*2.3. Batch Experiments*

For every soil six falcon tubes were used. One gram of soil was introduced to each falcon tube and 30 mL of the appropriate stock solution was added, resulting in a 1:30 w/v soil:solution ratio. Afterwards the falcon tubes were placed in an incubator with an adjusted steady temperature of 22 ± 1 ◦C and gently shaken at 120 rpm for 24 h on an end-to-end shaker. Then, the falcon tubes were centrifuged for 5 min at 3500 rpm and the supernatants were filtered through a Whatman paper No 42. Absorbed Se(IV) was calculated by the difference between the initial and the equilibrium solutions Se(IV) concentrations. Moreover, since pH plays an important role in Se behavior in the soil environment, the pH values of the initial Se(IV) solutions and of the equilibrium solutions were also recorded.

To desorb adsorbed Se(IV), 30 mL of 0.25 M KCl extractant solution was added in the falcon tubes containing the soil samples. Falcon tubes were placed again in an incubator with an adjusted steady temperature of 22 ± 1 ◦C, and gently shaken at 120 rpm for 24 h on an end-to-end shaker, centrifuged, and filtered through a Whatman paper No. 42, following the same procedure as described above. Desorbed Se(IV) was determined in the equilibrium solutions at the end of the process.
