Relevance of Sb(III), Sb(V), and Sb-containing nano-particles in urban atmospheric particulate matter

A quick and reliable analytical method for the separation and quantification of extractable Sb(III) and Sb (V) in atmospheric particulate matter (PM) by ion chromatography( IC)-inductively coupled plasma-mass spectrometry (ICP-MS) has been optimized, validated on pairs of real, equivalent PM10 samples and applied to a field monitoring campaign in a urban site. Both Sb(III) and Sb(V) forms were detected in real samples with Sb(III)/Sb(V) ratios up to 1.5. These two Sb species accounts only for a portion, of variable magnitude, of the total extractable Sb (10-70%); anyway, no other soluble Sb species were detected in the samples. The analysis of size-segregated samples collected by a 13-stage impactor showed that the recovery of [Sb(III) + Sb(V)] versus total extractable Sb is almost quantitative in the coarse fraction while it is below than 10% in the fine fraction. In the extracted solution from particles below 1 mu m we could highlight the presence of Sb-containing suspended solid nano-particles, which probably constitute the missing fraction. The contribution of nano-particles can be estimated as the difference between ICP-MS and IC-ICP-MS data, as small size solid bodies are able to pass through the nebulizer and reach the plasma torch, while they are retained by the chromatographic column. The aggregation state of these nano-particles seems to be easily altered when they are suspended in a water solution; a similar behavior could be hypothesized when in contact with biological fluids. It has been confirmed that brake pad abrasion is the prevalent source of Sb(III) in PM and that Sb(V) may be formed by oxidation during the braking processes. Differing from other environmental matrices, there is no evidence of any spontaneous oxidative conversion within the two species.