mineralization of organic contaminants in sludge-soil mixtures

698

Environmental Toxicology and Chemistry, Vol. 20, No. 4, pp. 698–705, 2001q 2001 SETAC

Printed in the USA0730-7268/01 $9.00 1 .00

MINERALIZATION OF ORGANIC CONTAMINANTS IN SLUDGE-SOIL MIXTURES

BO GEJLSBJERG, CHARLOTTE KLINGE, and TORBEN MADSEN*DHI Water and Environment, Department of Ecotoxicology, Agern Alle 11, DK-2970 Hørsholm, Denmark

(Received 28 April 2000; Accepted 8 August 2000)

Abstract—The mineralization of 14C-labeled linear alkylbenzene sulfonate (LAS), nonylphenol (NP), nonylphenol-di-ethoxylate(NP2EO), di-(2-ethylhexyl)phthalate (DEHP), pyrene, and 1,4-dichlorobenzene (DCB) was investigated in different sludge-soilmixtures and soils. Under aerobic conditions, the mineralization of LAS, NP, and NP2EO was between 50 and 81% of the addedamounts after two months, while DEHP and pyrene were mineralized more slowly. The mineralization of the model chemicals wasindirectly affected by the amount of sludge in the test mixtures. A higher content of sludge in the mixtures reduced the overallconcentration of oxygen, which resulted in a decrease of the mineralization of several of the model chemicals. In sludge-soilmixtures with predominantly anaerobic conditions, the mineralization was slower for all of the chemicals with the exception ofDEHP and DCB. The mineralization of DCB was enhanced in mixtures with a high sludge content. No pronounced difference inthe mineralization of the model chemicals (except DEHP) was observed when the sludge was mixed with three different agriculturalsoils.

Keywords—Organic contaminants Mineralization Sludge-amended soil Redox conditions

INTRODUCTION

Most of the domestic wastewater in Denmark is purified inwastewater treatment plants. During wastewater treatment,sludge is produced in the primary treatment (settling, filtration)and in the biological and chemical treatment processes. From1989 to 1996, the percentage of the wastewater in Denmarkpurified for organic matter, nitrogen, and phosphorous wasincreased from 10 to 72% [1]. In 1996, approximately 160,000tons dry weight sludge was produced. Since the sludge con-tains high amounts of nutrients, approximately 65% of thesludge is reused on agricultural land in Denmark [2]. Sludgecontains a large number of metals and different xenobioticorganic compounds (e.g., surfactants, plasticizers, polycyclicaromatic hydrocarbons [PAHs], and halogenated compounds)that are sorbed to the sludge during the wastewater treatmentprocesses [3]. Since some of the organic contaminants are notdegraded or are degraded only slowly under anaerobic con-ditions [4–6], they may accumulate in the sludge in the an-aerobic digester at the wastewater treatment plant. For theanionic surfactant linear alkylbenzene sulfonate, e.g., it hasbeen shown that between 15 and 20% of the linear alkylben-zene sulfonate (LAS) in the influent sewage was transferredto the sludge [4]. The Danish Ministry of Environment andEnergy (Copenhagen, Denmark) has set regulatory limits forselected organic compounds in sludge that is intended for ag-ricultural use. These organic compounds include LAS, non-ylphenol (NP) and nonylphenol-di-ethoxylates (NP2EO), di-(2-ethylhexyl)phthalate (DEHP), and PAHs [7].

The objective of the present study was to investigate themineralization of selected chemicals in laboratory mixtures ofsludge and soil and in soil without sludge amendment. Tosimulate sludge-amended soil dominated by either anaerobicor aerobic conditions, the sludge-soil ratio and the water con-tent of the sludge-soil mixtures was varied. Furthermore, theeffect of the soil type was investigated by mixing sludge with

* To whom correspondence may be addressed ([email protected]).

three different soils and by following the mineralization of thechemicals in soil without sludge addition.

MATERIALS AND METHODS

Chemicals

The radiolabeled chemicals produced by RISØ (Roskilde,Denmark) were 4-(2-dodecyl)benzene sulfonate, sodium salt(LAS; 0.294 mCi/mmol); linear 4-nonylphenol (NP; 0.107mCi/mmol); and linear 4-nonylphenol-di-ethoxylate (NP2EO;0.086 mCi/mmol). All of these were randomly marked with14C in the aromatic ring and had a radiochemical purity .95%.Di-(2-ethylhexyl)phthalate (DEHP; uniformly ring-labeled14C; 3.5 mCi/mmol), [4,5,9,10-14C]pyrene (32.3 mCi/mmol),and 1,4-dichlorobenzene (DCB; uniformly labeled 14C; 9.4mCi/mmol) were purchased from Sigma Chemical (St. Louis,MO, USA). The radiochemical purity of the chemicals pur-chased from Sigma Chemical was .98%.

Nonlabeled LAS, NP, and NP2EO identical to the 14C-la-beled chemicals were produced by RISØ (Roskilde, Denmark)and had a chemical purity of .99.9%. The DEHP (purity.99%) and pyrene (purity .99%) were purchased from SigmaChemical, while DCB (purity .99.9%) was purchased fromRidel de Haen (Seelze, Germany). All other chemicals werecommercially available and of analytical purity.

Soils

The soil used in most of the biodegradation experimentswas a coarse, sandy soil collected from the upper 20 cm ofan agricultural field in Jyndevad, Denmark. To compare thebiodegradation potentials in different soils, aerobic biodeg-radation experiments were also carried out with a sandy soilfrom Lundgaard (Denmark) and a more clayey soil from Askov(Denmark). None of the soils had previously received sludge.The characteristics of the soils are shown in Table 1. All soilswere air dried at room temperature for 20 h, sieved througha 2-mm mesh, and stored at 48C until use.

Mineralization of sludge contaminants Environ. Toxicol. Chem. 20, 2001 699

Table 1. Characteristics of the soils used in biodegradationexperimentsa

SoilCoarsesand

Finesand Silt Clay

Organicmatter pH

JyndevadLundgaardAskov

76.863.137.6

12.226.637.0

4.13.8

11.8

3.94.3

10.6

3.02.23.0

6.06.16.6

a All values of soil components are expressed as a percentage, afterHansen [24].

Sludge and pretreatment of sludge

Dewatered activated sludge was used in all experiments.The sludge was collected from a domestic wastewater treat-ment plant (Lundtofte, Denmark). The dry matter content ofthe sludge was approximately 28% (w/w). The sludge wassieved through a 4-mm mesh and distributed in thin layers incontainers with atmospheric air in order to maximize the extentof the aerobic surface layer. The sludge was then left for sta-bilization at 208C for 14 d to reduce the content of organiccontaminants. During this period, the sludge was moistenedto maintain constant water content. After the pretreatment, thesludge was stored at 48C until use.

Chemical-sludge and chemical-soil complexes

Chemical-sludge complexes were prepared 24 h before theexperiments were started. Each chemical was dissolved in anorganic solvent and was added to individual subsamples ofsludge. Stock solutions of LAS, NP, and NP2EO were preparedin methanol, while the stock solutions of DEHP, pyrene, andDCB were prepared in acetone. After the addition of a volumeof the stock solution to the subsamples of sludge, the solventwas allowed to evaporate for approximately 30 min at roomtemperature while flushing the spiked sludge with N2. For eachmodel chemical, all the sludge subsamples received the sametotal amount of model chemical. By varying the ratio between14C-labeled and nonlabeled chemicals, the variable radioactiv-ity in the chemical-sludge complexes made it possible toachieve the same radioactivity (;10,000 dpm/g wet wt) aftermixing the complexes with different amounts of soil. Afterevaporation of the solvent, the chemicals were allowed to sorbto the sludge for 24 h at 48C under an N2 atmosphere in orderto minimize biodegradation. Chemical-soil complexes wereprepared in the same way as chemical-sludge complexes ex-cept that the stock solution was added to soil instead of sludge.In this case, the compounds were added at a low concentrationin order to mimic the concentration a contaminant mightachieve in soil at a distance from the soil layers containingthe highest amount of sludge.

Biodegradation experiments

The effects of available oxygen on mineralization of themodel chemicals were examined. For each of the compounds,two subsamples of chemical-sludge complexes were thor-oughly mixed with Jyndevad soil to achieve homogeneoussludge-soil mixtures with sludge:soil ratios of either 1:20 or1:100 (dry wt). The mixtures were then divided into two por-tions and Milli-Q water (Bedford, MA, USA) was added toeither 40 or 80% of the water-holding capacity (WHC), re-spectively. A third subsample of chemical-sludge complex(;28% dry wt) was not mixed with soil. Quantities of 6 g wetweight of the mixtures (with ;60,000 dpm of model chemical)

were transferred into glass tubes (internal diameter, 12 mm;length, 50 mm) and were gently pressed into a core of ap-proximately 25 mm. Each tube was placed in a 200-ml glassjar with a gas phase of atmospheric air. A wet filter paper wasplaced in each jar to minimize the water loss from the mixtures,and the jars were closed with a gas-tight lid. A glass vialcontaining 2 ml of 0.5 N potassium hydroxide (KOH) wasplaced in each jar to trap the evolved 14CO2. The jars wereopened for approximately 10 min once or twice a week torestore the oxygen content of the headspace. All experimentswere carried out in the dark at 158C in four replicates and hada duration of two months.

An additional experiment was carried out with 14C-DCBspiked to 100% sludge or a sludge-soil mixture with a ratioof 1:20 at 80% of WHC by using a gas phase of oxygen-freeN2.

The effects of the soil type on mineralization of the modelchemicals were examined by using soil from Jyndevad, Lund-gaard, or Askov. Sludge-soil mixtures with a ratio of 1:100 orsoil without sludge amendment were moistened to 40% of theWHC and were incubated as described above. The radioactiv-ity added in this experiment varied between 10,000 and 20,000dpm (per replicate of 6 g wet wt), depending on the test chem-ical.

Determination of oxygen profiles

The oxygen concentrations in the sludge-soil cores weremeasured at the end of the experiments by using an oxygenmicroelectrode (outer diameter, 150 mm; Unisense, Arhus,Denmark) connected to a picoamperemeter (PA2000, Uni-sense). A two-point calibration of the electrode was performedin water amended with sodium-dithionite (anaerobic) and aer-obic water at 158C. The measurement, in picoamperes, wasthen calculated to an O2 saturation percent. Oxygen concen-trations were measured from the surface of the cores and atevery half millimeter by using a micrometer screw. The aerobicpart of the profile was defined by an O2 concentration .1%.At this O2 concentration, the average condition in the systemis aerobic, whereas aerobic respiration and denitrification com-pete as electron acceptors at lower O2 concentrations [8].

Chemical analysis

The effect of the aerobic pretreatment of the activatedsludge was determined by chemical analyses of selected or-ganic contaminants before and after the pretreatment. The con-centrations of eight PAHs, NP, nonylphenol-mono-ethoxylate(NP1EO), NP2EO, and DEHP were determined by gas chro-matography-mass spectrometry, while the concentration ofLAS was determined by high-performance liquid chromatog-raphy as described previously [3].

Recovery of added 14C

The mineralization of the radiolabeled compounds was fol-lowed by replacing the KOH in the CO2 absorbers at specificintervals. The KOH was then mixed with 15 ml of scintillationcocktail (Insta-Gel II Plus, Packard, Groningen, The Nether-lands), and the 14C activity was quantified by liquid scintil-lation counting on a Wallac 1409 b-counter (Turku, Finland).At the end of the experiment, the 14C remaining in the sludge,the sludge-soil mixtures, and the soil was quantified by com-bustion (6008C, 4 min) of 0.1-g subsamples in an excess ofoxygen. Before combustion, the samples were dried at 70 to1058C and then homogenized. Internal standards were used to

700 Environ. Toxicol. Chem. 20, 2001 B. Gejlsbjerg et al.

Fig. 1. Example of oxygen profiles in cores of test mixtures. Thesludge was used directly or mixed with soil at a ratio of 1:20 or 1:100 with added water to 40 or 80% of the water-holding capacity(WHC). — 100% sludge; □, sludge:soil (1:20, 40% of WHC); m,sludge:soil (1:20, 80% of WHC); m, sludge:soil (1:100, 40% ofWHC); 3, sludge:soil (1:100, 80% of WHC).

correct for color quenching and to correct for the efficiencyof the combustion.

Calculations

The maximum biodegradation rates were calculated by lin-ear regression as the percentage of the added 14C mineralizedper day for each replicate during the period in which the fastestrate was observed. The total mineralization after two monthswas calculated as the accumulated 14CO2, in percent, of theadded 14C.

The time to reach 50% of the final accumulated 14CO2 (t50)for LAS, NP, and NP2EO were calculated by fitting the 14CO2

accumulation curve to the first-order production model,

2ktPt 5 A(1 2 e )

where t50 5 ln 2/k, Pt 5 cumulative 14CO2 production, A 5percent of 14C recovered as 14CO2 after two months (when theaccumulation reached a stable plateau), k 5 first-order rateconstant, and t 5 time in days.

For aerobic mixtures of LAS, NP, and NP2EO, the cu-mulative 14CO2 production curve reached a stable plateau andthe t50 was therefore only calculated for these model chemicals.For statistics, a Student’s t test for unrelated samples was used.

RESULTS

Pretreatment of sludge

The chemical analyses of the sludge showed that the con-centrations of the sludge contaminants decreased during 14 dof aerobic stabilization. The largest decrease in the concen-tration of the parent compound was observed for LAS (82%loss), NP (93% loss), nonylphenol-mono- and -di-ethoxylates(87–90% loss), and DEHP (74% loss) (data not shown). Thepretreatment of the sludge reduced most of the PAHs to about50% of their initial concentrations, and, e.g., 55% loss of py-rene was observed after 14 d.

Biodegradation and effects of available oxygen

The higher water and sludge contents generally resulted inlower concentrations of O2 in the sludge-soil mixtures. Themixtures spiked with NP deviated from this pattern; they weredominated by aerobic conditions at the end of the experiment,which indicates that the mixtures had dried out during theexperiment. In tubes with 100% sludge, only the outer 1 to1.5 mm of the core was aerobic (corresponding to ,11% ofthe core) (Fig. 1 and Table 2). Typical oxygen profiles of thedifferent mixtures are shown in Figure 1. In all sludge-soilmixtures, the oxygen concentration was almost stable from 6mm below the surface to the center of the cores (data notshown).

The LAS, NP, and NP2EO were all rapidly mineralized inthe predominantly aerobic sludge-soil mixtures. The highestmineralization was generally seen in the mixtures with a watercontent corresponding to 40% of the WHC (p , 0.05), al-though this was not significant for NP with a sludge:soil ratioof 1:100. In most of these mixtures, the prevailing redox con-ditions were aerobic (Table 2). No clear relation was seenbetween the sludge content and the percentage of model chem-ical that was mineralized after two months. For LAS, therewas no significant difference between the 1:20 and the 1:100mixtures. For NP, there was only a difference for the mixturesat 80% WHC, where the sludge-soil mixture with a ratio of1:20 showed the higher mineralization (p , 0.05). For thesludge-soil mixtures with NP2EO at 80% WHC, the 1:100

mixture showed the highest mineralization (p , 0.01), but thiscould have been caused by the higher availability of O2 in thismixture (Table 2).

The mineralization of LAS, NP, and NP2EO under pre-dominantly aerobic conditions reached a stable plateau duringthe experiment, while the mineralization of all model chem-icals was slow under anaerobic conditions (Fig. 2a through band Table 2).

Only up to 21.8% of the added DEHP was mineralizedduring the two-month test period (Table 2, Fig. 2c). All themixtures resulted in almost the same level of mineralization(;20%), and there was no clear relation between the water orsludge content and the mineralization. Major mineralizationof pyrene (12.5 and 15%) was only seen in mixtures dominatedby aerobic conditions (Table 2). The mixture with a sludge:soil ratio of 1:20 at 40% WHC had a slightly but significantlyhigher mineralization (p , 0.05) than the 1:100 mixture at40% WHC.

Mineralization of DCB occurred in the mixtures with thehighest content of sludge, while the O2 concentration seemedto play a minor part (Table 2, Fig. 2d). The highest mineral-ization was seen in the sludge (31.8%). The mineralizationwas first observed after a lag phase of approximately 30 d.The sludge and the sludge:soil with a ratio of 1:20 showed amineralization of DCB between 12.4 and 21.6% after twomonths, while a negligible mineralization was observed in thepredominantly aerobic sludge-soil mixture with a ratio of 1:100. The mineralization of DCB was probably occurring inthe aerobic layers of the sludge-soil mixtures since only 0.4to 0.5% of the added 14C was mineralized to 14CO2 in 100%sludge or sludge:soil with a ratio of 1:20 during incubation at158C for 60 d under a N2 atmosphere (data not shown).

Effect of soil type

The mineralization of sludge-bound LAS during twomonths was lower when the sludge was mixed with Askovsoil compared with Jyndevad soil (p , 0.05) (Table 3). Whenno sludge was added, the mineralization of LAS was slightlylower in the Jyndevad soil compared with the two other soils(p , 0.05) (Table 3). Mineralization of NP and NP2EO wasnot affected by the soil type since the percentage of compoundmineralized after two months was not different between anyof the test mixtures.


Table 2. Mineralization of model chemicals in different sludge-soil mixturesa

Sludge-soil mixture,water contentb

Initialconcentration

in test mixture(mg/kg dry wt)c

Mineralized aftertwo months

(% of added 14C)Total recovery of 14C

(% of added 14C)Max rate (% of initial

concentration/day)Extension of aerobic

conditions (% of core)d

LASSludge1:20, 40%1:20, 80%1:100, 40%1:100, 80%

75936367.57.5

14.8 (6.7) A76.7 (4.2) B18.9 (4.1) C78.4 (2.9) B39.3 (13.4) A

90.1 (5.7)85.9 (1.9)80.0 (2.7)89.1 (0.2)80.9 (14.4)

0.24 (0.13) A7.7 (1.70) B0.39 (0.07) A8.8 (0.40) B1.3 (0.15) C

5.7 (8.6) A100 (0) B

19.3 (11.3) AC100 (0) B

31.3 (2.7) CNP

Sludge1:20, 40%1:20, 80%1:100, 40%1:100, 80%

1,154555511.411.4

28.5 (6.3) A63.2 (2.3) B56.0 (3.5) B58.4 (16.6) B44.2 (7.0) C

74.3 (2.9)95.3 (10.4)76.0 (6.6)84.0 (21.2)77.1 (10.3)

0.42 (0.10) A4.0 (0.16) B1.1 (0.11) A3.8 (0.94) B0.90 (0.20) C

6.1 (2.5) A96.0 (7.0) B91.6 (5.7) B96.4 (3.0) B68.8 (36.1) B

NP2EOSludge1:20, 40%1:20, 80%1:100, 40%1:100, 80%

2,002959519.819.8

14.8 (6.7) A61.4 (2.1) B12.4 (3.9) A70.2 (2.6) B43.4 (9.3) C

85.6 (7.7)89.2 (7.0)90.0 (14.8)96.7 (8.4)85.6 (9.0)

0.42 (0.19) A1.3 (0.02) B0.24 (0.07) A4.5 (0.38) C1.5 (0.41) B

9.1 (0.57) A100 (0) B

16.3 (5.71) A100 (0) B

37.5 (12.0) CDEHP

Sludge1:20, 40%1:20, 80%1:100, 40%1:100, 80%

643.03.00.630.63

17.3 (1.7) A19.7 (2.0) A21.8 (1.7) A20.3 (4.8) A17.8 (0.62) A

46.7 (2.5)41.6 (2.0)42.8 (5.4)54.9 (5.6)41.5 (0.8)

0.29 (0.03) A0.32 (0.04) AB0.36 (0.03) B0.33 (0.08) AB0.29 (0.01) A

9.3 (1.83) A28.3 (5.6) B16.3 (5.7) C67.0 (38.1) B53.0 (31.7) B

PyreneSludge1:20, 40%1:20, 80%1:100, 40%1:100, 80%

2.90.130.130.0280.028

1.30 (0.94) A15.0 (1.10) B2.23 (0.24) A

12.5 (0.81) C1.52 (0.81) A

102.2 (4.0)91.2 (11.1)83.8 (6.5)80.5 (7.5)80.0 (5.3)

0.008 (0.003) A0.40 (0.04) B0.04 (0.004) C0.20 (0.013) D0.03 (0.025) C

5.7 (2.9) A97.9 (4.1) B14.9 (0) C

100 (0) B81.8 (36.4) B

DCBSludge1:20, 40%1:20, 80%1:100, 40%1:100, 80%

7.20.340.340.0710.071

31.8 (7.75) A12.4 (9.72) ABC21.6 (2.26) A1.99 (0.21) B0.69 (0.20) C

34.3 (7.8)21.9 (13.6)31.1 (5.1)

5.9 (3.0)2.4 (0.3)

1.1 (0.18) A0.49 (0.66) ABCD0.66 (0.02) B0.03 (0.005) C0.01 (0.001) D

11.4 (0.3) A38.6 (41.1) AB

4.5 (0.2) A100 (0) B

30.6 (46.3) AB

a Values in parentheses are the standard deviations of four replicates. The differences are indicated individually for each compound. All values(for each compound) followed by one or more letters in common are not significantly different (p , 0.05). LAS 5 linear alkylbenzene sulfonate;NP 5 nonylphenol; NP2EO 5 nonylphenol-di-ethoxylate; DEHP 5 di-(2-ethylhexyl)phthalate; DCB 5 1,4-dichlorobenzene.

b Sludge-soil ratio based on dry weight; water content expressed as percent of water-holding capacity.c Initial concentration of test chemical expressed on a dry weight basis.d Defined as the percentage of the sludge-soil core, where the O2 concentration was above 1% of atmospheric saturation.

A higher mineralization of DEHP was seen for the sludgemixed with Jyndevad soil compared with sludge mixed withthe two other soils (p , 0.001). The DEHP mineralization inthe Jyndevad soil without sludge also exceeded the mineral-ization of DEHP in the Lundgaard and Askov soils (p , 0.001).For Jyndevad and Askov soil, the mineralization of DEHP wasslightly higher when the sludge was not added to the soil (p, 0.01), whereas the sludge did not affect the mineralizationof DEHP in Lundgaard soil (0.10 , p , 0.15). For pyrene,the mineralization in sludge mixed with Lundgaard soil waslower than in sludge mixed with the two other soils (p , 0.05).The mineralization of pyrene was slightly higher when theJyndevad soil was not mixed with sludge (p , 0.01).

The calculated t50 values for LAS, NP, and NP2EO in theprimarily aerobic mixtures are shown in Table 4. No pro-nounced difference was seen between the soils or soil:sludgemixtures with the exception of the longer t50 for the for NP2EOin 1:20 mixture at 40% WHC.

Recovery of 14C

The residual 14C after incubation was determined by com-bustion of subsamples of the sludge-soil mixtures or soils. The

recoveries are shown in Tables 2 and 3 and were highest forLAS (80–94%), NP (74–95%), NP2EO (80–97%), and pyrene(80–106%). The recoveries for DEHP (42–66%) and DCB (2–34%) were much lower, which was probably caused by vol-atilization of these test substances during the drying of thesamples before combustion and not by an inefficient 14CO2

absorption during the experiment. Thus, the total recovery of14C in mixtures with DCB was highest in the mixtures in whichthe highest mineralization occurred (31 and 34%). Later con-trol experiments in which an ethyleneglycol absorber wasplaced in each test flask with DCB (the most volatile of thetest chemicals) for trapping of volatile compounds showed thatapproximately 20% of the DCB was volatilized during the two-month mineralization experiment (data not shown).

DISCUSSION

LAS, NP, and NP2EO

The presence of oxygen was the most important parameterdetermining the mineralization of the model chemicals, andLAS, NP, and NP2EO were rapidly mineralized in aerobic


←

Fig. 2. Examples of mineralization curves. (a) LAS, (b) NP2EO, (c)DEHP, and (d) DCB. The sludge was spiked with test chemical andused directly or mixed with soil at a ratio of 1:20 or 1:100 with addedwater to 40 or 80% of the water-holding capacity (WHC) and themineralization to 14CO2 was followed. — 100% sludge; □, sludge:soil (1:20, 40% of WHC); m, sludge:soil (1:20, 80% of WHC); m,sludge:soil (1:100, 40% of WHC); 3, sludge:soil (1:100, 80% ofWHC). Note that different scales are used in the y-axes. LAS 5 linearalkylbenzene sulfonate; NP2EO 5 nonylphenol-di-ethoxylate; DEHP5 di-(2-ethylhexyl)phthalate; DCB 5 1,4-dichlorobenzene.

sludge-soil mixtures. The t50 values for these compounds in-dicate that they will rapidly attain a low concentration in theaerobic parts of sludge-amended soils (Table 4). The t50 forLAS in this study (7.0–8.5 d) corresponds well with fieldstudies of sludge-amended soils. Waters et al. [5] found a half-life (t1/2) between 7 and 22 d, while Holt and Bernstein [9]observed a t1/2 of 8.3 d in sludge-amended fields.

For LAS, NP, and NP2EO, only a minor part was miner-alized in the sludge that was dominated by anaerobic condi-tions. In some of the mixtures, the amount of model chemicalmineralized exceeded the amount of model chemical initiallypresent in the aerobic part of the sludge core. This may beexplained by a rapid aerobic mineralization in the outer layerof the sludge followed by a slow diffusion from the anaerobicto the aerobic part of the core. Alternatively, oxygen may havepenetrated into the interior of the sludge core, although thiswas not indicated by the O2 measurements.

The LAS, NP, or NP2EO is reported not to be mineralizedin anaerobic environments (e.g., sludge or sediments) [4,10–12]. As an exception to this, Ekelund et al. [13] observed thatNP was mineralized in an anaerobic marine sediment. Themineralization rate under anaerobic conditions was one thirdof the rate in aerobic sediment and seawater. However, a largevariation was observed between replicates. In our experiments,a large part of the NP was mineralized in all of the sludge-soil mixtures. Contrary to the typical situation for sludge-soilmixtures at 80% of WHC, the measurements of O2 concen-trations showed that the mixtures containing NP were domi-nated by aerobic conditions at the end of the experiment (Table2). This indicates a loss of moisture by evaporation during theexperiment. Only a minor mineralization of NP was seen inthe core with sludge that was mainly anaerobic (Table 2). Thevery low mineralization of NP2EO in mixtures with predom-inantly anaerobic conditions (Table 2) also confirms that NPwas not mineralized under anaerobic conditions since the twomodel chemical have very similar chemical structures.

DEHP

The DEHP was mineralized more slowly than LAS, NP,and NP2EO in the sludge-soil mixtures and in soil, althoughDEHP was readily biodegradable (80% in 28 d) in an aerobicscreening test for ready biodegradability with suspended ac-tivated sludge (Organisation for Economic Cooperation andDevelopment 301B) [14]. Sorption of DEHP to sludge andsoil apparently made DEHP less bioavailable to the degradingmicroorganisms in our study. Roslev et al. [15] also found arelatively slow mineralization rate for DEHP, although it washigher than the rate observed in our experiments. In the studyby Roslev et al. [15], approximately 35% of the DEHP wasmineralized under aerobic conditions during 60 d in sludge-soil mixtures. The dewatered sludge that was used in this ex-periment was mixed with soil in a ratio of 1:58 and water was


Table 3. Mineralization of model chemicals in sludge mixed with different soils and in soilsa

Sludge-soilmixtureb Soil

Initialconcentration

in test mixture(mg/kg dry wt)c

Mineralizedafter two months(% of added 14C)

Total recovery of 14C(% of added 14C)

Maximum rate(% of initial

concentration per day)

LAS1:1001:1001:100Soil onlySoil onlySoil only

JyndevadLundgaardAskovJyndevadLundgaardAskov

7.57.57.51.31.31.3

81.1 (5.49) A72.9 (5.14) AB68.8 (7.54) B61.8 (0.83) C67.1 (3.41) B64.4 (1.08) B

94.1 (7.8)88.6 (5.6)88.7 (15.6)89.1 (1.7)89.1 (6.7)88.4 (2.4)

8.53 (0.64) A4.71 (0.41) B4.04 (0.83) BC3.61 (0.11) C4.17 (0.24) B3.48 (0.10) C

NP1:1001:1001:100Soil only

JyndevadLundgaardAskovJyndevad

11.411.411.4

2.0

63.7 (12.01) A50.8 (13.77) A49.7 (8.11) A56.3 (0.42) A

81.4 (19.6)77.3 (17.6)79.5 (16.9)76.6 (7.1)

6.02 (0.93) A4.27 (1.15) BC3.86 (0.59) B4.73 (0.08) C

NP2EO1:1001:1001:100Soil only


19.819.819.8

4.4

63.0 (12.80) A62.4 (8.51) A55.2 (10.79) A64.4 (0.22) A

83.6 (8.7)86.7 (1.7)80.0 (11.6)84.4 (3.1)

2.0 (0.33) A2.0 (0.40) A1.8 (0.07) A6.09 (0.73) B

DEHP1:1001:100

JyndevadLundgaard

0.630.63

18.0 (2.16) A6.8 (2.30) BD

54.1 (10.4)61.8 (5.7)

0.29 (0.03) A0.11 (0.04) BD

1:100Soil onlySoil onlySoil only

AskowJyndevadLundgaardAskov

0.630.240.240.24

5.8 (0.46) B21.8 (1.25) C

9.43 (1.95) D8.46 (1.64) D

65.9 (12.5)48.8 (7.7)54.6 (0.4)58.2 (5.1)

0.09 (0.01) B0.37 (0.02) C0.16 (0.03) D0.14 (0.03) D

Pyrene1:1001:1001:100Soil only


0.070.070.070.01

4.0 (0.21) A2.2 (0.49) B4.8 (1.77) AC4.7 (0.11) C

105.5 (15.4)82.8 (8.0)98.4 (5.2)93.1 (8.9)

0.07 (0.003) A0.04 (0.008) B0.08 (0.03) AC0.08 (0.00) C

a All mixtures were entirely aerobic. Parentheses indicate the standard deviation of four replicates. The differences are indicated individually foreach compound. All values (for each compound) followed by one or more letters in common are not significantly different (p , 0.05). LAS5 linear alkylbenzene sulfonate; NP 5 nonylphenol; NP2EO 5 nonylphenol-di-ethoxylate; DEHP 5 di-(2-ethylhexyl)phthalate.

b Sludge soil ratio based on dry weight. The water content was 40% of the water-holding capacity in all sludge-soil mixtures and in soil.c Initial concentration of test chemical expressed on a dry weight basis.

Table 4. Time needed to reach 50% of the final accumulated 14CO2

(t50) for LAS, NP, and NP2EO in the aerobic sludge-soil mixtures andsoila

Sludge-soilmixtureb Soil

LASt50 (days)

NPt50 (days)

NP2EOt50 (days)

1:20, 40%1:100, 40%1:100, 40%1:100, 40%

JyndevadJyndevadJyndevadLundgaard

7.0 (0.55)8.5 (0.20)7.9 (0.19)7.9 (0.41)

7.3 (0.07)8.6 (0.16)9.8 (0.94)9.1 (0.18)

17.1 (0.84)8.5 (0.26)8.4 (0.19)7.8 (0.20)

1:100, 40%Soil onlySoil onlySoil only

AskovJyndevadLundgaardAskov

7.2 (0.14)8.2 (0.15)7.9 (0.14)7.5 (0.20)

8.8 (0.17)10.6 (0.26)

NDND

8.8 (1.40)10.0 (0.11)

NDND

a LAS 5 linear alkylbenzene sulfonate; NP 5 nonylphenol; NP2EO5 nonylphenol-di-ethoxylate; ND 5 not determined.

b Sludge-soil ratio based on dry weight. The water content was 40%of the water-holding capacity in all sludge-soil mixtures and in soil.Parentheses indicate the standard deviation of four replicates.

added to 75% of the WHC. The mineralization curve had aninitial phase that could be described by an exponential decreasein the concentration of DEHP. This initial phase was followedby a second phase of slow mineralization described by dif-ferent kinetics. With the use of model predictions, the authorsestimated that more than 40% of the DEHP was not miner-alized in sludge-soil mixtures after a year [15]. In our exper-iments, the mineralization of DEHP reached approximately

20% of the added 14C during two months in predominantlyaerobic as well as predominantly anaerobic sludge-soil cores(Table 2 and Fig. 2c). It is possible that DEHP was partiallymineralized under anaerobic conditions in our experiments.Ejlertsson and Svensson [16], however, found that DEHP wasnot degraded in methanogenic enrichment cultures during 330d. The mineralization of DEHP in our sludge-soil mixturesmay be the result of the presence of aerobic/anaerobic gra-dients. A denitrifying zone may have been present betweenthe aerobic outer layer and the anaerobic center of the cores,and transformation products may have diffused from the an-aerobic to the aerobic zone. New experiments performed undercontrolled anaerobic conditions have shown that 9.7% of theadded 14C[DEHP] was mineralized in 96 d under anaerobic,denitrifying conditions (B. Gejlsbjerg, unpublished data).

In our experiments, DEHP was mineralized more efficientlywhen sludge was mixed with Jyndevad soil compared with thetwo other soils (Table 3). This was probably caused by differentmineralization potentials of the soils since the highest min-eralization of DEHP in soils without sludge was also observedin the Jyndevad soil. However, Roslev et al. [15] found thatthe mineralization of DEHP in sludge was slightly higher thanwhen sludge was mixed with soil. However, as the total min-eralization in the experiments performed by Roslev et al. wasalso higher (35% after two months), it is likely that the sludgeused by Roslev et al. had a higher mineralization potential forDEHP than the sludge used in our experiments. Since the


concentration of organic carbon in the three soils used in ourstudy was similar and most of the DEHP is expected to sorbto the sludge or organic components in the soil, it is not likelythat the differences in mineralization were due to a differentsorption of DEHP in the three soils.

Pyrene

Pyrene was mineralized more slowly than the other modelchemicals and only under predominantly aerobic conditions(Table 2). Wild and Jones [17] observed that the degradationof pyrene and other PAHs attained a higher level in soil thathad previously received sludge (49% loss during 205 d) com-pared with nontreated soil (36% loss during 205 d). Grosseret al. [18] found as well that three coal tar-contaminated soilsmineralized pyrene significantly faster than nonexposed con-trol soils and between 10 and 48% of the pyrene was miner-alized after 60 d. When the degradation of pyrene is investi-gated, very different results are often found as a result ofwhether or not the soil has been preexposed to pyrene. Theslow mineralization of pyrene in our sludge-soil mixtures maybe explained by the fact that the soil had not previously re-ceived sludge and that the available concentration of pyrenein the sludge itself was too low to stimulate microbial deg-radation of the substance.

DCB

In contrast with the experiments with the other model chem-icals, a higher mineralization of DCB was seen in the mixturesthat contained the highest amount of sludge (Table 2). Thepoor 14C recovery in the assays with DCB does not allowstringent conclusions. However, several explanations for themore extensive mineralization of DCB in the sludge and inthe 1:20 sludge-soil mixture may be proposed. The observedmineralization of DCB was probably mediated by aerobic mi-croorganisms; an additional experiment performed understrictly anaerobic conditions showed that DCB was not min-eralized in sludge in the absence of molecular oxygen (datanot shown). However, the low oxygen content in the mixtureswith the highest amounts of sludge might have enhanced themineralization of DCB by a combination of anaerobic reduc-tive dechlorination and aerobic degradation processes. Reduc-tive dechlorination of DCB would lead to the formation ofmonochlorobenzene that may have been mineralized underaerobic conditions [19–21]. Another explanation for the highermineralization in mixtures with more sludge could be that moreDCB was sorbed to the sludge and therefore less DCB waslost by volatilization. Previous laboratory studies have indi-cated that volatilization was the main mechanism for loss ofDCB in an unacclimated, activated sludge system [22]. In astudy of the fate of different chlorinated benzenes in soil thathad been treated with sludge for 20 years, Wang et al. [23]found that 18% of the added DCB remained in the soil. Wanget al. also found that relatively minor amounts remained ofthe more volatile chlorinated benzenes, suggesting that thesesubstances were partially lost by volatilization.

CONCLUSIONS

When sludge is applied to agricultural soil, many of theorganic contaminants in the sludge may be rapidly mineralizedin the aerobic parts of the sludge-amended soil. Substanceslike LAS, NP, and NP2EO are rapidly mineralized under aer-obic conditions even in soils that have not previously been

exposed to xenobiotic organic compounds. The DEHP is min-eralized more slowly because the major part of the substanceis sorbed strongly to the soil and therefore is not bioavailable.For DCB, aerobic/anaerobic gradients in the soil can possiblehave a stimulating effect on the mineralization.

On the basis of these results, it is anticipated that substancesthat are ultimately biodegradable under aerobic conditions willalso degrade in sludge-amended soil as soon as the disinte-gration of larger sludge lumps exposes the chemicals to mo-lecular oxygen. The organic residues that resist biodegradationare probably bound to the organic fraction in the sludge andsoil, and therefore they are expected to be less toxic to soilliving organisms.

Acknowledgement—This work was supported by the Danish Envi-ronmental Research Programme, Centre for Sustainable Land Use andManagement of Contaminants, Carbon, and Nitrogen.

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mineralization of organic contaminants in sludge-soil mixtures

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