Stephanie Richins, an undergrad sophomore majoring in chemical engineering at NMSU, in conjunction with her faculty advisor, Dr. Hongmei Luo, also in the chemical engineering department at NMSU, have explored the route of mitigation through chemical conversion, enhanced by the use of a suitable catalyst and energy source. The research was supported in part by an NM WRRI Student Water Research Grant, and is summarized in a report titled: Oxidation of Arsenite by a Carbon Nitride Photocatalyst with Graphitized Polyacrylonitrile.
The methodology employed in this study has included the synthesis and analysis of a suitable catalyst, graphitic carbon nitride, and the use of a solar simulator to provide ultraviolet light to speed up the oxidation of Arsenic III to Arsenic V in the presence of the catalyst. Properties of the synthesized catalyst were established and confirmed using an array of instrumental techniques including X-ray diffractometer, infrared spectroscopy, transmission electron microscopy, and scanning electron microscopy combined with energy-dispersive X-ray analysis. These techniques permitted the identification of phase and structure of nanomaterials, as well as their morphology and particle size distributions, and the identification of elemental composition. More specifically, it revealed the concentration of active sites on the photocatalyst material, and confirmed, through the detected ratio of carbon to nitrogen in the material, that the synthesis of the catalyst was carried through to completion successfully.
Having established the properties of the catalyst, experiments were carried out to test the efficiency of conversion of arsenite to arsenate in water in the presence of the catalyst and ultraviolet light. The tracking of this chemical reaction as a function of time was accomplished using an inductively coupled plasma mass spectrometer. This device produces a very hot plasma that can ionize a small sample of solution, which then absorbs and emits light that is analyzed by the mass spectrometer; as a result the information in the light spectrum can be used to estimate the ratio of arsenic species in the water solution. It was found that roughly one half of the Arsenic III could be converted to Arsenic V within two hours of reaction time.
These results are very encouraging, and Stephanie hopes there will be an opportunity to pursue follow-on research, such as graphitic carbon nitride with modification or functionalization for large surface area and more active catalytic sites. As she notes, water agencies that work on arsenic contamination problems should be able to make use of this research. For example, the U.S. Environmental Protection Agency has restrictions on the amount of arsenic in water, and this research could help find a cost effective way to better control that contamination.
The final report for the project is on the NM WRRI website by clicking here.