| Hexachlorocyclohexane (HCH) and dichlorodiphenyltrichloroethane (DDT), which are recognized internationally as persistent organic pollutants (POPs) containing chlorine atoms, have caused serious negative influence on agricultural production, agricultural products quality safety and human health. HCH and DDT residues in soils are prone to cause the process of aging or solidifying, which not only result in decreased bio-availability and extractability, but also has great and even irreversible impacts on environment. At present, how to remove HCH and DDT safely and effectively from aging soils to ensure agricultural activities safety and sustainable utilization of land resource, have become an important research focus in the soil and environmental sciences. Phytoremediation, a new technology developed recently for the remediation of soil contaminated by organic chemicals, is thought as one of the most prospective practical method due to its high efficiency, security and economy properties. For these reasons, there historical residues of aging HCH and DDT were selected as the research objects in this paper, the OCPs (DDT and HCH) removal mechanisms of the farmland plants in-situ restoration technologies, the uptake and conversion of OCPs in plants and the degradation behaviors of OCPs in the plant rhizosphere soil were studied, and these may be of important theoretical and practical significance for finding new functional plants which can remove organic compounds from soils, indicating the degradation regularity of aging organic pollutants in the soils, utilizing limited land resources rationally, and protecting human health or the environment.On the basis of the theories and methods of Soil Science, Plant Science, Biogeochemistry, Pollution Ecology, and the new phytoremediation techniques, this work regards the transportation and transformation of HCH and DDT in soil-plant systems as key idea, and aims at removing ageing HCH and DDT which are historical residues. Taking the urban-rural conjunctive region of Hohhot as research regions with higher population and cultivation density, and land use function change, simulation and field experiments of plant self-remediation and food safety are carried out to study the processes and mechanism of phytoremediation in the soils contaminated by ageing pesticides. Based on the selection of functional plants, the interface migration of HCH and DDT in soil-plant system and the stress resistance of residual contaminant in plants are further discussed. The combined forms and the degradation regularity of ageing HCH and DDT in soils and in the surface or tissues of a living are characterized using the multi-level extraction technology. Further, the selected plants removal abilities for ageing HCH and DDT in situ soils were evaluated considering the difference between the potted and field experiments, which showed a new way on pesticides removal in-situ by phytoremediation. The main conclusions are as following:1) It is different that the soils used in all potted and field experiments were contaminated by ageing HCH and DDT in this work. In terms of research ideas, the results and conclusions would be more consist with the field conditions compared with previous studies feeding HCH and DDT on soils.2). Wheat and corn were selected as enriched plants according to the coefficient and capacity of bioaccumulation of the livings in the field with high residues of HCH and DDT. It maybe find a new way to phytoremediation for large scale soils polluted by ageing HCH and DDT, and develop the selection of plant species for phytoremediation.3). It was found that the interaction between plants rhizosphe and microorganism played the key role in phytoremediation mechanisms, which were derived from remediation parameters measured by60days potted experiments on the basis of mass conservation principle. While the contributions of absorption and degradation were less than1%.4). The migration and transformation of ageing HCH and DDT in soil-plant systems were quantified based on potted experiments. The results revealed the antioxidant activity of SOD, POD and CAT enzymes of wheat and corn can be inordinately activated to eliminate HCH and DDT toxicity; the selection absorptions of the two plants on HCH and DDT were impacted by the soil pollution levels and the roots absorption. The HCH concentration order in plants was root> leaf> stem, while DDT root> stem> leaf. The concentrations of HCH and DDT in stems and leaves showed an decreasing trend with plant height.5). The reduction effects of plants were simulated in vigorous growing stage in the rhizosphere soils polluted by ageing HCH and DDT, and the mechanism of fractions transformation of ageing HCH and DDT in rhizosphere soils were characterized by sequential extraction technique. The results illustrated that the cultivation of plants(wheat and corn) accelerate the reduction of extractable OCPs in the rhizosphere soil, and these reductions were simulated with ageing HCH and DDT activation and total concentrations reduction. It was found that ageing HCH and DDT combined with humic and fulvic acid in rhizosphere soil can transform to extracted fractions in process, and the concentrations variations of hexane-extracted HCH and DDT can effectively characterize the bio-availability of HCH and DDT in wheat or corn.6). Field experiments showed that the remediation effects on ageing OCPs were obvious after planting, and the bioremediation efficiency presented time accumulation process. Generally, the total extractable OCPs in the soils decreased22.6%and23.1%respectively after wheat and corn planting, and the hexane-extractable OCPs decreased32.7%for wheat planting and50.2%for corn planting, respectively. The accumulation of OCPs by wheat and corn experienced stages characterized a rapid increase and a stable absorption, and the remediation ability on soil pollutants are different in various growth stages. In order to improve the comprehensive remediation effects, the harvest periods of crops should be controlled reasonably. |
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