CEST2013 – Athens, Greece
Ref no: cest13_0664
A NEW INTEGRATED APPROACH TO REMOVE PAHs FROM HIGHLY CONTAMINATED SOIL: SOIL WASHING COMBINED TO ELECTRO-FENTON PROCESS AND POSSIBLE POST-BIOLOGICAL TREATMENT E. MOUSSET1, D. HUGUENOT1, E.D. VAN HULLEBUSCH1, N. OTURAN1, G. GUIBAUD2, G. ESPOSITO3, and M.A. OTURAN1 1
Université Paris-Est, Laboratoire Géomatériaux et Environnement, EA 4508, UPEMLV, 5 bd Descartes, 77454 Marne-la-Vallée Cedex 2, France. e-mail:
[email protected] 2 Université de Limoges, Faculté des Sciences et Techniques, Groupement de Recherche Eau Sol Environnement - EA 4330, 123 Avenue A. Thomas, 87060 Limoges Cedex, France. 3 University of Cassino and the Southern Lazio, Department of Civil and Mechanical Engineering, Via Di Biasio, 43 - 03043 Cassino (FR), Italy. EXTENDED ABSTRACT
In the present study, a new integrated process is suggested for soil remediation: soil washing (SW) process combined to electro-Fenton treatment and a possible postbiological treatment. The studied matrix is a real Polycyclic Aromatic Hydrocarbons (PAHs) and hydrocarbons contaminated soil. The total PAHs content and total hydrocarbons (C10-C40) content are 730 mg kg-1 and 850 mg kg-1, respectively. The pollutant monitored during extraction study are the following PAHs: acenaphthene (ACE) and phenanthrene (PHE) (3 rings), fluoranthene (FLA) and pyrene (PYR) (4 rings), benzo(a)pyrene (BAP) (5 rings) and benzo(g,h,i)perylene (DBP) (6 rings). Two extracting agents are compared in SW processes: a traditional surfactant such as Tween 80® (10 g L-1) and the most cost-efficient cyclodextrin, hydroxypropyl-beta-cyclodextrin (HPCD) (10 g L-1). Successive SW treatments are performed. The average extraction efficiency is much better in the case of Tween 80® (about 86%) compared to HPCD (about 5%). These SW solutions with Tween 80® and HPCD have a Chemical Oxygen Demand (COD) of 14,500 and 10,250 mg O2 L-1, respectively. The destruction of organic pollutants contained in these soils washing solutions needs the use of an efficient treatment process able to overcome the high COD values. An emerging Electrochemical Advanced Oxidation Process (EAOP) called electro-Fenton is suggested to perform the treatment. The mineralization of HPCD solutions is achieved after 20 h of electro-Fenton treatment, compared to 28 h in the case of Tween 80® solutions. The toxicity measurements have shown formation of higher toxic by-products during the first hours of treatment in both cases. On contrast to HPCD solution, still few toxic compounds are present in Tween 80® solutions after more than 98% of mineralization. However, electrochemical treatments can be energy consuming. A post-biological treatment could be considered after reaching a minimum threshold value of biodegradability during an electro-Fenton process as pretreatment. Usually, a biodegradability ratio expressed by Biochemical Oxygen Demand after 5 days (BOD5) / COD higher than 33% is enough to consider a biological treatment of an industrial effluent. This ratio is reached after 7 h and 20 h of electro-Fenton treatment of HPCD and Tween 80® solutions respectively. This leads to an energy consumption around 2.8 times higher with Tween 80® solution compared to HPCD ones. Knowing that the extraction efficiencies are 18 times higher with Tween 80® compared to HPCD and that Tween 80® is still cheaper than HPCD, it is still interesting to consider the use of Tween 80®. Keywords: Soil remediation, Surfactant, Tween 80®, Cyclodextrin, HPCD, Advanced oxidation processes, Chemical oxygen demand, Biodegradability, Microtox®.
Please do not use page numbers
1.
INTRODUCTION
Soil contamination by hydrophobic organic compounds (HOCs) such as Polycyclic Aromatic Hydrocarbons (PAHs) are a common concern. They are extremely difficult to remove since they are strongly bounded to soil. Moreover their potential toxicological impacts are significant (Chu and Chan, 2003). SW with extracting agents like Tween 80®, which is one of the most efficient non-ionic surfactant (Gómez et al., 2010), is widely studied and used in this process. Besides, cyclodextrin (host/guest molecules) that are known to be employed in pharmaceutical industry as a drug carrier, has been suggested as an alternative in the last decade (Mousset et al., in press). Since the enhanced SW processes only permit to extract pollutants but not to destroy it, a post-treatment is needed to treat these highly loaded solution (chemical oxygen demand higher than 10,000 mg O2 L-1). Advanced oxidation processes (AOPs), which involve the in-situ generation of a very powerful oxidizing agent such as hydroxyl radical (•OH), have shown promising results and are considered as environmentally friendly methods (Brillas et al., 2009). Electro-Fenton process can be a cost-effective post-treatment of highly loaded effluent. It is an electrochemical advanced oxidation process (EAOP), requiring Fenton’s reaction (Eq. 1): (1) Fe2+ + H 2O2 → Fe3+ + •OH + −OH It permits to avoid the use of chemical reagents (except a catalyst), since H2O2 is generated in situ and a catalytic amount of soluble iron is sufficient because it is continuously electro-regenerated at the cathode (Oturan, 2000). Thanks to these enhancements, higher degradation rate and mineralization degree of organic pollutants and no sludge production are observed. However, the treatment time until the complete COD removal can be long and energy consuming. In order to minimize the time of electrochemical treatment a post biological treatment could be considered if the BOD5/COD ratio is sufficient. Thus, the aim of this study is to remove and treat PAHs from real contaminated soil by combining SW techniques and electro-Fenton process with a possible additional biological treatment. The use of Tween 80® and HPCD agents are compared according to several factors: extraction efficiency, mineralization rate, toxicity and biodegradability during electroFenton treatment. The studied PAH were chosen as a function of number of rings: acenaphthene (ACE) and phenanthrene (PHE) (3 rings), fluoranthene (FLA) and pyrene (PYR) (4 rings), benzo(a)pyrene (BAP) (5 rings) and benzo(g,h,i)perylene (DBP) (6 rings). They are all listed in the Environmental Protection Agency of the United States (USEPA). 2.
MATERIAL AND METHODS
2.1. Chemicals Sodium sulfate, 2-(P-toluidino)naphthalene-6-sulfonic acid sodium (TNS), Tween 80® (polyoxyethylene (20) sorbitan monooleate), acetonitrile, ACE, PHE, FLA, PYR and BAP were purchased from Aldrich. Hydroxypropyl-beta-cyclodextrin (HPCD) was provided by Xi’an Taima Biological Engineering Company (China). Ammonium acetate, hydroxylamine hydrochloride and DBP were supplied by Acros at analytical grade. Analytical reagents like n-hexane, acetone and NaOH were provided by VWR. In all experiments, deionised water from a Millipore Simplicity 185 (conductivity < 6×10-8 S cm1 ) system was used. 2.2. Soil washing (SW) process
The polluted soil was sampled from a PAH and hydrocarbons contaminated site. The soil was sieved under 2 mm and homogenized by a sample divider (Retsch) prior to use. The soil characteristics obtained from an external certified laboratory gave the following particle size distribution: clay (< 2 µm): 19.7%, fine silt (2-20 µm): 23.3%, coarse silt (2050 µm): 7.5%, fine sand (50-200 µm): 12.3%, coarse sand (200-2000 µm): 37.1%. It has the other following characteristics: pH (water): 8.3, organic matter content: 4.71%, iron content: 69 mg kg-1. The amounts of pollutants in soils expressed in mg kg-1 are: ACE: 152, PHE: 308, FLA: 110, PYR: 80, BAP: 96, DBP: 23, total PAHs (16) content: 730 mg kg-1, total hydrocarbons (C10-C40) content: 850 mg kg-1. SW experiments were performed in a 500 mL bottle at a soil/liquid ratio equal to 10% (40 g / 400 mL). Solutions of Tween 80® (10 g L-1) or HPCD (10 g L-1) were used and the mixtures were rotated in a Rotoshake RS12 (Gerhardt) at 10 rotations per minute for 24 h. Then the particles settled for 12 h and the supernatants were filtered with a 0.7 µm glass microfiber filter. The supernatants were then used for PAH, TOC, HPCD and electro-Fenton treatments. The soil was used for respirometric assays. Successive SW by using fresh solution (Tween 80® or HPCD at 10 g L-1) were done by reusing each time the same soil and the same soil/liquid ratio. 2.3. Electrochemical treatment Electro-Fenton experiments were performed at room temperature (22 ± 1°C), in a 0.40 Lundivided glass electrochemical reactor at current controlled conditions. The cathode is a 150 cm2 carbon-felt piece (Carbone-Lorraine). The anode is a 5 cm x 4 cm BDD plate, which is centred in the cell and surrounded by cathode covering the inner wall of the cell. An inert electrolyte (Na2SO4 at 0.150 M) is added to the medium. Prior to each experiment containing HPCD, the solutions were saturated in O2 by supplying compressed air (10 min at 0.25 L min-1). Since too much foam is formed during bubbling system, the solutions containing Tween 80® are not saturated with O2. The electrochemical cell is monitored by a power supply HAMEG 7042-5 and applied current is set to 1000 mA. Solutions were stirred continuously by a magnetic stirrer. A heat exchanger system is provided to keep the solution at constant room temperature by using fresh water. The pH of initial solution was not adjusted to pH 3 as usual. It is considered that the initial iron content in soil is enough to be dissolved during SW with HPCD or Tween 80®. Consequently no iron salt was added. Quantification of iron in HPCD and Tween 80® solutions have shown that catalytic quantity of iron was solubilized at the respective concentrations 0.02 mM and 0.06 mM. 2.4. Analysis determination The amount studied PAHs in soil was determined by Soxhlet extraction (Behr, LaborTechnik). Two grams of dried soil were mix with 5 g of anhydrous sodium sulfate to prevent trace of humidity. A mixture of n-hexane/acetone (70 mL/70 mL) was then added. Extractions were performed for 16 h (4-5 cycles per h). The calculated amounts of PAH were compared to the values obtained by the external laboratory. The highest content was taken into account for each PAH. The PAH quantification in solution was followed by reversed phase with a high performance liquid chromatography (HPLC) coupled with an UV-absorbance and a fluorescence detectors (Merck, Hitachi). The UV detection was carried out at a wavelength of 254 nm. The fluorescence detection was performed at the following excitation/emission wavelengths: 275/350 nm for ACE and PHE, 270/440 nm for FLA and PYR and 290/430 nm for BAP and DBP. A RP-18 column (Lichrocart 250-4, Purospher®) placed in an oven set at 40°C was used. For ACE, PHE, FLA and PYR analysis, the mobile phase was a mixture of water/acetonitrile (35:65 v/v) and the flow rate was set at
1.0 mL min-1 (isocratic mode). For BAP and DBP analysis, the mobile phase was a mixture of water/acetonitrile (15:85 v/v) and the flow rate was set at 1.0 mL min-1 (isocratic mode). The HPCD concentration was determined by a fluorimetric technique based on enhancement of the fluorescence intensity of TNS, when they are complexed with the cyclodextrin (Hanna et al., 2005). A Kontron SFM 25 spectrofluorimeter was set out at 318 nm for excitation and 428 nm for emission. The total organic carbon (TOC) values were determined by catalytic oxidation using a Shimadzu VCSH TOC analyser. Calibrations were performed by using the potassium hydrogen phthalate solutions as standard. All samples were acidified to a pH value of 2 with H3PO4 (25%) to remove inorganic carbon. The injection volumes were 50 µL. All samples values are given with a coefficient of variance below to 2%. Toxicity assays were performed by using Microtox® standard method (ISO 11348-3) with marine bacteria Vibrio fischeri from LUMIStock LCK-487 (Hach Lange). A BERTHOLD Autolumat Plus LB 953 equipment was used. 22% of NaCl was added in each sample to insure an osmotic protection for bacteria. Before each toxicity measurement, all the samples were adjusted to circum-neutral pH and samples were filtered with RC filter (0.2 µm) to remove iron precipitates. In each batch test a blank without the compound studied was also measured and used for percentage of inhibition calculation based on 15 min exposure. The biodegradability was given by the ratio between Biochemical Oxygen Demand at 5 days (BOD5) and the Chemical Oxygen Demand (COD). BOD5 was determined by respirometric method with the OxiTop® control system (WTW). An aqueous solution containing a phosphate buffer solution and a saline solution was prepared according to Rodier et al. procedure (Rodier et al., 2009). This solution was then saturated in oxygen. As this solution and the samples are sterile, bacteria extracted with KCl at 9 g L-1 (30 mL with 3 g of soil) and a IKA-MS1 minishaker (1800 rpm during 1 min) from uncontaminated soil were added just before adding the samples. All the samples were adjusted to circumneutral pH. D(+)-Glucose•H2O was used as a reference and a blank with milli-Q water and the seed solution was prepared for each batch and taken into account for calculation. All the bottles containing the solutions were equipped by a rubber sleeve in which pure NaOH pellets were added to trap the CO2 formed during biodegradation. The samples were incubated at 20°C during 5 days. The BOD5 measured in each blank was not significant compared to the BOD5 of the samples, which causes no interference. COD measurements were achieved by adding 2 mL of samples in COD Cell test (Merck) and by heating at 148 °C during two hours with a Spectroquant® TR 420 (Merck). COD analyses were accomplished by a photometric method requiring a Spectroquant® NOVA 60 (Merck) equipment. 3.
RESULTS AND DISCUSSIONS
3.1. Extraction efficiency of soil washing (SW) processes with HPCD or Tween 80® The cyclodextrin (HPCD) and the traditional non-ionic surfactant (Tween 80®) are compared during SW experiments. Figure 1 illustrates extraction efficiency of successive SW processes by adding each time a fresh solution of HPCD (10 g L-1) or Tween 80® (10 g L-1). The percentages are given as a function of the initial concentration of pollutants in initial contaminated soil. It is obvious that the successive SW processes allow the extraction of higher quantity of PAHs with the use of Tween 80® compare to HPCD extractions. This is in accordance with the results obtained in some other articles (Gong et al., 2010; Maturi and Reddy, 2006).
(a)
Extraction / %
100 80 60 4.0
40 0.2
4.9 0.8
1.7
0.4
0.1
1.3
0.8
0.6
0.06
0.01
20
7.8
0.2
0.8
0 ACE
PHE
FLA
PYR
SW #1 SW #2 SW #3
DBP
(b) Extraction / %
100
60 40 20
32.8
25.1 14.8
49.3
46.1
43.6
80
17.1
29.1 16.5
43.0 22.3
18.1
31.6 23.1
14.7 12.3
30.0
13.7
SW #1 SW #2
SW #3
0 ACE
PHE
FLA
PYR
BAP
DBP
Figure 1. Extraction efficiency of three successive SW cycles in the presence of HPCD (10 g L-1) (a) or Tween 80® (10 g L-1) (b). By using HPCD as extracting agent, the total extraction efficiency after 3 SW cycles are: ACE (4.2%), PHE (0.3%), FLA (8.8%), PYR (7.4%) and DBP (2.9%). In the case of Tween 80®, the extraction efficiency after 3 SW cycles are: ACE (91%), PHE (96%), FLA (95%), PYR (83%), BAP (59%) and DBP (67%). After three successive SW, the averages of extraction efficiency by taking into account the same pollutant (ACE, PHE, FLA, PYR, DBP), are about 86% and 5% in the case of Tween 80® and HPCD, respectively. In that case Tween 80® is 18 times more efficient than HPCD. 3.2. Evolution of mineralization After SW process, EF treatments were performed in the aim to mineralize organic pollutants. Figure 2 depicts the evolution of mineralization in the presence of HPCD or Tween 80® solution during electro-Fenton treatment. It demonstrates that the end of mineralization is reached after 20 h of treatment in the presence of HPCD and 28 h of treatment in the presence of Tween 80®. Two main reasons can be evoked. The first one is that the electro-Fenton experiments with Tween 80® solution are performed without supplying O2 since too much foam is formed. The second reason is that the initial COD of Tween 80® solution (14,500 mg O2 L-1) is higher than with HPCD solution (10,250 mg O2 L-1). This initial COD value can be explained by the theoretical COD that is higher with Tween 80® (2 g O2 (g Tween 80®)-1) than with HPCD (1.28 g O2 (g Tween 80®)-1). The fact that Tween 80® can extract much more organic compounds (organic pollutants and organic matter) than HPCD, which is
confirmed by the extraction efficiency and the initial brown color of solution, is another reason of the higher initial Tween 80® COD value. Figure 2 also highlights the time-course of HPCD removal. After 14 h of treatment (70% of the total treatment), HPCD left in solution is negligible and the mineralization is about 88%. 100
100
(a)
90
80
80
70
70
60
60
50
50
40
40
30
30
20
20
10
10
0
Mineralization / %
HPCD removal / %
90
0
0
2
4
6
8
10
12
14
16
18
20
t/h
100
(b)
90
Inhibition / %
70 60 50 40
30 20
100
100
50
90
40
70
70
20
10 0 0
4
90
80
80
30
100
90
60
60
50
50
40
8 40 12 16 t / h Inhibition
2030
30
80
24
70 60
50 40
28
20
30
Figure 2. Evolution of mineralization ( 20)Biodeg and HPCD degradation ( ) during electro10 Inhibition 10 20 Fenton treatment of PAH-contaminated SW solution with different extracting agent: (a) 0 -1 ®0 -1 Biodeg 10 HPCD (10 g L 12 ), 10 (b) Tween 8020 (10 g L ). 0 4 8 16 0/ h treatment time 3.3. Evolution of biodegradability and toxicity 0
0
4
8
12
16
20
treatment / h during the It appears interesting to study the environmental impact of the SWtime solution electro-Fenton treatment and to carry out the possibility of a post-biological treatment. Figure 3 describes the evolution of toxicity (Microtox®) and biodegradability (BOD5/COD) during electro-Fenton treatment of SW solution containing Tween 80® or HPCD. In the case of HPCD, the toxicity increases until maximum (100% of inhibition) after the first hour of treatment and remain constant until 12 h of treatment. Then the toxicity decreases until the end of treatment. This behavior can be explained by the formation of more toxic oxidation by-products at the beginning, which is often observed in literature (Brillas et al., 2009). Then the toxicity decreases since there is an accumulation of carboxylic acids that
BOD5/COD / %
80
60
BOD5/COD / %
90
70
Inhibition / %
100
Mineralization / %
80
are usually known to be less toxic. The biodegradability results follow the trend of toxicity. The BOD5/COD ratio starts to increase only after 4 h of treatment and is negligible before. Then the ratio increases until 87% after 14 h of treatment. At this time the toxicity starts to decrease and the HPCD concentration in solution is negligible. These results are similar to those obtain in a previous study carried out in synthetic solutions (Mousset et al., 2012). 100 90
90
80
80
Inhibition / %
70
(a)
70
60
60
50
50
40
40
30
30
20
20
10
10
0
BOD5/COD / %
100
0
0
4
8
12
16
20
treatment time / h 100
Inhibition / %
70 60 50 40
80
80
(b) 100
60
90
50
80
40
70
30 20 10 0 0
4
8
12
16
20
treatment time / h
24
60
70
BOD5/COD / %BOD5/COD / %
80
90
Inhibition / %
90
90 70
Inhibition / %
100
100
100
100 60
90 50 40 80 30 70 20
50 60 10 40 50 0
302840
90 80 70 60
50 40 30 Inhibition
20 30 30 ® Figure 3. Evolution ) and biodegradability (BOD5/COD) ( )Biodeg Inhibition 10 20 20 of toxicity (Microtox ) ( during electro-Fenton treatment of SW of PAH-contaminated soil with different extracting 0 Biodeg 10 agent: (a) HPCD (10 g L-1), (b) 10g L-1). Tween 80® (10 0 4 8 12 16
0 0 treatment timeat/ h In Tween 80® solutions, a lower increase of toxicity is observed and remains stable 0 4 8 12 16 20 around 90% until the end of mineralization. At this time the TOC in the solution is time h one (3,570 mg C L-1). It seems that negligible (around 70 mg C L-1)treatment compared to the /initial still few toxic compounds remain in the solution. About the biodegradability curve, the ratio is negligible until 4 h of treatment and reaches around 33% after 20 h of treatment, which represents 70% of the time to reach the end of mineralization. Considering that a minimal BOD5/COD ratio of 33% is required to consider a post-biological treatment of industrial effluent (Rodier et al., 2009), the electro-Fenton treatment time would be 7 h and 20 h for HPCD and Tween 80® solutions respectively. In that case, Tween 80® SW solutions treatment lead to energy consumptions (in Kwh m-3) around 2.8 times higher than with HPCD solution.
20 10
0
20
4.
CONCLUSIONS
Tween 80® solutions were able to extract about 86% compare to 5% with HPCD after 3 successive SW cycles. Tween 80® is 18 times in average more efficient than HPCD by using the same mass concentration (10 g L-1). The electro-Fenton process suggested to treat these highly loaded solutions succeeds to completely mineralize the HPCD and Tween 80® solution after 20 h and 28 h, respectively. The toxicity results have shown formation of toxic intermediates during the first hours of treatment in both cases. Then the toxicity decreases after 14 h of treatment of HPCD solution until the end of mineralization. 14 h is the time when the HPCD concentration is negligible and the biodegradability becomes very high (87%). The toxicity remains constant (90%) in Tween 80® solution even after 98.4% of mineralization. Still few toxic compounds are in the solution. In the meantime, the BOD5/COD ratio increases until 33% after 20 h of treatment. By considering a biodegradability ratio of 33%, which is the threshold value to consider a biological treatment, the value is reached after 7 h of treatment with HPCD solution against 20 h in the case of Tween 80® solution. This represents an energy consumption 2.8 times higher than with HPCD solution. Regarding cost of products and extraction efficiency, Tween 80® is a better extracting agent than HPCD, even if the electro-Fenton treatment requires more time and is more energy consuming. REFERENCES Brillas E., Sirès I. and Oturan M. A. (2009) Electro-Fenton process and related electrochemical technologies based on Fenton’s reaction chemistry. Chem. Rev., 109, 6570–6631. Chu W. and Chan K. H. (2003) The mechanism of the surfactant-aided soil washing system for hydrophobic and partial hydrophobic organics. Sci. Total Environ., 307, 83–92. Gómez J., Alcántara M. T., Pazos M. and Sanromán, M. A. (2010) Remediation of polluted soil by a two-stage treatment system: desorption of phenanthrene in soil and electrochemical treatment to recover the extraction agent. J. Hazard. Mater., 173, 794–798. Gong Z., Wang X., Tu Y., Wu J., Sun Y. and Li, P. (2010) Polycyclic aromatic hydrocarbon removal from contaminated soils using fatty acid methyl esters. Chemosphere, 79, 138–143. Hanna, K., Chiron, S., & Oturan, M. a. (2005). Coupling enhanced water solubilization with cyclodextrin to indirect electrochemical treatment for pentachlorophenol contaminated soil remediation. Water Res., 39, 2763–2773. Maturi K., and Reddy K. R. (2006) Simultaneous removal of organic compounds and heavy metals from soils by electrokinetic remediation with a modified cyclodextrin. Chemosphere, 63, 1022–1031. Mousset E., Oturan M. A., van Hullebusch E. D., Guibaud G. and Esposito G. Soil washing / flushing treatments of organic pollutants enhanced by cyclodextrins and integrated treatments: state of the art. Crit. Rev. Env. Sci. Technol, in press. Mousset E., Oturan N., van Hullebusch E. D., Guibaud G., Esposito G. and Oturan M. A. (2012) Electro-Fenton treatment of soil washing solution of phenanthrene with cyclodextrin using different kind of anode materials: impacts on toxicity and biodegradability. DEEE'12 internationale conference, December 2-4 Paris, France. Oturan, M. A. (2000) An ecologically effective water treatment technique using electrochemically generated hydroxyl radicals for in situ destruction of organic pollutants: Application to herbicide 2,4-D. J. Appl. Electrochem., 30, 475–482. Rodier J., Legube B. and Merlet N. (2009) Analyse de l’eau (9 Ed.). Dunod, Paris.
