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Journal of Environmental Quality 31:1963-1971 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Organic Compounds in the Environment

Transport of Di(2-ethylhexyl)phthalate (DEHP) Applied with Sewage Sludge to Undisturbed and Repacked Soil Columns

H. de Jonge*,a, L. W. de Jongea, B. W. Blicherb and P. Moldrupb

a L.W. de Jonge, Danish Institute of Agricultural Sciences, Dep. of Crop Physiology and Soil Science, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark
b Dep. Environmental Engineering, Aalborg University, Sohngaardsholmvej 57, DK-9000 Aalborg, Denmark

* Corresponding author (Hubert.deJonge{at}agrsci.dk)

Received for publication October 16, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
Municipal sewage sludge is often used on arable soils as a source of nitrogen and phosphorus, but it also contains organic contaminants that may be leached to the ground water. Di(2-ethylhexyl)phthalate (DEHP) is a priority pollutant that is present in sewage sludge in ubiquitous amounts. Column experiments were performed on undisturbed soil cores (20-cm depth x 20-cm diameter) with three different soil types: a sand, a loamy sand, and a sandy loam soil. Dewatered sewage sludge was spiked with 14C-labeled DEHP (60 mg kg-1) and bromide (5 g kg-1). Sludge was applied to the soil columns either as five aggregates, or homogeneously mixed with the surface layer. Also, two leaching experiments were performed with repacked soil columns (loamy sand and sandy loam soil). The DEHP concentrations in the effluent did not exceed 1.0 µg L-1, and after 200 mm of outflow less than 0.5% of the applied amount was recovered in the leachate in all soils but the sandy loam soil with homogeneous sludge application (up to 3.4% of the applied amount recovered). In the absence of macropore flow, DEHP in the leachate was primarily sorbed to mobilized dissolved organic macromolecules (DOM, 30.3 to 81.3%), while 2.4 to 23.6% was sorbed to mobilized mineral particles. When macropore flow occurred, this changed to 16.5 to 37.4% (DOM) and 36.9 to 40.6% (mineral particles), respectively. The critical combination for leaching of considerable amounts of DEHP was homogeneous sludge application and a continuous macropore structure.

Abbreviations: DEHP, di(2-ethylhexyl)phthalate • DOM, dissolved organic macromolecules


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
SEWAGE SLUDGE may be applied to arable soil, as it is useful as a N and P fertilizer for agricultural purposes. However, it also contains hundreds of organic xenobiotic compounds that may pose a threat to ground water quality. The plastic softener di(2-ethylhexyl)phthalate (DEHP), discussed in the present work, is one of the ubiquitous compounds present in sewage sludge. DEHP belongs to the phthalate esters, and is used as a softener in industrial production of polyvinyl chloride (PVC) and in glues, lubricants, and dielectric fluids (Giam et al., 1984). DEHP is a priority pollutant with relatively low acute toxicity but suspected carcinogenic effects. It is a hydrophobic compound with a log Kow = 7.0 to 7.8, a maximum aqueous solubility of 0.003 mg L-1 (Staples et al., 1997). Non-specific van der Waals interactions are expected to dominate the sorption of DEHP in soils. DEHP can be mineralized in sludge and sludge–soil mixtures (Roslev et al., 1998), but part of the DEHP is resistant to biomineralization due to sorption interactions, and may therefore be subject to subsequent leaching.

Several papers have reported on the role of dissolved organic macromolecules (DOM) in the transport of hydrophobic organic contaminants (Kretzschmar et al., 1999, and references therein). Sewage sludge is also a source of DOM, which may enhance the mobility of hydrophobic compounds (Raber and Kögel-Knabner, 1997). We therefore expected DOM-facilitated transport as a likely transport mechanism for DEHP from sludge. Several papers report on the effect of organic amendments, including sewage sludge, on the sorption and leaching of organic compounds, mostly pesticides. Due to the content of highly condensed humic substances, organic amendments are reported to increase the sorption capacity for organic compounds (Senesi et al., 1997; Cox et al., 2000). Accordingly, reduced leaching rates were reported for diazinon in the presence of city refuse compost (Sánchez-Camazano et al., 1997), and for atrazine in the presence of municipal sewage sludge (Guo et al., 1991). However, Nelson et al. (1998) showed that leaching rates of napropamide in sludge-amended soils were twice as great as in reference soil, even though the center of mass of napropamide was delayed in sludge-amended soils, due to a higher sorption capacity. They attributed this to facilitated transport with DOM, which was supported by equilibrium dialysis experiments and the absence of macropores in the soil columns used. Petrovic et al. (1998) also reported higher concentrations of metalaxyl, a fungicide, in lysimeter effluents when organic waste materials were amended to the soil.

The purpose of the present work was to evaluate the potential for mobilization of DEHP when applied to the soil via pretreated sewage sludge. We focused on three aspects of the leaching risk: (i) the role of organic and inorganic colloids in the transport process, (ii) the role of sludge aggregate size, and (iii) the role of pore structure. This was evaluated by comparing experiments with three different soils having clay contents varying from 5 to 15%, and by comparing results from intact and repacked soil columns.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
DEHP
A stock solution was prepared by mixing 2.28 g 14C ring-labeled DEHP (Sigma, St. Louis, MO) with a specific activity of 130 MBq mmol-1 with 98.4 mL methanol. The 14C concentration in the stock solution was 18.3 MBq L-1.

Sludge
Aerobically treated dewatered sludge was used that originated from a municipal wastewater treatment plant in Denmark. Some chemical characteristics of the sludge were: pH = 6.8; dry matter content = 119 g kg-1; organic matter = 689 g kg-1 dry matter; C = 249, N = 72.5, P = 33.6, and K = 6.1 g kg-1 dry matter; Pb = 81, Cd = 1.6, Hg = 0.9, Ni = 20, Cr = 21, Zn = 488, Cu =199, and DEHP 19 mg kg-1 dry matter. The sludge was spiked with approximately 60 mg 14C-labeled DEHP (dissolved in methanol) per kg dry sludge and placed in a closed receptacle. The DEHP concentration of 60 mg kg-1 is the maximum allowable concentration for application of organic waste to arable land in Denmark. The dry matter content was determined from each spiked batch, to measure the exact DEHP concentration for each experiment. For the column experiments, the sludge was also spiked with 5 g bromide (as 0.1 M KBr) per kg dry sludge. Oxygen was removed by blowing free nitrogen in the headspace of the closed receptacle for 15 min. The sample was then stored at 2°C for 7 d before use in the batch or column experiments.

Soil Samples
The soils used in the experiments were sampled at three agricultural research stations in Denmark: Lundgaard, Askov, and Røgen (Table 1). The textural and taxonomical soil classifications are: sand, Orthic Haplohumod at Lundgaard; loamy sand, Typic Hapludalf at Askov; and sandy loam, Typic Hapludalf at Røgen. Undisturbed soil columns were collected from a depth of 2 to 22 cm (plow layer) in stainless steel rings (20-cm diameter x 20-cm length). The cylinders were pushed into the ground and excavated. The soil extending each end was carefully trimmed and no visual smearing or sealing occurred. The columns were stored at 2°C before use.


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  		Table 1. Characteristics of the soils used in the experiment.

 
Batch Desorption Experiments
An amount of 0.5 g spiked sludge (approximately 0.05 g dry weight, nine replicates) was weighed into 10 mL glass centrifuge tubes, and 8 mL of demineralized water was added. The samples were rotated end over end for 24 h at 20°C. After the first desorption step, the samples were centrifuged for 10 min at 5000 rpm, such that particles with diameter > 0.24 µm were removed from solution. Three milliliters from the supernatant was directly analyzed on a liquid scintillation counter (Packard [Downers Grove, IL] 2250 CA) to determine the total DEHP concentrations. Another 3 mL was used to quantify the amount of DEHP sorbed to DOM (described below). Two milliliters more of the supernatant was removed and discarded, and 8 mL of demineralized water was added. Three more consecutive desorption steps were performed with a 24-h equilibration time and a similar analysis of the supernatant as described above.

The DEHP sorbed to DOM was quantified by passing 3 mL supernatant, previously separated from the mineral colloids by centrifugation, over a solid-phase extraction column containing C18 material (Isolute C18; IST, Hengoed, UK). The vacuum was adjusted to control the flow rate at approximately 3 mL min-1. Free DEHP in solution was effectively retained (>92%) by the C18 column at concentrations under the maximum solubility of 0.003 mg L-1. We also observed that C18 columns were freely passed when DEHP concentrations exceeded the maximum solubility, indicating the presence of DEHP in the micellar form. It is not known whether mineral colloids smaller than the centrifuge cutoff diameter (0.24 µm) were retained by filtration over the solid-phase extraction (SPE) columns.

Column Experiments with Undisturbed Soil Cores
The experiment with intact soil columns had a two factorial design with three soil types and two sludge applications. The six treatments were performed with two replicates, except the Røgen soil (four replicates). Each soil column was water saturated from the bottom of the column with artificial soil water (0.652 mM NaCl, 0.025 mM KCl, 2.445 mM CaCl2, and 0.255 mM MgCl2) for a period of 4 d and then drained to a potential of -30 cm H2O (relative to the top of the column) for 3 d. Afterward, the spiked sludge was added to the columns. The heterogeneous sludge application (Treatment 1) consisted of five sludge aggregates that were placed in a symmetric cross with one aggregate in the center of the column. The four other aggregates were placed 2 cm from the edge of the column. For each aggregate, a hole was made (2.8 cm in diameter and 3.5 cm deep) in the soil with a 50-mL disposable syringe. About 1 cm of soil was retained in the syringe for covering the sludge aggregate. The spiked sludge was weighed into a syringe (14 g wet sludge, 1.47 g dry matter) and injected into the holes. For the homogeneous application (Treatment 2), 70 g spiked sludge was thoroughly mixed with soil removed from the upper 1-cm layer, and the mixture was placed evenly on the soil column surface. A total DEHP amount of approximately 400 µg 14C-DEHP was added to each column.

The system used for the column experiment is depicted in Fig. 1 . The water solution was pumped with a peristaltic pump from a reservoir to a stainless steel irrigation head equipped with 29 needles (0.5-mm inner diameter) placed 30 mm apart. The columns were automatically rotated throughout the experiment at three revolutions per hour to obtain a homogeneous irrigation pattern at the surface. Soil columns were placed on a steel screen, so that the lower boundary of the soil column was at atmospheric pressure. All columns were irrigated with an intensity of 10 mm h-1, except for one out of the two Lundgaard replicates, which was irrigated at an intensity of 20 mm h-1.



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  		Fig. 1. Schematic of the experimental column system.

 
Effluent was collected through a glass funnel into glass beakers, and subsequent measurements were performed immediately. Every 20 min, the effluent was analyzed for DEHP concentration, bromide, turbidity, electrical conductivity, and pH. The outflow rate was quantified by measuring the elapsed time and the mass of the effluent. The bromide was measured on an ion chromatograph (Metrohm, Herisau, Switzerland). The 14C activity was quantified with a liquid scintillation counter (Packard 2250 CA). Three fractions were measured: total DEHP, solution-phase DEHP, and DOM-complexed DEHP. Total DEHP was measured by pipetting 3 mL of the agitated effluent, including the dispersed colloids. A scintillation cocktail was added (17 mL Ultima Gold; Packard) and the 14C activity was measured directly. Solution-phase DEHP was measured by centrifuging a 10-mL subsample for 10 min at 5000 rpm and measuring 14C in the supernatant. DEHP sorbed to particles with a diameter smaller than 0.24 µm, the cutoff diameter of the centrifuging process, are herewith defined as solution phase (de Jonge et al., 1998). To determine DEHP sorbed to DOM, solid-phase extraction columns were used (described above). The particle-sorbed DEHP was calculated as the difference of the total DEHP and the solution-phase DEHP concentration.

The irrigation solution had a low ionic conductivity to mimic rain water and contained 0.012 mM CaCl2, 0.015 mM MgCl2, and 0.121 mM NaCl (Table 2). The columns were irrigated with rain water from t = 0 until t = 7 h, from t = 24 h until t = 31 h, and from t = 48 h until t = 49 h; continued by irrigation with 0.11 M NH4OH from t = 49 h until t = 50 h; resumed with rain water from t = 49 h until t = 55 h and from t = 72 h until t = 79 h. Accordingly, the experiments consisted of four measuring cycles of 7 h and three flow interruptions (from t = 7 until t = 24 h, from t = 31 h until t = 48 h, and from t = 55 until t = 72 h). The NH4OH pulse was added to study the effect of pH changes on DOM and DEHP mobilization.


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  		Table 2. Treatments of column experiments with intact soil columns.

 
Column Experiments with Disturbed Soil Cores
Experiments with repacked soil columns were performed with Askov and Røgen soil. The soils were air-dried and passed through a 2-mm sieve. Subsequently, they were packed in stainless steel rings (similar in size to the other column experiments) to a bulk density of 1.44 g cm-3 for the Askov soil and 1.60 g cm-3 for the Røgen soil, similar to the intact cores (Table 3). Before start of the experiment, the columns were saturated and drained similarly to the undisturbed soil cores. Spiked sludge was added to the columns similar to Treatment 1, as described above. Constant ponding was applied as the upper boundary condition. Only "rain water" (see above) was applied. Analysis of the effluent occurred as described above, though with less frequent intervals for the Røgen soil due to the lower flow rate. The duration of the experiment was 48 h for the Askov soil and 196 h for the Røgen soil. There were two flow interruptions for the Askov column (from t = 7 until t = 17 h and from t = 31 until t = 41 h), and one flow interruption for the Røgen column (from t = 100 until t = 170 h).


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  		Table 3. Soil physical parameters from intact soil columns.

 

   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSION
 REFERENCES
 
Desorption Isotherm
The desorption isotherm of DEHP on sludge revealed the strong sorption of DEHP to sewage sludge (Fig. 2) . Only 6.7% of the initially spiked material was desorbed after four desorption steps. The desorption isotherm was highly nonlinear, and the calculated Freundlich isotherm was:

where S is the sorbed concentration (mg kg-1) and Cw is the concentration in supernatant (mg L-1). The supernatant concentrations were higher than the maximum aqueous solubility of 0.003 mg L-1, hence the desorbed fraction was either colloidally bound or in the micellar phase. To quantify the DOM-complexed fraction, the supernatant was passed over a C18 solid-phase extraction column. Free DEHP is effectively retained by C18 column material, while DOM is not retained (Raber and Kögel-Knabner, 1997). We operationally defined DOM-complexed DEHP as the fraction of 14C passing the C18 column, but note that this fraction possibly includes DEHP that is in the micellar phase as long as the DEHP concentrations are over the maximum solubility. The fraction passing the C18 column was 94, 84, 77, and 55% for the four desorption steps, respectively. The absolute amounts desorbed were small for each additional desorption step (Fig. 2), and overall 87% of the DEHP passed the C18 columns. Hence, 87% of the "desorbed" fraction was therefore either DOM bound or in the micellar form. Possibly, part of the DEHP retained on the C18 columns was due to filtration of DEHP–particle complexes that were not centrifuged from the supernatant (diameter < 0.24 µm). On the basis of these results, we anticipated that DOM-facilitated transport would be important for DEHP transport in soil column experiments.



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  		Fig. 2. Desorption isotherm of di(2-ethylhexyl)phthalate (DEHP) from the sludge at 20°C. The line is the fitted Freundlich isotherm.

 
Column Experiments: Overview of Leached DEHP Amounts
An overview of cumulative DEHP amounts leached is shown in Table 4. The total amount of DEHP after 200 mm of outflow did not exceed 0.5% of the total DEHP input, with the exception of Røgen sandy loam soil, in which up to 3.4% was recovered. Variation in leaching from the Røgen columns was very large (Fig. 3) . Severe ponding occurred in five out of eight Røgen columns, which caused the experiment to be terminated (Columns 1a,b,d and 2a,b). This is reflected in low accumulated water volumes from these columns (Table 3). These major differences in the infiltration capacity were caused by the presence or absence of continuous macropores in the columns. The large variation of DEHP leaching (Fig. 3, note the inset graphs) was a direct consequence of this variation in the macropore structure (discussed below).


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  		Table 4. Overview of di(2-ethylhexyl)phthalate (DEHP) leaching from the three soils after cumulative outflow of 200 mm.

 


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  		Fig. 3. Cumulative leached fraction of di(2-ethylhexyl)phthalate (DEHP) for all the performed column experiments. Treatment 1 represents the heterogeneous sludge application, Treatment 2 represents the homogeneous case. Note the different scales used for the Røgen soil.

 
The DEHP concentrations in the column effluents never exceeded 1.0 µg L-1. Because this is below the maximum aqueous solubility, transport of DEHP in the micellar form is unlikely. Colloid-facilitated transport was the major pathway for DEHP transport. The contribution from mineral particles varied from 14 to 40% (average for two columns) for the intact soils columns, and clearly is highest for the Røgen soil (Table 4). For DOM, the relative contribution varied between 17 and 77%. It is likely that the source of DOM facilitating the transport of DEHP was mainly the sludge phase and not indigenous soil organic matter, because DEHP release from the sludge phase into true solution is low (only 13% in the batch desorption experiments). We believe that the "particle-bound" DEHP fraction resulted from binding of sludge-derived DOM to mineral particles that were subsequently mobilized in macropores.

We spiked the maximum allowed DEHP concentration in Denmark into the sludge (60 mg kg-1 sludge), but such a limit may not exist in other countries. Due to the primary transport mechanism facilitated by DOM, we expect that leached concentrations will be proportional with the amount of DEHP present in the sludge phase.

Influence of Sludge Distribution on DEHP Leaching Rates
The size distribution of sewage sludge aggregates when applied to the soil is usually in the range of 1 to 5 cm. Root and animal activity in the soil will gradually reduce the size of these aggregates with time, and we expected the leaching potential of compounds in the sludge to increase with decreasing size of the sludge aggregates, due to smaller diffusion distances and a larger interfacial area available for mass transfer. Hence, higher leaching rates were expected from Treatment 2 (homogeneous distribution) than from Treatment 1 (large aggregates, see Table 4). Indeed, we observed a delay, or time lag, in the mobilization of DEHP in Treatment 1 (Fig. 3 and 4) . The effluent DEHP concentrations in Treatment 2 increased almost immediately after the start of the experiment, while there was a delay of about 100 mm before the concentrations increased in Treatment 1. The same pattern holds for the Lundgaard soil. For the Røgen soil, this lag was not as obvious but there was a marked difference in the leaching rates between the heterogeneous and homogeneous treatment (note the different scale for the homogeneous treatment in Fig. 3). To explain the time lag, we assume that a low hydraulic conductivity of the sludge aggregates caused the mobile water to bypass the large sludge aggregates. Hence, the time lag may be explained by a rate limitation for exchange of DEHP from the large sludge aggregates to mobile soil water.



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  		Fig. 4. Concentration of 14C di(2-ethylhexyl)phthalate (DEHP) measured in the effluent of the Askov columns. Note the delay in the concentration increase for Treatment 1, which is absent for Treatment 2.

 
The same time-lag difference between Treatment 1 and 2 was not observed when examining bromide leaching rates, as the peak concentration arrived approximately simultaneously (Fig. 5) . Bromide is not influenced by sorption processes, but is subject to the same diffusion distances. Thus, it is likely that desorption nonequilibrium from sludge aggregates rather than diffusion caused the time lag for DEHP in Treatment 1. However, the analysis is more complicated because the sludge distribution had a distinct effect on the amounts of bromide recovered. For the Askov columns, the Br recovery in Treatment 1 was up to 80% after 250 mm of outflow, but less than 30% of the applied Br was recovered in the effluent of Treatment 2 (Fig. 5). We believe that this is caused by a decrease in the hydraulic conductivity of the soil and sludge–mixed surface layer of Treatment 2, which caused heterogeneous infiltration with part of the layer bypassed. Lessened contact with the soil matrix due to the preferential flow was apparently compensated for by faster DEHP mass transfer rates to mobile water, so that the DEHP final slope of the cumulative leaching curves were comparable for Treatments 1 and 2 (Fig. 3).



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  		Fig. 5. Cumulative leached fraction of Br tracer for all the performed column experiments. Treatment 1 represents the heterogeneous sludge application, Treatment 2 represents the homogeneous case.

 
Influence of Soil Structure and Flow Interruptions on DEHP Leaching Rates
The soil structure of intact columns had a clear effect on the leaching rates of DEHP. Recovered amounts of DEHP increased in the sequence Lundgaard < Askov < Røgen (Table 4). Hence, leaching risk increased with clay content of the soil (Table 1). The relative contribution of particle–DEHP complexes to DEHP leaching followed the same sequence (Table 4), while the order was reversed for the contribution of DOM: Røgen < Askov < Lundgaard. It appears likely that the soil structure determined the importance of macropore flow, in which rapid transport of mineral particles and associated contaminants may occur (de Jonge et al., 1998).

For the Lundgaard and Askov soil, peak concentrations of nonsorbing bromide (occurring at the highest slope in Fig. 5) were observed after 100 to 150 mm of outflow. For DEHP, concentrations steadily increased during the experiment (Fig. 3 and 4). Thus, DEHP leaching was slowed, presumably by sorption nonequilibrium of DEHP–DOM complexes. For the Røgen columns with continuous macropores, peak concentrations of both bromide and DEHP arrived almost immediately after the start of the experiments (Fig. 3 and 5, Replicates 1c,d), while DEHP breakthrough in Røgen Columns 1a,b was delayed (Fig. 3, Columns 1a,b). High DEHP leaching rates coincided with much higher initial turbidity values of the effluent (e.g., Fig. 6 , Røgen Column 1c) than columns with low DEHP leaching rates (Fig. 6, Røgen Column 1a). In addition, the electrical conductivity of leachate from these columns was lower, because there was less contact between moving water and the soil matrix. It is evident that DEHP–particle complexes were transported rapidly through Røgen columns in which continuous conducting macropores were active (Fig. 7B) , while particle-facilitated transport was insignificant in Røgen columns without such continuous macropores (Fig. 7A).



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  		Fig. 6. Turbidity, electrical conductivity, and pH in the effluent of some representative intact soil columns.

 


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  		Fig. 7. Cumulative leaching of di(2-ethylhexyl)phthalate (DEHP) from Røgen Columns 1a (A) and 1c (B). In Column 1a, the DEHP breakthrough is delayed, and the particle-facilitated contribution was small. In Column 1c, leaching occurred immediately after the start of the experiment, and the contribution from particle-facilitated transport is considerable.

 
The DOM-facilitated transport in Røgen columns was delayed in comparison with particle-facilitated transport (Fig. 7B). This may again be explained by sorption–nonequilibrium of DOM to mineral surfaces, similar to the Askov and Lundgaard soils, but also by size exclusion: larger mineral particles are excluded from smaller pores and are more rapidly transported in continuous macropores than DOM, which readily may diffuse into the soil matrix. However, the flow interruptions pointed to sorption–nonequilibrium: DOM-bound DEHP concentrations after the flow interruptions were lower than before the flow interruption, both for the homogenous and heterogeneous sludge distribution. More important, this effect was most pronounced for the Lundgaard soil without macropores, for which size exclusion may be ignored.

Repacking the Askov and Røgen columns led to the destruction of macropores, and lower leaching rates were anticipated. Smith et al. (1985) reported less leaching of Escherichia coli from repacked soil columns than undisturbed soil columns. Also, Camabreco et al. (1996) observed lower leaching rates of trace metals and DOM from repacked columns than intact soil columns. Our study confirms these results. We observed a delayed breakthrough of DEHP and Br in the repacked Røgen column (Fig. 3 and 5). The flow rate of the repacked Røgen soil column was 1.6 ± 0.4 mm h-1, while the intact columns exhibited higher variation (5.1 ± 2.7 mm h-1). The contribution from particles to DEHP breakthrough in the repacked columns was very low, while DOM-facilitated transport dominated the leaching of DEHP.

The outflow rate of the repacked Askov column was quite similar (12.9 ± 3.1 mm h-1) compared with the experiments with the intact Askov columns (9.4 ± 2.3 mm h-1). Hence, the delay in DEHP and Br breakthrough was a direct effect of the disruption of the pore structure (Fig. 3 and 5). The DEHP concentrations at the start were very low, and after 200 mm of outflow, more than 50% of the recovered DEHP in the effluent was not colloidally bound (Table 4). However, after 400 mm the leachate concentrations had increased and the DOM contribution was increased to 71.6%, while the particle-bound contribution was decreased to 2.0%. For both the Røgen and Askov repacked columns, DOM–DEHP concentrations were reduced after flow interruptions. This may again be interpreted as the result of sorption–nonequilibrium of the mobilized DOM–DEHP complexes.

Influence of Flow Rate and pH on DEHP Leaching Rates
The Lundgaard columns were irrigated at two different rates: 20 mm h-1 for Replicate a, and 10 mm h-1 for Replicate b. There were only small differences between the DEHP leaching rates from the two replicates (Fig. 3). Hence, it appears that the amount of water that passed the column determined the DEHP leached, regardless of the pore water velocity. Lower concentrations would be expected at higher flow rates if desorption–diffusion from sludge aggregates was rate limiting. This is indeed the case for Br, Treatment 1 (Fig. 5). However, we believe that reduced diffusion was counterbalanced by the higher porewater velocity and shorter equilibrium time for sorption equilibrium of the DOM–DEHP complexes. In accordance with this, Weigand and Totsche (1998) reported that breakthrough of DOM in goethite-coated silica was found to be proportional with the flow rate, and they attributed this to nonequilibrium sorption of DOM.

Application of basic agents may lead to higher mobility of DOM and associated organic compounds (Ballard, 1971; Smith and Willis, 1985). We applied a NH4OH pulse, and we observed some higher measured pH values in the effluent (Fig. 6) but no dramatic increases in the DEHP concentrations. On several occasions, the infiltration rate reduced after application of the NH4OH pulse and subsequent surface ponding occurred. This was due to dispersion of sludge colloids after the pH rise, followed by clogging of main water conducting pores. The pore clogging thus counterbalanced the anticipated higher mobility of the organic colloids under higher pH conditions.


   CONCLUSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
REFERENCES
 
The results show that DEHP was strongly bound to the sludge phase. However, facilitated leaching through the soil columns occurred along with mobile colloids, mostly DOM derived from the sludge phase. Mineral particles also facilitated the transport of DEHP, especially when continuous macropores were present. The relative amounts leached were mostly below 1%, and in one occasion up to 3.4% of the spiked amount. The critical combination for significant leaching to occur was the combination of homogeneous sludge application and continuous macropores. Hence, the leaching risk increased with the clay content of the three soils used in the present study. Because of the facilitated transport of DEHP, it is expected that absolute leachate concentrations will be proportional to the initial concentration in the sludge phase, and the loading rate of the sludge to the arable soil. Other strongly sorbing organic or inorganic compounds may similarly be leached when present in sewage sludge. Studies are needed to verify the leaching risk of strongly sorbing compounds from sludge applied to arable soils under realistic field conditions.


   ACKNOWLEDGMENTS
 
This work was supported by the Freja Research Program managed by the Danish Research Agency and the Danish Environmental Research Program (Center for Sustainable Land Use and Management of Contaminants, Carbon, and Nitrogen).


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





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