Utilização de esferas de alginato para extração e pré-concentração de chumbo em amostras de água

Abstract

Lead (Pb) is a toxic element and a potential contaminants in aquatic environments. World Health Organization (WHO) establishes a maximum limit of 10 µg L-1 of Pb in drinking water. The determination of Pb, in trace levels (< 1 mg L-1) using flame atomic absorption spectrometry (FAAS), generally requires a pre-concentration step. Due to this, various procedures of pre-concentration have been studied, mainly focusing on the development of alternative procedures that employ biopolymers. This work proposes the determination of Pb, in trace levels, in sample of river water by FAAS, after extraction procedure and pre-concentration with calcium alginate beads (CAB). Preliminary tests showed the high potential of CAB for extraction and pre-concentration of Pb in water samples. The follow parameters: mass of the beads, time of adsorption and desorption, influence of HCl concentration in desorption, influence of the mass/volume ratio of the beads and sample were optimized. In order to improve the performance of the CAB in the extraction process and pre-concentration of Pb in water, these were submitted to a stage of protonation, where there was an increase of 25% in the efficiency of extraction and pre-concentration. CAB reuse was assessed with extraction and pre-concentration capacity above 98% after 6 cycles. The morphological characterization of the CAB was performed by SEM-EDS while the FTIR was used to investigate the main functional groups involved in the extraction step. Mass of 100 mg of CAB showed the best results with adsorption and desorption times of 90 and 15 minutes, respectively, which are relatively smaller than that found in literature. The selected HCl concentration was 0.8 mol L-1, with a 200/200 mass/volume ratio (m/v). In the optimized conditions, after the protonation procedure the proposed method furnished a detection limit (LOD) of 2 µg L-1 and relative standard deviation (RSD) between 2.9 and 15%. An enrichment factor (EF) of 50 was obtained, demonstrating the efficiency of the procedure for pre-concentration of Pb, the accuracy was evaluated by means of recovery tests, with values obtained for well water (83.7 and 90.3 %) and ultrapure water (117 and 120%). The micrographs revealed that there was no significant difference in CAB before and after protonation, however, morphological changes were observed after the fourth pre-concentration cycle. The spectra obtained by FTIR indicate the presence of carboxylic groups, which may be associated with Pb biosorption. All water samples from the Pitangui and Tibagi River Basins were below the LOD. Thus, was proved that the proposed method is suitable for determining low concentrations of Pb in water samples.