Enhanced removal of high-As(III) from Cl(-I)-diluted SO4(-II)-rich wastewater at pH 2.3 via mixed tooeleite and (Cl(-I)-free) ferric arsenite hydroxychloride formation.

An advanced cost-saving method of removal of high-As(III) from SO4(-II)-rich metallurgical wastewater has been developed by diluting the SO4(-II) content with As(III)-Cl(-I)-rich metallurgical wastewater and then by the direct precipitation of As(III) with Fe(III) at pH 2.3. As(III) removal at various SO4(-II)/Cl(-I) molar ratios and temperatures was investigated. The results showed that 65.2‒98.2% of As(III) immobilization into solids occurred at the SO4(-II)/Cl(-I) molar ratios of 1:1‒32 and 15‒60 °C in 3 days, which were far higher than those in aqueous sole SO4(-II) or Cl(-I) media at the equimolar SO4(-II) or Cl(-I) and the same temperature. SO4(-II)/Cl(-I) molar ratio of 1:4 and 25 °C were optimal conditions to reach the As removal maximum. Mixed aqueous SO4(-II) and Cl(-I) played a synergetic role in the main tooeleite formation together with (Cl(-I)-free) ferric arsenite hydroxychloride (FAHC) involving the substitution of AsO33- for Cl(-I) for enhanced As fixation. The competitive complexation among FeH2AsO32+, FeSO4+ and FeCl2+ complexes was the main mechanism for the maximum As(III) precipitation at the SO4(-II)/Cl(-I) molar ratio of 1:4. Low As(III) immobilization at high temperature with increased Fe(III) hydrolysis was due to the formation of As(III)-bearing ferrihydrite with the relatively high Fe/As molar ratio at acidic pH.