Fig. 2. The K_d value for (upper) Se(IV) and (lower) Se(VI) for NE1 25–30 soil measured from a single Se concentration in the presence of increasing concentrations of sulfate (SO₄^2–) or calcium (Ca²⁺) relative to the K_d value measured from the same Se concentration in the absence of Ca²⁺ or SO₄^2– (KCl solution at I = 0.03). Initial concentrations were 17.55 µmol L^–1 and 6.74 µmol L^–1 for Se(IV) and Se(VI) with a soil mass/solution volume of 1:50 and 1:20, respectively. Constant ionic strength of 30 mM was achieved by adding appropriate amounts of KCl.