Article pubs.acs.org/JAFC
Sewage Sludge Polycyclic Aromatic Hydrocarbon (PAH) Decontamination Technique Based on the Utilization of a Lipopeptide Biosurfactant Extracted from Corn Steep Liquor Xanel Vecino, Lorena Rodríguez-López, Jose M. Cruz, and Ana B. Moldes* Chemical Engineering Department, School of Industrial Engineering (EEI), University of Vigo, Campus As Lagoas-Marcosende, 36310 Vigo, Pontevedra, Spain ABSTRACT: A decontamination technique based on the utilization of a lipopeptide biosurfactant extracted from corn steep liquor has been developed to eliminate polycyclic aromatic hydrocarbons (PAHs) from sewage sludge. High concentrations of PAHs were used during experiments observing that 408.3 mg/kg of naphthalene was almost completely mobilized and biodegraded, only 1.7% of naphthalene remained in the sewage sludge, whereas anthracene and pyrene were reduced up to 51.7 and 69.4%, respectively. The biodegradation of PAHs was fitted to several kinetic models (zero- and first-order kinetic models), observing good correlation coefficient values when biodegradation was described by the first-order kinetic model. Experimental results suggest that biosurfactant extracted from corn steep liquor may have great potential, as an ecofriendly washing agent, for the treatment of sewage sludge contaminated with PAHs. Therefore, in situ application of natural biosurfactants may be considered to be a good remediation alternative as they are not hazardous for water and soil organisms. KEYWORDS: sewage sludge, PAHs, biosurfactant, solubilization, biodegradation
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INTRODUCTION Sewage sludge is originated in the treatment of wastewater either after the physical−chemical treatment (primary sludge) or after biological digestion occurs (secondary sludge). The organic matter and nutrients present in sewage sludge make the dispersal of this kind of waste suitable as fertilizer or as organic soil improver; for instance, 68% of sewage sludge is used as soil amendment in Spain,1,2 whereas in other European countries, such as Poland, 40% of sewage sludge is applied in agriculture.3 However, the presence of emerging pollutants, such as heavy metals, polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins/dibenzofurans (PCDD/Fs), and PAHs (polycyclic aromatic hydrocarbons), in the sewage sludge could contaminate terrestrial and aquatic environment when the sludge is used in farming.2 Some authors4 have emphasized that PAHs can be concentrated in the sewage sludge during the treatment of water, achieving harmful environment concentrations of about 79 mg/kg. Furthermore, other authors5 support the hypothesis that total organic carbon plays an important role in the retention of PAHs in soil and that PAHs are often combined with black carbon during combustion emissions. Navalón and Valor1 have pointed out that 9.66 tons/year of PAHs and 0.07 tons/year of PCBs are released to environment in Spain due to the utilization of sewage sludge as soil amendment. These values change if all inputs of PAHs are taking into account; in this case 150 tons/year of PAHs are released in Spain to the environment. PAHs from sewage sludge represent about 6% of the total, soil and sediments being those environmental sites that support higher concentrations of PAHs (46%) and PCBs (53.6%). PAHs are included in the priority pollutants list of the U.S. Environmental Protection Agency (EPA)6 and in the European Union (EU) list of persistent organic pollutants (POPs).7 In © XXXX American Chemical Society
addition, several member states have legislated and implemented stricter limit values for heavy metals and set requirements for other contaminants in the sewage sludge. Therefore, limit values for PCBs, PDCDD/Fs, and adsorbable organic halogens (AOX) on sewage sludge are defined in German legislation. Thus, to prevent the accumulation of organic and recalcitrant pollutants in environmental positions, maximum concentrations of 6 mg/kg of PAHs in sewage sludge, as the sum of acenaphthene, fluorene, phenanthrene, fluoranthene, pyrene, benzo[b+j+k]fluoranthene, benzo[a]pyrene, benzo[ghi]perylene, and indeno[1,2,3-c,d]pyrene, have been regulated in the proposal for a Directive of the European Parliament and of the Council about dispersal of sludge on land.8 A recent study9 related with the PAH content in 32 sewage sludge samples from 15 EU countries showed that the lowest content of PAHs was observed in sewage sludge from Romania, whereas the highest values were observed in sewage sludge from France and the United Kingdom. Results from Italy were generally consistent with those reported from Poland, Czech Republic, or Turkey. On the other hand, biosurfactants are surface-active compounds with both hydrophobic and hydrophilic domains that are capable of lowering the surface tension and the interfacial tension of aqueous solutions. The low water solubility of PAHs prevents their biodegradation, which is a potential problem for the bioremediation of contaminated sites. Microbially produced surfactants can enhance the mobilization and solubilization of these hydrophobic compounds, allowing Received: May 20, 2015 Revised: July 20, 2015 Accepted: July 23, 2015
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DOI: 10.1021/acs.jafc.5b02346 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Table 1. Data Related to the Concentration of PAHs Solubilized, Mobilized, and/or Biodegraded from Sewage Sludge at the End of Treatment (27 Days) sample
naphthalene
anthracene
pyrene
initial concn of PAHs in sludge (mg/kg) concn of PAHs that can be achieved in the washing solution (solid/liquid ratio 1:10 v/v) (mg/L) concn of PAHs in washing solution after treatment (mg/L) AcN BS PAHs in the washing solution after treatment (%) AcN BS amount of PAHs that remain in sewage sludge after treatment (mg/kg) AcN BS PAHs that remain in sewage sludge after treatment (%) AcN BS amount of PAHs mobilized (mg/kg) AcN BS amount of PAHs mobilized (%) AcN BS amount of PAHs biodegraded (mg/kg) AcN BS PAHs biodegraded (%) AcN BS
408.34 ± 28.58 40.83 ± 2.86
368.83 ± 22.13 36.88 ± 2.21
561.30 ± 44.90 56.13 ± 4.49
17.78 ± 1.24 nda
36.90 ± 2.21 7.26 ± 0.44
56.20 ± 4.50 12.73 ± 1.02
43.55 ± 3.05 nd
100.05 ± 6.00 19.68 ± 1.18
100.13 ± 8.01 22.68 ± 1.81
230.52 ± 16.14 6.92 ± 0.48
nd 190.81 ± 11.45
nd 389.32 ± 31.15
56.45 ± 3.95 1.69 ± 0.12
nd 51.73 ± 3.10
nd 69.36 ± 5.55
177.81 ± 12.45 401.42 ± 28.10
368.83 ± 22.13 178.02 ± 10.68
561.30 ± 44.90 171.98 ± 13.76
43.55 ± 3.05 98.31 ± 6.88
100.00 ± 6.00 48.27 ± 2.90
100.00 ± 8.00 30.64 ± 2.45
nd 401.42 ± 28.10
nd 105.45 ± 6.33
nd 44.68 ± 3.57
nd 98.31 ± 6.88
nd 28.59 ± 1.72
nd 7.96 ± 0.64
a
nd, not detected.
processes will be performed using a low-cost biosurfactant extracted form corn steep liquor, and the biodegradation of the persistent organic pollutants will be studied by applying different kinetic models.
their biodegradation. Biosurfactants possess different chemical structures consisting of glycopeptides, lipopeptides, glycolipids, neutral lipids, and fatty acids and in comparison with chemical surfactants they are less toxic, more biodegradable, and effective at extreme operational conditions.10 Some biosurfactants also are able to stabilize emulsions between hydrophilic and hydrophobic solutions. For instance, biosurfactants produced by Lactobacillus pentosus exhibited a strong emulsion capacity when added to kerosene/water emulsions,11 gasoline/water emulsions,12 octane/water emulsions,13 rosemary oil/water emulsions,14 and fluorene/water emulsions.15 In the literature have been reported several works about the utilization of chemical surfactants for the bioremediation of contaminated soils16,17 because these are cheaper and more available to the consumer than biosurfactants produced biotechnologically. Therefore, few works deal with the utilization of biosurfactants for the biocorrection of contaminated soils or sludge.18 Shin et al.19 have used a rhamnolipid biosurfactant produced by the genus Pseudomonas to remediate phenanthrene-contaminated soil by the combination of solubilization and biodegradation processes, whereas Das and Mukherjee20 studied the utilization of pyrene as the sole source of carbon by Bacillus subtilis and Pseudomonas aeruginosa strains as well as the role of biosurfactants in enhancing the bioavailability of this organic contaminant, although most works focus on studying the solubilization of PAHs in aqueous solutions in the presence of biosurfactants.21,22 This work aims to study the bioremediation of sewage sludge provided by a municipal wastewater treatment plant and contaminated with high doses of PAHs. The bioremediation
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MATERIALS AND METHODS
Reagents. Anthracene (99%) and pyrene (98%) were purchased from Acros Organics (Morris Plains, NJ, USA). Naphthalene, acetonitrile (supergradient HPLC grade), and acetone (synthesis grade for contamination of sewage sludge) were supplied by Scharlau (Barcelona, Spain). Chloroform (stabilized with 50 ppm of amylene as preservative, 99.8%) was purchased from Panreac (Barcelona, Spain). Corn steep liquor, hexane (Chromasolv, HPLC, ≥97.0%), and acetone (puriss. p.a., ACS reagent, reag. ISO, reag. Ph. Eur., ≥99.5% for ASE extraction) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Nitrogen (Extrapure 3× (high purity, >99.998%)) was supplied by Praxair (Spain). Characteristics of the Sewage Sludge. Composition of Sewage Sludge. For the determination of inorganic carbon (IC) 0.5 g of sewage sludge was introduced in a muffle at 400 °C during 3 h to eliminate the organic matter contained in the sewage sludge samples, whereas for the determination of organic carbon (OC) and total nitrogen (TN) samples did not undergo this process. An Elemental Analyzer (LECO, CN-2000) was used for carbon and nitrogen determination using infrared (IR) detection and thermal conductivity detection (TCD), respectively. Scanning Electron Microscope (SEM) Images of Sewage Sludge. Samples were dried, covered with gold, and then observed using a scanning electron microscope (JEOL JSM-6700F FEG), operating at an acceleration voltage of 5.0 kV for secondary-electron imaging (SEI). Optical Images of Sewage Sludge. The macroviews of sewage sludge samples treated with water, acetonitrile, or biosurfactant B
DOI: 10.1021/acs.jafc.5b02346 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Journal of Agricultural and Food Chemistry solution were obtained with a regular digital camera (Sony optical SteadyShot) using an Optical Zoom 4× with 26 mm wide-angle lens and 14.1 megapixels. Contamination of Sewage Sludge. Sewage sludge used in this work was obtained after a secondary wastewater treatment in a local water treatment plant in Galicia (Spain), which was contaminated with a mixture of naphthalene, anthracene, and pyrene. Hence, 250 g of sewage sludge was contacted with different PAHs solubilized in acetone and stirred vigorously for 30 min to promote the homogeneous distribution of persistent organic pollutants. Before mobilization and biodegradation of PAHs was begun, the sewage sludge samples were left to rest for 48 h at 28 °C, to eliminate all of the acetone, in a shaker at 150 rpm. Table 1 includes the initial concentration of PAHs achieved in the sewage sludge after the acetone evaporation. Characteristics of the Biosurfactant Solution. The biosurfactant used in this work was obtained from a liquid stream of the corn milling industry usually called corn steep liquor (CSL). Prior to the preparation of washing solution, biosurfactant was extracted from CSL, with chloroform, using the methodology proposed by Vecino et al.23 The efficiency of a biosurfactant is determined by its critical micelle concentration (CMC), which is the point at which micelles start to form; hence, biosurfactant washing solution was formulated with concentration over 2.5 times the CMC (998.5 mg/L). Mobilization and Biodegradation of PAHs Contained in Sewage Sludge. Sewage sludge (10 g) samples contaminated with PAHs were placed in 250 mL Erlenmeyer flasks together with 100 mL of an aqueous solution containing the lipopeptide biosurfactant. The experiments were conducted in a shaker at 150 rpm, for 27 days at 30 °C. Moreover, acetonitrile (AcN) was used as solvent control extractant under the same conditions. Analysis of PAHs in the Sewage Sludge by Accelerated Solvent Extraction Process. Sewage sludge samples (around 5 g) were submitted to an extraction process using an accelerated solvent extraction system (Dionex-ASE 200). The empty spaces of extraction tubes were filled with diatomaceous earth. The cell contents (11 mL extraction cell) were extracted during four cycles of 5 min, at 100 °C and 1500 psi, with a hexane/acetone 1:1 (v/v) flush volume of 6 mL (60% of ASE cell capacity) and purged with nitrogen (60 s). The extracts were evaporated to dryness under a gentle stream of nitrogen gas in a TurboVap station (at 35 °C and 12 psi). Subsequently, samples were transferred by adding acetonitrile to reach a final volume of 5 mL, filtered through a 0.45 μm syringe filter, and injected into the HPLC-DAD-FLD system. Quantification of PAHs in the Liquid Phase by HPLC-DADFLD. To analyze the PAHs mobilized to the liquid phase, sewage sludge samples were removed from the washing solution consisting of acetonitrile or biosurfactant, by filtration (PTFE syringe filter). Subsequently, PAHs solubilized in the liquid phase were identified and quantified by high-performance liquid chromatography (HPLC) with diode array detection (DAD) and fluorescence detection (FLD) (Agilent Technologies 1200 series, Germany) using an Envirosep PP column (125 × 4.60 mm) (Phenomenex, Torrance, CA, USA) with the column oven kept at 35 °C. The mobile phase consisted of Milli-Q water/acetonitrile (1:1), the injection volume was 20 μL, and the flow rate was 2 mL/min.
Table 2. Elemental Analysis of Sewage Sludge at the Beginning of the Bioremediation Process and after Treatment with Acetonitrile (AcN) or Biosurfactant (BS) sample
total N (%)
inorganic C (%)
organic C (%)
sewage sludge contaminated sewage sludge treated with AcN sewage sludge treated with BS
5.39 ± 0.38
3.15 ± 0.22
29.75 ± 2.08
5.90 ± 0.47
3.19 ± 0.26
32.02 ± 2.56
4.51 ± 0.27
1.80 ± 0.11
28.39 ± 1.70
organic and inorganic carbon at levels lower than those achieved in the untreated sewage sludge. The same behavior was observed regarding the nitrogen percentage. Figure 1 shows the FTIR spectra of sewage sludge before and after treatment with acetonitrile or biosurfactant, showing a decrease in the main bands for treated samples in comparison with untreated sewage sludge. In addition Figure 2 shows SEM images of untreated and treated sewage sludge; samples containing biosurfactant gave a more homogeneous aspect, with less roughness in comparison with untreated sewage sludge or samples treated with acetonitrile. On the other hand, Figure 3 shows the macroview aspect of sewage sludge when it was introduced in water, acetonitrile, or biosurfactant solution. It was observed that biosurfactant allowed the homogenization of sewage sludge in water and stabilized the emulsion of the aqueous phase with the sewage sludge. This aspect is consistent with the pictures shown in Figure 2. Otherwise, when the sewage sludge was dissolved in acetonitrile, a clear separation between solid samples and the organic phase was observed (Figure 3b); similar behavior was detected when sewage sludge was dissolved in water in the absence of biosurfactant (Figure 3a). Bioremediation of Sewage Sludge Contaminated with PAHs. A bioremediation experiment for the decontamination of sewage sludge containing PAHs was developed during 27 days in the presence of a lipopeptide biosurfactant extracted from CSL. During the experiments, solid (sewage sludge) and liquid samples were obtained at several intervals of time to follow the solubilization and the kinetic biodegradation of PAHs. A control consisting of contaminated sewage sludge treated with acetonitrile was included in the study to compare the efficiency of treatment, using biosurfactant, with classical and less ecofriendly washing methodologies. Table 1 includes data about the amount of PAHs solubilized and biodegraded from sewage sludge as well as the amount of PAHs that remain in the sewage sludge at the end of the treatment (27 days), in comparison with the initial concentration of PAHs contained in the sewage sludge. The amount of PAHs mobilized from sewage sludge was calculated as the difference between the amount of PAHs contained in the sewage sludge at the beginning of the experiment and the amount of PAHs at specific interval of times that remained in the sewage sludge. In Table 1 are included the maximum amounts of PAHs mobilized after 27 days of treatment with acetonitrile and biosurfactant. Furthermore, Table 1 shows the maximum concentrations of PAHs that could be achieved in the washing solution. These values were calculated by taking into account the amount of PAHs in the sewage sludge before the bioremediation experiments and the liquid−solid ratio used during the bioremediation experiments.
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RESULTS AND DISCUSSION Physicochemical Characterization of Treated Sewage Sludge. In this work, sewage sludge was submitted to a bioremediation process in the presence of lipopeptide biosurfactant, extracted from corn steep liquor, for the removal of naphthalene, anthracene, and pyrene. Table 2 shows the elemental analysis of sewage sludge under different conditions. It was observed that contaminated samples treated with acetonitrile gave the highest percentages of organic and inorganic carbon, whereas the contaminated sewage sludge samples treated with biosurfactant reduced the concentration of C
DOI: 10.1021/acs.jafc.5b02346 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Figure 1. Comparison of the FTIR spectra of sewage sludge () without treatment, (- - -) treated with acetonitrile, and (···) treated with biosurfactant.
Figure 2. SEM images of the sewage sludge, at ×500 magnification, (a) without treatment, (b) treated with acetonitrile, and (c) treated with biosurfactant.
Figure 3. Optical images of sewage sludge (a) in water, (b) in acetonitrile, or (c) in biosurfactant solution.
sewage sludge. Thus, the biosurfactant, extracted from CSL, not only promoted the solubilization of PAHs from sewage sludge but also induced the biodegradation of these organic contaminants. For instance, from the 48.3% of anthracene removed from sewage sludge, 19.7% remained in the aqueous phase and 28.6% was biodegraded, whereas 100% of the solubilized naphthalene was biodegraded. In the current work, it was observed that acetonitrile was able to remove all anthracene and pyrene from sewage sludge, although they remained undigested in the aqueous phase, whereas acetonitrile was able to extract only 43.5% of naphthalene, probably due to the stronger adsorption ability of sewage sludge for naphthalene rather than anthracene and pyrene. Oluseyi et al.24 also observed that methanol and acetonitrile gave lower extraction yields for naphthalene in comparison with anthracene and pyrene.
In addition, Figures 4 and 5 show the variation in PAH percentages contained in the sewage sludge or in the washing solution, as well as the biodegraded PAH percentages in experiments carried out with acetonitrile or biosurfactant, respectively, at different intervals of time. At the end of the bioremediation experiments the concentration of PAHs in the sewage sludge, in the presence of biosurfactant, was reduced about 98.3, 48.3, and 30.6% for naphthalene, anthracene, and pyrene, respectively, after 27 days of treatment. Probably PAHs were removed from sewage sludge by combining desorption mechanisms induced by the ambiphilic nature of the biosurfactant and the biodegradative processes carried out by the microbial biomass contained in the sewage sludge. Biosurfactants reduce the surface tension between the aqueous phase of sewage sludge and the hydrophobic contaminants through the formation of micelles favoring the sequential solubilization and biodegradation of PAHs contained in the D
DOI: 10.1021/acs.jafc.5b02346 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Figure 4. Variation of PAH percentages in sewage sludge and in the washing solution as well as the biodegradation percentages of PAHs in samples treated with acetonitrile.
Figure 5. Variation of PAH percentages in sewage sludge and in the washing solution as well as the biodegradation percentages of PAHs in samples treated with biosurfactant.
Remediation methods relying on the immobilization of contamination or its removal may be carried out in place of its formation (in situ) or out of the place of its original location (ex situ).18 Organic solvents were the first agents used for extracting PAHs from soil, both in laboratory-scale studies and in field scale remediation projects.25 Some authors have proposed the utilization of physical−chemical methods to remove polycyclic aromatic hydrocarbons from sewage sludge,26 but in comparison with the utilization of biosurfactants13 or fermentation process,27,28 these methods are less ecofriendly. Bacteria able to form biosurfactants are of increasing interest as biosurfactants, have low toxicity, and are easily biodegraded.18 Moreover, it has been reported that some species, such as white rot fungi (e.g., Pleurotus ostreatus), allow high efficiency in the cleanup of soil contaminated with PAHs.29 There are few works in the literature dealing with the direct biodegradation of PAHs in soil or sewage sludge; thus, to discuss the results obtained in the current work, references related with the biodegradation of PAHs in aqueous streams
will be introduced. Once the PAHs contained in sewage sludge achieve the aqueous phase, the biodegradation of these pollutants could follow a behavior similar to that observed in aqueous streams, obviously with lower biodegradation kinetic rates. Thus, Lima et al.30 performed consecutive washings of soil contaminated with 200 mg/kg of phenanthrene using different lipopeptide biosurfactants, observing that 80−88% of phenanthrene from soil can be removed using lipopeptide biosurfactant as washing solution. The results obtained in the current study also are consistent with those achieved by other authors.31,32 Thus, Jacques et al.31 reported biodegradation values for pyrene between 24.8 and 36.8%, for phenanthrene between 31.4 and 48.2%, and for anthracene between 24.4 and 71.1% in the presence of Pseudomoma species isolated from a petrochemical sludge land farming site, which produced biosurfactants. Additionally, Obayori et al.32 found that pyrene degradation rates were increased when media were supplemented with CSL, probably not only because the presence of nutrients in CSL can induce the growth rate of Pseudomona but also due to the presence in E
DOI: 10.1021/acs.jafc.5b02346 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Journal of Agricultural and Food Chemistry CSL of biosurfactants that increased the solubilization and biodegradation of pyrene, achieving pyrene biodegradation percentages about 55.5% after 21 days of treatment. Biodegradation of PAHs: Kinetic Study. Microbial degradation of organic compounds can be defined following kinetic models, typically used to describe the consumption of reagents during a chemical reaction.33−36 Usually, microbial degradation of different carbon sources tends to be hyperbolic saturation functions and can be expressed by eq 1: rp = rpmax
C +C K
(1)
where rp is the biodegradation rate, rpmax is the maximum specific biodegradation rate, C is the pollutant concentration, and K is the half-saturation coefficient. Assuming that the pollutant concentration is much lower than the half-saturation coefficient (C ≪ K), eq 1 can be converted to eq 2: rp = rpmax
C K
Figure 6. Kinetic plots achieved for the biodegradation of different PAHs, contained in sewage sludge, in the presence of biosurfactant extracted from corn steep liquor.
(2)
related with the initial concentration of PAHs in the solid matrix. It was observed that naphthalene gave the lowest halflife value (around 5 days), whereas the half-life for biodegrading anthracene and pyrene was 47 and 182 days, respectively. Halflife values obtained in this work are higher than those achieved by other authors who studied the biodegradation of phenanthrene and pyrene contained in aqueous solutions in the presence of surfactants such as Tween 80 or Tween 20.36 However, it is necessary to take into consideration that in the current work the biodegradation occurs from a solid substrate (sewage sludge); thus, PAHs have to be first mobilized from sewage, solubilized, and then biodegraded and, therefore, it is obvious that the biodegradation of PAHs adsorbed in a solid matrix has to be slower than the biodegradation of any carbon source contained in an aqueous stream. Jianlong et al.33 also described the microbial biodegradation of phthalic acid esters, contained in mixed anaerobic digested sludge collected from a local wastewater treatment plant, by a first-order kinetic model. They observed that the biodegradation rate and biodegradability of three phthalates under anaerobic conditions appeared to be related to the length of the alkyl side chains. Thus, >90% of dimethyl and di-n-butyl phthalate were biodegraded, whereas the biodegradation of dinoctyl phtalate was limited, with a halflife value of 21 days. Some authors have reported that the half-life of hydrocarbons in biotic conditions was 17−126 days, whereas in abiotic conditions this ranged increased to 32−2048 days.3 In other work, Jianlong et al.37 studied the biodegradation of four phthalic acid esters (dimethyl phthalate, diethyl phthalate, di-n-butyl phthalate, and di-n-octyl phthalate) in soil supplemented with acclimated sludge, observing that those phthalic acid esters with a shorter alkyl chain were biodegraded more quickly than phthalic acid esters with higher alkyl chains. These results are consistent with the data achieved in the current work during the biodegradation of PAHs contained in sewage sludge, in the presence of biosurfactant, observing that those more complex PAHs were biodegraded more slowly. In the present work it was also observed that PAHs with a lower carbon chain length (naphthalene) gave higher biodegradation rates than anthracene and pyrene. Jianlong et al.37 also observed that the biodegradation of phthalic acid esters followed a first-order reaction kinetic
Furthermore, if K1 = rp/K, a linearized first-order kinetic reaction can be shown, eq 3: ln C = C0 + K1t
(3)
Hence, following the first-order kinetic reaction, the biodegradation half-life can be calculated according to eq 4: t1/2 =
ln 2 K1
(4)
On the other hand, taking into consideration eq 1, if it is assumed that the amount of PAHs contained in the sewage sludge is much higher than the half-saturation constants (C ≫ K), the biodegradation of PAHs could be described following eq 5, where the biodegradation rate is independent of the concentration of PAHs (rp = rpmax). Thus, increasing the concentration of PAHs will not increase the biodegradation rate of these pollutants; hence, the amount of PAHs biodegraded will be proportional to the time. If the biodegradation rate is constant, K0 = rp approaching a zero-order kinetic model is described by eq 5:
C = C0 + K 0t
(5)
where C0 is the initial concentration of PAHs and K0 is the zero-order kinetic constant. For a zero-order reaction the halflife is given by eq 6: t1/2 =
C0 2K 0
(6)
Figure 6 shows the description of naphthalene, anthracene, and pyrene biodegradation by first-order kinetic model, observing a good agreement between the experimental data and the theoretical results predicted by the model with relative high correlation coefficient values. Moreover, Table 3 includes the kinetic parameters (C0, K0, K1, and t1/2) predicted by the equation as well as the relative coefficients (R2). Otherwise, when the biodegradation of PAHs was adjusted to a zero-order kinetic model, a good concordance was not observed between experimental and theoretical data as can be deduced by the correlation coefficients included in Table 3. Thus, the biodegradation of PAHs in sewage sludge will be directly F
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Journal of Agricultural and Food Chemistry Table 3. Kinetic Parameters for Zero- and First-Order Kinetic Models first-order kinetic model
zero-order kinetic model PAH
C0 (mg/L)
K0 (day−1)
t1/2 (days)
R2
C0 (mg/L)
K1 (day−1)
t1/2 (days)
R2
naphthalene anthracene pyrene
213.14 364.15 559.81
−10.70 −4.61 −2.05
9.96 39.52 136.62
0.44 0.86 0.82
5.24 5.90 6.33
−0.136 −0.015 −0.004
5.08 47.51 182.43
0.88 0.88 0.82
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equation with a biodegradation half-life that decreased significantly with increasing carbon number of the alcohol moiety and correlated with the alkyl chain length and their octanol−water partition coefficients. Other authors36 proposed these models to explain the biodegradation of phenanthrene and pyrene in the presence of nonionic surfactants produced by chemical synthesis. These authors observed low correlation coefficient values when trying to describe the biodegradation of phenanthrene and pyrene in the presence of Tween 20 and Tween 80 at different concentrations applying zero-order kinetic model; however, they found relatively high correlation coefficient values when applying the first-order kinetic model to describe the biodegradation of both PAHs, and the same behavior was observed in the current work. Furthermore, related with the kinetic description of PAH biodegradation, Lu et al.35 found that the biodegradation of two- and three-ring PAHs can be described by a first-order kinetic model, whereas the biodegradation of four-ring PAHs followed a zero-order kinetic model. Biosurfactants have the ability to increase apparent aqueous solubility and bioavailability of PAHs, and they can be used to stimulate the biodegradation of these persistent contaminants contained in sewage sludge. The utilization of biosurfactants as washing agent not only has the advantage of allowing the solubilization of hydrophobic contaminants but also can promote their simultaneous biodegradation. Therefore, the elimination of PAHs from sewage sludge using biosurfactants could allow the safe use of this sewage sludge as fertilizer or as organic soil improver in agriculture.
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AUTHOR INFORMATION
Corresponding Author
*(A.B.M.) Phone: (34) 986812022. Fax: (34) 986812201. Email:
[email protected]. Funding
We thank the Spanish Ministry of Economy and Competitiveness (this work was funded by FEDER funds under Project CTM2012-31873). Notes
The authors declare no competing financial interest.
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