Kinetic release and fractionation of cobalt in some calcareous soils

Abstract

The availability of cobalt (Co) depends on its concentration in solution which is affected by kinetic release from solid phase, and also depends on its distribution between different soil constituents. There is very limited information on kinetic release of Co from calcareous soils. On the other hand, the lack of Co fractionation data in calcareous soils is a problem in determining mobility and availability of this element. The release kinetic of Co from 10 calcareous soils collected from different agricultural areas in Hamedan Province, western Iran, was investigated using 0.01 M calcium chloride (CaCl2) and ethylenediaminetetraacetic acid (EDTA) solutions in time intervals from 8 to 2084 h (8, 20, 44, 92, 164, 284, 452, 692, 1028, 1484 and 2084 h). The results showed that the general trends of Co release were similar for all studied soils and after 2084 h, 0.01 M EDTA released more Co than 0.01 M CaCl2. Different kinetic equations (parabolic diffusion, power function, Elovich and first order) adequately described the cumulative Co release and power function equation with the lowest Akaike Information Criterion (AIC) best described the Co release by 0.01 M CaCl2 and EDTA in studied soils. In the case of 0.01 M EDTA, the Co release rate parameters in parabolic diffusion, power function, Elovich equations were significantly correlated to pH (r = 0.80, 0.78 and 0.80, respectively; p < 0.01) and the percentages of clay (r = 0.87, 0.88 and 0.84, respectively; p < 0.01) and sand (r = −0.76, p < 0.01; −0.70 and −0.70, p < 0.05, respectively) that indicates the importance of these properties in relation to Co release from soils. Cobalt distribution between different soil constituents, before and after Co sorption in soils was investigated using sequential extraction procedure. Results indicated that the residual fraction (RES) was the dominant fraction both before and after Co sorption in soils. Sorption of Co in soils increased the proportion of Co associated with RES and carbonate-bound (CARB) fractions and decreased the proportions of exchangeable/solution (EXCH) and organically bound (OM) fractions, so it can be concluded that Co was mainly distributed between RES and CARB fractions and sorption included both adsorption and precipitation processes. Individual contamination factor (ICF) was used for Co pollution and leaching risk assessment in soils. According to results, the ICF in soil 7 (collected from Hesare Motahari) was higher than other soils, so this soil that is located close to a busy boulevard in vicinity of Hamedan city had the highest risk of Co pollution and leaching between studied soils.