Tungsten–humic substances complexation

Abstract

Tungsten (W) is an emerging contaminant whose environmental behaviors remain rather sketchy, narrow, and fragmentary. The mobility and fate of W in the aquatic environments may be influenced by naturally dissolved organic matter (DOM), nevertheless, no studies have addressed how W is bound to DOM. In this study, complexation behaviors and mechanisms of W(VI) with representative DOM, humic acid (HA) and fulvic acid (FA), were examined by batch adsorption, spectrometry, and isothermal titration calorimetry (ITC) under environmentally-relevant conditions. A higher W(VI) binding was observed at a lower pH. Compared to HA, FA showed a higher W(VI) complexing capability owing to the presence of more carboxylic groups. As shown in ITC, the carboxylic–W interaction was an endothermic process and driven by entropy, whereas the phenolic–W association was exothermic and driven by both entropy and enthalpy. The redox-active moieties within HA/FA molecules could reduce W(VI) to lower valence states species, predominantly W(V). The presence of Ca2+ not only promoted W–HA/FA complexation but also hindered W(VI) reduction. All in all, the role of dissolved organic matter in the complexation of W(VI) in the aquatic environments merits close attention. Graphical Abstract