**2. How molecules can escape degradation in the environment**

The adsorption of molecules, due to their affinity to other chemical components of the soil matrix or soil biosphere, protects them from chemical degradation as well biodegradation. Non adsorbed molecules are bioavailable and therefore are exposed to degradation. Experi‐ ments with soils weathered for long time with ethylene bromide contamination in which was added 14C-ethylene bromide, shows strong biodegradation of the new applied radioactive substance after sixty days at the same time that weathered contamination substance remains in the same concentration [1]. Microorganisms involved in biodegradation are active against a freshly applied radioactive molecule but are not efficient for the same substance entrapped in the soil. Weathered molecules by soil friction process in which the larger soil particles are broken into smaller ones results in the gradually desorption of the molecules with enhanced bioavalability [1]. Therefore it is a pitfall to conclude that the hazardous effects are only caused by pesticide concentrations in the soil [2].

The fate of pollutants in the soil depends on the soil properties and the physical chemical characteristics of these molecules. Essentially the uptake of pollutants in the soil can be understood to be the result of chemical attraction and bond strength. These processes in the soil are governed by a number of well described phenomena including: Van der Waals forces, hydrogen bonding, ion exchange, charge transfer mechanisms, lipophilic affinity, entrapment and covalent reactions to humic acids [3, 4]. In essence, pollutant molecules will move to the most attractive sites in the soil environment. Competition between plant roots, soil mesofauna, microbes and soil organic and inorganic components makes residue to bind in the most attractive site where residues will accumulate preferentially. Pollutants, such as pesticides, which are not held or bound to the living and non-living parts of the soil, will be leached through the soil, resulting in pollution of ground water, rivers and reservoirs or they will be volatilized.

#### **2.1. Where molecules adsorb**

In this section the factors that regulate the distribution of pollutants between the soil matrix and the biosphere including microorganisms, mesofauna and bioaccumulation in plants will be discussed. In aquatic environments, where polar water molecules predominate, lipophilic pollutants have affinity for organic matter and therefore always move toward the biosphere [5] (Figure 1). In this case the bioaccumulation factor depends on molecular size and correlates positively with a lipophilic character measured by the Kow value [6]. However, polarity on its own cannot explain to which compartment POP's move to in the soil.

Soils are a highly variable mixture of mineral and organic materials with living, dead and decaying biologic components. There is a lipophilic fraction as well that can adsorb lipophilic pollutants. The binding process is complex considering the diversity of compartments such as microorganisms, mesofauna and plants that compete for the uptake of lipophilic pollutants. Despite difficulties to standardize a methodology for lipid determination [7], this parameter is used to calculate the bioaccumulation factor [8, 9]. Soils can function as a filter when they adsorb the remaining residues, which then become unavailable for the biosphere. On the other hand low adsorption capacity pollutants remain bioavailable and can contaminate water, air, fodder plants, livestock and moving along the food chain up to humans. The soils may function as a filter or as a source of pollutants and this depends mainly on the kind of soil.

**Figure 1.** In aquatic systems the movement of lipophilic substance is unidirectional towards living forms, resulting in bioaccumulation

Despite enormous amounts of published scientific literature about bioaccumulation, the distribution of pollutants in the soil or biosphere is not well understood. Here the mechanisms of bioaccumulation in the soil environment will be discussed including the development of the proposed "preferential partition" concept.

#### *2.1.1. Lipophilic molecule uptake by microorganisms*

recalcitrant molecules that can not be controlled by remediation is the restriction of their use

The adsorption of molecules, due to their affinity to other chemical components of the soil matrix or soil biosphere, protects them from chemical degradation as well biodegradation. Non adsorbed molecules are bioavailable and therefore are exposed to degradation. Experi‐ ments with soils weathered for long time with ethylene bromide contamination in which was added 14C-ethylene bromide, shows strong biodegradation of the new applied radioactive substance after sixty days at the same time that weathered contamination substance remains in the same concentration [1]. Microorganisms involved in biodegradation are active against a freshly applied radioactive molecule but are not efficient for the same substance entrapped in the soil. Weathered molecules by soil friction process in which the larger soil particles are broken into smaller ones results in the gradually desorption of the molecules with enhanced bioavalability [1]. Therefore it is a pitfall to conclude that the hazardous effects are only caused

The fate of pollutants in the soil depends on the soil properties and the physical chemical characteristics of these molecules. Essentially the uptake of pollutants in the soil can be understood to be the result of chemical attraction and bond strength. These processes in the soil are governed by a number of well described phenomena including: Van der Waals forces, hydrogen bonding, ion exchange, charge transfer mechanisms, lipophilic affinity, entrapment and covalent reactions to humic acids [3, 4]. In essence, pollutant molecules will move to the most attractive sites in the soil environment. Competition between plant roots, soil mesofauna, microbes and soil organic and inorganic components makes residue to bind in the most attractive site where residues will accumulate preferentially. Pollutants, such as pesticides, which are not held or bound to the living and non-living parts of the soil, will be leached through the soil, resulting in pollution of ground water, rivers and reservoirs or they will be

In this section the factors that regulate the distribution of pollutants between the soil matrix and the biosphere including microorganisms, mesofauna and bioaccumulation in plants will be discussed. In aquatic environments, where polar water molecules predominate, lipophilic pollutants have affinity for organic matter and therefore always move toward the biosphere [5] (Figure 1). In this case the bioaccumulation factor depends on molecular size and correlates positively with a lipophilic character measured by the Kow value [6]. However, polarity on its

Soils are a highly variable mixture of mineral and organic materials with living, dead and decaying biologic components. There is a lipophilic fraction as well that can adsorb lipophilic pollutants. The binding process is complex considering the diversity of compartments such as

own cannot explain to which compartment POP's move to in the soil.

**2. How molecules can escape degradation in the environment**

or banning them altogether.

306 Applied Bioremediation - Active and Passive Approaches

by pesticide concentrations in the soil [2].

volatilized.

**2.1. Where molecules adsorb**

Soil microorganisms represent a large part of the living biomass but in general are not used for bioaccumulation studies since they cannot be separated from the soil to measure the pollutants. Nevertheless important information could be obtained from an experiment in which antibiotic resistant bacteria with bioaccumulated difocol were introduced into the soil [10]. Radioactive 14C-dicofol was bioaccumulated during the incubation of *Pseudo‐ monas fluorescens* (soil bacteria) resistant to kanamicyn and rifampycin. These bacteria can be measured in the soil using CFU counts with petri dishes containing the above men‐ tioned antibiotics able to suppress all other soil microorganisms. 14C-dicofol was incubat‐ ed and bioaccumulated in Pseudomonas fluorescens strains after which it was poured on top of a soil and subjected to a succession of simulated rainfall events. At the end of the simulated rainy period of 24h, 95% of the radioactive labeled insecticide remained in the upper 1cm of the soil, whereas 60% of the microorganisms had been transported 10 cm through the soil and were recovered in the leachate. Only 7% of the bacteria poured onto the soil remained within the first 1 cm of soil. In this experiment the acaricide dicofol moved away from the bacteria toward the soil particles [10] showing that it was more strongly attracted to the soil matrix than to the living cells. Therefore in this case the soil acted as a filter/sponge, protecting microorganisms, mesofauna, plant roots and prevent‐ ing ground water pollution. Molecules will move to the sites they find most attractive and in this case the soil matrix showed a higher uptake of the pesticide, thus reducing the dicofol content in the bacterial cell envelope. In organic matter rich soils, it is not un‐ common for many POP´s to show **preferential partition** towards soil organic matter ad‐ sorption rather than to plants or other living forms [11]. A good parameter to evaluate **preferential partition** of pollutants between soil and biosphere is to compare octanol/ water (Kow) ratios of each compartment. A higher octanol/water ratio (Kow) for organic matter than for the soil bacteria explains why the dicofol moves out of the cell and into the organic component of the soil [12, 13].

#### *2.1.2. Lipophilic molecule uptake by earthworms*

Earthworms, as a "living system" model, facilitate bioaccumulation studies in soil since they can be collected easily and analyzed for pollutant uptake. Papini and Andrea [14] working with simazine, a relatively non-polar (Kow 2-2.3) herbicide, [15, 16] and Paraquat, a highly polar herbicide, found that simazine did not bioaccumulate in the earthworm *Eisenia foetida* but Paraquat did. This result was the opposite of what was expected from the point of view that non polar substances bioaccumulate in the biosphere and polar substances do not. In a separate but similar study using an Argisol soil, the herbicide atrazine did not bioaccumulate in the earthworms *Pontoscolex corethrurus* either [17]. However, these results are not fully explained by the polarity of the pesticides. To interpret these results more precisely requires an under‐ standing of Kow as well as knowing the importance of organic matter in the soil. Soil organic carbon content (Koc) correlates positively with soil Kow and is an important factor to attract lipophilic substances [13, 18]. Given that Koc and Kow in general correlate positively, soil organic matter (OM) content can be used to select soils for study. Andréa and Papini used this method to compare how simazine and paraquat [19, 20] would behave in soil in the presence of the earthworm *Eisenia foetida* with different amounts of soil organic matter.

*ssp.* reduces bioaccumulation in the three species of earthworms. Similar experiments in the presence of *Curcurbita ovifera ssp.* showed a reduction as well as an increase of bioaccumulation in the different earthworm specie [21]. The authors observed that bioaccumulation in the plant C. pepo were enhanced with the three earthworm species which did not occur with *C. Ovifera* where only a slight increase was observed. In this set of experiments **preferential**

**Figure 2.** The movement of lipophilic pesticides in soil is influenced by the relative abundance of lipophilic sites in the

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An increase in the bioaccumulation in plants indicates higher bioavailability of this residue due to presence of the earthworm [22] which through chemolysis is able to change humic acid and increase the protein and carbohydrate moieties and degrade the carboxylic and aliphatic groups. In consequence the hydrophobic index HI = 0.0433 – 0.0811 in the soil decreases in the presence of earthworms and by *C. pepo* to 0.0231- 0.0286, a condition that reduces soil adsorp‐ tion and increases bioavailability and therefore bioaccumulation is enhanced in the plant [21]. Other data showed that the phytoextraction capacity of plants is related to the capacity of inorganic uptake from soil. Fertilizer amendment with N and P enhances phytoextraction and

Bioaccumulation of lipophilic substances such as chlordane is quite different between plant species, as observed by zucchini with a low and pumpkin with a high uptake [24]. These lipophilic substances are normally bioaccumulated in roots and only a small amount is translocated in a decreasing sequence to stems, leaves and fruits [21, 24, 25]. When Kow is higher than 5, plant uptake is considered to occur mainly via the air-to-plant route [26]. These data agree with the observations of Schnoor et al. [27] that plant uptake is very efficient for moderate hydrophobic organic chemicals with a Kow of 0.5 to 3. For a Kow higher than 3 these chemicals bind more and more strongly to the surface roots with decreasing translocation within the plant. However, translocation of chemicals such as terbuthylazine, with a Kow of 3, and atrazine can occur in high amounts [28, 29, 30]. Fairly soluble chemicals with a Kow lower than 0.5 are not sufficiently sorbed to roots and are not actively transported through plant membranes. Soil amendment with manure compost may reduce bioavailability by retaining the toxic organic chemicals in the organic matter and therefore reduce the hazardous effects [31] but the literature shows controversy data in which organic amendment can reduce adsorption of pesticides by increasing the desorption effects [32]. The increasing addition of sludge as the final disposal on soil introduces POPs in much higher amounts than air deposition [33].

**partition** occurs mainly toward plants and less to earthworms.

increases bioaccumulation in *C. Pepo* [23].

living and non living soil components.

Two soils, one with 12 g. L-1 organic matter (pH 5.7) and the other with 93 g. L-1 organic matter (pH 6.4), were treated with simazine and paraquat. Soils were incubated for 90 days with *Eisenia foetida* [19, 20] and the bioaccumulation coefficient factor (Bcf.) was determined. Simazine did not bioaccumulate (Bcf. 0.9) in the earthworms in the high organic matter soil [19]; however, in the low organic matter soil it did (Bcf. 6.9). With simazine, it seems likely that molecular polarity controlled the different distribution of pesticides between soil organic matter and biota. In the soil with low organic matter, **preferential partition** was toward earthworms that provided more attractive sites for the pesticide than the soil matrix. In the high organic matter soil, **preferential partition** was toward the soil organic matter, less polar than the earthworms and therefore chemically more attractive to lipophilic pesticides (Figure 2). The results at first appear to be at odds with one another, in terms of where the pesticide accumulated, until we recognize that pesticide movement is not necessarily unidirectional.

#### *2.1.3. Lipophilic molecule uptake between earthworms and plants*

Experiments with p,p`-DDE in soils with the earthworms *Ersenia fetida, Lumbricus terrestris* or *Apporectodea caliginosa* in the presence of different plant subspecies showed that *Curcurbita pepo*

ing ground water pollution. Molecules will move to the sites they find most attractive and in this case the soil matrix showed a higher uptake of the pesticide, thus reducing the dicofol content in the bacterial cell envelope. In organic matter rich soils, it is not un‐ common for many POP´s to show **preferential partition** towards soil organic matter ad‐ sorption rather than to plants or other living forms [11]. A good parameter to evaluate **preferential partition** of pollutants between soil and biosphere is to compare octanol/ water (Kow) ratios of each compartment. A higher octanol/water ratio (Kow) for organic matter than for the soil bacteria explains why the dicofol moves out of the cell and into

Earthworms, as a "living system" model, facilitate bioaccumulation studies in soil since they can be collected easily and analyzed for pollutant uptake. Papini and Andrea [14] working with simazine, a relatively non-polar (Kow 2-2.3) herbicide, [15, 16] and Paraquat, a highly polar herbicide, found that simazine did not bioaccumulate in the earthworm *Eisenia foetida* but Paraquat did. This result was the opposite of what was expected from the point of view that non polar substances bioaccumulate in the biosphere and polar substances do not. In a separate but similar study using an Argisol soil, the herbicide atrazine did not bioaccumulate in the earthworms *Pontoscolex corethrurus* either [17]. However, these results are not fully explained by the polarity of the pesticides. To interpret these results more precisely requires an under‐ standing of Kow as well as knowing the importance of organic matter in the soil. Soil organic carbon content (Koc) correlates positively with soil Kow and is an important factor to attract lipophilic substances [13, 18]. Given that Koc and Kow in general correlate positively, soil organic matter (OM) content can be used to select soils for study. Andréa and Papini used this method to compare how simazine and paraquat [19, 20] would behave in soil in the presence of the

Two soils, one with 12 g. L-1 organic matter (pH 5.7) and the other with 93 g. L-1 organic matter (pH 6.4), were treated with simazine and paraquat. Soils were incubated for 90 days with *Eisenia foetida* [19, 20] and the bioaccumulation coefficient factor (Bcf.) was determined. Simazine did not bioaccumulate (Bcf. 0.9) in the earthworms in the high organic matter soil [19]; however, in the low organic matter soil it did (Bcf. 6.9). With simazine, it seems likely that molecular polarity controlled the different distribution of pesticides between soil organic matter and biota. In the soil with low organic matter, **preferential partition** was toward earthworms that provided more attractive sites for the pesticide than the soil matrix. In the high organic matter soil, **preferential partition** was toward the soil organic matter, less polar than the earthworms and therefore chemically more attractive to lipophilic pesticides (Figure 2). The results at first appear to be at odds with one another, in terms of where the pesticide accumulated, until we recognize that pesticide movement is not necessarily unidirectional.

Experiments with p,p`-DDE in soils with the earthworms *Ersenia fetida, Lumbricus terrestris* or *Apporectodea caliginosa* in the presence of different plant subspecies showed that *Curcurbita pepo*

earthworm *Eisenia foetida* with different amounts of soil organic matter.

*2.1.3. Lipophilic molecule uptake between earthworms and plants*

the organic component of the soil [12, 13].

308 Applied Bioremediation - Active and Passive Approaches

*2.1.2. Lipophilic molecule uptake by earthworms*

**Figure 2.** The movement of lipophilic pesticides in soil is influenced by the relative abundance of lipophilic sites in the living and non living soil components.

*ssp.* reduces bioaccumulation in the three species of earthworms. Similar experiments in the presence of *Curcurbita ovifera ssp.* showed a reduction as well as an increase of bioaccumulation in the different earthworm specie [21]. The authors observed that bioaccumulation in the plant C. pepo were enhanced with the three earthworm species which did not occur with *C. Ovifera* where only a slight increase was observed. In this set of experiments **preferential partition** occurs mainly toward plants and less to earthworms.

An increase in the bioaccumulation in plants indicates higher bioavailability of this residue due to presence of the earthworm [22] which through chemolysis is able to change humic acid and increase the protein and carbohydrate moieties and degrade the carboxylic and aliphatic groups. In consequence the hydrophobic index HI = 0.0433 – 0.0811 in the soil decreases in the presence of earthworms and by *C. pepo* to 0.0231- 0.0286, a condition that reduces soil adsorp‐ tion and increases bioavailability and therefore bioaccumulation is enhanced in the plant [21]. Other data showed that the phytoextraction capacity of plants is related to the capacity of inorganic uptake from soil. Fertilizer amendment with N and P enhances phytoextraction and increases bioaccumulation in *C. Pepo* [23].

Bioaccumulation of lipophilic substances such as chlordane is quite different between plant species, as observed by zucchini with a low and pumpkin with a high uptake [24]. These lipophilic substances are normally bioaccumulated in roots and only a small amount is translocated in a decreasing sequence to stems, leaves and fruits [21, 24, 25]. When Kow is higher than 5, plant uptake is considered to occur mainly via the air-to-plant route [26]. These data agree with the observations of Schnoor et al. [27] that plant uptake is very efficient for moderate hydrophobic organic chemicals with a Kow of 0.5 to 3. For a Kow higher than 3 these chemicals bind more and more strongly to the surface roots with decreasing translocation within the plant. However, translocation of chemicals such as terbuthylazine, with a Kow of 3, and atrazine can occur in high amounts [28, 29, 30]. Fairly soluble chemicals with a Kow lower than 0.5 are not sufficiently sorbed to roots and are not actively transported through plant membranes.

Soil amendment with manure compost may reduce bioavailability by retaining the toxic organic chemicals in the organic matter and therefore reduce the hazardous effects [31] but the literature shows controversy data in which organic amendment can reduce adsorption of pesticides by increasing the desorption effects [32]. The increasing addition of sludge as the final disposal on soil introduces POPs in much higher amounts than air deposition [33]. Nevertheless lipophilic substances with a high octanol-water partition coefficient (log Kow) remain preferentially in soils and with little bioavailability they have low bioaccumulation in earthworms [8]. Radioactive atrazine applied on soil with low organic matter content previ‐ ously covered by cattle manure, showed a slower leachate speed compared to control but with a low retention capacity in the soil [34]. Soils modulate adsorptions and bioavailability and an inverse correlation occurs with a decrease of bioaccumulation in earthworms when Kow increases, which is different from an aqueous environment when there is a positive correlation between Kow and bioaccumulation [6].

#### *2.1.4. Cation bioaccumulation*

Many evidences indicate that the lipophilic character of soil organic matter is one of the most important factors for **preferential partition** of lipophilic substances toward soil; nevertheless this process can be carried out by chemical bonds such as ionic charged bonds of organic toxic chemicals. Below is a description of how **preferential partition** works between the soil and the living biosphere with cationic charged molecules. As far as the author know bioaccumu‐ lation in terms of cationic charged molecules between the soil and biosphere has not been reported in the literature before.

(molecular polarity) and availability of cation exchange sites (electrostatic bonds) in the soil

**Figure 3.** In preferential partition charged pesticides are attracted to anions between the abundance in soil and bio‐

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Ecotoxicology depends on soil organic matter. When the SOM is high the ecotoxicological effects are low and when the SOM is low the effects are high. In this latter case the residues remain bio-available and are intensely absorbed in the biosphere and therefore are hazardous to flora and fauna. Polarity and/or anion charge capacity of living organisms compete with the parameters in the soil organic matter. The highest attraction capacity of these different compartments defines the way in which molecules move in the environment. Molecules inserted in the soil matrix by "**Preferential Partition**" can be protected from hydrolysis,

All natural molecules can be biodegraded in the environment. An important constraint of xenobiotic biodegradation is the absence of microorganisms with efficient biodegradation capacity in a specific environment. Recently there has been an intense research effort to develop or transfer microbial efficient biodegradation genes to a microorganism that is adapted to a specific environment but lacks biodegradation capacity. Other constraints can occur when microorganisms do not enter in small soil microspores and therefore could not be present to

The link between bioavailable persistent substances and the spread of these substances in the environment, causing dissemination of hazardous effects, will be discussed in this section. Desorbed or non sorbed molecules are bio-available and can move into the food chain and can also get into ground/surface water or reach the atmosphere through volatilization and are thus be randomly disseminated in the biosphere (Figure 5) [36]. When persistent molecules are adsorbed in the soil they cause less hazardous effects than when they are bioavailable.

shows that the **Preferential Partition** concept can explain pesticide bioaccumulation.

oxidation/reduction reactions as well escape enzymatic action.

**3. Movement of persistent molecules in the environment**

**2.2. Microbial constraints of biodegradation**

promote biodegradation (Figure 4).

sphere.

Based on the hypothesis that polarity is the main factor controlling bioaccumulation, one would expect that a strongly polar pollutant, like paraquat, would be accumulated in the most polar parts of the soil. Thus in soils with low organic matter, one would expect no bioaccu‐ mulation in earthworms and paraquat would be bound within the soil matrix. Nevertheless, Papini and Andrea[14] found the opposite. **Preferential partition** moved this compound towards earthworms, depending on the amount of 14C-paraquat (1.2; 12 and 120 μg. a.i.g-1) applied to the soil [20]. To understand these results we have to note the importance of the relative abundance of charge-binding sites (attractive/exchange sites) in the biosphere and soil. In soils with low OM and low ion exchange capacities, the exchange sites, which sequester positive charged pesticides that gradually become saturated and consequently makes possible simultaneous available pesticides to bind anionic sites on or in earthworms [35]. This is a competition between the earthworms and attractive soil sites for paraquat. In this low SOM soil, paraquat bioaccumulation did not surpass Bcf 5 probably because the anionic sites on the surface of the earthworm cells were limited and already saturated (Figure 3).

In a high OM soil and with a low application of paraquat (1.2 μg a.i.g-1), the bioaccumulation factor (Bcf.) was 1.1 and increased with higher concentrations of applied paraquat up to Bcf. 3.8. With increasing paraquat concentrations (12 to 120 μg a.i.g-1) one would predict that as the soil charge sites gradually became saturated then gradually more paraquat would become attracted to the earthworms. From these experiments, we noted the predominance of electro‐ static binding in the soil and the importance of an abundance of exchange sites. In comparison, lipophilic attraction is driven by affinity without limits of concentrations which are different from the electrostatic bonds were the charged sites involve higher bond energy and therefore are predominant but have quantitative limitations with pesticides up to saturation with consequences in bioaccumulation. The correlation between Koc (soil organic matter) with Kow Persistence and Bioaccumulation of Persistent Organic Pollutants (POPs) http://dx.doi.org/10.5772/56418 311

**Figure 3.** In preferential partition charged pesticides are attracted to anions between the abundance in soil and bio‐ sphere.

 (molecular polarity) and availability of cation exchange sites (electrostatic bonds) in the soil shows that the **Preferential Partition** concept can explain pesticide bioaccumulation.

Ecotoxicology depends on soil organic matter. When the SOM is high the ecotoxicological effects are low and when the SOM is low the effects are high. In this latter case the residues remain bio-available and are intensely absorbed in the biosphere and therefore are hazardous to flora and fauna. Polarity and/or anion charge capacity of living organisms compete with the parameters in the soil organic matter. The highest attraction capacity of these different compartments defines the way in which molecules move in the environment. Molecules inserted in the soil matrix by "**Preferential Partition**" can be protected from hydrolysis, oxidation/reduction reactions as well escape enzymatic action.

#### **2.2. Microbial constraints of biodegradation**

Nevertheless lipophilic substances with a high octanol-water partition coefficient (log Kow) remain preferentially in soils and with little bioavailability they have low bioaccumulation in earthworms [8]. Radioactive atrazine applied on soil with low organic matter content previ‐ ously covered by cattle manure, showed a slower leachate speed compared to control but with a low retention capacity in the soil [34]. Soils modulate adsorptions and bioavailability and an inverse correlation occurs with a decrease of bioaccumulation in earthworms when Kow increases, which is different from an aqueous environment when there is a positive correlation

Many evidences indicate that the lipophilic character of soil organic matter is one of the most important factors for **preferential partition** of lipophilic substances toward soil; nevertheless this process can be carried out by chemical bonds such as ionic charged bonds of organic toxic chemicals. Below is a description of how **preferential partition** works between the soil and the living biosphere with cationic charged molecules. As far as the author know bioaccumu‐ lation in terms of cationic charged molecules between the soil and biosphere has not been

Based on the hypothesis that polarity is the main factor controlling bioaccumulation, one would expect that a strongly polar pollutant, like paraquat, would be accumulated in the most polar parts of the soil. Thus in soils with low organic matter, one would expect no bioaccu‐ mulation in earthworms and paraquat would be bound within the soil matrix. Nevertheless, Papini and Andrea[14] found the opposite. **Preferential partition** moved this compound towards earthworms, depending on the amount of 14C-paraquat (1.2; 12 and 120 μg. a.i.g-1) applied to the soil [20]. To understand these results we have to note the importance of the relative abundance of charge-binding sites (attractive/exchange sites) in the biosphere and soil. In soils with low OM and low ion exchange capacities, the exchange sites, which sequester positive charged pesticides that gradually become saturated and consequently makes possible simultaneous available pesticides to bind anionic sites on or in earthworms [35]. This is a competition between the earthworms and attractive soil sites for paraquat. In this low SOM soil, paraquat bioaccumulation did not surpass Bcf 5 probably because the anionic sites on the

surface of the earthworm cells were limited and already saturated (Figure 3).

In a high OM soil and with a low application of paraquat (1.2 μg a.i.g-1), the bioaccumulation factor (Bcf.) was 1.1 and increased with higher concentrations of applied paraquat up to Bcf. 3.8. With increasing paraquat concentrations (12 to 120 μg a.i.g-1) one would predict that as the soil charge sites gradually became saturated then gradually more paraquat would become attracted to the earthworms. From these experiments, we noted the predominance of electro‐ static binding in the soil and the importance of an abundance of exchange sites. In comparison, lipophilic attraction is driven by affinity without limits of concentrations which are different from the electrostatic bonds were the charged sites involve higher bond energy and therefore are predominant but have quantitative limitations with pesticides up to saturation with consequences in bioaccumulation. The correlation between Koc (soil organic matter) with Kow

between Kow and bioaccumulation [6].

310 Applied Bioremediation - Active and Passive Approaches

*2.1.4. Cation bioaccumulation*

reported in the literature before.

All natural molecules can be biodegraded in the environment. An important constraint of xenobiotic biodegradation is the absence of microorganisms with efficient biodegradation capacity in a specific environment. Recently there has been an intense research effort to develop or transfer microbial efficient biodegradation genes to a microorganism that is adapted to a specific environment but lacks biodegradation capacity. Other constraints can occur when microorganisms do not enter in small soil microspores and therefore could not be present to promote biodegradation (Figure 4).
