Selenite–selenate sorption in surface coal mine environment

Abstract

Information is limited on the sorption characteristics of selenite (SeO 3 2−) and selenate (SeO 4 2−) in reclaimed coalmine environments. The objectives of this research were to evaluate the adsorption of both Se species using various isotherm models, and to quantify the relationship between sorption parameters and different soil properties. Ten coalmine soils were studied using Se treatments ranging from 0 to 2.5 mg/l. The results indicated that the percent sorption varied from 48 to 99% for SeO 3 2−, and 44 to 95% for SeO 4 2−; however, for all soil samples, percent SeO 3 2− sorption was always greater than SeO 4 2−, indicating SeO 3 2− was the stronger adsorbate. The Langmuir model described SeO 3 2− sorption in eight samples, whereas adsorption of SeO 4 2− was described in only one sample by this model. The Freundlich model resulted in better statistical fitting of the data compared to the Langmuir model for both Se species; 10 and six samples conformed to adsorption of SeO 3 2− and SeO 4 2−, respectively. Adsorption of SeO 3 2− in nine soils and SeO 4 2− in six soils were described by the Temkin model. The best fitting of SeO 3 2− and SeO 4 2− adsorption (for all 10 soils) occurred with the Initial Mass isotherm. Higher K i values for SeO 3 2− in comparison to SeO 4 2− suggested that SeO 3 2− had a greater adsorption affinity for the coal mine soils. Difference in sorption strength of SeO 3 2− and SeO 4 2− was attributed to the structural difference of the two species based on the concepts of valence shell electron pair repulsion model and hybridization. Different adsorption indices, including Langmuir ( B l, K l B l), Freundlich ( n f), Temkin ( K t1 K t2), and Initial Mass ( K i) model parameters, were linearly related (at P of 0.05) to various soil properties. Phosphate extractable Se and total soil Se were correlated to mainly SeO 3 2− adsorption parameters including K l B l, K t1 K t2, and K i. Extractable Ca, Mn and Fe concentrations were correlated to B l and n f, indicating the presence of oxides or oxyhydroxides of the metals could result in greater Se retention in the mine soils. Correlations of n f with electrical conductivity, clay content, and specific surface area also suggested a possible dependence of Se adsorption on other soil properties.