| Soil aggregates are formed through binding of soil minerals with organic matter and/or soil organisms with varying strenghs as a process of interaction of mineral, organic matter and biota in soils.In response to soil organic matter turnover, soil aggregates may experience changes both in size fractions and chemical compositions. Soil aggregates of different fractions in size have been considered as the most basic and functional units of soil entity. Soil environmental processes in the filed may take place on the soil aggregate-water interface. Therefore, it is important to estimate environmental functions of size fractions to contaminations.Therefore, in this study, size fractions were separated from undisturbed paddy soils in Tai Lake region, China. And sorption and desorption behavior and factors of heavy metals and organic contaminations by size fractions were studied, in order to elucidate the transport and fate of contaminations by size fractions and well understand soil environmental process in field. The main results were as follows:1. Sorption of heavy metal (Cd2+) by different size fractions well fitted the Freundlich equation, with the highest sorbed capacity of clay fraction and coarse sand fraction while least of silt sand fraction. However, silt and fine sand fraction had highest contribution rates. Sorption capacities of Cd2+ by size fractions exceed to bulk soils, which demonstrated chemical studies with traditional treatments might lowly estimate transport and fate of heavy metals. Sorption capacities of Cd2+ increased in company with soil organic matter contents, and well correlated with on the contents of oxyhydrates and CEC in the size fractions. When desorped with CaCl2, compared to other size fractions, only 8.4% Cd2+ was desorbed from the clay fraction, indicating the clay fraction has the strongest capacity for Cd2+ sorption. 2. Heavy metal (Cu2+) sorption by different size fractions from two types of paddy soils fitted well the Freundlich and Langumir models, with the greatest sorption capacities of coarse sand and clay fraction. However, fine and coarse sand fractions were the dominator contributors to determine Cu2+ sorption of soils. Cu2+ sorption relations between size fractions and bulk soils depended on soil types. Cu2+ sorbed rates reached 100% by size fractions under lower Cu2+ concentrations, however, decreased in company with increased Cu2+ concentrations, especially in silt sand fraction from Huangnitu. Clay and coarse sand fraction could highly fix Cu2+, compared to other size fractions. Sorption capacities of Huangnitu were greater than Wushantu with the same size fractions. The sorption capacities of size fractions well correlated with the contents of organic matter and free iron oxyhydrates, however, SOM was the dominant factors affected Cu2+ sorption. When desorped with CaCl2, the clay fraction and coarse sand fraction had the Cu2+ strongest capacitis compared to other fractions.3. For Soil H, Cu2+ sorption capacity of the DCB-treated size fractions was decreased by 5.9% for fine sand fraction and by 40.4% for coarse sand fraction in comparison to a slight decrease by 2.9% for the bulk sample. However, Cu2+ sorption capacities of the H2O2-treated fractions were decreased by over 80% for the coarse sand fraction and by 15% for the clay-sized fraction in comparison to by 88% for bulk soil. However, for Soil W, Cu2+ sorption capacity of the DCB-treated size fraction was decreased by 30% for the coarse sand fraction and by over 75% for silt sand fraction in comparison to 44.5% for the bulk sample. While, Cu2+ sorption capacities of the H2O2-treated fractions were decreased by only 2.0% for the coarse sand fraction and by 15% for the fine sand fraction in comparison to by 3.4% for bulk soil. Small changes in desorption rates of size fractions existed after DCB treatment for both soils. However, Cu2+ desorption rates were much increased in the H2O2-treated samples over 80% except the clay-sized fraction (only 9.5%) for Soil H. While removal of SOM with H2O2 tended to increase desorption rate, being biggest at 5.2% for the silt-sized fraction for Soil W, decreased trends could be observed of Cu2+ retention capacity of size fractions with DCB- and H2O2-treated. Particularly for Soil H, there hardly remained Cu2+ retention capacity by size fractions after H2O2 treatment except for clay-sized fraction. These findings supported again the dominance of the coarse sand fraction in sorption of metals and the preference of absorbed metals bound to SOM in differently stabilized status among the size fractions. Thus, enrichment and turnover of SOM in paddy soils may have great effects on metal retention and chemical mobility in soil environment of paddy soils.4. Phenanthrene contents were enhanced in size fractions with employ of CF but decreased with CFM and CFS. In addition, similarly distribution characters were found in the size fractions after fertilization treatments, with more phenanthrene contents in clay and coarse sand fraction similarly to heavy metal distributions in size fractions. However, fine and coarse sand fraction had highest contribution rates. No effects on phenanthrene mobility of size fractions with long-time fertilizations, because absolutely no phenanthrene dissolve had been found with electrolyte solution.5. Sorption isotherm of phenanthrene by size fractions well fitted Freundlich equation, with the largest sorption capacities of the clay and the coarse sand fraction similarly to that of heavy metals. Fine and coarse sand fraction has the greatest contributor rates. However, near values of logKoc exist among different size fractions, which demonstrated SOM was the dominate factors affected phenanthrene sorption. Decreased sorption capacities of phenanthrene coupled with CF, but opposite phenomena with CFS and CFM. After removal of SOM and primary sorbed Cu2+ by size fractions, the sorption capacities of phenanthrene decreased and increased respectively. Irreversible sorption phenomena exist between bulk soils under different fertilization and the size fractions.6. Sorption of heavy metal and organic pollutants by different size fractions from paddy soils well fitted Freundlich equation, while Langumir could also be used for heavy metal sorption. In addition, desorption of heavy metal and organic pollutants by size fractions well fitted Freundlich and liner equation, respectively. Sorption capacities of heavy metal and organic pollutant by size fractions from Huangnitu exceed to bulk soils, with the highest sorbed capacity of clay fraction and coarse sand fraction. However, silt and fine sand fraction had highest contribution rates to heavy metals, which fine and coarse sand fraction to organic pollutants, which demonstrated different functional units to heavy metal and organic pollutants. On the other side, lower desorption rates implied special sorption was the dominate form of heavy sorption by size fractions. Comparatively, irreversible sorption phenomena exist between bulk soils under different fertilization and the size fractions. SOM was the dominant factor affected on sorption behavior to both heavy metal and organic pollutant by size fractions. Phenanthrene contents were decreased in size fractions with employ of organic fertilization, but enhanced sorption of adscititious organic pollutants, which revealed important function of biology. The same characters and dominate factors by size fractions exist between different type paddy soils, only differences in sorption capacities. However, sorption behavior of organic pollutants by size fractions from different type of paddy soils needs further study. |
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