Distribution of trace elements in sediments from a river of the Midwestern Argentina

SAMANIEGO, N.; FERNÁNDEZ, N.C.; PARDO, R.; VEGA, M.; DEBÁN, L.; CID, F. D.

Santiago de Compostela

Congreso; XX Reunión de la Sociedad Española de Química Analítica; 2015

SEQA

Heavy and trace elements are a very hazardous group of pollutants because of their toxic and accumulative characteristics. They are non-biodegradable and undergo global eco-biological cycles, so their circulation amongst the different environmental compartments is a main concern. River sediments can act as temporary reservoirs from which those elements could be released back, but the true environmental risk must consider the total concentrations as well as their mobility/availability. Chemical fractionation, using single or sequential extractions, is the usual approach to evaluate metal mobility in sediments and several leaching/extraction tests have been developed and applied for these purposes.Single extraction of metals with a strong acid such as concentrated HNO3 (US-EPA 3051 norm) allow a rapid evaluation of the pseudo-total content, which is an estimation of the total available metals. On the other hand, sequential extraction procedures provide more information about the different metal-solid phase associations. The BCR sequential procedure, now SM&TP, is the standard approach for these studies and uses three reagents: acetic acid, hydroxylamine hydrochloride and hydrogen peroxide, yielding three fractions of decreasing mobility/availability associated with metals bonded to (i) exchangeable and carbonate phases, (ii) iron/manganese oxide and hydroxide phases, and (iii) organic and sulphide phases. A fourth fraction, labeled Residual, can be calculated by difference from the ?pseudo-total? contents or by digestion of the last residue. The total concentrations can be determined after total digestion of the sediment, or by a non-destructive technique such as X-Ray Fluorescence (XRF).The Río Quinto (Argentina), an endorheic fluvial system is the main source of superficial water in the province of San Luis and its sediments have not be studied so far with regards to toxic metals. Sediment samples were collected at 10 different locations and characterized by X-Ray Difractometry (XRD) to ensure their mineral homogeneity. The total contents of As, Cd, Cr, Cu, Ni, Pb and Zn in the sediment samples were determined by XRF, whereas the pseudo-total ones were measured by ICP-OES in the extracts obtained after application of US-EPA 3051 norm. Chemical fractionation was carried out by means of the BCR SMT&P sequential extractions procedure, determining the metal contents in each fraction by ICP-MS. All the procedures were validated with the appropriated Certified Reference Materials. Pseudo-total contents (around 10 mg/kg for Cr and Zn and 1 mg/kg for the other elements) are below the reference values for environmental risk. The chemical fractionation results showed that the metals were mainly present in the Residual fraction. Results have been interpreted by multivariate statistical techniques: Hierarchical Clustering and normal and n-way Principal Component Analysis (PCA). In general, As, Pb and Zn showed a similar behavior, but different from that of Cd, Cr, Cu and Ni. It was also shown that Cu tends to concentrate in Fraction 3, while As and Pb tend to do it in Fraction 2. The best n-PCA model was the Tucker3 [2 2 1] with two significant interactions or factors, explaining the 66,1% of the information and confirming the above considerations.Therefore, it can be concluded that the studied toxic metals present in the sediments of the Río Quinto do not pose any environmental risk.