**2.1. Allophane microstructure**

pollutant for crops and water resources, and (3) the efficiency of the remediation process. The second factor is past and present **farming practices** through their effects on (1) the stock of pollutant and its components in the soil and its distribution at the plot and regional scales, and (2) the diffusion of the pollutant in the different compartments of the environ‐ ment. These two factors determine the contributing areas and their pollutant stock, the po‐ tential availability and fluxes of the pollutant, and hence its potential transfer to different

The studied soils come from Guadeloupe (16°15 N, 61°35 W) and Martinique (14°40 N, 61°00 W) in the French West Indies. These volcanic islands rise to 1467 m and 1388 m elevation respectively. Rainfall is high and ranges from 1000 to 10 000 mm/year depending on the ele‐ vation and geographical area. All primary minerals of andesitic rocks are weathered, so that soils have a high content of secondary minerals (clays): halloysite for nitisol, halloysite and Fe-oxihydroxides for ferralsol, and allophane for andosol, the three main soil types contami‐

These volcanic soils have a high infiltration capacity (saturated hydraulic conductivity greater than 60 mm h−1) [1, 10]. However in the ''clay'' matrix of andosols, there are pores smaller than 1 micron, where water and solute transfers are slow. All these soil types are acidic (pH = 4.5 -6), which prevents clay dispersion and sheet erosion [1]. Among these soils, the carbon content of andosol is particularly high, which may influence the retention of the

Indeed all soils are not equivalent in terms of pesticide contamination and in their ability to transfer the pollutant to water and to plants [17, 18]. For example, although andosols are highly polluted [1, 10, 16], data show they release less pesticide to percolating water and crops than other soils [1, 19]. In the case of CLD, one explanation for the retention effect re‐ ported in the literature is the high organic content of these soils and the high affinity of the

environmental compartments.

616 Environmental Risk Assessment of Soil Contamination

**Figure 1.** The chlordecone molecule (C10Cl10O)

**2. Influence of clay microstructure**

nated in French West Indies [1, 10, 16].

compound (stock) and its availability.

pesticide for soil organic carbon [1, 8].

Figure 2 shows the pore volume and specific surface area as a function of the allophane con‐ tent of a set of andosols. There was a clear increase in these two textural features with an increase in allophane content, showing that allophane clay favors larger porous features. The pore volume and the specific surface area were well correlated with allophane content (respectively P<0.0001 r2 =0.87and P <0.0001, r2 =0.80). The specific surface area was as high as 180 m2 .g-1 and pore volume close to 2.5 cm3 .g-1. This combination of high specific surface area and large pore volume suggests that the porous structure is made up of both microand mesopores.

**Figure 2.** Pore volume (empty circles, ○) and specific surface area (black squares, ∎) versus allophane content

Figure 3 shows the structure of amorphous clay in comparison with classical phyllosilicate clay: kaolinite or halloysite. These micrographs confirm the spongy structure of allophane clay.

The morphology of the allophane aggregates is peculiar [21]. Allophane has a very open structure made up of aggregated small particles (3-5 nm) that form clusters of around 10-20 nm. The clusters can stick together and form larger and larger aggregates up to ~ 100 nm in size. In comparison, the plate-like particles of phyllosilicate clay are 300-1000 nm in size (Figure 3). This aggregation mechanism is in agreement with results in the literature [22, 24].

**Figure 3.** Scanning electronic micrographs showing the structure of phyllosilicate clay and amorphous clay from [23]

Wada [22] describes allophane particles as nearly spherical, with diameters ranging from 3-5 nm. The aggregation mechanism corresponds to a fractal morphology. Several authors as‐ sumed that allophane could have a fractal structure [20, 22]. The structure of allophane can be studied at nanoscale using scattering experiments to quantify the fractal features of the soil samples. Small angle X-ray scattering (SAXS) experiments make it possible to calculate the fractal dimension *Df* (which expresses the compactness and tortuosity of the clusters) and the extent of the fractal aggregates (ξ) [25]. The fractal extent can be considered as the size of the tortuous "nano-labyrinth". SAXS curves [26] show that the fractal dimension *Df* is constant (2.5-2.7). Table 1 lists changes in ξ versus allophane content. Our results showed that the size of the fractal labyrinth increased with an increase in allophane content (P=0.001 and r2 =0.71).


**Table 1.** Fractal range (ξ) versus allophane content

These data (high specific surface area and pore volume, and fractal features) describe a high‐ ly tortuous microstructure and small mesopores, suggesting that accessibility inside the clay microstructure is reduced.

#### **2.2. Pesticide sequestration in allophane**

Allophane clay has a spongy structure comprising aggregated small particles that form a tortuous network with small pores. This peculiar structure influences the concentration of pesticide in the soil.

Figure 4 shows the marked increase in CLD concentration in soils with increasing allophane content (P<0.0001 and r2 =0.807). This finding confirms previous data in the literature [1, 16, 27] stating that allophanic soils are more contaminated than other kinds of tropical soils,

Diagnosis and Management of Field Pollution in the Case of an Organochlorine Pesticide, the Chlordecone http://dx.doi.org/10.5772/57263 619

**Figure 4.** Soil CLD contamination versus allophane content. CLD data have a confidence interval of 30%

which contain the usual crystalline clays (like halloysite and kaolinite). Figure 4 also clearly shows the link between CLD contamination of the soil and the allophane content of the soil and one can thus assume that the pesticide retention properties of the soil are partially de‐ pendent on the features of allophane.

Another interesting result of our previous study was that for similar soil CLD contents, crops cultivated on allophanic soils were much less contaminated than the same crops culti‐ vated on soils containing classical clays, [26]. We calculated the mean soil to plant transfer (expressed in µg.kg-1 of fresh matter / µg.kg-1 of dry soil), for different crops. Table 2 con‐ firms that, whatever the crop concerned [26], CLD soil to plant transfer was always higher in halloysite soils than in allophanic soils. For the three crops studied (lettuce, yam, and das‐ heen) the ratio was close to 3.


**Table 2.** Mean CLD transfers from halloysite and allophanic soils to lettuce, yam and dasheen, expressed in µg.kg-1 of fresh matter / µg.kg-1 of dry soil (standard deviation in brackets) [26]

#### **2.3. Trapping mechanism in allophane**

Wada [22] describes allophane particles as nearly spherical, with diameters ranging from 3-5 nm. The aggregation mechanism corresponds to a fractal morphology. Several authors as‐ sumed that allophane could have a fractal structure [20, 22]. The structure of allophane can be studied at nanoscale using scattering experiments to quantify the fractal features of the soil samples. Small angle X-ray scattering (SAXS) experiments make it possible to calculate

**Figure 3.** Scanning electronic micrographs showing the structure of phyllosilicate clay and amorphous clay from [23]

and the extent of the fractal aggregates (ξ) [25]. The fractal extent can be considered as the size of the tortuous "nano-labyrinth". SAXS curves [26] show that the fractal dimension *Df*

constant (2.5-2.7). Table 1 lists changes in ξ versus allophane content. Our results showed that the size of the fractal labyrinth increased with an increase in allophane content (P=0.001

Allophane (%) 0 3 5 8 10 12 13 15 18 22 ξ ( nm) 0 12 23 22-32 18 23-35 23 35 34-45 42-60

These data (high specific surface area and pore volume, and fractal features) describe a high‐ ly tortuous microstructure and small mesopores, suggesting that accessibility inside the clay

Allophane clay has a spongy structure comprising aggregated small particles that form a tortuous network with small pores. This peculiar structure influences the concentration of

Figure 4 shows the marked increase in CLD concentration in soils with increasing allophane

27] stating that allophanic soils are more contaminated than other kinds of tropical soils,

=0.807). This finding confirms previous data in the literature [1, 16,

(which expresses the compactness and tortuosity of the clusters)

is

the fractal dimension *Df*

618 Environmental Risk Assessment of Soil Contamination

**Table 1.** Fractal range (ξ) versus allophane content

**2.2. Pesticide sequestration in allophane**

microstructure is reduced.

pesticide in the soil.

content (P<0.0001 and r2

and r2

=0.71).

The results shown in Figure 4 and Table 2 may appear contradictory because one would ex‐ pect allophanic soils, which are more contaminated, to strongly pollute cultivated vegeta‐ bles. One explanation for the observed effect is that CLD is trapped in the microstructure of the allophane clay, thus reducing its transfer from the soil to the plant. The influence of soil allophane content on CLD retention is the signature of the peculiar microstructure of the al‐ lophane aggregate. The spongy structure influences transport inside the allophane aggre‐ gates. The SAXS data made it possible for us to propose a mechanism for the retention of pesticides in allophanic soils and also for the limited release of CLD to crops and water re‐ sources [1, 19]. At the scale of the allophane, accessibility is difficult because of the fractal structure and small pore size of allophane clay. CLD transfers within the soil depend on hy‐ draulic conductivity (*K*) and diffusion processes (*Di*) in the porous microstructure. The frac‐ tal structure allows an approximation of *K* and *Di* at the aggregates scale *l* [26] through the following equations:

$$\mathbb{K}\left(l\right) \approx \left[1 - \left\lfloor l/a \right\rfloor^{\text{D}\left(-3\right)}\right] l^2 \quad \text{and} \text{ Di} \approx \left(1 - \left(l/a\right)^{\text{D}\left(-3\right)}\right) \left\lfloor 1 - 2 \right\rfloor \left\{ 2 - \left(l/a\right)^{\text{D}\left(-3\right)} \right\} \left(l/a\right)^{\text{D}\left(-3\right)} l^{\text{D}2} \right\}. \tag{1}$$

**Figure 5.** Relative K (○) and Di (▲) versus the scale length, l (nm)

We calculated the transport properties (hydraulic conductivity and diffusion, inside the allo‐ phane fractal aggregate, i.e. between 3 and 60 nm (Table 2). Figure 5 shows changes in *K* and *Di* normalized to *K* and *Di* at *l* = 60 nm. Hydraulic conductivity decreased by 4 orders of magnitude and *Di* decreased by 20 orders of magnitude when *l* decreased from 60 to 4 nm.

The very low calculated *Di* and *K* suggest that CLD is trapped in the porosity. The trapping mechanism is favored by the large size of the fractal labyrinth. The higher the allophane content, the bigger the labyrinth and the higher the retention. Like nanoporous materials [28], the paradox of allophane clay is that it has large porosity but poor transport properties. In these fractal structures, possible reactions with chemical or biological species that could extract the pesticides are thus hindered; the pesticide remains trapped inside allophane clay and cannot be extracted.
