Arsenic mobility in Karnak soils after multi-year application of poultry litter containing roxarsone

2.1 Sampling

Karnak soil samples were collected from four row-crop fields in McLean County, Kentucky (Figure S1) prior to the “voluntary suspension” of ROX. The sampling sites are located in the northwestern part of the County within the Green River flood plain. This part of the County has very little surface relief and the Karnak soils are poorly drained [21]. The latitude and longitude coordinates of the four sampling sites are included for reference (Table S1). Field 1 was treated the heaviest with 4 tons of litter spread every 2 yrs for the past 8 yrs (Figure S2). Fields 2 and 3 had 4 tons applied the autumn previous to sampling (Figure S2). Field 4, the control, had not been recently amended with poultry litter (Figure S2).

Soil samples were collected from the Ap horizon (0–8 inches, strongly acidic [23]) from each of the four cultivated fields using a 7/8 in diameter soil probe. Enough 8-inch soil cores were collected from various areas throughout each field to loosely fill a 1-L polyethylene container. A grab sample of poultry litter was pulled from litter allowed to age open to the environment for an indeterminate length of time. Samples were stored capped at 4C until analysis.

2.2 Soil digestion for metal content

Digestions were conducted according to EPA method 3050B. Quadruplicate soil samples (1 g) were refluxed at 100C using 10 mL of 1:1 nitric acid and deionized water, multiple 5 mL aliquots of concentrated Omnitrace nitric acid (EMD), a 5 mL portion of 2:3 peroxide-to-water solution, 1 mL aliquots of 30% peroxide solution (Fisher; maximum of 10 mL), and 10 mL of Omnitrace hydrochloric acid (EMD).

2.3 Leaching arsenic mobility

Quadruplicate soil samples (1 g) were stirred in deionized water at pH 7 (20 mL) for 48 or 72 hr, digested, then filtered. EPA 3050B digestion method was adopted with slight modifications. Post-filtration, the samples were first diluted to 50 mL with water. Nitric acid (10 mL, 1:1) was added and the samples refluxed at 100C until total volume was approximately 50 mL. Next, nitric acid (5 mL, concentrated) was added and samples were refluxed until total volume was 15 mL before additional 5 mL portions of concentrated nitric were used to complete the digestion. Peroxide (30%) was added dropwise until effervescence subsided or a total of 10 mL had been added. Samples were allowed to reflux until volume was reduced to 10 mL. Finally, hydrochloric acid (10 mL, concentrated) was added and samples refluxed until volume was 15 mL before transferring to new digestion tubes and diluting to 50 mL with 1% nitric acid. Samples with visible solids remaining were plunge-filtered (0.45 μm) prior to analysis.

2.4 Instrumental analysis

The moisture content of the soil samples was calculated by drying triplicate samples (1.0 g) at 300C for three hours and measuring the mass change. The pH was measured following EPA method 9045D. Arsenic was analyzed at 193.7 nm using a Varian SpectrAA 880Z Zeeman graphite furnace atomic absorption spectrometer (GFAAS). A Varian Vista Pro CCD Simultaneous Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) was used to analyze for Ca, Cu, Fe, Mg, Mn, and Zn. A 1.0 ppm yttrium internal standard was used (371.029 nm) to evaluate and/or correct for matrix effects. Ion chromatography was performed on a Dionex ICS 2500 using an IonPac AS18 column preceded by an AS18 guard column Anions using EPA method 9056.. The 28 mM KOH eluent solution was maintained at a flow of 0.95 mL min^-1 for 18 min per sample. Zeta Potential and dynamic light scattering (DLS) were measured with a Brookhaven ZetaPals, Zeta Potential Analyzer. Zeta potential measurements were recorded in triplicate with 30 read cycles for each measurement. DLS was performed at an angle of 90° for 1 minute run times and repeated in triplicate. Stock solutions were made with 3 mg of soil with 20 mL of the solutions. All water was distilled and deionized (18 MΩ cm).

3.1 Soil moisture and pH of samples

The mean percent moisture was high for both the soils (22.97±1.71%) and litter (60.70±0.92%) because of the slow runoff of precipitation, which occurred several days prior to sampling. The pH of the Karnak soil in Mclean County ranges from 5.1–6.5 from a depth of 0–8 in. and from 5.1–7.8 at depths of 8–60 in. The soil pH values from the amended and unamended fields ranged from a low of 6.20 for Field 3 to a high of 6.90 for Field 1, indicating that the sampled fields were probably limed since the Karnak plow layer is expected to be strongly acidic (Table S2) [21]. The poultry litter was alkaline at pH 8.24 and not a contributing factor to the increased pH of the soils.

3.2 Digestion to determine metal content

The soil digestions using EPA method 3050B and litter application rates (for reference) are compiled in Tables 1 and 2, respectively. Information on the natural metal and metalloid content of Kentucky soils is limited [21, 22]. The natural arsenic concentration in soil is determined by the underlying bedrock composition [24]. The sampled region is located within the Western Kentucky Coal Field underlain by Pennsylvanian age bedrock [21]. High arsenic levels are not associated with soils developed from Pennsylvanian rocks from southeast Ohio [24, 25]. The mean arsenic topsoil concentration in the US is 6.6 ppm [26] with varying values ranging from a high of 1110 ppm to a low of 1 ppm [26]. The mean arsenic concentration from the sampled Karnak soils fields was 7.52±0.58 mg kg^-1, which is still higher than many reported mean arsenic levels [3, 27]. Of all the metals examined, total soil arsenic only demonstrated a strong (R² = 0.9355) relationship with the total manganese content of the Karnak soils. Manganese oxides play a pivotal role [28] in oxidizing As(III) to As(V) and to a much lesser extent manganese hydroxides have been shown to sorb arsenic [29]. However, this correlation is not likely the result of an increased propensity of manganese oxides to sorb arsenic. Rather, the oxidation of As(V) and it’s anionic charge likely increase the retention rates with oxides such as iron (oxy)hydroxides with demonstrated high affinity for sorbing trace metals, especially As(V) [30–39]. No other strong correlations with the other metals were observed.

The arsenic levels of the sampled poultry litter was slightly low, 5.84±0.72 mg kg^-1, in comparison to other reported litter values ranging from 12–50 mg kg^-1 [41–43]. However, not all broiler operations use the maximum dosage of ROX, and could be dosing anywhere along the approved levels ranging from 22.7–45.4 g ROX per ton feed [4]. Except for arsenic and iron, mean concentrations in the poultry litter were remarkably higher than those found in the soil samples. These compare to reported results where elevated concentrations of Cu, Mn, Zn, Ca and Mg have been reported in poultry litter samples [42, 43]. Increased metal concentrations are due to the formulation of the poultry rations [42, 43]. In addition to other antibiotics such as ROX, copper sulfate, iron (II) sulfate, zinc sulfate, dicalcium phosphate, and sodium chloride are also commonly added to balance the poultry ration and explain the unusually high levels in the litter [43, 44].

Surprisingly, the amount of poultry litter applied showed very little correlation (R² = 0.3074) to the concentrations of the arsenic in the soils. This was not expected from the application of tons of ROX derived poultry litter as fertilizer spanning multiple years. In addition, the unamended field 4 had arsenic levels higher than the mean soil arsenic value of 7.52 mg kg^-1. These results show that soil retention of inorganic arsenic had very little relationship to applied ROX litter. Several factors could be at play that help explain these results the most important of which is geographical location. Karnak soils typically lie in poorly drained areas with prevalent flooding during heavy precipitation periods [21]. Increased arsenic mobilization is likely a factor for the lack of retention compared to the unamended results. In addition, ton quantity litter application has been shown to create arsenic laden dust capable of inadvertently contaminating nearby locations [45]. A coal fired power plant, known to create particulates of toxic contaminants, is located just 3 miles northwest of the location of Field 4. However, these results are alarming as the mobilization of inorganic arsenic into the surrounding areas and aquifer is concerning. Soil correlation calculations demonstrated that total arsenic content of the soils had no relationship to the pH of the soil (R² = 0.0087). This was expected since pH is controlled by a dynamic process and not solely determinate to applied ROX concentrations.

3.3 Ion chromatography

The results for common anions found in soil as mg analyte per kg of soil are summarized in Table 3. The pKa values for arsenate are remarkably similar to those for phosphoric acid [46]. Due to the smaller size of the anion, phosphate binds more strongly to mineral surfaces than arsenate to the point of causing sorbed arsenate to be displaced from soils when concentrations of both are low, but comparable [47–49]. High levels of phosphate (715.6±53.3 ppm) were found in the poultry litter but not in the Karnack soils, most of which were below detection limits for three of the four fields. Thus, phosphate competition and remobilization of the soil bound arsenic should be negligible for the Karnak soil samples and does not help explain the similar arsenic concentrations of amended and unamended soils. The amount of arsenic present in the soils demonstrated a moderate relationship (R² = 0.6641) to the sulfate concentration. No other relationships between total arsenic content and anion content were found.

3.4 Particle size and zeta potential

The particle size and zeta potential for both the amended and unamended Karnak soils were measured from pH 2–10 (Figure S3-4). The zeta potentials for both soils were negative across pH 2–10. The Karnak soil contains high amounts of montmorillonite clay and compares similarly to other results with montmorillonite where zeta potentials were found to be negative from pH 3–11 [50]. The average particle size for the amended Karnak soil ranged over pH 2–10 from a low of 570.2 nm±37.5 to a high of 2889.2 nm±1744.2. The unammended Karnak soil ranged over pH 2–10 from a low of 925.2 nm±59.4 to a high of 3199.7 nm±924.8.

3.5 Leaching for arsenic mobility

The Karnak soil arsenic levels were all below the detection limit (5 ppb). High solids samples have a tendency to rupture 0.45 μm membrane syringe and plunge-type filters if samples are not pre-filtered with a larger-pore membrane first. This problem was circumvented in this batch study by using a Buchner filtration system to remove the bulk of the solids before re-filtering with 0.45 μm plunge filters. Furthermore, these results prove that soil arsenic is bound to soil particles larger than 0.45 μm, thus rendering them immobile. This is also supported by the average particle size determined by DLS for the amended Karnak soils, which were much larger than 0.45 microns. The arsenic species present is tightly associated with the particle surface and aqueous extraction did not leach any appreciable arsenic. These findings are also in line with a similar study where clay minerals were found to have less extractable arsenic than other soils, a testament to the highly charged nature of the high clay content in the Karnak soils with zeta potentials ranging from –12.07 to –45.2 mV from pH2–10 [51].

The poultry litter leaching found arsenic at 4.89±0.65 mg kg^-1 for 24 hr and 4.12±0.58 mg kg^-1 for 48 hrs. This represented 83.7% and 70.5% of the total 5.84 ppm arsenic available for leaching in the litter. These results are very similar to reported aqueous extraction concentrations of arsenic derived from broiler litter where similar extractions yielded 70% from just one extraction while 13 repeated extractions yielded 85% of the total arsenic [2]. This contrasts with the immobile arsenic found for ROX amended Karnak soils. No relationship between leaching of the Karnak amended soils and the litter was found.

The Karnak soil and litter copper proved readily mobile when leached with pH-neutral water, though the litter copper was over 100 times more concentrated than the soil mean of 1.54±0.39 mg kg^-1. Zinc was also easily mobilized (Table S3). Litter zinc concentrations were approximately 15 times more concentrated than the mean soils zinc at 21.6±5.9 mg kg^-1. Due to copper and zinc repeatedly appearing in litter leachates in this study and in the other studies in the literature, these two metals could serve as hallmarks pinpointing poultry litter application to agricultural fields as the source of observed arsenic contamination.