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Published online 1 May 2008
Published in J Environ Qual 37:963-971 (2008)
DOI: 10.2134/jeq2007.0416
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Waste Management

Controlling the Fate of Roxarsone and Inorganic Arsenic in Poultry Litter

Konstantinos C. Makrisa,*, Jason Salazara, Shahida Quazia, Syam S. Andraa, Dibyendu Sarkara, Stephan B. H. Bachb and Rupali Dattaa

a Environmental Geochemistry Lab., Univ. of Texas at San Antonio, TX 78249
b Dep. of Chemistry, Univ. of Texas at San Antonio, TX, 78249

* Corresponding author (konstantinos.makris{at}utsa.edu).

Received for publication August 6, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
A growing body of literature reports 3-nitro-4-hydroxyphenylarsonic acid (roxarsone) degradation in poultry litter (PL) to the more toxic inorganic arsenic (As). Aluminum-based drinking-water treatment residuals (WTR) present a low-cost amendment technology to reduce As availability in PL, similar to the use of alum to reduce phosphorus availability. Batch experiments investigated the effectiveness of WTR in removing roxarsone and inorganic As species from PL aqueous suspensions. Incubation experiments with WTR-amended PL evaluated the effects of WTR application rates (2.5–15% by weight) and incubation time (up to 32 d) at two incubation temperatures (23 and 35°C) on As availability in PL. Batch PL aqueous experiments showed the high affinity of As(V), As(III), dimethylarsinic acid (DMA), monomethylarsonic acid (MMA), and roxarsone for the WTR. The 10% WTR amendment rate decreased As availability in PL by half of that of the unamended (no WTR) PL-incubated samples. The reduction in dissolved As concentrations during incubation of WTR-amended PL samples was kinetically limited, being complete within 13 d. Parallel reductions in roxarsone, As(V), and DMA concentrations were observed with liquid chromatography–inductively coupled plasma mass spectrometry, whereas As(III) and MMA concentrations were always <5% of dissolved As. Incubation temperature did not significantly (p > 0.05) influence dissolved As concentrations in the WTR-amended PL. Potential formation of a copper-containing roxarsone metabolite was considered in PL aqueous suspensions with the aid of electrospray mass spectrometry. Further experiments in the field are necessary to ensure that sorbed As is stable in WTR-amended PL.

Abbreviations: CID, collision-induced dissociation • d-H2O, deionized H2O • DMA, dimethylarsinic acid • ES-MS, electrospray ionization mass spectrometry • GFAAS, graphite furnace atomic absorption spectroscopy • LC-ICPMS, liquid chromatography–inductively coupled plasma mass spectrometry • MMA, monomethylarsonic acid • PL, poultry litter • WTR, water-treatment residuals


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
CURRENT use of fresh, composted, or pelletized poultry litter (PL) as a fertilizer to agricultural land and home gardens may be impeded by recent evidence reporting 3-nitro-4-hydroxyphenylarsonic acid (roxarsone) degradation in PL (Bellows, 2005; Christen, 2006). Broilers are routinely fed roxarsone, an organoarsenical compound, for the control of coccidiosis and growth promotion (Blakley et al., 1990; Chapman and Johnson, 2002). A growing body of data suggests that roxarsone degradation in land-applied PL could contaminate crops and water bodies with toxic inorganic As (Jackson and Bertsch, 2001; Bednar et al., 2002; Sierra-Alvarez et al., 2006; Cortinas et al., 2006; Schaefer, 2007; Stolz et al., 2007). Total As concentrations in PL usually range from <1 to 40 mg kg–1 (Nicholson et al., 1999; Jackson and Bertsch, 2001; Garbarino et al., 2003; Rutherford et al., 2003; Anderson and Chamblee, 2001; Jackson et al., 2003).

Roxarsone and As(V) were the major As species in PL water extracts along with a number of unidentified roxarsone metabolites, while minor quantities of As(III), dimethylarsinic acid (DMA), monomethylarsonic acid (MMA), and 3-amino-4-hydroxyphenylarsonic acid were also detected (Garbarino et al., 2003; Rutherford et al., 2003; Jackson et al., 2003; Cortinas et al., 2006; Stolz et al., 2007). Environmental factors, such as moisture content, temperature, and incubation time, allow for certain microorganisms to degrade PL-containing roxarsone to inorganic As (Garbarino et al., 2003; Cortinas et al., 2006). Anaerobic conditions during PL composting favor bacteria of the genus Clostridium in selectively degrading roxarsone to inorganic As and other unidentified organoarsenical compounds (Stolz et al., 2007).

Chemical amendments to PL, such as aluminum sulfate (alum), have been used to control phosphorus (P) solubility and ammonia emissions via the formation of a mixed organo-aluminum-P precipitate (Moore et al., 1999; Smith et al., 2004). Substitution of aluminum-based drinking-water-treatment residuals (WTR) for alum could be a cost-effective practice to reduce soluble P concentrations in PL (Makris et al., 2005). Development of As-mitigating agents is deemed necessary to immobilize organic and inorganic As species in land-applied PL and making them unavailable for plant uptake or leaching to the groundwater. The WTR is the waste by-product of the drinking-water treatment process enriched in mixtures of Fe, Al hydr(oxides), or CaCO3, with organic carbon (C) reaching concentrations in the order of 200 g C kg–1 (Makris et al., 2005). Chemical similarities between P and As and the fact that both oxyanions are measured in PL led us to hypothesize that WTR could function as an effective As- and P-mitigating agent in PL (Makris et al., 2005). Batch experiments in our laboratory showed tremendous reactivity of WTR toward soluble As(V) and As(III), reaching As sorption capacities of ≥15,000 mg kg–1 (Makris et al., 2006). Land-applied WTR significantly (p < 0.001) increased the As sorption capacity of the soil compared with that of the unamended (no WTR) soil (Sarkar et al., 2007).

It was hypothesized that the WTR amendment could significantly decrease dissolved (<0.45 µm) roxarsone and inorganic As concentrations in PL. The objectives of this study were to (i) evaluate the effectiveness of WTR in removing roxarsone and its metabolites from PL aqueous suspensions during batch and incubation experiments, (ii) determine the effect of WTR application rates on reducing dissolved As concentrations in PL aqueous suspensions, and (iii) investigate the effects of incubation time and temperature on decreasing As availability in PL aqueous suspensions using liquid chromatography–inductively coupled plasma mass spectrometry (LC-ICPMS) and electrospray ionization mass spectrometry (ES-MS).


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Materials Characterization
The Al-based WTR was obtained from the drinking-water treatment plant in Bradenton, Florida. A homogenous PL sample that was taken from a PL storage facility was kindly supplied by Dr. J.H. Grove (Univ. of Kentucky). The pH and electrical conductivity of the WTR and the PL were measured in a 0.01 mol L–1 CaCl2 solution at a 1:10 solid/solution ratio after 1 d of reaction. Total C and N content was determined by combustion at 1010°C using a 2400 Series II CHNS/O Analyzer (PerkinElmer Life and Analytical Sciences Inc., Waltham, MA). Determination of percentage solids was performed by drying the materials at 105°C for 2 d (Rhoades, 1996). The WTR and PL samples were analyzed for total recoverable As, P, Fe, and Al concentrations by ICPMS after digestion according to the USEPA Method 3050B (USEPA, 2000). Oxalate (200 mmol L–1)-extractable As, P, Fe, and Al concentrations of the materials were determined by ICPMS after extraction (in the dark) at a 0.017 g mL–1 solid/solution ratio (McKeague et al., 1971).

As Sorption by the WTR in PL Aqueous Suspensions
Potential release of As from PL into solution was measured in batch using deionized H2O (d-H2O). Fresh PL samples (1 g dry weight) were shaken (85 g) with d-H2O in the dark at a solid/solution ratio of 0.1 g solid mL–1 for 0.16, 0.5, 1, 2, 4, 8, 24, 48, and 96 h at room temperature (23 ± 2°C). After shaking, tubes were centrifuged at 5000 g for 20 min, and the pH and redox potential of the supernatant was recorded. Samples were passed through 0.45-µm filters and analyzed for dissolved As and concentrations of several known As species using graphite furnace atomic absorption spectroscopy (GFAAS) and LC-ICPMS, respectively. The known As species in this study were As(III), As(V), DMA, MMA, and roxarsone. Quantification of the known As species was conducted according to the As speciation protocol by Bednar et al. (2004) with the aid of LC-ICPMS. Stock solutions for the As species were prepared in 0.25 mmol L–1 EDTA and preserved in the dark at 4°C. An LC system (Prostar 210; Varian, Inc., Palo Alto, CA) delivered a 50-µL sample to a Dionex column IonPac AS7 (Dionex, Sunnyvale, CA) (4 x 250 mm) and its respective guard column Dionex IonPac AG7 (4 x 50 mm) at a 1 mL min–1 flow rate. The LC was coupled to an ICPMS (Elan 9000; PerkinElmer) that was detecting As+ at m/z = 74.9 (ArCl interference was <1%). The mobile phase program consisted of 0 to 1 min 2.5 mmol L–1 HNO3 in 0.5% methanol, 1 to 3 min 2.5 to 50 mmol L–1 HNO3 in 0.5% methanol, 3 to 5.5 min 50 mmol L–1 HNO3 in 0.5% methanol, and 5.50 to 7 min 2.5 mmol L–1 HNO3 in 0.5% methanol.

Another batch experiment evaluated the effectiveness of the WTR in removing different As species from aqueous PL suspensions. The fresh PL aqueous suspension was prepared in bulk by shaking (85 g) with d-H2O for 2 h using a solid/solution ratio of 0.1 g mL–1. After 2 h of shaking, the suspension was allowed to settle for 30 min, and the supernatant was carefully separated from the settled PL particles. The PL supernatant was brought into contact with WTR to maintain a 0.1 g mL–1 solid/solution ratio and shaken (85 g) for 0.16, 0.5, 1, 2, 4, 8, 24, 48, and 96 h at room temperature (23 ± 2°C). After shaking, samples were centrifuged at 5000 g for 20 min, filtered (0.45 µm), and analyzed for dissolved As and the concentrations of known As species using GFAAS and LC-ICPMS, respectively.

After sorption, the supernatant was removed, and WTR-containing tubes were filled with 0.01 mol L–1 KCl solution (1 g WTR: 10 mL KCl) to determine As desorption from the WTR. Suspensions were shaken (85 g) for 1, 2, 4, 8, and 14 d in the dark at room temperature (23 ± 2°C). After shaking, samples were centrifuged (5000 g), filtered (0.45 µm), and analyzed for dissolved As and several known As species using GFAAS and LC-ICPMS, respectively. The amount of desorbed As from the WTR was calculated as the difference between sorbed As and As measured in solution after the desorption step, accounting for entrained dissolved As.

Incubation of PL with WTR
Four WTR application rates (2.5, 5, 10, 15% dry weight) were used to amend the fresh PL samples within the range of typical alum rates (2.5–10% by weight) used to control P availability in PL (Moore et al., 1999). Controls included PL without WTR and WTR with presorbed As (50 mg As-roxarsone kg–1) but with equal amount of bleached sand to maintain the predefined WTR rates in the absence of PL. The WTR rates were calculated on the basis of the desired Al/(As + P) molar ratios in the WTR and PL. The Al/(As + P) molar ratios were calculated using the oxalate-extractable Al concentrations of the WTR and the total recoverable P and As concentrations of the PL. The lowest (2.5%) and the highest (15%) WTR rates correspond to approximately 1 and 6.5 Al/(As + P) molar ratios, respectively. Litter was thoroughly mixed with the appropriate amounts of WTR to obtain the desired WTR rates. The water-holding capacity of both materials was used to calculate the volume of water needed to maintain the moisture content of the mixture at 70%. The effect of temperature during PL incubation was evaluated at two temperatures (23 and 35°C) in a humidified aerobic chamber. Moisture content of the samples was monitored every 3 d and adjusted accordingly. There was no attempt to exclude light from the samples during the incubation experiment. Periodically, samples were shaken (85 g) with d-H2O at a 0.1 g mL–1 solid/solution ratio for 2 h in the dark and analyzed for dissolved As and several known As species, such as As(V), As(III), DMA, MMA, and roxarsone. The decrease in dissolved As and the known As species concentrations in PL aqueous suspensions at a specific temperature were calculated as the ratio (x100) of the measured As species concentration (µg L–1) of the WTR treatment at the time interval (i) to the measured concentration (µg L–1) of the same As species at the same time interval (i) of the unamended (no WTR) PL. Deviations from 100% of the above-mentioned ratio were ascribed to a WTR effect. Dissolved As concentrations of the unamended (no WTR) PL samples increased from approximately 700 to 1150 µg L–1 within 13 d and remained unchanged up to 32 d of incubation for 23 and 35°C. Thus, the denominator of the ratio was represented by the actual As concentration measured at the specific time interval and not a time average.

Organoarsenical Isolation and ES-MS
Filtered (0.45 µm) PL water extracts were subjected to a liquid/liquid extraction procedure to isolate soluble and low-molecular-weight organic compounds from the hydrophobic, high-molecular-weight fraction of the PL aqueous suspension. Samples were mixed with toluene and 100 mmol L–1 sodium perchlorate at a ratio of 1:2:1 (by volume), shaken (85 g) for 2 h (23°C), and frozen for 2 h. The supernatant toluene phase was discarded, and the frozen aqueous phase was allowed to thaw. Arsenic analysis using GFAAS showed that arsenical compounds were fully recovered in the aqueous phase because dissolved As concentrations in the supernatant toluene phase were always less than the method detection limit (0.8 µg As L–1). The aqueous phase was passed through 0.2-µm Whatman filters, diluted x2 with the solvent mixture, and infused at 5 µL min–1 into a Finnigan LCQ Duo (Thermo Finnigan, San Jose, CA) electrospray ion-trap mass spectrometer with collision-induced dissociation (CID) capabilities. The solvent mixture consisted of chloroform and 70% acetonitrile/30% nano-pure water at a 1:20 (v/v) ratio.

Statistical Analysis
A full factorial experimental design was used to calculate the main and interaction effects of incubation time and WTR rate on the reduced dissolved As concentrations in PL during the incubation experiment, using the appropriate controls (Design-Expert, 2001). A test for normality was conducted, via the use of the normal probability plot that indicates whether the residuals follow a normal distribution, in which case the points follow a straight line. The normal probability graph plotted the studentized residual values versus the normal % probability for all pertinent data.


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
General Chemical Properties of the Materials
The PL was basic (pH 7.82) and characterized by a relatively high salt content as indicated by the high electrical conductivity (18 dS m–1) (Table 1 ). The Al-based WTR was acidic (pH 5.65) and had relatively low salt content (0.45 dS m–1). Total carbon (C) concentrations were high and typical for both materials (Table 1). On a dry-matter basis, total P concentrations for the PL were lower (14.4 g kg–1) than the average reported (25 g kg–1) (Dou et al., 2000). Total recoverable As for the PL was within average of the range reported for a suite of PL samples (<0.2–40 mg kg–1) (Nicholson et al., 1999; Garbarino et al., 2003). Total elemental analysis showed that the WTR was characterized by high Al and C content typical of the organo-mineral nature of Al-based WTR (Makris et al., 2005). Total recoverable As concentrations for the WTR were low (4.8 mg kg–1) and were attributed to small As concentrations in the raw water. Most of the total Al was oxalate extractable (95%), suggesting the amorphous nature of the WTR and its high reactivity toward oxyanions.


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  		Table 1. General chemical properties of the poultry litter and the water treatment residual (WTR) (oven-dry basis). Data are the average of two replicates ±1 SD.

 
As Availability and Speciation in PL Aqueous Suspensions
Dissolved As concentrations in the PL aqueous suspension initially (0.16–24 h) varied between 1200 and 1500 µg L–1 with no discernible trend, suggesting the absence of equilibrium (Fig. 1 ). Between 24 and 96 h, dissolved As concentrations gradually increased to approximately 2000 µg L–1 and remained constant thereafter (300 h) (Fig. 1). In earlier reports, the typical shaking times of PL with d-H2O ranged from 0.2 to 24 h, which may not represent equilibrium conditions, as our kinetic experiment revealed (Garbarino et al., 2003; Jackson et al., 2003; Arai et al., 2003).


Figure 1
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  		Fig. 1. Several dissolved known arsenic (As) species and total dissolved As concentrations (y axis) monitored in poultry litter aqueous suspensions with a 0.1 g mL–1 deionized H2O solid/solution ratio. Dissolved monomethylarsonic acid concentrations were always <7 ppb (not shown). Total dissolved As (triangles) data correspond to the right y axis. Total dissolved As data were obtained with graphite furnace atomic absorption spectroscopy, and dissolved As species concentrations were obtained with liquid chromatography–inductively coupled plasma mass spectrometry. Data are the average of two replicates ±1 SD. DMA, dimethylarsinic acid; ROX, roxarsone.

 
A simple d-H2O extraction of the PL samples exerted kinetically limited transformations on the initial As species according to LC-ICPMS data (Fig. 1). Roxarsone, DMA, and As(V) were the major As species in the PL aqueous suspension after 1 h of shaking, whereas negligible concentrations of MMA and As(III) were detected. Earlier studies showed that roxarsone, As(V), DMA, and 3-amino 4-hydroxyphenylarsonic acid were the major As species in PL aqueous suspensions (Garbarino et al., 2003; Jackson et al., 2003; Stolz et al., 2007). No discernible changes in roxarsone, DMA, and As(V) concentrations were observed by the end of 24 h, paralleling the trend in total dissolved As concentrations. Roxarsone concentrations gradually decreased after 24 h, reaching minimum concentrations (approximately 10 ppb) within 96 h (Fig. 1). Concentrations of DMA remained unchanged up to 96 h, but there was a significant increase between the 96- to 300-h equilibration period. Arsenate exhibited an initial increase during the first 24 h of the experiment but largely decreased thereafter (Fig. 1). Most importantly, As(III) concentrations gradually increased from 35 to 600 ppb within the 48-h period and remained constant (approximately 500 ppb) up to 300 h (Fig. 1), suggesting As(V) reduction to As(III). Redox potential measurements showed that anaerobic conditions (<–200 mV) were gradually established after 24 h of reaction, partially explaining the As(V)/As(III) redox coupling (data not shown). Microbial activity was responsible for the degradation of roxarsone to 4-hydroxy-3-aminophenylarsonic acid and As(V), which also produced As(III) after prolonged incubation of PL aqueous suspensions under anaerobic conditions (Cortinas et al., 2006).

The sum of known As species (As(III), As(V), DMA, and MMA) and roxarsone was always a fraction (mean value of 49%) of the total dissolved As, suggesting that roxarsone was transformed into a number of unidentified organo-As metabolites as well (Garbarino et al., 2003). Similar discrepancy between total dissolved As and the sum of known As species (LC-ICPMS) (mean of 43%) was also noted for a suite of PL samples (Garbarino et al., 2003; Jackson et al., 2003).

WTR Effectiveness in Reducing As Availability in PL
After 10 min of reaction, 50% of the initial dissolved As concentration disappeared from solution, illustrating the high affinity of As for the WTR (Fig. 2 ). Dissolved As concentrations continued to decrease with time, reaching 10% of the initial within 48 h, and the rest was removed by the end of 96 h (Fig. 2). The affinity of the WTR for dissolved As species was remarkable despite the relatively high dissolved organic C (DOC) content (approximately 20 g C kg–1 PL) (Makris et al., 2005) of the PL aqueous suspension, suggesting that As sorption by the WTR was relatively unaffected by the DOC content. The presence of organic C in the form of humic and fulvic acids may compete with inorganic As for sorption sites, impeding the overall As sorption capacity of a sorbent (Redman et al., 2002). Increasing methyl substitution to inorganic As-O bonds resulted in decreased As sorption by Fe (hydr)oxides (Lafferty and Loeppert, 2005).


Figure 2
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  		Fig. 2. Kinetic experiment monitoring the dissolved arsenic (As) concentrations in poultry litter aqueous suspensions for several known As species and for total dissolved As concentrations (y axis) in contact with an Al-based water treatment residual. Solid/solution ratio was 0.1 g mL–1 deionized H2O. C0 denotes the initial As concentration in the PL aqueous suspension before contact with the water treatment residual. Data are the average of two replicates ±1 SD. Monomethylarsonic acid was always <5 ppb and is not shown. DMA, dimethylarsinic acid; ROX, roxarsone.

 
Arsenate sorption by the WTR maximized (100%) within 1 h of reaction (Fig. 2). About 70 and 90% of the initial roxarsone concentration was removed by the WTR within 0.16 and 8 h, respectively. Dimethylarsinic acid concentrations decreased to <25% of the initial within 4 h and remained unchanged thereafter. Arsenite exhibited an interesting behavior: After 10 min, As(III) concentrations were 36% of the initial (approximately 60 ppb) and remained unchanged up to 24 h. After 24 h, As(III) concentration increased to approximately 70% of the initial, presumably due to roxarsone degradation because most of the As(V) (98–99%) was already sorbed by the WTR within 10 min. The slight increase in As(III) concentration did not affect the overall effectiveness of the WTR in sorbing As because As(III) concentration was only 8% of total dissolved As. The kinetically limited transformation of roxarsone to As(III) coincided with the relatively slow As(III) concentration increase during the PL aqueous suspension experiment in the absence of WTR. The establishment of anaerobic conditions was observed during the WTR/PL contact, especially after 24 h (–200 mV), but the majority of As species had already been sorbed by the WTR. An unidentified As peak with LC-ICPMS at 6.1 min (roxarsone appeared at 5.9 min) was observed in the WTR-amended PL, but this was not the case for the unamended (no WTR) PL sample.

The remarkable efficiency of the WTR in sorbing As species from the PL aqueous suspension was attributed to the high As sorption capacity of the WTR (Makris et al., 2006; Sarkar et al., 2007). Earlier batch experiments showed the high affinity of As(V) and As(III) for the WTR surfaces (Makris et al., 2006). Nearly all of the initially added roxarsone, MMA, and DMA (up to 10 mg As L–1) was sorbed by the WTR within 1 d (data not shown). Roxarsone and DMA exhibited a linear sorption isotherm, illustrating the high capacity of the WTR for these organoarsenical compounds. Arsenic speciation analysis with LC-ICPMS revealed that roxarsone and DMA did not convert to inorganic As during the sorption isotherm experiments (data not shown). Excellent As recovery (90–106%) was observed between the sum of As species and the total dissolved As concentrations.

After sorption, batch As desorption was initiated with a 0.01 mol L–1 KCl solution reacting with the As-containing WTR for 3 d, assuming that hysteretic type of desorption should occur. Roxarsone, MMA, or DMA desorption from the WTR surfaces was minimal because <1% As was desorbed back into solution (data not shown). Minimal As desorption (≤20%) was also observed in the batch WTR/PL suspensions with a 0.01 mol L–1 KCl solution up to 14 d of desorption (data not shown).

Incubation of PL with the WTR
After 32 d of incubation at room temperature (23°C), dissolved As concentrations for all rates (except the 2.5% rate) of the WTR-amended PL samples were significantly (p < 0.001) lower than those of the unamended (no WTR) samples (Fig. 3 ). Incubation time was an important factor that significantly (p < 0.001) interacted with the WTR rate treatment on the magnitude of reduction in As availability in PL (Fig. 3). The 2.5% WTR rate was inadequate in significantly (p < 0.05) decreasing dissolved As concentrations in PL, remaining constant at 100% of the unamended samples (no WTR) up to 25 d of incubation, followed by a slight reduction to approximately 85% after 32 d (Fig. 3). The 5% WTR rate was able to reduce As availability in PL to approximately 75% of the unamended within 4 d and remained constant thereafter. Major reduction in As availability in PL was observed with the 10 and 15% WTR rates (Fig. 3). Within 4 and 13 d of incubation, both rates were able to reduce dissolved As concentrations (As availability) to 60 and 50%, respectively, of those measured in the unamended (no WTR) PL (Fig. 3). No further reduction in As availability of PL was observed for the 10 and 15% WTR rate treatments up to 32 d of incubation.


Figure 3
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  		Fig. 3. Interactive effect of water treatment residuals amendment rate and incubation time on reducing As availability in water treatment residual–amended poultry litter samples at 70% moisture incubated at 23°C (top) and 35°C (bottom). Data are expressed as the mean of two replicates ±1 SD.

 
Pooling all WTR rate treatment data showed that changing the incubation temperature from 23 to 35°C did not result in a significant (p > 0.05) reduction in As availability (approximately 10%) of PL (Fig. 3). Similar to 23°C, there was a significant incubation time effect for all WTR rates, where major reduction in As availability occurred between 4 and 13 d and remained unchanged thereafter. Kinetic data from the incubation experiment parallel those obtained with the batch WTR/PL suspension experiment that showed nearly complete sorption of dissolved As by the WTR within 5 d of reaction (Fig. 2). Perhaps the relatively low moisture content (70%) of the incubated PL samples when compared with that of the batch experiment (0.1 g mL–1 solid/solution ratio) could be the limiting factor retarding the beneficial effect of WTR in reducing As availability in PL. Moisture content of PL largely influenced the magnitude of roxarsone degradation during composting (Garbarino et al., 2003).

Arsenic speciation analyses in the incubated samples showed that the number and forms of As species remained unaffected by changes in the WTR rates. Specifically for the 10% WTR rate at 23°C, As(III) concentrations were unaffected by incubation time (1–32 d), exhibiting a small decrease in As(III) concentration (approximately 90% of the unamended PL) (Fig. 4 ). However, As(III) concentrations in the unamended PL water extract were minimal, and they were always ≤20 ppb, or 4% of total dissolved As. Monomethylarsonic acid concentrations were reduced to approximately 40% within 4 d of incubation and remained unchanged thereafter. The 10% WTR rate was inadequate in reducing DMA concentrations in PL at 23°C because they gradually increased to those of the unamended PL after 13 d and remained unchanged thereafter (Fig. 4). Arsenate concentrations in the WTR-amended PL samples were reduced by half to those of the unamended samples after 13 d, paralleling decreases in total dissolved As concentrations (Fig. 3). Roxarsone concentrations decreased to 60% of the unamended within 1 d of incubation and reached 40% within 13 d. Roxarsone concentrations in the unamended PL decreased from 400 ppb at time 0 to approximately 200 ppb within 4 d and remained constant thereafter, whereas As(III) and MMA concentrations were minimal (<15 ppb) during the whole incubation period (32 d). This is in contrast to the PL aqueous suspension (0.1 g mL–1 solid/solution ratio) experiments that showed gradual disappearance of roxarsone from solution (Fig. 1). The relatively low moisture content (70%) coupled with the aerobic conditions maintained throughout the incubation period may be the limiting factor influencing the biotic transformation of roxarsone (Garbarino et al., 2003). With the exception of roxarsone, there was no significant (p > 0.05) reduction in As(III), DMA, and MMA, and As(V) concentrations by increasing the WTR rate to 15% at 23°C (data not shown).


Figure 4
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  		Fig. 4. Interactive effect of water treatment residual amendment rate and incubation time at 10% rate by weight on reducing known As species concentrations in water treatment residuals–amended poultry litter samples incubated at 23°C (top) and 35°C (bottom). Data are expressed as the mean of two replicates ±1 SD. DMA, dimethylarsinic acid; MMA, monomethylarsonic acid; ROX, roxarsone.

 
At 35°C, As(III) concentrations were unaffected by the 10% WTR treatment regardless of incubation time (Fig. 4). As with the 23°C treatment, As(III) concentration in the unamended PL samples incubated at 35°C were always <20 ppb, or 3.5% of the total dissolved As concentration. Within 13 d, MMA and DMA concentrations were reduced to 50 and 40%, respectively, of the corresponding concentrations in the unamended PL (Fig. 4). Increasing the incubation temperature did not influence the effectiveness of WTR in sorbing the major As species present in PL (i.e., As(V) and roxarsone, which decreased to 60 and 40%, respectively, of the unamended PL). At 35°C, there was a significant sorption of DMA by 10 or 15% WTR rates (50% of unamended), which improved the ability of the WTR in removing DMA from PL, relative to that at 23°C. Increasing the WTR rate to 15% did not further decrease As availability for all known As species (data not shown).

Electrospray Ionization–Mass Spectrometry Studies
Roxarsone (M) and As(III) at m/z equal to 262.0 and 124.9 for the [M-H]– and [As(III)(OH)3–H+]–, respectively, corroborated data obtained by LC-ICPMS and verified their presence in the PL aqueous suspension (Fig. 5 ). The peaks at m/z equal to 187.0, 248.9, and 324.8, which belong to ion pairs of copper-nitrate and roxarsone-nitrate, presumably arise during electrospray desolvation (Fig. 5). Wet chemical and ion-chromatographic analysis showed that the PL water extract contained 190, 360, 50, and 260 mg kg–1 of Cu, NO3–, NO2–, and NH4+, respectively. A copper (Cu) isotopic pattern coupled with the appropriate CID analysis (Table 2 ) led us to speculate that the peak at m/z = 352.0 could be assigned to an organometallic compound, which is composed of Cu directly bound to an As-cleaved nitrophenol structure that presumably originated from roxarsone (Fig. 5). The Cu seems to be complexed with three N2O molecules, according to the CID fragmentation pathway (Table 2). The presence of dissolved N2O in PL aqueous suspension was qualitatively verified with static-headspace gas chromatography coupled to a mass spectrometer. After 25 d of incubation, ES-MS spectra for the WTR-amended PL samples confirmed the LC-ICPMS data, showing significant reductions in roxarsone, As(III), and the proposed roxarsone metabolite at the 352.0 m/z ratio (data not shown).


Figure 5
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  		Fig. 5. Full electrospray ionization mass spectrometry graphs for the unamended (no water treatment residual) poultry litter water extract after 2 h. The OrgCu legend for the peak = 352.0 m/z refers to the speculated roxarsone metabolite containing one Cu and three N2O molecules. Rox, roxarsone.

 

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  		Table 2. Collision-induced dissociation analyses for most of the molecular ion peaks observed in the full mass spectrometry (MS) spectra (Fig. 5) of the poultry litter aqueous suspension after 2 h of shaking at a 0.1 g mL–1 solid/solution ratio. The molecular ion for the 352.0 m/z ratio peak is denoted as [OrgCu]. Principal ions observed (m/z, %) were [OrgCu]– (352.0, 22%), [Rox + NO3–]– (324.8, 50%), [Rox – H+]– (262.0, 100%), [Cu(II) + 3NO3–]– (248.9, 20%), [Rox – H2O ]– (244.0, 13%), [Cu(I) + 2NO3–]– (187.0, 23%), and [As(III)(OH)3 – H+]– (124.9, 9%).

 

   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
This study shows that As availability in PL can be reduced using a waste by-product (e.g., the WTR). Mixing or composting PL with WTR could minimize As mobility in PL-amended soils via surface runoff and leaching processes. Roxarsone degradation could increase soil As mobility in PL-amended soils with limited As sorption capacity (Brown et al., 2005). Repeated annual PL application to agricultural soils, especially those used for rice cultivation, may increase As build-up, which would lead to plant uptake and subsequent transfer to the human food chain (Wang et al., 2006; Williams et al., 2007) or leaching to groundwater (Brown et al., 2005). Our proposed PL management practice offers a low-cost solution to help minimize As availability in PL, reducing the risk associated with rainfall-induced As leaching from PL. The WTR can be obtained from drinking-water treatment plants at minimal or no cost, transported to poultry houses, and mixed with PL. This report shows that once sorbed by the WTR, dissolved As concentrations in PL did not increase for at least 32 d (an incubation experiment is ongoing). A separate control that was included in the incubation experiment using WTR with presorbed roxarsone (50 mg As kg–1 WTR) showed minimal As desorbability with water (<5 ppb As, 0.1 g mL–1 solid/solution ratio). Further experiments are needed to ensure that sorbed As by the WTR is stable under field conditions on land-application of WTR-amended PL.

The proposed amendment (WTR) to reduce As availability in PL has been successfully used to sorb dissolved phosphorus from PL aqueous suspensions (Makris et al., 2005), but alum is the conventional amendment adopted by farmers to reduce dissolved P concentrations in PL (Moore et al., 1999). The proposed WTR amendment could be a viable alternative to a well established amendment (alum), offering several advantages: (i) high roxarsone and inorganic As sorption capacity; (ii) lower purchasing cost because the WTR can be obtained at a minimal cost from the drinking-water treatment plants, assuming all other related costs constant; (iii) environmentally sustainable practice via the beneficial re-use of a waste (i.e., the WTR), which avoids its disposal in the cost-ineffective landfill space; and (iv) dual (P and As) oxyanion sorption capacity.

The negative-ion ES-MS analyses isolated roxarsone and a novel roxarsone metabolite at the 262.0 and 352.0 m/z ratios, respectively, in PL aqueous suspensions. The 352.0 peak was assigned to an As-cleaved, nitrophenol-based moiety that potentially contained Cu complexed to three N2O molecules. Questions arise regarding the chemistry behind such gas–metal coordination aqueous reactions with nitroaromatic compounds. Nitrous oxide gas solubility in water (1 mL mL–1 H2O at 5°C) is 50 times greater than that of N2 (Dowdell et al., 1979), suggesting that the N2O bubbling in water may allow for reactions with metals and other soluble constituents, similar to N2 aqueous reactions with metals during N2 fixation (Chatt and Richards, 1971). Nitrous oxide supersaturation of soil solutions (up to 10 mg N2O–N L–1) has been occasionally documented for organic soils or animal waste-amended soils (Heincke and Kaupenjohann, 1999). Denitrification is a major biotic process in soils and animal wastes leading to gradual formation of N2 gas. However, N2O build-up in solution may occur if the rate of nitrite reduction to N2O is greater than the rate of N2O reduction to N2 (Smith et al., 1997). Copper-containing organometallic compounds and free Cu may inhibit microbially driven N2O reduction to N2, accumulating N2O in anaerobic freshwater sediments (Flemming and Trevors, 1988). On the contrary, recent data collected in a mesotrophic lake water show a beneficial Cu effect on N2O reduction to N2 because Cu is required by denitrifying bacteria for the synthesis of N2O reductase (Knowles, 1982; Twining et al., 2007). Evidence from our study may suggest N2O binding by a Cu-containing aromatic compound, which seems to be a roxarsone metabolite. Ongoing studies will attempt to demonstrate in vitro synthesis of the proposed roxarsone metabolite and to elucidate the N2O-binding mechanism in PL aqueous suspensions.


   ACKNOWLEDGMENTS
 
We thank Mr. Conor P. Mullens for his valuable assistance and input during the ES-MS analyses.


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