Arsenic speciation in natural geological samples using synchrotron radiation-based X-ray techniques

BIA GONZALO L.; NIEVA NANCY E.; SOTO-RUEDA ELIANA; BORDA LAURA; GARCÍA M. GABRIELA; BORGNINO, LAURA

Campinas

Encuentro; 31th Annual Users Meeting; 2021

31th Annual Users Meeting

Along the Andes and the vast lowlands that extend towards the East, elevated concentrations of arsenic (As) in natural water reservoirs are mostly sourced from the weathering of volcanic materials, and its distribution depends on subsequent (bio)chemical reactions that are controlled by the system`s physicochemical and biological conditions.The Environmental Geochemistry Group in CICTERRA has been carrying out numerous studies aimed at identifying the sources of As and its dynamics in natural systems threatened by this hazardous geogenic contaminant for more than 10 years [1-4] The present review summarizes the results of our studies on As speciation (oxidation state and local chemical coordination) in amorphous and crystalline mineral phases that are responsible for the release and removal of As in natural systems affected by different environmental conditions. The solid speciation of As was determined by synchrotron XAS and µFRX, at the Brazilian Synchrotron Light Laboratory (LNLS Brazil). The analyzed samples are representative of the primary and secondary sources of As in the southern portion of South America and correspond to Patagonian volcanic ashes, metal sulfide mine wastes, volcanic glass-rich loess, and carbonates and borates collected from high altitude lakes in the Puna region.Synchrotron analysis allowed us discriminating three oxidation states of As in the studied samples: As(V) is the dominant oxidation state in secondary sources while in the primary sources, the reduced As(-1) and As(III) species were identified. As(V) is generally present in the form of arsenate ions adsorbed onto ferric (hydr)oxides and included in salts such as calcite, jarosite and ulexite, substituting the carbonate, sulfate, and borate ions respectively. We found that the As(-1) species is only associated with arsenopyrite or arsenical pyrite in all the samples where this species was detected, while As(III) is related to arsenite ions included as impurities within the glass structure.