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Removal of PAHs and pesticides from polluted soils by enhanced electrokinetic-Fenton treatment
Chemosphere xxx (2015) xxx–xxx
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Removal of PAHs and pesticides from polluted soils by enhanced electrokinetic-Fenton treatment Elvira Bocos, Carmen Fernández-Costas, Marta Pazos, M. Ángeles Sanromán ⇑ Department of Chemical Engineering, University of Vigo, Isaac Newton Building, Campus As Lagoas, Marcosende 36310, Vigo, Spain
h i g h l i g h t s The electrokinetic-Fenton technology has been used for the removal of PAHs and pesticide form soil. The metal precipitation was demonstrated as the main drawback of the treatment. The addition of citric acid enhances the treatment efficiency increasing the reached removals. After the electrokinetic-Fenton using citric acid the treated soil showed low phytotoxicity levels.
a r t i c l e
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Article history: Received 11 October 2014 Received in revised form 14 December 2014 Accepted 15 December 2014 Available online xxxx Handling Editor: E. Brillas Keywords: Citric acid Complexing agents Iron PAHs Phytotoxicity Pyrimethanil
a b s t r a c t In this study, electrokinetic-Fenton treatment was used to remediate a soil polluted with PAHs and the pesticide pyrimethanil. Recently, this treatment has emerged as an interesting alternative to conventional soil treatments due to its peculiar advantages, namely the capability of treating fine and low-permeability materials, as well as that of achieving a high yield in the removals of salt content and inorganic and organic pollutants. In a standard electrokinetic-Fenton treatment, the maximum degradation of the pollutant load achieved was 67%, due to the precipitation of the metals near the cathode chamber that reduces the electro-osmotic flow of the system and thus the efficiency of the treatment. To overcome this problem, different complexing agents and pH control in the cathode chamber were evaluated to increase the electro-osmotic flux as well as to render easier the solubilization of the metal species present in the soil. Four complexing agents (ascorbic acid, citric acid, oxalic acid and ethylenediaminetetraacetic acid) in the Fenton-like treatment were evaluated. Results revealed the citric acid as the most suitable complexing agent. Thereby its efficiency was tested as pH controller by flushing it in the cathode chamber (pH 2 and 5). For the latter treatments, near total degradation was achieved after 27 d. Finally, phytotoxicity tests for polluted and treated samples were carried out. The high germination levels of the soil treated under enhanced conditions concluded that nearly complete restoration was achieved. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction During the last decades, contamination in soils and groundwater has incremented. Unfortunately, this pollution is causing important environmental problems, particularly in soils, due to the persistence of the pollutants, which are strongly sorted there, becoming difficult to remove. It is well known that large amounts of organic pollutants, most of them arising from the anthropogenic action, are being released daily to the environment. Although some of these compounds are easily degraded by microorganisms (Liao ⇑ Corresponding author. Tel.: +34 986 812383; fax: +34 986 812380. E-mail address: [email protected] (M. Á. Sanromán).
and Li, 2012), many others such as Polycyclic Aromatic Hydrocarbons (PAHs) and pesticides are persistent in the environment owing to its low water solubility and low volatility. Over the past decade, the treatment of these compounds has become matter of interest for numerous authors. In this context, different technologies such as bioremediation, chemical treatments and physical treatments (Pazos et al., 2010) have been applied to remediate soils, sediments and groundwater. Polluted soils can be treated by ex situ and in situ methods. Ex situ technologies have proved to ameliorate decontamination of soils, whereas in situ treatments are preferred since they are not invasive techniques. Therefore, they cause less disruption to the environment and allow lowering the economic costs (Romantschuk et al.,
http://dx.doi.org/10.1016/j.chemosphere.2014.12.049 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Bocos, E., et al. Removal of PAHs and pesticides from polluted soils by enhanced electrokinetic-Fenton treatment. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2014.12.049
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E. Bocos et al. / Chemosphere xxx (2015) xxx–xxx
2000; Mulligan et al., 2001). Among the different in situ treatments, it is important to highlight the electrokinetic remediation. Since 1990, several studies have pointed out the cost-efficiency of this technology to remediate polluted soils. It has been also recognized as a promising method for the treatment of different kinds of organic and inorganic low-permeable soils. Electrokinetic treatments have been widely studied and already successfully applied on real polluted soils, water and sediments. In this study, the method used is based on the application of a low intensity current to an electrokinetic cell filled with a polluted soil, both of its extremes connected to two electrode chambers. This current promotes the movement of the pollutants from the anode chamber toward the cathode chamber, where they are finally collected and treated. In this case, the pollutants are mainly transported by the electro-migration of the ionic species in the pore fluid toward the opposite electrode and by the electro-osmotic flow generated between the anode and the cathode (Acar, 1993). Notwithstanding, one of the main disadvantages of this technology relies on the concentration of the pollutants in a small area of soil close to the cathode chamber due to the precipitation of several species present in the soil. Thus it is also necessary to treat the liquid of the electrolyte chamber in order to obtain the total pollutants degradation (Ren et al., 2014). In order to overcome these problems several technologies have been performed in combination to electrokinetic treatments. Over the last years, Advanced Oxidative Processes (AOPs) have attracted the interest of the scientific community, since they are environmentally friendly methods powerful in removing organic contaminants by the production of hydroxyl radicals as oxidizing agent. Besides, AOPs may enhance the treatment by the in situ degradation of pollutants, avoiding the necessity of treating the electrolytes collected from the chambers. As it was recently reported by Huang et al. (2014), chemical oxidation typically involves reduction/oxidation reactions that transform chemically hazardous contaminants into non-hazardous or less toxic, mobile or inert compounds. Habitually, the most common oxidants used are ozone, hydrogen peroxide, hypochlorites, chlorine, chlorine dioxide, potassium permanganate and Fenton’s reagent (hydrogen peroxide and iron). Over the last decades, Fenton process has been tested by several researchers in the remediation of soils (Kim et al., 2006; Alcántara et al., 2008) and marine sediments (Pazos et al., 2013). It is the case of some work developed by Yap et al. (2011), demonstrated the efficiency of the Fenton treatment for the remediation of Polycyclic Aromatic Hydrocarbons-polluted soils. This reaction is based on the generation of hydroxyl radicals by the combination of hydrogen peroxide and ferrous ions. Hydroxyl radicals are the strongest second oxidants after fluorine and they can react even with most of the organic persistent contaminants until their complete mineralization. However, the generation of these radicals is only effective under low pH (Valentine and Ann Wang, 1998). In the electrokinetic-Fenton treatment of soils Fenton’s reagent is used as a flushing agent, so that the removal of organic contaminants could be achieved by an in situ oxidation/destruction (Alcántara et al., 2008). Several researchers have already pointed out, the generation of oxidant species by the use of other Fenton’s reagents. In this context, some authors have reported the utility to use this combined technology when using permanganate in the treatment of different soils and contaminants. For example, Wu et al. (2012) tested the oxidative remediation of a typical organic contaminant (tetrachloroethene) by an electrokinetic in situ chemical oxidation, using as oxidant agent permanganate. These authors observed a reduction of the oxidant migration in low-permeability media under the electrokinetic process, which was ascribed to the increased oxidant consumption of competing reductors. Another interesting study was conducted by Yang and Yeh (2011), who
proposed the injection of persulfate in combination to an electrokinetic process for the destruction of trichloroethylene in a spiked sandy clay soil. They reported the enhancement of the electrokinetic treatment by the injection of persulfate from anode to cathode through the electro-osmotic flow, aiding the in situ chemical oxidation of trichloroethylene. Recently Ng et al. (2014) described different paths to enhance the electrokinetic-Fenton treatment such as the increase of the oxidant availability, electrolytes and H2O2 concentration. These authors also pointed out the importance of the type of soil and contaminants. They concluded that soils with low acid buffering capacity, adequate iron concentration, low organic matter content and low aromatic ring of the organic contaminants generally end up in higher efficient treatments. Ren et al. (2014) improved the remediation efficiency when they attained the desorption and/or solubilization of metals and organic matters in soil, which allows to enhance the conventional electrokinetic remediation. Enhanced techniques currently used in electrokinetic are mainly directed toward maintaining or bringing contaminants into solution by addition of facilitating agents and controlling pH. In this study, electrokinetic-Fenton treatment using H2O2 as flushing agent, as reported before by Tsai et al. (2010) was carried out. The remediation of a soil spiked with several persistent organic pollutants (PAHs and pyrimethanil) was studied and improved. In the former case, the effect of chemical properties (facilitating agents, pH control and iron content of the soil) was evaluated, in order to yield high degradation levels at the end of the treatment. Finally, the soil obtained after the most favourable treatment was evaluated in terms of phytotoxicity.
2. Materials and methods 2.1. Soil A real soil collected from a vineyard in the North-west of Spain (Ribeira Sacra, Galicia) was employed. Soil samples were collected from 0 to 30 cm depth by using an Eijkelkamp sampler, transported to the laboratory in polyethylene bags and sieved. Only the fraction containing particles smaller than 2 mm was selected. Some of the most typical characterization data of soil samples such as pH, conductivity, total organic carbon, total inorganic carbon, total carbon, organic matter and the presence of some metals are depicted in Table 1. In terms of the overall concentration of metals, iron was the most abundant metal found in the sample. This aspect is important because iron is known to play an important role in the degradation of organic compounds by Fenton reaction. Table 1 Chemical characterization of the studied soil. Parameters
Value
pH Hydraulic conductivity (cm s1) Inorganic C (%) Organic C (%) Total C (%) Organic matter (%) Cu (mg kg1) Zn (mg kg1) As (mg kg1) Ba (mg kg1) Cr (mg kg1) Pb (mg kg1) Ni (mg kg1) Fe (mg kg1) Co (mg kg1) Mn (mg kg1)
6.8 7 108 0.8 2.4 3.2 4.2 242 838.8 13.9 137.6 55.8 17.3 41.5 33,594 17.7 809.8
Please cite this article in press as: Bocos, E., et al. Removal of PAHs and pesticides from polluted soils by enhanced electrokinetic-Fenton treatment. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2014.12.049
E. Bocos et al. / Chemosphere xxx (2015) xxx–xxx
2.2. Spiked soil The pollutants (pyrimethanil, phenanthrene, anthracene, fluoranthene, pyrene benz[a]anthracene) and the hexane were provided by Sigma–Aldrich. Initially, 250 mg of each pollutant per kg were dissolved in 2.5 L of hexane. Afterwards, 2 kg of soil were spiked with this solution. This soil–pollutant–hexane mixture was placed beneath a ventilation hood for 9 d and stirred every day until the hexane was completely evaporated and the spiked soil was dried. Afterwards, a sample of the soil was taken to corroborate that the initial concentration of pollutants in the soil was the desired one, since a portion of the contaminants might have volatilized along with the solvent. 2.3. Soil treatments 2.3.1. Fenton-like treatment of the spiked soil The effect of several facilitating agents (ascorbic acid, citric acid, oxalic acid and ethylenediaminetetraacetic acid (EDTA)) on Fenton treatment of the polluted soil was performed following the experimental procedure described by Vicente et al. (2011). The Fenton-like treatment was carried out in glass tubes of 10 mL, where 3 g of sample were introduced. Finally, 6 mL of a solution of H2O2 (0.29 M) and 0.50 mM of each facilitating agent were mixed with the soil and keeping in agitation during 24 h. 2.3.2. Electrokinetic-Fenton treatment This treatment was performed in a cylindrical glass cell containing a sample compartment of 100 mm length and 32 mm of internal diameter. The two electrode chambers (filled with 300 mL of the process fluid) were placed at the end of the electrochemical cell. The sample compartment was isolated from the test matrix by means of a filter paper and porous stones. Graphite electrodes were used for both anode and cathode. Three auxiliary electrodes have allowed measurement of the electric field distribution along the sample. Electrode chambers were filled with a H2O2 (10% w/ v) solution and with Na2SO4 (0.1 M), this last was used to improve the conductivity of the solution and, hence, the current intensity (Kim et al., 2006). This fact, favours the electroosmotic flow due to the increase of the total ion concentration in the pore fluid. The electrolyte solution was renovated each 12 d in order to maintain its effect as buffering solution. As mentioned before, iron content in soil was elevated, in fact it was higher enough to activate the decomposition of H2O2, consequently enhancing the generation of hydroxyl radicals. A constant potential difference of 30 V was applied in all experiments. Readings of voltage drop, current, intensity and pH in the electrode compartments, were taken periodically (Pazos et al., 2012). 2.4. Analytical determinations At the beginning and at the end of each electrokinetic experiment, liquid samples were taken from the cathode and anode solutions as well as from the solid matrix for chemical analysis. Soil sample was divided into three equal sections (S1–S3, namely from anode to cathode) were analyzed for moisture content, pH and pollutant concentrations. Additionally, during the treatment liquid, samples were taken periodically from the electrolyte chambers, centrifuged at 10,000 rpm for 5 min and kept refrigerated to be analyzed. 2.4.1. PAHs and pesticide concentration To determine PAHs and pyrimethanil concentration in soil from each section (S1–S3), they were extracted from dry samples by a pressurized solvent extraction system using an OnePSE instrument (Applied Separations Inc.). Dry sample was thoroughly mixed with
3
pelletized diatomaceous earth. When a free-flowing powder was obtained, it was placed into an extraction vessel in the instrument. The extraction solution was composed of acetone/hexane (1:1, v/v). After four cycles of 5 min at 110 °C and 104 kPa, the extraction was complete and the collected sample was used to determine pollutant concentration. Pressurized solvent extraction can be used to replace soxhlet and sonication techniques and has been approved for use, following EPA Method 3545A. All analytical determinations were done in triplicate with an experimental error below 15%. Thus, the concentration of PAHs and of the pesticide in the extractions and the liquid samples collected at the end of the treatment was determined using an Agilent 1100 HPLC equipped with a XDB-C8 reverse phase column. The injection volume was set at 5 lL and the isocratic eluent (33:67 acetonitrile/water) was pumped at a rate of 1 mL min1.
2.4.2. Metal concentration The protocols used for the chemical extraction and analysis of metals were performed in accordance with EPA Methods 3010 and 3050 for the analysis of metals in liquid and solid samples, respectively. A mixture of concentrated nitric acid (5 mL) and the desired sample (100 mL liquid or 2 g soil) was refluxed in a covered Erlenmeyer flask. This step was repeated with additional portions of nitric acid until the mixture got light colour or until its colour was stabilised. After the digestate was brought to a low volume (3 mL), the sample was refluxed with 5 mL of hydrochloric acid (1:1) for 15 min. The sample was then filtered and adjusted to a final volume of 100 mL. Inductively Coupled Plasma–Optical Emission Spectrometer (ICP–OES) Perkin Elmer Optima 4300 DV was used to analyse metals. The experiments were performed at less twice with an experimental error, calculated as standard deviation, below of 15%.
2.4.3. pH The pH is one of the main parameters affecting the mobility of the contaminants. Therefore, at the end of each experiment, the pH value of the solid samples was measure by mixing 1 g of dry sample with 20 mL of deionized water. Afterwards, the mixture was kept in repose during 1 h and the pH was measured with a Sentron pH meter (model 1001).
2.4.4. Buffering capacity of the soil Buffering capacity is defined as the ability of a solid matrix to resist changes in pH. Acid buffering capacity of the sludge suspension was assayed by titration. A suspension of sludge sample in water (6.7% w/v) was stirred for 30 min and the pH was analyzed. Successive additions of 1 mL of 1 M HCl were made every 30 min and pH was measured thereafter. This procedure was repeated until the pH value was constant. The results were elucidated in a qualitative form, comparing the profiles corresponding to sludge sample with the obtained for the kaolin with low buffering capacity as it was previously reported (Pazos et al., 2012).
2.4.5. Electrical conductivity, moisture content and TOC Conductivity determination was accomplished adding 100 mL of distilled water to 20 g of the sample and shaking them for 30 min. After that, suspensions were filtered and conductivity was measured into the liquid fraction with a conductivity meter Crison Basic 30. The moisture content of the sediment was calculated with the loss of weight of the sample after heating at 105 °C during 24 h. Total organic carbon (TOC) was determined using the dichromate method proposed by Shumacher (2002).
Please cite this article in press as: Bocos, E., et al. Removal of PAHs and pesticides from polluted soils by enhanced electrokinetic-Fenton treatment. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2014.12.049
E. Bocos et al. / Chemosphere xxx (2015) xxx–xxx
3. Results and discussion 3.1. Soil characterization Initially, a detailed study of soil characteristics was necessary in order to test different strategies that may improve the efficacy of the electrokinetic treatment. Some factors, such as soil origin, pH, metal content, conductivity and buffering capacity, were analyzed. The analytical measures reveal that this soil possesses a slightly acid pH (6.76) and the highest metal concentrations correspond to Fe, Mn and Zn (Table 1). Since iron is one of the most important variables on the Fenton treatment, it is worth noting its high concentration, around 33594 mg of Fe kg1 of dried soil, which makes the sample appropriate to promote the Fenton’s reactions to take place. It is well known the great influence of the acid buffering capacity of the soils on the remediation process, since desorption of metals is favoured at low pH. Thus, after the analysis of our sample and comparing it with kaolin, this soil showed a similar behaviour than kaolin. This means that the sample has a low acid buffering capacity, which was attributed to its chemical composition. In addition, the hydraulic conductivity value can be used to extract information of the soil behaviour under the electric field action. Since the conductivity of the studied soil was high (Table 1), a low electrical resistance was expected, which will favour the electrokinetic treatment. 3.2. Electrokinetic–Fenton treatment Initially, unenhanced electrokinetic-Fenton treatment of polluted soil was carried out in 4 electrokinetic cells at constant potential difference of 3 V cm1 without addition of any facilitating agent. The electrode chambers were filled with H2O2 (10%) and, as in previous studies, 0.1 M Na2SO4 was used as electrolyte (Alcántara et al., 2008). Each cell was treated during 7, 14, 27 and 34 d in order to observe the degradation level of the contaminants trough the electrokinetic cell along the time. Table 2 depicted an average between the three sections of the soil for the obtained degradation levels of each organic pollutant. The results show a progressive degradation along the time of all pollutants; however after 27 d the maximum degradation values were reached
(67%) and not improvement was detected at higher treatment time. As it was described by Li et al. (1996), when the electrokinetic method is used to remove metals from soils, it is expected to find precipitate of hydroxides in the region of the soil where pH raises, limiting the remediation efficiency. The pH rise is caused by the generation of hydroxide ions as a result of electrolysis of water during the remediation. The presence of H2O2 in the electrokinetic cell normally avoids extremely high/low pH environments in the electrode chambers (Alcántara et al., 2008), however, in this study; the absence of strong buffering species in the soil did not allow this effect to last for long periods of treatment. As depicted in Fig. 1, after 34 d the pH was approximately 4 near the anode and amounted to pH 9.5 near the cathode. This fact suggests that a precipitated was formed at the end of the treatment in the proximities of the cathode chamber with the consequent blockage of the electro-osmotic flow and the restriction of the metals mobility. All the tested metals migrated toward the cathode. For example, Zn, Mn and Fe showed lower removal after 34 d of electrokinetic treatment. Only 18%, 27% and 14% of Zn, Mn and Fe, respectively were recovered in the cathode chamber, while the rest remained into the soil sample, mainly in the section close to the cathode (S3). On the other hand, as showed in Fig. 1, in terms of degradation the pollutants, almost total pollutants removal was obtained in the Section 1 however the removal was lower in Sections 2 and 3. This fact is a direct consequence of metals precipitation in the proximities of the cathode, (mainly Fe, which is essential in Fenton’s reaction). Besides, the electro-osmotic flow, which is the main responsible of the presence of H2O2 in the soil, was stopped with the blockage due to the precipitated. Thus, the degradation was reduced in the last sections due to both Fenton’s reagents were unable to react.
Table 2 Degradation levels of the different pollutants along the treatment time by the unenhanced electrokinetic-Fenton process. Compound
Degradation (%) 7 d
Degradation (%) 14 d
Degradation (%) 27 d
Degradation (%) 34 d
Pyrimethanil Phenanthrene Anthracene Fluoranthene Pyrene Benz[a]anthracene
24 26 40 14 9 12
28 28 48 18 23 12
58 49 59 67 48 52
59 48 60 67 48 51
1.0
10
0.8
8
0.6
6
0.4
4
0.2
2
pH
2.4.6. Phytoremediation assays After optimize the treatment, the phytotoxicity of soil was studied. Toxicity tests were accomplished following the methodology described by Macía et al. (2014) which is a modification of a standard method (OECD, 2000, 2006). Lolium perenne (ray grass) seeds were supplied by Fitó S.A. A culture with peat moss was employed as a control sample in the growth trials because it is only a physical support for plant growth; therefore, it allows establishing a reference of the germination and growth. A minimum germination of 80% in control samples was required to consider the test valid. The seedbeds used in the phytotoxicity assays had no holes in their bottom to avoid leaching of the contaminants and to evaluate the real toxicity of the soil samples. Every seedbed was filled with 50 g of fresh material and 0.3 g (around 150 seeds) of L. perenne were sown per seedbed and cultivated for 10 d. All tests were performed in triplicate. Germination and growth were carried out in an environmental control chamber (SANYO Versatile Environmental Test Chamber) at 25 ± 2 °C and at a relative humidity of a 60 ± 5%. They were kept in darkness for the first 48 h, then, a photoperiod of 16:8 light/dark (4000 lx of light) was established. Once per day, seedbeds were watered with distilled water, depending on irrigation requirements of the materials.
C/C 0
4
0.0
0 S1
S2
S3
Column section Fig. 1. Normalized PAHs and pyrimethanil concentration (C/C0) and pH profile after the electrokinetic-Fenton treatment in the three studied sections after 34 d. Pyrimethanil (black), phenanthrene (grey), anthracene (grey bar with lines), pyrene (white bar with black zigzag lines), fluoranthene (grey bars with black big circles) and benz[a]anthracene (grey bars with black point). pH is represented by the line that goes from S1 to S3 with 3 black points (pH at each section).
Please cite this article in press as: Bocos, E., et al. Removal of PAHs and pesticides from polluted soils by enhanced electrokinetic-Fenton treatment. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2014.12.049
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3.3. Fenton-like treatment of polluted soil Several studies have reported the significant influence of the pH in soil on the electrokinetic treatment, affecting the contaminant retention in the soil and the electro-osmotic flow (Pazos et al., 2008). Thus, many authors have followed different strategies to control soil pH (Lysenko and Mishchuk, 2009; Gomes et al., 2012) like polarity exchange technique (Pazos et al., 2006) and pH control in electrode chambers (Wong et al., 1997). On this context, the addition of complexing agents, bounds the contaminant particles to the soil surface and then form stronger water-soluble complexes. Thus, coordination compounds of a chelate-type, would be presented by two or more separate bonds among bidentate complexes or multidentate ligands, and removed through the enhanced electrokinetic remediation technology (Ren et al., 2014). Taking into account the aforementioned, different facilitating agents were tested in order to keep the metals in solution and increase the mobilization of the species in the soil. As it is wellknown, several facilitating agents have the ability to render contaminants desorbed and dissolved or solubilized on soil particle surfaces and/or in soil pores. They may be categorized in the three following groups i.e. surfactants, complexing agents and cosolvents (Mousset et al., 2014). Apart from boosting interaction between soil-solution and contaminants these compounds can also mobilize the solubilized pollutants (Ren et al., 2014). In this study, the behaviour of four complexing agents, used as facilitating agents was evaluated in order to keep the metals in solution. As it is shown in Fig. 2, the pesticide pyrimethanil was easily degraded
100
80
Pollutants removal (%)
This behaviour can be easily deduced, based on the metal characteristics, pH of the interstitial solution, the solubility constants of the corresponding metal hydroxides, and the stability constants for the formation of metal complexes. Under the effect of an electric field, metal cations move toward the cathode at a velocity that directly depends on its ionic mobility. Usually, ferric ion has the highest ionic mobility however in this study; it shows the lowest removal in comparison with the other metals. The retention of iron in the soil was owed to the formation of hydroxides and the pH of the interstitial fluid. Taking into account the solubility product of the metal hydroxides, and the pH value of their formation, it can be explained the initial precipitation of the iron hydroxides in S3 when the basic front penetrated into the soil from the cathode chamber, since it shows the lowest solubility product. The pH value in the soil was increasingly slower owing to the migration of OH from the cathode, and therefore, the other metals were also retained into the soil sample when the pH value of the interstitial fluid exceeded the pH of the formation of the corresponding metal hydroxide. In summary, the assay revealed the precipitation of metals in S3 in the following order: Fe, Zn and Mn. These results are in accordance with those obtained by Pazos et al. (2008), who also observed the precipitation of metals present in kaolinite samples followed in the order: Fe, Cu, Zn and Mn. As a result of the precipitated formed in S3, the mobility of the species was lower and the efficiency in the removal of the organic pollutants was reduced, reaching values of 59%, 48%, 60%, 67%, 48% and 52% of pyrimethanil, phenanthrene, anthracene, fluoranthene, pyrene and benz[a]anthracene removal respectively, after 34 d of treatment, barely the same amount of contaminant removed after 27 d of treatment. However, pollutants were not detected in the cathode chamber, which evidences the in situ degradation of these compounds. Additionally, the pollutants that could have reached the cathode chamber by electro-osmotic flow were oxidized by means of the presence of iron and hydrogen peroxide in the chamber, both combined; promote the Fenton reactions to take place, attaining their degradation.
60
40
20
0 Pyrimethanil
Phenanthrene Anthracene
Pyrene
Fluoranthene Benz[a]anthracene
Fig. 2. Degradation levels reached after 24 h of Fenton-like treatment of polluted soil in presence of the selected complexing agents: ascorbic acid (black), citric acid (white with black lines), oxalic acid (grey) and EDTA (white).
from the soil in all cases. Nevertheless, PAHs showed different behaviour and they were not completely removed. One of the disadvantages of the chelating agents is the possible oxidation of these compounds that can compete for hydroxyl radicals with the pollutant (Sillanpaa et al., 2011). Thereby, since PAHs are a complex class of organic compounds with low solubility, the lower degradation yields obtained for these compounds can be easily explained regarding the results obtained by Pardo et al. (2014). In this study, the authors demonstrated a directly relationship between the solubility of the pollutant and the activity of the chelant, hence the lower the solubility of the organic pollutant the higher the oxidant scavenging by the chelant. As it was expected, the degradation level was higher for phenanthrene and anthracene (with three fused aromatic rings) and lower for compounds with more complex structures such as fluoranthene, pyrene and benz[a]anthracene. The best results were obtained when citric acid and ascorbic acid were used as complexing agent. However, citric acid was, both economical and environmentally speaking, a friendly weak acid with demonstrated buffering capacity (Pazos et al., 2009, 2012). Based on previous studies developed by Pazos et al. (2012), the Fenton-like treatment was henceforth performed using citric acid (0.2 M) as complexing agent.
3.4. Enhanced electrokinetic-Fenton treatment by pH control with citric acid In the electrokinetic-Fenton treatment, the complexing agents are used to enhance the desorption of the entrapped pollutant, and solubilizing part of the iron from the soil (Reddy and Chandhuri, 2009). As it was previously demonstrated in Section 3.3, the addition of citric acid, as complexing agent, into the soil allows dissolving the iron metals and forms soluble Fe-citrate that remains available for the Fenton oxidation. These findings confirm those proved in previous studies (Xiu and Zhang, 2009) where citric acid was highly efficient for the recovery of Zn and Mn, those which generated the precipitate in the first experiments. Thereby, in this case, citric acid will allow pH to remain in an appropriated value along the soil in the electrokinetic cell. As it was mentioned above, this soil has low acid buffering capacity and so it can be easily acidified. For this reason, two experiments were carried out in order to evaluate desorption and oxidation of the absorbed organic contaminants. In both cases, electrode chambers were filled with the electrolyte, and citric acid (0.2 M) was used as pH control in the cathode chamber. In the former, a pH of 2 was kept whereas in the latter, it was maintained at 5.
Please cite this article in press as: Bocos, E., et al. Removal of PAHs and pesticides from polluted soils by enhanced electrokinetic-Fenton treatment. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2014.12.049
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E. Bocos et al. / Chemosphere xxx (2015) xxx–xxx
Therefore, the acid environment forces the pollutants to remain in the interstitial solution and then, they are transported toward the electrode chamber fostered by the action of the electric field (Ribeiro et al., 2005). At the end of both treatments, the pH of the soil becomes slightly acid in all sections, being 2.5 for the control assay at pH 2 and 4.0 for the one regulated at pH 5. Nevertheless, a treatment with a pH control at the cathode of 5 will cause less disruption in the natural soil properties, so that it appears to be more appropriated. As depicted in Fig. 3, even when the HPLC analysis revealed a near complete removal of the studied pollutants for both of the enhanced electrokinetic-Fenton treatments, the obtained degradation rates were higher for the experiment using controlled at pH 5 than for the experiment at pH 2 and much higher than those yields achieved by the unenhanced electrokinetic-Fenton. Besides, degradation products such as anthraquinone, which is a secondary product on the degradation of anthracene, were identified. These results confirm the approach of the in situ degradation of the compounds inside the soil due to the oxidation reactions that take place there. Moreover, it must be remarked that the addition of citric acid increased the electro-osmotic flow (around 18.7%) with respect to the unenhanced treatment, which had an average value of 7.1 mL d1, and that explains as well the higher average recoveries founded at the end of this treatment. To summarize, the enhanced electrokinetic-Fenton treatment improved the removal levels, in comparison with experiments performed without citric acid, due to the absence of the basic front in the soil. These results are in accordance with those found by other researchers, who have demonstrated that the presence of citric acid in the soil increased the electro-osmotic flow favouring metal removal (Eykholk and Daniel, 1994). Yuan and Chiang (2008), for example, found that citric acid can inhibit the precipitation of metal hydroxides when it is used as pH conditioner; consequently, when this compound was present the dissolution reaction became dominant in the electrokinetic system. The obtained results revealed that addition of citric acid to the cathode chamber has two effects: to neutralize the alkaline environment and to obtain ligands that form stable complexes with metals. 3.5. Phytotoxicity assays Phytotoxicity assays have been extensively used to understand the potential environmental impacts of treated soils (Giannis et al., 2008). Thereby, once the efficiency of the treatment was optimized, this study was performed to evaluate the effect of the different conditions used to treat the soil. 100
Pollutants removal (%)
80
60
40
20
Commercial substrate
Soil after enhanced treatment (pH 5)
Soil after unenhanced treatment
Polluted Soil
0
20
40
60
80
100
GI (%) Fig. 4. Phytotoxicity levels using as soil: commercial substrate (white), spiked soil (coarse), from unenhanced electrokinetic-Fenton treatment (black) and from enhanced electrokinetic-Fenton treatment with pH control 5 (grey).
Following the procedure described by Macía et al. (2014), after the cultivation period, plants were removed and germination and growth parameters of the seedlings were examined (number of germinated seeds and length of the roots). Germination and growth of the test L. perenne were studied through the germination index (GI) (Zucconi and de Bertoldi, 1987), according to Eq. (1):
GIð%Þ ¼ ½ðSG SLÞ=ðCG CLÞ 100
ð1Þ
where SG and CG are sample and control germination percentage, respectively; and SL and CL are lengths of the roots (cm) of the sample and control, respectively. A value of germination greater than 80% indicates no phytotoxicity, between 50% and 80% a moderate phytotoxicity and values less than a 50% a high phytotoxicity (Zucconi and de Bertoldi, 1987). Fig. 4 depicts the phytotoxicity levels of the soil treated with the electrokinetic-Fenton process enhanced with citric acid at pH control of 5, which exhibits low levels of phytotoxicity, attaining similar germination index than those reached by the control culture. However, unenhanced electrokinetic-Fenton shows moderate phytotoxicity while, as expected, polluted soil presented high levels of phytotoxicity, with a GI lower than 25%. These results evidence the reduction of the soil pollution, without negative effects on the soil, by an adequate enhancement of the electrokineticFenton process. 4. Conclusions In view of the obtained results, it can be concluded that the addition of citric acid to electrokinetic-Fenton treatment to a soil polluted with PAHs and the pesticide pyrimethanil seems to improve the degradation, promoting the regulation of pH of the soil. However, the pH control at 5 appears to be the most suitable at the end of the assays, since it causes less disruption of the final soil properties. Moreover, the phytotoxicity tests performed after different treatments of the soil were found to be an interesting alternative to determine its final conditions. These tests corroborate the efficiency of the enhanced treatment, which does not result in negative consequences in the soil, since similar results were obtained in comparison with those obtained in the control cultures. Thus, these results allow to conclude that when operating with the addition of citric acid as a flushing solution in an acid environment, the electrokinetic-Fenton treatment is a feasible way to fulfil the in situ remediation of a soil polluted with persistent contaminants. Acknowledgments
0 Pyrimethanil Phenanthrene Anthracene Fluoranthene
Pyrene Benz[a]anthracene
Fig. 3. Total removal percentages reached for each pollutant after 27 d of treatment by: unenhanced electrokinetic-Fenton (black), enhanced electrokinetic-Fenton with citric acid at pH 2 (white) and pH 5 (grey).
This research has been financially supported by the Spanish Ministry of Economy and Competitiveness and by ERDF Funds (Project CTM 2011-26423). The authors are grateful to the Spanish
Please cite this article in press as: Bocos, E., et al. Removal of PAHs and pesticides from polluted soils by enhanced electrokinetic-Fenton treatment. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2014.12.049
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Removal of PAHs and pesticides from polluted soils by enhanced electrokinetic-Fenton treatment Elvira Bocos, Carmen Fernández-Costas, Marta Pazos, M. Ángeles Sanromán ⇑ Department of Chemical Engineering, University of Vigo, Isaac Newton Building, Campus As Lagoas, Marcosende 36310, Vigo, Spain
h i g h l i g h t s The electrokinetic-Fenton technology has been used for the removal of PAHs and pesticide form soil. The metal precipitation was demonstrated as the main drawback of the treatment. The addition of citric acid enhances the treatment efficiency increasing the reached removals. After the electrokinetic-Fenton using citric acid the treated soil showed low phytotoxicity levels.
a r t i c l e
i n f o
Article history: Received 11 October 2014 Received in revised form 14 December 2014 Accepted 15 December 2014 Available online xxxx Handling Editor: E. Brillas Keywords: Citric acid Complexing agents Iron PAHs Phytotoxicity Pyrimethanil
a b s t r a c t In this study, electrokinetic-Fenton treatment was used to remediate a soil polluted with PAHs and the pesticide pyrimethanil. Recently, this treatment has emerged as an interesting alternative to conventional soil treatments due to its peculiar advantages, namely the capability of treating fine and low-permeability materials, as well as that of achieving a high yield in the removals of salt content and inorganic and organic pollutants. In a standard electrokinetic-Fenton treatment, the maximum degradation of the pollutant load achieved was 67%, due to the precipitation of the metals near the cathode chamber that reduces the electro-osmotic flow of the system and thus the efficiency of the treatment. To overcome this problem, different complexing agents and pH control in the cathode chamber were evaluated to increase the electro-osmotic flux as well as to render easier the solubilization of the metal species present in the soil. Four complexing agents (ascorbic acid, citric acid, oxalic acid and ethylenediaminetetraacetic acid) in the Fenton-like treatment were evaluated. Results revealed the citric acid as the most suitable complexing agent. Thereby its efficiency was tested as pH controller by flushing it in the cathode chamber (pH 2 and 5). For the latter treatments, near total degradation was achieved after 27 d. Finally, phytotoxicity tests for polluted and treated samples were carried out. The high germination levels of the soil treated under enhanced conditions concluded that nearly complete restoration was achieved. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction During the last decades, contamination in soils and groundwater has incremented. Unfortunately, this pollution is causing important environmental problems, particularly in soils, due to the persistence of the pollutants, which are strongly sorted there, becoming difficult to remove. It is well known that large amounts of organic pollutants, most of them arising from the anthropogenic action, are being released daily to the environment. Although some of these compounds are easily degraded by microorganisms (Liao ⇑ Corresponding author. Tel.: +34 986 812383; fax: +34 986 812380. E-mail address: [email protected] (M. Á. Sanromán).
and Li, 2012), many others such as Polycyclic Aromatic Hydrocarbons (PAHs) and pesticides are persistent in the environment owing to its low water solubility and low volatility. Over the past decade, the treatment of these compounds has become matter of interest for numerous authors. In this context, different technologies such as bioremediation, chemical treatments and physical treatments (Pazos et al., 2010) have been applied to remediate soils, sediments and groundwater. Polluted soils can be treated by ex situ and in situ methods. Ex situ technologies have proved to ameliorate decontamination of soils, whereas in situ treatments are preferred since they are not invasive techniques. Therefore, they cause less disruption to the environment and allow lowering the economic costs (Romantschuk et al.,
http://dx.doi.org/10.1016/j.chemosphere.2014.12.049 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Bocos, E., et al. Removal of PAHs and pesticides from polluted soils by enhanced electrokinetic-Fenton treatment. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2014.12.049
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2000; Mulligan et al., 2001). Among the different in situ treatments, it is important to highlight the electrokinetic remediation. Since 1990, several studies have pointed out the cost-efficiency of this technology to remediate polluted soils. It has been also recognized as a promising method for the treatment of different kinds of organic and inorganic low-permeable soils. Electrokinetic treatments have been widely studied and already successfully applied on real polluted soils, water and sediments. In this study, the method used is based on the application of a low intensity current to an electrokinetic cell filled with a polluted soil, both of its extremes connected to two electrode chambers. This current promotes the movement of the pollutants from the anode chamber toward the cathode chamber, where they are finally collected and treated. In this case, the pollutants are mainly transported by the electro-migration of the ionic species in the pore fluid toward the opposite electrode and by the electro-osmotic flow generated between the anode and the cathode (Acar, 1993). Notwithstanding, one of the main disadvantages of this technology relies on the concentration of the pollutants in a small area of soil close to the cathode chamber due to the precipitation of several species present in the soil. Thus it is also necessary to treat the liquid of the electrolyte chamber in order to obtain the total pollutants degradation (Ren et al., 2014). In order to overcome these problems several technologies have been performed in combination to electrokinetic treatments. Over the last years, Advanced Oxidative Processes (AOPs) have attracted the interest of the scientific community, since they are environmentally friendly methods powerful in removing organic contaminants by the production of hydroxyl radicals as oxidizing agent. Besides, AOPs may enhance the treatment by the in situ degradation of pollutants, avoiding the necessity of treating the electrolytes collected from the chambers. As it was recently reported by Huang et al. (2014), chemical oxidation typically involves reduction/oxidation reactions that transform chemically hazardous contaminants into non-hazardous or less toxic, mobile or inert compounds. Habitually, the most common oxidants used are ozone, hydrogen peroxide, hypochlorites, chlorine, chlorine dioxide, potassium permanganate and Fenton’s reagent (hydrogen peroxide and iron). Over the last decades, Fenton process has been tested by several researchers in the remediation of soils (Kim et al., 2006; Alcántara et al., 2008) and marine sediments (Pazos et al., 2013). It is the case of some work developed by Yap et al. (2011), demonstrated the efficiency of the Fenton treatment for the remediation of Polycyclic Aromatic Hydrocarbons-polluted soils. This reaction is based on the generation of hydroxyl radicals by the combination of hydrogen peroxide and ferrous ions. Hydroxyl radicals are the strongest second oxidants after fluorine and they can react even with most of the organic persistent contaminants until their complete mineralization. However, the generation of these radicals is only effective under low pH (Valentine and Ann Wang, 1998). In the electrokinetic-Fenton treatment of soils Fenton’s reagent is used as a flushing agent, so that the removal of organic contaminants could be achieved by an in situ oxidation/destruction (Alcántara et al., 2008). Several researchers have already pointed out, the generation of oxidant species by the use of other Fenton’s reagents. In this context, some authors have reported the utility to use this combined technology when using permanganate in the treatment of different soils and contaminants. For example, Wu et al. (2012) tested the oxidative remediation of a typical organic contaminant (tetrachloroethene) by an electrokinetic in situ chemical oxidation, using as oxidant agent permanganate. These authors observed a reduction of the oxidant migration in low-permeability media under the electrokinetic process, which was ascribed to the increased oxidant consumption of competing reductors. Another interesting study was conducted by Yang and Yeh (2011), who
proposed the injection of persulfate in combination to an electrokinetic process for the destruction of trichloroethylene in a spiked sandy clay soil. They reported the enhancement of the electrokinetic treatment by the injection of persulfate from anode to cathode through the electro-osmotic flow, aiding the in situ chemical oxidation of trichloroethylene. Recently Ng et al. (2014) described different paths to enhance the electrokinetic-Fenton treatment such as the increase of the oxidant availability, electrolytes and H2O2 concentration. These authors also pointed out the importance of the type of soil and contaminants. They concluded that soils with low acid buffering capacity, adequate iron concentration, low organic matter content and low aromatic ring of the organic contaminants generally end up in higher efficient treatments. Ren et al. (2014) improved the remediation efficiency when they attained the desorption and/or solubilization of metals and organic matters in soil, which allows to enhance the conventional electrokinetic remediation. Enhanced techniques currently used in electrokinetic are mainly directed toward maintaining or bringing contaminants into solution by addition of facilitating agents and controlling pH. In this study, electrokinetic-Fenton treatment using H2O2 as flushing agent, as reported before by Tsai et al. (2010) was carried out. The remediation of a soil spiked with several persistent organic pollutants (PAHs and pyrimethanil) was studied and improved. In the former case, the effect of chemical properties (facilitating agents, pH control and iron content of the soil) was evaluated, in order to yield high degradation levels at the end of the treatment. Finally, the soil obtained after the most favourable treatment was evaluated in terms of phytotoxicity.
2. Materials and methods 2.1. Soil A real soil collected from a vineyard in the North-west of Spain (Ribeira Sacra, Galicia) was employed. Soil samples were collected from 0 to 30 cm depth by using an Eijkelkamp sampler, transported to the laboratory in polyethylene bags and sieved. Only the fraction containing particles smaller than 2 mm was selected. Some of the most typical characterization data of soil samples such as pH, conductivity, total organic carbon, total inorganic carbon, total carbon, organic matter and the presence of some metals are depicted in Table 1. In terms of the overall concentration of metals, iron was the most abundant metal found in the sample. This aspect is important because iron is known to play an important role in the degradation of organic compounds by Fenton reaction. Table 1 Chemical characterization of the studied soil. Parameters
Value
pH Hydraulic conductivity (cm s1) Inorganic C (%) Organic C (%) Total C (%) Organic matter (%) Cu (mg kg1) Zn (mg kg1) As (mg kg1) Ba (mg kg1) Cr (mg kg1) Pb (mg kg1) Ni (mg kg1) Fe (mg kg1) Co (mg kg1) Mn (mg kg1)
6.8 7 108 0.8 2.4 3.2 4.2 242 838.8 13.9 137.6 55.8 17.3 41.5 33,594 17.7 809.8
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2.2. Spiked soil The pollutants (pyrimethanil, phenanthrene, anthracene, fluoranthene, pyrene benz[a]anthracene) and the hexane were provided by Sigma–Aldrich. Initially, 250 mg of each pollutant per kg were dissolved in 2.5 L of hexane. Afterwards, 2 kg of soil were spiked with this solution. This soil–pollutant–hexane mixture was placed beneath a ventilation hood for 9 d and stirred every day until the hexane was completely evaporated and the spiked soil was dried. Afterwards, a sample of the soil was taken to corroborate that the initial concentration of pollutants in the soil was the desired one, since a portion of the contaminants might have volatilized along with the solvent. 2.3. Soil treatments 2.3.1. Fenton-like treatment of the spiked soil The effect of several facilitating agents (ascorbic acid, citric acid, oxalic acid and ethylenediaminetetraacetic acid (EDTA)) on Fenton treatment of the polluted soil was performed following the experimental procedure described by Vicente et al. (2011). The Fenton-like treatment was carried out in glass tubes of 10 mL, where 3 g of sample were introduced. Finally, 6 mL of a solution of H2O2 (0.29 M) and 0.50 mM of each facilitating agent were mixed with the soil and keeping in agitation during 24 h. 2.3.2. Electrokinetic-Fenton treatment This treatment was performed in a cylindrical glass cell containing a sample compartment of 100 mm length and 32 mm of internal diameter. The two electrode chambers (filled with 300 mL of the process fluid) were placed at the end of the electrochemical cell. The sample compartment was isolated from the test matrix by means of a filter paper and porous stones. Graphite electrodes were used for both anode and cathode. Three auxiliary electrodes have allowed measurement of the electric field distribution along the sample. Electrode chambers were filled with a H2O2 (10% w/ v) solution and with Na2SO4 (0.1 M), this last was used to improve the conductivity of the solution and, hence, the current intensity (Kim et al., 2006). This fact, favours the electroosmotic flow due to the increase of the total ion concentration in the pore fluid. The electrolyte solution was renovated each 12 d in order to maintain its effect as buffering solution. As mentioned before, iron content in soil was elevated, in fact it was higher enough to activate the decomposition of H2O2, consequently enhancing the generation of hydroxyl radicals. A constant potential difference of 30 V was applied in all experiments. Readings of voltage drop, current, intensity and pH in the electrode compartments, were taken periodically (Pazos et al., 2012). 2.4. Analytical determinations At the beginning and at the end of each electrokinetic experiment, liquid samples were taken from the cathode and anode solutions as well as from the solid matrix for chemical analysis. Soil sample was divided into three equal sections (S1–S3, namely from anode to cathode) were analyzed for moisture content, pH and pollutant concentrations. Additionally, during the treatment liquid, samples were taken periodically from the electrolyte chambers, centrifuged at 10,000 rpm for 5 min and kept refrigerated to be analyzed. 2.4.1. PAHs and pesticide concentration To determine PAHs and pyrimethanil concentration in soil from each section (S1–S3), they were extracted from dry samples by a pressurized solvent extraction system using an OnePSE instrument (Applied Separations Inc.). Dry sample was thoroughly mixed with
3
pelletized diatomaceous earth. When a free-flowing powder was obtained, it was placed into an extraction vessel in the instrument. The extraction solution was composed of acetone/hexane (1:1, v/v). After four cycles of 5 min at 110 °C and 104 kPa, the extraction was complete and the collected sample was used to determine pollutant concentration. Pressurized solvent extraction can be used to replace soxhlet and sonication techniques and has been approved for use, following EPA Method 3545A. All analytical determinations were done in triplicate with an experimental error below 15%. Thus, the concentration of PAHs and of the pesticide in the extractions and the liquid samples collected at the end of the treatment was determined using an Agilent 1100 HPLC equipped with a XDB-C8 reverse phase column. The injection volume was set at 5 lL and the isocratic eluent (33:67 acetonitrile/water) was pumped at a rate of 1 mL min1.
2.4.2. Metal concentration The protocols used for the chemical extraction and analysis of metals were performed in accordance with EPA Methods 3010 and 3050 for the analysis of metals in liquid and solid samples, respectively. A mixture of concentrated nitric acid (5 mL) and the desired sample (100 mL liquid or 2 g soil) was refluxed in a covered Erlenmeyer flask. This step was repeated with additional portions of nitric acid until the mixture got light colour or until its colour was stabilised. After the digestate was brought to a low volume (3 mL), the sample was refluxed with 5 mL of hydrochloric acid (1:1) for 15 min. The sample was then filtered and adjusted to a final volume of 100 mL. Inductively Coupled Plasma–Optical Emission Spectrometer (ICP–OES) Perkin Elmer Optima 4300 DV was used to analyse metals. The experiments were performed at less twice with an experimental error, calculated as standard deviation, below of 15%.
2.4.3. pH The pH is one of the main parameters affecting the mobility of the contaminants. Therefore, at the end of each experiment, the pH value of the solid samples was measure by mixing 1 g of dry sample with 20 mL of deionized water. Afterwards, the mixture was kept in repose during 1 h and the pH was measured with a Sentron pH meter (model 1001).
2.4.4. Buffering capacity of the soil Buffering capacity is defined as the ability of a solid matrix to resist changes in pH. Acid buffering capacity of the sludge suspension was assayed by titration. A suspension of sludge sample in water (6.7% w/v) was stirred for 30 min and the pH was analyzed. Successive additions of 1 mL of 1 M HCl were made every 30 min and pH was measured thereafter. This procedure was repeated until the pH value was constant. The results were elucidated in a qualitative form, comparing the profiles corresponding to sludge sample with the obtained for the kaolin with low buffering capacity as it was previously reported (Pazos et al., 2012).
2.4.5. Electrical conductivity, moisture content and TOC Conductivity determination was accomplished adding 100 mL of distilled water to 20 g of the sample and shaking them for 30 min. After that, suspensions were filtered and conductivity was measured into the liquid fraction with a conductivity meter Crison Basic 30. The moisture content of the sediment was calculated with the loss of weight of the sample after heating at 105 °C during 24 h. Total organic carbon (TOC) was determined using the dichromate method proposed by Shumacher (2002).
Please cite this article in press as: Bocos, E., et al. Removal of PAHs and pesticides from polluted soils by enhanced electrokinetic-Fenton treatment. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2014.12.049
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3. Results and discussion 3.1. Soil characterization Initially, a detailed study of soil characteristics was necessary in order to test different strategies that may improve the efficacy of the electrokinetic treatment. Some factors, such as soil origin, pH, metal content, conductivity and buffering capacity, were analyzed. The analytical measures reveal that this soil possesses a slightly acid pH (6.76) and the highest metal concentrations correspond to Fe, Mn and Zn (Table 1). Since iron is one of the most important variables on the Fenton treatment, it is worth noting its high concentration, around 33594 mg of Fe kg1 of dried soil, which makes the sample appropriate to promote the Fenton’s reactions to take place. It is well known the great influence of the acid buffering capacity of the soils on the remediation process, since desorption of metals is favoured at low pH. Thus, after the analysis of our sample and comparing it with kaolin, this soil showed a similar behaviour than kaolin. This means that the sample has a low acid buffering capacity, which was attributed to its chemical composition. In addition, the hydraulic conductivity value can be used to extract information of the soil behaviour under the electric field action. Since the conductivity of the studied soil was high (Table 1), a low electrical resistance was expected, which will favour the electrokinetic treatment. 3.2. Electrokinetic–Fenton treatment Initially, unenhanced electrokinetic-Fenton treatment of polluted soil was carried out in 4 electrokinetic cells at constant potential difference of 3 V cm1 without addition of any facilitating agent. The electrode chambers were filled with H2O2 (10%) and, as in previous studies, 0.1 M Na2SO4 was used as electrolyte (Alcántara et al., 2008). Each cell was treated during 7, 14, 27 and 34 d in order to observe the degradation level of the contaminants trough the electrokinetic cell along the time. Table 2 depicted an average between the three sections of the soil for the obtained degradation levels of each organic pollutant. The results show a progressive degradation along the time of all pollutants; however after 27 d the maximum degradation values were reached
(67%) and not improvement was detected at higher treatment time. As it was described by Li et al. (1996), when the electrokinetic method is used to remove metals from soils, it is expected to find precipitate of hydroxides in the region of the soil where pH raises, limiting the remediation efficiency. The pH rise is caused by the generation of hydroxide ions as a result of electrolysis of water during the remediation. The presence of H2O2 in the electrokinetic cell normally avoids extremely high/low pH environments in the electrode chambers (Alcántara et al., 2008), however, in this study; the absence of strong buffering species in the soil did not allow this effect to last for long periods of treatment. As depicted in Fig. 1, after 34 d the pH was approximately 4 near the anode and amounted to pH 9.5 near the cathode. This fact suggests that a precipitated was formed at the end of the treatment in the proximities of the cathode chamber with the consequent blockage of the electro-osmotic flow and the restriction of the metals mobility. All the tested metals migrated toward the cathode. For example, Zn, Mn and Fe showed lower removal after 34 d of electrokinetic treatment. Only 18%, 27% and 14% of Zn, Mn and Fe, respectively were recovered in the cathode chamber, while the rest remained into the soil sample, mainly in the section close to the cathode (S3). On the other hand, as showed in Fig. 1, in terms of degradation the pollutants, almost total pollutants removal was obtained in the Section 1 however the removal was lower in Sections 2 and 3. This fact is a direct consequence of metals precipitation in the proximities of the cathode, (mainly Fe, which is essential in Fenton’s reaction). Besides, the electro-osmotic flow, which is the main responsible of the presence of H2O2 in the soil, was stopped with the blockage due to the precipitated. Thus, the degradation was reduced in the last sections due to both Fenton’s reagents were unable to react.
Table 2 Degradation levels of the different pollutants along the treatment time by the unenhanced electrokinetic-Fenton process. Compound
Degradation (%) 7 d
Degradation (%) 14 d
Degradation (%) 27 d
Degradation (%) 34 d
Pyrimethanil Phenanthrene Anthracene Fluoranthene Pyrene Benz[a]anthracene
24 26 40 14 9 12
28 28 48 18 23 12
58 49 59 67 48 52
59 48 60 67 48 51
1.0
10
0.8
8
0.6
6
0.4
4
0.2
2
pH
2.4.6. Phytoremediation assays After optimize the treatment, the phytotoxicity of soil was studied. Toxicity tests were accomplished following the methodology described by Macía et al. (2014) which is a modification of a standard method (OECD, 2000, 2006). Lolium perenne (ray grass) seeds were supplied by Fitó S.A. A culture with peat moss was employed as a control sample in the growth trials because it is only a physical support for plant growth; therefore, it allows establishing a reference of the germination and growth. A minimum germination of 80% in control samples was required to consider the test valid. The seedbeds used in the phytotoxicity assays had no holes in their bottom to avoid leaching of the contaminants and to evaluate the real toxicity of the soil samples. Every seedbed was filled with 50 g of fresh material and 0.3 g (around 150 seeds) of L. perenne were sown per seedbed and cultivated for 10 d. All tests were performed in triplicate. Germination and growth were carried out in an environmental control chamber (SANYO Versatile Environmental Test Chamber) at 25 ± 2 °C and at a relative humidity of a 60 ± 5%. They were kept in darkness for the first 48 h, then, a photoperiod of 16:8 light/dark (4000 lx of light) was established. Once per day, seedbeds were watered with distilled water, depending on irrigation requirements of the materials.
C/C 0
4
0.0
0 S1
S2
S3
Column section Fig. 1. Normalized PAHs and pyrimethanil concentration (C/C0) and pH profile after the electrokinetic-Fenton treatment in the three studied sections after 34 d. Pyrimethanil (black), phenanthrene (grey), anthracene (grey bar with lines), pyrene (white bar with black zigzag lines), fluoranthene (grey bars with black big circles) and benz[a]anthracene (grey bars with black point). pH is represented by the line that goes from S1 to S3 with 3 black points (pH at each section).
Please cite this article in press as: Bocos, E., et al. Removal of PAHs and pesticides from polluted soils by enhanced electrokinetic-Fenton treatment. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2014.12.049
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3.3. Fenton-like treatment of polluted soil Several studies have reported the significant influence of the pH in soil on the electrokinetic treatment, affecting the contaminant retention in the soil and the electro-osmotic flow (Pazos et al., 2008). Thus, many authors have followed different strategies to control soil pH (Lysenko and Mishchuk, 2009; Gomes et al., 2012) like polarity exchange technique (Pazos et al., 2006) and pH control in electrode chambers (Wong et al., 1997). On this context, the addition of complexing agents, bounds the contaminant particles to the soil surface and then form stronger water-soluble complexes. Thus, coordination compounds of a chelate-type, would be presented by two or more separate bonds among bidentate complexes or multidentate ligands, and removed through the enhanced electrokinetic remediation technology (Ren et al., 2014). Taking into account the aforementioned, different facilitating agents were tested in order to keep the metals in solution and increase the mobilization of the species in the soil. As it is wellknown, several facilitating agents have the ability to render contaminants desorbed and dissolved or solubilized on soil particle surfaces and/or in soil pores. They may be categorized in the three following groups i.e. surfactants, complexing agents and cosolvents (Mousset et al., 2014). Apart from boosting interaction between soil-solution and contaminants these compounds can also mobilize the solubilized pollutants (Ren et al., 2014). In this study, the behaviour of four complexing agents, used as facilitating agents was evaluated in order to keep the metals in solution. As it is shown in Fig. 2, the pesticide pyrimethanil was easily degraded
100
80
Pollutants removal (%)
This behaviour can be easily deduced, based on the metal characteristics, pH of the interstitial solution, the solubility constants of the corresponding metal hydroxides, and the stability constants for the formation of metal complexes. Under the effect of an electric field, metal cations move toward the cathode at a velocity that directly depends on its ionic mobility. Usually, ferric ion has the highest ionic mobility however in this study; it shows the lowest removal in comparison with the other metals. The retention of iron in the soil was owed to the formation of hydroxides and the pH of the interstitial fluid. Taking into account the solubility product of the metal hydroxides, and the pH value of their formation, it can be explained the initial precipitation of the iron hydroxides in S3 when the basic front penetrated into the soil from the cathode chamber, since it shows the lowest solubility product. The pH value in the soil was increasingly slower owing to the migration of OH from the cathode, and therefore, the other metals were also retained into the soil sample when the pH value of the interstitial fluid exceeded the pH of the formation of the corresponding metal hydroxide. In summary, the assay revealed the precipitation of metals in S3 in the following order: Fe, Zn and Mn. These results are in accordance with those obtained by Pazos et al. (2008), who also observed the precipitation of metals present in kaolinite samples followed in the order: Fe, Cu, Zn and Mn. As a result of the precipitated formed in S3, the mobility of the species was lower and the efficiency in the removal of the organic pollutants was reduced, reaching values of 59%, 48%, 60%, 67%, 48% and 52% of pyrimethanil, phenanthrene, anthracene, fluoranthene, pyrene and benz[a]anthracene removal respectively, after 34 d of treatment, barely the same amount of contaminant removed after 27 d of treatment. However, pollutants were not detected in the cathode chamber, which evidences the in situ degradation of these compounds. Additionally, the pollutants that could have reached the cathode chamber by electro-osmotic flow were oxidized by means of the presence of iron and hydrogen peroxide in the chamber, both combined; promote the Fenton reactions to take place, attaining their degradation.
60
40
20
0 Pyrimethanil
Phenanthrene Anthracene
Pyrene
Fluoranthene Benz[a]anthracene
Fig. 2. Degradation levels reached after 24 h of Fenton-like treatment of polluted soil in presence of the selected complexing agents: ascorbic acid (black), citric acid (white with black lines), oxalic acid (grey) and EDTA (white).
from the soil in all cases. Nevertheless, PAHs showed different behaviour and they were not completely removed. One of the disadvantages of the chelating agents is the possible oxidation of these compounds that can compete for hydroxyl radicals with the pollutant (Sillanpaa et al., 2011). Thereby, since PAHs are a complex class of organic compounds with low solubility, the lower degradation yields obtained for these compounds can be easily explained regarding the results obtained by Pardo et al. (2014). In this study, the authors demonstrated a directly relationship between the solubility of the pollutant and the activity of the chelant, hence the lower the solubility of the organic pollutant the higher the oxidant scavenging by the chelant. As it was expected, the degradation level was higher for phenanthrene and anthracene (with three fused aromatic rings) and lower for compounds with more complex structures such as fluoranthene, pyrene and benz[a]anthracene. The best results were obtained when citric acid and ascorbic acid were used as complexing agent. However, citric acid was, both economical and environmentally speaking, a friendly weak acid with demonstrated buffering capacity (Pazos et al., 2009, 2012). Based on previous studies developed by Pazos et al. (2012), the Fenton-like treatment was henceforth performed using citric acid (0.2 M) as complexing agent.
3.4. Enhanced electrokinetic-Fenton treatment by pH control with citric acid In the electrokinetic-Fenton treatment, the complexing agents are used to enhance the desorption of the entrapped pollutant, and solubilizing part of the iron from the soil (Reddy and Chandhuri, 2009). As it was previously demonstrated in Section 3.3, the addition of citric acid, as complexing agent, into the soil allows dissolving the iron metals and forms soluble Fe-citrate that remains available for the Fenton oxidation. These findings confirm those proved in previous studies (Xiu and Zhang, 2009) where citric acid was highly efficient for the recovery of Zn and Mn, those which generated the precipitate in the first experiments. Thereby, in this case, citric acid will allow pH to remain in an appropriated value along the soil in the electrokinetic cell. As it was mentioned above, this soil has low acid buffering capacity and so it can be easily acidified. For this reason, two experiments were carried out in order to evaluate desorption and oxidation of the absorbed organic contaminants. In both cases, electrode chambers were filled with the electrolyte, and citric acid (0.2 M) was used as pH control in the cathode chamber. In the former, a pH of 2 was kept whereas in the latter, it was maintained at 5.
Please cite this article in press as: Bocos, E., et al. Removal of PAHs and pesticides from polluted soils by enhanced electrokinetic-Fenton treatment. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2014.12.049
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Therefore, the acid environment forces the pollutants to remain in the interstitial solution and then, they are transported toward the electrode chamber fostered by the action of the electric field (Ribeiro et al., 2005). At the end of both treatments, the pH of the soil becomes slightly acid in all sections, being 2.5 for the control assay at pH 2 and 4.0 for the one regulated at pH 5. Nevertheless, a treatment with a pH control at the cathode of 5 will cause less disruption in the natural soil properties, so that it appears to be more appropriated. As depicted in Fig. 3, even when the HPLC analysis revealed a near complete removal of the studied pollutants for both of the enhanced electrokinetic-Fenton treatments, the obtained degradation rates were higher for the experiment using controlled at pH 5 than for the experiment at pH 2 and much higher than those yields achieved by the unenhanced electrokinetic-Fenton. Besides, degradation products such as anthraquinone, which is a secondary product on the degradation of anthracene, were identified. These results confirm the approach of the in situ degradation of the compounds inside the soil due to the oxidation reactions that take place there. Moreover, it must be remarked that the addition of citric acid increased the electro-osmotic flow (around 18.7%) with respect to the unenhanced treatment, which had an average value of 7.1 mL d1, and that explains as well the higher average recoveries founded at the end of this treatment. To summarize, the enhanced electrokinetic-Fenton treatment improved the removal levels, in comparison with experiments performed without citric acid, due to the absence of the basic front in the soil. These results are in accordance with those found by other researchers, who have demonstrated that the presence of citric acid in the soil increased the electro-osmotic flow favouring metal removal (Eykholk and Daniel, 1994). Yuan and Chiang (2008), for example, found that citric acid can inhibit the precipitation of metal hydroxides when it is used as pH conditioner; consequently, when this compound was present the dissolution reaction became dominant in the electrokinetic system. The obtained results revealed that addition of citric acid to the cathode chamber has two effects: to neutralize the alkaline environment and to obtain ligands that form stable complexes with metals. 3.5. Phytotoxicity assays Phytotoxicity assays have been extensively used to understand the potential environmental impacts of treated soils (Giannis et al., 2008). Thereby, once the efficiency of the treatment was optimized, this study was performed to evaluate the effect of the different conditions used to treat the soil. 100
Pollutants removal (%)
80
60
40
20
Commercial substrate
Soil after enhanced treatment (pH 5)
Soil after unenhanced treatment
Polluted Soil
0
20
40
60
80
100
GI (%) Fig. 4. Phytotoxicity levels using as soil: commercial substrate (white), spiked soil (coarse), from unenhanced electrokinetic-Fenton treatment (black) and from enhanced electrokinetic-Fenton treatment with pH control 5 (grey).
Following the procedure described by Macía et al. (2014), after the cultivation period, plants were removed and germination and growth parameters of the seedlings were examined (number of germinated seeds and length of the roots). Germination and growth of the test L. perenne were studied through the germination index (GI) (Zucconi and de Bertoldi, 1987), according to Eq. (1):
GIð%Þ ¼ ½ðSG SLÞ=ðCG CLÞ 100
ð1Þ
where SG and CG are sample and control germination percentage, respectively; and SL and CL are lengths of the roots (cm) of the sample and control, respectively. A value of germination greater than 80% indicates no phytotoxicity, between 50% and 80% a moderate phytotoxicity and values less than a 50% a high phytotoxicity (Zucconi and de Bertoldi, 1987). Fig. 4 depicts the phytotoxicity levels of the soil treated with the electrokinetic-Fenton process enhanced with citric acid at pH control of 5, which exhibits low levels of phytotoxicity, attaining similar germination index than those reached by the control culture. However, unenhanced electrokinetic-Fenton shows moderate phytotoxicity while, as expected, polluted soil presented high levels of phytotoxicity, with a GI lower than 25%. These results evidence the reduction of the soil pollution, without negative effects on the soil, by an adequate enhancement of the electrokineticFenton process. 4. Conclusions In view of the obtained results, it can be concluded that the addition of citric acid to electrokinetic-Fenton treatment to a soil polluted with PAHs and the pesticide pyrimethanil seems to improve the degradation, promoting the regulation of pH of the soil. However, the pH control at 5 appears to be the most suitable at the end of the assays, since it causes less disruption of the final soil properties. Moreover, the phytotoxicity tests performed after different treatments of the soil were found to be an interesting alternative to determine its final conditions. These tests corroborate the efficiency of the enhanced treatment, which does not result in negative consequences in the soil, since similar results were obtained in comparison with those obtained in the control cultures. Thus, these results allow to conclude that when operating with the addition of citric acid as a flushing solution in an acid environment, the electrokinetic-Fenton treatment is a feasible way to fulfil the in situ remediation of a soil polluted with persistent contaminants. Acknowledgments
0 Pyrimethanil Phenanthrene Anthracene Fluoranthene
Pyrene Benz[a]anthracene
Fig. 3. Total removal percentages reached for each pollutant after 27 d of treatment by: unenhanced electrokinetic-Fenton (black), enhanced electrokinetic-Fenton with citric acid at pH 2 (white) and pH 5 (grey).
This research has been financially supported by the Spanish Ministry of Economy and Competitiveness and by ERDF Funds (Project CTM 2011-26423). The authors are grateful to the Spanish
Please cite this article in press as: Bocos, E., et al. Removal of PAHs and pesticides from polluted soils by enhanced electrokinetic-Fenton treatment. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2014.12.049
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Please cite this article in press as: Bocos, E., et al. Removal of PAHs and pesticides from polluted soils by enhanced electrokinetic-Fenton treatment. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2014.12.049