Periodic Reporting for period 1 - CHROMA (Mobilization of chromium by organic matter in reduced systems)
Periodo di rendicontazione: 2022-07-01 al 2024-06-30
Chromium (Cr) has received considerable attention for its potential toxicity to human beings, as well as its utilisation as a tracer for redox processes operating in Earth’s critical zones and in ocean sediments. Chromium has two stable oxidation states, hexavalent Cr(VI) and trivalent Cr(III), which are respectively soluble and insoluble. Whilst reduction of Cr(VI) largely shapes the Cr distribution in natural environments (and has thus been adopted as a pollution mitigation strategy), redox transformations alone cannot fully explain the geochemical behaviour of Cr. For example, in surface waters, dissolved Cr concentrations are usually higher than predicted by the solubility of Cr minerals, suggesting additional ‘stabilisation’ mechanisms. We need to appraise the role of small organic molecules called ligands, which have been invoked to control the mobilisation, transport and fate of many heavy metals.
This project combines an interdisciplinary set of field sampling, controlled experiments, isotopic analysis alongside geochemical modelling, to ground-truth the hypotheses that (i) solid Cr can be effectively remobilised by organic ligands in reducing environment despite Cr reduction, and (ii) aqueous Cr can be stabilised in the form of organic complexes with distinct Cr isotopic signature. Addressing these questions is important to the geosciences community as well as the wider public. Firstly, as Cr is a known carcinogen, increased levels of organic ligands, such as in paddy field systems, will potentially cause increased ecological and health risks. Secondly, organic ligands play an overlooked role in modulating the input and removal processes of dissolved Cr to/from various environments (including weathering, benthic, and hydrothermal fluxes), which are poorly constrained to date. Lastly, Cr isotopes are proven as promising tool to fingerprint ligand-bound Cr(III) that has been difficult to characterise/quantify with conventional analytical approaches.
This project combines an interdisciplinary set of field sampling, controlled experiments, isotopic analysis alongside geochemical modelling, to ground-truth the hypotheses that (i) solid Cr can be effectively remobilised by organic ligands in reducing environment despite Cr reduction, and (ii) aqueous Cr can be stabilised in the form of organic complexes with distinct Cr isotopic signature. Addressing these questions is important to the geosciences community as well as the wider public. Firstly, as Cr is a known carcinogen, increased levels of organic ligands, such as in paddy field systems, will potentially cause increased ecological and health risks. Secondly, organic ligands play an overlooked role in modulating the input and removal processes of dissolved Cr to/from various environments (including weathering, benthic, and hydrothermal fluxes), which are poorly constrained to date. Lastly, Cr isotopes are proven as promising tool to fingerprint ligand-bound Cr(III) that has been difficult to characterise/quantify with conventional analytical approaches.
We performed field sampling from two localities, Veronica (Italy) and Sukinda (India), and characterised soil materials (Italy) and geologic materials (i.e. sediments, India) that were collected. Then, laboratory-controlled batch experiments were conducted, with two types of low-molecular weight organic acids and under both oxygenated and O2-free conditions, in order to understand the leaching behaviour of Cr and Cr isotopes. This was complemented by a flow-through column setup which simulates a more complicated natural system. Through collaboration, we have established methods for analysing Cr isotopes in high-matrix samples, and tested accuracy and precision of the method. We developed the first Cr isotope model for quantifying the effects of ligand-promoted dissolution.
We highlight the following results achieved over the time course of this project:
(1) A fraction of Cr(III), together with iron (Fe) and manganese (Mn), can be effectively released from the solid phase by citric and oxalic acid, due to ligand-cation complexation. The Cr-Fe-Mn continuum is clearly sensitive to the presence of oxygen, as well as types of ligands; for example, hydrolysis precipitation of Fe is enhanced in the presence of oxalic acid under oxic condition. Whilst formation of authigenic Fe particles means that a fraction of dissolved Cr is scavenged, such that solid phase Cr may be associated with Fe-(oxyhydr)oxides, Cr does not seem to be remobilised during reductive dissolution of Fe (and Mn).
(2) Cr isotope analyses show distinct δ53Cr values for the materials from Italy and India, respectively, as well as large variations (~ −1.4 to +0.15‰) at initial stages of the organic acid leaching, which can be primarily explained by kinetic fractionation and/or two-phase mixing models. For the first time, these results reveal the potential of Cr isotopes to discriminate mineral dissolution pathways, and to trace the transport and fate of ligand-bound Cr(III) (i.e. distinct from Cr(VI) anions) in natural waters.
We highlight the following results achieved over the time course of this project:
(1) A fraction of Cr(III), together with iron (Fe) and manganese (Mn), can be effectively released from the solid phase by citric and oxalic acid, due to ligand-cation complexation. The Cr-Fe-Mn continuum is clearly sensitive to the presence of oxygen, as well as types of ligands; for example, hydrolysis precipitation of Fe is enhanced in the presence of oxalic acid under oxic condition. Whilst formation of authigenic Fe particles means that a fraction of dissolved Cr is scavenged, such that solid phase Cr may be associated with Fe-(oxyhydr)oxides, Cr does not seem to be remobilised during reductive dissolution of Fe (and Mn).
(2) Cr isotope analyses show distinct δ53Cr values for the materials from Italy and India, respectively, as well as large variations (~ −1.4 to +0.15‰) at initial stages of the organic acid leaching, which can be primarily explained by kinetic fractionation and/or two-phase mixing models. For the first time, these results reveal the potential of Cr isotopes to discriminate mineral dissolution pathways, and to trace the transport and fate of ligand-bound Cr(III) (i.e. distinct from Cr(VI) anions) in natural waters.
The scientific impact of this project is twofold. First, we clearly demonstrate the role of organic acids in mobilising Cr even under reducing condition (and alkaline pH) where Cr is thought to be largely immobilised. The experiments were done in well-constrained systems to decipher the mechanism, and the impact can be extrapolated to various Earth’s environments, and to both the modern day and the geological past, where organic ligands are likely present throughout. Second, our results offer a validation of the power of isotope measurements for better quantifying the transfer of elements between different phases. The application of novel isotope tools to the study of biogeochemical processes is still at an early stage, the community will be inspired to build on this work.
Furthermore, this project will provide valuable information to policy makers on pollution mitigation strategy, and to the next generation on biogeochemical cycling operating in the Earth’s system.
Results from this project have been presented at international academic conferences as well as science engagement activities; a manuscript is being prepared for high-profile journal. The larger impact of this project will continue to be exploited and reported.
Furthermore, this project will provide valuable information to policy makers on pollution mitigation strategy, and to the next generation on biogeochemical cycling operating in the Earth’s system.
Results from this project have been presented at international academic conferences as well as science engagement activities; a manuscript is being prepared for high-profile journal. The larger impact of this project will continue to be exploited and reported.