Distributions and Leaching Behaviors of Toxic Elementsin Fly Ash

摘要

Fly ash usually contains a considerable amount of toxic elements that can be leached into the environment, thereby easily leading to serious contaminations. In this work, the leaching behaviors of poisonous elements including boron (B), phosphorus (P), vanadium (V), chromium (Cr), arsenic (As), selenium (Se), molybdenum (Mo), antimony (Sb), and tungsten (W) from fly ash were explored by sequential extraction. Importantly, the associations of these elements in fly ash were discussed based on their leaching and X-ray absorption near-edge structure (XANES) results. From the XANES results, it was observed that V(IV), Cr(III), As(V), Se(IV), and W(IV) were their main states of existence in fly ash. In terms of leaching results, large amounts of Mo and W were leached into pure water, which indicated their high mobilities. Furthermore, the occurrence of Mo in fly ash was mainly in the form of oxides, and W had complex associations including WX4 (X can be monovalent anions), its reduction state or association with the elements that can be oxidized, and existence in silicates. B was as easilyreleased into the environment as Mo and W. It can have several associationswith the other cations, such as Ca²⁺, Na⁺, andMg²⁺, and occurs in silicates. In contrast, most of theCr and Sb were locked in silicates, indicating that they were verystable in fly ash. In addition, P, V, and As can exist within thestructure of silicates as well. However, a considerable amount ofthem leached in the reduction step with a low pH. Hence, they canbe associated with Ca²⁺, Na⁺, Mg²⁺, or Fe³⁺. In terms of Se, oxidation processes playedan important role in controlling its leaching because of the oxidationof Se(IV) to Se(VI). Calcium selenite should be the predominant formof Se in fly ash.