Abstract
Selenium adsorption/desorption behavior was examined for eight Greek top soils with different properties, aiming to describe the geochemistry of the elements in the selected soils in terms of bioavailability and contamination risk by leaching. Four soils were acid and four alkaline, and metal oxides content greatly differed between the two groups of soils. The concentrations of Se(IV) used for the performed adsorption batch experiments ranged from 1 to 50 mg/L, while the soil to solution ratio was 1 g/0.03 L. Acid soils adsorbed significantly higher amounts of the added Se(IV) than alkaline soils. Freundlich and Langmuir equations adequately described the adsorption of Se(IV) in the studied soils, and the parameters of both isotherms significantly correlated with soil properties. In particular, both KF and qm values significantly positively correlated with ammonium oxalate extractable Fe and with dithionite extractable Al and Mn, suggesting that amorphous Fe oxides and Al and Mn oxides greatly affect exogenous Se(IV) adsorption in the eight soils. These two parameters were also significantly negatively correlated with soil electrical conductivity (EC) values, indicating that increased soluble salts concentration suppresses Se(IV) adsorption. No significant relation between adsorbed Se(IV) and soil organic content was recorded. A weak salt (0.25 M KCl) was used at the same soil to solution ratio to extract the amount of the adsorbed Se(IV) that is easily exchangeable and thus highly available in the soil ecosystem. A much higher Se(IV) desorption from alkaline soils was observed, pointing to the stronger retention of added Se(IV) by the acid soils. This result implies that in acid soils surface complexes on metal oxides may have been formed restricting Se desorption.
Highlights
Selenium (Se) is an essential micronutrient for humans and animals, but can lead to toxicity when taken in excessive amounts
Was used at the same soil to solution ratio to extract the amount of the adsorbed Se(IV) that is exchangeable and highly available in the soil ecosystem
The mobility and plant-availability of Se in soil is controlled by numerous chemical and biochemical processes, as follows: sorption, desorption, microbial activity, the formation of organic and inorganic complexes, precipitation, and dissolution and methylation to volatile compounds [6,7]
Summary
Selenium (Se) is an essential micronutrient for humans and animals, but can lead to toxicity when taken in excessive amounts. Plants are the main source of dietary Se, but the essentiality of Se for plants is still controversial, the beneficial effects of low doses of Se on plants have been reported in several studies [1,2,3]. The mobility and plant-availability of Se in soil is controlled by numerous chemical and biochemical processes, as follows: sorption, desorption, microbial activity, the formation of organic and inorganic complexes, precipitation, and dissolution and methylation to volatile compounds [6,7]. Depending on the oxidation state, Se is present in soil as selenide (Se2 − ), elemental selenium (Se0 ), selenite (SeO3 2− ), selenate (SeO2− ) and organic Se. The main factors controlling Se solubility and availability in soils are considered to be pH, oxidation-reduction potential (Eh), metallic oxy-hydroxides and clays, organic matter, microorganisms, and the presence
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