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 WX
4
(X can be
monovalent anions), its reduction state or association with the elements
that can be oxidized, and existence in silicates. B was as easily
released into the environment as Mo and W. It can have several associations
with the other cations, such as Ca
2+
, Na
+
, and
Mg
2+
, and occurs in silicates. In contrast, most of the
Cr and Sb were locked in silicates, indicating that they were very
stable in fly ash. In addition, P, V, and As can exist within the
structure of silicates as well. However, a considerable amount of
them leached in the reduction step with a low pH. Hence, they can
be associated with Ca
2+
, Na
+
, Mg
2+
, or Fe
3+
. In terms of Se, oxidation processes played
an important role in controlling its leaching because of the oxidation
of Se(IV) to Se(VI). Calcium selenite should be the predominant form
of Se in fly ash.
