*4.1. Selenium Adsorption*

The experimental data fitted well with Freundlich and Langmuir isotherms, in agreement with Dhillon and Dhillon's results [35] (Table 2). The calculated adsorption maxima (*qm*) from the Langmuir isotherm were higher for acid soils, as was in most cases the value of the bonding constant (*bL*), indicating the stronger Se(IV) retention by the acid soils.

**Table 2.** Parameters of the Langmuir and Freundlich models for Se(IV) sorption in the eight soils. 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 parameters of both isotherms, i.e., *KF* and 1/*n* from the Freundlich isotherm, and *qm* and *bL* from the Langmuir isotherm, showed significant correlations with soil constituents. Both *KF* and *qm* significantly positively correlated with ammonium oxalate extractable Fe and with dithionite extractable Al and Mn (*p* < 0.01, Table 3), underpinning the crucial role of amorphous Fe, Al and Mn oxides in the exogenous Se(IV) behavior of the studied soils. The ability of Fe (especially amorphous), Al and Mn oxides to control Se geochemical behavior has been highlighted in many studies, supporting thus the leading significance of metal oxides in regulating Se mobility in soils [6,10,34,39–41]. *KF* and *qm* were also significantly negatively correlated with EC (*p* < 0.05, Table 3) and negatively but not significantly with bonding constant (*bL*). These relations suggest that an increased soluble salts concentration suppresses both Se(IV) adsorption and strength of Se(IV) retention in soils, and leads to the increased availability of freshly added Se(IV) in the soil environment. This finding is also reported in the review of Natacha et al. [10] and in references therein. Furthermore, the bonding constant (*bL*) of the Langmuir isotherm significantly positively correlated with the Feo/Fed values of acid soils and with the eqCaCO3 content of alkaline soils (Table 3), pointing to the fact that in acid soils the fresh Se(IV) retention strength increases when amorphous Fe oxides constitute a larger part of free the Fe oxides, whereas in alkaline soils carbonates may possibly affect Se(IV) sorption. No significant correlation between the organic matter content and the initial or the adsorbed Se(IV) content was observed, a conclusion commonly reached by many researchers. Coppin et al. [42] did not find a direct relation between adsorbed Se and organic material, and suggest that Se may be indirectly sorbed on organic particles by forming associations with surface Fe oxides and clays. Additionally, Soderlund et al. [36] reported the limited importance of organic matter on Se retention compared to Fe and Al phases, even when the latter are incorporated in organic substances. Though clay is considered to affect Se sorption in soils [6,43], no significant correlations emerged between the clay content of the soils and the parameters of the Langmuir and Freundlich isotherms, or the distribution coefficient.

In Table 3, the correlation coefficients for Feo, Ald and Mnd and mean Kd (calculated from Kd values for each initial added Se concentration) relations are presented. The significant correlations between Kd values, ammonium oxalate extractable Fe and dithionite extractable Al and Mn (*p* < 0.05) further support that metal oxides govern Se(IV) sorption in the studied soils. The point of zero charge (PZC) of most Fe-oxides was shown to deviate slightly, ranging usually between pH 7 and 9, while the

pHpzc values for various Al oxides reported in the literature vary widely, with a median of 8.6. [44,45]. In the pH range of equilibrium solutions, the Fe and Al oxides are positively charged and can adsorb negatively charged Se species. At low pH values, Mn oxides may have offered additional positively charged sites, since the PZC for most Mn oxides usually occurs at pH < 5 [46,47], leading to the increased adsorption capacity of acid soils. Nakamaru et al. [48], by using 75Se as a tracer, found that the Kd values for selenite adsorption in Japanese soils were highly correlated with the active Al (Alo) and Fe(Feo) content of the soils. Premarantha et al. [49] reached the same conclusion for acid soils from rice-growing areas in Sri Lanka. However, Zhe Li et al. [50] did not observe any significant relation between Kd and Alo and/or Feo concentrations in 18 soils from China, and report only a strong negative correlation between Kd and soil pH values, indicating the stronger adsorption of selenite in acid soils. According to Table 3, the EC of soils was also significantly negatively correlated with mean Kd values (*p* < 0.05). Interestingly, Se availability was not only regulated by the absolute poorly crystallized iron oxides, but also by the relative Feo content in the free iron oxides, as can be deduced from the significant correlation between mean Kd and Feo/Fed values (*p* < 0.05, Table 3). Considering that the Feo/Fed ratio is used as an indicator for soil development, this result leads to the speculation that the stage of soil development can influence added Se(IV) behavior in the soil environment, and ultimately in the food chain. Nevertheless, the soils of the present study may have been formed from different parent materials, and such observations could be case specific, but may also be regarded as an indication for further research.

**Table 3.** Correlation coefficients, significant at *p* < 0.05 except *qm*-Mnd and *bL*-EC pairs (in italics) (*n* = 8).

