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Soil flushing and simultaneous degradation of organic pollutants in soils by electrokinetic-Fenton treatment
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Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Soil flushing and simultaneous degradation of organic pollutants in soils by electrokinetic-Fenton treatment Marius Popescu a , Emilio Rosales b , Ciprian Sandu a , Jéssica Meijide b , Marta Pazos b , Gabriel Lazar a , Ma Angeles Sanromán b,∗ a
Faculty of Engineering, “Vasile Alecsandri” University of Bacau, Calea Marasesti 157, 600115 Bacau, Romania Department of Chemical Engineering, University of Vigo, Isaac Newton Building, Campus As Lagoas, Marcosende, 36310 Vigo, Spain
b
a r t i c l e
i n f o
a b s t r a c t
Article history:
This study focuses on the evaluation of a combination of electrokinetic technology and
Received 28 December 2015
Fenton’s process to remediate a soil polluted with organic compounds. To determine the
Received in revised form 14 March
influence of the several variables such as hydrogen peroxide dosage, iron soil concentration
2016
and porosity, different experiments using kaolinite spiked by Rhodamine B were performed.
Accepted 17 March 2016
The use of this coloured sample permitted an easy monitoring of the oxidation reactions
Available online xxx
across the soil bed. From the obtained results, it is concluded that the highest colour removal rate was reached when a solution of hydrogen peroxide around 10% was used, and slight
Keywords:
influence of iron soil concentration was detected at the range of concentrations used in
Electrokinetic-Fenton
these experiments. In all cases, citric acid was added in the anolyte and catholyte solutions
Hydrogen peroxide
in order to solubilize the iron as Fe-citrate complex and to keep the pH in acid environment
Iron
favouring that the Fenton’s reactions take place into the soil. Based on these preliminary
Porosity
experiments, the electrokinetic-Fenton process was applied to total petroleum hydrocar-
Rhodamine B
bons (TPH) polluted soil. After 15 and 27 days of treatment, a homogeneous removal of
TPH
pollutants, around 54.4% and 58.2% of TPH removal efficiency, was reached, respectively. In addition, the Microtox bioassays confirmed the reduction of the Vibrio fischeri inhibition after the soil treatment. Summing up, in situ electrokinetic-Fenton treatment seems to be a suitable technique for the remediation of organics such as hydrocarbons present in polluted soils. © 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1.
Introduction
Soil contamination by organic compounds has significantly increased due to several activities and factors such as farming, accidental emissions of harmful pollutants, industrial wastewater and the landfill leachate (Cheng et al., 2016). The presence of these compounds in soil has been causing serious incidents of soil pollution. This fact has led to a grave decline in the quality of agricultural products and is evolving in a growing
interest among those involved in environmental remediation (Manz et al., 2001). Nowadays, extensive studies have been carried out for the development of soil remediation techniques such as thermal desorption, excavation or dredging, pumping and treating, surfactant enhanced aquifer remediation, solidification and stabilization, soil vapour extraction, bioremediation, nanoremediation or in situ oxidation (Ramírez et al., 2015a, 2015b; Cobas et al., 2013; Gómez et al., 2010; Pazos et al., 2010, 2011,
∗
Corresponding author. Tel.: +34 986 812383; fax: +34 986 812380. E-mail address: [email protected] (M.A. Sanromán). http://dx.doi.org/10.1016/j.psep.2016.03.012 0957-5820/© 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Popescu, M., et al., Soil flushing and simultaneous degradation of organic pollutants in soils by electrokineticFenton treatment. Process Safety and Environmental Protection (2016), http://dx.doi.org/10.1016/j.psep.2016.03.012
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2013; He et al., 2014; Huguenot et al., 2015; Mousset et al., 2016). Among these treatment techniques, advanced oxidation processes (AOPs) have the potential for rapidly treating or pretreating soils contaminated with toxic and biorefractory organic compounds (Cheng et al., 2016; Pazos et al., 2013; Watts et al., 2002). AOPs are chemical oxidation processes based on the in situ generation of hydroxyl radicals, which are very reactive and short-lived oxidants able to destroy target pollutants up to their mineralization or, at least, transform them into harmless or biodegradable products in short treatment times (Rosas et al., 2013). The main limitation in the AOP is the necessity of homogeneous distribution of the reactants into the soil to ensure the contact between the pollutants and reactive species, in order to reduction/oxidation reactions take place. The inclusion of the reactive agents into the soil must be carried out by soil flushing. However, it is impossible the application of this technique to fine and low-permeability soils. To overcome this problem, electrokinetic-Fenton treatment has been proposed as a solution (Oonnittan et al., 2013; Kang et al., 2014; Kim et al., 2005, 2009). There are several papers available in the literature in which the electrokinetic process with Fenton oxidation have been demonstrated as a useful method to remediate soils co-contaminated with organic pollutants and heavy metals (Bocos et al., 2015; Seo et al., 2015; Ng et al., 2014). In the electrokinetic treatment, a low-density direct current is applied by two electrodes inserted into wet-polluted soil. This current promotes the movement of the pollutants in the pore fluid towards the electrode chambers where they are finally collected and treated. Therefore, this technique is recommended for application into fine and low-permeability soils (Pazos et al., 2010; Lopez-Vizcaino et al., 2016) and it has been mainly applied to treatment of soils polluted by metals, due to the facility to move the ionic species (Robles et al., 2015; Acar, 1993). Regarding Fenton’s process, this technique has been evaluated to destroy different organic pollutants present in soil and wastewater over the last decades. The Fenton’s reaction is based on the generation of hydroxyl radicals by the combination of hydrogen peroxide and ferrous ions. Hydroxyl radicals can react even with most of the organics persistent pollutants until their complete mineralization (Mousset et al., 2016; Pardo et al., 2015; Sirés et al., 2014; Thiam et al., 2016; Oturan and Aaron, 2014). The main advantages of the process are: (i) pollutants can be destroyed operating in different modes (in situ, on-site or off-site); (ii) Fenton’s reagents are abundant, nontoxic, easy to handle and environmentally benign; and (iii) the process requires short treatment time in comparison to other technique such as bioremediation. In the Fenton’s treatment, iron is one of the most important variables and it can be present and available in soil. However, it is necessary to transport the hydrogen peroxide across the soil in order to promote the Fenton’s reactions. The hydrogen peroxide, transported in the soil through the electrokinetic phenomena and electro-osmotic flow, is decomposed by iron or other transition minerals into the active oxygen species which are capable of oxidizing pollutants (Watts and Stanton, 1999). The transport through the application of electric field enables a uniform and rapid transport of hydrogen peroxide into a soil with low permeability. The electro-osmotic flow could be enhanced by the presence of enhancing agents such as electrolytes, surfactants and chelating agents (citric acid, ethylenediaminetetraacetic acid, Na2 SO4 , NaNO3 , etc.) in the processing fluid and also applying pH control at the electrode
chambers (Rozas and Castellote, 2015; Masi et al., 2015). In addition, to improve the electrokinetic process, it is necessary to enhance pollutants desorption from soil and to create a favourable environment to their transport towards the electrode chambers. In the electrokinetic-Fenton treatment, the addition of complexing agents facilitates the desorption of the entrapped pollutants and the solubilization of the iron from soil (Reddy and Chandhuri, 2009). Recently, Bocos et al. (2015) reported that citric acid is the most suitable complexing agent, increasing iron solubilization as soluble Fe-citrate that remains available for the Fenton’s oxidation and enhancing the electro-osmotic flow (around 18.7%) with respect to the conventional electrokinetic remediation. In addition, the generation of hydroxyl radicals is only effective under low pH (Valentine and Ann Wang, 1998) and the buffer capacity of the citric acid will allow remaining in an appropriated pH value along the soil in the electrokinetic cell (Bocos et al., 2015). In terms of free radicals formation and consumption, there are three mechanisms whereby hydrogen peroxide is consumed: (i) namely reaction with Fe2+ ; (ii) reaction with Fe3+ and (iii) reactions with organic pollutants and/or free radicals. The efficacy of Fenton’s oxidation is therefore dependent on the hydrogen peroxide oxidant to iron catalyst ratio. Considering that scavenging effect exist within soil systems and the fact that the major cost of applying Fenton’s oxidation comes from the quantity of the main oxidant used, it is recommended that the reactant dosage and other operating parameters have to be carefully optimized before field applications (Venny et al., 2012). Therefore, in this study, the effect of different factors such as iron content, hydrogen peroxide dosage and soil porosity has been evaluated in order to reach a clear understanding of the complex interactions between all processes implicated in the soil electrokinetic-Fenton treatment.
2.
Materials and methods
2.1.
Spiked Rhodamine B kaolinite samples
Rhodamine B and kaolinite were purchased from Sigma–Aldrich. The kaolinite has a particle size average of 3 m and a specific surface of 13.5 m2 /g. The mineralogy analysis by X-Ray Diffraction indicated the presence of kaolinite clay 85%, mica 14% and quartz 1%. Polluted kaolinite was prepared mixing thoroughly 150 g of kaolinite clay spiked at different iron concentrations with a solution of Rhodamine B. This mixture was stood for more than 24 h to allow the sorption of the dye on the surface of kaolinite particles until a final concentration of 0.16 g dye/kg dry kaolinite. The initial pH of the mixture was around 4. In this sample, Rhodamine B was effectively adsorbed on the kaolinite particles since no dye was released in extraction tests with deionised water.
2.2.
Soil
TPH polluted soil samples were collected in the northwest of Spain from an area of high industrial activity. Samples were taken from 0 to 30 cm depth by using an Eijkelkamp sampler, transported to the laboratory in polyethylene bags and then sieved and homogenized. Only the fraction containing particles smaller than 2 mm was selected. Some of the most typical characterization data of used soil sample 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
Please cite this article in press as: Popescu, M., et al., Soil flushing and simultaneous degradation of organic pollutants in soils by electrokineticFenton treatment. Process Safety and Environmental Protection (2016), http://dx.doi.org/10.1016/j.psep.2016.03.012
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Table 1 – Chemical characterization of the studied soil. Parameters
Value
pH Hydraulic conductivity (cm/s) Inorganic C (%) Organic C (%) Total C (%) Organic matter (%) TPHs (g/kg) Fe (mg/kg) Mg (mg/kg) Mn (mg/kg) Pb (mg/kg) Zn (mg/kg)
5.7 6 × 10−8 0.7 9.1 9.8 15.7 80.4 7632 1981 135 12 72
metals, iron was the most abundant metal found in the sample which plays an important role in the degradation of organic compounds by Fenton’s reaction.
2.3.
Electrokinetic-Fenton treatment
This treatment was performed in a cylindrical glass cell containing a sample compartment of 110 mm length and 32 mm of internal diameter (Fig. 1). The two electrode chambers (filled with 300 mL of the processing 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 distributed along the sample have allowed the measurement of the voltage drop between different sections of the cell. Electrode chambers were filled with citric acid (0.1 M), and with Na2 SO4 (0.1 M), this last was used to improve the conductivity of the solution and, hence, the current intensity. Hydrogen peroxide solution was added in the anode chamber. A constant voltage drop of 3 V/cm was applied in all experiments with a power supply (HP model 6030A). For the acquisition of electrical parameters over time, automatic data acquisition software named EK-Data system was used. The purpose of this software is
3
the automatization of the data recording and storing tasks obtained by the measurement equipment. Thus, there is a continuous flow of information about the evolution of electrical parameters and electrode chamber pH in the systems. In addition, two video cameras were connected to the EK-Data system, in order to follow the colour reduction along the electrokinetic cell.
2.4.
Analytical determinations
In all cases, at the end of each electrokinetic-Fenton experiments solid matrix was divided into three equal sections (S1–S3, namely from anode to cathode) and they were analyzed for moisture content, pH and pollutant concentrations. Liquid samples were taken periodically from the electrode chambers, centrifuged at 10,000 rpm for 5 min and kept refrigerated to be analyzed.
2.4.1.
Rhodamine B concentration
Rhodamine B was measured spectrophotometrically with a UV-Vis Spectrophotometer (Unicam Helios , Thermo Electron Corp.). Previously, dye was taken out from the spiked kaolinite samples by ethanol extraction. The concentration of dye in these samples was determined based on the area under the curve at wavelength range among 400 and 750 nm. The evolution of the in situ removal of Rhodamine B was followed visually by means of recording system described before. After that, these images were recollected and treated with image software to measure the advance of the colour removal front.
2.4.2.
Total petroleum hydrocarbon (TPH) concentration
Determination of TPH concentration in the range of C10 to C40 was performed in accordance with EN ISO 16703:2011. TPH of soil was extracted from dry samples by a pressurized solvent extraction system using an OnePSE instrument (Applied Separations Inc.). Briefly, the dry sample was thoroughly mixed with pelletized diatomaceous earth and when a free-flowing powder was obtained it was placed into an
Fig. 1 – Schema of electrokinetic cell and EK-Data acquisition system. Please cite this article in press as: Popescu, M., et al., Soil flushing and simultaneous degradation of organic pollutants in soils by electrokineticFenton treatment. Process Safety and Environmental Protection (2016), http://dx.doi.org/10.1016/j.psep.2016.03.012
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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 sample was recollected. Sample purification for determinations of TPH was performed by preliminary clean-up, liquid-liquid extraction with hexane and florisil column fraction (Dual Layer Florisil/Na2 SO4 6 mL PP SPE Tube, SUPELCO). Hexane was removed in a concentrator (RVC 2-25, CHRIST) at 50 ◦ C and samples were dissolved in heptane. The samples were analyzed using a gas chromatograph GC Agilent 7820A that was equipped with a flame ionization detector (FID). The hydrocarbons were separated in a HP5 column (30 m × 0.32 mm i.d. × 0.25 m). The detector was scheduled at a temperature of 325 ◦ C, and the used thermal ramps were as follows: initial temperature of 40 ◦ C for 1 min followed by a gradient of 5 ◦ C/min until a temperature of 60 ◦ C to be maintained during 2 min, and then, another gradient of 10 ◦ C/min until a final temperature of 325 ◦ C maintained during 5 min. The used carrier gas was high purity air, hydrogen and nitrogen, with a flow of 200, 30 and 25 mL/min, respectively. Finally, the injection volume was 1 L and a split injection mode was employed. The measurements were performed at less twice with an experimental error, calculated as standard deviation, below of 15%.
2.4.3.
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 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 a light colour or until its colour was stabilized. 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 filtered, adjusted to a final volume of 100 mL and then measured in an Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES) Perkin Elmer Optima 4300 DV. The experiments were performed at less twice with an experimental error, calculated as standard deviation, below of 5%.
2.4.4.
Electrical conductivity 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. Total organic carbon (TOC) was determined using the dichromate method proposed by Schumacher (2002).
2.4.5.
pH
At the end of each experiment, the pH value of the solid samples was measured by mixing 1 g of dry sample with 2.5 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).
2.4.6.
Microtox bioassay
To perform the Microtox bioassay samples from the soil, extraction of initial and treated samples (S1–S3) were collected and tested. A Microtox model 500 analyzer manufactured by the SDI (Strategic Diagnostics Inc., Newark, DE, USA) was employed. All reagents, including Vibrio fischeri (freeze-dried
powder) reconstitution and dilution solutions, were purchased from SDI. The test was performed according to the standard procedure recommended by the SDI and every test set was performed in triplicate. The inhibition ratio, based on the test results, was calculated using the following equation: I=
1−
L S
LC
× 100
where I is the inhibition ratio (%), LS and LC are the luminescence level of sample and control after 15 min, respectively (Oh et al., 2015).
3.
Results and discussion
Soil electrokinetic-Fenton treatment consists of several processes with different typologies such as transport, reactive, physical and equilibrium (Risco et al., 2016). The efficiency of treatment depends on several critical factors, such as Fenton’s reagents concentrations. This study was structured in two stages. In the initial one, model samples of kaolinite clay spiked with dye Rhodamine B were used to evaluate the capability of the electrokinetic-Fenton process. Moreover, the effect of soil porosity, iron content and hydrogen peroxide dosage on the degradation of organic pollutants in soils was determined. After these experiments, the optimized electrokinetic-Fenton process was performed in a real soil with a high content of iron and polluted by TPH in order to determine its applicability to remediate hydrocarbon polluted soils by this in situ treatment.
3.1. Electrokinetic-Fenton process: preliminary experiments As it is shown in Fig. 2A, the unenhanced electrokinetic treatment, using water as processing fluid, did not result in any significant removal of Rhodamine B from the kaolinite sample. In this case, when water is used as processing fluid, the main reaction that takes place upon the electrodes is the electrolysis of water that yields O2 and H+ ions at the anode and OH− ions and H2 at the cathode. The H+ ions generated at the anode side move towards the cathode creating an acidic front favouring the acidification of the kaolinite. However, these reactions did not produce any effect on the dye fixed on kaolinite. The whole molecule of Rhodamine B is neutral; therefore, its transportation under the electric field is restricted to electro-osmosis. Nevertheless, in the conditions used in this study the dye was neither desorbed nor transported from sample to cathode chamber. The behaviour was completely different when in the anode chamber a solution of hydrogen peroxide at 10% was added. In Fig. 2B, the photos show the changes of colour from pink to white, which it is a clear proof of the removal front evolution for the experiment. Therefore, by application of the electrical potential the pore water with peroxide hydrogen was transported from the anode to the cathode chamber by electro-osmotic flow. Thus, the oxidant comes through the soil and attacks the organic pollutant. The rate of transport and availability of the hydrogen peroxide are primary requirements for a successful soil treatment using oxidation reactions. The operational conditions must be selected in order to deliver the oxidant into the soil in such a way so as to facilitate effective soil–oxidant interaction. Treatment time required to reduce the pollutant concentration to a desired value is an important criterion to consider a cost effective remediation process. The treatment
Please cite this article in press as: Popescu, M., et al., Soil flushing and simultaneous degradation of organic pollutants in soils by electrokineticFenton treatment. Process Safety and Environmental Protection (2016), http://dx.doi.org/10.1016/j.psep.2016.03.012
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Fig. 2 – Photos of the spiked Rhodamine B kaolinite into electrokinetic cell operating in (A): unenhanced electrokinetic treatment (left: initial; right: after 48 h) and (B): electrokinetic-Fenton treatment (initial and each 12 h). (For interpretation of the references to colour in the citation of this figure, the reader is referred to the web version of this article.) scavenging effect by an excess of hydrogen peroxide, thus, the application of diluted solution has a higher affinity for iron. The profiles of characteristic parameters obtained from the experiment carried out using a solution of hydrogen peroxide (10%) as anolyte are showed in Fig. 5. As it is depicted, there was a clear relationship between the current intensity, electroosmotic flow and the colour removal rate. Furthermore, as it is showed in Fig. 6, the overall voltage drop was kept constant (30 V). However, the measurement of the voltage drops using the three auxiliary electrodes allocated along the sample (see Fig. 1) permitted to determine the mobility of the species. The distribution of the voltage drops measured during the process can be associated with the oxidation reactions and transport of species into the soil matrix. The increase in voltage could be attributed to the migration of species across the soil matrix, such as hydrogen ions, ions introduced into the system Na2 SO4 , the addition of enhancing agent (citric acid), hydrogen peroxide and Fenton’s reactions. Thus, an increase in voltage between anode and P1 occurred at the initial time, that decreased when the voltage was increased between P1 and P2, and finally, similar profile was detected after 30 h between P3 and cathode. The flow across soil matrix follows a closer plug-flow model and no back mixing was detected with plugs of fluid passing through the soil matrix. This behaviour permitted the homogenous removal of Rhodamine B when the hydrogen peroxide arrived at a section of soil. As it is shown in
100
1.0
80
0.8
60
0.6
C/C0
Colour removal (%)
duration can be considerably shortened if the pollutant present throughout the soil section can be subjected to an effective oxidation by controlling oxidant species present into the soil. Therefore, several ratios of hydrogen peroxide:iron were evaluated in order to determine their effect on the treatment time to degrade the dye content into the electrokinetic cell. In Fig. 3, the profiles of the dye colour removal rate into the soil are showed. In the range of iron concentrations studied (3000–6000 ppm), it was not detected a clear effect of this parameter into the oxidation reactions. In all cases, iron removal was detected from S1 to S3. The profiles showed in Fig. 4 confirm the iron transportation towards the cathode chamber, which favours the Fenton’s reaction in the flow direction (Pazos et al., 2009). On the other hand, the effect of the hydrogen peroxide concentration added to the anode chamber was evaluated (2.5, 5, 7.5 and 10%). This range was selected in order to promote a safer working environment during the exothermic oxidation reactions. The results indicated that the best removals were obtained operating in a range of 7.5–10%. After two days, it was possible the total colour removal of dye in the soil (Fig. 5). These results are in concordance with those obtained by Kakarla et al. (2002), who reported that the use of diluted hydrogen peroxide improves the treatment efficiency during in situ remediation compared to concentrated solutions. This fact could be explained due to the reduction of free radical
Initial sample S1 S2 S3
0.4
40 3000 ppm 4000 ppm 5000 ppm 6000 ppm
20
0.2
0.0
0 0.0
0.5
1.0
1.5
2.0
2.5
Time (d)
Fig. 3 – Evolution of Rhodamine B colour removal at different iron concentrations (3000–6000 ppm).
3.0
3000
4000
5000
6000
Fe concentration (ppm)
Fig. 4 – Normalized iron concentration (C/C0 ) after the electrokinetic-Fenton treatment in the three studied sections after 3 days.
Please cite this article in press as: Popescu, M., et al., Soil flushing and simultaneous degradation of organic pollutants in soils by electrokineticFenton treatment. Process Safety and Environmental Protection (2016), http://dx.doi.org/10.1016/j.psep.2016.03.012
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100
0.040 Colour removal advance Intensity Electro-osmotic flow Anode pH Cathode pH
3.0
pH
2.5
2.0
1.5
0.035 80 0.030
8 0.025 6 0.020
0.5
40
4 0.015 20
2
1.0
60
Cumulative flow (mL)
Colour removal advance (cm)
10
Intensity (A)
3.5
0.010
0
0.005 0.0
0.5
1.0
1.5
0
2.0
Time (d) Fig. 5 – Profiles of Rhodamine B removal, electro-osmotic flow, current intensity and anode and cathode pH, obtained along the electrokinetic-Fenton treatment of spiked Rhodamine B kaolinite using as anolyte a solution of hydrogen peroxide 10% and soil bed porosity around 50%.
Fig. 2B, the colour removal of dye was detected in the direction of the electro-osmotic flow. Therefore, the cumulative electroosmotic flow was directly related to the advance of the colour removal front observed during the time (Fig. 5). In addition, it is clear that the profile of removal was related to the electrical current evolution. Initially, the electrical current increased slowly, reaching a peak within 32–36 h, and then stabilizing at a nearly constant value. Acidic Fenton’s process has been described as ideal for free radical generation. As it is mentioned before, during electrokinetic treatment an acidic front is generated at the anode due to water electrolysis, and migrates into the bulk of the soil causing desorption of metals from the soil. However, the basic front generated in the cathode increases the pH nearest this chamber, reducing the efficacy of the treatment. For this reason, in this process is necessary to apply strategies to control the soil pH. In this sense, a solution of citric acid (0.1 M) was added to the cathode and anode chamber with a dual benefit as chelating and pH control agent. As it was
30 Anode-Cathode Anode-P1 P1-P2 P2-P3 P3-Cathode
Votage drop (V)
25
20
15
10
5
0 0.0
0.5
1.0
1.5
2.0
Time (d)
Fig. 6 – Voltages drop profiles measured across the soil bed.
demonstrated in previous studies (Bocos et al., 2015; Vicente et al., 2011), this complexing agent enhances the desorption of the soil entrapped iron and forms soluble Fe-citrate complex that remains available for the Fenton’s oxidation. At the end of treatment, the pH of the soil becomes slightly acid in all sections. Therefore, the citric acid addition allowed remaining the pH in an appropriated value (below 5) along the soil, in the electrokinetic cell, favouring the oxidation reactions. According to the Helmholtz–Smoluchowski theory, the rate of electro-osmotic flow is influenced by the zeta potential of soils. Furthermore, this zeta potential has negative values under alkaline conditions at the surface of soil particles, and the rate of electro-osmotic flow increases as zeta potential decreases (Kim et al., 2005). It is remarkable that the citric acid also favoured the electro-osmotic flow and the cumulative volume of the effluent against the time, showing a constant flow that guarantee the effective soil–oxidant interaction. This fact is in accordance with Popov (1997) and Pazos et al. (2008) who reported that the presence of citric acid in the kaolinite sample modifies the superficial properties of clay particles increasing the total flux towards the cathode by the effect of the electric field. It is also known that the geomechanical behaviour of the soil has influence on the effectiveness of the electrokinetic processes. The results suggest that the main mechanism implicated in the colour removal of Rhodamine B is the dragging by electro-osmotic flow of hydrogen peroxide. For this reason, parameters such as porosity, defined as the ratio of the void volume to the total volume of the soil sample, is considered as a key factor (Lopez-Vizcaino et al., 2016; Wu et al., 2012; Chai et al., 2007). Compaction, which happens when unsaturated soils are compressed, affects the porosity and relates physical properties such as mechanical properties, and gas and water transport. For most soils, the compaction reduces the volume of large pores and consequently affects water retention properties and hydraulic conductivity in the range of high water potentials (Richard et al., 2001). To evaluate the influence of the soil porosity on the efficiency of the
Please cite this article in press as: Popescu, M., et al., Soil flushing and simultaneous degradation of organic pollutants in soils by electrokineticFenton treatment. Process Safety and Environmental Protection (2016), http://dx.doi.org/10.1016/j.psep.2016.03.012
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0.06
10
0.05
8
0.04
6
0.03
4
0.02
1.5
2
0.01
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0
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80
Intensity (A)
pH
2.5
Colour removal advance (cm)
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Colour removal advance Intensity Electro-osmotic flow Anode pH Cathode pH
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0.5
1.0
1.5
2.0
40
20
0.00 0.0
60
Cumulative flow (mL)
4.0
0
2.5
Time (d) Fig. 7 – Profiles of Rhodamine B removal, electro-osmotic flow, current intensity and anode and cathode pH, obtained along the electrokinetic-Fenton treatment of spiked Rhodamine kaolinite using as anolyte a solution of hydrogen peroxide 10% and soil bed porosity around 40%. electrokinetic-Fenton process, a comparative study using soil samples with different compactions was performed (porosity 40% and 50%). In Fig. 7, the profiles of the different studied parameters are showed for an electrokinetic-Fenton process using as anolyte a solution of hydrogen peroxide at 10% and a soil bed porosity of 40%. According to the Helmholtz–Smoluchowski theory, the rate of electro-osmotic flow is proportional to the available porosity of soils. For this reason, if the soil bed compaction was increased, the electroosmotic flow was reduced and this yielded the slowed down of the removal (Fig. 7). This fact highlights the importance of electro-osmotic flow in the electrokinetic-Fenton. Based on the reported results, it can be concluded that it is necessary (i) to keep an acid environment through the use of citric acid solution in both chambers and (ii) to use as anolyte a hydrogen peroxide solution of 10%, in order to increase the efficiency of this treatment in real soil.
3.2. soil
Electrokinetic-Fenton treatment of TPH polluted
Soil is the ultimate receptor of pollutants from anthropogenic activities, such as hydrocarbons, polycyclic aromatic hydrocarbons and polychlorinated biphenyls. Metals are attached
to different binding phases in soil, and the presence of these transition metals can be used to carry out the oxidation reactions. In this study, a TPH polluted soil was selected to validate the electrokinetic-Fenton treatment. Initially, the chemical characterization of collected soil samples, in terms of their heavy metal and TPH content, TOC, pH, conductivity was done (Table 1). It is noticeable the high concentration of iron, around 7600 mg of Fe/kg of dried soil, that makes this soil appropriate to the Fenton’s reactions take place. Another positive factor of this soil is its hydraulic conductivity value that will favour the transport of species in the interstitial fluid (Yeung, 1994). Thus, the studied soil will show a low electrical resistance and the electrical current will travel easily through the soil matrix, which favours the electrokinetic treatment. Initially, unenhanced electrokinetic-Fenton treatment of polluted soil was carried out at constant voltage drop of 3 V/cm and neither addition of facilitating agent nor hydrogen peroxide was done. In this experiment, the pH rise in the cathode chamber was caused by the generation of hydroxyl ions as a result of electrolysis of water during the remediation that also generated the apparition of a precipitate between S2 and S3. Then, the enhanced electrokinetic-Fenton was performed working with the optimal conditions obtained previously. In Table 2, the concentration of metals and TPH obtained by
Table 2 – TPH and metal concentrations and inhibition ratio of the initial sample and the three studied sections after the electrokinetic-Fenton treatment. Parameters
Fe Mg Mn Pb Zn TPH Microtox bioassay I
Initial (mg/kg)
7632 1981 135 12 72 80.4
After 15 d
After 27 d
S1 (mg/kg)
S2 (mg/kg)
S3 (mg/kg)
S1 (mg/kg)
S2 (mg/kg)
S3 (mg/kg)
6231.5 1631 39.8 8.5 33.5 35.8
6348 1691 42.4 12 37.5 36.5
6477 1743 44.5 12 42 38.1
6231.5 1455 36.4 7 21.5 32.5
6348 1496 40.4 11 22 31.5
6477 1544 42.3 12 23.5 34.8
Initial (%)
S1 (%)
S2 (%)
S3 (%)
S1 (%)
S2 (%)
S3 (%)
99.85
70.80
77.63
80.51
67.96
74.11
77.63
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electrokinetic treatment after 15 and 27 days are shown. The results indicated that a homogeneous removal of pollutants was achieved in the soil with TPH removal values around 54.4% and 58.2% after 15 and 27 days, respectively. Moreover, no TPH was detected in the electrolyte chamber. Because of the highest volatile hydrocarbon fractions were previously removed and the increase in soil temperature was negligible, hydrocarbon volatilization was not considered, and only the combination of electrokinetic and Fenton’s processes was assumed to be responsible for the TPH removal. The homogeneity of the TPH removal (S1–S3) and the slight increase in the removal after the longest treatment result from two aspects. First, the hydrogen peroxide was present throughout the soil matrix due to the electro-osmotic transport. In addition, after two weeks, the hydrogen peroxide in the anode chamber was decomposed and this fact explains the low degradation level obtained after 27 days. Finally, to determine the environmental effect of this treatment, the change in the toxicity of these samples using Microtox® bioassay was carried out with the initial and final samples obtained from each soil section after the electrokinetic-Fenton treatment. The mechanism of the Microtox® system is based upon sensing the respiration activity that is directly correlated with Vibrio fischeri cellular activity and its luminescence. In Table 2 the inhibition percentage of the different samples reveals that by electrokinetic-Fenton treatment, the THP soil content was reduced and then the toxicity effect was depleted in the same way. This fact confirms the efficiency of this treatment due to the in situ generation of hydroxyl radicals which were able to degrade TPH. Moreover, the application of this technique presents low environmental effect reaching a significant reduction of toxicity in the soil after the treatment.
4.
Conclusions
In this work, a deep analysis of the Electrokinetic-Fenton treatment was evaluated using kaolinite polluted with Rhodamine B as model sample. From these experiments, it was found that the iron concentration in the soil had a slight influence on the dye removal. However, the highest hydrogen peroxide dosage (7.5–10%) proportionated the faster colour removal rate. Furthermore, it was confirmed that the presence of citric acid favours the availability of the Fenton catalyst and the electroosmotic transport of the reactive species. Under the best operational conditions, the electrokinetic-Fenton treatment of a TPH polluted soil was efficiently carried out. Moreover, the Microtox analysis of the treated soil demonstrated the reduction of negative environmental effect and benevolence of the treatment. Summing up, the reported results confirmed that electrokinetic-Fenton treatment could be a suitable technique for the remediation of TPH polluted soils.
Acknowledgements This research has been financially supported by the Spanish Ministry of Economy and Competitiveness, Xunta de Galicia and ERDF Funds (Projects CTM2014-52471-R and GRC 2013/003). The authors are grateful to the Spanish Ministry of Economy and Competitiveness for the financial support of Marta Pazos under the Ramón y Cajal program and Xunta de Galicia for financial support of the researcher Emilio Rosales under a postdoctoral grant and to the European Union for the
financial support of Ciprian Sandu and Marius Popescu under the Erasmus+ program.
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Contents lists available at ScienceDirect
Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Soil flushing and simultaneous degradation of organic pollutants in soils by electrokinetic-Fenton treatment Marius Popescu a , Emilio Rosales b , Ciprian Sandu a , Jéssica Meijide b , Marta Pazos b , Gabriel Lazar a , Ma Angeles Sanromán b,∗ a
Faculty of Engineering, “Vasile Alecsandri” University of Bacau, Calea Marasesti 157, 600115 Bacau, Romania Department of Chemical Engineering, University of Vigo, Isaac Newton Building, Campus As Lagoas, Marcosende, 36310 Vigo, Spain
b
a r t i c l e
i n f o
a b s t r a c t
Article history:
This study focuses on the evaluation of a combination of electrokinetic technology and
Received 28 December 2015
Fenton’s process to remediate a soil polluted with organic compounds. To determine the
Received in revised form 14 March
influence of the several variables such as hydrogen peroxide dosage, iron soil concentration
2016
and porosity, different experiments using kaolinite spiked by Rhodamine B were performed.
Accepted 17 March 2016
The use of this coloured sample permitted an easy monitoring of the oxidation reactions
Available online xxx
across the soil bed. From the obtained results, it is concluded that the highest colour removal rate was reached when a solution of hydrogen peroxide around 10% was used, and slight
Keywords:
influence of iron soil concentration was detected at the range of concentrations used in
Electrokinetic-Fenton
these experiments. In all cases, citric acid was added in the anolyte and catholyte solutions
Hydrogen peroxide
in order to solubilize the iron as Fe-citrate complex and to keep the pH in acid environment
Iron
favouring that the Fenton’s reactions take place into the soil. Based on these preliminary
Porosity
experiments, the electrokinetic-Fenton process was applied to total petroleum hydrocar-
Rhodamine B
bons (TPH) polluted soil. After 15 and 27 days of treatment, a homogeneous removal of
TPH
pollutants, around 54.4% and 58.2% of TPH removal efficiency, was reached, respectively. In addition, the Microtox bioassays confirmed the reduction of the Vibrio fischeri inhibition after the soil treatment. Summing up, in situ electrokinetic-Fenton treatment seems to be a suitable technique for the remediation of organics such as hydrocarbons present in polluted soils. © 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1.
Introduction
Soil contamination by organic compounds has significantly increased due to several activities and factors such as farming, accidental emissions of harmful pollutants, industrial wastewater and the landfill leachate (Cheng et al., 2016). The presence of these compounds in soil has been causing serious incidents of soil pollution. This fact has led to a grave decline in the quality of agricultural products and is evolving in a growing
interest among those involved in environmental remediation (Manz et al., 2001). Nowadays, extensive studies have been carried out for the development of soil remediation techniques such as thermal desorption, excavation or dredging, pumping and treating, surfactant enhanced aquifer remediation, solidification and stabilization, soil vapour extraction, bioremediation, nanoremediation or in situ oxidation (Ramírez et al., 2015a, 2015b; Cobas et al., 2013; Gómez et al., 2010; Pazos et al., 2010, 2011,
∗
Corresponding author. Tel.: +34 986 812383; fax: +34 986 812380. E-mail address: [email protected] (M.A. Sanromán). http://dx.doi.org/10.1016/j.psep.2016.03.012 0957-5820/© 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Popescu, M., et al., Soil flushing and simultaneous degradation of organic pollutants in soils by electrokineticFenton treatment. Process Safety and Environmental Protection (2016), http://dx.doi.org/10.1016/j.psep.2016.03.012
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2013; He et al., 2014; Huguenot et al., 2015; Mousset et al., 2016). Among these treatment techniques, advanced oxidation processes (AOPs) have the potential for rapidly treating or pretreating soils contaminated with toxic and biorefractory organic compounds (Cheng et al., 2016; Pazos et al., 2013; Watts et al., 2002). AOPs are chemical oxidation processes based on the in situ generation of hydroxyl radicals, which are very reactive and short-lived oxidants able to destroy target pollutants up to their mineralization or, at least, transform them into harmless or biodegradable products in short treatment times (Rosas et al., 2013). The main limitation in the AOP is the necessity of homogeneous distribution of the reactants into the soil to ensure the contact between the pollutants and reactive species, in order to reduction/oxidation reactions take place. The inclusion of the reactive agents into the soil must be carried out by soil flushing. However, it is impossible the application of this technique to fine and low-permeability soils. To overcome this problem, electrokinetic-Fenton treatment has been proposed as a solution (Oonnittan et al., 2013; Kang et al., 2014; Kim et al., 2005, 2009). There are several papers available in the literature in which the electrokinetic process with Fenton oxidation have been demonstrated as a useful method to remediate soils co-contaminated with organic pollutants and heavy metals (Bocos et al., 2015; Seo et al., 2015; Ng et al., 2014). In the electrokinetic treatment, a low-density direct current is applied by two electrodes inserted into wet-polluted soil. This current promotes the movement of the pollutants in the pore fluid towards the electrode chambers where they are finally collected and treated. Therefore, this technique is recommended for application into fine and low-permeability soils (Pazos et al., 2010; Lopez-Vizcaino et al., 2016) and it has been mainly applied to treatment of soils polluted by metals, due to the facility to move the ionic species (Robles et al., 2015; Acar, 1993). Regarding Fenton’s process, this technique has been evaluated to destroy different organic pollutants present in soil and wastewater over the last decades. The Fenton’s reaction is based on the generation of hydroxyl radicals by the combination of hydrogen peroxide and ferrous ions. Hydroxyl radicals can react even with most of the organics persistent pollutants until their complete mineralization (Mousset et al., 2016; Pardo et al., 2015; Sirés et al., 2014; Thiam et al., 2016; Oturan and Aaron, 2014). The main advantages of the process are: (i) pollutants can be destroyed operating in different modes (in situ, on-site or off-site); (ii) Fenton’s reagents are abundant, nontoxic, easy to handle and environmentally benign; and (iii) the process requires short treatment time in comparison to other technique such as bioremediation. In the Fenton’s treatment, iron is one of the most important variables and it can be present and available in soil. However, it is necessary to transport the hydrogen peroxide across the soil in order to promote the Fenton’s reactions. The hydrogen peroxide, transported in the soil through the electrokinetic phenomena and electro-osmotic flow, is decomposed by iron or other transition minerals into the active oxygen species which are capable of oxidizing pollutants (Watts and Stanton, 1999). The transport through the application of electric field enables a uniform and rapid transport of hydrogen peroxide into a soil with low permeability. The electro-osmotic flow could be enhanced by the presence of enhancing agents such as electrolytes, surfactants and chelating agents (citric acid, ethylenediaminetetraacetic acid, Na2 SO4 , NaNO3 , etc.) in the processing fluid and also applying pH control at the electrode
chambers (Rozas and Castellote, 2015; Masi et al., 2015). In addition, to improve the electrokinetic process, it is necessary to enhance pollutants desorption from soil and to create a favourable environment to their transport towards the electrode chambers. In the electrokinetic-Fenton treatment, the addition of complexing agents facilitates the desorption of the entrapped pollutants and the solubilization of the iron from soil (Reddy and Chandhuri, 2009). Recently, Bocos et al. (2015) reported that citric acid is the most suitable complexing agent, increasing iron solubilization as soluble Fe-citrate that remains available for the Fenton’s oxidation and enhancing the electro-osmotic flow (around 18.7%) with respect to the conventional electrokinetic remediation. In addition, the generation of hydroxyl radicals is only effective under low pH (Valentine and Ann Wang, 1998) and the buffer capacity of the citric acid will allow remaining in an appropriated pH value along the soil in the electrokinetic cell (Bocos et al., 2015). In terms of free radicals formation and consumption, there are three mechanisms whereby hydrogen peroxide is consumed: (i) namely reaction with Fe2+ ; (ii) reaction with Fe3+ and (iii) reactions with organic pollutants and/or free radicals. The efficacy of Fenton’s oxidation is therefore dependent on the hydrogen peroxide oxidant to iron catalyst ratio. Considering that scavenging effect exist within soil systems and the fact that the major cost of applying Fenton’s oxidation comes from the quantity of the main oxidant used, it is recommended that the reactant dosage and other operating parameters have to be carefully optimized before field applications (Venny et al., 2012). Therefore, in this study, the effect of different factors such as iron content, hydrogen peroxide dosage and soil porosity has been evaluated in order to reach a clear understanding of the complex interactions between all processes implicated in the soil electrokinetic-Fenton treatment.
2.
Materials and methods
2.1.
Spiked Rhodamine B kaolinite samples
Rhodamine B and kaolinite were purchased from Sigma–Aldrich. The kaolinite has a particle size average of 3 m and a specific surface of 13.5 m2 /g. The mineralogy analysis by X-Ray Diffraction indicated the presence of kaolinite clay 85%, mica 14% and quartz 1%. Polluted kaolinite was prepared mixing thoroughly 150 g of kaolinite clay spiked at different iron concentrations with a solution of Rhodamine B. This mixture was stood for more than 24 h to allow the sorption of the dye on the surface of kaolinite particles until a final concentration of 0.16 g dye/kg dry kaolinite. The initial pH of the mixture was around 4. In this sample, Rhodamine B was effectively adsorbed on the kaolinite particles since no dye was released in extraction tests with deionised water.
2.2.
Soil
TPH polluted soil samples were collected in the northwest of Spain from an area of high industrial activity. Samples were taken from 0 to 30 cm depth by using an Eijkelkamp sampler, transported to the laboratory in polyethylene bags and then sieved and homogenized. Only the fraction containing particles smaller than 2 mm was selected. Some of the most typical characterization data of used soil sample 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
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Table 1 – Chemical characterization of the studied soil. Parameters
Value
pH Hydraulic conductivity (cm/s) Inorganic C (%) Organic C (%) Total C (%) Organic matter (%) TPHs (g/kg) Fe (mg/kg) Mg (mg/kg) Mn (mg/kg) Pb (mg/kg) Zn (mg/kg)
5.7 6 × 10−8 0.7 9.1 9.8 15.7 80.4 7632 1981 135 12 72
metals, iron was the most abundant metal found in the sample which plays an important role in the degradation of organic compounds by Fenton’s reaction.
2.3.
Electrokinetic-Fenton treatment
This treatment was performed in a cylindrical glass cell containing a sample compartment of 110 mm length and 32 mm of internal diameter (Fig. 1). The two electrode chambers (filled with 300 mL of the processing 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 distributed along the sample have allowed the measurement of the voltage drop between different sections of the cell. Electrode chambers were filled with citric acid (0.1 M), and with Na2 SO4 (0.1 M), this last was used to improve the conductivity of the solution and, hence, the current intensity. Hydrogen peroxide solution was added in the anode chamber. A constant voltage drop of 3 V/cm was applied in all experiments with a power supply (HP model 6030A). For the acquisition of electrical parameters over time, automatic data acquisition software named EK-Data system was used. The purpose of this software is
3
the automatization of the data recording and storing tasks obtained by the measurement equipment. Thus, there is a continuous flow of information about the evolution of electrical parameters and electrode chamber pH in the systems. In addition, two video cameras were connected to the EK-Data system, in order to follow the colour reduction along the electrokinetic cell.
2.4.
Analytical determinations
In all cases, at the end of each electrokinetic-Fenton experiments solid matrix was divided into three equal sections (S1–S3, namely from anode to cathode) and they were analyzed for moisture content, pH and pollutant concentrations. Liquid samples were taken periodically from the electrode chambers, centrifuged at 10,000 rpm for 5 min and kept refrigerated to be analyzed.
2.4.1.
Rhodamine B concentration
Rhodamine B was measured spectrophotometrically with a UV-Vis Spectrophotometer (Unicam Helios , Thermo Electron Corp.). Previously, dye was taken out from the spiked kaolinite samples by ethanol extraction. The concentration of dye in these samples was determined based on the area under the curve at wavelength range among 400 and 750 nm. The evolution of the in situ removal of Rhodamine B was followed visually by means of recording system described before. After that, these images were recollected and treated with image software to measure the advance of the colour removal front.
2.4.2.
Total petroleum hydrocarbon (TPH) concentration
Determination of TPH concentration in the range of C10 to C40 was performed in accordance with EN ISO 16703:2011. TPH of soil was extracted from dry samples by a pressurized solvent extraction system using an OnePSE instrument (Applied Separations Inc.). Briefly, the dry sample was thoroughly mixed with pelletized diatomaceous earth and when a free-flowing powder was obtained it was placed into an
Fig. 1 – Schema of electrokinetic cell and EK-Data acquisition system. Please cite this article in press as: Popescu, M., et al., Soil flushing and simultaneous degradation of organic pollutants in soils by electrokineticFenton treatment. Process Safety and Environmental Protection (2016), http://dx.doi.org/10.1016/j.psep.2016.03.012
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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 sample was recollected. Sample purification for determinations of TPH was performed by preliminary clean-up, liquid-liquid extraction with hexane and florisil column fraction (Dual Layer Florisil/Na2 SO4 6 mL PP SPE Tube, SUPELCO). Hexane was removed in a concentrator (RVC 2-25, CHRIST) at 50 ◦ C and samples were dissolved in heptane. The samples were analyzed using a gas chromatograph GC Agilent 7820A that was equipped with a flame ionization detector (FID). The hydrocarbons were separated in a HP5 column (30 m × 0.32 mm i.d. × 0.25 m). The detector was scheduled at a temperature of 325 ◦ C, and the used thermal ramps were as follows: initial temperature of 40 ◦ C for 1 min followed by a gradient of 5 ◦ C/min until a temperature of 60 ◦ C to be maintained during 2 min, and then, another gradient of 10 ◦ C/min until a final temperature of 325 ◦ C maintained during 5 min. The used carrier gas was high purity air, hydrogen and nitrogen, with a flow of 200, 30 and 25 mL/min, respectively. Finally, the injection volume was 1 L and a split injection mode was employed. The measurements were performed at less twice with an experimental error, calculated as standard deviation, below of 15%.
2.4.3.
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 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 a light colour or until its colour was stabilized. 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 filtered, adjusted to a final volume of 100 mL and then measured in an Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES) Perkin Elmer Optima 4300 DV. The experiments were performed at less twice with an experimental error, calculated as standard deviation, below of 5%.
2.4.4.
Electrical conductivity 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. Total organic carbon (TOC) was determined using the dichromate method proposed by Schumacher (2002).
2.4.5.
pH
At the end of each experiment, the pH value of the solid samples was measured by mixing 1 g of dry sample with 2.5 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).
2.4.6.
Microtox bioassay
To perform the Microtox bioassay samples from the soil, extraction of initial and treated samples (S1–S3) were collected and tested. A Microtox model 500 analyzer manufactured by the SDI (Strategic Diagnostics Inc., Newark, DE, USA) was employed. All reagents, including Vibrio fischeri (freeze-dried
powder) reconstitution and dilution solutions, were purchased from SDI. The test was performed according to the standard procedure recommended by the SDI and every test set was performed in triplicate. The inhibition ratio, based on the test results, was calculated using the following equation: I=
1−
L S
LC
× 100
where I is the inhibition ratio (%), LS and LC are the luminescence level of sample and control after 15 min, respectively (Oh et al., 2015).
3.
Results and discussion
Soil electrokinetic-Fenton treatment consists of several processes with different typologies such as transport, reactive, physical and equilibrium (Risco et al., 2016). The efficiency of treatment depends on several critical factors, such as Fenton’s reagents concentrations. This study was structured in two stages. In the initial one, model samples of kaolinite clay spiked with dye Rhodamine B were used to evaluate the capability of the electrokinetic-Fenton process. Moreover, the effect of soil porosity, iron content and hydrogen peroxide dosage on the degradation of organic pollutants in soils was determined. After these experiments, the optimized electrokinetic-Fenton process was performed in a real soil with a high content of iron and polluted by TPH in order to determine its applicability to remediate hydrocarbon polluted soils by this in situ treatment.
3.1. Electrokinetic-Fenton process: preliminary experiments As it is shown in Fig. 2A, the unenhanced electrokinetic treatment, using water as processing fluid, did not result in any significant removal of Rhodamine B from the kaolinite sample. In this case, when water is used as processing fluid, the main reaction that takes place upon the electrodes is the electrolysis of water that yields O2 and H+ ions at the anode and OH− ions and H2 at the cathode. The H+ ions generated at the anode side move towards the cathode creating an acidic front favouring the acidification of the kaolinite. However, these reactions did not produce any effect on the dye fixed on kaolinite. The whole molecule of Rhodamine B is neutral; therefore, its transportation under the electric field is restricted to electro-osmosis. Nevertheless, in the conditions used in this study the dye was neither desorbed nor transported from sample to cathode chamber. The behaviour was completely different when in the anode chamber a solution of hydrogen peroxide at 10% was added. In Fig. 2B, the photos show the changes of colour from pink to white, which it is a clear proof of the removal front evolution for the experiment. Therefore, by application of the electrical potential the pore water with peroxide hydrogen was transported from the anode to the cathode chamber by electro-osmotic flow. Thus, the oxidant comes through the soil and attacks the organic pollutant. The rate of transport and availability of the hydrogen peroxide are primary requirements for a successful soil treatment using oxidation reactions. The operational conditions must be selected in order to deliver the oxidant into the soil in such a way so as to facilitate effective soil–oxidant interaction. Treatment time required to reduce the pollutant concentration to a desired value is an important criterion to consider a cost effective remediation process. The treatment
Please cite this article in press as: Popescu, M., et al., Soil flushing and simultaneous degradation of organic pollutants in soils by electrokineticFenton treatment. Process Safety and Environmental Protection (2016), http://dx.doi.org/10.1016/j.psep.2016.03.012
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Fig. 2 – Photos of the spiked Rhodamine B kaolinite into electrokinetic cell operating in (A): unenhanced electrokinetic treatment (left: initial; right: after 48 h) and (B): electrokinetic-Fenton treatment (initial and each 12 h). (For interpretation of the references to colour in the citation of this figure, the reader is referred to the web version of this article.) scavenging effect by an excess of hydrogen peroxide, thus, the application of diluted solution has a higher affinity for iron. The profiles of characteristic parameters obtained from the experiment carried out using a solution of hydrogen peroxide (10%) as anolyte are showed in Fig. 5. As it is depicted, there was a clear relationship between the current intensity, electroosmotic flow and the colour removal rate. Furthermore, as it is showed in Fig. 6, the overall voltage drop was kept constant (30 V). However, the measurement of the voltage drops using the three auxiliary electrodes allocated along the sample (see Fig. 1) permitted to determine the mobility of the species. The distribution of the voltage drops measured during the process can be associated with the oxidation reactions and transport of species into the soil matrix. The increase in voltage could be attributed to the migration of species across the soil matrix, such as hydrogen ions, ions introduced into the system Na2 SO4 , the addition of enhancing agent (citric acid), hydrogen peroxide and Fenton’s reactions. Thus, an increase in voltage between anode and P1 occurred at the initial time, that decreased when the voltage was increased between P1 and P2, and finally, similar profile was detected after 30 h between P3 and cathode. The flow across soil matrix follows a closer plug-flow model and no back mixing was detected with plugs of fluid passing through the soil matrix. This behaviour permitted the homogenous removal of Rhodamine B when the hydrogen peroxide arrived at a section of soil. As it is shown in
100
1.0
80
0.8
60
0.6
C/C0
Colour removal (%)
duration can be considerably shortened if the pollutant present throughout the soil section can be subjected to an effective oxidation by controlling oxidant species present into the soil. Therefore, several ratios of hydrogen peroxide:iron were evaluated in order to determine their effect on the treatment time to degrade the dye content into the electrokinetic cell. In Fig. 3, the profiles of the dye colour removal rate into the soil are showed. In the range of iron concentrations studied (3000–6000 ppm), it was not detected a clear effect of this parameter into the oxidation reactions. In all cases, iron removal was detected from S1 to S3. The profiles showed in Fig. 4 confirm the iron transportation towards the cathode chamber, which favours the Fenton’s reaction in the flow direction (Pazos et al., 2009). On the other hand, the effect of the hydrogen peroxide concentration added to the anode chamber was evaluated (2.5, 5, 7.5 and 10%). This range was selected in order to promote a safer working environment during the exothermic oxidation reactions. The results indicated that the best removals were obtained operating in a range of 7.5–10%. After two days, it was possible the total colour removal of dye in the soil (Fig. 5). These results are in concordance with those obtained by Kakarla et al. (2002), who reported that the use of diluted hydrogen peroxide improves the treatment efficiency during in situ remediation compared to concentrated solutions. This fact could be explained due to the reduction of free radical
Initial sample S1 S2 S3
0.4
40 3000 ppm 4000 ppm 5000 ppm 6000 ppm
20
0.2
0.0
0 0.0
0.5
1.0
1.5
2.0
2.5
Time (d)
Fig. 3 – Evolution of Rhodamine B colour removal at different iron concentrations (3000–6000 ppm).
3.0
3000
4000
5000
6000
Fe concentration (ppm)
Fig. 4 – Normalized iron concentration (C/C0 ) after the electrokinetic-Fenton treatment in the three studied sections after 3 days.
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4.0
100
0.040 Colour removal advance Intensity Electro-osmotic flow Anode pH Cathode pH
3.0
pH
2.5
2.0
1.5
0.035 80 0.030
8 0.025 6 0.020
0.5
40
4 0.015 20
2
1.0
60
Cumulative flow (mL)
Colour removal advance (cm)
10
Intensity (A)
3.5
0.010
0
0.005 0.0
0.5
1.0
1.5
0
2.0
Time (d) Fig. 5 – Profiles of Rhodamine B removal, electro-osmotic flow, current intensity and anode and cathode pH, obtained along the electrokinetic-Fenton treatment of spiked Rhodamine B kaolinite using as anolyte a solution of hydrogen peroxide 10% and soil bed porosity around 50%.
Fig. 2B, the colour removal of dye was detected in the direction of the electro-osmotic flow. Therefore, the cumulative electroosmotic flow was directly related to the advance of the colour removal front observed during the time (Fig. 5). In addition, it is clear that the profile of removal was related to the electrical current evolution. Initially, the electrical current increased slowly, reaching a peak within 32–36 h, and then stabilizing at a nearly constant value. Acidic Fenton’s process has been described as ideal for free radical generation. As it is mentioned before, during electrokinetic treatment an acidic front is generated at the anode due to water electrolysis, and migrates into the bulk of the soil causing desorption of metals from the soil. However, the basic front generated in the cathode increases the pH nearest this chamber, reducing the efficacy of the treatment. For this reason, in this process is necessary to apply strategies to control the soil pH. In this sense, a solution of citric acid (0.1 M) was added to the cathode and anode chamber with a dual benefit as chelating and pH control agent. As it was
30 Anode-Cathode Anode-P1 P1-P2 P2-P3 P3-Cathode
Votage drop (V)
25
20
15
10
5
0 0.0
0.5
1.0
1.5
2.0
Time (d)
Fig. 6 – Voltages drop profiles measured across the soil bed.
demonstrated in previous studies (Bocos et al., 2015; Vicente et al., 2011), this complexing agent enhances the desorption of the soil entrapped iron and forms soluble Fe-citrate complex that remains available for the Fenton’s oxidation. At the end of treatment, the pH of the soil becomes slightly acid in all sections. Therefore, the citric acid addition allowed remaining the pH in an appropriated value (below 5) along the soil, in the electrokinetic cell, favouring the oxidation reactions. According to the Helmholtz–Smoluchowski theory, the rate of electro-osmotic flow is influenced by the zeta potential of soils. Furthermore, this zeta potential has negative values under alkaline conditions at the surface of soil particles, and the rate of electro-osmotic flow increases as zeta potential decreases (Kim et al., 2005). It is remarkable that the citric acid also favoured the electro-osmotic flow and the cumulative volume of the effluent against the time, showing a constant flow that guarantee the effective soil–oxidant interaction. This fact is in accordance with Popov (1997) and Pazos et al. (2008) who reported that the presence of citric acid in the kaolinite sample modifies the superficial properties of clay particles increasing the total flux towards the cathode by the effect of the electric field. It is also known that the geomechanical behaviour of the soil has influence on the effectiveness of the electrokinetic processes. The results suggest that the main mechanism implicated in the colour removal of Rhodamine B is the dragging by electro-osmotic flow of hydrogen peroxide. For this reason, parameters such as porosity, defined as the ratio of the void volume to the total volume of the soil sample, is considered as a key factor (Lopez-Vizcaino et al., 2016; Wu et al., 2012; Chai et al., 2007). Compaction, which happens when unsaturated soils are compressed, affects the porosity and relates physical properties such as mechanical properties, and gas and water transport. For most soils, the compaction reduces the volume of large pores and consequently affects water retention properties and hydraulic conductivity in the range of high water potentials (Richard et al., 2001). To evaluate the influence of the soil porosity on the efficiency of the
Please cite this article in press as: Popescu, M., et al., Soil flushing and simultaneous degradation of organic pollutants in soils by electrokineticFenton treatment. Process Safety and Environmental Protection (2016), http://dx.doi.org/10.1016/j.psep.2016.03.012
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0.06
10
0.05
8
0.04
6
0.03
4
0.02
1.5
2
0.01
1.0
0
2.0
80
Intensity (A)
pH
2.5
Colour removal advance (cm)
3.5
3.0
Colour removal advance Intensity Electro-osmotic flow Anode pH Cathode pH
100
0.5
1.0
1.5
2.0
40
20
0.00 0.0
60
Cumulative flow (mL)
4.0
0
2.5
Time (d) Fig. 7 – Profiles of Rhodamine B removal, electro-osmotic flow, current intensity and anode and cathode pH, obtained along the electrokinetic-Fenton treatment of spiked Rhodamine kaolinite using as anolyte a solution of hydrogen peroxide 10% and soil bed porosity around 40%. electrokinetic-Fenton process, a comparative study using soil samples with different compactions was performed (porosity 40% and 50%). In Fig. 7, the profiles of the different studied parameters are showed for an electrokinetic-Fenton process using as anolyte a solution of hydrogen peroxide at 10% and a soil bed porosity of 40%. According to the Helmholtz–Smoluchowski theory, the rate of electro-osmotic flow is proportional to the available porosity of soils. For this reason, if the soil bed compaction was increased, the electroosmotic flow was reduced and this yielded the slowed down of the removal (Fig. 7). This fact highlights the importance of electro-osmotic flow in the electrokinetic-Fenton. Based on the reported results, it can be concluded that it is necessary (i) to keep an acid environment through the use of citric acid solution in both chambers and (ii) to use as anolyte a hydrogen peroxide solution of 10%, in order to increase the efficiency of this treatment in real soil.
3.2. soil
Electrokinetic-Fenton treatment of TPH polluted
Soil is the ultimate receptor of pollutants from anthropogenic activities, such as hydrocarbons, polycyclic aromatic hydrocarbons and polychlorinated biphenyls. Metals are attached
to different binding phases in soil, and the presence of these transition metals can be used to carry out the oxidation reactions. In this study, a TPH polluted soil was selected to validate the electrokinetic-Fenton treatment. Initially, the chemical characterization of collected soil samples, in terms of their heavy metal and TPH content, TOC, pH, conductivity was done (Table 1). It is noticeable the high concentration of iron, around 7600 mg of Fe/kg of dried soil, that makes this soil appropriate to the Fenton’s reactions take place. Another positive factor of this soil is its hydraulic conductivity value that will favour the transport of species in the interstitial fluid (Yeung, 1994). Thus, the studied soil will show a low electrical resistance and the electrical current will travel easily through the soil matrix, which favours the electrokinetic treatment. Initially, unenhanced electrokinetic-Fenton treatment of polluted soil was carried out at constant voltage drop of 3 V/cm and neither addition of facilitating agent nor hydrogen peroxide was done. In this experiment, the pH rise in the cathode chamber was caused by the generation of hydroxyl ions as a result of electrolysis of water during the remediation that also generated the apparition of a precipitate between S2 and S3. Then, the enhanced electrokinetic-Fenton was performed working with the optimal conditions obtained previously. In Table 2, the concentration of metals and TPH obtained by
Table 2 – TPH and metal concentrations and inhibition ratio of the initial sample and the three studied sections after the electrokinetic-Fenton treatment. Parameters
Fe Mg Mn Pb Zn TPH Microtox bioassay I
Initial (mg/kg)
7632 1981 135 12 72 80.4
After 15 d
After 27 d
S1 (mg/kg)
S2 (mg/kg)
S3 (mg/kg)
S1 (mg/kg)
S2 (mg/kg)
S3 (mg/kg)
6231.5 1631 39.8 8.5 33.5 35.8
6348 1691 42.4 12 37.5 36.5
6477 1743 44.5 12 42 38.1
6231.5 1455 36.4 7 21.5 32.5
6348 1496 40.4 11 22 31.5
6477 1544 42.3 12 23.5 34.8
Initial (%)
S1 (%)
S2 (%)
S3 (%)
S1 (%)
S2 (%)
S3 (%)
99.85
70.80
77.63
80.51
67.96
74.11
77.63
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electrokinetic treatment after 15 and 27 days are shown. The results indicated that a homogeneous removal of pollutants was achieved in the soil with TPH removal values around 54.4% and 58.2% after 15 and 27 days, respectively. Moreover, no TPH was detected in the electrolyte chamber. Because of the highest volatile hydrocarbon fractions were previously removed and the increase in soil temperature was negligible, hydrocarbon volatilization was not considered, and only the combination of electrokinetic and Fenton’s processes was assumed to be responsible for the TPH removal. The homogeneity of the TPH removal (S1–S3) and the slight increase in the removal after the longest treatment result from two aspects. First, the hydrogen peroxide was present throughout the soil matrix due to the electro-osmotic transport. In addition, after two weeks, the hydrogen peroxide in the anode chamber was decomposed and this fact explains the low degradation level obtained after 27 days. Finally, to determine the environmental effect of this treatment, the change in the toxicity of these samples using Microtox® bioassay was carried out with the initial and final samples obtained from each soil section after the electrokinetic-Fenton treatment. The mechanism of the Microtox® system is based upon sensing the respiration activity that is directly correlated with Vibrio fischeri cellular activity and its luminescence. In Table 2 the inhibition percentage of the different samples reveals that by electrokinetic-Fenton treatment, the THP soil content was reduced and then the toxicity effect was depleted in the same way. This fact confirms the efficiency of this treatment due to the in situ generation of hydroxyl radicals which were able to degrade TPH. Moreover, the application of this technique presents low environmental effect reaching a significant reduction of toxicity in the soil after the treatment.
4.
Conclusions
In this work, a deep analysis of the Electrokinetic-Fenton treatment was evaluated using kaolinite polluted with Rhodamine B as model sample. From these experiments, it was found that the iron concentration in the soil had a slight influence on the dye removal. However, the highest hydrogen peroxide dosage (7.5–10%) proportionated the faster colour removal rate. Furthermore, it was confirmed that the presence of citric acid favours the availability of the Fenton catalyst and the electroosmotic transport of the reactive species. Under the best operational conditions, the electrokinetic-Fenton treatment of a TPH polluted soil was efficiently carried out. Moreover, the Microtox analysis of the treated soil demonstrated the reduction of negative environmental effect and benevolence of the treatment. Summing up, the reported results confirmed that electrokinetic-Fenton treatment could be a suitable technique for the remediation of TPH polluted soils.
Acknowledgements This research has been financially supported by the Spanish Ministry of Economy and Competitiveness, Xunta de Galicia and ERDF Funds (Projects CTM2014-52471-R and GRC 2013/003). The authors are grateful to the Spanish Ministry of Economy and Competitiveness for the financial support of Marta Pazos under the Ramón y Cajal program and Xunta de Galicia for financial support of the researcher Emilio Rosales under a postdoctoral grant and to the European Union for the
financial support of Ciprian Sandu and Marius Popescu under the Erasmus+ program.
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Please cite this article in press as: Popescu, M., et al., Soil flushing and simultaneous degradation of organic pollutants in soils by electrokineticFenton treatment. Process Safety and Environmental Protection (2016), http://dx.doi.org/10.1016/j.psep.2016.03.012