Raman spectroscopy to investigate the speciation and origin of hazardous elements associated to suspended particulate matter during a large flood event

Abstract

Hazardous elements are usually incorporated in natural fresh waters forming inorganic compounds or adsorbed onto organic matter as insoluble compounds in suspended particulate matters (SPMs). Considering the predominant transport of SPMs during heavy rainfalls, storms, or snow melting, such events require deep and exhaustive research to evaluate their environmental risk. The present work applies Raman spectroscopy for the analysis of SPM samples collected during a storm event (collected every 2 hr with an automatic water sampler) in a gauging station of an urban river (Deba River, one of the most polluted rivers in the Basque Country, Spain). In order to obtain revealing molecular information about the hazardous element distribution on the SPM during strong rainfall events micro‐Raman spectroscopy and Raman imaging were combined. The molecular characterization by Raman analysis determined that the main part of the compounds was from natural origin, such as, quartz, silicates, talc, calcite, etc. Contrary to oxides or oxi/hydroxides of lead, zinc, iron, titanium, or manganese, which were, probably, from anthropogenic origin. Those results were in agreement with energy‐dispersive X‐ray fluorescence and X‐ray diffraction analyses on SPM samples. Organic compounds such as calcium oxalate, amorphous carbon and β‐Carotene, and carbonates, such as, calcite, high‐Mg calcite, dolomite, strontianite, and graphite, as crystalline allotrope of carbon, were also found. Finally, Raman imaging analyses provided evidence enough to conclude that in the flood event studied, the SPMs firstly mobilized were rich in compounds with anthropogenic and/or natural origin, and the SPMs mobilized in the second part of the event were characterized by natural compounds coming from the lithogenic components of the area. Therefore, these discoveries support Raman imaging as a useful technique on the future sustainable management framework of the river.

GeoRaman

“…Hazardous elements are typically incorporated in natural aquatic systems, which form minerals that potentially adsorb onto organic matter as suspended particulate material. Gredilla et al [8] used Raman point spectra and imaging to characterize the minerals containing these elements. They showed after a storm event that most minerals were of anthropogenic in origin and the second part of the event mobilized natural components.…”