Enhancement of Arsenic
Adsorption during Mineral Transformation
from Siderite to Goethite: Mechanism and Application
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Abstract
Synthesized siderite was used to remove As(III) and As(V)
from
water solutions under anoxic conditions and oxic conditions. Results
showed that As adsorption on synthetic siderite under anoxic conditions
was around 10 mg/g calculated with Langmuir isotherm. However, the
calculated As adsorption on synthetic siderite under oxic conditions
ranged between 115 and 121 mg/g, which was around 11 times higher
than that under anoxic conditions. It was found that 75% siderite
was transformed into goethite during oxic adsorption. However, synthetic
goethite had lower As adsorption capacity than siderite under oxic
conditions, although its adsorption capacity was a little higher than
siderite under anoxic conditions. It suggested that the coexistence
of goethite and siderite bimineral during mineral transformation probably
contributed to the robust adsorption capacity of siderite under oxic
conditions. Results of extended X-ray absorption fine structure (EXAF)
spectroscopy indicated both As(III) and As(V) formed inner-sphere
complexes on the surface of As-treated solid regardless of substrates,
including the bidentate binuclear corner-sharing (<sup>2</sup>C) complexes
and the monodentate mononuclear corner-sharing (<sup>1</sup>V) complexes.
Monodenate (<sup>1</sup>V) and bidentate (<sup>2</sup>C) complexes
would be related to high As adsorption capacity of siderite under
oxic conditions. It showed that more Fe atoms were coordinated with
As atom in the monodentate complexes and the bidentate complexes of
As(V)/As(III)-treated siderite under oxic conditions, in comparison
with As(V)/As(III)-treated siderite under anoxic conditions and As(V)/As(III)-treated
goethite. Calcinations of natural siderite resulting in the coexistence
of goethite and siderite greatly increased As adsorption on the solid,
which confirmed that the coexistence of bimineral during mineral transformation
from siderite to goethite greatly enhanced As adsorption capacity
of siderite adsorbent. The observation can be applied for modification
of natural siderite for As removal from high As waters