Environmental Pollution 141 (2006) 459e468www.elsevier.com/locate/envpol

Degradation of polycyclic aromatic hydrocarbons (PAHs) in an agedcoal tar contaminated soil under in-vessel composting conditions

Blanca Antizar-Ladislao*, Joe Lopez-Real, Angus James Beck

Imperial College London, Wye Campus, Department of Agricultural Science, Wye, Ashford, Kent TN25 5AH, UK

Received 20 August 2004; accepted 26 August 2005

Maximum degradation of PAHs in an aged coal tar contaminated soil can be achieved usingoptimal operational conditions during composting.

Abstract

In-vessel composting of polycyclic aromatic hydrocarbons (PAHs) present in contaminated soil from a manufactured gas plant site was in-vestigated over 98 days using laboratory-scale in-vessel composting reactors. The composting reactors were operated at 18 different operationalconditions using a 3-factor factorial design with three temperatures (T, 38 �C, 55 �C and 70 �C), four soil to green waste ratios (S:GW, 0.6:1,0.7:1, 0.8:1 and 0.9:1 on a dry weight basis) and three moisture contents (MC, 40%, 60% and 80%). PAH losses followed first order kineticsreaching 0.015 day�1 at optimal operational conditions. A factor analysis of the 18 different operational conditions under investigation indicatedthat the optimal operational conditions for degradation of PAHs occurred at MC 60%, S:GW 0.8:1 and T 38 �C. Thus, it is recommended tomaintain operational conditions during in-vessel composting of PAH-solid waste close to these values.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Composting; Kinetics; Polycyclic aromatic hydrocarbons

1. Introduction

Composting of yard wastes, municipal wastewater sludges,and municipal solid wastes are long established, however, com-posting of soils contaminated with hazardous materials is stillan emerging ex situ biotreatment technology. Compostinghas been demonstrated to be effective in biodegrading polycy-clic aromatic hydrocarbons (PAHs) (Canet et al., 2001; Potteret al., 1999), chlorophenols (Laine et al., 1997), polychlori-nated biphenyls (Block, 1998), explosives (Gray, 1999) andpetroleum hydrocarbons (Namkoong et al., 2002) at the labora-tory and/or field-scales. Though these studies have shown thatcomposting can be effective, wide variations have beenreported due to the lack of control of key environmentalparameters (Jorgensen et al., 2000; Laine and Jorgensen,1997).

* Corresponding author. Tel.: C44 20 759 42779; fax: C44 20 759 42640.

E-mail address: b.antizar@imperial.ac.uk (B. Antizar-Ladislao).

0269-7491/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.envpol.2005.08.066

In recent years, the development and widespread use ofmore expensive in-vessel systems for the processing of greenbiowastes has resulted from legislative pressures on the safetyof the composting process and the subsequent use of the com-post product (EC, 2003). Such systems allow for much moreprecise control of the composting process particularly in termsof moisture and temperature control. Thus, current compostingapproaches and technologies tend to emphasize the use of hightemperatures (O70 �C) in order to meet regulatory require-ments for pathogen control (EC, 2003). Such temperaturesseverely inhibit the microbial diversity and hence enzymaticpotential of the system (Antizar-Ladislao et al., 2004, 2005).The use of in-vessel systems for the composting of contami-nated solid waste would therefore enable operators to selectthe adequate operating parameters to obtain maximum con-taminant degradation by a diverse microflora (Canet et al.,1999, 2001).

Previous research has suggested that operating parametersrelevant to composting efficiency include temperature, soil

460 B. Antizar-Ladislao et al. / Environmental Pollution 141 (2006) 459e468

to waste amendment ratio and moisture content but to datethere has been no systematic and holistic study of all threekey parameters together (Feitkenhauer et al., 2003; Namkoonget al., 2002; Potter et al., 1999). In addition to the influence ofeach operational variable on composting, these variables mayinteract in complex ways with uncertain effects. For example,Liang et al. (2003) reported that moisture content was moreimportant than temperature on composting. Previous investi-gations clearly indicate that the operational compostingparameters are of crucial importance during composting treat-ments of contaminated wastes, and thus their optimisation willresult in beneficial environmental and economic practices.Hence, this research sought to investigate the impact of tem-perature, soil to green waste ratio and moisture on the degra-dation of the 16 USEPA-listed PAHs at laboratory-scale and todetermine the optimal combination of operational parametersfor in-vessel composting of PAH-contaminated solid wastes.

2. Materials and methods

Eighteen experimental conditions were tested in triplicate using 360 labo-

ratory-scale composting reactors. The standard composting reactors comprised

three temperature levels (T, 38 �C, 55 �C and 70 �C) four soil to green waste

ratios (S:GW, 0.6:1, 0.7:1, 0.8:1 and 0.9:1 on a dry weight:weight basis) and

three moisture contents (MC, 40%, 60% and 80%). Control reactors consisted

of 1:0 S:GW ratio. The logistic approach to pinpoint the optimal operational

conditions for maximum degradation of PAHs was, first to investigate the in-

fluence of S:GW 0.6e0.9:1 at three temperature levels and MC 60%; then to

investigate the influence of MC 40e80% at three temperature levels and opti-

mal S:GW ratio. Using this logistic approach allowed to investigate 18 exper-

imental conditions at laboratory-scale rather than 36 possible combinations.

2.1. Contaminated soil

The coal tar contaminated soil was obtained from a manufactured gas plant

site commissioned in 1838 at Clitheroe, Lancashire, United Kingdom. An ex-

tensive description of the site and the procedures for soil sampling and prep-

aration were reported by Birngstingl (1997). Before the present study, the coal

tar contaminated soil was air-dried and homogenized by passing through a

5-mm sieve followed by a 2-mm sieve and stored in the laboratory at room

temperature. Before experimentation the coal tar contaminated soil was diluted

by homogenizing with silver sand (1:1) to provide a more homogeneous

distribution of the coal tar residue. The results presented here refer to post-

dilution coal tar contaminated soil, and all results are expressed as post-dilu-

tion. Thus, the soil contained 100.3 mg PAHs kg�1 soil, organic matter content

was 4.79 G 0.16% (wt/dry wt) and soil pHw was 7.3 G 0.1.

2.2. Green waste

The soil was conditioned with an artificial green waste for the composting

studies, which represents green waste to be used at field-scale based studies.

Artificial green waste was prepared by mixing foodstuff (mixture of carrots,

cucumber, lettuce, onions, potatoes and tomatoes in equal amounts)

(3%dw), sawdust (38%dw), leaves (17%dw), grass (27%dw) and wheat straw

(14%dw). Foodstuff, sawdust, wheat straw and leaves where separately blended

and grass was cut. The composition of the green waste satisfied the nutrient

requirement (C:N 40e50) according to the calculations using Cornell’s system

(Richard, 1995).

2.3. Reactor’s design

Three hundred and sixty glass composting reactors (200 ml) were con-

structed, which allowed conditions to be easily monitored and controlled

(Fig. 1). These fully enclosed bench-scale reactors each held about 65 g total

composting mixture. Composting reactors were placed in triplicates in temper-

ature-controlled incubators at 38 �C, 55 �C and 70 �C to simulate representa-

tive mesophilic and thermophilic microbiological stages during composting

processes (Antizar-Ladislao et al., 2004; Walter et al., 1992). For each glass

composting reactor, each ingredient of the artificial green waste and soil in

the particular S:GW ratio under investigation were thoroughly mixed in a glass

beaker (500 ml), and then the mixture introduced into the reactor. Initial mois-

ture content of the composting mixture was measured (see Section 2.4.1) and

double distilled water (DDW) was added when needed to reach the desirable

moisture content for the different experimental conditions under investigation.

Compost moisture content was measured (see Section 2.4.1) at intervals to en-

sure that it was maintained at the required level, and amended with DDW

when needed. The reactor units stood vertically with air flowing continuously

to avoid oxygen content limitation and vented outdoors to avoid volatiles ac-

cumulation in the composting reactors. Airflow up through the composting

mixture by means of a stainless steel air-delivery tube inserted into the bottom

of the composting reactors was provided by 100% oil free diaphragm pumps

(Model PXW-600-DIOV, VP1, Fisher Scientific). The air inlet was bubbled

through a DDW reservoir to avoid excessive water evaporation during

aeration.

2.4. Sample analysis

Destructive sampling, in triplicate, for each experimental treatment was

done after 0, 7, 21, 45, 56, 80 and 98 days. The contents of each composting

reactor were thoroughly mixed with a stainless steel spatula in a glass beaker

(500 ml), and sub-samples were collected for analysis.

2.4.1. Total organic matter (TOM) in composting mixtures

Ash content was determined using a loss-on-ignition (L.O.I.) procedure.

Triplicate 5 g samples were dried at 110 �C for 24 h (moisture content) and

then transferred to a muffle furnace at 550 �C for 12 h. Ash content was cal-

culated from the ratio of pre- and post-ignition sample weights. The residual

moisture of the samples was determined to produce the results on a dry matter

basis (110 �C).

2.4.2. PAHs in composting mixturesThe extraction of 16-E.P.A. Priority PAHs from composting mixtures and

soil was done by Accelerated Solvent Extraction (ASE� 200) and defined in

this investigation as total PAHs for simplicity. Briefly, glass fibre disks were

placed at the outlet end of the extraction cells and a 7-g sample of compost

was mixed with 3 g of sodium sulphate and 7 g of Hydromatrix� and intro-

duced into each extraction cell. Surrogate standards (1-fluoronaphthalene,

2-fluorobiphenyl, purity O 97%, Greyhound Chromatography & Allied

Airinlet

AirOutlet

Diaphragm Pump

Air/waterreservoir Composting reactors

Air inlet

Airreservoir

Air outlet

Temperature-controlled incubator

Fig. 1. Schematic laboratory set-up of composting reactors.

461B. Antizar-Ladislao et al. / Environmental Pollution 141 (2006) 459e468

Chemicals (UK)) were added to the cells prior to extraction to monitor PAH

losses. Extraction cells were placed into the auto-sampler tray with copper

turnings to remove sulphur. ASE� 200 conditions for PAH extraction were:

14 MPa (2000 psi), 100 �C, oven heat-up time Z 5 min, static time Z 5 min,

solvent dichloromethane/acetone (1:1) (v/v), flush volume Z 60% of extrac-

tion cell volume, nitrogen purge Z 1 MPa (150 psi) for 60 s. The extracts

were cleaned up on a column containing 1 g of activated-florisil (SiO2,

84.0%; MgO, 15.5%; Na2SO4, 0.5%; 60/100 mesh; 130 �C; 12 h) and 2 g

of anhydrous sodium sulphate. To remove hydrophobic impurities the column

was initially washed with 10 ml dichloromethane before eluting 5 ml of ex-

tract and then leaving to dry for 1 min. The PAHs were then eluted with

10 ml dichloromethane.

Internal standards (naphthalene-d8, acenaphthene-d10 in a mixture

with chrysene-d12, 1,4-dichlorobenzene-d4, perylene-d12, phenanthrene-d10,

purity O 97%, Greyhound Chromatography & Allied Chemicals (UK)) were

added to the clean extracts prior to analysis. A Hewlett Packard 6890 series

gas chromatograph with a 7673 series auto-sampler and a 5973 series mass se-

lective detector was used for the analysis. The GC inlet was operated in pulsed

(0.90 min, 30.0 psi) splitless mode at 270 �C with helium as carrier gas. The

injection volume was 1 ml and the inlet purged at 50 ml min�1 1 min after injec-

tion; the column flow was maintained at 1 ml min�1. Separation was achieved

using an HP-5MS column (19091S-433 30 m ! 0.25 mm ! 0.25 mm).

The temperature program comprised 70 �C for 2 min, 10 �C min�1 to

300 �C where it was held until the end of the analysis (10 min). The

MS transfer line was 280 �C providing conductive heating of the MS

source to about 230 �C. The instrument was tuned using perfluorotributyl-

amine. The MS was operated in selective ion monitoring (SIM) mode.

The 16 USEPA PAHs (quantification ion/confirmation ions), internal stand-

ards (quantification ion/confirmation ion) and surrogates (quantification

ion/confirmation ions) for SIM GCeMS mode were: naphthalene (128/

127, 129, 102), naphthalene-d8 (136/137, 134, 108), 1-fluoronaphthalene

(146/120, 125), 2-fluorobiphenyl (172/171, 170), acenaphthylene (152/

151, 153, 76), acenaphthene (154/153, 152), acenaphthene-d10 (164/162,

160, 163), fluorene (166/139, 165), phenanthrene (178/165, 163, 82,

176), anthracene (178/179, 176, 89), fluoranthene (202/200, 101, 203),

pyrene (202/200, 201, 101, 203), benzo[a]anthracene (228/226, 229),

chrysene (228/226, 230, 113), benzo[b]fluoranthene (252/250, 253, 126),

benzo[k]fluoranthene (252/253, 250, 126), benzo[a]pyrene (252/207, 253,

250, 126), indeno[1,2,3-c,d]pyrene (276/276, 279, 138), dibenzo[a,h]anthra-

cene (278/279, 139, 276), benzo[g,h,i]perylene (276/138, 137, 277). The

GCeMS system was calibrated prior to the analysis of samples using

seven calibration standards.

2.5. Data analysis

Comparison ofP

PAHs removal under different composting conditions

was investigated using paired t-tests. Factor analysis was used to simplify

the complex and diverse relationships that exist among the operational param-

eters and to investigate the structure in the relationship between the operational

parameters during in-vessel composting of PAH-contaminated solid waste.

Factor analysis was executed, with factors extracted by the principal compo-

nent method from the correlation matrix and varimax rotated factor loadings

using the latest version of StatistiXL.

3. Results and discussion

The initialP

PAH concentration in the investigated soil (1:1soil diluted with silver sand) was 100.3 mg PAHs kg�1 soil,which is lower than those concentrations of about 450 mgPAH kg�1 soil/sediment reported in a typical manufacturinggas plant site by Erickson et al. (1993), however, they wereabove the Dutch List action level of 40 mg PAH kg�1 air-driedsoil. The 16 USEPA-listed priority pollutant PAHs were groupedas two- and three-ring PAHs (naphthalene, acenaphthylene,acenaphthene, fluorene, anthracene, phenanthrene), four-ringPAHs (fluoranthene, pyrene, benzo[a]anthracene, chrysene)and five- and six-ring PAHs (benzo[b]fluoranthene, benzo[k]-fluoranthene, benzo[a]pyrene, dibenzo[a,h]anthracene, indeno-[1,2,3-c,d]pyrene, benzo[g,h,i]perylene) and thus defined assmall, medium and large PAHs, respectively, for ease ofdiscussion.

3.1. TOM evolution during composting

The TOM levels were high (w62%) at the beginning of thecomposting process and then decreased, reaching a value ofw40% after 98 days treatment at 38 �C, thus indicating theoccurrence of mineralization. At higher temperatures theTOM decrease was less, which may indicate that mineraliza-tion occurred to a lesser extent because higher temperaturesconstrained microbial growth (Antizar-Ladislao et al., 2005)(Fig. 2). The concentrations of the 16 USEPA-listed PAHs in-vestigated in the present study before treatment and after 98days at three different temperatures (T, 38 �C, 55 �C and70 �C), four soil to green waste ratio (S:GW, 0.6:1, 0.7:1,0.8:1 and 0.9:1) and three different moisture contents (MC,40%, 60% and 80%) as well as the control reactors PAH con-tent are presented in Table 1. PAH concentration in all reactors

A

30

40

50

60

70

0 15 30 45 60 75 90 105

Time, days

0 15 30 45 60 75 90 105

Time, days

0 15 30 45 60 75 90 105

Time, days

TOM

, %

30

40

50

60

70

TOM

, %

30

40

50

60

70

TOM

, %

MC60,S:GW60

MC60,S:GW70

MC60,S:GW80

MC60,S:GW90

MC40,S:GW80

MC80,S:GW80

B C

Fig. 2. Temporal profile of total organic matter in composting reactors. (A) 38 �C, (B) 55 �C and (C) 70 �C (Legend: MC e moisture; S:GW e soil to green waste

ratio).

462 B. Antizar-Ladislao et al. / Environmental Pollution 141 (2006) 459e468

Table 1

PAH concentrations (mg PAH/kg dry soil; % removal) after 98 days of continuous in-vessel composting treatment (PAH concentration (mg PAH/kg dry soil) at

start: 2 C 3 rings: 32.5 G 1.3; 4 rings: 46.5 G 3.9; 5 C 6 rings: 21.4 G 1.8;P

PAH: 100.3 G 3.2)

Case MC/S:GW PAH After 98 days

38 �C 55 �C 70 �C

60%/0.6:1 2 C 3 rings 0.6 G 0.4 (98.2%a) 15.3 G 0.3 (53.0%a) 27.0 G 0.4 (16.9%a)

4 rings 11.1 G 1.3 (76.2%b) 21.5 G 1.5 (53.7%b) 35.9 G 1.4 (22.7%b)

5 C 6 rings 12.1 G 1.3 (43.6%c) 11.2 G 0.7 (47.6%c) 15.1 G 1.7 (29.3%c)P

PAH 23.7 48.0 78.0

Removal (%) 76.4 52.2 22.3

60%/0.7:1 2 C 3 rings 2.0 G 0.2 (93.7%a) 13.2 G 0.3 (59.5%a) 21.3 G 0.6 (34.7%a)

4 rings 7.1 G 1.2 (84.8%b) 12.1 G 2.0 (74.0%b) 19.5 G 2.1 (58.0%b)

5 C 6 rings 9.0 G 1.2 (58.0%c) 7.2 G 0.8 (66.3%c) 13.4 G 0.5 (37.3%c)PPAH 18.1 32.4 54.2

Removal (%) 82.0 67.7 46.0

60%/08:1 2 C 3 rings 2.8 G 0.7 (91.3%a) 13.4 G 1.4 (58.9%a) 19.0 G 0.1 (41.6%a)

4 rings 10.3 G 5.6 (77.8%b) 15.7 G 3.7 (66.2%b) 11.8 G 2.4 (74.7%b)

5 C 6 rings 10.2 G 1.9 (52.3%c) 10.5 G 1.9 (50.7%c) 13.7 G 1.2 (35.9%c)P

PAH 23.4 39.6 44.5

Removal (%) 76.7 60.6 55.7

60%/0.9:1 2 C 3 rings 3.6 G 0.1 (88.9%a) 14.3 G 0.2 (56.0%a) 18.8 G 0.4 (42.2%a)

4 rings 11.3 G 1.4 (75.6%b) 13.7 G 1.0 (70.6%b) 12.9 G 1.9 (72.2%b)

5 C 6 rings 16.0 G 0.9 (25.0%c) 9.7 G 0.4 (54.5%c) 14.5 G 0.9 (32.3%c)PPAH 31.0 37.7 46.2

Removal (%) 69.1 62.43 54.0

40%/0.8:1 2 C 3 rings 5.3 G 0.5 (83.6%a) 16.3 G 0.9 (49.9%a) 18.6 G 0.3 (42.8%a)

4 rings 23.4 G 5.2 (49.5%b) 42.4 G 2.4 (8.7%b) 19.1 G 1.9 (59.0%b)

5 C 6 rings 32.23 G 2.1 (ec) 19.6 G 0.6 (8.3%c) 22.1 G 2.2 (ec)P

PAH 61.0 78.33 59.8

Removal (%) 39.2 21.94 40.4

80%/0.8:1 2 C 3 rings 3.4 G 0.6 (89.5%a) 13.4 G 1.0 (58.4%a) 20.0 G 0.2 (38.4%a)

4 rings 12.2 G 3.2 (73.7%b) 18.4 G 5.0 (60.3%b) 26.7 G 2.3 (42.5%b)

5 C 6 rings 16.1 G 3.7 (24.9%c) 17.1 G 2.2 (19.9%c) 19.1 G 3.0 (10.7%c)PPAH 31.7 49.0 65.8

Removal (%) 68.4 51.2 34.4

Control 0%/1.0:0 2 C 3 rings 24.4 G 0.3 (24.9%a) 20.1 G 0.4 (38.2%a) 11.0 G 0.6 (66.2%a)

4 rings 46.3 G 1.4 (0.43%b) 42.7 G 1.4 (8.17%b) 27.9 G 1.9 (40.0%b)

5 C 6 rings 20.1 G 1.4 (6.01%c) 20.6 G 0.7 (3.74%c) 18.3 G 0.7 (14.5%c)P

PAH 90.8 83.4 57.2

Removal (%) 9.5 16.9 43.0

a 2 C 3 rings percent removal: naphthalene, acenaphthylene, acenaphthene, fluorene, anthracene, phenanthrene.b 4 rings percent removal: fluoranthene, pyrene, benzo[a]anthracene, chrysene.c 5 C 6 rings percent removal: benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, dibenzo[a,h]anthracene, indeno[1,2,3-c,d]pyrene, benzo[g,h,i]perylene.

remained consistently distributed in the composting mixturesdue to an initial extensive blending of soil and green waste.TOC and

PPAH concentration decrease during composting

was in agreement and indicated that T, S:GW ratio and MChave an important influence on the behaviour of PAHs duringcomposting. Thus, a mechanistic and fundamental investiga-tion is necessary to optimise the operational parameters duringin-vessel composting of PAH-contaminated solid wastes.

3.2. Effect of temperature

The effect of temperature on PAH-contaminated waste deg-radation was initially investigated at 38 �C, 55 �C and 70 �Cfor S:GW 0.6:1 and MC 60%. Temperature was an importantfactor affecting the in-vessel composting of this PAH-

contaminated soil (Fig. 3). PAH losses were greater at 38 �C(76.8% removed) than at 55 �C or 70 �C, with differencesalso observed between the two higher temperature treatments(52.2% and 22.3% removed, respectively, at 55 �C and77 �C). Small PAHs were reduced by an average of 98.2%at 38 �C as compared to 53.0% and 16.9% at 55 �C and70 �C, respectively, at S:GW 0.6:1 and MC 60%. MediumPAHs were reduced by 76.2%, 53.7% and 22.7% at 38 �C,55 �C and 70 �C, respectively, and large PAHs were reducedby an average of 40% at 38 �C, 55 �C and 70 �C, S:GW0.6:1 and MC 60%.

Regarding concentrations of individual PAHs, chrysene,benzo[k]fluoranthene and benzo[a]pyrene were not removedat an S:GW ratio of 0.6:1 and MC 60% proving their recal-citrance nature. Benzo[g,h,i]perylene final concentration

463B. Antizar-Ladislao et al. / Environmental Pollution 141 (2006) 459e468

0

5

10

15

20

25

30

35

40

Nap

htha

lene

Acen

apht

hyle

ne

Acen

apht

hene

Fluo

rene

Phen

anth

rene

Anth

race

ne

Fluo

rant

hene

Pyre

ne

Benz

o(a)

anth

race

ne

Chr

ysen

e

Benz

o(b)

fluor

anth

ene

Benz

o(k)

fluor

anth

ene

Benz

o(a)

pyre

ne

Inde

no(1

,2,3

-c,d

)pyr

ene

Dib

enzo

(a,h

)Ant

hrac

ene

Benz

o(g,

h,i)p

eryl

ene

Con

cent

ratio

n, m

g/kg

38C 55C 70C Initial

0

40

80

120

0 7 21 45 56 80 98Time, days

% P

AHs

rem

aini

ng

2+3rings 4rings

5+6rings total

BA

Fig. 3. Effect of temperature on PAH removal at S:GW 0.6:1 and MC 60%. (A) Final concentration of 16 PAHs following 98 days of treatment. (B) Temporal

profile of 2 C 3 rings PAH, 4 rings PAH and 5 C 6 rings PAH at 38 �C.

appeared to be w55% of the initial concentration followingthe three temperature levels treatments, which may indicatethat the mechanisms of removal of this particular PAH maybe due to mechanisms other than biological degradation.Benzo[g,h,i]perylene is expected to be strongly bound to theorganic composting matrix due to its high hydrophobicity(log Kow 7.23) (Antizar-Ladislao et al., 2004) and volatilisationis not expected to be an important fate process, given itsHenry constant value (2 ! 10�7 atm m3 mol�1) (Antizar-Ladislao et al., 2004). In addition, adsorption to the organiccomposting matrix is expected to attenuate its potentialvolatilisation. However, under the investigated compostingconditions, benzo[g,h,i]perylene may partially exist in particu-late phase (vapour pressure 6 ! 10�8 Pa at 25 �C) (Antizar-Ladislao et al., 2004) or adsorbed onto fine particles, and itmay have been transferred to the atmosphere.

Large PAHs are often difficult to biodegrade and differentstrategies have been investigated in an attempt to find thebest conditions to remove these recalcitrant contaminants.Most of these strategies have been developed to overcomethe low bioavailability of large PAHs, such as an increase intemperature (Feitkenhauer et al., 2003) or use of surfactants(Steffen et al., 2003; Zheng and Obbard, 2002) combinedwith the use of specialized microorganisms which degradearomatic hydrocarbons (Bhatt et al., 2002; Canet et al.,2001; McFarland and Qiu, 1995). Current compostingapproaches and technologies tend to emphasize the use ofhigh temperatures (O70 �C) in order to meet regulatoryrequirements for pathogen control (EC, 2003). Such tempera-tures severely inhibit the microbial diversity and hence enzy-matic potential of the system (Antizar-Ladislao et al., 2005).What little research has been done suggests that the rangebetween 40 �C and 45 �C allows for such maximum

diversity to be expressed for both, bacteria, actinomycetes,and fungi (Antizar-Ladislao et al., 2004; Walter et al., 1992).It is of interest to note that this range represents an overlappoint at which both mesophilic and thermophilic microorgan-isms will be viable and active.

3.3. Effect of soil to green waste ratio

The effect of soil to green waste ratio on PAH-contaminatedwaste degradation was investigated at 38 �C, 55 �C and 70 �C(Fig. 4). A temperature of 38 �C proved to be the optimal forP

PAHs removal. Of the 12 experimental conditions (S:GW0.6e0.9:1, MC 60, T 38e70 �C),

PPAHs removal at 38 �C

was significantly higher thanP

PAHs removal at 55e70 �C(P ! 0.01) over time; and

PPAHs removal at 55 �C was as

well higher thanP

PAHs removal at 70 �C (P ! 0.1). Higherremoval rates were apparent at S:GW 0.7e0.8:1, although nosignificant (P O 0.1) differences in

PPAH removal were found

for different S:GW at MC 60%. Small PAHs and medium PAHswere removed by an average of 91.3% and 79.4%, respectively,at 38 �C, S:GW 0.7:1, 0.8:1, 0.9:1 and MC 60%. Averageremoval of large PAHs was 58.0% and 52.3% at S:GW 0.7:1and 0.8:1, respectively, as compared to 25.0% at S:GW 0.9:1,both at 38 �C and MC 60%. Increasing the S:GW ratio to0.9:1 resulted in a lower removal of benzo[b]fluoranthene, ben-zo[k]fluoranthene and indeno[1,2,3-c,d]pyrene. Crawford et al.(1993) reported a pilot study to assess the feasibility of treatingtwo- to four-ring PAH-contaminated soils (about 100 mg kg�1)by composting leaves in small windrows (about 19 m3 each).They found that the amendment ratio did not affect the extentof degradation of the PAHs. This study corroborates the resultsof Crawford et al. (1993) within the S:GW ratios investigated,

464 B. Antizar-Ladislao et al. / Environmental Pollution 141 (2006) 459e468

B CA

0

40

80

120

0 7 21 45 56 80 98

Time, days0 7 21 45 56 80 98

Time, days

0 7 21 35 56 80 98

Time, days

0 7 21 45 56 80 98

Time, days

0 7 21 45 56 80 98

Time, days

0 7 21 45 56 80 98

Time, days

0 7 21 33 56 62 98

Time, days0 7 21 45 56 80 98

Time, days

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2+3rings 4rings 5+6rings 2+3rings 4rings 5+6rings 2+3rings 4rings 5+6rings

S:GW 0.7:1 S:GW 0.7:1S:GW 0.7:1

S:GW 0.8:1 S:GW 0.8:1 S:GW 0.8:1

S:GW 0.9:1 S:GW 0.9:1 S:GW 0.9:1

Fig. 4. Effect of soil to green waste ratio on PAH-solid waste degradation at MC 60%. Temporal profile of 2 C 3 rings PAH, 4 rings PAH and 5 C 6 rings PAH.

(A) 38 �C, (B) 55 �C and (C) 70 �C.

however, S:GW ratio may influence the rates of removal ofPAHs (see Section 3.6).

3.4. Effect of moisture content

The effect of moisture content on PAH-contaminated wastedegradation was investigated at 40%, 60% and 80% for anS:GW 0.8:1 and T 38 �C, 55 �C and 70 �C.

PPAHs removal

was significantly lower (P ! 0.05) at an MC of 40% as com-pared to 60% within the investigated temperature range,although no significant difference was found between MC60% and 80%. As aforementioned, while removal of PAHswas also observed at 70 �C and MC 40e80%, this occurredto a lesser extent than at 38 �C or 55 �C. Small PAHs were re-moved by an average of 88.1% at 38 �C within the MC rangeunder investigation. Medium PAHs were removed by an aver-age of 75.8% at MC 60% and 80% as compared to 49.5% atMC 40% and 38 �C, and large PAHs were removed by52.3% and 24.9% at MC 60% and 80% as compared to no re-moval at MC 40% and 38 �C. Potter et al. (1999) investigatedthe degradation of 19 PAHs from a soil during in-vessel com-posting at the laboratory-scale, using 70% soil and 30% corn-cob mixture on a dry weight basis, amended with cow manure,a modified OECD fertilizer, or activated sludge to adjust thenutrient content (CNP 100:5:1). They maintained an MC of30e35%, aerobic conditions, and temperature range 41e53 �C. Potter et al. (1999) reported that two- to three-ringPAHs were removed by an average of 87% in all composters

after 84 days of treatment, and four-ring were reduced by anaverage of 61%; however, none of the amendments conditionsappeared effective in degrading five- to six-ring PAHs. This in-vestigation is in agreement with Potter et al. (1999) results,and emphasizes the importance of MC optimisation duringin-vessel composting-remediation of contaminated wastes.

3.5. Optimal conditions for in-vessel compostingof a PAH-contaminated soil

The factor analysis result using 18 operational conditionsindicated that 3 factors could explain 99.8% of the total vari-ance (Table 2). For a simpler and easier interpretation, factorrotation was performed using varimax rotation after factorextraction, and rotation was normalized (Table 3). Factor 1 ex-plains the largest proportion (72.3%) of the total variance, thereare high positive loadings for the six operational conditions at38 �C (Table 4). This factor is associated with the removal of

Table 2

Eigenvalues of factors extracted by the principal component method from the

correlation matrix for removal of small, medium, large and total PAHs under

the 18 different in-vessel composting following 98 days of continuous

treatment

Value Factor 1 Factor 2 Factor 3

Eigenvalue 2.894 0.797 0.300

% of Var. 72.347 19.924 7.499

Cum% 72.347 92.271 99.770

465B. Antizar-Ladislao et al. / Environmental Pollution 141 (2006) 459e468

small (eigenvalue Z 0.955) and medium (eigenvalue Z 0.302)PAHs; in other words, Factor 1 is globally indicative on theoptimal operational conditions of S:GW, MC and T for degra-dation of small and medium PAHs. Factor 2 explains 19.9% ofthe total variance and accounts for the major part of the varian-ces of removal of large PAHs (eigenvalue Z 0.957), with highpositive loadings for operational conditions at MC 60%, S:GW0.7e0.8:1 and 38e55 �C (Table 4). It seems probable thatFactor 2 may be indicative on the optimal operational condi-tions for degradation of large PAHs. Factor 3 explains 7.5%of the total variance and accounts for the major part of the var-iances of removal of medium PAHs (eigenvalue Z 0.907), withhigh positive loadings for operational conditions at MC 40e60%, S:GW 0.8e0.9:1 and 70 �C (Table 4). It seems probablethat Factor 3 may be indicative on the operational conditionsfacilitating physicochemical losses of PAHs.

On the basis of the above analyses, the dominant mecha-nisms or ongoing processes affecting the removal of PAHs

Table 3

Eigenvalues of varimax rotated (normalized) factor loadings extracted by the

principal component method from the correlation matrix for removal of small,

medium, large and total PAHs under the 18 different in-vessel composting

following 98 days of continuous treatment

Value Factor 1 Factor 2 Factor 3

Small PAHs 0.955 0.057 0.291

Medium PAHs 0.302 0.292 0.907

Large PAHs 0.070 0.957 0.283PPAH 0.549 0.468 0.683

during in-vessel composting may be of biotic or abiotic nature.Nevertheless, optimal operational conditions for degradationof PAHs will be sought after at high positive Factors 1 and2 for lowest Factor 3. This situation would occur at MC60%, S:GW 0.8:1 and T 38 �C. In this study, it appears that38 �C enhances the biological removal of PAHs, which mightoccur due to a promotion of the native microbial population

Table 4

Casewise factor scores extracted by the principal component method from the

correlation matrix for removal of small, medium, large and total PAHs under

the 18 different in-vessel composting following 98 days of continuous

treatment

Case Factor 1 Factor 2 Factor 3

MC 60%; S:GW 0.6:1; T 38 �C 1.526 0.361 0.217

MC 60%; S:GW 0.7:1; T 38 �C 1.181 1.033 0.580

MC 60%; S:GW 0.8:1; T 38 �C 1.186 0.829 0.261

MC 60%; S:GW 0.9:1; T 38 �C 1.040 �0.717 0.699

MC 40%; S:GW 0.8:1; T 38 �C 1.265 �1.621 �0.597

MC 80%; S:GW 0.8:1; T 38 �C 1.105 �0.687 0.563

MC 60%; S:GW 0.6:1; T 55 �C �0.204 0.959 �0.653

MC 60%; S:GW 0.7:1; T 55 �C �0.234 1.677 0.236

MC 60%; S:GW 0.8:1; T 55 �C �0.154 0.915 0.033

MC 60%; S:GW 0.9:1; T 55 �C �0.369 1.052 0.299

MC 40%; S:GW 0.8:1; T 55 �C 0.415 �0.495 �2.849

MC 80%; S:GW 0.8:1; T 55 �C �0.147 �0.764 0.262

MC 60%; S:GW 0.6:1; T 70 �C �1.390 0.430 �1.795

MC 60%; S:GW 0.7:1; T 70 �C �1.193 0.323 0.114

MC 60%; S:GW 0.8:1; T 70 �C �1.192 �0.118 1.179

MC 60%; S:GW 0.9:1; T 70 �C �1.124 �0.281 1.074

MC 40%; S:GW 0.8:1; T 70 �C �0.923 �1.906 0.813

MC 80%; S:GW 0.8:1; T 70 �C �0.789 �0.991 �0.434

0 7 21 45 56 98

MC 60%

MC 40% MC 40% MC 40%

MC 60% MC 60%

MC 80% MC 80% MC 80%

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Time, days0 7 21 45 56 98

Time, days

0 7 21 45 56 98

Time, days0 7 21 45 56 98

Time, days

0 7 21 45 56 80 98

Time, days0 7 21 45 56 80 98

Time, days0 7 21 45 56 80 98

Time, days

0 7 21 45 56 80

Time, days

0 7 21 45 56 80

Time, days

2+3rings 4rings 5+6rings 2+3rings 4rings 5+6rings 2+3rings 4rings 5+6rings

A B C

Fig. 5. Effect of moisture content on PAH-solid waste degradation at S:GW 0.8:1. Temporal profile of 2 C 3 rings PAH, 4 rings PAH and 5 C 6 rings PAH.

(A) 38 �C, (B) 55 �C and (C) 70 �C.

466 B. Antizar-Ladislao et al. / Environmental Pollution 141 (2006) 459e468

A

0.0000.002

0.0040.006

0.0080.010

0.0120.014

0.016

65

70

75

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85

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k1, d

ay-1

soil, % tempe

rature

, 0C

0.0000.0020.0040.0060.0080.0100.0120.0140.016

B

0.0000.002

0.0040.006

0.0080.010

0.012

0.014

0.016

4045

5055

6065

7075

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k1, d

ay-1

moisture content, %

tempe

rature

, 0C

0.0000.0020.0040.0060.0080.0100.0120.0140.016

Fig. 6. Optimal soil to green waste ratio and moisture content during in-vessel composting of PAH-contaminated solid waste at 38e70 �C. (A) first order deg-

radation rate constant k1 (day�1) vs. soil content (%) at a constant moisture content 60%; (B) First order degradation rate constant k1 (day�1) vs. moisture content

(%) at a constant soil to green waste ratio equal to 0.8:1.

and activity (Antizar-Ladislao et al., 2005; Liang et al., 2003).Temperature thus is the factor that more strongly affects theremoval of PAH under in-vessel composting conditions. In ad-dition, higher temperatures may facilitate PAHs volatilisationas observed in the control composting reactors in this investi-gation and others (Lazzari et al., 1999; McFarland and Qiu,

1995) and result in a combination of various removal mecha-nisms, thus higher temperatures should be avoided to mini-mise the transfer of PAHs from the solid phase to theatmospheric phase. Moisture content is the second most im-portant factor, and thus water amendments may be normallynecessary during composing processes. It has been previously

467B. Antizar-Ladislao et al. / Environmental Pollution 141 (2006) 459e468

reported that the amendment of PAHs contaminated soils mayenhance PAH removal (Haderlein et al., 2001). Soil to organicamendment ratios has proved to be necessary for in-vesselcomposting optimisation. For an elevated operation yield(i.e. higher amount of solid waste treated), a higher S:GWratio will be advantageous, although an S:GW ratio too highmay show inhibitory effects on the residential microbialpopulation.

3.6. Kinetics of degradation of PAHs during composting

Most of the PAH losses occurred within the first 45 days oftreatment with a plateau forming by 98 days. These temporalprofiles suggested typical biphasic kinetics indicating relativelyrapid removal of PAHs during the initial phase of treatmentfollowed by slower removal during the later phase (Figs. 3e5).The rate of removal of PAHs were estimated by linear regressionof the temporal profiles of PAH concentration over 98 days ofcontinuous treatment. The kinetics of removal of

PPAHs at

the different conditions investigated in this study are graphicallysummarized in Fig. 6. First order constants of

PPAH reached

a maximum of about 0.014 day�1 during optimal compostingconditions at S:GW 0.8:1, MC 60% and T 38 �C. The removalrates reported in this investigation are in agreement with thosereported by McFarland and Qiu (1995) and Potter et al.(1999) during composting of PAH-contaminated wastes. Thereduction in degradation over time in the kinetic study can be ex-plained by reduced bioavailability of PAHs due to immobilisa-tion in micropores or changes in binding forms (Beck et al.,1996; Ehlers and Luthy, 2003; McFarland et al., 1992).

4. Conclusion

This investigation indicated that controlled in-vessel com-posting of PAH-contaminated wastes can reduce PAHs con-centration in the soil. Removal of small, medium and largePAHs was achieved by optimising the operational parametersof in-vessel composting at laboratory-scale. TOC and

PPAH

concentration decrease during composting was in agreementand indicated that T, S:GW ratio and MC have an importantinfluence on the behaviour of PAHs during composting. Thisinvestigation indicated that for small, medium and largePAHs maximum achievable degradation during compostingoptimal temperature is in the surroundings to 38 �C, optimalsoil to green waste ratio is in the range of 0.7:1e0.8:1 at38 �C, and optimal moisture content is in the range of 60e80% at 38 �C. A factor analysis of the 18 different operationalconditions under investigation indicated that the dominantmechanisms processes affecting the removal of PAHs duringin-vessel composting was of biotic nature at optimal opera-tional conditions MC 60%, S:GW 0.8:1 and T 38 �C. Thekinetics of degradation of PAHs followed first order kineticsreaching 0.015 day�1 at optimal operational conditions.Thus, operational conditions close to those indicated here willbe maintained for maximal PAH degradation. Future challengesfor research on composting of PAH-contaminated wastesinvolve increasing the complete removal of plateaus mainly

promoting further the removal of large PAHs, and up-scalingthis investigation to a field-scale for the bioremediation ofcoal tar contaminated soils and green waste, and thus extrapola-tion of the optimal operational parameters obtained in the pres-ent investigation. If complete removal of PAHs is not achieved,post-composting polishing steps such as land farming will needto be considered.

Acknowledgments

We are grateful to Cleanaway Ltd. and London Remade forproviding support for this study through the Entrust scheme.We also thank Dr. Jeremy Birnstingl for providing the coaltar contaminated soil and Miss. Katarina Spanova for herassistance in the chemical analysis.

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