Environmental Fate And Ecologic Effects Of Typical Pollutants In Agricultural Soils

Soil is a major sink for pollutants released to the environment. In soil, pollutants could transport and transform in components and interfaces, threatening the safety of soil, creature, atmosphere and groundwater. Reactions and processes of pollutants in soil, plants, and other individual components or interfaces, stretching from tops of plants to the saturated aquifer, have been characterized to some extent. But few studies have examined behaviors of pollutants in agricultural field as a whole system, especially studies carried outdoor for long period. Performed over years in outdoor lysimeters with rice-wheat rotation planting, our studies offered a systemic insight into the fate and ecologic effect of PBDEs (BDE-209), nanomaterials (nano-ZnO and nano-TiO2) and PAHs (phenanthrene and naphthalene) in agricultural soil. Gaseous losses, leaching losses, residue in soil, vertical transport, plant uptake were all quantified. Differences in soil enzymes between blank soil and spiked soil over years aging were also assessed to evaluate their actual effects on soil quality. The main findings arethe followings:1. BDE-209was demonstrated to be persistent with low degradation rate in field, and the half-life of BDE-209was calculated to be12years. By restraining plant biomass (by8.3%for overgroud part, by5.2%for spike), elevating the urease activity and inhibiting the protease activity, BDE-209still had influences on crop growth and soil quality after4years of spiking. BDE-209in the soil decreased mainly through leaching losses. Among the debrominated congeners (BDE-28,47,99,100,153,154,183), BDE-47, BDE-153and BDE-183contributed the highest percent in soil. BDE-209and seven lower brominated PBDEs were all detected in plants. There might be further debromination of BDE-209inside plants and low brominated congeners were more readily to be taken up by plants for lower Kow. Vertical trend of BDE-209over years showed its significant vertical migration in the soil column and its net average migration rate was calculated to be1.54mg·m-2·yr-1.2. Ecological effects of the two naopartciles (TiO2, ZnO) were evaluated for short-term and long-term (2.5yrs). In early period, both of the nanoparticles reduced the biomass of wheat. The mechanisms of toxication caused by the two NPs differed in this study:the toxicity of the TiO2NPs might result from the presence of the NPs in cells or accumulated on the cell walls, and the phytotoxicity of ZnO NPs to wheat was due to the dissolved Zn. The TiO2nanoparticles were retained in the soil for long periods and primarily adhered to cell walls of periderm cells. Some small-sized TiO2NPs penetrated through the cell wall, which were dark particles found in cortex cells. The ZnO nanoparticles dissolved in the soil, thereby enhancing the uptake of toxic Zn by wheat. The applied NPs clearly affected the soil environment in early period, as evidenced by the changes in soil enzyme activities, but after2.5years the changes were unsignificant. After2.5years, there was no difference of crop biomass between growth in control and soil spiked with nano-ZnO, but crop growth in soil spiked with nano-TiO2was increased.3. Four years after spiking,17.7%and9.6%of14C-chemical applied were detected in soil spiked with naphthalene and phenanthrene, respectively, and1.3%of C-chemical applied was found as parent compound. Residual concentration of PAHs in soil can not be ignored, restraining the crop growth and grain yeild. Residue of naphtalene in soil after4years has no significant effect on soil enzyme activities, but phenathrene residue still affected the soil quality by influencing enzyme activities.4. Colloid-facilitated transport of PAHs found in lysimeter experiment was further investigated. Soil column experiments and adsorption test were carried out to investigate release of colloid and its transport of phenanthrene in vadose zone. Colloid particles of different sizes were of different elemental composition and also had different adsorption abilities of phenanthrene. Colloid particles with smaller size (<0.45μm) had stronger adsorption ability, but2-0.45μm colloid particles contributed to the most adsorption capacity due to their absolute advantage in number. Under steady conditions, alkaline and low ionic strength conditions were more in favor of the release of colloid particles, but acidic and high ionic strength conditions were more in favor of the adsorption of phenanthrene on colloid particles. Under change conditions, changers of ionic strength, pH value and alternating of drying and wetting, all promoted the release of colloid. Decrease in ionic strength and changes of pH value from7to9were in favor of the release of dissolved phenanthrene. Increase in ionic strength and changes of pH value from7to5were in favor of the release of adsorptive phenanthrene on colloid. The experiment confirmed colloid-facilited transport of phenanthrene, which was effected by changing conditions.These results on long-term fate and ecological effects of typical contaminants in agricultutal field contributed to assessment influences of soil pollution on groundwater and food safety. Study on colloid release and its effects on contaminant transport in vadose zone provided basic data and method support for further large scale studies of colloid-facilitied transport of pollutants. Environment behaviors of typical pollutants especially emerging contaminants in earth’s critical zone still need future work.