Published in J. Environ. Qual. 33:1568-1574 (2004).
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Photodegradation of the Endocrine-Disrupting Chemical 4-Nonylphenol in Biosolids Applied to Soil
Kang Xiaa,* and
Chang Yoon Jeongb
a Department of Crop and Soil Sciences, 3111 Plant Sciences Building, University of Georgia, Athens, GA 30602
b Department of Renewable Resources, University of Louisiana, P.O. Box 44650, Lafayette, LA 70504
* Corresponding author (kxia{at}uga.edu).
Received for publication September 9, 2003.
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ABSTRACT
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There is increasing concern about the environmental fate and impact of biosolids-associated anthropogenic organic chemicals, among which 4-nonylphenol (4-NP) is one of the most studied chemicals. This is primarily because 4-NP is an endocrine disruptor and has been frequently detected in environmental samples. Due to its high hydrophobicity, 4-NP has high affinity for biosolids. Land application of 4-NP–containing biosolids could potentially introduce large quantities of this chemical into the environment. A laboratory experiment was conducted to investigate the effect of artificial sunlight on 4-NP degradation in biosolids applied to soil. When exposed to artificial sunlight for 30 d, the top-5-mm layer of biosolids showed a 55% reduction of 4-NP, while less than 15% of the 4-NP was degraded when the biosolids were kept in the dark. Our results indicate that sensitized photolysis reaction plays an important role in reducing the levels of 4-NP in land-applied biosolids. Surface application rather than soil incorporation of biosolids could be effective in reducing biosolids-associated organic chemicals that can be degraded through photolysis reactions. However, the risks of animal ingestion, foliar deposition, and runoff should also be evaluated when biosolids are applied on the soil surface.
Abbreviations: HPLC, high performance liquid chromatography • 4-NP, 4-nonylphenol • NPnEO, nonylphenol polyethoxylate
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INTRODUCTION
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THE ENDOCRINE DISRUPTOR 4-NP is one of the major anaerobic degradation metabolites of nonylphenol polyethoxylates (NPnEOs), nonionic surfactants that are widely used as industrial detergents, emulsifiers, wetting agents, and dispersing agents (Maguire, 1999; Thiele et al., 1997). Detailed molecular structures for 4-NP and NPnEOs can be found in a review article by Maguire (1999). Due to its high hydrophobicity (log KOW = approximately 4.48, log KOC = approximately 3.97) (Ahel and Giger, 1993; Rolf-Alexander et al., 2002), large quantities of 4-NP are found in biosolids, which consist of high levels of organic matter. The levels of 4-NP in biosolids were found to be from a few mg kg–1 up to several thousand mg kg–1 (Maguire, 1999; Guardia et al., 2001; Keller et al., 2003; Xia and Pillar, 2003). Land application of biosolids is one of the most common ways of biosolids disposal and is expected to increase as other disposal options become more expensive or heavily regulated (USEPA, 1999). Given that the annual production of biosolids in the United States is projected to increase sharply to about 47 million Mg (50% of which will be land-disposed) within the next decade (USEPA, 1999), several thousand Mg of 4-NP could be released to the environment through land application of biosolids. 4-Nonylphenol has been frequently detected in a wide range of environmental samples (Shang et al., 1999; Dachs et al., 1999; Solé et al., 2000; Kolpin et al., 2002; Ferguson et al., 2003). Significant levels (50–200 µg kg–1) of 4-NP have been found in sediments of rivers that receive surface runoff from biosolids-amended land (Solé et al., 2000). Marcomini et al. (1989) observed an 80% reduction of 4-NP in the top 5 cm of soil 30 d after biosolids were spread on the soil surface (13.5 dry Mg ha–1 yr–1). The remaining 4-NP in the soil stayed at a fairly constant level (100 µg kg–1) even 320 d after the application. The levels of 4-NP in deeper soil profiles were not investigated in the study by Marcomini et al. (1989). Vikelsøe et al. (2002) investigated 4-NP levels along soil profiles to a depth of 60 cm in a Danish field receiving biosolids application (17 dry Mg ha–1 yr–1) for 25 yr. Even six years after ceasing biosolids application in this field, they found significant concentrations of 4-NP, ranging from 500 to 5000 µg kg–1, along the soil profiles. Those levels exceeded the current recommended Danish soil quality criteria of 10 µg kg–1 for 4-NP (Jensen et al., 1997). Different from the study by Marcomini et al. (1989) in which biosolids were surface-applied, biosolids were incorporated into soil through conventional cultivation during the application in the field investigated by Vikelsøe et al. (2002). The plow depth was not noted in the study by Vikelsøe et al. (2002). None of the above-cited studies attempted to explore the mechanisms for the transformation of 4-NP in soil systems. We hypothesize that biosolids application methods may have a significant impact on the fate of 4-NP in soil.
Surface application and soil incorporation are frequently used for biosolids disposal in the United States (USEPA, 1999). Compared with soil incorporation, biosolids are exposed to more sunlight and oxygen when they are surface-applied. Research (Faust and Holgné, 1987; Pelizzetti et al., 1989; Ahel et al., 1994) has shown that under aerobic conditions 4-NP in natural water degrades rapidly mainly due to sensitized photolysis by dissolved organic matter, while direct photolysis is comparatively slow. Sensitized (indirect) photolysis is a transformation of a given xenobiotic compound initiated through light absorption by other chemicals present in the system. Direct photolysis is a process in which a given compound undergoes transformation due to its absorption of light (Schwarzenbach et al., 1993). It is believed that dissolved organic matter–derived organic peroxy radicals (ROO·) formed in natural water under sunlight can react with 4-NP (Faust and Holgné, 1987; Schwarzenbach et al., 1993), a sensitized photolysis reaction. The half-life of 4-NP in the surface layer of natural waters was estimated in the range of 0.6 to 29 d (Faust and Holgné, 1987; Ahel et al., 1994). Research by Pelizzetti et al. (1989) demonstrated a complete photocatalytic degradation of 4-NP within an hour after it was exposed to UV light (wavelength <340 nm) and TiO2 in water. Although research has shown photodegradation of 4-NP in aqueous systems, no information has been found on how sunlight affects the degradation of 4-NP in biosolids that are applied to soil. Photodegradation may contribute to the fast reduction of 4-NP observed by Marcomini et al. (1989) in soils receiving biosolids through surface application. The objective of the present study was to use laboratory-constructed soil profiles to investigate the potential of 4-NP photodegradation in biosolids spread on the soil surface, incorporated with soil, and applied below the soil surface.
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Materials and Methods
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Biosolids, Compost, and Soil Samples
Freshly produced biosolids and compost of biosolids were collected from a wastewater treatment plant located in northeastern Kansas. This treatment plant, operated using activated sludge systems, serves a city with 150000 people. It also receives wastewater from several medium-scale industries. The wastewater treatment capacity of the plant is approximately 4.5 x 104 m3 d–1 (12 million gallon d–1). The activated sludge is partially dewatered on a belt filter press, producing approximately 10000 kg wet biosolids per day. The biosolids produced are immediately transferred to lagoons and composted for up to two months before they are applied on land. The average water contents in the biosolids and compost are 85 and 23%, respectively. The biosolids were collected on three different days and then composited. Compost samples were collected from different compost lagoons and then composited. Biosolids and compost samples were kept frozen until the conduction of 4-NP photodegradation experiments. Soil used for this study was a Kennebec silt loam (fine-silty, mixed, mesic Cumulic Hapludolls), an agricultural soil collected from Manhattan, KS. The organic matter content of the soil is 2.8%. The soil consists of (weight percent) montmorillonite (37%), kaolinite (8%), mica (27%), and montmorillonite-mica (27%).
Experimental Setup
Appropriate amounts of biosolids, compost, or biosolids and soil mix (1:1 weight ratio, equivalent to application of biosolids at a rate of approximately 120 dry Mg ha–1 to the top 1 cm of soil) were distributed homogeneously in the cell shown in Fig. 1. Two types of cells (6- and 11-mm-thick) were constructed. One cell could hold a sample with a 5-mm thickness and the other could hold a sample with a 10-mm thickness. The cell loaded with a 5-mm-thick sample was placed on top of a cell, which was loaded with a 10-mm-thick sample to form a cell unit. The two cells were pressed together by fold-back clips placed along the border of the cells. Each cell unit was irradiated with the cell containing a 5-mm-thick sample facing the light for 0.5 h, 12 h, 4 d, 10 d, 20 d, and 30 d at 25°C in a temperature-controlled growth chamber fitted with lamps simulating the September sunlight radiation (approximately 2 kWh m–2 d–1) as measured in Manhattan, KS. A sheet of aluminum foil was attached beneath each cell unit to avoid irradiation through the bottom plate by scattering light. Dark controls, cell units completely wrapped with aluminum foil, were run simultaneously. One-millimeter headspace was kept in each cell and air was pumped at a constant rate through the headspace to maintain aerobic conditions. The outgoing air from each cell was bubbled through a small bottle containing 10 mL hexane to trap volatilized 4-NP. Every day, the hexane in each bottle was collected for analysis of 4-NP. Ten milliliters of fresh hexane was immediately added into each bottle after the collection. Each collected hexane solution was evaporated to dryness under N2, redissolved in 0.5 mL methanol, then analyzed for 4-NP using high performance liquid chromatography (HPLC). The water contents (10% weight base) of the samples were monitored by weighing each cell unit daily and kept at their original levels by adding water when needed. For comparison, photodegradation of 4-NP in a 10-mL solution containing 6.6 mg L–1 4-NP and 5 mg L–1 fulvic acid (International Humic Substances Society Standard IS103F) was also investigated under similar experimental conditions as that for biosolids. The carbon concentration in the fulvic acid solution was similar to that in typical surface water (Faust and Holgné, 1987). All experiments were run in triplicate.
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Fig. 1. Schematic diagram for the cell used in the photodegradation study (modified from Balmer et al., 2000). Pyrex glass is adequate for our experiment in which near-soil-surface sunlight (terrestrial light) (wavelength > 290 nm; Schwarzenbach et al., 1993) is of interest. Pyrex glass does not completely block UV light between 290 and 325 nm. It is more transparent to UV light with wavelength > 325 nm than quartz.
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Sample Extraction and Cleanup
Before extraction for 4-NP and NPnEOs, solid samples taken from each layer of the cell units were freeze-dried and ground to a fine powder. Loss of 4-NP from the samples did not occur during freeze-drying and grinding. Freeze-dried samples (2–5 g) were extracted with hexane and acetone (1:1, volume ratio) on an accelerated solvent extraction system (Model 200ASE; Dionex, Sunnyvale, CA) using a single static cycle (20 min, 100°C, 10342 kPa [1500 psi]). Water samples were extracted for 4-NP with 10 mL hexane using a liquid–liquid extraction method. Extracts of solid samples and water samples were then evaporated to dryness under N2 (50°C), redissolved in 1 mL methanol, and stored at –10°C until analysis. For both solid samples and water samples, 4-tert-butylphenol, sublimed (Sigma Chemical, St. Louis, MO) and 2,4,6-tribromophenol (Sigma Chemical) were used as surrogate standard and internal standard, respectively, for quality control purposes.
Analysis of 4-Nonylphenol and Nonylphenol Polyethoxylates by High Performance Liquid Chromatography and Gas Chromatography–Mass Spectrometry
The concentrations of 4-nonylphenol and NPnEOs were analyzed, respectively, via reverse phase and normal phase HPLC with a diode array detector (DAD) and a fluorescence detector (FLD). The presence of 4-NP and NPnEOs (n = 1–4) in each sample was confirmed using gas chromatography with mass spectrometry detector (GC–MS). Technical-grade 4-NP purchased from Sigma Chemical, pure NP1EO and NP2EO purchased from Ehrenstorfer Labs (Augsburg, Germany), and Surfonic N-95 donated by Mr. Carter Naylor (Huntsman Corp., Austin, TX) were used as standards. The Surfonic N-95, with an average of 9.5 ethoxy units, consists of a mixture of NPnEOs with n = 2 to 16 (Keller et al., 2003).
A Hewlett-Packard (Palo Alto, CA) 1050 HPLC equipped with a DAD and a FLD was used for sample analysis. Injections (5 µL) were passed through a 25-µL sample loop. The analytical column was kept at 40°C. The DAD was operated under the following conditions: signal = 277 nm, bandwidth = 40 nm, and reference = 350 nm. Data were collected from the FLD at excitation 
= 230 nm, emission = 301 nm, and pmtgain = 6. A 124- x 4-mm LiChrospher 100-RP-18e column with a particle size of 5 µm (Agilent Technologies, Santa Clarita, CA) was used for the reverse-phase HPLC. A methanol and water mixture (8:2) was used as the mobile phase at a flow rate of 1.5 mL min–1. Normal phase HPLC used a 4.6- x 100-mm Hypersil APS column (Agilent Technologies) with a particle size of 5 µm. A flow rate of 1.5 mL min–1 was used for the mobile phase (hexane to water to isopropanol ratio = 78:2:20, 50:3:47, and 0:3:97 at 0–3, 3–22, and 22–23 min, respectively).
A Hewlett-Packard 6890 Series GC–MS was used to confirm the presence of 4-NP and NPnEOs (n = 1–4) in the biosolids and water extracts. The GC–MS used a Model 5972 quadrupole mass selective detector and was operated in the electron impact mode using helium as the carrier gas (88.9 kPa [12.9 psi]; 1.1 mL min–1). A 30-m x 0.25-mm x 0.25-µm HP-5MS column was used under the following conditions (Marcomini et al., 1989; De Voogt et al., 1997). The initial column temperature was held at 100°C for 0.5 min and then increased to 320°C at a rate of 10°C min–1. The temperature was finally maintained at 320°C for 5 min. Injections (1 µL) were in the splitless mode with the injector temperature at 200°C and interface line temperature at 250°C. Published spectra (Stephanou and Ginger, 1982), 4-NP, NP1EO, and NP2EO standards, and commercial surfactant mixtures were used in the confirmation of 4-NP and NPnEOs (n = 1–4) in the extractants.
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Results and Discussion
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The initial concentrations of 4-NP in the biosolids, compost, and biosolids and soil (1:1) mixture used for this experiment were 937, 125, and 430 mg kg–1, respectively. Our results suggest that volatilization due to continuous air flow through each cell during the entire experimental period was insignificant. Figure 2 shows that when the cell units were kept in the dark for 30 d the levels of 4-NP decreased slowly, only about 10 to 15% of the initial concentrations, in the surface- and bottom-layer biosolids (top 5 mm and bottom 10 mm, respectively). A rapid decrease was observed for 4-NP in the top-5-mm layer of biosolids when the cell units were exposed to artificial sunlight. The 4-NP concentration in this layer dropped about 55% within 30 d of light exposure and seemed to continue to drop with time. Contrary to what was observed for the top-5-mm layer, the 4-NP concentrations in the bottom-10-mm layer biosolids in the cell units that were exposed to light decreased at the same slow rate as that in the biosolids that were not exposed to artificial sunlight (Fig. 2). Similar results were observed for 4-NP in the compost (Fig. 3). Figure 4 shows that artificial sunlight had no effect on the degradation of the parent compounds (NPnEOs) of 4-NP in the top-5-mm layer of biosolids and, therefore, no new 4-NP was formed in our samples during the experimental period. Our results are in agreement with the findings from the study conducted by Ahel et al. (1994), in which significant photolysis reactions were not detected for NPnEOs. The insignificant photodegradation of NPnEOs may be due to their lack of reactivity with dissolved organic matter–derived organic peroxy radicals (ROO·) (Ahel et al., 1994).
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Fig. 2. Concentrations of 4-nonylphenol (4-NP) in biosolids in top-5-mm and bottom-10-mm layers exposed (solid circle) and unexposed (solid triangle) to artificial sunlight. The term C/Ci is the concentration ratio of 4-NP at each sampling point to its initial concentration.
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Fig. 3. Concentrations of 4-nonylphenol (4-NP) in compost in top-5-mm and bottom-10-mm layers exposed (solid circle) and unexposed (solid triangle) to artificial sunlight. The term C/Ci is the concentration ratio of 4-NP at each sampling point to its initial concentration.
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Fig. 4. Levels of nonylphenol polyethoxylates (NPnEOs) in the top-5-mm layer of biosolids exposed (open triangle) and unexposed (open circle) to artificial sunlight.
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Previous studies have indicated that certain microorganisms could degrade 4-NP in pure culture when 4-NP was the only carbon and energy source (Tanghe et al., 1999; Fujii et al., 2000; Vallini et al., 2001). The half-life of 4-NP in these microbial culture varied from 4 to 7 d. However, in biosolids the role of microorganisms may not be as significant as in pure culture because of the large quantities of other available carbon sources for microorganisms in biosolids. No information has been found on whether 4-NP can be cometabolized with the presence of other available carbon sources. In addition, 4-NP may be sorbed tightly onto the organic matter in biosolids and, therefore, is less available for microorganisms. Our study has shown a Kd value of approximately 2000 mL g–1 for 4-NP on the biosolids used in this study. Our observed degradation rates of 4-NP in samples that were not exposed to artificial sunlight were much slower than what have been presented in several recent studies in which soil samples, soil and uncontaminated biosolids mixture samples, or marine sediment samples were spiked with 4-NP (Topp and Starratt, 2000; Hesselsøe et al., 2001; Gejlsbjerg et al., 2001; Ying and Kookana, 2003). The 4-NP reductions varying from 30 to 95% of original levels within the 30-d period were observed in these studies. It has been well-documented that faster degradation rate is in general observed for an organic chemical when it is freshly added to a soil matrix than when it is sequestered in a soil due to prolonged chemical–soil contact time (aging) (Hatzinger and Alexander, 1995; Kelsey et al., 1997; Alexander, 2000). During the wastewater treatment processes, 4-NP molecules may have moved into sites within the biosolids matrix (an "aging" process) that are not readily accessed by microorganisms, resulting in the slower microbial degradation observed in our study compared with that observed in the above-cited experiments, which used freshly spiked samples.
Our results suggest that sunlight plays an important role in degrading 4-NP in biosolids. The 4-NP degradation rate in the top-5-mm layer of biosolids exposed to artificial sunlight was almost five times as fast as that in the samples without light exposure. It has been shown that photolysis depth in soils is only limited to a depth up to 2 mm (Hebert and Miller, 1990) and, therefore, artificial light had almost no impact on 4-NP in the bottom-10-mm layer biosolids. When the biosolids were incorporated with soil by mixing with soil at 1:1 weight ratio, 30% of 4-NP was degraded in the top-5-mm layer within 30 d of light exposure (Fig. 5), a rate slower than that for the biosolids-only samples (Fig. 2). This may have been due to the fact that soil particles blocked some of the light from reaching biosolids particles, resulting in less photolysis reaction for 4-NP in biosolids. Previous research has shown sensitized photolysis of 4-NP by dissolved organic matter in natural waters (Faust and Holgné, 1987; Ahel et al., 1994). Our results shown in Fig. 6 further prove this reaction. A complete reduction of 4-NP was achieved within 6 d (144 h) in a solution containing 5 mg L–1 fulvic acid when the solution was exposed to artificial sunlight. Although microbial degradation was likely to be retarded due to the strong sorption and "aging" of 4-NP in biosolids, the association of 4-NP with organic matter microsites in biosolids might have increased the effectiveness of sensitized photolysis reaction when the 4-NP–containing biosolids were exposed to light.
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Fig. 5. Concentrations of 4-nonylphenol (4-NP) in soil and biosolids mixture (1:1 weight ratio) in the top-5-mm layer exposed (solid circle) and unexposed (solid triangle) to artificial sunlight. The term C/Ci is the concentration ratio of 4-NP at each sampling point to its initial concentration.
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Fig. 6. Photolysis of 4-nonylphenol (4-NP) in a solution containing fulvic acid at 5 mg L–1. The term C/Ci is the concentration ratio of 4-NP at each sampling point to its initial concentration.
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Our laboratory study suggests that surface application of biosolids on soil could be effective in reducing biosolids-associated organic chemicals that can be degraded through photolysis reactions. However, since water is an important factor for the photolysis reaction, wet rather than dry biosolids should be applied. Surface-broadcasting on sunny days might be a better approach than broadcasting on overcast days. The results from the field investigation by Marcomini et al. (1989) support the conclusion from our laboratory study. An initial rapid decrease of 4-NP within 30 d was observed in a field on which biosolids were thinly spread onto the surface of the soil at multiple times per year. Sunlight could rapidly degrade 4-NP before it has a chance to be incorporated into soil and/or leached down the soil profile. 4-Nonylphenol was observed to be more persistent when biosolids were incorporated into soils through cultivation (Vikelsøe et al., 2002).
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ACKNOWLEDGMENTS
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The experiments of this research were conducted at the Department of Agronomy at Kansas State University. The research was financially supported by the Kansas Center for Agricultural Resources and the Environment (KCARE) and the Kansas Agricultural Experiment Station.
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TOP
ABSTRACT
INTRODUCTION
