Aging Process Of Exogenic Selenate And Selenite In Soil

Adsorption of exogenous selenium on soil is crucial in perspective of its environmentfate and bioavailability, and has connection with selenium distribution in each fractionation,which will vary at different time. Therefore, this research focused on the adsorption,desorption and aging of exogenous selenite and selenate in18types of agriculture soils fromChina, analyzed the adsorption and desorption isotherm and changes of selenium fractionationin soil, and elaborated the relationship between soil selenium fractionation and soil physicaland chemical properties. The main results were conveyed as follows:1. Langmiur and Freundlich empricial equation fitted well to the soil adsorption onselenite from the range of0.15to1.82mg/kg. On the contrary, in the3.14mg/kg selenitetreated soil, the actual adsorbance fell above the fitting results from Langmiur equation whileit was consistent with Freundlich fitting results. Soil pH excerted significant effect on seleniteadsorption on soil, while amorphous ferric had positive effect (P<0.05). Organic mattershowed indirect effect when1.82mg/kg or more selenite present in soil. And also, carbonateimposes indirect effect on selenite adsorption on soil. The soil and selenite bind strengthsignificantly correlated to soil pH (P<0.05), but had no correlation with other soil properties.2. The selenite desorption ratio was the minimum at1.82mg/kg selenite treated soil, andshowed reverse trend compared with adsorption. Soil pH had positive effect on selenitedesorption ratio (P<0.05) while amorphous ferric showed negative effect (P<0.05); desorptionratio had no significantly correlation with the intensity of adsorption form Langmiur equation,but had significantly posotive effect on adsorption maxima. K_dhad significantly correlationwith soil pH(P<0.01) and decreased to the bottom when selenite concentration increased from0to1.82mg/kg with a slightly rebounce in3.14mg/kg selenite treated soil. Seleniteadsorption and desorption in the18typical soils were mainly affected by soil pH, amorphousiron, organic matter and others.3.18types of agriculture soils assembled with selenate adsorption which showednegligible variation in spite of the significant difference in soil physical and chemicalproperties. Adsorbance ranged from below detect limitation to18mg/kg as exogenousselenate increased from0.5mg/L to3.75mg/L in soils except for Yunnan Terra cotta soil andHunan Krasnozems, where absorbance were0.23mg/kg and0.49mg/kg at0.05mg/L selenium treatment and26.9mg/kg and29.0mg/kg at3.75mg/L selenium treatment,respectively.4. Exogenous selenite and selenate differed in soil selenium fractionation. Exogenousselenite adsorbed on soil proponents after added in soil, resulting in the low soluble seleniumconcentration. Exchangeable and carbonate bound selenium accounted for the most part ofselenium fractionation (60%), followed by iron and manganese oxides bound selenium andorganic matter bound and elementary selenium, which composed of20%-40%and10%-20%of total selenium. Exchangeable and carbonate bound selenium concentration was also themaximum among the selenium fractions in selenate treatment, while it was relatively lowerthan that in the selenite treatment but the increase with time was more significant. SolubleSe6+decrease dramatically with aging time, however, iron and manganese oxides boundselenium increased and organic matter bound and elementary selenium decreased. After141day, selenium fraction in soil has been basically stable. Iron and manganese oxides boundselenium concentration was lower in the selenate treatment than selenite treatment, whileorganic matter bound and elementary selenium was on the opposite. Soli seleniumfractionation expressed significantly correlation with soil properties such as pH, organicmatter, carbonate, amorphous iron, CEC, electric conductivity and clay at some extent.5. Liaoning burozem had the minimum Uts value compared with other soils addedexogenous selenium, which reached the bottom, which was1.04and0.36in the selenite andselenate treatment at the141th and last day, respectively. The day when Uts value was themaximum varied in selenite and selenate treatment. In selenite treatment, Uts values ofJiangxi Krasnozems increased with aging time and reached maximum at17.4withsubsequently fluctuant. In selenate treatment, Uts value of Shandong Fluvo-aquic reachedmaximum at17.4at the18th day and reduced thereafter. While that of Jiangxi Krasnozemsincreased with fluctuant up to17.4and kept steady thereafter. IRvalue decreased afterexogenous selenite and selenate in soil,and in Yunnan Terra cotta soil it is higher than othersoils.