*4.2. Selenium Desorption*

Selenium desorption, presented as the percentage of the adsorbed Se(IV) concentration found in the equilibrium desorption solutions, increased as the added Se(IV) amounts increased (Figure 5). Much lower Se% desorption from the acid than from the alkaline soils was observed, indicating a stronger retention of fresh Se(IV) by the acid soils. In fact, the mean Se% desorption (the average of Se% values for each initial added Se concentration) from the acid soils was significantly lower than the mean Se% desorption from alkaline soils (*p* < 0.01). Acid soils provided more active sites for the adsorption of negatively charged Se(IV) forms, since when lowering the pH positive charges on soil colloids increase, i.e., there is a higher protonation of surface hydroxyl groups, such as Fe-OH and Al-OH functional groups [36]. However, the stronger retention of Se(IV) by acid soils over the whole concentration range implies the involvement of different sorption mechanisms by the two groups of soils. It is probable that surface complexes may have been formed between Se(IV) species and oxides that lowered the reversibility of sorption process in acid soils. As is shown in Figure 4, for acid soils the pH of equilibrating solutions was very low, supporting the claim that stronger acidic conditions may have occurred close to the surfaces of active soil colloids that could lead to the formation of Se species preferably sorbed on such sites [6,10,40]. On the contrary, Se on the active surfaces of alkaline soils may have been retained mostly as easily exchangeable, thus leading to higher Se desorption by KCl. Numerous studies support the claim that low soil pH favors the higher sorption of Se (independently of Se speciation in equilibrating solutions) [8,51–53] but much less has been done on the evaluation of freshly added Se(IV)'s desorption behavior in acid and alkaline soils. The dominant role of metal oxides in the sorption–desorption behavior of Se(IV) under the conditions of the performed batch

experiments is also supported by the significant negative correlations between mean Se% desorption values and oxides concentrations (Table 3).

**Figure 5.** Percentage Se desorption by 0.25 M KCl from alkaline and acid soils. Contact time 24 h, agitation rate 125 rpm, sorbent/solution ratio 1 g/0.03 L, temperature 22 ◦C.
