- Email: [email protected]
Influence of anolyte and catholyte composition on TPHs removal from low permeability soil by electrokinetic reclamation
Electrochimica Acta 54 (2009) 2119–2124
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Influence of anolyte and catholyte composition on TPHs removal from low permeability soil by electrokinetic reclamation B. Murillo-Rivera a , I. Labastida a , J. Barrón a , M.T. Oropeza-Guzman b,∗ , I. González a , M.M.M. Teutli-Leon c a
Universidad Autónoma Metropolitana-Iztapalapa, Departamento de Química, Av. San Rafael Atlixco No. 186, Col. Vicentina C.P. 09340, Iztapalapa, México D.F., Mexico Centro de Graduados del Instituto Tecnológico de Tijuana, Bvd. Industrial s/n, Col. Otay. C.P. 22500, Tijuana, B. C. México, Mexico c Benémerita Universidad Autónoma de Puebla, Facultad de Ingeniería, Edificio 123, Ciudad Universitaria, C.P. 72570, Puebla, Pue. México, Mexico b
a r t i c l e
i n f o
Article history: Received 29 April 2008 Received in revised form 17 September 2008 Accepted 17 September 2008 Available online 8 October 2008 Keywords: Electroreclamation TPHs desorption Electroosmotic flow Soil contamination Electrokinetics
a b s t r a c t Removal of TPHs from polluted soil by electrokientic reclamation was done by using different electrolytes (anolyte and catholyte). The initial concentration of TPHs in soil was 23,000 ppm and removal efficiencies reached almost 90% for a combination of 0.04 M NaOH and 0.1 M Na2 SO4 in the anode and cathode chambers, respectively. Electroosmotic flow and TPHs desorption were measured under galvanostatic conditions (1.95 mA cm−2 and electric field <10 V cm−1 ). The study is supported on the electrokinetic transport model for low permeability soils. Electrolytes (anolyte and catholyte) were maintained at constant ionic composition to keep constant boundary conditions, thus launch a pseudostationary state for fluid and charge transport throughout the soil. It was also observed that electrolyte concentration favored TPHs desorption as well as their transport throughout the soil by electroosmotic flow from anode to cathode. Both, electrolytes concentration and wetting solution helped to maintain a constant pH profile during electroreclamation, thus a sustained fluid flow from anode to cathode. © 2008 Published by Elsevier Ltd.
1. Introduction In situ reclamation processes for total petroleum hydrocarbon , TPHs, like soil flushing or soil washing are physicochemical methods that use surfactants and/or cosolvents to improve pollutants solubility [1–3]. It should be considered that even if desorption and solubilization were improved by surfactants and solvents, this effect competes with the increase of fluid viscosity indirectly distressing soil hydraulic conductivity. Electrokinetic technology has aided in soil flushing by increasing soil–fluid interaction in low permeability regions thereby diminishing tailing, remediation time, and costs [4]. The most general knowledge of electrokinetics, as a soil electroreclamation process, states the application of an electric field under 10 V cm−1 (necessary to attaining low electrical current densities) using a pair of electrodes introduced in a humidified low permeable soil. The electric perturbation create an electrochemical potential gradient that may become visible as an electroosmotic flow, TPHs desorption, ion electromigration and electrophoretic transport of colloids and/or bacteria [5].
∗ Corresponding author. Tel.: +52 664 623 4043; fax: +52 664 623 4043. E-mail address: [email protected] (M.T. Oropeza-Guzman). 0013-4686/$ – see front matter © 2008 Published by Elsevier Ltd. doi:10.1016/j.electacta.2008.09.054
Published articles in soil electroreclamation, mentioned that a successful process design should be based on knowledge of (a) (b) (c) (d) (e) (f) (g)
soil characteristics; contaminant properties like concentration and mobility; voltage and current distribution into the electrokinetic cell; effluent chemistry; electrode material and geometric configuration; energy consumption; costs [6–8].
Also, several authors mention that soil exhibits a complex dynamic behavior under the action of an electric field, needing a careful analysis of all process variables; as it is shown in papers focused on physicochemical mathematical models [5,6,9]. Among the simplified models concerning electrokinetics in porous media, constant boundary conditions are a key factor to ensure a pseudostationary regime for water transport phenomena (disregarding the TPHs mass balance that is non-stationary) [9]. As a consequence, constant electrochemical potential gradient may ensure constant electroosmotic flow. It is also important to remark that constant chemical and electric potential gradient between catholyte and anolyte chambers leads to see the soil as a large membrane.
2120
B. Murillo-Rivera et al. / Electrochimica Acta 54 (2009) 2119–2124
Several soil electroreclamation papers have showed that organic compounds as aromatic hydrocarbons (benzene, toluene, ethylbenzene and xylene (BTEX), phenol and pentachlorophenol) can be successfully removed [10–13]. Also polyaromatic hydrocarbons like phenantrene [11,12] have served as models to prove the electroreclamation efficiency in low permeability soils. Some authors pointed out that pH control is a key factor for promoting electroosmotic flow [8,13]. For example, for phenol and pentachlorophenol removal, Kim and co-workers [13] used 0.1 M NaCl and 0.1 M NaOH as electrolytes. The same authors proposed to wet soil previously with 0.1 M NaOH. Their experimental setup allowed an alkaline pH along the cell, and a substantial increase in the observed electroosmotic flow. Other authors [7,14] have argued that sustaining extreme pH values along the cell can provoke a decrease of the electroosmotic flow, and even its disappearance. These phenomena being attributed to soil surface charge neutralization by excess in protons (acid pH) or hydroxyl ions (alkaline pH). On the other hand, the well known Helmholtz–Smoluchowski equation for the electroosmotic velocity, veo
veo =
−DEz
shows that changes in soil surface potential, (zeta potential), electric potential at the a differential space of the cell, Ez , as well as fluid dielectric constant D, and viscosity , will influence the electroosmotic flow EOF (EOF = S veo , with S: transversal cell area) throughout a capillary channel [4]. Furthermore, to keep the electroosmotic flow towards the cathode, the soil particles–fluid system needs to ensure high pH and a negative surface charge and potential ( < 0). These conditions will keep the soil particles far from the zero charge potential [4]. Since the electrolysis reaction at the anode may lowering the pH below the pHpzc, in that region, a cessation of electroosmotic flow may occur. It should be remembered though, that many factors such as the pH, potential, pore pressures, and electrical gradient often vary along length of the soil sample during the electroreclamation [7,15]. A combination of these factors may turn surface charge to a positive value ( > 0). This condition may stop or reverse the electroosmotic flow [7,16,17]. Even if the relationship between electroosmotic fluid flow direction and pore fluid physicochemistry still need to be fully understood, it is true that electroosmotic advection is a mechanical displacement of contaminants associated with electroosmotic fluid flow. This dragging mechanism is applicable to all contaminants desorbed from soil particles and remains in the porous media [18]. Then again, supposing that electroosmotic flow may drag hydrocarbons out of the soil, it should be considered their water solubility. Non-soluble hydrocarbons removal would be limited, while soluble ones strongly depends on desorption (needs an increase in local soil potential by changing the excess surface charge) [6]. Also, an enhancement scheme like an alkaline purge acts increasing the rate of electroosmotic flow and hydrocarbons removal [8]. This paper reports an experimental strategy to remove hydrocarbon from a real weathered and polluted soil within a lab designed cylindrical cell. Experiments were focused on getting evidence about how electrolyte in each electrode chamber can produce a maximum TPHs desorption and transport as a result of its dissolution and dragging forces provoked by the electroosmotic flow.
mixing. The mean initial TPHs concentration was 23,000 ppm. The soil sample was classified as sandy soil (97.6% sand, 2% silt and 0.4% clay) by the Unified Classification System procedure (based on ASTM D-2487) with 6.17% of organic matter (OM), a cation exchange capacity (CEC) of 24.2 mmol kg−1 ; 52.92 and 128 mg kg−1 of Fe and Mg, respectively. Soil preconditioning solutions and electrolytes were prepared with either sodium hydroxide (Baker A. R.) or sodium sulphate (Baker A. R.). Adsorbed TPHs extraction was accomplished by using tetrachlorethylene HPLC analytical grade as a solvent in an ultrasonic bath Branson 2510. Due to the complexity of the weathered soil, it was not possible to perform a detailed analysis for assessing which hydrocarbon species were present; therefore, initial and final concentration of soil TPHs were determined following standard procedures (EPA3550b, 418.1). TPHs concentration was measured by the gravimetric method. The initial TPHs concentration was measured to all preconditioned soil samples, before assembling the cell. After each electrokinetic experiment, soil specimen was partitioned in three sections named anodic, middle and cathodic. Each section was hand-mixed again and two representative samples were taken for analysis of TPHs final concentration. Since water electrode reactions (oxidation and reduction) should be performed under optimal conditions to avoid extra resistance to electric current through the cell, electrode materials have been chosen among those named dimensionally stable anodes (DSA) that are made of Ti covered with RuO2 . This type of electrode allows for a faster water oxidation/reduction, and at the same time they exhibit a high corrosion resistance [19]. In this work, electrodes of Ti–RuO2 were prepared using a well-known method [20] and used as cathode and anode. Both of them have an estimated cross-section area of 12.84 cm2 . All experiments were performed in a cylindrical cell (radius = 1.8 cm and length = 20 cm), divided in three compartments: anodic, soil sample and cathodic chambers. A flow diagram of the experimental device is shown in Fig. 1. Cell is easy assembling and handling, it has several sampling ports for introducing pH electrodes, as well as circulates fresh solution in both electrode chambers [21]. Electrokinetic experiments were conducted under galvanostatic conditions using a potentiostat–galvanostat from Princeton Applied Research model EG&G PAR 173 and measuring
2. Experimental 20 kg of hydrocarbon polluted soil was provided by a Mexican company working in soil remediation. Soil was obtained from a region nearby an oil refinery and it turned homogeneous by hand
Fig. 1. Electrokinetic setup uses in this work. Each element is indicated in the figure.
B. Murillo-Rivera et al. / Electrochimica Acta 54 (2009) 2119–2124
2121
Table 1 Experimental conditions for electrokinetic experiments of low permeability sandy soil polluted with total petroleum hydrocarbons (TPHs). Applied current density (mA cm−2 )
Experiment ID
Soil humectation (wt%)
Electrolytes Wetting solution
Anode chamber
Cathode chamber
T1 T2
10
0.1 M Na2 SO4 0.1 M Na2 SO4
0.1 M Na2 SO4 0.1 M Na2 SO4
0.1 M Na2 SO4 0.1 M Na2 SO4
0.00 1.95
T3 T4 T5 T6 T7 T8
20
0.1 M Na2 SO4 0.1 M NaOH 0.1 M Na2 SO4 0.1 M Na2 SO4 0.1 M Na2 SO4 0.1 M Na2 SO4
0.01 M NaOH 0.1 M NaOH 0.1 M NaOH 0.1 M NaOH 0.05 M NaOH 0.04 M NaOH
0.01 M NaOH 0.1 M NaOH 0.1 M NaOH 0.1 M NaOH 0.1 M Na2 SO4 0.1 M Na2 SO4
1.95 1.95 3.90 1.95 1.95 1.95
the cell potential with a high impedance multimeter model PROTEK 506. A peristaltic pump with double head from Cole Parmer was used for electrolyte circulation at each electrode chamber. The rate of circulation was fixed at 80 mL h−1 . pH was measured in situ with a HANNA soil pH electrode HI2031. This type of electrode avoids the “suspension effect” due to an unstable potential in the junction of the reference liquid part. Zeta potential measurements were done using a laser dispersion electrophoretic cell in a NS Malvern zetameter ZEN3500. 3. Results The efficiency of hydrocarbon reclamation from polluted soils depends in the conditions for maximum TPHs desorption and transportation. Parameters that can be easily manipulated in any electrokinetic experiment are: soil humidity, electric current and electrolyte composition. In Table 1 tested conditions are reported. To ensure that no pressure gradient was developed inside the electrokinetic cell, recirculation of each electrolyte was maintained during 48 h. Soil sample was previously humidified at 10% (under its field capacity) with 0.1 M Na2 SO4 and packed into the cell. Initial and final soil TPHs concentration showed very similar concentration. So far, the electrolyte by itself had shown none effect on releasing TPHs from the soil matrix. Voltage gradient for T2 was higher (7.7–8.9 V cm−1 ) than that one observed in metal reclamation (1–1.5 V cm−1 ). Low wetting condition produced soil overheats, exhibited by raising temperature from 23 to 35 ◦ C. After 18 h, cumulative cathodic fluid volume increased abruptly providing 11 mL in 4 h and after 22 h it went downward. In this experiment the maximum temperature matched with the higher electroosmotic flow producing soil moisture loss at extremely fast rates. Pursuing to overcome soil moisture loss due to overheating and lowering the exhibited voltage gradient, soil-wetting condition was modified to 20%. Solutions at electrode chambers were replaced by 0.01 M NaOH (T3). With this new setup, a smaller increase in temperature was registered (23–26 ◦ C in 4 h), remaining stable after that. Also, electric potential gradient was lower than T2 exhibiting 2 V cm−1 during 7 h, later lectures showed a steady raise reaching 10 V cm−1 at 21 h. At this point, pH profiles in T2 and T3 showed a typical electroreclamation trend (acid in the anodic region and alkaline in the cathodic region). Quantitatively, at similar times (21 h), the cumulative cathodic fluid in T3 was shorter (14 mL) than the one obtained in T2 (23 mL), but T3 was more effective on diminishing TPHs soil concentration. Fig. 2 shows a comparison of final pH profile and TPHs residual concentration for T2 (Fig. 2a) and T3 (Fig. 2b). From this comparison, the pH soil profile for T2 (lower wetting), Fig. 2a exhibits an acidic condition in the anodic region, with an abrupt transition for reaching a variable alkaline condition from the middle to the cathode. For T3 (higher humidity), Fig. 2b, a less acidic
condition was reached from the anode to the middle and a smoother transition to reach alkaline pH in the cathodic region. It is also important to mention that even if electroosmotic flow is theoretically justified only for clayed soils (large cation exchange capacity) also sandy soil can develop water flow under the effect of an electric field. As shown in this study alkaline conditions throughout the soil, as well as electric field may induce soil surface structure deformation that increase its superficial cation exchange capacity and promotes an electroosmotic flow. Correlating TPHs residual concentrations with the exhibited pH profiles, it can be established that in acidic condition (anodic region) TPHs removal is lower than those for alkaline conditions (cathodic region) (Fig. 2); also, the smoother pH profile exhibited by T3 (Fig. 2b) seems to be favorable for higher TPHs removal. Nev-
Fig. 2. pH and TPHs profiles at the end of electroreclamation experiments (a current of 1.95 mA cm−2 was imposed for 22 h). Soil humidity was changed from 10% (T2, Table 1) to 20% (T3, Table 1).
2122
B. Murillo-Rivera et al. / Electrochimica Acta 54 (2009) 2119–2124
Fig. 3. Image of the final state electrokinetic reclamation (T4, Table 1) showing bidirectional TPHs flow.
ertheless, it must be remarked that higher TPHs concentration was presented in the middle section of the cell. In order to facilitate an alkaline condition within the soil matrix, the 0.1 M NaOH electrolyte was used for both soil humidification and electrode chambers (T4, Table 1). This change seems to work well because in 48 h overheating was almost negligible, temperature was almost constant (26–28 ◦ C), and electric potential gradient was barely stable with values oscillating between 3.5 and 3.8 V cm−1 . The pH profiles confirmed that soil got an alkaline condition throughout the cell, and an almost stable state at the cathodic region. At 48 h it became obvious the acid front penetration in the anodic region, although the alkaline condition prevailed all over the cell. This effect impacted positively on the electroosmotic flow, which exhibited almost a linear trend (slope = 1.1 mL h−1 and R2 = 0.96) with 54 mL collected in 48 h. Observation of electrode chambers, while running the experiment, allowed detecting TPHs presence in both electrode chamber, this phenomenon became evident through changes in color. At 24 h the anodic chamber solution turned out turbid and at 48 h color in the anodic chamber was yellow, while the cathodic chamber was brown (Fig. 3). This result suggested a bidirectional TPHs transport, first of all due to TPH desorption and then due to TPHs diffusion toward electrode chambers as well as TPHs dragged by electroosmotic flow from anode to cathode. All these phenomena appeared when the electrokinetic processes in sandy soil took place at excessively alkaline conditions. A comparison of residual TPHs from T3 to T4 suggested that alkaline condition permitted higher removal of TPHs in the anodic region and a similar amount in the cathodic region. However, in the middle of the cell an accumulation of TPHs took place, and detected concentration was very close to the initial one. This condition was visually confirmed because soil in the middle section showed a “dark line”. Due to the bidirectional TPHs flow exhibited in T4, it was reconsidered to change the wetting solution to 0.1 M Na2 SO4 . While the electrode chamber solutions remain with 0.1 M NaOH (T5, Table 1), the applied current was twice the previous one (3.9 mA cm−2 ). This new setup allowed to keep a minimal overheating, temperature varied from 27 to 30 ◦ C in 4 h, and after that became constant. The electric potential gradient went from 3.0 to 3.5 V cm−1 in 6 h and after that remains stable. Electroosmotic flow exhibited a linear trend (slope = 3.8 mL h−1 and R2 = 0.99) providing 84 mL in 19 h. Removal of TPHs became more uniform in the whole cell (final concentration were: anodic 9.5, middle 10.8, and cathodic 8.8 g L−1 ); so far the excessive accumulation in the middle section was overcome.
Fig. 4. Variation of permeated solution volume through the electrokinetic cell (from anode to cathode) during the time elapsed in the electroreclamation experiments (Table 1): (a) T6 (anodic chamber: 0.1 M NaOH), (b) T7 (anodic chamber: 0.05 M NaOH) and (c) T8 (anodic chamber: 0.04 M NaOH). Solid lines represent linear regressions.
The deficiencies of this treatment were: (a) the process still exhibited a bidirectional TPHs flow evidenced by the coloration of the anodic well solution and (b) when the cell was disassembled soil consistency was muddy, phenomena that could imply a disruption of the soil matrix. In order to protect soil structure and enhances TPHs flow from anode to cathode, experimental conditions were modified lowering again the applied current density to 1.947 mA cm−2 (T6, Table 1). By doing so, similar conditions for temperature (24–27 ◦ C) and voltage gradient (3.5–3.8 V cm−1 ) were observed; the electroosmotic flow exhibited an almost linear trend (slope = 1.6 mL h−1 and R2 = 0.99) providing 75 mL in 48 h (Fig. 4a). In this treatment (T6), pH profiles exhibited an almost neutral value in the anodic region, and highly alkaline conditions were favored in the middle and cathodic regions (Fig. 5a). Like in T4, the alkaline state produced a bidirectional TPHs flow, but removals in the anodic and middle regions were lowered (Fig. 5a). Although good results were obtained in T6, the bidirectional TPHs flow still persisted. A new proposal was to diminishing the concentration of electrolyte in the anodic chamber, from 0.1 to 0.05 M NaOH (T7, Table 1); since less concentrated electrolytes may favor changes in potential of soil particles, thus increasing desorption phenomena and TPHs removal [22]. The cathodic chamber electrolyte was 0.1 M Na2 SO4 . Finally, the soil wetting ratio (20 wt%) and current density (1.947 mA cm−2 ) (T7, Table 1) were kept the same as in T6. In a first instance the electroosmotic flow follows an almost linear trend (1.4 mL h−1 and R2 = 0.98) providing 69 mL in 48 h (Fig. 4b); while temperature oscillated between 25 and 29 ◦ C, and electric potential gradient exhibited a singular behavior raising from 1.8 to 4.8 and then decreasing to 2.0 V cm−1 . Final pH profile and TPHs concentration matched in a better way since the soil alkalinity ensured not only a constant electroosmotic flow, but also a more homogeneous TPHs removal (Fig. 5b). Global TPHs removal efficiency increased enough to improve any other previous treatment. In the experiment T7, the anodic chamber
B. Murillo-Rivera et al. / Electrochimica Acta 54 (2009) 2119–2124
2123
Fig. 6. Zeta potential measurement of different soil dispersions, in deionized water. The analyzed soils were obtained after the electrokinetic treatment T6 and T8 (Table 1), the non-treated soil is included.
Fig. 5. Profiles at the end of electroreclamation experiments (a current of 1.95 mA cm−2 was imposed for 48 h). The experimental setup is indicated in Table 1. The electrolytes in the anodic and cathode chamber were varied: T6, anodic 0.1 M NaOH and cathodic 0.1 M NaOH; T7, anodic 0.05 M NaOH and cathodic 0.1 M Na2 SO4 ; T8, anodic 0.04 M NaOH and cathodic 0.1 M Na2 SO4 . (a) Final pH profile and (b) final TPHs profile.
electrolyte never changed its color implying that the bidirectional TPHs flow was overcome. A last refinement was tried using electrolyte 0.04 M NaOH at the anodic chamber, with all other conditions as in T7. In this treatment (T8, Table 1), electroosmotic flow exhibited a delay of 10 h, after that a nearly linear trend (2.6 mL h−1 and R2 = 0.99) providing 85 mL at 48 h. A slight overheating was present and temperature oscillated between 23 and 27 ◦ C, while the electric potential gradient went from 1.8 to 6.0 and then decreasing to 3.6 V cm−1 . pH profiles indicated that an alkaline condition was rapidly obtained and sustained along the experiment (Fig. 5a). This conditions in pH also overcome the bidirectional TPHs flow which was confirmed by the following facts: (a) electrolyte in the anodic chamber remained almost unaffected, color solution exhibited only a slight turbidity and (b) the attained TPHs removal is highest than that obtained in T7 (Fig. 5b). Results obtained in the test T7 are encouraging since by applying a 48-h treatment, at low current density, it was possible to assure alkaline conditions throughout the soil at any time. By doing so any possibility of bidirectional TPHs flow was eliminated, and TPHs removal was high enough to consider a clean-up greater than 90% in the anodic section, and greater than 75% for the middle and cathodic ones. Also a high sensitivity of the electroreclamation process was patent since the anodic chamber electrolyte concentrations vary
in a very wide range in treatments T6 and T8 (from 0.1 to 0.04 M NaOH). Finally, potential measurements of T6 and T8 soil samples, after electrokinetic test, were done by dispersing 5 mg of soil in 50 mL of deionized water. Fig. 6 shows negative values for the potential in all cases. The negative potential induces electroosmotic flow towards the cathode during electrokinetic process [22] On the other hand, Fig. 6 also shows how the potential (interphase property defined by charge arrangement between solid and fluid phases) was more negative for T8 soil in all the cell sections. Zeta potential values under −30 mV implied that soil after electrokinetic test for T8, reached a better dispersion ensuring greater water electroosmotic permeability (Smoluchovsky model). Moreover to reach more negative potential, soil particles have to exhibit larger negative surface charge confirming the action of alkaline media on the soil surface structure. All these arguments are a clear confirmation that electrolyte composition is one of the most important variables of electrokinetics as a soil remediation method, since it changes surface soil conditions in such a way that can promote contaminants desorption and increase soil permeability. 4. Conclusions The reported experimental data allowed to confirm that hydrocarbon polluted soils can be successfully electroremediated, even its complexity, by setting up the right conditions in soil wetting, electrolyte solution and an adequate enhancement scheme. Soil humectation of 20% showed to be sufficient for getting electrokinetic remediation without overheating the soil. This value is quite close to the water field capacity of the sandy soil used in this study (23%), indicating that electrokinetics does not require water saturated soils. Unless the field water capacity is more than enough to start a soil electrokinetic remediation. Electrolyte used for soil humidification must be selected to avoid soil overheating (Joul effect), as well as any possibility of soil disruption. For sandy soil used in this study the best solution was the 0.1 M Na2 SO4 . This result certainly was combined with the extremely low conductivity of soil sample. However, it is not possible to generalize the use of electrolyte solutions for soil preconditioning. More research in this sense needs to be done in order to obtain a suitable conductivity range for a successful electrokinetic reclamation. The mixed enhancement scheme including 0.1 M NaOH at the anode chamber, and 0.1 M Na2 SO4 at the cathode chamber, neutralized protons generated at the anode as well as establishing an input of hydroxyl ions, such that an alkaline pH was reached
2124
B. Murillo-Rivera et al. / Electrochimica Acta 54 (2009) 2119–2124
and sustained. On the other hand, extremely alkaline conditions have favored soil surface modification allowing TPHs desorption as well as an increase in surface ion exchange capacity manifested in greater soil permeability under the action of an electric field. High sensitivity of electroosmotic flow to the anodic chamber electrolyte concentration was shown. The last experiment (T8, Table 1) joined the best condition to get an electroosmotic flow, with a residence time enough for TPHs desorption and their incorporation into the fluid phase that was transported from anode to cathode. The experimental conditions covered in this work are useful to refine soil electrokinetics setup, showing that it is possible to perform an electroreclamation having optimal conditions in soil conductivity, soil pH, maximum desorption and unidirectional transport of TPHs. References [1] S.E. Manahan, Environmental Chemistry, CRC Press LLC, Boca Raton, 2000, p. 22.
[2] M.L. Davies, S.J. Masten, Principles of Environmental Engineering and Science, McGraw Hill Science/Engineering/Math, New York, 2004, p. 26. [3] E. Riser-Roberts, Remediation of Petroleum Contaminated Soils, Biological, Physical and Chemical Processes, CRC Press LLC, Boca Raton, 1998, p. 99. [4] R.E. Saicheck, K.R. Reddy, Crit. Rev. Environ. Sci. Technol. 35 (2005) 115. [5] A.N. Alshawabkeh, A.T. Yeung, M.R. Bricka, J. Environ. Eng. 125 (1999) 27. [6] R.F. Probstein, R.E. Hicks, Science 260 (1993) 498. [7] G.R. Eykholt, D.E. Daniel, J. Geotech. Eng. ASCE 120 (1994) 797. [8] M.M. Page, Ch.L. Page, J. Environ. Eng. 128 (2002) 208. [9] M.M. Teutli-León, M.T. Oropeza, I. González, A. Soria, AIChE J. 51 (6) (2005)1822. [10] G. Maini, A.K. Sharman, C.J. Knoules, G. Sunderland, S.A. Jackman, J. Chem. Technol. Biotechnol. 75 (2000) 657. [11] A. Li, K.A. Cheung, K.R. Reddy, J. Environ. Eng. 126 (2000) 527. [12] R.K. Reddy, E.R. Saichek, J. Environ. Sci. Health A94 (5) (2004) 1189. [13] S.O. Kim, S.H. Moon, K.W. Kim, J. Environ. Technol. 21 (1999) 417. [14] A.P. Shapiro, R.F. Probstein, Environ. Sci. Technol. 27 (1993) 283. [15] G.R. Eykholt, J. Hazard. Mater 55 (1997) 171. [16] Y.B. Acar, et al., J. Hazard. Mater 40 (1995) 117. [17] L.M. Vane, G.M. Zang, J. Hazard. Mater. 55 (1997) 1. [18] A.T. Yeung, Environ. Eng. Sci. 23 (2006) 202. [19] S. Trasatti, Electrochim. Acta 45 (2000) 2377. [20] R.G. Freitas, et al., J. Power Sources 171 (2007) 373. [21] B. Murillo-Rivera, M.S. Thesis, Universidad Autonóma Metropolitana, México, 2006. [22] A.Z. Al-Hamdan, K.R. Reddy, Geotech. Geolog. Eng. 24 (2006) 1679.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Influence of anolyte and catholyte composition on TPHs removal from low permeability soil by electrokinetic reclamation B. Murillo-Rivera a , I. Labastida a , J. Barrón a , M.T. Oropeza-Guzman b,∗ , I. González a , M.M.M. Teutli-Leon c a
Universidad Autónoma Metropolitana-Iztapalapa, Departamento de Química, Av. San Rafael Atlixco No. 186, Col. Vicentina C.P. 09340, Iztapalapa, México D.F., Mexico Centro de Graduados del Instituto Tecnológico de Tijuana, Bvd. Industrial s/n, Col. Otay. C.P. 22500, Tijuana, B. C. México, Mexico c Benémerita Universidad Autónoma de Puebla, Facultad de Ingeniería, Edificio 123, Ciudad Universitaria, C.P. 72570, Puebla, Pue. México, Mexico b
a r t i c l e
i n f o
Article history: Received 29 April 2008 Received in revised form 17 September 2008 Accepted 17 September 2008 Available online 8 October 2008 Keywords: Electroreclamation TPHs desorption Electroosmotic flow Soil contamination Electrokinetics
a b s t r a c t Removal of TPHs from polluted soil by electrokientic reclamation was done by using different electrolytes (anolyte and catholyte). The initial concentration of TPHs in soil was 23,000 ppm and removal efficiencies reached almost 90% for a combination of 0.04 M NaOH and 0.1 M Na2 SO4 in the anode and cathode chambers, respectively. Electroosmotic flow and TPHs desorption were measured under galvanostatic conditions (1.95 mA cm−2 and electric field <10 V cm−1 ). The study is supported on the electrokinetic transport model for low permeability soils. Electrolytes (anolyte and catholyte) were maintained at constant ionic composition to keep constant boundary conditions, thus launch a pseudostationary state for fluid and charge transport throughout the soil. It was also observed that electrolyte concentration favored TPHs desorption as well as their transport throughout the soil by electroosmotic flow from anode to cathode. Both, electrolytes concentration and wetting solution helped to maintain a constant pH profile during electroreclamation, thus a sustained fluid flow from anode to cathode. © 2008 Published by Elsevier Ltd.
1. Introduction In situ reclamation processes for total petroleum hydrocarbon , TPHs, like soil flushing or soil washing are physicochemical methods that use surfactants and/or cosolvents to improve pollutants solubility [1–3]. It should be considered that even if desorption and solubilization were improved by surfactants and solvents, this effect competes with the increase of fluid viscosity indirectly distressing soil hydraulic conductivity. Electrokinetic technology has aided in soil flushing by increasing soil–fluid interaction in low permeability regions thereby diminishing tailing, remediation time, and costs [4]. The most general knowledge of electrokinetics, as a soil electroreclamation process, states the application of an electric field under 10 V cm−1 (necessary to attaining low electrical current densities) using a pair of electrodes introduced in a humidified low permeable soil. The electric perturbation create an electrochemical potential gradient that may become visible as an electroosmotic flow, TPHs desorption, ion electromigration and electrophoretic transport of colloids and/or bacteria [5].
∗ Corresponding author. Tel.: +52 664 623 4043; fax: +52 664 623 4043. E-mail address: [email protected] (M.T. Oropeza-Guzman). 0013-4686/$ – see front matter © 2008 Published by Elsevier Ltd. doi:10.1016/j.electacta.2008.09.054
Published articles in soil electroreclamation, mentioned that a successful process design should be based on knowledge of (a) (b) (c) (d) (e) (f) (g)
soil characteristics; contaminant properties like concentration and mobility; voltage and current distribution into the electrokinetic cell; effluent chemistry; electrode material and geometric configuration; energy consumption; costs [6–8].
Also, several authors mention that soil exhibits a complex dynamic behavior under the action of an electric field, needing a careful analysis of all process variables; as it is shown in papers focused on physicochemical mathematical models [5,6,9]. Among the simplified models concerning electrokinetics in porous media, constant boundary conditions are a key factor to ensure a pseudostationary regime for water transport phenomena (disregarding the TPHs mass balance that is non-stationary) [9]. As a consequence, constant electrochemical potential gradient may ensure constant electroosmotic flow. It is also important to remark that constant chemical and electric potential gradient between catholyte and anolyte chambers leads to see the soil as a large membrane.
2120
B. Murillo-Rivera et al. / Electrochimica Acta 54 (2009) 2119–2124
Several soil electroreclamation papers have showed that organic compounds as aromatic hydrocarbons (benzene, toluene, ethylbenzene and xylene (BTEX), phenol and pentachlorophenol) can be successfully removed [10–13]. Also polyaromatic hydrocarbons like phenantrene [11,12] have served as models to prove the electroreclamation efficiency in low permeability soils. Some authors pointed out that pH control is a key factor for promoting electroosmotic flow [8,13]. For example, for phenol and pentachlorophenol removal, Kim and co-workers [13] used 0.1 M NaCl and 0.1 M NaOH as electrolytes. The same authors proposed to wet soil previously with 0.1 M NaOH. Their experimental setup allowed an alkaline pH along the cell, and a substantial increase in the observed electroosmotic flow. Other authors [7,14] have argued that sustaining extreme pH values along the cell can provoke a decrease of the electroosmotic flow, and even its disappearance. These phenomena being attributed to soil surface charge neutralization by excess in protons (acid pH) or hydroxyl ions (alkaline pH). On the other hand, the well known Helmholtz–Smoluchowski equation for the electroosmotic velocity, veo
veo =
−DEz
shows that changes in soil surface potential, (zeta potential), electric potential at the a differential space of the cell, Ez , as well as fluid dielectric constant D, and viscosity , will influence the electroosmotic flow EOF (EOF = S veo , with S: transversal cell area) throughout a capillary channel [4]. Furthermore, to keep the electroosmotic flow towards the cathode, the soil particles–fluid system needs to ensure high pH and a negative surface charge and potential ( < 0). These conditions will keep the soil particles far from the zero charge potential [4]. Since the electrolysis reaction at the anode may lowering the pH below the pHpzc, in that region, a cessation of electroosmotic flow may occur. It should be remembered though, that many factors such as the pH, potential, pore pressures, and electrical gradient often vary along length of the soil sample during the electroreclamation [7,15]. A combination of these factors may turn surface charge to a positive value ( > 0). This condition may stop or reverse the electroosmotic flow [7,16,17]. Even if the relationship between electroosmotic fluid flow direction and pore fluid physicochemistry still need to be fully understood, it is true that electroosmotic advection is a mechanical displacement of contaminants associated with electroosmotic fluid flow. This dragging mechanism is applicable to all contaminants desorbed from soil particles and remains in the porous media [18]. Then again, supposing that electroosmotic flow may drag hydrocarbons out of the soil, it should be considered their water solubility. Non-soluble hydrocarbons removal would be limited, while soluble ones strongly depends on desorption (needs an increase in local soil potential by changing the excess surface charge) [6]. Also, an enhancement scheme like an alkaline purge acts increasing the rate of electroosmotic flow and hydrocarbons removal [8]. This paper reports an experimental strategy to remove hydrocarbon from a real weathered and polluted soil within a lab designed cylindrical cell. Experiments were focused on getting evidence about how electrolyte in each electrode chamber can produce a maximum TPHs desorption and transport as a result of its dissolution and dragging forces provoked by the electroosmotic flow.
mixing. The mean initial TPHs concentration was 23,000 ppm. The soil sample was classified as sandy soil (97.6% sand, 2% silt and 0.4% clay) by the Unified Classification System procedure (based on ASTM D-2487) with 6.17% of organic matter (OM), a cation exchange capacity (CEC) of 24.2 mmol kg−1 ; 52.92 and 128 mg kg−1 of Fe and Mg, respectively. Soil preconditioning solutions and electrolytes were prepared with either sodium hydroxide (Baker A. R.) or sodium sulphate (Baker A. R.). Adsorbed TPHs extraction was accomplished by using tetrachlorethylene HPLC analytical grade as a solvent in an ultrasonic bath Branson 2510. Due to the complexity of the weathered soil, it was not possible to perform a detailed analysis for assessing which hydrocarbon species were present; therefore, initial and final concentration of soil TPHs were determined following standard procedures (EPA3550b, 418.1). TPHs concentration was measured by the gravimetric method. The initial TPHs concentration was measured to all preconditioned soil samples, before assembling the cell. After each electrokinetic experiment, soil specimen was partitioned in three sections named anodic, middle and cathodic. Each section was hand-mixed again and two representative samples were taken for analysis of TPHs final concentration. Since water electrode reactions (oxidation and reduction) should be performed under optimal conditions to avoid extra resistance to electric current through the cell, electrode materials have been chosen among those named dimensionally stable anodes (DSA) that are made of Ti covered with RuO2 . This type of electrode allows for a faster water oxidation/reduction, and at the same time they exhibit a high corrosion resistance [19]. In this work, electrodes of Ti–RuO2 were prepared using a well-known method [20] and used as cathode and anode. Both of them have an estimated cross-section area of 12.84 cm2 . All experiments were performed in a cylindrical cell (radius = 1.8 cm and length = 20 cm), divided in three compartments: anodic, soil sample and cathodic chambers. A flow diagram of the experimental device is shown in Fig. 1. Cell is easy assembling and handling, it has several sampling ports for introducing pH electrodes, as well as circulates fresh solution in both electrode chambers [21]. Electrokinetic experiments were conducted under galvanostatic conditions using a potentiostat–galvanostat from Princeton Applied Research model EG&G PAR 173 and measuring
2. Experimental 20 kg of hydrocarbon polluted soil was provided by a Mexican company working in soil remediation. Soil was obtained from a region nearby an oil refinery and it turned homogeneous by hand
Fig. 1. Electrokinetic setup uses in this work. Each element is indicated in the figure.
B. Murillo-Rivera et al. / Electrochimica Acta 54 (2009) 2119–2124
2121
Table 1 Experimental conditions for electrokinetic experiments of low permeability sandy soil polluted with total petroleum hydrocarbons (TPHs). Applied current density (mA cm−2 )
Experiment ID
Soil humectation (wt%)
Electrolytes Wetting solution
Anode chamber
Cathode chamber
T1 T2
10
0.1 M Na2 SO4 0.1 M Na2 SO4
0.1 M Na2 SO4 0.1 M Na2 SO4
0.1 M Na2 SO4 0.1 M Na2 SO4
0.00 1.95
T3 T4 T5 T6 T7 T8
20
0.1 M Na2 SO4 0.1 M NaOH 0.1 M Na2 SO4 0.1 M Na2 SO4 0.1 M Na2 SO4 0.1 M Na2 SO4
0.01 M NaOH 0.1 M NaOH 0.1 M NaOH 0.1 M NaOH 0.05 M NaOH 0.04 M NaOH
0.01 M NaOH 0.1 M NaOH 0.1 M NaOH 0.1 M NaOH 0.1 M Na2 SO4 0.1 M Na2 SO4
1.95 1.95 3.90 1.95 1.95 1.95
the cell potential with a high impedance multimeter model PROTEK 506. A peristaltic pump with double head from Cole Parmer was used for electrolyte circulation at each electrode chamber. The rate of circulation was fixed at 80 mL h−1 . pH was measured in situ with a HANNA soil pH electrode HI2031. This type of electrode avoids the “suspension effect” due to an unstable potential in the junction of the reference liquid part. Zeta potential measurements were done using a laser dispersion electrophoretic cell in a NS Malvern zetameter ZEN3500. 3. Results The efficiency of hydrocarbon reclamation from polluted soils depends in the conditions for maximum TPHs desorption and transportation. Parameters that can be easily manipulated in any electrokinetic experiment are: soil humidity, electric current and electrolyte composition. In Table 1 tested conditions are reported. To ensure that no pressure gradient was developed inside the electrokinetic cell, recirculation of each electrolyte was maintained during 48 h. Soil sample was previously humidified at 10% (under its field capacity) with 0.1 M Na2 SO4 and packed into the cell. Initial and final soil TPHs concentration showed very similar concentration. So far, the electrolyte by itself had shown none effect on releasing TPHs from the soil matrix. Voltage gradient for T2 was higher (7.7–8.9 V cm−1 ) than that one observed in metal reclamation (1–1.5 V cm−1 ). Low wetting condition produced soil overheats, exhibited by raising temperature from 23 to 35 ◦ C. After 18 h, cumulative cathodic fluid volume increased abruptly providing 11 mL in 4 h and after 22 h it went downward. In this experiment the maximum temperature matched with the higher electroosmotic flow producing soil moisture loss at extremely fast rates. Pursuing to overcome soil moisture loss due to overheating and lowering the exhibited voltage gradient, soil-wetting condition was modified to 20%. Solutions at electrode chambers were replaced by 0.01 M NaOH (T3). With this new setup, a smaller increase in temperature was registered (23–26 ◦ C in 4 h), remaining stable after that. Also, electric potential gradient was lower than T2 exhibiting 2 V cm−1 during 7 h, later lectures showed a steady raise reaching 10 V cm−1 at 21 h. At this point, pH profiles in T2 and T3 showed a typical electroreclamation trend (acid in the anodic region and alkaline in the cathodic region). Quantitatively, at similar times (21 h), the cumulative cathodic fluid in T3 was shorter (14 mL) than the one obtained in T2 (23 mL), but T3 was more effective on diminishing TPHs soil concentration. Fig. 2 shows a comparison of final pH profile and TPHs residual concentration for T2 (Fig. 2a) and T3 (Fig. 2b). From this comparison, the pH soil profile for T2 (lower wetting), Fig. 2a exhibits an acidic condition in the anodic region, with an abrupt transition for reaching a variable alkaline condition from the middle to the cathode. For T3 (higher humidity), Fig. 2b, a less acidic
condition was reached from the anode to the middle and a smoother transition to reach alkaline pH in the cathodic region. It is also important to mention that even if electroosmotic flow is theoretically justified only for clayed soils (large cation exchange capacity) also sandy soil can develop water flow under the effect of an electric field. As shown in this study alkaline conditions throughout the soil, as well as electric field may induce soil surface structure deformation that increase its superficial cation exchange capacity and promotes an electroosmotic flow. Correlating TPHs residual concentrations with the exhibited pH profiles, it can be established that in acidic condition (anodic region) TPHs removal is lower than those for alkaline conditions (cathodic region) (Fig. 2); also, the smoother pH profile exhibited by T3 (Fig. 2b) seems to be favorable for higher TPHs removal. Nev-
Fig. 2. pH and TPHs profiles at the end of electroreclamation experiments (a current of 1.95 mA cm−2 was imposed for 22 h). Soil humidity was changed from 10% (T2, Table 1) to 20% (T3, Table 1).
2122
B. Murillo-Rivera et al. / Electrochimica Acta 54 (2009) 2119–2124
Fig. 3. Image of the final state electrokinetic reclamation (T4, Table 1) showing bidirectional TPHs flow.
ertheless, it must be remarked that higher TPHs concentration was presented in the middle section of the cell. In order to facilitate an alkaline condition within the soil matrix, the 0.1 M NaOH electrolyte was used for both soil humidification and electrode chambers (T4, Table 1). This change seems to work well because in 48 h overheating was almost negligible, temperature was almost constant (26–28 ◦ C), and electric potential gradient was barely stable with values oscillating between 3.5 and 3.8 V cm−1 . The pH profiles confirmed that soil got an alkaline condition throughout the cell, and an almost stable state at the cathodic region. At 48 h it became obvious the acid front penetration in the anodic region, although the alkaline condition prevailed all over the cell. This effect impacted positively on the electroosmotic flow, which exhibited almost a linear trend (slope = 1.1 mL h−1 and R2 = 0.96) with 54 mL collected in 48 h. Observation of electrode chambers, while running the experiment, allowed detecting TPHs presence in both electrode chamber, this phenomenon became evident through changes in color. At 24 h the anodic chamber solution turned out turbid and at 48 h color in the anodic chamber was yellow, while the cathodic chamber was brown (Fig. 3). This result suggested a bidirectional TPHs transport, first of all due to TPH desorption and then due to TPHs diffusion toward electrode chambers as well as TPHs dragged by electroosmotic flow from anode to cathode. All these phenomena appeared when the electrokinetic processes in sandy soil took place at excessively alkaline conditions. A comparison of residual TPHs from T3 to T4 suggested that alkaline condition permitted higher removal of TPHs in the anodic region and a similar amount in the cathodic region. However, in the middle of the cell an accumulation of TPHs took place, and detected concentration was very close to the initial one. This condition was visually confirmed because soil in the middle section showed a “dark line”. Due to the bidirectional TPHs flow exhibited in T4, it was reconsidered to change the wetting solution to 0.1 M Na2 SO4 . While the electrode chamber solutions remain with 0.1 M NaOH (T5, Table 1), the applied current was twice the previous one (3.9 mA cm−2 ). This new setup allowed to keep a minimal overheating, temperature varied from 27 to 30 ◦ C in 4 h, and after that became constant. The electric potential gradient went from 3.0 to 3.5 V cm−1 in 6 h and after that remains stable. Electroosmotic flow exhibited a linear trend (slope = 3.8 mL h−1 and R2 = 0.99) providing 84 mL in 19 h. Removal of TPHs became more uniform in the whole cell (final concentration were: anodic 9.5, middle 10.8, and cathodic 8.8 g L−1 ); so far the excessive accumulation in the middle section was overcome.
Fig. 4. Variation of permeated solution volume through the electrokinetic cell (from anode to cathode) during the time elapsed in the electroreclamation experiments (Table 1): (a) T6 (anodic chamber: 0.1 M NaOH), (b) T7 (anodic chamber: 0.05 M NaOH) and (c) T8 (anodic chamber: 0.04 M NaOH). Solid lines represent linear regressions.
The deficiencies of this treatment were: (a) the process still exhibited a bidirectional TPHs flow evidenced by the coloration of the anodic well solution and (b) when the cell was disassembled soil consistency was muddy, phenomena that could imply a disruption of the soil matrix. In order to protect soil structure and enhances TPHs flow from anode to cathode, experimental conditions were modified lowering again the applied current density to 1.947 mA cm−2 (T6, Table 1). By doing so, similar conditions for temperature (24–27 ◦ C) and voltage gradient (3.5–3.8 V cm−1 ) were observed; the electroosmotic flow exhibited an almost linear trend (slope = 1.6 mL h−1 and R2 = 0.99) providing 75 mL in 48 h (Fig. 4a). In this treatment (T6), pH profiles exhibited an almost neutral value in the anodic region, and highly alkaline conditions were favored in the middle and cathodic regions (Fig. 5a). Like in T4, the alkaline state produced a bidirectional TPHs flow, but removals in the anodic and middle regions were lowered (Fig. 5a). Although good results were obtained in T6, the bidirectional TPHs flow still persisted. A new proposal was to diminishing the concentration of electrolyte in the anodic chamber, from 0.1 to 0.05 M NaOH (T7, Table 1); since less concentrated electrolytes may favor changes in potential of soil particles, thus increasing desorption phenomena and TPHs removal [22]. The cathodic chamber electrolyte was 0.1 M Na2 SO4 . Finally, the soil wetting ratio (20 wt%) and current density (1.947 mA cm−2 ) (T7, Table 1) were kept the same as in T6. In a first instance the electroosmotic flow follows an almost linear trend (1.4 mL h−1 and R2 = 0.98) providing 69 mL in 48 h (Fig. 4b); while temperature oscillated between 25 and 29 ◦ C, and electric potential gradient exhibited a singular behavior raising from 1.8 to 4.8 and then decreasing to 2.0 V cm−1 . Final pH profile and TPHs concentration matched in a better way since the soil alkalinity ensured not only a constant electroosmotic flow, but also a more homogeneous TPHs removal (Fig. 5b). Global TPHs removal efficiency increased enough to improve any other previous treatment. In the experiment T7, the anodic chamber
B. Murillo-Rivera et al. / Electrochimica Acta 54 (2009) 2119–2124
2123
Fig. 6. Zeta potential measurement of different soil dispersions, in deionized water. The analyzed soils were obtained after the electrokinetic treatment T6 and T8 (Table 1), the non-treated soil is included.
Fig. 5. Profiles at the end of electroreclamation experiments (a current of 1.95 mA cm−2 was imposed for 48 h). The experimental setup is indicated in Table 1. The electrolytes in the anodic and cathode chamber were varied: T6, anodic 0.1 M NaOH and cathodic 0.1 M NaOH; T7, anodic 0.05 M NaOH and cathodic 0.1 M Na2 SO4 ; T8, anodic 0.04 M NaOH and cathodic 0.1 M Na2 SO4 . (a) Final pH profile and (b) final TPHs profile.
electrolyte never changed its color implying that the bidirectional TPHs flow was overcome. A last refinement was tried using electrolyte 0.04 M NaOH at the anodic chamber, with all other conditions as in T7. In this treatment (T8, Table 1), electroosmotic flow exhibited a delay of 10 h, after that a nearly linear trend (2.6 mL h−1 and R2 = 0.99) providing 85 mL at 48 h. A slight overheating was present and temperature oscillated between 23 and 27 ◦ C, while the electric potential gradient went from 1.8 to 6.0 and then decreasing to 3.6 V cm−1 . pH profiles indicated that an alkaline condition was rapidly obtained and sustained along the experiment (Fig. 5a). This conditions in pH also overcome the bidirectional TPHs flow which was confirmed by the following facts: (a) electrolyte in the anodic chamber remained almost unaffected, color solution exhibited only a slight turbidity and (b) the attained TPHs removal is highest than that obtained in T7 (Fig. 5b). Results obtained in the test T7 are encouraging since by applying a 48-h treatment, at low current density, it was possible to assure alkaline conditions throughout the soil at any time. By doing so any possibility of bidirectional TPHs flow was eliminated, and TPHs removal was high enough to consider a clean-up greater than 90% in the anodic section, and greater than 75% for the middle and cathodic ones. Also a high sensitivity of the electroreclamation process was patent since the anodic chamber electrolyte concentrations vary
in a very wide range in treatments T6 and T8 (from 0.1 to 0.04 M NaOH). Finally, potential measurements of T6 and T8 soil samples, after electrokinetic test, were done by dispersing 5 mg of soil in 50 mL of deionized water. Fig. 6 shows negative values for the potential in all cases. The negative potential induces electroosmotic flow towards the cathode during electrokinetic process [22] On the other hand, Fig. 6 also shows how the potential (interphase property defined by charge arrangement between solid and fluid phases) was more negative for T8 soil in all the cell sections. Zeta potential values under −30 mV implied that soil after electrokinetic test for T8, reached a better dispersion ensuring greater water electroosmotic permeability (Smoluchovsky model). Moreover to reach more negative potential, soil particles have to exhibit larger negative surface charge confirming the action of alkaline media on the soil surface structure. All these arguments are a clear confirmation that electrolyte composition is one of the most important variables of electrokinetics as a soil remediation method, since it changes surface soil conditions in such a way that can promote contaminants desorption and increase soil permeability. 4. Conclusions The reported experimental data allowed to confirm that hydrocarbon polluted soils can be successfully electroremediated, even its complexity, by setting up the right conditions in soil wetting, electrolyte solution and an adequate enhancement scheme. Soil humectation of 20% showed to be sufficient for getting electrokinetic remediation without overheating the soil. This value is quite close to the water field capacity of the sandy soil used in this study (23%), indicating that electrokinetics does not require water saturated soils. Unless the field water capacity is more than enough to start a soil electrokinetic remediation. Electrolyte used for soil humidification must be selected to avoid soil overheating (Joul effect), as well as any possibility of soil disruption. For sandy soil used in this study the best solution was the 0.1 M Na2 SO4 . This result certainly was combined with the extremely low conductivity of soil sample. However, it is not possible to generalize the use of electrolyte solutions for soil preconditioning. More research in this sense needs to be done in order to obtain a suitable conductivity range for a successful electrokinetic reclamation. The mixed enhancement scheme including 0.1 M NaOH at the anode chamber, and 0.1 M Na2 SO4 at the cathode chamber, neutralized protons generated at the anode as well as establishing an input of hydroxyl ions, such that an alkaline pH was reached
2124
B. Murillo-Rivera et al. / Electrochimica Acta 54 (2009) 2119–2124
and sustained. On the other hand, extremely alkaline conditions have favored soil surface modification allowing TPHs desorption as well as an increase in surface ion exchange capacity manifested in greater soil permeability under the action of an electric field. High sensitivity of electroosmotic flow to the anodic chamber electrolyte concentration was shown. The last experiment (T8, Table 1) joined the best condition to get an electroosmotic flow, with a residence time enough for TPHs desorption and their incorporation into the fluid phase that was transported from anode to cathode. The experimental conditions covered in this work are useful to refine soil electrokinetics setup, showing that it is possible to perform an electroreclamation having optimal conditions in soil conductivity, soil pH, maximum desorption and unidirectional transport of TPHs. References [1] S.E. Manahan, Environmental Chemistry, CRC Press LLC, Boca Raton, 2000, p. 22.
[2] M.L. Davies, S.J. Masten, Principles of Environmental Engineering and Science, McGraw Hill Science/Engineering/Math, New York, 2004, p. 26. [3] E. Riser-Roberts, Remediation of Petroleum Contaminated Soils, Biological, Physical and Chemical Processes, CRC Press LLC, Boca Raton, 1998, p. 99. [4] R.E. Saicheck, K.R. Reddy, Crit. Rev. Environ. Sci. Technol. 35 (2005) 115. [5] A.N. Alshawabkeh, A.T. Yeung, M.R. Bricka, J. Environ. Eng. 125 (1999) 27. [6] R.F. Probstein, R.E. Hicks, Science 260 (1993) 498. [7] G.R. Eykholt, D.E. Daniel, J. Geotech. Eng. ASCE 120 (1994) 797. [8] M.M. Page, Ch.L. Page, J. Environ. Eng. 128 (2002) 208. [9] M.M. Teutli-León, M.T. Oropeza, I. González, A. Soria, AIChE J. 51 (6) (2005)1822. [10] G. Maini, A.K. Sharman, C.J. Knoules, G. Sunderland, S.A. Jackman, J. Chem. Technol. Biotechnol. 75 (2000) 657. [11] A. Li, K.A. Cheung, K.R. Reddy, J. Environ. Eng. 126 (2000) 527. [12] R.K. Reddy, E.R. Saichek, J. Environ. Sci. Health A94 (5) (2004) 1189. [13] S.O. Kim, S.H. Moon, K.W. Kim, J. Environ. Technol. 21 (1999) 417. [14] A.P. Shapiro, R.F. Probstein, Environ. Sci. Technol. 27 (1993) 283. [15] G.R. Eykholt, J. Hazard. Mater 55 (1997) 171. [16] Y.B. Acar, et al., J. Hazard. Mater 40 (1995) 117. [17] L.M. Vane, G.M. Zang, J. Hazard. Mater. 55 (1997) 1. [18] A.T. Yeung, Environ. Eng. Sci. 23 (2006) 202. [19] S. Trasatti, Electrochim. Acta 45 (2000) 2377. [20] R.G. Freitas, et al., J. Power Sources 171 (2007) 373. [21] B. Murillo-Rivera, M.S. Thesis, Universidad Autonóma Metropolitana, México, 2006. [22] A.Z. Al-Hamdan, K.R. Reddy, Geotech. Geolog. Eng. 24 (2006) 1679.