Dimensional distribution and environmental importance of antimony(III) and antimony(V) in PM

During the last decade there has been a growing attention towards the environmental diffusion of antimony, because of its potentially harmful effects. Antimony concentration has been suffering high increase in environmental matrices and some recent studies show that antimony is now the most present trace element in the urban atmosphere. As with other elements, the Sb oxidation state influences the physiological and toxicological behavior of antimony. Particularly, Sb(III) compounds are about 10 time more toxic than Sb(V) species. All the studies on the separation of the two inorganic antimony forms in soils and waters report the Sb(V) prevalence and suggest the spontaneous oxidation of Sb(III) in these matrices. Identification and quantification of the antimony species in PM samples has been attempted only in a few research works, in which a very low number of real samples were considered. Furthermore, no studies were addressed to the dimensional distribution of the two species and to identification of their emission sources. Our work concerned the optimization and the validation of a quick and sensitive analytical method able to separate and quantify Sb(III) and Sb(V) in particulate matter (PM) by ion chromatography - inductively coupled plasma - mass spectrometry (IC-ICP-MS). The procedure has been applied to PM10 and size segregated samples. Both Sb(III) and Sb(V) species were detected; contrasting with the results from other environmental matrices, in some PM10 samples the Sb(III) concentration resulted higher than the Sb(V) one and ratios Sb(III)/Sb(V) ranged from 0 to 1.5. Furthermore, the spontaneous oxidative conversion between the two species in the atmosphere resulted negligible. The analysis of the size segregated samples, collected by a 13 stages impactor, allowed us to draw some conclusions on the Sb(III) and Sb(V) sources. These two chemical forms are detectable only in particles with aerodynamic diameter higher than ca. 1 µm, that are generally related to mechanical abrasion mechanisms. The most probable source for coarse antimony is related to brake pads abrasion. Preliminary tests on these materials showed a high variability of the Sb(III)/Sb(V) ratio as a function of the blend production and, in some cases, Sb(III) resulted the predominant species. Recovery percentages of the IC-ICP-MS analysis of PM10 samples, calculated with respect to the total Sb content determined on the same solution by direct ICP-MS analysis, showed an high variability, with values ranging from 30% to 70%. A series of experimental tests were addressed to the comprehension of the reason of these low and variable recovery values. All the tests suggested the presence in the analysed solutions of nano-particles aggregates, that are able to reach the plasma torch in the case of direct ICP-MS analysis, but are detained by the chromatographic column when the IC-ICP-MS method is applied. Furthermore, the aggregation state of these nano-particles seems to be easily altered when they are suspended in the water solution. A similar behaviour could be hypothesized when the aggregates are in contact with biological fluids and their health effect would then deserve further investigations.