Magnetite (Fe3O4) nanoparticles have shown great potential for sorption of arsenic in contaminated soil and water because of the small particle size, large specific surface area, and high sorption capacity and affinity. To prevent nanoparticles from aggregating, and thus, to enhance sorption capacity and soil deliverability, starch has been chosen to act as a stabilizer in preparation of the Fe3O4 particles. However, a molecular level understanding has been lacking. A team from Research Center for Eco-Environmental Sciences (RCEES), CAS, has gained insight into bridge this knowledge gap. Their research has been published in August, 2011 in Environmental Pollution.

The team synthesized magnetite nanoparticles by co-precipitation method, using starch as a stabilizer. They found that the adsorption characteristics of starch-stabilized magnetite nanoparticles (SMNP) and their particle sizes could be manipulated by adjusting the concentration of starch. Starch can not only control the particle size of SMNP, but the adsorption affinity for arsenate. Such information is critical for predicting arsenic fate and transport in the environment as well as for guiding the design of more effective nano-scale sorbents for arsenic removal.

The presence of starch leads to the formation of more effective adsorbing sites on magnetite particle surfaces. Arsenate is adsorbed on starch stabilized magnetite nanoparticles mainly as inner-sphere bidentate and monodentate complexes. The coordination numbers of As-Fe binding increases with increasing starch concentration, which indicates that the arsenate is more firmly adsorbed at higher starch concentrations. The XAFS analyses were performed on the Beamline 4W1B of BSRF.

The research finds the application of starch as a stabilizer in preparation of the Fe3O4 particles is able to effectively reduce particle aggregation, and thus, particle size, resulting in much greater specific surface area and adsorption sites. XAFS spectroscopy was employed to examine the local structure of the arsenate complexes and the local structure of the average Fe atoms. Synchrotron-based XAFS can yieldchemical and structural information for specific elements in a variety of unmodified—even hydrated—samples f surface species. The information provided by XAFS includes an element’s oxidation state and coordination chemistry (in the form of the average number, distance, and atomic species of elements surrounding the element of interest). Synchrotron capabilities hold great promise for more sophisticated studies and increased understanding of metal transformations at water-mineral interfaces. We hope that we can carry out time-resolved XAFS experiment in the near future to study of dynamic chemical systems in non steady-state environments, and, for instance, the elucidation of the structure of intermediates involved in chemical processes.

Article:

Meiyi Zhang, Gang Pan*, Dongye Zhao, Guangzhi He,XAFS study of starch-stabilized magnetite nanoparticles and surface speciation of arsenate, Environ Pollut. 2011, 159(12), 3509-14.