Study of Arsenic Speciation by means of X-ray Raman Scattering Spectroscopy

H.J. SÁNCHEZ; JUAN JOSÉ LEANI; C.A. PEREZ

Santa Marta

Congreso; XIII Latin-American Seminar of Analysis by X-Ray Techniques (SARX 2012); 2012

Universidad Industrial de Santander

The toxicity of arsenic species is widely known. The sources of arsenic can be of natural origin, such as volcanic ash or geological material, or artificial as pesticides, mine tailings, coal fly ash, etc. In arsenic-rich areas, pollution of crops, drinking waters and soils frequently occurs and it contaminates the food chain of cattle and human beings. A realistic evaluation of the risk posed by As depends on accurate determination of As speciation, because its toxicity and mobility varies with oxidation state and chemical environment. The most toxic species are inorganic As (III) and As (V) called respectively arsenite or trivalent arsenic, and arsenate or pentavalent arsenic. For example, in vivo studies indicate that the toxicity of reduced As compounds such as gallium arsenide (and by analogy, elemental arsenic (As) and arsenopyrite (FeAsS)) are significantly lower than inorganic As compounds containing As(III) and As(V), mainly because of their lower solubility. However, upon ingestion reduced arsenic compounds are slowly converted to inorganic As(III) and or As(V). Recently, x-ray Resonant Raman Scattering (RRS) spectroscopy has been successfully employed to determine the oxidation state of metals. This procedure can be attained with conventional XRF spectrometers or with TXRF systems, although the best performance is obtained when synchrotron sources are used. The X-ray resonant Raman scattering is an inelastic process that presents significant differences compared to other scattering interactions between X-rays and atoms; when the energy of the incident photon approaches from below to the absorption edge of the target element, a strong resonant behaviour takes place contributing to increase the RRS cross section. A realistic evaluation of the risk posed by As depends on accurate determination of As speciation, because its toxicity and mobility varies with oxidation state and chemical environment. In this work we use RRS spectroscopy to perform arsenic speciation, showing that resonant Raman scattering spectroscopy can be used to analyse arsenic species. Mineral samples of As in different oxidation states (As(III) and AS(V)), and two biological forms of arsenic (monomethylarsonic acid (MMA(V) and dimethylarsinic acid DMA(V)) were measured in the XRF station of the Brazilian synchrotron. The used method is very simple and reliable. The most important feature of this method relies in the possibility of using the same spectrometer of XRF analysis or TXRF analysis. This work is pioneer in the area and an important work of interpretation is necessary in the future. Nevertheless, this behaviour in the residuals of compounds could be used to identify the oxidation state of the elements under study, offering a possibility of chemical environment determination using RRS spectroscopy.