Arsenic-contaminated red mud (As-RM) is a hazardous waste with limited recycling approaches. Generally, through reductive roasting and magnetic separation, RM could be transformed into Fe-rich concentrate for Fe resource recovery. However, due to the poor thermostabilization of As species, reductive roasting of As-RM would cause severe As volatilization pollution together with high As leaching risks from heated residue. Herein, a novel johnbaumite-based As thermostabilization strategy is developed for clean Fe resources recycling from As-RM. We found that in the presence of Ca(OH) 2 , the As species in As-RM could be immobilized as thermostable and insoluble johnbaumite (Ca 5 (AsO 4 ) 3 OH) at 900 °C, effectively enhancing the As thermostability and insolubility. Introducing 1.5% Ca(OH) 2 into As-RM suppressed the As volatilization ratio from 60.3% to 15.7% during reductive roasting. Meanwhile, the As leaching concentration of the reduced residue was reduced to < 100 µg/L, thus satisfying the Japanese wastewater discharge standard. A concentrate with approximately 67.5% total iron grade was obtained from As-RM through this clean reductive roasting and magnetic separation. Overall, the approach introduced in this work effectively reduces the As diffusion pollution deriving from As-RM thermal reduction, which could contribute to hazardous As-RM reutilization, clean Fe resources recovery, and As pollution mitigation.
Qimei Hu,Li Li,Jie Li,Xin Sun,Congqi Yan,Minlin Mao,Zhang Lin,Weizhen Liu
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.
Xin Sun,Minlin Mao,Kaibin Lu,Qimei Hu,Weizhen Liu,Zhang Lin
High-level arsenic-containing wastewater (HAW) causes serious environmental pollution . Chemical precipitation is the most widely used technology for treating HAW. However, chemical precipitation generates huge amounts of hazardous solid wastes, which leads to secondary pollution. In this work, an efficient method, producing no secondary pollution was developed for one-step complete removal of As(V) from HAW using a hazardous solid waste namely arsenic-bearing gypsum (ABG). After the treatment, ABG was transformed into highly stable and environment-friendly mineral Johnbaumite. Meanwhile, the arsenic concentration in the wastewater decreased from 10,000 mg L −1 to 0.22 mg L −1 under optimized hydrothermal conditions (ABG dosage of 50 g L −1 , solution pH of 13.5, temperature of 150 °C for 12 h). The mechanism mainly included the following processes: (i) The phase transformation of ABG resulted in the release of calcium and hydrogen arsenate ions in ABG, (ii) Hydrogen arsenate ions transformed into arsenate ions in alkaline environment , and (iii) Under alkaline conditions, calcium ions combined with arsenate ions to form Johnbaumite, whereas the hydrothermal conditions accelerated the crystal growth of Johnbaumite. This study provides a new idea for the synchronous treatment of toxic heavy metal-containing wastewaters and hazardous solid wastes.
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