Effects Of Cu-se Combined Pollution On Copper And Selenium Bioavailability And Releated Mechanism

With the development of industry and agriculture, heavy metal pollution becomesincreasingly serious. Coexistence of two or more metals may have different chemicalprocesses such as adsorption-desorption and precipitation effects, which may affect thebioavailability of themselves. So how to indicate the bioavailability of heavy metal have beenhot topics in the study of environmental chemistry. Pot experiments and laboratory analysiswere used to investigate the effects of single and combined Cu-Se pollution on bioavailability,through plant growth, metal uptake, transformation of fractions and soil enzyme activities;combined with the results of adsorption-desorption of Cu and Se, the indicators ofbioavailability had been put forward. The main results are as follow:1. A proper amount of Se (≤2.5mg·kg^-1) could promote the growth of pak choi,increasing the SPAD value （relative content of chlorophyll） of pak choi leaves, root and shootlengths and root biomass. But high concentrations of Se (≥10.0mg·kg^-1) could inhibit growthof pak choi. Exogenous Cu and Cu-Se combined inhibited the growth of pak choi significantly.The SPAD value, root length, stem length and biomass of shoots and roots had significantnegative correlation with exogenous Cu concentration （P<0.05）. In the combined Cu-Setreatment, with the same concentration of Cu, the content of Cu in shoots of pak choiincreased with the concentration of Se in soil, while the Cu content in roots was highest withSe at10mg·kg^-1; when Se content was fixed, the biomass of shoots and roots of pak choiincreased first and then decreased with increasing exogenous Cu concentration and the Secontent was highest for the treatment with200mg·kg^-1Cu. The proper content of Se (≤2.5mg·kg^-1) had antagonistic effects with Cu; but when the exogenous Se concentration wasover10mg·kg^-1, Se exerted synergetic toxic effects with Cu. Selenium could promote transferof copper from the root system of pak choi to the aerial part, while Cu could keep moreselenium in the roots of pak choi.2. Cu mainly existed as residual bound form, while selenium was present mainly asorganic bound and residual form in the uncontaminated soil. In the contaminated soil, coppermainly bounded to hydrate oxides of iron and manganese, while Se was as exchangeable andcarbonate forms. After one month growing season, Cu tended to transfer into organic boundfractions, while Se tended to bound to hydrated oxides of iron and manganese. The IRvalue of Cu decreased with the copper and selenium concentrations increasing gradually, while the IRvalue of Se decreased with Se concentration increasing and had nothing to do with theconcentration of exogenous Cu for both before pakchoi planting and after it harvest. Theparameters estimated by S curve fitting showed that, the suitable amount of Se (≤10mg·kg^-1)could promote the Cu uptake of pakchoi, whereas certain amount of Cu (≤400mg·kg^-1)could promote Se absorption of pakchoi. Both the IRvalue for Cu and Se had similar trend asthe Cu, Se concentration in pakchoi, which meant that the IRvalue could be used to evaluatethe bioavailability of heavy metals in soil. Partial correlation analysis showed that soilexchangeable and organic bound of Cu and Se had better bioavailability for pakchoi.Therefore, the IRvalue of elements in soils and the change of elements before pakchoiplanting and after it harvest can be used as indicators of evaluation on bioavailability of heavymetals.3. Four soil enzyme activities were inhibited by Cu and Se pollution either individuallyor combined in different degrees, following the order as nitrate reductase> urease> catalase> alkaline phosphatase. Growing plants could stimulate soil enzyme activity in a similar trend,compared with treatments without plants. The joint effects of Cu and Se on catalase activityshowed synergism at low concentrations and antagonism at high concentrations, whereas itwas the opposite on urease activity. However, nitrate reductase activity showed synergism forboth with and without plant treatments. The half maximal effective concentration (EC₅₀) ofexchangeable fractions had a similar trend with the EC₅₀of total content, and was lower thanthat of total content. The EC₅₀of nitrate reductase and urease activities were significantlylower for both Se and Cu （P<0.05）, which indicated that they were more sensitive than thetwo other enzymes.4. Soil urease and nitrate reductase activities were inhibited by Cu and Se pollutioneither individually or combined in different degrees, following the order nitratereductase>urease. The significant correlation between the IRvalue and soil enzyme activitiessuggests that this value could be used to evaluate the bioavailability of heavy metals in soil.Path analysis showed that the variations in exchangeable Cu and organic-sulphidematter-bound and elemental Se had direct effects on the activities of the two enzymes,suggesting their high bioavailability. Therefore, the IRvalue and the transformation of metalsin soil could be used as indicators in evaluating the bioavailability of heavy metals.5. The adsorption-desorption curve of Cu and Se in three kinds of soil were same,Heilongjiang soil and Yangling soil had stronger ability to adsorb Cu, while Yangling soil hadweakly ability to adsorb Se. The Langmuir model is more suitable for the adsorption of Cu,and Se in soil. Different order could effect the the adsorption of Cu and Se. After adsorption of Cu, the Se was harder to adosorpe on soil; and after sorption of Se, the effect showed aspromotion at low concentration and inhibition at high. When Cu and Se coexist in soilsolution, Se could promote the adsorption of Cu, while high concentration of Cu may inhibitthe adsorption of Se.6. The combined pollution inhibited the SPAD value, root length, stems length andbiomass of shoots and roots significantly; the IRvalue of heavy metal in soils and the changeof elements before pakchoi planting and after it harvest can be used as indicators of evaluationon bioavailability of heavy metals.