*3.2. Selenium Adsorption*

Acid soils showed a much higher retention of added Se(IV) than alkaline soils, in accordance with many studies [6,34–36]. In particular, Se(IV) adsorption ranged between 8.52 and 234 mg kg−<sup>1</sup>

<sup>\*</sup> D.L.: Detection Limit.

(Figure 1a) for alkaline soils, while the corresponding range for acid soils was 19.2–558.9 mg kg−<sup>1</sup> (Figure 1b).

**Figure 1.** Se(IV) sorption on the studied soils (**a**) alkaline and (**b**) acid. Contact time 24 h, agitation rate 125 rpm, sorbent/solution ratio 1 g/0.03 L, Se(IV) concentrations at start time from 1 to 50 mg/L, temperature 22 ◦C.

The distribution coefficient (Kd) is a measure of the occupation of available sorption sites in relation to the concentration of the added element. Depending on added Se(IV) concentrations, the Se(IV) Kd values were within the ranges 2.6–36.7 and 3.5–1091.5 L/kg for alkaline and acid soils, respectively. Over the whole range of added Se(IV) concentrations, the Kd values of acid soils were considerably higher than those of the alkaline soils (Figure 2). The observed Kd values for the acid soils were noticeably higher than those reported by Soderlund et al. [36] for selenite adsorption on mineral soils (0.4–240 L/kg), while the highest Kd values are close to those determined by Sheppard et al. [37] for indigenous selenium (800–1500 L/kg). A decreasing trend of Kd values is commonly observed as the concentration of the element in solution increases, indicating that proportionally less of the added element is adsorbed by the soil colloids. Indeed, for all studied soils, Kd decreased as the Se(IV) solution concentration increased (Figure 2), and the higher to lower Kd ratio ranged between 4.6 and 9.1 for alkaline soils, whereas the corresponding range for acid soils was 10–90.2.

**Figure 2.** Values of Se(IV) Kd (L/kg) for the studied soils (**a**) alkaline and (**b**) acid. Contact time 24 h, agitation rate 125 rpm, sorbent/solution ratio 1 g/0.03 L, Se(IV) concentrations at start time from 1 to 50 mg/L, temperature 22 ◦C.
