Abstract. 1 Introduction

Transcription

1 In Situ Electrokinetic Remediation of Aquifer Contaminated by Heavy Metal S. Shiba and Y. Hirata Department of Chemical Engineering, Osaka University, Toyonaka, Osaka 560, Japan Abstract Ground water is utilized as one of important water resources as well as surface water. However, ground water in many aquifers has been seriously contaminated due to the migration of hazardous organic and inorganic chemicals from the disporsal of solid and liquid wastes. In order to investigate the characteristics of nonsteady process of the electrokinetic remediation, a mathematical model based on physicochemical mass transport theory has been developed. The electrokinetic remediation is relatively new and is an innovative in situ technique to remove the pollutants from contaminated ground water and soil. With use of the model the removal of heavy metal (Cu) from pore water is simulated numerically. The results of the simulation have theoretically proved that the electrokinetic remediation is effective to remove heavy metals from contaminated water and soil. 1 Introduction New development and recycling of water resources are required more and more. Then, groundwater are utilized as an important water resources. However, recently the contamination of groundwater due to migration of organic and inorganic chemicals has become serious in many aquifers. The sites need to be cleaned up soil and groundwater are increasing. The large potential hazard to groundwater quality is the disposal of industrial liquid and solid wastes. The leachate from the wastes percolates into the aquifer and

2 124 Computer Methods in Water Resources XII many chemicals react with the aquifer particles or the ground water. The reaction may produce undesirable changes in water and soil characteristics. For the remediation of the aquifers contaminated, many new techniques have been presented. Among them, as an in situ process, the electrokinetic method is considered one of the most cost-effective technologies especially for the aquifer of low hydraulic conductivity. The electrokinetic method is executed by burying electrodes into the treatment zone of the aquifer saturated with water and applying a direct current between electrodes. The contaminated water percolated to cathode is drawn out and purge water is injected into anode. Contaminants (e.g., heavy metals) in the aquifer are accelerated to move toward cathode due to electrokinetic driving force superposed on the advective pore water flowing in the same direction. In civil engineering electroosmotic technology has been used for dewatering from clays, silts, and fine sands for many years, although application of the electrokinetic remediation method to decontamination of chemical wastes is relatively new. Therefore, both the theoretical and experimental studies on this method have not been accumulated enough and only a few studies ([1], [2], [3], [4], [5], and [6]) can be utilized to construct the useful mathematical model. However, the experimental study by Nekrasova and Korolev [7] shows that the maximum effect of soil remediation by electrokinetic method is achieved in the cleaning of a natural loam from the pollution with ions of heavy metals as Zn, Pb, and Cu. 2 Governing Equations Application of a direct current across the soil between anode and cathode mobilizes charged species in pore water of soil. In pore water cations migrate to cathode and anions migrate to anode. Under steady-state uniform flow, the equation of mass conservation, which considers the advection, dispersion, and sorption for chemical species k in saturated homogeneous isotropic materials, can be described as follows [6]: where Ck = concentration of species k (mol/l); r - tortuosity determined experimentally ( ); T>\^ - diffusion coefficient of species k in pore water (cm^/s); nek = electromigration velocity (cm/s); us - velocity of pore flow (cm/s); and R^ - molar rate due to chemical reactions (mol/l/s).

3 Computer Methods in Water Resources XII 125 Electromigration velocity of species k is given by: F ^ <9</) %ek- ^^Dk M where F = Faraday's constant (C/mol); R = universal gas constant (J/K/mol); T = liquid temperature (degrees Kelvin); z% = charge number of species k (-); and 0 = electric potential (V). The reaction term R^. is composed of homogeneous chemical reaction R^ in pore water and heterogeneous adsorption-desorption reaction R^ between pore water and soil as: + ( 3) Because homogeneous reactions in the pore water R^ are usually quick dissociation-association reactions, in dissolved species the chemical equilibrium conditions are assumed to be established. Sorption reaction ^ can be expressed as: where p - density of soil solid (g cm *); n = porosity of soil ( ); and s\^ - absorbed concentration of species k per unit mass of soil solid (mot g~*). As typical absorption reaction achieves equilibrium rapidly, the adsorption isotherms between 5k and C'k are general!) expressed by a linear function. Therefore, the next equation can be taken: <^^k (5) where Kdk is called the distribution coefficient of species k (Lg~*). When adsorption is fast and reversible, the retardation factor Rdk is defined as: = Kdk ( 6) n Using Eqs.(l)-(6), the governing equation can be represented as: C/^ r)^ ^^^ ^^ ^ C/Z" )_// where > and u*^ are effective diffusion coefficient and effective electromigration velocity, respectively. They are denned as: %>k = k : <k = 4«ek ( 8). ( 9) 7)

4 126 Computer Methods in Water Resources XII The initial conditions for the governing equation Eq.(7) is given by: Ck = Cko at Z = 0 (10) The boundary conditions at anode and cathode must describe the electrochemical reactions which enable to pass the electric current through the electrodes. At anode, H^O gives electrons to electrode. Then the principal reaction is the production of oxygen gas and H+ as: IHzO - 4e- -^ 02(g) + 4H+ (11) Therefore, in the vicinity of anode, H+ concentration becomes high and ph decreases in its value. At cathode, H^O takes electrons from the electrode and releases hydrogen gas and OH~ as: 2HzO 4-2e- -» %(g) + 2OH" (12) Therefore, the concentration of OH~ is increased near cathode. The production rate of H+ at anode and OH~ at cathode can be expressed in terms of their fluxes J%H and JCOH as follows: (13), ( where i is the electric current applied between anode and cathode. The boundary conditions can be represented in terms of fluxes of chemical species as (ZH = 4-l;andzoH = -1): +te_,i)cl = (<*$ + *. for H* dz \ usc, lor others (15) \),,, JusCk-f. for OH~ \ ^("'S-'W'ek)^k = S ^i.., } at Z = L dz \ t/sck. tor others J (16) The governing equation Eq.(7) can be integrated numerically using the initial condition Eq.(lO) and the boundary conditions Eqs.(15) and (16). For the convenience of numerical calculation, the dimensionless variables and constants are introduced as: k = ; t = -2 ; z = : $ = - 0 (17), (18), (19), (20) Co nrdol^ L RT ^ = ; us = us ; ulk - 4 (21), (22), (23)

5 Computer Methods in Water Resources XII 127 r i (24). (25). (26) /oo where L = distance between anode and cathode (cm); and subscript 0 denotes that the quantity attached the subscript is used for the basis of normalization. The dimensionless form of the governing equation can be given as: (27) The initial condition and the boundary conditions are also transformed into the dimensionless forms by using the dimensionless variables and constants. 3 Chemical Reactions The term of aqueous phase chemical reaction R^ in the governing equation varies its form from pollutant to pollutant. In this study, as an example, the remediation of the aquifer contaminated by copper sulfate (heavy metal) is simulated. The chemical reactions in pore water may be represented as follows: CuS04 ^ Cu^+SO^- (Id) : HzSC^ ^ H++HSQ- (Kz) (28), (29) HSO[ ^ H+ + SO^ (IQ) ; %() ^ H+ + OH" (Kw) (30), (31) where KI, I<2, IQ, and KW are dissociation constants. The concentrations of chemical species used above chemical reactions are defined as: ], [H+], [OH"]) (32) As I\2 > 1 M, from the dissociation reaction given by Eqs.(28)-(31) the reaction terms R^ in Eq.(7) are obtained as: - ki.czcs (33). (34)

6 128 Computer Methods in Water Resources XII where ki +, ki_, k^_^, k^_, kw+, and kw- are reaction rate constants satisfying the next equations: Ki = ki+/ki_ ; K4 = k4+/k4_ ; Kw = kw+/kw- (39), (40), (41) Although the three constants Kfs are evaluated easily, the precise values of the six constants ki+'s and kj_'s cannot be taken easily. Therefore, to form the closed system of model equations, it is necessary for governing equations to be coupled with another three equations which contain C\, 62, Cy, 64, Cs^andCe. These three equations are the equilibrium equations which obtained from the dissociation reactions Eqs.(28), (30), and (31) as: KiCi = CzCg ; K4Q = CsCs ; Kw = C^Cc (42), (43), (44) Eqs.(42)-(44) are solved at each time step of integration of governing equation Eq. (7). 4 Numerical Simulation The one-dimensional model simulations were carried out under the following conditions: column length (anode and cathode distance) = 40 cm; cross-sectional area of column = 100 cm^; porosity = 0.4; tortuosity 1.5; and flow rate = 100 cc/s. The purge water is pure but a little acidic (ph = 6) and is injected into anode (z 0). The applied voltage between anode and cathode (z L) was changed from 25 V to 100 V. Because anode was set at the injection point (i.e., upstream point), the electric current goes in the same direction of the advective flow of the purge water. Some of the results of the numerical simulation are shown in Figs. 1 and 2. The numerical integration of the governing equation were done with use offiniteelement method. According to the results of numerical simulations, the removal (i.e., decrease in the concentration) of Cu^+ at anode (z 0) increses with lapse of time. Also the accumulation (i.e., increase in the concentration) of Cu^+ at cathode (z 1) increases with lapse of time. These results indicate that the electrokinetic process makes steady progress in the remediation of the aquifer with lapse of time. The amount of the removal and that of the accumulation are increased with increase of the applied voltage. Depicted in Fig. 1 is the distribution of the copper ion concentration Ci in the pore water of the one-dimensional aquifer at t 8 x 10~^ (118.5 h). The distributions are shown parametrically in the applied voltage A0.

7 Computer Methods in Water Resources XII 129 The concentration of Cu^+ (Qz) increases with increase in z (distance from anode). This result accords with the experimental results on heavy metals, which were executed with use of one-dimensional column by N-ekrasova and Korolev [7]. Because the advective flow and the electromigration are the same direction, the cation Cu^+ is enhanced to move toward cathode. Then, the concentration of Cu^+ gets the maximum at cathode and the minimum at anode. This means that the remediation of the polluted site progresses from anode to cathode. In other words, the contaminant uniformly distributed in the polluted site is removed near anode and accumulated near cathode with lapse of time. The accumulated contaminant may be able to be removed with the effluent of the purge water from cathode. As is supposed, the higher the applied valtage is, the larger the migration amount of Cir+ becomes (i.e., the removal near anode and the accumulation near cathode are incresed) F1 t = 8x10" T = 1l8-5h) (VI <u u_ O 0-01 o 0-00 h- < en D1MENSIONLESS DISTANCE Z Figure. 1. Variation of Distribution of 62 with Applied Voltage Fig.2 shows the time variation of ph distribution in the pore water. The initial ph is 6 and the applied voltage is 25 V It can be seen that ph is low

8 130 Computer Methods in Water Resources XII at anode and high at cathode (i.e., H+ concentration is high at anode and low at cathode). This result coincides with general observation on the ph distribution ([8] and [9]). The base front (the shoulder of ph jump which faces to anode) is well reproduced by the numerical simulation. The base front proceeds to anode with lapse of time. However, the acid front (the foot of ph jump which faces to cathode) is not so clearly reproduced. The base front travels along the column in opposite direction of the advective flow, because the negative electrmigration velocity of [OH~] is greater than the positive advective velocity of purge water and as a result OH~ is accelerated toward anode. On the other hand, such traveling of the acid front in flow direction is not so clear. The traveling speed of the base front is faster in early stage (t 10""^ - 10~^) of process but it gradually slows down after about f = 2 xlo-\ F i D1MENSIONLESS DISTANCE Z 1-0 Figure. 2. Time Variation of ph Distribution in One-Dimensional Column 5 Conclusions Under electric field, heavy metals in pore water of soil are affected by electromigration, advection, diffusion, aqueous phase chemical reaction (dissociation and association), and sorption between pore water and soil. Taking these various phenomena into account, a one-dimensional mathe-

9 Computer Methods in Water Resources XII 131 matical model for the simulation of the electrokinetlc remediation (removal of heavy metals) of aquifer has been constructed to investigate the characteristics of the remediation. From the numerical simulation of the decontamination process of the heavy metal (copper sulfate), it may be concluded that: ( 1) the electrokmetic remediation is theoretically proved to be effective for removal of heavy metals; (2) the higher the applied voltage to electrodes is, the higher the remediation efficiency becomes; (3) the traveling of the base front from cathode to anode is clearly reproduced, although the acid front from anode to cathode is obscure; and (4) the traveling speed of the base front gradually slows down. References 1. Alshawabkeh, A. N., and Acar, Y. B. (1992), Removal of Contaminants from Soils by Electrokinetics: A Theoretical Treatise, Environ. z, Vol.A27, No.7, pp Bruell, C. 1, Segall, B. A., and Walsh, M. T. (1992), Electroosmotic Removal of Gasoline Hydrocarbons and TCE from Clay, Journal of, ASCE, Vol.118, No.l, pp Eykholt, G. R., and Daniel, D. E. (1994), Impact of System Chemistry on Electroosmosis in Contaminated Soil, Journal ofgeotechni-, ASCE, Vol.120, No.5, pp Rodsand, T., Acar, Y. B., and Breedveld, G. (1995), Electrokinetic Extraction of Load from Spiked Norwegian Marine Clay, Geoenvironment 2000 (Edited by Acar, Y. B., and D. E. Daniel), Vol.2, pp Segall, B. A., and Bruell, C. J. (1992), Electroosmtic Contaminant- Removal Processes, Journal of Environmental Engineering, ASCE, Vol.118, No.l, pp Shapiro, A. P., Renaud, P. C., and Probstein, R. F. (1989), Prelimmary Studies on the Removal of Chemical Species from Saturated Porous Media by Electroosmosis, Physico Chemical Hydrodynam,- z'cj, Vol. 11, No.5/6, pp

10 132 Computer Methods in Water Resources XII 7. Nekrasova, M. A., and Korolev, V. A. (1997), Electrochemical Cleaning of Polluted Soils, E/igmzgrmg Gzo/ogy arzdf f/zz EMwmrzmzfif (Edited by Marinos, P. G. et al.), Vol.2, pp Penn, M., and Savvidou, C. (1997), Temperature Effects on Electrokinetic Remediation of Contaminated Soil, Engineering Geology and the Environment (Edited by Marinos, P. G. et al.), Vol.2, pp Taha, M. R., Acar, Y. B. and Gale, R. J. (1997), Electrokinetic Enhancement Tests on TNT Contaminated Soil, Engineering Geology and the Environment (Edited by Marinos, P. G. et al.), Vol.2, pp