Stabilization of Arsenic Sulfide Sludge to Form Stable Johnbaumite by Alkaline-Oxidative Hydrothermal Treatment

The arsenic sulfide sludge (ASS) generated from the treatment of acidic arsenic-containing wastewater by sulfide precipitation causes severe environmental pollution due to its potential to release highly toxic As(III). This work describes an alkaline-oxidative hydrothermal method that converted ASS to the highly stable johnbaumite mineral (Ca5(AsO4)3(OH)) in which arsenic exists as the less toxic As(V). Under the optimal reaction conditions, i.e., with H2O2 (ω = 3.3 wt %) and Ca(OH)2 (mCH/mASS = 1:1) at pH = 14.18 and 180 °C for 18 h, the arsenic leaching was reduced from 762.85 mg/L for the original ASS down to 0.04 mg/L for the hydrothermally treated product, far below the 1.2 mg/L limit set in the Chinese standard GB 18598-2019. The stabilization of arsenic mainly involved three steps: (i) As(III) in ASS was released into the alkaline solution and oxidized to AsO43–, (ii) the free AsO43– was captured by Ca2+ to form the Ca3(AsO4)2 and Ca5(AsO4)3(OH), (iii) the intermediate Ca3(AsO4)2 was further converted to johnbaumite under hydrothermal condition, and eventually AsO43– was stably encapsulated in the johnbaumite lattice. This study thus provided an effective strategy for the treatment and safe disposal of ASS and revealed the mechanisms underlying arsenic stabilization via johnbaumite.