Assessing chromate availability in tropical ultramafic soils using isotopic exchange kinetics
Résumé
Background, aim, and scopeThe presence of labile chromate in the soils is an environmental problem because of its high toxicity. The isotopic exchange kinetics (IEK) methods have been shown to be a useful tool to measure the phytoavailability of major (P, K) and trace elements (Cd, Zn, Ni, Pb) in soils. This study focused on the potential of applying IEK for chromate to characterize its availability in two tropical ultramafic Ferralsols.Materials and methodsTwo Ferralsols (NIQ II and NIQ III) of the ultramafic complex of Niquelândia (Goias, Brazil), known to have a high content of extractable chromate, were investigated. We adapted IEK for chromate in order to distinguish different pools of available chromate according to their rate of exchange kinetic.ResultsThe extractable Cr(III) ranged from 9 to 132 mg kg−1, whereas extractable Cr(VI) ranged from 64 to 1,014 mg kg−1. The intensity factor, i.e., concentration of soluble Cr, ranged from 78 to 231 μg L−1 in profile NIQ II and from 6 to 141 μg L−1 in profile NIQ III. The highest concentrations were found in both topsoils and in the NIQ II-5 horizon. Most of the Cr(VI) was labile in short (E0−1 min) or medium-term (E1 min-24 h) in both soils. The E0−1 min and E1 min-24 h represented 39 to 83% of labile Cr(VI) in NIQ II and 69 to 80% in NIQ III. A high quantity of Cr(VI) was thus extremely labile and highly available, particularly in NIQ II. Moreover, both soils had a high buffering capacity of soluble Cr(VI) by labile pools.DiscussionThe Cr(VI) availability is large and varied significantly among the soil profiles. The r1/R parameter has long been considered as an indicator of the soil “fixing capacity” for ions like P. The values of r1/R for Cr(VI) measured on the two studied soils are among the lowest ever reported for any element, especially in the organic matter-poor and iron oxide-rich horizons (r1/R in the 0.001-0.003 range). But, considering the high proportion of labile CrVI in these soils, it is more appropriate to relate r1/R to the buffer capacity. The latter was extremely high and probably due to labile Cr(VI) retained in its majority by low-energy bonds on the surface of colloids. The quantity of readily labile Cr (E0-1min) was significantly correlated (r = 0.96, p < 0.01) with the quantity of Cr associated to amorphous or poorly crystallized Fe-oxides. Thus, amorphous Fe oxides control the Cr availability in these Ferralsols. The correlation between E parameters and clay content has to be carefully considered. Indeed, these soils contain mainly fine and discrete clay-sized Fe oxides, particularly goethite. Despite different data supporting the idea of the formation of inner-sphere surface complexes of chromate on goethite, the high quantity of readily labile Cr(VI) and the high buffer capacity observed for these soils are consistent with low-energy bonds on the surface of colloids in agreement with the formation of outer-sphere complexes.ConclusionsThe two studied Ferralsols contain a large quantity of labile Cr(VI), which is controlled by amorphous Fe oxides and pH. IEK for chromate allows distinguishing different pools of available chromate according to their rate of exchange kinetic. The buffer capacity of these soils is extremely high and probably with a majority of low-energy bonds on the surface of colloids. The study highlighted a high chromate availability in those soils and, consequently, potential chromate toxicity on soils organisms.Recommendations and perspectivesIEK could be a powerful tool to quantify chromate availability in soils. Our attempt to apply the IEK for chromate seems to be a success and the IEK for Cr consequently seems to have future. However, the robustness and the limit of the IEK for chromate have to be examined in more detail for a large diversity of soils.