Chromium (Cr) levels surpassing regulatory limits remain a pressing environmental challenge worldwide, especially in natural waters associated with nickel (Ni) laterite ore deposits developed from ultramafic rocks. While Cr persists in the environment as Cr(III), an essential micronutrient for human nutrition, public concern lies with Cr(VI), the more mobile, toxic, and carcinogenic form of Cr. It is, therefore, crucial to quantify the speciation and mobility of Cr in Ni laterites and associated mine tailings and waters to predict possible pathways for its release in surrounding environments. However, the incomplete understanding of the interaction between Cr and prevalent iron (Fe) (oxyhydr)oxides in Ni laterites renders such quantification challenging. Although Fe (oxyhydr)oxides, as well as redox-active manganese (Mn) oxides, have been known to control the mobility of Cr in the environment, mechanistic evidence of their role in the release and sequestration of Cr during weathering of ultramafic rocks into laterites remains lacking. In addition, the mining and processing of worldwide Ni laterite ore deposits are expected to increase exponentially because of the global energy transition. Therefore, it is not only important to elucidate the fate of Cr during lateritization, but also to quantify its ultimate fate after mining and processing of the Ni laterite ores. To bridge these knowledge gaps, I conducted a holistic study combining field observations and detailed experimental evaluations based on a suite of highly complementary laboratory- and synchrotron-based analytical approaches, ultimately tracing the fate of Cr from source to sink.
Through laboratory mineral synthesis and dissolution experiments, I optimized a Cr sequential extraction procedure (SEP) that allows the quantitative assessment of the partitioning and, thus, the potential mobility of Cr in iron-rich laterites. I validated the new method using complementary mineralogical, geochemical, and synchrotron-based X-ray absorption spectroscopy (XAS) methods, which showed that my new SEP more efficiently and accurately quantifies the partitioning of Cr in natural Ni laterite samples compared to existing SEPs. With the optimized SEP, easily mobilizable Cr fractions, most especially adsorbed Cr(VI) oxyanions, could be better quantified compared to existing methods, improving our ability to evaluate the potential environmental impacts of Cr. Moreover, my new SEP more adequately leaches important Cr host phases like Fe (oxyhydr)oxides by considering the properties of different Cr species and the ability of Cr to stabilize crystal structures.
To trace the evolution of Cr from the weathering of ultramafic rocks to the formation of Ni laterites, I investigated the changes in Cr speciation along several Ni laterite profiles from the Philippines. By combining high energy resolution fluorescence detection (HERFD) and total fluorescence yield (TFY) Cr, Fe, and Mn K-edge XAS, I documented that Cr(III) released from the weathering of ultramafic minerals (e.g., olivine, chromite) is redistributed to the saprolite zone by structural incorporation or adsorption onto secondary phyllosilicates such as serpentine and smectite. I also captured the Cr sequestration mechanism by secondary Fe (oxyhydr)oxides – starting from the polymerization of Cr(III) with Fe(III) to form poorly crystalline precursors to their crystallization, and their subsequent transformation to Cr(III)-substituted Fe (oxyhydr)oxides (e.g., goethite, hematite) in the overlying limonite zone. There, the redox dynamics between Cr(III) and Mn(IV/III) (oxyhydr)oxides also oxidized up to 13% of the total Cr into the toxic Cr(VI), resulting in detectable Cr(VI) concentrations ranging from 128 to 2,713 mg kg⁻¹, which is up to 80 times higher than the average Cr composition of the upper crust. My data documents that Cr(VI) could be adsorbed by Fe (oxyhydr)oxides as mononuclear edge-sharing (2E) and monodentate mononuclear (1V) inner-sphere surface complexes or leached downstream due to its high mobility, explaining the elevated Cr(VI) concentrations waterbodies associated with Ni laterites. By comparing Ni laterite profiles from different localities, my work also highlighted the role of environmental factors in mobilizing Cr, especially in tropical areas where the rate of weathering is enhanced.
Finally, I followed the fate of Cr after mining and high pressure acid leaching (HPAL) of Ni and Co from the laterite ores, revealing how and in what form Cr ends up in the mine tailings. Specifically, I employed nano-scale characterization using scanning transmission electron microscopy (STEM) and synchrotron-based XAS integrated with the optimized Cr SEP to investigate Cr in the liquid and solid phases of mine tailings collected from active and rehabilitated impoundments. I unraveled that Cr is undetectable in the liquid phase because it became immobilized in the form of Cr(III) in the structure of recalcitrant minerals. Secondary hematite that precipitates early in the HPAL process serves as the main Cr trap by structurally incorporating up to 61% of the total Cr. Meanwhile, the remaining Cr is primarily hosted in chromite residues from the source laterite ores. These results emphasized the crucial role of Fe (oxyhydr)oxides in the sequestration of Cr in highly stable sinks, preventing potential remobilization in the natural environment.
Overall, my doctoral work delivered new and significant insights into the release and sequestration of Cr in Ni laterite areas affected by geogenic (e.g., weathering) and anthropogenic (e.g., processing) activities. This data now allows us to better predict and monitor Cr pathways, contributing far-reaching implications for the environmental management and sustainable development of these important mineral resources.
