**1. Introduction**

Some organic pesticides are the most widespread POPs (Persistent Organic Pollutants) used in agriculture worldwide. Other POPs such as volatilized industrial pollutants as well as pollutants from fuel combustion can also contaminate large areas due to dispersion by wind and rainfall. Non biodegradable molecules persist in the environment and the soil serves as a large sink for them. These pollutants are able to move in the environment and in low concen‐ trations over a long period of time, but through bioaccumulation they can have hazardous effects on the biota of the region. This article focuses mainly on pesticides and some other POPs that contaminate large areas. The chemical characteristics of persistent molecules that are resistant to biodegradation will be discussed in the context of the environmental conditions that enhance persistence.

Adsorption or not of a molecule in the soil is a key step to define bioavailability. The concept of "preferential partition" helps us understand the competitive mechanisms between the soil and biosphere. Bioaccumulation occur when concentrations in the biosphere are much higher than those found in the surrounding environment. The uptake of persistent molecules to levels of high concentrations can have hazardous effects on flora and, through the food chain, on fauna and human health.

Xenobiotics not adsorbed by the soil can become mobile and when the molecule is persistent it may be dispersed by water or air through the biosphere, polluting large geographical areas and affecting the biodiversity of the flora and fauna. A brief overview of this dispersion will be necessary to show how recalcitrant molecules in the environment make remediation inefficient. The application of pesticides over large areas can only be sustainable if their molecules biodegrade naturally. Remediation can be applied usefully to accidents in restricted areas but it is not feasible over large areas due to the high costs. The only efficient policy for

© 2013 Langenbach; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

recalcitrant molecules that can not be controlled by remediation is the restriction of their use or banning them altogether.

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

Persistence and Bioaccumulation of Persistent Organic Pollutants (POPs)

http://dx.doi.org/10.5772/56418

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

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

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‐

bioaccumulation

the proposed "preferential partition" concept.

*2.1.1. Lipophilic molecule uptake by microorganisms*

as a filter or as a source of pollutants and this depends mainly on the kind of soil.
