Accepted Manuscript Stabilization of sewage sludge by different biochars towards reducing freely dissolved polycyclic aromatic hydrocarbons (PAHs) content Patryk Oleszczuk, Anna Zielińska, Gerard Cornelissen PII: DOI: Reference:
S0960-8524(14)00010-8 http://dx.doi.org/10.1016/j.biortech.2014.01.003 BITE 12855
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
29 October 2013 29 December 2013 2 January 2014
Please cite this article as: Oleszczuk, P., Zielińska, A., Cornelissen, G., Stabilization of sewage sludge by different biochars towards reducing freely dissolved polycyclic aromatic hydrocarbons (PAHs) content, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech.2014.01.003
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STABILIZATION OF SEWAGE SLUDGE BY DIFFERENT BIOCHARS TOWARDS REDUCING FREELY DISSOLVED POLYCYCLIC AROMATIC HYDROCARBONS (PAHs) CONTENT
Patryk Oleszczuk1*, Anna Zielińska1, Gerard Cornelissen2,3,4
1
Department of Environmental Chemistry, Maria Curie-Skłodowska University, 3 Maria
Curie-Skłodowska Square, 20-031 Lublin, Poland; 2
Department of Environmental Engineering, Norwegian Geotechnical Institute NGI, Oslo,
16 Norway; 3
Department of Applied Environmental Sciences (ITM), Stockholm University, Stockholm,
Sweden; 4
Institute for Plant and Environmental Sciences, University of Life Sciences (UMB), 5003 Ås,
Norway
Correspondence to Patryk Oleszczuk (phone: +48 81 5248160; fax: +48 81 5248150; e-mail: [email protected])
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Abstract
The objective of the study was to identify the effect of various biochars on the content of freely dissolved (Cfree) PAHs in sewage sludge. Apart from the evaluation of biochars obtained from various materials, the study also included the determination of the effects of biochar particle sizes and biochar production temperature on their ability to bind PAHs in sewage sludge. Increase in biochar dose caused a gradual reduction of Cfree PAHs content, but only up to the biochar dose of 5%. Depending on the kind of initial material from which the biochar was produced, the reduction of Cfree PAHs content in sewage sludge varied from 17.4 to 58.0%. Both the temperature and the particle size of biochar had an effect on PAH free concentration reduction. Biochars characterised by a low polarity index (O/C or (O+N)/C) reduced the level of Cfree PAHs better than biochars with a higher polarity index value.
Keywords: biochar; sewage sludge; PAHs; availability;
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1. INTRODUCTION
Biochar is charcoal created by the pyrolysis of biomass (Lehmann and Joseph, 2009). Application of biochar to soil has recently been recognized as having great potential to sequester carbon and reduce greenhouse gas emission. Thus, potentially these substances can be used to help combat global warming by holding carbon in soil and by displacing fossil fuel use. In recent years it can be observed the application of different adsorbents for the remediation of soils and sediments contaminated by hydrophobic organic compounds (Ghosh et al., 2011). Activated carbon is the most applied for this purpose (Rakowska et al., 2012). An alternative to activated carbon can also be the biochar (Oleszczuk et al., 2012a). Biochars has smaller surface area compared to activated carbon, however they are able to effectively reduce the bioavailability of polycyclic aromatic hydrocarbons (PAHs), pesticides and heavy metals (Beesley and Marmiroli, 2011; Hale et al., 2011; Pignatello et al., 2006; Tatarková et al., 2013). An additional advantage of biochar is its lower cost compared to activated carbon. It was estimated that biochar is about 10 times cheaper than activated carbon (2 USD/kg). Because of reducing the bioavailability of contaminants in soils and sediments, biochar can also be used to other contaminated matrices. Sewage sludge can be an example. Sewage sludges, due to their content of organic matter and nutrients, can be interesting material (Epstein, 2003) that can be used for the improvement of the properties of soils, or for their restoration. However, contaminants present in sludges (e.g. PAH) may restrict that application due to their toxic, mutagenic and carcinogenic character. The application of sludges containing contaminants of that kind into soil may lead to their migration to various environmental compartments (surface and ground waters, accumulation by soil organisms and plants). Immobilisation of PAHs in sewage sludges through e.g. the application of strong
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adsorbents may be a method of limitation of the mobility of dangerous contaminants and, at the same time, of expanding the possibilities of utilisation of sewage sludges. In addition, enrichment of sewage sludges by biochar may improve their fertiliser properties. Material obtained in this manner may be used e.g. for soil remediation, combining in one treatment the limitation of contaminant mobility with benefits resulting from the fertiliser properties of sewage sludge (Epstein, 2003) and biochar (Glaser et al., 2002). The problem addressed here has an extremely important practical aspect in the context of the continually increasing production of sewage sludges, and thus the constant need of their rational utilisation. Our earlier research demonstrated that after the addition of biochar (produced from corn stove) the bioavailability of PAHs in sewage sludges can be reduced by as much as 57%, depending on the doses used (0.5-10%) (Oleszczuk et al., 2012a). Those promising results necessitated more accurate determination of the following factors, which are the focus of the present work: (1) the effect of biochar properties and biochar feedstock on the binding of PAHs in sewage sludges, (2) the effect of the pyrolysis temperature and biochar particle size of PAH immobilization expressed as reduction in freely dissolved (Cfree) PAHs content. PAHs were selected for the study, as those compounds often are indicators for sludges quality.
2. MATERIALS AND METHODS
2.1. Chemicals
Heptane, acetone, methanol and hexane (all supra solv quality), dimethylformamid (DMF), silica gel 100 (0.063–0.200 mm), sodium sulphate (pro analysis) and sodium azide (NaN3) were purchased from VWR International AS (Kalbakken, Norway). Alpha Q water
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purification system was from Millipore SA (Molsheim, France). External PAH standard (PAH Mix 2) was from Supelco (Belleforte, PA, USA) and internal standard containing deuterated phenanthrene, fluoranthene, pyrene, benzo[a]pyrene and benzo[ghi]perylene from Cambridge Isotope Laboratories Inc. Polyoxymethylene (POM) was purchased in 1 kg cylinder-shaped blocks from Astrup AS (Oslo, Norway) and cut in 1.5 cm wide and 55 lm thick slices with a lathe equipped with a high-precision razor blade (Cornelissen et al., 2008).
2.2. Materials
Sewage sludge was collected during summer 2010 from a municipal sewage treatment plant localized in southeastern part of Poland (Biłgoraj, +50° 32' 52.37", +22° 43' 36.26"). The sludge sample (about 15 kg) was collected at the end point, after the sewage sludge digestion process. A few representative subsamples (5 x 50 g) were taken for the present experiments. Sewage sludge was characterised by neutral pH (7.2). The content of the total organic carbon and organic nitrogen was at the level of 157.2 and 22.1 g/kg, respectively. The sewage sludge was characterized by a high content of available phosphorus (27.2 mg/kg) and very low content of potassium (3.40 mg/kg). The cation exchange capacity (CEC) was 97.6 mmol/kg. Ca2+ and Mg2+ were the predominant base cations. Total contents of Pb, Cd, Cr, Cu, Ni and Zn were 8.8, 1.1, 19.9, 46.3, 7.2, 1350 mg/kg, respectively. EU standards were not exceeded in any case. This indicates that sewage sludge used in the experiment can be applied in agriculture and for the purpose of soil re-cultivation for agricultural purposes. The total content of PAH in the sewage sludge was at the level of 10.6 mg/kg. Phenanthrene (24%), fluoranthene (23%) and pyrene (14.6%) were the predominant PAHs in the sludge (electronic annex, Fig. S1A). Taking into consideration EU standards concerning
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the maximum content of the 11 PAHs sum in sewage sludge (6 mg/kg), the above value was exceeded. Six biochars obtained from different materials were used in the present research. Maize stover biochar (MG) was produced from corn stover residues Zea mays L. at 600oC using a slow pyrolysis (BEST energies reactor, Australia) method in a continuous unit with a residence time of 20 min. Pine wood (Pinus ponderosa) (PW) was obtained from Denver, Colorado and air-dried prior to use. Samples were charred at temperature of 600oC for 8 hours. Switchgrass (Panicum virgatum) (SC) was collected at the Split Landscape Study, Clemson University, Clemson, South Carolina. The sample was dried and ground before shipment. Samples were charred at temperature 600oC for 8 hours. Chars (PW or SC) were placed in a preheated muffle furnace with nitrogen flow sufficient to produce three volume changes per minute within the furnace. The nitrogen used was ultra high purity (UHP) grade with less than 1 parts per million (ppm) oxygen. Food waste (FW) was collected from the Cornell University (Ithaca, NY) campus as dining waste which was composed on site, and charred by BEST energies reactor via a slow pyrolysis without a catalyst or a fluidised bed. The reactor was purged with N2 to remove O2 prior to pyrolysis and the pyrolysis temperature (600oC) was maintained for 30 minutes. Digested dairy manure was obtained from AA Farms (Candor, NY) which is a large confined animal feeding operation. Raw barn floor manure was anaerobically digested, the liquid portion was removed and the solids were collected and charred by BEST energies via a slow pyrolysis without a catalyst or a fluidised bed. The reactor was purged with N2 to remove O2 prior to pyrolysis and the desired pyrolysis temperaturę (350, 400, 500 and 600oC) was maintained for 30 minutes. Paper mill waste biochar (PMW) was produced from paper mill waste which was collected from Mohawk Paper Company (Waterford, NY, USA). The water was removed from the pulp waste with a
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coagulant and flocculent before fibers were collected. The PMW was charred via a slow pyrolysis batch system at 350, 400, 500 and 600oC (Daisy reactor, Best Energies, Inc., Cashton, WI, USA). Properties of biochars used in the present experiment are presented in electronic annex (Table S1)
2.3. Sewage sludge-biochar mixture preparation
Mixing of sewage sludge with biochar for all parameters testing (effect of biochar dose, feedstock, size and pyrolysis temperature) was carried out in the laboratory in the batch experiment. Wet sewage sludge samples and dry biochar samples were added to 50 mL glass flasks with glass lids. The sewage sludge (2 g) and biochar (depending on the dose used) were thoroughly mixed with a glass spatula and rolled end over end for 30 days (in the dark, room temperature) at 10 rpm. In order to evaluate the biochar dose on freely dissolved PAHs content reduction, sewage sludge was spiked with biochar MG at the dose of 0.5%, 2%, 5%, 10% and 20% (w/w). All calculations were based on dry weight of the materials. In the experiment involving the influence of the particle diameter of biochar on reduction of freely dissolved PAHs content, dried biochar MG was sieved with a diameter of 300 μm, 300–500 μm and >500 μm. Then the separated material was mixed with sewage sludge (at the dose of 5%) according to the procedure described above. In the experiment where the influence of the biochar properties and pyrolysis temperature was investigated on Cfree PAHs content reduction, biochar was added to sewage sludge (according to the method described above) at the dose of 5%.
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2.4. Freely dissolved PAHs (Cfree) extraction and quantification
The freely dissolved (Cfree) concentration of PAH in soil/water suspensions was determined as described previously (Cornelissen et al., 2008). Polyoxymethylene (POM) passive samplers were precleaned with methanol (1 day), heptane (1 day), Millipore water (1 day) and rinsed with Millipore water prior to use. Sewage sludge (2 g, dry weight) with and without biochar was shaken in 50 mL glass flasks with glass lids end over end with 200 mg POM (55 μm thick) and 40 mL AlphaQ-water. The system was sterilized with 0.2% NaN3. After 6 weeks, POM strips were taken out, wiped off with a paper tissue and extracted in 20 mL heptane:acetone mixture (4:1, v/v) by horizontal shaking (48 h). A shaking time of six weeks was considered sufficient since equilibration time of powdered activated biocharsediment systems was less than 31 d. Deuterated PAHs (fluoranthene, pyrene, benzo[a]pyrene and benzo[ghi]perylene) in heptane (20 ng each) were added before extraction as internal standard. An aliquot of 20 mL of the organic phase (after POM extraction) was evaporated to 0.5 mL on a rotational vacuum concentrator (RVC 2-25CD plus, Martin Christ, Germany) and was subjected to partitioning between DMF/hexane for clean-up as described in Brandli et al. (2006). The recollected phase was reduced again and applied to an open micro glass column (150 mm × 7 mm i.d.) filled with (from bottom to top) glass wool, deactivated silica gel (10% milli-Q water, 3 cm), waterfree sodium sulphate, and prewashed with 5 ml heptane. The extract was eluted with 10 ml of heptane. Concentration of the eluate to a volume of 0.5 ml was again performed with a rotational vacuum concentrator. The extract was transferred to GC-vial.
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Determination of the total PAHs content included extraction of samples via a Soxhlet method with toluene (90 ml) for 6 h at 160oC, followed by the cleaning up of the concentrated extracts by DMF/hexane and microcolumn filled with silica gel as described above. POM–water distribution ratios (KPOM) were used to calculate the Cfree PAH concentration in the water from the PAH concentration measured in the POM (Hawthorne et al., 2011). PAH mass balances in the used closed solid phase/water systems were found to be satisfactory in an earlier studies for sediments (Cornelissen et al., 2006) and consequently only POM was analyzed in the present study. PAHs were separated on a Agilent 6850 Gas Chromatograph equipped with a Agilent DBXLB Column (length 30 m, 0.25 mm id and 0.1 lm film thickness, TeknoLab, Kolbotn, Norway) with a flow of 1 mL/min and the following temperaturę program: 2 min at 50oC, to 150oC at 10oC/min, to 280oC at 5oC/min, 9 min at 280oC, to 310oC with 40oC/min, at 310oC for 8 min). Detection was performed with an Agilent 5973 mass spectrometer in the electron impact mode with a 70 eV ionisation energy and a dwelling time of 25 ms. Identification of the PAH was assured by using two compound-specific ions: a quantifier ion corresponding to the respective molecular weight (m/z = M+) and a qualifier ion ([M-2H]+ for analytes and [M-2D]+ for internal standards) with a mass ratio similar to the one determined in the calibration. Four external calibration standards (1–1000 µg/mL of analyte) containing constant amounts of internal standards (100 µg/mL) were run before each series. Detection limits were 0.02 µg/kg per single PAH in sewage sludge and around 50 pg PAH, i.e. 0.2 ng/g in POM. Blanks were run with each series and levels were below detection limits in all POM experiments and below 5% in soil analysis.
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Data analysis
To check the effect of biochar-sewage sludge amendment on TOC-water distribution coefficients (KTOC) was calculated by the following equation: KTOC=Cs/(fTOC×Cw,free) where Cs is the PAH concentration in sewage sludge (ug/kg dw), fTOC the fraction of organic carbon in the sewage sludge and CW,free the freely dissolved concentration in the water (ug/L). Sorption to biochars in biochar-amended sewage sludge was calculated with a nonlinear Freundlich isotherm according to the equation proposed by Oen et al. (Oen et al., 2011). The calculated values were presented in Tables S2–S5.
3. RESULTS AND DISCUSSION
3.1. Effect of biochar dose on freely dissolved PAHs content
The sum of Cfree PAHs in the sewage sludge was 59.2 ng/L. As in the case of total PAH content, the composition of the Cfree PAHs fraction was dominated by phenanthrene (46%), fluoranthene (19%) and pyrene (12%). However, phenanthrene constituted a notably larger contribution in the Cfree PAHs fraction compared to the total PAH content (electronic annex, Fig. S1A and B). This was due primarily to a lower content of 5 and 6-ring PAHs (electronic annex, Fig. S1B). The addition of biochar to sewage sludge reduced Cfree PAHs content, which was depended on the biochar dose. Increase of the dose of biochar caused a reduction of Cfree PAHs concentration (Fig. 1). A statistically significant (P≤0.05) difference was, however,
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observed only upon the addition of BC at the dose > 2%. For example, at the dose of 2 and 5% biochar the content of Cfree PAHs decreased by 20% and 26%, respectively, relative to sludge without any biochar content. Further increase of biochar dose (to 10 and 20%) did not cause any significant further effect on the Cfree PAH concentration. Irrespective of the dose applied no trend was found between the range of reduction and the molecular weight of the individual compounds. This confirms that the systems were at equilibrium, otherwise the largest PAHs would have shown the lowest effect due to slow sludge-to-biochar mass transfer. Figure 1 shows the effect of the biochar dose on representative PAHs: phenanthrene (PHEN), pyrene (PYR) and benzo[a]pyrene (BaP). The relationship between biochar dose and the reduction of Cfree PAHs was only observed for PHEN. In the case of PYR and BaP the lowest value of the reduction was reached at biochar dose of 5% (Fig. 1). Biochar has been shown previously to reduce the bioavailability of pesticides and other contaminants present in soils and sediments (Ghosh et al., 2011). In contrast, there is a complete lack of information on the reduction of the bioavailable fraction in sewage sludge. Reduction of Cfree PAHs concentration at the level of 20.2-28.4%, depending on the dose applied, obtained in this study is lower than the results obtained by Beesley et al. (2010) for soils contaminated with PAHs. Those authors observed reduction of PAHs fraction extracted with cyclodextrin (potentially bioavailable) at levels of over 50% in the presence of biochar added to the soil at dose of 5%. Whereas, studies conducted by Gomez-Eyles et al. (2011) indicated a considerably lower reduction of the bioavailable fraction. The observed level of reduction varied from 20 to 40% and was comparable to the results obtained in this study. The effect of reduction is related with the phenomenon of PAH sorption in the presence of biochar and it most likely depends on the properties of the biochars. What is surprising is the lack of a relationship between the dose and the observed range of reduction in the case of 5 and 6-ring
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PAHs. That trend appears in the case of 3-ring PAHs, next – for 4-ring compounds – it is observed within the range from 1 to 5%, and then, for 5 and 6-ring compounds that trend no longer exists. It is possible that 5- and 6-ring compounds are bound so strongly with the sewage sludge matrix that the addition of biochar has only a slight effect on their bioavailability. However, substantiation of that possibility requires additional research. Also surprising is the lack of relation between increasing doses of biochar and the level of reduction of the Cfree PAHs content. In our earlier studies, concerning both the estimation of Cfree PAHs content (Oleszczuk et al., 2012a), and the toxicity of sewage sludge (Oleszczuk et al., 2012b) and sediment (Jośko et al., 2013) the dose of 5% was the maximum at which statistically significant reduction of toxicity or bioavailability of contaminants was observed. It is intriguing the lack of significant reduction or only a minor reduction of Cfree PYR and BaP after use of biochar in a dose above of 5% (Fig. 1). It is possible that some PAHs were desorbed from biochar and adsorbed by the POM strips what increased the Cfree PAHs content. However a similar phenomenon was not observed for PHEN. It is possible that PHEN because of its properties can be adsorbed faster by biochar, then PYR and BaP which need a longer time for a full adsorption. It is known that in general biochar does not have as high a sorption capacity for organic pollutants as compared to AC. For example the log AC-water partitioning coefficient for PHEN to anthracite-based AC was reported as 7.8 L kg-1 (Hale and Werner, 2010) while the sorption of PHEN on soot, charcoal and coal was less strong with log Kd values between 5.27 and 6.57 L kg-1 (Jonker and Koelmans, 2002). Comparing these values to others for PHEN and for different biochars as shown in Table S2-S5, it can be seen that the values reported for biochars are somewhat lower (Kong et al., 2011; Wang and Xing, 2007) than for the other carbonaceous geosorbents. However the reported partitioning coefficients for biochars
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obtained from field wood samples (mixed wood and Populus spp. biochar from wildfires in Western Canada) (James et al., 2005), were in good agreement for those reported for soot, charcoal and coal (Jonker and Koelmans, 2002).
3.2. Effect of biochar feedstocks on freely dissolved PAHs content
The effect of various biochars on Cfree PAHs concentration is presented in Fig. 2. Previous research has shown (Chen et al., 2008; Kookana, 2010), that the affinity of organic contaminants to biochar may depend on feedstock material and pyrolysis conditions. These parameters have a direct impact on the elemental composition and specific surface area (SSA) of biochar. The addition of biochars obtained from various materials to sewage sludge caused a significant reduction of Cfree PAH content. Depending on the kind of biochar, the range of reduction of PAHs varied from 17.4 to 58.0%. The highest effectiveness, clearly divergent from the mean value (31.8%), was obtained for biochar produced from switchgrass (SC). In the case of that biochar the reduction of Cfree PAHs content varied, for the individual PAHs, from 38.3 to 69.0% (58.0% for total PAHs). Relatively large reductions in Cfree were obtained also for biochar produced from pinewood (PW) (from 30.8 to 56.7% for individual PAHs, 37.0% for total PAHs). The poorest effectiveness was observed for biochars produced from digested dairy manure (DDM) (from 1.9 to 34.0%, 17.4% for total PAHs) and from food waste (FW) (from 4.2 to 55.9%, 21.1% for total PAHs). As in the case of the dose, also after the application of various biochars the reduction of Cfree PAHs content was specific for the individual PAHs and did not depend on the molecular weight of the compounds studied. Figure 2 shows a reduction of Cfree of PHEN, PYR and BaP. In the case of PHEN and PYR variation between biochars was very similar to those observed for the total PAHs content.
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However the freely dissolved BaP content did not differ between the majorities of the investigated biochars. DDM was only exception for which freely dissolved content did not differ significantly from sewage sludge without biochar. It is assumed that the parameters that may significantly determine the binding of organic contaminants, including PAHs, by geosorbents include carbon content, specific surface area and atomic ratio (Koelmans et al., 2006). Table S6 presents calculated coefficients of correlation between the properties of the biochars and the range of reduction of freely dissolved PAHs content. In no case statistically significant correlations were found between the content of C, H, N and SSA and the reduction of Cfree PAHs content. It should be emphasised that biochars characterised by the largest surface area (SG and PW) displayed also the highest levels of reduction of Cfree PAHs content (Fig. 2). However, the biochar characterised by the highest SSA (PW) reduced Cfree PAHs content only slightly better than biochars with notably smaller surface area (e.g. PMW or MG). Previous studies showed that pore-filling is one of dominant sorption mechanisms for HOCs to pyrolyzed chars (Chun et al., 2004). The absence of a distinct relationship, in this study, between SSA and reduction of Cfree PAHs content does not preclude a significant role of pore-filling and may be due to the small differences in SSA among the biochars (2-13 m2/g for FW, MG, DDM and PMW). Sewage sludge contains considerable amounts of organic matter and other substances that may significantly reduce the effectiveness of biochars through blocking access to the pores. With small values of SSA pores could have been clogged rapidly, limiting the role of that type of binding for contaminant sorption. In this situation a greater role may be played by surface effects. It has been also reported that the polarity and structure of polar aromatic compounds play an important role in the binding of contaminants by biochars. The elemental composition of each biochar was used to calculate atomic ratios as a predictor of their polarity
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(Table S1). It has earlier been found that biochars with low values of O/C and (O+N)/C will be characterised by low polarity, and thus will effectively bind hydrophobic contaminants such as PAHs (Chun et al., 2004). This hypothesis is supported in this study, where the low values of O/C and (N+O)/C (0.06 and 0.07, respectively) were noted for biochar SC which was the most effective in reducing freely dissolved PAHs content. The next in this respect, biochar PW, was also characterised by low values of O/C and (N+O)/C (0.05 and 0.06, respectively) compared to the remaining biochars studied (Table S1). An important role of polarity in the binding of HOC is also supported by the studies by Chen et al. (2008). Those authors also observed that with lowering polarity there was an increase of the partition coefficient of HOC (naphthalene, nitrobenzene and m-nitrobenzene). In that case, the lack of distinct correlations for all of the biochars studied may, as in the case of SSA, result from small differences in the values of O/C and (N+O)/C among the remaining biochars. Tables S2-S5 present the values of log KBC of native compounds (thirteen PAHs) in sewage sludge amended with biochars (5%), determined for all the variants of the experiment. Log KBC values were 2 order higher than those for non-amended sewage sludge TOC for native PAHs (Fig. 3). Theoretically freely dissolved PAHs content reduction should be much higher than that observed in the present study. It is likely that the matrix (sewage sludge) has a strong effect on the sorptive properties of biochars. Depending on the properties of biochars, components of sewage sludge (including organic matter) may variously interact with biochars of diverse properties. It has been observed earlier for soils and sediments that the presence of a matrix notably reduced the sorptive properties of AC (Amstaetter et al., 2012; Cornelissen et al., 2006). The values of log KBC (Table S2-S5) obtained in this study in the presence of sewage sludge for the particular PAHs were lower by 1-2 orders of magnitude compared to the values determined for AC by (Amstaetter et al., 2012). This supports our earlier
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observations where AC adsorbed PAHs more effectively than various BCs (Oleszczuk et al., 2012a).
3.3. Effect of biochars diameter on freely dissolved PAHs content
Due to the fact that the particle size of adsorbents may be significant in the estimation of their effectiveness in binding organic contaminants (Zimmerman et al., 2005), biochar MG was divided into fractions with particle sizes below 300 μm, from 300 to 500 μm, and above 500 μm. The effectiveness of reduction of freely dissolved PAHs content was clearly related to the size fraction (Fig. 4). Particles characterised by the smallest size reduced Cfree PAHs content at the level of 35.8% relative to sludge with no biochar content. That fraction (500 μm, were notably worse in this respect. After adding the medium size fraction (300-500 μm) to sewage sludge, the reduction of PAHs did not differ significantly from biochar not separated into the size fractions and amounted to 14.0%. The coarsest fraction (>500 μm) showed the same level of reduction of Cfree PAHs content. In the case of individual PAHs similar relationship as for the sum was observed only for PHEN. The freely dissolved PYR and BaA content was significantly reduced only by the smallest fraction of biochar (500 μm) Cfree PAHs content was not significantly different from sewage sludge without biochar (Fig. 4). As in earlier studies, no relation was noted between the molecular weight of the particular compounds and the reduction of Cfree PAHs content. As inferred from this lack of dependence of biochar effectiveness of PAH molecular size (discussed above), we rule out incomplete
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equilibrium as a reason for the observation that smaller size fractions result in higher reductions of Cfree content. In contrast, a significant relation was observed between carbon content and the reduction of Cfree 4-ring PAHs content (Fig. 4). The effect of polarity observed earlier was also confirmed. In this case a significant relation was noted between reduction of Cfree PAHs content and the values of O/C and (O+N)/C ratios (Fig. 5). Studies conducted so far, concerned with particle size, are focused mainly on AC. It has been demonstrated (Lebo et al., 2003; Zimmerman et al., 2005) that particles characterised by smaller size are more effective in reducing the concentration of contaminants in aqueous phase than larger particles. Carbon materials with particle diameter from 75 to 300 µm, that can be considered as small particles of granulated active carbon, proved to be the most effective in immobilising HOCs (Brändli et al., 2008; Zimmerman et al., 2005). Better binding of contaminants by smaller particles is attributed primarily to their larger specific surface area, when most of the SSA is residing in internal pores (Pignatello et al., 2006). Values of SSA assayed in this study for biochar MG were lower in the case of smaller particles than in the case of larger ones (Table S1), but in spite of that it was still observed that the smaller particles (as in studies by other authors) displayed better binding of PAHs. The relation obtained (Fig. 5) indicates that lowering of polarity (increase of hydrophobicity) may be an important factor responsible for better effectiveness of the biochar in binding PAHs. In addition, better binding of PAHs by smaller biochar particles can be attributed to the fact that a greater number of particles are contained in a unit of mass of the sludge, and thus more particles have contact with the sludge (Brändli et al., 2008; Lebo et al., 2003).
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3.4. Influence of pyrolysis temperature on freely dissolved PAHs content
Previous research has shown that the affinity of organic contaminants to biochar may vary depending on feedstock material and pyrolysis conditions, mainly temperature (Chen et al., 2008; Keiluweit et al., 2010; Sun et al., 2012). The temperature at which the biochars were produced also had a significant effect on the content of Cfree PAHs (Fig. 6). The addition of biochars produced at 350oC to the sludge had no significant effect on the level of Cfree PAHs content. Also, no significant effect was noted in the case of biochar PMW obtained at temperature of 400oC. Whereas, at temperature of 400oC a significant reduction (by 15.9%) of Cfree PAHs content was observed after the application of biochar DDM to the sludge. The effect observed most probably results from the higher content of carbon in DDM compared to PMW (Table S1). The application to the sludge of biochars produced at temperatures of 500 and 600oC caused significant reduction of Cfree PAHs content, by 22.9% (DDM) and 27.3% (PMW) and by 17.4% (DDM) and 31.0% (PMW), respectively. Those values did not differ significantly among the particular biochars for a given temperature. Biochars made at low temperatures have amorphous structure and contain considerable proportions of ‘soft’ aliphatic domains (Chun et al., 2004). Under such conditions, partitioning is the main mechanism of binding HOC (Sun et al., 2012). Increase of temperature (to 500 and 600oC) causes a change from the more flexible aliphatic phase to a more rigid and condensed aromatic phase (Chen et al., 2008), which is confirmed by the decreasing value of H/C (Table S1, for PMW). It is likely, therefore, that the reduction of Cfree PAHs content is related primarily to the binding of PAHs present in sewage sludge mainly by the condensed aromatic phase. Bonds of that type are strong (Pignatello and Xing, 1996), which from the practical point of view may be an argument for the effectiveness of the method.
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4. CONCLUSIONS
The increasing production of sewage sludges and the necessity of finding a safe solution regarding their utilisation requires the search for new alternative methods. The presence of organic contaminants in sludges may create a threat to the environment and restrict their utilisation. The addition of biochar to sludge, reduces the freely dissolved PAHs content. The reduction of the bioavailable fraction of sewage sludge, responsible for the toxic effect and at the same time mobile, will expand the possibilities of its potential utilisation. This may find an application in situations where sewage sludge is used for the reclamation of degraded areas.
Acknowledgements
The project was funded by the National Science Centre granted on the basis of the decision number DEC-2012/07/E/ST10/00572. Johannes Lehmann and Kelly Hanley (Cornell University) and David W. Rutherford (US Geological Survey, Denver) are thanked for kindly providing biochar samples. GC thanks the Research Council of Norway for a FRIPRO personal stipend (217918/F20). Sarah E. Hale (NGI) is thanked for fruitful discussion and advice in the laboratory.
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Brändli, R.C., Hartnik, T., Henriksen, T., Cornelissen, G., 2008. Sorption of native polyaromatic hydrocarbons (PAH) to black carbon and amended activated carbon in soil. Chemosphere 73, 1805–1810.
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Chen, B., Zhou, D., Zhu, L., 2008. Transitional adsorption and partition of nonpolar and polar aromatic contaminants by biochars of pine needles with different pyrolytic temperatures. Environ. Sci. Technol. 42, 5137–5143.
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Chun, Y., Sheng, G., Chiou, C.T., Xing, B., 2004. Compositions and sorptive properties of crop residue-derived chars. Environ. Sci. Technol. 38, 4649–4655.
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Cornelissen, G., Breedveld, G.D., Kalaitzidis, S., Christanis, K., Kibsgaard, A., Oen, A.M.P., 2006. Strong sorption of native PAHs to pyrogenic and unburned carbonaceous geosorbents in sediments. Environ. Sci. Technol. 40, 1197–1203.
10. Epstein, E., 2003. Land application of sewage sludge and biosolids. Lewis Publishers,
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Boca Raton, Fla. 11. Ghosh, U., Luthy, R.G., Cornelissen, G., Werner, D., Menzie, C.A., 2011. In-situ sorbent amendments: a new direction in contaminated sediment management. Environ. Sci. Technol. 45, 1163–1168. 12. Glaser, B., Lehmann, J., Zech, W., 2002. Ameliorating physical and chemical properties of highly weathered soils in the tropics with charcoal - a review. Biol. Fertil. Soils 35, 219–230. 13. Gomez-Eyles, J.L., Sizmur, T., Collins, C.D., Hodson, M.E., 2011. Effects of biochar and the earthworm Eisenia fetida on the bioavailability of polycyclic aromatic hydrocarbons and potentially toxic elements. Environ. Pollut. 159, 616–622. 14. Hale, S., Hanley, K., Lehmann, J., Zimmerman, A., Cornelissen, G., 2011. Effects of chemical, biological, and physical aging as well as soil addition on the sorption of pyrene to activated carbon and biochar. Environ. Sci. Technol. 45, 10445–10453. 15. Hale, S.E., Werner, D., 2010. Modeling the mass transfer of hydrophobic organic pollutants in briefly and continuously mixed sediment after amendment with activated carbon. Environ. Sci. Technol. 44, 3381–3387. 16. Hawthorne, S.B., Jonker, M.T.O., van der Heijden, S.A., Grabanski, C.B., Azzolina, N.A., Miller, D.J., 2011. Measuring picogram per liter concentrations of freely dissolved parent and alkyl PAHs (PAH-34), using passive sampling with polyoxymethylene. Anal. Chem. 83, 6754–6761. 17. James, G., Sabatini, D.A., Chiou, C.T., Rutherford, D., Scott, A.C., Karapanagioti, H.K., 2005. Evaluating phenanthrene sorption on various wood chars. Water Res. 39, 549–558. 18. Jonker, M.T.O., Koelmans, A.A., 2002. Sorption of polycyclic aromatic hydrocarbons and polychlorinated biphenyls to soot and soot-like materials in the aqueous
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environment: mechanistic considerations. Environ. Sci. Technol. 36, 3725–3734. 19. Jośko, I., Oleszczuk, P., Pranagal, J., Lehmann, J., Xing, B., Cornelissen, G., 2013. Effect of biochars, activated carbon and multiwalled carbon nanotubes on phytotoxicity of sediment contaminated by inorganic and organic pollutants. Ecol. Eng. 60, 50–59. 20. Keiluweit, M., Nico, P.S., Johnson, M.G., Kleber, M., 2010. Dynamic molecular structure of plant biomass-derived black carbon (Biochar). Environ. Sci. Technol. 44, 1247–1253. 21. Koelmans, A.A., Jonker, M.T.O., Cornelissen, G., Bucheli, T.D., Van Noort, P.C.M., Gustafsson, Ö., 2006. Black carbon: The reverse of its dark side. Chemosphere 63, 365– 377. 22. Kong, H., He, J., Gao, Y., Wu, H., Zhu, X., 2011. Cosorption of phenanthrene and mercury(ii) from aqueous solution by soybean stalk-based biochar. J. Agric. Food Chem. 59, 12116–12123. 23. Kookana, R.S., 2010. The role of biochar in modifying the environmental fate, bioavailability, and efficacy of pesticides in soils: A review. Aust. J. Soil Res. 48, 627– 637. 24. Lebo, J.A., Huckins, J.N., Petty, J.D., Cranor, W.L., Ho, K.T., 2003. Comparisons of coarse and fine versions of two carbons for reducing the bioavailabilities of sedimentbound hydrophobic organic contaminants. Chemosphere 50, 1309–1317. 25. Lehmann, J., Joseph, S., 2009. Biochar for environmental management: science and technology. Earthscan, London ; Sterling, VA. 26. Oen, A.M.P., Janssen, E.M.L., Cornelissen, G., Breedveld, G.D., Eek, E., Luthy, R.G., 2011. In situ measurement of PCB pore water concentration profiles in activated carbonamended sediment using passive samplers. Environ. Sci. Technol. 45, 4053–4059.
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27. Oleszczuk, P., Hale, S.E., Lehmann, J., Cornelissen, G., 2012a. Activated carbon and biochar amendments decrease pore-water concentrations of polycyclic aromatic hydrocarbons (PAHs) in sewage sludge. Bioresour. Technol. 111, 84–91. 28. Oleszczuk, P., Rycaj, M., Lehmann, J., Cornelissen, G., 2012b. Influence of activated carbon and biochar on phytotoxicity of air-dried sewage sludges to Lepidium sativum. Ecotoxicol. Environ. Safe. 80, 321–326. 29. Pignatello, J.J., Kwon, S., Lu, Y., 2006. Effect of natural organic substances on the surface and adsorptive properties of environmental black carbon (char): attenuation of surface activity by humic and fulvic acids. Environ. Sci. Technol. 40, 7757–7763. 30. Pignatello, J.J., Xing, B., 1996. Mechanisms of slow sorption of organic chemicals to natural particles. Environ. Sci. Technol. 30, 1–11. 31. Rakowska, M.I., Kupryianchyk, D., Harmsen, J., Grotenhuis, T., Koelmans, A.A., 2012. In situ remediation of contaminated sediments using carbonaceous materials. Environ. Toxicol. Chem. 31, 693–704. 32. Sun, K., Jin, J., Keiluweit, M., Kleber, M., Wang, Z., Pan, Z., Xing, B., 2012. Polar and aliphatic domains regulate sorption of phthalic acid esters (PAEs) to biochars. Bioresour. Technol. 118, 120–127. 33. Tatarková, V., Hiller, E., Vaculík, M., 2013. Impact of wheat straw biochar addition to soil on the sorption, leaching, dissipation of the herbicide (4-chloro-2methylphenoxy)acetic acid and the growth of sunflower (Helianthus annuus L.). Ecotoxicol. Environ. Safe. 92, 215–221. 34. Wang, X., Xing, B., 2007. Sorption of organic contaminants by biopolymer-derived chars. Environ. Sci. Technol. 41, 8342–8348. 35. Zimmerman, J.R., Werner, D., Ghosh, U., Millward, R.N., Bridges, T.S., Luthy, R.G.,
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2005. Effects of dose and particle size on activated carbon treatment to sequester polychlorinated biphenyls and polycyclic aromatic hydrocarbons in marine sediments. Environ. Toxicol. Chem. SETAC 24, 1594–1601.
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Freely dissolved PAHs sum content (ng/L)
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Figure 1. The effect of biochar dose on freely dissolved PAHs content in sewage sludge Freely dissolved PAHs concentration [ng/L]
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Figure 2. Influence of different biochars (5% dose) on freely dissolved PAHs content in sewage sludge. Sewage sludge (SL), corn stove (MG), paper mill waste (PMW), manure waste (DDM), food waste (FW), pinechar (PC) and switchgrass (SC).
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Log KBC
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KTOC-SL LFER 2 2
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Figure 3. Kowvs KTOC for sewage sludge and KBC for different biochars calculated with a nonlinear Freundlich isotherm (details about calculations are presented in supporting information)
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SC
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Freely dissolved PAHs content [ng/L]
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Figure 4. Influence of biochar diameter (biochar MG) on freely dissolved PAHs content in sewage sludge
O/C or (O+N)/C ratio
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O/C (O+N)/C yO/C=3.929x-0.106 y(O+N)/C=4.006x-0.108
0 0
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Figure 5. The relationship between reduction of freely dissolved PAHs content and polarity index of different particle size of biochar
Freely dissolved PAHs concentration [ng/L]
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Figure 6. Effect of biochar temperature preparation on freely dissolved PAHs content in sewage sludge. * statistically significant different (P
S0960-8524(14)00010-8 http://dx.doi.org/10.1016/j.biortech.2014.01.003 BITE 12855
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
29 October 2013 29 December 2013 2 January 2014
Please cite this article as: Oleszczuk, P., Zielińska, A., Cornelissen, G., Stabilization of sewage sludge by different biochars towards reducing freely dissolved polycyclic aromatic hydrocarbons (PAHs) content, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech.2014.01.003
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
STABILIZATION OF SEWAGE SLUDGE BY DIFFERENT BIOCHARS TOWARDS REDUCING FREELY DISSOLVED POLYCYCLIC AROMATIC HYDROCARBONS (PAHs) CONTENT
Patryk Oleszczuk1*, Anna Zielińska1, Gerard Cornelissen2,3,4
1
Department of Environmental Chemistry, Maria Curie-Skłodowska University, 3 Maria
Curie-Skłodowska Square, 20-031 Lublin, Poland; 2
Department of Environmental Engineering, Norwegian Geotechnical Institute NGI, Oslo,
16 Norway; 3
Department of Applied Environmental Sciences (ITM), Stockholm University, Stockholm,
Sweden; 4
Institute for Plant and Environmental Sciences, University of Life Sciences (UMB), 5003 Ås,
Norway
Correspondence to Patryk Oleszczuk (phone: +48 81 5248160; fax: +48 81 5248150; e-mail: [email protected])
1
Abstract
The objective of the study was to identify the effect of various biochars on the content of freely dissolved (Cfree) PAHs in sewage sludge. Apart from the evaluation of biochars obtained from various materials, the study also included the determination of the effects of biochar particle sizes and biochar production temperature on their ability to bind PAHs in sewage sludge. Increase in biochar dose caused a gradual reduction of Cfree PAHs content, but only up to the biochar dose of 5%. Depending on the kind of initial material from which the biochar was produced, the reduction of Cfree PAHs content in sewage sludge varied from 17.4 to 58.0%. Both the temperature and the particle size of biochar had an effect on PAH free concentration reduction. Biochars characterised by a low polarity index (O/C or (O+N)/C) reduced the level of Cfree PAHs better than biochars with a higher polarity index value.
Keywords: biochar; sewage sludge; PAHs; availability;
2
1. INTRODUCTION
Biochar is charcoal created by the pyrolysis of biomass (Lehmann and Joseph, 2009). Application of biochar to soil has recently been recognized as having great potential to sequester carbon and reduce greenhouse gas emission. Thus, potentially these substances can be used to help combat global warming by holding carbon in soil and by displacing fossil fuel use. In recent years it can be observed the application of different adsorbents for the remediation of soils and sediments contaminated by hydrophobic organic compounds (Ghosh et al., 2011). Activated carbon is the most applied for this purpose (Rakowska et al., 2012). An alternative to activated carbon can also be the biochar (Oleszczuk et al., 2012a). Biochars has smaller surface area compared to activated carbon, however they are able to effectively reduce the bioavailability of polycyclic aromatic hydrocarbons (PAHs), pesticides and heavy metals (Beesley and Marmiroli, 2011; Hale et al., 2011; Pignatello et al., 2006; Tatarková et al., 2013). An additional advantage of biochar is its lower cost compared to activated carbon. It was estimated that biochar is about 10 times cheaper than activated carbon (2 USD/kg). Because of reducing the bioavailability of contaminants in soils and sediments, biochar can also be used to other contaminated matrices. Sewage sludge can be an example. Sewage sludges, due to their content of organic matter and nutrients, can be interesting material (Epstein, 2003) that can be used for the improvement of the properties of soils, or for their restoration. However, contaminants present in sludges (e.g. PAH) may restrict that application due to their toxic, mutagenic and carcinogenic character. The application of sludges containing contaminants of that kind into soil may lead to their migration to various environmental compartments (surface and ground waters, accumulation by soil organisms and plants). Immobilisation of PAHs in sewage sludges through e.g. the application of strong
3
adsorbents may be a method of limitation of the mobility of dangerous contaminants and, at the same time, of expanding the possibilities of utilisation of sewage sludges. In addition, enrichment of sewage sludges by biochar may improve their fertiliser properties. Material obtained in this manner may be used e.g. for soil remediation, combining in one treatment the limitation of contaminant mobility with benefits resulting from the fertiliser properties of sewage sludge (Epstein, 2003) and biochar (Glaser et al., 2002). The problem addressed here has an extremely important practical aspect in the context of the continually increasing production of sewage sludges, and thus the constant need of their rational utilisation. Our earlier research demonstrated that after the addition of biochar (produced from corn stove) the bioavailability of PAHs in sewage sludges can be reduced by as much as 57%, depending on the doses used (0.5-10%) (Oleszczuk et al., 2012a). Those promising results necessitated more accurate determination of the following factors, which are the focus of the present work: (1) the effect of biochar properties and biochar feedstock on the binding of PAHs in sewage sludges, (2) the effect of the pyrolysis temperature and biochar particle size of PAH immobilization expressed as reduction in freely dissolved (Cfree) PAHs content. PAHs were selected for the study, as those compounds often are indicators for sludges quality.
2. MATERIALS AND METHODS
2.1. Chemicals
Heptane, acetone, methanol and hexane (all supra solv quality), dimethylformamid (DMF), silica gel 100 (0.063–0.200 mm), sodium sulphate (pro analysis) and sodium azide (NaN3) were purchased from VWR International AS (Kalbakken, Norway). Alpha Q water
4
purification system was from Millipore SA (Molsheim, France). External PAH standard (PAH Mix 2) was from Supelco (Belleforte, PA, USA) and internal standard containing deuterated phenanthrene, fluoranthene, pyrene, benzo[a]pyrene and benzo[ghi]perylene from Cambridge Isotope Laboratories Inc. Polyoxymethylene (POM) was purchased in 1 kg cylinder-shaped blocks from Astrup AS (Oslo, Norway) and cut in 1.5 cm wide and 55 lm thick slices with a lathe equipped with a high-precision razor blade (Cornelissen et al., 2008).
2.2. Materials
Sewage sludge was collected during summer 2010 from a municipal sewage treatment plant localized in southeastern part of Poland (Biłgoraj, +50° 32' 52.37", +22° 43' 36.26"). The sludge sample (about 15 kg) was collected at the end point, after the sewage sludge digestion process. A few representative subsamples (5 x 50 g) were taken for the present experiments. Sewage sludge was characterised by neutral pH (7.2). The content of the total organic carbon and organic nitrogen was at the level of 157.2 and 22.1 g/kg, respectively. The sewage sludge was characterized by a high content of available phosphorus (27.2 mg/kg) and very low content of potassium (3.40 mg/kg). The cation exchange capacity (CEC) was 97.6 mmol/kg. Ca2+ and Mg2+ were the predominant base cations. Total contents of Pb, Cd, Cr, Cu, Ni and Zn were 8.8, 1.1, 19.9, 46.3, 7.2, 1350 mg/kg, respectively. EU standards were not exceeded in any case. This indicates that sewage sludge used in the experiment can be applied in agriculture and for the purpose of soil re-cultivation for agricultural purposes. The total content of PAH in the sewage sludge was at the level of 10.6 mg/kg. Phenanthrene (24%), fluoranthene (23%) and pyrene (14.6%) were the predominant PAHs in the sludge (electronic annex, Fig. S1A). Taking into consideration EU standards concerning
5
the maximum content of the 11 PAHs sum in sewage sludge (6 mg/kg), the above value was exceeded. Six biochars obtained from different materials were used in the present research. Maize stover biochar (MG) was produced from corn stover residues Zea mays L. at 600oC using a slow pyrolysis (BEST energies reactor, Australia) method in a continuous unit with a residence time of 20 min. Pine wood (Pinus ponderosa) (PW) was obtained from Denver, Colorado and air-dried prior to use. Samples were charred at temperature of 600oC for 8 hours. Switchgrass (Panicum virgatum) (SC) was collected at the Split Landscape Study, Clemson University, Clemson, South Carolina. The sample was dried and ground before shipment. Samples were charred at temperature 600oC for 8 hours. Chars (PW or SC) were placed in a preheated muffle furnace with nitrogen flow sufficient to produce three volume changes per minute within the furnace. The nitrogen used was ultra high purity (UHP) grade with less than 1 parts per million (ppm) oxygen. Food waste (FW) was collected from the Cornell University (Ithaca, NY) campus as dining waste which was composed on site, and charred by BEST energies reactor via a slow pyrolysis without a catalyst or a fluidised bed. The reactor was purged with N2 to remove O2 prior to pyrolysis and the pyrolysis temperature (600oC) was maintained for 30 minutes. Digested dairy manure was obtained from AA Farms (Candor, NY) which is a large confined animal feeding operation. Raw barn floor manure was anaerobically digested, the liquid portion was removed and the solids were collected and charred by BEST energies via a slow pyrolysis without a catalyst or a fluidised bed. The reactor was purged with N2 to remove O2 prior to pyrolysis and the desired pyrolysis temperaturę (350, 400, 500 and 600oC) was maintained for 30 minutes. Paper mill waste biochar (PMW) was produced from paper mill waste which was collected from Mohawk Paper Company (Waterford, NY, USA). The water was removed from the pulp waste with a
6
coagulant and flocculent before fibers were collected. The PMW was charred via a slow pyrolysis batch system at 350, 400, 500 and 600oC (Daisy reactor, Best Energies, Inc., Cashton, WI, USA). Properties of biochars used in the present experiment are presented in electronic annex (Table S1)
2.3. Sewage sludge-biochar mixture preparation
Mixing of sewage sludge with biochar for all parameters testing (effect of biochar dose, feedstock, size and pyrolysis temperature) was carried out in the laboratory in the batch experiment. Wet sewage sludge samples and dry biochar samples were added to 50 mL glass flasks with glass lids. The sewage sludge (2 g) and biochar (depending on the dose used) were thoroughly mixed with a glass spatula and rolled end over end for 30 days (in the dark, room temperature) at 10 rpm. In order to evaluate the biochar dose on freely dissolved PAHs content reduction, sewage sludge was spiked with biochar MG at the dose of 0.5%, 2%, 5%, 10% and 20% (w/w). All calculations were based on dry weight of the materials. In the experiment involving the influence of the particle diameter of biochar on reduction of freely dissolved PAHs content, dried biochar MG was sieved with a diameter of 300 μm, 300–500 μm and >500 μm. Then the separated material was mixed with sewage sludge (at the dose of 5%) according to the procedure described above. In the experiment where the influence of the biochar properties and pyrolysis temperature was investigated on Cfree PAHs content reduction, biochar was added to sewage sludge (according to the method described above) at the dose of 5%.
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2.4. Freely dissolved PAHs (Cfree) extraction and quantification
The freely dissolved (Cfree) concentration of PAH in soil/water suspensions was determined as described previously (Cornelissen et al., 2008). Polyoxymethylene (POM) passive samplers were precleaned with methanol (1 day), heptane (1 day), Millipore water (1 day) and rinsed with Millipore water prior to use. Sewage sludge (2 g, dry weight) with and without biochar was shaken in 50 mL glass flasks with glass lids end over end with 200 mg POM (55 μm thick) and 40 mL AlphaQ-water. The system was sterilized with 0.2% NaN3. After 6 weeks, POM strips were taken out, wiped off with a paper tissue and extracted in 20 mL heptane:acetone mixture (4:1, v/v) by horizontal shaking (48 h). A shaking time of six weeks was considered sufficient since equilibration time of powdered activated biocharsediment systems was less than 31 d. Deuterated PAHs (fluoranthene, pyrene, benzo[a]pyrene and benzo[ghi]perylene) in heptane (20 ng each) were added before extraction as internal standard. An aliquot of 20 mL of the organic phase (after POM extraction) was evaporated to 0.5 mL on a rotational vacuum concentrator (RVC 2-25CD plus, Martin Christ, Germany) and was subjected to partitioning between DMF/hexane for clean-up as described in Brandli et al. (2006). The recollected phase was reduced again and applied to an open micro glass column (150 mm × 7 mm i.d.) filled with (from bottom to top) glass wool, deactivated silica gel (10% milli-Q water, 3 cm), waterfree sodium sulphate, and prewashed with 5 ml heptane. The extract was eluted with 10 ml of heptane. Concentration of the eluate to a volume of 0.5 ml was again performed with a rotational vacuum concentrator. The extract was transferred to GC-vial.
8
Determination of the total PAHs content included extraction of samples via a Soxhlet method with toluene (90 ml) for 6 h at 160oC, followed by the cleaning up of the concentrated extracts by DMF/hexane and microcolumn filled with silica gel as described above. POM–water distribution ratios (KPOM) were used to calculate the Cfree PAH concentration in the water from the PAH concentration measured in the POM (Hawthorne et al., 2011). PAH mass balances in the used closed solid phase/water systems were found to be satisfactory in an earlier studies for sediments (Cornelissen et al., 2006) and consequently only POM was analyzed in the present study. PAHs were separated on a Agilent 6850 Gas Chromatograph equipped with a Agilent DBXLB Column (length 30 m, 0.25 mm id and 0.1 lm film thickness, TeknoLab, Kolbotn, Norway) with a flow of 1 mL/min and the following temperaturę program: 2 min at 50oC, to 150oC at 10oC/min, to 280oC at 5oC/min, 9 min at 280oC, to 310oC with 40oC/min, at 310oC for 8 min). Detection was performed with an Agilent 5973 mass spectrometer in the electron impact mode with a 70 eV ionisation energy and a dwelling time of 25 ms. Identification of the PAH was assured by using two compound-specific ions: a quantifier ion corresponding to the respective molecular weight (m/z = M+) and a qualifier ion ([M-2H]+ for analytes and [M-2D]+ for internal standards) with a mass ratio similar to the one determined in the calibration. Four external calibration standards (1–1000 µg/mL of analyte) containing constant amounts of internal standards (100 µg/mL) were run before each series. Detection limits were 0.02 µg/kg per single PAH in sewage sludge and around 50 pg PAH, i.e. 0.2 ng/g in POM. Blanks were run with each series and levels were below detection limits in all POM experiments and below 5% in soil analysis.
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Data analysis
To check the effect of biochar-sewage sludge amendment on TOC-water distribution coefficients (KTOC) was calculated by the following equation: KTOC=Cs/(fTOC×Cw,free) where Cs is the PAH concentration in sewage sludge (ug/kg dw), fTOC the fraction of organic carbon in the sewage sludge and CW,free the freely dissolved concentration in the water (ug/L). Sorption to biochars in biochar-amended sewage sludge was calculated with a nonlinear Freundlich isotherm according to the equation proposed by Oen et al. (Oen et al., 2011). The calculated values were presented in Tables S2–S5.
3. RESULTS AND DISCUSSION
3.1. Effect of biochar dose on freely dissolved PAHs content
The sum of Cfree PAHs in the sewage sludge was 59.2 ng/L. As in the case of total PAH content, the composition of the Cfree PAHs fraction was dominated by phenanthrene (46%), fluoranthene (19%) and pyrene (12%). However, phenanthrene constituted a notably larger contribution in the Cfree PAHs fraction compared to the total PAH content (electronic annex, Fig. S1A and B). This was due primarily to a lower content of 5 and 6-ring PAHs (electronic annex, Fig. S1B). The addition of biochar to sewage sludge reduced Cfree PAHs content, which was depended on the biochar dose. Increase of the dose of biochar caused a reduction of Cfree PAHs concentration (Fig. 1). A statistically significant (P≤0.05) difference was, however,
10
observed only upon the addition of BC at the dose > 2%. For example, at the dose of 2 and 5% biochar the content of Cfree PAHs decreased by 20% and 26%, respectively, relative to sludge without any biochar content. Further increase of biochar dose (to 10 and 20%) did not cause any significant further effect on the Cfree PAH concentration. Irrespective of the dose applied no trend was found between the range of reduction and the molecular weight of the individual compounds. This confirms that the systems were at equilibrium, otherwise the largest PAHs would have shown the lowest effect due to slow sludge-to-biochar mass transfer. Figure 1 shows the effect of the biochar dose on representative PAHs: phenanthrene (PHEN), pyrene (PYR) and benzo[a]pyrene (BaP). The relationship between biochar dose and the reduction of Cfree PAHs was only observed for PHEN. In the case of PYR and BaP the lowest value of the reduction was reached at biochar dose of 5% (Fig. 1). Biochar has been shown previously to reduce the bioavailability of pesticides and other contaminants present in soils and sediments (Ghosh et al., 2011). In contrast, there is a complete lack of information on the reduction of the bioavailable fraction in sewage sludge. Reduction of Cfree PAHs concentration at the level of 20.2-28.4%, depending on the dose applied, obtained in this study is lower than the results obtained by Beesley et al. (2010) for soils contaminated with PAHs. Those authors observed reduction of PAHs fraction extracted with cyclodextrin (potentially bioavailable) at levels of over 50% in the presence of biochar added to the soil at dose of 5%. Whereas, studies conducted by Gomez-Eyles et al. (2011) indicated a considerably lower reduction of the bioavailable fraction. The observed level of reduction varied from 20 to 40% and was comparable to the results obtained in this study. The effect of reduction is related with the phenomenon of PAH sorption in the presence of biochar and it most likely depends on the properties of the biochars. What is surprising is the lack of a relationship between the dose and the observed range of reduction in the case of 5 and 6-ring
11
PAHs. That trend appears in the case of 3-ring PAHs, next – for 4-ring compounds – it is observed within the range from 1 to 5%, and then, for 5 and 6-ring compounds that trend no longer exists. It is possible that 5- and 6-ring compounds are bound so strongly with the sewage sludge matrix that the addition of biochar has only a slight effect on their bioavailability. However, substantiation of that possibility requires additional research. Also surprising is the lack of relation between increasing doses of biochar and the level of reduction of the Cfree PAHs content. In our earlier studies, concerning both the estimation of Cfree PAHs content (Oleszczuk et al., 2012a), and the toxicity of sewage sludge (Oleszczuk et al., 2012b) and sediment (Jośko et al., 2013) the dose of 5% was the maximum at which statistically significant reduction of toxicity or bioavailability of contaminants was observed. It is intriguing the lack of significant reduction or only a minor reduction of Cfree PYR and BaP after use of biochar in a dose above of 5% (Fig. 1). It is possible that some PAHs were desorbed from biochar and adsorbed by the POM strips what increased the Cfree PAHs content. However a similar phenomenon was not observed for PHEN. It is possible that PHEN because of its properties can be adsorbed faster by biochar, then PYR and BaP which need a longer time for a full adsorption. It is known that in general biochar does not have as high a sorption capacity for organic pollutants as compared to AC. For example the log AC-water partitioning coefficient for PHEN to anthracite-based AC was reported as 7.8 L kg-1 (Hale and Werner, 2010) while the sorption of PHEN on soot, charcoal and coal was less strong with log Kd values between 5.27 and 6.57 L kg-1 (Jonker and Koelmans, 2002). Comparing these values to others for PHEN and for different biochars as shown in Table S2-S5, it can be seen that the values reported for biochars are somewhat lower (Kong et al., 2011; Wang and Xing, 2007) than for the other carbonaceous geosorbents. However the reported partitioning coefficients for biochars
12
obtained from field wood samples (mixed wood and Populus spp. biochar from wildfires in Western Canada) (James et al., 2005), were in good agreement for those reported for soot, charcoal and coal (Jonker and Koelmans, 2002).
3.2. Effect of biochar feedstocks on freely dissolved PAHs content
The effect of various biochars on Cfree PAHs concentration is presented in Fig. 2. Previous research has shown (Chen et al., 2008; Kookana, 2010), that the affinity of organic contaminants to biochar may depend on feedstock material and pyrolysis conditions. These parameters have a direct impact on the elemental composition and specific surface area (SSA) of biochar. The addition of biochars obtained from various materials to sewage sludge caused a significant reduction of Cfree PAH content. Depending on the kind of biochar, the range of reduction of PAHs varied from 17.4 to 58.0%. The highest effectiveness, clearly divergent from the mean value (31.8%), was obtained for biochar produced from switchgrass (SC). In the case of that biochar the reduction of Cfree PAHs content varied, for the individual PAHs, from 38.3 to 69.0% (58.0% for total PAHs). Relatively large reductions in Cfree were obtained also for biochar produced from pinewood (PW) (from 30.8 to 56.7% for individual PAHs, 37.0% for total PAHs). The poorest effectiveness was observed for biochars produced from digested dairy manure (DDM) (from 1.9 to 34.0%, 17.4% for total PAHs) and from food waste (FW) (from 4.2 to 55.9%, 21.1% for total PAHs). As in the case of the dose, also after the application of various biochars the reduction of Cfree PAHs content was specific for the individual PAHs and did not depend on the molecular weight of the compounds studied. Figure 2 shows a reduction of Cfree of PHEN, PYR and BaP. In the case of PHEN and PYR variation between biochars was very similar to those observed for the total PAHs content.
13
However the freely dissolved BaP content did not differ between the majorities of the investigated biochars. DDM was only exception for which freely dissolved content did not differ significantly from sewage sludge without biochar. It is assumed that the parameters that may significantly determine the binding of organic contaminants, including PAHs, by geosorbents include carbon content, specific surface area and atomic ratio (Koelmans et al., 2006). Table S6 presents calculated coefficients of correlation between the properties of the biochars and the range of reduction of freely dissolved PAHs content. In no case statistically significant correlations were found between the content of C, H, N and SSA and the reduction of Cfree PAHs content. It should be emphasised that biochars characterised by the largest surface area (SG and PW) displayed also the highest levels of reduction of Cfree PAHs content (Fig. 2). However, the biochar characterised by the highest SSA (PW) reduced Cfree PAHs content only slightly better than biochars with notably smaller surface area (e.g. PMW or MG). Previous studies showed that pore-filling is one of dominant sorption mechanisms for HOCs to pyrolyzed chars (Chun et al., 2004). The absence of a distinct relationship, in this study, between SSA and reduction of Cfree PAHs content does not preclude a significant role of pore-filling and may be due to the small differences in SSA among the biochars (2-13 m2/g for FW, MG, DDM and PMW). Sewage sludge contains considerable amounts of organic matter and other substances that may significantly reduce the effectiveness of biochars through blocking access to the pores. With small values of SSA pores could have been clogged rapidly, limiting the role of that type of binding for contaminant sorption. In this situation a greater role may be played by surface effects. It has been also reported that the polarity and structure of polar aromatic compounds play an important role in the binding of contaminants by biochars. The elemental composition of each biochar was used to calculate atomic ratios as a predictor of their polarity
14
(Table S1). It has earlier been found that biochars with low values of O/C and (O+N)/C will be characterised by low polarity, and thus will effectively bind hydrophobic contaminants such as PAHs (Chun et al., 2004). This hypothesis is supported in this study, where the low values of O/C and (N+O)/C (0.06 and 0.07, respectively) were noted for biochar SC which was the most effective in reducing freely dissolved PAHs content. The next in this respect, biochar PW, was also characterised by low values of O/C and (N+O)/C (0.05 and 0.06, respectively) compared to the remaining biochars studied (Table S1). An important role of polarity in the binding of HOC is also supported by the studies by Chen et al. (2008). Those authors also observed that with lowering polarity there was an increase of the partition coefficient of HOC (naphthalene, nitrobenzene and m-nitrobenzene). In that case, the lack of distinct correlations for all of the biochars studied may, as in the case of SSA, result from small differences in the values of O/C and (N+O)/C among the remaining biochars. Tables S2-S5 present the values of log KBC of native compounds (thirteen PAHs) in sewage sludge amended with biochars (5%), determined for all the variants of the experiment. Log KBC values were 2 order higher than those for non-amended sewage sludge TOC for native PAHs (Fig. 3). Theoretically freely dissolved PAHs content reduction should be much higher than that observed in the present study. It is likely that the matrix (sewage sludge) has a strong effect on the sorptive properties of biochars. Depending on the properties of biochars, components of sewage sludge (including organic matter) may variously interact with biochars of diverse properties. It has been observed earlier for soils and sediments that the presence of a matrix notably reduced the sorptive properties of AC (Amstaetter et al., 2012; Cornelissen et al., 2006). The values of log KBC (Table S2-S5) obtained in this study in the presence of sewage sludge for the particular PAHs were lower by 1-2 orders of magnitude compared to the values determined for AC by (Amstaetter et al., 2012). This supports our earlier
15
observations where AC adsorbed PAHs more effectively than various BCs (Oleszczuk et al., 2012a).
3.3. Effect of biochars diameter on freely dissolved PAHs content
Due to the fact that the particle size of adsorbents may be significant in the estimation of their effectiveness in binding organic contaminants (Zimmerman et al., 2005), biochar MG was divided into fractions with particle sizes below 300 μm, from 300 to 500 μm, and above 500 μm. The effectiveness of reduction of freely dissolved PAHs content was clearly related to the size fraction (Fig. 4). Particles characterised by the smallest size reduced Cfree PAHs content at the level of 35.8% relative to sludge with no biochar content. That fraction (500 μm, were notably worse in this respect. After adding the medium size fraction (300-500 μm) to sewage sludge, the reduction of PAHs did not differ significantly from biochar not separated into the size fractions and amounted to 14.0%. The coarsest fraction (>500 μm) showed the same level of reduction of Cfree PAHs content. In the case of individual PAHs similar relationship as for the sum was observed only for PHEN. The freely dissolved PYR and BaA content was significantly reduced only by the smallest fraction of biochar (500 μm) Cfree PAHs content was not significantly different from sewage sludge without biochar (Fig. 4). As in earlier studies, no relation was noted between the molecular weight of the particular compounds and the reduction of Cfree PAHs content. As inferred from this lack of dependence of biochar effectiveness of PAH molecular size (discussed above), we rule out incomplete
16
equilibrium as a reason for the observation that smaller size fractions result in higher reductions of Cfree content. In contrast, a significant relation was observed between carbon content and the reduction of Cfree 4-ring PAHs content (Fig. 4). The effect of polarity observed earlier was also confirmed. In this case a significant relation was noted between reduction of Cfree PAHs content and the values of O/C and (O+N)/C ratios (Fig. 5). Studies conducted so far, concerned with particle size, are focused mainly on AC. It has been demonstrated (Lebo et al., 2003; Zimmerman et al., 2005) that particles characterised by smaller size are more effective in reducing the concentration of contaminants in aqueous phase than larger particles. Carbon materials with particle diameter from 75 to 300 µm, that can be considered as small particles of granulated active carbon, proved to be the most effective in immobilising HOCs (Brändli et al., 2008; Zimmerman et al., 2005). Better binding of contaminants by smaller particles is attributed primarily to their larger specific surface area, when most of the SSA is residing in internal pores (Pignatello et al., 2006). Values of SSA assayed in this study for biochar MG were lower in the case of smaller particles than in the case of larger ones (Table S1), but in spite of that it was still observed that the smaller particles (as in studies by other authors) displayed better binding of PAHs. The relation obtained (Fig. 5) indicates that lowering of polarity (increase of hydrophobicity) may be an important factor responsible for better effectiveness of the biochar in binding PAHs. In addition, better binding of PAHs by smaller biochar particles can be attributed to the fact that a greater number of particles are contained in a unit of mass of the sludge, and thus more particles have contact with the sludge (Brändli et al., 2008; Lebo et al., 2003).
17
3.4. Influence of pyrolysis temperature on freely dissolved PAHs content
Previous research has shown that the affinity of organic contaminants to biochar may vary depending on feedstock material and pyrolysis conditions, mainly temperature (Chen et al., 2008; Keiluweit et al., 2010; Sun et al., 2012). The temperature at which the biochars were produced also had a significant effect on the content of Cfree PAHs (Fig. 6). The addition of biochars produced at 350oC to the sludge had no significant effect on the level of Cfree PAHs content. Also, no significant effect was noted in the case of biochar PMW obtained at temperature of 400oC. Whereas, at temperature of 400oC a significant reduction (by 15.9%) of Cfree PAHs content was observed after the application of biochar DDM to the sludge. The effect observed most probably results from the higher content of carbon in DDM compared to PMW (Table S1). The application to the sludge of biochars produced at temperatures of 500 and 600oC caused significant reduction of Cfree PAHs content, by 22.9% (DDM) and 27.3% (PMW) and by 17.4% (DDM) and 31.0% (PMW), respectively. Those values did not differ significantly among the particular biochars for a given temperature. Biochars made at low temperatures have amorphous structure and contain considerable proportions of ‘soft’ aliphatic domains (Chun et al., 2004). Under such conditions, partitioning is the main mechanism of binding HOC (Sun et al., 2012). Increase of temperature (to 500 and 600oC) causes a change from the more flexible aliphatic phase to a more rigid and condensed aromatic phase (Chen et al., 2008), which is confirmed by the decreasing value of H/C (Table S1, for PMW). It is likely, therefore, that the reduction of Cfree PAHs content is related primarily to the binding of PAHs present in sewage sludge mainly by the condensed aromatic phase. Bonds of that type are strong (Pignatello and Xing, 1996), which from the practical point of view may be an argument for the effectiveness of the method.
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4. CONCLUSIONS
The increasing production of sewage sludges and the necessity of finding a safe solution regarding their utilisation requires the search for new alternative methods. The presence of organic contaminants in sludges may create a threat to the environment and restrict their utilisation. The addition of biochar to sludge, reduces the freely dissolved PAHs content. The reduction of the bioavailable fraction of sewage sludge, responsible for the toxic effect and at the same time mobile, will expand the possibilities of its potential utilisation. This may find an application in situations where sewage sludge is used for the reclamation of degraded areas.
Acknowledgements
The project was funded by the National Science Centre granted on the basis of the decision number DEC-2012/07/E/ST10/00572. Johannes Lehmann and Kelly Hanley (Cornell University) and David W. Rutherford (US Geological Survey, Denver) are thanked for kindly providing biochar samples. GC thanks the Research Council of Norway for a FRIPRO personal stipend (217918/F20). Sarah E. Hale (NGI) is thanked for fruitful discussion and advice in the laboratory.
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Freely dissolved PAHs sum content (ng/L)
70
ΣPAHs
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PHEN
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Figure 1. The effect of biochar dose on freely dissolved PAHs content in sewage sludge Freely dissolved PAHs concentration [ng/L]
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ΣPAHs
a 60
35
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8 c
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Sewage sludge with biochar
PMW
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Sewage sludge with biochar
SL
MG
PMW
DDM
FW
PW
Sewage sludge with biochar
Figure 2. Influence of different biochars (5% dose) on freely dissolved PAHs content in sewage sludge. Sewage sludge (SL), corn stove (MG), paper mill waste (PMW), manure waste (DDM), food waste (FW), pinechar (PC) and switchgrass (SC).
8
7
Log KBC
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KBC-PMW KBC-DDM
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KTOC-SL LFER 2 2
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Figure 3. Kowvs KTOC for sewage sludge and KBC for different biochars calculated with a nonlinear Freundlich isotherm (details about calculations are presented in supporting information)
25
SC
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ΣPAHs
Freely dissolved PAHs content [ng/L]
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Biochar diameter
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Figure 4. Influence of biochar diameter (biochar MG) on freely dissolved PAHs content in sewage sludge
O/C or (O+N)/C ratio
4
3
2
1
O/C (O+N)/C yO/C=3.929x-0.106 y(O+N)/C=4.006x-0.108
0 0
10
20
30
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% of freely dissolved PAHs content reduction
Figure 5. The relationship between reduction of freely dissolved PAHs content and polarity index of different particle size of biochar
Freely dissolved PAHs concentration [ng/L]
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DDM PMW 60
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PYR
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Temperature of biochar preparation [ C]
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Figure 6. Effect of biochar temperature preparation on freely dissolved PAHs content in sewage sludge. * statistically significant different (P
