’ (as determined by aqua regia digestion) concentrations of As, Fe, Cu and Zn, soil test P, and SOM. Similar to the results of the soluble extractions, the treated soils have higher concentrations of P, Cu and Zn than do the control soils. It is interesting to note that although all soils contained at least 1 wt% of total
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Table 3 Arsenic sequential extraction results Extraction step
As (mg/kg), Ap-control
As (mg/kg), Ap-treated
Extraction procedurea
1. Exchangeable
bdl
0.067
2. Easily reducible metal oxide
bdl
0.086
4. Organic Matter
5.642
7.242
4. Crystalline Fe oxide
0.256
0.212
1 M MgCl2 (25 mL, pH 7); 4 h (25 C); shaker 0.1 M NH2 OH Æ HCl (10 mL) + 0.01 M HNO3 (15 mL); 30 min (25 C); shaker Add 0.5 g KClO4 and 10 mL of concentrated HCl; 45 min (25 C) intermittent shaking 0.11 M (NH4)2 C 2O4 (10 mL) + 0.09 M C2 H2O4 (10 mL); 45 min (80 C), add 0.4 g Na2 S2 O4 ; 45 min (80 C), cool, rotate overnight, centrifuge
Sequential Extraction Total Aqua Regia Digestion Total
5.898 6.58
7.607 7.03
Addition of Steps 1–4 2 h digestion with aqua regia
Notes: bdl, below detection limit of 0.0056 mg/kg.Values represent averages of triplicate measurements. a Method outlined by Turpeinen et al. (1999), modified by Brown (2003).
Fe, soluble Fe was extracted from the Ap soils but not the Bt soils. This is perhaps due to the higher concentrations of DOC in the Ap soils, which can complex Fe and increase its solubility in water. Total As concentrations do not appear to be significantly higher in the treated soils than in the control soils, but additional sampling would be needed to confirm or deny this. 4.2. Sequential extractions Results of the sequential extraction of the Ap soils are presented in Table 3. Summation of the As concentrations from the sequential extraction procedure is within 10% of the ‘‘total’’ (aqua regia digestion) concentration, suggesting that the procedure was successful in extracting As from all solid phases within these soils. Due to the multiple steps required in this procedure, only one sample each of the Ap-control and Ap-treated samples was extracted, and thus, results cannot be used to show statistically significant differences between the two. However, the results can be used to qualitatively show that although the total As in these two soils are similar (6.58 for control and 7.03 mg/kg for treated), the distribution of As within solid phases in which As is sequestered differs. Arsenic is associated within organic matter and crystalline Fe-oxide in both soils. In the Ap-treated soil, however, additional As is found in more mobile forms, including an exchangeable fraction and a fraction bound to easily reducible metal oxides. 4.3. Adsorption kinetics The first adsorption experiment was designed to evaluate the time required for roxarsone adsorption on
Frederick series soils to reach steady state. Ap- and Bt-treated soils were spiked with 100 lg/L of roxarsone as As and were allowed to equilibrate at pH 5 for up to 12 days (Fig. 1). Results show several interesting trends: (1) roxarsone adsorption is greater to Bt soils than to Ap soils; (2) roxarsone adsorption to the Bt-treated soils does not change significantly with time; (3) in the live Ap-treated soils, there was considerably more variability in concentration within each sampling period, and the overall trend shows decreasing solution concentrations, or increasing adsorption, over time. 4.4. Initial biotransformation experiments Results of the adsorption kinetics experiment (Fig. 1) demonstrated a kinetic control on adsorption of what was thought to be roxarsone on the Ap soils. An initial biotransformation experiment, utilizing live and killed Ap- and Bt-treated soils, was conducted to determine if the kinetic control was impacted by microbial activity. Arsenic speciation analysis (Fig. 2) indicated that in the live Ap-treated soils, approximately 5% of the roxarsone was transformed to As(V) over a three day period. Transformation was not observed in killed Ap-treated soils nor was it observed in live or killed Bt-treated soils. Although these results demonstrate that roxarsone biotransformation can occur in the live Ap soils, the adsorption isotherm experiments were conducted using live (instead of killed) soils for the following reasons: (1) the biotransformation was relatively minor (5%) over the 3-day period chosen for the adsorption isotherm experiments; (2) sterilization would alter soil physicochemical properties, which could significantly impact adsorption characteristics; and (3) live soils are more representative of field conditions.
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Fig. 1. Results of kinetic adsorption experiments at pH 5, showing adsorbed total As (C*, lg As/g soil) vs time for live treated Ap and Bt soils after 100 lg/L roxarsone-As spike. Results represent averaged concentrations from duplicate experiments.
4.5. Bt adsorption isotherms The isotherms for the Bt soils show strong adsorption of roxarsone at pH 5 as compared with the Ap soils (Fig. 3). Because the relationship between solution and sorbed roxarsone is linear at the concentrations used in these experiments, partition coefficients (Kd, in L/g) were calculated using the equation C* = KdC, where C* = mass of solute sorbed per dry weight of solid (lg/g) and C = concentration of solute in solution in equilibrium with the mass of solute sorbed onto the solid (lg/L). The calculated Kd values for Bt1-treated and Bt1-control soils at pH 5 are 0.165 and 0.259 L/g, respectively (Table 4). 4.6. Ap adsorption isotherms It should be noted that the isotherm data were collected assuming that all As was present as roxarsone. However, as described above, minor biotransformation of roxarsone occurred in the Ap soils over the threeday experimental period, so the isotherms represent adsorption characteristics of a roxarsone–As(V) mixture. Even with this limitation, the data show interesting trends. First, similar to the Bt soils, the adsorption isotherms for the Ap soils at pH 5 show a linear response over the range in roxarsone concentrations used; however there is much more variation than for the Bt soils (Fig. 3). Adsorption of roxarsone was stronger to the Ap-control soils than to the Ap-treated soils; this difference is shown in the calculated Kd values (pH 5), which are 0.001 and 0.005 L/g for Ap-treated and Ap-control soils, respectively (Table 4). 4.7. Biotransformation kinetics To further examine the potential for roxarsone biotransformation, a kinetic experiment using Ap soils
was conducted over a 12-day period; results are shown in Figs. 4 and 5. The live Ap-control soil shows an increase in adsorption of total As over time (Fig. 4). In contrast, adsorption does not change in killed Ap-treated soil. Analysis of As speciation in these experiments (Fig. 5) shows that in the live soils, roxarsone concentrations are declining over time, corresponding with an increase in As(V). In the killed soils, however, roxarsone concentrations do not change over time, and very little, if any, As(V) is detected.
5. Discussion 5.1. Impact of poultry litter application on soil and soil water chemistry Although the sample size is small, the extraction results for total and soluble components of the Ap soils (Fig. 2) suggest that litter-treated soils have higher concentrations of organic matter, P, Cu and Zn than the control soils. Similar results have been reported from a variety of surface soils to which poultry litter has been applied (e.g., Kingery et al., 1994; Gupta and Charles, 1999; Jackson et al., 2003; Rutherford et al., 2003). The increased P loading in watersheds where poultry litter is applied is a concern for water quality, as P increases have been linked to eutrophication (Sims and Wolf, 1994; Mullins, 2000). Phosphorus loading is also a concern from the standpoint of As transport, as between 21% and 58% of total P in animal manure is inorganic (Barnett, 1994); inorganic phosphate can compete with As oxyanions for adsorption sites on mineral surfaces. The presence of higher concentrations of soluble Cu, Fe, Zn, P, and DOC in the Ap-treated soils than in the Ap-control soils suggests these components are leaching from poultry litter. DOC can enhance the
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Fig. 3. Adsorption isotherms for roxarsone to live Ap and Bt soils at pH 5. Error bars represent standard deviation of triplicate experiments. Solid lines represent linear regressions (not through origin) of averaged data. Regression is not forced through zero because initial concentration of As in soils is not zero in all cases (see Table 2). Table 4 Calculated Kda values for roxarsone at pH 5 Soil type Ap-control Ap-treated Bt1-control Bt1-treated
Kd (L/g)
Kd (L/g) (SOM removed) b
0.005 ± 0.002 0.001 ± 0.001b 0.260 ± 0.077 0.165 ± 0.048
0.003 ± 0.0003 0.004 ± 0.0003 NM NM
Notes: SOM, soil organic matter; NM, not measured. Values based on the averaged data of triplicate experiments. ± = standard error. a Calculated using a linear regression method where the regression line was forced through zero. Statistical analysis demonstrated no significant difference between this method and one where the regression line was not fit through zero, indicating that leaching of low concentrations of soluble As in the Ap-treated soils (see Table 2) does not affect the calculated Kd value. b Because minor biotransformation of roxarsone to As(V) occurred in the Ap soils, these values represent the adsorption characteristics of a mixture of roxarsone and As(V). Fig. 2. Results of initial biotransformation experiments, showing As speciation chromatographs (area counts vs. step time) of HPLC-HG-ICP-AES analysis after 3-day reaction of roxarsone with: (a) live Ap-treated soil; (b) killed Ap-treated soil; (c) live Bt1-treated soil.
transport of litter-associated metals, such as Cu and Zn, through the soil column, increasing the potential for leaching into ground and surface water (e.g., Li and Shuman, 1997; Gupta and Charles, 1999; Aldrich et al., 2002; Jackson et al., 2003). Soluble (0.01 M NaCl) As was only detected in Aptreated soils and poultry litter (Table 2). Results of sequential extraction (Table 3) of the Ap soils show similar patterns; the treated soil has a higher concentration
of exchangeable As than does the control. These results are consistent with those of Jackson et al. (2003) and Rutherford et al. (2003), who found that 70–75% of As in poultry litter is water soluble. Extractions of soluble components from Bt soils (Table 2) show fewer differences between treated and control soils. It is interesting to note that despite the presence of Fe and As (total digested) in Bt soils, the soluble fractions of these elements are low (Table 3). 5.2. Roxarsone adsorption to Frederick series soils Roxarsone adsorbs strongly to Bt soils (Kd at pH 5 of 0.165 and 0.259 L/g, for control and treated soils,
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Fig. 4. Adsorbed total As (C*, lg As/g soil) at pH 5 for live and killed Ap-control soils after 100 lg/L roxarsone-As spike. Error bars represent standard deviation of triplicate experiments.
Fig. 5. Concentration of As species (arsenate and roxarsone; arsenite was not detected) in 12-day biotransformation kinetic experiments for: (a) live Ap-control soils; (b) killed Ap-control soils.
respectively). Several lines of evidence suggest that the adsorption patterns of roxarsone to Bt soils are controlled by adsorption of the As(V) functional group on
roxarsone to Fe oxides. Results of the total and soluble extractions show that although As is present in the Bt soils (control: 5.1 mg/kg; treated: 10.58 mg/kg), the As is not soluble. In addition, total As in the Bt soils appears to be associated with higher total Fe concentrations (Table 2). As has been discussed in other studies, As(V) forms inner-sphere complexes with Fe oxides; removal of this strongly-bound As requires dissolution of the Fe oxides or addition of a competitive anion such as PO4. Additional support for As(V) bonding to Fe oxides is provided by Jackson and Miller (2000), who found similar desorption patterns of roxarsone and p-arsanilic acid from Fe oxides. Because both of these compounds have As(V) functional groups bound to benzene rings through As–C bonds, but differ with respect to other functional groups (p-arsanilic acid has an amino group, while roxarsone contains nitro and hydroxyl groups), the results suggest that the As(V) oxyanion is the functional group that is actively interacting with the Fe oxide surface. The adsorption patterns of roxarsone to the Ap soils display two important features: weaker adsorption in comparison with Bt soils, and weaker adsorption to the treated than to the control soils. These patterns are likely controlled by a number of factors, including low availability of strong sorbents (e.g., Fe oxides) in the Ap soils. It is well documented that arsenate anions have a strong adsorption affinity for Fe oxides and clays (see Stollenwerk, 2003, for review). The Ap soils have significantly lower concentrations of clays (Table 1) and solid phase Fe (proxy for Fe oxides; Table 2) than do the Bt soils, which is the most likely explanation for the weaker adsorption characteristics. The differences in behavior of roxarsone adsorption to the Ap-treated and Ap-control soils could be caused by a number of factors, including differences in grain size, concentrations of SOM, or concentrations of competing anions. Although there are differences in grain size distribution between the Ap-treated and Ap-control soils, which is most likely caused by differences in agricultural management practices, the specific surface areas for these soils are similar (5.17 and 6.33 m2/g for Ap-control and treated, respectively), suggesting that differences in grain size is not the main control on the adsorption differences. SOM concentrations are higher in the treated (2.8%) than in the control soils (1.4%); because As(V) adsorbs more strongly to mineral surfaces than to organic matter, the weaker adsorption of roxarsone to the treated soils could be impacted by differences in SOM. Phosphate and phytic acid, which are present in poultry litter (Barnett, 1994), have been shown to be strong competitors with arsenate for adsorption sites (e.g., Manning and Goldberg, 1996; Jackson and Miller, 2000; Seaman et al., 2001). The Ap-treated soils have significantly higher soil test P than the Ap-control soils (298 vs. 9 mg/kg; Table 2). Further evidence for the pres-
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Fig. 6. Concentrations of selected chemical parameters (As, Fe, DOC, PO4) of soil water collected from lysimeters at Muddy Creek site in May 2003. The pH of soil water ranges from 5.5 to 6.0: (a) Data from upper site, treated with poultry litter; (b) Data from lower site, not treated with poultry litter. Arsenic and Fe at lower site are below detection (detection limits: 2 lg/L (As) and 10 lg/L (Fe)). Note break in depth on y-axis. Delineation of Ap and Bt horizons is approximate.
ence of competing anions in the treated soils is from the soil water data collected from the Muddy Creek site (Fig. 6). Both PO4 and DOC are present in higher concentrations in soil water from the litter-treated field than in a control field to which litter was not applied. 5.3. Roxarsone biotransformation to As(V)
6, the lysimeter results are not directly comparable to experimental data (pH 5), but they do demonstrate that As can be leached from solid surfaces through the upper soil column. These results have implications for As cycling within agricultural watersheds in the Shenandoah Valley and other poultry-dominated agricultural regions, outlined in a conceptual model in Fig. 7. In surface soils (and
Results of the study demonstrate that roxarsone is biotransformed to As(V) in Ap soils, which is the most probable cause of the kinetic behavior of adsorption of As to these soils. As(V) is adsorbed much more strongly than roxarsone to the Ap soils; thus, as roxarsone is biotransformed to As(V) over time, the apparent adsorption increases. The initial three-day experiment did not show buildup of As(V) in Bt-treated soils; however, additional experiments are being conducted to examine the potential for roxarsone biotransformation over a longer time period. 5.4. Implications for roxarsone transport in agricultural watersheds Although the laboratory experiments demonstrate strong adsorption of low concentrations of roxarsone to Bt soils (0.4–4 lg roxarsone-As/g soil) at pH 5, field monitoring of lysimeters show As present in soil water underlying fields to which poultry litter has been applied (Fig. 6). Because soil water pH values range from 5.5 to
Fig. 7. Conceptual model of roxarsone and As(V) transport in vadose zone of agricultural watersheds in which poultry litter is applied.
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likely within poultry litter), roxarsone can be biotransformed to As(V), which is a more toxic form of As. Roxarsone adsorbs weakly to the Ap soils and can thus leach into the Bt soils. In the Bt horizon, roxarsone can either undergo further biotransformation (which was not observed in the experiments over a 3-day period), or be adsorbed. Although the low concentrations of As(V) produced from biotransformation should adsorb strongly to mineral surfaces, as has been shown in many other studies, the lysimeter data demonstrate that As(V) is detected in soil water in the Ap ands Bt horizons underlying a field to which poultry litter has been applied, suggesting that other biogeochemical processes, such as competitive adsorption from PO4 or organic acids, may allow for enhanced transport through the vadose zone and into ground water systems. Additional experiments are being conducted to address these questions. Acknowledgements This work was supported by funding from the USDA Watershed Processes Program and the Virginia Water Resources Research Center. The authors thank Greg Mullins, Mike Brosius, Jody Smiley, Nancy Frank, John Garbarino, and Tracy Hancock for assistance with sampling and analysis. Comments by Alan Welch, Robert Wershaw and Donald Sparks greatly improved the manuscript.
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