Desenvolvimento de método para determinação de Al, Ca, Cr, Cu, Fe, K, Mg, Mn, V, Ni e Zn em amostras de lodo de estação de tratamento de água por MIP OES após a digestão ácida com sistema de dedo frio sob refluxo

Abstract

This study presents the method development for sample preparation of Drinking Water Treatment Sludge (DWTS) samples, through decomposition in semi closed digestion system with cold finger for the determination of Al, Ca, Cu, Cr, Fe, K, Mg, Mn, V, Ni and Zn by microwave induced plasma optical emission spectrometry (MIP OES). This system was used to decompose three different DWTS samples, with different characteristics. Sample preparation using the cold-finger reflux system was optimized by using statistical tools, being the compromise conditions sample mass of 250 mg, 5 mL of HNO3, 1 mL of H2SO4, and heat at 225 °C for 150 minutes, using 1% of LaCl3 and 0,1% of CsCl, as ionization buffer solution. To ensure sample mineralization and investigate the concentration of the analytes in the remaining precipitate after acid digestion in semi closed system with cold finger and reflux (a silicate-like solid), the sample was transferred to a polypropylene tube and 1 mL of HF was added to it for an ultrasonic (US) bath digestion step. It was possible to verify the mineralization of the precipitate, although all analytes were extracted during the first step of the digestion procedure, without the US step with HF. The accuracy of the method was ensured through the use of a sediment certified reference material (NIST 1646a), the standard addition technique. Additionally, the availability of these elements in solubilized extracts of the DWTS samples were evaluated, following the norm NBR 10006:2004, to obtain the solubilized solid waste extract, and afterwards the solubilized extracts were digested according to the US EPA 3010a method (acid digestion of aqueous samples and extract for total metals for analysis by FLAA and ICPIX spectroscopy). This experiment showed that the available levels of these elements are higher than the maximum levels allowed for Al, Fe and Mn in the norm ABNT NBR 10004:2004 (Classification of solid residues), in order to classify the DWTS as an non-hazardous and inert waste. Thereafter, the possibility of immobilization of these elements from the liming method was evaluated, widely used to increase pH of agricultural soils, and thus to reduce the availability of these elements. For that, different proportions of DWTS and limestone were used, for 28 days of incubation. The outcome from this experiment was the reduction of the available concentrations of Al, Fe and Mn, although just for one sample, the concentration of Al decreased until the limits of the norm NBR 10004:2004. For all sample the concentration of Fe and Mn did not reach the desirable limits, required by the norm NBR 10004:2004 in order to classify the DWTS as an non-hazardous and inert waste. In addition, the available concentration of these elements in four soils, from the state of Rio Grande do Sul was also evaluated, which were undergone assay for obtaining of solubilized extract of solid waste according to the norm ABNT NBR 10006:2004 and then digested according US EPA 3010a method, and for some of the evaluated soils, the contents of Al, Fe and Mn were above the maximum level of the norm NBR 10004:2004, which classifies these soils as nonhazardous and non-inert, just like the DWTSs. Therefore, for the DWTS usage in agricultural soils, one must first evaluate the soil itself, in order to verify the levels of potentially toxic elements that both, soil and DWTS, might make available to the plants and therefore, find the adequate and safe proportion of the soil of interest and the DWTS.