Arsenic (As) and antimony (Sb) have multiple oxidation states (-3, 0, +3, +5) and their behaviour in environments (e.g. toxicity, mobility, adsorption capacity) can be varied depending on their speciation. Accordingly, the importance of studying on removal mechanism under the various oxidation or reduction conditions has been recognized.
This study was aimed to investigate the removal mechanism of As(Ⅲ), As(Ⅴ), Sb(Ⅲ), Sb(Ⅴ) with mackinawite (FeS), a reduced iron sulfide, by conducting batch experiments such as equilibrium experiments and adsorption envelope experiments under a strictly anaerobic condition. The transformation of oxidation state during the reaction was also investigated in aqueous phase through the speciation methods and in solid phase using spectroscopic methods such as X-ray Photoelectron Spectroscopy (XPS), X-ray Absorption Spectroscopy (XAFS), Scanning Electron Microscope-Energy Dispersive Spectroscopy (SEM-EDS).
The results of batch studies demonstrate that the important parameters controlling the reaction of As or Sb with mackinawite are the oxidation state of As or Sb in aqueous phase and pH conditions in reaction batches. The removal efficiencies of As(Ⅲ) and Sb(Ⅲ) are much higher than those of As(Ⅴ) and Sb(Ⅴ). The removal efficiencies of As or Sb by mackinawite is also highly pH-dependent showing the rapid decrease of removal under neutral or alkaline pH conditions. These behaviors can be explained by the low solubility of mackinawite at pH more than 6. At this pH range, the surface reaction is dominantly responsible to the removal of As or Sb and negative surface charges of both mackinawite and oxyanions of As or Sb seemed to cause electrostatic repulsion between the surfaces. Based on the results of spectroscopic study, the information on the reaction species of As or Sb with mackinawite in solid phase was obtained. In the case of As, regardless of initial oxidation states, As was found to be precipitated as As2S3 at pH 5 whereas absorbed on the surface of mackinawite at pH 7 and 9. Similar to the reaction between As and mackinawite, Sb seemed to be precipitated as Sb2S3 under pH 5 by reacting with S2- ion dissolved from mackinawite (FeS), whereas, under pH 7 and 9, it was suggested that the predominant reactions for Sb removal were the adsorption of Sb on the mackinawite and the precipitation of Sb2O5 due to low solubility of Sb2O5. The results of this study demonstrated the analogous reaction mechanism of As and Sb species with mackinawite under anaerobic environments.