Triethylamine-assisted Mg (OH)₂ coprecipitation/preconcentration for determination of trace element concentrations in rainwater from Sicily, Italy

Studies on trace elements composition of rainwater are an emerging tool for environmental monitoring of pollution of the atmosphere, soils, and water resources. A magnesium hydroxide (Mg(OH)₂) coprecipitation method for the determination of 17 trace elements (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Mo, Cd, Sn, Sb, Te, Pb, U) in rainwater samples was applied. Rainwater samples were collected through bulk collectors, from March 2021 to November 2021, at 15 sampling sites located in different contexts and divided into two 50 mL aliquots. The first was filtered (0.45 μm) and acidified with ultrapure HNO₃. The second one was filtered with the same filters and then enriched adding 500 μL of MgCl₂x6H₂O and subsequently 200 μL of Triethylamine (an alkylamine, weak base) to the solution; this procedure triggers the precipitation of Mg(OH)₂ scavenging a large suite of trace elements from the rainwater. The solutions were centrifuged at 6000 rpm for 20 minutes, the supernatant was discarded, and the precipitate was dissolved first in 1.5 mL of 7% HNO₃, and then in 3.0 mL of 1% HNO₃. Both aliquots were analysed with ICP-MS (Agilent 7800ce). In the co-precipitated aliquot, fewer samples resulted below the detection limit, especially for Cd (0.18%), Sn (61%), Te (42%), Pb (22%), and U (18%). Important differences in the concentrations (on a median basis) of trace elements were observed between the two aliquots, up to +233% for Pb (0.058 μg/L and 0.135 μg/L for the first and the second aliquot, respectively). For Te, Sn, Co, U, and Cu the recovery by co-precipitation was partial, up to -62% for Te. The magnesium hydroxide coprecipitation method can therefore be a valuable enrichment tool for better determination of trace elements’ concentration in highly diluted matrices such as rainwater.