Distribution of trace elements in filtered and non filtered aqueous fractions: Insights from rivers and streams of Sardinia (Italy)

Abstract

The distribution of several trace elements in different aqueous fractions has been studied in running waters from Sardinia (Italy). Trace elements and major components were determined in water samples collected at high- and low-discharge from rivers (90 samples) and streams (70 samples). At selected sites, total (non filtered samples) and dissolved (0.4 μm and 0.015 μm pore-size filtered samples) amounts of trace elements were determined, and the composition of the solid matter retained on the filters was investigated for estimating the eventual interrelationship. The elements B, Li, Rb, Sr, Ba, As, Sb, Mo, Tl and U in the studied waters showed small differences between total and dissolved amounts; dissolved concentrations were higher under low flow conditions, when the contribution of rainwater to the rivers was minimum; their concentrations were often correlated with total dissolved solids (TDS), and appeared to be related to the intensity of water-rock interaction processes. The elements Al, Fe, Mn, Pb, Zn, Cd, Cu, Co, Ni, Cs, Y, REE and Th were not related to TDS and/or major ions; they showed higher concentrations under high flow conditions; marked differences occurred between total and dissolved amounts; much lower concentrations were generally observed in the water filtered through 0.015 μm than in the water filtered through 0.4 μm, especially when sampling was carried out after heavy rain events that enhanced the load of solid matter in the water. These observations indicate an aqueous transport via sorption processes on very fine particles, such as Fe-oxide/hydroxide and clay mineral particles, which have been inferred by SEM-EDX analyses of the matter retained on the filters. Results on modelled aqueous speciation of trace elements, aimed to assess the role of sorption processes, suggest that: (i) those elements speciated as oxyanions (e.g. As(V), Mo(VI)) or as negatively charged complexes (e.g. UO 2 ( CO 3 ) 3 4 - and UO 2 ( CO 3 ) 2 2 - ) are either poorly or not adsorbed onto oxide/hydroxide minerals under neutral to alkaline conditions as observed in the studied waters; (ii) dissolved concentrations of metals prevalently speciated as free, positively charged ions (e.g. Zn, Cd, Ni) are strongly affected by the amount of Fe-oxide/hydroxide particles retained on 0.015 μm filters; (iii) trace metals occurring as neutral carbonate complexes (e.g. PbCO 3 ° and CuCO 3 ° ) are affected (Pb) or not affected (Cu) depending on the extent of adsorptive affinity of these aqueous species to form strong ternary complexes to oxide/hydroxide surfaces. The results of this study could be used for understanding the aqueous dispersion of trace elements and the seasonal variability in concentrations, which in turn could allow estimation of the risk of exposure to the toxic and harmful elements.