Phytoremediation

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The uptake of pesticides by plants depends on the physicochemical properties of the compound, mode of application, soil type, climatic factors and plant species. Once in the roots, the chemical may be translocated to shoot via xylem. The permeation from plant roots to xylem is optimal for moderately hydrophobic molecules, with a log Kow between 0.5 and 3.5. More hydrophobic chemicals tend to bind with lipid membranes or oil possibly present in plant roots. Translocation of non-ionic pesticides thus varies greatly between plant species and depends on the properties of the chemical. Therefore, the uptake and translocation of hydrophobic compounds (log Kow > 4) is limited, and consequently so is their phytodegradation. On the other hand, the transformation of pesticides by rhizosphere microorganisms could result in metabolites more efficiently absorbed and translocated by plants. Thus any factor enhancing rhizospheric microbial activity should also increase the overall efficiency of pesticide phytoremediation.

With regards to HCH isomers, phytoextraction should not be favoured, since they are hydrophobic chemicals, theoretically bound to soil particles and also to plant roots, thus preventing plant uptake [61-64]. However, it has been recently shown that the predicted partition of lindane with lipids using the log Kow is notably lower than the measured sorption in roots and shoots of ryegrass and wheat seedlings, due to underestimation of the plant lipid contents and to the fact that octanol is less effective than plant lipids as a partition medium [65].

Nevertheless, the persistence of all isomers was found to be lower in cropped plots of maize (Zea mays), wheat (Triticum sp.) or pigeon pea (Cajananus cajan) than in uncropped plots, therefore sustaining the idea of remediation of polluted sites with plants [66]. Revision of available literature concerning contamination of the aerial part of plants reveals that the a, P and y isomers have been detected in many plants, including Lactuca sativa (lettuce), Sesamum indicum (sesame), Hydrilla verticillata (hydra), Lagernia siceraira (bottle gourd), Memordica charantia (bitter gourd), Luffa cylindrical (sponge gourd), Citrullus varifistulosus (tinda punjab), Spinacia oleracea (spinach) and Brassica campestris (rape). These species were not selected for testing in hydroponics however, since the detected residues were due to a direct contact with lindane, and not as a result of translocation from roots to shoots. Barriada-Pereira et al. [67] also attributed the presence of lindane in the shoots of Rubus ulmifolius and Paranthropophytia to atmospheric deposition and not translocation. However, chilli (Capsicum annuum) and coriander (Coriander sativum) cultivated in lindane-contaminated soil were reported to contain y-HCH residues in their aerial parts [43]. This fact was considered as a clue for the selection of plants able to take up lindane from a hydroponic experimental system. Within 9 days, lindane concentration in the medium decreased by 70% with chilli and 86% with coriander (Figure 5). Adsorption on roots of chilli and coriander accounted for 29% and 40% of y-HCH disappearance, respectively. The 23% and 30% remaining loss was called "plant effect", which could include the increasing of pH from 5.0 to 6.8, leading to enhanced hydrolysis, and a possible uptake of y-HCH in unknown quantities with the transpiration flux [43, 68]. However, the translocation of lindane from roots to shoots should be low, due to lindane hydrophobicity, unless special molecules are produced by plants, able to increase the apparent aqueous solubility of hydrophobic pollutants, such as those shown by Campanella and Paul [69] for dioxin absorption by zucchini (Cucurbita pepo L.).

In further experiments, vetiver plants were grown in Hoagland solution with 14C lindane (2 mg/l). At the end of 30 days, it was observed that 12% of lindane disappeared from the solution. Since lindane was found to accumulate in vetiver roots where essential oil is produced, an attempt was made to use a plant, which has oil in the shoot as well. Lemon grass (Cymbopogan citrates) was thus grown in Hoagland solution for 19 days in the presence of 14C lindane, but again more 14C residues were found in roots (11.4%) than in shoots (4.2%). All the plants tested for uptake of 14C lindane, Iris sp., Sesuvium portulacastrum, Zea mays and Cymbopogan sp. showed that 14C residues were retained in roots and there was no significant translocation to shoots.

Another study has shown that the concentration of lindane in ryegrass, cultivated in hydroponics, slowly increases with uptake time to reach a plateau after a few days, indicating that plant metabolism and formation of bound residues are minimal in such a system [70]. Other authors [71, 72] aimed to evaluate the bioaccumulation of HCH isomers in plants growing on areas surrounding a production centre and to investigate

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