Chromium can be used as a tracer of redox sensitive environmental processes. In soils Cr (III) is inert, immobile and resides predominantly in minerals, clays and oxides. Cr (VI) is toxic, soluble and mobile and is usually lost from the soil to local run off. Chromium isotopes have been shown to fractionate under both reducing and oxidizing conditions [1, 2]. Recent studies on d53Cr isotopes in laterite soils show that oxidative weathering of Cr-bearing rocks is accompanied by an isotopic fractionation, where by the lighter isotopes are retained in the residual soil and the heavier isotope is enriched in local runoff [1]. This study aims to quantify the stable Cr isotope composition of two modern basaltic weathering profiles, to help better understand the processes that oxidize inert Cr (III) to toxic Cr (VI). We sampled basaltic weathering profiles and associated river waters from areas of two contrasting climates; 1) Northern Ireland, a marine temperate climate, and 2) Uruguay, a sub-tropical climate.
The oxidation of Cr in soils can be broken down into two important stages,
(i) The breaking down of Cr-bearing minerals in the basalt, which make Cr (III) accessible to oxidizing processes.
Cr3+ + 2H2O Cr (OH) 2+ + 2H+ (1)
(ii) The continuing oxidation of Cr(III) and Cr(OH)2+ to Cr(VI) oxyanions, CrO42-, HCrO4- and Cr2O72-.
Cr3+ +H2O HCrO4- + 7H+ +3e- (2)
Cr(OH)2+ + 2H2O HCrO4- + 5H+ + 3e- (3)
This process predominates in the soil horizons and is accompanied by an isotopic fractionation. As reaction (2) and (3) proceeds Cr (VI) is lost from the system to local run off.
However soils are complex heterogenous systems, even in the presence of oxygen other factors such as pH and the presence of reducers for example Fe2+ [3] and organic matter can impede Cr(III) oxidation. The d53Cr value of a soil can be used as a measure of Cr(III) oxidation in a soil, and can therefore give a good indiction of the extent to which toxic Cr (VI) is lost from the soil to local run off.
[1] Crowe et al., 2013, EPSL in press [2] Ellis et al., 2002. Science v 295 p. 2060-2062 [3] Døssing et al., 2011. Chem Geol. v. 285 p. 157-166.