Effect of phosphate addition on the sorption–desorption reaction of selenium in Japanese agricultural soils

Desorption levels of soil-sorbed selenium (Se) were studied by adding phosphate to 22 typical Japanese agricultural soils. Soil–soil solution distribution coefficients of Se (Kd-Se) were measured using a batch process as an index of Se sorption level, adding 75Se as a tracer. After the Kd measurement, extraction of soil-sorbed 75Se with a 0.1 M or 1 M Na2HPO4 solution followed to determine the amount of 75Se desorbed by the phosphate. When the 0.1 M Na2HPO4 solution was used, 18–70% of soil-sorbed Se was extracted (average: 47%). However, when the 1 M Na2HPO4 solution was used, 27–83% of soil-sorbed Se was extracted (average: 57%). The observed 75Se desorption percentage indicated the maximum Se removability by phosphate addition. The desorption percentage of Se with 1 M Na2HPO4 correlated with Kd-Se values, suggesting that the soil sample with higher Kd-Se contained more reactive components for phosphate-sorption than the soil sample with lower Kd-Se. To evaluate the effect of phosphate concentration on the Se sorption, the Kd-Se was measured for two typical soils under different levels of phosphate (0.1–10 mM PO4). The Kd values were decreased by phosphate addition for both soils. The Kd decrease was observed even for just 1 mM PO4. The phosphate addition with 1 mM PO4 is the same level as in P fertilizer applied to paddy fields in Japan. Therefore, it was suggested that Se desorption should occur in Japanese soils due to the phosphate input.