**2. Procedures for simultaneous decontamination of polluted soils**

This chapter presents a review of the processes and technologies that allow the simultaneous removal/destruction/immobilization of more than one class of contaminants in soils, focusing on dual decontamination of at least two different pollutants, one being an inorganic, the second an organic compound. Relevant papers were retrieved using screening of the scientific literature using Scopus, ISI Web of Knowledge and Google Scholar.

As previously mentioned, the most important class of inorganic pollutants is represented by heavy metals [21], while among the organic pollutants various classes of compounds can be mentioned [22] (e.g. pesticides, polyaromatic hydrocarbons, polychlorinated compounds, etc.) and it is more than often that either rural or urban soils are polluted with both types (organic and inorganic). In order to realize a simultaneous cleansing of the soil, the method used must be suitable and effective for both classes of compounds. Indeed, some of the methods applied separately for organic or inorganic contaminants proved to be successful when both types of compounds were present in soils. These methods can be classified as follows:

– Washing using a proper solvent mixture and eventually a surfactant (including flotation processes)


#### **2.1. Washing processes**

Chronologically speaking, elution techniques were the first used to simultaneously clean contaminated soil with both heavy metals and organic compounds, as early as the end of the twentieth century. The main problem was identifying the proper solvent mixtures.

One of these early papers in the field of dual separation techniques investigated the ability of aqueous cyclodextrin solutions to simultaneously remove heavy metals and low-polarity organic compounds from contaminated soil [23]. In that purpose, Brusseau and co-workers used three types of soil spiked with the model organic compound (phenanthrene) and the model heavy metal (cadmium). Previously, Dunn et al. had used surfactants in a micellarenhanced ultrafiltration, which is a separation process using surfactants and membranes, removing dissolved organic solutes or multivalent ions from water with high rejections [24]. Through this procedure, mixtures of pollutants containing phenol or *o*-cresol and Zn2+ and/or Ni2+ were separated from contaminated soils, using an anionic surfactant. Micellar-enhanced ultrafiltration was subsequently used by other authors to remove either Cu and phenol [25] or chlorinated aromatic hydrocarbons, nitrate and chromate ions [26]. Modified cyclodextrin (as glycine-β-cyclodextrin) was later successfully used in the study of the desorption behaviour of phenanthrene and lead from co-contaminated soil [27]. The authors showed that glycineβ-cyclodextrin had good solubilization properties for both phenanthrene and lead carbonate (900 g/L for phenanthrene, respectively, 2945 mg/L PbCO3).

A parallel process used a combination of 2.5 N sulphuric acid and isopropyl alcohol (in a 4:9 ratio), with a dilution of 5 part solution to 1 part soil, the separation being made by ultrafiltration [28]. The contaminants removed in these experiments were heavy metals (Cd, Ag and Cu), volatile organic compounds (ethyl benzene and methyl iso-butyl ketone), halogenated compounds (chloroethene and tetrachloroethylene) and pesticides, herbicides and insecticides (lindane, methoxychlor and endrin). However, the acidic treatment cannot be applied to other types of soils than sandy ones. The ultrafiltration methods are still at the laboratory-scale level and therefore are not yet suitable for *in situ* applications. Thus, researchers tended to deepen the knowledge in the field of surfactant use.

mentioned [22] (e.g. pesticides, polyaromatic hydrocarbons, polychlorinated compounds, etc.) and it is more than often that either rural or urban soils are polluted with both types (organic and inorganic). In order to realize a simultaneous cleansing of the soil, the method used must be suitable and effective for both classes of compounds. Indeed, some of the methods applied separately for organic or inorganic contaminants proved to be successful when both types of

– Washing using a proper solvent mixture and eventually a surfactant (including flotation

Chronologically speaking, elution techniques were the first used to simultaneously clean contaminated soil with both heavy metals and organic compounds, as early as the end of the

One of these early papers in the field of dual separation techniques investigated the ability of aqueous cyclodextrin solutions to simultaneously remove heavy metals and low-polarity organic compounds from contaminated soil [23]. In that purpose, Brusseau and co-workers used three types of soil spiked with the model organic compound (phenanthrene) and the model heavy metal (cadmium). Previously, Dunn et al. had used surfactants in a micellarenhanced ultrafiltration, which is a separation process using surfactants and membranes, removing dissolved organic solutes or multivalent ions from water with high rejections [24]. Through this procedure, mixtures of pollutants containing phenol or *o*-cresol and Zn2+ and/or Ni2+ were separated from contaminated soils, using an anionic surfactant. Micellar-enhanced ultrafiltration was subsequently used by other authors to remove either Cu and phenol [25] or chlorinated aromatic hydrocarbons, nitrate and chromate ions [26]. Modified cyclodextrin (as glycine-β-cyclodextrin) was later successfully used in the study of the desorption behaviour of phenanthrene and lead from co-contaminated soil [27]. The authors showed that glycineβ-cyclodextrin had good solubilization properties for both phenanthrene and lead carbonate

A parallel process used a combination of 2.5 N sulphuric acid and isopropyl alcohol (in a 4:9 ratio), with a dilution of 5 part solution to 1 part soil, the separation being made by ultrafiltration [28]. The contaminants removed in these experiments were heavy metals (Cd, Ag and Cu), volatile organic compounds (ethyl benzene and methyl iso-butyl ketone), halogenated compounds (chloroethene and tetrachloroethylene) and pesticides, herbicides and insecticides (lindane, methoxychlor and endrin). However, the acidic treatment cannot be applied to other types of soils than sandy ones. The ultrafiltration methods are still at the laboratory-scale level

twentieth century. The main problem was identifying the proper solvent mixtures.

compounds were present in soils. These methods can be classified as follows:

– Electrokinetic methods (derived from the washing techniques)

– Miscellaneous (including thermal or chemical methods)

(900 g/L for phenanthrene, respectively, 2945 mg/L PbCO3).

– Bioremediation (including phytoremediation)

236 Soil Contamination - Current Consequences and Further Solutions

– Combinations of the previous

**2.1. Washing processes**

processes)

A first step was to widen the variety of surfactants used. Thus, Marshall and co-workers, after testing the efficiency of a non-ionic surfactant (polyethylene oxide (PEO) of chain length 7.5 (Triton X-114), 9.5 (Triton X-100), 30 (Triton X-305) or 40 units (Triton X-405)), combined with iodide salts [29], introduced the use of ethylenediaminetetraacetate (EDTA) to simultaneously extract heavy metals and polychlorinated biphenyl compounds from a field-contaminated soil [30]. Since both cyclodextrin and EDTA were effective, the use of their combination was a naturally occurring step, took again by Marshall and his team at the McGill University [31]. Thus, ultrasonication was used to mix field-contaminated soil with a combination of cyclodextrin solution (10%, w/v) and 2 mmol EDTA, in the same time mobilizing polychlorinated biphenyls and much of the metals (Cd, Cr, Cu, Mn, Ni, Pb and Zn). The authors observed that a combination of randomly methylated or hydroxypropyled β-cyclodextrin with EDTA did not alter the polychlorinated biphenyls extraction efficiency nor did the presence of cyclodextrin change the efficiency of mobilization of most heavy metals (Al, Cd, Cr, Fe, Mn, Ni and Zn) but it did increase the recovery of Cu and Pb. Three sonication washings with the same charge of reagents mobilized appreciable quantities of polychlorinated biphenyls (40–76%) and quantitatively extracted the labile fraction of Cd, Cu, Mn and Pb. However, due to the low degree of biodegradability in soil [32], Marshall opted for another complexing reagent (ethylenediaminedisuccinic acid, [S,S]-EDDS). Thus, the same authors evaluated the efficacy of soil washing with a non-ionic surfactant (Brij98) in combination with [S,S]-EDDS for the simultaneous mobilization of heavy metals and polycyclic aromatic hydrocarbons from a fieldcontaminated soil [33, 34]. Moreover, they extended their procedure to the remediation of polluted soils with arsenic, chromium, copper, pentachlorophenol (PCP), polychlorinated dibenzo-*p*-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) [35]. The highest level of mobilization/detoxification was achieved in three soil washes with a mixture of 0.1 M [S,S]-EDDS and 2% Brij98 at pH 9 with 20 min of ultrasonication treatment at room temperature. This combination mobilized 70% of As, 75% of Cr, 80% of Cu, 90% of pentachlorophenol and 79% of PCDDs and PCDFs.

In order to render the process even more environmentally friendly, many authors turned their attention to naturally occurring surfactants. Thus, starting from the idea that biosurfactants are potentially less toxic to soil organisms than other chemical agents, Lima and co-workers studied the efficiency of a combination of iodide salt ligands and surfactants produced by different bacterial species in the simultaneous removal of cadmium and phenanthrene in a Haplustox soil sample [36]. For their part, Zhu et al. from Zhejiang University used saponin, a plant-derived biosurfactant, for the dual removal of phenanthrene and cadmium from contaminated soils [37]. Another Chinese team successfully treated soils from a contaminated electronic waste site that contained both polybrominated diphenyl ethers and heavy metals using common sunflower oil in a mixture with carboxymethyl chitosan [38].

Another Canadian team, led by Blais from the Université du Quebec, chose to combine alkaline-washing process with flotation in acidic solutions (in the presence of a surfactant such as cocamidopropyl betaine). They succeeded in decontaminating various types of soils from mixtures of heavy metals, pentachlorophenol and polychlorodibenzo-*p*-dioxins and furans (PCDD/F) [39]. Blais' process is mainly based on physical techniques, such as crushing, gravimetric separation and attrition. In another such study, Blais' team demonstrated that it is possible to attain removal efficiencies of 49–73% for Cu and from 43 to 63% for Zn, whereas a removal yield of 92% was measured for total PAHs [40, 41]. The results were improved replacing cocamidopropyl betaine with cocamidopropyl hydroxysultaine (up to 90% PAHs and Pb removal, in three different polluted soil types) [42]. By carefully choosing the acidic species (hydrochloric, nitric, sulphuric and lactic acids and ethanol) for leaching metals from soil in combination with non-ionic, ionic and amphoteric surfactants, Blais and co-worker studied the simultaneous removal of heavy metals and pentachlorophenol by flotation [43]. Thus, removal yields of 82–93, 30–80, 79–90 and 36–78% were obtained from As, Cr, Cu and PCP, respectively.

A different approach was undertaken by a team from the Hebrew University of Jerusalem, under the form of a sediments remediation phase transition extraction [44]. This process is based on using partially miscible solvent mixtures in which specific organic soluble chelating agents are dissolved. Extraction efficiency is improved by a phase transition cycle induced by temperature variation. With this technology, up to 90% of cadmium ions were removed within approximately 15 min, as well as practically all the organic matter, including PAHs.

#### **2.2. Electrokinetic processes**

A second family of technologies that allow concurring decontamination of co-contaminated soils are the electrokinetics processes (*aka* electrokinetics). Electrokinetics are a group of emerging techniques that are intended to separate and extract heavy metals, radionuclides and organic contaminants from various types of soils, sludges, sediments and even groundwater [45]. The goal of electrokinetic remediation is to realize the migration of subsurface contaminants in an imposed electric field via electro-osmosis, electromigration and/or electrophoresis. These phenomena occur when the soil is electrically charged with a low-voltage current. The fundamental configuration for all three processes involves the application of an electrical potential between electrode pairs that have been implanted in the ground on each side of a contaminated soil mass. There are even some emerging *in situ* electrokinetic soil remediation technologies, such as Lasagna™, Elektro-Klean™ or Electrobioremediation.

Even from the first attempts to simultaneously eliminate both heavy metals (including lead, zinc, manganese, copper and arsenic) and organic pollutants (PAHs, benzene, toluene, ethylbenzene and xylene) from co-contaminated soils, it was clear that migration occurred on the straight line between electrodes and that the process is a lengthy one, from 23 to 112 days, using a current density of 3.72 A/m2 [46].

Pioneering studies on electrokinetics were performed at the University of Illinois in Chicago by Professor K.R. Reddy whose results demonstrated the effectiveness of this method on soils polluted either with heavy metals [47, 48] or organic contaminants [49, 50]. In an obvious continuation, electrokinetics was applied to soils co-contaminated with both heavy metals and organic pollutants [51]. As required by the theory of electrokinetic remediation technologies, the soil was acidified, using 1 M citric acid, which acted also as a chelating agent, along with ethylenediaminetetraacetic acid, and some surfactants (Igepal or Tween). The best results of the sequential extraction test were obtained with citric acid 1 M combined with Igepal CA-720 (5%) or Tween (5%). However, the authors observed that the presence of surfactants tended to reduce the electric conductivity of the soil and the electro-osmotic flow compared with the stages where citric acid was used as flushing solution. The results confirmed previous studies, in which the ability of surfactants, cosolvents, cyclodextrins, chelating agents and organic acids to remove Ni and phenanthrene from kaolin soil was tested [52, 53]. EDTA was confirmed to produce complications during electrokinetic experiments [54].

mixtures of heavy metals, pentachlorophenol and polychlorodibenzo-*p*-dioxins and furans (PCDD/F) [39]. Blais' process is mainly based on physical techniques, such as crushing, gravimetric separation and attrition. In another such study, Blais' team demonstrated that it is possible to attain removal efficiencies of 49–73% for Cu and from 43 to 63% for Zn, whereas a removal yield of 92% was measured for total PAHs [40, 41]. The results were improved replacing cocamidopropyl betaine with cocamidopropyl hydroxysultaine (up to 90% PAHs and Pb removal, in three different polluted soil types) [42]. By carefully choosing the acidic species (hydrochloric, nitric, sulphuric and lactic acids and ethanol) for leaching metals from soil in combination with non-ionic, ionic and amphoteric surfactants, Blais and co-worker studied the simultaneous removal of heavy metals and pentachlorophenol by flotation [43]. Thus, removal yields of 82–93, 30–80, 79–90 and 36–78% were obtained from As, Cr, Cu and

A different approach was undertaken by a team from the Hebrew University of Jerusalem, under the form of a sediments remediation phase transition extraction [44]. This process is based on using partially miscible solvent mixtures in which specific organic soluble chelating agents are dissolved. Extraction efficiency is improved by a phase transition cycle induced by temperature variation. With this technology, up to 90% of cadmium ions were removed within

A second family of technologies that allow concurring decontamination of co-contaminated soils are the electrokinetics processes (*aka* electrokinetics). Electrokinetics are a group of emerging techniques that are intended to separate and extract heavy metals, radionuclides and organic contaminants from various types of soils, sludges, sediments and even groundwater [45]. The goal of electrokinetic remediation is to realize the migration of subsurface contaminants in an imposed electric field via electro-osmosis, electromigration and/or electrophoresis. These phenomena occur when the soil is electrically charged with a low-voltage current. The fundamental configuration for all three processes involves the application of an electrical potential between electrode pairs that have been implanted in the ground on each side of a contaminated soil mass. There are even some emerging *in situ* electrokinetic soil remediation

Even from the first attempts to simultaneously eliminate both heavy metals (including lead, zinc, manganese, copper and arsenic) and organic pollutants (PAHs, benzene, toluene, ethylbenzene and xylene) from co-contaminated soils, it was clear that migration occurred on the straight line between electrodes and that the process is a lengthy one, from 23 to 112 days,

Pioneering studies on electrokinetics were performed at the University of Illinois in Chicago by Professor K.R. Reddy whose results demonstrated the effectiveness of this method on soils polluted either with heavy metals [47, 48] or organic contaminants [49, 50]. In an obvious continuation, electrokinetics was applied to soils co-contaminated with both heavy metals and organic pollutants [51]. As required by the theory of electrokinetic remediation technologies, the soil was acidified, using 1 M citric acid, which acted also as a chelating agent, along with

approximately 15 min, as well as practically all the organic matter, including PAHs.

technologies, such as Lasagna™, Elektro-Klean™ or Electrobioremediation.

[46].

PCP, respectively.

**2.2. Electrokinetic processes**

238 Soil Contamination - Current Consequences and Further Solutions

using a current density of 3.72 A/m2

Surfactants, cosolvents and cyclodextrins (same as in washing technique) generally yielded better results for phenanthrene removal, whereas chelating agents and organic acids yielded better removal for Ni [55].

To some extent, the EK process can be applied to sediments of harbour waterways, for the rapid elimination of heavy metals (Cd, Cr, Cu, Zn and Pb) and PAHs [56]. Beside citric acid, nitric acid (which is not recommended in case of *in situ* remediation) was also tested to avoid the formation of an alkaline front into the sediment and favour the metals removal. As surfactants, sodium dodecyl sulphate (as an anionic surfactant) and Tween 20 (as a non-ionic surfactant) were used to solubilize and mobilize PAHs. However, for achieving an almost complete removal of heavy metals and PAHs, the process needs ca 10 days. The EK process was extended to real-life polluted soil, deriving from a dismissed industrial site, contaminated with several metals: Hg, Ni, Co, Zn, Pb, Cu, Cr, As and organic substances [57]. Using a Ti/Pt-Ir anode and a stainless steel cathode, the procedure allowed a fair to good removal of most of heavy metals and PAHs over an interval of 10–15 days. An addition to the previous procedure was the presence of an oxidizing leaching agent, electrochemically produced. A similar approach, by using an oxidizing agent such as H2O2 (hydrogen peroxide was also present in a study by Reddy and co-workers [58]), NaClO, KMnO4 or Na2S2O8 in a controlled pH (3.5 or 10), was applied in a so-called enhanced-EK remediation technology to decontaminate a heavy metal-organic compound co-contaminated soil [59]. Over ca 14 days of applying 1.0 V/cm, the results showed that there was significant migration of pyrene (favoured by the presence of oxidizing reagent such as KMnO4 or Na2S2O8) and Cu from the soil (favoured by low pH), and that the removal percentage of soil pyrene and Cu varied in the range of 30–52 and 8–94%, respectively.

Shorter elution times were obtained using vertical electrokinetic cell (just 6 days, using smaller diameter or shorter-height cells and 0.01 M HNO3 solution as cathode chamber flow) [60], but it is hard to expect that such procedure could be adapted for *in situ* applications. Nonetheless, removal efficiencies of phenanthrene, *p*-xylene, Cu and Pb were 67, 93, 62 and 35%, respectively.

An interesting twist to the technique was brought up by Ma and co-workers, who used bamboo charcoal as adsorbent, in a bench-scale experiment conducted to investigate the simultaneous removal of 2,4-dichlorophenol and Cd from a sandy loam (artificially spiked), at different periodic polarity reversals [61]. After ca 11 days of operating, about 75% of Cd and 55% of the phenol were removed at intervals of 24 h (about half for intervals of 12 h), at soil pH values ranging from 7.2 to 7.4.

The idea of combining EK process with adsorption was investigated also by Lukman from the King Fahd University of Petroleum and Minerals, who used a locally produced granularactivated carbon from date palmpits in the treatment zones. Natural saline-sodic soil, spiked with contaminant mixture (kerosene, phenol, Cr, Cd, Cu, Zn, Pb and Hg), was submitted to a 21-day period of continuous electrokinetics-adsorption experimental run, the efficiency for the removal of Zn, Pb, Cu, Cd, Cr, Hg, phenol and kerosene being 26.8, 55.8, 41.0, 34.4, 75.9, 92.49, 100.0 and 49.8%, respectively [62]. Lukman also demonstrated the importance of the processing fluids (anolytes and catholytes), which are rapidly degrading depending on the applied voltage gradient, ultimately leading to an eventual rise in the cost of operating the remediation process [63].

A significant improvement was brought to EK process by combining it with ultrasonication. This led to an enhancement of the remediation rate of soils co-contaminated with Pb and phenanthrene [64]. The migration of water and contaminants in the porous soil media is permitted through the actions of electro-osmotic flow and electromigration by electric power and acoustic flow by ultrasonic waves. The accumulated outflow and contaminant-removal rate were higher by the addition of vibration, cavitation and sonication effects. However, if efficiency seemed to improve (the removal rates of Pb and phenanthrene were average 88 and 85% for electrokinetic test and average 91 and 90% for electrokinetic and ultrasonic test, respectively), the duration was not reduced—it still needed ca 15 days of treatment, which means that by using both EK and ultrasonication the costs are higher.

A comprehensive review on the status of *in situ* applicable electrokinetic processes (Electro-Klean™ Electrical Separation, Electrokinetic Bioremediation, Electrochemical GeoOxidation (ECGO), Electrochemical Oxidative Remediation of Groundwater, Electrochemical Ion Exchange (EIX), Electrosorb™ and Lasagna™ process) was presented by E.M. Morales, from PGATech [65].

#### **2.3. Bioremediation processes**

Bioremediation generally uses living organisms (usually microbial metabolism), in the presence of optimum environmental conditions and sufficient nutrients, to break down soil organic and inorganic contaminants. Since it represents an attractive method due to the ease of *in situ* applications, bioremediation methods have been reviewed over the past few years, mostly depending on the nature of the polluting agent (heavy metals, PAHs, polychlorinated compounds, pesticides, etc.) or on the contaminated matrix (soil, sediments, groundwater, etc.) [66–70]. Since the method proved effective for both types of contaminants, attempts for simultaneous decontamination did not take long to appear.

A first approach was the so-called 'bioaugmentation': metal-contaminated soils were enriched with metal-detoxifying microorganisms while organic-contaminated soils were supplemented with organic-degrading microorganisms [71]. In such of the first examples, a co-contaminated soil with both Cd and 2,4-dichlorophenoxiacetic acid (2,4-D), the degradation of both contaminants was realized by introducing specific microorganisms for each contaminant (*Ralstonia eutropha* JMP134 for 2,4-D and *Arthrobacter*, *Bacillus* and *Pseudomonas* species for Cd) [72]. Contaminated soil with sulphide ore ashes and aromatic hydrocarbons from a historical industrial site underwent sequential leaching by 0.5 M citrate and microbial treatments [73]. The acidic-leached soil was bioaugmented with *Allescheriella*, *Stachybotrys*, *Phlebia*, *Pleurotus pulmonarius* and *Botryosphaeria rhodina*, which proved to be the most effective, leading to a significant depletion of the most abundant contaminants, including 7-H-benz[*d,e*]anthracene-7-one, 9,10-anthracene dione and dichloroaniline isomers. Simultaneously, the overall metal content was sensibly diminished under the action of *P. pulmonarius*.

The idea of combining EK process with adsorption was investigated also by Lukman from the King Fahd University of Petroleum and Minerals, who used a locally produced granularactivated carbon from date palmpits in the treatment zones. Natural saline-sodic soil, spiked with contaminant mixture (kerosene, phenol, Cr, Cd, Cu, Zn, Pb and Hg), was submitted to a 21-day period of continuous electrokinetics-adsorption experimental run, the efficiency for the removal of Zn, Pb, Cu, Cd, Cr, Hg, phenol and kerosene being 26.8, 55.8, 41.0, 34.4, 75.9, 92.49, 100.0 and 49.8%, respectively [62]. Lukman also demonstrated the importance of the processing fluids (anolytes and catholytes), which are rapidly degrading depending on the applied voltage gradient, ultimately leading to an eventual rise in the cost of operating the remediation

A significant improvement was brought to EK process by combining it with ultrasonication. This led to an enhancement of the remediation rate of soils co-contaminated with Pb and phenanthrene [64]. The migration of water and contaminants in the porous soil media is permitted through the actions of electro-osmotic flow and electromigration by electric power and acoustic flow by ultrasonic waves. The accumulated outflow and contaminant-removal rate were higher by the addition of vibration, cavitation and sonication effects. However, if efficiency seemed to improve (the removal rates of Pb and phenanthrene were average 88 and 85% for electrokinetic test and average 91 and 90% for electrokinetic and ultrasonic test, respectively), the duration was not reduced—it still needed ca 15 days of treatment, which

A comprehensive review on the status of *in situ* applicable electrokinetic processes (Electro-Klean™ Electrical Separation, Electrokinetic Bioremediation, Electrochemical GeoOxidation (ECGO), Electrochemical Oxidative Remediation of Groundwater, Electrochemical Ion Exchange (EIX), Electrosorb™ and Lasagna™ process) was presented by E.M. Morales, from

Bioremediation generally uses living organisms (usually microbial metabolism), in the presence of optimum environmental conditions and sufficient nutrients, to break down soil organic and inorganic contaminants. Since it represents an attractive method due to the ease of *in situ* applications, bioremediation methods have been reviewed over the past few years, mostly depending on the nature of the polluting agent (heavy metals, PAHs, polychlorinated compounds, pesticides, etc.) or on the contaminated matrix (soil, sediments, groundwater, etc.) [66–70]. Since the method proved effective for both types of contaminants, attempts for

A first approach was the so-called 'bioaugmentation': metal-contaminated soils were enriched with metal-detoxifying microorganisms while organic-contaminated soils were supplemented with organic-degrading microorganisms [71]. In such of the first examples, a co-contaminated soil with both Cd and 2,4-dichlorophenoxiacetic acid (2,4-D), the degradation of both contaminants was realized by introducing specific microorganisms for each contaminant (*Ralstonia eutropha* JMP134 for 2,4-D and *Arthrobacter*, *Bacillus* and *Pseudomonas* species for Cd) [72]. Contaminated soil with sulphide ore ashes and aromatic hydrocarbons from a historical

means that by using both EK and ultrasonication the costs are higher.

simultaneous decontamination did not take long to appear.

process [63].

240 Soil Contamination - Current Consequences and Further Solutions

PGATech [65].

**2.3. Bioremediation processes**

The discovery of the dissimilatory metal reduction [74] under the action of heavy metals reducing bacteria provided the idea for the next step. It was soon afterwards that it became clear that the reduction can occur only in the presence of a hydrogen donor, usually water or an organic compound. Thus, Cr(VI)-reducing bacteria may utilize a variety of organic compounds as electron donors for Cr(VI) reduction, though the organic compounds are generally limited to natural aliphatics, mainly low-molecular-weight carbohydrates, amino acids and fatty acids [75]. Thus, why not have the organic pollutant as hydrogen donor, resulting thus in its oxidation? A good hydrogen donor, and also a well-known organic pollutant, is phenol. Indeed, soils co-polluted with a heavy metal such as Cr(VI) and phenol can be decontaminated in the same time by using strains of *P. aeruginosa* [76]. Another strain of *Pseudomonas*, *P. fluorescens*, allowed a drastic reduction of the concentration of heavy metals (Cu2+, Cd2+, Ni2+ and Pb2+) but also of phenols and various pesticides (hexachlorobenzene, mancozeb or 2,4-D) in water [77]. Not only *Pseudomonas* strains were able to perform dual Cr(VI) reduction/phenol degradation, but also *Stenotrophomonas* species [78].

Similar examples of metal-reducing bacteria used in dual decontamination procedures are summarized in the following. *Geobacter metallireducens* was successfully used for the biodegradation of toluene and bioleaching of As, and in the same time for an accelerated degradation rate of toluene with reductive dissolution of Fe and co-dissolution of As [79]. In another paper, *Actinobacteria*, in a mixture with some *Streptomyces* spp. *Amycolatopsis tucumanensis*, were used to remediate soil co-contaminated with Cr(VI) and lindane [80]. It is interesting that the incubation period was of only 14 days.

At a certain level, phytoremediation can be considered as a bioremediation [81, 82]. According to the same rationale, if the phytoremediation process can be applied separately to heavy metals [83] and organic pollutants [84, 85], then it can be applied to co-contaminated soils with both species [86]. However, due to its limitations (phytoremediation is applicable only to rootdeep soil horizons), there are very few examples of co-decontamination.

One such example is the use of willows for a dual decontamination Cd—oil [87] and another is the use of maize for soil polluted with Cd and pyrene [88].

It is interesting to mention the fact that in a recently published review on the applicability of phytoremediation of soils with mixed organic and heavy metal contaminants, Reddy (*vide supra*) and co-workers identified the most suitable species that proved to be effective separately for heavy metals and organic contaminants, but that were never investigated in simultaneous phytoremediation.

An alternate route was taken by an Italian team led by Baldi, from the Ca Foscari University of Venice, consisting of a first step of bioprecipitation followed by fungal degradation of organic pollutants from contaminated soils [89]. Thus, the contaminated soil was leached with 0.5 M citric acid leading to a good removal of metals and a low removal of organic contaminants (12%). The leachate was then incubated with a metal-resistant *Klebsiella oxytoca* strain, capable of using residual citrate to produce an iron gel that co-precipitated metals. In the same time, the leached solid waste was bioaugmented with a fungus strain of *Allescheriella* to complete the degradation of several organic contaminants, including trichlorobenzene, naphthalene, dichloroaniline and pentachloroaniline.

#### **2.4. Miscellaneous processes**

In this area, there are two possibilities: processes that combine aspects of the above procedures (washing, EK and bioremediation) or processes that are completely different, belonging to the immobilization/sorption techniques or to the purely chemical or thermal types of technologies.

Thus, either washing or electrokinetic process can be improved if realized with the augmentation of the naturally occurring microbial activity. In a recent report, PAHs and heavy metalpolluted soil from an abandoned coking plant was in a first step cleaned up by using a methylβ-cyclodextrin solution to enhance ex situ extraction of PAHs and metals simultaneously, followed by the addition of PAH-degrading bacteria (*Paracoccus* sp. strain) and supplemental nutrients to treat the residual soil-bound PAHs [90]. Elevated temperature (50°C) in combination with ultrasonication was also needed. In the second case, the authors studied the benefits of integrating electrokinetic remediation with biodegradation for decontaminating soil cocontaminated with crude oil and Pb, in laboratory-scale experiments lasting for 30 days [91].

Immobilization techniques imply the adsorption of both heavy metals and organic contaminants on a solid support, usually biochar [92]. Cao et al. demonstrated that incubated biochar (prepared from dairy manure) for 210 days was effective for immobilization of both atrazine and Pb (its effectiveness was enhanced by increasing incubation time and quantity) [93]. After treatment, soils to which ca 5.0% biochar was added showed more than 57 and 66% reduction in Pb and atrazine concentrations, respectively.

On the chemical side of dual degradation processes, there are mentions of some techniques that use a photochemical activation. The problem encountered by photocatalytic processes is that they need the *a priori* formation of a solution. Thus, in an aqueous solution, it was possible to simultaneously reduce Cr(VI) and oxidize benzoic acid, in a suspension of N-F-co-doped TiO2 [94]. Therefore, photocatalytic process must follow a washing step [95].

In a purely chemical process, Mitoma and co-workers used a nano-size mixture of metallic Ca and CaO that played a double role: in combination with a hydrogen donor (naturally occurring moisture) can hydrodechlorinate harmful dioxin compounds and during mixing can immobilize heavy metals in a cement-like matrix [96]. Thus, a soil contaminated with both heavy metals and dioxins (the most common type of polluted soil from reclaimed factories) can be safely treated.

The thermal type of co-decontamination process is illustrated by only one example, in which PCDD/Fs, pentachlorophenol and mercury are simultaneously removed [97]. This is understandable since other than Hg, all the other heavy metals are thermally quite stable (Hg has a boiling point of 356.73°C).
