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Thiol groups controls on arsenite binding by organic matter: New experimental and modeling evidence

Abstract : Although it has been suggested that several mechanisms can describe the direct binding ofAs(III) to organic matter (OM), more recently, the thiol functional group of humic acid (HA)was shown to be an important potential binding site for As(III). Isotherm experiments onAs(III) sorption to HAs, that have either been grafted with thiol or not, were thus conducted toinvestigate the preferential As(III) binding sites. There was a low level of binding of As(III) toHA, which was strongly dependent on the abundance of the thiols. Experimental datasetswere used to develop a new model (the modified PHREEQC-Model VI), which defines HA asa group of discrete carboxylic, phenolic and thiol sites. Protonation/deprotonation constantswere determined for each group of sites (pKA = 4.28 ± 0.03; ΔpKA = 2.13 ± 0.10; pKB = 7.11 ±0.26; ΔpKB = 3.52 ± 0.49; pKS = 5.82 ± 0.052; ΔpKS = 6.12 ± 0.12 for the carboxylic, phenolicand thiols sites, respectively) from HAs that were either grafted with thiol or not. The pKSvalue corresponds to that of single thiol-containing organic ligands. Two binding models weretested: the Mono model, which considered that As(III) is bound to the HA thiol site asmonodentate complexes, and the Tri model, which considered that As(III) is bound astridentate complexes. A simulation of the available literature datasets was used to validate2the Mono model, with log KMS = 2.91 ± 0.04, i.e. the monodentate hypothesis. This studyhighlighted the importance of thiol groups in OM reactivity and, notably, determined theAs(III) concentration bound to OM (considering that Fe is lacking or at least negligible) andwas used to develop a model that is able to determine the As(III) concentrations bound toOM.
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Charlotte Catrouillet, Mélanie Davranche, Aline Dia, Martine Bouhnik-Le Coz, Mathieu Pédrot, et al.. Thiol groups controls on arsenite binding by organic matter: New experimental and modeling evidence. Journal of Colloid and Interface Science, Elsevier, 2015, 460, pp.310-320. ⟨10.1016/j.jcis.2015.08.045⟩. ⟨insu-01187067⟩

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