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Transformation of arsenic in offshore sedimentunder the impact of anaerobic microbial activities
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The stability of sedimentary arsenic significantly depend on many processes, such as adsorption, desorption, oxidation, reduction, dissolution, precipitation or co-precipitation. And thus the arsenic species will changed between different speciation with those chemical processes and redistributed in different phases. Although the arsenic speciation under microbial activity was controlled by many factors, the redox conditions within the sediment appear to directly affect arsenic speciation. Microbial activity strongly affects the redox conditions in sediment, speciation transformation of iron and sulfate, and thus ties to the stability of arsenic in sediments. It is important to shed more light on the behavior and the fate of arsenic in offshore sediment of seawater systems. A team from Institute of Applied Ecology, Chinese Academy of Sciences has gained insight into the fate and pathways of arsenic speciation conversion from oxyhydroxide-associated to sulfide-associated forms. Their research has been published on Desember 15th, 2011 in Water Research.

In present work, the fate of arsenic was discussed based on quantitative analysis of aqueous and solid arsenic and iron, and qualitative characterization using X-ray absorption near edge spectroscopy (XANES). It is proposed from XANES evidence that orpiment-like mineral is the major As sulfide phase precipitated. This indicates that the arsenic in offshore sediment has undergone a transformation process from oxyhydroxide associated arsenate to sulfide associated forms due to anaerobic microbial activities. Arsenic was released and reduced upon development of anoxic conditions but was resequestered by authigenic minerals later.

 

Left: Arsenic XANES spectra (A) arsenate. (B) arsenite. (C) arsenopyrite. (D) realgar. (E) orpiment. (F) as-received sediment. (G) the sediment incubated at 30 °C for 20 days. (H) the sediment incubated at 30 °C for 60 days (I) the sediment stored in tightly sealed bottle at 4 °C for 1.5 year. Right: Sulfur XANES spectra (A) disordered mackinawite. (B) pyrite. (C) arsenopyrite. (D) realgar. (E) orpiment. (F) elemental sulfur. (G) ferrous sulfate. (H) ferric sulfate. (I) the sediment incubated at 30 °C for 20 days. (J) as-received sediment.

Arsenic K-edge XANES spectra were collected on beamline U7C (XAFS station) at National Synchrotron Radiation Laboratory (NSRL) of China. Sulfur K-edge XANES spectra were collected on the mid-energy X-ray station at Beijing Synchrotron Radiation Facilities. In present work, it is obtained the transformation of As and S in offshore sediment under the anaerobic microbial activities. Sulfate was transformed from S (+VI) to S(0), S(-1) and S(-II), while the formation of orpiment-like mineral was the major mechanism for As sequestration in an estuarine sediment.

The present work documented a transformation process of Fe oxyhydroxides associated As to sulfide associated As upon environment shifting from oxic-suboxic to anoxic-sulfidic condition in a sulfur-rich system, e.g. offshore sediment.

The authors proposed that the arsenic speciation and behavior under microbial activity depend strongly upon specific environmental conditions, the fate was probably different with different scenarios. Realgar was identified as the dominant As species in the aquifer sediment infiltrated with marine water of San Francisco Bay (O’Day et al., 2004), whereas the fate of arsenic in the sediment of Jinzhou Bay in China is the orpiment-like mineral.

 

Article:

Liying Xu, Zhixi Zhao, Shaofeng Wang, Rongrong Pan, Yongfeng Jia*.Transformation of arsenic in offshore sediment under the impact of anaerobic microbial activities. Water research, 2011, 45,6181-6188.

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