Laboratory studies of the remediation of polycyclic aromatic hydrocarbon contaminated soil by in-vessel composting

Waste Management 25 (2005) 281–289 www.elsevier.com/locate/wasman

Laboratory studies of the remediation of polycyclic aromatic hydrocarbon contaminated soil by in-vessel composting Blanca Antizar-Ladislao *, Joseph Lopez-Real, Angus J. Beck Department of Agricultural Sciences, Imperial College London, High Street, Wye campus, Wye, Ashford, Kent TN25 5AH, UK Accepted 11 January 2005

Abstract The biodegradation of 16 polycyclic aromatic hydrocarbons (PAHs), listed as priority pollutants by the USEPA, present in a coal-tar-contaminated soil from a former manufactured gas plant site was investigated using laboratory-scale in-vessel composting reactors to determine the suitability of this approach as a bioremediation technology. Preliminary investigations were conducted over 16 weeks to determine the optimum soil composting temperature (38, 55 and 70
C). Three tests were performed; firstly, soil was composted with green-waste, with a moisture content of 60%. Secondly, microbial activity was HgCl2-inhibited in the soil greenwaste mixture with a moisture content of 60%, to evaluate abiotic losses, while in the third experiment only soil was incubated at the three different temperatures. PAHs and microbial populations were monitored. PAHs were lost from all treatments with 38
C being the optimum temperature for both PAH removal and microbial activity. Calculated activation energy values (Ea) for total PAHs suggested that the main loss mechanism in the soil-green waste reactors was biological, whereas in the soil reactors it was chemical. Total PAH losses in the soil-green waste composting mixtures were by pseudo-first order kinetics at 38
C (k = 0.013 day
1, R2 = 0.95), 55
C (k = 0.010 day
1, R2 = 0.76) and at 70
C (k = 0.009 day
1, R2 = 0.73).  2005 Elsevier Ltd. All rights reserved.

1. Introduction There are three main reasons for the growth of the composting industry in the UK: legislation for biodegradable municipal solid waste, environmental benefits and economic benefits. Green-waste comprised the majority (92% in 1998) of municipal wastes produced in the United Kingdom. The three main regulatory drivers for composting are the EU landfill directive (EC, 1999), the UK Waste Strategy 2000 (DETR, 2000) and the EU Animal Byproducts Regulations (EC, 2003). These have increased interest in composting of garden, tree, and food-processing organic wastes. Composting of yard wastes, municipal wastewater sludges, and mu-

*

Corresponding author. Tel.: +44 20 759 42779; fax. +44 20 759 42640. E-mail address: [email protected] (B. Antizar-Ladislao). 0956-053X/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2005.01.009

nicipal solid wastes are long established; however, composting of soils contaminated with hazardous materials is still an emerging ex situ biotreatment technology. Composting conditions differ from other ex situ soil treatment systems in that bulking agents are added to the compost mixture to increase porosity and serve as sources of easily assimilated carbon for biomass growth (Haug, 1993). Aerobic metabolism generates heat, resulting in significant temperature increases that bring about changes in the microbial population and physiology in the compost mixture. The conventional aerobic compost process passes through four major microbiological phases identified by temperature: mesophilic (30–45
C), thermophilic (45–75
C), cooling, and maturation. The greatest microbial diversity has been observed in the mesophilic stage. The thermophilic stage is characterised by spore-forming bacteria and thermophilic fungi. Microbial recolonisation during the cooling phase is characterised by the appearance of mesophilic

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fungi whose spores withstand the high temperatures of the thermophilic stage. In the final compost stage (maturation), most digestible organic matter has been consumed by the microbial population, and the composted material is considered stable (Epstein, 1997; Sela et al., 1998). Composting has been demonstrated to be effective in biodegrading PAHs (McFarland and Qiu, 1995; Potter et al., 1999; Canet et al., 2001), chlorophenols (Laine and Jørgensen, 1997), polychlorinated biphenyls (PCBs) (Block, 1998), explosives (Gray, 1999) and petroleum hydrocarbons, especially diesel fuel (Namkoong et al., 2002) at both the laboratory and field scales. It is widely accepted that temperature is an important environmental variable in composting efficiency (Joshua et al., 1998; Namkoong et al., 2002). Temperature affects not only the physiological reaction rates and population dynamics of microbes, but also most of the physicochemical characteristics of the environment. Temperature increase within composting materials is a function of initial temperature, metabolic heat evolution and heat conservation. Temperatures of composting material below 20
C have been demonstrated to significantly slow or stop the composting process (Paul and Clark, 1996). Temperatures in excess of 60
C have also been shown to reduce the activity of the microbial community, and microbial activity declines when the thermophilic optimum of microorganisms is exceeded. If the temperatures reach 82
C, the microbial community is severely inhibited (Paul and Clark, 1996). MacGregor et al. (1981) found that optimum composting temperatures, based on maximising the decomposition of raw sewage sludge mixed with woodchips were in the range of 52–60
C. However, some researchers have found that such high temperatures are not required to produce a high quality product (Miller et al., 1990). Other studies have indicated that lower temperatures might allow more microbial activity (Liang et al., 2003). The objectives of this study were to: (i) determine the potential for losses of the 16 USEPA-listed PAHs from a coal-tar-contaminated soil during composting, (ii) elucidate the impact of temperature on the (bio)degradation of these 16 PAHs, (iii) study the rates of (bio)degradation of 16 PAHs at different temperatures, and (iv) monitor the changing microbial populations in relation to temperature.

2. Materials and methods Nine experimental conditions were tested in triplicates using 189 laboratory-scale composting reactors. The standard composting reactors comprised a soil to green-waste ratio of 0.6:1 on a dry weight basis. The HgCl2-inhibited composting reactors comprised a soil to green-waste ratio of 0.6:1 on a dry weight basis with

2% HgCl2 used as a microbiological inhibitor. The control reactors consisted of 100% soil. Batches of 63 reactors were placed in three different incubators at a constant temperature equal to 38, 55 and 70
C, respectively. 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 extensive description of the site and the procedures for soil sampling and preparation is provided by Birnstingl (1997). The soil samples were selected and composited from several areas on site. Stones and oily materials were removed, the soil was then air-dried and homogenised by passing through a 5-mm sieve followed by a 2-mm sieve and stored in the laboratory at room temperature. Before experimentation the soil was diluted by homogenizing with silver sand (sharp fine sand of silvery appearance) (1:1) to provide a more homogeneous distribution of the coal-tar residue. Soil organic content was 4.79 ± 0.16% (wt/dry wt); soil pHw was 7.3 ± 0.1. The soil was conditioned with green-waste at a ratio of 0.6:1 on a dry weight basis. The 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 (18% dw), grass (27% dw) and wheat straw (14% dw) (Table 1). 2.2. Reactors design One hundred and eighty nine 200 ml glass composting reactors were made to provide closely monitored and controlled conditions (Fig. 1). These fully enclosed bench-scale reactors each held about 65 g total compost mixture. The reactor units stood vertically with air, saturated with water vapour, flowing continuously up through the compost mixture. Constant air-flow to the composting reactors was provided by 100% oil-free diaphragm pumps (Model PXW-600-DIOV, VP1, Fisher Scientific) and vented outdoors. In order to maintain Table 1 Physicochemical properties of the green-waste used Green waste

Moisture content (%)

Incinerable matter (%)

Foodstuff, which contains: carrot (16.7%), cucumber (16.7%), lettuce (16.7%), onion (16.7%), potato (16.7%), tomato (16.7%). Sawdust Leaves Grass Wheat straw

90.6 ± 0.2

99.4 ± 0.0

10.4 ± 0.10 46.5 ± 4.9 64.3 ± 19.7 9.9 ± 0.5

99.7 ± 0.0 97.3 ± 0.0 97.0 ± 0.1 94.5 ± 0.4

Foodstuff, leaves and wheat straw were blended, grass was cut.

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Fig. 1. Design of laboratory-scale composting reactors.

similar air-flow in the 189 reactors, they were separated in batches of 63 reactors per incubator (and temperature), 21 standard-composting reactors, 21 HgCl2-inhibited composting reactors, and 21 soil reactors. Air was pumped to an air/water reservoir kept at the same temperature as the reactors (i.e., 38, 55 or 70
C) where it was saturated with water. The air/water reservoir had 42 exits, which were connected to each reactor (standard and HgCl2 reactors). Soil reactors were not aerated, but open to the aerobic atmosphere. Compost moisture content was measured weekly to ensure it was maintained at 60%. The air inlet was bubbled through a water reservoir to avoid excessive water evaporation during aeration. The cylindrical reactor design permitted a better distribution of the air flow inside the reactors, preventing the creation of anaerobic pockets in the compost mixture. Streams of inlet and exhaust gas were occasionally monitored for carbon dioxide production as evidence of aerobic biodegradation. 2.3. Sample analysis Destructive sampling (in triplicate) for each treatment occurred at time 0 and after 7, 21, 35, 54, 66, 102, 111 d for PAH analyses, and after 21, 54 and 102 d for biomass analyses. Ash content was determined using a loss-onignition procedure. Triplicate 5 g samples were dried for 24 h at 110
C (moisture content) and then transferred to a muffle furnace at 550
C for 12 h to burn the organic matter. Moisture content was expressed on a wet basis, defined as the mass of the water in a sample divided by the total wet mass of the sample (Agnew and Leonard, 2003). Ash content was calculated from the ratio of preand post-ignition sample weights. 2.4. PAH Analysis PAH extraction from compost mixtures and soil was by Accelerated Solvent Extraction (ASEe) 200,

283

with 22 mL stainless steel extraction cells meeting the requirements for the extraction of PAHs from solid waste as described in the USEPA Method 3545. 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 Hydromatrixe and introduced into each extraction cell. Surrogate standards (1-fluoronaphthalene, 2-fluorobiphenyl, purity >97%, Greyhound Chromatography & Allied 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. ASEe 200 conditions for PAH extraction were: 14 MPa (2000 psi), 100
C, oven heat-up time = 5 min, static time = 5 min, solvent dichloromethane/acetone (1:1), (v/v), flush volume = 60% of extraction cell volume, nitrogen purge = 1 MPa (150 psi) for 60 s. The extracts were purified on chromatographic columns packed with 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 Na2SO4. In order to remove hydrophobic impurities, the columns were washed with 10 ml dichloromethane, then 5 ml of extracts (or more according to the removal rates) were eluted, and left 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, 1,4dichlorobenzene-d4, perylene-d12, phenanthrene-d10, purity >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 selective detector was used for the analysis. Data acquisition and processing was achieved using a Hewlett– Packard MS Chemstation (G1034C Version C.02.00). 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 ll and the inlet purged at 50 ml min
1 1 min after injection; inlet pressure was varied by electronic pneumatics control (EPC) to maintain a constant column flow of 1 ml min
1. Separation was achieved using an HP-5MS column (19091S-433 30 m · 0.25 mm · 0.25 lm). The temperature program comprised 70
C for 2 min, 10
C min
1 to 300
C, which was maintained for 10 min to allow late eluting peaks to exit the column. The MS transfer line was 280
C providing conductive heating of the MS source to about 230
C. The instrument was tuned using perfluorotributylamine. The MS was operated in selective ion monitoring (SIM) mode. The GC–MS system was calibrated prior to the analysis of samples using seven calibration standards. The calibration was frequently checked during the analysis of samples by the repeated analysis of quality control standards. The 16 USEPA

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Table 2 Quantification and confirmation ions of 16 USEPA PAHs, internal standards and surrogates Compound

Quantification ion

Confirmation ions

Naphthalene Naphthalene-d10 1-Fluoronaphthalene 2-Fluorobiphenyl Acenaphthylene Acenaphthene Acenaphthene-d10 Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Benzo[a]anthracene Chrysene Chrysene-d12 Benzo[b]fluoranthene Benzo[k]fluoranthene Benzo[a]pyrene Indeno[1,2,3-c,d]pyrene Dibenzo[a,h]anthracene Benzo[g,h,i]perylene

128 136 146 172 152 154 164 166 178 178 202 202 228 228 240 252 252 252 276 278 276

127, 137, 120, 171, 151, 153, 162, 139, 165, 179, 200, 200, 226, 226, 236, 250, 253, 207, 277, 279, 138,

129, 134, 125 170 153, 152 160, 165 163, 176, 101, 201, 229 230, 241 253, 250, 253, 279, 139, 137,

102 108

76 163 82, 176 89 203 101, 203

ene, pyrene, benzo[a]anthracene, chrysene) and fiveand 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 as small, medium and large PAHs, respectively, for ease of discussion. The initial total PAH concentration in the investigated soil after dilution with silver sand (100 mg PAH kg
1 air dried soil) was lower than those concentrations (about 450 mg PAH kg
1 soil/sediment) reported in a manufactured gas plant site by Erickson et al. (1993), however, they are above the Dutch List action level of 40 mg PAH kg
1 air dried soil and thus they should be treated. 3.1. Removal of PAH

113 126 126 250, 126 138 276 277

The concentrations of the 16 USEPA-listed priority pollutant PAHs investigated in the standard reactors before treatment and after 111, 107 and 105 d at 38, 55 and 70
C, respectively, (as mg PAH kg
1 dry soil) are presented (Table 3, Fig. 2(a)). Losses of total PAH were observed during all temperature treatments, although PAH

PAHs, internal standards and surrogates for SIM GC– MS mode are summarised in Table 2.

Table 3 PAH concentrations (mg PAH kg
1 dry soil) in reactors at the beginning and end of treatment (% removal in parenthesis)

2.5. Biomass

Compound

Analysis of bacteria, fungi and actinomycetes were by the dilution and spread-plate method following the ‘‘Standard Methods for the Examination of Water and Wastewater’’ (APHA-AWWA-WPCF, 1998) with minor modifications. Briefly, 10 g of the soil green-waste mixture sample were mixed with 90 ml of RingersÕ solution and shaken for 10 min. Consecutive 1:10 dilutions were prepared, starting with 1 ml of sample to produce eight dilutions of each sample. Then 0.1 ml of each dilution were spread onto five plates of nutrient agar (with cycloheximide) for bacteria, five plates of starch casein (with cycloheximide) for actinomycetes and five plates of potato dextrose agar (with rose bengal) for fungi. Cycloheximide was used to inhibit the growth of fungi from the soil, and rose bengal was used to suppress the growth of bacteria. Samples from the soil greenwaste mixtures treated at 38, 55 and 70
C were incubated at 38, 55 and 70
C, respectively, for 72 h. Following incubation, plates were counted.

Initial

Temperature 38
C

55
C

70
C

111 d

107 d

105 d

6.1 (71.4%c)

8.4 (72.8%a) 13.2 (71.7%b) 6.2 (70.9%c)

5.9 (81.9%a) 18.4 (60.3%b) 11.7 (45.1%c)

19.2 80.9%

28.2 71.9%

36.1 64.1%

1.5 (95.5%a) 5.9 (87.3%b)

6.5 (69.4%c)

5.5 (82.9%a) 11.8 (74.7%b) 5.4 (74.6%c)

36.4 63.4%

22.7 77.3%

10.0 90.0%

10.7 (67.1%a) 30.6 (34.0%b) 16.9 (21.0%c)

3.7 (88.7%a) 19.1 (58.8%b) 11.0 (48.6%c)

58.2 42.0%

33.8 66.3%

Standard composting reactors 2 + 3 rings 32.5 2.7 (91.8%a) 4 rings 46.4 10.4 (77.6%b) 5 + 6 rings Total PAHs Percent removal

21.4 100.3

HgCl2-composting reactors 2 + 3 rings 32.5 5.7 (82.4%a) 4 rings 46.4 24.5 (47.3%b) 5 + 6 rings Total PAHs Percent removal

21.4 100.3

Soil reactors 2 + 3 rings

32.5

20.0 (38.6%a)

4 rings

46.4

41.2 (11.3%b)

5 + 6 rings

21.4

18.3 (14.3%c)

3. Results and discussion The 16 USEPA-PAHs (total PAHs) under investigation were grouped as two- and three-ring PAHs (naphthalene, acenaphthylene, acenaphthene, fluorene, anthracene, phenanthrene), four-ring PAHs (fluoranth-

Total PAHs Percent removal a b c

100.3

79.5 20.8%

2 + 3 rings percent removal. 4 rings percent removal. 5 + 6 rings percent removal.

2.7 (87.6%c)

B. Antizar-Ladislao et al. / Waste Management 25 (2005) 281–289

285

(a) Standard-composting reactors 120

80 60 40 20 0

120

550C

100

Concentration, mg/Kg

380C

100

Concentration, mg/Kg

Concentration, mg/Kg

120

80 60 40 20 0

0

20

40 60 80 Time, days

100

120

700C

100 80 60 40 20 0

0

20

40 60 80 Time, days

100

120

0

20

40 60 80 Time, days

100

120

(b) HgCl2-composting reactors 120

120

38 C

100 80 60 40 20 0

0

Concentration, mg/Kg

0

Concentration, mg/Kg

Concentration, mg/Kg

120

55 C

100 80 60 40 20

20

40 60 80 Time, days

100

120

80 60 40 20 0

0 0

700C

100

0

20

40 60 80 Time, days

100

0

120

20

40 60 80 Time, days

100

120

(c) Soil reactors 120

80 60 40 20 0

120

550C

100

Concentration, mg/Kg

380C

100

Concentration, mg/Kg

Concentration, mg/Kg

120

80 60 40 20 0

0

20

40 60 80 Time, days

100

120

700C

100 80 60 40 20 0

0

20

40 60 80 Time, days

100

120

0

20

40 60 80 Time, days

100

120

Fig. 2. Evaluation of temporal concentrations of small (e), medium (h), large (n) and total PAHs at 38, 55 and 70
C in (a) standard-composting reactors, (b) HgCl2-composting reactors, and (c) soil reactors. Plots show average values for triplicate reactors.

losses decreased with increasing hydrophobicity of the PAHs. Large PAHs have higher octanol–water partition coefficients and lower water solubilities than medium and small PAHs (Antizar-Ladislao et al., 2004), thus bioavailability (Carriere and Mesania, 1995; Potter et al., 1999; Lee et al., 2001) and toxicity (Sverdrup et al., 2002) may have limited their (bio)degradation, resulting in their persistence. The majority of small PAHs were removed by the end of the composting treatment resulting in a concentration removal of 91.8% at 38
C, 72.8% at 55
C and 81.9% at 70
C. Medium and large PAHs were also removed to a great extent at 38
C, as compared to their removal at 55 and 70
C (Table 3). Increasing the temperature from 38 to 70
C resulted in a significant decrease in total PAHs removal (P < 0.01), from 80.9% to 64.1%, respectively.

Comparing the final removal of PAHs in the three different types of composting reactors at 38
C (Table 3), the highest removal percentage of total PAHs was observed in the standard composting reactors (80.9%). The concentration of total PAH in the HgCl2-inhibited composting reactors remained constant during the first 21 d of treatment at 38
C (Fig. 2(b)), and then fell, culminating in 63.4% removal of total PAH after 111 ds of continuous composting treatment. In the soil reactors, a 20.8% removal of total PAH occurred over 111 d, mainly due to the removal of small PAHs. At 55
C (Fig. 2), the temporal concentration of total PAH started to decline in the standard and HgCl2-inhibited composting reactors after 21 d of composting treatment resulting in similar final removals of total PAH (74.6% average) in both reactor types after 107 d of continuous

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composting treatment. In the soil reactors, a 42% final removal of total PAH occurred, mainly due to the removal of small and medium PAHs (Table 3). At 70
C (Fig. 2), the temporal concentration of total PAH varied in the standard and HgCl2-inhibited composting reactors during the length of the experiment resulting in a final higher removal of total PAH in the HgCl2-composting reactors (90.0%) than in the standard composting reactors (64.1%). In the soil reactors a removal of 66.3% occurred (Table 3). Increases in PAH concentration (mg PAH kg
1 dry soil) during composting were occasionally observed in the reactors. The experimental variation of moisture content or flow rate during the composting treatment would potentially affect the biodegradation extent and rate of PAHs in the composting mixtures, although they would not explain an increase in PAH concentration. Thus, an occasional increase in PAH concentration might be a consequence of a selective biodegradation of organic matter within the soil to green waste mixture, where components of the green waste would have degraded faster than components in the soil, changing the ratio of soil to green waste in the mixture and therefore in the calculation of the concentration of PAHs in the mixture. Removal of PAHs observed in the HgCl2-inhibited composting reactors may indicate that the biocidal effects of 2% HgCl2 were reduced over time, thus some foreign microorganisms may have been able to colonise the medium again. Difficulties found with the use of a chemical inhibitor in this and previous studies (Canet et al., 2001), suggest that a better option might be the use of non-amended soil as an abiotic control. Thus, removal of PAHs from the original aged-soil without green waste, water or air supply amendment at different temperatures would better represent the abiotic losses in this type of experiments. In the soil reactors 20.8%, 42.0% and 66.3% removal of total PAH was achieved at 38, 55 and 70
C, respectively, which clearly showed a direct temperature influence on the removal of total PAHs. In order to predict the relative contributions of chemical and biological processes to the removal of PAHs, activation energy values (Ea) were calculated from data obtained in the reactors using the Arrhenius equation,

30 kJ mol
1are likely to represent biological mechanisms, whereas values greater than 60 kJ mol
1have been reported for chemical reactions in soil (Taylor-Lovell et al., 2002). To explain this, it is assumed that catalysed reactions such as enzyme-mediated biological processes have a lower activation energy requirement, causing them to be less responsive to temperature compared to chemical reactions. The activation energy in this study indicates that biological mechanisms govern the removal of PAHs from composting mixtures in the standard-composting reactors (Ea =
6.43 kJ mol
1, R2 = 0.99) and that chemical reactions lead the mechanisms of removal in the soil reactors (Ea = 32.25 kJ mol
1, R2 = 0.99). Additionally, at the highest temperature investigated, most of the microorganisms would be rendered inactive (Antizar-Ladislao et al., 2004), and thus, the removal of PAHs would occur mainly due to volatilisation. This would indicate that the leading mechanism of removal at 38
C was biological, whereas at 70
C it was volatilisation (Table 3), and most likely a combination of these two mechanisms at 55
C. Other authors have also reported removal of PAHs from contaminated wastes due to a combination of abiotic and biotic mechanisms (Civilini, 1994; McFarland and Qiu, 1995). Nevertheless, abiotic losses are more important for the small, more volatile PAHs than for larger PAHs. McFarland and Qiu (McFarland and Qiu, 1995) reported no loss of benzo(a)pyrene (large PAH) through volatilisation or mineralization during composting of soil with corn cobs at 39
C, which is consistent with our findings at 38
C. Thus, temperature plays an important role in the removal of PAHs during composting. In this study it appears that a temperature of 38
C enhances the biological removal of PAHs, which might occur due to a promotion of the native microbial population and activity. In addition, higher temperatures may facilitate desorption (Lee et al., 1998) and volatilisation (Lazzari et al., 1999) of PAHs. Desorption of PAHs at higher temperatures from the soil-composting matrix may have increased their availability to the present thermophiles but also may enhance inhibition of biological activity as reported elsewhere (Carriere and Mesania, 1995).

lnðrÞ ¼ lnðAÞ
ðEa =RT Þ;

Most of the PAH losses occurred within the first 21 d of treatment, slowing thereafter with little change being observed by the end of the composting treatments. The pseudo-first-order kinetic approximation was applied using the linear integrated form of

where r is the removal of PAHs (%), A is an empirical constant, T is temperature (K), R is the universal gas constant (8.3145 J K
1 mol
1) and Ea is expressed in kJ mol
1. The percent removal (%) calculated at each temperature in standard composting reactors and soil reactors (Table 3) was used to determine Ea. On the basis of regression of the percent removal with temperature, an Ea was calculated for the removal of total PAHs in all reactors. Previous studies have suggested that Ea values less than

3.2. Kinetics of removal

lnðC=C 0 Þ ¼
k  t; where C is the concentration at time t, C0 is the concentration at t = t0, k is the first-order constant of removal (obtained by linear regression) and t is time. First-order kinetic analyses were performed for the standard-compo-

B. Antizar-Ladislao et al. / Waste Management 25 (2005) 281–289

of treatment at 38
C (k1 = 0.030 day
1, R2 = 0.94), 55
C (k1 = 0.023 day
1, R2 = 0.79) and 70
C (k1 = 0.022 day
1, R2 = 0.76) and after the first three weeks of treatment at 38
C (k2 = 0.013 day
1, R2 = 0.98), 55
C (k2 = 0.004 day
1, R2 = 0.35) and 70
C (k1 = 0.004 day
1, R2 = 0.34) where found using the two-phase model. The model of Admon et al. (2001) did not improve the fitting of pseudo-first order kinetics to our experimental results when considering only the first phase, while the fitting of the second phase at 55 and 70
C was very poor. However, the use of their suggested two-phase model indicated that approximately 2.4 times higher removal rates might be found during the first three weeks of treatment as compared to the use of the one-phase model. The reduction in biodegradation over time in the kinetic study can be

0

0.0

-0.5

-0.5

ln(C/C0)

ln(C/C0)

sting mixtures (Fig. 3). A good fit was observed at 38
C (k = 0.013 day
1, R2 = 0.95), 55
C (k = 0.010 day
1, R2 = 0.76) and at 70
C (k = 0.009 day
1, R2 = 0.73) for the removal of total PAHs. Removal rates of small, medium and large PAHs were also investigated (Table 4), which indicated that a higher removal rate at 38
C was mainly due to the approximately two times faster removal rate of small PAHs at 38
C (k = 0.028 day
1, R2 = 0.83) than at 55
C (k = 0.011 day
1, R2 = 0.75) or 70
C (k = 0.012 day
1, R2 = 0.57). Other investigators have reported better fitting of pseudo-first order kinetics using two separate regression analysis in the two apparent phases (Admon et al., 2001). This two-phase model approach was additionally tested in the standard-composting mixtures (Fig. 3). Differences in the removal rate during the first three weeks

-1 -1.5

-1.0 -1.5

-1

k = 0.013 day , R = 0.95

0

k 2 = 0.013 day , R = 0.98

20 40 60 80 100 120 time, days

0

0.0

0.0

-0.5

-0.5 ln(C/C0)

ln(C/C0)

2

2

-2.0

(a)

-1.0 -1.5

40 60 80 100 120 time, days

-1

2

k 1 = 0.023 day , R = 0.79 -1 2 k 2 = 0.004 day , R = 0.35

2

k = 0.010 day , R = 0.76

-2.0

20

-1.0 -1.5

-1

-2.0 0

20

(b)

40 60 80 100 120 time, days

0

0.0

0.0

-0.5

-0.5 ln(C/C0)

ln(C/C0)

-1

-1

k 1 = 0.030 day , R = 0.94

2

-2

-1.0 -1.5

40 60 80 100 120 time, days

-1

2

k 1 = -0.022 day , R = 0.76 -1 2 k 2 = -0.004 day , R = 0.34

2

k = -0.009 day , R = 0.73

-2.0

20

-1.0 -1.5

-1

-2.0 0

(c)

287

20

40 60 80 100 120 Time, days

0

20

40 60 80 100 120 Time, days

Fig. 3. Kinetics of the removal of total PAHs in the standard-composting reactors at (a) 38
C, (b) 55
C and (c) 70
C. k represents one-phase rate constant (+), and k1 and k2 represent the first () and second () phase rate constants, respectively.

288

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Table 4 Degradation rate constants in the standard-composting reactors at 38, 55 and 70
C

Table 5 Colony forming units in the standard-composting reactors at 38, 55 and 70
C

Compound

Microorganisms

21 d

54 d

111 d

38
C Bacteria Actinomycete Fungi

2.9 · 108 3.9 · 108 5.1 · 107

3.2 · 108 n.d. 1.4 · 107

n.d. n.d. 3.1 · 107

21 d

51 d

107 d

6.7 · 106 1.9 · 106 9.1 · 102

3.9 · 104 n.d. n.d.

n.d. n.d. n.d.

21 d

54 d

105 d

n.d. n.d. n.d.

n.d. n.d. n.d.

n.d. n.d. n.d.

Temperature 38
C

55
C

70
C

2 + 3 rings 4 rings 5 + 6 rings

0.028 (0.83) 0.010 (0.78) 0.011 (0.86)

0.011 (0.75) 0.009 (0.52) 0.012 (0.80)

0.012 (0.57) 0.004 (0.15) 0.008 (0.68)

Total PAHs

0.013 (0.95)

0.010 (0.76)

0.008 (0.53)

2

k represents one-phase rate constant, and R is the correlation coefficient obtained for the regression analyses (R2 in parenthesis).

explained by reduced bioavailability of PAHs due to immobilisation in micropores or changes in binding forms (McFarland et al., 1992). Although data has been analysed using the one-phase and two-phase models, the variable nature of compost complicated the fitting of the second phase of the twophase model. Thus, the use of the one-phase model is more appropriate in the present study, and recommended to be used to fit short-term experimental data. First-order kinetics proves convenient since the rate of degradation is proportional to the amount of substrate available, allowing a half-life time, to describe the degradation pattern over the entire duration of decay of a given substance. For this reason, regulatory agencies often favor this approach even when more complex mechanistic models fit the data more closely (Wolt et al., 2001). 3.3. Biomass During composting, the amount of biomass was higher in the reactors incubated at 38
C than at 55
C, and at 70
C no biomass was detected using the dilution and spread plate method (Table 5). Additionally, the greatest amount of biomass appeared within the first three weeks of composting treatment at 38
C. Higher biomass population at 38
C supports our assertion that PAH biodegradation was greater at 38
C than at 55 or 70
C. No biomass was apparently present at 70
C using the dilution and spread plate method, indicating that the removal of PAHs at this temperature was mainly due to abiotic mechanisms. However, only a small fraction (possibly <0.1%) of the soil microbial community is amenable to investigation using traditional culturing techniques using a variety of culture media designed to maximize the recovery of diverse microbial populations (van der Merwe et al., 2002). To overcome these problems, other methods such as phospholipid fatty acids (PLFA) analysis may prove more appropriate to study a greater proportion of the soil microbial community, and they are currently being applied in the investigation of the rapidly changing microbial community in active composting mixtures (Baath and Anderson, 2003;Ranneklev and Baath, 2003).

55
C Bacteria Actinomycete Fungi

70
C Bacteria Actinomycete Fungi

n.d., not detected. Data show average values for triplicate reactors.

4. Conclusions This study used laboratory-scale in-vessel composting reactors to investigate the (bio)degradation of 16 USEPA-listed PAHs from coal-tar-contaminated soil. Our findings indicated that in-vessel composting can reduce PAH concentration in a contaminated soil, and thus it might have useful potential as a bioremediation technology. Optimal removal occurred at 38
C where the highest microbial activity was also observed. The main mechanism of removal of PAHs in the standard composting reactors at 38
C was biological, although abiotic mechanisms also played a role. Additionally, the use of the one-phase model is recommended to describe the degradation pattern of PAHs in short-term studies. The highest removal rate of total PAHs during in-vessel composting was observed at 38
C (k = 0.013 day
1, R2 = 0.95). Future challenges for research on in-vessel composting of PAH contaminated soils involves understanding how other parameters such as moisture content or soil to green-waste ratio may also influence the optimal environmental conditions for maximum removal. These questions will be addressed in future experiments.

Acknowledgements We are grateful to Cleanaway Ltd and London Remade for providing support for this study through the Entrust scheme. We also thank Miss Jennifer Gosling for the biomass analysis, and Dr. Jeremy Birnstingl for providing the coal-tar-contaminated soil.

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