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Published in J. Environ. Qual. 33:1210-1216 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Bioremediation and Biodegradation

Application of a Slow-Release Fertilizer for Oil Bioremediation in Beach Sediment

R. Xua, N. L. A. Laub, K. L. Ngb and J. P. Obbarda,*

a Department of Chemical and Biomolecular Engineering, 4 Engineering Drive 4, National University of Singapore, Singapore 117576
b Tropical Marine Science Institute, 14 Kent Ridge Road, Singapore 119223

* Corresponding author (chejpo{at}nus.edu.sg).

Received for publication October 30, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A 105-d field experiment was conducted to determine the potential of the slow-release fertilizer, Osmocote (Scotts, Marysville, OH), to stimulate the indigenous microbial biodegradation of petroleum hydrocarbons in an oil-spiked beach sediment on an intertidal foreshore in Singapore. Triplicate microcosms containing 80 kg of weathered sediment, spiked with 5% (w/w) Arabian light crude oil and 1.2% (w/w) Osmocote pellets, were established, together with control microcosms minus Osmocote. Relative to the control, the presence of the Osmocote sustained a significantly higher level of nutrients (NH4+–N, NO3––N, and PO43––P) in the sediment pore water over the duration of the experiment. The metabolic activity of the indigenous microbial biomass, as measured using an intracellular dehydrogenase enzyme assay, was also significantly enhanced over the duration of the experiment in amended sediments. The loss of total recoverable petroleum hydrocarbons (TRPH) and biodegradation of total n-alkanes (C10–C33), branched alkanes (pristane and phytane), as well as total target polycyclic aromatic hydrocarbons (PAHs) (two- to six-ring), in both the control and Osmocote-amended sediments, followed a first-order biodegradation model. The first-order loss rate of total recoverable petroleum hydrocarbons was 2.57 times greater than that of the control. The hopane-normalized rate constants for total n-alkane, branched alkane, and total target PAH biodegradation in the Osmocote-treated sediments were 3.95-, 5.50-, and 2.45-fold higher than the control, respectively. Overall, the presence of Osmocote was able to significantly enhance and accelerate the biodegradation of aliphatics and PAHs in oil-contaminated sediments under natural field conditions in an intertidal foreshore environment.

Abbreviations: ALCO, Arabian light crude oil • DHA, dehydrogenase activity • INTF, 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazoliumchloride-formazan • PAH, polycyclic aromatic hydrocarbon • TRPH, total recoverable petroleum hydrocarbons


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SINGAPORE IS ONE of the busiest shipping ports in the world and has the world's third largest petroleum refining industry, capable of processing more than 1.3 million barrels of crude oil each day. Intermittent marine oil spillages, of various magnitudes, occur on a semiregular basis. Cleanup operations were undertaken by the Maritime and Port Authority of Singapore using booms, skimmers, and absorbants to recover the oil from the sea surface. Typically, in Singapore, oil reaching the foreshore environment is physically removed, leaving behind residual hydrocarbons to be broken down naturally by the indigenous microbial biomass. Although physical collection techniques are usually the primary choice for emergency response teams following an oil spillage, they are known to have limited application for the removal of residual hydrocarbons from foreshore environments (Prince et al., 1999).

It is well known that microbial bioremediation can be an effective and environmentally benign option for the cleanup of shorelines contaminated with oil (Head and Swannell, 1999). To date, bioremediation techniques are not yet routinely used in Singapore, although the potential of the indigenous microbial biomass to degrade hydrocarbons has previously been established (Mathew et al., 1999). Singapore's tropical climate, typified by high temperatures and precipitation, as well as the pre-exposure of the microbial biomass to oil-spillage events, renders the intertidal beach sediments of Singapore conducive to bioremediation. However, in Singapore, as elsewhere, the natural biodegradation process is constrained by the availability of nutrients to the microbial biomass. In the open foreshore environment, essential metabolic nutrients are scarce as a result of heavy leaching by tidal inundation and wave action (Head and Swannell, 1999).

Numerous experimental studies have shown that amendment of nutrients to beach sediments can result in the enhanced biodegradation of petroleum hydrocarbons (Mearns, 1997; Lee and Merlin, 1999; Head and Swannell, 1999; Xu and Obbard, 2003; Xu et al., 2003). Wrenn et al. (1997) showed that indigenous biodegradation rate of hydrocarbons in beach sediments is a function of the nutrient concentration in pore water. In addition, Young et al. (2001) demonstrated that nutrient levels need to be present in sufficient concentrations throughout the entire bioremediation program to support maximal growth rates of hydrocarbon degrading microorganisms. However, amendment of nutrients to open beach environments is often impractical as water-soluble nutrients can be rapidly diluted and leached out of the sediment profile (Lee and De Mora, 1999). Slow-release inorganic fertilizers are designed to release nutrients continually or intermittently over a period of time on contact with water (Lessard et al., 1995). Pelletized slow-release fertilizers such as Customblen (Sierra Chemical Co., Sparks, NV) (Pritchard et al., 1992; Lessard et al., 1995) and Max Bac (a product derived from the Customblen used in Alaska by Grace-Sierra Chemicals) (Sveum and Ramstad, 1995; Wright et al., 1996; Oudot et al., 1998) have been applied to oil-contaminated beach sediments to stimulate and maintain indigenous biodegradation rates.

In this study, the potential of the slow-release fertilizer Osmocote to enhance indigenous biodegradation of petroleum hydrocarbons in beach sediments in Singapore was investigated in a field trial conducted on an intertidal foreshore. Osmocote, which has been developed principally for agricultural and horticultural use, comprises a semipermeable membrane with a resin-coated prill containing water-soluble nutrients. As water diffuses into the prill, the coating swells and ruptures the membrane, releasing the nutrients into the surrounding medium. Osmocote has been utilized with some success to accelerate indigenous biodegradation rates in an oil-contaminated mangrove ecosystem (Ramsay et al., 2000). Its beneficial use for stimulating biodegradation of alkanes and polycyclic aromatic hydrocarbons (PAHs) has also been demonstrated in our laboratory-based bioremediation experiments on oil-spiked beach sediments (Xu and Obbard, 2003; Xu et al., 2003). It was hypothesized that the application of Osmocote to the open foreshore environment of Singapore may provide a sustained release of essential nutrients to support the sustained biodegradation of aliphatic hydrocarbons by the indigenous microbial biomass on the intertidal foreshore environment.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Setup
Six free-draining, stainless steel (grade 316) microcosms, each measuring 75 x 75 x 60 cm, were placed between the upper and lower tidal limits of an isolated beach on a small island 8 km south of the Singapore main island. The open bases of the microcosms were inserted to a depth of 25 cm into the beach sediment. The experiment included three replicate microcosms containing oil-spiked sediments amended with Osmocote, and three oil-spiked unamended controls (details below). Control and treated microcosms were alternated with a spatial separation of 2 m between each microcosm. As the tidal range in Singapore is restricted (typically around 2–3 m) and wave action is limited, the risk of physical damage to the microcosms was low. The top of each microcosm was fitted with a cover of fine stainless steel wire mesh (mesh size 100) in between two sheets of coarser mesh (mesh size 24) to allow the free exchange of seawater, but prevent loss of sediment.

A total of 480 kg of beach sediment (dry weight equivalent, 71.03% sand, 28.85% silt, and 0.12% clay) was spiked (5% wet weight) with an Arabian light crude oil (ALCO). The concentrations of NH+4–N, NO–3–N, and PO3–4–P in clean sediment pore water were 0.72, 0.36, and 0.13 mg L–1, respectively. The organic content of the uncontaminated sediments was 0.54 g kg–1 dry sediment. The ALCO is comprised of 87.4% carbon, 13.1% hydrogen, and 1.2% nitrogen. Oil-spiked sediments were thoroughly homogenized by physical mixing and then weathered for 2 wk to allow physical adsorption of petroleum hydrocarbons to sand particulates, as well as the loss of volatile organic compounds. Each microcosm contained 80 kg of ALCO-spiked, weathered sediment, where three of the microcosms were amended with 1.2% (w/w) Osmocote and thoroughly mixed just before the start of the field trial. The Osmocote used contains water-soluble N–P–K at concentrations of 18, 4.8, and 8.3% (w/w), respectively, with no other trace constituents. Microcosms were naturally inundated with seawater twice daily by the tide, and left undisturbed between sample episodes. There was no physical agitation of sediments other than by natural wave and current action. During the experiment, 100-g sediment samples were taken randomly from eight different points to a depth of 15 cm in each microcosm and mixed together for biological and chemical analyses. Sampling was conducted on Days 0, 7, 15, 21, 28, 40, 56, 69, 77, and 105, when the experiment was terminated.

Nutrients in Sediment Pore Water Extracts
Interstitial pore water was extracted from the sediment subsamples (50 g dry wt. equivalent) using 250 mL of deionized water on a rotary shaker (150 rpm) for 90 min at 25°C. Any Osmocote pellets present in the sediment were removed from the treated samples before the extraction of the sediment pore water. The extracted solutions were then analyzed on a Hach (Loveland, CO) DR2000 direct reading spectrophotometer using Hach proprietary reagents. Ammonia was determined using the Nessler method (Eaton et al., 1995a, p. 4-76 to 4-78; Hach Company, 1995a); nitrate by the cadmium reduction method (Eaton et al., 1995b, p. 4-87 to 4-88; Hach Company, 1995b); and phosphate by the PhosVer 3 (ascorbic acid) method (Eaton et al., 1995c, p. 4-113 to 4-114; Hach Company, 1995c). Nutrient concentrations were expressed in mg L–1 of sediment pore water.

Dehydrogenase Activity
The metabolic activity of the indigenous microbial biomass in the sediment samples was determined by the measurement of dehydrogenase activity (DHA), based on the method optimized by Mathew and Obbard (2001). Cultural methods for enumeration of microorganisms are inherently inaccurate due to the heterogeneity in distribution of the microbial population and the adherence of viable cells to the substrate matrix (Oberbremer and Muller-Hurtig, 1989; Torstensson, 1997). The measurement of DHA as an indicator of the overall intensity of respiratory metabolism by the microbial biomass is a robust and accurate technique, as the enzymes are intracellular and are rapidly degraded following cell death (Rossel et al., 1997; Lee et al., 2000).

Analysis was initiated on the day of sampling by adding 2.5 mL deionized water and 1 mL of 0.75% freshly prepared 2-p-iodophenyl-3-p-nitrophenyl-5 phenyltetrazoliumchloride (INT) solution (pH 7.9) into 5 g of sediment (dry weight equivalent). The sample was incubated in the dark at 27°C for 22 h, and the 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazoliumchloride-formazan-formazan (INTF) formed was extracted by the addition of 25 mL of methanol. The tube was inverted 12 times, and then further incubated in the dark at 27°C for 2 h. The extracted INTF was filtered through Whatman (Maidstone, UK) autovials (0.45 µm) and measured for absorbance at {lambda}max = 428 nm on a PerkinElmer (Wellesley, MA) UV/VIS Spectrophotometer Lambda 20. The spectrophotometer was calibrated with INTF standards prepared in methanol. Dehydrogenase activity was expressed as mg INTF formed kg–1 dry sediment h–1.

Hydrocarbon Analyses
The percentage loss of total recoverable petroleum hydrocarbons (TRPH) in sediments was measured using USEPA Method 3540 (Eaton et al., 1995d, p. 5-34 to 5-35). Sediment samples were dried overnight at 60°C (Korda et al., 1997) and 5 g of sediment was then extracted with a 165-mL hexane and acetone (1:1) mixture using Soxhlet extraction. The extract obtained was cooled and filtered through grease-free glass microfiber filter discs (Whatman) into a tared flask (USEPA Methods 413.3 and 418.8; Eaton et al., 1995d, p. 5-34 to 5-35). The filtrate was then rotary evaporated (Eyela; Fisher Scientific, Hampton, NH) for solvent removal at 68.8°C, the boiling point of hexane. The flask, with residue, was then dried and cooled in a dessicator for 12 h before weighing. Total recoverable petroleum hydrocarbons was calculated per kg dry weight of sediment.

A Hewlett-Packard (Palo Alto, CA) 6890 gas chromatograph equipped with a HP 6890 series mass selective detector (MSD; Model 5972A) and an HP6890 autosampler was used for analysis of straight (C10–C33) and branched alkanes (pristane and phytane), PAHs (two- to six-ring PAHs and the C1 to C4 alkyl homologs of two- and three-ring PAHs), as well as the conservative biomarker, C30–17{alpha}(H), 21ß(H)-hopane. Samples were prepared by dissolving the residues obtained for TRPH measurement in 100 mL of 1:1 v/v hexane and acetone. An HP 19091S-433, HP-5MS 5% phenyl methyl siloxane 30.0-m-long x 250-µm-i.d. (0.25-µm film) capillary column was used for hydrocarbon separation, with helium as the carrier gas at a flow rate of 1.6 mL min–1. The injector and detector temperatures were set at 290 and 300°C, respectively. The temperature program for aliphatics was set as follows: 2-min hold at 50°C; ramp to 105°C at 8°C min–1; ramp to 285°C at 5°C min–1, and 3-min hold at 285°C. The temperature program for C30–17{alpha}(H), 21ß(H)-hopane was set as follows: 2-min hold at 50°C; ramp to 105°C at 8°C min–1; ramp to 300°C at 5°C min–1, and 5-min hold at 300°C. The temperature program for target PAHs was: 1-min hold at 90°C; ramp to 160°C at 25°C min–1; ramp to 290°C at 8°C min–1, and 15-min hold at 290°C. The temperature program for the alkyl homologs of PAHs was: 2-min hold at 50°C; ramp to 300°C at 6°C min–1, and 16-min hold at 300°C. A 1-µL aliquot of solvent was injected into the gas chromatography–mass spectrometer using a splitless mode with a 6-min purge-off. The MSD was operated in the scan mode to obtain spectral data for identification of hydrocarbon components, and in the selected ion monitoring (SIM) mode for quantification of target compounds. Ions monitored included: alkanes at m/z of 71 and 85; pristane at m/z of 97 and 268; phytane at m/z of 97 and 282; and hopanes at m/z of 191, 177, 412, and 397 (Wang et al., 1994). All data were normalized with respect to the biomarker, C30–17{alpha}(H), 21ß(H)-hopane. The PAHs monitored were two-ring PAHs (naphthalene and its C1 to C4 alkyl homologs); three-ring PAHs (phenanthrene, dibenzothiophene, fluorene, and their C1 to C4 alkyl homologs); four-ring PAHs [fluoranthene, benzacenaphthylene, pyrene, benzo(a)anthracene, chrysene, triphenylene, naphthacene]; five-ring PAHs [benzo(ghi)fluoranthene, cyclopenta(cd)pyrene, benzofluoranthene, benzo(a)pyrene, benzo(e)pyrene, perylene, dibenzoanthracene, benzo(b)chrysene]; and six-ring PAHs [indeno(123-cd)pyrene, benzo(ghi)perylene, antanthrene].

Statistical Analysis and First-Order Biodegradation Modeling
Venosa et al. (1996)(1997) proposed a first-order hopane-normalized model for oil biodegradation, as follows:

[1]
where C is the concentration of analyte, CH is the concentration of hopane, k is the first-order biodegradation rate constant for the analyte, (C/CH) is the time-varying hopane normalized concentration of the analyte, and (C/CH)0 is the theoretical value of that quantity at the onset of biodegradation. Simplifying Eq. [2] gives the following relationship:

[2]
where y = C and y0 = C0 for the decline of TRPH and y = C/CH and y0 = (C/CH)0 for the biodegradation of target oil components.

Nonlinear regression analysis, using Eq. [2], was used to estimate the first-order rates (k), the coefficients of determination (r2), and the y intercepts (y0) of the decline of TRPH and the biodegradation of oil components in each sediment treatment.

A one-way analysis of variance (ANOVA) test was used to determine the statistical significance of nutrient concentrations in sediment pore water extracts, DHA values, and the concentration of petroleum hydrocarbons in oil-spiked control and Osmocote-treated sediments over time. Differences in experimental and theoretical k or y0 values of target oil components were determined by multiple comparisons using Tukey's procedure at a family error rate of 5% (Walpole et al. 2002, p. 479–480). Data were considered to be significantly different between two values if p < 0.05. All statistical analyses were performed using Minitab Release 13.20 (Minitab, 2000).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Nutrients in Sediment Pore Water Extracts
The nutrient concentrations (i.e., NH+4–N, NO–3–N, and PO3–4–P) in pore water extracts of the control and treated sediments over the duration of the field trial are shown in Fig. 1. Nutrient levels in the ALCO-spiked control sediment were low and stable throughout the experiment. Initially, the rate of NH+4–N, NO–3–N, and PO3–4–P released from the Osmocote pellets was higher than the rate of nutrient leaching and immobilization into the microbial biomass leading to a net accumulation of nutrients in sediment. The concentration of NO–3–N increased until Day 21 (maximum of 37.8 mg L–1), and NH+4–N and PO3–4–P up to Day 28 (maxima of 29.0 and 26.2 mg L–1, respectively). Subsequently, nutrient levels decreased gradually to the range of 3.7 to 7.6 mg L–1. Relative to the control, the presence of the Osmocote was able to sustain a significantly elevated level of nutrients in naturally leached beach sediments over the entire duration of the 105-d experiment. It is possible that Osmocote acted as both a direct nutrient source, as well as indirectly via the uptake of nutrients into the biomass. The biomass represents a labile reservoir of temporarily immobilized nutrients, which are released slowly as a result of cell lysis on biomass attrition (Santas et al., 1999).



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  		Fig. 1. Concentrations of NH+4–N, NO–3–N, and PO3–4–P in sediment pore water extracts during the 105-d period experiment. Error bars represent ±1 standard deviation unit. C, control samples; Os, Osmocote-treated samples.

 
Dehydrogenase Activity of Microbial Biomass
Dehydrogenase activity of the indigenous microbial biomass over the duration of the experiment is shown in Fig. 2. The DHA in the oil-spiked control sediment was relatively low and showed no major variance over the 105-d experiment, ranging between 0.22 and 1.56 mg INTF formed kg–1 dry sediment h–1. In contrast, the DHA of the Osmocote-treated sediments was increased by a factor of 45 times in the first 28 d (i.e., from 0.32 to 14.5 mg INTF formed kg–1 dry sediment h–1). The DHA subsequently declined to 2.75 mg INTF formed kg–1 dry sediment h–1 at the termination of the field trial on Day 105, but remained significantly higher than that in the control (p < 0.05). As the nutrient levels in the Osmocote-treated sediments remained high after Day 28 (Fig. 1), the reduction of DHA from Day 28 onward was probably due to a reduction in readily degradable hydrocarbons (see below) rather than a nutrient limitation on metabolic activity. Therefore, the addition of Osmocote as a nutrient source can significantly enhance the microbial biomass activity in the oil-contaminated beach sediments in the intertidal foreshore environment at a concentration of 1.2% (w/w).



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  		Fig. 2. Dehydrogenase activity of microbial biomass in oil-spiked control (C) and Osmocote-treated (Os) sediments. Error bars represent ±1 standard deviation unit.

 
Hydrocarbon Losses
Table 1 summarizes the first-order rate constants (k), coefficients of determination (r2) for goodness of fit, and y intercepts (y0) as calculated by nonlinear regression of the TRPH, total straight alkanes, total target branched alkanes, total target PAHs (i.e., two- to six-ring PAHs and the C1 to C4 alkyl homologs of two- and three-ring PAHs), as well as total target PAHs with individual ring numbers (2–6) separately, in the oil-spiked control and Osmocote-treated sediments. All concentrations of the target analytes were normalized to sediment hopane (C/CH) concentration to identify losses due only to biodegradation (Venosa et al., 1996), except TRPH values. The four y intercepts (i.e., experimentally measured as well as theoretically estimated y intercepts on Day 0 in the control and treatment) of each group of target analytes were not significantly different from each other (Table 1; p > 0.05). According to the r2 values (0.862–0.997), TRPH as well as all the hopane-normalized concentrations of target analytes declined in close approximation to the first-order model (Eq. [2]). Figures 3, 4, and 5 show the first-order loss of TRPH, total straight alkanes, and total target PAHs as representative parameters for the loss of oil and its components from the sediments.


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  		Table 1. First-order rate constants (k), coefficients of determination (r2), and y intercepts (y0,T for the theoretically estimated value using the first-order biodegradation model and y0,E for the experimentally measured value) for the degradation of total n-alkanes (C10–C33), branched alkanes (pristane and phytane), and polycyclic aromatic hydrocarbons (PAHs) (two- to six-ring PAHs and the C1 to C4 alkyl homologues of two- and three-ring PAHs).

 


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  		Fig. 3. First-order decline in total recoverable petroleum hydrocarbons (TRPH). Error bars represent ±1 standard deviation unit. C, control samples; Os, Osmocote-treated samples.

 


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  		Fig. 4. First-order decline in total n-alkanes. Error bars represent ±1 standard deviation unit. C, control samples; Os, Osmocote-treated samples. C/CH, hopane-normalized concentration of total n-alkanes.

 


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  		Fig. 5. First-order decline in total target polycyclic aromatic hydrocarbons (PAHs) (i.e., two- to six-ring PAHs and C1 to C4 alkyl homologs of two- and three-ring PAHs). Error bars represent ±1 standard deviation unit. C, control samples; Os, Osmocote-treated samples. C/CH, hopane-normalized concentration of total n-alkanes.

 
Total Recoverable Petroleum Hydrocarbons Loss
The losses of TRPH from oil-spiked control and Osmocote-treated sediments are shown in Fig. 3. The initial concentrations of TRPH estimated by Eq. [2] were equivalent for the oil-spiked control and Osmocote-treated sediments (Table 1) at around 23 g TRPH kg–1 dry sediment (p = 0.59), but first-order rates differed significantly (p = 0.02). In this case, y represents TRPH concentration throughout the experiment, and y0 represents TRPH concentration on Day 0. The rate constant for Osmocote-treated sediment was approximately 2.57 times that of the control. At the end of the 105-d experiment, the TRPH in Osmocote-treated sediments was approximately 40% less than the oil-spiked control (7.57 versus 12.13 g kg–1 dry sediment). The presence of Osmocote significantly enhanced both the rate and total loss of TRPH relative to the unamended control sediment.

Loss of Aliphatic Hydrocarbons
The hopane-normalized concentration (C/CH) of total n-alkanes (i.e., C10–C33) decreased significantly with time (Fig. 4; p < 0.05) in both the oil-spiked control and Osmocote-treated sediments and dropped to a similar low level (i.e., 266–374 hopane units) at the end of the 105-d experiment (p > 0.05). However, their declination rates differed significantly (p = 0.00). The hopane-normalized rate constant of total n-alkanes for Osmocote-treated sediments was approximately 3.95-fold higher than the control (Table 1). This is impressive when compared with previously reported enhancements of 1.58 to 2.15 in sediments amended with soluble nutrient sources (Venosa et al., 1996, 1997). Alkanes in Osmocote-amended sediments degraded rapidly from Days 0 to 28 when the loss was almost total. The loss of n-alkanes in the control was substantially and significantly lower, where 40% of the total alkanes remained at Day 28.

The loss rate constant for branched alkanes in Osmocote-treated sediments was 5.5-fold higher than the control (Table 1). Comparing the first-order rate constants for the n-alkanes and branched alkanes, it was found that the loss rates of total branched alkanes were significantly lower than total n-alkanes in both the control and treated sediments (p < 0.05). This confirms the relatively higher and expected recalcitrance of the branched alkanes relative to the n-alkanes in the marine environment. After the 105-d field trial, approximately 97% of the branched alkanes were lost from the Osmocote-treated beach sediments, compared with only 41% in the control. Therefore, this confirms that pristane and phytane are not reliable to serve as conservative biomarkers for monitoring of oil biodegradation in beach sediments (Prince et al., 1994). Overall, it was evident that Osmocote was able to significantly enhance and accelerate the biodegradation of aliphatic hydrocarbons in oil-contaminated sediments under natural field conditions.

Loss of Total Target Polycyclic Aromatic Hydrocarbons
Figure 5 shows the biodegradation loss of total target PAHs. The loss of total target PAHs was exactly compliant with the first-order rate model kinetics since the r2 values for both the control and Osmocote-treated sediments were greater than 0.97 (Table 1). The loss rate of total PAHs in Osmocote-treated sediments was approximately 2.45-fold higher than the control as a result of biodegradation. This is impressive compared with the previously reported enhancements of 1.5- to 2-fold in sediments amended with soluble nutrient sources (Mearns, 1997; Venosa et al. (1996) and 1997). At the end of the 105-d experiment, only 1% of total target PAHs remained in sediment treated with Osmocote, significantly lower than the 11% in the control (Fig. 5; p = 0.00). A 90% loss of total target PAHs was achieved in Osmocote-treated sediment after 28 d, and after 56 d in the control sediment. Therefore, addition of Osmocote significantly enhanced both the biodegradation loss and rate of total target PAHs under natural field conditions.

Loss of Polycyclic Aromatic Hydrocarbons with Individual Ring Number
All target PAHs, of various benzene ring number, followed the first-order loss model (Eq. [1]). Our previous laboratory studies on PAH biodegradation in oil-spiked sediments using an "open" irrigation system showed a different pattern of biodegradation kinetics for three- to six-ring PAHs in the oil-spiked sediments (Xu and Obbard, 2004). The difference may be related to induced microbial stress as a result of sediment excavation and exposure to laboratory conditions. The loss rates of two-ring PAHs (i.e., naphthalene and its C1–C4 alkyl homologs) for both the control and Osmocote-treated sediments were higher than their corresponding alkane loss rates (Table 1). This result was consistent with our previous laboratory studies, which also used Osmocote as a nutrient source (Xu and Obbard, 2004). The difference may be caused by the higher susceptibility of two-ring PAHs to biodegradation. Fedorak and Westlake (1981a)(1981b) also reported that the biodegradation of simple aromatics (e.g., naphthalene and 2-methylnaphthalene) was faster than n-alkanes in Prudhoe Bay crude-oil-contaminated sediments and seawater.

It can be noted from Table 1 that the first-order rate constant of PAHs generally decreased with increasing benzene ring number in both treatments. This is consistent with early studies that have reported high ring number PAHs are more recalcitrant and less bioavailable than those of a lower ring number (Atlas and Bartha, 1992; Mueller et al., 1996). The hopane normalized first-order rate constants of PAHs with ring number from two to six in Osmocote-treated sediments were 2.71-, 2.48-, 3.27-, 2.14-, and 2.20-fold higher than the control, respectively. Therefore, the addition of Osmocote to the oil-contaminated beach sediments significantly enhanced and accelerated the biodegradation of target PAHs with individual ring numbers from two to six.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
In this study, the sustained release of nutrients from Osmocote to leached, oil-spiked beach sediments under natural field conditions dramatically increased the metabolic activity of the indigenous microbial biomass, and significantly accelerated the biodegradation of oil hydrocarbons (i.e., aliphatics and PAHs). This finding supports and extends our previous laboratory studies where Osmocote was applied to oil-contaminated beach sediments to stimulate hydrocarbon biodegradation under controlled conditions (Xu and Obbard, 2003; Xu et al., 2003). The ability of Osmocote to supply a sufficiently high level of nutrients over a 105-d period to the biomass in an open, leached foreshore environment, at a reasonably low application rate of 1.2% (w/w), is encouraging. Furthermore, we found that the nutrient level in the Osmocote-treated sediments was still significantly higher than in the unamended oil-spiked control after one year. This means we need only to apply Osmocote once or twice per year to achieve elevated nutrient condition. This significantly decreases the cost of oil bioremediation in an oil spill. In summary, Osmocote can be regarded as an effective slow-release fertilizer for the bioremediation of petroleum hydrocarbons in oil-contaminated beach sediments on the intertidal foreshore in the tropical environment of Singapore.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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JEQ 2004 33: 1177-1182. [Full Text]  




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