Understanding of (bio)geochemical processes which control chromium release, speciation and isotopic fractionation in ultramafic environments impacted by mining activities
Ultramafic systems are often synonym with high chromium (Cr) content in rocks, which is naturally leached to surface and groundwater. Due to this natural enrichment, in ultramafic areas chromium and nickel are widely mined. The mining process includes exploitation and beneficiation activities, in which large amounts of metal-rich residues, such as overburden, waste rock and tailings, stored in open air, are produced. These processes may considerably increase the amount of both trivalent (Cr(III)) and hexavalent chromium (Cr(VI)) released to the environment. The later (Cr(VI)) is known to be highly soluble in water, bioavailable and toxic. Within these mining residues, many chemical and biological processes may take place, which will control Cr speciation, mobility and availability. The present work was conducted on i) the nickel exploitation and metallurgic area of Barro Alto (BA, Goiás state, Brazil); ii) the historical chromite exploitation area of Cromínia (CA, Goiás State, Brazil) and iii) the current chromite mine in the Sukinda valley (Odisha, India). The main focus of this research is the identification of the impact of mining activities (nickel and chromium mining) on Cr mobility and availability in ultramafic environments, through the use of isotopic techniques.
The chemically and isotopically exchangeable pools of Cr(VI) (ECr(VI)) were higher in BA ore samples, where the enrichment in light chromium isotopes (-0.76 to -0.16‰) was attributed to the loose of isotopically heavy and exchangeable Cr(VI) during weathering. Astonishingly, heavy δ53Cr values were also found in saprolitic ores in BA and in mining-affected soils in CA up to +3.9‰, strongly enriched in Cr(III). The main causes are attributed to the existence of hydrothermal chromite in CA and/or to natural weathering followed by Cr(VI) reduction that induces reprecipitation of mobile and isotopically heavy Cr. In such a mining context, accelerated weathering would play an important role in this process.
Chromium in the leachate of BA lateritic and saprolitic ores samples was present as Cr(VI), isotopically enriched in heavy isotopes (up to +4.84‰), consistent with the exchangeable Cr (ECr(VI)) (up to +4.37‰). These values were in the same range as isotopic compositions measured in the fresh waters (streams and ponds) in the ultramafic area. These results imply that Cr is mainly released as the toxic Cr(VI) species, whose availability increases from i) the soil profile to ii) the ores and iii) the mining residues. This also suggests that δ53Cr could be used as a tracer of Cr leaching in environmental studies in the dissolved phase.
In bioleaching tests on tailings with Acidithiobacillus thiooxidans (pH ~ 2) or Pseudomonas putida (pH ~ 9), Cr was initially extracted as Cr(VI) and later reduced to Cr(III). In the experiments with A. thiooxidans Cr reduction is due to the production of a series of sulfur compounds with high reducing power, while for P. putida probably uses a variety of electron acceptor for chromate reduction, enhanced by the presence of extracellular polymeric substances. Those mechanisms together with the increase of natural organic matter (NOM) and mineral carbonation, could explain the lower exchangeable pool of Cr(VI) in stockpiled chromite tailings compared to fresh tailings.
Through the use of isotopic exchange, the contribution of suspended particulate matter (SPM) to the transport of exchangeable Cr and the impact of colloids containing Cr-bearing phases on the determination of the exchangeable pool of Cr associated to SPM (EWCr) was highlighted. Larger particles (>0.2 μm) were dominant in the impacted area, while chromium was mainly associated with colloids (1 kDa–500 kDa) in the pristine area. The presence of organic and inorganic colloids containing non-exchangeable Cr induces an overestimation of the EWCr values, which can be over-come with a worst-case scenario correction.