3 research outputs found
Enhancement of Arsenic Adsorption during Mineral Transformation from Siderite to Goethite: Mechanism and Application
Synthesized siderite was used to remove AsĀ(III) and AsĀ(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 AsĀ(III) and AsĀ(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
AsĀ(V)/AsĀ(III)-treated siderite under oxic conditions, in comparison
with AsĀ(V)/AsĀ(III)-treated siderite under anoxic conditions and AsĀ(V)/AsĀ(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
Stimulation of Fe(II) Oxidation, Biogenic Lepidocrocite Formation, and Arsenic Immobilization by <i>Pseudogulbenkiania</i> Sp. Strain 2002
An anaerobic nitrate-reducing
FeĀ(II)-oxidizing bacterium, <i>Pseudogulbenkiania</i> sp.
strain 2002, was used to investigate
As immobilization by biogenic Fe oxyhydroxides under different initial
molar ratios of Fe/As in solutions. Results showed that FeĀ(II) was
effectively oxidized, mainly forming lepidocrocite, which immobilized
more AsĀ(III) than AsĀ(V) without changing the redox state of As. When
the initial Fe/As ratios were kept constant, higher initial FeĀ(II)
concentrations immobilized more As with higher As<sub>immobilized</sub>/Fe<sub>precipitated</sub> in biogenic lepidocrocite. EXAFS analysis showed that variations of initial FeĀ(II) concentrations
did not change the AsāFe complexes (bidentate binuclear complexes
(<sup>2</sup><i>C</i>)) with a fixed AsĀ(III) or AsĀ(V) initial
concentration of 13.3 Ī¼M. On the other hand, variations in initial
As concentrations but fixed FeĀ(II) initial concentration induced the
co-occurrence of bidentate binuclear and bidentate mononuclear complexes
(<sup>2</sup><i>E</i>) and bidentate binuclear and monodentate
mononuclear complexes (<sup>1</sup><i>V</i>) for AsĀ(III)
and AsĀ(V)-treated series, respectively. The coexistence of <sup>2</sup><i>C</i> and <sup>2</sup><i>E</i> complexes (or <sup>2</sup><i>C</i> and <sup>1</sup><i>V</i> complexes)
could contribute to higher As removal in experimental series with
higher initial FeĀ(II) concentrations at the same initial Fe/As ratio.
Simultaneous removal of soluble As and nitrate by anaerobic nitrate-reducing
FeĀ(II)-oxidizing bacteria provides a feasible approach for in situ
remediation of As-nitrate cocontaminated groundwater
Sulfur Cycling-Related Biogeochemical Processes of Arsenic Mobilization in the Western Hetao Basin, China: Evidence from Multiple Isotope Approaches
The role of sulfur cycling in arsenic
behavior under reducing conditions
is not well-understood in previous investigations. This study provides
observations of sulfur and oxygen isotope fractionation in sulfate
and evaluation of sulfur cycling-related biogeochemical processes
controlling dissolved arsenic groundwater concentrations using multiple
isotope approaches. As a typical basin hosting high arsenic groundwater,
the western Hetao basin was selected as the study area. Results showed
that, along the groundwater flow paths, groundwater Ī“<sup>34</sup>S<sub>SO4</sub>, Ī“<sup>18</sup>O<sub>SO4</sub>, and Ī“<sup>13</sup>C<sub>DOC</sub> increased with increases in arsenic, dissolved
iron, hydrogen sulfide and ammonium concentrations, while Ī“<sup>13</sup>C<sub>DIC</sub> decreased with decreasing Eh and sulfate/chloride.
Bacterial sulfate reduction (BSR) was responsible for many of these
observed changes. The Ī“<sup>34</sup>S<sub>SO4</sub> indicated
that dissolved sulfate was mainly sourced from oxidative weathering
of sulfides in upgradient alluvial fans. The high oxygenāsulfur
isotope fractionation ratio (0.60) may result from both slow sulfate
reduction rates and bacterial disproportionation of sulfur intermediates
(BDSI). Data indicate that both the sulfide produced by BSR and the
overall BDSI reduce arsenic-bearing ironĀ(III) oxyhydroxides, leading
to the release of arsenic into groundwater. These results suggest
that sulfur-related biogeochemical processes are important in mobilizing
arsenic in aquifer systems