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 of As(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 on As(III) sorption to HAs, that have either been grafted with thiol or not, were thus conducted to investigate the preferential As(III) binding sites. There was a low level of binding of As(III) to HA, which was strongly dependent on the abundance of the thiols. Experimental datasets were used to develop a new model (the modified PHREEQC-Model VI), which defines HA as a group of discrete carboxylic, phenolic and thiol sites. Protonation/deprotonation constants were 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, phenolic and thiols sites, respectively) from HAs that were either grafted with thiol or not. The pKS value corresponds to that of single thiol-containing organic ligands. Two binding models were tested: the Mono model, which considered that As(III) is bound to the HA thiol site as monodentate complexes, and the Tri model, which considered that As(III) is bound as tridentate complexes. A simulation of the available literature datasets was used to validate the Mono model, with logKMS=2.91±0.04, i.e. the monodentate hypothesis. This study highlighted the importance of thiol groups in OM reactivity and, notably, determined the As(III) concentration bound to OM (considering that Fe is lacking or at least negligible) and was used to develop a model that is able to determine the As(III) concentrations bound to OM.

Does As(III) interact with Fe(II), Fe(III) and organic matter through ternary complexes?

“…Because As(III) can bind to OM thiol groups, the modeling calculations were performed using a modified version of the PHREEQC/Model VI allowing this particular binding to be taken into account [21]. In the modified PHREEQC-Model VI, the ions complexation occurs through 12 discrete sites: four carboxylic groups (sites A), four phenolic groups (sites B) and four thiol groups (sites S).…”

“…The abundances, intrinsic acidity constant for A, B and S sites and their distribution term are denoted as n A , n B , n S , pK A , pK B , pK S , ΔpK A , ΔpK B and ΔpK S , respectively. Only monodentate complexes of As(III) with thiols are defined [21]. The fraction of proton sites that can form bidentate and tridentate complexes are named f B and f T , respectively [34].…”

“…More recently, several studies demonstrated that OM may directly bind As(III,V). Different mechanisms were put forward to describe As-OM binding, including As(III, V) complexation with OM carboxylic and phenolic groups [17,18], or As(III) binding with OM thiol groups [19][20][21]. However, most of the As bound to OM generally occurs as As-Fe-OM ternary complexes in several systems, such as peatland, riparian wetlands, streams, groundwaters [13,[22][23][24][25][26][27].…”

Metal and Metalloid Speciation in Aquatic Environments: Concepts, Techniques and Applications

“…19,20 Os principais sítios complexantes das SH incluem grupos carboxilas, fenólicos, carbonilas e tióis, que são responsáveis pela sua interação com metais e metaloides. 3,18,21,22 As suas características e reatividade são dependentes da sua origem, o que também vai afetar o comportamento de metais-traços em meio aquático. 23,24 As SH são conhecidas por formar homoagregados (agregação com mesmo tipo de coloide) ou heteroagregados (agregados com mais de um tipo de coloide), levando à formação de diferentes formas físico-químicas dos coloides, nas quais metais e metaloides podem interagir.…”

XAFS analysis of Arsenic bound in holocellulose extracted from organic-rich contaminated sediments

“…This material is one of the precursors of humic substances, which consist of a heterogeneous mixture with no single structural formula. The arsenic in humic acid has been reported bound via inorganic elements such as iron, alumina, calcium, sulphur, or directly bound with amino, carboxyl, and phenolic functional group in humic substances [3][4][5]. The process whereby arsenic is incorporated into humic substances determines its stability in organic sediments and the evaluation of its toxicity.…”