Colloids control the environmental fate of redox-sensitive trace elements – COLOSSAL

The COLOSSAL team, made up of members from the Institut de Physique du Globe de Paris and Géosciences Rennes, will tackle these challenges by developing three interdependent workpackages (WP).

WP1 aims to determine a relationship between the composition/structure of colloids and the redox mechanisms of ETM onto their surface. A combination of cutting-edge techniques, including spectroscopy, isotopy, microscopy, nanometrology, and analytical tools, will be used to study the processes, both at the natural water-colloid interface and in aqueous solutions. Studying colloidal suspensions under controlled redox conditions is a challenge that we have taken up, for example by: (i) modifying the Fe(II) content of magnetite nanoparticles as desired for the study of reducing environments, (ii) modifying the average oxidation state of Mn in MnO2 nanoparticles for oxidizing conditions, and (iii) selecting ETM according to their redox properties and various colloids (inorganic and organic).

WP2 will study the dynamic behavior of colloids under the influence of various environmental factors (microbiology, Eh, pH, T, salinity fluctuations or gradients, etc.) and the concomitant changes in the redox speciation of ETM. Simple abiotic/biomimetic experiments, microbiology, and soil/sediment incubation and leaching experiments will be set up to differentiate the effects of the many processes involved.

WP3 will develop a new generation of surface complexation models, with a coherent description of the kinetic and thermodynamic aspects of ETM redox speciation onto heterogeneous colloidal surfaces. The model will be tested and validated using data from natural soils and sediments collected under different biogeochemical and pedoclimatic conditions. The new model will be developed in a popular geochemical speciation code for optimal dissemination of results in both the public and private sectors.

In an opinion paper [1], we propose a mathematical equation (WP3) linking the surface redox speciation of an ETM to: (i) the redox potential in solution and (ii) the affinity of each of its oxidation states for surface sites (i.e., describing the left side of the illustration). The first phase of the project aimed to verify the principles of this equation by focusing on redox processes involving ETM at the surface of colloids (WP1). Thus, we were able to observe by synchrotron X-ray absorption spectroscopy (XAS) that Cr(III) is stable on the surface of many minerals [2] and, onto the surface of magnetite, Cr(VI) is very easily reduced to Cr(III), even by highly oxidized magnetites (Fe(II)/Fe(III) = R = 0.1) [3]. On the other hand, Cu(II) and U(VI) remain stable for R = 0.1, are reduced to Cu(I)/U(V) for 0.1 < R < 0.5, and even to Cu(0)/U(IV) under the most reducing conditions (R = 0.5) [4,5]. Under many conditions, the analysis of redox speciation in solution does not coincide with that at the surface. For example, Cu(II) and U(IV) are predominant at the surface, while only Cu(I) or U(VI) are detected in solution. This result strongly supports our new model, which predicts that Cu(II) and U(IV) will be stabilized by adsorption onto colloids. Similarly, organic colloids stabilize Ce(III) because it forms stronger complexes at near-neutral pH than Ce(IV), which prefers to hydroxylate [6]. Furthermore, different faces of the same colloid (here, the sides and termination of goethite particles; an Fe oxyhydroxide) do not stabilize the same oxidation state of an element (here Ce(III) and Ce(IV)), thus confirming the link between the redox speciation of an ETM and the affinity of each of its oxidation states for surface sites [7].

The processes described above are highly dependent on the physicochemical conditions of the environment, as demonstrated in the case of Co with magnetite [8,9]. But, this also affects the structure and surface of colloids and, consequently, the interactions between ETM and colloids. Thus, we have demonstrated that Cr(III) [3] and Ni(II) [10] form complexes onto the surface of oxidized magnetite (quasi-maghemite, R = 0.1) but, incorporate into Fe(II)-rich magnetite. These are our first results demonstrating the impact of colloid dynamics, under the influence of environmental factors (in this case redox conditions), on their interactions with ETM (WP2).

[1] Marsac et al., COCIS, 2024 ; [2] Nguyen et al., Coll. Surf. A, 2024 ; [3] Scaria et al., ES&T, 2025 ; [4] Scaria et al., soumis à ES&T ; [5] Yomogida et al., soumis à ChemCom ; [6] Kiththangodage et al., en préparation ; [7] Iqbal et al., ES&T, 2024 ; [8] Fablet et al., Environ. Sci. : Nano, 2024 ; [9] Fablet et al., en préparation ; [10] Fablet et al., Environ. Sci. : Nano, 2025.

After studying redox processes on the surface of pure colloidal phases, the final question to be answered by WP1 will be: “What is the redox speciation of contaminants onto the surface of heterogeneous colloids?” To do this, experiments involving organo-mineral colloids will be carried out. These data should enable us to complete the first stage of WP3, by developing and calibrating a predictive model of ETM redox speciation on the surface of colloids based on our first equation [1].

The impact of colloidal dynamics on ETM-colloid interactions and the redox speciation of ETM, under the influence of bio/hydro/pedological and climatic conditions (WP2), will be addressed from different angles and in different contexts, notably by taking advantage from new collaborations and complementary co-financing sources (e.g., INRAE, Brittany Region). Firstly, we will study these effects in an agricultural context in order to answer the following question: “What is the effect of fresh organic matter input (spreading), which is likely to evolve over time, on ETM-organic colloid interactions?” Secondly, the colloidal compartment may be affected by alternating floods and droughts under the influence of climate change. Soil incubation experiments will be carried out to study the evolution of the colloidal compartment and the redox speciation of associated ETM. Thirdly, we will address the role played by the colloidal phase in the mechanisms of ETM transfer at the soil/plant interface (pH modification, solubilization of root and bacterial exudates, etc.).

The experimental data from WP2 will be used to develop empirical equations describing and predicting changes in the surface reactivity of colloids in relation to their composition, size, structure, and morphology. These equations will be integrated into the model combining the results of WP1 and our first equation [1]. This semi-empirical approach will enable the prediction of the redox speciation of ETM in dynamic environments.

Our model will therefore have applications in various fields and contexts, such as ecotoxicology, risk assessment (e.g., underground waste storage, mining activities), and the development of remediation strategies (e.g., contaminated soils and waters mitigation). These tools will therefore be useful for decision-making, particularly in the political sphere. In addition, redox processes at the nanoparticle-water interface are at the heart of a wide range of applications such as nanotechnologies (e.g., nanomedicine, energy storage and conversion), catalysis (e.g., processes and development of new catalysts), and (bio)hydrometallurgy (e.g., gentle alternatives for extracting ETMs from raw materials and waste).

Trace elements (TE; e.g. As, Cr, U, Cu) are responsible for serious threats to the environment in both developing and industrialised countries. Although the mechanisms controlling their speciation have been extensively studied, we are still unable to correctly predict their behaviour and fate in natural systems. This is particularly true for redox-sensitive TE, whose geochemical behaviour and toxicity are primarily determined by the prevailing oxidation state. The COLOSSAL project hypothesizes that the poor understanding of the combined role of colloids and redox conditions on TE speciation is a major reason for this scientific failure. Colloids are ubiquitous in environmental systems and have been reported as major TE vectors in many environmental systems due to their extreme reactivity towards TE, and high stability in aqueous suspensions. Although recent breakthroughs have demonstrated the need to account for the TE redox reactions occurring at the colloidal surfaces, further progress is hampered by our inability to capture the extreme physical (e.g. size, shape) and chemical (e.g. composition) heterogeneities of colloids, as well as their highly dynamic behaviour and transformability due to their metastable structures. The aim of the COLOSSAL project is to advance our understanding and prediction of TE redox speciation in the environment by decoupling the slow evolution of the colloidal compartment in response to prevailing bio/hydro/pedo/climatological conditions from the fast TE redox reactions promoted by TE binding onto the colloidal surface. The COLOSSAL team will address these challenges in three interrelated work packages (WPs). WP1 will establish a relationship between the composition and structure of colloids and the reaction mechanism responsible for TE redox transformation. This will be done using a combination of state-of-the-art spectroscopy (in particular in situ and operando), isotopy, microscopy, nanometrology and analytical tools to study complex redox processes occurring both at natural (heterogeneous) colloid-water interfaces and in aqueous solutions. WP2 will investigate the dynamic behaviour of colloids under the influence of varying environmental factors (e.g. microbiology, Eh, pH, T, salinity fluctuations or gradients) and the concomitant changes in TE redox speciation. This will include simple abiotic/biomimetic experiments, microbiology experiments and soil/sediment incubation and leaching experiments to disentangle the effects of the different operational processes. WP3 will develop a new generation of mechanistic surface complexation models with a consistent description of both kinetic and thermodynamic aspects of TE redox speciation on heterogeneous colloid surfaces. This will allow appropriate interpretation of the results throughout the project, especially in WP2 where colloid characterisation and TE redox speciation results will be confronted with modelling to decouple the effects of colloids and environmental factors and thus achieve a comprehensive understanding of TE redox behaviour in natural systems. The model will be tested and validated using data from natural soils and sediments collected under different biogeochemical and pedoclimatic conditions. The new model will be developed in a popular geochemical speciation code to allow wide dissemination of the results to both the public and private sectors. By shedding light on unforeseen aspects of the biogeochemical behaviour of colloids and redox-sensitive TE, COLOSSAL will provide a key understanding and valuable tool for potential environmental application (e.g., risk assessment, development of remediation strategies, ecotoxicology), but also far beyond, as redox processes at colloid and nanoparticle-water interfaces are central to a wide range of applications (e.g., nanotechnologies, catalysis, hydrometallurgy).