Using Chromium Stable Isotope Ratios To Quantify Cr(VI) Reduction:  Lack of Sorption Effects

Abstract

Chromium stable isotope values can be effectively used to monitor reduction of Cr(VI) in natural waters. We investigate effects of sorption during transport of Cr(VI) which may also shift Cr isotopes values, complicating efforts to quantify reduction. This study shows that Cr stable isotope fractionation caused by sorption is negligible. Equilibrium fractionation of Cr stable isotopes between dissolved Cr(VI) and Cr(VI) adsorbed onto gamma-Al2O3 and goethite is less than 0.04 per thousand (53Cr/52Cr) under environmentally relevant pH conditions. Batch experiments at pH 4.0 and pH 6.0 were conducted in series to sequentially magnify small isotope fractionations. A simple transport model suggests that adsorption may cause amplification of a small isotope fractionation along extreme fringes of a plume, leading to shifts in 53Cr/52Cr values. We therefore suggest that isotope values at extreme fringes of Cr plumes be critically evaluated for sorption effects. A kinetic effect was observed in experiments with goethite at pH 4 where apparently lighter isotopes diffuse into goethite clumps at a faster rate before eventually reaching equilibrium. This observed kinetic effect may be important in a natural system that has not attained equilibrium and is in need of further study. Cr isotope fractionation caused by speciation of Cr(VI) between HCrO4- and CrO4(2-) was also examined, and we conclude that it is not measurable. In the absence of isotope fractionation caused by equilibrium speciation and sorption, most of the variation in delta53Cr values may be attributed to reduction, and reliable estimates of Cr reduction can be made.

Theoretical and Experimental Principles

“…In contrast Fe (II) sorbed on goethite particles is isotopically heavy compared to dissolved Fe (II) . For Cr, a negligible isotope fractionation is reported (Ellis et al 2004).…”

The marine biogeochemistry of chromium isotopes

“…First, the Cr isotope system was used to track the reduction of toxic and soluble Cr(VI) to benign and immobile Cr(III) in groundwater (Ellis et al, 2004. These studies observed the preferential reduction of isotopically light chromate/ CrO 4 2-, which resulted in the remaining Cr(VI) pool becoming isotopically heavier as the reduction proceeded.…”

Chromium isotope stratigraphy of Ediacaran cap dolostones, Doushantuo Formation, South China

“…The light Cr isotope ( 52 Cr) is assumed to be preferentially retained with Cr(III) within the weathered soil whereas the oxidized and mobilized Cr(VI) is enriched in the heavy 53 Cr isotope (Izbicki et al, 2008;Crowe et al, 2013;Frei and Polat, 2013;Berger and Frei, 2014) and transported -potentially without significant isotope effect during adsorption onto particles (Ellis et al, 2004) -via river water to the open ocean (Bonnand et al, 2013;Frei et al, 2014;Paulukat et al, 2015). There, oxygenated water stabilizes the transported Cr signal (Table 1); the position of samples presented in this figure correlates to their position in the schematic stratigraphic column and to their deposition on the Yangtze Platform (see Fig.…”

“…During transport to the oceans, Cr(VI) is potentially again partially reduced (Ellis et al, 2002;Sikora et al, 2008;Døssing et al, 2011;Kitchen et al, 2012;Basu et al, 2014), which would result in an additional enrichment of the remaining dissolved Cr(VI) in 53 Cr, whereas 53 Cr-depleted Cr(III) is likely to be precipitated and/or adsorbed (Ellis et al, 2002(Ellis et al, , 2004Crowe et al, 2013). The overall Cr isotope composition of seawater is potentially affected if the influence of the mobile 53 Cr-enriched Cr(VI) input is large enough (Crowe et al, 2013).…”

“…Of the environmentally stable Cr(III)-Cr(VI) redox pair, Cr(VI) is highly soluble whereas Cr(III) is highly particle-reactive and forms insoluble Crhydroxide complexes . The oxidation of Cr(III) to Cr(VI), catalyzed by Mn-oxyhydroxides (Eary and Rai, 1987;Fendorf and Zasoski, 1992), is accompanied by an isotopic fractionation where the mobilized Cr(VI) is enriched in the heavier isotope 53 Cr (Ellis et al, 2004(Ellis et al, , 2002Zink et al, 2010) while no significant isotope effect was observed during adsorption of Cr(VI) onto particles (Ellis et al, 2004). This means, that the weathered soil remains isotopically lighter in respect to Cr while oxidative atmospheric weathering enriches the run-off in 53 Cr ( Izbicki et al, 2008;Crowe et al, 2013;Frei and Polat, 2013;Berger and Frei, 2014).…”