Modelling of Metal-thiol interactions in the environment – Me-Thiol

Some particularly toxic metals, such as Hg, Ag, Cd and emerging contaminants like Pt and Pd, have a strong affinity for thiol functional groups (-SH) in organic matter (OM). Despite, -SH groups are omitted in models that predict the fate and bioavailability of metals. Indeed, -SH groups have long been considered to be scarce and unstable over time, unlike the carboxylic (-COOH) and phenolic (-PhOH) groups in OM. Recent studies have demonstrated the presence of thiol compounds in many natural environments at sufficiently high concentrations (nM-µM) to control the speciation of toxic metals. Furthermore, although many thiol compounds are rapidly (hours/days) degraded in the presence of O2, some thiol compounds persist in reduced form even under extreme purification conditions. Furthermore, -SH groups are produced continuously (i) under metallic stress by living organisms and (ii) by thiolation of OM, which occurs in environments rich in OM and natural or anthropogenic sulphides. This suggests a cryptic -SH cycle. The Me-Thiol project therefore aims to gain a better understanding of the mechanisms that control the speciation of the most toxic metals by assessing how metal-SH interactions influence their fate.
But 4 major challenges need to be overcome: (i) the reference OMs currently used are depleted in -SH (because of the purification protocols), which prevents a complete study of metal-SH interactions. (ii) The OMs contain -COOH, -PhOH and -SH functions that complex metals and it is difficult to distinguish between these different complexation sites, which is crucial for studying the speciation of metal mixtures. (iii) -SH functions, unlike -COOH and -PhOH functions, are redox-sensitive because they reduce certain metals (by oxidation of thiols), which is not taken into account in speciation models. (iv) The -SH functions promote the formation of nanoparticles, whether they are of metal in the native state or via the formation of metal sulphides, produced from metal-thiol complexes. It is therefore necessary to study these mechanisms simultaneously (complexation, redox and precipitation) in order to study metal-thiol interactions.
To this end, biological thiol compounds (FOM) will be produced and wetland soil OM (COM) will be extracted, in order to provide thiol compounds representative of natural environments (WP1). Thiol compounds (cysteine, used as a model molecule, FOM and COM) will be used to deconvolute the various metal-thiol interaction mechanisms (complexation, redox and precipitation). The targeted metals are Cu(0, I, II), Cd(II) and Ag(0, I), but the list of metals will be extended for cysteine (WP2), which will be used as a reference compound to extrapolate the results of this project to other metals. Finally, a predictive model of metal speciation in the environment, taking into account metal-SH interactions, will be developed from experimental data by providing a database of the complexation, redox kinetics and precipitation constants available to all (WP3). A linear relationship between metal-cysteine and metal-COM/FOM complexation constants will be proposed in order to estimate complexation constants for a wider range of metals.
This new model will be developed in a popular, open-access speciation code so that the results can be widely disseminated to the public and private sectors. The results of Me-Thiol will provide knowledge and a model for potential applications in risk assessment, the development of remediation methods, ecotoxicology, the development of more efficient systems of methanisation, and health.