Electrokinectic soil remediation

Electrokinectic soil remediation

Uncorrected Proof ARTICLE IN PRESS 1 2 3 Ž. The Science of the Total Environment xxx 2001 xxx᎐xxx 4 5 6 7 8 910 Electrokinetic soil remediation ᎏ critical overview 11 12 Jurate Virkutyte a, U , Mika Sillanpaa a , Petri Latostenmaa b ¨¨ 13 14 a Uni¨ersity of Oulu, Water Resources and En¨ironmental Engineering Laboratory, Tutkijantie 1 F 2, 90570 Oulu, Finland15 b ¨ Finnish Chemicals Oy, P.O. Box 7, FIN-32741 Aetsa, Finland ¨ 16 17 Received 28 May 2001; accepted 31 August 200118 19 20 21 Abstract22 23 In recent years, there has been increasing interest in finding new and innovative solutions for the efficient removal24 of contaminants from soils to solve groundwater, as well as soil, pollution. The objective of this review is to examine25 several alternative soil-remediating technologies, with respect to heavy metal remediation, pointing out their26 strengths and drawbacks and placing an emphasis on electrokinetic soil remediation technology. In addition, the27 review presents detailed theoretical aspects, design and operational considerations of electrokinetic soil-remediation28 variables, which are most important in efficient process application, as well as the advantages over other technologies29 and obstacles to overcome. The review discusses possibilities of removing selected heavy metal contaminants from30 clay and sandy soils, both saturated and unsaturated. It also gives selected efficiency rates for heavy metal removal,31 the dependence of these rates on soil variables, and operational conditions, as well as a cost᎐benefit analysis. Finally,32 several emerging in situ electrokinetic soil remediation technologies, such as Lasagna TM , Elektro-Klean TM , elec-33 trobioremediation, etc., are reviewed, and their advantages, disadvantages and possibilities in full-scale commercial34 applications are examined. ᮊ 2001 Published by Elsevier Science B.V. 35 36 Keywords: Electrokinetic soil remediation; Heavy metals37 38 39 1. Introduction 40 41 Every year, millions of tonnes of hazardous 42 waste are generated in the world. Due to ineffi-43 cient waste handling techniques and hazardous 44 waste leakage in the past, thousands of sites were 45 46 47 4849 U Corresponding author.50 . 51 contaminated by heavy metals, organic com- 52 pounds and other hazardous materials, which 53 made an enormous impact on the quality of 54 groundwater, soil and associated ecosystems. Dur- 55 ing the past decades, several new and innovative 56 solutions for efficient contaminant removal from 57 soils have been investigated and it is strongly 58 believed that they will help to solve groundwater 59 and soil pollution. Despite numerous promising 60 laboratory experiments, there are not many suc- 61 0048-9697r01r$ - see front matter ᮊ 2001 Published by Elsevier Science B.V.62 Ž. PII: S 0 0 4 8 - 9 6 9 7 0 1 01027-0 Uncorrected Proof ARTICLE IN PRESS () J. Virkutyte et al. rThe Science of the Total En ¨ironment xxx 2001 xxx᎐xxx2 63 cessfully implemented in situ soil-treatment tech-64 niques yet. Because of uncertainty, lack of ap-65 propriate methodology and proven results, many66 in situ projects are currently under way. It is67 likely that there will not be a single universal in68 situ soil-treatment technology. Instead, quite a69 large variety of technologies and their combina-70 tions suitable for different soil remediation situa-71 tions will be developed and implemented.72 Although the successful and environmentally73 friendly soil treatment technologies have not been74 completely investigated and implemented, there75 are several techniques which have attracted in-76 creased interest among scientists and industry77 officials. These are:78 7980 ⅷ Bioremediation ᎏ despite a demonstrated81 ability to remove halogenated and non-82 halogenated volatiles and semi-volatiles, as 83 well as pesticides, this technique has failed to84 show efficient results in removing heavy met-85 als from contaminated soils.8687 ⅷ Thermal desorption ᎏ this treats halogenated88 and non-halogenated volatiles and semi-vola-89 tiles, as well as fuel hydrocarbons and pesti-90 cides. It has failed to demonstrate an ability to91 remove heavy metals from contaminated soils.9293 ⅷ Soil vapour extraction ᎏ there are several94 promising results in reducing the volume of95 treated heavy metals. Nevertheless, this tech-96 nique cannot reduce their toxicity.9798 ⅷ Soil washing ᎏ this technique has demon-99 strated potential effectiveness in treating100 heavy metals in the soil matrix.101102 ⅷ Soil flushing ᎏ according to laboratory-scale103 experiments, this is efficient in removing heavy104 metals from soils, despite the fact that it can-105 not reduce their toxicity. 106107 ⅷ Electrokinetic soil remediation.108 109 As none of the other in situ soil remediation 110 techniques has demonstrated the efficient re-111 moval of heavy metals, there was a necessity to 112 develop other methods to remediate soil contami-113 nated by heavy metals. 114 Electrokinetic soil remediation is an emerging115 technology that has attracted increased interest116 among scientists and governmental officials in the 117 last decade, due to several promising laboratory 118 and pilot-scale studies and experiments. This 119 method aims to remove heavy metal contami- 120 nants from low permeability contaminated soils 121 under the influence of an applied direct current. 122 However, regardless of promising results, this 123 method has its own drawbacks. First of all, the 124 whole electrokinetic remediation process is highly 125 dependant on acidic conditions during the appli- 126 cation, which favours the release of the heavy 127 metal contaminants into the solution phase. How- 128 ever, achieving these acidic conditions might be 129 difficult when the soil buffering capacity is high. 130 In addition, acidification of soils may not be an 131 environmentally acceptable method. Second, the 132 remediation process is a very time-consuming ap- 133 plication; the overall application time may vary 134 from several days to even a few years. There are 135 some other limitations of the proposed technique 136 that need to be overcome: i.e. the solubility of the 137 contaminant and its desorption from the soil ma- 138 trix; low target ion concentration and high non- 139 target ion concentration; requirement of a con- 140 ducting pore fluid to mobilise contaminants; and 141 heterogeneity or anomalies found at sites, such as 142 large quantities of iron or iron oxides, large rocks 143 Ž. or gravel, etc. Sogorka et al., 1998 . 144 According to the experiments and pilot-scale 145 studies conducted, metals such as lead, chromium, 146 cadmium, copper, uranium, mercury and zinc, as 147 well as polychlorinated biphenyls, phenols, 148 chlorophenols, toluene, trichlorethane and acetic 149 acid, are suitable for electrokinetic remediation 150 and recovery. 151 152 2. Theoretical, design and operational 153 considerations 154 155 2.1. Theoretical aspects 156 157 The first electrokinetic phenomenon was 158 observed at the beginning of the 19th Century, 159 when Reuss applied a direct current to a 160 Ž clay᎐water mixture Acar and Alshawabkeh, 161 . 1993 . However, Helmholtz and Smoluchowski 162 were the first scientists to propose a theory deal- 163 ing with the electroosmotic velocity of a fluid and Uncorrected Proof ARTICLE IN PRESS () J. Virkutyte et al. r The Science of the Total En ¨ironment xxx 2001 xxx᎐xxx 3 164 the zeta potential under an imposed electric gra-165 Ž.Ž . dient ␨ Acar and Alshawabkeh, 1993 . Sibel166 Pamukcu and her research group have derived167 the following Helmholtz᎐Smoluchowski equation: 168 ␧␨ Ѩ␾ Ž. u s 1 EO ␮Ѩx 169 where u is the electroosmotic velocity, ␧ is the EO170 dielectric constant of the pore fluid, ␮ is the171 viscosity of the fluid and Ѩ␾rѨx is the electric172 gradient.173 When DC electric fields are applied to con-174 taminated soil via electrodes placed into the175 ground, migration of charged ions occurs. Positive176 ions are attracted to the negatively charged cath-177 ode, and negative ions move to the positively178 charged anode. It has been experimentally proved179 that non-ionic species are transported along with180 the electroosmosis-induced water flow. The direc-181 tion and quantity of contaminant movement is182 influenced by the contaminant concentration, soil 183 type and structure, and the mobility of contami-184 nant ions, as well as the interfacial chemistry and185 the conductivity of the soil pore water. Electroki-186 netic remediation is possible in both saturated187 and unsaturated soils.188 Electrokinetic soil treatment relies on several189 interacting mechanisms, including advection,190 which is generated by electroosmotic flow and191 externally applied hydraulic gradients, diffusion192 of the acid front to the cathode, and the migra-193 tion of cations and anions towards the respective194 Ž. electrode Zelina and Rusling, 1999 . The domi- 195 nant and most important electron transfer reac-196 tions that occur at electrodes during the elec-197 trokinetic process is the electrolysis of water: 198 q Ž. y HOª 2H q1r2Ogq2e 22199 200 yy Ž. Ž. 2H O q2e ª 2OH qHg 2 22 201 The acid front is carried towards the cathode202 by electrical migration, diffusion and advection. 203 The hydrogen ions produced decrease the pH204 near the anode. At the same time, an increase in205 the hydroxide ion concentration causes an in-206 crease in the pH near the cathode. In order to 207 solubilise the metal hydroxides and carbonates 208 formed, or different species adsorbed onto soils 209 particles, as well as protonate organic functional 210 groups, there is a necessity to introduce acid into 211 the soil. However, this acid addition has some 212 major drawbacks, which greatly influence the ef- 213 ficiency of the treatment process. The addition of 214 acid leads to heavy acidification of the contami- 215 nated soil, and there is no well-established method 216 for determining the time required for the system 217 to regain equilibrium. 218 The main goal of electrokinetic remediation is 219 to effect the migration of subsurface contami- 220 nants in an imposed electric field via electro- 221 osmosis, electromigration and electrophoresis. 222 These three phenomena can be summarised as 223 follows: 224 225226 ⅷ Electroosmosis is the movement of soil mois- 227 ture or groundwater from the anode to the 228 cathode of an electrolytic cell. 229230 ⅷ Electromigration is the transport of ions and 231 ion complexes to the electrode of opposite 232 charge. 233234 ⅷ Electrophoresis is the transport of charged 235 particles or colloids under the influence of an 236 electric field; contaminants bound to mobile 237 particulate matter can be transported in this 238 manner. 239 240 The phenomena occur when the soil is charged 241 with low-voltage direct current. The process might 242 be enhanced through the use of surfactants or 243 reagents to increase the contaminant removal 244 rates at the electrodes. Upon their migration to 245 the electrodes, the contaminants may be removed 246 by electroplating, precipitationrco-precipitation, 247 pumping near the electrode, or complexing with 248 ion exchange resins. 249 Electromigration takes place when highly solu- 250 ble ionised inorganic species, including metal 251 cations, chlorides, nitrates and phosphates, are 252 present in moist soil environments. Electrokinetic 253 remediation of soils is a unique method, because 254 it can remediate even low-permeability soils. 255 Other mechanisms that greatly affect the elec- 256 trochemical remediation process are electroosmo- 257 sis, coupled with sorption, precipitation and disso- Uncorrected Proof ARTICLE IN PRESS () J. Virkutyte et al. r The Science of the Total En ¨ironment xxx 2001 xxx᎐xxx4 258 Ž. lution reactions van Cauwenberghe, 1997 . This259 is the reason why all the appropriate processes260 should be taken into consideration and investi-261 gated before implementation of the technique262 can take place.263 Once the remediation process is over, extrac-264 tion and removal of heavy metal contaminants265 are accomplished by electroplating at the elec-266 trode, precipitation or co-precipitation at the267 electrode, pumping water near the electrode, or268 complexing with ion exchange resins. Adsorption269 onto the electrode may also be feasible, as some270 ionic species will change their valency near the271 Ž. electrode depending on the soil pH , making 272 Ž them more likely to adsorb van Cauwenberghe,273 . 1997 . 274 Prediction of THE decontamination time is of275 great importance in order to estimate possible276 power consumption and to avoid the occurrence277 of reverse electroosmotic flow, i.e. from the cath-278 Ž ode to the anode, during the process Baraud et279 . al., 1997, 1998 . The phenomenon of reverse elec- 280 troosmotic flow is not well understood and should281 be further investigated.282 Decontamination velocity depends on two283 Ž. parameters Baraud et al., 1997, 1998 : 284 285286 ⅷ Contaminant concentration in the soil solu-287 tion, which is related to the various possible288 Ž solidrliquid interactions adsorptionrdesorp- 289 tion, complexation, precipitation, dissolution,290 . etc. and to the speciation of the target species.291292 ⅷ Velocity in the pore solution when species are293 in the soil solution and not engaged in any294 reactions or interactions. The velocity depends295 Ž on different driving forces electric potential296 gradient, hydraulic head differences and con- 297 . centration gradient and is not closely related298 to soil properties, except for the electroosmo-299 sis phenomenon.300 301 The success of electrochemical remediation de-302 pends on the specific conditions encountered in 303 the field, including the types and amount of con-304 taminant present, soil type, pH and organic con- 305 Ž. tent Acar and Alshawabkeh, 1993 .306 For in situ conditions, the contaminated site307 itself and the immersed electrodes form a type of 308 electrolytic cell. Usually, the electrokinetic cell 309 design in laboratory experiments consists of an 310 open-flow arrangement at the electrodes, which 311 permits injection of the processing fluid into the 312 porous medium, with later removal of the con- 313 Ž taminated fluid Sogorka et al., 1998; Reddy and 314 Chinthamreddy, 1999; Reddy et al., 1997, 1999; 315 . Zelina and Rusling, 1999 . 316 It seems that there is a controversy as to where 317 electrodes should be placed to obtain the most 318 reliable and efficient results. It is obvious that 319 imposition of an electrical gradient by having 320 inert electrodes results in electroosmotic flow to 321 the cathode. Many authors propose that position- 322 ing of the electrodes directly into the wet soil 323 Ž mass produces the most desirable effect Sims, 324 1990; Acar and Alshawabkeh, 1993; Reddy et al., 325 . 1999; Sogorka et al., 1998 . Through seeking im- 326 provements in experiments, some researchers tend 327 to place the electrodes not directly into the wet 328 soil mass, but into an electrolyte solution, at- 329 tached to the contaminated soil, or else to use 330 Ž different membranes and other materials van 331 Cauwenberghe, 1997; Baraud et al., 1998; Bena- 332 . zon, 1999 . In order to maintain appropriate 333 process conditions, a cleaning agent or clean wa- 334 ter may be injected continuously at the anode. 335 Thus, contaminated water can be removed at the 336 cathode. Contaminants at the cathode may be 337 removed by electrodeposition, precipitation or ion 338 exchange. 339 Electrodes that are inert to anodic dissolution 340 should be used during the remediation process. 341 The most suitable electrodes used for research 342 purposes include graphite, platinum, gold and sil- 343 ver. However, for pilot studies, it is more ap- 344 propriate to use much cheaper, although reliable, 345 titanium, stainless steel, or even plastic elec- 346 trodes. Using inert electrodes, the electrode reac- 347 tions will produce H q ions and oxygen gas at the 348 anode and OH y ions and hydrogen gas at the 349 cathode, which means that if pH is not controlled, 350 an acid front will be propagated into the soil 351 pores from the anode and a base front will move 352 out from the cathode. 353 It has been proved by experiments that when 354 heavy metals enter into basic conditions, they 355 adsorb to soil particles or precipitate as hydrox- Uncorrected Proof ARTICLE IN PRESS () J. Virkutyte et al. r The Science of the Total En ¨ironment xxx 2001 xxx᎐xxx 5 356 ides, oxyhydroxides, etc., and in acidic conditions,357 those ions desorb, solubilise and migrate. 358 Another important parameter in the electroki- 359 netic soil-remediation technique is the conductiv- 360 ity, since this, together with soil and pore fluid,361 affects the electroosmotic flow rate. 362 The conductivity of soil depends on the concen- 363 tration and the mobility of the ions present, i.e.364 contaminant removal efficiencies decrease with a 365 Ž reduction in contaminant concentration Reddy 366 et al., 1997, 1999; Reddy and Chinthamreddy, 367 . 1999; Zelina and Rusling, 1999 . This is due to 368 hydrogen ion exchange with cationic contami- 369 nants on the soil surface, with release of the 370 contaminants. As the contaminant is removed,371 the hydrogen ion concentration in the pore fluid 372 increases, resulting in an increasing fraction of 373 the current being carried by the hydrogen ions374 rather than by the cationic contaminants. 375 It is possible to conclude that the variables 376 which have impact on the efficiency of removing377 contaminants from soils are: 378 379 380 ⅷ Chemical processes at the electrodes; 381382 ⅷ Water content of the soil; 383384 ⅷ Soil type and structure; 385386 ⅷ Saturation of the soil; 387388 ⅷ pH and pH gradients; 389 390 ⅷ Type and concentration of chemicals in the 391 soil; 392393 ⅷ Applied current density; and 394395 ⅷ Sample conditioning. 396 397 In addition, insoluble organics, such as heavy 398 hydrocarbons, are essentially not ionised, and the 399 soils in contact with them are not charged. The 400 removal of insoluble organics by electric field is 401 limited to their movement out of the soil by 402 electroosmotic purging of the liquid, either with 403 water and surfactant to solubilise the compounds, 404 or by pushing the compounds ahead of a water 405 Ž. front Probstein and Hicks, 1993 . 406 Ionic migration is the movement of ions sub- 407 jected to an applied DC electric field. Electromi- 408 Ž gration rates in the subsurface depend upon van 409 . Cauwenberghe, 1997 : 410 411412 ⅷ Soil porewater current density; 413414 ⅷ Grain size; 415416 ⅷ Ionic mobility; 417418 ⅷ Contaminant concentration; and 419420 ⅷ Total ionic concentration. 421 422 The process efficiency is not as dependent on 423 the fluid permeability of soil as it is on the pore- 424 water electrical conductivity and path length 425 Ž. Fig. 1. Electroosmosis and electromigration of ions adapted from Acar et al., 1994, 1996; Acar and Alshawabkeh, 1996 . Uncorrected Proof ARTICLE IN PRESS () J. Virkutyte et al. r The Science of the Total En ¨ironment xxx 2001 xxx᎐xxx6 426 through the soil, both of which are a function of 427 the soil moisture content. As electromigration 428 does not depend on the pore size, it is equally 429 Ž applicable to coarse and fine-grained soils van430 . Cauwenberghe, 1997 . 431 Electroosmosis in water-saturated soil is the 432 movement of water relative to the soil under the 433 influence of an imposed electric gradient. When 434 there is direct current applied across the porous 435 media filled with liquid, the liquid moves relative 436 to the stationary charged solid surface. When the 437 surface is negatively charged, liquid flows to the 438 Ž. cathode. Acar et al. 1994, 1996 have conducted 439 numerous experiments and found that this process 440 Ž. works well in wet i.e. water-saturated fine- 441 grained soils and can be used to remove soluble 442 pollutants, even if they are not ionic. The dis- 443 solved neutral molecules simply go with the flow. 444 Fig. 1 shows a schematic representation of this445 process. 446 An excess negative surface charge exists in all 447 kinds of soil. For example, many clays are anionic, 448 colloidal poly-electrolytes. The surface charge 449 density increases in the following order: sand- 450 silt - kaolinite - illite - montmorillonite. Injec- 451 tion of clean fluid, or simply clean water, at the 452 anode can improve the efficiency of pollutant 453 removal. For example, such a flushing technique 454 using electroosmosis has been developed for the 455 removal of benzene, toluene, trichlorethane and456 m-xylene from saturated clay. 457 According to that stated above, the main fac- 458 tors affecting the electroosmotic transport of con-459 taminants in the soil system are as follows:460 461 462 ⅷ Mobility and hydration of the ions and charged 463 particles within the soil moisture; 464465 ⅷ Ion concentration; 466 467 ⅷ Dielectric constant, depending on the amount 468 of organic and inorganic particles in the pore469 solution; and 470471 ⅷ Temperature.472 473 Most soil particle surfaces are negatively 474 charged as a result of isomorphous substitution 475 Ž and the presence of broken bonds Yeung et al.,476 . 1997 . 477 Experiments have determined the dependence 478 of the zeta potential of most charged particles on 479 solution pH, ionic strength, types of ionic species, 480 Ž temperature and type of clay minerals Vane and 481 . Zang, 1997 . For water-saturated silts and clays, 482 the zeta potential is typically negative, with values 483 measured in the 10᎐100-mV range. 484 However, if ions produced in the electrolysis of 485 water are not removed or neutralised, they lower 486 the pH at the anode and increase it at the cath- 487 ode, accompanied by the propagation of an acid 488 front into the soil pores from the anode and a 489 base front from the cathode. This process can 490 Ž significantly effect the soil zeta potential drop in 491 . zeta potential , as well as the solubility, ionic state 492 and charge, level of adsorption of the contami- 493 Ž. nant, etc. Yeung et al., 1997 . 494 In addition, different initial metal concentra- 495 tions and sorption capacity of the soil may pro- 496 duce soil surfaces that are less negative, which at 497 the same time may become positive at a pH of 498 approximately the original zero-point charge 499 Ž. Yeung et al., 1997 . Similarly, chemisorption of 500 anions makes the surface more negative. 501 Electroosmotic flow from the anode to the 502 cathode promotes the development of a low-pH 503 environment in the soil. This low-pH environment 504 inhibits most metallic contaminants from being 505 sorbed onto soil particle surfaces and favours the 506 formation of soluble compounds. Thus, electro- 507 osmotic flow from the anode to cathode, resulting 508 from the existence of a negative zeta potential, 509 enables the removal of heavy metal contaminants 510 by the electrokinetic remediation process. 511 The pH of the soil should be maintained low 512 enough to keep all contaminants in the dissolved 513 phase. Nevertheless, when the pH becomes too 514 low, the polarity of the zeta potential changes and 515 Ž reversed electroosmotic flow i.e. from the cath- 516 . ode to the anode may occur. In order to achieve 517 efficient results in removing contaminants from 518 soils, it is necessary to maintain a pH low enough 519 pH to keep metal contaminants in the dissolved 520 phase and high enough to maintain a negative 521 Ž. zeta potential Yeung et al., 1997 . Despite this 522 apparently easily implemented theory, simultane- 523 ous maintenance of a negative zeta potential and Uncorrected Proof ARTICLE IN PRESS () J. Virkutyte et al. r The Science of the Total En ¨ironment xxx 2001 xxx᎐xxx 7 524 dissolved metal contaminants remains the great-525 est obstacle in the successful implementation of526 the electrokinetic soil remediation process.527 528 2.2. Design considerations529 530 In order to obtain efficient and reliable results,531 electrokinetic remediation of soil should be im-532 plemented under steady-state conditions. It is533 obvious that during the remediation process, other534 reactions, such as transport and sorption, and535 precipitation and dissolution reactions, occur and536 affect the remediation process.537 There have been numerous indications of the538 importance of heat and gas generation at elec-539 trodes, the sorption of contaminants onto soil540 particle surfaces and the precipitation of contami-541 nants in the electrokinetic remediation process542 Ž Acar and Alshawabkeh, 1993; Lageman, 1993; 543 . Zelina and Rusling, 1999 . These processes should544 be further investigated, because it is believed that545 they may weaken the removal efficiency for heavy546 metal contaminants. It is reported that different547 physicochemical properties of the soil may influ-548 ence the removal rates of heavy metal contami-549 nants, due to changed pH values, hydrolysis, and550 oxidation and reduction reaction patterns.551 In order to enhance the electrokinetic remedia-552 tion process, several authors recommend the use553 of a multiple anode system, which is shown in Fig.554 2.555 556 2.3. Operational considerations557 558 As there are several experimental techniques559 to remediate coarse-grained soils, in situ elec-560 trokinetic treatment has been developed for con-561 taminants in low-permeability soils. Electrokinet- 562 ics is applicable in zones of low hydraulic conduc-563 tivity, particularly with a high clay content. 564 Contaminants affected by electrokinetic565 processes include:566 567568 ⅷ Heavy metals;569570 Ž ⅷ Radioactive species Cs , Sr , Co , ura- 137 90 60571 . nium ; 572573 Ž. ⅷ Toxic anions nitrates and sulfates ;574575 Ž. ⅷ Dense, non-aqueous-phase liquids DNAPLs ; 576 Fig. 2. Multiple anodes system US EPA, 1998. 577 578 ⅷ Cyanides; 579580 Ž ⅷ Petroleum hydrocarbons diesel fuel, gasoline, 581 . kerosene and lubricating oils ; 582583 ⅷ Explosives; 584585 ⅷ Mixed organicrionic contaminants; 586587 ⅷ Halogenated hydrocarbons; 588589 ⅷ Non-halogenated pollutants; and 590591 ⅷ Polynuclear aromatic hydrocarbons. 592 593 Heavy metal interactions in the soil solution 594 Ž are governed by several processes, such as Sims, 595 . 1990 : 596 597598 ⅷ Inorganicrorganic complexation; 599600 ⅷ Acid᎐base reactions; 601602 ⅷ Redox reactions; 603604 ⅷ Precipitationrdissolution reactions; and 605606 ⅷ Interfacial reactions. 607 608 The choice of appropriate soil for electroki- 609 netic remediation process should be made with 610 extreme caution and possible soil pre-treatment 611 experiments should be carried out. 612 Soils that may be used for the electrokinetic 613 Ž. remediation process should have Sims, 1990 : 614 615616 ⅷ Low hydraulic conductivity; 617618 Ž ⅷ Water-soluble contaminants if there are any 619 poorly soluble contaminants, it may be essen- 620 . tial to add solubility-enhancing reagents ; and Uncorrected Proof ARTICLE IN PRESS () J. Virkutyte et al. r The Science of the Total En ¨ironment xxx 2001 xxx᎐xxx8 621 622 ⅷ Relatively low concentrations of ionic materi-623 als in the water.624 625 It is reported that with applied electric fields,626 the most suitable soils for heavy metal remedia-627 Ž. tion are kaolinite, clay and sand Sims, 1990 . As628 recommended, clay has low hydraulic conductiv-629 ity, reducing redox potential, slightly alkaline pH630 Ž which is suitable for the remediation of several631 . heavy metal contaminants , high cation exchange632 capacity and high plasticity. Under normal condi-633 tions, migration of ions is very slow, but is en-634 hanced by electrical fields or hydraulic pressure.635 The highest degree of removal of heavy metals636 Ž. over 90% of the initial contaminant has been637 achieved for clayey, low-permeability soils,638 whereas for porous, high-permeability soils, such639 as peat, the degree of removal was only 65%640 Ž. Chilingar et al., 1997 . Laboratory results showed 641 that electrokinetic purging of acetate and phenol642 from saturated kaoline clay resulted in greater643 than 94% removal of the initial contaminants.644 However, this methodology needs to be further645 investigated, because phenol has been reported to646 be toxic to humans and the environment.647 648 3. Removal of metals649 650 If heavy metal contaminants in the soil are in651 ionic forms, they are attracted by the static elec-652 trical force of negatively charged soil colloids.653 The attraction of metal ions to the soil colloids654 primarily depends on the soil electronegativity655 Ž and the dissociation energy of ions Sah and656 . Chen, 1998 . If there are appropriate pH condi-657 tions, heavy metals are likely to be adsorbed onto 658 the negatively charged soil particles. The main659 sorption mechanisms include adsorption andror 660 ion exchange. Desorption of cationic species from661 clay surfaces is essential in extraction of species662 from fine-grained deposits with high cation- 663 exchange capacity.664 As Acar and his research group have indicated 665 Ž Acar and Alshawabkeh, 1993, 1996; Acar et al.,666 . 1994, 1996 , the sorption mechanisms depend on667 the surface charge density of the clay mineral, the668 characteristics and concentration of the cationic 669 species, and the presence of organic matter and 670 carbonates in the soil. The mechanism is also 671 significantly dependent on the pore fluid pH. The 672 higher the content of carbonates and organic 673 material in soils, the lower the heavy metal re- 674 moval efficiency, which is why the former should 675 be further investigated and taken into the con- 676 sideration. 677 During numerous experiments, a decrease in 678 Ž current density was observed Acar and Al- 679 shawabkeh, 1993, 1996; Acar et al., 1994, 1996; 680 . Sah and Chen, 1998 . The possible reasons might 681 be as follows: 682 683684 Activation polarisation: during the electroki- 685 Ž netic remediation process, gaseous bubbles O 2 686 . and H cover the electrodes. These bubbles 2 687 are good insulators and reduce the electrical 688 conductivity, subsequently reducing the cur- 689 rent. 690691 Resistance polarisation: after the electrokinetic 692 remediation process, a white layer was observed 693 on the cathode surface. This layer may be the 694 insoluble salt and other impurities that were 695 not only attracted to the cathode, but also 696 inhibited the conductivity, with a subsequent 697 decrease in current. 698699 Concentration polarisation: the H q ions gener- 700 ated at the anode are attracted to the cathode 701 and the OH y ions generated at the cathode 702 are attracted to the anode. If acid and alkaline 703 conditions are not neutralised, the current also 704 drops. 705 706 It is possible to conclude that soil containing 707 heavy metal contaminants influences the conduc- 708 tivity. 709 Interaction of the pollutants with the soil also 710 affects the remediation process. In order to in- 711 crease the solubility of complexes formed, or to 712 improve electromigration characteristics of speci- 713 fic heavy metal contaminants, an enhancement 714 solution may be added to the soil matrix. 715 Sometimes electroosmotic flow rates are too 716 low, and it may be necessary to flush the elec- 717 trodes with a cleaning agent, or simply clean tap 718 Ž. water Probstein and Hicks, 1993 . In addition, 719 the electrode may be surrounded by ion-exchange Uncorrected Proof ARTICLE IN PRESS () J. Virkutyte et al. r The Science of the Total En ¨ironment xxx 2001 xxx᎐xxx 9 720 material to trap the contaminant and prevent its721 precipitation. It is essential to know the buffering722 capacity of the soil in order to alter the pH with723 suitable solutions or clean water. Many ground-724 waters contain high concentrations of bicarbon-725 ates, which consume added hydrogen ions to form726 carbonic acid, or hydroxyl ions to form carbonate727 ions. It is vital to draw attention to the limited728 solubility of metal carbonates, as well as the need729 for evaluation of sulfide, sulfate, chlor-730 ide and ammonia effects, which may occur when731 these compounds are introduced into the soil732 Ž system during the remediation process Probstein733 . and Hicks, 1993 .734 New alternatives have been suggested for the735 remediation of heavy metals from soils without 736 Ž having low pH conditions Probstein and Hicks, 737 . 1993 . When the metal enters the region of high738 pH near the cathode, it may adsorb onto the soil,739 precipitate, or form hydroxy complexes. At higher740 pH values, the solubility increases because of the741 increasing stability of soluble hydroxy complexes.742 Despite favourable soluble complexes, the disso-743 lution process may be time-consuming and too744 slow to be successfully implemented.745 Concerning the process of transport of con-746 taminants and their derivatives, two major pheno-747 Ž. mena were indicated Chilingar et al., 1997 :748 749750 1. The flow of contaminant solution through a751 solid matrix due to Darcy’s law and electroki-752 netics; and753754 2. Spatial redistribution of dissolved substances755 with respect to the moving liquid due to the756 diffusion and migration of charged particles.757 758 The total movement of the matter of the con-759 taminant solution in the DC electric field can be 760 expressed as the sum of four components761 Ž. Chilingar et al., 1997 : 762 763764 ⅷ The hydrodynamic flow of liquids driven by765 the pressure gradient;766767 ⅷ The electrokinetic flow of fluids due to inter-768 action of the double layer with the DC field;769770 ⅷ The diffusion of components dissolved in the771 flowing solution; and772773 ⅷ The migration of ions inside moving fluids due 774 to the attraction of charged particles to the 775 electrodes. 776 777 The very questionable concept that removal of 778 heavy metals in the direct current field is effective 779 was also expressed, because electromigration of 780 ions is rapid and does not depend on the zeta 781 potential. In order to prove or disapprove this, 782 further investigations of this concept should be 783 carried out. Despite some disagreements, it was 784 agreed that in order to obtain efficient and reli- 785 able results and control the remediation process, 786 there is a need to provide continuous control of 787 Ž the pH in the vicinity of the electrodes Acar and 788 Alshawabkeh, 1993, 1996; Acar et al., 1994, 1996; 789 . Chilingar et al., 1997 . One possible way to achieve 790 this is periodic rinsing of the cathode with fresh 791 water. 792 Experiments have proved that electrical field 793 application in situ leads to an increase in temper- 794 ature, which in turn reduces the viscosity of hy- 795 Ž drocarbon-containing fluids Chilingar et al., 796 . 1997 . The reduction in fluid viscosity leads to an 797 increase in the total flow rate. 798 Ž. It is reported Chilingar et al., 1997 that in 799 order to accelerate the fluid transport in situ, 800 electrical properties of soils, such as electrical 801 resistivity and the ionisation rate of the flowing 802 fluids that can affect the total rate flow, should 803 consider. In an applied DC field, some soil types 804 showed an increase in their hydraulic permeabil- 805 ity, which allows us to conclude that direct cur- 806 rent may accelerate fluid transport. However, this 807 method is not applicable to some clays, because 808 under the DC field, those clays become amor- 809 phous. It is possible to avoid such a transforma- 810 tion if interlayer clay water is trapped and is not 811 able to leave the system. 812 From the numerous laboratory and field experi- 813 ments and studies conducted, it is possible to 814 conclude that migration rates of heavy metal ions 815 Ž. i.e. removal efficiencies are highly dependent on 816 soil moisture content, soil grain size, ionic mobil- 817 ity, pore water amount, current density and con- 818 Ž taminant concentration Acar and Alshawabkeh, 819 1993, 1996; Acar et al., 1994, 1996; Chilingar et 820 . al., 1997; Sah and Chen, 1998 . Also, in order to 821 assure the efficient and successful heavy metal Uncorrected Proof ARTICLE IN PRESS () J. Virkutyte et al. r The Science of the Total En ¨ironment xxx 2001 xxx᎐xxx10 822 removal from soils, one of the main drawbacks of823 this process must be solved, which is premature824 precipitation of metal species close to the cathode825 compartment. 826 827 3.1. Limitations of the technique 828 829 The removal of heavy metals from soils using830 electrokinetic remediation has some limitations,831 which have been widely discussed among many832 scientists and researchers. For example, the sur-833 face of the electrode attracts the gas generated834 from the electrolytic dissociation process and in-835 creases the resistance, which significantly slows836 Ž down the remediation process Sah and Chen,837 . 1998 . It is obvious that soil resistance is lower in838 the earlier stages of the electrokinetic process,839 and therefore a lower input voltage is required.840 When the electrokinetic process continues, gas841 bubbles from electrolytic dissociation cover the842 whole cathode surface and the resistance in-843 creases. To continue the soil remediation process,844 the input voltage must be increased to maintain 845 the same current, which also increases the voltage846 gradient. OH y ion that are formed react with847 cations and form a sediment, which plugs the848 spacing between soil particles, subsequently hin-849 dering the electrical current and decreasing the850 diffusive flow over time when the voltage is ap-851 Ž. plied Sah and Chen, 1998 . 852 853 3.2. Enhancement and conditioning 854 855 To overcome the premature precipitation of856 ionic species, Acar and his research group have 857 recommended using different enhancement tech-858 niques to remove or to avoid these precipitates in859 the cathode compartment. Efficient techniques 860 should have the following characteristics:861 862863 ⅷ The precipitate should be solubilised andror864 precipitation should be avoided.865866 ⅷ Ionic conductivity across the specimen should 867 not increase excessively in a short period of868 time to avoid a premature decrease in the869 electroosmotic transport. 870 871 ⅷ The cathode reaction should possibly be de- 872 polarised to avoid the generation of hydroxide 873 and its transport into the specimen. 874 875 ⅷ Depolarisation will decrease the electrical po- 876 tential difference across the electrodes, which 877 would result in lower energy consumption. 878 879 ⅷ If any chemical is used, the precipitate of the 880 metal with the new chemical should be per- 881 fectly soluble within the pH range attained. 882 883 ⅷ Any special chemicals introduced should not 884 result in any increase in toxic residue in the 885 soil mass. 886 887 ⅷ The cost efficiency of the process should be 888 maintained when the cost of enhancement is 889 included. 890 891 It is obvious that an enhancement fluid in- 892 creases the efficiency of contaminated soil treat- 893 ment; however, there is a lack of data which 894 would clarify further soil and contaminant inter- 895 actions in the presence of this fluid. 896 Ž. As a depolariser i.e. enhancement fluid in the 897 cathode compartment, it is possible to use a low 898 Ž concentration of hydrochloric or acetic acid Acar 899 and Alshawabkeh, 1993, 1996; Acar et al., 1994, 900 . 1996 . The main concern with hydrochloric acid 901 as the depolariser is that due to electrolysis, the 902 chlorine gas formed may reach the anode, as well 903 as groundwater, and increase its contamination. 904 Acetic acid is environmentally safe and it does 905 not fully dissociate. In addition, most acetate salts 906 are soluble, and therefore acetic acid is preferred 907 in the process. 908 The anode reaction should also be depolarised, 909 because of the dissolution and release of silica, 910 alumina and heavy metals associated with the clay 911 mineral sheets over long exposure to protons 912 Ž Acar and Alshawabkeh, 1993, 1996; Acar et al., 913 . 1994, 1996 . 914 In order to accomplish both tasks successfully, 915 it is better to use calcium hydroxide as the en- 916 hancement fluid to depolarise the anode reaction, 917 and hydrochloric acid as the enhancement fluid to 918 depolarise the cathode reaction. 919 The use of an enhancement fluid should be 920 Ž examined with extreme care to prevent Yeung et 921 . al., 1997 : [...]... 1994;44Ž2.:239᎐254 Acar YB et al Enhance soil bioremediation with electric fields Chemtech 1996;26Ž4.:40᎐44 Baraud F, Tellier S, Astruc M Ion velocity in soil solution during electrokinetic remediation J Hazard Mater 1997; 56:315᎐332 Baraud F, Fourcade MC et al Modelling of decontamination rate in an electrokinetic soil processing Int J Environ Anal Chem 1998;68:105᎐121 Benazon N Soil remediation Hazard Mater Manage... removed before the electrokinetic remediation process begins, metallic mercury would inhibit the overall remediation process due to its electric conductivity 5.4 Remo¨ al of zinc and copper re 1600 ct 1610 Electrokinetic remediation of Hg-contaminated soils is very difficult because of the low solubility of Hg in most natural soils The predominant species of insoluble Hg in the soils are HgS, HgŽI and Hg 2... trokinetic soil remediation was on saturated, fine-grained soils and clays, which led to the misconception that electrokinetics was not suitable for unsaturated, sandy soils Laboratory experiments proved that with appropriate technology and well-designed methods, it is possible to remediate heavy metals from unsaturated and sandy soils ŽMattson and Lindgren, 1995 The treatment of unsaturated soils has... electrokinetic soil remediation processes J Hazard Mater 1997;55:1᎐22 Viadero RCJr et al A laboratory-scale study of applied voltage on the electrokinetic separation of lead from soils Sep Sci Technol 1998;33Ž12.:1833᎐1859 Yeung A et al Physicochemical soil ᎏ contaminant interactions during electrokinetic extraction J Hazard Mater 1997;55:221᎐237 Zelina JP, Rusling JF Electrochemical remediation of soils,... the amount of water being added to the soil from the ceramic castings Anode ceramic casting would be suitable for long-term electrokinetic remediation processes if it was ensured that electroosmotic flow occurred from the surrounding soil towards the interior of the anode casting ŽMattson and Lindgren, 1995 As efficient electrokinetic remediation in unsaturated soils depends on the water amount at the... site soils contaminated with arsenic or chromium: evaluation of the electrokinetic method Environ Technol 1998;19:1095᎐1102 Hicks RE, Tondorf S Electrorestoration of metal-contaminated soils Environ Sci Technol 1994;28Ž12.: 2203᎐2210 Ho SV et al Scale-up aspects of the LasagnaTM process for in situ soil decontamination J Hazard Mater 1997;55:39᎐60 Ho SV et al The Lasagna technology for in situ soil remediation. .. subsurface soils http:rrgeotech civen.okstate.edurejgerppr9703rindex.htm Pamukcu S, Wittle JK Electrokinetic removal of selected heavy metals from soil Environ Prog 1992;11Ž3.:241᎐250 Probstein RF, Hicks RE Removal of contaminants from soils by electric fields Science 1993;260:498᎐503 Puppala SK, Alshawabkeh AN, Acar YB, Gale RJ, Bricka M Enhanced electrokinetic remediation of high sorption capacity soils... Reddy K, Chinthamreddy S Electrokinetic remediation of heavy metal-contaminated soils under reducing environments Waste Manage 1999;19:269᎐282 Reddy K, Parupudi US et al Effects of soil composition on the removal of chromium by electrokinetics J Hazard Mater 1997;55:135᎐158 Reddy K, Donahue M, Sasaoka R Preliminary assessment of electrokinetic remediation of soil and sludge contaminated with m ixed... The electrical conductivity of soil depends on the moisture content ŽMattson and Lindgren, 1995 During electroosmotic migration through the soil, the water content near the anode is reduced As the moisture content decreases, the soil conductivity becomes too low for the electrokinetic remediation application In order to control the hydraulic flux of water in the treated soil, the use of porous ceramic... of hydroxides wCdŽOH 2 , PbŽOH 2 x and carbonates ŽCdO3 , PbCO 3 Soil pH determines the concentrations of hydroxide and carbonate in the soil solution, which play a crucial role in the formation of heavy metal complexes in soil ŽSah and Chen, 1998 In order to understand the migration of Pb and Cd between electrified vs non-electrified soil samples under different times, locations and solu- 1288 1289 . soils, despite the fact that it can-105 not reduce their toxicity. 106107 ⅷ Electrokinetic soil remediation. 108 109 As none of the other in situ soil remediation 110 techniques. 252 present in moist soil environments. Electrokinetic 253 remediation of soils is a unique method, because 254 it can remediate even low-permeability soils. 255 Other