Long-term stabilization of arsenic-alkali residues via adjusting As redistribution and mineralogical transformation

Increasingly stringent environmental standards pose a huge challenge to the treatment of arsenic alkali residue (AAR). This work proposed an alternative method for stabilizing AAR based on lab and field-scale study. Three methods were discussed by introducing bleach and/or FeCl3 in different dose and order to obtain the optimistic procedure. The redistribution and mineralogical transformation of As in AAR were illustrated based on stoichiometric, XRD, and XPS analyses. The leachate As concentration decreased significantly with high dose of bleach addition (>4%) via supplying sufficient hypochloric acid and labile Ca. The high stability of AAR was obtained through the oxidation of (CaHAsO3)-O-III and then formation of high alkaline-dependent Ca-4(OH)(2)((AsO4)-O-V)(2)center dot 4H(2)O minerals (pH >= 12). However, the leachate pH failed to meet the standard for landfill (pH12). In contrast, synchronously introducing low dose of bleach (3 %) and FeCl3 (3 %-5%) could achieve the expected standard of leachate As concentration and pH value. The oxidation efficiency of (CaHAsO3)-O-III by bleach was facilitated with FeCl3 addition via the accelerated dissolution of (CaHAsO3)-O-III. And then the generation of stable Ca - Fe arsenates (Ca2Fe3((AsO4)-O-V)(3)O-2 center dot 3H(2)O) was promoted via creating an appropriate Ca/Fe/As ratio and pH circumstance (8.23-9.61). Compared with the synchronous treatment, the obvious rebound of leachate As concentrations was observed with sequentially adding bleach and FeCl3. The stability of the formed Ca - As-V minerals with bleach treatment was destroyed by the sequent addition of FeCl3 due to the generated CO2 from CaCO3 dissolution and decreased pH values (10.2-10.9). The redistribution and transformation of As in AAR were remarkably affected by the dose and order of adding remediation agent. The formation of Ca - Fe arsenates with high As capacity and stability was curial for AAR stability via synchronous treatment, which provided an alternative option for AAR stabilization.