Chemosphere 90 (2013) 2240–2246
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Influence of mature compost amendment on total and bioavailable polycyclic aromatic hydrocarbons in contaminated soils Guozhong Wu a,b, Cedric Kechavarzi b, Xingang Li a,c, Hong Sui a,c, Simon J.T. Pollard b, Frédéric Coulon a,b,⇑ a
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Department of Environmental Science and Technology, Cranfield University, Cranfield MK43 0AL, UK c National Engineering Research Centre for Distillation Technology, Tianjin 300072, China b
h i g h l i g h t s " Compost addition resulted in >90% loss of
P
16PAHs
irrespective of soil types.
" Type and rate of compost were less significant than soil type and contact time. " Degradation contributed more than desorption to the PAH loss in soils. " PAH bioavailability correlated with log Kow and the number of benzene rings.
a r t i c l e
i n f o
Article history: Received 18 July 2012 Received in revised form 9 October 2012 Accepted 10 October 2012 Available online 9 November 2012 Keywords: Bioavailability Compost PAHs Degradation Desorption
a b s t r a c t A laboratory microcosm study was carried out to assess the influence of compost amendment on the degradation and bioavailability of PAHs in contaminated soils. Three soils, contaminated with diesel, coal ash and coal tar, respectively, were amended with two composts made from contrasting feedstock (green waste and predominantly meat waste) at two different rates (250 and 750 t ha 1) and incubated for 8 months. During this period the treatments were sampled for PAH analysis after 0, 3, 6 and 8 months. Total and bioavailable fractions were obtained by sequential ultrasonic solvent extraction and hydroxypropyl-b-cyclodextrin extraction, respectively, and PAHs were identified and quantified by GC–MS. Bioavailability decrease due to sorption was only observed at the first 3 months in the diesel spiked soil. After 8 months, compost addition resulted in over 90% loss of total PAHs irrespective of soil types. Desorption and degradation contributed to 30% and 70%, respectively, of the PAH loss in the spiked soil, while PAH loss in the other two soils resulted from 40% enhanced desorption and 60% enhanced degradation. Compost type and application rates had little influence on PAH bioavailability, but higher PAH removal was observed at higher initial concentration during the early stage of incubation. The bioavailable fraction of PAH was inversely correlated to the number of benzene rings and the octanol–water partition coefficient. Further degradation was not likely after 8-month although over 30% of the residual PAHs were bioavailable, which highlighted the application of bioavailability concept during remediation activities. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Composting capacity in the UK has recently seen a rapid annual expansion of about 25% (Defra, 2006) as a result of the implementation of the EU Landfill Directive 99/31/EC (EC, 1999) and Waste Strategy for England (EFRAC, 2010). This requires the UK to divert large quantities of organic wastes from landfill, including most biodegradable organic fractions of municipal solid waste, food and kitchen waste and wood waste (EFRAC, 2010). As a result, the ⇑ Corresponding author at: Department of Environmental Science and Technology, Cranfield University, Cranfield MK43 0AL, UK. Tel.: +44 1234 754 981; fax: +44 1234 754 036. E-mail address: f.coulon@cranfield.ac.uk (F. Coulon). 0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2012.10.003
possibility of using compost as an amendment to return degraded or contaminated land to productive use has received increasing attention (Puglisi et al., 2007; Lord et al., 2007; Tejada et al., 2008; Gandolfi et al., 2010). Addition of compost is beneficial to soil in terms of physical properties, nutrient availability and microbial activity and it can help sustain plant growth (Wanas and Omran, 2006; Tejada et al., 2008; Gandolfi et al., 2010). There have been several trials in the UK aimed at establishing biomass crops on neglected land using compost (Paulson et al., 2003; Lord et al., 2007). High compost application rates (between 250 and 750 t ha 1) were used to overcome the nutrient deficiency, the phytotoxicity of the contaminants and the poor soil physical conditions. However, the sustainability and viability of such practice will depend on its impact on the long-term management of risk from contaminants and
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notably on the influence of compost on the fate of organic contaminants. The main concern of using such large amounts of compost is that mixing compost with contaminated soil may result in a far greater quantity of contaminated material. This uncertainty and the relative paucity of information on the toxicity, distribution and bioavailability of such contaminants in compost amended soils may therefore result in the derivation of overly stringent soil assessment criteria with concomitant remediation cost implications (Latawiec et al., 2011). Several studies (Semple et al., 2001; Namkoong et al., 2002; Reid et al., 2002; Sayara et al., 2010) have demonstrated the efficiency of bioremediation by means of compost amendment, however our understanding of the effects of compost on the bioavailability of organic contaminants such as PAHs is still limited. Puglisi et al. (2007) found that the addition of green compost to soils spiked with phenanthrene increased phenanthrene sequestration thus reducing its bioavailability; but in the meantime the degradation was enhanced due to the native microorganisms present in the compost. However, it remains unclear to which extent these two processes influence the phenanthrene bioavailability. Moreover, there is a need to investigate authentic contaminated soils where the fate and transport of complex mixture of PAHs are controlled by a number of factors such as ageing time, mineral and organic matter content, aqueous solubility, polarity, hydrophobicity, lipophilicity and molecular structure of contaminants (Reid et al., 2000). The term ‘bioavailability’ refers to the fraction of a chemical in a soil that can be taken up, or transformed, by living organisms or the degree to which a compound is free to move into or onto an organism (Stokes et al., 2006). The bioavailability of contaminants has significant implications for the risk assessment and remediation of contaminated media (Latawiec et al., 2011). If it can be demonstrated that greater levels of contamination can be left in soil without additional risk, lower costs and smaller remediation volumes may be realised and an opportunity for less intrusive remedial approaches exists (Stokes et al., 2006; Latawiec et al., 2011). This implies that a contaminated site where the bioavailable concentrations of the chemicals of concern are below the relevant regulatory concentrations may be deemed to present an acceptable risk even though their total concentrations exceed the target values in soils. However, most of the current models used for assessing human health risk posed by contaminated land defined the end-point of remedial activities based only on the chemical concentration likely to pose significant risk and do not account for bioavailability. This is largely due to the uncertainties and complexity of assessing the bioavailability of complex hydrocarbon mixtures and a lack of substantive bioavailability data (Latawiec et al., 2011). These identified gaps highlight the need for further research investigating the interrelationships between hydrocarbon bioavailability, toxicity and mass transport in contaminated soils. In this study, we characterised the influence of large amount of compost addition on the bioavailability of 16 PAHs from spiked and authentic contaminated soils with PAH concentration ranging from 4 to 211 mg kg 1. The main objectives were to (i) identify the changes in total and bioavailable PAH concentrations after compost addition; (ii) quantify the contribution of degradation and desorption to bioavailability change; and (iii) compare the behaviour of individual PAH in terms of bioavailability. 2. Materials and methods 2.1. Chemicals All reagents were purchased from Sigma–Aldrich (UK) and all solvents were HPLC grade. QTM standard PAH mix in dichloromethane (DCM) was used for quantification of 2-bromonaphthalene (BNap) and the following fifteen US EPA Priority PAHs (USEPA,
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1989): acenaphthene (Ace), acenaphthylene (Acy), naphthalene (Nap), anthracene (Ant), phenanthrene (Phe), fluorine (Flu), fluoranthene (FL), chrysene (Chr), pyrene (Pyr), benzo[a]anthracene (BaA), benzo[b] fluoranthene (BbF), benzo[ghi]perylene (BgP), benzo[a]pyrene (BaP), dibenzo[a, h]anthracene (DbA) an dindeno[1, 2, 3-cd]pyrene (InP). Deuterated PAH standard mixture containing naphthalene-d8, anthracene-d10, chrysene-d12 and perylene-d12 in DCM was used as internal standard and added to extracts at 0.5 lg mL 1. 2.2. Soils and composts Three soils and two types of compost were used. A sandy loam soil (Soil A) was initially spiked with diesel at 12.5 g kg 1 and then stored outside, covered with tarpaulin and left to age for 3 year by mixing periodically to keep it aerated. The two other soils (Soils B and C), historically contaminated with coal tar and coal ash, respectively, were collected from brownfield sites in County Durham, UK. The two composts applied were green waste compost (Compost A) and Compost B composed of approximately half green waste and half catering meat waste (hereafter referred to as green and meat composts, respectively). Both were certified PAS100 which is the British Standards Institution’s publicly available specification for compost material (BSI, 2011). For each soil and compost, the physicochemical properties including pH, moisture, loss on ignition (LOI), total carbon (TC), organic carbon of soil (% Org C), available phosphorus (AP), total phosphorus (TP), total nitrogen (TN) and particle size distribution were determined on triplicate samples using standard methods (ISO, 1994, 1995, 2000, 2001, 2010a,b). 2.3. Incubation process The soils and composts were screened through a 19.5 and 12.5 mm sieve, respectively, to remove large stones and debris. Each soil was amended in duplicate with two rates of compost equivalent to field application rates of 250 and 750 t ha 1 with an incorporation depth of 30 cm and similar to the application rates used in field trials by Lord et al. (2007). Soils without compost were prepared to provide unamended controls. The amended soils and unamended controls were packed into cylindrical cores (1.29 L, Fig. SM-1 in Supplementary Material (SM)). The samples were stored at room temperature on plant pot saucers and covered with parafilm with small holes to reduce evaporation. To ensure the soils were kept at the same moisture content, water was added at 7-d intervals to saturate the first few cm of the soils which drained to the field capacity without leaching. Subsamples (10 g) were collected from the upper part of each cylinder directly at the onset of the experiment (T0), and after 3 (T3), 6 (T6) and 8 (T8) months. The sampled soils were ground using a mortar, screened through a 1 mm sieve and stored at 4 °C before analysis. 2.4. PAH analysis The total PAHs in soils was determined by sequential ultrasonic solvent extraction (Risdon et al., 2008). Briefly, soils (5 g) were chemically dried with 5 g anhydrous Na2SO4 in 50 mL Teflon centrifuge tubes. Acetone (4 mL) was added and sonicated for 2 min at 20 °C. Acetone (6 mL) and hexane (10 mL) were added to the samples and sonicated for 10 min, followed by manually shaking to mix the solvent and soil. This step was repeated twice followed by centrifugation for 5 min at 750 rpm. After passing the supernatant through a filter column fitted with glass receiver tube, a sequential step series, including resuspension of the samples in 10 mL of acetone/hexane (1:1), sonicated for 15 min at 20 °C, centrifugation for 5 min at 750 rpm, and decantation into a filter column, was repeated twice. The final extract volume was adjusted to 40 mL with a mixture of acetone/hexane (1:1) and homogenised
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by manual shaking. The silica gel column clean-up was performed by passing the extracts through a column filled with florisil. Bioavailable PAHs in soil were extracted using hydroxypropylb-cyclodextrin (HPCD) extraction method previously described by Oleszczuk (2008). Aqueous HPCD solution was prepared using deionised water to make up a concentration of 50 mM. Soil samples (2.5 g) were weighed into 50 mL Teflon centrifuge tubes. After adding 50 mL of HPCD solution, the tubes were sealed and shaken on a side-to-side shaker at room temperature at 150 rpm for 20 h. The tubes were centrifuged at 2000 rpm for 30 min and the supernatant was discarded. After manually shaking with deionised water, the residue soil slurry was centrifuged again and the supernatant was then discarded. The residual PAHs were determined by ultrasonic solvent extraction method as described previously. The difference between the total and the residual PAHs after HPCD extraction was assumed to be the bioavailable fraction of PAHs. PAHs were identified and quantified by GC-MS. Details of GCMS operational conditions are available in the SM. 2.5. Statistical analysis The ANOVA with Tukey test was performed to compare the differences in the physicochemical properties between soils and composts. Differences in the PAH concentration between different treatments were compared using ANOVA by LSD test. The difference was recognised as significant where P < 0.05. All tests were performed using SPSS 17.0. Similarity between the 16 target PAHs in terms of their bioavailable and non-bioavailable concentration profiles were identified by the hierarchical cluster analysis using PRIMER 6.0 (Primer-E, UK). 3. Results and discussion 3.1. Soils and composts physicochemical properties General physicochemical properties of the soils and composts are presented in Table 1. The textual analysis showed that all soils were sandy loam, while Soil C had a higher percentage of silt. The pH of the three soils were slightly alkaline (pH value: 7.9–8.2) and within the preferable ranges for bioremediation (Wilson and Jones, 1993). The largest differences in the physicochemical properties were observed between Soil A and the two other soils especially in terms of LOI, TC and % Org C, suggesting that the PAH-coated soil solids phase may be less important than in the two other soils due to its lower carbon content (Coulon et al., 2010). According to the theoretical C:N:P ratio 100:10:1 of a microbial cell (Leys et al., 2005), the N and P concentrations in the three soils were not optimal (Table SM-2). The addition of compost to the soils
Table 1 Physicochemical characteristics of the three contaminated soils and two compost types (average values ± standard error, n = 3. Values followed by the same letter are not statistically different from each other. Tukey test, P < 0.05). Property pH Moisture (%) LOI (%) TC (%) % Org C AP (ppm) TP (ppm) TN (%) % Sand % Silt % Clay
Soil A
Compost A
Compost B
7.9 ± 0.0a 31 ± 1a
Soil B 8.2 ± 0.1a 26 ± 4
Soil C 8.2 ± 0.0a 32 ± 3
8.1 ± 0.0a 56 ± 5b
2.6 ± 0.6b 8 ± 0.1
6 ± 1a 3 ± 1a 3 ± 0.6a 21 ± 0.4a 509 ± 4a 0.2 ± 0.0a 72 ± 5a 9 ± 4a 19 ± 1a
24 ± 2b 17 ± 2b 16 ± 2b 37 ± 4b 422 ± 6b 0.5 ± 0.0b 74 ± 5a 7 ± 5a 19 ± 1a
21 ± 3b 17 ± 1b 16 ± 1b 30 ± 1b 332 ± 8c 0.5 ± 0.0b 56 ± 6b 30 ± 6b 14 ± 1b
31 ± 2c 20 ± 0.1b 18 ± 0.3b 150 ± 14c 1806 ± 17d 2.1 ± 0.1c
54 ± 3d 32 ± 0.4c 31 ± 0.3c 124 ± 25c 2547 ± 13e 3.2 ± 0.0d
reduced the unbalanced C:N ratio. This suggested that compost addition would change the nutrient dynamics by reducing C:N ratio which determined whether mineralisation or immobilisation dominated in the early stages of organic matter decomposition in the soils (Cambardella et al., 2003). 3.2. Initial PAH concentrations in the unamended soils The initial PAH concentrations before compost amendment are reported in Table 2. PAHs were grouped as 2-ring (Nap and BNap), 3-ring (Acy, Ace, Flu, Phe and Ant), 4-ring (FL, Pyr, Chr and BaA), 5ring (BbF, BaP and DbA) and 6-ring (InP and BgP) for discussion. Results show that the PAH content in Soil A was about one-order of magnitude lower than that in the other two soils. The 4-ring PAHs were the most abundant PAHs in the three soils (Table 2). However, information on PAH distribution alone is inadequate to predict risk because it ignores the changes in contaminants’ bioavailability which often results in overestimating the level of cleanup needed at contaminated sites (Shor et al., 2004; Coulon et al., 2010). Table 2 indicates that there are only slight differences in the bioP availability of 16PAHs among the three soils. PAH bioavailability decreased with an increased number of aromatic rings and molecular weight due to higher octanol–water partition coefficients (log Kow). In particular, the contribution of bioavailable fractions in 6-ring PAHs was 57% lower than that of 2-ring PAHs in Soil C. 3.3. PAH bioavailability changes and degradation The changes in PAH concentration following compost addition P are presented in Table 3. Less than 40%w/w of 16PAHs was lost in the three unamended soils over the 8-month duration. The PAH loss in Soil A was approximately 48% higher than in other soils, probably due to the more recalcitrant PAHs in the genuinely contaminated soils than in the spiked soil (Table 2). The most significant decrease in the unamended controls was observed for the 2-ring and 3-ring PAH group after 8 months, but little (<20 wt.%) degradation was observed for the 6-ring PAHs. In Soil B and C, the percentage of total PAH loss was at least 50% less than that of the bioavailable fractions. For example, less than P 25 wt.% of 16PAHs was lost in unamended Soil C (Table 3) P although over 50 wt.% of 16PAHs was bioavailable (Table 4) throughout the incubation process. This suggests that bioavailability was not the limiting factor. Instead, the slow biotransformation may be attributed to the high C:N:P ratio which may potentially reduce the biodegradative activity of PAH-degrading bacteria (Leys et al., 2005). 3.4. Effect of compost amendment on PAH bioavailability and degradation Overall, soil amendment with the two composts was beneficial for PAH removal (Table 3). The 3-ring PAHs were characterised by the largest decrease (>94%) in concentration after 8-month independent of treatments. The smallest decrease was observed for the 6-ring PAHs, as up to 45%, 17% and 8% of their initial content was still present in Soil A, B and C, respectively after 8 months. The enhancement in degradation by compost addition may be moderated by a decrease in bioavailability after organic amendment (Kästner et al., 1995; Pignatello, 1998; Huang et al., 2003), because compost can serve as a major compartment for sorption which sequestrates PAHs within the organic matrix (i.e. the lignin–cellulosic residues). On the other hand, studies have showed that humic acid-like compounds in the compost behaved as biosurfactants and enhanced desorption by decreasing surface tension, forming micelles and incorporating PAHs in the micelles cores
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G. Wu et al. / Chemosphere 90 (2013) 2240–2246 Table 2 Initial PAH distribution and bioavailability profiles in soils (average ± standard error, n = 2). Soil A Total (mg kg Nap Acy BNaP Ace Flu Phe Ant FL Pyr Chr BaA BbF BaP InP DbA BgP P 2 Rings P 3 Rings P 4 Rings P 5 Rings P 6 Rings P 16PAH
Soil B 1
)
0.1 ± 0.00 0.1 ± 0.00 0.6 ± 0.00 0.2 ± 0.02 0.4 ± 0.01 0.3 ± 0.00 0.4 ± 0.02 0.5 ± 0.00 0.4 ± 0.03 0.2 ± 0.03 0.3 ± 0.07 0.3 ± 0.05 0.2 ± 0.00 0.1 ± 0.00 0.1 ± 0.01 0.2 ± 0.01 0.6 ± 0.00 1.5 ± 0.2 1.4 ± 0.07 0.6 ± 0.00 0.3 ± 0.00 4.5 ± 0.3
Bioavailable (mg kg 1)
Bioavailable (%)
Total (mg kg
0.1 ± 0.00 0.1 ± 0.00 0.3 ± 0.00 0.2 ± 0.01 0.3 ± 0.01 0.3 ± 0.01 0.2 ± 0.01 0.3 ± 0.00 0.3 ± 0.04 0.1 ± 0.01 0.2 ± 0.06 0.2 ± 0.02 0.1 ± 0.00 0.0 ± 0.00 0.0 ± 0.00 0.1 ± 0.00 0.4 ± 0.00 1.1 ± 0.2 0.9 ± 0.06 0.3 ± 0.00 0.2 ± 0.00 2.9 ± 0.2
76 74 59 78 86 83 51 65 71 59 56 51 56 35 40 51 62 73 64 51 45 64
1±0 0±0 0±0 1±0 1±0 17 ± 4 18 ± 2 22 ± 3 17 ± 1 15 ± 4 12 ± 0 20 ± 4 12 ± 0 8±1 1±0 6±1 1±0 37 ± 3 66 ± 9 33 ± 4 13 ± 2 150 ± 12
1
)
Soil C
Bioavailable (mg kg 1)
Bioavailable (%)
Total (mg kg
0±0 0±0 0±0 0±0 0±0 11 ± 2 13 ± 1 15 ± 1 6±0 11 ± 2 8±1 10 ± 2 4±0 2±0 0±0 2±0 0±0 24 ± 3 40 ± 5 14 ± 2 4±0 82 ± 10
60 72 60 56 70 64 68 72 37 71 62 48 32 25 40 38 60 66 61 42 31 55
3±0 0±0 0±0 3±0 2±0 16 ± 1 23 ± 0 17 ± 5 25 ± 1 28 ± 2 18 ± 1 24 ± 2 23 ± 2 15 ± 0 1±0 11 ± 1 3±0 44 ± 1 89 ± 6 49 ± 0 26 ± 1 211 ± 5
1
)
Bioavailable (mg kg 1)
Bioavailable (%)
2±0 0±0 0±0 2±0 2±0 12 ± 0 14 ± 1 11 ± 3 16 ± 2 17 ± 1 10 ± 0 14 ± 1 10 ± 0 5±0 1±0 4±0 3±0 30 ± 1 55 ± 2 26 ± 1 10 ± 0 124 ± 1
88 84 53 77 76 76 62 66 64 62 55 59 45 35 48 38 83 69 62 52 36 59
Table 3 Percentage of the initial PAHs lost during incubation in relation to the number of aromatic rings (negative values represent percentage increase in PAH concentrations). 3-Month S
6-Month A1
A2
B1
B2
8-Month
S
A1
A2
B1
B2
S
A1
A2
B1
B2
Soil A 2-Ring 3-Ring 4-Ring 5-Ring 6-Ring P 16PAH
32 39 26 19 11 29
44 42 33 4 21 29
26 4 48 18 61 13
47 51 23 30 70 22
78 41 12 68 115 3
52 55 34 18 14 40
87 92 87 91 94 90
88 93 87 84 90 89
81 94 86 81 91 88
83 95 76 79 92 85
46 56 39 18 14 38
93 95 92 92 95 91
91 96 91 82 92 89
95 97 93 92 94 94
90 95 91 84 94 90
Soil B 2-Ring 3-Ring 4-Ring 5-Ring 6-Ring P 16PAH
16 12 9 10 2 10
13 37 30 27 10 29
21 30 32 0 2 22
6 27 23 18 6 21
42 32 32 25 4 28
73 28 19 16 13 20
69 79 74 74 92 77
73 90 87 85 93 88
81 86 80 78 94 82
82 89 85 85 96 87
81 32 26 22 14 26
87 95 88 87 93 89
89 95 91 92 94 92
88 96 89 89 94 90
90 98 93 93 91 94
Soil C 2-Ring 3-Ring 4-Ring 5-Ring 6-Ring P 16PAH
41 14 14 4 4 11
45 35 46 56 37 45
60 62 69 68 73 68
55 52 60 63 67 60
70 70 75 82 81 76
87 25 20 12 13 19
83 89 86 87 92 85
89 95 90 92 95 92
83 86 86 85 95 86
90 97 96 96 93 95
94 32 24 15 17 24
94 96 90 92 92 92
96 98 95 96 95 96
95 96 86 89 95 90
96 98 91 95 93 93
S = unamended soils; A1 = soils amended with Compost A at 250 t ha B2 = soils amended with Compost B at 750 t ha 1.
1
; A2 = soils amended with Compost A at 750 t ha
(Janzen et al., 1996; Quagliotto et al., 2006; Montoneri et al., 2009). The sorption process drives the PAHs from accessible compartments (e.g. aqueous phase) into less accessible or inaccessible compartments (e.g. micro- or nano-pores of soil organic matter) thereby increasing the non-bioavailable fractions, while desorption process enhances water solubility of hydrophobic molecules therefore reducing the non-bioavailable fractions. The contribution of sorption/desorption (non-bioavailable concentration changes) and degradation (difference between total concentration loss and non-bioavailable concentration changes) to the overall PAH loss after compost addition is shown in Fig. 1. In Soil A, the PAH degradation and sorption changed in a biphasic manner. During the first 3 months, compost addition was adverse
1
; B1 = soils amended with Compost B at 250 t ha
1
;
for PAH removal as the overall PAH loss in the compost amended soils was 26–89% less than that in the unamended controls (Table 3). This was attributed to the decreased bioavailability resulting from strong sorption (Fig. 1). Table 3 further demonstrated that the compost addition restrained the removal of high molecular (5- and 6-ring) PAHs. Similar to Oleszczuk (2007a,b), there was initially an increase of the PAH content adsorbed to the soils and thereby temporarily less solvent extractable. It is of particular interest to observe that the addition of green compost at 250 t ha 1 had no influence on the loss of total PAH as the decreased bioavailable fractions were completely transformed into non-bioavailable fractions rather than degraded. After 6 months, the PAH removal in the compost amended soils reached
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Table 4 Percentage of bioavailable fractions in total concentration of PAHs during incubation process. 3-Month
6-Month
8-Month
S
A1
A2
B1
B2
S
A1
A2
B1
B2
S
A1
A2
B1
B2
Soil A 2-Ring 3-Ring 4-Ring 5-Ring 6-Ring P 16PAH
43 58 41 47 47 48
35 31 41 37 23 34
45 17 32 14 16 23
43 50 36 41 29 39
44 37 37 27 21 32
35 46 33 37 32 37
34 40 26 27 19 31
22 30 16 12 11 19
32 37 31 29 21 31
31 30 24 23 19 25
33 41 29 35 27 34
34 25 19 14 8 16
15 18 12 11 6 12
31 33 24 27 21 26
35 27 22 23 17 24
Soil B 2-Ring 3-Ring 4-Ring 5-Ring 6-Ring P 16PAH
49 60 56 40 30 51
42 33 39 49 44 41
19 53 53 52 40 51
31 42 41 52 46 44
59 62 62 46 42 56
54 63 57 39 30 51
43 50 43 32 22 41
59 60 59 52 30 56
47 53 54 56 29 53
50 59 57 50 38 55
40 55 53 37 26 47
43 31 33 29 28 31
52 43 39 21 20 32
40 40 26 35 37 31
30 48 29 39 42 35
Soil C 2-Ring 3-Ring 4-Ring 5-Ring 6-Ring P 16PAH
41 62 58 50 34 54
25 43 48 34 36 42
37 54 59 58 43 55
27 60 50 42 29 48
40 61 55 50 42 54
36 62 57 50 33 53
53 53 46 47 12 47
37 59 67 50 34 60
49 64 48 50 12 51
70 60 52 53 26 54
39 58 55 47 30 50
26 43 50 30 29 42
36 65 53 44 34 49
17 28 44 44 31 41
41 51 55 41 20 47
S = unamended soils; A1 = soils amended with Compost A at 250 t ha B2 = soils amended with Compost B at 750 t ha 1.
1
; A2 = soils amended with Compost A at 750 t ha
P Fig. 1. Contribution of desorption ( ) and degradation ( ) to the 16 PAH 1 loss in soils amended with A1 (Compost A, 250 t ha ), A2 (Compost A, 750 t ha 1), B1 (Compost B, 250 t ha 1) and B2 (Compost B, 750 t ha 1). The negative values indicate sorption of PAHs to the compost amended soils.
90%. Approximately 30% of the total loss was attributed to the enhanced desorption and 70% attributed to the enhanced degradation after compost addition (Fig. 1). In Soil B, compost addition resulted in up to 30% PAH loss in the first 3 months and further increased to 94% after 8 months. Similar
1
; B1 = soils amended with Compost B at 250 t ha
1
;
trends were found in Soil C but the PAH loss was up to 76% in the first 3 months. In most cases for Soil B and C, approximately 40% of the PAH loss was due to desorption and 60% due to degradation (Fig. 1). An exception was observed when the composts were applied at 250 t ha 1 where, in the first 3 months, desorption was less important and contributed only to 2–35% of the total loss. Sorption due to compost addition in these soils was not obviously observed, probably because the PAHs in heavy coal tar or coal ash in the soils were not able to overcome the mass transfer limitations to move to the compost matrix. In general and irrespective of soil type, further loss of 6-ring PAHs was unapparent after 8 months although over 30% and 10% of the total mass was bioavailable in the unamended and amended soils, respectively (Table 4). In contrary, the concentration of 6-ring PAHs increased at the end of the experiment (Table 3), which was particularly obvious in the diesel spiked soil. Therefore, it is inferred that the primary process limiting the removal of residual PAHs (especially high molecular PAHs) at the end of incubation was more likely degradation rather than desorption. The increase in PAH concentration in green compost amended soils was also observed by Antizar-Ladislao et al. (2005). This observed phenomenon can be explained by the fact that the strength of the bonds between the PAHs initially adsorbed to the compost–soil mixture were weaken during organic matter mineralisation (Oleszczuk, 2009), which led to an increase in the bioavailability of the PAHs that were not bioavailable earlier (Oleszczuk, 2007a,b). This finding emphasises that it is incorrect to automatically assume that the residual PAHs after extensive bioremediation treatment are recalcitrant and thereby may be left in place without creating environmental risks. This is consistent with Huesemann et al. (2004). Overall, influences of compost on desorption and degradation varied with soil types. Degradation predominated in the real soils throughout the incubation process. In the diesel spiked soils, sorption was the main process limiting PAH removal at the initial stage but degradation contributed more than desorption to the PAH loss at final stage. Increasing the compost ratio did not necessarily increase PAH removal. However, the initial PAH concentration levels seemed to strongly influence removal rates, especially at the early P stage of the experiment. The initial 16PAHs in Soil A, B and C was
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Fig. 2. Comparison of the mass distribution profiles (bioavailable vs. non-bioavailable) of individual PAHs. (a) Dendogram from hierarchical cluster analysis; (b) schematic of PAH molecular structure. log Kow is presented in brackets.
4.5, 150 and 211 mg kg 1, respectively. Lower removal after 3 months was observed in Soil A (3–29%), followed by Soils B (21–29%) and C (45–76%). This agreed with Sayara et al. (2010), who demonstrated that at low concentrations it was difficult to keep the required activity of microorganisms in soils amended with municipal composts. 3.5. Behaviour of individual compounds Fig. 2 indicated that the bioavailable fraction of each PAH is inversely correlated to the number of benzene rings and the log Kow. There was no obvious discrimination between the 3-ring and 4-ring compounds as their similarity was >70%. The most significant differences were between two PAHs (BNap and DbA) and the remaining compounds (similarity <40%). The formation of insoluble aggregates of HPCD–BNap complexes at high HPCD concentration (>4 M), and the higher log Kow of DbA (6.8) compared to that of the other PAHs (3.3–6.7) might contribute to the atypical behaviour of these compounds in terms of solubility and bioavailability (ATSDR, 1995; Shixiang et al., 1998). Another explanation was the extraction mechanism by HPCD, which is a cyclic oligosaccharide with a hydrophilic exterior and a toroidal shaped hydrophobic interior where PAHs are incorporated to form a water soluble inclusion complex (Cuypers et al., 2002). We infer that both BNap and DbA could only achieve partial inclusion with HPCD which made them ‘less bioavailable’. For example, the molecular length of DbA (1.32 nm) was much greater than the diameter of HPCD cavity (0.78 nm) rendering it difficult to be wholly encompassed in the HPCD interior (Song et al., 1999; Shundo et al., 2005). Although the BNap has a smaller size (molecular length: 0.74 nm), the much polar bromo-substitute would be exposed out of the HPCD cavity into the water phase with the hydrophobic parts inserted in the interior of HPCD (Song et al., 1999). These findings implied that HPCD extraction might not be a good choice for predicting the bioavailability of PAHs with a high molecular size or polar substituent which may potentially form partial inclusion with HPCD.
4. Conclusions This study showed that the slow biodegradation (<40%) of PAHs in unamended soils was potentially due to the lack of inorganic nutrients (N and P) rather than a limited bioavailability. In the diesel spiked soil, compost addition initially decreased PAH removal by up to 89% because of the decreased bioavailability resulting from strong sorption irrespective of compost type. But as time increased, compost amendment enhanced PAH removal by more than 2-fold compared with unamended control, to which 30% was contributed by desorption and 70% by degradation. In coal tar and coal ash contaminated soils, compost addition was beneficial overall for enhancing PAH removal up to 94% and 40% of the total loss was due to the enhanced desorption. Cluster analysis demonstrated the correlation between bioavailability and log Kow and also suggested the unsuitability of HPCD method for predicting bioavailability of PAHs with large molecular size or polar substituent. The overall results suggest compost addition is an effective approach for enhancing PAHs removal from soils, but increasing the ratio of compost added does not necessarily help to increase removal. Enhanced removal by compost addition seems more effective for higher initial PAH concentrations. Further treatments may be necessary for completely removing PAHs because over 30% of the residual PAHs were still bioavailable.
Acknowledgement This research was funded by the Program for Changjiang Scholars and Innovative Research Team in University (IRT0936).
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2012.10.003.
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