Leaching behaviors and chemical fraction distribution of exogenous selenium in three agricultural soils through simulated rainfall.

To clarify the leaching risk of selenium (Se) in agricultural soils, a laboratory column experiment was conducted to study the characteristics of leaching and chemical fractions of Se in three different soils treated with different levels of exogenous selenate under simulated local rainfall. Results demonstrated that the Se concentration in leachates of all tested soils decreased rapidly at the beginning of leaching and slowly decreased thereafter. After leaching, Se concentrations in leachates of all tested soils at 1, 3, and 6 mg/kg exogenous Se concentrations were 0.06-0.24, 0.25-0.84, and 0.60-1.65 mg/L, respectively, which exceeded the standard limit of the Chinese Environmental Quality Standards for Groundwater (<0.01 mg/L) (GB/T 14848-2017). The cumulative leached Se amount accounted for 51.27-86.22% of the total Se. Those results indicated the high risk of Se leaching in the tested soils. The Elovich model could better describe Se leaching processes in krasnozem, while the leaching processes of Se in black soil and loess soil accorded with the power function model. Se mainly existed in soluble fraction (61.33-81.05%) before leaching and residual fraction (48.91-68.04%) after leaching. The soluble and exchangeable Se fractions were the main contributors of Se in leachates. In addition, the parameters of the U and I values could well describe the distribution of Se fractions in soil during leaching. In general, more attention should be placed on the assessment of Se leaching in soil.